August, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
The factor of heat conductivity in both the processing and the testing of a material has been discussed (4). It should be of definite interest to process and test two like materials differing only in the heat conductivity of the filler used. The particle shape and size of these two fillers should be the same. Preliminary tests on a bronze powder and a clay indicate that the difference in heat conductivity between fillers is of minor importance.
Conclusion 1. The inability to obtain a consistent correlation in the rating of a series of molding powders as t o plasticity by using various test methods has been shown.
975
2. The effect of cure on the plasticity of a molding powder depends on the test method used. 3. The fillers used in compounding have a marked effect on the plasticity of the resultant molding powder.
Literature Cited (1) Burns, R.,Proc. Am. SOC.Testing Materials, 40, 1283 (1940).
(2) Farrer, M., Brit. Plastics, 4, 19, 51 (1932). (3) Krahl, M., Ibid., 6, 235 (1934). (4) Leokvitzky, A. N., Plasticheskie M a s s y , 3, 43 (1934). (5) Norton, A. J., Plastics & Molded Products, 7, 271 (1931). (6) Peakes, G . L., Plastic Products, 10, 53, 93, 132 (1934). (7) Rossi, L. M., and Peakes, G. L. (to Bakelite Corp.), U.S. Patent 2,066,016 (Deo. 29, 1936).
Phenolic Resins for Plywood The historical development of phenolic resins for use in plywood is traced for the period from 1900 to the present day. The various physical forms in which the resin has been used as a plywood adhesive are described-i. e., as a powder, in organic and aqueous solution, as a dispersion, and finally in film form. New products, watersoluble phenolic powders of good stability and rapid curing propertie#, are discussed. Development work covering the use of phenolic resins as impregnants to produce products such as Improved Wood and wood with better dimensional stability are covered. The fields of application for resinbonded plywood with special emphasis to aircraft construction are touched briefly.
URING the last decade, one of the outstanding developments in the resin field, which witnessed many important advances during this period, was the growth of resin-bonded plywood ( I , 9, 16, 17, 18, 81). This development, because it took place in somewhat unspectacular fashion, has not perhaps been appreciated by those not directly interested in plywood. Moreover, resin-bonded plywood is not readily identifiable as such, and there are many applications where it is employed without the fabricator or ultimate user being aware of its identity. There are in the United States today approximately 150 hot presses. If this figure is compared with less than a dopen in 1934, some idea of the rapid growth during the last five or six years may be obtained. Compared t o the total of approximately 7000 hot presses in the molding industry, the figure of 150 is not particularly impressive. However, the capacity of a plywood hot press is generally high. Practically all of them have multiple openings; they may have as many as twenty platens, and they range in size up to 100 X 150 inches. This permits ii productive capacity %measuredin many thousands of square feet per day for a single press. Unfortunately it is not pos-
D
LOUIS KLEIN The Resinous Products & Chemical Company, Philadelphia, Penna.
sible to obtain an accurate figure on the total volume of resinbonded plywood, but it runs to many million square feet per month. The actual total of resin used in the manufacture of plywood is not accurately known, but it is estimated to be between 5 and 10 million pounds per year, a figure that is relatively small compared to the use of phenolic resins in such fields as molding powders, laminating varnishes, and oleoresinous varnishes. However, again it must be pointed out that in the manufacture of plywood, the phenolic resin is a relatively small proportion of the total weight, generally less than 10 par cent. The growth of phenolic resin bonding from the beginning of the century t o the present time will be reviewed here, first, from 1901 to 1934, and secondly, from 1934 to the present. 1934 is chosen as the division point because Tego resin film was first manufactured in the United States in that year. From that point on, the growth of resin bonding proceeded rapidly. I n referring t o phenolic resins, we mean not only iesins manufactured from phenol itself, but also products made from cresols, xylenols, and other substituted phenols. For practical purposes, however, the resins used commercially are prepared almost exclusively by reacting formaldehyde with phenol or commercial cresylic acid. No attempt will be made to cover the chemical reactions involved in the preparation of the resins or their behavior during the process of hot pressing. These are similar to those involved in the manufacture and curing of the thermosetting phenolic resins used in molding and are fully described elsewhere ( I 1,83). There are two general theories concerning the nature of the adhesive forcesacting between resin and wood. One postulates that the action is entirely mechanical and that the resin is embedded in the pores of the wood, the strength of the bond depending in part on the degree of penetration and the cohesive strength of the cured resin. The second or polar theory emphasizes the inhence of secondary valence forces, and there is considerable experimental evidence to substantiate its conclusions (8,?21).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Historical Background Wood has been used by man for many centuries. It is plentiful, cheap, and easily worked to produce useful articles. It has excellent mechanical properties and is light in weight, and certain types, because of inherent beauty of figure and texture, find use in decorative application. Together with these advantages, wood does have certain definite limitations. It is nonuniform, swells on absorbing moisture, shrinks on
Vol. 33, No. 8
necessity of conditioning the plywood after its preparation. A much more serious drawback was the poor resistance of the glue line t o water and mold growth. Definite improvements in water resistance resulted from the introduction of glues made from casein and blood albumin, which came later. However, only with the introduction of synthetic resins was it found possible t o obtain a glue line which could be truly called waterproof and moldproof.
