Characteristics of Thermal Decomposition of Energetic Materials in a Study of Their Initiation Reactivity

Chapter 14

Characteristics of Thermal Decomposition of Energetic Materials in a Study of Their Initiation Reactivity Svatopluk Zeman University of Pardubice, Pardubice, Czech Republic

14.1 INTRODUCTION During the 1980s, relationships were found between the results of nonisothermal differential thermal analysis (DTA, i.e., low-temperature thermal decomposition) and the detonation characteristics of polynitro compounds (see Ref. [1] and quotations therein). A more detailed analysis of these results shows that their classification, in the sense of the relationships found, is given primarily by (a) steric conditions and (b) the electron configuration in the ground state of the reaction center of the molecule [1–8], with a logical emphasis on their molecular similarity (see, e.g., papers [1–3,8–13]). At the same time, such facts represent one of the basic principles of the organic chemistry approach in dealing with reactivity problems in general [3]. Recently, the basic knowledge from that earlier period has been extended by the study of the primary fission mechanism in other types of initiation of energetic materials (EMs) [1–8] including the use of quantum mechanics in addressing these problems (see, e.g., Refs. [6–13]), and by performance studies of such materials (see Ref. [8] and references therein). It is appropriate to include in this list a study by Tsyshevsky et al., according to which the thermal decomposition processes play a major role in defining the sensitivity (initiation reactivity) to detonation initiation of such EMs [14]. The characteristics of these processes are easily available and, when these correspond to a non-autocatalyzed mechanism, it is possible to extrapolate them to the conditions of detonation [1–5,7]. The facts outlined above are a part of this chapter, which also includes certain possibilities for determining the reaction centers in molecules. Handbook of Thermal Analysis and Calorimetry, Vol. 6. https://doi.org/10.1016/B978-0-444-64062-8.00006-1 © 2018 Elsevier B.V. All rights reserved.

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14.2 THE MAIN SOURCES OF THERMAL DECOMPOSITION DATA The course of the primary fission processes in thermal decomposition of EMs is generally monitored by studying their secondary effects, that is, the type and quantity of gaseous products, and/or thermal effects, and/or the mass decrease accompanying the reaction. The thermal decomposition of individual EMs can thus be divided into two limiting types [9]: (a) a decomposition into products with a low molecular weight, the mechanism of which does not involve too many intermediate stages (e.g., nitramines and polynitroaliphatic compounds) and (b) a decomposition with relatively stable intermediates where the induction period need not necessarily be linked with a more pronounced development of gaseous products (e.g., polynitroarenes and polynitroheteroarenes). Reaction kinetics for the first type of decomposition can be studied by methods based on monitoring the quantity of reaction products over time. Reaction kinetics for the second type can best be studied by methods based on monitoring the thermal effects of the reaction, methods which may, in addition, be used with advantage for the first kind of decomposition. In practice, these gasometric and thermoanalytical methods are the most widespread in today’s study of the thermal stability of EMs [8,9]. The strong dependence of the corresponding kinetic parameters on temperature, pressure, and the construction materials in contact with the decomposed sample is well known in defining the kinetics and mechanism of thermal decomposition of EMs [2,3,5,9,15]. A correlation of results from thermal analyses of these materials using different methods (gasometric versus thermoanalytical) and/or different types of apparatus of different origin is relatively rare (see Refs. [1–3,8,9,13] and quotations therein), even though in the past few years it has been possible to observe a certain consolidation [15–17]. So far, the most reliable results in this area are both the theoretical and the practical findings obtained by Russian scientists using their manometric method (a special version of the isothermal vacuum stability test— see Refs. [1–3,15,18] and quotations therein). The data obtained by this method, in the past also referred to as SMM, are known to correspond to the non-autocatalyzed stage of thermal decomposition of the given material (i.e., to molecular structure), and also to the absolute values of the corresponding Arrhenius parameters [1–3,9,15]. For thermal decomposition in the condensed state, the results of some methods used in differential scanning calorimetry (DSC) (e.g., Refs. [2,3,9,19,20]) might be directly comparable with the results of SMM [9,10,15,19] and somewhat less good than those from thermogravimetric analysis (TGA) (e.g., Refs. [19–23]); in the open system of the TGA, measurement evaporation (sublimation) has a certain influence [22].

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The kinetic parameters of the thermal decomposition of EMs have also been predicted and give results very similar to those obtained using SMM [24,25]. The first of these methods [24] assumes that the activation energy of some 86 energetic compounds with different molecular structures and with the general formula CaHbNcOd can be expressed as a function of an optimized elemental composition as well as the contribution of specific molecular structural parameters. The second method [25] uses a new computer program, named EDPHT 2.0, which also embraces the prediction of the crystal density and enthalpy of formation. Paper [1] describes the history of using simple nonisothermal DTA in the study of detonation initiation. The choice of the methodology for evaluating the DTA recordings was originally based on the fact that a higher brisance of an individual explosive is usually connected with a steeper DTA exothermal curve for its decomposition [1]. Another verified source of reactivity data [1,26] is the evaluation using the well-known Kissinger method [27] represented by the equally well-known relationship:       f Ea 1 AR (14.1)  + ln ln 2 ¼  R T T Ea The thermal reactivity was expressed by the EaR1 slopes from this relationship [1,3,8,26]. This method is used with advantage for a study of heterogeneous explosive mixtures such as the powdered and emulsion explosives or plastic bonded explosives (PBXs) [3,26,28].

14.3 STRATEGY AND REASONS FOR THE VARIOUS APPROACHES 14.3.1 Approach Based on Primary Fission Similarity 14.3.1.1 Detonation The classical hydrodynamic theory of detonation [29] deals with the initial and final state of this transformation and thus cares nothing about the chemical mechanism of initiation of this transformation. A survey of models of the detonation initiation by shock in presented in a monograph [3], including the most interesting approaches to this problem in the 1993–2004 period. Zeman has summarized the published significant views on this kind of initiation (except for the model of physical kinetics) as follows [3]: the influence of shock on EMs results in adiabatic compression of the molecular layer struck. According to Klimenko and Dremin [30–33], the kinetic energy of the shock in this compression is accumulated, through translational– vibrational relaxation processes, by translational and vibrational modes of molecular crystals of the material within 1013–1012 s. This causes a considerable quasi-overheating (20,000–40,000 K [32,33]), especially of vibrational modes. A nonequilibrium state is established with concomitant

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primary fission of the EM into radicals [3,30–33]. Chemical reactions of these active particles cause the shock front to spread and evoke a second equilibrium stage of detonation behind the front. This or similar ideas of transformation of low-frequency vibrations of crystal lattice (acoustic phonons) into high-frequency vibrations (vibrons), with subsequent spontaneous localization of vibrational energy in the explosophore groupings [3,34,35], have been applied by a number of authors in their studies of shock reactivity of EMs. Concerning the thermal decomposition namely of polynitro and polynitroso compounds, its dependence on reaction conditions was already mentioned; especially the construction materials, which are in contact with the sample, have significant affect on the kinetics here [9,36] what is a clear evidence of the homolytic primary fission in this decomposition. The homolytic character of primary fission in both the detonation and low-temperature thermal decompositions of EMs was a reason for using the Evans-Polanyi-Semenov equation (E-P-S) [37] to study the chemical micromechanism governing initiation of EMs [1,3,8,9,26]. The original E-P-S equation describes a relationship between activation energies E of most substitution reactions of free radicals and their corresponding heats of reaction DH [37]: E ¼ B + a0∗ DH

(14.2)

where B is a constant. Eq. (14.2) is valid for closely related molecular structures and indicates that the strength of the bond being split is a decisive factor in the given reaction [28]. Substitutions of DH by heat of explosion Q and E by activation energy Ea of the low-temperature thermal decomposition led to a first version of the modified E-P-S equation [1,3,8,9,19] with the general shape (here C is a constant): Ea ¼ C + a∗ Q

(14.3)

which is applicable to the study of the chemical micromechanism of initiation of the individual EMs [1,3,8,9,26]. A better correlation is obtained for “real” heats of explosion, Qreal, which should correspond to the experimental value determined in a calorimeter under isochoric combustion conditions [38] (see Figs. 14.1–14.3) and whose value depends on the given EM density. Taking the well-known relationships between heat of explosion and the square of detonation velocity, D2 (see monograph [39]): Q¼

D2 2ð g 2  1 Þ

(14.4)

it is possible to rewrite Eq. (14.3) to form the following equation [1,3,8,9,26] (see Figs. 14.4 and 14.5): Ea ¼ C0 + b∗ D2

(14.5)

FIG. 14.1 Graphical representation of the modified E-P-S equation (14.3) [3,9] for polynitro arenes with a hydrogen atom at the g-position with respect to the nitro group (see Refs. [2,3,5,9,10] and quotations therein)—the polychlorinated derivatives of 1,3,5-trinitrobenzene—TNB (i.e., CTB, DCTB, and TCTB) are exceptions. Their thermal decomposition, however, represents a certain analogy with the decomposition of the polymethyl derivatives of TNB [3,9]; the straight line for polyamino TNBs represents a transition between decomposition from the liquid state (PAM) to the solid state (TATB, with sublimation in DSC measurement while under conditions for the SMM method, decomposes as if in liquid state). Reproduced from S. Zeman, Modified Evans–Polanyi–Semenov relationship in the study of chemical micromechanism governing detonation initiation of individual energetic materials, Thermochim. Acta 384 (2002) 137–154.

