Butyl rubber is produced by polymerization of about 98% of isobutylene with about 2% of isoprene. Structurally, polyisobutylene resembles polypropylene, having two methyl groups substituted on every other carbon atom. Butyl rubber has excellent impermeability, and the long polyisobutylene segments of its polymer chain give it good flex properties.
The abbreviation IIR stands for Isobutylene Isoprene Rubber. Butyl is impermeable to air and has good flex properties with a high level of heat resistance, as well as, excellent low-temperature flexibility. BK-1675N butyl rubber is impermeable to air and used in many applications requiring an airtight rubber.