Study On Liquid Oxygen Compatibility And Cryogenic Mechanical Properties Of Epoxy Resins

Developing composite liquid oxygen tanks is the consensus of aerospace industrial and academic communities to further enhance the payload and lower the launch costs for launch vehicles.Currently,composite liquid oxygen tanks have been successfully developed and are now on ground tests at abroad.In our country,the development is still in its infancy.Making the composites compatible with liquid oxygen and possess excellent cryogenic mechanical properties are the key technologies of developing composite liquid oxygen tanks.In the present study,based on the mechanism of liquid oxygen compatibility and molecular design,liquid oxygen compatible epoxy resin system was obtained by introducing phosphorus and silicon containing functional groups into bisphenol A and F epoxy resins and then curing them by high-temperature aromatic amines,4,4'-diaminodiphenyl methane and 4,4'-diaminobisphenol sulfone.Cryogenic mechanical properties of the modified epoxy resins were also checked.Furthermore,under the premise of no lowering liquid oxygen compatibility,nano-Zr02 was used to improve cryogenic mechanical properties of epoxy resins.The main research contents and results are follows:(1)Phosphorus containing epoxy resins were designed and synthesized by introducing DOPO into bisphenol A and F epoxy resins to improve liquid oxygen compatibility of epoxy resins.The results of the liquid oxygen impact test prove that the introduction of DOPO could significantly improve liquid oxygen compatibility of the epoxy resins and make them compatible with liquid oxygen.The enhancement of thermal stability and flame retardancy is favorable to improve liquid oxygen compatibility for the epoxy resins.The initial thermal stability also produces certain influences on liquid oxygen compatibility.Bad initial thermal stability would induce the rapid decomposition of the epoxy resins at relatively lower temperature,resulting in the incompatible reaction in the case that phosphorus has not played a role in a timely manner.After the liquid oxygen impact test,the DOPO modified epoxy resins are still in the oxidation stage,while the pure ones have already been in the carbonization stage,which is attributed to that phosphorus works in both gas and condensed phases to inhibit the reaction between the epoxy resins and liquid oxygen.The results of the mechanical property testing show that the modified bisphenol F epoxy resins cured by DDM have the best mechanical properties.At room temperature(RT)and liquid oxygen temperature(90 K),compared with the pure epoxy resins,tensile strength is improved by 27%and 15%,fracture strain is improved by 80%and 37%and fracture toughness is improved by 44%and 23%.(2)ODOPB is a derivative of DOPO and possesses one more benzene ring and thus higher thermal stability compared with DOPO.ODOPB containing epoxy resins were designed and synthesized to further enhance liquid oxygen compatibility of epoxy resins.The results show that the ODOPB modified epoxy resins possess obviously enhanced liquid oxygen compatibility compared with the DOPO modified ones.Compared with the pure epoxy resins,the ODOPB modified ones have much lower weight loss rate and higher decomposition temperature during the whole degradation stage under oxygen atmosphere,which results in their significantly improved liquid oxygen compatibility.Besides,the ODOPB modified epoxy resins have higher initial thermal stability than the DOPO modified ones.The increase of the phosphorus content also does not lower the initial thermal stability,thus the ODOPB modified epoxy resins do not show decreased liquid oxygen compatibility with the increase of the phosphorus content.After the liquid oxygen impact test,the oxidation degree of the DDM cured system is obviously lower than that of the DDS cured system.The mechanical properties at both RT and 90 K would achieve the optimum at 1 wt.%phosphorus content.(3)A novel phosphorous/silicon containing epoxy-based hybrid was designed and synthesized by introducing GLYMO prepolymer into the DOPO modified epoxy resin.The results of the liquid oxygen impact test show that the hybrid is compatible with liquid oxygen.Its Tg is as high as 190 0C.Under oxygen atmosphere,its 5%weight loss temperature is 381?,and char residue at 600? is as high as 41.5%.After the liquid oxygen impact test,the hybrid has the lowest oxidation degree compared with the pure epoxy resin and the ones only containing phosphorus or silicon.A lot of PO3 and Si-O groups are generated on the surface of the hybrid after the liquid oxygen impact test,indicating the existence of polymetaphosphate and silicon dioxide.They work together to inhibit the reaction between the hybrid and liquid oxygen.The hybrid has a higher crack resistance under 90 K and demonstrates improved ductility and fracture toughness at both room temperature and 90 K.(4)Nano-ZrO2 was surface modified by(3-aminopropyl)triethoxysilane(APTES)and then introduced into the epoxy resin to enhance its cryogenic mechanical properties.The results of Fourier transform infrared spectroscopy(FTIR)verify that APTES could be successfully grafted onto the surface of nano-ZrO2.APTES modification is favorable to the dispersion of nano—ZrO2 in an epoxy matrix.The introduction of APTES modified nano-Zr02 could raise glass transition temperature of the epoxy resin and does not affect the curing degree evidently.The results of the mechanical property testing show that tensile strength and failure strain of the modified nano-ZrO2/epoxy composites are at most increased by 30%and 50%at RT,while their highest enhancements at 90 K are 26%and 21%,respectively,compared with those of the neat epoxy resin.Besides,fracture toughness at RT and 90 K are at most increased by 53%and 39%,respectively.These enhancements are mainly attributed to the strong modified nano-ZrO2/epoxy interfacial bonding and the better dispersion of the modified nano-ZrO2 in the epoxy matrix.When subjected to the external load,the modified nano-ZrO2 would deflect the crack propagation to enhance the mechanical properties of the epoxy matrix.(5)Crack formation and propagation of the carbon fiber reinforced composites prepared by the unmodified and modified epoxy resins BA/DDM and BA-P2/DDM was simulated by extended finite element method in commercial finite element analysis software Abaqus.Their crack resistance were compared.The results show that the composite prepared by BA-P2/DDM with higher mechanical properties has better resistance to crack formation and propagation.The load corresponding to the crack initiation is higher.The length of crack propagation is shorter in same load.This verifies that the modified epoxy resins developed in the present study have better application foreground in the manufacture of liquid oxygen tanks.