The effect of lactose particle size on the extrusion properties of microcrystalline cellulose-lactose mixtures.

Ram extrusion has been used to assess and compare the flow characteristics and quality of wet powder masses formed from mixtures of water with microcrystalline cellulose and two different particle size samples of lactose. The force-displacement profiles can reveal poor flow properties. Steady state flow was achieved for the mixture containing fine lactose which produced an extrudate of uniform moisture content at all extrusion rates except the lowest (5 cm min-1). Increasing the lactose particle size significantly altered the extrusion properties of the formulation with forced flow predominating and high extrusion pressures observed. Formulations showing that type of extrusion are to be avoided as they produce poor quality extrudate which may be unsuitable for spheronization. Flow visualization studies showed that problems associated with such mixtures are caused by failure to maintain a constant angle of convergence during extrusion, which is essential for maintenance of steady state flow. With the mixture made with coarse lactose, the extrudate quality could be improved by extruding at high velocities, information that may be of significance in the development of formulations for large-scale production. Differences between formulations may be expressed quantitatively by plotting the apparent shear stress-shear rate relationship. The curves are modified by the particle size of lactose included in the mixtures. Such curves have implications in predicting the suitability of an extrudate, produced under particular experimental conditions, for spheronization.