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Preparation of ammonium nitrate-based solid composite propellants supplemented with polyurethane/nitrocellulose blends binder and their thermal decomposition behavior

Sabri Touidjine

^a

, Moulai Karim Boulkadid

^a^,^*

, Djalal Trache

^b

, Samir Belkhiri

^a

, Abderrahmane Mezroua

^b

, Mohamed Islam Aleg

^a

, Afaf Belkebiche

^a

aEnergetic Propulsion Laboratory, Teaching and Research Unit of Energetic Processes, Ecole Militaire Polytechnique, BP 17, Bordj-El-Bahri, 16046, Algiers, Algeria

bEnergetic Materials Laboratory, Teaching and Research Unit of Energetic Processes, Ecole Militaire Polytechnique, BP 17, Bordj-El-Bahri, 16046, Algiers, Algeria

a r t i c l e i n f o

Article history:

Received 1 November 2021 Received in revised form 6 December 2021 Accepted 24 December 2021 Available online 31 December 2021

Keywords:

Composite propellants Polyurethane Nitrocellulose Ammonium nitrate Decomposition kinetics Thermal decomposition

a b s t r a c t

To improve the performance of solid composite propellants (SCPs) supplemented with ammonium nitrate (AN) as an oxidizer, the incorporation of energetic ingredients such as explosives, energetic binders or catalysts is a common effective approach. For this purpose, polyurethane (PU), a typical inert binder, was mixed with nitrocellulose (NC) as an energetic polymer. Numerous composite solid propellant compositions based on AN and NC-modiﬁed polyurethane binder with different NC ratios were prepared.

The prepared formulations were characterized using Fourier transform infrared spectroscopy (FTIR), RAMAN spectroscopy, X-ray diffraction (XRD), electron densimetry, thermogravimetric (TG) analysis, and differential scanning calorimetry (DSC). A kinetic study was then performed using the iterative Kissinger- Akahira-Sunose (It-KAS), Flynn-Wall-Ozawa (It-FWO), and non-linear Vyazovkin integral with compensation effect (VYA/CE) methods. The theoretical performances, such as theoretical specific impulse, adiabaticflame temperature, and ideal exhaust gaseous species, were also determined using the NASA Lewis Code, Chemical Equilibrium with Application (CEA). Spectroscopic examinations revealed the existence of NC and full polymerization of PU in the prepared propellants. According to density tests, the density of the propellant increases as the nitrocellulose component increases. According to the thermal analysis and kinetics study, the increase in NC content catalyzed the thermal decomposition of the AN-based composite solid propellants. Based on the theoretical study, increasing the amount of NC in the propellant increased the specific impulse and, as a result, the overall performance.

licenses/by-nc-nd/4.0/).

1. Introduction

Nowadays, the development of environmentally friendly solid propellants has become a need that has generated immense interest in ammonium nitrate (AN) - based propellants [1,2]. A solid composite propellant is composed of a fuel and an oxidizer bound together by a matrix of a synthetic or plastic binder. This category of propellants is considerably employed to produce propulsive energy for diverse applications in rockets, satellites and cruise missiles.

Ammonium nitrate (AN) has several uses in theﬁeld of energetic materials. Indeed, it is the main component of several explosives, including Ammonals, Ammonites, Dynamites and ANFO [3]. It also plays the role of oxidizer in solid composite propellants (SCPs) [4e12]. AN has different advantages over other oxidizers, such as, availability, low cost, low hazard, strong gas production, ecoefriendly combustion products, and comparatively low sensi- tivity. Nevertheless, AN possesses several disadvantages as an oxidizer like room temperature crystallographic phase transition, hygroscopicity, and low burning rate.

Several investigations have been accomplished to ameliorate the burning properties of AN-based propellants, including the utilization of catalysts [13e15], the introduction of metals [16e18],

*Corresponding author.

E-mail address:[email protected](M. Karim Boulkadid).

Peer review under responsibility of China Ordnance Society

Contents lists available atScienceDirect

Defence Technology

j o u r n a l h o m e p a g e :w w w . k e a i p u b l i s h i n g . c o m / e n / j o u r n a l s / d e f e n c e - t e c h n o l o g y

https://doi.org/10.1016/j.dt.2021.12.013

2214-9147/©2022 China Ordnance Society. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC- ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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and the utilization of energetic binders [19]. The use of an energetic binder is an efﬁcient method with respect to the enhancement of the performance of AN-based propellants. Nonetheless, the synthesis procedures of energetic binders are complex and expensive, and consequently, it is complicated to produce these binders industrially. Thus, the development and the assessment of new systems is crucial.

