Non-isothermal Decomposition Kinetics,Specific Heat Capacity and Adiabatic Time-to-explosion of Cu(pn)2(FOX-7)2

GONG Xiang, SUN Qian, XU Kang-zhen, SONG Ji-rong, ZHAO Feng-qi

(1. School of Chemical Engineering, Northwest University, Xi′an 710069, China; 2. Xi′an Modern Chemistry Research Institute, Xi′an 710065, China)

1 Introduction

1,1-Diamino-2,2-dinitroethylene (FOX-7) is a high-energy material with high thermal stability and low sensitivity to impact and friction[1-2]. Since first reported in 1998, FOX-7 has been considered as a research emphasis of energetic materials and will be used in insensitive ammunition and solid propellant. FOX-7 is a representative “push-pull” nitro-enamine, which possesses a highly polarized carbon-carbon double bond with positive and negative charges being stabilized by the amino group and nitro group respectively, and presents certain acidic properties[3-9]. Many researches have been studied on the synthesis[5-6], mechanism[4], molecule structure[2], theoretical calculation[11], thermal behavior[12], explosive performance and application[28]of FOX-7. Existing in manifold tautomers and resonances, FOX-7 can react with some nucleophiles to prepare many new energetic derivatives[11]. Some energetic salts, such as potassium salt, rubidium salt, cesium salt and guanidine salt, have been reported[11-12]. Other salts and metal complexes of FOX-7 also can be synthesized through replacement reaction, such as Cu(NH3)2(FOX-7)2, Cu(CH3NH3)2(FOX-7)2, [Cu(en)2(FOX-7)2(H2O)]·H2O, [Cu(phen)2FOX-7]Cl·3H2O, Zn(NH3)2(FOX-7)2and Zn(en)2(FOX-7)2[14-17].

Many energetic Cu(Ⅱ) complexes were often used as detonating explosive or combustion catalyst of solid propellant[18-21], so we hope that Cu-FOX-7 complexes can also be used as energetic catalyst. Cu(pn)2(FOX-7)2is a new typical FOX-7 complex, and its synthesis and crystal structure have been reported[16]. In this paper, we studied the decomposition kinetics of Cu(pn)2(FOX-7)2, determined specific heat capacity and calculated adiabatic time-to-explosion for further estimating its thermal stability.

2 Experimental

2.1 Synthesis

All chemicals used in synthesis were analytical-grade commercial products. FOX-7 came from Xi'an Modern Chemistry Research Institute (purity>99%). K(FOX-7)·H2O was prepared according to the ref.[13].

Cu(pn)2(FOX-7)2(pn=1,3-diaminopropane) was prepared according to ref.[16] as follows: K(FOX-7)·H2O (2 mmol) and Cu(NO3)2·3H2O (1 mmol in 3 mL water) were stirred in 1,3-diaminopropane solution (78 mmol) for 30 min to give a clear solution at room temperature. Gradually purple crystals slowly appeared and were identified as Cu(pn)2(FOX-7)2. (yield 46%, 0.233 g). IR (KBr,ν/cm-1): 3408, 3294, 3226, 2931, 2359, 2025, 1659, 1500, 1344, 1281, 1132, 1029, 926, 829, 741, 681, 501. Anal. Calcd.(%) for C10H26N12O8Cu: C 23.21, H 5.32, N 33.68; found: C 23.14, H 5.57, N 33.22.

2.2 Physical Measurements

The DSC experiments were performed using a DSC200 F3 apparatus (NETZSCH, Germany) under a nitrogen atmosphere at a flow rate of 80 mL·min-1. The heating rates were 5.0, 7.5, 10.0 ℃·min-1and 12.5 ℃·min-1from ambient temperature to 350 ℃, respectively. The TG/DTG experiment was performed using a SDT-Q600 apparatus (TA, USA) under a nitrogen atmosphere at a flow rate of 100 mL·min-1. The heating rate was 5.0 ℃·min-1from ambient temperature to 350 ℃. The specific heat capacity was determined using a Micro-DSCⅢ apparatus (SETARAM, France). The heating rate used was 0.15 ℃·min-1from 10 ℃ to 80 ℃. The sample mass was 115.7 mg.

The impact sensitivity was determined by using a ZBL-B impact sensitivity instrument (NACHEN,China). The mass of drop hammer is 2.5 kg. The sample mass for every test is 30 mg.

3 Results and Discussion

3.1 Thermal Decomposition Behavior

DSC curves of Cu(pn)2(FOX-7)2at various heating rates are shown in Fig.1. TG-DTG curve of Cu(pn)2(FOX-7)2sample at a heating rate of 5.0 ℃·min-1is given in Fig.2.

