Eg ON ts ee ee Ae ST ana a ote oo 
 at a ae fe re Fate 28 Beings >See ai 
 Big a re vans ias 
 et AOE eee 
 sea ee ee 
 
 
 
 = wes mi 
 op aS se eS aT SRT ee pom ne 
 ois aan = i ie 
 ~ See ata ene - 
 et er 2 poe axe 
 f Tp) ae SS See 
 
 
 
 
 
 
 
 
 
 an ee 
 
 = 
 ee 
 pte 
 
 
 
 WahA 
 = 
 " 
 prette POSE Ca Wet 
 > wat. we tatet Aes ate Cee 
 
 ANT 
 My . 
 4 
 
 i} 
 
 HRMS 
 Abr eats 
 
 fist why a 
 
 ee ee 
 se Se ae na ae a ge 
 es = | = 
 
 
 
 . 
 = 
 
 ry 
 
 ives yi 
 HANDS FAIL 
 a pecapinny sea r Seer rer arte it er, 
 See : : ms , Eo acre Sa me by 
 YAM REAR REGEN MA TES BRO ARAL TE hPa NS } 
 
 TRAP ON ASCH WAS AAL 
 
 
 
 ( Be 
 « 
 ‘= 
 i # 
 fe Le 
 oT iz 
 =< AS 
 = oe 
 es 1s 
 as td 5 
 ~~ 
 ee . ~ 
 Fe ix : .z 
 < 
 Ne ee y + 
 = 4 ~ 
 —s = See: —— Sn = Ol re ee P| 
 eo ed = 2 TE ng ney PO Fe Tg Oe See Pe a 
 . < SNe a xp se 38 a ee ee ae > at — ea 
 Pas ” * = “ So ee _—— (a ees ee ng ORE a a ae agreeing ene ee ee a = eta ee ge a 
 a ae —— I ee Oe ae enon ea et I wl ee te See eagle a ad 
 ae j . pinata A a ao ee a - ~ re ce ape ee Se ne na ae Fa Gp hans en ne rn ag ee ee a ae - 
 Pees Sy Ee atin” Is - pte Bee “ “ 2 om nat A i Soca OT EY oa Pn af ee ig ae emer, on eaten of Se re SS ee Sa Te Pe — ee ge a apie 3 
 OS ee . Ss ang hE Bl ng aS aah eg Oe ea aE, fe ee a GE ag ‘ os 5 gta a - Oe OS TE ag A peta aN, Np rte aia ee apermPue nie ere neat ee ey 
 “~ 2 oe ee i a ng a AED pl nag IAI BO ae z ~ eae ee PO pe ae SO pee a ee en, pate nnn ng 
 PO ee cima Se OI IPT Ni ag og BET ie: a i Se oe pee ig nag Oe nea nn Sa Poa ge a PM pe. 
 SS 3 SS eee : SIF 
 na Pe at oe eo — << = ~ ’ a > —_ ‘-= = - OS oF a ee at eal ag = . 
 we, Oe, 2p age A ta Sr Se ae gag . ~ - el P = w ee ae oP ne Sera Sa ga a ap ae eee or —- = ER ana © 
 Sy a esa asm i gee age Ss - Falta “ ng . x 7" ne PN nel OP a ae a an ee oe Pe am =e —_—~ — 
 a. wees Seat Se a ee age 7 SS Sz 3 , gen ee ee Se oe era ee OS eee ete = 
 ore aes np ora aie ae I a foe By Ae 2 J ae ‘ : ; os A I eg a ee gl a a ln a Oa es —— men 
 ste PI wana we Pe TORS Se a nO tae Nets ee mah a cites ee of an al na ees See eo Te 1 —— eS eR ae et ee ee SS SS <i 
 ee ~ being .- <— ete, Ona - aed O ~ be « ae es —_ = = PO - aoe en. = = > —_ >. eS Set seal oan me —, = Pd <_ A et ~ 
 Git FS SO PO AE Ow a PPM a no eat - ——— ee an Se gat ee Pe ae SED = a Na IP “~ > 
 iene wa Gt pn ag ee em SO ee era Co OOK. x . Ra ne ga eee et SE te a Se ee ae ee 
 Es Re Ea a a eg mae a ann 5 ene ee a Sa ee ea ae a eae epee Sea ON eg aa agen nae pe ME ee oe tmee: 303 
 rae SS Se SRE LET IO atl ah age i eee ee ean gg gg A tS eS Sc me SE S 
 i ee Fa Fe es ee pe oie Oe IT So ag Se EE ge Cae Sn eI ren ae ae Sh — 
 os Me, Meee tna oe -—_ Fe Pane Sse iow rea See ae as eS Any rat i cata aie Rea te teen On sie - ae = pS es 3 = 
 SS ee ap ein pore ee a ee oo on i 3 Se cong nn en OE gC Ee og eS Og ee 
 = RaaP aya Fe < nat in * > = ee oe a Seated ae : eweera heey in ae Ss - % - =~ > 
 nos Lea a at er ga a ne ee Ee ee ke ern ee Fa an re a a I Nees a 
 na | oe ee — - ae aS ame 4 — “tgp pe orig " - ee oneal Se pe ee a me, ace ee a ans “ sO genn ye 
 ay Seas: FeO ae SITES yg ole eg Oe re eg at erage eet SE PR pr ee Rn at ae en i =, 
 one a ol oo - sat Ce Se TF aes gt le cat a ae ON aie ye gio higcele ve ee eae ip = Se a lp ea Se ath ee aaa i re Tea 
 a oe Se a ee eal i a Le perpen a eS at eee age ee a we RN a ad eS re Sparano Nae ee ee 
 SA rn at Gg Sa a St eee 
 or ea at al lS an Pree oe ant a a ey es th aS tte tig es ap a Ng lng a gr ae a at tt Prey, apt eh 
 Sak.” dae 3 re Ae Aa bn ne ee eo 4 SS Pe FSi ee ae a eS a eer rence soe Pe en ~ 
 =< ate Ten ag a a ee cB ELT mia a gO — Sars Ore I ein Pa nee ee renee toon ee ate oes cp aga —* agate, Ta 
 = OT a ne Pa te nip Tg aD A cat ae Ee OR eae CFR een A gyal pgg ~o SS SNE ig ae a gee Se ee pa nn ss « 
 mat TE eT er ga SF a ROO Rape ll ON ag atin. . > neo Senators Pe Se SP ee Se =e 
 na 5 a nn a te ee cane Se ge nh iat need ne enh Np = ane Pps OES a" get a TE i oer gn, a a ns 
 tS Se ee ee om Se a oP pee a Oe See ee 
 Ee eee ig a eaeateed Pa Sp eae a I Fo a ah wenn hg” ee a cane eS eR aE ee rae St aS ee Se a a rc RS ra 
 a2 3 a a ee a a np pg SI Sp ae ge ge 
 SS ae a ee See eae or en a ae a eee as IE Se NOES TO ice a ete Ree ee ee 
 er gS oR a Oe oat eg A cr a age ae aa ee en ar ie out . . —— a aan ad eee a ee ~ fo ag oa oe a 
 ral set Na reine ste a Rhee, case ne OO wi eset NES Se an, Se oes paar ee i Oe aap eee Cage ea ee ee Se 
 ow on ; nee cst ona Se ea ae aioe on Sap a ~ Sere : a 4 Se ee Teele ewe a he — 
 rat nk Se PS ae i wg Sh ee Pep NE ore oe Oat Sees IN pen We te SR ae oe ee ES, ee an ae ee oe Sic a ee eS me 
 aa oe * i ye ie “ ~ ~ _- “ss a = “ a ama “a wena |=. = < ee ant waste Fg eae OS ey a a = Le 
 OE eS PP Dp a ag Aa Not a a8 vi . - <= Se ee Steak eee ee ae i PEO oe. a ne 
 ae ate tO ae in AO, Se ag er oe ASS ae panties, Sa ee ee ee Spe ag a ne a a an 
 Sy 2 ee es ee eng pe gpl er apt aN eI ng a TT Sree a le ey ee 
 ro renee gr fg aes a 5 gt ale as ae iy teen A ee ee semaet SS en ie — Nee Suir An esa oe i ee a ae ee ng ete hg pene ee ee ae 
 Bie, —— Fo pty 2 gt OS 5 net a, a oP runs —_ ma! - ee AS ee een el = et a ~ “~ 
 wa a Ole ro aia A Se SNaR mel agetae a a le on a Ww ee a tl a <r a ese <——- 
 tte oe et ‘ol Se eee ae . ee es ote oo — eee ~ sae ome = SS eS . ~~ 
 A a ne ee ee a SOR RS SS 
 - hes sai RE Sem ea oe ee ae al : ~ “ an a= = ae ve “. Ve - - ke es eter oe ee ee 
 Taree — ee ere <i re alin pee ae ee oo a y ae; S s . “ = Se) Ae Se ee Sane varie: SKS | «a eS 
 — OS Se Pt i ge aa ne “ angel m « oe Lin, Nam ed hig a ea my 
 —— > Oe gt ages Ee a Pag > ie ries s ; +! 
 r a i ate A Ee aS ~ eee = re Pens i & - . IT Ren 4 eee TG, Seg “OR 
 so ee ee pa eae a Ne ion: 4 e ay | Poe MN a ee eee Ce YO I eRe er . 
 —_ TOE LE IE PN 1 le te Ama - a Aa 8 Se PMS La > cer 
 ~ en a gl ON ta nay : 
 “. ape nag t nell pecan tt me yg Sn ‘ a ey ah rrn a cae na af 
 an ee - wite Keeedh oe ee $2 aa Cae a a: > 
 ee Seen et ae yee - . ‘ 
 Saee | 7 , 7 In sre pe eee ee wen , = 
 v= 
 
 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Cia 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 THE GETTY CENTERLIBRARY 
 
vanes Reha! see 
 i ote: e. “ i 
 
 
 

 
CELLULOSE ESTER VARNISHES 
 
OIL & COLOUR CHEMISTRY MONOGRAPHS 
 Edited by R. S. Morrell, M.A., Ph.D., F.C. 
 
 UNIFORM WITH THIS VOLUME 
 
 BLACKS AND PITCHES 
 
 By H. M. LANGTON, M.A., B.Sc., A.L.C. (Messrs. 
 ¥. B. Walker €3 Co., Ltd.). 
 
 THE CHEMISTRY OF DRYING OILS 
 By R. 8. MORRELL, M.A., Ph.D., F.LC. (Messrs. 
 Mander Bros.), and H. R. WOOD (Messrs. Storey, 
 Smithson &F Co., Lid.). 
 
 IN PREPARATION 
 
 THE CHEMISTRY AND MANUFACTURE OF 
 PIGMENTS AND PAINTS 
 
 2vols. By C. A. KLEIN, M.Sc. (Brimsdown Lead Com- 
 pany), and W. G. ASTON (Messrs. W. Symonds & Co., 
 Lid.). 
 
 THE PROBLEMS OF PAINT AND VARNISH 
 FILMS 
 By H. H. MORGAN, Ph.D., B.Sc. (Messrs. Naylor Bros., 
 Slough). 
 VARNISH ‘THINNERS 
 By NOEL HEATON, B.Sc. (Messrs. R. W. Greef &F Co.). 
 
 THE ANALYSIS OF PIGMENTS, PAINTS 
 
 AND VARNISHES 7 ; 
 By J. J. FOX, O.B.E., F.I.C., and T. H. BOWLES, 
 Fac 
 
 RESINS: NATURAL AND SYNTHETIC 
 By T. HEDLEY BARRY (Editor, Fournal of the Oil and 
 Colour Chemists’ Association), and R. S$. MORRELL, 
 MMA PLD, FAG 
 
OIL & COLOUR CHEMISTRY MONOGRAPHS 
 Edited by R. S. Morrell, M.A. Ph.D., FIC. 
 
 CELLULOSE ESTER 
 VARNISHES 
 
 BY 
 
 F. SPROXTON, B.Sc., FLL. 
 (British Xylonite Company, Ltd.) 
 
 NEW YORK 
 D. VAN NOSTRAND COMPANY 
 EIGHT WARREN STREET 
 1925 
 

 

 
 
 
 
 Adar 
 
 ish Siann DMS. : 
 
 \ 
 
~ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ‘ rd i 
 rs ee) 
 » ¥ 
 ‘ 
 ; y) 
 / > a 
 - ‘ 5 
 : ye 
 r L 
 « 
 fe 4 
 ‘a! : . 
 » > i} ’ 
 ay 
 Sy 
 t 
 pit ‘ 
 1 
 ‘ 
 é - aa 
 ? - a, 9 
 ‘ + 
 ~' 
 r 
 “ 
 q ~ —_ 
 3 \ 
 ~s 
 : “2 
 ™ 
 . 
 yam hr 
 : , 4 7 
 \ 
 :* 
 hy * 
 ¥ ‘ *» 
 , s » 
 “ 
 ¥ . 
 a ive) 
 1 
 j 
 —_— 
 wy 
 Py 3 : . 
 . 
 Lio 
 rs “ 
 
 
 
AUTHOR’S PREFACE 
 
 THE primary purpose of a preface is to disarm the critic, but the 
 author of a book is usually so keenly aware of its shortcomings 
 that only the major criticisms can be anticipated before it passes 
 into the hands of readers. Worden’s immense volumes contain 
 such a complete account of the facts known about the derivatives 
 of cellulose, that an author writing on the same subject, or any part 
 of it, must be to some degree a plagiarist. There seems to be room, 
 however, for a small book on the cellulose ester varnishes which, 
 while not dealing exhaustively with any part of the subject, may 
 nevertheless be individual enough in outlook to be something more 
 than a guide to the larger reference books. I have thought it 
 possible that such a book might be useful to the manufacturer 
 of the varnishes, particularly to chemists entering the laboratories 
 of such manufacturers; to the user of the varnishes, who may be 
 interested to know more of the nature of, and the problems asso- 
 ciated with, the materials with which he works; and, lastly, to the 
 student, who may obtain a bird’s-eye view of a growing industry 
 based on the principles of colloid seine and who may, perhaps, 
 find some suggestions for research. 
 
 Many references will be found to the excellent specifications 
 for aircraft materials issued by the British Engineering Standards 
 Association, which should be in the hands of all manufacturers of 
 these varnishes. A few minor criticisms of detail will be found in 
 the text of the book. The only general criticism I wish to make 
 is that where the information in the specifications is derived from 
 chemical literature, the principal references might be quoted. 
 This practice would frequently assist the chemist who applies the 
 specifications to materials in the laboratory; it would also ease the 
 conscience of an author anxious not to infringe copyright. 
 
 Patent literature has been scantily used, and then only faute 
 de mieux. <A perusal of the patent literature of the cellulose esters 
 generates more heat than light, and it is frequently difficult to separ- 
 ate the wheat from the chaff. Wherever possible, the attention 
 of the reader has been drawn to processes which have actually 
 been employed on a large scale. It will be evident from the list 
 of references that no single journal ministers to the entire needs 
 of theindustry. The excellent abstracts of the Journal of the Society 
 of Chemical Industry, Chemistry and Industry, will keep the reader in 
 touch with all new developments, but the original papers must often 
 be consulted. Another useful English publication is the Journal 
 of the Oil and Colour Chemists’ Association. Industrial develop- 
 ments can be followed in Kunststoffe, and scientific developments 
 
 Vil 
 
vill Author’s Preface 
 
 in colloid chemistry in the Kolloid Zeitschrift. Useful information 
 often appears in Le Caoutchouc et La Gutta Percha, but is sometimes 
 not original. 
 
 Nomenclature is a difficult problem in a scientific book written 
 on a technical subject. The word “ nitro-cellulose,’ for example, 
 is scientifically incorrect ; yet it is firmly established in the industry 
 and it is accepted by the learned Society which at one time used to 
 alter the substantive “‘ smell ”’ to “‘ odour,” and presumably thought 
 the patriarch’s description of Esau indelicate. ‘‘ Nitro-cotton ” 
 is scientifically worse, but actually less objectionable (if applied 
 to nitrated cotton cellulose) since it makes no pretence of scientific 
 derivation. On the whole, the best plan seems to be to use the terms 
 ‘cellulose acetate’ and “‘ cellulose nitrate ’’ where scientific com- 
 parison or contrast is drawn, and elsewhere to use the customary 
 names indiscriminately. Similar considerations apply to some of 
 the solvents. 
 
 My thanks are due to the directors of the British Xylonite 
 Company, Ltd., for permission to write this book; to my colleague, 
 Mr. A. M. Hutchison, B.Sc., who has read the manuscript and made 
 many valuable suggestions; and to the following bodies for per- 
 mission to reproduce, in some instances with slight modifications, 
 drawings which have appeared in their publications :— 
 
 The Council of the Chemical Society for Figs. 3 and 4. 
 
 The Council of the Society of Chemical Industry for Fig. 2. 
 
 The Council of the Faraday Society for Fig. 8. 
 
 The Aeronautical Research Committee for Figs. 5 and 6. 
 
 The Council of the Franklin Institute for Fig. 7. 
 
 ¥.. 8, 
 
 BRANTHAM WORKS, 
 Nr. MANNINGTREE, 
 Essex 
 
CONTENTS 
 
 PAGE 
 
 PREFACE : r : : : ; ; e j Vii 
 
 CHAPTER I 
 
 INTRODUCTION : : : : pe? 1S 
 
 Cellulose Nitrate and Acetate—Solubility in Volatile Solvents— 
 Polymerised Structure—Nomenclature of the Solutions—History of 
 Cellulose Nitrate—Schonbein, Hadow, Scott Archer, Alexander Parkes 
 —Stevens and Amyl Acetate—Search for Technical Solvents—Com- 
 mercial Developments—Economic Considerations—History of Cellulose 
 Acetate—Schiitzenberger to Cross and Bevan—Miles’s Process of 
 Partial Hydration—War Expansion—Aeroplane Dopes—Shortage of 
 Solvents—Post-war Developments. 
 
 CHAPTER II 
 
 CELLULOSE : ; ; j : : : . : ee 2 
 
 Cellulose—Elementary Formula—Cotton Cellulose—Sources—Chemical 
 Reactions—Chemical Constitution—Formula of Irvine and Hirst— 
 Views of other Investigators of Cellulose—X-Ray Evidence—Viscosity of 
 Cellulose and its Esters in Solution—Significance of High Viscosity. 
 
 CHAPTER III 
 
 NITRATION OF CELLULOSE , , .. 80 
 
 Reaction between Cellulose and Mixed Acids—Specification of Acids— 
 Acid Balance—Conditions of Nitration influencing Solubility—Ratio of 
 Acid to Cellulose—Composition of Acid Bath—Time and Temperature— 
 Preliminary Treatment of Cotton—Necessity for Strict Control—Varieties 
 of Cotton Used—Cotton Specifications—Degree of Nitration—Nomencla- 
 ture of Cellulose Nitrates—Nitration Plant—Direct Dipping—Centrifugal 
 Nitration—Displacement Process—Nitration of Linters in United States 
 —Removal of Acid—Boiling—Poaching—Bleaching—Drying of Nitro- 
 cellulose—Dehydration of Nitrocellulose—Properties and Specifications 
 of Nitrocellulose. 
 
 CHAPTER IV 
 
 ACETYLATION OF CELLULOSE . ; ; : eee 2: 
 
 Reaction between Cellulose and Acetylating Bath—Differences from 
 Nitration Process—Catalysts—Non-inflammable Cinema Films—Low 
 Viscosity of Acetates—War Research on Dopes—Principal Manufac- 
 turers and Outline of their Patented Processes—Investigations on 
 Partial Hydration—Acetylation Processes in which the Acetate does 
 not Dissolve—Acetylation Plant—Properties and Specification of 
 Cellulose Acetate. . 
 
 CHAPTER V 
 
 CELLULOSE EstTER SOLUTIONS. SomE Properties. Part I. 358 
 
 Phenomena accompanying Dissolution — Viscosity — Movement of 
 Liquids in Capillary Tubes—Falling Sphere Viscometer—Dispersion 
 —Solubility Limit—Solvent Power—Temperature Effect—Constitution 
 of the Esters—Disintegration of Structure during Esterification— 
 Fractional Precipitation of Solutions—Dimensions of Cellulose Hydro- 
 lysed by Acids—Contrast with Alkaline Treatment—Chemical Con- 
 stitution of Solvents of Cellulose Esters—Chiefly Carbonyl Derivatives— 
 Cellulose Nitrate in Single Solvents—Baker’s Researches on Concentra- 
 tion and Viscosity—Later Researches—Mixed Solvents—Acetone and 
 Water—Ether and Alcohol—Cellulose Acetate in Binary Mixtures 
 containing Acetone — Mardles’ Researches — Association and De- 
 a of the Solvents—Plastic Flow (Bingham)—The Tyndall 
 ect. J 
 
 ix 
 
x Contents 
 
 CHAPTER VI PAGE 
 
 CELLULOSE Ester SoOLuTIONS. Some Properties. Part II. 84 
 
 Swelling—Researches of Knoevenagel and his collaborators—Distribu- 
 tion of Solvent between Swollen Ester and Liquid—Volume Change on 
 Dissolution—Dielectric Capacity—Discussion of Evidence on Constitution 
 of Cellulose Ester Solutions. 
 
 CHAPTER VII 
 
 INGREDIENTS OF CELLULOSE ESTER VARNISHES ; : ; 93 
 
 Inflammability of Solvents—Inflammability of Coatings—Choice of 
 Solvents and Diluents—Application of Laboratory Results—Variation 
 in the Esters—Commercial Solvents of Esters—Approximate Classifica- 
 tion—Specifications for Acetone, Methyl Ethyl Ketone, Methyl Acetone, 
 Ethyl Alcohol, Methyl Alcohol and Wood Spirit, Ether, Ethyl Acetate, 
 Butyl Acetate, Amyl Acetate, Ethyl Lactate, Acetone Oil, Diacetone 
 Alcohol, Tetrachloroethane, Benzene, Toluene, Butyl Alcohol, Amyl 
 Alcohol, Triacetin, Benzyl Alcohol, Triphenyl Phosphate, Castor Oil— 
 ti nat Et a Mastic, Copal, Sandarac, Dammar—Typical Resin 
 Formule. 
 
 CHAPTER VIII 
 
 MANUFACTURE AND APPLICATION ‘ : : / : « its 
 
 Manufacture of the Varnishes—Mixers—Measurement of Ingredients— 
 Addition of Solvents to Esters—Incorporation of Pigments—Clarification 
 and Filtration—Aeroplane Dopes—Effect on Strength of Fabric—Composi- 
 tion of Dopes—Doping Schemes—Method of Application—Discussion of 
 Typical Dopes—Differences in Allied Practices—Protection from Sun- 
 light—Metal Coatings—Application by Brushing, Dipping and Spray- 
 ing — Transparent Lacquers — Enamels — Aluminium and _ Bronze 
 Mediums—Motor-car Enamels—Imitation Leather—Leather Dressing— 
 Patent Kid— Patent Leathers—Treatment of Motor-car Upholstery 
 Leather—Coatings for Wood, Paper—Preservation of Antiques—Mis- 
 cellaneous Applications—Collodion. 
 
 CHAPTER IX 
 
 MISCELLANEOUS E : d ; ‘ ? ‘ : . sys 
 
 Precautions Necessary in using Cellulose Ester Varnishes—Safety Pre- 
 cautions—Cause of Blooming—Humidity of Atmosphere—Measurement 
 of Humidity—Effect of Raising or Lowering Temperature—Settling of 
 Pigments—Cleanliness of the Work—Evaporation Losses—Analysis of 
 Cellulose Nitrate Solutions; Odour, Total Solids, Nitrogen Determina- 
 tions—Precipitation of Nitrocellulose with Fusel Oil (Lorenz)—Precipi- 
 tation of Nitrocellulose with Chloroform (Conley)—Precipitation of Nitro- 
 cellulose with Aqueous Electrolytes—Identification of Solvents— 
 Viscosity—Analysis of Cellulose Acetate Varnishes—Solvent Recovery— 
 Scientific Application of Cellulose Ester Solutions—Collodion Membranes 
 —Interference Colours of Thin Films—Dimensions of Thinnest Obtain- 
 able Films—Density of Thin Films—Chromatic Emulsions—Transport of 
 Cellulose Ester Solutions on British Railways. 
 
 INDEX . f : i : ; : : 5 ; Pa by 
 
LIST OF DIAGRAMS 
 
 FIG. PAGE 
 1. ILLUSTRATIONS OF SHEARING STRAIN . ; ; ‘ 59 
 
 2. RELATIONS BETWEEN VISCOSITY, SOLVENT PowEeR NUMBER 
 AND COMPOSITION, IN BINARY SOLVENT MIXTURES 
 
 (Mardles) .. ; : een 
 8. RELATION BETWEEN VISCOSITY AND CONCENTRATION OF 
 SOLUTIONS OF CELLULOSE NITRATE (Baker) . , meer aS 
 
 4. RELATION BETWEEN VISCOSITY AND WATER CONTENT IN 
 ACETONE SOLUTIONS OF CELLULOSE NITRATE (Masson 
 and McCall) ; ; : ; ; : ie are 
 
 5. RELATION BETWEEN ViscosIry AND WATER CONTENT IN 
 ACETONE SOLUTIONS OF CELLULOSE ACETATE (Barr and 
 Bircumshaw) . : : ; ; ‘ : pga a 
 
 6. RELATION BETWEEN VISCOSITY AND BENZENE CONTENT IN 
 ACETONE SOLUTIONS OF CELLULOSE ACETATE (Barr and 
 Bircumshaw) ; : : ‘ ; Peehy 
 
 7. (a) RELATION BETWEEN PRESSURE AND OUTFLOW, AND (b) 
 BETWEEN YIELD VALUE AND TEMPERATURE, OF A 
 SOLUTION OF CELLULOSE NITRATE (Bingham and Hyden) 80 
 
 8. RELATION BETWEEN TYNDALL NUMBER AND CONCENTRATION 
 OF A SOLUTION OF CELLULOSE ACETATE (Mardles) eee 
 
 xi 
 

 
CELLULOSE ESTER VARNISHES 
 
 CHAPTER I 
 
 INTRODUCTION 
 
 Cellulose Nitrate and Acetate—Solubility in Volatile Solvents—Polymerised 
 Structure—Nomenclature of the Solutions—History of Cellulose Nitrate 
 —Schénbein, Hadow, Scott Archer, Alexander Parkes—Stevens and 
 Amyl Acetate—Search for Technical Solvents—Commercial Develop- 
 ments—Economic Considerations—History of Cellulose Acetate— 
 Schiitzenberger to Cross and Bevan—Miles’s Process of Partial Hydra- 
 tion—War Expansion—Aeroplane Dopes—Shortage of Solvents—Post- 
 war Developments. 
 
 CELLULOSE is familiar to everyone in various forms, such as cotton 
 wool, cotton thread, calico, and linen. The paper on which most 
 newspapers are printed contains wood pulp, an impure form of 
 cellulose. Cellulose itself is not easily dissolved, and its solutions 
 are not suitable for use as varnishes. Under certain conditions, 
 however, it may be made to combine with acids, and the compounds 
 formed are termed “ cellulose esters.”’ 
 
 Only two esters of cellulose are of any importance in the varnish 
 industry, namely, the nitrate and the acetate, formed by the com- 
 bination of cellulose with nitric acid and acetic acid respectively. 
 These substances are soluble in certain mixtures of organic liquids 
 which have a comparatively high rate of evaporation, and when 
 the solutions are spread on solid surfaces they rapidly become dry, 
 leaving the cellulose ester in the form of a continuous film. By 
 making certain additions to the original solutions, the hardness 
 and adhesiveness of the films may be modified. Owing to the 
 rapidity with which the solutions dry, they may be applied at ordinary 
 temperatures, although a warm room is advantageous. As a rule 
 a coating is dry in a few hours. 
 
 All organic protective coatings consist of highly polymerised 
 structures. The chief distinction between the cellulose ester var- 
 nishes and the oil varnishes is that, in the former, this polymerised 
 structure already exists, whereas the polymerisation of oil varnishes 
 takes place to a large extent after application. The degree to which 
 the original cellulose is de-polymerised or degraded in structure 
 before and during the manufacture of the ester determines to a 
 considerable extent the properties of the resulting varnish. 
 
 Properties of Cellulose Ester Varnishes. 
 
 The cellulose ester varnishes occur in commerce as more or less 
 viscous solutions, which may be either transparent or opaque. 
 
 The transparent varnishes may be coloured in practically any tint, 
 13 
 
14 Cellulose Ester Varnishes 
 
 so as to give a coloured transparent coating.» These are usually 
 termed lacquers. The opaque varnishes may also be obtained 
 in any colour and are usually termed enamels. As a rule they are 
 made so as to dry with a bright surface, but the enamels are some- 
 times required to dry with a dull or matt surface. It is becoming 
 customary to include all solutions of cellulose esters used for pro- 
 tective or decorative coatings under the general term of “ cellulose 
 varnishes.””’ The name is not a happy one, as the materials do 
 not resemble the oil varnishes very closely, and they are not solutions 
 of cellulose. To those accustomed to scientific terminology, “‘ cellu- 
 lose ester varnish”’ and “cellulose ester solution ’’ would bear 
 somewhat similar meanings, but “ cellulose solution ’’ would denote 
 something very different. However, it is difficult to dislodge a 
 trade name, which in this instance probably dates back to the 
 discovery of “‘ cellulose ”’ by the public in 1916. 
 
 As all these varnishes are made with volatile organic solvents, 
 they have characteristic smells which disappear when the coating 
 dries. They may be applied by brush, but better results are obtained 
 by dipping or by use of an air-spray. Under proper conditions, 
 the solutions dry with a high polish, and the films resist water, 
 petrol, soap, turpentine, weak acids and weak alkalies. 
 
 Cellulose Nitrate Solutions—Historical. 
 
 Although. cellulose has been known from time immemorial, 
 the history of cellulose nitrate solutions scarcely begins until the 
 latter half of the nineteenth century. Pelouze,! in 1838, investi- 
 gated the action of nitric acid on paper, linen, and cotton wool, but 
 the importance of the products was first recognised by Schénbein,? 
 who in 1845—6 prepared nitrates of cotton cellulose by a process 
 which he kept secret. Otto? discovered and published the method 
 of production a little later, and Knop 4 is said to have been the first 
 to make use of a mixture of sulphuric and nitric acids, thereby 
 laying the foundation of the modern cellulose nitrate industry. 
 The papers in which these early discoveries are described are avail- 
 able in an English translation by MacDonald.’ Schénbein and his 
 collaborator Béttger, although they observed the solubility of their 
 product in various organic liquids such as ethyl acetate, were chiefly 
 interested in it as a substitute for gunpowder. An interesting 
 review of the current state of knowledge of cellulose nitrate is 
 given in Gmelin’s “ Handbook of Chemistry ” (1872 edition),® 
 although no notice is taken of such technical developments as those 
 
Introduction 15 
 
 of Parkes (v. infra). It was known by then that the properties 
 of the product varied largely according to the composition of the 
 acid mixture, the time and the temperature of nitration. Directions 
 are given for the preparation of (a) an explosive pyroxylin, (b) a 
 collodion-wool soluble in ether-alcohol, and for the preliminary 
 treatment of the cotton with boiling soda solution and bleach. 
 Various analyses of the product, by Schénbein, Crum, Pelouze, 
 Gladstone and others, are given, showing percentages of nitrogen 
 ranging from 9-3 to 14:26, while among the solvents quoted are 
 ether—alcohol, ether alone, alcohol, ethyl acetate, glacial acetic acid, 
 acetone, and wood spirit. The first important researches on the 
 solubility of cellulose nitrate were carried out by Domonte and 
 Ménard ? (1846), who investigated the difference between ether- 
 soluble and ether-insoluble products. Hartig § in 1847 stated that 
 cellulose nitrate formed dark brown solutions with common camphor, 
 oils, waxes and resins. This appears to be the first occasion on 
 which cellulose nitrate and camphor were brought together, but his 
 observations apply to heated mixtures and not to any product 
 resembling either a modern celluloid varnish or solid celluloid. 
 
 The researches of Hadow ® in 1855 on the relation between the 
 composition of the acid mixture and the properties of the cellulose 
 nitrate produced are of interest as the first of a long series of similar 
 experiments by various chemists. The value of the observations 
 recorded is in many cases discounted by the fact that the cellulose 
 which formed the starting point is not rigidly defined. This aspect 
 of the nitration of cellulose will be briefly developed later in the book. 
 
 The first attempts to find practical applications for solutions 
 of cellulose nitrate were made in 1848 by Maynard ?° and Bigelow,}! 
 who suggested independently the use of a solution of cellulose nitrate 
 in ether—alcohol (collodion) as a protective dressing for wounds. 
 F. Scott Archer 12 in 1851 was the first successfully to use collodion 
 in photography, and there is an interesting description of the pre- 
 paration and use of collodion for this purpose in Robert Hunt’s 
 “Manual of Photography’ (Griffin, 1853).* All of the early 
 workers were confronted with the difficulty that these solutions 
 contracted considerably on drying, causing puckering and uneven- 
 ness, while at times the films would be more or less white and opaque 
 instead of transparent. 
 
 Alexander Parkes, the greatest of the pioneers in the develop- 
 ment of the industrial uses of cellulose nitrate, began work on the 
 
 ; * The writer is indebted to Mr. A. M. Hutchison for the loan of this 
 ook | 
 
16 Cellulose Ester Varnishes 
 
 subject in the early ‘fifties. Although concerning himself chiefly 
 with the manufacture of plastic solids, he investigated many liquid 
 solvents and foresaw the value of the material as a base for varnishes. 
 He recognised that the opacity frequently manifested by films from 
 ether-alcohol solutions was due to precipitation of amorphous 
 cellulose nitrate by absorption of atmospheric moisture, and was 
 the first to employ anhydrous solvents. An early patent of his 
 describes the dehydration of wood spirit (crude methyl alcohol) 
 by distillation with calcium chloride.* This patent makes special 
 reference to the use of solutions as protective coverings, and fore- 
 shadows many later developments, thus: “ For a transparent 
 and colourless substance I use a solution of guncotton alone or with 
 gums or resins that will set transparent with it and this is applicable 
 as a coating to silk tinsel articles or other textile fabrics, sewing 
 cotton thread, worsted, string, felted goods, leather, plaster or wood.”’ 
 In the same specification Parkes also suggests the use of metallic 
 bronzes with cellulose nitrate solutions. He recommends using 
 thick solutions and spreading machinery, thereby producing films 
 which can be used for bookbinding or button manufacture. Lastly, 
 in order to render the film non-inflammable he recommends the 
 incorporation of ammonium phosphate, magnesium phosphate, 
 cadmium iodide, mercuric iodide, calcium oxalate, talc or alum. 
 
 Parkes realised that satisfactory films would not be obtained 
 under ordinary atmospheric conditions from solutions of cellulose 
 nitrate in solvents of low boiling point, and he investigated aniline 
 and nitrobenzene. These, however, failed for the opposite reason, 
 their vapour pressure being so low at ordinary temperatures that 
 solutions would not dry in a reasonable time. The poisonous 
 nature of the vapours was also a drawback. Acetic acid was 
 obviously unsuitable on account of its unpleasant and corrosive 
 vapour. 
 
 In 1864 Parkes 1 disclosed the strong solvent power possessed 
 by solutions of camphor in oil of turpentine or in wood spirit. He 
 was therefore the first to use camphor in useful conjunction with 
 cellulose nitrate, and in a lecture given before the Royal Society 
 of Arts in 1865 he describes the use of camphor as an important 
 improvement in the manufacture of solid masses from pyroxylin, 
 since it renders them more uniform and less contractile. 
 
 Pellen’s patent of 185615 for the use of collodion for covering 
 toys and balloons is of interest in view of the immense Maino 6i3 
 of aircraft dopes sixty years later. 
 
 The most interesting of Spill’s 1° patents in reference to besa 
 
Introduction 17 
 
 of cellulose nitrate is that in which he makes known the solvent 
 power of a mixture of ethyl alcohol and benzene, each constituent 
 being a non-solvent when employed alone. The study of these 
 mixed solvents is one of the most interesting problems in the chemistry 
 of cellulose nitrate. A mixture of ether and alcohol is one of the 
 commonest solvents of cellulose nitrate, containing from 11:0 to 
 12-4% nitrogen, although neither constituent alone is a solvent at 
 ordinary temperatures. It was employed by nearly all the early 
 experimenters and was investigated by Saillard in 1859.17 A care- 
 ful but inconclusive research was carried out by Baker 18 in 1912, 
 while as recently as 1921 Bancroft ! has described it as one of the 
 present problems of colloid chemistry. 
 
 Amyl Acetate. 
 
 Until the year 1882, the development of a large industry in 
 solutions of cellulose nitrate was prevented by the fact that there 
 were no cheap and otherwise desirable solvents and solvent mixtures 
 known intermediate in boiling point between the group of low- 
 boiling liquids such as methyl alcohol, acetone, ethyl acetate, ethyl 
 alcohol, ethyl ether and benzene, and the high-boiling slow-drying 
 solvents such as aniline and nitrobenzene. The gap was filled in 
 1882 by the discovery of the solvent power of amyl acetate 
 by Stevens.” Crude amyl alcohol, the so-called fusel oil, is a by- 
 product in the production of ethyl alcohol by fermentation, and for 
 many years was of so little value that it was burned in lamps in 
 the distilleries in place of paraffin oil. Stevens found that the ace- 
 tate of amyl alcohol (strictly speaking, of the mixture of various 
 amyl and other alcohols which fusel oil contains) possessed the blend 
 of properties required for his purpose, namely, high solvent power 
 for a large range of cellulose nitrates, a vapour pressure at ordinary 
 temperatures low enough to prevent unduly rapid evaporation 
 but high enough not to prolong the drying of the film, and very small 
 affinity for water. 
 
 Search for Technical Solvents. 
 
 The search for solvents of cellulose nitrate has continued right 
 down to the present day, but since the discovery of the solvent 
 power of amyl acetate there have not been very many additions to the 
 list which have had much influence on the varnish industry. The 
 acetins (acetates of glycerol) were patented by Schiipphaus in 1889.7} 
 F, oe 22 patented the use of acetone oil (a mixture of indefinite 
 
18 Cellulose Ester Varnishes 
 
 composition remaining from the fractionation of acetone) in 1892, 
 and a large number of compounds, including some esters which have 
 had a limited application in the varnish industry, were patented 
 by A. Nobel, from 1889 onwards. 
 
 The earliest commercial development of cellulose nitrate var- 
 nishes took place in the United States. The Stevens patents 
 belonged to the Celluloid Company of New Jersey, while Hale and 
 Crane formed the Crane Chemical Company and began the manu- 
 facture of varnishes and lacquers in 1886. The usefulness of the 
 lacquers is greatly increased by the incorporation of resins, such as 
 shellac, mastic and copal. Experiments in this direction date back 
 at least to the 1855 patent of Parkes already quoted, but the first 
 products of the type of modern lacquers, in which cellulose nitrate 
 is blended with a resin in a mixture of solvents of graded volatility, 
 were due to Field.?8 Such varnishes are more adhesive than those 
 prepared with cellulose nitrate alone, and can be made with a 
 higher solid content for the same degree of fluidity. At the present 
 time they form a large proportion of the total output. 
 
 Economic Considerations. 
 
 In deciding the suitability of any substance as an industrial 
 solvent for cellulose nitrate, economic considerations must come 
 first. So much is known about the chemical groupings which impart 
 solvent power to an organic molecule that it is safe to say an expert 
 organic chemist could, if it were worth while, make extensive 
 additions to the list of cellulose nitrate solvents; but only those 
 substances which can compete in price with the known solvents 
 have any chance of industrial application. Occasionally the develop- 
 ment of chemical industry may result in a group of solvents, hitherto 
 of academic interest only, becoming industrially available. For 
 example, the fermentation of starch to butyl alcohol and acetone 
 was originally developed to yield butyl alcohol for the manufacture 
 of artificial rubber.24 The acetone was absorbed in the explosives 
 and varnish industries. During the Great War, acetone became the 
 more important product, and as artificial rubber became of negligible 
 account in England, butyl alcohol and butyl acetate became available 
 as substitutes for amyl alcohol and amyl acetate. Since amyl 
 acetate tends to become increasingly scarce, owing to the demands 
 of the film and varnish industry and the diminution of spirit manu- 
 facture in the United States, butyl acetate has become a normal 
 solvent in the cellulose ester industry. 
 
 
 
 f 
 J 
 
Introduction 19 
 
 Unless, however, the discoverer of a new solvent for celluiose 
 esters sees some probability of the material being available at a price 
 not greater than that of the usual solvents, it is not worth while 
 for him to seek patent protection. — 
 
 Cellulose Acetate—Historical. 
 
 Shortly after the time when Parkes began working on the indus- 
 trial possibilities of cellulose nitrate, another industrial ester of 
 cellulose, the acetate, was first prepared by Schiitzenberger.?5 
 The principal difference between the nitration and the acetylation 
 of cellulose is that whereas cellulose nitrate is insoluble in the 
 “spent acid,” 7.e., the weakened mixture of sulphuric and nitric 
 acid remaining after nitration of the cellulose, cellulose acetate is 
 soluble in acetic acid. Hence, unless special indifferent diluents 
 are added to the acetylation mixture, the cellulose passes into solu- 
 tion as acetylation proceeds, yielding a viscous mixture from which 
 cellulose acetate is precipitated by the addition of water. In 
 Schiitzenberger’s first paper, he described the preparation by heating 
 cellulose with acetic anhydride in sealed tubes to 130—140°C. In 
 a later publication he recommended a temperature of 180°C., 
 obtaining a product insoluble in alcohol or ether, and easily saponified 
 by caustic alkalies. 
 
 The use of acetic anhydride under pressure was avoided by 
 Franchimont’s discovery of the so-called catalytic action of sul- 
 _ phuric acid or zinc chloride.26 This step really marks the foundation 
 of the modern cellulose acetate industry, Cross and Bevan used 
 the zinc chloride method in their investigations of the constitution 
 of the ligno-celluloses.2” These investigators took out a patent in 
 1894 2° for a method of acetylation involving the use of zinc chloride. 
 The product was soluble in chloroform, yielding films which were 
 suggested as substitutes for collodion in surgery. It is with this 
 patent that the industrial history of cellulose acetate may be said 
 to begin. 
 
 The technical development of cellulose acetate was much hindered 
 by the slowness of the reaction between cellulose and the acetylating 
 agents, and the limited range of solubility of the acetates produced. 
 Investigators were therefore led to experiment on the acetylation 
 of modified forms of cellulose, 7.e., cellulose which has been rendered 
 more open to attack by partial degradation of its structure. Girard 2° 
 had applied the name of hydrocellulose to the derivatives obtained 
 by the mild acid hydrolysis of cellulose, and Lederer *° found that 
 
20 Cellulose Ester Varnishes 
 
 hydrocellulose could be acetylated more smoothly and at a lower 
 temperature than unmodified cellulose. F. Bayer & Co. #} 
 patented a similar process almost at the same time. 
 
 So far the cellulose acetates produced had not been soluble in 
 any of the useful organic solvents unless hydrolysis had been carried 
 so far that the films left on evaporation were so brittle as to be 
 technically useless. A great advance was made by Miles,*? who 
 found that by a partial hydrolysis of the primary product of acetyla- 
 tion, an acetate soluble in acetone was produced. In 1906 the 
 Bayer Co.** patented an adaptation of Miles’s process, and in 1907 
 they placed on the market varieties of cellulose acetate described as 
 Cellite L and Sericose. These produced moderately good films when 
 evaporated on glass, and were intended as textile finishes. The 
 solvents employed for Cellite L were acetone or a mixture of ethyl 
 acetate and alcohol. Sericose was insoluble in anhydrous acetone, 
 but soluble in aqueous acetone or in acetic acid; either concentrated 
 or somewhat dilute. It is evident that these substances were acetyl 
 derivatives of a highly depolymerised cellulose. The acetic acid 
 was not firmly held, as both the solid materials and their solutions 
 rapidly developed a smell of free acetic acid. Owing to their high 
 price (about seven shillings per pound) they never came into common 
 use. 
 
 Kichengriin discovered the solubility of certain varieties of cellu- 
 lose acetate in a hot mixture of benzene and alcohol and did much to 
 develop the acetate varnishes. The later history of the acetate and 
 its manufacture on a commercial scale by the Usines du Rhone, 
 Dreyfus, and F. Bayer & Co., will be briefly sketched in a later 
 chapter. 
 
 War Expansion. 
 
 An enormous development in the use of cellulose ester varnishes 
 was brought about by the Great War of 1914—18, particularly in 
 regard to cellulose acetate solutions. It had been found previous 
 to the war *4 that these solutions, when applied to textile fabrics 
 stretched on a frame, possessed the valuable property of shrinking 
 and drawing the fabric taut. This property was utilised for making 
 taut the wings of aeroplanes and protecting the fabric from the 
 weather. It was subsequently found that by adding appropriate 
 ingredients to the solutions the destructive effect of bright sunlight 
 on the fabric could be considerably diminished. Similar effects can 
 be produced with solutions of cellulose nitrate, but since the total 
 thickness of the coatings applied to the fabric is quite considerable, 
 
Introduction 21 
 
 and the fabric itself is combustible, the use of cellulose nitrate 
 alone adds appreciably to the fire danger, especially when the machines 
 are used on active service. 
 
 Cellulose acetate was not manufactured in this country before 
 the war, and the needs of the Government after the outbreak of war 
 were met for a time by the importation of the material, chiefly from 
 Switzerland and France. This arrangement, however, had finally 
 to come to an end, owing to the great expansion in the demand, 
 and a factory for the manufacture of cellulose acetate base and 
 varnishes was established with Government assistance near Derby. 
 Since the conclusion of the war, the demand for cellulose acetate 
 dopes has naturally diminished, and the manufacturers have given 
 their chief attention in this country to the production of artificial 
 silk (rayon) from cellulose acetate. 
 
 This, however, was by no means the end of the difficulties. 
 The principal solvent for cellulose acetate was acetone, a product 
 of the distillation of wood, and during the early part of the war 
 acetone was needed, not only for cellulose acetate varnishes, but 
 also for the still more important material cordite. Certain wood 
 distillation factories were established in this country, and the 
 fermentation process by which acetone (and butyl alcohol) are pro- 
 duced from starch was developed. Nevertheless, the bulk of the 
 country’s requirements in acetone was imported, and since the 
 demand exceeded the supply, and there were many other claims 
 on the available shipping capacity, it became imperative to restrict 
 the use of acetone as much as possible. Hence a great deal of research 
 was carried out in two or three years which would probably have 
 been spread over many years in times of peace. These researches 
 had for their object to find the most economical way of using 
 imported solvents, and involved numerous investigations into the 
 changes of solvent power caused by mixing different liquids together, 
 and into the resulting variations in viscosity of solutions of cellulose 
 acetate made up from the mixtures. 
 
 In the explosives industry, the shortage of acetone required 
 for cordite was still more severely felt, and led in time to the radical 
 alteration of service cordite.*> Instead of guncotton, which is a 
 nitrate of cellulose containing about 13-0°% of nitrogen, soluble in 
 acetone but insoluble in any other available low-boiling solvent, 
 the new cordite (R.D.B.) was made from a collodion cotton, con- 
 taining about 12-4% of nitrogen, soluble in a mixture of ether and 
 alcohol, and resembling a varnish nitro-cotton. Since ether is 
 made from alcohol, and alcohol is cheaply and easily made from 
 
22 Cellulose Ester Varnishes 
 
 molasses or any material containing starch, the use of acetone 
 or any other product of the distillation of wood in the manufacture of 
 cordite was thus entirely avoided. This change of solvent, however, 
 involved, not only investigations into the best proportions of ether 
 and alcohol to be used as the solvent, but also fundamental work 
 on cotton cellulose itself, since it was found that cotton from different 
 sources might give nitrates so different in solubility that not only 
 the consumption of solvent but also the output of the new cordite 
 were gravely affected. 
 
 The war also caused a considerable expansion in the use of 
 solutions of cellulose nitrate for cementing the joints of celluloid 
 accumulator cases, and for fixing glass to celluloid in the eyepieces 
 of gas masks. 
 
 Since the war, some of the information gained in the Government 
 factories has been published, and is influencing the development 
 of industry. The most interesting advance is in the use of cellulose 
 nitrate solutions of much greater fluidity than formerly. It must 
 be understood that the high viscosity of these solutions imposes 
 a practical limit on the amount of cellulose nitrate that can be 
 dissolved in the solvents, since it is obvious that they must be fluid 
 enough to flow somewhat like a paint. By preparing cellulose 
 nitrate which yields solutions of greatly increased fluidity, it is 
 possible to dissolve more of the ester than before without exceeding 
 this practical limit, and a varnish is thus obtained which yields a 
 much thicker coating for the same expenditure of labour and solvent. 
 These very fluid cellulose nitrate varnishes have been developed 
 chiefly in the United States, but the process of manufacture has 
 been kept secret.?® 
 
 REFERENCES AND BIBLIOGRAPHY. 
 
 1 Pelouze, Comptes rend., 1838, 7, 713. 2 Schdénbein, Pogg. Ann., 1847, 
 70, 320. * F. Otto, Augsburger Allgem. Zeitung, 1846. * Knop, Comptes 
 rend., 1846, 23, 808. 5 MacDonald, ‘‘ Historical Papers on Modern Ex- 
 plosives,” 1912. ® Gmelin, ‘‘ Handbook of Chemistry,’ 1872, Vol. XV., 
 168-181. 7 Domonte and Ménard, Comptes rend., 1846, 28, 1087 and 1847, 
 24, 87, 390. &® Hartig, ‘ ‘ Untersuchungen tiber ‘Schiessbaumwolle, ”* 1847. 
 9 Hadow, Chem. Soc. Quarterly Journal, 1855, 7, 201. 1° Maynard, Boston 
 Med. and Sci. J., 1848, 38, 266. 11 Bigelow, ibid., 1848, 38, 178. 12 F. 
 Scott Archer, The Chemist, New Series, 1851, 2, 257. 13 A. Parkes, E.P. 
 2359/1855. 14 A. Parkes, E.P. 2675/1864. 15 Pellen, E.P. 2256/1856. 
 16D), Spill, E.P. 1739/1875. 27 Saillard, E.P. 444/1859. 18 F. Baker, 
 Chem. Soc. Trans., 1912, 102, 1409. 1% W. D. Bancroft, J. Ind. Eng. Chem., 
 1921, 13, 260. %° J. H. Stevens, U.S.P. 269,340. #1 R. Schiipphaus, U.S.P. 
 410,208/1889. 2? F. Crane, E.P. 6543/1892. °° W. D. Field, U.S.P. 
 422,195/1890. #4 W. H. Perkin, jun., J. Soc. Chem. Ind., 1912, 31, 616-624. 
 25 Schiitzenberger, Comptes rend., 1865, 60, 485-486. 26 A. Franchimont, 
 Comptes rend., 1879, 89, 711; ibid., 1881, 92, 1053. 27 Cross and Bevan, 
 
 
 
Introduction 28 
 
 Chem. News, 1889, 60, 163, 254. 8 Cross and Bevan, E.P. 9676/1894. 
 29 Girard, “‘ Mémoire sur |’Hydrocellulose et ses dérivées,’? Paris, 1881. 
 80 Lederer, G.P.’s 118,538/1899 and 120,713/1900. °1 F. Bayer & Co., 
 F.P. 317,007/1901. %* G. W. Miles, U.S.P.’s 733,729/1903 and 838,350/1905. 
 33 FY. Bayer & Co., E.P. 24067/06. 34 J. E. Ramsbottom, ‘‘ Tech. Report 
 of Advisory Comm. for Aeronautics,’”’ 1913-14, p. 426. %5 R. Robertson, 
 Trans. Faraday Soc., 1921, 16, 66-71. °° G. E. Condé, Canad. Chem. and 
 Metall., 1924, 8, 219-221. | 
 
 See also E. Fischer, Kunststoffe, 1912, 2, 21, 48, 64; 1914, 4, 102, 123 
 (chronological list of patents and brief abstracts). Clément et Riviére, <bid., 
 1914, 4, 148-152, 166-168 (translated from Comptes rend. with annota- 
 tions by W. Vieweg). C. P. Schwalbe, J. Soc. Dyers and Colorists, 1914, 
 30, 13-15. E. C. Worden, J. Soc. Chem. Ind., 1919, 38, 370-374r. P. 
 Drinker, J. Ind. Eng. Chem., 1921, 18, 831-835; ‘‘ European Practice in 
 Cellulose Acetate and Dopes during the War.”’ Abel Caille, Chim. et Ind., 
 1924, 12, 441-448 (an historical review of the manufacture of cellulose 
 acetate). E.C. Worden, “‘ Technology of Cellulose Esters,’’ Vol. I., Pt. IV. 
 E. C. Worden, ‘“ Nitrocellulose Industry,” Vol. I., chap. x. F. Ullmann, 
 **Enzyklopadie der technischen Chemie,”’ Vol. III., 309-316. Clément et 
 Riviére, “‘ Matiéres Plastiques et Soies Artificielles ’’ (Bailliére et Fils), 1924, 
 Pt. I., chap. ii.; Pt. I1., chaps. i. and ii. 
 
CHAPTER II 
 
 CELLULOSE 
 
 Cellulose—Elementary Formula—Cotton Cellulose—Sources—Chemical Re- 
 actions—Chemical Constitution—Formula of Irvine and Hirst—Views 
 of other Investigators of Cellulose—X-Ray Evidence—Viscosity of 
 Cellulose and its Esters in Solution—Significance of High Viscosity. 
 
 In this volume it is only necessary to discuss cellulose in relation 
 to its esters and the solutions made from them. Cellulose may be 
 described as the chief constituent of the skeleton of all vegetable 
 bodies. Elementary analysis shows it to belong to the group of 
 organic bodies known as the carbohydrates, and to have the com- 
 position C,H,,0;. Its physical and colloidal characteristics indicate 
 that we must regard it as a polymer of the simple formula, and it 
 is usually written as (C,H, )0;)n. The latest researches on its 
 constitution show that there may be two varieties of polymerisation 
 in the cellulose structure, so that we might write it as [(C,H,)0;)n|m, 
 n being probably a small integer and m a large and probably variable 
 number. 
 
 The kind of cellulose used commercially for the manufacture 
 of esters is almost entirely cotton cellulose, although wood cellulose 
 has been used, particularly for the manufacture of cellulose nitrate, 
 when supplies of cotton have been short } 2%. Cotton is the purest 
 form of cellulose produced by nature in large quantities, and con- 
 tains about 90% of cellulose. The coniferous woods which form 
 the source of wood cellulose contain about 60°%,4 and therefore 
 require a greater degree of purification to isolate the cellulose. 
 
 Cotton is cultivated industrially chiefly in the United States 
 and in India. The cotton fibres or hairs are attached to seeds 
 contained in the “‘ boll”’ or seed-pod of the cotton plant. When 
 the bolls ripen they burst open, and the white mass of cotton fibre, 
 after being freed from the seeds, forms the raw material of the 
 spinning industry. 
 
