Thermal kinetic and thermal safety of Shuangfang-3 gun propellant based on thermal decomposition characteristics

ZHAO Yuxin, TAN Yingxin, MA Jiangeng, ZHANG Huarong, LIU Haihong, GUO Jiaxin

(School of Environment and Safety Engineering， North University of China， Taiyuan 030051， China)

Abstract： Differential scanning calorimetry (DSC) was used to investigate the thermal decomposition and thermal safety characteristics of Shuangfang-3(SF-3) gun propellant. The kinetic calculation of the DSC curve was carried out by Kissinger and Friedman models, and the time to the maximum rate under adiabatic conditions and the self-accelerating decomposition temperature were calculated by using the AKTS thermal analysis software in combination with the heat balance equation. The thermal history experiment was carried out to further analyze the autocatalytic properties of SF-3. The results show that the initial decomposition temperature, decomposition peak temperature, and decomposition completion temperature of SF-3 all move to the high temperature direction with the increase of heating rate, and the average decomposition heat is 1 521.4 J/g. The kinetic model showcased that SF-3 has different reactions in different reaction stages, and its apparent activation energy is 168.2 kJ/mol. When the times to maximum rate under adiabatic conditions are 2.0 h, 4.0 h, 8.0 h, 24.0 h, respectively, the corresponding temperatures are 130.7 ℃, 124.8 ℃, 119.2 ℃ and 110.5 ℃, respectively. When the masses are 5.0 kg, 15.0 kg, 25.0 kg, 50.0 kg, 100.0 kg, respectively, the corresponding self-accelerating decomposition temperatures are 110.0 ℃, 105.0 ℃, 102.0 ℃, 99.0 ℃ and 96.0 ℃, respectively. As the packaging mass increases, it is more difficult to exchange the liberated heat into the surrounding environment and its safety would be further reduced. The thermal history experiment demonstrates that the thermal decomposition of SF-3 is an n-stage reaction and does not have autocatalytic properties. Therefore, the size and ventilation conditions of the sample have a certain impact on the storage stability of SF-3. In the actual production, usage, storage and transportation, sample size and ventilation conditions should be controlled, and practical and effective measures should be taken according to the actual situation.

Key words： explosive； thermal decomposition； autocatalytic properties； double-base propellant

0 Introduction

In the field of high-energy propellant, researchers usually use high-energy solid particles such as hexogen and nitroguanidine in the nitrocellulose/nitroglycerin matrix to increase the energy of the propellant. Double-base propellant is a generally recognized representative propellant. It has the characteristics of high energy, strong ablation, good mechanical properties, and excellent economy.Compared with single-base propellant, it has the advantages of high energy, low moisture absorption, high physical stability and ballistic stability. Therefore, it has larger sizes and more complex shapes. However, its disadvantages are high explosion temperature, severe ablation to the barrel, and greater danger during production[1-3]. There are three major factors for the chemical instability of the double-base propellant： thermal decomposition of nitrate； decomposition acceleration of thermal accumulation； catalytic effect of H+. Scholars have conducted a lot of research on them[4-5]. Based on thermogravimetric differential thermogravimetry (TG-DTG), differential scanning calorimetry (DSC) and other analytical instruments, Kissinger and Ozawa methods were adopted to study the combustion heat and thermal decomposition reaction kinetics of double-base propellant[6-9]. Pourmortazavi et al.[10]studied the catalytic effects of nano-MgO nanoparticles on the thermal behavior and decomposition kinetics of double-base propellants by TG-DTG and DSC. Chen et al.[11]studied the thermal safety of ShuangFang-3(SF-3) gun propellant through spontaneous combustion delay period, ignition temperature and kinetic parameters, and found that the key factor affecting the thermal spontaneous combustion reaction of the propellant was heat accumulation. Kou et al.[12]proposed a method to evaluate the thermal safety of solid propellant charges by measuring the temperature and pressure of the propellant sample in the cooking test to determine reaction kinetics.

