An
explosive material, also called an
explosive, is a reactive substance that contains a great amount of potential energy that can produce an
explosion if released suddenly, usually accompanied by the production of
light,
heat,
sound, and
pressure. An
explosive charge is a measured quantity of explosive material.
This potential energy stored in an explosive material may be
Explosive materials may be categorized by the speed at which they expand. Materials that
detonate (explode faster than the
speed of sound) are said to be "high explosives" and materials that
deflagrate are said to be "low explosives". Explosives may also be categorized by their
sensitivity. Sensitive materials that can be initiated by a relatively small amount of heat or pressure are
primary explosives and materials that are relatively insensitive are
secondary explosives.
Chemical
An explosion is a type of spontaneous chemical reaction that, once initiated, is driven by both a large exothermic change (great release of heat) and a large positive
entropy change (great quantities of gases are released) in going from reactants to products, thereby constituting a thermodynamically favorable process in addition to one that propagates very rapidly. Thus, explosives are substances that contain a large amount of energy stored in
chemical bonds. The energetic stability of the gaseous products and hence their generation comes from the formation of strongly bonded species like carbon monoxide, carbon dioxide, and (di)nitrogen, which contain strong double and triple bonds having bond strengths of nearly 1 MJ/mole. Consequently, most commercial explosives are organic compounds containing
-NO2,
-ONO2 and
-NHNO2 groups that, when detonated, release gases like the aforementioned (e.g.,
nitroglycerin,
TNT,
HMX,
PETN,
nitrocellulose).
[1]An explosive is classified as a low or high explosive according to its rate 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.
[.]Decomposition
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.
[.]Deflagration
Main article:
DeflagrationIn deflagration, the decomposition of the explosive material is propagated by a flame front which moves slowly through the explosive material, in contrast to
detonation. Deflagration is a characteristic of
low explosive material.
[.]Detonation
This term is used to describe an explosive phenomenon whereby the decomposition is
propagated by an explosive
shock wave traversing the explosive material. The shock front is capable of passing through the high explosive material at great speeds, typically thousands of metres per second.
[.]Exotic
In addition to chemical explosives, there are a number of more exotic explosive materials, and exotic methods of causing explosions. Examples include
nuclear explosives,
antimatter, and abruptly heating a substance to a
plasma state with a high-intensity
laser or
electric arc.
Laser- and arc-heating are used in laser detonators,
exploding-bridgewire detonators, and
exploding foil initiators, where a shock wave and then detonation in conventional chemical explosive material is created by laser- or electric-arc heating. Laser and electric energy are not currently used in practice to generate most of the required energy, but only to initiate reactions.
.]Properties of explosive materials
To determine the suitability of an explosive substance for a particular use, its
physical properties must first be known. The usefulness of an explosive can only be appreciated when the properties and the factors affecting them are fully understood. Some of the more important characteristics are listed below:
[.]Availability and cost
The availability and cost of explosives are determined by the availability of the raw materials and the cost, complexity, and safety of the manufacturing operations.
[..]Sensitivity
Sensitivity refers to the ease with which an explosive 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 relate to:
- Impact — Sensitivity is expressed in terms of the distance through which a standard weight must be dropped onto the material to cause it to explode.
- Friction — Sensitivity is expressed in terms of what occurs when a weighted pendulum scrapes across the material (it may snap, crackle, ignite, and/or explode).
- 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.
[.]Sensitivity to initiation
The index of the capacity of an explosive to be initiated into detonation in a sustained manner. It is defined by the power of the detonator which is certain to prime the explosive to a sustained and continuous detonation. Reference is made to the
Sellier-Bellot scale that consists of a series of 10 detonators, from n. 1 to n. 10, each of which corresponds to an increasing charge weight. In practice, most of the explosives on the market today are sensitive to an n. 8 detonator, where the charge corresponds to 2 grams of
mercury fulminate.
[.]Velocity of detonation
The velocity with which the reaction process propagates in the mass of the explosive. Most commercial mining explosives have detonation velocities ranging from 1800 m/s to 8000 m/s. Today, velocity of detonation can be measured with accuracy. Together with density it is an important element influencing the yield of the energy transmitted for both atmospheric overpressure and ground acceleration.
[.]Stability
Stability is the ability of an explosive to be stored without
deterioration.
