Network Polyesters for Surface Coatings
The original polyesters for coatings were prepared from phthalic anhydride and glycerol and were referred to as glyptals or glyptal resins:
The products from above polyesterifications are brittle materials. They are, therefore, modified with oils, either drying or nondrying. Such oil-modified resins bear the names of alkyds. While glycerol is widely used, other polyhydroxy compounds (polyols) are also utilized. These may be trimethylolpropane, pentaerythritol, sorbitol, or others. Phthalic anhydride is usually used in alkyd preparations. Other dicarboxylic compounds, however, may also be included for modification of properties. Common modifiers might be isophthalic, adipic, or sebacic acids, or maleic anhydride. In addition, many other acid modifiers are described in the patent literature. The oils in alkyd resins are usually of vegetable origin. They are classified by the type and amount of residual unsaturation into drying, semidrying, and nondrying oils. The drying oils contain most of residual unsaturation, while the nondrying ones contain mostly saturated fatty acids. Alkyds are also classified by the quantity of modifying oil that they contain into short, medium, or long oil alkyds. Short oil alkyds contain 30–50% oil and are usually baked to obtain a hard dry surface. Medium oil (50–65%) and long oil (65–75%) alkyds will air dry upon addition of metal dryers. There are two main methods for preparation of alkyd resins. In the first one, called fatty acid process, a free fatty acid is coesterified directly with the dibasic acid and the polyol at 200–240C. The reaction may be carried out without a solvent by first heating in an inert atmosphere. At the end, an inert gas may be blown into the resin from the bottom of the reaction kettle to remove water and unreacted materials. As a modification of this, a small quantity of a solvent may be used to remove water of esterification continuously by azeotropic distillation with the aid of moisture traps. In the second method, known as the alcoholysis process, the drying oil is heated with the glycerol in the first stage of the reaction, at about 240C. This is usually done in the presence of a transesterification basic catalyst to form monoglycerides
After the first stage is complete, phthalic anhydride, with or without another dibasic acid, is added and a copolyesterification is carried out in the same manner as in the first stage of the reaction. The conditions under which the reaction is carried out and the rate at which the temperature is raised during the condensation affect the molecular weight distribution of the final product. In addition, the monoglyceride content of the original reaction mixture determines the microgel content of the alkyd and the dynamic properties of the dried film [31]. The finished alkyd resin can be illustrated as follows:
