Production and characterization of ablative aerogel composites containing biomonomers for aerospace applications

Materials used in aerospace missions are exposed to high temperatures and pressures due to severe aerodynamic heating during re-entry into the atmosphere. Thermal protection systems (TPS) provide thermal insulation by preventing the aerodynamic heat generated on the outer surface of the spacecraft from being transmitted to the interior of the vehicle and the loads (people and cargo) from being affected by this heat. In this thesis, composites were produced by developing matrix material for ablative polymeric composites used in thermal protection systems. The ablation properties and ablation mechanisms of the composites were investigated. Resorcinol-furfural resin (FR) was first synthesized with different weight percentages of hexamethylene tetramine in order to understand the effect of crosslink ratio, and then matrix materials were developed by modifying with phenylboronic acid in different weight percentages. The matrix materials were characterized by BET (Brunauer-Emmett-Teller), FTIR (Fourier transform infrared spectroscopy), TGA (Thermogravimetric analysis) and DSC (Differential scanning calorimetry) methods. When the BET analysis of the developed materials was examined, it was found that it increased with increasing crosslink ratio but decreased with increasing phenylboronic acid modification. When the FTIR peaks were examined, it was observed that RF resins had dense aromatic rings, while new boronate bonds were formed in phenylboronic acid modified resins. Modification of the matrix material with boron increased the char yield. While the char yield of RF8 polymer was 41.23% at 1000 °C, the char yield of the matrix material prepared by modifying with 25 wt% phenylboronic acid (RFB25) was increased to 46.63%. The prepared matrix materials were impregnated with polyacrylonitrile (PAN) based needle-punched carbon fiber felts and composites were produced. When SEM images of the prepared composites were examined, cracks and pitting were observed in composites with RF matrix, while these formations were not observed in composites with RFB matrix. In addition, free phenylboronic acid particles were observed on the carbon fiber felts in the composites with RFB matrix. The ablation properties of the composites were determined using oxyacetylene flame test. The ablation resistance was increased by modifying the composites with phenylboronic acid. While the average linear ablation rate (LAR), mass ablation rate (MAR) and carbonization rate (CR) of the composite prepared with RF8 matrix were 0.009 mm/sec, 0.032 g/sec and 0.235 mm/sec, respectively, the LAR, MAR and CR values of the K-RFB25 composite, which provided the best ablation resistance, were reduced to 0.007 mm/sec, 0.024 g/sec and 0.165 mm/sec, respectively. In the oxyacetylene test of the composites, the back surface temperatures remained in the range of 25.3-27.5 °C despite the temperature value of 2733.8-2070.9 °C on the front surfaces of the composites and provided a heat barrier.