New plastic is self-healing, recyclable and non-toxic

Researchers from Groningen have developed an alternative to epoxy resin that can repair itself when damaged. The material is also fully recyclable a limited number of times, and does not contain Bisphenol A, a substance that can disrupt the endocrine system. Rodrigo Andrés Araya Hermosilla will defend his PhD thesis on Monday 17 October on the development of this alternative.

Epoxy resin is an important ingredient in thermosetting plastics. Plastics generally consists of a polymer and a filler. The filler largely determines the properties of the final product. Mix different fillers with epoxy resin and you get plastics ranging from the inside of bottle tops or beer cans and coatings for cars, oars, wind turbines and electronics to glues.

‘One problem with these plastics is that you can’t return them to their original components’, says Francesco Picchioni, Professor of Chemical Product Technology at the Engineering and Technology Institute of the University of Groningen and supervisor of Andrés Araya Hermosilla. Long polymer chains bond together to form plastics. With thermosetting polymers, so much energy is needed to break these bonds that the chains themselves break too. ‘In 2009, we showed that we could make a thermosetting polymer where you could break the bonds without damaging the chains.’

Andrés Araya Hermosilla’s work builds upon this. ‘Our first study focused on polymers only. This time we studied them in combination with fillers’, explains Picchioni. In the study, they altered the combination of polymer and filler one step at a time until they produced a good product. ‘The material was too brittle, for instance, but the addition of rubber made it tougher.’

Nanotubes

The material that they produced is mainly based on carbon nanotubes. These give the material good mechanical properties, but also make it a good conductor. And this has an interesting application, says Picchioni. ‘If we send electricity through the material, it heats up. And when it heats up, the bonds between the polymers gradually slacken and become somewhat fluid. In this state mild damage disappears as if by magic.’ An electrical charge therefore induces the material to repair itself.

If you increase the heat, the material breaks down completely, leaving the original polymer chains. ‘So you can use it again. You therefore have full recycling, from cradle to cradle as it is known.’ Until now, this was not possible with thermosetting plastics. ‘You can only reuse them in various applications as plastic beads, or you can try to regain some of the energy from the material by burning it.’

Electronics

This form of recycling is still limited. ‘We have only managed to reuse the material once. If we try to reuse it again, the results are no longer as good.’ It may be possible to improve on this, but being able to reuse it once already represents progress.

Andrés Araya Hermosilla did not study specific applications for his material. ‘The most obvious one is to replace epoxy resins’, says Picchioni. These are used in all sorts of plastics, glues and coatings, as well as in electronics. ‘At present, none of these can be fully recycled.’ Furthermore, epoxy resin often contains Bisphenol A, which can mimic the female hormone oestrogen. Health risks appear to be associated with its large-scale use.

Picchioni’s alternative is recyclable, self-healing and does not contain Bisphenol A. Are there no disadvantages? Picchioni laughs. ‘Our plastic is made with polyketone, a polymer that was developed in the 1980s but that is not yet commercially available.’ But change appears to be afoot, with the recent opening of a polyketone factory in South Korea. Perhaps this is the route to market domination for the Groningen invention?

Andrés Araya Hermosilla defended his thesis on October 17, 2016.