INDUSTRIAL AND ENGINEERING CHEMISTRY
1002
the McCabe-Thiele method (3). The number of theoretical plates was then graphically determined to be ten above the feed and nine below. In order to avoid obscuring the curve, these plates are not represented in Figure 3. Assuming a plate efficiency of 75 per cent, the fractionating column to give the stipulated separation of furfural and furfuryl alcohol would consist of fourteen plates in the rectification section and twelve plates in the exhausting section. The foregoing data indicate that a continuous vacuum still would be satisfactory for the separation of furfural and furfuryl alcohol. I n such a still the time of exposure of furfural
VOL. 32, NO. 7
and furfuryl alcohol to heat would be comparatively short. As illustrated in Figure 2, the time element is important.
Literature Cited (1) Dunlop, A. P., and Trimble F., IND.ENG.CHEM.,Anal. Ed., 11, 602 (1939). (2) Lecat, M., Ann. 8oc. sd.BTuxe&8,45, 169, 284 (1926); 47B,21, . 45,620 (1926). 63, 109 (1927); Rec. t m ~chim., (3) McCabe, W. L., and Thiele, E. W.. IND.ENG.CEIEN.,17, 605 (1925). (4) Mains, G. H., C h m . & Met. Eng., 31, 307 (1924). (5) Othmer, D. F., IND.ENG.CHEM.,20, 743 (1928).
Protein Plastics from Soybean Products J
Thermoplastic formal de h y de h ar deaed soybean meal is converted into a thermosetting, resinous molding plastic with greatly reduced water absorption by mixture with phenolic resin. This is a new type of plastic, definitely superior to any previously suggested modified protein plastic, and it holds good possibilities for future development.
-
Influence of Phenolic Resins or Phenolic Molding Compounds on Formaldehyde-
Hardened Protein Material' GEORGE H. BROTHER AND LEONARD L. MCKINNEY U. S. Regional Soybean Industrial Products Laboratory, Urbana, Ill.*
N ORDER for a molding plastic to fit into
I
the present American industrial picture, it must be either sufficiently thermoplastic to work in injection molds or thermosetting enough to permit removal from hot dies without blistering. Modern production schedules demand speed undreamed of a few years ago. It is not unusual for an injector to operate on a 30-second cycle, or for a thermosetting resin that formerly required a 10-15 minute cycle, to cure in less than 1minute. We showed previously ('7) that protein material may be hardened to produce the base for a thermoplastic molding powder. We showed further (8) that, although polyfunctional alcohols exert some plasticizing action on formaldehydehardened protein, no plasticizer has yet been found to render this material fluid enough to work in injector dies. Research is indicated in this direction since, if a suitable plasticizer can be found, the protein plastic resulting would find wide application, both in present fields in competition with more expensive plastics and in new fields opened up by the cheaper plastic. However, it will be slow work to find a suitable plasticizer. Little is known of the fundamental structure of protein and somewhat less of the protein-formaldehyde complex; Previous papers in this series appeared in 1938 (pages 437 snd 1236) and in 1939 (page 84). 1 A cooperative organization participated in by the Bureaus of Agricultural Chemistry and Engineering and of Plant Industry of the United States Department of Agriculture, and the Agricultural Experiment Stations of the North Central States of Illinois, Indians, Iowa, Kansas. Michigan, Minnesota, Missouri. Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin. 1
therefore, about the only possible approach a t present is that of trial and error (10). There are hundreds of commercial plasticizers to be investigated with additions being made constantly, as well as possibilities in changing chemically the protein-formaldehyde complex. The other alternative is to impart sufficient thermosetting properties to the formaldehyde-hardened protein material to permit removal of the molded piece from the heated die without blistering. It has been shown (8) that ethylene glycol appears to react with protein-formaldehyde to produce a thermosetting plastic. This reaction, however, under such conditions as have been studied, appears too slow to be of practical value. Mixtures of the thermoplastic formaldehyde-hardened protein and commercial molding resins were studied (Q),and it was found that the protein material was compatible with urea and phenolic resinous molding powders and incompatible with cellulose acetate, ethylcellulose, and vinyl and methyl methacrylate resins. The fact was noted that the compatible resins form thermosetting molding material, and mixtures of these resins with formaldehyde-hardened protein material were further studied. The present investigation is limited to the study of mixtures or combinations of formaldehyde-hardened protein material with phenolic resins or resinous materials.
