Protein Plastics f rorn Soybean Products J
LAMINATED MATERIAL’ GEORGE H. BROTHER, LEONARD L. McICINNEY, AND W. CARTER SUTTLE, U. S. Regional Soybean Industrial Products Laboratory2,Urbana, Ill.
A protein laminated plastic material pre-
hyde dispersions. On a large production basis the less expensive solvent-extracted soybean meal would be used. The soybean “alpha”-protein was dispersed in a concentrated solution containing 10 per cent powdered borax and 8 per cent strong ammonium hydroxide solution (both on the dry protein weight). This was heated to about 65” C. to obtain a clear dispersion, cooled, and diluted to 10 per cent protein with water to which had been added formaldehyde in amount to give 4 to 5 per cent in the final solution. The protein will not be hardened by formaldehyde in this solution a t a pH within the isoelectric range of the protein, as was found most advantageous for protein plastic material (@, This was shown to be impractical (8). However, the protein is hardened a t as low a pH as possible. The resultant product will be designated “formaldehyde-hardened soybean protein salts” to differentiate it from the formaldehyde-hardened soybean protein which is properly hardened within the isoelectric range of the protein. The dispersion would be made in a n analogous manner from the solvent-extracted soybean meal, except that it would be necessary to hydrolyze the protein by preliminary treatment with caustic solution or by other means before proceeding to disperse it. It is doubtful whether it would be necessary to remove the insolubles, as they would cause no difficulty in making the laminated material. I n this way difficult filtration would be avoided. Several types of unsized or saturating paper used in the production of present commercial laminated plastics were investigated. Included were samples of saturating rag, alphacellulose, and bleached and unbleached kraft papers, in thicknesses ranging from 0.003 to 0.022 inch. It was found that a regular saturating kraft paper, 0.011 inch in thickness (elevenpoint), gave satisfactory results. Since this was also the least expensive paper investigated, it was adopted.
pared from unsized kraft paper impregnated with formaldehyde-hardened thermoplastic soybean protein salt compared favorably with similar commercial materials as regards impact and flexural strength and modulus of elasticity, but not as regards water resistance. By placing a single sheet of phenolic- or urea-impregnated paper on each exposed face before pressing, a product resulted with the water resistance and other desirable properties of present commercial products, except for the edges, in materially reduced time of pressing. It is proposed to extend this investigation to cellulose fibers before sheeting and to the preparation of fabric laminated material. H E N it was found that formaldehyde-hardened soybean protein was thermoplastic @),the possibility of developing an aqueous dispersion of formaldehydehardened soybean protein was considered. With such a dispersion it would be possible to impregnate fibrous materials such as sheets of saturating kraft paper, textiles, and similar materials. Upon drying, these could be united by the thermoplastic hardened protein into laminated plastic material under the action of heat and pressure. This material should have good strength and be considerably less expensive in materials and in process of manufacture than the phenolic and urea laminated plastic materials now in use.
Soybean Protein-Formaldehyde Dispersion Accordingly, a dispersion of soybean protein in aqueous formaldehyde solution was developed, which, in concentrations up to 10 per cent protein, remained stable for long periods. This development is described in detail elsewhere (9), so it is sufficient merely to outline here the method for its preparation. For this investigation, conducted as it was on a small laboratory scale, the commercial soybean “alpha”protein was used in preparing the soybean protein-formaldeI Previous papers in this aeries appeared in 1938 (pages 437 and 1236). in l93Q (page 841, and in 1940 (page 1002). 1 A oooperative organization participated in by the Bureaus of Agricultural Chemistry and Engineering and of Plant Industry of the U. 8. Department of Agriculture, and the Agriaultural Experiment Stations of the North Central States of Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisoonsin.
