(400z) Industrially Relevant Powder Characterisation Using a Uniaxial Powder Tester

During production, rapid and reliable evaluation of process intermediates is essential to ensure high efficiency and final product quality. It is therefore necessary that any powder characterisation methods utilised are simple and quick, and that the resulting data is easy to interpret. Whilst methods such as Angle of Repose (AOR) and Tapped Density can fulfil these criteria, they are often poorly repeatable, highly operator dependent and typically lack differentiation. These techniques may also be unsuitable for testing highly cohesive, and therefore typically problematic, powders. The recent development of an Advanced Uniaxial Powder Tester (AUPT) provides a tool with the ease and simplicity of the more traditional techniques with the added benefits of a robust, semi-automated and well defined operating procedure designed to deliver high levels of repeatability and significantly reduce operator variability.

The AUPT measures the uniaxial Unconfined Yield Stress (uUYS) - the stress required to fail a free-standing powder column which has been consolidated at a known Major Principal Stress (MPS). In addition to measuring uUYS, the AUPT also provides compressibility and bulk density evaluation. Adjustment of the MPS and/or consolidation time also enables assessment of the powder flow properties across a range of storage conditions.

In this study, 12 different powders representing a range of industries, including food and pharmaceutical materials, were evaluated using the AUPT. Each sample was tested at low, intermediate and high MPS values, with repeat measurements conducted for each data point. Clear and repeatable differences were demonstrated between the samples, especially when subjected to higher consolidation stresses. The highly repeatable compressibility, bulk density and uUYS values demonstrate that consistent consolidation was achieved at each MPS for all of the samples tested. Comparable results were also obtained between different users, demonstrating minimal influence of the operator on this technique.

A subset of the powders was also subjected to longer consolidation periods using off-line consolidation stations. These devices can be used to assess the impact of long storage periods, ranging from a few hours to several months, as well as the influence of elevated temperatures and relative humidity. This enables the effect of environmental conditions during long term storage of powders in IBC’s, hoppers and silos to be accurately quantified. In this study, the samples were consolidated using a range of stress levels for time periods between 1 and 16 hours. Clear and repeatable differences were again observed between the samples with extended storage times shown to have a considerable impact on flowability of the powders.

The AUPT was therefore demonstrated to a reliable and sensitive instrument for both rapid evaluation and long term studies for a diverse range of materials from different applications, across a wide range of consolidation levels.