Influence of temperature on mechanical stimulation threshold of typical liquid propellant

JIANG Yuxuan,JIN Shanpin,2,JIANG Rongpei,TANG Yulin, WU Xingliang,XU Feiyang,XU Sen,LIU Dabin

(1. School of Chemical Engineering,Nanjing University of Science and Technology,Nanjing 210094,China； 2. Dafang County Public Security Bureau of Guizhou Province,Bijie 551600，China； 3. Beijing Institute of Aerospace Technology,Beijing 100074,China； 4. Shanghai Space Propulsion Technology Research Institute,Shanghai 201109,China)

Abstract：In order to study the variation of temperature to mechanical stimulation threshold of typical liquid propellants (ADN-based HAN-based and nitromethane),the critical impact energy and critical friction of three propellants under different temperatures were studied by using BAM fall hammer impact sensitivity tester and BAM friction sensitivity tester.Experiments show that under 80 ℃,60 ℃,40 ℃ and 20 ℃,the critical impact energy of HAN-based are 20 J,15 J,15 J,15 J；the critical impact energy of nitromethane are 2 J,2 J,2 J,2 J；and the critical impact energy of ADN-based are <1 J,3 J,7.5 J,15 J.It reveals that HAN-based propellant has the highest critical impact energy,while nitromethane propellant has the lowest critical impact energy.ADN-based propellant has a notable decrease on its critical impact energy with temperature decreasing,indicating that temperature has a significant effect on impact sensitivity of ADN-based propellant.The critical friction of three samples are all higher than 360 N at 80 ℃,60 ℃,40 ℃ and 20 ℃,which shows that the samples are not sensitive to friction,and temperature has no significant effect on the critical friction of three samples.The mechanical stimulations that may be encountered during the production and use of liquid propellants are analyzed,which takes certain working conditions and the temperature coupling effect into consideration,thereby providing support for safety management of liquid propellants during production and storage process.

Key words：liquid propellant;hydroxylamine nitrate (HAN-based)；nitromethane；ammonium dinitramide (ADN-based)；impact sensitivity；friction sensitivity；temperature

0 Introduction

Hydroxylamine nitrate (HAN-based),nitromethane and ammonium dinitramide (ADN-based)propellant are widely used in different fields.Wang et al.[1]invented a stabilizer for HAN,which helps to improve the safety of storage and transportation of HAN.Liu et al.[2]added Fe3+into HAN to find out the effect of metal ions on the stability of HAN.Lu[3]made a research on the decomposition mechanism of nitromethane to make an explanation for the reaction process.Ye et al.[4]measured and calculated the detonation temperature of nitromethane,the results were similar to those recorded.Lei et al.[5]studied KDN crystal category and found out that it is mechanically insensitive.However,mechanical collisions such as friction and impact are inevitable during storage and transportation,which will lead to heat accumulation withinin a narrow space,sometimes even cause accidents[6].Therefore,it is necessary to study the mechanical sensitivity of propellants sysmatically,thereby providing data support for propellants classification.

Mechanical sensitivity is one of the key factors for evaluating the safety of energetic materials.Scientists all over the world have made a great deal of research on sensitivity characteristics of energetic materials[7-12].Liu et al.[13]made a research on manufacturing process and found that the smaller particle size,the narrower particle distribution,the more regular surface morphology and shape contribute to lower impact sensitivity of HNIW.Wang et al.[14]investigated the influence of manufacturing temperature on the coating process and impact sensitivity of F2604/HMX composite particles,and concluded that with the rise of temperature,the coating effect of the F2604/HMX system first went up and then decreased.In addition,the coating effect is the best at 50 ℃,and the impact sensitivity is also the lowest.Gao et al.[15]studied the influence of polymer additives on the impact sensitivity and functional capacity of explosive B.The results show that adding resin can significantly improve the performance of explosive B,even reduce its impact sensitivity.The researchers also found that specific energetic materials have a higher mechanical sensitivity under high temperatures[16-17],so it is necessary to study the effect of temperature on energetic materials.

