Testing the Circular Quality of Polymers

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A specific limiting aspect of recycling plastics and, more concretely, thermoplastics is the loss of properties of plastics during mechanical recycling, the only well-established technological reprocessing of plastic waste material. This loss is mainly due to chain scission, hydrolysis, and thermal degradation during melt processing, while other aspects may also negatively influence this performance. This aspect might be crucial when deciding among the materials to be used and for calculating the circularity value creation principles established by the Ellen MacArthur Foundation.

In order to investigate the difference between two bio-based (PLA and PHB) and two fossil-based polymers, researchers from the UNESCO Chair in Life Cycle and Climate Change at ESCI-UPF (Ilija Sazdovski, Dr. Sahar Azarkamand, Dr. Alba Bala,  Dr. Pere Fullana-i-Palmer) and from the LEPAMAP at the University of Girona (Dr. Leonidas Milios supported by Dr. Ferran Serra Parareda and Dr. Marc Delgado-Aguilar) developed a scientific paper published at MDPI’s Polymers journal.

The factual circularity of materials requires the utilisation of materials that keep their quality properties after going through recycling to minimize the inflow of virgin materials in the technosphere.

Within the Product Environmental Footprint (PEF) methodology, and based on the economic model approach, the European Commission provides default parameters for the quality changes of the polymers after recycling, but solely for some fossil-based polymers. This study provides a test-based example for the calculation of technical substitutability for 2 fossil-based polymers (HDPE and PET) and 2 bio-based polymers (PLA and PHB) based on mechanical, processing, and optical properties.

The results show that the economic substitutability method gives very different results compared to those obtained by considering real technical quality changes of the polymers in multiple-cycle recycling. HDPE proved to have superior circular properties compared to any other polymer under research. In addition, the recent practice of substituting fossil-based with bio-based polymers will need to be re-evaluated after additional research related to the quality change of bio-based polymers in recycling.

This study highlights significant limitations in the current PEF methodology regarding the quality factors (Qs/Qp) used for assessing fossil-based polymers. It demonstrates that the default Qs/Qp values—0.9 for both PET and HDPE—do not reflect the actual quality loss observed during mechanical recycling, with empirical tests revealing much lower values (0.328 for PET and 0.671 for HDPE).

This discrepancy suggests that current PEF assumptions may unjustifiably favour certain polymers, especially PET, over others. Moreover, the lack of Qs/Qp values for bio-based polymers underlines the need for a more comprehensive and physically based methodology. The study also raises concerns about the recyclability of bio-based plastics like PLA, which show rapid degradation across cycles, challenging the assumption that bio-based plastics always equate to better circular performance.

Further, the research underscores the importance of expanding PEF guidance to include more accurate, application-specific Qs/Qp factors based on physical properties rather than market proxies. This is particularly relevant for materials where aesthetics, such as optical properties, play a minor functional role but dominate product design decisions. The current «visually flawless» industry standard may need rethinking to align with circular economy goals.

The findings advocate for a shift towards test-based protocols tailored to specific end uses, which would enhance the accuracy and relevance of LCAs. This work provides a foundation for future methodological improvements and calls for broader discussions in the LCA community and with European policymakers to refine how quality degradation is assessed in polymer recycling.