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How to improve the solubility of acid dyes?

Date:Dec 31, 2020

Acid dyes, direct dyes and reactive dyes are all water-soluble dyes. The output in 2001 was 30,000 tons, 20,000 tons and 45,000 tons, respectively. However, for a long time, my country's dyestuff enterprises have paid more attention to the development and research of new structural dyes, while the research on the post-processing of dyes has been relatively weak. Commonly used standardization reagents for water-soluble dyes include sodium sulfate (sodium sulfate), dextrin, starch derivatives, sucrose, urea, naphthalene formaldehyde sulfonate, etc. These standardization reagents are mixed with the original dye in proportion to obtain the required strength Commodities, but they can not meet the needs of different printing and dyeing processes in the printing and dyeing industry. Although the above-mentioned dye diluents are relatively low in cost, they have poor wettability and water solubility, making it difficult to adapt to the needs of the international market and can only be exported as original dyes. Therefore, in the commercialization of water-soluble dyes, the wettability and water solubility of the dyes are issues that need to be resolved urgently, and the corresponding additives must be relied on.

Dye wettability treatment

   Broadly speaking, wetting is the replacement of a fluid (should be a gas) on the surface by another fluid. Specifically, the powder or granular interface should be a gas/solid interface, and the process of wetting is when liquid (water) replaces the gas on the surface of the particles. It can be seen that wetting is a physical process between substances on the surface. In dye post-treatment, wetting often plays an important role. Generally, the dye is processed into a solid state, such as powder or granule, which needs to be wetted during use. Therefore, the wettability of the dye will directly affect the application effect. For example, during the dissolution process, it is undesirable for the dye to be difficult to wet and float on the water. With the continuous improvement of dye quality requirements today, wetting performance has become one of the indicators to measure the quality of dyes. The surface energy of water is 72.75mN/m at 20℃, which decreases with the increase of temperature, while the surface energy of solids is basically unchanged, generally below 100mN/m. Usually metals and their oxides, inorganic salts, etc. are easy to wet Wet, called high surface energy. The surface energy of solid organics and polymers is comparable to that of general liquids, which is called low surface energy, but it changes with the solid particle size and degree of porosity. The smaller the particle size, the greater the degree of porous formation, and the surface The higher the energy, the size depends on the substrate. Therefore, the particle size of the dye must be small. After the dye is processed by commercial processing such as salting out and grinding in different media, the particle size of the dye becomes finer, the crystallinity is reduced, and the crystal phase changes, which improves the surface energy of the dye and facilitates wetting.

Solubility treatment of acid dyes

   With the use of small bath ratio and continuous dyeing technology, the degree of automation in printing and dyeing has been continuously improved. The emergence of automatic fillers and pastes, and the introduction of liquid dyes require the preparation of high-concentration and high-stability dye liquors and printing pastes. However, the solubility of acidic, reactive and direct dyes in domestic dye products is only about 100g/L, especially for acid dyes. Some varieties are even only about 20g/L. The solubility of the dye is related to the molecular structure of the dye. The higher the molecular weight and the fewer sulfonic acid groups, the lower the solubility; otherwise, the higher. In addition, the commercial processing of dyes is extremely important, including the crystallization method of the dye, the degree of grinding, the particle size, the addition of additives, etc., which will affect the solubility of the dye. The easier the dye is to ionize, the higher its solubility in water. However, the commercialization and standardization of traditional dyes are based on a large amount of electrolytes, such as sodium sulfate and salt. A large amount of Na+ in water reduces the solubility of the dye in water. Therefore, to improve the solubility of water-soluble dyes, first do not add electrolyte to commercial dyes.

Additives and solubility

  ⑴ Alcohol compound and urea cosolvent

   Because 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 carry a certain amount of negative charge. When a cosolvent containing a hydrogen bond forming group is added, a protective layer of hydrated ions is formed on the surface of the dye ions, which promotes the ionization and dissolution of the dye molecules to improve the solubility. Polyols such as diethylene glycol ether, thiodiethanol, polyethylene glycol, etc. are usually used as auxiliary solvents for water-soluble dyes. Because they can form a hydrogen bond with the dye, the surface of the dye ion forms a protective layer of hydrated ions, which prevents the aggregation and intermolecular interaction of the dye molecules, and promotes the ionization and dissociation of the dye.

  ⑵Non-ionic surfactant

Adding a certain non-ionic surfactant to the dye can weaken the binding force between the dye molecules and between the molecules, accelerate ionization, and make the dye molecules form micelles in water, which has good dispersibility. Polar dyes form micelles. The solubilizing molecules form a network of compatibilization between the molecules to improve the solubility, such as polyoxyethylene ether or ester. However, if the co-solvent molecule lacks a strong hydrophobic group, the dispersion and solubilization effect on the micelle formed by the dye will be weak, and the solubility will not increase significantly. Therefore, try to choose solvents containing aromatic rings that can form hydrophobic bonds with dyes. For example, alkylphenol polyoxyethylene ether, polyoxyethylene sorbitan ester emulsifier, and others such as polyalkylphenylphenol polyoxyethylene ether.

