Page 657 - Corrosion Engineering Principles and Practice

610 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 611

molecular weight will generally be reacted in situ. Binders can be
classified according to their essential chemical reactions.
Oxygen-Reactive Binders. Oxygen-reactive binders are generally low
molecular weight resins that are only capable of producing coatings
through an intermolecular reaction with oxygen. This reaction is
often catalyzed by metallic salts of cobalt or lead. Examples are

• Alkyds: Alkyds are produced by chemically reacting natural
drying oils to form a synthetic resin with good film curability,
chemical resistance, and weather resistance.
• Epoxy esters: Epoxy resins react chemically with drying oils to
form epoxy esters. The drying oil part of the molecule determines
the basic properties of the epoxy ester coatings. The coating
dries by oxidation in the same manner as an alkyd.
• Urethane alkyds: Epoxy resins are also chemically combined
with drying oils as part of the molecule that further reacts with
isocyanates to produce urethane alkyds. Upon application as a
liquid coating, the resin-oil combination converts by oxidation
to a solid.
• Silicone alkyds: Alkyd resins are combined with silicone
molecules to form an excellent weather-resistant combination
known as silicone alkyds.

Lacquers. Lacquers are coatings that are converted from a liquid
material to a solid film by the evaporation of solvents alone. Lacquers
have generally a low volume of solids. Examples are
• Polyvinyl chloride polymers: This principal corrosion-resistant
lacquer is made from polyvinyl chloride copolymers. The
vinyl molecule is relatively large and will effectively dissolve
in solvent in the 20 percent range.
• Chlorinated rubbers: To be effective, chlorinated rubbers have
to be modified by other resistant resins to obtain higher solids,
decreased brittleness, and increased adhesion.
• Acrylics: Acrylics are also of high molecular weight and may
be combined with vinyls to improve exterior weatherability
and color retention.
• Bituminous materials: Bituminous asphalts and coal tars are
often combined with solvents to form lacquer-type films.
They can provide good corrosion resistance but can only be
applied where appearance is not a factor.
Heat-conversion binders. Examples of heat-conversion binders are

• Hot melts: Hot melts normally involve asphalt or coal tar and
are melted and applied as 100 percent solids in the hot-liquid
condition.