Sunday, March 27, 2011

After Treatment And Stripping of Reactive Dye

Washing-off of unfixed reactive dyes after dyeing:
Removal of hydrolysed and un-reacted dye from the goods is a vital step after dyeing. The amount of unfixed dye remaining in a cotton fabric dyed with reactive dyes may have to be less than 0.002% owf.

Both batch and continuous washing process involves three stages.
- Initially, the goods are rinsed in cold and warm water. This is a dilution stage in that removing as much salt and alkali from the goods as possible.
- Secondly, soaping stage much more efficient since at lower electrolyte concentrations the substantivity of the dyes less, making its desorption easier.
- Finally, again a warm rinsing to dilute the final dye solution adhering to the fibres to the point that the amount of unfixed dye carried over to the final drying is minimal. This residual quantity of dye will be deposited on the fibre surface on evaporation of the water during drying and will be easily removed by later washing. Obviously, the amount must be as small as possible.

After-treatment of Remazol dyes:
There are two factors are important during the aftertreatment of Remazol dyeing. The dyeing should not be soaped at the boil under alkaline conditions. This is ensured by neutralizing with acetic acid before soaping. The unfixed (hydrolysed) dye is best removed by boiling with a detergent solution at the boil. It is important to note that the substantivity of the unfixed dye is reduced at higher temperatures and at the boil, the dye has practically no affinity for the fibre and the loosely held dye rapidly diffuse out. If this dye is removed completely, it is not necessary to treat the dyeing with a cationic dye fixing agent to achieve the optimum perspiration and water fastness.

If the boiling after-treatment is carried out under alkaline conditions, some amount of the dye already fixed (reacted with the fibre) is separated by the rupture of the dye - fibre bond, since this bond is not stable to alkali especially at the boil.

When sodium silicate is used as the alkali for fixing the dye, neutralization with acetic acid should not be done. If it is neutralized insoluble silicic acid may get deposited in the fibre producing a harsh feel. In this case, an overflow rinsing with warm water should be given before theboiling after-treatment. (Sodium silicate is more easily washed off than caustic acid). Sodium hexametaphosphate 2 g/l is to be added to the boiling after-treatment bath. 

Stripping of dyed materials:
Partial stripping:
The material is treated in a solution containing 5 to 10 ml glacial acetic acid per 1000 ml. water at 85-95°C until the shade is reduced to the desired extent. In the case of viscose rayon 2.5-10 ml of formic acid (85%) is used in place of acetic acid. The material is then rinsed and soaped at boil for 15 minutes.

Full stripping:
It is difficult to strip fully fixed dyeing completely to a white, but they can be reduced to a form suitable for re-dyeing to dark shades. The material is treated at boil in a solution containing 5 gms of sodium hydrosulphite and 2 gms of soda ash per litre of water for 20 minutes. It is then washed and treated at room temperature in l°Tw sodium hypochlorite, soured, rinsed and soaped at boil for 1 0 - 1 5 minutes.

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