Mesophase pitches as matrix precursor of carbon fiber reinforced carbon .2. Stabilization of mesophase pitch matrix by oxygen treatment

The stabilization of unidirectional (UD) composites fabricated with non-surface-oxidized and surface-oxidized HT carbon fibers, mesogenic pitches and mesophase pitches with oxygen was studied by variation of oxygen pressure (up to 2 MPa), temperature (160-200 degrees C), and stabilization time (up to 36 hours). The oxygen uptake is diffusion controlled and can be described by Fick's second law. By stabilization, swelling of the composites during carbonization is diminished, and the coke yield is increased. At a pitch-specific oxygen uptake, a minimum swelling and simultaneously a maximum coke yield were found. This specific oxygen uptake was assigned as optimum stabilization. This conclusion is confirmed by a decreased rate of oxygen uptake by further treatment. The oxygen uptake can be strongly accelerated by increased oxygen pressure, but like the maximum temperature for optimum stabilization, there is a maximum oxygen pressure. Both parameters are pitch-specific. In contrast to the optimum stabilization, it was surprisingly found that the bulk density of the carbonized composites can be increased by over-stabilization. Copyright (C) 1996 Elsevier Science Ltd