 The chemical reactions which the constitutional formula of 
 cellulose must explain are conveniently summarised by Green 5 
 and are quoted by Hall.* It is not necessary to give the complete 
 list here, but the following properties have a bearing on the specifica- 
 tion of cotton and its use in the cellulose ester industry :— 
 
 (a) The highest stage of nitration or acetylation is a tri- 
 ester calculated on a C, formula. 
 
 (b) Cellulose does not contain free carbonyl (aldehydic 
 or ketonic) groups, but such groups are formed by simple 
 hydrolysis. 
 
 ~ 24 
 
 ~ 
 
Cellulose 25 
 
 (c) The ultimate product of hydrolysis is dextrose. (The 
 rigid proof of this is much more recent.) 
 
 (d) The oxidation of cellulose yields oxycellulose, which 
 is markedly acidic. 
 
 The proof of the chemical constitution of cotton cellulose has 
 been given by Irvine and Hirst,’ based principally on their own work, 
 that of Denham and Woodhouse,’ and of Haworth and Leitch.® 
 The original papers must be read for a detailed account. The 
 methods used were chiefly— 
 
 (1) hydrolysis and graded acetolysis (i.e. simultaneous 
 hydrolysis and acetylation) of cellulose, and 
 (2) alternate methylation and hydrolysis. 
 
 The researches involved identification of the products of these 
 reactions, and careful determinations of the yields in which they 
 were obtained. 
 
 It was shown that cellulose contains the dextrose grouping 
 
 and next that the residues B and C were also themselves entirely 
 composed of groups having the dextrose configuration. There are 
 several ways in which such a structure might be built up, but Irvine 
 prefers the symmetrical formula :— 
 
 CH,OH 
 
 | ee Oe 
 -CH—0—CH—CH—CHOH—CHOH—CH 
 
 j (HOH 
 
 eyes O 
 
 Sa ae a 
 HH '————-o————_~ 
 
 It may be noted that one step in the proof depends on the isolation 
 from cellulose by Haworth and Hirst of a crystalline derivative 
 of a sugar containing two C, groups in the molecule (cello-biose). 
 It will be seen that Irvine’s formula could only yield one molecule 
 of a cello-biose (C,,) from one unit (C,,) of cellulose, and calculation 
 shows that the maximum yield of cello-biose from cellulose would 
 
26 Cellulose Ester Varnishes 
 
 be 70%. The highest recorded yields are 50—60%, and a yield 
 of more than 70% would disprove Irvine and Hirst’s symmetrical 
 C,, formula. Irvine and Hirst exclude the possibility of cello- 
 biose being formed in a secondary reaction by the condensation 
 of two molecules of dextrose. 
 
 The formula represents cellulose as possessing three hydroxyl 
 groups for every six carbon atoms. Hence unless cellulose is hydro- 
 lysed (with formation of new hydroxyl groups) only three ester 
 radicals can be introduced into each C, unit. It may be remarked 
 in passing that two of the hydroxyl groups are present in the form 
 of secondary alcoholic groups, and the other as a primary alcoholic 
 eroup.!° The formula does little, however, to explain those pro- 
 perties of cellulose and its derivatives which are of chief interest 
 to the varnish manufacturer. Why, for example, is cellulose, with 
 its high percentage of hydroxyl groups, insoluble in water? Why 
 have cellulose and its esters such a high viscosity in solution ? 
 Irvine and Hirst realised these difficulties, and their own words 
 may be quoted. The formula is “the simplest expression of a 
 molecule which, polymerised in unknown numbers, would represent 
 cellulose as a chemical entity, but we recognise that any odd number 
 of anhydroglucose residues from three upwards can be arranged so 
 as to fulfil the conditions. Our work gives no indication of the degree 
 of polymerisation undergone by the molecular unit, but the extreme 
 insolubility of trimethyl! cellulose compared with the ready solubility 
 of methylated starch, inulin and glycogen points to the idea that 
 cellulose is the most highly polymerised of the polysaccharides.” 
 It has been remarked by Dorée that the purely chemical and the 
 
 colloidal investigations of cellulose have proceeded almost indepen- — 
 
 dently, and it is of interest to place next to the foregoing quotation 
 from Irvine the views of some of those who have been chiefly 
 interested in cellulose as a colloid. 
 
 W. Harrison 1! suggests that the cellulose fibre itself may be 
 regarded as the molecule. Schwalbe and Becker 1* found that when 
 cellulose is mechanically converted into a mucilage by minute 
 subdivision in water, its power of reducing copper is materially 
 increased, presumably because the mechanical rupture of linkages 
 exposes fresh oxidisable atomic groupings. W. L. Balls ® by the 
 microscopic examination of sections of swollen cotton fibres was 
 able to show well-defined growth rings, and in a later paper 
 concludes that the wall of the hair is probably a sponge-like structure 
 with free air-spaces. Pierce has measured both the rigidity 15 and 
 the plasticity 1° of the cotton fibre. On the other hand, C. F. Cross 1 
 
Cellulose 27 
 
 regards the cellulose fibre as resembling a liquid system : “‘ Cellulose 
 (reacts) as a labile complex of groups of varying dimensions repre- 
 senting a state of matter somewhat analogous to that of a solution 
 of a saline electrolyte—that is, it reacts rather as a solution—aggre- 
 gate than by a succession of molecular combinations; the masses 
 actually reacting following the stoichiometrical ratios proper to 
 the dimensions of these ultimate groups, and retaining their relation- 
 ships in the aggregate, which is thus progressively modified by the 
 entrance of the new groups.” 
 
 S.S. Napper 1° regards Cross’s suggestion as tending in the same 
 direction as Harrison’s, and as being in harmony with the view that 
 cellulose is built up by the condensation of soluble constituents of 
 plant juices. J. Boeseken, J.C. van den Berg and A. H. Kerstjens,?9 
 from a study of the graded acetylation of cellulose, concluded that 
 if the formula is written as (C,H,,)0,;),, ~ must be at least 45. 
 Duclaux and Wollmann *° fractionally precipitated an acetone 
 solution of cellulose nitrate and obtained fractions of identical 
 elementary composition but of such widely different viscosity that 
 they probably represented large variations in molecular size. 
 
 Any theory of cellulose structure built up from Irvine and Hirst’s 
 formula must take account of these views. The polymerisation 
 of the units during growth must take place in three dimensions, and 
 it is not difficult to imagine that the method of condensation may not 
 be uniform. There may be denser packing of C, units in some 
 parts of the same fibre than in others, the density depending on 
 atmospheric conditions during growth, such as temperature and 
 humidity, which would probably influence the degree of internal 
 anhydride formation. On the other hand, we must assume that 
 the structure is sufficiently open to allow penetration of cupram- 
 monium solvent, for example, which under controlled conditions 
 will dissolve cellulose without degradation of the structure, or of 
 nitrating acid which will convert cellulose into its nitrate with com- 
 paratively little degradation. It may be that the somewhat con- 
 flicting evidence on the yield of cello-biose obtainable from cellulose 
 is due to the fact that the condensation of C, groups in the structure 
 proceeds irregularly. 
 
 Rontgen Ray Evidence. 
 
 For work on the application of von Laue’s X-ray method to the 
 elucidation of the structure of cellulose, reference must be made to 
 the papers of Herzog and Jancke.?!_ These investigators state that 
 cellulose from cotton, ramie and wood all showed evidence of con- 
 
28 Cellulose Ester Varnishes 
 
 taining a crystalline substance with rhombic symmetry, with ratios 
 a:b:c = 0-6935:1:0-4467. There is also evidence that the 
 particles are regularly orientated or arranged in the direction of the 
 length of the fibre. The dimensions of the elementary cellulose 
 particle are found to be 7:9: 8-45: 10:2 x 10°§ cm., and Herzog 
 and Jancke deduce that it contains four dextrose residues, or two 
 cello-biose residues. Irvine and Hirst, from chemical evidence, 
 prefer to assume three dextrose residues, or one cello-biose and one 
 dextrose residue, in the unit. 
 
 It should be explained that the terms crystal and crystalline 
 refer here solely to the existence of a repeated regular structure in 
 cellulose, causing it to produce a regular pattern by the diffraction 
 of X-rays. The popular criteria of the crystalline state, 7.e., visible 
 geometrical uniformity, some degree of hardness, and unscattered 
 reflection of light, are evidently absent. 
 
 Significance of Viscosity of Cellulose Ester Solutions. 
 
 The most important property of cellulose and its esters from the 
 point of view of the varnish manufacturer is the viscosity of their 
 solutions. This property, which is the inverse of fluidity, is a 
 measure of their resistance to flow. It will be treated more fully 
 in a later chapter, but it is necessary to clear up at this point a 
 misapprehension that may arise. It is often stated that the vis- 
 cosity of cellulose ester solutions is an indication of a highly polymer- 
 -ised molecular structure, and that degradation of the structure 
 reduces viscosity and vice versa. On the other hand, it will be shown 
 later that the same sample of cellulose ester will yield solutions of 
 very different viscosity in different solvents and combinations of 
 solvents. It is natural to ask whether the degree of polymerisation 
 of the structure differs in different solvent media? A full answer 
 to this question cannot be given until the constitution of these 
 colloidal solutions has been cleared up, but it may be taken as cer- 
 tain that the difference in viscosity between solutions of the same 
 sample of ester in different solvents does not indicate a difference 
 in the molecular structure of the ester particles, but in the degree to 
 which they are surrounded and swollen by solvent molecules. It 
 follows that it is not safe to draw conclusions as to structural degra- 
 dation of cellulose esters from viscosity determinations in any 
 single solvent. To put this perhaps more correctly, it is not safe 
 to deduce a high degree of polymerisation from the fact that a 
 sample of ester has a high viscosity in one particular solvent or mixture 
 
Cellulose 29 
 
 of solvents. From the fact that acetone dissolves a very wide range 
 of the nitrates and acetates of cellulose, it has become almost 
 customary to draw conclusions as to the magnitude of the molecular 
 structure from viscosity determinations in acetone solutions. It 
 would be safer to employ a range of solvents, including aqueous 
 acetone and other binary mixtures, and to base deductions as to 
 the structure on the lowest viscosity measured. 
 
 REFERENCES AND BIBLIOGRAPHY. 
 
 1 C. G. Schwalbe and A. Schrimpf, Z. angew. Chem., 1914, 27, 662. 
 2 B. Rassow, zbid., 1924, 37, 792. *% G. Leysieffer, Koll. Chem. Beth., 1918, 
 10, 145. 4 A. J. Hall, ‘‘ Cotton Cellulose,’ 1924, 25. 5 A. G. Green, Zeit. 
 Farben u. Textil.-Chem., 1904, 3, 97. & A. J. Hall, loc. cit., pp. 182-183 (see 
 also H. Hibbert, J. Ind. Hing. Chem., 1921, 18, 256-260, 334-342). 7 J.C. 
 Irvine and E. L. Hirst, Chem. Soc. Trans., 1923, 123, 518. 8 W.S. Denham 
 and H. Woodhouse, ibid., 1913, 108, 1735. ® W. N. Haworth and G. C. 
 Leitch, 7b7d., 1918, 113, 188. 1° H. Hibbert, loc. cit.; also J. Amer. Chem. 
 Soc., 1923, 45, 734. 11 W. Harrison, 2nd Report on Colloid Chemistry, 
 1919, 55. 12 C. G. Schwalbe and E. Becker, Z. angew. Chem., 1919. 32, 
 265. 4 W. L. Balls, Proc. Roy. Soc., 1919, B90, 542. 14 Idem, ibid., 1923, 
 B95, 72. 15 F. T. Pierce, J. Text. Inst., 1923, 14, lr. 1° Idem, ibid., 390r. 
 17 C. F. Cross and E. J. Bevan, ‘‘ Researches on Cellulose,’’ 1900-1905, 7. 
 18 §. S. Napper, ‘“‘ Applied Chemistry Reports,’ 1919, 121. 1% J. Boeseken, 
 J. C. van den Berg and A. H. Kerstjens, Rec. trav. chim., 1916, 35, 320-345. 
 20 J. Duclaux and E. Wollmann, Bull. Soc. chim., 1920, 27,414. 21 R. O. 
 Herzog and W. Jancke, Ber., 1920, 588, 2162-2164; Z. Phys., 1920, 3, 
 196-198; Z. angew. Chem., 1921, 34, 385-387. 
 
 Additional References. 
 
 C. F. Cross and E. J. Bevan, ‘‘ Cellulose’; ‘‘ Researches on Cellulose,”’ 
 1895-1900, 1900-1905, 1905-1910. C.F. Cross and C. Dorée, ibid., 1910-1921. 
 A. J. Hall, ‘* Cotton Cellulose.”’ 
 
CHAPTER, III 
 
 NITRATION OF CELLULOSE 
 
 Reaction between Cellulose and Mixed Acids—Specification of Acids—Acid 
 Balance—Conditions of WNitration influencing Solubility—Ratio of 
 Acid to Cellulose—Composition of Acid Bath—Time and Temperature— 
 Preliminary Treatment of Cotton—Necessity for Strict Control— 
 Varieties of Cotton Used—Cotton Specifications—Degree of Nitration— 
 Nomenclature of Cellulose Nitrates—Nitration Plant—Direct Dipping— 
 Centrifugal Nitration—Displacement Process—Nitration of Linters 
 in United States—Removal of Acid—Boiling—Poaching—Bleaching— 
 Drying of Nitrocellulose—Dehydration of Nitrocellulose—Properties 
 and Specifications of Nitrocellulose. 
 
 CELLULOSE may be nitrated by nitric acid alone, but in practice 
 
 a mixture of sulphuric and nitric acid is always used. The process 
 
 consists essentially in bringing cotton cellulose into contact with 
 
 the mixed acid, and, after nitration is complete, washing out the 
 acid. The cotton does not alter in appearance during the process, 
 but it gains considerably in weight and becomes harsher to the 
 touch and more resistant to water. It does not dissolve in the 
 acids, so that the rate of nitration is determined by the amount 
 of surface exposed to the acid, and the rate of diffusion of the acid 
 mixture through the cotton hairs. The nitration process takes 
 
 place according to the typical equation 
 R°OH -- OH'NO, = R:0:NO, + H,0, 
 
 but this does not express the whole of the conditions. The sulphuric 
 acid plays a much more active part than merely to absorb the water 
 formed during nitration. It certainly reacts with the cellulose, 
 since combined sulphuric acid is always found in the product, and it 
 may be that the nitration process consists partly of a replacement of 
 combined sulphuric acid by combined nitric acid. One of the 
 effects of the sulphuric acid is to act hydrolytically by opening up 
 fresh hydroxyl groups, so that there is always a partial depolymerisa- — 
 tion during nitration. 
 
 Specification of the Acids. 
 
 The acids used are of the usual commercial quality, and specifica- 
 tions usually quote the percentage strength and the specific gravity. 
 The nitric acid should not contain too much nitrous acid, and both 
 acids should be reasonably free from suspended matter and dissolved 
 solids. In practice they cannot be obtained entirely free from 
 dissolved and suspended compounds, chiefly of iron and lead, 
 derived from the materials used in the plant in which they are 
 
 made. 
 30 
 
Nitration of Cellulose 31 
 
 Acid Balance. 
 
 The nitration of cellulose never exhausts the acid bath of its 
 nitric acid, so that the spent acid recovered after the nitration can 
 be “ revivified ” by the addition of stronger acid and brought up 
 to its original strength ready for use again. 
 
 For example, in the preparation of cellulose nitrate of high 
 solubility in ether—alcohol, 140 tons of cellulose nitrate was prepared 
 from 2795 tons of an acid having the following composition :— 
 
 H,S O, ° . . . 6 I . 7 % 
 HNO, . . ° ° 23: aq % 
 H,O . ’ . . 14:6% 
 The spent acid recovered from the nitration contained :— 
 HSO, . : 60-:9% 
 HNO, ° : . . 20-2% 
 H,O . ° ° . 1 8: 9 wt 
 The revivifying acids available were mixed acid containing :— 
 H,SO, . . . . 60:5% 
 HNO, .. “atid . 31-:8% 
 H,O . ’ . ° 7: 7 we 
 and concentrated sulphuric acid containing :— . 
 H,SO, . x 92% 
 H,O : ' 8% 
 
 To produce the original weight of nitrating bath of the same com- 
 position it is necessary to mix :— 
 
 Containing tons 
 
 Acid. Weight. 
 oo a. ae 1717 tons 
 Revivifying acids : 
 8) Ce eae 994 ,, 
 (6) Sulphurie acid ...... 84, 
 Ce ae een 2795 tons 
 Percentages 
 
 
 
 The subject of acid balance is a very important one, and has 
 been dealt with exhaustively in the Technical Records of the 
 Ministry of Munitions,! from which the example given above is 
 taken. 
 
82 Cellulose Ester Varnishes 
 
 Conditions of Nitration influencing Solubility. 
 
 If potassium nitrate is to be made by treating potassium 
 hydroxide with nitric acid, a small excess or deficit of nitric acid is 
 not, speaking qualitatively, a serious matter, since it is easy to 
 obtain pure potassium nitrate from the product. The nitration 
 of cellulose is a much more difficult matter. Cellulose nitrate is not 
 a definite chemical individual, and the composition and properties 
 of the product of nitration depend on a number of factors: (a) the 
 ratio of acid to cellulose, (b) the composition of the acid bath, (c) the 
 time and (d) the temperature of nitration. To these factors may be 
 added a fifth, (e) the physical state of the cellulose before nitration, 
 for neither is cellulose a definite chemical individual. A variation 
 in any one of these conditions leaves its permanent impress on the 
 properties of the cellulose nitrate produced. 
 
 (a) Ratio of Acid to Cellulose—Cotton is a bulky material, and 
 in order to make it as susceptible as possible to the action of acids, 
 it is always nitrated in a very open condition. A large quantity of 
 acid is therefore necessary if only to bring all the cotton in contact 
 with acid, and the ratio of acid to cotton in practice varies from 
 about 30/1 to 50/1. Since nitration only takes place in com- 
 paratively strong acid mixtures, a considerable excess of acid is 
 always used, and the cellulose nitrate comes into some kind of 
 equilibrium with the spent acid. It follows that if a still higher 
 ratio of acid to cellulose is used, the final cellulose nitrate will be 
 in equilibrium with a stronger spent acid and will therefore be more 
 highly nitrated. The literature of nitration would have been much 
 simplified if it had always been recognised that the degree of nitration 
 depends on the final equilibrium and not on the original strength 
 of the nitrating bath. 
 
 (b) Composition of Acid Bath—Numbers of researches have 
 been carried out to trace the influence of the composition of the 
 acid bath on the properties of the cellulose nitrate produced 7°. 
 The percentage of nitrogen increases with the strength of the acid 
 bath, 7.e. roughly speaking, as the proportion of water in the acid 
 bath rises, the percentage of nitrogen taken up by the cellulose falls. 
 In all acid baths used on the large scale, the percentage of sulphuric 
 acid exceeds that of nitric acid, and under these conditions the 
 viscosity of the cellulose nitrate increases with increasing nitric 
 acid content in the bath. The composition of a typical bath for 
 producing cellulose nitrate with high solubility in ether-alcohol has 
 already been given. 
 
Nitration of Cellulose 33 
 
 _ (ce) and (d) Time and Temperature.—These factors are conveni- 
 ently considered together. The time of nitration must not be 
 unduly curtailed, as time is necessary for the complete penetration 
 of the cotton hairs by the acid mixture. As far as the combination 
 with nitric acid is concerned, as shown by determination of nitrogen 
 percentage, there is little to be gained by prolonging nitration 
 beyond half an hour. Extra time spent in nitration naturally 
 diminishes the output of the plant, but it also reduces the viscosity, 
 and modifies the solubility relations of the product, so that the 
 time of nitration is not fixed by considerations of output alone. 
 
 A high temperature also reduces viscosity, but a practical limit 
 is placed to the employment of this factor by the tendency of the 
 cellulose to oxidise if temperature is too high. This occurrence is 
 known as “‘ fuming off’ and leads to the total destruction of the 
 cellulose, and the loss of a considerable quantity of nitric acid in 
 the form of oxides of nitrogen. 
 
 (e) Cellulose Purification.—The purification of cotton for nitra- 
 tion consists chiefly in treatment with hot alkali solutions in closed 
 vessels. Sometimes a bleach is also given. Either treatment if 
 - carried too far will begin to break down the cellulose structure and 
 reduce its viscosity. Cellulose from wood yields in general less 
 viscous esters than those from cotton. 
 
 Need for Strict Control. 
 
 It will be evident from the preceding paragraphs that in order 
 to manufacture cellulose nitrate of uniform quality, strict control 
 must be exercised over the properties of the original cellulose, the 
 composition of the acid bath and all the conditions of nitration. 
 Even when all this has been done, small variations will occur, 
 perhaps the most difficult factor to control being the temperature 
 in the interior of masses of reacting cellulose. These variations 
 must be averaged by careful blending at a later stage of the 
 
 manufacture. 
 
 Cellulose for Nitration Purposes. 
 
 In the early days of the manufacture of cellulose nitrate for 
 explosive purposes, several disasters occurred owing to the insuffi- 
 cient stability of the product. One explanation advanced to account 
 for these explosions was the presence in the nitrated cotton of 
 unstable nitro-derivatives of impurities in the original cotton. To 
 diminish the danger from this source, cotton in the purest available 
 form, namely, hanks of yarn, was adopted as the raw material. 
 Researches by Abel? in the ’sixties and seventies proved. that if 
 
 3 : . 
 
34 Cellulose Ester Varnishes 
 
 the nitrated cotton were pulped, the purification effected by the 
 washing process was greatly improved, and with the introduction 
 of this process it was found possible to employ cotton waste from 
 the spinning mills in place of the expensive hanks of yarn. The 
 utilisation of cotton waste from the spinning mills is an industry 
 in itself, and new methods of treating it were and are continually 
 being discovered.’ Hence the cellulose nitrate industry has always 
 had to contend with difficulties arising from variations in the quality 
 of its principal raw material. Cellulose nitrate for explosive 
 purposes, when the very highest stability is required, is made from 
 sliver (waste made at the card, comber and draw frames) and from 
 cop bottoms (the last portion of spun thread remaining on the 
 spindles). For the preparation of varnishes, linters (the short hairs 
 remaining attached to the cotton seed after the long hairs have been 
 removed) are largely used, especially in the United States.? Lastly, 
 there are the mixed waste swept up in the cotton mills, which 
 contains miscellaneous dirt and oil, and hull fibre,!° which consists 
 of the short hairs left on the seed after removal of the linters, and is 
 obtained from the seed by the processes known as decortication 
 and defibrination. All these materials require preparation before 
 they can be used for nitration, and the greater the impurity the more 
 drastic the treatment must be. In general, it consists of an alkali 
 boil under pressure, thorough washing with water, sometimes a 
 bleach, followed by acidification and washing again. 
 
 Important factory investigations on the preparation of cotton 
 for nitration were carried out during the war, and have been 
 described by Punter.!! It was found that treatment with dilute 
 alkalies under pressure tended to bring cellulose, derived from 
 several different sources, down to an approximately constant entity 
 yielding solutions of approximately uniform viscosity. By applying 
 the results obtained to the treatment of cotton supplied to explo- 
 sives factories during the war, it was possible to obtain a much 
 more uniform output from the cordite machines, and to economise 
 in the consumption of the ether—alcohol used as solvent. 
 
 In the celluloid industry, cotton cellulose is frequently nitrated 
 in the form of a uniform cotton tissue paper, which is capable of 
 undergoing a very uniform nitration owing to the rapid and thorough 
 manner in which it comes into contact with the nitrating acids. 
 
 Wood cellulose has been nitrated for explosives purposes in 
 Germany. It does not give a nitrate of so good a colour as cotton 
 cellulose, but it may find application in the varnish industry. It 
 is usually nitrated in the form of tissue. 
 
Nitration of Cellulose 35 
 
 Cotton Specifications. 
 
 A typical specification may be quoted from Piest, Stich and 
 Vieweg.4* The cotton must be clean and white. It must not con- 
 tain impurities such as sand, paper, untorn threads, knots or seed. 
 It must be very loose and suitable for nitration without further 
 treatment. It should not contain much dust, and the fibre must 
 be long and not overbleached. Moisture must not exceed 9%, 
 fat content 0-5%, ash 1% and wood gum 2%. It must not contain 
 more than traces of chlorine. One gramme of the cotton pressed 
 lightly by hand must sink in water within three minutes. 
 
 Schwarz ® gives the following specification: Maximum ash 
 0-6%, fat 0-4%, water 6%. A white colour, freedom from chlorine, 
 acidity, dust and vegetable impurities. | 
 
 In considering these and similar specifications it must be remem- 
 bered that cellulose is not a chemical individual, but a residue which 
 has resisted certain chemical treatments in the course of its 
 manufacture (7.e., alkaline boiling, acidification and possibly bleach- 
 
 ing). It is required for its properties as a colloid. It would be 
 useless to specify that it must agree with a certain elementary 
 analysis, since that would give no information about its suitability 
 for the manufacture of esters possessing the required physical 
 properties. The specifications given deal only with possible 
 impurities, for the following reasons :— 
 
 Moisture.—Cellulose is hygroscopic and absorbent, and a limit 
 to the moisture content must be fixed for commercial reasons. 
 Moreover, a high avidity for atmospheric moisture often denotes 
 degradation of structure resulting from too drastic purification, 
 while the nitration of damp cellulose increases the evolution of heat 
 and the risk of fuming-off. 
 
 Ash.—The mineral matter in cellulose is partly derived from 
 the original inorganic content of the cotton hairs, partly from the 
 water and alkali used in the preliminary treatment, and partly from 
 adventitious impurity. The limit imposed is a safeguard against 
 artificially weighted cellulose and faulty pre-treatment. 
 
 Fat.—This is usually termed, more correctly, ether extract. 
 Greasy, oily and resinous impurities may be derived from the 
 original cotton or from external contamination. If from the former, 
 the hot alkaline treatment of the cellulose has been insufficient. 
 These impurities hinder the wetting of the cellulose by the acid, 
 
 ~ and may leave unstable nitro-bodies in the product. The time of 
 
36 Cellulose Ester Varnishes 
 
 sinking in water is a rough but useful factory test for the same 
 impurity. . 
 
 Additional tests frequently agreed between buyers and sellers 
 are the copper number,!5 staining tests,1° e.g., with a solution of 
 methylene-blue, and determinations of loss of weight on boiling 
 with alkali under specified conditions. The exact significance of 
 some of these tests is not yet completely understood, but they 
 indicate the extent to which reducing and acidic groups have been 
 produced in the cellulose structure, and they can frequently be 
 correlated with the behaviour of the cellulose after its conversion 
 into nitrate. 
 
 The most valuable test of cellulose for the varnish manufacturer 
 is undoubtedly its viscosity. Cellulose is dissolved by cupram- 
 monium solution, and a method has been worked out by Gibson, 
 Spencer and McCall 1’ for standardising the procedure, and obtaining 
 comparisons of the viscosity of cellulose from different sources. They 
 also showed that the ‘viscosity of esters prepared from cellulose 
 under controlled conditions is governed by that of the original 
 cellulose. Varnish manufacturers would probably prefer in most 
 instances to carry out viscosity determinations on the ester instead 
 of the cellulose, since the procedure is simpler and the results can 
 be directly translated into factory practice. Nevertheless, occasions 
 may arise when the application of a direct viscosity test to the 
 cellulose may be of great value. 
 
 Degree of Nitration and Nomenclature. 
 
 It has already been pointed out that although theoretically 
 cellulose should be capable of nitration in three distinct stages 
 corresponding to the mono-, di- and tri-nitrate, in practice it is found 
 from the determination of nitrogen percentage that the composition 
 rarely accords with any one of these stages. It is customary 
 therefore to classify cellulose nitrates according to the amount of 
 nitrogen which they contain, and the terms mono-nitrate, etc., have 
 been largely abandoned. The classification by nitrogen percentage 
 has one great advantage in that it indicates roughly the degree of 
 inflammability of the material, those with high nitrogen percentages, 
 e.g.. from 12:4 to 13:0%, being the varieties used as explosives. 
 Its disadvantage lies in the fact that it tells nothing of the other 
 properties of the material. Hence a rough and exceedingly arbitrary 
 nomenclature has come into existence in the industry, and it must be 
 emphasised that it rests on no scientific basis and is characterised 
 
Nitration of Cellulose 37 
 
 by no sharp limits. The range of commercial cellulose nitrates is 
 as follows :— } 
 
 Nitrogen 
 
 Percentage. Names and Uses. 
 
 10-2-11-2% Sometimes called xyloidin in England. Pyroxylin in U.S. 
 Characterised by solubility in camphor-alcohol. Some of 
 the new low-viscosity nitrates said to have nitrogen as 
 low as 11:0%. 
 
 1]-2-12-4% Collodion cotton, nitro-cotton, pyroxylin. Chiefly used for 
 lacquers and enamels. High solubility in ether—alcohol. 
 Cellulose nitrate used in service cordite during latter part 
 of war had about 12-2—12-4% nitrogen. 
 
 12-4-13:0% | Guncotton. Characterised by diminishing solubility in 
 
 ether—alcohol as nitrogen content rises, but still soluble 
 in acetone. 
 
 
 
 Nitration Plant. 
 
 The conditions to be satisfied as far as practicable in any plant 
 for the treatment of cellulose with nitrating acid are as follows :-— 
 
 (1) It should ensure thorough contact between the cellulose 
 and the acid. 
 
 (2) It should be adapted to maintain the acid balance 
 between spent and revivified acid, with minimum loss of acid. 
 
 (3) Fumes should be minimised, with the object of diminish- 
 ing the discomfort to the workers, reducing losses of acid, and 
 avoiding the corrosive effect of the fumes on other plant and 
 fittings. 
 
 (4) Output should be high, so as to reduce labour charges. 
 
 (5) Repairs should be low. 
 
 Obviously, in all nitration systems, a compromise has to be 
 made between these conditions when they come into conflict. 
 
 For a detailed account of processes of nitration the literature 
 must be consulted. A good description is given by Nathan.1® The 
 methods may be roughly classified as follows :— 
 
 (1) Nitration in pans by direct dipping of the cotton in the 
 acid. | 
 (2) Nitration in centrifugals. 
 (3) Nitration by displacement process. 
 
 (1) Nitration by direct dipping is historically the oldest process. 
 A weighed quantity of cotton is immersed portion by portion in a 
 
38 Cellulose Ester Varnishes 
 
 measured quantity of the mixed acid. Special precautions are taken 
 to withdraw fumes from the workers, who wear protective clothing 
 and sometimes hoods supplied with compressed air. The bulk 
 of the acid may be removed by hydraulic pressure, but more 
 frequently by spinning in a centrifugal machine. The resulting 
 mass is then quickly drowned by sudden immersion in a large 
 quantity of water. 
 
 The chief disadvantages of this method are low output, occasional 
 fuming-off, a great deal of acid fume to be dealt with by ventila- 
 tion and somewhat heavy repair and replacement charges. 
 
 (2) A logical development from the last system is nitration in 
 the centrifugal itself, so as to avoid the transfer of the cotton and 
 acid from the dipping pans to the centrifugal. This process was 
 developed chiefly in Germany. It gives a high output per man, 
 and therefore a low labour charge. The capital cost of the plant 
 is high, and repairs somewhat heavy, as the machines are working 
 in a very corrosive atmosphere. 
 
 (3) The displacement process was developed very considerably 
 during the war to supply nitro-cotton soluble in ether—alcohol for 
 cordite manufacture. The fundamental difference between this 
 process and those previously described lies in the manner in which 
 the cotton is freed from acid after nitration. Instead of a sudden 
 drowning of the mixture of acid and cotton in a large volume of 
 water, an extremely slow displacement is effected, by allowing water 
 to flow in slowly at the top of the nitrating pan while the spent acid 
 is withdrawn at the same rate below. ‘The aim in each process is 
 the same, namely, to remove the acid from the cotton without the 
 great rise in temperature which occurs when a strong nitrating 
 acid is mixed with a moderate quantity of water. The displace- 
 ment process is carried out in wide, shallow earthenware pans. 
 The measured charge of acid is first introduced, and the cotton in 
 as bulky and open a condition as possible is fed in slowly by hand 
 and pushed under the surface with a fork. While this operation 
 is proceeding, the pan is covered with an aluminium hood con- 
 nected to a fume draught. After the cotton has all been introduced 
 and packed as evenly as possible, perforated earthenware plates in 
 segments are carefully placed on top of the mass so as to cover the 
 whole surface and a small quantity of cold water is allowed to run 
 gently over the plates. If the ratio of acid to cotton, the original 
 mechanical preparation of the cotton, and the packing of the pan are 
 correctly judged, the acid should meet the water layer at a level 
 between the upper and lower surfaces of the earthenware covers, 
 
ig Nitration of Cellulose 39 
 
 7.€., in the perforations, so that the actual surface of contact of 
 the water and acid is at a minimum. As soon as the film of water 
 covers the plates, all fuming from the acid is prevented, and the 
 hoods may be removed, ‘The time of nitration may vary from one 
 to four hours, according to the properties desired in the product. 
 When nitration is completed, acid is slowly drawn off at the bottom 
 of the pan and water is run in at the same rate above. The density 
 of the acid is 1-6 to 1-7, so that there is little tendency for the acid 
 and water to mix, and the acid is slowly displaced by the water. 
 The composition of the spent acid issuing below is checked by means 
 of a hydrometer, the readings of which remain practically constant 
 until the zone of acid and water, which results from a slight mixing 
 of the layers, reaches the discharge pipe. This zone can also be 
 recognised by its higher temperature. The strong spent acid is 
 collected in one set of receivers, and a proportion of the weak spent 
 acidin another. The point at which the collection of weak spent acid 
 is stopped is fixed by the economical consideration of whether it pays 
 to recover the acid. 
 
 The following advantages are claimed for the displacement 
 process :— 
 
 (1) Economy of Acid.—The recovery of acid is more complete 
 than is obtained by the use of centrifugal wringing or by hydraulic 
 pressure. It is usual to revivify the greater portion of this, and to 
 return the rest to the acid plant to be treated for the recovery of 
 the nitric and sulphuric acid which it contains. 
 
 (2) Output.—This naturally depends on the time of nitration, 
 but it compares favourably with the output of the other processes 
 (except the centrifugal process) making the same product. The 
 following figures are given by Nathan :— 7 
 
 Output per Man per Week. 
 
 Abel process (Waltham Abbey) 458 lb. 
 
 Direct dipping (Ardeer) . ; 1112 Ib. 
 
 Nitrating centrifugal : j (3000) lb. (Marshall) 
 Displacement (Waltham Abbey) 1742 lb. 
 
 The number of hours worked per week is not stated, but is presum- 
 ably the same for each process. 
 
 (3) There is no handling of acid nitro-cotton. The acid is 
 removed in the original nitrating vessel and the final product is wet 
 with water only. 
 
 (4) The loss of nitro-cotton and acids by fuming-off is much 
 diminished. 
 
40 Cellulose Ester Varnishes 
 
 (5) There are savings in power, water and repairs. On the 
 other hand, the initial capital cost of the plant is high, and the full 
 advantage of acid economy cannot be realised unless the process is 
 used in conjunction with plant for treating the excess of waste 
 acid. 
 
 Kirkpatrick has given an interesting description of the purifica- 
 tion and nitration of linters for lacquer manufacture. The bales of 
 linters are opened, broken up, teased and freed from dust. The 
 cotton is then blown into autoclaves, in which it is boiled with a 
 dilute alkali solution, the final conditions being a two-hour boil at 
 100—110 lb. pressure maintained with live steam. The product 
 is thoroughly washed, then bleached under carefully controlled con- 
 ditions, acidified and washed again: The purified cotton is dried 
 down to a moisture content of 14°%,and in this condition it is nitrated. 
 The nitration plant is a development of the direct dipping method. 
 The nitrating pans are steel drums with conical bottoms, 24 ft. 
 in diameter and 4ft.in depth. Each is stirred by revolving paddles. 
 35 lb. of cotton are nitrated with 1500 lb. of mixed acid for 25 
 to 30 minutes. The charge is then tipped into a centrifugal machine 
 to remove the bulk of the spent acid, which is collected and revivified 
 for future use. The remainder of the acid is removed in a drowning 
 vat. 
 
 Removal of Acid from Nuitro-cellulose. 
 
 The stability of nitrocellulose depends almost entirely on the 
 completeness with which it is purified after nitration. There are 
 two sources of acidity which may give trouble, (a) acid held mechanic- 
 ally in the cotton; (b) sulphuric acid chemically combined with the 
 cellulose as a sulphuric acid ester. The former may be entirely 
 removed by repeated washing with cold water, preferably a hard 
 water containing calcium and magnesium bicarbonates. The 
 removal of free acid from the interior of the fibres is a diffusion 
 process involving time, and the washing must not therefore be unduly 
 hurried, even if the expenditure of water remains the same. For 
 many purposes, a nitro-cotton so purified is sufficiently stable, 
 particularly if it is to be used in enamels containing basic pigments 
 such as zinc oxide. If, however, the highest stability is required, 
 it is necessary to remove the greater part of the combined 
 sulphuric acid also. The combination between the sulphuric acid 
 radical and cellulose is not a stable one, particularly in the presence 
 of traces of moisture and in a warm atmosphere. Free sulphuric 
 acid is slowly liberated, and this has a hydrolysing action on the 
 
Nitration of Cellulose 41 
 
 nitric ester, liberating nitric acid and weakening the film. The 
 removal of combined sulphuric acid is carried out by boiling with 
 several changes of water. The combined nitric acid is scarcely 
 affected by this process of the reaction if the water is kept slightly 
 acid. The boiling process has long been used in the stabilisation 
 of gun-cotton,!* but was first put on a scientific basis by the 
 researches of Robertson.® It may require local modifications 
 depending on the hardness of the water supply and the amount of 
 combined sulphuric acid to be removed, the latter depending again 
 on the kind of cellulose nitrated and the composition of the acid- 
 bath. 
 
 The boiling process is followed by a cold washing process to 
 ensure the complete neutralisation of the liberated acid. It should 
 be noted that the boiling process has a marked influence on the 
 solubility relations of nitro-cotton and tends to make it less viscous. 
 
 Washing is carried out in vats resembling the poachers used 
 by paper-makers. These are provided with a revolving drum 
 furnished with knives bearing against similar knives on a bed- 
 plate, so that the nitrocellulose is circulated, and at the same time 
 cut up. A poacher may deal with a large charge of nitro-cotton— 
 five hundredweight or more—and therefore effects a blend of a 
 number of separate nitrations. For varnish manufacture this blend- 
 ing is sufficient. In the manufacture of explosives, blending must 
 be carried further, since the uniformity of explosive power depends 
 on keeping the percentage of nitrogen constant. 
 
 Bleaching.—For most purposes it is unnecessary to bleach the 
 nitrocellulose, but if a nearly colourless lacquer is required, it is 
 advantageous to bleach with bleaching powder solution in a faintly 
 acid solution. The excess of bleach is usually removed with sulphur- 
 ous acid, followed by a thorough washing. 
 
 Removal of Water: Drying and Dehydration. 
 
 The bulk of the water in the wet cotton so obtained is wrung out 
 in simple centrifugal machines, yielding a product containing 
 usually from 35—40% of water. If the nitro-cotton is to be trans- 
 ported to a distance before use, it is usual to pack it in this form, 
 which is recognised by the railway companies as safe. 
 
 The complete drying of the nitro-cotton is a somewhat dangerous 
 process. It is carried out in stoves heated to a temperature of about 
 40° C., and ventilated to carry off the moisture-laden air. Factories 
 carrying on this process come under the provisions of the Explosives 
 Acts, and the most stringent safety precautions must be taken. 
 
4.2 Cellulose Ester Varnishes 
 
 Fortunately for the cellulose nitrate varnish industry, alcohol 
 is a constituent of all the ordinary varnishes, and nitro-cotton may — 
 be dehydrated in safety by means of alcohol. The process is essen- 
 tially the same as the displacement nitration process, since water is 
 displaced from the wet cotton by the lighter liquid alcohol. There 
 are several variants of the process. Nitro-cotton for cordite 
 (R.D.B.) was dehydrated during the war in the Dupont press, by 
 passing a stream of alcohol under pressure through a mass of damp 
 nitrocellulose in a hydraulic press, the spent alcohol passing away 
 through perforations in the face of the lower ram. This process is 
 much quicker than displacement under atmospheric pressure, or 
 under the usual low pressure of compressed air, and is economic 
 in alcohol. Another method is by spraying with alcohol in a centri- 
 fugal machine, but this needs a larger quantity of alcohol. In all 
 of these processes, the spent alcohol is collected, and redistilled to 
 bring it up to its original strength for re-use, and in passing it may 
 be mentioned that this process may not be carried on except 
 under the supervision of officers of the Board of Customs and 
 Excise. 
 
 The requirements of the varnish manufacturer in regard to 
 dehydration are that the water shall have been effectively removed 
 and replaced by alcohol of such a strength that the nitro-cotton 
 will dissolve in the chosen solvents and film without blooming. 
 The amount of alcohol present in the material must, of course, be 
 taken into account when weighing the nitro-cotton and measuring 
 the liquid ingredients of the varnish. 
 
 Description of the Dehydrated Nitro-cotton. 
 
 Nitro-cotton made by the above processes closely resembles 
 the original cotton, but the fibre is shorter and feels harsher. It 
 has a tendency to ball together into “ pills,’ which, however, are 
 easily opened out again. The properties of chief interest to the 
 manufacturer, and those which are usually incorporated in a 
 specification, are the following :— 
 
 (1) Solubility. —This is measured in a specified solvent or mixture 
 of solvents. If, for example, butyl acetate solutions are of most 
 interest to the manufacturer, he may specify a certain maximum 
 quantity of insoluble matter in a solution in butyl acetate of given 
 concentration. When the solvent specified has a high boiling point 
 such as that of butyl acetate, the amount of insoluble matter is 
 determined usually by centrifuging the solution for a certain time 
 
Nitration of Cellulose 43 
 
 under defined conditions, and measuring the volume of insoluble 
 matter thrown down to the bottom of the tube. If the solvent 
 is readily volatile, a determination can easily be made of the actual 
 amount of nitro-cotton in solution, by evaporating a measured 
 volume to constant weight or by weighing the insoluble residue. 
 The centrifugal method is quicker. 
 
 (2) Viscosity —This property is measured at a standard con- 
 centration in a specified liquid at a definite temperature, either by 
 
 (a) measurement of the rate of flow in a capillary tube, or 
 (b) by the rate of fall of a small steel ball in a column of 
 the liquid. 
 
 Method (a) is most suitable for a highly fluid solution (low 
 viscosity), since the rate of flow of viscous solutions in a capillary 
 tube is so slow that the measurements take too much time. 
 Method (b), on the other hand, is difficult to apply with accuracy to 
 a fluid solution, since the time of fall is too rapid to be measured 
 with accuracy, but it is admirably adapted to viscous solutions. 
 It is desirable that measurements of viscosity should be made in 
 solutions approaching as nearly as is practicable the concentrations 
 of the finished varnishes. Hence, in general, the falling sphere 
 is to be preferred to the Ostwald instrument for technical purposes, 
 and it is rather surprising that the British Engineering Standards 
 Association should have adopted the Ostwald viscometer in its 
 specification. 
 
 In addition to the obvious limitations as to moisture content, 
 ash and acidity, the following properties may also be specified. 
 
 (3) Stability—Imperfectly purified nitro-cotton tends to become 
 acid on keeping, and although in the varnish trade (unlike the explo- 
 _ sives industry) this defect is not attended with danger, it is never- 
 theless often desirable that a maximum rate of decomposition should 
 be fixed. This is particularly so when the varnish films are required 
 to withstand severe atmospheric conditions, but it is also advantage- 
 ous when the varnish is to be used in contact with any metals, 
 either as a protective lacquer, or as a medium for applying metal 
 powder. 
 
 Several methods of testing the stability of cellulose nitrate are 
 known. Since even the most unstable product is very lasting at 
 ordinary temperatures, the tests are always applied to the nitro- 
 cotton heated to a somewhat high temperature, and the assumption 
 is made that the stability at this high temperature affords an 
 estimate of the stability, or the length of useful life, at ordinary 
 
4A Cellulose Ester Varnishes 
 
 temperatures. The two commonest tests are the Abel heat test, 
 and the Bergmann and Junk test. 
 
 The Abel heat test depends on the fact that when cellulose 
 nitrate is exposed to a temperature of 170° F. (76-7° C.) it is slowly 
 decomposed and evolves nitrous fumes. The time is taken for these 
 fumes to reach a concentration at which a piece of moistened filter- 
 paper treated with starch and potassium iodide and suspended in 
 the tube containing the sample, first begins to show discoloration. 
 The significance of the heat test was discussed by Robertson and 
 Smart in 1910,?° and the method of conducting the test is described 
 in the ‘“‘ First Report of the Departmental Committee on the Heat 
 Test as applied to Explosives ’’ (1914). 
 
 The Bergmann and Junk test originated in 1904.21 In this 
 test the nitro-cotton is heated to a much higher temperature (132° C.) 
 for two hours, and the acid fumes evolved are collected in water and 
 estimated. In the original form of the test, the amount of nitrogen 
 in the water was determined by what is known as the Schultze- 
 Tiemann method. The British Engineering Standards Association 
 have adopted the test, but estimate the collected nitrogen acids by 
 a simple acidimetric titration. 
 
 (4) It has already been mentioned that the instability of nitro- 
 cotton is greatly increased by the presence in it of combined 
 sulphuric acid, and that the efficacy of the boiling process as a 
 stabilising agent depends on the reduction in the percentage of this 
 ingredient. Hence an additional but indirect stability test is made 
 by determining the amount of sulphur in the nitro-cotton. This 
 is carried out by oxidising a weighed quantity with nitric acid and 
 potassium or sodium chlorate, and precipitating the sulphuric acid 
 thus produced as barium sulphate. The B.E.8.A. limit for nitro- 
 cellulose for aircraft dope is 0-1% (calculated as H,SO,). For 
 varnish for ordinary purposes, the limit need not be so small, and 
 often no limit is specified. 
 
 (5) Although the nitrogen content of nitro-cotton has no bearing 
 on its usefulness as a base for varnish, it is a useful check against 
 irregularities in nitration and is so easily applied that it should not 
 be omitted. The best method of determination is the measurement 
 of the nitric oxide evolved when the sample is boiled with a 
 solution of ferrous chloride in hydrochloric acid (Schultze-Tiemann). 
 Perhaps the most widely-used method is that of Lunge, in which is 
 measured the volume of nitric oxide evolved when the sample is 
 shaken with mercury and sulphuric acid. Details of both methods 
 are given in standard books on analysis. 
 
Nitration of Cellulose 45 
 
 The B.E.8.A.” specification for nitro-cotton for aeroplane 
 varnish is contained in No. 2, D. 8 of Nov. 1921, Par. 6 and appendix. 
 This specification is more stringent than is necessary for ordinary 
 varnish work, but strictness is necessary on account of the very severe 
 conditions to which aeroplane varnishes are subjected. 
 
 REFERENCES AND BIBLIOGRAPHY. 
 
 1 “Tech. Records of Explosives Supply No. 4. Theory and Practice of 
 Acid Mixing.”” # G. de Bruin, Rec. trav. chim., 1921, 40, 632. %° G. Lunge, 
 J. Amer. Chem. Soc., 1901, 28, 257; Z. angew. Chem., 1906, 19, 2051-2058. 
 4 C. Piest, ibid., 1909, 22, 1215-1224. 5 C. N. Hake and M. Bell, J. Soc. 
 Chem. Ind., 1909, 28, 457-464. ® C. Piest, Z. angew. Chem., 1910, 23, 1009— 
 1018. 7 F. Abel, Trans. Roy. Soc., 1867, 181-253. §® T. Thornley, ‘‘ Cotton 
 Waste.” °* S. K. Kirkpatrick, Chem. and Met. Eng., 1924, 31, 178-182. 
 10 #}. C. de Segundo, J. Roy. Soc. Arts, Feb. 5,1919. 11 R. A. Punter, J. 
 Soc. Chem. Ind., 1920, 39, 3337. 1% C. Piest, E. Stich and W. Vieweg, ‘‘ Das 
 Celluloid,” p. 17. 1% R. Schwarz, Oester. Chem. Zeit., 1919, 22, 50-52, 57— 
 60. 15 D. A. Clibbens and A. Geake, J. Text. Inst., 1924, 115, 27. 18 C. 
 Birtwell, D. A. Clibbens, and B. P. Ridge; ibid., 1923, 14, 2977. 17 W. H. 
 Gibson, L. Spencer and R. McCall, Trans. Chem. Soc., 1920, 117, 479; see 
 also R. Robertson, ‘‘ Aeron. Research Committee, Rep. and Memor.,”’ No. 734, 
 Sept. 1920; and R. A. Joyner, Trans. Chem. Soc., 1922, 121, 1511, 2395. 
 18 fF, L. Nathan, J. Soc. Chem. Ind., 1909, 28, 177-187. 19 R. Robertson, 
 ibid., 1906, 25, 624. °° R. Robertson and B. J. Smart, zbid., 1910, 29, 130- 
 138. 71 E. Bergmann and A. Junk, Z. angew. Chem., 1904, 9, 982, 1018 and 
 1074. *2 To be obtained from the secretary, 28 Victoria Street, S.W., 
 price 2d. post free. 
 
 Additional References. 
 
 A. Marshall, ‘‘ Explosives’? (1915), chaps. vii.xii. E. C. Worden, 
 ** Nitrocellulose Industry,’’ Vol. I., chaps. i.-iii. EE. C. Worden, ‘*‘ Technology 
 of Cellulose Esters,” Vol. I. H. Barthélemy, Rev. des prod. chim., 1921, 
 24, 301-306 (purification and bleaching of cotton linters). J. O. Small and 
 C. A. Higgins, Chemical Age (New York), June 1920 (‘‘ Notes on the Manu- 
 facture of Nitrocellulose,” with photographic illustrations of plant). W. 
 MacNab, “‘ Lectures on Explosives,”’ Institute of Chemistry, 1914. M. A. W. 
 Sapojenikoy, ‘‘ Theory of the Nitration of Cellulose,” Seventh International 
 Congress of Applied Chemistry, Section IIIs, ‘ Explosives”’ (Partridge & 
 Cooper, 1910). F.C. Axtell, J. Ind. Hing. Chem., 1913, 5, 38-47 (a description 
 of the plant of the Sakai Celluloid Co., Japan). J. R. Dupont, Chem. Met. 
 Hing., 1922, 26, 11-16 (‘‘ Manufacture of cellulose nitrate for pyroxylin 
 plastics ’’). Most of these articles are accompanied by illustrations and 
 drawings of plant. 
 
CHAPTER IV 
 ACETYLATION OF CELLULOSE 
 
 Reaction between Cellulose and Acetylating Bath—Differences from Nitra- 
 tion Process—Catalysts—Non-inflammable Cinema Films—Low Viscosity 
 of Acetates—War Research on Dopes—Principal Manufacturers and 
 Outline of their Patented Processes—Investigations on Partial Hydra- 
 tion—Acetylation Processes in which the Acetate does not Dissolve— 
 Acetylation Plant—Properties and Specification of Cellulose Acetate. 
 
 CELLULOSE acetate is produced when cellulose combines with 
 
 acetic acid in accordance with the typical equation of acetylation :— 
 R°OH + HOOC’CH, = R°0°CO’CH, + H,0. 
 
 In order to promote the reaction, it is necessary for a so-called 
 catalyst to be present, and the substance almost always used for 
 this purpose is sulphuric acid. Nominally, its function is to com- 
 bine with the water produced during the esterification. Actually, 
 as in the parallel instance of nitration, it must play a more important 
 part, and the final product always contains combined sulphuric 
 acid. Further, cellulose cannot be acetylated by acetic acid alone. 
 It is necessary to use the more powerful acetylating agent acetic 
 anhydride. This substance alone is capable of taking up all the 
 water produced in the reaction, which is additional evidence that 
 the sulphuric acid is required for some other purpose. 
 
 Differences from Nitration Process. 
 
 Although nitration and acetylation are analogous in theory, 
 in practice some important differences are found. The most obvious 
 arises from the fact that cellulose acetate is soluble in the mixture 
 of acetic acid, acetic anhydride and sulphuric acid used for the 
 acetylation. Hence as the cellulose is acetylated it passes into 
 solution, and the final product of the reaction is a viscous mass 
 from which the ester is obtained by precipitation with water. In 
 the next place, the time required for acetylation is much longer 
 than the time required for nitration. Lastly, the composition 
 of the acid bath is very different from that used for nitration, 
 the percentage of sulphuric acid being very much smaller. 
 
 Catalysts. 
 
 Many substances have been investigated with the object of 
 substituting them for sulphuric acid as the catalyst of the reaction. 
 Sulphuric acid has a strong hydrolytic effect on cellulose, and owing 
 to the slowness of the acetylation process, the cellulose always 
 undergoes a considerable degradation in structure simultaneously 
 with acetylation. One of the principal subjects of research in the 
 
 manufacture has been to find a means of reducing this degradation 
 46 
 
Acetylation of Cellulose Av 
 
 to the smallest limits so as to produce a more viscous acetate. This 
 consideration has not been of much importance in the manufacture 
 of cellulose acetate for solutions, where low viscosity is, within 
 limits, often advantageous, but it has been recognised that the tough- 
 ness and strength of ordinary celluloid are closely related to the high 
 viscosity of its solutions, and that cellulose acetate is not likely 
 to replace cellulose nitrate in celluloid manufacture until it can 
 be made to give solutions of equally high viscosity. 
 
 It has been pointed out? that all substances which assist the 
 reaction between cellulose and acetic anhydride are solvents of 
 both components. Perhaps it would be more correct to say that 
 the catalyst is capable of being adsorbed by the cellulose, and is 
 also miscible with aceti¢ anhydride, so that the latter is brought 
 into intimate contact with swollen cellulose, exposing a large sur- 
 face to acetylation. There is probably also a secondary reaction 
 involved in which the sulphuric acid ester of cellulose is converted 
 into the acetic acid ester. The rate of formation of cellulose acetate 
 is governed, not by the rate of acetylation of the hydroxyl groups 
 of the cellulose, but by the rate of diffusion of the acetylating mixture 
 into the cellulose, which is much slower. Many of the substitutes 
 for sulphuric acid which have been patented are compounds of sul- 
 phuric acid in which the hydrolytic action of the acid is presumed 
 to be diminished, e.g., sodium bisulphate, sulphates of organic 
 amines, metallic sulphates such as copper sulphate, chamber crystals 
 (nitrosyl sulphuric acid), sulphonyl chloride. It may be doubted 
 whether the inhibition of the hydrolysing power of the acid does 
 not correspondingly retard acetylation, and the use of sulphuric 
 acid in the manufacture on the commercial scale is believed to be 
 almost universal. 
 
