Alkyd resins are thermoplastic polyester resins made by heating polyhydric alcohols* with polybasic acids or their anhydrides. They are used in making protective coatings with good weathering properties and are important ingredients in many synthetic paints due to their versatility and low cost. They are commonly produced by polycondensation of an alcohol (polyol), and a dicarboxylic acid or its anhydride - for instance, glycerol and phthalic anhydride react to form the polyester glyptal.
The principal polybasic acids used in alkyd preparation include phthalic anhydride, isophthalic acid, maleic anhydride, and fumaric acid, among many others. The principal types of polyols used in alkyd synthesis are glycerol, trimethylolethane, trimethylolpropane, pentaerythritol, ethylene glycol, and neopentyl glycol. The overwhelming majority of the monobasic acids used in alkyd resins are long-chain fatty acids of natural occurrence. The selection of each of the aforementioned ingredients (polyols, mono-, and polyacids) affect the properties of the resin and may also affect the choice of manufacturing processes. For example, long fatty acids greatly improve the flexibility of the coating products and an increasing degree of unsaturation the “drying” speed. However, unsaturated acids such as Linolenic acid also increase the yellowing tendency of alkyds whereas alkyds made with nondrying oils (low degree of unsaturation) or their fatty acids have excellent color and gloss stability.
One of the important attributes of alkyd resins is their good compatibility with many other coating polymers. For example vinyl resins, i.e., copolymers of vinyl chloride and vinyl acetate which contain hydroxyl groups can be formulated with alkyd resins to improve their application properties and adhesion. These products are often used for making marine top-coat paints. Very long emulsified oil-drying alkyds** are also added to synthetic latex house paints to improve adhesion to chalky painted surfaces, whereas medium to short oil alkyd resins are sometimes blended with silicone resins with high phenyl content for air-dried or baked coatings to improve heat or weather resistance. Some commercially important chemical modifications of alkyd resins include vinylated alkyds, silicone alkyds, urethane alkyds, phenolic alkyds, and polyamide alkyds.
With the ever-increasing awareness of the need of environment protection, there has been a strong trend to increase the solids content of all coating products, including alkyds, to reduce solvent vapor emission. In several cases, solvent-borne coatings, including alkyds, have been successfully replaced with water-borne coatings to eliminate the solvent vapor emission and to improve safety against fire and health hazards of organic solvents. These efforts led to a clear gradual decline in the market share of alkyd resins in favor of acrylic, polyurethane and epoxy resins. Furthermore, some of the ingredients, such as phthalic anhydride, maleic anhydride, certain solvents, and several of the vinyl monomers, are known irritants or skin sensitizers. Nevertheless, alkyd resins, as a family, are still important in the coatings industry and have been for decades.
COMMERCIAL Alkyd Resins
Alkyd resins are important ingredients in many synthetic paints, varnishes and enamels and find also applications as thermosetting plastics that can be molded. The principal applications are furniture and architectural coatings, product finishes, special-purpose coatings and automotive refinishing primers. Alkyd-amino resin blends are probably the most important group of 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 paints, concrete floor, and swimming pool paints. Applications of alkyd modified silicon resins include heat-resistant paints, and marine coatings and insulation varnishes.
Notes:* Alcohols of the fatty alkyl series that contain several hydroxyl groups (OH) in the molecule.
**A drying oil is an oil that hardens to a solid film after exposure to air. The oil hardens through a chemical reaction in which the components crosslink (polymerize) by the action of oxygen. Some commonly used drying oils include linseed, tung and walnut oil.