Factors Affecting The Coloring Of Cationic Dyes

Cationic dyes have a strong affinity for acrylic fibers. During dyeing, due to their rapid adsorption and slow diffusion, uneven dyeing is prone to occur. Once uneven dyeing happens, it is difficult to correct it by extending the dyeing time. When dyeing with cationic dyes, to achieve uniform dyeing results, the dyeing rate should be appropriately reduced. In addition to the type of acrylic, the factors influencing the dyeing rate of cationic dyes also include the following aspects:

Temperature

Temperature is an important factor in controlling uniform dyeing. When dyeing acrylic with cationic dyes, the dye uptake is very little below 75℃. However, when the dyeing temperature reaches the glass transition temperature of the fiber (75-85℃), the dye uptake rate increases rapidly. Therefore, when the dyeing temperature reaches the glass transition temperature of the fiber, it should be increased slowly, generally by 1 ℃ every 2 to 4 minutes. It can also be maintained at 85 to 90 degrees Celsius for dyeing for a period of time, and then the temperature can be further increased to boiling.

pH value of the dye bath

Adding acid to the dye bath can inhibit the dissociation of acidic groups in acrylic fibers, reduce the number of anionic groups on the fibers, decrease the Coulombic force between the dye and the fibers, and lower the dyeing rate. The influence of pH value on the dyeing rate is more significant for acrylic fibers containing carboxylic acid groups, while the dyeing rate of acrylic fibers containing sulfonic acid groups is less affected by the pH value of the dyeing bath. The pH value of the dyeing bath should be reasonably controlled during dyeing. Cationic dyes are generally not resistant to alkali. The optimal pH value for dyeing is usually 4 to 4.5. When dyeing dark colors, the pH value of the dye bath can be higher, while for light colors, the dyeing should be carried out at a lower pH value. The pH value of the dye bath is generally adjusted with acetic acid. Acetic acid can not only lower the pH value of the dye bath but also increase the solubility of the dye. Adding sodium acetate simultaneously to the dye bath can stabilize the pH value of the dye bath within the required range.

Electrolyte

Adding electrolytes such as mirabilite and table salt to the dye bath can reduce the loading rate of cationic dyes and has a retarding effect. The electrolyte has no obvious retarding effect on dyes with a K value of 1 to 1.5, but it does have a retarding effect on dyes with a K value of 3 to 5. The retarding effect of electrolytes 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.), but it is not necessary when dyeing dark colors.

Dye retarder

In cationic dye dyeing, retarders are often added to reduce the dyeing rate and achieve uniform dyeing results. There are two types of retarders for cationic dyes: cationic retarders and anionic retarders.

Cationic retarders are the most commonly used retarders in cationic dye dyeing. Most of them are cationic surfactants, such as 1227 surfactant (leveling agent TAN) and 1631 surfactant (leveling agent Ⅳ state). Cationic retarders have an affinity for acrylic fibers. For cationic retarders with smaller molecules and lower affinity for fibers, due to their fast diffusion rate, they first occupy the dye seats on the fibers during dyeing. Once the dye cations enter the fibers, as the affinity of the retarder for the fibers is less than that of the dye for the fibers, they will gradually be replaced by the dye, thereby reducing the dyeing rate. The dosage of this type of dye retarder should not be too large; otherwise, the dye will be concentrated in the later stage of dyeing, which will instead cause uneven dyeing. For cationic retarders with complex molecular structures and strong affinity for fibers, they can compete with cationic dyes during dyeing, thereby reducing the dye uptake rate of cationic dyes. However, since cationic retarders will occupy a certain dyeing position in the fibers, the dye uptake percentage of cationic dyes will decrease. The higher the dosage of this type of cationic dye retarder, the more significant the retarding effect, but the color of the dyed substance becomes lighter.

The dosage of cationic retarder is determined by the nature and concentration of the dye used. When dyeing dyes with a low K value or light colors, the dosage of cationic retarder is higher; when dyeing dyes with a large K value or deep and intense colors, the dosage of retarder is lower. Cationic retarders also have saturation values for acrylic fibers, and there are also compatibility issues with cationic dyes. For cationic dyes with higher affinity, cationic retarders with higher affinity should be used. In the dyeing prescription, the combined dosage of cationic retarder and cationic dye should not exceed the dyeing saturation value of the fiber.

In addition to the above-mentioned cationic retarders, there is another type of cationic retarder, namely the polymeric cationic retarder, such as retarder A. This type of dye retarder has relatively large molecules with a degree of polymerization of several hundred. Each large molecule contains several hundred cationic groups. It cannot enter the interior of the fiber but can only cover the surface of the fiber, significantly reducing the Coulombic attraction between cationic dyes and acrylic fibers, thereby lowering the dye uptake rate. The retarding ability of this type of retarder is stronger than that of cationic surfactants, and it does not occupy the dye spots in the fibers, thus having no impact on the dyeing saturation value of acrylic.

Most anionic retarders are negatively charged aromatic sulfonates, which can combine with cationic dyes to form complexes with lower solubility and be suspended in the dye bath with the dispersion effect of nonionic auxiliaries. The formed complexes have a relatively small affinity for fibers. After adding anionic dye retarders to the dye bath, the concentration of free dye cations is reduced, which slows down the dye uptake rate. As the dyeing temperature increases, the complex gradually decomposes, releasing free dye cations, which gradually raises the dyeing rate and thus achieves the purpose of uniform dyeing. Due to the low solubility of the complex, to prevent precipitation, nonionic surfactants need to be added simultaneously with anionic dye retarders. The reduction in the dye uptake percentage is more significant when using anionic retarders than when using cationic retarders.

After using anionic retarders, cationic dyes and anionic dyes can be dyed in the same bath, creating conditions for one-bath dyeing of acrylic blended fabrics with cationic dyes/acid dyes and cationic dyes/reactive dyes.