Bleaches
Bleaches are used to eliminate color imperfections such as grayness from natural fibers, including wool and cotton, and to impart a pleasing white, laundered effect. They are also used to lighten the color of flour and other foodstuffs; to treat wood pulp prior to its manufacture into paper; as disinfectants; to sanitize swimming pools; to remove mold; as deodorants; and as cleaning agents. Although bleaches are commonly used to remove color from clothing, there are other ways, such as exposure to sunlight, to achieve the same effect. The bleaching action is accomplished by oxidation of the colored impurities by the bleach.
There are two basic types of bleaches: chlorine (hypochlorite) bleaches, and peroxy type bleaches. The hypochlorite bleaches tend to be more powerful oxidizers than the peroxy compounds.Sodium bromide is sometimes used in combination with the hypochlorites to bleach cellulose materials.
Sodium hypochlorite, which is now sold commercially as a liquid bleach in household (5.25 wt % NaClO) and industrial (13 wt % NaClO) strengths, has been available since 1787.Calcium hypochlorites, which are produced by passing chlorine gas over slaked lime, have been used since 1799 as bleaching powders. Other commonly used chlorine compounds include potassium dichloroisocyanurate and chlorinated trisodium phosphate.
The chlorine bleaches deliver the whitening action of chlorine without requiring the direct handling of liquid or gaseous chlorine. When properly used, chlorine bleach is a simple and effective disinfectant. However, this type of bleach and its vapors are irritating to the skin, eyes, nose, and throat. Dermatitis may result from direct skin contact. Ingestion can cause esophageal injury, stomach irritation, and prolonged nausea and vomiting. And chlorine bleaches, when mixed with acidic substances such as ammonia, toilet bowl cleaners, drain cleaner, or vinegar, form toxic gases that may prove fatal if inhaled.
Chlorine bleach is a highly reactive chemical that rapidly converts, through oxidative reactions, chiefly into NaCl (table salt), oxygen, and water. Chlorine bleach manufacturers, therefore, claim that problems with biodegradability will not arise when this bleach is used in the correct manner and in recommended quantities,. But critics are quick to point out that, legally, a product can be said to be biodegradable if at least 80% of it biodegrades in 90-180 days, and this definition can leave as much as another 20% in the environment for longer periods of time as toxic waste.
Of the peroxy type bleaches, sodium perborate (NaBO34H2O) is the least expensive. Organic peroxides such as dibenzoyl peroxide, are used in the bleaching of flour.Hydrogen peroxide is generally made from nonrenewable resources, but decomposes into hydrogen and oxygen. It is safer than chlorine (hydrogen peroxide is often used as a topical antiseptic and mouthwash). It is used to bleach cotton, wool, and groundwood pulp, and is a component in hair bleaching preparations.
Optical bleaches (or fluorescent whitening agents) are organic chromophores that absorb ultraviolet light and then emit part of this energy as visible light. They thus enhance the appearance of laundered fabrics by causing the fabric to appear brighter and whiter.
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