Objectives

Extrapolation of data beyond its measured range and its quality.

Prediction of the behaviour of explosives at lower temperatures.

Explosives

1. Introduction

An explosion is a physical or chemical phenomenon in which energy is
released in a very short time, usually accompanied by formation and
vigorous expansion of a very large amount of hot gas:

Mechanical explosions are caused by the sudden breaking of a vessel
containing gas under pressure

Chemical explosions are caused by decomposition or very rapid
reaction of a product or a mixture

Nuclear explosions are caused by fission or fusion of atomic nuclei

Electrical explosions are caused by sudden strong electrical
currents that volatilize metal wire (exploding wire)

Only chemical explosions are treated in this article.

For an explosion to occur, the reaction must be exothermic ; a large
amount of gas must be produced by the chemical reaction and vaporization
of products ; and the reaction must propagate very fast. For example,
gasoline in air burns at a rate of ca. 10–6 m/s ; a solid
propellant, at ca. 10–2 m/s ; and an explosive detonates at a
rate of ca. 103 – 104 m/s (detonation velocity).

The two different modes of decomposition are deflagration and
detonation. Deflagration exhibits two characteristics : 1) the combustion
is very rapid (1 m/s up to a few hundred meters per second) and 2) the
combustion rate increases with pressure and exceeds the speed of sound in
the gaseous environment, but does not exceed the speed of sound in the
burning solid. The materials are often powdered or granular, as with
certain pyrotechnics and black powder. Detonation is chemically the same
as deflagration, but is characterized by a shock wave formed within the
decomposing product and transmitted perpendicularly to the decomposition
surface at a very high velocity (several thousand meters per second)
independent of the surrounding pressure (see Chap. 2. Physical Properties
and Chemical Reactions).

Explosive substances can be divided into three classes. Members of the
first class detonate accidentally under certain conditions. These are
explosible substances, some of which are used in industry as catalysts
(e.g., peroxides), dyes, and fertilizers. This class includes products or
mixtures whose formation must be avoided or controlled, e.g., firedamp, or
peroxides in ethers. In the second class are products normally used for
their quick burning properties but which may detonate under some
circumstances, e.g., pyrotechnic compositions, propellants, and some kinds
of hunting powder. In the third class are substances intentionally
detonated for various purposes.

For reasons of safety, acquaintance with the first group of materials
is necessary. The second group is described elsewhere (Propellants;
Pyrotechnics). Pure substances and mixtures of the third class are
described here.

The distinctions between these three classes are not clear-cut because
most explosives burn smoothly if they are not confined. However, if some
fine hunting powder burns under certain confined conditions, combustion
may become detonation. Dry nitrocellulose fibers can easily detonate, but
this tendency is significantly lower in the gelatinized form. Some
compositions, such as mixtures of cyclotrimethylenetrinitramine (RDX) with
a binder, can be used as a propellant, gunpowder, or high explosive,
depending on the type of initiation.

The third class consists of primary and secondary explosives. Primary
explosives (initiator explosives) detonate following weak external
stimuli, such as percussion, friction, or electrical or light energy.
Secondary explosives are much less sensitive to shock. However, they can
detonate under a strong stimulus, such as a shock wave produced by a
primary explosive, which may be reinforced by a booster composed of a more
sensitive secondary explosive. The various secondary explosives are used
militarily or industrially as shown in Figure (1).

Figure 1

Functions and Constraints. Explosives can be either pure
substances or mixtures. They function in such systems as munitions, where
they are a component of a complex firing system or as firing devices in
quarries or metal forming equipment. With such systems, the ingredients
must fulfill one or more functions, while meeting various constraints
arising in manufacture or use. Therefore, tests that represent these
functions and constraints are required.

When an explosive detonates, it generates a shock wave, which may
initiate less sensitive explosives, cause destruction (shell fragments,
blast effect, or depression effect), split rocks and soils, or cause
formation of a detonation wave. A detonation wave of special geometry
(hollow charge effect) may modify materials by very rapid generation of
high pressure ; for example, shaped charges, metal hardening, metal-powder
compaction, or transformation of crystalline forms. A shock wave may be
used to transmit signals, e.g., for safety devices or in seismic
prospecting.

In general, constraints are related to safety, stability, compatibility
with other elements of the system, vulnerability, toxicity, economics,
and, more recently, environmental and disposal problems [21].

History  [1][2][3][4]. Explosives were probably first used
in fireworks and incendiary devices. The admixture of saltpeter with
combustible products such as coal and sulfur produced black powder, for
which a fabrication process was described by the Chinese in 808 A.D. [82].
Its use as an explosive charge in a shell — also by the Chinese —
was recorded as early as 1044 A.D., but in Europe only around 1580 (siege
of Berg-op Zoom). However, the difficulties of initiation upon impact
against the target were not overcome until 1820 when fulminate caps were
developed. In the early 1600s, black powder was used for the first time to
break up rocks in a mine in Bohemia. This technique spread throughout
western Europe during the 1600s. Ammonium perchlorate was discovered in
1832.

The development of organic chemistry after 1830 led to new products,
although their explosive properties were not always immediately
recognized. These include nitrocellulose and nitroglycerin.

The very important contributions of ALFRED NOBEL (1833 – 1896) include
the use of mercury fulminate for the initiation of explosives (1859 –
1861), the development of dynamites (1873), and the addition of 8 %
nitrocellulose to nitroglycerin (blasting gelatine, 1876).

Many new products were developed between 1865 and 1910, such as
nitrated explosives, mixtures in situ of an oxidizer and a fuel,
explosives safe in the presence of firedamp, chlorate explosives, and
liquid oxygen explosives. Organic nitro compounds for military uses
included tetryl, trinitrophenol, and trinitrotoluene.

Between the two world wars, RDX, pentaerythritol tetranitrate (PETN),
and lead azide were produced. After 1945 cyclotetramethylenetetranitramine
(HMX), 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), and hexanitrostilbene
(HNS) were developed (see Chap. 5. Secondary Explosives), as well as "fuel
– air explosives." Ammonium nitrate – fuel oil (ANFO) explosives and
slurries were developed for industrial uses ; some were improved by adding
bubbles or micropores.

