Influence of the Physicochemical Variability of Magnesium Stearate on Its Lubricant Properties: Possible Solutions

The variability of the physicochemical properties of 13 commercial batches of magnesium stearate (from three vendors) were determined using various physicochemical tests. Differences observed were related to the crystal lattice and the hydration state of the samples as well as the impurities contained in their matrices. A formulation model containing 2% of magnesium stearate was used to determine the lubricant properties of 6 of the 13 magnesium stearate lots received. The tablet press used was a Stoks® Single Station Instrumented F Press. The different lubricant properties observed were related to the particle size of the magnesium stearate lot used. The influence of the crystalline structure on the lubricant properties of magnesium stearate was also shown whereas the influence of the adsorbed water did not appear to determine process capabilities. Two possible solutions were evaluated to reduce the lubricant property differences among the lots tested. By decreasing the particle size of a lot of magnesium stearate, it has been possible to significantly improve its lubricant properties. Magnesium stearate in association with talc also presented an improvement of its lubricant properties.