Sustainable catalyst for polyester recycling could revolutionise the fashion industry.
Researchers in Denmark have cooked up a solution for recycling mixed polyester fabric by heating the textile with hartshorn salt, ammonium bicarbonate NH4HCO3 – a traditional leavening agent, also known as bakers ammonia.
The team at the University of Copenhagen opted to tackle blended cotton and polyethylene terephthalate (PET) – a complex material that is difficult to recycle (ACS Sustainable Chemistry & Engineering, doi: 10.1021/acssuschemeng.3c03114).
In 2020 alone, over 109m t of textile fibre were made, with polyester accounting for 52% and cotton 24% of volume. Yet less than 20% of textile waste is recycled, the researchers note. Cotton-polyester blends are especially challenging, interweaving a natural plant material with a synthetic polymer.
Chemical recycling is one way to solve this problem, says JiWoong Lee, the organic chemist at Copenhagen who led the research. Most chemical recycling involves elevated temperatures of 160 to 220°C and metal catalysts – rather than more benign enzymes – which then must be removed from the process during a purification step.
‘We use simple compounds like ammonia and carbon dioxide, says Ji-Woong Lee. ‘Both are gas molecules and so after the reaction they evaporate. This allowed the researchers to capture and re-use these simple catalysts.
The researchers submerged a t-shirt in ethylene glycol – a nonpurified solvent that can be bought in a hardware store. They added ammonium bicarbonate and heated the reagents for 18-24 hours under mild conditions of pH 7-8 and 160°C.
The CO2 produced triggers the activation of the ester functional groups on PET, while NH3 activates the hydroxyl groups on ethylene glycol. This is the first demonstration, say the researchers, of a synergistic effect of catalytic CO2 and an amine in glycolysis.
The combination causes glycolytic breakdown of PET to generate high purity bis(hydroxyethyl)terephthalate (BHET), an oligomer of terephthalic acid monomer and ethylene glycol. BHET can be used in existing PET manufacturing processes or for making polyurethane foams, copolyesters or acrylic coatings.
According to the study's author, this approach paves the way for large-scale chemical recycling and the upcycling of blended composites and PET-containing waste. Especially attractive was that the cotton fabric was not significantly hydrolysed; usually, under acidic conditions cotton and other cellulose-based materials are prone to degradation via hydrolysis.
Lee says that the group now aims to lower the reaction temperature, to generate higher quality cotton, and to replace ammonia with a superior base catalyst.
‘This work has two interesting features, says Susannah Scott, Chair of sustainable catalytic processing at the University of California, Santa Barbara, US. ‘The catalytic depolymerisation of the PET component leaves the cotton mostly intact, and the catalyst itself does not contaminate the BHET product because both ammonia and CO2 are highly volatile and easy to remove.’
There is a lot of interest in recycling mixed fabrics. Japanese fibre manufacturer Teijin Frontier reported earlier in 2023 it had developed a new processing agent to remove polyurethane from used polyester clothing.
‘A lot of people are working on chemical recycling of PET, but the catalyst they are using here is a little bit different, says Fergal Byrne, a chemist and cofounder of Addible, a startup company in Ireland that focuses on green chemistry for recycling processes. He describes the conversion rate with the ammonium biocarbonate catalyst of roughly 80% as good, though not great.
‘Im skeptical of this entire approach, says Byrne. ‘These are multi-step chemical processes that are expensive and require a lot of energy, while PET itself is very cheap. Companies are continuing to focus on such catalytically driven recycling processes for PET.
‘I dont think it will be commercialised for any significant volumes, although it might be useful for making particularly high-quality PET, adds Bryne. His company is partnering with a textile company to advance its own recycling strategy for stretchy polyurethane fabrics using an in-house database of green solvents that can be blended to achieve desired properties.
Scott is more impressed. ‘Bases like ammonia are actually well-known to promote PET transesterification, she notes. ‘It's surprising that including CO2, an acid, makes the ammonia work so much better. Nonetheless, ‘the sheer volume of textiles we need to recycle will require catalysts that are robust, inexpensive, selective, and fast. The authors' catalyst combination is all of these except fast’, says Scott. ‘A commercial process will need to be faster.’