The low-cost process
According to ORNL, more than 90 percent of the energy needed to manufacture advanced composites is consumed in manufacturing the carbon fiber itself. Reduction in energy consumption in manufacturing will enable earlier net energy payback – energy savings gained in using products made from lighter weight material compared to the energy consumed in making the material. A detailed analysis compared the new process to a published baseline for conventional carbon fiber production. Cost factors were considered for nine major process steps, starting with the precursor and pre-treatment and finishing with surface treatment, sizing, winding, inspection and shipping.
Carbon fiber is produced by converting a carbon-containing polymer precursor fiber to pure carbon fiber through a carefully controlled series of heating and stretching steps. In current commercial practice, the precursor – polyacrylonitrile, or PAN – is chemically modified and optimized to maximize the mechanical properties of the end product. The high cost of specialty precursor materials and the energy and capital-intensive nature of the conversion process are the principal contributors to the high cost of the end product.
However, acrylic fiber of similar chemistry is produced on a commodity basis for clothing and carpets – a high-volume product that costs roughly half as much as the specialty PAN used in the carbon fiber industry. ORNL researchers believed textile-grade PAN was a pathway to lower-cost carbon fiber, but laboratory- scale experiments could not fully explore its potential at a pro- duction scale. In order to provide that capability, the US Department of Energy’s Advanced Manufacturing and Vehicle Technologies offices have funded research and operations at ORNL’s Carbon Fiber Technology Facility, a highly instrumented, semi-production scale carbon fiber conversion plant. Extensive mechanical property tests have been performed on carbon fiber from the new process, and several automotive manufacturers and their suppliers received quantities suitable for prototyping, with encouraging results.
The Carbon Fiber Technology Facility at ORNL was designed, manufactured and installed by Harper International – a company specializing in thermal processing solutions and technical services essential for the production of advanced materials. The 118-meter (390-foot) process line has been designed to be flexible and highly instrumented to demonstrate advanced technology scalability and produces market development volumes of prototypical carbon fibers, and serves as a key step before commercial production scale. With a production capacity of 4.3 kilograms per hour, it allows industry to validate conversion of carbon fiber precursors at semi- production scale.
With a rated capacity of 25 tonnes per year based on 24k PAN tows, the carbon fiber line is configured for PAN, polyolefins, lignin and pitch precursors, as well as being upgradable for rayon and high-modulus carbon fibers. Internally, it has been designed with high degree of corrosion resistance for alternative precursors. The facility is designed for 3k to 80k tows and web up to 300 mm wide by 12.7 mm loft. An oxidation temperature of 4008C is possible with airflow configurable for parallel, cross or down flow, and driven pass-back rollers are installed for slip prevention at low loading. Optimized for faster oxidation through elimination of the chimney effect, there is improved velocity uniformity and range, as well as an assurance of temperature uniformity at a variety of flow rates. Harper says that flexible internal design throughout the line allows material processing in either tow (unsupported) or web (supported by belt) formats. Low temperature carbonization up to 1000 C is possible with the capability to produce structural or micro/Nano-porous fibers. High-temperature carbonization to 2000 C can also be achieved. A post-treatment is system designed for compatibilizing fibers with performance or commodity resins.