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Role of inorganic additives on the ballistic performance of gun propellant formulations
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Journal of Hazardous Materials 154 (2008) 888–892
Role of inorganic additives on the ballistic performance of gun propellant formulations R.S. Damse ∗ , A.K. Sikder High Energy Materials Research Laboratory, Sutarwadi, Pune 411021, India Received 6 June 2007; received in revised form 30 October 2007; accepted 31 October 2007 Available online 23 November 2007
Abstract This paper explores the possibility of increasing the ballistic performance of gun propellant with the addition of inorganic additives viz. aluminium and ammonium perchlorate. Compositions based on propellant NQ containing additional aluminium and ammonium perchlorate in different parts were studied theoretically and experimentally. Performance in respect of ballistic parameters, sensitivity, thermal characteristics, thermal stability and mechanical properties are evaluated and compared with that of the conventional triple base propellant NQ. Experimental data on comparative study indicate that the compositions containing aluminium and ammonium perchlorate are superior to propellant NQ in respect of energy. © 2007 Elsevier B.V. All rights reserved. Keywords: Aluminium; Ammonium perchlorate; Pressure exponent; Burning rate characteristics; Sensitivity
1. Introduction Conventional triple base propellants such as flash-less propellant N, consisting of NC (12.95% N) 18.80%, NG 18.60%, picrite 55.00%, carbamite 7.60%, and K2 SO4 0.30 parts, high cal-val flash-less propellant NQ consisting of NC (12.95% N) 20.80, NG 20.60, picrite 55.00, carbamite 3.60 and K2 SO4 0.30 parts, and the multi-perforated propellant M-30 consisting of NC (12.6) 28.00, NG 22.50, picrite 48.00, carbamite 1.50, K2 SO4 0.30 parts and graphite 0.30 parts have been extensively used for the tank gun ammunitions. The ballistic requirements of the propellant for tank gun ammunitions are: higher force constant to achieve the highest possible muzzle velocity, lower flame temperature (Tf < 3273 K) to minimize the gun barrel erosion, lower burning rate characteristics (pressure exponent, α < 1.0) to ensure the safety of gun and (linear rate of burning co-efficient, β1 < 0.15 cm/(s MPa)) to achieve the loadability, easy manufacture and consistent performance of the propellant. In addition, sensitivity (h50%Expl > 25 cm, friction > 20 kg), thermal stability (Abel heat test >10 min, Methyl violet test > 40 min, B&J Test < 5cm3 /5g) and mechanical properties (tensile strength > 100 kgf/cm2 and % compression > 10) are
∗
Corresponding author. Tel.: +91 20 25869303; fax: +91 20 25869316. E-mail address: [email protected] (R.S. Damse).
0304-3894/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2007.10.103
also important parameters to assess the suitability of propellant for the tank gun ammunition [1]. The conventional triple base propellants though fulfill the requirements of flame temperature and burning rate characteristics, their energy output in terms of force constant is limited to 1100 J/g. Hence, to propel the advanced projectiles with higher muzzle velocity, propellants having higher force constant are required to be developed. In view of this, extensive research is going on globally to improve upon the force constant of the propellant by using the novel energetic ingredients. In this context, plasticizers like glycidyl azide polymer (GAP, MW ≈ 400), 1,5-diazido3-nitrazapentane(DANPE), N-n butyl-N-(2-nitroxyethyl) nirtamine (Bu-NENA), ethylene glycol bis-azido-acetate (EGBAA), energetic additives like hexanitro hexa aza isowurtizane (HNIW or CL-20), 1,1 diamino 2,2 dinitro ethylene (FOX-7) and binders like poly 3,3-bis(azidomethyl)oxetane (BAMO),3-azidomethyl-3-methyl oxetane(AMMO), poly-3nitratomethyl-3-methyloxetane (NIMMO) have been attempted by number of researchers[2–5]. However, it has been observed that the incorporation of energetic additives though increase the level of force constant, it boosts the flame temperature of the propellant beyond the desired level of tank gun ammunition. In this paper, attempts have been made to increase the force constant of the conventional triple base propellant NQ with the addition of inorganic additives viz. aluminium (Al) and ammonium perchlorate (AP). Eventhough aluminium has
R.S. Damse, A.K. Sikder / Journal of Hazardous Materials 154 (2008) 888–892
been successfully used for boosting the performance of composite and composite modified double base rocket propellants, very scanty information is available on its use into solid gun propellants. Pokhol et al. attempted to evaluate the burning mechanism of aluminium into propellant N and the model mixtures based on ammonium per chlorate or potassium per chlorate containing various oxidizable compounds. They found out that the metal particles undergo agglomeration with the decomposed products of nitrocellulose and got fused at the inner surface of the cartridge. In case of the AP/KP based compositions the exothermic reactions occur on the metallic surface which increase the gun erosion [6]. It has recently been reported that addition of aluminium into conventional gun propellants increases the energy. However, the optimum content of aluminium was found to be only about 5–8% [7]. Similarly, ammonium perchlorate is considered to be the workhorse oxidizer for solid rocket propellants but no experimental work is reported on the gun propellant formulations containing ammonium perchlorate as an oxidizer. Ammonium dinitramide (ADN) and hydrazinium nitroformate (HNF) have emerged as energetic oxidizers but are not available in required quantity [8]. In contrast, ammonium nitrate is available in plenty but badly suffers from hygroscopicity and phase transformations [9]. Thus, ammonium perchlorate, in view of its easy availability, higher oxygen balance (OB100 = +34%) and heat of formation (Hf = −70 kcal/mole) has been identified as a suitable candidate of inorganic oxidizer for the present study. The formulations based on NQ propellant containing additional aluminium and ammonium perchlorate have been studied extensively so as to select the suitable compositions for the advanced tank gun ammunitions. 