A Study of Sulfenamide Acceleration

Abstract

Abstract Sulfenamide—sulfur curing systems containing zinc oxide and stearic acid respond to both ionic and radical reaction conditions. Scorch delay and rate of the vulcanization are determined by the basicity and steric structure of the amine forming the amide, and by the accelerator-sulfur ratio. Sulfenamides derived from heterocyclic amines which form stable sulfides showed progressive improvement in scorch delay as the concentration of the sulfenamides was increased at constant sulfur level. In contrast to this group, the acceleration rates of sterically hindered sulfenamides were slow and their scorch delay decreased with increased accelerator concentration. The assumption that the initial step in sulfenamide acceleration is the dissociation of the sulfenamide into benzothiazolyl sulfenamide and amine radicals does not satisfy these experimental results. Consideration of the rate of sulfenamide-sulfur reaction and the stability of these intermediates has been suggested as an alternate explanation. Higher concentrations of accelerators in general resulted in vulcanizates with higher modulus and a larger number of effective sulfur crosslinks. About 1.5–2 atoms of sulfur per effective crosslink were found for vulcanizates where one mole of sulfenamide was used per atom of sulfur. It was also found that at this high concentration of sulfenamide a considerable fraction of the benzothiazolyl sulfenyl groups was initially bound to the rubber molecule and later released as extractable 2-mercaptobenzothiazole derivatives. Part of these groups which remained bound to rubber were reducible to form 2-mercaptobenzothiazole. Part of the true combined sulfur is released as ZnS in later stages of cure.