FTIR Spectroscopic Studies on Curing of Brominated Poly(Isobutylene-co-4-Methylstyrene)

Abstract

Abstract The curing of brominated poly(isobutylene-co-4-methylstyrene) or BIMS with ZnO/HSt, ZnO/Zn(St)2 or Zn(St)2 was investigated using FTIR spectroscopy. Changes of carbonyl absorption were monitored by FTIR during the course of curing, which revealed a four-step curing process: 1) formation of zinc salts, 2) esterification to generate Zn-containing Lewis-acid catalysts, 3) Friedel-Crafts alkylation, and 4) post cure. It was found that for the ZnO/HSt system, the ZnO concentration had to be high enough to meet the ZnO/Br molar ratio of 0.9 for complete conversion of curing functionalities. HSt was an effective accelerator, which reduces the induction time and increases the curing rate. HSt of 2 to 3 phr was recommended to achieve optimum acceleration results. Over use of HSt would lead to dramatic reduction of crosslinking density, due to the fact that significant amounts of curing functionalities were consumed to form stearate esters instead of crosslinks. The curing with ZnO/Zn(St)2 was similar to the ZnO/HSt system but faster due to the fact that zinc salt was directly added. Zn(St)2 alone was effective to initiate a cure, but the reversion of curing was observed at high concentrations, probably due to the presence of large amount of acids, such as HSt and HBr both of which were generated during curing, and the stearate esters. Addition of ZnO eliminates the reversion through neutralization of the acids.