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Non-Isothermal Kinetic Study of the Thermal Decomposition ofMelamine 3-Nitro-1,2,4-triazol-5-one Salt

Mostafa Najafi,* Ayatallah Kalantari Samangani

Faculty of Science, Department of Chemistry, Imam Hossein University, P.O. Box 16597-55673, Tehran, Irane-mail: mnajafi2000@yahoo.com

Received: November 30, 2010; revised version: May 1, 2011

DOI: 10.1002/prep.201000153

Abstract

Study on thermal behavior of 3-nitro-1,2,4-triazol-5-one(NTO) salts was required to obtain important data for applica-tion purposes. These compounds have been shown to be usefulintermediates for gun propellant ingredients, high energetic bal-listic modifiers for solid propellants and other potential applica-tions. In this paper, thermal decomposition and non-isothermalkinetics of melamine 3-nitro-1,2,4-triazol-5-one salt (MNTO)were studied under non-isothermal conditions by DSC and TGmethods. The kinetic parameters were obtained from analysis ofthe DSC and TG curves by Kissinger and Ozawa methods. Thecritical temperature of thermal explosion (Tb) was 574 K. The re-sults show that MNTO is thermally more stable than NTO whencompared in terms of the critical temperature of thermal explo-sion. Finally, the values of DS#, DH#, and DG# of its decomposi-tion reaction were calculated.

Keywords: Melamine 3-nitro-1,2,4-triazol-5-one Salt,Thermolysis, Non-isothermal Kinetics, Decomposition

1 Introduction

3-Nitro-1,2,4-triazol-5-one (NTO) is a potential high per-formance explosive with low sensitivity to impact andfriction [1,2]. Manchot and Noll first prepared NTO in1905 by nitration of 1,2,4-triazol-5-one (TO) although thestructural assignment was incorrect [3]. Major interest inthe explosive characteristics of NTO, however, developedin 1980s by other groups [4, 5]. Owing to the unique prop-erties, a lot of applications have been proposed for NTO,such as military munitions, plastic bonded explosives [6]and non-azide automobile inflatable air bag systems[7, 8]. The acidic nature of NTO favors the formation of alarge number of salts by deprotonation of the fourth posi-tion of the NTO ring. NTO salts can be classified intometal salts and amine salts. Singh and Felix [9] have re-viewed the preparation, characterization and applications

of more than 50 salts of NTO. Since these salts are alsoinsensitive and high energetic in nature, they have manyspecial features and some potential applications [9–16].We have recently reported [17] the synthesis of five newamine salts of NTO. In this paper, non-isothermal reac-tion kinetics and thermodynamic properties of melamine3-nitro-1,2,4-triazol-5-one salt (MNTO, Scheme 1) werereported. This is quite useful in the evaluation of its ther-mal stability under non-isothermal conditions and in thestudy of its thermal changes at high temperature, NTOsalts can be a good candidate as gun propellant ingredientand high energetic ballistic modifier for solid propellantsand other potential applications.

2 Experimental

2.1 Materials

NTO was prepared by refluxing semicarbazide hydrochlo-ride with formic acid to obtain triazolone followed by ni-tration with nitric acid as previous reported [18]. Itspurity was more than 99.5%. The melamine salt of NTOwas synthesized and characterized by FT-IR and 13CNMR spectroscopy [17]. Other chemicals required for thesynthesis of MNTO were analytical grade and used as re-ceived.

Scheme 1. Structural formula of MNTO.

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2.2 Experimental Equipment

The STA curves (TG & DSC) were obtained by DuPontSimultaneous Thermal Analyzer model STA�1500 Rheo-metric Scientific, in temperature range of 298–673 K atdifferent heating rates (2.5, 5, 10 and 20 K min�1), underargon atmosphere with the flow rate of 30 mL min�1. Thesample holder was made of aluminum open cup and in-strument was calibrated using pure indium and tin parti-cles as standard samples. The mass of sample used wasabout 5 mg.

