September, 1933
I N D U S T R I A L ,4N D E N G I N E E R I N G C H E 31 I S T R Y
.LITERATURE CITED (1) Barre, C o ~ ~ prend., t. 148, 1604-6 (1909); A n n . chim. p h y s . , 24, 145-256 (1911). (2) Clarke, Davidson, a n d Storch, Bur. Mines, Rept. Investigations 3061 (1931). (3) Conley, Fraas, a n d Davidson, Ibid., 3167 (1932). (4) D'Ans, 2. anorg. a.ZZgem. Chem., 62, 129-67 (1908). ( 5 ) International Critical Tables, Vol. IV, p . 353, McGriiw-Hill, 1926. ( 6 ) Storch, B u r . Mines, R e p t . Investigations 3032 (1930). ( 7 ) Storch, IND.EXQ.C H E W , 22, 934 (1930); S t o r c h a n d Fragen, I b i d . , 23, 991 (1931).
1009
(8) Storch a n d Clarke, Bur. Mines, Rept. Investigations 3002 (1930). (9) Unpublished d a t a obtained a t this station (to be submitted for publication during 1933). (10) V a n ' t Hoff, Voerman, a n d Blasdale, Sitrher. preuss. A k a d . n-iss., 1905, 305. (11) V a n ' t Hoff, a n d Wilson, Ibid., 1900, 1142. RECEIVED February 28, 1933. Presented before the Division of Industrial and Engineering Chemistry at the 85th Meeting of the American Cheniical Society, Washington, D. C., Rlarch 26 t o 31, 1933. Published by permission of the Director, U . 6 . Rureau of \lines. ( N o t subject t o copyright.)
React ions during Vulcanization 111. The .Multiple-Accelerator Effect' H. C. JONES, The New Jersey Zinc Company, Palmerton, Pa. Diphenylguanidine functions as a n activator for rnercaptobenzothiazole when the two accelerators are compounded in a stock dejicient in fatty acids. Diphenylguanidine functions as a n accelerator when compounded with mercaptobenzothiazole in a stock with a normal fatty acid content.
T
Butyraldehyde aniline in coniunction with mercaptobenzothiazole behaves in the same manner as diphenylguanidine. 15'ithin the limits investigated, the amount of soluble zinc arailable for reaction has a direct bearing on the ultimate tensile strength of diphenylguanidine and mercaptobenzothiazole stocks.
has definite limitations because it could not apply in the case of two accelerators of the acidic type, such as tetramethylthiuram disulfide with mercaptobenzothiazole, a combination of which shows the two-accelerator effect. As a result of some experimental work with the accelerator mixture, diphenylguanidine and mercaptobenzothiazole, a partial explanation of the two-accelerator effect suggests itself. In a stock deficient in fatty acids (contains 40 per TWO-ACCELERATOR EFFECT cent as much fatty acid as first quality rubber) accelerated The salient facts :ire well known. Perhaps the most with 0.5 per cent of mercaptobenzothiazole, increments of common examule is the combination of mercaptobenzothia- diphenylguPnidire increase the tensile strength (Figure 2 A ) . zole and dipheiylguanidine. From When the same base stockis accelerFigure 1 it is evident that a mixated with 0.5 per cent of diphenylguanidine, and increments of merture of 0.5 per cent of mercaptobenzothiazole and 0.5 per cent of captobenzothiazole are added, the diphenylguanidine is a more effecrate of cure increases as shown by tive accelerator than one per cent the decreased time to the optimum of e i t h e r accelerator alone. The cure, despite the fatty acid depresence of stearic acid in the merficiency of the stock (Figure 2B). captobenzothiazole formula and its Where there is a deficiency in fatty omission with diphenylguanidine is acids, diphenylguanidine r e n d e r s to the advantage of each accelerathe zinc oxide effective for the actor, as will be shown later. tivation of the second accelerator Khitby and C a m b r o n ( 5 ) , in (mercaptobenzothiazole), perforniseeking an e x p l a n a t i o n for this ing the same function as the fatty phenomenon, came to the concluacids, if they mere present. These sion that the soaps formed by the results are in agreement with the r e a c t i o n of the basic accelerator observations of Sebrell and Vogt (4) and the resin acids in rubber functhat all accelerators require soluble tioned as very active d i s p e r s i n g zinc to produce the best physiT M E O F C U R E IN M I N U T E S A T I 3 0 B C . agents and would assist in the reaccal properties and that diphenylFIGURE1. (zOMP.4RISON OF RATEOF CURE OF tion between caoutchouc and sulfur guanidine reacts directly or through >IERC.4PTOBESZOTHIAZOLE, DIPHENYLGUAR'IDINE, during vulcanization. Although it some sulfur reaction p r o d u c t s to ANI) h f I X T U R E S OF THE T W O may be that R h i t b y and Cambron's render zinc oxide soluble for actiFORMUL~ e x p l a n a t i o n has some b e a r i n g Pale crepe vation. 100 100 100 Stearic acid ... 3.0 ... on v u l c a n i z a t i o n , it obviously Three per cent of stearic acid was Mercaptobensorhiazole 0 5 1.0 ... Diphenylguanid ine 0 5 ... 1.0 added to each of the series of stocks 1 Part I, IND. ENG. CHEM..23, 1467 Zinc oxide 12 2 12.2 12.2 Sulfur (1931); Part 11, I b z d , 24, 565 (193?). described above. I n the first seijes 3.0 3.0 3.0 ECHKICAL interest in the two-accelerator or multipleaccelerator effect has received considerable impetus within the past several years owing to improvements in factory processing methods which permit the utilization of its advantages. However, the literature records very little of scientific interest on the phenomenon.
