Basic Knowledge of Cationic Dyes

2023-09-27

Basic Information

Cationic dyes are a type of textile dyes, also known as basic dyes and salt-based dyes. Dissolved in water in a cationic state, cationic dyes are soluble in water and ionize in aqueous solution to produce dyes with positively charged colored ions. The cation of the dye can combine with the acidic group of the third monomer in the fabric to dye the fiber. It is a special dye for acrylic fiber dyeing, with the advantages of high strength, bright color, good light fastness and so on.
Cationic dyestuff is produced with the development of petrochemical industry. With the appearance of acrylonitrile fiber in 1950s, Geigy Company of Switzerland and Bayer Company of Germany developed cationic dyestuff for acrylonitrile fiber successively. In the early 1960s, China also developed the first generation of cationic dyestuffs, and in the 1970s, the second generation of X-type cationic dyestuffs was developed, which enlarged the chromatographic range, improved the dyeing performance, and took into consideration of the compatibility between ternary colors, and in the 1970s, the third generation of M-type cationic dyestuffs was produced on a trial basis, which is called migratory cationic dyestuffs, and it can be applied to dyeing of various kinds of acrylonitrile fibers.
In recent years, both domestic and foreign countries have devoted themselves to the research and development of new varieties with excellent dyeing performance, specialization and new structure, as well as the improvement of old products and energy saving and consumption reduction.

Cationic dyestuffs can be categorized according to two classification methods: application performance and chemical structure type. According to the application performance, China categorizes the existing dyes into common type, X type and M type.

(1) Isolated cationic dyes: the positive charge in the dye molecule is connected to the conjugate system of the dye chromophore through the isolation group, and the positive charge is fixed on the nitrogen atom of the quaternary ammonium salt. Cationic dyes are characterized by a slightly lower amount of color giving, not very bright color light, but heat, sun, acid and alkali resistance, good stability. For example, C.I.11085 Cationic Red GTL is a dark red powder, dissolved in water, dark red, reddish orange in concentrated sulfuric acid, red after dilution, dyeing acrylics is dark red. It is produced by hydroxyethylation of N-ethyl aniline, chlorination, ammoniation; diazotization of 2-chloro-4-nitroaniline and coupling with the above ammoniation, then methylation, filtration and drying. It is used for the printing and dyeing of acrylic and its blended fabrics, and can also be used for the dyeing of modified acrylic and polyester, and the printing of acetate fabrics.

(2) Conjugated cationic dyestuffs: The positive charge in the dye molecule is part of the conjugated system of the dye chromophore, and the positive charge is not fixed on a certain atom, but can be shifted. This type of dyes is characterized by bright color and high coloring power, but medium fastness to sunlight. For example, C.I. 48056 Cationic Yellow X-6G is a brownish-yellow powder, slightly soluble in cold and hot water, and prints and dyes acrylics in a brilliant greenish glossy yellow color. It is produced by condensing 1,3,3-trimethylindoline acetaldehyde with p-aminoanisole, filtered and dried. It is used for the printing and dyeing of acrylic and its blended fabrics, and can also be used for the printing of acetate fiber, PVC fiber and modified acrylic fiber.

Cationic dyes are special dyes for acrylonitrile fibers, the dyeing method is in a weakly acidic (pH value of 4.5 or so) bathtub, acetic acid and sodium acetate as a buffer, in the presence of dielectrics and the right amount of additives, close to boiling dyeing. The dyeing mechanism can be divided into three steps:

(1) Dye cations in the dye bath are adsorbed on the fiber surface.

(2) The dye cations diffuse from the fiber surface to the interior.

(3) The dye cations are salt-bonded to the anionic groups contained in the fiber.

Under normal dyeing conditions, the salt formation in the last step is an irreversible reaction, which can lead to high fastness and uneven dyeing. The first and last steps react fast, the second step diffusion slow, it determines the whole dyeing speed. Due to the high affinity between dyes and fibers, in order to get even dyeing effect, the temperature of dyeing bath, pH value, electrolyte, retarder, etc. must be strictly controlled.

When dyeing with cationic dyestuffs, the dyeing rate should be appropriately reduced in order to obtain uniform dyeing results.

1. Temperature
When acrylics are dyed with cationic dyes, the temperature is seldom below 75°C. When the temperature reaches the glass transition temperature of the fiber (75-85°C), the dyeing rate increases rapidly. Therefore, when the dyeing temperature reaches the glass transition temperature of the fiber, the temperature should be increased slowly, generally every 2-4 min l ℃. Can also be held at 85 ~ 90 ℃ dyeing for a period of time, and then continue to raise the temperature to boiling.

2. pH value of dyeing bath
Adding acid in the dyeing bath can inhibit the dissociation of acidic groups in acrylic, reduce the number of anionic groups on the fiber, so that the Coulombic gravitational force between the dye and the fiber decreases, and the dyeing rate decreases. pH value has a more significant effect on the dyeing rate of carboxylic acid-containing acrylics. The pH value of the dyeing bath should be reasonably controlled. Cationic dyestuffs are generally not alkali resistant, the best pH value is generally 4-4.5, the pH value of the dyeing bath can be higher when dyeing dark colors, and the light colors should be carried out at a lower pH value. The pH value of the dyeing bath is generally adjusted with acetic acid, which can not only reduce the pH value, but also improve the solubility of the dyestuff. Adding sodium acetate in the dye bath at the same time can stabilize the pH value of the dye bath within the required range.

3. Electrolyte
Adding electrolytes, such as metaminic powder and table salt can reduce the dyeing rate of cationic dyestuffs and have the effect of slowing down the dyeing. Electrolyte has no obvious retarding effect on dyestuffs with K-value 1~1.5, and has retarding effect on dyestuffs with K-value 3~5. The retardation effect of electrolyte decreases with the increase of dyeing temperature. When dyeing light color, the dosage of electrolyte can be higher, about 5%~10% (o.w.f.), but not when dyeing dark color.

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