Courtesy, Gunnzaon Houszno Csrporatzon
THE AVAILABILITYOF RESIK-BONDED PLYWOOD OF UNLIMITED EXTERIOR DURABILITY Has ADDEDIMPETUS TO THE MASUFACTURE OF PREFABRICATED HOUSES
losing it, warps badly, and is flammable; its strength characteristics vary enormously with and against the grain. There is evidence that as early as 2000 years ago the Chinese made the first veneered woods in an effort t o overcome some of these limitations. Undoubtedly, they were primarily interested in decorative effect rather than mechanical properties. Probably plywood, as it is known today, first came into general use about 1870 (25). This development was a natural outgrowth of the attempts t o use wood t o greatest advantage. It was found that many of the defects of solid wood could be overcome by cutting it into relatively thin sheets (generally called “veneers”), cross layering the veneers, and gluing them together. These veneers generally vary in thickness from about to l / 4 inch, although veneers as thin inch or less are often cut for special purposes. In as general, best results are obtained from the so-called balanced construction-that is, the cross layering of an odd number of plies, such as three, five, seven, etc. Many of the common types of wood are widely used in the manufacture of plywood, although certain species can be used t o greater advantage than others. Mora ($6)listed the following characteristics in which plywood is superior t o lumber: Plywood is slightly heavier and harder; it shows less volume change and warping with variations in humidity; tensile, compressive, and impact strengths are more uniform than in lumber; plywood shows less splitting; weight for weight, plywood is stronger. I n the development of plywood it soon became evident that the limiting factor was the glue line. I n the early days the adhesive was generally an animal or fish glue. Later these were replaced to a large extent by the cheaper vegetable glues made chiefly from tapioca starch. All these adhesives suffered from serious drawbacks. Because of the large quantities of water used in the preparation of the adhesive, careful control was necessary to prevent warping and swelling. Moreover, the process was slow and laborious owing to the
Probably the first record of an attempt t o use phenolic resins in the manufacture of plywood is given in a British patent, which issued in 1901 (68). Presumably the possible uses of their resins in this field was very much in the mind of Baekeland and his co-workers in the years between 1900 and 1910. I n 1910 a French patent (14) issued covering the use of a phenolic resin !?dm in the manufacture of plywood. In 1918 a patent was granted to McClain (26) on the use of a phenolic resin in film form, but there is no evidence that this process was ever widely used. It seems clear, therefore, that in the period from 1901 through the first World War, nothing of commercial importance took place. The best evidence that resin-bonded plywood was a rarity in this period is that plywood made for aircraft during the war was glued with casein and blood. This fact later proved to be a serious obstacle t o the subsequent development of plywood, because of the delamination of large quantities of surplus military plywood stored for several years after the war. I n the latter part of the decade between 1920 and 1930, active development work took place in the field. There seems to be little question that the products investigated initially were solutions or dispersions of phenolic resins. One company marketed for some time. a dispersion of a phenolic resin in water. This process never became a great success chiefly because of the difficulty of controlling spread and adjusting moisture content a t the time of gluing. Attempts were made to use phenolic resins in alcoholic solutions, the form in which they were most generally available. However, a hazard resulted from the presence of the somewhat flammable alcohol, and here again the difficulties of uniform spread and proper control of flow proved difficult hurdles t o surmount. Considerable work was also carried out with a phenolic resin applied to the veneer in the form of a powder (IO). I n order to obtain adequate flow, it was necessary to add water after coating the veneers; this introduced an added operation. The equipment and care required to operate this
August, 1941
INDUSTRIAL A N D ENGINEERING CHEMISTRY
process proved insurmountable obstacles, and it fell by the wayside. While work of this nature was going on in this country, in Europe there was active development on the production of a phenolic resin in film form (33). In 1932 the four available resin forms were discussed in a Symposium on Glues for Wood Products (32). At that time, it was too early to predict which type would prove to be most generally useful, although experience in Europe already pointed toward the advantages of the film. I n the following year Sorensen (29) described more fully the resin film which was at that time manufactured in Europe. I n 1934 manufacture was begun in this country, and from that time on, the growth of resin-bonded plywood advanced rapidly.
