I. Definition of cationic dyes and their usage
Cationic dyes are a kind of textile dyes, which are in a cationic state when dissolved in water. Cationic dyes are soluble in water and ionized in aqueous solution to generate dyes with positively charged colored ions. Cationic dyes form salts with acid anions such as chloride, acetate, phosphate, methylsulfate, etc. in solution to dye polyacrylonitrile fibers. In actual dyeing, several cationic dyes are commonly used to form a specific color. The cation of the dye can combine with the acid group of the third monomer in the fabric to dye the fiber. It is a special dye for acrylic fiber dyeing. It has the advantages of high strength, bright color and good light fastness.
II. Properties of cationic dyes
1. Solubility:
Salt-forming alkyl groups and anionic groups in cationic dye molecules affect the solubility of dyes. 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 ordinary cationic dyes and acidic, reactive, and disperse dyes, otherwise precipitation will occur. Anti-precipitation agents are generally added to solve such problems.
2. Sensitivity to pH value:
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-donating group 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 ammonium base, or the structure may be destroyed. The dye has precipitated, changed color or faded. For example, oxazine dyes are converted into non-cationic dyes in alkaline media, and lose their affinity for acrylic fibers and cannot be dyed.
3. Light fastness performance:
The light fastness of the dye is related to its molecular structure. The cationic group in the conjugated cationic dye molecule is a relatively sensitive part, which is easily activated from the position of the cationic group after being affected by light energy, and then transmitted to the entire color hair system, causing it to be damaged and faded. The cationic group in the isolated cationic dye molecule is separated from the conjugated system by a linking group. Even if it is activated under the action of light energy, it is not easy to transfer energy to the conjugated system that develops color, so the light fastness is better than conjugated cationic dyes.
III. Applications of cationic dyes
Cationic dyes are mostly used for dyeing polyester fibers and acrylic fibers. It can also be used for dyeing if acid groups are introduced into diacetate fibers and vinyl chloride fibers. For some blended products, when cationic dyes are used, carrier dyeing or high temperature dyeing can be used. For some blends (such as polyester fibers and wool fibers), cationic dyes, acid dyes, direct dyes, reactive dyes, etc. are used for combined dyeing. In this case, precipitation and tar formation occur, and anti-sedimentation agents can be used at this time.
IV. Synthesis of cationic dyes
There are many ways to prepare cationic dyes. For example: the preparation method of methine dyes and azomethine dyes is the condensation reaction of aromatic aldehydes with active methylene and aromatic amines, or the condensation reaction of aromatic diazonium salts with active methylene. Azomethine dyes are generally prepared by azo coupling reaction. Triphenylmethane dyes utilize the condensation of aromatic aldehydes and aromatic amines. Kanazine dyes are condensations of aromatic nitroso compounds and aminophenols.
V. Instructions for use of cationic dyes
Cationic dyes are mostly bright-colored, and the three primary colors can be used to dye various color spectrums. Most of the commercial forms are powder, soluble in water, acetic acid and alcohol can speed up the wetting of the dye, and heating can help dissolve it. The solubility of dyes varies greatly, so it must be completely dissolved before dyeing.
Cationic dyes are usually dyed in a weakly acidic dye bath; some species are sensitive to acid and alkali, and often cause changes in shade when encountering alkali.
Cationic dyes have a strong affinity for acrylic. The dyeing depth is related to the fiber saturation value, and the dosage of dyes and auxiliaries has certain limits. In the application of color matching in production, the three primary colors with the same compatibility value are usually used together, and bright color dyes are used to adjust the shade.
