Test methodProcessing degradation index (PDI) - A quantitative measure of processing stability of polypropylene

Multiple extrusion, simulating thermo-mechanical processing conditions on the laboratory scale, has commonly been accepted as a tool for assessing the processing stability of polypropylene. The graphical output of multiple extrusion is the processing curve (MFR vs. number of extrusions), the slope and position of which provide information on the extent of polymer degradation. Processing degradation index (PDI) is proposed as the way of numerical processing of this curve, which allows expressing its position by means of a single parameter. The application of PDI then allows direct construction of plots showing the dependence of processing stability on variables such as additive concentration, matrix composition, temperature or simply the origin of the polymer. PDI is a relative parameter and will always be related to the given machinery, temperature and number of extrusions.