Adhesion of charged powders to a metal surface in the powder coating process

Abstract

Substrate-particle adhesion of electrostatically charged, nonconducting particles deposited on electrically grounded substrates is discussed. Glass microspheres of diameters ranging from 25.5-74.1 /spl mu/m, charged by corona and tribe-charging, were deposited in a monolayer on conducting stannic-oxide coated surfaces of glass plates (NESA). The total force of adhesion due to 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 0.013 m. The adhesive force was measured as a function of particle size and charge. The net average charge on the particles was measured using a Faraday cup. The experimental values agree well with the calculated force of adhesion for a single layer deposition. The charge decay of the particles was studied using a noncontact electrostatic voltmeter. The charge relaxation time of the deposited powder was found to increase with time. A physical model of the adhesion of charged powder paints deposited on a grounded metal substrate is presented. The role of the forces acting on a spherical polymer particle deposited on the surface of a uniform coating of powder paint is investigated as a function of particle diameter and charge. The particles are assumed to be unipolarly charged and deposited uniformly on the substrate. The relative magnitudes of the electrostatic attractive and repulsive forces are analyzed as functions of powder film thickness and particle size.