Oxidative bleaching

Oxidative bleaching Oxidative carbonylation Oxidative coupling... 

Table 6. Equivalent Weight and Efficiency of Oxidizing Bleaching Agents ...

There are two types of bleachiag oxidative and reductive. Oxidative bleaching cleaves carbon—carbon double bonds thus destroyiag chromophores. Reductive bleaching reduces carbon—carbon double bonds to single bonds. The loss of extended conjugation ia the chromophore leads to loss of color. 

Whiter fabrics are produced if an oxidative bleaching is followed by a reductive one this is often referred to as full bleaching. One such process involves a hydrogen peroxide oxidative bleaching, followed by addition of thiourea to the Hquor to generate thiourea dioxide for a reductive bleach in situ (115). 

Further Preparative Reactions. When pulps are to be used in the production of materials that do not retain the original fiber stmcture, such as rayon or ceUulose acetate film, the lignin, hemiceUulose, and other components must be reduced to the lowest possible concentrations. A surfactant (ionic or nonionic) is often added during a hot, weakly alkaline extraction step after chlorination. Another approach, sometimes used in addition to the surfactant step, is to treat the pulp with 6—10% NaOH after most of the oxidative bleaching is finished. This treatment removes most of the hemiceUulose. In most purification plants the final stage includes use of sulfuric acid chelators are optional. 

Sulfur dioxide is a reducing bleach and can counter the effects of oxidizing bleaches, thus preserving color in fruits dried in the sun. The combination of fruit acids and ultraviolet light would otherwise react with the colorful compounds, making the fruit pale. 

First, oxidizing bleaches such as sodium hypochlorite break the molecules at the double bond. This results in either a shorter molecule that does not absorb visible light, or a molecule whose chromophore is either shorter or nonexistent. A shorter chro-mophore will absorb light of a shorter wavelength than visible light (such as ultraviolet light), and so does not appear colored. 

However, the most famous hair bleach is hydrogen peroxide. Unlike sunlight and lemon juice, peroxide is an oxidizing bleach, and its effects are less easily undone. 

Oxidizing bleaches kill microbes by reacting with cell membranes and cell proteins. The most widely used is sodium hypochlorite for household and hospital uses, and calcium hypochlorite for drinking water and swimming pool disinfecting. 

In traditional peroxide bleaching, hydrogen peroxide is activated by alkali. Acids, both inorganic and organic, can also be used to activate peroxide by the formation of a peracid. Peracids can be effective oxidative bleaching agents and, at least potentially, offer an alternative to the environmentally sensitive chlorine bleaches. Although known for quite... 

Oxidative bleaching of wool is invariably carried out with hydrogen peroxide. The active species involved is likely to be the same as on cellulosic substrates but specific reactions with wool amino acid residues must be considered. The primary reaction is oxidation of cystine disulphide bonds leading to the formation of cysteic acid residues (Scheme 10.41). The rupture of disulphide crosslinks, with attendant increase in urea-bisulphite and alkali solubility values, adversely affects fibre properties. As the severity of bleaching conditions increases, the urea-bisulphite solubility remains little changed but the relationships between alkali solubility and cysteic acid (Figure 10.36) and between cystine and cysteic acid (Figure... 

Reductive bleaches are generally less costly than oxidative bleaches but tend to give a greenish white compared with the reddish white tones from peroxide. Combining the two in an oxidation/reduction sequence gives a more neutral white, this being known as a full... 

As an alternative to oxidative bleaching with sodium chlorite, acrylic fibres may be given a reductive bleach using sodium bisulphite in the presence of oxalic acid. This method is... 

Chemical bleaching is used to remove colored non-washable soils and stains adhering to fibers and is accomplished by oxidative or reductive decomposition of chromophoric systems. Only oxidative bleaches are used in laundry products to a great extent. 

Since 1927 chromic acid has been used for the oxidative bleaching of Montan waxes. Hoechst has been operating the electrochemical regeneration of chromic acid for more than 90 years. The newly developed electrochemical process (as enlargement and partial replacement for the existing plant) is based on the... 

In the oxidative bleaching processes, the decoloration of p- and o-quinones and of coniferaldehyde structures also seems to be involved. In the case of coniferaldehyde, the removal of the conjugated side chain is probably involved (Figure 3.20). 

Oxidation/bleaching Treatment with peracetic acid and/or hydrogen peroxide, or sodium hypochlorite or sodium chlorite, or sulfur dioxide, or potassium permanganate or ammonium persulfate... 

Oxidation/bleaching Low viscosity, high clarity, and low Used in batters and breading for... 

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