FIG. 14.2 Modified E-P-S equation (14.3) for the relationship between the activation energies, Ea, for low-temperature thermal decomposition of nitramines (mostly from the Russian manometric method), and the real heats of explosion, Qreal—an extended original figure from paper [3,9] about the HNIW data. The data for this nitramine on the straight line for cyclic nitramines are from a Russian study and those with the lower Ea values were obtained by TGA, probably on technical samples (see Refs. [8,19,20] and quotations therein).

FIG. 14.3 Modified E-P-S equation (14.3) for azides and fulminates [9]. For azides, the literature gives activation energy values, Ea, for thermal decomposition within various temperature ranges; however, only those Ea values which correspond to the lowest experimental temperature ranges correlate well [3,9]. Reproduced from S. Zeman, Modified Evans–Polanyi–Semenov relationship in the study of chemical micromechanism governing detonation initiation of individual energetic materials, Thermochim. Acta 384 (2002) 137–154.

FIG. 14.4 Relationship (14.5) between the squares of the calculated detonation velocities and experimental activation energies of the thermal decomposition for nitramines [19]; in comparison with Fig. 14.2, the set of nitramines is extended by including the following compounds [19]: Aurora, 4,8,10,12-tetranitro-2,6-dioxa-4,8,10,12-tetraazaisowurtzitane; BCHMX, cis-1,3,4,6tetranitrooctahydro-imidazo-[4,5-d]imidazole; DINGU, 1,4-dinitrotetrahydroimidazo[4,5-d]imidazol-2,5-(1H,3H)-dione; TETROGEN, 1,3-dinitro-1,3-diazetidine; TNAZ, 1,3,3-trinitroazetidine; TNIO, 3,4,6-trinitrohexahydro-2H-imidazo[4,5-d][1,3]oxazole; TNOTZ, 3,5,7-trinitro-1-oxa3,5,7-triazocane. Reproduced from S. Zeman, Q.-L. Yan, M. Vlcˇek, Recent advances in the study of the initiation of energetic materials using characteristics of their thermal decomposition part I. Cyclic nitramines, Cent. Eur. J. Energetic Mater. 11(2) (2014) 173–189 with permission.

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FIG. 14.5 Graphical representation of the modified E-P-S relationship (14.5) between the EaR1 slope of the Kissinger relationship (14.1) and the square of the calculated detonation velocity of nitramines; an original figure from papers [1,3] extended with BCHMX data, taken from Ref. [26]—molecular structural similarity is strongly emphasized here.

The square of detonation velocity, D2, is here a representative of performance of the given EM; the advantage of this approach rests in the relatively easy experimental availability of the D values (see Fig. 14.6). A comparison of Figs. 14.1 and 14.2 shows a difference between primary fission via a five- or six-membered transition state in derivatives of polynitro arenes (see Refs. [2,3] and citations therein) and direct homolysis of the N–N bonds in nitramines (different straight-line slopes). Decomposition of ionic azides and fulminates (Fig. 14.3) is a chain reaction, that is, not going through cyclic transition states [40] (and results of Aluker et al., published in Ref. [3]). Trends in Figs. 14.1 and 14.3 correspond to increasing sensitivity due to an increase in performance of the EMs [8]. In Fig. 14.2, it is the opposite. In the latter case, a stabilizing influence of the crystal lattice mainly in cyclic nitramines may well be a reason for this distinction. Relationship (14.5) for a set of nitramines is described by Fig. 14.4 [19]. In contrast with Fig. 14.2, the heat of explosion, Qreal, has here been substituted by the square of detonation velocity, D2, and several other EM molecular structures have been included in the set (BCHMX, Aurora, TNIO, TNOTZ) [19]. This substitution leads to separating the nitramine group into several subgroups according to the specific influence on their thermal decomposition of certain physicochemical properties of the individual nitramines studied [19]: straight-line A corresponds to decomposition in the solid state, and straight-line B combines nitramines liquidized during thermal decomposition due to their dissolution in the decomposition products. Decomposition in the

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FIG. 14.6 The graphical representations of relationship (14.5) between the squares of the experimental detonation rates of explosive mixtures and the EaR1 slopes of the Kissinger relationship (14.1)—figure from paper [19] extended by adding the PBXs data from [26]—straight-line D: oxidizing systems for groups A, B, C and TATP/AN explosives are based on ammonium nitrate (AN) and its mixtures with sodium nitrate (SN), potassium nitrate (PN), calcium nitrate (CN), aluminum nitrate (AlN), lithium nitrate (LiN), sodium chloride-sodium nitrate (NaCl/SN), nickel(II) nitrate (NiN), copper(II) nitrate (CuN), and cobalt(II) nitrate (CoN). The black points on straightline A are dynamites from the Explosia Co. The oxidizing system in Alc/HP liquid mixtures is hydrogen peroxide, whereas the fuels are ethanol, glycol, and glycerine; Code Comp means demilitarized mixture RDX/TNT 60/40. Straight-line D combines plastic bonded explosives with 9% by wt of binders: V9, Viton A; PA, polymethylmethacrylate; C4, polyisobutylene.

liquid state is characteristic of compounds on line C, and line D associates nitramines with crowded molecules, decomposed mostly in the liquid state. The correlation of the g-HNIW data with this line clearly shows that this polymorphic modification substitutes for a liquid phase of HNIW. Relationships (14.3), (14.5) are applicable not only to the study of the micromechanism of detonation [1–5,9,19] and assessment of the mutual compatibility of activation energies from the different authors and methods [2,3,5,9,19], but also to the assessment of the physicochemical changes in the crystals of EMs in the temperature region close to that for their thermal decomposition [19]. An interesting relationship results from the comparison of the rate constants, k, of unimolecular thermal decomposition, calculated at 230°C (k230), and detonation velocities, D, at maximum theoretical crystal densities of the nitramines in the study [19]. This newly found logarithmic relationship (14.6) generally shows these velocities decreasing with increasing k(t) values (calculated for a certain temperature t) [19]: ln kðtÞ ¼ a  ln D + b

(14.6)

A key part of this relationship, in the nitramines studied here, rests in the equivalence of the primary fission processes in the low-temperature thermal

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decomposition and in the initiation and growth of detonation. Splitting the compounds according to this relationship is similar to the split in Fig. 14.4. Specifically, the influence of the molecular structure is slightly lower, whereas the influence of the physical state in thermal decomposition is clear and, in addition, the influence of detonation thermochemistry is observed [19]. This relationship, together with those in Fig. 14.4, confirms the problems in the kinetic specification of thermal decomposition encountered with HMX and HNIW [19]. 14.3.1.1.1 Generalization of Validity of the Relationships Found Subsequent research has shown that, in both Eqs. (14.3), (14.5), the activation energy, Ea, is not the only parameter for the activation process that can be used [1,3,8–10,19,26]. It can be substituted by the EaR 1 slope of the Kissinger relationship (14.1) [1,3,11,19,26,36], the electric spark energy, EES [3,41], drop energy, Edr [13], or by the half-wave polarographic potential [42]. Similarly, the Q or D2 variables might be substituted by detonation pressure [28] (Fig. 14.7) or by the product of density and the square of detonation velocity [26] (Fig. 14.8), by the charge, qN, at the nitrogen atom of the most reactive nitro group [1,11,13,43,44] in the molecule (see Figs. 14.3, 14.17, and 14.18), by the net charge of this nitro group, QNO2 [13] or, recently, by

FIG. 14.7 Relationship between thermal reactivity, expressed as the slopes of the Kissinger relationship (14.1), and calculated detonation pressure (by Explo5 code) of the explosive mixtures containing urea and/or peroxides [28]. The codes used here are: AN, ammonium nitrate; U, urea; UHP, urea-hydrogen peroxide adduct; AP, ammonium perchlorate; UN, urea nitrate; Al, aluminum; ANFO, mixture of AN and fuel oil; TATP, 3,3,6,6,9,9-hexamethyl-1,2,4,5,7,8-hexoxonane; the suffix “react” means “reacted in C-J plane.” Reproduced from A.K. Hussein, S. Zeman, M. Su
ceska, M. Jungova´, Relative explosive strength of some explosive mixtures containing urea and/or peroxides, Chin. J. Propellants Explos. 39(5) (2016) 22–27 with permission.