2. Experimental 2.1. Propellant formulations and processing The triple base propellant NQ has been used as the control composition for the present study. Atomized spherical aluminium powder of average particle size 18 m (M/s. MAPCO, Madurai) and ammonium perchlorate having particle size 10 m (Tamilnadu Chlorates Ltd., Madurai) were used as the additives for the present formulations. The chemical composition of propellant NQ was modified with the addition of aluminium powder in five parts and ammonium perchlorate to the extent of 12 parts, and accordingly different compositions were formulated. Ballistic parameters in respect of force constants and flame temperatures were computed using ‘THERM” programmer [10]. The compositions were processed by the standard solvent method (1 kg batch size) using 18% solution of a mixture of acetone and water in the ratio of 92.5:7.5 and extruded into multitubular (heptatubular) configuration [11]. The multitubular strands were cut with L/D = 1.7, using a rotary cutting machine. The propellant grains were dried in an oven at 45 ◦ C till the volatile matter reduced to 1%. The dried propellant samples were tested for physical measurements like web size, density, etc and finally subjected to evaluation tests.
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2.2. Propellant test methods The ballistic performance of the propellants in respect of force constant, flame temperature and burning rate characteristics (β1 and α) was evaluated on the basis of closed vessel firings conducted in 700 cm3 closed vessel at 0.20 g/cm3 loading density. Impact sensitivity was determined by a standard “Fall Hammer” set-up according to the ‘Bruceton Staircase’ approach [12] whereas friction sensitivity was assessed on the Julius Peters apparatus [13] by incrementally increasing the load from 0.2 to 36 kg till there was no ignition/explosion in five consecutive test samples. Deflagration temperature was recorded in the Julius Peters apparatus using 5 mg sample with gradually raising the temperature at the rate of 5 ◦ C/min. Thermal behavior of the compositions was studied using NETZCH, a German make differential thermal analyzer (DTA). The DTA curves were recorded in an inert atmosphere using a 10 mg sample in alumina crucible at heating rate of 10 ◦ C/min. The calorimetric value of the propellant compositions was determined in Julius Peter’s adiabatic bomb calorimeter. Stability aspects were studied on the basis of gaseous nitrogen oxide evolved during the heating of propellant sample, by applying both qualitative (Abel’s heat test & Methyl violet test) and quantitative (Bergmann and Junk test) methods as per standard procedure [14]. Mechanical properties of the propellants were determined on Universal Testing Machine (Instron-1185). 3. Results and discussion It is reported that the addition of aluminium powder into standard triple base propellant NQ increases the energy of the propellant. However, to meet the ballistic requirements of tank gun ammunition, the addition of aluminium is restricted to five parts [15]. Therefore, the present compositions have been formulated with the addition of aluminium to the extent of five parts only. Attempts have been made to increase the combustion potential of the propellant with the addition of ammonium perchlorate as an oxidizer. Theoretical calculations of ballistic parameters indicate that the addition of aluminium to the extent of five parts increases the force constant of propellant NQ from 1025 J/g to 1090 J/g and flame temperature from 2760 K to 3085 K whereas successive addition of AP at the rate of two parts increases the force constant only by 4 J/g. However, the flame temperature increases with 30–40 K (Table 1). It has been observed that only those compositions containing five parts of aluminium along with ammonium perchlorate up to four parts are processable. Because, addition of ammonium perchlorate excess over four parts make the propellant too sensitive to process under normal conditions. Hence, only those compositions containing ammonium perchlorate up to four parts were processed. The experimental results obtained from the closed vessel test are presented in Table 1. It has been observed that the experimentally determined values of force constant are in good agreement with the theoretically calculated values. The energy increase of the conventional propellant NQ with the addition of aluminium powder is attributed to the formation of lower molecular weight
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Table 1 Ballistic performance of the propellant compositions Propellant composition
Propellant NQ NQ + Al(5p) NQ + Al(5p) + AP(2p) NQ + Al(5p) + AP(4p) NQ + Al(5p) + AP(6p) NQ + Al(5p) + AP(8p) NQ + Al(5p) + AP(10p) NQ + Al(5p) + AP(12p)
Force constant (J/g) Theoritical
Experimental
1025 1090 1095 1100 1104 1108 1112 1115
1024 1090 1094 1098
Flame temperature (K)
Linear rate of burning co-efficient β1 , (cm/(s MPa))
Pressure exponent (α)
2760 3085 3110 3155 3195 3235 3275 3315
0.12 0.13 0.14 0.15
0.68 0.73 0.73 0.73
Propellant NQ (chemical composition): NC (12.95 N%): 20.80 + NG: 20.60 + Picrite: 55.00 + corbamite: 3.60 + K2 SO4 : 0.30 parts, Al, aluminium, AP, ammonium perchlorate, p, parts.
