Go Hn,Hou Xio-ting,Ke Xing,Liu Jie,Ho Gzi,Xio Lei,Chen Teng,Zuo Ying-ying,Jing Wei,*

aNational Special Superfine Particle Engineering Research Center,Nanjing University of Science and Technology,Nanjing 210094,Jiangsu,China

bShanxi North Xing'an Chemical Industry Co.Ltd,Taiyuan 030008,Shanxi,China

Effects of nano-HMX on the properties of RDX-CMDB propellant:Higher energy and lower sensitivity

Gao Hana,Hou Xiao-tingb,Ke Xianga,Liu Jiea,Hao Gazia,Xiao Leia,Chen Tenga,Zuo Ying-yingb,Jiang Weia,*

aNational Special Superfine Particle Engineering Research Center,Nanjing University of Science and Technology,Nanjing 210094,Jiangsu,China

bShanxi North Xing'an Chemical Industry Co.Ltd,Taiyuan 030008,Shanxi,China

1.Introduction

Ammonium nitrate explosives such as RDX,HMX and CL-20 get notonly the increase in energy release rate and the enhancement of detonation stability,but also the reduction in mechanical sensitivities such as impact,friction and shock after ultrafine treatment or nanocrystallization[1-7].Therefore they have good prospects for application[8-12].

There are many kinds of energetic additives,such as RDX,HMX,DNTF,ADN and CL-20,which possess high energy,low signature,low corrosion and low residue in composite modified double base(CMDB)propellants[13].Unfortunately,these additives also bring worse mechanical performance,low burning rate,high pressure exponents and harsh process conditions.

[14]In our previous study,nano-HMX was proved to have better detonation performance but lower mechanical sensitivity than RDX.So it is promising for nano-HMX to replace the raw RDX in an original CMDB propellant to obtain better comprehensive performance.Nano-HMX was used to replace the raw RDX in the formulation gradually by 10%mass fraction increase of each sample.Then the burning rate,mechanical property and mechanicalsensitivity of propellant strands with different masses of nano-HMX were tested.

2.Materials and preparation methods

2.1.Materials

The raw RDX(D50=100 μm)and HMX(D50=120 μm)were produced by Gansu Yinguang Chemical Industry Group Co.Ltd.Nano-HMX(D50=62.5 nm)was fabricated on a bi-directional mill designed by Li(Chinese Patent:CN2766956).Other components of CMDB propellant were provided by Shanxi North Xing'an Chemical Industry Co.Ltd.

2.2.Preparation of CMDB propellant

The CMDB propellant samples were manufactured by a solvent free ironing roll calender.RDX mass content was 48.5%,the binder system(NC+NG)was 41.5%,and the aluminum powder,catalyst and other components were 10%altogether.The specific formulas of samples are listed in Table 1.

2.3.Characterization

A constant pressure static burning rate meter was used fordetermining the burning rate of CMDB propellant with target line method according to GJB770A-97 706.1[15].The burning rates of six strips were measured at each pressure,and the mean values were put into the Vieille burning rate equation u=u0pnto calculate the pressure exponent n.

Table 1 Test formulas of samples.

An Autograph DCS material testing machine was employed to test the tensile strength and elongation rate at high temperature(50°C),room temperature(20°C)and low temperature(-20°C)according to the Chinese military standards GJB770A-97 413.1.

Impact sensitivity test was carried out with a WL-1 H3.5-10 W drop hammer instrument according to GJB770A-97 601.2 test method(drop hammer weight:2 kg;sample mass:30±1 mg).Friction sensitivity was tested by a WM-1 friction apparatus according to GJB770A-97 602.1 test method(swinging angle:66°;gage pressure:2.45 MPa).

Explosion heat of each sample was tested using adiabatic method according to GJB770A-97 701.1.

The morphologies of samples were characterized using Hitachi S-4800.

3.Results and discussion

3.1.SEM

As shown in Fig.1(a)and(b),raw RDX tends to have uneven size and a very wide size distribution range.Most of the particles are 50μm or more in diameter.Nano HMX has a D50 about 62.5 nm which was tested by a laser particle size analyzer.The massive RDX particles can be observed from Fig.1(c)obviously but not from Fig.1(d).

3.2.Burning rate

Burning rates of each sample at different pressures are listed in Table 2.Burning rate coefficient and Pressure exponent of each sample were calculated according to the Vieille burning rate equation,and the calculated results are listed in Table 3.It can be seen from Table 3 that the burning rate of each propellant increases with the increase in the pressure and the mass content of nano-HMX.To be specific,compared with sample 1,the burning rate of sample 2,with 10%nano-HMX increased slightly at 6-10 MPa,but decreased at 12-16 MPa along with the pressure exponent.Sample 3 has a roughly equivalent performance with sample 1.The burning rates of sample 4 and 5 all increased at different pressures,and the increase in burning rate of sample 5 was much more obvious.Unfortunately,the processing property of sample 5 was poor,which caused the pressure exponent to increase.

