Pyrolysis of thermal protection system materials: molar yields of volatile products derived from in situ mass spectrometric measurements

Abstract

Mass spectrometric techniques have been developed to measure the molar yields of pyrolysis products from ablative resins and composite materials at heating rates that are relevant to flight conditions. Thermal decomposition mechanisms of phenolic and an epoxy-novolac resin systems are reviewed. New insights into the thermal decomposition mechanisms of PICA (Phenolic Impregnated Carbon Ablator) and epoxy-novolac D.E.N. 438 (Dow Epoxy-Novolac) are proposed and are based on the measurements of molar yields from these materials. Molar yield data have been provided in the appendices of this thesis for use in material response models. The thermal decomposition of phenolic impregnated carbon ablator (PICA) has been investigated with the objective of measuring molar yields of pyrolysis products at heating rates that are relevant to MSL flight conditions. The relative molar yields of 14 pyrolysis gases were obtained in conjunction with mass loss measurements. These measurements allowed for the calculation of absolute molar yields and mass yields as a function of temperature and heating rate, as well as the simulation of TGA curves. Pyrolysis product yields change as a function of heating rate, and this behavior has been attributed to two mechanisms that compete during the initial stages of thermal decomposition. The results of this study are now available for use in material response models. The thermal decomposition of an epoxy-novolac resin system has also been investigated. Samples of D.E.N. 438 were cured using NMA (methyl-5-norbornene-2,3-dicarboxylic anhydride) as a crosslinking agent and BDMA (N-benzyldimethylamine) as a catalyst. A radiative heating method was developed to minimize experimental uncertainties that may emerge from thermal gradients that are established across the samples as they experience high rates of heating. The molar yields of the six dominant pyrolysis products were measured at a heating rate of 8°C s -1. The molar yields of pyrolysis products provide new insight, and a new thermal decomposition mechanism is proposed.