Poly(ethylene terephthalate)/Aluminum Inclusions as Critical Defects in Recycled Polypropylene Derived from Municipal Waste: Synchrotron-Based Visualization and Fracture Analysis

Recycled polypropylene (rPP) sourced from postconsumer packaging waste is a promising material for sustainable polymer applications. However, its mechanical performance is often unpredictable, which restricts its wider adoption. In this study, we investigated the origins of the mechanical degradation exhibited by rPP produced from household plastic waste collected by Japanese municipalities and reprocessed by commercial recycling companies. Tensile testing of 10 rPP samples revealed significant variability in their ductility, with 60% of the samples exhibiting brittle fractures at or before the yield point, whereas the others retained moderate-to-high degrees of elongation. Synchrotron-based X-ray computed tomography (CT), mapping X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy analyses revealed large foreign inclusions comprising poly(ethylene terephthalate) (PET), aluminum (Al), or their multilayer composites at the fracture origin points. These inclusions typically exhibited film- or foil-like morphologies, with embedded voids near the crack surfaces. Importantly, the in-plane sizes of these inclusions were highly inversely correlated with the fracture strain, regardless of their compositions. A normalized size threshold of ∼0.2 was identified, beyond which ductility decreased sharply and brittle fracture dominated. These findings demonstrate that the inclusion size, rather than chemical identity, mainly governs elongation at break in container-grade rPP. The persistence of large, film-type contaminants highlights a critical limitation of the existing recycling practices and highlights the need for advanced separation technologies that can remove oversized multilayer residues. While these results are specific to Japan’s municipal recycling system, they offer insight into the embrittlement mechanisms of polyolefin recyclates and inform future strategies for attaining increased material performance.