Pyrolysis schedule optimization of benzoxazine-derived carbon/carbon composites through reaction rate optimization

Abstract

Carbon-carbon composites (CCCs) are a form of carbon-fiber reinforced materials that exhibit excellent thermomechanical properties under extreme environmental conditions. To expand the applicability of CCCs, the fabrication process must be modified to reduce the cost or processing time. An optimization of this fabrication process was proposed for a thermoset benzoxazine-derived carbon-carbon composite and resulted in a 7 – hour pyrolytic schedule. This abbreviated schedule was achieved using a multi-stage nth – order kinetics model to limit individual reaction rates. These imposed limits reduced the internal pressures generated during thermal processing preventing layer separations and fiber rupture. The results of this modification were evaluated post heat treatment, via X-Ray 3D Computed Tomography, to ensure that the porous microstructure was fully interconnected with minimal closed void volume. Considering the absence of sample failure and closed void volumes of <1%, the pyrolysis schedule optimization was deemed successful in terms of producing a shortened cycle for a thermoset-derived CCC. To define the limitations of the optimization's applicability, a 1D model was proposed to predict the internal pressure generated during the final ramp as a function of decomposition kinetics, the through-thickness length, and the air permeability. Analysis of these predicted pressures resulted in a design chart that provided the upper bounds of the optimization protocol as it relates to sample thicknesses ≤50 mm and applied ramp rates ≤40 °C min−1.