A more effective way to recycle plastic waste

Plastic pollution is a global crisis. Very little plastic waste is recycled; most ends up in landfill, some is incinerated, and some enters the environment where it can break down into microplastics. Microplastics are now found in every habitat on Earth, as well as inside our bodies and brains, with potential long-term health impacts. By recycling plastic, we could reduce use of fossil fuel resources and limit the amount of plastic that enters our environment.

But plastic waste recycling is difficult; the polymers that form plastics resist recycling treatments and can degrade after each use, making them difficult to reuse. Polypropylene, the second most-commonly-used plastic, can be recycled, but widely used mechanical recycling techniques do not produce high quality outputs. This is especially true of non-woven polypropylene, such as that used in disposable face masks, which have a fibrous structure and often include additives and colourants that can contaminate the recycled plastic.

Led by London South Bank University with collaborators from The Open University, ISIS and Loughborough University, the researchers investigated whether ionic liquids could provide an alternative route for recycling polypropylene. Unlike mechanical recycling, which can degrade the polymers that make up the plastic, chemical recycling can remove impurities and recover higher-quality polymers. However, some current methods require high temperatures or toxic, flammable and non-renewable solvents.

Ionic liquids are salts with low melting points, and many are liquid at room temperature. They are attractive for recycling applications because they are often less flammable, can show high thermal stability, and researchers can tailor their chemical structures for specific uses. For their project, the team designed an ionic liquid tailored to polypropylene by combining an organic base with an organic acid.

The researchers first identified the optimal conditions for dissolving new polypropylene in the ionic liquid; at around 160 ºC, the plastic dissolved in less than one minute. The process was then repeated with polypropylene from disposable facemasks. Initially, the recovered polypropylene was tinted blue from colourants in the face masks, but the researchers found they could remove the contamination using a hot filtration step.

Using quasi-elastic neutron scattering, the researchers showed that the dissolution process happened in stages. As the temperature increased, interactions between the polymer and the ionic liquid became stronger, increasing the mobility of the polymer chains and helping the polypropylene dissolve. The process was similar for both virgin polypropylene and polypropylene recovered from the masks. Further analysis showed the properties of the recycled plastic closely matched that of the new plastic.

Overall, the process recovered around 99% of the polypropylene from the mask material. The team also showed the ionic liquid remained thermally and chemically stable during use, suggesting it could be reused.

The process could also be applied to other polypropylene-containing waste streams beyond disposable face masks. More broadly, the ability to chemically tailor ionic liquids means that similar approaches could be developed for other types of plastic waste.

Related publication: Rapid polypropylene recovery using ionic liquid: Achieving 99% recycling efficiency with mechanistic insight from neutron scattering – ScienceDirect