Recyclable: A New Epoxy Resin

Researchers at Swiss Federal Laboratories for Materials Science and Technology – Empa have developed an epoxy resin-based plastic that is fully recyclable, repairable and flame retardant. They are now looking for industrial partners.

Epoxy resins are tough and versatile polymers. In combination with glass or carbon fibers, they are used, for example, to manufacture components for aircraft, cars, trains, ships and wind turbines. Such epoxy-based fiber-reinforced polymers have excellent mechanical and thermal properties and are much lighter than metal. Their weakness: They are not recyclable – at least not yet. Now Empa researchers led by Sabyasachi Gaan at Empa’s Advanced Fibers laboratory seem to have found a solution. As reported, the new material is fully recyclable, repairable and also flame retardant – all while retaining the favorable thermomechanical properties of epoxy resins.

“Recycling epoxy resins is anything but trivial, because these plastics are so-called thermosets,” Empa underlined. In this type of polymer, the polymer chains were closely crosslinked. These chemical crosslinks would make melting impossible. Once the plastic had hardened, it could no longer be reshaped. “This is not the case for thermoplasts, such as PET or polyolefins. Their polymer chains lie close together but are not chemically linked to each other. When heated, these polymers can be melted and formed into new shapes. However, because of the lack of crosslinks, their mechanical properties at elevated temperatures are generally not as good as those of thermosets.”

A new kind of polymer
According to Empa, the new epoxy resin that has been developed in collaboration with national and international partners is technically a thermoset, but can be reshaped like a thermoplast. The key is the addition of a very special functional molecule from the class of phosphonate esters into the new resin matrix. “We originally synthesized this molecule as a flame retardant,” co-inventor of this technology and Empa scientist Wenyu Wu Klingler was cited. However, the bond the molecule forms with the polymer chains of the epoxy resin is dynamic and can be broken under certain conditions. This would loosen the crosslinking of the polymer chains so that they can be melted and reshaped.

Not usually possible for thermosets: The cut in the material can be repaired by applying heat and pressure (Photo: Empa)

Such materials, also known as vitrimers, are considered particularly promising. “Today, fiber-reinforced composites are not recyclable at all, except under very harsh conditions, which damage the recovered fibers,” Wu Klingler informed. “Once they have reached the end of their service life, they are incinerated or disposed of in landfills. With our plastic, it would be possible for the first time to bring them back into circulation again.”

As underlined by Sabyasachi Gaan, the vision for the future “is a composite material, in which both the fibers and the plastic matrix can be completely separated and reused.” The researcher sees an opportunity in carbon-fiber-reinforced plastics in particular, which are used in the construction of airplanes, trains, boats, cars, bicycles and more. “The production of carbon fibers requires a lot of energy and releases an enormous amount of CO2. If we could recycle them, their environmental footprint would be a lot better – and the price a lot lower.” Moreover, the recovery of valuable elements like phosphorus connected to the matrix polymer would be possible.

A material for numerous applications
Fiber-reinforced composites are not the only application for the new polymer. For example, it could be used to coat wooden floors, as a transparent, resistant layer that has good flame-retardant properties – and where scratches and dents can be “healed” with a little pressure and heat. According to Gaan, flame retardancy, recyclability and repairability are a given. Additionally, the Empa researchers could “optimize all other properties depending on the intended use”.

The researchers are looking for industrial partners to pursue further applications of the material. They are convinced that the chances of commercial success are good. As stated, in addition to all its other advantageous properties, the modified epoxy polymer is inexpensive and easy to manufacture.

(Published in GLOBAL RECYCLING Magazine 3/2023, Page 10, Photo: Empa)