polyesters

Polyesters are a family of polymers characterized by ester functional groups in their main chains. They range from synthetic thermoplastics used in packaging and textiles to specialty resins, and include natural polyesters such as cutin found in plant cuticles. The most commercially important polyesters are thermoplastic, processable by conventional plastic methods.

Most synthetic polyesters are formed by condensation polymerization between diols and dicarboxylic acids, releasing water. An alternative route uses transesterification of monomer esters (for example dimethyl terephthalate) with glycols. The repeating unit contains the ester linkage -O-CO-O-. Typical monomers include ethylene glycol and terephthalic acid, which give polyethylene terephthalate (PET).

Major polyesters include PET, polybutylene terephthalate (PBT) and glycol-modified PET (PETG). Polyesters like polyethylene naphthalate (PEN) offer higher barrier properties. Unsaturated polyesters are used in resin systems and composites. These materials are usually thermoplastics but differ in crystallinity, Tg, and melting behavior.

Properties commonly shared by polyesters are good strength and stiffness, chemical resistance, and processability by melt methods. They tend to be hygroscopic, absorbing moisture that can affect dimensional stability and mechanical performance. PET is noted for clarity and excellent barrier properties in packaging applications.

Applications span textile fibers and apparel, beverage bottles, food and retail packaging, films, and engineering plastics for automotive, electronics, and consumer goods. Unsaturated polyesters are widely used in fiber-reinforced composites and coatings.

Recycling and environmental aspects: PET is one of the most recycled plastics, using mechanical and chemical recycling routes. Recycling challenges include contamination and collection infrastructure. Polyesters are generally not biodegradable, contributing to persistent waste; life-cycle assessments emphasize energy use, resource efficiency, and potential for chemical recycling to improve sustainability.