Some properties of PAN-based carbon fiber

In the late 19th century, T. A. Edison, a great inventor, applied a carbon filament made from bamboo fibers grown in Kyoto to an incandescent lamp, wh ich was one of the earliest technical applications of carbon fiber. The nee d for carbon fiber for use in military aircraft originated shortly after Wo rld War II in the U.S.A. as carbon fiber reinforced structural materials. S ince the early 1950s, many varieties of reinforcing fibers have been develo ped and the UCC demonstrated high-performance carbon fiber made from rayon. However, its tensile strength and elastic modulus were not particularly hi gh. A technical and commercial breakthrough for high-performance carbon fib ers occurred in the mid 1960s-the production of carbon fibers from polyacry lonitrile (PAN) precursor fibers. This process proved to be more economical due to the lower cost of the PAN precursor fiber and the simpler process r equired to convert PAN fiber to carbon fiber. Shindo first reported the use of PAN as a precursor for carbon fibers. At that time almost no military a pplication of engineering materials was considered in Japan. The history of these carbon fibers can be found in "Carbon Reinforcements and Carbon/Carb on Composites," by E. Fitzer and L. M. Manocha, Springer-Verlag (1998). Shindo's invention appeared as Japanese Patent No. 304892 (1963, applied fo r in 1959). His claim was a method of manufacturing carbon or graphite mate rials, which comprises heating PAN polymer up to 350 degrees C in an oxygen -rich atmosphere followed by heating to a temperature above 800 degrees C. The heating PAN fiber at lower temperatures in an oxidizing atmosphere does not cause melting or deformation of the fiber during subsequent heating. T his heat treatment was referred to as a stabilization process and became a fundamental process for conversion of organic precursors to inorganic fiber s. Shindo's paper describes, (1) the growth of crystallites in two kinds of PA N-based carbon fibers, (2) changes in the mechanical properties and (3) cha nges in the electrical resistance by heat treatment in the temperature rang e, 1000 to 3000 degrees C. From the viewpoint of the remarkable development of carbon fiber applications subsequently, the available data on mechanica l and crystallite growth must be significant. High grade of preferred orien tation of graphite layer planes parallel to the fiber axis, which is shown in X-ray diffraction photos and electron diffraction pattern, could be due to the high-strength and high-modulus of the fiber. Furthermore, after heat treatment at a temperature as low as 1000 degrees C, the fiber assumed sig nificantly higher preferred orientation. This shows that a high-modulus car bon fiber can be more easily manufactured from PAN fiber than rayon-based f iber. The density of the carbon fibers vs, heat treatment temperature curve (Fig. 8) shows a fairly large increase above 2000 degrees C. The electrical resi stivity vs. heat treatment temperature curve (Fig. 9) shows three regions: a steep decrease below 1000 degrees C, a slight decrease from 1000 to 2300 degrees C and a very small decrease above 2300 degrees C. Figure 10 shows t hat the tensile strength of the fibers decreases steadily from 5000-10000 t o 2000-5000 kgf/cm(2) with raising heat: treatment temperature from 1000 to more than 2500 degrees C. The elongation at the breaking point decreases f rom 1 to 0.3% with raising heat treatment temperature, as shown in Fig. 12. Young's modulus, as determined from these curves, increases from about 110 00 kgf/mm(2) at a heat treatment temperature of 1000 degrees C to 15000 kgf /mm(2) at 3000 degrees C with a maximum at 2000 degrees C, as shown in Fig. 13. The content of the Shindo's paper appeared in Govt. Res. Inst., Osaka, Report No. 317 (1961) in English, which has been widely cited in many arti cles concerning carbon. Now commercial carbon fibers are mainly manufactured from three precursor m aterials: rayon, PAN and pitch. Japan makes 70% of the world production of carbon fibers. For applications carbon fiber is rarely used alone. Carbon f ibers are widely used due to their high specific property: tensile strength and elastic modulus divided by density, Carbon fiber reinforced carbon mat rix composites (c/c composites) are now applied to spacecraft and aircraft, and also used in brake pads, sporting goods, structural components, medica l parts and so on. Finally a brief personal history of Dr. Shindo: He was born in 1926, acquir ed a position in the Government Industrial Research Institute, MITI in 1952 after graduating from Hiroshima University and retired from GIRI in 1987. Re was awarded numerous prizes such as the Chemical Society of Japan Award for Technical Development in 1977, a Purple Ribbon Medal from the Japanese Government in 1977, and the Centennial Award from the Ceramic Society of Ja pan for Advancement of Ceramic Technology in 1991, among others.