(44d) Industrially Relevant Powder Characterisation Using a Uniaxial Powder Tester
The AUPT measures the load required to break/fail a free-standing powder column which has been consolidated at a known Major Principal Stress (MPS). In addition to these measurements, the AUPT can also determine Bulk Density and Compressibility Percentage data. Powder flow properties can also be evaluated for a range of storage conditions by adjusting MPS and/or consolidation time.
Twelve diverse powders were evaluated using the AUPT, with each tested at three different MPS values. Highly repeatable values for uniaxial Unconfined Yield Stress (uUYS), Compressibility and Bulk Density were generated for each of the materials at each MPS, demonstrating how uniaxial powder testing can be used to quantify and rank the flow properties of diverse materials representing a wide range of industries. Comparable results were also generated between different users demonstrating low operator variability, unlike many other simple techniques.
All twelve samples displayed high sensitivity to the different consolidation loads utilised. This is most prominently observed for a sample of detergent which generated lower uUYS values at lower MPS values but was significantly more sensitive to applied stress than many of the other materials, for example limestone, generating higher uUYS values at 100 kPa. As such, this material may display more problematic process performance in higher stress operations such as hopper discharge.
The highly repeatable Compressibility values generated for each of the materials indicates consistent consolidation across the powder column over repeat runs. The data also illustrates that bulk properties such as Compressibility should not be relied on alone to assess flowability. For example, despite MCC and a sample of carboxymethyl cellulose (CMC) generating comparable Compressibility profiles, clear differences in uUYS were observed.
Powders were also consolidated for extended periods of time in controlled environments using off-line consolidation stations to study the impact of long-term storage as well as elevated temperatures and humidity levels. Clear and repeatable differences were observed between samples after being consolidated for 1 or 16 hours over a range of stress levels.
This study has demonstrated that the AUPT can be used to rapidly quantify a defined set of parameters to improve process efficiency and obtain repeatable data in long term studies. The technique displays the speed and ease of use associated with more traditional characterisation methods with the additional benefits of having a well-defined operating procedure, automation and low operator dependency.