SULFUR DYE

Sulfur dyes are the most commonly used dyes manufactured for cotton in terms of volume. 
They are inexpensive, generally have good wash-fastness, and are easy to apply. 
Sulfur dyes are predominantly black, brown, and dark blue.
Red sulfur dyes are unknown, although a pink or lighter scarlet color is available.

Chemistry
Sulphur linkages are the integral part of chromophore in sulfur dyes. 
They are organosulfur compounds consisting of sulphide (–S–), disulphide (–S–S–) and polysulphide (–Sn–) links in heterocyclic rings. 
They feature thiazoles, thiazone, thianthrene, and phenothiazonethioanthrone subunits. 
Being nonionic, sulphur dyes are insoluble in water.

Process
Dyeing includes a few stages, viz. reduction, dyeing, washing, oxidation, soaping and final washing. 
The anion is developed on reducing and solubilising at boil when it shows affinity for cellulose. 
Sodium sulphide (Na2S), the reducing cum solubilising agent perform both reduction and solubilisation producing thiols and then to sodium salt of thiols or thiolates, which are soluble in water and substantive towards cellulose. 
Higher rate of exhaustion occurs at 90-95oC in presence of electrolyte. 
Dyed cellulosics exhibit tendering effect on storage under humid atmosphere due to presence of excess free sulphur and aftertreatment with sodium acetate is required to suppress that. 
H2S liberated during dyeing forms corrosive metal sulphide and this restricts use of metal vessels except those made of stainless steel. 
Fe + H2S → FeS + H2

Production, past and present
The forerunner of sulfur dyes is attributed to "Cachou de Laval," which is prepared by treating wood products with sulfide sources. 
Subsequently, the so-called Vidal Blacks were produced by reactions of various aniline derivatives with sulfur. 
These experiments demonstrated that deeply colored materials could be readily produced by combining aromatic compounds and sulfur sources.

The most important member of the class is Sulfur Dye. 
Sulfur Dye is produced by the reaction of 2,4-dinitrophenol and sodium sulfide in hot water.
Like many sulfur dyes, details on the chemical reactions are poorly understood. 
Sulfur Dye is accepted that the sulfide reduces the nitro groups to aniline derivatives, which are thought to form indophenol-containing intermediates that are further crosslinked by reaction with sulfur. 
The result are insoluble, high molecular weight species. 
Sulfur Dye is imperfectly understood, and the material is probably heterogeneous. 
Sulfur Dye is speculated to be a polymer consisting of thianthrene and phenothiazine subunits. 
The so-called sulfur bake dyes are produced from 1,4-diaminobenzene and diaminotoluene derivatives. These dyes are proposed to consist of polymers with benzothiazole subunits. 
Members of the sulfur bake dyes class are Sulfur Orange 1, Sulfur Brown 21, and Sulfur Green 12.

Application method
Sulfur dyes are water-insoluble. 
In the presence of a reducing agent and at alkali pH's at elevated temperature of around 80 °C, the dye particles disintegrate, which then becomes water-soluble and hence can be absorbed by the fabric. 
Sodium sulfide or sodium hydrosulfide are suitable reducing agents. 
Common salt facilitates the absorption. 
After the fabric is removed from the dye solution, it is allowed to stand in air whereupon the dye is regenerated by oxidation. The regenerated parent dye is insoluble in water. 
Oxidation can also be effected in air or by hydrogen peroxide or sodium bromate in a mildly acidic solution.

The low water solubility is the basis of the good wash-fastness of these dyed fabrics. 
These dyes have good all round fastness except to chlorine bleaches. 
Because the dye is water-insoluble, it will not bleed when washed in water and will not stain other clothes. 
The dye, however, may have poor fastness to rubbing. The dyes are bleached by hypochlorite bleach.

Environmental issues
Due to the highly polluting nature of the dye-bath effluent, sulfur dyes are being slowly phased out in the West but they are used on a large scale in China.
Recent advances in dyeing technologies have allowed the substitution of toxic sulfide reducing agents. 
Glucose in basic solution is now used and both low sulfide and zero sulfide products are available. 
Future developments in the field of reducing dye levels by means of electro-chemical processes are promising.

Sulfur dye, any of a group of sulfur-containing, complex synthetic organic dyes applied from an alkaline solution of sodium sulfide (in which they dissolve) to cellulose, where they become substantive to the fibre. 
On exposure to air, the dyes in the fibre are oxidized back to their original insoluble form. 
Sulfur dyes are fast to washing, perspiration, and light but have poor resistance to chlorine bleach. 
Most colours are available, especially subdued and deep shades, rich blacks, and navy, but not bright red and orange.

