Adhesion Measurements for Electrostatic Powder Coatings

Abstract

The total force holding a layer of electrostatically charged powder onto a work piece results from a combination of effects; an interfacial electrochemical phenomenon at the surface, inter particle/particle cohesion and a bulk electrostatic attractive force. Attempts have been made to obtain the total integrated effect by assessing the force that is required to selectively remove the powder layer from a metal substrate. This is a destructive measurement that disrupts the powder layer. All the measurements are undertaken on the powder layer before fusing. Powder adhesion measurements have been obtained from three different techniques consisting of (i) the centrifugal spinning of a powder coated cylinder, (ii) measuring the mass of powder removed when a powder coated sample is subject to an impulse from a drop test and (iii) selectively removing powder under computer control with a series of micro air pulses. The results tend to be in agreement and from these different test procedures they assist in modelling the structure of a deposited powder layer. Spray tests using different spraying systems indicate that the outer layer of the coating tends to have higher adhesive characteristics when the specific charge (Q ∗) is < 2 μC/g. When the specific charge is > 3.5 μC/g the adhesion of the outer layer is reduced.