Peroxide bleaching

Polcin and Rapson [92] found that remission difference spectra for peroxide bleaching of groundwood pulp from spruce or hemlock were qualitatively similar to those recorded for reduction with borohydride or TP+. The difference spectra showed broad, featureless changes in the visible and a discrete maximum at about 360 nm. Peroxide caused larger decreases in absorption coefficient than the reducing agents. 

Heitner and Min [48] compared the chromophore content of TMP and CTMP prepared from black spruce wood. The scattering coefficients of these two pulps were quite different therefore, the remission function was unsuitable to compare the chromophore changes and the absorption coefficients were calculated explicitly. 

FIGURE 3.10 Absorption difference spectra for peroxide bleaching of softwood TMP, Ak = - kf Q. The shape of the difference spectrnm for pnlps bleached less than 5 honrs is 

FIGURE 3.11 Difference in absorption between softwood TMP bleached with alkaline hydrogen peroxide for longer than 5 hours and that bleached for 5 hours, 5 - 

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The most common chemical bleaching procedures are hypochlorite bleach for cotton hydrogen peroxide bleach for wool and cotton sodium chlorite bleach for cotton, polyamide, polyester, and polyacrylonitrile and reductive bleaching with dithionite for wool and polyamide. 

This indicates alkaline conditions for peroxide bleaching. However, an upper pH limit exists as a result of competing, nonreversible reactions ... 

Worldwide siUcate production is estimated to be approximately 4 x 10 t/yr. The 1995 prices for typical products are given in Table 4. The price for a hquid product, normalised to a 100% soHds basis, is usually close to the price of the corresponding bulk glass. International trade of sibcates, especially bquid products, is limited by the cost of transportation rather than production costs. Production of sibcates has increased in the 1990s (Table 5). This growth is primarily attributed to an increase in the use of sibcate to make derivatives and also to use in peroxide bleaching. 

W. C. Eroass, Interactions of Calcium, Magnesium and Silicate Under Peroxide Bleaching Conditions, M.S. Dissertation, SUNY College of Environmental Science and Eorestry, Syracuse, N.Y., June, 1991. 

A smaller but important use for sulfur dioxide is for stabilization of pulp (qv) brightness after hydrogen peroxide bleaching of mechanical pulps. Sulfur dioxide neutralizes the alkalinity and destroys any excess hydrogen peroxide, which if left in the pulp would cause it to lose brightness. 

Hydrogen peroxide bleaching is performed in alkaline solution where part of the hydrogen peroxide is converted to the perhydroxyl anion (eq. 14). 

When bleaching is requited, a reductive bleach with sodium hydrosulfite and sodium metabisulfite is used. Cotton blends may requite a hydrogen peroxide bleach at pH 9.0—9.5 prior to or iastead of the normal reductive bleach. Chlorine-type bleaches which damage elastomeric fibers are avoided. 

In fastness to peroxide bleaching, ISO 10S-N02, the specimen is immersed ia a standard bleaching solution containing hydrogen peroxide (or sodium peroxide for viscose) where the composition of the bleaching Hquor is dependent on the fibers used ia the test specimen as are the pH and time of exposure (1—2 h). The objective of the test is to assess the colorfastness usiag typical bulk bleaching conditions for the fiber under test. 

Super-leitfahigkeit, /. superconductivity, -oxyd, n. superoxide (peroxide), -oxyd-bleiche, /. peroxide bleach, superoxydecht, a. fast to peroxide. Super-oxydhydrat, n. hydrated peroxide, -phosphatschlempe, /. a fertilizer made by incorporating superphosphate with molasses residue. 

Sodium carbonate peroxide is added to peroxide toothpaste as a whitener. It breaks down into sodium carbonate (washing soda) and hydrogen peroxide. The hydrogen peroxide bleaches the teeth and kills germs. 

Thus the detailed mechanism of peroxide bleaching is not yet finally resolved. It should be borne in mind, however, that the work of Dannacher and Schlenker was carried out at 60 °C on tea-stained cotton, whereas Spiro and Griffith studied the decolorisation of individual colorants at 21-25 °C in the absence of a textile substrate. 

In an optimally controlled process free from transition-metal ions hydrogen peroxide bleaching is remarkably safe, there being no reported detrimental effects of bleaching at around 100 °C or for more than several hours [143]. Under such conditions, most of the peroxide appears to be consumed in the oxidation of chain end units of the cellulose macromolecule. The other major effect on the substrate is oxidation of secondary hydroxy to keto groups, accompanied by the formation of very few aldehyde or carboxyl groups [235]. 

Table 10.14 Recommended conditions for batchwise peroxide bleaching [143]...

Table 10.16 Recommended conditions for cold pad-batch peroxide bleaching...

The final step after peroxide bleaching is to ensure that the goods do not contain residual peroxide. Reducing agents have been used traditionally for this purpose. However, the possibility of using environmentally friendly catalase enzymes should not be overlooked [87-89]. 

Table 10.17 Recommended conditions for TAED-activated peroxide bleaching [244]...

The mechanism of hypochlorite bleaching appears to be considerably less controversial than peroxide bleaching. The pH-related active species in sodium hypochlorite are shown in Figure 10.29 and Scheme 10.33. The pH range 9-11 is the most suitable for hypochlorite bleaching. The active bleaching species is the hypochlorite anion CIO. In fact bleaching... 

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... 

Figure 10.36 Relationship between alkali solubility and cysteic acid content of peroxide-bleached wool [259,261]...

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