(1) Drying alkyd resins. (a) Unmodified. (b) Modified with natural materials. (c) Modified with other synthetic resins. (d) Modified with other synthetic resins and oil extended. (2) Semidrying alkyd resins. (3) Nondrying alkyd resins. (4) Miscellaneous modified alkyd resins. (5) Alkyd resins in water dispersion. (6) Alkyd resins in molding compositions. At least 75 percent of the alkyd resin finishes used at present are of the drying type and about 15 percent of the nondrying type. _Unmodified drying alkyd resins._—This class of alkyd resins consists of a series of compounds made from polyhydric alcohols, polybasic acids, and fatty acids in chemical combination. The alcohol is usually glycerin, and the polybasic acid largely phthalic anhydride or acid, although others, such as maleic anhydride (acid) are increasing rapidly in importance. The fatty acid or oil used may be linseed, tung, perilla, hempseed, soybean, sunflower, safflower, or other drying oil. It is believed that tung oil and perilla oil are the most important at this time. Unmodified drying alkyd resins are characterized by excellent durability but limited resistance to water in air-dried finishes. Both in air-dried and in baked finishes they are outstanding as to flexibility, quick drying, long luster life, and permanent adhesion. Their principal uses are in finishes for interior walls and woodwork, automobiles, coatings on steel such as for refrigerators, railway equipment, bridges, advertising signs, and lithographed containers. In these applications the products of this type compete with nitrocellulose lacquers and the older types of varnishes and paints. While the initial cost is higher, greater durability is obtained together with faster drying, flexibility, and hardness. Probably the largest field for surface coatings is outdoor wood finishes. Several attempts have been made to adapt pure alkyd finishes to this use but with limited success because the hard and non-porous finish does not permit the escape of moisture contained in the wood and the pressure developed from vaporization of the moisture by the sun’s rays tends to lift the coating from the wood surface. Recently it has been found practicable to incorporate from 15 to 20 percent alkyd resins in conventional types of outdoor paints for wood. Here the use of alkyds has contributed greater durability and retention of fresh appearance over a longer period. Paints of this type are now on the retail market. _Drying alkyd resins modified with natural materials._—This type of alkyd resin is modified principally with natural resins, such as rosin, damar, mastic, shellac, or copal. The use of these natural resins imparts hardness to the resin but shortens its durability. They make the product less expensive, permit easier incorporation of the drying oil, and in some instances increase the water resistance.