 The brief sketch in the first chapter brought the history of 
 cellulose acetate up as far as the important hydration process of 
 G. W. Miles, by which it was proved that the solubility relations 
 of cellulose acetate hitherto known could be entirely modified. 
 In following the subject from this point to the present day it must 
 be constantly borne in mind that the main objective of the investi- 
 gators and manufacturers up till the outbreak of war was not the 
 development of cellulose acetate varnishes. A much more valuable 
 prize was waiting for the manufacturer who could produce from cellu- 
 lose acetate a film comparable with the celluloid film which was 
 beginning to be used in such enormous quantities for cinema 
 projection. A more difficult problem still was the substitution of 
 cellulose acetate for cellulose nitrate in the manufacture of celluloid 
 
. 
 
 ‘é 
 48 Cellulose Ester Varnishes 
 
 articles, and it may be remarked parenthetically that few of the 
 patentees of the period seemed to know that celluloid cinemato- 
 graphic films are made from an entirely different variety of cellulose 
 nitrate from that used in celluloid articles; or, if they knew, they 
 did not appreciate the importance of the fact, for one continually 
 meets with the tacit assumption that a variety of cellulose acetate 
 suitable for films would also be adapted to the manufacture of 
 celluloid articles. 
 
 The most evident difference between the early varieties of cellu- 
 lose acetate and the cellulose nitrate used in cinema films lay in the 
 lower viscosity of the former and the weakness and brittleness of 
 the films made from it. Dreyfus is usually credited with having 
 been the first to correlate these two facts, and to realise that in order 
 to obtain a stronger film, a more viscous ester must be made, but 
 the deduction was so obvious that it must have been made by many 
 of those engaged in the celluloid film industry as soon as cellulose 
 acetate appeared on the market. From about 1907, therefore, 
 until 1914, the energies of chemists were bent on producing cellulose 
 acetates of higher viscosity without sacrificing the wider range of 
 solubility which the Miles process had made possible, 
 
 After the outbreak of war, the direction of research was gradually 
 altered. It has already been shown how cellulose acetate was 
 needed for the doping of aeroplane wings, and how the manufacture 
 of the ester for this purpose had to be increased enormously. The 
 need for high viscosity was now not so great, as satisfactory dopes 
 could be made from an ester of medium viscosity which would not be 
 suitable for making a celluloid substitute. It is somewhat para- 
 doxical that the cellulose nitrate manufacturers since the war should 
 have directed their efforts to obtaining an ester of low viscosity, 
 which the acetate manufacturers before the war were doing their 
 best to avoid. | 
 
 Information on the progress of the industry during these periods 
 is almost entirely confined to the patent literature, which is unusually 
 intricate and obscure, fully deserving the caustic comment of Briggs.? 
 It will therefore be most informative to give the names of those 
 firms who were actually manufacturing cellulose acetate in quantity 
 at the outbreak of war, and to outline briefly the lines of advance 
 indicated by their patents. According to Drinker,’ these were :— 
 
 In France :—The Société Chimique des Usines du Rhone. 
 In Germany :—The Bayer Company at Leverkusen. 
 In Switzerland :—The Cellonite Company (Dreyfus Bros.) at Bale. 
 
Acetylation of Cellulose 49 
 
 Worden ® mentions also that small amounts were being made by 
 a firm at Lyons in France, and by another near Boston, Mass., 
 for the manufacture of artificial silk. 
 
 The Société Chimique des Usines du Rhone patented in 1911 ° 
 an entirely novel procedure in acetylation, in which the vapour of 
 acetic anhydride is the active agent. Since the boiling point of 
 acetic anhydride at atmospheric pressure is too high, partial vacuum 
 is employed so that the acetylation takes place at about 50—55°, 
 The cellulose may be either dry, damp with water or with a dilute 
 acid. The patent describes as an example the acetylation of cotton 
 impregnated with sulphuric acid. This is placed in a vessel con- 
 nected with a condenser, receiver and vacuum pump. The vessel 
 is heated to 55°, while the pressure is reduced to 30 mm. The vessel 
 is then connected with a heated still containing acetic anhydride, 
 which boils under this pressure at 50—55°. The vapour of acetic 
 anhydride passes through the mass of cotton and acetylates it, any 
 excess of anhydride being condensed and collected. 
 
 In 1913 the same company patented a process 7 on more con- 
 ventional lines, in which the cellulose is given a preliminary treat- 
 ment which recalls the earlier processes in which the cellulose is 
 modified by treatment with mineral acids previous to acetylation. 
 It differs from them in that acetic anhydride is present during the 
 initial treatment. For example, 10 parts of cellulose is soaked 
 for several hours at 30° in a mixture of 60 parts of glacial acetic acid, 
 4 parts of acetic anhydride, and 0-5 part of sulphuric acid. Twenty- 
 one parts of acetic anhydride is then added, and the cellulose is 
 rapidly acetylated and dissolved. It is recovered by precipitation 
 with water in the usual way and yields an ester soluble in chloroform, 
 very slightly soluble in alcohol, and insoluble in nitrobenzene, 
 acetone or ether. Partial hydrolysis modifies the solubility and may 
 be made to yield esters soluble in acetone. 
 
 In 1914,8 the Usines du Rhone patented a process of acetylation 
 in the presence of trioxymethylene, the treatment with which may 
 either precede or accompany acetylation. For example, 10 parts 
 of cellulose is treated for some hours at 30° with 60 parts of glacial 
 acetic acid, 4 parts of acetic anhydride, 0-5 part of 100% sulphuric 
 acid and 1 part of trioxymethylene. Twenty-one parts of acetic 
 anhydride is added, and acetylation is carried on at 40°. 
 
 A variant of this process was patented later in the same year ® 
 in which the trioxymethylene is first converted into methylene sul- 
 phate, by treatment with fuming sulphuric acid, and the methylene 
 ieumert is used as catalyst without the addition of free sulphuric 
 
50 Cellulose Ester Varnishes 
 
 acid. In another patent in the same year,!° the process described 
 in F.P, 473,399 (q.v.) is modified by the use of a little nitric acid 
 in the preliminary treatment. The product of acetylation is described 
 as a nitroacetate, and may contain, for example, 0:5°% of combined 
 nitrogen. 
 
 In 1920 another variation of the process was patented 11 in 
 which the preliminary treatment of the cellulose before the actual 
 acetylation is carried out at a lower temperature (25—30°) and with 
 less sulphuric acid present (3—5°%). The esters thus obtained are 
 insoluble in chloroform. If partially hydrolysed before precipita- 
 tion, the ester may be obtained in a form insoluble in chloroform 
 and soluble in acetone, or soluble in ethyl acetate. 
 
 The Bayer Company first appear in the patent list in 1901. 
 In their earliest patent 12 they describe the preparation of an ester 
 soluble in alcohol, by acting on a hydrated cellulose with acetic 
 anhydride, acetic acid and sulphuric acid. Later they bought Miles’s 
 patent, covering the production of acetone-soluble cellulose acetate 
 by the hydration process. Worden? describes an acetylation 
 by this process as follows: 100 parts of dry cellulose is acetylated 
 with a mixture of about 300 parts of acetic anhydride and 400 parts 
 of glacial acetic acid with 5 to 9 parts of sulphuric acid as catalyst. 
 The temperature is first kept at about 40°, but may later rise to 
 50—60°. After from 36 to 40 hours, the cellulose should have passed 
 entirely into solution and a sample on precipitation should be soluble 
 in cold chloroform, but not in acetone. The partial hydrolysis 
 which is characteristic of Miles’s process is then begun, by adding 
 slowly, without interrupting the kneading process, a mixture of 60 
 to 65 parts of water and 60 parts of acetic acid. The product of 
 the original reaction approximates to a triacetate, 7.¢e., an ester con- 
 taining three acetyl groups to each ©, unit. After the addition 
 of water, test samples are taken at intervals. It is found that the 
 solubility in chloroform gradually lessens, while that in acetone 
 increases. The process is stopped when the solubility in acetone 
 is complete. At this point, the solubility in chloroform has dimin- 
 ished until the acetate merely swells in that liquid without dispersing. ~ 
 This process of allowing the cellulose acetate to stand while the hydra- 
 tion and solubility changes are proceeding is usually spoken of as 
 ripening. It may need from 12 to 16 hours at a temperature of 40— 
 50°, and during this time the mass is removed from the kneaders 
 and allowed to ripen in separate vessels, which are not usually 
 stirred. ‘The cellulose acetate so produced yields stronger films 
 than those obtained from a solution of the original acetate in chloro- 
 
Acetylation of Cellulose 51 
 
 form. It should be noted that if the hydration of the acetate is 
 allowed to proceed beyond this stage, the product becomes unaffected 
 by chloroform, and the acetone solution will bear the addition of 
 more and more water without precipitation of the ester.® 
 
 A variation of Miles’s process is described by Bayer in 1906, 
 in which dry cellulose acetate is soaked in hydrochloric acid until 
 it is soluble in acetone. 
 
 The course of the hydration process has been investigated by 
 Ost and the results were published in a series of papers in 1919.15 
 It is interesting to note that the hydration process bears some 
 resemblance to the stabilising of cellulose nitrate by boiling water, 
 in that combined sulphuric acid is eliminated from the ester with 
 proportionately little disturbance to the combined acetic acid. For 
 example, in one of Ost’s experiments, a primary acetylation was 
 allowed to “‘ripen’’ with the addition of a little water for four 
 days, during which the combined sulphuric acid dropped from over 
 3% to 0-17%, and the acetic acid from 57-4% to 50-9%. 
 
 Ost classified the cellulose acetates as follows :— 
 
 (1) The primary products of acetylation are acetates soluble 
 in chloroform, preferably with the addition of a little alcohol. 
 They dissolve in acetone, but yield unsatisfactory films which 
 cannot be entirely redissolved. 
 
 (2) The secondary acetates are derived from the primary 
 acetates by mild hydrolysis (hydration). They are easily 
 soluble in acetone and yield good films which can be entirely 
 redissolved. 
 
 He found that the limits of acetone ening lay between 50-9 
 and 57-69% of combined acetic acid. 
 
 Generalisations on the solubility of cellulose esters are always 
 unsafe, as there are so many factors which can be varied in their 
 preparation. The solubility relations of primary acetates prepared 
 by the later Dreyfus processes do not agree with Ost’s conclusions, 
 as they are insoluble in chloroform and can be redissolved in acetone 
 indefinitely.1* Nevertheless, Ost’s work contains much useful 
 information on the influence of various catalysts on the course of 
 the acetylation process, and has thrown much light on an obscure 
 and difficult subject. 
 
 In 19101” F. Bayer and Co. describe an acetylation process 
 in which aniline bisulphate is used as catalyst instead of sulphuric 
 acid. This process falls into the category mentioned earlier in 
 the chapter of those in which an attempt is made to depress the 
 
52 Cellulose Ester Varnishes 
 
 hydrolytic activity of sulphuric acid by combining it with another 
 substance—in this instance an aromatic base. 
 
 The Dreyfus patents are the most important of all from the 
 technical point of view. The first patent appeared in 1911,'® 
 and it has been followed by a long series of patents right up to the 
 present time, dealing with all aspects of the manufacture and utilisa- 
 tion of cellulose acetates. A complete list is given by Worden.® 
 The conditions to be controlled are (a) the ratio of the weights of 
 acetylating agent and of condensing (or catalysing) agent to the weight 
 of cellulose; (b) the ratio of the weight of condensing agent to the 
 humidity of the cellulose, (c) the temperature. As an example, 
 the directions in the French Patent 478,023 of H. Dreyfus 1° may be 
 briefly abstracted: A mixture of 300—400 kilos. of glacial acetic 
 acid, 250 kilos. of acetic anhydride and 10—15 kilos. of concentrated 
 sulphuric acid is cooled to below 0°. One hundred kilos. of cotton 
 or paper of normal humidity (3—6%) is stirred into the mixture. 
 After a time the temperature rises to 5—15° and then falls again to 
 5—10°. The cooling may then be discontinued, and the tempera- 
 ture may be allowed to rise to 15—20°. If it is allowed to rise higher 
 than 25—35°, the viscosity of the product will be diminished. 
 Cooling is then started again with constant stirring until the tem- 
 perature of the mass begins to fall, and the mixture is then allowed 
 to stand until all the fibres have disappeared. At this stage water 
 may be added and the mixture allowed to stand in order to modify 
 the solubility of the product (cf. Miles’s process). This process is 
 said to produce cellulose acetate of maximum viscosity insoluble in 
 chloroform, and great stress is laid on the accurate observance of 
 the limits of temperature allowed. 
 
 Processes in which the Cellulose Acetate does not Dissolve. 
 
 Mention should also be made of processes of acetylation in which 
 the original form of the cellulose is preserved. This is achieved by 
 diluting the acetylation mixture with a liquid in which cellulose 
 acetate is insoluble, but which is miscible with the combined acetyl- 
 ating reagents. The earliest patents on this subject are those of 
 Lederer 2° and Mork.?!_ Lederer uses carbon tetrachloride as the 
 non-solvent diluent, while Mork uses benzene. This process appears 
 to have at least two great advantages over the usual process in 
 which the cellulose acetate goes into solution. In the first place, 
 the greater bulk of liquid and the greater ease with which the mass 
 can be stirred must greatly facilitate temperature control. In 
 the second place, the spent acetylating bath can be separated from 
 
Acetylation of Cellulose 538 
 
 the acetate without having to use precipitation by water (which 
 destroys any unused acetic anhydride and dilutes the acetic acid). 
 One would expect, therefore, that it would be possible to revivify 
 the spent bath in much the same way as a nitration bath is brought 
 up to its original strength. The process has certainly been used 
 on a commercial scale in the United States and the writer has examined 
 cellulose acetate made in this way which showed excellent solubility. 
 However, the method has not been widely adopted, and there is 
 probably some technical disadvantage attached to it which does 
 not appear on the surface. 
 
 Very little information has been published about the plant used 
 in the acetylation of cellulose, nor is it possible to tell from the patent 
 literature what are the processes actually used in the factories. The 
 outline is as follows : The kneading of the cellulose with the acetyl- 
 ating mixture of acetic anhydride, acetic acid and sulphuric acid 
 takes place in bronze-lined kneaders fitted with two shafts bearing 
 curved blades, designed to shear the contents and keep them con- 
 tinuously in motion. The kneaders are jacketed so that either 
 warm or cold water, or refrigerating brine, may be circulated to 
 control the temperature. Although the Dreyfus patents lay great 
 stress on keeping the temperature below certain limits when working 
 for cellulose acetate of high viscosity, this condition is not so impor- 
 tant when the acetate is required for the manufacture of dope, and 
 it will be noticed that the Rhone processes do not employ brine: 
 refrigeration. If cold acetylation is required, the acetylating mix- 
 ture is given a preliminary cooling in a copper or bronze vessel 
 provided with coils for the circulation of brine, before it is introduced 
 into the kneader. 
 
 The Dreyfus factories are said to employ cellulose in the form 
 of a special cotton tissue paper, and the Usines du Rhone a special 
 long staple cotton. The British Engineering Standards Associa- 
 tion 2? specify for cellulose used in the manufacture of cellulose 
 acetate that it must be neutral in reaction and free from starch. It 
 must not contain more than 8% of moisture, 0-5% soluble in ether, 
 0-5% soluble in water, and it must not yield more than 0-5% of ash. 
 When boiled with 3% caustic soda under specified conditions it 
 must not lose more than 4% in weight. 
 
 After the cellulose has dissolved in the acetylating mixture, it 
 undergoes a ripening process, and for this purpose it is transferred 
 to copper or enamelled vessels which are maintained in a room kept 
 at the appropriate temperature, usually from 30° to 45°. When 
 the examination of samples shows that the right degree of solubility 
 
54 Cellulose Ester Varnishes 
 
 has been attained (which usually means complete solubility in 
 anhydrous acetone), the cellulose acetate is precipitated by pouring 
 it into a large volume of water. This is done in a wooden vat fitted 
 with stirring gear. The aim of this process is to produce a precipi- 
 tate in as fine a form as possible, so as to facilitate the washing-out 
 of all traces of free acid, and it is difficult to control the conditions 
 so as to avoid the formation of large clots. The liquor now con- 
 tains the excess of acetic anhydride and acetic acid in the form 
 of dilute acetic acid together with a small percentage of sulphuric 
 acid. This is treated for the recovery of acetic acid, usually as 
 sodium acetate, which can be employed for the manufacture of — 
 acetic anhydride or glacial acetic acid. The precipitate of cellulose 
 acetate is thoroughly washed, with a preliminary grinding if there 
 are large lumps in it. It is then separated off by filtration or cen- 
 trifuging, dried in warm stoves at 40—50° and ground up. It is 
 particularly necessary that the ester should be in a fine condition 
 so as to dissolve more readily. 
 
 Properties of Cellulose Acetate. 
 
 Cellulose acetate as it usually occurs in commerce is a white 
 and somewhat dusty powder, not entirely free from small, tough 
 lumps which were originally clots formed in the precipitation 
 by water. The product of those processes in which the cellulose 
 is not allowed to dissolve (Mork and Lederer) retains the original 
 form of the cotton. Cellulose acetate, like cellulose nitrate, is much 
 more resistant to water than the cellulose from which it is made. 
 
 It can be distinguished from cellulose nitrate by its solubility 
 in a mixture of chloroform and alcohol, or of tetrachlorethane and 
 alcohol, and by the slowness with which it burns when heated on 
 porcelain or platinum. 
 
 Several methods have been proposed for the estimation of com- 
 bined acetic acid, of which the simplest and best is probably that of 
 Kberstadt,?? although Fenton and Berry *4 prefer Ost’s method.1® 
 Both depend on the saponification of a weighed quantity of cellulose 
 acetate by alkali. 
 
 Kberstadt’s 25 method depends on the fact that cellulose acetate 
 swells in a mixture of alcohol and water, and in the swollen state 
 is saponified rapidly and completely by caustic alkali. 0-2—0-3 
 gramme of cellulose acetate is moistened with a small quantity 
 of alcohol, to which 10 c.c. of N-alkali is added. The flask is 
 shaken occasionally, and at the end of an hour the excess of alkali 
 is titrated with normal acid.*¢ 
 
Acetylation of Cellulose 55 
 
 Alkaline saponifications are preferred by most authorities to 
 the acid methods in which the acetate is hydrolysed by sulphuric 
 or some other non-volatile acid, and the acetic acid distilled off in 
 steam. 
 
 Barnett 1 describes another variation of the alkaline method 
 in which the sample is dissolved in acetone, and saponified by shaking 
 it with standard caustic soda in a stoppered flask. He indicates 
 a slight correction due to neutralisation of soda by the cellulose. 
 
 Barr and Bircumshaw 2’ prefer Barthélemy’s 28 method of hot 
 saponification with normal aqueous sodium hydroxide at 85°, 
 stating that they have obtained more concordant results by this 
 method than by older ones, although the results are not necessarily 
 more accurate. 
 
 Determination of Sulphur. 
 
 The presence of combined sulphuric acid in cellulose acetate 
 has the same detrimental influence on the stability of the ester as 
 it has on cellulose nitrate. It may be determined by oxidising a 
 weighed quantity with aqua regia, or with a mixture of nitric acid 
 and potassium chlorate, and precipitating the barium chloride as 
 sulphate with the usual precautions. 
 
 Entat and Vulquin ?® hydrolyse with water under pressure at 
 125°, and estimate the sulphuric acid by electrometric titration. 
 They state that the percentage of combined sulphuric acid in a 
 good sample should not exceed 0-:03°%. This is a somewhat stringent 
 limit, as a good sample analysed by Barr and Bircumshaw (loc. cit.) 
 contained 0:04%. 
 
 Charring Test. 
 
 A second useful stability test developed during the war was the 
 determination of the temperature at which charring is first noticed. 
 The charring is chiefly due to the liberation and concentration of 
 liberated traces of sulphuric acid. Barr and Bircumshaw 2? mention 
 the test and it has been standardised by the British Engineering 
 Standards Association. 
 
 A third stability test measures the loss of acetic acid when the 
 sample is heated to a high temperature for a fixed time (compare 
 the Bergmann-Junk test for nitrocellulose). In its earlier form,?’ 
 this was carried out by passing air through the acetate and collecting 
 in standard alkali the acetic acid vapour carried off. In its standard- 
 ised form the sample is heated with water in a sealed tube for a 
 specified time and the acid set free is titrated with standard alkali. 
 
56 Cellulose Ester Varnishes 
 
 Solubility is determined in a chosen solvent by dissolving the 
 acetate in the ingredients of the solvent under specified conditions, 
 spinning the solution in a centrifuge and reading off the volume 
 of insoluble matter. If volatile solvents are chosen, such as acetone 
 or methyl acetate, the weight of ester in an aliquot portion drawn 
 off from the clear layer after the solution has stood for some hours, 
 may be found directly by evaporation. 
 
 Viscosity. 
 
 Viscosity is usually the most important property to be measured, 
 and this test is carried out on a solution of fixed concentration in a 
 specified solvent. The British Engineering Standards Association *° 
 issue specifications for two varieties of cellulose acetate, one described 
 as ‘“‘ Cellulose Acetate’? (2D. 6) and the other as “ Medium Vis- 
 cosity Cellulose Acetate ’’ (D. 50). The solvent in each instance 
 is made up from :— 
 
 14 ¢.c. ethyl alcohol, 
 14 c.c. benzene, 
 10 c.c. methyl ethyl ketone, 
 60 c.c. acetone, 
 2 c.c. benzyl alcohol, 
 
 and in this mixture 6 grammes of the sample must be dissolved. 
 The viscosity at 25° is determined in an Ostwald viscometer and 
 must lie between 30 and 40 for “‘ cellulose acetate,’ and between 
 15 and 21 for “ medium viscosity cellulose acetate,” the standard 
 substance being glycerol, which is given a viscosity in empirical 
 units of 100. Barr 31 has investigated the suitability of glycerol as a 
 standard in viscometry and has drawn attention to the precautions 
 necessary to secure uniformity of results. 
 
 The other routine tests are those for water content, acidity, 
 ash and filming capacity. They do not call for special comment. — 
 
 REFERENCES AND BIBLIOGRAPHY. 
 
 1 W. L. Barnett, J. Soc. Chem. Ind., 1921, 40, 81-107. 2% J. Boeseken, 
 J. C. van den Berg, and A. H. Kerstjens, Rec. trav. chim., Pays-Bas, 1916, 
 35, 320-345. §% J. F. Briggs, Ann. Rep. Soc. Chem. Ind., 1918, 190. # P. 
 Drinker, J. Ind. Eng. Chem., 1921,18, 835. 5 E. C. Worden, J. Soc. Chem. Ind., 
 1919, 38, 3707-3747. & Soc. Chim. des Usines du Rhone, E.P. 25,893/1911. 
 * Ibid., F.P. 473,399/1914 (Int. Conv. 1913). 8 Ibid., E.P. 7,773/1915 (Int. 
 Conv. 1914). % Ibid., E.P. 10,822/1915 (Int. Conv. 1914). 1 Jbid., E.P. 
 8,046/1915 (Int. Cony. 1914). 11 Ibid., E.P. 146,092/1920 (Int. Conv. 1919). 
 12 F. Bayer & Co., E.P. 21,628/1901. 1% E. C. Worden, ‘‘ Technology of 
 Cellulose Esters,” Vol. VIII., 2567-2575 (see also M. Deschiens, Chim. et 
 Ind., 1920, 8, 591-607). 14 F. Bayer & Co., E.P. 24,067/1906. 45 H. Ost, 
 
Acetylation of Cellulose 57 
 
 Z. angew. Chem., 1919, 32, 66-70, 76-79, 82-89. 1° C. F. Cross, J. Soc. 
 Dyers and Col., 1920, 36, 19. 17 F. Bayer & Co., E.P. 14,271/1910. 18 H. 
 Dreyfus, E.P. 20,977/1911. %#® H. Dreyfus, F.P. 478,023/1914. % L. 
 Lederer, E.P. 3,103/1907. #1! H. 8S. Mork, U.S.P. 854,374/1907. 2? British 
 Engineering Standards Association, Specifications 2D. 6 and D. 50.* 23 E. 
 Knoevenagel and O. Eberstadt, Koll. Chem. Bethefte, 1921, 13, 194-212. 
 24H. J. H. Fenton and A. J. Berry, Proc. Camb. Phil. Soc., 1920, 20, 16-22. 
 25 See also E. Knoevenagel and K. Koenig, Cellulosechemie, 1922, 3, 113-122. 
 26 Q. Torii, J. Chem. Ind. Japan, 1922, 25, 118-131. 2? G. Barr and L. L. 
 Bircumshaw, Advis. Comm. for Aeronautics, Rep. and Mem. No. 303, June 
 1916—-Nov. 1917. 78 Barthélemy, Monit. Scient., 1913, 78, 549. 9 M. Entat 
 and E. Vulquin, Ann. Chim. Analyt., 1922, 4, 131-135. %° British Engineer- 
 ing Standards Association, Specifications 2D. 6 and D. 50.* 31 G. Barr, 
 Aer. Rev. Committee, Rep. and Mem. No. 755, Sept. 1920. 
 
 Additional References. 
 
 L. Clément and C. Riviére, ‘* La Cellulose ’’ (1920), 87-95. 
 
 * Specifications published by the British Engineering Standards Associa- 
 tion may be obtained from the Secretary of the Association, 28 Victoria Street, 
 Westminster, 8.W. 1, price 2d. post free. 
 
CHAPTER V 
 CELLULOSE ESTER SOLUTIONS: SOME PROPERTIES. PART I 
 
 Phenomena accompanying Dissolution—Viscosity—Movement of Liquids 
 in Capillary Tubes—Falling Sphere Viscometer—Dispersion—Solu- 
 bility Limit—Solvent Power—Temperature Efféct—Constitution of the 
 Esters—Disintegration of Structure during Esterification—Fractional _ 
 Precipitation of Solutions—Dimensions of Cellulose Hydrolysed by 
 Acids—Contrast with Alkaline Treatment—Chemical Constitution of 
 Solvents of Cellulose Esters—Chiefly Carbonyl Derivatives—Cellulose 
 Nitrate in Single Solvents—Baker’s Researches on Concentration and 
 Viscosity—Later Researches—Mixed Solvents—Acetone and Water— 
 Ether and Alcohol—Cellulose Acetate in Binary Mixtures containing 
 Acetone—Mardles Researches —Association and De-association of the 
 Solvents—Plastic Flow (Bingham)—The Tyndall Effect. 
 
 CoLLOIDAL NATURE OF CELLULOSE EsTER SOLUTIONS 
 Phenomena accompanying Dissolution. 
 
 Ir a small quantity of dry cellulose nitrate or acetate is covered 
 with a suitable solvent, e.g., acetone, the ester rapidly turns to a 
 transparent jelly, which if left undisturbed will remain at the 
 bottom of the vessel almost indefinitely. If the mass is stirred or 
 shaken, the jelly is gradually distributed in the solution until the 
 latter becomes homogeneous to the naked eye, except for small 
 particles of insoluble impurities, such as incompletely esterified 
 cellulose and dust, which are seldom entirely absent from the technical 
 esters. If the solution is centrifuged at an early stage of the mixing, 
 a considerable quantity of undispersed jelly may be thrown down 
 to the bottom of the tube, but as the mixing becomes more com- 
 plete the amount becomes less, and a well-mixed solution of ester 
 of good solubility when whirled in a centrifuge will deposit little 
 except the insoluble impurities already mentioned. The features 
 accompanying the dispersion of nitrocellulose in ether—alcohol 
 and acetone in particular are described by Masson and McCall,! 
 and a very thorough investigation of the preliminary swelling of 
 cellulose acetate in mixtures of solvent and non-solvent liquids was 
 made by Knoevenagel and his pupils (v. infra). 
 
 The most noticeable property of the solutions is their increased 
 viscosity when compared with the original solvents. This is roughly 
 demonstrated by tipping or inverting the vessels containing them. 
 The name of “colloid”’ was originally given by Thomas Graham 
 to those substances which, like glue, form viscous solutions, in which 
 the dissolved substance possesses a very low rate of diffusion com- 
 pared with crystalline bodies. The cellulose. esters form viscous 
 colloidal solutions, known as “sols” (also after Graham), while 
 under some conditions they may deposit some or all of their colloid 
 
 content as “gels.” The nomenclature of colloid chemistry is not 
 58 
 
Cellulose Ester Solutions: Some Properties 59 
 
 yet quite firmly established, and colloid terms will not be used in 
 this chapter more than is necessary. 
 
 Viscosity. 
 
 If we imagine two plane surfaces in a liquid, each of unit area 
 (one square cm.), and one centimetre apart, forming as it were two 
 opposite surfaces of a cube of the liquid, it is evident that to keep 
 these surfaces moving relatively to each other, each remaining in its 
 own plane, will require more force in some liquids than in others. 
 The inherent property in a liquid which opposes this motion is 
 called its viscosity, and is defined as the tangential force on a unit 
 area of either of two horizontal 
 planes at a unit distance apart 
 required to move one plane 
 with unit velocity in reference 
 to the other plane, the space (a) 
 between being filled with the 
 viscous substance (Bingham,? 
 from Clerk-Maxwell). If the 
 force applied is F’, the velocity 
 of one plane relative to the (b) 
 other is v, and their distance 
 apart s, the coefficient of vis- 
 
 : F's 
 cosity 7 = pe 
 
 The viscosity of a liquid is 
 sometimes called its internal 
 friction, and the analogy with 
 solid friction will be apparent, 
 although it must not be taken 
 too far. The viscosity of cellu- 
 lose ester solutions increases at a very rapid rate with the con- 
 centration, so that in plotting the viscosity—concentration curve 
 it is usual to plot the logarithm of the viscosity instead of the 
 actual number. This has led to the deduction of some interest- 
 ing empirical equations connecting viscosity and concentration 
 (v. infra). 
 
 The simplest mechanical illustration of layers of liquid sliding 
 over one another is given by deforming a pack of cards by a shearing 
 stress from the shape illustrated in Fig. 1 (a) to the shape illustrated 
 in Fig. 1 (b). The experimental measurement of viscosity, how- 
 ever, by a method reproducing the conditions laid down in the 
 definition, would be extremely difficult, and the classic method is by 
 
 (c) 
 Fia. 1. 
 
60 Cellulose Ester Varnishes 
 
 the movement of the liquid in a capillary tube. It is not easy at 
 first to see how this movement realises the theoretical idea of layers 
 of liquid sliding over one another, but if we take as an illustration, 
 instead of a pack of cards, a rolled-up tape measure, and deform it by 
 pushing the centre out (cf. Fig. 1 (c)), we see how the movement of 
 liquid in a capillary tube may be regarded as being made up of a 
 series of concentric layers moving relatively to each other, the velocity 
 being zero at the circumference (where the liquid touches the tube) 
 and greatest at the centre. 
 
 The equation connecting the radius of the tube, the head of 
 pressure on the liquid, the rate of flow and the viscosity of the liquid 
 
 2 
 i897 == sai in which 
 = pressure on the liquid. 
 g = acceleration due to gravity. 
 R = radius of capillary. 
 1 = length of tube. 
 
 I = velocity of flow (c.c. per sec.). 
 
 It was derived experimentally in another form by Poiseuille in 1842. 
 Poiseuille’s original publications are not readily available, but his 
 results have been republished by Bingham, and should be con- 
 sulted by all those interested in the subject. The accuracy of the 
 agreement with the derived equation is on the whole remarkable. 
 
 The measurement of rate of flow in a capillary tube is still per- 
 haps the most widely used method of determining viscosity. For 
 dealing with solutions of very high viscosity, however, there are 
 more convenient methods. One depends on measurement of the 
 rate of fall of a small steel ball through the solution, and makes use 
 of the original mathematical investigation of Stokes. It was 
 developed for cellulose ester solutions by Sheppard * and by Gibson 
 and Jacobs.* Others measure the actual friction when one metal 
 cylinder is rotated inside another concentric with it, the space 
 between being filled with the liquid under investigation. Measure- 
 ments of the viscosity of cellulose esters for technical purposes are 
 usually made either with the Ostwald (capillary tube) (B.E.S.A.) 
 or the falling sphere instruments. 
 
 Dispersion. 
 
 Strictly speaking, the term solution should be applied only to 
 those mixtures in which the components are subdivided down to 
 molecular limits. Viscous colloids are believed to subdivide into 
 particles much larger than molecules, and they are therefore said to 
 
Cellulose Ester Solutions: Some Properties 61 
 
 disperse rather than to dissolve, and to form dispersions rather than 
 solutions. The term “ colloid solution,’? however, it used even in 
 purely scientific literature when the context provides against mis- 
 conception. It is becoming customary to distinguish between 
 “solution ’’ and “ dissolution,” the latter referring to the process of 
 dissolving, and the former to the product. ‘‘ Dispersion” at 
 present carries both meanings, but “ spersion ’’ may arrive in due 
 course. 
 Solubility Limit. 
 ~ When a crystalline substance, e.g., sodium chloride, is dissolved 
 in water, it is common knowledge that if the quantity of salt is 
 increased, a point is reached at which the liquid, at any fixed tempera- 
 ture, will dissolve no more of the solid. An alteration in temperature 
 will usually cause an alteration in the quantity that can be dis- 
 solved, but the limiting concentration is just as sharp. The cellulose 
 esters behave differently. If we continue to add cellulose nitrate 
 to acetone, it will disperse with more and more difficulty owing to 
 the mechanical difficulty of stirring the viscous mass, but we do 
 not reach a point at which the acetone is saturated with cellulose 
 nitrate and cannot dissolve any more. The limit appears to be set 
 only by the mechanical strength of our apparatus. Hence we cannot 
 compare the technical advantages of different solvents of a cellulose 
 ester by measuring the concentration of the ester at saturation 
 point, as we should do if we were dealing with a crystalline sub- 
 stance; other methods have to be used. The most obvious one is 
 suggested by the earlier part of this paragraph. If the dispersion 
 of the ester in the solvent is limited only by the mechanical strength 
 of the apparatus, that solvent is the best (in regard to solvent power 
 only) which at equal concentration gives the least viscous, and 
 therefore most easily workable, solution. Hence one way of 
 comparing solvent power is to measure the viscosity of solutions 
 of the same cellulose ester at the same concentration. 
 Another method arises indirectly from economic considerations. 
 It is often possible, under conditions which will be discussed later, 
 to dilute solutions of a cellulose ester in a solvent with a cheap 
 non-solvent. For example, a solution of cellulose nitrate in amyl 
 acetate may be diluted with petroleum spirit to a considerable 
 extent without causing precipitation of the dissolved ester. Hence 
 one may compare two solvents, such as, for example, amyl acetate 
 and ethyl acetate, by preparing from each a solution of cellulose 
 nitrate of identical concentration, and determining what proportion 
 
62 Cellulose Ester Varnishes 
 
 of petroleum spirit may be added to each, at any chosen tempera- 
 ture, before precipitation begins. 
 
 Correlation of Solvent Power and Viscosity of Cellulose Ester 
 Solutions. 
 
 This experiment determines the ‘‘ solvent power number,” which 
 has been defined by Mardles ® as the volume in cubic centimetres 
 of a miscible non-solvent required to start precipitation of a cellulose 
 ester from 1 gramme of a 5% solution at 20°. — 
 
 NS 
 
 Viscosity —> 
 aS © 
 - ——— Solvent Power Number. 
 
 A) 
 
 
 
 80° 70 60 580° 40° 30 202-7 O 
 Alcohol per cent. ~ 
 
 Fie. 2 (Mardles).—The curves connecting viscosity and solvent power number 
 with the composition of the solvent are here plotted for a solution of nitrocellulose 
 in ether-alcohol. Note that each curve displays a decided minimum at approxi- 
 mately 50—60% of alcohol in the solvent, and that viscosity is measured upwards 
 and solvent power number downwards. Thus minimum viscosity corresponds 
 approximately with maximum solvent power, 7.e., maximum resistance to pre- 
 cipitation by a diluent. 
 
 It has been shown by Mardles © that there is a near relation 
 between the viscosity of a solution of cellulose ester and the solvent 
 power, and if these two properties are measured for solutions of 
 cellulose esters in mixtures of two liquids in varying proportions, 
 the curves expressing the results are generally similar in shape, 
 z.e., these two methods of estimating the solvent value of a liquid 
 
Cellulose Ester Solutions: Some Properties 68 
 
 usually show moderately good agreement (see Fig. 2). Mardles 
 points out that on theoretical grounds the best solvent mixture 
 should be the least viscous (1.e., the most fluid), since high fluidity 
 indicates a considerable degree of dispersion and therefore a close 
 approach to true solution. The industrial value of these measure- 
 ments is evident from the fact that the solvents of highest solvent 
 power are those which dissolve the esters most rapidly, and which 
 give solutions of required viscosity with the smallest volume of 
 solvent, while the solutions are the clearest and have the least 
 tendency to coagulate. 
 
 Effect of Temperature. 
 
 Although there is no definite limit to the amount of cellulose 
 ester which can be dissolved in a given solvent, there exists for many 
 substances a temperature above which they are solvents and below 
 which they are not. This critical temperature has been termed the 
 transition temperature.’ It is not quite sharp, probably owing to 
 the heterogeneity of commercial cellulose esters, and it varies 
 slightly with the concentration, so that the concentration of the 
 solution should be stated when quoting it. The usual concentration 
 employed for the determination is 59%. Increase of temperature 
 always lowers the viscosity of solutions. 
 
 Chemical Constitution of the Cellulose Esters. 
 
 Before considering the results obtained in the investigation of 
 single and mixed solvents, a few words should be said about the 
 constitution of the esters themselves. 
 
 It will be recalled that cellulose has been proved to be built up 
 of glucose anhydride residues, having the formula :— 
 
 The groups B and C are themselves built up of similar glucose 
 units. This formula contains three hydroxyl groups in every C, unit, 
 and we may write it as [C,H,O,(OH)3)n. These hydroxyl groups may 
 theoretically be esterified in three stages, yielding the compounds 
 
 C,H,0,(0H),(OB) 
 C,H,0,(0H) (OR), 
 C.H,0,(OR), 
 
64 Cellulose Ester Varnishes 
 
 R being CH,:CO in the acetic esters and NO, in the nitric esters. 
 In the following table are shown the percentages of hydroxyl group, 
 of acetyl group as acetic acid and of nitryl group as nitrogen in the 
 theoretical cellulose group as the hydroxyl groups are successively 
 esterified :— 
 
 
 
 
 
 % OH % acetic On ASEL YN 
 in acid in in in 
 acetate. acetate. nitrate. nitrate. 
 Celloee |; crises cag ccteabendates [31-5] nil [31-5] nil 
 - mono-ester ......... 16-7 29-4 16-4 6:8 
 oe EEE ORES ae a ates 6:9 48-8 6°7 11-1 
 ve, ETS OMEN! eB ak nil 62-5 nil 14-1 
 
 It must be emphasised that these figures refer only to a theoretical 
 esterification in which there has been no simultaneous hydrolysis of 
 anhydride oxygen (—O—) atoms in the cellulose. 
 
 Thus the theoretical cellulose di-nitrate contains 11:1% of 
 nitrogen and 6:7% of hydroxyl. The corresponding di-acetate con- 
 tains acetyl groups equivalent to 488% of acetic acid and 6-9% of 
 hydroxyl, while the tri-acetate contains 62-5°% of acetic acid and no 
 hydroxyl. It is evident therefore that, except in the instance of 
 completely esterified cellulose, there is sufficient free hydroxyl in 
 the ester to lead us to expect some evidence of its presence in the 
 properties of the material. We know, moreover, that esterification 
 of cellulose is always accompanied by some degree of hydrolysis 
 which increases the percentage of hydroxyl group to more than 
 the theoretical value. 
 
 It has already been mentioned that the products of nitration and 
 acetylation of cellulose do not indicate directly the esterification of 
 exactly one, two or three hydroxyl groups. Stable cellulose nitrates 
 may be obtained with any percentage of nitrogen from about 10% 
 to nearly 14%. This has been explained on the hypothesis that the 
 ester groups are not chemically combined with the cellulose, but 
 adsorbed. This theory, however, has had little or no support, and 
 the usual view is that all the C, units need not necessarily be esterified 
 to the same extent. Some, for example, may be di-nitrates, others 
 tri-nitrates, the average percentage of nitrogen for the nitrate in 
 bulk corresponding to something between a di-nitrate and a tri- 
 nitrate. Another consideration which has been lost sight of to 
 some extent is that since hydrolysis takes place simultaneously with 
 esterification, the percentage of nitrogen must thereby be altered. 
 
Cellulose Ester Solutions: Some Properties 65 
 
 For instance, if an oxygen linkage between two neighbouring C, 
 units is hydrolysed by addition of water, simultaneously with the 
 esterification of two hydroxyl groups in each unit, the percentage 
 of nitrogen in the two units considered together will not be 11-1%, 
 as in the theoretical di-nitrate, but 10-4%. This is, of course, an 
 extreme case, but it is evident that some degree of hydrolysis dis- 
 tributed over a comparatively large conglomerate of C, units will 
 cause the percentage of nitrogen in a true di-nitrate to diverge 
 appreciably from 11:1%. 
 
 Disintegration of Cellulose Structure during Esterification. 
 
 We have already seen that the degree of polymerisation of the 
 cellulose unit, or, in other words, the value of m in the formula 
 (C,H,)0;). is unknown. It has been suggested that the entire 
 cotton hair may be a single ‘“‘ molecule”’ of cellulose, and it must 
 be admitted that the larger the structure grows, the more difficult 
 it is to see what internal forces can suddenly come into play to 
 stop growth. The limit is probably enforced by conditions external 
 to the plant, such as temperature and especially humidity. Thus 
 it has recently been shown ® that there is a large variation in the 
 viscosity of nitrocellulose prepared from wood cellulose which has 
 been extracted from concentric rings of the same section of the 
 trunk of a poplar tree. The central zone yielded nitrocellulose 
 with the lowest viscosity and the outer zone nitrocellulose with 
 
 the highest viscosity. 
 The high initial viscosity of a solution of cotton cellulose which 
 has not been exposed to light or oxidation is attributed by Barr ° 
 to the existence of an outer cuticle of cellulose more highly poly- 
 merised than the rest of the cotton hair. 
 
 It is natural to ask to what extent the structure is disintegrated 
 during esterification. It is not likely that, even if the structure 
 of cotton cellulose is coterminous with the cotton hair, it will nitrate 
 as a hair, although experiments are on record in the celluloid 
 industry in which transparent celluloid gradually developed cloudi- 
 ness due to the appearance of fibres.1° Prolonged nitration or 
 acetylation always give products of low viscosity, and there is 
 little doubt that hydrolysis and esterification begin simultaneously 
 with the immersion of the cotton in the acids. The product of a 
 nitration probably consists of fragments of the original cellulose, 
 not of uniform size, of which the smaller ones have probably under- 
 gone a greater proportionate degree of hydrolysis than the larger. 
 The Sua picture of the esterification process is assisted if we 
 
66 Cellulose Ester Varnishes 
 
 remember that according to Balls 41 the wall of the cotton hair is 
 probably a sponge-like structure containing free air spaces, and 
 therefore presenting a large surface to the esterifying acids. 
 
 Fractional Precipitation of Solutions. 
 
 The presence of units of different dimensions in cellulose esters 
 is clearly shown by the experiments of Duclaux and Wollman,” — 
 who fractionally precipitated a solution of cellulose nitrate in 
 acetone by means of aqueous acetone, and found that although 
 the various fractions were practically identical in nitrogen per- 
 centage, they differed greatly in viscosity, the earliest fraction 
 (i.e., the first fraction to precipitate) having the highest viscosity, 
 and succeeding fractions diminishing progressively in viscosity. 
 Since it is unlikely that the method of precipitation could have 
 any influence on the molecular dimensions of the dissolved particles, 
 we must conclude that these various cellulose nitrates of different 
 viscosity co-exist in the original solution, and that the least viscous 
 are the most soluble in aqueous acetone. Cellulose acetate has 
 been found in the writer’s laboratory to behave similarly. 
 
 The acetylation of cellulose has been the subject of interesting 
 research from a similar point of view by Béeseken, van den Berg, 
 and Kerstjens.1® The formula for cellulose may be written 
 
 (CH 120,4)n — (n — 1)H,0, 
 
 in which n diminishes as the structure is hydrolysed, while the 
 percentage of acetic acid rises from 62-5% for cellulose tri-acetate 
 to 77% (for dextrose pentacetate, 7.e., after complete hydrolysis to 
 dextrose). Bdéeseken and his colleagues first found that if acetyla- — 
 tion were stopped before the cellulose had gone into solution, the 
 acetate found in solution was the tri-acetate, and the undissolved 
 cellulose was practically unesterified. They therefore concluded 
 that when acetylation occurred the tri-acetate was formed imme- 
 diately. When cellulose (C,H,)0;)n is hydrolysed by the addition 
 of one molecule of water, its molecular weight becomes 
 (C,H 1)0;)2 + 18, and the number of hydroxyl groups becomes 
 3n + 2. Hence 3n + 2 molecules of acetic acid will be required 
 for the acetylation to tri-acetate, and, since one molecule of water 
 is lost for each hydroxyl acetylated, the molecular weight of the 
 
 product will be given by the formula . 
 
 [(CgH 4 05)n + 18] + (8” + 2)(CH;;COOH) — (3n + 2)(H,0) = 
 3 [288m + 102] 
 
Cellulose Ester Solutions: Some Properties 67 
 
 The percentage of acetic acid in this tri-acetate is 
 
 100 x 60(387 + 2) _18,000n + 12,000 
 
 288m +102 ~~ 288n + 102 
 
 
 
 and since this constituent can be determined analytically, the value 
 of m can be found; it gives the average number of condensed 
 dextrose groups in the products of hydrolysis. When n is very 
 large, the percentage of acetic acid in the product closely approaches 
 
 ale 2 or 62:5%. Asn becomes smaller, the value approaches 77%. 
 
 
 
 It was found by following the course of acetylation that the experi- 
 mental number did not rapidly increase above 62:5%, showing 
 that the cellulose had probably at first hydrolysed into fragments 
 approximately equal in size. This was confirmed by the fact that 
 only a small proportion of the product dissolved in ether—alcohol, 
 which dissolves the acetates of the carbohydrates of low molecular 
 weight. When once the increase in combined acetic acid begins, 
 however, it becomes rapid, and indicates that the cellulose structure 
 is being disintegrated quickly. 
 
 Barnett 14 found that cellulose acetates contain free ketonic 
 groups which react in the normal way with phenylhydrazine or 
 p-bromophenylhydrazine, often forming compounds of definite 
 melting point. By determining the percentage of nitrogen or 
 bromine in these compounds he was able to deduce values for n, 
 which he found to vary from 2 to 12 for the phenylhydrazones 
 and from 9 to 36 for the bromophenylhydrazones. The method is 
 useful for showing progressive degradation of the cellulose structure 
 as the acetylation proceeds, but many of the acetates studied appear 
 to have been considerably further depolymerised than those used 
 in industry. 
 
 [It is interesting to compare with these results those obtained 
 by Irvine and Hirst 15 in the methylation of cellulose, in an 
 alkaline medium by the method of Denham and Woodhouse. 
 Fourteen methylations yielded a product containing 43-89% OCH, 
 and twenty more methylations produced no further rise in the 
 percentage of methoxyl groups. Irvine concluded that methylation 
 caused no hydrolysis or oxidation of the cellulose molecule. With 
 this conclusion may be compared the results obtained by Punter 14 
 and his collaborators on a very different scale, in the technical 
 treatment of cotton required to produce uniform viscosity. They 
 found that hot alkaline treatment of different varieties caused 
 reduction of viscosity, and therefore presumably of structure, but 
 
68 ~ Cellulose Ester Varnishes 
 
 after a time all varieties appeared to be reduced to the same more 
 stable structure, the viscosity of which was not altered by further 
 treatment of the same kind, as long as no chemical breakdown 
 took place. Something of the same nature may have happened 
 to Irvine’s methylated cellulose in the early methylations, so that 
 the formula which he derived may be that of a cellulose modified 
 by the alkaline treatment. ] 
 
 The evidence therefore suggests that the acetylation or nitration 
 of cellulose yields a product which consists of fragments of the 
 original cellulose structure, probably not esterified uniformly as 
 regards the number of ester groups in each C, unit, not of uniform 
 dimensions, and therefore not uniformly hydrolysed, but perhaps 
 not differing very largely in this respect. 
 
 Chemical Constitution of Solvents. 
 
 It has been mentioned in the first chapter of this book that 
 the earlier developments of cellulose nitrate solutions were much 
 hampered by the lack of suitable solvents. As soon as it was 
 realised that there were vast commercial possibilities in such solu- 
 tions, a large number of substances were tested and many of them 
 patented. De Mosenthal 1’ gives a long list and Worden 1* one 
 still longer. It has been said quite recently by a critic that the 
 method of research seems to be one of trial and error. There are, 
 however, some regularities which give indications of the constitution 
 and properties which confer solvent power for cellulose esters. 
 Cellulose nitrate is dissolved by the lower ketones, e.g., acetone, 
 methyl ethyl ketone, diethyl ketone; by the methyl, ethyl, propyl, 
 butyl and amy] esters of formic, acetic, propionic and butyric acids ; 
 and by various derivatives of acetanilide. Cellulose acetate is 
 dissolved by acetone, methyl acetate, methyl and ethyl formate, 
 formic and acetic acids. We find, however, an extremely inter- 
 esting class of solvents prepared by mixing liquids which, by them- 
 selves, have little or no solvent power. For example, for cellulose 
 acetate: chloroform and alcohol, tetrachloroethane and alcohol, 
 benzene and alcohol (hot). For cellulose nitrate: ethyl ether and 
 alcohol, benzene and alcohol, toluene and alcohol. Since, for 
 technical purposes, cellulose esters are almost always employed — 
 in mixed and not single solvents, the investigation of this pheno- 
 menon is of great technical interest, and during the war, owing to 
 the shortage of solvents and the imperative necessity of reducing 
 the demands on shipping, the research became a national necessity. 
 If we examine the lists of cellulose ester solvents, we notice that 
 
Cellulose Ester Solutions: Some Properties 69 
 
 single solvents always contain some atomic grouping of marked 
 reactivity. Consider, for example, the list of cellulose nitrate 
 solvents given above. These substances are all either ketones, 
 esters, or anilides, the distinctive groupings of which are 
 
 R,—CO—R, 
 
 R,—CO—OR,, 
 
 Ar—NH—CO—R, 
 
 respectively, the carbonyl group —CO— being common to all of 
 them. 
 
 Cellulose Nitrate in Single Solvents. 
 
 The first systematic examination of single solvents for cellulose 
 nitrate was made by Baker,!® who studied the relation between 
 viscosity and concentration, using three different typical samples 
 of the material. The solvents used were :— 
 
 Acetone, CH,°CO-CH; ; 
 
 Ethyl formate, H-COO-C,H,; ; 
 Methyl acetate, CH,-COO-CH;, ; 
 Ethyl acetate, CH,-COO-C,H,; ; 
 Propyl acetate, CH,°COO-C,H, ; 
 Amy] acetate, CH,-COOC,H,, ; 
 Ethyl butyrate, C,H,;->COOC,H,; ; 
 
 vA Nee Cols (the phenyl analogue is 
 Aceto ethyl-o-toluidide,|_ [~ ~CO°CH, mannol) 
 neg 
 
 CH, 
 /\wc eh 
 Ethyl-o-tolylethyl carbamate, | CO-OC,H; 
 | Se 
 
 db \w Cells 
 Phenyl ethyl urethane,| [| ~CO°OC,H; 
 I 
 
 COOC,H,; 
 
 Ethyl phthalate, | =| ; 
 “\C00-0,H, 
 
 Baker found that if concentration were plotted against viscosity 
 
 at constant temperature, the curve for each solvent could be 
 represented by the empirical formula 
 
 1 = N (1 + ac) 
 n = viscosity of solution at concentration ¢ 
 
 No = Viscosity of solvent 
 a, k = constants, different for each solvent. 
 
 where 
 
70 Cellulose Ester Varnishes 
 
 A convenient expression for comparing different solvents is 
 given by the differential 
 
 d log of A constant 
 dc 
 
 The value of this constant differs for the same nitrocellulose in 
 different solvents and hence the effect of cellulose nitrate on the 
 
 viscosity of its solutions varies with the nature of the solvent. 
 Probably also the condition of the nitrate is not the same in different 
 
 
 
 5 
 
 Log (7x 10°) 
 ® 
 
 
 
 O OS aor 15 2:0 
 Concentration per cent, 
 
 Fia. 3 (Baker).—Concentration of nitrocellulose plotted against the logarithm 
 of the viscosity, for a solution in equal volumes of benzyl alcohol and ethyl ether. 
 
 If n =n (1 + ac) 
 eect Cael ee 
 
 and ak = 2-303 tan a, where a is the angle between the tangent and the concentration 
 axis. The value of ak is a characteristic function of each curve (Baker, loc. cit.). 
 
 liquids. Hence the act of solution produces some change in the 
 cellulose nitrate other than a merely physical change of state. 
 Baker concluded that the cellulose nitrate becomes associated with 
 the solvent, so that the true solute is not cellulose nitrate, but a 
 complex of cellulose nitrate and the solvent. He accepts a sug- 
 gestion previously put forward by Schwarz *° from a study of the 
 viscosity of cellulose nitrate in camphor and alcohol, that the 
 fluidity (7.e., the inverse of viscosity) of a solution of cellulose 
 
\ 
 
 Cellulose Ester Solutions: Some Properties ‘71 
 
 nitrate in a solvent is a criterion of the gelatinising or solvent power 
 of the liquid. 
 
 Results similar to Baker’s were obtained by C. Visser 24 by the 
 examination of cellulose acetate from three different sources in the 
 simple solvents acetone, methyl acetate and ethyl formate. Her 
 equation, which is also derived empirically from an examination 
 of the curves, is of the same form as Baker’s, namely, 
 
 Le no( 1 H a) 
 
 where P and Q are constants, and she adopts the suggestion of 
 Baker and of Hatschek ** that the aggregates in the solution 
 consist of the dissolved ester combined in some way with part 
 of the solvent, so that the disperse phase is a complex depending 
 on the grade of cellulose acetate and the nature of the solvent. 
 This paper gives a brief discussion of the formule suggested PY 
 
 Hinstein (Ann. Physik, 1906, (iv), 19, 289). 
 
 Hatschek (Z. Chem. Ind. Koll., 1910, 7, 301-304; ibid., 1911, 
 8, 34-39). 
 
 Bancelin (Compt. rend., 1911, 152, 1382). 
 
 Kendall (Medd. K. Vet. Nobelinst., 1913, 2, No. 25, 1-16). 
 
 Arrhenius (Z. Phys. Chem., 1887, 1, 288). 
 
 Baker (T'rans. Chem. Soc., 1913, 108, 1653-1675, v. supra). 
 
 Smoluchowski (Koll. Z., 1916, 18, 190). 
 
 connecting viscosity and concentration. 
 
