Value‐Upcycling of Waste Polyethylene Terephthalate ( <scp>PET</scp> ) Textiles via Solid‐State Shear Milling for Reinforcing Recycled Polypropylene Composites

ABSTRACT The low recycling rate of waste polyethylene terephthalate (PET) textiles presents significant environmental challenges, primarily due to difficulties in preserving their fibrous morphology and achieving strong interfacial adhesion in composites. This study introduces a novel upcycling strategy by employing solid‐state shear milling (S 3 M) technology to convert waste PET textiles into high‐value reinforcement for recycled polypropylene (R‐PP). The S 3 M process simultaneously and precisely controls the fiber morphology and induces mechanochemical surface activation. An optimal fiber aspect ratio ( L / D ≈ 25–40) was achieved after 1–2 milling cycles, generating abundant reactive terminal groups (‐COOH, ‐OH) on the fiber surfaces, as confirmed by both Fourier transform infrared spectroscopy (FTIR) and quantitative x‐ray photoelectron spectroscopy (XPS) analysis. When incorporated into an R‐PP matrix with maleic anhydride‐grafted polypropylene (PP‐g‐MAH) compatibilizer, these activated fibers demonstrated significantly enhanced interfacial adhesion, evidenced by a transition from fiber pull‐out to fiber fracture in scanning electron microscopy (SEM) analysis. Furthermore, the PET fibers acted as potent nucleating agents, increasing the crystallization temperature of R‐PP by 3.4°C and achieving a peak crystallinity of 47.7% at an optimal 10% PET content, as revealed by DSC. The composites displayed a homogeneous, single‐stage thermal decomposition profile, distinct from the dual‐phase degradation of the non‐S 3 M control, confirming the crucial role of S 3 M in achieving coupled degradation kinetics (TGA). The synergistic effects of morphology control, interfacial engineering, and enhanced crystallization resulted in superior mechanical properties. The optimal composite (10% PET, 2 co‐milling cycles) exhibited a 17.6% increase in flexural modulus, a 55% increase in tensile strength, and a remarkable 130% improvement in impact strength compared to neat R‐PP. This work provides a feasible and efficient “fiber‐to‐composite” upcycling pathway, transforming mixed waste textiles into high‐performance composites with potential applications in automotive, durable goods, and construction.