acrylonitrile adhesive aflas butyl rubber chloroprene cross-link diene epdm rubber elasticity elastin epichlorohydrin ethylene ethylene-vinyl acetate ethylene propylene rubber fkm glass transition temperature halogenation hydrogenation isobutylene isoprene kalrez monomer neoprene noncovalent bonding polybutadiene polymer polysulfide polyurethane protein resilin silicone rubber styrene-butadiene tecnoflon thermoplastic elastomer viton vulcanization