Isotropic pitch fibers

Much less ordered than PAN-based high-strength CFs are the isotropic CFs. They are produced by the carbonization of isotropic pitch fibers (or other fibrous precursors such as phenolic resins or cellulose, including rayon), without any attempt to obtain a preferred orientation of the polyaromatic molecules in the fiber direction. Consequently, they have a random nanotexture and belong to the low modulus class of CFs [16]. Rather than being used for high-performance reinforcement purposes, they find their application as thermal insulators for furnaces or as reinforcements for cement [1]. Another important use of isotropic CFs is as a feedstock for the production of activated carbon fibers, a material dealt with in Section 2.4.4. 

Figure 7.8 Effect of final heat treatment temperature on tensile strength of PAN, mesophase and isotropic pitch fibers. PAN A MP1 [41] MP2 IP [42], Source Reprinted from MatsumotoT, Mesophase pitch and its carbon fibers, Pure AppI Chem, 57(11), 1533,1985, Shen Z, Guo H et al, Carbon and Carbonaceous Composite Materials Structure-Property Relationship, Abstr and Proc, Malenovice, Czech Republic, 31, Oct 10-13, 1995.

ISOTROPIC PITCH BASED CARBON FIBERS are CARBON FIBERS obtained by CARBONIZATION of isotropic pitch fibers after these have been stabilized (i.e. made non-fusible). LAMP BLACK is a special type of CARBON BLACK produced by incomplete combustion of a fuel rich in aromatics that is burned in flat pans. LAMP BLACK is characterized by a relatively broad particle size distribution. 

Certain pitches can be spun directly into isotropic pitch fibers with only minor devolitization. Carbonized isotropic pitch fibers cannot be graphitized and develop mechanical and thermal properties that are substantially inferior to those produced by other precursors. However, isotropic pitch fibers are very inexpensive and have found commercial applications in areas that do not require the exceptional mechanical and thermal properties of mesophase pitch-based carbon fibers. Isotropic pitch-based carbon fibers are used in filters, brake pads, activated carbons, and as substrates for chemical vapor deposition (23). Table 3 summarizes properties of isotropic pitch-based carbon fibers. 

Pitch-based fibers can be divided in two groups (a) the isotropic pitch fibers, which have low mechanical properties and relatively low cost, and (b) he mesophase-pitch fibers, which have very high modulus but are more expensive. Pitch-based fibers are produced by a simple process and their cost should eventually rival that of glass fibers. Their potential is yet to be fully reetlized. 

Isotropic pitch-based carbon fibers are carbon fibers obtained by carbonization of isotropic pitch fibers after these have been stabilized (i.e. made non-fusible). 

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. 

Low density, carbon fiber-carbon binder composites are fabricated from a variety of carbon fibers, including fibers derived from rayon, polyacrylonitrile (PAN), isotropic pitch, and mesophase pitch. The manufacture, structure, and properties of carbon fibers have been thoroughly reviewed elsewhere [3] and. therefore, are... 

A recently developed adsorbent version of ORNL s porous carbon fiber-carbon binder eomposite is named carbon fiber composite molecular sieve (CFCMS). The CFCMS monoliths were the product of a collaborative researeh program between ORNL and the University of Kentueky, Center for Applied Energy Researeh (UKCAER) [19-21]. The m.onoliths are manufactured in the manner deseribed in Section 2 from P200 isotropic pitch derived fibers. While development of these materials is in its early stages, a number of potential applieations can be identified. 

For rayon fiber based composites (Sections 3 and 4) the fiber and powdered resins were mixed in a water slurry in approximately equal parts by mass. The isotropic pitch carbon fiber composites (Section 5) were manufactured with less binder, typically a 4 1 mass ratio of fiber to binder being utilized. The slurry was transferred to a molding tank and the water drawn through a porous screen under vacuum. In previous studies [2] it was established that a head of water must be maintained over the mold screen in order to prevent the formation of large voids, and thus to assure uniform properties. The fabrication process allows the manufacture of slab or tubular forms. In the latter case, the cylinders were molded over a perforated tubular mandrel covered with a fine mesh or screen. Moreover, it is possible to mold contoured plates, and tubes, to near net shape via this synthesis route. 

Isotropic fibrous materials, 11 176-177 Isotropic microporous membranes, 15 798 Isotropic moldings, 23 397 Isotropic pitch-based carbon fibers, 26 734-735... 

PAN-based carbon fibers Isotropic-pitch-based carbon fibers... 

Characterization of Pore Distribution in ACF Prepared from Isotropic Pitch-Based Carbon Fiber... 

Rayon-based carbon fibers, on the other hand, have low thermal conductivity and are, therefore, used for thermal insulation purposes. Examples include the nose cone and leading edges of the U S Space Shuttle Orbiter. Isotropic pitch-based carbon fibers are also used for insulating purposes because of their low thermal conductivity. 

As we can observe in Table 4, the results agree with those obtained by other authors However, we have not observed the shortest lifetime, probably due to the small size of the graphite microcrystals existing in isotropic pitch-based carbon fibers. 

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. ... 

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