Sustainable Chemistry Method to Improve the Wash-off Process of Reactive Dyes on Cotton

Reactive dyes are extensively used for coloration of cellulosic fibers because of their excellent wash fastness (stability to washing with aqueous detergent solutions), which arises from covalent bond formation between dye and fiber. However, up to 40% of the dyestuff may hydrolyze in the dyeing process; this hydrolyzed dye has affinity for the fiber via hydrogen bonding and van der Waals interactions, but is not covalently bonded and as such exhibits poor wash fastness. Accordingly, these reactive dyeings require a multistep “wash-off” process after dyeing, involving various aqueous rinses and washings, in order for the dyeing to achieve the characteristic very high wash fastness. Wash-off and subsequent effluent treatment can account for up to 50% of the total cost of reactive dyeing and consumes significant amounts of water and energy; from a sustainable chemistry and engineering perspective, consumption of water and energy are arguably the biggest issues in textile dyeing. Existing and developmental dye transfer inhibiting (DTI) polymers were employed to remove unfixed (hydrolyzed) dyes. It was found that the use of DTI in the wash-off of reactive dyes enables a much more efficient, economical and sustainable process to be developed, which significantly reduces operation time, water consumption and energy consumption. Different DTI polymer types varied in their efficacy of dye removal; poly(vinylpyridine-N-oxide) polymers were the most effective with respect to the level of hydrolyzed dye removal closely followed by the poly(vinylpyridine betaine) polymers, which were also highly efficient in hydrolyzed dye removal, with poly(vinylpyrrolidone) polymers being the least effective. These differences were attributed to variations in the magnitude and delocalization of positive electrostatic potential of the functional moieties in the polymers and the extent of and propensity for hydrogen bonding with the hydrolyzed reactive dyes