(Mw) of the combustion gases. This is due to the conversion of CO2 (Mw = 44) to CO (Mw = 28) and H2 O (Mw = 18) to H2 (Mw = 2) taking place by the oxidation of Al up to the condensed phase Al2 O3. The exothermic heat produced during the reactions increases the flame temperature (Tf ). Thus, the addition of aluminium powder increases the number of moles of the combustion products per gram of the propellant (‘n’ value) as well as the flame temperature (Tf ) and thereby the force constant. Force constant of the propellant can be presented as F = nRTf = RTf /M, where, R is the universal gas constant. In addition, higher density (2.7 g/cm3 ) and atomized spherical particle size of aluminium also contribute to exploit the combustion potential of the propellant. However, further addition of AP decreases the value of mole number (n) with the formation of relatively higher molecular weight (Mw) of the combustion gases. This is due to the reconversion of CO (Mw = 28) to CO2 (Mw = 44) and H2 (Mw = 2) to H2 O (Mw = 18). The re-conversion reactions are favored by the addition of ammonium perchlorate. As a consequence, only A virtual increase in force constant (∼4 J/g) could be achieved with a significant rise in to flame temperature (30–40 K). Therefore the optimum content of AP has been fixed to four parts. Because, beyond this limit, the mean molecular weight of the combustion gases goes on increasing to such an extent that it offsets the advantage of higher force constant (Table 1). It has also been found out that the addition of aluminium to the extent of five parts increases the burning rate characteristics, i.e. β1 from 0.12 cm/sec/MPa to 0.13 cm/sec/MPa and α from 0.68 to 0.73. This may be due to the exothermic reactions taking place between aluminium and the combustion gases evolved. The heat released due to the conversion of CO2 to CO and H2 O to H2 is the main source of energy produced in the condensed zone of propellant during the decomposition process which contributes to enhance the burning rate characteristics. It has been observed that further addition of AP to the extent of four parts increases β1 from 0.13 cm/sec/MPa to 0.15 cm/sec/MPa without changing the value of pressure exponent (α = 0.73). The combustion models developed for the composite modified double base propellant reveal that the higher concentration of AP gives greater heat feedback from the gas phase to the burning surface with increase in condensed phase temperature so as the burning rate. However, proximity of the flame to the pyrolysing surface reduces the dependence of burning rate on
pressure and thus the constant value of pressure exponent (α) is observed with the increase in linear burning rate co-efficient, β1 [16]. It has been observed that the addition of aluminium up to five parts does not change the sensitivity of the propellant NQ with a significant value. However, subsequent addition of ammonium perchlorate at the rate of two parts increases the sensitivity with a significant margin as shown by the height for 50% explosion and dead load required to explode the sample under the influence of friction (Table 2). This is quite consistent with the increasing order of oxygen balance followed by the compositions. (OB100 = −31.04 to −29.00%). However, the values of sensitivity are within the usable limit of gun propellant. Experimental data obtained on the combustion characteristics indicates that the addition of aluminium to the extent of five parts does not affect the deflagration and decomposition temperature of the propellant in a significant way. However, addition of AP to the extent of four parts decreases the deflagration temperature from 177 ◦ C to 154 ◦ C and decomposition temperature from 185 ◦ C to 170 ◦ C (Fig. 1). The steady state behavior of the propellant in respect of deflagration and decomposition temperature is attributed to the typical combustion characteristics of the metallic powder such as ratio of molar volume, co-efficient of thermal expansion of the solid metal/metal oxide and solubility of metal oxide in molten metal. Aluminium is basically a non-volatile metal because of its high boiling point. An oxide layer formed on the metal surface serves as an effective barrier to mass diffusion of reactants and energy transfer due to the low solubility of aluminium oxide in molten aluminium. As a result, the combustion is impeded till the boiling point of aluminium or melting point of aluminium oxide, Al2 O3 (2318 K/2823 K) is achieved. As a consequence, the deflagration and decomposition temperatures do not increase appreciably. However, the addition of ammonium perchlorate lowers the deflagration and decomposition temperature to the considerable extent. It has been reported by Galway et al. that the decomposition process of ammonium perchlorate is initiated with the proton and electron transfer mechanism generating the nitryl perchlorate (NO2 ClO4 ) as an active intermediate [15]. The subsequent rupture of the covalent bond O2 NO–ClO3 in the nitryl perchlorate triggers the ignition process of the propellant. It has been reported by the same authors that the energy (31 kcal/mole) required to break the covalent bond O2 NO–ClO2,
