Product Code Database
An alkyd is a polyester resin modified by the addition of and other components. Alkyds are derived from and organic acids including dicarboxylic acids or carboxylic acid anhydride and triglyceride. The term alkyd is a modification of the original name "alcid", reflecting the fact that they are derived from alcohol and organic acid. The inclusion of a fatty acid confers a tendency to form flexible coatings. Alkyds are used in , and in moulds for casting. They are the dominant resin or binder in most commercial oil-based coatings. Approximately 200,000 tons of alkyd resins are produced each year.. Published online: 15 January 2003. The original alkyds were compounds of glycerol and phthalic acid sold under the name Glyptal. These were sold as substitutes for the darker-colored copal resins, thus creating alkyd that were much paler in colour. From these, the alkyds that are known today were developed.
For the drying resins, triglycerides are derived from polyunsaturated fatty acids (often derived from plant and , e.g. linseed oil). These drying alkyds are cured by the oxygen in air. The drying speed and the nature of the coatings depends on the amount and type of drying oil employed (more polyunsaturated oil means faster reaction in air) and presence of catalysts, the so-called oil drying agents. These catalysts are that catalyze crosslinking of the unsaturated sites. Cobalt salts are particularly effective and widely used. Because of the alleged carcinogenicity of cobalt, some researchers are attempting to develop non-cobalt catalysts as replacements in alkyd coatings; none have been found yet that are as effective as cobalt.
Alkyd resins are produced in two processes: the fatty acid process and the alcoholysis or monoglyceride process. Higher-quality, higher-performance alkyds are produced in the fatty acid process, in which the composition of the resulting resin may be more precisely controlled. In this process, an acid anhydride, a polyol and an unsaturated fatty acid are combined and cooked together until the product has achieved a predetermined level of viscosity. Pentaerythritol based alkyds are made this way. More economical alkyd resins are produced from the alcoholysis or glyceride process, in which end-product quality control is not as paramount. In this process, raw vegetable oil, high in unsaturated component, is combined with additional polyol and heated to cause transesterification of the triglycerides into a mixture of mono- and diglyceride oils. Soybean oil is often used. Acid anhydride is added to the resulting mixture to build the molecular weight of the resin into roughly the same product as with the fatty acid process. However, the alcoholysis process produces a more randomly oriented structure. To remove the water produced as a by-product and to increase the reaction rate, surplus phthalic anhydride is added. Water is thus removed with the unreacted acid by heating the bulk to a specific temperature. The reaction is not as controllable as would be desired, so a new process was introduced in which xylene is added to produce an azeotrope with the water. This gives greater control at a lower temperature and also produces resins at a lower viscosity, useful in making high-solids paints. This is known as the AZO process. In both cases, the resulting product is a polyester resin to which pendant group drying oil groups are attached. At the conclusion of each process the resin is purified, diluted in solvent and sold to paint and varnish makers."Additives for Coatings" J. H. Bielman, Ed. Wiley-VCH, 2000, Weinheim. .
Because the major components of an alkyd coating, i.e. fatty acids and triglyceride oils, are derived from low cost renewable resources, the cost of alkyd coatings has remained very low despite the ever-increasing cost of petroleum, which is the predominant raw-material source of most other coatings such as vinyl polymer, Acrylic paint, epoxy and . Typical sources of drying oils for alkyd coatings are tung oil, linseed oil, sunflower oil, safflower oil, walnut oil, soybean oil, fish oil, corn oil, dehydrated castor oil (in which dehydration transforms certain of its fatty acids' single bonds into double bonds, some of them conjugated, creating a semi-drying oil) and tall oil (resinous oil by-product from pulp and paper manufacturing). Non-drying/plasticizer resins are made from castor, palm, coconut oils and Cardura (the proprietary name of a synthetic ester of versatic acid). Dehydrated castor oil was at one time the only oil permitted in resin manufacture in India; no edible oils were allowed.
They may be used to formulate flame-retardant coatings.
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