(alkyds), polymers obtained by the interaction of monatomic alcohols with poly basic acids. The most widely distributed alkyd resins are glyptal resins (based on phthalic anhydride and glycerine) and pentaphthalic resins (based on phthalic anhydride and pentaerythritol).
Unmodified alkyd resins (solid products) are not widely used because of their poor solubility. They form brittle films and are used mainly in making insulating varnishes and glues.
Alkyd resins can be modified during the process of obtaining them by the introduction into the reaction mix of (1) rosin, which gives the resin the ability to mix with oils and gives greater hardness and luster to the varnish and paint coating based on the alkyd resin, or (2) unsaturated fatty acids (especially in the form of vegetable oils such as linseed, tung, or cottonseed oils), which give the coatings greater light resistance as well as the ability to dry in the air at temperatures of approximately 20°C.
Alkyd resins modified with rosin are solid, brittle products that melt at temperatures from 80° to 150°C. They are used only as additives to various film-forming substances in order to increase the hardness of the coating. The properties of alkyd resins modified by oils depend upon their oil content—that is, their “oiliness.” Thus, glyptal resins are divided into long-oil (more than 60 percent oil), medium-oil (45–60 percent), short-oil (35–45 percent) and supershort-oil (less than 35 percent) resins. Pentaphthalic resins are made with no less than 58 percent oil content. As the oil content of the alkyd resin increases, the elasticity and atmosphere resistance increase, but drying is retarded and the hardness, luster, and resistance to the action of solvents and lubricating oils are decreased. Pentaphthalic varnishes and enamels dry more quickly in cold drying than glyptal resins modified with the same oils, and they also form more water-resistant coatings.
Drying alkyd resins, obtained from drying oils dehydrated by castor, linseed, tung, and other oils, solidify at approximately 20°C or by drying in the 60°-80°C range. Coatings with better properties are obtained in the latter case. Drying alkyd resins are used as independent film-forming agents (glyptal and pentaphthalic varnish and enamels) or in a mixture with other film-forming agents. Nondrying alkyd resins contain nondrying oils (cocoa or raw castor oil). They do not dry, even at temperatures exceeding 120°C. For this reason they are used only when mixed with other film-forming materials.
Varnishes and enamels based on modified alkyd resins are used for painting railroad cars, airplanes, agricultural machines, and so forth. Enamels for painting automobiles, bicycles, and various instruments and equipment are obtained from compositions of alkyd resins with urea-formaldehyde and melamine-formaldehyde resins. Enamels based on nondrying alkyd resins mixed with nitrocellulose are used to paint automobiles (“autonitroenamels”), industrial equipment, and leather. Alkyd resins are also used for preparing printer’s ink, glues, and other substances.