Wet granulation end point prediction using dimensionless numbers in a mixer torque rheometer: Relationship between capillary and Weber numbers and the optimal wet mass consistency

Abstract The optimal wet mass consistency during wet granulation is often determined using the hand squeezing test. In this study, torque values recorded inside the wet mass were measured using a mixer torque rheometer (MTR) via multiple additions of liquid. The main objective of this work was to predict the optimal wet mass consistency of pharmaceutical powders using the modified capillary ( Ca ∗ ) and Weber ( We ∗ ) dimensionless numbers. The results show that the optimal wet mass consistency versus Ca ∗ (or We ∗ ) can be fitted with a power–law function, whereas the improved capillary number C a ′ proposed in this work gives different relationships and behaviors depending on the spreadability and wettability of the blend. The wettability was obtained by measuring the contact angle between the liquids and the pharmaceutical powders. The surface free energy and the polar and dispersive parts of a liquid’s surface energy were obtained from Young’s equation and the Owens–Wendt–Rabel–Kaelble (OWRK) model. This study demonstrated the importance of the interfacial energy σ b - s and the pore radius, R pore in the establishment of a dimensionless number, Ca ∗ , that can satisfactorily predict with an R 2 of 0.80, the optimal wet mass consistency of pharmaceutical powders measured by the MTR.