Builders are employed principally to improve the cleaning performance of detergents by removing water hardness.
Some builders are able additionally to buffer pH and contribute directly to soil and stain removal.
Borates possess good all-round builder-assist properties, and when combined with certain organic builders interact
synergistically and often out-perform alternatives across the range of desired builder properties.
Sequestration
Water hardness
Hard water is detrimental to washing. Ions commonly responsible are calcium and, to a lesser extent, magnesium: they
have to be eliminated from the process or rendered inactive. Ion exchange, using standard zeolites for example, is
employed and recent advances have led to the development of new zeolites better able to deal with magnesium ions.
Alternatively, the calcium can be precipitated out of solution or it may be sequestered by a soluble compound that
does not interfere with detergent activity.
Obviously it is important to retain anionic surfactants (e.g. dodecylbenzenesulfonate, DBS; or linear alkylbenzenesulfonate,
LABS) in solution, and hence active, rather than being precipitated as a result of water hardness effects such as
Ca2+ + 2DBS-0 Ca(DBS)29 This precipitation is apparent when, without an added builder or non-ionic surfactant, Ca2+
concentration in solution exceeds approximately 50ppm at a surfactant concentration of 1.5g/l.
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| Fig B1: calcium ion sequestration by borax (0.025M) |
Sodium tripolyphosphate (STPP) is the classic detergent sequestrant builder, but has seen a decline in use. Borates
such as borax have a mild sequestrant ability, due to their divalent cation binding property. A 1:1 complex of the
borate with calcium or magnesium ions is believed to form:
Ca2+ + B(OH)4-0 CaB(OH)4+
with the stability constant KCa=18 (pKCa=1.25) at 40oC-.
KCa = [CaB(OH)4+] ÷
[Ca2+] [B(OH)4-]
When borates are present they behave as monovalent electrolytes and calcium ion sequestrants which combine to give
the surfactant much improved tolerance to calcium. Performance in this context is dependent on the tetrahydroxy borate
anion concentration in solution.
Soda ash (sodium carbonate, Na2CO3) is used in laundry detergents, acting as a precipitant builder to form insoluble
calcium carbonate. Consequently polymeric additives must be used in order to prevent excessive precipitation and build-up
of CaCO3 deposits on tableware in dishwashers, or on fabrics where they can contribute to mechanical damage of the fibers.
This is more likely to be a problem with sodium percarbonate than sodium perborate.
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Other ions
Borates are also able to complex with the transition, heavy and alkali metal ions as well as alkaline earth metal ions
such as Ca2+ and Mg2+, via the reaction:
Mn+ + B(OH)4-0 MB(OH)4(n-1)+
However, stronger complexing agents are generally added to detergent formulations to remove transition metal ions, for
example, to prevent the wasteful decomposition of the bleach species.
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| Fig B2: anionic surfactant (DBS- ion) precipitation; no salt, with borax, and with PBS1 |
Borate-polyhydroxycarboxylate complexes
These complexes have been found to have significantly enhanced calcium ion sequestering ability compared with that
exhibited by either compound alone: a synergy resulting from complexation. Some have also been observed to exhibit
secondary builder properties such as better soil dispersion. They offer availability from cheap, renewable organic
raw materials, and chemicals of this class are often biodegradable and present lower environmental impact.
Perborate-saccharate
A very high complexing capacity for calcium ions is known for mixtures of these: with sequestration almost equal to
STPP or NTA (nitrilotriacetic acid) on an equal weight basis. Analysis indicates that this is due to the formation
of a diester complex with a 1:2 borate:saccharate stoichiometry to which up to two Ca2+ ions can be bound.
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| Fig B3: calcium sequestration as function of sequestrant concentration |
Fig B4: the structure of a spiroboron complex (borate-saccharate) showing the sequestration of two Ca2+ ions |
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