Acid dyes are highly water soluble, and have better light fastness than basic dyes. The textile acid dyes are effective for protein fibers such as silk, wool, nylon and modified acrylics. They contain sulphonic acid groups, which are usually present as sodium sulphonate salts. These increase solubility in water, and give the dye molecules a negative charge. In an acidic solution, the -NH2 functionalities of the fibres are protonated to give a positive charge: -NH3+. This charge interacts with the negative dye charge, allowing the formation of ionic interactions. As well as this, Van-der-Waals bonds, dipolar bonds and hydrogen bonds are formed between dye and fibre. As a group, acid dyes can be divided into two sub-groups: acid-leveling or acid-milling.
Chemical structure of acid dyes
These dyes are normally very complex in structure but have large aromatic molecules, having a sulphonyl or amino group which makes them soluble in water. Most of the acid dyes belongs to following three main structural molecules,
2.Azo dye type
Different types of acid dyes
The basic dyes are classified into several groups , based on the leveling properties, economy of the dyeing and fastness properties, however generally these are classified into these three classes,
1. Neutral acid dyes :-
These are supra milling or fast acid dyes, having medium to good wet fastness properties , some of the dyes have poor light fastness in pale shades . many of the dyes are used as self shades only. These are applied to the fiber in a weakly acid or neutral pH.
2. Weak acid dyes
These dyes belongs to the milling class of dyes. These dyes have good fastness properties but light fastness is moderate to poor.
3. Strong acid dyes
These dyes are applied in a strongly acidic medium and also called leveling dyes, however there wet fastness properties is a limitation. These dyes are very good to produce the combination shades.
Classification according to dyeing characteristics
Acid dyes are commonly classified according to their dyeing behaviour, especially in relation to the dyeing pH, their migration ability during dyeing and their washing fastness. The molecular weight and the degree of sulphonation of the dye molecule determine these dyeing characteristics. The original classification of this type, based on their behaviour in wool dyeing, is as follows:
1.Level dyeing or equalising acid dyes;
2.Fast acid dyes;
3.Milling acid dyes;
4.Super-milling acid dyes.
Milling is the process in which a woollen material is treated, in weakly alkaline solution, with considerable mechanical action to promote felting. Dyes of good fastness to milling are essential to avoid colour bleeding during the process.
Properties of acid dyes
Since these are sold as a sodium salt, therefore these form a large anion in the aqueous medium.
1.These dyes are anionic in nature.
2.These dyes are suitable for wool, silk, polyamide and modified acrylics.
3.These are applied from a strongly acidic to neutral pH bath.
4.These dyes have no affinity for cotton cellulose’s , hence not suitable for cellulosics.
5.These dyes combine with the fiber by hydrogen bonds , vander waals forces or through ionic linkages.
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