Unsaturated polyester resins are resins formed by the reaction of dibasic organic acids and polyhydric alcohols. Polyesters resins used in sheet molding compound, bulk molding compound and the toner in laser printers. Wall panels made of polyester resin reinforced with glass fiber - known as fiberglass reinforced plastic (GRP) - are typically used in restaurants, kitchens, toilets and other areas washable walls require little maintenance.

Unsaturated polyesters are condensation polymers formed by the reaction of polyols (also known as polyols), organic compounds containing multiple alcohol or hydroxy-functional groups, a saturated or unsaturated dibasic acids. Typical used polyols are glycols such as ethylene glycol, acids used are phthalic acid and maleic acid. Water, a byproduct of esterification reaction is continuously removed, driving the reaction to complete. The use of unsaturated polyesters and additives such as styrene lowers the viscosity of the resin. The initially liquid resin is converted into a solid by cross-linking chains. This is done by creating free radicals on unsaturated bonds, which move in a chain reaction of other unsaturated in adjacent molecules, linking them in the process. The first free radicals caused by adding a compound that readily decomposes into free radicals. This connection is usually and wrongly known as the catalyst. Substances that are used are generally organic peroxides such as benzoyl peroxide.

Polyester resins are thermosetting resins and, as with other, exothermic cure. The use of excessive catalyst thus lead to charring or ignition during the curing process. Excessive catalyst can also lead to product failure or a rubber-like material forms