2.3 Theoretical Background for Non-isothermal Kinetics

Solid-state kinetics reactions were extensively studied bythermal analyses. Because of their complexity in the pro-cesses involved in solid-state reactions, two or more ki-netic models were applied to evaluate kinetic parameters.Two thermal analysis kinetic methods were jointly em-ployed in this study to process the DSC data to calculatethe Arrhenius parameters (the activation energy (Ea) andthe pre-exponential factor (A)) for the compound stud-ied. These methods are as follow:

(a) Kissinger Equation [19]:

lnbi

Tpi2

� �¼ ln

AKREK

� �� EK

RTpið1Þ

Where, b is the heating rate, Tpi the maximum peaktemperature, and R the gas constant.

A straight line was obtained by plotting ln (bi/Tpi2) vs.

reciprocal of the temperature (T�1). This plot exhibitsthat the mechanism of thermal decomposition of thiscompound is of first order [20]. The slope of the line isequal to EK/RTpi. The activation energy and pre-exponen-tial factor were calculated from the slope and intercept ofthe plot, respectively.

b) Ozawa–Flynn–Wall Equation [21]:

logb ¼ logAEo

RGðaÞ�2:315� 0:4567Eo

RTð2Þ

where G(a) is a conversion functional relationship andthe other terms have usual meanings. The degree of con-version is defined as a= (m0�m)/(m0�mf), where m0, m,mf are the initial, actual and final mass of the sample re-spectively. This isoconversional method allows evaluatingthe dependence of the activation energy on the degree ofconversion without the knowledge of the explicit form ofG(a) [22]. At the peak temperature, the degree of con-version (a) at different heating rates would be at a con-stant value [23]. Therefore, Doyle�s approximation [24],Equation (2) was simplified and rewritten by Flynn, Walland Ozawa as:

lnb ¼ const:�0:4567Eo

RTð3Þ

A straight line was obtained by plotting lnb vs. recipro-cal of the temperature (T�1). The overall activationenergy and pre-exponential factor were calculated fromthe slope and intercept of the plot, respectively.

3 Results and Discussion

3.1 Thermogravimetric Analysis Data

Figure 1 shows the TG and DSC curves of MNTO at20 K min�1. The thermal decomposition of MNTO can bedivided into two main stages as were observed by TGcurve. The mass loss with maximum rate was in thesecond stage. On the other hand, in the DSC curve, thereappear one endothermic and one exothermic process,which agree to TG curve. The TG and DSC data profilesat different heating rates has been summarized in Table 1.As seen from Figure 1 and Table 1, the first stage of thethermal decomposition of MNTO salt is connected withits dehydration process in the temperature range 389–473 K with mass loss of 6.7 %. These results are in goodagreement with its respective calculated value of 6.6 %for MNTO·H2O corresponding to the loss of 1 mol H2O.Further decomposition of the MNTO salt occurs between473–623 K accompanied with 63.1 % mass loss that agreeswith the calculated amount of 62.8% (1 mol melamineand 1 mol NO2). The inset of Figure 1 shows that the exo-thermic peak of MNTO at 589.68 K is caused by a rapiddecomposition reaction. From the above analysis, thethermolysis mechanism of MNTO·H2O salt can be postu-lated to proceed as follows:

MNTO �H2O 389�473 K�����! MNTO 473�623 K�����! residue

The DSC traces for NTO are very similar, with a singleexothermic peak with a maximum between 526 and 552 K

Figure 1. TG curve of MNTO salt at a heating rate of20 K min�1. Inset is DSC curve of MNTO salt at similar condi-tions.

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Full Paper M. Najafi, A. K. Samangani

depending on the experimental conditions [4,16, 18,25, 26]. Data reported on the thermal characteristics ofmetal and amine salts of NTO in the literature [9, 27–29],show that NTO derivatives have usually one exothermicpeak in DSC curve, which is similar to that of NTO, butthe peak temperature is lower than that of NTO. Howev-er, the exothermic peak of MNTO in DSC curve is ob-served at higher temperatures than of NTO and other re-ported NTO salts.

