RDX-based solid propellant

Thermochimica Acta 537 (2012) 51–56

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Thermal decomposition kinetics of GAP ETPE/RDX-based solid propellant Jong Sung You a , Shin Chun Kang a , Soon Kil Kweon b , Ho Lim Kim c , Yong Hwan Ahn c , Si Tae Noh a,∗ a

Department of Chemical Engineering, Hanyang University, Ansan, Gyeonggi-do 426-791, Republic of Korea Agency for Defense Development, Daejeon 305-152, Republic of Korea c Hanwha Corporation, Yeosu, Jeollanam-do 550-240, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 10 November 2011 Received in revised form 28 February 2012 Accepted 29 February 2012 Available online 7 March 2012 Keywords: GAP RDX ETPE Thermal decomposition Propellants

a b s t r a c t We performed a kinetic study applying model fitting and the iso-conversional method to investigate the thermal decomposition behaviors of GAP ETPE/RDX-based solid propellant (ETPE propellant). The ETPE propellant consists of GAP ETPE and RDX was processed by an extrusion method. The surfaces of the ETPE propellant exhibited heterogeneous surface structure with noticeable RDX particles. The DSC results showed that the GAP ETPE played a role in lowering the thermal decomposition temperature of the ETPE propellant. The TGA results demonstrated that the decomposition process of the ETPE propellant progressed in one stage in the temperature range of 150–250 ◦ C. From the kinetic analysis, with the GAP ETPE in the ETPE propellant, the activation energy was maintained up to ˛ < 0.2 at 170 kJ/mol, preventing evaporation of liquid RDX. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nitroamine explosives (such as RDX and HMX) have been widely used in propellants due to their high energy performance and low cost [1–6]. However, they are sensitive to impact and friction stimuli also produce a high temperature of flame. There are many researches to mix polymer binder to overcome this defect [7–10]. Moreover, the polymer binder play an important role in improving the properties of the base explosive, such as improving mechanical properties, enhancing chemical and thermal stability, reducing environment impact, and so on. Various polymers have been used to solid propellants in the past [11–13]. However, these polymers are often inert, and result in significant reductions in performance. Therefore, inert polymers are being replaced with energetic polymers such as glycidyl azide polymer (GAP). Especially, GAP-based energetic thermoplastic elastomers (GAP ETPEs) have recently been studied in solid propellant applications due to their easy processing and environmental characteristics [14–16]. In fact the thermoplastic elastomer can be extracted from the GAP ETPE/RDX propellant by dissolution into an appropriate solvent. The RDX will then precipitate and may be used again and again to produce new recyclable propellants. Consequently, the recyclable property of a GAP ETPE/RDX based propellant would make it environmental friendly.

∗ Corresponding author. E-mail address: [email protected] (S.T. Noh). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2012.02.032

GAP ETPEs can increase the specific impulse of solid propellants due to their high heat of formation and capability of evolving gaseous decomposition products [17,18]. In a previous study, energetic binder plays an important role in thermal decomposition and combustion. Oyumi [19] showed that the presence of bis(azidomethyl)oxetane/tetrahydrofuran (BAMO) initiated and accelerated the rate of HMX thermal decomposition in a composite propellant. Singh et al. [20] studied the role of binders during thermolysis of propellants with RDX and concluded that the type of binder that is used affects the thermal stability of the polymer bonded explosives (PBXs). Recently, Pisharath et al. [21] investigated the thermal decomposition of a mixture of GAP and nitramine oxidizer (4,10-dinitro-2,6,8,12tetraoxa-4,10-diazaisowurtzitane (TEX)) and concluded that the decomposition temperature of TEX is unaffected by the presence of GAP in the GAP/TEX mixture. Although GAP ETPEs offer the potential of producing a new generation of advanced propellants, there has been little research on the thermochemical properties of mixtures containing energetic binders and nitramine (RDX or HMX). Most of the nitramine-based solid propellants used in previous studies contained inert binders and other inert propellant ingredients. Therefore, an understanding of the effects of the energetic binder is important for solid propellant systems. Especially, kinetic parameters are important for the prediction of the safety and efficiency of solid propellant system because they are key factors in understanding ignition and combustion process and product distribution in explosion. So, we focused on thermal decomposition kinetic analysis of GAP ETPE and ETPE propellant.

