4.

Representative chemical-modification routes for flame-retardant cellulose-based materials. (a) Mechanochemical phosphorylation of cellulose nanofibrils, followed by melamine incorporation to construct a phosphorus–nitrogen synergistic flame-retardant system. Adapted with permission. Copyright 2020, American Chemical Society. (b) Molecular grafting of amino-acid-derived phosphorus–nitrogen flame retardants onto cellulose through stable P–O–C linkages. Adapted with permission. Copyright 2021, Elsevier. (c) Borate-based chemical modification of cellulose composite foams through boric acid/borate incorporation, forming borate-related covalent structures that enhance condensed-phase protection. Adapted with permission. Copyright 2021, Elsevier. (d) Homogeneous phosphorylation/esterification of cellulose chains for the uniform introduction of phosphorus-containing groups and controlled molecular design of flame-retardant functionality. Adapted with permission. Copyright 2019, Springer.