Powder Flow Measurement Comparison between two ring shear testers

Shear testers are the internationally recognized means to measure the flow properties of cohesive bulk solids. They were used lately for assessment of the flow behaviour of different pharmaceutical excipients and active ingredients in many pharmaceutical research groups. In this work two automated Schulze ring shear testers were compared. The Schulze testers are direct shear testers with rotational displacement and unlimited strain. The Schulze shear tester RST-01.pc (larger tester) and the Schulze shear tester RST-XS (smaller tester) were compared. In this study a 200 ml shear cell was used for the measurements carried out on the RST-01.pc tester while a 30 ml shear cell was used for the measurements on the RST-XS tester. A set of binary mixtures of different active ingredients and excipients were prepared and a total of 189 measurements were carried out on each tester. Both testers led to low ffc values with cohesive mixtures and high ffc values with easy flowing mixtures. However, ffc values more than 10 were not used because these values were highly fluctuating; besides they still refer to a free flowing mixture. The easy flowing mixtures showed a broader ffc scatter while the more cohesive powders showed a narrower scatter on both testers. Comparing the ffc values of both testers showed that the results were well correlated with a correlation coefficient r = 0.97. However, the smaller (RST-XS) tester showed slightly lower ffc values compared to the larger (RST-01.pc) tester. For comparative tests this effect does not play a role as long as the same ring shear tester with the same shear cell size is used throughout the measurements. However, an advantage of the smaller (RST-XS) tester is that a smaller amount of bulk solid is required for the test.