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Explosives

An explosive is defined as a material (chemical or nuclear) that can be initiated to undergo very rapid, self-propagating decomposition that results in the formation of more stable material, the liberation of heat, or the development of a sudden pressure effect through the action of heat on produced or adjacent gases. All of these outcomes produce energy; a weapon's effectiveness is measured by the quantity of energy - or damage potential - it delivers to the target.

Modern weapons use both kinetic and potential energy to achieve maximum lethality. Kinetic energy systems rely on the conversion of kinetic energy to work, while potential energy systems use explosive energy directly in the form of heat and blast, or by accelerating metal as a shaped charge, EFP or case fragments to increase their kinetic energy and damage volume.

Energy may be broadly classified as potential or kinetic. Potential energy is energy of configuration or position, or the capacity to perform work. For example, the relatively unstable chemical bonds among the atoms that comprise trinitrotoluene (TNT) possess chemical potential energy. Potential energy can, under suitable conditions, be transformed into kinetic energy, which is energy of motion. When a conventional explosive such as TNT is detonated, the relatively unstable chemical bonds are converted into bonds that are more stable, producing kinetic energy in the form of blast and thermal energies. This process of transforming a chemical system's bonds from lesser to greater stability is exothermic (there is a net production of energy).

A chemical explosive is a compound or a mixture of compounds which, when subjected to heat, impact, friction, or shock, undergoes very rapid, self-propagating, heat- producing decomposition. This decomposition produces gases that exert tremendous pressures as they expand at the high temperature of the reaction. The work done by an explosive depends primarily on the amount of heat given off during the explosion. The term detonation indicates that the reaction is moving through the explosive faster than the speed of sound in the unreacted explosive; whereas, deflagration indicates a slower reaction (rapid burning). A high explosive will detonate; a low explosive will deflagrate. All commercial explosives except black powder are high explosives.

Low-order explosives (LE) create a subsonic explosion [below 3,300 feet per second] and lack HE's over-pressurization wave. Examples of LE include pipe bombs, gunpowder, and most pure petroleum-based bombs such as Molotov cocktails or aircraft improvised as guided missiles.

A High Explosive (HE) is a compound or mixture which, when initiated, is capable of sustaining a detonation shockwave to produce a powerful blast effect. A detonation is the powerful explosive effect caused by the propagation of a high-speed shockwave through a high explosive compound or mixture. During the process of detonation, the high explosive is largely decomposed into hot, rapidly expanding gas.

The most important single property in rating an explosive is detonation velocity, which may be expressed for either confined or un-confined conditions. It is the speed at which the detonation wave travels through the explosive. Since explosives in boreholes are confined to some degree, the confined value is the more significant. Most manufacturers, however, measure the detonation velocity in an unconfined column of explosive 1- i/4 in. in diameter. The detonation velocity of an explosive is dependent on the density, ingredients, particle size, charge diameter, and degree of confinement. Decreased particle size, increased charge diameter, and increased confinement all tend to increase the detonation velocity. Unconfined velocities are generally 70 to 80 percent of confined velocities.

The confined detonation velocity of commercial explosives varies from 4,000 to 25,000 fps. With cartridge explosives the confined velocity is seldom attained. Some explosives and blasting agents are sensitive to diameter changes. As diameter is reduced, the velocity is reduced until at some critical diameter, propagation is no longer assured and misfires are likely.

Relative effectiveness factor (R.E. factor) is a measurement of an explosive's power for military demolitions purposes. It measures the detonating velocity relative to that of TNT, which has an R.E. factor of 1.00. TNT equivalent is a measure of the energy released from the detonation of a nuclear weapon, or from the explosion of a given quantity of fissionable material, in terms of the amount of TNT (trinitrotoluene) which could release the same amount of energy when exploded. The twelve-kiloton Hiroshima atomic bomb had had a blast effect alone equivalent to some twenty-five million pounds of TNT-that's million.

