ADHESION MEASUREMENTS FOR ELECTROSTATIC POWDER COATINGS

The total force holding a layer of electrostatically charged powder on to a work piece results from a combination of effects; an interfacial electrochemical phenomenon at the surface, inter particle/particle coh esion and a bulk electrostatic attractive force. Attempts have been ma de to obtain the total integrated effect by assessing the force that i s required to selectively remove the powder layer from a metal substra te. 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 coat ed cylinder, (ii) measuring the mass of powder removed when a powder c oated sample is subject to an impulse from a drop test and (iii) selec tively removing powder under computer control with a series of micro a ir pulses. The results tend to be in agreement and from these differen t test procedures they assist in modelling the structure of a deposite d powder layer. Spray tests using different spraying systems indicate that the outer layer of the coating tends to have higher adhesive char acteristics when the specific charge (Q) is < 2 mu C/g. When the spec ific charge is > 3.5 mu C/g the adhesion of the outer layer is reduced .