Polyurethane (PU) elastomers are largely used in the field of high-energy composites such as SCPs and high-energy polymer- bonded explosives (PBXs) due to their distinguished characteristics [20e23]. Conventional PU binders are mostly non-energetic materials, and consequently reduce the energy performance signifi- cantly. Nitrocellulose (NC) is an energetic polymer widely used as an ingredient in propellants, explosives,fireworks, and gas gener- ators [24,25], which may be introduced in PU-based compositions to overcome their performance drawback. Therefore, it could be an efficient binder to improve the burning properties of composite propellants. Additionally, during these last two decades, different research teams [26e31] mixed the polyurethane with NC in order to improve the mechanical and energetic properties. Subsequently, it is predicted that a PU/NC blend could be a promising binder component for increasing the performance of AN-based composite propellants. To the best of our knowledge, the preparation, characterization and thermal decomposition kinetics investigation of AN-based SCPs have never been accomplished.

Therefore, in the present work, we prepared and studied a series of AN - based SCPs supplemented with PU/NC. The physicochemical and thermal properties of the obtained SCPs were investigated. Additionally, based on NASA Lewis Code, Chemical Equilibrium with Application (CEA) [32,33], comparative exami- nation of the theoretical speciﬁc impulse, of the adiabaticﬂame temperature, as well as of the ideal exhaust gaseous species has been done.

2. Experimental and methods

2.1. Preparation of samples

This part of the experimental work consists in developing different formulations of composite solid propellant with a total mass of 7.5 g, containing ammonium nitrate, aluminum (Al) and PU, as well as PU mixed with nitrocellulose.

The size distributions of the AN and Al powders used in the SCPs formulations were determined by the CILAS 1190 laser particle size analyzer. The average diameters of AN and Al are, respectively, equal to 160 and 28m^m.

In order to develop the reference composite propellant (SCP- REF) based on the PU binder, several steps were followed in a careful manner, namely, the preparation of the binder, preparation of the solidﬁller, mixing of the ingredients and the molding followed by the curing process. The PU binder is composed of several ingredients, the resin, the crosslinking agent, the plasticizer, the surfactant and the chain extender. The resin used in this study is a hydroxyl terminated polyester prepolymer (HTPS), the crosslinking agent is isophorone diisocyanate (IPDI), the plasticizer is desavin, the surfactant is silicon oil and the chain extender is glycerin. An amount of the plasticizer was added to the HTPS resin to reduce its viscosity, followed by the addition of the surfactant (silicon oil), the chain extender (glycerin) and the isophorone diisocyanate (IPDI).

These components are mixed at a temperature of (60Ce80C) until a homogeneous mixture is obtained. The mixing operation is an essential step in the manufacture of composite propellants. It must be long enough to create a homogeneous paste ready for pouring. This operation was carried out at a temperature of (60Ce80C) by mixing the prepared binder with the solidﬁller,

which is added in small quantities in a Teﬂon beaker until a homogeneous mixture is obtained (a duration of 40 min is necessary).

The propellant pastes were then casted under vacuum in a Craw- ford mold. Subsequently, the mold was left at room temperature for 18 h and then placed in an oven at 60C for 2 days. The purpose of this step is to accelerate the reaction of the crosslinking of the binder. The cooling of the propellant was carried out gradually to avoid thermal shock as shown inFig. S1(Supporting Information).

Table 1summarizes the mass fractions of the components used for the preparation of SCP-REF.

To obtain the solid composite propellant formulations based on the polyurethane mixed with the NC (SCP-NCxx, where the symbol

“xx”corresponds to the percentage of NC in the formulations), we followed several steps, namely, the preparation of the modified binder with the NC, preparation of the solidfiller, mixing of the ingredients and the molding followed by the curing. It is worthy to note that the only difference with the preparation method of SCP- REF is the step of preparing the binder, this is why in the following we will only detail this step. For the preparation of PU-NC mixtures, NCfibers without stabilizers (having a nitrogen content equal to 13.0%) were solubilized in acetone with stirring at a speed of 750 rpm in a Teflon beaker. Then the PU binder ingredients were added in the following order (prepolymer, plasticizer, chain extender, surfactant, diisocyanate) to the solubilized NC. The mixture was subsequently stirred (750 rpm) for 10 min to ensure homogenization as shown inFig. S2(Supporting Information). The mass fractions of the NC as well as those of the ingredients of the PU binder are given inTable 2. It is noted that for the chain extender, surfactant and plasticizer, we used a mass fraction of 1% for each component.

