The importance of humidity control in powder rheometer studies

The importance of humidity as a factor affecting powder flow and caking is well established. Traditionally, powders are stored in a humidity-controlled chamber prior to powder flow characterization, so the actual flow rheology experiment is not conducted under humidity-controlled conditions. In this study, a new method is demonstrated for directly introducing humidity into a powder rheometer to precisely determine powder flow response to humidity in-situ. Our study confirms that for non-conducting powders, humidity-induced changes in powder flowability due to electrostatics are substantial. It also demonstrates that for powders that are susceptible to humidity induced agglomeration/caking, the powder flow energy changes with both time and as a function of humidity. This study anticipates the introduction of humidity-controlled powder rheometers to be the starting point of more sophisticated experimental investigations of powder flow behaviour. Such instrumentation would facilitate the establishment of direct correlations of ambient humidity levels, and thus moisture content of powders, with powder flow energy and particle properties including surface energy, surface conductivity and particle shape.