Tribo-electrification of powders due to dispersion

Abstract Powder processing operations can give rise to tribo-electrification of particles. The resulting electrostatic force can cause segregation/agglomeration of particles as well as their adhesion to the walls of the processing equipment, adversely affecting process operations and the end quality of the product. Current methods for evaluation of the tribo-electric charging of powders are unsuitable for testing small quantities of sample. Moreover, limited work has been reported on the tribo-electrification of powders due to aerodynamic dispersion, which is an area of great interest, e.g. in particle characterisation processes and dry powder inhalers. A simple methodology is reported here using a commercially available dispersion unit to investigate the tribo-electric charging of powders. A small quantity of powder, typically a few milligrammes, is sandwiched between two thin metal foils in the dispersion unit. An air pressure pulse is applied to burst the foils and the rapid penetration of air through the packed bed of powder disperses it. The unit is mounted on a conventional Faraday cup, connected to an electrometer to measure the net charge transfer following powder dispersion. Tribo-electric charging of model particles (glass beads) is reported here. The results show that the tribo-electric charging of glass beads is independent of the pressure used within the range tested, i.e. 150–600 kPa (0.5–5 barg) for a single thickness of aluminium foil (6 μm) and the charge transfer is linearly proportional to the surface area of the beads. The surface materials of the dispersion device and the bursting foils used to sandwich the powder affect the transferred charge. The methodology developed in this work has the potential to be used to characterise tribo-electric charging of small quantities of powders. The operating procedure is less time consuming as compared to other current charge characterisation devices. The methodology is potentially suitable for testing active pharmaceutical ingredients (APIs), which are sparse in the early development stages.