Polyester

Polyester is a category of polymers that contain one or two ester linkages 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 some natural products, such as those found in plants and insects. Natural polyesters and a few synthetic ones are biodegradable, but most commercial polyesters are not. Polyesters are used extensively in clothing. Synthetic 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.

Polyester is a category of polymers that contain one or two ester linkages 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 some natural products, such as those found in plants and insects. Natural polyesters and a few synthetic ones are biodegradable, but most commercial polyesters are not. Polyesters are used extensively in clothing. Synthetic 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== picture of a bend in a high-surface area polyester fiber with a seven-lobed cross section]] Polyesters can contain one ester linkage per repeat unit of the polymer, as in polyhydroxyalkanoates like polylactic acid, or they may have two ester linkages per repeat unit, as in polyethylene terephthalate (PET). Polyesters are one of the most economically important classes of polymers, driven especially by PET, which is counted among the commodity plastics; in 2019 around 30.5 million metric tons were produced worldwide. There is a 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 (M n >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 (M n 1000 tonnes/day and can easily reach 2500 tonnes/day. Besides the above-mentioned large processing units to produce staple fiber or yarns, there are ten thousands of small and very small processing plants, so it can be estimated that polyester is processed and recycled in more than 10 000 plants around the globe. This is without counting all the companies involved in the supply industry, beginning with engineering and processing machines and ending with special additives, stabilizers and colors. This industry complex is still growing by 4–8% per year, depending on the world region. ==Synthesis== Synthesis of polyesters is generally achieved by a polycondensation reaction. The general equation for the reaction of a diol with a diacid is: (n+1) R(OH) 2 + n R'(COOH) 2 → HO[ROOCR'COO] n ROH + 2n H 2 O. Polyesters can be obtained by a wide range of reactions of which the most important are the reaction of acids and alcohols, alcoholysis and or acidolysis of low-molecular weight esters or the alcoholysis of acyl chlorides. The following figure gives an overview over such typical polycondensation reactions for polyester production. Furthermore, polyesters are accessible via ring-opening polymerization. Azeotrope esterification is a classical method for condensation. The water formed by the reaction of alcohol and a carboxylic acid is continually removed by azeotropic distillation. When melting points of the monomers are sufficiently low, a polyester can be formed via direct esterification while removing the reaction water via vacuum. Direct bulk polyesterification at high temperatures (150 – 290 °C) is well-suited and used on the industrial scale for the production of aliphatic, unsaturated, and aromatic–aliphatic polyesters. Monomers containing phenolic or tertiary hydroxyl groups exhibit a low reactivity with carboxylic acids and cannot be polymerized via direct acid alcohol-based polyesterification. ⁠ In the case of PET production, however, the direct process has several advantages, in particular a higher reaction rate, a higher attainable molecular weight, the release of water instead of methanol and lower storage costs of the acid when compared to the ester due to the lower weight. ===Alcoholic transesterification=== Transesterification: An alcohol-terminated oligomer and an ester-terminated oligomer condense to form an ester linkage, with loss of an alcohol. R and R' are the two oligomer chains, R is a sacrificial unit such as a methyl group (methanol is the byproduct of the esterification reaction). The term "transesterification" is typically used to describe hydroxy–ester, carboxy–ester, and ester–ester exchange reactions. The hydroxy–ester exchange reaction possesses the highest rate of reaction and is used for the production of numerous aromatic–aliphatic and wholly aromatic polyesters. The transesterification based synthesis is particularly useful for when high melting and poorly soluble dicarboxylic acids are used. In addition, alcohols as condensation product are more volatile and thereby easier to remove than water. The high-temperature melt synthesis between bisphenol diacetates and aromatic dicarboxylic acids or in reverse between bisphenols and aromatic dicarboxylic acid diphenyl esters (carried out at 220 to 320 °C upon the release