In this discussion, we have indicated that carbon blacks display both chemical and catalytic activity which appear to be sufficient to radically alter the chemistry of vulcanization. Much of the chemical reactivity results in removal of molecules or reactive intermediates which might otherwise produce crosslinks. However, in some cases the chemical reactivity seems to be associated with catalytic activity. When carbon black is heated with rubber, as during hot mixing or during cure, the following types of reaction are considered possible: a) Alkylation (in a broad sense) of carbon black by rubber molecules. This would be a case of alkylation of an aromatic material by an olefin. b) Isomerization of the double bond structure of rubber molecules resulting in conjugation. c) Chemisorption of rubber molecules with dissociation—dehydrogenation—and chemical combination of the rubber-free radicals so formed with the carbon black surface or with themselves, a type of polymerization. Insofar as addition to carbon black is involved, the results of a) and c) are, for practical purposes, identical. During vulcanization, the action of carbon black will be dependent upon the nature of the rubber, the nature of the curing system and the presence of other compounding ingredients. During cure, carbon black may be considered to act in some, if not all, of the following ways: 1) As a catalyst for dehydrogenation by sulfur. 2) As a catalyst for the oxidation of —SH intermediates to —S—S— crosslinks. 3) As a catalyst to convert polysulfides to disulfides or prevent polysulfide formation. (This particular activity has not been demonstrated ; it is suggested in this review only as a possibility.) 4) As a catalyst in activating accelerators by breaking —S—S— linkages, as in TMTD, —S—(S)x—S— linkages in the MBT polysulfide product of Dogadkin and Tutorskii˘, —S—N— linkages in Santocure, NOBS Special, etc. 5) As a catalyst for hydrogen sulfide formation (associated with 1, above) which is apparently necessary, at least under some conditions, to activate curing systems. 6) As a catalyst in the presence of oxidizing agents for the conversion of hydrogen sulfide (and sulfanes) to sulfur. 7) As a catalyst which promotes a type of decomposition of TMTD, and no doubt other systems, in a manner which is efficient in producing crosslinks. In the initial phases of vulcanization, its activity as a dehydrogenation catalyst is of considerable importance. This probably involves chemisorption with dissociation of α-methylene hydrogen atoms. This activity directs the chemical reactions of vulcanization to the α-methylene carbon atom and may lead directly to coupling rather than addition reactions at the double bond. After the dehydrogenation step, polymerization reactions, as described in this issue by Craig, should also be considered as possible. Agglomeration of carbon black particles plus high crosslink density seems to be strongly indicated. This would certainly result in heterogeneous crosslink distribution which would manifest itself in physical properties and possibly in some “chemical properties” of the reinforced vulcanizates.

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Uncertainty as to the nature of the crosslinks in vulcanized rubber has been a tremendous handicap to chemical studies of vulcanization. A direct result is that physical methods have been the most successful in providing definite information about the nature of vulcanized rubber. However, these physical methods also possess serious limitations. A brief review may be in order on the methods which have provided insight into the chemical nature of the crosslinks. A tremendous amount of information has been obtained on the reactions between sulfur and various olefins at vulcanizing temperatures. Much, but not all, of this work has been carried out in the laboratories of the British Rubber Producers' Research Association. These studies are too extensive to review in detail here. But some of the impressions gained from studying them are: (1) There is no evidence that reaction with sulfur produces any carbon-to-carbon bonds. (2) The linkages produced between different molecules (intermolecular) are monosulfides, disulfides and polysulfides. The dominant linkage and the amount of polysulfide sulfur depends on the nature of the olefin, the reaction conditions and the presence of auxiliary materials such as accelerators, zinc oxide and fatty acids. (3) In the initial stages of the reaction, more polysulfide groups are present than in the later stages. Some of the sulfur in these polysulfide groups appears to react with the rubber to form five- and/or six-membered rings, and is referred to as “cyclic sulfur”. The intermolecular sulfur-containing groups which survive contain less sulfur on the average than those present during the early part of the reaction.

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