Vulcanization Structures and Their Influence on the Heat Resistance and Fatigue of Rubber

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Abstract
(1) The phenomenon of stress relaxation in vulcanizates, due to thermal decomposition of the vulcanization structures, is discussed. The process follows the equation: σt=σ0⋅e−kt, where k is the kinetic constant of decomposition of the vulcanization structures, and therefore can serve as a measure of the heat resistance of a vulcanizate. (2) The amounts of polysulfide and disulfide bonds in various vulcanizates were determined. (3) It is established that the heat resistance of vulcanizates depends on the total quantity and also on the proportions of bonds of the type : R1—C—C—R2, R1—C—S—C—R2, R1—C—S—S—C—R3, and R1—C—Sn—R2. (4) It is shown that the relative contents of various vulcanization structures depends on the type of rubber and on the nature of the vulcanizing group. Thus, vulcanizates with tetramethylthiuram disulfide do not contain any polysulfide bonds, whereas vulcanizates with elemental sulfur (without accelerator) contain predominantly polysulfide bonds. (5) It is established that recombination of the radicals formed during thermal decomposition of a vulcanizate leads to the formation of both intramolecular bonds and new stable bridge bonds. (6) It is suggested that thermal decomposition of sulfur bridges is one of the causes of the phenomenon of a vulcanization optimum (reversion). (7) A relation is established between the nature of vulcanization structures and the resistance of vulcanizates to repeated deformation.

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