Explosive material

An explosive material is a material that either is chemically or otherwise

energetically unstable or produces a sudden expansion of the material usually
accompanied by the production of heat and large changes in pressure (and typically
also a flash and/or loud noise) upon initiation; this is called the explosion. An
explosive charge is a measured quantity of explosive material.

Contents

 1 Chemical explosives
o 1.1 Explosive compatibility groupings
 2 Exotic explosives
 3 Low explosives
 4 High explosives
 5 Detonation of an explosive charge
 6 Composition of the material
o 6.1 Mixtures of an oxidizer and a fuel
 6.1.1 Chemically pure compounds
 7 Chemical explosive reaction
o 7.1 Evolution of heat
o 7.2 Rapidity of reaction
o 7.3 Initiation of reaction
o 7.4 Sensitiser
 8 Military explosives
o 8.1 Availability and cost
o 8.2 Sensitivity
o 8.3 Velocity of detonation
o 8.4 Sensitivity to initiation
o 8.5 Stability
o 8.6 Power
o 8.7 Brisance
o 8.8 Density
o 8.9 Volatility
o 8.10 Hygroscopicity
o 8.11 Toxicity
o 8.12 Water resistance
 9 Measurement of chemical explosive reaction
o 9.1 Oxygen balance (OB%)
o 9.2 Heat of explosion
o 9.3 Balancing chemical explosion equations
o 9.4 Volume of products of explosion
o 9.5 Explosive strength
o 9.6 Example of thermochemical calculations


Chemical explosives

Explosives are classified as low or high explosives according to their rates of burn: low
explosives burn rapidly (or deflagrate), while high explosives detonate. While these
definitions are distinct, the problem of precisely measuring rapid decomposition makes
practical classification of explosives difficult.

The chemical decomposition of an explosive may take years, days, hours, or a fraction
of a second. The slower processes of decomposition take place in storage and are of
interest only from a stability standpoint. Of more interest are the two rapid forms of
decomposition, deflagration and detonation.

The latter term is used to describe an explosive phenomenon whereby the

decomposition is propagated by the explosive shockwave traversing the explosive
material. The shockwave front is capable of passing through the high explosive
material at great speeds, typically thousands of meters per second.

Explosives usually have less potential energy than petroleum fuels, but their high rate
of energy release produces the great blast pressure. TNT has a detonation velocity of
6,940 m/s compared to 1,680 m/s for the detonation of a pentane-air mixture, and the
0.34-m/s stoichiometric flame speed of gasoline combustion in air.

Explosive force is released in a direction perpendicular to the surface of the explosive.

If the surface is cut or shaped, the explosive forces can be focused to produce a
greater local effect; this is known as a shaped charge.

In a low explosive (which deflagrates), the decomposition is propagated by a flame

front which travels much more slowly through the explosive material.

The properties of the explosive indicate the class into which it falls. In some cases
explosives can be made to fall into either class by the conditions under which they are
initiated. In sufficiently large quantities, almost all low explosives can undergo a
Deflagration to Detonation Transition (DDT). For convenience, low and high explosives
may be differentiated by the shipping and storage classes.

[-] Explosive compatibility groupings

Shipping labels and tags will include UN and national, e.g. USDOT, hazardous material
Class with Compatibility Letter, as follows:

 1.1 Mass Explosion Hazard

 1.2 Non-mass explosion, fragment-producing
 1.3 Mass fire, minor blast or fragment hazard
 1.4 Moderate fire, no blast or fragment: a consumer firework is 1.4G or 1.4S
 1.5 Explosive substance, very insensitive (with a mass explosion hazard)
 1.6 Explosive article, extremely insensitive

A Primary explosive substance (1.1A)

B An article containing a primary explosive substance and not containing two or more
effective protective features. Some articles, such as detonator assemblies for blasting
and primers, cap-type, are included. (1.1B, 1.2B, 1.4B)

C Propellant explosive substance or other deflagrating explosive substance or article

containing such explosive substance (1.1C, 1.2C, 1.3C, 1.4C)
D Secondary detonating explosive substance or black powder or article containing a
secondary detonating explosive substance, in each case without means of initiation and
without a propelling charge, or article containing a primary explosive substance and
containing two or more effective protective features. (1.1D, 1.2D, 1.4D, 1.5D)

E Article containing a secondary detonating explosive substance without means of

initiation, with a propelling charge (other than one containing flammable liquid, gel or
hypergolic liquid) (1.1E, 1.2E, 1.4E)

F containing a secondary detonating explosive substance with its means of initiation,

with a propelling charge (other than one containing flammable liquid, gel or hypergolic
liquid) or without a propelling charge (1.1F, 1.2F, 1.3F, 1.4F)

