The effect of particle size on electrostatic powder coatings

In the electrostatic powder coating industry an increase in deposition efficiency and a controlled decrease in film thickness are constantly strived for. If adjustments to the size distribution of powders are made in order to improve the process in any way it is important that the excellent physical properties of the coating are not sacrificed. The aim of this study is to gain an understanding of the effects of particle size on the packing and final film properties of electrostatically sprayed coatings so that-recommendations can be made to increase the effectiveness of the process. Experimental studies involved the production of stoved sample coatings of various thicknesses obtained by spraying narrow size ranges of a commercial powder using a reproducible coating method. A testing programme was developed to assess the effect of particle size on the physical properties of the films produced. A three way comparison of physical property, size of sprayed powder and coating thickness has been compiled and results are discussed. It was observed that particle size had no significant physical effect on film properties. A method for determining a spreading factor, representative of the flow characteristics of a given powder, was developed. Observations from these stoving experiments suggested that the excellent flow properties of the powder used in earlier experiments accounted for the small variations in those results. However photomicrographs of typical coatings showed that large particles were dominant in the upper layers of packings. A loss of fines was identified from size analysis of various powders on coated plates compared to the original feed materials. Computer plots simulating the packing of particles on a substrate showed similar trends. Integration of the trajectory equation for individual particles, represented by means of a force balance, was carried out with consideration of all interparticulate electrostatic forces of the packed layer together with field and aerodynamic forces. Simulations of packings of monosized and size distributed powders sprayed at various transport air velocities show that, as a packing increases in thickness, various sizes behave differently. The results from these theoretical experiments give indications for the explanation of previous results.