Sulfur dye, a class of dye containing sulfur, has more complex molecular structure. 
Sulfur dye is generally heated sulfur or sodium sulfide together with certain aromatic amines, amino phenols and other organic compounds, namely vulcanization. 
Most sulfur dyes are insoluble in water.
In dyeing, sulfur dye needs to be dissolved in a sodium sulfide solution or alkaline hydrosulfite solution to reduce to its hidden color, and then absorbed and oxidized by fiber to show color. 
According to the desired dyeing conditions, sulfur dye can be divided into sulfur dye and sulfide vat dye. 
The former use sodium sulfide as reducing agent; the latter hydrosulfite as reducing agent, and its fastness and properties is between sulfur dye and sulfide vat dye, so that it is a higher sulfur dye.

In 1873, the initial sulfur dye is made out from melting sodium by Groissant. 
In 1918, sulfur dye began to be produced in our country, mainly used for plant fiber dyeing. 
Sulfur dye is characterized by containing a sulfur atom in the molecule, becoming into water-soluble hydrocolloid after the reaction with a reducing agent such as sodium sulfide, dyeing the fibers, and then the dye turns  insoluble through the air oxidation. 
The sulfur dye is not very bright, but generally washable and light fast, enables the fiber brittle. 
There is also soluble sulfur, which is soluble in water, and more convenient to use. The sulfur dye is a mixture generating from every reaction stage.
In 1950, the  continuous dyeing method of fine particles of sulfur appeared, but it occurred unsalable after the 1950s, because of the successive appearance of reactive dyes and advanced direct dyes, lightfastness direct dye especially. Still, owe to its budget and other factors, it is still in use.

Sulfur dyes is more popular with rural customers for its relatively simple manufacturing process and low cost, also, it is washable, light fast, not bright shade therefore can be dyed dark, adapted plant fibers and stained.  
In addition to color is not bright, the dye chromatography insufficiency, lacking of good red and purple, and low color strength, larger dosage, therefore sulfur dyes is limited in application, but so far it still is the largest output and applied widely dyes in China.

Sulfur dyes are synthetic organic dyes that are formed by the thionation or sulphurisation of organic intermediates containing nitro or amino groups. 
Sulfur dyes have sulfur bonds in their molecules. 
Sulfur dyes are highly colored water-insoluble dyes. 
Some sulfur dyes are partially soluble in water. 
They have no direct affinity for cellulose fibers. 
To make them substantive, they are converted to soluble forms by treatment with a reducing agent. 
Sulfur dyes have good lightfastness. 
Due to the strong alkaline conditions, they are not suitable for wool. 
Sulfur dyes are cheap and easy to manufacture. 
Sulfur dyes have moderate to good heat resistance and chemical resistance. 
Due to their poor fastness to chlorine, they are not suitable for products that are bleached with hypochlorite.

Application

Sulfur dyes are mainly used for the dyeing of textile cellulosic materials, or blends of cellulosic fibers with synthetic fibers such as acrylic fibers, polyamide and polyester. 
They can also be used for a limited number of silk and paper applications. 
Soluble sulfur dyes are used for the dyeing of certain types of leather. 
Low water solubility of sulfur dye is the basis for good wash fastness of these dyed fabrics. 
Since the dye is usually insoluble in water, it does not seep when washed in water and does not stain other clothes. 
Most colors are available, especially deep shades, rich blacks, and navy, but not bright red and orange.

Sulfur dyeing mechanism

Sulfur dyes are highly colored, water-insoluble compounds that must be converted into water-soluble forms before being applied to textile materials. 
The conversion is performed by treatment with a reducing agent such as aqueous Na2S solution. 
They are absorbed on the fiber surface and then reconverted back to the original water-insoluble dye form by oxidation. 
In addition to cellulose fibers, sulfur dyes can also be used to dye staple fibers and yarn.

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:

CI sulphur dyes
CI leuco sulphur dyes
CI solublized sulphur dyes
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).

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.

Sulphur 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:

The first Sulphur dyes where made in 1873 heating saw dust, caustic soda and Sulphur. 
Sulfur Dye 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.
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.
In 1897 Kalischer produced Immedial Black FF by heating 2, 4-dinitro-4-dihydroxy diphenylamine with Na-poly sulphide.
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.

