Corrosion of Paint
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7.7.3.1 Air-Drying
Air-drying coatings cure at room temperatures, forming films. The films are
formed by one of three mechanisms:
1. Solvent evaporation. Shellac and lacquers such as nitrocellulose,
acrylic, styrene-butadiene, and cellulose acetate thermoplastic resins
form films by solvent evaporation.
2. Conversion. As solvents evaporate, films are formed that are cured
by oxidation, catalysis, or crosslinking. Thermosetting coatings
crosslink to form films at room temperature by catalysis or oxidation. The addition of catalysts will accelerate the oxidative curing
of drying oils and oil-modified resins. Monomeric materials can
form films and cure by crosslinking with polymers in the presence of catalysts, as in the case of styrene monomers and polyester
resins. Epoxy resins will crosslink with polyamine resins to form
films and cure. Airborne moisture starts a reaction in the vehicle
of polyurethane resin coating systems, resulting in film formation
and cure.
3. Coalescing. Emulsion or latex coatings, such as styrene-butadiene,
acrylic ester, and vinyl acetate acrylic, form films by coalescing and
dry by solvent evaporation.
7.7.3.2 Baking
Coatings requiring baking will form films at room temperature but require
elevated temperatures of 300 to 372°F (150 to 200°C) to cure. Curing is either
by conversion or phase change.
Conversion.
•
Heating accelerates the cure of many oxidative thermosetting coatings. In resin systems such as thermosetting acrylics and alkyd melamines, the reactions will not take place below a
temperature of 275°F (135°C). Coatings that require heat for curing
are generally tougher than air-drying coatings. In some cases, the
cured films are so hard that they must be modified with other
resins.
Phase change.
•
Polyolefins, waxes, and polyamides are thermoplastic
coatings that form films by phase changes — generally from solid to
liquid, then back to solid. Plastisols and organosols undergo phase
changes during film formation. Fluidized-bed powder coatings, both
thermoplastic and thermosetting, also undergo changes during film
formation and cure.
239
7.7.3.1 Air-Drying
Air-drying coatings cure at room temperatures, forming films. The films are
formed by one of three mechanisms:
1. Solvent evaporation. Shellac and lacquers such as nitrocellulose,
acrylic, styrene-butadiene, and cellulose acetate thermoplastic resins
form films by solvent evaporation.
2. Conversion. As solvents evaporate, films are formed that are cured
by oxidation, catalysis, or crosslinking. Thermosetting coatings
crosslink to form films at room temperature by catalysis or oxidation. The addition of catalysts will accelerate the oxidative curing
of drying oils and oil-modified resins. Monomeric materials can
form films and cure by crosslinking with polymers in the presence of catalysts, as in the case of styrene monomers and polyester
resins. Epoxy resins will crosslink with polyamine resins to form
films and cure. Airborne moisture starts a reaction in the vehicle
of polyurethane resin coating systems, resulting in film formation
and cure.
3. Coalescing. Emulsion or latex coatings, such as styrene-butadiene,
acrylic ester, and vinyl acetate acrylic, form films by coalescing and
dry by solvent evaporation.
7.7.3.2 Baking
Coatings requiring baking will form films at room temperature but require
elevated temperatures of 300 to 372°F (150 to 200°C) to cure. Curing is either
by conversion or phase change.
Conversion.
•
Heating accelerates the cure of many oxidative thermosetting coatings. In resin systems such as thermosetting acrylics and alkyd melamines, the reactions will not take place below a
temperature of 275°F (135°C). Coatings that require heat for curing
are generally tougher than air-drying coatings. In some cases, the
cured films are so hard that they must be modified with other
resins.
Phase change.
•
Polyolefins, waxes, and polyamides are thermoplastic
coatings that form films by phase changes — generally from solid to
liquid, then back to solid. Plastisols and organosols undergo phase
changes during film formation. Fluidized-bed powder coatings, both
thermoplastic and thermosetting, also undergo changes during film
formation and cure.