Previous Investigations Attempts to produce plastic mixtures of unhardened protein material with phenol-formaldehyde resins or resin mixtures were made as early as 1907. In the majority of cases, casein from milk was used (1, 4, 14, 16,17, 18, 90, 9497-32, S 6 , 4 l , 44, 46, C Y ) , although mention is made of other proteins such
JULY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
as glue or gelatin (2, 3, 33, 3 4 , leather (I.%'), corn gluten and zein (6, 19, 21, 25, 44), animal or vegetable ivory (6, 48, 60, 61), soybean protein (16, 43), and albumin (23, 49). These mixtures were made either by adding the protein to phenol in which it was partially soluble and treating this mixture with formaldehyde, or by adding the protein to the phenolic resin previously prepared. I n some cases it was specified that the resin must liberate free formaldehyde on heating and thus harden the protein after i t had been formed to shape. The only formaldehyde-hardened protein suggested was powdered scrap galalith, casein-formaldehyde (13, 26),and that for filler purposes. Although the unhardened protein doubtless filled a more important role than that of mere filler material, these mixtures can hardly be considered mixed plastics. Either the phenolic resin predominated and the protein merely modified its properties, imparting somewhat greater toughness and at the same time materially decreasing the water resistance, or the protein predominated and the phenolic resin was used as an agent for delayed liberation of formaldehyde to harden the protein. Even when the substitution of protein for part of the inert filler made it possible to reduce somewhat the amount of phenol in the mixture and thus cheapen the product, it could be considered only a modified phenolic mixture since the amount of protein used was considerably less than the amount of phenolic resin. So far as can be determined, only one such modified phenolic plastic has been commercially successful. A phenolic molding compound modified by approximately 25 per cent soybean meal (11, 24, 35, 45) has found such application to the molded parts of automobiles as to increase the consumption of soybean meal in this mixture from 4000 pounds in 1934 to 311,750 pounds in 1937.
Mixtures of Formaldehyde-Hardened Protein Material with Phenolic Molding Compounds The present investigation deals with an entirely different material in that all proteins used in the mixtures studied, with such exceptions as will be noted, were formaldehydehardened proteins or protein materials. The objective is to plasticize and render thermosetting the thermoplastic dried formaldehyde-hardened protein by the incorporation of the smallest amount of phenolic resin or resinous mixture that will accomplish the desired result. It will be shown that, with formaldehyde-hardened protein, material may be produced superior to that made with the type of unhardened protein previously used in these mixtures. The protein material utilized for the major part of this investigation was either solvent-extracted soybean meal or Prosoy G, both low-cost products. In addition, for purposes of comparison commercial soybean Alpha Protein, self-soured casein, and zein were also studied. All except the zein were hardened with formaldehyde as previously described ( 7 ) and dried t o 2 per cent moisture content before being mixed with the phenolic material. When not treated with formaldehyde, zein was shown ( 7 ) to possess better plastic properties; therefore it was merely dried to 2 per cent moisture content before mixing. Analyses and descriptions of all protein materials used except Prosoy G may be found in earlier studies ( 7 ) . Prosoy G is a commercial product which contains 40 to 50 per cent protein, the balance being insoluble fiber and carbohydrate material which may be considered equivalent to filler. All phenolic resins and resinous molding compounds used were supplied by manufacturers. The identifying letters and numerals listed in the tables are those placed on the samples by the suppliers. The mixing consisted of grinding the protein and phenolic material together in a ball mill overnight followed, in the case of the resins, by milling on heated calender rolls. Since this is a preliminary investigation into the possibilities that may lie in these mixtures, plastic flow measurements and water absorption data are considered adequate criteria for comparison. A later investigation dealing with the development of commercial possibilities of this plastic will report on (comparative strength tests.