2.2. Characterization of the samples

The prepared formulations were characterized through, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), density measurements and differential scanning calorimetry (DSC).

The FTIR spectra were obtained using a Fourier transform spectrometer of the“PerkinElmer”type Spectrum One model. The spectra were obtained by preparing pellets containing a mixture of powders of dried potassium bromide and 1% w/w of the studied Table 1

The mass fractions of the components used for the preparation of SCP-REF.

Components Role Mass fraction/%

AN Oxidizer 46

Al Combustible 24

HTPS Resin 16.2

IPDI Curing agent 10.8

Dessavin Plasticizer 1

Glycerin Chain extender 1

Silicon oil Surfactant 1

Table 2

The mass fractions of the NC as well as those of the ingredients of the PU binder for different prepared SCPs formulations.

SCPs formulations NC/% Resin HTPS/% IPDI/%

SCP-REF 0 60.0 40.0

SCP-NC10 10 52.2 34.8

SCP-NC15 15 49.2 32.8

SCP-NC20 20 46.2 30.8

SCP-NC30 30 40.2 26.8

SCP-NC40 40 34.2 22.8

SCP-NC50 50 28.2 18.8

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systems. Noting that to obtain the samples in powder form, they were ground. The spectra were collected in the transmission mode with a resolution of 4 cm¹, in the range of 4000e600 cm¹, where the main characteristic peaks of polyurethane and nitrocellulose are found. An accumulation of 32 scans, to achieve an acceptable signal-to-noise ratio, was taken within this interval.

The RAMAN measurements were carried out using a RAMAN spectrometer (Thermo Fisher Scientiﬁc) equipped with a laser source with an excitation wavelength of 532 nm. The samples were analyzed in the frequency range of 500e3500 cm¹.

The XRD diffractograms were acquired employing a PAN- alyticalX'pert PRO MultiPurpose diffractometer with Cu Ka^radia- tion. An X'celerator detector was exploited to obtain data over an angular range of 5e80/2qwith a step size of 0.017/2qand a count time of 50.1650 s at each step. The voltage was selected to be 45 kV with a current of 40 mA. The specimens were charged on aluminium sample containers by employing a side loading tool.

The densities of all samples were also determined employing an electronic densimeter. The densimeter apparatus comprises a computer command unit, a bottle of helium gas and a densimeter type gas pycnometer Accupyc 1340 II. Three measurements were performed for each sample.

The thermogravimetric (TG) analysis was carried out by thermogravimetric analyzer model Q500. Each sample mass (about 5 mg) was heated under nitrogen atmosphere from 50 to 350C at 10C/min.

The differential scanning calorimeter measurements were accomplished by employing a DSC 8000 (PerkinElmer), within a temperature domain of 50e300C, at distinct heating rates (5, 10, 15, and 20C/min) under nitrogen atmosphere (20 cm³/min). Small mass of samples, around 1.5 mg with less than 10% variation between sample masses, was charged in closed aluminium pans and employed for each analysis. The calibration of the DSC has been made following the procedure described in the technical guide provided by the manufacturer. We followed the different calibration phases, namely the calibration of the baseline, the temperature and the enthalpy of reaction. To evaluate the precision and accuracy of the instrument, analysis with high purity indium and zinc were performed. The uncertainty of the measured temperatures were less than 0.2C. Moreover, data acquisition and processing were completed by Pyris software version 10.1.

2.3. Theoretical performance analyses

To evaluate the performance of the prepared composite solid propellants, the NASA-CEA chemical equilibrium software was employed,ﬁxing the chamber pressure value at 7 MPa, the ideal expansion to 0.1 MPa, and the supersonic section ratioA_e/A_tat 10, whereas the initial temperature is 298 K. The main assumption to calculate the theoretical performances are detailed elsewhere [34].

