Coupling coloration of meta-aramid fabric utilising diazonium salts from weakly basic aromatic amines

Meta-aramid fibre exhibits poor dyeing performance when treated using conventional dyeing methods. In this study, coloration of meta-aramid fabrics was performed at room temperature utilising diazonium salts from weakly basic aromatic amines and was achieved via in situ construction of azo chromophores on the meta-phenylenediamine moiety. To promote diazonium-salt penetration into the meta-aramid fibre, the pretreatment of meta-aramid fabrics with dimethyl sulfoxide at 55 degrees C for 30 min and the utilisation of methanol were involved during the coloration process. As a result, the fabrics could be coloured with deep, levelled and firm shades of brown series. The azo chromophore is derived from the skeleton of the polyamide chain. Therefore, the coloured fabrics exhibit good resistance to organic solvents as well as good to excellent colour fastness to soaping, rubbing and sublimation. A study of the coloration mechanism revealed that the terminal and middle meta-phenylenediamine moieties serve as the main coupling reaction sites. When treated with a diazonium salt, the highly reactive terminal meta-phenylenediamine moiety reacted first and generated chromophores with an absorption peak in the long-wavelength region. However, such chromophores only contributed a limited colour depth owing to the limited number of the terminals. If excess diazonium salts were used, the main hue can be attributed to the azo chromophores based on the massive middle meta-phenylenediamine moieties, although these moieties are less reactive than the terminal meta-phenylenediamine moieties.