1003
TABLE I. MIXTURES OF FORMALDEHYDE-HARDENED INDUSTRIAL PROTEINS WITH COMMERCIAL PHENOLIC MOLDINGCOMPOUNDS IN 50-50 PROPORTION Phenolic Molding Compound
Flow at 1500 Lb./Sq. In. Inches
Wate: Absorption Molding Time (48 Hr.) Minutes Per cent Soybean Meal 3 8.6 5 8.1
Resinox 1811
1.35
Bakclite 120
0.52
3
Durez 3630
5
7.6 7.6
0.80
3 5
9.5 Blistered
Appearance on Drying
Very rough Very rough Very rough Rough
....
Prosoy G Resinox 1811
0.65
Bakelite 120
0 14
Durez 3630
0.37
Reisnox 1811
0.85
Bakelite 120 Durez 3630
3 5 3 5
8.8 7.3 6.5
5.9 3 5.1 5 3.7 Soybean Alpha Protein 9
5.2
0.64
5 2
4.9 S O 5.0
0.72
2
2.7 2.0
5
5
Self-soured Casein 2 7.1
Resinox 1811
0.44
Bakelite 120
0.18
2
0.23
2
1.50a 1.50 0 28
2b
4.6
5b
3.6 3.2
Durez 3630
Resinox 1811 Bakelite 120 Durez 36.10
5
5.6 6.6
5.1 4.2 3.3 Zein (Not Formaldehyde-Hardened) 1.50a 2b 3.5 5 5
5b
2
5
3.5
2.9
Cracked Checked Cracked Checked Cracked Checked
Checked Checked Checked Checked Checked Checked Cracked Cracked Cracked Cracked Rough Rough Rough surface Rough surface Rounh surface Rough surface Rough surface Rough surface
Commercial phenolic molding compound Bakelite 120wood flour, 50-
Dull 0.15 5 3.5 Pressure 800 pounds per square inch. b It war! necessary t o chill these pieces in the die under pressure because the material remained thermoplastic even after a 10-minute cure. c Given for comparison. 50c
Q
The plastic flow was measFred on a Rossi-Peakes flow tester at 302" * 0.9" F. (150 * 0.5' C.) and at the pressures given with the data. The test pellet weighed 0.645 gram and was l / g inch high X 3/g inch in diameter. The duration of each test was 1 minute, and the curves were automatically plotted by the apparatus, the distance of flow in inches along the abscissa and the time in seconds along the ordinate. A complete description and outline of the procedure may be found in the literature (3740,42). Water absorption was determined with disks 2 inches in diame ter and l / g inch thick by a modified A. S. T. M. method. These disks were all molded at 325" F. (163" C.) and 3000 pounds per square inch pressure. The conditioning was changed to 24 hours at room temperature in a desiccator instead of at 50" C. (122' F.) in an oven, because it was proposed t o use the water absorption data as a measure of the curing time. The weight of the disks at the end of the conditioning period was taken as the dry weight. The disks were immersed in distilled water at room temperature for 48 hours, superficially dried, and weighed again. The gain in weight was taken as the amount of water absorbed. The disks were dried at 50" C. for 48 hours and inspected, and their appearance was noted and recorded. (48)
For the purpose of this study 50-50 mixtures of formaldehyde-hardened meal and phenolic molding compounds were found satisfactory and were adopted. The mixture did not flow so readily as the phenolic molding powder alone. It required a somewhat longer time (cure) in the die, had considerably greater water absorption, and was much more affected by the action of water as shown by the cracking and checking of the water absorption test pieces on drying (Table I). This was probably the result of the action of lime in the phenolic molding powder on the protein. Different molding powders gave varied results in these tests. The molded pieces showed
