Mechanical recycling of HDPE-based packaging: Interplay between cross contamination, aging and reprocessing

Abstract

Achieving effective mechanical recycling strategies for polyolefins remains a major challenge for several reasons. Firstly, the thermo-mechanical degradation underwent during reprocessing, as well as the different degradation forms experienced during the service life, cause a severe modification of polyolefin microstructure, ultimately leading to a progressive deterioration of their performance. On the other hand, due to non-fully accurate sorting technologies, low levels of cross-contamination are commonly encountered in recycled polyolefins. All these features result in the obtainment recyclates with a heterogeneous and complex morphology, which significantly affects their final properties, often limiting their possible future applications. This work aims at addressing these issues, evaluating the combined effect of cross-contamination and of the degradation undergone by the polymers during service life and reprocessing for high-density polyethylene (HDPE) containing low amounts of polypropylene (PP) and polyethylene terephthalate (PET) as contaminants. In particular, pristine and cross-contaminated HDPE were subjected to photo-oxidative or thermo-oxidative treatments and the aged materials were reprocessed, aiming at simulating the real conditions of a typical mechanical recycling process. The obtained results demonstrated that cross-contamination minimises the functional degradation of HDPE, especially under photo-oxidative conditions. Conversely, the microstructural characterization pointed out that different microstructures can be achieved depending on the level of cross-contamination and on the aging treatment. Finally, it was shown that the presence of PP and PET as contaminants has a detrimental impact on the HDPE ductility, especially under thermo-oxidative conditions, while for photo-oxidised materials exerts a marginal role.