| Reactive dyes have become the preferred usage in cellulose fibers dyeing since their introduction in the market, depending on their excellent dyeing properties. However, it is very easy for the reactive dyes to hydrolyze in the conventional dyeing system. So there is considerable loss of reactive dyes and the fixation rate is40～85%. Additionally, in order to achieve the desired level of color fastness, a thorough wash-off treatment is required for reactive cotton dyeings to remove hydrolyzed and/or unfixed dyes distributed inside and outside fibers after dyeing. Hence, many researches have focused on the issue of reactive high fixation dyeing methods and the way to wash off these dyes generated from reactive cotton dyeings.Although dye utilization was improved by pure solvent dyeing, this dyeing method was not popular for the limit of dye species, solvent safety and dyeing process. Little-liquor dyeing method was also rarely used, which was affected by many factors, such as dye solubility, special dyeing machine, dyeing unevenness and the surface of fabric scratched easily. In addition, the conventional wash-off treatment after dyeing consumes a huge amount of water and heating energy as well as sewage discharge. This research aims to investigate application of ethanol-water system acted as medium in reactive cotton dyeing and wash-off.At the beginning of this paper, based on the characteristics of reactive dyes and solubility parameters, the solubility of reactive dyes in different solvents were studied. According to both the physical properties of conventional organic solvents and security of using, ethanol-water system was selected to act as medium in reactive cotton dyeing and wash-off after dyeing. The solubility of reactive dyes and the dye up-take with the increase of the volume fraction of ethanol in ethanol-water system were discussed. It was demonstrated that the solubility of reactive dyes decreased and the dyeing rate increased with the increase of the volume fraction of ethanol in ethanol-water system, which was consistent with the variation of the bath ratio in traditional dyeing system. The exhaustion of the three reactive dyes (RY84, RR120and RB171) in ethanol-water (9:1v/v) system without salt was99%, and that of RB71was more than82%. The maximum exhaustion of the four dyes was achieved when the concentration of sodium carbonate in ethanol-water (9:1v/v) system was optimized to5g/L.The exhaustion and fixation in ethanol-water (9:1v/v) system in the absence of salt with cotton fabrics pretreated in sodium carbonate solution was higher than that of conventional dyeing system in the presence of salt. When the concentration of reactive dyes was1%, the exhaustion of RY84, RR120, RB171and RB71increased by13.74%,17.59%,11.87%and39.77%, respectively. The fixation of RY84, RR120, RB171and RB71increased by19.67%,23.62%,8.90%and39.26%, respectively. So, it can be seen that salt-free dyeing with reactive dyes in ethanol-water system is achievable, and the ethanol-water (9:1v/v) dyeing method improves the utilization of reactive dyes.On the basis of the kinetics of reactive dyeing in ethanol-water (9:1v/v) system, the diffusion coefficient decreased with extension of dyeing times. The diffusion coefficient and dyeing rate constant were increased with the increase of temperature, but half-dyeing time was decreased. The diffusion coefficient on cotton fabrics pretreated in basic solution was larger than that on cotton fabrics pretreated in water solution, because the diameters of cotton fiber pretreated in basic solution were larger than that pretreated in water solution. At the beginning period of reactive dyeing in ethanol-water (9:1v/v) system, the fixation of cotton fabrics pretreated in caustic soda solution was higher than that in sodium carbonate solution. However, the fixation declined gradually with the extension of dyeing times, because of covalent bonds between dyes and fibers hydrolyzed in high concentration of caustic soda.By comparing with dyeing processes of reactive dyes in ethanol-water system, it showed that the optimum dyeing process was that cotton fabrics were immersed in the solution with dyes and one fourth of ethanol and dyed at40℃for10minutes, then the remaining ethanol was added to the solution, which dyed at60℃for30minutes, and finally dyed at80℃for30～60minutes after sodium carbonate was added in ethanol-water (8:2v/v) system. As indicated in this paper for exhaustion and fixation, not only did the ethanol-water (8:2v/v) dyeing method increase the utilization of reactive dyes4.87～23.41%, compared to the conventional water dyeing method, but also reduced the amount of water and sewage discharge by means of distilling the dyeing effluent to recover ethanol. In addition, it is very important that the destruction of environment is prevented because of no salt in the ethanol-water dyeing process. The ethanol-water (8:2v/v) dyeing method is worth promoting widely.Additionally, the factors influencing desorption of hydrolyzed dyes in ethanol-water system acted as wash-off medium after reactive dyeing were investigated, which included volume fraction of ethanol, pH and temperature. The maximum desorption was achieved when the volume fraction of ethanol was optimized to40%. The desorption was increased with the increase of pH and temperature. The kinetic curves of hydrolyzed dyes desorption in ethanol-water (4:6v/v) system at three different temperatures fit the two-step kinetic model, which presents a multi-step desorption process:a rapid desorption step followed by a slow one. When desorption reached equilibrium, the dye desorpability at60℃in ethanol-water system was close to that by using conventional water washing in the presence of detergent at95℃. This study has concluded that the wash-off in ethanol-water system can be considered as an alternative, more effective process for removing hydrolyzed dyes from reactive cotton dyeings.The detergents were added in ethanol-water (4:6v/v) system in order to study the synergy effect between ethnaol and detergents. It showed that when the detergent of XC-W was added in ethanol-water (4:6v/v) system, the inhibition of the adsorption of hydrolyzed reactive dyes to cotton fabrics and fastness of reactive cotton dyeings were increased effectively. The longer wash-off time lasted, the higher the concentration of dye in residue remained, indicating that wash-off time could increase washing efficiency. But wash-off time was unfavorable to inhibit the re-adsorption of hydrolyzed reactive dyes to cotton fabrics. So the wash-off time should not keep too long.After comparing wash-off methods for reactive cotton dyeings, the optimum ethanol-water wash-off method was that cotton fabric were treated successively in the ethanol-water (4:6v/v) system with the detergent of XC-W at80℃for20minutes, in the ethanol-water (4:6v/v) system at60℃for10minutes, then in the water solution at room temperature for10minutes. As indicated in this paper, not only does the optimum ethanol-water (4:6v/v) wash-off method offer potential savings in water and energy consumptions, compared to the conventional water wash-off method, but also reduces dye effluents in wash-off. |
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