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FIG. 14.8 Relationship, for PBXs based on cyclic nitramines, between the product of loading density and the square of experimental detonation velocities, on the one hand, and the slope of the Kissinger relationship (14.1), on the other [26]: here suffix C4 means 9% by wt of the polyisobutylene binder, suffix PA represents 9% by wt of the polymethyl methacrylate binder, and V9 9% by wt of the vinylidene fluoride-hexafluoropropene copolymer (Viton A). The nitramines represented are: 1,3,5-trinitro-1,3,5-triazinane (RDX), b-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imidazole (BCHMX), and e-2,4,6,8,10,12hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20 or HNIW). Reproduced from S. Zeman, Q.-L. Yan, A. Elbeih, Recent advances in the study of the initiation of energetic materials using the characteristics of their thermal decomposition. Part II. Using simple differential thermal analysis, Cent. Eur. J. Energetic Mater. 11(3) (2014) 285–294 with permission.

the summed positive and negative extremes of the electrostatic surface potentials, VS,S (Eq. 14.8 and Fig. 14.20) [20]. 14.3.1.1.2

Thermal Reactivity as a Kissinger Slope Relationship

The relationship between thermal reactivity, expressed by the Kissinger slope (14.1), EaR1, and performance as a square of detonation velocity, D2, is represented by Fig. 14.5 for individual nitramines [1,3] and by Fig. 14.6 for explosive mixtures [19]. In the first case, data for all nitramines, with the exception of TEX, reside on straight lines which have a logical common intersection point—1,3,5-trinitro-1,3,5-triazepane (HOMO)—this kind of relationship is typical for Physical Organic Chemistry. The difference between the eand a-HNIW molecules is interesting here (it is caused by mutually different intermolecular interactions in their crystal lattices as demonstrated by the data comprising the corresponding straight lines). For a study of thermal reactivity of heterogeneous explosive mixtures, such as the PBXs or industrial explosives, the simple DTA is more suitable, with its results being evaluated by the Kissinger method [27] (see

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Figs. 14.6–14.8). The main component of the oxidizing systems in the W/O explosive materials from Fig. 14.6 is ammonium nitrate (AN)—a most reactive component (with the exception of the peroxide mixtures Alc/HP and TATP/ AN). Here line B corresponds to the classical W/O emulsion explosives, line A to the W/O emulsions fortified by the addition of at least 30% (w/w) of the high explosives RDX and PETN (black points represent dynamites containing 28%–40% of nitric esters), and line C to the W/O mixtures fortified by demilitarized composition B (RDX/TNT in a ratio of 60/40). The group around line A indicates that a critical amount of admixtures of nitrate esters or nitramines exists for the mixtures (i.e., about 30% by wt). It has been stated that in the explosive mixtures studied the thermal reactivity of the oxidizing system and/or its mixture with a high explosive replaces the primary thermal reactivity of explosophore groups in the individual EMs [19]. The opposite performance trend shown in lines B and D, in comparison with the other relationships in Fig. 14.6, corresponds to the difference in amount of the usable oxygen in the CJ-plane in these two groups of explosives. In the first case, it is the negative effect of another inorganic nitrate admixture in the W/O mixture on the consumption of AN in the reaction zone of the detonation wave (AN behaves partially as if inert in the presence of inorganic nitrates) [47], whereas in the second case this oxygen is removed by the binder (polyfluorinated Viton A logically has the smallest negative effect here). The version of Eq. (14.3) with calculated detonation pressure, P, instead of heat of explosion, is represented by Fig. 14.7 for the explosive mixtures containing urea and/or peroxides [28]. Here the trends, corresponding to lines C and D, are in agreement with analogous trends in PBXs [26]. The different positions of these lines in the grid system of Fig. 14.7 are caused by the fact that urea can substantially increase the thermal stability of AN (line C in Fig. 14.7 corresponds to this) without reducing the ability to instigate detonation [47]. The position of the data for the UHP/AN/Alreact mixture (point M1, practically on the straight-line D) might mean that aluminum in the presence of peroxides could succumb (at least partially) to oxidation in the CJ plane of the detonation wave. Line A combines mainly groups of mixtures based on the urea-hydrogen peroxide adduct (UHP), and line B those mixtures with urea nitrate (UN) or TATP, both with decreasing activation energies with increasing performance. Thus, on the basis of this relationship, it is possible to split the mixtures into those with the ionic primary fission (the most reactive admixture is AN) and those with the radical fission (homolysis of the O–O bond in peroxides) in their initiation [28]. Instead of using detonation pressure, product loading density and the square of detonation velocity (r  D2) can be used, as shown in Fig. 14.8 [26]. Here explosives CL-20-PA and CL-20-C4 ought to be with the PBXs centered on the straight-line I and, similarly, explosive CL-20-V9 should be incorporated into the group on the straight-line II. From the study of the role of polyfluorinated binders during the detonation process [48], it was found that, from a certain pressure and temperature, detonation of the corresponding

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nitramine’s PBXs ought to cause a shift in the fluorine chemistry in this process in comparison with analogous PBXs, and this assumption should be the reason for the CL-20-V9 data placement in Fig. 14.8. However, the already mentioned place of the CL-20-PA and CL-20-C4 data in this figure can be seen as a proof of similar changes in the case of other PBX binders. Fig. 14.8 also confirms the advantage of using polyfluorinated binders from the point of view of the performance of the corresponding PBXs.

14.3.1.2 Impact Reactivity (Sensitivity) Following a review of some recent studies concerning the prediction of impact sensitivity for different classes of energetic crystals, Zhu and Xiao have stated that the first-principles band gap criterion is applicable to different series of energetic crystals with a similar structure or with similar thermal decomposition mechanisms [49]. Already in an earlier paper [12] concerning these materials, these same authors found that the bond order of the trigger bonds and the activation energy of the breaking were directly related to the impact sensitivity. They have proposed two criteria for impact sensitivity of EMs with similar structures: (a) the smaller the bond order, the more sensitive is the EM and (b) the higher the activation energy, the less sensitive is the material. The principle of the smallest bond order is in most cases equivalent to the activation energy criterion [12]. Comparing the activation energies, Ea, of thermal decomposition under the conditions of the Russian manometric method with the experimentally determined impact sensitivities of nitramines, determined as “the first reaction,” a semilogarithmic relationship [50] was specified according to which the increasing Ea values correspond to decreasing sensitivity (i.e., the opposite of the observation from Zhu and Xiao [12,49]). A similar trend was found between the EaR1 slopes of the Kissinger relationship (14.1) and impact sensitivities of nitramine-based PBXs, detected by noise [51]. Activation energies for pure nitramines from the Russian manometric method (and other comparable energies) [19,20] correlate with the impact sensitivity for these compounds, as detected by noise [52,53] and as shown in Fig. 14.9. The division of this nitramine group into several subgroups is due to intermolecular interaction in the corresponding crystals and/or melts (see also Fig. 14.4) during thermal decomposition (see discussion in Refs. [19,20]). The above-mentioned slope of this relationship [12,49] is conserved in Fig. 14.9, even though roughly parallel opposite relationships are also visible here, characterized by a certain molecular structural similarity. Some relationships in this figure are linked to the problem with the kinetic specifications for the thermal decomposition of HMX, and especially HNIW [19,20], which, in the case of HNIW, can also be linked with the complications associated with HNIW polymorph transitions [19,54] and its purity. Such facts suggest a need for studying the intermolecular interactions, not only in the nitramine molecular crystals, but also in their melts and in the initial phase of their initiation [20].

