1 Introduction

The development of energetic materials contributed enormously to the progress and prosperity of mankind^[1,2,3]. Meanwhile, heavy casualties caused by their accidental explosion and serious environmental pollution always plague the researchers, thus, the application of greener and more insensitive explosives is required^[4,5,6]. Generally, insensitive explosives are divided into two categories based on their preparation processes. One is “cast‑cured explosives”, since the preparation is achieved through cast cured process^[7]. The other is “melt‑cast explosives”, which is melted and cast into shells during their preparations^[8]. Compared with melt‑cast explosives, cast‑cured ones are more difficult to process and recycle, and more expensive^[9], therefore, there are increasing interests for insensitive melt‑cast explosives in the past decades^[10,11].

2,4,6‑Trinitrotoluene (TNT) acts as an ideal organic solvent in melt‑cast process due to its capability in the dissolution of numerous energetic materials, which results in an acceptable processing viscosity^[12]. Currently, TNT based melt‑cast explosives are no doubt the most important “melt‑cast explosives” with intensive studies^[13], however, strong toxicity and high vapor‑pressure of TNT often lead to fatal harmful effect and the relatively high sensitivities cause significant risks during the production and application processes of TNT^[14]. In recent years, developing new insensitive explosive to replace TNT has become a key research direction in the field of melt‑cast explosives^[15,16].

Nitroguanidine (NQ) is widely applied and known for its inherent insensitivity. Propyl‑nitroguanidine(PrNQ) is an alkylated analog of NQ, with a propyl moiety effectively reduces the melting point while nitroguanidine portion of the molecule provides the explosive power. PrNQ has a detonation velocity of 7.76 km·s^-1 with a density of 1.64 g·cm^-3, which is higher than TNT^[17]. In contrast, the ERL impact and BAM friction sensitivity data are higher than 100 J and 360 N, which are lower than those of TNT (88 J and 220 N)^[18]. The good detonation and insensitivity performances make PrNQ an important potential candidate to replace TNT. However, studies on its thermal behaviors and compatibilities with other high explosives, which are crucial for its further applications, are still limited. Herein, in this work, we reported the thermal behaviors, non‑isothermal kinetics of thermal decomposition, and decomposition mechanism of PrNQ as well as its compatibility with some important high explosives. The thermal decomposition process of PrNQ was studied by DSC‑TG method while the compatibilities of PrNQ with 1,3,5‑trinitroperhydro‑1,3,5‑triazine (RDX), 1,3,5,7‑Tetranitro‑1,3,5,7‑tetrazoctane (HMX), hexanitrohexaazaisowurtzitane(CL‑20), 5,5′‑bistetrazole‑1,1′‑diolate(TKX‑50) were investigated by DSC experiments. The intermediates formed during the pyrolysis process were also characterized through in‑situ FTIR spectroscopy technologies to reveal the intrinsic decomposition mechanism of this novel nitroguanidine derivative.

2 Experimental

2.1 Sample

The sample of PrNQ used in this work was prepared through one step synthesis from nitroguanidine (NQ) and propylamine (Scheme 1).

Scheme 1 Preparation of PrNQ from NQ

2.2 Apparatus and Measurements

The thermal analysis experiments of PrNQ were performed on a model TG‑DSC STA 449C instrument (NETZSCH, Germany) as well as a DSC Q200 instrument (TA, America). The operation conditions were: sample mass, 0.6 mg; atmosphere, dynamic nitrogen, 50 mL·min^-1; aluminum cell was used.

IR spectra were recorded on a Nicolet 60SX FTIR spectrometer employing an HgCdTe detector. The in‑situ FTIR spectroscopy experiment was carried out with a Nicolet 60 SXR FTIR spectrometer. The operation conditions were: sample mass, 0.6 mg; heating rate, 10 ℃·min^-1; 17.8 file·min^-1 and 16 scans·file^-1 were recorded at a resolution of 4 cm^-1; temperature range, 20-450 ℃.

3 Results and Discussion

3.1 Thermal Behaviors of PrNQ

Thermal decomposition behavior is regarded as one of the most important indexes on evaluating the quantity of explosives and studies of thermal properties of PrNQ were first carried out through DSC measurements. From the DSC curve in Fig.1, an endothermic peak with summit peak at 99.39 ℃ could be clearly observed and the highly sharp shape of this peak indicates that it most likely to be a physical change. To clarify this physical process, TG‑DTG technology was carried out. It finds that the mass of PrNQ maintains during the process, which demonstrates that PrNQ melted around 99.39 ℃, which is an ideal melting point for the preparation of melt‑cast explosives. With the rise of the heating temperature, a major exothermic decomposition peak near 236.10 ℃ appears in DSC curve. It is clear that the mass of PrNQ lost completely around this temperature in the TG‑DTG curve (Fig.2). The discrepancy between melting temperature and decomposition temperature of PrNQ is about 137 ℃, which is large enough to guarantee the safety of the melt‑cast process as the heating temperatures are generally under 150 ℃ in the preparation of melt‑cast explosives.