Resin Film The adoption of resin film by the plywood manufacturer was the result of several advantages: (a) A uniform spread is assured which permits uniform bonding; (b) it is a d r y process, and therefore no swelling results from the introduction of water; ( c ) the process is rapid, simple, and clean; ( d ) light-colored thin veneers can be used without danger of staining; (e) the bond is moldproof and waterproof, being resistant even to boiling water. I n the preparation of the film, phenol and formaldehyde are first reacted in molecular proportions in the presence of a small amount of sodium hydroxide as a catalyst. The resulting product, without being carried to the point of water separation, is impregnated on a tissue paper carrier and subsequently dried. I n commercial practice the paper is 0.001 inch thick and the resultant film is slightly over 0.002 inch thick. The film as supplied today contains approximately two parts of resin to one part of paper (SO). With the development of satisfactory equipment, it has become possible to produce this film commercially in widths of 74 inches or more a t a cost to permit its utilization even where economy is an important factor. The film has excellent stability and under normal conditions may be kept for approximately a
MECHANICPREPARING. RESIN-BONDED PLYWOOD FOR WINUS O F THE CABIN AIRPLANE AT THE REAR
THE
971
year before use. Cresol may be substituted wholly or in part for the phenol. The differences produced are slight; cresol resins give slightly slower cure and a slight odor, and the shear value is somewhat lower. In the process of manufacturing plywood using resin film, the film is first cut to size and then slipped between veneers held a t a moisture content of approximately 8 to 10 per cent (27). The assembly is then loaded into the hot press (mechanical loading devices are often used), pressure is applied for a few minutes, and the panels are removed from the hot press and allowed to cool or, in some cases, dipped in water. The panel is now ready for trimming and sanding and other operations without further delay. The temperatures generally used are in the range 275-300" F. The time in the press may vary from 5 to 20 minutes, depending on the thickness of the panel. Normal pressures vary from 125 to 250 pounds per square inch, depending on the type of wood, higher pressures being used for the harder, denser woods, and lower pressures for the softer woods, to minimize compression. The method of manufacturing plywood using the resin 61m is simple, although it requires careful control of four variablesmoisture content of the veneer, temperature, pressure, and time. 1934 witnessed the beginning of the rapid growth of resinbonded plywood. For the peiiod of 1934 through 1937, the resin film was practically the sole type of resinous adhesive used. One important exception was the Douglas fir plywood industry. Here it was never possible to work out application conditions for the film to meet the demands of this mass production industry. Moreover, largely because of rough veneer cutting and nonuniform density of the species, the film itself did not appear to be entirely suitable for this type of wood. I n this field, therefore, during the period discussed, only one or two important manufacturers produced resinbonded plywood, and they used the more conventional alcohol-soluble phenol-f ormaldehyde resin. As the number of hot presses in the plywood plants grew, thought was given to the question of producing a cheaper and more economical plywood. For many applications the boilproofness and high shear value of the resin film was not needed. For exterior panels, aircraft, two-ply faces, and other applications where maximum durability or special characteristics were required, the resin film held its own. However, there were a number of uses where maximum durability was not essential, and for such applications aqueous urea-formaldehyde resins proved satisfactory. I n 1937 ureaformaldehyde resins were successfully used for the first time on a commercial scale. Resins of this type had important advantages. They were cheap, could be mixed with flour and other inert extenders, and could be used a t temperatures around 230" F. as compared to 285-300" F. for the phenolic film. By 1939 the use of urea-formaldehyde resins had apparently grown to the point where the quantity of plywood manufactured had already equaled that made from phenolic resin. While these important developments were taking place in the field of urea-formaldehyde resins, intense activity was in progress in the phenolic resins. On the Pacific Coast where the production of Douglas fir plywood was growing by leaps and bounds, improvements in the alcoholic type resin were vitally necessary. If shipped and stored in the alcoholic solution, its storage characteristics were poor. If shipped in lump form for dissolving in alcohol a t the plywood plant, the operation was messy and cumbersome. Moreover, the use of alcohol is always disadvantageous in a woodworking plant. Ofreal importance during 1940 was the announcement of three new phenolic resins in powder form for use in the manufacture of plywood. They have certain factors in common: They are
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 33, No. 8
