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(8) Bostrom, Siegfried, Kolloddchem. Beihefte, 26, 439 (1928) ; Rubber Chem. Tech., 2, 259 (1929). (9) BostrBm, Siegfried, Kolloidchem. Beihefte, 26, 467 (1928). (10) Bruson, H. A., Sebrell, L. B . , and Calvert, W. C., IND. ENG. CHEM.,19, 1033 (1927). (11) Carothers, W. H., Williams, Ira, Collins, A. M., and Kirby, J. E., J. Am. Chem. Soc., 53, 4203 (1931). (12) Conant, J. B., and Tongberg, C. O., Ibid., 52, 1659 (1930). (13) Fisher, H. L., IND.ENG.CHEM.,19, 1325 (1927). (14) Hada, Fukaya, and Nakajima, Rubber Chem. Tech., 4, 507 (1931). (16) Hada and Nakajima, Ibid., 3, 56 (1933). (16) Hardman, A . F., and White, F. L., IND. EKG.CHEM.,19, 1037 (1927). (17) Hartner, Fritz, Kolloidchem. Beihefte, 30, 83 (1929) ; Rubber Chem. Tech., 3, 215 (1930). (18) Hock, Lothar, Ibid., 2, 275 (1929). (19) Jessup, J. Research Natl. Bur. Standards, 20, 589 (1938). (20) Jessup and Cummings, Ibid., 13, 357 (1934); Rubber Chem. Tech., 8, 44 (1935). (21) Kirchhof, Gummi-Zfg., 39, 892 (1925). (22) Kirchhof and Wagner, Ibid., 39, 537, 572 (1925). (23) McPherson, A. T., and Bekkedahl, N . , IND. ENG.CHEM.,27 597 (1935). (24) Menadue, F. B., Rubber Chem. Tech., 6, 442 (1933). (25) Parks, G. S.,and Huffman, H. M., “Free Energies of Some Organic Compounds”, New York, Chemical Catalog Co., 1932.
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(26) Perks, A. A, J. SOC.Chem. I n d . , 45, 142T (1926). (27) Shepard, N. A., and Krall, S., J . IND. ENG. CHEM.,14, 951 (1922). (28) Spear, E. B., Colloid Symposium Monograph, 1, 321 (1923). (29) Spence, D . , and Ferry, J. D., J. Am. Chem. SOC.,59,1648 (1927) ; Rubber Chem. Tech., 11, 54 (1938). (30) Staudinger, H., and Geiger, E . , Helv. Chim. Acta, 9, 549 (1926). (31) Stevens and Stevens, J. Soc. Chem. Ind., 51, 44T (1932) ; Rubber Chem. Tech., 5, 645 (1932). (32) Toyabe, Ibid., 3, 384 (1930). (33) Walker, H. W., unpublished data from Jackson Lab., E. I . du Pont de Nemours & Co., Inc. (34) Weber, C. O., “Chemistry of India Rubber”, p. 104, London, Griffin and Co., 1902. (35) Wiegand, W. B . , Rubber Chem. Tech., 10, 407 (1937). (36) Williams, Ira, “Polymerisation”, p. 174, New York, Reinhold Pub. Corp., 1937. (37) Williams, Ira, IND.ENG.CHEM.,26, 1190 (1934); Rubber Chem. Tech., 8, 102 (1935). (38) Williams, Ira, Ibid., 12, 191 (1939); Proc. Rubber Tech. Conf., London, 1938, 304. (39) Williams, Ira, and Beaver, D . J., IND. ENG. CHEM.,15, 255 (1923). (40) Williams, Ira, and Starkweather, H. W., in Davis and Blake’s “Chemistry and Technology of Rubber”, A. C. S.Monograph 74, p. 253, New York, Reinhold Pub. Corp., 1937. (41) Williams, Ira, and Walker, H. W., IND. ENG. CHEM.,25, 201 (1933).
Theories of Acceleration L. B. SEBRELL The Goodyear Tire & Rubber Company, Akron, Ohio
.
The various theories that have been advanced over the past thirty-one years to explain the action of organic accelerators and the mechanism of acceleration are given. Wherever consistent, a chronological order of discussion has been adopted, beginning with the work of Erdmann in
1908. Earlier theories were confined for the most part to a purely chemical consideration for an explanation of acceleration. Recently the trend is toward a physical or physicochemical method to investigate and correlate the facts concerning the action of accelerators.