1004
INDUSTRIAL AND ENGINEERING CHEMISTRY
a homogeneous, resinlike conchoidal fracture on breaking, which was entirely different from that of the phenolic-wood flour molding plastics with their heterogeneous porous centers and hard resinous surfaces. From many angles the material appears to be a modified protein plastic. I n order to compare formaldehyde-hardened soybean meal directly with wood flour, two test mixtures were made; plastic flow was determined at 1500 pounds per square inch and 302' F. with the results shown in Figure 1. Not only does the wood flour greatly reduce the plastic flow, but it retards the cure to the extent that no cure is indicated during the period of the test. The sharp breaks in curves 2 and 4 indicate the completion of cure; i. e., the material ceases to be thermoplastic and thermosets. The point of cure has been defined (39) as the point where the flow distance fails to increase by as much as 0.01 inch in 5 seconds a t 302 O F. I n order to continue a comparison begun in a preceding study (7), commercial soybean Alpha Protein, Prosoy G, selfsoured casein, and zein were treated in a manner analogous to the soybean meal except that, as noted before, zein was not treated with formaldehyde. The untreated zein and the formaldehyde-hardened soybean Alpha Protein, Prosoy G, and casein were all dried to 2 per cent moisture content, mixed 50-50 with phenolic molding compounds, and ground in a ball mill overnight. The results of tests made on these materials are listed in Table I. These data show that the molding mixture containing the formaldehyde-hardened soybean Alpha Protein flowed more readily, cured more rapidly, and was somewhat lower in water absorption than the mixture with the formaldehyde-hardened casein. Two of the phenolic molding compounds in a 50-50 mixture with zein produced very free-flowing plastic materials, but they lacked thermosetting properties. The third phenolic molding powder gave an entirely different product with zein, which was less free flowing but had thermosetting properties. All the zein-phenolic plastics were affected less by the alkali in the molding mixture than the other proteinphenolic plastics, none checking or cracking on drying after the 48-hour water immersion.
VOL. 32, NO. 7
Properties I n order to determine in a more complete and practical manner the properties of soybean protein-phenolic plastic material, the formaldehyde-hardened soybean protein material was compounded with phenolic resins. The phenolic resins were furnished by industrial concerns and were designated as shown in Table 11. As the result of a number of experiments the following formulas were established for mixing materials prior to milling on the rolls: Materials Commercial phenolic resin Hexamethylenetetramine Formaldehyde-hardened soybean meal or Prosoy G Lime Calcium stearate Spirit-soluble dyestuff
Two-Stage Resins Parts 40 4 66 0.75 1 2
One-Stage Resins Parts 40
...
