Unsaturated polyester resin

Unsaturated polyester resin is a thermosetting polymer made from polyesters containing carbon–carbon double bonds, which crosslink during curing, often around reinforcing fibers.

Polyesterification: A condensation reaction that forms polyester chains from polyfunctional acids and alcohols. This reaction builds the polyester backbone and leaves sites for later crosslinking.
Unsaturated polyester: A polyester containing carbon–carbon double bonds in its molecular backbone, capable of further crosslinking. It is the reactive polymer component that gives the resin its name.
Fiberglass: A composite material made by embedding glass fibers in a polymer matrix. Unsaturated polyester is a widely used matrix for fiberglass products.
Epoxy resin: A family of reactive polymers that cure into crosslinked materials, often through reaction with amine hardeners. Epoxy generally offers stronger adhesion and lower cure shrinkage, but costs more.
Volatile organic compound: An organic chemical that readily evaporates into the air under ordinary conditions. Styrene emissions from polyester resin contribute to workplace and environmental VOC exposure.
Styrene: An aromatic vinyl compound used as a reactive diluent and crosslinking comonomer in many polyester resins. It lowers viscosity and reacts with polyester double bonds during cure.
Maleic anhydride: A cyclic acid anhydride used to introduce unsaturated units into polyester chains. Its double bond provides reactive sites for curing many polyester formulations.
Hand lay-up: An open-mold composite process in which reinforcement and liquid resin are placed by hand. Polyester resin’s workable cure time suits this simple laminating method.
Vinyl ester resin: A thermosetting resin made by reacting epoxy oligomers with unsaturated acids, then crosslinking with a reactive diluent. It often improves chemical resistance and toughness over polyester at higher cost.
Styrene exposure: Human contact with styrene through inhalation, skin absorption, or ingestion. Open-mold polyester work can expose workers to styrene vapor.

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