Plant-Based Epoxy Enables Recyclable Carbon Fiber
Ten times stronger than steel, nearly half the weight of aluminum, and far stiffer than fiberglass, carbon fiber carries a package of advantages, making it a preferred material for use in luxury sedans and Formula 1 race cars alike. But it still needs perfecting to become economical for mass-market vehicles, according to National Renewable Energy Laboratory (NREL) scientist Nicholas Rorrer. "Carbon fiber is expensive," he explained. "It is also energy intensive to make, so it is not exactly greenhouse-gas (GHG) friendly. Making carbon fiber readily recyclable could help in both these regards."
Recyling process triggered at room temperatureThanks to recent advances in bio-based material design, recycling carbon fiber at an industrial scale could already be close at hand. Through a project supported by the US Department of Energy's Vehicle Technologies Office, under the Composites Core Program, Rorrer and other NREL researchers have shown that making carbon-fiber composites with bio-based epoxies and anhydride hardeners makes the material fully recyclable by introducing linkages that are more easily degraded. In fact, the recycling process — called methanolysis — can be selectively triggered at room temperature without degrading the quality or orientation of the fibers. That could represent a strong step toward a circular material, which can make carbon fiber cheaper and greener when used across multiple lives.
At once strong and featherweight, carbon fiber's advantages come from its layered design. It is a composite material of both long filaments of pure carbon and a glue-like epoxy coating known as a thermoset. When curing, molecules in the liquid resin bind with each other and around the woven carbon filaments, hardening into a strong and rigid lattice.
However, the thermoset-nature of the cured epoxy makes those superior products difficult to break apart, especially without severely damaging the carbon filaments. Products made from carbon fiber — despite their premium price — often head to the landfill at the end of their lives, along with any efficiency benefits they may have earned.
Indeed, although carbon fiber could cut the weight of a typical passenger car in half — boosting its fuel efficiency by as much as 35% — any efficiency benefits are effectively offset by the GHG-intensive energy used to manufacture it. Synthesizing carbon fiber involves temperatures greater than 1,000°C.
That reality got Rorrer thinking: "Is there a way to reuse carbon fiber over multiple material lives to reclaim that fiber and get more value and environmental benefits?"
Experimenting with biomassRorrer and teammates began experimenting with the chemistry of biomass to understand if it could enable a new epoxy designed for recyclability. Compared to the hydrocarbons in petroleum, biomass contains higher levels of oxygen and nitrogen, offering a different set of chemical possibilities.
"We essentially redesigned the epoxy amine resins — today's thermosets in carbon fiber — with epoxy and anhydrides synthesized from biomass, predominantly from the biological and chemical conversion of sugars," Rorrer explained. "We have shown that [the] reformulated resin can maintain and/or exceed all the same properties as in today's epoxy amine resins, but also make them recyclable by design, and at room temperature."
Using a special catalyst, the NREL team was able to break down the bio-based resin at room temperature, a process known as "depolymerization." That allowed them to recover the carbon filaments while maintaining their quality and alignment.
No downcycling involved"We can actually maintain the fiber quality over at least three material lives," Rorrer said. "So not only are we able to recycle it, we are able to recycle it without any detriment to properties. We are not downcycling the material at all."
Combined with NREL's research into low-cost, bio-based acrylonitrile as a carbon-fiber precursor, which earned an R&D 100 award in 2018, the breakthrough in epoxy could go a long way in making carbon-fiber composites more cost effective and environmentally friendly.
Being able to extract and recycle the carbon fiber could make the material more economical for mass-market electric vehicles, freeing up weight and space for batteries. It would also lower the material's GHG footprint by 20 to 40%. Better yet, it could achieve all that without increasing manufacturing costs, as Rorrer estimates NREL's epoxy could be produced for roughly the same price as today's petroleum-based epoxy-amine resins.
"By using bio-based feedstocks instead of petrochemical feedstocks, we don't have to use extra energy to dramatically retool their chemistries," Rorrer added. "That allows us to more precisely, cheaply, and effectively design advanced materials with performance and environmental advantages."