bleaching


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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 Grass bleaching has been combined with or superseded by chemical methods, which are deleterious unless rigidly controlled.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.

Bleaching

In wood finish, cleansing or whitening by the use of acid.

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.

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.

bleaching

A chemical or photochemical reaction which whitens or removes color from a surface.
References in periodicals archive ?
I do buy it for cleaning the tea stains from my sink and occasionally for cold bleaching white linen and tea towels.
For laser bleaching, a dentist applies a bleaching agent to the teeth, then uses an argon laser to activate it.
Global warming may cause large tracts of Australia's World Heritage-listed Great Barrier Reef to die from coral bleaching within 30 years, according to a report released by Greenpeace in Brisbane on Tuesday.
That is what happened with the big eco-catastrophe story of 1997-98, the global spread of coral bleaching.
Temperature control is vital to maximization of the peroxide bleaching process.
Simple peroxide bleaching is the most popular and one of the cheapest forms of professional whitening, according to Jeffrey Lissot, a partner in Woodland Hills-based Aesthetic Dentistry Associates.
An additional $20-million project completed at Fort Frances will allow the mill to increase the substitution of chlorine dioxide for chlorine in the bleaching process.
com/research/1a186115/environmentally_be) has announced the addition of Elsevier Science and Technology's new report "Environmentally Benign Approaches for Pulp Bleaching.
com/research/fmxdns/activated) has announced the addition of the "Activated Bleaching Earth (CAS 70131-50-9) Market Research Report" report to their offering.
Scientists at the Southeast Dairy Foods Research Center evaluated the influence of starter cultures, whey solids, composition and spray-drying on the efficacy, or efficiency, of the bleaching process.
Although the contributors to this issue examine the phenomenon from a variety perspectives, all draw attention to the impact of global White supremacy on valuations of skin color and the extent to which skin bleaching, as a social practice, is functionary of white domination.
Washington, October 2 (ANI): Scientists have found that bleaching can make corals more susceptible to disease and, in turn, coral disease can exacerbate the negative effects of bleaching.