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Supercritical Fluid Extraction for High Thermal Conductivity Carbon Fibers

Abstract : The goal of this project was to evaluate the potential of supercritical extraction (SCE) for producing a high-quality mesophase suitable for conversion into high-performance carbon fibers with excellent thermal and/or mechanical properties. Mesophase pitch fractions were produced by fractionation of an isotropic pitch with supercritical toluene in a region of liquid-liquid equilibrium. Results indicate that SCE provides greater product flexibility and uses less solvent than conventional solvent extraction. In particular, several 100% mesophase fractions with significantly different yields, softening points, and chemical compositions were made. Selected supercritically extracted mesophase fractions were melt-spun into both round and ribbon-shaped fibers. To facilitate the evaluation of these mesophases, a new method (i.e., Z-values by WAXD) was investigated for predicting the properties of final fibers from those of as-spun fibers. Lower Z-values of as-spun fibers yielded, upon identical heat treatment, improvements in tensile properties, crystallographic parameters, and thermal conductivities for the associated final fibers. Even though the heat treatment of the as-spun fibers was not optimized, final fibers with thermal conductivities equal to Amoco's P- 120 fibers were produced by graphitizing at only 2400 deg C. Finally, a preliminary design analysis indicates that producing mesophase by SCE is economically competitive with today's best commercial processes.