At present, for ShangFang-3 (SF-3), a double-base gun propellant, there are few studies on its thermal safety characteristics, such as apparent activation energy, the time to maximum rate under adiabatic conditions, etc. For this reason, we conducted a linear heating experiment by DSC to study the changes of thermal decomposition of SF-3 under different heating rate conditions. Based on the DSC data, the advanced thermal analysis kinetics software (AKTS) is used to calculate the thermal analysis kinetics, the time to maximum rate under adiabatic conditions, and self-accelerating decomposition temperature (SADT) parameters of SF-3, so as to provide a certain supplement to the performance of the existing SF-3 thermal safety.

1 Experiment

1.1 Samples

Samples： SF-3 was made in the laboratory, and its main components (mass fraction) include nitrocellulose (NC), 56.0%; nitroglycerin (NG), 26.5%; and the second centralite, 3%.

1.2 Instruments and experimental process

The DSC131 was produced bySetaram, France. The crucible was made of alumina. Both shielding gas and purge gas were high-purity nitrogen with a flow rate of 30.0 mL/min.

DSC linear temperature rise experiment was carried out as follows： the sample mass was 0.5 mg, the heating rates were 5.0 ℃/min, 10.0 ℃/min, 15.0 ℃/min, 20.0 ℃/min, respectively; and the temperature rose to 400.0 ℃.

Thermal history experiment： the sample mass was 0.5 mg, the heating rate was 10.0 ℃/min, and the interruption rescanning temperatures were 170.0 ℃, 180.0 ℃, 190.0 ℃ and 195.0 ℃, respectively.

2 Results and discussion

2.1 Thermal decomposition behavior of SF-3

Differential scanning calorimeter was used to test SF-3 at different heating rates. The DSC test curve and data are shown in Fig.1 and Table 1, respectively. In the dynamic heating experiment, the initial decomposition temperature can be determined by selecting the intersection of the horizontal baseline and the tangent at the maximum slope of the decomposition temperature or the temperature point where the decomposition temperature just deviates from the baseline. The latter method is adopted in this work for selection of the initial decomposition temperature.

Fig.1 DSC experimental curves of SF-3

Table 1 Experimental results of thermal decomposition characteristics of SF-3 under DSC

From Fig.1 and Table 1, it can be seen that the initial decomposition temperature (Tonset) of SF-3 is within 156.9 ℃-169.0 ℃, the decomposition peak temperature (Tpeak) is within 201.2 ℃-216.2 ℃, the decomposition completion temperature (Toffset) is within 258.6 ℃-283.6 ℃, and the average decomposition enthalpy is 1 521.4 J/g. The initial decomposition temperature, decomposition peak temperature, and decomposition completion temperature move toward high temperature with the increase of heating rate. The reason is short of heat exchange between the sample and the environment. Therefore, the phenomenon of thermal hysteresis[14]occurs.

2.2 Thermal decomposition kinetic analysis

The calculation of kinetics is to study reaction process and reaction mechanism. By analyzing the calorimetric test data with the help of various mathematical methods, we can obtain the kinetic model and parameters, thereby providing a scientific basis for quantitative description of chemical reaction process, thermal risk assessment, industrial amplification prediction, etc. Based on the DSC dynamic experimental results of SF-3 in section 2.1, the Kissinger model[14](Eq.(1)) and Friedman model[15](Eq.(2)) were used to analyze the thermal decomposition kinetics of SF-3.

(1)

(2)

whereβis the different heating rate, ℃/min；αis the conversion rate；Tis the reaction temperature, K；Ais the pre-factor, s-1；Eais the activation energy, kJ·mol-1；Ris the universal gas constant, J·mol-1·K-1；f(α) is the reaction mechanism function;Tpis the peak temperature, K.

Kissinger method to is used calculate the activation energyEaof SF-3, and the result is 168.2 kJ/mol, as shown in Fig.2.

Fig.2 Fitting line of apparent activation energy (Kissinger)

Fig.3 Relationship between apparent activation energy, ln[Af(α)] and conversion rate (Friedman)

Although this method cannot calculatef(α) value, the calculation result is more universal. In the initial stage of the reaction, due to the presence of environmental factors such as noise, it is easy to cause the instability of the instrument signal. Therefore, when calculating the conversion rate, the curve data range is 0.1-0.9, and the step size is 0.1. The analysis shows that SF-3 is relatively gentle at the beginning of the reaction, and rapidly decreases when the conversion rate is in the range of 0.2-0.4, and then abates with the reduction of the conversion rate until the end of the reaction. In order to compare the activation energies of the two calculations, the two methods were plotted in the same graph, as shown in Fig.4.

