Investigation of Polyurethane-Modified Phenolic Resin Pyrolysis Mechanism via Experiments and MD Simulation

Thermal blocking effect serves as the core mechanism of heat dissipation for ablative thermal protection materials. Modifying the molecular structure of the ablative matrix to regulate the pyrolysis temperature range, along with investigating the pyrolysis mechanism and gas-release behavior during the thermal decomposition process, is crucial for the design of novel advanced ablative materials. Herein, we focus on lowering the low-temperature pyrolysis range by innovatively introducing polyurethane (PU) structures into the molecular framework of phenolic resins (PRs) via covalent bonding and successfully obtained two types of polyurethane-modified phenolic resins (PR-PUs) with different ratios of soft and hard segments. The gaseous products and pyrolysis mechanism of PR-PUs were monitored and analyzed by a combination of experimental characterization and molecular dynamics (MD) simulation. Results of TGA indicated that the incorporation of PU advances the initial thermal decomposition of PR, and the initial decomposition temperature (Td5%) of PR-PUs decreased from 425 °C for PR to a lowest of 358 °C for 250-PR-PU-15% (a 17.7% decrease). During the initial stage at 250 °C-400 °C of PR-PUs pyrolysis, the cracking of carbamate generates CO2 and ethene. Between 400 and 600 °C, the degradation of soft segments of PU releases small molecules of aldehydes, ketones, and cyclic ethers. Furthermore, oxygen-containing radicals generated from PU promote the thermal decomposition of PR repeating units. The introduction of PU has profoundly altered the pyrolysis mechanism and gas-release behavior of PR, particularly in 250–600 °C. This study proposes novel insights and theoretical foundations for designing novel ablation-resistant PRs.