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Plastic from plants called costly

A biodegradable plastic can be created from cells contained in fermented plants such as corn.  

August 27, 1999
Web posted at: 12:52 p.m. EDT (1652 GMT)


It is chemically possible to turn plants into plastics through the process of fermentation — it's just energy intensive and causes a lot of pollution, according to research presented Monday at a meeting of chemists in New Orleans, La.

Plastics derived the old-fashioned way, via petroleum, are more environmentally benign, Tillman Gerngross, a professor of engineering at Dartmouth College, said at the American Chemical Society meetings.

Gerngross examined the process by which corn is turned into plastics called polyhydroxyalkanoates (PHAs). The renewable resource-based plastic has similar properties to polystyrene — a widely used plastic that keeps fast-food warm and compact disks safe.

Of the alternative plastics that have been developed, PHAs are one of the leading candidates to replace conventional plastics on a large, industrial scale. They are made from plants and are 100 percent biodegradable.

"If we can get away from a plastic that is based on petrochemicals to something that is renewable, that would be fantastic," said Gerngross. "I did not expect this to have this sort of outcome."

To find out why PHAs are not the dream plastic, Gerngross looked at what it would take to actually produce the product. "If you add it all up, there is an enormous amount of energy."

The corn must be grown, harvested and transported to a processor where its glucose is extracted and fermented into cells containing PHA. The cells are washed, spun in a centrifuge and broken apart to release the PHA, which is again washed and centrifuged, then concentrated and dried to a powder.

Gerngross found that this process consumes 19 times more electricity, 22 percent more steam and seven times more water than the chemical method of manufacturing polystyrene.

"Using plants as the basis of polymer production sounds like a sustainable solution, but in this particular case the increased energy consumption has offset any benefits from switching to a renewable raw material," he said.

He found that the total energy needed to produce one pound of PHA is equivalent to the consumption of 2.39 pounds of fossil resources. Producing the same amount of polystyrene using chemical manufacture requires only 2.26 pounds of oil.

"In the fermentation process, the entire 2.39 pounds would have to be burned to produce energy; whereas in the chemical process only 1.26 pounds would be burned. So the polluting effects of the 'green' approach are also greater," he said. "Your net carbon dioxide emissions are going to be twice as high."

With international campaigns to reduce emissions of carbon dioxide on the horizon, such as the Kyoto Protocol, Gerngross believes the scientific community has lost perspective on just how much energy it takes to produce the renewable products.

Nevertheless, he is not saying that the use of plants to make plastics should be abandoned.

"It would be most unfortunate if this study were viewed as a general indictment of biological processing. We now have the tools to look at the environmental impact of biological processes. We expect some processes to fail this analysis but we anticipate others will demonstrate superior performance. We will be looking for both."

Copyright 1999, Environmental News Network, All Rights Reserved

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Tillman Gerngross
American Chemical Society
American Plastics Council
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