PAPER-HCNEyCCMD CCRES FCR 
 STRUCTURAL SANDWICH PANELS 
 
 March I )t/ 
 
 Of M 
 
 ill II FU 
 
 
 No. R1918 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE 
 
 FOREST SERVICE 
 FOREST PRODUCTS LABORATORY 
 Madison 5, Wisconsin 
 
 In Cooperation with the University of Wisconsin 
 
Digitized by the Internet Archive 
 
 in 2013 
 
 http://archive.org/details/honeoOOfore 
 
PAPER-HO" '-' ~ 
 
 Bv 
 
 . J. SLIDE, Chemical Engineer 
 
 Forest Produe al 'oratory,- Forest Service 
 U. S. Department ox Agriculture 
 
 Sum mary 
 
 Bonding of paper-honeycomb cores between thin and strong facing sheets results 
 in sandwich panels that are becoming increasingly Important for structural 
 applications. The panels are strong, stiff, light, and economical of raw 
 materials. They can be made by processes that lend themselves to mass produc- 
 tion from materials that are produced in large volume. 
 
 This paper summarizes briefly some of the research at the U. S. Forest 
 Products Laboratory on sandwich building panels having cores made from 
 corrugated paper of low resin content. Extensive testing of these panels in 
 the wet and dry condition has shown that satisfactory performance could be 
 expected, and a considerable amount of data has been obtained to aid in the 
 structural design of panels. For use in dwelling units, strength tests have 
 indicated that a suitable sandwich wall panel is comparable to conventional 
 house construction in bending strength and resistance to vertical loads. With 
 proper treatment, the cores have reasonably good decay resistance. Thermal 
 insulation values are relatively good and depend on the core density and 
 specific construction; for some uses, the thickness of the panel may be 
 determined by the thermal requirements. Insulation values can be further 
 improved by filling cells with materials such as foams or loose insulation. 
 
 Bowing of panels due to differential moisture and thermal conditions was 
 found to be of approximately the same order as that of insulated, stressed- 
 facing panels made of plywood glued to wood frames. The fire resistance of 
 
 -Presented at a meeting of the Plastics Committee of the Technical Association 
 of the Pulp and Paper Industry, Syracuse, "I. Y., Nov. 9, 1951. 
 
 2 
 
 Acknowledgment is made of information used in this report based on the work 
 
 of various investigators at the Forest Products Laboratory and on work done 
 
 by the Laboratory in coooeration with the Housing and Home Finance Agency. 
 
 3 
 
 ""Maintained at Madison, Wis., in cooperation with the University of 
 
 Wisconsin. 
 
 Rept. No. R1918 -1- Agriculture-Madison 
 
honeycomb panels is appreciably higher than nearly hollow panels with like 
 facings. Filling of cells with foamed resin further increases fire 
 resistance. Results of accelerated aging tests are favorable. Durability 
 and bowing under outdoor conditions are under study in a sandwich- panel test 
 unit, which was built in 19U7 and is still in very good condition. 
 
 Introduction 
 
 The principle of the sandwich panel was undoubtedly put to effective use for 
 many years before it was defined by engineers and recognized as a separate 
 type of construction dominated by certain mathematical principles. General 
 recognition occurred during the accelerated search for high- strength, light- 
 weight materials for aircraft in World "Jar II. Simply stated, a sandwich 
 panel consists of an appreciably thick core of a low-density material bonded 
 on each side to a thin sheet of strong, stiff material. A great variety of 
 facing materials, such as wood, hardboard, asbestos board, aluminum, and 
 plastic laminates, have been bonded to many lightweight core materials 
 such as balsa wood, rubber, plastic foams, and formed sheets of cloth, 
 metal, or paper. 
 
 The new and ingenious materials, constructions, and ideas for use multiplied 
 rapidly. Stringent demands for strength and special characteristics were set 
 by designers to meet needs for wartime aircraft. As the development progressed, 
 a distinction between highly specialized aircraft panels and other general 
 building panels became evident. Obviously, a radical change in the outlook 
 regarding materials, properties, and economics was indicated, although the 
 principle remained unchanged. Demands for strength appeared less exacting 
 and requirements for durability and thermal insulation were perhaps more 
 critical for building panels than for aircraft panels. Problems of supply 
 and cost were paramount for building panels. Cores made of sheet materials, 
 such as cloth, nonwoven fabric, or paper formed into cellular configurations 
 of one kind or another, became more important. Then careful consideration 
 was given to economics, availability, and other properties, it became in- 
 creasingly clear that paper would play a very important part in the large- 
 scale development of honeycomb. It may indeed be only a slight overstatement 
 to say that paper is almost the only material apparent at present for the 
 large-scale development of high-strength honeycomb panels. 
 
 In 19hl, following several years of research with lightweight, paper-core 
 constructions and sandwich panels at the Forest Products Laboratory, the 
 status of the work was reviewed from the standpoint of panel applications in 
 building. Many of the most promising ideas were gathered together and 
 crystallized in the form of a sandwich- panel test building unit for outdoor 
 
 exposure (11).— 
 
 K, 
 
 nderlined numbers in parentheses refer to literature cited at end of report, 
 Rept. No. R1918 -2- 
 
Since the erection of the test unit, a continuing interest in the structural 
 sandwich panel has developed, as indicated by the large number of inquiries 
 received from an unusual variety of sources. Kany are interested in only 
 certain phases of the development, such as fabrication of the core or press- 
 
 of the panels. Pulp and paper mills endeavoring to increase the number 
 of products possible from their raw material consider the sandwich panel as 
 an additional long-term outlet for paper. Some resin manufacturers consider 
 th?t manufacture of sandwich panels provides a promising use for their 
 products in core treatment and in plastic sandwich facings. A few boxboard 
 mills have observed the similarity between their regular products an 
 honeycomb and arc. following the development closely, thinking perhaps of 
 the amplication of boxboard experience to honeycomb and, conversely, honeycomb 
 principles to boxboards and containers. 
 
 Plywood and veneer mills are interested in sandwich pan< Is as a means of 
 increasing the monetary yield from a supply of logs. ! etal compan: 
 ask about the feasibility of usin^ metal as facing v rial and see in the 
 sandwich p-ncl an opportunity to use thin metal sheets for applications 
 where they previously were not suitable. The advantages of sandwich con- 
 struction in furniture and doors arc evident to of 1 'oducts. 
 Architects and builders interested in n Lais inquire about 
 
 strength, fire resistance, and thermal insulation afforded 'oy the hen ycomb 
 core, feasibility of using sandwich construction in buildings, and possible 
 sources of sandwich panels. 
 
 hakers of hardboards end composition boards visual! n :w and int ] I Lag 
 uses for their products. The continuous support that 4 
 fiberboards themselves suitable for items such as walls, partitions, and 
 doors. 'hen fiberboards are used as facings, the possibility of maki 
 high- strength structural panels from pulpwood and woods residues is evident. 
 
 Basically, the problem of making honeycomb from paper can be viewed as one 
 of "bl< -up" a certain volume of a dense paper to yiel i uch lar 
 volume of a low-density cellular structure with certain properties. After 
 the spacing between sandwich facings has been determined by design, the 
 honeycomb must fill all of the volume between the facir . Density of core 
 is therefore one of the key factors in economy, as the materials are rela- 
 tively costly on a weight basis, and they must be made to provide the greatest 
 possible volume consistent with adequate- property 
 
 Paper can be "increased in volume" to produce core in numerous ways. Three 
 of the more nrominent types used for sa: :h panels might be describee as 
 "expanded," "figure 8," and "corrugated," In the expanded type, sheets of 
 treated paper are laid up and interspaced with parallel and uniformly spaced 
 strips of adhesive. Successive strips of adhesive are placed so that the 
 centers of any one layer are positioned at midpoint between the strips of 
 the preceding and the succeeding layer. The blanks after bonding are ex- 
 panded to form hexagonal cell sections, as shown in figure 1. There has been 
 an increasingly large interest in honeycomb structures of this typ 
 (1, 2, h s 5, 7). 
 
