New Insights into the Chemical Compatibility of Nitrochitosan with Potential Energetic Molecules

Abstract

This study provides new insights into the compatibility of a promising energetic polysaccharide, called nitrochitosan (NCS), with energy-rich ammonium perchlorate (AP), ammonium nitrate (AN), and hydrazine 3-nitro-1,2,4-triazol-5-one (HNTO) molecules, in order to survey their application prospects in solid rocket propellants and explosives. For this purpose, differential scanning calorimetry (DSC) and thermogravimetric (TGA) analyses were carried out to accurately evaluate the chemical compatibility of NCS with the selected energetic molecules following the STANAG 4147 criterion. Fourier transform infrared spectroscopy (FTIR), as a non-thermal complementary technique, was also performed to further elucidate the eventual structural alterations occurring in the physical mixtures (NCS/AP, NCS/AN, and NCS/HNTO). Based on DSC results, the maximum exothermic peak temperature difference between NCS (T_peak = 164.7 °C) and the as-prepared NCS/AP (T_peak = 164.3 °C), NCS/AN (T_peak = 204.3 °C), and NCS/HNTO (T_peak = 197.0 °C) admixtures is found to be lower than 4 °C. Moreover, TGA experiments showed that the observed mass losses of the physical mixtures are lower than the sum of the weight losses of their respective individual compounds. Therefore, thermal results demonstrated the excellent chemical compatibility of NCS with the corresponding energetic molecules. In addition, FTIR measurements highlighted the absence of chemical interactions between NCS and the selected AP, AN, and HNTO. Therefore, a deep investigation into the characteristics of such energetic composites and their real-world applications will be among the main focuses of the postulated next stage of research.

1. Introduction

Recently, considerable interest has been devoted to developing and characterizing new energetic formulations for potential applications in several industrial (e.g., membranes and cosmetic products) and military (e.g., gun and rocket propellants, explosives, and pyrotechnics) fields owing to their tailorable functionalities and outstanding performance, such as excellent mechanical properties, flammability, and explosiveness [1,2,3]. At present, nitrocellulose (NC) is still employed as the main nitrate ester binder for the manufacture of conventional gun powders and homogenous solid propellants owing to its inherent features of high energy and good compatibility with various additives [4,5]. Recently, new NC-based composites have been investigated, such as NC/energetic binder (e.g., GAP: glycidyl azide polymer) [6], NC/energetic plasticizer (e.g., DEGDN: diethylene glycol dinitrate) [7], NC/oxidizers (e.g., AP: ammonium perchlorate, AN: ammonium nitrate) [8,9], and NC/energetic fillers (e.g., RDX: cyclotrimethylenetrinitramine, HNTO: hydrazine 3-nitro-1,2,4-triazol-5-one) [10,11], for eventual use in high-performance rocket propellants and explosive systems. Nevertheless, some serious concerns with NC-rich formulations, including their high impact sensitivity, poor chemical stability, and inappropriate combustion performance, have pushed the energetic materials community toward alternative energetic polymers, which can satisfy the required sensitivity features, thermal stability, and energy level of munitions [12,13]. In this context, a new class of insensitive and high-energy dense cellulosic materials has been designed by incorporating nitrate esters and other nitrogen-rich groups, such as azido, tetrazole-acetate, N-nitrocarbamate, and aminoethyl-nitramine, providing new opportunities to fabricate innovative energetic composites with enhanced features [14,15,16,17]. More recently, high-nitrated chitosan (NCS) has been outlined as an outstanding energetic nitrogen-rich polysaccharide with great potential to replace the conventional NC in advanced energetic formulations (e.g., solid propellants and composite explosives) with high density, good thermal stability, insensitivity, and high-energy performance [18,19,20,21]. Indeed, Li et al. [20] were the first authors who reported the preparation of a high-substitute NCS using a mixture of anhydrous acetic and fuming nitric acids in a specific charging sequence. This investigation was subsequently followed by the work of Tarchoun et al. [18], who studied the characteristics of NCS, derived from shrimp shells, and its blends with NC. In fact, the obtained findings demonstrated that this emergent nitrate ester polymer shows attractive characteristics, such as great nitrogen content (Nc = 16.79%), high density (ρ = 1.708 g/cm³), moderate impact sensitivity (15 J), great exothermic decomposition enthalpy (2285 J), and increased detonation velocity (7788 m/s), which are significantly better than those of the traditional NC [19]. Therefore, these outstanding properties of NCS have motivated us to evaluate its ability to be used with the state-of-the-art AP and AN oxidizers, as well as emergent nitrogen-rich HNTO salt. The choice of these energetic molecules as co-formers to NCS is attributed to their useful benefits in terms of meeting the high energy and better performance of the designed energetic composites [2,10,22]. For this purpose, testing the chemical compatibility of the components of the energetic mixture, defined as their aptitude to preserve their original properties when mixed together, is a prime important step in developing modern energetic formulations [23,24]. This crucial safety aspect had to be deeply investigated during the elaboration, storage, and large-scale use of new energetic mixtures, since the lack of compatibility may lead to self-ignition and/or unexpected explosions [25,26]. Examples of the most frequently employed techniques and the requirements to scrutinize the compatibility of energetic mixed components can be found in the NATO standardization agreement (STANAG) 4147 [27]. Moreover, non-thermal methods, such as Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), have recently been recommended as efficient complementary tools for the initial checking of incompatibility issues in the energetic mixture by elucidating possible interactions between the components or eventual changes in the molecular structure [28,29].

