Influence of moisture content on die filling of pharmaceutical powders

Flow performance of pharmaceutical powders dictates various manufacturing processes, such as blending, filling and tableting, and is determined by not only material properties but also environmental conditions, such as relative humidity and temperature. In this study, the effect of moisture content on flow performance of two commonly used pharmaceutical excipients (mannitol and dicalcium phosphate anhydrous) during die filling was investigated. A rotary die filling system was employed and the die filling efficiency was assessed using the concept of critical filling velocity. Powder flowability was also characterised using a dynamic flow tester and a shear cell tester. It was found that, with increasing moisture content, the flow parameters, including the basic flow energy, the average pressure drop and the cohesion, increased first and then decreased from a maximum at 50% relative humidity (RH). For both mannitol and dicalcium phosphate anhydrous (DCPA), when the relative humidity is high (say >50%) and the cohesion due to the formation of liquid bridges dominates, the critical filling velocity increases as the moisture content increases. While at a low relative humidity (e.g. 15%), a strong electrostatic attraction between particles was observed, which has a significant impact on the flow properties and the die filling performance of the powders.