Dye :
Organic coloured substance that has the properties of fixing itself on the substrate to give it permanent coloured appearance and when fixed is not fugitive e.g. fast to light and not washable by water, dilute acid or alkali is called dye.Dye is usually in the form of aquous solution.
History of Dyeing
The first dyes were Turkey red and indigo blue. A breakthrough event in the production of dyes was the chemical synthesis of aniline purple carried out by William Henry Perkin in 1856. For the purpose of the synthesis, benzene was nitrated to nitro-benzene and then reduced to aniline, which was further oxidized.
Classification of dyes
Chemical classification of dyes
1)Nitro dyes
2)Nitraso dyes
3)Azo dyes
4)Diphenyle methane dyes
5)Triphenyle methane dyes
6)Xanthan dyes
7)Diphenyl amine dyes
8)Heterocyclic dyes
9)Sulpher dyes
10)Pthalocyanin dyes
11)Anthraquinoid dyes
Dyers classification
1. Acid dyes
2. Basic dyes
3. Direct dyes
4. Vat dyes
5. Developed dyes
6. Sulphur dyes
7. Mordant dyes
Requisites of a dye:
1. It must have a stable colour.
2. It must have attractive colour.
3. It must be attached itself to material form solution or to be capable to fixation it.
4. It must be soluble in the application media.
5. The substrate to be dyed must have a natural affinity for an appropriate dye and must be able to absorb it form solution of concentration, temperature and pH.
6. When a dye is fixed to a substrate it must be fast to washing, dye cleaning, perspiration, light, heat and other agencies
7. The shade and fastness of a given dye may vary depending of the substrate due to different interactions of the molecular orbital of the dye with the substrate.
Objective of dyeing
1. To provide colour.
2. To obliterate the surface.
3. To develop newer design as per the market requirement
4. To improve the weathering properties and durability.
Acid dyes:
Acid dyes are generally sodium or potassium salts of organic coloured acid and ionize in solution as follows
NaD = Na+ D- (Coloured acid)
Basic dyes:
Basic dyes are generally chlorides or hydrochlorides of coloured bases and ionize in the solution as follows
BCl = B+ + Cl-
Direct dyes:
Direct dyes are also sodium or potassium salts of coloured acid and behave the same ways as acid dyes. This dyes can directly used to impart colour to the fabric and hence the name.
NaD = Na+ + D-
Vat dyes
Vat dyes are suitable for cotton dyeing. These dyestuffs are insoluble in water but when reduced in an alkaline medium become soluble but practically colourless. The colour again restored on oxidation by the atmospheric air or nay suitable oxidizing material. Before dyeing the dye solution called a vat, is prepared by dissolving the dyestuff in lukewarm water containing caustic soda and reducing agent. Then the material to be dyed is impregnated with this solution, it is squeezed and hung up for oxidation to develop colour. For heavy colour the procedure may have to be repeated several times.
Preparation of leather for dyeing
Splitting, shaving, washing and degreasing in some case in order to remove the still remaining material greases and where necessary wetting back which is practiced in retanning of suede’s, nubacks splits.
The next preparatory operation considered to be the most important is the adjustment of an appropriate pH or the ionic charge in the leather in order to suit the subsequent retanning operation to be followed. For chrome or mineral tanned leather the process is known as neutralization, is the removal of protein bound acid from the surface as well as interior of leather up to certain depth depending on penetrating material used in the retanning system.
For vegetable tanned leather the preparatory wet operation for adjustment of suitable pH or ionic charge in the leather starts from acidification which commences after usual washing or stripping and degreasing where necessary.
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