Thermodynamic Properties, Non-isothermal Reaction Kinetics and Safety of RDX-based Combustible Materials

YANG Wei-tao, ZHANG Yu-cheng, LIU Shao-wu, ZHANG Yuan-bo, WANG Qiong-lin

(Xi′an Modern Chemistry Research Institute, Xi′an 710065,China)

Thermodynamic Properties, Non-isothermal Reaction Kinetics and Safety of RDX-based Combustible Materials

YANG Wei-tao, ZHANG Yu-cheng, LIU Shao-wu, ZHANG Yuan-bo, WANG Qiong-lin

(Xi′an Modern Chemistry Research Institute, Xi′an 710065,China)

TG and DSC were used to investigate the thermal behaviors of the RDX-based microcellular combustible materials. The exothermic reaction kinetics was studied by non-isothermal DSC. The Kissinger and Ozawa methods were applied to investigate the thermal decomposition kinetics of combustible materials. The compatibility between components was evaluated based on the Kissinger method. The evaluation of heat resistance and moisture absorption were conducted. The results show that the decomposition is viewed as a two-step reaction, the first stage is the endothermic melting reaction without mass loss, the second stage is the exothermic reaction with mass loss. The activation energy obtained by Kissinger and Ozawa methods is similar, and activation energies of combustible materials are lower than that of pure RDX. The compatibility between binders (PMMA and CA) and RDX is good, level 1. The critical temperature of thermal explosion is similar to RDX. Other characteristics of new microcellular combusitble materials are superior to the old ones.

combustible materials; thermal decomposition performance; thermal decomposition kinetics; RDX

Received date:2016-10-21； Revised date:2017-01-09

Biography:YANG Wei-tao (1987-), male,Ph.D,research field: application of energetic materials. E-mail: njyangweitao@163.com

Introduction

Microcellular combustible materials based on polymer bonded RDX possess several advantages such as an adjustable energy content, high burning rate and low vulnerability[1-3]. The materials can burn out in a reduced time for a substantial internal surface area, leaving no burn residue. These materials have immense potential for application in combustible cartridge cases and caseless ammunition[4-5]. Furthermore, the fixing of ammunition components by foam and gradient charges can also be achieved in the future applcations such as compact or modular charges.

Thermal decomposition is a fundamental process of combustion. Meanwhile, since the combustible object is always exposed to action of heat when it was in processing, storage or service, the decomposition performance contributes much to the actual security and vulnerability. Thermal analysis is a frequently used tool in propellant research[6-7]. Thermogravimetry (TG) and differential scanning calorimetry (DSC) were extensively employed to investigate the decomposition and safety performance assessing of energetic materials. Kissigner[8], Ozawa[9]and Flynn[10]demonstrated that the DSC technique, based on the linear relation between peak temperature and heating rate, can be used to determine the kinetic parameters of a thermal decomposition (activation energy, rate constant). The Ozawa method is one of the most popular methods for estimating activation energies by linear heating rate and it is the so-called isoconversional method. Simultaneously, compatibility between RDX and a binder, stability of combustible objects and, spontaneous ignition temperature can also be obtained by thermal decomposition analysis.

In the present work, the decomposition of combustible materials composed of polymethylmethacrylate/Hexogeon (PMMA/RDX) and cellulose acetate/Hexogeon (CA/RDX) was studied. Simultaneously, Kissinger and Ozawa dynamic method were used to determine the kinetic parameters of these materials. The calculated activation energy value was used to evaluate the compatibility of components, the stability and the spontaneous ignition temperature.

1 Experimental

1.1 Materials

Cellulose acetate (CA) was supplied by Xi′an North Hui′an Chemical Industries Co., Ltd. Ethanol (AR, Nanjing Chemical Reagent Co., LTD). Poly(methyl methacrylate) (PMMA) was AltuglasV 040. The average diameter of RDX particles is 10 μm. RDX and binders were blended and kneaded in a sigma blender using acetone as a solvent, and extruded by a vertical hydraulic press. The propellants were dried in a water bath oven until the volatile matter was brought to the level of 0.5%.

1.2 Experimental techniques

TG curves were obtained on a TGS-2 in the temperature range of 323-723 K, under dynamic nitrogen atmosphere (20 mL/min). Samples masses were about 1.0 mg, and the heating rate was 20 K/min.

DSC curves were obtained on a Mettler-Toledo HP DSC 827ein the temperature range of 323-573 K, under dynamic nitrogen atmosphere (30 mL/min). Samples masses were about 1.0 mg, and each sample was heated in aluminium oxide pans. Three different heating rates of 10, 20 and 30 K/min were used.

