The Polyester resin is an unsaturated, thermosetting resin produced by reaction between several organic acids and polyvalent alcohols. It is most often used in building reinforced fibers and composite products. The polyester resin is a viscous liquid requiring the addition of catalysts and accelerators to complete the curing process.. Unsaturated polyester resins are used in the manufacture of composite details by the hand-contact method or by spray. They do not need pressure to harden. Although these products have several different disadvantages compared to other commonly used composite resins, they still offer an attractive balance between ease of use, low prices and positive physical characteristics.

The most commonly used polyester resins are pale colored, fairly viscous liquids, consisting of a solution of polyester and styrene. The styrene added to reduce resin viscosity makes it run better and plays a vital role in the curing process. Resin manufacturers include several other additives in polyester resin products, depending on their intended use. These include pigments, UV stabilizers, fillers and fire-resistant or chemically resistant substances. These additives can be added in different quantities to produce customized products tailored to the needs of consumers.One of the most comprehensive single applications of polyester resins is in the production of glass fiber reinforced details. Polyester resin and fiberglass reinforced details are used in automotive, shipbuilding, leisure parks, construction and the food industry. Combined with mineral fillers, polyester resins serve for the production of countertops, sinks, car repair, mortars and bonding pastes.