Studies on thermoplastic elastomers based RDX-propellant compositions

Abstract

This paper presents the results obtained during studies on 80% RDX propellant systems based on thermoplastic elastomers (TPEs) namely ethylene-vinyl acetate (EVA), triblock copolymers of styrene-butadiene/styrene-isoprene (Kraton), poly-urethane-ester-MDI (Estane) and copolymer of polybutylene terephthalate - polyether glycol (Hytrel) as binders, Dioctyl phthalate (DOP), triacetin (TA) and glycidyl azide polymer (GAP) were incorporated as plasticizers in the formulations. An attempt has been made to correlate structural features of TPEs with mechanical properties as well as glass transition temperature (Tg). Results obtained suggest that TPE-based RDX-propellants have the advantage of high insensitivity to impact and friction stimuli vis-à-vis nitrte ester based conventional propellants. EVA based propellents gave the best results in this regard. Ignition temperature for all the compositions was >200°C. EVA, Hytrel and Estane based formulations were found to be more energetic than Kraton based formulations. Incorporation of GAP resulted in the improvement in ballistics (Impetus and burn rates) as compared to DOP plasticised formulations. TA based compositions gave an intermediate value. Thermal decomposition pattern was determined by applying Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC). An attempt has been made to explain the trends observed on the basis of the evidences generated during this study and theories proposed by other researchers.     

Studies on Nonaluminized High Burning Rate AP-Composite Propellants

This paper discusses the effect of replacement of hydroxyl terminated poly butadiene (HTPB) by butacene on the burning rates of a nonaluminized ammonium perchlorate (AP)–based composite propellant. A remarkable burning rate enhancement (60–100%) was observed on replacement of HTPB by butacene to the extent of 25%. Replacement of 50–75% HTPB by butacene led to further increase in burning rate by merely 10–20%, and no incremental effect was observed on complete replacement of HTPB. Butacene-based propellants also exhibited a lower pressure exponent (n) of the burning rate. The results obtained are in line with the findings of other researchers on aluminized AP composite propellants. Addition of transition metal oxides, ferric oxide (FO), and copper chromite (CC) to butacene-based formulations led to further enhancement of the burning rates, albeit to a less extent than in the case of HTPB-based systems. However, the burning rates of the former were higher than those of the latter in the case of ballistically modified compositions as well. DSC results for butacene-based formulations revealed lower activation energy (Ea) for decomposition than that for HTPB-based propellants. TGA studies brought out that butacene-based compositions decompose following a pattern similar to that of HTPB-based formulations. However, lower decomposition temperatures (1st and 2nd stages) were observed for butacene-based propellants.     

Screening of polymer-plasticizer systems for propellant binder applications: an experimental and simulation approach

ABSTRACT

Composite propellants of advanced propulsion systems demand high energy, sufficient structural integrity, and low temperature capability. Polymers and plasticizers contribute to all these three important characteristics of propellants and they need to be screened rigorously to achieve the most suitable combination. In this investigation, experimental and simulation studies were performed to predict the miscibility of polymers and plasticizers. Solubility parameter is the measure of miscibility of any two material of interest, evaluated by equilibrium swelling and intrinsic viscosity experiments. Further, molecular dynamic simulations were also used to evaluate the solubility parameter for a few important polymers and plasticizers that are used in rocket propellants. Flory-Huggins interaction parameter (χ) was calculated for identifying the suitable pair. Simulated and experimental data proved that energetic plasticizers are not miscible with hydroxyl terminated polybutadiene (HTPB), while n-Butyl-nitratoethylnitramine (Bu-NENA) was found to be the most suitable plasticizer for polymers containing nitrogen atoms. Therefore, this study finds its importance in identifying an appropriate plasticizer for any specific polymer, which is the foremost critical step for achieving successful propellant formulations with desired characteristics.     

