Preliminary Study on Characteristics of NC/HTPB-Based High-Energy Gun Propellants

: This study mainly explored the characteristics of NC/HTPB-based high-energy gun pro-pellants with RDX, CL-20 or TKX-50 by experimental method. Three series of test samples were prepared referring to the formulation of M1 single-base gun propellant (M1 SBP). The thermochemical characteristics, chemical stability, explosion heat, impact and friction sensitivities of prepared samples were determined by simultaneous differential scanning calorimetry–thermogravimetric analysis (STA DSC–TGA), vacuum stability tester (VST), bomb calorimeter (BC), BAM fallhammer and BAM friction tester, respectively, and compared with those of the reference sample M1. The experimental results indicated that the thermochemical characteristics of NC/HTPB-based high-energy gun propellants were similar to those of M1 SBP. The NC/HTPB-based high-energy gun propellants had good chemical stability and were superior to M1 SBP. The explosion heat of NC/HTPB-based high-energy gun propellants was close to and slightly larger than that of M1 SBP. In addition, the NC/HTPB-based high-energy gun propellants had lower impact and friction sensitivities than the M1 SBP. Therefore, the NC/HTPB-based high-energy gun propellants have the potential to replace the M1 SBP. The combustion performances of NC/HTPB-based high-energy gun propellants will be continuously studied and veriﬁed in the future.


Introduction
Gun propellants are designed to produce very large amounts of gas to propel the projectile at high velocity.The velocity of the projectile depends on the rate at which the gas is produced.This, in turn, depends on the amount of chemical energy released [1].Traditional gun propellants are broadly classified as a single-base propellant (SBP), doublebase propellant (DBP), and triple-base propellant (TBP).SBPs contain nitrocellulose (NC) as the main energetic ingredient, while DBPs contain both NC and nitroglycerine (NG) and TBPs contain nitroguanidine (NQ) in addition to NC and NG [2].In view of the problem that traditional gun propellants are highly prone to accidental ignition, low vulnerability ammunition (LOVA) gun propellants with low sensitivity have been continuously developed.This has been primarily achieved by removing NG and reducing the amount of NC in the formulation, replacing them with inert and energetic plasticizers and cyclic nitramines [3][4][5][6][7][8].
M1 SBP is composed of 85% NC, 10% 2,4-dinitrotoluene (2,4-DNT, flash inhibitor), 5% dibutylphalate (DBP, burn rate modifier and plasticizer) and an additional 1% diphenylamine (DPA, stabilizer), which is used in 105 mm, 155 mm, and 8 inch Howitzers [21].M1 SBP has a lower flame temperature and therefore less erosion, and its burning rate can be controlled by coating the burning rate modifier.However, irregular burning is a major drawback due to the hygroscopic nature of M1 SBP.In addition, the temperature sensitivity of M1 SBP is relatively high in the range of operating ambient temperatures, which has also attracted the attention of researchers.Several additives can be considered for addition to M1 SBP to improve its combustion behavior, such as stabilizers, plasticizers, and phlegmatizers [22].
In this study, the characteristics of NC/HTPB-based high-energy gun propellants with RDX, CL-20 or TKX-50 were preliminarily explored by experimental method.The formulation of M1 SBP was selected as a reference formulation.Three series of test samples were designed and prepared, and their thermochemical characteristics, chemical stability, explosion heat and mechanical sensitivity (impact and friction) were determined by simultaneous differential scanning calorimetry-thermogravimetric analysis (STA DSC-TGA), vacuum stability tester (VST), bomb calorimeter (BC), BAM fallhammer and BAM friction tester, respectively, and compared with those of the reference sample M1.In addition, the feasibility of using NC/HTPB-based high-energy gun propellants to replace the M1 SBP was also analyzed and evaluated.

Materials
Nitrocellulose (NC) with a nitrogen content of 13.15% produced by the 205th Arsenal in Taiwan was used as an energetic ingredient in the preparation of NC/HTPB-based high-energy gun propellants.Hydroxyl-terminated polybutadiene (HTPB) was an organic polymer used as an inert and energetic binder, which was obtained from the National Chung Shan Institute of Science and Technology (NCSIST) in Taiwan.1,3,5-Trinitroperhydro-1,3,5triazine [RDX, (CH 2 NNO 2 ) 3 ] with a purity of 99.9% was also obtained from the 205th Arsenal, which has been actually used in various military applications.with a purity of 99% and diphenylamine [DPA, (C 6 H 5 ) 2 NH] with a purity of 99% were purchased from Sigma-Aldrich Corporation (St. Louis, MO, USA) and used as additives to improve the combustion performance of gun propellants.Ethanol (C 2 H 5 OH, 99.8%) and diethyl ether [(C 2 H 5 ) 2 O, 99.7%] were also purchased from Sigma-Aldrich and used as solvents without further purification.In addition, all aqueous solutions were prepared using deionized water.

