In addition to their chemical properties, borates can be used to adjust the physical characteristics of detergent formulations.
Carriers
Incorporating liquids into powder formulations often requires a carrier to maintain flowability. It may also be desirable
for stability in storage to separate some components from others by means of a physical barrier. Puffed or expanded borax
penta- or decahydrate have this ability, and may also be used as low-cost bulking agents which will benefit the wash in
many other ways.
When heated above 62oC borax first dissolves in its own water of crystallization and then, as the water is driven off, it
swells into a voluminous, spongy mass. Its bulk density when puffed can be from 64 to 640 kg/m3.
Puffed borax has a honeycomb structure, largely void, depending on the degree of expansion and can be processed into a
powder of particle size 0.25 - 0.5mm. Surface area and porosity increase as bulk density decreases. The particles have
a reasonable resistance to mechanical breakdown which is much improved when other substances are absorbed into the void spaces.
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| Fig H1: puffed borax volume is many times that of the original borate. Each cylinder contains 25g borax. |
The particles also have a large surface area which aids their eventual dissolution rate in water.
Large amounts of liquids and solids can be loaded into puffed borax, both resulting in a dry free-flowing powder. Liquids
may be applied directly; solids can be melted and then applied or introduced via a volatile solvent. Because of the aqueous
solubility of borax, the use of water as the applied liquid is limited to the equivalent (original) amount of water of
crystallization of borax.
Puffed borax can accept high loadings of materials: loadings of 90% trichloroethylene, 50 per cent cyclohexane and 15 to
40 per cent higher viscosity liquids such as nonionic surfactants have been reported. Non-ionic, cationic and anionic
surfactants can all be successfully loaded.
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The loaded product has excellent flow properties, and borax itself will also dissolve quickly to provide useful buffering,
solubilization, stabilization and other functions.
Density modification
A relatively small quantity of puffed borax can enable the formulator to tailor product density, with borates first being
incorporated into detergent granules using dry mixing agglomeration methods.
Subsequent ‘puffing’ of the granules can then give a final low-density product similar to that produced by traditional
spray-drying technology.
Borates can also function as binding agents, for example in the production of detergent tablets.
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| Fig H2: scanning electron micrograph of puffed borax |
Rheology control
Control of viscosity or the development of viscoelastic properties is necessary in liquid detergent formulations to confer
appropriate aesthetic qualities: thickness and ‘richness’ are perceived in the market to be related to each other and
desirable. More scientifically, structured liquids contain insoluble particles in suspension, such as zeolites. It is
essential, therefore, that in storage these materials are prevented from settling out by controlling the rheological
characteristics of the medium.
Once in contact with water however, built-in viscosity must be destroyed so the ingredients can disperse quickly and hence
become active. The unique reversibility of the borate-induced thickening phenomenon facilitates this.
The increased viscosity found in borate systems for liquid detergent formulations results from their ability to cross-link
with polyhydroxy polymers, particularly those with an appreciable number of cis-hydroxyl groups such as guar gum, xanthan
gum and polyvinyl alcohol.
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