Effect of externally sprayed lubricant combinations on ejection force and physical characteristics of tablets

Lubricants are essential for tablet ejection, but internal use of magnesium stearate (MgSt) can impair tablet tensile strength and disintegration. This study evaluated an external lubrication approach using binary 1:1 (w/w) lubricant mixtures to reduce the MgSt level. Sodium stearyl fumarate (SSF) and ground sodium lauryl sulfate (SLS), the latter used for the first time externally, were applied individually and in combinations during compression (5-10 kN). Mean ejection force (EF) and dynamic EF profiles were recorded across 100 tablets. From the individual lubricants, SSF produced the lowest EF, while SLS showed the highest EF with the widest fluctuations. Binary mixtures had comparable EFs to, and sometimes lower EF than, those of individual lubricants, demonstrating that MgSt usage can be halved without loss of lubrication efficiency. Dynamic EF profiles revealed force-dependent behaviours: hydrophobic MgSt systems stabilised at higher force, attributed to better film formation, whereas hydrophilic lubricants were more stable at lower forces and less stable at higher forces. Issues such as inconsistent spraying of SLS due to non-reproducible surface area of particles or instability of the lubricant layer in hydrophobic-hydrophilic lubricant combinations were explored as explanations for inconsistent ejection force profiles. Tablet properties were influenced by lubricant type, with SSF and SLS systems outperforming MgSt, effectively mitigating the negative impact on mechanical strength while ensuring good disintegration. The presence of lubricants on the tablet surfaces was confirmed by Raman mapping. These findings support binary external lubrication as a viable strategy to reduce MgSt-induced mechanical defects of tablets while maintaining effective ejection.