2 Results and Discussion

2.1 Thermal decomposition studies

Figure 1 shows the TG and DTG curves of samples with RDX contents of 60%, 65% and 70%, compared with pure RDX and the binder. As the figure reveals, the decomposition process was composed of the decomposition of RDX with a wide temperature range of 469 to 523 K and the decomposition of binder from 543 to about 693 K. In addition, compared with the curve of pure RDX, the decomposition of the samples was delayed, which may be influenced by the nearby binder around the RDX particles.

As Figure 1 indicated, the decompositions of PMMA/RDX and CA/RDX mixture are a continuous physical and chemical change and comprised of four stages:

Stage 1 (below 473K): volatilization of volatile;

Stage 2 (473-523K): decomposition of energetic component;

Stage 3 (523-688K): decomposition of non-energetic component;

Stage 4 (above 688K): residues which are hard to decompose.

There are some wiggles in Mass loss rate curves between Stage 2 and Stage 3 resulting from the decomposition of inert binder.

Fig.1 TG and DTG curves of PMMA/RDX and CA/RDX composites

The DSC curves show that the first stage is endothermic and the second stage is exothermic. Figure 2 is the DSC curves of PMMA/RDX and CA/RDX composites. From Figure 2, one can find that with the increased RDX content from 60% to 100%, the endothermic peak temperature rises, the endothermic and exothermic heat presents an increasing trend in value; while the exothermic peak temperature keeps almost constant. As Figure 2 shows, the endothermic temperature rises with the increased RDX content with almost similar endothermic heat; the exothermic peak rises slightly with increased RDX content, and the exothermic heat presents an opposite tendency. Comparing the two combustible objects with the same RDX content, CA-based mixture releases more heat during decomposition process.

Comparing Figure 1 with Figure 2, little mass loss is observed during the endothermic event. Thus, the endothermic reaction can be viewed as a melting process of the solid sample. The decomposition is viewed as a two-step reaction sequence A→B→C, where A is the unreacted solid, B is the molten product of the endothermic reaction without mass loss, and C is the product of the exothermic reaction.

Fig.2 DSC curves of PMMA/RDX and CA/RDX composites

2.2 Thermal decomposition kinetics

The method used in the analysis of composite samples was based on DSC experiments in which the temperatures of maximum heat flow were determined from the resulting measured curves for exothermic reactions. DSC curves at different heating rates,β, for RDX and two composite samples are shown in Figure 3.

In order to determine the kinetic parameters of the decomposition step, Ozawa[11]and Kissinger′s methods[12]were applied. They were both derived from the basic kinetic equations for heterogeneous chemical reactions and therefore have a wide application, as it is not necessary to know the reaction order or the conversional function to determine the kinetic parameters. The activation energy determined by applying these methods is the sum of the activation energies of the chemical reactions and physical processes in the thermal decomposition, therefore it is called an apparent activation energy.

The temperatures of the exothermic peaks,Tp, can be used to calculate the kinetic parameters by the Ozawa method. These parameters are the activation energy,E, and the pre-exponential factor,A, relatives to the decomposition process.

Fig.3 DSC curves at different heating rates for pure RDX, PMMA/RDX and CA/RDX composites

A linear relationship between the heating rate (logβ) and the reciprocal of the absolute temperature,T-1p, may be found and the following linear equation can be established:

(1)

Assuming that the rate constant follows the Arrhenius law and that the exothermic reaction can be considered as a single step process, the conversion at the maximum conversion rate is invariant with the heating rate when this is linear. Making these assumptions in account, Eq.1 may be applied to the exothermic peak maximum temperature considering different heating rates. Thus carrying out several experiments at different heating rates, a plot of logβvs. 1/Tpmay be produced and the activation energy can be estimated directly from the slope of the curve.

With the above assumptions, the Kissinger method may also be used to calculate the activation energy and the pre-exponetial factor from the maximum rate condition which will occur at the maximum exothermic peak temperature,Tp.

(2)

Fig.4 Ozawa plot and Kissinger plot for samples at different heating rates

Table 1Eand lnAvalues of RDX, PMMA/RDX and CA/RDX

Samples E/(kJ·mol-1)ln(A/min-1)OzawamethodKissingermethodPureRDX263.4268.463.2PMMA/RDX246.6250.859.1CA/RDX258.4263.161.9

2.3 Evaluation of compatibility between RDX and polymer binders

The DSC method provides data that can be used to evaluat the compatibility for RDX and polymer binder.The levels of compatibility for explosives and contacted materials are listed in Table 2. Level 1: safe for use in any explosive design; Level 2: safe for use in testing, when the device will be used in a very short period of time, not to be used as a binder material, or when long-term storage is desired; Level 3: not recommended for use with explosive items; Level 4: hazardous, do not use under any conditions.

