MIL-HDBK-1110
Coatings that cure by solvent or water evaporation are
unchanged chemically during curing. They are said to be
thermoplastic, because they can be softened by heat or by
solvent. Chemically curing coatings are said to be
thermosetting, because they are not softened by heating or by
solvent. Air-oxidizing coatings (oil-based paints) are
thermoplastic after initial curing. Upon further curing (e.g.,
6 months or more), the additional polymerization (cross-linking
of polymers) slowly converts them to thermosetting coatings.
This is shown below:
Thermoplastic Coatings
Thermosetting
Lacquers
Chemically Reacting Products
Latex Products
Oil-Based Products (After Aging)
Oil-Based Products (Initially)
2.2.1
Air Oxidation of Drying Oils. For coatings that cure
by air oxidation of drying oils (usually vegetable), oxygen from
the air reacts with unsaturated fatty acids in their drying oils.
By this reaction, liquid resins are converted to solid films.
Metal driers are usually incorporated into formulations of drying
oil coatings to accelerate their normally slow curing.
a) Examples of coatings that are cured by this
mechanism are:
(1)
Unmodified drying oils
(2)
Alkyds
(3)
Silicone alkyds
(4)
Epoxy esters
(5)
Oleoresinous phenolics
b) Such coatings wet surfaces very well and generally
perform well in mild atmospheric environments, but they have
limited durability in chemical environments, particularly
alkaline environments. Epoxy esters provide some additional
chemical resistance. They should not be confused with higher
performance, two-component, chemically reacting epoxies.
Oleoresinous phenolic coatings are the only oil-based coatings
that can be used successfully in water immersion service.
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