Sulphur Dyes: Properties, Classification, Mechanism, Stripping & Defects

What is Sulphur Dyes?
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.

The sulphur dyes are named so because of the presence of sulphur atoms in their molecules. Like direct dyes, sulphur dyes are also quite cheap for dyeing cellulosic textiles with limited colour fastness properties. Different types of sulphur dyes include:

  1. CI sulphur dyes
  2. CI leuco sulphur dyes
  3. CI solublized sulphur dyes
  4. CI condensed sulphur dyes

Sulphur 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 Sulphur 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).

sulphur dyes

The reducing agents converts the “S” in dye in to –SH group and the Sulphur linkages. Then inside the material the thiols containing –SH groups are oxidized & thus reconverted to original form of dye.

This is shown in the bellow:

sulphur dyes reactionSulphur gives best result (Bright Tone) when they are used to produce black, Black and brown shades but red shades cannot be obtained by Sulphur dyes.

History of Sulphur Dyes
The history of Sulphur dyes may be summarized as below:

  1. The first Sulphur dyes where made in 1873 heating saw dust, caustic soda and Sulphur. It occurred by chance when a reaction vessel containing Na2S was leaking and the saw dust was used to wipe the solution coming out. Later a cotton fabric come in contact with this contaminated sawdust and become stained.
  2. The real pioneer of Sulphur dyes was vidal who produce vidal black (Name of Sulphur dye) by fusing para-phenylene diamine with Na2S & Sulphur in 1893.
  3. In 1897 Kalischer produced Immedial Black FF by heating 2, 4-dinitro-4-dihydroxy diphenylamine with Na-poly sulphide.
  4. In 1896 Read Holliday introduced a range of grey, brown and black Sulphur dyes by the action of Sulphur, alkali sulphides and many organic compounds.

Method of Manufacture of Sulphur Dye

…………………………………………………Thionation Sulphorization
Aromatic Hydro Carbon —————————————————————-→ Sulphur Dye
Containing Hydroxy amino or nitro group

Trade Names

Trade Names Name of Manufacturer Country of origin
Calcogen Dyes Dept. American Cyanamid Co. USA
Pyrogene Ciba Switzerland
Thional Sandoz Switzerland
Solfo ACNA Italy
Sulfogene Du pont USA
Thional Imperial UK
Mitsui Sulphur Mitsui Chemicals Ind. Co. Ltd Japan

Properties 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.
  8. Important for producing a wide range of shades on a varity of cotton and rayon.
  9. Sulphur dyes are suitable for heavy & durable shades
  10. Available in powder and soluble form
  11. Sulphur dyes are cheap & easy to manufacture.
  12. Heat and chemical resistance of Sulphur dyes are moderate to good. They have poor fastness to chlorine and are not applied to goods which are bleached with hypochlorite.

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.

Chemical Nature of Sulphur Dyes
The manufacturing method of Sulphur dyes may be shown as below:-

…………………………………………………Thionation Sulphorization
Aromatic Hydro Carbon —————————————————————-→ Sulphur Dye
Containing Hydroxy amino or nitro group

This reaction is carried out in a closed vessel in the presence or absence of solvents. For this (Thionisation/Sulphorisation) purpose Sulphur or Na- polysulphide is used. The features of the products of thionation are controlled by organic compounds, conditions of reaction (time, temp etc). Condition of separating dyes from reaction mixture etc.

After the reaction is over the dye is precipitated acidification or oxidation or both.

Generally Sulphur dyes are marketed in forms of powder pastes or liquid solutions. The dyes are amorphous colloidal materials of high molecular weight and variable composition. Their exact chemical composition is not yet established. However they are complex in structure. Some amorphous present in Sulphur dyes are as below:

amorphous present in Sulphur dyes

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 fibres. 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.

Sulpher dyes contain Sulpher linkage within their moleculesThis 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.

water soluble luco form of Sulphur dyeSulphur 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

thiozine ringA portion of Sulphur dye molecule is shown below:

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

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.

These in turn are subdivided in to 6 sub classes, depending on the method of application. Namely–

  1. Method i
  2. Method ii
  3. Method iii
  4. Method iv
  5. Method v
  6. Method vi

Method-i:
Conventional Sulphur dyes (i.e. water insoluble Sulphur dyes) are dyed in method i. They have slight affinity to textile fibers of cellulosic origin. Method is as below:-

Dye pasted with cold water

Add required amount of Na-sulphide

Dissolved by adding boiling water

Boiled for 100 minutes to complete desolation

Taken in dye bath containing soda ash

Material immersed in dye bath

Raising temperature & adding Glauber’s salt for exhaustion.

