Cleaning up the plastic act

THE growing use-and-throw urban lifestyle has spurred the demand for plastics, which, unfortunately, are non-biodegradable substances that pile up in landfills. Moreover, making plastic requires hazardous chemicals and solvents that end up as large quantities of pollutants and toxic wastes.

This where a "split-personality" molecule has come in help. Joseph DeSimone and his colleagues at the University of North Carolina in Chapel Hill have taken the lead with a new method that obviates the need for harmful solvents (Science, Vol 265, No 5170).

In conventional methods of making plastic, organic solvents or water is used. The main environmental problem is posed by the leftovers in the solvent and even if the original solvent is ordinary water, it gets contaminated and cleaning it up is a major problem.

DeSimone's team uses a new kind of solvent, called super critical carbon dioxide, which eases the cleaning stage by escaping like a gas. Transforming the inexpensive gaseous carbon dioxide into its supercritical form can be done at room temperature.

Although the scientists have proved the merit of their innovation on fluoropolymers, a common plastic, their method has run into a bottleneck with other polymers, which have poor solubility in carbon dioxide. Say DeSimone and his colleagues, "Most heavier polymers are not soluble to any appreciable extent, fluoropolymers and silicones being exceptions."

DeSimone's group has now found an interesting alternative to overcome this problem by adding a molecule that retains polymer in the solution. The scientists designed a detergent-based "stabiliser" molecule with a "split-personality", part of which is attracted towards the carbon dioxide and the other with an affinity for polymer molecules. This dual character enables the stabilisers to hold the polymer in the carbon dioxide.

"This strategy is not only cleaner than conventional techniques but also gives chemists more control over polymer size," says Keith Johnston, chemical engineer at the University of Texas. Polymer growth in other solvents has a tendency to go random when a stray solvent molecule binds to the end of the growing polymer chain. Moreover, says Johnston, "carbon dioxide can slip in between the molecules of the growing polymer and make it like a sponge full of holes".

Although carbon dioxide is one of the cheapest solvents, a disadvantage with this technique is the need to run the polymerisation at high pressure. Says DeSimone, "High pressure is always capital-intensive. But if the clean-up savings outweigh the added capital costs, there would be zero emission of hazardous volatile organic compounds in the atmosphere."

Science & Technology