A Comparative Tg-Ms Study of the Carbonization Behavior of Different Pitches

The purpose of this work was to study the formation of mesophase spherules from a low-temperature coal tar pitch under carbonization conditions. For comparison, the carbonization of a high-temperature coal tar pitch and a petroleum pitch were also considered. Different operating conditions during the carbonization process were used in an attempt to cover different degrees of mesophase formation and development for each pitch. The parent pitches and the semicokes thus obtained were analyzed by elemental analysis, optical microscopy, and Fourier transform infrared spectroscopy (FT-IR). More significantly, the samples were subjected to thermal decomposition under well-controlled operating conditions from room temperature to 850 °C in a thermogravimetric analyzer (TG). The use of a mass spectrometer linked to the TG (TG-MS) provides additional data about the devolatilization process, yielding information about the evolution of different volatile products and about possible chemical reactions occurring during thermal decomposition. Thus an insight into the process of mesophase formation is obtained. The results from FT-IR, elemental analysis, and the TG-MS tests were compared with the different extents of mesophase formation, checked by optical microscopy. According to the results, several stages can be distinguished as temperature increases in the carbonization process of the pitches. In the low-temperature coal tar pitch, the devolatilization of light components, especially phenols, accounts for the most significant weight loss. Moreover, cross-linking contributes greatly to the formation and development of mesophase, resulting in the predominance of bulk mesophase in a relatively short time in the case of the low-temperature coal tar pitch.