In the 1880s BERTHELOT described the phenomenon of detonation. About
the same time the hollow-charge effect was discovered, and the foundations
of the hydrodynamic theory of detonation were established. In 1906, the
first accurate measurements of velocities of detonation were made. After
World War II, the science of detonation was further developed and
perfected [17].

Recently, new explosive compositions of low vulnerability (LOVA = low
vulnerability ammunitions) are being developed. Major efforts are being
made to use predicted properties (e.g., density, DHf, sensitivity, themal
stability) to avoid unnecessary experiments [18][19][20], [87], [94].

[21]  Life Cycle of Energetic Materials, Conferences Proceedings
(1993, 1994, 1996), Los Alamos Lab.

2. Physical Properties and Chemical Reactions

2.1. Detonation

The detonation process needed for most uses is characterized by a shock
wave that initiates chemical reactions as it propagates through the
explosive charge. The shock wave and reaction zone have the same
supersonic velocity ; a fraction of the chemical energy is used to support
the shock.

2.1.1. Ideal Detonation

A model of ideal detonation (ID) is shown in Figure (2), with steady
flow to the end of the reaction zone.

Stationarity requires the plane corresponding to the end of the
reactions to be locally sonic. This condition, termed the Chapman –
Jouguet (CJ) condition [23], [24], yields the relation D = u +
c  needed to solve the equations of conservation of flow (where
D = detonation velocity, u = particle velocity, and c
= sound velocity).

Figure 2

The structure of the reaction zone of the ID plane detonation can be
ignored, and the mechanical and thermodynamic data can be calculated by
solving equations between the nonreacted and fully reacted states. The
description of the ideal detonation is given by the model [25][26][27]
represented in Figure (3) by the p – V plane (Fig. (3) A) and the
pressure p vs. distance x profile at a given instant of time (Fig. (3) B).
This model relates the explosive at rest (V0), the shocked
nonreacted explosive (ZND spike (*)), and the end of the reaction zone (CJ
plane (  ^ )).

In Figure (3) A the three states are located on a straight line
(Rayleigh line), with slope equal to
-D2/
V02. The loci of the shocked
states are termed Hugoniot curves : H0 for the
unreacted explosives, and H for the completely reacted explosives. Some
relations at the CJ plane can be expressed as a function of D and the
polytropic coefficent of the detonation products :

These relations are valid for most condensed organic explosives
&Gamma; &asymp; 3 , with the assumptions that
p0 = 0 and u0
= 0.

2.1.2. Deflagration and Detonation

In a thermodynamic diagram such as Figure
(3) A, there is another point that satisfies the Chapman – Jouguet
condition in the region p < p0 (not drawn in
the figure). It represents the Chapman – Jouguet deflagration, in contrast
to the detonation :

Unlike the detonation wave, the deflagration wave is subsonic, and
consequently a precursor shock is propagated in front of the reaction
zone. Its intensity and velocity depend on the chemical energy released
and on the boundary conditions ; in contrast to detonation, a specific
explosive does not provide a unique solution for deflagration.

A precursor shock that is strong enough can, in addition to compressing
the explosive, also heat it sufficiently to initiate reactions just behind
its front ; a progressive buildup of a completely stationary process
identical with the ZND model of the detonation is observed. Consequently,
a detonation is equivalent to a shock followed by a deflagration.

[23]  D. L. Chapman, Phil. Mag. 47 (1899) 90.

2.2. Prediction of Detonation Data

2.2.1. Complete Calculation

[28]
The quantities D, p, û, V, and T in the Chapman
– Jouguet state are needed to evaluate further the effectiveness of the
explosive on the surroundings in a given action. The calculation of these
quantities requires the equation of decomposition, the heats of reaction,
and an equation of state for the reaction products, which may be
theoretical (virial expansion) [29] , [30] , semiempirical [31], empirical
[32], or a constant &Gamma; law. In addition, the equilibrium
constants for the reaction given as a function of V and T
are needed.

For organic explosives, the distinction between oxygen-positive or
weakly oxygen-negative explosives, which give only gaseous products, and
the strongly oxygen-negative explosives, which also give free carbon, is
important. In fact, the assignment to one class or the other is determined
by the values of the equilibrium constants for (V,T) determined
after a first calculation.

Inasmuch as (V,T) are a function of the loading density
r0 , some explosives, for instance,
pentaerythritol tetranitrate (PETN), produce free carbon at high loading
densities and only gaseous products at moderate values of
r0 . Because of the complexity of the
calculation, the number and formulation of the equations depend on the
final result.

The thermochemical equations are solved with a priori (V,T)
pairs to give gas product composition. The mechanical equations are then
applied at the Chapman – Jouguet plane with the geometric condition
represented on Figure (3) A : at the CJ point, the isentropic and the
Hugoniot curves have the same tangent, with a slope equal to
-D2/
V02.

Development of the ideal detonation
A) The p-V plane; B) Th
epressure vs. distance profile; p presure; V specific
volume; D detonation velocity; H0 Hugoniot
curve of nonreacted explosives; H Hugoniot curve of reaction
products; (0) explosive at rest; (1) reaction zone of length a, an
arbitrary quantity; (2) isentropic release of the detonation product

Figure 3

2.2.2. Approximation Methods

A first prediction of the CJ data can be given by using methods valid
for condensed explosives.
Chemical Potential. For many organic
explosives, D and p can be expressed simply as a function of
a parameter F defined as [33] :

O + H  ->  H2O
O + C  -> CO2

i.e., the formation of water is considered before the formation of
carbon oxides.
The following relations are then proposed :

Molecular masses and atomic composition for explosive mixtures must be
derived as sums of mass-average molecular mass and of elemental mole
fractions. The prediction of the detonation velocity is ca. 95 % accurate.

[28]  Détonique Théorique, Edition de l'Ecole Nationale Sup. des
Techniques Avancées, Paris 1981.

