Processing stability of polypropylene impact-copolymer during multiple extrusion - Effect of polymerization technology

Three commercially available polypropylene impact-copolymers (ICPP) produced by Innovene (INN), Spheripol (SPH) and Unipol (UNI) technologies were subjected to multiple extrusion using a twin-screw extruder W&P ZSK25 at 220 °C. Processing stability and changes in properties induced by extrusion were investigated. The materials were of similar MFR ∼6 dg/min, similar ethylene contents ∼7.5 wt. % and the same type and level 1200 ppm of phenol/phosphite stabilizer system was used. Ranking INN < UNI < SPH in processing and long-term (LTHA) stabilities observed was primarily related to the reactivity of catalyst residues rather than to other factors, such as contents of ethylene, quantity of extractables, EPR phase composition, levels of ash or individual elements in it. The mechanically demanding multiple extrusion conditions and consequently different extent of processing degradation, however, induced only minimum changes in morphology and impact strength of the solid ICPP matrix. Thus, regardless of changes in melt-flow properties induced by extrusions, all the three grades even after 5th extrusion at 220 °C exhibited Charpy notched impact strength at 23 °C only minimally changed.