Effects of Water on Powder Flowability of Diverse Powders Assessed by Complimentary Techniques

The storage of bulk powders in suboptimal conditions can affect their handling, processing, and performance. The aim of this work was to assess the influence of storage conditions on powder flowability, which is a crucial property in powder processing. The 7 model compounds were common tablet excipients with varying crystallinity ranging from amorphous to partially crystalline, and 2 crystalline materials. The water sorption-desorption isotherms of all the excipients were analyzed with dynamic vapor sorption, and the kinetics of the water sorption at 75% relative humidity were investigated using isothermal microcalorimetry. In addition, the powders were conditioned at 3 relative humidities 23%, 43%, and 75% at ambient temperature, and their flowability was measured using 3 different techniques: ring shear testing, basic flowability energy, and flow through an orifice. All 3 methods were able to detect the storage-related change in flowability induced by varying storage conditions. With increasing storage humidity, a trend toward a decrease in powder flowability could be detected with the chosen instruments. Furthermore, the same rank order of powder flowability could be observed using the flow through an orifice and ring shear testing method.