A new type of plastic that turns into a gas when heated and reforms into a solid when cooled has been developed by scientists at the University of Surrey, opening up a new approach to processing and recycling polymers.

In a study published in Macromolecules, researchers introduce a novel polymer that could simplify how materials are applied, removed and recycled by eliminating several of the complex processing steps used today.

Conventional plastics such as polyethylene, used in everyday products such as carrier bags, food packaging and shampoo bottles, are persistent and not suitable for chemical recycling. While other chemically recyclable polymers have been developed, they typically require much higher temperatures of around 150–200 degrees Celsius to break down, and the recovered material often needs additional chemical processing before it can be turned back into a plastic.

The Surrey team's new material is similar to polyethene in that it is soft, insoluble and hydrophobic, but it can be heated to just 90 degrees Celsius, where it undergoes efficient depolymerisation – breaking down into its individual building blocks, known as monomers. Unlike most depolymerisation processes, where the recovered monomers are liquids, Surrey’s material forms a gas instead. As the vapour cools, it spontaneously reforms into the original waterproof polymer with the same properties as before.

Dr Peter Roth, Senior Lecturer in Applied Organic and Polymer Chemistry at the University of Surrey and corresponding author of the study, said:

"Most plastics are designed to be stable, which is exactly what makes them difficult to recycle or remove. We've shown that it’s possible to create a material that behaves very differently – one that can transform into a vapour at relatively low temperatures before naturally rebuilding itself into the same polymer.
"This isn't a replacement for conventional plastics, and it's certainly not a solution to the global plastic waste problem. But it does introduce a new concept that could inspire an entirely new generation of circular materials."

To demonstrate the concept, the Surrey team laid out three potential applications.

First, they created waterproof polymer coatings by allowing the polymer vapour to condense onto a surface. They then showed that these coatings could be removed simply by reheating them, causing the polymer to evaporate. Finally, the researchers demonstrated that a contaminated polymer could be purified through sublimation, separating the polymer from a model additive before it reformed as a clean solid.

Touseef Kazmi, postgraduate researcher at the University of Surrey and lead author of the study, said:

"The exciting part is that we've demonstrated a principle that wasn't previously available. If we can learn how to tailor this chemistry, it could eventually lead to new materials that are easier to apply, remove and recycle than many of today's plastics."

The work provides a proof of concept for a new class of circular materials and could eventually enable new ways of applying and removing waterproof coatings, particularly for applications where liquid coatings struggle to cover complex surfaces.

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