Rheological properties of polypropylene during multiple extrusion

Polypropylene (PP) degradation during multiple extrusion at different temperatures was studied by following changes in rheological properties. Dynamic and transient experiments were carried out in a rheometer with parallel plate geometry. In addition, a statistical experimental design (22, with replicates) was employed using die zone temperature (X1) and number of extrusion cycles (X2) varying at two levels. The system degradation was modeled using melt flow index (MFI) as a control variable. A rapid increase of chain scissions of the PP macromolecules and, consequently, a molecular weight reduction, was observed through abrupt increase of MFI and reduction of properties such as complex viscosity (η*) and elasticity of the molten polymer.