Triboelectric charging of polymer powders in fluidization and transport processes

Steady flow of powder at a desired rate is a necessity for controlling thickness and uniformity of the deposited powder layer in electrostatic spray painting. In most powder coating applications, the polymer powder is fluidized to transport the powder to the spray gun using a powder pump. The powder delivery tube is often long; sometimes in excess of 10 m. During fluidization and transport, the powder particles acquire electrostatic charge due to triboelectrification. This tribocharging of powder can present some problems: (1) agglomeration of powder due to bipolar charging, (2) deposition of charged powder on the inner wall of the transport tube, and (3) biasing of the final charge-to-mass ratio (q/m) distribution of powder exiting the corona gun. In this study, we investigate the role of tribocharging in fluidization, flowability, and q/m distribution as functions of particle size distribution (PSD), fluidization time and transport tubes of different materials. A particle size distribution shift due to the polarity of the bipolarly charged particles is shown. A brief discussion is presented on minimizing the adverse effects due to powder tribocharging, therefore improving spray uniformity.