New Stabilisers
Although existing stabilisers are effective,
their cost makes them unsuitable for large-scale commercial applications.
They are polymers containing silicon or fluorine, and are expensive
to prepare.
A team of researchers at the University of Pittsburgh, lead by Professor
Eric Beckman, have produced stabilisers made from commonly available
materials like propene, ethene and carbon dioxide. Not only are they
much cheaper to make, they can be used at lower pressures and are biodegradable.
These materials will allow a wider range of substances to be dissolved
in scCO2, paving the way for extending its use as a solvent
for polymer production, pharmaceutical manufacture and general synthesis.
The significance of this development was recognised by the 2002 Presidential
Green Chemistry Award in the US.
Poly(ether-carbonate) is one of the new stabilisers produced to help
extend the use of scCO2.
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Supercritical
carbon dioxide (scCO2) is far from being a universal
solvent. It will not easily dissolve materials with high molecular
weight (like polymers), but the addition of stabilisers allows polymerisation
to be carried out in scCO2.
The stabiliser works very much like a detergent - one end of the
stabiliser molecule will attach to the polymer, but is insoluble
in scCO2 (often described as CO2-phobic).
The other end is soluble in scCO2 (and is described as
CO2-philic). This means a cluster of stabiliser molecules
surrounds each polymer particle, enabling it to remain dissolved
in the scCO2.
After removing the solvent, the result is a fine polymer powder,
ideal for use in a range of moulding processes. The use of scCO2
in this type of polymer manufacture will reduce the use of hydrocarbon
(organic) solvents, an important aim for greener chemistry. |
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