This paper reports a detailed investigation of the molecular mechanism of crosslink formation in sulfur vulcanization accelerated by mercaptobenzothiazole accelerators, such as bis(mercaptobenzothiazolato)zinc(II) (ZMBT), mercaptobenzothiazole disulfide (MBTS) and N-cyclohexyl-2-benzothiazole sulfenamide (CBS). First, the current state of knowledge regarding the mechanism of crosslink formation in these systems is briefly reviewed.
Subsequently, Reaction-Stage Modeling experiments are reported with the model-crosslink precursor (2,3-dimethyl-2-butene-1-yl)(mercaptobenthiazolato)sulfide (dmb-S2-Bt). These experiments reveal that in the presence of bis(cyclohexylamine)bis(mercapto benzo-thiazolato)zinc(II) (ZMBT·(H2NC6H11)) the precursor dmb-S2-Bt is transformed into a disulfidic crosslink and MBTS via symmetric disproportionation reactions. Also sulfuration and isomerization reactions, leading to a hitherto unknown type of pendent group, namely 2-(2,3-dimethyl-2-buten-1-yl)-1,2-benzisothioazolin-3-thione, were shown to occur. The zinc-amine complex was found to have a pronounced catalytic effect on the crosslink reaction; in its absence, hardly any crosslinks are formed. Reactions performed in the presence of additional MBTS showed an inhibitory effect on crosslink formation, suggesting that the disproportionation reaction is in fact an equilibrium reaction. Indeed, it appeared possible to transform disulfidic model crosslinks into crosslink precursors in the presence of both MBTS and the ZMBT-amine complex. These results have lead to determine the prevailing mechanism by which crosslinks are formed during mercaptobenzothiazole vulcanization, namely through an equilibrated, ZMBT-catalyzed disproportionation reaction of crosslinks precursors. This mechanism not only explains the observations in the present model study, but is also in line with results obtained earlier in real rubber experiments.