Kinetic analysis of pyrolysis of nitrocellulose and 5,5’-bistetrazole-1,1’-diolate (TKX-50)

For the sake of exploring the decomposition characteristics and kinetic of nitrocellulose and TKX-50 mixtures, Friedman, KAS, and OFW kinetic analysis methods were used to investigate the mixture of nitrocellulose and TKX-50 mixtures. It shows that nitrocellulose and TKX-50 promote each other during the thermal decomposition process. Therefore, this study provides a reference for NC-A and TKX-50 in the application field of explosives.


Introduction
The energy density and safety performance of materials with high energy are associated with the performance of weapons.Azole compounds containing massive chemical bonds relate to element N, which owns abundant nitrogen as well as positive enthalpy.In recent years, many new types of energetic materials have emerged, and azole compounds have attracted the attention of relevant researchers [1][2][3].The most concerning are tetrazolium compounds, which contain more than 80.0% nitrogen.It is currently the ring structure unit with the highest nitrogen content that can stably exist.Among these tetrazole compounds, dicyclotetrazole and its derivatives, have received great attention and extensive research from researchers around the world, because they have good density, better enthalpy and excellent stability.In 2012, Klapotke et al [4] designed and synthesized TKX-50.
Compatibility is the primary consideration when formulating gun propellant, otherwise, the components will react with each other, which may accelerate aging or change their stability, leading to unexpected results.Several research studied the compatibility of TKX-50 with other commonly used materials.Zhang et al. [5] studied the change of (H 2 dabco) [NH 4 (ClO 4 ) 3 ], TKX-50 and DAP-4 mixture by solid in situ infrared technology, and finally inferred their thermal decomposition mechanism.Wang et al. [6] studied the influence of AP on TKX-50 and believed that AP and TKX-50 produced a strong interaction under thermal stimulation, which can increase the pyrolysis temperature of AP/TKX-50 without affecting the complete decomposition.Zhao [7] found the interaction between TKX-50 and energetic compounds such as HMX under thermal action and discovered that HMX can decrease the peak temperatures of TKX-50 at two thermal decomposition stages by 4 K and 15 K, respectively.Furthermore, the final decomposition was incomplete with a residual rate of 11%, which affected the release of energy for TKX-50.

Theoretical background
2.1 DSC analysis NETZSCH DSC 204 F 1 was used in this study.The heating rates were 1, 2, 5, 10 K min −1 , and the temperature was from 50 to 305°C.The sample mass was about 0.6 mg.

Kinetic analysis
For estimating kinetic parameters, there are many methods.Among them, integral OFW [8], KAS [9] and Friedman [10] methods are the most widely used.These methods have been already introduced in the reported references and their calculation formulas are listed in Table 1.In previous research, these model-free methods were always used for decomposition kinetics, because of the complexity brought about by the conversion evaluation of each point in kinetic parameter analysis, this complexity makes the E a and pre-exponential a function of α.
Eq. (3)  In Figure 1 (a), the DSC curves of nitrocellulose are not very sharp in the exothermic region at different heating rates.At 1, 2 K min -1 , the exothermic peak is about 190°C, while at 5, 10 K min -1 , the exothermic peak is around 200°C.The entire exothermic process starts at about 170°C and ends at about 250°C.The decomposition of TKX-50 is roughly two parts, as shown in Figure 1 (b).At 1, 2 K min -1 , the first exothermic peak is about 220°C, while at 5, 10K min -1 , the first exothermic peak is about 240°C.The whole exothermic process starts at about 200°C and ends at about 300°C.In Figure 2, the decomposition of a 30% TKX-50 + 70% nitrocellulose mixture has three stages, but the third stage is not significant when at 1 or 2 K min -1 .The first one is about 170-195℃, then comes 185-210℃ and the final is about 230-250℃.The mixture has two obvious decomposition ranges.The low peak temperature range is about 170-194℃ and the high peak temperature range is about 180-207℃.Considering the DSC thermal decomposition curves of pure nitrocellulose and TKX-50, it can be inferred that the first range is similar to the decomposition of nitrocellulose, and the next range can be attributed to the decomposition of TKX-50.It is obvious that compared to the pure components, the decomposition temperature range of the mixture has advanced.

Model-free methods analysis
The E a was figured out by OFW, KAS and Friedman methods.The curves of E a with conversion α are shown in Figure 3. Firstly, the E a of nitrocellulose has a tendency of increasing, then stabilizing, and finally decreasing during the heating process, while the changing trend of E a of TKX-50 is first increasing, then reducing, and finally growing.But the changing trend of the mixture was to increase continuously and then decreased.It can also be seen that for all researched methods, there is a steady increase from α = 0.10 to 0.50, with E a increasing from 185 to 245.Then E a reaches a maximum figure of approximately 320 at α = 0.80.When α of is small, the E a of the mixture is about 20% smaller than that of nitrocellulose, indicating that TKX-50 has an advanced impact on the decomposition of nitrocellulose.According to the above analysis, the decomposition of nitrocellulose has been advanced during the decomposition of the mixture, and when TKX-50 begins to decompose, some changes have already taken place.From Table 2, it was worth noting that these three model-free methods gave very close results, the average E a of KAS and OFW methods is around 212 kJ mol -1 , while Friedman is about 234 kJ mol -1 .Therefore, in practical applications, the differences caused by calculations can be ignored.

Conclusion
(1) In this paper, through relevant thermal analysis experiments, it was discovered that TKX-50 advanced the thermal decomposition of nitrocellulose, but it has an impact on the entire decomposition of nitrocellulose, while the addition of little nitrocellulose would change the process of TKX-50.
(2) The E a of the mixture is lower than nitrocellulose and higher than TKX-50 in the initial reaction stage, so TKX-50 advances the decomposition of nitrocellulose in the initial stage, and TKX-50 is not likely to start the reaction at the current temperature.
(3) It can be known that OFW, KAS and Friedman methods gave similar results, therefore, in practical applications, the differences caused by calculations can be ignored.
(4) This work provides theoretical guidance for future applications in the field of explosives, and the compatibility of the two is not as good as expected, and the connection between the compatibility of the two can be further explored.