Solubility Treatment of Acid Dyes

With the use of small liquor ratio and continuous dyeing process, the degree of automation of printing and dyeing has been continuously improved, the emergence of automatic fillers and pulp mixing, and the introduction of liquid dyes require the preparation of high-concentration and high-stability dye liquors and printing pastes. However, at present, the solubility of acid, reactive and direct dyes in domestic dye products is only about 100g/L, especially acid dyes, and some varieties are even only about 20g/L. The solubility of dyes is related to the molecular structure of dyes. The larger the molecular weight and the fewer sulfonic acid groups, the lower the solubility; otherwise, the higher the solubility. In addition, the commercial processing of dyes is extremely important, including the crystallization mode, grinding degree, particle size, and addition of additives of the dyes, which will affect the solubility of the dyes. The more easily the dye is ionized, the higher its solubility in water. However, the commercialization and standardization of traditional dyes is to apply a large amount of electrolytes, such as yuanming powder and table salt. A large amount of Na+ in water reduces the solubility of dyes in water. Therefore, to improve the solubility of water-soluble dyes, first, no electrolyte is added to commercial dyes.

Additives and Solubility

(1) Alcohol compounds and urea cosolvents 

Since water-soluble dyes contain a certain number of sulfonic acid groups and carboxylic acid groups, the dye particles are easily dissociated in aqueous solution and have a certain amount of negative charges. When a cosolvent containing a hydrogen bond forming group is added, a hydrated ion protective layer is formed on the surface of the dye ion, which promotes the ionization and dissolution of the dye molecule to improve the solubility. Polyols such as diethylene glycol ether, thiodiethanol, polyethylene glycol, etc. are usually used as cosolvents for water-soluble dyes. Because they can form hydrogen bonds with dyes, the surface of dye ions forms a protective layer of hydrated ions, which prevents dye molecular agglomeration and intermolecular interactions, and promotes dye ionization and dissociation.

(2) Nonionic surfactants

Adding a certain nonionic surfactant to the dye can weaken the binding force between the dye molecules and molecules, accelerate the ionization, and make the dye molecules form micelles in water with good dispersibility. Polar dyes form micelles. The solubilizing molecules form network compatibilization to improve solubility, such as polyoxyethylene ether or ester. However, if there is a lack of strong hydrophobic groups in the cosolvent molecule, the dispersing and solubilizing effects on the micelles formed by the dye are weak, and the solubility improvement is not significant. Therefore, try to choose an aromatic ring-containing solvent that can form a hydrophobic bond with the dye. For example, alkylphenol polyoxyethylene ether, polyoxyethylene sorbitan ester emulsifier, and others such as polyalkylphenylphenol polyoxyethylene ether.

(3) Lignosulfonate dispersant

The dispersant has a great influence on the solubility of the dye. Choosing a good dispersant according to the structure of the dye is of great help to improve the solubility of the dye. In water-soluble dyes, in order to prevent the mutual adsorption (van der Waals force) and agglomeration between dye molecules, it has a certain effect. Among the dispersants, lignosulfonate is the most effective, and there are domestic studies on it.