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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 be
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 oftenammonium) 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 versatile.