Pyrolysis of composites
prepared from a phenol-formaldehyde resin and carbon fibres (CFRC) or oxidized PAN fibres (OFRC) has been investigated up to 1270 K. Two temperature regions can be distinguished in the pyrolysis of the CFRC: below 770 K the carbon fibres hold the composite structure resisting the tendency of the resin to produce composite shrinkage; above 770K the CFRC volume and porosity abruptly increase due to a weakening of fibre/resin adhesion. During the pyrolysis of the OFRC a chemical reaction at the fibre/resin interface apparently takes place, beginning at 570 K and producing important changes in the material at 87&970 K. Somewhat smaller weight losses and markedly lesser shrinkage of the pyrolysed composite with respect to its separately treated components have been found. The fibres and the resin coalesce into a product having relatively high density and low porosity, the latter mainly consisting of narrow pores. Hydrogen content of the product is lower than that of the separately treated constituents.
The reflections of both the resin and the oxidized PAN fibres are seen only in the X-ray patterns of the OFRC treated at low temperature, but above 670 K only the reflection due to the fibres appears. The interlayer spacing of the fibres in the composite is greater than in the fibres alone treated under the same conditions.
According to the IR spectra, the CN groups and aromatic CH groups disappear from the composite at lower temperature than from its components treated separately.
The existence of the reaction between the resin and the oxidized PAN fibres, leading to the destruction of the fibre materials, means that the co-carbonization product at 1270 K is not a composite and will not have the necessary mechanical properties.
Therefore, this study must continue to look for fibres and matrices which are compatible during co-carbonization to 1270 K.