Phosphorus‑nitrogen synergistic flame retardancy in lyocell fiber composites: Mechanistic investigation

Lyocell fibres, as eco-friendly, renewable cellulose-based materials, have seen growing production capacity. However, their flammability limits compliance with fire safety requirements. The present study aimed to enhance the flammability resistance of lyocell fibres through the incorporation of a synthesized phosphorus‑nitrogen-based polymer (FR-P) flame retardant. The flame-retardant lyocell fibres (FR-L) were prepared using an addition blending method. Analyses of NMMO solvent confirmed minimal solvent decomposition and stable rheological properties of the spinning solution. Thermal decomposition studies indicated that FR-P modified cellulose degradation by promoting dehydration and carbonization, reducing the release of flammable volatiles, thereby increasing char residue, and lowering both the peak heat release rate (PHRR) and total smoke release (TSR). Microstructural analysis revealed that a higher FR-P content elevated the limiting oxygen index (LOI) but decreased mechanical strength, increased internal microporosity, and worsened pore alignment relative to the fiber axis. Among the flame-retardant fiber samples, FR-L3 (with FRP 25.37 % content) exhibited optimal performance: after 50 washes, the LOI was retained at 28.6 % (down from the initial 31.4 %), and the mechanical strength remained at 2.86 cN/dtex. This work demonstrates the feasibility of producing high-performance flame-retardant lyocell fibres through blending modification, providing valuable insights for industrial production and performance optimization.