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

Sulphur dyes have Sulphur linkage within their molecules.
Sulphur 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) Sulphur 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.
Important for producing a wide range of shades on a varity of cotton and rayon.
Sulphur dyes are suitable for heavy & durable shades
Available in powder and soluble form
Sulphur dyes are cheap & easy to manufacture.
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

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

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:-

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

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:-

Na-Sulphide.
Na-Hydro Sulphide.
Thioglycolic Acid.
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. 
Sulfur Dye 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-

Potassium dichromate
Na-Perborate.
Na-percarborate.
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-

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.

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.

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

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.
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 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.

Sulphur Dyeing In Denim

This is a guest post by Harry Mercer. 
Sulfur Dye tells about the  best practices in Sulphur dyeing for fashion denim and was  a part of the presentation made at Dhaka  by him on a seminar on Sulphur Dyes by Fenazol

The Nature of Sulfur Dyes
Sulfur dyes are a form of vat dyes – they are water-insoluble and in order to apply to fibers must be made water-soluble through the process of reduction. 
Reduction is a chemical process in which hydrogen is liberated. The hydrogen reacts with the dye and permits a water molecule to attach to the dye. The dye is transported into cotton fiber by the water.
This reduced dye must then be oxidized. Oxygen reacts with the hydrogen producing water. Removing the hydrogen makes the dye insoluble, which results in the dye becoming physically trapped inside the fiber.
The Uses of Sulfur Dyes
Sulfur dyes are commonly used for black, blue, brown, khaki and green colors.
Sulfur dyes are applied to cellulosic fibers and blends of cellulosics with polyester, nylon and acrylics.
Sulfur dyes can be applied with little difficulty and with excellent results at a relatively low cost.
With good application methods, sulfur dyes are extremely wash-fast, however, they are usually not fast to chlorine bleaching.
Dark and medium colors are the most common with sulfur dyeing. Light colors can be produced by special procedures involving blended reducing agents, blends of surfactants, controlled oxidation and preparation methods like Mercerizing and bleaching.

Dark, dull sulfur dyeing
•Sulfur blacks are commonly applied in the 1st box on Indigo machines.

•The sulfur black dye molecule exists as very large polymers which require high temperatures for penetration into fibers (90-95 degrees).

•The maximum amount of sulfur black dye that can be efficiently applied is 4%on weight of cotton of a pure sulfur powder or 20% of a standard liquid (20% dye strenght). 
Darker sulfur blacks can be achieved by adding a sulfur blue.

•Sulfur blacks are best oxidized with an air passage like Indigo.

Light Sulphur Colors
Sulfur dyeing procedures have been used to create a full range of colors including browns, yellows, greens, violets, reds, orange and neutrals  (light/medium greys)
Producing special colors with these processes involves the use of true sulfur dyes alone or in combination with sulfurized vats or special direct dyes that  can be applied with special sulfur dyeing procedures.
For success in fashion markets with these colors, marketing is focused on color “themes” such as “earth tones”, “urban”, sport or university colors.
Control Of Light Sulphurs
In order to match specific colors, dyes with greatly different affinity factors are mixed.
Different affinity factors result in these dyes being picked up at different rates, so special methods are necessary to avoid color variation.
Sulfur colors should be applied from 30 to 60 degrees to avoid color variation and must be chemically oxidized.

Sulphur Bottoming
Bottoming of Indigo dyed yarns produces a darker, brighter effect with Indigo.
The purpose of bottoming originally was to produce a darker color with less Indigo.
While large amounts of sulfur dye can be applied in the 1st box, most of the sulfur is removed by hydrosulfite in the Indigo dye.
Unlike other sulfur dye applications, a pH of 12 with sodium hydroxide is helpful to retard the dyeing for color consistency.
A temperature of 60 degrees or less should be used to lower the affinity factor for consistent color.

Black-on-Black Dyeing
Sulfur black dyes are low-intensity dyes that require large amounts for dark shades.
The amount of sulfur black that will produce the darkest shade when applied properly is 4% of a 100% powder or 18% of a 20% liquid.
Sulfur Dye is difficult for cotton to easily absorb that quantity of dye in a way that produces a very dark shade. 
One method that has been used to produce darker, more colorfast sulfur blacks on yarn is to apply 50% of the dye, then dry the cotton, air oxidize and without washing, proceed to a second dye-box where the dye is applied again, dried oxidized with air again and washed.