Fig.4 Comparison of activation energies of SF-3 calculated by different methods

When the conversion rate is low, the activation energy calculated by the Friedman method is larger. When the reaction enters the stable stage, and the activation energy is basically stable, while the activation energy calculated by the Kissinger method is a fixed value, indicating whether the reaction mechanism can be assumed to have a certain influence on the activation energy when performing kinetic calculations.

2.3 The time to maximum rate under adiabatic conditions

The time to maximum rate under adiabatic conditions (tTMR,ad) refers to the time for a substance to reach the maximum reaction rate from the initial state, and it is an important parameter to measure safety accidents in the industrial production process[17]. When calculatingtMR,adunder adiabatic conditions, we regard the experimental curve as composed of many primitives, that is, its apparent activation energy is a function that varies with conversion rate in the process of SF-3 reaction. Assuming that all the heat generated during the thermal decomposition of SF-3 heats the sample, AKTS software is used to calculate the correlation coefficient (R=0.990 1) between the experimental curve and the simulation curve of the thermal decomposition reaction process of SF-3, the results are shown in Fig.5.

Fig.5 Experimental and simulation curves of SF-3 reaction

It indicates that the kinetic parameters have been verified. Combining with the thermal decomposition characteristics of SF-3, when evaluating its thermal safety, we set the time to maximum rate under adiabatic conditions of 2.0 h (TD2), 4.0 h (TD4), 8.0 h (TD8) and 24.0 h (TD24) as an important safety parameter. The adiabatic temperature history of SF-3 is further obtained, as shown in Figs.6-7.

It can be seen from Fig.6 that when the initial temperature of SF-3 is higher thanTD2, the slope of the curve tends to increase, which means the time to maximum rate under adiabatic conditions is less affected by high temperatures. If the starting temperature is lower thanTD24, the slope of the curve is smaller, which means that the time to maximum rate under adiabatic conditions is greatly affected by at low temperatures.TD24of SF-3 is 110.5 ℃, which is much lower than the initial decomposition temperature of 156.9 ℃ in Section 2.1.

Fig.6 tMR,ad of SF-3 at different temperatures

Fig.7 Adiabatic temperature simulation of SF-3 at different temperatures

From the above analysis, it can be seen that during the storage of SF-3, if the temperature is lower than the initial decomposition temperature, such asTD24, it undergos slow thermal decomposition. When the generated heat is not diffused, it will gradually accumulate and cause explosion of SF-3. Therefore, for the thermal safety of SF-3,TD24has more practical reference significance than the initial decomposition temperature. On this basis, the adiabatic temperature history of SF-3 is calculated by simulation software. As shown in Fig.7, the thermal decomposition of SF-3 reaches the maximum reaction rate after about 24 h, and the reaction is out of control, resulting in an explosion. In the range of 130.7 ℃-110.5 ℃, if the initial decomposition temperature of SF-3 further increases, the time to maximum rate under adiabatic conditions gradually decreases. When the temperatures are 110.5 ℃, 119.2 ℃, 124.8 ℃ and 130.7 ℃, respectively, the times for SF-3 to reach the maximum reaction rate are 24.0 h, 8.0 h, 4.0 h and 2.0 h, respectively. It means that the reaction time of thermal runaway lessens rapidly with the increase of temperature, and the danger further augments. Therefore, during the storage of SF-3, we should to avoid putting it in an adiabatic environment or large mass accumulation to prevent heat from being unable to dissipate, forming an adiabatic environment inside it and causing thermal explosion. For safety reasons, the storage environment of SF-3 should be kept ventilated, and the time to maximum rate under adiabatic conditions should be regarded as an important safety parameter.

2.4 Self-accelerating decomposition temperature

The SADT under different packaging quality is calculated using the DSC experimental curve and kinetic parameters obtained in Section 2.1[17-19], SADT means that the actual packaging items happen within 7 days since the accelerate the decomposition of the minimum ambient temperature. Because it is related to the physical and chemical properties of the reactants as well as the packaging quality and the mass of packaging materials, it has important reference significance in the thermal risk assessment, storage and transportation of energetic materials and hazardous chemicals.

