Bleaching

bleaching, process of whitening by chemicals or by exposure to sun and air, commonly applied to textiles, paper pulp, wheat flour, petroleum products, oils and fats, straw, hair, feathers, and wood. Chemical methods include oxidation, as by hypochlorites, ozone, and the per-compounds; reduction, as with sulfur dioxide; and adsorption, as by bone charcoal used to decolorize sugar solutions. Textiles have long been whitened by grass bleaching, a method virtually monopolized by the Dutch from the time of the Crusades to the 18th cent. They developed a technique in which goods were alternately soaked in alkaline solutions and grassed, or crofted, a procedure in which they are exposed to air and sunlight; the goods were then treated with sour milk to remove excess alkali. Later they substituted dilute sulfuric acid for the milk. In 1785 the French chemist Claude Berthollet suggested the commercial application of chlorine for bleaching, and in 1799 the Scottish chemist Charles Macintosh invented bleaching powder, or chloride of lime, the first of the modern chemical bleaches. Bleaching processes vary for different fibers. Cotton, naturally a grayish yellow, contains waxy and oily impurities that interfere with the action of dyes. It must be scoured and boiled in huge kettles (kiers) before bleaching. Grass bleaching has been combined with or superseded by chemical methods, which are deleterious unless rigidly controlled. Four degrees, ranging from quarter to full bleach, are recognized in the industry. Full bleach is reputed to weaken the fiber as much as 20%. Since chlorine bleaches react with the protein of animal fibers, silk and wool are commonly bleached with hydrogen peroxide. Although sulfurous acid or sulfur dioxide are also used for wool, they do not permanently whiten it. For effective bleaching, wool must first be scoured and silk must be degummed. Common bleaching agents used domestically are Javelle water, which is sodium hypochlorite in water, and other chlorine-based mixtures.

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bleaching [′blēch·iŋ]

(graphic arts)

An afterprocess in the production of direct positive photographs, in which an oxidizing solution dissolves the negative silver.

(optics)

A decrease in the optical absorption of a medium, produced by radiation or by external forces.

(textiles)

A process in which natural coloring matter is removed from a fiber, yarn, or fabric to make it white.

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bleaching

A chemical or photochemical reaction which whitens or removes color from a surface.

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Bleaching 

chemical processes used to remove impurities and eliminate any undesired color from various materials to give them a white color or prepare them for dyeing (textiles, wood pulp, wax, and so on).

Bleaching is most widely used in the textile industry. Preliminary removal of impurities involves treatment of the material with chloramine, weak acid or alkaline solutions, and enzyme preparations, and also boiling in alkaline solution. Oxidizing agents (sodium or calcium hypochlorite, hydrogen peroxide, sodium chlorite, or potassium permanganate) or reducing agents (sulfur dioxide or sodium hydrosulfite) are used in the actual bleaching process. Bleaching is done in a bleaching plant equipped with continuous-operation steaming units and with scouring machines, in which the textiles are processed with solutions of alkalies, acids, and oxidizing agents. The textiles are washed with water between bleaching operations and upon completion of the process. Organic compounds are also used in bleaching.