Lightweight, highly durable fiber-reinforced epoxy composites consisting of glass fiber or carbon fiber embedded in a polymer matrix are high-performance materials essential for manufacturing automotive, ship, aircraft, and wind turbine blades.
By 2025, about 25,000 tons of wind turbine blades will reach their operational life every year. Traditionally, wind turbine blades have been difficult to recycle due to the chemical properties of epoxy, which is an elastic substance and is considered a component that cannot be broken down into reusable materials. Epoxy resins are not biodegradable and release toxic gases when burned, ultimately leading to landfill as the main way to dispose of them.
Landfill of wind turbine blades has been banned by several European countries due to its inefficiency and unsustainability, and is expected to be implemented by more countries in the future. Therefore, there is an urgent need for viable recycling strategies for epoxy resins and their composites.
The currently newly discovered process is a proof-of-concept of the recycling strategy and can be applied to the vast majority of existing wind turbine blades and blades currently in production, as well as other epoxy-based materials.
The results were published in the leading scientific journal Nature and Aarhus University, together with the Danish Institute of Technology, applied for a patent for the process.
Specifically, the researchers showed that by using A ruthenium-based catalyst and solvents isopropanol and toluene, they could separate the epoxy matrix and release one of the original structural units of the epoxy polymer, bisphenol A, and intact glass fibers in a single process.
However, the method is not immediately scalable because the catalytic system is not efficient enough for industrial implementation -and ruthenium is a rare and expensive metal. Scientists at Aarhus University are therefore continuing to improve the method.
"Nonetheless, we see this as a major breakthrough in developing durable technologies that can create a circular economy for epoxy-based materials. This is the first publication of chemical processes that can selectively decompose epoxy resin composites and isolate one of the most important materials. "Epoxy polymers, as well as important components of glass or carbon fibres, do not damage the latter in the process," said Troels Skrydstrup, one of the lead authors of the study.
Troels Skrydstrup is a professor at the Department of Chemistry and the Center for Interdisciplinary Nanosciences (iNANO) at Aarhus University. The research was supported by the CETEC project (Circular Economy for Thermoset Epoxy Composites), a partnership between Vestas, Oilon, the Danish Institute of Technology and Aarhus University.
In this study, the researchers used a Ru-catalyzed dehydrogenation/bond breaking/reduction tandem reaction to break the most common C(alkyl)-O bond in the polymer, which can be used to break the C(alkyl)-O single bond adjacent to the BPA matrix. The researchers demonstrated the application of the method to unmodified amine-cured epoxy resins and commercial composite materials, including the casing of wind turbine blades. The results of the researchers show that chemical recovery of thermoset epoxies and composites is feasible.
The catalytic deconstruction experiment of epoxy resin showed that 81% of BPA could be recovered after 4 days of catalytic reaction.
With a general approach that can be used for molecular decomposition of amine-cured epoxies, the investigators turned to investigate the applicability of the protocol for the decondification of fiber-reinforced epoxies that contain fibers by high weight percentage in addition to the polymer matrix. After 3 days, the composites clearly separated into loose fibers without any pretreatment. Decantation reaction mixture; After washing, 57wt% of carbon fiber was recovered, and 13wt% of BPA was isolated from the solution.
It then tested the casing of a state-of-the-art retired wind turbine blade. This commercial composite sample was catalytically decomposed thoroughly, resulting in 50wt% glass fiber and 19wt% BPA.
In conclusion, for the components recovered from end-of-life composites, a circular economy can be considered. The highly purified bisphenol A obtained from recycling can theoretically be reused in existing production chains such as epoxy resins, polycarbonate or polyester, replacing the original BPA produced from petroleum feedstock. The researchers' catalytic process can be seen as a proof-of-concept, demonstrating that it is feasible to achieve a circular economy for these valuable and relevant materials.
