Polymerization catalyst is key: Predicting molecular changes during mechanical recycling of phillips and ziegler-natta catalyst high-density polyethylene (HDPE)

• Polymerization catalyst for HDPE determines performance after mechanical recycling • Ziegler-Natta HDPE shows only chain scission during mechanical recycling • Long-chain branching in recycled Ziegler-Natta HDPE at higher recycling temperature • Phillips HDPE shows branching/crosslinking during mechanical recycling • State-of-the-art characterization with CFC/SEC/TREF and rheology We present a systematic approach to predict the molecular degradation of high-density polyethylene (HDPE) during mechanical recycling based on the polymerization catalyst. HDPE represents about 12.5 wt % of the polymer world production and is industrially mainly produced by Phillips (P-HDPE) and Ziegler-Natta catalyst (ZN-HDPE). Two blow-moulding P- and ZN-HDPE with identical melt flow indices (MFI) were subjected to mechanical recycling at an extrusion temperature of 170 °C and 210 °C, a screw speed of 180 rpm, and recycling times from 10 to 240 min. Chemical and rheological characterization revealed, that despite similar initial melt properties, P-HDPE and ZN- HDPE exhibited fully contrary degradation mechanisms: for recycled P- HDPE at 170 °C and 210 °C, the complex viscosity (|η*|) and the molecular weight drastically increased after 10 min of recycling time due to star-like branching, followed by chain scission at 170 °C, but crosslinking at 210 °C, resulting in unprocessable, rubber-like material. In contrast, recycled ZN-HDPE exhibited a continuous drop in molecular weight and in |η*| across all conditions. This work provides a polymerization catalyst-specific framework to predict and engineer thermo-mechanical molecular degradation pathways in HDPE recyclates, paving the way to tailored recycling strategies to obtain value-added materials.