60 0.75 1 2
The amount of hexamethylenetetramine used in the formula with the two-stage resin varied somewhat with the resin used. The producer of the resin furnished this information. I n order to obtain satisfactory results with one-stage resins, i t was found essential that all the excess formaldehyde be removed from the formaldehyde-hardened protein material. The entire formula was mixed in a ball mill 16 to 20 hours and then milled on a 12 x 6 inch laboratory calender roll. The faster roll was maintained a t approximately 210' F. (99' C.) and the slower roll at 160" F. (71' C.). As soon as the resin melted, i t penetrated the formaldehyde-hardened protein material, plasticized it, and caused it to adhere and Fork well on the rolls. It was essential that this material be milled long enough to ensure the penetration of the phenolic resin into every particle of hardened protein in order to plasticize the material uniformly and ensure homogeneous molded pieces. A minimum time of 2 minutes of milling on the rolls was essential. All mixtures which set up in less than 2 minutes were discarded. Resins with softening points of 140' to 16O'F. (60' to 71' C . ) were found to be necessary unless a larger proportion I of the resin was used, and this was undesirable from economic as well as other angles. The data obtained from plastics preTABLE11. PLASTIC MATERIALMADE FROM FORMALDEHYDEHARDENED SOYBEANMEAL COMPOUNDED WITH COMMERCIAL pared from formaldehyde-hardened soybean meal and several PHENOLIC RESINS phenolic resins are given in Table 11. Formaldehyde-hardWater ened Prosoy G substituted for the meal gave material with Flow at 800 Molding Absorption Appearance Phenolic Resin Lb./Sq. In. Time (48 Hr.) on Drying water absorption ranging from 5.3 to 1.8 per cent. Inches Minutes P m cent A comparison of some of these 60-40 formaldehyde-hardened Resinox L-4448 0.73 1 10.4 Checked soybean and phenolic resin mixtures with commercial phe7 4.6 Checked Resinox L-4449 0.45 1 6.8 Cracked nolic wood flour molding compounds is shown in Figure 2. 7 4.6 Cracked Resinox L-4450 0.47 1 6.8 Cracked Curves 1, 2, and 3 were produced by three different com7 3.8 Cracked mercial phenolic molding compounds, the three best of those Glidden VM-422 0.70 1 7.1 Cracked 5 5.8 Cracked used in this investigation; curves 4, 5 , and 6 were produced Glidden VM-22-63-2 1.48 2" 6.2 Cracked 7 3.6 Cracked by 60-40 mixtures of formaldehyde-hardened soybean meal N o t plastic enough t o work on the rolls Glidden VM-22-74-1 and Resinox L-4449, Resinox L-4448, and Glidden VM-22N o t plastic enough to work on the rolls Bakelite 1922 63-2, respectively (Table 11). The pressure in every test 100% phenolic molding powder (Bakelite was 800 pounds per square inch and the temperature, 302' F. 120) b 0.56 5 0.9 Good It is apparent that the soybean plastics compare favorably Two-minute cure,, blistered when removed from die. b Given for comparison. with these commercially established molding compounds, both in regard to plastic flow and curing time. (1
A comparison between the product of the method here developed and that of preceding investigators was made by preparing a 50-50 mixture of unhardened soybean Alpha Protein and Resinox 1811 exactly as the analogous mixture with the formaldehyde-hardened soybean Alpha Protein. Test pieces molded from the unhardened mixture gave water absorption figures of 35 and 29 per cent, respectively, for 3- and 5-minute cures, which compare with 5.2 and 4.9 per cent, respectively, 'for the hardened mixture (Table I).
Effect of Alkali It has been indicated that the presence of lime or other alkali in the molding mixture has a n adverse effect on the protein. However, lime has been incorporated in order to obtain a smooth polished surface. To demonstrate the effect of lime, the following formula was compounded by the usual procedure : 40 parts Glidden VM-422 resin, 3.6 paraformaldehyde, 65 formaldehyde-hardened Prosoy G , 2 aluminum
JULY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
1005
from both hardened and unhardened Prosoy G cracked, but those from the unhardened Prosoy G not only shattered much worse but warped badly. Pieces molded from the 40-60 mixtures of phenolic resin-formaldehyde-hardened protein material are definitely superior in appearance and water resistance, as well as in plastic flow and strength, to those molded from 50-50 mixtures of phenolic molding powders-formsldehyde-hardened protein material. However, they could not be considered commercial because they check on drying after water immersion, and are too resinous and brittle. To overcome these handicaps, different proportions of fibrous material such as wood flour were incorporated into the mixture. A good practical working formula was found to be 37.5 per cent formaldehyde-hardened sovbean meal. 37.5 Dhenolic resin. and 25 wood 2 . CO\.lP.IRISON O F PL.4sTIC FIGURE 1. COMPARISON O F PL.4STIC FIGURE flo"ur. These'matekals were miied in a ball O F 60-40 M I X T U R E S O F FORMFLOW OF 50-50 MIXTURES OF FORM-FLOW mill and then milled on the calender rolls, as ALDEHYDE-HARDENED SOYBEAN ALDEHYDE-HARDENED SOYBEAN described above, for a t least 2 minutes. MEAL AND PHENOLIC RESIN, WITH M E A L AND P H E N O L I C RESIS, W I T H Molded pieces had nearly, if not quite, the COMMERCIAL P H E K O L I C W O O D FLOURCOMMERCIAL P H E X O L I C MOLDING COMPOUXDS MOLDING COMPOUNDS impact and flexural strength of regular phenolic 1, 2, 3. T h r e e commercial phenolic moldmolding materials, and water absorption of 1. 