Wet air oxidation of energetics and chemical agent surrogates

Abstract

Wet air oxidation studies have been conducted on a number of energetic materials and wastewaters derived from energetic materials to demonstrate high destruction levels of specific energetic components. Triple-base propellant, OTTO Fuel (used as a torpedo propellant) and hydrazine-based rocket fuel were energetics of interest. Triple-base propellant contain nitrocellulose, nitroglycerin, and nitroguanidine. OTTO Fuel contains substantial amounts of propylene glycol trinitrate. Hydrazine based rocket fuel contains hydrazine and 1,1-dimethylhydrazine (asymmetrical dimethyl hydrazine or UDMH). A bench scale wet air oxidation study on alkaline hydrolyzates of triple-base propellants indicated that essentially complete destruction of the reactive nitrogen components could be achieved at an oxidation temperature of 280°C. Bench scale wet air oxidation studies on OTTO Fuel wastewaters indicated that a 99+ percent destruction of propylene glycol dinitrate can be achieved at an oxidation temperature of 280°C. Processing OTTO Fuel wastewaters in a continuous flow, full scale wet air oxidation unit achieved even higher destruction levels.

Bench scale wet air oxidation studies on hydrazine-based rocket fuel wastewaters indicated that a 99.8 percent destruction of hydrazine and a >99.0 percent destruction of 1,1-dimethylhydrazine can be achieved at an oxidation temperature of 280°C. Again, processing of hydrazine-based rocket fuel wastewaters in a continuous flow, full scale wet air oxidation unit achieved similar destruction levels.

The application of wet air oxidation for the destruction of chemical agents has been made by the extrapolation of data from the wet air oxidation of compounds with similar chemical structures or of surrogate compounds. Sarin and V-agents are nerve agents which have an organo-phosphorus structure similar to that of certain commonly used pesticides. Pesticides such as glyphosate and malathion, which have a similar organo-phosphorus structure, are essentially completely destroyed (>99 percent destruction) by wet air oxidation in the temperature range of 200 to 280°C.

The chemical agent surrogate, dimethyl methyl phosphonate (DMMP) was wet air oxidized at temperatures of 220 to 280°C. Alkaline hydrolyzed DMMP was wet air oxidized at 280°. All of the oxidized effluents showed a >97.5 percent destruction efficiency for the DMMP.

The blister agent, mustard (HD) is a chlorinated sulfide, bis(2-chloroethyl) sulfide. Organic sulfides such as mercaptans can be destroyed by wet air oxidation at 260 to 280°C.

It is concluded that the wet air oxidation process is a promising alternative to incineration for disposal of energetics and chemical warfare agents.     

Moisture absorption by JA2 gun propellant under extreme conditions

Abstract

JA2 propellant downloaded from a 120-mm tank round that had been subjected to conditioning at high temperature and humidity was observed to be discolored and covered with an unknown liquid. Analysis of the propellant indicated a decrease in the nominal diethylene glycol dinitrate (DEGDN) plasticizer content. The nitroglycerine (NG) content was not observed to deviate significantly from its nominal level. The cause of the discoloration of the propellant was traced to its high moisture content. The liquid covering the material was determined to be a mixture of water and DEGDN. In follow-up experiments it was     

Hydrazinium nitroformate (HNF) and HNF based propellants: A review

Abstract

This paper reviews the studies carried out so far on HNF, which is emerging as potential oxidizer for futuristic propellant systems. Methods of synthesis of HNF including efforts to obtain HNF of desired particle size/shape have been discussed in detail. As purity of HNF has bearing on its thermal, physical and chemical characteristics, process parameters are being optimized to obtain high quality HNF with reproducible characteristics. Potential of HNF based propellants as propulsive force to missiles and space vehicles is also discussed in this paper. Problem areas in processing HNF based propellants have also been identified.     

“Clean burning” low flame temperature solid gun propellants

Abstract

In an attempt to eliminate nitrogen oxides (NOx's) from the combustion and pyrolysis products of solid propellants, an investigation into the use of “De-NOx” agents was performed. The agents selected were all compounds which thermally decompose to generate amines; participation of amines in the thermal De-NOx process results in the reduction of nitrogen oxides to nitrogen. Of all the compounds screened, it was found that urea was the best suited for use as a De-NOx agent in solid gun propellants. Use of urea resulted in a significant decrease in NOx production. Furthermore, urea is thermally stable up to a temperature of 130°C, and is compatible with nitrocellulose-based formulations. Compared to neat JA2 (a double-base propellant), a JA2/urea formulation containing 10 wt-% urea generates 60% less NOx. Calculations indicate that the incorporation of 4% urea results in a 200°K decrease in flame temperature and a decrease in impetus and velocity of approximately 4% and 2%, respectively.     