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2. Experimental 2.1. Materials Glycidyl azide polyol (GAP-diol; Mw = 5500; Hepce Chem. Co.), poly(tetramethylene ether) glycol (PTMG; Mw = 2000; BASF), and 1,5-pentanediol (1,5-PD; Mw = 104; Aldrich) were used after vacuum drying for 8 h at 60 ◦ C. 4,4 -Methylenebis(phenylisocyanate) (MDI; Mw = 250; Kumho Mitsui Chem. Co.) and dibutyl tin dilaurate (DBTDL; Aldrich Chemical Co.) were used without further purification. Dimethylformamide (DMF; Samchun Chemical Co.) was dried with 4 A˚ molecular sieves for 7 days before use. Cyclotrimethylene trinitramine (RDX, Hanwha Co.) was used after drying (water content <0.05%). Ethyl centralite (ECL, stabilizer) were dried under reduced pressure for 8 h at 60 ◦ C before use. Ethyl acetate (EA; Samchun Chemical Co.) was dried with 4 A˚ molecular sieves for 7 days before use.

2.2. Synthesis and preparation of ETPE propellant GAP ETPE was synthesized as our previous publication [15]. The synthesis process presents in Scheme 1. GAP (250.43 g) dissolved in DMF (250 ml) was added to the 5000 ml beaker-type flask equipped with a mechanical stirrer, thermocouple, and nitrogen gas inlet system. The reactant solution was heated and stirred at 50 ◦ C, and then DBTDL (0.001%) and MDI (111.88 g) were added. The reaction mixture was stirred at 80 ◦ C for 8 h, and PTMG (188.38 g) dissolved in DMF (333 ml) was added at 60 ◦ C and kept for 2 h at 60 ◦ C. Finally the 1,5-PD (29.52 g) dissolved in DMF (292 ml) was added at 60 ◦ C. An additional DMF (486 ml) was added in the reactants and continued for 3 h at 60 ◦ C. The completion of the reaction for each step was monitored by FTIR through the disappearance of the hydroxyl band at 3450 cm−1 or the isocyanate band at 2270 cm−1 . The reaction mixture was precipitated into a large amount of distilled water and then washed with methanol. ETPE propellant was made in a sigma blade incorporator by the solvent method. RDX in ethyl acetate (EA) was mixed with ethylcentralite (ECL, stabilizer) in a sigma blade mixer for 10 min. Then,

OCN

NCO

CH2

+

MDI

Fig. 1. Process of ETPE propellants.

after adding the GAP ETPE/EA solution and carbon graphite/EA suspension, the mixtures were made into compounds for 1 h at 60 ◦ C. After evaporation of the EA at 50 ◦ C, ETPE propellant was extruded in a heptatubular configuration using a suitable die/pin assembly in a vertical hydraulic press. The extruded ETPE propellant was cut into grains of suitable length using a cutting machine and was then dried by blowing hot air at 45 ◦ C for 48 h (Fig. 1). 2.3. Characterization Infrared spectrum was obtained on a BioRad FTS-7 Fourier transform infrared (FTIR) spectrometer (USA) with a wave-number

H

OCHCH2

OCH2CH2O

CH2N3 n

DBTDL

GAP-diol

CHCH2O CH2N3

H

n

80oC, 8h, DMF O OCN

O

NCO

CH2

OC N

(GAP)

H x

H

CH2

NCO

PTMG 60oC, 2h, DMF O OCN

O

*

CH2

O

O

NCO (GAP) OC N H H x

CH2

NCO (GAP) H

O

O

OC N

H x

CH2

O

NCO (PTMG) O C N H H y Chain extender (1,5-PD) 60oC, 2h, DMF CH2

O

O

NCO (PTMG) O C N H

H y

Soft Segment

CH2

NCO RX H

Hard Segment

Energetic Thermoplastic Polyurethanes Elastomers (ETPEs) *RX : Chain Extender Unit Scheme 1. Synthesis process of GAP ETPE.

