Halogenation of Butyl Rubber

The low degree of unsaturation of butyl rubber, whilst having certain advantages, has the consequences of a low cure rate when using the conventional vulcanization systems used in diene rubbers. In turn this 

Bromination appears to involve substitution rather than addition. Baldwin and Kuntz (I960) report that bromination of butyl rubber with elemental bromine is accompanied by the formation of large amounts of HBr. Using ozonolysis techniques it was found that unsaturation did not greatly change with bromination. It has thus been inferred (Makowski, 1%9) that the reaction proceeds by a free radical allylic substitution rather than by an ionic mechanism, e.g. 

On the other hand Feniak et al. (1974) suggest that the continuous process now being operated by Polysar proceeds by the following ionic mechanism  

They consider that essentially all of the bromination takes place at the unsaturated isoprenic moiety. The somewhat hindered nature of the double bond favours substitution rather than addition and NMR studies indicate that up to 90% of the bromine is allylic to the double bond with the isomer shown above being predominant. 

Brominated butyl rubbers are of interest for truck tyre inner tubes because of their heat resistance, in giant type inner tubes because they may be blended with natural rubber to give a useful combination of properties, and for a variety of other uses. 

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Isobutylene and isoprene are in a ratio of approximately 50 1. Chlorobutyl rubber and bromobutyl rubber are produced by the halogenation of butyl rubber. Butyl rubber and halobutyl rubber are highly impermeable to air and show very low water absorption, and good heat, oxygen and ozone resistance. As noted earlier, they therefore find extensive use in liners of radial tires, covers and insulation of high-voltage electric cables, and automobile engine and radiator hoses. 

Halobutyl rubbers at the stages of halogenation of butyl rubber, neutralising and washing of the polymerisate, and the introduction of stabilisers. 

Butyl and Halobutyl Rubber. Butyl rubber is made by the copolymerization of isobutylene with 1-3% isoprene (eq. 5), which is added to provide sites for curing. It is designated HR because of these monomers. Halogenation of butyl rubber with bromine or chlorine increases the reaction rate for vulcanization (eq. 6). It is estimated that of the 200,000 kt of butyl (HR) and halobutyl (HIIR) rubber manufactured in North America, over 90% is used in tire applications. The halogenated polymer is used in the innerhner of tubeless tires. 

Vukov R (1984) Halogenation of butyl rubber — a model compound approach. Rubber Chem Technol... 

Forms of BR and polyisobutylene. The properties of butyl rubber and polyisobutylene depend on their moleeular weight, degree of unsaturation, nature of the stabilizer incorporated during manufacture and, in some cases, chemical modification. It is common to produce halogenated forms of butyl rubber to increase polarity and to provide a reactive site for alternate cure mechanisms [6],... 

Chemical reactions are used to modify existing polymers, often for specialty applications. Although of considerable importance for plastics, very few polymer reactions (aside from crosslinking) are important for elastomers. Chlorination and bromination of Butyl rubber to the extent of about one halogen atom per isoprene unit yields elastomers which are more easily crosslinked than Butyl rubber. Substitution occurs with rearrangement to yield an allylic halide structure... 

Typical halogenation processes for making halobutyl rubbers involves the injection of chlorine or bromine into a solution of butyl rubber. The reactants are mixed vigorously in the halogenation reactor with a rather short resident time, typically less than 1 min, followed by the neutralization of the HC1 or HBr and removal of the unreacted halogen (13). The procedures of halogenation have been described in detail elsewhere (41,42). 

The chloride is used to manufacture silicones, tetramethyl lead and triptane (2,2,3 trimethylbutane). Lesser uses include the manufacture of butyl rubber, higher halogenated methanes, methyl cellulose, quaternary ammonium compounds, methyl mercaptan, methionine, fungicides and pesticides (primarily the Me-arsenate herbicides). Recently the chlorinated fluorocarbons have replaced CH3CI as high volume refrigerants and propellants (ref. 32) Tables 12 and 13 list the chemical and physical properties and potential numbers of workers exposed to the monohalomethanes. 

