Chemical and rheological characterization of air-blown coal-tar pitches

A commercial impregnating coal-tar pitch was air-blown at 275 degrees C for 10, 18, 25 and 30 hours. Isotropic pitches were produced with softening points ranging from 140 to 210 degrees C. The parent and treated pitches were characterized by elemental analysis, Fourier transform infrared, solubility in toluene and N-methylpyrrolidone, iodine adsorption and reflected light optical microscopy. The viscoelasticity of the pitches also was investigated using transient shear and controlled strain oscillatory rheometry. The chemical characterization of the pitches suggested that, as air-blowing proceeded, larger molecules formed through dehydrogenative polymerization (aromatization/condensation) and cross-linking of the pitch molecules. This is consistent with the rheological characterization of the pitches. The results obtained from transient shear and oscillatory rheometry showed that the parent impregnating pitch exhibited a viscous behavior (typical of fluids composed of small molecules). In contrast, the air-blown pitches, which contained large aromatic molecules, showed viscoelastic properties. (C) 1998 Elsevier Science Ltd. All rights reserved.