Studies on energetic compounds part 15 : Transition metal salts of NTO as potential energetic ballistic modifiers for composite solid propellants

Abstract

The combustion of hydroxyl terminated polybutadiene (HTPB) - ammonium perchlorate (AP) composite solid propellants has been studied using transition metal (Mn, Fe, Co, Ni, Cu and Zn) salts of 5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one (NTO) as energetic burning rate additives. The steady burning rate (r) was considerably enhanced with Cu(NTO)₂ and Fe(NTO)₂ whereas moderately enhanced with Zn(NTO)₂ and Co(NTO)₂ at low concentration (2% by wt.). Activity of these salts has been observed during isothermal decomposition of AP at 260°C. The values of ignition delay (t_iJ), ignition temperature (T_ign.) and activation energy for ignition (E?) for AP has also been lowered when these salts are added to it at 2% wt. concentration. The processing parameters as well as mechanical properties of the propellants with Cu(NTO)₂ as additive have been studied in detail. The r of the propellants (both highly aluminized and less aluminized) with Cu(NTO)₂ as additive at various concentrations, has been determined at high pressures, also shows its activity during combustion, The condensed phase activity of Cu(NTO)₂ during propellant decomposition has also been studied using TG-DTG techniques.     

Influence of metal salts of 4-(2, 4, 6-trinitroanilino) benzoic acid on the burnining rate of double base propellants

Abstract

Cobalt, Nickel, Copper and Lead salts of 4-(2, 4, 6-trinitroanilino) benzoic acid have been evaluated as ballistic modifiers in double base propellant formulations. Measurements showed considerable increase in burning rate over the control propellant, in presence of salts at all pressures in the range 3.43 – 8.82 MPa. The effect of the lead salt, however, was more pronounced and showed a burning rate increase of 50 – 60%; the lower pressure ranges showing higher burning rate enhancement. The salts decompose exothermically: cobalt salt at 270°C (initiation), nickel salt at 300°C, copper salt at 240°C and lead salt at 260 °C.     

Detonation properties of ammonium nitrate/nitramine-based composite propellants

ABSTRACT

Ammonium nitrate (AN) propellants have attracted attention because of their low cost and ecofriendliness despite certain major disadvantages such as low burning rate, poor ignitability, low energy, and volume change due to phase transition of AN. The addition of nitramine to AN propellants is one of the approaches to overcome these disadvantages. However, AN/nitramine propellants are characterized by easy initiation, high sensitivity, and high detonation velocity because of the high energy of nitramine. The detonation properties of AN/nitramine propellants were investigated in this study. It was observed that the AN/nitramine propellant required a booster to detonate the propellants, as detonation did not occur while using only a commercial electric blasting cap. It was further observed that the detonation velocity (D) increased linearly as the mass of nitramine per unit propellant volume was increased, and AN had a negligible effect on the detonation properties. An approximate equation was derived from the relationship between D and the mass per unit propellant volume of nitramine. The dominant factors that can be used to estimate the detonation/no-detonation boundaries of the propellants were identified, and the experimental boundary equations were determined using these factors.     

Burning Rate Studies of Metal Powder (Ti,Ni)–Based Fuel-Rich Propellants

This paper reports the burning rate results of titanium (Ti) and nickel (Ni)–based fuel-rich propellants with hydroxyl terminated polybutadiene (HTPB) and double-base (DB) matrix as binder. The results are discussed in comparison to aluminum (Al) and zirconium (Zr)–based formulations studied earlier by the authors. While 20% Ti containing composition with HTPB as binder gave burning rates comparable to those of aluminized formulation, superior burning rates were obtained for Ti-based formulations with higher metal loading (30–60%). 50–60% Ti-containing compositions gave higher burning rates than those for even zirconium (Zr)–based formulations. Fuel-rich formulations with a DB matrix produced much higher burning rates than corresponding HTPB-based compositions in the pressure range of 3.4–8.8 MPa. In these systems, Ti-incorporating formulations exhibited higher burning rates than aluminized compositions and lower than those for Zr compositions. Interestingly, Ni propellants gave burning rates close to those for Ti-based compositions and exhibited superior combustion characteristics in lower pressure ranges in systems with a DB matrix. Formulations with a GAP plasticized DB matrix exhibited 2–3 times higher burning rates than corresponding DEP plasticized compositions.     