Reducing Agents For Sulphur Dyes
Because sulfur dyes cannot be dissolved in water directly, reducing agents must be utilized to produce hydrogen which allows the dye to attach to water.
Any standard reducing agent can be used to dye sulfurs including reducing sugars (dextrins), sodium hydrosulfite, sodium bisulfite, sodium sulfide, sodium polysulfide and sodium hydrosulfide.
Selection of reducing agents
Reducing agents for sulfur dyes should be selected according to the type of dyeing (batch or continuous), temperature of dyeing (cold or hot) and depth of color (light or dark).
The reduction potential measured in millivolts for continuous sulfur dyeing as on Indigo machines should be at least -600 for dark, dull shades and at least -700 for bright, colorfast shades.
The best sulfur dyeing for bright, consistent and colorfast shades requires blends of reducing agents.
Advances in sulfur dyeing
Cold dyeing of sulfurs is possible on Indigo machinery in a full range of colors.
This procedure has long been utilized by craft dyers in Asia.
Cold dyeing of sulfurs have the advantage of requiring only 50% as much sulfur dye,
Energy savings, excellent colorfastness, no polluting dye waste and less cotton damage for a soft touch and higher weaving efficiency.
Special Notes
Dyeing with 100% sulfur blacks on yarn results in lower weaving efficiency. 
Normal yarn break levels with black can be achieved with improved washing and the use of buffers in dyeing.
Sulfur blacks can be dyed with special procedures that provide wet rub-fastness of 4 and good washfastness even after 20 launderings.
Caution And Care
The recommendations of many sulfur dye suppliers result in dye waste of 50% or more, which increases dyeing costs by twice that is necessary. 
Also, most of the additional chemicals used in sulfur dyeing, especially for blacks, are unnecessary.
The use of fixatives after sulfur bottoming are often recommended to increase dye shades, but these are destroyed by the hydrosulfite in the Indigo boxes.

Sulphur dyeing is used for dyeing of cotton and in market it is available in powder and liquid form also. 
In exhaust dyeing maximum powder form is used in dyeing of cotton material.

In sulphur black dye main advantages it can cover immature cotton available in fabric portion and get lot of fashion shade. 
Even in Fabric Dyeing general fashion is there if shade wise any major problem is there convert it sulphur black shade.

Here in this article we are going to show the sulphur dye used in different machinery.

Sulfur dyes are the biggest volume dyes manufactured for cotton. 
They are cheap, generally have good wash-fastness and are easy to apply. 
The dyes are absorbed by cotton from a bath containing sulfide and are insolubilised within the fibre by oxidation.
During this process the dyes form complex larger molecules which is the basis of their good wash-fastness. 
These dyes produce robust dyeings that have all round fastness except to chlorine. 
Due to the high polluting nature of the let out dye-baths, slowly sulfur dyes are being phased out. 
Sulfur dyes are primarily used for dark colors such as blacks, browns, and dark blues. 
The deep indigo blues of denim blue jeans are a product of sulfur dyes.

Sulpher dyes
Sulpher dyes: Sulphur dyes are synthetic organic substantive dyes produced by thionation or sulphurisation of organic intermediates containing nitro and amino groups and are customarily applied to cotton and other cellulose fibre substances from reduced bath containing sodium sulphide.
The dyes are so called as they contain Sulphur linkage within their molecular structure but dissolve in a solution of Na- 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.

History of Sulphur dye: The first sulphur dye was made in 1873 by heating sawdust with sulphur and caustic Soda. 
This substance was adsorbed from alkaline liquor by cotton and subsequent Oxidation with potassium dichromate produced a brown shade of quite good Wet-fastness. 
This particular colour, however, never achieved commercial success and the real pioneer of sulper dyes was Vidal, who in 1893, produced the Vidal blacks by Fusing paraphenylenediamine or para-aminophenol, with sodiumsulphide and sulphur. 
In 1897 Kalischer prepared Immedial Black FF by heating 2:4-dinitro-4’ –hydroxyl Diphenylamine with sodium polysulphide.

Classification of Sulphur Dyes: Essentially there are three classes of sulphur dyes
Conventional or water insoluble dyes.
Leuco sulphur dyes.
Solubilised sulphur dyes.

Properties of Sulphur dyes:
They contain sulphur linkage within their molecule.
Water Insoluble dyes.
Heat reduction and oxidation of the development in the fibre.
Wet fastness good.
Light fastness satisfactory.
Wash fastness excellent.
Limited range of colour normally black.
Chorine fastness fair.
Shading is not more than 10%.
Unhygienic for environment.
Very cheap.
Available in the market.
S-dye makes colloidal solution.
S-dye produces Hydrogen Sulphide to decompose.

Mechanism of sulphur dyeing: The sulphur dyes contain sulphur linkage within their molecules.
They are insoluble in water but can be reduced to become soluble in water by treating with reducing agents and shows substantivity towards cellulose. 
Sodium sulphide acts as reducing agent that breaks the sulphur linkage and break down the molecules into simpler components which can easily penetrate the fabric surface.

