© 2006 by Taylor & Francis Group, LLC

12 Corrosion Control Through Organic Coatings
and cured binder, through which water finds its easiest route to the metal surface.
It can also cover pores in the bulk of the coating, blocking this means of water
transport. Because this healed surface is very thin, however, its ability to entirely
prevent water uptake is greatly limited. Generally, it succeeds much better at limiting
transport of oxygen. The ability to absorb, rather than transmit, UV radiation is
polymer-dependent; acrylics, for example, are for most purposes impervious to
UV-light, whereas epoxies are extremely sensitive to it.
The binders used in anticorrosion paints are almost exclusively organic polymers.
The only commercially significant exceptions are the silicon-based binder in inorganic
ZRPs sil oxanes, and high-temperature silicone coatings. Many of the coating’s
physical and mechanical properties — including flexibility, hardness, chemical
resistances, UV-vulnerability, and water and oxygen transport — are determined
wholly or in part by the particular polymer or blend of polymers used.
Combinations of monomers and polymers are commonly used, even if the
coating belongs to one generic polymer class. Literally hundreds of acrylics are
commercially available, all chemically unique; they differ in molecular weights,
functional groups, starting monomers, and other characteristics. A paint formulator
may purposely blend several acrylics to take advantage of the characteristics of each;
thus a methacrylate-based acrylic with its excellent hardness and strength might be
blended with one of the softer polyacrylates to impart flexibility to the cured paint.
Hybrids, or combinations of different polymer families, are also used. Examples
of hybrids include acrylic-alkyd hybrid waterborne paints and the epoxy-modified
alkyds known as epoxy ester paints.
2.2.1 EPOXIES
Because of their superior strength, chemical resistance, and adhesion to substrates,
epoxies are the most important class of anticorrosive paint. In general, epoxies have
the following features:
• Very strong mechanical properties
• Very good adhesion to metal substrates
• Excellent chemical, acid, and water resistance
• Better alkali resistance than most other types of polymers
• Susceptibility to UV degradation
2.2.1.1 Chemistry
The term epoxy refers to thermosetting polymers produced by reaction of an epoxide
group (also known as the glycidyl, epoxy, or oxirane group; see Figure 2.1). The
ring structure of the epoxide group provides a site for crosslinking with proton
donors, usually amines or polyamides [1].
FIGURE 2.1 Epoxide or oxirane group.
© 2006 by Taylor & Francis Group, LLC
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