Applications of Resin-Bonded Plywood In consequence of the availability of a superior plywood resulting from the use of synthetic tesins, the growth of plywood in fields ng, radios, automosure, commercia1,and alty application field such as trays, sporting goods, etc., has been tremendous. Perhaps the three most important fields where plywood is directly used are those of housing, marine, and aircraft construction. I n the housing field prefabricated units of plywood are an important application. In the marine field the torpedo boats, about which so much has been written, are constructed from large panels of resin-bonded plywood. In the work on plywood already described, the actual penetration of the resin into the plies is very slight , and the weight of resin is only a small percentage of the total. It was felt that the disadvantages resulting from the fibrous nature of wood might be reduced if the resin content were increased. Investigation of this basic idea resulted in the production of so-called Improved Wood (.$,GI 9,18,19). A multiple assembly of thin plies (approximately 1 / ( 0 inch thick) with sheets of resin film interleaved between the plies, is subjected t o a pressure of approxiSIXTEEX-OPENING HYDRAULIC HOTPRESS FOR THE MANUFACTURE OF RESINmately 400 to 500 pounds per square inch. BOKDED PLYWOOD AT THE PLANT OF THE MENGEL COMPANY. LAUREL, MISS. I n some cases it is more advantageous t o use the resin in liquid form. The assembly often contains as many as forty or fifty plies. It has been found advantageous in some cases to have only every all supplied in powder form, they dissolve readily in water, and they make waterproof , boilproof bonds comparable in quality tenth ply cross-grained rather than to cross grain alternate plies as is the case with the usual plywood. As a result of the with those obtained from resin film. One of the types is penetration of the resin into the wood, the final product has intended for temperatures about 300" F. and is generally properties far superior not only to wood itself but also to resinrecommended for use without the addition of any extender. bonded plywood. Table I (6) indicates the advantages of ImThe second type is suggested for utilization with fairly large proved Wood over resin-bonded plywood in the case of both quantities of blood or other protein matter at about 240" F., beech and birch. Worth noting particularly is the high increase and an excellent bond is obtained particularly with Douglas in strength across the grain,the large increase in specifiogravity, fir. The third product is most interesting because it repthe reduction in water absorption, and the decrease in swelling. resents an important advance in the synthesis of phenolic So far the chief application of Improved Wood is in airresins. For the first time a product is available which comcraft construction where it is used in making struts, supportbines numerous advantages. This new resin can be used at ing members, etc. Its chief advantages are freedom from temperatures of 230-250" F., is available as a powder of good knots and imperfections as well as unity in change of dimenstability which dissolves readily in water, and is stable over a sions on exposure to water. Other applications are in the wide range of pH, and the bonds have outstanding mold and water resistance (even to boiling water). More will be manufacture of specialty materials, sporting goods, forms, casters, gun stocks, etc. A recent paper gave important data said about this resin in connection with applications in the aircraft field. on the mechanical properties of Improved Wood (4). As to resins intended for use in the cold-bonding procedure, Aircraft Construction this description is somewhat misleading. When we refer Phenolic resins enter into the construction of the modern to the "cold bonding" of plywood, we generally mean that plywood plane in four different ways: as a flat sheet which is the material was not made by the simultaneous application of heat and pressure. Often the adhesive is applied t o the subsequently formed and shaped, as high-density plywood (Improved Wood), in the so-called plastic plane, and as the surfaces of the veneer and subsequently put in a kiln at a cold-bonding adhesive used in assembly work. Advantages somewhat elevated temperature. This particular field of resulting from the use of plywood are as follows: speed of proapplication is important in aircraft because in the assembly duction, high stiffness per unit of weight (approximately of a plywood plane it is not possible to apply simultaneously heat and pressure. So far, urea-formaldehyde resins are eighteen times that of Dural) , improved product (better widely used and, despite their poor resistance to boiling water, streamlining, etc.), decreased cost and weight, reduction in the number of skilled men required for fabrication, and simare very satisfactory and have been approved by the Civil Aeronautics Authority. At least one large aircraft manuplification of machinery. Another indirect advantage is the release of such strategic metals as aluminum and magfacturer is using a phenolic resin presumably catalyzed by a nesium for other purposes. Disadvantages resulting from strong acid for this application (12).
