Crystallization of vulcanizates. II. Low‐temperature crystallization as a function of extent of cure for polybutadiene vulcanized with dicumyl peroxide, tetramethylthiuram disulfide, 2‐bisbenzothiazole‐2,2′‐disulfide, and zinc–accelerator complexes

Abstract

Polybutadiene compounds, vulcanized to various degrees of cure, were crystallized in a density column at Ϫ16°C. The percentage crystallinity of vulcanizates was also determined by differential scanning calorimetry where samples, precooled at a programmed rate, were reheated. Curing with peroxides has little effect on either the rate or the extent of crystallization, except at very high crosslink densities, although the induction period prior to crystallization increases progressively with increased crosslink density. Tetramethylthiuram disulfide (TMTD)/sulfur and 2-bisbenzothiazole-2,2Ј-disulfide (MBTS)/sulfur vulcanizates, cured for progressively longer periods, were found to have lower densities, a result attributed to an increase in free volume occasioned by the formation of accelerator-terminated pendent groups on the polymer chain. The induction period for crystallization increases and both the rate and the extent of crystallization decrease with extent of cure. These changes are more marked for MBTS vulcanizates that do not crystallize once a gel has formed. Formulations with zinc stearate develop higher crosslink densities and crystallize to a greater extent on cooling, showing the effect of zinc stearate in the crosslinking of pendent groups. The densities of both zinc dimethyldithiocarbamate [Zn 2 (dmtc) 4 ]-and zinc mercaptobenzothiazole [Zn(mbt) 2 ]-accelerated sulfur vulcanizates increase with cure time, a result attributed to the formation of ZnS in the compounds. Zn 2 (dmtc) 4 compounds crystallize extensively on cooling, pointing to limited main-chain modification. It is suggested that main-chain modification in these vulcanizates may comprise cyclic sulfide formation. Zn(mbt) 2 compounds crystallize less readily than Zn 2 (dmtc) 4 compounds, but to a greater extent than MBTS/sulfur compounds. The crystallization of the vulcanizates is discussed in terms of vulcanization reactions that give rise to crosslinking with the different formulations used.