Halogenated butyl rubbers processing

During the production process of halogen butyl rubbers relatively few additives are necessary and the chance that additives are leached from the rubber into the pharmaceutical preparation is less when compared to natural rubber or butyl rubber. 

Elemental chlorine or bromine is brought into intimate contact with butyl rubber in a hydrocarbon solvent solution. The hydrochloric or hy-drobromic acid by-product is neutralised with dilute caustic soda, and the halogenated butyl rubber is recovered from the hydrocarbon solution by conventional steam stripping. The resulting slurry of halobutyl in water is screened, dried and compacted into bales. Halogenation is rapid and the commercial process is continuous. 

The differences in the cure reactivity of chlorobutyl and bromobutyl rubbers are sufficiently great so that one cannot usually be substituted directly into compounds designed specifically for the other. Bromobutyl substituted for chlorobutyl in a chlorobutyl compound would be likely to prove unacceptable for factory processing because of a greatly increased risk of scorch. Chlorobutyl substituted for bromobutyl in a bromobutyl compound would be unlikely to reach an adequate state of cure in a reasonable time. In other respects, however, the compounding principles and practices used for unmodified butyl rubbers apply also to the halogenated butyl rubbers. 

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). 

Butyl rubber is produced at very low temperature (below — 90°C) to control the rapid exotherm, and to provide high molecular weight. The process consists of charging isobutylene along with isoprene (2-4%) with an inert diluent such as methyl chloride to a reactor to which a Friedel-Crafts catalyst is added. The polymerization is very rapid, and the polymer forms in a crumb or slurry in the diluent. Heat is removed via the reactor jacket. The slurry is steam-stripped to remove all volatiles. The catalyst is neutralized, and antioxidants are added to the slurry prior to drying.53 The halogenated derivatives are produced by the direct addition of the halogen to a solution of the isobutylene-isoprene polymer. 

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. 

An alternative solution process, developed in Russia, uses a C5-C7 hydrocarbon as solvent and an aluminum alkyl halide as the initiator. The polymerization is conducted in scraped surface reactors at -90 to —50°C. The solution process avoids the use of methyl chloride, which is an advantage when butyl rubber is to be halogenated. However, the energy costs are higher than for the slurry process because of the higher viscosity of the polymer solution. Consequently, it is imlikely that the well-established slurry process will be displaced. 

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. 

An alternative process is bulk-phase halogenation (74-76). Dry butyl rubber is fed into a specially designed extruder reactor and contacted in the melt phase with chlorine or bromine vapor. By-product halogen acids are vented directly, avoiding the need for a separate neutralization step. Halogenated rubbers comparable in composition and properties to commercial products can be obtained. 

Halogenated Butyl. Butyl elastomers have a low density of unsaturation, which results in a low cure rate when conventional vulcanization systems are used. Butyl copolymers containing a small amount of combined chlorine or bromine are vulcanized more quickly than normal butyl rubber because the halogen atoms provide additional sites for the cross-linking process. Butyl rubber has poor adhesive properties to metals and other rubbers because of the lack of polar groups. This is the reason for using halogenation. 

Almost all polymer and oligomer syntheses by eationic polymerization, both in industry and in the laboratory, employ MX -type initiators, typically AICI3 and BF3 complexes. Non-MX initiators (0x0 acids and halogens) have been used much less frequently, only for basic studies on polymerizations of styrenes and vinyl ethers. In industry, most cationic polymerization processes [e.g., polyisobutene (butyl rubber) and petroleum resin production] utilize MX initiators perhaps the only one involving non-MX initiators is the synthesis of oligo(isobutene) with strong protonic acids. The very rare use of non-MX initiators may be due to a lack of information on the characteristics of these unique compounds. 

These products are almost exclusively made from butyl rubbers. A typical formulation is shown in Table 8. Resin curing provides excellent resistance to wet and dry heat. Polychloroprene supplies the halogen needed to activate the resin. High structure black provides processing ease and high thermal conductivity. 

The manufacture of halobutyl rubbers such as Bromobutyl, Chlorobutyl, and Exxpro [bromopoly(isobutylene-co-/j-methylstyrene)] requires a second chemical reaction the halogenation of the polymer backbone. This can be achieved in two ways, the finished polymer produced in the butyl plant can be dissolved in a hydrocarbon solvent such as hexane or pentane, or a solvent replacement process can be used to dissolve the polymer from the slurry leaving the reactor. A schematic flow diagram of the halogenation process is shown by Figure 2. 

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