The temperature gradient of the mechanical sensitivity of different energetic materials is also of interest to researchers.Xie and Lu et al.[18]used spray crystallization to refine HNIW (Hexanitrohexaazaisowurtzitane or CL-20)and tested the mechanical sensitivity of the refined samples,and figured that compared to coarse-grained HNIW,the ultrafine HNIW has lower impact sensitivity,while the friction sensitivity between both sides has little difference.Jiang et al.[19]tested mechanical sensitivity of mixed pyrotechnic agents under different temperatures.The results show that the impact sensitivity of the agent increases as the temperature increases,while the friction sensitivity varies from one agent to another.Tan et al.[20]made an analysis on how temperature,humidity,particle size and other factors affect the mechanical sensitivity of pyrotechnic agents.The results show that the mechanical sensitivity of the medicine is proportional to temperature,and its rate depends on the composition of the medicine.

The domestic explosives industry uses the impact and friction sensitivity test methods of former Soviet Union to test the sensitivity of explosives.The test standard is GJB772A[21]，including the explosion frequency method and drop height method.These methods can compare the safety of different energetic materials,thus contributing to domestic blasting fields.However,it is impossible to obtain the critical response energy of energetic materials under mechanical stimulation with these methods,which also leads to the fact that current data containing the sensitivity characteristics of energetic materials cannot help to provide sufficient theoretical basis for the safe production process of explosives.At the same time,these two methods are only suitable for solid energetic materials.In the aspect of liquid energetic materials,foreign researchers have carried out a large number of studies by using BAM impact sensitivity and friction sensitivity instrument[22].

This paper uses BAM impact sensitivity instrument and friction sensitivity instrument to investigate the mechanical sensitivity of the three typical liquid propellants,and studies their critical reaction energy to compare the sensitivity at different temperatures,thus providing direct data support for production,storage,transportation,use and disposal processes.

1 Test samples and devices

1.1 Test samples

Set test samples are as follows:reference sample,recrystallized cyclotrimethylenetrinitramine (RDX)；sample 1,HAN-based bipropellant；sample 2,nitromethane；sample 3,ADN-based bipropellant.The physical and chemical properties of HAN and ADN are shown in Table 1.

Table 1 Physical and chemical properties of HAN and ADN

The samples were placed in an oven at 20 ℃,40 ℃,60 ℃,75 ℃ and 80 ℃ for 3 h.

1.2 Test device and method

1.2.1 Impact sensitivity test

The German BAM impact sensitivity tester of Test 3(a)(ii)in “Proposal on the Transport of Dangerous Goods,Test and Standard Manual”[21]was used to carry out the critical impact energy test of the samples.

1.2.2 Friction sensitivity test

The German BAM friction sensitivity tester of Test 3(b)(i)in “Proposal on the Transport of Dangerous Goods,Test and Standard Manual”[21]was used to carry out the critical friction test of the samples.

2 Results and discussion

2.1 Test results and analysis of critical impact energy

The results of the four samples are shown in Tables 2-5.The critical impact energy value of the RDX after recrystallization is 5 J,as shown in Table 2,which keeps pace with “Proposal on the Transport of Dangerous Goods,Test and Standard Manual”[21],indicating that the test system is accurate.

Table 2 Critical impact energy test results of standard RDX

Table 3 Critical impact energy test results of sample 1

It can be seen from Table 3 that the critical impact energy of sample 1 is 15 J at 20 ℃,40 ℃,60 ℃ and 75 ℃.In this temperature interval,as temperature increases,the impact sensitivity remains the same.However,when the temperature goes up to 80 ℃,the critical impact energy of sample 1 suddenly comes up to 20 J.The reason for this phenomenon is that the sample needs to be kept under 80 ℃ for 3 h before testing,but hydroxylamine nitrate has poor thermal stability,so sample 1 is partially decomposed before the test,which leads to an increase on the critical impact energy of sample 1.According to the analysis,the main components of sample 1 are water and hydroxylamine nitrate (composed of the reducing component NH3OH and the oxidizing component NO3[23-24]).When it comes to the constant temperature process in the sample cell,the amount of sample is so little that the water vapors fast.After the volatilization,the remaining hydroxylamine nitrate in liquid sample 1 is decomposed.The specific reaction is as follows[25-28]

HAN→NH2OH+HNO3→NOx+H2O.

(1)

NOxexists as gas among the decomposition products,and it will overflow the sample during the heat preservation process,while the H2O in the decomposition product exists as liquid and remains in the sample.H2O reduces its initial reaction energy to a certain extent when sample 1 is stimulated by impact,causing the impact energy of sample 1 increase.In general,no matter how temperature changes,the impact sensitivity of sample 1 does not change.

The test results of sample 2 are shown in Table 4.

