US researchers use microwave technology to increase energy efficiency in plastic upcycling

Researchers at West Virginia University (WVU) in the US are working to convert PP back into its building block, propylene, which can be reused in new plastic products. The US Department of Energy is funding the project with a US$1 million grant.

Yuxin Wang, chemical engineer and assistant professor at WVU, tells Packaging Insights: “Our previous research indicates the advantages of microwave heating technology in certain manufacturing processes, such as lowering reaction temperatures, accelerating reaction rates, improving selectivity and enabling unique reaction pathways — such as ammonia synthesis, natural gas conversion and CO2 conversion.”

“These benefits encourage us to use microwave heating technology to recover propylene from polyolefins. We want to generate chemicals that can not only be used to manufacture PP again but can also be valuable in other reactions.” 

Efficient pathway for recycling

Traditional methods to recycle PP, such as pyrolysis, heat the plastic in an oxygen-free environment. The research, supported by Srinivas Palanki, chair of WVU’s Department of Chemical and Biomedical Engineering, utilizes microwave technology instead. 

“We make 150.3 million metric tons of propylene every year through energy-intensive processes like ethane steam cracking,” says Palanki.

“If we can recover usable propylene from PP, we’ll reduce energy and emissions throughout the lifecycle of these plastics, and we’ll improve productivity and efficiency for US manufacturers.” 

In the latest research, scientists use microwave radiation to first heat an intermediary catalyst, which then transfers the heat to the PP waste. This technique requires temperatures around 300°C — far lower than the 600-700°C needed in conventional thermal recovery methods. 

According to the scientists, the pyrolysis of PP typically results in a propylene yield of less than 25% with an optimized process. Moreover, the small amount that is recycled is confined to a certain set of uses.

“In recycling, you crush a water bottle into small pieces and then use those pieces to build a bottle again. Recycling limits your utilization,” says Wang.

Turning waste into material

According to the scientists, the challenge right now is to use microwaves to do it in a way that is cost-competitive and can be practically implemented.

APS has cutting-edge technology that we will use in this project (Image credit: APS at Argonne National Laboratory).“We need to diversify our strategies for dealing with the abundance of plastic waste. Currently, PP has a notably low recovery rate of only 1%, meaning 99% of PP products become garbage. We want to change that by using microwave irradiation to recover propylene from PP,” highlights Wang

The team’s ultimate goal is to “upcycle” PP by breaking it down into its original chemical components rather than grinding the tiny pieces of plastics into new products.

“It is a highly energy-efficient process. The propylene produced through this approach can sustainably meet the demand for polymers and other materials, reducing reliance on fossil resources,” he continues.

Another advantage of using microwaves as an energy source is that the technology enables precise and selective control over the process.

In the future, WVU will further the study with resources from the Advanced Photon Source (APS) facility at the Argonne National Laboratory in Illinois, US.

“As part of this study, WVU students will be able to use new techniques developed at APS for use in the Argonne ‘synchrotron,’ a kind of particle accelerator,” he says.

Earlier this year, US researchers developed a new chemical process capable of vaporizing plastics and converting them into hydrocarbon building blocks for new materials. This catalytic method breaks down PP into monomers.