| Carbon fibers reinforced epoxy resin matrix composites have the advantage of high specific strength,high specific stiffness and flexible design.The weight savings in launch vehicles can be 20%-40%,if composites are used in cryogenic propellant tank,compared with the existing metal tank.However,ordinary epoxy resin-based composites are incompatible with liquid oxygen.When they are used under liquid oxygen environment,they are prone to violent chemical reactions with liquid oxygen,resulting in structural failure,which may cause serious accidents for launch vehicles.Relevant literatures have proved that carbon fiber is compatible with liquid oxygen.Therefore,the design and development of an epoxy resin matrix compatible with liquid oxygen is the key to preparing liquid oxygen compatible composites.This will have far-reaching effects on promoting the application of composites in liquid oxygen storage tanks.Based on free radical theory and hot spot theory,this article attampts to improve the liquid oxygen compatibility of the resin matrix from both chemical and mechanical perspectives.From a chemical point of view,blending technology,chemical grafting technology and nanoparticle-composite technology were used to prepare multiple modified thermosets.The liquid oxygen compatibility,flame retardancy and mechanical properties of the modified epoxy resins at low temperature were systematically studied.The mechanism of phosphorus/nitrogen synergy on the liquid oxygen compatibility of epoxy resin was revealed.From a mechanical point of view,the crosslink density of the modified epoxy was changed by adjusting the epoxy/amine equivalent ratio,and the effect of deformability of the resin at low temperature on the liquid oxygen compatibility was studied.The finite element numerical analysis method was used to simulate and verify the test results.The specific research content is as follows:(1)Firstly,taking the compatibility of epoxy resin and liquid oxygen into account,a series of epoxy resin thermosets(including triepoxy alicyclic epoxy resin(TDE85),triepoxy aminophenol epoxy resin(AFG-90H),bisphenol A epoxy resin(E51),bisphenol F epoxy resin(E59)and nitrogen-containing heterocyclic hydantoin epoxy resin(HE),all cured by 4,4’-diaminodiene methane(DDM))were tested and screened through thermal stability analysis,liquid oxygen compatibility test and liquid oxygen oxidation resistance test.The results showed that all epoxy resins were incompatible with liquid oxygen.Among the thermosets,HE/DDM showed relatively good compatibility with liquid oxygen,which had a lower liquid oxygen impact sensitivity coefficient(IRS),a higher thermal decomposition temperature and the lowest surface carbonyl group increment after soaked in liquid oxygen.(2)Secondly,HE,as the basic resin,was modified by blending with the additive phosphorus-containing compound(hexaphenoxy cyclotriphosphazene(HP)and tris(2-chloropropyl)phosphate(TP)).The correlation between the liquid oxygen compatibility and flame retardant properties of resins was revealed.The results showed that adding HP could significantly improve the liquid oxygen compatibility of epoxy resin.The introduction of HP and TP significantly improved the oxygen index(LOI)and flame retardant grade of the resin.The highest LOI value of the modified resin could reach 36.5% and 35.4%,respectively.The corresponding flame retardant grade reached V-0.Comparing the liquid oxygen compatibility and flame retardancy test results of epoxy resin,the index of the flame retardant grade of the resin could not be directly used to judge the liquid oxygen compatibility of the resin.However,the flame extinguishing time after the initial ignition of the resin,the blowing-out effect during combustion,and the morphology of the carbon layer after combustion might be indirectly used to evaluate the liquid oxygen compatibility of the epoxy resin.Combined with Fourier transform infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS)analysis,the liquid oxygen compatibility mechanism of HP modified resin included: the capturing effect of the highly active radicals(H·,HO· and ·O·)by phosphorus containing radicals(P·,PO· and·PO2 free radicals)and the dilution effect of nitrogen-containing non-combustible gases decomposed from HP in the gas phase;and the carbonization effect of HP and barrier effect of the compact carbon in the deposition phase.However,HP had disadvantages of poor dispersibility in the resin and easy precipitation.