 Rept. No. R1918 -3- 
 
To produce the figure-8 core, a ribbon of paper is looped and bonded to form 
 circular cells resembling a figure 8 in cross section, as shown in figure 2. 
 For the production of either the figure-8 type or the expanded type, special 
 machines are required. 
 
 To produce the corrugated type of honeycomb, the cell or flute is made by 
 hot forming paper between fluted rolls on equipment of the type used in making 
 corrugated container board. The corrugated sheet, with or without uncorrugated 
 interleaving sheets, can be assembled in many ways. The three types discussed 
 chiefly in this report are shown in figures 3, h, and 5>. 
 
 Each type of core has advantages and disadvantages peculiar to its construc- 
 tion. The differences lie in equipment needed for production, limitations in 
 resins and adhesives, relative difficulty of accurate cutting, strength, thermal 
 insulating properties, ease of shipment, and other factors. As far as the base 
 material and resin treatment are concerned, such problems as fiber, resin, wet 
 strength, and durability are substantially common to all types. It was there- 
 fore intended that the results of this work should apply to any honeycomb struc- 
 ture made from paper treated with phenolic resin, although most of the data 
 were obtained from sandwich panels having corrugated paper cores. 
 
 This report summarizes some of the research that has been conducted at the 
 Forest Products Laboratory on the structural type of corrugated-paper-honeycomb 
 cores for use in building panels, as distinguished from use in aircraft. 
 Consideration is given to the resin treatment of the base material and to 
 the fabrication of the core and sandwich panels in the interest of low- cost 
 building material. Strength properties, bowing, thermal properties, dura- 
 bility, and fire resistance of the sandwich panels are discussed. Figure 6 
 shows a structural sandwich wall panel. 
 
 Base Paoer 
 
 For most experimental work on sandwich panels at the Laboratory, an unsized, 
 neutral kraft paper was selected, because it was strong, potentially available 
 in large quantities, and had favorable indications of permanence. The paper 
 contained no sizing in order to permit better resin penetration and to favor 
 permanence. 
 
 A ^0-pound (weight per 3*000 square feet) paper was usually employed for cores 
 having cells of A- flute size. Tests were made to explore the possibility of 
 reducing the weight of paper without undue sacrifice of strength in the sand- 
 wich panel. The use of lighter-weight papers results not only in an economy 
 of base material but also in a reduction in the amount of saturating resin 
 required to produce a given volume of core. Results showed that sandwich 
 panels with surprisingly high strength properties could be made with paper 
 weighing only 30 pounds per 3*000 square feet, which yielded core of a 
 density of about 2.5 pounds per cubic foot. Thermal insulation data obtained 
 on these core constructions showed that, for a particular core construction, 
 an improvement in thermal values could be achieved by reducing the weight of 
 the paper. The lower limit of paper weight was not determined, as it was felt 
 
 Rept. No. R1918 -I- 
 
that paper of much lower weight than 30 pounds per 3,000 square feet would be 
 difficult to handle in the corrugating and fabricating process. 
 
 No information is available on the minimum strength requirements of the base 
 sheet for satisfactory core structures. The relation between ordinary 
 properties of paper and the properties of cores is obscure, and studies 
 should be made to overcome this deficicn . ",\ r et strength can indicate the 
 effectiveness of the resin treatment, ring compression of the sheet may have 
 some s. sance, and accelerated agin; ma: ir - icatc permanence. Compres- 
 sion, tensile, and shear tests on assembled core are informative but are 
 costly and occur too late to assist paper makers in producing suitable papers. 
 
 Until the requirements of paper for use in core can be defined in a more 
 usable form, it will remain difficult to determine the best papers for par- 
 ticular uses. At present, it is certainly safe to assume that any of the 
 ordinary chemical pulps arc suitable. For many applications, especially those 
 involving short time expectancy for which maximum permanence is not essential, 
 a high- yield or scmichcmical pulp may be acceptable or even preferable for 
 structural uses. Reclaimed paper may also provide a suitable fiber for cer- 
 tain panels. Few data are available on the permanence of the high-yield or 
 semichemical pulps. The permanence of the p-per may not, however, be critical, 
 since the core mat rial in the sandwich panel is protected from ultraviolet 
 rays, which contribute to the loss of strength of paper upon aging. 
 
 Re sin- treated P aper 
 
 One of the first problems common to any paper core is that of resin treatment 
 of the paper. Although in some cases paper without resin might be acceptable, 
 it is assumed that for general usage only resin-treated papers can be con- 
 sidered. Since panels are likely to be subjected to damp or wet conditions, 
 the presence of resin in the paper is necessary to yisld a product that is 
 permanently strong and stiff in the wet condition. Even small amounts of cer- 
 tain resins can be effective in this respect. In terms of tensile strength, 
 it is not difficult to produce a paper that retains over 7^ percent of its 
 strength when dry after water soaking that would cause untreated paper to 
 lose almost all of its strength. 
 
 The work reported relates to the use of phenol- formaldehyde resins of several 
 types. Two of these are water-soluble and alcohol- soluble types and, although 
 both are suitable, they have inherent differences that need to be considered 
 when they are used with cellulose. With solutions of water-soluble resins, 
 the swelling effect of the water on the fiber permits the resin to penetrate 
 the fiber itself and through a "bulkina effect" (10) to maintain the fiber 
 in a swollen state. This also causes a reduction in the equilibrium moisture 
 content of the sheet with water vapor. iVhen alcohol- soluble resin is applied 
 in the absence of a swelling solvent, the effect appears to be chiefly one of 
 coating the fibers. The equilibrium moisture content of such paper, when 
 based on the oven-dry weight of fiber alone, is nearly as high as that of 
 untreated paper (3). Both may have high levels of strength in the wet 
 condition. 
 
 Rept. Mo. R1918 -5- 
 
Additional evidence that the water-soluble resin penetrates the fiber better 
 than alcohol- soluble resin was indicated by decay tests made on paper treated 
 with both types of resin (8). The results showed that a paper treated with 
 15 percent of water-soluble resin has considerable resistance to deca3>- fungi, 
 as measured by strength, while the paper containing an equal amount of alcohol- 
 soluble resin lost most of its strength under similar exposure conditions. 
 I'Jhile the water-soluble phenolic resins are not especially good as fungicides, 
 they can reduce the equilibrium moisture content of the paper and thus dis- 
 courage the grox-rt-h o r fungi. Inherent brittleness may be a disadvantage of 
 paper impregnated with water-soluble resin. 
 
 A resin treatment of 1$ percent of water-soluble resin (based on weights of 
 resin and fiber) was found to be adequate for providing paper of good strength 
 when wet, decay resistance, and handling characteristics during corrugation 
 and subsequent fabrication. Resin content in excess of about 15 percent does 
 not seem to produce a gain in strength commensurate with the increased 
 quantity of resin required. The tensile strength of a kraft paper treated 
 with 15 percent of water-soluble phenolic resin ma;> be greater when wet 
 after prolonged soaking in x^rater than the strength of the untreated paper 
 x<rhen dry. 
 
 Although more than 15 percent of resin may be required for certain uses, it 
 is promising that even a lower resin content will be acceptable for most 
 applications. In one series of tests, paper xdth as little as 5 percent of 
 x^ater- soluble resin was satisfactory in strength but showed less resistance 
 to decay organisms than a paper with 15 percent of resin. In this case, the 
 addition of 2 percent of pentachlorophenol in the paper overcame the 
 deficiency in decay resistance. The pentachlorophenol was dissolved in a 
 small amount of ethyl alcohol and added to the resin. The concentration of 
 the solution was further reduced by adding a mixture of 50 percent each of 
 water and alcohol. Using this mixture, a kraft paper was treated with 5 
 percent of a water-soluble phenolic resin and 2 percent of fungicide. The 
 pentachlorophenol did not seem to affect the cure of the resin as indicated 
 by the good strength values of the treated paper and honeycomb structures 
 when wet. Negligible losses in strength resulted from paper impregnated with 
 5 percent of resin plus 2 percent of pentachlorophenol after 2 months' expo- 
 sure to two types of wood- destroying fungi. No data are available to indi- 
 cate the minimum amount of pentachlorophenol required for good decay resist- 
 ance, but it is likely to be less than 2 percent. 
 
