Why reactive dyes have low fixation rate and poor color fastnessⅠ

Reactive dye molecules contain active groups, which can be covalently bonded with carboxyl, amino and hydroxyl groups on fibers, and can be used for fiber dyeing such as cellulose fibers, protein fibers and nylon. There are also some unsatisfactory aspects of reactive dyes, such as the fact that the fixing rate of reactive dyes is not very high, and it is easy to be hydrolyzed; the dyed products of reactive dyes will have discoloration, fading or staining during testing, use and storage; Soaping fastness, light fastness, chlorine bleaching fastness, wet rubbing fastness, etc. are not very ideal. How to overcome or improve these defects of reactive dyes, the author tries to analyze the reasons for the low fixation rate and poor color fastness of reactive dyes from the aspects of the dyeing mechanism and dyeing process of reactive dyes, and discusses how to improve the fixation rate and color fastness of reactive dyes. Some of the methods and approaches to the degree.

Fixation and hydrolysis of reactive dyes

Reactive dyes are composed of four parts: dye parent, bridge group, active group and water-soluble group in molecular structure, and are mainly used for dyeing cellulose fibers. Under alkaline conditions, the fibers are hydrolyzed to generate cellulose negative ions, which can undergo nucleophilic reaction with the reactive groups of reactive dyes to form covalent bonds to generate dye-fiber macromolecules (called bonding reaction or color-fixing reaction). At the same time as the fixation reaction occurs, the hydroxide ion OH▔ in the dye liquor can also undergo a nucleophilic reaction with the dye to generate a hydrolyzed dye (called a hydrolysis reaction). This hydrolyzed dye loses its activity and can no longer undergo a fixation reaction with the fiber. The reactive dyes that have reacted with cellulose, the dye-fiber covalent bond is not very stable, and may also be hydrolyzed under certain conditions, resulting in the breakage of the dye-fiber covalent bond. The resulting hydrolyzed dyes have little affinity for fibers and are easily washed away (the hydrolyzed dyes have water-soluble groups), resulting in fading of the dyed product. The more hydrolyzed dyes, the lower the fixation rate, the worse the color fastness and the more serious the pollution.

Factors affecting the fixation rate and color fastness of reactive dyes

2.1 Factors Affecting Fixation Rate

In general, the rate of dye fixation reaction is much higher than that of hydrolysis reaction, but when the reaction conditions are not controlled properly, the dye hydrolysis will be aggravated and the fixation rate will decrease. There are several factors that affect the fixation rate.

2.1.1 Directness of dyes

The directness of the dye is the main factor affecting the dye adsorption capacity of the fiber. In general, the fixation rate of reactive dyes increases with the increase of the directness of the dyes, but when the directness increases to a certain extent, the increase of the fixation rate is not obvious, but it will make the hydrolyzed dyes not easy to be washed off, affecting the dyeing fastness and dyeing. uniformity. Therefore, the directness of the dye should be appropriate, and the directness of the dye mainly depends on the dye parent.

2.1.2 pH and temperature of dye solution

Under normal circumstances, when the temperature of the dye liquor increases, the pH value increases, and the dye fixing reaction rate and the hydrolysis reaction rate increase at the same time, but the hydrolysis reaction rate increases faster than the fixing reaction rate, and the color fixing rate decreases instead. Therefore, a reasonable fixing temperature and pH value should be determined according to the reactivity of the dye to avoid a decrease in the fixing rate.

2.1.3 Dyeing time

Extending the fixing time, on the one hand, can make the dye fix more fully, which is beneficial to improve the fixing rate; but on the other hand, the reactive dyes with poor alkali resistance (or acid resistance) of the dye-fiber bond can prolong the fixing time. , on the contrary, the fixed dye will be hydrolyzed from the fiber, resulting in a decrease in the fixing rate. Therefore, the fixing time should also be appropriate.