This requires use of a polymer precursor which, when heated, is stripped of its hydrogen and oxygen atoms to leave a carbon residue. There are two main types of precursor used:
Thermosetting resins (eg phenolics and furans) are used because they are processible by current industrial forming routes and readily impregnate preform structures. Thermosetting resins polymerise at low temperatures (250°C), to form a highly-cross linked, glassy solid which does not graphitise, even at temperatures up to 3000°C. The carbon yield from these precursors is up to 60%, and hence density of the composite may be adversely affected by the low density of the matrix formed.
Thermosetting resins are used to impregnate fibrous substrates, which are then taken through a carbonisation cycle. To build up density, it is frequently necessary to repeat the impregnation/carbonisation stage.
Pitches (eg coal-tar and petroleum pitch) are used because they have a low melt viscosity, high carbon yield and a tendency to form graphitic structures. Impregnation under high pressure also forces the pitch into the pores, thereby increasing overall density; however, carbonisation must be performed slowly, to allow the gaseous products to evolve without blowing the component apart.
The most widely used approach for introducing a carbon matrix into the fibrous preform is through impregnation with a liquid pitch or resin precursor. This is followed by carbonisation in an inert atmosphere to convert the impregnant to coke. This conventional technique is usually repeated several times to improve density. Use of high isostatic pressure during the impregnation and coking stages of densification results in a more efficient process. Coke yields for pitch increase from 50% at ambient pressure to 85% at 70MPa.