Résumé
The formulation of a mixture for obtaining pellets by extrusion/spheronisation is based on experience and trials and not on a complete scientific understanding of the process. It has not been possible to exploit the progress made in rheology for characterizing the wet mixtures designed for use with this process. This is no doubt because of the difficulty of studying them by using existing rheometry apparatus in the traditional manner for substances possessing specific properties. For the essential conditions for conducting a conventional rheological study, i.e. the presence of a laminar flow with assumed perfect die wall adhesion, represents a difficult challenge to take up when working with heterogeneous materials whose density changes under the impact of pressure. This article is based on the use of an apparatus, the compresso-rheometer, which enables the extrusion process to be simulated on a reduced scale. It presents an original, precise study protocol resulting in mechanical characterisation through the use of two indices (visco-elasticity and plasticity) for microcrystalline cellulose/water mixtures. These new indices were established for the purpose of forecasting the behaviour of raw materials mixed with a liquid when subjected to a shaping process, and also for indirectly addressing the problems of formulation and dissolution.