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Butyl rubber - also known as polyisobutylene (C4H8)—is a synthetic rubber, a homopolymer of 2-methyl-1-propene. Polyisobutylene is produced by polymerization of about 98% of isobutylene with about 2% of isoprene. Structurally, polyisobutylene resembles polypropylene, having two methyl groups substituted on every other carbon atom. It has excellent impermeability, and the long polyisobutylene segments of its polymer chains give it good flex properties. Polyisobutylene is a colorless to light yellow elastic semi-solid or viscous substance. It is generally odorless and tasteless, though it may exhibit a slight characteristic odor.
A synthetic rubber, or elastomer, polyisobutylene is impermeable to air and used in many applications requiring an airtight rubber. Polyisobutylene is used in making adhesives, agricultural chemicals, fiber optic compounds, caulks and sealants, cling film, electrical fluids, lubricants (2 cycle engine oil), paper and pulp, personal care products, pigment concentrates, for rubber and polymer modification, and as a gasoline/diesel fuel additive. The first major application of butyl rubber was tire inner tubes. This remains an important segment of its market even today.
In 1950s and 1960s, halogenated butyl rubber (halobutyl) was developed, in its chlorinated (chlorobutyl) and brominated (bromobutyl) variant, providing significantly higher curing rates and allowing covulcanization with other rubbers like natural rubber and styrene-butadiene rubber. Halobutyl is today the most important material for inner tubes. The butyl rubber is vulcanized by elemental sulfur. Zinc oxide can be used instead of halobutyl.
Usage as a fuel and lubricant additive
Polyisobutylene (in the form of polyisobutylene succinimide) has interesting properties when used as an additive in lubricating oils and motor fuels. Polyisobutylene added in small amounts to the lubricating oils used in machining results in a significant reduction in the generation of oil mist and thus reduces the operator's inhalation of oil mist. It is also used to clean up waterborne oil spills as part of the commercial product Elastol. When added to crude oil it increases the oil's viscoelasticity when pulled; causing the oil to resist breakup when it is vacuumed from the surface of the water.
As a fuel additive, polyisobutylene has detergent properties. When added to diesel fuel, it resists fouling of fuel injectors, leading to reduced hydrocarbon and particulate emissions.   Polyisobutylene is manufactured by BASF (as well as its competitors) and blended with other detergents and additives to make a "detergent package" that is blended into gasoline and diesel fuel to resist buildup of deposits and engine knock.  Because fuel additive formulas are closely held trade secrets, it is impossible to know which additives any particular brand of gasoline may contain.