© 2006 by Taylor & Francis Group, LLC

14 Corrosion Control Through Organic Coatings
Chalking also occurs to some extent with several other types of polymers. It
does not directly affect corrosion protection but is a concern because it eventually
results in a thinner coating. The problem is easily overcome with epoxies, however,
by covering the epoxy layer with a coating that contains a UV-resistant binder.
Polyurethanes are frequently used for this purpose because they are similar in
chemical structure to epoxies but are not susceptible to UV breakdown.
2.2.1.3 Variety of Epoxy Paints
The resins used in the epoxy reactions described in section 2.2.1.1 are available in a
wide range of molecular weights. In general, as molecular weight increases, flexibility,
adhesion, substrate wetting, pot life, viscosity, and toughness increase. Increased
molecular weight also corresponds to decreased crosslink density, solvent resistance,
and chemical resistance [2]. Resins of differing molecular weights are usually blended
to provide the balance of properties needed for a particular type of coating.
The number of epoxide reactions possible is practically infinite and has resulted
in a huge variety of epoxy polymers. Paint formulators have taken advantage of this
variability to provide epoxy paints with a wide range of physical, chemical, and
mechanical characteristics. The term “epoxy” encompasses an extremely wide range
of coatings, from very-low-viscosity epoxy sealers (for penetration of crevices) to
exceptionally thick epoxy mastic coatings.
2.2.1.3.1 Mastics
Mastics are high-solids, high-build epoxy coatings designed for situations in which
surface preparation is less than ideal. They are sometimes referred to as “surface tolerant”
because they do not require the substrate to be cleaned by abrasive blasting to Sa2 1/2.
Mastics can tolerate a lack of surface profile (for anchoring) and a certain amount of
contamination (e.g., by oil) that would cause other types of paints to quickly fail.
Formulation is challenging, because the demands placed on this class can be
contradictory. Because they are used on smoother and less clean surfaces, mastics
must have good wetting characteristics. At the same time, viscosity must be very
high to prevent sagging of the very thick wet film on vertical surfaces. Meeting both
of these requirements presents a challenge to the paint chemist.
Epoxy mastics with aluminium flake pigments have very low moisture permeations
and are popular both as spot primers or full coats. They can be formulated with weak
solvents and thus can be used over old paint. The lack of aggressive solvents in mastics
means that old paints will not be destroyed by epoxy mastics. This characteristic is
needed for spot primers, which overlap old, intact paint at the edge of the spot to be
coated. Mastics pigmented with aluminium flake are also used as full-coat primers.
Because of their very high dry film thickness, build-up of internal stress in the
coating during cure is often an important consideration in using mastic coatings.
2.2.1.3.2 Solvent-Free Epoxies
Another type of commonly used epoxy paint is the solvent-free, or 100% solid,
epoxies. Despite their name, these epoxies are not completely solvent-free. The
levels of organic solvents are very low, typically below 5%, which allows very high
film builds and greatly reduces concerns about volatile organic compounds (VOCs).
© 2006 by Taylor & Francis Group, LLC