Salt-Free Dyeing Of Cotton Knit Fabric With Reactive Dyes

Reactive dyes can form strong covalent bond with cotton fibres during the dyeing process, which leads to its good color fastness, expecially its fastness to wet rubbing. In addition, reactive dyes havg-a wide color and low cost of production. But there exist a prominent problem with its dyeing. During the dyeing proces both the fibers and dyes are negatively charged and the repulsion is very big. So it need lots of inorganic salt to promote its dye rate and to increase its utilization ratio. Large amout of salts not only increase the cost of the dyeing mills, but also cause huge pollution on the environment. So it is imperative to dye cotton without such environmentally-unfriendly salt.This research uses cationic reagent to modify the cotton knit fabric, after which the nagtive charge on the surface of the fibre have decreased or changed to positive charge. The modified cotton knit fabric is dyed by reactive dyes without salt. The optimum salt-free dyeing condition under different dye concentration is obtained through response surface methodology. Fistly, elemental analysis is used to illustrate if the cationic reagent has formed covalent bond with cotton fibres and Zeta potention is used to detect the potention change of the cotton fibre before and after modification. Secondly, the dyeing rate of the three-primary colours on the modified knit cotton is tested, which is to confirm if the modified knit cotton have no harsh requirment on reactive dyes. Thirdly, four factors affecting the dyeing process of the salt-free dyeing are obtained by single-factor experiment. Meanwhile the levels of the four factors are also obtained by this experiment.The four factors are dyeing temperature, liquor ratio, dyeing time, dosage of sodium carbonate. Fourthly, on the basis of single-factor experiment, response surface methodology (RSM), based on a three level, four variables is employed to obtain the best possible combination of dyeing temperature, liquor ratio, dyeing time, dosage of sodium carbonate. This is applied to determine the optimal conditions of salt-free dyeing under different dye concentration. Lastly it is verification and comparation test that is testing the K/S value and the levelling property and the color fastness of the dyed fabric in the optimal salt-free dyeing conditions under different dye concentration. Besides, those best dyed fabics are compared to other dyed fabics obtained through the conventional dyeing process. The levelling property and the color fastness of the best dyed fabric is better than the fabric dyed through the conventional dyeing process.