2.3. Nonideal Detonation Waves and Explosives

A detonation wave is nonideal if the geometry and dimensions of the
charge are such that the reaction zone is affected by lateral shock or
rarefaction waves. Consequently, the nonideality depends strongly on the
dimensions of the charge and the explosive composition characterized by a
given reaction zone length.

An explosive composition consisting of components with different
reaction kinetics does not satisfy the ideal detonation model, which
assumes all the exoenergetic reactions to be completed simultaneously in
the sonic Chapman – Jouguet plane. Such compositions are nonideal
explosives. The ideal detonation model applies only as an approximation to
most multicomponent explosives ; however, with data on reaction kinetics
under pressure often lacking, calculations are commonly based on the ideal
detonation model.

2.3.1. Nonideal Explosive Compositions

An example of an organic explosive with a metallic nonexplosive
additive is a dispersion of aluminum grains or flakes in a cast organic
explosive. The process sequence is shown in Figure (4). The energy
effectively supporting the detonation wave has been delivered at time
tA . The absorption of energy by aluminum reduces
p and D, but the late combustion of the aluminum allows the
pressure to be sustained behind the CJ plane, thereby increasing the total
impulse transmitted to the surroundings.

Figure 4

The possible sequence of an organic explosive (a) with a negative
oxygen balance cast or pressed with a binder containing oxygen (b) is
shown in Figure (5). The relatively slow decomposition of the binder
produces oxygen gas, which shifts the reaction of the organic explosive
toward better oxygen balance, thus increasing energy production.

Figure 5

A typical example of a mixture of two explosive components with
different reaction kinetics is a mixture of trinitrotoluene (TNT) and
ammonium nitrate (AN). The slower reaction of AN produces energy and an
excess of oxygen, which shifts the equilibrium of the TNT reaction to a
higher energy release. Because the detonation of the nonideal explosives
involves heat transfer between components, the specific surface areas
become important. Moreover, because the total reaction zone is relatively
long, detonation depends on the dimensions of the charges.

2.3.2. Detonation of Cylindrical Cartridges

In the detonation of cylindrical cartridges, the reactive flow is
two-dimensional, stationary, and relatively easy to model. Many military
or engineering applications use cylindrical geometry.

The basic phenomenon is the interaction of the inwardly propagating
rarefaction fan, originating at the outer surface as it is reached by the
shock wave, with the reaction zone. The first consequence of this
interaction is freezing of the reactions by cooling, which reduces the
released energy. A second effect is a curvature of the detonation front
and a loss in energy through a diverging flow. The reaction zone length
and the radius of the cartridge are two quantities that play opposite
roles in the process.

The experimental determination of the detonation velocity as a function
of the diameter produces a curve limited in two ways :

As the diameter increases, the detonation velocity D
approaches a constant value Di equal to that of
the ideal detonation wave.

Below a given diameter d, called the critical diameter,
dcr , the detonation is no longer
self-supporting and fails.

Many analytical expressions  D/Di
(d)  have been proposed in which the explosive is characterized
by its ideal reaction zone length. The function D (d) and the
critical diameter dcr depend on both the length and the structure of the
reaction zone, i.e., on factors that affect this zone, such as the loading
density r0 , grain size g, initial
temperature t0 , and composition, especially in
the case of mixtures.

As an example of the effects of these factors on ideal explosive
compositions, the influence of r0 is shown in
Figure (6) for the conventional military composition 60 RDX/40 TNT
(composition B) [35] . In the case of energy-rich explosives, the effect
of the cylinder diameter increases with decreasing density, whereas the
critical diameter decreases with increasing density and increasing
temperature [36]. The influence of mean grain size, e.g., that of powdered
TNT, is shown in Figure (7) [28], and most nonideal explosive compositions
and some pure explosives exhibit a particular behavior (see Chap. 6. High
Explosive Mixtures).

Figure 6

Figure 7

2.3.3. Low- and High-Order Detonation Velocity

Some explosives exhibit two different detonation velocities, depending
on the diameter of the cartridge and the initial ignition energy. Such
explosives always have a relatively low energy and loading density, with
great sensitivity to shock waves [35] . Conventional examples are the
dynamites and highly porous explosives [37] .

The high sensitivity to shock waves tends to allow the detonation to be
sustained. This is true even when the development of the reactions is
seriously limited by the size of the cartridge. Thus, under steady
conditions, a state of shock plus reaction would correspond to that
appearing in a transient regime in the buildup of heterogeneous explosives
to detonation.

2.3.4. The Effect of Confinement

Confinement usually delays the arrival of the expansion waves on the
axis. A confined charge is equivalent to a charge having a large diameter.
However, two anomalous effects should be noted : if the velocity of sound
in the confinement exceeds the detonation velocity for the given diameter,
a foreshock is propagated ahead of the wave. This foreshock can accelerate
the wave (increase the density) or stop the detonation, i.e., desensitize
a porous explosive by compaction, the explosive becoming homogenous. An
analogous desensitizing effect can be generated if there is an air gap
between the explosive and the container. The expanding reaction products
adiabatically compress the porous explosive. This effect can stop the
detonation of mining charges embedded in boreholes.

[35]  C. H. Johansson, P. A. Persson : Detonics of High
Explosives, Academic Press, London-New York 1970.

2.4. The Buildup of Detonation

2.4.1. Combustion – Deflagration – Detonation Transition (DDT)

Deflagration can generate a shock wave, which is propagated in the
unreacted medium, and if strong enough, can initiate reactions and become
a detonation wave. This transition can occur only if the explosive charge
is confined or is of such a size that expansion waves do not prevent
formation of a shock wave. Accident reports have shown that DDT affects a
variety of explosives and propellant charges.

