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A model formulation, comprising ibuprofen and direct compression lactose (Tablettose 80) was used to assess the influence of two lubricants, magnesium stearate and stearic acid, on punch tip adherence. Lubricant concentrations were varied from 0.25% to 2% w/w. Formulations in the presence and absence of 0.5% w/w colloidal silica (Aerosil 200) were examined, to assess the influence of the glidant on the anti-adherent effects of the lubricants. Differential scanning calorimetry (DSC) was used to examine the effect of the lubricants on the melting temperature of ibuprofen. Tablets were compacted using a single punch tablet press at 10 kN using hard chrome-plated punches or at 40 kN using uncoated steel punches, tooling was 12.5-mm diameter in each case. The upper punch faces were characterized by obtaining Taylor Hobson Talysurf surface profiles. Following compaction, ibuprofen attached to the face was quantified by spectroscopy. At low concentrations of each lubricant, the levels of sticking observed were similar. Whilst sticking increased at magnesium stearate concentrations above 1%, sticking with stearic acid remained relatively constant at all concentrations. DSC revealed that the melting temperature of ibuprofen was lowered by the formation of eutectic mixtures with both lubricants. However, the onset temperature of melting and melting point were lowered to a greater extent with magnesium stearate compared with stearic acid. When using uncoated tooling at 40 kN, the deleterious effects of magnesium stearate on the tensile strength of the tablets also contributed to sticking. When using chrome-plated punches at 10 kN, the tensile strength reduction by the presence of magnesium stearate was less pronounced, as was the level of sticking.
Roberts et al. (Mon,) studied this question.