From waste to high-performance materials: Processing routes for upcycling and downcycling of recycled polymer composites

The widespread adoption of polymer composite materials in automotive, aerospace, construction, energy, and consumer applications are driven by their high specific strength, stiffness, and durability. However, the end-of-life (EOL) management of carbon, glass, and natural fiber–reinforced thermoset and thermoplastic composites remains a critical challenge, as conventional disposal routes such as landfilling and incineration are environmentally unsustainable. In response, diverse recycling technologies have been developed to recover fibers and polymer matrices, yet their impacts on material performance vary significantly. This review critically analyses mechanical, thermal, chemical, and emerging recycling methods for polymer composites, with emphasis on how processing parameters influence fiber quality, matrix degradation, and interfacial characteristics. Mechanical properties, physicochemical characterization, surface morphology, and life cycle assessment (LCA) considerations are discussed with virgin and recycled composites. Key challenges and future research directions are highlighted to support value-oriented composite recycling. Recycled composites are classified based on property retention and performance potential, enabling differentiation between high-value structural applications (upcycling) and lower-value secondary (downcycling) uses. The novelty of this work lies in the systematic segregation of upcycling and downcycling pathways for recycled polymer composites, an aspect rarely addressed explicitly in existing literature.