© 2006 by Taylor & Francis Group, LLC

50 Corrosion Control Through Organic Coatings
2.4.2 REACTIVE REAGENTS
Reactive reagents generally aid in film formation, forming bonds to the substrate,
crosslinking, and curing. Examples of this class of additives include metallic driers,
such as zinc or tin salts, to aid in crosslinking [10,18]; curing catalysts and accelerators;
photoinitiators; and adhesion promoters.
2.4.3 CONTRA-ENVIRONMENTAL CHEMICALS
As their name implies, contra-environmental chemicals are a group of additives that
are intended to provide the coating with protection against its service environment.
Examples of this type of additive include [128]:
• Performance enhancers (antiskinning agents, antioxidants, light stabilizers,
nonpigmental corrosion inhibitors)
• Thermal controllers (freeze-thaw controllers, heat stabilizers)
• Biological controllers (biocides, antifouling agents)
Antioxidants and light stabilizers are used to provide topcoats with thermooxidative
and UV stabilization, thus increasing service life in outdoor applications.
For thermo-oxidative stabilization, phenolic antioxidants and aromatic amines are
generally used [129]. Hindered amine light stabilizers (HALS; for example, Hostavin
N30 TM , Goodrite 3150 TM , Chimassorb 944 TM ) or UV absorbers (for example,
Cyasorb UV-531 TM ) [130] are added to the coating mostly for UV protection and,
to some extent, for thermo-oxidative stabilization. A mixture of antioxidants and
light stabilizers is frequently used; this must be carefully formulated because both
positive and negative effects have been reported from combining these additives
[131,132]. Barret and colleagues suggest that the phenol in the antioxidant prevents
the conversion of HALS to a stabilizing nitroxide [133]. Another mechanism may
be that the radicals of different stabilizers interact.
The term corrosion inhibitors is not meant to include anticorrosion pigments in
this section. These additives are completely soluble in order to provide the maximum
possible corrosion protection immediately upon application of the paint. Pigments
have a much more controlled solubility rate in order to have an effect over a long
period. Corrosion inhibitors are commonly used for preventing spot or “flash”
rusting. Sodium nitrate, for example, is sometimes added to waterborne coatings to
prevent flash rusting [3]. These corrosive-inhibiting additives are used in addition
to, rather than as a substitute for, anticorrosion pigments. Corrosion inhibitors and
anticorrosion pigments must be chosen with care if used together, so as not to
adversely affect the in-can stability of the formulation [3].
Biocides prevent microbial growth in coatings, both in-can and in the cured
paint. They are more important in waterborne coatings than in solvent-borne
coatings.
Antifouling agents prevent the growth of mussels, sea urchins, and other
marine life on marine coatings. They are used exclusively in topcoats, rather than
in the primers that provide the corrosion protection to the metal substrate.
© 2006 by Taylor & Francis Group, LLC