Sample
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Sample
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Thermochemical Characteristics Test
The thermochemical characteristics of three series of NC/HTPB-based high-energy gun propellant samples were studied by thermal analysis technique and compared with those of the reference sample M1.The thermal decomposition temperature and mass loss of the test samples during the decomposition reaction were measured by STA DSC-TGA.The test sample was placed in a ceramic crucible using a sample weight of about 3-5 mg.All experiments were carried out at a heating rate of 10 • C/min under a nitrogen flow of 20 mL/min.In addition, STA DSC-TGA was also employed to measure the thermal decomposition temperatures of the test samples at heating rates of 1, 2, 5 and 10 • C/min, and the experimental data were used to calculate the activation energies by the Kissinger [23] and Ozawa [24] methods.

Chemical Stability Test
The chemical stability of three series of NC/HTPB-based high-energy gun propellant samples was determined by VST according to the MIL-STD-1751A method 1061 [25] and compared with that of the reference sample M1.The VST was assembled by our laboratory, which consists of a heating block and glass tubes with temperature and pressure sensors.A 5 g test sample was placed in the glass tube and closed by a head with pressure and temperature transducers.The amount of gas released by thermal decomposition was measured under vacuum at a temperature of 100 • C for 40 h.The gas release required by the MIL-STD-1751A standard must be less than 2 mL/g.

Explosion Heat Test
The explosion heat of three series of NC/HTPB-based high-energy gun propellant samples was measured by BC and compared with that of the reference sample M1.The BC consists of a strong cylindrical stainless container (called bomb) which can withstand high pressure when the material is burnt in it [26].About 0.5 g of the sample was placed in the bomb, which was then filled with nitrogen.Afterwards, the bomb was placed in the bucket with 2 L of water, and the bucket was placed inside the calorimeter, which was surrounded by an air-jacket to prevent heat loss due to radiation.Finally, the sample was ignited by a fuse wire to measure the heat of explosion.

Mechanical Sensitivity Tests
The impact sensitivity of three series of NC/HTPB-based high-energy gun propellant samples was determined by BAM fallhammer according to the MIL-STD-1751A method 1015 [25] and compared with that of the reference sample M1.The Bruceton method [27] was used to evaluate the impact sensitivity, which was based on a statistical analysis by determining the drop height (H 50 ) at which there was 50% probability of obtaining an ignition.Each sample was tested utilizing a 5 kg drop weight for 30 time to obtain a H 50 .The impact energy (E 50 ) was calculated using the formula E 50 (Joule) = mgH 50 , where m is the drop weight mass [kg], g is the acceleration due to gravity [ms −2 ], and H 50 is the drop height [m].
The friction sensitivity of three series of NC/HTPB-based high-energy gun propellant samples was determined by BAM friction tester according to the MIL-STD-1751A method 1024 [25] and compared with that of the reference sample M1.The 1 of 6 method was used to evaluate the friction sensitivity, which was defined as the smallest load at which an audible or visible decomposition reaction is obtained from at least one out of six trials.The measurement range of friction load was from 0.5 to 360 N.

Calculation of Activation Energy
In kinetic analysis, it is generally assumed that the rate of reaction can be described by two separable functions, k(T) and f (α), such that where dα/dt is the rate of mass loss, α is the fractional decomposition at any time, and k(T) is the temperature-dependent rate constant.The term f (α) is a function of α given by: where n is an order of reaction.The temperature dependence of the reaction rate is commonly described by the Arrhenius equation: where E a is the activation energy, A is the pre-exponential factor, and R is the universal gas constant.By combining Equation (1) with Equation (3), the following expression is obtained:

Kissinger Method
Because the maximum rate occurs when d 2 α/dt 2 = 0, differentiation of Equation ( 4) gives: where T P is the temperature peak of the DSC curve at linear heating rate β = dT/dt.The Kissinger method [23,28] assumed that the product n(1−α) n−1 is independent of β.Therefore, the following expression is derived: The value of activation energy can be calculated from the slope of the approximately straight line ln β/T 2 P versus (1/T P ).

Ozawa Method
At linear heating rate β = dT/dt, Equation ( 4) can be written as: The Ozawa method [24,28] assumed that A, f(α) and E a are independent on T, whereas A and E a are independent on conversion rate α.By separating and integrating Equation ( 7), the resulting Ozawa equation is: The straight line obtained by plotting logβ against (1/T P ), and E a values can be determined from the slope (−0.4567E a /R).

