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butyl rubber [′byüd·əl ‚rəb·ər]
Butyl Rubber
a synthetic rubber that is a product of the copolymerization of isobutylene (I) and a small quantity (1-5 percent) of isoprene (II); it has the general formula
Butyl rubber is obtained by the cationic copolymerization of monomers in a solution of methyl chloride or ethyl chloride at temperatures of about -100° C (the catalyst is aluminum chloride). Butyl rubber is a product of light-yellow color with a density of 920 kg/m3 (0.92 g/cm3), insoluble in alcohols, ethers, ketones, dichlorethane, aniline, and nitrobenzene and resistant to the action of water. Butyl rubber is characterized by low gas permeability, second in this respect only to polysulfide rubber: the coefficients of gas permeability for butyl rubber by hydrogen, oxygen, and nitrogen are 55 × 10-18, 9.9 × 10-18, and 2.47 × 10-18 m2/(sec . newton/m2), or 5.5 × 10-8, 0.99 × 10-8, and 0.247 × 10-8cm2/(sec. kg-force/cm2), respectively. The absence of double bonds in the majority of mers of the macromolecules of butyl rubber accounts for the resistance of butyl rubber to the action of oxygen, ozone, and light and, at the same time, also for the retarded vulcanization of the rubber. (So-called chlorbutyl rubber and brombutyl rubber, which significantly exceed initial butyl rubber in rapidity of vulcanization, are obtained by halogenization of butyl rubber.)
Butyl rubber is vulcanized by means of sulfur (in this case ultra-accelerators are used), dinitroso compounds, and alkyl-phenol-formaldehyde resins.
The capacity of butyl rubber to crystallize upon stretching makes it possible to obtain high-strength pure resins from it. Upon the introduction of active fillers (mainly carbon black), the strength is unchanged or in some cases even decreased, but the other physicochemical properties of cured rubbers made from butyl rubber are increased (see Table 1).
| Table 1. Properties of pure and black-filled cured rubbers made from Soviet-made butyl rubber | ||
|---|---|---|
| Pure | Black-filled | |
| 1 TM-2 is a Soviet instrument for measuring microhardness | ||
| Tensile strength [MN/m2 (kgf/cm2)]............. | 23 (230) | 23 (230) |
| Modulus when stretched 500% [MN/m2 (kgf/cm2)]............. | 1.2(12) | 11 (110) |
| Tear resistance (kN/m or kgf/cm) | 9 | 85 |
| Hardness according to TM-2 (arbitrary units)1 | 30 | 65 |
The principal merits of cured rubbers made from butyl rubber are their resistance to the action of many aggressive mediums, including alkalies, hydrogen peroxide, and certain vegetable oils; high dielectric properties [specific volume electric resistance, 1014 ohms m (1016 ohms cm); dielectric permeability, 2.1-2.3], which are maintained after prolonged immersion of the cured rubbers in water; gas impermeability; and heat resistance. The cured rubbers made from butyl rubber that have the greatest heat resistance are those that are vulcanized with aklyl-phenol-formaldehyde resins. The disadvantages of cured rubbers made from butyl rubber are low rebound elasticity at room temperature (˜ 10 percent), high residual compressive strain, and high heat buildup under dynamic influences.
Butyl rubber is manufactured in the form of briquettes weighing about 30 kg. The commercial brands of butyl rubber include BK (USSR), Enjay Butyl and Bucar Butyl (USA), Esso Butyl (England), and Plastugil Butyl (France). The most important field of usage of butyl rubber is in tire production. In addition, butyl rubber is used in the manufacture of rubberized fabrics, various rubber products that are resistant to high temperatures and aggressive mediums, and so on. The production capacity for butyl rubber in the capitalist countries in 1970 was about 360, 000 tons.
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