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FIG. 14.9 Relationship between activation energy of thermal decomposition and impact sensitivity of nitramines, expressed as drop energy. Chart built here from published activation energies [19,20] and impact sensitivities [52,53], detected by noise.

A statistical approach to the relationship between the activation energies of thermal decomposition and the impact sensitivity of polynitro compounds has been published by Keshavarz et al. [47]. Here the basis for deriving the corresponding relationships is the ratio of the numbers of hydrogen and oxygen atoms in the molecule and the contribution of specific molecular structural parameters.

14.3.1.3 Friction Reactivity (Sensitivity) Determining the friction sensitivity of EMs can be heavily influenced by “human variability.” Nevertheless, carefully carrying out the correct measurements by a single researcher provides results showing the correct relationships and correlates well with the results from other stability and physicochemical tests [45,55,56]. A decisive factor governing the crystal structure of nitramines is the dipole– dipole interaction of the oxygen atoms of nitro groups with nitrogen atoms of nitro groups in neighboring nitramine molecules in the crystal [57–59]. This type of interaction will act against the shear slide during friction and will depend on the conformation of the molecule. Comparison of friction sensitivity with heats of fusion, DHm,tr, shows [55] that an increase in the DHm,tr values is more or less connected with a decrease in friction sensitivity. From this, a relationship is found between the activation energies of thermal decomposition (from SMM or comparable methods) and friction force, as is shown in Fig. 14.10. Here straight-lines A and B correspond to cyclic nitramines and lines C and D represent the linear ones. Solid N,N0 -dimethylnitramine (DMNA) behaves here as a cyclic compound, in the same way as in a study of the physical stability of nitramines [54].

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FIG. 14.10 Relationship between activation energy of thermal decomposition and friction sensitivity for nitramines; reconstructed and extended (by the BCHMX data [59]) figure, originally published in paper [45].

A predictive method for the relationship between friction sensitivity and activation energy of thermal decomposition of nitramines has recently been published by Keshavarz et al. [60]. As in the case of impact sensitivity, this is a statistical method based on the number of oxygen, nitrogen, and carbon atoms as well as on two nonadditive structural parameters.

14.3.1.4 Sensitivity to Electric Spark As stated in review [8] (see also quotations therein), there exist several different approaches and types of apparatus for measuring and solving electric spark sensitivity (EES). The various EES values obtained reflect the considerable diversity of approaches, and often differ by an order of magnitude (see Ref. [8] and references therein). It seems that, in Europe at least, the Czech Defense Standard (a part of STANAG [45]) is gaining acceptance, and thus the readings from instrument recordings are used here according to this particular standard [43,61,62]. It has been found that, for nitramines, increasing the N–N bond dissociation energies decreases the electric spark sensitivity [43]. This should mean that nitramine compounds, owing to their mesomeric equilibrium, might preferentially decompose via anionic states in an electric field. However, Fig. 14.11 shows that initiation of nitramines by an electric spark shows some similarity to initiation by heat—a straight line with a negative slope which is otherwise in agreement with the relationship with dissociation energies mentioned earlier, but the data for this line are not as close as for the other lines— as if “derived” from parts of other lines in the figure. In Fig. 14.11 also, the dual behavior of e-HNIW is again visible. The point marked as e-HNIWTGA

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FIG. 14.11 Relationship between activation energy of thermal decomposition of nitramines and their electric spark sensitivity, expressed as the energy of the electric spark for a 50% probability of initiation; constructed here from thermal analysis data [19] and the corresponding sensitivity data [43].

should correspond to common (technical) quality and/or to melts of this compound as has been seen in other kinds of similar relationships [8,19,20]. Fig. 14.12 shows the same kind of relationship for polynitroarenes. Here again, most of the partial relationships in this figure demonstrate a direct proportion between activation energy of thermal decomposition and electric spark sensitivity. Nevertheless, compounds around the straight-line A should decompose via C–NO2 bond hemolysis, those around the lines B, D, E, and F by a cyclic “trinitrotoluene mechanism” (typical for derivatives of polynitroarenes with a hydrogen atom in the g-position vis-à-vis the nitro group—see Refs. [2,3,5] and quotations therein). In those compounds belonging to line C, the reaction is initiated by fission of the C–NO2 bond, while in the remaining compounds it might be the “trinitrotoluene mechanism” for decomposition, a splitting off of the OH radical (see Refs. [2,3,5] and quotations therein). Decomposition of polynitroarenes, belonging to straight-line F, should take place in the liquid state, and those belonging to line E in a “solid-liquid” transition state. Taking the activation parameters of thermal decomposition of polynitroarenes under conditions of the Russian manometric method, or comparable ones, thermal stability thresholds were obtained for thermal exposure over 6 h (for the calculation methodology, see Refs. [16,63]. These correlate with the logarithm of electric spark sensitivity for these compounds [64]; the set of polynitroarenes studied is thus split into four subgroups according to the

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FIG. 14.12 Relationship between activation energy of thermal decomposition of polynitro arenes and their electric spark sensitivity, expressed as energy of the electric spark for a 50% probability of initiation; constructed here from thermal analysis data, mainly from the Russian manometric method [9,15,53], and corresponding sensitivity data [61,62].

logarithm of the rate constants of the thermal decomposition. The important conclusion here is that increasing thermal stability is connected with an increase in electric spark sensitivity [64]. A statistical approach to the relationship between activation energies of thermal decomposition and electric spark sensitivity for nitramines has been described by Keshavarz et al. [65]. Here the basis for a relationship is again the ratio of the numbers of hydrogen and oxygen atoms in the molecule and the contribution of specific molecular structure parameters.

14.3.1.5 Note Concerning the Use of the Czech Vacuum Stability Test In the preceding sections, the results of the Russian manometric method (SMM) have already been mentioned. This uses the Bourdon compensating gauge [18,66] and it has already been stated that the SMM outputs are so realistic that they can be considered as standards. Another widely used procedure is an American vacuum stability test [67,68], which is one of the methods used for technical assessment of explosives quality. Here the conditions of use enable the recording of secondary and subsequent thermolytic processes, and thus its results are not completely comparable with those from the SMM method. The Czech STABIL system [68,69] can, to a certain extent, be considered as an automated version of the American vacuum stability test. This has recently been applied to the study of the kinetics and

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thermal decomposition mechanism of EMs [70–72], including their relationship to sensitivity [71,72]. Monitoring of thermal decomposition of the 14 most important industrial explosives at 110°C using the STABIL system has shown that in the first 6 h the gaseous product evaluation is roughly a zero-order reaction [70]. Subsequently, a semilogarithmic relation has been found between the rate of gaseous evolution and the explosion temperature (i.e., temperature in the CJ plane during detonation). On the basis of comparing this relationship with the Eyring equation for the specific rate constant, it has been stated that there exists a ratio between the rates of the processes of the low-temperature thermal decomposition and the reaction rate in the reaction zone of the detonation wave [70]. Using a similar approach, a study of the thermal reactivity has been performed on the following PBXs [70,72] at 120°C with active fillers: 1,3,3-trinitro-1,3,5-triazinane (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imidazole (BCHMX), and e-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (e-CL-20 or e-HNIW). These nitramines were bonded by 13% by wt of an oily material (Formex matrix)-softened styrene-butadiene rubber [71,73] and by 9% by wt of polyisobutylene softened by dioctyl sebacate and oil (C4-matrix) [72]. Owing to the small number of points obtained, a relationship was originally suggested between the corresponding zero-order reaction velocities (the specific rate constants) and experimental detonation velocities as an approximate linear relationship [71–73], but after a reevaluation in the sense of Eq. (14.6) Fig. 14.13 was obtained (this is not basically different from the original versions [70,72]). This figure partly documents the difference between HNIW with reduced sensitivity (marked RS—see more in papers [74,75]) and those “common” quality nitramines, and partly the influence of its own binder on detonation chemistry (performance). Sterically crowded HNIW and BCHMX molecules gave PBXs which have a tendency to form their own group in the sense of the relationships in this figure. As expected, relationships also exist between the impact and electric spark sensitivities of the pure nitramine fillers, on the one hand, and the specific rate constants of the zero-order thermal decomposition of the PBXs studied, on the other [71]. Fig. 14.14 shows the combined results of the impact sensitivity study from papers [71,72] which show different influences of the binders used on both the impact sensitivity and on the rate constant of thermal decomposition. The behavior of RDX and HMX with the C4 matrix is essentially dissimilar compared with those in styrene-butadiene rubber. The presence of the benzene nucleus in the rubber might be a reason for this behavior. Data for Formex P1 in Fig. 14.14 do not correlate with the nitramine’s PBXs because its filler, pentaerythritol tetranitrate (PETN), represents another chemical entity (primary homolysis of the O–NO2 bond during its initiation).

