Ultrahigh modulus carbon fibers

L Steel J (ceramic fiber) Ultrahigh modulus carbon fiber... 

Pyrolysis of pitch fibers yields low cost fibers with low strength (1 GPa) and low modulus (<40 GPa). An advance in the 1970s afforded ultrahigh modulus carbon fibers (>380 GPa) from mesophase pitch. Mesophase is a liquid crystal phase formed in the transformation of organic precursors to carbon. The chemical pretreatment of the pitch is complex and the potential cost advantage of using a pitch precursor is still not fully realized [1-8]. 

Fig. 1. Specific strength and Young s modulus of various engineering materials where CF = carbon fiber HM/UHM = high modulus/ultrahigh modulus ...

Carbon fibers are generally typed by precursor such as PAN, pitch, or rayon and classified by tensile modulus and strength. Tensile modulus classes range from low (<240 GPa), to standard (240 GPa), intermediate (280—300 GPa), high (350—500 GPa), and ultrahigh (500—1000 GPa). Typical mechanical and physical properties of commercially available carbon fibers are presented in Table 1. 

Fibers produced from pitch precursors can be manufactured by heat treating isotropic pitch at 400 to 450°C in an inert environment to transform it into a hquid crystalline state. The pitch is then spun into fibers and allowed to thermoset at 300°C for short periods of time. The fibers are subsequendy carbonized and graphitized at temperatures similar to those used in the manufacture of PAN-based fibers. The isotropic pitch precursor has not proved attractive to industry. However, a process based on anisotropic mesophase pitch (30), in which commercial pitch is spun and polymerized to form the mesophase, which is then melt spun, stabilized in air at about 300°C, carbonized at 1300°C, and graphitized at 3000°C, produces ultrahigh modulus (UHM) carbon fibers. In this process tension is not requited in the stabilization and graphitization stages. 

For a wide range of apphcations, composites reinforced with continuous fibers are the most efficient structural materials at low to moderate temperatures. Consequently, we focus on them. Table 5.3 presents room-temperature mechanical properties of unidirectional polymer matrix composites reinforced with key fibers E-glass, aramid, boron, standard modulus (SM) PAN (polyacrylonitrile) carbon, intermediate modulus (IM) PAN carbon, ultrahigh modulus (UHM) PAN carbon, ultrahigh modulus (UHM) pitch carbon, and ul-trahigh thermal conductivity (UHK) pitch carbon. The fiber volume fraction is 60 percent, a typical value. 

One classification scheme for carbon fibers is by tensile modulus on this basis, the four classes are standard, intermediate, high, and ultrahigh moduli. Fiber diameters normally range between 4 and 10 p-m both continuous and chopped forms are available. In addition, carbon fibers are normally coated with a protective epoxy size that also improves adhesion with the polymer matrix. 

Naito, K., Tanaka, Y, Yang, J.M., and Kagawa, Y, Tensile Properties of Ultrahigh Strength PAN-Based, Ultrahigh Modulus Pitch-Based and High Ductility Pitch-Based Caibon Fibers , Carbon, 46, 189-195, 2008. 

---