ADHESION OF CHARGED POWDERS TO A METAL-SURFACE IN THE POWDER COATING PROCESS

Substrate-particle adhesion of electrostatically charged, nonconductin g particles deposited on electrically grounded conducting substrates i s discussed, Glass microspheres of diameters ranging from 25.5-74.1 mu m, charged by corona and tribe-charging, were deposited in a monolaye r on conducting stannic-oxide coated surfaces of glass plates (NESA(R) ). The total force of adhesion due to electrostatic, van der Waals, an d gravitational forces was measured by observing the removal of partic les by applying a known electric held between the particle coated surf ace 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 functio n 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, T he charge decay of the particles was studied using a noncontact electr ostatic voltmeter, The charge relaxation time of the deposited powder was found to increase with time, A physical model of the adhesion of c harged powder paints deposited on a grounded metal substrate is presen ted, The role of the forces acting on a spherical polymer particle dep osited on the surface of a uniform coating of powder paint is investig ated as a function of particle diameter and charge, The particles are assumed to be unipolarly charged and deposited uniformly on the substr ate, The relative magnitudes of the electrostatic attractive and repul sive forces are analyzed as functions of powder film thickness and par ticle size. The model shows that: 1) to obtain a desired film thicknes s in an electrostatic powder coating process, there is an optimum part icle size distribution and 2) as film thickness increases, the accumul ated surface and volume charge prevents the deposition of small partic les below a certain size, The powder film continues to increase in thi ckness, allowing deposition of particles of successively larger size, thereby making the pre-cured film coarse.