 Duclaux and Wollman ™ arrived at a still simpler equation | 
 connecting concentration and viscosity, namely :— 
 
 1) = 19 - 10", 
 and state that & is almost independent of the solvent for the same 
 nitrocellulose, but differs greatly with different nitrocelluloses so 
 that it forms a kind of index of the viscosity of cellulose nitrate. 
 It will be noted that if the viscosity of a sample of cellulose nitrate 
 is 7’ in solvent A, and 7’ at the same concentration in solvent B, 
 then 
 Meet Le Ge as Oe me ere 
 
 meta, A i ees op a ee aay en 
 
 If & is independent of the solvent, we obtain by dividing equation 
 (1) by equation (2) 
 
 and 
 
72 Cellulose Ester Varnishes 
 
 1.€., the viscosities of solutions of equal concentrations are in the 
 same ratio as the viscosities of the solvents. This relation would 
 certainly not hold over a wide range of different solvents and 
 different nitrocelluloses, but the results given by Duclaux and 
 Wollman are of considerable interest, and it would be a valuable 
 research to determine the limits of the applicability of the equation. 
 If it was derived only from the examination of the products of the 
 fractional precipitation of solutions of cellulose nitrate in acetone, 
 it suggests that these fractions may show regularities (perhaps 
 due to more restricted range of dimensions) which are not shown 
 by samples derived from entirely different processes of nitration. 
 
 CELLULOSE Esters In MIxED SOLVENTS. 
 Cellulose Nitrate in Acetone and Water. 
 
 This system was examined by Masson and McCall,1 who found 
 that the viscosities of solutions in anhydrous acetone are com- 
 paratively high, but are lowered by the presence of small quantities _ 
 of water. A minimum value exists which corresponds to a per- 
 centage of water which is slightly different for different samples 
 of nitrocellulose, and also varies somewhat with the concentration 
 for the same samples of nitrocellulose (see Fig. 4). For example, 
 using cellulose nitrate containing 12-39% of nitrogen, the minimum 
 viscosity for a 5% solution occurs at 8—9% of water, and for a 
 10% solution at 9—10% of water. Using cellulose nitrate con- 
 taining 13:0% of nitrogen (gun-cotton), the minimum viscosity 
 for a 5% solution occurs at 6—7% of water and for a 10% solution 
 at 7°%% of water. If the amount of water is increased much beyond 
 these figures, a point is reached at which gelatinisation still occurs, 
 but the cellulose nitrate will not disperse. 
 
 As a matter of interest it may be mentioned that at still higher 
 concentrations of water a mixture is reached which when shaken 
 with fibrous cellulose nitrate will powder it without dissolving it 
 (although a small loss of weight occurs). This process is an 
 approach to the fractionation process of Duclaux and Wollman ™@ 
 from the opposite direction. 
 
 Ether and Alcohol. 
 
 When it is found that certain varieties of cellulose nitrate 
 dissolve in a mixture of ether and alcohol, neither of which is a 
 solvent alone at ordinary temperatures, nor contains an atomic 
 grouping which would lead us by analogy to expect that it would 
 
Cellulose Ester Solutions: Some Properties 78 
 
 be a solvent, it is natural to speculate whether, when the two solvents 
 are mixed, any complex is formed which rearranges their affinities. 
 Baker 7 investigated pure ether—alcohol mixtures from this point 
 of view, using the sensitive property of viscosity as a test for 
 complex formation. He concluded that mixtures of alcohol and 
 ether contain 
 
 (a) non-associated ether, 
 (>) ether—alcohol complex, 
 (c) non-associated alcohol, 
 (d) associated alcohol, 
 
 
 
 ye 
 
 ‘Scosi 
 
 e 
 4 
 
 V, 
 
 O 2 4 6 8 10 
 Per cent Water in Acetone. 
 
 Fic, 4 (Masson and McCall).—These curves show the viscosity of three solutions 
 of nitrocellulose in mixtures of acetone and water, plotted against the composition 
 of the solvent. Note that, in each example, the lowest viscosity is shown, not in 
 anhydrous acetone, but in acetone containing an appreciable quantity of water. 
 The concentration of nitrocellulose is constant along each curve. 
 
 ‘ar 1.—10 grammes of nitrocellulose (12:39% nitrogen) in 100 grammes of 
 solvent. 
 
 ere. 2.—8:5 grammes of nitrocellulose (12-39% nitrogen) in 100 grammes of 
 solvent. 5 
 
 rings 3.—10 grammes of nitrocellulose (13:0% nitrogen) in 100 grammes of 
 solvent. 
 
 and that the solvent power of the mixture is due to the complex. 
 Baker rejected the view that the solvent power was due to 
 unassociated alcohol molecules, and that the effect of the ether 
 was merely to increase the number of those molecules, on the following 
 grounds :— : 
 
re! Cellulose Ester Varnishes 
 
 (a) It is not true that the higher the degree of association 
 in a homologous series of alcohols, the less is the solvent power. 
 The opposite is the case. 
 
 (6) If increase in the proportion of unassociated alcohol 
 increases the solvent power, any indifferent liquid added to 
 alcohol should increase the solvent power, but this is not so. 
 
 He also attributed the increased solvent power of ether—alcohol 
 at low temperatures *4 to the increase in the proportion of complex 
 at these temperatures. 
 
 Gibson and McCall 2° investigated the effect of variations in the 
 proportions of ether—alcohol on the viscosity of solutions of cellulose 
 nitrate, and found that the proportions required to produce the 
 best solvent (i.e., as shown by the lowest viscosity) depended on 
 the nitrogen content of the nitrocellulose, and that the higher the 
 nitrogen content the greater the proportion of ether required in 
 the solvent (Fig. 2). 
 
 From the discussion given in the earlier part of this chapter 
 it will be evident that variation in nitrogen content may be, and 
 probably always is, accompanied by independent variation in— 
 
 (a) the degree of hydrolysis of the cellulose, and therefore 
 in the percentage of hydroxyl groups, 
 
 (b) the average dimensions, and the range of dimensions, 
 of the nitrated cellulose particles, 
 
 (c) the percentage of the minor functions of the cellulose, 
 of which only the sulphuric acid ester group need be mentioned 
 here. 
 
 Therefore the proportion of ether required to produce the best 
 solvent may be influenced, not only by variation in the nitrogen 
 content, but also by variation in any of these accompanying factors. 
 If, for instance, one sample of cellulose nitrate contains a higher 
 percentage of hydroxyl groups than another, and if we assume 
 that these attract alcohol molecules in preference to ether molecules 
 from the ether—alcohol mixture, we should have a reason why 
 the sample with higher hydroxyl content should require a solvent 
 with a higher proportion of alcohol, and vice versa. This conclusion 
 is not inconsistent with Baker’s theory that the active solvent is an 
 ether—alcohol complex. Highfield ** criticises this view on several 
 grounds. In the first place, working with ether—alcohol solutions 
 containing from 10 to 60% of ether, the variation in viscosity is 
 much less when only 2% of water is present than when 7% is 
 present, so that variation in the amount of ether—alcohol complex 
 
 # 
 
Cellulose Ester Solutions: Some Properties 75 
 
 is not a predominating factor. Secondly, Gibson and McCall found 
 that a 4% solution of cellulose nitrate containing 11-8° of nitrogen 
 required a solvent of 50% alcohol to yield a solution of minimum 
 viscosity, while if the nitrogen content were 12:5%, the required 
 proportions were 70% ether and 30% alcohol. This implies that an 
 increase of 0-7% in the nitrogen content involves 20% of the alcohol 
 in these 4°% solutions, so that in making more concentrated solutions 
 so much alcohol would be used up in saturating the hydroxyl groups 
 of the cellulose nitrate that little would be available for producing 
 the ether-alcohol complex. Therefore it should be difficult to 
 make these more concentrated solutions, which is not the case. 
 Thirdly, if part of the alcohol is required to saturate hydroxyl 
 groups, the composition of the best solvent mixture should vary 
 with the amount of cellulose nitrate to be dissolved, but, in fact, 
 for the same sample, the proportions of ether—alcohol required 
 to give the lowest viscosity are independent of the concentration. 
 Lastly, if solvent power depends on an ether—alcohol complex, one 
 would expect dry ether—alcohol to yield solutions of lower viscosity 
 than those containing water, whereas, in fact, dry ether—alcohol 
 mixed with cellulose nitrate produces stiff jellies which become 
 much more fluid on the addition of a little water. 
 
 These objections must be taken into account in constructing 
 any picture of the dissolution of cellulose nitrate by ether—alcohol. 
 It should be pointed out, however, that the suggestion of the 
 selective adsorption of alcohol from ether—alcohol by the hydroxyl 
 groups of dissolved cellulose nitrate forms no part of Baker’s theory 
 of a solvent ether—-alcohol complex. The influence of the affinity 
 of cellulose nitrate for alcohol molecules, however, cannot be dis- 
 regarded. Everyone with practical experience of the dehydration 
 of cellulose nitrate with alcohol knows what differences there are 
 between different types of material in the tenacity with which 
 alcohol is retained. Obviously, however, this affinity is only 
 one of several co-existing in equilibrium in an ether—alcohol solution 
 containing water. If one sample of nitrocellulose attracts alcohol 
 molecules more than another, the equilibrium will be displaced 
 in such a direction that a higher proportion of alcohol will be 
 required in the best solvent mixture. This view is qualitatively 
 in agreement with the theory of the existence of an ether—alcohol 
 complex, but is quite independent of that theory, and we are far 
 from possessing sufficient data to express the changes in equilibrium 
 quantitatively. 
 
 Masson 2? points out that the influence of temperature must 
 
76 Cellulose Ester Varnishes 
 
 be considered in any general theory. Alcohol or, still better, 
 aqueous alcohol at — 80° will gelatinise cellulose nitrate, whereas 
 ether will not. In an ether—alcohol solution, we do not know how 
 the solvent is distributed between the dispersed cellulose nitrate 
 and the dispersing medium, and we cannot deduce the composition 
 of the phases from the initial composition of the solvent. There 
 is some mutual action between ether and alcohol which favours 
 absorption of these liquids by cellulose nitrate, but at present its 
 nature is undetermined. 
 
 A different aspect of ether—alcohol solutions is opened up by 
 
 3 
 a 
 
 Relative Viscosity. 
 a 
 
 “=~ 
 i) 
 H 
 
 
 
 20 30 40 
 Molecular Percentage of Water. 
 
 Fic 5, (Barr and Bircumshaw).—This curve shows the viscosity of a sample of 
 Dreyfus cellulose acetate in solvents consisting of various mixtures of acetone and 
 water, the concentration of cellulose acetate being maintained constant at 5%. 
 The viscosities are measured in empirical units and are here only of relative value. 
 Note that the minimum viscosity is shown in a mixture containing about 20 mole- 
 Soe water %. Compare behaviour of cellulose nitrate in acetone and water, 
 
 ig. 4. 
 
 Kugelmass,?° who exposed a sample of cellulose nitrate containing 
 11:9% to the action of alcohol and ether separately at low tem- 
 peratures. In each case a sol was formed above the layer of 
 undissolved nitrocellulose. The opalescent ether sol contained 
 nitrocellulose with 13-75% of nitrogen. It was centrifuged until 
 no more solid was deposited, and the cellulose nitrate obtained 
 from it contained 11-20% of nitrogen. The experiment with 
 alcohol carried out in the same way yielded a product with 14-02% 
 of nitrogen. The theoretical di-nitrate requires 11-13%, and the 
 tri-nitrate 14-17%, so that apparently at low temperatures, ether 
 disperses the one and alcohol the other. 
 
a see 
 
 / 
 
 Cellulose Ester Solutions: Some Properties ‘ii 
 
 Cellulose Acetate in Mixed Solvents. 
 
 The solubility relations of cellulose acetate have probably 
 been studied more completely than those of cellulose nitrate. This 
 is due to two causes: (1) the shortage of appropriate solvents during 
 the war, (2) the more restricted range of solubility shown by cellulose 
 acetate as compared with cellulose nitrate. 
 
 1500 
 
 : 
 
 1000 
 
 750 
 
 Relative Viscosit Y: 
 
 
 
 Oo 
 
 & ra) 15 ZO 25 
 Molecular Percentage of Benzene. 
 
 Fic, 6 (Barr and Bircumshaw).—This curve is directly comparable with that 
 shown in Fig. 5, and shows the viscosity of the same sample of cellulose acetate in 
 solvents consisting of various mixtures of acetone and benzene, the concentration 
 of cellulose acetate being maintained constant at 59%. Note that there is no minimum 
 viscosity. Benzene, unlike water, acts throughout merely as a diluent and increases 
 the viscosity of the solution. 
 
 Barr and Bircumshaw *° examined the changes in viscosity of 
 solutions of cellulose acetate in acetone to which varying quantities 
 of water (Fig. 5), alcohol and benzene (Fig. 6) had been added. 
 The concentration of acetate in the solutions was 5%. The addition 
 of alcohol causes a considerable fall in the viscosity, reaching a 
 minimum at about 6% of alcohol and then remaining moderately 
 steady up to a concentration of 40%. Aqueous acetone yielded 
 solutions which had a well-marked minimum viscosity when the 
 
78 Cellulose Ester Varnishes 
 
 acetone contained 19% (mol.) of water. The addition of benzene 
 to acetate solutions, on the other hand, caused a steady increase 
 in the viscosity of solutions, the curve showing no minimum. 
 
 In a number of instances, observers have found minima in the 
 viscosity—concentration curves which approximated closely to a 
 simple molecular combination of the constituents of the mixture, 
 €.9.; 
 
 1 mol. of tetrachloroethane + 1 mol. of alcohol 
 2 mols. of epichlorohydrin -+ 1 mol. of alcohol 
 
 1 mol. of » + 2 mols. of acetic acid ‘ 
 1 ,, of ethyl formate + 1 mol. of acetone 
 1 ,, of mesityl oxide +1 ,, of alcohol. 
 
 Mardles * has therefore examined several examples of mixed solvents 
 in which different degrees of affinity would be expected between 
 the constituents. Aniline and acetic acid, for example, form a 
 solvent mixture for cellulose acetate, but also, as a basic and acid 
 compound respectively, react with each other forming compounds 
 which can be isolated. A viscosity—concentration curve for the 
 solvents alone shows a marked peak, 7.e., a maximum viscosity 
 at a position close to, but not coinciding exactly with, the point 
 corresponding to a compound of 2 molecules of acetic acid with 
 1 molecule of aniline. If the same curve is plotted for similar 
 mixtures containing a fixed amount of cellulose acetate in solution, 
 the viscosity maximum is much exaggerated, and is shifted a little 
 nearer to the position of molecular ratios. The curve connecting 
 solvent power number with concentration shows a similar maximum. 
 In this solvent mixture there is no doubt that a complex is formed 
 between the two constituents of the mixture, and that the solvent 
 power is diminished by the formation of this complex. 
 
 The corresponding curves for a mixture of benzyl alcohol and 
 cyclohexanone show a sag and not a peak, and again the char- 
 acteristic of the curve is much exaggerated when the mixtures 
 contain cellulose acetate, the sag becoming a V. 
 
 The mixture of acetic acid and water shows a well-marked 
 maximum solvent power for cellulose acetate at a concentration 
 of about 36% of water for a 5% solution of cellulose acetate, and 
 the mixture of acetic acid and methyl alcohol a similar maximum 
 at a concentration of about 24% of methyl alcohol. Nevertheless, 
 both of these mixtures show evidence of the formation of a com- 
 plex between the two ingredients. Hence we have the anomaly 
 that a non-solvent, water, added to a solvent, acetic acid, (a) forms 
 a complex with the acid, but (b) increases its solvent power. At 
 
Cellulose Ester Solutions: Some Properties 79 
 
 first sight this appears to bring us back to Baker’s theory of 
 the solvent power being due to the complex, but in view of all the 
 evidence, Mardles concludes that there are instances in which the 
 addition of one liquid to another may result both in complex 
 formation and in molecular simplification (de-association). The 
 increased solvent power is due to the solvent action of the simple 
 molecules produced by de-association of associated molecules. 
 Complex-formation decreases the solvent power, but when complex- 
 formation and de-association occur simultaneously, the favourable 
 influence of the later phenomenon may entirely mask the unfavour- 
 able influence of the former. 
 
 A few words may be said about other physical properties the 
 study of which has not yet given results comparable with those 
 obtained by studies of viscosity and solvent power in so far as 
 the commercial utilisation of cellulose ester solutions is concerned, 
 but which will have to be considered in any general theory of their 
 constitution. 
 
 Viscosity and Plastic Flow. 
 
 The experimental and mathematical investigations of Bingham 
 in the United States have stimulated research on the properties 
 actually concerned in viscosity measurements of colloids. It 
 follows from the equation given on p. 60 that if the same liquid is 
 used in the same capillary tube, 
 
 ip == bf, 
 i.e., the curve connecting pressure with outflow is a straight line 
 passing through the point of origin. Bingham and his collaborators 
 have investigated a large number of colloidal solutions in visco- 
 meters permitting variation of pressure, and have found that the 
 p/I curve at low pressures is not linear. At higher pressures it 
 becomes linear, and the straight line if produced cuts the pressure 
 axis at a small positive value, 7.e., the linear part of the curve is 
 represented by a modified equation (P — p) = k I, where P is the 
 total pressure applied, and p, the intercept on the pressure axis, is a 
 fraction of the total pressure, which is used up, according to Bing- 
 ham, in overcoming an internal friction or resistance to viscous flow. 
 This behaviour distinguishes such solutions from pure liquids. 
 Solutions of cellulose nitrate in acetone were studied by Bingham 
 and Hyden,®° and were found to exhibit the property just described. 
 At temperatures from 5° to 40° the isotherms connecting pressure 
 and outflow are straight lines which by extrapolation cut the pressure 
 axis at points representing small positive values. These small. 
 pressures, which must be applied before viscous flow begins, are 
 
80 Cellulose Ester Varnishes 
 
 termed the yield values. The yield value is not the same for 
 solutions of different concentration, and varies with the temperature. 
 If, however, yield value is plotted against temperature, the curve 
 is again a straight line, which when produced cuts the temperature 
 
 
 
 
 
 
 
 0-010 
 . yee 
 doce) anes 
 ssigey HALLS 
 HE 
 Ls) 
 E o. 
 x aie al eR 
 fees ee ee 
 tonto 
 Gj / 
 PV MRE A a 
 AGE RMmn mr ss 
 0 300 600 $00 
 Shearing Stress (Pressure). 
 (Qa) 
 
 & 
 
 Aa 
 
 8 
 
 Q 
 
 8 
 
 
 
 0 W 20 30.40 50 60 70 80 90 
 Yield Value. 
 (b) 
 
 Fic. 7 (Bingham and Hyden).—Graph (a) shows the relation between the shearing 
 stress (pressure) and rate of flow through a capillary tube, of a dispersion of a certain 
 sample of nitrocellulose in acetone. Concentration 7:-708%. Temperatures 5°, 
 20°, 35°. The intercept on the pressure axis is friction or yield value f, which Bing- 
 ham defines as the shearing stress at the wall of the capillary necessary to start the 
 flow. Shearing stress is measured in dynes per square centimetre, and efflux in 
 millilitres per second. Note that as the temperature rises, the yield value diminishes. 
 Graph (6b) shows the yield value of the same solution plotted against the temperature. 
 Note that the curve is linear, and on extrapolation cuts the temperature axis at 
 about 43°, This behaviour suggests that this particular solution becomes a true 
 liquid at 43°, since it is one characteristic Of pure liquids that they are deformed by 
 any finite pressure, however small. At lower temperatures the flow of the dis- 
 persion is partly a ‘‘ plastic flow,’’ characteristic of plastic solids. 
 
 axis at a point which indicates the temperature at which yield value 
 should be zero, i.e., at which the solution begins to behave like a 
 
 true liquid (see Fig. 7). It was not definitely settled whether this 
 temperature depends on the concentration. 
 
Cellulose Ester Solutions: Some Properties 81 
 
 These observations are of the greatest interest to all chemists 
 working with solutions of cellulose esters, and it is greatly to be 
 desired that they may be continued and extended. f 
 
 It is impossible in a paragraph to do justice to Bingham’s work, 
 and the reader is referred to the bibliography at the end of the 
 chapter for fuller accounts. His view that viscosities of colloid 
 solutions as usually measured are not true viscosities does not 
 invalidate the technical applications of such measurements under 
 fixed conditions in the examination of cellulose esters, but he has 
 already done much to distinguish and define the properties which 
 give these solutions their special qualities, and his work may lead 
 to a more rigid and scientific system for the definition and classi- 
 fication of viscous colloids in general. Bingham’s interpretation 
 of viscosity investigations has been vigorously criticised, e.g., by 
 de Waele, who traverses the physical conceptions which Bingham 
 has developed, and adduces interesting experimental evidence on 
 the flow of petroleum jelly in tubes under low pressure. He also 
 finds departure from linearity in pressure/outflow curves at greater 
 ranges of pressure, and derives an empirical equation 
 
 p=kit 
 connecting pressure and outflow, ¢ being a constant less than unity. 
 Bingham’s and Green’s reply to the criticism (ibid.) should also be 
 read. It seems to the writer that since de Waele’s experiments show 
 a marked difference in adhesion between the colloid and the wall of 
 the capillary according to the pressure applied, a complete investiga- 
 tion of the process must take account of the distribution of pressure 
 in a semi-solid, which cannot take the same simple form as in a 
 liquid. 
 Tyndall Effect. 
 
 A beam of light passing through an air space completely free 
 from dust is invisible when looked at in a direction at right 
 angles to the direction of the beam. The same is true of pure 
 liquids when completely free from dust (“optically empty ”’). 
 Liquids which contain particles in suspension, however, show the 
 path of the light, even in many instances in which the particles 
 are so small or so near in refractive index to the medium that the 
 liquid appears transparent to the naked eye. This property is 
 known from its chief investigator as the Tyndall effect, and has 
 been applied largely to the study of colloidal solutions, particularly 
 in ultramicroscopy. Solutions of cellulose esters show usually 
 a weak Tyndall effect, because the swollen particles have a refractive 
 index ed near to that of the medium in which they are suspended. 
 
82 Cellulose Ester Varnishes - 
 
 Mardles *! examined the variation of Tyndall effect with concen- 
 tration, using solutions of cellulose acetate, and made the 
 interesting discovery that as the concentration of cellulose acetate 
 increases, the effect rises to a maximum and then falls off (see 
 Fig. 8). For example, solutions of a certain cellulose acetate in 
 benzyl alcohol gave a marked maximum effect at a concentration 
 of 6%. The peak to the curve is much less marked at higher 
 temperatures than at low. The physical significance of this is 
 uncertain. Since the effect is due to the difference between the 
 refractive index of the particles and that of the solvent, when the 
 effect increases there must be either an increase in the difference of 
 
 dall Number. 
 
 Tyr 
 § 
 
 
 
 0.2 4 6 8,0 2 4 Biome ae 
 Concentration in Grms. per 100c.c. 
 
 Fic. 8 (Mardles).—These curves show the change of Tyndall number with con- 
 centration for a sample of cellulose acetate in benzyl aleohol. Note that there is a 
 very marked maximum at about 6% concentration, but this maximum is much less 
 marked with rise of temperature. At 35° it has almost disappeared. The Tyndall 
 numbers in which the scattering of light is measured are multiples of an arbitrary unit 
 representing the scattering of light by a certain sample of castor oil used as a standard. 
 The comparison was made photometrically. } 
 refractive index or an increase in the amount of surface bounding 
 the two phases, or both. The first condition would be brought 
 about if the additions of cellulose acetate were taken up mainly 
 by particles already existing; the second, either by the change 
 in size or shape of the existing particles, or by formation of 
 additional particles of swollen ester, or by subdivision of existing 
 particles. The latter is so unlikely that it may be left out of 
 account. If we assume the simplest behaviour of all, namely, 
 that the cellulose acetate when dissolved in benzyl alcohol disperses 
 
 into spheres of uniform size, the maximum surface will be exposed 
 
Cellulose Ester Solutions: Some Properties 88 
 
 just before the boundaries of the spheres begin to touch, 7.e., when 
 the spheres occupy 71% of the total volume of the solution. If 
 more cellulose acetate is added, the boundaries will begin to 
 coalesce, and the scattering of light to diminish. An explanation 
 on these lines involves the assumption that in a 6% solution of 
 cellulose acetate in benzyl alcohol the swollen particles occupy 71% 
 of the total volume. A paper by Hatschek and Humphry 2 on 
 agar sols and gels should be read in relation to this subject. It 
 was pointed out by Porter in the discussion on the paper that the 
 scattering of light need not indicate any great difference in com- 
 position between the two phases, since it varies with the square 
 of the relative refractive index. Hatschek agreed, but said that; 
 on the other hand, it might require a considerable difference in 
 composition between the two phases to cause even a slight alteration 
 in the refractive index. 
 
 REFERENCES AND BIBLIOGRAPHY. 
 
 1 J. Masson and R. McCall, Trans. Chem. Soc., 1920, 117, 819-823. 
 2 EK. C. Bingham, “ Fluidity and Plasticity.”” ° 8S. E. Sheppard, J. Ind. 
 Eing. Chem., 1917, 9, 523. 4 W. H. Gibson and L. M. Jacobs, Trans. Chem. 
 Soc., 1920, 117, 473-477. 5 E. W. J. Mardles, ibid., 1924, 125, 2244-2259. 
 6 KE. W. J. Mardles, J. Soc. Chem. Ind., 1923, 42, 207-21llT. 7 E. W. J. 
 Mardles, A. Moses and W. Willstrop, Advis. Comm. for Aeronautics, Rep 
 No. 568, Dec. 1918. *® L. Meunier and A. Breguet, Rev. Gén. des Colloides, 
 1924, 2, 289-294. °® G. Barr, J. Soc. Chem. Ind., 1924, 48, 1107. 2° J. N. 
 Goldsmith, ibid., 1904, 28, 297 (it is possible that the fibres were originally 
 invisible, owing to the refractive index being identical with that of the 
 celluloid, and became visible later owing to a change in the refractive index 
 of the latter brought about by loss of volatile solvent). 11 W. L. Balls, 
 Proc. Roy. Soc., 1923, B, 95, 72. 1% J. Duclaux and E. Wollman, Bull. 
 Soc. chim., 1920, 27, 414. 1° J. Boeseken, J. C. van den Berg, and A. H. : 
 Kerstjens, Rec. trav. chim. Pays-Bas, 1916, 35, 320-345. 14 W. L. Barnett, 
 J. Soc. Chem. Ind., 1921, 40, 61-637r. 15 J. C. Irvine and E. L. Hirst, Trans. 
 Chem. Soc., 1923, 128, 518. 1° R. A. Punter, J. Soc. Chem. Ind., 1920, 39, 
 3337. 17 H. de Mosenthal, zbid., 1904, 28, 295. 18 E. C. Worden, ‘‘ Nitro- 
 cellulose Industry,” Vol. I., chap. iv. 1° F. Baker, Trans. Chem. Soc., 1913, 
 103, 1653-1675. ?° H. Schwartz, Z. Chem. Ind. Koll., 1913, 12, 32. 21 C. 
 Visser, Aeronaut. Research Comm., Memo. No. 758, Aug. 1920. ”% E. 
 Hatschek, Z. Chem. Ind. Koll., 1912, 11, 284-286. #3 F. Baker, Trans. 
 Chem. Soc., 1912, 101, 1409-1416. 4 E. Berl and R. Klaye, Zetisch. Schiess 
 Sprengstoffe, 1907, 2, 381. 25 W. H. Gibson and R. McCall, J. Soc. Chem. 
 Ind., 1920, 39, 1727, 2 A. Hi ghfield, Trans. Faraday Soe. .» 1921, 16, 94. 
 a 35 I. O. Masson, ibid., 1921, 16, 95. ®8 I. N. Aas OR ti Rec. trav. chim. 
 Pays-Bas., 1922, 44, 751-763. 29 G. Barr and L Bircumshaw, Advis. 
 Comm. for Aeronautics, Rept. No. 663, Nov. 1919. *4 A. de Waele, J. 
 Oil and Colour Ohem. Assoc., 1923, 4, 33-88. 30 FE. C. Bingham and W. L. 
 Hyden, J. Franklin Inst., 1922, Dec., 731-740. 31 EK. W. J. Mardles, T'rans. 
 Faraday Soc., 1923, 18, 318-326. %2 E. Hatschek and R. H. Humphry, ibid., 
 1924, 20, 18-29. 
 
 | Additional. References. 
 
 E. Hatschek, (1) ‘“‘ Introduction to Physics and Chemistry of Colloids,” 
 (2) Colloid Reports of British Association, No. 1, pp. 2—5 (a short report 
 with a valuable bibliography). British Engineering Standards Association, 
 ** Determination of Viscosity in Absolute Units.” 
 
CHAPTER VI 
 CELLULOSE ESTER SOLUTIONS: SOME PROPERTIES. PART II 
 
 Swelling—Researches of Knoevenagel and his collaborators—Distribution of 
 Solvent between Swollen Ester and Liquid—Volume Change on Dis- 
 solution—Dielectric Capacity—Discussion of Evidence on Constitution 
 of Cellulose Ester Solutions. 
 
 Swelling. 
 
 Brrore cellulose esters disperse to form what are usually termed 
 solutions, they undergo swelling, evidently by absorption of one or 
 more constituents of the solvent. They also undergo swelling in 
 some liquids in which they do not disperse. This behaviour was 
 investigated by the late E. Knoevenagel and his collaborators in a 
 series of papers which probably rank with those of Mardles as being 
 the most important of recent contributions to the study of the 
 chemistry and physics of cellulose esters. 
 
 The material employed was in all cases cellulose acetate in the 
 form of threads resembling horse-hair. The most interesting experi- 
 ments were those in which weighed quantities of this material 
 were treated with different combinations of liquids, and the distribu- 
 tion of the ingredients between the swollen ester and the supernatant 
 liquid determined by analytical methods, in many instances specially 
 devised. The results obtained can only be indicated very briefly 
 here, and should be consulted in the original papers, which are worth 
 republishing separately with the correction of some misprints. 
 
 Cellulose acetate does not swell either in water or in absolute 
 alcohol, but in mixtures of the two it swells considerably.1 The 
 degree of swelling is determined by the composition of the mixture, 
 and exhibits a maximum. It is not changed if, after equilibrium is 
 reached, the organic portion of the solvent is entirely displaced by 
 water. Swollen cellulose acetate is dyed by methylene-blue, or 
 saponified by alkali, much more readily than the unswollen material, 
 and these two processes run parallel with each other and with the 
 degree of swelling. , 
 
 When cellulose acetate ? is shaken with different aqueous solu- 
 tions of aniline, phenol or ethyl tartrate, these ingredients are found 
 after 24 hours to be distributed between the ester and the water 
 according to the Distribution Law :— 
 
 C4 = kCg 
 
 where C'4 = the concentration of, say, aniline in the swollen acetate, 
 C's = concentration of aniline in the liquid layer, and k = constant. 
 This Law of Distribution is usually applicable to the equilibrium 
 
 between two immiscible solutions of a common solute (e.g., to the 
 84 
 
Cellulose Ester Solutions: Some Properties 85 
 
 partition of picric acid between benzene and water in contact). 
 Hence these experiments make it probable that when cellulose 
 acetate swells, it takes up molecules as if it were a solvent of them, 
 and not by adsorption at the surface. 
 
 Measurements were next made? of swelling in the binary mix- 
 tures benzene—alcohol, nitrobenzene—alcohol, carbon tetrachloride— 
 alcohol. If the pure liquids were employed, the order of swelling, 
 in decreasing values, was nitrobenzene, benzene, alcohol and carbon 
 tetrachloride, of which only nitrobenzene could be called a powerful 
 swelling agent. ‘The surface tensions of these pure liquids decrease 
 in the following order—nitrobenzene, benzene, carbon tetrachloride, 
 alcohol. In binary mixtures, alcohol always caused a lowering of 
 surface tension, and a qualitative relation was iound between the 
 degree of swelling and the lowering of surface tension. The greater 
 the reduction of surface tension caused by the addition of alcohol, 
 the greater the swelling action of the mixture. Since, however, 
 alcohol alone has no swelling action on cellulose acetate, a maximum 
 of swelling effect is reached at a certain concentration. 
 
 A relation was then sought* between the degree of swelling 
 (which precedes dispersion) and the viscosity of the solutions after 
 dispersion. The viscosity—concentration curves were plotted for 
 various mixtures of ethyl alcohol and nitrobenzene, and it was found 
 that the viscosity curve for a liquid of weaker swelling power always 
 lies below that for a liquid of stronger swelling power. An interesting 
 regularity observed was that for any two mixtures of alcohol and 
 nitrobenzene, the ratio of the viscosities at equal concentrations of 
 cellulose acetate was constant. It was also found that the con- 
 centration—viscosity data for any one solvent mixture were approxi- 
 mately fitted by the equation 
 
 concentration = constant x log. of viscosity. 
 
 (The statement in the original paper, Koll. Chem. Beihefte, 1921, 
 13, 267, “ Die Viskositat steigt im gleichen bindiren Lésungsgemisch 
 bei zunehmenden Azetylzellulosegehalt annahernd proportional mit 
 den Logarithmen der inneren Reibung an,”’ is an evident slip.) 
 Further investigations into the system cellulose acetate—nitro- 
 benzene-alcohol > confirmed previous work by showing that the 
 nitrobenzene over a range of concentrations obeyed the Distribution 
 Law (Henry’s Law), i.e., there was a constant ratio between the 
 concentration of nitrobenzene in the liquid and in the swollen solid. 
 It was also found that alcohol was taken up in constant amount by 
 the cellulose acetate through a considerable range of mixtures 
 
 \ 
 
86 Cellulose Ester Varnishes 
 
 containing small concentrations of nitrobenzene. The experiments 
 were extended ® to the mixtures acetic acid—water, acetic acid— 
 benzene, acetone—-benzene, nitrobenzene-isopropyl alcohol, ethyl 
 alcohol—benzene, nitrobenzene—benzene, ethyl alcohol—water, acetic 
 acid—nitrobenzene, acetone—nitrobenzene, methyl alcohol-nitro- 
 benzene, acetone—methyl alcohol, acetic acid-camphor. It now 
 begins to be difficult to follow the deductions which Knoevenagel 
 made from the experimental results. Consider, for example, the 
 system cellulose acetate—benzene-acetone. It is stated that there is 
 a constant ratio of cellulose acetate to benzene in the swollen acetate ; 
 also that both benzene and acetone are distributed between the 
 liquid and the swollen acetate according to Henry’s Law. Using 
 his nomenclature, we have :— 
 mols. of acetone per cent. = a. 
 mols. of benzene per cent. = b. 
 
 mols. of acetone per cent. = c. 
 In the swollen acetate jmol of benzene per cent. = d. 
 
 mols. of cellulose acetate per cent. = e. 
 (assuming a C, unit of cellulose acetate). 
 
 In the liquid { 
 
 It is stated (p. 186) that © = constant (mean value 1-05, range 0-91 
 to 1-18). 
 
 » (mean value 1-11, range 0-85 
 to 1-26). 
 
 » (mean value 5-09, range 4-31 
 
 to 7:42). 
 If these three relations are all true, it follows that 
 
 | 
 
 | 
 
 QO} Qi Qo 
 
 b 
 : = constant. 
 
 Actually : varies from 8-8 to 1-7, so that it appears that the con- 
 
 stancy of one or more of the three ratios mentioned above has been 
 too readily assumed. It is greatly to be desired that this matter 
 should be cleared up, preferably by some of Knoevenagel’s collabora- 
 tors, as the experimental work forming the basis of this series of 
 researches is invaluable. It is right to add that Knoevenagel 
 admitted the existence of deviations in this series, probably owing 
 to the chemical relations between acetone and benzene, and the 
 solubility of cellulose acetate in acetone. 
 
 His general conclusion is that alcohol, benzene and water are 
 taken up in constant quantities from various mixtures, and this 
 
 an ee 
 
Cellulose Ester Solutions: Some Properties 87 
 
 suggests that swelling is due to chemical action and not to surface 
 adsorption. The affinities concerned are probably of a different 
 order from those concerned in typical molecular compounds. 
 
 In the next paper,’ similar experiments are described, from which 
 it is concluded that from binary mixtures of alcohol, nitrobenzene 
 and benzene, liquid is absorbed by cellulose acetate in molecular 
 proportions. The sum of the number of molecules of each liquid 
 taken up from a binary mixture by cellulose acetate is constant. 
 Determinations of heat change support the view that swelling is a 
 molecular process. 
 
 In the last publication, ® it is shown that cellulose acetate which has 
 been brought into equilibrium with a solvent in which it swells 
 considerably, will, if placed in a solvent with smaller swelling power, 
 shrink until it possesses the same degree of swelling as it would have 
 had if it had been placed originally in the second solvent. This is 
 in contrast with the behaviour noticed with water, which can entirely 
 replace an aqueous—organic swelling mixture such as alcohol and 
 water, without changing the degree of swelling. Probably the water, 
 which has no swelling action whatever, hardens the original swollen 
 structure and sets it. 
 
 Volume Change on Dissolution. 
 
 Mardles ® found that when cellulose acetate is dispersed in a 
 solvent, a slight contraction in volume takes place. With simple 
 solvents, the greatest contraction takes place with the best solvents. 
 With mixed solvents, the contraction is greatest with the mixture 
 of highest solvent power. It is proportionately greater at low 
 concentrations and at high temperatures, 7.e., under those conditions 
 in which the dispersion approaches most closely to a molecular 
 solution. 
 
 It should be noted that these observations refer to the system 
 as a whole, whereas the “ swelling’ studied by Knoevenagel and 
 his collaborators refers only to the colloid. Thus the colloid swells 
 (by absorption of solvent molecules), but the system as a whole 
 contracts. Further, Knoevenagel found that solvent mixtures in 
 which the swelling was small yielded solutions of lower viscosity 
 than those in which swelling was great. It is generally agreed now 
 that solutions of low viscosity indicate high solvent power, and it 
 would be expected that when the swelling of the colloid was least, 
 the contraction of the whole system would be greatest. Hence we 
 see a qualitative connection between low swelling of the colloid, 
 high contraction of the system, and low viscosity of the solution. 
 
88 Cellulose Ester Varnishes 
 
 Dielectric Capacity. 
 
 Fenton and Berry 1° thought from the investigation of the solvent 
 power of a large number of simple liquids for cellulose acetate that 
 there was some relation between the dielectric constant and solvent 
 action, although there were admittedly exceptions. Mardles 14 
 quotes a number of these exceptions and points out that the dielectric 
 constant diminishes with rise of temperature, whereas solvent 
 action increases. 4 
 
 Conclusion. 
 
 The preceding discussion on some of the properties of solutions 
 of cellulose esters which are most nearly related to their industrial 
 application, furnishes material for provisional views of their nature. 
 
 The dispersion of a cellulose ester by a simple or mixed solvent 
 obviously depends in some way on the attraction exerted by the 
 molecules of the solvent, or some groups in the molecules, on some 
 of the distinctive groups in the esterified cellulose. Nevertheless, 
 it must always be remembered that any picture based on this con- 
 sideration alone will be incomplete, since on this view cellulose, with 
 its high percentage of hydroxyl groups, should be soluble in water. 
 The secondary affinities, which bind together the Cj, units of cellulose 
 proposed by Irvine, still exist in the cellulose ester, although probably 
 they are weakened. 
 
 Ksselen 1 was apparently the first to suggest the application of 
 Langmuir’s theory of film adsorption to the dispersion of cellulose 
 _esters. Considering the example of cellulose acetate dispersing in 
 certain mixtures of chloroform and alcohol, he assumes that the 
 surface of cellulose acetate attracts the hydroxyl groups of the 
 alcohol and adsorbs the alcohol in such a way as to hold the hydroxyl 
 groups next to the acetate, leaving the hydrocarbon (ethyl) radical 
 projecting into the solution. These hydrocarbon groups may 
 attract the hydrocarbon end of the chloroform molecules. Both of 
 these processes would induce swelling and tend to disperse the 
 cellulose acetate. Esselen further suggests that the composition 
 of the best solvent mixture may coincide with the point at which all 
 the secondary valencies at the surface of the acetate are just saturated 
 by the alcohol. With technical cellulose acetate this point corre- 
 sponds with 25 to 30% alcohol in the solvent. 
 
 As a second example, Esselen considers the solubility of some 
 varieties of cellulose acetate in a warm mixture of alcohol and 
 benzene, although they are soluble in neither constituent alone nor 
 in the mixture when cold. In this instance he supposes a similar 
 
Cellulose Ester Solutions: Some Properties 89 
 
 adsorption of alcohol by the ester, and an attraction between the 
 ethyl radical of the alcohol and the benzene hydrocarbon. That a 
 paraffin hydrocarbon does not behave similarly he explains by the 
 known fact that the paraffin hydrocarbons and alcohol are only very 
 slightly miscible, so that if the ester only dissolves in the alcohol— 
 benzene mixture with difficulty, one would not expect the alcohol- 
 paraffin mixture to possess any solvent power at all. 
 
 Esselen then extends the theory to compounds possessing the 
 aromatic nucleus and the hydroxyl group in one molecule and 
 suggests that this may explain the high solvent power of the phenols. 
 [Perhaps a better example still would be benzyl alcohol. ] 
 
 This theory appears to contain the germ of a rational mode of 
 viewing the dispersion of cellulose esters, and it is worth examining 
 in the light of the evidence summarised in previous pages. If the 
 best solvent mixture corresponds to a point at which the surface 
 is entirely saturated with alcohol, the composition of the best solvent 
 mixture should vary with the concentration of the ester. On this 
 point there is conflict of evidence. Highfield states that, in the 
 instance of cellulose nitrate and ether—alcohol, the composition of the 
 best mixture is independent of the concentration, while Masson and 
 McCall, working with acetone and water, found slight variations 
 with the concentration. It does not seem to be necessary to assume 
 that the whole of the alcohol in the best solvent mixture is adsorbed 
 at the surface of the ester and it is not clear that this is Esselen’s 
 assumption. -Itis more reasonable to assume an equilibrium between 
 the alcohol so adsorbed and the alcohol in the surrounding medium. 
 A change in the concentration of the ester would displace this 
 equilibrium, but would not lead to a quantitative adsorption of more 
 ethyl alcohol exactly proportionate to the fresh surface of ester 
 supplied. Another factor which may enter here is that the swelling 
 of the ester may itself open up fresh surface to the adsorption of 
 alcohol, and if this is inhibited in any way at higher concentrations, 
 the amount of surface will not be proportional to the concentration. 
 
 The dispersion of the ester would therefore be.due to the affinity 
 of one or more constituents of the solvent mixture for groups on the 
 surface of the particle of cellulose ester, some of these groups being 
 formed by the particle selectively adsorbing constituents from the 
 solvent mixture itself. Hence, in a restricted sense, the formation 
 of a complex between the constituents of a binary mixture may be 
 related to its solvent power, provided that one constituent of the 
 complex is also selectively adsorbed by the particles of cellulose 
 ester. This constituent acts as a link between the cellulose ester 
 
90 Cellulose Ester Varnishes 
 
 and the solvent medium, and through it the molecular motion of 
 the latter is transmitted to the ester as a dispersive influence. Since 
 vapour pressure is a manifestation of molecular energy, we see why 
 substances of low boiling point are more active constituents of solvent 
 mixtures than their homologues of higher boiling point, and it is 
 interesting that chloroform and ether, which when mixed with 
 alcohol form solvents for cellulose acetate and nitrate respectively, 
 are both liquids of high vapour pressure. Possibly this property is 
 also a factor in the use of the volatile substance carbon di-sulphide 
 in the viscose reaction. 
 
 Single solvents must be regarded as combining in one substance 
 the function of being adsorbed by the cellulose ester, and of being 
 attracted also by the molecules of their own kind in the remainder 
 of the solvent medium. Thus, the solvent action of benzyl alcohol 
 on cellulose acetate would be explained by the selective adsorption 
 of the hydroxyl group by the particles of cellulose acetate, and 
 the attraction between the remainder of the molecule, namely, 
 C,H;°CH,°, and the rest of the benzyl alcohol in the solvent. Such an 
 attraction would be expected to manifest itself in a certain degree 
 of association in the pure solvent, and we have a speculative 
 explanation of the fact that associated liquids are frequently good 
 solvents. 
 
 The increased solvent power frequently imparted by small 
 quantities of water in solvents may be due to a similar attraction 
 between hydroxyl groups in the ester and water molecules, and the 
 complex so formed may then adsorb some water-soluble constituent 
 of the rest of the solvent. Worden } states that if the acid hydrolysis 
 of cellulose acetate is unduly prolonged, the resulting product will 
 take more and more water in its solution in acetone without precipita- 
 tion. There is an undoubted relation between these factors, and 
 also a possibility that the ester groups themselves may have an 
 inherent attraction for water molecules, since all the inorganic 
 nitrates are soluble in water, and most of the acetates. 
 
 It is not profitable, however, in the present state of our knowledge 
 to push the theory of selective solvent absorption or adsorption 
 too far. There are specific influences at work also, which must 
 explain why alcohol and chloroform disperse cellulose acetate and 
 not cellulose nitrate, while ether and alcohol disperse cellulose 
 nitrate and not cellulose acetate; in each case we certainly have 
 adsorption of alcohol and some affinity between the alcohol and the 
 other ingredient; yet entirely different behaviour. Perhaps the 
 underlying attraction of the theory of selective absorption is that it 
 
Cellulose Ester Solutions: Some Properties 91 
 
 suggests an analogy with selective permeability of membranes, and 
 a possibility of applying a modification of Donnan’s theory of 
 membrane equilibria to the swelling of cellulose esters, thus bringing 
 them into line with gelatin. 
 
 Against the suggestion that the dispersion of cellulose esters in 
 a solvent may be brought about by selective adsorption of constituent 
 groups of the solvent mixture at the surface of the ester particles, 
 must also be placed the weight of evidence adduced by Knoevenagel. 
 If solvent molecules are absorbed by the ester so that the ratio of 
 _ the number of molecules to the number of C, units is constant, 
 Knoevenagel appears to be justified in assuming the formation of 
 some kind of molecular compound, and in asking the question—What 
 has become of the surface? Perhaps, after all, this is only a restate- 
 ment of an old difficulty. What structure is it that prevents 
 cellulose dissolving in water and yet allows the penetration of a 
 nitrating acid mixture (presumably) to every C, unit in the mass ? 
 A great deal more experimental evidence is required, particularly 
 along the lines indicated by Knoevenagel, before these questions 
 can be answered. 
 
 In regard to the molecular dimensions of the particles in dis- 
 persions of cellulose esters, there is also contradictory evidence. 
 Béeseken and his collaborators think that there is not a great 
 range of dimensions, 7.e., that » does not vary much in the 
 particles of cellulose acetate when acetylation and the accompanying 
 hydrolysis are not pushed too far. Duclaux and Wollman, on the 
 other hand, find a wide range of viscosities in the products of the 
 fractional precipitation of cellulose nitrate, and the same fact holds 
 for cellulose acetate. Since nitration is a shorter process than 
 acetylation, one would expect the range of dimensions of cellulose 
 nitrate to be at any rate no greater than that of cellulose acetate. 
 One way of reconciling the two views is to assume that a com- 
 paratively small difference in the average value of n causes a large 
 difference in the viscosity of the solutions. 
 
 The peak in the curve connecting Tyndall effect and concentra- 
 tion (Mardles) requires much more investigation, using different 
 esters and different solvents. Since the cellulose esters usually 
 differ in refractive index from their solvents, a dispersion of the ester 
 in the solvent would be expected to show a Tyndall cone, as long as 
 the particles were not too small. If, however, the particles swell 
 very considerably by adsorption of solvent, the difference in refrac- 
 tion between the two phases must diminish and the Tyndall cone 
 become less marked, unless we assume that orientated benzyl 
 
92 Cellulose Ester Varnishes 
 
 alcohol molecules, 2.e., as produced by a shell of C,H,;-CH,* groups 
 forming the outside of a cellulose acetate particle which has selec- 
 tively adsorbed the ‘OH groups, have a different refractive index 
 from normal benzyl alcohol. It has already been suggested that the 
 peak may represent the maximum development of surface before 
 envelopes of solvent, surrounding ester particles, begin to coalesce. 
 If so, there should be a relation between this curve and the curve 
 connecting concentration with yield value (Bingham). An investiga- 
 tion on these lines would be more likely to yield results of value if 
 the particles were of approximately uniform size, and this condition 
 could be partly realised by using the products of a fractional precipi- 
 tation (Duclaux) as working material. Further, Mardles found that 
 the peak diminished and disappeared with rise of temperature, while 
 Bingham (with a different ester and solvent) found that solutions 
 showing zero fluidity approached and finally attained the status of 
 true liquids with rise of temperature. A relation should be sought 
 between these phenomena, to see whether the temperature at which 
 a cellulose ester dispersion becomes a true solution is coincident with, 
 or related to, the temperature at which the peak in the Tyndall- 
 concentration curve disappears. 
 
 This short survey of recent researches on the properties of cellu- 
 lose ester solutions is enough to show that much remains to be done, 
 and a great deal of the work can best be carried out in academic 
 research laboratories. Many of the measurements required, though 
 comparatively simple and involving no great manipulative skill, 
 are tedious and require for their performance a degree of freedom from 
 the claims of other interests which can seldom be realised in a factory 
 laboratory. Apart from their scientific interest, there is always the 
 possibility of discovering some fact of immediate technical value. 
 
 REFERENCES. 
 
 1 KH. Knoevenagel and O. Eberstadt, Koll. Chem. Beihefte, 1921, 18, 194- 
 212. 2? E. Knoevenagel and R. Motz, ibid., 1921, 13, 233-241. 3 'E. Knoe- 
 venagel and A. Bregenzer, <zbid., 1921, 13, 249-261, 4 Idem, abid., 1921, 
 18, 262-271. 5 Idem, ibid., 1921, 14, 1-24, ° KE. Knoevenagel, J. Hogrefe, 
 and F. Mertens, zbid., 1922, 16, 180-214. * E. Knoevenagel and E. Volz, 
 tbid., 1923, 17, 51-71. ® E. Knoevenagel, abid., 1923, 18, 39-43. ® E. W. J. 
 Mardles, Trans. Faraday Soc., 1923, 18, 365-385. 1° H, J. H. Fenton and 
 A. J. Berry, Proc. Camb. Phil. Soc., 1920, p. 16. 11 E. W. J. Mardles, J. 
 Soc. Chem. Ind., 1923, 42, 127-1367. 1% G. J. Esselen, jun., J. Ind. Eng. 
 Chem., 1920, 12, 801-803. 18 E. C. Worden, J. Soc. Chem. Ind., 1919, 38, 
 370-3747. 
 
CHAPTER VII 
 
 INGREDIENTS OF CELLULOSE ESTER VARNISHES 
 
 Inflammability of Solvents—Inflammability of Coatings—Choice of Solvents 
 and Diluents—Application of Laboratory Results—Variation in the 
 Esters—Commercial Solvents of Esters—Approximate Classification— 
 Specifications for Acetone, Methyl Ethyl Ketone, Methyl Acetone, Ethyl 
 Alcohol, Methyl Alcohol and Wood Spirit, Ether, Ethyl Acetate, Butyl 
 Acetate, Amyl Acetate, Ethyl Lactate, Acetone Oil, Diacetone Alcohol, 
 Tetrachloroethane, Benzene, Toluene, Butyl Alcohol, Amyl Alcohol, 
 Triacetin, Benzyl Alcohol, Triphenyl Phosphate, Castor Oil—Resins— 
 Shellac, Mastic, Copal, Sandarac, Dammar—Typical Resin Formule. 
 
 THE processes and considerations involved in the manufacture 
 and application of cellulose nitrate and cellulose acetate varnishes 
 are so similar that it is best to treat them together. The choice 
 between the two is mainly that of price, but occasionally other 
 considerations occur, such as the inflammability of the coating or 
 the resistance of the coating to particular solvents. 
 
 In regard to inflammability, misapprehensions have frequently 
 arisen as to the comparative dangers of the two esters. Dry cellulose 
 acetate itself, the base of the acetate varnishes, is usually termed 
 non-inflammable—generally as a distinction from cellulose nitrate— 
 because although it will burn (7.e., is combustible), it burns quietly 
 and presents no special fire danger. Dry cellulose nitrate, on the 
 other hand, burns fiercely, and in the higher nitrations is explosive 
 under certain conditions. The solutions of these esters which are 
 employed as varnishes, however, invariably contain volatile inflam- 
 mable solvents or diluents such as acetone, methyl alcohol, ethyl 
 alcohol, benzene and ethyl acetate, to which they owe their property 
 of drying quickly. The danger incurred in the application of these 
 varnishes is therefore practically equal for both esters, since it 
 arises only from the accumulation of inflammable vapours in the 
 working rooms, unless simple precautions are taken in regard to 
 ventilation. These inflammable vapours in certain ranges of 
 concentration form explosive mixtures with air, and even when all 
 precautions are taken, naked lights, fires or smoking should never 
 be allowed in the work-rooms. Exactly the same precautions are 
 ' necessary with the so-called spirit varnishes, which consist chiefly 
 of resins dissolved in methylated spirit. The danger during appli- 
 cation has nothing to do with the solid materials dissolved in the 
 solvents; it arises from the properties of the solvents alone. 
 
 When the volatile solvents have dried out, leaving the film of 
 cellulose ester, with or without softeners, resins or pigments, a new 
 set of conditions arises. ‘The film of cellulose nitrate, taken alone 
 
 and away from any support, is still highly inflammable. If, however, 
 93 
 
94, Cellulose Ester Varnishes 
 
 the film is attached to a support—as in practice it always is—the 
 danger due to its inflammability in most cases disappears. On a 
 metal surface, the film cannot be made to ignite, as the application 
 of a flame only causes it to blister and decompose without flaming. 
 Wood coated with the varnish is no more easy to ignite than uncoated 
 wood. In fact, one may say that in general the fire danger of 
 materials coated with a cellulose nitrate varnish is the same as that 
 of the materials alone. 
 
 There is, however, one important class of exceptions—that of 
 textiles covered with a thick protective coating of cellulose ester. 
 In this instance we have an inflammable fabric, together with a 
 coating which forms an appreciable fraction of the total weight of the 
 coated material. Here a cellulose nitrate coating is an added 
 danger, and it is from this fact that the use of cellulose acetate 
 varnishes for aeroplane fabrics has become so extensive. Even 
 cellulose acetate varnishes for this purpose often have special 
 ingredients added so as to reduce their non-inflammability to incom- 
 bustibility, since the film from a varnish not so treated is usually 
 appreciably more inflammable than the original cellulose acetate 
 base, on account of the inflammable organic softeners and solvents 
 of high boiling point which it contains. 
 
 Choice of Solvents. 
 
 In nearly all technical varnishes based on cellulose esters, the 
 solvent is a mixture of several liquids. The reason for this is obvious. 
 The chief advantage possessed by these varnishes over oil varnishes 
 is their rapid rate of drying, due to the volatility of their solvents. 
 If, however, a varnish is made up entirely from a solvent of low 
 volatility, e.g., acetone, it is found that the rapid rate of drying 
 entails such a marked fall in temperature at the surface of the film 
 that the atmosphere begins to deposit moisture upon it, causing 
 precipitation of the ester and sometimes pitting of the film. This 
 tendency imposes a practical limit on the rapidity of drying which 
 can be allowed in a commercial varnish and will be considered in 
 more detail in a later section. 
 
