How do reactive dyes form covalent bonds with cotton fibres?

Reactive dyes form covalent bonds with cotton fibres through specific chemical reactions that anchor the dye molecule directly to the cellulose structure. This process ensures the colorant becomes a permanent part of the fibre, providing superior wash fastness compared to dyes that merely adhere by adsorption.12

The bonding mechanism varies by dye type:

• Triazine-based dyes (e.g., Procion MX): These dyes contain reactive chlorine atoms on a triazine ring. Under mild alkaline conditions, a chlorine atom is displaced by a hydroxyl group on the cotton fibre, creating a stable covalent link.1
• Vinylsulfone-based dyes (e.g., Remazol): These dyes are applied as sulfate esters. When treated with a mild base, they generate a vinylsulfone group, which then reacts with the cellulose to form a unique dye-fibre bond.1
• Phosphonate-based dyes: Some specialized series, such as Procion T, utilize a phosphonate ester to bond the dye to the fibre.1

Because hydrolysis with water in the dyebath can compete with this fixation process, modern developments often include dyes with multiple reactive groups to improve efficiency.3