Pitch precursors

Pitch is a general name for the tarry substance, which is solid at room temperature and can be obtained from one of several sources  

Petroleum refining, normally called bitumen, or asphalt in the U.S.A. 

Pyrolysis of ring compounds, such as naphthalene and anthracene 

In general, Riggs et al [178] considered a pitch to be composed of four main classes of chemical compounds  

Naphthene aromatics—low molecular weight aromatics and saturated ring structures 

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For nosetip materials 3-directional-reinforced (3D) carbon preforms are formed using small cell sizes for uniform ablation and small pore size. Figure 5 shows typical unit cell dimensions for two of the most common 3D nosetip materials. Carbon-carbon woven preforms have been made with a variety of cell dimensions for different appHcations (27—33). Fibers common to these composites include rayon, polyacrylonitrile, and pitch precursor carbon fibers. Strength of these fibers ranges from 1 to 5 GPa (145,000—725,000 psi) and modulus ranges from 300 to 800 GPa. 

More than 95% of current carbon fiber production for advanced composite appHcations is based on the thermal conversion of polyacrylonitrile (PAN) or pitch precursors to carbon or graphite fibers. Generally, the conversion of PAN or pitch precursor to carbon fiber involves similar process steps fiber formation, ie, spinning, stabilization to thermoset the fiber, carbonization—graphitization, surface treatment, and sizing. Schematic process flow diagrams are shown in Eigure 4. However, specific process details differ. 

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. 

Further improvements in the properties of PAN-based carbon fibers are likely to emerge through improved stabilization, that is, by creating the ideally cross-linked fiber. On the other hand, as purer pitch precursors become available, further improvements in mesophase pitch-based carbon fibers are likely to arise from optimized spinnerette designs and enhanced understanding of the relationship between pitch chemistry and its flow/orientation behavior. Of course, the development of new precursors offers the potential to form carbon fibers with a balance of properties ideal for a given application. 

The coal-derived pitch precursors for WVU-1, WVU-2, and WVU-3 test graphites were de-ashed by the combined centrifugation and filtration method while all of the other pitches were de-ashed by centrifugation alone (2000 G, 90 minutes). 

Since PAN-based carbon fibers tend to be fibrillar in texture, they are unable to develop any extended graphitic structure. Hence, the modulus of a PAN-based fiber is considerably less than the theoretical value (a limit which is nearly achieved by mesophase fibers), as shown in Fig. 9. On the other hand, most commercial PAN-based fibers exhibit higher tensile strengths than mesophase-based fibers. This can be attributed to the fact that the tensile strength of a brittle material is controlled by structural flaws [58], Their extended graphitic structure makes mesophase fibers more prone to this type of flaw. The impure nature of the pitch precursor also contributes to their lower strengths. 

Carbon/graphite fibers are prepared from either a polyacrylonitrile or rayon precursor fiber or from a pitch precursor 22,23). In either case, the fibers are treated at high... 

Carbon/carbon composites fabricated by multiple cycles of liquid impregnation and recarbonization are a typical example of modern petroleum derived carbons. In the 1975 ACS Symposium on Petroleum Derived Carbons (JL), papers were presented on carbon/carbon composite materials formed by pyrolytic infiltration processes (2 ) or by liquid impregnation with petroleum pitch (3,4), on fabrication processes for high-modulus carbon fibers based on polyacrylonitrile (PAN) or pitch precursors ( 5 ), and on the use of carbon materials for thermostructural (6 ) as well as biomedical applications (1 ) ... 

Amoco has developed a family of ultra high modulus continuous graphite fibers and preforms with axial thermal conductivity to llOOW/mK. The extremely high thermal conductivity is a direct result of an extremely high degree of crystallinity during carbonization of the mesophase pitch precursor fiber. Table... 

Two categories of pitch-based fiber exist isotropic carbon fiber produced from an isotropic pitch precursor, and an oriented, anisotropic fiber produced from a mesophase pitch precursor. Isotropic fibers were developed from low melting point isotropic pitches The precursor was melt-spun into fibers, which were oxidized to render them infusible, and then carbonized. Their low strengths and moduli make these fibers unsuitable for use in advanced composites. Orientation was accomplished by a hot-stretching process (>2200°C), but it is accompanied by the same processing difficulties encountered in the rayon precursor process. A different approach was suggested by the discovery of carbonaceous mesophase. ... 

Pitch precursor carbon fibers have the potential of providing good mechanical properties at low cost. 

Solvent-Extracted Pitch Precursors for Carbon Fiba ... 

A "desirable" carbon fiber feed from a pitch precursor should satisfy at least four main criteria. These criteria include the following ... 

The carbonized fibers from the solvent extracted pitch precursor described in the previous section generally exhibited textures similar to those shown in Figures 9a-b. Radial textures predominated, although random textured fibers and hybrids (random/radial) were occasionally found. Fiber splits (or missing wedges) were rarely observed. The final texture obtained in the fibers was found to be a... 

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