Theoretical Investigation of Char Formation from Polymeric Ablative Materials for Spacecraft Thermal Protection Systems

As the commercial space industry continues to grow, polymeric ablative materials (PAM) for thermal protection systems (TPS) for re-entry need to be able to meet the higher demands of various spacecraft. Novolac phenolic resin (NPR) is the most common PAM, which possesses a high thermal decomposition temperature (TD) of 500 °C and char yield (YC) of 55 %wt. to resist high temperatures during the fight, as well as dissipate the heat from the rocket’s surface. However, this conventional PAM possesses drawbacks in high thermal conductivity and substantial water uptake owing to its hydroxyl functional group and highly crosslinked nature. In this regard, the present study looks for a novel polymeric for PAM that mitigates such drawbacks of NPR while holding comparable pyrolysis characteristics through theoretical analysis. First, we analyzed the pyrolysis mechanism of NPR using density functional theory (DFT) calculation to understand the underlying physics that leads to a high YC of 55 %wt. As a result, we found that the radical species from NPR’s initial pyrolysis step undergoes intramolecular cyclization, forming tricyclic intermediates (such as 9H-xanthene), irrespective of the initiation mechanism, whether bond fission or hydrogen abstraction. Second, we compared NPR’s pyrolysis pathway to those of two other potential PAMs – poly(dimethyl phenol) (PPO) and poly(diphenyl phenol) (PPPO) – which possess an improved water resistance and thermal conductivity while yielding less char upon pyrolysis (25 %wt. for PPO and 45 %wt. for PPPO). A mechanistic study on PPO pyrolysis indicates that it forms tricyclic intermediates (1,3,5-trimethyl-4a,9a-dihydro-9H-xanthen-2-ol) only if initiated by hydrogen abstraction. The bond fission of PPO has a substantial energy barrier to the formation of the tricyclic intermediate, explaining the lower YC of PPO than other polymers. Meanwhile, despite the similarity in the molecular structure of PPO and PPPO, the bond fission of PPPO forms tricyclic intermediates owing to the substituted phenyl group facilitating intermolecular cyclization. In other words, we found that a polymer’s YC depends on its structural capacity to form radicals that cyclize to form a polycyclic structure. The present study provides theoretical insights into structure-property relationships of polymer’s YC that can guide the accelerated discovery of novel PAM with superior pyrolysis characteristics for TPS application.