Novel Sulfur Vulcanization Accelerators Based on Mercapto-Pyridine, -Pyrazine, and -Pyrimidine

Sulfur vulcanization accelerators derived from 2-mercaptobenzothiazole (MBT) have been a staple for the rubber processing industry for over 65 years. Most noteworthy are the sulfenamide derivatives, which provide various combinations of scorch delay and cure rate, depending upon the basicity and steric nature of the sulfenamide N-substituent(s). While the effect of the amine moiety on benzothiazole sulfenamide accelerator performance has been well documented, the effect of the S-linked heterocyclic moiety of the sulfenamide has been investigated to a much lesser extent. Thus, to gain a better understanding of the role of the heterocycle, a variety of novel disulfides and sulfenamides based on pyridine have been synthesized and tested. The performance of the pyridine based accelerators have been compared to each other and to the various benzothiazole analogues. Until recently, the corresponding sulfenimides (compounds containing two divalent thiol moieties attached to an amine nitrogen ) seem to have received little attention in the rubber industry. Sulfenimides based on pyridine, pyrazine and pyrimidine have therefore also been synthesized and tested. The differences in preparation and vulcanization activity for the benzothiazole sulfenimides and sulfenamides are described, as well as some rudimentary differences which have appeared in the early stages of mechanistic work. It has been discovered in the course of this work that other heterocyclic thiols may be substituted for 2-mercaptobenzothiazole in the basic sulfenimide structure to gain advantages both in faster cure rate and extended scorch delay. The behavior of these novel sulfenimides in common rubber cure systems is presented together with representative physical properties of the rubber vulcanizates.