MIL-HDBK-1110
(1) The base component is mixed to disperse
settled pigment. If necessary, some of the thin, upper portion
may be poured off before stirring to make it easier to disperse
the pigment. When the upper portion is poured off, it must be
mixed back with the bottom portion before the two components are
mixed together.
(2) While continuing to stir, the two components
are slowly mixed together. No more than a few gallons should be
mixed at a time, or no more than that specified by the coating
manufacturer, since heat is usually generated upon mixing because
of the chemical cross-linking reaction. Excessive heat may lead
to premature curing of the coating, reducing the pot life.
(3) The two combined parts are agitated until they
are of smooth consistency and of uniform color. (Often the color
of the two components is different.)
b) Induction. Some two-component paints must stand
for approximately 30 minutes after mixing before application.
This time is called the induction time. During induction, the
chemical reaction proceeds to such an extent that the paint can
be successfully applied. However, some formulations of two-
component paints do not require any induction time and can be
applied immediately after mixing the two components. Material
specifications and manufacturer's recommendations must be
followed carefully. Induction time will depend on temperature of
the paint.
c) Pot Life. Pot life is the time interval after
mixing in which a two-component paint can be satisfactorily
applied. Paints low in VOC content often have a reduced pot
life. The chemical reaction that occurs when two component
paints are mixed accelerates with increasing temperature. Thus,
a paint's pot life decreases as the temperature increases. Above
90 degrees F, the pot life can be very short. (Curing time of
the applied coating is also faster at higher temperatures.) Pot
life is also affected by the size of the batch mixed, because the
chemical reaction produces heat. The larger the batch, the more
the heat produced and the faster the curing reaction proceeds.
Thus, the shorter the pot life. Paint must be applied within the
pot life. The coating manufacturer's recommendations must be
followed carefully. Mixed two-component paint remaining at the
end of a shift cannot be reused and must be discarded. Lines,
spray pots, and spray guns must be cleaned during the pot life of
the paint.
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