Butyl rubber is a copolymer of isobutylene and isoprene, hence IIR. Its grades vary in isoprene content and viscosity, which is related to molecular weight. If a halogen, such as chlorine or bromine, is introduced into the polymer architecture, it becomes CIIR or BIIR, respectively. IIR has some properties similar to those of EPDM, such as good mineral acid and base resistance (like EPDM some concentrated mineral acids are a problem), and weather resistance which is similar to that of EPDM. IIR has excellent resistance to permeability by gases. For example, Fusco mentions its permeability to air being as low as 10% that of NR, at 65°C. Like EPDM, the polarity of IIR is low which means poor resistance to petroleum oils and conversely low swell in many polar solvents, such as ketones. Resilience is poor, which translates to good damping ability. The upper continuous heat aging temperature limit is around 121°C, which can be distinctly improved with IIR compounds containing resin (polymethylol-phenol) cure systems. For low temperature properties the vulcanizate becomes stiff and leathery at around -18°C, although it is not brittle until around -70 0C, Applications naturally following from these properties include mounts and bumpers for vibration and shock prevention, roof and tank linings, curing bladders and inner tubes for tires. A significant use is inner liners for tubeless tires, where halobutyl is preferred due to improved interply adhesion with the rest of the inner tire. Halobutyls can be blended with unsaturated elastomers such as NR, whereas for IIR it is not recommended. Blending is not recommended for IIR since the rate of cure of the 'other elastomer' in the blend is often much faster than the rate of cure of the IIR, resulting in under cured IIR in the blend. IIR and halobutyl are used for pharmaceutical closures using high purity zinc oxide as the curative. Zinc oxide, is 'generally regarded as safe' by the United States Food and Drug Administration, i.e., has historically been in common use in contact with food or skin for many years without ill effect. Recent elastomer modifications (from Exxon) are p-methylstyrenelisobutylene copolymers, which have the low permeability and high damping of IIR with the environmental and aging resistance of EPDM, and 'star branched' butyls, which have improved processing properties, prior to cure.
Reference: An Introduction to Rubber Technology by Andrew Ciesielski 1999