Vitrimer-Enabled Circularity: Upcycling Mixed Polyolefin Waste from Milk Packets into High-Value 3D Printing Feedstock

The global plastic waste crisis is exacerbated by the formidable challenge of upcycling mixed polymer streams, particularly the incompatible polyolefin phases of polyethylene (PE) and polypropylene (PP). Herein, we present a transformative strategy to address this issue by engineering a dynamic covalent vitrimeric system from post-consumer recycled polypropylene (PCR PP). By crosslinking maleated PCR-PP with a tetrafunctional epoxy, we fabricated a recyclable thermoset, or vitrimer, capable of fundamentally altering the material properties of mixed plastic waste. Our results demonstrate that this vitrimeric network significantly improves the interfacial compatibility between otherwise immiscible PCR PP and post-consumer polyethylene from milk packets (PCR PE). Characterization through rheology, gel content analysis, and SEM confirms the formation of a robust covalent adaptable network, which confers superior mechanical properties, including optimal tensile strength and enhanced dimensional stability under creep deformation. Critically, these vitrimeric blends retain their morphological and mechanical integrity across three reprocessing cycles, affirming their potential for genuine circularity. Furthermore, we successfully demonstrate the application of this upcycled material for additive manufacturing via Fused Granulate Fabrication (FGF), positioning a traditionally low-value waste stream as a high-value feedstock for 3D printing. The underlying molecular mechanism of this compatibilization is elucidated by DFT simulations, which substantiate the radical grafting and PE scavenging pathways, providing a theoretical foundation for our empirical observations. This work introduces a powerful and versatile platform for valorizing complex polyolefin waste, offering a scalable solution for transforming waste into functional and endlessly recyclable resources.