Controlling the Oligomeric Composition of Carbon‐Fiber Precursors by Dense‐Gas Extraction

Abstract

Dense‐gas extraction of M‐50 petroleum pitch with the solvent toluene was carried out in a continuous, countercurrent‐flow, packed column at temperatures from 330° to 380°C, pressures from 42 to 84 bar, and a solvent‐to‐pitch ratio of 5:1. Both one‐ and two‐column configurations were used to fractionate raw pitches into top and bottom cuts of controlled oligomeric distribution. Using matrix‐assisted laser desorption–ionization mass spectroscopy for absolute molecular weight determination, the effect of changes in pitch oligomeric distribution on both the softening point and the degree of anisotropy (i.e., isotropic–mesophase equilibrium) was established. For example, pitch fractions rich in dimer and trimer species, and with essentially all monomer removed, exhibit the anisotropic microstructure typically found in precursors for high‐performance carbon fibers.