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

Chlorinated poly(isoprene-coisobutylene) (CIIR or chloro-butyl) was charact erized by Nuclear Magnetic Resonance. Compounds with ZnO, ZnCl2, and ZnO/ Z nCl2 were vulcanized in a DSC at a programmed heating rate and isothermally at 150 degreesC in a press. Crosslink densities were determined by swellin g and extracted ZnCl2 analyzed by atomic absorption. ZnCl2 formation preced es crosslinking, and at equilibrium only 50% of the chlorine in the rubber is extractable as ZnCl2. ZnCl2 promotes crosslinking, and its addition to f ormulations decreases but does not eliminate the induction period prior to crosslinking. Dehydrohalogenation, which is catalyzed by ZnCl2 in an autoca talytic 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 deh ydrohalogenation but not crosslinking, while ZnO does not promote either re action. CIIR can be crosslinked to polybutadiene and to polyisoprene, no in duction period applying when ZnCl2 is present in the formulation, and high crosslink densities develop in blends precipitated from solution. Severe re version occurs in formulations where there is insufficient ZnO to trap all of the HCl evolved. (C) 2000 John Wiley & Sons, Inc.