However, different phenomena, which depend on the mechanical properties
of the medium, may occur [38] , [39] : if the explosive is porous or
exhibits poor mechanical behavior, the gas formed and subjected to
pressure is injected between the grains or into cracks, increasing the
combustion surface. The resulting acceleration stops only when detonation
occurs. The path versus time of the observed ionization front is
continuous. In the case of explosives stiff enough to withstand an elastic
wave, a steady deflagration develops as soon as the pressure makes the
medium impervious to the gas. The transition to detonation occurs as the
pressure of the gas formed behind the deflagration zone becomes high
enough to generate a shock wave. Detonation occurs when the shock wave
reaches the front of the deflagration wave. As a consequence, a
discontinuity is observed in the path versus time of the ionization front.

2.4.2. Shock-to-Detonation Transition (SDT)

If the end of an explosive charge is subjected to a shock wave, the
steady detonation appears only at a distance s and with a delay t inside
the explosive charge [40] . For a given composition, s and t are inversely
proportional to the intensity of the shock wave. There are two SDT
processes : one for homogeneous (for instance, a liquid) and one for
heterogeneous composition.

Homogeneous Explosives. The shock wave (a) in Figure (8)
propagated at constant velocity initiates decomposition reactions that are
first completed along the entrance side (b) at time t, the detonation
appearing at point A. The detonation wave (c) travels in a compressed
medium heated by shock wave (a). Therefore, the detonation wave (c)
travels with a velocity exceeding the normal detonation velocity, which is
attained at point B (wave d) as soon as wave c has reached wave a.

Figure 8

Heterogeneous Explosives. In Figure (9), the shock wave (a) is
gradually accelerated by the energy released upon its passage. Its
acceleration ends with the appearence of a detonation wave at point B.
This wave is characterized by its luminosity, as seen on optical records.
Sometimes a second wave starts from point B and moves backward in the
explosive, which has reacted only partially (retonation wave). The
macroscopic analysis can be explained by the microscopic heterogeneities
of the explosive [41] .

Figure 9

The energy of the shock wave is converted into heat energy by the
implosion of occluded gas bubbles, the impact of microjets, or the
adiabatic shearing of the powder grains. All the observations allow for
two successive phases, ignition and buildup, that are accomodated by
numerical models assuming inward or outward combustion of the grains [42]
.

2.4.3. Shock and Impact Sensitivity

The sensitivity of the explosives to an applied load is measured by the
maximum load at which no detonation occurs, or at which there is a 50 %
occurrence of detonation, i.e., a 50 % chance of failure. The sensitivity
to shock loading depends on the pressure p* and its action time
t*. Sensitivity is defined with the aid of the curve p*
(t*), which separates detonation points from failure points.

For a variety of explosives and for a certain pressure range, the
sensitivity is defined by the relation  , where e is a constant that
depends on the composition and exhibits the dimension surface energy per
unit area [43] .

Figure (10) shows that the concept of the energy threshold is an
acceptable approximation for solid-state compositions [44] . For liquid
explosives, which exhibit different behavior, the relation t* (p*),
based on an Arrhenius-type kinetic theory (with initial temperature
T* expressed as a function of pressure p*) accurately
reproduces the experimental findings. The critical time t* is
equal, for a given shock pressure, to the time of the shock-to-detonation
transition (see Fig. (8)). Except for primary explosives, the sensitivity
to shock-wave action and the shock-to-detonation transition are both
dependent on the homogeneity of the composition. Sensitivity is affected
by grain size [95] and internal grain defects [96]. The reaction of an
explosive to impact loading depends on several factors. If the projectile
is a large plate, the reaction is identical with that observed in the
preceding case : the shock wave induced by the impact depends on the
velocity and acoustic impedance, whereas the action time is proportional
to the plate thickness. In tests of sensitivity to shock-wave action, the
impacting plates are driven by explosives. If the projectile is a small
sphere or cylinder, the interaction with the target becomes complex [40].
If the shock wave cannot induce a shock – detonation transition, it
degenerates into an adiabatic compression. Frictional and shearing effects
can cause ignition. Finally, a phenomenon analogous to the deflagration –
detonation transition (see Section 2.4.1. Combustion – Deflagration –
Detonation Transition (DDT)) is observed, in which the mechanical
properties of the explosive play a part.

Figure 10

[38]  R. R. Bernecker et al., Symposium (International) on
Detonation, 7th, Office of Naval Research Report NSWC MP 82 – 334(1981).

2.5. Classification of Explosives

For many years the behavior of cylindrical cartridges, as described in
Section 2.3.2. Detonation of Cylindrical Cartridges, was regarded as
typical of ideal explosives. A detailed analysis has shown, however, that
classification of explosives into two groups is possible, taking into
account the influence of the loading density r0
on the detonation velocity as a function of the cartridge diameter and the
critical diameter [45] .

The first group includes a variety of organic explosives : TNT, RDX,
HMX, and their mixtures (see Chap. 5. Secondary Explosives). The second
group includes mixtures of an explosive and a combustible nonexploding
constituent (nonideal composition). This group also includes some pure
explosives such as hydrazine mononitrate (HN), nitroguanidine (NG),
ammonium nitrate (AN), dinitrotoluene (DNT), dinitrophenol (DNP), and
ammonium perchlorate (AP). The behavior of the second group is illustrated
for ammonium perchlorate in Figure (11).

Detonation behavior of ammonium perchlorate, eta = percent of
theoretical maximum density

Figure 11

[45]  D. Price, Symp. (Int.) Combust. 11th (1967) 963.

2.6. Functional Groups

Certain groups impart explosive potential.

2.6.1. Nitro Group

The nitro group is present in the form of salts of nitric acid,
ONO2 derivatives (nitrate esters), CNO2 derivatives
(aliphatic or aromatic nitro compounds), and NNO2 derivatives
(N-nitro compounds such as nitramines, nitroureas, etc.).

Salts of Nitric Acid. Salts of nitric acid include the alkali
metal and alkaline earth metal nitrates, ammonium nitrate, and the
nitrates of methylamine, urea, and guanidine. These are usually
low-density, water-soluble, sometimes hygroscopic compounds. With the
exception of some salts of hydrazines, e.g., triaminoguanidine and
hydrazine nitrates, they are insensitive to impact and friction.