When the masses are taken as 5.0 kg, 15.0 kg, 25.0 kg, 50.0 kg and 100 kg, respectively, the SF-3 has the corresponding self-accelerating decomposition temperatures of 110.0 ℃, 105.0 ℃, 102.0 ℃, 99.0 ℃ and 96.0 ℃ within 7 days, as shown in Fig.8. As the mass of the packaging increases, the unit area and environmental heat dissipation area become smaller.

Fig.8 Relationship between SF-3 self-accelerating decomposition temperature and packaging mass

The heat generated by the decomposition of the sample is more difficult to exchange into the external environment, which is prone to heat accumulation. The self-accelerating decomposition temperature gradually abates, and its safety also reduces. Therefore, in actual applications, small mass packaging should be adopted as much as possible, and a good ventilation environment should be ensured for safety during production, transportation and storage. This calculation method can quickly determine the SADT value, avoiding the time-consuming and high cost of such large-scale experiments.

2.5 Thermal history experiment

The thermal history experiment is based on the linear heating experiment. The general process is as follows: first, the sample is heated to the interrupted temperature, then cooled to room temperature, and finally heated again to complete decomposition. Through this method, we can judge whether the sample has autocatalytic properties and analyze the risk of the sample. In the thermal history experiment, if the initial decomposition temperature and the decomposition peak temperature are both lower than those of the linear heating experiment, it is determined that the sample has autocatalytic properties[20]. The interrupted temperature is betweenTonsetandTpeakin this section, andTpeak1(206.6 ℃) corresponds to a conversion rate of 0.426 7. As shown in Fig.9, we use a thermal history experiment to study the autocatalytic properties of SF-3. The specific experimental conditions and results are shown in Table 2.

Fig.9 Thermal history curve of SF-3

It can be seen from Fig.9 and Table 2 that under the conditions of linear heating and no thermal history, the initial decomposition temperature and decomposition peak temperature of SF-3 are 163.3 ℃ and 206.6 ℃, respectively. After the thermal history experiment, when the interruption rescanning temperatures are 170 ℃, 180 ℃, 190 ℃, 195 ℃, respectively, the initial decomposition temperature range is 161.4 ℃-170.7 ℃, and the decomposition peak temperature range is 200.8 ℃-208.6 ℃. According to the judgment of the thermal history method, the thermal decomposition of SF-3 is ann-stage reaction and does not have autocatalytic properties. This indicates that even though NC is the main component of SF-3 and NC has autocatalytic properties[21], because NG is dispersed in NC, SF-3 does not have autocatalytic properties.

Table 2 Thermal history test results of SF-3

3 Conclusions

TheTonset,Tpeak, andToffsetobtained from the dynamic DSC experiment of SF-3 all move to the high temperature direction with the increase of the heating rate. The ranges ofTonset,TpeakandToffsetare 156.9 ℃-169.0 ℃, 201.2 ℃-216.2 ℃ and 258.6 ℃-283.6 ℃, respectively, and the average heat of decomposition is 1 521.4 J·g-1. The apparent activation energy of SF-3 calculated by Kissinger method is 168.2 kJ·mol-1. According to the Friedman method, the thermal decomposition process of SF-3 is more complicated, with different reactions in different conversion processes, and its activation energy first decreases rapidly and then gradually lessens with the change of conversion rate.

At 2.0 h, 4.0 h, 8.0 h and 24.0 h, SF-3 reaches the maximum reaction rate, and its adiabatic temperatures are 130.7 ℃, 124.8 ℃, 119.2 ℃ and 110.5 ℃, respectively. The thermal runaway reaction timelessens rapidly with the increase of temperature. With the increase of packaging mass (5 kg, 15 kg, 25 kg, 50 kg, 100 kg), it is more difficult for the heat generated to exchange into the external environment. Therefore, during the storage process, we should avoid putting it in an adiabatic environment or mass accumulation. Besides, the thermal history experiment shows that the thermal decomposition of SF-3 is ann-stage reaction, which does not have autocatalytic properties.