Pyrolysis of Phenolic Resin Investigated by Molecular Beam Mass Spectrometry

Most pyrolyzing ablative heat shields contain phenolic resin; therefore, an understanding of the thermal decomposition mechanisms of this material can provide a foundation upon which to develop improved materials response models, which can, in turn, lead to better heat shield design and performance prediction. With the goal of obtaining high-fidelity data on the thermal decomposition mechanisms of phenolic materials, a baseline study of the gaseous pyrolysis products from the phenolic resin derived by curing Durite SC-1008 has been conducted. Product yields were detected by a mass spectrometer as a function of sample temperature, over the range of 25-1200 °C, at five linear temperature gradients with respect to time (0.83, 3.03, 5.83, 11.54, and 23.11 °C s^-1) with the use of a molecular beam mass spectrometry sampling method similar to the method used in earlier work by [Bessire, B. K.; Minton, T. K. <i>ACS Appl. Mater. Interfaces</i> 2017, 9, 21422-21437]. Quantitative molar and mass yields of 15 gaseous products have been determined as a function of temperature, and thermogravimetric analysis curves have been synthesized from the temperature-dependent mass yield data and measurements of total mass loss. Both the molar and mass yields exhibit temperature-gradient dependencies, as decomposition mechanisms compete and the residence time at each temperature may be comparable to or shorter than mass diffusion and heat conduction time scales. The total mass loss increases with an increase in the rate of temperature change, as a result of the competition between condensation and methylene bridge scission reactions, which was proposed previously, and this competition combined with the direct breakdown of backbone phenolic groups contributes to the changing molar yields. The new high-fidelity pyrolysis data provide mechanistic insight into the thermal decomposition of phenolic resin that can be used as a benchmark for understanding and modeling the pyrolysis of carbon phenolic composites that are used in thermal protection systems on space vehicles.