Bromination of the polyisobutylene backbone

Halogenated butyl · improved cure

Bromobutyl rubber starts from the same isobutylene-isoprene copolymer backbone as standard butyl rubber, with bromine atoms introduced at the isoprene unsaturation sites. This bromination provides more reactive cross-linking sites during vulcanisation, allowing faster cure cycles and more consistent cross-link density across production batches - directly improving compression set consistency and seal reliability lot-to-lot.

Gas permeability coefficient (relative)

Oxygen / water vapour barrier

The saturated polyisobutylene backbone has no double bonds available for oxygen attack along the chain (unlike natural rubber or many synthetic elastomers), giving butyl/bromobutyl compounds the lowest gas and water vapour permeability of any elastomer class commonly used in pharmaceutical closures. This protects oxygen-sensitive and moisture-sensitive formulations stored under the stopper seal.

Chemical inertness - low extractables

Minimal leachables · drug compatibility

The saturated backbone structure also makes bromobutyl chemically resistant to attack by most pharmaceutical formulation components - acids, bases, and many organic solvents. This translates to low extractable and leachable substance profiles, a critical requirement for injectable and oral liquid pharmaceutical products where any extracted rubber-derived compound represents a patient safety consideration.

Thermal & UV stability

Long-term storage · autoclave compatible

Bromobutyl compounds maintain mechanical properties across a wide temperature range and resist UV-driven degradation better than many alternative elastomers - important both for long-shelf-life pharmaceutical products and for stoppers that must survive steam autoclave sterilisation cycles (typically 121°C) without losing their sealing geometry or elastic recovery properties.

USP Plastics Class VI compliant formulations

Biocompatibility basis · regulatory accepted

Pharmaceutical-grade bromobutyl compounds are formulated to meet USP Plastics Class VI biological reactivity requirements and the broader USP <381> Elastomeric Closures for Injections testing framework - the established regulatory basis accepted across global pharmaceutical markets for injectable and oral product closures.

Elastic recovery & compression set resistance

Maintains seal force over shelf life

The vulcanised cross-link network gives bromobutyl rubber strong elastic memory - after compression under the aluminum cap, the material continuously exerts a restoring force rather than permanently deforming. Compression set (the percentage of compression that becomes permanent rather than recoverable) is formulated to remain low across the product's full shelf-life temperature excursion range.