  ⑶ lignosulfonate dispersant

   dispersant has a great influence on the solubility of dyes. Choosing a good dispersant according to the dye structure will greatly help improve the solubility of dyes. In water-soluble dyes, it plays a certain role in preventing mutual adsorption (van der Waals force) and aggregation among dye molecules. Among the dispersants, lignosulfonate is the most effective, which has been studied in China.

The molecular structure of disperse dyes does not contain strong hydrophilic groups, but only weakly polar groups, so it has only weak hydrophilicity, and the actual solubility is very small. Most disperse dyes can only dissolve in water at 25℃. 1~10mg/L.

  The solubility of disperse dyes is related to the following factors:

  Molecular Structure

"The solubility of disperse dyes in water increases as the hydrophobic part of the dye molecule decreases and the hydrophilic part (the quality and quantity of polar groups) increases. That is to say, the solubility of dyes with relatively small relative molecular mass and more weak polar groups such as -OH and -NH2 will be higher. Dyes with larger relative molecular mass and fewer weakly polar groups have relatively low solubility. For example, Disperse Red (I), its M=321, the solubility is less than 0.1mg/L at 25℃, and the solubility is 1.2mg/L at 80℃. Disperse Red (II), M=352, solubility at 25℃ is 7.1mg/L, and solubility at 80℃ is 240mg/L.

  Dispersant

In    powdered disperse dyes, the content of pure dyes is generally 40% to 60%, and the rest are dispersants, dustproof agents, protective agents, sodium sulfate, etc. Among them, the dispersant accounts for a larger proportion.

The dispersant (diffusion agent) can coat the fine crystal grains of the dye into hydrophilic colloidal particles and disperse it stably in water. After the critical micelle concentration is exceeded, micelles will also be formed, which will reduce part of the tiny dye crystal grains. Dissolved in micelles, the so-called "solubilization" phenomenon occurs, thereby increasing the solubility of the dye. Moreover, the better the quality of the dispersant and the higher the concentration, the greater the solubilization and solubilization effect.

It should be noted that the solubilization effect of dispersant on disperse dyes of different structures is different, and the difference is very large; the solubilization effect of dispersant on disperse dyes decreases with the increase of water temperature, which is exactly the same as the effect of water temperature on disperse dyes. The solubility has the opposite effect.

   After the hydrophobic crystal grains of the disperse dye and the dispersant form hydrophilic colloidal particles, its dispersion stability will be significantly improved. Moreover, these dye colloidal particles play the role of "supplying" dyes during the dyeing process. This is because after the dye molecules in the dissolved state are absorbed by the fiber, the dye "stored" in the colloidal particles will be released in time to maintain the dissolution balance of the dye.

  The state of disperse dye in the dispersion

  1-dispersant molecule

  2-Dye crystallite (solubilization)

  3-dispersant micelle

  4-Dye single molecule (dissolved)

  5-Dye grain

  6-dispersant lipophilic base

  7-dispersant hydrophilic base

  8-sodium ion (Na+)

   9-aggregates of dye crystallites

   However, if the "cohesion" between the dye and the dispersant is too large, the "supply" of the dye single molecule will lag or the phenomenon of "supply exceeds demand". Therefore, it will directly reduce the dyeing rate and balance the dyeing percentage, resulting in slow dyeing and light color.

  It can be seen that when selecting and using dispersants, not only the dispersion stability of the dye should be considered, but also the influence on the color of the dye.

  (3) Dyeing solution temperature

  The solubility of disperse dyes in water increases with the increase of water temperature. For example, the solubility of Disperse Yellow in 80°C water is 18 times that at 25°C. The solubility of Disperse Red in 80°C water is 33 times that at 25°C. The solubility of Disperse Blue in 80°C water is 37 times that at 25°C. If the water temperature exceeds 100°C, the solubility of disperse dyes will increase even more.

   Here is a special note: this dissolving property of disperse dyes will bring hidden dangers to practical applications. For example, when the dye liquor is heated unevenly, the dye liquor with high temperature flows to the place with low temperature. As the water temperature decreases, the dye liquor becomes supersaturated, and the dissolved dye will precipitate, causing the growth of dye crystal grains and the decrease of solubility , Resulting in reduced dye uptake.

   (four) dye crystal form

  Some disperse dyes have the phenomenon of "isomorphism". That is, the same disperse dye, due to the different dispersion technology in the manufacturing process, will form several crystal forms, such as needles, rods, flakes, granules, and blocks. In the application process, especially when dyeing at 130°C, the more unstable crystal form will change to the more stable crystal form.

   It is worth noting that the more stable crystal form has greater solubility, and the less stable crystal form has relatively less solubility. This will directly affect the dye uptake rate and dye uptake percentage.

  (5) Particle size

  Generally, dyes with small particles have high solubility and good dispersion stability. Dyes with large particles have less solubility and relatively poor dispersion stability