Tyre innerliners take the majority, in bromobutyl form, followed by inner tubes for bicycle, motorcycle and agricultural tyres. Pharmaceutical closures — vial stoppers, syringe plungers, infusion seals — are a smaller but high-value segment where butyl is effectively the reference material. Beyond those it goes into insulating glass edge sealants and construction tapes, tyre curing bladders, vibration and acoustic damping products, and chemical protective clothing.

Because there is almost nowhere for a gas molecule to go. Every second carbon in the backbone carries two methyl groups, and that dense, symmetrical substitution packs the chains together with very little free volume between them. Gas permeation through a rubber happens by molecules hopping between transient gaps opened by segmental motion, and butyl's crowded structure both reduces the gaps and damps the motion that creates them. The result is air permeability roughly ten times lower than natural rubber and water vapour permeability lower still. The same crowding is why butyl has such high damping — it absorbs vibration rather than returning it, which is useful in mounts and unhelpful in a tyre tread.

Halobutyl is butyl rubber that has been reacted with chlorine or bromine, which attaches a halogen at the isoprene units and creates a far more reactive cure site. The reason it matters is co-vulcanisation. A tyre innerliner is a thin butyl layer bonded to a carcass built from natural rubber and SBR, and the bond has to be a chemical crosslink across the interface rather than mere adhesion. Plain butyl cures too slowly and by too different a mechanism to crosslink with the general purpose rubbers next to it, so the liner would separate. The halogenated cure site cures fast enough and by a compatible enough mechanism to bridge the interface. Essentially every modern tubeless tyre depends on this, and it is why bromobutyl rather than butyl is the high-volume product.

Because isobutylene polymerises cationically, and cationic polymerisation is dominated by chain transfer. At ordinary temperatures the growing cation transfers a proton to monomer almost as readily as it adds monomer, so chains terminate early and the product is a low molecular weight oil rather than a rubber. Chain transfer has a higher activation energy than propagation, so cooling the reaction suppresses transfer far more than it suppresses growth. At −95 to −100 °C, in a methyl chloride slurry with aluminium chloride, propagation wins and high molecular weight polymer forms. Maintaining that temperature at industrial scale is energy intensive and capital intensive, which is a substantial part of why butyl is made at only a handful of plants worldwide.

Several properties converge. A vial stopper has to maintain a seal and keep moisture and oxygen out for the shelf life of the product, and butyl's permeability is the lowest available in a mouldable elastomer. It also has to be clean — low in extractables and leachables that could migrate into an injectable — and butyl's saturated, non-polar structure with sparse unsaturation gives it a simpler and more stable extractables profile than unsaturated rubbers, which need heavier antioxidant loadings. It resists the water, steam and repeated autoclaving involved in sterilisation, and it reseals after needle puncture. Halobutyl grades are usual, and pharmaceutical grades are made with additional controls on particulates and on the cure system used.

Every shipment includes a Certificate of Analysis covering Mooney viscosity with test conditions stated, unsaturation as mol% isoprene, halogen content for chlorobutyl and bromobutyl, volatile matter, ash and stabiliser type; a Safety Data Sheet to GHS and REACH format; REACH polymer compliance confirmation; pharmacopoeial and extractables documentation for closure grades; an origin certificate; and destination-market certificates where required.