bleaching

Decolorization of coloured materials. The two main types of bleaching agent are oxidizing bleaches, which bring about the oxidation of pigments and include the ultraviolet rays in sunshine, hydrogen peroxide, and chlorine in household bleaches; and reducing bleaches, which bring about reduction and include sulphur dioxide.

Bleach is used in industry to lighten or whiten fabrics, yarns, or fibres. Bleaching processes have been known from antiquity, mainly those acting through sunlight. Both natural and synthetic pigments usually possess highly complex molecules, the colour property often being due to only a part of the molecule. Bleaches usually attack only that small part, yielding another substance similar in chemical structure but colourless.

In the fabric industry, hydrogen peroxide is usually used to whiten cotton, which in its natural state is a yellowish-white. Chlorine dioxide is used to bleach polyester, acrylic, and blends of synthetic fibres. Sodium hydrosulphite is used to bleach wool. Specialist bleaches can be added to washing detergents to improve stain removal and brighten fabrics. Optical bleach, which is added to many detergents, is not really bleach, but fluorescent dye which coats fabric, making it brighter; the dye also makes the fabric glow under ultraviolet light.