Halogenated butyl rubbers heat resistance

Peroxide cross-linked terpolymers have a lower compression set than conventional butyl rubber vulcanizates and better ozone resistance than butyl rubbers of comparable unsaturation. Suitably stabilized, these systems have a heat resistance comparable to phenolic resin cured conventional butyl rubbers but with faster cure rates and higher compatibility with other types of rubber (Walker et ai, 1974). Some of these latter properties may also be shared with halogenated butyl rubber vulcanizates. 

Halobutyl rubbers can be crosslinked with phenolic resins by the same mechanism postulated for regular butyl rubbers. The usual association with zinc oxide ensures a high state of cure and the vulcanizates have excellent dry heat resistance. However, they are inferior to resin cured regular butyl rubber in resistance to steam and superheated water. Since halogen is present in the polymer itself, another halogen donor is not neeessary in the resin cure of halobutyl rubbers. 

Where there is a need for very rapid cures coupled with optimum resistance to heat, weather and ozone, or for maximum adhesion to highly unsaturated elastomers, the halogenated butyl rubbers should be considered. Table 3 lists examples of appropriate selections of elastomers by unsaturation level, for a variety of applications. 

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. 

Phenolic resins are effective in conjunction with zinc oxide as cross-linking agents. Fast cures are possible with bromobutyl rubber without the need for adding halogen-containing compounds as in conventional butyl rubber. The products in this case have good dry heat resistance but inferior steam ageing resistance compared with the vulcanizates of unmodified butyl. 

Exceptional heat resistance and low compression set can be obtained by curing butyl rubbers with dimethylol phenol resins. The curing reaction is very slow even at high temperatures and when activated by halogens. Stannous chloride, and combinations of a halogenated polymer (such as neoprene, halobutyl or brominated resin) with zinc oxide, are the most commonly used halogen-bearing activators. The systems shown in Table 5 are typical. 

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. 

Resin cures utilise phenol-formaldehyde resins with reactive methylene groups and a small added amount of either a chlorinated rubber, e.g., polychloroprene, or stannous chloride. If halogenated phenolic resins are used the additional source of a halogen may not be required. Resin cures give butyl compounds excellent heat stability and are used to good effect where this is required, e.g., in tyre curing bags which have to resist service at 150 °C in a steam atmosphere. 

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