The following factors affect the stability of an explosive:
- Chemical constitution. In the strictest technical sense, the word "stability" is a thermodynamic term referring to the energy of a substance relative to a reference state or to some other substance. However, in the context of explosives, stability commonly refers to ease of detonation, which is concerned with kinetics (i.e., rate of decomposition). It is perhaps best, then, to differentiate between the terms thermodynamically stable and kinetically stable by referring to the latter as "inert." Contrarily, a kinetically unstable substance is said to be "labile." It is generally recognized that certain groups like nitro (–NO2), nitrate (–ONO2), and azide (–N3), are intrinsically labile. Kinetically, there exists a low activation barrier to the decomposition reaction. Consequently, these compounds exhibit high sensitivity to flame or mechanical shock. The chemical bonding in these compounds is characterized as predominantly covalent and thus they are not thermodynamically stabilized by a high ionic-lattice energy. Furthermore, they generally have positive enthalpies of formation and there is little mechanistic hindrance to internal molecular rearrangement to yield the more thermodynamically stable (more strongly bonded) decomposition products. For example, in lead azide, Pb(N3)2, the nitrogen atoms are already bonded to one another, so decomposition into Pb and N2.[1] is relatively easy.
- Temperature of storage. The rate of decomposition of explosives increases at higher temperatures. All standard military explosives may be considered to have a high degree of stability at temperatures from –10 to +35 °C, but each has a high temperature at which its rate ofdecomposition rapidly accelerates and stability is reduced. As a rule of thumb, most explosives become dangerously unstable at temperatures above 70 °C.
- Exposure to sunlight. When exposed to the ultraviolet rays of sunlight, many explosive compounds containing nitrogen groups rapidly decompose, affecting their stability.
- Electrical discharge. Electrostatic or spark sensitivity to initiation is common in a number of explosives. Static or other electrical discharge may be sufficient to cause a reaction, even detonation, under some circumstances. As a result, safe handling of explosives andpyrotechnics usually requires proper electrical grounding of the operator.
[.]Power, performance, and strength
The term power or performance as applied to an explosive refers to its ability to do work. In practice it is defined as the explosive's ability to accomplish what is intended in the way of energy delivery (i.e., fragment projection, air blast, high-velocity jet, underwater shock and bubble energy, etc.). Explosive power or performance is evaluated by a tailored series of tests to assess the material for its intended use. Of the tests listed below, cylinder expansion and air-blast tests are common to most testing programs, and the others support specific applications.
- Cylinder expansion test. A standard amount of explosive is loaded into a long hollow cylinder, usually of copper, and detonated at one end. Data is collected concerning the rate of radial expansion of the cylinder and the maximum cylinder wall velocity. This also establishes the Gurney energy or 2E.
- Cylinder fragmentation. A standard steel cylinder is loaded with explosive and detonated in a sawdust pit. The fragments are collected and the size distribution analyzed.
- Detonation pressure (Chapman-Jouguet condition). Detonation pressure data derived from measurements of shock waves transmitted into water by the detonation of cylindrical explosive charges of a standard size.
- Determination of critical diameter. This test establishes the minimum physical size a charge of a specific explosive must be to sustain its own detonation wave. The procedure involves the detonation of a series of charges of different diameters until difficulty in detonation wave propagation is observed.
- Infinite-diameter detonation velocity. Detonation velocity is dependent on loading density (c), charge diameter, and grain size. The hydrodynamic theory of detonation used in predicting explosive phenomena does not include the diameter of the charge, and therefore a detonation velocity, for an imaginary charge of infinite diameter. This procedure requires the firing of a series of charges of the same density and physical structure, but different diameters, and the extrapolation of the resulting detonation velocities to predict the detonation velocity of a charge of infinite diameter.
- Pressure versus scaled distance. A charge of a specific size is detonated and its pressure effects measured at a standard distance. The values obtained are compared with those for TNT.
- Impulse versus scaled distance. A charge of a specific size is detonated and its impulse (the area under the pressure-time curve) measured as a function of distance. The results are tabulated and expressed as TNT equivalents.
- Relative bubble energy (RBE). A 5 to 50 kg charge is detonated in water and piezoelectric gauges measure peak pressure, time constant, impulse, and energy.
-
- The RBE may be defined as Kx 3
- RBE = Ks
- where K = the bubble expansion period for an experimental (x) or a standard (s) charge.
.]Brisance
In addition to strength, explosives display a second characteristic, which is their shattering effect or brisance (from the French meaning to "break"), which is distinguished and separate from their total work capacity. This characteristic is of practical importance in determining the effectiveness of an explosion in fragmenting shells, bomb casings,
grenades, and the like. The rapidity with which an explosive reaches its peak pressure (
power) is a measure of its brisance. Brisance values are primarily employed in France and Russia.
The sand crush test is commonly employed to determine the relative brisance in comparison to TNT. No test is capable of directly comparing the explosive properties of two or more compounds; it is important to examine the data from several such tests (sand crush,
trauzl, and so forth) in order to gauge relative brisance. True values for comparison require field experiments.
