Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include naturally occurring chemicals, such as in plants and insects, as well as synthetics such as polybutyrate. Natural polyesters and a few synthetic ones are biodegradable, but most synthetic polyesters are not. Synthetic polyesters are used extensively in clothing. Polyester fibers are sometimes spun together with natural fibers to produce a cloth with blended properties. Cotton-polyester blends can be strong, wrinkle- and tear-resistant, and reduce shrinking. Synthetic fibers using polyester have high water, wind and environmental resistance compared to plant-derived fibers. They are less fire-resistant and can melt when ignited. Liquid crystalline polyesters are among the first industrially used liquid crystal polymers. They are used for their mechanical properties and heat-resistance. These traits are also important in their application as an abradable seal in jet engines. Types Polyesters are one of the most economically important classes of polymers, driven especially by PET, which is counted among the commodity plastics; in 2000 around 30 million tons were produced worldwide. There is great variety of structures and properties in the polyester family, based on the varying nature of the R group (see first figure with blue ester group). Natural Polyesters occurring in nature include the cutin component of plant cuticles, which consists of omega hydroxy acids and their derivatives, interlinked via ester bonds, forming polyester polymers of indeterminate size. Polyesters are also produced by bees in the genus Colletes, which secrete a cellophane-like polyester lining for their underground brood cells earning them the nickname "polyester bees". Synthetic The family of synthetic polyesters comprises Linear aliphatic high molecular weight polyesters (Mn >10,000) are low-melting (m. p. 40 – 80 °C) semicrystalline polymers and exhibit relatively poor mechanical properties. Their inherent degradability, resulting from their hydrolytic instability, makes them suitable for applications where a possible environmental impact is a concern, e.g. packaging, disposable items or agricultural mulch films⁠ or in biomedical and pharmaceutical applications. Aliphatic linear low-molar-mass (Mn < 10,000) hydroxy-terminated polyesters are used as macromonomers for the production of polyurethanes. hyperbranched polyesters are used as rheology modifiers in thermoplastics or as crosslinkers in coatings due to their particularly low viscosity, good solubility and high functionality Aliphatic–aromatic polyesters, including poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), poly(hexamethylene terephthalate)(PHT),