Influence of primary QI on pitch pyrolysis with reference to unidirectional C/C composites

The pyrolysis of four pitches of increasing quinoline insolubles (QI) content (0.5–11 wt%), prepared by blending, in different proportions, a commercial binder pitch with varying amounts of the same filtered pitch (almost QI free), was studied by hot stage microscopy and thermogravimetric analysis. Pyrolysis products of the pitches, obtained at 450 °C for 30 minutes, were examined by optical microscopy to monitor the influence of QI on mesophase formation. The optical texture, crack development, porosity and fibre/matrix bonding of the composites were considered in relation to the QI content of the pitchmatrix precursor. Bulk density, flexural strength and interlaminar shear strength of composites were explained in regard to their microstructure and the composition of the pitch precursors. The restricted development of mesophase in the individual pitches was associated with the presence of QI and lead to the generation of intramatrix mosaics in the C/C composites, domains always being orientated around the fibre. QI improved fibre/matrix bonding and the overall strength of the composites, but the composites became more brittle.