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What is the cause of color spots and stains in reactive dyeing?

Date:Mar 26, 2021

Reactive dyes have a very good solubility in water. Reactive dyes mainly rely on the sulfonic acid group on the dye molecule to dissolve in water. For meso-temperature reactive dyes containing vinylsulfone groups, in addition to the sulfonic acid group, the β -Ethylsulfonyl sulfate is also a very good dissolving group.


In the aqueous solution, the sodium ions on the sulfonic acid group and the -ethylsulfone sulfate group undergo hydration reaction to make the dye form anion and dissolve in the water. The dyeing of the reactive dye depends on the anion of the dye to be dyed to the fiber.


The solubility of reactive dyes is more than 100 g/L, most of the dyes have a solubility of 200-400 g/L, and some dyes can even reach 450 g/L. However, during the dyeing process, the solubility of the dye will decrease due to various reasons (or even completely insoluble). When the solubility of the dye decreases, part of the dye will change from a single free anion to particles, due to the large charge repulsion between the particles. Decrease, the particles and particles will attract each other to produce agglomeration. This agglomeration firstly gathers dye particles into agglomerates, then turns into agglomerates, and finally turns into flocs. Although the flocs are a kind of loose assembly, they are The surrounding electric double layer formed by positive and negative charges is generally difficult to decompose by the shear force when the dye liquor circulates, and the flocs are easy to precipitate on the fabric, resulting in surface dyeing or staining.


Once the dye has such agglomeration, the color fastness will be significantly reduced, and at the same time it will cause different degrees of stains, stains, and stains. For some dyes, the flocculation will further accelerate the assembly under the shear force of the dye solution, causing dehydration and salting out. Once salting out occurs, the dyed color will become extremely light, or even not dyed, even if it is dyed, it will be serious color stains and stains.



Causes of dye aggregation


The main reason is the electrolyte. In the dyeing process, the main electrolyte is the dye accelerant (sodium salt and salt). The dye accelerant contains sodium ions, and the sodium ion equivalent in the dye molecule is much lower than that of the dye accelerant. The equivalent number of sodium ions, the normal concentration of the dye accelerator in the normal dyeing process will not have much influence on the solubility of the dye in the dye bath.


However, when the amount of dye accelerant increases, the concentration of sodium ions in the solution increases accordingly. Excess sodium ions will inhibit the ionization of sodium ions on the dissolving group of the dye molecule, thereby reducing the solubility of the dye. After more than 200 g/L, most dyes will have different degrees of aggregation. When the concentration of the dye accelerator exceeds 250 g/L, the degree of aggregation will increase, forming agglomerates first, and then in the dye solution. Agglomerates and floccules are formed quickly, and some dyes with low solubility are partially salted out or even dehydrated. Dyes with different molecular structures have different anti-agglomeration and salt-out resistance properties. The lower the solubility, the anti-agglomeration and salt-tolerant properties. The worse the analytical performance.


The solubility of the dye is mainly determined by the number of sulfonic acid groups in the dye molecule and the number of β-ethylsulfone sulfates. At the same time, the greater the hydrophilicity of the dye molecule, the higher the solubility, and the less hydrophilic. The lower the solubility. (For example, dyes with an azo structure are more hydrophilic than dyes with a heterocyclic structure.) In addition, the larger the molecular structure of the dye, the lower the solubility, and the smaller the molecular structure, the higher the solubility.