Table 2 The levels of compatibility for explosive and contacted materials

Change of exothermic peak temperature was calculated by Eq.(3):

ΔTp=Tp1-Tp2

(3)

Where,ΔTpis the change value of temperature, K;Tp1is the exothermic peak temperature of mixture, K;Tp2is the exothermic peak temperature of RDX, K.

The relative change rate of apparent activation energy was calculated by Eq.(4)：

(4)

Where,ΔEis the change of activation energy, J/mol;Eais the activation energy of RDX, J/mol;Ebis the activation energy of mixture, J/mol.

Table 3 Compatibility results

2.4 Self-accelerating decomposition temperature (Tp0) and critical temperature of thermal explosion (Tpe)

The critical temperature of thermal explosion is an important parameter required to ensure safe storage and process operations involving explosives, propellants and pyrotechnics. It is by definition the lowest temperature to which a specific charge may be heated without undergoing thermal runaway[13-14].

The values of peak temperature (Tp0) of the sample corresponding toβ→0 can be obtained by substituting theTpandβshown in Figure 4[15]and the results are listed in Table 3.

(5)

whereb,c,dare coefficients.

The corresponding critical temperature of thermal explosion (Tpe) are obtained using Eq. 6 taken from[13, 15], and recorded in Table 4.

(6)

WhereRis the gas constant;Ekis the value ofEcalculated using the Kissinger method.

Table 4 Thermal property data of the samples

2.5 Heat resistance

In this section, the cook-off test[16, 17]was used to investigate the heat resistant properties of microcellular combustible materials composed of RDX and inert binders. 12 mm×12 mm×2 mm slides were placed on a metal plate at the desired temperature and the ignition delay time (tτ) was recorded. Figure 5 presents the values oftτof different formulations compared with traditional felted combustible case material with NC 62%, kraft fibers 24.5%, adhesive and addition agent 13.5%. As Figure 6 indicates, thetτvalues for RDX-based samples decreased rapidly to 2s when temperature exceeded 300℃. Furthermore, the microcellular material was not ignited when the temperature was 200℃. Accordingly, a substitution of RDX for NC could improve the heat resistance performance.

Fig.5 Cook-off time for different formulations

2.6 Moisture absorption

The tested samples were placed in humidity chamber with relative humidity of 98% for 10 hours, the moisture absorption amounts for two basic formulations compared with traditional felted combustible case material were recorded and compared in Figure 6. As Figure 6 reveals, the moisture absorption amount for microcellular material is much lower than that of felted material. For felted cases, the high amounts of hydrophilic substance and high open porosity leads to high moisture absorption amount. In addition, the microcellular combustible materials possess superior performance low moisture absorption, 0.5% for PMMA-based and 1.6% for CA-based samples, respectively.

Fig.6 Hygroscopicity curves of microcellular and feleted samples

3 Conclusions

(1) This article studied the thermal decomposition and performance evaluation of microcellular combustible materials. The DSC method is effective in the study of the thermal decomposition of propellants.

(2) The thermal decomposition behaviour of the propellant is affected by the heating rates used in the experiments.

(3) Kinetic and thermodynamic data obtained allowed a detailed investigation of the thermal stability of the different propellants.

(4) Heat resistance and moisture absorption behavior of the microcellular material is improved compared with traditional felted combustible case material.

Fortunately, to date, many excellent studies focused on energetic materials have been conducted and proposed by scientists all around the world. Therefore, it is anticipated that this study can help the relevant plants or processes to avoid thermal accidents and also provide safer munitions that can be used on the battlefield due to the longer cook-off times.

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RDX基可燃材料的热力学性能，非等温反应动力学及安全性研究

杨伟涛，张玉成，刘少武，张远波，王琼林

(西安近代化学研究所，陕西 西安 710065)

用TG和DSC研究了RDX基微孔可燃材料的热行为，用非等温DSC研究了其放热反应动力学，用Kissinger和Ozawa方法计算了可燃材料的动力学参数；基于Kissinger方法，研究了组分间的相容性；采用耐热和吸湿试验进行了微孔可燃材料的耐热性和吸湿性评价。结果表明，可燃材料的分解被看作是两步反应：第一阶段是吸热熔融反应，无质量损失，第二阶段是放热反应，有质量损失；用Kissinger和Ozawa方法得到的活化能相近，可燃材料的活化能均低于纯RDX的分解活化能；黏结剂PMMA、CA与固体填料RDX相容性好，相容性等级为1级；热爆炸临界温度与RDX相近。新型微孔可燃材料的其他性能优于传统可燃材料。

可燃材料；热分解性能；热分解动力学；RDX

10.14077/j.issn.1007-7812.2017.02.003

TJ55;TQ562 Document Code：A Article ID：1007-7812(2017)02-0019-05