Recycling bulletproof vests in the microwave

The consortium Closing Carbon Cycles with Renewable Amines (3CRA) aims to investigate how valuable amines can be extracted from biomass and recyclate. ‘Amines are important building blocks for the chemical industry,’ says Vincent Voet, Professor of Circular Plastics at NHL Stenden University of Applied Sciences. ‘You can extract them from residual streams, but also from plastics and materials such as the aramid fibres in Twaron.’ Together with colleagues, the group of Professor Katja Loos at the University of Groningen, and the company Teijin Aramid (which manufactures Twaron), they joined forces and pooled their expertise to investigate whether aramid fibres can be recycled in this way.

‘It’s really a kind of KNCV paper,’ says Loos with a laugh. ‘Bert Gebben is principal scientist at Teijin Aramid, but also chair of the KNCV Macromolecules section. Vincent is treasurer, and Joël Benninga, the doctoral researcher who carried out the study, is the webmaster of the board. On top of that, I myself am chair of the KNCV Soft Matter section. So we know each other well and already had a solid basis of trust for this project.’ The far-reaching collaboration between a university and a university of applied sciences is also rather unusual. ‘That doesn’t happen very often. But this more practical approach and collaboration is something universities could do with seeing more of — we need to engage more with societal challenges.’

Fifteen minutes

Aramid fibres are found in a wide range of applications, including cut-resistant gloves, wear-resistant tyres, and bulletproof vests. But recycling is still in its infancy, and when the material is worn out it is usually downcycled. Voet: ‘In theory you could put the fibres straight into the microwave, but we started with the powder form of the polymer, poly(p-phenyleneterephthalamide).’ With some water and NaOH in the microwave at 250 °C, the polymer is hydrolysed into the monomers p-phenylenediamine and (a salt of) terephthalic acid within a quarter of an hour.

Once the splitting of the polymer into its monomers had been achieved, the greatest challenge began: how do you obtain them in pure form? ‘Joël looked for the right way to separate them from each other and from the residual substances,’ says Voet. ‘That is always a challenge in recycling: depolymerisation often works well enough, but purification requires a great deal of investigative work.’ Benninga succeeded by using a combination of extraction and crystallisation.

‘If you are considering a process for industry, you also have to think about such practical steps from the outset,’ says Loos. ‘It works in the lab, but how will it fare on a large scale?’ Voet adds: ‘Teijin Aramid naturally has a great deal of experience with the chemistry behind aramid fibres and is keen to make progress in recycling. That is why the company came on board early to be genuinely involved in the process. That is what I find so rewarding about this kind of public-private partnership.’

Radiation

A number of questions remain. ‘In particular, what the mechanism of degradation is,’ Voet notes. Loos: ‘Simply heating in an oil bath is far slower, so what exactly happens during depolymerisation?’ Several papers examining microwave reactions appear to suggest that the microwave radiation itself influences the process, and that is something the researchers intend to explore further. ‘In science you always roll from one question into the next,’ says Voet.

The next step will also be to recycle not the powder form, but actual fibres. ‘After that you look at fibres with additives, and so progressively towards greater levels of contamination,’ Loos explains. ‘It is a high-tech application, especially compared with other plastics such as PET. Once the technology has been developed, it is up to industry to do something with it.’ The researchers are confident, in any case, that the method has a future.

Black-and-white thinking

Criticism from the field might hold that aramid fibres are harmful to the environment. ‘But most products containing aramid are readily collectable, so they need not end up in the environment. Even so, you have to be mindful of what you use any plastic for,’ says Loos. ‘You have to use plastics consciously, because you cannot know in advance what a material might do once it enters the environment.’

‘Beyond that, as a researcher I don’t really believe in the black-and-white thinking that suggests there is, for instance, one best fibre or one best method of recycling,’ Voet adds. ‘That is the kind of polarisation you see throughout society. We are not claiming to have the best solution, but if you want a sustainable future, you have to answer these kinds of questions [about recycling and environmental impact, eds.]. As researchers, we must work together with our partners to supply the puzzle pieces needed to complete the circular puzzle of the future.’

Benninga, J. et al. (2025) JACS 147(9), DOI: 10.1021/jacs.4c17791