O-Nitro Derivatives, Nitrate Esters
(RONO2). The low molecular mass
representatives are liquids or low-melting solids. They are sensitive to
impact when the number of carbon atoms and –ONO2 groups are
equal or nearly equal. Densities are in the medium range, except for
symmetrical molecules such as pentaerythritol tetranitrate (PETN). Heat
stability is moderate ; they are subject to hydrolysis and autocatalytic
decomposition.

C-Nitro Derivatives (CNO2). Aliphatic
and cycloaliphatic nitro compounds differ greatly from the aromatic and
heteroaromatic series. Members of the first group have limited explosive
properties, except when the number of NO2 groups equals or
exceeds the number of carbon atoms, as in some gem-dinitro compounds and
derivatives of nitroform, HC(NO2)3. The latter are
often shock sensitive, with limited heat stability. The compounds
RCF(NO2)2 are stable to heat and rather insensitive
[6] , [7] . Members of the second group containing two or three
NO2 groups per ring are valuable ; they are often dense and
insensitive to impact, with good hydrolytic and thermal stability.

N-Nitro Derivatives (NNO2).
N-Nitro derivatives are often difficult to synthesize. They exhibit high
densities and detonation velocities, with some sensitivity to impact. RDX
and HMX are representative.

2.6.2. Other Groups

Organic chlorates and perchlorates, peroxides, metal salts of some
organic compounds (acetylides and nitronates), and some organic compounds
with three-membered rings or chains of nitrogen atoms or triple bonds have
only limited application, except as primary explosives. Most are very
dangerous to handle.

Nitroso (NO) compounds are usually unstable. So-called
hexanitrosobenzene is an exception ; however, it is actually not a nitroso
compound but a furoxan derivative (benzotrifuroxan). It is of interest
because it is free of hydrogen (zero-hydrogen explosive). Other furoxanes
and also furazanes have explosive properties [83]

The difluoroamine group imparts explosive character, increased density
and volatility, a lower melting point and detonation velocity, and often
much higher impact sensitivity. These compounds resemble primary
explosives.

Many metal azides are primary explosives. Some organic azido
derivatives are being studied because of their high density and stability.
However, polyazido compounds can be very sensitive.

[6]  Kirk-Othmer, 9, 561 – 620, and 15, 841 – 853.

3. Application

3.1. Energy Transfer from the Explosive to the Surroundings

The energy available in the gaseous reaction products is transferred to
the surroundings by shock waves. The mechanical effects depend on the
geometry of the charge and the surroundings, on the distance from the
charge, and on the acoustic impedance of the media. The explosive energy
is used either to create compression and tension for engineering
applications or to accelerate projectiles for military applications.

3.1.1. Shock and Blast Waves

The highest dynamic pressures are produced at the exit end of the
charge, where the detonation shock is transferred to the inert material.
The shock pressure is a function of the shock impedance. It is defined by
the so-called shock polar curve of the pressure (p) vs. particle
velocity (u). By a graphical method (Fig. (12)) the induced
pressure can be determined at the intersection of a shock polar curve with
the detonation gas-product curve passing through the CJ point.

Figure 12

Even higher pressures can be obtained by using converging geometries
or, indirectly, by a two-stage device. The explosive accelerates a thin
metal plate, which generates an intense shock in the solid specimen as it
strikes it in free flight. During wave propagation in the surrounding
medium, the intensity of a shock wave decreases and the pressure profile
changes. At a given distance from the charge, compression alternates with
tension. The mechanical effects are a function of the maximum pressure and
of the positive and negative impulses comprising the so-called blast wave.
The maximum and minimum pressures are represented in Figure (13) as a
function of the reduced distance R/m1/3 from the charge.

Front pressure pf and minimum pressure
pmin in air vs. reduced distance
R/m1/3 from TNT in air, where R is the
distance in meters and m the mass in kilograms.

Figure 13

3.1.2. Casing and Liner Acceleration

A casing or liner in contact with an explosive charge is accelerated by
a three-step process :

u1 given by the shock wave

u2 = 2 u1 given by
reflection of the shock wave in a release wave at the free surface

u3 given by the further expansion of the
gaseous detonation products

The Gurney formula gives an approximate u3 for
a casing filled with an explosive characterized by the quantity E,
which is given in J/kg [46] :

where
M = mass of the casing, kg
C = mass of the
explosive, kg
a = 0.5 for a cylindrical charge and
a
= 0.6 for a spherical charge

[46]  S. J. Jacobs, Naval Ordnance Laboratory Technical Report 74
– 86.

3.2. High Compression of Solids

Explosives provide the most powerful means for compressing solids in
spite of the fact that at high pressure the heating limits the volume
reduction.

Production of New Crystal Phases.  The large pressure generated by
shock waves may be used to transform one crystal phase into another
(polymorphic transformation). The best known example of this technique is
the transformation of graphite into diamond. Unfortunately, the short
duration of the shock limits this technique to the production of small
crystals used as abrasives.

Powder Compaction. The compression of powders by shock waves
produced by explosives creates high pressures and temperatures
simultaneously, resulting in grain welding. The main problem, which has
been solved only recently, is the explosion caused by the interaction of
release waves, which follow the shock ; special geometries are required
[47] . Rapidly solidified amorphous and metastable microcrystalline
materials and ultrahigh-strength ceramics are expected to be produced by
this technique.

Shock Hardening.  The detonation of a thin sheet of
explosive covering a piece of steel creates great surface hardness by a
sequence of rapid compression, heating, and cooling.
[47]  M. L.
Wilkins, UCRL Report 90 142 (1983).

3.3. Metal Forming and Welding

The detonation of an explosive charge is used to form a metal plate ;
the shock wave is moderated by a liquid transmitting medium (Fig. (14)).
Techniques of free forming (Fig. (14) A) or bulkhead forming (Fig. (14) B)
may be used.

Figure 14

A grazing detonation may weld two metal plates with diffusion of metal
through the interface (Fig. (15)) [48] . The required collision velocity,
vc , depends on the materials. This process is
called explosive welding or cladding.

