Electrostatic effects on first pass transfer efficiency in the application of powder coatings

Two important process conditions desired in most powder coating applications are: a high first pass transfer efficiency (FPTE) and uniformity of the powder layer covering the surface to be coated. Both of these desired outcomes are influenced by the electrostatic properties of powder and electrostatic parameters involved in the spraying process. An FPTE, greater than 90 percent may eliminate the need for recycling of the overspray in some applications, thereby, permitting fast color changes. Many industries are unable to switch from solvent based coatings to powder coatings because of the long color-change time required in current powder coating processes, To obtain a high FPTE, a clear understanding of the electrostatic phenomena involved in the process is desired, but the complexity of the process, particularly such conflicting requirements between high FPTE and strong adhesion of the powder layer, between high current density for charging and elimination of back corona require optimization of the process based on theoretical and experimental investigations. This paper describes relationships between FPTE and (1) ion current density and (2) particle size distribution (PSD). Theoretical relationships between process parameters and electrostatic effects and how process conditions can be optimized for a high FPTE and uniformity of the powder layer are discussed with experimental results. Possible empirical steps that can be used to optimize FPTE are presented.