What is Sulphur Dye?
Sulphur dyes are complex heterocyclic molecules or mixtures formed by melting or boiling organic compounds containing amino or nitro groups with Na-polysulphide and Sulphur. Sulphur dyes are so called as they all contain Sulphur linkage within their molecules.

Sulphur dyes are highly colored, 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 Sulphur dye is substantive to cellulosic materials. They are absorbed on the fiber 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). Sulphur gives best result (Bright Tone) when they are used to produce black, Black & brown shades but red shades cannot be obtained by Sulphur dyes. The cotton textile materials dyeing with Sulphur dyes can be topped with basic dyes for brightening the shades.

Characteristics of Sulphur Dyes:
The main properties and characteristics features of Sulphur dyes are mentioned below:-

1. Sulphur dyes have Sulphur linkage within their molecules.
2. Sulphur dyes are highly colouerd water insoluble dyes. Some dyes are partially soluble in water.
3. 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)
4. Sulphur dyes have good light fastness with rating about 4. This light fastness may be improved by an after treatment with metallic salt.
5. 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.
6. They are not applicable to wool due to strong alkaline condition.
7. They are exclusively amorphous, few of them show crystallinity.

Classification of Sulphur Dyes:
Sulphur dyes may be classified in many ways. But according to their solubility there are essentially 3 classes of Sulphur dyes as below:

1. Conventional or Water insoluble dyes
2. Leuco Sulphur dyes (Partially soluble) and
3. Solubilised Sulphur dyes.

Features of Sulphur Dye

1. Amorphous Colloidal materials.
2. High molecular weight with various composition
3. Complex molecular structure –heterocyclic molecules containing Sulphur linkage.
4. Decomposed by acids, with the liberation of H₂S.
5. Characterized by thiozine ring, containing Sulphur atom.

Chemistry of Dyeing with Sulphur Dye:
The Sulphur dyes contain Sulphur 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 fibers. Na₂S acts as reducing agent that breaks the Sulphur 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 Sulphur dye in to previous water insoluble form which has a very good wash fastness property.

Sulphur 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.

Chemical Structure:
Sulphur dye contain Sulphur atom in their molecule and is characterized by the thiozine ring

A portion of Sulphur dye molecule is shown below:

The structure formula is incomplete because the complete composition and structure of Sulphur dye is mot known.

Reducing Steps of Sulphur Dyes:
Reducing step is the most important in the application of Sulphur dyes. Unless the dyes are converted in to the completely soluble form, the full colour value cannot be achieved. The solubility of reduced Sulphur dyes varies appreciably from dye to dyes. For example yellow Sulphur dyes are the last soluble dyes (i.e. most insoluble dyes)

The reducing agents are used for Sulphur dyes are:

1. Na-Sulphide.
2. Na-Hydro Sulphide.
3. Thioglycolic Acid.
4. Pseudo-Thio-Hydantion.

But Na Sulphide is the most widely used reducing agent for dissolving Sulphur 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 Sulphur 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-

1. Potassium dichromate
2. Na-Perborate.
3. Na-percarborate.
4. Na-peroxide.

The method of oxidizing selection plays an important role in the development of correct shades & their optimum fastness 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-

1. Use of perborate or percartborate in presence of acetic acid gives brighter shade.
2. Treating dyed material with dichromate without rinsing reduces colour losses but causes dull shade.

The chemistry of oxidization is as below:-

Typical Recipe:
The Typical recipe of sulphur dyes for dyeing with cotton is as below:-

• Sulphur 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

Improving of Fastness Properties:
The light fastness of cotton dyeing with Sulphur 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)
• CH₃COOH (60%)                   →        1-2%
• Temperature                            →        70⁰C
• Time                                       →        20-30 Minutes

Or

• Na₂Cr₂O₇                                →        1-1.5 % (on the weight of goods)
• CuSO₄ Solution                       →        0.5-1%
• CH₃COOH (60%)                   →        1-2%
• Temperature                            →        70⁰C
• Time                                       →        20-30 Minutes

The latter one also improves the washing fastness of Sulphur dyed textile materials.

Defects of Sulphur Dyeing:
They are mainly two defects which are common in Sulphur dyes which dyeing with textile materials:-

1. Bronziness or Dullness of shades
2. Sulphur Black tendering

Causes for the Popularity of Producing Black Shades with Sulphur Dyes:

1. Low cost
2. Fair to good light fastness
3. High Wash fastness
4. Easy to apply
5. Low energy required
6. Chemical resistance is moderate to good.
7. Wide range of shades especially on heavy, durable shades on apparel fabric.