A strategy for extending the processing temperature for polypropylene in foam extrusion and its theoretical validation

The quite narrow PP foaming temperature window is the main challenge for continuous extrusion foaming of polypropylene (PP) using supercritical CO2. In this study, high impact polystyrene (HIPS) is added to PP to widen the polypropylene foaming temperature range by reducing the temperature sensitivity of the melt strength of the blend. The behavior of crystalline, dynamic rheology, and extensional rheological of PP/HIPS blends are analyzed. The results show that the crystallinity and crystallization temperature of PP/HIPS blends decreased significantly, the temperature dependence of the blends’ extensional viscosity and melt strength became weaker, and the activation energy of extensional viscosity and melt strength activation energy decreased, while the elastic modulus and viscosity and the system relaxation time increased. It means that the addition of HIPS reduces the crystallinity of PP, improves the cell morphology and to a certain extent overcomes the problem of a sharp decrease in PP melt strength with increasing temperature. The addition of HIPS extends the foaming temperature range of the PP material from 4 K to a maximum of 12 K. Furthermore, we simply estimated the temperature window for extruded foams using the Arrhenius equation. The estimated values have the same trend as the experimental results and are analyzed.