Cationic dyes have a high affinity for acrylic fibers, and because of fast absorption and slow diffusion during dyeing, uneven dyeing is easy to occur. Once uneven dyeing occurs, it is difficult to correct it by extending the dyeing time. When dyeing with cationic dyes, in order to obtain uniform dyeing results, the dyeing rate should be appropriately reduced. In addition to the types of acrylic fiber, the factors affecting the dyeing rate of cationic dyes.
Temperature is an important factor in controlling levelling. When acrylic fiber is dyed with cationic dyes, it is rarely dyed below 75°C. When the dyeing temperature reaches the glass transition temperature of the fiber (75-85°C), the dye uptake rate increases rapidly. Therefore, when the dyeing temperature reaches the glass transition temperature of the fiber, the temperature should be increased slowly, generally 1°C every 2 to 4 min. It can also be kept at 85～90℃ for dyeing for a period of time, and then continue to heat up to boiling.
Adding acid in the dyeing bath can inhibit the dissociation of acidic groups in the acrylic fiber, reduce the number of anionic groups on the fiber, reduce the Coulomb attraction between the dye and the fiber, and reduce the dyeing rate. The effect of pH on the dyeing rate is more significant for acrylic fibers containing carboxylic acid groups, and the dyeing rate of acrylic fibers containing sulfonic acid groups is less affected by the pH of the dyeing bath. The pH value of the dye bath should be controlled reasonably during dyeing. Cationic dyes are generally not alkali-resistant, and the optimal pH value for dyeing is generally 4-4.5. When dyeing dark colors, the pH value of the dye bath can be higher, and dyeing light colors should be performed at a lower pH value. The pH value of the dye bath is generally adjusted with acetic acid. Acetic acid can not only reduce the pH value of the dye bath, but also improve the solubility of the dye. Adding sodium acetate to the dye bath at the same time can stabilize the pH value of the dye bath within the required range.
Adding electrolytes, such as sodium sulfate, table salt, etc. to the dyeing bath, can reduce the dyeing rate of cationic dyes and have a slow dyeing effect. Electrolyte has no obvious slowing effect on dyes with K value of 1~1.5, and has slowing effect on dyes with K value of 3~5. The slow dyeing effect of electrolyte decreases with the increase of dyeing temperature. When dyeing light colors, the amount of electrolyte can be higher, about 5% to 10% (o.w.f.), and not adding when dyeing dark colors.
In cationic dyeing, a retarder is often added to reduce the dyeing rate and obtain a uniform dyeing effect. Cationic dye retarders include cationic retarders and anionic retarders.
Cationic retarder is the most commonly used retarder for cationic dyes, most of which are cationic surfactants, such as 1227 surfactant (leveling agent TAN), 1631 surfactant (leveling agent IV). Cationic retarder has affinity for acrylic fiber. For cationic retarders with small molecules and low affinity with fibers, due to the fast diffusion rate, they first occupy the dye seat on the fiber during dyeing. After the dye cation enters the fiber, the affinity between the retarder and the fiber is less than that of the dye and fiber. The affinity of the dye will gradually be replaced by the dye, thereby reducing the dyeing rate. The amount of this kind of retarder should not be too large, otherwise the dyeing will be concentrated in the later stage of dyeing, which will cause uneven dyeing. For cationic dye retardants with complex molecular structure and greater affinity with fibers, they can compete with cationic dyes during dyeing, thereby reducing the dye uptake rate of cationic dyes. However, because cationic retarders occupy a certain dyeing position in the fiber, As a result, the percentage of cationic dye uptake is reduced. The higher the dosage of this kind of cationic retarder, the more significant the retarding effect, but the color of the dyed material becomes lighter. The amount of cationic retarder depends on the nature and concentration of the dye used. For dyes with a low K value or dyeing light colors, the amount of cationic retarder is higher. For dyes with larger K values or dyeing dark colors, The amount of retarder is lower. Cationic dye retardants also have saturation values for acrylic fibers, and there is a problem of compatibility with cationic dyes. Cationic dyes with higher affinity should be used with cationic dyes with higher affinity. In the dyeing prescription, the sum of the amount of cationic retarder and cationic dye should not exceed the dyeing saturation value of the fiber.
In addition to the cationic retarders mentioned above, there is another type of cationic retarders, namely polymeric cationic retarders, such as retarder A. This kind of retarder has a large molecule with a degree of polymerization of several hundred. Each macromolecule contains several hundred cationic groups. It is impossible to enter the fiber and can only cover the surface of the fiber, which makes the coulomb attraction between the cationic dye and the acrylic fiber great. Reduce, thereby reducing the dye uptake rate. This kind of retarder has stronger retarding ability than cationic surfactants, and does not occupy the dye seat in the fiber, and does not affect the dye saturation value of acrylic fiber. Most of the anionic retarders are negatively charged aromatic hydrocarbon sulfonates, which can be combined with cationic dyes to form a complex with low solubility, and are suspended in the dye bath with the aid of the dispersing effect of non-ionic additives. The resulting complex affects the fiber The affinity is less. After adding anionic slow dyeing vitex to the dyeing bath, the dye uptake rate is slowed down due to the reduction of the free dye cation concentration. As the dyeing temperature increases, the compound gradually decomposes and releases free dye cations, so that the dyeing rate gradually increases, so as to achieve the goal of leveling. Due to the low solubility of the compound, in order to avoid precipitation, it is necessary to add a nonionic surfactant while adding an anionic retarder. The use of anionic retarder can reduce the percentage of dye uptake more significantly than with cationic retarder. After using anionic retarder, cationic dyes and anionic dyes can be dyed in the same bath, which creates conditions for the one-bath dyeing of acrylic fiber blended fabrics with cationic dyes/acid dyes and cationic dyes/reactive dyes.