The nature of cationic dyes

Solubility

The salt-forming alkyl and anionic groups in the cationic dye molecule affect the solubility of the dye. In addition, if there are anionic compounds in the dyeing medium, such as anionic surfactants and anionic dyes, they will also combine with cationic dyes to form a precipitate. Wool/nitrile, polyester/nitrile and other blended fabrics cannot be dyed in the same bath with ordinary cationic dyes and acidic, reactive, and disperse dyes, otherwise precipitation will occur. Generally add anti-settling agent to solve this kind of problem.

Sensitivity to pH

Generally, the stable pH range of cationic dyes is 2.5-5.5. When the pH value is low, the amino group in the dye molecule is protonated, changing from an electron donor to an electron withdrawing group, causing the color of the dye to change; if the pH value is high, the cationic dye may form a quaternary amine base, or the structure may be destroyed. Dyes precipitate, change color or fade. For example, oxazine dyes are transformed into non-cationic dyes in alkaline medium and lose their affinity for acrylic fibers and cannot be dyed.

Compatibility

Cationic dyes have a relatively high affinity for acrylic fibers, and have poor migration properties in the fibers, making it difficult to level dye. Different dyes have different affinities for the same fiber, and their diffusion rate inside the fiber is also different. When dyes with a large difference in dyeing rate are mixed, the color change and uneven dyeing are prone to occur during the dyeing process; When dyes with similar speeds are mixed, their concentration ratio in the dyeing bath is basically unchanged, so that the color of the product remains consistent and the dyeing is more uniform. The performance of this type of dyeing is called the compatibility of dyes.

For ease of use, people use numerical values to express the compatibility of dyes, usually expressed by K value. Use one set of yellow and blue standard dyes, each set consists of five dyes with different dyeing rates, and a total of five compatibility values (1, 2, 3, 4, 5), the dye compatibility value with high dyeing rate Small, poor dye migration and level dyeing. Dyes with low dyeing rate have a large compatibility value, and dye migration and level dyeing are better. Combine the dyes to be tested and the standard dyes one by one, and then evaluate the dyeing effect and determine the compatibility value of the dyes to be tested.

There is a certain relationship between the compatibility value of the dye and its molecular structure. The introduction of hydrophobic groups into the dye molecule will decrease the water solubility, increase the affinity of the dye to the fiber, increase the dyeing rate, decrease the compatibility value, decrease the migration and leveling properties on the fiber, and increase the amount of color. Certain groups in the dye molecule cause steric obstacles due to their geometric configuration, which also reduces the affinity of the dye to the fiber and increases the compatibility value.

Lightfastness

The light fastness of the dye is related to its molecular structure. The cationic group in the conjugated cationic dye molecule is a more sensitive part. It is easily activated from the position of the cationic group after being acted on by light energy, and then transferred to the entire color system, causing it to be damaged and faded. Conjugated triarylmethane The light fastnesses of polymethines, polymethines and oxazines are not good. The cationic group in the isolated cationic dye molecule is separated from the conjugated system by the linking group. Even if it is activated under the action of light energy, it is not easy to transfer the energy to the conjugated system of the color, making it well preserved, so it is resistant The light fastness is better than the conjugate type.