Effects of electrostatic charge on particle adhesion, powder cohesiveness and its alternative influences on powder flow properties

Characterising powder flowability for handling process is important but can be particularly challenging if only a small quantity of samples is available. A novel method developed at the Wolfson Centre uses only a few milligrams of samples to predict powder flow properties by Bond number - a representation of powder cohesiveness at the median size of particles by measuring particle adhesion. A good agreement between this method and the results using conventional shear cell testers has been found across various powders and formulations. However, recent investigations on acetaminophens revealed a discrepancy: predictions based on the Bond number did not align with the shear cell test results, suggesting the presence of additional contributing forces during the Bond number measurement. As the Bond number is determined by assessing particle adhesion, it was hypothesised that electrostatic forces could influence the adhesion results and therefore the Bond number. This study focused on the electrostatic charge measurements of two grades of acetaminophen (dense and micronised) with differing particle sizes. For a comparison, common excipients such as lactose, magnesium stearate, and calcium carbonate were also evaluated, all of which previously exhibited good predictive correlations. Results show that acetaminophen samples exhibited charge levels up to 20 times higher than the excipient materials. It is inferred that electrostatic forces can strongly influence particle adhesion, if charge is significant, though their effect appears negligible in shear cell testing. The study concludes that electrostatic forces can significantly contribute to particle adhesion and impact powder flow behaviour particularly at low consolidation stresses.