Vulcanization. Part I. Fate of Curing System during the Sulfur Vulcanization of Natural Rubber Accelerated by Benzothiazole Derivatives

Abstract Several investigators have proposed mechanisms for the delayed action sulfur vulcanization of rubbers which postulate that a number of intermediate compounds containing fragments of the accelerator are formed during the vulcanization cycle. Although a number of workers have measured the change in initial accelerator and sulfur concentration in the rubber during vulcanization, none have identified and determined the intermediate compounds which appear to be formed during vulcanization. The purpose of this paper is to first confirm the existence of these intermediates and subsequently to identify and quantitatively follow their concentrations throughout the vulcanization cycle. In our initial studies we have limited our investigations to natural rubber cured with sulfur and accelerated with (i) MBT (2-mercaptobenzothiazole), (ii) (MBTS)[2,2′-dithiobisbenzothiazole)], and (iii) 2-(4-morpholinothio)benzothiazole. In order to reduce the complexity of the system, other normally used additional curing agents, notably ZnO and stearic acid, have been deleted.