Acid Dyes | Properties of Acid Dyes | Mechanism of Dyeing with Acid Dyes

Acid dyes
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,
  1. Anthraquinon type
  2. Azo dye type
  3. Triphenylmethane 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, there fore these form a large anion in the aqueous medium.

The main properties of acid dyes are ,
  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.
Mechanism of dyeing with acid dyes
Dissolution of dyes in aqueous solvent, produces a colored anion,
The protein and polyamide fibers produce cationic sites in water under acidic conditions, as the acidity of the solution is increased more cationic sites are produced under these strongly acidic conditions. These cationic sites are thus available for the acid dye anions to combine with through hydrogen bonding, vander waals forces or ionic bonding. These linkages are strong enough to break , and thus dyeing produced are fast .

Reaction between an acid dye and wool can be represented by following equation
On Wool
Electrolyte in the acid dye bath acts as a retarding agent because of chlorides ions attracted by the positive sites at the fiber and in the competition between. Addition of acid acts as a n exhausting agent , because strongly acidic conditions makes more cationic sites available and thus available dye anions got combined with these.

Dyeing temperature

The dyeing is generally carried out at boiling temperature for 30- 60 minutes depending upon the depth of the shade and dyestuffs used.

Dyeing leveling agents
In the case dyeing with acid dyes , mainly cationic agents such as ethoxylated fatty amines are used as leveling agents.

Heating rates
Heating rate is generally kept 1-30C/Min

Washing off process
A typical dyeing cycle of nylon filament dyeing with acid dyes is shown in the above chart,

Wool dyeing method with acid dyes

Method 1
At A set bath at 50° with:
4% Sulphuric Acid (96%)
5% Glaubers Salt anhydrous,
pH 2.5 to 3.5

At B add required amount to dyestuff.

Method 2
At A set bath at 50° with:
2% Formic Acid (85%)
5% Glaubers Salt anhydrous,
pH 3.5 to 4.5

At B add required amount of dye.

At C add 2% Sulphuric Acid (96%) or 2% Formic Acid (85%).

Thoroughly rinse after dyeing to remove loose colour.

A dyeing cycle for nylon filament dyeing

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 :- 
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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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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