50-50 Resinox 1811-wood flour ing materials 2 . 50-50 Resinox 1811-formaldehyde-hardabout 3 per cent in 48 hours; and the 4 5 6. GO-40 Mixtures of formaldehydeened soybean meal hHrdened soybean meal with Resinox L3. 5C-50 Bakeljte 120-wood flour molded test pieces did not check or crack on 4449, Resinox L-4448, a n d Glidden VSI4. 5C-50 Bakelite 120-formaldehyde-hard22-63-2, respectively ened soybean meal drying. The cure mas little, if any, slower than that of commercial phenolic molding compounds, and the plastic flow, although of a different type, was as practical as that of the average stearate, 0.5 dyestuff. Formaldehyde-hardened Prosoy G phenolic molding compounds. Protein is recognized as was chosen for this experiment because molded plastics made being a good base for dyeing so that this new molding with it were less permanent after immersion. It is more mixture may be expected to furnish a good range of bright acid than the formaldehyde-hardened soybean meal, the pH colors. Detailed investigation of this material is under way a t of the water extracts being 4.6 and 6.5, respectively. Except present. for the absence of any alkali, the above formula is comparable to the one used in the preceding study. The water absorpSummary tion for the alkali-containing material which cured in 3 Mechanical mixtures of formaldehyde-hardened soybean minutes was 3.3 per cent, and the test piece cracked on drying. meal and phenolic molding compounds possess thermosetting The water absorption for material molded under the same characteristics. Such a mixture in a 50-50 proportion proconditions from the above formula was 1.9 per cent, and the duces pieces which clear the fully heated die without blistertest piece did not crack or check on drying. The alkali-free ing, when molded under the same conditions as those required material worked more satisfactorily on the rolls and showed by the phenolic molding compounds alone and with the less tendency to set up quickly. However, pieces molded same curing time. from this material, even when given a 15-minute molding Formaldehyde-hardened soybean meal in mixture with period, lacked the bright glossy surfaces of pieces made from phenolic resins cannot be considered a filler material such as molding powder containing lime. The difference in the wood flour. It is more correct to consider the phenolic appearance of the surfaces of the alkaline and acid material resin as a plasticizer and a modifier of the hardened protein is sometimes attributed to different types or degrees of plastic. polymerization. Mixtures of 60 per cent formaldehyde-hardened soybean Attempts were made to find a catalyst which would satismeal and 40 per cent phenolic resins, compounded on heated factorily cure the phenolic resin but which would not be calender rolls, produce a good thermosetting molding plastic, alkaline enough to affect the protein seriously. Borax, but one inclined to be somewhat resinous. The alkali comalum, oxalic acid, and stannous chloride were tried, but monly used to catalyze the cure of the phenolic resin appears molded pieces made from powder containing these catalysts to weaken the water resistance of the hardened protein. when discharged from the die were not so highly polished as Incorporation of 25 per cent wood flour with equal parts those made from powder containing lime. of formaldehyde-hardened soybean meal and phenolic resin In order to compare further the results of this present work produces thermosetting plastics with as fine a finish, as great with those of earlier investigators, a 60-40 mixture of unstrength, as short a cure, as permanent as average phenolic hardened Prosoy G and Glidden VM-422 resin was complastics, and with greater possibilities for color and shade pounded and milled on the rolls in exactly the same way as production. the mixture of formaldehyde-hardened Prosoy G used to produce the test material mentioned above. Test pieces Literature Cited were molded a t 3 and 5 minutes. The water absorption per(1) Albert and Berend, French Patent 436,720 (April 3, 1912). centages of these pieces were 27.7 and 17.8, respectively, (2) Allgemeine Elektricitsts-Ges., British Patent 445,839, not which should be compared with 3.4 and 3.3 per cent for the granted. test pieces made from the molding powder containing form(3) Berend, L., C . S. Patent 962,724 (March 22, 1910). aldehyde-hardened Prosoy G. On dryihg, the test pieces (4) Ibid., 1,040,850 (Oct. 8. 1912).