 Since resin is the costly item in the honeycomb structures, a series of tests 
 xtfas made to compare the properties of cores containing 5, 10, and 15 percent 
 of water-soluble phenolic resin and 15 percent of alcohol-soluble resin. 
 These cores, of the XM type (fig. h) with a density of about 2.5 pounds per 
 cubic foot, were bonded to plywood facings, and the panels tested for shear 
 and compressive strength in ^oth the dry and wet condition, 'Then tested dry, 
 panels with 5 percent resin content had a shear stress of 52 pounds per 
 square inch and a compressive strength of 35 pounds per square inch. In- 
 creases in saturating resin content of the core resulted in an increase in 
 both strength properties of the panels. Increasing the water-soluble resin 
 content of the core from 5 to 15 percent increased the shear strength of the 
 panels about I4O percent in the dry condition and about 300 percent in the wet 
 
 Rept. Mo. R1918 -6- 
 
condition. The effect of resin content on compressive strength was not so 
 great as on shear strength. The use of the alcohol-soluble resin as a 
 saturant resulted in sliphtly higher shear stress developed in bending and 
 compressive strength for dry panels but lower strength for panels tested wet. 
 
 Impregnating paper with resin on a separate impregnating machine is likely to 
 be somewhat costly, as it involves a secondary operation. Because the resin 
 content of the paper in the honeycomb core is in the low rane-e, it is reason- 
 ably simple to apply resin durin? the process of paper manufacture as a 
 means of reducing costs if the tonnare required can justify such a process. 
 It is possible to add resin during paper manufacture by: 
 
 1. Applying it at the paper-machine size oress or similar device. 
 
 2. Blending of fiber and resin in water suspension in the beater and retain- 
 
 this resin in the sheet durin' paper making. 
 
 3. Continuous addition of liquid resin at the machine headbox. 
 
 On the experimental paper machine at the Forest Products Laboratory, capers 
 with a wide range of resin content were readily prepared by use of the size- 
 press equipment. These papers were comparable in most respects to 
 matched sheets treated on a separate impregnation machine with the same 
 resin. From numerous experimental trials, it appeared that the size-press 
 treatment was one logical step toward the realization of a lower-cost core 
 material. 
 
 In recent years, suitable resins were developed for addition to both the 
 beater and the paper-machine headbox. The retention of early beater-added 
 phenolic resins was very poor, operating difficulties x^rere numerous, and 
 large chemical addition was required to properly precipitate the resin. 
 Several resins exist today that are suitable for this type of addition, and 
 experimental paper-machine runs have been made with promising results. In 
 one series, the addition of 12 or 20 percent of this resin to the pulp slurry 
 tended to free the stock on the machine wire and indicated no special operat- 
 ing difficulties. This is in sharp contrast to the "slowing" of stocks by 
 resin addition in earlier years. The paper after cure of the resin was con- 
 siderably less brittle than sheets treated with water-soluble phenolic resin 
 on a re sin- impregnating machine or size press. An indication of the tough- 
 ness of the sheet was given by sharply creasing the sheet by hand and then 
 obtaining a tensile strength value across the folded area. The paper re- 
 tained a surprisingly hirh percentage of its original strength. Microscopic 
 examination of the papers containing beater- added resin, using an Ultropak 
 microscope equipped with fluorescent lighting, showed only a faint haze of 
 resin, indicating very small particle size and excellent dispersion. 
 
 Core Fabri cation 
 
 Paper for the corrugated type of honeycomb used in this study was corrugated 
 on a 50- inch machine at the Forest Products Laboratory. The A- size fluted 
 
 Rept. No. R1918 -7- 
 
rolls common to the box industry were used. The procedure and weight of 
 paper were varied as needed, depending on the nature of the core desired. 
 Simplest from the standpoint of adaptability to existing equipment is the 
 PNL type of construction (fig. 3). This consists of alternate corrugated 
 and flat sheets and is made by cutting the continuous web of single-faced 
 corrugated paper and bonding these sheets into blocks of considerable thick- 
 ness, almost exactly as is now done in making blocks for insulation or 
 cushioning in packaging. The only difference, in fact, is in the use of 
 treated instead of untreated paper. In order to make the PNL type of core 
 with A flutes and a density of 2.5 to 3 pounds per cubic foot, it was neces- 
 sary to use a paper of about 30-pound basis weight (per 3*000 square feet). 
 This core had certain attractive features, such as excellent strength, a 
 slight compressibility in one direction, which aids in fitting pieces of 
 core into a panel of predetermined width, and ease of manufacture. 
 
 The XN type of core (fig. h) was used for most of the data given in this 
 report. To fabricate this type, corrugated sheets were assembled with the 
 principal flute directions of adjacent sheets at right angles and bonded at 
 the crests to form a block of core material. No uncorrugated facing sheet 
 was used for this construction. The base psper, a 50-pound (per 3,000 square 
 feet) kraft paper impregnated with approximately 15 percent of water-soluble 
 phenolic resin, j^ielded cores with a density of about 2.5 pounds per cubic 
 foot. To assure a good bond between adjacent cross-bonded sheets of corrugated 
 paper in the core, a phenolic adhesive was used in an amount equal to about 
 10 percent of the weight of the core. 
 
 The core was cut and assembled in the sandwich panel with one-half of the flutes 
 perpendicular to the facings. The flutes in the other half of the corrugated 
 sheets were parallel to the facings and contributed very little to the strength 
 of the core, but served rather as spacers for the flutes that carry most of the 
 load. It was demonstrated that the basis weight of the spacer sheets could be 
 reduced from 50 pounds to 30 pounds per 3,000 square feet with very little 
 effect on strength properties of the panel, A further economy in cost could 
 also be realized by reducing the resin content of this lighter paper. 
 
 The production of large volumes of this type of core would require certain 
 changes in existing commercial corrugating equipment. Pasting of a flat 
 sheet to the corrugated web is omitted, and more care is needed in handling 
 the web without stretching the corrugations. The crossing of the corrugated 
 sheets would require special but relatively simple equipment. 
 
 Either the PNL or the XN type of core can be assembled in the panel so that all 
 the flutes are parallel to the facings of the panel instead of perpendicular. 
 This results in a great improvement in the thermal insulation but a loss in 
 strength. The principal disadvantage of this flatwise construction is that 
 the integrity of the panel itself depends on each glue line between the sheets 
 as well as on the glue line betx^een core and facings. 
 
 The PN construction illustrated in figure 5 is similar to the PNL type except 
 that the flat sheet is omitted. The PN type is probably the most difficult of 
 the various corrugated types to fabricate, since it involves placing all 
 corrugated sheets parallel, the crests of each sheet being in direct contact 
 
 Rept. No. R191B 
 
with the crests of adjacent sheets. To make experimental amounts of this core, 
 segments of ordinary soda straws inserted at the four corners of each sheet 
 were used to achieve good crest-to-crest alinement. This type of core has 
 certain ideal features for studies of materials and design factors. A con- 
 siderable amount of study was based on this type of core, particularly as 
 related to specialized aircraft parts (6, 9). 
 
 Most of the early work in making cores involved the use of costly adhesives 
 to bond paper sheets to each other. In order to better accommodate existing 
 machines and to provide greater economy, a study of the effect of the quality 
 of the bond between sheets of paper on the properties of the sandwich was made. 
 In the assembly of ordinary corrugated board, a nonwater-resistant adhesive is 
 usually employed. It was hypothesized that the bond between individual sheets 
 within the honeycomb core was not critical, since the ends of each flute were 
 bonded to both facings with a high-quality, water-resistant adhesive. The cost 
 of the adhesive and the equipment necessary to use such an adhesive are uneco- 
 nomical features in the process, unless the high-quality bond is required. 
 
