After Treatment Processes of Direct Dye | Stripping Process of Direct Dyes

After Treatment Processes of Direct Dye:
The wet fastness properties (particularly washing, water and perspiration test) of virtually all dyeing of direct dyes are inadequate for many end uses but notable improvements can be brought about by after treatments. All such treatments, however, incur increased processing costs because of the extra time, energy, labor and chemicals involved. 
  1. Diazotization and development.
  2. Metal salt treatments
  3. Cationic fixing agents
  4. Formaldehyde treatment
  5. Cross linking agents and resin treatments. 
Direct dye
Diazotization and Development:
Certain direct dyes have a structure similar to the base of azoic dyes. This enables these dyes to be treated with naphthols as in the case of azoic dyes. These increase the size of dye molecule within the polymer system of the cellulose fibre and improve the dyes wash fastness from poor to good.

Metal Salt Treatments:
1. Treatment with acidified copper salt solution (0.25-2% copper sulphate and 1% acetic acid for 20-30 minutes at 60C) results in a marked improvement in the light fastness of certain direct dyes.

2. Washing and alkaline treatments removes the copper and light fastness becomes normal.

Cationic Fixing Agents:
1. These compounds interact with the sulphonate groups present in direct dyes, conferring increased wet fastness in all tests at temperatures below 60C

2. They will also precipitate direct dyes from solution and therefore the dyed material must be cleared of loosely held dye before treatment.

3. Color changes may occur and in some cases light fastness may be reduced.

Formaldehyde Treatment:
1. Treatment of certain direct dyes mainly blacks, with 2-3% formaldehyde (30%) 1% acetic acid (30%) for 30 minutes at 70-80C improves the wet fastness to both water and washing.

2. A drop of light fastness may occur.

Cross Linking Agents and Resin Treatments:
1. Improvements in wet fastness properties can be ensured by treatment with cellulose reactants or amide-formaldehyde resins

2. Subsequent removal of resin by acid hydrolysis (formic acid at 90C or HCL at 60C) leaves the unfixed direct dye on the fiber with its originally low level of wet fastness. 

3. Treatment with cross linking agents in resin finishing improves the wet fastness but color and light fastness may be affected.

Stripping of Direct Dyes:
1. It is very easy to strip the direct dyes if these have not been after treated.

2. The colors can be destroyed by boiling with sodium dithionite Na2S2O4.H2O

3. Or by bleaching with hypochlorite solution containing 1-2gm/l of available chlorine.

4. If a cationic agent has been used as an after treatment, it is removed by boiling with 1-2% formic acid before destroying color with either reduction or oxidation.

5. For materials after treated with metal salts, the metal is first removed by boiling with a metal sequestering agent like sodium salt of ethylene diamine tetra acetic acid (EDTA) in a concentration of 3g/l and then the dye is decomposed by the usual treatment with hydros or chlorine.

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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