Four Steps to Effective Pigment Dispersions
August 2017 www.coatingsworld.com Coatings World | 45
in solvents. Due to their stable chemical structures, most inorganic pigments have better weatherability, dispersibility and
opacity than organic pigments, however they will typically have
lower chromaticity and tinctorial strength.
Pigment Impurities
Pigments with the same color index identification produced by
different manufacturers or different manufacturing processes
could potentially have different performance in spite of being
coloristically similar. This is due to impurities which can be
the result of unreacted raw materials, by-products, inorganic
salts, and contaminants in the water. These impurities may
cause adverse effects on weatherfastness, dispersion stability,
and solvent fastness.
In order to provide assurance that pigments do not have
excessive impurities, the pigment manufacturer should test for
pH, conductivity, oil absorption, and viscosity in specific testing systems. They may even consider doing additional viscosity stability and dispersibility testing for certain pigments as
needed. In addition to approving a pigment for color, coatings
formulators should always verify the stability of the final coating in determining the proper pigment or pigments for a given
system or application.
Factors Affecting Color Development
Pigment color development is dependent on five variables:
dispersibility, energy input, process dwell time, system
components, and pigment interactions. Paying proper atten-
tion to these variables will provide the highest probability of
developing a stable formula.
Dispersibility
Pigments must be properly wetted, de-agglomerated/dis-persed and uniformly distributed and stabilized (Figure 3) in
order to achieve maximum color intensity, gloss, and hiding
power. Stabilization of a pigment dispersion requires time
and energy. Dispersed pigments have a strong tendency to
return to their initial agglomerated state. Due to this strong
tendency, proper selection of wetting and dispersing agents
is critical in obtaining a stable formula. Surfactants or wetting additives are normally defined as amphiphilic chemistries with low molecular weight while dispersing additives
are oligomers able to stabilize the pigments and avoid re-agglomeration. The different types of wetting and dispersing
agents are described in Table 1.
Wetting
To properly wet a pigment, the air/solid interface needs to be
replaced by a liquid/solid one. Therefore, the less air entrapped
in the system, the more effective the wetting will be. It’s all
about surface tension! For a liquid to wet a solid, its surface
tension must be lower than the free surface energy of the solid.
Therefore, liquids with low surface tension are more effective at
wetting and this is why wetting additives are so valuable to the
formulator. They will reduce the surface tension and will adhere
to the surface and coat the pigment to create an additive/liquid
interface. Generally, solvent-based systems more readily wet a
pigment due to the lower surface tension of solvents when compared to the higher surface tension of water (Table 2).
Figure 1. Solubility of PY 74 in different solvents.
Figure 2. Pigment selection criteria.
Table 1. Wetting and dispersing agents – chemical types.
Figure 3. Steps for a stable pigment dispersion.
