Microwave tech targets polypropylene waste

According to the West Virginia University (WVU) team behind the technology, the method relies on microwave radiation to heat an intermediary ‘catalyst’ material, which then transfers the heat to the polypropylene waste. Rather than breaking the material down into tiny pieces, the process converts polypropylene back into propylene, which can be reused in new plastics or other products.

“Currently polypropylene has a notably low recovery rate of only one per cent, meaning 99 per cent of polypropylene products become garbage,” said research lead  Yuxin Wang, WVU chemical engineer and assistant professor at the university’s Benjamin M Statler College of Engineering and Mineral Resources. 

“We want to change that by using microwave irradiation to recover propylene from polypropylene.”

Used in everything from food packaging to car parts, polypropylene is the second most abundant plastic after polyethene. And with recycling rates so low, the WVU team claims new processes are essential to reduce the material’s environmental impact. 

“We make 150.3 million metric tons of propylene every year through energy-intensive processes like ethane steam cracking,” said Srinivas Palanki, chair of WVU’s Department of Chemical and Biomedical Engineering.

“These processes require very high temperatures. If we can recover usable propylene from polypropylene, we’ll reduce energy and emissions throughout the lifecycle of these plastics, and we’ll improve productivity and efficiency for US manufacturers.”

Efforts to date to separate propylene from polypropylene waste have largely relied on thermochemical approaches such as pyrolysis, where the material is heated in an oxygen-free environment. But according to Wang, polypropylene has largely resisted those efforts, which is why his research has focused on microwave energy as an alternative. As well as operating at a much lower temperature – 300°C versus 600-700°C for pyrolysis – the microwave tech also allows more precise control over the recovery process.

“The pyrolysis of polypropylene typically results in a propylene yield of less than 25 per cent even with an optimised process,” said Wang. “Our challenge is to use microwaves to do it in a way that is cost competitive and can be practically implemented.”