In reactive dyeing, although the fixation reaction rate is much higher than the hydrolysis reaction rate, if the process conditions are not properly controlled, such as too high pH value of dye liquor or too high temperature, it will accelerate the hydrolysis of dyes, reduce the fixation rate, increase the floating color on dyed fabrics, increase the post-treatment burden, and increase the dyeing cost and the treatment burden of printing and dyeing wastewater. Therefore, improving the fixation rate of reactive dyes is an important issue in reactive dyes dyeing.01 dye directness
The precondition of fixing color is dyeing fiber products with dyes, and the directness of dyes is the main factor affecting the dyeing rate. In general, the fixation rate of reactive dyes increases with the increase of directness. When the directness of dyes is low, the fixation rate increases rapidly with the increase of directness. When the directness reaches a certain level, the fixation rate increases little. This is because the dye with high directness has poor diffusion performance, which causes the dye to be fixed on the fiber surface. Therefore, the dye with high directness can not improve the fixation rate, but will reduce the levelness of the dye, and cause the hydrolyzed dye to be difficult to clean, thus affecting the color fastness. The directness of reactive dyes is generally low, which is one of the reasons for the low fixation rate of reactive dyes. Therefore, in order to improve the directness of reactive dyes, too high temperature and pH value should not be used for dyeing, because with the increase of dyeing temperature, the directness of reactive dyes decreases. At the same time, under the condition of higher pH value, the ionization degree of cellulose fiber increases, which repels the dye anion, and as a result, the dye adsorption amount decreases, as shown in the following figure.
02 dyeing PH value and temperature
The main factors affecting the reactivity of reactive dyes are dyeing temperature and pH value. In general, the dye reactivity (including reaction with fiber and reaction with water) increases with the increase of dye temperature and pH value. Because the temperature rises, the reaction rate of dyes increases; With the increase of pH value, the expansion degree of fiber and the diffusion rate of dye increase, which is beneficial to the reaction between dye and fiber. However, when pH value is too high, the ratio of [cell-o-]/[oh-] decreases, which is not conducive to the reaction between dye and fiber. In addition, too high temperature and pH value will accelerate the hydrolysis of dyes and reduce the fixation rate. Therefore, in order to ensure the normal fixation of dyes and minimize the hydrolysis of dyes, the fixation temperature and pH value should be reasonably determined according to the reactivity of dyes, and the pH value should generally be controlled at 10 ~ 11.
03 dyeing time
Dyeing with reactive dyes can be divided into two stages: dyeing and fixing. Prolonging the dyeing stage can make the dye fully diffuse and penetrate, improve the dyeing rate, and be beneficial to leveling. However, after fixation of reactive dyes, the dyes and fibers are covalently bonded, and the dyes have become a part of the fibers, which can no longer be transferred. Therefore, prolonging fixation time has little effect on leveling. For those dyes with weak reactivity, prolonging the fixation time can make the dye fixation more complete and help to improve the fixation rate. For dyes with poor alkali resistance of dye-fiber bond, prolonging fixation time will also lead to hydrolysis of the fixed dyes.
04 addition of electrolyte
In the dyeing process of reactive dyes, adding a proper amount of neutral electrolyte to promote dyeing is beneficial to improve the fixation rate. This is because adding neutral electrolyte can reduce the potential of the electrokinetic layer on the fiber surface and increase the directness of dye, thus improving the fixation rate. Its dyeing promotion principle and influencing factors are similar to those of direct dyeing.
05 dyeing bath ratio
The directness of reactive dyes is small. When exhaustion dyeing is adopted, the small bath ratio is beneficial to improve the dye uptake rate, increase the probability of reaction between dyes and fibers, reduce the hydrolysis of dyes and improve the fixation rate.
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