© 2006 by Taylor & Francis Group, LLC

Composition of the Anticorrosion Coating 15
An interesting note about these coatings is that many of them generate significant
amounts of heat upon mixing. The cross-linking is exothermic, and little solvent is
present to take up the heat in vaporization [2].
2.2.1.3.3 Glass Flake Epoxies
Glass flake epoxy coatings are used to protect steel in extremely aggressive environments.
When these coatings were first introduced, they were primarily used in
offshore applications. In recent years, however, they have been gaining acceptance
in mainstream infrastructure as well. Glass flake pigments are large and very thin,
which allows them to form many dense layers with a large degree of overlap between
glass particles. This layering creates a highly effective barrier against moisture and
chemical penetration because the pathway around and between the glass flakes is
extremely long. The glass pigment can also confer increased impact and abrasion
resistance and may aid in relieving internal stress in the cured coating.
2.2.1.3.4 Coal Tar Epoxies
Coal tar, or pitch, is the black organic resin left over from the distillation of coal.
It is nearly waterproof and has been added to epoxy amine and polymide paints to
obtain coatings with very low water permeability. It should be noted that coal tar
products contain polynuclear aromatic compounds, which are suspected to be carcinogenic.
The use of coal tar coatings is therefore restricted or banned in some countries.
2.2.2 ACRYLICS
Acrylics is a term used to describe a large and varied family of polymers. General
characteristics of this group include:
• Outstanding UV stability
• Good mechanical properties, particularly toughness [3]
Their exceptional UV resistance makes acrylics particularly suitable for applications
in which retention of clarity and color are important.
Acrylic polymers can be used in both waterborne and solvent-borne coating
formulations. For anticorrosion paints, the term acrylic usually refers to waterborne
or latex formulations.
2.2.2.1 Chemistry
Acrylics are formed by radical polymerization. In this chain of reactions, an initiator
— typically a compound with an azo link (⎯N=N⎯) or a peroxy link (⎯ 0–0⎯)
— breaks down at the central bond, creating two free radicals. These free radicals
combine with a monomer, creating a larger free-radical molecule, which continues
to grow as it combines with monomers, until it either:
• Combines with another free radical (effectively canceling each other)
• Reacts with another free radical: briefly meeting, transferring electrons and
splitting unevenly, so that one molecule has an extra hydrogen atom and
one is lacking a hydrogen atom (a process known as disproportionation)
© 2006 by Taylor & Francis Group, LLC