Dyeing Of Cotton Fabric With Sulpher Dyes
Sulpher dyes are complex heterocyclic molecules or mixtures formed by melting or boiling organic compounds containing amino or nitro groups with Na-polysulphide and Sulpher. Sulpher dyes are so called as they all contain Sulpher linkage within their molecules.
Sulpher dyes are highly coloured, water insoluble compounds and have to be converted in to water soluble substantive forms (lucoforms) before application to the textile materials. This conversion is carried out by a treatment with a reducing agent like dilute aqueous Na2S. Since this lucoform of Sulpher dye is substantive to cellulosic materials. They are absorbed on the fibre surface. Then they are reconverted original water insoluble form of dye by oxidation. This oxidation is carried out by “airing” (exposure to air) or by using an oxidizing agent like Na-dichromate (Na2Cr2O7).Sulpher gives best result (Bright Tone) when they are used to produce black, Black & brown shades but red shades cannot be obtained by Sulpher dyes.
Method of Manufacture of Sulpher dye:
Aromatic Hydro Carbon Containing Hydroxy amino or nitro group ⇒ Thionation Sulphorization ⇒ Sulpher Dye
|Trade Names||Name of Manufacturer||Country of origin|
|Calcogen||Dyes Dept. American Cyanamid Co.||USA|
|Mitsui Sulpher||Mitsui Chemicals Ind. Co. Ltd||Japan|
Characteristics of Sulpher Dyes:
The main properties and characteristics features of Sulpher dyes are mentioned below:-
- Sulpher dyes have Sulpher linkage within their molecules.
- Sulpher dyes are highly colouerd water insoluble dyes. Some dyes are partially soluble in water.
- They have no direct affinity towards cellulosic fibres. To make them substantive they are to be converted in to soluble lucoform by treating them with reducing agents (Like dilute Na2S solution)
- Sulpher dyes have good light fastness with rating about 4. This light fastness may be improved by an after treatment with metallic salt.
- These dyes have excellent wash fastness with rating about 3-4. This good wash fastness is due to its larger molecular size & insolubility in water.
- They are not applicable to wool due to strong alkaline condition.
- They are exclusively amorphous, few of them show crystallinity.
Classification of Sulpher Dyes:
Sulpher dyes may be classified in many ways:- But according to their solubility there are essentially 3 classes of Sulpher dyes as below:-
- Conventional or Water insoluble dyes
- Leuco Sulpher dyes (Partially soluble) and
- Solubilised Sulpher dyes.
These in turn are subdivided in to 6 sub classes, depending on the method of application. Namely–
- Method i
- Method ii
- Method iii
- Method iv
- Method v
- Method vi
Features of Sulpher Dye :
- Amorphous Colloidal materials.
- High molecular weight with various composition
- Complex molecular structure –heterocyclic molecules containing Sulpher linkage.
- Decomposed by acids, with the liberation of H2S.
- Characterized by thiozine ring, containing Sulpher atom.
Chemistry of dyeing with Sulpher Dye:
The Sulpher dyes contain Sulpher linkage within their molecules. They are insoluble in water but can be made soluble in water by treating them with reducing agents. This also makes them substantive towards cellulosic fibres. Na2S acts as reducing agent that breaks the Sulpher linkage and break down the longer molecules in to simple components which can penetrate the material (fiber/fabric) surface easily.
This thios containing the –SH groups are readily oxidized by the action of atmospheric O2 or any other oxidizing agents. This reconverts the water soluble luco form of Sulpher dye in to previous water insoluble form which has a very good wash fastness property
Sulpher dyes are negatively ionized. No Vander wall’s force effect on them is activated. Addition of salt improves efficiency of dyeing by increasing physical force.
Sulpher dye contain Sulpher atom in their molecule and is characterized by the thiozine ring.A portion of Sulpher dye molecule is shown below:
The structure formula is incomplete because the complete composition and structure of Sulpher dye is mot known.
Some oxidizing and reducing agent is written below :
|Oxidizing Agent||Reducing Agent|
|Potassium Dichromate(K2Cr2O7)||Sodium Sulphide (Na2S)|
|Acetic Acid(CH3COOH)||Sodium Hydro sulphide(NaHSO3)|
|Sodium Perborate||Thioglycolic acid|
|Sodium per carborate||Thio Salicylic acid|
|Sodium peroxide(NaO2)||Pseudo thiohydantion|
Reducing Steps of Sulpher Dyes:
Reducing step is the most important in the application of Sulpher dyes. Unless the dyes are converted in to the completely soluble form, the full colour value cannot be achieved. The solubility of reduced Sulpher dyes varies appreciably from dye to dyes. For example yellow Sulpher dyes are the last soluble dyes (i.e. most insoluble dyes)
The reducing agents are used for Sulpher dyes are:-
- Na-Hydro Sulphide.
- Thioglycolic Acid.
But Na Sulphide is the most widely used reducing agent for dissolving Sulpher dyes. They may be replaced by Na-Hydrosulphide in some cases. But the latter is a more powerful reducing agent. So, over reducing of the dye may take place, leading to a product having lower affinity for cellulosic fibres. Hence lower colour yields are obtained along with wastage of dyestuff. In some cases lower wash fastness results if Na-Hydrosulphide is used. It also decreases the life time of the vessel.
Oxidation Step of Sulpher Dye:
After dyeing the reduced water soluble form of the dyes have to be converted in to the original water insoluble form by oxidation. The commonly used oxidizing agents are-
- Potassium dichromate
The method of oxidizing selection plays an important role in the development of correct shades & their optimum fast5ness properties. Rinsing off the dyeing should be done rapidly to remove as much as Na-Sulphide as possible. If even after washing some Na-sulphide or Soda-ash stays on fabric, we use CH3-COOH to neutralize them.
Using of different oxidizing agents may give following results-
- Use of perborate or percartborate in presence of acetic acid gives brighter shade.
- Treating dyed material with dichromate without rinsing reduces colour losses but causes dull shade.
The chemistry of oxidization is as below:-
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)|
|Soda Ash (NaCO3)||7 gm/litre|
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: –
|CuSO4 Solution||1-2% (on the weight of goods)|
|Na2Cr2O7||1-1.5 %( on the weight of goods)|
The latter one also improves the washing fastness of Sulpher dyed textile materials.
Defects of Sulpher Dyeing:
They are mainly two defects which are common in Sulpher dyed textile materials:-
- Bronziness or Dullness of shades
- Sulpher Black tendering
Causes for the Popularity of Producing Black Shades with Sulpher Dyes:
- Low cost
- Fair to good light fastness
- High Wash fastness
- Easy to apply
- Low energy required
- Chemical resistance is moderate to good.
- Wide range of shades especially on heavy, durable shades on apparel fabric.
You May Look :
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- Dyeing Of Cotton Fabric With Reactive Dyes