A team of US chemists has found a way to convert unrecyclable mixtures of plastic into useful chemicals.
Mixed materials usually end up in landfill because they are very difficult to recycle. A shirt made from 40% polyester and 60% cotton, for instance, has plastic and cotton fibres wound very tightly together. The fibres must be laboriously unwound, or taken through a number of energy-intensive chemical reactions before the plastic and cotton can be separated and used again.
It leaves behind a soup of carbon-based chemicals which can then be turned back into plastic, or into other materials.
The research is published in Materials Horizons.
But the researchers successfully removed and recycled the plastic in a number of mixed materials in a single reaction, including in a fabric that’s 40% polyester and 60% cotton.
The catalyst is an “organocatalyst”: made mostly from carbon and hydrogen atoms, and lacking the precious metals that many catalysts rely on. It has the catchy name TBD:TFA.
When heated to specific temperatures (between about 100-200°C, depending on the substances), the catalyst could break down four different plastics the researchers tested: polycarbonate, PET, polyurethane and polyamide in under 2 hours.
In each case, it rendered “monomers” of each plastic: the individual molecules that, when linked together, form the long polymer chains that give plastic its properties. These monomers could be turned back into plastics or used to make other chemicals, like dyes or pharmaceuticals.
The catalyst left behind other polymers, like the cellulose that makes up cotton. It could also be re-used with ease.
“The selective deconstruction of mixed plastics by TBD:TFA provides a feasible path for implementing in a practical large pilot scale deconstruction plant,” write the researchers in their paper.
They estimate that using their catalyst could lead to “a more than 80% reduction in energy and carbon footprint”.
“This concept offers highly efficient and low-carbon chemical recycling of plastics and presents a promising strategy toward establishing closed-loop circularity of plastics,” says corresponding author Dr Tomonori Saito, a chemist at Oak Ridge National Laboratory, US.