Experimental and Multi-Scale Modelling to Understand Electrostatic Behavior of Pharmaceutical Powders

In pharmaceutical formulation processes, particle charging is often a nuisance and can cause problems in the manufacture of products, such as affecting powder flow, and reducing fill and dose uniformity. For a fundamental understanding of the powder triboelectrification, it is essential to study charge transfer under well-defined conditions. Hence all experiments were conducted in a humidity controlled glove box at 20%rh.  To understand tribocharging materials, different surface viz. PVC, aluminum, Teflon, PMMA were used along with two pharmaceutical excipients and two API’s. The experiments were performed in V-blender at a prescribed speed of 13 rpm. A triboelectric series of pharmaceutical materials was generated. For the pharmaceutical materials the work function was determined using MOPAC. MOPAC is a general-purpose semi-empirical molecular orbital package for the study of solid state and molecular structures and reactions providing band gap and ionization potential of molecules. In addition a discrete element method (DEM), including charge transfer and screened coulombic forces was developed for the V-blender system. There are different competing mechanisms involved in tribocharging. The two most important ones are electron transfer and ion transfer. Electron transfer has been well established for metal-metal and to some extent for insulator-metal contacts. There is still an ongoing debate for insulator-insulator charging whether the charge carriers are electrons or ions. As per work function hypothesis or established tribo-electric series powders should charge negatively against Alumimium, PMMA surfaces and positively against Teflon surfaces and PVC surfaces. According to the experiments in V-blender both the excipients (lactose and MCC) and API’s charged negatively against PMMA and Aluminum and positively against Teflon and PVC surfaces.