I discussed pigment dispersion and its effect on defects and appearance in the September 2006 issue of JCT CoatingsTech. However, this article is concerned more with the dispersion process itself and how to improve it. Pigment dispersion may appear to be a simple operation, but actually is a complicated one. The process consists of three steps: wetting, deagglomeration, and stabilization. Neither wetting nor deagglomeration is very difficult. What is difficult is stabilizing the pigment properly so that it does not flocculate in the paste, on letdown, or when the paint is shipped and stored. Dispersants are used to help with wetting and deagglomeration, but their main purpose is stabilization. The dispersant adsorbs on the pigment surface and either imparts a charge to it so that particles repel each other (charge stabilization in waterborne pastes and paints) or builds up an adsorbed layer to repel other particles (steric stabilization in solventborne pastes and paints). The letdown can be thought of as a fourth step, one that can undo all the good work in the first three. An excellent dispersion can be ruined by poor letdown procedures or by letdown solvents that strip the dispersant from the pigment.
All dispersion steps are helped by shearing, which separates particles and opens fresh areas to wetting and adsorption adsorption, adhesion of the molecules of liquids, gases, and dissolved substances to the surfaces of solids, as opposed to absorption, in which the molecules actually enter the absorbing medium (see adhesion and cohesion). . The degree of shear depends on the equipment used, the amount of energy applied, and the viscosity of the pigment paste. If the viscosity is too low, there will be little or no shearing action; if it is too high, there will be a lot of shear, but also high energy use, high temperature, and possibly destabilization and jamming of the mill. The viscosity will depend on the pigment itself, its concentration in the paste, and how well it is wet. Shearing and viscosity control also are necessary for effective letdown and mixing.
Although pigment dispersions are developed in the lab, what counts is how they behave in the plant and the quality of the paint that they produce. Ease and efficiency of dispersion, paste quality, and letdown effectiveness may differ considerably between the lab and the paint plant. It does not help that formulators generally do not understand the manufacturing process and its requirements. Fortunately, training and good communication between the lab and plant can alleviate these problems.
Some dispersion and letdown recommendations:
1. Optimize the total paint, not just the paste. It does no good to have a pigment paste that is easy to manufacture and well-dispersed, but flocculates on letdown or otherwise hurts paint properties.
2. Use dispersing resins, but minimize the amounts. Dispersing resins tend to degrade the physical and mechanical properties of coatings.
3. Optimize the order of addition to give the thickest and most stable adsorption layer. Make sure that the dispersing resin contacts the pigment before the other components do.
4. For solventborne systems, use solvents with less affinity for the pigment surface than for the dispersing resin (want good solvent for resin, poor solvent for pigment). There is a simple test for the affinity/compatibility of solvents for/with pigments: add a small amount of pigment to the surface of solvent in a beaker or graduated cylinder and observe what happens. If the pigment floats, then it is not wet by the solvent and is not very compatible with it. If the pigment sinks, then there must be some wetting and affinity. That solvent may displace the dispersing resin from the surface of the pigment.
Paste Mixing and Letdown
1. Reduce the gel strength of highly thixotropic or gel-like pastes by high speed agitation and/or dilution with vehicle before letting down.
2. Equalize the vehicle solids of the first portion of the letdown and the pigment paste as closely as possible.
3. Add the initial letdown portion slowly and steadily to the paste as the latter is being agitated
4. Keep temperatures and viscosities of pastes and letdown solutions as close as possible.
5. Equalize the pH values and surface tensions of waterborne pastes and their letdowns as closely as possible.
6. Use high shear post-mixing.
By Clifford K. Schoff, Schoff Associates