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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TABLEI. COMPARISON OF RESIN-BONDED PLYWOOD AND IMPROVED WOOD( 6 ) Property Specific gravity Moisture content, Tensile strengtha
Yo
Compressive strengtha Bending strength“ Shear strengtha Modulus of elasticity= (+OOO)
Direction of Stress
.....
.....
Longitudinal Transverse Longitudinal Transverse Longitudinal Transverse Longitudinal Transverse Longitudinal Transverse Longitudinal
Modulus of rigidity* (+OOO) Water absorption after 50-hr. immersion, yo ... Swelling after 50-hr. water immersion, % Thickness Width a Pounds per square inch.
. .
Resin-bonded plywood 0.65 9.0 19,900 1,420 8,530
....
18,500 1,710 2,840 1,560 2,130 130 131 38.5 5.8 (3.5
the use of plywood are poor flammability characteristics and poor structural properties in certain respects. Plywood finds its chief use in the construction of military trainers. There are still good reasons for the use of metal in bomber, transport, pursuit, and scout planes, although certain sections of the larger planes such as the floor, bomb-bay doors, etc., are being built of plywood. Flat sheets of plywood are employed in the manufacture of spars, wing coverings, ribs, gussets, floors, doors, etc. I n most cases these sheets are manufactured from resin film. High-density plywood finds its chief application in the production of reinforcing plates and spars, as well as propellers. The resistance t o shear and nail-holding power are far superior to that of solid wood. The name of the so-called plastic plane (7, 13, 3ff) is somewhat misleading. The plastic plane actualIy contains about 90 per cent of wood veneer and 10 per cent of synthetic resin on a weight basis. Phenol-formaldehyde, urea-formaldehyde, and thermoplastic resins, such as the polyvinyl acetals, have been used as the bonding agents. All of these types have some advantages as well as certain disadvantages, but the phenolic resin has already found the widest application and will probably continue to dominate the field. The plastic plane is one in which the veneers are resin-bonded over a form or die instead of in the usual flat condition so that the resultant plywood is already shaped. According to
PHENOL-FORMALDEHYDE RESINSIN SHEETFORMARE WIDELY USEDIN THE MANUFACTURE OF WATERPROOF PLYWOOD FOR AIRCRAFT, MARINE, ARCHITECTURAL, AND OTHER USES
Beech Im roved Woo$ 45 ply 0.94 6.0 18,300 3,060 19,200 7,680 31,300 5,120 4,550 2,840 2,630 480 213 8.8
1.05
..
Increase,
%
+ 45 -
35 -7.9 +115 +I25
...
++20069 ++ 6082 ++27823 + 63 - 77 -...82
Birch Resin-bonded Improved plywood Wood, 50 ply 0.67 1.00 10.0 6.5 19,600 18,200 1,070 5,450 9,950 16,300 1,280 7,680 19,900 31,300 1,420 7,820 2,840
....
2,280 830 114 43.3 5.18 6.5
.... ....
Increase,
%
+ 49 -
35 -7.3 +400 64 +500 57 +450
+
+
... ...
3,060 6,830
++72834
8.0 1.85 0.18
--
....
...
