Four Steps to Effective Pigment Dispersions
August 2017 www.coatingsworld.com Coatings World | 47
Amount of Surfactant Selection
Too much or too little of a surfactant can be detrimental to the
stability of a pigment dispersion. The determination of the optimal level is based on the rule of 2-2.5 mg of polymeric dispersant
needed per square meter of pigment surface (Figure 5). This rule
must be applied initially, after which the formulator is advised to
do a ladder study to determine the optimal level based on viscosity changes, the occurrence of flooding and floating, oven stability
and freeze-thaw stability evaluations when necessary.
Dispersing Agent Selection
The recommended type of dispersant adhesion group depends
on the pigment surface. The following suggestions may help formulators select the most effective dispersant chemistry for the
pigment being evaluated.
• Organic pigments (aromatic surface treatment) - Dispersants
containing phenyl or naphthyl groups are recommended.
•Inorganic pigments (oxides, sulfides, silicates, etc.) -
Dispersants containing acidic groups, i.e, phosphate, car-boxy, or sulfate are recommended.
• Carbon blacks (diazonium surface treatment) - Dispersants
containing nitrogen.
Pigment Dispersibility – Co-Grinding vs.
Single Pigment Dispersion
Pigment chemistries vary in texture and the amount of energy
required to achieve full color development will differ from one to
another. A good way to determine the pigment’s optimal dispersion time is by doing a dispersibility study in which the formulator will determine the time and energy needed to develop the
pigment to its full strength by evaluating its coloristic properties.
Figure 6 shows the dispersibility of selected PB 15:2, PG 7, PY 74
and PY 65 pigments. These pigments were evaluated for strength
and color in 30 minute intervals, where the dispersion made with
15 minutes of grind time was used as the standard. Pigments were
ground beyond their optimal level which can be confirmed when
a loss of color strength is observed. This type of evaluation is important in determining the optimal processing time for a pigment
in a selected formulation. As can be seen in Figure 6, some pigments develop their strength and color faster than others. This is a
key factor to consider when co-grinding pigments as it may not be
possible to achieve optimum color development and consistency
by this means. It is for this reason that single pigment dispersions
are recommended as the best means of obtaining the full color
development value of a given pigment.
Additionally, not all pigments have the same behavior in a specific coatings formula due to variations in their physical (i.e., surface area, oil absorption, particle size) and chemical properties (i.e.,
chemical structure, surface treatment) as shown in Table 3. It should
be noted that there is no direct correlation between the physical
Figure 5. Selecting the amount of surfactant.
Figure 6. Dispersibility study.
