Efficient and effective
pigment dispersion is necessary in order to obtain optimum tinctorial strength,
cleanliness of shade and good gloss from the final coating. High quality coatings
of high brilliance and color strength are characterized by a perfect pigment dispersion,
optimal pigment particle size and long-term stabilization of the dispersed particle
in the formulation. Most organic pigments show better transparency as dispersion
improves, while in the case of the larger particle size inorganic pigments, opacity
is improved by good dispersion.
The dispersion process consists of the permanent breaking down of agglomerates into,
as far as possible, primary particles.
There are four aspects to the dispersion process:
- Deagglomeration is the breaking down of the agglomerates and agregates by
the shear forces of the equipment being employed. A mixture of crushing action and
mechanical shearing force is necessary.
- Wetting out occurs at the surface of a pigment when a binder (or surface
active agent) sticks to the pigment's surface and acts as a connection between the
pigment and the binder. The air and moisture are displaced from the surface. Between
the particles of the pigment aggregates, agglomerates are replaced by the resin
Wetting out time depends on the viscosity. Heat produced by the mechanical shearing
process causes the temperature of the mixture to rise, thus helping the wetting
out process. This increase in temperature reduces the viscosity as well as the effectiveness
of deagglomeration which is a well known phenomenon.
- Distribution demands the pigment to be equally dispersed throughout the binder
system. A lower viscosity tends to lead to a more even pigment distribution.
- Stabilization prevents the pigments from re-agglomerating. The pigment dispersion
is stabilized by dispersing agents in order to prevent the formation of uncontrolled
flocculates. The resultant suspension is stabilized due to the adsorption of binder
species or molecules at the pigment surface.