Why Sulphur dye is so called?
The dyes are so called as they contain Sulpher linkage within their molecular structure but dissolve in a solution of Na-sulphide which acts as a reducing agent, breaking the Sulpher linkage and converting the molecules into simpler components which are soluble in water and substantive toward cellulose.
Water insoluble Sulpher dye molecule
Classes of Sulpher dye:
Three classes of Sulphur dye:
- Conventional or water insoluble dyes
- Leuco sulphur dyes
- Solubilized Sulpher dyes
Commercial forms of sulphur dye:
- Disperse powders
- Disperse pastes
- Water soluble brands
General Chemical Structure of sulphur dye:
General formula of a reduced sulpher dye:
D-(Sn –S– )m .mNa
Here D is a chromophore or reduced chromophore. n is zero to five and m is greater than or equal to one.
Chemical structure of Sulphur dye:
One of the reactions which takes place when sulpher is heated with P-toluidine is the formation of dehydrothiotoluidine:
The dehydrothio-toluidine can react with another molecule of toluidire a process which may be repeated until quite a complex molecule of toluidine a process which may be repeated until quite a complex molecule has been built up:
The dye sulpher Yellow 4 is built up of units possessing the structure:
Properties of sulphur dyes:
- Sulphar dye is water insoluble dye
- It needs solubilization for application (with reducing agent)
- Oxidation is needed for final color development
- Sulphur dye is applied in alkaline condition
- Electrolyte can be added in the dye to enhance exhaustion of dyes
- Fastness properties are average
- Best suitable for producing black and brown shade on the textile materials
- It is mainly applied on cotton,viscose and staple fiber.
- Its exhaustion properties are good
- Most of the times dyeing is carried out in 80-95 degree Celsius
- Sulphur dye is comparatively cheap in price.
Chemistry of dyeing with S-dyes OR Mechanism of dyeing:
The sulphur dye contain sulpher 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 sulpher linkage and break down the molecules into simpler components which can easily penetrate the fabric surface.