The thiols, containing the -SH groups are readily oxidized in the fibre to the original insoluble sulphur dyes by oxidizing agent and gives a colour with very good wet fastness properly. 
Dyes are negatively ionized and no Vander Waals force effect on them. 
Salt improves dyeing efficiency by increasing physical force.

Auxiliaries used in Sulphur Dyeing:

1. Reducing Agent: The two most important reducing agents for Sulphur dyes are Sodium sulphide (Na2S) And Sodium Hydro sulphide (NaHS). 
The sulphur dyes contain sulphur linkage within their molecules. 
They are Insoluble in Water but can be reduced to become soluble in water by treating with reducing agents and shows substantantivity towards cellulose. 
Sodium sulphide acts as reducing agent that breaks the sulphur linkage break down the molecules into simpler components can easily penetrate the fabric surface.

2. Oxidizing agents:After dyeing, the reduced, water soluble form of the dye has to converted into the original water-insoluble form by oxidation using a wide range oxidizing agents. 
Dichromate Acetic acid, Hydrogen per-oxide, ammonium persulphate used as oxidizing agents. 
The thiols containing the – SH groups are readily oxidized in the fibre to the original insoluble Sulphurdyes by oxidizing agent and gives a colour with very good wet fastness properly.

3. Sequestering agents: Sequestering agents based on sodium hexametaphosphate the sodium salt of ethylene diaminetertra acetic Acid (EDTA) are widely used in dyeing with Sulphur dyes where the water quality is low. 
Wetting agents- majority of Sulphur dyes are unaffected by most wetting agents. 
These are usually Non-ionic surfactants and should be avoided, both in pre-scouring and in the dye bath itself.

Fastness Properties: The fastness properties of Sulphur dyes fall between those of direct dyes and Vat dyes. 
As with most ranges the fastness varies from dye to dye but those of higher fastness, e.g. CI Sulphur Green 14 and Black 11, behave similarly to those vat dyes having hot least impressive fastness performance.

Fastness to Light: The fastness to light increases throughout the range from lowest in the yellows and oranges to highest in the yellows and oranges to highest in black s and navy blues.

Fastness to wet treatments:

Washing: Sulphur dyeing in general exhibit good fastness to washing tests based on soap (tests C01-C05), but are less resistant to laundering with detergents and perborate.

Perspiration: The fastness to Perspiration (test E04), especially at PH 5.5, is particularly good. 
Alkaline Perspiration Is generally good to excellent, some exceptions being CI Sulphur Red6, Blue 15 and Green3.

Rubbing: The fastness to rubbing (test method X12) is so greatly dependent on the fabric itself, its preparation and the dyeing process especially the efficiency to rinsing before oxidation. 
The fastness to dry rubbing is normally quite good. Even in heavy depths ratings of 4-5. 
Fastness to wet rubbing in heavy depths is not usually as good and stain ratings of 2-3 are on dark navy and black dyeing.

Bleaching: Most Sulphur colours are destroyed by sodium hyprochlorite (test N01). 
The effect of per oxide bleaching (test N02) is not quite as severe as hypochlorite. 
MercerizingThe fastness in mercerizing (test X04) is good.

Improving Fastness properties: The light fastness of sulphur dyeing, which is generally good, can be improved by after-treatment with certain metallic salts. 
Thus a treatment with copper sulphate and acetic acid in the presence of sodium or potassium dichromate 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.

The dyed and rinsed goods are treated with solution containing copper sulphate (1 to 2% of the weight of the goods) and acetic acid 60% (1 to 2%) at 700C for 20 to 30 min. 
The combined treatment is given in a solution containing sodium dichromate (1 to 1.5%), copper sulphate (0.5 to 1%) and acetic acid 60% (1 to 2%) at 70°C for 20 to 30 min.

Disadvantage or Defects of Sulphur dyes: Mainly two defects which common in sulphur are dyeing
            Bronziness or Dullness of Shades.
           Tendering.

Bronziness or dullness of shades:
 Sulfur Dye is a common defect found in sulphur dyed textile materials.
Causes:
Excessive delay between lifting of the material out of the dye bath and washing off.
The presence of excessive dyestuff on the material.
Insufficient amount of sodium sulphide in the dye bath.
Exposure of goods to air while dyeing.
Excessive heat.

Remedies:
Good washing and dilute solution of Na2S (1%) at 30°C or,
A treatment with boiling soap solution or strong Na2S solution or,
A treatment with a solution containing 10% of saponified palm oil at 60°C

2. Tendering: If sulphur dyed textile materials are stored for a prolonged period, tendering effect is seen on cellulose and hence losses strength on storage.

