Chemistry of Dyeing

An acid dye is a dye which is a salt of a sulfuric, carboxylic or phenolic organic acid. The salts are often sodium or ammonium salts. Acid dyes are typically soluble in water and possesses affinity for amphoteric fibers while lacking direct dyes' affinity for cellulose fibers. When dyeing, ionic bonding with fiber cationic sites accounts for fixation of colored anions in the dyed material. Acids are added to dyeing baths to increase the number of protonated amino-groups in fibers.

Some acid dyes are used as food colorants.

 

Fibers

In the laboratory, home, or art studio, the acid used in the dye-bath is often vinegar (acetic acid)[3] or citric acid.[4] The uptake rate of the dye is controlled with the use of sodium chloride. In textiles, acid dyes are effective on protein fibers, i.e. animal hair fibers like wool, alpaca and mohair. They are also effective on silk. They are effective in dyeing the synthetic fiber nylon, but of minimum interest in dyeing any other synthetic fibers.

 

Description

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 as well as the process used.

Acid dyes affix 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 tendency of the dye to remain dissolved in water over fixation to the fiber.

 

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:

• Level dyeing or equalising acid dyes;

• Fast acid dyes;

• Milling acid dyes;

• Super-milling acid dyes.