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FIG. 14.13 Relationships between experimental detonation velocities and specific rate constants of thermal decomposition of the PBXs studied—combined and reconstructed figures for PBXs with Formex [71] and C4 [72] matrixes: here RS-e-HNIW-C4 corresponds to a nitramine with reduced sensitivity [74,75] and e-HNIW-C4 with e-HNIW-formex was prepared from “common” (technical) e-HNIW.

FIG. 14.14 Logarithmic relationship between impact sensitivities and specific rate constants of the zero-order thermal decomposition of the PBXs studied—combined results for the Formex [71] and C4 [72] matrixes; point Formex P1 means an explosive based on pentaerythritol tetranitrate (PETN) bonded by 13% by wt of Formex-matrix [70], RS-e-HNIW-C4 corresponds to a nitramine with reduced sensitivity [74,75], and e-HNIW-C4 together with e-HNIW-formex were prepared from “common” (technical) e-HNIW.

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14.3.2 Approach on the Basis of Electron Structure in the Reaction Center Both the electron configuration and steric situation in the reaction center of the molecule can be characterized from the standpoint of physical organic chemistry as explained in [1–5,7,8]: (a) the NMR chemical shifts of key atoms of the reaction center of the molecule (semiempirical determination of the nitramino or nitro groups that are first to react in the initiation) [1–5,7,8,10,76–79] and (b) direct correlation of reaction characteristics with electron charges at the nitrogen atoms of these first-reacting nitro groups [1–5,13,44,80,81] or with the net charges of these nitro groups [13,43].

14.3.2.1 NMR Chemical Shifts—Specification of the Reaction Center in the Molecule It is well known that using 13C and, particularly, 15N NMR chemical shifts in the study of initiation of polynitro compounds by shock [2–4,10,76–78], impact [2,3,5,8,52], friction [79], electric spark [2,3,8,10,43], or heat [1–5,7,8,10] can give very valuable results. It can be argued that an NMR study in solution neglects important crystal-lattice effects that are vital to the determination of explosive properties [82]. It is partially true, but from our several recent papers [1–5,7,8,10,45,52,55,59,76,83] it can be seen that this objection has no fundamental significance for studies of the chemical micromechanism of initiation of EMs—the conformation of molecules in solution is far more close to reality than those of optimized and isolated molecules in the DFT methods. Fig. 14.15 presents the relationship between the activation energies, Ea, of thermal decomposition of nitramines and the 15N NMR chemical shifts, dN, of the nitrogen atoms in the most reactive nitro groups [3,7]. Here straight-line A corresponds to decomposition in a solid state, line B in a liquid state, and straight-line C combines data of the crowded nitramines, decomposed in a liquid state. All these groups of nitramines should be characterized by the N–N bond homolysis as the first fission [18]. On the other hand, nitramines on the straight-line D decompose with negative activation entropy, DS‡. Taking the kinetic data from Ref. [7] and, for g-HNIW from Ref. [19], these entropies for liquid EDNA, liquid DMNA, RDX, HMX, and g-HNIW are, respectively (in J mol1 K1): 153.0, 111.9, 42.2, and 109.4 (fission thus goes through a transitional cyclic or bimolecular state). EDNA and DMNA mainly decompose by a bimolecular mechanism [84] (due to an association of their molecules by hydrogen bonds [85]), and the remainder of the nitramines in this group might decompose by the mechanism mentioned and, in the case of g-HNIW, the presence of some impurities cannot be excluded (of course, the choice of measurement technique and results evaluation might also have

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FIG. 14.15 Relationship between the activation energies, Ea, of nitramine decomposition, resulting from the Russian manometric method, and the 15N NMR chemical shifts of nitrogen atoms of the nitro groups (nonisochronous molecule HNIW correlates through the shift of the nitro-nitrogen atom in position 2)—extended and modified graphical presentation from Refs. [3,7] taking data from Refs. [19,52].

an influence on the results). Data for HMX being practically on the intersection of the straight-lines A and B, together with the results of an analysis of the several published papers about thermal decomposition of HMX (for analysis and corresponding quotations, see Ref. [7]), have tended to support the opinion that the liquefaction of HMX at temperatures above 553 K is connected with its dissolving in the products of its thermal decomposition. In the polynitroarene series, a relationship similar to the previous one exists as is documented by Fig. 14.16. This figure clearly demonstrates the difference between primary homolysis of the C–N bond in polynitro polyphenylenes and fission by a cyclic “trinitrotoluene mechanism” in derivatives of such compounds. The most reactive positions in these molecules were determined on the basis of the Russian manometric method (see, e.g., references in papers [1–3]), nowadays used in combination with the DFT methods [3,13,44,76,80] together with the X-ray crystallographic study [76]. The following compounds— 1-(2,4,6-trinitrophenyl)-5,7-dinitrobenzotriazole (BTX), 2,20 ,200 ,4,40 ,400 ,6,60 , 600 -nonanitro-1,10 :30 ,100 -terphenyl (NONA) and 2,20 ,200 ,2000 ,4,40 ,400 ,4000 ,6,60 ,600 , 6000 -dodecanitro-1,10 :30 100 :300 ,1000 -quaterphenyl (DODECA)—should have more than one reaction center in their molecules. For BTX, this would be at position 7 in the benzotriazole part and at position 2 in the picryl section; for NONA, this would be at positions 40 and 60 , and for DODECA this would be at positions 40 and 400 . Using the approaches based not only on 15N NMR chemical shifts, but also on 13C NMR ones (the latter relating to the bearers of the most reactive nitro groups), results were obtained which, in the same way, logically correlate not only with the thermal decomposition parameters, but also with the

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FIG. 14.16 Relationship between the activation energies, Ea, of the decomposition of polynitroarenes and the 15N NMR chemical shifts of the nitrogen atoms of the nitro groups (the numbers in parentheses denote the position in the molecule): a general scheme of the primary step of chemical decomposition is presented close to each line; extended and modified graphical presentations from Refs. [3,77].

characteristics of detonation [1–5,61,83], impact [1–5,8,52], and electric spark [8,43] sensitivities.

14.3.2.2 Correlation of Reaction Characteristics With Electron Charges This approach is derived from the 15N NMR chemical shift correlation with initiation reactivity parameters of polynitro compounds [1–3,8,11,13,44,52,76,79]. As has already been stated, the abilities of nitramine groupings to participate in primary initiation processes are different [2,3,5]. This fact is documented also by the electronic charges at nitrogen atoms of the nitramines calculated on the basis of the Mulliken population analysis of electron densities, qN, obtained ab initio by the DFT B3LYP/6-31G** method [11]. The relationships found between these charges at the nitrogen atoms of the primary leaving nitro groups and thermal reactivity of a set of nitramines are documented in Fig. 14.17. The logical relationships of this figure are discussed in Ref. [11] as follows (there is a certain similarity with Fig. 14.5): the charge value on the nitrogen atom of the nitro group at position 2 in the molecules of a-HNIW and e-HNIW correlates with both the lines A and B. This nitro group is the primary leaving group on initiation of HNIW [2,3,5]. In practice, the thermal decomposition of both polymorphous modifications of HNIW proceeds only after their transition to g-HNIW [11]. Chukanov

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FIG. 14.17 Relationship between the slopes of the Kissinger relation (14.1), EaR1, and Mulliken B3LYP/6-31G** charges, qN, at the nitrogen atoms of the primary reacting nitro groups in nitramino groupings in the molecule (given in parentheses are the positions in the molecule which correlate with the nonisochronous molecules)—an original figure from papers [3,11] augmented by the BCHMX data (the EaR1 value taken from [26]). The intersection point on the straightlines A and C corresponds to the EaR1 value of BCHMX. Molecular structural similarity is strongly highlighted here.

