Contact electrification phenomena and powder surface treatments

Abstract

Powder surface treatments and/or modifications are reviewed in relation to contact electrification between dissimilar materials. The cascade and blow-off methods are shown as representative measuring techniques for contact charging. The origin of contact electrification is described from a recent viewpoint of acid-base properties for both inorganic and organic materials. Examples of particle surface modification with the adhesion of smaller particles are given in powder-substrate, powder-powder and molecule-powder systems. Powders presented include toners for electrophotography, phosphors for cathode ray tubes and fluorescent lamps, white and black pigments, barium ferrites, and colloidal silica. It is shown that (a) the adhesion strength between dissimilar materials is governed by the difference in their charging tendency; (b) surfaces can be modified reasonably when the materials are selected properly, according to their charging tendency; and (c) the degree of modification can be evaluated by measuring the change in the charging tendency of the surfaces. The significance of the surface modification in practical applications is also described in such properties as adherence strength to a substrate, homogeneity in powder mixtures and dispersion in liquids.