 A series of tests was made to compare panels having cores in which individual 
 sheets were bonded with (a) phenolic resin, (b) urea resin, (c) sodium 
 silicate, and (d) no adhesive. Although the assembly of corrugated sheets in 
 a panel without the use of a sheet-to-sheet adhesive is impractical, it was 
 reasoned that the resultant properties would establish the lower limits of 
 strength. The nature of the crest adhesive appeared to have only a slight 
 effect on the shear strength of panels tested in either the dry or wet condi- 
 tion if the web of the corrugation was parallel to the span. Panels with the 
 silicate adhesive had slightly higher strengths when dry and slightly lower 
 strengths when wet than those with the phenolic -re sin adhesive. These tests 
 demonstrated that it was desirable to have the web of the corrugation parallel 
 instead of perpendicular to the span to obtain maximum strength, regardless 
 of adhesive used. 
 
 Sufficient commercial trials have been made to demonstrate the feasibility of 
 running impregnated papers on corrugating machines, and such papers have been 
 corrugated at ordinary commercial speeds. The ordinary size of flutes used in 
 the box industry produces satisfactory core, but, for economy, it is desirable 
 to use a much lighter paper than the usual corrugating medium for boxes. 
 Another and possibly better approach would be to use larger flutes and heavier 
 papers, resulting in a faster build-up of core of a given density. A few 
 existing machines have larger flute sizes, and any new machine designed 
 expressly for this purpose undoubtedly should use larger flutes. 
 
 Panel Manufacture 
 
 Facings 
 
 Any honeycomb core is satisfactory only in relation to the facings it supports 
 and, conversely, the suitability of any facing may depend on the core. Per- 
 haps one of the best long-term advantages of the sandwich panel is the great 
 latitude it provides in choice of facings and the opportunity to use thin 
 
 Rept. No. R1918 -9- 
 
sheet materials because of the nearly continuous support by the core. The 
 stiffness, stability, and, to a large extent, the strength of the sandwich 
 are determined by the characteristics of the facings. There are a wide 
 variety of sheet materials suitable for facings or skins on honeycomb cores. 
 Some of the different types that have been used include plywood or single 
 veneers overlaid with a resin-treated paper; hardboards; asbestos board; 
 metals, such as aluminum, enameled steel, stainless steely or magnesium sheet; 
 wallboards; fiber-reinforced plastics or laminates, and veneer bonded to metal. 
 
 Plywood is a versatile facing material, and the performance of panels with 
 such facings can now be well predicted. It has good dimensional stability 
 characteristics and strength properties end can be dependably bonded to core 
 with proven adhesives. T -Jhen exposed to outdoor conditions, some plywoods, 
 such as Douglas-fir, may check considerably, and grain raising becomes 
 apparent. These defects may be eliminated to a large extent by applying a 
 resin- treated paper- overlay sheet to the outer face of the panel, resulting 
 in a smooth surface and uniform base for painting. 
 
 Since the facing is securely bonded to the cere in the sandwich panel, a two- 
 ply veneer facing, with or without an overlay, can be used as in making flush 
 doors. Although each facing is unbalanced, the panel itself is in balance. 
 Sandwich panels with dissimilar facings may also be suitable for certain uses 
 if the proper unbalance is selected. These might be considered in cases where 
 the exposure conditions on each side are not in balance. Unbalanced panels, 
 however, should be used only with considerable caution. Panels with facings 
 of hardboard or veneer x^rith paper overlays are promising panels from the stand- 
 point of being lightweight and economical. Excess dimensional movement may 
 limit use of this type in some cases. 
 
 Cuttin g of Core 
 
 In assembling the core for fabrication of the sandwich panels, the cutting of 
 strips of core material to accurate thickness is essential to avoid difficulty 
 in making or using the panels. Core material may be reduced to the desired 
 thickness by sawing on a circular saw, band saw, vibrating knife, or, in some 
 types , by a guillotine cutter. It may also be made to the desired thickness 
 to avoid cutting. An accurate cut can usually be made with a circular saw, 
 but this requires a rather thin section and therefore more cuts. With the 
 proper blade, speed, and technique, a band saw will reduce the core blocks to 
 proper size within +0.01 5- inch tolerance, which is probably sufficient for 
 panels of 2 inches or more in thickness. A band saw with five teeth per inch, 
 operating at a blade speed of 3,^00 feet per minute, was found to be suitable 
 for most of the test panels, A vibrating-knife type of cutter may be suit- 
 able for cutting honeycomb core, because it eliminates the saw-cut losses. 
 For commercial production, it may be desirable to cut the core initially to 
 a broader tolerance and control the thickness accurately with a sander or 
 other machine. It may be sufficient to simply roughen or slit the edges of 
 the core, so that, when pressure is applied to the panel in the press, the 
 effective thickness will be uniform. 
 
 Rept. No, R1918 -10- 
 
Pressing 
 
 Satisfactory performance of the sandwich panel depends to a great extent ( • 
 the bond between the core anc 5 facings. The development of sandwich panels was 
 made possible, in fact, by the introduction of high-quality adhesives in 
 recent years, 
 
 A study was made of the durability of a number of typical exterior resin 
 adhesives. Paper- honeycomb core made from paper treated with 15> percent of 
 resin was bonded to plywood and tested in tension and shear after exposure to 
 a number of extreme temperature end moisture cycles. The following adhesiv 
 produced sufficiently strong bonds between the core and facings to force a 
 high percentage of failure in the core: 
 
 (a) An acid-catalyzed, high- temperature-setting, phenol- resin adhesive. 
 
 (b) An alkaline- catalyzed, intermediate- temperature- setting, phenol-resin 
 
 adhesive. 
 
 (c) An alkaline-catalyzed, room- temperature- setting resorcinol-resin 
 
 adhesive. 
 
 (d) An alkaline- catalyzed, high- temperature-setting, phenol-resin adhesive. 
 
 The bonding of most of the experimental sandwich panels was done in hot-press 
 equipment. For laboratory work, the short pressing cycle and the long assembly 
 time favored the acid-catalyzed, high-temperature- setting, phenol-resin adhe- 
 sive. Adhesive was applied to both the core and facings with a rubber roller 
 or an ordinary paint roller at the rate of about 22 grams per square foot of 
 surface, one-half to the core and one-half to the facings. It was also 
 demonstrated that it may not be necessary to apply adhesive to the core in 
 order to produce a satisfactory bond. Core and facings were allowed to stand 
 after spread of adhesive to permit the evaporation of the solvent. Panel 
 components were then assembled and placed in a hot press at a temperature 
 of 230° F. (for the aoid-catalyzed phenolic adhesive) to cure the adhesive. 
 Pressures ranging from 15 to 50 pounds per square inch were used, depending 
 on density and type of core construction. Test panels up to lh feet in 
 length and 6 inches in thickness were made in a h by U-foot laboratory press 
 using step-pressinr techniques. Flat sandwich panels were produced with this 
 method. 
 
 Special presses are indicated for sandwich- panel manufacture. The pressures 
 required are usually lower than can be obtained in the range of good pressure 
 control on ordinary plywood or plastic presses. Because pressure require- 
 ments are low, simple and perhaps less costly presses could be used. Contin- 
 uous roller presses or bag-molding equipment may also be suitable. Certain 
 special problems arise in the pressing of sandwich panels, but their manufac- 
 ture is basically not complicated. 
 
 Pressing of sandwich panels having dissimilar facings in hot presses has been 
 difficult because of unequal dimensional movement of the facings due to mois- 
 ture or thermal changes. In such cases, cold-pressing may be necessary. This 
 of course creates its own problems with respect to adhesive limitations, 
 assembly time, wat< r introduced with adhesive, long time under pressure and, 
 in some cases, durability of the bond. Ordinary cold-setting adhesives, 
 
 Rept. No. R1918 -I 1 - 
 
however, are undoubtedly adequate for a host of prospective sandwich- 
 panel applications. 
 
