Polyester resins are unsaturated synthetic resins formed by the reaction of dibasic organic acids and polyhydric alcohols. Maleic Anhydride is a commonly used raw material with diacid functionality. Polyester resins are used in sheet moulding compound, bulk moulding compound and the toner of laser printers. Wall panels fabricated from polyester resins reinforced with fiberglass—so-called fiberglass reinforced plastic (FRP)—are typically used in restaurants, kitchens, restrooms and other areas that require washable low-maintenance walls. They are also used extensively in cured-in-place pipe applications. Departments of Transportation in the USA also specify them for use as overlays on roads and bridges. In this application they are known as PCO Polyester Concrete Overlays. These are usually based on isophthalic acid and cut with styrene at high levels—usually up to 50%. Polyesters are also used in anchor bolt adhesives though epoxy based materials are also used.  Many companies have and continue to introduce styrene free systems mainly due to odor issues.
Unsaturated polyesters are condensation polymers formed by the reaction of polyols (also known as polyhydric alcohols), organic compounds with multiple alcohol or hydroxy functional groups, with saturated or unsaturated dibasic acids. Typical polyols used are glycols such as ethylene glycol; acids used are phthalic acid, isophthalic acid and maleic acid. Water, a by-product of esterification reactions, is continuously removed, driving the reaction to completion. The use of unsaturated polyesters and additives such as styrene lowers the viscosity of the resin. The initially liquid resin is converted to a solid by cross-linking chains. This is done by creating free radicals at unsaturated bonds, which propagate in a chain reaction to other unsaturated bonds in adjacent molecules, linking them in the process. The initial free radicals are induced by adding a compound that easily decomposes into free radicals. This compound is usually and incorrectly known as the catalyst. Initiator is the more correct term. Cobalt salts are usually used as a true catalyst. Substances used are generally organic peroxides such as benzoyl peroxide or methyl ethyl ketone peroxide.
Polyester resins are thermosetting and, as with other resins, cure exothermically. The use of excessive initiator especially with a catalyst present can, therefore, cause charring or even ignition during the curing process. Excessive catalyst may also cause the product to fracture or form a rubbery material.
Lichens have been shown to deteriorate polyester resins, as can be seen in archaeological sites in the Roman city of Baelo Claudia Spain.
Polyester resin offers the following advantages:
- Adequate resistance to water and variety of chemicals.
- Adequate resistance to weathering and ageing.
- Low cost.
- Polyesters can withstand a temperature up to 80°C.
- Polyesters have good wetting to glass fibres.
- Relatively low shrinkage at between 4-8% during curing.
- Linear thermal expansion ranges from 100-200 x 10-6 K-1.
Polyester resin has the following disadvantages:
- Strong styrene odor
- More difficult to mix than other resins, such as a two-part epoxy
- The toxic nature of its fumes, and especially of its catalyst, MEKP, pose a safety risk if proper protection isn't used
- Not appropriate for bonding many substrates
- The finished cure is most likely weaker than an equal amount of an epoxy resin
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- "2K Polymer Systems Ltd: Bonded Anchors: P - Polyester". www.2kps.net. Retrieved 2018-04-05.
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- Francesca Cappitelli; Claudia Sorlini (2008). "Microorganisms Attack Synthetic Polymers in Items Representing Our Cultural Heritage". Applied and Environmental Microbiology. 74: 564–9. doi:10.1128/AEM.01768-07. PMC 2227722. PMID 18065627.