Thermal degradation of polypropylene reprocessed in a co-rotating twin-screw extruder: kinetic model and relationship between Melt Flow Index and Molecular weight

Thermo-mechanical degradation of polypropylene (PP) during reprocessing causes significant changes in its properties and presents a challenge to recycling. In this study, PP was reprocessed multiple times (1 to 5 cycles, at 240, 260, or 280 °C), and the physical and structural properties of the reprocessed material were evaluated. In comparison to virgin PP, the reprocessed polymer presented a higher carbonyl index, higher melt flow index, lower average molecular weight, and narrower polydispersity. Changes in these parameters became more pronounced as the processing time and temperature increased. Additionally, two different behavior regions were identified, the first one spanning between 240 - 260 °C, and the second one comprising higher temperatures (280 °C). Moreover, a kinetic model that calculates the Mw of PP reprocessed at different temperatures was evaluated. Finally, adjusted parameters for the relationship between MFI and Mw were obtained, and the MFI of reprocessed PP was also calculated.

Keywords: polypropylene recycling, polypropylene degradation, kinetic model, melt flow index, molecular weight.