Polyester Reaction and Preparation of The Resin

Both the saturated and unsaturated types of polyester resin can be made by a number of different methods, but the one most commonly used is the direct reaction at high temperature between a polycarboxylic acid or anhydride and a polyhydric alcohol, preferably in an inert atmosphere. When making unsaturated polyesters, a substance which inhibits ethylenic polymerisation, for example hydroquinone, may be added to the reactants.

With both types the molecular ratio used is such that the numbers of carboxyl and hydroxyl groups are approximately equal. The esterification can be catalysed by addition of an acidic substance. If the acid and the alcohol are difunctional, the resulting polymer is a linier polymer and therefore thermoplastic, an example is polyethylene terephtalate. If, however, either the acid or the alcohol contains more than two functional groups the resulting polyester is capable of crosslinking and therefore potentially thermosetting; examples of this type are the polyesters known by the name of Glyptals (from glycerol and phthalic anhydride), and the unsaturated polyester.

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