Butyl rubber consists mostly of isobutylene (95-98%) and about 2-5% isoprene units. 1 The isoprene unit is halogenated by either chlorine or bromine to obtain the corresponding halobutyl rubbers. Despite the superior elastomeric properties of halobutyl, the elastomer can easily undergo dehydrohalogenation leading to crosslinfang, and the isoprene unsaturation is subject to ozone cracking. To remedy these problems and to improve the halobutyl properties, a new class of elastomer poly(isobutylene-co-p-methylstyrene) [poly (IB-PMS)] was developed. Unlike butyl rubber, it contains no double bonds and therefore cannot be crosslinked unless otherwise functionalized. The chemical structures of butyl rubber and poly (IB-PMS) copolymers are shown below. 

Halogenation and hydrohalogenation of elastomers have been reported extensively in the literature [26]. The main problems with these reactions are the cyclization and chain scission that occur parallel to the halogenation reaction. These introduce difficult problems in the characterization of the resulting products. Despite these problems, several products have been prepared and commercialized. Chlorination of poly(l, 4-butadiene) to prepare a product similar to poly(vinyl chloride) has been reported by several workers [27]. This process had extensive side reactions and chain degradation. The chlorination of butyl rubber and conjugated diene-butyl rubbers gives end products that are used in the tire industry as inner liners for air retention. 

Halogenated butyl rubber greatly extended the usefulness of butyl rubber by providing much higher vulcanization rates and improving the compatibility with highly unsaturated elastomers, such as natural rubber and styrene-butadiene rubber (SBR). These properties permitted the production of tubeless tires with chlorinated or brominated butyl innerliners. The retention of air pressure (5) and low intercarcass pressure (6) extended tire durability. 

The halogenation process begins with the preparation of a hexane soln-tion of butyl rubber with the desired molecular weight and unsaturation. Slnrry from a butyl polymerization reactor is dissolved (69-71) by transferring it into a drum containing hot liqnid hexane that rapidly dissolves the fine slnrry particles. 

A study was made of the ozonolysis of butyl rubber and its halogenated derivatives, as well as their model compounds in hexane. The effect of various additives, such as diphenylamine, triisopropyl phosphite, hexylamine and 2,6-di-tert-butyl cresol, on chain scission and rearrangement of bromobutyl rubber, was also evaluated. It was found while the ozonolysis of butyl and chlorobutyl mbbers involved normal cleavage of olefmic double bonds, the mechanism of ozonolysis of bromobutyl mbber was more complicated. 8 refs. 

Halogenated Butyl Rubber. The halogenation is carried out in hydrocarbon solution using elemental chlorine or bromine in a 1 1 molar ratio with enchained isoprene. The reactions ate fast chlorination is faster. Both chlorinated and brominated butyl mbbers can be produced in the same plant in blocked operation. However, there are some differences in equipment and reaction conditions. A longer reaction time is requited for hromination. Separate faciUties are needed to store and meter individual halogens to the reactor. Additional faciUties are requited because of the complexity of stabilising brominated butyl mbber. 

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

The standard polymers used for rubber linings consist of materials that are cross-linkable macromolecules which, on mixing with suitable reactants that form strong chemical bonds, change from a soft deformable substance into an elastic material. These polymers include natural rubber and its corresponding synthetic, c/s-polyisoprene, styrene-butadiene rubber, polychloroprene, butyl rubber, halogenated butyl rubbers, acrylonitrile-... 

Butyl rubber (a copolymer of isobutylene and 1-3 mole per cent isoprene) and its halogenated derivatives have unsaturation in the carbon-carbon backbone and consequently do not have as good aging properties as EPDM. There are also reports (9-12) that ozone-resistant butyl rubber with a high degree of unsaturation can be prepared by copolymerization of isobutylene with either cyclopentadiene or 9-pinene. 

Resin cures utilise the same resins that are used for butyl rubber, but more resin (ca. 10-12 phr) and a halogen donor (10 phr), typically bromobutyl, or polychloroprene, are required. Although heat stability is slightly improved by resin curing when compared to sulphur cures, the effect is not as marked as in the resin curing of butyl. 

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