Sulfur Group Analyses in Natural Rubber Vulcanizates

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Abstract
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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