Figure 15

[48]  V. D. Linse, H. E. Otto, P. Pocaliko : "Explosion Welding,"
Welding Handbook, vol. 3, Am. Welding Soc., Miami 1980.

3.4. Rock Blasting [35]

In rock blasting the charges are placed in drill holes, which are
usually parallel to the rock surface. With strong confinement, most of the
explosive energy is usefully employed, even though much of it is released
by afterburning. Equally good results are given by nitroglycerin in high
(6000 – 8000 m/s) or low (2000 – 3000 m/s) explosives. Various types of
wedge cuts are used for tunnel blasting.

[35]  C. H. Johansson, P. A. Persson : Detonics of High
Explosives, Academic Press, London-New York 1970.

3.5. Perforators, Shaped or Hollow Charges

An explosive can be used to produce a thin, high-speed metallic
projectile capable of perforating armor [49] . Nonmilitary applications
include oil recovery.

A cavity in the explosive (Fig. (16)) is lined with a metal, usually
copper. The detonation divides the liner into two parts that move along
the axis at different velocities. Most of the mass of the liner forms the
so-called slug at a velocity of several hundred meters per second. The
remainder forms a thin projectile which is elongated because of the
difference of velocity between the first formed elements near the apex
(ujet max. » 8000 – 11 000 m/s) and those formed last (ujet min. = 1500 –
2000 m/s). The ultimate length of the projectile depends on the ductility
of the liner and can be > 10 times the diameter of the charge.

Figure 16

When the projectile strikes a solid or liquid target, it drills a deep,
narrow hole. The hole depth P is given approximately by the
expression

where L is the length of the projectile and
rj and rt are the
densities of the projectile and target.

[49]  M. Défourneaux, Sciences et Techniques de l'Armement, Paris
2 (1970) 293.

4. Primary Explosives

Under low-intensity stimulus of short duration, primary explosives,
even in thin layers, decompose and produce a detonation wave ; the
activation energy is low. The stimulus may be shock, friction, an electric
spark, or sudden heating. The deflagration – detonation transition occurs
within a distance often too short to be measured. The released energy and
the detonation velocity of primary explosives are small. Their formation
is often endothermic.

The main function of primary explosives is to produce a shock wave when
the explosive is stimulated by percussion, electrically, or optically
(laser), thus initiating a secondary explosive. Primary explosives are the
active detonator ingredients. Some are used in primer mixtures to ignite
propellants or pyrotechnics. Because of their sensitivity, primary
explosives are used in quantities limited to a few grams, and are
manufactured under special precautions to avoid any shock or spark. To be
used in industry, primary explosives must have limited sensitivity and
adequate stability to heat, hydrolysis, and storage. Mercury fulminate,
azides, and diazodinitrophenol are among the few products that meet these
requirements and that can detonate a secondary explosive. Others, e.g.,
lead styphnate or tetrazene, initiate burning or act as sensitizers.

Properties of primary explosives are given in Table (1).

Table 1

Mercury fulminate [628-86-4] , Hg(ONC)2,
Mr 284.65, was first prepared in the 1600s and was used by
Nobel in 1867 to detonate dynamites. It is prepared by the reaction of
mercury with nitric acid and 95 % ethanol, in small quantities because the
reaction is difficult to control. Mercury fulminate is a gray toxic
powder, which lacks the stability for storage. It reacts with metals in a
moist atmosphere ; in most industrial countries its use has been
abandoned.

Lead azide [13424-46-9] , Pb(N3)2,
Mr 291.26, discovered by CURTIUS (1891) [13], was developed
after World War I and is now the most important primary explosive. It is
produced continuously by the reaction of lead nitrate or acetate with
sodium azide in aqueous solution under basic conditions to avoid formation
of hydrazoic acid, which explodes readily. The crystal size must be
carefully controlled, large crystals being dangerous, by controlling the
stirring and by using wetting agents. Demineralized water must be used.
With thickeners such as dextrin, sodium carboxymethylcellulose, or
poly(vinyl alcohol), purities from 92 % to 99 % are possible, the former
containing 3 % dextrin and 4 – 5 % Pb (OH)2. The lower
sensitivity of the 92 % lead azide to impact and friction compared to
purer lead azides facilitates detonator loading. Lead azide has good
stability to heat and storage. Contact with copper must be avoided because
copper azide is extremely sensitive ; aluminum is preferred. Silver azide
is used at high temperature or in miniaturized pyrotechnic devices. It is
prepared from aqueous silver nitrate and sodium azide.

Diazodinitrophenol [28655-69-8] , DDNP,
C6H2N4O5 , Mr
210.06, is obtained by diazotizing picramic acid and purified by
recrystallization from acetone. It is sparingly soluble in water,
nonhygroscopic, and sensitive to impact, but not as sensitive to friction
or electrostatic energy. It is less stable to heat than lead azide. It is
most often used in the United States.

Lead styphnate [15245-44-0] , lead trinitroresorcinate,
C6HN3O8Pb, Mr 450.27, is
produced continuously by the aqueous reaction of the magnesium salt with
lead acetate, sometimes in the presence of agents that promote the
formation of the correct crystalline form. Especially sensitive to
electrostatic discharge, it is most frequently used to sensitize lead
azide and in primer compositions to initiate burning.

Tetrazene [31330-63-9] ,
C2H8N10O, Mr 188.07, is
obtained by the reaction of sodium nitrite with a soluble salt of
aminoguanidine in acetic acid at 30 – 40 °C. It decomposes in boiling
water. Its greatest value is for the sensitization of priming
compositions.

[13]  H. D. Fair, R. F. Walker : Energetic Materials. Inorganic
Azides, Plenum Press, New York 1977.

5. Secondary Explosives

5.1. Production

Common high explosives are usually made by liquid-phase nitration. The
overall mechanism is believed to be ionic, with NO2+
generally the reactive species. In some cases, N2O4
may be added to a double bond or to an epoxy group. It is also possible to
nitrate gas-phase hydrocarbons. The most important nitrating agent is
nitric acid.

