New epoxy resin resists flames and reduces waste 

New epoxy resin resists flames and reduces waste 

Epoxy resins are tough 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. Such epoxy-based, fibre-reinforced polymers have excellent mechanical and thermal properties and are much lighter than metal. Their weakness? They are not recyclable … at least, not yet.

Researchers in Switzerland, led by Sabyasachi Gaan at 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 favourable thermomechanical properties of epoxy resins. They have published their findings in the Chemical Engineering Journal.

Recycling epoxy resins is anything but trivial because these plastics are so-called thermosets. In this type of polymer, the polymer chains are closely crosslinked. These chemical crosslinks make melting impossible; once the plastic has hardened, it can 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, so 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.

The unique epoxy resin that the Empa researchers have developed is technically a thermoset. Unlike other thermosets, however, it 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 synthesised this molecule as a flame retardant,” says co-inventor and Empa scientist Wenyu Wu Klingler. However, the bond the molecule forms with the polymer chains of the epoxy resin is dynamic and can be broken under certain conditions. This loosens the crosslinking of the polymer chains so that they can be melted and reshaped.

The existence of such materials, also known as vitrimers, has only been known for about 10 years and they are considered particularly promising. “Today, fibre-reinforced composites are not recyclable at all, except under very harsh conditions, which damage the recovered fibers,” explains Wu Klingler. “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.”

“Our vision for the future is a composite material, in which both the fibres and the plastic matrix can be completely separated and reused,” adds Gaan, who sees an opportunity in carbon-fibre-reinforced plastics in particular, as they’re commonly used in the construction of airplanes, trains, boats, cars, bicycles, and more.

“The production of carbon fibres requires a lot of energy and releases an enormous amount of CO2,” he explains. “If we could recycle them, their environmental footprint would be a lot better – and the price a lot lower.”

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