3.2 Effect of Heating Rate

Figure 2A and Figure 2B show TG and DSC curves ofthe decomposition of MNTO salt at several heating ratesrespectively. It was found that, with increasing the heatingrate, the dehydration peaks and the decomposition tem-peratures of the MNTO salt shifted to higher tempera-tures. However, peak separation in DSC curves for thedehydration step is smaller than the decomposition step.On the other hand, by increasing the heating rate, DSCcurves clearly appear. Table 1 shows TG profile data cor-responding to increasing heating rate. It can be seen fromTable 1 by increasing heating rate the observed masslosses were close to calculated mass losses.

3.3 Non-Isothermal Kinetic Studies

Non-isothermal DSC studies on MNTO salt have alsobeen done and the corresponding thermograms areshown in Figure 2. The kinetic parameters for the thermaldecomposition of MNTO salt (the apparent activationenergy (Ea) and pre-exponential factor (A)) and the mostprobable kinetic model function were obtained by Kis-singer (Equation (1)) and Ozawa (Equation (2) andEquation (3)) methods. The data needed for the equa-tions, i, ai, b, Ti, Te (onset temperature), and Tp are ob-tained from the DSC curves and summarized in Table 2.

Figure 3 shows the distribution of activation energieswhen degrees of conversion were in the range 0.0032 to0.9960 for thermal decomposition of the MNTO salt. Asseen from Figure 3 activation energies are distributedwithin the range 114.40 to 216.76 kJ mol�1. They show ac-tivation energy making great changes with the increase of

Table 1. TG and DSC profile data of MNTO salt.

Onset TGA DSC Peak Temperatures /K

temperature sdt fdt Temperature Mass loss /%

b/K min�1 Te /K /Ka) /Kb) ranges /K observed calculated Endothermic Exothermic

2.5 548 390 573 390–438 3.9 6.6 412 –438–573 58.7 62.8 – 555

5 562 390 586 390–444 4.5 6.6 419 –444–586 59.1 62.8 – 566

10 569 390 603 390–464 5.1 6.6 427 –464–603 59.5 62.8 – 583

20 587 389 623 389–473 6.7 6.6 436 –473–623 63.1 62.8 – 590

a) Starting decomposition temperature, b) Final decomposition temperature.

Figure 2. (A) Non-isothermal TG thermograms of MNTO salt.(B) DSC thermograms of MNTO salt at different heating rates:(– –) 2.5; (–·–·) 5; (—) 10; and (····) 20 K min�1.

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Thermal Decomposition of Melamine 3-Nitro-1,2,4-triazol-5-one Salt

conversion degree except for the section of 0.1–0.7 (a).The average activation energy we obtained according tothe degree of conversion in the pyrolysis of MNTO saltwas 151.42 kJ mol�1. In this section, activation energychanges faintly, which means that the decompositionmechanism of the process does not transfer in essence orthe transference could be ignored. Therefore, it is feasibleto study the reaction mechanism and kinetics in this sec-tion. The values of kinetic parameters (Ea and log A) andlinear correlation coefficient (r) at various heating ratesof 2.5, 5, 10 and 20 K min�1 are listed in Table 3. FromTable 3, we can find that the activation energy of the exo-thermic reaction is relatively low, showing that this reac-tion easily took place in the temperature range of 573–613 K. Comparing the results obtained by using twomethods show that the values calculated for MNTO byOzawa method are slightly higher than those of Kissinger

method. However, both methods reveal the same trend ofactivation energies for the whole conversion range stud-ies.

The value (Tp0) of the peak temperature (Tpi) corre-sponding to b!0 obtained by Equation (4) taken fromRef. [30] is 488.38 K for MNTO salt.

Tpi ¼ Tp0 þ bbi þ cbi2 þ dbi

3, ð4Þ

where b, c and d are coefficients.The critical temperature of thermal explosion (Tb) ob-

tained from Equation (5) taken from Ref. [22] is 574.13 Kfor MNTO salt.

Tb ¼Eo�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiEo

2�4EoRTp0

p2R

ð5Þ

where E0 is the apparent activation energy obtained byOzawa�s method and R is the gas constant.