Fig.1 DSC curve of PrNQ at a heating rate of 10 ℃·min^-1

Fig.2 TG‑DTG curve of PrNQ at a heating rate of 10 ℃·min^-1

For better evaluation of application prospect, DSC experiments of TNT under same conditions were also carried out. TNT melted at a relatively lower temperature of 80 ℃. However, with the rise of heating temperature, a spontaneous endothermic volatilization process of TNT was observed, which means a large amount of TNT vapor was generated when the molecule was heated, which could also be observed in TG‑DTG studies of TNT^[19]. The existence of TNT vapor obviously will lead to high toxicity and serious safety issues during the melt‑cast process of explosives. In comparison of the previous research, it can be concluded that PrNQ has a much lower toxicity and vapor pressure than TNT, showing a good application prospect as the new melt‑cast explosives carrier for the replacement of TNT.

3.2 Kinetic Study of PrNQ

In order to obtain the kinetic parameters of the thermolysis process, investigations on the non‑isothermal kinetics of thermal decomposition of PrNQ were carried out and DSC curves at different heating rates were employed. Seen from Fig.3, the peak temperatures increase when the heating rate increases, and the decomposition peak temperatures are 215, 227, 235 ℃ and 248 ℃ at the heating rates of 2.5, 5, 10 ℃·min^-1 and 20 ℃·min^-1, respectively. On this basis, we choose both Kissinger′s model and Ozawa′s model^[20,21] to further calculate the apparent activation energy (E _a) of the decomposition reaction of PrNQ. The results show that two methods coincide quite well and the reaction activation energy of PrNQ is approximately 129 kJ·mol^-1(128.1 kJ·mol^-1 from Kissinger′s model and 129.8 kJ·mol^-1 from Ozawa′s model).

Fig.3 DSC curves of PrNQ at different heating rates

3.3 Structural Transformations During the Decomposition Process of PrNQ

The studies of structural transformations over thermal decomposition process will provide rich information for the decomposition mechanisms of energetic molecules. To better understand the thermal decomposition mechanisms of PrNQ, detailed structural transformations during the pyrolysis process were carefully characterized through in‑suit FTIR spectroscopy. Fig.4 demonstrates the in‑suit FTIR spectroscopy of PrNQ at 20-450 ℃. There are no great changes on the peak positions within the first 20 min. With the further rise of heating temperature, the IR peaks are changing rapidly. Seen from Fig.4, the intensity of IR peaks of —NH₂ and —NH— units are first weakened, followed by the intensity of IR peaks of —NO₂ unit and —Pr unit, which indicates that structure unit connected by amine are the most unstable moiety under heating conditions. The new formed IR peak at about 2200-2600 cm^-1 is very likely caused by the formation of —CN units during the major exothermic decomposition.

Fig.4 In‑suit FTIR spectroscopy of PrNQ at heating temperatures

3.4 Compatibility of PrNQ with Some High Explosives

As a potential formula carrier for melt‑cast explosives, it requires not only excellent thermostability but also good compatibility with existing high explosives. From application perspectives, compatibility of PrNQ with other energetic materials is one of the most important characters. RDX, HMX, CL‑20 and TKX‑50 are among the most potent chemical explosives manufactured and the compatibilities of PrNQ with RDX, HMX, CL‑20, TKX‑50 were investigated by means of DSC technique. The experiments were carried out according to method 602.1 in GJB 772A-1997, and the results are shown in Fig.5 and Table 1.

a. RDX

b. HMX

c. TKX‑50

d. CL‑20

Fig.5 DSC curves describing the compatibility of PrNQ with existing high explosives.

The major exothermic peaks of RDX at 241 ℃ and PrNQ at 236 ℃ are caused by the rapid decomposition reactions. Affected by the decomposition process of each other, the major exothermic peak of the mixture moves forward to 231 ℃. The decrease of exothermic peak value of the mixture demonstrates the incompatibilities of PrNQ with RDX. Similarly, when PrNQ mixes with CL‑20, the major exothermic peak of the mixture at 192 ℃ is much lower than the exothermic peak of PrNQ and CL‑20, indicating a poor compatibility between them. In contrast, the major exothermic peak of PrNQ‑HMX mixture is almost the same with the major exothermic peak of PrNQ, showing a good compatibility of PrNQ with HMX. When PrNQ is mixed with TKX‑50, there are two major exothermic peaks of the mixture. One major exothermic peak of the mixture is exactly same with TKX‑50 and a little higher than PrNQ while the other major exothermic peak of the mixture is higher than both of TKX‑50 and PrNQ. The higher exothermic peak value of the mixture shows an ideal compatibility of PrNQ with TKX‑50. The experiment data are summarized in Table 1. In conclusion, PrNQ is compatible to HMX and TKX‑50 from a heat standpoint, while not compatible with RDX and CL‑20, of which the compatibility with CL‑20 is the worst.

4 Conclusions

Thermal behaviors and decomposition process of PrNQ were studied with DSC‑TG and in‑situ FTIR spectroscopy methods. Conclusions are obtained as follows:

(1) According to the DSC‑TG analysis, PrNQ is melted around 99.39 ℃, which has an ideal melting point as the potential formula carrier for melt‑cast explosives.

(2) The major exothermic decomposition peak of PrNQ is near 236.10 ℃ and the discrepancy between melting temperature and decomposition temperature is about 136 ℃, which is large enough to guarantee the safety of the melt‑cast process.

(3) Studies on kinetics and decomposition mechanism of PrNQ show that this energetic molecule has excellent thermostability. Furthermore, from studies of compatibilities of PrNQ with other high explosives, PrNQ is proved to be compatible to HMX and TKX‑50 from a heat standpoint, while not compatible with RDX and CL‑20, of which the compatibility of PrNQ with CL‑20 is the worst.

（责编: 张 琪）

Reference