Various pretreatments of coal tar pitches are used in the formulation of matrix precursors for carbon-carbon composites, in particular air blowing or gas sparging has been widely employed to produce pitches of increased softening point. The observed hardening may involve a number of mechanisms. Volatilisation of low molecular weight species, molecular growth in the residuum and oxidative cross-linking are all considered likely. There is a particular need for clarification of the role of oxygen in gas sparging, it is unclear to what extent, if any, oxidation of the pitch influences hardening.
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