New plastic protects against flames
Empa researchers have developed an epoxy resin that can be repaired and recycled - and is also flame retardant and mechanically resistant. Possible applications range from coatings for parquet floors to composite materials for trains and aircraft.
Epoxy resins are resistant and versatile plastics. In combination with glass or carbon fibres, they are used, for example, to manufacture components for aircraft, cars, trains, ships and wind turbines. Such fibre-reinforced epoxy-based plastics 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 from Empa’s Advanced Fibers Laboratory have developed an epoxy resin-based plastic that is fully recyclable, repairable and also flame retardant – while retaining the favourable thermomechanical properties of epoxy resins. They have published their results in the Chemical Engineering Journal.
Recycling epoxy resins is anything but trivial, because these plastics belong to the so-called duromers. In this type of plastic, the polymer chains are closely cross-linked. These chemical bonds make melting impossible. Once the plastic has hardened, it can no longer be deformed.
The situation is different with thermoplastics, such as PET or polyolefins. Their polymer chains are close together but not bonded. Under the influence of heat, these plastics can be melted and formed into new shapes. The only problem is that due to the lack of cross-linking, their mechanical properties at elevated temperatures are generally not as advantageous as those of duromers.
A new type of plastic
The special epoxy resin that Empa researchers have developed in collaboration with national and international partners is actually a duromer – but unlike other duromers, it can certainly be melted like a thermoplastic. The key is the addition of a special functional molecule from the class of phosphonic acid esters to the resin matrix. “We originally synthesised this molecule as a flame retardant,” says Empa scientist Wenyu Wu Klingler, who co-invented the technology. However, the bond that the molecule forms with the polymer chains of the epoxy resin is reversible, i.e. it can be dissolved again under certain conditions. This loosens the cross-linking of the polymer chains so that they can be melted and shaped.
Such materials, also called vitrimers, have only been known for about ten years and are considered particularly promising. “Today, fibre-reinforced plastics are practically impossible to recycle, except under extreme conditions that damage the fibres,” explains Wu Klingler. “Once they have had their day, they are incinerated or disposed of in landfills. With our plastic, it would be possible for the first time to put them back into the material cycle.”
Their vision for the future, adds group leader Sabyasachi Gaan, is “a composite material in which the fibres and the plastic matrix can be completely separated and reused.” The researcher sees a particular advantage in carbon fibre-reinforced plastics, for example, as used in the construction of planes, 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 ecological footprint would be a lot better – and the price a lot lower.” In addition, valuable additives such as phosphorus could be recovered from the polymer matrix.
Tailor-made material
Fibre-reinforced plastics are not the only application for the new plastic. 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 damage can be “healed” again with a little pressure and heat.
“We didn’t develop a single material for a specific purpose, but rather a toolbox,” Gaan explains. “The flame retardancy, recyclability and repairability are all there. We can optimise all other properties depending on the intended use.” For example, he says, flow properties are particularly important for the production of fibre-reinforced plastics, while exterior wood coatings must also be weather-resistant.
To pursue these and other applications of the material, the researchers are now looking for industrial partners. The chances of commercial success are good: because in addition to all its other advantageous properties, the modified synthetic resin is also cheap and easy to manufacture.