Carbon Fiber Manufacturing and Properties
Carbon fiber and carbon fiber cloth consist of bulk, chopped fibers, continuous strands or woven cloth forms of carbon or graphite. Carbon and graphite are used in reinforcing composites as well as other specialized electrical and thermal applications. Carbon fiber is made by charring synthetic polymer fibers made of polyacrylonitrile (PAN) by using an oxidation or thermal process. Carbon monofilament is composed of many long thin sheets of carbon molecules. The filaments are then spun into thread and woven into cloth, rope or braided tubes. Alternatively, carbon filaments are chopped and then blended into resins or formed into nonwoven cloth.
Carbon materials have a unique combination of exceptional properties:
- High strength (σUTS or σMOR) and stiffness (E) with low density (ρ) - High specific strength (σ /ρ) and high specific modulus (E/ρ)
- Chemical or corrosion resistance
- Refractoriness or elevated temperature resistance (use temperature is limited by resin matrix)
- High electrical (ɣ) and thermal conductivity (λ)
- Low coefficient of thermal expansion (CTE or α)
- Good thermal shock resistance (thermal shock resistance Rs = (λ x σMOR) / (α x E)
Carbon fiber reinforced plastic (CFRP) composites have an unexcelled combination of specific strength and specific modulus. The specific strength vs. specific modulus Ashby chart in figure 2 (from material selection charts and software from Grant Design) demonstrates CFRP's high specific strength (σ /ρ) and high specific modulus (E/ρ) compared to other materials. Material and fabrication costs continue to impede the wider adoption of carbon fiber in many applications.
Fabricating Carbon Fiber Products
Carbon fiber used in structural applications is typically a carbon fiber composite or carbon fiber reinforced plastic (CFRP). Composites and composite materials are formed through a variety of processes. Examples include resin formulation, solution pre-pegging, wet filament winding, automated tape lay-up, resin infusion (liquid molding, SCRIMP), pultrusion, compression molding and injection molding.
Pre-pegging involves the application of formulated resin products to the carbon fiber reinforcement material. Carbon fiber reinforcements in roving, woven cloth, cordage, braided sleeve or nonwoven mats are saturated with a composite matrix resin and then partially cured to allow easier handling and processing into carbon fiber parts.
Wet filament winding draws continuous carbon fiber reinforcement materials through a container of resin mixture. The wet filaments are wound around a mandrel to form lightweight, high-strength pressure vessels and gas cylinders. Automated tape lay-up feeds a pre-peg carbon fiber tape material through an automated tape machine. Pultrusion pulls continuous roving carbon fiber strands from a creel through a strand-tensioning device into a resin bath.
Molding is one of the oldest methods for producing composites. Carbon fiber sheet molding compounds (SMC) and bulk molding compounds (BMC) consist of mixing chopped carbon fibers, epoxy or other resins and additives such as curing agents or accelerators to achieve specific performance parameters. Sheet molding compounds consists of a thin layer of the uncured composite between sheets of release liners. The carbon fiber molding compounds with thermoset resins are formed into products using compression molding, which applies heat and pressure. Carbon fiber reinforced thermoplastic molding compounds can be injection molded. The CFRP molding compound is heated to melt the resins and then pressure is used to inject the CFRP compound into a metal mold. Injection molding can be easily automated. However, thermoplastic resins may not provide the thermal, mechanical and cost performance of thermoset resins required for certain applications.
The vacuum bagging and SCRIMP (Seemann Composites Resin Infusion Molding Process) are resin infusion processes to mold very large products such as tanks or boat hulls. In SCRIMP, carbon fiber is laid up in a mold by hand, covered with a plastic bag and sealed and then the air in carbon fiber is evacuated using a vacuum. Finally, resins are infiltrated into the carbon fiber and thermally cured. Liquid molding or resin transfer molding (RTM) is a production infusion molding process where a dry carbon fiber preform in a closed metal or composite mold is infused with resin. The dry preforms used in RTM are typically less expensive than pre-preg material and do not require refrigerated storage. Resin transfer molding produces thick, near-net shape parts, which require minimal post fabrication or finishing.
Carbon Fiber Applications
Carbon fiber has been used for years in many aerospace products where the cost of the carbon fibers is not a concern. Carbon fiber reinforcement is also used in leisure products such as skis, tennis rackets, sails, fishing rods, golf clubs, camera tripods, laptop computer, yachts and sports cars. The high material costs are not an issue because consumers are willing to pay a premium for the carbon fiber’s real or perceived light weight and high strength.
A load bearing carbon fiber monocoque chassis is a key component in both high-end sports cars and Formula 1 race cars. A monocoque is a structural cell that typically consists of carbon fiber skin layers surrounding a honeycomb core. Reducing vehicle weight can increase performance and fuel economy. The Department of Energy indicates a 10% reduction in vehicle weight translates into a 6 to 7% increase in fuel economy. General Electric’s GE9X jet engine uses carbon fiber fan blades as well. The lighter, stronger blades increase air flow and improve fuel efficiency by 5% compared to similar engines. The reduced fossil fuel consumption in the automotive and aerospace application contributes to a cleaner environment.