 One of the most persistent difficulties in the use of sandwich panels is 
 in the problems caused by the necessity for edges, inserts, and connectors 
 for panels. In some cases, the problem involves tying together thin fac- 
 ing materials without severe stress concentrations and, in other cases, 
 such as furniture, the problem is caused by u show-through" of core or 
 inserts through decorative facings. These problems, probably as much as 
 any other factor, have restricted the development and use of honeycomb 
 core on a larger scale and should be studied from several standpoints. 
 The differential dimensional movement between core and insert materials 
 should be at a minimum, including the rate as well as the degree of move- 
 ment. Adl'.esives and moisture introduced by adhesives would be a factor in 
 this study. Examples of materials to consider for edges or inserts would 
 be end-grain wood, plywood on edge, part honeycomb and part vood, metal, 
 dense honeycomb, and mastics or fillers© Another approach would be to 
 study engineering design factors for getting panels into the ultimate 
 product without the use of molded-in inserts; this has certain ideal 
 features from the standpoint of sandwich-panel manufacture, and it 
 simplifies pressing. 
 
 Properties of Sandwich Panels 
 
 Strength 
 
 Strength data were obtained on both large and small sandwich panels compris- 
 ing paper-honeycomb core with facings of veneer, plywood, hardboard, asbestos 
 board, aluminum, or other materials. These tests included static bending, 
 impact bending, and column tests. The most common test conducted was the 
 static bending test, which consisted of supporting the panels at the ends 
 and applying an increasing load at two quarter-span points. The amount of 
 deflection was recorded at various loads to the design load or until failure 
 occurred. This test not only produced useful information on stresses 
 developed in the facings and on stiffness of the assembly but gave infor- 
 mation indicative of the shear strength of the cores and the quality of the 
 bond between core and facings. 
 
 In the impact bending test, a 10-inch-diameter sandbag, weighing 60 pounds, 
 was dropped on the center of a large sand/rich panel, beginning at a height 
 of 1 foot and increasing by increments of 1 foot to a height of 10 feet or 
 until failure occurred. Panels were supported at each end, and both, 
 instantaneous and permanent deflections were measured. The vertical load 
 test was made to determine the ability of a panel to meet certain column 
 structure requirements. The shortening of the panel in the vertical 
 direction and its lateral deflections were measured. 
 
 Strength tests conducted on large-size sandwich wall panels, 3 inches 
 
 thick and having plyvood facings, indicated higher shear strengths developed 
 
 in bending and greater resistance to vertical loads than found in conventional 
 
 Rept. No. R191B -12- 
 
house construction. These panels withstood maximum loads in bending of at 
 least 12 times the design loads of 20 pounds per square foot sometimes used 
 for house panels. 
 
 A considerable amount of design data has been obtained to make it possible to 
 predict the strength of sandwich panels or to design them to meet certain 
 strength requirements. The core of the sandwich may be considered as only a 
 means of separating and stabilizing the facings to produce a member similar 
 to an I-beam. The core simulates the web and the facing the flanges of an 
 I-beam, wince the core in the sandwich is subjected to shear, as is the web 
 of the I-beam, it is essential that the cor sufficiently strong to wi 
 stand the shearing stresses imposed. It is assumed, however, that the core 
 itself adds no stiffness to the sandwich construction. 
 
 The following formulas are used for predicting the behavior of sandwich con- 
 structions subjected to various loads. No consideration is given to concen- 
 trations that may occur at joints and fastenings. Each type of fastening 
 will have to be considered individually insofar as its effectiveness in trans- 
 mitting loads and its strength are concerned. 
 
 The constructions arc assumed to be well bonded with an adhesive and strong 
 and durable enough to perform adequately for the conditions of use. 
 
 The mean stresses in the facings of sandwich construction under flatwise 
 bending loads are given by 
 
 - ^ (i) 
 
 1,2 f x 2 (h+c)b 
 
 where cr ^ ? = facing stresses in facings 1 or 2 
 
 f, , = thicknesses of facings 1 or 2 
 
 I: = moment 
 
 h = total sandwich thickness 
 
 c = core thickness 
 
 b = width of sandwich 
 
 The shear stress in the core of sandwich construction und - loads is 
 given by 
 
 2V 
 
 T = (FT^ (2) 
 
 where r = core she:ir stress 
 
 V = shear load 
 Rept. No. R191B -13- 
 
The stiffness of sandwich construction having facings of the same material and 
 equal in thickness is given by 
 
 E r (h 3 -c 3 )b 
 
 D = (3) 
 
 12X 
 
 where D = stiffness 
 
 E = modulus of elasticity of the facings 
 F 
 
 ^ =0.99 for wood or plywood facin-s 
 
 ^ =0.91 for isotropic facings 
 
 The maximum deflection of a sandwich construction having facings of the 
 same material and equal in thickness under a uniformly applied load and 
 simply supported at the ends is given by 
 
 C Pa 3 I" -rr 2 cfE^ j 
 
 3oU D 2 
 
 2 \ a G 
 L c 
 
 where A = maximum deflection 
 
 P = total load on the sandwich 
 
 G = shear modulus of the core material 
 c 
 
 a = span 
 
 Note: An approximation of the deflection may be obtained by neglecting the 
 last term in the brackets in formula h. This term will be less than 10 percent 
 for most constructions on long spans but may become appreciable for short spans. 
 
 Strength data for sandwich panels of various thicknesses and comprising 
 different facings are given in table 1. Most of the information on the 3- inch- 
 thick panels was obtained by experimentation, while values for the 1- and 2- 
 inch thicknesses were calculated mathematically with the above formulas. Values 
 in the table show that reducing, the thickness of the panel from 3 inches to 
 1 inch would decrease its stiffness 10 to 1$ times and decrease the maximum 
 load that it will suoport 3 to h times. To meet a span- deflect ion ratio of 
 270 or more under a uniform load of 20 pounds per square foot, a structural 
 sandwich wall panel on a span of 96 inches would have to be more than 2 inches 
 thick if its facings were of l/'i-inch Douglas-fir plywood. Certain properties 
 of the honeycomb core may be varied considerably with only a mild deviation in 
 stiffness of the resultant sandwich panel. 
 
 Rept. No. R1918 -1.' - 
 
Dimensional stabi l ity and Moving of P anels 
 
 In a structure such as a sandwich panel in which two facings are bonded to a 
 core to form an integral panel, any Lonal mov< i one facing has an 
 effect on the entire panel. A differential movement of facings causes bow- 
 ing on an unrestrained panel. If dimensional change of both facings is equal, 
 the length and width dimensions of the panel will increase or decreas< , but 
 bowing will not result. This is important for many uses. The problem is 
 chiefly related to the facings because the core does not have enough stiffness 
 to cause ' of the panel or to cause it to remain flat, laboratory tests 
 have demonstrated that the dimensional stability of a panel is not affected by 
 the type of core. The magnitude of the bowing effect, however, depends on the 
 thickness of core. The use of dissimilar facings is often desirable from an 
 economic standpoint, yet dimensional instability of facings during panel manu- 
 facture or exposure may rule out possible benefits, as indicated earlier in 
 this report. 
 
 For many applications, such as integral housing panels or what might be des- 
 cribed as conventional prefabricated house panels, bow i : almost always a 
 consideration. During exposures in housing when temperature and relative 
 humidity conditions are approximately equal on both sides of the panel, the 
 panel will remain reasonably stable. As col< Dps, the moisture 
 
 content on the inner facing of roscopic mat rial decreases and that of 
 the outer facing increases, causing an oui f ' ow. The effects of winter 
 temperatures produce opposite curvature . r v the summertime, the bowing 
 is less pronounced, since the moisture differential is less. This character- 
 istic bowing pattern has been experienced by producers of stressed- skin, ply- 
 wood prefabricated panels for many years. Although shrinkage on ansion 
 in plywood with moisture changes is slight, the diff i . is enough to cause 
 a detectable bow. In wood, hnrdboard, or other hygroscopic materials, warp- 
 can be attributed to both moistur ai temperature differences, while in 
 al- faced sandwich panels, only temperature differences cause dimensional 
 changes, and these changes are due to thermal expansion or contraction. 
 
