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PROTECTIVE COATINGS 207

and cools, solidifying into a protective film. The most common flame-spray is
polyethylene, which is ground into a powder and flocked through a flame that
converts polyethylene into a molten state. This type of coating can be remelted
or dissolved in a suitable solvent.
Coalescence Coatings. In this type of coating, tiny particles of resin are encap-
sulated in a soap-like material and then dispersed in water, which acts as a
diluent rather than as a true solvent. This type of blend is known as an emul-
sion. When the water evaporates, the resin particles fuse (coalesce) to form a
stable, cured coating film. These coatings, once cured, cannot be redissolved
in water, although other organic solvents may dissolve them. These coatings
consist of acrylic latex suspensions and epoxy emulsions.

4.2.1.2 Convertible Coatings Most convertible coatings cure by polymerization.
Polymerization occurs when two or more resin molecules combine to form a sin-
gle, more complex molecule. The resin molecules may be monomers or short chain
polymers that react to form polymers. The four main types of polymerization used
in coating technology are oxygen-induced, chemically induced, heat-induced, and
hydrolysis.

Oxygen-induced Polymerized Coating: Alkyds. These are oil-based primers and
topcoats. Alkyds are based on fish or vegetable oils blended with pigments
and catalysts in a solvent. The film is formed when the oil reacts with oxygen
assisted by the catalyst followed by the evaporation of the solvent. Most paints
sold in cans are alkyds. Drying oils consist of penetrating oils and lacquers that
form a thin protective film.
Chemically Induced Polymeric Coatings. Epoxies are the preferred corrosion con-
trol coating for severe environments. Epoxies are a generic class of materials
based on the presence of an epoxide polymer side group. Epoxies exhibit supe-
rior adhesion and chemical resistance but are susceptible to weathering degra-
dation (by chalking) and are often top-coated to shield them from ultraviolet
light.
Polyurethanes. Polyurethanes are used for color retention and weathering. These
are extensively used over steel for long-term decorative corrosion protection.
Polyurethanes vary widely in chemistry and can be formulated to be very flex-
ible elastomers, rigid foams, or dense brittle films.
Heat-Induced Polymerized Coatings. Polyesters and vinylesters are based on
styrene monomers with a very reactive catalyst. These may be classified
as chemically induced curing polymers; however, the actual reaction is
heat-induced. The catalytic reaction generates a great deal of heat, which
polymerizes the styrene monomer and the ester groups. They are used as tank
linings and form the basis of many freestanding fiberglass structures.
Phenolics. These are thin films, which form by evaporation of solvent followed by
∘
baking at, or greater than, 204 C. Phenolics form a very strong, hard chemical
and temperature-resistant film used for storage of strong acids and solvents.