D. D. EDIE Coal-tar-derived mesophase Petroleum-derived mesophase Fig. 7. Typical polynuclear aromatic hydrocarbons in mesophases produced from coal-tar and petroleum [321 ferred fiber formation process less complicated spinning proce as sel avoids Chwastiak [ 32] found that if an inert gas spurge was that this used to agitate the pitch during thermal polymeriza tion, a spinnable 100% mesophase product could be potential low cost of the precursor, would make produced. A single-phase precursor is preferable for pitch-based fibers a low-cost alternative to PAN melt-spinning processes because it avoids the stability based carbon fibers. While this may come to pass problem associated with two-phase extrusion eventually, the economics of pitch fiber processing Nevertheless, based on a recent TEM study by Fitz are not quite this simple. Also, as researchers have Gerald et al. [33 it would appear that some commer discovered, the fundamental structure of pitch-based cial pitch-based fibers are still produced from a mixed carhon fibers is very different from that of PAN mesophase/isotropic pitch precursor based carbon fibers and each structure offers certain Diefendorf and Riggs developed an alternative advantages. Like PAN-based fibers, the structure of technique, solvent extraction that produced a spinn pitch-based fibers is largely developed during fiber feed [34. In their process a portion of highly aro- matic pitch was extracted using a solvent mixture 3.1 Production of mesophase pitch such as benzene and toluene. The extraction step Like PAN-based carbon fibers, the peculiarities of removes the smaller disordering molecules and con- iLch-based fibers are the direct result of the precursor cenTrales the higher molecular weight mat and the process used to convert it to fiber form. In higher molecular fraction can be converted to 100% this case the precursor is mesophase pitch, a liquid mesophase by heating it to between 230 and 400C rystalline material consisting of large polynuclear for only 10 minutes. In either the thermal polymeriza romatic hydrocarbons, The properties of meso- hase. its formation, an the subject of numerous articles[25-28 The first commercial mesophase precursors were produced by Union Carbide using a thermal polyme Pitch ization process. Evidently, the original process pro Nitrogen, zone 1: zone2:zone3Ⅲ可 duced a mixture of isotropic and mesophase pitcl [29, 30]. These early patents claim that small amounts a0onldnooddala f isotropic pitch are needed to reduce the viscosit of the polymerized mesophase and, therefore, the Fllter Extruder spinning temperature. These mixed precursors were prepared by thermally polymerizing a highly aromatic isotropic pitch feed (originating from either petro- leum or coal tar) at temperatures of 400-410'C for as long as 40 hours [29, 30. Coal tar pitch produ Quench Air mesop ith higher aromaticity whereas petroleum pitch yields a mesophase product with a more open structure and a higher content of ( see Fig.7)19.31 by Lewis[30], agitation during heat treatment pre duces a lower molecular weight mesophase and cre- Fig 8 Schematic of melt-spinning prooess used lo produce pitch, making the material easier to spin. Later mesophase pitch precursor fibers [39]
350 D. Eon Coal-tar-derived mesophase Petroleum-derived mesophase Fig. 7. Typical polynuclear aromatic hydrocarbons in mesophases produced from coal-tar and petroleum [32] ferred fiber formation process because it avoids solvent-related issues. Initially, it was felt that this less complicated spinning process, combined with the potential low cost of the precursor, would make pitch-based fibers a low-cost alternative to PANbased carbon fibers. While this may come to pass eventually, the economics of pitch fiber processing are not quite this simple. Also, as researchers have discovered, the fundamental structure of pitch-based carbon fibers is very different from that of PANbased carbon fibers, and each structure offers certain advantages. Like PAN-based fibers, the structure of pitch-based fibers is largely developed during fiber formation. 3.1 Production of’mesophase pitch Like PAN-based carbon fibers, the peculiarities of pitch-based fibers are the direct result of the precursor and the process used to convert it to fiber form. In this case, the precursor is mesophase pitch, a liquid crystalline material consisting of large polynuclear aromatic hydrocarbons. The properties of mesophase, its formation, and mode of growth have been the subject of numerous articles [25-281. The first commercial mesophase precursors were produced by Union Carbide using a thermal polymerization process. Evidently, the original process produced a mixture of isotropic and mesophase pitch [29,30]. These early patents claim that small amounts of isotropic pitch are needed to reduce the viscosity of the polymerized mesophase and, therefore, the