Processing Stability of Carbon Nanofiber-Reinforced Glass Fiber/Polypropylene Composites Under Repeated Extrusion for Mechanical Recycling

Glass fiber-reinforced polypropylene (PP/GF) is used widely in lightweight automotive applications, but it is affected adversely by fiber breakage and matrix degradation during recycling. This study investigates the effects of carbon nanofiber (CBNF) addition on the recyclability of PP/GF composites subjected to repeated extrusion. Homo-type PP was compounded with GF and CBNFs and was processed for up to nine extrusion cycles. Melt viscosity, fiber morphology, flexural properties, interfacial shear strength, and notched Charpy impact strength were evaluated. Neat PP showed a pronounced increase in the melt volume rate (MVR) with cumulative cycles, indicating molecular degradation. By contrast, CBNF-containing composites exhibited superior viscosity stability, with MVR increasing only 2.9-fold after nine cycles compared with 5.4-fold for GF-only systems. Fiber length was well maintained (96–98% retention). The flexural strength and modulus were preserved, respectively, as greater than 92% and 95%. The interfacial shear strength remained stable, whereas the impact strength decreased moderately but retained 84% of its initial value. These results underscore that a slight addition of CBNFs (5 wt%) suppresses viscosity loss effectively and stabilizes mechanical performance, offering a viable strategy for sustainable recycling of PP/GF composites in transportation applications.