Numerous end products of nitration are soluble in concentrated
HNO3 , and may be recovered by dilution. However, dilution
below ca. 55 wt % acid is not economic.

To increase the NO2+ content (3 wt % in pure
HNO3), lower the solubility of end products, reduce oxidative
side reactions, and facilitate the treatment of spent acids, mixtures of
sulfuric and nitric acids (mixed acid) are used. The water content of a
mixed acid may be reduced by adding oleum. In 50 : 50 wt % mixed acid, the
nitric acid is ca. 15 % dissociated.

However, sulfuric acid is difficult to remove from products by washing.
Furthermore, some substances (e.g., nitramines, nitriles, etc.) are
decomposed. Orthophosphoric acid or polyphosphoric acid may be used
instead of sulfuric acid ; however, these phosphoric acids are more
expensive and more difficult to recover.

Mild nitration or nitrolysis can be conducted in mixtures of nitric
acid and acetic anhydride or acetic acid ; the reactive species may be
CH3COONO2H+. Mixtures of nitric acid and
acetic anhydride containing between 30 and 80 wt % HNO3 can
detonate [50] . A high concentration of acetic anhydride avoids this
danger. Recently developed new nitration methods use
N2O5 as a solution in pure nitric acid ("nitric
oleum") or in chlorinated solvents (e.g., CH2Cl2).
Three processes for the production of nitric oleum are in operation or in
development [85] :

Oxidative electrolysis of a N2O4 –
HNO3 mixture

Ozonation of N2O4

Distillation of an oleum (H2SO4 +
SO3) – NH4NO3 mixture [84]

Nitric oleum allows yields to be improved and permits some syntheses to
be performed that are not possible by other routes ; the organic
N2O5 solutions allow nitration to be performed under
very mild and selective conditions [16],[85]. These nitrating agents must
be used at their site of production.

Nitration. Reaction is exothermic. Dilution of the nitric and
mixed acids with water liberates heat [51]. Normally, nitration is rapid.
However, 70 – 85 wt % nitric acid is also an oxidant, and for this reason
the reaction is best conducted continuously to limit the contact time of
the product with the reactive medium. If the medium is free of solid
particles, a tubular reactor can be used ; otherwise, reactors in cascade
with efficient stirring are employed. These reactors are made of highly
polished stainless steel. Reactors and stirrers must be carefully designed
to avoid dead zones and friction with crusts of explosives. Reactors are
often provided with a valve that opens quickly to discharge reactants into
a dilution vessel in an emergency.

Product Isolation. If a precipitate is not highly sensitive to
friction, it may be centrifuged. For safe continuous filtration, the
product must be stable in its mother liquor, and the filter design must
avoid introduction of the explosive between moving and fixed parts. The
precipitate is washed in two stages. The first uses only a small amount of
water, which is recovered and mixed with spent acid. The acid is recovered
from this mixture.

Purification.  Treatment with boiling water is sometimes
sufficient to hydrolyze impurities and wash out nitric acid.
Crystallization eliminates sulfuric acid and produces the desired grain
size. The solvent can be diluted with water or removed by steam
distillation. Very fine crystals are obtained by dilution with high-speed
stirring or by grinding in water or an inert liquid. A fluid energy mill
may also be used. Size separation can be effected by sieving under flowing
water or in a classifier. Drying is usually done as late as possible in
the process.

Recovery of Spent Acids. Acid recovery in an explosive plant is
of great importance in controlling production costs. These acids include
55 wt % HNO3, 63 – 68 wt % H2SO4 from
nitric acid concentration, and spent H2SO4
containing some nitric acid and nitration products.

To concentrate nitric acid, water is removed by countercurrent
extraction with 92 – 95 % H2SO4. At the top, 98 – 99
% HNO3 is produced. At the bottom, 63 – 68 %
H2SO4 is obtained, which can be concentrated to 93 %
by stripping with combustion gases, or to 96 – 98 % by vacuum
distillation. Nitric acid can also be concentrated by distillation over
magnesium nitrate.

Sulfuric acid is freed of nitric acid and nitro compounds by heating or
steam injection.

Pollution Problems. Gaseous pollution may occur during
nitrations, with evolution of red nitrous fumes. These can be absorbed in
columns by recycling water or dilute nitric acid to provide 50 – 55 wt %
acid.

Liquid and solid pollution is created by washing. Usually acids are
transferred to decanting and settling basins, where product and other
solid particles settle and liquids are neutralized. Some liquid wastes
require special treatment ; for example, the red liquors from TNT are best
destroyed by combustion.

Safety [9] , [11] . Accidents caused by detonation are usually
very severe. In addition to the normal precautions required in acid
handling, the production and use of explosives obviously involves special
risks. Loss of life and destruction of property by an accidental explosion
must be prevented at all costs, and the detonation of explosives stored
near populated areas must be avoided. Minimum distances between plant and
nearby structures are regulated.

The transmission of detonation by pipes or feeding devices must also be
prevented by appropriate arrangements. The danger of detonation by an
accidental fire can be mitigated by limiting vessel size. Detonations by
shock and friction in pipes, pumps, or valves are prevented by suitable
measures.

The following principles apply : partition of risks (e.g., specific
buildings for specific operations) ; limitation of risks (limited number
of persons present and limited quantities of explosives) ; and
installation of two or three independent safety devices. Some of these
measures are taken by the manufacturers under the supervision of
professional organizations following government regulations.

[50]  J. Dubar, J. Calzia, C.R. Seances Acad. Sci. Sér. C 266
(1968) 1114 – 1116.

6. High Explosive Mixtures

Explosives are used for global or directed destruction (bombs,
torpedoes, mines, reactive armors, and warheads) or for pyrotechnics.
Among pure compounds, only TNT and trinitrophenol are sufficiently
insensitive to be loaded in large quantities by casting. Small quantities
of other pure compounds are sometimes used for detonators or cutting or
transmitting cords.

