It may seem strange to link sustainability with pigment dispersion, but an important strategy for improving coatings sustainability is to make higher quality coatings that last longer and to make them more efficiently. Increasing the effectiveness of pigment dispersion saves time and money, improves appearance, reduces the frequency of surface defects, cuts down on pigment usage, and is likely to give greater durability. The initial aim is to produce pigment particles of the optimum size for the type of paint, whether it is a primer or a topcoat, high gloss or low gloss, transparent, or needing excellent hiding. At the same time, pigment flocculation must be kept at a low level to guard against defects such as poor gloss, poor leveling, flooding and floating, Bénard cells, and poor color development. Of course, we want to do this using a minimum of energy and time. However, we should not stop there. The letdown process also is important. I have seen too many examples of excellent dispersions being spoiled by letdown processes where the one component was added to the other too rapidly, or there was a large temperature and/or viscosity difference between the paste and letdown.
How can we get more effective pigment dispersion and letdown? Let us begin with dispersion. Many readers will know that the dispersing of pigment involves three steps: wetting, deagglomeration, and stabilization. Good wetting depends on the dispersing medium (solvent, resin solution, or surfactant dispersion) being compatible with and having a lower surface tension than the pigments. Deagglomeration is done by shearing the pigment-dispersant mixture. The process usually is not difficult, but attention must be paid to the design of the mill base, particularly in terms of viscosity. Too low a viscosity results in little or no shearing and a poor dispersion. Too high a viscosity leads to the mill heating up and can cause kick-out of the paste and clogging of the mill. High solids paints can be a problem because the formulator is allowed so little solvent to work with that it may not be possible to truly optimize the mill base.
The key step is to stabilize 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 to stabilize the particles (to do that they must stay on the pigment—no matter what). Dispersants adsorb on the particles and either impart a charge to the surface so that particles repel each other (charge stabilization—waterborne pastes and paints) or build up an adsorbed layer to repel other particles (steric stabilization—solventborne systems). A wide range of dispersants for solventborne and waterborne coatings are described in the literature.
The letdown process is deceptively simple. You either stir paste into the letdown solution or dispersion or the letdown is stirred into the paste. Occasionally, the two components are not compatible with each other, and they separate into two phases instead of mixing. I recall working on a dispersion problem where I wanted to observe letdown effects under the microscope. I put a small drop each of paste and letdown next to each other on a microscope slide and placed a cover slip over them. Normally that would result in mixing. In this case, they immediately ran away from each other, ending up on opposite sides of the drop like girls and boys at a 7th grade dance. It was quite a shock to see it. Solvent changes had to be made in the mill base vehicle and letdown to make them more compatible.
A letdown solvent blend can cause another problem if it is such a good solvent for the dispersing resin that it strips it from the pigment surface resulting in flocculation. The result can be a wrong color on application due to floating of one pigment to the surface. For example, a medium blue coating spray applied as a light blue. Rub-up on the wet surface gave the correct color; light microscopy showed pigment flocculation, particularly of the blue. Much of the white pigment was tending to float to the top of the film. Initial attempts to solve the problem produced dark blue (blue float) or even lighter blue (white float), but not the right color. Eventually, measuring the solubility parameters of the dispersing and film forming resins, identifying solvents and nonsolvents for each one, and working up a letdown solvent package that did not remove dispersing resin from either of the pigments solved the problem.
The technique that I use for characterizing the quality of pigment dispersions in terms of particle size, size distribution, and degree of flocculation is optical microscopy at 200–500x with transmitted light and dark field. Pastes can be examined directly but are difficult to evaluate because there are so many particles jammed together. Pastes can be diluted for viewing (dilution with the dispersant vehicle is highly recommended), but if the diluted specimen turns out to be flocculated, it is not possible to tell whether the dilution caused the flocculation or if that is the true state of the paste. It is more useful to view a paint made with that paste since the quality of the dispersion in the paint rather than the paste is the key to performance anyway. For more on optical microscopy, see JCT CoatingsTech, 4 (8) 72, (2007).
Good luck with upgrading pigment dispersion to improve sustainability!
CoatingsTech | Vol. 16, No. 4 | April 2019