Hydrogen peroxide, bleaches

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. 

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. 

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. 

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]. 

FBA may be applied by exhaustion from the same bath. If the polyester portion of the blend is to be bleached with sodium chlorite, the cotton is usually brightened in a second step since most FBAs for cotton are destroyed by sodium chlorite. Both types of FBA are normally compatible with a hydrogen peroxide bleaching process. 

Methods. A typical hydrogen peroxide bleaching operation consists of treatment of a 20% HPL solution in 60-75% aqueous ethanol with 25-50% hydrogen peroxide (by weight) on lignin derivative at 80°C. The reaction medium is adjusted to pH 2 by the addition of HC1, or to pH >12-13 by NaOH. Color loss is determined by UV/VIS absorbance measurements on suitably diluted samples. 

The effect of sodium borohydride on the light-induced yellowing of hydrogen peroxide bleached TMP is different from that of newsprint made from unbleached mechanical pulp. Treatment of bleached TMP with sodium borohydride does not change the extent or the rate of light-induced yellowing (15). 

Figure 5. Relative yields (peak heights) of diarylpropane and stilbene structures from spruce wood (S), unbleached GW (PI), unbleached CTMP (P2), hydrogen peroxide bleached GW (P3) and hydrogen peroxide bleached CTMP (P4) after acid hydrolysis.

The natural content of stilbenes is considered to be very low in spruce wood lignin and this was confirmed in the present work (Figure 5). Analysis of the aqueous extracts after mild acidic hydrolysis revealed, however, that from wood to unbleached pulps and further to bleached pulps a stepwise conversion of diarylpropane to diguaiacyl stilbene structures takes place (Figure 5). The content of diarylpropane structures was found to be 25% less in the unbleached pulp samples than in the wood with a simultaneous increase in the content of stilbene structures. After hydrogen peroxide bleaching, the content of diarylpropane structures further decreased and the... 

The effects of these dose rates were studied on an unbleached TMP and on a hydrogen peroxide bleached TMP (P-TMP). The effects of dose from 500 to 3000 krad in increments of 500 krad on these TMP and P-TMP were also studied. This was done so as to compare with the 2.5 Mrad dose reported in the literature (8-10) and used in the industry to irradiate medical papers (12). 

The curve marked by a DOT represents the bleaching effect of alkaline hydrogen peroxide. The difference in the absorption coefficient at 457 nm is about -2.0 m2/kg and associated with an increase of 4.1 in the ISO brightness as the result of peroxide bleaching. Upon y irradiation of an alkaline hydrogen peroxide bleached pulp (P-TMP), one still attacks the phenolic hydroxyl groups or coniferaldehyde groups around 350 nm as indicated by the curved marked with a DIAMOND. 

The traditional conditions for alkaline hydrogen peroxide bleaching have called for modest temperatures in the region of 70-80 °C, but temperatures as... 

Cystine, which contains a disulfide bond, is reported to be the most numerous and reactive amino acid present in hair keratin. Disulfide bonds in cystine are reduced by mercaptans and phosphines, and oxidized by perborates, bromates, and bleach. These reactions result in structural rearrangements within keratin which may affect the physiochemical properties of hair, since disulfide bonds in cystine contribute to the stability of hair. For example, hydrogen peroxide bleaching of hair is an oxidative process which occurs readily in an alkaline medium. This results in the formation of perhydroxy anions which have been proposed to react with cystine to form cysteic acid residues. The process of bleaching results in the loss of approximately 15% of the cystine bonds originally present in keratin and may explain the increased permeability of bleached hair to chemicals. - ... 

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