The effect of water content on the porosity and liquid saturation of extruded cylinders

Abstract

The preparation of isodiametrical and nearly monodispersed pellets by extrusion-spheronization requires an extruded mass which shows a complex balance between deformability and its ability to break up into smaller units. The possibility of extruding the mass through an orifice is mainly related to its plasticity or deformability. Such mechanical properties of moist compacts are related to both the porosity and the degree of liquid saturation. In this study the effect of water addition has been studied in this context. Mixtures of dicalcium phosphate dihydrate (DCPD) and Avicel PH 101 (70:30%) were massed with water (moisture contents of 50, 60, 70, 80 w/w %). The masses were extruded in a ram extruder (25.0 mm in diameter), initially with a 10.0 mm die and then without the die. The moist extrudates were cut to isodiametrical cylinders and the porosity and the degree of liquid saturation were determined. An increased moisture content decreased the extrusion force and increased the porosity of the extrudates. The degrees of liquid saturation were similar and high for all extruded masses. It is suggested that such a nearly complete filling of the pore space with a liquid component is a prerequisite for the wet mass to yield and thus extrude. The mechanism behind the yielding process is probably an enhanced possibility for particle movement and rearrangement due to a reduced particle-particle attraction when the pore space is filled with a liquid.