et al. stated [86] that the e-g polymorphic transition changes the conformation in the HNIW molecule, the stressed aza atom appears, and the colinear nitrogen lone electron pair is thus differently oriented here (it ought to relate to a change in reactivity). This change is preceded by an induction period with the formation of active microregions which are loaded with a mechanical stress gradient [86]; this polymorphic transition leads to the appearance of cracks in the HNIW crystal [19,20]. The difference between the mechanisms for the a-HNIW and e-HNIW transitions is thus probably caused by a difference in arrangement of defects in the crystal lattice in particular (i.e., a difference in intermolecular forces) in the resulting g-modifications [11]. The different concentrations of crystal defects in the g-modifications of HNIW, which have different origins, should make themselves felt in their different reactivity (in general). An analogous tight relationship, especially in the light of their molecular structure, is also valid for polynitro aromatic compounds, as Fig. 14.18 shows [13]. The likely reaction center in molecules here might be localized by means of the electronic charges on the nitrogen atom of the primary reacting nitro group or by the net charges of such nitro groups (see paper [76]. In addition, X-ray techniques were used). A possible reason for the existence of a group “TATB, NTFA, and TACOT-Z” in this figure might be the presence of the

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FIG. 14.18 Relationship between slopes EaR1 of the Kissinger relationship and charges on nitrogen atoms q(NO2)N, the primary reacting nitro groups in initiation (in parentheses are found the respective position numbers in the molecule). Modified figure from S. Zeman, Z. Friedl, M. Rohac, Molecular structure aspect of initiation of some highly thermostable polynitro arenes, Thermochim. Acta 451 (2006) 105–114.

nitrogen atom in the ortho position toward the nitro group (its probable influence on primary fission—analogous to Fig. 14.16) together with a stronger intermolecular interaction in the corresponding molecular crystals. There exists an interesting relationship between the charges, qN, at the nitrogen atom of the primary reacting nitro group and the onset of thermal decomposition, TD, of polynitro arenes as shown in Fig. 14.19. This might be connected with the electrostatic interaction of “instantaneous point dipoles” at the reaction center of their molecules (assuming the existence of electric microfields in the vicinity of these dipoles [11]). For an explanation of this, the Einstein-Nernst relationship [78] was used in a rewritten and modified form [11]:
2
(14.7) qN ¼ kB  s  F1  n1 TD where n is the number of defects, kB is the Boltzmann constant, and TD is the onset temperature of exothermic decomposition from nonisothermal DTA. The charge qN at the nitrogen atom of the primary reacting nitro group is here considered as the representative of Coulombic electrostatic interaction during formation of the activated complex of thermal decomposition of the nonionic polynitro arenes under study. The compounds investigated can be divided into several groups according to the mechanism of the primary splitting

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FIG. 14.19 Relationship between onsets, TD, from nonisothermal DTA and square of the Mulliken B3LYP/6-31G** charges, qN, at the nitrogen atoms of the primary reacting nitro groups in polynitroarenes and their derivatives. The suffix “solut” means solution in 1,3,5-trinitrobenzene. Codes of the genuine polynitro arenes are written in bold font. The tris(3-methyl-2,4,6-trinitrophenyl)-1,3,5-triazine (code TMPM) molecule has two potential reaction centers [80] (see also Scheme 14.1). Modified (reduced) figure from S. Zeman, Z. Friedl, Relationship between electronic charges a nitrogen atoms of nitro groups and onsets of thermal decomposition of plynitroarenes. Cent. Eur. J. Energet. Mater. 1 (1) (2004) 3–21.

(groups A–C) and also to the influence of their physical state at the beginning of decomposition; the physical state becomes dominant in the case of polynitro arenes exhibiting a very distinct stabilizing influence on the crystal lattice (compounds in group D) [11]. For the purpose of evaluating this influence in the compounds mentioned, it is advantageous to use the TD values determined in their solutions in 1,3,5-trinitrobenzene (TNB), even though TNB cannot be considered to be an inert solvent [11,63]. Data for such solutions of 1,4,5,8-tetranitronaphthalene (TENN) and NONA [63] correlate with straight-line C with which data for pure 2,20 ,4,40 ,6,60 -hexanitrobiphenyl (HNB), DODECA, and TNB also correlate well. In the case of the large molecule of DODECA, this is understandable, taking into account the results of X-ray studies of its conformation, according to which dihedral angles and the mutual orientation of phenyl rings led us to the idea of comparing this type of molecule to helicenes, for example, where a similarly screwed molecular structure can be observed [76]. We have also used our new approach using molecular surface electrostatic potentials (SEP) for a comparison with each of the Arrhenius parameters from the different resources with those from the Russian manometric method [20]. The imbalance between the VS,max and VS,min SEP extremes (obtained by calculation in the DFT B3LYP/6-311 + G(d,p)//6-311 + G(d,p) level) have been used

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as a criterion here. Their sum has been derived—VS,S (Eq. 14.8 [20])—and used as a new simple characteristic for SEPs, which has been used for the first time in a study of nitramine detonation [46] and whose values have close relationships to the Arrhenius parameters of thermal decomposition of the nitramines studied [20]: VS, max + Vs, min ¼ VS,S

(14.8)

The correlation of the VS,S values with activation energies, Ea (Fig. 14.20), allows making recommendations about the physicochemical behavior of the nitramines studied during their thermal decomposition, because the summed SEPs have also a direct logical relation to the heats of fusion of these nitro compounds. A similar statement is possible about the semilogarithmic correlation of the unimolecular rate constant for a given temperature (kt) with these VS,S values according to which increasing the VS,S values leads to decreasing values for these rate constants and to increasing physical stability in the nitramines discussed in paper [20]. In Figs. 14.2, 14.4, 14.13, and 14.20, Ea values for HNIW are shown below 180 kJ mol1, which probably correspond to the thermal decomposition of this particular nitramine in melts and/or in solution, especially as it is likely to be in the presence of impurities (NATO STANAG-4566 allows the

FIG. 14.20 Relationship between the activation energies of thermal decomposition from different authors and the VS,S values [23]. Line I corresponds to decomposition in the solid state, line II to decomposition in the liquid state, line III to decomposition during the “solid-liquid” transition state, and line IV corresponds to crowded molecules decomposed in the liquid state [23]. For interpretative notes to the nitramine codes, see Figs. 14.2, 14.4, and 14.5. Modified version of figure from [23] with extension by data of 2,4-dinitro-2,4-diazapentane (OCPX) and 1,4-dinitro-1, 4-diazabutane (EDNA) (their Ea values were taken from Ref. [45] and their VS,S values from Ref. [46]).

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maximum content in the final HNIW to be 3%, w/w). Of course, the influence of the technique used (TGA) can appear here, including the cracking of the HNIW crystals during heating [19,20] and possible sublimation of the nitramine.

14.4 COMMENT 14.4.1 Why Are Low-Temperature Thermal Decomposition Data Important? The study of energetic systems by theoretical methods has accelerated dramatically over the past 25 years. These activities provide a considerable insight into the understanding of certain factors affecting their behavior but, on the other hand, they are still bedeviled by a great deal of opacity and guesswork [3]. The main problem with quantum chemical calculations and simulations is that they labor under the strong opinions held by their authors, and the results reflect this “fact.” Another problem is that previously abundant results from a study of the primary fission of polynitro and polynitroso compounds by means of the Russian manometric method (see, e.g., citations in Refs. [2,3,15,18,66]) are on the decline. Similarly, other valuable results are rarely mentioned, such as X-ray photoelectron spectroscopy (XPS) evidence of the primary fission of the N–NO2 bond in 1,3,5-trinitro-1,3,5-triazinane (RDX) [88] or the furoxanes and furazanes formed from TATB [58,59,89], both exposed to shock. In addition, the DKIE study of the thermal and shock primary fission of TNT [90,91] has not been cited for a long time, and a similar lack of interest is shown in the application of forensic analysis to the study of the primary fission, by heat and shock, of the polynitro arenes with the hydrogen atom in the g-position toward the nitro group [3,5,87,92]. From the earlier parts of this chapter, it follows that the characteristics of low-temperature decomposition can correlate not only with the characteristics of reaction centers in the molecule but also directly with the detonation parameters and/or sensitivity data [2,3,5]. It has been found that the primary fragmentation mechanism at low temperature (up to 600 K) should be entirely different from the high temperature variant. In the case of the unimolecular decomposition of RDX, it can be stated that elimination of the NO2 group by homolysis of one N–N bond is observed for all reaction conditions, whereas the triazinane ring fission (depolymerization to 1-nitro-1-azaethylene) occurs predominantly not only in the gas phase thermal decomposition of this nitramine but also in the decomposition of HMX, that is, at higher temperatures (see Refs. [93–96] and quotations therein). This intercorrelation between detonation and low-temperature decomposition characteristics is very well demonstrated by the inorganic azides in which only those activation energy values (from among those published—see Fig. 14.3) that correspond to the lowest temperature ranges of their thermal

