Bleaching Agents, Survey

Abstract

Abstract A bleaching agent is a material that lightens or whitens a substrate through chemical reaction. The bleaching reactions usually involve oxidative or reductive processes that degrade color systems. These processes may involve the destruction or modification of chromophoric groups in the substrate as well as the degradation of color bodies into smaller, more soluble units that are more easily removed in the bleaching process. The most common bleaching agents generally fall into two categories: chlorine and its related compounds (such as sodium hypochlorite) and the peroxygen bleaching agents, such as hydrogen peroxide and sodium perborate. Reducing bleaches represent another category. Bleaching agents are used for textile, paper, and pulp bleaching as well as for home laundering. Chlorine‐containing bleaching agents are the most cost‐effective bleaching agents known. They are also effective disinfectants, and water disinfection is often the largest use of many chlorine‐containing bleaching agents. They may be divided into four classes: chlorine, hypochlorites, N ‐chloro compounds, and chlorine dioxide. Except to bleach wood pulp and flour, chlorine itself is rarely used as a bleaching agent. The principal form of hypochlorite produced is sodium hypochlorite, NaOCl. Other hypochlorites include calcium hypochlorite and bleach liquor, bleaching powder and tropical bleach. The principal solid chlorine bleaching agents are the chlorinated isocyanurates, eg, sodium dichloroisocyanurate dihydrate. Other N ‐chloro compounds include halogenated hydantoins, and sodium N ‐chlorobenzenesulfonamide (chloramine B). Chlorine dioxide, ClO 2 , is a gas that is more hazardous than chlorine. Large amounts for pulp bleaching are made by several processes in which sodium chlorate is reduced with chloride, methanol, or sulfur dioxide in highly acidic solutions by complex reactions. For most other purposes chlorine dioxide is made from sodium chlorite. Peroxygen compounds contain the peroxide linkage ( \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\relbar \kern-5pt{\relbar}\kern-7pt{\relbar}}{\rm{O}}{\relbar \kern-5pt{\relbar}\kern-7pt{\relbar}}{\rm{O}}{\relbar \kern-5pt{\relbar}\kern-7pt{\relbar}}}$\end{document} ) in which one of the oxygen atoms is active. Hydrogen peroxide is one of the most common bleaching agents. It is the primary bleaching agent in the textile industry, and is also used in pulp, paper, and home laundry applications. Hydrogen peroxide reacts with many compounds, such as borates, carbonates, pyrophosphates, sulfates, etc, to give peroxy compounds or peroxyhydrates. Compounds include perborates, sodium carbonate peroxyhydrate, and peroxymonosulfate. Peracids are compounds containing the functional group \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\relbar \kern-5pt{\relbar}\kern-7pt{\relbar}}{\rm{OOH}}}$\end{document} derived from an organic or inorganic acid functionality. Peracids have superior cold water bleaching capability versus hydrogen peroxide because of the greater electrophilicity of the peracid peroxygen moiety. Lower wash temperatures and phosphate reductions or bans in detergent systems account for the recent utilization and vast literature of peracids in textile bleaching. The reducing agents generally used in bleaching include sulfur dioxide, sulfurous acid, bisulfites, sulfites, hydrosulfite (dithionites), sodium sulfoxylate formaldehyde, and sodium borohydride. These materials are used mainly in pulp and textile bleaching. Bleaching is a decolorization or whitening process that can occur in solution or on a surface. The color‐producing materials in solution or on fibers are typically organic compounds that possess extended conjugated chains of alternating single and double bonds and often include heteroatoms, carbonyl, and phenyl rings in the conjugated system. The portion of molecule that absorbs a photon of light is referred to as the chromophore. Bleaching and decolorization can occur by destroying one or more of the double bonds in the conjugated chain, by cleaving the conjugated chain, or by oxidation of one of the other moieties in the conjugated chain. The molecule then absorbs light in the ultraviolet region, and no color is produced. Chlorine bleaches react with more chromophores than oxygen bleaches. The mechanism of bleaching of hydrogen peroxide is not well understood. It is generally believed that the perhydroxyl anion ( \documentclass{article}\pagestyle{empty}\begin{document}${{\rm{HOO}}{^{-}}}$\end{document} ) is the active bleaching species. Peracids undergo a variety of reactions which result in bleaching, eq., they can add an oxygen across a double bond to give an epoxide, which can undergo further reactions including hydrolysis to give a vicinal diol. Reducing agents are thought to work by reduction of the chromophoric carbonyl groups in textiles or pulp. The most widely used bleach in the United States is liquid chlorine bleach, an alkaline aqueous solution of sodium hypochlorite. This bleach is highly effective at whitening fabrics and also provides germicidal activity at usage concentrations. Dry and liquid bleaches that deliver hydrogen peroxide to the wash are used to enhance cleaning on fabrics. They are less efficacious than chlorine bleaches but are safe to use on more fabrics. The dry bleaches typically contain sodium perborate in an alkaline base whereas the liquid peroxide bleaches contain hydrogen peroxide in an acidic solution. The worldwide decreasing wash temperatures, which decrease the effectiveness of hydrogen peroxide‐based bleaches, have stimulated research to identify activators to improve bleaching effectiveness. Tetraacetylethylenediamine is widely used in European detergents to compensate for the trend to use lower wash temperatures. TAED generates peracetic acid in the wash in combination with hydrogen peroxide. TAED has not been utilized in the United States, where one activator nonanoyloxybenzene sulfonate (NOBS) has been commercialized and incorporated into several detergent products. NOBS is claimed to provide superior cleaning in contrast to perborate bleaches. In industrial and institutional bleaching, either liquid or dry chlorine bleaches are used because of their effectiveness, low cost, and germicidal properties. Bleaching agents are used in hard surface cleaners to remove stains caused by mildew, foods, etc, and to disinfect surfaces. Disinfection is especially important for many industrial uses. Alkaline solutions of 1–5% sodium hypochlorite that may contain surfactants and other auxiliaries are most often used for these purposes. In‐tank toilet cleaners use calcium hypochlorite, dichloroisocyanurates, or N ‐chloro compounds to release hypochlorite with each flush. The primary role of bleach in automatic dishwashing and warewashing is to reduce spotting and filming. Many textiles are bleached to remove any remaining soil and colored compounds before dyeing and finishing. Cotton is the principal fiber bleached today, and almost all cotton is bleached. Rayon is bleached similarly to cotton but under milder conditions since the fibers are more easily damaged. Most synthetic fibers are sufficiently white and do not require bleaching. Wool must be bleached carefully, in order to avoid fiber damage. It is usually bleached with 1–5% hydrogen peroxide solutions. Hydrogen peroxide is the most satisfactory bleaching agent for human hair. During the refining of sugar, sulfur dioxide is added to remove the last traces of color. Flour can be bleached with a variety of chemicals including chlorine and chlorine dioxide.