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FIGERE 2 A Rubber deficient in fatty acids Mercaptobenzothiazole Diphenylguanidine Zinc oxide Sulfur
100
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pendent upon the amount and distribution of zinc in a soluble form. Depew ( 2 ) determined the amount of soluble zinc formed when rubber vulcanizing agents react in a lorn-viscosity medium. The following method is in many respects similar to that described by Depew: To 150 cc. of solvent (12.5 per cent methanol and 87.5 per cent benzene) were added the compounding ingredients along with 16 cc. of the solvent saturated with hydrogen sulfide. The mixture was placed in a constant-temperature bath (46' C.) for 15 hours, after which the clear solution was filtered off and the residue thoroughly washed with portions of the solvent me di u m . After evaporation of the solvents the organic material was decomDosed with nitric and sulfuric acids and the zinc residue titrated by a method described by Kolthoff and Pearson (3).
I t is clear, therefore, that fatty acids improve the solution in which mercaptobenzothiazole was held constant a t 0.5 per cent, increments of diphenylguanidine increase the rate of cure of zinc with mercaptobenzothiazole, but in the presence of (Figure 3A). Likewise when corresponding amounts of mer- diphenylguanidine they inhibit the solution of zinc. These captobenzothiazole were added to the second series with 0.5 observations are in agreement with compounding experiments per cent of diphenylguanidine (Figure 3B), the rate of cure which show increased ultimate tensile properties (Figure 6) was increased. These experiments indicate that in the pres- when increments of stearic acid are added to 0.5 per cent mercaptobenzothiazole ence of f a t t y a c i d s , a stock. Conversely, mixture of d i p h e n y l w h e n 3 p e r c e n t of guanidine and mercaptostearic acid is added to benzothiazole f o r m s a a stock accelerated with new accelerator which is diphenylguanidine (Figmuch more active than ure 7 ) , stearic acid deeither one alone. Therec i d e d l y r e d u c e s the f o r e , it is c l e a r that tensile properties of the diphenylguanidine in the compound. p r e s e n c e of mercaptoT h e benzene-methbenzothiazole has a dual anolexperimentsinTable action, functioning as an I1 explain the series of activating agent in the rubber compounds a b s e n c e of fatty acids shown in Figures 2 and 3 . and as an a c c e l e r a t o r ddditions of diphenylwhen these materials are guanidine increase the already present. FIGURE 3 amount of zinc in soluButyraldehyde aniline A . Same a6 Figure 2A + 3.0 parts stearic acid B. Same as Figure 2B + 3.0 parts stearic acid tion (Table 11. samdes (Figures 4 and 5) func1-31. indicatine t h a t t h e tions in the same manner as diphenylguanidine when compounded with mer- improvement in tensile properties in the compounds (Figure captobenzothiazole. Since butyraldehyde aniline is a more 2d) was due to the greater amount of soluble zinc available powerful accelerator than diphenylguanidine, smaller amounts for reaction. With additions of mercaptobenzothiazole to a constant amount of diphenylguanidine (Table 11, samples of this material were used in the accelerator mixture. While these results indicate that some relationship exists 4-6) the zinc in solution remains the same, although the between the two-accelerator effect and the solubility of zinc compounds shorn an increase in rate of cure (Figure 2B). in the compound, these conclusions are, after all, largely hypothetical and can be proved only by resorting to some system which allows a separation of the zinc in solution. I
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EFFECT OF SOLUBLE ZINC Bedford and Sebrell ( I ) , in describing the mechanism of vulcanization, state that zinc oxide reacts with the accelerator to form the zinc salt of that compound, which in turn reacts with sulfur to form polysulfides with the zinc salt of the accelerator. The unstable polysulfides decompose, l i b e r a t i n g sulfur available f o r v u l c a n i z a t i o n , along with the reformation of the zinc salt of the accelerator. Side reactions decompose and regenerate the zinc accelerator c o m p o u n d continually, the c o n c e n t r a t i o n of the zinc accelerator compound a t any time being de-
FIGURE 4 Rubber deficient in fatty acid Mercaptobenzothiazole Butyraldehyde aniline Zinc oxide Sulfur
A
B
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September, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
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FIGURE6 . IXFLUENCE OF STEARIC ACID ON A STOCK ACCELERATED WITH MERCAPTO-
FIGURE5 A.