3.3.Mechanical sensitivity

The impact and friction sensitivities of each sample are listed in Table 4.In addition,the mechanical sensitivities of raw RDX,raw HMX and nano-HMX were also tested under the same conditionsfor comparison.As listed in Table 4,the sensitivities of nano-HMX were lower than those of raw HMX,even lower than RDX,which provides a theoretical basis for the design and improvement of CMDB propellant.One can figure out that the impact sensitivity decreases as the mass content of nano-HMX increases.When the mass content of nano-HMX ran up to 40%,the impact sensitivity was 26%,which was lower than that of sample 1,and the friction sensitivity reached 0%.In a word,sample 5 had the best mechanical sensitivities only judged from Table 4.

Table 2 Burning rates of samples at different pressures.

Fig.1.SEMs of(a)raw RDX,(b)nano-HMX,(c)section of 1 and(d)section of 4.

Table 3 The calculated burning rate coefficients and pressure exponents of samples.

Table 4 Impact and friction sensitivities of samples.

3.4.Mechanical property

The tensile strength and elongation rate of CMDB propellant at high temperature(+50°C),room temperature(+20°C)and low temperature(-20°C)are listed in Table 5.The results indicate that Samples 2-5 have higher tensile strengths than those of sample 1 at both high temperature and room temperature,and in the same level with sample 1 at low temperature.Equally,the tensile strength grew when the mass content of nano-HMX was enlarged.For the elongation rate,the regularity was not obvious.Sample has the best elongation rate at room temperature but not the same at other temperature.One can judge that the sample 4 has the best mechanical properties.

3.5.Explosion heat

The data of explosion heat of samples are listed in Table 6.From the view of theory,the more HMX the propellant contains,the higher explosion heat it has.In addition to the components of propellant,the processing performance also matters.As listed in Table 6,only raw RDX is contained in Sample 1 and it was wellprocessed,so the explosion heat of Sample 1 was slightly higher than those of Sample 2 and 3.In the case of Samples 4 and 5,the content of nano-HMX was enough high to offset the loss of energy during processing.

Table 5 The results of tensile strength and elongation rate of each sample.

Table 6 Explosion heat of samples.

3.6.Possible mechanism of action

The mass content of nano-HMX increases gradually to cause the increase in specific surface area of solid filler.It can be found from Fig.2 that the diameter of NC is about 20-30μm,which is smaller than D50 of RDX,but bigger than that of nano-HMX.In case of this,Nano-HMX can make good contact with the binder NC.The sample which contained nano-HMX has better mechanical property.But if the mass content of nano-HMX reaches 40%(Sample 5),the absorbent powder is hard to form a propellant strand.The strand surface of Sample 5 was obviously crispy through observation.

From the burning rate test,one can figure out that the burning rate of propellant with nano-HMX has slightly higher burning rate than that of Sample 1 at the same pressure.This is because nano-HMX has a bigger specific surface area which brings larger contact area with other components in the propellant.When the propellant combusts,the large contact area enlarges the combustion area and improves the heat conductivity,which is favorable for combustion of the propellant.On the other hand,RDX is used as a rate reducer of the original formula.As the mass content of RDX decreases,the burning rate increases.Moreover,HMX has higher energy than RDX with more heat release which has potential of improving the burning reaction of CMDB propellant.

4.Conclusions

Nano-HMX was used in a CMDB propellant by gradually replacing method.The Sample 4 which contains 30%mass content of nano-HMX has the best comprehensive performance through the characterization of burning rate,mechanical sensitivity,mechanical property and explosion heat.Compared with Sample 1,the drop height(H50)is increased by 24.2%,and the friction probability(P)is reduced by 83.3%.Moreover,the burning rate is increased by 5%at the same pressure.At last,the mechanism and the reasonwhy Sample 5 did not have the best properties was explained.The applications of nano-HMX in CMDB propellant will be promising in the future.

Acknowledgements

This work was financially supported by the Youth Science and Technology Innovation of China North Chemical Industry Group Co.,Ltd.,Natural Science Foundation of China(Project No 50972060 and No 51606102),the Weapon Research SupportFund(62201070804),Qing Lan Project,Environmental Protection Scientific Research Project of Jiangsu Province(2016056),a Project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions,the Shanghai Aerospace Science and Technology Innovation Fund(SAST2015020),Basic Product Innovation Technology Research Project of Explosives.

Fig.2.SEM of(a)NC,(b)Absorbent powder of Sample 1,and(c),(d)Absorbent powder of Sample 4.

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A R T I C L E I N F O

Article history:

17 January 2017

in revised form

22 March 2017

Accepted 29 March 2017

Available online 29 March 2017

Nano-HMX

CMDB propellant

Comprehensive performance

Batch preparation of nano-HMX was achieved via a mechanical trituration method.The morphology and particle size of nano-HMX and raw RDX were characterized using SEM.Then nano-HMX was used in a formulation of composite modified double base propellant containing RDX.The method is to use nano-HMX to replace the RDX in the formulation by 10%gradually with the total mass content of RDX and HMX unchanged.The burning rate,mechanical sensitivity and mechanical property of propellant strands with different mass content of nano-HMX were tested.The results indicate that the 30%content of nano-HMX has the best comprehensive performance which can be used as an improvement of the existing formula.A possible mechanism of action was discussed.

?2017 Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

*Corresponding author.

E-mail address:superfine_jw@126.com(J.Wei).

Peer review under responsibility of China Ordnance Society.