R.S. Damse, A.K. Sikder / Journal of Hazardous Materials 154 (2008) 888–892
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Table 2 Suitability parameters of the propellant compositions for tank gun ammunition Parameter
Propellant NQ
NQ + Al(5p)
NQ + Al(5p) + AP(2p)
NQ + Al(5p) + AP(4p)
Sensitivity H50%Expl (cm) Friction (kg) Ignition Temp (◦ C) Cal-val (Cal/g) Abel heat test (min) Methyl violet test (min) B&J test (cm3 /5g) Tensile strength (kgf/cm2 ) Elongation (%) Flexural test displacement (mm) Compression (%)
38 24.0 176 880 17 55 2.5 250 6.7 4.2 11.2
37 24.0 177 925 17 60 2.0 207 7.0 3.2 11.2
36 21.6 157 930 17 70 1.1 173 7.4 2.9 11.0
31 21.6 154 935 17 75 1.0 116 7.4 2.9 11.0
is responsible for the deflagration and decomposition process of the propellant (Table 2). It is also observed that the calorimetric value of the propellant goes on increasing with the successive addition of aluminium (Table 2). This can be attributed to the exothermic reactions taking place between the aluminium powder and the evolved combustion gases of the propellant. Successive addition of AP increases the oxygen balance of the propellant and thus higher heat output is obtained (Table 2). Results of thermal stability tests like Abel heat test, Methyl violet test and Bergmann and Junk tests indicate that the propellants are thermally stable (Table 2). Experimental data on mechanical properties indicates that the tensile strength, percentage elongation, flexural properties and percentage compression of the propellants are reasonably good and within the acceptable limit of the gun propellant (Table 2). However, the addition of aluminium and ammonium perchlorate decreases the mechanical properties of the propellant with a narrow margin. This can be attributed to the relatively lower reinforcing power gained by the heterogeneous propellant dough.
Fig. 1. DTA curves.
4. Conclusions The addition of inorganic additives viz. aluminium and ammonium perchlorate into standard triple base propellant NQ increases the energy of the propellant. However, to meet the ballistic requirements of tank gun ammunitions, the optimum content of aluminium and ammonium perchlorate found to be five parts and four parts respectively. The propellants containing the inorganic additives also exhibit reasonably good thermal stability and mechanical properties. Acknowledgement The authors are thankful to Dr. A. Subhananda Rao, Director, HEMRL for his, encouragement. References [1] R.S. Damse, H. Singh, Nitramine-based high energy propellant compositions for the tank guns, Def. Sci. J. 50 (1) (2000) 75–81. [2] R.S. Damse, H. Singh, Glycidyl azide polymer based high energy gun propellants, in: Proceedings of the second conference (International) and exhibit, IIT Madras (Chennai), India, 1998, p. 346. [3] M.M. Joshi, C.R. Dayanandan, M.J. Kohadkar, A.G.S. Pillai, A. Singh, Study of energetic plasticizer DANPE in triple base gun propellant, in: Proceedings of the 29th International pyrotechnic seminar, Colorado, USA, 2002, p. 643. [4] B. Wardle, B.H. Robert, L.V. Raule, Wallac, High energy oxetane/HNIW gun propellants, in: Proceedings of the 27th Annual Conference of ICT, Karlsruhe, Germany, 1996, p. 52.01. [5] R.L. Simmon, Guidelines of high energy gun propellants, in: Proceedings of the 27th international conference of ICT on energetic materials, Germany, 1996, p. 22.01. [6] P.F. Pokhol, V.M. Maltesv, V.S. Logachev, V.A. Seleznev, Combustion of aluminium particles in a tongue of flame of condensed systems, Goreniyavzryva, Moscow, USSR 7 (1) (1971) 51–57. [7] I.G. Assovskii, O.T. Chizhevskii, V.V. Sergeev, Thermodynamic and ballistic characteristics of gun propellants and charges containing metallic additives, in: Proceedings of the seminar on chemical physics, ARS, Kosygin str. 4, Moscow, 1995, p. 11.00/88. [8] S. Borman, Advanced energetic materials emerge for military and space applications, Chem. Eng. News 72 (1994) 18–27. [9] R.S. Damse, Waterproofing materials for ammonium nitrate, Def. Sci. J. 54 (4) (2004) 483–492. [10] K.P. Rao, Calculation of thermo chemical constants of propellant, Def. Sci. J. 29 (1) (1979) 21–26.
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[11] S. Singh, Double base propellants, CI(ME) Report 1 (76) (1976) 124. [12] D.H. Mallory (Ed.), Development of impact sensitivity tests at Explosive Research Laboratory, Bruceton, Pennsylvania, NAVORD, Report No. 4236, 1956. [13] J.K.G. Peters, Proceedings of the production programmer of Julius Peter Company for Members of M.B.B. Course-81, Berlin, 1921, p. 14.
[14] A.B. Bofors, Analytical Method for Powders and Explosives, Nobel Krute, Bofors, Sweden, 1960, p. 21. [15] A.K. Galway, P.W. Jacobs, Trans. Far. Soc. 55 (1959) 1165– 1175. [16] E. Santasesaria, A. Morini, S. Carra, Ammonium perchlorate decomposition in the presence of metallic oxides, Combus. Flame 31 (1978) 17–23.
