Preparation of Mesophase Pitches

Both isotropic and mesophase pitch carbon fibers are made from the same feedstock. 

A mesophase pitch suitable for producing high performance carbon fiber must have the following properties [228]  

Not contain insolubles, which must be removed by filtration, otherwise would interfere with the spinning and also lower mechanical properties. 

Must not undergo polymerization during spinning as this would increase the melt viscosity, which in turn would require higher process temperatures and could generate gases and leave bubbles in the spun fiber. 

The mesophase portion must be able to undergo orientation during the spinning process. 

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Mesophase is susceptible to chemical reactions other than those induced by pyrolysis. Modifications to enhance fusibility or solubility for easier spinning (see Preparation of Mesophase Pitch) and to induce thermosetting for carbonization without deformation are both practical steps in carbon fiber manufacture. 

In those days, pitch chemistry had not advanced sufficiently to understand fundamentally the foregoing phenomena. The preparation of mesophase pitch with low softening point (the so-called "soft mesophase pitch") was based on direct experiment. Nevertheless through extensive and serious efforts, it became possible to prepare soft mesophase pitches from naphtha tars, decant oils from fluidized catalytic crackers (FCC), atmospheric-reduced crude oils, and other pitch-like materials. A typical example of these preparation procedures is the following. A purified FCC or naphtha pitch is heated at 400°C for one hour under methane to convert the pitch to a mesophase content of 23.6% (27,28). The mesophase separated by sedimentation has a softening point of 226°C it is spun at 320°C, and the fiber is stabilized in air and finally carbonized by rapid heating at 100 to 1600°C/min (29). 

Reprinted with permission from Hornby J, Kearsey HA, Carbon fibres from pitch precursors 2. The preparation of mesophase pitches suitable for spinning into fibres, AERE Harwell report AERE-M 3029, Oct 1979. Copyright 1979, AEA Technology pic. 

Mochida I, Shimizu K, Korai Y, Otsuka H, Sakai Y, Fujiyama S. Preparation of mesophase pitch from aromatic hydrocarbons by aid of HF/BF3. Carbon 1990 28(2-3) 311-319. 

Scheme 6 Steps involved in the preparation of mesophase pitch by the hydrogenation method...

Materials used in the activation with KOH include high volatile bituminous coal C, coal semi-coke CS, pitch mesophase PM, pitch semicoke PS and commercial activated carbon AC. The semi-cokes CS and PS were produced by the heat treatment of corresponding parent materials at 520°C with a heating rate of 5°C/min and 2 hours soaking time. The preparation of mesophase PM comprised the soaking of coal-tar pitch at 450°C for 7 h with a continuous stirring. All the treatments were performed under argon in a vertical Pyrex retort of 45 mm diameter. 

Summary of Known Methods of Ereparation of Mesophase. At the time of writing, it appears that a major objective of research into the chemistry of mesophase formation is to obtain mesophase at as low a temperature as possible and to have a soluble mesophase, e.g. in 1,2,4 trichlorobenzene (112). The applications of such mesophase(s) must be in the production of graphitizable pitch fibres. However, this direction of research could have applications in many other areas of carbon/graphite production. Therefore it is appropriate, now, to make a list of known methods of preparation of mesophase (113). 

The technology of mesophase-pitch-based carbon fiber has stimulated the rapid development of the chemistry of mesophase behavior and preparation. The carbonization schemes and mechanisms leading to optical anisotropy via the mesophase, the control of carbonization with emphasis on the preparation of spinnable mesophase, and the mesophase transition and reactivity in relation to the structure of its constituent molecules are summarized in this paper. 

Figure 8. Viscosity of mesophase pitches prepared by the Kyukoshi method, with schematic microstructures of fibers spun at temperatures in the range of 300 to 400°C.

Fig. 4. Typical X-ray diffraction patterns of mesophase pitch, fluorinated pitch and (CF) (A) mesophase pitch (B) solid type fluorinated pitch prepared at 70°C (C) graphite fluoride, (CF) . (Quoted from H. Fujimoto, A. Mabuchi, T. Maeda, Y. Matsumura, N. Watanabe, H. Touhara, Carbon, 30 (1992) 851, with permission).

Our previous papers [15,16] and the current work show that die imprinting of mesophase pitch particles with colloidal silica is an efficient technique to prepare mesoporous carbons with uniform spherical pores as well as carbons with bimodal pore size distributions. These carbons exhibit negligible amount of micropores, which can be further eliminated during graphitization process. If micropores are need, they can be created by controlled oxidation analogous to that used in the preparation of activated carbon fibers. The possibility of tailoring the size of uniform spherical mesopores is of great importance for catalysis, adsorption and other advanced applications such as die manufacture of hi -quaiity electrochemical double-layer capacitors, fuel cells and lidiium batteries. 

The effect of mesophase pitches on the tensile modulus of the resultant pitch based carbon fiber has been studied [245] and carbon fiber prepared from an isotropic pitch containing mesophase spheres has also been studied [246],... 

Scheme 5 Preparation of pitch by separation of mesophase pitch from isotropic phase...

Hu et al. have synthesised ordered mesoporous and macroporous carbon monoliths by template method using silica monoliths as template. In this preparation, the mesophase pitch was used as the carbon precursor [54]. The silica monolith templates were filled with a 10 wt.% solution of mesophase pitch in tetrahydrofuran solvent and then carbonised at different temperatures. The physical characteristics of these various mesoporous and macro-porous carbon are presented in Table 7.8, which demonstrates these carbons have more prominent graphitic structures as the carbonisation temperature increases. SEM images of the carbon monoliths obtained at carbonisation temperature of 700°C and 2500°C are shown in Figure 7.59. 

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