(380d) Powder Permeability As a Measurement Surrogate for Triboelectric Charging

Triboelectric charging is common for many powders in solids handling and other processing applications, sometimes intentional as with laser printing but more often than not unintentional and a processing nuisance. Such particle charging can lead both to wall deposition and increased interparticle agglomeration with substantial increases in minimum bin opening sizes to prevent stable arching and ratholing.

Furthermore, the exact nature of triboelectric charging remains poorly understood, with severe limitations in readily made characterization to provide reliable engineering and process guidance. For example, powders may charge by surface interactions (traditional triboelectric charging) or by particle-particle interactions (self-charge by frictional rubbing). Generation of frictional charge can also lead to (i) a net bulk powder charge, (ii) distributions in particle net charge, or even (iii) surface distributions in charge across single particles. Net bulk charge is readily measured through transfer or Faraday cups; however, the latter two mechanisms involving charge distribution are more likely relevant to powder agglomeration, and far more difficult to assess.

Recent additive manufacturing research has shown that bed permeability is readily impacted by the extent of powder handling and related charging. Bulk powder permeability is the relationship between interstitial gas velocity and the resulting bed pressure drop as typically governed by Darcy’s law [1,2]. This paper presents a novel approach to using powder bed permeability as a rapid surrogate measurement to the extent of powder triboelectric charging. Permeability is shown to track with the extent of drying, humidity conditioning, and time of handling. Limited confirming measurements are also made by Faraday cup for varying processing time and transfer material, as well as by shear cell measurements of minimum opening size and friction.

References:

[1] Darcy, H. (1856). Les fontaines publiques de la ville de Dijon. Paris: Dalmont.

[2] Ennis, B.J. et al. (2008) Section 21: Solids-Solids Operations & Processing. Perry’s Chemical Engineers’ Handbook. Perry, R. H., & Green, D. W. (eds.), New York: McGraw-Hill.