Synthesis And Characterization Of Energetic Thermoplastic Polyurethane Elastomers

Energetic thermoplastic polyurethane elastomers(ETPUE), based on Glycidyl azide polymer(GAP) and 3,3-bis(azidomethyl)oxetane-tetrahydrofuran(BAMO-THF) copolymer(PBT) and Polyglycidyl nitrate(PGN) as soft segments, were synthesized in this paper. A serial of ETPUE with different chain extenders were synthesized, respectively. The chain extenders includes ordinary extender(1, 4-butanediol,BDO) and the extenders containing groups with bonding functions, such as Diethyl Bis(hydroxymethyl)malonate(DBM) and N-(2-Cyanoethyl) diethanolamine(CBA). GPC, FTIR, DSC, SAXS, TGA, tensile test techniques and contact angle test were applied to study the properties. Through comparative analyses, suitable ETPUE are selected.First, GAP-based ETPUE were synthesized using BDO, DBM and CBA as extenders, respectively. The surface free energy of ETPUE was obtained by measurement of contact angle for the ETPUE and the work of adhesion(Wa) between nitramine solid ingredients(RDX) and ETPUE were calculated. Test results showed that the Wa value between RDX and ETPUE extended by DBM and CBA are greatly improved compared with ETPUE extended by BDO. This showed that GAP-based ETPUE extended by DBM and CBA have bonding functions. As the ETPUE extended by pure DBM and CBA poor mechanical properties, so it has a value using mixture of chain extenders including DBM/BDO and CBA/BDO. Test results showed that with the percentage of DBM or CBA increasing, the Wa between RDX and ETPUE increased and the maximum tensile strength(σm) of ETPUEs decreased on the other hand. The mechanical properties of propellant are affected by multiple factors, including mechanical properties of ETPEs and the Wa between solid ingredients and ETPUE. The model propellants ETPUE/RDX were prepared by means of melt blending and the mechanical properties of them were tested to consider the results of two factors. Results showed that ETPUEs extended by CBA had higher positive formation enthalpy and higher.Wa with RDX. When the percentage of CBA in mixed chain-extenders is 50%, the synthetic GAP-CBA-BDO-50 can effectively prevent the dewetting and then improve the mechanical properties of propellants(σm=3.8MPa,εb=11.8%).Second, PBT-based ETPUE were synthesized using BDO, DBM and CBA as extenders, respectively. The surface free energy of ETPUE and Wa between RDX and ETPUE was obtained. Test results showed that the Wa value between RDX and ETPUE extended by DBM and CBA are greatly improved compared with ETPUE extended by BDO. This showed that PBT-based ETPUE extended by DBM and CBA have bonding functions. In order to prepare ETPUE with good mechanical properties as well as bonding functions, we used mixed chain-extenders(DBM/BDO and CBA/BDO). Test results showed that with the percentage of DBM or CBA increasing, the Wa between RDX and ETPUE increased and the maximum tensile strength(σm) of ETPUEs decreased on the other hand. In order to test the combining results, the model propellants ETPUE/RDX were prepared by means of melt blending and the mechanical properties of them were tested. Results showed when the percentage of CBA in mixed chain-extenders is 50%, the synthetic PBT-CBA-BDO-50 can effectively prevent the dewetting and then improve the mechanical properties of propellants(σm=5.92 MPa,εb=16.5%).Third, PGN-based ETPUE were synthesized using BDO and DBM as extenders, respectively. The CBA extender would fail to work in PGN system as the interaction between –CN and –ONO2, so CBA are not available as extender for PGN-based ETPUE. Through analysis of ETPUE extended by BDO and DBM, we can find that the Wa value between RDX and ETPUE extended by DBM are greatly improved compared with ETPUE extended by BDO. This showed that PGGN-based ETPUE extended by DBM have bonding functions. In order to prepare ETPUE with good mechanical properties as well as bonding functions, we used mixed chain-extenders(DBM/BDO). Test results showed that with the percentage of DBM, the Wa between RDX and ETPUE increased and the maximum tensile strength(σm) of ETPUEs decreased on the other hand. The model propellants ETPUE/RDX were prepared and the mechanical properties of them were tested. Results showed when the percentage of DBM in mixed chain-extenders is 75%, the synthetic PGN-DBM-BDO-75 can effectively prevent the dewetting and then improve the mechanical properties of propellants(σm=4.40 MPa, εb=5.2%).Finally, by comparing the characteristic of three optimized energetic thermoplastic polyurethane elastomers(GAP-CBA-BDO-50, PBT-CBA-BDO-50 and PGN-DBM-BDO-75), ETPUE with good comprehensive performance were selected. Results showed that compared with GAP-CBA-BDO-50 and PBT-CBA-BDO-50, PGN-DBM-BDO-75 had negative formation enthalpy and its glass transition temperature of soft segment was higher(-33.4℃). The elongation at break point of model propellants prepared by PGN-ETPUE were very low(εb=5.2%). PBT-CBA-BDO-50 have lower glass transition temperature(-43.6℃) and the model propellants prepared by PBT-CBA-BDO-50 have good mechanical property(σm=5.92 MPa, εb=16.5%). But the formation enthalpy was lower than GAP-CBA-BDO-50, and the rate of reactions of PBT-ETPUE were so fast that it’s difficult to handle. Finally, the GAP-ETPUE have lower glass transition temperature(-38.2℃), highest positive formation enthalpy and the model propellants prepared by GAP-CBA-BDO-50 have good mechanical property(σm=3.8MPa, εb=11.8%). In addition, the reaction was smooth, easy to handle and could be applied to industrialization production. Comprehensive analysis showed that the GAP-CBA-BDO-50 extended by CBA/BDO mixed chain-extenders had good general properties and good application prospect.