Pyrolysis behaviour of mesophase and isotropic phases isolated from the same pitch

Four thermally treated coal-tar pitches, with different mesophase content, were filtrated to separate the mesophase from the isotropic phase. The evolution of each fraction during carbonisation was studied from their weight loss (TGA and DTG) and differential scanning calorimetry (DSC). Thermogravimetric analyses of the isotropic phases showed that while the weight loss in the isotropic phase is mainly associated with the distillation of light compounds, in the anisotropic fractions it is related with thermal reactions. The DSC curves corroborated the thermogravimetric results, showing an endothermic band due to distillation around 300 degreesC. Two exothermic peaks at 400 and 450 degreesC were characteristic of all the isotropic phases and whole pitches, while only the first one was present in the mesophase samples. Both were attributed to polymerisation reactions, the first one between the molecules forming the mesophase and the second one between molecules that do not easily form mesophase. Both processes involve loss of weight as they were also observed by thermogravimetry. The cokes obtained after carbonisation of the anisotropic phases at 900 degreesC had an optical texture similar to that of the thermally treated parent pitches, coarse mosaics and small domains, while those obtained from the isotropic phases showed an optical texture of flow domains. (C) 2002 Elsevier Science B.V. All rights reserved.