MODELING OF THE APPARENT VISCOSITY OF PITCHES AND MESOPHASES AT LINEAR TEMPERATURE INCREASE UP TO 500-DEGREES-C

During the heating of pitch or mesophase, the viscosity first decreases and then increases. The viscosity decrease is a net physical effect and reversible. The viscosity increase is caused by chemical changes and thus it is irreversible. This viscosity behaviour with increasing temperature is important for the spinning of mesophase, for the infiltration of carbon fibre preforms and especially for the sintering of pressed mesophase powders. By modeling the viscosity changes apparent activation energies for the reversible viscosity decrease E(rev) and also for the irreversible viscosity increase E(irr) were obtained. The E(irr) values were in the range between 120 and 160 kJ mol-1, and thus not strongly different from one material to another. Significant differences were found for the E(rev) values, namely from 160 to 250 kJ mol-1, and it is explained that a low E(rev) value is very important for the sintering properties of a mesophase.