In-situ synthesis of a novel acid dye based on phosphonitrilic chloride trimer to develop coloured and flame-retardant wool

•Novel flame retardant acid dye for dyeing of wool.•Characterisation of dyed wool.•Enhancement in flame retardancy of dyed wool.•Novel compound for affecting thermal degradation and stability of wool.•New route to finish wool for preparing protective textiles. Nowadays, novel routes to develop flame retardant textiles have become a research hotspot. In the present study, an acid dye was developed to provide a wash-durable flame-retardant protection to wool. A phosphorus and nitrogen-based compound containing multiple amino groups was synthesised and characterised through nuclear magnetic resonance spectroscopy (1H, 31P, and 13C). The amine was also analysed by electrospray ionisation mass spectrometry (ESI-MS) analysis. The amine was diazotised and reacted with H-acid (4-Amino-5-hydroxy-2,7-naphthalenedisulfonic acid) at an acidic pH (3-4) to synthesise the acid dye in-situ on wool fabric. The flame retardancy provided by dyed wool was determined using the limiting oxygen index (LOI), vertical flammability test, and cone calorimetry analysis. The colour appearance, colour values, and fastness of dyed wool were determined to check the dyeing properties of the acid dye on wool. The dyed fabrics were also characterised through FTIR, SEM, ICP-MS, and TGA. The flame-retardancy mechanism of dyed wool was also assessed by characterising the char through FTIR, Raman spectroscopy, and SEM analyses. Thus, the newly developed acid dye provided wash-durable colouration and flame-retardant effects to wool. A maximum LOI value of 35.5 was obtained, and dyed wool displayed efficient flame-retardancy.