fil WCCD AND PAPER-BASE PLASTICS November 1943 All 3RANCH No. R1438 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin - : st p , ure uction \re bee or as plastic rr«- - ■ . • - id flour and wood pulp fillers for bhe for of paper for a' . Is now finding , resin-treated, compr f eg) • The bituent is being icelluloses ' I re smaller quantities of critic: - - ' phenolic rnol- ' 's, its res' Pact that the ligni is a plas* ' n-impregnated paper-base lamii ier c ommercial e of former , - sesses for ry, and Impreg A number of desirable properties can be imp;:,-'- bo wood by the form- ing of synthetic resins throughout the structure from resin-forming constituents of low molecular weight that have an affinity for wood. Although the hardness and compressive strength properties of wood can be improved by mechanically depositing any solid material within the structure, permanent dimensional stability and related properties hsr r e been successfully imparted to the wood only with a few specific resinoids under specific treating con- ditions. Tins is due to the fact that materials such as fats, waxes, natural resins, and appreciably preformed synthetic resins, because of their large molecular size an^ their lack of polarity, under normal treating con- ditions show no tendency to penetrate the cell -wall structure of wood and no tendency to bond to the wood. Forest Products Laboratory tests have shown that any water resistance these materials ?ive to wood is entirely mechanical in nature. Thev cut down the rate at which water can traverse the structure but they do not change the final amount taken up and, conse- quently, do not reduce the equilibrium swelling. Their failure to accomplish this is primarily due to their lac v of affinity for wood. One slight flaw in an internal coating will allow water to work its wav between the fibrous Structure and the coating and disperse itself throughout the continuous structure. It thus becomes apparent that a treating material with an affinity for the wood greater than that of wood for water should be used. Unfortunately, all such materials in themselves have a high affinity for water. This difficulty has been solved with treating agents that are selectively adsorbed within the intimate cell-wall structure of wood and can subsequently be converted to water-insoluble resins within the wood structure while remaining permanently bonded to the structure. The most effective treating agent thus far found is a phenol- formaldehyde, water-soluble resinoid that is not advanced beyond the phenol- alcohol stage. Resorcinol can be substituted for the phenol or furfural for the formaldehyde without loss of effectiveness. All urea-formaldehyde resinoids tried have proved to be too highly prepolymerized to penetrate the structure adequately, with the exception of demethylol urea. Even this material when polymerized within the structure reduced the swelling and shrinking on an equilibrium basis to only SO percent of normal in contrast to reductions to 30 percent of normal effected by phenol-formaldehyde resi' . None of the thermoplastic resins or thermoplastic resin-forming systems thus far tried have effectively reduced the swelling and shrinking of wood, presumably because none of them have the desired affinity for the wood. It was shown that the chemical affinity of wood for a resin-forming system can be gauged by the iegree to which the resin-forming solution swells wood beyond the swelling caused by water alone. A 40 percent aqueous solution of a phenol-formaldehyde re.r' will swell wood about 20 percent more than does water, I ' ly swollen wood is dried and the resin is formed within the structure by the application of heat, considerably less than normal shrinkage occurs. The oven-dry treated wood s a volume about equal to that of the green untreated wood. °n an R1438 -2- rat to 30 per ry weig ss practical] ' , " rmed In the structure, the specific is increased only about 16 - - than -, ■ ty ha.- >n I t »re ; cals throughout the structure of | tr practical only for veneer. The value of •s of vene r Loh is normally built up into plywo~ is that, in cross-tar. iywood, the fiber iire< I of one ply restrains the across-the-f iber dimension chaj ;es >f , thus mecha>- ' Ly reducing such changes. Swelling and shr cannot, however, be prevented mechanically. The mechanical restrai merely changes I eel on of swelling and shrinking. If •vented from swelling normally in the si ras it w: the thickness :ir~ction or internally into the f" ' . normal plywood takes up and then loses moisture, the plies e lly , as a result of th° u , face is more s°rious than in solid boards. Resin treatment, - swelling and shrinking to about 30 percent of 1, reduces - such an extent that checkin practically elimi of fancy crotch veneer for use in furniture ■ reduced 'eatment with a pher Lc resin. The treatment of wood with stabilizing resins also imparts appreciable d^cay and termite resistance. Figure 1 shows a 3-ply piece is-fir plywood with treated faces and an u sore, en was immersed for 6 months to a depth