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.
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