Impact and shock sensitivities

It seems reasonable to expect that expls that are sensitive (eg, sensitive to impact, friction, sparks, etc) will also be sensitive to shock. Generally this expectation is borne out by experience, at least in a qualitative way. Under comparable conditions PETN is certainly more shock-sensitive than TNT. and so on. An example of the parallelism in impact and shock sensitivity of four common expls is shown in the tabulation below. The comparisons are obviously qualitative in that the expls are ranked in descending order of sensitivity. It must be emphasized that reversals in sensitivity ranking can occur if comparisons are made of sensitivity data obtained under non-comparable test conditions... 

A key point, with regard to both issues, is the decomposition process of the compound What are its energetics and how readily does it occur Our emphasis in this chapter shall be upon factors that influence the ease with which decomposition can be initiated by unwanted external stimuli, i.e. sensitivity. These stimuli may be of various types, including impact, shock, friction, heat and electrostatic charge [8]. Relative vulnerabilities to these different effects need not be the same for example, the onset temperatures for the thermal decomposition of TNT (1) and HMX (2) are quite similar, but the latter is much more likely to undergo detonation upon impact [8]. It has been shown, however, that there is a general correlation between impact and shock sensitivities [10], which are the ones upon which we will focus. 

Impact and shock sensitivities depend upon a variety of factors chemical, structural and physical. We shall limit our discussion to the effects of molecular structure (i.e. chemical composition and molecular geometry). 

Reflecting this emphasis upon the C—NO2 and N-NO2 bonds, several studies have sought to correlate impact and shock sensitivities with properties of these bonds, particularly various measures of their stabilities. These have included the lengths of the bonds [38,39], the electrostatic potentials at their midpoints [38,40,41], and their polarities in excited states of the molecules [42]. Taking a different approach, Kohno et al have argued that the impact sensitivities of nitramines can be related to the differences between the N-NO2 distances in the gas phase and in the crystal [43,44]. 

It should be apparent from even this very brief discussion that impact and shock sensitivities can depend upon a variety of chemical and structural factors. Any generalizations should be made cautiously, and are likely to be subject to qualifications and limitations. 

We have presented an overview of various attempts to relate the impact and shock sensitivities of energetic materials to their molecular structures. The objectives of such efforts are to better understand the chemical and structural determinants of these sensitivities, and to develop a predictive capability to facilitate the evaluation of new and proposed energetic compounds. 

BENSULFOID (7704-34-9) Combustible solid (flash point 405°F/207°C). Finely divided dry materia forms explosive mixture with air. The vapor reacts violently with lithium carbide. Reacts violently with many substances, including strong oxidizers, aluminum powders, boron, bromine pentafluoride, bromine trifluoride, calcium hypochlorite, carbides, cesium, chlorates, chlorine dioxide, chlorine trifluoride, chromic acid, chromyl chloride, dichlorine oxide, diethylzinc, fluorine, halogen compounds, hexalithium disilicide, lampblack, lead chlorite, lead dioxide, lithium, powdered nickel, nickel catalysis, red phosphorus, phosphorus trioxide, potassium, potassium chlorite, potassium iodate, potassium peroxoferrate, rubidium acetylide, ruthenium tetraoxide, sodium, sodium chlorite, sodium peroxide, tin, uranium, zinc, zinc(II) nitrate, hexahydrate. Forms heat-, friction-, impact-, and shock-sensitive explosive or pyrophoric mixtures with ammonia, ammonium nitrate, barium bromate, bromates, calcium carbide, charcoal, hydrocarbons, iodates, iodine pentafluoride, iodine penloxide, iron, lead chromate, mercurous oxide, mercury nitrate, mercury oxide, nitryl fluoride, nitrogen dioxide, inorganic perchlorates, potassium bromate, potassium nitride, potassium perchlorate, silver nitrate, sodium hydride, sulfur dichloride. Incompatible with barium carbide, calcium, calcium carbide, calcium phosphide, chromates, chromic acid, chromic... 

Study of EM sensitivity has become an implicit aspect of understanding the primary chemical processes of their initiation, hi this respect, initiation by heat (low-temperature thermal decomposition) is the most frequently studied to date. With regard to the above-mentioned dominant interest in the study of impact and shock sensitivities within the mentioned period of nine years, and also with regard to the importance of these sensitivities in technical practice, the topic of splitting of EM molecules by the said stimuli have been included in Sect. 3 on Impact and Shock Sensitivities. 

Some authors approach the study of impact and shock sensitivities as if it were a single type of initiation. However, from the standpoint of the way the impulse energy is transferred to the reaction centre of the molecule there are two different initiation processes (Sect. 4). On the basis of his multi-phonon up-pumping model of initiation of EMs, Dlott considers the impact initiation to be a kind of low velocity initiation (LVI) [1,74]. 

Studies of the sensitivity of EMs are mostly directed to impact and shock sensitivities (Sect. 3). A primary fragmentation of the given EM until adiabatic compression of a thin layer of its molecules by impact or shock wave is the unambiguously dominant view in this case. Physical Kinetics Model [97] was a typical representative of this view of mechanical spht-ting of the molecule at any dehberate bonds between its atoms by shock wave. One of the best developed models is Dlott s Model of the Multiphonon... 

TABLE 1. The impact and shock sensitivities of a variety of common explosives and their mixtures. 

We have shown a relationship between impact and shock sensitivity and illustrated how a sensitivity index based on oxygen balance can be used to estimate sensitivity in closely related series of molecules. It is shown that the critical temperature of an explosive calculated by the Frank-Kamenetskii equation correlates fairly well with the shock sensitivity of the material. This supports the idea that the shock or impact initiation of an explosive is primarily a thermal event and not dominated by pressure driven chemistry. The concept of the "trigger linkage" in explosives is discussed and it is pointed out that insensitive explosives will require early chemistry that is thermochemically neutral or endothermic and leads to the build-up of later strongly exothermic chemistry. 

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