Dye Basics: Cationic Dyes

Cationic dyes are special dyes for polyacrylonitrile fiber dyeing, and can also be used for the dyeing of modified polyester (CDP). Today, I will share the basic knowledge of cationic dyes.

An overview of cationic dyes

1. History
Cationic dyes are one of the earliest synthetic dyes produced. The aniline violet synthesized by W.H.Perkin in the United States in 1856 and the subsequent crystal violet and malachite green are all cationic dyes. These dyes were formerly known as basic dyes, which can dye protein fibers and cellulose fibers treated with tannin and tartar. They have bright colors, but are not lightfast, and were later developed by direct dyes and vat dyes. and acid dyes.

After the industrial production of acrylic fibers in the 1950s, it was found that on polyacrylonitrile fibers, cationic dyes not only have high directness and bright color, but also have a much higher color fastness than protein fibers and cellulose fibers. arouse people's interest. In order to further adapt to the application of acrylic fibers and other synthetic fibers, many new varieties with high fastness have been synthesized, such as polymethine structure, nitrogen-substituted polymethine structure and pernalactam structure, etc., so that cationic dyes become polyacrylonitrile. A class of main dyes for fiber dyeing.

2. Features:
Cationic dyes generate positively charged colored ions in solution, and form salts with acid anions such as chloride ion, acetate group, phosphate group, methyl sulfate group, etc., thereby dyeing polyacrylonitrile fibers. In actual dyeing, several cationic dyes are commonly used to form a specific color. However, the mixed dyeing of cationic dyes is often difficult to evenly dye into the same color light, resulting in mottled and layered. Therefore, in the production of cationic dyes, in addition to expanding the variety and quantity, we must also pay attention to the matching of dye varieties; in order to prevent dyeing, we must pay attention to developing varieties with good levelness, and also pay attention to improving the steam fastness of cationic dyes. and light fastness.

Second, the classification of cationic dyes

The positively charged group in the cationic dye molecule is connected with the conjugated system in a certain way, and then forms a salt with the anionic group. According to the position of the positively charged group in the conjugated system, cationic dyes can be divided into two categories: isolated and conjugated.

1. Isolated cationic dyes
The isolating cationic dye precursor and the positively charged group are connected through the isolating group, and the positive charge is localized, similar to the introduction of quaternary ammonium group at the molecular end of disperse dyes. It can be represented by the following formula:

Due to the concentration of positive charges, it is easy to combine with fibers, and the dyeing percentage and dyeing rate are relatively high, but the levelness is poor. Generally, the shade is dark, the molar absorbance is low, and the shade is not strong enough, but it has excellent heat resistance and light fastness, and high fastness. It is often used in dyeing medium and light colors. Typical varieties are:

2. Conjugated cationic dyes
The positively charged group of the conjugated cationic dye is directly connected to the conjugated system of the dye, and the positive charge is delocalized. The color of this type of dye is very bright and the molar absorbance is high, but some varieties have poor light fastness and heat resistance. Among the types used, the conjugated type accounts for more than 90%. There are many varieties of conjugated cationic dyes, mainly including triarylmethane, oxazine and polymethine structures.

3. New cationic dyes

1. Migration cationic dyes
The so-called migratory cationic dyes refer to a class of dyes with relatively simple structure, small molecular weight and molecular volume, and good diffusivity and leveling performance, which have now become a large category of cationic dyes. Its advantages are as follows:

It has good migration and leveling properties, and has no selectivity to acrylic fibers. It can be applied to different grades of acrylic fibers and better solve the problem of uniform dyeing of acrylic fibers. The amount of retarder is small (from 2 to 3% to 0.1 to 0.5%), and it is even possible to dye single color without adding retarder, so the use can reduce the cost of dyeing. It can simplify the dyeing process and greatly shorten the dyeing time from (the original 45 to 90 minutes to 10 to 25 minutes).

2. Cationic dyes for modification:
In order to adapt to the dyeing of modified synthetic fibers, a batch of cationic dyes was screened and synthesized. The following structures are suitable for modified polyester fibers. Yellow is mainly conjugated methine dyes, red is triazole-based or thiazole-based azo dyes and isolating azo dyes, and blue is thiazole-based azo dyes and azo dyes. Oxazine dyes.

