Polyethylene terephthalate (PET) is a semicrystalline thermoplastic polyester synthesized primarily through the esterification of purified terephthalic acid (PTA) with monoethylene glycol (MEG), followed by melt polycondensation to form high-molecular-weight polymer chains containing aromatic terephthalate units linked by ester functional groups (–COO–). Its molecular architecture combines rigid aromatic rings with flexible ethylene segments, governing chain mobility, crystallization behavior, tensile strength, thermal stability, and gas barrier performance. PET typically has a glass transition temperature of approximately 70-80°C and a melting temperature of 245-260°C, depending on molecular structure and thermal history. Commercial PET resins are differentiated by intrinsic viscosity (IV), crystallinity, molecular weight, and processing requirements, with bottle-grade, fiber-grade, and film-grade materials engineered for distinct applications. PET is also a major mechanically recycled thermoplastic, particularly within established post-consumer beverage bottle recycling systems.