New physical forms of perborate
A recognized benefit of sodium percarbonate (PCS) is its higher available oxygen density (mass of available
oxygen in a given volume) compared with either of the perborates (mono- or tetrahydrate). This has tended to favor
sodium percarbonate usage in premium super compact heavy duty laundry powders and tablets, where low volume dosing
is an important feature.
Studies carried out by Borax and by other groups have, however, led to the development on a laboratory scale of forms
of perborate which have improved Avox densities, while maintaining other vital properties such as dissolution rate,
bleaching performance, moisture uptake and stability. The physical properties of the densified perborate and those of
the existing commercial persalts are shown in the table below:
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| Fig A10: Avox content and bulk density of PBS1, PBS4, PCS and densified PBS |
The peroxy compound choice
Stain removal remains the consumer’s key requirement in laundering. Peroxygen bleaches (perborate and percarbonate)
are employed for this purpose in heavy duty laundry powders. When compared on an equivalent available oxygen basis,
reference detergents containing perborate/TAED and percarbonate/TAED deliver very
similar bleaching performances under controlled test conditions. Only at temperatures below 40ºC is there some evidence
of a minor performance benefit from percarbonate.
Moreover, studies conducted by Borax reveal that under TAED-activated, unbuffered conditions where pH might vary,
sodium percarbonate performance against the tea stain falls short of both PBS4 and PBS1. This is due to the higher
pH of PCS wash liquors (pH 10 for PCS vs. pH 9.5 for PBS). Stain removal with peracids declines at high pH for many
stains and this explains the poorer performance of PCS under these conditions.
Nor is percarbonate able to match the versatility of perborate activity (as builders, stabilizers, solubilizers and
pH buffers) described in this publication.
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| Fig A11: tea stain bleaching with TAED-activated PBS and PCS; unbuffered conditions |
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| Fig A12: effect of pH on peracid bleaching performance |
Storage stability
It is important for a detergent to maintain its available oxygen on storage. In tests under stressed conditions,
perborate demonstrates the superiority of its true peroxygen bonding over percarbonate, whose H2O2 is held in the
crystal lattice. Here, 50:50 (wt.%) mixtures of commercial persalts with zeolite A builder were stored at 30ºC and 70%
relative humidity (RH). Measurement of percentage decomposition (residual available oxygen) over time reveals that
although coated percarbonate is more stable than uncoated samples, it fails to match the performances of both sodium
perborate mono- and tetrahydrate. Better results are reported with phosphate builders, but these are not currently
used in detergents on a Europe-wide basis.
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| Fig A13: stability of PBS and PCS in detergent powders under stressed conditions |
Non-oxidative bleaching
In conjunction with peroxide/peracid bleaching activity, borates other than sodium perborate can act as important
nonoxidative bleaches. This has been found to be due to the ability of borate to solubilize stains by forming
negatively charged complexes. These are more water soluble, and as the fabric surface is also negatively charged
in the high-pH wash liquor, electrostatic repulsion helps remove the stain from the fabric. Additionally, borates
have the ability to inhibit the adsorption of stain back onto the fabric surface.
Many stains are of vegetable origin and comprise, in the main, polyphenolic components with polyhydroxy functionality.
These are known to interact with the tetrahydroxy borate anion in solution to form borate ester complexes, and because
these are negatively charged, they will be electrostatically repelled from the similarly charged fabric surface. This
is particularly evident in red wine stain removal.
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