 The varnish must therefore contain only a limited quantity of 
 highly volatile solvents if it is to dry in a normal atmosphere with a 
 bright surface and a coherent structure. To lower the rate of 
 evaporation we must add a solvent of higher boiling point, and quite 
 satisfactory varnishes for many purposes can be made from a simple 
 mixture of solvents with boiling points varying from low (e.g., about 
 50°) to high (about 130°). Here, however, considerations of cost 
 
 Se ys 
 
Ingredients of Cellulose Ester Varnishes 95 
 
 enter. Solvents of the cellulose esters are usually comparatively 
 expensive materials, and we have to find in what way we may dilute 
 these expensive ingredients with cheaper materials without inter- 
 fering with their properties as varnishes. These diluents must, of 
 course, be miscible with the solvents used. They must not precipi- 
 tate the cellulose ester from the solution, and this condition must 
 hold not only in the original varnish, but at all stages of the evapora- 
 tion of the volatile ingredients. For example, a solution of cellulose 
 nitrate in acetone could easily be diluted with a considerable quantity 
 of xylene without precipitating the ester. If, however, such a 
 solution were poured on to a glass plate and allowed to evaporate, 
 even in a perfectly dry atmosphere so that no deposition of moisture 
 was to be feared, it would not form a satisfactory film. The acetone 
 would evaporate much more rapidly than the xylene, and the point 
 would soon be reached at which the cellulose nitrate would begin 
 to separate from the solution. The final product would be an 
 opaque, flaky and brittle deposit of no value. 
 
 If, however, a diluent is chosen having a suitable rate of evapora- 
 tion, it is possible to make use of it. For example, a solution of 
 cellulose nitrate in amyl acetate may be considerably diluted with a 
 light petroleum spirit, e.g., one which distils between 50° and 120°, 
 without precipitating the ester, and during the evaporation the amyl 
 acetate may be continuously present in sufficient quantity to keep 
 the ester in solution. Under these circumstances a clear tough film 
 is formed. 
 
 The preceding chapters have shown, however, that there are 
 other cheap diluents which may be used in combinations which 
 actually assist the true solvents. Thus alcohol has been shown to 
 reduce the viscosity of solutions of both cellulose acetate and 
 cellulose nitrate, when used in certain combinations. For example, 
 alcohol and toluene together have a solvent action on some grades 
 of cellulose nitrate, and alcohol and benzene a weaker solvent action 
 on cellulose acetate, and it is for this reason that the laboratory 
 researches on the solvent power of these mixed liquids are so impor- 
 tant. Since alcohol and the coal tar distillates are cheaper than 
 most of the true solvents, the use of certain combinations of them as 
 diluents of the true solvents will often achieve considerable economy 
 without detriment to the varnish. 
 
 Application of Laboratory Results. 
 These combinations must, of course, be chosen with considerable 
 care, and much experience is necessary in applying the results of 
 laboratory work to technical manufacture. Apart from the 
 
96 Cellulose Ester Varnishes 
 
 necessity of avoiding excessively rapid evaporation and the precipi- 
 tation of the ester at any stage of evaporation, three other considera- 
 tions enter :— 
 
 (1) Most laboratory investigations are rightly carried out with 
 chemically pure solvents. Technical solvents are seldom 
 chemically pure and in some cases the degree of impurity may be 
 somewhat large. The influence of these impurities is irregular. 
 Often it may be favourable, as in the presence of traces of 
 acetone in methyl alcohol (as a cellulose nitrate solvent), or of 
 acetone in methyl ethyl ketone (as a cellulose acetate solvent). 
 Whether favourable or unfavourable, however, the presence of 
 these impurities complicates the direct sae of the 
 results of research. 
 
 (2) When a solvent contains three or more ingredients 
 (and in practice simpler mixtures are rare) the application of the 
 results of the investigation of binary mixtures becomes very 
 complicated, even if the ingredients are chemically pure. Take 
 for instance a mixture of amyl acetate, ethyl alcohol, benzene 
 and acetone. It could be regarded as a mixture of two binary 
 mixtures, (1) amyl acetate + ethyl alcohol, (2) benzene + 
 acetone, and the solvent properties of these separate mixtures 
 might be known, but it would be impossible to forecast the 
 solvent properties of the complete mixture by making use of 
 this knowledge alone. Each one of these ingredients gives 
 characteristic viscosity and solvent power curves with each of 
 the other three, and the mutual interaction of all these mani- 
 festations of affinity cannot be accurately gauged. 
 
 (3) A point which is repeatedly overlooked in patent and 
 other literature is that no two samples of cellulose ester can be 
 guaranteed to be exactly alike. Profound differences are possible 
 in solubility, in viscosity and in chemical composition. Even 
 with the most careful control of manufacture small variations 
 occur in cellulose esters made by the same process and in the 
 same factory, and between one manufacturer’s product and 
 another there may be considerable differences due to variations 
 in the raw material, composition of the esterifying mixture and 
 the time and temperature of esterification. . 
 
 It follows from the last paragraph that the value of long lists of 
 formule of cellulose ester varnishes is doubtful, especially since 
 the advent of cellulose nitrates of unusually low viscosity. For 
 instance, two separate samples of cellulose nitrate varnish might 
 
Ingredients of Cellulose Ester Varnishes 97 
 
 be made up to exactly the same formula. One might yield a 
 stiff jelly which would not flow appreciably when the vessel 
 containing it was inverted. The other might give a solution 
 more fluid than glycerine. The chief interest and value of 
 published formule lie in the manner of combination of the 
 solvents. 
 
 In a recent circular, Gardner and Parkes! discuss the rate of 
 evaporation of solvents from pyroxylin lacquers, on the whole 
 along well-known lines; there are, however, some points of interest. 
 Too rapid evaporation of the solvent causes the faults variously 
 described as pin-holing, goose-fleshing and the orange-peel effect. 
 Tests on the rate of evaporation of individual solvents alone, at 
 ordinary temperatures, show that the order in which the solvents 
 are placed by this test does not exactly coincide with the order of 
 the boiling points; for example, ethyl alcohol evaporates into the 
 air more slowly than ethyl! acetate, although the boiling points are 
 almost identical. This was to be expected, since the rate of evapora- 
 tion at ordinary temperatures depends on the vapour pressure at 
 that temperature, and the vapour pressure curves of different 
 substance may cross one another; the boiling point is only a rough, 
 but nevertheless useful, indication of the rate of evaporation at 
 ordinary temperatures. Curves are given for the rate of drying of 
 solutions of nitrocellulose in ethyl acetate, denatured alcohol (using 
 a special alcohol-soluble nitrocellulose), diethyl carbonate, butyl 
 acetate and amyl acetate. The time taken for 87% of the solvent 
 to evaporate is given in the following table :— 
 
 Ethyl acetate. ; : ‘ : . 380 mins. 
 Ethyl alcohol ; ; . : ey ne Oe 
 Diethyl | 110 
 Butyl acetate . ie 
 Amyl acetate . ; ; : : Been bei" ae 
 
 The test with ethyl alcohol is hardly comparable with the others, 
 since it was carried out with a special nitrocellulose, and the nature 
 of the dissolved ester undoubtedly influences the rate of evaporation. 
 The other four tests were carried out with the same sample of 
 nitrocellulose. 
 
 Curves are also given showing the rate of evaporation of butyl 
 acetate from solutions of different concentration of the same nitro- 
 cellulose. These all appear to reach approximately constant weight 
 in 120 minutes on filming, although the amount of solvent left in the 
 films shi from 3% for the thinnest film to 26% for the thickest 
 
98 Cellulose Ester Varnishes 
 
 film. Closer investigation would, no doubt, show that the thicker 
 films were still losing solvent at a slow rate. ‘The writer’s experience 
 in the celluloid industry is that there is a rapid loss of solvent from 
 unseasoned material in the first few hours which is practically the 
 same per unit of surface whether the material is thick or thin. ‘The 
 rate quickly diminishes, but thick material will continue to lose 
 weight long after thin material is practically dry. 
 
 Commercial Solvents for Cellulose Nitrate. 
 
 The principal solvents in use for cellulose nitrate varnishes are 
 acetone, methyl alcohol, ethyl acetate, butyl acetate and amyl 
 acetate. Of these, ethyl acetate is used more in the United States 
 than in this country. Acetone and methyl alcohol are both highly 
 volatile at ordinary temperatures, and solutions in either of these 
 solvents will not give clear films unless special precautions are taken 
 to reduce the humidity of the air and the rate of evaporation. Butyl 
 acetate and amyl acetate are used in nearly all the technical var- 
 nishes, as, owing to their low vapour pressure, they retard the rate 
 of evaporation and diminish the tendency of the film to absorb 
 moisture from the atmosphere. They can be used with diluents 
 having a vapour pressure only slightly less than their own, as long 
 as they are present in sufficient amount throughout the evaporation 
 to prevent separation of the cellulose nitrate. According to Wiesel,? 
 diethyl carbonate is being introduced as a solvent in the United 
 States. Cyclohexanone and cyclohexanyl acetate have also been 
 suggested, but have not yet come into general use. The chief 
 diluents are ethyl alcohol, benzene, toluene, butyl alcohol and amyl 
 alcohol (or fusel oil). Ethyl alcohol is present in practically all 
 cellulose nitrate varnishes, since it is used to expel water from the 
 original cellulose nitrate in the dehydration process. It is not 
 simply a diluent, since it develops the solvent power of some other 
 solvents. Benzene and toluene are both coal-tar fractions. They 
 have no solvent power of their own, but are partial or complete 
 solvents when mixed with alcohol in certain proportions. When 
 toluene, which has a lower vapour pressure than benzene, is used, 
 special care must be taken that there is sufficient butyl or amyl 
 acetate present to counteract its lack of solvent power when the 
 bulk of the alcohol has evaporated. 
 
 Finally, it may occasionally be economical to dilute a solution 
 with an entirely indifferent liquid which has no solvent power, either 
 alone or in combination, at ordinary temperatures. The only 
 liquid of this class used commercially is a light petroleum spirit. 
 
 | 
 | 
 
Ingredients of Cellulose Ester Varnishes 99 
 
 The heavier paraffin distillates are too slow in evaporation, and 
 would not satisfy the condition that the true solvents must be in 
 excess throughout the evaporation. 
 
 Cellulose Acetate Solvents. 
 
 These may be considered under practically the same headings as 
 cellulose nitrate solvents. Clément and Riviére® classify the 
 acetate solvents as follows :— 
 
 (1) Direct volatile solvents : Acetone, methyl acetate, methyl 
 and ethyl formates, diacetone—-alcohol, nitromethane, formic 
 and acetic acids, pyridine, ethyl acetoacetate, benzyl alcohol, 
 furfurol. 
 
 (2) Indirect volatile solvents: Hot alcohol and benzene, 
 chloroform and alcohol, tetrachloroethane and alcohol. 
 
 (3) High-boiling gelatinising solvents: Acetins, particularly 
 triacetin, diphenyl ethers of glycerin, glycerin benzoate, glycol— 
 chlorohydrin, resorcin diacetate, eugenol, butyl and amyl 
 tartrates, dimethyl phthalate. 
 
 One may say roughly that, in the above list, acetone, methyl acetate 
 and methyl and ethyl formates correspond to the highly volatile 
 sc ~°nts used in cellulose nitrate varnishes. To these may be 
 addea methyl ethyl ketone, which, although not a solvent when 
 chemically pure,* always contains, in its technical form, sufficient 
 acetone to make it one. Benzyl alcohol, eugenol and the acetins 
 are similar in function to butyl and amyl acetates in cellulose nitrate 
 solutions; while alcohol and benzene, alcohol and chloroform, 
 _ alcohol and tetrachloroethane are binary mixtures possessing definite 
 solvent power and not to be classified as mere diluents. Entirely 
 indifferent diluents such as petroleum spirit do not appear ever to 
 be used in cellulose acetate solutions. ‘The solvents used in the Air 
 Ministry dopes are acetone, methyl ethyl ketone, alcohol, benzene 
 and benzyl alcohol. Many of the additional substances in Clément 
 and Riviére’s list were used to a greater or less extent during the war, 
 but have been entirely displaced by cheaper solvents since. 
 
 Specifications of Cellulose Nitrate and Acetate Solvents. 
 
 Specifications for all of the usual solvents have been published 
 by the British Engineering Standards Association in accordance 
 with the best commercial practice. They are copyright, and can be 
 obtained from the Secretary, 28 Victoria Street, London, 8.W.1, at. 
 2d. each, post free. 
 
100 Cellulose Ester Varnishes 
 
 These specifications refer generally to materials required for 
 Air Ministry Dopes, and in most cases it may be advisable to make 
 them more or less stringent for other commercial purposes. On 
 the whole, however, they form a reasonable basis for the examination 
 of solvents. 
 
 In general, it may be said that a high standard of chemical purity 
 is not necessary for the ingredients used in the preparation of 
 cellulose ester varnish, since it follows from what has been written 
 before that the solvent power of mixtures is often greater than that 
 of either ingredient singly; but it is clearly necessary for the buyer 
 to know what he is buying, and to have the mixing under his control. 
 Further, there are certain classes of impurities which must be 
 stringently guarded against. One of these is acidity. Both the 
 acetate and the nitrate of cellulose are sensitive to strong acids, and 
 even weak acids such as acetic acid may be harmful in their effects 
 on containers, pigments, or fabrics. A second harmful impurity is 
 water. Although water in small amounts increases the solvent power 
 of some solvents, in large amounts it acts as a precipitant and 
 destroys the tenacity of the coating. It must also be remembered 
 that considerable cooling takes place when a cellulose ester coating 
 is drying, and this always results in a certain amount of moisture 
 being absorbed from the atmosphere, even if the original ingredients 
 were anhydrous. A third undesirable impurity is solid foreign 
 matter, either suspended or dissolved in the solvent. Suspended 
 solids can be removed by filtration, but dissolved solids will be left 
 in the varnish coating. 
 
 Hence specifications of cellulose ester solvents usually contain 
 the following clauses: Water content, acidity and residue on 
 evaporation are strictly limited. Other impurities are limited by 
 determinations of one or more physical properties such as specific 
 eravity, range of temperature of distillation, refractive index, 
 miscibility with specified liquids or solutions, and colour. Usually 
 also, if practicable and convenient, there is a chemical determination 
 of the percentage of the pure chemical compound in the sample. 
 
 Specifications for Cellulose Nitrate and Cellulose Acetate Solvents. 
 The following are typical specifications for some of the commoner 
 solvents and diluents used in the cellulose ester varnishes :— 
 
 Acetone.—Until the introduction of the new cordite (R.D.B.) 
 during the war, acetone was used principally as a solvent for gun- 
 cotton, and the best quality was supplied to the British Government 
 
 
 
Ingredients of Cellulose Ester Varnishes 101 
 
 Specification (usually abbreviated B.G.S.). This specification is 
 still used almost universally in commerce. 
 
 B.G.8. acetone must be colourless and transparent, and must 
 mix with distilled water in any proportions without showing tur- 
 bidity. It must leave no residue when evaporated to dryness. Its 
 specific gravity at 15°/15° must not exceed 0-800. If 100c.c. of the 
 acetone is mixed with 1 c.c. of a 0-1% solution of potassium perman- 
 ganate the colour must not be discharged in less than 30 minutes 
 in the dark at a temperature of 15°. This last test, known as the 
 permanganate test, is the one on which acetone is most frequently 
 rejected. In 1904, as a result of researches by Marshall,' a clause 
 was added to the specification to ensure that the acetone should 
 not contain more than 0-002% of carbon dioxide, and should be 
 otherwise quite neutral. Carbon dioxide has no harmful effect in the 
 manufacture of lacquers, and it is usual only to limit the percentage 
 of acidity and basicity (v. infra). 
 
 A useful additional test is the amount of discoloration on exposure 
 to light. This is important if the acetone is to be used in colourless 
 or delicately tinted lacquers. 
 
 The B.E.S.A. specification differs in some particulars from the 
 B.G.8. The limits of specific gravity are given as 0-769 and 0-801 
 at 15°, and a distillation test is included providing that 95° must 
 distil between 55° and 60° at 760 mm. pressure. ‘Titration tests are 
 included to ensure that the acidity and alkalinity do not exceed 
 0-01% calculated as acetic acid and as caustic soda respectively. 
 The time in the permanganate test is reduced to 10 minutes, and the 
 amount of acetone in the sample is estimated by a titration method. 
 
 Until shortly before the war, acetone was derived entirely from 
 the distillation of wood, and the separation of pure acetone from the 
 crude distillate, containing water, methyl alcohol, acetic acid, 
 various other ketones and pyroligneous impurities, is technically 
 difficult. Large-scale experiments on the fermentation of starch 
 by a special ferment, yielding butyl alcohol and acetone, were begun 
 in 1912, and the process was largely developed during the war. 
 According to Tunison,® contamination difficulties in the fermentation 
 process have only been successfully overcome during the last two 
 years, and the increased production from this source is likely to 
 reduce the price of acetone in future.’ 
 
 Methyl Ethyl Ketone, CH,:CO-C,H;—This substance is the 
 homologue of ordinary acetone, which is dimethyl ketone, and is 
 derived from the products of the distillation of wood. It has usually 
 a pale yellow colour, which should not be too pronounced. Its 
 
- 102 Cellulose Ester Varnishes 
 
 specific gravity varies from 0-810 to 0:815 at 15°, and on distillation 
 95% should distil between 70° and 81°. It should be miscible with 
 an equal volume of benzene and with three volumes of water, and 
 should be neutral and free from non-volatile impurity. The 
 B.E.8.A. specification (2D. 1) includes a test for refractive index, 
 solubility in a saturated salt solution, and content of ketone, 
 determined by a titration method based on oxime formation with 
 hydroxylamine hydrochloride. Methyl ethyl ketone always con- 
 tains some acetone, which is estimated with it and calculated as 
 methyl ethyl ketone. This explains why it is possible to fix the 
 minimum content as high as 100%. Pure methyl ethyl ketone is 
 not a solvent for cellulose acetate at ordinary temperatures. 
 
 Methyl Acetone—Methyl acetone is a crude fraction of wood 
 distillate containing acetone and methyl alcohol as its principal 
 ingredients. It varies very considerably in properties, as the following 
 results of distillation will show :— 
 
 Range. No. 1. No. 2. No. 3. No. 4. No. 5. 
 
 Up to 50° a few a few a few a few a few 
 
 drops drops drops drops drops 
 
 50—55 2% 2% 31% 2% 35% 
 55—60 44 54 49 36 54 
 60—65 49 34 15 6 9 
 65—70 4 9 4 1] 1 
 Above 70 — — — 44 — 
 
 The colour is usually yellow or yellowish-green and the odour 
 very rank. Sometimes both the colour and the odour improve 
 slightly after an exposure to light for several days. 
 
 Higher grades can be obtained, however, and the material has 
 been standardised by the British Engineering Standards Association 
 (No. D. 2). It should be colourless or not darker than a faint 
 yellow; 95% should distil between 50° and 70°. (It will be noted 
 that all but No. 4 of the samples described above comply with this 
 distillation test.) The acidity or alkalinity to phenolphthalein must 
 not exceed 0-02%, and it must be miscible with water in all propor- 
 tions or with its own volume of carbon disulphide at 25°. 
 
 This specification is of particular interest, as it makes use of a 
 determination of solvent power number (q.v.). Two solutions are 
 made up containing 6 grammes of cellulose acetate (D. 6), one with 
 ‘ pure dry acetone ”’ and the other with the sample of methyl acetone 
 under examination. A measured volume of each solution is placed 
 in a flask immersed in a thermostat at 25°, and alcohol (2D. 9) is run 
 
 “a 
 
 
 
Ingredients of Cellulose Ester Varnishes 103 
 
 into each from a burette, with constant shaking, until a faint perma- 
 nent opalescence is obtained. It is evident that if acetone is the 
 better solvent, it will bear a greater dilution with alcohol than the 
 methyl acetone before the cellulose acetate begins to precipitate ; 
 and that the solvent power is measured by the volume of alcohol 
 added. If the acetone solution has taken a c.c. of alcohol and the 
 methyl acetone solution b c.c. of alcohol, then 
 
 Solvent power of sample = : x 100. 
 
 The material is graded according to the result of this test. 
 
 Grade I has solvent power not below 65, 
 99 II 9 9 99 50, 
 29 ITI 29 29 ” 30. 
 
 Methyl acetone in Grade I must also contain at least 55°% of acetone, 
 determined by Messinger’s method (Ber., 1888, 21, 3366). 
 
 It will be observed that the solvent power test only determines 
 the value of methyl acetone as a solvent for cellulose acetate, and this 
 is roughly proportional to its content in acetone. Since the other 
 main ingredient is methyl alcohol, which is a solvent of nitrocellulose, 
 methyl acetone is a better solvent for cellulose nitrate than for 
 cellulose acetate. It is not a very desirable material, however, on 
 account of its variability and its usually unpleasant odour. 
 
 Ethyl Alcohol, C,H;-OH.—Ethyl alcohol, usually termed simply 
 alcohol, and sometimes “ spirit,” is a constituent of nearly all cellulose 
 nitrate lacquers, since they are made chiefly from nitrocellulose 
 which has been dehydrated with alcohol. As is well known, it is 
 produced by the fermentation of dextrose, derived either from 
 starchy materials or from molasses. Other methods of manufacture, 
 such as by the addition of the elements of water to the gas ethylene, 
 CH,:CH,, have been and are being investigated, but have not yet 
 had any influence on the market. 
 
 Alcohol for use in the manufacture of varnishes is denatured 
 (rendered non-potable) by the addition of 5% of wood naphtha, and 
 is bought free of duty as “ Industrial Methylated Spirits.’ The 
 presence of this quantity of wood naphtha, which is an impure form 
 of methyl alcohol, confers slight solvent power on the alcohol. 
 Alcohol should be colourless, free from appreciable non-volatile 
 residue, and miscible with distilled water in all proportions. The 
 concentration of alcohol is usually ascertained from its density, 
 which is conveniently determined by a delicate hydrometer. A 
 special form of hydrometer, known as Sikes’s, is made for this 
 
104 Cellulose Ester Varnishes 
 
 purpose, provided with a series of counterpoises by the use of which 
 it is possible to measure with considerable accuracy the density 
 of any concentration of alcohol from zero to 100%. If only strong 
 alcohol is dealt with, this instrument is not necessary. The best 
 specific gravity tables to consult in ordinary laboratory work are 
 those of Thorpe. 
 
 Industrial methylated spirit should have a proof strength of not 
 less than 66 O.P., which denotes a liquid having a specific gravity 
 of 0:8171 at 15-6°/15-6°, and containing 91:99, of ethyl alcohol 
 (including the methyl alcohol used as denaturant) by weight, and 
 94-7°%, by volume. A higher specific gravity shows the presence of 
 an excess of water, and such a spirit should not be accepted. 
 Occasionally a somewhat stronger spirit with a lower specific gravity 
 may be encountered, and is, of course, unobjectionable. 
 
 A distillation test should show a fraction of not less than 95% 
 distilling between 76° and 79° at 760 mm. pressure, and the alcohol 
 should be neutral in reaction. The B.E.S.A. specification (2D. 9) 
 gives limits for acidity to phenolphthalein and alkalinity to methyl 
 red, and states that the spirit should have an agreeable odour, a 
 stipulation with which the Government Chemist would probably 
 not agree. 
 
 Methyl Alcohol, CH;,-OH.—Methyl alcohol is a useful low-boiling 
 solvent of the lower nitrates of cellulose. It is produced with 
 other substances in the dry distillation of wood, and whether its 
 solvent power is due to the presence of traces of acetone is a 
 question that has never been authoritatively settled. At any 
 rate the purest form occurring in commerce is a powerful solvent. 
 This grade is seldom, if ever, used in varnishes, as it is dutiable. 
 There is a lower grade usually sold as wood spirit, and the lowest 
 grade of all is the so-called “ miscible wood naphtha,” selected 
 for its nauseousness and used for the denaturing of ethyl alcohol 
 (v. supra). 
 
 Wood spirit cannot usually be used for transparent or delicately 
 tinted lacquers, as it is seldom free from colour and often 
 colours still more strongly on exposure to light. If it is required 
 for a purpose where colour is of importance, this property should 
 be tested with the samples submitted. It should leave no appreci- 
 able residue on evaporation and on distillation should give 95% 
 between 63° and 68°. The specific gravity 15°/15° should not 
 exceed 0-810, and the spirit should be neutral. 
 
 It should be noted that both industrial methylated spirit (ethyl 
 alcohol) and wood spirit (methyl alcohol) invariably contain a small 
 
Ingredients of Cellulose Ester Varnishes 105 
 
 percentage of water, so that neither is miscible with petroleum 
 spirit without cloudiness. According to Tunison,® the use of 
 anhydrous ethyl alcohol for the manufacture of lacquers is being 
 developed in the United States, with the object of reducing as far 
 as possible the original water-content of the lacquer. 
 
 Hiher.—Ether, or diethyl ether, (C,H,),0, is made by the inter- 
 action of sulphuric acid and alcohol; hence it is sometimes termed 
 “sulphuric ether.” It is not an important commodity in the 
 varnish industry, as it enters only into the manufacture of the 
 ether—alcohol collodions, which represent a very small proportion 
 of the total output. In the explosives industry it takes a much 
 more important place. 
 
 Ether is a colourless, mobile, volatile liquid, boiling at 34-5°, and 
 is the most volatile solvent used in the industry. It is only partially 
 miscible with water. The specific gravity of the commercial article 
 varies from 0-72 to 0-73, according to the alcohol and water content. 
 The specific gravity of pure ether is 0-7195 at 15°, and if the chief 
 impurity is alcohol, a specific gravity of 0-730 corresponds with a 
 material containing about 92% of ether, and a specific gravity 
 of 0-720 with one containing 98—99% of ether. Anhydrous 
 _ ether should show no cloudiness when mixed with an equal volume 
 of carbon disulphide at 15°; the usual commercial article is miscible 
 with alcohol, benzene and chloroform. Ether sometimes contains 
 a small quantity of residue with an unpleasant odour, which is 
 disadvantageous in collodion manufacture. This is most conveni- 
 ently detected by allowing a few cubic centimetres to evaporate in 
 an open dish. 
 
 Although ether is so volatile, its vapour is so heavy, on account 
 of its comparatively high molecular weight, that it may travel along 
 the ground for a considerable distance in sufficient concentration to 
 propagate a flame. Hence special care must be taken both in 
 manufacturing and applying solutions in which ether is used. 
 Running motors are the most likely source of sparks near the ground, 
 and these should not be used in the same room as ether. 
 
 Kithyl Acetate.—Ethy] acetate has hitherto been used much more 
 in the United States than in this country, and Tunison ® refers to 
 it as the most important nitrocellulose solvent. It is now made in 
 this country from industrial methylated spirit under special Customs 
 regulation, and the consumer must give a guarantee that it will be 
 used for solvent purposes only, and not for such purposes as flavour- 
 ing, where it would displace a dutiable article. The following is a 
 typical specification :— 
 
106 | Cellulose Ester Varnishes 
 
 It should be a clear, colourless liquid, with specific gravity at 
 
 15° not less than 0-890 or more than 0-900. 90% should distil 
 between 70° and 85°. (The boiling point of pure ethyl acetate is 
 77°.) Its acidity calculated as acetic acid should not exceed 0-01%, 
 and it should be miscible at 15° with double its volume of benzene. 
 
 The ester content is determined by saponification with standard | 
 
 alcoholic caustic potash in the usual way and should not be less 
 than 85% calculated as ethyl acetate. 
 
 Ethyl acetate has a higher specific gravity than ethyl alcohol 
 (from which it is made) and a lower specific gravity than water. 
 Hence a high figure for the specific gravity does not necessarily 
 indicate a high ester content. It may indicate dissolved water and 
 should be checked by the miscibility test. A low specific gravity 
 usually indicates excess of unesterified ethyl alcohol. 
 
 The B.E.S.A. specification (D. 19) is less stringent than that just 
 given in regard to acidity, as it fixes the maximum at 0-1%. It is 
 more stringent in regard to distillation (95% between 70° and 80°) 
 and ester content (90%). 
 
 A curious sample of American origin analysed in the writer’s 
 laboratory a few years ago gave an ester content of 93:4%, was 
 free from water and satisfactory in regard to acidity. The dis- 
 tillation, however, gave only 71% up to 85°, 20% from 85—100°, 
 and 7°% from 100—113° (total 98%). Apparently the higher boiling 
 fraction consisted chiefly of a homologous ester. 
 
 Butyl Acetate-——This material is displacing amyl acetate to a 
 very considerable extent as the chief high-boiling solvent con- 
 stituent of lacquers. This is largely due to its greater stability of 
 price, since amyl acetate is generally to be preferred on account of 
 its somewhat slower rate of evaporation and smaller affinity for 
 water. It is made by the acetylation of butyl alcohol. 
 
 Butyl acetate should be a clear, colourless liquid, free from 
 appreciable non-volatile residue and having a specific gravity at 
 15° between 0:875 and 0-880. On distillation 95% should boil 
 between 110° and 130°. It should be miscible in all proportions 
 with benzene, although the commercial article will sometimes turn 
 cloudy with 4 volumes of petroleum spirit. The ester content 
 should be at least 87-5% calculated as butyl acetate. 
 
 The B.E.S.A. specification is 2D, 4. 
 
 Amyl Acetate——It was explained in an earlier chapter ise the 
 discovery by Stevens of the valuable properties of amyl acetate as a 
 solvent for nitrocellulose laid the foundation of the lacquer industry. 
 Unfortunately, ever since the rise of the cinema film industry, amyl 
 
Ingredients of Cellulose Ester Varnishes 107 
 
 acetate, or rather the fusel oil from which it is made, has been a 
 favourite commodity for the speculator. Its sole commercial 
 source is as a by-product in the fermentation of sugar to alcohol 
 and there is no competitive method of manufacture. The effect 
 of the rapid fluctuations in price has been greatly to stimulate the 
 manufacture of butyl alcohol and butyl acetate. 
 
 Technical amyl acetate should be a clear, colourless, neutral 
 liquid, free from appreciable residue on evaporation. The per- 
 centage of ester calculated as amyl acetate should not be less than 
 90, and 98% should distil between 110° and 145°. The specific 
 gravity should not be less than 0-870 nor more than 0-875; and the 
 ester should be miscible with benzene or toluene in all proportions. 
 The B.E.S.A. specification is a little stricter in regard to distillation 
 ranges, which is laid down as 95% between 125° and 145°. In effect 
 this reduces the permissible amount of lower homologues, derived 
 from the presence of propyl and butyl alcohols in the original fusel 
 oil. 
 
 Hthyl Lactate—According to Tunison,® the use of ethyl lactate 
 as a nitrocellulose solvent is increasing rapidly. 
 
 Technical ethyl lactate is a colourless liquid boiling from 145° 
 to 155°, and its specific gravity is 1-037 at 20°. It is therefore 
 somewhat heavier than amyl acetate, and evaporates more slowly. 
 It will bear heavier dilution with water than most solvents without 
 losing its solvent power. According to Gardner,’ solutions of 
 nitrocellulose in ethyl lactate have a rather higher viscosity than 
 those of equal concentration in butyl acetate, and dry more slowly. 
 They will, however, bear considerably more dilution with non- 
 solvents such as benzene. 
 
 Acetone Oils.—The materials appearing in commerce under the 
 name “acetone oil” are fractions, varying in composition, of 
 higher boiling point than acetone, obtained in the wood distillation 
 industry after the removal of the acetone. They consist of higher 
 ketones, and a good sample may contain a considerable quantity of 
 low-boiling ketones such as methyl ethyl ketone and methyl propyl 
 ketone. Such solvents are known as light acetone oils and are 
 useful in the industry. Heavy acetone oils are of much smaller 
 value ; not only do they dry very slowly, but the smell is sometimes 
 difficult to tolerate. 
 
 No standard specification can be suggested, owing to the varia- 
 bility of the material, and acetone oil cannot be commended as a 
 solvent for the same reason. All that can be said is that the value 
 of the oil is governed chiefly by its content in low-boiling material, its — 
 
108 Cellulose Ester Varnishes 
 
 colour, and its odour, provided that the usual tests for neutrality 
 and residue are satisfied. The acetone oils were first used in nitro- 
 cellulose lacquers by Crane. 
 
 Diacetone alcohol was patented by Doerflinger © in 1911 as a 
 solvent for cellulose acetate and nitrate. It is prepared by the 
 condensation of two molecules of acetone by sodium hydroxide, and 
 has the formula 
 
 (Hs 
 HO—C—CH,—CO—CH,, 
 CH, 
 
 and the boiling point is 164°. A sample examined in the writer’s 
 laboratory gave the following figures :— 
 
 Specific gravity at 15° 0-9037. 
 Completely miscible with water. 
 
 Distillation— 
 Up to 65° : MP eS 
 65— 80° ; ; : - ee 
 86-1409 
 140-—164%° 
 164-1709 ee 
 
 The sample contained a considerable quantity of acetone. A 
 solution of nitrocellulose in the sample gave a clear film on glass, 
 which dried very slowly owing to the low vapour pressure of the 
 bulk of the solvent. It is also a solvent of cellulose acetate, but has 
 been used to a larger extent in the United States than in Europe. 
 
 Tetrachloroethane, C,H,Cl,.—Tetrachloroethane is made by the 
 combination of acetylene and chlorine. Acetylene tetrachloride 
 and ‘‘ westron’’..are synonymous. It has the somewhat high 
 boiling point of 146°, and is a good solvent for cellulose acetate when 
 mixed with alcohol. Its specific gravity is 1:6 at 15°. In the 
 presence of moisture it is liable to develop traces of hydrochloric acid, 
 and it should be tested by shaking it with water, acidifying the 
 aqueous layer with nitric acid and adding silver nitrate. It is not 
 a solvent for cellulose nitrate under any conditions, and is not an 
 ingredient of the official cellulose acetate aeroplane dopes. The 
 odour is characteristic and easily recognisable. Whenever it is 
 used, special attention must be paid to the ventilating arrangements, 
 as the continued inhalation of the vapour is injurious and a number 
 of cases of toxic jaundice among workers in the Benin por doping 
 sheds in 1915—16 were traced to its use.11 
 
Ingredients of Cellulose Ester Varnishes 109 
 
 Benzene, C,H,.—This is a coal tar distillate, to be carefully dis- 
 tinguished from benzine, a paraffin distillate. To prevent the con- 
 fusion arising from the identity of pronunciation, the former is usually 
 known by its commercial name benzol, and the latter is frequently 
 called petroleum spirit. Benzene approximates to a pure aromatic 
 hydrocarbon, C,H, while benzine is a mixture of several homologous 
 saturated hydrocarbons. Benzene forms with alcohol a mixture 
 having weak solvent power for the lower nitrates of cellulose. 
 Benzine is a non-solvent under all conditions. 
 
 There is no need to stipulate for a high degree of chemical purity 
 in benzol required for varnish manufacture, since its next homologue, 
 toluene, C,H,-CH,, is also a useful diluent which forms a similar 
 solvent mixture when mixed with alcohol. If required for clear or 
 delicately coloured lacquers, it should itself be practically colourless. 
 It should be free from acidity, and should leave no appreciable 
 residue on evaporation. Its specific gravity should be not less than 
 0-883 or more than 0-886 at 15°. When shaken with pure concen- 
 trated sulphuric acid it should not give more than a pale yellow 
 coloration. The B.E.S.A. specification (2D. 10) limits the amount 
 of carbon disulphide, and includes a test to prove the absence of 
 paraffin hydrocarbons. Carbon disulphide is an unpleasant impurity, 
 as it not only affects the workers using the lacquers, but may also 
 tarnish metal work by forming metallic sulphides. In fact, a 
 rough but useful test is to note the time for the development of a 
 stain on a clean silver coin. Paraffin hydrocarbons may be present 
 as adulterants, since they are cheaper than benzol, but the sulphona- 
 tion test described is not necessary with every consignment from a 
 reputable supplier. 
 
 Toluene, C,H,-CH,.—Toluene comes next to benzene in the series 
 of aromatic hydrocarbons derived from coal tar, and the specifica- 
 tion runs on similar lines. It can be obtained commercially in a 
 high state of purity, perhaps higher than is necessary in the majority 
 of varnishes. The distillation of such toluene should give at least 
 95% boiling between 110° and 112°. The specific gravity should be 
 not less than 0-867, not more than 0:869 at 15°. The colour developed 
 on shaking with concentrated sulphuric acid should not be darker 
 than straw-yellow. Toluene was prepared on a large scale during 
 the war by the fractional distillation of Asiatic petroleum. Toluene 
 of this origin should be tested by the sulphonation test (B.E.S.A. 
 2D. 10) for the presence of saturated hydrocarbons.'* This test 
 is unnecessary for toluene derived from coal tar. 
 
 Commercial distillates can be obtained consisting chiefly of a 
 
110 ~ Cellulose Ester Varnishes 
 
 mixture of benzene and toluene, and these can frequently be used in — 
 varnishes. The specification in this instance should be based on a 
 distillation test and specific gravity test, designed to limit undue 
 variation in the proportions of the ingredients and hence in the 
 rate of evaporation. 
 
 Butyl Alcohol.—Buty] alcohol is a useful modern addition to the 
 ingredients of cellulose nitrate varnishes. Although not a solvent 
 itself it forms solvent mixtures with organic esters and with aromatic 
 hydrocarbons, and on account of its low vapour pressure it checks 
 the rate of evaporation, thereby preventing the deposition of water, 
 and producing a smoother and tougher film. It should be a clear 
 colourless neutral liquid, having a specific gravity at 15° not less 
 than 0-810 or more than 0-820. 95% should distil between 95° 
 and 120°. The B.E.S.A. specification (D. 17) contains a deter- 
 mination of the percentage of butyl alcohol present, based on 
 acetylation with pure acetic anhydride in a pyridine medium and 
 titration of the acetic acid produced :— 
 
 (CH,-CO),:0 +- C,H,-OH — C,H,:O-CO-CH, ae CH,-COOH 
 
 This method is due to Verley and Bélsing,!* whose original paper 
 should be consulted. It is, of course, not specific for butyl alcohol. 
 Spiers 14 describes a method of determination of butyl alcohol by 
 fractional distillation. 
 
 Amyl Alcohol.—Crude amy] alcohol from the distilleries is known 
 as fusel oil, and when purified by rectification is called amyl alcohol. 
 These substances have similar properties to butyl alcohol, which is 
 largely displacing them. 
 
 Both should be neutral colourless liquids. The specific gravity 
 of fusel oil ranges from 0-813 to 0-820, and of amyl alcohol from 
 0-815 to 0-819. Fusel oil should distil between 125° and 132°, 
 amyl alcohol between 128° and 132°. They should mix with an 
 equal volume of toluene without cloudiness. 
 
 Triacetin is the triglyceryl ester of acetic acid, 
 
 3 CH,"0:CO:CH, 
 CH:0-CO-CH,; , 
 CH,°0-CO-CH, 
 
 and is used as a softening agent both in cellulose acetate and in 
 cellulose nitrate dopes, but chiefly in the former. The B.H.S.A. 
 specification is 2D. 11. 
 
 Triacetin should be a clear, colourless liquid having a specific 
 gravity at 15° not less than 1-16 nor more than 1:17. It should be 
 
Ingredients of Cellulose Ester Varnishes 111 
 
 miscible with twice its volume of benzene at 0° without showing 
 cloudiness (freedom from glycerin and water). The acidity should 
 be less than 0-15°% calculated as acetic acid. The makers will 
 guarantee a maximum of 0-:1%. The percentage of ester is deter- 
 mined by saponification with normal aqueous sodium hydroxide, 
 and should be not less than 97% calculated as triactin. 
 
 Benzyl alcohol, C,H;-CH,OH, is used as a high-boiling solvent 
 and softener in cellulose acetate dopes, and occasionally in nitro- 
 cellulose solutions. It is made by the saponification of benzyl 
 chloride, C,H;-CH,Cl, and it must therefore be tested for free 
 hydrochloric acid and for combined chlorine. The B.E.S.A. speci- 
 cation is 2D. 7. 
 
 Benzyl alcohol should be a clear, colourless liquid with a specific 
 gravity at 15° not less than 1-050 and not more than 1-055. On 
 distillation, 95°% should be collected between 200° and 210°. Rather 
 a higher tolerance than usual is allowed for non-volatile residue, 
 namely 0-1 gramme (maximum) from 10 c.c. Free acid, calculated 
 as benzoic acid, must not exceed 0-1%. It must give up no soluble 
 chloride to distilled water on shaking. The chlorine content is 
 determined by boiling under reflux with alcoholic potash and estimat- 
 ing the soluble chloride formed; this method assumes that the 
 chlorine is present in a form removable by hydrolysis (e.g., as benzyl 
 chloride). Benzyl alcohol is determined by acetylation with acetic 
 anhydride in pyridine solution (v. butyl alcohol). 
 
 Triphenyl Phosphate.—This material is used both as a softener 
 and as a flame-resister in cellulose acetate dopes. It has rather an 
 interesting history. It was patented as far back as 1901 by Zthl? 
 as a substitute for camphor in ordinary celluloid, but was never used 
 on the large scale. Shortly before the war it was adopted as a 
 constituent of acetate dopes, and was manufactured in very large 
 quantities for that purpose. After the war, attempts were made to 
 work off stocks by using it as a camphor substitute in ordinary 
 celluloid in accordance with Ziihl’s original patent, but it has several 
 drawbacks, and its use for this purpose is diminishing. There is 
 some conflict of evidence as to its solvent properties. For some 
 varieties of nitrocellulose, a solution of triphenyl phosphate in 
 ethyl alcohol has practically no solvent power whatever. 
 
 The B.E.S.A. specification is 8D. 12. Triphenyl phosphate 
 should be a white, dry powder or crystalline substance, which 
 should not darken on exposure to sunlight. It should be com- 
 pletely soluble in amyl acetate, butyl acetate, acetone, benzene and 
 alcohol. The melting point should lie between 45° and 48°. The 
 
ita Cellulose Ester Varnishes 
 
 acidity should be less than 0-01%, calculated as the equivalent of 
 caustic soda. It should give a clear solution in chloroform (absence 
 of water) and should be free from chlorides, sulphates and inorganic 
 phosphates. The ash should be less than 0-05%. 
 
 Castor oil is a valuable softening agent for nitrocellulose coatings, 
 since its solubility in alcohol facilitates its incorporation in solutions, 
 and when of good quality it has only a faint yellow colour. The 
 best oil is that known as “ cold drawn.” 
 
 The B.E.S.A. specification is 2D.5. The specific gravity should 
 be not less than 0-963 nor more than 0-966 at 15°. The free acid 
 determined by sodium hydroxide titration with phenolphthalein 
 as indicator, should not exceed 0-75%, calculated as oleic acid. The 
 iodine value should lie between 84 and 89. The solubility test in 
 alcohol described in 2D. 5 is useful, but it is not clear why it is 
 entitled ‘‘ Critical Solution Temperature in Alcohol,” since no 
 critical temperature is determined. 
 
 According to this test, 1 volume of the material wroule be 
 completely soluble in 5 volumes of 59-6 O.P. alcohol at 20° and the 
 solution should remain clear when cooled to 0°. (59:6 O.P. alcohol 
 has the required specific gravity of 0-8303 at 60° F.) The refractive 
 index at 20° should lie between 1-4772 and 1-4791. 
 
 (Note.—The miscibility test described above appears to be based 
 on that of Finkener. Archbutt and Deeley 1* recommend a variation 
 of this test which has been of considerable service in the writer’s 
 laboratory :—‘“* Measure exactly 10 c.c. of castor oil in a graduated 
 stoppered test cylinder, add 50 c.c. of alcohol of specific gravity 
 0-834 (= 89-9% by vol. = 57-6 O.P.) and well mix. If as little as 
 5% of foreign oil be present the liquid will remain strongly turbid 
 even on warming to 20°.’’) 
 
 Castor oil is an expensive material, and unless it is bought from 
 firms of the highest standing, adulteration is not uncommon. In 
 doubtful instances it will be found useful to consult Lewkowitsch.*’ 
 
 Resins.—The chemistry of the resins is unusually complicated, 
 and for fuller details than can be given here, the reader is referred 
 to the literature mentioned in the bibliography. 
 
 Resins are incorporated with nitrocellulose varnishes for the 
 purpose of increasing the body or solid content, and producing a 
 harder, more elastic and more adhesive coating. The presence of 
 the nitrocellulose renders the film tougher and more resistant than 
 one derived from resins alone. The resins chiefly used in the 
 industry are shellac, mastic, copal, sandarac, and dammar. 
 
 Shellac—Shellac resin is the secretion of the lac insect, 
 indigenous to India, and the production of shellac is an important 
 
 ‘\ 
 
Ingredients of Cellulose Ester Varnishes 113 
 
 native industry in that country. It is a valuable resin on account 
 of its hardness, adhesiveness and elasticity. The standard grade 
 
 is that sold as <n, which usually contains 3% of added rosin. 
 
 Garnet lac, which has a deeper colour, may have 10% of rosin, while 
 the form known as button lac, consisting of small plates or discs of 
 the resin, varies in composition from pure shellac to a product which 
 is about half rosin. The addition of rosin (a cheap rosin which forms 
 the residue from turpentine distillation) was at one time thought to 
 assist the manufacture, and although it is no longer necessary, the 
 existence of the adulteration is recognised in the industry. A 
 bleached shellac is made by the action of alkaline hypochlorite on 
 
 shellac. It is much paler in colour than <a> shellac, but more 
 
 friable and not quite so soluble in methylated spirit. 
 
 Shellac contains about 4% wax (insoluble in alcohol), the re- 
 mainder being the resin. Garnet lac is free from wax and therefore 
 gives a clear solution in alcohol, but against this advantage must be 
 placed the higher percentage of adulterant. 
 
 The pure resin is soluble in ethyl alcohol, methyl alcohol and 
 amyl alcohol; partly soluble in ether, ethyl acetate and acetone; 
 only slightly soluble in benzene, toluene and petroleum spirit. 
 
 Mastic_—Mastic resin (commonly called gum mastic) is an 
 exudation from a tree growing abundantly on the shores of the 
 Mediterranean. It is a hard resin, but softens below 100°, and is 
 pale yellow or greenish in colour. It is completely soluble in amyl 
 alcohol, benzene and ether and partly soluble in most of the other 
 usual ingredients of nitrocellulose varnishes, but is insoluble in 
 petroleum spirit. It is, however, difficult to blend with solutions 
 of other gums or with nitrocellulose on account of a tendency to 
 precipitate, and considerable experience is necessary to obtain 
 satisfactory results. It is sometimes adulterated with rosin or 
 sandarac resin, as it is an expensive material. | 
 
 Copal.—Copal is the name given to a large variety of tropical 
 resinous. products, the best and hardest being of fossil origin. It 
 occurs in East and West Africa, the East Indies, New Zealand and 
 parts of South America. The colour varies from red to yellow. 
 Zanzibar (East African) copal is usually considered the best, and 
 New Zealand (kauri) is also very good. The best and hardest 
 varieties, however, are not used in nitrocellulose varnishes, on 
 account of their poor solubility. Softer copals are more soluble. 
 For example, Zanzibar copal may contain 80—90% insoluble in 
 geno, while the softer Manila copals are soluble to the extent of 
 
114 Cellulose Ester Varnishes 
 
 about 95% in alcohol, and also dissolve in amy! alcohol and in amyl 
 acetate. 
 
 Sandarac.—Sandarac is the resin derived from a tree flourishing 
 in north-west Africa, and another variety comes from Australia. 
 It is a yellow, somewhat friable resin, occurring in commerce in 
 small pieces or transparent drops of a yellow colour. It is soluble 
 in ether, ethyl alcohol, acetone and amy] alcohol, but only partially 
 soluble in benzene, toluene and petroleum spirit. It is moderately 
 hard, and is used in the preparation of negative and label varnishes. 
 
 Dammar.—Dammar is obtained from various trees growing in 
 the Federated Malay States, Sumatra and the Dutch Hast Indies. 
 It is a fairly hard resin, but it softens considerably below 100°. It 
 is completely soluble in benzene and in oil of turpentine, and nearly 
 so in ethyl alcohol. Itis partially soluble in ether and acetone. 
 
 The resins show some properties resembling those of the cellulose 
 esters, particularly in regard to their solubility relations. ‘They are 
 frequently more soluble in a mixture of two liquids than in either 
 alone, and this fact must be kept in mind in consulting the published 
 information on their solubility data. 
 
 Esselen notes that when lacquers are made up from solutions of 
 cellulose esters and resins, the finished product is different according 
 to whether the cellulose ester or the resin is added to the solvent. 
 In some instances, if the resin is dissolved in the solvent first, the 
 cellulose ester will not dissolve, while if the ester is added first, the 
 resin will easily dissolve afterwards. He suggests that the cellulose 
 ester and the solvent together form the dispersion medium, and the 
 resin the disperse phase. 
 
 The proportions in which the resins are used differ very consider- 
 ably, according to the purpose for which the varnish is required. A 
 typical formula given by Field, the originator of this kind of lacquer, 
 was :— 
 
 Amy] acetate : ; 4 . 650 gallons (U.S.) 
 Oil of turpentine . ‘ ; . eae 
 Methyl alcohol . ‘ ; a ee. 
 Nitrocellulose : : ; . 87-5 Ib. 
 Shellac . ; ; : f <a 
 
 Worden remarks that the turpentine was omitted later, on account 
 of its slow-drying properties. 
 
 Two other formule may be quoted from Worden to illustrate the 
 use of the resins. Both are for bronze paints having moderate 
 resistance to heat, and therefore suitable for low- ‘hope 
 radiators and hot water pipes. 
 
Ingredients of Cellulose Ester Varnishes 115 
 
 Formula 1. Formula 2. 
 
 TE ls s soca sacsnsennesceatesienasastasyes css 40—50 35—45 
 STR oli acc ica yc ccd es sets ace aescoeceusacncs« 4—5 20—30 
 ge ES a 35—50 15—25 
 NS ks ccs reves as s4lsccedacanowsenge vanes 35—50 30—45 
 IEE evo c cin icrvisccsgssksteicsagcvcceteeyece — 4t—6 
 Nitrocellulose (not defined ).............cceceeceseseeeees 2-75 3:0 
 Ee eu Gk ltkwn bys cnhuaesovecnccves¥enceceokanease 1:5 — 
 eta ee cnc cco seSiuse vse te varscessapsscacets — 4-0 
 Aluminium and bronze powder .............sceseeeeeee (added before 10-0 
 use) 
 
 The increased proportion of amyl alcohol in the second formula 
 is probably necessary to keep the copal in solution, and the use of 
 linseed oil is said to diminish the tendency of the paint to gelatinise, 
 a drawback of bronze mediums to which reference will be made 
 (p. 132). On the other hand, the addition of linseed oil diminishes 
 the rate of drying considerably, and the film from such a solution 
 will feel tacky long after one from a solution similar to formula 1 
 will have set hard. 
 
 REFERENCES. 
 
 1H. A. Gardner and H. C. Parkes, Paint Manufacturers’ Assoc. U.S. 
 Educational Bureau, Circular No. 218. 2 J. C. Wiesel, Chem. Age (New 
 York), 1924, 32, 439. ° L. Clément and C. Riviére, Chimie et Ind., 1921, 
 6, 283-295. * E. W. J. Mardles, J. Soc. Chem. Ind., 1923, 42, 1287. 
 5 A. Marshall, J. Soc. Chem. Ind., 1904, 238, 645-648. ®B. R. Tunison, Chem. 
 and Met Hng., 1925, 32, 93-94. 7 E. W. Blair, T. S. Wheeler, and J. Reilly, 
 J. Soc. Chem. Ind., 1923, 42, 367 (‘‘ On the Fermentation of Maize Starch 
 to n-Butyl Alcohol and Acetone’’). 8 Sir T. E. Thorpe, ‘‘ Aleoholometric 
 Tables”? (Longmans, Green). °® H. A. Gardner, Paint Manufacturers’ Assoc. 
 U.S., Cire. 225, Jan. 1925. 19W.F. Doerflinger, U.S.P. 1,003,438/1911. 141 (a) 
 W. H. Willcox, Lancet, 1915, 188, 544; (6) W. H. Willcox, B. Spilsbury, 
 and T. Legge, Trans. Med. Soc. London, 1915, 38, 129; (c) Annual Report 
 of Chief Inspector of Factories, 1917 (Cd. 9108), p. 18 (Report on Doping in 
 Aircraft Works, by W. H. Smith). 12 H. G. Evans, J. Soc. Chem. Ind., 
 1919, 38, 402-405r (‘“‘On the Estimation of Saturated Hydrocarbons in 
 Toluene ’’). 1% Verley and Bolsing, Ber., 1901, 31, 3354. 14 C. H. Spiers, 
 J. Soc. Chem. Ind., 1924, 48, 251-2527. 15 EK. Zihl, E.P. 8072/1901. 
 16 T,, Archbutt and R. M. Deeley, ‘‘ Lubrication and Lubricants ”’ (1912). 
 17 J. Lewkowitsch, “‘Chemical Analysis of Oils, Fats, and Waxes” 
 (Macmillan). 
 
 | Additional References to Solvents. 
 
 J. R. Lorenz, J. Amer. Leather Chem. Assoc., 1919, 14, 548-556. <A. D. 
 Conley, J. Ind. Hng. Chem., 1915, 7, 882. 
 
 Bibliography on the Resins. 
 
 E. J. Parry, ‘‘ Gums and Resins”’ (Pitman). R. 8S. Morrell, “‘ Varnishes 
 and their Components ’’ (Oxford Technical Publications). Allen’s ‘‘ Com- 
 mercial Organic Analysis”? (Churchill), Vol. IV. G. J. Esselen, jun., 
 ‘Colloidal Behaviour” (edited by R. H. Bogue) (McGraw-Hill). E. C. 
 Worden, “ Nitrocellulose Industry,”’ Vol. I, chap. x. T. H. Barry and R.S%. 
 Morrell, “‘ Resins: Natural and Synthetic”’ (Benn). 
 
 British Engineering Standards Association Specifications. 
 
 Methyl Ethyl Ketone, 2D. 1. Butyl Acetate, 2D. 4. Castor Oil, 2D. 5. 
 Benzyl Alcohol, 2D. 7. Nitrocellulose Syrup, 2D. 8. Alcohol, 2D. 9. 
 Benzol, 2D. 10. Triphenyl Phosphate, 3D. 12. Acetone, 2D. 22. Cellulose 
 Acetate, D. 50.—To be obtained from the Secretary of the Association, 
 28 Victoria Street, Westminster, S.W. 1., price 2d. post free. 
 
CHAPTER VIII 
 MANUFACTURE AND APPLICATION 
 
 Manufacture of the Varnishes—Mixers—Measurement of Ingredients— 
 Addition of Solvents to Esters—Incorporation of Pigments—Clari- 
 fication and Filtration—Aeroplane Dopes—Effect on Strength of Fabric 
 —Composition of Dopes—Doping Schemes—Method of Application— 
 Discussion of Typical Dopes—Differences in Allied Practices —-Protec- 
 tion from Sunlight—Metal Coatings—Application by Brushing, Dipping 
 and Spraying—Transparent Lacquers—Enamels—Aluminium and 
 Bronze Mediums—Motor-car Enamels—Imitation Leather—Leather 
 Dressing—Patent Kid—Patent Leathers—Treatment of Motor-car 
 Upholstery Leather—Coatings for Wood, Paper—Preservation of Antiques 
 —NMiscellaneous Applications—Collodion. 
 
