- Explosive material
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
- chemical energy, such as nitroglycerine or grain dust
- pressurized compressed gas, such as a gas cylinder or aerosol can
- nuclear, such as fissile isotopes of uranium-235 and plutonium-239
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.
Type of reaction
An explosion is a type of spontaneous chemical reaction (once initiated) that 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 very 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 1,000 kJ/mole. Consequently, most commercial explosives are organic compounds containing -NO2, -ONO2 and -NHNO2 groups that when detonated release gases like the aforementioned ones (e.g., nitroglycerin, TNT, HMX, PETN, nitrocellulose).
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.
In 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.
This term is used to describe an explosive phenomenon whereby the decomposition is propagated by the 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.
In addition to chemical explosives, there exist varieties of more exotic explosive material, and theoretical 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 practice 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 practically used directly for the majority of the energy, 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 is determined by the availability of the raw materials and the cost, complexity, and safety of the manufacturing operations.
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.
Sensitivity to initiation
The index of the capacity of the 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 the 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 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 is commonly used to refer to the ease of detonation, which is concerned with kinetics (i.e. the 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 with respect to decomposition. Kinetically, there exits a low activation barrier to the decomposition reaction. Consequently, these compounds exhibit a high sensitivity to flame or mechanical shock. The chemical bonding in these compounds is characterized by being predominantly covalent in nature 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 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 there is relatively easy decomposition into Pb and N2.
- Temperature of storage. The rate of decomposition of explosives increases at higher temperatures. All of the standard military explosives may be considered to have a high degree of stability at temperatures of -10 to +35 °C, but each has a high temperature at which the rate of decomposition rapidly accelerates and stability is reduced. As a rule of thumb, most explosives become dangerously unstable at temperatures exceeding 70 °C.
- Exposure to the sun. If exposed to the ultraviolet rays of the sun, many explosive compounds that contain nitrogen groups will rapidly decompose, affecting their stability.
- Electrical discharge. Electrostatic or spark sensitivity to initiation is common to a number of explosives. Static or other electrical discharge may be sufficient to inspire detonation under some circumstances. As a result, the safe handling of explosives and pyrotechnics almost always requires 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 jets, 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 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 diameter of the charge, and therefore a detonation velocity, for an imaginary charge of Infinite diameter. This procedure requires a series of charges of the same density and physical structure, but different diameters, to be fired and the resulting detonation velocities extrapolated to predict the detonation velocity of a charge of infinite diameter.
- Pressure versus scaled distance. A charge of specific size is detonated and its pressure effects measured at a standard distance. The values obtained are compared with that for TNT.
- Impulse versus scaled distance. A charge of specific size is detonated and its impulse (the area under the pressure-time curve) measured versus distance. The results are tabulated and expressed in TNT equivalent.
- 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 = bubble expansion period for experimental (x) or standard (s) charge.
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 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 one used is 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 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: liberating ammonia, nitrogen dioxide and hydrogen peroxide. Plus, ammonium nitrate is hygroscopic, susceptible to damp, hence the above.
Due to their chemical structure, most explosives are toxic to some extent. Explosive product gases can also be toxic.
Another property of explosive material is where it exists in the explosive train of the device or system. An example of this 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. Since a molar volume is the volume of one mole of gas, one mole of nitroglycerin produces 3 + 2.5 + 1.5 + 0.25 = 7.25 molar volumes of gas; and these molar volumes at 0°C and atmospheric pressure form an actual volume of 7.25 × 22.4 = 162.4 liters of gas.
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.
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)
Oxygen 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. 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.
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 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.
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.
- 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.
Classification of explosive materials
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.
Examples of primary high explosives are:
- Acetone peroxide
- Ammonium permanganate
- Copper acetylide
- Hexamethylene triperoxide diamine
- Lead azide
- Lead styphnate
- Lead picrate
- Mercury(II) fulminate
- Nitrogen trichloride
- Nitrogen triiodide
- Silver azide
- Silver acetylide
- Silver fulminate
- Sodium azide
- Tetraamine copper complexes
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 explosives, also called blasting agents, are so insensitive to shock that they cannot be reliably detonated by practical quantities of primary explosive, and instead require an intermediate explosive booster of secondary explosive. These are primarily used in large-scale mining and construction operations, and in terrorism.
ANFO is an example of a tertiary explosive.
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 wave of 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.
A low explosive is usually a mixture of a combustible substance and an oxidant that decomposes rapidly (deflagration); however, they burn more slowly than a high explosive which has an extremely fast burn rate.