1006
INDUSTRIAL AND ENGINEERING CHEMISTRY
(5) Berlin, H., Ibid., 1,988,475(Jan. 22, 1935). (6) Bormans, E.,Ibid., 1,885,563(Nov. 1, 1932). (7) Brother, G. H.,and McKinney, L. L., IND.ENO.CHEM.,30, 1236-40 (1938). (8) Ibid., 31, 84-7 (1939). (9) Brother, G. H., and McKinney, L. L., Modern Plastics, 16,‘41-3 (1938). (10) Burk, R. E.,Thompson, H. E., Weith, A. J., and Williams, I., “Polymerization”, A. C. S. Monograph 75,p. 215,New York, Reinhold Pub. Corp., 1937. (11) Chase, H.,Brit. Plastics, 7,516 (April, 1936). (12) Collet, R.,French Patent 734,335 (Oct. 19, 1932). (13) Ellis, Carleton, “Chemistry of Synthetic Resins”, p. 415. New York, Reinhold Pub. Corp., 1935. (14) Frood, H.. British Patent 176,405(1920). (15) Fuhrmann, L. J., U. S. Patent 2,006.736(July 2, 1935). (16) Goldsmith, B. B., British Patent 14,098 (June 18, 1907). (17) Zbid., 412 (Jan. 7, 1910). (18) Goldsmith, B. B., U. S. Patent 840,931(Jan. 8,1907). (19) Ibid., 924,057(June 8,1909). (20) Ibid., 964,964(July 19, 1910). (21) Zbid., 965,137(July 19. 1910). (22) Ibid., 1,027,122(May 21, 1912). (23) Ibid., 1,076,417(Oct. 21, 1913). (24) Grodzinski, P.,K u m t s t o f e , 26, 141-4 (1936). (25) Hansen, D.W.,U. S. Patent 2,047,961(July 21, 1936). (26) Herald Akt. Ges., German Patent 530,134 (Dec. 29, 1927). (27) Hotta, K., and Nakajimi, K., Japanese Patent 42,075 (March 24, 1922).
VOL. 32, NO. 7
(28) I. G. Farbenindustrie Akt.-Ges., British Patent 282,635 (Dec. 23, 1926). (29) I. G. Farbenindustrie Akt.-Ges., French Patent 700,411 (Aug . 11, 1930). (30) Jaroslaw’s Erste Glimmerwaren-Fabrik, British Patent 272,947 (June 17, 1926). (31) Jaroslaw’s Erste Glimmerwaren-Fabrik, French Patent 635,745 (June 10,1927). (32) Kuhl, H., German Patent 280,648 (Aug. 21, 1913). (33) Laurin, L., and Bidot, E., French Patent 751.798 (SeDt. 9.. , 1933). (34) Laurin, L.,end Bidot, E., U.S. Patent 1,969,932(Aug. 14,1934). (35) Lougee, E.F., Modern Plastics, 13,13-15 (April, 1936). (36) Pabst, F., French Patent 652,615(April 12,1928). (37) Peakes, G. L.,Modern Plastics, 14, 39-41 (1937). (38) Peakes, G. L.,Plastic Products, 10,53-7 (Feb., 1934). (39) Zbid., 10,93-8 (March, 1934). (40) Ibid., 10, 133-6 (April, 1934). (41) Plauson, H., U. S. Patent 1,395,729(Nov. 1, 1921). (42) Rossi, L.M., and Peakes, G. L., Zbid., 2,066,016(Deo. 29,1936). (43) Satow, S.. Ibid., 1,245,980(Nov. 8 , 1917). (44) Sturken, O.,Ibid., 2,053,850(Sept. 8, 1936). (45) Taylor, R. L.,Chern. & Met. Eng., 43, 172 (1936). (46) Tripet, R.,French Patent 808,497(Feb. 8, 1937). (47) Weygang, C.,British Patent 111, 171 (Nov. 22, 1916). (48) Wiechmann, F. G.. U. S. Patent 1,061,346(May 13. 1913). (49) Ibid., 1,080,188(Dec. 2, 1913). (50) Zbid., 1,135,340(April 13,1915). (51) Ibid., 1,218,146(March 6 , 1917).
