The materials are commonly called polyesterresins but this simple name does not distinguishbetween at least two major classes of commercialmaterials. Also, the same name is usedwithin the unsaturated class to designate boththe cured and uncured state. These plastics maybe defined by identifying the materials as “unsaturatedpolyester resins which, when cured, yieldthermoset products as opposed to thermoplasticproducts.” The latter, as exemplified by Dacronand Videne, are saturated polyesters.
Unsaturated polyester resins of commerce arecomposed of two major components, a linear,unsaturated polyester and a polymerizablemonomer. The former is a condensation-type ofpolymer prepared by esterification of an unsaturateddibasic acid with a glycol. Actually, mostpolyesters are made from two or more dibasicacids.The most commonly used unsaturated acidsare maleic or fumaric with limited quantities ofothers used to provide special properties. Thesecond acid does not contain reactive unsaturation.Phthalic anhydride is most commonlyused but adipic acid and, more recently, isophthalicacid are employed. The properties ofthe final product can be varied widely, fromflexible to rigid, by changing the ratios and componentsin the polyester portion of the resin.
Polyester resins may contain numerousminor components such as light stabilizers andaccelerators, but must contain inhibitory componentsso that storage stability is achieved.Otherwise, polymerization will take place atroom temperature.
