US researchers fully recycle CNT fibers to expand material lifecycle

Researchers at William Marsh Rice University (Rice) in Houston, US, have successfully demonstrated the full recyclability of carbon nanotube (CNT) fibers. These findings make CNTs a viable alternative to traditional materials like metals and polymers, which contain larger and more difficult to recycle carbon fibers.

“Recycling has long been a challenge in the materials industry — metals recycling is often inefficient and energy intensive, polymers tend to lose their properties after reprocessing, and carbon fibers cannot be recycled at all, only downcycled by chopping them up into short pieces,” says corresponding author Matteo Pasquali, director of Rice’s Carbon Hub and a professor of Chemical and Biomolecular Engineering, Materials Science and NanoEngineering and Chemistry.

“As CNT fibers are being scaled up, we asked whether and how these new materials could be recycled in the future so as to proactively avoid waste management problems that emerged as other engineered materials reached large-scale use,” he continues.

“We expected that recycling would be difficult and would lead to significant loss of properties. Surprisingly, we found that CNT fibers far exceed the recyclability potential of existing engineered materials, offering a solution to a major environmental issue.”

The study findings were recently published in the journal Carbon.

Recycling CNTs

The research team created solution-spun CNT fibers by dissolving fiber-grade commercial CNTs in chlorosulfonic acid, a common industrial solvent. 

The researchers find that CNT fibers “far exceed” the recyclability potential of existing materials (Image credit: Rice).The scientists set out to assess the effect of multiple material sources on the fiber manufacturing process and fiber properties because end-of-life recycling brings together materials manufactured by different companies using different processes.

Initially, the team divided fibers made from different types of CNTs produced by different manufacturers into separate single-source virgin fibers, then recycled them by mixing in chlorosulfonic acid.

The researchers were surprised to find that mixing the two fibers led to complete redissolution. There was no indication left that the two source materials were separated. The resulting redissolved material was spun into a mixed-source recycled fiber with the structure and alignment of virgin fiber.

Co-first author Michelle Durán-Chaves, a graduate student in chemistry, says: “By using two different sources of carbon nanotubes, we ensured that our recycling process was representative of real-life conditions.”

“Remarkably, the recycled fibers demonstrated equivalent mechanical strength, electrical conductivity, thermal conductivity and alignment, which is unprecedented in the field of engineered materials.”

Extended lifecycle

The team says that CNT fibers are a “promising” sustainable material, offering full recyclability and no loss in quality during the  recycling process.

When scaled, the CNT recycling process will reduce waste, energy consumption and CO2 emissions (Image credit: Rice).Co-first author Ivan R. Siqueira says: “This preservation of quality means CNT fibers can be used and reused in demanding applications without compromising performance, thus extending their lifecycle and reducing the need for new raw materials.”

The researchers explain that material quality is vital to ensuring the efficiency of the recycling process. Their study demonstrates that CNT fiber recycling is more efficient than traditional recycling methods for metals and polymers, which tend to involve high energy use, hazardous chemicals or labor-intensive sorting.

CNT fibers can be recycled without sorting as fibers from various sources can be combined to produce high-quality recycled materials. Once these materials reach scale, this simple recycling process will significantly reduce waste, energy consumption and carbon dioxide emissions associated with materials manufacturing.

Durán-Chaves concludes: “The ability to fully recycle CNT fibers has broad implications for industries like aerospace, automotive and electronics, We hope this could pave the way for fully recyclable composites in aircraft, vehicles, civil infrastructures and more, ultimately reducing environmental impacts across a wide range of sectors.”