MIL-HDBK-1110
b) Air-oxidizing coatings have limited solvent
resistance. They continue to oxidize and cross-link after
initial drying and curing. Thus, with time, they become harder,
more brittle, and less soluble in solvent. That is, they become
more like thermosetting coatings and are harder to recoat and
maintain.
c) Although alkyds have long been the most widely used
type of protective coating, their use is dropping rapidly because
of difficulties in preparing formulations with: low VOC content,
a brushable viscosity, and good film properties. Exempted
halogenated hydrocarbons are presently being used to produce
limited low-VOC alkyd formulations, but this exemption is
expected to be withdrawn in the near future.
Alkyd and Most Other Air-Oxidizing Coatings
Advantages
Limitations
Easy
to apply/repair/topcoat
Relatively high in VOCs
Good
initial flexibility possible
Poor performance in severe
Good
surface wetting/adhesion
environment
Good
gloss retention
Poor chemical/solvent
resistance
Relatively inexpensive
Poor immersion resistance
Based on renewable source
Poor alkali resistance
Poor heat resistance
Become brittle with extended
aging
2.3.2
Water Emulsion (Latex) Coatings. Water emulsion
coatings, commonly called latex coatings, have been successfully
used for many years to coat wood and masonry structures. The
porous nature of their films allows water vapor to pass through
them, i.e., they are breathing. This porosity reduces their
durability on steel. Thus, much effort is being made to develop
more durable products because of the great advantages of their
low VOC contents and ease of application and clean-up. In
addition, water-emulsion coatings have excellent flexibility and
low cost, and are easily topcoated and repaired. Drawbacks
include poor solvent and heat resistance (as with all
thermoplastics), poor immersion resistance, and difficulty in
bonding to smooth oil/alkyd coatings and chalky surfaces. The
poor bonding is due to insufficient content of organic solvents
to soften and wet the binder in the existing paint film. Because
of this limited adhesion, it is necessary to sand smooth enamels
and/or use a surface conditioner before topcoating with latex
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