Stress distribution in lubricated vs unlubricated pharmaceutical powder columns and their container walls during translational and torsional shear testing

In this work, it has been shown that, in a FT4 torsional type powder shear tester, when the powder formulation contains a lubricant/glidant agent, such as magnesium stearate, the shearing process does not take place within the bulk solid in certain low wall friction conditions. Instead, the powder acts as a monolithic block and the shear process occurs between the powder cake surfaces and the vessel walls. In these cases, the FT4 shear tester fails to measure yield loci properly. Introducing a grooved surface at the bottom of the shear cell is proposed to solve this problem. With it solved, shear test comparisons between Jenike and FT4 shear testers revealed that there was good agreement for some bulk solids (i.e. Xylitol), whereas in other cases (i.e. microcrystalline cellulose (MCC) and dicalcium phosphate (DCP)), there was not. Two numerical stress state models proposed in the literature—Janssen analysis and method of characteristics—have been used to theoretically calculate the relationship between normal stress applied at the surface of the powder column and the measured torque. Good agreement between experimental torque measurement and simulations was observed for Xylitol, whereas significant differences were obtained in the cases of MCC and DCP. The reasons for (a) the discrepancies found for some powders submitted to translational (Jenike) and rotational (FT4) shear testing, and (b) differences with theoretical simulation are still poorly understood.

Graphical AbstractIn a torsional type powder shear tester, at certain low wall friction conditions, the shearing process does not take place within the bulk solid. Instead, the powder acts as a monolithic block and the shear process occurs between the powder cake surfaces and the vessel walls, thus leading to erroneous yield loci. A geometry modification is proposed to solve this problem.Figure optionsDownload full-size imageDownload as PowerPoint slide