Journal of Hazardous Materials 154 (2008) 888–892
Role of inorganic additives on the ballistic performance of gun propellant formulations R.S. Damse ∗ , A.K. Sikder High Energy Materials Research Laboratory, Sutarwadi, Pune 411021, India Received 6 June 2007; received in revised form 30 October 2007; accepted 31 October 2007 Available online 23 November 2007
Abstract This paper explores the possibility of increasing the ballistic performance of gun propellant with the addition of inorganic additives viz. aluminium and ammonium perchlorate. Compositions based on propellant NQ containing additional aluminium and ammonium perchlorate in different parts were studied theoretically and experimentally. Performance in respect of ballistic parameters, sensitivity, thermal characteristics, thermal stability and mechanical properties are evaluated and compared with that of the conventional triple base propellant NQ. Experimental data on comparative study indicate that the compositions containing aluminium and ammonium perchlorate are superior to propellant NQ in respect of energy. © 2007 Elsevier B.V. All rights reserved. Keywords: Aluminium; Ammonium perchlorate; Pressure exponent; Burning rate characteristics; Sensitivity
1. Introduction Conventional triple base propellants such as flash-less propellant N, consisting of NC (12.95% N) 18.80%, NG 18.60%, picrite 55.00%, carbamite 7.60%, and K2 SO4 0.30 parts, high cal-val flash-less propellant NQ consisting of NC (12.95% N) 20.80, NG 20.60, picrite 55.00, carbamite 3.60 and K2 SO4 0.30 parts, and the multi-perforated propellant M-30 consisting of NC (12.6) 28.00, NG 22.50, picrite 48.00, carbamite 1.50, K2 SO4 0.30 parts and graphite 0.30 parts have been extensively used for the tank gun ammunitions. The ballistic requirements of the propellant for tank gun ammunitions are: higher force constant to achieve the highest possible muzzle velocity, lower flame temperature (Tf < 3273 K) to minimize the gun barrel erosion, lower burning rate characteristics (pressure exponent, α < 1.0) to ensure the safety of gun and (linear rate of burning co-efficient, β1 < 0.15 cm/(s MPa)) to achieve the loadability, easy manufacture and consistent performance of the propellant. In addition, sensitivity (h50%Expl > 25 cm, friction > 20 kg), thermal stability (Abel heat test >10 min, Methyl violet test > 40 min, B&J Test < 5cm3 /5g) and mechanical properties (tensile strength > 100 kgf/cm2 and % compression > 10) are
∗
Corresponding author. Tel.: +91 20 25869303; fax: +91 20 25869316. E-mail address: [email protected] (R.S. Damse).
0304-3894/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2007.10.103
also important parameters to assess the suitability of propellant for the tank gun ammunition [1]. The conventional triple base propellants though fulfill the requirements of flame temperature and burning rate characteristics, their energy output in terms of force constant is limited to 1100 J/g. Hence, to propel the advanced projectiles with higher muzzle velocity, propellants having higher force constant are required to be developed. In view of this, extensive research is going on globally to improve upon the force constant of the propellant by using the novel energetic ingredients. In this context, plasticizers like glycidyl azide polymer (GAP, MW ≈ 400), 1,5-diazido3-nitrazapentane(DANPE), N-n butyl-N-(2-nitroxyethyl) nirtamine (Bu-NENA), ethylene glycol bis-azido-acetate (EGBAA), energetic additives like hexanitro hexa aza isowurtizane (HNIW or CL-20), 1,1 diamino 2,2 dinitro ethylene (FOX-7) and binders like poly 3,3-bis(azidomethyl)oxetane (BAMO),3-azidomethyl-3-methyl oxetane(AMMO), poly-3nitratomethyl-3-methyloxetane (NIMMO) have been attempted by number of researchers[2–5]. However, it has been observed that the incorporation of energetic additives though increase the level of force constant, it boosts the flame temperature of the propellant beyond the desired level of tank gun ammunition. In this paper, attempts have been made to increase the force constant of the conventional triple base propellant NQ with the addition of inorganic additives viz. aluminium (Al) and ammonium perchlorate (AP). Eventhough aluminium has
R.S. Damse, A.K. Sikder / Journal of Hazardous Materials 154 (2008) 888–892
been successfully used for boosting the performance of composite and composite modified double base rocket propellants, very scanty information is available on its use into solid gun propellants. Pokhol et al. attempted to evaluate the burning mechanism of aluminium into propellant N and the model mixtures based on ammonium per chlorate or potassium per chlorate containing various oxidizable compounds. They found out that the metal particles undergo agglomeration with the decomposed products of nitrocellulose and got fused at the inner surface of the cartridge. In case of the AP/KP based compositions the exothermic reactions occur on the metallic surface which increase the gun erosion [6]. It has recently been reported that addition of aluminium into conventional gun propellants increases the energy. However, the optimum content of aluminium was found to be only about 5–8% [7]. Similarly, ammonium perchlorate is considered to be the workhorse oxidizer for solid rocket propellants but no experimental work is reported on the gun propellant formulations containing ammonium perchlorate as an oxidizer. Ammonium dinitramide (ADN) and hydrazinium nitroformate (HNF) have emerged as energetic oxidizers but are not available in required quantity [8]. In contrast, ammonium nitrate is available in plenty but badly suffers from hygroscopicity and phase transformations [9]. Thus, ammonium perchlorate, in view of its easy availability, higher oxygen balance (OB100 = +34%) and heat of formation (Hf = −70 kcal/mole) has been identified as a suitable candidate of inorganic oxidizer for the present study. The formulations based on NQ propellant containing additional aluminium and ammonium perchlorate have been studied extensively so as to select the suitable compositions for the advanced tank gun ammunitions. 