Effects of liquid phase migration on extrusion of microcrystalline cellulose pastes.

The behaviour of water-based microcrystalline cellulose pastes undergoing ram extrusion has been investigated. Factors affecting the redistribution of water within the extruding paste and the upstream barrel compact, such as the initial water content, extrusion rate and die geometry, have been considered. The rates of dewatering for these given systems were characterised by the gradients of the extrusion pressure-ram displacement profiles. A linear relationship between the pressure-displacement gradient and the inverse square root of the paste velocity was obtained for a given paste and extrusion geometry. At velocities where water migration was significant, the extrudate was found to have a higher water content than that of the paste in the barrel at any given time; both the extrudate and the barrel paste decreased in water content with increasing ram displacement. Spheronisation of extrudate samples has shown that the redistribution of liquid during extrusion is an important factor affecting the quality of the spheres. A paste flow model, incorporating pseudo-plastic and shear deformation terms, was used to predict the change in extrusion pressures caused by liquid phase migration. The model parameters were obtained as functions of water content and gave good agreement with the experimental extrusion profiles.