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Why is indigo firmly trapped in cotton after vat dyeing?
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dye
Examples of anthraquinone pigments.
Indigo is firmly trapped in cotton during vat dyeing because of its physical state rather than a chemical affinity for the cotton fiber.1
The process works through the following steps:
- Reduction: Indigo is naturally insoluble in water, so it is first reduced to a soluble, colorless form known as leucoindigo.21
- Application: The fabric is treated in a bath of this soluble leucoindigo, which can penetrate the pores of the cotton fiber.1
- Oxidation: Once the fabric is exposed to air, atmospheric oxygen oxidizes the leucoindigo, regenerating the original, insoluble indigo dye.21
- Entrapment: Because the regenerated indigo molecules form water-insoluble aggregates that are larger than the pores of the cotton fiber, they become physically locked or trapped within the fabric structure.1