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What are the precautions in the application of acid dyes?

Date:Dec 17, 2020

Acid Dyes is a kind of water-soluble dyes with acidic groups on the structure, which are dyed in acid medium. Most acid dyes contain sodium sulfonate, which is soluble in water, bright in color and complete chromatogram. It is mainly used for dyeing wool, silk, nylon, etc., but also for leather, paper, ink, etc. Generally no tinting power to cellulose fibers.

Acid dyes have complete chromatograms, bright colors, and light fastness and wet processing fastness vary greatly depending on the dye variety. Compared with direct dyes, acid dyes have a simple structure, lacking long conjugated double bonds and coplanar structures, so they lack directness to cellulose fibers and cannot be used for dyeing cellulose fibers. Different types of acid dyes have different dyeing properties and different dyeing methods due to different molecular structures.


application

It is mainly used for dyeing wool, silk and nylon, but also for leather, paper, ink, etc. Generally no tinting power to cellulose fibers.


classification

(1) Strong acid dyes

The earliest developed acid dye requires dyeing in a strong acid dye bath. It has a simple molecular structure, low molecular weight, contains sulfonic acid or carboxyl groups, has little affinity for wool, and can move evenly on wool. It is evenly dyed, so it is also called acid level dyeing dye, but the shade is not deep, the washing fastness is poor, and the wool is damaged during dyeing, and the dyed wool feels poor. Such as Acid Red G (ie C.I. Acid Red 1).


(2) Weak acid dyes

In strong acid dyes, weak acid dyes are formed by increasing the molecular weight, introducing groups such as arylsulfone groups, or introducing long carbon chains. The molecular structure is more complex, and it has a greater affinity for wool. It can dye wool in a weak acid medium without damage to the wool. The shade is darker and the fastness is improved, but the dyeing is uneven. Such as weak acid brilliant blue RAW (ie C.I. Acid Blue 80).


(3) Acid mordant dye

After treated with certain metal salts (such as chromium salts, copper salts, etc.) as mordants, acid dyes that form metal complexes on the fabric. Mordant dyeing procedures are more complicated, but dyes with better properties such as light fastness, washing fastness and rubbing fastness can be obtained, such as acid mordant black T (ie C.I. mordant black 11).


(4) Acid complex dyes

It is formed by complexing some acid dyes with metals such as chromium and cobalt. It is soluble in water, and its dyed products have excellent light fastness and light fastness. Its dye matrix is similar to acid mordant dyes, but in the preparation of dyes, metal atoms have been introduced into azo dye molecules, and the ratio of metal atoms to dye molecules is 1:1, so it is also called 1:1 metal complex dye. There is no need to use mordant for dyeing. Such as acid complex yellow GR (ie C.I. Acid Yellow 99).


Selection Guide

Acid dyes are only suitable for fibers with positive charges, such as: dyeing polyamide fibers in an acid bath, but cotton fibers can also be dyed with acid dyes after cationization.

When two dyes are used for dyeing, the structure of the dye and the structure of cationic cotton play a dominant role.

Increasing the number of cationic groups on cationic cotton can get better washing fastness.

In any case, the overall color fastness of cationic cotton dyed with acid dyes is equivalent to that of nylon 6 dyed with the same acid dyes.


Precautions

1. Choose dyes carefully

Metal complex dyes containing copper, chromium and nickel are dyes that are pre-made into metal complexes with dyes with specific structures and metal complexing agents. There are 93 acid metal complex dyes. EU regulations: when it is used for dyeing related fibers, the amount of dye liquor discharged into wastewater for treatment after dyeing with each metal complex dye used should be less than 7%, that is, the coloring rate of metal complex dyes should be less than 7%. More than 93%; meanwhile, copper or nickel should not exceed 75ms/ks fiber, and chromium should not exceed 50m mineral ks fiber. Therefore, the selected dye needs a certain degree of purity.


2. Dyeing water

The water hardness for acid dyes is preferably 100-150 mg/kg. If the water hardness is too high, 0.5-2s/L chelating agent can be added to improve water quality and ensure dyeing. However, chelating agents such as phosphates or surfactants will destroy the integration of the dye itself to varying degrees, and affect the vividness and fastness of colors. The test found that the chelating agent 88 provided by ORTHO Chemical Co., Ltd. has little effect on the integration of metal ions in the dye. Therefore, chelating agents must be selected when dyeing, and should not be used blindly.


3. Chemical method

Because the solubility of acid complex dyes is inconsistent, it is necessary to treat differently when compounding. Generally, it is first adjusted into a slurry with cold water, and then diluted with hot or boiling water to dissolve it. Sometimes it is difficult to dissolve the commercial dyes because of the fillers added. If necessary, they need to be boiled and stirred until they are fully dissolved. Finally, they are sieved and slowly poured into the tank to prevent the formation of color spots (spots).


4. Fabric pretreatment

Because there are grease, slurry, impurities and natural pigments in the fabric. Sometimes it is not only necessary to remove the slurry, but even after half-bleaching before dyeing. Practice has proved that the pre-treatment of pure textiles is easier, while blended textiles are difficult, but they must have good penetration (wetting) before dyeing, depending on the specific variety.


5. The role of acid

Regardless of sulfuric acid, formic acid (formic acid) and acetic acid, they are all accelerants for acid and acid complex dyes. Sometimes in order to further dye up (exhaust) the dye, the amount of acid is often added in the middle and later stages of dyeing. However, it needs to be diluted in advance when adding, close the steam valve, stop heating, and add people slowly to prevent staining.


6. The role of Yuanmingfen

Sometimes acid complex dyes need to be dyed with a pH below 4, so sodium sulfate can be used as a dye retardant. Conversely, if the pH value of the dye bath medium is above 4, sodium sulfate should not be used because it will increase the aggregation tendency of the dye. Instead, use non-ionic surfactants such as Pingping O and emulsifier OP as the retarding agent. . The dosage should be determined according to the color depth and dyeing equipment. Excessive dosage will make wool felt and feel rough, which will affect the exhaustion of dyes in the dyeing liquor. The foam will increase on the liquid flow machine, which will affect the normal operation of the circulation exchange system; less dosage will result in poor results.


7. Neutralization

Alkali agents are mainly used for the neutralization treatment after dyeing in strong acid medium, the purpose is to restore the ionized amino groups to non-ionized amino groups after dyeing, so that the fiber can continue to be further combined with the dye. The dosage can be determined according to the actual pH value of the dye before neutralization. Generally, ammonia water is 1mL/L and soda ash is 2-3S/L. Treat it for 20-30 minutes at a temperature of 40-50 bar, after neutralization is completed, and then wash thoroughly.


8. When the dyeing is terminated, the temperature of the washing out of the vat should not be too low. Generally, it should be controlled at 50-60°C to prevent the fabric from wrinkling, especially once the wrinkles of wool and nylon are difficult to repair.


9. Fixing agent treatment

For fabrics with high color fastness requirements, after dyeing with acid complex dyes, cationic fixing agent (powder) 2%-6% (owf), bath ratio 1:20-30, temperature 55 T, immersion 30 min. Especially for silk fabrics, through fixation, the general color fastness can be improved by 0.5 to 1.5. Tests show that nylon fabric does not need fixing treatment.


10. Acid metal complex dyes have different degrees of heat sensitivity, especially red and blue. Therefore, the temperature should not be too high during the drying process to prevent the discoloration from recovering.