Method-ii:
These dyes are also insoluble in water and are called specialized conventional Sulphur dyes. Its application method is as below:-

Pasting of dye with 25-30 times soft water and reducing agent

Dissolved dye added to dye bath (mixture of soda ash & rongalite C) and boiled for sometime

Material put in dye bath and dyeing carried out for 20 minutes

For exhaustion common salt or Glauber salt is added and dyeing continued for 30-40 minutes.

Method-iii:
This method is used for leuco Sulphur dyes; they have distinct affinity for cellulosic fibres. These Sulphur dyes are dyed as in the case of method –I except that little or no Na-sulphide is required for their volatilization.

Method-iv:
These dyes are soluble in water having substantivity to cellulosic fibres. They are mixture of Sulphur dyes and Na- sulphide. The dyeing method is as below:

Dye pasted with little amount water at 300C

Diluted with 10-20 times water at same temperature

Dissolved dye is added to required quantity of water in dye vessel

Common or Glauber Salt and soda ash is added in dye bath

Material is put in dye bath and kept at 400-600C for 25-30 minutes.

Method-v:
These dyes are also soluble in water & are substantive to cellulosic fibres. They are dry mixture of the dyes and Rongallite-C. The dyeing method is as below:

Dye pasted with water & dissolved by boiling at 10-20 times soft water

Solution is added to dye bath containing required amount of water

Temperature is raised to 900C

Material is entered in to the dye bath after 20 minutes

Glauber salt is added & dyeing continued for some times.

Method-vi:
These dyes are solubilized Sulphur dyes. They have no affinity towards cellulosic fibres. Chemically they are thio-sulphuric acid derivatives and are applied either by Na-sulphite method or by Na-Hydrosulphite method.

Some oxidizing and reducing agent

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
Hydrozen peroxide (H2O2)

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.

For dissolving the Sulphur dyes, reducing agent is taken according to the weight of dye. In practice, they are taken at same weight. For example, 10 gm dye is mixed with 10 gm reducing agent. Then the mixture is boiled after adding necessary amount of water to get a good solution. For this wooden vessel or enameled iron vessel are used but not iron vessels. Because iron reacts with Sulphur.

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:-

chemistry of oxidizationDyeing of Cellulosic Fibres with Sulphur Dyes:
The sequence of dyeing with Sulphur 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:-

  • 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

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 Sulphur 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:
The dyed substance is soaped which makes the colour bright and makes shade permanent.

The light fastness property of 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 improves the light fastness. When the dichromate is also present the washing fastness is slightly improved.

The following recipe is used for after treatment:

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

Precaution in the dyeing process:
The precautions in the dyeing process with the Sulphur dyes are mentioned below:

  1. In the application of Sulphur dye, the dye bath should not contain Ca- Salts. If they are present in the form of insoluble co-compounds with the Sulphur dyes which precipitated easily in closed machines and form sludge when restrict the circulation of the dye liquor.
  2. The dye should be dissolved with soda.
  3. 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.
  4. Fabric must not to expose to air during dyeing to prevent precipitation & oxidation. This will cause uneven dyeing.
  5. 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.
  6. To prevent the broziness the dye liquor should not be very worm.
  7. Goods dyed with Sulphur black should be after treated with a solution of K2Cr2O7 followed by rinsing. This will prevent Sulphur black tendering.

Control of Dyeing:
Reduced Sulphur 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 Sulphur dyes are relatively cheap this wastage is not considered. Especially in case of medium & light shades.

Topping of Sulphur Dyes:
Sulphur dyes act as mordents for the basic dyes. The cotton textile materials dyed with Sulphur dyes can be topped with basic dyes for brightening the shades. So, topping of Sulphur dyes is one kind of re dyeing by which required brighter shades are obtained.

For topping of Sulphur dyes, at first dyed materials (dyed with Sulphur dyes) is washed thoroughly to remove the alkali from the fabric. Then the fabric is treated with basic dye in a second dye bath containing acetic acid of alum. In this bath temperature is raised about 600C to control the exhaustion of basic dye.

Improving of Fastness Properties:
The light fastness of the 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

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 dyed textile materials:-

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

Now they are Described below:-

1. Bronziness of Shades:
This is a common defect found in Sulphur dyed textile materials.

Causes:
The causes are as below:-

  • Excessive delay between lifting of the material from the dye bath and washing off.
  • Exposure of goods to air while dyeing.
  • Too much use of common salts as exhausting agents.
  • Insufficient Na2S (i.e. reducing agent) in dye bath. As a result of which dye is incompletely reduced.
  • Strong dye liquor in the dye bath.

