Polyester resin are unsaturated synthetic resins formed by the reaction of di-basic organic acids and polyhydric alcohols. They are used in sheets moulding and bulk casting. Wall panels fabricated from polyester resins are reinforced with fibre glass mats. This are mostly used in restaurants, kitchens, restrooms, extensive bathrooms and tubs. Polyester resin comes in different types and every batch could be factory produced depending on the nature of application or industrial work. The figure below shows the idealized chemical structure of a typical polyester. Note the positions of the ester groups (CO - O - C) and the reactive sites (C* = C*) within the molecular chain.

Most polyester resins are viscous, pale coloured liquids consisting of a solution of a polyester in a monomer which is usually styrene. The addition of styrene in amounts of up to 50% helps to make the resin easier to handle by reducing its viscosity. The styrene also performs the vital function of enabling the resin to cure from a liquid to a solid by 'cross-linking' the molecular chains of the polyester, without the evolution of any by-products. These resins can therefore be moulded without the use of pressure and are called 'contact' or 'low pressure' resins. Polyester resins have a limited storage life as they will set or 'gel' on their own over a long period of time. Often small quantities of inhibitor are added during the resin manufacture to slow this gelling action. Polyester resin offers the following advantages:

1. Resistance to water and variety of chemicals
2. Adequate resistance to weathering and ageing
3. Low cost
4. Polyester can withstand a temperature up to 80 degree
5. It has good wetting to fibre glass
6. It shrinks between of 4-8 percentage during curing
7. Thermal expansion ranges from 108-200