Few materials used in manufacturing today are as versatile and desirable in performance applications as carbon fiber. Also known as graphite fiber and carbon graphite, carbon fiber is made up of thin strands of carbon that vary in diameter from four to ten microns, dependent on manufacturer and use. Do not let the size of these fibers fool you. Carbon fiber is regarded as one of the lightest and strongest materials on Earth. Compared to a unit of steel, carbon fiber is up to ten times stronger, two times stiffer, and 66% lighter.
These characteristics make carbon fiber an ideal material for use in the aerospace industry. Its lighter weight, higher fatigue strength and temperature tolerance, and superior corrosion resistance give carbon fiber a distinct advantage over metals historically used in the industry, such as steel and aluminum.
Take fatigue strength as an example. Carbon fiber’s high fatigue strength translates to a much lower loss of strength (20%) compared to aluminum (50%) under similar stresses. Its performance over large temperature ranges is also advantageous. Unlike metals that expand and contract under extreme temperature changes, carbon fiber maintains a much tighter tolerance and does not stress as easily. In the long run, using carbon fiber in aircraft components can save money in terms of lower maintenance costs due to more durable, longer-lasting equipment, and reduced fuel consumption due to lighter aircraft.
Carbon fiber is already making waves in the aerospace industry. Carbon fiber reinforced composites are being used in the construction of aircraft parts such as fuselages, doors, wings, and tails. The potential exists to increase its use across a wider range of applications, especially in areas where shaving weight is a primary consideration. Despite its potential and obvious appeal, the high cost of raw materials and the energy and capital-intensive nature of its production process limit widespread adoption of carbon fiber beyond current applications in the industry.
Traditional Carbon Fiber Production Process
Today’s carbon fiber manufacturing process has not changed much since commercialization began in the 1960s. The process involves converting bundles (called tows) of polyacrylonitrile (PAN), a carbon-containing polymer fiber, to pure carbon fiber through a carefully controlled series of heating and stretching steps. These steps include:
Spinning – PAN is chemically modified using a proprietary mix of ingredients to maximize the resulting carbon fiber’s mechanical properties. It is then spun into fibers that are washed and stretched.
Oxidation – Strands of PAN fibers are fed through a series of heated ovens, which facilitate chemical conversion of the polymer’s nitrile groups.
Carbonization – Once the fibers are stabilized, they are heated to progressively higher processing temperatures (up to 2,000°C) in an inert (oxygen-free) environment, typically nitrogen, to remove the fibers’ remaining hydrogen and nitrogen molecules, creating hexagonal “ladder-like” graphitic planes.
Surface Treatment and Sizing – Fibers are processed through various treatments (dependent on the manufacturer) to improve the bonding properties of the fibers’ surface in preparation for their formation into composite materials.
Of these steps, the oxidation process poses the greatest challenge to scaling carbon fiber production. The processing time required for fiber stabilization of industrial tow sizes (24k and above) typically ranges from 80 – 110 minutes for conventional methods, accounting for approximately 97% of the entire carbon fiber production process. The amount of time spent in the oxidation stage makes it the most energy-intensive step, using up to 7.5 megawatts per hour. The energy expended during oxidation is used to maintain a temperature range of 200 - 300°C and requires a high rate of atmosphere-turnover to maintain a non-combustible atmosphere during processing.
New Rapid Oxidation Technology
Carbon fiber manufacturers now have access to a new technology that decreases oxidation process time from 80 – 110 minutes down to less than 15 minutes, making it possible to produce 360% more carbon fiber by volume in less time and with lower capital and energy costs. The technology was developed by Carbon Nexus, an open access carbon fiber and composite research facility located at Australia’s Deakin University, and it has since been licensed to LeMond Carbon.
While the conventional method requires long process times to allow oxygen molecules to penetrate the PAN fibers allowing for controlled crosslinking of the polymer’s nitrile groups, this new technology creates an environment where the formation of nitrile groups is obtained in a controlled manner by creating a “catalytic-like” reaction. This “catalytic-like” effect creates a chain reaction and speeds up chemical conversion of the fiber’s nitrile groups. The new technology uses the same PAN fiber precursor that is currently employed by carbon fiber manufacturers. This allows for a carbon fiber product that not only fits into most manufacturers’ applications with little to no retooling required but possesses the same advantageous properties that manufacturers have come to expect, with values for tensile strength, tensile modulus, and strain to failure exceeding 3.5 Gpa, 270 Gpa, and 1% respectively.
There are multiple benefits associated with this new technology. First, there is the energy savings. Due to a shorter processing time, the oxidation stage uses up to 75% less energy compared to the conventional approach. This translates to both lower energy use and lower gaseous emissions produced during processing. This technology also has a smaller infrastructure footprint, employing up to 75% less process equipment compared to a conventional carbon fiber manufacturing facility, per kilogram of carbon fiber produced. This smaller footprint, along with reduced energy and capital costs, will enable manufacturers to localize their operations and ramp up production so that they can keep up with market demand. These factors are key for broadening carbon fiber’s applications in the aerospace industry.
Third-Party Verified Technology
In July 2019, LeMond Carbon sought third-party verification of its carbon fiber rapid oxidation technology. The independent technical audit was conducted on the 100-ton per annum pilot line at Deakin University’s Carbon Nexus facility by Bureau Veritas (BV), a world leader in laboratory testing, inspection, and manufacturing processes. During the audit, BV measured oxidation times, assured process traceability, and oversaw fiber sampling, packaging, and shipping of audit samples to BV’s laboratories in Pessac, France for extensive testing. Composite tow tests of the fiber were completed according to ASTM D 4018-17 standards.
Audit results were publicly released in December 2019. Ultimately, BV measured total oxidation times of sub-15 and sub-20 minutes over two separate production campaigns of 24K standard modulus carbon fiber, achieving fiber tow properties in excess of 270 GPa tensile modulus and 3,500 MPa tensile strength. BV’s findings provide validation that this rapid oxidation technology delivers on its promise to significantly reduce oxidation process times while continuing to produce a high performing carbon fiber end product.
LeMond Carbon views BV’s third-party verification as a critical step in obtaining industry validation of this new technology and its ability to revolutionize how carbon fiber is manufactured. The company is currently producing samples on Deakin University’s Carbon Nexus pilot line for trials with target customers. It hopes to increase customer sampling and testing in the coming months to show potential customers how the fiber produced from this rapid oxidation technology can easily integrate into their current manufacturing processes and composite components.
Advanced materials and composites will continue to play a major role as the aerospace industry seeks new ways to create lighter, safer, more durable, and environmentally friendly aircraft. Access to low-cost, high-quality carbon fiber will undoubtedly pave the way for further industry innovation, which is currently encumbered due to high carbon fiber costs. By speeding up the oxidation process and lowering cost, LeMond Carbon’s rapid oxidation technology is opening the door to greater adoption and innovation of carbon fiber products, not tomorrow, but today.
This article was written by David Church, Chief Technology Officer, LeMond Carbon (Oak Ridge, TN). For more information, visit here .