Thermochemical Characteristics Analysis
The thermochemical characteristics of three series of NC/HTPB-based high-energy gun propellant samples were measured by STA DSC-TGA at a heating rate of 10 • C/min under nitrogen atmosphere and compared with those of the reference sample M1.Each experiment was repeated three times, and the reported data were the average value of three measurements.The maximum standard deviations for peak temperature and weight loss measured by STA TG-DSC were 0.5 • C and 0.03%, respectively.The DSC and TG curves of reference sample M1 and three series of NC/HTPB-based high-energy gun propellant samples are shown in Figures 2 and 3, respectively.Curve in Figure 2a and curve in Figure 3a are the DSC and TG curves of reference sample M1, respectively.It can be seen that the DSC curve exhibits a broad exothermic peak in the range of 166-253 • C with a peak temperature at 209.4 • C, and the corresponding TG curve reveals a weight loss of 95.40%, which can reasonably be attributed to the decomposition reaction of reference sample M1.Roduit et al. [29] have also reported similar experimental results.Curves in Figure 2b-g and curves in Figure 3b-g are the DSC and TG curves of three series of NC/HTPB-based high-energy gun propellant samples, respectively, which are similar to those of reference sample M1.However, the decomposition temperature and weight loss of three series of NC/HTPB-based high-energy gun propellant samples are slightly lower than those of reference sample M1.Furthermore, Figure 4 presents the DSC curves of reference sample M1 at heating rates of 1, 2, 5 and 10 • C. The decomposition activation energies calculated by the Kissinger and Ozawa methods are 184.0 and 182.4 kJ/mol, respectively, as shown in Table 2.The DSC curves of three series of NC/HTPB-based high-energy gun propellant samples at heating rates of 1, 2, 5 and 10 • C are shown in Figure 5, and the calculated decomposition activation energies are also listed in Table 2.The activation energies of three series of NC/HTPB-based high-energy gun propellant samples can be compared with that of the reference sample M1.The test samples with RDX (NHR05 and NHR10) or TKX-50 (NHT05 and NHT10) have lower decomposition activation energies than reference sample M1.However, the decomposition activation energies of test samples with CL-20 (NHC05 and NHC10) are higher than that of the reference sample M1.It is also found that the decomposition activation energy of a test sample with lower RDX or TKX-50 content (NHR05 or NHT05) is higher than that of a test sample with higher RDX or TKX-50 content (NHR10 or NHT10).However, the test sample with lower CL-20 content (NHC05) has a lower decomposition activation energy than the test sample with higher CL-20 content (NHC10).Previous literatures [30][31][32] have reported that the activation energies of RDX, CL-20 and TKX-50 are 139.7-148.3kJ/mol, 186-188 kJ/mol and 168-184 kJ/mol, respectively.Therefore, the decomposition activation energy of test samples with CL-20 is higher than that of test samples with RDX or TKX-50, which can be reasonably attributed to the fact that the decomposition activation energy of CL-20 is higher than that of RDX and TKX-50.
with a peak temperature at 209.4 °C , and the corresponding TG curve reveals a weight loss of 95.40%, which can reasonably be attributed to the decomposition reaction of reference sample M1.Roduit et al. [29] have also reported similar experimental results.Curves in Figure 2b-g and curves in Figure 3b-g are the DSC and TG curves of three series of NC/HTPB-based high-energy gun propellant samples, respectively, which are similar to those of reference sample M1.However, the decomposition temperature and weight loss of three series of NC/HTPB-based high-energy gun propellant samples are slightly lower than those of reference sample M1.Furthermore, Figure 4 presents the DSC curves of reference sample M1 at heating rates of 1, 2, 5 and 10 °C .The decomposition activation energies calculated by the Kissinger and Ozawa methods are 184.0 and 182.4 kJ/mol, respectively, as shown in Table 2.The DSC curves of three series of NC/HTPB-based highenergy gun propellant samples at heating rates of 1, 2, 5 and 10 °C are shown in Figure 5, and the calculated decomposition activation energies are also listed in Table 2.The activation energies of three series of NC/HTPB-based high-energy gun propellant samples can be compared with that of the reference sample M1.The test samples with RDX (NHR05 and NHR10) or TKX-50 (NHT05 and NHT10) have lower decomposition activation energies than reference sample M1.However, the decomposition activation energies of test samples with CL-20 (NHC05 and NHC10) are higher than that of the reference sample M1.It is also found that the decomposition activation energy of a test sample with lower RDX or TKX-50 content (NHR05 or NHT05) is higher than that of a test sample with higher RDX or TKX-50 content (NHR10 or NHT10).However, the test sample with lower CL-20 content (NHC05) has a lower decomposition activation energy than the test sample with higher CL-20 content (NHC10).Previous literatures [30][31][32] have reported that the activation energies of RDX, CL-20 and TKX-50 are 139.7-148.3kJ/mol, 186-188 kJ/mol and 168-184 kJ/mol, respectively.Therefore, the decomposition activation energy of test samples with CL-20 is higher than that of test samples with RDX or TKX-50, which can be reasonably attributed to the fact that the decomposition activation energy of CL-20 is higher than that of RDX and TKX-50.