2. Experimental 2.1. Propellant formulations and processing The triple base propellant NQ has been used as the control composition for the present study. Atomized spherical aluminium powder of average particle size 18 m (M/s. MAPCO, Madurai) and ammonium perchlorate having particle size 10 m (Tamilnadu Chlorates Ltd., Madurai) were used as the additives for the present formulations. The chemical composition of propellant NQ was modified with the addition of aluminium powder in five parts and ammonium perchlorate to the extent of 12 parts, and accordingly different compositions were formulated. Ballistic parameters in respect of force constants and flame temperatures were computed using ‘THERM” programmer [10]. The compositions were processed by the standard solvent method (1 kg batch size) using 18% solution of a mixture of acetone and water in the ratio of 92.5:7.5 and extruded into multitubular (heptatubular) configuration [11]. The multitubular strands were cut with L/D = 1.7, using a rotary cutting machine. The propellant grains were dried in an oven at 45 ◦ C till the volatile matter reduced to 1%. The dried propellant samples were tested for physical measurements like web size, density, etc and finally subjected to evaluation tests.
889
2.2. Propellant test methods The ballistic performance of the propellants in respect of force constant, flame temperature and burning rate characteristics (β1 and α) was evaluated on the basis of closed vessel firings conducted in 700 cm3 closed vessel at 0.20 g/cm3 loading density. Impact sensitivity was determined by a standard “Fall Hammer” set-up according to the ‘Bruceton Staircase’ approach [12] whereas friction sensitivity was assessed on the Julius Peters apparatus [13] by incrementally increasing the load from 0.2 to 36 kg till there was no ignition/explosion in five consecutive test samples. Deflagration temperature was recorded in the Julius Peters apparatus using 5 mg sample with gradually raising the temperature at the rate of 5 ◦ C/min. Thermal behavior of the compositions was studied using NETZCH, a German make differential thermal analyzer (DTA). The DTA curves were recorded in an inert atmosphere using a 10 mg sample in alumina crucible at heating rate of 10 ◦ C/min. The calorimetric value of the propellant compositions was determined in Julius Peter’s adiabatic bomb calorimeter. Stability aspects were studied on the basis of gaseous nitrogen oxide evolved during the heating of propellant sample, by applying both qualitative (Abel’s heat test & Methyl violet test) and quantitative (Bergmann and Junk test) methods as per standard procedure [14]. Mechanical properties of the propellants were determined on Universal Testing Machine (Instron-1185). 3. Results and discussion It is reported that the addition of aluminium powder into standard triple base propellant NQ increases the energy of the propellant. However, to meet the ballistic requirements of tank gun ammunition, the addition of aluminium is restricted to five parts [15]. Therefore, the present compositions have been formulated with the addition of aluminium to the extent of five parts only. Attempts have been made to increase the combustion potential of the propellant with the addition of ammonium perchlorate as an oxidizer. Theoretical calculations of ballistic parameters indicate that the addition of aluminium to the extent of five parts increases the force constant of propellant NQ from 1025 J/g to 1090 J/g and flame temperature from 2760 K to 3085 K whereas successive addition of AP at the rate of two parts increases the force constant only by 4 J/g. However, the flame temperature increases with 30–40 K (Table 1). It has been observed that only those compositions containing five parts of aluminium along with ammonium perchlorate up to four parts are processable. Because, addition of ammonium perchlorate excess over four parts make the propellant too sensitive to process under normal conditions. Hence, only those compositions containing ammonium perchlorate up to four parts were processed. The experimental results obtained from the closed vessel test are presented in Table 1. It has been observed that the experimentally determined values of force constant are in good agreement with the theoretically calculated values. The energy increase of the conventional propellant NQ with the addition of aluminium powder is attributed to the formation of lower molecular weight
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Table 1 Ballistic performance of the propellant compositions Propellant composition
Propellant NQ NQ + Al(5p) NQ + Al(5p) + AP(2p) NQ + Al(5p) + AP(4p) NQ + Al(5p) + AP(6p) NQ + Al(5p) + AP(8p) NQ + Al(5p) + AP(10p) NQ + Al(5p) + AP(12p)
Force constant (J/g) Theoritical
Experimental
1025 1090 1095 1100 1104 1108 1112 1115
1024 1090 1094 1098
Flame temperature (K)
Linear rate of burning co-efficient β1 , (cm/(s MPa))
Pressure exponent (α)
2760 3085 3110 3155 3195 3235 3275 3315
0.12 0.13 0.14 0.15
0.68 0.73 0.73 0.73
Propellant NQ (chemical composition): NC (12.95 N%): 20.80 + NG: 20.60 + Picrite: 55.00 + corbamite: 3.60 + K2 SO4 : 0.30 parts, Al, aluminium, AP, ammonium perchlorate, p, parts.