Causes:
Gradual oxidation of sulphur to sulphuric acid on storage.
After treatment with copper salt causes rapid tendering.
Presence of iron as an impurity causes rapid tendering.
The method of oxidation for the reconversion to insoluble form influence tendering.

Remedies:
Treatment of dyed material with a little sodium acetate so that H2SO4 may be converted in to harmless acetic acid.
Treatment of dyed material with a solution of potassium or sodium dichromate followed by rinsing.

Advantages of S-dye/ S-dyes are so popular in producing black shade:
Low cost.
Fair to good light fastness.
High wash fastness.
Easy to apply.
Low energy required.
Chemical resistance in mode rate to good.
Wet fastness good.
Chlorine fastness fair.

Uses of S-dye:
Used to dye of black umbrella cloth.
Used for lining boat.
Used to dye of rubber materials.

Precaution in Sulphur Dyeing:
1. In the application of Sulphur dyes, the dye bath should not contain calcium salts. 
If calcium salts are present, they form inert, insoluble calcium compounds with the Sulphur dyes, which precipitate, especially in closed machines, form sludges which restrict the circulation of the dye liquor and cause much trouble. 
Calcium salts may come in the dye bath from the water used or the salt used for exhausting.

2. The tendering of the fibres dyed with Sulphur black when subjected to conditions of humidity and high temperature may be inhibited by an aftertreatment with certain organic amino compounds. 
Sulphur black reacts with the compounds to form a more stable molecule, which resists decomposition and the formation of tendering substances (acids).

Stripping:
Unevenly dyed shades on cellulosic materials with Sulphur dyes may be corrected by a treatment with a warm solution of sodium sulphide in the presence of Albigen A (Polyvinyl pyrrolidone).
If this method is found to be ineffective, the susceptibility of many of the Sulphur dyes to over-oxidation (by sodium hypochlorite) may be utilized for destroying the dye taken up the material.
The uneven dyeing may be treated with a solution of soldium hypochlorite or bleaching powder (2 to 3 g/litre of available chlorine), when the dye is completely oxidized and the material may be re-dyed.
In some cases, bleaching with potassium permanganate solution may be effectively carried out.
In still other cases, the dyeing may be treated with warm sodium hydrosulphite solution in the presence of sodium hydroxide.

Sulfur dyes are synthetic organic substantive dyes, produced by thionation or sulfurization of organic intermediates containing nitro and amino groups. 
The main characteristic feature of this dye class is that they all contain sulfur linkages within their molecules. 
They are like vat dyes, which are highly colored, water-insoluble compounds, and they need to be converted into substantive leuco form before application to the textile materials. 
This conversion needs reducing agents, which sever the sulfur linkage and break down the molecules into water-soluble components (leuco form) that have an affinity toward cellulosic fibers.

Definition of sulfur dye
: any of a class of sulfur-containing dyes that are made by heating various organic compounds (such as aromatic polyamines or indophenols) with sulfur or alkali polysulfides and are used chiefly in dyeing cotton and other cellulose fibers

Sulfur dyes
Sulfur dyes are non-ionic, possess sulfur bonds, are water-insoluble, and produce shades that are reasonably cheap and washable, and moderate fastness properties. 
The dyes are applied to the cellulosic in the reduced and dissolved anionic state. 
These dyes are mainly used in the cellulosic fabric. 
Sulfur Dye gives different types of dark shades like black. 
Examples of sulfur dyes are indophenol, sulfur black, sulfur blue dye, sulfur brilliant green, etc.

Description of Sulfur dyes
Sulfur dyes made for cotton are the most widely used dyes in terms of volume. These are inexpensive, usually have wash fastness, and are easy to apply. Sulfur dyes are mainly black, brown, and dark blue. Red sulfur dyes are unknown, although pink or light scarlet colors are available.

The sulfur dye is usually heated by sulfur or sodium sulfide together with specific aromatic amines, amino phenols, and other organic compounds. 
During dyeing, the sulfur dye should be dissolved in sodium sulfide solution or alkaline hydrosulfite solution to reduce its hidden color and then absorbed and oxidized by fiber to show the color.

I. Sulfur dyes are commonly used for black, blue, brown, khaki, and green.

ii. Sulfur dyes are applied to cellulosic fibers and to cellulosic blends with polyester, nylon, and acrylics.

iii. Sulfur dyes can be applied with great results with little difficulty and relatively low cost.