NCO

CH2

O OC N H z

*

J.S. You et al. / Thermochimica Acta 537 (2012) 51–56

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resolution of 4 cm−1 and a single average of 64 scans at room temperature. The sample for infrared analysis was prepared by depositing a thin film of GAP ETPE sample dissolved in THF on the surfaces of a KBr window. Thermal decomposition analysis was carried out in a DSC (DSC 2010, TA instrument). In each run, 2–3 mg samples in sealed aluminum pans were conducted with a heating rate of 2 ◦ C/min under a dry nitrogen atmosphere over the range of 30–250 ◦ C. The thermo gravimetric analysis was carried out in a TGA (TA-500, TA Instruments). Approximately 2 mg of samples were used for each run. The samples were heated from room temperature to 400 ◦ C (ETPE propellant) or 600 ◦ C (GAP ETPE) with heating rates of 1 ◦ C, 2 ◦ C, 4 ◦ C, 6 ◦ C, and 8 ◦ C in an open platinum crucible under nitrogen atmosphere. Fig. 2. FTIR spectrum of GAP ETPE.

2.4. Kinetic analysis The decomposition kinetic evaluation of ETPE propellant can be expressed as follows: d˛ = kf (˛) dt

(1)

g(˛) = kt

(2)

Here f(˛) is the differential reaction model and g(˛) is the integral reaction model. The temperature dependence of the rate constant (k) is represented by the Arrhenius equation k = A e−Ea /RT

(3)

where A is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the absolute temperature. Substitution of Eq. (3) into Eqs. (1) and (2) results in the following equations for the differential model and for the integral model respectively: d˛ = A e−Ea /RT f (˛) dt

(4)

g(˛) = A e−Ea /RT t

(5)

3. Results and discussion GAP ETPE (GAP wt%:PTMG wt%:hard segment wt% = 43:33:24) was synthesized with 4,4 -diphenylmehane diisocyanate (MDI) and 1,5-pentanediol (1,5-PD) by solution polymerization in dimethyl formamide (DMF). Fig. 2 shows typical FTIR spectrum of synthesized GAP ETPE. The spectrum did not show absorption bands at 3350 and 2270 cm−1 associated with OH and N C O groups. In addition, new urethane characteristic bands represents at 3340, 1725 and 1540 cm−1 corresponding to NH stretching, amide I (C O stretching) and amide II (CNH bending vibration) mode, respectively. Also, new characteristic peaks related to azide ( N3 ) group were clearly observed at 1280 and 2080 cm−1 . ETPE propellant of GAP ETPE and RDX were processed by an extrusion method. The composition of the propellant was 80% RDX and 19.6% GAP ETPE, along with additives including 0.4% ethylcentralite (ECL) and 0.2% graphite. Generally, propellants with high RDX content tend to be brittle. GAP ETPE with elasticity can reduce brittleness of the propellant. ECL as stabilizer prevents the auto-ignition.

The kinetic parameters of the integral iso-conversional method (integral reaction model) are based on the assumption that the reaction model in Eq. (2) is not dependent on temperature [22]. According to this method, for a set of n experiments performed at various heating rates (ˇ), the activation energy (Ea ) at any specific value of decomposition (˛) can be determined by minimizing the function ϕ(Ea ): ϕ(Ea ) =

n n   J[Ea˛ , T (t˛ )] i

i=1 j = / i

(6)

J[Ea˛ , Tj (t˛ )]

In Eq. (6), the temperature integral is represented as follows:



t˛

J[Ea˛ , T (t˛ )] =



exp − 0

Ea˛ RT (t)

 dt

(7)

Vyazovkin developed the advanced iso-conversional (AIC) method [23]. This method involves integration over smaller time intervals. Therefore, Eq. (7) can be altered to yield the following equation:



t˛

J[Ea˛ , T (t˛ )] =



exp − t˛ −˛

Ea˛ RT (t)

 dt

(8)

We calculated the activation energy of the GAP ETPE and ETPE propellant using Arrhenius kinetics (differential M), the iso-conversional Vyazovkin method (VYZ M), and an advanced isoconversional method (AIC M) using MATLAB program.

Fig. 3. SEM image of ETPE propellant.

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Table 1 Thermochemical data of propellants. Propellants

Ballistic performance

JA2a LOVAa ETPE/RDX

Impetus (J/g)

Flame temp. (K)

1135 1150 1203

3372 2993 2796

a ATK Final Technical Report: “Environmentally Friendly Advanced Gun Propellants”.

