Mesophase pyrolysis

Synthetic Resins. Various polymers and resins are utilized to produce some specialty carbon products such as glassy carbon or carbon foam and as treatments for carbon products. Typical resins include phenoHcs, furan-based polymers, and polyurethanes. These materials give good yields of carbon on pyrolysis and generally carbonize directly from the thermoset polymer state. Because they form Httle or no mesophase, the ultimate carbon end product is nongraphitizing. 

Fitzer, E., Kompalik, D. and Mayer, B., Influence of additives on pyrolysis of mesophase pitch. In Carbon 86 Proceedings of International Carbon Conference, Baden-Baden, Germany, 1986, pp. 842 845. 

As the pyrolysis of model, polynuclear hydrocarbon compounds represents, possibly, the ultimate in ability to form the largest and most stable of mesophase molecules leading to the domains (Table 1) of optical texture in cokes, then smaller sizes of optical texture can be explained by processes which restrict or inhibit polymerization to larger molecule sizes. Conversely, it may be possible to increase the size of the optical texture of coke by suitable ameliorative treatments to a pitch. 

Composition of Parent Pitch. Once the chemical composition of the carbonizing system moves away from the comparative simplicity of polynuclear aromatic hydrocarbons to that of industrial pitches, then the pyrolysis chemistry incorporates effects caused by the presence of heteroatoms (0, N and S) and alkyl and naphthenic groups. In general terms, the system becomes more Reactive creating higher concentrations of radicals detectable by ESR. This in turn, leads to enhanced cross-linkages and polymerization of molecular constituents of any mesophase which is formed, and this causes enhanced viscosity and a reduction in size of optical texture. 

Thus, one approach to understanding the chemistry of pyrolysis of pitch leading to mesophase is not to make a complete molecular analysis but to solvent fractionate the pitch using solvents of increasing solubility parameters (58). An early study of fractionation and NMR analysis of fractions is that of Smith et al. (61). 

Miyazawa et al. (92) related rates of decrease of aliphatic hydrogen protons during pyrolysis of ethylene tar pitch to formation of mesophase. Yokono et al, (93) used the model compound anthracene to monitor the availability of transferable hydrogen. Co-carboniza-tions of pitches with anthracene suggested that extents of formation of 9,10-dihydroanthracene could be correlated with size of optical texture. The method was then applied to the carbonization behaviour of hydrogenated ethylene tar pitch (94). This pitch, hydrogenated at 573 K, had a pronounced proton donor ability and produced, on carbonization, a coke of flow-type anisotropy compared with the coarse-grained mosaics (<10 ym dia) of coke from untreated pitch. 

The presence of inerts can override chemical factors within pitches undergoing pyrolysis. In fact, Marsh et al. (110) postulate that the chemistry of polymerization within mesophase may be dependent upon the flow properties of the mesophase. Hence, inerts may influence pyrolysis chemistry because they can restrict movement of mesophase. 

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. 

As outlined earlier, the rheological properties of pitch and mesophase pitch are important in the processing of these materials to carbon products. The rheology of isotropic pitch will be considered briefly and then the effects of pyrolysis to mesophase will be described. 

Mesophase Pitch. Mesophase pitch is a generic term for those products of pyrolysis that contain mesophase, or are entirely mesophase, but which on reheating still pass through a fluid phase prior to the formation of semi-coke. The following are areas of scientific or industrial interest where the rheological properties of mesophase pitch are of interest. ... 

Sedimentation of the mesophase from the isotropic pitch due to its slightly higher density is dependent, at least partly, on the viscosity of the isotropic phase. (Under the conditions of normal pyrolysis other factors, such as turbulence or gas evolution, may also play major roles.)... 

The experimental diagram resembles the schematic one except that the decomposition temperature is relatively insensitive to the degree of pyrolysis until the volatile content is reduced below about 10%. This point corresponds approximately to that at which the residue is entirely mesophase, which is also the point at which the Tg of the mesophase equals the decomposition temperature. This means that it is not possible to produce, under these conditions of pyrolysis, a chemically stable single-phase mesophase pitch in the fluid state (although this may be possible by other techniques). 

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