Butyl rubber

Butyl rubber and other isobutylene polymers of technological importance iaclude various homopolymers and isobutylene copolymers containing unsaturation achieved by copolymerization with isoprene. Bromination or chlorination of the unsaturated site is practiced commercially, and other modifications are beiag iavestigated. 

Isobutjiene was first polymerized ia 1873. High molecular weight polymer was later synthesized at I. G. Farben by decreasiag the polymerization temperature to —75°, but the saturated, unreactive polymer could not be cross-linked iato a useful synthetic elastomer. It was not until 1937 that poly(isobutylene- (9-isoprene) [9010-85-9] or butyl mbber was iavented at the Standard Oil Development Co. (now Exxon Chemical Co.) laboratories (1). 

The first sulfur curable copolymer was prepared ia ethyl chloride usiag AlCl coinitiator and 1,3-butadiene as comonomer however, it was soon found that isoprene was a better diene comonomer and methyl chloride was a better polymerization diluent. With the advent of World War II, there was a critical need to produce synthetic elastomers in North America because the supply of natural mbber was drastically curtailed. This resulted in an enormous scientific and engineering effort that resulted in commercial production of butyl mbber in 1943. 

Prior to butyl mbber, the known natural and synthetic elastomers had reactive sites at every monomer unit. Unlike natural mbber, polychloroprene, and polybutadiene, butyl mbber had widely spaced olefin sites with aHyUc hydrogens. This led to the principle of limited functionahty synthetic elastomers that was later appHed to other synthetic elastomers, eg, chlorosulfonated polyethylene, siUcone mbber, and ethylene—propylene terpolymers. 

The first pubHshed information on the halogenation of butyl mbber was provided by B. F. Goodrich Co. (2). Brominating agents such as /V-bromosuccinimide were used the bromination occurred ia a bulk reaction. This technology was commercialized ia 1954, but withdrawn ia 1969 (3). Exxon Chemical researchers pursued the chlorination of butyl mbber ia hexane solution usiag elemental chlorine, and a continuous process was commercialized ia 1961 (4). Currentiy, both chlorination and bromination are carried out ia continuous-solution processes. 

Butyl rubber is a copolymer of isobutylene (97.5%) and isoprene (2.5%). The polymerization is carried out at low temperature (below 

The product is a linear random copolymer that can he cured to a thermosetting polymer. This is made possible through the presence of some unsaturation from isoprene. 

Butyl ruhher vulcanizates have tensile strengths up to 2,000 psi, and are characterized hy low permeahility to air and a high resistance to many chemicals and to oxidation. These properties make it a suitable rubber for the production of tire inner tubes and inner liners of tubeless tires. The major use of butyl rubber is for inner tubes. Other uses include wire and cable insulation, steam hoses, mechanical goods, and adhesives. Chlorinated butyl is a low molecular weight polymer used as an adhesive and a sealant. 

Butyl rubber was developed by the research department of Standard Oil Company [4], New Jersey in the 1930s which later became a constituent of the Exxon Corporation. 

Butyl rubber like Hypalon, Neoprene or nitrile rubber is a speciality polymer which can be compounded for a soft, deformable elastic vulcanisate similar to the other elastomers, but having certain distinctive characteristics, like low permeability to all gases and resistance to ageing and ozone cracking. Butyl has poor oil resistance and medium low temperature flexibility. 

Zinc dithiocarbamate can be used in small quantities (0.25-0.75 phr) with zinc oxide cures used in heat resistant and improve compression set. 

For zinc oxide at, for example, levels of 15 to 45 phr, good filler dispersion is essential for better dimensional stability in calendered sheets. Butyl sheets are used in storage tanks, digesters and other large equipment handling phosphoric acid, hydrochloric acid and sulfuric acid very effectively in fertiliser and chlor-alkali plants. When high temperatures are encountered, carbon bricks or acid resistant bricks are lined over the rubber lining. 

Butyl rubber is used in specialty application such as reservoir or canal linings, tank linings, pharmaceuticals and sealing caulks. The low unsaturation and low permeability of the rubber molecule contribute to the chemical inertness of the butyl rubber. 

Butyl rubber is a copolymer of isobutylene and isoprene. It is a product of research carried out in both Germany and the United States in the 1930s. 

Poiyisobutylene rubber (IM) was a precursor to butyl rubber, which was first developed in Germany. However, this polymer had no unsaturation it could not be cured with a sulfur-based system. In 1937 Standard Oil (now ExxonMobil) developed the copolymer version that is used today in high volume. A small amount of isoprene provides the unsaturation that enables conventional curing with sulfur or other crosslinkers. 

The world productive capacity for butyl rubber (and its related cousin halobutyl) is about 2 billion pounds annually from over 12 production plants scattered around the world. This volume represents 7% of the world production of synthetic elastomers. It is the fifth largest volume rubber used today and the highest volume specialty elastomer. In fact, the consumption of butyl rubber is comparable in volume to that of the general-purpose elastomers discussed earlier. 

While there are butyl rubber plants around the world, there appears to be a disproportionately high level of capacity in North America. However, because of the relatively higher prices for butyl rubber compared to general-purpose elastomers, the shipment costs are only a small fraction of the total costs. Therefore, a significant quantity of butyl rubber is exported from North America to many other countries worldwide. 

Isobutylene j( g fractionation of refinery gases to obtain isobutylene 

Butyl rubber is one of the most widely used thermoset elastomers and is typically used for inner tubes and other industrial gas bladders. 

Butyl rubber is also available in a halogenated form with either bromine or chlorine and this often increases its thermal performance while retaining the low gas permeability to gas and moisture. 

Butyl rubbers can be bonded with most engineering adhesives and cyanoacrylates show high strength with the substrate failing before the adhesive bond (Table 4.1). Some solvent-based adhesives show good strength on butyl rubber, especially if the rubber is cleaned with a chlorinated solvent prior to bonding. 

All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. 

The only important commercial elastomer prepared by a cationic polymerization is butyl rubber, i.e., a copolymer of isobutene and isoprene. The latter 

Isoprene is used as the comonomer in butyl rubber (0.5-2.5 mol%) (Kresge et al., 1987) because the isobutene-isoprene reactivity ratios are more favorable for inclusion of the diene than those of the isobutene-butadiene pair (Kennedy, 1968). Thus, for the former pair, the r (isobutene) = 2.5, and r (isoprene) = 0.4 (Kennedy, 1968). It should be noted, however, that as discussed previously, such r values can be markedly influenced by the nature of the initiator and solvent used in the polymerization. The values just quoted are applicable to the commercial butyl rubber process, as described earlier. It has been shown that the isoprene unit enters the chain predominantly in a 1,4-conflguration (Chen and Field, 1967). 

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