PET (polyester)

PET, short for polyethylene terephthalate, is the polyester of drink bottles and fleece jackets. It is strong, clear and stable when warm, and it can be made into bottles, fibers or very thin films.

It's built from two small molecules that alternate along a long chain: Terephthalic acid and ethylene glycol. They are mixed into a paste and heated. The acid groups on one react with the alcohol groups on the other, joining them and releasing water. At around 280 °C under vacuum, the short pieces keep joining until the chains are very long. The molten plastic is cooled and cut into pellets.

To make film, pellets are melted and pressed through a slot onto a chilled roller, then stretched in both directions while warm. This lines up the chains and makes the film tough, flat and clear. Mylar is a well-known film made this way.

In a TV, PET film is the base of the diffuser sheet, prism sheet and reflector sheet. It's laminated around the copper strips of a flat flexible cable, and as tape it insulates the windings inside a transformer. British chemists John Whinfield and James Dickson patented PET in 1941.