The aim of the current research is to provide new insights into the potential ability of NCS to be used with the selected energetic AP, AN, and HNTO molecules. The as-prepared physical mixtures (NCS/AP, NCS/AN and NCS/HNTO), with a mass ratio of 1:1, and their pure components were firstly characterized by FTIR to look for eventual molecular interactions between the mixed samples. The effect of AP, AN, and HNTO compounds related to the chemical compatibility of NCS was further examined using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), as recommended by the STANAG 4147 standard. This study shows a good prospect of nitrochitosan in the research of modern energetic formulations.

2. Experimental Section

2.1. Materials and Sample Preparation

NCS, with a nitrogen content of 16.79%, and HNTO were synthesized in our laboratory according to the procedures reported elsewhere [19]. High-purity AP, with a mean diameter of 200 µm, and AN oxidizer were acquired from VWR-Prolabo and used without any further purification. In order to ensure effective mixing and a high level of physical contact between ingredients during testing, our samples (NCS, AP, AN, and HNTO) were ground and passed through a sieve with a 2 mm opening.

2.2. Compatibility Assessment

In order to study the compatibility of NCS with the selected energetic molecules, binary mixtures (NCS/AP, NCS/AN and NCS/HNTO), with equal mass fractions, were prepared via the physical mixing of 100 mg of NCS powder with 100 mg of each ingredient (AP, AN, and HNTO), as illustrated in Figure 1. According to the STANAG 4147, the 1:1 mass ratio increases the probability of detecting any occurred interactions between the components of the physical mixture. The prepared admixtures and their pure compounds were firstly analyzed via FTIR spectroscopy, performed using a Perkin Elmer 1600 spectrometer using the attenuated total reflection (ATR) mode, in the wavenumber range of 4000–400 cm⁻¹, by averaging 64 scans at a resolution of 4 cm⁻¹. Compatibility was screened by comparing the spectra of pure samples with those of their admixtures. The appearance/disappearance of new/old vibrational bands is assumed to result from incompatibility issues. After that step, the compatibility status was confirmed using DSC and TGA techniques, following the requirements and criteria of the STANAG 4147. DSC experiments were recorded using a Perkin-Elmer DSC 8000 analyzer. A sample mass of 1 mg was placed into a sealed aluminum crucible, while another identical pan was used as a reference. TGA analyses, for about 1 mg samples, were carried out via a TGA 8000 instrument using platinum pans. Thermal experiments were performed within the temperature range of 30–450 °C, at 50 mL/min of flowing nitrogen atmosphere. It is worth noting that while the STANAG 4147 requirements call for thermal analyses to occur at 2 °C/min, a heating rate of 10 °C/min was taken to minimize time analysis, as already reported in the literature [28,30]. In addition, a series of runs was performed, and an analysis of the data showed no discernible difference in the results. The main factor involved in determining chemical compatibility, according to the DSC-based compatibility standard, is the shift in the maximum peak temperature (ΔT_p), which is calculated using Equation (1) [28,31].