Thermal Decomposition Behaviors and Burning Characteristics of Composite Propellants Prepared Using Combined Ammonium Perchlorate/Ammonium Nitrate Particles

The thermal decomposition behaviors and burning characteristics of propellants prepared with combined ammonium perchlorate (AP)/ammonium nitrate (AN) particles greatly depended on the AN content (χ) of the AP/AN sample. The thermal decomposition behaviors of the propellants prepared with the combined samples almost matched those of the propellants prepared by physically mixing AP and AN particles, while their burning characteristics differed. The use of combined AP/AN particles decreased the heterogeneity of the combustion waves of the AP/AN propellants because of the difference in the combustion wave structure. In contrast, the addition of Fe₂O₃ caused unsteady combustion of the propellants prepared using samples with χ values lower than 8.1%.     

Tuning the thermal,mechanical, and combustion properties of NC-TEGDN-RDX propellants via incorporation of graphene nanoplates

ABSTRACT

Graphene nanoplates (GNPs) were incorporated into a solid composite propellant (NC-TEGDN-RDX) to tune the thermal, mechanical, and combustion properties of the material. Physical, thermal, and combustion properties of NC-TEGDN-RDX with <2 wt% addition of GNPs were characterized using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), tensile/compressive/impact strength testing, and constant volume combustion experiments. Microstructure of the composite propellants examined using SEM demonstrated uniform dispersion of the GNPs at low-weight percent additives (<1 wt%), but began to show large agglomerations of the additives at higher additive content. Decomposition enthalpy of the propellant with 1 wt% GNPs increased by ~130 J/g compared to neat propellants. Moreover, the maximum burning rate was observed for samples containing 1 wt% GNPs, with values of 19 cm/s at 20°C and 17 cm/s at ?40°C. Dynamic vivacity of the propellant achieved a maximum upon addition of 1 wt% GNPs. The pressure exponent of the propellant decreased with the addition of GNPs, as well. The mechanical properties including tensile, compressive, and impact strength were improved at 20°C and ?40°C. These results demonstrate that the addition of GNPs may offer new methods by which to tune and improve thermal decomposition, thermal conductivity, combustion performance, and mechanical properties of the NC-TEGDN-RDX propellants.     

Examination of some proposed relations among HE sensitivity data

Abstract

Recent data collected from the literature were used to test the general applicability of proposed empirical correlations of sensitivity data of high explosives (HE) and propellants. It was found that the relationships were not applicable to all explosives. In particular, insensitive high explosives and propellants did not conform to the empirical equations.     

Comparative study of two methods used for the characterization of the degradation of the polymer binder in gap-based propellants

Abstract

Reliability assessment for composite propellants calls for effective shelf-life predicting tools, implying the existence of appropriate accelerated-aging procedures and of quantitative methods for the characterization of the degradation process. Ideally, one would like to investigate stabilizer depletion as well as the mechanical integrity of the polymer network used as the binder. There are many available methods that can be combined to achieve this goal. In the present work, a comparative study of two of these methods has been performed. The first procedure makes use of FTIR spectroscopy to characterize the binder's degradation and of HPLC to follow stabilizer depletion. The other method, based on ¹H NMR spectroscopy allows the measurement of the polymer network degradation, the depletion of the stabilizer and the loss of plasticizer in a single step. Finally, the pros and cons of both procedures have been evaluated from the analysis of experimental data related to accelerated aging at 40, 60 and 80°C of GAP-based composite propellant formulations.     

Relationship between Slivering Point and Gas Generation Rules of 19-Perforation TEGDN Propellants with Different Length/Outside Diameter Ratios and Perforation Diameters

