Factors influencing structure formation and phase size in an immiscible polymer blend of polycarbonate and polypropylene prepared by twin-screw extrusion

The modification of the morphology of an immiscible binary blend of polypropylene (PP) and polycarbonate (PC), in a co-rotating, intermeshing, twin-screw extruder has been studied. It was found that the proximity to the die wall, high composition and high viscosity/elasticity ratios resulted in preferential fibre formation for the dispersed phase. The phase size and size distribution increase significantly with composition. The point of phase inversion is difficult to determine precisely due to the presence of extended morphologies over a wide composition range. Composite blend morphologies (occlusion formation) were observed at 60% PC (weight). The influence of the viscosity ratio on phase size was studied over a wide range and was found to be a minor effect at 10% PC (volume) composition. The effect of viscosity ratio becomes more pronounced at 20% PC. Comparison of the viscosity ratio effect on phase size between the twin-screw and an internal mixing chamber indicated a significantly coarser dispersed phase in the latter case at high viscosity ratio. At a lower viscosity ratio the phase size and size distribution were identical in the two processing environments. The minor-phase morphology in the centre of the extruded strand was not found to be highly sensitive to the screw speed and the volumetric flow rate. This indicates that the minor phase in these blends is not highly dependent on the shear stress and energy imposed on the material before the die. The above results are contrasted with previous detailed studies carried out in an internal mixing chamber.