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decomposition [3,9,10] can be used. The similarity is also reflected in a relationship between the kinetics of low-temperature thermal decomposition of EMs and the reaction rates in the reaction zone in their detonation [70,97]. From this, it follows that temperature (i.e., thermal decomposition), in the classical sense, can play no part in the process of initiating detonation of EMs by shock or impact [1–3,5,10]. It also means that the primary fragmentation of EMs in their detonation transformation proceeds in milder conditions than those present at the front of the detonation wave or in its reaction zone, which means that the detonation transformation itself of the given substance should be preceded by an induction period (already during the EM compression—before the front of detonation) [2,3,10]. It is very interesting that, already in 1956, on the basis of a review of the chemical mechanisms which are presumed to occur in, or immediately behind, the detonation wave, Taylor found [98] that such reaction mechanisms must be operative before a detonation wave front and that they also govern the sensitiveness of high explosives. This statement is in perfect harmony with the findings of this chapter. In the light of the chemical physics of explosion, Dremin also postulated the necessity of such an induction period [99] and the Multidimensional Reactive Flow model based on the Non-Equilibrium Zeldovich-von Neumann-D€ oring theory [100,101] also takes such a period into account.

14.4.2 Primary Fission—Reaction Center of the Molecule Very often, authors of theoretical papers about the fission of EMs are putting forward the primary fission of molecules of such materials along with the subsequent fission processes concerning the original resulting fragments from the primary fission (see, e.g., Ref. [15]). This approach might be suitable for processes under low pressures and temperatures but it does not reflect the reality of detonation—the path of the subsequent reactions of the primary produced fragments very strongly depends on the pressure and temperature in which such fragments are situated. If these reaction conditions allow an explosive reaction to develop (i.e., a sufficient production of primary fragments), thereafter the pressure produced by this reaction can attain the critical initiation pressure, P*, and this reaction evolves itself by a homogenous mechanism. In the opposite sense, a “hot spots” mechanism comes into effect with the possibility of extinguishing the reaction [99,102]. The influence of the detonation conditions on the chemistry of this transformation is well documented by Volk’s studies about the composition of detonation products [103–105]. This chemistry is affected also by the strength of initiation [103,106]. The above-mentioned extinguishing of detonation leads to the stabilization of the primarily formed fragments into stable compounds, and to the preservation of further secondary products, which might be best demonstrated by the thermal and shock initiation of TNT [3,5,92]—the first fragment in both

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cases is stabilized by cyclization into 4,6-dinitro-2,1-benzisozaxole and other stable secondary products derived from TNT by oxidation of its methyl group, that is, the corresponding acid, aldehyde, and alcohol, and also 1,3,5trinitrobenzene [92]. Regarding the primary fission findings, that is, about the specification of the reaction center in the molecule, the 13C and 15N NMR chemical shifts in correlation with characteristics not only of thermal decomposition (see Figs. 14.15 and 14.16 and Refs. [2,3,8,10,43]) but also of shock [2–4,10,76–78], impact [2,3,5,8,52], friction [79], and electric spark [2,3,8,10,43] can produce very valuable results. This approach also uses the DFT methodology (Figs. 14.17–14.20) and has been supplemented by the X-ray studies for polynitro compounds [59,76]. Using these three methods, and taking into account also the considerable knowledge obtained by the Russian colleagues between the 1960s and the 1980s concerning the thermal decomposition of polynitro and polynitroso compounds, it is possible to find the most reactive grouping in the corresponding molecules. Scheme 14.1 presents examples of substances with the most reactive positions

SCHEME 14.1 The most reactive positions in molecules of 2,4,6,8-tetranitro-2,4,6, 8-tetraazanonane (OHMX), 1,3,5-trinitro-1,3,5-triazepane (HOMO), 2,4,6,8,10,12-hexanitro2,4,6,8,10,12-hexaazaisowurtzitane (HNIW), 3,5-dinitro-N,N0 -bis(2,4,6-trinitrophenyl)pyridine2,6-diamine (PYX), 2,20 ,200 ,2000 ,4,40 ,400 ,4000 ,6,60 ,600 ,6000 -dodecanitro-1,10 :30 100 :300 ,1000 -quaterphenyl (DODECA) and tris(3-methyl-2,4,6-trinitrophenylamino)-1,3,5-triazine (TMPM). In the case of PYX, a fission in the sense of “channel 1” leads to the stable intermediates 4,6-dinitrobenzofurazane, 2,4,6-trinitroaniline and N-(2,4,6-trinitrophenyl)-2,4,6-trinitroaniline [3,87]; in the TMPM molecule, “channel 4” should be preferred in thermal decomposition (see Fig. 14.19).

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specified in their molecules. After the incomplete initiation of PYX by thermal and shock impulses, the remaining substances were subjected to forensic analysis, as were the substances remaining after similar treatment of several other N-(2,4,6-trinitrophenyl)-substituted amino derivatives [3,87]. On the basis of these results, it has been stated that the chemical micromechanism of the primary fragmentation of their low-temperature decomposition should be the same as in the case of their initiation by shock [87]. This is in excellent agreement with analogous studies of TNT [92] and older results, obtained for TNT using the deuterium kinetics isotope effect (DKIE) [90,91]. If a molecule contains several types of constituents, it can contain several potential reaction centers (e.g., the TMPM and PYX molecules in Scheme 14.1). The initiation proper can then be achieved by the molecule simultaneously participating with several or only with a single center in a given type of initiation [2,3]. On the basis of the above-mentioned facts, the existence of relationships such as in Figs. 14.15–14.19 is understandable. It is important to point out here the mutually different courses of relations between activation energies and 15N shifts for the homolytic or cyclic progress of fission as is shown in Fig. 14.16, because this difference is reflected also in the other relationships (e.g., Figs. 14.1 and 14.2). As was presented in Section 14.3.1.1.1, subsequent research into the primarily found relationships [1,9–11], demonstrated by Figs. 14.1–14.3, has led to the discovery that the characteristics of the reaction centers in molecules of EMs correlate with the characteristics of sensitivity and detonation parameters of such materials [1–3,5,8,10,13,42,45,51,52,55,56,59,64,71–73,76,78,79]. In other words, the primary fragmentation processes of molecules of EMs in the low-temperature thermal decomposition should be identical with those where initiation takes place with any impact, friction, electric spark, or shock [2,3,5]. This means that the primary fragmentation of EMs in their detonation transformation might be proceeded with in milder conditions than those present at the front of the detonation wave or in its reaction zone [1–3,5], as was already mentioned. In this connection, it is good to remember here the significant Multidimensional Reactive Flow model based on the Non-Equilibrium Zeldovichvon Neumann-D€ oring theory of detonation, which within the mathematical description works with experimental data of thermal explosion and, therefore, considers the induction period of the process [100,101]; however, this period of the EM decomposition in the front of the detonation wave should make the front kinetically unstable and pulsating [102]. If this induction period is limited mainly on the molecules excitation and only on partial endothermic fission, then the mentioned instability and pulsation might not be observed. As for the above-mentioned finding, it is important to mention a logarithmic relationship (14.6) between the rate constants of the unimolecular thermal

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SCHEME 14.2 Two ways of nitromethane thermal decomposition—on detonation, the hydroxyl radical formation route will lead to an increasing reaction rate in the reaction zone of the detonation wave (hydroxyl radical is more active than the nitro analogue).

decomposition and the detonation velocities of EMs (see also Fig. 14.13)—if a given material is able to decompose by different mechanisms with different kinetics, then each mechanism should have its own detonation velocity. This is a case with decomposition of nitromethane about which a lot of papers have been written in the past (see quotations in Refs. [3,6]). Depending upon pressure, its low-temperature thermal decomposition in the condensed state can follow two reaction mechanisms [107], with one mechanism changing to the other [108] at 5 GPa pressure or, at 4 GPa pressure, at a temperature higher than 130°C (the latter mechanism has a negative activation volume [109]). An admixture of about 2% water or, better, about 0.05% by wt of diethylenetriamine [6] should be sufficient to activate nitromethane. This nitromethane detonation proceeds through a hydroxyl radical formation as the primary and very reactive fragment [6], which leads to increasing detonation velocity and an increase in detonation pressure from the original 13 to 20 GPa (see quotations in Ref. [6]). Similar behavior is known for nitroglycerine which also has two kinetics for thermal decomposition (with dependence on the filling of the reaction volume) [110] and for which there are two detonation velocities— 1100–2000 m s1 and, in a steel tube, 8000–8500 m s1 [111] (Scheme 14.2).

