Recyclable, repairable and robust: New epoxy resin for the circular economy

Epoxy resins are used in combination with glass or carbon fibers to manufacture components for aircraft, cars, trains, ships and more. The fiber-reinforced plastics based on epoxy resins have excellent mechanical and thermal properties, are lighter than metal, but until now have not been recyclable. Empa researchers have now developed the first fire-safe epoxy resin-based plastic that makes just this possible.

In an interview with K-MAG, Dr. Wenyu Wu Klingler and Dr. Sabyasachi Gaan from the Empa "Advanced Fibers" laboratory talk about the new plastic, its advantages and the chemical processes involved in its manufacture.

Can you tell me about the new plastic you developed together?

Wenyu Wu Klingler: There are two types of plastics, thermoplastics and thermosets. The thermoplastic can be melted and reformed. In contrast, thermoset becomes fixed when it’s cured. Epoxy resin is a thermoset and very robust up to a certain temperature and normally not recyclable.

In our work, using a specific phosphorus chemistry the bond that the molecule forms within the cured epoxy polymer chains becomes reversible, meaning that it can be broken and re-joined again under certain conditions. This loosens the crosslinking of the polymer chains so that it can be "soften/melted" and reshaped. Therefor it can be repaired after use. The material we created has a phosphorus functionality, which means that it is also fire-safe and mechanically resistant.

What was the most exciting aspect of the development for you?

Sabyasachi Gaan: When we started working with epoxy, we were researching its flammability. It was not our primary goal to make it recyclable. We have a lot of experience in developing fire protecting materials and made use of the reactive monomer to develop flame retardant epoxy materials.

We then observed the material seemed to soften when it is heated, and we began to research the recyclability of the material. We changed the concentration of the monomer and were able to not only make it flame-retardant but also recyclable.

Why has the development of recyclable epoxy resin been a challenge up to now?

Klingler: What is special about what we developed is that we compensate the recyclability without losing the mechanical performance, in addition to fire safety. There are other groups working on this field, however, it is difficult to achieve feasibility, scalability, material performance, and recyclability at the same time. Our material is offering a very balanced performance using simpler approach.

Gaan: Epoxies contain bonds which cannot be broken. And for recycling you need to have bonds which can be cleaved and reformed again. Thermosets consist of a cross-linking network and to make it recyclable, you need to replace the strong bonds of the thermosets with some weak and reversible bonds. They are a part of a bigger concept called "covalent adaptable networks". The challenge is going from a completely strong bond system to a mix of strong and weak bonds. Depending on what kind of weak bonds you put in, you'll you have to deal with other challenges for example losing mechanical performance. To sum up, it has always been challenging to balance the chemistry with the performance of the material for a longer period.

What is your recyclable epoxy resin particularly suitable for and what is it not suitable for?

Klingler: It is suitable for fiber-reinforced composites, which can be used in transportation, aviation, building construction and more, where fire safety is important concern. In addition to that, it can be used for the coating of wood. It is transparent, self-healable, and protective coating, which is also fire safe. In addition, such materials can also be used in printed circuit boards, where at the end of life precious metals can be recovered by debonding them. Advanced materials combining thermoplastics and such vitrimers in structural application could also be of interest.  We did not develop a single material for a specific purpose. The flame retardancy, recyclability and repairability allows it to be optimized depending on the intended use.

How do you contribute to more a sustainable plastic industry with your development?

Gaan: Most thermosets are either landfilled or incinerated. Therefor all the material including very valuable resources are lost.

What we offer is a new material that can be recycled by a thermal mechanical means, self-healing and chemical recycling. Our dream is to take apart all the components, which are used in building the thermoset material and to recover each, component and reuse it, and by this create a circular economy.

We offer this while also addressing the functionality of the material. Other research groups have also developed recyclable thermosets but do not address fire protection at the same time. We differentiate ourselves by offering both in one solution and for many applications. This is quite important as when used in a public place, fire protection is always necessary.

Do you have any future projects planned that you can tell me about?

Klingler: We are currently working on one project where we are looking into developing a natural fiber reinforced composite for different applications, including different transportation sectors.

Gaan: And we have another project ongoing where we are working on fiber (glass and carbon) reinforced plastics too.

The chemistry we developed is versatile and hopefully we can translate the same idea to other kinds of thermosets, for example, polyurethanes that are also used for coating ad glue applications and unsaturated polyesters, which are mostly used for making composite applications.