Cleaner cationization of cotton fabrics by reusing modification bath for salt-free reactive dyeing

Cotton cationization is an important method to realize its salt-free dyeing, but the high dosage and low utilization of cationic agents result in severe environmental problems. This study synthesized a cationic polymer (quaternized poly(2-(dimethylamino)ethyl methacrylate) (QPDMAEMA)) for the reusable modification bath to significantly reduce the chemicals consumption during the cationization and dyeing of cotton fabric. The effect of polymer molecular weight, quaternization degree and pH value of the modification bath to the polymer adsorption and diffusion behavior on cotton fiber was studied. The results showed that under the pH value of 13, the low molecular weight and high charge density benefit the diffusion of cationic polymers in cotton fibers, so that the modified cotton fabrics can achieve salt-free dyeing performance similar to that of conventional dyeing. Meanwhile, well-distributed polymers can attract the dyes to diffuse more evenly within the fiber compared with high molecular polymers, thus ensuring better fastness of dyed fabrics. Through the optimization of polymer structure and application condition, about 80% cationic agent remaining in the residual bath can be successfully reused by replenishing a small amount of chemicals, and the modification and salt-free dyeing performance are relatively stable in 10 times of cationization bath reuse. Compared with conventional dyeing and salt-free dyeing with a normal modified process, the reused modified and salt-free dyeing process reduces by 97.98% and 71.02% chemicals consumption per kilogram of dyed cotton fabric, respectively.