Effect of mechanical recycling on molecular structure and rheological properties of blends of Phillips and Ziegler-Natta catalyst High density polyethylene (HDPE)

High-density polyethylene (HDPE) represents a major fraction of global plastic production and waste. Industrially, HDPE is mainly produced using Ziegler-Natta (ZN) and Phillips (P) catalysts, which yield distinctly different molecular features such as terminal vinyl groups in P-HDPE. As shown recently by our group for two blow moulding grade ZN- and P-HDPE, ZN- and P-HDPE have complementary degradation pathways during mechanical recycling. ZN-HDPE mainly undergoes chain scission, whereas P-HDPE tends to form star-like branched to crosslinked structures due to the terminal vinyl groups. This results in completely different processing properties, i.e., the complex viscosity at low frequencies increases for P-HDPE but decreases for ZN-HDPE. In realistic post-consumer waste streams, HDPE mixtures derived from both catalyst systems are inevitably, underlining the need to understand the degradation pathways of their blends. We investigate the molecular and rheological evolution of blends of ZN-HDPE and P-HDPE with varying compositions during simulated mechanical recycling in a twin-screw extruder at 210 °C and 180 rpm for 10, 20 and 30 min. ... mehrFourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), cross-fractionation chromatography (CFC), melt flow index (MFI), small amplitude oscillatory shear (SAOS), extensional rheology as well as mechanical solid-state properties were used to analyse the recycled materials. The analysis reveals that increasing the P-HDPE to ZN-HDPE ratio leads during mechanical recycling to an increased content of star-like branched/crosslinked chains. This is quantified by a decreasing degree of crystallinity and weight average molecular weight, whereas viscosity at low frequencies drastically increased. For instance, the low-frequency viscosity of rZN-HDPE after 10 min increased by 30 % compared to vZN-HPDE, but the addition of 10 or 30 wt% of P-HPDE resulted in a viscosity increase of about 300 % and 600 %, respectively. The here presented findings provide mechanistic insight into degradation pathways in mixed-catalyst HDPE post-consumer waste streams and offer guidance for improving recycling and upcycling strategies for more realistic polyolefin waste stream mixtures.