Preparation,Stucture And Performance Of Polybenzoxazine Aerogel For Thermal Insulation

Thermal-insulating materials with lightweight,micro-ablation,high-efficiency thermal insulation,and long-term anti-oxidation are required for the thermal protection system in advanced high-speed near-space vehicles.Polybenzoxazine aerogel（PBO）is a new type of aerogel ablation thermal insulation material with low density,low thermal conductivity,and high strength.In this paper,the effects of preparation technology on PBO aerogels’microstructure and properties were studied,and PBO aerogels with adjustable density,microstructure,thermal conductivity,and mechanical properties were obtained.The PBO aerogel was modified by silicon to improve the thermal stability,thermal insulation,and oxidation resistance properties.The effects of silicon source type and content on the microstructure and properties of PBO aerogel were studied,and the modification scheme suitable for thermal insulating materials was optimized.PBO aerogel combined with quartz fiber needle felt reinforced SiO₂ aerogel composites were proposed for the first time,and PBO aerogel thermal insulation composites with excellent comprehensive properties were successfully prepared.The microstructure and properties of PBO aerogel composites were studied systematically,and the mechanisms of thermal insulation and anti-oxidation ablation were analyzed.The main conclusions are as follows:（1）The effects of monomer concentration of benzoxazine,ambient drying method and supercritical drying method on the microstructure and properties of PBO aerogel were studied,and the process parameters for preparing PBO aerogel by atmospheric drying were determined.The change rule of thermal conductivity of PBO aerogel under different pressure and atmosphere was revealed,and the heat transfer mechanism of gas molecules in different pore structure of PBO aerogel was clarified.PBO aerogel with thermal conductivity as low as 0.0335 W/（m·K）was prepared.（2）In order to improve the thermal stability and oxidation resistance of PBO aerogels,PBO aerogels were modified by silicon-containing compounds for the first time.Polyvinylmethyl dimethoxysilane（PVMDMS）,SiO₂ sol and SiO₂ aerogels modified PVMDMS-PBO,PBO/SiO₂ and SiO₂-PBO aerogels were prepared.The formation process and chemical structure of the three modified PBO aerogels were analyzed,and the microstructure and properties were studied systematically.The mass residual rate of the prepared organic-inorganic hybrid SiO₂-PBO aerogel was as high as57.83%after thermogravimetric test in air atmosphere at 800℃,which significantly improved the thermal stability and oxidation resistance of PBO aerogel.（3）The microstructure and chemical structure changes and the flame retardant and heat insulation mechanism of PBO,PBO/SiO₂ and SiO₂-PBO aerogels before and after combustion were revealed.PBO aerogel combustion forms macroporous carbon residue structure,which accelerates oxygen diffusion and heat transfer rate.During the combustion of PBO/SiO₂ and SiO₂-PBO aerogels,SiO₂ nanoparticles are filled in the combustion carbon layer to form small pores and play a better role of thermal insulation.At the same time,SiO₂ has its own non-flammability and high temperature resistance,which can effectively restrain the diffusion of heat flux and oxygen into the aerogel and make it have better flame retardant and thermal insulation properties.（4）A new type of PBO aerogel thermal insulation composites was prepared for the first-time combined quartz fiber needle felt reinforced SiO₂ aerogel composite with PBO aerogel.The structure and properties of two kinds of composites and the variation of thermal conductivity with temperature were studied.The two materials have similar micro-morphology,the aerogel matrix is thoroughly and uniformly filled between the fiber pores,and the interface is well bonded.In the thermal insulation composites,the aerogel matrix has a tighter three-dimensional network structure,thus showing better mechanical properties.The compressive strength（10%ε,2.58 MPa）and bending strength（3.03 MPa）of SiO₂ aerogel composites are better than those of SiO₂ aerogel composites,and the impact crater size and depth after impact test are smaller than those of SiO₂ aerogel composites.The high-temperature thermal conductivity of thermal insulation composites（1100℃,0.079 W/（m·K））is lower than that of SiO₂ aerogel composites（1100℃,0.095 W/（m·K））.The quartz lamp’s one-sided heating test further verified that the thermal insulation composite material had better thermal insulation performance at high temperatures.After the examination at 1100℃and 1800 s,the cold surface temperature decreased from 381.3℃to 307.2℃,which decreased by 74.1℃.（5）The ablation retrogression rate and mass ablation rate of PBO aerogel thermal insulation composites with ablation temperature and time were studied.In the same ablation time,both of them increase with the increase of temperature;At the same temperature,the probability of contraction in the range of 1100°C and 1200℃,increases in advance with the ablation time in the range of 1300°C.The mass ablation rate decreases with the increase of ablation time,which indicates that the thermal insulation composites are more suitable for the long-term oxidation environment at medium and low temperatures.The thickness line shrinkage and mass loss rate after ablation at different temperatures and different times are analyzed.The thickness line shrinkage is less than 2%when ablated at 1100-1200℃,while the mass loss rate is10.84-16.03%.High mass loss rate and low shrinkage rate show that the material has good dimensional performance,and the thermal insulation performance is further enhanced by a mass loss to dissipate surface heat.（6）The micromorphology,chemical structure,and crystal phase structure of the ablation surface and different depth ablation layer of PBO aerogel thermal insulation composites and their ablation/thermal insulation mechanism in the long-term aerobic environment were studied.After ablation,the PBO aerogel thermal insulation composites is still amorphous,and the microstructure is unchanged.There are three elements C,O,and Si in each region,and the structure of Si-O-Si and Si-O-C is formed.The formation of the Si-O-C structure can effectively reduce the content of pyrolytic free carbon and further improve the materials’oxidation resistance and temperature resistance.PBO thermal desorption,the release of small molecules to take away the heat,pyrolytic carbon scattering to reduce radiation heat transfer,and high-temperature resistance and thermal insulation of SiO₂ aerogel,Si-O-C structure formation to improve oxidation resistance,PBO aerogel thermal insulation composites have excellent thermal insulation properties.