New epoxy resin is recyclable and flame retardant

The new material, which is said to retain the favourable thermomechanical properties of epoxy resins, is detailed in Chemical Engineering Journal.

Epoxy resins are so-called thermosets, a type of polymer with closely crosslinked polymer chains that prevent melting or reshaping once the plastic has hardened.

Thermoplasts contain polymer chains that lie close together but are not chemically linked. When heated, these polymers can be melted and formed into new shapes. Because of the lack of crosslinks, their mechanical properties at elevated temperatures are generally not as good as those of thermosets.

The epoxy resin developed at Empa in collaboration with national and international partners is a thermoset that can be reshaped like a thermoplast. This was made possible with the addition of a functional molecule from the class of phosphonate esters into the new resin matrix.

"We originally synthesised this molecule as a flame retardant," said Empa scientist Wenyu Wu Klingler.

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According to Empa, 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. Such materials – vitrimers - have been known for about ten years and are considered particularly promising.

"Today, fibre-reinforced composites are not recyclable at all, except under very harsh conditions, which damage the recovered fibres," said 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," said research lead Sabyasachi Gaan from Empa's Advanced Fibres laboratory.

Gaan sees an opportunity for the material in carbon-fibre-reinforced plastics due to their widespread use in applications including airplanes, trains, boats, cars, and bicycles.

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

The recovery of elements like phosphorus connected to the matrix polymer would be possible also, the researchers said.

As well as fibre-reinforced composites the new polymer 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.

"We didn't develop a single material for a specific purpose, but rather a toolbox," said Gaan. "Flame retardancy, recyclability and repairability are a given. We can optimise all other properties depending on the intended use."

The researchers are now looking for industrial partners to pursue applications of the material.