Investigations in the Field of Rubber Vulcanization. IV. Phenomenon of Vulcanization Optimum in Mixtures with a Large Proportion of Constructive Agent

1. In mixtures with a large content of constructive (vulcanizing) agent, the vulcanization-optimum phenomenon is connected with the formation of dense space-lattices in the structure of the vulcanizate, which inhibit the formation of crystalline or highly oriented segments during elongation. 2. In contrast to mixtures with a small content of vulcanization agent, the vulcanization optimum of which is explained by superimposition of oppositely directed constructive and destructive processes, the case noted in the preceding paragraph shows a maximum only on the kinetic curve representing the change in strength. The kinetic curves representing relative elongation, swelling, and solubility are monotonic in character. 3. The maximum on the kinetic curve representing the change in strength in mixtures with a large content of constructive agent occurs before complete combination of the latter with the rubber. 4. In consequence of the absence of oppositely directed actions on the part of sulfur and oxygen, the vulcanization of sodium-butadiene rubber mixtures shows a maximum only on the kinetic curves representing the change in strength. 5. The position of the strength maximum in mixtures with a large content of constructive agent depends on the rate of deformation. As the rate of deformation is decreased, the maximum appears after more prolonged vulcanization periods of the mixtures. 6. The strength of natural rubber vulcanizates at the vulcanization optimum is a linear function of the quantity of crystalline phase formed during elongation.