Interfacial Phosphorus-to-Nitrogen Balance Engineering of Chitosan-Phytic Acid Multilayers for Fire-Safe and Wash-Durable Cellulose Paper

Sustainable flame-retardant paper must balance coating loading, structural integrity, and aqueous stability. We constructed a low-cycle chitosan/phytic acid (CS/PA) interface on cellulose paper through sequential assembly and pressing. Optimization identified the optimal chitosan–phytic acid-coated cellulose paper (CS1.0-PA20/CP), which formed a continuous, defect-minimized coating with homogeneous N and P distributions. The treatment reduced afterflame time to 1.96 ± 0.30 s, increased the limiting oxygen index from 19.2% to 27.4%, and prolonged ignition from 13 to 16 s. Total smoke release and CO yield decreased by 42.9% and 74.4%, respectively. After five washes, the coating retained 95.5% of its mass and a limiting oxygen index of 25.8%. Post-combustion analyses support a condensed-phase mechanism involving PA-catalyzed dehydration, P–N-assisted char stabilization, and P-O-C/polyphosphate formation. These findings show that interfacial P/N balance and defect control enable effective fire protection through an aqueous, low-temperature, three-cycle process.