Effect of ambient relative humidity on charge decay properties of polymer powder and on the occurrence of back corona in powder coating

Back corona on a highly resistive polymer powder layer deposited via the electrostatic powder coating process using corona guns has a strong influence on the texture of the cured powder film often evidenced by orange peel, micro-dents and pinholes. The surface resistivity of the sprayed polymer powder governs the charge decay process, and therefore the onset of back corona as the thickness of the powder layer increases. The polymer powders generally used in powder coating are highly insulative, with surface resistivity often greater than 10/sup 11/ /spl Omega//. Surface resistivity can be altered by the adsorption of moisture on the surfaces of the powder particles. By changing the relative humidity (RH) of the ambient air during the powder spraying process, it was observed that the surface resistivity could be varied by orders of magnitude. For example, the surface resistivity for an acrylic powder decreased from 1.77/spl times/10/sup 15/ /spl Omega// at 25% RH to 5.7/spl times/10/sup 13/ /spl Omega// at 70% RH. The effects of variation of REI on the texture of the powder surface layer and on the cured film are presented for an acrylic polymer powder. The film texture was characterized by microstructural surface analysis using an optical scanning instrument. Methods for decreasing surface resistivity by using hygroscopic surfactants or by plasma treatment are briefly discussed.