Explosion Heat Analysis
The explosion heat analysis of three series of NC/HTPB-based high-energy gun propellant samples was carried out and compared with that of the reference sample M1.Each experiment was repeated three times and the reported data were the average value of three measurements.The maximum standard deviation for explosion heat measured by BC was 26 J/g.All experimental results are also listed in Table 3 for comparison.It is found that the explosion heat of three series of NC/HTPB-based high-energy gun propellant samples is close to and slightly larger than that of the reference sample M1.Previous literatures [33,34] have reported that the explosion heats of RDX, CL-20 and TKX-50 are 5294 J/g, 6084 J/g and 5984 J/g, respectively.Therefore, the explosion heat of test samples with CL-20 (NHC05 and NHC10) is higher than that of test samples with RDX (NHR05 and NHR10) or TKX-50 (NHT05 and NHT10), which can be reasonably attributed to the fact that the explosion heat of CL-20 is higher than that of RDX and TKX-50.This also means that the NC/HTPB-based high-energy gun propellants have the potential to replace the M1 single-base gun propellant.

Mechanical Sensitivity Analysis
The impact and friction sensitivities of three series of NC/HTPB-based high-energy gun propellant samples were determined and compared with those of the reference sample M1.All experimental results are also listed in Table 3.The impact and friction sensitivities of the reference sample M1 are 12.74 J and 240 N, respectively.Three series of NC/HTPBbased high-energy gun propellant samples have lower impact and friction sensitivities than the reference sample M1.This also means that adding HTPB and energetic material (RDX, CL-20 or TKX-50) to the M1 single-base gun propellant can help reduce the mechanical sensitivity.In addition, the mechanical sensitivity of test samples with CL-20 (NHC05 and NHC10) is higher than that of test samples with RDX (NHR05 and NHR10) or TKX-50 (NHT05 and NHT10).

Conclusions
In this study, the characteristics of three series of NC/HTPB-based high-energy gun propellants were explored experimentally and compared with those of the M1 single-base gun propellant.Based on the experimental results and analysis, the following conclusions are drawn: (1) The analysis of thermochemical characteristics indicates that the NC/HTPB-based high-energy gun propellants with RDX or TKX-50 have lower decomposition activation energy than the M1 single-base gun propellant.However, the decomposition activation energy of NC/HTPB-based high-energy gun propellants with CL-20 is higher than that of M1 single-base gun propellant.

Figure 6 .
Figure 6.Pressure-time curves of reference sample M1 and three series of NC/HTPB-based highenergy gun propellant samples obtained by VST with 100 • C/40 h.

( 2 )
The analysis of chemical stability shows that the NC/HTPB-based high-energy gun propellants with RDX, CL-20 or TKX-50 have good chemical stability and are superior to M1 single-base gun propellant.(3) The analysis of explosion heat reveals that the explosion heat of NC/HTPB-based high-energy gun propellants with RDX, CL-20 or TKX-50 are close to and slightly larger than that of M1 single-base gun propellant.(4) The analysis of mechanical sensitivity indicates that the NC/HTPB-based high-energy gun propellants with RDX, CL-20 or TKX-50 have lower impact and friction sensitivities than the M1 single-base gun propellant.Adding HTPB and energetic material (RDX, CL-20 or TKX-50) to the M1 single-base gun propellant can help reduce the mechanical sensitivity.(5) Based on the above conclusions, the NC/HTPB-based high-energy gun propellants have the potential to replace the M1 single-base gun propellant.The combustion performances of NC/HTPB-based high-energy gun propellants will be continuously studied and verified in the future.

Table 1 .
Compositions of test samples.

Table 1 .
Compositions of test samples.

Table 2 .
Activation energy values of decomposition reactions of test samples calculated by the Kissinger and Ozawa methods.

Table 3 .
Experimental results of VST, BC and sensitivity tests.