Functional, Peroxide-Curable Isobutylene-Rich Elastomers

Isobutylene-rich elastomers are valued materials due to their excellent barrier properties, thermal stability and resistance to oxidation. Materials such as butyl rubber (polyisobutylene-co-isoprene, IIR) support a range of cure chemistries including conventional sulfur, accelerated sulfur and phenolic resin crosslinking; the range of cure chemistries supported is further extended through the used of halogenated derivatives such as brominated butyl rubber (BIIR). Peroxide cure chemistry provides thermoset elastomers with carbon-carbon crosslinks that provide good thermal stability and compression set resistance, and can also lead to reduced leachables and extractables compared to conventional cures. Isobutylene-rich copolymers such as butyl rubber tend to undergo cleavage reactions under the action of peroxides, so that the molecular weight losses through macro-radical fragmentation are greater than the molecular weight gains through macro-radical combination. In order to address this problem we have made a series of derivatives of BIIR in which a pendant peroxide-polymerisable group has been incorporated into the elastomer to generate macro-monomers. These derivatives have been prepared by displacement of bromide by carboxylate nucleophiles using thermally rearranged BIIR to enhance reactivity.

The first series of derivatives evaluated incorporated acrylate, vinylbenzoate (styrenic), methacrylate and maleimido groups as the polymerisable entity. Dynamic shear modulus measurements were carried out on an APA 2000 rheometer at 170°C using dicumyl peroxide (DCP) to evaluate the extent of cure. For the second series of derivatives, half-esters of maleic acid and itaconic acid were prepared by reacting a functional alcohol (or amine) with the corresponding acid anhydride. Subsequent modification of BIIR with the carboxylate salts of these half-esters (or amidic acids) was used to generate functional macro-monomers of butyl rubber. The extent of peroxide cure was similarly evaluated with DCP.