Dyeing of Cellulosic Fibres with Sulpher Dyes | Sulpher Dyeing Process of Natural Fibers

The sequence of dyeing with Sulpher dyes is as following:

Goods Preparation

Dye Solution preparation

Dyeing

Oxidation

After treatment

Dyed Goods


Typical Recipe:
The Typical recipe for dyeing is as below:-
  • Sulpher Dye : 10% (On the weight of the fabric)
  • Na2S (Reducing Agent) : 1.5%(on the weight of the Dye)
  • Salt : 8 gm/litre(NaCl)
  • Soda Ash (NaCO3) : 7 gm/litre
  • Temperature : 1000C
  • Time : 90 minutes
  • Material: Liquor : 1:20
Sometimes a chelating agent EDTA is added to chelate any metal salt extracted from the cotton goods & effectively remove these ions from the dye bath. This is necessary to prevent a harsh texture forming on a textile.

Good Preparation:
Only normal pretreated fabric (scoured, bleached) is required. Mercerized cotton goods causes an increased colour yield of 30-40%, But goods bleached by Na-Hypochlorite should not be dyed by Sulpher dyes:

Dye solution preparation or Reducing Step:
Required amount of dye, soda ash & Na2S are taken in a beaker and a little amount of cold water is added to make a paste. In a separate vessel the rest of the water is boiled and the paste is added to it. Then the mixture is boiled for 5 minutes to make the dye completely reduced.

Dyeing:
At first dye solution is taken in a dye bath & the fabric is immersed in that solution. Then it is heated for 10-15 minutes. Then half of the total NaCl solution is added and temperature is raised to boil & then the rest amount of NaCl is added. Dyeing at this temperature is continued for about 30 minutes. Then the steaming is turned off and the application continued in cooling liquor for a further half hour. This steaming helps to diffuse the dye in to the fabric before fixing. Retained dye liquor should be rinsed out immediately from the surface of the fabric.
Dyeing Curve
Oxidation:
After dyeing the reduced water soluble form of the dye should be make insoluble by oxidation, to fix it on the fabric permanently. For this oxidation, we may use K2Cr2O7, CH3COOH. Na-perborate, Na2O2 etc.

The typical recipe of oxidation is as below:
  • Na-Perborate ........... 0.5-1 gm/litre
  • Temperature .......... 40-500C
  • Time ............ 20 minutes.
After Treatment:
  1. The dyed substance is soaped which makes the colour bright and makes shade permanent.
  2. The light fastness property of Sulpher dyes which is generally good can be improved by after treatment with certain metallic salts. Thus a treatment with CuSO4 & CH3COOH in presence of K2Cr2O7 improves the light fastness. When the dichromate is also present the washing fastness is slightly improved.
The following recipe is used for after treatment:
  1. CuSO4 Solution ......... 1-2% (on the weight of goods)
  2. CH3COOH (60%) ......... 1-2%
  3. Temperature .......... 700C
  4. Time .......... 20-30 Minutes
Precaution in the dyeing process:
The precautions in the dyeing process with the Sulpher dyes are mentioned below:-
  • In the application of Sulpher dye, the dye bath should not contain Ca- Salts. If they are present in the form of insoluble co-compounds with the Sulpher dyes which precipitated easily in closed machines and form sludge when restrict the circulation of the dye liquor. 
  • The dye should be dissolved with soda. 
  • Great care should be taken during reduction process because over reduction should lower the affinity of dye towards the fibre. As a result dull shade will appear and a lot of dye stuff will go to waste. 
  • Fabric must not to expose to air during dyeing to prevent precipitation & oxidation. This will cause uneven dyeing. 
  • To prevent uneven dyeing in the fabric selvedge more amount of Na2S or NaOH and salt should be added in case of the jigger dyeing machine. 
  • To prevent the broziness the dye liquor should not be very worm. 
  • Goods dyed with Sulpher black should be after treated with a solution of K2Cr2O7 followed by rinsing. This will prevent Sulpher black tendering.
Control of Dyeing:
Reduced Sulpher dyes are similar to direct dyes. Because they can also be exhausted by adding common salts (NaCl) or Glauber’s salt (Na2SO4.10H2O) to the dye bath.

Sodium bi sulphite (NaHSO3) and ammonium sulphate {(NH4)2SO4} may also act as exhausting agents. Especially in dyeing combination shades. NaHSO3 is an acidic salt and (NH4)2SO4 is an acid liberating agent. They reduce the pH of dye bath and produce 15-20% deeper shades than common salt, when used as the exhausting agent. But the shades produced in presence of them are less bloomy and dull.

When NaHSO3 is used alone the rate of exhaustion is low but the produced shade is brighter and bloomier than common salt (NaCl) dyeing.

The use of the exhausting agents causes wastage of dye but as the Sulpher dyes are relatively cheap this wastage is not considered. Especially in case of medium & light shades.

Improving of Fastness Properties:
The light fastness of the Sulpher dyes which is generally good can be improved by after treatment with certain metallic salts. Thus a treatment with CuSO4 & CH3COOH in presence of K2Cr2O7 or Na2Cr2O7 improves the light fastness. When the dichromate is also present the washing fastness is slightly improved. In actual practice the following after treatments are given: -
  1. CuSO4 Solution à 1-2% (on the weight of goods)
  2. CH3COOH (60%) à 1-2%
  3. Temperature à 700C
  4. Time à 20-30 Minutes
The latter one also improves the washing fastness of Sulpher dyed textile materials.

0 comments:

Comment here

Textile Learner is the largest Textile Blog over the net. It is an ultimate reference for textile students. It describes textile articles in comprehensive. It also supplies news on latest textile technology, educational institute news of the world.