Application of shear analysis to predict flow and lubrication properties of consolidated powder beds

Abstract

The production and use of fine powders in the pharmaceutical industry requires reliable methods for testing the flow properties of a powder. Poor flow of drugs and excipients can lead to high weight and dose variations, especially during scale-up. There are no reported studies that explain changes in powder flow properties with an increase in batch size. The hypothesis of this work is that the flow and lubricity of a powder are influenced by changes in consolidation state of a powder bed. It is also hypothesized that the change in flow and lubrication is influenced by powder properties like porosity, particle size and shape. An annular shear cell was designed and validated, to study the flow behavior of powders at various consolidation loads. Shear analysis was found sensitive to particle size and shape, and could discriminate between cohesive and free flowing powders. The values obtained were also sensitive to the addition of lubricants and glidants. Wall friction stresses measured using a modified annular shear cell correlated well with tablet ejection forces. The flow properties of some drugs and excipients studied, depended on the consolidation state of the powder bed. The effect of consolidation on flow was significant in the case of cohesive powders, especially when the particle was needle shaped. A change in particle size did not have as significant an effect during a change in consolidation load. The addition of a lubricant and a glidant reduced the effect of applied consolidation load. The flow properties of binary mixtures of drugs and excipients were intermediate to those of neat materials. Although the angles of yield loci depended on the composition of the mixture, the values obtained deviated from predicted ideal mixture values. This anomaly could not be explained by the particle size, shape or density of the individual components. There was no direct correlation between composition of the blend and the consolidation load applied on the powder bed. In summary, the annular shear cell proved to be an important tool for studying the flow and lubricational properties of pharmaceutical powders.