The analyzed parameters are as follows: the speciﬁc impulse (I_sp), the adiabaticﬂame temperature (T_f), as well as the decomposition product mole fractions. The physical and thermodynamic data of distinct components employed are showed inTable 3.

3. Results and discussion

3.1. Characterization 3.1.1. FTIR analysis

To identify the chemical groups of the different constituents of the propellants produced, we performed FTIR analysis for the

Table 3

Physical and thermodynamic data of propellant components utilized for thermochemical evaluation.

Ingredient Formula r/(g$cm³) DHf/(kJ$mol¹) References

AN NH4NO3 1.720 365.00 [35]

PU^a C_4$763H_7$505O2.131N0.088 1.210 550.00 [36]

NC C6H_7$42O_10$16N2.58 1.770 543.61 [37]

aPU based on HTPS prepolymer.

Fig. 1.FTIR spectra of nitrocellulose, polyurethane, aluminum, pure ammonium nitrate and composite propellants.

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different samples.Figs. 1e3show, respectively, the results obtained by FTIR of nitrocellulose, polyurethane, aluminum and pure ammonium nitrate as well as those of composite propellants (SPC- REF, SPC -NC10, SPC-NC15, SPC-NC20, SPC -NC30, SPC-NC40 and SPC-NC50). All the characteristic bands are found and correspond to what available in the literature [35e38].The spectra of the elaborate composite propellants show the appearance of the characteristic urethane peak at 1730 cm¹and the absence of the isocyanate peak at about 2276 cm¹, which conﬁrms the complete crosslinking of the binder, generated by the polymerization reaction between the IPDI and the HTPS prepolymer. From the superposition of the spectra, we can clearly verify that the characteristic bands of AN, pure Al are all found in the spectra of the produced propellants. Obviously, the characteristic peak at 1388 cm¹ corresponding to the asymmetric elongation vibration of NH4þ and NO3, the peak at 830 cm¹of the out-of-plane deformation of NO3

and the peak at 3124 cm¹are assigned to the symmetrical elongation of NH₄^þand the vibration of Al a 1045 cm¹. Moreover, from these results, we note that the characteristic bands of the characteristic groups of the NC such as the asymmetric and symmetrical elongation vibrations of NO₂, which appeared, respectively, at 1280 cm¹and 1680 cm¹, and the vibration corresponding to the ﬂexion of the OeNO2group at 824 cm¹, can be easily seen in the SPC-NCxx, without appearing in the PPC-REFs. These results conﬁrm the presence of NC in the propellants produced with a mixture of PU/NC binders.

3.1.2. XRD analysis

XRD analysis is used to determine the crystallographic structure of samples of nitrocellulose, ammonium nitrate, aluminum, the

reference propellant formulation (SCP-REF) and other formulations of propellant based on the binders composed of the mixture of PU/

NC. Thus, an identiﬁcation of the effect of the interaction during mixing by superimposing the diffractograms is carried out.Fig. 2 represents the diffractograms obtained by the XRD analysis for the different samples: SCP-REF, SCP -NC10, SCP -NC15, SCP -NC20, SCP -NC30, SCP -NC40 and SCP -NC50, as well as that of pure NC, AN and Al. The characteristic peaks of AN as oxidizer at around of 53.3, 24.9, 23.8and 19.5are all preserved in the diffractograms of the produced composite propellant formulations. The obtained results show that the SCP-NCxx formulations display a characteristic peak at 19.0, which is attributed to the amorphous region of cellulosic chain [39], indicating the presence of nitrocellulose in the propellants supplemented with a mixture of PU/NC binders. Further- more, all the characteristic peaks of aluminum at 38.4and 44.8 are found in the XRD patterns of composite propellants.

Fig. 2.X-ray diffractograms of (a) nitrocellulose, aluminum, pure ammonium nitrate and (b) composite propellants.

Fig. 3.RAMAN spectra of (a) nitrocellulose, pure ammonium nitrate and (b) composite propellants.

Table 4

The densities of the composite propellants produced with the pure polyurethane binder and those with mixtures of PU-NC binders.

