For airplanes and trains Researchers Develop Recyclable Fiber Composite 

Airplanes and passenger trains must meet strict safety requirements. This calls for materials that are flame-retardant, lightweight, durable, and scalable. Researchers at Empa, in collaboration with their industry partner Elantas, have now succeeded for the first time in making such a composite material fully recyclable.

“No flying machine will ever fly from New York to Paris,” aviation pioneer Orville Wright once said. What even the visionary Wright brothers considered impossible is now a reality—thanks in part to new materials that make modern airplanes and engines possible in the first place. 

In the more than 100 years since the Wright brothers’ first flight attempts, aviation has not only become more powerful but, fortunately, also significantly safer. The materials used in aircraft construction must meet the strictest safety requirements, including fire safety standards. At the same time, they must be lightweight and mechanically robust—and, increasingly, recyclable as well.

Bringing all these properties together under one roof is a challenge. “For example, when you make a material flame-retardant, you always end up changing its other properties as well,” explains Empa researcher Sabyasachi Gaan from the Advanced Fibers Laboratory.  

Flame-retardant and recyclable fiber-reinforced composite material

Gaan and his team took on this challenge together with their industry partner Elantas—a subsidiary of the German specialty chemicals group Altana—as part of a project supported by Innosuisse. Their focus was on a specific composite material used in the interior design of airplanes and trains, such as for the floor of the passenger cabin.

The material is a multilayer sandwich. In the center is a honeycomb structure made of aramid, a heat-resistant plastic. This sturdy, lightweight core is covered on the top and bottom with several flat-woven layers of glass or carbon fibers. Epoxy resin serves as the binder, as it does in many composite materials. Normally, this polymer cannot be recycled either chemically or thermally. “Composites containing epoxy end up in landfills or are incinerated today,” says Gaan.

This is exactly where the Empa researchers have an ace up their sleeve. They have developed an additive for epoxy resin that makes the material flame-retardant—and recyclable at the same time. If this phosphorus-containing molecule is added to the epoxy during production, the material can be softened and reshaped under certain conditions after it has cured—a process known as thermomechanical recycling (FAQ box), which until now had been impossible with epoxy.

All components recovered

In the Innosuisse project, however, the partners pursued a different goal: the complete recycling of the composite material. With the right solvent and a little heat, the sandwich structure can be broken down into its individual components: the honeycomb structure and the woven fibers. 

Aramid honeycombs, in particular, but also carbon fibers, are relatively expensive. Recycling them is therefore also economically attractive. “In principle, it is also possible to recover the epoxy resin itself from the solution. We plan to tackle this in future projects,” said Gaan.

The recyclable epoxy resin was developed at Empa. Working with Elantas, the researchers have now explored its potential for industrial application for the first time. “Our material complies with fire safety regulations and achieves nearly the same beneficial mechanical properties as conventional epoxy,” says Gaan. However, it allows for the complete recycling of the composite material for the first time.

Also of interest to the construction and energy sectors

The project partners are satisfied with how the project is progressing. The next step is to further scale up production and recycling. “For the aerospace industry, it is crucial to combine fire protection, lightweight construction, and recyclability. This epoxy composite system demonstrates that this is now possible for the first time,” says Fiorenzo Lenzi, Head of the Aerospace/Ballistic Product Line at Elantas.

At the same time, Gaan and his team are already conducting research into further applications for the recyclable, flame-retardant plastic, such as in the energy sector and the construction industry. The scientist emphasizes that the fact a material has come this far is also thanks to basic research. “Before we can work on the applications, we need to have a very good understanding of the material’s properties.”