International. Empa researchers develop an epoxy resin that can be repaired and recycled, as well as being flame retardant and mechanically resistant. Potential applications range from wood floor coverings to composites in aerospace and railways.
Epoxy resins are strong and versatile polymers. In combination with glass or carbon fibres, they are used, for example, to manufacture components for aircraft, cars, trains, ships and wind turbines. These epoxy-based fiber-reinforced polymers have excellent mechanical and thermal properties and are much lighter than metal. But they are not recyclable.
Now, Empa researchers led by Sabyasachi Gaan in Empa's Advanced Fibers laboratory have developed an epoxy resin-based plastic that is fully recyclable, repairable and also flame retardant, all while retaining the favorable thermomechanical properties of epoxy resins.
Recycling epoxy resins is not trivial, because these plastics are called thermosets. In this type of polymer, the polymer chains are closely crosslinked. These chemical cross-links make fusion impossible. Once the plastic has hardened, it can no longer be reshaped.
The unique epoxy resin that Empa researchers have developed in collaboration with national and international partners is technically thermoset, but unlike other thermosets, it can be remodeled as a thermoplastic. The key is the addition of a very special functional molecule of the phosphonate ester class to the new resin matrix.
"We originally synthesized this molecule as a flame retardant," said Wenyu Wu Klingler, an Empa scientist and co-inventor of this technology. However, the bond formed by the molecule with the polymer chains of the epoxy resin is dynamic and can be broken under certain conditions. This loosens the crosslinking of polymer chains so that they can melt and reshape.
These materials, also known as vitrimeros, have been known for about ten years and are considered especially promising.
"Today, fiber-reinforced composites are not recyclable at all, except under very harsh conditions, which damage the recovered fibers," Wu Klingler explained.
"Once they have reached the end of their useful life, they are incinerated or disposed of in landfills. With our plastic it would be possible for the first time to put them back into circulation," he said.
"Our vision for the future," said Sabyasachi Gaan, leader of the group.
"It is a composite material in which both the fibers and the plastic matrix can be separated and completely reused. The production of carbon fibers requires a lot of energy and releases an enormous amount of CO2," he said.