Fig.1 DSC curves of Cu(pn)2(FOX-7)2

Fig.2 TG/DTG curve of Cu(pn)2(FOX-7)2at 5 ℃·min-1

Fig.1 shows that the DSC curves of Cu(pn)2(FOX-7)2exhibit two exothermic peaks, which are in agreement with the results of TG/DTG, and the peak temperatures go up with the increase of heating rate. Fig. 2 illustrates that the thermal decomposition of Cu(pn)2(FOX-7)2can be divided into two decomposition processes. The first is an intense decomposition process, which occurs at 140-185 ℃ with a mass loss of 35.30%. The extrapolated onset temperature, peak temperature and heat of decomposition are 155.47 ℃, 156.49 ℃ and 816.5 J·g-1at the heating rate of 5.0 ℃·min-1. The second stage is a slow decomposition process at the temperature range of 185-270 ℃ with a mass loss of about 14.19%. The peak temperature is 215.8 ℃ at a heating rate of 5.0 ℃·min-1. The final residue at 350 ℃ is about 41.74%. Comparing with the thermal decomposition of Cu(NH3)2(FOX-7)2[14], they exhibits similar thermal decomposition processes, but the thermal stability of Cu(pn)2(FOX-7)2is slightly lower than that of Cu(NH3)2(FOX-7)2, which is due to the introduce of long carbon chain.

3.2 Non-isothermal Decomposition Kinetics

In order to obtain the kinetic parameters(the apparent activation energy (E) and pre-exponential factor (A)) of the first exothermic decomposition process, Kissinger method[21]and Ozawa method[22]were employed. The determined values of the beginning temperature (T0), extrapolated onset temperature (Te) and peak temperature (Tp) at the different heating rates are listed in Table 1. The values ofT00andTe0[22]corresponding toβ→0 obtained by Eq. (1) are also listed in Table 1.

T0i or ei=T00 or e0+nβi+mβi,i=1-4

(1)

wherenandmare coefficients.

The calculated kinetic parameters (EandA) in Table 1 show that theEobtained by Kissinger method is consistent with that by Ozawa method. The linear correlation coefficients (r) are all close to 1. So, the result is credible.

Tversusα(the conversion degree) curves at different heating rates are shown in Fig.3. The values ofEOfor any given value ofαwere obtained and shown in Fig.4. The values ofEOsteadily distribute from 142 to 158 kJ·mol-1in theαrange of 0.175-0.875, and the average value ofEOis 151.9 kJ·mol-1, which is in approximate agreement with that obtained by Kissinger method and Ozawa method from only peak temperature values (163.5 and 162.3 kJ·mol-1, respectively). So, the values were used to check the validity ofEby other methods.

The integral equations (The general integral equation, The universal integral equation, MacCallum-Tanner equation,atava-esták equation and Agrawal equation) were cited to obtain the values ofE,Aand the most probable kinetic model function [f(α)] from each DSC curve[24]. Forty-one types of kinetic model functions taken from Ref. [24] and experimental data form each DSC curve were put into the above five integral equations for calculating, respectively. The values were obtained and shown in Table 2. So, the most probable kinetic model function is classified asf(α)=3α2/3(No. 23 equation, Mampel power law,n=1/3), according to the unanimity rule of calculation results from each model equation[24]. The kinetic equation can be described as:

Table 1 The values ofT0,Te,Tp,T00,Te0and kinetic parameters of the first exothermic decomposition process for Cu(pn)2(FOX-7)2determined from DSC curves at various heating rates (β)

β/℃·min-1T0/℃Te/℃Tp/℃T00/℃Te0/℃EK/kJ·mol-1log(A/s-1)rKEO/kJ·mol-1rO5.0147.3155.5156.67.5150.4159.3160.110.0152.9161.7162.012.5155.1163.6165.2139.8145.6163.517.830.9908162.30.9915

Note: Subscript K, data obtained by Kissinger method; subscript O, data obtained by Ozawa method.

(2)

Fig.3Tvsαcurves for the decomposition reaction of Cu(pn)2(FOX-7)2at different heating rates

Fig.4EOvsαcurve for the decomposition reaction of Cu(pn)2(FOX-7)2by Ozawa method

Table 2

β/℃·min-1Eq.E/kJ·mol-1log/(A/s-1)r5.0thegeneralintegralequation189.921.00.9665theuniversalintegralequation187.319.40.9656MacCallum-Tannerequation189.820.90.9688?atava-?estákequation187.420.70.9688Agrawalequation189.921.00.96657.5thegeneralintegralequation180.519.80.9694theuniversalintegralequation178.018.20.9687MacCallum-Tannerequation180.419.80.9716?atava-?estákequation178.519.60.9716Agrawalequation180.519.80.969410.0thegeneralintegralequation154.716.70.9785theuniversalintegralequation152.215.10.9778MacCallum-Tannerequation154.416.60.9803?atava-?estákequation154.016.60.9803Agrawalequation154.716.60.978512.5thegeneralintegralequation133.714.10.9743theuniversalintegralequation131.312.60.9734MacCallum-Tannerequation133.314.00.9768?atava-?estákequation134.114.10.9768Agrawalequation133.714.10.9743mean163.917.5

3.3 Self-accelerating Decomposition Temperature and Critical Explosion Temperature

The self-accelerating decomposition temperature (TSADT) and critical temperature of thermal explosion (Tb) are two important parameters required to ensure safe storage and process operations for energetic materials and then to evaluate the thermal stability[24-25].TSADTandTbcan be obtained by Eq. (3) and Eq. (4), respectively.