.]Density
Density of loading refers to the mass of an explosive per unit volume. Several methods of loading are available, including pellet loading, cast loading, and press loading, the choice being determined by the characteristics of the explosive. Dependent upon the method employed, an average density of the loaded charge can be obtained that is within 80–99% of the theoretical maximum density of the explosive. High load density can reduce
sensitivity by making the
mass more resistant to internal
friction. However, if density is increased to the extent that individual
crystals are crushed, the explosive may become more sensitive. Increased load density also permits the use of more explosive, thereby increasing the power of the
warhead. It is possible to compress an explosive beyond a point of sensitivity, known also as
dead-pressing, in which the material is no longer capable of being reliably initiated, if at all.
[.]Volatility
Volatility is the readiness with which a substance
vaporizes. Excessive volatility often results in the development of pressure within rounds of ammunition and separation of mixtures into their constituents. Volatility affects the chemical composition of the explosive such that a marked reduction in stability may occur, which results in an increase in the danger of handling.
.]Hygroscopicity and water resistance
The introduction of
water into an explosive is highly undesirable since it reduces the sensitivity, strength, and velocity of detonation of the explosive.
Hygroscopicity is used as a measure of a material's moisture-absorbing tendencies. Moisture affects explosives adversely by acting as an inert material that absorbs heat when vaporized, and by acting as a solvent medium that can cause undesired chemical reactions. Sensitivity, strength, and velocity of detonation are reduced by inert materials that reduce the continuity of the explosive mass. When the moisture content evaporates during detonation, cooling occurs, which reduces the temperature of reaction. Stability is also affected by the presence of moisture since moisture promotes decomposition of the explosive and, in addition, causes corrosion of the explosive's metal container.
Explosives considerably differ from one another as to their behavior in the presence of water. Gelatin dynamites containing nitroglycerine have a degree of water resistance. Explosives based on
ammonium nitrate have little or no water resistance due to the reaction between ammonium nitrate and water, which liberates ammonia, nitrogen dioxide and hydrogen peroxide. In addition, ammonium nitrate is hygroscopic, susceptible to damp, hence the above concerns.
[.]Toxicity
Due to their chemical structure, most explosives are
toxic to some extent. Explosive product gases can also be toxic.
[.Explosive train
Explosive material may be incorporated in the
explosive train of a device or system. An example is a pyrotechnic lead igniting a booster, which causes the main charge to detonate.
[.Volume of products of explosion
Avogadro's law states that equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules, that is, the
molar volume of one gas is equal to the molar volume of any other gas. The molar volume of any gas at 0°C and under normal atmospheric pressure is very nearly 22.4 liters. Thus, considering the nitroglycerin reaction,
- C3H5(NO3)3 → 3CO2 + 2.5H2O + 1.5N2 + 0.25O2
the explosion of one mole of nitroglycerin produces 3 moles of CO2, 2.5 moles of H2O, 1.5 moles of N2, and 0.25 mole of O2, all in the gaseous state. A mole of nitroglycerin thus produces a total of 7.25 moles of gas, the volume of which at 0 °C and atmospheric pressure would be 7.25 × 22.4 = 162.4 liters.
Based upon this simple beginning, it can be seen that the volume of the products of explosion can be predicted for any quantity of the explosive. Further, by employing
Charles' Law for perfect gases, the volume of the products of explosion may also be calculated for any given temperature. This law states that at a constant pressure a perfect gas expands 1/273.15 of its volume at 0 °C, for each
degree Celsius of rise in temperature.
[citation needed]Therefore, at 15 °C (288.15
kelvin) the molar volume of an ideal gas is
- V15 = 22.414 (288.15/273.15) = 23.64 liters per mole
Thus, at 15 °C the volume of gas produced by the explosive decomposition of one mole of nitroglycerin becomes
- V = (23.64 l/mol)(7.25 mol) = 171.4 l
[..]Oxygen balance (OB% or Ω)
Main article:
Oxygen balanceOxygen balance is an expression that is used to indicate the degree to which an explosive can be oxidized. If an explosive molecule contains just enough oxygen to convert all of its carbon to carbon dioxide, all of its hydrogen to water, and all of its metal to metal oxide with no excess, the molecule is said to have a zero oxygen balance. The molecule is said to have a positive oxygen balance if it contains more oxygen than is needed and a negative oxygen balance if it contains less oxygen than is needed.
[2] The sensitivity,
strength, and
brisance of an explosive are all somewhat dependent upon oxygen balance and tend to approach their maximums as oxygen balance approaches zero.
[.]Chemical composition
[edit]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.
The above compositions may describe most of the explosive material, but a practical explosive will often include small percentages of other substances. 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.
[.]Mixture of oxidizer and 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.
[.]Classification of explosive materials
[.]By sensitivity
[.]Primary explosive
A
primary explosive is an
explosive that is extremely sensitive to stimuli such as
impact,
friction,
heat,
static electricity, or
electromagnetic radiation. A relatively small amount of energy is required for
initiation. As a very general rule, primary explosives are considered to be those compounds that are more sensitive than
PETN. As a practical measure, primary explosives are sufficiently sensitive that they can be reliably initiated with a blow from a hammer; however, PETN can usually be initiated in this manner, so this is only a very broad guideline. Additionally, several compounds, such as
nitrogen triiodide, are so sensitive that they cannot even be handled without detonating.
