More Plastics May Be Recycled with Plasma

In most municipalities, it is difficult, if not impossible, to recycle common plastics used in cling wrap and retail bags. Those types of plastics are mainly made up of polyolefins, which are thermoplastics derives from oil, natural gas, or renewable sources such as sugar cane, according to Plastics Europe, a trade association that advocates for change within the plastics industry.

They’re called single-use plastics—things like plastic bags, coffee stirrers, and plastic packaging—that are only used once before they’re thrown away. The world produces more than 300 million tones of plastic each year, half of which is single-use items, according to the Natural Resources Defense Council (NRDC).
 
Other types of single-use plastics, such as soda bottles, break down over years into tiny particles and release toxic chemicals as they break down, according to the NRDC.

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Researchers at Iowa State University are looking at a new way to recycle by studying how to use plasma technology to turn plastic film waste into biodegradable polymers that return naturally to the environment. Plastics are, after all, chains of synthetic polymers.

The method would also cost less than the few ways currently available to recycle the films, said Xianglan Bai, a professor of mechanical engineering at Iowa State who led the project.  

The researchers’ nonthermal plasma-based technology breaks the films down into base chemicals, most of which can be converted into high-value chemicals, Bai said.

Nonthermal plasma—also called cold plasma—is unique because it mainly produces hot, highly energetic electrons by applying an electric field to a gas. The resulting particles are great for blasting apart the chemical bonds of plastic, said researchers at the University of Newcastle, England, who are also studying cold-plasma recycling.

Because of its high chemical reactivity, it doesn’t need much energy to create chemical reactions, Xianglan added.

Plasma has already been used for plastic-waste recycling, but that process occurs at very high temperatures above 3,000 °C. It also requires a complex and energy intensive cooling system, said Ahn Phan, a professor of chemical engineering at Newcastle University who led a cold-plasma recycling project there.

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The Iowa State researchers are investigating the most efficient plastic-films recycling technique by combining their plasma technique with traditional biological recycling methods, Xianglan said.  

“Cold-plasma and traditional recycling can help cut the amount of plastic waste clogging landfills,” she added.

At Newcastle, Phan and her group are investigating a process for cold plasma pyrolysis that operates at 500 °C to 600 °C. It combines conventional heating and cold plasma, which means the process requires much less energy. Pyrolysis is the process in which materials are combusted in a low-oxygen environment and converted into gases such as hydrogen, methane, and ethylene.

The process can be used to recover ethylene from high-density polyethylene (HDPE), which is used in objects such as plastic bottles and piping, Bia added.

Both hydrogen and methane can be used as clean fuels as they only produce minimal amounts of harmful compounds such as soot, unburnt hydrocarbons, and carbon dioxide. Ethylene is also a building block in most plastics used today, Phan added.

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They recently tested the results of their cold plasma technique on plastic bags, milk, and bleach bottles collected by a local recycling facility in Newcastle, Phan said.

The Newcastle researchers found that 55 times more ethylene was recovered from HDPE using their cold-plasma method as compared to conventional pyrolysis. About 24 percent of plastic weight was converted from HDPE directly into valuable products, she added.

With cold plasma plastics recycling, it may yet be possible to realize the true value of plastic waste, and turn it into something clean and useful, Phan said.

Jean Thilmany is a technology writer in Saint Paul, Minn.