Design,Preparation And Properties Of Epoxy/benzoxazine System For Liquid Oxygen Environment

Liquid oxygen(LOX)is still the most widely used oxidant of the fuels for spacecraft.It will be of great significance to develop light-weight materials for the LOX tanks for the weight reduction,increasing the payload ratio and reducing the launch cost.Polymer-based carbon fiber reinforced composites are considered to be one of the most promissing options to manufacturing the LOX tank due to its high specific strength.However,the reliability and safty must be placed at the first place considering the great importance of the space activities.The aim of this paper is to improve the thermal stability and the ability of capturing free radicals of epoxy resin.Benzoxazine structure was introduced into the epoxy network to improve the thermal stability of epoxy resin and phosphorus-containing monomer was to capture the free radical during the decomposition prcoess.The mechanic behavior of the epoxy/benzoxazine copolymer was promoted through the introduction of flexible polysiloxane segments and modified nanoparticle.The main research contents and conclusions are as follows:1.The synthesized DOPO derivative(DOPO-TVS)was introduced into the side chain of the general bisphenol A epoxy resin via the Si-O covalent bond through the sol-gel method without consuming the epoxy group.The toughening effect and the synergistic effect of phosphorus-silicon system on the thermal properties and LOX compatibility were investigated.The results showed that the glass transition temperature increased slightly after covalently introduced the Si-O network.Moreover,the additional Si-O-Si network may increase the crosslink density of the cured epoxy matrix.The thermal stability and flame retardancy of the modified epoxy improved obviously.The limit oxygen index(LOI)of the cured resin increased from 22.5 to 32.0 when the content of DOPO-TVS reached to 10%and the char yield in the thermogravimetric analysis in N2 atmosphere increased form 21.7%to 31.7%in 600 ?.The maximum weight losing rate around 400 ? decreased by more than 50%compared to the unmodified epoxy.The LOX impact sensiti,ve index decreased from 23.5%to 6.7%with the increasing of the phosphorus content and it remained static after the phosphorus content went beyond 1%.The XPS analysis of the surface element after the LOX impact test showed that there was a new bonding state at 135.1 eV of phosphorus which was attributed to the oxidation of phosphorus.Moreover,the migration of silicon was also observed after the LOX impact.The result of tensile test showed that the tensile strength and elongation were promoted by 10%and 47%respectively.2.The cured benzoxazine has excellent thermal stability and mechanical strength due to the abundant aromatic heterocyclic structure and intermolecular hydrogen bonding in the crosslinking network.Trialkoxy-terminated benzoxazine monomer was synthesized.Subsequently,epoxy/benzoxazine prepolymer was prepared using the synthesized benzoxazine monomer and the modified epoxy discussed in the last chapter through sol-gel method.The epoxy/benzoxazine prepolymer was finally cured with the Bisphenol A benzoxazine monomer to obtain the polysiloxane reinforced epoxy/benzoxazine copolymer.The impact strength increased by 10.8%?26%and the fracture toughness increased by 22%after introducing the Si-O-Si.Thermal gravimetric analysis(TGA)and dynamic mechanical analysis(DMA)showed that the organic-inorganic ternary copolymer possesses promoted thermal stability compared with the unmodified epoxy/benzoxazine matrix.The char yield of the ternary copolymer reached to 34.7%which was 22%?26%higher than the pure epoxy/benzoxazine copolymer.The char residues of the ternary copolymer after decomposition test reveal a dense surface layer and unbroken original dimension which is in accordance with the TGA results.3.The introduction of benzoxazine into epoxy resin can significantly improve the thermal stability of cured products.Phosphorous-containing benzoxazine monomer(PBA)was obtained through the addition reaction between 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO)and a synthesized alkene-terminated monofunctional benzoxazine monomer(A-BA).Epoxy/benzoxazine copolymer with chemically-bonded DOPO was prepared using the synthesized PBA.DMA results showed that polymer with the chemically-bonded DOPO possessed higher modulus and crosslink density than that with physical blended DOPO.The modulus in the room temperature was 12?18%higher than that with the blended DOPO.The char yield of the specimens with chemically bonded DOPO at 700 ? was 6.8%while that of the epoxy/amine specimen was zero.The char residue of the phosphorus containing specimen was 10?20 ? lower than the pure in the first weight losing stage while the weight losing rate was 9%?25%lower.The LOX sensitivity index of the epoxy/benzoxazine copolymers were much lower(5.3%)than that of the epoxy/amine ones(15.3%)and the epoxy/benzoxazine copolymer including PBA exhibited promoted stability in the impact test.The sensitivity index of EP-PBA2 reached 0 which met the standard of ASTM D2512-95.4.Polymeric materials exhibit great brittleness at liquid oxygen temperature even though they may have good toughness at room temperature.The greater the brittleness.the more serious the stress concentration at the crack tip and the easier the crack propagates.It is the most direct and effective way to enhance polymer using at ultra-low temperature by filling nanoparticles to inhibit crack propagation.Firstly,a monofunctional benzoxazine(BS)was synthesized using pentadecyl phenol.Subsequently.modified amorphous zirconia(ZrO2)nanoparticles(NPs)were obtained through sol-gel method with the synthesized BS monomer.The results of DLS showed that the average diameter of the modified NPs was 19.5nm with the multidispersion coefficient(PDI)of 0.1 1 8.Transmission electron microscopy(TEM)showed that the dispersibility of nanoparticles prepared by BS was significantly better than that of the comparative group.Zeta potential went beyond 40 mV after the BS amount was more than 10 mol%.The surface energy of nanoparticles was studied by two-liquid method.The results showed that the surface energy of modified nanoparticles decreased significantly.The standard deviation derived from the tensile strength values increased from 2.63 Mpa(for the prue resin)to 7.31 Mpa(for the composite with 5 wt.%unmodified nanoparticle),while that for BS3-5 was just 2.64 MPa.The maximum tensile strength was 105.7 MPa derived from the BS3-3 group while that for the U3 group was just 73.5 MPa.The mechanical behavior of specimens with the modified nanoparticle proved to be more stable and with stronger enhancement.The critical point of the tensile stress moved toward higher NPs content for the composites including the BS modified NPs.The impact strength of BS1 and BS3 reinforced composites increased by 69%and 71.4%at room temperature and 90 K temperature,respectively,compared with those of unmodified composites.The critical mass fraction of the tensile strength of the cured resin reinforced by modified nanoparticles is higher and the variability of the results is smaller.