Experimenta1 Procedure The size of the panels of laminated soybean plastic prepared was limited by the equipment available. A %-ton press with 12 X 12 inch platens waa used. This restricted the work to a laboratory basis, and the results obtained must be so construed. The method used for impregnatin the paper with the soybean protein-formaldehyde salt waa on a faboratory scale. -The &e& were dip d in the dispersion and hung up in a well-ventilated hood to g i n and dry at room temperature. The paper absorbed about one third of its weight of formaldeh de-hardened soybean protein salt in the fist dip and about one gfth more in each succeeding dip. From 3 to 5 hours were required to dry the paper between treatments. On a commercial scale the paper would be run directly from rolls throu h the soybean protein-formsldehyde salt dispersion strip e8, and then run through heated forced-draft dryers. ft ooulxbe treated in this way as many times as necessary, and was cut into sheets when finally ready t o be loaded into presses. Since the dispersing medium was water, there are no valuable solvents to be recovered; the amount of formaldehyde that might be recovered is too small to have economic significance. Air from the dryerswould have to be exhausted
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outside, however, since the formaldehyde would be a health hazard in the factory. In order to determine the proportion of formaldehydehardened soybean rotein salt to celluose in the sheets that would produce the best laminated plastic m a t e r i a l , v a r i a t i o n s were made ranging from 3 0 / 7 0 t o 50/50. These variations were controlled by the number of times the sheets were dipped, the 50 per cent hardened protein impregnation requiring four to DEFLECTION IN INCHESfive t r e a t m e n t s . After the final treatFIGURE1. LOAD-DEFLECTION CURVE ment, the sheets were dried to a moisture content of 9 to 10 per cent, which required from 16 to 20 hours at room temperature. This amount of moisture was found to be adequate t o plasticize the hardened soybean Drotein salt satisfactorily in the hot press. The final stage in the process consisted in pressing the sheets between the heated datens of a hvdraulic Dress so that thev united to form the liminated panel." A temperature of 250' tb 280' F. gave satisfactory results. The time of the cycle de ends upon the thickness of the panel, but since the formaldeiydehardened soybean protein salt is thermoplastic, no curing time is necessary. The time, therefore, is merely that necessary to heat the material uniformly. This is a marked improvement in efficiencyover laminated materials as now produced, the curing time of some requiring as much as a 2-hour cycle. The soybean laminated material, as is the case with phenolic and urea laminated, should be cooled while under pressure t o prevent subsequent warping. The surface will be that of the plates between which it is pressed; in order t o roduce material with highly polished faces, highly polished steef plates should be used in the press.
P
Impact and Flexural Strength Tests I n order to obtain data on the most practical pressure to employ in the preparation of soybean laminated plastic material, a series of test pieces were made with twenty sheets of paper impregnated with 30 to 50 per cent formaldehydehardened soybean protein salt. This gave sheet material averaging 0.17 inch thick, and requiring 3 to 4 minutes of hot pressing to laminate properly. Pressures of 500, 1000, and 2000 pounds per square inch were used, and all were pressed a t 280' F. Test pieces were prepared from this material, and tests were run according to A. S. T. M. standards as described below. The results are given in Table I. Less than
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1 per cent moisture was lost during the pressing operation, leaving 8 to 9 per cent in the material. At the end of 3 months the moisture content was found to be about 8 per cent, showing that this was the equilibrium moisture content of the material. The test pieces were conditioned 24 hours a t 50" C . This conditioned material was found to have about 10 per cent less strength, both impact and flexural, than the nonconditioned. Test pieces were sawed from sheet material and milled to the proper dimensions. Care was taken not to burn the material by friction while preparing the samples. Impact tests were made according to the A. S. T. M. standard method, D-256-38, for sheet material. The cantilever beam method (Izod) was used. A standard notch was milled in the specimen, and two pieces were used as a composite sample, making a total thickness of about 0.35 inch along the notch. The test pieces were broken edgewise, and the averages are reported as foot-pounds per inch of notch. Each figure recorded in Table I for impact strength is the average of five determinations. Flexural strength, correctly called "modulus of rupture" (7), was determined by A. 8. T. M. standard method D-22938T, using a machine designed in the Soybean Laboratory (4) and built to A. S. T. M. specifications. A composite of two specimens was used, the pieces being fastened together by small bolts a t each end. This gave a test piece about 0.34 inch thick, and uniform results were obtained by breaking the samples edgewise. The average maximum fiber stress (modulus of rupture) is recorded in Table I in pounds per square inch; each figure represents the average of three determinations. The data in Table I show that the strength of the material, both flexural and impact, is a direct function of the molding pressure. This is in accord with expectation, as the higher the molding pressure, the tighter the lamination and the thinner the panel produced. Panels produced at pressures of less than 1000 pounds per square inch are inclined to warp too readily. Also, both the flexural and impact strengths of the material vary inversely with the percentage of protein in the sample. This again is in accord with expectation, since the cellulose fiber (paper) is less brittle than the hardened protein salt. Apparently the strongest material is that prepared with just enough protein to bind the paper into a unified whole, which from these data is about 35 per cent. The strength of this material compares favorably with present commercial laminated material, Bakelite (1).
