Lignin migration-driven construction of reed fibers with phosphate-rich interphases and their application in flame-retardant PLA composites

Natural fiber reinforced polylactic acid (NF/PLA) composites are attractive sustainable materials, yet their broader engineering use is limited by weak interphase load transfer and rapid, dripping combustion. A lignin migration driven interphase phosphorylation strategy was proposed to construct a continuous phosphate-rich interphase on reed fibers, enabling simultaneous reinforcement and flame retardancy in PLA. A mild and efficient solvent activation step induces in- situ lignin deposition and generates a reactive, adhesive fiber surface, followed by aqueous phosphorylation to covalently introduce phosphate functionalities. Crucially, this strategy preserves the intrinsic structural integrity of the macroscopic fibers while simultaneously tailoring the interface for enhanced adhesion and flame retardancy. As a result, the reconstructed interphase strengthens fiber matrix adhesion and promotes condensed phase charring while improving melt resistance to dripping. At a 30 wt% modified fiber loading, the composite reaches a limiting oxygen index of 33.0% and achieves a UL-94 V-0 rating with no cotton ignition. Cone calorimetry shows 38% and 27% reductions in peak heat release rate and total heat release relative to neat PLA. Meanwhile, tensile strength increases by about 15% and notched impact toughness by about 270%, evidencing effective load transfer. The lignin enriched, phosphate functionalized interphase synchronizes rapid melt stabilization with the formation of a compact protective char, thereby reconciling reinforcement with fire safety. This scalable interphase-engineering strategy offers a sustainable route to high performance PLA composites for transportation, electronics, packaging, and related applications.