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 containing silane-treated basalt fiber, lignin-pyrolyzed biocarbon, ocean-recycled PA 6, and 15 wt% non-halogenated flame retardant (melamine cyanurate) were developed via melt extrusion and 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 strong filler–matrix adhesion and a stable viscoelastic network, characterized by the lowest C-factor (0.438) and the highest entanglement density (∼14 × 10 4 mol m -3 ). Moreover, the non-flammable hybrid composites with 50% sustainability and much lighter (>50%) than metallic counterparts showed strong potential for lightweight EV battery enclosures and structural automotive components. • Non-flammable hybrid polyamide composites using recycled PA 6, basalt fiber (BF) and lignin biocarbon (BC) were developed. • Synergistic BF-BC reinforcement resulted in a 179% increase in tensile modulus and 194% increase in flexural modulus. • The hybrid composites exhibited UL-94 V-0 rating and zero horizontal burning rate at low flame-retardant loading. • The hybrid composites also demonstrated HDT > 200 °C, 53% reduction in CLTE, and good thermal and rheological properties. • Composites with 50% sustainability retained good processability and were much lighter than metals for EV components.