| Thermal Protection System(TPS)plays an important role in protecting the internal structure of aircraft when the spacecraft passes through the atmosphere.As ablative insulation materialsilicone rubber has many advantages such as low density,good thermal stability,excellent thermal insulation performance and high elasticity.However,char yield is low for silicone rubber.A more dense and rigid char layer was very resistant to peel-off under high pressure and shear forces of ablative gases during the ablation process,which had reduced the penetration of heat flux from the hot oxygen-acetylene flame.Boron phenolic resin has high carbon residue yield and good thermal stability due to introduce B atom to polymer chain.However,there are also some disadvantages such as high brittleness.In this paper,boron phenolic resin was introduced into silicone rubber matrix to prepare boron phenolic/silicone rubber composites for anti-ablation applications.The properties and mechanism of ablative insulating materials were studied systematically.Determination of basic formula and investigation of vulcanization process:the optimal formula and vulcanization process of ablative heat insulation material were studied by using Mooney vulcanizing instrument and plate vulcanizing machine.The results showed that methyl vinyl silicone rubber(VMQ),methyl vinyl phenyl silicone rubber(PVMQ),carbon-white and boron phenolic resin were used as raw materials.Vulcanization process:(1)The vulcanizing temperature is 150 C;(2)The pressure is 5 MPa,(3)Vulcanization time is 38 min;(4)Secondary vulcanization temperatures is 170 ?,pressure of 5 MPa,vulcanization time 30 mins.(5)The third curing temperature 190 C,pressure is 5 MPa and vulcanization time is 2 h.The times are two for deflated process(each for 1min).The effects of carbon fiber and ZrB2 on the properties of boron phenolic/silicone rubber based ablative materials were investigated.The ablation and heat insulation mechanism of ablative heat insulation material at high temperature was discussed by thermodynamic analysis using HSC Chemistry software.The results showed that the composites weren't significantly enhanced when carbon fiber and ZrB2 were added compared with that of carbon-white.However,ablation resistance increased with the adding of the carbon fiber and ZrB2.The linear ablation rate was reduced to the lowest 0.223mm/s when the content of boron phenol resins is 30 phr.The ablation mechanism of the ablation resistant material was obtained by thermodynamic analysis.During the ablation process,ZrBZ in the surface ceramic layer first reacts with O2 at high temperature to form ZrO2.The ceramic compound formed by the combination of ZrO2 and molten SiO2 blocks the further transfer of heat.Carbon fiber plays a role in skeleton connection makes the ceramic layer difficult to break and peel off.ZrB2 in the pyrolysis layer reacts with CO,CO2 and H2O produced by the pyrolysis of boron phenolic resin,and they generate ZrO2 and other products.Carbon fiber cloth was pretreated with silane coupling agent and boron phenolic resin to prepare boron phenolic/silicone rubber based ablative materials.The structure and ablation mechanism of the material were studied by means of FTIR,EDS,XRD and thermodynamic analysis.The results show that the stripping strength of the carbon fiber cloth with 3%KH570 surface modification reaches 2.1KN/m.The tensile strength and linear ablation rate of the ablation-resistant adiabatic material strengthened by CFRP treated with 4%KH570 are the best.Its tensile strength is 16.3MPa,and its linear ablation rate is 0.065mm/s.The ablation mechanism is that the carbonization-ceramic coupling of ablative materials occurs during the ablation process. |
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