Carbon fiber wind turbine blades improve the wind energy to electric power conversion efficiency, which creates more green electrical power. Carbon fiber provides a weight savings of 25% over fiberglass blades, but carbon fiber blades cost 28% more compared to fiberglass, according to Christopher Monk, engineering manager for StrucTeam Ltd., in Carbon Fibre: Challenges and Benefits for use in Wind Turbine Blade Design [.pdf]. Light weighting also enables the use of thinner sections in the turbine and tower components, which provides a cost savings to counter the higher cost of carbon fiber. Carbon fiber blades can be found in Vestas, Gamesa, Enercon, AREVA and GE wind turbines.
Carbon fiber is also important in other renewable energy segments. Carbon fiber battery felt, bipolar plates and graphite foils are important in the electrical energy storage and fuel cell industries. Carbon fibers have extreme corrosion resistance and can withstand the acids and electrolytes with a battery. Carbon fiber paper is being developed as a gas diffusion electrode layer within fuel cells. Battery and fuel cell electric vehicles will have a large impact on reducing greenhouse gases (CO2), NOx and particulates.
The refractoriness and chemical resistance of carbon is employed in high temperatures furnaces and heat exchangers for silicon and solar cell materials processing. NASA has developed a new class of phenolic and carbon fiber reinforced phenolic composites for thermal protection systems or ablative heat shields, which will enable spacecraft to return to earth without burning up. The high heat resistance of carbon fiber reinforced carbon (CFRC) composites is utilized in rocket nozzles, leading edges of missiles, friction materials in aircraft brakes, glass melt processing equipment, vacuum furnaces and chemical processing systems parts (stirrers, feed pipes, column packing, etc.).
Overcoming the Carbon Fiber Cost Barrier
The cost and availability of carbon fiber has impeded the wider option and use of carbon fiber or CFRP composites. Many recent developments in carbon fiber technology are coming together to make low cost fiber a reality in the near future. Carbon Fiber Technology Facility (CFTF) at Oak Ridge National Laboratory (ORNL) has developed new technology for manufacturing low-cost, high-volume carbon fiber for transportation, energy and infrastructure industries. The ORNL process uses textile grade acrylic fiber as a precursor to produce 400,000 to 600,000 tow carbon fiber with tensile strength of 400 ksi, tensile modulus (E) of 40 Msi and strain to failure of 1%. The tow is the count of individual carbon filaments in a bundle or strand used to weave carbon fabric. A 400,000 tow means there are 380-400,000 carbon filaments in the tow.
The ORNL technology can reduce carbon fiber production costs by more than 50 percent, as well as reduce energy consumed during production by up to 60 percent. ORNL has also developed an advanced carbon fiber plasma oxidation process, which can dramatically shorten production times.
“In conventional systems, it generally takes between 80 and 120 minutes for oxidation,” said ORNL co-inventor Felix Paulauskas. “We found a way to cut the time by a factor of 2.5 to 3 times, so we can process fiber in 25 to 35 minutes.”
RMX Technologies has licensed the plasma oxidation process and expects to sell its first plasma oxidation oven before the end of the year. An RMX subsidiary, 4M Industrial Oxidation, will jointly manufacture and license the technology with C.A. Litzler, a manufacturer of ovens, dryers and carbon fiber production equipment. 4M Industrial Ovens slogan is “The Gatekeeper to Low-Cost Carbon Fiber.” They claim that, "The patented atmospheric plasma technology produces an equal or better fiber. Our oven is 1/3rd the size of conventional technology for the same production capacity, and utilizes 75% less energy. The technology reduces the cost of carbon fiber by approximately $3 per pound.” 4M has extensive experience in the oxidation process for carbon fiber production.
LeMond Composites has licensed the ORNL technology and has also signed agreements to license Deakin University’s low cost carbon fiber process. Deakin University’s process oxidizes carbon fiber precursors faster with 75% lower energy input and less equipment.
Greg LeMond, founder of LeMond Composites, has said, "Carbon fibre, up until recently, has been a dream material for automotive and for so many different industries but the cost has been prohibitive.”
The typical precursor for carbon fiber is polyacrylonitrile (PAN), which is a petrochemical. Southern Research has develop a “green” carbon fiber precursor. Amit Goyal, a research manager at Southern Research said, “… we have developed an innovative, thermocatalytic process that converts second generation sugars obtained from biomass to acrylonitrile." The Southern Research process could economically produce a sustainable replacement for petroleum acrylonitrile, which would reduce greenhouse gas (GHG) emissions by up to 40 percent.
The benefits to the environment must be weighed against the environmental impact of producing carbon fibers and carbon fiber products such as:
- Fugitive emissions during the conversion of PAN resins to carbon fibers
- Airborne carbon fiber particulate emissions
- Fugitive emission of catalysts, solvents and resin fumes during CFRP molding or fabrication
Carbon fiber manufacturers and processors must comply with the EPA regulations and guidelines to monitor and restrict pollutants. Carbon fiber products reduce fuel consumption through weight reduction and are a critical component in many green energy products, so carbon fiber products are a major asset to the environment and economy. Cost and time reduction in carbon fiber production and new composite manufacturing technologies are disruptive technologies that will enable rapid increase in carbon fiber materials in many new product applications.