Silicones have superior low-temperature flexibility, but this is achieved at a much higher price and at a sacrifice in other properties such as tensile strength, tear resistance, and general durability. A large volume of polybutadiene is used in blends with other polymers to enhance their resilience and reduce heat buildup. It is also used in products requiring high resiliency over a broad temperature range such as industrial tires and vibration mounts. The two types of rubber in this category are both based on crude oil. The first is polyisobutylene with an occasional isoprene unit inserted in the polymer chain to enhance vulcanization characteristics.
Butyl rubbers have outstanding impermeability to gases and excellent oxidation and ozone resistance.
Butyls lack the toughness and durability, however, of some of the general-purpose rubbers. The attribute responsible for the high-volume use of butyl rubber in automotive inner tubes and tubeless tire interliners is its excellent impermeability to air. Like the butyls, the EP rubbers are of two types. EPDM is chemically reactive and is capable of sulfur vulcanization. The copolymer must be cured with peroxide.