 Manufacture of the Varnishes 
 
 THE varnishes are made in any convenient type of mixing 
 apparatus. Enclosed mixers provided with stirring blades are very 
 efficient. Various types of revolving churns can be employed, or 
 earthenware or lead-lined vessels with metal or wooden stirrers. 
 Rapid stirring is neither necessary nor desirable, as the rate of 
 dispersion of the esters is slow. Since in all instances some at least 
 of the solvents are highly volatile, the mixer must be built as airtight 
 as possible to avoid loss of solvent and change of composition during 
 mixing. It is advantageous if the mixer can be easily cleaned, but 
 it is preferable to use a separate mixer for solutions of each colour. 
 If it is ever necessary to use a machine for an entirely different 
 colour from that for which it has previously been used, it should 
 be cleaned out by hand as completely as possible, and then washed 
 out for several hours with a strong solvent mixture. 
 
 The solid constituents of the varnish are weighed out. Cellulose 
 acetate is usually weighed dry. Cellulose nitrate is generally weighed 
 wet with alcohol from the dehydration process. The required 
 weight must then be calculated from the laboratory determination 
 of the amount of dry ester in the sample, and the quantity of alcohol 
 in the ester must be deducted from the quantity indicated by the 
 formula, when the liquid ingredients are added. 
 
 The liquids are usually measured by volume, although, owing 
 to their high coefficient of expansion, this method is not very 
 accurate unless the temperature of the shop is moderately constant. 
 There is a great deal to be said for measuring all the liquid ingredients 
 by weight. Unfortunately, there is no uniformity of practice in 
 the trades dealing in solvents. The tendency is for the cheaper 
 liquids, alcohol, benzene, toluene, xylene and petroleum spirit, 
 to be sold by the gallon, and more expensive liquids, such as acetone, 
 ethyl, butyl and amyl acetates, by the ton. If they were all sold 
 by weight, the varnish manufacturers would probably adopt the 
 
 116 
 
Manufacture and Application 117 
 
 same system and ensure, not only a more accurate and uniform 
 manufacture of their goods, but also considerable simplification in 
 their stock-taking and book-keeping. 
 
 It is sometimes good practice not to add all of the solvent 
 mixture to the cellulose ester at one time. No general rule can be 
 laid down, but there is frequently a saving in time if the whole of 
 the non-solvents (if any) and only a part of the true solvents are 
 placed in the mixer at starting. The remainder of the true solvent 
 is added at intervals determined by experience. There are two 
 possible explanations of the saving of time which sometimes results 
 from this procedure. In some, in fact in most instances, the ‘‘ non- 
 solvents ” include ingredients which increase the solvent power of 
 the true solvents. These ingredients diffuse rapidly into the 
 ungelatinised ester, probably causing it to swell without dissolving, 
 and this may assist the later penetration of the true solvent. 
 There may also be a physical disintegration of the ester when the 
 composition of the liquids is just approaching the point at which 
 they become a solvent, and such disintegration will expose a large 
 surface to the action of the solvent. This may be illustrated on a 
 laboratory scale by shaking cellulose nitrate with aqueous acetone 
 containing enough water to possess no solvent action. If the 
 proportion of acetone is gradually increased, a stage is reached at 
 which the cellulose nitrate disintegrates to a powder, and the con- 
 tinued addition of acetone leads to rapid dispersion. On the other 
 hand, when pure acetone is used as the solvent, it is difficult to 
 prevent the formation of large clots, enveloped in gelatinised ester, 
 and exposing so little surface to the action of the solvent that 
 dispersion is greatly delayed. 
 
 _ No general rule can be laid down as to the time required for the 
 manufacture of cellulose ester solutions, since this varies according 
 to the viscosity of the cellulose ester, the nature of the solvents, the 
 order in which they are added, the final concentration and the 
 temperature of the shop. Usually, however, batches begun on 
 one day and kept in movement overnight are ready the next day. 
 Pigmented varnishes (enamels) may be made directly from pig- 
 mented celluloid, by dissolving the celluloid in an appropriate 
 mixture of solvents. Celluloid, however, is made from a cellulose 
 nitrate of high viscosity, and a solution made from it will yield a 
 thinner coating for the same consumption of solvent than one made 
 from an ester of low viscosity. Another property of varnishes 
 made directly from celluloid which may be disadvantageous is their 
 comparative softness arising from the presence of camphor. 
 
118 Cellulose Ester Varnishes 
 
 When enamels have to be made from the nitrocellulose base, 
 great care is needed to ensure the complete incorporation of the 
 pigment. A mere stirring of the pigment into the finished solution 
 is rarely successful; the particles of pigment tend to adhere together 
 and ‘‘ ball up,” so that they do not disperse uniformly in the solu- 
 tion. The film from such a solution, whatever the solvent, will 
 be rough and gritty, and the pigment will rapidly settle out of the 
 solution on standing. It must be remembered that to ensure that 
 every particle of pigment is surrounded by an envelope of solution 
 requires the performance of a considerable amount of work against 
 surface tension, and this work cannot easily be done by stirring. 
 To obtain a satisfactory result, the solution and pigment should 
 be ground together in a ball mill, preferably with porcelain balls, 
 or the ester should be gelatinised with part of the solvent and ground 
 on steel rollers with the pigment. The latter process is expensive 
 in solvent, since there is a considerable amount of evaporation 
 during the grinding. 
 
 Clarification of the Varnishes. 
 
 Solutions of commercial nitrocellulose or cellulose acetate are 
 never quite transparent, owing to, the presence of undissolved 
 fibres, ‘clots or adventitious impurities. Jor many purposes, the 
 presence of a small quantity of insoluble matter does not affect 
 the usefulness of the varnish. In making enamels, for example, 
 the process of grinding in the pigment comminutes the impurities 
 also, and any kind of clarification is unnecessary. Sometimes, 
 however, it is necessary to obtain solutions of high transparency, 
 and some treatment is necessary. 
 
 According to Kirkpatrick,! in the Parlin works of E. I. du Pont 
 de Nemours and Company, filtration is carried out in frame filter- 
 presses, through filter-paper backed with a heavy canvas filter- 
 cloth. This process is only feasible when the solution to be filtered 
 is of low viscosity, and even then it is advantageous to allow some 
 of the insoluble material to settle by standing. A solution of 
 high viscosity filters with extreme slowness through a press even 
 when free from gelatinous impurities. The cellulose ester solutions, 
 however, always contain impurities of a fibrous nature, frequently 
 swollen by the action of the solvent mixture, and these felt together 
 and form an increasing hindrance to the filtration. 
 
 The impurities in cellulose ester solutions can be thrown out 
 satisfactorily by treatment in a centrifugal machine, but unless 
 special precautions are taken there is a heavy loss of solvent during 
 the process on account of the rapid movement of air over the surface. 
 
Manufacture and Application 119 
 
 The method which gives the most satisfactory results is 
 undoubtedly settling under the influence of gravity. This is 
 simply carried out by allowing the solution to stand in tall, narrow, 
 cylindrical vessels, preferably of earthenware, until the impurities 
 have fallen to the bottom. Where special transparency is required 
 this process may take weeks or months. The disadvantages are 
 (1) that losses by evaporation may be heavy unless strict pre- 
 cautions are taken, (2) that a great deal of money is locked up 
 while the settling is taking place. Clément and Riviére? state 
 that solutions settled in this manner for eight months are optically 
 pure. The writer cannot quite confirm this, but the path of a beam 
 of light through such a solution is certainly very nearly invisible. 
 On the other hand, solutions clarified by filtration, and especially 
 by centrifugal treatment, give an unsatisfactory optical test, and 
 will sometimes, on standing, deposit a flocculent gel which will take 
 longer to settle out completely than the original impurities which 
 have been removed. Apparently in the clarification of these 
 solutions time is a factor which cannot be completely ignored. For 
 general purposes, however, optical purity is not required in varnishes, 
 and subject to the criticisms made above, filtration and centrifugal 
 treatment are satisfactory. | 
 
 Ayres ® mentions an instance of a nitrocellulose lacquer, which 
 contained a cloud of finely-divided cellulose impurities, being 
 filtered satisfactorily after the addition of a small percentage of 
 tricalcium phosphate precipitated in alcohol. Other filtering agents, 
 such as kieselguhr, fuller’s earth, bone black, and barium sulphate, ° 
 were found useless. 
 
 APPLICATIONS OF CELLULOSE ESTER VARNISHES. 
 
 Aeroplane and Airship Dopes. 
 
 The properties required in an aeroplane dope are that it shall 
 increase the tensile strength of the fabric on which it is deposited, 
 and cause a permanent increase in the tension. It should possess 
 maximum resistance against water, oil and petrol, it should protect 
 the fabric against the destructive effect of strong sunlight and 
 atmospheric exposure, and it should be incombustible. 
 
 It has already been pointed out in an earlier chapter that the 
 need for fulfilling the last condition has led to the adoption of 
 cellulose acetate rather than nitrate as the basis of aeroplane doping 
 systems. 
 
 Dopes for use on the fabrics of machines lighter than air should 
 
oe 
 
 120 Cellulose Ester Varnishes 
 
 possess the additional property of being impermeable to gases, and 
 they should be poor conductors of heat. 
 
 Clément and Riviére,? who have made a special study of the 
 mechanical properties of doped fabrics, give the following figures 
 to show the order of increase in strength brought about by doping. 
 The figures refer to breaking strain calculated per linear metre of 
 the fabric; the thickness is not stated :— 
 
 Warp. Weit. 
 Material. pe ee fe ee ee 
 Before After Before After 
 coating. coating. coating. coating. 
 LOOT Feat on stag Seerees Oye 1,730 k. 1,860 k. 1,800 k. 2,660 k. 
 COLOR hao < vait ae wae 940 1,080 1,010 1,180 
 ELS Stn lia srasam sou anan 1,200 1,500 1,490 1,740 
 
 A number of tables are given by Ramsbottom ‘ in the Technical 
 Report of the Advisory Committee for Aeronautics for 1913-14, 
 from which the following is taken :— 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Fabric | Fabric 
 
 Fabric , ; 
 Tests on dope A. before with with 
 doping. 5 coats | 7 coats 
 dope. 
 Weight, gmake PREF cicentae ant 138-5 312-5 
 MOpe /1.? ye sicdvess — (174) 
 Strength, = i to ees are 1275 1775 
 Increase in strength due to 
 GODS, 4 [icokstaes eek sacs we 500 
 Specific strength of fabric... 9-2 5-7 
 
 Specific strength due to 
 Saieedehinustaneesemnves — ’ ‘ ne 2:87 
 
 These tests are made in the direction of the warp only, as the fabric 
 is more uniform in that direction. The results given on the last 
 line for the specific strength due to the dope were calculated to 
 show what is the increase in strength for a given added weight of 
 dope. The values represent :— 
 
 Increase in strength (kilos/m.) 
 Weight of dope (grammes/m?.) 
 
 
 
 They show that the influence of successive coatings of dope becomes 
 rapidly less, so that a compromise is adopted between the standard 
 of strength required in the coated fabric, and the weight of the 
 fabric. 
 
Manufacture and Application 121 
 
 According to Drinker,’ the fabrics used by the Allies for 
 aeroplanes during the war were :— 
 
 Great Britain . : ; . Irish linen, cotton. 
 United States . ; . Mercerised cotton. 
 Italy : ‘ ; ; oa, SEL 
 
 - France. : ; : . Silk and cotton. 
 
 Composition of Aeroplane Dopes. 
 
 The French air service used acetate dopes on all their aeroplanes. 
 The British used a nitrocellulose dope on aeroplanes used for 
 training, and on the war machines an acetate dope with a final 
 coating of nitrocellulose varnish to protect the under-coats from 
 the action of light. 
 
 If a dope is made up from cellulose acetate dissolved only in 
 volatile solvents, it dries with considerable contraction to a hard 
 film, which easily cracks. It is possible to introduce high-boiling 
 softening agents into the solution which counteract this hardness, 
 and produce a flexible film, at the expense of the contractile power. 
 Since the fabric is stretched tightly on the frames before it is doped, 
 only a moderate degree of contraction is necessary in the dope. It 
 is therefore possible to incorporate a certain amount of a softener 
 and thereby increase the resistance of the film to sudden shock. 
 Of the softeners used the most important are triphenyl phosphate, 
 tricresyl phosphate, phenol, acetanilide, methyl acetanilide, triacetin, 
 benzyl alcohol. 
 
 A large number of softeners were tried during the war, but 
 owing to their high price the use of many of these has been dis- 
 continued. 
 
 Non-inflammability. 
 
 The term non-inflammable is a somewhat vague word applied 
 to those substances which do not burn freely. Cellulose acetate is 
 a non-inflammable substance, so called because if in the massive 
 form it is ignited, the flame usually expires as soon as the source 
 of heat is removed. It must be remembered that the property of 
 non-inflammability depends to some extent on the physical state 
 of the material. Cotton wool, for instance, is an inflammable 
 material; but a highly compressed bale of cotton or cotton paper 
 in the form of a book would be difficult to ignite. é 
 
 The term incombustible is reserved for substances having a 
 higher degree of non-inflammability. Even this term is perhaps 
 
122 Cellulose Ester Varnishes 
 
 only relative, but in common language it is applied to those sub- 
 
 stances which exhibit no fire risk whatever. 
 
 Although cellulose acetate is non-inflammable it is not incom- 
 bustible, and when applied to a fabric in conjunction with an organic 
 ‘“‘ softener,” the fire risk may become sufficiently great to become 
 a danger on aeroplane fabrics. It is usual, therefore, to incorporate 
 some substance which will reduce the inflammability of the film to 
 a state approaching incombustibility. Triphenyl phosphate is the 
 material most commonly used for this purpose. It combines the 
 functions of softening agent and reducer of inflammability. Although 
 it contains a considerable percentage of carbon, the phosphate 
 residue is an efficient check on combustion. 
 
 Application of Dopes. 
 
 British practice in the application of cellulose acetate and 
 nitrate dopes is described in the following specifications issued by 
 the British Engineering Standards Association.* 
 2p. 101, Feb. 1923, “‘ Properties of Aeroplane Doping Scheme.” 
 2p. 104, Dec. 1922, * Air Ministry Doping Schemes.” 
 4p. 100, Feb. 1923, “‘ Air Ministry Cellulose Acetate Dopes.” 
 
 D. 23, July 1918, “‘ Nitrocellulose Syrup for the Manufacture of 
 Dope.” 
 
 D. 102, June 1918, “‘ Nitrocellulose Dope.” 
 
 2p. 103, March 1922, “ Air Ministry Nitro Dope Coverings and 
 Identification Colours.” 
 
 Dp. 105, Feb. 1923, “ Pigmented Nitrocellulose Dopes for Use on 
 Aeroplanes.”’ 
 
 These valuable publications should be consulted by anyone interested 
 in the application of cellulose ester varnishes, whether in reference 
 to aeroplane protection or not, as many useful hints are given and 
 tests described. Only a brief summary with comments can be 
 given here. 
 
 2p 104 describes the purpose of the doping schemes, namely, 
 the production of a taut, smooth, air-tight surface. Special coatings 
 are required for protection of the doped fabric from light, for 
 increasing the reflection of heat and light rays, and for better 
 protection from observation during night flying. Directions are 
 given for the application of the dope, which follow closely the 
 usual practice in the industry. The shop should be dry and at a 
 
 * To be obtained from the Secretary of the Association, 28 Victoria Street, 
 Westminster, S.W. 1, price 2d., post free. 
 
 a 
 
Manufacture and Application 123 
 
 fairly constant temperature of 65-70° F. Humidity, or the preva- 
 lence of cold draughts, causes ‘‘ blooming ”’ of the films through the 
 deposition of moisture. The standard of ventilation is a high one, 
 namely, that the air should be completely changed thirty times 
 per hour. This standard was laid down after the exhaustive inves- 
 tigation into the subject of ventilation carried out as the result of 
 the mishaps with tetrachlorethane in 1915-16 (v. supra). 
 
 The dope is applied with a flat brush of stiff bristle, and the first 
 coat must be well pressed into the fabric. The fabric must not 
 be damp or cold. It must be free from weaving defects, and should 
 be lightly and evenly stretched on the frames. It has been already 
 pointed out from Ramsbottom’s figures that the first coat gives 
 the greatest proportionate increase in strength. This is doubtless 
 due to the penetration of the dope into the fabric in such a way 
 as to surround the fibres. A minimum of one hour is allowed before 
 the next coat is applied. Usually in the industry a longer time 
 than this is given, particularly when the solution is brushed on, 
 as there is a great tendency for the second coat to disturb the first. 
 Brush-work with cellulose ester varnishes is entirely different from 
 brush-work with turpentine-linseed oil paints. The rate of evapor- 
 ation is very rapid, and there must be no attempt at smoothing-out. 
 The planes are doped preferably in a horizontal position, which 
 favours an even application, and the brush should move in the 
 direction of the strands of the fabric. 
 
 The allowance of dope per 100 square yards of fabric (not 
 including waste) is from 14 to 18 gallons, for four coats, which 
 corresponds with rather more than 2 oz. per square yard when the 
 volatile solvent has evaporated. 
 
 2p. 101 defines four classes of doping schemes, the requirements 
 of a dope being that it must tauten the fabric, protect it from 
 injurious light rays and from moisture. Different schemes are 
 possible according to whether those requirements are met by one 
 dope or different combinations of two dopes. This specification 
 describes useful tests for determining tautening properties, adhesion, 
 elasticity, brittleness, weight, inflammability and rate of burning. 
 
 4p. 100 gives the formule of four different types of cellulose 
 acetate dope, namely : 
 
 (1) A clear dope for use in temperate climates (A.D.) 
 
 (2) 9 ” ” 39 ” tropical 99 (A.D.T.) 
 (3) ,, pigmented ,, ,, temperate __,, (A.D.P.) 
 Bere i oi ey ay)” * SeOpions io (DP) 
 
124 Cellulose Ester Varnishes 
 
 It will be worth while to consider one of these specifications in 
 some detail, as it will show how much standardisation is necessary 
 in order to ensure practical uniformity in the dope. 
 
 The formula for dope A.D. is as follows :— 
 
 Cellulose acetate . : 5 ; oo TBE 
 Acetone : , : . 60 gallons. 
 Methyl ethyl Keiene, , « - Aes 
 Alcohol . : : ‘ : : Si Pee 
 Benzol . A ; : t ; Maes te 
 Benzyl! alcohol ; : ; } j se ie ae 
 Triphenyl phosphate ; ; , : 15 I, 
 
 The literature of cellulose esters abounds with formule stated in a 
 similar form to this, and most of them are almost worthless, because 
 the properties of the ingredients are not defined. In this instance, 
 however, every one of the ingredients has been standardised by 
 previous specifications. Leaving out what one may call the obvious 
 limitations on acidity, basicity and non-volatile residue on evapor- 
 ation, these previous specifications fix the following properties on 
 which the behaviour of the dope entirely depends :— 
 
 (1) Cellulose Acetate. The required solubility of this material 
 in a test solvent made up in the same proportions as in dope 
 A.D. has been strictly defined (2D. 50), and, most important of 
 all, the viscosity of this solution between narrow limits. 
 
 (2) The five liquid solvents, acetone, methyl ethyl ketone, 
 alcohol, benzol and benzyl alcohol, have all been specified in 
 regard to allowable water content, specific gravity and range 
 of boiling point. These determinations limit the amounts of 
 impurities which might otherwise be present in commercial 
 materials, and therefore ensure to a satisfactory degree that 
 the viscosity of the dope when made up will fall between the 
 same limits as were observed in the sample. 
 
 (3) The specification for the triphenyl phosphate employed 
 ensures uniformity in the hardness of the film produced and 
 in the degree of combustibility. 
 
 Without the rigid specification of all these ingredients, the 
 final dope might vary so enormously in its properties that it would 
 be entirely a matter of chance whether it would be suitable for the 
 purpose in view. 
 
 It will also be of interest to consider the same formula in relation 
 
Manufacture and Application 125 
 
 to the boiling points of the volatile solvents, as these will give an 
 approximate idea of the changes occurring during evaporation :— 
 
 
 
 Ingredient. Quantity. Range of B. p. 
 PRC rcerepestessescravcccsese 60 gall. 95% from 55° to 60° 
 Methyl ethyl ketone ............ 49): 9575 =». 10° to-81° 
 SM uansinsisens oransessece if.) 95% ., 76° to. 79° 
 aida Ss vipndeas coeds st Maes 95% , 75° to 85° 
 a ee Te 95%. ,, 200° to 210° 
 Triphenyl phosphate ............ 15 Ib. M. p. 45° to 48° 
 
 In the first place it must be noted that two of the ingredients, 
 alcohol and benzol, are, when taken alone, non-solvents of cellulose 
 acetate, even when hot. When mixed together, however, although 
 they are still non-solvents in the cold, they form a solvent mixture 
 on warming. Evidently therefore they are of more value when 
 present together than either would be if present alone. In the 
 next place, the most volatile constituent is evidently the acetone, 
 and this will evaporate at a faster rate than the other constituents. 
 It is, however, present in much larger quantity than the others, 
 so that acetone is likely to be present in slight excess right up to 
 the end of drying. Alcohol and benzol, which are chiefly of value 
 when present together, and which have approximately equal rates 
 of evaporation, are included in equal proportions, and their total 
 volume, 28 gallons, is sufficiently small to ensure that they will 
 not be left in excess when the true solvents have evaporated. The 
 methyl ethyl ketone helps to diminish the rate of evaporation of 
 the solvents, and in the presence of acetone is itself a strong solvent. 
 It will be noted, however, that practically all of the volatile solvents 
 boil below 85°, and that there is nothing intermediate between these 
 and the benzyl alcohol, which will be left when the former have 
 evaporated. The vapour pressure of benzyl alcohol at ordinary 
 temperatures is extremely small so that its rate of evaporation is 
 low, and it is left almost intact in the film with the non-volatile 
 triphenyl phosphate. 
 
 This is a comparatively simple formula to study in this way, 
 owing to the large excess of strong solvents in it. When the pro- 
 portion of non-solvents or latent solvents is higher, it becomes 
 much more difficult to forecast what is likely to happen as the 
 solvents evaporate. Skill in this exercise is of great assistance in 
 the preparation of new formule. 
 
 2p. 103 specifies various nitrocellulose dope coverings, applied 
 
126 Cellulose Ester Varnishes 
 
 for protective coverings over acetate dopes. One of these may 
 be examined in a similar way :— 
 
 Transparent Covering V. 114 
 
 260 Ib. of Nitrocellulose : : : .  62-4b. 
 Nitrocellulose {Aleta : : : ; ~~ 2 
 Syrup 2p.8 (Butyl acetate : ; : . 1 
 Butyl or amyl acetate . : . 184 gallons. 
 Alcohol ; : ; } : Ga 
 Benzol ; ; ; ; . ee 
 Castor oil . , ‘ / Jee 
 
 It will be noted that this formula involves a previous specification 
 2D. 8 in which the quantities are expressed by weight. The reason 
 for this is that the nitrocellulose syrup is highly viscous and difficult 
 to measure by volume. 
 
 The same remarks apply to this formula as to the previous 
 acetate dope formula. It would be valueless for its purpose if the 
 solubility of the cellulose nitrate, and its viscosity in the specified 
 solvents, were not fixed within narrow limits, and if the proportion 
 of the solvents were not defined so as to exclude the presence of 
 impurities which even in small quantities might have a marked 
 effect on the properties of the dope. 
 
 The boiling points of the ingredients may be tabulated in the 
 same way :— 
 
 Ingredient. Quantity. Range of B. p. 
 Bnbyl acetate) ics cess s'exudeveeees 38) galls. 95% from 100° to 130° 
 PARC ID alah s sles x oue yea elemieamaes af OD, as 76° to 79° 
 BOT Sy ings ha Sh kcaikectoneee 994 ee 9695" 45 75° to 85° 
 LIAR DONO |. Vous cas. tidavkee dee tawe gan 77. Ib, 
 
 This is an even simpler mixture than the last. The non-solvents 
 are again alcohol and benzol, but they form a solvent when mixed. 
 Alcohol also assists the solvent power of butyl acetate. Benzol is 
 the ingredient present in smallest amount, and there is no risk of 
 it being present in excess at any stage of the evaporation. If amyl 
 acetate is used instead of butyl acetate, the rate of drying will. 
 be somewhat slower. 
 
 Castor oil, being non-volatile, is entirely left in the nitrocellulose 
 coating of which it will form more than half. 
 
Manufacture and Application 127 
 
 Note on Bibliography. 
 
 Interesting comparisons between the practice of various countries 
 are given in the papers by Drinker 5 and by Clément and Riviere.? 
 The Germans used lighter dope films than the Allies, the weight of 
 dope being usually from 1 to 1:5 oz. per square yard, whereas 
 British experience preferred a coat of 2 oz. The formation of white 
 patches during doping, known as “ blushing ” in the United States 
 and as “ blooming ”’ in this country, was due to the use of damp 
 or draughty shops for the doping process. There is a very high 
 percentage of low-boiling solvents in the standard aeroplane dope 
 and very considerable cooling takes place as the solvents evaporate. 
 Hence there is every probability of an excess of moisture depositing 
 on the film and precipitating the cellulose acetate unless special 
 precautions are taken to keep air dry. In this country, where 
 doping processes and shops had to be hurriedly improvised, it was 
 not possible to insist on the control of humidity in the doping 
 sheds, but in the United States advantage was taken of the experi- 
 ence of the Allies and such control was enforced. 
 
 Although some experimental work was carried out on doping 
 by means of a spray, or air-brush, the process was carried on by 
 hand-labour throughout the war. 
 
 In French practice, the first or “‘ scratch ” coat contained only 
 from 3-5% of cellulose acetate, and was made up with a high 
 proportion of low-boiling solvent in order to give a greater tautening 
 effect on the fabric. The second and third coats contained from 
 8-9°% of acetate, with 2° of mineral pigments or metallic powders. 
 Hither eugenol or triacetin was used as the softening agent in these 
 coats. The last coat was made with 8% acetate, and with a solvent 
 having a high proportion of low-boiling solvent, as in the first coat. 
 The French used chiefly methyl acetate as low-boiling solvent, and 
 the British chiefly acetone, but the British Air Ministry endeavoured 
 to minimise the consumption of products of wood distillation by 
 the use of alcohol and benzene as diluents. 
 
 The French dopes did not contain triphenyl phosphate, but 
 employed a variety of other softeners such as eugenol, benzyl 
 alcohol, triacetin and glyceryl benzoate. None of these had a 
 fire-resisting action similar to that of triphenyl phosphate. The 
 Italian used a small percentage of phenol as the softening agent. 
 The United States service used in the first coat a small percentage 
 of phenol or naphthalene, and also an ant-acid, either urea or 
 dicyandiamide. Three additional coats were put on, which con- 
 
128 Cellulose Ester Varnishes 
 
 tained 7°% of diacetone-alcohol to prevent “ blushing,” and small 
 quantities of benzyl acetate or benzyl benzoate as softening agents. 
 
 The destructive action of sunlight on aeroplane fabric was 
 investigated by Aston ® (of isotope fame) and was found to be due 
 chiefly to ultra-violet light in the region of the spectrum lying 
 between 2950 and 4000 wu. These rays could be absorbed by 
 certain pigmented coatings, and a special protective dope was 
 developed known as PC 10. The pigment in this covering consists 
 of a mixture of yellow ochre and carbon black, in a nitrocellulose 
 and castor oil medium. 
 
 The various pigments employed to produce the colours used on 
 British aeroplanes for the purposes of identification and protection 
 are detailed in B.E.S.A. 2p. 103. Dopes which gave a matt black 
 surface were adopted in France, at the suggestion of Clément and 
 Riviére,? for machines used for night-bombing, and were almost 
 invisible in the beam of a searchlight. Later suggestions from the 
 same source were ‘clair de lune,” blue-violet and deep red. 
 
 Metal Coatings. 
 
 The application of special enamels made from nitrocellulose of 
 low viscosity to motor-car bodies, as has already been pointed out, 
 is a recent development which has taken place only in the last two 
 years. For many years, however, an industry has flourished in 
 the protection and decoration of small metal articles with nitro- 
 cellulose lacquers. The use of lacquers and enamels of higher 
 viscosity results in thinner coatings for the same consumption of 
 solvent, or, conversely, a large number of applications to produce 
 a thick coating. As most of the articles so coated are intended 
 only for indoor use, where they do not encounter severe weather 
 conditions, a thin coating is generally adequate for the purpose in 
 view. 
 
 There are three general methods of application, (1) brushing, 
 (2) dipping, (3) spraying. 
 
 Brushing is the least satisfactory process. Nitrocellulose lacquers 
 dry so rapidly that any attempt to smooth out brush marks generally 
 defeats its purpose. If used for a second application, it invariably 
 disturbs the first coat, and the lacquer rapidly becomes sticky 
 enough to draw bristles from the brush. Nevertheless, it may 
 sometimes be necessary to apply a lacquer or enamel to a surface 
 too large to be dipped conveniently, and in a situation where a 
 compressed air spray is not available. Under these circumstances, 
 
Manufacture and Application 129 
 
 fair work may be done if the operator resolutely banishes from his 
 mind all preconceived notions derived from the application of 
 ordinary linseed oil paints or varnishes. The lacquer should be 
 quickly applied to the surface without brushing out, keeping the 
 brush well moistened. Whenever work is stopped, if only tem- 
 porarily, the brush should be immersed in a solvent mixture to keep 
 it from drying out. Lacquers intended for brushing require a 
 higher percentage of high boiling solvent, so as to delay the rate 
 of drying. 
 
 Dipping is a very satisfactory method of coating small articles. 
 The lacquer is placed in a receptacle of convenient size, and the 
 article is completely immersed and withdrawn. It is allowed to 
 drain for a few seconds and is then transferred to a rack to dry. 
 The two chief precautions to be taken are (1) to immerse the article 
 in such a way that no bubbles of air are drawn under the surface ; 
 the procedure depends on the shape of the article and can usually 
 be determined after one or two experiments; (2) to drain the 
 article in such a way that no “‘ drip ”’ is formed at corners or edges; 
 in some cases it may be advisable to apply a slow rotation mechanic- 
 ally to the articles while drying. A recent patent taken out by 
 B. D. Baker’ claims that drip may be avoided by hanging the 
 articles in a vessel of the lacquer, and slowly drawing off the lacquer 
 from the bottom of the vessel. 
 
 Spraying is a comparatively modern development, and is carried 
 out with apparatus similar to that used in painting. Various types 
 of air sprays are on the market, such as the Aerograph and the Aero- 
 style. The principle of the apparatus is that a fine jet of air under 
 somewhat high pressure draws the lacquer from a reservoir by its 
 injector action, and propels it in the form of a fine spray towards 
 the article to be coated. In some instances, the reservoir feeding 
 the spray is also under pressure. The conditions which may be 
 varied are the pressure of the air, the viscosity and composition of 
 the lacquer, the ratio of air to lacquer, the temperature of the 
 workshop and the distance of the spraying apparatus from the 
 work. Most of these are under the control of the operator, but the 
 composition of the lacquer is the concern of the manufacturers. 
 The latter can, however, assist the operator by supplying the lacquer 
 in a slightly more concentrated form than is required for the spray, 
 and supplying separately a ‘‘ thinnings”’ or diluent, consisting of 
 solvent only, which may or may not have the same composition as 
 the liquid solvent in the lacquer. A few trials will usually show 
 what . the best proportion in which to mix the lacquer and the 
 
130 Cellulose Ester Varnishes 
 
 thinnings, and great care should be taken to ensure that the two 
 are thoroughly mixed. 3 
 
 Flowing is really a variant of the dipping process, and is some- 
 times convenient when large articles are to be covered. The lacquer 
 is allowed to flow from an orifice over the surface of the article, the 
 excess which drains off being returned to the containing vessel. 
 
 Another process of metal covering is intermediate between the 
 use of solutions and the use of solid celluloid coverings. The 
 lacquer or enamel is made in such a high concentration as to be a 
 plastic semi-solid. This is applied to the article sometimes by 
 mechanical means, sometimes by hand with a palette knife, and 
 after the coating has partly dried it is moulded to the desired shape 
 in a specially designed press. 
 
 Coatings for metals may be classified as follows :— 
 
 (1) Transparent lacquers, either colourless or coloured. 
 
 (2) Enamels, white, black or coloured. 
 
 (3) Metal-powder coatings, such as bronze or aluminium 
 paints. 
 
 These usually contain resins in addition to the nitrocellulose. 
 The resins add body to the solution, and promote adhesion. 
 
 Transparent Lacquers. 
 
 Gardner quotes as a typical solvent mixture :— 
 
 Butyl acetate ' : : . Mn hy = 
 Ethyl acetate : . «2, 
 Benzene : : A ; ~ Oe 
 
 to a gallon of which would be added 6 to 8 oz. of nitrocellulose 
 (viscosity not specified). It will be noted that this formula contains 
 no alcohol, and would have to be made up from stove-dried nitro- 
 cellulose. As it stands, it contains the solvents butyl and ethyl 
 acetate in a sufficient proportion to ensure that they will be in 
 excess of the non-solvent benzene at all stages of the evaporation. 
 If nitrocellulose dehydrated with alcohol were employed, as is 
 usual, the 8 oz. of nitrocellulose would introduce about 4 oz. of 
 alcohol, which would not alter the properties of the lacquer appre- 
 ciably. In fact, half of the benzene in the above formula might 
 be replaced by alcohol. For use as a protective coating, Gardner 
 recommends the introduction of 5-7% of castor oil or treated tung 
 oil, giving a more “elastic”’ film. The term elastic is here used 
 in its popular sense of ‘‘ extensible.” The film is actually softer 
 
Manufacture and Application 181 
 
 and more plastic, and less likely to strip away from the metal by 
 shrinkage. 
 
 Wiesel suggests the following as a typical lacquer for highly 
 polished surfaces :— 
 
 Nitrocellulose (4 sec.) . . 5% by weight 
 Alcohol . ; ; ae, 
 
 Benzol ; : ; - . 20 
 
 Butyl acetate ; ; jf . 40 
 
 Butyl alcohol ; ; : ik 
 
 Ethyl acetate : : ; vee 
 
 Castor oil . : ; cate tere 
 
 Here the viscosity of the nitrocellulose is indicated by the expression 
 “4 sec.” This expresses the result of an empirical test of viscosity 
 carried out by timing the fall of a steel ball of fixed weight through 
 a distance of 10 inches in a solution of fixed concentration in a given 
 solvent mixture. Such a test is quite definite as long as the con- 
 ditions are observed, but there is much to be said for expressing 
 the results in recognised scientific units. 
 
 In this formula alcohol is present to the extent of 20%, so that 
 the solution could be readily made up from nitro-cotton dehydrated 
 in the usual way with alcohol. The precipitating tendency of 
 benzol is amply safeguarded by the presence of alcohol, with which 
 it forms a solvent mixture, and by the combined solvent power of 
 the ethyl and butyl acetates. The butyl alcohol helps to slow the 
 rate of evaporation, and assists the solvent power of the acetate. 
 Also, like the ethyl alcohol, it forms a weak solvent mixture with 
 the benzol. The mutual influences on solubility of the ingredients 
 in this mixture are thus somewhat complicated. 
 
 If either of these formulz were required for a colourless lacquer, 
 attention would have to be paid to the colour of all the ingredients 
 and of the original nitrocellulose. Either could be transformed 
 into a coloured lacquer by dissolving in the liquid solvent the 
 appropriate spirit-soluble aniline dye, or mixture of dyes. These 
 lacquers are largely used in the fancy metal-ware industry, both 
 for protection from corrosion and for decoration. 
 
 Enamels. 
 
 Similar formule with the addition of the requisite pigments and 
 resins give the enamels. The pigments should be ground up either 
 in the castor oil or in the plastic mass formed when the nitrocellulose 
 is gelatinised with part of the solvent. 
 
182 Cellulose Ester Varnishes 
 
 Enamels of this kind are used as waterproof paints on domestic 
 metal-work, and on small articles such as motor-car accessories, 
 scientific instruments, and sanitary goods. When the best results 
 are required, it is usual to apply two or three coats, and to buff or 
 rub down with fine glass-paper between the coats. The best effect 
 of all is produced by giving a final dip or spray with a transparent 
 celluloid lacquer, which gives a beautiful gloss to the surface. 
 
 A matt surface is obtained by increasing the quantity of pigment 
 and/or varying the proportion of resins. This type of surface 
 appears to be due to the pigment particles not being completely 
 surrounded by an envelope of the nitrocellulose, as they are in a 
 glossy enamel. It is a somewhat uncertain process in practice, 
 as a matt enamel may under some conditions of temperature and 
 humidity dry brighter or duller than usual. A good matt enamel 
 should not give up colour when rubbed with the finger. 
 
 Aluminium and Bronze Mediums. 
 
 These invariably contain resins in addition to the nitrocellulose, 
 and they are applied to metal as much for protection as for decor- 
 ation. The metal powders must be very fine, so as not to clog the 
 jet in spraying work. 
 
 When bronze powders are worked into nitrocellulose solutions 
 it is sometimes noticed that after a few hours or days a green colour 
 develops and the nitrocellulose begins to separate out in large clots. 
 This is usually attributed to instability of the nitrocellulose, but it 
 sometimes occurs with nitrocelluloses of proved stability, and 
 when the solvents are apparently in good order. It is a phenomenon 
 which requires investigation. It is better not to stock these solutions 
 made up ready for use, but to work in the metal powder immediately 
 before the paint is required. These paints are frequently applied 
 with a brush, and have been used to a larger extent on outside 
 work than any other nitrocellulose solution previous to the new 
 motor-car finishes. 
 
 Motor-car Enamels. 
 
 The use of nitrocellulose enamels for covering the bodies of” 
 motor-cars has been developed chiefly in the United States, and is 
 competing with the existing methods, which are classified by 
 Mougey ?° as follows :— | 
 
 (1) Methods employing oil and resin varnishes, applied in 
 a similar way to coach-builders’ varnishes. ‘The applications. 
 
Manufacture and Application 133 
 
 may consist of one priming coat, any number up to seven 
 surfacing coats, from one to five colour coats and a glossy 
 finishing coat. 
 
 (2) The use of stoving enamels, which require heating after 
 application to temperatures varying from 60° to 200°. These 
 may be applied in three coats, a primer, a colour coat and a 
 finish. Black enamels containing bitumen are the commonest 
 of the stoving enamels. 
 
 Both of these methods may give excellent results, but they 
 have certain drawbacks. Varnishing in accordance with the first 
 method is costly in time and labour. The coats take several hours 
 to dry, and there is a considerable amount of rubbing down to be 
 done if the best results are required. As many as twenty-eight days 
 might be required for the complete operation. 
 
 Stoving enamels give a hard coating with a high lustre. Both 
 processes, however, yield a coating which may have poor resistance 
 to atmospheric conditions. The varnish coatings may develop 
 large cracks, and the stove enamels, though more durable, some- 
 times become covered with a vast number of very small intersecting 
 cracks. Both finishes are easily scratched, and a varnish finish is 
 sensitive to petrol, oils and tar. 
 
 _ By the use of nitrocellulose enamels it is claimed that a finish 
 may be obtained which is equal to that of the black stoved enamel, 
 but without the need for stoving. Practically any colour can be 
 obtained, and the solution can be applied by an air-spray. Com- 
 pared with the first method, the number of coats can be considerably 
 reduced, since an enamel of high nitrocellulose content is used, 
 giving a thick coating which is dry in a very few minutes, and which 
 can be polished to any desired degree. 
 
 According to Flaherty,1! nitrocellulose enamels were first. used 
 on motor-cars in some of the re-finishing shops in California, but 
 at that time the amount of nitrocellulose in the solution was so small 
 that several coats had to be used to give sufficient thickness of film 
 and depth of colour. The process was therefore very expensive 
 and not adapted to general use. 
 
 The later development in motor-car enamels has been brought 
 about entirely by the discovery of a method of producing stable 
 nitrocelluloses of much lower viscosity than was formerly thought 
 possible, so that the manufacturer can obtain a much higher solid 
 content without making the enamel too viscous to spray. 
 
 The coatings obtained by the new process are said to be more 
 
134 Cellulose Ester Varnishes 
 
 stable than those made from oil/resin paints and varnishes, and to 
 possess better resistance to tar and petrol. They can be cleaned 
 down with a wet mop without becoming scratched, and although 
 the original gloss is not so good as that obtained by the best coach- 
 builders’ varnish or stoved black enamel, it can be brought up by 
 rubbing and improves with age. The extra resisting power of 
 nitrocellulose coatings is chiefly due to the fact mentioned in the 
 first chapter, that the polymerisation of a drying-oil takes place 
 slowly after the application of the varnish, whereas the nitrocellulose 
 enamels already contain the polymerised base in the form of the 
 cellulose ester. 
 
 The saving in time is remarkable. Kirkpatrick states that the 
 time required to finish the body of one particular make of car has 
 been reduced from 336 hours to 134. The number of operations 
 has been reduced by one-third, and the number of bodies in process 
 at one time has been reduced from 2,400 to 600. Condé 3° states 
 that the same firm were able to reduce the area of floor space given 
 up to painting operations by 30,000 sq. feet, which in itself repre- 
 sented a large capital saving. . 
 
 The method of manufacture of low viscosity nitrocellulose 
 enamels follows the ordinary procedure so far as details have been 
 published, but the precise way in which the viscosity is kept so low | 
 has been kept secret. The solvents employed are said to be chosen 
 from amyl, butyl and ethyl acetates, amyl and butyl alcohols, 
 xylene, toluene, benzene, ethyl alcohol and acetone, while among 
 the gums mentioned in this connexion are shellac, soft copals and 
 dammar. 
 
 The method of application of the enamels has been described 
 by Flaherty.1!_ Any previous coating of varnish or paint must be 
 removed, e¢.g., by use of the blow lamp, as the nitrocellulose solvents 
 _are liable to form soft spots over places where the previous varnish 
 coat is not completely oxidised. When thoroughly cleaned, the 
 metal is coated with an ordinary oxide primer, which must be 
 allowed to oxidise fully before the enamel is sprayed on. The 
 nitrocellulose enamel is then applied. 
 
 Respirators are worn by the workmen while they are spraying, 
 and in British practice the car body stands in front of a concave 
 metal shield with an extraction fan in the centre designed to with- 
 draw the fumes. As a rule, a fairly high air pressure is used, some- 
 times as much as 80 lb. per square inch, but custom varies consider- 
 ably in this respect. 
 
 The use of a glossy oil-resin varnish over the nitrocellulose 
 
Manufacture and Application 135 
 
 enamel is not recommended, as if a high gloss is required it may be 
 obtained by friction. 
 
 Failure of the enamels in service may be due to any one of four 
 causes: (1) Improper preparation and oxidation of the under 
 coats. (2) Too heavy a coating of enamel. (3) Spraying the 
 enamel too dry, either through using too high a pressure, or keeping 
 at too great a distance from the work. (4) Allowing the enamel 
 to settle out in the container, for want of stirring. 
 
 Imitation Leather. 
 
 The imitation leather industry originated in the process of 
 Wilson and Story (1884), who coated cloth with a solution of nitro- 
 cellulose in a mixture of amyl acetate and castor oil.!2 The later 
 developments have been chiefly in the use of more economical solvent 
 mixtures, and more suitable varieties of nitrocellulose. 
 
 The fabric coated is usually cotton, commonly a canvas duck, 
 which should be free from weaving defects giving an irregular 
 surface, such as irregularity in the thickness of the thread. Light 
 fabrics are usually given a dressing to weight them. Heavy fabrics 
 are lightly fluffed so as to assist adhesion. 
 
 The solutions are applied to the moving fabric through an 
 adjustable slot set to deliver the correct amount. The solvents 
 chiefly used are acetone oil and ethyl acetate, diluted with alcohol, 
 benzene and petroleum spirit. Oil must be added to ensure plia- 
 bility of the coating. Castor oil is still the best, on account of its 
 light colour and non-drying properties. It is said that blown 
 linseed oil, colza oil and cotton-seed oil are also used, but they are 
 liable to turn acid. The pigments are ground in the oil before 
 introducing them into the solution. Resins are not employed. 
 
 The number of coatings may vary from three to thirty, according 
 to the thickness of material desired, but it is likely that the advent 
 of nitrocellulose of lower viscosity will reduce the number required 
 for thick material. The first coatings usually contain only a small 
 quantity of oil, as they are required primarily to secure adhesion. 
 The intermediate layers, which give body, contain more oil, and the 
 final coatings, which give the gloss, contain neither oil nor pigment. 
 The cloth must be dried between the applications. 
 
 Tucker 1 discusses the question of viscosity. If this is expressed 
 by the time taken for a steel ball, 5/16 inch in diameter, to fall 
 through a distance of 10 inches in a solution made up from 16 oz. 
 of nitrocellulose to 1 (U.S.) gallon of solvent, 70% ethyl acetate and 
 30% benzene, the viscosity of the nitrocellulose used for artificial 
 
186 Cellulose Ester Varnishes 
 
 leather is usually a 20-second cotton. When this is made up into 
 the correct formula, including the diluents, the viscosity is about 
 40 seconds. 
 
 Deschiens 15 has described the preparation of artificial leather 
 from cellulose acetate, but the higher cost of this ester limits its 
 employment for this purpose. It could only compete if there were 
 special reasons for producing a material less inflammable than the 
 nitrocellulose leather. 
 
 The imitation of the grain of real leather is put on by passing 
 the coated cloth through engraved rollers under pressure, and 
 sometimes the cloth is then rubbed by hand with a pad soaked in 
 solvent containing a slightly darker pigment than that used in the 
 coating solution. This pigment collects in the channels of the grain 
 and produces an imitation of old leather. 
 
 Leather Dressing. 
 
 Nitrocellulose solutions are used to impart a high gloss in the 
 so-called patent or enamelled leathers. 
 
 Patent Kid. 
 
 The processes used in making this material are described by 
 Crockett.1® Three coats of linseed oil varnish are usually applied, 
 and the skins are stoved between the applications. A nitrocellulose 
 solution is sometimes used instead of the first coat or “ daub,” and 
 is said to be a 3% solution in amyl acetate. The viscosity is not 
 specified, but it must not be too low, or the solution will penetrate 
 the leather too much and harden it. The daub is applied by means 
 of a brush on the grain (outer) side of the leather, which has been 
 previously cleaned with petroleum spirit to remove grease. The 
 other two coats are applied above the nitrocellulose coat. They 
 consist of linseed oil, blue and black pigments, and driers, boiled 
 for several hours at 500° F. 
 
 Patent and Enamelled Leathers. 
 
 These are now frequently prepared entirely with nitrocellulose 
 solutions, the older linseed oil varnishes having been largely super- 
 seded. The leather should be thoroughly cleansed from grease, 
 and quite dry. Three coats are applied, the first by means of a 
 stiff brush, with which the solution is worked well into the leather. 
 The second coat is applied by means of a woollen sheep-skin swab, 
 made by tacking to a wooden handle a piece of sheepskin tanned 
 with the wool on. The third coat is applied in the same way as 
 
Manufacture and Application 137 
 
 the second, and must be applied thinly and evenly. Between the 
 applications the skins are dried at 65-100° F. 
 
 According to Clément and Riviére,? the solvents employed in 
 the first coat are :— 
 
 Amyl acetate : . 1800 parts 
 Petroleum spirit . - ume DOO 3, 
 Castor oil 
 
 or 900 _—,, 
 Boiled linseed oil 
 
 The second coat contains less oil and the third none, so that a high 
 gloss is obtained. 
 
 The colour is usually a soluble (aniline) black. The leather 
 may be embossed under a hydraulic press in order to give it any 
 desired configuration. - 
 
 Motor-car Upholstery Leather. 
 
 Herrmann and Radel !” describe the treatment of this material 
 in an article very fully illustrated by reproductions of photographs. 
 The split hides are doped with nitrocellulose varnish. In this process 
 the skins are stretched out on tables, and the first coat is put on 
 with the same type of brush as that used by bootblacks. The later 
 coats are applied with brushes made of Chinese hog bristle, six or 
 seven inches long, having a wooden peg or “ dutchman”’ in the 
 middle so as to allow an extra quantity of material to be carried 
 and to give a spread to the brush. It is stated that this spread is 
 essential to produce a good finish. 
 
 The leather is sometimes finished with nitrocellulose dope alone, 
 sometimes entirely with a japanning finish of boiled linseed oil, 
 turpentine, naphtha, driers and pigments, and sometimes with a 
 combination finish consisting of undercoats of nitrocellulose and 
 finishing coats of a linseed oil japan. 
 
 Wood Coatings. 
 
 The use of nitrocellulose varnishes for the protection and decor- 
 ation of wood at first sight presents a great advantage over French 
 polishing in the great saving in time involved. Up to the present, 
 however, there has not been great development in this direction. 
 This may be partly due, as Clément and Riviére suggest, to the 
 conservatism of the skilled French polisher, which is not unnatural 
 in view of the excellent work which he turns out. Probably the 
 
138 Cellulose Ester Varnishes 
 
 chief obstacle, however, has been the large consumption of solvent 
 in the process, due to the absorption of the solution by the wood, 
 and the first problem is undoubtedly to find a cheap coating to act 
 as filler and primer. This is made possible by the use of modern 
 low viscosity varnishes, which deposit much more solid content for 
 the same expenditure of solvent.1® It is sometimes advantageous 
 to begin by rubbing in a pigment of appropriate colour damped with 
 methylated spirit, e.g., for a light coloured wood such as sycamore, 
 plaster of Paris, finely-divided silica, chalk, or starch. Spirit 
 varnishes (resins) may also be used as first coats, but the final 
 effect is not usually so good. 
 
 Wood covered with a thick coating of white nitrocellulose paint 
 is much used for sanitary purposes, and for bathroom, surgery and 
 hospital shelves. This is given a succession of applications, allowing 
 a few hours for drying between each, and with frequent rubbing 
 down with glass paper to ensure smoothness. The final coats should 
 be given with an enamel of the purest white obtainable, and some- 
 times a transparent coating is applied last of all to give a high gloss. 
 Wood so treated is proof against damp and can be washed down 
 without taking harm. 
 
 Picture frames and other decorative articles in wood are fre- 
 quently coated with bronze or aluminium paints. Matt black 
 varnishes are employed for the interior of cameras and other optical 
 instruments. 
 
 Paper Coatings. 
 
 Paper is sometimes coated with nitrocellulose varnish either for 
 protective purposes or for decoration. Cardboard price labels for 
 use in retail shops are frequently coated with a transparent solution 
 drying with a high gloss. Cheap fancy articles may be made by 
 coating stout paper in much the same way as in the manufacture 
 of imitation leather, using a solution containing castor oil as softener, 
 and soluble colours of pigments. The paper may then be embossed 
 with any desired pattern by passing it through engraved rollers. 
 
 Photographic papers are sometimes enamelled by coating with 
 a transparent nitrocellulose varnish. Posters and time tables may 
 be similarly treated to protect them against the weather. 
 
 Preservation of Antiques. 
 Solutions of cellulose acetate and cellulose nitrate have been 
 largely used by Lucas 1° for the preservation of antiques from the 
 recently opened Egyptian tombs. For this purpose he employed 
 
Manufacture and Application 139 
 
 either ordinary celluloid dissolved in acetone, amyl acetate, or a 
 mixture of the two; or cellulose acetate in acetone. No mention is 
 made of any special precautions taken to ensure the stability of 
 the cellulose ester base, a matter which should not be overlooked 
 when treating valuable specimens. 
 
 These solutions were employed for all kinds of materials—faience, 
 glass, inlaid work, pottery, stone, wood, alabaster, amber and ivory. 
 They were usually applied by means of a small camel’s-hair brush 
 or a piece of pointed wood. Porous materials, such as faience or 
 pottery, need thorough soaking with the solution at the broken 
 edges before making the joint. Fabrics showing signs of disintegra- 
 tion were sprayed with the solution. 
 
 For details of the procedure the original must be consulted. 
 There is a curious fact noted on p. 76 that silver articles sometimes 
 absorbed a large amount of the solution. 
 
 Miscellaneous Applications. 
 
 There are a large number of miscellaneous applications of cellulose 
 ester solutions which need not be entered into in detail since the 
 principles involved are the same as in the applications already 
 - described. 
 
 : Cement Floors. 
 
 Gardner finds that nitrocellulose lacquers form good priming 
 coats for certain types of cement floors which have to be painted. 
 By using them, waterproofing greases or other materials which may 
 be in the cement are insulated from action on the enamels subse- 
 quently applied. 
 
 Imitation of Snakeskin. 
 
 Under certain conditions of evaporation, depending partly on 
 the solvent mixture, the film left by varnishes breaks up into small 
 irregular polygonal areas on drying. Clément and Riviére have 
 utilised this property in an ingenious way by applying such a varnish 
 over a coloured base and obtaining an imitation of snake’s skin. 
 A similar effect can be obtained on penholders, pencils, cartons, 
 etc., and can be made permanent by application of a transparent 
 ~ coating.? 
 
 Crépe Stiffeners. 
 
 Crépe is stiffened by the application of a nitrocellulose varnish 
 containing resins and black pigment. The varnish must give a 
 dull, matt effect, but it must not give up pigment when rubbed or 
 crushed. | 
 
140 Cellulose Ester Varnishes 
 Hats. 
 
 Hats are stiffened with a nitrocellulose or resin solution, which 
 gives a waterproof stiffening. Straw hats are sometimes water- 
 proofed with a nitrocellulose solution, but without resin, which 
 would darken the colour. 
 
 Incandescent Mantles. 
 
 These are strengthened for transport by coating them with 
 nitrocellulose slightly softened with castor oil. A high viscosity 
 nitrocellulose is needed for this purpose so as to fill the mesh in 
 the fabric. 
 
 Glass. 
 
 Glass may be coloured by coating it with transparent solutions 
 coloured with various dyes, and this method is used for coating 
 incandescent electric bulbs, street lamps and photographic lamps. 
 A frosted appearance may be obtained by the incorporation of 
 certain resins. 
 
 Very dilute solutions of either cellulose nitrate or acetate may 
 be made which leave a film so thin that it shows interference colours. 
 
 Such films are, however, very fragile, and it is difficult to protect 
 
 them without destroying them. 
 
 Compressed Cork. 
 
 Slabs of compressed cork used as bath-room mats and floor 
 coverings are made by incorporating cork dust with a nitrocellulose 
 solution and pressing the product in moulds into the required shape. 
 
 Gilding Celluloid. 
 
 Transparent celluloid may be gilded on one side by spraying 
 with a solution containing fine bronze powder. Such material is 
 used very effectively in the button trade. 
 
 Imitation Gold Leaf. 
 
 A similar solution sprayed on to glass and stripped forms a 
 cheap substitute for gold leaf, and is largely employed in the 
 bookbinding trade. 
 
 Collodions. 
 
