Polystyrene has proven valuable in numerous packaging and other applications but also contribute to the persistence of plastics in the environment. Efficient routes are required to recycle these plastics, reduce dependence on virgin materials and advance low-carbon economies. A chemical process based on pyrolysis is being promoted to deliver these varied benefits.

The pyrolysis process devised by researchers from Massachusetts-based Worcester Polytechnic Institute and the University of Bath, U.K., subjects the material to temperatures exceeding 450° C in an oxygen-free chamber, ensuring that it does not ignite. The polystyrene is decomposed into monomers, which can then be purified and reconstituted into virgin polystyrene. Creating 1 kg of the new material consumes less than 10 megajoules of energy, which is enough to power a microwave for around 30 minutes.

The technology described in Chemical Engineering Journal relies on a pyrolysis reactor, a heat exchanger and a pair of distillation columns that separate the parts of polystyrene into monomer grade styrene to be reformed into polystyrene. Light and heavy petroleum-like by-products are also produced for reuse in other ways.

For every 1 kg of used polystyrene, 600 g of 99% pure monomer grade styrene are left available to manufacture new polystyrene, decreasing the consumption of fossil fuels. This translates into a 60% yield and reduced carbon dioxide emission control costs relative to other recycling options.

According to University of Bath researcher Bernardo Castro-Dominguez, “Chemical recycling techniques are a major focus within chemical engineering right now, and cost- and energy-efficient ways to breakdown plastics to their primary building blocks such as polystyrene are urgently needed. Less than 5% of polystyrene is recycled at present — our work shows that as much as 60% of all polystyrene used today could be replaced by chemically recycled styrene.”