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CATEGORIES (articles) > Chassis & Bodywork Construction > Finishings > Polyester as used in Powder Coating

Polyester as used in Powder Coating


Close-up of a polyester shirt
SEM picture of a bend in a high surface area polyester fiber with a seven-lobed cross section

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 and culture

The most common general usage of the word polyester is usually in reference into the fiber. Polyester clothing is considered to have a "less natural" feeling to it in comparison to natural fibers. Quite frequently, polyester fibers are spun together with fibers of cotton, producing a cloth with some of the better properties of each.

Since before World War II, American middle-class households have had the blessing of having an electricity powered washing machine to assist in the cleaning of clothes. Still a tedious process, many Mondays were set aside to wash, dry on the line, and then sprinkle and roll clothes and place them in the vegetable bin of the refrigerator (so there was time to starch and iron them when time permitted). When Dacron blouses appeared in 1956, women were instantly drawn to the fact that this new material dried fast and required very little attention with an iron. Women felt liberated and even though the new man-made fibers were not a huge trend in the fashion capitol Paris, designers in New York, California, Milan, the Riviera, and London encouraged the production of the sythetic fibres and fabrics. Not only was polyester a fabric that was easy to manage,but because of the way non-absorbent synthetic interacts with synthetic dyestuffs it had the effect of seeming almost fluorescent in color. Green, yellow, and pink were among the colors that were intensified to bring adventure to clothing apparel.

Applications

  • Fibers (and microfibers) for fabric
  • Bottles
  • Films such as Mylar, often aluminized
  • Photographic film (after cellulose triacetate, polyester is the most important substrate)
  • A common matrix for glass-reinforced plastic (commonly called "fiberglass") and other composite materials.
  • Liquid crystal displays
  • Holograms
  • Filters

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]
leftrightarrow
          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.




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