Closing in on a carbon fiber solution
Stronger than steel and a third its weight, carbon fiber is a hot commodity—not so much for what it does, but for what it could do.
Today carbon fiber is found in fast cars, jetliners and specialty sporting goods from bicycles to bass boats. Because it's a lot more expensive than steel, however, it hasn't been able to make the jump to the broader consumer market for inexpensive cars and other everyday items.
What would cheaper carbon fiber mean? One quick example: The US Department of Energy estimates that competitively priced carbon fiber could reduce the weight of key vehicle components by more than 60 percent—dramatically increasing gas mileage. The appeal of more economical transportation is undeniable, so it's not surprising that DOE is working with auto companies and other manufacturers to make cheaper carbon fiber a reality.
That's where ORNL comes in. The laboratory has been investigating various methods of reducing the cost and increasing the strength of carbon fiber for years. One of these efforts uses lignin, a common manufacturing byproduct, as the raw material or "precursor." Lignin is a rigid, woody material that allows trees and other plants to stand upright. It's also churned out in huge quantities by paper mills and biofuel refineries.
"We're investigating lignin because 50 percent of the cost of manufacturing conventional carbon fiber is the cost of the precursor, and lignin is relatively inexpensive," ORNL materials scientist Amit Naskar explains. "Today 90 percent of carbon fiber is manufactured from polyacrylonitrile, or PAN—a material that is chemically similar to the synthetic acrylic fabric used in clothes. Although versatile, PAN is relatively expensive, and it's petroleum-based."
Part of the mission for Naskar and his colleagues from the laboratory, the University of Tennessee, and the Georgia Institute of Technology and industrial partners from the Carbon Fiber Composite Consortium has been to find a lower-cost, renewable alternative to PAN.
Naskar describes the project as an "integrated interdisciplinary program" that includes researchers from ORNL's BioEnergy Science Center, who are investigating what genetic characteristics yield the best lignin for creating carbon fiber; polymer chemists, who are developing ways of chemically modifying the lignin to provide better carbon fiber; and composite researchers, who are developing protocols for conventional composite fabrication and printing composites at ORNL's newly built manufacturing demonstration facility.
"An interdisciplinary team provides a means of attacking a problem from a number of different sides," Naskar explains. "Creating cheaper, stronger carbon fiber is our goal, so anyone who has a new idea for improving the process or the product is welcome to work on that. Each research group has its own goals, but we share information and developments with one another."
"For instance," he says, "bioscience colleagues recently told us that they had isolated a different type of lignin and an associated gene that was behaving differently from others in a particular species of tree. We suggested extracting the lignin and chemically analyzing it to examine the interconnections among molecules to see if it might have applications in carbon fiber production. If we weren't working together, I would never have known about this development. My polymer science colleagues found a composition that provides a higher carbon yield after modification of the lignin, and we are working on producing fiber from such compositions."
Quite a few of the lab's interdisciplinary resources have also been focused on the problem of revealing and refining lignin's molecular structure to make it a better fit for the carbon fiber production process.
Naskar describes lignin as "a very difficult molecule." Lignin has an irregular, threedimensional nature that is problematic because the raw material for carbon fiber must first be extruded into spaghetti-like filaments called "fiber tow" or spun into mats of interconnected thread-like fibers that are eventually converted to almost pure carbon through a sequential heating process. "It's not easy to process lignins into a common form that can be spun into fiber," Naskar explains. "We need to determine how much we can do, in terms of chemistry, to make it more suitable for the spinning process."
Naskar notes that the task of analyzing and understanding lignin's structure is made easier by the lab's extensive resources. He and his colleagues are using two DOE user facilities at ORNL. Both the unique characterization tools at the Center for Nanophase Materials Sciences and the neutron scattering capabilities of the High Flux Isotope Reactor have been vital to understanding the various configurations of lignin molecules.
"Sometimes people ask us why we are trying to understand lignin on the molecular level rather than spending our time developing the carbon fiber, " Naskar says. "I always say these activities are interconnected. The structure of lignin molecules has important implications for the process of creating carbon fiber. Once we demonstrate the feasibility of producing carbon fiber using conventional techniques, then we can investigate the advanced processing methods that are being developed by other ORNL researchers."
The team is also working with biofuel refineries and paper mills to modify their processes to create lignin with properties that are better suited to carbon fiber processing. This "high-quality" lignin will theoretically require less filtering and chemical modification.
"We have evaluated 27 lignin samples, both from biorefineries and pulping operations," Naskar says, "and we are continuing to work with those who can provide us with large quantities that might enable us to scale lignin fiber production to a level that it could be used as a commercially viable feedstock for carbon fiber."
A challenging task
Naskar emphasizes that developing lowcost carbon fiber will be a high priority for years to come—not only for the laboratory and DOE, but also for dozens of businesses and industries.
"We have a target of demonstrating a path for improvement in the properties of this material within two or three years," he says. "Then we can work with industry to scale up the process. This would offer an excellent opportunity to extensively use the Carbon Fiber Technology Facility that DOE's Office of Energy Efficiency and Renewable Energy built at ORNL to demonstrate that we can produce industrial quantities of low-cost carbon fiber."
To ensure that interaction with industry and research partners remains at a high level, the laboratory also started the Oak Ridge Carbon Fiber Composites Consortium, which includes more than 50 industrial partners, including carbon fiber manufacturers, automotive companies, paper companies, and other businesses related to biomass and lignin production, all dedicated to promoting innovation in the carbon fiber production process.
"It is a very challenging task," Naskar admits, "but we're an integrated team, and we're working together to solve it." —Jim Pearce