Remedies:
The remedies are as below:-

  1. Good washing and dilute solution of Na2S (0.1%) at 300C or,
  2. A treatment with boiling soap solution or a strong  Na2S solution or,
  3. A treatment with a solution containing 10% salponified palm oil at 600C or,
  • Ammonia                    :           125-200 gm/100 litre or, 1.25-2 gm/litre
  • T.R Oil                         :           200-300gm/100litre or, 2-3 gm/litre.
  • Time                            :           15 minutes
  • Temperature                :           Water Bath

2. Sulphur Black Tendering:
If Sulphur dyed textile materials are stored (weakening) tentering effect is seen on the cellulose. Due to storage a part of the Sulphur colour converts in to H2SO4 by oxidizing which in turn causes degradation of cotton by hydrolysis. It is a serious problem of Sulphur dyed goods.

Causes: 
The causes are as below:-

  • Gradual oxidation of Sulphur to H2SO4 on storage.
  • After treatment with copper salts causes rapid tendering.
  • Presence of iron as an impurity causes rapid tendering.
  • The method of oxidation for the recon version to insoluble form influence tendering.

Remedies:
The remedies are as below:-

  • Treatment of dyed material with 1-3% K2Cr2O7 and 1-3% CH3COOH at 600C temperature for 30 minutes followed by through rinsing.
  • Treatment with a little CH3COOH so that H2SO4 may be converted in to harmless acetic acid.
  • Using 5 gm/litre soda ash after dyeing followed by drying without rinsing.

Stripping of Sulphur Dyes:
Unevenly Dyed shades on cellulosic materials with Sulphur dyes may be corrected by a treatment with a worm solution of Na2S in the presence of Albigen A (polyvinyl pyrolodine)

If this method is found to be in effective then the uneven dyed material may be treated with a solution of a NaOCl or bleaching powder (2-3 gm/Litre of available chlorine).

In some cases bleaching with KMnO4 solution may be effectively carried out. In other cases the dyed material may be treated with worm NaOCl solution in the presence of NaOH.

The uneven dyeing of material causes due to the following reasons:

  • The oxidation of dye during dyeing when the material comes in contact with air.
  • Presence of gummy material on the fabric.
  • Defective Dye.

Uses of Sulphur Dye:

  1. To dye Umbrella cloth with Sulphur black.
  2. To dye cotton fabric eg-lining boot.
  3. To dye rubber material hence after treatment by CuSO4 is necessary

S-Radical in Sulphur dye:
In case of tendering of Sulphur, due to storage lose of strength occurs and forms to Sulphur radical. Three types of Sulphur radical are present in Sulphur dyes as below:-

  • Mechanically free Sulphur: Air+S= H2SO4
  • Stable Sulphur: 70%-80%, not easily oxidized.
  • Chemically active Sulphur : 20-25%

Colors/Shades found from Sulphur dyes:

  • Red                             : Not good, not pure red color.
  • Black, blue, brown      : Excellent Shade.
  • Others                        : Olive, Orange, green etc.

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.

Comparison between Sulphur & Vat Dyes:

Subject Sulphur Dye Vat Dye
Molecular Size Dye molecules are very Large Smaller than Sulphur
Shade Gives dull shade due to varying conditions Gives bright shades
After reduction Smaller size molecules are obtained

D–S–S–D+[H] → D–SH + SH–D

Vat dyes are reduced as below

Vat dyes are reduced

Fabric Dyed Cotton, Rayon, Nylon & P/C Cellulose Rayon etc
Hydrolysis Under high temperature and Humidity Susceptible to hydrolysis i.e. easy to Hydrolysis.
Wash Fastness Good to very good Excellent
Price Cheaper than Vat dye Very costly dye
Light fastness Good to very good Excellent for anthraquinone.
Sulphur linkage Contain Sulphur linkage No Sulphur linkage is present.
Ionization The dye molecules are negatively ionized after reduction Same

Reasons for why so called?
The dyes are so called as they contain Sulphur linkage within their molecules but dissolved in a solution of sulphide which acts as a reducing agent, breaking the Sulphur linkage and converting the molecules into simpler components which are soluble in water and substantive towards cellulose.

Dye–S–S–Dye (Water insoluble Sulphur dye molecule)

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