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 used.

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 DyesAn 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 DyesEqualising/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.