ΔT_P = T_S − T_M

(1)

where T_S and T_M refer, respectively, to the maximum peak temperatures of the resulting exotherms of the less thermally stable single compound and the binary mixture. Based on the instructions outlined in the STANAG 4147 criteria, the admixture is considered as compatible if ΔT_P ≤ 4 °C and incompatible if ΔT_P ≥ 20 °C. Otherwise, a value of ΔT_P between 4 and 20 °C suggests that additional compatibility measurements should be performed.

In the case of the TGA method, the difference between the observed weight loss of the admixture and the total calculated mass loss of the pure substances is determined at the lowest temperature of the occurrence exotherm reaction peak selected via the DSC measurement [27]. According to the STANAG 4147 criteria, a weight difference of less than 4% indicates that the tested system is compatible. Differences within the range of 4–20% point to the uncertain compatibility, whereas a change higher than 20% reveals that the admixture is incompatible.

3. Results and Discussion

3.1. Non-Thermal Characterization

Infrared spectroscopy was performed as a screening tool for the preliminary evaluation of the compatibility status of the admixture. As can be perceived from the obtained FTIR spectra plotted in Figure 2, the three investigated energetic admixtures exhibit the most characteristic vibrational bands of NCS, including O-H stretching at 3600–3200 cm⁻¹, C-O stretching in pyranose cycles at 1020–1080 cm⁻¹, and the stretching and bending vibrations of NO₂, N-NO₂, and O-NO₂ groups in the spectra region of 1800–500 cm⁻¹ [18,20]. The representative peaks of AP are also identified in the spectrum of NCS/AP, which are assigned to the N-H elongation at 3280 cm⁻¹, N-H bending at 1420 cm⁻¹, and ClO₄⁻ stretching at 1080 cm⁻¹ and 625 cm⁻¹ [32]. Moreover, the NCS/AN spectrum shows the well-known functional groups of AN, comprising stretching and flexural vibrations of N-H at 3280 cm⁻¹ and 1420 cm⁻¹, respectively, and NO₃⁻ deformation at 830 cm⁻¹ [9,33]. These outcomes suggest the absence of mutual interactions between NCS and the corresponding oxidizers. In the case of NCS/HNTO, it is noted that the most typical bands of HNTO, such as N-H of hydrazine and triazole ring at 3350 cm⁻¹ and 2730 cm⁻¹, respectively, and the vibrations corresponding to the C-NO₂ at 1505 cm⁻¹ and 1315 cm⁻¹, can be clearly observed [10,34]. However, some peak alterations and broadening are detected in the spectrum of NCS/HNTO mixture; thus, intermolecular interactions can be suspected between NCS and HNTO. As such, the interactions of NCS with the reported energetic molecules and other additives required to develop the next generation of nitrochitosan-based energetic formulations should remain among the main foci of the postulated next stage of research into those promising materials.

