Sustainable and non-flammable hybrid composites based on ocean-recycled polyamide 6 reinforced with surface-modified basalt fiber and lignin-derived biocarbon

The growing demand for lightweight, sustainable and fire-resistant materials in electric vehicles (EVs) and the automotive sectors necessitates the development of advanced polymer composites with balanced mechanical, thermal and flammability performance. In this work, non-flammable and high-performance polyamide 6 (PA 6) hybrid composites with silane-treated basalt fiber, lignin-pyrolyzed biocarbon, ocean-recycled PA 6, and a low loading of non-halogenated flame retardant (melamine cyanurate) were prepared by melt extrusion followed by injection molding. The silane-treated basalt fiber provided excellent stiffness, while lignin biocarbon produced at 950 °C imparted effective condensed-phase char formation. The synergistic hybrid reinforcement strategy significantly enhanced stiffness, heat stability and flame retardancy while maintaining good processability. The optimized hybrid composites with 10 wt% biocarbon and 20 wt% recycled PA 6 exhibited a 179% increase in tensile modulus and 194% increase in flexural modulus compared to neat PA 6. A heat deflection temperature above 200 °C and 53% reduction in the coefficient of linear thermal expansion were also observed for these composites. These sustainable composites achieved a UL-94 V-0 rating and zero horizontal burning rate while retaining melt flow index values above 10 g/10 min. Dynamic mechanical and rheological analyses revealed high entanglement density, strong filler-matrix adhesion, and a stable viscoelastic network. Moreover, hybrid composites with 50% sustainability and much lighter (>50%) than metal composites showed strong potential for lightweight EV battery enclosures and structural automotive components.