G Pyrotechnic substance or article containing a pyrotechnic substance, or article

containing both an explosive substance and an illuminating, incendiary, tear-producing
or smoke-producing substance (other than a water-activated article or one containing
white phosphorus, phosphide or flammable liquid or gel or hypergolic liquid) (1.1G,
1.2G, 1.3G, 1.4G)

H Article containing both an explosive substance and white phosphorus (1.2H, 1.3H)

J Article containing both an explosive substance and flammable liquid or gel (1.1J,
1.2J, 1.3J)

K Article containing both an explosive substance and a toxic chemical agent (1.2K,
1.3K)

L Explosive substance or article containing an explosive substance and presenting a

special risk (e.g., due to water-activation or presence of hypergolic liquids, phosphides
or pyrophoric substances) needing isolation of each type (1.1L, 1.2L, 1.3L)

N Articles containing only extremely insensitive detonating substances (1.6N)

S Substance or article so packed or designed that any hazardous effects arising from
accidental functioning are limited to the extent that they do not significantly hinder or
prohibit fire fighting or other emergency response efforts in the immediate vicinity of
the package (1.4S)

[-] Low explosives

A low explosive is usually a mixture of a combustible substance and an oxidant that

decomposes rapidly (deflagration), as opposed to most high explosives, which are
compounds[citation needed].

Under normal conditions, low explosives undergo deflagration at rates that vary from a
few centimeters per second to approximately 400 metres per second. It is possible for
them to deflagrate very quickly, producing an effect similar to a detonation. This
usually occurs when ignited in a confined space.

Low explosives are normally employed as propellants. Included in this group are gun
powders, pyrotechnics such as flares and illumination devices .
[-] High explosives

High explosives normally are employed in mining, demolition, and military warheads.
High explosive compounds detonate at rates ranging from 3,000 to 9,000 meters per
second, and are, conventionally, subdivided into two explosives classes, differentiated
by sensitivity:

 Primary explosives are extremely sensitive to mechanical shock, friction, and

heat, to which they will respond by burning rapidly or detonating. Examples
include mercury fulminate, lead styphnate and lead azide.
 Secondary explosives, also called base explosives, are relatively insensitive
to shock, friction, and heat. They may burn when exposed to heat or flame in
small, unconfined quantities, but detonation can occur. These are sometimes
added in small amounts to blasting caps to boost their power. Dynamite, TNT,
RDX, PETN, HMX, and others are secondary explosives. PETN is the benchmark
compound; compounds more sensitive than PETN are classed as primary
explosives.

Some definitions add a third category:

 Tertiary explosives or blasting agents, are insensitive to shock, they

cannot be reliably detonated with practical quantities of primary explosive, and,
instead, require an intermediate explosive booster, of secondary explosive, e.g.
ammonium nitrate/fuel oil mixture (ANFO) and slurry (wet bag) explosives that
are primarily used in large-scale mining and construction.

Note that many, if not most, explosive chemical compounds may usefully deflagrate
and detonate, and are used in high- and low-explosive compounds. Thus, under the
correct conditions, a propellant (for example nitrocellulose) might deflagrate if ignited,
or may detonate if initiated with a detonator.

[-] Detonation of an explosive charge

The explosive train, also called an initiation sequence or firing train, is the
sequence of charges that progresses from relatively low levels of energy to initiate the
final explosive material or main charge. There are low- and high-explosive trains. Low-
explosive trains are as simple as a rifle cartridge, including a primer and a propellant
charge. High-explosives trains can be more complex, either two-step (e.g., detonator
and dynamite) or three-step (e.g., detonator, booster of primary explosive, and main
charge of secondary explosive). Detonators are often made from tetryl and fulminates.

[-] Composition of the material

An explosive may consist of either a chemically pure compound, such as nitroglycerin,

or a mixture of an oxidizer and a fuel, such as black powder.

[-] Mixtures of an oxidizer and a fuel

An oxidizer is a pure substance (molecule) that in a chemical reaction can contribute

some atoms of one or more oxidizing elements, in which the fuel component of the
explosive burns. On the simplest level, the oxidizer may itself be an oxidizing element,
such as gaseous or liquid oxygen.
  Black powder: Potassium nitrate, charcoal and sulfur
 Flash powder: Fine metal powder (usually aluminium or magnesium) and a
strong oxidizer (e.g. potassium chlorate or perchlorate).
 Ammonal: Ammonium nitrate and aluminium powder.
 Armstrong's mixture: Potassium chlorate and red phosphorus. This is a very
sensitive mixture. It is a primary high explosive in which sulfur is substituted for
some or all phosphorus to slightly decrease sensitivity.
 Sprengel explosives: A very general class incorporating any strong oxidizer
and highly reactive fuel, although in practice the name most commonly was
applied to mixtures of chlorates and nitroaromatics.
o ANFO: Ammonium nitrate and fuel oil.
o Cheddites: Chlorates or perchlorates and oil.
o Oxyliquits: Mixtures of organic materials and liquid oxygen.
o Panclastites: Mixtures of organic materials and dinitrogen tetroxide.