 In conventional construction, it is desirable to install vapor barriers, usually 
 of asphalt-impregnated paper or metal foil, to block the migration of vapor to 
 the cold side of a wall. Various experiments were conducted or proposed to 
 improve vapor resistance of sandwich panels, such as bonding of metal foil; 
 blending aluminum flake with resin bonding adhesives; use of plastic vapor 
 barriers between veneers, overlay papers, and special finishes; and, of course, 
 metal or plastic facings. jause added cost is likely, some of these should 
 not be resorted to unless their need has been demonstrated. Panel edges may 
 also be sealed by various methods if this is indicated. As a generalization, 
 the bowing of sandwich panels is probably neither greater nor less than 
 stressed-skin housi aving similar facings. 
 
 In one series of tests of bowing under laboratory conditions, six sandwich 
 panels, each 3 inches thick, subjected to s 1 ire differences 
 
 to determine their bowing characteristics. proximately 20 by 72 inches 
 
 were placed in openings between two rooms, one of which was held at a tei - 
 
 ture of -20 ?. and the other at +70' 7. The edges of the panels were 
 sealed with aluminum fail to prevent the entrance of moisture. The facings 
 
 Rept. Uo. rtl9l8 -1 - 
 
consisted of three-ply, l/U-inch Douglas- fir plywood with and without paper 
 overlay* two-ply, 1/5;- inch plywood with paper overlay; and overlaid, l/8-inch 
 veneer. One of the plywood- faced panels had two coats of aluminum paint on 
 the warm side only. 
 
 All panels deflected slightly toward the warm side immediately after being in- 
 stalled, as the facing on the cold side underwent thermal contraction. As the 
 test progressed, the deflection became less prominent, because the moisture 
 content of the cold side increased. The overlaid, three- and two-ply construc- 
 tions after 9 weeks' exposure became straight. At the end of the exposure, the 
 overlaid veneer panel showed the greatest deflection, although the overlaid, 
 three-ply panel showed the greatest initial change. After the panels were re- 
 moved from the wall and exposed to 70° F. on both sides for 3 hours, the panels 
 reversed themselves, bowing toward the cold wall. This was probably caused by 
 the plywood on the cold side undergoing thermal expansion and possibly, in part 
 by the absorption of moisture from the melting of the frost crystals that had 
 formed inside the panels during the exposure. Although these test conditions 
 are not exactly typical of ordinary service conditions, the amount of deflec- 
 tion observed in these panels would not be considered objectionable. Accurate 
 data on the dimensional change of sheet facing materials due to moisture or 
 thermal changes is very important to a study of panel bowing. 
 
 It is possible to calculate mathematically the bowing of a sandwich construc- 
 tion if the percent expansion of each facing is known. The maximum deflection 
 due to bowing caused by the expansion of one facing resulting from tempera- 
 ture or moisture differential is given approximately by 
 
 ^ = ka2 
 
 30 Oh 
 
 where k = percent expansion of one facing as compared to the opposite facing 
 
 a = length of panel 
 
 h = total sandwich thickness 
 
 Using this formula, the approximate bowing deflection was calculated for 
 hardboard- and plywood- faced sandwich panels of various thicknesses. This 
 information is given in table 1. The bowing deflection due to moisture or 
 temperature differences is greatly dependent on thickness of the panel. As 
 an example, a 96-inch-long, plywood- faced panel 3 inches thick bows about 
 one-third as much as a similar panel 1 inch thick. 
 
 Thermal Conductivity 
 
 The basic values determined in establishing the thermal conductivity of a 
 material or combination of materials used in a structure can be defined as 
 follows: 
 
 Rept. No. K1918 -1 '- 
 
k - thermal conductivity: The tine rate of heat flow through 
 a homogeneous material or one of uniform structure under steady 
 state conditions, expressed in British thermal units per hour 
 per square foot per inch of thickness per degree difference in 
 temperature between surface of the material. 
 
 U - ove^-all coefficient of heat transmission, air to air: 
 The time rate of heat flow expressed in British thermal units 
 per hour per square foot per degree difference in temperature. 
 
 The term U applies to the combination of materials used in a 
 construction; for example, both facings and core of a sandwich, 
 and includes standard values for fj_ and f for still air on 
 
 the warm side and air moving at 15 miles per hour on the cold 
 side. 
 
 The usual method of comparing insulating values for wall construction is by 
 comparison of heat transmission coefficients or U values. The U value for a 
 construction is found from the relation 
 
 4 
 
 where the values of k are for the several constituents of the wall and t 
 the thickness of these constituents. 
 
 To compare insulating characteristics of various core constructions, tests 
 were made on 1- inch- thick specimens using a guarded hot-plate apparatus. The 
 data are summarized in table 2. The conductivity value of the core is 
 affected by: cell size, density, resin content, construction, and type of 
 material. Thermal insulation was significantly affected by the type of 
 construction; values for k varied from as low as 0.30 to as high as 0.55 with 
 variations in core structure. A k value of 0.U5 and 0,'t: British thermal 
 units per hour per square foot per inch of thickness ocr °7. was obtained 
 with cross-corrugated (XW) or the parallel- corrugated (P'l) structures having 
 flutes perpendicular to facings. By placing these same structures in the test 
 so that flutes were parallel instead of perpendicular to the plates, the 
 value was reduced to 0.30 to 0.3!.. " n ut these structures have other disadvan- 
 tages, as mentioned earlier in this report. For any particular core construc- 
 tion, the density of the structure affects the insulation value. As an ex- 
 ample, a PIJL core having a density of about 5.5 pounds per cubic foot had a k 
 value of about 0.59, while a similarly assembled core, with a density of 3.3~> 
 pounds per cubic foot, gave a k value of about 0.U7. 
 
 An actual U value of 0.1 50 British thermal units per square foot per hour per 
 °F. was obtained on a 3-inch- thick, large-size panel having the cross- 
 corrugated (XM) type of core and l/h-inch, three-ply Douglas-fir plywood 
 facings. This panel was exposed in a wall between two rooms, one controlled 
 at 72° F. and U0 percent relative humidity and the other at -20° F. Th.is 
 0.150 value compares reasonably well with a calculated U value of O.lUl for 
 the same panel. If the thickness of this oanel was reduced to 1 inch, the 1 
 value could be expected to increase to about 0.U2. - 
 
 Rept. No. R1915 -17- 
 
An improvement in the insulation value of the sandwich construction can be 
 realized by filling the honeycomb core with insulation or a foamed- in-place 
 resin. A reduction in the k value of a corrugated core from 0,U6 to O.hO 
 British thermal units per hour per square foot per inch of thickness per °F. 
 was obtained when a phenolic resin was foamed into the core. A slightly 
 lower value was obtained through the use of fill insulation. Foaming of 
 resins into honeycomb appears very promising as a means of improving thermal 
 insulation and fire resistance. 
 
 F ire Resistance 
 
 Sandwich panels were tested for fire resistance by two methods designed 
 for housing materials: (1) Exposing one face of the panel to a standard 
 flame that approximates conditions of fire in a house in which furnishings 
 are being burned; and (2) introducing flame through a hole in the facing, as 
 might occur at openings for electrical conduit or other house equipment. 
 
 The first method gives fire-resistance values for sandwich construction that 
 are comparable with those accepted for other types of house construction. 
 The critical factor in this test is the ability of the facings and the bond 
 between the facings and core to resist the high temperature without developing 
 construction failures. Obviously when the facings or bond have failed, the 
 construction is gravely weakened, since the facings are the principal load- 
 carrying elements of the sandwich. At the Laboratory, this test was con- 
 ducted in a gas- fired furnace according to the exposure conditions specified 
 in American Society for Testing Materials (ASTH) Specification No. E-119-li7. 
 The fire resistance of the wood- faced sandwich panels was appreciably higher 
 than hollow panels faced with the same thickness of plywood. When aluminum- 
 faced panels were exposed to the gas, a rather rapid buckling of the facing 
 and failure of the bond vsually occurred. In most cases, the cores were badly 
 charred after the test but retained their original form to a considerable 
 extent. 
 
 The fire resistance of the sandwich panel can be increased considerably by 
 incorporating in the core foamed resin or an intumescent coating material, 
 such as certain types of sodium silicates. It is conceivable that a material 
 could be developed for deposition in the core that would serve the triple 
 function of bonding the paper sheets, providing foam for fire resistance, 
 and also improving, thermal insulation. 
 
