Although the color fixing reaction rate is much higher than the hydrolysis reaction rate, if the process conditions are not properly controlled, such as the dye pH value is too high or the temperature is too high, the dye hydrolysis will be aggravated, the color fixing rate will be reduced, the floating color on the dyed fabric will be increased, the post-treatment burden will be increased, the dyeing cost will be increased, and the dyeing wastewater treatment burden will be increased. Therefore, improving the fixation rate of reactive dyes is an important problem in the dyeing of reactive dyes.

01 Dye directness

The premise of color fixation is to dye the fiber products, and the directness of the dye is the main factor affecting the dyeing rate. Under normal circumstances, the color fixation rate of reactive dyes increases with the increase of directness. Taking vinyl sulfone reactive dyes as an example, the relationship between directness and color fixation rate is shown in the figure.

Figure the relationship between the directness of reactive dyes and color retention

It can be seen from the curve in the figure that when the directness of the dye is low, the color fixation rate increases rapidly with the increase of the directness, and when the directness reaches a certain degree, the increase of the color fixation rate is not obvious.

This is because the dye with high directness has poor diffusion performance, so that the dye is fixed on the fiber surface, so the dye with high directness can not improve the color fixation rate, but reduce the evenness of the dye, and cause the hydrolyzed dye is not easy to wash, affecting the dyeing fastness.

The directness of reactive dyes is generally small, which is one of the reasons for the low color fixation rate of reactive dyes. Therefore, to improve the directness of reactive dyes, it is not appropriate to use too high temperature and pH value on dyeing, because with the increase of dyeing temperature, the directness of reactive dyes decreases, as shown in the figure below.

Figure 1 shows the relationship between temperature and the directness and reactivity of dyes

1 - Directness 2 - Reactivity

At the same time, under the condition of higher pH value, the degree of ionization of cellulose fiber is increased, which repelling the dye anion, resulting in a decrease in the dye adsorption capacity, as shown in the figure below.

Figure pH in relation to the directness and reactivity of the dye

1 - Directness 2 - Reactivity

02 Dyeing pH and temperature

The main factors affecting the reactivity of reactive dyes are dyeing temperature and pH value. In general, the temperature of the dye solution increases, the pH value increases, and the reactivity of the dye is enhanced (including the reaction with the fiber and the reaction with water). As shown in the figure.

As the temperature increases, the reaction rate of the dye increases. With the increase of pH value, the swelling degree of fiber and the diffusion rate of dye increase, which is conducive to the reaction of dye and fiber. However, the pH value is too high, and the [Cell - O-]/[OH-] ratio decreases, which is not conducive to the reaction between dyes and fibers. And too high temperature and pH value will accelerate the dye hydrolysis, reduce the color fixation rate. Therefore, in order to ensure the normal color fixation of the dye and minimize the hydrolysis of the dye as far as possible, the color fixation temperature and pH value should be reasonably determined according to the reactivity of the dye itself, and the general pH value is controlled at 10 to 11.

03 Dyeing time

Reactive dye dyeing is divided into two stages of dyeing and fixing. Prolongation of dyeing time can make the dye fully diffuse and permeate, and increase dyeing rate, which is conducive to even dyeing. However, after the reactive dye is fixed, the dye and the fiber have covalent bonding, and the dye has become a part of the fiber, and can not be transferred. Therefore, lengthening the fixation time has little effect on levelling. For those dyes with weak reactivity, extending the color fixation time can make the dye more fully fixed, which is conducive to improving the color fixation rate. For dyes with poor alkaline resistance of dye-fiber bond, prolongation of color fixation time will also lead to hydrolysis of dyes that have been fixed.

04 Electrolyte addition

In the process of dyeing reactive dyes, adding an appropriate amount of neutral electrolyte to promote dyeing is conducive to improving the color fixation rate. This is because the addition of neutral electrolyte can reduce the electrokinetic layer potential on the fiber surface, increase the directness of the dye, and thus improve the color fixation rate. The dyeing promotion principle and influencing factors are similar to those of direct dyeing.

05 Dye bath ratio

The direct activity of reactive dyes is small, when the use of dry dyeing, small bath ratio is conducive to improving the dyeing rate of dyes, and can increase the probability of reaction between dyes and fibers, reduce the hydrolysis of dyes, improve the color fixation rate.