Polyester

  Polyester (aka Terylene) is a category of polymers which contain the ester functional group in their main chain. Although there are many forms of polyesters, the term "polyester" is most commonly used to refer to polyethylene terephthalate (PET). Other forms of polyester include naturally occurring cutin of plant cuticles as well as synthetic polyesters such as polycarbonate and polybutyrate.

Polyester may be produced in numerous forms. For example, polyester as a thermoplastic may be heated and processed into different forms, e.g., fibers, sheets, and three-dimensional shapes. While combustible at high temperatures, polyester tends to shrink away from flames and often self-extinguishes.

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Applications

Polyester is the most widely used manufactured fiber in the United States. Woven polyester fabrics are used for apparel and home furnishings. These include bed sheets, bedspreads, curtains and draperies. Polyester fiberfill is also used to stuff pillows, comforters and cushion padding.

Polyester fabrics sometimes have a "less natural" feel when compared to similarly woven fabrics made from natural fibers, e.g., cotton. However, polyester fabrics may exhibit other advantages over natural fabrics, e.g., improved wrinkle resistance. As a result, polyester fibers are often spun together with natural fibers, e.g., cotton, to produce a cloth with blended properties.  

Polyesters are also used to make bottles, films, tarpaulin, liquid crystal displays, holograms, filters, dielectric film for capacitors, film insulation for wire and insulating tapes.

Liquid crystalline polyesters are among the first industrially used liquid crystalline polymers. In general they have extremely good mechanical properties and are extremely heat resistant. For that reason, they can be used as an abradable seal in jet engines.

Thermosetting polyester resins are generally copolymers of unsaturated polyesters with styrene. The unsaturation in the polyester is generally governed by maleic acid or fumaric acid. Another important family is the group of vinyl esters. Here the unsaturation is found in the alcohol part of the polyester. The double bond of the unsaturated polyester reacts with styrene resulting in a 3-D crosslinked structure, the thermoset material. The cross-linking is initiated through an exothermic reaction involving an organic peroxide, such as methyl ethyl ketone peroxide or benzoyl peroxide. Unsaturated polyesters are commonly used as casting materials, fiberglass laminating resins, and non-metallic auto-body fillers. Fiberglass reinforced unsaturated polyesters find wide application in bodies of yachts and as body parts of cars.

Polyester is also widely used as a finish on high-quality wooden products like guitars, pianos and vehicle/yacht interiors (Burns Guitars, Rolls Royce and Sunseeker are examples of companies that use polyester on their products). The thixotropic properties of the sprayable form of polyester make it ideal for use on open grain timbers as it can quickly fill the grain and has a high build film thickness per coat. The cured polyester can then be sanded and polished to a high-gloss, durable finish.

Synthesis

Synthesis of polyesters is generally achieved by a polycondensation reaction. See "condensation reactions in polymer chemistry". 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]_nROH + 2n H₂O

Azeotrope esterification

In this classical method, an alcohol and a carboxylic acid react to form a carboxylic ester. To assemble a polymer, the water formed by the reaction must be continually removed by azeotrope distillation.

Alcoholic transesterification

See main article on transesterification.

O \\ C - OCH3 + OH[Oligomer2] / [Oligomer1]		O \\ C - O[Oligomer2] + CH3OH / [Oligomer1]
(ester-terminated oligomer + alcohol-terminated oligomer)		(larger oligomer + methanol)

Acylation (HCl method)

The acid begins as an acid chloride, and thus the polycondensation proceeds with emission of hydrochloric acid (HCl) instead of water. This method can be carried out in solution or as an enamel.

Silyl method

In this variant of the HCl method, the carboxylic acid chloride is converted with the trimethyl silyl ether of the alcohol component; trimethyl silyl chloride is produced.

Acetate method (esterification)

Silyl acetate method

Ring-opening Polymerization

Aliphatic polyesters can be assembled from lactones under very mild conditions, catalyzed anionically, cationically or metallorganically.

References

• Textiles, by Sara Kadolph and Anna Langford. 8th Edition, 1998.