of half ! field where decay and termite action on wood sign of decay, but plenty of termite action, attack found the resin-treated faces not to their a flank attack and, as a result, practically cl Liar specimens that were edge coated with re 11 plies were treated were still sound after a 2- 3 re. red reduction in decay a] on, it . is due r fact that t reated wood will not take up enough w cell-wall structure to support decay than to . r?atment of wood with stabilize ^creases its trical resis* is a result of the reduce roscc; is an excellent electrical ' tor, but it loses its resists Ly with an increase in moisture con rat, resistance of the treated w ' I ss , v Lie ai arc« 'is ab - . , re si start. also increase: ce. 20 percent - Impact. s one pr perty that ' 3 bed, the resin content of wood is increased, it becomes more brittle. Likewi more un] For le resin e "ittlene Unfortunately, the hn ~- i treatment from the s1 of stabilization 1 poorest from the standpoint of brittleness. ^ormal birch has an Izod \lue of 9 to 10 foot-pounds .per inch of notch u --^ when treated abilizing re. ' value drops to'-onlv 2 to 3 foe 4 -- rids per notch* To summarize, impreg has the following advantages over normal woe (l) reduced swelling:; (2) reduced chec^.in?, and surface degrade; (3) improved ; stance to decay and termites; (4) improved electrical resistance; acid resistance; and (6) improved compressive strength and 'nessi These improvements are secured at the expense of decreased toughness. C om pre g > Compreg is re-in-troated wood that is compressed while the resin is f ormed-within its structure. Although a number of different resins have tried in making this material, none has proved as successful as phenol- formaldehyde. There are two types of compreg: (l) the older form, * developed in Europe, which is treated with a spirit-soluble pr.enolic rep' prepoly.nerized to the stage that it does not tend to penetrate the cell- wall structure and bond to the polar croups of the wood and-, as a result, does not stabilize wood aonreciably; (?) the form developed by the Forest ucts Laboratory which is. treated with a water-soluble, phenol- for yde resinoid, as in the case of impreg, so as to form the resin out the cell-wall, structure of the wood and v ond it to the act' polar Toups of the wood. The latter form of c is much more stable than the former but tends to v e more brittle; like impreg," it has ^ood dec?. r ' u e resistance and food electrical resistance. ^orest Products Lai ratory compreg can be compressed to virtually the ultimate compression (specif ic . p-.ravity of 1.3 to 1*4) under a pressure of 1,000 pounds per square inch, using practically an ,r species of woe . The urn e form of comr. . - the other hand, require re of 2,500 to 3,000 pounds per square inch to compress the wood to the same -ree. There is a still greater difference in the pressures required to compress the wood of the stable and unstable forms to intermediate e rees of compression. Practically all the softwoods (coniferous woods) and the sof^ - p rdwoods (deciduous woods) such as Cottonwood, basswood, and aspen, can, when treated with a stabilizing resin, be co-pressed to about one-half Lr original thickness under pressures as law as 250 pounds per square inch. Phis makes possible the compression of co ; 'aces ' eir simultaneous as ly with an untrea' e ~ ~ed and precured core with but slight compression of tie core. This type of material, whic v #s great promise for postwar uses, cannot be made in one operation when the plies are treated with an appreciably polymerized resin, sre is little differential compressibility between such treated plies and the untreated plies. 138 -4- surface of ■face. ■ . ?he r ■ La hi ■ >hol an >tone, • of For • r _ >r le i 4 c^ >d immersion } • . ■ than ong in Lber -- noisture absor La greatest — e T ; room bemperatur onten- ... ' > >f compr . hickness as the a table form, lose a large part of its compression. Thi Fact that wat as a result water structure much more ;. • swe. ■ion may be as much aa 20 to -thirds • ' recovery frc .3 ion. 51 ' ihanica] - i - rma ol r ilar id, i ;eneral, vary abou- ' en wooi sompreased to ne-thir riginal volu . rength, lus 0: , of ela. •■• are about trebled, irreapective oi lent prior - -• 1 ' •• Lrec- " ' • ■ ' onal Lty. ' - r ■ _ _ ..,,., lore brittle. Under carefully controlled on- litions, le form >f ' \ from birch with an J.". ae o ' foot-] t inch of notch. The unstable com- -, en the other hand, will hare an Izod value if P to 9 foot-poun • inch of notch. An important feature of compreg is that it can bf made from a great variety of woods, including such normally inferior species as cot- - , and obtain a ict with properties which approach the optimum values . only species to be avoided are the naturally resinous woods, such as southern pine, and those that are extremely iifficult to treat, such as oak, preg can be machined easily with metal-working tools but not . woodworking tgbls. Because of this, it is desirable to rough out ■ shape of objects prior to compression, using