Laundry bleaching, the whitening, lightening and brightening of fabrics and other articles by chemical means, is a
combination of oxidative and non-oxidative processes.
Sodium perborate is recognized as an outstanding oxidative bleaching agent and has been widely used as such for almost
a century. What is less well appreciated is that perborates and other borates also provide significant non-oxidative
bleaching properties.
This section concentrates on powders for fabric washing, but notes on liquids, dishwasher formulations and household
cleaners are appended.
Different machine washing practices around the world - derived either from the wash temperature, detergent
concentration, wash time, or user willingness to add bleach separately from the detergent - have in the main
determined the bleaching agent used.
As oxidative bleaches, hydrogen peroxide and sodium hypochlorite are very effective, albeit over different
temperature ranges, but cannot be incorporated directly into washing powders. Sodium perborate (PBS, NaBO3.nH2O
where n=1 or 4) can readily be incorporated. It has been described as a stable, solid form of hydrogen peroxide
allowing its introduction into the wash at the same time as the detergent. Sodium perborate is a gentler bleach
than sodium hypochlorite, causing less damage to fabrics and dyes, but by itself is only effective at high (>60ºC)
temperatures. Although solid chlorine bleaches exist, they are rarely used in laundry detergents.
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| Fig A1: effect of temperature on the bleaching performance of PBS with and without TAED (a typical activator) |
In practice today, the effectiveness of sodium perborate as a source of available oxygen at more moderate temperatures
(40 - 60oC) is regained by the use of activators. Activators such as TAED (tetraacetylethylenediamine) are precursors
for peracid bleaches formed in situ during the washing process. In general peracids, which are more powerful bleach
species, cannot be formulated directly into detergent powders for stability reasons and have to be formed in situ.
However, at the even lower wash temperatures found outside - and increasingly in - Europe, encouraged by energy
considerations, the performance of activated perborate falls away. It is expected however that new technologies,
based on bleach activators or catalysts, will in future promote successful perborate bleaching at these lower temperatures.
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The chemistry of sodium perborate bleaching
Sodium perborate, formed by the reaction of borax pentahydrate, sodium hydroxide and hydrogen peroxide, is commercially
available as the tetrahydrate (NaBO3 .4H2O, or PBS4) or the monohydrate (NaBO3 .H2O, or PBS1, formed by heating PBS4),
which dissolves faster than PBS4 at lower temperatures. They contain true peroxygen bonds, unlike, for instance, sodium
percarbonate or sodium perphosphate. In the case of percarbonate for example, the lack of a peroxocarbonate bond results
in a product which is inherently less stable. To enable this material to be safely handled in bulk, and to be used in
certain super compact heavy duty laundry powders, requires the use of various stabilizing coatings. Without these,
thermal run-away reactions and rapid loss of available oxygen would occur.
In the crystalline form, PBS exists as a dimeric cyclic peroxodiborate salt which in aqueous media rapidly
hydrolyzes to form an equilibrium solution of hydrogen peroxide, tetrahydroxy borate anions, and one or more
peroxoborate species. The equilibrium constants determining the hydrolysis imply that in practice sodium perborate
can indeed be regarded as a solid, stable source of hydrogen peroxide bleach.
Peroxoborate species such as (HO)3BOOH-, that is those in which the peroxo group remains coordinated to boron,
are stable in solution. Calculations show that under European laundry conditions with 0.01M borate and 0.01M
available oxygen, about one sixth of the available oxygen and borate is contained in peroxoborate species,
principally (HO)3BOOH-.
Peroxoborates may be better bleaching agents than hydrogen peroxide itself. The electrophilic nature of the
peroxygen groups is increased by the coordination to the boron centre. So, sodium perborate can be a superior
bleaching agent to hydrogen peroxide at equivalent, high temperatures.
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