At-line estimation of powder lubrication using rotating drum: Linking specific volume kinetics to tensile strength loss.

This work introduces a rapid, at-line and automated method to quantify powder lubrication using a rotating drum equipped with real-time imaging. As powders mix, shear forces promote the delamination of magnesium stearate (or alternative lubricants), enabling it to spread across host particle surfaces and progressively densify the blend. The rotating drum captures this densification through changes in normalized specific volume (SVN), which follows an exponential decay. By fitting this measure, the method extracts two key parameters: (i) rate constant γ, and (ii) magnitude β. Comparison against tablet tensile strength data demonstrates that the same kinetic rate constant governs both densification and tablet tensile strength loss, enabling a simplified two-point calibration: tensile strengths from unlubricated and fully-lubricated blends, combined with the SVN-derived γ, accurately reconstruct the full lubrication curve (R² ≈ 0.86-0.97). Across excipient systems (2:1 MCC:Lactose, 2:1 MCC:DCP), lubricant concentrations, lubricant grades, and particle-size variations, the rotating-drum method distinguishes lubrication efficiency with high sensitivity. Results align with conventional lubrication models, but the method requires a small amount of material (∼25 g), minimal tablet compaction and hardness testing (two sets only), is low-cost and simple to use, and provides measurements of lubrication state with high temporal resolution.