They are made by the esterification of a polybasic acid with a polyhydric alcohol, and have the characteristics of homogeneity and solubility that make them especially suitable for coatings and finishes, plastic molding compounds, calking compounds, adhesives, and plasticizers for other resins. The resins have high adhesion to metals; are transparent, easily colored, tough, flexible, heat-and chemical-resistant; and have good dielectric strength. They vary greatly with the raw materials used and with varying percentage compositions, from soft rubbery gums to hard brittle solids. Phthalic anhydride
imparts hardness and stability. Maleic acid makes a higher melting-point resin. Azelaic acid gives a softer and less brittle resin. The long-chain dibasic acids, such as adipic acid, give resins of great toughness and flexibility. In place of glycerol the glycols yield soft resins, and sometimes the glycerol is modified with a proportion of glycol. The resins are reacted with oils, fatty acids, or other resins such as urea or melamine, to make them compatible with drying oils and to impart special characteristics.
Since alkyd resins are basically esterification products of innumerable polybasic acids and polyhydric alcohols, and can be modified with many types of oils and resins, the actual number of different alkyd resins is unlimited, and the users’ specification is normally by service requirements rather than composition. Short-oil alkyds, with 30 to 45% nonoxidizing oils, are used in baking enamels, while the long-oil alkyds, with 56 to 70% oxidizing oils, are soluble in mineral spirits and are used for brushing
enamels.

Alkyd coatings are used for such diverse applications as air-drying water emulsion wall paints and baked enamels for automobiles and appliances. The properties of oil-modified alkyd coatings depend upon the specific oil used as well as the percentage of oil in the composition. In general, they are comparatively low in cost and have excellent color reten-
tion, durability, and flexibility, but only fair drying speed, chemical resistance, heat resistance, and salt spray resistance. The oil-modified alkyds can be further modified with other resins to produce resin-modified alkyds. The resin is sometimes added during manufacture of the alkyd and becomes an integral part of the alkyd, or the modifying resin is
blended with the alkyd when the paint is formulated. When mixed with urea formalehyde or melamine resin, harder and more resistant baked enamels are produced. Alkyds blended with ethyl cellulose are used as tough flexible coatings for electric cable. Other resins blended with alkyds to produce special or improved properties include phenolic, rosin, vinyl,
and silicone.
 

Alkyd plastics molding compounds are composed of a polyester resin and usually a diallyl phthalate monomer plus various inorganic fillers, depending on the desired properties. The raw material is produced in three forms-granular, putty, and glass-reinforced. As a class, the alkyds have excellent heat resistance up to about 300°F (150°C), high stiffness, and moderate tensile and impact strength. Their low moisture absorption combined with good dielectric strength makes them particularly suitable for electronic and electrical hardware such as switchgear, insulators and parts for motor controllers and automotive ignition systems. They are easily molded at low pressures and cure rapidly.
 

Amberlac and Duraplex are alkyd resins of the Rohm & Haas Co. in a wide range of formulations. The Paraplex resins are oil-modified soft resins used for coatings for textiles and paper, and for blending with cellulose plastics to give better adhesion on lacquers. Paraplex, P-43HV, however, is a polyester-styrene copolymer supplied as a clear liquid for laminating, molding, or casting at low pressure. Aquaplex, of the same company, is a group of oil-modified resins for use in water paints. Teglac resins, of the American Cyanamid Co., used for indoor finishes and clear-coat varnishes, are hard alkyd resins made with natural resin acids as blending agents. Cycopol, of the same company, is an alkyd-styrene copolymer resin for fast-drying enamels, giving high gloss and durability. Beckosol 1341, of Reichhold Chemicals, Inc., is a phenol-modified alkyd resin for fast-drying enamels. Petrex resin, of Hercules, Inc., is a series of alkyd resins used in lacquers, varnishes, adhesives, and inks. Aroplaz 1248-M, of U.S. Industrial Chemicals, Inc., is a high-gloss phthalic alkyd resin, soluble in mineral spirits, for industrial finishes. The Iso Burnok 32-0, of the T. F. Washburn Co., used as a vehicle for thixotropic paints, is a soybean oil isothalic alkyd. It has high clarity and is odorless. Dyal, of the Sherwin-Williams Co., is an alkyd resin for finishes, as are also Amalite and Amavar, of the American Alkyd Industries.
 

Plaskon Alkyd, of the Allied Chemical Corp., is a mineral-filled alkyd molding powder used for electrical parts having good arc resistance and heat resistance to 350°F (176°C). Neolyn resins, of Hercules, Inc., are alkyd resins produced from rosin. They are used as modifiers for nitrocellulose and for vinyl resins in lacquers and adhesives to add toughness to the film. Cellolyn 501, of the same company, is a lauric acid-pentaerythritol alkyd used in durable, color-stable nitrocellulose lacquers. When maleic anhydride or fumaric acid is reacted with rosin and then esteriiied with glycerol or other polyhydric alcohol, a series of alkyd resins is produced. Isocyanate resins are linear alkyds lengthened by isocyanates and then treated with a glycol or a diamine to cross-link the molecular chain. Plastics made from them are noted for good abrasion resistance.