Polyester resins are condensation type polymers formed by the reaction of organic compounds having multiple alcohol or hydroxy functional groups commonly known as polyhydric alcohols or polyols, with saturated or unsaturated dibasic acids. Formation of polyester resin is an esterification reaction with water as by product which is continuously removed to drive the reaction to completion.
Polyester resin, initially liquid resin is converted to a solid by cross-linking chains. This is achieved by obtaining free radicals at unsaturated bonds, these are pushed in a chain reaction to other unsaturated bonds in adjoining molecules thereby, linking them in the process. The initial free radicals are generated by adding a compound that easily decomposes into free radicals.
Polyester resins cure exothermically and are thermosetting resins. The use of excessive free radical donor compound especially with a catalyst present can cause charring or even ignition of polyester resin while curing. Alternately, the solid polyester resin may fracture or form a rubbery material if excessive catalyst is used.