[-] Chemically pure compounds

Some chemical compounds are unstable in that, when shocked, they react, possibly to
the point of detonation. Each molecule of the compound dissociates into two or more
new molecules (generally gases) with the release of energy.

 Nitroglycerin: A highly unstable and sensitive liquid.

 Acetone peroxide: A very unstable white organic peroxide.
 TNT: Yellow insensitive crystals that can be melted and cast without
detonation.
 Nitrocellulose: A nitrated polymer which can be a high or low explosive
depending on nitration level and conditions.
 RDX, PETN, HMX: Very powerful explosives which can be used pure or in
plastic explosives.
o C-4 (or Composition C-4): An RDX plastic explosive plasticized to be
adhesive and malleable.

The above compositions may describe the majority of the explosive material, but a
practical explosive will often include small percentages of other materials. For example,
dynamite is a mixture of highly sensitive nitroglycerin with sawdust, powdered silica, or
most commonly diatomaceous earth, which act as stabilizers. Plastics and polymers
may be added to bind powders of explosive compounds; waxes may be incorporated to
make them safer to handle; aluminium powder may be introduced to increase total
energy and blast effects. Explosive compounds are also often "alloyed": HMX or RDX
powders may be mixed (typically by melt-casting) with TNT to form Octol or Cyclotol.

[-] Chemical explosive reaction

A chemical explosive is a compound or mixture which, upon the application of heat or

shock, decomposes or rearranges with extreme rapidity, yielding much gas and heat.
Many substances not ordinarily classed as explosives may do one, or even two, of
these things. For example, at high temperatures (> 2000°C) a mixture of nitrogen and
oxygen can be made to react with great rapidity and yield the gaseous product nitric
oxide; yet the mixture is not an explosive since it does not evolve heat, but rather
absorbs heat.

N2 + O2 → 2NO - 43,200 calories (or 180 kJ) per mole of N2

For a chemical to be an explosive, it must exhibit all of the following:

 Rapid expansion (i.e.,. rapid production of gases or rapid heating of

surroundings)
 Evolution of heat
 Rapidity of reaction
 Initiation of reaction

[-] Initiation of reaction

A reaction must be capable of being initiated by the application of shock or heat to a

small portion of the mass of the explosive material. A material in which the first three
factors exist cannot be accepted as an explosive unless the reaction can be made to
occur when desired.

[-] Military explosives

To determine the suitability of an explosive substance for military use, its physical
properties must first be investigated. The usefulness of a military explosive can only be
appreciated when these properties and the factors affecting them are fully understood.
Many explosives have been studied in past years to determine their suitability for
military use and most have been found wanting. Several of those found acceptable
have displayed certain characteristics that are considered undesirable and, therefore,
limit their usefulness in military applications. The requirements of a military explosive
are stringent, and very few explosives display all of the characteristics necessary to
make them acceptable for military standardization. Some of the more important
characteristics are discussed below:

[-] Sensitivity

Regarding an explosive, this refers to the ease with which it can be ignited or
detonated—i.e., the amount and intensity of shock, friction, or heat that is required.
When the term sensitivity is used, care must be taken to clarify what kind of sensitivity
is under discussion. The relative sensitivity of a given explosive to impact may vary
greatly from its sensitivity to friction or heat. Some of the test methods used to
determine sensitivity are as follows:

 Impact Sensitivity is expressed in terms of the distance through which a

standard weight must be dropped to cause the material to explode.
 Friction Sensitivity is expressed in terms of what occurs when a weighted
pendulum scrapes across the material (snaps, crackles, ignites, and/or
explodes).
 Heat Sensitivity is expressed in terms of the temperature at which flashing or
explosion of the material occurs.

Sensitivity is an important consideration in selecting an explosive for a particular

purpose. The explosive in an armor-piercing projectile must be relatively insensitive, or
the shock of impact would cause it to detonate before it penetrated to the point
desired. The explosive lenses around nuclear charges are also designed to be highly
insensitive, to minimize the risk of accidental detonation.