How are azo dyes formed directly within fabric during ingrain dyeing?

In ingrain dyeing, azo dyes are synthesized directly within the fiber through the reaction of two chemical precursors: an azoic diazo component and a coupling component.12

The process typically follows these steps:

• Application of Components: The fabric is first treated with a solution of the coupling component (such as Naphtol AS, which has an affinity for cotton). Alternatively, the fabric may be treated with the diazo component before it undergoes diazotization.32
• Formation of the Dye: The fabric is then immersed in a solution containing the second component. When the diazonium salt reacts with the coupling component within the fiber, an irreversible chemical change occurs, producing an insoluble dye.12
• Bonding: The resulting dye molecules attach to the fiber, often through interactions with the hydroxyl or amino groups of materials like cotton, silk, or wool.1

Because this synthesis often occurs at ice temperatures, these colorants are sometimes referred to as ice colors. This method results in improved wetfastness compared to other application techniques.312