Butadiene

Butadiene, properly 1,3-butadiene, is a colorless gas with the formula C₄H₆. Its four carbon atoms sit in a chain with two double bonds, one at each end. Those double bonds make it ideal for building rubber: it links into long, flexible chains.

Its carbon and hydrogen come from oil and gas. Most butadiene is a by-product of steam cracking, the same process that makes ethylene and propylene. Naphtha or other hydrocarbons are heated very briefly to about 800 °C with steam, which breaks them into smaller molecules. Among the mixture is a stream of four-carbon molecules, and butadiene is pulled out of it with a solvent that grabs it selectively, then distilled. Where cracking makes too little, it can also be made by stripping hydrogen from butane or butenes.

Most butadiene goes into synthetic rubber, especially for tires. For ABS plastic, it is first linked into polybutadiene rubber particles, which give ABS its toughness so it doesn't crack. It boils at about −4 °C, so it is stored as a liquid under pressure. It is a known carcinogen, handled in closed systems.