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BENZOTHIAZOLE
++ 3.0 3 . 0 parts stearic acid parts stearic acid
Rubber deficient in f a t t y acid Stearic acid Mercap tobenzothiazole Zinc oyide Sulfur
I n the presence of fatty acids (Table 11, samples 7-12) the amount of zinc in solution is reduced in all the mixtures of niercapt'obenzothiazole and diphenylguanidine.
100 Variable 0.6
12.2 3.0
action 1) balances the increased concentration of fatty acid in the equilibrium reaction (2). The benzene-methanol experiin-Tables IMand I1 are to T~~~~ 1. D~~~~o:?TB ~ ~ E~~~~~~~~~~~ ~ ~ mental ~ data reported ~ ~ not comparable ~ ~ those published by Depew (1' since these tests were run in (All samples contained 5 grams U. S. P. zinc oxide and 2 grains of d f u r ) the presence of hydrogen sulfide which more nearly simuMERCAPTOBENZODIPHENYLTHIAZOLE GUANIDINE FATTY AcIDa IN SOLN. lates rubber vulcanization conditions. Bedford and Sebrell Grams Grams Grams M Q. 1 2.0 ... ... None detectabie 2 2.0 2.0 3 4 5 ... 6 Coconut oil fatty acids.
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TABLE11. DATAON BENZENE-METHANOL EXPERIMENTS (All samples contained 5 grams U. S. P. zinc oxide and 2 grams sulfur) MERCAPTOBENEODIPHENYLSANPLE THIAZOLE GUANIDINE FATTY ACID ZINC I N QOLN. Grams Grams Gram .MQ. 1 1.0 0.5 6.2 2 1.0 1.0 9.2 1.0 2.0 16.8 3 4 0.5 1.0 8.7 5 1.0 1.0 ... 9.4 6 2.0 1.0 9.3 7 1.0 0.5 0.3 1.2 8 1.0 1.0 0.3 4.5 9 1.0 2.0 0.3 13.6 10 0.5 1.0 0.3 4.7 11 1.0 1.0 0.3 4.9 12 2.0 1.0 0.3 4.0
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Depew ( 2 ) has suggested that an equilibrium exists between mercaptobenzothiazole, its zinc compound, fatty acid, and zinc soap, and has presented evidence to show that an increase in concentration of soluble zinc increases rate of cure. A cursory examination of the data in Figure 6 would indicate a violation of the mass action principle. However, a more detailed study of the following generally accepted rubber reaction will serve to show that the present data do not conflict with that principle: ZnO
+ fatty acid +zinc soap + H20 + mercaptobenzothiazole e zinc benzothiazole + fatty acid
Zn zoap
mercapto-
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Depew selected a compound with just sufficient zinc: oxide to react with all the fatty acid so that further addition of fatty acid tended to displace the equilibrium t o the left (reaction a), resulting in a retardation in rate of cure. The compounds in Of zinc Oxide present so that the Figure had a definite reaction ( 2 ) Was not by addition of fatty acid. I n this case, the formation of additional zinc soap (re-
OF CURE i N MINUTLS A T IZS'C.
FIGURE7. INFLUENCE OF STEARIC ACID ON STOCKACCELERATEDWITH DIPHENYLGUANIDINE Rubber deficient in fatty acid 100 Stearic acid Variable Diphenylguanidine 1 .O Zinc oxide 12.2 Sulfur 3.0
(1) state that hydrogen sulfide is formed during vulcanization by the action of sulfur on the non-rubber constituents of crude rubber.
(1) (2)
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LITERATURE CITED Bedford and Sebrell, J. IND. ENQ.CHEY.,14, 2 5 (1922). Depew, H. A., Ibid., 24, 565 (1932). Kolthoff a n d pearson,Ibid., ~ ~E d . ,~4 , 147 1 (1932). . ENG.CHEM.,16, 792 (1924). Sebrell a n d Vogt, IND. ( 5 ) Whitby, G. S., a n d C a m b r o n , A . , J. SOC.Chem. Ind.. 42, 333T (1923). (1) (2) (3) (4)
RECEIVED March 30, 1933. Presented before the Division of Rubber Chemistry a t the 85th Meeting of the American Chemical Society, Washington. D. c., March 26 to 31, 1933.
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