Flowability of Pharmaceutical Powders: Impact of Humidity

Abstract

Surrounding conditions impact the behavior of many powders. Depending on the geographical location, bulk solids exhibit differing flowability due to different environmental conditions, e.g. Caracas (30°C, 80 %rH) vs. Boulder/Colorado (25°C, 35 %rH). The powder adjusts dynamically to its environmental condition until reaching saturation at a certain point (1). Besides that, many powders show a “sticky point”. At this point, a significant change in flow and adhesion behavior has been reported (2). Granular media generally tend to increase their cohesive character when surrounded by moisture. This is attributed to several mechanisms whereas formation of adhesive films on the grain surface, liquid bridging between the grains, or if the cohesive behavior decreases, a “lubrication” of the interstitials between grains are considered as the main underlying mechanisms (3). The investigated powder is a pharmaceutical cold and flu hot drink which comprises high amounts of vitamin C, citric acid, sugar and pharmaceutically active materials. The powder exhibits a tendency to cake and time consolidate. This is most likely due to a partial dissolution in condensed water as Groen et al. suggests for a pure citric acid (2). This partial dissolution is desired as the powder is supposed to dissolve in water. As the investigated powder contains a large amount of citric acid as well, a similar “sticky point” is expected. However, this property results in the forming of knots and clumps which is problematic for storage and processing. Thus, the aim of the presented work is to investigate changes in the powder’s flowability behavior after exposure to different relative humidity levels as well as to investigate the difference between consolidated and non-consolidated samples with the help of powder shear cell measurements. 2. Methods