Flame-Retardant Paper from Phosphorylated Cellulose Fibers: Preparation, Characterization, and Fire Performance Evaluation

Cellulose fibers are abundant, renewable biopolymers with significant potential to replace petroleum-based materials, yet their high flammability limits their use in fire-sensitive applications such as protective packaging, electrical insulation, and interior building materials. In this study, softwood kraft fibers were phosphorylated without solvent or chemical pre-treatment using a molten urea/phosphoric acid system at 160 °C for 3 h (molar ratio of 6:3:1 for urea, H3PO4, and cellulose anhydroglucose units). After water quenching, disintegration, and washing, the modified fibers were blended with unmodified kraft fibers at various ratios (0, 20, 40, 60, 80, and 100 wt.%) to prepare standard paper handsheets (85–90 g/m2) dried at 120 °C. Fiber characterization confirmed successful phosphate ester grafting, yielding a phosphorus content of 5.9%, a nitrogen content of 8.7%, and a total charge increase from 92 to 4676 mmol/kg, while preserving fiber length (decreasing slightly from 2.04 to 1.93 mm). Thermogravimetric analysis revealed enhanced thermal stability, generating a residual char mass of 31.12 wt.% at 900 °C compared to 6.9 wt.% for untreated fibers. Handsheet apparent density varied from 0.260 g/cm3 for KFP60 to 0.316 g/cm3 for KFP100. Inter-fiber phosphate crosslinking during hot drying significantly improved mechanical performance, increasing maximum load from 10.27 N to 42.29 N and Young’s modulus from 39.77 to 99.78 MPa. Vertical flammability tests demonstrated that handsheets containing ≥60 wt.% phosphorylated fibers preserved their structural integrity after 30 s of flame exposure, whereas untreated handsheets burned completely. These results highlight phosphorylation as a scalable, halogen-free, and effective strategy for producing mechanically robust, fire-resistant cellulose materials.