Alkyd

An alkyd is a polyester modified by the addition of fatty acids and other components. They are derived from polyols and a dicarboxylic acid or carboxylic acid anhydride. The term alkyd is a modification of the original name "alcid", reflecting the fact that they are derived from alcohol and organic acids. The inclusion of the fatty acid confers a tendency to form flexible coating. Alkyds are used in paints 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. 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 varnishes which were much paler in colour. From these, the alkyds we know today were developed.

There are two types of alkyd resins, drying (including semi drying) and nondrying. Both types are typically produced from dicarboxylic acids or anhydrides, such as phthalic anhydride or maleic anhydride, and polyols, such as trimethylolpropane, glycerine, or pentaerythritol. Alkyds are also used in items such as resins and paints.

For the "drying" resins, triglycerides are derived from polyunsaturated fatty acids (often derived from plant and vegetable oils, e.g. linseed oil). These drying alkyds are cured 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 use of metal salts, the so-called oil drying agents. These are metal complexes that catalyze crosslinking of the unsaturated sites.

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