Temperature-Dependent Pyrolysis of PICA-D: Quantification of Product Yields

Abstract The thermal and mass transport between a pyrolyzing ablative heat shield and the boundary layer gas during atmospheric entry depends on the ablation rate, chemical composition of gaseous products, and quantity of gaseous products produced during the pyrolysis of the heat shield material. Material response models are used to predict the ablation rate and thermal transport; however, discrepancies exist between state-of-the-art models and flight data, reducing the reliability of such models. Accurate data on the temperature-dependent pyrolysis gas yields for relevant heat shield materials are therefore needed. We have thus obtained high-fidelity data on the pyrolysis products for the ablative heat shield material PICA-D (phenolic impregnated carbon ablator−domestic), a carbon-phenolic composite, as a function of sample temperature over the range, 25−1200 °C, at five linear temperature gradients with respect to time (1.08, 3.15, 5.92, 12.55, and 24.67 °C s−1), using molecular beam mass spectrometry. Absolute and normalized molar and mass yields of 15 gaseous products have been determined as a function of sample temperature, and thermogravimetric analysis (TGA) curves have been synthesized from the absolute mass yields. The yields and TGA curves exhibit temperature-gradient dependencies; however, these dependencies are not nearly as pronounced as those seen in our analogous earlier studies of cured SC-1008 pure phenolic resin. Comparison of the yields of PICA-D and cured SC-1008 resin have led to the identification of condensation reactions between the resin and oxidized species on the carbon fibers in PICA-D. These high-fidelity data have shown that the specific environment of the cured SC-1008 phenolic resin in the composite material, PICA-D, including density, thermal conductivity, and presence of carbon fibers, influences the temperature-dependent pyrolysis processes.