Rheological characterization of pharmaceutical powders using tap testing, shear cell and mercury porosimeter

Most of the pharmaceutical processes involved in the manufacturing of so lid dosage forms are connected with powder flow properties, at least for some of the intermediate steps. Powder flow characteristics are commonl y investigated by various measurements, such as handling angles, tap tes ting, shear cell measurements, etc. All these approaches allow the calc ulation of indices characterising powder flowability. Unfortunately, th ese methodologies are highly product consuming, which is a limitation in the first steps of a novel drug development, when only a small amount of product is available. The use of mercury porosimetry to evaluate compre ssibility and flow properties of powders could be a new and alternative approach to obtain insight in the rheological properties of granular med ium by the interpretation of the first part of programs (low pressures) . We have developed such an evaluation and compared the results obtaine d with those given by tap testing and shear cell measurements, applied t o four excipients for direct tabletting and three different drugs. Merc ury porosimetry turned out to be a sensitive technique, able to providequantitative information about powder flow properties, complemen ted by an evaluation of particles micro porosity and size distribution, in a single step. These characterisations are obtained with only approx imately 250mg of bulk powder compared to high quantities ( >100g) needed for other methods.