Formulations Density/(g$cm³)

SCP-REF 1.6956±0.0005

SCP-NC10 1.7203±0.0019

SCP-NC15 1.7266±0.0041

SCP-NC20 1.7653±0.0014

SCP-NC30 1.7757±0.0081

SCP-NC40 1.8142±0.0042

SCP-NC50 1.8614±0.0081

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3.1.3. Raman analysis

Raman spectroscopy, which is a complementary technique to infrared spectroscopy, also measures vibrational energy levels. The characterizations by RAMAN spectroscopy of the reference formulation (SCP-REF) as well as the other developed formulations (SCP -NC10, SCP -NC15, SCP -NC20 and SCP -NC30) were carried out in order to highlight the different links that exist and to conﬁrm the

structure of the chemical mixture obtained with nitrocellulose and ammonium nitrate. The results obtained from this analysis for the various composite propellants produced, nitrocellulose and ammonium nitrate are shown in Fig. 3. The absorption bands indicative of the allocation of the functional groups of the NC, the Fig. 4.TG/DTG curves of the produced composite propellants: (a) SCP-REF; (b) SCP-

NC30 and (c) SCP-NC50 atﬁxed heating rate (b¼10C/min).

Fig. 5.DSC thermograms of the produced composite propellants: (a) SCP-REF; (b) SCP- NC30 and (c) SCP-NC50 at different heating rates.

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AN and the composite propellants produced, are deﬁned from previous studies [38,40e43]. From the results obtained by RAMAN, we notice that the characteristic bands of polyurethane, AN are all found. As regards the formulations of propellants based on binders prepared on the basis of admixture PU/NC (SCP-NC10, SCP -NC15, SCP -NC20 and SCP -NC30), the presence of peaks characteristic of NC is noted. In addition, the 3100 cm¹absorption band is due to the elongation vibrations of the NH group. The band around 2933 cm¹is attributed to the CH group (Vibration of elongation of the CeH group). The absorption band at 1732 cm¹ is due to a stretching vibration of the carbonyl (amide band I) of the urethane bonds. The symmetrical elongation vibrations of the NO2group are attributed to the 1285 cm¹band. The absorption band at 853 cm¹ is assigned to the symmetrical elongation vibrations of the CeOeC ester group. The peaks at 1043 cm¹and 714 cm¹are, respectively, related respectively to the elongation of the NeO group and to the strain vibration of the NeH group, which are present in the AN. The results obtained by RAMAN are in agreement with those derived from the FTIR. Indeed, the presence of urethane functions conﬁrms the complete polymerization of PU in the prepared propellant formulations. In addition, they contain nitro groups due to nitrocellulose and AN.

3.1.4. Density analysis

The densities of the composite propellants produced with the pure polyurethane binder as well as those with mixtures of PU-NC binders were measured. The values obtained are presented in Table 4. The results showed that as the content of the NC increases, the density of the propellant increases; this trend can be explained by the higher density of nitrocellulose compared to the substituted polyurethane, revealing the positive effect of NC on the overall density of propellant since it, closely affect theﬁnd performance.

3.1.5. TG analysis

Fig. 4shows TG/DTG curves of SCP-REF, SCP-NC30 and SCP-NC50 samples at heating rate of 10C/min. It can be observed that the introduction of NC in the formulation catalyze the thermal decomposition of the solid composite propellant. In fact, according to DTG curves, it is obvious that the decomposition of SCP-NCxx took place at a lower temperature than that of SCP-REF. This may be due to the effect of the exothermic decomposition of the nitrocellulose, which could promote the decomposition of the propellant.

3.1.6. DSC analysis

In order to study the thermal properties of the produced propellants, we used the DSC analysis. The obtained results after

performing the tests at different heating rates (b¼5, 10, 15 and 20 C/min) for the several formulations which are SCP-REF, SCP -NC30 and SCP-NC50, are illustrated inFig. 5. According to the obtained thermograms, for SCP-REF, we note the appearance of a single exothermic peak around 255C, which corresponds to the decomposition of the propellant. For the SCP-NC30 and SCP-NC50 formulations, the appearance of two overlapping exothermic peaks is observed; theﬁrst peak corresponds to the decomposition of the NC, while the second peak is related to the main decomposition of the propellant. The superposition of the thermograms al- lows to notice a shift towards low temperatures with the increase in the percentage of the NC in the propellant binder. Therefore, the higher amount of NC promotes the decomposition of the propellant. The early decomposition of SCP-NC30 and SCP-NC50 samples may be due to the effect of exothermic decomposition of nitrocellulose, which could promote the decomposition process of the propellant. It is also important to mention that some endothermic peaks appeared as well. They, correspond to the solid-solid tran- sitions of AN as well as its melting.

