Color variations of anthraquinone and azo reactive dyes on cellulose caused by nitrogen oxides under wet conditions

The effects of nitric acid and nitrous acid on the color variations of aminoanthraquinone and azo dyes on cellulose film were examined. The immersion of dyed films in an acidic aqueous nitrite solution led to color variations caused by the diazotization of the amino groups in the anthraquinone and azo reactive dyes examined in addition to the dye–fiber bond scission by acid hydrolysis. When the dyed films immersed in an acidic nitrite solution were exposed to light, the photodecomposition of the diazotized dyes occurred to generate the corresponding phenols via dediazotization, together with the products generated by the photofading of the original dye. No nitrosation of the imino groups was observed for many reactive dyes with imino groups. The color fastness of reactive dyes to nitrogen oxide is primarily attributed to the irreversible color change caused by the apparently direct transformation of amino groups into hydroxyl ones through these processes. Dyes that show larger color changes at higher rates possess lower color fastness to nitrogen oxide.