Hydrolysis of Reactive Dye | Removing Process of Hydrolysis of ReactiveDye | Why Low Affinity Reactive Dyes are Preferred for Dyeing?

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,
 D-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.

Why low affinity reactive dyes are preferred for dyeing?

If the reactivity of the dye is increased considerably, the rate of reaction with the fibre increases. There fore, the dyeing can be carried out in a short time. However in this case the rate of dye also increases, leading to deactivation of a part of the dye. This results in wastage of the dye. If on the other hand the reactivity of the dye is decreased, the extent of hydrolysis can be reduced considerably. However this results in the slower rate of reaction with the fibre also. The ultimate object of dyeing is to react as much of the dye ass possible with the fibre and minimize the hydrolysis of the dye. This is achieved in practice in two stages. The dyeing is first started from the aqueous medium under neutral conditions when the dye does not react either with the fibre or with water. 

Then gluber salt or common salt is added to exhaust the dye onto the fibre as much as possible. In this respect, this stage of dyeing (exhaustion) resembles the dyeing of direct dyes on cotton. Then the second step (that of fixation or reaction with the fibre) is carried out by adding the alkali (usually used soda ash). Since the exhausted dye is already on the fibre, it is more likely that the exhausted dye reacts with the fibre in preference to water. However the dye present in the dye bath (which contains a substantial amount of the reactive dye) can now react with water since it is under alkaline condition. It is already stated that the hydrolyzed dye cannot further react with the fibre but dye to the affinity forces; it is absorbed by the fibre and is retained in it. 

During the subsequent washing or soaping the substantivity held hydrolyzed dye gets stripped into the washing bath thereby reducing the washing fastness of the dyeing. If the affinity of the original dye is reduced to a very low value, this problem will not arise and a rigorous treatment of the dyeing with boiling soap or detergent solution removes almost all hydrolyzed dye. 

However if the affinity is very low, exhaustion of the dye bath prior to fixation cannot be achieved substantially. This results in a larger amount of the reactive dye remaining in the dye bath and getting hydrolyzed when alkali is added subsequently. If the dye has high affinity for cellulose like a direct dye, it becomes difficult to remove the hydrolyzed dye from the dyeing since it is also absorbed by and retained in the fibre by fairly strong affinity forces, through not as strong ass the covalent bond formed between the dye and the fibre. Hence in actual practice low affinity dyes are selected for converting in to reactive dyes.