Rheological study of wet powder masses: application to a pharmaceutical grade of microcrystalline cellulose

Control over the quality of products made by the extrusion/spheronisation p rocess calls for knowledge of the rheological properties of the materials t o be shaped. These are the result of mixing powders with a liquid. In addit ion to the actual physicochemical properties of the powders, the quantity o f liquid added to the powders to produce a wetted mass determines the feasi bility and/or the physical characteristics of the finished product. It is i n this context that the Compresso-rheometer was developed to establish the rheological behaviour of microcrystalline cellulose (Avicel PH101) wit vary ing levels of water content. This study revealed th shear-thinning behaviou r of these flowing wetted masses, with a consistency index which fell as th e level of liquid increased. This behavior could be explained by the fibrou s form of th cellulose particles under test, which oriented themselves alon g the flow streamlines in line wit the increase in the shear rate.