Textile Knowledge : Sulfur dye

Sulfur dye

Sulfur dyes are complex heterocyclic molecule’s or mixtures formed by melting or organic compounds containing amino or nitro groups with na-poly sulfide and/or sulfur. Sulfur dyes are so called as they all contain sulfur linkage within their molecules.

Sulfur dyes are highly colored, water insoluble compound and have to be converted into water soluble substantive forms before application to textile materials. This conversion is carried out by a treatment with a reducing agent like dilute aqueous na₂s. Since this leuco form of sulfur dye is substantive to cellulosic material they are absorbed on the fibre surface. Then they are reconverted to original water insoluble form of dye by oxidation. This oxidizing is carried out by ‘airing’ or by using an oxidizing agent like Na-dichromate (Na2Cr₂O₇).

The reducing agent converts the ‘S’ in dye into-SH group and break the sulfur linkages. Then inside the material the thiols containing –SH group are oxidized and thus reconverted to original form of dye.

This is shown as below :-

Dye-S-S-Dye + 2[H] ^{reducing agent} Dye-SH + HS-Dye

water insoluble water soluble

Dye-SH + HS-Dye + [O] ^{oxidizing agent} Dye-S-S-Dye + H₂O

water soluble water insoluble

Sulfur dyes gives best result(bright tone) when they are used to produce block, blue and brown shades. But ‘red’ shade can’t be Obtain by sulfur dyes.

Properties of sulfur dyes :

1) Sulfur dyes are water insoluble dyes, some are partially soluble

2) They have sulfur linkages within their molecules

3) They have no direct affinity toward cellulosic fibre.

4) They have good light fastness.

5) These dyes have excellent wash fastness about 3-4

6) Highly amorphous and few crystalline.

7) Suitable for heavy and durable shades

8) Unhygienic for environment.

9) Cheap and easy manufacture

Trade name	Name of manufacturer	Country of origin
1. Calcogene	Dyes dept. American Cyanamid co.	U.S.A
2. Pyrogene	Ciba	Switzerland
3. Thional	Sandoz	Switzerland
4. Solfo	ACNA	Italy
5. Sulfogene	DU pont	U.S.A

Classification of sulfur dye :

Sulfur dyes are classified in many ways. But according to their solubility, there are three types :

1) Conventional or water insoluble dyes

2) Leuco sulfur dyes (partially soluble)

3) Solubilised sulfur dyes

Reducing step of sulfur dyes :

Reducing step is the most important in the application of sulfur dyes. Unless the dyes are converted into the completely soluble form, the full color value can’t be achieved. The solubility of reduced sulfur dyes varies appreciably from dye to dyes. For example yellow sulfur dyes are the least soluble dyes(i.e. Most insoluble dyes).

The reducing agents used for sulfur dyes are:

1) Na-sulfide

2) Na-hydrosulfide

3) Thio glycolic acid

But Na-Sulfide is most widely used reducing agent for dissolving sulfur dyes. They may be replaced by Na-hydrosulfide in some cases. But the latter is a more powerful reducing agent. So over reduction of the dye may take place, leading to a product having lower affinity for cellulosic fibre. Hence lower color yields are obtain along with wastage of dye stuff. In some cases lower wash fastness results if Na-hydrosulfide is used. It also reduce the lifetime of vessel.

For dissolving the sulfur 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 vessels are used but not iron vessels. Because iron reacts with sulfur.

Oxidation step of sulfur dyes :

After dyeing the reduced, water-soluble form of the dye has to be converted into original water insoluble form by oxidation. the commonly used oxidizing agents are:

1) Potassium dichromate (K₂Cr₂O₇₎ and acetic acid (CH₃-COOH )

2) Na-per carbonate

3) Na-peroxide

The method of oxidizing selected plays an important roll in the development of correct shades and their optimum fastness properties. Rinsing off the dye should be done rapidly to remove as much Na-Sulfide as possible. if even after washing some Na sulfide or soda-ash stays on fabric. We used CH₃-COOH to neutralize them. Using of different oxidizing agents may give following Result :

1. Use of carbonate or per carbonate in presence of acetic acid gives bright shade.

2. Treating dyed material with dichromate without rinsing reduces color losses but causes dull shade.

The chemistry of oxidization is as below :

D-S-S-D + 2[H] ^reduction D-SH + HS-D

original molecule Thiols

2D-SH + [O] ^oxidation D-S-S-D+ H₂O

original dye

Dyeing of cellulosic fibres with sulfur dyes :

The sequence of dyeing with sulfur dyes is as follows :

Goods preparation (fabric preparation)

Dye solution preparation or reducing step

Dyeing

Oxidation

After treatment

Dyes goods

Chemistry of dyeing of sulfur dyes :

The sulfur dyes contain sulfur linkage within their molecules. They are insoluble in water but can be made soluble in water by treat them with reducing agents. This also make them substantive towards cellulose fibres. Na₂s acts as reducing agent that breaks sulfur linkage and break down the longer molecules into simple components which can penetrate the material (fibre/fabric) surface easily.

Dye-S-S-Dye + 2[H] ^{reducing agent} Dye-SH + HS-Dye

water insoluble water soluble

The thiols containing the –SH group are reading oxidize by the action of atmospheric O₂ or any other oxidizing agent. This reconverts the water soluble leuco form of sulfur dye into previous water insoluble form which have a very good wash fastness property.

Dye-SH + HS-Dye + [O] ^{oxidizing
agent} Dye-S-S-Dye + H₂O

water soluble water insoluble

Sulfur dyes are negatively ionized. No van der-waals force effect on them. Addition of salt improves efficiency of dyeing by increasing physical force.

Sulfur dyeing steps :

Goods preparation (fabric) :

Only normal pretreated fabric is required. Mercerized cotton goods cause an increased color yield of 30-40% . but goods bleached by Na hypo chloride should not be dyed by sulfur dyes.

Dye solution preparation:

Required amount of dye, soda ash and Na₂S are taken in a breaker and a little amount of cold water is added to makes a paste. in a separate vessel to it. Then the mixture is boiled and the paste is added to it. Then the mixture is boiled for 5 minutes to make the dye completely required.

Dyeing:

At first dye solution is taken in a dye bath and fabric is immersrd in that solution. Then it is heated for 10 – 15 minutes. Then half of the total NaCl solution os added and temperature is raised to boil and then the rest amount of NaCl is added. Dyeing at this temperature is continued for 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 fabric.

Oxidation:

After dyeing the reduced water soluble form of dye should be make insoluble bu oxidation. tox it on the fabric permanently. for this oxidation we may use K₂Cr₂O₇, CH₃-COOH , Na-per borate, Na₂O etc.

After treatment of sulfur dyeing :

I. The dyes substance is soaped which makes the color bright and makes the shade permanent.

II. The light fastness properties of sulfur dyes, which is generally good, can be improve by after treatment with certain metallic salts. Thus a treatment with CuSO₄ and CH₃-COOH in presence of K₂Cr₂O₃ improves the light fastness. When the dichromate is also present the washing is slightly is also present the washing fastness is slightly improved.