H E use of accelerators in connection with the vulcanization of rubber has been common practice since the beginning of the industry. That Charles Goodyear employed white lead to accelerate the vulcanization of the first sample of rubber is well known. In the manufacture of rubber articles for various uses, it was early recognized that certain powders or fillers showed definite effects in giving desirable properties to the finished product. Tensile strength, elongation, hardness, abrasion resistance, and many other properties could be controlled by the proper compounding of fillers. Accordingly, within a few years many inorganic and some organic substances were mixed into rubber and their effects studied. It was soon found that
T
certain materials had a marked effect on the rate of vuloanization, and quicker curing stocks with better physical properties were the result. Such materials included various lead compounds other than the basic carbonate. Lead thiosulfate or “Black Hypo”, litharge, red lead, etc., lime, magnesia, and antimony pentasulfide were also used extensively; the sulfides of the alkali and alkaline earth metals were also used but to a lesser extent. These compounds were known as sulfur carriers, the term “accelerator” being introduced some time later. Considerable work on the action of lead compounds as sulfur carriers was disclosed by Weber (20)in his classical treatise on rubber. He contended that only the sulfides of the heavy metals act as sulfur carriers and that they act through the formation of polysulfides. He states: “The action of these sulfides, or sulfur carriers, would appear, therefore, to consist in the acceleration of the splitting up of the complex sulfur molecules. . It is much more probable that the catalytic action of these sulfur carriers rather depends upon the matter of chemical equilibrium, and that their presence tends to render it unstable, thus hastening progress toward a lower potential, which in this case is represented by polyprene sulfide.”
...
Thiozone Theory The first serious attempt to explain the action of certain inorganics was that of Erdmann (6) who considered an active form of sulfur to be produced in hot vulcanization; this form was designated as thiozone: XaZS 3s NaS /s
+
>Sn
I+
R&-S RaC-S
+ HzO
+ s + RaC-S I + ZnS R3C
>s
RaC
+s
The mercapto accelerator first reacts with zinc oxide to form the zinc salt. Added sulfur reacts with the zinc salt to form a disulfide and zinc sulfide. The disulfide then liberates the
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active sulfur that brings about vulcanization. The action of thioureas and mercaptobenzothiazole was explained in a similar way. However, the disulfides are not so active as the acids and mercaptans from which they are prepared. It would seem from this theory of disulfides that the true accelerating agent-the disulfides-would show more activity than a compound that must be first converted into a disulfide before the vulcanization action begins. Much sound criticism has been advanced against this theory.
what is termed “power acceleration” or “multiple activation” and is illustrated in the formation of orthothiozonides. Such compounds are considered to be the salts of the hypothetical acid:
Action of Disulfides
celerator would show the zinc-replacing hydrogen from the orthothiosonate thus:
Langenbeck and Rheim (10) have a somewhat different idea of the action of disulfides from that expressed by Bruni and Romani. Whereas the latter assume that disulfides give up one atom of sulfur in an active form and are converted to monosulfides, Langenbeck and Rheim consider the action to be somewhat different. They explain the activity of disulfides to be due to the formation of addition compounds of disulfides and sulfur, and they showed the existence of loosely bound combinations of accelerating disulfides and sulfur by their additive solubilities in pyridine and by thaw-fusion curves which indicated definite maxima for varying mixtures of disulfides and sulfur. They contend, therefore, that sulfur is activated by the presence of.disulfides to form combinations of disulfides and sulfur.
SH A zinc salt of this type derived from a carbosulfhydryl ac-
-C+
\
Then with an activator (in this case ammonia), an amide is formed in this way:
-c=s
k
Theory of Feuchter Until 1925, when Feuchter (6)advanced his theory of acceleration, investigators had all assumed that, by some means or other, accelerators brought about the formation of active sulfur which disengaged itself from the accelerator-sulfur complex and united with rubber to form the vulcanizate. The various theories, therefore, were concerned with the different ways and means by which this could be accomplished. Feuchter presented the theory that both accelerator and sulfur are joined chemically to rubber during vulcanization, and the vulcanizate is such a combination. The bonding of sulfur to rubber is supposed to be preceded by a salt formation. This is a reaction of the acidic “accelerator” of the mercapto type and an activator which is a salt-forming oxide or base. A zinc salt of dithiocarbamic acid would represent such a salt and is known as the accelerator system. Naturally, such a salt may be formed before addition to a rubber mix, but its existence as such is considered necessary before vulcanization starts. A vulcanizate using a zinc dithiocarbamate would be represented thus : \ / --r“ I I
-c=s
\
\ -Zn (1)
$-
”=’