In order to investigate the relationship between the slivering point and burning progressivity, a set of 19-perforation propellants containing triethylene glycol dinitrate (TEGDN) with different lengths/outside diameter ratios and perforation diameters was prepared and tested in a closed vessel. The mass fraction of burnt propellant was derived from the recorded pressure–time history of 19-perforation TEGDN propellants in the closed vessel according to the gas state equation and the form function of tested propellants. Based on the form function calculation and the mass fraction of burnt propellant, instantaneous burning surface area and the burning rate were obtained. The influence of length/outside diameter ratios and perforation diameters on the progressive combustion performance is studied through the dynamic vivacity method. With an increase in the length/outsider diameter, the slivering point occurs earlier and the slivering process lasts longer. Further, the burning progressivity of surface area can be improved. For propellants with same length/outside diameter ratio, with a decreasing of perforation diameter, the slivering point lags behind and the burning progressivity becomes greater. The slivering point corresponds to the instantaneous burning area, which is related to the form function and total burning process as well. However, the total burning progressivity of propellant is a very comprehensive result of propellant under multiple actions, including the mass fraction of burnt propellant, grain size and burning rate at different pressure regions. The correlation between them can boost a better understanding on the interaction between grain size, slivering burning process and burning progressivity.     

Hexammine metal perchlorates as energetic burning rate modifiers

Abstract

Four transition metal hexammine perchlorates namely, [Cu(NH₃)₆](ClO₄)₂, [Co(NH₃)₆](ClO₄)₂, [Ni(NH₃)₆](ClO₄)₂ and [Zn(NH₃)₆](ClO₄)₂ have been prepared, characterized and used as ballistic modifiers in the combustion of hydroxy terminated polybutadiene (HTPB)-Ammonium perchlorate (AP) composite solid propellants. Burning rate was considerably enhanced with [Co(NH₃)₆](ClO₄)₂ and [Cu(NH₃)₆](ClO₄)₂ whereas moderately with [Ni(NH₃)₆](ClO₄)₂ and [Zn(NH₃)₆](ClO₄)₂ at low concentration (2% by wt). [Co(NH₃)₆](ClO₄)₂ was found to accelerate the burning rate by three fold at two percent concentration and it can be exploited as potential energetic burning rate modifier for HTPB-AP propellants. Further, ignition delay studies showed that the deflagration of propellants and AP was accelerated by these additives.     

Burning rate of artificially aged solid double-base gun propellants

ABSTRACT

Nitrocellulose (NC) and nitroglycerin (NG) are conventional energetic ingredients of solid double-base gun propellants. NC and NG are aliphatic nitrate ester compounds that are manufactured by nitrating through immersion in acid. Solid propellants are used in a wide range of both civilian and military applications, for example, as gas generators in airbags, as fuels in rocket motors or to shoot projectiles in guns. In all illustrations, the function of solid propellant is to generate gas, which is then employed to do mechanical work. The maximum pressure and the projectile velocity are important parameters of the interior ballistic process. The goal of the interior ballistic design of the gun is to attain a higher muzzle velocity under the smaller maximum chamber pressure. The burning rate of the propellant directly affects the formation rate of burning gases, which affects the ballistic parameters, either by diminishing the ballistic effectiveness or raising the safety hazard to the operator during firing. Note that the effect of the change of burning rate is not always negative; the fact is that we could change/control the burning rate to get better ballistic performances, includin ballistic efficiency as well as launching safety. Nevertheless, nitrate ester propellants are subjected to physical and chemical degradation during storage and this can modify the burning rate. Here we describe the effect of aging on the burning rate of spherical double-base propellants both qualitatively and quantitatively. We experimented unaged propellant, and propellant that was artificially aged at 71°C and 80ºC. The durations of heating at 71ºC were 10, 20, 30, 37, 48 and 62.5 days, respectively. The durations of heating at 80ºC were 1.8, 3.63, 5.41, 7.22, 9.02 and 10.83 days, respectively. The burning rate of investigating propellant was obtained by using closed-vessel experiments. The results show that towards low pressures, the burning rate increased with aging. Towards high pressure, the burning rate decreased as a function of aging. The effect of aging was quantified by the determination of defined aging sensitivity coefficients.     

Investigating the effect of chemical ingredient modifications on the slow cook-off violence of ammonium perchlorate solid propellants on the laboratory scale

ABSTRACT

Rocket propellants that contain ammonium perchlorate (AP) often have a violent response during slow heating. Two laboratory tests have been developed to determine the reaction violence of these materials. The first test was the Combustion Rate Analysis of a Slowly Heated Propellant (CRASH-P) test, which used dynamic pressure sensors to quantify the violence of the reaction. The second test used x-rays to measure physical changes in the propellants before ignition and their reaction violence through the deformation speed of the sample container. For this study, four propellant formulation modification categories were investigated. The propellants were investigated for their reaction violence and sample expansion before ignition.