Adhesion of charged powders on metal surface in powder coating process

Substrate-particle adhesion of electrostatically charged, nonconducting particles deposited on electrically grounded conducting substrates is examined. Glass microspheres of diameters ranging from 25.5 to 74.1 μm charged by corona and tribocharging were deposited, in a monolayer, on a conducting stannic-oxide coated surface of glass plates (NESA). The total adhesive force, (the vector sum of the electrostatic, van der Waals, and gravitational forces) was measured by observing the removal of particles by applying a known electric field between the particle coated surface and a clean surface of a second NESA glass, placed parallel to it at a distance of 0.0125 m. The charge decay rate or the effective relaxation time constant was found to increase with time. A physical model of the adhesion of charged powder paints deposited on a grounded conducting substrate is presented to analyze the role of the electrostatic and van der Waals forces on spherical polymer particles with uniform unipolar surface charge density, deposited in a multilayer film. The model shows that, for obtaining a desired film thickness in an electrostatic powder coating process, there is an optimum particle size distribution, and as film thickness increases, the accumulated surface charge prevents deposition of small particles, and the powder film continues to increase allowing deposition of particles of successively larger size