Epoxy resins are resilient and functional polymers. For instance, they are utilized to create parts for wind turbines, trains, vehicles, and airplanes when combined with glass or carbon fibers. These fiber-reinforced polymers with epoxy bases offer superior mechanical and thermal characteristics and weigh substantially less than metal. However, they are currently not recyclable.
The addition of a phosphonate ester into the resin matrix allows the new epoxy resin to be melted and reshaped under certain conditions. Image Credit: Empa
Researchers at Empa’s Advanced Fibers laboratory, under the direction of Sabyasachi Gaan, have now created an epoxy resin-based material that is completely recyclable, repairable, and flame-resistant while keeping the advantageous thermomechanical characteristics of epoxy resins. Their study has been published in the Chemical Engineering Journal.
Since these plastics are so-called thermosets, recycling epoxy resins is everything but simple. The polymer chains in this type of polymer are tightly crosslinked. Melting is not feasible because of these chemical cross-links. Plastic cannot be reshaped once it has solidified.
Thermoplasts like PET and polyolefins are an exception to this. Although their polymer chains are near together, they are not chemically connected. These polymers can melt and take on various forms when heated. Their mechanical capabilities at high temperatures, however, are often inferior to those of thermosets due to the lack of crosslinks.
A New Kind of Polymer
Although officially a thermoset, the special epoxy resin that Empa researchers created in partnership with local and international partners can be molded like a thermoplast. The secret is incorporating a highly unique functional molecule from the group of phosphonate esters into the newly created resin matrix.
We originally synthesized this molecule as a flame retardant.
Wenyu Wu Klingler, Co-Inventor and Scientist, Empa
The molecule’s link with the epoxy resin polymer chains is dynamic, though, and can be disrupted under specific circumstances. The polymer chains’ crosslinking is loosened as a result, allowing them to be melted and reformed.
These materials, also known as vitrimers, are extremely new and are regarded as highly promising.
Klinger added, “Today, fiber-reinforced composites are not recyclable at all, except under very harsh conditions, which damage the recovered fibers. 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.”
Group leader Sabyasachi Gaan stated, “Our 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, which are extensively utilized in the production of aircraft, trains, boats, automobiles, bicycles, and other vehicles.
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.”
Furthermore, it would be feasible to retrieve precious elements like phosphorus that are attached to the matrix polymer.
A Material Made to Measure
The new polymer is not just useful in fiber-reinforced composites. For example, it could be used to cover hardwood floors as a transparent, durable layer with high flame-retardant capabilities—and where scratches and dents can be “healed” with just enough pressure and heat.
Gaan added, “We didn’t develop a single material for a specific purpose, but rather a toolbox. Flame retardancy, recyclability and repairability are a given. We can optimize all other properties depending on the intended use.”
Flow qualities, for example, he explains, are extremely important in producing fiber-reinforced plastics, while exterior wood coatings should also be weather-resistant.
The researchers are now searching for industrial partners to investigate these and other applications of the material. The likelihood of commercial success is high: Aside from its numerous advantages, the modified epoxy polymer is also economical and simple to create.
European Meeting on Fire Retardant Polymeric Materials 2023
Sabyasachi Gaan and Wenyu Wu Klingler are members of Empa’s Advanced Fibers laboratory, which has been researching flame retardants for fabrics, plastics, and wood for 15 years. On July 26–29th, 2023 the lab successfully held the European Meeting on Fire Retardant Polymeric Materials (FRPM) 2023 at the Empa Academy, bringing together 250 specialists from academia and industry for multidisciplinary presentations and discussions. This project was also presented during the conference.
Journal Reference:
Klingler, W. W., et al. (2023) Recyclable flame retardant phosphonated epoxy based thermosets enabled via a reactive approach. Chemical Engineering Journal. doi:10.1016/j.cej.2023.143051