Sustainable and efficient dyeing method for cotton fibers: A diazo-based pre-activation strategy

Cotton fabrics, as both a polysaccharide and a high-molecular-weight polymer, are typically dyed with reactive dyes, which require a high amount of salt and produce significant colored effluent, increasing treatment complexity. This paper presents a novel and revolutionary reactive dyeing technique based on diazo pre-activation and coupling coloration of cotton fibers, which is distinct from conventional reactive dyeing methods. It demonstrates a room-temperature, salt-free reactive dyeing method to address problems like high energy consumption, significant salt usage, wastewater treatment challenges, the long duration from dye preparation to application, and the difficulty of recycling cotton fibers in reactive dyeing processes. This method combines dye preparation and dyeing by amination, diazotization, and coupling. It produces salt-free, room-temperature dyeing in <4 h, which is substantially less time than a standard reactive dyeing procedure that requires both dye synthesis and application. Furthermore, cotton fabrics can be endowed with bright, deep, and diverse colors, achieving superior colorfastness with ratings of 3-4 or more and K/S values exceeding 12. Moreover, dyed cotton fabrics can be easily decolorized by alkaline washing, producing alkaline washed cotton (AWC), while preserving their good whiteness and recyclable nature.