Production of recyclable plastic step closer

A recently discovered group of recyclable plastics seems to be a promising alternative to the hard plastic that is difficult to recycle. However, researchers at TU Eindhoven think that these so-called vitrimers cannot yet be used in products because they are difficult to process. This is due to the fact that not only temperature, but also density affects the processability of this new form of plastic. New insights bring the production of recyclable plastics closer; it could lead to recyclable dashboards for the automotive industry, among other things.

“Plastic is known to be difficult to recycle and therefore highly harmful to the environment,” wrote Eindhoven University of Technology on its website in late November. “However, this mainly concerns so-called hard plastics, or thermosets, such as billiard balls or your car’s dashboard. Another group of plastics, thermoplastics – which the industry uses to make cling film, coffee cups or toys – are actually very easy to recycle but, unfortunately, much less strong. In 2011, a new group of plastics was developed – the so-called vitrimers – which are as strong as hard plastics but are recyclable. In 2015, the European Patent Office (EPO) named the discoverer of the material ‘European Inventor’ (video).”

This is all very promising, according to the scientists, but a commercial application of vitriol is still a long way off. To produce with it, manufacturers need to know how to give the material the desired properties so that they can be incorporated into products in the desired way. However, even scientists have yet to figure out how the material works.

TU/e physicist Liesbeth Janssen, PhD student Simone Ciarella and BSc student Rutger Biezemans now present a computer model that clarifies how temperature and density deform vitrimers. Janssen: "Now that we better understand the material's physics, we hope to actually produce recyclable plastics for industry in a follow-up project."

Their model shows that the fragility of the vitrimers determines the processability of the plastic in the factory. Ciarella: "The fragility represents the rate at which a material changes from a solid to a liquid (melting) and vice versa (solidifying). The lower the fragility value is, the easier the plastic is to form and therefore the more recyclable the material is."

Besides understanding and being able to predict how this fragility affects it, Liesbeth Janssen and colleagues even managed to 'tune' the fragility of the plastic. It turns out, for instance, that the fragility can be changed by adjusting the density of the material. This allows the manufacturer to precisely set the desired behaviour in the future.

The star-shaped vitrimers swap cross-links.

To explain how they work, we need to zoom in on the physical properties of all three types of plastics. Plastics consist of very long strands, also called polymers. Thermosets are so strong because of the many connections between these strands. And that directly explains the poor recyclability. This is because heating causes these bonds to break irreversibly. Thermoplastics have no bonds, and are therefore easily deformed by raising the temperature. But that also makes them immediately less strong.

Ciarella: 'The interesting thing about vitrimers is that they do have many cross-links, but heating allows them to open left and later close again in a different place. At room temperature, these vitrimers therefore behave as strongly as thermosets. But when heated, they actually behave like easily deformable thermoplastics. These plastics therefore retain their quality when remelted.'

The findings from this research will serve as the theoretical basis for the Horizon 2020 International Training Network project known as VITRIMAT, which is due to start in January. As part of this project, TU/e will work with eight international partners towards the practical application of vitrimation. One of the objectives is to design recyclable dashboards for the automotive industry.

Fifteen PhD students will soon be recruited for this project. One of them will be based in Eindhoven, under the supervision of Liesbeth Janssen, Kees Storm and Wouter Ellenbroek. Janssen’s focus in this regard is on fundamental physics. Her field of research is, in fact, much broader than plastics. She studies glass-like materials. Vitrimers are one example of this. But Janssen also conducts research into the glass phase of epithelial cells in your alveoli or in computer bits. Read more about Janssen’s glass research in this background story.

Source: TU Eindhoven