Polybutylene terephthalate (PBT) is a thermoplastic engineering polymer that is used as an insulator in the electrical and electronics industries. It is a thermoplastic (semi-)crystalline polymer, and a type of polyester. PBT is resistant to solvents, shrinks very little during forming, is mechanically strong, heat-resistant up to 150 °C / 302 °F or 200 °C / 392 °F with glass-fiber reinforcement, and can be treated with flame retardants to make it non-combustible.
PBT is closely related to other thermoplastic polyesters. Compared to PET (polyethylene terephthalate), PBT has slightly lower strength and rigidity, slightly better impact resistance, and a slightly lower glass transition temperature. PBT and PET are sensitive to hot water above 60 °C / 140 °F. PBT and PET need UV protection if used outdoors, and most grades of these polyesters are flammable, although additives can be used to improve their UV and flammability properties.