Mixed, unsortable polyester waste. Credit: Fabrice Esnault, ESPCI Paris
Researchers have developed a new method to break down complex polyester waste like fibres and textiles into reusable building blocks, without having to sort it beforehand.
These building blocks can then be repolymerised into high-quality plastics, contributing to creating a circular polyester economy.
“Looking to the future, our finding … suggests that blending rather than sorting, which is often costly or impossible, may be the way forward,” write the authors of the study published in Proceedings of the National Academy of Sciences (PNAS).
The approach adds a new step to “enzymatic depolymerisation” – an environmentally friendly technology which uses enzymes, water-based media and mild conditions to break down polyesters.
Enzymatic depolymerisation has proved useful for the industrial-scale recycling of easily sortable and unformulated polyethylene terephthalate (PET) waste, like PET bottles. However, it does not work for unsortable, mixed waste such as PET fibres and textiles, or other common polyesters such as polybutylene terephthalate (PBT).
Researchers in France have now found that a simple melting step can fix this problem.
The new method takes advantage of catalysts which are used in the polyester manufacturing process and remain within the waste as residues. When heated 270°C the residual catalyst in the waste remained active and transformed a blend of different polyester wastes into “transesterified random copolymers”.
These copolymers could then be broken down using enzymatic depolymerisation to produce polyester building blocks suitable for the manufacture of new plastic products.
Longer mixing times and the addition of an epoxy cross-linking agent improved the technique’s effectiveness to achieve yields as high as 90%.
“We have shown that it is possible to depolymerise particularly fast-crystallising, unsortable wastes consisting of a mixture of different polyesters, such as those from textiles made of different or composite fibres, or mixtures of fibres used to fill coats or furniture,” the authors write.
“This finding is relevant in the context of today’s use of blended polyester fibres to ensure comfort, durability, and performance on the one hand and the use of recycled textile fibres as fillers on the other.”