Table S1(Supporting Information) presents a comparative sur- vey of the onset and peak decomposition temperatures. Compared to the reference propellant formulation (SCP-REF), it can be seen that the prepared PU/NC binders promoted effectively the thermal decomposition of composite solid propellant with lower decomposition temperature. Additionally, the energetic performance of the SCPs have been enhanced. Indeed,Fig. S3(Supporting Infor- mation), which exhibits the heat release at different heating rates, revealed that as the content of the NC increases the energy release increases.

3.2. Kinetic parameters

Different models of thermal kinetic analysis are available to assess the kinetic triplet namely, activation energy, pre-exponential factor and reaction model. In the framework of this study, we applied the iterative methods of Kissinger-Akahira-Sunose (It-KAS), Flynn-Wall-Ozawa (It-FWO) and the nonlinear Vyazovkin integral with compensation effect (VYA/CE), using the results provided by DSC to investigate the decomposition kinetics of the prepared composite propellants based on PU and the mixture of PU-NC binders. More details regarding the kinetic approaches employed can be found in a previous study [39,45]. In addition, all the cal- culations, which will be mentioned hereafter, were carried out using a kinetic calculation interface compiled under the MATLAB programming software. It is worthy to mention that to well un- derstand the effect of the variation of the NC content in the binder employed for the different propellant formulations, we have

Fig. 6.Kinetic parameters and their errors bars as function of conversion rate (a) for the SCP-REF sample determined by It-FWO and It-KAS methods.

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selected three formulations that contain a higher and medium content, in addition to the benchmark based propellant.

3.2.1. SCP-REF

Figs. 6 and 7give the variation of the kinetic parameters as a function of the conversion rate (a) for the SCP-REF.

3.2.2. SCP-NCxx

In the case of SCPs prepared on the basis of PU/NC mixtures, two thermal decomposition peaks were observed (Fig. 5). Consequently, the determination of the kinetic parameters was carried out separately for each peak. For this purpose,Fig. 8shows an example

of the deconvolution of the decomposition peaks of the SCP-NC30 by the Gaussian method, taking the correlation coefﬁcient R²>0.98 for all the peaks. The same method was followed for the sample SCP-NC50.

3.2.2.1. First peak of decomposition. Figs. 9 and 10show the variation of kinetic parameters as a function of conversion ratea^for samples SCP-NC30 and SCP-NC50.

3.2.2.2. Second peak of decomposition. Figs. 11 and 12exhibit the change of the kinetic properties as a function of the conversion rate a corresponding to the second decomposition peak of the propellants SCP-NC30 and SCP-NC50.

3.2.3. Discussions

Tables S2, S3, S4, S5, S6 and S7(Supporting Information) sum- marize all the results obtained for the propellants SCP-REF, SCP- NC30 and SCP-NC50, calculated by the three isoconversional methods, namely, It-KAS, It-FWO and Vyazovkin/CE.

The comparison between the reference propellant and the propellants based on PU/NC binders, reveals that the mean Ea of the SCP-REF, which is equals to 125 kJ/mol, is lower than that of SCP- NCxx. Indeed, the activation energy decreases to 55 kJ/mol for SCP-NC30 and to 41 kJ/mol for SCP-NC50. It can be therefore concluded that the addition of NC has a catalytic effect on the decomposition of the propellant, caused mainly by the earliest decomposition of NC.

The evaluation of the g (a) decomposition model showed that the formulations SCP-REF, SCP-NC30 and SCP-NC50 decompose according to similar mechanisms for the three calculation methods.

The values of Ea and Log (A) vary in the same way. In addition, the results of the activation energies are almost similar regardless of the employed model, which shows that the adopted approach is Fig. 7.Kinetic parameters and their errors bars as function of conversion rate (a) for the SCP-REF sample determined by Vyazovkin's method.

Fig. 8.Deconvolution of the decomposition peak of the SCP-NC30 at heating rate (b¼20C/min).

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Fig. 9.Kinetic parameters and their errors bars as function of conversion rate (a) for samples SCP-NC30 and SCP-NC50 determined by It-FWO and It-KAS methods (First peak).

