EFFECT OF PRESSURE ON THE COKING YIELDS OF COAL-TAR PITCHES

Pyrolysis of five coal tar pitches with wide ranging characteristics, made from the same coal tar precursor, has been studied under nitrogen pressures of 10(5), 50 x 10(5), 90 x 10(5) and 160 x 10(5) Pa, at a temperature of 550-degrees-C. The residues were further heat-treated to 900-degrees-C to obtain the ultimate normal (10(5) Pa) and pressure coking yields of these pitches. The literature states that for pitches with relatively lower softening points the carbonization pressure not only increases the coking yield but also lowers the temperature at which the pyrolysis is complete. This is seen to hold true for the present set of pitches, having a much wider range of softening points. Further, one of the pitches, earlier reported by us to be a good preforming pitch for carbon-carbon composites, gave an ultimate coking yield of 88% on subjection to a nitrogen pressure of 160 x 10(5) Pa at 550-degrees-C followed by ambient pressure carbonization to 900-degrees-C. It thus appears that a carbonization pressure of 160 x 10(5) Pa for a suitable preforming pitch can act as a reasonably good alternative to the expensive hot isostatic pressure impregnation carbonization technique employed in the production of carbon-carbon composites.