Insolubility
A pigment must be insoluble in the vehicle (the medium in which it is dispersed),
and it must not react with any of the components of the paint, such as crosslinking
agents. Pigments are required to retain these properties even when the paint is
being dried, which is frequently carried out at elevated temperatures. Once in the
dried film, the pigment must also remain unaffected by the substrate and to agents
with which it comes into contact, including water, which may simply be in the form
of condensation, or acidic industrial atmospheres.
Under certain conditions, pigments may dissolve, leading to application problems.
Organic pigments may dissolve to a limited extent in organic solvents, and inorganic
pigments may be affected by other components. Solubility of a pigment generates
the following problems:
Blooming
If the pigment dissolves in the solvent, as the paint dries, the solvent comes to
the surface and evaporates, leaving crystals of the pigment on the surface in the
form of a fine powder. As solubility increases with temperature, this phenomenon
is made worse at elevated temperatures.
Plate out
The effect of plate out looks similar to blooming, but occurs in plastics and powder
coatings. However, it is not due to the pigment dissolving, but rather to the surface
of the pigment not being properly wetted out. It usually occurs mainly with complex
pigments and once wiped from the surface does not reappear.
Bleeding
Pigments in a dried paint film may dissolve in the solvent contained in a new coat
of paint applied on top of the original film. If the topcoat is a different color,
particularly a white or pale color, the result can be disastrous. Again elevated
temperatures exacerbate the problem.
Recrystallization
This phenomenon was almost unknown until the introduction of beadmills. During the
milling stage, heat is generated, which dissolves a portion of the pigment. Over
a period of time, the dissolved "pigment" starts to precipitate out, losing
brilliance and color strength. This becomes especially noticeable in the case of
paints containing two differently colored pigments that have different solubility
characteristics. The more soluble pigment dissolves and then as it comes out of
solution and precipitates, the paint will take the shade of the second pigment.
Recrystallization can even take place in aqueous systems. It can be avoided by using
less soluble pigments and/or by controlling the temperature during the dispersion
process.
