1.Principle of color stripping
Stripping uses chemical action to destroy the dye on the fiber, causing it to lose its color.
There are two main types of chemical strippers:
One type is reductive stripping agents, which achieve the purpose of fading or decolorizing by destroying the color system in the molecular structure of dyes. For example, dyes with azo structures, their azo groups may be reduced to amino groups and lose color. However, the damage caused by reducing agents to dye color systems with certain structures is reversible, so their fading can be restored, such as the color system with anthraquinone structure. Insurance powder and white powder are commonly used reducing stripping agents.
The other type is oxidative stripping agents, of which hydrogen peroxide and sodium hypochlorite are commonly used. Under certain conditions, oxidants can cause damage to certain groups that make up the color system of dye molecules, such as azo group decomposition, amination oxidation, hydroxymethylation, and detachment of complex metal ions. These irreversible structural changes lead to fading or decolorization of the dye, so theoretically speaking, oxidative stripping agents can be used for complete color stripping. This method is particularly effective for dyes with anthraquinone structures.
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 sequestering agent (2 g/L EDTA). Then rinse thoroughly before alkaline reduction or oxidation stripping treatment. Complete color stripping is usually done at high temperature in alkali and insurance powder for 30 minutes. After restoration and stripping, rinse thoroughly. Then cold bleach in sodium hypochlorite solution.
2.2 Stripping of sulfur dyes
To correct fabrics dyed with sulfur dyes, they are usually treated 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 material before heavy dyeing. color. In serious cases, sodium hypochlorite or insurance powder must be used.
2.3 Stripping of acid dyes
Use ammonia (2O to 30 g/L) and anionic wetting agent (1 to 2 g/L) and boil for 30 to 45 minutes. Before ammonia treatment, use insurance powder (10 to 20 g/L) at 70°C to help complete color stripping. Finally, oxidation stripping method can also be used.
Under acidic conditions, adding special surfactants also has a good color stripping effect. There are also methods that use alkaline conditions for color stripping.
2.4 Stripping of vat dyes
Generally, in a mixed system of sodium hydroxide and insurance powder, the fabric dye is reduced at a relatively high temperature. Sometimes it is necessary to add polyvinylpyrrolidine solution, such as BASF's AlbigenA.