Vulcanization of Chlorobutyl Rubber. III. Reaction Mechanisms in Compounds Containing Combinations of Zinc Dimethyldithiocarbamate, Tetramethylthiuram Disulfide, Sulfur, and ZnO

摘要

Poly(isoprene-co-isobutylene) (llR or butyl) and chlorinated poly(isoprene- co-isobutylene) (CIIR or chlorobutyl) compounds containing combinations of zinc dim- ethyldithiocarbamate [Zn_2(dmtc)_4], tetramethylthiuram disulfide (TMTD), sulfur, and ZnO were vulcanized at 150°C, the reaction was stopped at various points, crosslink densities were determined by swelling, and the concentrations of residual curatives and extractable reaction intermediates and products were determined by high-performance liquid chromatography and atomic absorption (ZnCI2). 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 crosslinks. As found with ZnO/ZnCI_2 formulations, both ZnCl_2 and conjugated diene butyl are essential precur- sors to crosslink formation. Zn_2(dmtc)_4 can trap HCl, thus preventing reversion and may also initiate dehydrohalogenation. When the equilibrium crosslink density is reached, 50% of the chlorine originally present in the rubber is extractable as ZnCl_2 and the remainder as dimethylthiocarbamic acid chloride. A mechanism to account for dehydrochlorination and crosslinking in the presence of Zn_2(dmtc)_4 is presented. In compounds with sulfur, crosslinking occurs via accelerated sulfur vulcanization and chlorine abstraction, leading to higher crosslink densities than is achieved with either curative system on its own. Carbon-carbon crosslinks predominate, the slower, accel- erated sulfur reaction, making a lesser contribution to the overall reaction.