spinning temperature. These mixed precursors were prepared by thermally polymerizing a highly aromatic isotropic pitch feed (originating from either petroleum or coal tar) at temperatures of 400-410°C for as long as 40 hours [29,30]. Coal tar pitch produces a mesophase product with higher aromaticity. whereas petroleum pitch yields a mesophase product with a more open structure and a higher content of aliphatic side chains (see Fig. 7) [9.31]. As revealed by Lewis [30], agitation during heat treatment produces a lower molecular weight mesophase and creates an emulsion of the mesophase and isotropic pitch, making the material easier to spin. Later Chwastiak [32] found that if an inert gas spurge was used to agitate the pitch during thermal polymerization, a spinnable 100% mesophase product could be produced. A single-phase precursor is preferable for melt-spinning processes because it avoids the stability problem associated with two-phase extrusion, Nevertheless, based on a recent TEM study by Fitz Gerald et (I/. [33], it would appear that some commercial pitch-based fibers are still produced from a mixed mcsophase/isotropic pitch precursor. Diefendorf and Riggs developed an alternative technique, solvent extraction. that produced a spinnable 100% mesophase precursor from an isotropic feed [34]. In their process a portion of highly aromatic pitch was extracted using a solvent mixture such as benzene and toluene. The extraction step removes the smaller disordering molecules and concentrates the higher molecular weight material. The higher molecular fraction can be converted to 100% mesophase by heating it to between 230 and 400°C for only 10 minutes. In either the thermal polymerizaQuench Air Variable Speed Winder - Fig. 8. Schematic of melt-spinning process used to product mesophase pitch precursor fibers [39]
Effect of processing on carbon fibers 351 Fig. 9. Predicted influence of major process variables during mesophase melt spinning [41 tion or the solvent extraction process, a free radica but uses large amounts of solvents, Supercritical mechanism is believed to be responsible for polymer- extraction and catalytic polymerization produce rela ization of the carbonaceous material. Although tively uniform products with w molecular solvent extraction does reduce the molecular weight weight distributions. However, supercritical extrac distribution somewhat prior to heat treatment, this tion has yet to be proven on a commercial scale, and reaction mechanism still tends to create a product catalysT with a relatively broad molecular weight distribution. to increasing environmental regulations Recently, Thies and coworkers [35] developed a The mesophase products produced by these four variation of this solvent-extraction process that uses processes ditter considerably, but they also exhibit a supercritical fluid instead of a conventional liquid many similarities. For instance, each process yields a solvent. In this process, an aromatic isotropic feed product with a different molecular weight distribution tch is initially dissolved in an aromatic solvent, and a different concentration of aliphatic side chains such as toluene at supercritical conditions, The n the individual mesophase molecules. Consequently resulting homogencous solution is then fractionated their viscous characteristics differ and, as will be nventional manner, using changes in either explained later, their rate of stabilization diffe temperature or pressure to produce pitch fractions well. However all of these mesophase products ce f relatively narrow molecular weight distribution. tain a range of molecular weights, with an average Recent tests have demonstrated that the process can from 800 to 1200. Because of this these mesophases desired molecular weight and softening point(3b a reach a viscosity of 200 Pas-i ur lower well below their degradation temperature. Also, although some- Mochida [37] also has developed a process which what irregular, the individual mesophase molecules oduces a spinnable 100% mesophase precursor with are, in general, disc-like in shape. Recent work by a relatively narrow molecular weight distribution. Korai and Mochida [38] indicates that mesophase This proccss, reccntly commercialized by Mitsubishi molccules can form a substructure and that the Gas Chemical Company, uses a strong Lewis acid chemical nature of the molecule influences the size catalyst(HF-BF3) to catalyze a pure chemical feed, of this substructure methyl-naphthalene, to a While coalesced mesophase can exhibit compli- 100% mesophase product. As Mochida has shown, cated extinction patterns caused by disclinations, no the use of HF BF, greatly reduces the molecular grain boundaries appear to be present [ 39]. In other weight distribution of the mesophase product com- words, one might expect bulk mesophase to behave pared to that produced by thermal polymerization. as