High explosives are explosive materials that detonate, meaning that the explosive shock front passes through the material at a supersonic speed. High explosives detonate with explosive velocity rates ranging from 3,000 to 9,000 meters per second. They are normally employed in mining, demolition, and military applications. They can be divided into two explosives classes differentiated by sensitivity: Primary explosive and secondary explosive. The term high explosive is in contrast to the term low explosive, which explodes (deflagrates) at a slower rate.
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:
- Plastic or polymer bonded
- Putties (AKA plastic explosives)
- Blasting agents
- Slurries and gels
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.
Examples of national markings would include United States Department of Transportation (U.S. DOT) codes.
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 the primary hazard associated with a substance using the minimum amount of markings.
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 of the 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 Para 3-8 and 3-9 from the 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).
C: Propellant explosive substance or other deflagrating explosive substance or article containing such explosive substance (1.1C, 1.2C, 1.3C, 1.4C). These are bulk propellants, propelling charges, and devices containing propellants with or without means of ignition. Examples include single-, double-, triple-based, and composite propellants, solid propellant rocket motors and ammunition with inert projectiles.
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).
Legacy article contents
Contents below this heading have not been incorporated into the regrouped article.
Explosives usually have less potential energy than petroleum fuels, but their high rate of energy release produces a 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.
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.
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.
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
A sensitiser is a powdered or fine particulate material that is sometimes used to create voids that aid in the initiation or propagation of the detonation wave. It may be as high-tech as glass beads or as simple as seeds.
Measurement of chemical explosive reaction
The development of new and improved types of ammunition requires a continuous program of research and development. Adoption of an explosive for a particular use is based upon both proving ground and service tests. Before these tests, however, preliminary estimates of the characteristics of the explosive are made. The principles of thermochemistry are applied for this process.
Thermochemistry is concerned with the changes in internal energy, principally as heat, in chemical reactions. An explosion consists of a series of reactions, highly exothermic, involving decomposition of the ingredients and recombination to form the products of explosion. Energy changes in explosive reactions are calculated either from known chemical laws or by analysis of the products.
For most common reactions, tables based on previous investigations permit rapid calculation of energy changes. Products of an explosive remaining in a closed calorimetric bomb (a constant-volume explosion) after cooling the bomb back to room temperature and pressure are rarely those present at the instant of maximum temperature and pressure. Since only the final products may be analyzed conveniently, indirect or theoretical methods are often used to determine the maximum temperature and pressure values.
Some of the important characteristics of an explosive that can be determined by such theoretical computations are:
- Oxygen balance
- Heat of explosion or reaction
- Volume of products of explosion
- Potential of the explosive
Balancing chemical explosion equations
In order to assist in balancing chemical equations, an order of priorities is presented in table 1. Explosives containing C, H, O, and N and/or a metal will form the products of reaction in the priority sequence shown. Some observation you might want to make as you balance an equation:
- The progression is from top to bottom; you may skip steps that are not applicable, but you never back up.
- At each separate step there are never more than two compositions and two products.
- At the conclusion of the balancing, elemental nitrogen, oxygen, and hydrogen are always found in diatomic form.
Table 1. Order of Priorities Priority Composition of explosive Products of decomposition Phase of products 1 A metal and chlorine Metallic chloride Solid 2 Hydrogen and chlorine HCl Gas 3 A metal and oxygen Metallic oxide Solid 4 Carbon and oxygen CO Gas 5 Hydrogen and oxygen H2O Gas 6 Carbon monoxide and oxygen CO2 Gas 7 Nitrogen N2 Gas 8 Excess oxygen O2 Gas 9 Excess hydrogen H2 Gas 10 Excess carbon C Solid
- C6H2(NO2)3CH3; → : 7C + 5H + 3N + 6O
Using the order of priorities in table 1, priority 4 gives the first reaction products:
- 7C + 6O → 6CO with one mol of carbon remaining
Next, since all the oxygen has been combined with the carbon to form CO, priority 7 results in:
- 3N → 1.5N2
Finally, priority 9 results in: 5H → 2.5H2
The balanced equation, showing the products of reaction resulting from the detonation of TNT is:
- C6H2(NO2)3CH3 → 6CO + 2.5H2 + 1.5N2 + C
Notice that partial moles are permitted in these calculations. The number of moles of gas formed is 10. The product carbon is a solid.
Example of thermochemical calculations
The PETN reaction will be examined as an example of thermo-chemical calculations.
- PETN: C(CH2ONO2)4
- Molecular weight = 316.15 g/mol
- Heat of formation = 119.4 kcal/mol
(1) Balance the chemical reaction equation. Using table 1, priority 4 gives the first reaction products:
- 5C + 12O → 5CO + 7O
Next, the hydrogen combines with remaining oxygen:
- 8H + 7O → 4H2O + 3O
Then the remaining oxygen will combine with the CO to form CO and CO2.
