Exploring pharmaceutical powder cohesion through the Warren Spring cohesion test

Understanding bulk powder flow properties is crucial in pharmaceutical manufacturing to prevent material handling issues and ensure consistent product quality. Cohesion, a key factor influencing powder flow, is typically evaluated indirectly through methods such as the angle of repose and shear cell testing. This investigation explores the Warren Spring cohesion test, less common in the pharmaceutical industry, that directly measures powder cohesion. The test determines powder shear strength by rotating a paddle with vanes in a pre-compacted powder bed and applying torque without normal stress on the shear plane. The resulting Warren Spring cohesion strength, $S_{\mathrm{WSC}}$, was normalized by the conditioned bulk density, showing a strong linear correlation with the penetration force of the paddle. The slope of this relationship allows for a direct comparison of cohesion across materials. Notably, the $S_{\mathrm{WSC}}$ was unaffected by pre-compaction stresses, demonstrating the consistency of this method. By controlling the penetration force, a simpler and faster method for testing cohesion across different powders was established. The correlation between the Warren Spring tester and the Brookfield powder flow tester was confirmed, highlighting the advantages of quicker assessments with the Warren Spring cohesion test, despite the shear cell’s greater sensitivity in certain situations.