Quantitative Determination of Particle Dispersion in a Paint Film
Degree of particle dispersion in a liquid paint or dry paint film affects practically every important property of these 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). We detail a technique, based on electron microscopy and image analysis, of quantifying degree of TiO2 dispersion in a paint film. Results clearly indicate flocculation in some paints and, more importantly, put an upper limit on the potential for improvement of pigment dispersion.
Measuring the degree of particle dispersion in a paint film is problematic. Since poor dispersion results in poor hiding and low tint strength, these two measurements are often used as a proxy for degree of dispersion. This approach has the advantages that (a) it directly measures the attribute of interest (coatings users are interested in hiding power, not particle distribution per se), and (b) it is relatively easy to do. In addition, correlations can be drawn between dispersion parameters (energy, ingredients, equipment, etc.) and degree of dispersion, and empirical relationships developed that allow for performance optimization and for the development of theories explaining dispersion behavior. However, poor dispersion is possible by a number of mechanisms (poor initial dispersion, flocculation on drying, crowding due to other solid particles, etc.), and a detailed understanding of the state of dispersion is not possible through hiding power measurements alone. Instead, microscopic examination of the paint film is required.