Butyl Rubber

Butyl Rubber is a copolymer of isobutylene, present at well over 95% of the backbone, with a small amount of isoprene — typically only a few percent — included purely to provide the minimal number of reactive sites needed for vulcanisation. The overwhelming majority of the chain is therefore fully saturated, built from closely packed methyl side-groups along the isobutylene-derived backbone, and it is this tightly packed, saturated microstructure, more than any other elastomer's, that gives butyl rubber its defining property: an exceptionally low permeability to gases, water vapour and many chemicals, well beyond what general-purpose diene rubbers achieve.

As supplied, raw Butyl Rubber is a pale cream to grey, flexible solid, shipped in standard bales for compounding. Its very low level of unsaturation means it cures more slowly under conventional sulphur vulcanisation than most other rubbers, and it is correspondingly also far more resistant to ozone and weathering attack than the general-purpose diene rubbers whose backbones carry far more unsaturation.

Where it fits in the value chain

Butyl Rubber's gas impermeability has made it the standard material for the innerliner of pneumatic tyres — the layer that actually holds the pressurised air or nitrogen inside the tyre carcass — a role in which no other elastomer is close to being a practical substitute at scale. The same impermeability, combined with good flex-fatigue and heat-ageing resistance, makes it the preferred rubber for pharmaceutical vial stoppers and syringe plungers, where moisture and gas barrier integrity are critical to product shelf life, and for sealants, caulks and adhesives where its low permeability and weathering resistance both matter. Halogenated derivatives (chlorobutyl, bromobutyl), produced by a further halogenation step on regular butyl rubber, offer faster, more compatible cure chemistry with other rubbers and are especially widely used in multi-layer tyre innerliners.

Grades we supply

Chelora Chemicals offers regular Butyl Rubber across standard and low-unsaturation grades (the latter intended as feedstock for the buyer's own halogenation step) and a range of Mooney viscosities. Each grade is supplied with a batch-wise certificate of analysis covering Mooney viscosity and unsaturation level.

Butyl Rubber is produced by cationic copolymerisation of isobutylene with a small proportion of isoprene, carried out in solution at very low temperature (typically around −90 to −100 °C) using a Lewis acid catalyst such as aluminium chloride together with a proton-donating co-catalyst. The low reaction temperature is essential: cationic polymerisation of isobutylene is extremely fast and exothermic, and without aggressive cooling the reaction would run away and produce low-molecular-weight, poorly controlled product rather than the high-molecular-weight elastomer the market needs.

After polymerisation, the rubber cement is coagulated, typically with hot water and steam that simultaneously strip residual solvent and unreacted monomer, then dried and baled. Halogenated grades (chlorobutyl, bromobutyl) are produced by a separate downstream step in which regular butyl rubber is reacted with chlorine or bromine in solution, substituting a halogen atom at the isoprene-derived unsaturation site to create a more reactive, faster-curing cure site without materially changing the gas-barrier performance of the base polymer.