(3)Thirdly,in order to overcome the disadvantages of poor dispersibility and easy precipitation caused by phosphorus-containing compound,reactive compound such as cyanate ester(CE),phosphorus-containing compound(DOPO)and DOPO-triazole containing compound(DPAT)were selected to modifiy HE.FTIR results showed that both CE and DOPO were successfully grafted into the molecular backbone of HE.The liquid oxygen compatibility results of CE/DOPO modified epoxy resin showed that phosphorus and nitrogen had a synergistic effect on improving the liquid oxygen compatibility of epoxy resin.The synergistic effect of phosphorus/nitrogen was manifested in promoting the aggregation of phosphorus in the carbon layer,increasing the content of P-C bonds in the carbon layer and inhibiting the breaking of C-O bonds in the molecular structure.However,CE/DOPO modified resin failed to pass the liquid oxygen compatibility test.Thus,an new DOPO-triazole containing compound(DPAT)was synthesized and used to further improve the liquid oxygen compatibility of epoxy resin.The molecular structure of DPAT was characterized by FTIR and nuclear magnetic spectroscopy analysis.The IRS of the modified resin decreased with the increase of DPAT content.When the DPAT content was 6 wt.%,the modifed resin was compatible with liquid oxygen.The liquid oxygen compatibility mechanism of DPAT modified resin was similar to that of CE/DOPO modified resin.However,when the content of DPAT in the modified resin exceeded 6 wt.%,the tensile strength,failure strain,and impact strength of the resin at room temperature and low temperature all showed a downward trend obviously.(4)Then,aiming at the shortcomings of poor cryogenic mechanical properties of phosphorous/nitrogen grafted epoxy resin,surface-modified nanoparticles were selected to toughen the resin.Firstly,the influence of silane coupling agent on the dispersibility of silica nanoparticles and mechanical properties of modified resin were studied.Transmission electron microscopy(TEM)analysis showed that particles modified by the epoxy-containing silane coupling agent had the lowest degree of agglomeration and stacking in the resin.The mechanical properties test results showed that the epoxy-containing silane coupling agent was more conducive to the toughening effect of nanoparticles.Secondly,to take the liquid oxygen compatibility,cryogenic mechanical properties and the dispersibility of nanoparticles into account,a nanoparticle-composite technology was used to design and prepare a needle-like nano-boehmite(Al OOH-DOPO-GPTS)grafted with phosphorus,silicon and epoxy group.TEM results implied that Al OOH-DOPO-GPTS had good dispersibility in the resin.The introduction of an appropriate amount of Al OOH-DOPO-GPTS could make the epoxy resin compatible with liquid oxygen.Its mechanism included: the endothermic effect of boehmite(Al OOH),the dilution effect of Al OOH in the gas phase with the barrier effect in the deposition phase,the free radical capture effect of phosphorus in the gas phase with the carbonization effect in the deposition phase,and the synergistic effect between Al OOH and phosphorus.The Al OOH-DOPO-GPTS modified resin compatible with liquid oxygen had good cryogenic mechanical properties and the corresponding tensile strength and failure strain were126.1 MPa and 1.57%,respectively.(5)Finally,in order to clarify the internal energy distribution of resins during impact at cryogenic temperature,the effect of cryogenic deformability of resins on the liquid oxygen compatibility was studied from a mechanical point of view.The finite element analysis software(LS-DYNA)was used to numerically simulate the structure-thermal coupling process of epoxy resin under mechanical impact at liquid oxygen temperature.The results showed that thermosets with higher deformability at cryogenic temperature(with lower modulus and crosslinking density,higher failure strain)passed the liquid oxygen compatibility test,which was attributed to the fact that the better deformability of resin at low temperature was conducive to the buffering and dissipation of impact energy,which reduced the probability of hightemperature hot spots.Simulation results showed that the failure mode of numerical simulation results of resins was consistent with the actual test results.The total energy of the system during impact process followed the conservation of energy.Comparing with resins under offset condition,the internal energy increment of resins was higher under vertical conditon.The internal energy increment and the corresponding rate of the resin with higher deformability was relatively low under vertical condition.The corresponding temperature rise of the resin was relatively low along with a uniform temperature distribution.So the probability of forming high-temperature hot spots in the resin was lower than others,which resulted in a better compatibility between the resin and liquid oxygen. |
