An Efficient Approach to Dispersing Pigments
suitable for use in waterborne formulations with high dielectric charges. The
dispersing additive adsorbs onto the pigment surface and dissociates into an anionic and cationic part. This creates an
electric double layer that prevents pigments from flocculating together via an
electrostatic repulsion of like-charges. 3
In contrast to electrostatic stabilization, steric stabilization uses polymeric
side chains to keep pigment particles
stable in a dispersion. When pigment
particles come closer together, the polymeric side chains restrict their movement
and decrease entropy. 4 The outcome is
a repulsive force between the two particles. These interactions also restrict the
movement of the particles that give rise
to the viscosity.
In some cases, using only electrostatic
or only steric stabilization is not enough.
Pigment dispersions have complex demands, therefore it is sometimes necessary to combine the two stabilizations to
create electrosteric stabilization.
Wetting and Dispersing
Agents
The terms wetting agent, wetting additive, dispersant, or wetting and dispersing additive are often used without
full understanding of their exact definitions. There are very important differences in the chemical structures and
functions of each.
Wetting agents are low molecular
weight, amphiphilic molecules with a hydrophilic and hydrophobic segment. They
aid in reducing surface tension and wetting out surfaces, but most of the time,
pigment stabilization is not achieved.
Dispersing agents are oligomers or polymers that aid in the stabilization of pigments and fillers. At the core of every
dispersing agent, there must be a wetting
agent in order to facilitate the first step in
the dispersion process – wetting out the
pigment. The major difference between
these two technologies, is that dispersing
agents utilize anchor groups and polymeric side chains to stabilize pigments.
Polymeric Side Chains
Polymeric side chains aid in the adsorp-
tion of the dispersing agent onto the
pigment surface. The side chains must
be soluble in the medium. If they are not
soluble, they could collapse onto the pig-
ment surface causing flocculation.
Polymers of medium molecular
weight are optimal. If the mass is too
low, in the case of wetting agents, then
there might not be enough of an effect
to stabilize pigments. On the other hand,
if the mass is too large, then it could be
incompatible and cause viscosity to rise.
Anchor Groups
Anchor groups are placed at the end of
the polymeric chains to attach to the surface of the pigment. Without these anchor
groups, the polymeric side chains would
not be useful. Specific chemical groups are
Figure 5. Mechanisms of pigment stabilization.1
Table 1. Yellow iron oxide formulations.
Table 2. Pigment violet formulations.
