Single-molecular engineering and design of azo-colourant for dyeing and flame retardancy of cotton with durable performance

A novel functionalized azo-dye acts as a single-molecule strategy for simultaneously dyeing and flame-retarding cotton fabric as reported through the synthesis and application of a novel phosphorodiamidate-azo dye, HAPD-AZO. Starting from H-acid (1-amino-8-naphthol-3,6-disulfonic acid) and phosphoryl trichloride, a phosphorodiamidate dimer (HAPD) was synthesized in which two H-acid units are covalently bridged through a central phosphorodiamidate P N linkage and subsequently coupled with diazotized sulfanilic acid. The resulting HAPD-AZO carries two P N bonds, six sulfonate groups and an azo chromophore within a single covalent scaffold, characterized by 31 P NMR, 1 H NMR, FTIR, UV–Vis, XRD, XPS, and TGA. Exhaust dyeing of cotton fabric at 2–12% owf yielded deep red-magenta colouration (λ max = 526 nm; K/S = 34.66 at 12% owf) with wash fastness grade 4. HAPD-AZO functions as a direct-type dye on cotton, binding through non-covalent hydrogen bonding and van der Waals interactions. Flame retardancy increased progressively with dye loading; the optimum formulation FR-AZO-7 (12% owf) achieved a Limiting oxygen index (LOI) of 41.9 ± 1.1%, approximately 2.3 times that of untreated cotton, with complete self-extinction. It sustains flame for more than 300 s in Continuous flame test (CFT) also it demonstrates excellent results in vertical flammability testing (VFT) (char length 3.6 cm), there was a 93% reduction in peak heat release rate (7.03 vs. 100.83 kW/m 2 ) and total heat release (1.25 vs. 18.13 MJ/m 2 ) by cone calorimetry. The flame-retardant mechanism operates through a dominant condensed-phase pathway, as evidenced by constant phosphorus retention (5.9 wt%) in EDX analysis before and after combustion, with a secondary gas-phase contribution from radical scavenging. This work establishes covalent P N integration into an azo dye scaffold as a proof-of-concept, single-step route to coloured, durable and self-extinguishing cotton textiles.