3.2. Thermal Analyses

3.2.1. DSC-Based Compatibility

Figure 3 presents the DSC thermograms of the binary mixtures and their neat compounds obtained at β = 10 °C/min. As can be seen, NCS displays two consecutive exothermic processes, with the first event corresponding to the partial thermolytic cleavage of explosophoric N-NO₂ and O-NO₂ functional groups at 135–173 °C, while the second decomposition, between 173 and 250 °C, is assigned to the complete homolytic splitting of residual N-NO₂ and O-NO₂ groups, accompanied by the thermo-oxidative destruction of the polymeric framework [18,35]. Regarding the investigated oxidizers, three exothermic processes are observed in the case of AP, as illustrated in Figure 3a, including the endothermic crystallographic transition from an orthorhombic to a cubic structure at around 249.7 °C, accompanied by two overlapped exothermic peaks, between 343 and 382 °C, related to the low and high-temperature decomposition steps [36,37]. AN, however, undergoes three consecutive endothermic events within the temperature ranges of 50–58 °C, 125–132 °C, and 165–173 °C, corresponding to an orthorhombic–orthorhombic transition, a tetragonal–cubic transition, and the melting phenomenon of AN, respectively [9,38]. These processes are then followed by an exothermic decomposition event occurring at a maximum peak temperature of 259.2 °C, as depicted in Figure 3b. In the case of HNTO salt, presented in Figure 3c, two consecutive exothermic peaks are observed at T_peak = 188.0 °C and T_peak = 224.9 °C, which are related, respectively, to the low and high decomposition stages of HNTO [34,39]. On the other hand, it can be clearly noticed from Figure 3 that the maximum peak temperatures of the primary exothermic reactions of NCS/AN (T_peak = 204.3 °C) and NCS/HNTO (T_peak = 197.0 °C) admixtures shift toward higher temperatures compared to the first exotherm of NCS (T_peak = 164.7 °C), while the maximum exothermic peak temperature difference between NCS and NCS/AP is found to be 0.4 °C. The findings demonstrate that mixtures of NCS and AP, AN, and HNTO are chemically compatible according to the STANAG 4147. This statement obtained via DSC further confirms the potential use of the reported energetic molecules in nitrochitosan-based composite explosives and rocket propellants without detriment to the safety and reliability of the energetic formulation. Therefore, the next stage of research will be the determination of the optimal composition of such energetic composites, followed by a deep investigation of their molecular structures, crystallinity, morphological characteristics, thermo-kinetic behavior, and energetic performance.

3.2.2. TGA-Based Compatibility

To further elucidate the chemical compatibility between NCS and the selected energetic compounds, TGA experiments were carried out, and the recorded thermograms are depicted in Figure 4. It is obvious from the plotted TGA curves that NCS, which exhibits two overlapping decomposition stages, has the highest weight loss (21%), measured at the lowest exothermic decomposition peak selected via DSC, compared to the prepared physical mixtures. Indeed, it can be seen in Figure 4a that AP does not display any mass loss at T_peak = 164.7 °C. This selected temperature corresponds to the exothermic decomposition peak of NCS/PA admixture, which lost 15% of its initial weight. Similarly, Figure 4b shows that AN, identified with a single mass loss event that happened at a higher temperature than NCS, presents a mass loss of 7% at T_peak = 164.7 °C, while the observed weight loss of NCS/AN mixture is found to be 15%. Additionally, based on Figure 4c, it is clear that HNTO salt lost only 2% of its initial mass at the maximum peak temperature of the first exotherm event of NCS (T_peak = 164.7 °C). However, the NCS/HNTO admixture, which decomposes in two steps, shows a weight loss of 13% at the above-mentioned temperature. According to the above-mentioned findings and the STANAG 4147 standard criteria, we can conclude that the observed mass losses of NCS/HNTO (13%), NCS/AN (15%), and NCS/AP (15%) are lower than the sum of the weight losses of their respective individual compounds, providing further evidence of the extremely good chemical compatibility between NCS and the selected energetic molecules. Thus, such composites can be effectively used to develop the next generation of energetic formulations for potential use in military applications.

4. Conclusions

In summary, the chemical compatibility of NCS with three commonly used energetic molecules, namely AP, AN, and HNTO, was comprehensively investigated using non-thermal and thermal methods. FTIR findings of the raw compounds and their equi-mass ((wt%) = 50:50) physical mixtures revealed the absence of chemical interactions between NCS and the corresponding energetic molecules. DSC and TGA results showed that the thermal stability levels of NCS/AN and NCS/HNTO were significantly improved compared to that of NCS, while there was no notable change in the thermal stability of NCS/AP. As it arises from the STANAG 4147 criteria, the developed energetic NCS polysaccharide possessed excellent chemical compatibility with all the selected energy-rich compounds. Therefore, the outstanding findings obtained from this work may offer a fundamental theory for and data supporting the farsighted application of nitrochitosan-based composites, as well as address the challenges that will be encountered. These challenges involve investigating chitosan alternative sources to enhance sustainability, optimizing nitrochitosan-based formulations, and conducting assessments of their energetic performance.