Sulfur dyes are compounds of high molecular weight that are obtained from the reaction of sulfur or sulfide with phenols and amines. 
There are plenty of colorants on the market that contains sulfur but only those dyes that are soluble in water after reacting with sodium sulfide in alkaline conditions are known as sulfur dyes.

Sulfur black dye is available in powder, granules and liquid form has a higher degree of property than other cotton colors on the market. 
Good color fastness properties like lightfastness, wash fastness, and cost-effectiveness makes sulfur black an important class of dye. 
This dye can be applied by clothing, fabric extraction, semi-continuous, or continuous dyeing methods.

Sulfur dyes manufacturing process (Navy Blue)
I. The first step in this process is-

The preparation of para-nitroso phenol that is formed by the action of sodium nitrite in the phenol in an acid solution at a temperature of around 0 °C. 
These para-nitroso phenols are filtered, grated in an extractor, analyzed, and stored in a cool place. Sulfur Dye is quite flammable and combustible and must be used with care to manage and preserve it.

ii. The second step is-

To prepare the color base or indo-phenol by the concentration at low temperatures in a solution of sulfuric acid, para-nitroso phenol, and ortho-toluidine. 
When this extremely delicate reaction is complete, the mass is driven into a mixed solution of soda ash and the acid is neutralized with the resulting precipitation of indo-phenol that is filtered and analyzed but not dyed, as the color base is quite unstable and must be used fresh.

iii. The third step is-

To prepare sodium polysulphide by heating the solution and some amount of sodium sulfide together in sulfur. 
Indo-phenol is dissolved in sodium polysulphide, the solution is brought to a certain boiling point by evaporation or reduction and is run in boiling state in a closed tank or kettle equipped with a reflux condenser for a period of 12 to 100 hours. 
During the fusion cycle, substantial evolution of hydrogen sulfide occurs. 
The strength, ratio of polysulphide from sulfide to sulfur, temperature, and fusion reaction times are different to create all the desired shades. 
Also, specific solvents and fusion auxiliaries are sometimes employed to produce specific shade or solubility results. These include assistant’s alcohol, glycerin, and their derivatives.

iv. In the fourth step, is completed the fusion -

The dissolution of the dyestuff from the mixture and the sulfide solution means that the dyestuff continues to flow through the air by means of acid, acid salts, or a solution of oxidation of sulfide.

v. The fifth step is filtering-

The precipitated dyestuff drying, grinding, aging, and standardization by mixing different batches and diluting with salt, sodium sulfate, etc., and then marketing.

Properties of sulfur dyes
These are water insoluble dyes and have no affinity for the cellulosics as such, but solubilised when treated with a weak alkaline solution of sodium sulphide or any other reducing agent to form a leuco compound. These leuco compounds are water soluble and have affinity for the cellulosic materials such as cotton, viscose, jute and flex etc. 
These dyes are absorbed by the cellulosic material in the leuco form from aqueous solution and when oxidized by suitable oxidizing agents, got converted into insoluble parent dye, which is fast to normal color fastness parameters.
Main properties of the sulfur dyes are as follows,
1.Economical dyeing with excellent tinctorial value and good build up properties.
2.Good overall colorfastness properties such as wash fastness, light fastness, perspiration fastness etc. 
Moderate fastness to crocking and poor fastness to chlorines bleaching agents such as bleaching powder and sodium hypochlorite.
3.Limited shade range to produce only dull shades and there is no true red dye in the range.
4.These dyes can be applied by exhaust, semi continuous or continuous dyeing methods on garment, yarn, knits, fabric as well as loose stock etc.
5. Available in powder, granules and liquid forms.
6. Sulphur black 1 is the major black dye used world vide for dyeing of cellulosics.
7.The conventional dyeing process is not environment friendly due to pollution problems of sodium sulphide as well as sod/pot. Dichromates.
8.When dyed by using non polluting reducing and oxidizing agents the process is environment friendly.
Types of sulfur dyes
There are three classes of sulfur dyes, which are available commercially,
1.Conventional water insoluble dyes which have no substantivity to cellulosics.
2.Solubilised sulfur dyes, which are water soluble and non substantive to cellulosics.
3.Pre-reduced sulfur dyes, in the stabilized leuco compound form, which are substantive to cellulosics.

