Diffusion of oxygen into pitch

Abstract

The rate of diffusion of molecular oxygen into pitch was determined by monitoring the effect of paramagnetic oxygen on the relaxation behavior of the electron paramagnetic resonance (EPR) signals from the naturally-occurring free radicals in pitch. The materials investigated included mesophase pitch particles, as-spun mesophase and isotropic pitch fibers, and partially and completely thermoset mesophase pitch fibers. The diffusion was studied at temperatures between − 50 °C and +150 °C. A dimensionless mathematical model was derived and used to determine the oxygen diffusion coefficients and fractional oxygen concentrations for as-spun mesophase pitch fibers. A room-temperature oxygen diffusion coefficient of 2 × 10 −9cm 2s −1 and activation energy of 8.0 kcal mol −1 were obtained.