Pigment dispersion and its effect on defects and appearance.

The quality of the pigment dispersion in a paint can have a considerable effect on the appearance of the resultant coating and the frequency of defects. Partially dispersed pigment often appears as seeds and/or gives a rough, low gloss surface. The color may be different from what is expected. In other cases, the pigment may be well dispersed initially, but become flocculated on further grinding, during storage of the paste, on letdown, or on storage as the finished paint. Flocculation can contribute to a number of defects, including poor gloss, poor leveling, flooding and floating, Benard cells, and poor color development (see the March 2006 issue of JCT COATINGSTECH for more information on flocculation).

The dispersing of pigment involves three steps: wetting, deagglomeration, and stabilization. Good wetting depends on the dispersing medium (solvent, resin solution, or surfactant dispersion) having a lower surface tension than that of the pigments, but surprisingly good deagglomeration can be done by brute force shearing even if wetting is not good. Deagglomeration is not difficult. What is difficult 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. We use dispersants 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 pastes and paints).

Most discussions of pigment dispersion assume single pigment grinds, but composite grinds containing several pigments are common and can save time and money. They also can be very frustrating because they may produce well dispersed and badly dispersed pigments in the same paste. The dispersion process often must be done in stages with difficult pigments added at the beginning and easily dispersed ones added toward the end.

Materials such as clays, silicas and other extenders, and thixotropes that are used to raise low shear viscosity sometimes suffer a problem where they are wet and dispersed so well that they are unable to form the network structures needed to provide the high viscosity. Early in my career I learned the hard way that, although a super-dispersed master batch of fumed silica was very easy to stir into the paint and gave unusually good gloss, it was a failure because it had little ability to prevent sag.

Although I have never been able to prove the connection, over the years I have seen a number of cases where paint with flocculated pigment or seeds showed solvent popping, but a better dispersed batch did not. In a few cases, the popping disappeared after the paint was sheared to break up some of the clumps or chunks. We theorized that the clumps of pigment trapped solvent that did not release immediately, but blew out after the film had formed. A somewhat similar thing has been seen with cratering, both in the lab and in production. Running a batch that craters through the mill again often stops the cratering. However, the disappearance of the craters probably involves the absorbance of surface active components or contaminants onto the pigment during the extra milling rather than the breaking up of clumps.

The best technique for characterizing the quality of pigment dispersions is optical microscopy at 200-400x with transmitted light and dark field. Pastes can be examined, but are difficult to evaluate because there are so many particles jammed together (as shown in Figure 1). It is more useful to view the paint since the quality of the dispersion in the paint rather than the paste is the key to performance. Figure 2 shows the paint made from the paste in Figure 1. (For more on optical microscopy, see Coatings Clinic in the February 2006 issue of JCT COATINGSTECH.)

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"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].