FVK Flame-Retardant and Recyclable Fiber-Reinforced Composite Material

Airplanes and passenger trains must meet strict safety requirements, including those related to fire protection. This calls for materials that are flame-retardant, lightweight, durable, and scalable. Empa researchers, in collaboration with their industry partner Elantas, have developed a fully recyclable composite material.

The material is a multilayer “sandwich.” At its center is a honeycomb structure made of aramid, a heat-resistant plastic. This sturdy, lightweight core is covered on 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—in other words, it cannot be melted down or dissolved. “Composite materials containing epoxy currently end up in landfills or are incinerated,” says Empa researcher Sabyasachi Gaan from the “Advanced Fibers” laboratory.
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, which was previously impossible for epoxy.

Our material complies with fire safety regulations and offers nearly the same beneficial mechanical properties as standard epoxy.

Sabyasachi Gaan

Together with its industry partner Elantas—a subsidiary of the German specialty chemicals group Altana—the Empa team pursued another goal in this project: the complete recycling of the composite material. With the right solvent and a little heat, the “sandwich” can be broken down again 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’s also possible to recover the epoxy resin itself from the solution. We plan to tackle that in future projects,” says Gaan.
The recyclable epoxy resin was developed at Empa. Together 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 advantageous mechanical properties as conventional epoxy,” says Gaan. “But for the first time, it allows for complete recycling of the composite material.”
The next step is to further scale up production and recycling. “For the aerospace industry, it is crucial to combine fire safety, lightweight construction, and recyclability. This epoxy composite system shows 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, Sabyasachi Gaan and his team are already researching 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.”