Acid Dyes are used for dyeing protein fibers. The main protein
fibers used for which acid dyes are used are wool, angora, cashmere and silk.
Apart these, milk protein fibers like Silk Latte, Soya Protein etc., can also
The chemistry of acid dyes is quite complex. Dyes are normally very large
aromatic molecules consisting of many linked rings. Acid dyes usually have a
sulfo or carboxy group on the molecule making them soluble in water. Water is
the medium in which dyeing takes place.
Acid dyes are generally divided into three classes which depend on fastness
requirements, level dyeing properties and economy. The classes overlap and
generally depend on type of fiber to be colored and also the process used.
Acid dyes are thought to fix to fibers by hydrogen bonding, Van der Waals
forces and ionic bonding. They are normally sold as the Sodium salt therefore
they are in solution anionic. Animal protein fibers and synthetic Nylon fibers
contain many cationic sites therefore there is an attraction of anionic dye
molecule to a cationic site on the fiber. The strength (fastness) of this bond
is related to the desire/ chemistry of the dye to remain dissolved in water
over fixation to the fiber.
Characteristics of Acid Dyes
An acid dye is a dye, in chemical regard a sodium (less
often–ammonium) salt of a sulfonic, carboxylic or phenol organic acid. Acid dye
is soluble in water and possesses affinity for amphoteric fibers while lacking
direct dyes' affinity for cellulose fibers.
It has a scary sound to some novices that seems to be due to strong caustic
soda. But, in reality these dyes are non-caustic acids and in some cases, these
are also non-toxics also. In fact in some cases mild acids like vinegar are
used in the dyeing process. Some such mild acids are more toxic than the fiber
reactive dyes and some are less toxic and even safer to eat.
Technical Characteristics of Acid Dyes
Chemical Name : Acetamide
Molecular Formula: C2H5NO
Molecular Weight: 59.07
CAS Registry Number: 60-35-5
Other Names: Acetic Acid Amide, Acetamide Acid, Ethanamide
Categories of Acid Dyes
Equalising/Levelling Acid Dyes
The different trade names of levelling Acid Dyes are Kiton, Erio, Sandolan and
Amacid, among others. But, it is hard to say that which specific Acid Dyes fall
into which of these dye classes.
This kind of acid dyes produce similar, single color solid effects. These have
highest level dyeing properties, combined in trichromatic shades. The condition
is that items that are dyed with levelling acid dyes should not be machine dyed
or should not be washed in warm water. Instead of this, these should be washed
in cool water through dry-cleaned or hand washed.
Milling Acid Dyes
These dyes are mainly used for self shading. The wet fastness of these dyes are
medium to high range. Some Milling Acid Dyes have bad light fastness in pale
shades. These dyes are generally not combined with other dyes.
Super Milling Acid Dyes
Super Milling Acid dyes have higher neutral dyeing affinity, better coverage of
dye uptake differences in the substrate. Milk protein fibers like Silk Latte,
Soya Protein etc., can be used for dyeing through these Super milling acid
dyes. These dyes are least toxic among all others Acid Dyes and extremely