Researchers at the National Renewable Energy Laboratory (NREL) have shown that making carbon fiber composites with bio-based epoxies and an anhydride hardener makes the material fully recyclable by introducing linkages that are more easily degraded.
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. When produced with a mold, the material can take a range of shapes for a variety of applications, from car bumpers to wind turbine blades and more.
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.
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 of more than 1,000 °C.
NREL scientist Nicholas Rorrer 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 amines resins—today’s thermosets in carbon fiber—with epoxy and anhydrides synthesized from biomass, predominantly from the biological and chemical conversion of sugars. We have shown that that 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.—Nicholas Rorrer
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 the recovery of the carbon filaments while maintaining their quality and alignment.
We can actually maintain the fiber quality over at least three material lives. 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.—Nicholas Rorrer
Combined with NREL’s research into low-cost, bio-based acrylonitrile as a carbon fiber precursor, 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%–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.