Although the fixation reaction rate is much greater than the hydrolysis reaction rate, if the process conditions are not properly controlled, such as the pH value of the dye solution is too high or the temperature is too high, the hydrolysis of the dye will be intensified, the fixation rate will be reduced, and floating colors on the dyed fabrics will appear. Increase, increase the burden of post-processing, increase dyeing costs, increase the burden of printing and dyeing wastewater treatment, etc. Therefore, improving the fixation rate of reactive dyes is an important issue in reactive dyeing.
01 Dye directness
The prerequisite for color fixation is dyeing fiber products. The directness of dye is the main factor affecting the dyeing rate. In general, the fixation rate of reactive dyes increases with the increase of substantivity. Taking vinyl sulfone reactive dyes as an example, the relationship between substantivity and fixation rate is shown in the figure.
Figure the relationship between the substantivity and fixation rate of reactive dyes
It can be seen from the curve in the figure that when the directness of the dye is low, the fixation rate increases rapidly with the increase of directness. When the directness reaches a certain level, the increase in fixation rate is no longer obvious.
This is because dyes with high substantivity have poor diffusion properties, causing the dye to be fixed on the surface of the fiber. Therefore, dyes with too high substantivity will not only fail to improve the fixation rate, but will actually reduce it. The leveling properties of dyes and the difficulty of washing hydrolyzed dyes affect the dye fastness.
Reactive dyes are generally less direct, which is one of the reasons for the low fixation rate of reactive dyes. Therefore, to improve the directness of reactive dyes, it is not advisable to use too high a temperature and pH value for dyeing, because as the dyeing temperature increases, the directness of reactive dyes decreases, as shown in the figure below.
The relationship between temperature and the directness and reactivity of dyes in the above figure
1—Directness 2—Reactivity
At the same time, under higher pH conditions, the degree of ionization of cellulose fibers increases, which repels dye anions. As a result The amount of dye adsorbed is reduced, as shown in the figure below.
Figure pH value and dye The relationship between directness and reactivity
1—directness 2—reactivity
02 Dyeing pH value and temperature
The main factors that affect the reactivity of reactive dyes are dyeing temperature and pH value. Generally speaking, as the temperature of the dye liquor increases, the pH value increases, and the dye reactivity increases (including reaction with fiber and reaction with water). as the picture shows.
Because the temperature increases, the reaction rate of the dye increases; when the pH value increases, the degree of puffing of the fiber increases, and the diffusion rate of the dye increases, which is beneficial to the reaction between the dye and the fiber. However, if the pH value is too high, the [Cell—O-]/[OH-] ratio decreases, which is not conducive to the reaction between dye and fiber. And too high temperature and pH value will accelerate the hydrolysis of the dye and reduce the color fixation rate. Therefore, in order to ensure the normal fixation of the dye and minimize the hydrolysis of the dye, the fixation temperature and pH value should be reasonably determined based on the reactivity of the dye itself. Generally, the pH value should be controlled between 10 and 11.
03 Staining time
Reactive dye dyeing is divided into two stages: dyeing and fixation. Extending the time of the dyeing stage can fully diffuse and penetrate the dye, increase the dyeing rate, and is conducive to leveling dyeing. However, after the reactive dye is fixed, the dye and the fiber are covalently bonded, and the dye has become part of the fiber and can no longer migrate. Therefore, extending the fixation time has little effect on leveling. For those dyes with weak reactivity, extending the fixation time can make the dye fixation more complete and help improve the fixation rate. For dyes with poor alkali resistance in the dye-fiber bond, prolonging the fixation time will also lead to hydrolysis of the fixed dye.
04 Adding electrolytes
During the dyeing process with reactive dyes , adding an appropriate amount of neutral electrolyte to promote dyeing will help improve the color fixation rate. This is because adding neutral electrolytes can reduce the potential of the electrodynamic layer on the fiber surface, increase the directness of the dye, and thus improve the color fixation rate. Its dyeing promotion principle and influencing factors are similar to those of direct dye dyeing.
05 Dyeing bath ratio
Reactive dyes are less direct. When exhaust dyeing is used, a small liquor ratio is beneficial to improving the dye uptake rate, and can increase the probability of the reaction between the dye and the fiber, reduce the hydrolysis of the dye, and improve the color fixation rate. </p
