Vulcanization of chlorobutyl rubber. I. The identification of crosslink precursors in compounds containing ZnO/ZnCl2

Chlorinated poly(isoprene-coisobutylene) (CIIR or chloro-butyl) was characterized by Nuclear Magnetic Resonance. Compounds with ZnO, ZnCl2, and ZnO/ZnCl2 were vulcanized in a DSC at a programmed heating rate and isothermally at 150°C in a press. Crosslink densities were determined by swelling and extracted ZnCl2 analyzed by atomic absorption. ZnCl2 formation precedes crosslinking, and at equilibrium only 50% of the chlorine in the rubber is extractable as ZnCl2. ZnCl2 promotes crosslinking, and its addition to formulations decreases but does not eliminate the induction period prior to crosslinking. Dehydrohalogenation, which is catalyzed by ZnCl2 in an autocatalytic process, is accompanied by the formation of conjugated diene butyl on the polymer chain. It is demonstrated that both species, not only ZnCl2, are necessary precursors to crosslink formation. Moist ZnCl2 catalyzes dehydrohalogenation but not crosslinking, while ZnO does not promote either reaction. CIIR can be crosslinked to polybutadiene and to polyisoprene, no induction period applying when ZnCl2 is present in the formulation, and high crosslink densities develop in blends precipitated from solution. Severe reversion occurs in formulations where there is insufficient ZnO to trap all of the HCl evolved. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2290–2301, 2000