In the next few years, the sales revenue of China's dye industry will continue to grow steadily. By 2018, the sales revenue of China's dye industry will exceed 100 billion yuan. Speaking of the dyes that are most commonly used in dyeing, reactive dyes have to be mentioned. The popularity of reactive dyes is due to their moderate price, high color yield, and appropriate color fastness. The only drawback is the hydrolysis of the dye.
The meaning of hydrolysis: The dye is usually fixed on cotton under alkaline conditions, and this basicity promotes the reaction of the dye with water, resulting in its deactivation. Inactivated dyes, also known as hydrolyzed dyes, cannot react with cotton, resulting in considerable loss of dye. The hydrolyzed dye physically adheres to the substrate until it is washed out during the water wash process, thus causing problems with the wash fastness. In addition, hydrolyzed dyes can also cause human waste, resulting in increased pollution loads.
The reactive dye dyes the cellulose fiber matrix in three stages:
1. Dye the dye in a neutral medium with salt or Yuan Ming.
2. After fixing the base with the dye, add a covalent bond between the dye and the fiber.
3. Thoroughly remove dyes that loosely adhere to fabrics or fabrics.
The above third stage is very necessary. This is because the reactive dye in the dyeing bath is mostly dyed on the fiber, and partially reacts with water under alkaline conditions, loses its reactivity with the fiber, and thus has low affinity with the fiber. If this step is not performed properly, due to the presence of hydrolyzed dyes, poor wash fastness of the dyed product will result, while dyes reacting with the fiber will have high wash fastness.
The reaction of the reactive dye with water is not the only factor that affects the color yield of the dye. The application properties of the dyes are also closely related to factors such as the storage stability of the dyestuffs, the stability of the padding liquor or the printing paste, and the change in the concentration of the reactive dyes in the dyestuff formulation during the thermal dissolution process.
Hydrolysis of dyes begins to occur from the time of production and continues during drying and grinding. The degree of hydrolysis of the dye depends on the active groups and chromophores in its molecule.
Hydrolysis dyes, except that the unstable atoms or groups in the original reactive dyes are replaced by hydroxyl groups, and their diffusion and adsorption properties are very similar to those of the original reactive dyes, and therefore they easily diffuse into the fibers and adsorb the surface.
At the end of the alkaline dyeing stage, when the reactive dyes are completely consumed, whether they react with water or with fibers, a new balance is established, ie, the distribution balance of the hydrolyzed dyes in the fiber and the dyeing bath. The distribution of reactive dyes in neutral solutions is similar. At this stage, the fibers contain a large amount of chemically bound dye. In order to obtain a dye product with high wet fastness, the hydrolyzed dye must be washed off the fibers during the final water wash. In the dyeing of reactive dyes, on average, about 70% of the dyes are fixed to the fibers, and the remainder is hydrolyzed. This part of the dyes is at least 30%.
In actual reaction systems, there are many factors that affect the rate of hydrolysis of reactive dyes, such as bath pH, ​​temperature, dye concentration, and electrolyte concentration.
(1) Influence of the pH value of the dyebath The relationship between the pH value of the medium and the degree of dissociation of the cellulose was found to increase the alkali concentration so that the pH of the solution exceeded 10-11. This is because at this time all accessible hydroxyl groups in cellulose are ionized, and then increased in pH, it will only lead to hydrolysis.
(2) Influence of Temperature Increasing the temperature will result in increased hydrolysis of reactive dyes.
(3) Effect of Dye Concentration Increasing the concentration of reactive dye in the solution will promote its agglomeration and thus decrease the rate of hydrolysis.
(4) Effect of electrolyte concentration The higher the electrolyte concentration in the dye bath, the more dyes are hydrolyzed.
It has been found that the reactivity of the reactive dye with water is also related to the chromophore in the dye molecule, because the chromophore influences the electron cloud density in the reaction center. If the negative ion group is formed in the chromophoric system, the density of the electron cloud in the reaction center increases, thereby reducing the reactivity of the reactive dye with water.
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