Modulus of Elasticity
Load-deflection curves were plotted for the samples tested, and Figure 1 is typical. The curve approximates Hooke's law, the deflection being approximately proportional to the load. Above the 200-pound load the curve deviates much more sharply; this indicates that shearing stresses within the material exceed the cohesive strength of the protein binder, the failure progressing until the piece ruptures. TABLEI. STRENGTHOF LAMINATED MATERIAL AS DETERMINED BY Modulus of elasticity was calculated from the FLEXURAL AND IMPACT TESTS~ load-deflection curves by the formula and method Percentage of Formaldehyde-Hardened Soybean Protein Salt: 36 44 47 suggested by Hopkins (6) : 29 31
Molding Pressure
Flexurd
Flexural
Flexural
Impact
Flexural
Impact
Flexural
Lb,/sQ. an.
Lb,/sq. zn.
Lb,/sq. zn.
Lb,/eq. m.
Ft,-Eb./ In. notch
Lb,/sq. an.
Ft,-lb./ zn. notch
Lb,./sq. an.
500 b 21,850 22.400 0.45 13,530 0.39 23,500 1000 21,930 23,300 0.49 20,730 0.44 2000 24,400 24,220 25,510 0.57 21,230 0.47 a The average flexural strength of paper-base Bakelite laminated ia square inch (1). b Test uiece did not laminate at this uressure.
Impact
Ft,-lb./
m.
notch 0.41
17,030 18,250 0.41 20,170 0.42 20,000 pounds per
where E
modulus of elasticity
load, pounds w == length of test bar, inches
L =
D = deflection of bar, inches B = width of test bar, inches ~
H = height of test bar, inches
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weight was recorded as water absorption (a pending A. S. T. M. tentative method). Laminated material containing 47 per cent formaldehyde-hardened soybean protein salt gave 77 per cent water absorption, and that containing 36 per cent gave 57 per cent water absorption. In both cases the material swelled considerably, and, on drying, fractured on the edges, but the laminations did not Beparate; neither could they he pried apart readily. For this material to have any extended commercial application, tlie water resistance wodd have to he materially improved. It was shown (3) that tliermoplastic formaldehyde-hardened soybean protein is perfectly compatible with phenolic and urea resins. Use was made of this fact, and a single sheet oE phenolic-resin-~pregnated paper was placed on the top and on the bottom of the pile when the sheets were introduced into the press for lamination. The result was a panel with exposed faces of phenolic and center of soybean-protein binder. The water absorption of this material fell to 24 per cent, with swelling only a.round the edges for a distance of Z/,O inch. For areas larger than the test pieces this percentage would be reduced. and with the nrotection of the edges (a simple and economick matter) the water resistance is t,he same as that of the phenolic.
W O O D G a h l N AXD ALACK-ORYX EFFECTS
Wand D represent the loads and deflections correspontling to 20 and 70 per cent OS the maxiiniim load. Mvtlulus of elmticity computed in this manner for trvent,yime samplw gave a range of 900,000 to 1,600,ooOponnils per squart~incli. For Bakelite laminated, the midullis
