Butyl Rubber

The defining feature of butyl rubber is its impermeability. If natural rubber is a screen door, butyl rubber is a bank vault vault.

National Institutes of Health (.gov)https://pmc.ncbi.nlm.nih.gov butyl rubber

A solid piece of butyl rubber (isobutylene-isoprene rubber, IIR) is a dense, black or dark-gray elastomer known for its very low gas permeability, high dampening, and excellent resistance to weathering, ozone, and chemicals. The defining feature of butyl rubber is its impermeability