Research On The Mechanical Properties Of Azide Polyurethane Elastomers And Its Propellant

In recent decades,azide polyether propellants based on azide polymers,like glycidyl azide polymer(GAP),3,3-bis(azidomethy)oxetane-r-tetrahydrofuran copolyether(BAMO-THF),are recognised as an important direction of the development of composite solid propellant,duo to the inherent advantages of azide propellant,like high energy,high burning rate,low signature,and so on.However,the existence of bulky pendant group(-CH2N3)in polymer backbone reduces intermolecular forces,and then worsen the mechanical properties of formula,resulting in the range of application in high-energy formulations is limited.In this study,a series of thermoplastic or crosslinked azide polyurethane elastomers were prepared by copolymerization method.Besides,the relationship of structure and performance and effect of the nitrate ester plasticizer on the elastomers were also investigated.Moreover,the BAMO-THF with good cryogenic mechanical performance was selected to prepare azide polyether propellant.The mechanical properties under wide temperature range and complex load were systematic studied.The preparation and phase structure of GAP copolymers.In order to improve the mechanical performance of GAP,the prepolymers like poly(oxytetramethylene glycol)(PTMG)and polycaprolactone(PCL)were incorporated into GAP-based elastomers to promote a sufficient chemical incompatibility with various blend ratios.The tensile strength and elongation at break of thermoplastic elastomers(TPEs)based on GAP-PCL could reach to 13.5 MPa and 1 650%,respectively.After slightly corss-linking,the tensile strength and elongation at break of GAP-PCL copolymers were up to 27.1 MPa and 750%,respectively.Meanwhile,the tensile result of GAP-PTMG based TPEs could also reached to 16.1 MPa and 1 456%,respectively.Compared to the pure GAP elastomers,the mechanical performance of GAP-based block copolymers were enhanced clearly.Moreover,we focus on exploring the addition influence of PTMG on microphase separation of GAP-based elastomers.SAXS provided clear evidence for the microphase separated structure in the TPEs and the approximate domain size and average domain spacing can be estimated to be 10 and 30nm,respectively.The average domain spacing show a decreasing tendency as the increases of PTMG content,indicating the enhancement of ordered microstructure.Furthermore,the estimate of Porod invariant indicates that the microphase separation of the elastomers is likely induced by the crystallization of flexible PTMG segments.These results suggest that modifying the polymer backbone by copolymerization of flexible prepolymers into the GAP-based elastomers can be an effective method to tailor their phase separated microstructure and mechanical performance.A series of TPEs and slightly cross-linked elastomers were prepared by one-step melt polyaddition with high content of BAMO-THF.For the TPEs,the increase of hard segments helps to improve the strength and modulus,while the raise of soft segment fraction is benefit to the elongation and low-temperature mechanical properties.And among the obtained TPEs,D20 exhibits satisfactory mechanical properties,in which the desired soft segment(BAMO-THF)reaches to 93.3%and hard segment decreases to 6.7%,its tensile strength reach to 3.61 MPa while elongation at break also attain 1 277%along with the glass transition temperature(Tg)of-23.4℃.Besides,due to the linear structure,these TPEs exhibit strong degree of creep under load stress and test temperature.For the slightly cross-linked elastomers,the isocyanate curing agent and chain extender have a strongly influence on the mechanical performance of BAMO-THF based elastomers.The BAMO-THF with good cryogenic mechanical performance was selected to prepare azide polyether propellants with various formulas.Results show that the calculated impulse of the azide polyether propellant is up to 268 s,with the density of 1.78 g/cm3.The raise of plasticizer content or the introduction of butyl nitroxyethyl nitramine can effectively lower the value of Tg and viscosity of slurry,leading to an good cryogenic mechanical and processing performance.Meanwhile,it also reduces the modulus at normal and high temperature.The diamine chain extender,which forms the urea hard segment structure by reaction,can enhance the modulus and strength of propellants at normal and high temperature,but also leads to the deterioration of toughness at low temperature.The application of diamine/diol chain extender in azide polyether propellant has a good effect on the integrated mechanical properties.Its tensile strengths at-55℃,20℃,and70℃ are3.3 MPa,1.1 MPa and 0.6MPa,respectively,with the corresponding maximum elongations of 39.3%,45.5%and 41.2%.The Tg of the formula is-51.0℃ by differential scanning calorimeter(DSC)or-42.4℃ by dynamic mechanical analyzer(DMA).To investigate the embrittlement of azide polyether propellant in the ignition process under low temperature and high loading frequency,dynamic mechanical analyzer,impact loading test and X-ray micro-tomography were employed to investigate the dynamic mechanical properties,impact fracture and damage characteristics.Results show that not only testing temperature but also loading frequency strongly influences the viscoelasticity of the azide polyether propellant at low temperature.The brittleness parameter adopted can directly reflect the influences of structure,loading frequency and testing temperature on the cryogenic transient fragility of azide polyether propellant in the ignition process.Moreover,the results of impact loading test in-20℃ differ from the results in-40℃;all samples are not broken in the former test,while samples show a brittle fracture except for T-2 in the latter test,which is attributed to the significant increase of brittleness parameter as the test temperature drop down.The impact stress concentrate on the larger ammonium perchlorate(AP)grain,which caused the microcracks on the surface of the particles.With the increase of the brittleness parameter,the crack is more obvious.In the samples with brittleness parameter of 0.13,the AP particles appeared seriously damaged;the crack coalescence between AP and glassy matrix lead to the macroscopic fracture of azide polyether propellant.For analyzing high-temperature softening problem of azide propellant,the high-temperature viscoelastic behavior of azide polyether propellant was investigated by means of creep and stress relaxation test.The results reveal that creep behavior of propel.lant sample has close relationship with temperature and stress,especially displays obvious loading effect.Under the condition of 50℃ and 0.1 MPa,the creep strain of T-1,T-2,T-3 and T-4 are 7.0%,7.8%,18.4%,and 19.9%,respectively.And among the fitted parameters,it can be mentioned that the proportion of high elastic deformation is obviously larger than those of instantaneous deformation and viscous flow deformation,which indicate the elastic performance of solid propellant has great influence on their high-temperature mechanical properties.Besides,the creep curves of azide polyether propellant do not present accelerated creep failure stage,which can be confirmed by the modest increase of creep compliance in the prediction time(108s).The trend of stress relaxation of azide polyether propellants at different temperatures is basically the same,with obvious stress relaxation in the initial stage.The influence of loading conditions(testing temperature and loading stress)is mainly reflected in the second stage.The smaller the strain level or the higher the temperature,the attenuation of relaxation modulus is more obvious.Based on the uniaxial tensile test,the mechanical properties of the samples with different network structures and different plasticizers were studied.The mechanical properties of azide polyether propellant showed a significant correlation between temperature and loading rate.The critical strain,which deviates from actual stress-strain curve,serves as ultimate of linear characteristic strain.The ultimate of linear characteristic strain of azide polyether propellant samples could estimate to 8.2%,7.1%and 9.2%for-40℃,20℃,and 50℃.The time temperature equivalence principle can apply in nonlinear mechanical stage of azide polyether propellant.Based on this,principal stress-converted time curves under different strain level were established.And the principal curves can be used to predict the stress response curves under different tensile conditions.