woodworking tools, an then compress them to* the final shape in some form of mold. A technique ■ doing this has been developed at the Forest Products Laboratory. Treated, uncompressed plies are glued up into a blank of the correct size with a phenolic glue under conditions such that the treating resin is noi cured and the bonding resin is but slightly cured. The shearing strength of such a block is not great but it is sufficiently strong so that it can be carved or turned in such a manner that the final i lensions ar 3 obtained in one plane but the thickness at right angles to this plane is 1.5 to 3 is the final dimensions. The carved blank is then pressed in a split Id in the ' s direction. A ,• ' i-gan company is using this method to mol i propellers for the ground testing "> p airplane motors (fig. 2) and airplane aerial masts. An airplane tail wheel has been successfully molded in this way so as to pass all static tests requirements (fig. 3, left). The technique could ' 3 readily applied in the molding of pulley and rear wheels v y stamping out the correct sections in the plan a of \ wheels from the individual plies and rotating these with respect to each other in ^h assembly as desired* Although wood is not moldable in the sense that a molding powder is, it is surprisingly subject to molding under proper conditions, A recently developed process of which nothing can at present be divulged makes possible the production of a highly stable form of com- presse vood without the use of any impregnating resin. Hydroxylin Lignin is Nature's plastic which cements the cellulose fibers of wcor! together, A mild hy-irolysis treatment breaks the cellulose-lignin id of woc r i, frying the lignin so that it can be used to rel ond the cellulose fibers together. Wood waste, preferably hardwood sawdust or mill waste, car. < ydrolyzed by several different methods. The oroc c dure which has received the greatest attention at the Forest Products Laborat is a hydrolysis with dilute sulfuric acid in. a rotary digester at a steam pressure of 135 to 200 pounds per square inch for 10 to 30 minutes* H143 8 •oaring the cellulose-lignin bond, t . - 'ts thr selluloses to sugars. Thes- 3ut of the 1. . zed wot bed to -rain alcohol, ig a valuable bypr- . Phe - anstitfcte of 1 1 be original wood. As a e remove.] o* of .llulose, the lignin content is increased *:o 5! to 40 per After , the hydrolyzed wood is q ' ly to a powder, preferably Df 40 to 100 mesh, lyzed wood can be made to f ntly for th< -^.e objects by • adding small amounts of water b at ., the flow is not adequate t" T ive a product that is suffici coherent to stand long water . similar results were -^sinous plasticizerr for : ; -ere used of w •, though they did reduce the moldi: re. It was hence for; necessary ^o | ? s or plastic-f • with a iticiaer for limin, not also serve as such. The most suitabl I -rial - the ee : worV function- ire of 8 perc- percent furfur ] , ether with 84 perc •11 amount of mold lubricant such as zinc stee » Molded Products with good Ld defi n, water resistance, acid resistance, sctrical ai Lanioal properties can be obtained by pressing at 300° P, for (in the case of small objects) at 3,000 to 4,000 per Because the product is semither s-tic, it ust be cooled the mc", . flow of this molding powder is not so great af the - urpose commercial mol iing powders, . bher with ct t the nroduct cannot be drawn r e pre? , on the of hydrolyzed wood. The best ed have been with a r 75 perc<- . s orb i nation, pre Les of the pr ict -re of gener L- compounds containing -in and 5C of wood flour. The fact th i nolic resin is required with the hydrolyz* that the 13 lyzed wood Li "astic • rties to I produc . 3d wc ' -" 'ed products wi1 cural brei - -: • ng from 8, , a ] »r square inch, water — ^ ons of onlv 0.2 tc ,3 perc r 48 hours 1 ■ .- , ind ex T r . . • L stance. It po; rial into jeneral- rcia] ' - . ributes, t is now 1 ; : of sizeable objects of rial rtai ce. If chips : - wdust ar ' ^he uct is al r to a fiber r r< >r, it R1438 - - i be fv Into a sheet on the paper machine. These sheets, with only mount of phenolic resin, can be compressed together into thick els# The panels have considerably higher flexural strengths than panels te from the molding powder because of the reinforcing action of the much longer cellulose fiber. Papre g ■minates treated with phenolic resins have been made for rsi They have been used chiefly for electrical insulating panels and for other nonstructural uses which do not require exceptional mechanical properties. The manufacturers, in developing these materials, have approached the problem primarily from the resin standpoint. It was hence felt at the Forest Products Laboratory that further development of paper- base laminates, from the standpoint of finding the most suitable paper for the purpose, was a promising field of research. This proved to be the case. Within 6 months after the research was started, a paper-base laminate was developed that possessed several properties double those of the former i nates. The types of paper