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Oranges a building block for 'greener' plastic

A promising plastic without petroleum

By Marsha Walton

Limonene makes up about 95 percent of the oil in an orange peel.
Cool Science
Synthetics and Plastics
Cornell University

ATLANTA, Georgia -- Cornell University chemists are looking for ways to take the petroleum out of plastics. And nature has provided one green alternative, in the form of oranges.

To create their new plastic, known as polylimonene carbonate, the scientists used molecules from two sources, carbon dioxide and limonene oxide, which comes from the peel of citrus fruits and other plants.

"Limonene has been around forever, carbon dioxide as well. What we've been able to do is devise this catalyst for the first time that allows these two small molecules to come together to make a plastic," said Cornell chemistry professor Geoffrey Coates.

The catalyst used in the chemical process is a compound that contains a small amount of zinc. The zinc in the catalyst is the type found in the common sunscreen zinc oxide.

The limonene oxide liquid, the CO2 gas, and the catalyst are combined to make a white powder. That powder can be melted into things typically made from plastic, such as disposable cups.

This new plastic has some properties similar to polystyrene, a petroleum-based plastic that's in hundreds of household products.

The Cornell research team, including Scott Allen and Chris Byrne, are testing to see how sturdy it is and how it might hold up to heat and cold. They are also evaluating its biodegradable properties.

Details of this orange-based alternative are published in the Journal of the American Chemical Society.

The research is funded by the National Science Foundation.

Coates says the discovery is stirring a lot of commercial inquiries.

"I'm getting deluged by people wanting to know more," Coates said. "It's fun to talk to people envisioning other uses for this."

Researchers in Florida say they are always looking for beneficial uses for citrus byproducts.

"Basically when an orange comes into a juice processing plant, 50 to 60 percent is juice, the rest is waste," said Robin Bryant, research manager for the Florida Department of Citrus.

Bryant says sometimes orange peel is dried and added to cattle feed. Limonene already is an ingredient in many cleaning products, she said, and scientists are researching ways to create ethanol from citrus waste.

Polylimonene carbonate could possibly be used as a less toxic ingredient in making cast iron and cast aluminum, or as an ingredient in some herbicides or pesticides.

But even with growing commercial possibilities for renewable resources, Coates cautions the route from lab to store is long.

"In terms of a timeline for discoveries being converted into commercial products, these numbers can vary from months to years, or in some cases, never," he said.

And it is unlikely that these green alternatives will ever completely replace the huge quantities of oil-based plastics in the world.

"Last year the world production (of limonene) was around 150 million pounds. This will never replace polymers such as polyethylene. Last year just in North America we made 42 billion pounds of polythylene," said Coates.

But nature seems to have an abundance of other possible building blocks for new polymers, from fruits to corn to pine trees.

"We're really excited about not only using a byproduct from the orange juice industry but also from the paper industry," said Coates. When timber is processed into paper, one byproduct is a compound called pinene, which gives pine cleaning products their smell.

The scientists are also trying to find out if they can combine carbon dioxide with compounds made from vegetable oil, which is abundant and very inexpensive.

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