Chemical Composition of Polyester Resin

Written by andrew breslin
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Chemical Composition of Polyester Resin
Polyester resins are widely used in boat manufacture and maintenance. (Brand X Pictures/Brand X Pictures/Getty Images)

Polyester resins are a marvel of modern chemistry. They are liquids that can be formed into a desired shape and, through the addition of chemical catalysts, harden into a rigid solid material. They are used widely in consumer and industrial products, including textiles, luggage and vehicles, especially boats.

Ester Groups

An ester is a specific type of chemical structure. A polyester is a large molecule made up of a number of individual molecules containing ester groups, linked together. An ester group contains a carbon double bonded to an oxygen. The carbon forms a single bond with either a single hydrogen atom or an "R" group containing carbon and hydrogen. The same carbon atom is single bonded to an oxygen, which forms another single bond with another "R" group. The basic diagram is: R-CO-O-R

Benzene Rings

The two primary types of polyester resins have benzene rings as components of their R groups. Benzene features six carbon atoms arranged in a hexagon. If the benzene ring has an ester group linked on two adjacent carbons, that is an orthophthalic polyester resin. If the ring has ester groups linked on two carbon atoms opposite one another on the benzene ring, that's an isophthalic polyester resin.


Styrene, C8H8, enables individual polyester resin monomers to polymerise. Active sites of the individual monomers contain carbon atoms double boned to one another as part of their R groups. In the presence of the proper chemical catalyst, (usually methyl ethyl ketone peroxide, or MEKP) a styrene molecule reacts at the active sites of two different monomers, linking them together. Many individual monomers can be attached together to form very large molecules. When this polymerisation happens, the material's properties change dramatically and it becomes a solid.


In spite of the complexity of esters, polyesters, and polyester resins, there are only three different chemical elements involved. These are carbon, hydrogen and oxygen. Carbon's ability to form four separate bonds allows these very large chains of polymers to be built up from relatively small and simple molecules.

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