Dyes are coloured compounds capable of being fixed to a fabric. A dye must be ‘fast’ or chemically stable so that the colour will not wash with soap and water, fade on exposure to sunlight etc. During the process of dyeing, the dye molecules form an uncut chemical bond with the fiber molecules. The chromophore group in the dye is what decides the dye colour while the auxochrome group causes the dye-fiber reaction.

Historically, dye substances have been extracted from a plethora of natural substances. However, they have been largely replaced by man-made dyes due to the inherent advantages that the man-made dyes have:-

• They can be made in a much wider variety of colors and shades of each color.
• They are more reliable than natural dyes.
• They are cheaper.

Man-made dyes, such as direct dyes, acid dyes, basic dyes, reactive dyes, mordant dyes, vat dyes, metal complex dyes, disperse dyes, etc, are derived from organic or inorganic compounds, and are known as synthetic dyes.

Reactive Dyes react with the cellulosic fiber of the textile to form a covalent bond – the reactive group of the dye combines directly with the hydroxyl or amino group of the fiber. Because of the chemical reaction the colour is fast and has a very long life. An example of this bonding is as shown here for Reactive Blue 5:-

 

Thus reactive dyed fiber has an extremely high wash fastness property. Cotton, wool, silk, viscose, nylon, etc can be dyed in this manner.