Epichlorohydrin

Epichlorohydrin is a small, very reactive molecule: three carbons, one chlorine atom and a tight three-membered ring of two carbons and an oxygen, called an epoxide. It's a colorless liquid with a garlic-like smell. It's hazardous to handle, but in finished plastics it's locked away as part of the structure.

The classic route starts with propylene and chlorine. At around 500 °C, a chlorine atom swaps onto the end of propylene, giving allyl chloride. That reacts with chlorine in water, giving dichlorohydrin, which carries two chlorines and an -OH group. Finally, a base such as sodium hydroxide or lime pulls off one chlorine and a hydrogen, and the oxygen snaps shut into the epoxide ring. Some newer plants instead start from glycerol, a byproduct of biodiesel, treated with hydrogen chloride.

Nearly all of it goes into epoxy resin. When it reacts with bisphenol A, its chlorine leaves and its epoxide ring ends up capping each end of the molecule. Those rings are the reactive ends that later open to cure the resin.