What are the methods for stripping various fabrics in dyeing and finishing?

1. Stripping principle

Stripping is the use of chemical action to destroy the dye on the fiber and make it lose its color.

There are two main types of chemical stripping agents:

One type is reductive stripping agent, which achieves the purpose of fading or decoloring by destroying the color system in the molecular structure of the dye. For example, in the dye of the azo structure, the azo group may be reduced to an amino group and lose its color. However, the damage of the reducing agent to the color system of certain dyes is reversible, so the fading can be restored, such as the color system of the anthraquinone structure. Sodium sulfonate and white powder are commonly used reductive peeling agents.

The other is oxidative stripping agents, among which the most commonly used are hydrogen peroxide and sodium hypochlorite. Under certain conditions, oxidants can cause damage to certain groups that make up the dye molecular color system, such as decomposition of azo groups, oxidation of amino groups, methylation of hydroxyl groups, and detachment of complexed metal ions. These irreversible structural changes result in the fading or decolorization of the dye, so theoretically, the oxidative stripping agent can be used for complete stripping treatment. This method is particularly effective for dyes with anthraquinone structure.

2. Stripping of commonly used dyes

2.1 Stripping of reactive dyes

Any reactive dye containing metal complexes should first be boiled in a solution of metal polyvalent chelating agent (2 g/L EDTA). Then wash thoroughly with water before alkaline reduction or oxidation stripping treatment. The complete stripping is usually treated at high temperature for 30 minutes in alkali and sodium hydroxide. After the peeling is restored, wash thoroughly. Then it is cold bleached in sodium hypochlorite solution.

Process example:

Examples of continuous stripping process:

Dyed cloth → padding reducing solution (caustic soda 20 g/l, soda powder 30 g/l) → 703 reduction steamer steaming (100℃) → washing → drying

Example of dyeing vat peeling process:

Color defect cloth→reel→2 hot water→2 caustic soda (20g/l)→8 peeling color (sodium sulfide 15g/l, 60℃) 4 hot water→2 cold water scroll→normal sodium hypochlorite level bleaching Process (NaClO2.5 g/l, stacked for 45 minutes).

2.2 Stripping of sulfur dyes

Sulfur dye-dyed fabrics are usually corrected by treating them in a blank solution of reducing agent (6 g/L full-strength sodium sulfide) at the highest possible temperature to achieve partial peeling of the dyed fabric before re-dyeing. color. In severe cases, sodium hypochlorite or sodium hypochlorite must be used.

Process example:

Light color example:

Into the cloth → more soaking and rolling (sodium hypochlorite 5-6 grams liters, 50 ℃) → 703 steamer (2 minutes) → full water washing → drying.

Dark example:

Color imperfect fabric → rolling oxalic acid (15 g/l at 40°C) → drying → rolling sodium hypochlorite (6 g/l, 30°C for 15 seconds) → fully washing and drying

Examples of batch processes:

55% crystalline sodium sulfide: 5-10 g/l; soda ash: 2-5 g/l (or 36°BéNaOH 2-5 ml/l); temperature 80-100, time 15-30, bath ratio 1:30-40 .

2.3 Stripping of acid dyes

Boil for 30 to 45 minutes with ammonia water (2O to 30 g/L) and anionic wetting agent (1 to 2 g/L). Before ammonia treatment, use sodium sulfonate (10 to 20 g/L) at 70°C to help complete peeling. Finally, the oxidation stripping method can also be used.

Under acidic conditions, the addition of special surfactants also has a good peeling effect. There are also those that use alkaline conditions for peeling.

Process example:

Examples of real silk peeling process:

Reduction, peeling and bleaching (soda ash 1g/L, flat addition of O2g/L, 2-3g/L of sodium hydroxide, temperature 60℃, time 30-45min, bath ratio 1:30) → pre-media treatment (ferrous sulfate heptahydrate 10g /L, 50% hypophosphorous acid 2g/L, formic acid adjust pH 3-3.5, 80°C for 60min)→rinse (80°C wash for 20min)→oxidation stripping and bleaching (35% hydrogen peroxide 10mL/L, pentacrystalline sodium silicate 3 -5g/L, temperature 70-8O℃, time 45-90min, pH value 8-10)→clean

Example of wool stripping process:

Nifanidine AN: 4; Oxalic acid: 2%; Raise the temperature to boiling within 30 minutes and keep it at the boiling point for 20-30 minutes; then clean it.

Example of nylon stripping process:

36°BéNaOH: 1%-3%; Ping Ping O: 15%-20%; Synthetic detergent: 5%-8%; Bath ratio: 1:25-1:30; Temperature: 98-100°C; Time: 20-30min (until all decolorization).

After all the color is stripped, the temperature is gradually lowered, and then washed with water to clean, then use 0.5mL/L acetic acid, 30℃, 10min to fully neutralize the alkali remaining on the nylon, and then rinse with water.

2.4 Stripping of vat dyes

Generally, in a mixed system of sodium hydroxide and sodium hydroxide, the fabric dye is reduced again at a relatively high temperature. Sometimes it is necessary to add polyvinylpyrrolidine solution, such as BASF's AlbigenA.

Examples of continuous stripping process:

Dyed cloth → padding reducing solution (caustic soda 20 g/l, soda powder 30 g/l) → 703 reduction steamer steaming (100℃) → washing → drying

Example of intermittent peeling process:

Pingping O: 2-4 g/l; 36°BéNaOH: 12-15 ml/l; Sosine: 5-6 g/l; the temperature during stripping treatment is 70-80℃, and the time is 30-60 minutes. The bath ratio is 1:30-40.

2.5 Stripping of disperse dyes

The following methods are usually used to strip disperse dyes on polyester:

Method 1: Sodium formaldehyde sulfoxylate and carrier, treated at 100°C and pH4-5; the treatment effect is more significant at 130°C.

Method 2: Sodium chlorite and formic acid are processed at 100°C and pH 3.5.

The best result is the first treatment followed by the second treatment. Over-dye black as much as possible after treatment.

2.6 Stripping of cationic dyes

The stripping of disperse dyes on polyester usually uses the following methods:

In a bath containing 5 ml/liter monoethanolamine and 5 g/liter sodium chloride, treat at boiling point for 1 hour. Then clean it, and then bleach in a bath containing 5 ml/L sodium hypochlorite (150 g/L available chlorine), 5 g/L sodium nitrate (corrosion inhibitor), and adjust the pH to 4 to 4.5 with acidic acid. 30 minute. Finally, the fabric is treated with sodium chloride sulfite (3 g/L) at 60°C for 15 minutes, or 1-1.5 g/L sodium hydroxide at 85°C for 20 to 30 minutes. And finally clean it.

Using detergent (0.5 to 1 g/L) and a boiling solution of acetic acid, and treating the dyed fabric at pH 4 for 1-2 hours can also achieve a partial peeling effect.