Rheological stability as the missing criterion in HDPE circularity: A critical review and a new decision framework
Maja Csapó
, József Gábor Kovács
, József Gábor KovácsVol. 20., No.4., Pages 414-434, 2026
DOI: 10.3144/expresspolymlett.2026.32
DOI: 10.3144/expresspolymlett.2026.32
Corresponding author: József Gábor Kovács 
GRAPHICAL ABSTRACT
ABSTRACT
The greatest obstacle to recycling post-consumer high-density polyethylene (PCR-HDPE) is typically the degradation of properties caused by impurities and heterogeneity. However, a critical analysis of the literature reveals that the real bottleneck is not the material composition, but rather rheological stability, which simultaneously determines the degradation history of the waste stream, melt behavior, and processability at the cycle level. This review proposes a new perspective: the decision among mechanical, chemical, and energetic recycling is better made based on a unified rheological stability index (RSI), which integrates carbonyl index, viscosity change after multiple instances of melting, melt flow index (MFI) instability, in-mold pressure fluctuation, and the degree of polymer incompatibility. RSI enables the prediction of the processability of PCR-HDPE and identifies which recycling path a fraction is most suitable for. The study demonstrates how an RSI-based approach can reduce quality risk, improve cycle stability, and support circular decision-making in an industrial environment.
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