2-Mercaptothiazole and Derivatives as Vulcanization Accelerators

In this paper the subject of rubber vulcanization accelerated by 2-mercapto-benzothiazole and its derivatives has been reviewed. The technical literature from 1945 through 1960 and patents from 1932 through 1960 have been covered. Topics include: methods of synthesis and manufacture of these accelerators; application and compounding data on their use in rubber processing; and studies of the modes of action and mechanisms for the chemical reactions involved during accelerated vulcanization. Much disagreement exists concerning the mechanism of accelerated vulcanization and the action of thiazole accelerators. However, most of the conflict lies not in the experimental data collected, but in the interpretation of the meaning of the data. It is well documented that MBT and activators (zinc stearate, or zinc oxide and stearic acid) undergo an initial reaction; that these reaction products then react with sulfur and/or rubber hydrocarbon to form intermediate compounds; and that these intermediates then react in some manner to form sulfur crosslinks. Not known, for the most part, are the precise reaction steps involved; the sequence in which these reactions occur; the individual mechanisms, whether ionic, free radical, or neither, by which these reactions proceed; and the side reactions involved, if any (except in natural rubber, in which the non-crosslink forming cyclization reaction is well documented) which might lead to erroneous conclusions from the experimental data, particularly from kinetic studies. These same conclusions apply in general to thiazole sulfenamide accelerated sulfur vulcanization, with the exception that, in contrast to MBT, which has a free thiol group available for immediate reaction, the sulfenamide must first decompose or react in some manner before acceleration occurs. Sulfur and divalent sulfur compounds readily undergo both radical and ionic reactions, depending only on co-reactants and reaction conditions. At present, the reactions of sulfur with hydrocarbons and accelerators are not sufficiently well understood to draw concrete conclusions about the mechanism of acceleration. Further progress on elucidation of the mechanism will come with a better knowledge of the chemistry and mechanisms of sulfur reactions.