Production and characterization of ablative polymeric composites for thermal protection systems

Materials used extensively in many fields such as aerospace and nuclear engineering are exposed to serious temperatures and pressures. Vehicles such as re-entry space vehicles, missiles and space capsules used in the field of aerospace cause the surface of the vehicles to be exposed to high temperatures and pressure as a result of severe aerodynamic heating during re-entry into the atmosphere. Thermal protection systems (TPS) act as a thermal barrier by preventing the aerodynamic heat generated on the surface of the spacecraft from being carried inside the spacecraft and reaching the payloads (humans and cargo). 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 produced composites were investigated. Matrix materials have been developed by modifying resorcinol-formaldehyde resin (RR) with different weight percentages of phenylboronic acid and filling with titanium diboride (TiB2) at different weight percentages. Matrix materials were characterized by DSC (Differential scanning calorimetry), FTIR (Fourier Transform Infrared Spectroscopy), NMR (Nuclear magnetic resonance), SEM (Scanning electron microscopy), TGA (Thermogravimetric analysis), XPS (X-ray photoelectron spectroscopy) and XRD (X-Ray diffraction) methods. New boronate bonds were included in the structure of the developed phenylboronic modified resorcinol-formaldehyde resins (RRB), and boronate bonds were transformed into B2O3 structure during the pyrolysis reactions. Modifing with boron and filling with TiB2 of the matrix material increased the thermal stability and char yield. While the char yield of the RR polymer was 47.9% at 1000 °C, the char yield of the matrix material (RRBT20), which was prepared by modifying with 6% wt of phenylboronic acid and filling with 20% by weight of TiB2, was increased to 88.2%. Composites were produced by impregnating the prepared matrix materials into polyacrylonitrile (PAN) based needle punched carbon fiber felts. Composites were subjected to a three-point bending test, modification of the resin with boron and filling increased the flexural strength of the composites. The ablation properties of the composites were determined using the oxyacetylene flame test. Increasing the titanium diboride content in the composites increased the ablation resistance. While the mean linear ablation rate (LAR), mass ablation rate (MAR) and charring rate (CR) of the composite prepared with pristine RR matrix provided values of 0.0273 mm/sec, 0.0652 g/sec and 0.2120 mm/sec, respectively, the LAR, MAR and CR values of the K-RRBT20 composite, which provides the best ablation resistance, were reduced to 0.0053 mm/sec, 0.0195 g/sec and 0.0543 mm/sec, respectively. The ablation mechanisms of the composites were derived from the inferences obtained from EDS (Energy dispersive spectroscopy), SEM (Scanning electron microscopy), XPS (X-Ray photoelectron spectroscopy) and XRD (X-ray diffraction) analyzes of char layers formed after exposure to oxyacetylene flame. Titanium diboride in the composites oxidized under high temperature conditions, resulting in the formation of B2O3 (diboron trioxide) and TiO2 (titanium dioxide) structures. TiC (titanium carbide) was formed by the conversion of carbon fiber and resin into carbon char at high temperature and then reacting with TiO2. These new structures have enabled the ablation resistance of the composites to increase and the composite to form a heat barrier. Despite the 2650-2700 °C temperature value on the front surfaces of the composites in the oxyacetylene test, the back surface temperatures remained in the range of 24.8-26.8 °C, providing a heat barrier.