Butyl rubber (isobutyl, isoprene rubber). A polymer of isobutyl and isoprene with good chemical resistance, good ageing and heat resistance, and low gas permeability. A detrimental property is its very low elasticity at room temperature, especially when subjected to dynamic loading. Contact with mineral oils and greases, fuels and chlorinated hydrocarbons causes heavy swelling. Material characteristics materials for secondary seals.