Dyes are chemical substances used to impart color to textiles, leather, paper, etc.. These are organic colored compound. Dyes are resistant to washing or cleansing action and are stable towards light. Dyes are often confused with pigments but both these are different things. Dyes are soluble in water and organic solvents whereas pigments are insoluble in water and organic solvents. Dyes impart color due to selective absorption of light. These can be classified in many ways. On the basis of their application dyes are classified as reactive, acid, direct, disperse, vat and basic dyes.
Reactive dyes : These are used for fabrics and fibers. It reacts with the fiber to form colors. They form strong covalent bond with the fiber. These are water soluble dyes and require alkaline conditions for dyeing such as in the presence of sodium carbonate or urea. Reactive dyes used for cotton is PROCION, for silk and wool is PROCILAN. These dyes are cheap in price.
Acid dyes : These type of dyes are actually acidified basic dyes. They are used on protein fibers, nylon and acrylics. It is applied at low pH and are anionic in nature. They combine through hydrogen bonding or weak forces. Some of them are used as food colorants. Azoanthraquinone and triarylmethane are the types of acid dyes.
Direct dyes : These dyes are anionic substantive to cellulosic fibers. They are easy to apply, water soluble, cheap and not widely used as compared to reactive dyes. Dyeing is carried out in alkaline conditions. Congo red – 1-naphthylamine-4-sulfonic acid is a direct dye.
Disperse dyes: These dyes are substantive for hydrophobic fibers such as cellulose acetate, nylon, polyester. They require a carrier. The dye is printed or painted on to paper and heat pressed onto fabric. Disperse dyes have strong bright colors such as disperse red 11 and disperse blue 3.
Vat dyes : These are the fastest dyes for cellulose fibers. They are soluble in alkali. The fabric is first dyed and then oxidized in the air to achieve true color. Synthetic indigo is a characteristic vat dye.
Basic dyes : These are very bright cationic dyes used for dying wool, silk and acrylic fiber. They are sparingly soluble in water. Basic dyes are also used for making inks, typewriter ribbons and dyeing leather. Methylene blue, crystal violet are some of the basic dyes.