Oxygen-containing bleaches

M Ek, J Gierer, K Jansbo, T Reitberger. Smdy on the selectivity of bleaching with oxygen-containing species. Holzforschung 43 391-396, 1989. 

Coimnon bleach activators are not very weight- and volume-effective, as they work stoichiometri-cally with peroxide. This drawback is overcome by using bleach catalysts, normally transition metal complexes with nitrogen- or oxygen-containing ligands. 

Economic reductive bleaching of secondary fibers requires an oxygen-free stock as dithionite is sensitive to the oxygen contained in the air. Therefore the stock has to be deaerated which can be done sufficiently at a medium consistency of about 10 to 15 %. FAS is less sensitive, thus operating consistency can be as high as 30% and a Disperger may be used for admixing of FAS. Best results are obtained when... 

Finally, we look at an example where environment-friendly catalytic oxidation is desired. In the paper industry, bleaching of wood pulp by hypochlorite is an important step. The chloride-containing effluent of the bleaching process increases the E factor and is of environmental concern. The heteropolyoxo anion [AlVWjjO J has been proposed as a potential homogeneous catalyst for a chloride-free, oxygen-based bleaching process. 

Earlier formulations contained mainly chlorine bleach, metasiUcates, triphosphate, and nonionic surfactants. Modem manufacturers have switched to more compHcated formulations with disiUcates, phosphates or citrate, phosphonates, polycarboxylates, nonionic surfactants, oxygen bleach, bleach activator, and enzymes. The replacement of metasiUcates by disilicates lowers pH from approximately 12 to 10.5, at 1 g ADD/L water. The combined effect of decreased pH, the absence of hypochlorite, and the trend toward lower wash temperatures has paved the way for the introduction of enzymes into ADDs. Most ADD brands in Europe are part of the new generation of ADD products with enzymes. The new formulations are described in the patent hterature (55—57). 

Sodium hypochlorite is usually sold in solution in water, where it makes a greenish-yellow liquid. (It is too hygroscopic—it absorbs water from the air—to be used conveniently in solid form.) Household bleaches usually contain sodium hypochlorite in a 3 percent to 6 percent solution. Some sodium hydroxide (lye) is added to keep the pH high to avoid decomposition. If the solution is made more acidic, sodium hypochlorite will dissociate, producing chlorine gas and oxygen. 

Colored substances contain molecules with chromophores, areas of the molecule that have double bonds between carbon atoms or oxygen atoms. A good example is beta-carotene, and that section goes into more detail on how molecules become colored. Bleaches attack these chromophores in one of two ways. 

Choose oxygen bleach over chlorine bleach if the product contains enzymes or perfumes. 

In the iodate detection system (ASTM D1552), the sample is burned in a stream of oxygen at a sufficiently high temperature to convert about 97% by weight of the sulfur to sulfur dioxide. The combustion products are passed into an absorber containing an acidic solution of potassium iodide and starch indicator. A faint blue color is developed in the absorber solution by the addition of standard potassium iodate solution. As combustion proceeds, bleaching the blue color, more iodate is added. The sulfur content of the sample is calculated from the amount of standard iodate consumed during the combustion. 

Peroxide. Compounds containing —0—0— linkage. Peroxides are extremely reactive and give up atomic oxygen readily. They are used industrially as oxidizing or bleaching agent. 

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