Influence of processing on the flow properties of long-chain branched polypropylene

Abstract

Long-chain branched polymers have high melt strength and show strain hardening in the elongational flow. These characteristics are important quantities for certain processing procedures, but are strongly affected by thermomechanical treatment. In this study, different rheological methods and material functions in shear and elongation are applied to determine the flow behavior that is affected by certain processing procedures. For this purpose, a series of blends of linear polypropylene (L-PP) and long-chain branched polypropylene (LCB-PP) is prepared using different processing methods (mixing chamber with kneading elements, twin-screw extruder, foaming) and varying processing parameters. The decrease of dynamic moduli and of flow activation energy depends on the number of processing stresses. Equilibrium states are created by solution treatment, and in this way process-modification can be separated into reversible and irreversible parts. It is concluded that the irreversible process-modification does not reduce the density of entanglements, but tears chains.