3. Disperse cationic dyes:
In order to adapt to the dyeing of modified synthetic fibers, a batch of cationic dyes was screened and synthesized. The following structures are suitable for modified polyester fibers. Yellow is mainly conjugated methine dyes, red is triazole-based or thiazole-based azo dyes and isolating azo dyes, and blue is thiazole-based azo dyes and azo dyes. Oxazine dyes.

4. Reactive cationic dyes:
Reactive cationic dyes are a new class of cationic dyes. After the reactive group is introduced into the conjugated or isolated dye molecule, this kind of dye is given special properties, especially on the blended fiber, it not only maintains the bright color, but also can dye a variety of fibers.

Fourth, the properties of cationic dyes

1. Solubility:
The salt-forming alkyl and anionic groups in the cationic dye molecule have been described above to 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 precipitates. Wool/nitrile, polyester/nitrile and other blended fabrics cannot be dyed in the same bath with common cationic dyes and acid, reactive and disperse dyes, otherwise precipitation will occur. Anti-precipitation agents are generally added to solve such problems.

2. Sensitivity to pH:
Generally, cationic dyes are stable in the pH range of 2.5 to 5.5. When the pH value is low, the amino group in the dye molecule is protonated, and the electron-donating group is converted into an electron-withdrawing group, causing the color of the dye to change; Precipitation, discoloration, or fading of the dye occurs. For example, oxazine dyes are converted into non-cationic dyes in an alkaline medium, which loses their affinity for acrylic fibers and cannot be dyed.

3. Compatibility:
Cationic dyes have a relatively large affinity for acrylic fibers, and have poor migration performance in fibers, making it difficult to level dye. Different dyes have different affinities for the same fiber, and their diffusion rates inside the fiber are also different. When dyes with greatly different dyeing rates are mixed together, color changes and uneven dyeing are likely to occur during the dyeing process. When the dyes with similar rates are mixed, their concentration ratio in the dye bath is basically unchanged, so that the color of the product remains consistent and the dyeing is more uniform. The performance of this dye combination is called the compatibility of dyes.

For the convenience of use, people use numerical values ​​to express the compatibility of dyes, usually expressed as K value. One set of yellow and blue standard dyes is used, each set is composed of five dyes with different dyeing rates, and there are five compatibility values ​​(1, 2, 3, 4, 5), and the compatibility value of the dye with the largest dyeing rate Small, the migration and levelness of the dye are poor, and the dye with a small dyeing rate has a large compatibility value, and the migration and levelness of the dye are better. The dye to be tested and the standard dye are dyed one by one, and then the dyeing effect is evaluated to determine the compatibility value of the dye to be tested.

There is a certain relationship between the compatibility value of dyes and their molecular structures. Hydrophobic groups are introduced into the dye molecules, the water solubility decreases, the affinity of the dye to the fiber increases, the dyeing rate increases, the compatibility value decreases, the migration and levelness on the fiber decrease, and the color supply increases. Some groups in the dye molecule cause steric barriers due to geometric configuration, which also reduces the affinity of the dye to fibers and increases the compatibility value.

4. Lightfastness:

The light fastness of dyes is related to its molecular structure. The cationic group in the conjugated cationic dye molecule is a relatively sensitive part. It is easily activated from the position of the cationic group after being acted by light energy, and then transferred to the entire chromophore system, causing it to be destroyed and faded. Conjugated triarylmethane The light fastness of oxazine, polymethine and oxazine is 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, so that it is well preserved. The light fastness is better than the conjugated type.

5. Extended reading: Cationic fabrics
Cationic fabric is 100% polyester fabric, which is woven from two different all-polyester raw materials, but contains modified polyester fiber. This modified polyester fiber and ordinary polyester fiber are colored with different dyes and dyed twice. Color, one-time polyester dyeing, one-time cationic dyeing, generally use cationic yarn in the warp direction, and ordinary polyester yarn in the weft direction. Two different dyes are used when dyeing: ordinary disperse dyes for polyester yarns, and cationic dyes for cationic yarns (also known as cationic dyes). Disperse cationic dyes can be used), the cloth effect will have a two-color effect.

Post time: Jul-21-2022