Supercritical Fluid Extraction: A New Process for Producing High-Performance, Low-Cost Carbon Fibers

Supercritical fluid SCF extraction has been investigated for the production of the mesophase pitch used to spin high-performance carbon fibers. A bench-scale, continuous-flow apparatus was designed and constructed for fractionating petroleum pitch with SCF solvents at temperatures and pressures to 400 degC and 350 bar respectively. A heat-soaked, isotropic petroleum pitch has been fractionated with supercritical toluene in a region of liquid-liquid equilibrium at temperatures and pressures from 300-380 degC and 30-140 bar respectively. Bottom-phase pitch fractions produced at 340 degC, 70 bar, and a toluene-to-pitch ratio of 31 consist of a 100 bulk mesophase. This mesophase has been used to produce carbon fibers with strengths and moduli 3.3 and 820 GPa, respectively equal to or better than the best pitch-based fibers currently available. Since SCF extraction can be used to produce mesophase on a continuous basis, production costs could be as low as 0.50lb. For high production rates of carbon fiber, this processing technique would have a significant cost advantage over alternative methods for producing mesophase.