A copolymer of about 98% isobutylene and 2% isoprene. It has the poorest resistance to petroleum oils and gasolines of any rubber. Excellent resistance to vegetable and mineral oils: to solvents such as acetone, alcohol, phenol and ethylene glycol; and to water and gas absorption. Heat resistance is above average. Sunlight resistance is excellent. It's abrasion resistance is not as good as natural rubber. Usually low permeability to gases.