Powder charging relative humidity

Mackin, L.A. Rowley, G. Fletcher, E.J. An investigation of carrier particle type, electrostatic charge and relative humidity on in-vitro drug deposition from dry powder inhaler formulations. Pharm. Sci. 1979, 3, 583-586. 

High relative humidity can reduce the resistivity of some powders and increase the rate of charge decay from bulk powder in grounded metal containers. However, in most cases this will only be effective if a relative humidity in excess of 65% is maintained. 

Fig. 2 is a schematic representation of the electric field lines from the gun to a part that has a protrusion at its center. In this case, the electric field lines will concentrate at the protrusion. The charged powder and free ions will build up faster here and contribute to back-ionization. Because the powder particles are strong dielectrics, their charge will be retained for many hours, depending on a number of factors such as powder resistivity, as well as the ambient relative humidity. 

Chi Lip Kwok P, Chan HK (2008) Effect of relative humidity on the electrostatic charge properties of dry powder inhaler aerosols. Pharm Res 25 (2) 277-288... 

Proteins in the body liquids may be considered as a colloidal electrolyte solute in a water solvent. Contact with water is the natural state of a protein. In more or less dry form, a protein powder loses some of its electrolytic character it loses the charged double layer on the surface and behaves electrically very differently from protein with water. Such materials may well be mixed conductors—electronic in the dry state and ionic with water content. Keratin is a more or less dry protein found in the natural state of no longer living biological materials such as hair, nails, and the stratum corneum. The water content of such materials is dependent on the relative humidity of the ambient air. The question of ionic or electronic conductivity in proteins is important, and an electronic conduction mechanism must be considered in many cases. 

Corona charging is very widely used due to its ability to charge a wide range of powders at fairly high throughput. This charging method is relatively insensitive to ambient humidity, plus the equipment is robust for a wide variety of manufacturing processes. 

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