Study On The Synthesis And Thermal Resistance Mechanism Of Phenolic Resin Containing Boron And Silicon

Fiber reinforced polymeric ablatives as thermal protection materials have attracted much attention and been widely developed owing to their good stability,safety and reliability,low cost,easy shaping,short processing cycle,good thermal insulation and high adaptability to the changeable external environment.Among them,polymer resin matrix plays an important role in the ablative properties of fiber reinforced polymeric ablatives.Due to the existence of a large number of benzene rings in the backbone chains,phenolic resin can easily form a carbon layer in a high-temperature environment,leading to an excellent ablation performance.Besides,because of its low cost and simple molding process,phenolic resin is commonly used in fiber reinforced polymeric ablatives as polymer matrix.However,with the development of aeronautics and astronautics industries,as well as the weapon equipment,the working environment for the materials become more complex and harsher,it is urgent to produce thermal protection materials with higher requirements.Phenolic resin has been used for more than 100 years,but due to its inherent structure features,thermal properties and char yield cannot meet the needs.Against the above background,the current investigation would mainly focus on the development of a new type of phenolic resin.According to the structure design,researches have been made,as well as the thermal resistance mechanism.The main contents and conclusion are as follows.Boron-containing high-orho novolac resin?BHON?has been prepared by adding phenyl boric acid into phenolic resin intermediate.The structure analyses show that boron element was introduced via B–O–C structure.The char yield of cured BHON has been improved in comparison to high-ortho novolac resin?HON?,and the structure evolution has been studied.According to the analyses,the reasons for improved char yield are as follows:Formation of B–O–C can reduce the content of phenolic hydroxyl,which helps decrease weight loss;B₂O₃is also formed,and it could prevent the releasing of carbon oxides;incorporation boron helps to decrease interlayer distance and improve graphitization degree;owing to the existence of boron,N could form stronger interaction force with H in phenolic hydroxyl group,leading to the formation of thermal stable structure C=N.1,1'-bis?dimethylsilyl?ferrocene was introduced to synthesize modified high-ortho novolac resin containing silicon and ferrocene structure?Fe HON?.The thermal properties and structural evolution of cured Fe HON were studied to analyze the reasons for high char yield of cured Fe HON.Compared with HON,thermal stability of cured Fe HON has been improved,and the char yield of cured Fe HON at 800^oC can be increased to?70%with addition of7.5wt%modifier,which is higher than HON??61%?.The results show that silicon atom and ferrocene contribute to the high char yield of cured Fe HON.On one hand,the thermal stable structure Si–O can be kept in the system during the whole pyrolysis process,which could help to reduce the weight loss.On the other hand,introducing ferrocene into HON promotes the graphitization degree and graphite crystallites of pyrolyzed resins.Moreover,the pyrolyzed products possess different magnetic properties owing to the formation of iron nanocrystal,Si C and Fe₅Si₃ nanocrystal,and carbon nanowires can be in-situ formed after the cured Fe HON heat treated at 1200^oC.In order to overcome the phase separation between polysiloxane and phenolic resin using solvent-free method to modify phenolic resin?PR?,a silicon-containing phenolic resin?MSi PR?was prepared by using methyl triethoxysilane as silicon source.Through curing process,silicone oligomers were copolymerized into PR structure by Si–O–C structure.Char yield of cured MSi PR at 800^oC is?71%.According to the structure analyses during pyrolysis,the effect of silicon on the char yield has been discussed.On one hand,the formation of Si–O–C structure can reduce the number of phenyl hydroxyl groups,which contributes to the reduced weight loss.On the other hand,incorporation silicon can introduce thermal stable structure Si–O–Si which can exist during the whole pyrolysis,and partially form into Si O₂.Considering the positive roles of boron and silicon in improving the thermal stability of phenolic resin,siloxane and boric acid were used together to modify PR.Based on this,using methyltriethoxysilane as the silicon source and boric acid as the coupling agent instead of silane with a long alkane chain,phenolic resin containing boron and silicon?BSi PR?was prepared by a facile one-pot method.The results show that boron and silicon elements are introduced into BSi PR via B–O–C and B–O–Si structures.The char yield of resulting resin at800°C is improved to?76%.The reasons for higher char yield are investigated.The formation of B–O–C can reduce the content of phenolic hydroxyl,which helps to decrease the weight loss.B₂O₃ is also formed at 400°C,and it could prevent the release of carbon oxides.Moreover,thermal stable B–O–Si and Si–O structures remain stable during the pyrolysis.Incorporation of boron and silicon elements cannot help to improve the graphite degree of carbonized products.In addition,the mechanical and ablative properties of high-silica glass fiber?HSF?/BSi PR composites exhibit enhanced flexural strength,the interlaminar shear strength,and ablation resistance.This route offers an innovative and solvent-free method to prepare silicon-containing PR using boric acid instead of silane coupling agents.