Sulpher dyes & its mechanism

 

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.

 Dye-S – S-Dye

Water insoluble sulphur dye molecule

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.

Dyeing of cellulosic fibre/cotton by Sulphur dyes: Method Application of S-dyes:

Fabric preparation

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Preparation of dye solution

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Dyeing

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Oxidation

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After treatments

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Dyed goods

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

1.             Bronziness or Dullness of Shades.
2.            Tendering.

Bronziness or dullness of shades:

 It 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.