Chemical engineers in US, UK developing polystyrene recycling process

A team of chemical engineers based at the University of Bath in the United Kingdom and Worcester Polytechnic Institute in Massachusetts are developing a technique for recycling polystyrene (PS) that could make the material reusable.

The engineers also say their process could be the first to make recycling PS both economically viable and energy efficient.

Explained in a new research paper published in the Chemical Engineering Journal, the technique uses pyrolysis to break down PS into parts which can be reformed into new pieces of the material.

“Chemical recycling techniques are a major focus within chemical engineering right now, and cost- and energy-efficient ways to break down plastics to their primary building blocks such as polystyrene are urgently needed," says Bernardo Castro-Dominguez, a senior lecturer in chemical engineering at the University of Bath and a co-director of the Center for Digital, Manufacturing & Design (dMaDe).

“Less than 5 percent of polystyrene is recycled at present. Our work shows that as much as 60 percent of all polystyrene used today could be replaced by chemically recycled styrene.”

The engineers say PS can be chemically recycled using heat, but repeated treatments downgrade the material, causing it to lose strength and flexibility. They claim that because the process requires specialized facilities, most recycling centers do not accept PS, and because of its bulk, high transport costs mean it is rarely moved to these facilities. Consequently, very little PS is recycled at present.

They add that pyrolysis involves exposing a material to very high temperatures of more than 450 degrees Celsius in an oxygen-free chamber, meaning it cannot ignite. Instead, PS breaks down into monomers, which can then be purified and subsequently reconstituted into virgin PS. Creating 1 kilogram of the new material requires less than 10 megajoules of energy—roughly enough to power a typical microwave for around 30 minutes.

“Our analysis finds polystyrene to be an ideal candidate for a chemical recycling process," says Michael Timko, professor of chemical engineering at the Worcester Polytechnic Institute. "Surprisingly, the process is energetically efficient and potentially economically competitive. In terms of emissions, investing in this process has the potential to be equivalent to simple measures such as energy conservation in terms of the amount of emissions reduction that can be achieved for a given investment.”

The researchers say that policies to incentivize consumers to recycle PS or divert it from landfills would help make the process even more economically attractive.