Classification of Dyes:
Natural dyes obtained from animals, plants and minerals without any chemical processing. Natural dyes are vat dyes, substantive or mordant dyes. There are some advantages of natural dyes, such as: these dyes need no special care, wonderful & rich in tones, act as health cure, have no disposal problems, have no carcinogenic effect, easily biodegradable, require simple dye house to apply on matrix and mild reactions conditions are involved in their extraction & application. But there are also some limitations of natural dyes which includes, lesser availability of colors, poor color yield, complex dyeing processing, poor fastness properties and difficulty in blending dyes. Synthetic dyes are a class of highly colored organic substances, primarily utilized tinting textiles that attach themselves through chemical bonding between the molecules of dye & that of fiber. There are several ways of classification of dyes. We can classify dyes in the following ways; classification of dyes according to application, classification of dyes according to structure, classification of dyes according to constitution etc. Synthetic dyes are classified on the basis of chemical structure or on the basis of methods of application to the material.
Innumerable synthetic organic dyes are employed in the textile industry. (Certain inorganic dyes such as sodium dichromate are also employed to obtain certain specific colors). The organic dyes are generally classified according to their chemical constitution. However in textile processing, dyes are classified based on their applications. Some prominent types of dyes are discussed briefly.
These dyes more properly referred as “fiber reactive dyes” are relatively new dyes. They are water-soluble anionic compounds. This class of dyes is the largest dye class and used extnesively on cellulosics (cotton) when bright shades are desired. They can be very well applied to nylon, silk and wool. The auxiliaries required are sodium chloride, urea, sodium carbonate, caustic soda and buffers.
Disperse dyes are water insoluble dyes mainly employed for dyeing polyester. They find minor use in dyeing synthetic fibers such as cellulose acetate and polyamides. Disperse dye molecules are the smallest dye molecules among all dyes. A disperse dye molecule is based on an azobenzene (as Disperse Red 1 or Disperse Orange 37) or anthraquinone molecule with nitro, amine, hydroxyl etc., groups attached to it. The dye is generallyapplied under pressure at a temperature of about 1300C.
These dyes are exclusively used in dyeing of wool and other animal fibers. Acid dyes are widely used on nylon when high wash fastness is required and they are seldom used on cotton or linen as they require mordants. Acid dyes are sodium salts of sulfuric acid or carboxylic or other organic acids. Acid dyes are water soluble anionic compounds. The following auxiliaries are required for satisfactory dyeing with acid dyes: sodium sulfate, sulfuric acid, formic acid, acetic acid, ammonium acetate, ammonium sulfate, ammonium phosphate and levelling agents.
These dyes are water insoluble dyes containing sulfur or sodium sulfide. They require dissolution in an alkaline solution before application. The dyes are usually applied at high temperatures (600C to 1000C). They are applied in a soluble reduced form from a sodium sulfide solution, which are then reoxidized to insoluble form on the fiber. Sulfur dyes are mainly applied to cotton for economical dark shades. These dyes have excellent resistance to washing but poor resistance to sunlight. They generally give very poor fastness to chlorine. Auxiliary chemicals for satisfactory dyeing include sodium sulfide, sodium carbonate, acetic acid, hydrogen peroxide, sodium chloride, sodium sulfate and sodium dichromate.
These dyes are also known as “cationic dyes”. They are soluble in water. This class of dyes gives bright colors. These dyes exhibit good fastness and are mainly used on acrylic fibers and polyester and on wool, and silk. (Cellulose fibers (cotton) have no affinity for basic dyes). These dyes are usually hydrochlorides or salts of organic bases and applied along with organic acids such as formicacid, tannic acid, acetic acid and oxalic acid. Sodiumsulfate and sodium acetate are also employed.
These are also termed as neutral dyes or ‘substantive dyes’ and are sodium salts of sulfonic acids and are almost invariably azo compounds. They are water soluble anionic compounds and used mainly for dyeing cotton. Most of the small dyeing houses use direct dyes only, as they are cheaper, easy to apply and the requirement of auxiliary chemicals is much less. The auxiliaries include sodium chloride, sodium nitrite, sodium sulfate, sequestering agents, hydrochloric acid and aromatic amines. Though the color with direct dyes is stable for washing, their color fastness is not so appreciable.
This category consists of both natural and synthetic dyes. Since they do not have affinity for textile fibers, they are applied after mordanting the fiber with a metallic oxide. The most commonly employed mordant is chromium oxide and these dyes are also referred to as chrome dyes. Other chemicals required are acetic acid, formic acid or sulfuric acid, sodium sulfate, sodium (or potassium) dichromate, ammonium sulfate and penetrating agents.
These dyes are widely used on cotton, linen, rayon, silk, wool and sometimes nylon. Vat dyes are considered to be the most resistant dyes to both washing and sunlight. These are water insoluble dyes made from indigo, anthraquinone and carbazole. They are made soluble on treatment with reducing agents and alkali and then applied to fiber and reoxidized to the original insoluble form. Additional chemicals required for dyeing with vat dyes are caustic soda, sodium hydrosulfite, dispersing agents, perborate, hydrogen peroxide and acetic acid.
These dyes contain azo component (–N=N-), used for dyeing of cotton fabrics. In the dyeing process fiber is first treated with coupler followed by application of azo dye. This type of dye is extremely fast to light.
These dyes are variants of mordant dyes including metallic oxide (usually chromic) in their structure, thus eliminating the need for addition of dichromates in the process. The auxiliaries used are the same as for mordant dyes. Permetallized dyes are used on nylon, silk and wool.