Acid Dyes | Properties of Acid Dyes | Classification of Acid Dyes

The Acid dye stuff is so called mainly due to two reasons. In the first place these classes of dyestuff were applied in a bath containing mineral or organic acids like sulphuric, acetic or formic acid and secondly most of them are sodium salts of organics acids.
 
 Properties of Acid Dyes:

1. Acid dyestuff is soluble in water.
2. These dyes are easily applied on wool, silk and nylon fibres,
3. These dyes are generally applied in the presence of acids like sulphuric, acetic or formic acid.
4. They are in much case soluble in alcohol.
5. When acid dyestuffs are treated with a reducing agent they are generally decolorized.
6. They are usually combined with basic dyes.
7. The dyed acid colors have good light fastness and moderate washing fastness. And leveling characteristics:
 
Classification of Acid Dyes
Acid dyes are divided into three Groups according to the leveling characteristics
 
A. Acid dyes with Good leveling characteristics
This type of acid dye is responsible for their good leveling characteristics. As the dye molecules have less attraction for the fibre they will migrate only slowly into the polymer systems of wool or nylon fibres. However to obtain sufficient substantive and to ensure, adequate exhausting agent (sulphuric acid) is added to the dye liquor, their lack of substantive is evidence by their poor wash fastness. However the light fastness is very good to excellent.
 
B. Acid dyes with average leveling characteristics
The moderate substantive of this type of acid dye is responsible for average leveling characteristics. To obtain sufficient substantive and to and ensure it, adequate exhausting agent, a week acid (Acetic acid or Formic acid) is added to the dye both. The washing fastness of these dyes is fair, whilst their light – fastness is good to very good.
 
C.  Acid dyes with poor leveling characteristics
These dyes are also known as fast acid dyes, and milling dyes or Natural dyeing acid dyeing acid dyes. They have the best substantive of all the acid dyes, but have relatively poor leveling characteristics. Unless care is taken during, their relatively good substantive for the fibre may result in too rapid uptake and consequently unleveled dyeing.

The excellent substantive of these dyes require neutral bath in order to obtain slower exhaustion and more level dyeing. The wash fastnesses of these dyes are good to very good, while their light fastness is fair to good. The better wash-fastness, compared with the other two type of acid dyes, is due to the greater number of sodium sulphate groups.


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