Butyl tape doesn’t dry, doesn’t cure, and never really sets. It just keeps flowing quietly for decades, which is exactly why it seals better than almost anything else. The material is butyl rubber, a copolymer first synthesised at Standard Oil in 1937 by William Sparks and Robert Thomas. It’s built from long chains of polyisobutylene with a tiny fraction of isoprene (typically under 2%) blended in to provide the occasional cross-linking site. That backbone is the whole story. Every second carbon along the chain carries two methyl groups. Packed tightly together, they create steric hindrance that throttles segmental motion, the wriggling that lets gas molecules diffuse through a polymer. Choke that movement and you choke permeability. Butyl is roughly an order of magnitude less permeable to air than natural rubber, which is why it lines the inside of nearly every tubeless tyre on the road. As a tape, it works by never fully settling: - It’s a viscoelastic pressure-sensitive adhesive,...