14.4.3 Relationships Between Decomposition Activation Energies and Performance of EMs This kind of relationship might belong to those between sensitivity and performance (an increase in performance corresponds to an increase in sensitivity—the subject of the paper [8]). However, a waveform for such relationships for pure compounds only corresponds to this assumption in the case of Figs. 14.1 and 14.3, whereas for nitramines this trend is the reverse (Figs. 14.2, 14.4, and 14.5). It seems that decomposition through a five- or six-membered transition state [2,3] (organic compounds in Fig. 14.1), on the one hand, and the homolytic fission of trigger bonds (Figs. 14.2, 14.4, and 14.5), on the other, is one reason for this difference. These facts merit a more detailed thermodynamic analysis of the kinetic data of decomposition: for

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example, activation entropy, DS6¼, for homolytic fission should show that the degree of disorder of the activated complexes is higher than those of the reactant molecules, whereas in derivatives of polynitro arenes it should be the opposite. Besides modern thermoanalytical techniques, simple nonisothermal DTA would appear still to have its use, mainly for heterogenous mixtures. DTA, in combination with the Kissinger method [27] for analyzing results, represents a relatively quick, reliable, and easily available method. By means of this approach, it is possible to study reactivity differences in the HNIW polymorphs (Figs. 14.5 and 14.17), to make some conclusions about the chemistry of detonation, such as the influence of inorganic nitrates or binders on detonation W/O explosives and PBXs, respectively (Fig. 14.6), and the behavior of aluminum in the CJ-plane in the presence of hydrogen peroxide (Fig. 14.7) or binders in the detonation of HNIW (Fig. 14.8). In all these relationships, the intensity of intermolecular interaction is highlighted (solid versus liquid phase—Figs. 14.1, 14.4, 14.15, and 14.19, or linear versus cyclic nitramines—Figs. 14.2, 14.5, and 14.17).

14.4.4 Mechanical and Electric Spark Sensitivities in Connection With Thermal Decomposition As Fig. 14.9 shows, the impact sensitivity (connects with uniaxial compression) of nitramines increases with a decrease in decomposition activation energies. A more detailed thermodynamic analysis of the corresponding kinetic data and the use of DFT methodology are required to explain this relationship. As expected, this relationship for friction sensitivity (shear slide with a fixed volume) has a different shape for linear nitramines, compared with that for cyclic ones (Fig. 14.10). Also, the relationship of spark sensitivity to activation energies of decomposition corresponds basically to expectations (Figs. 14.11 and 14.12), with only two partial relationships being excluded— these are formed by data from the thermal decomposition of the corresponding substances in the liquid state.

14.4.4.1 Toward the Initiation Reactivity of 2,4,6,8,10,12Hexanitro-2,4,6,8,10,12-Hexaazaisowurtzitane (HNIW) The facts presented here have helped to gain an insight into the problem of the high sensitivity of HNIW, information which is widespread in the literature [75,112]. Ou Yuxiang et al. [113] have described pure e-HNIW with an impact sensitivity of 13.2 J and for the rest of the pure polymorphs they found the following sensitivities: a-HNIW of 10.1 J, b-HNIW of 11.9 J, and g-HNIW of 12.2 J. As yet, all these values are practically ignored in the literature. Nevertheless, technological practices have already been published, according to which the products have an impact sensitivity of 8–12 J [73,75]

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(RS-e-HNIW). Results of the thermal analysis study also correspond to the allegedly highly sensitive HNIW. From Figs. 14.2, 14.4, 14.11, 14.15, and 14.20, the Ea values of 172–176 kJ mol1 and of 196.0–216.9 kJ mol1 can be seen for HNIW. While the first of these ranges comprise the “usual” values, the second range of values represents relatively recent Russian data [54,114]. Both these groups of data are subordinate to the molecular structural relations (see the corresponding figures). From the above-mentioned facts, it is possible to state that HNIW with the lower Ea values should correspond to its solutions (to the cocrystals) in impurities and/or in products of its decomposition. It is a pity that the authors mostly did not specify the quality of HNIW used in taking the measurements.

14.5 CONCLUSION Original relationships between low-temperature thermal decomposition and detonation characteristics of EMs [1], elaborated over time into quite a number of new relationships of the type “initiation reactivity—molecular structure,” provide the possibility of using them not only in the study of the chemistry of the primary initiation processes but also for making some statements about detonation chemistry and for explaining the behavior of some EMs. Specifically, the use of the 13C and 15N NMR chemical shifts and of the results from DFT methodologies in constructing these relationships have increased the available information for individual EMs. Above all, for energetic mixtures, a simple DTA, in combination with the Kissinger method for evaluating its result, still seems to be the best for obtaining the necessary characteristics of thermal decomposition. It is possible to summarize the existing results from research in this area as follows: The endothermic primary fragmentation of EMs in their detonation transformation might take place in milder conditions than those present at the front of the detonation wave or in its reaction zone [1–3,5,10]. This means that the detonation transformation itself of the given substance should be preceded by an induction period which is what many scientists studying the physics of explosions assume [1–3,5,97–101] even if it would be excitation of molecules [100,101] with their subsequent only partial endothermic decomposition; the course of the fragmentation mentioned is not random but is characterized by a chemical mechanism, which is most likely identical to that of the primary fragmentation of the compounds during their low-temperature thermal decomposition [2,3,5,91]. One of the continuing problems in the study of initiation reactivity is the absence of clear information concerning the mechanism of the mechanical or electrical initiation impulses transfer into the reaction center of EMs molecules. Although considerable efforts have been expended in this area (see a review of some theories in Ref. [3]), the question is still open.

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ACKNOWLEDGMENTS This chapter is based upon work supported by the Faculty of Chemical Technology, University of Pardubice. The author is indebted to Mrs. Monika Sˇubrtova´ from the Institute of Energetic Materials at the Faculty of Chemical Technlo´logy for her help with the DTA and VST measurements and with the figures arrangement. The author thanks also to Prof. Thomas Klap€oetke from LMU Mu˝nchen for his invitation to prepare a part [3] of his monograph which become a meaning groundwork for this chapter.

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FURTHER READING [1] L. Tan, L.-H. Xia, Q.-J. Wu, X. Sen, D.-B. Liu, Effect of urea on detonation characteristics and thermal stability of ammonium nitrate, J. Loss Prev. Process Ind. 38 (2015) 169–175. [2] N. Zohari, M.H. Keshavarz, S.A. Seyedsadjadi, A link between impact sensitivity of energetic compounds and their activation energies of thermal decomposition, J. Therm. Anal. Calorim. 117 (2014) 423–432. [3] Anonymous Writer, North Atlantic Council: STANAG4490 Ed.1, Explosives, ESD sensitivity  Compatibility of Ammunitests, adapted Czech Def. Standard No. COS137601—Chemical ´ NMZ, October 4, 2010. tion Components with Explosives (Non Nuclear Applications), U [4] B. Fidanovski, M. Dimi
c, A. Milojkovi
c, V. Rodi
c, Determination of chemical stability of propellants using the vacuum stability test method, Sci. Tech. Rev. 66 (1) (2016) 18–22. [5] S. Zeman, A. Elbeih, A. Hussein, T. Elshenawy, M. Jungova, Q.-L. Yan, A modified vacuum stability test in the study of initiation reactivity of nitramine explosives, Thermochim. Acta 656 (2017) 16–24.