Prediction of the growth kinetics and agglomeration mechanisms using a mixer torque rheometer

Although high-shear wet granulation is widely used, the knowledge of this operation is not yet fully clarified and the process design is particularly difficult. The evolution of this process depends on strength and plasticity of the wet masses, consequently the purpose of this study was to develop a rheological method able to predict the granule growth. To pursue this goal, the torque evolution over the time of three different wet powder formulations was recorded in a mixer torque rheometer (MTR3, Caleva, UK). The torque profiles were then analyzed to find a characteristic torque value that could be correlated to the strength and the deformability of the wet granules. The first selected formulation was composed of 100% (w/w) of microcrystalline cellulose (MCC100), the second consisted in a mixture 1:1 of sucrose and microcrystalline cellulose (SUCR50) and the third contained 5% (w/w) of xanthan gum and 95% (w/w) of microcrystalline cellulose (XG5). Granulation experiments (material exchange experiments and extended granulation experiments) were performed using different experimental conditions (different amount of liquid binder and impeller speed) to identify the growth mechanism of three formulations studied. Results have shown that the maximum torque peak (Tpk) developed by the wet masses over the time can be successfully used to predict the granule growth mechanisms. In particular, for formulations that developed relatively high Tpk values (>0.2 Nm/g), consolidation and growth of the granules were difficult, and an induction growth mechanism could be observed (formulation XG5). If Tpk values lower than 0.2 Nm/g, the deformability of the wet mass was high, and the growth regime corresponded to the steady growth (formulations MCC100 and SUCR50). In addition, experiments have shown that the higher the Tpk values, the faster the growth kinetic. In conclusion the rheological characterization of the wet masses demonstrated to be useful to predict granule growth behaviour in high shear wet granulation and the developed method could represent a valuable tool in the process design and development.