Bipolar charging of poly-disperse polymer powders in fluidized beds

It is well known that many common industrial powder handling operations such as pneumatic transport, mixing, and fluidization may produce a net charge on the powder particles. However, it is less well known that the net charge is often the result of a bipolar charge distribution in which the smaller particles acquire charges of a polarity, opposite to those on the larger. This suggests contact charging between particles having the same chemical makeup. Very little quantitative data exist in the literature concerning this observation and no acceptable explanation currently exists. The purpose of this paper is to review the previous published work and to describe some results showing bipolar charging using polymer powders in fluidized beds. A new measurement system is described for measuring the bipolar charge distribution. This consists of a vertical array of seven Faraday pail sensors, which can selectively detect different charge components based upon particle size (gravity segregation) and charge (space-charge repulsion). For the experiments reported here the charge and mass values were measured for each sensor allowing the calculation of charge-to-mass ratio (Q/M). In addition, size distribution and surface analyses were carried out for representative samples of the powder components. Data are presented for several types of polymer powders (surface area, mean diameter < 100 mum). The results show that, for a given powder, even though the net charge may be positive or negative, the smaller particles show a negative charge and the coarser particles positive. These results are compared under several possible hypotheses. Each of these possibilities is examined using the measurements of the Q/M for different size fractions and the results of surface analysis and particle size distributions of these fractions.