Interphase-anchored cellulose nanohybrids enabling mechanically robust, anti-dripping flame-retardant PLA composites

Poly(lactic acid) (PLA) offers a bio-based origin and established melt processability, but its broader use is limited by high flammability, severe melt dripping, and low ductility. Here, a silica-phosphate‑nitrogen nanohybrid (SCPU) was developed by constructing a silica-containing, phytic-acid/urea-functionalized interphase on cellulose nanofibers and incorporating the hybrid into PLA through maleic anhydride/dicumyl peroxide (MA/DCP)-assisted reactive melt processing. Relative to neat PLA, the optimized composites retained flowable melt behavior while delivering a 13% increase in tensile strength, a 4-fold increase in elongation at break, and a 23% improvement in impact strength. Fire performance was also markedly improved, with a transition from UL-94 failure to V-0 behavior, suppression of dripping, and an increase in limiting oxygen index from 18.5% for neat PLA to 34.3% for 10-SCPU. Cone calorimetry showed a 45% reduction in peak heat release rate and a 17% reduction in total heat release. Residue morphology, SEM-EDS elemental mapping, XPS, Raman spectroscopy, and temperature-resolved FTIR indicate that flame retardancy is governed mainly by condensed-phase stabilization through the formation of a cohesive silica-phosphate-reinforced char barrier, while a minor gas-phase contribution from nitrogen- or phosphorus-containing volatile fragments cannot be completely excluded. These results highlight a scalable route toward mechanically robust, anti-dripping PLA composites for durable thermoplastic applications.