Capillary extrusion of polypropylene/high-density polyethylene immiscible blends as studied by rheo-particle image velocimetry

The capillary extrusion of polypropylene (PP) and high-density polyethylene (HDPE) immiscible blends was studied in this work by rheo-particle image velocimetry (Rheo-PIV). The PP/HDPE blends were prepared by single screw extrusion and extruded through a transparent capillary die at a temperature of 200 degrees C and concentrations of 80/20, 60/40, 40/60 and 20/80 wt%, respectively. PIV measurements described accurately the flow behavior of PP/HDPE blends and revealed continuous velocity profiles in the die, without macroscopic phase separation, for all the blends in the resolution range of the PIV technique. The flow behavior of all the blends was shear-thinning (power-law) type and their viscosities laid in between the values corresponding to the neat polymers and increased in an exponential way along with the concentration of the highest viscosity component in the blend (HDPE). Also, it was found that the extruded blends acquired a stratified morphology and that HDPE mitigates extrudate distortions in PP, meanwhile PP eliminates slip and flow instabilities in HDPE by migrating to the region of highest shear stresses in the die. Migration of PP to the capillary wall was corroborated by Raman spectroscopy measurements on the periphery of solid extrudates. Finally, via calculations of the density of the molten blends under flow using the velocity profiles in the die, we show that the homopolymers are compatible in the molten state and follow a simple inverse relation for their density, and an exponential one for their viscosity.