Plastics such as epoxy resins are an integral part of our everyday lives – but their disposal is an unsolved environmental problem. At the end of their use, all that remains is incineration or landfilling, which puts a strain on ecosystems and the climate for decades. But now, for the first time, researchers have found a way to make epoxy resin recyclable. Their method allows the plastic to be recycled and even chemically broken down into fiber composites.
The durable plastic epoxy resin is widely used, on the one hand in its pure form, for example in the form of coatings or adhesives, and on the other hand as part of fiber-reinforced materials, where epoxy resin is used in combination with carbon or glass fibers for everything from aircraft and car parts to sports equipment and wind turbines. Researchers at the Swiss Federal Materials Testing and Research Institute (Empa) have now succeeded in developing a recyclable epoxy resin. Not only can their polymer be recycled using various methods, it is also flame retardant and easy to produce, which paves the way for industrial use.
Phosphorus makes it possible
The element that makes all of these properties possible is phosphorus. “Phosphorus-based additives are popular flame retardants,” says lead author Arvindh Sekar from Empa. “Typically they are simply mixed into the epoxy resin as a powder.” He and his team now went one step further and added a phosphorus-containing polymer to the resin before it hardened. This reacts with the epoxy. The flame-retardant effect of the phosphorus is retained, as are the advantageous mechanical properties of the epoxy.
However, the phosphorus polymer means that the crosslinks between the polymer chains in the cured epoxy can rearrange themselves under the influence of heat. After use, the plastic can simply be ground into powder and pressed into a new shape using heat, causing the connections to rearrange themselves: so-called thermomechanical recycling. “We carried out ten such rounds of recycling and the epoxy did not lose any significant mechanical resistance,” says Sekar.
Recycling despite fibers
But what should you do if the epoxy is part of a composite material with fibers and cannot be easily ground up? The new material also scores points here because, in addition to thermomechanical recycling, it can also be chemically dissolved. This allows the fibers embedded in the resin to be recovered without significant damage – a step that was previously almost impossible. “In addition to the fibers, we can also recover over 90 percent of the epoxy and phosphorus,” reports Sekar.
In contrast to thermomechanical recycling, however, chemical recycling requires a lot of energy and larger amounts of solvents – just like the chemical recycling of other polymers. “Chemical recycling should always be the last step. Thermomechanical recycling is preferred wherever possible,” says Sekar. However, there is no alternative to this chemical solution for fiber-reinforced epoxy resins.
Lots of possible applications
The researchers have been working on their recyclable epoxy resin for several years. They have now been able to improve the manufacturing process to the point where it can be scaled up to an industrial scale. “We are looking for industrial partners who would be interested in commercializing the flame-retardant and recyclable epoxy,” says Sekar. The first areas of application that could be considered, for example, are interior and exterior coatings. Here the material has the additional advantage that, thanks to the addition of phosphorus, it has improved color stability and yellows less quickly than conventional epoxy resin.
Another field of application would be, for example, as an adhesive in the construction of wind turbines. “Wind turbines are vulnerable to fire incidents, whether due to short circuits or lightning strikes,” said Sekar. “In addition to improving fire safety, our material would facilitate maintenance and replacement of components as it can be reformed even after curing under the right conditions.” In the meantime, the researchers want to add the phosphorus polymer to other plastics in order to make them fire-resistant and recyclable.
Source: Empa – Federal Materials Testing and Research Institute; Specialist article: Chemical Engineering Journal, doi: 10.1016/j.cej.2025.165779