Acid dyes, direct dyes and reactive dyes are all water-soluble dyes, and the output in 2001 was 30,000 tons, 20,000 tons and 45,000 tons respectively. However, for a long time, my country's dye enterprises have paid more attention to the development and research of new structural dyes, while the research on the post-processing of dyes is relatively weak. Standardizing reagents commonly used for water-soluble dyes include sodium sulfate (yuanming powder), dextrin, starch derivatives, sucrose, urea, naphthalene formaldehyde sulfonate, etc. These standardizing reagents are mixed with the original dye in proportion to obtain the required strength. Commodities, but they cannot meet the needs of different printing and dyeing processes in the printing and dyeing industry. Although the cost of the above-mentioned dye thinners is relatively low, the wettability and water solubility are relatively poor, so it is difficult to meet 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 urgent problems, and corresponding auxiliaries must be relied on.
Wetting treatment of dyes
In a broad sense, wetting is the replacement of one fluid (should be a gas) on a surface by another. Specifically, the powder or granular interface should be a gas/solid interface, and the process of wetting occurs when the liquid (water) replaces the gas on the particle surface. It can be seen that wetting is a surface physical process between substances. In the post-processing of dyes, wetting often plays an important role. Generally, dyes are processed into solid forms, such as powder or granules, which need to be wetted when used. 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 surface. With the continuous improvement of the quality requirements of dyes today, wetting performance has been used as one of the indicators to measure the quality of dyes. The surface energy of water is 72.75mN/m at 20°C, which decreases with the increase of temperature, while the surface energy of solid is basically unchanged, generally below 100mN/m. Usually metals and their oxides, inorganic salts, etc. are easy to wet. wet, known as 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 varies with the particle size of the solid and the degree of porosity. The higher the energy, its size depends on the substrate. Therefore, the particle size of the dye must be small. After the dye is commercialized by salting out and grinding in different media, the particle size of the dye becomes finer, the crystallinity decreases, and the crystal phase changes, which increases the surface energy of the dye and facilitates wetting.
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 pastes, 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.
⑶ 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.
The molecular structure of disperse dyes does not contain strong hydrophilic groups, but only weak polar groups, so it has only weak hydrophilicity, and the actual solubility is very small. Most disperse dyes can only dissolve 0. 1～10mg/L.
The solubility of disperse dyes is related to the following factors:
The solubility of disperse dyes in water increases with the decrease of the hydrophobic part and the increase of the hydrophilic part (mass and quantity of polar groups) in the dye molecule. That is, dyes with relatively small molecular weight and more weak polar groups such as -OH and -NH2 have higher solubility. The relative molecular mass is larger and the dyes with less weak polar groups have relatively low solubility. Such as disperse red (I), its M=321, the solubility at 25°C is less than 0.1mg/L, and the solubility at 80°C is 1.2mg/L. Disperse red (Ⅱ), its M=352, the solubility at 25℃ is 7.1mg/L, and the solubility at 80℃ is 240mg/L.
In powdery disperse dyes, the content of pure dyes is generally 40% to 60%, and the rest is dispersant, dustproof agent, protective agent, Yuanming powder, etc. Among them, the proportion of dispersants is larger.
The dispersing agent (diffusing agent) can not only coat the fine grains of the dye into hydrophilic micelles and stably disperse them in water, but also form micelles after the critical micelle concentration is exceeded, which will disperse some of the tiny dye grains. Dissolved in micelles, the so-called "solubilization" phenomenon occurs, thereby increasing the solubility of dyes. Moreover, the better the quality of the dispersant, the higher the concentration, the greater the solubilization and solubilization effect.
It should be noted that the solubilization effect of dispersant on disperse dyes with 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 that of water temperature on disperse dyes themselves. Solubility has the opposite effect.
After the hydrophobic crystal grains of the disperse dye and the dispersant form hydrophilic colloidal particles, the dispersion stability will be significantly improved. Moreover, these dye micelles also "supply" the dye during the dyeing process. Because, after the dye molecules in the dissolved state are adsorbed by the fibers, the dyes "stored" in the colloidal particles will be released in time to maintain the dissolution balance of the dyes.
The state of existence of disperse dyes in dispersion liquid
1 - Dispersant molecule
2- Dye crystallites (solubilized)
4-Dye Monomolecules (Dissolved)
5- Dye Grains
6-Dispersant lipophilic group
7-Dispersant hydrophilic group
8-Sodium ion (Na+)
9 - Aggregates of dye crystallites
However, if the "cohesion force" between the dye and the dispersant is too large, the "supply" of the dye single molecule will lag or the phenomenon of "in short supply" will occur. Therefore, it will directly reduce the dyeing rate and the equilibrium 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 dyes, but also the influence on the coloring of the dyes should be considered.
(3) Temperature of dye solution
The solubility of disperse dyes in water increases with the increase of water temperature. For example, the solubility of disperse yellow in water at 80°C is 18 times that at 25°C. The solubility of disperse red in water at 80°C is 33 times that at 25°C. The solubility of disperse blue in water at 80°C is 37 times that at 25°C. If the water temperature exceeds 100°C, the solubility of disperse dyes will increase more greatly.
A special reminder here: this dissolving characteristic 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. Due to the decrease of water temperature, the dye liquor becomes supersaturated state, and the dissolved dyes will be precipitated, resulting in the growth of dye crystals and the decrease in solubility. , resulting in a decrease in the uptake rate.
(4) Dye crystal form
Some disperse dyes have the phenomenon of "isomorphism". That is to say, the same disperse dye will form several crystal forms, such as needle-like, rod-like, flake-like, granular, block-like, etc. During the application process, especially when dyeing at 130 °C, those less stable crystal forms will transform into more stable crystal forms.
It is worth noting that the more stable crystal forms have greater solubility, and the less stable crystal forms have relatively less solubility. This directly affects the dye uptake rate and percentage uptake.
(5) Particle size
Generally, dyes with small particles have high solubility and good dispersion stability. The dyes with large particles have low solubility and relatively poor dispersion stability.
At present, the particle size of domestic disperse dyes is generally 0.5 to 2.0 μm (Note: Dispersion dyeing requires a particle size of 0.5 to 1.0 μm).
In summary, there is still a certain gap between the technical level and application research of dye production in my country and developed countries. Therefore, while researching the new structure of dyes, we must also pay attention to the research on the post-processing of dye commercialization, and continuously develop products with excellent application performance. Dissolved products and post-treatment processes, thus promoting the continuous development of my country's dye industry.