| The development of heavy launch vehicles is a prerequisite for humans to achieve interstellar travel and deep space exploration.The cryogenic fuel tank is the largest component in the launch vehicle,and its light weight level determines the overall performance of the rocket.Among many candidate materials for the new generation of lightweight cryogenic fuel tanks,carbon fiber reinforced epoxy resin matrix composites are preferred due to their excellent overall performance.However,as a structural material of the cryogenic propellant tank,the carbon fiber composite must face severe ultra-low temperature environment(liquid oxygen/liquid hydrogen:-183/-253 ℃).In this work,in view of the shortcomings of epoxy resin in the cryogenic environment,such as brittleness,poor toughness and impact resistance,it is expected to enhance the cryogenic mechanical properties of epoxy resin by incorporating the soft molecular chains and nanoparticles reinforcing phase.However,in view of the particularity of liquid oxygen(the liquid oxygen compatibility of epoxy resin),NPEF-170 epoxy resin is selected as the matrix resin from four commercial epoxy resins due to its better liquid oxygen compatibility.Using NPEF-170 epoxy resin as matrix resin,a method for introducing flexible polysiloxane(-Si-O-Si-)molecular chains into the molecular structure of epoxy resin is proposed to improve the cryogenic mechanical properties of epoxy resin.At the same time,the Br is incorporated into the molecular structure of epoxy resin to study its liquid oxygen compatibility mechanism.Then,in order to avoid the phase separation of the polysiloxane in the epoxy resin,the epoxy group and the phenyl functional group are introduced into the molecular structure of the polysiloxane to improve the compatibility with epoxy resin.In addition,in order to overcome the shortcomings of flexible polysiloxane decreasing the tensile strength and elastic modulus of epoxy resin at RT,a methord is proposed to improve the tensile strength,elastic modulus,elongation at break and fracture toughness and impact strength of epoxy resin at RT and cryogenic temperature throuth simultaneously introducing flexible polysiloxanes and Nano-SiO2 into the molecular structure of epoxy resin.The strengthening and toughening mechanism of flexible molecular chains and nanoparticles on epoxy resin are discussed.At last,the carbon fiber reinforced epoxy resin matrix composites are prepared by vacuum injection molding with neat and modified epoxy resin as the matrix and T300 carbon fiber orthogonal woven fabric as the reinforcement.In addition,the cryogenic mechanical properties and the influence mechanism of cryogenic immersing treatment and RT/cryogenic environment thermal cycle treatment on the mechanical properties of composites is studied.The main research contents and conclusions are as follows:(1)The liquid oxygen compatibility of bisphenol A epoxy resin,NPEF-170 epoxy resin.TDE-85 epoxy resin and AFG-90 epoxy resin was evaluated by 98J liquid oxygen impact test.The test results indicate that the NPEF-170 epoxy resin has a relatively better liquid oxygen compatibility,and the liquid oxygen impact sensitivity of these four epoxy resins is as follow:NPEF-170 epoxy resin<AFG-90 epoxy resin<TDE-85 epoxy resin<bisphenol A epoxy resin.The 98J impact test results indicated that the Br can improve the liquid oxygen compatibility of epoxy resin.(2)A flexible epoxy-functionalized polysiloxane(PSE-I)is synthesized by hydrolysis condensation reaction of siloxanes.The molecular structure of PSE-I is characterizated by Fourier Transform Infrared and 1H-NMR spectrum.The mechanical test results indicated that PSE-I can improve the elongation at break,fracture toughness KIC and impact strength of epoxy resin,which increases by 70.94%,8.66%and 7.16%,respectively,compared with the neat resin(6.16%,1.27MPa·ma1/3,73.05kJ/m2).At cryogenic environment,the tensile strength,elongation at break,fracture toughness KIC and impact strength of PSE-I modified epoxy resin increases by 18.60%.25.70%,11.11%and 22.28%,respectively,compared with the neat resin(174.10MPa,2.49%,2.34MPa·m1/2,35.33kJ/m2).However,PSE-I could weaken the tensile strength and elastic modulus of modified epoxy resin at RT.The cryogenic environment significantly reduces the elongation at break and impact strength of epoxy resin,while the KIC value is significantly increased.At the same time,as the PSE-I content increases,the phase separation of the polysiloxane gradually occurs in the epoxy resin.(3)In order to avoid the phase separation of polysiloxane in epoxy resin,a novel polysiloxane(PSE-II)containing both epoxy and phenyl functional groups was synthesized.Phenyl and epoxy groups can significantly improve the compatibility of polysiloxane with epoxy resin.The mechanical test results indicated that PSE-II can improve the elongation at break,fracture toughness KIC and impact strength of epoxy resin,which increases by 71.56%,31.50%and 12.60%,respectively,compared with the neat resin.At cryogenic environment,the tensile strength,elongation at break,fracture toughness Kic and impact strength of PSE-I modified epoxy resin increases by 11.16%,33.78%,30.34%and 14.77%,respectively.Similar to the case of PSE-Ⅰ,PSE-Ⅱ could reduce the tensile strength and elastic modulus of modified epoxy resin at RT.(4)In order to overcome the-shortcomings of PSE-Ⅰ/Ⅱ decreasing the tensile strength and elastic modulus of epoxy resin,a PSE-II/Nano-SiO2 nanomaterial is prepared by in-situ polymerization with PSE-II as the organic phase and Nano-SiO2 as the inorganic phase.The TEM images of cured epoxy resin shows that the Nano-SiO2 have a good dispersibility in the epoxy resin.The mechanical test results showed that the PSE-II/Nano-SiO2 can simultaneously improve the tensile strength,elongation at break and elastic modulus of epoxy resin at RT.At RT,the tensile strength,elastic modulus,the elongation at break,fracture toughness KIC value and impact strength of modified epoxy resin increases by 9.30%,25.50%,54.55%,48.82%and 21.36%,respectively,compared with neat epoxy resin.At cryogenic environment,the tensile strength,elastic modulus,the elongation at break,fracture toughness KIC value and impact strength of modified epoxy resin increases by 17.07%,4.30%,26.91%,33.33%and 28.45%,respectively.(5)Carbon fiber reinforced epoxy resin matrix composites are prepared using neat and PSE-II/nano-SiO2 modified epoxy resin as matrix and T300 carbon fiber orthogonal woven fabric as reinforcement.The mechanical test results show that the mechanical properties of CFRP-Si epoxy are better than the CFRP-Neat epoxy at RT and cryogenic environment.The tensile strength of composites at cryogenic environment is lower than that of at RT However,the tensile modulus and flexural modulus is increased compared to the case of RT.At RT and cryogenic environment,the bending failure of composites first occurs on the tensile side of material,accompanied by the delamination of the interlayer fibers and the fiber pull-out failure.The liquid nitrogen immersing 30 days and RT/cryogenic environment thermal cycling treatment 80 times,to a certain extent,will improve the mechanical properties of composites at-RT and cryogenic environment. |
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