Based on the kinetic parameters (Ea and A), the ther-modynamic parameters of activation could be calculated.The entropy of activation (DS¼6 ), the enthalpy of activa-tion (DH¼6 ), and the free energy of activation (DG¼6 ), ob-tained by Equation (6), Equation (7), and Equation (8)[31,32] are 394.01 J mol�1K�1, 127.51 kJ mol�1 and87.71 kJ mol�1, respectively.

Table 2. Data for the decomposition process of MNTO salt determined by DSC at different heating rates.

Data a T/K

point b=2.5 K min�1 b=5 K min�1 b=10 K min�1 b=20 K min�1

1 0.0032 521.05 533.25 537.87 540.402 0.0064 526.61 538.74 544.73 549.503 0.0177 530.94 544.32 550.18 557.924 0.0411 536.81 548.62 552.03 566.815 0.1270 542.33 549.88 560.01 575.586 0.2200 543.95 555.26 564.70 578.587 0.3400 546.49 555.33 565.98 580.878 0.4710 548.76 559.55 568.65 584.159 0.5800 552.15 560.66 571.41 587.58

10 0.6680 552.46 566.04 573.26 589.8311 0.7200 554.19 568.52 578.61 592.6912 0.7900 557.45 571.34 579.40 597.6513 0.8430 560.05 574.02 582.71 601.5014 0.8900 561.62 574.71 584.75 605.0515 0.9290 564.92 577.22 586.37 609.9016 0.9730 568.36 586.07 595.17 618.8117 0.9960 573.91 594.65 606.67 626.72

– – 569.52(Te) 586.71(Te)552.46(Tp) 566.04 (Tp) 582.71(Tp) 589.83(Tp)

Figure 3. The Ea/a curve obtained by Ozawa�s method.

Table 3. Kinetic parameters obtained for the decomposition pro-cess of MNTO.

Method E /kJ mol�1 logA /s�1 r

Ozawa 135.45 11.97 0.9876Kissinger 132.95 11.66 0.9858

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A ¼ ðkBT=hÞ expðDS6¼=RÞ ð6Þ

A expð�Ea=RTÞ ¼ ðkT=hÞ expðDS6¼=RÞexpð�DH 6¼=RTÞð7Þ

DG 6¼ ¼ DH 6¼�TDS 6¼ ð8Þ

where T=Tp0, obtained by Equation (4), 488.38 K; Ea =Ek, the value of Ea calculated by Kissinger�s method,132.95 kJ mol�1; A=Ak, the value of A calculated by Kis-singer�s method, 1011.66 s�1; kB, Boltzmann constant,1.3807 �10�23 J K�1; h, Plank constant, 6.626 � 10�34 J s�1.

Because the kinetic parameters may suffer from thecomplexities imposed by the experimental conditions; adirect comparison of thermal stability of NTO and NTOsalts based on values of Ea is not possible. However, a le-gitimate comparison is possible as far as the conditions arethe same. The critical temperature for thermal explosionof NTO has been obtained using various methods [33]. Huet al. [34] reported that the critical temperature of thermalexplosion for NTO is 528.22 K based on non-isothermalDSC method. Based on the values of thermal stability re-ported in the literature, the thermal stability of NTO de-creases when it is converted into its salts [9]. However, theresults show that the heat resistance of MNTO salt(574.13 K) was higher than NTO and other reported saltsof NTO.

4 Conclusion

The thermal behavior of the melamine 3-nitro-1,2,4-tria-zol-5-one salt (MNTO) was investigated by simultaneousthermogravimetry and differential scanning calorimetry(TG/DSC) at various scan rates. From the DSC and TGcurves, we can clearly see that the decomposition processof MNTO salt can be divided into two steps, which are inaccordance with the dehydration and decomposition pro-cesses. The values of Arrhenius parameters (Ea and A)for decomposition of MNTO that were calculated by theKissinger and Ozawa methods are in close agreementwith each other and other NTO salts values. It was alsofound that the calculated value of the critical temperatureof thermal explosion (Tb) of MNTO is higher than that ofNTO. Thus, MNTO could be used for applications, whichrequires high insensitivity, high thermal stability, and highpercentage of nitrogen.

Acknowledgments

The authors wish to acknowledge the support of the Imam Hos-sein University of Tehran.

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