A quantitative approach to assess the mixing ability of single-screw extruders for polymer extrusion

Models to predict the evolution of the morphology of immiscible liquid-liquid systems and of solid agglomerates dispersion in single-screw extruders were adapted to compute global distributive and dispersive mixing indices. Using these indices, the direct assessment of the mixing ability of a given screw or a comparison of distinct screws becomes readily available. In the case of liquid-liquid systems, the degree of distributive mixing is computed from the extent of deformation of the individual drops, whereas the degree of dispersive mixing is determined by the reduction in drop size. For solid-liquid systems, distributive mixing is quantified by a normalized Shannon entropy, whereas dispersive mixing is defined from the extent of size reduction. The effect of material properties, operating conditions, and geometry of the screw and die is investigated. Finally, the indices are used to determine (via optimization) the operating conditions and the screw geometry that maximize the mixing efficiency.The financial support of the Portuguese Science Foundation (FCT - Fundacao para a Ciencia e Tecnologia) under grants POCTI CTM/48448/2002 and SFRH/BD/19605/2004 is gratefully acknowledged