Acid dyes
Acid dyes are
highly water soluble and have better lightfastness 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 belong to following three main structural molecules,
The basic dyes are classified into several groups, based on the leveling properties, an economy of the dying 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 belong 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 behavior, 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 behavior in wool dyeing, is as follows:
Properties of acid dyes
Since these are sold as a sodium salt, therefore these form a large anion in the aqueous medium.
The main properties of acid dyes are,
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 belong to following three main structural molecules,
- Anthraquinon type
- Azo dye type
- Triphenylmethane type.
The basic dyes are classified into several groups, based on the leveling properties, an economy of the dying 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 belong 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 behavior, 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 behavior in wool dyeing, is as follows:
- Level dyeing or equalizing acid dyes;
- Fast acid dyes;
- Milling acid dyes;
- Super-milling acid dyes.
Properties of acid dyes
Since these are sold as a sodium salt, therefore these form a large anion in the aqueous medium.
The main properties of acid dyes are,
- These dyes are anionic in nature.
- These dyes are suitable for wool, silk, polyamide and modified acrylics.
- These are applied from a strongly acidic to neutral pH bath.
- These dyes have no affinity for cotton cellulose ’s, hence not suitable for cellulosic.
- These dyes combine with the fiber by hydrogen bonds, van der Waals forces or through ionic linkages.
Fastness properties of acid dyes
The wet and light fastness properties of the acid dyes varies from poor to excellent , depending upon the molecular structure of the dyes.
The fastness properties as per the category are as follows
Neutral acid dyes:-since these dyes have very good leveling and migration properties ,and have a low affinity for the fiber, therefore the wet fastness properties of this class are generally poor.
Weak acid dyes or half milling dyes :-
The wet and light fastness properties of the acid dyes varies from poor to excellent , depending upon the molecular structure of the dyes.
The fastness properties as per the category are as follows
Neutral acid dyes:-since these dyes have very good leveling and migration properties ,and have a low affinity for the fiber, therefore the wet fastness properties of this class are generally poor.
Weak acid dyes or half milling dyes :-
These
dyes have a medium to good affinity for the fiber and are generally
applied in a weakly acidic bath, shows medium to good wet fastness
properties. Strong acid dyes or super milling dyes :- These dyes have
poor exhaustion properties, therefore applied under very strong acidic
condition , exhibit good fastness properties.
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