Precision of pharmaceutical powder flow measurement using ring shear tester: High variability is inherent to powders with low cohesion

The purpose of this study is to assess the precision of pharmaceutical powder flow measurement using ring shear tester, and to understand whether certain types of powders are more error-prone than others. To answer these questions, we presented the nonlinear mathematical model which enables the calculation of flow function from raw experimental data from the ring shear tester. The variance of the flow function and its origin were evaluated through the first order Taylor expansion of the flow function model. These analyses indicated that the precision of flow function measurement can be compromised by powders with inherently low cohesion values. A marked increase in flow function variability is seen when the cohesion value of a powder is below 100Pa (in measurements with pre-consolidation stress of 1kPa). The variability in flow function measurement is attributed predominantly by cohesion, whereas friction angles have only minor contribution. The study also showed that the precision of flow function measurement cannot be improved by testing at higher pre-consolidation stresses. All the results were verified experimentally using six pharmaceutical powders exhibiting diverse flow behaviors. These results suggested that one must exercise caution when testing and analyzing powders with low cohesion values using ring shear tester. For such materials, an alternative testing method is recommended to corroborate the powder flow data from ring shear testing.