Dyeing and printing are finishing processes that provide lasting beauty and delight to the textile fabrics by adding colour. Dyeing and printing differs in the method by which colour is applied to the fabric. In the dyeing process; fibre, yarn or fabric is impregnated with a dyestuff. In printing, a pattern or a design is generally imprinted on the fabric in one or more colours by using dyes in paste form.
CLASSIFICATION OF DYES
Dyes are classified as Natural and Synthetic dyes.
Natural dyes are taken from three sources namely plants, animals and minerals.
Vegetable Dyes : Around 4000 years back Egyptians have used Indigo dyes, that are obtained from stems and leaves of a particular plant. Alizarin dyes are taken from roots of madder plant. Logwood dyes are extracted from the trees which give black colour to silk and cotton fabrics.
Animal Dyes : Cochineal dye was extracted from an insect - Coccus Cacti. The dye was taken after killing the female insects. These dyes were used for imparting red and orange colours in silk and wool fabrics. Tyrian purple dye was made out of shell fish.
Mineral Dyes : Natural minerals yield certain varieties of dyes for example Iron Buff.
Synthetic dyes were first derived from coal tar in 1856. Later innumerable dye compounds were made from coal tar, and are constantly being improved as to beauty of colour and colourfastness. They are as follows :
Direct Dye or Salt Dye : Direct dye can be applied to animal as well as vegetable fabrics but are generally applied to cotton and are known as direct cotton dye. These dyes are soluble in water and are chiefly composed of amines and phenols. Because a little salt is added to the solution while dyeing with direct dyes, these are also called salt dyes. A further treatment with acetic acid and sodium dichromate is necessary to make them fast to washing. The dye colours often have only fair fastness to light, poor fastness to washing and are not very bright.
Basic Dyes :The first coal-tar dye was a so-called basic dye. Basic dyes are salts of organic colour bases. It was developed to give many bright shades of silk and wool. Basic dyes are otherwise known as cationic dyes, the same are used with a mordant, Tannic acid, for cotton, linen, acetate, nylon, polyester and acrylics. This dye gives beautiful colour but is not fast to sunlight, washing and perspiration.
Acid (Anionic) Dyes : Acid dyes are the sodium or calcium salts of colour organic acids. They are used mostly on wool and silk. Acid dyes are inexpensive and fairly fast to light, but they are not fast to washing. Soap containing alkali if used will change the colour.
Mordant or Chrome Dyes : Sodium or Potassium dichromate mordant is added in the dye bath. This mordant along with dyes will penetrate into the fabric. These are used to dye wool and also for printing cotton. These are fast to light, washing and perspiration.
Developed Dyes : This process requires a base to be dyed on the goods. This is followed by a diazotizing process, whereby the dye is chemically changed and treated with a fresh set of chemicals, called developers, that form the completed dye. Developed dyes are fairly fast to washing because they have been literally built into the fibre.
Sulphur Dyes : Sulphur dyes, first made in 1879, are used for cotton and linen. Sulphur dyes are insoluble in water and must be made soluble with the aid of caustic soda and sodium sulphide. These dyes are fast to washing, light and perspiration, but excessive chlorine will strip the colour.
Vat Dyes : The first vat dye was an Indigo created in 1879. Vat dyes are the fastest dyes for cotton, linen and rayon. Vat dyes are resistant to light, acids, alkali as well as to oxidizing bleaches. Vat dyes are insoluble pigments, but are made soluble in water by the use of a strong reducing agent, such as hydrosulphite developed in the alkali sodium hydroxide. The fabric is immersed in this solution. Subsequent exposure to air or immersion in an oxidizing bath (bichromate) restores the dye to its insoluble form as a part of the fibre.
Reactive Dyes : Reactive dyes were developed in 1957. These dyes react with fibre molecules to form a chemical compound. These dyes were first designed for cellulose fibres, now available for wool, silk, nylon, acrylics and blends of these fibres. Advantages of reactive dyes are their excellent fastness to light and washing. They give very deep and brilliant colours.
Pigment Dyes : Pigment dyes are not true dyes because they have no affinity for the fibre and if applied and held to the fabric with resins, which are then cured at high temperature. The colours, confined to light shades, bright colours, such as metallic colours as gold are usually applied to cotton cloth but are also used in fabrics of wool and manmade fibres.
Optical Brighteners (Colourless Dyes) : These so-called dyes are also called fluorescent whiteners or optical brighteners. The whiteness is really caused by absorption of ultraviolet light and reflection of visible blue light. Optical brighteners are available for cotton, acrylics, wool, acetate and nylon. They may be applied during bleaching, before resin finishing or with the resin.