APPLICATION
Mechanism of the sulfur dyeing
The application of the sulfur dyes involves several steps, which are described as given below,
1.dissolving the dyestuff.
The dye is taken in an SS vessel ( size of the vessel should be selected as per the quantity and solubility of the dyes) and pasted well with a good alkali stable wetting agent and small quantity of soft water. 
A required quantity of soda ash may be added to neutralize any acid formed in the dyestuff during storage.
(if the acid is not neutralized, it will react with the sodium sulphide, resulting into formation of H2S gas, which will result into incomplete and poor reduction of the dyes). 
Sulfur Dye is very important that the dye dissolution must be complete otherwise particles of the undissolved dyes may deposit on the surface of the substrate resulting into patchy dyeing and poor rubbing / washing fastness.
2.reducing the dyes to form a leuco compound.
Chiefly sodium sulphide is used as a reducing agent for the sulfur dyeing. 
The quantity of the reducing agent depends upon the shade depth and M:L of the bath. 
For complete reduction the required quantity of the sodium sulphide is dissolved in a separate container and solution is allowed to settle for 10-15 min. 
before decanting the clear solution into the dye dissolving vessel. Further boiling water is to be added to make up the required volume, then heated to boil for 10-15 minutes either by live steam or indirect heating, for complete reduction of the dyestuff.
3.dyeing with the reduced dyes.
Sulfur Dye is advantageous that the goods are scoured well before dyeing,to have a satisfactory absorbency for better penetration. 
The dye bath is kept ready with small quantity of the alkali stable and aompatible wetting agent, a dye bath stabilizer, sodium sulphide and caustic soda or soda ash to maintain the alkalinity of the dye bath. 
The dye solution is then added through a filter cloth slowly over 15-25 minutes and then run for another 15 minutes at 40-50 oC, then temperature is raised to 60 oC and electrolyte is added in at least 3 portions. 
The quantity of salt added is depends upon the type of shade, depth and dyestuffs, however a maximum quantity does not exceed more than 15 gpl.
The temperature is then raised to above 80 oC or even boil depending upon the dyes and kept for sufficient time to get the desired shade.

Sulfur dyes are dyes that must be dissolved in alkali sulfide. 
They are mainly used for dyeing cotton fibers, but also for cotton/dimensional blended fabrics. 
The cost is low, and the dyed products are generally washable and light-resistant, but the color is not bright enough. 
Commonly used varieties are sulfur black, sulfur blue and so on.

Sulfur dyes are one of the most widely used dyes. 
According to reports, the world's output of sulphur dyes reaches hundreds of thousands of tons, and the most important variety is sulphur black dyes. 
The output of sulphur black accounts for 75%-85% of the total output of sulphur dyes. 
Because of its simple synthesis, low cost, good fastness, and non-carcinogenicity, it is favored by various printing and dyeing manufacturers. 
Sulfur Dye is widely used in the dyeing of cotton and other cellulose fibers, with black and blue series being the most widely used.

 

There are two industrial production methods for sulfur dyes:

(1) Baking method, baking amines, phenols or nitro compounds of raw materials aromatic hydrocarbons and sulfur or sodium polysulfide at high temperature to prepare yellow, orange and brown sulfur dyes.

(2) Boiling method, heating and boiling the amines, phenols or nitro compounds of raw aromatic hydrocarbons and sodium polysulfide in water or organic solvents to prepare black, blue and green sulfur dyes.

 

Sulfur dyes are a type of sulfur-containing dyes. 
The molecules contain sulfur bonds composed of two or more sulfur atoms. 
When used, they are reduced to leuco body, dissolved in water, and dyed fibers. 
The dyeing characteristics of sulfur dyes vary depending on the type of dye. 
Sulfur dyes have high washing fastness and strong applicability. Although the rubbing fastness and vividness are not as good as reactive dyes, their color fastness and light fastness are better than reactive dyes. 
Sulfur dyes use less salt and consume less water. less. 
Sulfur dyes are organic compounds containing nitro and amino groups, most of which are formed by reacting with sulfur and sodium sulfide at high temperatures. 
Many sulfur dyes do not have a certain chemical formula. The dyeing principle of sulfur dyes is similar to that of vat dyes. 
They form a water-soluble leuco body dyeing fiber with affinity through chemical reduction reaction, and then bind tightly to the fiber through an oxidation process.

 

Sulfur dyes are insoluble in water. 
Sodium sulfide or other reducing agents are needed to reduce the dyes to soluble leucos. 
Sulfur Dye has affinity for fibers and dyes the fibers, and then restores its insoluble state after oxidation and color development and is fixed on the fibers.
So sulfur dye is also a kind of vat dye. Sulfur dyes can be used for dyeing cotton, linen, viscose and other fibers. 
Its manufacturing process is simple and low in cost. 
Sulfur Dye can dye single colors and can also match colors. 
Sulfur Dye has good light fastness and poor abrasion fastness. 
The color spectrum lacks red and purple, and the color is dark, suitable for dyeing deep colors.