Vulcanization of Elastomers. 40. Vulcanization of Natural Rubber and Synthetic Rubber with Sulfur in Presence of Sulfenamides. III

Abstract

Vulcanization of natural rubber with sulfur was studied in presence of six sulfenamides, to determine the effect of the chemical constitution of the sulfenamide on sulfur decrease and on crosslinking. The results can be condensed as follows: (1) The kinetics of sulfur disappearance is in every respect qualitatively independent of the chemical constitution of the sulfenamide. (2) For the sulfenamides investigated, the smallest and largest rate constants for sulfur decrease differed only by a factor of two. (3) Greater differences are encountered in the induction times for sulfur decrease and for crosslinking. The latter are notably longer than those for sulfur disappearance. (4) The same activation energy, 23 kcal/mole, is derived from the temperature dependence of the induction times for all the sulfenamides. (5) The dissociation of sulfenamides in solution and their reaction with mercaptobenzothiazole were investigated further. The results provide the basis for a proposed reaction mechanism, which is presented in detail and can account for a number of the features typical of sulfenamide-accelerated vulcanization. (6) The drop in sulfur concentration goes at practically the same rate, if one introduces, instead of N, N-dicyclohexyl-2-benzothiazolesulfenamide, the corresponding ammonium mercaptide in equimolar concentration.

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“…According to De et al,1 the devulcanized rubber contains active cross‐linking gel, which reduces the scorch time. Scheele and Helberg19 reported that the CBS first decomposes to amine and dibenzothiazole disulfide. The dibenzothiazole disulfides then dissociate to form MBT, which is only responsible for cross‐linking.…”

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“…The shortness or complete absence of the induction period indicates that crosslinking reactions may start immediately upon heating 7. It is reported that sulfenamides (thiazole derivatives, CBS, and TBBS) first decompose to form an amine and benzothiazole disulfide, which then dissociates to form benzothiazoles (MBT) 17. Crosslinking can occur only after MBT formation.…”

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“…Though retarder has been added to revulcanized rubber but maximum rheometric torque, rate of curing and scorch time were lower than that of the original rubber blends. The revulcanized rubber contains more [23] explained that the sulfonamide first decomposes to an amine and dibenzothiazole disulfide, which than dissociates and forms 2-mercaptobenzothiazole (MBT). After MBT generation only crosslinking occurs.…”