Rheological behaviour of a high-melt-strength polypropylene at elevated pressure and gas loading for foaming purposes

The rheological properties of a long chain branched polypropylene (LCB-PP) were investigated at the processing conditions of foam extrusion, namely high pressure, gas loading and high shear rates, as well as in elongational deformation. For measuring the rheological properties of PP at moderate and high deformation rates, an in-line rheometer was used. Comparison of the results to standard rotational rheometry showed good agreement. The effect of the processing parameters was quantified using shift factors for temperature, pressure and gas concentration. The influence of pressure on the shear viscosity was found to be of minor importance. In contrast, the shear viscosity was distinctly affected by CO₂ concentration, reducing it to one third of its gas-free value at a concentration of 6 wt% at a specific shear rate. The change of viscosity by a variation of temperature is similar in magnitude compared to the variation due to dissolved blowing agent. Furthermore, thermo-rheological complex behaviour was observed. In the foaming process, thermo-rheological complexity could contribute to a better morphology control of long chain branched polymers compared to linear ones. The elongational viscosity was measured using both a hyperbolic die and a film stretching tool (UXF) for comparison. It is almost three decades higher than the shear viscosity in the non-linear region, due to pronounced strain hardening of the melt.