What are the conditions for using vat dyes?

The use of vat dyes should meet the following key conditions: 1. Reductive dissolution: the dye itself is insoluble in water and needs to be reduced to soluble leuco sodium salt under the action of strong reducing agent (such as sodium hydrosulfite Na₂S₂O₄) and alkaline medium (sodium hydroxide). The reduction potential should be stable (-700mV to -900mV) to ensure full dissolution. 2. Dyeing environment: Dyeing temperature is usually controlled at 50-60, too high will lead to excessive reduction or dye decomposition. The alkaline pH(12-14) must be maintained to prevent the leuco from hydrolysis failure. 3. Oxidation fixation: After dyeing, it is necessary to use oxidant (hydrogen peroxide, air, etc.) to re-oxidize the hidden color weight into insoluble dye to restore the original color. Oxidation conditions should be mild and uniform to avoid local over-oxidation. 4. Post-treatment: soaping (over 90%) removes floating color on the fiber surface, and improves color fastness and brightness. Applicability: mainly for cellulose fibers such as cotton and hemp, and can also be used for vinylon. The advantages are excellent washing fastness and light fastness, but the process is complicated, and it is necessary to accurately control the dosage of reducing agent, temperature and oxidation time, otherwise it is easy to produce color difference or decrease the fastness. In addition, sodium hydrosulfite is unstable and needs to be prepared now, and environmental protection substitutes (such as thiourea dioxide) are gradually popularized.