82 64 97
Perry ( W ) ,“The principles involved in molding plywood are old and the only novel features were the methods of applying the pressure as well as the unusually large size of the molded unit. The fundamental principle involved is that of using an inflated or deflated rubber bag as one of the halves of a pair of molding dies. There is not only a saving in matching up the dies where the intermediate distance between the halves must be accurately determined, but in many cases the dies are wholly eliminated; in any event, the rubber bag pressure is of the order of fluid pressure and a t substantially right angles to any surface that is under pressure”. There are a number of different processes, the three best known being the Duramold, Vidal, and Timm. Although much has been written about them, many of the actual details still remain secret. There are obvious advantages in the plastic plane. It presents smooth exteriors and thus exerts less aerodynamic drag; it can be made rapidly and inexpensively. AB illustrative of the number of mechanical problems, a series of a t least thirty patents describes variations in fuselage construction. Comparing the advantages and disadvantages of the three general types of adhesives indicated above, the following factors warrant consideration. Temperature is extremely important because the life of the rubber bag, whether it be natural or synthetic, depends chiefly on the temperature to which it is subjected. I n this characteristic the urea-formaldehyde and thermoplastic resins have a definite advantage compared to the conventional phenol-formaldehyde resin of the past because they can be used a t lower temperatures. However, both of these resins have serious disadvantages. There is a serious question concerning the durability of plywood bonded with urea-formaldehyde resins when subjected to extreme weather conditions. The use of a thermoplastic resin raises serious questions regarding the behavior of the product under conditions of extreme heat. With the development of low-temperature phenol-formaldehyderesin in powder form described earlier in this paper, it would seem that the one serious disadvantage of the phenolic resin has been overcome-namely, the necessity of operating a t a high temperature. As time goes on, the plastic plane will probably increase in importance. The mechanical difficulties of operatfon can undoubtedly be overcome, and it seems that we are well on the road to a satisfactory and ideal adhesive. Discussion of the plywood plane would be incomplete without mention of the methods of assembling the various resin-bonded structures. I n the past the adhesives have been casein and animal glue. Obviously it would be foolish to build a plane of resin-bonded plywood unless the joints were equally well bonded. For this purpose a urea-formaldehyde resin is generally used with an acid catalyst. The durability
I N D U S T R I A L A N D E N G I: N E E R I N G C H E M I S T R Y
980
characteristics in this type of application seem ample for the conditions of service. An interesting development in the field of equipment has been that of high-frequency electrostatic heating devices (3). Although this type of equipment has not, to the best of our knowledge, been applied to the production of assembly bonds, use in this field is obviously indicated as it would permit the application of heat and pressure to thick assemblies and irregular shapes and forms.
Conclusions Despite the more impressive applications of phenolic resins
in the field of molding and coatings, they play an important part in the manufacture of plywood. The importance of plywood has grown tremendously in the last few years and is of great importance at the present time in our program of national defense. There seems little doubt that the importance of plywood in aircraft construction is just beginning to be appreciated in an industry which has heretofore devoted its attention almost exclusively to metals. With the availability of a fast curing, easily handled, powdered phenolic resin to supplement the phenolic resin film, the major adhesive problems seem already well on the way to solution.
Bibliography Anonymous, Modern Plastics, 17, 25 (1940). Anonymous, Modern Plastics Catalog, pp. 414-18 (1941). Berkness, I. R., Can. Wood Worker & M f r . , 40, 9-10, 24 (1940). Bernhard, R. K . , Perry, T. D., and Stern, E. G., Mech. Eng., 62, 189-94 and 748-51 (1940). Brenner, P., Aircraft Eng., 10, 129-34 (May, 1938). Brenner, P., and Kraemer, O., “Improvement of Wood by Synthetic Resin Gluing”, Mitt. d. Fachaussch. fur Holzfragen, Vol. 11. Berlin, 1935. Clark, V. E., Aero Digest, 35, 101-5 (1939).
