Other mechanisms may be in play. The reaction of the peracid with its anion, promoted at a pH equal to the pKa of the
peracid, may lead to the formation of active oxygen species. It has been speculated that singlet oxygen is responsible
for the observed bleaching with sodium perborate/NOBS systems.
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| Fig A7: reaction between nucleophilic stains and peroxide |
Pre-soak practices
In some markets, notably the People’s Republic of China and South America, it is usual to soak laundry before washing at
ambient temperatures in a detergent that typically does not contain a bleach.
Although a perborate/activator combination (e.g. PBS1/TAED) will not produce effective bleaching by washing at these
temperatures, Borax has shown that after soaking overnight using a detergent containing PBS and TAED, followed by a machine
wash, a variety of stains are effectively bleached, especially the tea stain. These results demonstrate that the
performance of PBS/TAED is not only dependent on temperature, but also upon the contact time between stained fabric and
bleach solution. Activated perborate is slow to react at low temperatures, but given long enough, is able to deliver
excellent results.
Beyond activators: metal catalysis
Activators effectively increase the reactivity of available oxygen from PBS.
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It is undeniable that commercially available bleach activators do not perform well below 40ºC. New activators
are under development, but other approaches are being considered: ions of the transition metals (manganese
has attracted much attention) for example can also catalyze peroxy reactions to promote the formation of
reactive peroxy or oxy derivatives at low temperatures.
The attractions of a catalytic route to expose the stain to available oxygen are obvious: as effective as
activators, but with less bulk, at less cost and with greater economy of chemical usage. The invention of a
transition metal based bleach catalyst was first commercialized in the early 1990s, but resulted in
unacceptable dye and fabric damage under some washing conditions.
Bleaching
Despite the setback, transition metal catalyst systems are under active scrutiny. An effective catalyst-based
formulation, apart from being kind to fabrics and dyes, must:
- be hydrolytically and oxidatively stable e.g., not form colored insoluble metal oxides or hydroxides
- promote useful rather than wasteful release of the available oxygen
- be safe in use and as an effluent
- be economical to produce
The parameters of peroxygen bleaching
Adjustable perborates
Of the many forms of sodium perborate known, the two most familiar in detergents are the mono- and tetrahydrates.
Their dissolving rates vary with temperature, and because of this, and their different available oxygen content,
‘tailored’ intermediate perborate mixtures enable attainment of the optimum available oxygen (concentration vs. time)
profiles.
Other means of altering solution rates exist. Polyhydroxycarboxylates capable of forming complexes with perborate
increase the perborate dissolution rate (see Builders section). Specific bleach activity is measured by the available
oxygen content. The theoretical limit of PBS4 is 10.4 wt.% available oxygen, and PBS1 up to 16 wt.%.
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