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Unsaturated polyester resins are the workhorse of the composites industry.

There is a large range of polyesters made from different acids, glycols and monomers, all having varying properties. The polymer is dissolved in a reactive monomer such as styrene to give a low viscosity liquid. Unsaturated polyesters are divided into classes depending upon the structures of their basic building blocks. Some common examples would be orthophthalic (ortho), isophthalic (iso) and dicyclopentadiene (DCPD). Ortho resin is the standard ‘general purpose resin’. Iso resin is the preferred material in industries such as boat building, where its superior water resistance is desirable. Care must be taken when mixing the resin prior to moulding. The resin and any additives must be carefully stirred to disperse all the components evenly before the catalyst is added. It is also important to add the accelerator and catalyst in carefully measured amounts to control the polymerization reaction to give the best material properties. Too much catalyst will cause too rapid a gelation time, whereas too little catalyst will result in under-cure.

Colouring of the resin mix can be carried out with pigments. The choice of a suitable pigment material, even though only added at about 3% resin weight, must be carefully considered as it is easy to affect the curing reaction and degrade the final laminate by use of unsuitable pigments. Fillers are often added in quantities up to 50% of the resin weight although such addition levels will reduce the flexural and tensile strength of the laminate.

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