The extrusion of various formulations of microcrystalline celluloses

The process of extrusion is fundamental to many of the dosage forms produced by modern pharmaceutical industry. Often, the excipient of first choice in pharmaceutical extrusion is microcrystalline cellulose. The work described indicates that the extrusion behavior of formulations based on microcrystalline cellulose can be studied and compared effectively using various techniques to assess the relationship betwen force and displacement during ram extrusion through a single-holed die. The results show that the grade of microcrystalline cellulose upon which a formulation is based affects its extrusion in terms of the rheological parameters measured and the quality of the extrudate produced. The differences appear to be due to physical differences in the powders themselves (eg particle size) and to physico-chemical differences in the interaction of the microcrystalline cellulose with the continuous phase. The work demonstrates that microcrystalline cellulose formulations are able to respond to shear rate by the formation of a lubricating layer at the die wall, which facilitates stable flow and yields a product with a smoother surface. There is an optimum quantity of water required to lubricate flow. The origin of imperfections in the flow of these systems is identified. Declining or uneven Steady State Flow Stages are indicative of barrel wall interaction and unstable flow respectively. Pressure overshoot is indicative of difficulty in overcoming capillary resistance at the yield stage, but is not necessarily related to ease of flow after yield has occurred. Forced Flow is shown to arise at the entrance to the die, whereas extrudate surface impairment is seen to originate downstream of the die-entry region