Fully biobased materials for flame-retardant modification of cotton fabric through layer-by-layer strategy.

The development of halogen- and phosphorus-free flame-retardant strategies is critical for advancing the safety of natural cotton textiles. This study reports an eco-friendly finishing method based on the layer-by-layer assembly of the biobased compounds sodium alginate, chitosan and calcium chloride. The structural features, thermal stability, combustion behavior, flame-retardant efficiency, and underlying mechanism of the coated cotton fabrics were systematically investigated. The coated cotton displayed a markedly reduced damaged length of 8.6 cm and an increased limiting oxygen index of 35.0%, confirming excellent flame resistance. Cone calorimetry revealed an 85% reduction in peak heat release rate, substantially lowering fire hazards. Thermogravimetric analysis demonstrated earlier decomposition and a pronounced increase in char yield, with residues rising from 8.4% to 42.4% under nitrogen. Raman spectroscopy further indicated enhanced graphitization of the residues, consistent with the formation of a compact carbonaceous structure. Mechanistic studies revealed that the synergistic interaction of sodium alginate, chitosan and calcium ions catalyzed dehydration and carbonization, producing a robust char barrier that effectively suppressed heat transfer. Importantly, the layer-by-layer coating induced only minor changes in tensile strength, handle and whiteness, thereby preserving the fabrics' practicality.