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 alc ohol and organic ac id s. The inclusion of the fatty acid confers a tendency to form flexible coating. Alkyds are used in paints and in moulds for casting.