Quantitative Determination of Particle Dispersion in a Paint Film

Steven De Backer, DuPont Titanium Technologies, Belgium

Being a very complex mixture of a lot of ingredients, the hiding power of a paint is an important but complex phenomenon. Dupont found a way to describe the 8 fundamental factors influencing the opacity of a coating. An experimental protocol is described to determine each of these factors. In this paper the quantification of one of these eight fundamental factors is described: degree of particle dispersion. This parameter affects practically every important property of paint systems, including viscosity, touch-up, rub-up, scrub resistance, porosity, gloss, appearance of defects, and, for opacifying particles like TiO2, hiding power and tinting strength. Numerous methods have been devised to quantify degree of dispersion, but most are comparative rather than absolute, especially those that pertain to dispersion in the dry film (e. g., comparing hiding power to a standard) or give information about the large undispersed particles (eg hegmann gage) where we know that these have a small influence on total hiding power. We detail a technique where a picture taken with electron microscope is analysed with easy to obtain software (freeware). This image analysis allows to quantifying the degree of TiO2 dispersion in a paint film. Results clearly indicate flocculation in some paints and, more importantly, it helps the formulator to improve the state of dispersion and to optimize the paint formula.