 The term collodion is usually, though not invariably, applied to 
 solutions of cellulose nitrate in a mixture composed mainly of ether 
 and alcohol. Several types of collodion occur in the pharninconaem 
 of which the following may be quoted :— | 
 
Manufacture and Application 141 
 Collodion (official) B.P. synonym Contractile Collodion. 
 
 Pyroxylin é' ie lL part 
 Alcohol (90%) . 12 parts 
 _ Ether (sp. gr. 0-735). : } pO O..,, 
 
 Acetone may be used instead of ether—alcohol, but the film 
 obtained is opaque. 
 _ The United States Pharmacopeeia formula is :— 
 
 Pyroxylin : ; : ; . 4 parts 
 Ether. : EN | aFeage 
 Alcohol . ; : = Bie 
 
 Note that the B.P. preparation is about 2% and the U.S.P. 4%. 
 The B.P. Codex has the following :— 
 
 Collodium Acetonum. 
 
 Pyroxylin Sw Atiai ; . 5 parts 
 Clove Oil : : f : aah ae ee 
 Amy]! Acetate . Ciaah Rex, 
 
 ' Benzol . : : ; : alts) ape 
 Acetone to ‘ ; : , gr O0 =. 
 
 This dries more slowly than an ether—alcohol collodion, owing to 
 the presence of amyl acetate. 
 
 Martindale and Westcott speak highly of their ‘ Collodium 
 Aceto-Aethericum,” which is a 5% solution of pyroxylin in ethyl 
 acetate. 
 
 Collodium Flexile (official). 
 This preparation is made to yield a flexible film, instead of the 
 
 “contractile” film of the above-quoted official formula, by the 
 incorporation of castor oil :— 
 
 Contractile collodion ‘ : . 48 parts 
 Canada turpentine . : mp + aa 
 Castor oil (by weight) : kus A pak 
 
 Various other formule are given in the Extra Pharmacopeia. 
 
 Celloidin. 
 
 This is a form of nitrocellulose prepared by the clarification and 
 evaporation of an ether—alcohol solution. It dissolves again in 
 ether-alcohol, yielding a solution of high transparency which is 
 used as an embedding agent in microscopy, and also in surgery. 
 It usually occurs in the form of small irregular cubes or flakes. 
 
 _— 
 
142 Cellulose Ester Varnishes 
 
 REFERENCES. 
 
 1 §. D. Kirkpatrick, Chem. and Met. Eng., 1924, 31, 178-182. ? L. 
 Clément and C. Riviére,' Chimie et Ind., 1921, 6, 283-295. 3 A. E. Ayres, 
 Chem. and Met. EHng., 1916, 14, 502. 4 fo E. "Ramsabottoun Annual Report 
 Advis. Comm. for Aeronautics for 1913-14, 426-432. 5 P. Drinker, J. Ind. 
 Eng. Chem., 1921, 18, 831-835. ® F. W. Aston, Advis. Comm. for Aero- 
 nautics Rep. and Mem., No. 585 (Feb. 1919). See PC. 10 in B.E.S.A., 
 2D. 103. 7 B. D. Baker, U.S.P. 1,330,550. ® H. A. Gardner, Paint Manu- 
 facturers’ Association of U.S. Educational Bureau, Circular No. 65, June 
 1919. ® J. C. Wiesel, Chem. Age (New York), 1924, 32, 439. 1° H. C. 
 Mougey, Paint, Oil and Chemical Review (Chicago), 1924, 78, 10-12. 11 E. M. 
 Flaherty, Journ. Soc. Automotive Hng., 1924, 14, 352-353. 12 Caoutchouc 
 et Gutta-Percha, 1921, 18, 10,849-10,851. 1% Joseph R. Lorenz, J. Amer. 
 Leather Chemists’ Assoc., 1919, 14, 548. 14 W.K. Tucker, J. Ind. Eng. Chem., 
 1921, 18, 623. 15 M. Deschiens, Chemical Age (New York), 1921, 29, 270. 
 16 H. G. Crockett, ‘‘ Practical Leather Manufacture.” 17 K. L. Herrmann 
 and F. J. Radel, J. Soc. Automotive Engineers, 1924, 14, 327-334. 18 G. E. 
 Condé, Canad. Chem. and Met., 1924, 8, 219-220. 19 A. Lucas, ‘‘ Antiques ; 
 their Restoration and Preservation > (Edward Arnold, 1924). 
 
 Additional Reference. 
 
 The British Engineering Standards Association publish a pamphlet 
 entitled ‘‘ British Standards of Reference for Aircraft Dope and Protectin 
 Covering,’’ which includes specifications for each ingredient and method of 
 application. ‘To be obtained from the Secretary, 28 Victoria Street, London, 
 S.W. 1, price ls. 2d., post free. 
 
CHAPTER IX 
 MISCELLANEOUS 
 
 Precautions Necessary in using Cellulose Ester Varnishes—Safety Precautions 
 —Cause of Blooming—Humidity of Atmosphere—Measurement of 
 Humidity—Effect of Raising or Lowering Temperature—Settling of 
 Pigments—Cleanliness of the Work—Evaporation Losses—Analysis 
 of Cellulose Nitrate Solutions; Odour, Total Solids, Nitrogen Determina- 
 tions—Precipitation of Nitrocellulose with Fusel Oil (Lorenz)—Pre- 
 cipitation of Nitrocellulose with Chloroform (Conley)—Precipitation of 
 Nitrocellulose with Aqueous Electrolytes—lIdentification of Solvents— 
 Viscosity—Analysis of Cellulose Acetate Varnishes—Solvent Recovery 
 —Scientific Application of Cellulose Ester Solutions—Collodion Mem- 
 branes—Interference Colours of Thin Films—Dimensions of Thinnest 
 Obtainable Films—Density of Thin Films—Chromatic Emulsions— 
 Transport of Cellulose Ester Solutions on British Railways. 
 
 Precautions Necessary in using Cellulose Ester Varnishes. 
 THESE may be classified under two headings :— 
 
 (1) Precautions necessary for the safety and comfort of the 
 workers. 
 
 (2) Precautions required in order to get the best results. 
 
 (1) The chief precautions to be taken in the employment of 
 cellulose ester varnishes are those required against fire. It is an 
 essential feature of these varnishes that they dry quickly, and 
 that being so, the air must rapidly become laden with the inflam- 
 mable vapour of the solvents unless special arrangements are 
 made to renew it. When shops are being specially constructed for 
 industries using these solutions, it is a comparatively simple matter 
 to plan the arrangements so that fans may draw the vapours away 
 from the workpeople. All the vapours are heavier than air, so 
 that a fan is most efficient when placed near the ground. Direct- 
 coupled motor-driven fans are not advisable on account of the risk 
 of sparking, particularly if running on direct current. A belt drive 
 is to be preferred. 
 
 No general rule can be laid down as to the rate at which the 
 air ought to be changed. This is governed more by the nature of 
 the solvents than by the actual fire danger. In the aeroplane 
 doping sheds during the war, the air was changed every two minutes, 
 i.€., the total amount of air moved by the fans in two minutes was 
 equal to the cubi¢ capacity of the shop.1 This is perhaps the maxi- 
 mum requirement in ventilation, and it was brought about by 
 earlier fatalities and illness caused by the poisonous vapour of 
 tetrachlorethane. None of the solvents now in general use is as 
 dangerous. Those of high vapour pressure, such as acetone, benzene 
 and alcohol, require watching most closely, since the concentration 
 of vapour which can be reached is naturally higher. Of these, 
 
 benzene is most likely to give trouble, but with good ventilation it 
 143 
 
144 Cellulose Ester Varnishes 
 
 can be used with perfect safety. It may be remarked that if the 
 ventilation is sufficiently good to prevent any feeling of discomfort 
 to those working in the shop, the fire danger due to the accumulation 
 of vapour is also non-existent, provided that the extraction fans 
 are placed low. If the fans are placed too high, there is always a 
 risk of accumulation of heavy vapours near the floor. 
 
 Extraction fans (vacuum system) are to be preferred to fans 
 blowing fresh air into the shop (plenum system). 
 
 When much spraying work is done, it is advisable for the oper- 
 atives to wear respirators, since the dried spray contains particles 
 of the solid contents of the lacquers, in a very fine state of division. 
 
 (2) The chief cause of failure met with in the use of cellulose 
 ester varnishes is “‘ blooming.” This term is applied to the whitish 
 deposit which sometimes appears on the surface of the work during 
 drying. It is usually accompanied by brittleness of the coating, and 
 when it occurs with white solutions, on which the milkiness does not 
 show, it is sometimes called ‘‘ chalkiness.’”’ Another way in which 
 the same fault may be manifested is by pitting of the coating. 
 
 These faults are all due to the deposition of atmospheric moisture 
 on the film during drying. If a thermometer, preferably one with 
 a long bulb, is plunged into a nitrocellulose solution and withdrawn, 
 it will be noticed that the mercury immediately begins to drop, 
 and may fall as low as 10-15° below the temperature of the sur- 
 rounding atmosphere. As a rule, before it has ceased to fall, beads 
 of moisture will be seen on the film as it dries on the bulb. In fact, 
 this rough test will, in the hands of an observant foreman, give 
 useful information about atmospheric conditions. The cooling is 
 due to the rapid evaporation of the volatile solvents, and the 
 deposition of moisture to the fact that air of our climate always 
 contains moisture, and that it does not hold so much when it is 
 cold as when it is warm. Scientific readers must pardon the writer 
 if he discusses this question in a very elementary way, as it is fre- 
 quently a source of perplexity to the users of cellulose ester varnishes. 
 
 The simplest way in which to approach the subject of water 
 vapour in the atmosphere is to consider the analogy with an aqueous 
 solution of a crystalline salt, say potassium nitrate. The following 
 table shows how much potassium nitrate will dissolve in 100 lb. of 
 water at ¢° :— 
 
 i Lb. of Potassium Nitrate 
 LO Ass ; : ae 
 20° 31 
 30° 45 
 40° 64 
 50° 86 
 
Miscellaneous 145 
 
 In a similar way, we may tabulate the weight of water vapour 
 which will saturate 100 lb. of air at ¢#° and normal atmospheric 
 pressure 2 :— 
 
 t° Lb. of Water Vapour 
 10° ; : ; oe TT 
 20° 1-48 
 30° 2:75 
 40° 4-89 
 50° 8-68 
 
 If we take a solution saturated with potassium nitrate at 40°, 
 we see from the first table that it will contain 64 lb. of potassium 
 nitrate to every 100 lb. of water. If we allow the temperature to 
 fall to 30°, we see that 100 lb. of water can then only hold 465 lb. 
 of potassium nitrate, so that 19 lb. of the salt must be deposited 
 from the solution in the solid form. 
 
 Similarly, if we have the air of a room saturated with water 
 vapour at 40°, we see from the second table that it will contain 
 4-89 lb. of water vapour for every 100 lb. of air. If we allow the 
 temperature to fall to 30°, we see that 100 lb. of air can then only 
 hold 2-75 lb. of water vapour, and the remainder, 2:14 lb. must 
 be deposited. This moisture separates in fact as a dew or fog. 
 
 Conversely, if we warm up a solution of potassium nitrate, 
 saturated at 30°, from 30° to 40°, we can dissolve in it an additional 
 19 lb. of potassium nitrate for every 100 lb. of water present. Simi- 
 larly, if we warm up the air of a room, which is saturated with 
 moisture at 30°, from 30° to 40°, we can make it take up an additional 
 2-14 lb. of water vapour for every 100 lb. of dry air present. 
 
 The fact expressed in the last sentence is the source of the 
 misapprehension so often encountered, that “ warming the air 
 dries it.” It does not. Warming the air enables it to take up an 
 additional quantity of water vapour 7f it 1s maintained at the higher 
 temperature, but if it is allowed to fall in temperature to its original 
 state, all this extra water vapour must be deposited again. For 
 example, it is of no use to draw air over steam pipes by means of 
 a fan and deliver it into a cold shop. Steam pipes inside the shop 
 are useful if great care is taken that all the joints are sound. A 
 small leak of steam will speedily saturate the air of a room with 
 moisture. Steam pipes outside the shop are only useful if the 
 volume of air delivered over them, and the heat supplied to the 
 air, are sufficient to warm the whole atmosphere of the shop. 
 
 The outer atmosphere is not often entirely saturated with 
 moisture. What is usually called the “ humidity ” is the proportion 
 “simon as a percentage) between the amount of water vapour 
 
146 Cellulose Ester Varnishes 
 
 actually present and the amount that would be present if the air 
 were saturated with it. If, for example, 100 lb. of air at a certain 
 temperature contains 1 lb. of water vapour, but is capable of holding 
 1-5 lb., its humidity is 
 
  & 0 
 
 15 * 100 = 66-:7%. 
 The difference between humidity and 100%, therefore, expresses 
 the drying power of the air at its existing temperature. 
 
 Since the atmosphere always contains moisture, it is always 
 possible, by reducing the temperature sufficiently, to reach a point 
 at which it is saturated, 7.e., at which dew begins to deposit. This 
 temperature is known as the dew point. Returning to our original 
 experiment with the thermometer dipped into a nitrocellulose 
 solution and then withdrawn, we see that the appearance of the 
 beads of moisture on the film is due to the fact that the film has 
 cooled (by evaporation of the solvent) below the dew point of the 
 atmosphere. 
 
 Measurements of humidity and dew point are useful data for 
 those continuously employing cellulose ester solutions. They can 
 be simply determined by means of an apparatus known as the 
 “wet and dry bulb thermometer,” which consists of two identical 
 thermometers mounted side by side on the same stand. One of 
 the bulbs—the ‘“‘ wet bulb ’—is covered with a piece of muslin, 
 which is connected by a few strands of cotton to a small reservoir 
 containing water. The capillary action of the cotton keeps the 
 muslin continually damp with water. When the air is very dry, 
 the moisture evaporates rapidly from the muslin, and lowers the © 
 temperature of the mercury. If the air is damp, evaporation of 
 the water is hindered, and the lowering of temperature is less. 
 Hence the difference between the temperatures recorded by the 
 two thermometers is governed by and measures the humidity of 
 the surrounding air. Tables, originally complied by Glaisher,® are 
 available from which the humidity of the air can be read directly 
 from the temperatures of the wet bulb and the dry bulb. 
 
 A modern variation of the wet and dry bulb thermometer is 
 made in which the thermometers can be whirled on an axle, so that 
 the air is in rapid motion past the bulbs. Instruments known as 
 hygrometers are also made which indicate the humidity directly 
 on a dial. They depend on the influence of atmospheric moisture 
 on the length of animal fibres such as horse-hair. 
 
 The following monthly averages of atmospheric temperature 
 (dry bulb), wet bulb temperature, humidity and dew point may be 
 
Miscellaneous 147 
 
 of interest. They were taken outside the writer’s laboratory 
 (Suffolk) at mid-day on working days, and show the kind of variation 
 which occurs between summer and winter weather :— 
 
 
 
 1924. Dew Humidity 
 point. ss 
 Eee 60-0° F. 75-6 
 OS eae 52:6 64-2 
 September .............5. 52-2 67-4 
 SS re 49-4 74-7 
 PRovemiber: ..0.....-...+. 42-5 75-2 
 se 40:7 76:8 
 
 The chief point to observe is that the dew point in the open air 
 is lower in winter than in summer, and this is the temperature at 
 which the atmosphere begins to deposit dew. Supposing that, 
 without any change in the actual weight of the water vapour content, 
 the air defined in the above table were drawn into a shop at a 
 temperature of 55° F. The dew point in July averaged 60° F.; 
 therefore this air at 55° F. would be below its dew point and would 
 become foggy and deposit dew. The other five samples would 
 remain clear. 
 
 The usual working temperature of a shop is about 65° F., and 
 this is a fairer temperature to take. If the above six samples of 
 air were drawn, without change of water content, into a shop at 
 65° F., what effect would it have on their drying properties? This 
 can easily be ascertained from Glaisher’s tables. Since the water 
 content has not changed, the dew point of each sample must be 
 unchanged also. We have to find therefore what would be the 
 humidity of six samples of air, whose dry bulb temperature was in 
 each case 65° F., but whose dew points were 60-0, 52-6, 52-2, 49-4, 
 42-5 and 40-7 respectively. The answers are given in the following 
 table :— 
 
 
 
 1924. Humidity if at 65° F. 
 NE irs ay a sinengiobesakgnnaninns 84% 
 MIE ie wentasecrsava setae 64 
 BODUGIADOT 55. sscyiesvedacsees . 63 
 SOB ois hncsasedewigtns 57 
 PHOVGIMDG? So accocssvecccd cess 44 
 - PAGCOR a ooo sd dads bo vhs eds 41 
 
 It will be noticed that the humidity of the July air has been increased, 
 that of August has hardly changed, while the average air of the 
 other four months has all been reduced in humidity by bringing it 
 
148 Cellulose Ester Varnishes 
 
 to the uniform temperature of 65° F. The content of water vapour 
 has not been altered, The change has occurred in the proportion 
 between the actual water content, and the water that the air could 
 hold if saturated; in other words, the drying power of the air has 
 been changed, although its water content has not. 
 
 It follows from this that if work is carried on in a shop main- 
 tained at a uniform temperature throughout the year, trouble with 
 blooming is more likely to occur in summer, when the dew point of 
 the outer air is high, than in winter, when it is low. In practice, 
 conditions are not quite so simple as they have been assumed to be 
 in this example. For example, the presence of human beings in a 
 room alters the humidity conditions, owing to the moisture in 
 expired air; but the main conclusions hold. 
 
 Solutions sold by manufacturers as drying bright contain 
 sufficient high boiling solvent to prevent blooming under all usual 
 atmospheric conditions, but when consumers ask for specially 
 quick-drying solutions, blooming becomes a frequent problem. 
 
 It is evident that there are three ways of preventing the deposition 
 of atmospheric moisture on the film :— 
 
 (1) Drying the air which is being drawn into the shop by 
 the exhaust fans. 
 
 (2) Checking the rate of evaporation, so as to diminish the 
 cooling of the film. 
 
 (3) Carrying out the drying operation in a chamber at a 
 slightly higher temperature than the work-room. 
 
 A combination of (2) and (3) is the usual remedy for blooming. 
 Drying the air, the first remedy, is a rather more complicated process — 
 than most solution-users care to undertake. It may be carried out 
 by passing the incoming air over refrigerating coils, when the 
 moisture in the air is deposited on the pipes as ice, or by drawing 
 the air through a drying tower fed with sulphuric acid. Sometimes 
 trays of calcium chloride in lumps are placed in the drying chamber, 
 thereby combining methods (1) and (3). If the work, after having 
 been sprayed or dipped, is promptly transferred to a warm com- 
 partment, at a temperature of 80° to 90° F., solutions made with 
 ordinarily volatile solvents will usually dry bright. In such a 
 compartment, the rate of evaporation can be checked by means of 
 ventilators, thus applying method (2). The air under these con- 
 ditions becomes highly charged with vapour, and precautions 
 against fire must be rigorously enforced. 
 
 Varnishes containing high-boiling solvents do not often give 
 trouble through blooming, but it must be remembered that the 
 
 % 
 
Miscellaneous 149 
 
 high-boiling solvents are the expensive ones, and keen competition 
 will compel manufacturers to reduce the percentage of these solvents 
 to the lowest limit. 
 
 Setiling.—The rate of settling of the pigment in a well-made 
 cellulose ester varnish is extremely slow, but nevertheless it does 
 occur when the pigment is heavy or present in large amount. Such 
 varnishes should therefore be well stirred before use, and should 
 not be stocked for long, if this can be avoided. If the varnish is 
 allowed to settle, the top layers will be deficient in covering power, 
 and the bottom layer will give brittle or rough coatings. 
 
 Cleanloness.—It is particularly necessary that surfaces, especially 
 metallic surfaces, to be coated with either a cellulose nitrate or 
 cellulose acetate varnish, should be quite clean. Grease or oil is 
 specially to be avoided, as it prevents proper adhesion. Washing 
 the surface with petroleum spirit, benzol or toluol is recommended 
 when practicable, but the surface must be quite dry again before 
 the first coating is applied. 
 
 Evaporation Losses.—It is the custom in many factories using 
 cellulose ester solutions to distribute the solutions about the shops 
 in small vessels or containers, for the conveniente of the work- 
 people. Unless these can be closed at night with an air-tight cover, 
 they should always be emptied back into the main storage vessel. 
 Otherwise a great deal of solvent is wasted, some of the varnish is 
 spoilt, the air of the shop is needlessly contaminated with solvent 
 vapours and the fire risk while the shop is unoccupied is unneces- 
 sarily increased. 
 
 Analysis of Cellulose Nitrate Varnishes.—In view of the fact that 
 very few of the ingredients of cellulose ester varnishes are, or even 
 approximate to, chemically pure substances, it is evident that the 
 deduction of a formula by the analysis of a sample is always a matter 
 of great difficulty, and sometimes an impossibility. In fact, as a 
 rule more can be learned about a sample in a short time by 
 close observations of its behaviour on filming, its odour and its 
 price, than can be learned in the same time from its chemical 
 behaviour. ; 
 
 Certain methods are available, however, and yield results which 
 are of considerable assistance. 
 
 (1) Odour.—The constituents of the solvent can often be identi- 
 fied by the smell of the sample. Amyl acetate is unmistakable, 
 but may mask butyl acetate. Acetone, benzene, wood spirit and 
 ethyl acetate can usually be recognised, even when present together. 
 Petroleum spirit is less easily distinguished and ethyl alcohol is 
 difficult to detect. There are, of course, considerable personal 
 
150 Cellulose Ester Varnishes 
 
 differences in the acuteness of the sense of smell and the power of 
 distinguishing the constituents of mixtures. 
 
 (2) Total Solids—The percentage of total solids is determined 
 by the evaporation of a small weighed quantity to constant weight. 
 The solution should be spread out in as thin a film as possible. 
 Conley 5 recommends using just enough of the solution to cover 
 the bottom of the weighing bottle, but this limits the quantity 
 rather stringently. On the other hand, weighing on a clock glass 
 is inadmissible on account of the rapid loss of solvent during weighing. 
 A preferable method is to weigh the sample in a wide-mouthed bottle 
 or sample tin; the rounded bottom of a stout weighed glass tube is 
 then dipped into the solution and withdrawn, rotating it in the 
 fingers if there is a tendency to drip. The sample is weighed again, 
 and the difference in weight is the amount transferred to the outside 
 of the tube. The latter is kept in a warm place until free from the 
 smell of solvent and is then weighed at intervals until of sufficiently 
 constant weight. The appearance and manner of drying of the 
 film can be very conveniently studied in this way, and the coating 
 is easily removed as a thimble by putting the tube in hot water 
 for a few seconds. 
 
 Nitrogen Determinations.—Writers on the subject do not appear 
 to have realised the value of a nitrogen determination in a portion 
 of the film obtained from a total solid determination of a cellulose 
 nitrate solution. This is best carried out by the Schultze-Tiemann 
 method on a weighed quantity of about 0-3 gramme of the sample. 
 [It will probably be noticed that there is a distinct odour of solvent 
 when this piece is boiled in the flask—a sign that the estimation of 
 solid content has given a higher figure than the true one.] From 
 the nitrogen figure it is possible to obtain immediately an approxi- 
 mate indication of what proportion of the solid content is noé nitro- 
 cellulose. The nitrocelluloses used in varnish manufacture usually 
 contain from 11-7 to 12-2% of nitrogen. Hence if the nitrogen is 
 much lower than this, there must be an appreciable quantity of 
 other ingredients and the percentage of nitrocellulose in the film 
 will be roughly 
 
 a 
 
 O 
 75 X 100% 
 
 of the weight of the film, x being the determined percentage of 
 nitrogen. 
 
 If the film is transparent and nearly colourless, the other con- 
 stituent may be castor oil, which imparts its characteristic odour 
 to the film and makes it supple. Ifthe film is transparent, somewhat 
 
Miscellaneous 151 
 
 dark in colour and hard, resins are indicated. If opaque, there are 
 probably pigments present and a determination of ash should be 
 made to compare with the value calculated from the percentage 
 of nitrocellulose. The ash may be analysed by the ordinary methods 
 of inorganic analysis. The presence of oil in the film is often shown 
 by drops floating on the surface of the liquor remaining in the flask 
 after the completion of the nitrogen determination. Resins, oil 
 and pigment may all be present together, or any two of them, and 
 the problem becomes complicated. Extraction with an organic 
 solvent which does not dissolve nitrocellulose, such as chloroform 
 (best), benzene or petroleum spirit, will often take out the resins 
 and/or oil, leaving pigment and nitrocellulose together. A fresh 
 nitrogen determination at this stage will check the loss of weight 
 of the film and the weight of the oil extracted. When weighing 
 an oil extract which is being taken to approximately constant 
 weight, it is usually advantageous to employ a small porcelain 
 dish resting on a watch glass or small clock glass, so that any oil 
 creeping over the side of the dish is not lost. 
 
 Precipitation with Fusel Oil—For a mixture containing nitro- 
 cellulose, resin and pigment, amyl acetate, amyl alcohol, methyl 
 alcohol and petroleum spirit, Lorenz* recommends a different 
 method, which has its advantages. To 100 c.c. of the varnish are 
 added gradually with continuous stirring 200 c.c. of fusel oil. The 
 nitrocellulose is precipitated as a voluminous gel. Fifty c.c. of the 
 clear liquor are drawn off and saponified with standard alcoholic 
 potash. If the only ester present is amyl acetate, the amount in the 
 50 ¢.c. is one-sixth of that originally present in 100 c.c. of the 
 varnish. 
 
 [If butyl or ethyl acetate is present as well as amyl acetate, it 
 is only possible to obtain the total percentage of combined acetic 
 acid in the original varnish, a figure that may be quite useful.] 
 
 The remaining 250 c.c. from the previous determination are 
 transferred to a tared filter and washed with warm fusel oil until 
 no more solid matter is extracted. The object of this is to extract 
 all the resins (of which amyl alcohol is perhaps the most general 
 solvent), leaving the nitrocellulose on the filter-paper. An aliquot 
 portion of the filtrate is taken to dryness to give the percentage of 
 resins. Lorenz remarks that ‘‘ the nature of the gum resins and 
 camphor which constitute the residue may be determined by the 
 usual methods.” If camphor is present, the writer would much 
 prefer to get rid of it before weighing the resins, since the amount 
 of camphor left after evaporating to dryness 150 c.c. of fusel oil (as 
 Lorenz recommends) would be much less than the camphor originally 
 
152 Cellulose Ester Varnishes 
 
 present, because of its high volatility. The writer would suggest 
 moistening the residue of resins with toluene, and taking to dryness 
 again, repeating this operation until sufficient constancy of weight 
 is reached. The presence of small amounts of camphor in varnish 
 samples is a much more awkward analytical problem than the 
 estimation of camphor in solid celluloid. The only safe method of 
 camphor estimation is by the polarimeter (which, of course, fails 
 when optically inactive camphor is used), and it is not easy to get 
 from a sample of varnish a solution of the camphor (in a pure solvent) 
 of sufficient concentration to give an accurate polarimeter measure- 
 ment of its optical activity. The writer would prefer to proceed 
 on the following assumptions :— 
 
 (a) If camphor is detected qualitatively, and the nitrogen 
 content of the nitrocellulose is not greater than 11%, the 
 varnish has probably been made from ordinary celluloid, and 
 should be matched by the use of ordinary celluloid based on 
 the nitrocellulose content of the latter. 
 
 (b) If the nitrogen content of the nitrocellulose is about 
 12% and camphor is present, the varnish has probably been 
 made from waste cinema film, and should be matched by the 
 use of cinema film based on its nitrocellulose content. 
 
 The assumption underlying this procedure is that camphor is 
 not likely to be added to a varnish made up from fresh nitrocellulose, 
 and its presence usually indicates the use of a form of celluloid 
 scrap. Admittedly exceptions occur. In any case, the proportion 
 of camphor is not likely to exceed 25% of the weight of nitrocellulose. 
 
 Returning to the analytical scheme of Lorenz, the mass on the 
 filter-paper now consists of nitrocellulose and adhering fusel oil, 
 together with pigment. It is washed with a mixture of equal parts 
 of acetone and amyl acetate, which dissolves the nitrocellulose, 
 and the solution passing through the filter is collected. On 
 evaporation to dryness of the whole of this (or an aliquot portion), 
 the nitrocellulose is obtained and can be weighed. The writer’s 
 experience is that the weight of nitrocellulose so obtained is likely 
 to be too high. (See p. 154.) 
 
 The residue on the filter-paper is the pigment, the weight of 
 which is obtained by drying to constant weight at 100° and deducting 
 the weight of the filter. It is the writer’s opinion that with many 
 of the finely-ground pigments used in present-day lacquers, a certain 
 amount would pass through the filter with the nitrocellulose, so 
 that the figure would be only approximately correct. Some method 
 such as this, however, is the only practicable one if the pigment is 
 
Miscellaneous 153 
 
 some form of carbon, since it cannot be ignited without loss. If 
 the pigment is a permanent oxide, such as zinc oxide, it is better 
 to determine pigment by ignition of the film obtained in the estima- 
 tion of total solid contents, as already described. 
 
 Petroleum spirit is determined by adding 75 c.c. of concentrated 
 sulphuric acid to 50 c.c. of the solution in a graduated cylinder, 
 stirring vigorously. After several hours, the petroleum spirit, 
 which is the only ingredient insoluble in the acid, will have risen 
 to the top and the volume may be read. 
 
 Lorenz estimates the amounts of methyl and amyl alcohol by 
 distilling 100 c.c. of the original sample, using a fractionating column, 
 and assuming that the volume of distillate coming over below 75° 
 is equal to the content of methyl alcohol. Of the four liquid 
 constituents of the mixture, amyl acetate, amyl alcohol, methyl 
 alcohol and petroleum spirit, three have thus been determined, and 
 the amyl alcohol is calculated by difference. 
 
 Conley’s Scheme of Analysis.—Conley 5 abandons the method of 
 precipitation by fusel oil, since some nitrocellulose remains in 
 solution. He prefers to measure out 10 c.c. of the solution, and 
 precipitate by the gradual addition of chloroform, with continuous 
 shaking. If no resins or pigments are present, the precipitate is 
 pure nitrocellulose, and may be filtered off, dried, and weighed. If 
 resins are present in considerable amount, the first precipitate of 
 nitrocellulose will be contaminated with resin. It is then necessary 
 to decant the clear liquor as completely as possible, to redissolve 
 the nitrocellulose in the smallest possible quantity of solvent, and 
 to reprecipitate with chloroform. Conley does not suggest a solvent 
 for this purpose, but the writer would suggest a low-boiling solvent 
 such as acetone, so as to diminish the risk of contaminating the final 
 precipitate with a high-boiling solvent such as amyl acetate. 
 
 Resins or oil are estimated by evaporation to dryness of an aliquot 
 portion of the combined filtrate. Castor oil is recognised by its 
 smell and its relative insolubility in petroleum spirit. It is almost 
 impossible to identify the resins obtained in this way, since their 
 solubility is frequently altered by the treatment they have received. 
 
 The writer’s experience is that no method of precipitation of 
 nitrocellulose by organic solvents is cleanly or easy to carry out, 
 although in view of the fact that the analysis of a nitrocellulose 
 varnish is at best a very approximate affair, the method is probably 
 good enough. From the researches of Knoevenagel and his 
 collaborators on the swelling of cellulose acetate, it is probable that 
 the precipitated nitrocellulose contains absorbed solvent, in some 
 kind of equilibrium with the surrounding liquid, and it is difficult to 
 
154 Cellulose Ester Varnishes 
 
 remove this in reasonable time by drying. If a precipitate so 
 obtained, after having been taken down to approximately constant 
 weight, is transferred to a test-tube and boiled with a little water, 
 there is an immediate odour of solvent, showing that the nitro- 
 cellulose is not pure. 
 
 Precipitation with Aqueous Electrolytes—The following method 
 is much cleaner, and has been employed in the writer’s laboratory 
 for several years, if a more accurate determination is necessary. 
 A film is first obtained directly from a weighed or measured quantity 
 of the original solution, as in the determination of solid content. 
 The amount of film should be enough to yield about 0-5 gramme of 
 nitrocellulose. If oils or resins are present, they are extracted 
 under reflux or in a Soxhlet, with chloroform or any other appropriate 
 solvent. If they are not present, this step is unnecessary. The 
 _ film is then dissolved in 50 c.c. of acetone, and diluted with aqueous 
 
 alcohol (50%). The quantity cannot be stated definitely, since it 
 differs according to the solubility of the nitrocellulose, but it must 
 be sufficient to convert the solution to an opalescent suspensoid 
 colloid, blue by reflected light and red by transmitted light. Such 
 a solution would pass unchanged through filter-paper. On adding 
 to it a dilute aqueous solution of an electrolyte, such as 2% 
 ammonium chloride, it is coagulated to a manageable precipitate. 
 Here also the required conditions are variable. Sometimes it is 
 better to precipitate hot, sometimes cold. A little experience is 
 the best guide. Under favourable conditions, the precipitate is as 
 easy to manipulate as silver chloride. The liquid is brought to 
 the boil, allowed to settle, and filtered off through a Gooch crucible 
 under suction, the first few washings being decanted off. Washing 
 is complete when the filtrate contains no chloride. The precipitate 
 is dried in the usual way at about 40°, and when constant weight 
 is obtained, it may be washed directly into the Schultze-Tiemann 
 apparatus for nitrogen determination. The greater purity of the 
 nitrocellulose so obtained is shown by the fact that it always gives 
 
 a higher nitrogen content than those obtained by precipitation with | 
 
 an organic liquid, and the apparatus is left quite clean. 
 
 This method is a modification of one first suggested by Dubovitz,® 
 who precipitates an acetone solution of nitrocellulose directly with 
 a solution of ammonium chloride. The modification is a little more 
 elastic, and the precipitated nitrocellulose is less likely to contain 
 enclosed impurities. The hot aqueous solution of acetone and 
 alcohol is very efficacious in keeping camphor and other organic 
 solvents in solution. 
 
 Idenittfication of the Solvents —Conley recommends dry distillation 
 
 # 
 
Miscellaneous 155 
 
 up to 120° of 100 c.c. of the varnish from a distilling flask heated 
 in a paraffin bath. Above 120°, water is added, and a steam distil- 
 lation is substituted. The product of dry distillation will contain 
 all the low-boiling solvents present, e.g., methyl and ethyl alcohols, 
 acetone, methyl ethyl ketone, ethyl acetate, benzene and part of 
 the petroleum spirit. The product of the steam distillation will 
 contain, ¢.g., amyl acetate, butyl acetate, higher acetone oils, and 
 the remainder of the petroleum spirit. 
 
 The rigid separation and identification of these in the limited 
 time available in a technical laboratory is impossible. The low- 
 boiling solvent should be fractionated, and a selection from the 
 following tests applied to the fractions: odour, specific gravity, 
 miscibility with water, miscibility with salt solution, miscibility 
 with concentrated sulphuric acid, iodoform reaction, reaction of 
 immiscible liquids with nitric acid. The product of the steam 
 distillation, after salting out dissolved solvents from the water 
 layer, may be similarly treated. From the indications obtained it 
 is usually possible after some experience to match fairly closely the 
 composition of the solvent mixture. 
 
 Viscosity. 
 
 This is the most important property of all. It should be 
 measured by a method giving results comparable with those of 
 other investigators, and the writer strongly recommends the use 
 of the falling sphere viscometer as standardised by Gibson and 
 Jacobs.? The instrument should be calibrated against a standard 
 material such as glycerine, and the result expressed either in absolute 
 C.G.8. units, or in a unit easily convertible into the scientific unit. 
 A modification of the falling sphere viscometer due to Mardles 
 can be used for opaque solutions in which a weight is attached to 
 a fine wire on the other end of which is a counterpoise.* The wire 
 is hung over a low-friction pulley and the weight is allowed to fall 
 in the solution. The rate of fall is then measured by the rate of 
 rise of the counterpoise. The B.E.S.A. specifications favour the 
 Ostwald viscometer. Their standardisation of the temperature 
 of 25° for viscosity determinations is to be commended. Although 
 this temperature is a little higher than that at which most solutions 
 are used, it is one that can be maintained in a thermostat in hot 
 weather without difficulty. 
 
 Analysis of Cellulose Acetate Varnishes. 
 
 There is no literature on the subject of the analysis of cellulose 
 acetate varnishes, probably because there has been very little need 
 
156 Cellulose Ester Varnishes 
 
 for such analyses up to the present. The writer suggests the 
 following scheme as a working basis :— 
 
 (a) Determination of solid content by filming-off a weighed 
 quantity of solution. The solid content consists of cellulose 
 acetate and the softeners, such as triphenyl phosphate, tri- 
 acetin, benzyl alcohol, acetanilide. Extraction of the film 
 with low-boiling non-solvents such as ethyl ether. Weight of 
 cellulose acetate remaining, and determination of acetyl group 
 if of interest. Resins are not found in cellulose acetate varnishes. 
 
 (b) Identification of the softener by precipitating a larger 
 quantity of the dope with ether, filtering off the precipitated 
 cellulose acetate and evaporating to approximately constant 
 weight. Test the residue for organic phosphate and for the 
 phenyl, glyceryl, benzyl, acetyl and amino-radicals. Quanti- 
 tative determination is likely to be difficult or impossible. 
 Estimations of phosphate and of nitrogen are most likely to 
 yield useful information. 
 
 (c) Dry distillation to 120° followed by steam distillation, 
 as in Conley’s scheme for nitrocellulose varnishes, should give 
 much information about the volatile solvents. Tetrachloro- 
 ethane is not likely to be found in a modern acetate varnish, 
 but is easily recognised by the smell. The solvents for which 
 a look-out should be kept are acetone, methyl ethyl ketone, 
 alcohol, benzol, methyl acetate. Fractionation of the dry 
 distillate and miscibility tests of the fractions would show the 
 presence of benzol, acetone and methyl ethyl ketone (note 
 smell and boiling point). Quantitative determinations are not- 
 likely to be of much use, but a saponification to determine ester 
 (methyl acetate) and an acetylation to determine free hydroxyl 
 (ethyl! alcohol and possibly methyl alcohol) may be worth while. 
 
 Addendum to Analysis. 
 
 In converting proportions by weight to proportions by volume, 
 or vice versa, it is necessary to know the specific gravity of the 
 solution. ‘There is no advantage to be gained by carrying out this 
 determination in a pyknometer with the precautions usual in 
 scientific work, and it is difficult to do so on account of the high 
 viscosity and volatility of the solution. A better way is to weigh 
 25 or 50 c.c. in a tared graduated cylinder. The accuracy obtained 
 in this way is much greater than the probable accuracy of any 
 determinations of individual solvents which may be attempted. 
 
 The cultivation of a sense of proportion is very necessary in 
 
Miscellaneous 157 
 
 technical analysis if high accuracy is unattainable, and will often 
 lead to much saving of time. The accuracy of an analysis is limited 
 by that of its least accurate step, and it is to the improvement of 
 the latter that care and attention should always be given. 
 
 Solvent Recovery. 
 
 In view of the fact that a large proportion of the weight of a 
 cellulose ester varnish consists of volatile liquids which evaporate 
 from the coating in the course of drying, there would appear to be 
 considerable scope for the application of processes of solvent 
 recovery. Actually, however, there are few industries using these 
 varnishes in which solvent recovery has been installed. Perhaps 
 the chief reason for this is that the consumption of cellulose ester 
 varnishes is spread over a large number of small users, and solvent 
 recovery is much more profitable for large quantities of solvent 
 than for small, on account of the high overhead charges on small 
 plants. Another factor is the considerable range of different solvents 
 employed, and the difficulty of ascertaining the exact composition 
 of a mixed recovered solvent—knowledge which is essential before 
 the solvent can be employed again for a similar purpose. However, 
 very great strides were made in the theory and practice of solvent 
 recovery during the war, and it is possible that there may be 
 considerable developments during the next few. years. 
 
 The processes of solvent recovery fall into three principal 
 classes :— 
 
 (1) Absorption of the solvent vapour in a suitable liquid or 
 combination of liquids. 7 
 
 (2) Adsorption of the solvent vapour on the surface of solids. 
 
 (3) Direct condensation of the vapours, using refrigeration. 
 
 The third method is only economical when the concentration of 
 the vapour is very high, or in certain processes where partial vacuum 
 is employed to hasten evaporation. Since the vapour coming from 
 work-rooms in which cellulose ester solutions are used is necessarily 
 dilute, this method need not be further considered. 
 
 The first method is the one which has been used to the greatest 
 extent in industry, and usually resolves itself into a search for a 
 suitable absorbent. When alcohol vapours alone have to be cap- 
 tured, as in the final stages of manufacture of certain soaps, water 
 is an excellent absorbent. In the Chardonnet process of artificial 
 silk manufacture, in which an ether—alcohol solution of nitro- 
 cellulose is formed into threads, large quantities of ether—alcohol 
 
158 Cellulose Ester Varnishes 
 
 are set free, and much of the earlier work was carried out in con- 
 nexion with this industry. Although water is an excellent absorbent 
 for alcohol, it is not so for ether, and the only absorbents which 
 were at all successful in capturing both solvents on a large scale 
 before the war were amy] alcohol, and, better, sulphuric acid. When 
 the manufacture of R.D.B. cordite, which used ether-alcohol in 
 place of acetone as the solvent, was undertaken on an enormous 
 scale in Great Britain during the war, the question of recovery 
 became pressing, and absorption in sulphuric acid was carried on 
 for some time at Gretna. A peculiarity of this process is that ethyl 
 alcohol reacts with sulphuric acid to form ethyl hydrogen sulphate, 
 which reacts reversibly with either alcohol or water to form ether 
 or alcohol respectively, according to the equations :— 
 
 (1) C,H,-H:SO, +- C,H;-OH =— C,H,-0-C,H; - H,SQ,. 
 (2) C,H,-H-SO, + H,O — C,H, OH + H,S0,. 
 
 Hence the proportions of ether and alcohol in the recovered solvent 
 altered according to the concentration conditions when the absorbent 
 was distilled. The equilibria in this process were studied by Masson 
 and McEwan.® The disadvantage of this process was the excessive 
 corrosion of plant brought about by the sulphuric acid. 
 
 The use of cresol as an absorbent was patented by Brégeat in 
 1917,1° and this substance was found to be an excellent absorbent 
 for both ether and alcohol. A large experimental plant working 
 on this system was erected at Gretna, and a complete plant for 
 dealing with the whole of the vapour from the cordite stoves was — 
 erected, but not used owing to the termination of the war. In 
 this plant the vapours were drawn by immense fans through Whessoe 
 scrubbers through which cresol was circulated in counter-current. 
 The saturated cresol was steam distilled to remove the alcohol and 
 ether, and was sent back, after being cooled, to the scrubbers. The 
 crude distillate was distilled over soda to remove cresol and frac- 
 tionated for the recovery of alcohol and ether. Some of the 
 information obtained at Gretna in the design and working of the 
 plants has been published in the Technical Records of the Ministry 
 of Munitions.14 
 
 Since cresol is miscible in all proportions with the common 
 cellulose ester solvents, it is probable that it would, under favourable 
 conditions, effect a good recovery of the mixed vapours with which 
 it would have to deal. According to Drinker,’ methyl, ethyl and 
 amyl alcohols and acetates, acetone, benzene, toluene, xylene and 
 chloroform can all be recovered in satisfactory yield, but it is not 
 stated how large a plant is necessary before a satisfactory return 
 
Miscellaneous 159 
 
 on the capital outlay is assured. The rapid fluctuations in the 
 prices of solvents since the war do not make the problem any 
 easier. 
 
 The adsorbents used in the second method are activated charcoal 
 and silica gel, the former, as its name implies, a specially prepared 
 carbon, and the latter a form of precipitated silica. War-time 
 experience with charcoal used in the box respirator as an absorbent 
 proved the efficacy of this substance as a means of removing vapours 
 of substances of high molecular weight from air, and its use for 
 solvent recovery is based on this absorbent power. Silica gel 
 possesses similar properties, and the rival claims of these two 
 absorbents are still under discussion. Neither appears to have been 
 used yet in this country for the recovery of solvents from varnishing 
 ‘operations, but each promises to be a serious rival to the processes 
 using liquid absorbents. 
 
 Scientific Applications. 
 
 Collodion Membranes.—The properties of collodion membranes 
 have interested scientific investigators ever since the middle of the 
 nineteenth century, chiefly on account of their restricted permeability. 
 There is a useful historical summary in Worden’s “‘ Nitrocellulose 
 Industry.” #2 The interest in this subject has increased rather 
 than diminished during the last few years, owing to the prominence 
 which the subjects of permeability and osmotic pressure have 
 assumed in biology, medicine and chemistry. The writer is not 
 qualified to give an extended account of this work, which would in 
 any case be out of place in this volume, but a short reference should 
 be made to it in so far as cellulose nitrate solutions are concerned. 
 
 Collodion membranes are made by allowing a solution of nitro- 
 cellulose in ether alcohol to evaporate, leaving a film. If, for example, 
 a test-tube is dipped into such a solution, withdrawn and rotated, 
 the solution rapidly thickens to a firm jelly and will in time dry up 
 to a tenacious film. Before that stage is reached, however, the 
 tube is dipped into water, which stiffens or “sets ” the film, which 
 can then be withdrawn from the outside of the tube as a thimble or 
 sac. Such membranes may be made, by methods to be mentioned 
 later, to vary considerably in permeability and can be used to filter 
 off substances which would pass through any ordinary filtering 
 medium such as filter-paper. To this process Bechhold applied the 
 word. “ ultra-filtration.”’ The collodion membrane may be made 
 to withstand somewhat high pressures. 
 
 Walpole has published a considerable amount of information on 
 this subject.14 The use of these membranes depends on the 
 
160 Cellulose Ester Varnishes 
 
 different porosity of the gel of the membrane to molecules of different 
 sizes. If, instead of using the membrane as a filter, the filtrate side 
 of the membrane is bathed in a solution other than the filtrate, the 
 process becomes dialysis, and Walpole finds many advantages in 
 dialysis under pressure, using collodion bags which will stand a 
 pressure of 2—3 atmospheres. Following earlier work by Martin 
 and Cherry, he found that the toxins of both tetanus and diphtheria 
 were held back by collodion membranes, the filtrates or dialysates 
 of the sera being free from them. 
 
 Walpole found that collodion membranes were more uniform 
 than parchment, and proposed to standardise them. The pro- 
 perties of these films depend on a large number of factors, beginning 
 with the origin of the cellulose and its treatment before nitration, 
 and ending with the pressure applied to the membrane when it is 
 in use. They can, however, be characterised by two factors :— 
 
 (1) The weight of dry nitrocellulose per square centimetre 
 of the film. 
 
 (2) The wetness of the film, which is defined as the ratio of 
 the wet weight of the film (after being soaked in distilled 
 water until it is saturated) to the weight of nitrocellulose which 
 it contains. 
 
 The wetness of the film is a measure of its porosity,.and it 
 diminishes as the time allowed for evaporation in the preparation 
 of the membrane increases. Thus the permeability of a film which 
 has dried up completely before it is detached from the glass support 
 is very low. 
 
 The weakness of this method of characterisation seems to the 
 writer to be that a film containing comparatively few large pores, 
 and another containing comparatively many small pores, might hold 
 the same amount of water, 7.e., be equally wet, and yet differ con- 
 siderably in permeability. Films made from the same solution of 
 nitrocellulose with different times of drying might give repro- 
 ducible results if standardised in this way, but it seems doubtful 
 whether the results could be repeated if the films were made from 
 an ether—alcohol solution of an entirely different sample of nitro- 
 cellulose. However, Walpole obtained very interesting results, 
 and gives the characteristic factors for films which would pass all 
 simple molecules, but retain quantitatively all antigens; he also 
 gives data on the permeability of the membranes to various enzymes. 
 
 Keeble 15 found that the hormone of the sensitive plant Mimosa 
 pudica, by which the excitation of the tissue is transmitted, will 
 diffuse across a film of collodion without losing its potency. 
 
Precautions Necessary 161 
 
 W. Brown !¢ has studied the technique of membrane preparation. 
 ‘The permeability of the film may be altered by soaking it in various 
 mixtures of water and alcohol, and Brown suggests an alcohol index, 
 denoting the strength of alcohol with which a film comes into 
 equilibrium. Membranes can be made that will hold back so 
 simple a molecule as copper sulphate, or that will pass so large a 
 molecule as aniline blue. He expresses the process of membrane 
 formation in a generalised form. If we have a “ restraining liquid ”’ 
 A which is not imbibed by the membrane, and a “ swelling liquid ”’ 
 B which is strongly imbibed by the membrane, then if A and B are 
 miscible in all proportions, it is possible to obtain grades of per- 
 meability by steeping the films in mixtures of A and B, subsequently 
 washing them in A. The higher the proportion of B in the steeping 
 liquid the greater the permeability obtained. In this way, Brown 
 has prepared membranes of cellulose nitrate, cellulose acetate, agar 
 and gelatin. } 
 
 | Farmer 1’ also found that the permeability varied according to 
 the time of drying, and Eggerth 18 that it depended on the proportion 
 of alcohol in the original collodion. Gans found that the per- 
 meability could be varied by the use of acetic acid. Looney *° 
 obtained more flexible membranes by using a proportion of ethyl 
 acetate in the collodion, but this increased flexibility would probably 
 not be permanent. 
 
 These researches are recalled by the work of Knoevenagel (q.v.) 
 on swelling, which is directly related to them. 
 
 Wegelin 2! used solutions of nitrocellulose in acetic acid instead 
 of ether—alcohol for the preparation of the membranes, and found. 
 that the film from a 7-5% solution was ten times as porous as a 
 film from a 15% solution. Experimental details are given. The 
 membranes were used for the dialytic purification of various well- 
 known inorganic colloids, and he suggests using them for the measure- 
 ment of the size of particles in colloidal solutions and for the purifica- 
 tion of precipitates not easily washed by ordinary methods. 
 
 S. L. Bigelow ?? in 1907 studied the permeability of collodion 
 membranes, gold-beaters’ skin and parchment paper, to pure liquids, 
 and concluded that the rate of passage of the liquids through them 
 was governed by the same laws as the flow of liquids through 
 capillary tubes. Duclaux and Errera 2? in 1924 found that the 
 velocity of filtration of water, aqueous solutions and certain organic 
 liquids through collodion membranes was approximately propor- 
 tional to the viscosity, and thus support Bigelow’s conclusions. 
 Bartell and Carpenter,?4 however, found examples of anomalous 
 gamed which they attributed to the influence of the electrical 
 
162 Cellulose Ester Varnishes 
 
 properties of the membrane system. It is of interest to note that 
 they prepared their films on the surface of mercury. Preuner 
 and Roder 25 also found abnormal osmosis, which they explained 
 by the existence of a potential difference between the walls of the 
 membrane, differing according to the concentration of the solutions. 
 
 Duclaux ** has patented the preparation of continuous lengths 
 of membranes for ultra-filtration by spreading a solution of a cellulose 
 ester on a moving strip of fabric and coagulating the film in a suitable 
 liquid. 
 
 Various papers have been published during the last few years 
 dealing with the preparation of membrane filters, and their employ- 
 ment in analytical operations, among which those of Zsigmondy and 
 Bachmann,?? Bachmann,?§ Jander,2® Kling and Lassieur,®® and 
 Bechhold and Gutlohn *! may be mentioned. 
 
 Miscellaneous Scientific Uses. 
 
 Several investigators have interested themselves in the properties 
 of extremely thin films of collodion, but the materials used in their 
 experiments are sometimes described somewhat vaguely. Wood 
 prepared collodion films of soap-bubble thickness by diluting 
 ‘ordinary collodion ’’ with about ten parts of ether, pouring the 
 mixture on a glass plate and draining it off immediately. If a 
 square is ruled on the glass with the point of a sharp knife, and the 
 plate is lowered into water, the square piece of film may be floated 
 off. It may be picked off the water by lowering on to it the well- 
 greased edge of a cylindrical support such as the bottom of a 
 cylindrical lamp glass. . 
 
 The same observer noticed *% that a thin collodion film deposited 
 on a bright silver surface shows brilliant colours in reflected light, 
 which, however, were not obtained if the commercial ether in the 
 collodion were replaced by pure ether. Similar films on glass did 
 not give the same effect. This phenomenon attracted the attention 
 of the late Lord Rayleigh.*4 
 
 Boys *° used a solution of celluloid in amyl acetate to abbade 
 permanent iridescent films. He quotes directions given by Glew. 
 Amy] acetate is boiled for a few minutes to remove all moisture 
 from it. It is then used to make up a solution of celluloid in the 
 proportion of 1 gramme in 14-6 c.c. A drop of this solution is 
 allowed to fall on the surface of clean cold water in a large basin, 
 and the amyl acetate is allowed to evaporate. The film may be 
 lifted from the water on a wire ring, and becomes more brilliant as 
 it dries in the air. Glew states that the films may be thrown into 
 
Precautions Necessary 163 
 
 nodes and loops by sounds of short wavelength; the effects may be 
 studied by reflecting sunlight from the film on to a sheet of white 
 card. Whistling or singing near the film then causes a charming 
 combination of motion and colour on the screen. 
 
 Barton and Hunt °° describe how extremely thin films of “ cellu- 
 loid ’’ were obtained at the U.S. Bureau of Standards. One gramme 
 of celluloid was dissolved in 400 grammes of amyl acetate, and a 
 drop was placed on the surface of clean water and allowed to 
 evaporate. ‘The film was transferred to glass and its thickness was 
 found to be 30 Angstrém units (3 x 10-7 cm.). Films obtained 
 similarly from a solution of 1 gramme in 800 grammes of amyl 
 acetate would just hold together, but films from a solution of 1 
 gramme in 1200 grammes of amyl acetate broke up as the amyl 
 acetate evaporated. If the thickness of the films decreased in the 
 same ratio as the dilution of the solution, the thickness of the films 
 just too thin to hold together would be 10 Angstrém units, and this 
 agrees with the thickness calculated from the density of celluloid, 
 the concentration of the solution, and the area of the film. Hence 
 the molecular complex of celluloid is not more than 10 Angstrom 
 units (= 10°’ cm.) in diameter.* This agrees with results of similar 
 experiments on films of oil and other organic substances. 
 
 From the description of the material as “ celluloid,’ the authors 
 probably employed either ordinary celluloid or cinema film in these 
 experiments, 2.e., a complex of cellulose nitrate and camphor. The 
 final film, however, would probably be cellulose nitrate alone, since 
 camphor is sufficiently soluble in water to be completely extracted 
 from such a film in a very short time. It is probable that measure- 
 ments of the thickness of such films would yield valuable results if 
 they were correlated with the viscosity of the nitrocellulose, which is 
 believed to be a function of the complexity of its molecular structure. 
 The problem of deciding how the film is built up from the various 
 sizes of particles believed to be present in these solutions is, however, 
 much more difficult. 
 
 Experiments by Laird 3’ throw some doubt on the validity of 
 the calculations by which Barton and Hunt checked the thickness 
 of the films. She measured the thickness of very thin films by an 
 interferometer method, and assuming that the optical thickness is 
 the true thickness, found that the density of the celluloid remained 
 
 -approximately constant at 1-41 down to a thickness of 400 uu(= 4 x 
 10° cm.). Below this thickness, the density begins to increase. In 
 the neighbourhood of 60 uu (6 x 10°%cm.) it is about 2 and at 30 wu 
 (3 x 10cm.) it is about 2-5. 
 