(Mw) of the combustion gases. This is due to the conversion of CO2 (Mw = 44) to CO (Mw = 28) and H2 O (Mw = 18) to H2 (Mw = 2) taking place by the oxidation of Al up to the condensed phase Al2 O3. The exothermic heat produced during the reactions increases the flame temperature (Tf ). Thus, the addition of aluminium powder increases the number of moles of the combustion products per gram of the propellant (‘n’ value) as well as the flame temperature (Tf ) and thereby the force constant. Force constant of the propellant can be presented as F = nRTf = RTf /M, where, R is the universal gas constant. In addition, higher density (2.7 g/cm3 ) and atomized spherical particle size of aluminium also contribute to exploit the combustion potential of the propellant. However, further addition of AP decreases the value of mole number (n) with the formation of relatively higher molecular weight (Mw) of the combustion gases. This is due to the reconversion of CO (Mw = 28) to CO2 (Mw = 44) and H2 (Mw = 2) to H2 O (Mw = 18). The re-conversion reactions are favored by the addition of ammonium perchlorate. As a consequence, only A virtual increase in force constant (∼4 J/g) could be achieved with a significant rise in to flame temperature (30–40 K). Therefore the optimum content of AP has been fixed to four parts. Because, beyond this limit, the mean molecular weight of the combustion gases goes on increasing to such an extent that it offsets the advantage of higher force constant (Table 1). It has also been found out that the addition of aluminium to the extent of five parts increases the burning rate characteristics, i.e. β1 from 0.12 cm/sec/MPa to 0.13 cm/sec/MPa and α from 0.68 to 0.73. This may be due to the exothermic reactions taking place between aluminium and the combustion gases evolved. The heat released due to the conversion of CO2 to CO and H2 O to H2 is the main source of energy produced in the condensed zone of propellant during the decomposition process which contributes to enhance the burning rate characteristics. It has been observed that further addition of AP to the extent of four parts increases β1 from 0.13 cm/sec/MPa to 0.15 cm/sec/MPa without changing the value of pressure exponent (α = 0.73). The combustion models developed for the composite modified double base propellant reveal that the higher concentration of AP gives greater heat feedback from the gas phase to the burning surface with increase in condensed phase temperature so as the burning rate. However, proximity of the flame to the pyrolysing surface reduces the dependence of burning rate on
pressure and thus the constant value of pressure exponent (α) is observed with the increase in linear burning rate co-efficient, β1 [16]. It has been observed that the addition of aluminium up to five parts does not change the sensitivity of the propellant NQ with a significant value. However, subsequent addition of ammonium perchlorate at the rate of two parts increases the sensitivity with a significant margin as shown by the height for 50% explosion and dead load required to explode the sample under the influence of friction (Table 2). This is quite consistent with the increasing order of oxygen balance followed by the compositions. (OB100 = −31.04 to −29.00%). However, the values of sensitivity are within the usable limit of gun propellant. Experimental data obtained on the combustion characteristics indicates that the addition of aluminium to the extent of five parts does not affect the deflagration and decomposition temperature of the propellant in a significant way. However, addition of AP to the extent of four parts decreases the deflagration temperature from 177 ◦ C to 154 ◦ C and decomposition temperature from 185 ◦ C to 170 ◦ C (Fig. 1). The steady state behavior of the propellant in respect of deflagration and decomposition temperature is attributed to the typical combustion characteristics of the metallic powder such as ratio of molar volume, co-efficient of thermal expansion of the solid metal/metal oxide and solubility of metal oxide in molten metal. Aluminium is basically a non-volatile metal because of its high boiling point. An oxide layer formed on the metal surface serves as an effective barrier to mass diffusion of reactants and energy transfer due to the low solubility of aluminium oxide in molten aluminium. As a result, the combustion is impeded till the boiling point of aluminium or melting point of aluminium oxide, Al2 O3 (2318 K/2823 K) is achieved. As a consequence, the deflagration and decomposition temperatures do not increase appreciably. However, the addition of ammonium perchlorate lowers the deflagration and decomposition temperature to the considerable extent. It has been reported by Galway et al. that the decomposition process of ammonium perchlorate is initiated with the proton and electron transfer mechanism generating the nitryl perchlorate (NO2 ClO4 ) as an active intermediate [15]. The subsequent rupture of the covalent bond O2 NO–ClO3 in the nitryl perchlorate triggers the ignition process of the propellant. It has been reported by the same authors that the energy (31 kcal/mole) required to break the covalent bond O2 NO–ClO2,
