Fabrication And Thermal Pyrolysis Behavior Of BPFR Based Composites

Resin based ablative materials are kinds of materials which depend on the mass loss of polymers caused by thermal oxidation, while absorbing heat to meet the requirements of thermal protection. Boron-containing phenol-formaldehyde resin(BPFR) becomes the best resin matrix of ablative composites, because of high carbon yield and low ablation rate. With the rapid development of aerospace and other fields, it is urgent to modify BPFR to develop the maximum using temperature of ablative materials.The current research focused on improving thermal decomposition temperature of organic structures in BPFR based composites, but the thermal decomposition temperature was limited, and failed to form a substantial breakthrough. In this present study, the ablative temperature was improved by protecting the inorganic structures formed from organic structures. B2O3 and B4 C were used as reinforcing materials to increase the ablative performance of BPFR based composites in temperature period. Field Emission Scanning Electron Microscope(FESEM), Fourier Transform Infrared Spectroscopy(FTIR) and X-ray Diffraction(XRD) were used to analyze the pyrolysis behavior in-depth.BPFR had serious deterioration of mechanical properties after pyrolysis at different temperatures, the flexural strength were only 3.1 MPa, 4.3 MPa, 1.9 MPa after pyrolyzed at 600 oC, 800 oC, 1000 oC. Modification effect by adding B2O3 was mainly occurred between 600 oC and 800 oC, and when the B2O3 content was 40 wt.%, the composites had best mechanical properties, the flexural strength were 14.7 MPa, 15.7 MPa, 8.0 MPa after pyrolyzed at 600 oC, 800 oC, 1000 oC, respectively. Adding B4 C had a good modification effect mainly above 800 oC, and when the B4 C content was 20 wt.%, the composites also had best mechanical properties after pyrolyzed at 600 oC, 800 oC, 1000 oC, the flexural strength were 7.9 MPa, 30.1 MPa, 24.4 MPa, respectively. Finally, selecting B4 C and B2O3 as mixed reinforcing materials and the content of these two particles were 10 wt.%, respectively. The flexural strength of BPFR/B2O3/B4 C composites were 15.8 MPa, 33.1 MPa, 33.2MPa, respectively. The composites had a greatly improvement on flexural strength compared to pure BPFR. It could be seen that B2O3 and B4 C could improve the ablative performance of BPFR based composites in temperature period.The flexural strength of BPFR/B2O3/B4 C composites after pyrolyzed at each temperature reached at high level, the molten B2O3 could fill holes and cracks formed in resin matrix after pyrolysis, wrap part of the active groups, enhance the mechanical properties and oxidation resistance. When the temperature was above 800 oC, because of serious loss of B2O3, the modification effect was not obvious. At the same time, part of B4 C on the surface could be converted into B2O3 resulting from oxidation, forming a protective layer to prevent oxygen from difusing into the interior of materials. B4 C could also react with the amorphous carbon, generating a more compact overall to improve the mechanical properties of the composites.