© 2006 by Taylor & Francis Group, LLC

Composition of the Anticorrosion Coating 23
terephthalic acid, fumaric acid, and maleic acid. Alcohol reactants used in condensation
include bisphenol A, neopentyl glycol, and propylene glycol [17]. The combinations
of alcohol and organic acids used determine the mechanical and chemical
properties, thermal stability, and other characteristics of polyesters.
2.2.4.2 Saponification
In an alkali environment, the ester links in a polyester can undergo hydrolysis —
that is, the bond breaks and reforms into alcohol and acid. This reaction is not
favored in acidic or neutral environments but is favored in alkali environments
because the alkali forms a salt with the acid component of the ester. These fatty acid
salts are called soaps, and hence this form of polymer degradation is known as
saponification.
The extent to which a particular polyester is vulnerable to alkali attack depends
on the combination of reactants used to form the polyester prepolymer and the
unsaturated monomer with which it is crosslinked.
2.2.4.3 Fillers
Fillers are very important in polyester coatings because these resins are unusually
prone to build up of internal stresses. The stresses in cured paint films arise for two
reasons: shrinkage during cure and a high coefficient of thermal expansion.
During cure, polyester resins typically shrink a relatively high amount, 8 to 10
volume percent [17]. Once the curing film has formed multiple bonds to the substrate,
however, shrinkage can freely occur only in the direction perpendicular to the
substrate. Shrinkage is hindered in the other two directions (parallel to the surface
of the substrate), thus creating internal stress in the curing film. Fillers and reinforcements
are used to help avoid brittleness in the cured polyester film.
Stresses also arise in polyesters due to their high coefficients of thermal expansion.
Values for polyesters are in the range of 36 to 72 × 10 –6 mm/mm/°C, whereas
those for steel are typically only 11 × 10 –6 mm/mm/°C [17]. Fillers and reinforcements
are important for minimizing the stresses caused by temperature changes.
2.2.5 ALKYDS
In commercial use since 1927 [18], alkyd resins are among the most widely used
anticorrosion coatings. They are one-component air-curing paints and, therefore, are
fairly easy to use. Alkyds are relatively inexpensive. Alkyds can be formulated into
both solvent-borne and waterborne coatings.
Alkyd paints are not without disadvantages:
• After cure, they continue to react with oxygen in the atmosphere, creating
additional crosslinking and then brittleness as the coating ages [18].
• Alkyds cannot tolerate alkali conditions; therefore, they are unsuitable for
zinc surfaces or any surfaces where an alkali condition can be expected
to occur, such as concrete.
© 2006 by Taylor & Francis Group, LLC