Butyl Rubber is a solution copolymer of isobutylene with a small proportion (from 1 to 4%) of isoprene. Polyisobutylene by itself is fully saturated, and the isoprene is included to provide sufficient double bonds to allow vulcanisation with sulphur. Its outstanding property is the very low permeability to air and other gases. Low resilience, very good resistance to sunlight, ozone and aging, and relatively good resistance to high temperature are the other specialities. Butyl tends to be incompatible with other rubbers, but this difficulty can be overcome by modifying the polymer with chlorine and bromine to give chlorobutyl and bromobutyl rubbers. The main application for all forms of butyl rubber is to make inner tubes for tyres, and more recently is being used as linings for tubeless industries. Its high heat resistance has lead to its utilization for applications such as steam hose and the curing bags used in tyre vulcanisation. Sunlight and weather resistance of butyl is exploited in roofing membranes and reservoir sheeting. Its chemical resistance is made use of in anticorrosion linings.