Fig. 10.Kinetic parameters and their errors bars as function of conversion rate (a) samples SCP-NC30 and SCP-NC50 determined by Vyazovkin's method (First peak).

Fig. 11.Kinetic parameters and their errors bars as function of conversion rate (a) for samples SCP-NC30 and SCP-NC50 determined by It-FWO and It-KAS methods (Second peak).

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correct, with a slight difference due to the nature of the used approximations.

3.3. Theoretical performance analyses

The results of calculation by the code CEA-NASA of the speciﬁc impulse (Isp), adiabatic temperature of the ﬂame (Tf) and the

average molar mass (Mm) as well as the mole fractions of the decomposition products of SCPs based on the PU binder modiﬁed with the NC, are grouped together inTables 5 and 6.

One can observe that the composite propellants based on PU-NC binder have better performance parameters. In fact, an increase of 14.64 s in the value ofIspis noted following the addition of 50% by mass of NC in the formulation of the binder. Additionally, it can be seen that the fraction of CO has slightly increased with the increase in the percentage of NC introduced into the composition of the propellants for the percentages 10, 15, 20 and 30% of NC, while for 40 and 50% of NC, the CO fraction decreased, unlike the CO₂fractions where a decrease and then a considerable increase is observed in the case of SCP-NC40 and SCP-NC50. Also, the molar Fig. 12.Kinetic parameters and their errors bars as function of conversion rate (a) samples SCP-NC30 and SCP-NC50 determined by Vyazovkin's method (Second peak).

Table 5

CEA calculated values of Isp, Tfand Mmof the composite propellants produced with the pure polyurethane binder and those with mixtures of PU-NC binders.

Formulations ISP/s Tf/K Mm/(g$mol¹)

SCP-REF 239.26 2356.89 21.681

SCP-NC10 241.59 2431.97 22.258

SCP-NC15 242.43 2514.18 22.498

SCP-NC20 244.36 2589.86 22.773

SCP-NC30 248.84 2762.88 23.175

SCP-NC40 252.19 2953.86 23.792

SCP-NC50 253.90 3110.43 24.523

Table 6

The calculated mole fractions of combustion products for propellants produced with the pure polyurethane binder and those with mixtures of PU-NC binders.

Formulations Mole fractions/%

CO CO2 H2 H2O N2 Other

SCP-REF 21.728 0.005 47.913 0.029 12.431 17.893

SCP-NC10 23.383 0.003 47.812 0.019 13.067 15.717

SCP-NC15 24.248 0.003 47.753 0.016 13.399 14.581

SCP-NC20 25.139 0.002 47.689 0.013 13.743 13.413

SCP-NC30 27.005 0.002 47.548 0.010 14.465 10.970

SCP-NC40 26.350 0.313 45.447 1.981 15.258 10.649

SCP-NC50 24.987 0.651 42.443 4.779 16.087 11.053

Fig. 13.Oxygen balance of solid composite propellant formulations.

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fraction of H2O increases unlike that of H2, which decreases with the increase in the mass percentage of NC in the formulations.

These trends are explained by a more complete combustion after the addition of the NC, especially for SCP-NC40 and SCP-NC50, which brings an additional quantity of oxygen to the compositions as shown in Fig. 13. The fraction of N2 increases with the percentage of NC added in the formulation of the propellant. This trend is explained by the additional supply of nitrogen due to the addition of nitrocellulose.

4. Conclusions

The synthesis and characterization of ammonium nitrate-based composite solid propellants supplemented with a blend of PU-NC binders were the focus of this research. FTIR, XRD, RAMAN, electronic densimetry, TG, and DSC instruments were utilized to char- acterize the various samples. A theoretical performance study was also completed using the NASA-CEA algorithm. The spectroscopic results show that the chemical structure of the NC has not changed within the composite propellant through the creation of the polyurethane network. The density of propellants based on PU/NC blends enhances as the NC ratio rises. The results of a study on the non-isothermal degradation and kinetics revealed that the introduction of NC in the formulation of SCPs catalyzed the overall thermal decomposition of the propellants. The theoretical characterization implies that hypothetical AN-based composite solid propellant formulations containing PU/NC blends exhibited better performance. Finally, the current research will undoubtedly pave the way for the development of high-performance solid propellants based on ammonium nitrate as oxidizer.

Declaration of competing interest

The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have appeared to inﬂuence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dt.2021.12.013.

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