Stepwise room-temperature aqueous N/P functionalization of lignocellulose nanofibers for flame-retardant multifunctional PVA films.

Imparting flame retardancy to lignocellulose nanofibers (LCNFs) while retaining their reinforcing function remains challenging. Here, preformed LCNFs were functionalized through a stepwise room-temperature aqueous process. 3,5-Diamino-1,2,4-triazole (DAT) was introduced by treatment with formaldehyde, followed by immobilization of polyphosphoric acid, likely through ionic association, to obtain halogen-free N/P-functionalized LCNFs (PDLCNFs). Mannich grafting involving residual lignin was proposed as a contributing pathway for DAT immobilization. This stepwise design suppressed bulk aggregation of N/P components. Spectroscopic and elemental analyses supported N/P incorporation and retention after repeated washing, while FE-SEM and WAXS showed largely preserved nanofibrous morphology and cellulose Iβ structure. Self-standing PDLCNF sheets exhibited a limiting oxygen index (LOI) of 58.0 ± 0.3% and rapid self-extinguishing behavior. Thermal and combustion-residue analyses supported the formation of a porous N/P-containing char layer that could act as a thermal barrier. When incorporated into poly(vinyl alcohol) (PVA) at 20 wt%, PDLCNF increased the LOI from 19.0 ± 0.8% to 31.1 ± 0.4%, enabled self-extinction within 0.5 s after flame removal, and reduced the peak heat release rate and total heat release by approximately 92% and 79%, respectively. Tensile strength increased by 17% and 11% for vacuum-dried and humidity-conditioned specimens, respectively. Despite slightly higher water uptake, Young's modulus after humidity conditioning increased from 21.0 ± 1.1 to 342 ± 26 MPa, approximately 16 times the neat PVA value. The composite films also exhibited UV shielding. These results establish a mild route to structure-retaining, multifunctional LCNF additives through stepwise N/P functionalization.