The fusion stage in twin-screw extruders plays a major role in the dispersion of the minor phase in a PE/PS blend. Right after fusion, the PE minor phase is in a mixed droplet/fiber morphology. Surprisingly, fibers and droplet diameters are already in the micron range at that early stage. In the following conveying and kneading sections, the minor phase progressively goes from a partially fibrillar to a nodular morphology. During that period however, the average fiber diameter remained unchanged and the droplet diameter dropped only by a factor between 2 and 4. For low PE concentrations (i.e. 2% and 5%), the kneading section was very efficient for breaking fibers. The use of two different PE/PS systems has shown that the evaluation of the viscosity ratio at processing temperature is not sufficient. In fact, since most of the dispersed phase deformation occurs right after fusion, the viscosity ratio at temperatures much lower than processing (i.e. barrel) temperature are needed to assess the ability to disperse. For the PE/PS system the viscosity ratio, is very low right after PE fusion and increases rapidly with temperature. Therefore, it was possible to disperse the relatively viscous PE-2 down to the 10 micron range even though the PE-2/PS system has high viscosity ratio at normal processing temperatures.
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