The Mechanism of Vulcanization in the Presence of 2-Mercaptobenzothiazole

Abstract

Abstract The mechanism of the action of one of the most widely used vulcanization accelerators, 2-mercaptobenzothiazole (MBT), has not been sufficiently investigated. It was demonstrated by Wistinghausen, and also Dogadkin and Pevzner (1951) that MBT is consumed during vulcanization, but the quantitative relationships involved in this reaction could not be established due to the lack of a method for determining the disappearance of the MBT. For the present study we made use of MBT labeled with the radioisotope S35 in the thiazole ring. The synthesis of MBT was carried out in the following manner. First, MBT labeled in both sulfur atoms, was prepared from phenly isothiocyanate and elementary S35. The MBT thus obtained was then heated in sealed ampules with excess ordinary sulfur at 140° for 6 hours. Under these conditions, the interchange reaction between the elementary sulfur and the sulfhydryl sulfur leads to an equilibrium. There was a twofold drop in the activity of the synthesized MBT. That is the way in which the labeling of the benzothiazole radical alone was accomplished, so that the radioisotope S35 introduced into the MBT molecule was not exchanged with elementary sulfur under the vulcanization conditions that obtained. This now made possible simultaneous measurements of the rate of sulfur addition and the rate with which the accelerator is added to the rubber.