Science: King of the solvent substitute
Sunday 13 December 1998
"The marketing men probably decided that the word supercritical would frighten the general public," says Dr Steve Howdle, a research chemist at Nottingham University. "But carbon dioxide is far healthier than the dry cleaning solvents which were once used instead."
Howdle and his colleagues are examining what happens when carbon dioxide is compressed to about 70 times normal atmospheric pressure. Under those conditions it becomes "supercritical" and the gas takes on some of the properties of a liquid. It can then be used as a solvent to carry out chemical reactions and make new materials. As an alternative to harmful organic compounds like carbon tetrachloride, it has potential applications in a range of industrial processes. Extracting the caffeine out of coffee beans is one of the first to be used on a commercial scale, but there are many others in the pipeline.
The big advantage of carbon dioxide is that when the taps on a pressure vessel are opened it changes back into a gas and floats harmlessly away, leaving behind a clean, dry product. When liquid organic solvents are used a small residue always remains. The characteristic smell of a new car is caused by the solvents used in making the plastic trimmings. The residue may be harmful to human health and the ozone layer.
Using supercritical carbon dioxide does not contribute to the Greenhouse effect, and much of the gas used is a natural by-product of the brewing industry. In the Nottingham team there are 30 scientists looking at new applications for the technology.
Howdle became interested in this field while an undergraduate in Manchester 12 years ago. He heard a lecture on new solvents given by Martyn Poliakoff. He had been hoping to do some form of postgraduate study and went up to Professor Poliakoff after the lecture and asked for a job.
He said: "What I like about it that I am taking real problems that people are having to deal with and looking for new ways round them." As a Royal Society University research fellow, Howdle is encouraged to collaborate with scientists abroad. He is working with a group of Russian scientists in using carbon dioxide to make plastics for medical implants. Some people fitted with dental plates for false teeth, he points out, find they are unable to wear them because residues of conventional solvents can cause allergies.
Increasingly stringent legislation has led to bans on some solvents and will increase the pressure on industry to develop more environmentally friendly processes. "The challenge is not just to develop a greener process, we must also try to make it better, cheaper, easier or more controllable."
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