The exact mechanism of bleaching by available oxygen is subject to conjecture. Stains susceptible to oxidative
bleaching comprise chemicals with a degree of unsaturation. The conjugated double bonds in such compounds can
give rise to the color of such components: these double bonds may be disrupted by epoxidation leading to their
decolorization. The epoxides may be hydrolyzed to 1,2-diols with consequent increase in water solubility, hence
facilitating the removal of the bleached substance from the stained surface.
Another effect may be the removal of chemical bonds that bind the stain to the fabric, without necessarily
disrupting the bonds in the chromophore - especially in the case of aged stains.
The perhydroxyl anion (HOO-) is believed to be an important, perhaps the most important, bleaching species.
However others, including peroxoborates and singlet oxygen, may also be involved. Recent work has indicated
that the superoxide radical, O2- ., might act as the active oxygen species in some circumstances.
Hydrogen peroxide can react with both nucleophiles and electrophiles, and a given chromophore may well contain sites susceptible to either kind of attack. The available oxygen also has an important microbiocidal function
[see Sanitization section].
Activating sodium perborate for low temperature performance
Many bleach activators have been described that react with hydrogen peroxide to generate peracid, with its more
reactive form of available oxygen, in situ. Most are acyl donors, generally attached to a phenol or secondary/tertiary
nitrogen source since the resultant compounds are less stable in alkaline solution than simple alcohol esters.
They acylate the perhydroxyl anion to form peracids and peracyl anions – the perhydrolysis reaction.
An essential feature of activators is the presence of a good leaving group (pKa ~ 8 - 10). Two widely used activators
are tetraacetylethylenediamine (TAED) and sodium nonanoyloxybenzenesulfonate (NOBS).
TAED is favored in Europe, while in the U.S., where washes are typically at lower temperatures with shorter wash times
and more dilute detergent, NOBS is favored. This has been attributed to the surfactancy of the pernonanoic acid generated
by NOBS, which is interfacially active and more effective on stains, particularly so for lipophilic types. Peracetic acid
(PAA) from TAED is less interfacially active, i.e. more hydrophilic, and better suited to European conditions.
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The activator should not act on the perborate until they are both present in the wash liquor, and so TAED and NOBS are
encapsulated as prills or granules for powder formulations. The usual package composition is 2 - 6wt.% TAED with 12 - 25wt.%
perborate (Europe), or 1 - 2wt.% NOBS (U.S.) with 2 - 5wt.% PBS1.
The ratio of perborate to activator is important. The rate of peracid generation is enhanced by a higher peroxide:activator
ratio, higher pH and higher temperature; however peracid bleaching is more effective at a lower pH. Conveniently, the
pH profile during the wash changes: falling from 10 to 10.5 at the outset to about 9 after perhydrolysis. (see figs A4 - A6).
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| Fig A4 & A5: tea stain bleaching; effect of pH and activation |
Fig A4-A6:
• PBS simulated by use of H2O2 in borate buffer (to eliminate PBS dissolving rate effects)
• 8.6mM Avox |
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| Fig A6: effect of pH and temperature on PAA profiles with activated PBS |
The bleaching mechanism responsible, where a reaction between nucleophilic stains and peroxide is involved, is the
heterolysis of the peroxidic bond. Electrophilic activation of hydrogen peroxide can be achieved by the replacement
of the existing leaving group OH- by a weaker base (RO-), for example CH3CO2-. The result is a much faster bleaching
reaction, activating the otherwise slow kinetics of hydrogen peroxide.
This is exactly what is achieved by converting hydrogen peroxide into peracetic acid in TAED-activated perborate bleaching.
Much of the effort in developing improved low temperature peroxygen bleaching technologies is focused on maximizing the
rate of this reaction.
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