Bio-based coating of phytic acid, chitosan, and biochar for flame-retardant cotton fabrics

• Biochar was uniformly finished on the cotton fabric. • PA/CH/BC-coating improved the thermal stability and degradation of cotton. • PA/CH/BC (7.5%)-coated cotton fabric has excellent flame retardant performance. • PA/CH/BC (7.5%)-coated cotton fabric has good durable flame retardancy. A flame retardant coating containing phytic acid (PA), chitosan (CH), and biochar (BC) was prepared by the layer-by-layer assembly, and its flame retardancy after application to cotton fabrics was evaluated. The surface morphology, combustion properties, thermal degradation, and flame retardant behavior of untreated and coated cotton fabrics were investigated by scanning electron microscopy, cone calorimetry, and thermal gravimetric analysis coupled online with a mass spectrometer (TGA-MS). Results showed that the biochar was well dispersed in viscous chitosan and uniformly finished on the cotton fabric. Using the biochar, phytic acid, and chitosan (PA/CH/BC) coating significantly improved the char yield, protected the underlayer cotton fabric from degradation, and improved the thermal stability of the treated cotton through a condensed phase flame retardant mechanism. Cone calorimetry results confirmed that the PA/CH/BC-coated cotton fabric formed a thermally stable char layer during combustion, and this layer effectively inhibited oxygen and heat transfer. The peak heat release rate (pkHRR) and total heat release rate (THR) of PA/CH/BC-coated cotton fabric were significantly reduced compared with those of untreated cotton fabric. Compared to untreated fabric, cone calorimetry showed that cotton fabric treated with PA/CH/BC (7.5%) exhibited pkHRR and THR values that were decreased by 88.66% and 88.69%, respectively, and a limiting oxygen index that increased from 18.6% to 64.1%, indicating that the treated fabric has excellent flame retardant performance. Thermal degradation stability and thermal oxidation performance tests were conducted on treated materials subjected to ten laundering cycles to evaluate the durability of the flame retardant in practical application. TG analysis showed that the treated cotton exhibits favorable thermal degradation and thermal oxidation stability, and cone calorimetry suggested that approximately 60% of the flame retardant properties were retained after washing.