Westinghouse chose mixtures of organic, ethylenically unsaturated (containing C=C bonds) dicarboxylic acids, which contain reactive free carboxyl groups (–COOH), to be reacted with polyhydric alcohols having only reactive hydroxyl (–OH) groups. When substantially molar equivalents of these materials are mixed and heated in a closed reaction vessel with an esterification catalyst, which may be mineral acids, the esterification reaction takes place. Removal of the water formed in the reaction, to increase the degree of esterification, may be accomplished by azeotropic distillation when the reaction is carried out in the presence of a volatile organic liquid such as toluene, xylene, or the like. The starting materials and the degree of esterification were chosen so that the final product was a solvent-free, syrupy polyester resin at room temperature or when heated.