Copolyester

Copolyester is a category of polyesters in which two or more different ester monomer units are linked to form the polymer chain. The backbone is typically built from diols and diacids or diesters, with additional monomer units introduced to create copolymers. By varying the type and ratio of comonomers, copolyesters can be engineered to exhibit a range of characteristics, including glass transition temperature, crystallinity, clarity, toughness, and chemical resistance.

Common forms include random, alternating, block, and graft copolymers. A well-known example is glycol-modified polyethylene terephthalate (PETG), which is PET with glycol units that disrupt crystallinity and improve clarity and impact resistance. Other copolyesters incorporate different dicarboxylic acids such as isophthalic or sebacic acids, or use alternative diols, to further tailor properties. Copolyesters can be amorphous or semi-crystalline, depending on composition and processing.

Key properties of copolyesters include good clarity when amorphous, tunable mechanical strength and flexibility, and adjustable heat resistance. They often offer improved impact resistance and processability compared with certain homopolyesters, though barrier properties and chemical resistance can vary with composition. Processing methods include injection molding, extrusion, film blowing, and thermoforming. Some copolyesters are used in applications requiring clear, tough plastics, such as packaging, beverage bottles, films, medical devices, and consumer electronics housings.

Production is typically by condensation polymerization or transesterification, combining diols and diacids or diesters under catalysts with removal of byproducts to achieve high molecular weight. Recycling of copolyesters may be compatible with PET streams for some formulations, but others require separate processing due to differences in crystallinity and Tg.