Changes in Polysulfide Chain Length in Sulfur Structures of Vulcanizates by Light

Abstract

In previous work we showed that sulfur vulcanizates of polybutadiene rubber (polysulfide and thiuram vulcanizates), which differ in their type of crosslinks, also differ in their stability to light. The best light stability is shown by a polysulfide vulcanizate. The action of ultraviolet light in this case is accompanied by decomposition of sulfur bonds with the formation of a type of free sulfur which, in contrast to elemental sulfur, does not take part in photovulcanization processes, i.e., remains “inactive”. Just what the nature of this inactive sulfur is, and the change in length of the polysulfide chains in the sulfur structures of the above-mentioned vulcanizates under the action of light, had not yet been established. Hence it was impossible to give a well-defined answer to the highly important question of why a polysulfide vulcanizate has greater stability to light than a thiuram vulcanizate. The present work is devoted to a study of this question. Although the study of the types of sulfur bonds in vulcanizates is of considerable importance, no reliable methods for their determination are given in the literature. Ascertaining the nature of the bonds in a vulcanizate by chemical methods involves great difficulties because of the impossibility of separating out the sulfur structures with different polysulfide chain lengths, as well as identical sulfur structures combined with the polymer in varying forms.