Oxidative Stabilization of Mesophase Pitch: Permeabiltity Measurements and Kinetics

Mesophase pitch is a precursor material for both high quality carbon spun fibers, as well as the matrix in high temperature carbon composites. In both cases, to prevent foaming and local expulsion of the pitch during the high temperature carbonization and graphitization steps of the manufacturing process, the pitch must be stabilized (thermoset) by crosslinking with oxygen. This is accomplished by exposing the solid mesophase pitch fiber or composites to oxygen at a fixed temperature.Stabilization is one of the most time consuming steps in the production of carbon-carbon composites. The oxidative stabilization reaction of pitch solid matrix for those composite materials is believed to be oxygen diffusion limited. The main problem however is the very small value of oxygen permeability into the matrix which makes it extremely difficult to measure directly with conventional analytical methods. Although many works on carbon-carbon composites have been done in the past 30 years, the transport and kinetics of the oxidative stabilization are complex and still not completely understood.Resin Transfer Molding (RTM) composites are obtained from a high pressure melted liquid pitch impregnation process and is used for high matrix densities in carbon composite brakes. Our goal is to study experimentally and theoretically the transport phenomena and kinetics of the oxygen stabilization of a synthetic mesophase pitch matrix in a carbon-carbon composite sample obtained by Resin Transfer Molding (RTM). Oxygen concentration profiles in unreacted and reacted RTM samples are obtained from Auger Spectroscopy. Information about the chemistry and products, such as carbon loss and the rate of the total gaseous oxygen reacted to the oxygen uptake by the solid pitch, are established through chemical elemental analysis and gas chromatography product analysis performed in parallel on synthetic Mitsubishi AR mesophase pitch powder of different particle sizes.The data obtained from Auger Spectroscopy, elemental analysis and gas chromatography for constant temperature stabilization reaction, are used to estimate the permeability of RTM, 4.09 x 10-14 cm2/sec and 1.33 x 10-14 cm2/sec at 170 ÌøåÀå_C and 195 ÌøåÀå_C respectively.Experimental permeability values are used to develop a semi-empirical mathematical model analyzing the diffusion and kinetics process of isothermal oxidative stabilization of solid mesophase pitch carbon composite matrices. A comparison between the experimental data and oxygen profiles predicted by the model is performed to check the validity of the model and agreement is obtained.