Acid Dye (Full PDF)

Acid Dye (Full PDF)

1. 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. Acid dye class is a
water soluble class of dyes with anionic properties. The textile acid dyes are effective for protein
fibers such as silk, wool, nylon and modified acrylics. Acid dyes fix to the fibers by hydrogen
bonding, vander waals forces and ionic linkages.

Properties of Acid Dyes:

Since these are sold as a sodium salt, there fore 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. Acid dyestuff is soluble in water.
5. These dyes are generally applied in the presence of acids like sulphuric, acetic or formic
acid.
6. They are in much case soluble in alcohol.
7. When acid dyestuffs are treated with a reducing agent they are generally decolorized.
8. They are usually combined with basic dyes.
9. The dyed acid colors have good light fastness and moderate washing fastness. And
leveling characteristics:
10. These dyes have no affinity for cotton cellulose’s , hence not suitable for cellulosics.
11. These dyes combine with the fiber by hydrogen bonds , van der waals forces or through
ionic linkages

2. 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

3. 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.

4. 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.


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

5. 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.

6. 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.