Regular Butyl Rubber

https://www.yushengmax.com/products/regular-butyl-rubber/

Regular butyl rubber IIR is a copolymer rubber of isobutylene and isoprene. The synthetic butyl rubber has excellent impermeability to gas diffusion, and the long polyisobutylene segments of its polymer chains give it good flex properties. It is generally odorless and tasteless, though it may exhibit a slight characteristic odor.