Assessing powder flowability at low consolidation stresses.

Abstract

The inability of cohesive powders to flow consistently and reliably is a major cause of process downtime and product wastage across a wide range of powder processing industries. Extensive work has been carried out characterising powder flowability using a wide array of techniques, with the most established method of powder flow measurement at moderate to high stresses being shear cells, with theories developed for silo and hopper design. Many processes of great industrial interest though, such as filling and dosing of powders in capsules and dispersion in dry powder inhalers (DPI), expose particles to very low consolidation stresses (≤ 1 kPa), at which the determination of unconfined yield strength by shear testing is often marred with inconsistencies in the measurement, or in comparison to the observed behaviour. Therefore, there is a need for an alternative approach in order to develop an understanding of powder flow for weakly consolidated powders, such as ball indentation, which is a penetration technique capable of assessing powder flowability down to very low stresses, whilst requiring only a very small amount of material. In this technique, a powder bed is consolidated to any desired stress that provides a reasonably flat surface, and then penetrated by a spherical indenter. The flowability is determined by calculating the hardness of the powder bed, from the force-displacement response of the bed. Hardness can be linked to the unconfined yield strength, commonly derived by shear testers, via the constraint factor, which is dependent on particle properties, although cannot yet be determined a priori. In this work, the constraint factor of a broad class of powders was quantified from indentation and shear cell experiments at moderate to high stresses and was found to be generally independent of the applied stress, while for four out of the twenty-five materials it exhibited fluctuations. In order to infer the unconfined yield strength from hardness measurements at low stresses, it was assumed that the constraint factor remains constant at lower stresses. Distinct Element Method (DEM) modelling was also utilised to simulate the ball indentation system, allowing the powder bed internal failure stresses to be realised in order to elucidate the behaviour of the constraint factor at low stresses. The simulations validated the assumption that the constraint factor remains constant throughout the applied stress range. Furthermore, the applicability and reliability of both ball indentation and the FT4 shear cell were assessed in a wide range of both low and moderate to high stresses. Ball indentation gave very repeatable results throughout the whole range of stresses applied, whilst the FT4 shear cell was deemed unreliable for most materials at pre-shear normal stresses of 1 kPa and below. For all materials except the three powders that remain very cohesive throughout the stress range tested, the increase of hardness (and therefore also the unconfined yield strength inferred from ball indentation) with stress was observed to be much steeper at low stresses, as compared to higher stresses, due to a more rapid increase in packing fraction. For all model glass beads tested, except for the 0 - 20 μm samples, hardness was found to be independent of penetration depth in a certain depth range. In contrast, for most ‘real’ materials, plus the aforementioned very fine model glass beads, hardness was found to continually increase with depth, with a gradient that is independent of the applied stress and similar for all materials tested. The powders that are prone to stick-slip deviated from the above behaviour and exhibited a fluctuating force response. The influence of a variety of particle properties on the constraint factor, and subsequently powder flowability, was also determined both experimentally and computationally. The effects of particle size, size distribution, and single particle and agglomerate shape were investigated experimentally, whilst static and rolling friction, and plastic yield stress were varied in DEM simulations. The influence of interface energy was studied both experimentally and by DEM modelling. All properties were found to affect the constraint factor and flow behaviour to some extent, denoting the complexity of powder flow behaviour.