Acid dyes are generally aromatic sulfonic acid sodium salts, containing sulfonic acid groups, hydroxyl groups and other groups, a few are carboxylic acid sodium salts ionized in water to form pigment acid anions, which can be combined with amino or amide groups on the fiber. Therefore, it can be used for dyeing and printing of protein fibers such as wool and silk and polyamide synthetic fibers. Acid dyes have bright colors and good dye fastness. Most of these dyes are dyed in acid solutions, so they are called acid dyes.
Classification
Acid dyes include acid dyes, acid mordant dyes and acid mordant dyes. Some metal mordant dyes with a 1:2 complex structure can be used to dye nylon and vinylon. The color is darker.
Dyeing with acid dyes and 1:2 acid mordant dyes.
Acid dyeing
Acid dyes have complete chromatograms, bright colors, and light fastness and wet processing fastness vary greatly depending on the type of dye. Compared with direct dyes, acid dyes have a simple structure, lacking long conjugated double bonds and coplanar structures, so they lack directness to cellulose fibers and cannot be used for dyeing cellulose fibers. Different types of acid dyes have different dyeing properties and different dyeing methods due to their different molecular structures. According to the dyeing performance and dyeing method, acid dyes can be divided into strong acid and weak acid.
strong acid
Acid dyes for bath dyeing have a relatively simple molecular structure. Sulfonic acid groups occupies a large proportion in the entire molecular structure of the dye, so the solubility of the dye is relatively high. It exists in the dye bath in anionic form. When dyeing, the fiber must be dyed well in a strong acid dye bath, so it is called strong acid dye. This type of dye is combined with fiber in the form of ionic bond. It has good level dyeing performance and bright color, so it is also called level dyeing acid dye, but the wet processing fastness and perspiration fastness are both low. The addition of electrolyte can slow the dyeing effect.
weak acid
Acid dyes for bath dyeing The structure of this type of dye is more complex, and the proportion of sulfonic acid groups in the dye molecular structure is small, so the solubility of the dyes is low, and they have a greater tendency to aggregate in the solution. In addition to ionic bonding with fibers, intermolecular forces and hydrogen bonds play an important role when these dyes are dyed. When dyeing, it can be dyed in the weak acid dyeing margin, so it is called weak acid acid dye. The wet treatment fastness of this type of dye is higher than that of strong acid dyes, but the level dyeing property is not as good as strong acid dyes. These dyes are used in the dyeing of wool, silk and nylon.
Performance of various acid dyes Sulfuric acid Acetic acid Ammonium acetate
PH value of dye solution 2~4 4~66~7
Dye solubility high, medium and low
Level dyeing good fair poor
Wet fastness is poor, good, good
Directness to fiber Low Moderately High
Bonding mode with fiber Ionic bond Ionic bond, intermolecular force and hydrogen bond Intermolecular force and hydrogen bond
1:2 type acid mordant dyeing
The dyeing fastness of the acid mordant dye head material is better, the dyeing operation is simple, and the method is similar to that of acid dyes, but the leveling property is not as good as that of acid dyes. This type of dye is formed by complexation between ortho-, ortho-dihydroxy and azo compounds and trivalent chromium salts of organic acids (such as copper and chromium). According to the complexing ratio of dye and metal and the different complexing process, acid mordant dyes can be divided into 1:1 type and 1:2 type. Among them, 1:1 acid mordant dyes must be dyed in a strong acid bath. Damage, 1:2 acid mordant dye dyes in weak acid or neutral medium, so it is also called neutral dye.
The 1:2 acid mordant dye is formed by the complexation of two dye molecules and one metal atom. The dyeing of this kind of dye is the same as the acid dye of neutral bath dyeing. It can dye wool, silk, nylon, vinylon in slightly acid and neutral medium. Since the dye structure does not contain ionized hydrophilic groups, the dye has low hydrophilicity and poor solubility; the dye has a large molecular weight and poor level dyeing properties. When the pH value is low during dyeing, the combination of the dye and the protein fiber is dominated by ionic bonds, when the pH value is high, the combination with the fiber is dominated by hydrogen bonds and intermolecular attraction.
Acid dyes for neutral bath dyeing These dyes have a smaller proportion of sulfonic acid groups in the molecular structure, and they can dye fibers in a neutral dye bath, so they are called neutral acid dyes. When this type of dye is dyed, the bond between the dye and the fiber is mainly caused by intermolecular forces and hydrogen bonds. The neutral salt such as table salt and sodium sulfate does not slow dyeing but promotes dyeing. These dyes have poor level dyeing properties, but have good wet fastness, and they are also used for dyeing silk and wool.
This kind of dye does not need to be dyed in a strong acid medium. Ammonium acetate and ammonium sulfate are commonly used as auxiliary dyes. The dyeing time is short, the dyed product has better gloss and soft hand feeling, but the price is higher, and the color is not as good as acid dyes and 1:1 Type acid mordant dyes are bright and dark in color.
Dyeing at room temperature, heating to boiling within 30 to 45 minutes, and boiling for 30 minutes. Then wash and dry. The dye uptake rate of this kind of dyes varies greatly with temperature, and the temperature should be carefully controlled to obtain a higher dye uptake rate and leveling effect.
1:2 acid mordant dyes are widely used in the dyeing of vinylon and its blended fabrics. This type of dye has a higher direct dyeing effect on vinylon, but the dyeing rate is faster and it is not easy to level dye. In order to achieve level dyeing, when dyeing vinylon, the pH of the dyeing bath can be controlled at 4 to 5. For example, dyeing/cotton can control the pH of the dyeing bath from 6 to 7.5. At the same time, the temperature should not be too fast. When dyeing vinylon with 1:2 acid mordant dye, the color of the finished product is similar to that of wool. The dyeing fastness is higher and the light fastness is more prominent. Other fastnesses such as soaping, perspiration, friction and ironing are basically the same as those of dyeing. The results for wool are similar.