Vulcanization of chlorobutyl rubber. III. Reaction mechanisms in compoundscontaining combinations of zinc dimethyldithiocarbamate, tetramethylthiuram disulfide, sulfur, and ZnO

Poly(isoprene-co-isobutylene) (IIR or butyl) and chlorinated poly(isoprene- co-isobutylene) (CIIR or chlorobutyl) compounds containing combinations of zinc dimethyldithiocarbamate [Zn-2(dmtc)(4)], tetramethylthiuram disulfide (TMTD), sulfur, and ZnO were vulcanized at 150 degreesC, the reaction was s topped at various points, crosslink densities were determined by swelling, and the concentrations of residual curatives and extractable reaction inter mediates and products were determined by high-performance liquid chromatogr aphy and atomic absorption (ZnCl2). In compounds that did not contain zinc, CIIR crosslinked more slowly than IIR and crosslinking could be explained by the same mechanism as applies to the vulcanization to highly unsaturated rubbers like polyisoprene. In zinc containing compounds, CIIR crosslinked faster because of dehydrohalogenation reactions that led to carbon-carbon c rosslinks. As found with ZnO/ZnCl2 formulations, both ZnCl2 and conjugated diene butyl are essential precursors to crosslink formation. Zn-2(dmtc)(4) can trap HCl, thus preventing reversion and may also initiate dehydrohaloge nation. When the equilibrium crosslink density is reached, 50% of the chlor ine originally present in the rubber is extractable as ZnCl2 and the remain der as dimethylthiocarbamic acid chloride. A mechanism to account for dehyd rochlorination and crosslinking in the presence of Zn-2(dmtc)(4) is present ed. In compounds with sulfur, crosslinking occurs via accelerated sulfur vu lcanization and chlorine abstraction, leading to higher crosslink densities than is achieved with either curative system on its own. Carbon-carbon cro sslinks predominate, the slower, accelerated sulfur reaction, making a less er contribution to the overall reaction. (C) 2000 John Wiley & Sons, Inc.