Fig. 3 shows the surface structure of the ETPE propellant system. The surface of the ETPE propellant exhibits a heterogeneous surface structure with noticeable RDX particles spreading over the entire surface. This characteristic is similar to the Nitrocellulose-RDX system [7]. As indicated in Table 1, ETPE propellant exhibits the impetus (1203 J/g) at flame temperature about 3071 K. The thermochemical data of ETPE propellant compared to JA2 propellant (double base propellant) or RDX LOVA (Low Vulnerability Propellant) showed a somewhat higher impetus at a lower flame temperature. Fig. 4 shows the DSC thermograms of the GAP ETPE and ETPE propellant. The GAP ETPE represents a single exothermic peak observed at around 232 ◦ C due to the elimination of N2 from the decomposition of N3 bonds within the azide polymers. The decomposition enthalpy of 1067 J/g was found for GAP ETPE. The ETPE propellant exhibits a single exothermic peak at around 219 ◦ C and a decomposition enthalpy of 2197 J/g. Generally, the DSC thermograms of pure RDX show one sharp endotherm (Tm = 204 ◦ C) corresponding to its melting, followed by an exothermic peak at around 223 ◦ C (heating rate: 2 ◦ C/min) due to the decomposition of RDX [24,25]. However, the ETPE propellant did not show a melting transition related to RDX, but only a single exothermic peak at around 219 ◦ C due to decomposition of mixtures of GAP ETPE and RDX. Previous reports have shown that the decomposition temperature of ETPE propellant decreases when they are mixed with polymer binder. Singh et al. [26] reported that the reduction in the decomposition temperature of HMX takes place in ETPE propellant which coated an energetic binder. Our results also indicate that the decomposition temperature (219 ◦ C) of ETPE propellant decreases below that of GAP ETPE (232 ◦ C) and pure RDX (223 ◦ C) at a 2 ◦ C/min heating rate. TGA experiments were performed at slow heating rates (<10 ◦ C) due to thermal runaway generated in the energetic materials. Fig. 5 shows the derived conversion–temperature plots and the

219oC / 2197 J/g

Endotherm

GAP ETPEs ETPE propellants

232oC /1067J/g

50

100

150

200

250

o

Temperature ( C) Fig. 4. DSC thermograms of GAP ETPE and ETPE propellant.

300

Fig. 5. Conversion versus temperature plots of GAP ETPE generate from TGA curves.

derivative of the weight loss–temperature curves obtained from the TGA curve of the GAP ETPE. In the conversion–temperature plots, the curves shifted to higher temperatures as the heating rate increased. This general feature occurs because of the shorter time required for a sample to reach a given temperature at a faster heating rate [27]. In Fig. 5, GAP ETPE shows two major steps of mass loss. The first decomposition stage occurs at ˛ < 0.3, which starts at 200 ◦ C and ends at around 250 ◦ C. This corresponds to the N2 released from the azide groups by scission of the azide bonds. The second stage occurring at 0.6 < ˛ < 1 involves the decomposition of the polymer backbone between 320 ◦ C and 450 ◦ C. Fig. 6 shows the derived conversion–temperature plots and the derivative of the weight loss–temperature curves obtained from the TGA curves of the ETPE propellant. The decomposition process of the ETPE propellant occurs in one stage, unlike the two-stage decomposition process of the GAP ETPE. The decomposition stage occurs from 150 to 250 ◦ C. This implies that there is a strong interaction between RDX and GAP ETPE. The interaction between the compounds causes more heat release and, consequently, occurs decomposition with one stage. This result corresponds to the TGA results of GAP–RDX mixtures [28]. The purpose of thermal decomposition experiment was to gain kinetic parameters including the activation energy to predict the thermal decomposition process. Because all high-energy materials are not thermodynamically stable, their existence is made possible by kinetic factors. The rate constant of decomposition in the gas phase is the most accurate characteristic of molecule stability. Therefore, kinetic parameters are important for the prediction of the safety and efficiency of ETPE propellant. In addition, the shape of the activation energy (E˛ ) acquired in the graphs over the range of temperature could be used to infer the reaction type such as competing, independent, consecutive, reversible reactions, and reactions complicated by diffusion [29].

