Abstract

Butyl rubber and other isobutylene polymers in wide use include various homopolymers and copolymers. Low levels of unsaturation for subsequent cross-linking are incorporated by copolymerization with isoprene. Halogenation at the unsaturation site provides additional cross-linking mechanisms and covulcanization with highly unsaturated elastomers. Polymers containing star branches, conjugated-diene sites, divinylbenzene, and brominated p-methylstyrene have also been synthesized. Isobutylene polymers are polymerized by a cationic mechanism, using a Brønsted acid as an initiator and a Lewis acid as a coinitiator, eg, water and aluminum chloride. Molecular weights range from several hundred for terminally unsaturated polybutenes to over a million for elastomer grades.

The T_g of polyisobutylene is about –70°C, and the polymer is amorphous in the unstrained state. The polymers exhibit a broad damping peak and a low plateau modulus. They display an exceptionally low permeability to gases, and are widely used as air retention barriers in tires. Optimum physical properties are obtained by compounding with carbon black and other fillers. Various mechanisms are used to cross-link the compounds into elastomeric vulcanizates. Sulfur is used to provide polysulfidic cross-links, while resin and zinc oxide cure systems give carbon–carbon cross-links. Besides inner tubes and innerliners, butyl rubbers are used in blends to improve cut-growth resistance and wet-skid resistance. Blends with thermoplastic resins improve toughness. Polyisobutylenes enjoy extensive use as adhesives and caulks, and are chemically modified for use in lubricants as dispersants.