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. Most polyester resins are viscous, pale coloured liquids consisting of a solution of a polyester in a monomer which is usually styrene..
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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