J.S. You et al. / Thermochimica Acta 537 (2012) 51–56

Fig. 6. Conversion versus temperature plots of ETPE propellant generate from TGA curves.

55

Figs. 7 and 8 show the results of kinetic analysis for the thermal decomposition of the GAP ETPE and ETPE propellant, respectively. The Arrhenius integration (differential M) reveals strong variation of the activation energy as a function of the extent of reaction due to regular Arrhenius integration from 0 to t˛ with integration over small time segments. The conventional model fitting approach fails to reveal the complexities of the thermal decomposition of energetic materials. On the other hand, in the iso-conversional (VYZ M) and AIC (AIC M) methods, the errors become smaller since fewer data points are used to estimate the activation energies for each given ˛. Therefore, we performed the data analysis using the VYZ M. In Fig. 7, the VYZ M graph shows two decomposition stages with varying activation energy. For GAP ETPE, the activation energy slightly increases to 190 kJ/mol at conversions of 0 < ˛ < 0.2 and levels off from 190 kJ/mol to 130 kJ/mol in the conversion range of 0.2 < ˛ < 0.45. In the second stage, the activation energy increases from 130 kJ/mol to 180 kJ/mol at 0.45 < ˛. For the ETPE propellant, the activation energy of the initial stage slightly increases to 170 kJ/mol at conversions of 0 < ˛ < 0.2 and the second stage shows a decreasing trend of the activation energy from 170 kJ/mol to 140 kJ/mol at ˛ > 0.2. The activation energy of the decomposition of the ETPE propellant is approximately 160–200 kJ/mol [30–32]. Our obtained activation energy (170 kJ/mol) of the ETPE propellant decomposition is consistent with previously reported values. In addition, the activation energy shows an increasing curve in the first stage of thermal decomposition. Long et al. [33] reported variation of the activation energy of RDX. They explained that the liquid state thermal decomposition of RDX occurs through three major steps: vaporization, liquid phase decomposition, and gas phase decomposition, which have activation energies of ∼100, ∼200, and ∼140 kJ/mol, respectively. Therefore, the initial stage of decomposition of ETPE propellant is controlled by liquid phase decomposition of RDX, which is the main constituent of ETPE propellant. Singh et al. [26] reported that the role of binder in the ETPE propellant is to prevent evaporation of liquid RDX and thus, thermal decomposition proceeds in the melt phase itself. Our system demonstrated similar properties in which the activation energy increased to 170 kJ/mol for ˛ < 0.2 and slightly decreased to 140 kJ/mol for ˛ > 0.2. The contributions of vaporization and gas phase decomposition increase as the reaction progresses and the activation energy is reduced.

4. Conclusions

Fig. 7. Dependence of activation energy on the extent of conversion for GAP ETPE.

Fig. 8. Dependence of activation energy on the extent of conversion for ETPE propellant.

GAP ETPE was successfully synthesized with glycidyl azide polyol (GAP), poly(tetramethylene ether) glycol (PTMG), methylenebis(phenylisocyanate) (MDI) and chain extender. ETPE propellant was prepared with GAP ETPE, cyclotrimethylene trinitramine (RDX) and ethyl-centralite (ECL) by the solvent method. From thermal analysis result, GAP ETPE showed a single exothermic peak around 232 ◦ C due to the elimination of N2 from the decomposition of N3 bonds within azide polymer. Also, ETPE propellant exhibit a single exothermic peak observed around 219 ◦ C related to the homolytic cleavage of N NO2 fission in the RDX molecule. This result indicates that the decomposition temperature (219 ◦ C) of ETPE propellant decreases below that of GAP ETPE (232 ◦ C) and pure RDX (223 ◦ C) at a 2 ◦ C/min heating rate. Therefore, GAP ETPE performed main role decreasing the decomposition temperature of ETPE propellant. Thermal decomposition kinetic study of GAP ETPE and ETPE propellant was investigated by model fitting methods and iso-conversional method. The decomposition process of ETPE propellant showed one stage of decomposition in the temperature range of 150–250 ◦ C. The obtained activation energy of the ETPE propellant was 170 kJ/mol for ˛ < 0.2. The GAP ETPE seems to affect the thermal decomposition reaction

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