and modifications in processing cannot be given hero. It can be stated, however, that suitable papers are now being produced by several different paper mills. Table 2 gives the readily obtainable properties of parallel- inated papreg as of November 1942, The term "parallel laminated" indicates that in all the sheets making up the panel the machine direction of the paper runs the same way. Paper made on a paper machine is always stronger in the machine direction than across the machine direction. The difference may be as much as twofold. When isotropic properties are sought in the Laminate, alternate sheets are crossed as in plywood. Cross- banded papreg differs from plywood and impreg and compreg with plywood construction in that the strength properties ot so seriously reduced V^low the values for parallel -lamina ted mater^- 1. Due to the fact that veneer is from 20 to 40 times as strong in tension in the fiber direction as across the fiber direction, the tensile strength of plywood depends almost entirely on the longitudinal plies. Cross-banded papreg has strength properties ranging from two-thirds to three-fourths of those for rallel-3 3d material, in contrast to the strength properties of cross-banded compreg, which are only about one-half as high as its parallel-laminated values. Fany strength Properties of parallel-laminated compreg and papreg are much the same. Cross-banded papreg is superior in almost all stre "roperties to cross-banded compreg. Papreg has rength properties quate for s large number of semi structural uses and some structural uses. fcn stural material, its brittleness seems to be its most serious handicap. Compared to ordi- nary plastics, it has quite good Izod values, but it is definitely ' infer- ior in t .is respect to fabric and "•lass fabric '. tes« It is, however, superior to fabric laminates in practically all other gth properties, R1438 -8- lo , r Lly molded to • . iccei , ■ , Lng of bh but ;oring , ■ ->th nfl synthetic, ' p r- ; 3" f 'rom thr of ch ng the Lng up the to- .» without sacrifice in water i ther mec. ] proper!" s« Detail te of the wor Lven isent« Conclusions It is obvious from this r of product? I I wood is making oe for itself in the ; cs field. Although woe stituents serve mostly as the structural or filler part of thr tics, wood and wood products show promise of ing the res 4 '\nsol (a rosin-purif icati or r Luentst It is also of interest that phenols, furfur other I -tituents are obtainable from woo ?structi"e c ( hydro - cesses. It does not require to self-contained wood industry that uses ost exc ve\y in the manufi ) sties* R1438 ile l.-- Kormal appr pr opertie s of parallel -laminated b irch c ompreg with a specific gri vity of 1.35-L Property lue Lb« per sq. in, 22,000 32,000 3,500,000 Tension: Stress tit proportional limit, imum strength , . . . lulus of elasticity Flexure : Stress at proportional limit.... Modulus of rupture . . . , . . . lulus of elasticity........... Compression parallel to grain: Stress at proportional limit.... .ximum strength. ............... -lulus of elasticity. ......... . Johnson double shear, parallel to grain and perpendicular to laminati ons . . . < Izod impact :£. Face -notched. Edee-notched. • • e • 21,000 36,000 3,500,000 16,000 24,000 3,500,000 7,000 Ft.- . 'erin. 3 to 9 2 to 7 —The properties, gi^en, with the exception of impact rength, are about the same for "both stabilized ad unstabilized compreg °nd do not vary ~ppre- bly between species, .iThree to " foot-pounds per inch of notch for stabilized compreg (face notched). Six to 9 foot-pounds per inch of notch for unstabilized compreg (face-notched). R1438 .-- ..-- re r - r-rtj.^ of '. ? 1 - i re6 Property ' Tensio' : ngth ulus oi Flexure : ulus of rupture . . . . >lus of elasticity, .38 Lb« pe 36,000 3, , x C om] r ' dii : rallel to grain* •■ . . Flatwise • iicular to s;rain.« Edgewi •" ] r I : . . . i Johnson double sher,r, parallel to , perpendicular to ana , . . . ,...'- Izod impr.ct: F \ce-notched, Edge-notched, iness (Rockwell) Vfe.ter absorption (24 .100 ,300 13,000 Ft. - . j . 0.8 100 6 y R1439 Z M 51063 r Figure 1. — Action of termites on 3-ply res In-bonded Douglas- fir plywood with faces treated with 30 percent by weight of synthetic resin (on the basis of the dry weight of the un- treated wood) and an untreated core that was immersed to half its length in a termite infested field. The core has been almost completely eaten out up to the ground line while the faces are perfectly sound. I V. 51063 " Figure 2. — A molded compreg propeller for testing airplane motors that Is being commercially produced bv a "lchlgan com- pany using the process developed at the Forest Products Laboratory. Figure 3. — Left to right, half of an airplane tail wheel molded of compreg; a compreg specimen varying in specific gravity from end to end (1.3 to 0.6); a model airplane pro- peller molded of compreg; a cut panel of birch compreg sanded and buffed to show that the finish exists throughout the structure. Z M 51i:64 I UNIVERSITY OF FLORIDA ■II|I1IIIIIII 3 1262 08925 4329