The Thermal Processing and Rheological Behaviour of Pitch

Abstract

Pitch is the most common and most important precursor for the production of engineering carbon and graphite materials, because it is an excellent source of graphitisable carbon, both readily available and cheap. It is the basis of both the filler and binder phases in polygranular materials, such as electrodes, nuclear graphite and mechanical carbons; it can be converted into isotropic or high performance (anisotropic) fibres; it can be used to provide the matrix in carbon-carbon composites and it can be processed into ‘sinterable’ particles used in the manufacture of high strength carbons/graphites. In addition to its ability to form graphitisable carbon, a key characteristic of pitch is that it is thermoplastic. Pitches show similar behaviour to other glassy/thermoplastic systems, exhibiting viscoelastic behaviour in the region of the glass transition and a wide variation in viscosity at temperatures above Tg. The rheological properties vary tremendously with pitch composition and, in addition, there is a limit to the temperature range over which each pitch composition is thermally stable. This, therefore, imposes limits to the rheological behaviour that different materials may display. These factors are critical in the selection of pitches for a proposed process and will be discussed here. The chapter will focus on the general relationship between rheological properties and composition and how these change with the heat treatment that is given to the pitch to convert it to carbon. Of particular significance is the carbonaceous mesophase, which appears as an intermediate state in the pyrolysis process and is essential to the graphitisability of the carbon material that is formed during pyrolysis.