Discharging and stripping processes for common dyes

Color removal, also known as stripping, generally uses chemical reactions to destroy the combination of dyes on fibers and fibers, destroy the dye structure on fibers, so that the dyes are stripped from fibers and cannot color.

Commonly used color removal agents are mainly divided into two categories: reducing color removal agents and oxidizing color removal agents.

Reducing color removal agents: commonly used ones are insurance powder and Rongalite. They achieve the purpose of fading and decolorization by destroying the color development system in the dye molecular structure. For example, the azo group of azo-structured dyes can be reduced to amino groups and cannot color. However, the damage of reducing agents to the color development system of certain structural dyes is reversible, such as the color development system of anthraquinone structure, so its fading is recoverable.

Oxidizing color removal agents: commonly used ones are hydrogen peroxide and sodium hypochlorite. Oxidants can destroy certain groups that constitute the color development system of dye molecules under certain conditions, such as decomposition of azo groups, oxidation of amino groups, methylation of hydroxy groups, and detachment of complex metal ions. These irreversible structural changes can cause the dye to fade and lose color, so in theory, oxidative color removal agents can achieve complete color removal, and this type of color removal agent is particularly effective for dyes with anthraquinone structures.

Discharge process of different dyes

1 Discharge of reactive dyes

Light color: discharge of insurance powder under alkaline conditions, such as: caustic soda 2g/L + insurance powder 4g/L 95℃ 30min→ hot wash → water wash

Dark color: reduction discharge + oxygen bleaching, such as: caustic soda 4g/L + insurance powder 8g/L 95℃ 40min→ hot wash → oxygen bleaching → hot wash → water wash

2 Discharge of sulphur dyes

For the correction of sulphur dyed fabrics, it is usually placed in a reducing agent blank solution (6g/L full-strength sodium sulfide) and treated at the highest possible temperature to partially discharge the dyed fabric and then re-dye it. In severe cases, sodium hypochlorite or insurance powder is required.

Color discharge process for light-colored fabrics:

Color defective fabrics → soak and roll (sodium hypochlorite 5-6g/L, 50℃) → 703 steamer (2min) → fully washed → dried

Color discharge process for dark-colored fabrics:

Color defective fabrics → roll oxalic acid (15g/L, 40℃) → dry → roll sodium hypochlorite (6g/L, 30℃×15s) → fully washed → dried

Intermittent color discharge process:

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

3 Color discharge of acid dyes

Use ammonia (20-30g/L) and anionic wetting agent (1-2g/L), boil for 30-45min. Before treatment, use insurance powder (10-20g/L) at 70℃ to treat, which is conducive to complete color removal. Oxidation color removal can also be used.

Under acidic conditions, adding special surfactants can have a good color removal effect, and alkaline conditions can also be used for color removal.

Silk color stripping process:

Reduction stripping bleaching (soda ash 1g/L, 2g/L of sodium bicarbonate, 2-3g/L of hydrosulfite, 60℃, 30-45min, bath ratio 1:30) → pre-treatment (FeSO4, 7H2O 10g/L, 50% hypophosphorous acid 2g/L, formic acid to adjust PH = 3-3, 5, 80℃×60min) → rinsing (80℃×20min) → oxidation stripping bleaching (35% hydrogen peroxide 10mL/L, NaSiO3.5H2O 3-5g/L, 70-80℃, 45-90min, PH=8-10) → washing

Wool color stripping process;

Nifandine AN4%; oxalic acid 2%; heat to boiling within 30min, keep for 20-30min, then wash.

Nylon color removal process:

36°NaOH 1%～3%; Perchlorate 15%～20%; synthetic detergent 5%～8%; bath ratio 1:25～30; 98～100℃; 20～30min (until all color is removed). After all color is removed, gradually cool down, wash thoroughly with water, then use 0.5ml/L acetic acid at 30℃ for 10min to fully neutralize the alkali remaining on the nylon, and then wash with water.

4. Dye discharge of reduced dyes

Generally, in a mixed system of sodium hydroxide and hydrosulfite, the dyes on the fabric are reduced at a higher temperature. Sometimes, polyvinyl pyrrolidone solution needs to be added, such as BASF's ALigen A.

Continuous color discharge process:

Color defect cloth is dipped in reduction solution (caustic soda 20g/L, hydrosulfite 30g/L) → 703 reduction steamer steaming (100℃) → washing → drying

Intermittent color discharge process:

Pingping O 2~4g/L; 36°NaOH 12~15mL/L; hydrosulfite 5~6g/L; 70~80℃; 30~60min; bath ratio 1:30~40.

5. Dye discharge of disperse dyes

The methods for discharging disperse dyes on polyester fibers are:

⑴ Treat with Rongbai powder and carrier at 100℃ and pH 4~5. If treated at 130℃, the effect is more significant.

⑵ Treat with sodium chlorite and formic acid at 100℃ and pH 3.5.

To achieve the best effect, the best method is to treat with method ⑴ and then with method ⑵, and dye black as much as possible after treatment.

6 Cationic dye discharge

There are usually the following methods for cationic dye discharge on acrylic fiber:

Boil for 1 hour in a treatment bath containing 5ml/L monoethanolamine and 5g/L sodium chloride; after cleaning, bleach for 30 minutes in a treatment bath containing 5ml/L sodium hypochlorite (150g/L effective chlorine), 5g/L sodium nitrate (corrosion inhibitor), and pH 4-4.5; finally, treat with sodium sulfite (3g/L) at 60℃ for 15min or treat with 1-1.5g/L insurance powder at 85℃ for 20-30min, and clean.

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

7 Color removal of insoluble azo dyes

Use 5-10ml/L 38° caustic soda, 1-2ml/L thermal stability dispersant and 3-5g/L insurance powder for treatment, and add 0.5-1g/L anthraquinone powder as an indicator. If the insurance powder and caustic soda are sufficient, anthraquinone will turn the color stripping solution red; if the color stripping solution turns yellow or brown, caustic soda and insurance powder must be further added. The fabric should be thoroughly washed after color removal.

8 Paint stripping

The paint is difficult to strip, and generally only potassium permanganate can be used to strip it lightly.

Process: Color defective cloth → rolling potassium permanganate (18g/L) → water washing → rolling oxalic acid 20g/L, 40℃) → water washing and drying