Vulcanization of Rubber Compounds Effect of Hydrogen E. W. BOOTH AND D. J. BEAVER Monsanto Chemical Company, Nitro, W. Va.
M
Sulfide on the Rate of Vulcanization
ANY articles in the literature give theories of the vul-
canization of rubber; some have claimed that the formation of hydrogen sulfide during the cure is essential, others have denied the formation of any hydrogen sulfide during soft rubber vulcanization. I n 1923 Bedford and Gray (8) presented a summary of previous theories and stated that hydrogen sulfide is liberated during vulcanization, which r e acts with zinc oxide present to form zinc sulfide. They further state that the accelerating action of zinc dithiocarbamates are retarded by hydrogen sulfide. I n 1936 Fisher and Schubert (4) stated that their data “appear to show that in the formation of hard rubber, when the amount of sulfur mixed with the rubber is approximately the theoretical (32.02 per cent), i t adds to the rubber hydrocarbon until saturation is complete; very little, if any, substitution takes place”. If, then, saturation with sulfur must be complete before substitution can take place, little or no hydrogen sulfide can be formed during soft rubber vulcanization. In 1939 Fisher (3) proposed a new theory of vulcanization, the basis of which is that hydrogen sulfide adds to the double bond of rubber to form a mercaptan. Should this theory represent the actual mechanism of the vulcanization process, then the addition of hydrogen sulfide to a compounded stock should increase the rate of vulcanization and possibly produce an improved vulcanizate. Therefore, the tests which form the basis for this paper were carried out to determine the effect of hydrogen sulfide on the rate of vulcanization of accelerated rubber compounds.
The rubber compounds used in these tests were milled, vulcanized, and tested according to the specified procedure given by the A. S. T. M. ( I ) . The uncured compounds were divided into two portions, and from each portion sheets were prepared (4.5 X 16.5 X 0.25 cm.). Half of the sheets were placed on shelves of 1-cm. wire screen in a closed drum, and hydrogen sulfide was passed through. I n most cases the time of treatment was 15 hours at room temperature (25” C.), which was sufficient to saturate the rubber compounds. Tests were also carried out in which the time of treatment was varied from 2 to 72 hours and the temperature from 25” to 100” C. To obtain reproducible results, the treated sheets must be vulcanized immediately after treatment because of the rapid loss of hydrogen sulfide on standing. Table I shows the results obtained with rubber compounds containing practically every commercial type accelerator, many of which are used in combination with diphenylguanidine. All are retarded in rate of vulcanization by the hydrogen sulfide treatment. However, the physical properties of a majority of the compounds are not permanently affected; that is, a longer time of vulcanization of the treated compounds has produced maximum modulus and tensile figurer:, comparable with the untreated compounds. Aldehyde-amines, dithiocarbamates, thiuram sulfides, and litharge are permanently affected; -that is, the maximum modulus and tensile