Primary explosives are often used in
detonators or to
trigger larger charges of less sensitive
secondary explosives. Primary explosives are commonly used in
blasting caps and
percussion caps to translate a physical shock signal. In other situations, different signals such as electrical/physical shock, or in the case of laser detonation systems, light, are used to initiate an action, i.e., an explosion. A small quantity, usually milligrams, is sufficient to initiate a larger charge of explosive that is usually safer to handle.
[3]Examples of primary high explosives are:
[.]Secondary explosive
A secondary explosive is less sensitive than a primary explosive and require substantially more energy to be initiated. Because they are less sensitive they are usable in a wider variety of applications and are safer to handle and store. Secondary explosives are used in larger quantities in an explosive train and are usually initiated by a smaller quantity of a primary explosive.
Examples of secondary explosives include
TNT and
RDX.
[.]Tertiary explosive
ANFO is an example of a tertiary explosive.
[.By velocity
[.]Low explosives
Low explosives are compounds where the rate of decomposition proceeds through the material at less than the
speed of sound. The decomposition is propagated by a flame front (
deflagration) which travels much more slowly through the explosive material than a
shock waveof a
high explosive.
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 can happen under higher
pressure or
temperature, which usually occurs when ignited in a confined space.
[.]High explosives
[.]By composition
[.]Priming composition
Priming compositions are primary explosives mixed with other compositions to control (lessen) the sensitivity of the mixture to the desired property.
For example,
primary explosives are so sensitive that they need to be stored and shipped in a wet state to prevent accidental initiation.
[..]By physical form
Explosives are often characterized by the physical form that the explosives are produced or used in. These use forms are commonly categorized as:
[5][.]Shipping label classifications
Shipping labels and tags may include both
United Nations and national markings.
United Nations markings include numbered Hazard Class and Division (HC/D) codes and alphabetic Compatibility Group codes. Though the two are related, they are separate and distinct. Any Compatibility Group designator can be assigned to any Hazard Class and Division. An example of this hybrid marking would be a consumer
firework, which is labeled as 1.4G or 1.4S.
[.]United Nations Organization (UNO) Hazard Class and Division (HC/D)
The Hazard Class and Division (HC/D) is a numeric designator within a hazard class indicating the character, predominance of associated hazards, and potential for causing personnel casualties and property damage. It is an internationally accepted system that communicates using the minimum amount of markings the primary hazard associated with a substance.
[6]Listed below are the Divisions for Class 1 (Explosives):
- 1.1 Mass Detonation Hazard. With HC/D 1.1, it is expected that if one item in a container or pallet inadvertently detonates, the explosion will sympathetically detonate the surrounding items. The explosion could propagate to all or the majority of the items stored together, causing a mass detonation. There will also be fragments from the item’s casing and/or structures in the blast area.
- 1.2 Non-mass explosion, fragment-producing. HC/D 1.2 is further divided into three subdivisions, HC/D 1.2.1, 1.2.2 and 1.2.3, to account for the magnitude of the effects of an explosion.
- 1.3 Mass fire, minor blast or fragment hazard. Propellants and many pyrotechnic items fall into this category. If one item in a package or stack initiates, it will usually propagate to the other items, creating a mass fire.
- 1.4 Moderate fire, no blast or fragment. HC/D 1.4 items are listed in the table as explosives with no significant hazard. Most small arms and some pyrotechnic items fall into this category. If the energetic material in these items inadvertently initiates, most of the energy and fragments will be contained within the storage structure or the item containers themselves.
- 1.5 mass detonation hazard, very insensitive.
- 1.6 detonation hazard without mass detonation hazard, extremely insensitive.
To see an entire UNO Table, browse Paragraphs 3-8 and 3-9 of NAVSEA OP 5, Vol. 1, Chapter 3.
]Class 1 Compatibility Group
Compatibility Group codes are used to indicate storage compatibility for HC/D Class 1 (explosive) materials. Letters are used to designate 13 compatibility groups as follows.
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).
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). Examples include Flares, signals, incendiary or illuminating ammunition and other smoke and tear producing devices.
H: Article containing both an explosive substance and white phosphorus (1.2H, 1.3H). These articles will spontaneously combust when exposed to the atmosphere.
J: Article containing both an explosive substance and flammable liquid or gel (1.1J, 1.2J, 1.3J). This excludes liquids or gels which are spontaneously flammable when exposed to water or the atmosphere, which belong in group H. Examples include liquid or gel filled incendiary ammunition, fuel-air explosive (FAE) devices, and flammable liquid fueled missiles.
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). Damaged or suspect ammunition of any group belongs in this group.
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).
