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Chemical Composition
Butyl rubber is 98 percent polyisobutylene and 2 percent isoprene. Polyisobutylene is also called an elastomer, or a type of rubber that can be stretched many times its length and spring back to its original shape. Isoprene is a colorless liquid that is highly volatile. It's commonly used in the production of synthetic rubber. The isoprene used in making butyl rubber contains a double bond that allows for the crosslinking during vulcanization (the process of treating rubber with sulfur at a high temperature to improve elasticity).
Structure and Polymerization
Butyl rubber has a similar structure to polyetyhylene. The only exception is that the second carbon atom in the double bond is bonded to two methyl groups. The polymer of butyl rubber is formed during a process called cationic vinyl polymerization. The cation (a positively charged ion) pulls a pair of electrons from the double carbon bond, creating a single bond with the cation. Typically, this polymerization occurs at temperatures of -100 Celsius because if higher temperatures existed, the reaction would occur too fast.
Properties
Butyl rubber is exceptionally airtight and gas impermeable. Being gas impermeable means that butyl rubber is the only rubber that is able to hold air for lengthy periods of time. Often, balloons that typically go flat after a few days are made with polyisoprene, a natural rubber polymer that is not gas impermeable. Butyl rubber is also quite flexible, has good weathering resistance, is ozone resistant and is biocompatible. Additionally, butyl rubber is heat resistant, making it crucial in the manufacturing of tires.
Uses
Other than tires, inner tubes and curing bladders, butyl rubber is also used for the inner linings of footballs and basketballs, medicine bottle stoppers, sealants and adhesives, joint replacements, chewing gum, tank and pond liners and speaker surrounds.