Borate cross-linked layer-by-layer assembly of green polyelectrolytes on polyamide 66 fabrics for flame-retardant treatment

Borate cross-linked thin coatings made from green polyelectrolytes, chitosan (CS) and phytic acid (PA), were deposited on polyamide 66 (PA66) fabrics via layer-by-layer (LbL) assembly to develop a durable flame retardant method. The surface and morphological characterization showed that CS and PA were successfully deposited onto the PA66 fabrics and cross-linked by borate. The presence of the coating slightly changed the limiting oxygen index (LOD values, but suppressed flame spread and completely stopped melt-dripping of PA66 in UL 94 vertical burning test. The CS-PA coating also decreased the peak heat release rate of PA66 observed in cone calorimeter tests, and a maximum reduction of 31% was observed for PA66-10BL fabric sample. The addition of CS, PA and borate moieties lowered the initial decomposition temperature of PA66, but the char yield and char quality were improved. Especially, the char of the PA66 fabrics treated by borate exhibited better thermal resistance in thermogravimetric analysis (TGA) tests. More importantly, it was noticed that the stability of deposited CS-PA coating in sodium dodecyl sulfate (SDS) solution was enhanced significantly after cross-linking by borate, which would be favourable to durable flame-retardant treatment.