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Table 2 Suitability parameters of the propellant compositions for tank gun ammunition Parameter
Propellant NQ
NQ + Al(5p)
NQ + Al(5p) + AP(2p)
NQ + Al(5p) + AP(4p)
Sensitivity H50%Expl (cm) Friction (kg) Ignition Temp (◦ C) Cal-val (Cal/g) Abel heat test (min) Methyl violet test (min) B&J test (cm3 /5g) Tensile strength (kgf/cm2 ) Elongation (%) Flexural test displacement (mm) Compression (%)
38 24.0 176 880 17 55 2.5 250 6.7 4.2 11.2
37 24.0 177 925 17 60 2.0 207 7.0 3.2 11.2
36 21.6 157 930 17 70 1.1 173 7.4 2.9 11.0
31 21.6 154 935 17 75 1.0 116 7.4 2.9 11.0
is responsible for the deflagration and decomposition process of the propellant (Table 2). It is also observed that the calorimetric value of the propellant goes on increasing with the successive addition of aluminium (Table 2). This can be attributed to the exothermic reactions taking place between the aluminium powder and the evolved combustion gases of the propellant. Successive addition of AP increases the oxygen balance of the propellant and thus higher heat output is obtained (Table 2). Results of thermal stability tests like Abel heat test, Methyl violet test and Bergmann and Junk tests indicate that the propellants are thermally stable (Table 2). Experimental data on mechanical properties indicates that the tensile strength, percentage elongation, flexural properties and percentage compression of the propellants are reasonably good and within the acceptable limit of the gun propellant (Table 2). However, the addition of aluminium and ammonium perchlorate decreases the mechanical properties of the propellant with a narrow margin. This can be attributed to the relatively lower reinforcing power gained by the heterogeneous propellant dough.
Fig. 1. DTA curves.
4. Conclusions The addition of inorganic additives viz. aluminium and ammonium perchlorate into standard triple base propellant NQ increases the energy of the propellant. However, to meet the ballistic requirements of tank gun ammunitions, the optimum content of aluminium and ammonium perchlorate found to be five parts and four parts respectively. The propellants containing the inorganic additives also exhibit reasonably good thermal stability and mechanical properties. Acknowledgement The authors are thankful to Dr. A. Subhananda Rao, Director, HEMRL for his, encouragement. References [1] R.S. Damse, H. Singh, Nitramine-based high energy propellant compositions for the tank guns, Def. Sci. J. 50 (1) (2000) 75–81. [2] R.S. Damse, H. Singh, Glycidyl azide polymer based high energy gun propellants, in: Proceedings of the second conference (International) and exhibit, IIT Madras (Chennai), India, 1998, p. 346. [3] M.M. Joshi, C.R. Dayanandan, M.J. Kohadkar, A.G.S. Pillai, A. Singh, Study of energetic plasticizer DANPE in triple base gun propellant, in: Proceedings of the 29th International pyrotechnic seminar, Colorado, USA, 2002, p. 643. [4] B. Wardle, B.H. Robert, L.V. Raule, Wallac, High energy oxetane/HNIW gun propellants, in: Proceedings of the 27th Annual Conference of ICT, Karlsruhe, Germany, 1996, p. 52.01. [5] R.L. Simmon, Guidelines of high energy gun propellants, in: Proceedings of the 27th international conference of ICT on energetic materials, Germany, 1996, p. 22.01. [6] P.F. Pokhol, V.M. Maltesv, V.S. Logachev, V.A. Seleznev, Combustion of aluminium particles in a tongue of flame of condensed systems, Goreniyavzryva, Moscow, USSR 7 (1) (1971) 51–57. [7] I.G. Assovskii, O.T. Chizhevskii, V.V. Sergeev, Thermodynamic and ballistic characteristics of gun propellants and charges containing metallic additives, in: Proceedings of the seminar on chemical physics, ARS, Kosygin str. 4, Moscow, 1995, p. 11.00/88. [8] S. Borman, Advanced energetic materials emerge for military and space applications, Chem. Eng. News 72 (1994) 18–27. [9] R.S. Damse, Waterproofing materials for ammonium nitrate, Def. Sci. J. 54 (4) (2004) 483–492. [10] K.P. Rao, Calculation of thermo chemical constants of propellant, Def. Sci. J. 29 (1) (1979) 21–26.
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[11] S. Singh, Double base propellants, CI(ME) Report 1 (76) (1976) 124. [12] D.H. Mallory (Ed.), Development of impact sensitivity tests at Explosive Research Laboratory, Bruceton, Pennsylvania, NAVORD, Report No. 4236, 1956. [13] J.K.G. Peters, Proceedings of the production programmer of Julius Peter Company for Members of M.B.B. Course-81, Berlin, 1921, p. 14.
[14] A.B. Bofors, Analytical Method for Powders and Explosives, Nobel Krute, Bofors, Sweden, 1960, p. 21. [15] A.K. Galway, P.W. Jacobs, Trans. Far. Soc. 55 (1959) 1165– 1175. [16] E. Santasesaria, A. Morini, S. Carra, Ammonium perchlorate decomposition in the presence of metallic oxides, Combus. Flame 31 (1978) 17–23.