Explaining electrostatic charging and flow of surface-modified acetaminophen powders as a function of relative humidity through surface energetics.

Powder flow involves particle-particle and particle-vessel contacts and separation resulting in electrostatic charging. This important phenomenon was studied for uncoated and dry-coated micronized acetaminophen (MAPAP) as a function of relative humidity. The main hypothesis is that by modifying powder surface energy via dry coating of MAPAP performed using magnetically assisted impaction coating, its charging tendency, flow can be controlled. The examination of the relationship between electrostatic charging, powder flow, and the surface energies of the powders revealed that an improvement in flow because of dry coating corresponded to a decrease in the charging of the particles. A general trend of reduction in both electrostatic charging and dispersive surface energy with dry coating and relative humidity were also observed, except that a divergent behavior was observed at higher relative humidities (≥55% RH). The uncoated powder was found to have strong electron acceptor characteristic as compared with the dry coated. The adhesion energy between the particles and the tubes used for the electrostatic charging qualitatively predicted the decreasing trend in electrostatic charging from plastic tubes to stainless steel. In summary, the surface energies of the powders and the vessel could explain the electrostatic charging behavior and charge reduction because of dry coating.