Chemical Reactions during Vulcanization

Direct addition or bridging of sulfur seems to occur at the double bonds of the rubber in sulfur vulcanization without accelerators (Figure 5, a and b). In accelerated stocks the fact that sulfur combines in excess of one atom of sulfur saturating one double bond of the rubber hydrocarbon suggests that sulfur may also combine in a dehydrogenation reaction (Figure 5, c), and that this type of sulfur linkage is most effective in producing high tensile strengths. The excess loss in unsaturation on overcure indicates that direct polymerization without sulfur (Figure 5, d), as well as reactions of oxygen at the double bonds, may occur after most of the sulfur has combined, but that these chemical changes are not definitely associated with changes in the vulcanized structure as measured by tensile strength. In the vulcanization of a simple rubber-sulfur compound, infrared absorption data indicate that oxygen is combining with the rubber during overcure to produce a degradation in the hydrocarbon structure which is accompanied by a decided loss in tensile strength.