Mesophase pitch, coalescence

When the elementary spherules of mesophase coalesce to form anisotropic domains, the microtexture of the carbonaceous mesophase becomes more complex. Disdinations (rotational defects) in the arrangement of the discotic molecules are often present. Disdinations in 2-D media play a role similar to that of dislocations in crystals and the evolution of gas bubbles results from condensation reactions in a medium which is still fluid. Disdinations, which occur during the mesophase formation, remain after carbonization and are key to understanding the relationships between the microtexture and the properties of carbon fibers formed from mesophase pitches. 

Figure 4.24 Streaky mesophase formed by coalescing spheres (polarized light xlOO). Source 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.

MESOPHASE PITCH is a PITCH with a complex mixture of numerous, essentially aromatic hydrocarbons containing anisotropic liquid crystalline particles (CARBONACEOUS MESOPHASE), detectable by optical microscopy and capable of coalescence into the BULK MESOPHASE. 

Mesophase grows by coalescence and sohdifies into mosaics. In pure mesophase A (e.g., A240 pitch) coalescence introduces defects (dischnations) (Figure 1.33a). Their combinations form nodes [99]. Because the material is still plastic... 

Fig. 12. A, Schematic representation of parallel arrays of polynuclear aromatic hydrocarbon molecules in a mesophase sphere. B, a) isolated mesophasc spheres in an isotropic fluid pitch matrix b) coalescence of mesophase c) structure of semi-coke after phase inversion and solidification.

In the course of liquid-phase carbonization of pitches, optically anisotropic spheres are formed first, which are called mesophase spheres. By further heating, the spheres grow and coalesce with... 

FIGURE 2.23 (a) Formation, (b) growth and (c) coalescence of mesophase spheres in a pitch. 

Pressure of Carbonization. The effect of a pressurized carbonization is to create a closed system preventing loss of volatile materials. Hence, carbon yields increase. Further, the material normally lost as volatiles in open systems is now retained and the effect of this, by reducing turbulence and bubble formation, is to enhance the size of resultant optical textures. Hiittinger and Rosenblatt (54) report such effects when gas pressures up to 15 MPa pressure (150 bar) were applied to the carbonization of a coal-tar pitch. If higher pressures are used, the pressure being applied hydraulically to the carbonization system, then the effect of pressures at, say, 300 MPa, is to enhance the viscosity of the total system and this prevents coalescence of the mesophase. The resultant appearance of the carbon has been described as botryoidal (55, 56) and an example is Figure 8. 

This is a serious misnomer as these inert constituents of pitch are certainly not inert during the carbonization processes. It is well-established that the size of the optical texture of a coke can be reduced by the presence within the pitch of primary QI material (102-105). The QI material within the pitch becomes adsorbed on the surfaces of the growth units of mesophase. This thereby prohibits coalescence of these growth units into the larger sized optical textures. When this process is viewed by hot-stage optical microscopy (106) this lack of coalescence is seen to reduce markedly the flow characteristics of the mesophase - it becomes almost static. 

This influences the structural features of the mesophase which remains more disordered, a point made by Cranmer et al. (43). Stadelhofer (107) found that the presence of QI did not change rates of formation of mesophase. Romovacek et al. (108) consider that pyrolytic particles in pitch (primary QI) retard the development of mesophase and suppress coalescence. Decrease in size of optical texture, as brought about by mechanical modification as distinct from chemical modification of pitch properties can increase both the strength and reactivity to oxidising gases of the resultant coke, as recently put forward by Markovic et al. (109). ... 

Coalescence of mesophase is often said to be determined by the mesophase viscosity. This aspect requires much further investigation. However, it is clear that, amongst other factors, the rheological behaviour (including viscoelastic effects) of each phase is important in mesophase growth and coalescence. Diffusion of molecular species through the isotropic pitch to the mesophase spheres is likely to be related to the viscosity of the isotropic med i urn. 

The preferred orientation of the matrix parallel to the fiber bundles appears to be affected by processing pressure during pyrolysis (400-500°C), becoming more random as impregnation pressures are increased. Even transverse lamellar orientations can result (Figure lb). Because pitch mesophase coalescence has been inhibited by the pressure, grains of matrix may be transversely oriented to the filaments. 

This phase transition is initially reversible, as demonstrated by Lewis (10), but as carbonization progresses chemical cross-linking makes it irreversible The liquid crystal droplets grow and coalesce until all the isotropic melt has undergone the phase transition This new phase is then termed the liquid crystal mesophase, i e the phase intermediate between the isotropic fluid pitch and solid semi-coke ... 

This research focused on answering three fundamental questions What solvents can be used to extract a fraction from a commercial pitch (such as A-240) which is capable of forming a coalesced mesophase rapidly upon melting, and what criteria will predict their effectiveness ... 

Both examples Illustrate that solvent blends can be used to extract mesophase forming fractions from the A-240. Figure 9 suggests several other solvents may form successful blends. Solvents normally inappropriate should be blended to shift the solubility parameters of the mixture closer to the edge of the enhanced solubility region where the blend can extract a pitch fraction capable of complete coalescence. Solvent blends can also be used to moderate solvent costs without impairing solvent effectiveness. 

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