Polyester

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Polyester is a category of polymers, or, more specifically condensation polymers, which contain the ester functional group in their main chain. Although polyesters do exist in nature, polyester generally refers to the large family of synthetic polyesters (plastics) which includes polycarbonate and above all polyethylene terephthalate (PET). PET is one of the most important thermoplastic polyesters.

The first synthetic polyester, glycerine phthalate, was used in the First World War for waterproofing. Natural polyesters have been known since around 1830.

Common usage

A common usage of the word polyester refers to cloth woven from polyester fiber. Polyester clothing is considered to have a "less natural" feeling to it compared to natural fibers. Polyester fibers are often spun together with fibers of cotton, producing a cloth with some of the better properties of each.

Applications

• Fibers (and microfibers) for fabric
• Bottles
• Films such as biaxially oriented PET film, often aluminized
• Photographic film (after cellulose triacetate, polyester is the most important substrate film base)
• A common matrix for glass-reinforced plastic (commonly called "fiberglass") and other composite materials.
• Liquid crystal displays
• Holograms
• Filters
• Dielectric film for capacitors and 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 commonly used as casting materials, fiberglass laminating resins, and non-metallic auto-body fillers. In such applications, polymerization and cross-linking are initiated through an exothermic reaction involving an organic peroxide, such as methyl ethyl ketone peroxide or benzoyl peroxide.

Synthesis

Synthesis of polyesters is generally achieved by a polycondensation reaction. See "condensation reactions in polymer chemistry".

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]

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.