Table 4 Critical impact energy test results of sample 2

It can be seen from the table that the critical impact energy of sample 2 is 2 J at different temperatures.No matter how the temperature changes,the impact sensitivity of sample 2 remains at 2 J,which shows that sample 2 is quite sensitive to impact compared to sample 1,and the sample sensitivity is always at a high level,despite how the temperature changes.This is due to the pyrolysis mechanism of nitromethane.Also,the temperature change has little effect on the generation of free radicals,and the impact can break the chemical bonds in the molecule and generate free radicals.The speed-determining step of sample 2 is the cleavage of C-N bond to form diradicals or isomerization to generate CH3ONO.Among them,the cleavage possibility of C-N bond is greater[29-30].The impact can provide initial energy for the disconnection of the C-N bond,thereby stimulating the occurrence of the reaction.Therefore,sample 2 shows high sensitivity on impact at different temperatures.The C-N bond dissociation energy barrier of sample 2 is 87.8 kcal/mol[31].Shao et al.[32]also found that the self-heating decomposition temperature of sample 2 is 161 ℃,which is much higher than the test temperature (80 ℃)in this experiment,and the fracture of the bond and C-N bond has little effect on its decomposition temperature.Therefore,the impact sensitivity of nitromethane is the same at different temperatures.

The test results of sample 3 at different temperatures are shown in Table 5.

Table 5 Critical impact energy test results of sample 3

It can be found that the critical impact energy of sample 3 is relatively high at room temperature,but the critical impact energy decreases rapidly when the temperature rises.The critical impact energy is 15 J under 20 ℃,7.5 J under 40 ℃,3 J under 60 ℃.The critical impact energy at 80 ℃ is even lower than the minimum scale (1 J).This indicates that sample 3 has a relatively low sensitivity at homeothermy.Once the temperature of the sample increases,the critical impact energy of the sample will rapidly decrease and the sample becomes quite sensitive,indicating that the temperature has a significant impact on the impact sensitivity of sample 3.The analysis results point out that,on the one hand,the increase of temperature will cause the methanol volatilization of the sample and increase the concentration of ADN in sample 3,thereby improving its impact sensitivity.On the other hand,the increase of temperature can promote the activation and decomposition of dinitramide,thereby generating gaseous nitrogen oxides.As the temperature increases,the rate of this conversion will increase remarkably,so the impact sensitivity increases significantly with the temperature.The specific reaction equation[33]is as follows

(2)

(3)

In the reaction process,N2O and NO2exist as gases,and they will gradually volatilize during the preservation process to push the balance to the right side,and then push the remaining substances to further decompose and make the reaction continue.

2.2 Test results and analysis of critical friction

Based on the recrystallized RDX,the accuracy of the friction sensitivity instrument is verified.The critical friction value of the RDX obtained by this experimental instrument after recrystallization is 128 J,which is consistent with the “Proposal on the Transport of Dangerous Goods,Test and Standard Manual”[21],and the test results of the reaction device can be considered accurate.

The critical friction test results of the three samples are shown in Table 6 and Fig.1.

Table 6 Critical friction energy test results of calibration sample and three samples

Fig.1 Critical friction test results of three samples

It can be seen from Table 6 that the critical friction of the three samples at 20 ℃,40 ℃,60 ℃ and 80 ℃ are all greater than 360 N,which shows that the three samples are relatively stable to frictional stimulation,hard to react and have good friction sensitivity.Analysis indicates that the sample is in a liquid state and has certain fluidity.During the friction test,the flow of the liquid sample reduces the frictional contact surface,so it is not sensitive to frictional stimulation.

3 Conclusions

1)Ranging from 20 ℃ to 80 ℃,the temperature has little effect on the impact sensitivity of HAN-based samples because the critical impact energy of HAN-based samples is always 15 J.

2)Ranging from 20 ℃ to 80 ℃,the critical impact energy of nitromethane is 2 J,indicating that nitromethane is quite sensitive to impact,but the temperature will not significantly affect the impact sensitivity of nitromethane.

3)Ranging from 20 ℃ to 80 ℃,the critical impact energy of ADN-based samples reduces from 15 J to under 1 J,which indicates that temperature has a significant influence on the impact sensitivity of ADN-based samples.

4)The friction sensitivities of the three samples at different temperatures are all greater than 360 N,indicating that the three samples are not sensitive to friction,and the temperature has no effect on the friction sensitivity of the three samples.