Surface Charge Buildup During Electrostatic Spraying

In industrial electrostatic spray coating operations, liquid paint is applied to substrates of varying electrical characteristics. In many cases a metal substrate is chemically pretreated and/or primed prior to electrostatic spraying. A previously rejected part may have already received a number of thoroughly cured topcoats prior to the final spray application. Typical primers and topcoats have large surface resistivities (10 8 to 10 10 ohm.cm -2 ) and exhibit slow surface potential decay rates. A surface potential measurement system has been developed to determine the time dependent electrical properties of various substrates when coated with commercial water base and organic paint systems. It has been observed that during some electrostatic recoat operations, the charged paint particles appear to be repelled from the surface to be coated. It is found that this phenomenon can be predicted when conditions are such that adequate charge leakage paths to ground are not available due to high surface and bulk resistivities of the film resulting in a repelling charge and potential buildup on the surface. It has been found that the charge removal process cannot be described as a simple exponential decay, but as a superposition of three exponential decay processes each with its own decay constant τ.