Production of High-Solid-Content Fire-Retardant Phosphorylated Cellulose Microfibrils

Phosphorylated cellulosic micro(nano)fibrillated materials are increasingly considered for flame-retardant applications as a biobased alternative to their halogen-based counterparts. Most of the reported cellulose functionalization strategies, however, are realized at low solids contents and/or involve energy-intensive fiber disintegration methods. In this perspective, we propose an alternative concept of phosphorylated microfibrillated cellulose production with notably high (25 wt %) solids content and low (0.6 MWh/t) energy consumption. Here, an enzyme-aided pulp disintegration upon mild mechanical treatment was combined with an effective mixing of the fibrillated material with (NH4)2HPO4 in the presence of urea. Subsequently, the obtained slurry was cured at elevated temperature to enable cellulose phosphorylation, which was redispersed afterward in water. The morphology of the obtained phosphorylated micro(nano)fibrillated cellulose materials was extensively characterized by optical microscopy, a fiber analyzer, SEM, and AFM. The presence of phosphate groups in the cellulose structure was validated by ATR-FTIR as well as 31P and 13C NMR spectroscopy. The casted films prepared from phosphorylated cellulose bearing a charge of 1540 μmol/g, which was the highest among the prepared samples, demonstrated noticeably improved flame retardancy, leaving ∼89% of the material after burning as well as self-extinguishing properties when the samples were subjected to a butane flame for 3 s.