Acid dyes are water-soluble dyes with acidic groups in their structure, which are dyed in acidic media. Most acid dyes contain sodium sulfonate, which is soluble in water with bright colors and complete chromatograms. Mainly used for dyeing wool, silk and nylon, but also for leather, paper and ink. Generally speaking, it has no ability to color cellulose fibers.

Acid dyes have a complete chromatogram and bright colors, and the light fastness and wet fastness of different dyes vary greatly. Compared with direct dyes, acid dyes have a simple structure and lack a longer coplanarity and coplanarity structure. Therefore, they lack directness to cellulose fibers and cannot be used for dyeing cellulose fibers. Different types of acid dyes have different dyeing properties and dyeing methods due to different molecular structures.

application

Mainly used for dyeing wool, silk and nylon, but also for leather, paper and ink. Generally speaking, it has no ability to color cellulose fibers.

classification

(1) Strong acid dyes

The first acid dyes developed need to be dyed in a strong acid dye solution. Its molecular structure is simple, low molecular weight, containing sulfonic acid group or carboxyl group, and has low affinity for wool. It can move evenly on wool and dye evenly, so it is also called acid leveling agent. However, its color is not dark, its washing resistance is poor, it is harmful to wool during dyeing, and the dyed wool has a poor hand feeling. For example, acid red g (ie c.i. acid red 1).

(2) Weak acid dyes

Weak acid dyes can be produced in strong acid dyes by increasing molecular weight, introducing arylsulfone groups, or introducing long carbon chains. The molecular structure is complex and it has a strong affinity for wool. It can dye wool in a weakly acidic medium, and has no damage to the wool, with deep shade and improved fastness, but the dyeing is uneven. For example, Weak Acid Brilliant Blue RAW (ie c.i. Acid Blue 80).

(3) Acid mordant dye

Some metal salts (such as chromium salts, copper salts, etc.) are used as mordants to form acid dyes with metal complexes on fabrics. The mordant dyeing process is 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 chromium, cobalt and other metals. It is easily soluble in water, and its dyed products have excellent light fastness and light fastness. Its dye matrix is ​​similar to acid mordant dyes, but when preparing dyes, metal atoms have been introduced into the azo dye molecules, and the ratio of metal atoms to dye molecules is 1:1, so it is also called 1:1 metal Complex dyes. Dyeing does not require mordant treatment. For example, Acid Yellow GR (ie, c.i. Acid Yellow 99).

Selection guide

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

When dyeing with two dyes, the dye structure and cationic cotton structure play a leading role.

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

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

Related matters

1. Choose your dye carefully

Metal complex dyes containing copper, chromium and nickel are dyes prepared into metal complexes with dyes of specific structures and metal complexing agents in advance. There are 93 kinds of acid metal complex dyes. The EU stipulates that when dyeing related fibers, the amount of dye liquor discharged into wastewater treatment after dyeing with each metal complex dye should be less than 7%, that is, the coloring rate of metal complex dye should exceed 93%; at the same time, copper or nickel It should not exceed 75 ms/ks fiber, and chromium should not exceed 50 m ore ks fiber. Therefore, the selected dye needs a certain degree of purity.

2. Dyeing water

The hardness of the acid dyeing water should be 100-150 mg/kg. If the water hardness is very high, you can add 0.5-2 sec/L chelating agent to improve water quality and ensure dyeing. However, chelating agents, such as phosphates or surfactants, can damage the binding of dyes to varying degrees and affect brightness and color fastness. It was found that the chelating agent 88 provided by Ortho Chemical Co., Ltd. had little effect on the binding of metal ions in the dye. Therefore, chelating agents must be screened during dyeing and cannot be used blindly.

3. Chemical methods

Because the solubility of acid complex dyes is inconsistent, it is necessary to treat them differently. Generally speaking, it is made into a slurry with cold water first, and then diluted with hot water or boiling water to dissolve. Sometimes, because commercial dyes are full of fillers, they are difficult to dissolve. If necessary, boil and stir until completely dissolved. Finally, it is filtered and slowly injected into the cylinder to prevent the formation of stains (spots).

4. Fabric pretreatment

Due to the presence of grease, slurry, impurities and natural pigments in the fabric. Sometimes it is not only necessary to remove the pulp, but also to semi-bleach before dyeing. Practice has proved that the pretreatment of pure textiles is easy, while blended textiles are difficult, but they must have good permeability (wettability) before dyeing, which depends on the specific variety.

5. The role of acid

Whether it is sulfuric acid, formic acid or acetic acid, they are all dyeing accelerators for acid and acid composite dyes. Sometimes, in order to further dye (exhaust), it is usually necessary to add acid in the middle and late stages of dyeing. However, it should be diluted in advance, the steam valve should be closed, the heating should be stopped, and people should be added slowly to prevent the flowers from staining.

6. The role of anhydrous sodium sulfate

Sometimes acid composite dyes need to be dyed at a pH lower than 4, so sodium sulfate can be used as a retarder. On the contrary, if the pH of the dye bath medium is higher than 4, sodium sulfate should not be used because it will increase the tendency of dye aggregation. Non-ionic surfactants such as Pingjia and emulsifier OP should be used as dye blockers. The dosage should be determined according to the color depth and dyeing equipment. Excessive amount will make the wool felt rough and affect the exhaustion of the dye in the dye liquor. At the same time, due to the increase of foam on the liquid flow machine, it will also affect the normal operation of the circulation exchange system; if the amount is small, the effect is not good.

7. Neutralization

Alkali agents are mainly used for neutralization after dyeing in a strong acid medium. The purpose is to reduce 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 of the dye before neutralization. Ammonia water is generally 1 ml/liter, soda ash is 2-3 g/liter, treated at a temperature of 40-50 bar for 20-30 minutes. After neutralization, wash thoroughly with water.

8. At the end of dyeing, the washing temperature should not be too low, but generally should be controlled at 50-60 ℃, mainly to prevent the fabric from wrinkling, especially when wool and nylon are wrinkled, which is difficult to repair.

9. Fixing agent treatment

For fabrics with high color fastness requirements, after dyeing with acid composite dyes, cationic fixing agent (powder) 2%-6% (OWF) can be used, bath ratio 1: 20-30, temperature 55 T, and immersion for 30 min. Especially for silk fabrics, through fixing, the general color fastness can be improved by 0.5-1.5 level. Experiments show that nylon fabrics do not need fixing treatment.

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