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High-Performance Structural Fibers for Advanced Polymer Matrix Composites
DEVELOPMENT OF HIGH-PERFORMANCE FIBERS
The development pathways of high-performance carbon and organic fibers have been driven by decidedly different cost and performance requirements. These different pathways strongly affect the future prospects for military and commercial applications of these fibers.
High-Performance Carbon Fibers
Carbon fibers are typically produced by spinning and then thermally carbonizing one of three types of precursor fibers: polyacrylonitrile (PAN), pitch, or rayon. Depending on the type of precursor and the processing method, the finished carbon fiber will have a different microstructure and therefore different properties. PAN-based fibers have a disordered microstructure that typically confers higher tensile and compressive strengths, while pitch-based fibers have a more crystalline microstructure that results in a higher tensile modulus and much higher (100 times) thermal conductivity. In general, PAN-based fibers dominate applications where strength is critical, and pitch-based fibers dominate applications where heat transfer or stiffness (i.e., more than 80 Mpsi fiber modulus) is important. Around 90 percent of all commercial carbon fibers are produced by the thermal conversion of PAN precursor fibers.4
As the carbon fiber industry matured during the 1980s and costs began to decrease, a variety of commercial applications for high-performance composites emerged, including sporting goods, commercial aircraft, and industrial applications. In 2003, the world market demand for PAN-based carbon fiber was approximately 38 million pounds, divided almost equally (10 million to 13 million pounds each) among North America, Europe, and Asia. Aerospace applications predominate in North America, while industrial and aerospace applications are emphasized in Europe, and sports and leisure are the primary applications in Asia.5
As a result, DoD usage, which dominated the U.S. requirements in the 1970s and 1980s, became a smaller part of the total market. In 2003, the lowest prices observed for carbon fibers were $5.25 per pound for a standard-modulus (32 Mpsi) fiber and $17 per pound for an intermediate-modulus (42 Mpsi) fiber. The DoD market was just under 10 percent of the total U.S. carbon fiber market and 4 percent of the world carbon fiber market.
Until the late 1980s, special acrylic fiber (SAF) precursor was used for nearly all PAN-based carbon fibers. This precursor fiber is produced in filament bundle (or tow) sizes of 3,000 (3k), 6,000 (6k), and 12,000 (12k) filaments, and is normally supplied on spools without applying a crimp or twist. The majority of current defense applications are based on the performance and consistency of properties provided by these small-tow, SAF-based carbon fibers.
In response to the emergence and projected growth of such commercial carbon fiber markets as sporting goods, construction, and transportation, a sector of the carbon fiber industry has evolved to produce fibers in large volumes at relatively low cost. In one sector, commercially available PAN fibers are used as precursors to produce carbon fibers in large tow counts, having up to 24k filaments. These precursor fibers are normally used in the textile industry and draw on an approximately 5-billion-pound annual market to bring about lower material costs.
Another sector of the fiber supply base has emerged to meet the increasing demand for commercial applications such as Boeing’s 7E7 aircraft using SAF fiber precursors. Japanese companies are establishing domestic capacity for both SAF precursor plants and fiber lines. For example, prices for T700 12k and 24k tows of $5.25 per pound, a historic low, were not uncommon in 2003. At this price, they represent a cost-competitive alternative to the large-tow-count textile-based fibers, with performance comparable to that of the current SAF fibers approved for use by DoD. The combination of lower cost potential and equivalent performance makes these fibers very attractive for future DoD systems. Today, there is another shortage of SAF-based carbon fiber and prices have risen, demonstrating that $5.25 is not a sustainable price for SAF aerospace-grade carbon fiber.
goal set by Dr. Sikkema was to develop a high-strength synthetic fiber that excelled as both a ballistic fabric and a composite material.
Intertech. 2004. The Global Outlook for Carbon Fiber. Proceedings of a conference in Hamburg, Germany, October 18-20. Portland, Me.: Intertech Corporation.
Fiber Organon. 2004. U.S. Manufactured Fiber Capacity, Production & Utilization Review. January, p. 7.