TSADT=Te0

(3)

(4)

TSADTandTbfor Cu(pn)2(FOX-7)2are 145.6 ℃ and 146.7 ℃, respectively, which are similar with those of Cu(NH3)2(FOX-7)2as 145.5 ℃ and 156.2 ℃[26], but much lower than those of FOX-7 as 206.0 ℃ and 207.1 ℃[27]. Admittedly, the thermal stability of FOX-7 all declines when it becomes salts or complexes, and the decomposition process also becomes severe.

3.4 Specific Heat Capacity

Figure 5 shows the result of Cu(pn)2(FOX-7)2measured by a continuous specific heat capacity mode of Micro-DSCⅢ. In determined temperature range, specific heat capacity presents a good quadratic relationship with temperature. Specific heat capacity equation of Cu(pn)2(FOX-7)2is :

cp=-2.6824+1.9441×10-2T-2.0494×10-5T2

(285.0 K

(5)

wherecpis the specific heat capacity in J·g-1·K-1.

The molar heat capacity of Cu(pn)2(FOX-7)2is 653.79 J·mol-1·K-1at 298.15 K.

Fig.5 Determination results of the continuous specific heat capacity of Cu(pn)2(FOX-7)2

3.5 Adiabatic Time-to-explosion

The adiabatic time-to-explosion[24, 28]is also an important parameter for evaluating the thermal stability of energetic materials and can be calculated by Eqs. (6) and (7).

(6)

(7)

whereTis the absolute temperature in K,tis the adiabatic time-to-explosion in s,Qis the exothermic values in J·mol-1,Ais the pre-exponential factor in s-1,Eis the apparent activation energy of the thermal decomposition reaction in J·mol-1,Ris the gas constant in J·mol-1·K-1,f(α) is the most probable kinetic model function andαis the conversion degree.

The adiabatic time-to-explosion equation is:

(8)

where the limit of temperature integration is fromT00toTb.

In fact, the value ofαof energetic materials from the beginning thermal decomposition to thermal explosion in the adiabatic conditions is very small, and it is very difficult to obtain the most probable kinetic model function [f(α)] at the process. So, Power-low model [Eq.(9)], Reaction-order model [Eq.(10)] and Avrami-Erofeev model [Eq.(11)] were separately used to estimate the adiabatic time-to-explosion[24, 29]. The calculation results are listed in Table 3.

f(α)=nα(n-1)/n

(9)

f(α)=(1-α)n

(10)

f(α)=n(1-α)[-ln(1-α)](n-1)/n

(11)

Table 3 The calculation results of adiabatic time-to-explosion

equationrateordermodeltime/sEq.9n=1f(α)=125.55n=2f(α)=2α1/266.86n=3f(α)=3α2/377.39n=4f(α)=4α3/476.48Eq.10n=0f(α)=125.55n=1f(α)=1-α26.51n=2f(α)=(1-α)227.52Eq.11n=1f(α)=1-α26.51n=2f(α)=2(1-α)[-ln1-α()]1/268.74n=3f(α)=3(1-α)[-ln1-α()]2/379.31n=4f(α)=4(1-α)[-ln1-α()]3/478.26

From Table 3, we can see that the calculation results exhibit some deviation and the decomposition model has big influence on the estimating result of adiabatic time-to-explosion. Form the results, the adiabatic time-to-explosion of Cu(pn)2(FOX-7)2is calculated to about 77 s. The time can be proved credible according to the change of DSC curves in the exothermic decomposition process.

3.6 Sensitivity

The experimental results indicate that the characteristic drop height (H50) of Cu(pn)2(FOX-7)2is 71 cm (about >14 J). Explosion probability for friction sensitivity is 40 % (25 time experiments). So, Cu(pn)2(FOX-7)2is relatively less sensitive. Moreover, the impact sensitivity of Cu(pn)2(FOX-7)2is lower than that of RDX (>7.5 J), but higher than that of FOX-7 (>24.7 J)[30].

4 Conclusions

(1) The thermal decomposition of Cu(pn)2(FOX-7)2exhibits two exothermic processes. The non-isothermal decomposition kinetic equation of the first process is dα/dT=(1017.83/β)3α2/3exp(-1.635×105/RT). The self-accelerating decomposition temperature and critical temperature of thermal explosion are 145.6 and 146.7 ℃, respectively.

(2) Specific heat capacity equation of Cu(pn)2(FOX-7)2iscp=-2.6824+1.9441×10-2T-2.0494×10-5T2(285.0 K14 J) (RDX>7.5 J).

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