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Making Plastics from Plants May Waste Energy

An analysis presented at the meeting of the American Chemical Society in New Orleans Monday shows that in some cases, "green" manufacturing may be more energy-consuming -- and more polluting -- than traditional methods. The study was done by Assistant Professor Tillman Gerngross of the Thayer School of Engineering at Dartmouth College.

Conserving enough natural resources to support future generations is the goal of sustainability. An approach that has intuitive appeal is substituting renewable resources for finite ones.

Petroleum -- a finite resource -- is the basis for polymers known as plastics. That fundamental dependence on fossil fuels has caused critics to call the manufacture of plastics unsustainable and prompted the industry to explore manufacturing processes that substitute renewable resources -- such as plants -- for petroleum. Plants can be turned into plastics through fermentation.

Gerngross examined the process by which maize, or corn, is turned into plastics called polyhydroxyalkanoates, or PHAs. These renewable resource-based plastics have properties similar to polystyrene, a conventional plastic used extensively in the food packaging and fast food industry.

Of the alternative polymers that have been developed, PHAs have been considered one of the leading candidates to replace conventional plastics on a large industrial scale. In addition to being synthesized biologically, the final products are fully biodegradable.

To make polyhydroxyalkanoates, maize 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," Gerngross says. "By focusing on the origin of the raw materials we have lost track of the energy it takes to move materials through a process."

To arrive at these figures, Gerngross calculated the indirect or latent use of fossil fuels for generating the power used in manufacturing, and the net energy required to produce the fertilizer, insecticide and herbicides used in growing maize as well as the energy required to harvest and process the crop.

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. And the difference doesn't stop there.

"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 says.

Should manufacturers abandon the use of plants in the production of plastics? "Absolutely not," he says. "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."

[Contact: Tillman Gerngross ]

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