Semicrystalline thermoplastic plastic. It has a polymeric structure similar to the one of Polyethylene terephtalate. Both are based on an aromatic monomere and are typical polyesters. Also the blend of both materials PET/PBT has practical relevance.
Anyhow properties and applications of materials are significantly different. While PET can be dominantly amorphous (in particular in its use for bottles, PBT allways has a mainly crystalline character. Therefore it ist characterized by higher hardness and stiffness and never is transparent.