| High energy and low vulnerability is being the trend of gun propellant development. Traditional Nitrocellulose (NC) binder based gun propellants has high energy, however the survival ability of weapon system is strongly affected by the vulnerability of the gun propellants, which has the possibility of deflagration-to-detonation upon stimulation such as heat, mechanical force, blast wave and efflux jet. Gun propellants which are based non energetic binders (such as cellulose, crosslinking or thermoset binder, and thermoplastic elastomer) have satisfactory mechanical properties but give relatively low energy. Glycidyl azide polymer (GAP) is a novel energetic binder being considered as the key component of gun propellants due to its insensitivity and high energy. The pendant azide group of GAP main chain results in low mechanical properties of gun propellants. So it is necessary to improve the mechanical properties of GAP binder. In this paper, a series of energetic thermoplastic elastomers with GAP prepolymer were synthesized and characterized for improving the micro domain of crystallization and then the mechanical properties of the binders. A novel blend with NC and GAP-TPE is further prepared to modify the GAP-TPE's drawback of modulus decreasing as temperature increasing. At last, the thermal decomposition kinetics of GAP-TPE and NC/GAP-TPE blends was thoroughly studied. The main work is as follows:Energetic thermoplastic polyurethane elastomer is synthesized using GAP with number averaged molecular weight of 3000 as soft segment,4,4'-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO) as hard segments. The results showed that GAP-TPE synthesized by melt-prepolymerization has better mechanical properties, having the -NCO/-OH mole ratio (R) of 0.98 and reacting for 2h. The mechanical properties of GAP-TPE are better when reducing the chain extension temperature. The bulk viscosity of the mixture decreased as the reaction temperature of the chain extension decreased. Using BDO as chain extender, the GAP-TPE showed higher tensile strength and elongation at break at higher content of the hard segment.In order to improve the mechanical properties of GAP-TPE, GAP-TPEs were synthesized with different chain extenders while the hard segment content was fixed as 35%. The studied chain extenders included EDO, BDO and HDO, having the structure of HO(CH2CH2)n OH. FTIR, GPC, Elemental analysis, DMA and Density are applied to characterize the synthesized elastomers. The elastomers have the same structure characteristic of polyurethane. The GAP-TPE using BDO as chain extender has better mechanical properties, tensile stress (σb) of 13.4 MPa, elongation at break (εb) of 362%, storage modulus (E',T=50℃) of 22.6 MPa. Its density is 1.35 g·cm-3. the number averaged molecular weight is 76630, and the glass-transition temperature (Tg) is -22.3℃. SEM photographs showed the homogeneous microstructure of the GAP-TPE.Further using DEG as chain extender, the synthesized GAP-TPE with hard segment of 35% has better properties than those of HO (CH2CH2)n OH extended GAP-TPE. DEG extended GAP-TPE has a density of 1.37 g·cm-3, number averaged molecular weight of 84530,σb, of 14.6 MPa,εb of 414%, storage modulus (E',T=50℃) of 69MPa, and Tg of -24.9℃. DEG extended GAP-TPE also has higher degree of hydrogen bonding and micro-phase separation.To improve the storage modulus of GAP-TPE at high temperature, NC/GAP-TPE blends are prepared by the step of chain extension, blending and aging. XRD and density analysis show that the densities of NC/GAP-TPE blends increase with the increase of NC content. The DMA analysis shows that NC/GAP-TPE (9/95,10/90, mass ratio, the same as follows) owns only one glass transition temperature, while two glass transition temperatures of NC/GAP-TPE (20/80,40/60,60/40,80/20) are observed and they shift closer to each other with the increase of NC content, implying that the blend was partially miscible. Theσb andεb of NC/GAP-TPE (20/80) are 20.4MPa and 139% whereas theσb andεb of NC/PEG-TPE (5/95) are 10.4MPa and 264%, respectively. When the mass ratio are 5/95,10/90,20/80,40/60, 60/40 and 80/20 for NC/PEG-TPE, the storage modulus (E', T=50℃) are 14,16,42,236, 1747 and 2650 MPa, respectively. Thus, the composite energetic binder, NC/GAP-TPE, combined the high modulus of NC with the good toughness of GAP-TPE, having the potential to adjust the properties of NC/GAP-TPE by changing the mass ratio of NC and GAP-TPE.Thermogravimetric analysis (TG) and derivative thermogravimetry (DTG) are employed to evaluate the thermal decomposition behaviors of GAP-TPE and NC/GAP-TPE. The peak separation is performed to separate the thermal decomposition peak of GAP-TPE into several stages according to the characteristic of the experimental differential mass loss curve. The activation energy calculated with Ozawa methods for each decomposition stage has good agreement with the result calculated with Kissinger method. The activation energy of GAP-TPE in the three stages are 223,235 and 57 kJ·mol-1, respectively. The activation energy of NC/GAP-TPE in the three stages are 163,166 and 292 kJ·mol-1. The thermal decomposition activation energy of pendant azide group decreased and the hard segment carbamate increased after blending. In addition, the three stage's mechanism functions of GAP-TPE calculated by the Coats-Redfen method are [-ln(1-α)]1/3,-ln(1-α) and -ln(1-α), respectively. The mechanism functions of NC/GAP-TPE in three stages calculated by the Coats-Redfen method are all a2.
|
PDF Full Text Request