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updated 12/6/2010 6:48:45 PM ET 2010-12-06T23:48:45

Plant-based plastics appeal to green-minded consumers thanks to their renewable origins, but their production carries environmental costs that make them less green than they may seem, research says.

"The main concern for us is that these plant-derived products have a green stamp on them just because they're derived from biomass," said study author Amy Landis of the University of Pittsburgh. "It's not true that they should be considered sustainable. Just because they're plants doesn't mean they're green."

The work, by Landis and others and published in Environmental Science & Technology, traces the full impact of plastic production all the way back to its source for several types of plastics.

The technique considered each process's footprint for 10 different types of environmental impacts, from global warming potential to release of human carcinogens to smog to algae-choked lakes or eutrophication.

For bioplastics, the tracing incorporates, among other things, the environmental consequences of the chemicals and the energy required to grow the corn, soybeans or sugarcane that often comprise the plastics. For standard, petroleum-based plastics, the analysis includes the environmental impacts of extracting and refining the oil.

The researchers found that while making bioplastics requires less fossil fuel and has a lower impact on global warming, they have higher impacts for eutrophication, eco-toxicity and production of human carcinogens.

These impacts came largely from fertilizer use, pesticide use and conversion of lands to agricultural fields, along with processing the bio-feedstocks into plastics, the authors reported.

"I don’t want people to take away from this that biopolymers are bad. The problem is that there are problems with them," Landis said. "There is a lot of research going on to create biopolymers based on non-corn feedstocks," which would reduce these impacts, she said.

Using cellulosic sources of biomass, such as corn stalks, grasses, or woody plant parts would be better, agreed Michael Griffin of Carnegie Mellon University in Pittsburgh. "If instead of using something like corn, you move to something like cellulosic, that immediately gets you a reduction of impact. A lot of these cellulosic feedstocks use fewer chemicals and you get higher yield," he said.

Bio-based plastics face an uphill battle, because they are competing against a highly streamlined production process for petroleum.

"It is an amazingly efficient process in getting petroleum from the ground and into a feedstock," Griffin said. "If you then take the petroleum feedstock and convert it very efficiently into the polymer, it can have a very low environmental footprint."

One important limitation of the study is that the comparison did not include the relative impacts of disposal, Landis said, because the necessary data for these calculations was not available. This means bio-plastics did not get "credit" for being biodegradable, nor do certain types of petroleum plastic for being easily recyclable.

"That could drastically change the ranking of these polymers," Griffin said.

According to the study, polypropylene topped the team's list as having the least life-cycle impact. Polypropylene is used in a wide array of plastic products including textiles and medium-duty objects like remote controls, large plastic tubs or chairs.

High-density polyethylene and low-density polyethylene were the second and third place polymers. These polymers are widely used for plastic bottles and bags.

The biopolymer polyhydroxyalkanoate came next on the list. Polyvinyl chloride (PVC) and polyethylene terephthalate (PET) were ranked last.

But taking the rankings at face value has its limits, Griffin noted. The rankings weigh each environmental impact equally, but different manufacturers or consumers may have different priorities for what is most important, whether it be global warming potential or toxin release.

"They're important to different people," Griffin said. "People are going to make different decisions based on how they weigh things."

© 2012 Discovery Channel

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