Unsaturated polyester resins (UP) are mainly used in fibre-reinforced plastics and as insulating casting systems and moulding compounds in prototype construction. They also form the basis in many cases for reactive, reinforced moulding compounds such as SMC or BMC.
The chemical basis for the production of components is the double bond contained in the unsaturated polyesters. These polyesters are usually dissolved in vinyl monomers, often styrene, and together form the starting component of the resin. Substances are used as hardeners which decompose into so-called radicals under the influence of heat or light or when further accelerators are added. In UP resin systems, organic peroxides are often used for this purpose. The resulting radicals trigger the chain reaction (radical polymerization) to crosslink the polyester/vinyl mixture.
As the cross-linking reaction of polyester resins is based on a chain reaction, the mixing ratio of resin and hardener is not too decisive. Instead, the molecular structure of the polyester and the crosslinker determine the subsequent properties. Unfilled polyester resins usually exhibit high volume shrinkage during curing. For this reason, correct design and knowledge, especially in the field of fibre-reinforced plastics, is of great importance in order to avoid warpage in the component.