 By the second method, the likelihood of flame spread in the core was investi- 
 gated. This was done by cutting a small hole in one facing, holding the panel 
 vertically, and applying a gas flame to a small area for U minutes. In panels 
 having flutes perpendicular to the facings, such as in the expanded figure-8 
 and the corrugated PN and PNL types of core, only slight flame spread 
 occurred. Burning was restricted to the honeycomb material in contact with 
 the flame. When the flame was removed, flaming stopped immediately. Some 
 glow persisted for an additional 1 to 2 minutes. In the case of the cross- 
 corrugated XN type of core in which one-half of the flutes are parallel to 
 the length of the panel, the spread of flame occurred in the vertical direction 
 due to the open channels. This could no doubt be readily improved by placing a 
 
 Rept. Mo. R1918 -18- 
 
barrier sheet at the top of the panel or at intervals in the panel height, or 
 perhaps by simply turning the length of the core blocks at 90 c to the vertical 
 direction. 
 
 The re sin- treated core in itself is not fire-resistant, but its use between 
 sandwich facings does not seem to be hazardous. It is possible to add fire- 
 resistant chemicals to the paper or to dip or spray the core assembly with 
 such chemicals, but this is believed to be unnecessary when wood-based facings 
 are employed in the panel construction. Such treatments could be effected, if 
 necessary, but might create gluing and moisture- absorption problems, and their 
 effects on long-time aging characteristics of paper are not well understood. 
 
 Sand wi c h-panel Exterior Test Unit 
 
 In 19U7* an experimental sandwich unit about 12 by U0 feet in size was built 
 on the Laboratory grounds for long-time exposure tests. Incorporated in this 
 unit are various types of sandwich wall, floor, partition, and roof panels. 
 All have paper-honeycomb cores and are faced with veneer, plywood, overlaid 
 plywood, hardboard, asbestos board, or aluminum for comparative purposes. 
 The unit on the inside is equipped with heating coils and is controlled dur- 
 ing the winter at a temperature of 72° F. and a relative humidity of U0 per- 
 cent. The outside of the unit is exposed to the variable outdoor temperatures 
 of the Madison, Wis., area. 
 
 Before the wood-faced and aluminum- faced panels were installed in the test 
 unit, they were tested to determine their deflection and span-deflection 
 ratios at design loads. The 3- inch-thick, wood- faced wall panels met the 
 requirement that the span-deflection ratio be not less than 270 under a design 
 load of 20 pounds per square foot. Floor panels of 6- inch thickness with fac- 
 ings of 3/8- inch, five-ply Douglas- fir plywood had a span-deflection ratio of 
 about 800 under a load of k0 pounds per square foot (11). 
 
 After 16 months' exposure, four wall panels were removed from the experimental 
 unit for test. Two panels had facings of l/h-inch Douglas- fir plywood, and 
 two had facings of 1/8- inch Douglas- fir veneer overlaid with paper. Cores were 
 of the cross- corrugated XM type. On removal, the panels showed no visible 
 signs of deterioration in either the core or the facings. The stiffness of 
 the exposed panels was equal to or slightly greater than their stiffness prior 
 to installation. This increase in stiffness may possibly be due to the 
 further cure of the resin upon aging. Maximum loads obtained on these panels 
 varied from 13 to 20 times the design load of 20 pounds per square foot. 
 
 To obtain additional information on the effect of continuous weathering on 
 appearance and strength, sandwich panels having 1- inch-thick, paper-honevcomb core 
 and various wood facings with and without paper overlays were subjected to the 
 following conditions in the Laboratory: 
 
 1. Immersed in water at 122° F. for 1 hour. 
 
 2. Sprayed with wet steam at 19h° to 200'- F. for 3 hours. 
 
 3. Stored at 10° F. for 20 hours. 
 
 Ii. Heated in dry air at 212° F. for 3 hours. 
 
 Rept. No. R1918 -19- 
 
5. Sprayed with wet steam at 19U* to 200° F. for 3 hours. 
 
 6. Heated in dry air at 212° F. for 18 hours. 
 
 This sequence of exposures was continued through six cycles, after which 
 appearance of the specimens was noted, and bending tests were made to determine 
 any change in strength properties, results of these tests were compared with 
 those of tests made on control specimens not subjected to the aging tests. The 
 reduction in shear stress developed in the cores of the aged specimens was 
 about 20 to 30 percent as compared with that of the control specimens. Reduc- 
 tion in stiffness was about 20 percent as obtained from a comparison of load- 
 deflection ratios. The sandwich specimens were exceptionally straight, and 
 no visual defects were apparent in the core. Although accelerated aging tests 
 are never completely satisfactory, the performance of such specimens plus the 
 observations made on the actual exposure unit indicate that good performance 
 could be expected. 
 
 One of the reasons for building the test unit was to obtain measurements of 
 the actual bowing under outdoor conditions. Some data were obtained during 
 the first year of exposure of these panels. Shortly after the unit was con- 
 structed in June 19^7, deflection data showed a tendency of the panels with 
 wood facings to bow slightly inward. In November when the heating system 
 was turned on, the wood panels reversed their movement and bowed outward. 
 The outward bowing was due to the lower outside temperature, which caused an 
 increase in moisture content of the outer facing, and the shrinkage of the 
 inner facings due to the heat. The bowing increased progressively in the 
 wood-faced panels as the average outdoor temperature decreased and continued 
 until late in March when the outdoor temperature began to rise again. The 
 maximum bowing recorded in plywood- faced panels was about one-fourth inch. 
 Aluminum- faced panels, not being affected by moisture, bowed toward the in- 
 side as the temperature of the outside dropped. On a hot day with a high 
 surface temperature on the outside, an outward bow could be noted. Results 
 of the first year indicated that bowing of sandwich panels with facings of 
 three-ply plywood is consistent with that of panels with three-ply stressed 
 facings. Whether the inner surface was untreated, had aluminum paint, or had 
 an overlay appeared to be unimportant in panels with three-ply facings. 
 
 As the development of the sandwich structural panel grows, the need for more 
 data on the behavior of such panels under ordinary exposure conditions will 
 increase. It is hoped that this test unit will be helpful in resolving some 
 of the problems regarding large-scale use of sandwich panels. Although there 
 are many other uses for sandwich panels, the evaluation of these panels under 
 housing conditions provides information that applies to most other products 
 in which use of sandwich panels might be considered. 
 
 No attempt is made in this report to detail actual or proposed uses for sand- 
 wich panels, but these include partitions, doors, spandrell panels, and other 
 constructions in houses, trailers, shelter buildings, warehouses, and farm 
 buildings, lightweight shipping containers, and furniture. Because of the 
 inherent structural strength of these panels, the greatest total benefit can 
 probably be realized by using them to carry the principal loads in a construc- 
 tion, not just to provide coverage. The general trend toward the use of sheet 
 materials, both on the inner and outer surfaces of buildings, also points to 
 long-term importance of sandwich panels as building materials, 
 
 Rept. No. R1918 -20- 
 
Literature Cite d 
 
 (1) AUTHOR UNKNOWN 
 
 19U6. EXPANDED STRUCTURAL PLASTICS. Bakelite Review, July. 
 
 T55T. HONEYCOMB COKES OF AGE. Modern Plastics Vol. 28, No. 11. 
 
 (3) BAIRD, P. K., SEIDL, R. J., AND FAHEY, D. J. 
 
 19U9. EFFECT OF PHENOLIC RESINS ON PHYSICAL PROPERTIES OF KRAFT 
 PAPER. Forest Products Lab. Rept. R17SO, 10 pp., illus. 
 
 (U) GRAHAM, P. H. 
 
 1951. HONEYCOMB CORE GIVES BEST WEIGHT-STRENGTH RATIO. Veneers 
 & Plywood Vol. U5, No. 8. 
 
 (5) LINCOLN, J. D. 
 
 19U6. PRODUCTION OF HONEYCOMB CORES. Modern Plastics Vol. 23, No. 9. 
 
 (6) NORRIS, C. B. AND MACKIN, G. E. 
 
