Hydrolysis of reactive dye & prevention of hydrolysis

Hydrolysis of reactive dye:

    Under alkaline condition reactive dyes react with the terminal hydroxyl group of cellulose. But if the solution of the dye is kept for long time its concentration drops. Then the dye react with the hydroxyl group of water. This reaction of dye with water is known as hydrolysis of reactive dye. After hydrolysis dye cannot react with fibre. So hydrolysis increases the loss of dyes.

This hydrolysis occurs in two stages. At first the concentration of dye initially increases and then begins to decrease. Where as the concentration of hydroxyl compound increases continuously. Then the hydroxyl compound cannot react with dye.

1.    Hydrolysis of halogen containing reactive dye,

     R-Cl  +  H-OH     →     D-R-OH  +  H-Cl

2.    Hydrolysis of activated vinyl compound containing dye,

                D-F-CH₂-CH₂-OSO₃H  +  H-OH   →   D-F- CH₂-CH₂-OH + H₂SO₄

For preventing hydrolysis the following precautions are taken—

1.    As hydrolysis increases with increasing temperature during dissolving and application temperature should not be more than 40°C.

2.    Dye and alkali solution are prepared separately and mixed just before using.

3.    Dye and alkali should not be kept for long time after mixing.