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What is most afraid of reactive dyes? The color difference caused by hydrolysis?

Date:Jan 03, 2021

Reactive dyes are the most commonly used dyes in cotton dyeing, and their consumption is also on the rise. It is expected that this momentum will continue in the next few years. Reactive dyes are popular because of their moderate price, high color yield, and suitable color fastness. The only disadvantage is the hydrolysis of the dye.


The meaning of hydrolysis


The dye is usually fixed on the cotton under alkaline conditions, and this alkalinity promotes the reaction of the dye with water, leading to its inactivation. Inactivated dyes, that is, hydrolyzed dyes, cannot react with cotton, causing a considerable amount of dye to be lost. The hydrolyzed dyes physically adhere to the substrate until they are washed off during the washing treatment stage, thus causing washing fastness problems. In addition, hydrolyzed dyes will also flow into the waste liquid, resulting in increased pollution load.


Reactive dyes dye the cellulose fiber substrate in three stages:


The dye is dyed in a neutral medium with salt or sodium sulfate.


The dyeing dye is fixed after adding alkali, forming a covalent bond between the dye and the fiber.


Thoroughly wash away any loosely adhering dyes to or in the fabric.


The third stage mentioned above is very necessary. This is because most of the reactive dyes in the dyeing bath are dyed on the fiber, and some of them react with water under alkaline conditions and lose the reactivity with the fiber, so the affinity with the fiber is low. If this step is not performed properly, the presence of hydrolyzed dyes will result in poor washing fastness of dyed products; while dyes that react with fibers have high washing fastness.


The reaction of reactive dyes with water is not the only factor that affects the amount of dyeing. The application performance of the dye is also closely related to the following factors, such as the storage stability of the dye, the stability of the padding liquid or printing paste, and the change of the concentration of the reactive dye during the thermal dissolution of the dye formulation.


The hydrolytic decomposition of the dye starts from the production, and continues during the drying and grinding. The degree of dye hydrolysis depends on the active groups and chromophores in the molecule.


Hydrolyzed dyes, but the unstable atoms or groups in the original reactive dyes are replaced by hydroxyl groups, and its diffusion and adsorption performance is very similar to the original reactive dyes, so it is easy to diffuse into the fiber and adsorb its surface.


At the end of the alkaline dyeing stage, when the reactive dyes are completely consumed whether they react with water or the fiber, a new balance is established, that is, the distribution balance of the hydrolyzed dye in the fiber and the dye bath, and its mode is The distribution of reactive dyes in neutral solutions is similar. At this stage, the fibers contain a large amount of chemically bound dyes.


In order to obtain dyed products with high wet fastness, the hydrolyzed dyes must be washed off the fibers in the final washing process. In reactive dyeing, an average of about 70% of the dye is fixed on the fiber, and the rest is hydrolyzed and wasted. This part of the dye is at least 30%.


In the actual reaction system, there are many factors that affect the hydrolysis rate of reactive dyes, such as the pH value of the dye bath, temperature, dye concentration and electrolyte concentration.


(1) The influence of the pH value of the dyeing bath. By studying the relationship between the pH value of the medium and the degree of cellulose dissociation, it is found that the alkali concentration cannot be increased and the pH value of the solution exceeds 10-11. Because, at this time, all accessible hydroxyl groups in the cellulose are ionized, and increasing the pH will only cause its hydrolysis.


(2) The influence of temperature Increasing temperature will lead to more hydrolysis of reactive dyes.


(3) The influence of dye concentration Increasing the concentration of reactive dyes in the solution will promote their aggregation and reduce the rate of hydrolysis.


(4) The influence of electrolyte concentration The higher the electrolyte concentration in the dye bath, the more dye will be hydrolyzed.


"Research found that the reactivity of reactive dyes with water is also related to the chromophore in the dye molecule, because the chromophore affects the electron cloud density of the reaction center. If anionic groups are formed in the chromophoric system, the reaction center The electron cloud density increases, thereby reducing the reactivity of reactive dyes with water.