 19U8. AN INVESTIGATION OF ! ' fIC:\L PROPERTIES OF HONEYCOMB STRUC- 
 TURES MADE OF RESIN- IMPREGNATED PAPER. Natl. Advisory Committee 
 for Aeronautics. Tech. Note No. 1529. 
 
 (7) RINGELSTETTER, L. A., VOSS, A. ''., A 'ORRIS, C. B. 
 
 1950. EFFECT OF CELL SHAPE ON COMPRESSIVE STRENGTH OF HEXAGONAL 
 HONEYCOMB STRUCTURES. Natl. Advisory Committee for 
 Aeronautics. Tech. Note No. 22U3. 
 
 (8) SEIDL, R. J., KUENZI, E. W., FAHEY, D. J., AMD MOSES, C. S. 
 
 1951. PAPER-HONEYCOMB CORES FOR STRUCTURAL BUILDING PANELS: EFFECT 
 OF RESINS, ADHESIVES, FUNGICIDES, AND 'EIGHT OF PAPER ON 
 STRENGTH AND RESISTANCE TO DECAY. Forest Products Lab. Rept. 
 No. R1796, 16 pp., illus. 
 
 (9) ______ FAHEY, D. J., AND VOSS, A. W. 
 
 1~TT PROPERTIES OF HONEYCOMB CORES AS AFFECTED BY FIBER TYPE, FIBER 
 ORIENTATION, RESIN TYPE AND AMOUNT. Natl. Advisory Committee 
 for Aeronautics. Tech. Note No. 256U. 
 
 (10) TARKOW, HAROLD 
 
 19U9. THE SWELLING AND SHRINKING OF WOOD, PAPER, AND COTTON TEXTILES 
 AND THEIR CONTROL. Tappi Vol. 32, No. 5. 
 
 (11) U. S. FOREST PRODUCTS LABORATORY 
 
 19U8. PHYSICAL PROPERTIES AND FABRICATION DETAILS OF EXPERIMENTAL 
 
 HONEYCOMB-CORE SANDWICH HOUSE PANELS. Housing k Home Finance 
 Agency Tech. Paper No. 7. 
 
 Rept. No. R1918 -21- 
 
Table 1. — Stiffness and stre ngth of structural sandwich panels on a 96 -inch 
 
 _ 
 
 span- 
 
 Facings 
 
 : Panel 
 • thick- 
 ness 
 
 : Center 
 
 p 
 
 : deflection^ 
 
 : Span- 
 : deflection 
 
 ratio— 
 
 Maximum : 
 
 uniform : 
 
 load : 
 
 Approximate 
 
 bowing _ 
 
 deflection— 
 
 
 In. 
 
 3 
 2 
 1 
 
 3 
 2 
 1 
 
 : 3 
 2 
 
 1 
 
 3 
 2 
 1 
 
 3 
 2 
 1 
 
 : In. 
 
 : 0.18U 
 
 J:53 
 : 2.390 
 
 : .207 
 : .U83 
 : 2.1).|0 
 
 : .169 
 : .395 
 : 1.810 
 
 : .202 
 
 : .U98 
 
 2.630 
 
 .313 
 
 : .735 
 
 3.350 
 
 521 
 : 212 
 
 Uo 
 U6U 
 
 : 199 
 
 : U5 
 
 553 
 
 : 2U2 
 
 53 
 
 1*75 
 : 193 
 
 : 36 
 
 : 306 
 : 130 
 : 29 
 
 Lb . per : 
 
 
 In. 
 
 
 sq. ft. : 
 
 
 l/):-inch Douglas-fir 
 plywood 
 
 Two l/10-inch Douglas- 
 fir veneers with 
 
 300 : 
 
 192 : 
 
 82 : 
 
 263 • 
 
 175 : 
 
 85 : 
 
 332 • 
 216 • 
 
 : 101 : 
 
 3UU ! 
 218 : 
 
 9U : 
 
 269 • 
 
 176 : 
 
 82 : 
 
 0»38 
 58 
 
 i!i5 
 
 
 
 paper overlay 
 One l/3-inch Douglas- 
 
 
 
 
 
 fir veneer with 
 
 
 
 paper overlay on 
 each side 
 
 l/U-inch tempered 
 hardboard 
 
 1/8- inch tempered 
 
 
 
 
 1.00 
 
 i.5o 
 3.oo 
 
 hardboard 
 
 
 
 
 
 
 
 
 
 "All cores of the cross- corrugated XN type were made from 50-pound paper 
 treated with 15 percent of resin. The core density was 2.5 pounds per 
 cubic foot. 
 
 2 
 
 -Deflection under a uniform load of 20 pounds per square foot on a span of 
 
 96 inches. 
 
 3 
 
 -i.idspan deflection computed from the differential expansion of the two 
 
 facings obtained by exposure of one facing to a relative humidit}' of 97 
 
 percent, the other to a relative humidity of 30 percent for 30 days. 
 
 Rept. No. R1918 
 
Table 2 . — Effect of core con struction, de nsity, and filler on thermal 
 conductivity of core ma''e from re sin- treated paper 
 
 Core 
 
 : Filler 
 
 Density 
 
 k 
 
 1 
 
 construction 
 
 in core 
 
 
 
 k 
 
 
 
 . per cu. ft. 
 
 B.t.u. per hr. per 
 
 
 
 sq, ft. per' inch 
 
 
 
 
 
 . .... v. 
 
 
 XN 
 
 None 
 
 2,75 
 
 o.U5 
 
 : 2.22 
 
 
 do 
 
 2.9h 
 
 M6 
 
 2.1 7 
 
 
 do 
 
 5.U7 
 3.35 
 
 .58 
 ,U7 
 
 1.73 
 
 
 
 : 2.13 
 
 PNL 
 
 do 
 
 <.$o 
 
 .59 
 
 : 1. 
 
 Figure 8 
 
 do 
 
 2,89 
 
 .53 
 
 1.89 
 
 pn 
 
 Foamed resin 
 
 : 5.36 
 
 J 
 
 ■ 2. 
 
 Figure C 
 
 
 1. 
 
 .31 
 
 3.23 
 
 : 
 
 Fill insulation 
 
 m.72 
 
 .37 
 
 2.70 
 
 Rept. Mo. R1918 
 
Figure 1 . - -Expanded type of core consisting of sheets of paper inter- 
 spaced with parallel strips of adhesive and expanded to form hexa- 
 gonal cell sections. 
 
 Z M 87220 F 
 
JK JK ^»: JL ->: -J 
 
 -*LJB -rff; -*> -<*• ■ J* 
 
 Ajb-M -*: -* -dtf jil -*' -j* ^ 
 &*l-tLgM ~M *# jl ^>: _*. ^ 
 
 to«A jfiUkjKJL** ****** 
 *iJ*-M 
 
 JK. ^ ^* ^ 
 
 K J* 4* ^Jk ->■ 
 
 — *^^ -ai^ 1 -JK* -^T* -^»* '^^r* ,^ 
 
 *. ^+: jtk J0- -#■ *+• JM 
 
 ^: ^* <jic -^ ^ J* ,-* 
 
 *d*' ~* *j*' ~J* +4* 
 
 Figure 2. --Looped or figure-6 type of core consisting of sheets of 
 paper looped and bonded to form circular cells. 
 
 Z M 87221 F 
 
Figure 3 .--Corrugated type of core designated as PNL. It consists of 
 corrugated sheets of paper assembled parallel to each other and 
 separated by a single treated, and uncorrugated sheet. 
 
 Z M S7223 F 
 
^*^> 
 
 &!&&§." 
 
 Figure k. — Corrugated type of core designated as XN. It consists of 
 corrugated sheets assembled with principal flute directions of adja- 
 cent sheets at right angles . 
 
 Z M 87222 F 
 
2m 888-H F 
 
 Figure 5. — Corrugated type of core designated as PN. It consists of 
 corrugated sheets assembled parallel to each other and bonded at 
 the crests . 
 
Figure 6. — Structural sandwich wall panel tested 
 under vertical load. 
 
 Z M 79770 F 
 
UNIVERSITY OF FLORIDA 
 
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 3 1262 08866 6044