Rheological behavior of porous pharmaceutical materials: linking torque profiles during wet massing to water diffusion coefficients and penetration time

Abstract Wet massing is commonly performed during the wet granulation process after the addition of a liquid binder. Torque values were measured using a Mixer Torque Rheometer (MTR) by the method of multiple additions of water and variable mixing time. The main objective of this work is to link water diffusion coefficients and penetration time in pharmaceutical powders to torque profiles generated in an MTR. The diffusion coefficients were calculated by fitting the experimental data to an exponential decay function and by minimizing the Root Mean Square Error between the fitted curve and the solution of Fick’s 2nd law. Water penetration times were measured and calculated using experimental drop penetration and Washburn methods, respectively. The main research hypothesis is that the temporal change in torque profiles after each water addition step is related to the mass diffusion coefficients and water penetration time in the powder of varying degrees of water content. The results show that for the microcrystalline cellulose (MCC) powders at varying granulating fluid levels, capillary diffusion seems to be more adequate to explain the rheological behavior observed compared with the Fickian diffusion model.