Vol. 33, No. 8
(8) De Bruyne, N. A,, Aircraft Engineer (supplement to Flight), 18, 61-4 (1939). (9) Decat, R., Aero Digest, 38, 146, 149, 194 (1941). (10) Dike, T. D. ( t o Laminating Patents Corp.), U.S. Patent 1,922,668 (Aug. 15, 1935). (11) Ellis, C. L., “Chemistry of Synthetic Resins”, Vol. 1, p. 291, New York. Reinhold Publishing Coro.. 1936. (12) Haut, H . N.,’ Chem. Industries, 48; 26-9 (1941). (13) Hayward, C. H., W o o d , 4, 443-7 (1939). (14) Hulot and Mouninou-Doumichaud, French Patent 414,045 (1910). (15) Knight, E. V., and Wulpi, M., “Veneers and Plywood”, New York, Ronald Press, 1927. (16) Koch, R.. M o d ~ r nPlastics, 16, 264-5, 270 (1938). . . (17) Ibid., 17, 416-20 (1939). (18) Kollmann, F., “Technology of Wood”, p. 539, Berlin, Julius Springer, 1936. (19) Kuech, W., Jahrbuch der deutsche Luftfahr Forschung, pp. 4473 (1938). (20) Laucks, I. F., Modern Plastics, 15, 294-8 (1937). (21) McBain, J. W., and Lee, X7.B., J . SOC.Chem. Znd., 46, 321-4T (1927). (22) McClain, J. R. (to Westinghouse Electric & Mfg. Co.), U. S. Patent 1,299,747 (April 8, 1919). (23) Megson, N. J. L., “Phenomena of Condensation and Polymerization”, p. 336, London, Gurney & Jackson, 1935. (24) Moon, H. P., Auiation, 40, 44 (1941). (25) Mora, A , “Plywood, Its Production, Use and Properties”, London, Timber and Plywood, 1932. (26) Perry, T. D., J . A E T O ~ Sci., UU~ 8,.204-16 (1941). (27) Perry, T. D., and Bretl, 31. F., T ~ a n s Am. . SOC.Mech. Engrs., Wood Ind., 60, No. 3, 59-68, 682 (1938). (28) SOC.Derepas FrBres, Brit. Patent 17,327 (1901). (29) Sorensen, R., Trans. Am. SOC.lwech. E ~ T s .Wood , Ind., 56, NO.1, 37-48 (1934). (30) Sorensen, R., and Klein, L., Hardwood Record, 72, 15 (Oct., 1934). (31) Spencer, H. S., Modern Plastics, 14, 296 (1936). 132) Truax, T. R., Trans. Am. SOC.Afech. Engrs., Wood Ind., 54, N o . 1 (1932). (33) Weber, J., and Hengstebeck, E‘. (to Th. Goldschmidt Corp.), U. S . Patents 1,960,176-7 (May 22, 1934).
Cottonseed Hulls in Phenolic Plastics FRITZ ROSENTHAL University of Tennessee, Knoxville, Tenn.
HE processing of cottonseed results in the production of four primary commodities: cottonseed oil, an important edible vegetable oil; cottonseed meal, a cattle feed which is extremely high in protein content; cotton linters, a valuable source of alpha-cellulose; and cottonseed hulls. The hulls have been used so far as roughage for beef and dairy cattle, and whenever the price of hulls fell below a certain level they were burned as fuel by the oil mills. The applications of the four cottonseed oil mill commodities indicate that cottonseed hulls are the least profitable product of all of them. This fact has been the incentive for many diversified research efforts to find better uses for the hulls. 9 recent article by Musser and Nickerson (6) contains a complete review of research and development work devoted to the utilization of cottonseed hulls. Cottonseed hulls have been suggested as a filler for phenolic molding compounds. This is not surprising in view of the spectacular development of phenolics within the last twenty years. It is surprising, however, how little information is available on the utilization of cottonseed hulls as fillers in phenolic plastics. Hurst (4) obtained a patent on a lowcost molding compound comprising 70 to 80 per cent cotton-
T
seed hulls and 20 to 30 per cent phenolic resin. He called attention to the nonabsorbency of resin by cottonseed hulls and reduced the resin content by such amount as is ordinarily required to saturate the filler, and accordingly reduced the cost of the molding compound. Hurst’s claim with respect to nonabsorbency is of dubious value in the light of a publication by Meharg (6),who has given comprehensive thought to filler requirements. He discerns six primary and eight secondary requirements a material has to meet in order to be a satisfactory filler in thermosetting molding compounds. One of his primary requirements is the property of being easily wetted by resins, which he states is necessary to good bonding arid to good finish of the molded piece.
Absorbency The first problem to be solved was to decide whether cottonseed hulls are nonabsorbing, as claimed by Hurst, or whether they have a high absorbing power and therefore possess a primary requirement for a good filler. The absorbing power of any filler material, be it wood flour, cottonseed hulls, or cotton flock, is a function of the particle size of the