For flexible wrapped cords, the pure explosive (e.g., PETN) flows
continuously through a funnel to form a flowing stream that is wrapped in
a thin plastic band. Thread is immediately woven on the plastic-enclosed
explosive, and this is again encased with plastic by drawing through a
die, which also helps adjust the loading density.

For metallic cords, the high explosive (PETN, RDX, HMX, or HNS) is
introduced into a metallic tube (lead, copper, or silver), which is drawn
through dies until the desired size and cross section are obtained.

Loading Processes. In casting processes, the explosive is
employed as a solution or a liquid suspension ; it is cast into the shell
or in the mold, where it crystallizes on cooling (physical process) or
solidifies by the cross-linking of a polymer (chemical process). In
pressing processes, the explosive is introduced in granular form into a
mold or shell and pressed with a piston at a pressure of 10 – 300 MPa ;
conditions depend on the munitions size, the binder, and the required
mechanical properties. Special shapes are prepared by laminating or
calendering.

Flexibility is increased by the use of blends, which can enhance
explosive performances while preserving safety and reducing cost. Special
effects, such as delayed detonation, are possible with improved safety and
high-temperature stability.

7. Industrial Explosives

For more than 350 years explosives have been employed to mine ores and
minerals. The annual consumption of explosives by each of the largest
Western European consuming countries is around 40 000 t. World annual
consumption of industrial explosives is at least 5 × 106 t, 75
% of which is ammonium nitrate – fuel oil (ANFO).

During the first 250 years of this period, only black powders were
known and used, but fundamental changes occurred in the 1860s (invention
of dynamite and blasting cap by ALFRED NOBEL), 1950s (ANFO), and 1980s
(emulsions). The search continues for less expensive products and safer
techniques for production and field use in mines, quarries, and road,
tunnel, and dam construction. At the same time, the introduction of new
products is restrained by the cost of existing investments and by safety
and environmental regulations.

8. Test Methods

Tests of secondary explosives are designed to study detonation
phenomena, to approve a product, and to verify that manufactured lots meet
the requirements (i.e., quality control).

Detonation phenomena (10–9 to 10–12 s) are
observed by laser interferometry, streak cameras, and X-ray flashes, all
electromagnetic methods. The very high instantaneous pressures (10 – 40
MPa) are measured, e.g., with low-impedance piezoresistive manganin gauges
(Cu–Mn–Ni alloy) [74] or with fluorinated piezo polymers as shock sensors
[93]. Phenomena are modelled based on Becker – Kistiakowsky – Wilson (BKW)
[31] or Jones – Wilkins – Lee (JWL) [32] equations developed in the United
States at the Los Alamos Scientific Laboratories (LASL) and the Lawrence
Livermore National Laboratory (LLNL).

Tests to approve a product measure detonation velocity and pressure,
energy released, ability to initiate or transmit detonation, and critical
diameter. They also determine the safety characteristics in use, storage,
and transportation.

The verification of a lot is usually based on the determination of
physical properties (melting point, particle size, specific area) and
chemical analysis. Safety and storage characteristics must match those of
the approved product.

Codified tests for military use are published in the United States as
military specifications (MIL), in France as Manuel des modes opératoires,
in the Federal Republic of Germany as VTL, in the United Kingdom as DEF.
STAN, and at NATO as STANAG. Industrial uses are the responsibility of the
National Bureau of Standards in the United States, CERCHAR in France, and
the Bundesanstalt für Materialprüfung (BAM) in the Federal Republic of
Germany. Japan issues Japanese Industrial Standards [75]. Unified and
standardized European explosives tests (EXTEST) are published in Explosifs
(Belgium, 1960 – 1970), in Explosivstoffe (FRG, 1971 – 1973), in
Propellants and Explosives (FRG, 1977 to 1981), and in Propellants,
Explosives, Pyrotechnics (1982 to date).

[74]  M. J. Ginsberg, A. R. Anderson, J. Wackerle, Symposium sur
les jauges et matériaux piezoresistifs, 1st, Arcachon, France, 1981.L. M.
Erickson et al., ibid.

9. Legal Aspects and Production

Governments regulate the possesion, production, storage, packaging,
shipping, and trading of explosives. Periodic inspections and labeling
prevent theft. Tagging the explosive permits tracing its origin.

10. Toxicology and Occupational Health

Raw Materials. Both acids (HNO3 and
H2SO4) and benzene derivatives (toluene and
chlorobenzene) are of concern [4] . The usual safety measures must be
followed to protect eyes, skin, and respiratory tract. Red fumes of
nitrogen oxides, formed normally or by accident, can induce lung edema two
days after exposure ; the use of a gas mask is recommended.

Explosives. Some primary explosives are toxic (mercury fulminate
and certain lead salts), but the toxic hazards are far below those arising
from high sensitivities to detonation. Among the secondary explosives,
nitroglycerin and other liquid nitrates exhibit hypotensive action
accompanied by headaches and, in chronic cases, by methemoglobinemia. A
tolerance to nitroglycerin can be developed. These substances are usually
absorbed through the skin rather than by inhalation. For nitroglycerin and
glycol dinitrate, TLV = 1.5 mg/m3. PETN, which like
nitroglycerin is prescribed for heart diseases, has a very low vapor
pressure and is not readily absorbed by the skin.

The toxic effects of TNT include blood changes, cyanosis,
methemoglobinemia, and toxic hepatitis. It is introduced by inhalation,
ingestion, and skin absorption ; at inhalation level, TLV = ca. 1
mg/m3. Dust in the plant is to be avoided.

Tetryl  and hexyl can induce an allergic dermatitis ; sensitivity
to hexyl is more widespread. For this reason there is a tendency to
abandon the use of these explosives.

Both RDX and HMX have negligible vapor pressures. They do not penetrate
the skin and their toxicity is very low. However, there have been reports
of epileptiform convulsions, without liver involvement, or carcinogenic or
mutagenic effects. Dust formation should be avoided and masks should be
worn.

[4]  B. T. Fedoroff et al.: Encyclopedia of Explosives and Related
Items, U.S. Army and National Technical Information Service, Springfield,
Va., 1960 – 1983.