Definition of Alkyd

Alkyd coatings are a class of polyester coatings derived from the reaction of an alcohol (alkohol) and an acid or acid anhydride hence the term alk-yd from "alcohol and acid or anhydride]" and are the dominant resin or "binder" in most "oil-based" coatings sold to the consumer market. Alkyd coatings today are typically manufactured from acid anhydrides such as phthalic anhydride  or maleic anhydride and polyols such as glycerine or pentaerythritol and are modified with unsaturated fatty acids (from plant and vegetable oils) to give them air drying properties. The unsaturated oils react with oxygen from the air which cause the oils to polymerize or crosslink with each other. The drying speed of the coatings depends on the amount and type of drying oil employed (more unsaturated oil means faster reaction with oxygen from the air) and use of organic metal salts or "driers" which catalyze crosslinking between the unsaturated oils used to modify the base polyester resin. Traditionally organic lead salts were employed but have been replaced with cobalt and other alternative driers such as zirconium, zinc, calcium, and iron. Most driers are colorless but cobalt is a deep blue purple color (iron driers are reddish orange) thus the colored driers are typically employed only in paint colors where the blue color of cobalt or reddish orange color of iron can be easily masked.