of acetic acid) is, besides the acyl chloride based synthesis, the preferred route to wholly aromatic polyesters. ===Silyl method=== In this variant of the HCl method, the carboxylic acid chloride is converted with the trimethyl silyl ether of the alcohol component and production of trimethyl silyl chloride is obtained ===Acetate method (esterification)=== ===Ring-opening polymerization=== Aliphatic polyesters can be assembled from lactones under very mild conditions, catalyzed anionically, cationically, metallorganically or enzyme-based. A number of catalytic methods for the copolymerization of epoxides with cyclic anhydrides have also recently been shown to provide a wide array of functionalized polyesters, both saturated and unsaturated. Ring-opening polymerization of lactones and lactides is also applied on the industrial scale. ===Other methods=== Numerous other reactions have been reported for the synthesis of selected polyesters, but are limited to laboratory-scale syntheses using specific conditions, for example using dicarboxylic acid salts and dialkyl halides or reactions between bisketenes and diols. Instead of acyl chlorides, so-called activating agents can be used, such as 1,1'-carbonyldiimidazole, dicyclohexylcarbodiimide, or trifluoroacetic anhydride. The polycondensation proceeds via the in situ conversion of the carboxylic acid into a more reactive intermediate while the activating agents are consumed. The reaction proceeds, for example, via an intermediate N-acylimidazole which reacts with catalytically acting sodium alkoxide: The use of activating agents for the production of high-melting aromatic polyesters and polyamides under mild conditions has been subject of intensive academic research since the 1980s, but the reactions have not gained commercial acceptance as similar results can be achieved with cheaper reactants. K_C = \frac{[...\ce{-C(O)O -}...][\ce{H2O}]}{[\ce{-C(O)OH}][\ce{-OH}]} The equilibrium constant of the acid-alcohol based polyesterification is typically K C ≤ 10, what is not high enough to obtain high-molecular weight polymers (DP n ≥ 100), as the number average degree of polymerization (DP n ) can be calculated from the equilibrium constant K C . DP_n ~ = ~ \sqrt[2]{K_C} + 1 In equilibrium reactions, it is therefore necessary to remove the condensation product continuously and efficiently from the reaction medium in order to drive the equilibrium towards polymer. The condensation product is therefore removed at reduced pressure and high temperatures (150–320 °C, depending on the monomers) to prevent the back reaction. With the progress of the reaction, the concentration of active chain ends is decreasing and the viscosity of the melt or solution increasing. For an increase of the reaction rate, the reaction is carried out at high end group concentration (preferably in the bulk), promoted by the elevated temperatures. Equilibrium constants of magnitude K C ≥ 10 4 are achieved when using reactive reactants (acid chlorides or acid anhydrides) or activating agents like 1,1′-carbonyldiimidazole. Using these reactants, molecular weights required for technical applications can be achieved even without active removal of the condensation product. ===Cross-linking=== Unsaturated polyesters are thermosetting polymers. They are generally copolymers prepared by polymerizing one or more diols with saturated and unsaturated dicarboxylic acids (maleic acid, fumaric acid, etc.) or their anhydrides. The double bond of unsaturated polyesters reacts with a vinyl monomer, usually styrene, resulting in a 3-D cross-linked structure. This structure acts as a thermoset. The exothermic cross-linking reaction is initiated through a catalyst, usually an organic peroxide such as methyl ethyl ketone peroxide or benzoyl peroxide. ===Pollution of freshwater and seawater habitats=== A team at Plymouth University in the UK spent 12 months analysing what happened when a number of synthetic materials were washed at different temperatures in domestic washing machines, using different combinations of detergents, to quantify the microfibres shed. They found that an average washing load of 6 kg could release an estimated 137,951 fibres from polyester-cotton blend fabric, 496,030 fibres from polyester and 728,789 from acrylic. Those fibers add to the general microplastics pollution. ==Safety== ===Fertility=== Ahmed Shafik was a sexologist, who won a Ig Nobel Prize on his research regarding how polyester underwear cause temporary oligospermia of rats, dogs, and men. Bisphenol A which is an endocrine disrupting chemical may be used in the synthesis of polyester. ==See also== Epoxy Glycerine phthalate Microfiber Oligoester Polyamide Rayon Viscose

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