an ideal liquid crystalline fuid-a single-domain Each of these processes are being explored, and liquid crystal. As will be seen, this appears to be to varying degrees, used on a commercial scale. While both true and false ach process and its phase product offer certain advantages, they also suffer from some disadvan- 3.2 Production of mesop se pitch precursor tages. Thermal polymerization avoids the use of fibers solvents, but produces a product with a broad molec- As previously mentioned, the mesophase pitches ular weight distribution form carbon fibers soften and How well below phase mixture). Solvent extraction produces a pro- their degradation temperature. Therefore, they can duct with a narrower molecular weight distribution be melt spun into fiber form. The schematic for a
Effect of processing on carbon fibers 351 Quench Cross Air winder Spinning Temperature Velocity Velocity Temperature Fig. 9. Predicted influence of major process variables during mesophase melt spinning [41]. tion or the solvent extraction process, a free radical mechanism is believed to be responsible for polymerization of the carbonaceous material. Although solvent extraction does reduce the molecular weight distribution somewhat prior to heat treatment, this reaction mechanism still tends to create a product with a relatively broad molecular weight distribution. Recently, Thies and coworkers [35] developed a variation of this solvent-extraction process that uses a supercritical fluid instead of a conventional liquid solvent. In this process, an aromatic isotropic feed pitch is initially dissolved in an aromatic solvent, such as toluene, at supercritical conditions. The resulting homogeneous solution is then fractionated in a conventional manner, using changes in either temperature or pressure, to produce pitch fractions of relatively narrow molecular weight distribution. Recent tests have demonstrated that the process can be used to produce 100% mesophase fractions of a desired molecular weight and softening point [ 361. Mochida [37] also has developed a process which produces a spinnable 100% mesophase precursor with a relatively narrow molecular weight distribution. This process, recently commercialized by Mitsubishi Gas Chemical Company, uses a strong Lewis acid catalyst (HF-BF,) to catalyze a pure chemical feed, such as naphthalene or methyl-naphthalene, to a 100% mesophase product. As Mochida has shown, the use of HF-BF, greatly reduces the molecular weight distribution of the mesophase product compared to that produced by thermal polymerization. Each of these processes are being explored, and, to varying degrees, used on a commercial scale. While each process and its mesophase product offer certain advantages, they also suffer from some disadvantages. Thermal polymerization avoids the use of solvents, but produces a product with a broad molecular weight distribution (and perhaps even a twophase mixture). Solvent extraction produces a product with a narrower molecular weight distribution, but uses large amounts of solvents. Supercritical extraction and catalytic polymerization produce relatively uniform products with narrow molecular weight distributions. However, supercritical extraction has yet to be proven on a commercial scale, and processes using HF-BF, catalysis are being subjected to increasing environmental regulations. The mesophase products produced by these four processes differ considerably, but they also exhibit many similarities. For instance, each process yields a product with a different molecular weight distribution and a different concentration of aliphatic side chains on the individual mesophase molecules. Consequently their viscous characteristics differ and, as will be explained later, their rate of stabilization differs as well. However, all of these mesophase products contain a range of molecular weights, with an average from 800 to 1200. Because of this these mesophases reach a viscosity of 200 Pa s-l or lower well below their degradation temperature. Also, although somewhat irregular, the individual mesophase molecules are, in general, disc-like in shape. Recent work by Korai and Mochida [38] indicates that mesophase molecules can form a substructure and that the chemical nature of the molecule influences the size of this substructure. While coalesced mesophase can exhibit complicated extinction patterns caused by disclinations, no grain boundaries appear to be present [ 391. In other words, one might expect bulk mesophase to behave as an ideal liquid crystalline fluid-a single-domain liquid crystal. As will be seen, this appears to be both true and false. 3.2 Production of mesophase pitch precursor jibers As previously mentioned, the mesophase pitches used to form carbon fibers soften and flow well below their degradation temperature. Therefore, they can be melt spun into fiber form. The schematic for a