- 5CO + 3O → 2CO + 3CO2
Finally the remaining nitrogen forms in its natural state (N2).
- 4N → 2N2
The balanced reaction equation is:
- C(CH2ONO2)4 → 2CO + 4H2O + 3CO2 + 2N2
(2) Determine the number of molar volumes of gas per mole. Since the molar volume of one gas is equal to the molar volume of any other gas, and since all the products of the PETN reaction are gaseous, the resulting number of molar volumes of gas (Nm) is:
- Nm = 2 + 4 + 3 + 2 = 11 Vmolar/mol
(3) Determine the potential (capacity for doing work). If the total heat liberated by an explosive under constant volume conditions (Qm) is converted to the equivalent work units, the result is the potential of that explosive.
The heat liberated at constant volume (Qmv) is equivalent to the heat liberated at constant pressure (Qmp) plus that heat converted to work in expanding the surrounding medium. Hence, Qmv = Qmp + work (converted).
- a. Qmp = Qfi (products) − Qfk (reactants)
- where: Qf = heat of formation (see table 1)
- For the PETN reaction:
- Qmp = 2(26.343) + 4(57.81) + 3(94.39) − (119.4) = 447.87 kcal/mol
- (If the compound produced a metallic oxide, that heat of formation would be included in Qmp.)
- b. Work = 0.572Nm = 0.572(11) = 6.292 kcal/mol
- As previously stated, Qmv converted to equivalent work units is taken as the potential of the explosive.
- c. Potential J = Qmv (4.185 × 106 kg)(MW) = 454.16 (4.185 × 106) 316.15 = 6.01 × 106 J kg
- This product may then be used to find the relative strength (RS) of PETN, which is
- d. RS = Pot (PETN) = 6.01 × 106 = 2.21 Pot (TNT) 2.72 × 106
Though early thermal weapons, such as Greek fire, have existed since ancient times, the first widely used explosive in warfare and mining was black powder, invented in 9th century China ( see the history of gunpowder ). This material was sensitive to water, and evolved lots of dark smoke. During the 19th century black powder was replaced by nitroglycerine, nitrocellulose, smokeless powder, dynamite and gelignite (the two latter invented by Alfred Nobel). World War II saw an extensive use of new explosives ( see explosives used during World War II ). In turn, these have largely been replaced by modern explosives such as trinitrotoluene and C-4.
The increased availability of chemicals has allowed the construction of improvised explosive devices.
The legality of possessing or using explosives varies by jurisdiction.
- ^ W. W. Porterfield, Inorganic Chemistry: A Unified Approach, 2nd ed., Academic Press, Inc., San Diego, pp. 479-480 (1993).
- ^ Meyer, Rudolf; Josef Köhler, Axel Homburg (2007). Explosives, 6th Ed.. Wiley VCH. ISBN 3-527-31656-6.
- ^ Primary Explosives. Globalsecurity.org. Retrieved on 2010-02-11.
- ^ PowerLabs Lead Picrate Synthesis
- ^ Cooper, Paul W. (1996). "Chapter 4: Use forms of explosives". Explosives Engineering. Wiley-VCH. pp. 51–66. ISBN 0-471-18636-8.
- ^ Table 12-4.—United Nations Organization Hazard Classes. Tpub.com. Retrieved on 2010-02-11.
- ^ Federal Explosives Law and Regulations, U.S. Department of Justice, Bureau of Alcohol, Tobacco, Firearms and Explosives
- ^ Special provisions relating to black powder
- Army Research Office. Elements of Armament Engineering (Part One). Washington, D.C.: U.S. Army Materiel Command, 1964.
- Commander, Naval Ordnance Systems Command. Safety and Performance Tests for Qualification of Explosives. NAVORD OD 44811. Washington, D.C.: GPO, 1972.
- Commander, Naval Ordnance Systems Command. Weapons Systems Fundamentals. NAVORD OP 3000, vol. 2, 1st rev. Washington, D.C.: GPO, 1971.
- Departments of the Army and Air Force. Military Explosives. Washington, D.C.: 1967.
- USDOT Hazardous Materials Transportation Placards
- Swiss Agency for the Environment, Forests, and Landscap. "Occurrence and relevance of organic pollutants in compost, digestate and organic residues", Research for Agriculture and Nature. 8 November 2004. p 52, 91, 182.
- Youtube video demonstrating blast wave in slow motion
- Blaster Exchange - Explosives Industry Portal
- Explosive information and guides
- Why high nitrogen density in explosives?
- The Explosives and Weapons Forum
- Military Explosives
- UN Hazard Classification Code at GlobalSecurity.org
- Explosives at GlobalSecurity.org
- Class 1 Hazmat Placards
- Journal of Energetic Materials
- Explosives info
Wikimedia Foundation. 2010.