 BEG ye 28; 
 
164 Cellulose Ester Varnishes 
 
 Chromatic Emulsions. 
 
 In experimenting on the production of transparent emulsions, 
 by emulsifying two immiscible liquids of equal refractive index, 
 Holmes and Cameron °° succeeded in obtaining some very striking 
 chromatic emulsions using a solution of cellulose nitrate in amyl 
 acetate as the emulsifying agent. Their directions for a lecture 
 experiment as are follows: 4 volumes of glycerol and 4 volumes 
 of a 2 to 4% solution of cellulose nitrate in amyl acetate are shaken 
 together to form an emulsion. To this is added 5—10 volumes of 
 benzene (toluene may be substituted), then more glycerol until the 
 emulsion is somewhat viscous, and finally benzene again in small 
 quantities until the colours appear. The order in which the pairs 
 of colours appear with the addition of benzene is yellow and blue, 
 pink and green, peacock-blue and pale yellow. If more benzene is 
 added, the colours disappear and the emulsion becomes milky, but 
 the addition of amyl acetate will bring them back in the reverse 
 order. Chromatic emulsions have been known for many years, 
 and can be produced, for example by emulsifying oil of turpentine 
 and glycerol. Holmes and Cameron’s emulsions, however, are more 
 beautiful and permanent. If the emulsions are allowed to stand, 
 a cream separates which shows the colours very clearly, and this 
 can be separated from the lower layer. A quantity of this emulsion 
 made by the writer nearly two years ago has retained its chromatic 
 properties without change, except that the cream is now a semi- 
 solid gel showing brilliant pink and green interference colours, 
 altering with change of temperature. 
 
 The theory of chromatic emulsions is discussed by Holmes and 
 Cameron (loc. cit.). They are due to the presence of two phases 
 nearly equal in refractive index but differing considerably in optical 
 dispersive power. : 
 
 Ne 
 
 Transport of Cellulose Ester Solutions on British Railways.* 
 
 Cellulose ester varnishes are classified by the British railways 
 under the general heading of Inflammable Liquids, Class A, and 
 the regulations controlling the traffic are printed in the “ General 
 Railway Classification of Goods” published by the Railway Clearing 
 House. It is possible to find variations in the classification, but 
 the policy of the railway companies, after having safeguarded the 
 interests of passengers and of the owners of other merchandise in 
 transit, is to meet the requests of traders as far as they can. The 
 
 * Mr. J. H. B. Jenkins, Chief Chemist of the London and North-Eastern 
 
 Railway, has kindly read this section and checked the accuracy of the facts ; 
 for any comments or opinions the author alone is responsible. 
 
Precautions Necessary 165 
 
 variations, therefore, arise from the fact that different industries 
 have asked for different facilities, which have sometimes been 
 granted. The writer is glad to testify to the sympathetic hearing 
 which manufacturers are given when they appear before a combined 
 meeting of the chief chemists of the British railways. 
 
 The page references in the paragraphs which follow are to the 
 current issue of the General Railways Classification of Goods, 
 dated Ist April, 1924. 
 
 ‘The varnishes appear in the classification under some unfamiliar 
 names, derived sometimes from the trade names of the products 
 made by the manufacturers who carried on the pioneer negotiations 
 with the railway companies. Thus the cellulose nitrate varnishes 
 appear as N.E. xylonite solutions (pp. 355 and 356), collodion 
 (pp. 357 and 359), xylonite solution (p. 357), nitrocellulose solution 
 (pp. 358 and 359), the initials N.E. signifying that the solution 
 contains no ether. Solvents supplied for the dilution of the varnishes 
 are described as N.E. xylonite thinnings, xylonite thinnings and 
 collodion thinners. Aeroplane varnish or dope appears on p. 357, 
 no distinction being made between nitrate and acetate dopes in 
 view of the fact that the solvents in each constitute a similar fire 
 danger. | 
 
 The distinguishing feature of the inflammable liquids in Class A 
 is that their flash point is below 73° F., the determination of the 
 flash point being carried out in accordance with the directions of 
 the first Schedule to the Petroleum Act of 1879. The instructions 
 are reproduced on pp. 538-541. 
 
 The regulations may be summarised as follows: Cellulose esters 
 under Class A were for a period subdivided (in effect) into those 
 which contained ether and those which did not. The high vapour 
 pressure of ether was regarded by the railway companies as justi- 
 fying an extra precaution, which took the form of limiting the 
 capacity of the containers, in which the solution might travel, to 
 10 gallons. Solutions not containing ether (N.E.) were allowed to 
 travel in approved drums or barrels of anything up to 100 gallons 
 capacity. This distinction has now been effaced by the entry on 
 p. 359, allowing collodion also to travel in approved drums or barrels 
 up to 100 gallons capacity. Since all solutions containing ether are 
 usually grouped under the term collodion, the effect of this entry 
 is in practice to put solutions which contain ether on the same basis 
 as those which do not. Collodion thinners, however, p. 358, are 
 still subject to the 10-gallon limit. ‘ 
 
 The specification for drums of 20 to 100 gallons capacity for this 
 traffic is given on pages 527-528, and may be summarised thus : 
 
166 Cellulose Ester Varnishes 
 
 They must be made of best soft iron or mild steel, with joints either 
 riveted, soldered or autogenously welded; strengthened at each end 
 by a strong iron or steel hoop shrunk on to the body, and provided 
 with two rolling hoops welded or shrunk on. An alternative to 
 the rolling hoops is the provision of two corrugations in the body 
 protected by cover strips to specification. Barrels must have a 
 bilge proportional to their length, the 33-inch barrel requiring a 
 21-inch bilge; they must also have end hoops, but need not have 
 rolling hoops. The minimum thickness of the metal varies according 
 to the size of the barrel according to the following table :— 
 
 Capacity. Body. Ends. 
 20 galls 14 B.G 16 B.G 
 25—35_ ,, i ae 15 ,, 
 36—65_ ,, 10 ,, 
 
 it hs 
 12 (barrels) 
 66—100 ,, 9 5, 10 (drums) 
 
 Each drum or barrel must have a well-fitting screw bung, which 
 must not project above the rolling hoops when screwed in. The 
 boss must be autogenously welded or riveted and soldered to the 
 body or end of the barrel. The drums must stand a hydraulic or 
 air test of 10 lb. without leaking; this test to be repeated annually 
 and whenever they show signs of damage. They must also be 
 repainted or regalvanised as required to protect them from rust. 
 The last two provisions are important—an air space of 5% must 
 be allowed in filling and the bungs of empty barrels and drums must 
 be securely screwed in before they are returned. The provision of 
 the air space is to prevent bursting of full drums by rise of tem- 
 perature, and the regulation about bunging the empties guards against 
 the danger of accidental ignition of inflammable vapours remaining 
 in the drum. 
 
 The specification for smaller drums, p. 524, of 1 to 15 gallons 
 capacity is on similar lines, the differences being as follows: Streng- 
 thening hoops at each end are only required if the drum exceeds 
 12 gallons in capacity. The minimum thickness is 20 B.G. The 
 screw bung and boss must be faced, and a washer provided, to 
 ensure a good joint. (The bungs on these small drums are frequently 
 wider than on the larger drums and barrels.) Drums for the export 
 trade may have a bung or shive driven into a plain cylindrical neck, 
 and covered with a soldered metal cap. In this instance the neck 
 must be not more than 1} inches in diameter, and the top must be 
 from 4 inch to } inch below the level of the chime. 
 
Precautions Necessary 167 
 
 The containers smaller than 1 gallon are used chiefly in the 
 retail trade, and for a description of these reference must be made 
 to the original classification. The chief safeguard consists in packing 
 them in cases with sawdust. 
 
 Aeroplane dopes or varnish may travel in drums or taper-neck 
 cans of 10 gallons maximum capacity (p. 529), and otherwise in 
 wide-mouthed drums and taper-neck cans of 5-gallon and 10-gallon 
 capacity (p. 533). 
 
 The thickness of metal is given in the following table :— 
 
 5 gall. — 10 gall. 
 NRE A UAE 8 sje os 5 nin d's ve ss'tecvisvcgss 22 B.G. 20 B.G. 
 Body, shoulder and neck of cans...... = aa 
 es 0 20 B.G. 
 
 BPG COTE OEICAIS 0 ogi ss daceeedeuse os ver. 
 The ends of drums and the bottoms of cans must be stamped 
 out with a flange of 3 inch to 1 inch, and placed on the body with 
 the flange outward. These ends must either be swaged and welded, 
 or welded and hooped as directed. 
 
 Two methods of closing the drums are allowed: (1) A cast-iron 
 plug not exceeding 24 inches in diameter screwed with a leather 
 washer into a flanged mild steel collar welded to the head of the 
 drum at one side; (2) a well-fitting cork bung in a smooth neck of 
 4 inches diameter, with a metal capsule reeled over the wired edge 
 of the neck. 
 
 Drums must be provided with handles welded, or riveted and 
 welded, to the heads, but the handles and the screw bungs must 
 not project beyond the end of the drums. 
 
 The taper-neck cans must have the shoulders autogenously 
 welded to the cylindrical body, and the necks similarly attached 
 to the shoulders. A 3-inch steel or brass ring must be welded or 
 soldered respectively to the neck after galvanising. The ring is 
 screwed to receive a screwed cast-iron plug with a leather washer. 
 Two wrought-iron handles are welded on, one on each side of the 
 shoulder. 
 
 A method is described for sealing the drums by means of a 
 wire passing through the bung and the rim of the drum. Alternative 
 arrangements are sanctioned. 
 
 The principal special regulations for the traffic are that each 
 package must bear a conspicuous label “‘ Highly Inflammable ”’ ; 
 drums must be painted red at each end with the words “ Highly 
 Inflammable ”’ in white letters; loading must be performed in day- 
 
168 Cellulose. Ester Varnishes 
 
 light, and the traffic must not be stored in any of the railway 
 companies’ enclosed sheds and warehouses. A special form of 
 consignment note is necessary. 
 
 In regard to these regulations in general, it may be said that 
 they are consistent with the principle mentioned in an earlier 
 chapter of this book, that the danger of cellulose ester solutions is 
 a function only of the inflammable solvents which they contain, 
 and not of the solid contents. On the surface they appear somewhat 
 onerous, but prospective users of such solutions should note that 
 whatever burden they may place on the industry is borne almost 
 entirely by the supplier and not by the user, except in so far as special 
 regulations for the traffic must add something to the cost of the 
 commodity. All that the regulations call on the consumer to do 
 is to employ the special consignment note when returning empty 
 containers to the supplier, and to replace the screw bungs securely. 
 
 REFERENCES. 
 
 _ Precautions necessary in using Cellulose Hster Varnishes.—! Annual 
 Report of Chief Inspector of Factories, 1917 [Cd. 9108], p. 18 (Report on 
 Doping in Aircraft Works by W. H. Smith, with advice on ventilation). 
 2 EK. Hausbrand, ‘‘ Drying by means of Air and Steam ”’ (Scott Greenwood). 
 %,. J. Glaisher, Hygrometrical Tables (Taylor and Francis). 
 
 Analysis of Cellulose Ester Varnishes.—* J. R. Lorenz, J. Amer. Leather 
 Chem. Assoc., 1919, 14, 548-556. 5 A. D. Conley, J. Ind. Eng. Chem., 1915, 
 7, 882. ® H. Dubovitz, Chem. Zeit., 1906, 30, 936-937. 7 W. H. Gibson 
 and L. M. Jacobs, Trans. Chem. Soc., 1920, 117, 473. ® E. W. J. Mardles,- 
 Trans. Faraday Soc., 1923, 18, 337. 
 
 Solvent Recovery.—® J. I. O. Masson and T. L. McEwan, J. Soc. Chem. 
 Ind., 1921, 40, 32-387. 1° J. Brégeat, B.P. 128,640 (1917). 41 Technical 
 Records of Explosives Supply No. 8, ‘‘ Solvent Recovery.” 1% P. Drinker, 
 J. Ind. Eng. Chem., 1921, 18, 831-835. 
 
 See also C. S. Robinson, ‘‘ Recovery of Volatile Solvents’? (Chemical 
 Catalog Co., 1922). M. Deschiens, ‘‘ Procédés industriels de récuperation 
 des dissolvants volatils,’’ Rev. des Prod. Chim., 15/5/1920. M. Ponchon, 
 Chim. et Ind., 1918, 1, 481-491 (‘‘ Theory of Refrigeration Methods ’’). 
 E. C. Worden, ‘‘ Technology of Cellulose Esters,” Vol. I., Pt. IV (List of 
 patents). W. D. Milne, Chem. Age (New York), 1923, 31, 201-205 (Deals 
 with fire dangers in solvent recovery, with some description of various pro- 
 cesses). J. H. Wild, India-Rubber Journal, 1923, 65, 313-322 (primarily an 
 account of solvent recovery in the rubber industry). P. Brasseur, Fed 
 Ind, Chim. Belg., 1922, 1, 155-167, 315-327. A. Djeinem, ‘‘ Le Caoutchouc et 
 La Gutta-Percha,’’ 1919, 16, 9980-9984. H. Carstens, Z. angew. Chem., 1921,. 
 34, 389-392 (an account of the Bayer Co.’s process of solvent recovery by 
 activated charcoal). E. C. Williams, J. Soc. Chem. Ind., 1924, 48, 97-1127 
 (‘‘ Benzol Recovery by Adsorption by Silica-gel ”’). 
 
 Scientific Applications.—'* E. C. Worden, ‘‘ Nitrocellulose Industry,” 
 Vol. I., p. 812. 14 G. 8. Walpole, Biochem. J., 1915, 9, 284-308. 15 FP. 
 Keeble, Nature, 1924, 114, 56. 1° W. Brown, ibid., 1915, 8, 591-617; 1915, 
 9, 320; 1917, 11, 40-57. 17 C. J. Farmer, J. Biol. Chem., 1917, 32, 447-— 
 453. 18 A. H. Eggerth, ébid., 1921, 48, 203-221. 1% R. Gans, Ann. Physik, 
 1920, [iv], 62, 327-330. #9 J. M. Looney, J. Biol. Chem., 1922, 50, 1-21, 
 21 G. Wegelin, Koll.-Z., 1918, 18, 225-239. 2 §. L. Bigelow, J. Amer. 
 Chem. Soc., 1907, 29, 1675-1692. 3 J. Duclaux and J. Errera, Rev. gén, 
 Collotdes, 1924, 2, 130-139 (see J. Soc. Chem. Ind., 1924, 43, B, 815). % F. E. 
 
 ght aati 
 
Precautions Necessary 169 
 
 Bartell and D. C. Carpenter, J. Phys. Chem., 1923, 27, 101-116. 25 G. 
 Preuner and O. Roder, Z. Elektrochem., 1923, 29, 54-64. 2° J. Duclaux, 
 B.P. 203,714/1922. 27 R. Zsigmondy and W. Bachmann, Z. angew. Chem., 
 1918, 11, 413; J. pr. Chem., 1918, 108, 119-128. #8 W. Bachmann, Koll.-Z., 
 1920, 37, 138; Z. angew. Chem., 1919, 32, 616. °° G. Jander, Z. angew. 
 Chem., 1922, 35, 721; <dbid., 1923, 36, 505. °° A. Kling and A. Lassieur, 
 Chim. et Ind., 1923, 10, 42. °! H. Bechhold and L. Gutlohn, Z. angew. 
 Chem., 1924, 37, 494-497. 
 
 See also C. F. Tinker, Trans. Faraday Soc., 1917, 37, 133-140. Wo. 
 Ostwald, ibid., 1921, 16, 89-93. 
 
 Miscellaneous Scientific Uses.—** R. W. Wood, Physical Optics (1911), 
 p. 97. 33 Idem, ibid. (1914), p. 172. 34 Lord Rayleigh, Phil. Mag., 1917, 
 34, 423-428. %5 C. V. Boys, “‘ Soap Bubbles ”’ (S.P.C.K.), 117-120. °° V. P. 
 Barton and F. L. Hunt; ’Noture, 1924,,114, 861.°:%7 Elizabeth, R. Laird, 
 Physical Review, 1922, 19; &84.; 738 °H:.N. Beimes ‘artd?)/ H.:Camieron, :/. 
 Amer. Chem. Soc., 1922, 44, *73574, 2 233 230235 7% 0553 7252 2 8 3 ae 8 Sao 
 
 » 28 ra ,990 ©>. 9 ae 
 ) eo % "3 5 ) 1 9 
 oe 
 

 
INDEX OF NAMES 
 
 ABEL, F., 33, 44 
 
 Archbutt, oe ap Deeley, R. M., 112 
 
 Archer, F, = 
 
 Aston, FE, W., pa 
 
 Ayres, A. E., 119 
 
 Bachmann, W., 162. See also Zsig- 
 mondy. 
 
 Baker, F., 17, 69, 73 
 
 Balls, W. L., 26, 66. 
 
 Bancroft, W. D., 17 
 
 Barnett, W. L., 55, 67 
 
 Barr, G., 56 
 
 Barr, G., and Bircumshaw, L. L., 55, 77 
 
 Bartell, F. E., and Carpenter, D. C., 161 
 
 Barthélemy, i. 45, 55 
 
 Barton, V. P., and Hunt, F. L., 163 
 
 Bayer, F. & Co., 20, 48, 50 et seq. 
 
 _Bechhold, H., and Gutlohn, L., 162 
 
 Becker, E. See Schwalbe, C. G. 
 
 Bell, M. See Hake, C. N. 
 
 Berg, van den, J. C. See Boeseken, J. 
 
 Bergmann, E., and Junk, A., 44 
 
 Berry, A. J. See Fenton, H. J. H. 
 
 B.E.8.A. See British Engineering 
 Standards Association. 
 
 Bevan, E. J. See Cross, C. F. 
 
 Bigelow, J. See Maynard, F. 
 
 Bigelow, S. L., 161 
 
 Bingham, E. C., 59, 60, 79, 92 
 
 Bingham, FE. fi, and Hyden, W. S., 80 
 
 Bireumshaw, L. L. See Barr, G. 
 
 Birtwell, C., Clibbens, D. A., and Ridge, 
 B. P., 45 
 
 Boeseken, J., van den Berg, J. C., and 
 Kerstjens, A. H., 27, 66, 91 
 
 Bolsing, Fr. See Verley, A. 
 
 Bottger, R., 14 
 
 Boys, C. V., 162 
 
 Brasseur, P., 168 
 
 Brégeat, J., 158 
 
 Bregenzer, A. See Knoevenagel, E. 
 
 Breguet, A. See Meunier, L. 
 
 Briggs, J. F., 48 
 
 British Engineering Standards Associa- 
 tion, vii, 43, 44, 45, 53, 55, 56, 60, 99, 
 101, 102, 104, 106, 107, 109, 110, 
 111, 112, 122, 155 
 
 British Pharmacopeceia, 141 
 
 Brown, W., 161 
 
 Bruin, de, G., 45 
 
 Caille, A., 23 
 
 Cameron, D. H. Sac Holmes, H. N. 
 
 Carpenter, D.C. See Bartell, F. E. 
 
 Carstens, H., 168 
 
 Cellonite Co., 48 
 
 Celluloid Co., 18 
 
 Clément, L., and Riviére, C., 23, 99, 119, 
 120, 128, 137, 139 
 
 Clibbens, D. A., and Geake, A., 45 
 
 Clibbens, D. A. See also Birtwell. 
 
 Condé, G. E., 23, 134, 142 
 
 Conley, A. D., 153 
 
 Crane Chemical Co., 18 
 
 Crane, F., 17 
 
 Crockett, H. G., 136 
 
 Cross, C. F., 26, 27, 57 
 
 Cross and Bevan, 19, 29 
 
 Cross and Dorée, 29 
 
 Crum, W., 15 
 
 Custom and Excise, Board of, 42 
 
 De Waele, A., 81 
 
 Deeley, R. M. See Archbutt, L., 
 
 Denham, W. S., and Woodhouse, H., 25 
 
 Departmental Committee on Heat Test, 
 44 
 
 Deschiens, M., 136, 168 
 
 Djeinem, A., 168 
 
 Doerflinger, W. F., 108 
 
 Domonte, F., and Menard, 15 
 
 Donnan, F. G., 91 
 
 Dorée, C., 26. See also Cross, C. F. 
 
 Dreyfus, Bros., 48 
 
 Dreyfus, H., 20, 48, 51, 52 et seq. 
 
 Drinker, P., 23, 48, 121, 158 
 
 Dubovitz, H., 154 
 
 Duclaux, J., 162 
 
 Duclaux, J., and Errera, J., 161 
 
 Duclaux, J., and Wollmann, E., 27, 66, 
 Tl, 72, 91 
 
 Eberstadt, O., 54. 
 
 venagel, E. 
 Eggerth, A. H., 161 
 Eichengrin, A., 20 
 Entat, M., and Vulquin, E., 55 
 Errera, J. See Duclaux, J 
 Esselen, G. J., Jnr., 88, 114 
 Evans, H. G., 115 
 
 See also Knoe- 
 
 Factories, Chief Inspector of, 115, 143 
 
 Farmer, C. J., 161 
 
 Fenton, H. J. H., and Berry, A. J., 
 54, 88 
 
 Field, W. D., 18 
 
 Finkener, 112 
 
 Fischer, E., 23 
 
 Flaherty, E. M., 133, 134 
 
 Franchimont, A., 19 
 
 Gans, R., 161 
 
 Gardner, H. A., 130 
 
 Gardner, H. A., and Parkes, H. C., 97 
 
 Geake, A. See Clibbens, D. A. 
 
 Gibson, W. H., and Jacobs, S. M., 60, 155 
 
 Gibson, W. H., and McCall, R., 74, 75 
 
 Gibson, W. H., Spencer, L., and McCall 
 R., 36 
 
 Girard, A., 19 
 
 Gladstone, J. H., 15 
 
 Gmelin, L., 14 
 
 Goldsmith, J. N., 83 
 
 Graham, T., 58 
 
 Green, A. G., 24 
 
 Gutlohn, L. See Bechhold, H., 
 
 Hadow, E., 15 
 
 Hake, C. N,, and Bell, M., 45 
 Hall, A. J., 24 
 
 Harrison, W., 26, 27 
 
 171 
 
172 
 
 Hartig, 15 
 
 Hatschek, E., 83 
 
 Hatschek, E., and Humphry, R. H., 
 83 
 
 Hausbrand, E., 168 
 
 Haworth, W. N., and Hirst, E. L., 25 
 
 Haworth, W.N., and Leitch, G. C., 25 
 
 Herrmann, K. L., and Radel, F. J., 137 
 
 Herzog, R. O., and Jancke, W., 27 
 
 Higgins, C. A. See Small, J. O. 
 
 Highfield, A., 74 
 
 Hirst, E. ‘ See Irvine, J. C., and 
 Haworth, W.N. 
 
 Hogrefe, J. See Knoevenagel, E. 
 
 Holmes, H. N., and Cameron, D. H., 164 
 
 Humphry, R. H. See Hatschek, E. 
 
 Hunt, F. L. See Barton, V. P. 
 
 Hunt, R., 15 
 
 Hutchison, A. M., 15 
 
 Hyden, W.8. Sce Bingham, BE. C. 
 
 Irvine, J. C., and Hirst, E. L., 25, 26, 
 27, 67 
 
 Jacobs, 8. M. See Gibson, W. H. 
 Jancke, W. See Herzog, R. O. 
 Jander, G., 162 
 
 Joyner, R. ‘fh. 45 
 
 Junk, A. See Bergmann, E. 
 
 Kerstjens, A. H. See Béeseken, J. 
 
 Kirkpatrick, S. D., 40, 118, 134 
 
 Kling, A., and Lassieur, A., 162 
 
 Knoevenagel, E., 84, 91, 161 
 
 Knoevenagel, E., and Bregenzer, A 5 92 
 
 Knoevenagel, E., and Eberstadt, O., 54, 
 92 
 
 Knoevenagel, E., Hogrefe, J., and 
 Mertens, F.., 92 
 
 Knoevenagel, E,, and Motz, R., 92 
 
 Knoevenagel, E., and Volz, E., 92 
 
 Knop, W., 14 
 
 Kugelmass, I. N., 76 
 
 Laird, E., 163 
 
 Lassieur, A. See Kling, A. 
 Laue, von, 27 
 
 Lederer, L., 19, 52 
 
 Legge, T. "See Willcox, W. H. 
 Leitch, G. C. See Haworth, W. N. 
 Lewkowitsch, J., 112 
 Leysieffer, a. 29 
 
 Looney, J. M., 161 
 
 Lorenz, J. R., 151 
 
 Lucas, A., 138 
 
 MacDonald, G. W., 14 
 
 MacNab, W., 45 
 
 Mardles, E. W. J., 62, 78, 82, 87, 88, 91 
 
 Marshall, A., 45, 101 
 
 Masson, J. I. O., 75 
 
 Masson, J. I. O., and McCall, R., 58, 72 
 
 Masson, J. I. O., and McEwan, T. L., 
 158 
 
 Maxwell, C., 59 
 
 Maynard, Jui and Bigelow, S., 15 
 
 Index of Names 
 
 McCall, R. See Gibson, W. H., and 
 Masson, J.I. O. 
 
 Menard. See Domonte, F. 
 
 Mertens, F. See Knoevenagel, E. 
 
 Meunier, L., and Breguet, A., 83 
 
 Miles, G. W.., 20, 47, 50, 51 
 
 Milne, W. D.; 168 
 
 Ministry of Munitions, Technical Records 
 of Explosives Supply, 31, 158 
 
 Mork, H. §., 52 
 
 Morrell, R. S., 115 
 
 Mosenthal, de, H., 68 
 
 Motz, R. See Knoevenagel, E. 
 
 Mougey, H. C., 132 
 
 Napper, S., 27 
 Nathan, F., 37 
 Nobel, A., 18 
 
 Ost, H., 51 et seq., 54 
 Ostwald, W., 43, 60 
 Ostwald, Wo., 169 
 Otto, F., 14 
 
 Parkes, A., 15, 16, 18 
 
 Parkes, H.C. See Gardner, H. A. 
 
 Parry, E. J., 115 
 
 Pellen, M., 16 
 
 Pelouze, J., 14, 15 
 
 Perkin, W. H., Jnr., 22 
 
 Pierce, F. T., 26 
 
 Piest, C., 45 
 
 Piest, C., Stich, E., and Vieweg, W., 
 35 
 
 Poiseuille, 60 
 
 Ponchon, M., 168 
 
 Porter, A. W., 83 
 
 Preuner, G., and Roder, O., 162 
 Punter, R. A., 34 
 
 Radel, F. J. See Herrmann, K, L. 
 Ramsbottom, J. E., 23, 120 
 Rassow, B., 29 
 
 Rhéne. See Usines du Rhéne. 
 Ridge, B. P. See Birtwell, C. 
 Riviere, C. See Clément, L. 
 Robertson, R., 23, 41 
 
 Robertson, R., and Smart, B. J., 44 
 Robinson, C. S., 168 
 
 Roder, O. See Preuner, G. 
 
 Saillard, B., 17 
 
 Sapojenikov, M. A. W., 45 
 Schonbein, OC. F., 14, 15 
 
 Schrimpf, A. See Schwalbe, C. G. 
 Schultze-Tiemann, 44 
 Schiipphaus, R., 17 
 Schiitzenberger, P., 19 
 
 . Schwalbe, C. G., 23 
 
 Schwalbe, C. G., and Becker, E., 26 
 Schwalbe, 0. G., and Schrimpf, Ag 29 
 Schwarz, H., 35, 70 
 
 Segundo, de, E. P., 45 
 
 Sheppard, S. E., 60 
 
 Small, J. O., and Higgins, C. A., 45. 
 Smart, B. a; See Robertson, R. 
 
Index of Names 173 
 
 Smith, W. H., 115, 168 
 Spencer, L. See Gibson, W. H. 
 Spiers, C. H., 110 
 
 Spill, D., 16 
 
 Spilsbury, B. See Willcox, W. H. 
 
 Stevens, J. H., 17, 18 
 Stich, E. See Piest, C. 
 
 Thornley, T., 45 
 
 Tinker, C. F., 169 
 
 Tucker, W. K., 135 
 Tunison, B. R., 101, 105, 107 
 
 Ullmann, F., 23 
 United States Pharmacopoeia, 141 
 
 Usines du Rhéne, 20, 48, 49 et seq. 
 
 Verley, A., and Bolsing, Fr., 110 
 Vieweg, W. See Piest, C. 
 Visser, C., 71 — 
 
 Volz, KE. See Knoevenagel, E. 
 Vulquin, E. See Entat, M. 
 
 Waele, de, A., 81 
 
 Walpole, G. 8., 159, 160 
 
 Wegelin, B., 161 
 
 Wiesel, J. C., 131 
 
 Wild, J. H., 168 
 
 Willcox, W. H., 115 
 
 be ee W.H., Spilsbury, B., and Legge, 
 
 -» 115 
 
 Williams, E. C., 168 
 
 Wollmann, E. See Duclaux, J. 
 
 Wood, R. W., 162 
 
 Woodhouse, H. See Denham, W. 8S. 
 
 Worden, E. C., vii, 23, 45, 49, 50, 52, 
 68, 90, 115, 159, 168 
 
 Zsigmondy, R., and Bachmann, W., 169 
 Zihl, E., Lil 
 
INDEX OF SUBJECTS 
 
 ACCUMULATOR cases, 22 
 Acetic acid, 15, 16 
 Acetic anhydride, 19, 46 
 Acetins, 17 
 Acetoethyl-o-toluidide, 69 
 Acetone, 15, 18, 21, 27, 37, 69, 71 
 British Government Specification, 100 
 manufacture, 101 
 viscosity determination in, 29 
 Acetone oil, 17 
 specification of, 107 
 Acetylene tetrachloride. 
 chloroethane. 
 Acetylation process, 48 et seq. 
 plant for, 53 
 Acid balance, 31 
 Acid ratio in nitration, 32 
 Acid revivification, 31 
 Aeroplane dopes, 16, 20, 48, 94 
 allied practice, 121, 127 et seq. 
 adhesion test, 123 
 British practice, 122 
 brittleness, 123 
 doping schemes, 122 
 elasticity, 123 
 formule, discussion of, 124 et seq. 
 German practice, 127 
 inflammability, 122, 123 
 manufacture, 119 et seq. 
 for night bombers, 128 
 protective, 128 
 rate of burning, determination, 123 
 softness, 121 
 tautening test, 123 
 transport, 165 et seq. 
 Aeroplane fabrics 
 action of sunlight on, 128 et seq. 
 enumeration of, 121 
 specific strength of, 120 
 strength after doping, 120 et seq. 
 Air, moisture in, 144 et seq. 
 Alabaster, preservation of, 139 
 Alcohol. See Ethyl alcohol. 
 Alum, 16 
 Aluminium mediums, 132 et seq. 
 Aluminium paints, 138 
 Amber, preservation of, 194 
 Ammonium phosphate, 16 
 Amy] acetate, 17, 18, 69 
 determination in varnish, 161 
 properties, 17, 106 
 specification, 107 
 Amy] alcohol, 17, 18 
 specification for, 110 
 Aniline, 16 
 Antiques, preservation of, 138 et seq. 
 
 See Tetra- 
 
 Barrels, 166 
 approved for transport, 166 
 labelling, 167 
 Bathroom fittings, 138 
 Benzene, 17 
 as diluent in acetylation, 52 
 specification for, 109 
 Benzine. See Petroleum spirit. 
 Benzol. See Benzene. 
 
 174 
 
 Benzyl alcohol, specification, 111 
 Blooming, 144 et seq. 
 prevention of, 148 et seq. 
 Blushing. See Blooming. 
 Bronze mediums, 16, 132 et seq. 
 paints, 138 
 powder, action of, on nitrocellulose, 
 
 Brushing lacquers, 129 
 
 Butyl acetate, 18 
 specification for, 106 
 
 Butyl alcohol, 18, 21 
 manufacture of, 101 
 specification for, 110 
 
 Cadmium iodide, 16 
 Calcium oxalate, 16 
 Camphor, 16 
 and cellulose nitrate, 15 
 in varnishes, significance of, 152 
 Camphor-alcohol solvent, 37 
 Carbon tetrachloride, 52 
 Carbonyl group and solvent power, 69 
 Cardboard varnishes, 138 
 Castor oil, in varnishes, 130 
 specification for, 112 
 Cellite L., 20 
 Cellobiose, 25, 27 
 Celloidin, 439 
 Celluloid, cements, 22 
 gilding, 140 
 Cement floors, primers, 139 
 Cellulose, 24 et seq. 
 acetolysis of, 25 
 acetylation of, 19 et seq. 
 constitution of, 25 et seq. 
 constitution of, Rontgen ray, 27 
 depolymerisation of, 28 
 formula of, 24-26 
 formula, polymerisation of, 26 
 hydrolysis of, 25 
 methylation of, 25 
 molecule, 27 
 molecule, dimensions of, 28 
 mucilage, 27 
 nitration of, 14, 30 et seq. 
 nitration of, acid bath, 32 
 nitration of, conditions affecting solu- 
 bility, 32 
 nitration of, control of, 33 
 nitration, processes of. See Nitration 
 processes. 
 nitration of, temperature of, 33 
 nitration of, time of, 33 
 oxidation of, 25 
 reactions of, 24 et seq. 
 triacetate, 25 
 trinitrate, 25 
 varieties of, 13 
 varnishes. See Cellulose. ester var- 
 nishes. 
 viscosity of, 28 
 viscosity, and growth of, 65 
 viscosity of, specification, 36 
 Cellulose acetate (s), acetyl content, 
 _ determination, 54 
 
Index of Subjects 175 
 
 Cellulose acetate (s), (conid.)— 
 
 acetyl content, theoretical, 64 
 
 analysis, 54 
 
 benzyl alcohol and cyclohexanone, 78 
 
 bromophenylhydrazones, 67 
 
 charring test, 55 
 
 classification (Ost), 51 
 
 dopes. See Aeroplane dopes. 
 
 history, 19 et seq. 
 
 hydration of, 20 
 
 hydroxyl content of, theoretical, 64 
 
 inflammability of, 93 
 
 manufacture of, in Europe, 21, 48 
 
 phenylhydrazones, 67 
 
 properties of, 54 
 
 solubility of, 20 
 
 solubility of, in aqueous acetone, 51 
 
 solubility of, modifications of, 51, 
 52 et seq. 
 
 solubility of, test of, 56 
 
 solutions, in acetic acid and water, 78 
 
 solutions in aniline and acetic acid, 78 
 
 solutions, fractional precipitation of, 66 
 
 solutions of, inmixed solvents, 77 et seq. 
 
 solutions of, Tyndall effect in, 81 
 
 solutions of, viscosity of, 48, 77 et seq. 
 
 solutions of, viscosity, acetone and 
 water, 77 
 
 solutions of, viscosity, acetone and 
 benzene, 77 
 
 solutions of, viscosity, acetone and 
 alcohol, 77 | 
 
 solvents of, 68 
 
 solvents of, classification of, 99 
 
 specification of, 54 et seq. 
 
 stability tests of, 55 et seq. 
 
 sulphur determination in, 55 
 
 swelling of, 84 et seq. 
 
 swelling of, equilibrium, 87 
 
 swelling of, heat change, 87 
 
 swelling of, surface tension relations, 
 85 
 
 swelling of, viscosity relations, 85 
 
 swollen, dyeing of, 84 
 
 swollen, saponification of, 84 
 
 swollen, and solvent, distribution of 
 components, 84 et seq. 
 
 varnishes. See Aeroplane dopes. 
 
 varnishes, analysis of, 155 et seq. 
 
 varnishes, for artificial leather, 136 
 
 viscosity test, 56 
 
 viscosity test, solvents for, 56 
 
 volume change on dispersion, 87 
 
 Cellulose esters (not specified). 
 
 association with solvent, 88 et seq. 
 
 constitution of, 63 et seq. 
 
 dissolution, phenomena accompanying, 
 58 
 
 hydrolysis during formation, 65 
 particles, dimensions of, 91 
 variations in, 96 
 
 Cellulose ester solutions. See also 
 
 Cellulose ester varnishes. 
 constitution of, 88 et seq. 
 inflammability of, 93 
 nomenclature of, 14 
 
 Cellulose ester solutions (contd. )— 
 
 properties of, 13 et seq. 
 
 viscosity of, 21, 28 
 
 viscosity and concentration, 69 et seq. 
 viscosity and temperature, 63 
 viscosity, significance of, 28 
 
 Cellulose ester varnishes, 
 
 absorption by silver, 139 
 
 analysis of, 149 et seq. 
 
 application, methods of, 128 et seq. 
 
 applications of, 119 et seq. 
 
 brushing, 128 
 
 centrifugalisation of, 118 
 
 clarification of, 118 et seq. 
 
 dipping, 129 
 
 filtration of, 118 
 
 flash point of, 165 
 
 flowing, 130 
 
 ingredients, measurement of, 116 
 
 manufacture of, 116 et seq. 
 
 manufacture of, time required in, 117 
 
 moulding, 130 
 
 settling, 118 
 
 spraying, 129 
 
 transport of, on British Railways, 
 164 et seq. 
 
 Cellulose nitrate (s), 
 
 and alcohol at low temperature, 76 
 blending of, 41 
 
 boiling, 41 
 
 dehydration of, centrifugal, 42 
 dehydration of, Dupont press, 42 
 determination of, 151 et seq. 
 disintegration of, by acetone and water, 
 
 drying, 42 
 
 and ether, at low temperatures, 76 
 
 explosive, 15 
 
 films, thin, density of, 163 
 
 films, thinnest, dimensions of, 163 
 
 ‘fuming off,” 33 
 
 heat tests of, 43 et seq. 
 
 history of, 14 et seq. 
 
 hydroxyl content of, theoretical, 64 
 
 inflammability of, 36, 93 
 
 manufacture of. See Nitration pro- 
 cesses. 
 
 nitrogen percentage, 15, 36 
 
 nitrogen percentage, determination of, 
 of 
 
 nitrogen percentage, determination of, 
 in lacquers, 150 
 
 nitrogen percentage, theoretical, 64 
 
 nomenclature of, 37 
 
 poaching, 41 
 
 precipitation of, by aqueous electro- 
 lytes, 154 
 
 properties of, 42 et seq. 
 
 solubility of, 15, 37, 42 
 
 solutions. See also Cellulose nitrate 
 varnishes. 
 
 solutions, fractional precipitation of, 
 27, 66 
 
 solutions, viscosity of, 27 
 
 solutions, viscosity of, acetone and 
 water, 72 et seq. 
 
176 
 
 Cellulose nitrate (contd.)— 
 solutions, viscosity and concentration 
 of, 69 et seq. 
 solutions, viscosity, determination of ,43 
 solutions, viscosity, ether-alcohol, 14 
 et seq. 
 solutions, viscosity low, 22, 133 et seq. 
 solvents, 68 et seq., 98 
 solvents, mixed, 72 et seq. 
 ‘specification of, 42 et seq. 
 stability of, 43 et seq. 
 sulphur determination in, 44 
 varnishes, 16 
 varnishes, analysis of, 149 et seq. 
 viscosity, determination of, 43 
 washing, 41 
 Chalkiness, 144 
 Charcoal, activated, 159 
 Chloroform, 19 
 use in analysis of varnishes, 153 
 Chromatic emulsions, 164 
 Cinema films, non-inflammable, 47 
 Cleanliness of work, 149 
 Collodion, 15, 16, 140 
 cellulose acetate, 19 
 contractile, 141 
 films, thin, colours of, 162 et seq. 
 films, thin, density of, 163 
 films, thin, dimensions of, 163 
 membranes, 159 et seq. 
 membranes, alcohol index, 161 
 membranes, osmosis, anomalous, 161 
 membranes, permeability of, 160 et seq. 
 membranes, properties of, 160 et seq. 
 membranes, standardisation of, 160 
 photographic, 15 
 surgical, 15 
 surgical, formule of, 141 
 Collodion cotton. See Cellulose nitrate. 
 Collodium, aceto-zthericum, 141 
 flexile, 141 
 Colloids, 58 
 solubility of, 61 
 Colza oil, 135 
 Cop bottoms, 34 
 Copal, 18, 113 
 Cordite, 21, 37, 42 
 output, relation to cotton treatment, 34 
 recovery of solvents from, 158 
 Cork, compressed, 140 
 Cotton cellulose, action of alkali on, 34, 
 
 67 
 
 alkali soluble, 36 
 ash, 35 
 copper number of, 36 
 cultivation of, 24 
 ether extract, 35 
 growth of, 27 
 methylation of, 25, 67 
 moisture in, 35 
 purification of, 33 et seq. 
 sources of, 24, 34 
 specification, for acetylation, 53 
 specification, for nitration, 35 
 staining test for, 36 
 
 Cotton fibre. See Cotton hair. 
 
 Index of Subjects 
 
 Cotton hair, appearance in 
 celluloid, 65 
 growth rings, 26 
 plasticity, 26 
 rigidity, 26 
 Cotton-seed oil, 135 
 Cotton tissue, nitration of, 34 
 Cotton waste, 34 
 Crépe stiffening, 139 
 Cresol, for solvent recovery, 158 | 
 Cuprammonium solvent, 27, 36 
 
 Dammar, 114 
 Daub, 136 
 Dew point, 146 
 determination of, 146 
 Dextrose, 25, 28 
 Diacetone alcohol, 108 
 Dispersion, definition of, 60 
 Dissolution, definition of, 61 
 Distribution Law, 84 et seq. 
 Drums, approved, for railway transport, 
 165 et seq. 
 approved, labelling, 167 
 Drying air, 148 et seq. 
 
 Emulsions, chromatic, preparation of, 
 164 
 Enamels, 14, 128, 131 et seq. 
 Ether, 15, 21 
 dispersion of nitrocellulose in, 76 . 
 inflammability of, 105 
 solutions containing, 
 165 
 specification of, 105 
 Ether-alcohol, 15, 16, 17, 21, 34, 37 
 complex, 72 et seq. 
 recovery of, 157 et seq. 
 solvent power of, 90 
 viscosity, 73 
 Ethyl acetate, 15, 69 
 specification of, 105 
 Ethyl alcohol, 14, 17, 21 
 solvent power for cellulose nitrate, 76 
 specification of, 103 
 Ethyl] alcohol-benzene, solvent power of, 
 88 et seq. 
 alcohol-chloroform, solvent power of, 
 88 et seq. 
 Ethyl butyrate, 69 
 formate, 69, 71 
 lactate, specification of, 107 
 phthalate, 69 
 Ethyl-o-tolyl-ethyl carbamate, 69 
 Evaporation, of solvents, 97 Lead 
 losses due to, 149 
 Explosives, 21 
 Explosives Acts, 41 
 
 transport of, 
 
 Faience, protection of, 139 
 Fans, electric, 143. 
 Faults, recognition of, 144 et seq. 
 prevention of, 144 et seq. 
 Felt, 16 ; 
 Films, non-inflammable, 47. 
 thin, properties of, 162 - 
 
Index of Subjects 
 
 Fire dangers, 143, 148 
 Flash point, 165 
 Fluidity, 22, 28 
 and Tyndall effect, 92 
 Fractional precipitation, of solutions, 27, 
 66 
 
 French polishing, 137 
 Friction, internal, 59 
 Fusel oil, 17 
 
 in analysis of varnishes, 151 
 
 Gas masks, 22 
 
 Glass, antique, preservation of, 139 
 decoration of, 140 
 
 Glucose. See Dextrose. 
 
 Glycerol, as viscosity standard, 56 
 
 Glycogen, 26 
 
 Gold leaf imitation, 140 
 
 Guncotton, 37 
 
 Hats, stiffening, 140 
 
 Henry’s Law, 86 
 
 Hospital fittings, 138 
 
 Hull fibre, 34 
 
 Humidity, atmospheric, 16, 94 
 atmospheric, controlled, 127 
 atmospheric, definition of, 145 
 atmospheric, measurement of, 146 
 atmospheric, table of, 147 
 
 Hydrocellulose, acetylation of, 19 
 
 Hygrometer, 146 
 
 Incandescent mantles, 
 140 
 
 Incombustibility, definition of, 121 
 
 Inlaid work, protection of, 139 
 
 Inulin, 26 
 
 Ivory, protection of, 139 
 
 strengthening, 
 
 Kid, patent. See Leather, patent. 
 
 Laboratory results, application of, 96 et 
 
 seq. 
 Lacquers, 14, 130 
 coloured, 131 
 formule of, 130 et seq. 
 resin, formule of, 114 et seq. 
 Leather, coatings, 16, 136 
 dressing, 136 et seq. 
 imitation, 135 et seq. 
 imitation, fabric for, 135 
 imitation, from cellulose acetate, 136 
 imitation, viscosity of varnish, 135 
 patent, manufacture of, 136 et seq. 
 upholstery, manufacture of, 137 et seq. 
 Linseed oil, 135 
 Linters, 34 — 
 nitration of, 40 
 
 Magnesium phosphate, 16 
 
 Mannol, 69 
 
 Mastic, 18, 113 
 
 Matt surface, 132 
 
 Mercuric iodide, 16 
 
 Metals, coating of, 128 et seq. 
 
 Metal lacquers, classification of, 130 
 
 I2 
 
 177 
 
 Metal powder mediums, 130 
 
 Methyl acetate, 69, 71 
 
 Methyl acetone, specification for, 102 
 
 Methyl alcohol. See also Wood spirit. 
 determination of, in varnish, 151 
 specification for, 104 
 
 Methylene sulphate, 49 
 
 ions pe ethyl ketone, specification for, 
 
 ] 
 
 Mixed acid, 14, 15, 31 
 
 Mixers for varnish manufacture, 116 
 
 Motor-car enamels, 132 et seq. 
 application of, 134 
 failure of, 135 - 
 properties of, 133 
 
 Motor-car oil varnishes, 132 
 
 Motor-car stoving enamels, 133 
 
 Nitration processes, 37 et seq. 
 American method, 40 
 comparison of, 39 
 centrifugal, 38 
 direct dipping, 37 
 displacement, 38 
 requirements of, 37 
 
 Nitric acid, specification for, 30 
 
 Nitro-benzene, 16 
 
 Nitro-cotton, 37. 
 
 nitrate. 
 
 Non-inflammability, definition of, 121 
 
 See also Cellulose 
 
 Optical instruments, 138 
 Oxycellulose, 25 
 
 Paper varnishes, 138 
 Patent leather. See Leather, patent. 
 Petroleum Act (1879), 165 
 Petroleum spirit, 109 
 determination of, 153 
 Phenol, solvent power of, 89 
 Phenyl-ethyl urethane, 69 
 Photographic paper varnishes, 138 
 Picture frames, 138 
 Pigments, determination of, in varnishes, 
 151 et seq. 
 incorporation of, 118 
 settling of, 149 
 Pitting, 144 
 Plaster, 16 
 Plastic flow, 80 
 Polymerised coatings, 13 
 Posters, protection of, 138 
 Pottery, protection of, 139 
 Precautions in using varnishes, 93, 143 
 Propyl acetate, 69 
 Pyroxylin, 15, 16, 37. 
 nitrate. 
 Railway Classification, 164 et seq. 
 Railway, transport of varnishes on, 
 164 et seq. 
 Ramie cellulose, 27 
 Refractive index and Tyndall effect, 82, 
 83 
 
 See also Cellulose 
 
 Resins, 16, 18,-112 et seq., 130 
 determination of, 151 et seq. 
 Resin lacquers, formule of, 114 
 
178 
 
 Resin solutions, blending with nitro- 
 cellulose, 114 
 
 Rontgen rays, and cellulose structure, 
 27 
 
 Rubber, artificial, 18 
 
 Sandarac, 114 
 
 Sericose, 20 
 
 Settling, 149 
 
 Shellac, 18, 112 
 
 Silica, 138 
 
 Silica gel in solvent recovery, 159 
 
 Silver, collodion films on, 162 
 absorption of solutions by, 139 
 protection of, 139 
 
 Sliver, 34 
 
 Snakeskin, imitation of, 139 
 
 Solids, total, determination of, 150 
 
 Sols, 58 
 
 Solvent power, 21 
 definition of, 62 
 temperature relation, 63, 74, 76 
 use in solvent specification, 102 
 viscosity relation, 62 
 
 Solvents (not specified), 18 
 anhydrous, 16 
 blending of, 94 et seq. 
 choice of, 94 et seq. 
 comparison of, 61 
 complexes in mixed, 73 et seq. 
 constitution of, 68 et seq. 
 dielectric capacity of, 88 
 dilution of, 61 
 economic considerations, 18 
 evaporation of, 97 
 history of, 17 et seq. 
 identification of, 154 
 inflammability of, 93 
 mixed, 72 et seq. 
 mixed, complexes in, 73 et seq. 
 mixed, de-association in, 79 et seq. 
 mixed, dispersion in, 88 et seq. 
 mixed, in varnishes, 95 
 odour of, 149 
 recovery of, 157 et seq. 
 specifications of, general clauses, 100 
 swelling power of, 84 et seq. 
 vapour pressure of, 97 
 
 Spraying, 129 
 pressure used in, 134 
 
 Starch, 18, 26 
 
 Stone, protection of, 139 
 
 Sulphuric acid, for acetylation, 19, 46 
 for nitration, 30 
 specification of, 30 
 
 Sunlight, action of, on fabric, 20 
 
 Surgery fittings, 138 
 
 Swelling in solvents, 84 et seq. 
 chemical nature of, 87 
 
 Tale, 16 
 
 Temperature and solvent power, 63 
 
 Tetrachloroethane, specification of, 108 
 toxicity of, 108 
 
 Index of Subjects 
 
 Thermometers, wet and dry bulb, 146 
 Thinners, 129 
 Thinnings, 129 | 
 Thread, protection of, 16 
 Time-tables, protection of, 138 
 Tinsel, protection of, 16 
 Toys, decoration of, 16 
 Transport of cellulose ester varnishes, 
 164 
 Triacetin, specification for, 110 
 Trimethyl] cellulose, 26 
 Trioxy methylene, 49 
 Triphenyl phosphate, history of, 11] 
 as rétarder of combustion, 122 
 specification for, 111 
 Toluene, specification for, 109 
 Tung oil, in lacquers, 130 
 Turpentine oil, 16 
 Tyndall effect, 81, 91 
 and fluidity, 92 
 and yield value, 91 
 
 Ventilation, 93, 108, 143 
 in doping schemes, 123 
 plenum and vacuum, 144 
 Viscose reaction, 90 
 Viscometer, falling sphere, 43, 60 — 
 Ostwald, 43, 60 
 Viscosity, 28, 59, 131, 135 
 and concentration, equation connect- 
 ing, 69 et seq., 85 
 and fractional precipitation, 66 
 and solvent power, 62 
 and swelling power, 85 
 and time of esterification, 65 
 definition of, 59 
 measurement of, 60 
 measurement of, in analysis, 155 
 measurements, validity of, 81 
 minima in mixed solvents, 72 et seq. 
 Volume change on dispersion, 87 
 
 Westron. See Tetrachloroethane. 
 Wood, antiques, protection of, 139 
 coatings, 16, 138 et seq. 
 distillation of, 21 
 fillers, 138 
 spirit, 15, 16. 
 alcohol. 
 spirit, specification for, 104 
 Wood cellulose, 24, 27, 33 
 nitration of, 34 
 viscosity and growth, 65 
 Worsted, protection of, 16 
 
 See also Methyl 
 
 X-Rays. See Rontgen rays. 
 
 Xyloidine, 37 
 
 Xylonite, solutions, 165 
 thinnings, 165 
 
 Yield value, 80 
 and Tyndall effect, 92 
 
 Zinc chloride, 19 
 
 a ae 
 a ee ee 
 

 
 D.VAN NOSTRAND COMPANY | 
 
 
 
 are prepared to supply, either from 
 their complete stock or at 
 short notice, 
 
 Any Technical or 
 Scientific Book 
 
 In addition to publishing a very large 
 and varied number of SCIENTIFIC AND 
 ENGINEERING Books, D. Van Nostrand 
 Company have on hand the largest 
 assortment in the United States of such 
 books issued by American and foreign 
 publishers. 
 
 All inquiries are cheerfully and care- 
 fully answered and complete catalogs 
 sent free on request. 
 
 
 
 8 WARREN STREET - - 5 New YorxkK 
 

 
roxton, Foster. 
 
 : GETTY CENTER LIBRARY CONS 
 ~g $77 1925a BKS 
 etdies ester mama a 
 
 3 3125 00192 3099 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
fee ena 
 
 — ge ee ae ae, Pn ete 
 a EA Le NO TNT yO ee tt eee ee es 
 
 
 
 rl a ibn, Ui ne mE tHE Et et Se et - 
 ° Bp ne, ~- 
 
 “ Cm : 6 Pas 
 sae ae = ~ ~ - ~ nts ni «gat a ag 
 
 q “a0 
 
 ee ee ES IE eed 
 I te NI etme 
 ere 
 
 ve . 7 ae’ 
 Ee ag ee 
 
 Leey es 
 
 ne eeetcn OF, 
 
 We 
 
 m aren = and z 
 eo 
 SS Ng lar 
 
 WERE WHI DAL 
 
 ON a 
 
 PRR RAMA le a 
 
 So 
 
 Pete Ot Em 
 
 e ~ _ . se te 
 
 ee mal ai ¥ wien : F ~ ; ~ < oe ne ~ heen os ~ aie es * 
 
 eas yt genre - we pC ona! int AE wars mera cred gg i, onal = oe PD tir Ae Mg OO eh ers = Orgter nes 
 Pad - Me we vs 
 ota 
 
 hd 
 
 NRL TERN We ttee tag Cot baw UR Mirns! 
 PTAA CN ARCA RAL RL Ould a ‘ 
 
 AEs BA 
 Dares 
 
 i 
 7 
 
 pee ie 4 
 
 re Fort oe OIE ig 
 
 a 
 
 Bo ein gf nae ah 
 ~— mene ee, 
 
 oe Pane 
 
 PRECRLOCUNA GLE RR AER CR IO GE Os EMEA UE OU AOR OEEO RAE S NS ZRPAS AEA RNAGI AEA OMA PCRS 
 
 - WDE NA LID LRA TR WEES Fete Be 
 
 — Poe 
 
 ta 
 
 PAO ER EA Cee 
 
 Hanes 
 
 pee > 
 
 { 
 
 Hat” OS fra pals 
 Hip ea nl te cee A ETN 
 
 , 
 
 ty 
 
 4 \ ue 
 
 ae if 
 
 we Dale hr aveblesianwm sawn bane i8ie 
 Rn Y eee i 8 . 4 J 
 
 NUR GRIMES A ERAS AN BLU CEA TOKURA GURL CLAY CTE 
 
 LY e ape me, %, mie 
 , a eee) a p - 
 Se — - Sl an Stn is 
 “ ei ae "A eit SPE Chek SP Le Mn gues ae - 
 
 ns i ASSO Nt ce Et OE Bi Pant eA NE AEE ci ee cnet hi ale 
 
 o 
 rT Ps "’