Alkyd Resins
Authors: Lin K. F. 
Published: August 2004

In spite of challenges from many new coating resins developed over the years, alkyd resins as a family have maintained a prominent position for two principal reasons, their high versatility and low cost. The main reactions involved in alkyd resin synthesis are polycondensation by esterification and ester interchange. There is usually some residual acidity as well as free hydroxyl groups left in the resin molecules. Alkyd resins are usually referred to by a brief description based on certain classification schemes based on the nature of the fatty acid and oil length. The oil length of an alkyd resin has profound effects on the properties of the resin. The three principal components of alkyd resins are the polybasic acids, the polyols, and the monobasic acids. The selection of each of these ingredients affects the properties of the resin and may affect the choice of manufacturing processes. The principal polybasic acids used in alkyd preparation include phthalic anhydride, isophthalic acid, maleic anhydride, and fumaric acid, among others. The principal types of polyol used in alkyd synthesis are pentaerythritol, glycerol, trimethylolpropane, trimethylolethane, ethylene glycol, and neopentyl glycol. The overwhelming majority of monobasic acids used in alkyd resins are long‐chain fatty acids of natural occurrence. They may be used in the form of oil or free fatty acid. The drying property of fats and oils is related to their degree of unsaturation, and hence, to iodine values. Linolenic acid is responsible for the high yellowing tendency of alkyds based on linseed oil fatty acids. Alkyds made with nondrying oils or their fatty acids have excellent color and gloss stability and are frequently the choice for white industrial baking enamels and lacquers. The process of alkyd resin designing should begin with the question, “What are the intended applications of the resin?” The application dictates property requirements, such as solubility, viscosity, drying characteristics, compatibility, film hardness, film flexibility, acid value, water resistance, chemical resistance, and environmental endurance. With the targets in mind, a selection of oil length and a preliminary list of alternative choices of ingredients can then be made. For commercial production, the raw material list is screened based on considerations of material cost, availability, yield, and potential hazard to health, safety, and the environment. Once the oil length and ingredients are chosen, the first draft of a detailed formulation for the resin can be made. A simple molecular approach is favored by some alkyd chemists for deriving a starting formulation. Different chemical procedures may be used for the synthesis of alkyd resins, including the alcoholysis process, the fatty acid process, the fatty acid–oil process, and the acidolysis process. The processing method may be varied with different mechanical arrangements to remove the by‐product, water. Methods include the fusion process and the solvent process. The manufacturing of alkyd resins involves a wide variety of organic ingredients, most of them relatively mild and of low toxicity. Some, such as phthalic anhydride, maleic anhydride, solvents, and many of the vinyl (especially acrylic) monomers, are known irritants or skin sensitizers and are poisonous to humans. With the ever‐increasing awareness of the need of environment protection, the emission of solvent vapors and organic fumes into the atmosphere should be prevented by treating the exhaust through a proper scrubber. One of the important attributes of alkyds is their good compatibility with a wide variety of other coating polymers. Nitrocellulose‐based lacquers often contain short or medium oil alkyds to improve flexibility and adhesion. The principal applications are furniture coatings, top lacquer for printed paper, and automotive refinishing primers. Amino resins are probably the most important modifiers for alkyd resins. Many industrial baking enamels, such as those for appliances, coil coatings, and automotive finishes, are based on alkyd‐amino resin blends. Chlorinated rubber is often used in combination with medium oil drying‐type alkyds. The principal applications are highway traffic paint, concrete floor, and swimming pool paints. Vinyl resins, ie, copolymers of vinyl chloride and vinyl acetate which contain hydroxyl groups, may be formulated with alkyd resins to improve their application properties and adhesion; they are primarily used in making marine top‐coat paints. Synthetic latex house paints sometimes contain emulsified long oil or very long oil drying alkyds to improve adhesion to chalky painted surfaces. Silicone resins with high phenyl contents may be used with medium or short oil alkyds as blends in air‐dried or baked coatings to improve heat or weather resistance. Applications include insulation varnishes, heat‐resistant paints, and marine coatings. Chemically combining the desired modifier into the alkyd structure eliminates compatibility problems and gives a more uniform product. Several such chemical modifications of the alkyd resins of commercial importance include vinylated alkyds, silicone alkyds, urethane alkyds, phenolic alkyds, and polyamide alkyds. There has been a strong trend in recent years to increase the solids content of all coating materials, including alkyds, to reduce solvent vapor emission. Replacing solvent‐borne coatings with water‐borne coatings reduces solvent vapor emission and improves safety against the fire and health hazards of organic solvents. Alkyd resins, as a family, have remained the workhorse of the coatings industry for decades. Because of the efforts of the coatings industry to reduce solvent emission, there has been a clear gradual decline in the market share of alkyds, but their versatility and low cost will undoubtedly help maintain their significance in coatings. A significant portion of their raw material, fatty acids, is renewable.