Denser explosives usually give higher detonation velocities and pressures. A dense explosive may be desirable for difficult blasting conditions or where fine fragmentation is required. Low-density ex-plosives will suffice in easily fragmented or closely jointed rocks and are preferred for quarrying coarse material.

Energetic materials are made in two ways. The first is by physically mixing solid oxidizers and fuels, a process that, in its basics, has remained virtually unchanged for centuries. Such a process results in a composite energetic material such as black powder. The second process involves creating a monomolecular energetic material, such as TNT, in which each molecule contains an oxidizing component and a fuel component. For the composites, the total energy can be much greater than that of monomolecular materials. However, the rate at which this energy is released is relatively slow when compared to the release rate of monomolecular materials. Monomolecular materials such as TNT work fast and thus have greater power than composites, but they have only moderate energy densities-commonly half those of composites. Greater energy densities versus greater power-that's been the traditional trade-off.

Ingredients of high explosives are classified as explosive bases, combustibles, oxygen carriers, antacids, and absorbents. Some ingredients perform more than one function. An explosive base is a solid or liquid which, upon the application of sufficient heat or shock, decomposes to gases with an accompanying release of considerable heat. A combustible combines with excess oxygen to prevent the formation of nitrogen oxides. An oxygen carrier assures complete oxidation of the carbon to prevent the formation of carbon monoxide. The formation of nitrogen oxides or carbon monoxide, in addition to being undesirable from the standpoint of fumes, results in lower heat of explosion and efficiency than when carbon dioxide and nitrogen are formed. Antacids increase stability in storage, and absorb-ents absorb liquid explosive bases.

Explosives are classified as primary or secondary based on their susceptibility to initiation. Primary explosives, which include lead azide and lead styphnate, are highly susceptible to initiation. Primary explosives often are referred to as initiating explosives because they can be used to ignite secondary explosives. Secondary explosives, which include nitroaromatics and nitramines are much more prevalent at military sites than are primary explosives. Because they are formulated to detonate only under specific circumstances, secondary explosives often are used as main charge or bolstering explosives.

Secondary explosives can be loosely categorized into melt-pour explosives, which are based on nitroaromatics such as TNT, and plastic-bonded explosives which are based on a binder and crystalline explosive such as RDX.

Propellants include both rocket and gun propellants. Most rocket propellants are composites based on a rubber binder, ammonium perchlorate oxidizer, and a powdered aluminum fuel; or composites based on a nitrate esters, usually nitroglycerine or nitrocellulose and nitramines. If a binder is used, it usually is an isocyanate-cured polyester or polyether. Some propellants also contain combustion modifiers, such as lead oxide. One group of gun propellants are called "single base" (principally nitrocellulose), "double base" (nitrocellulose and nitroglycerine), or "triple base" (nitrocellulose, nitroglycerine, and nitroguanidine). Some of the newer, lower vulnerability gun propellants contain polymer binders and crystalline nitramines.

Pyrotechnics include illuminating flares, signaling flares, colored and white smoke generators, tracers, incendiary delays, fuses, and photo-flash compounds. Pyrotechnics usually are composed of an inorganic oxidizer and metal powder in a binder. Illuminating flares contain sodium nitrate, magnesium, and a binder. Signaling flares contain barium, strontium, or other metal nitrates.

Explosive and incendiary (fire) bombs are further characterized based on their source. "Manufactured" implies standard military-issued, mass produced, and quality-tested weapons. "Improvised" describes weapons produced in small quantities, or use of a device outside its intended purpose, such as converting a commercial aircraft into a guided missile. Manufactured (military) explosive weapons are exclusively HE-based. Terrorists will use whatever is available - illegally obtained manufactured weapons or improvised explosive devices (also known as "IEDs") that may be composed of HE, LE, or both. Manufactured and improvised bombs cause markedly different injuries.

Plastic explosive means an explosive material in flexible or elastic sheet form formulated with one or more high explosives which in their pure form has a vapor pressure less than 10-4 Pa at a temperature of 25 deg. C., is formulated with a binder material, and is as a mixture malleable or flexible at normal room temperature.

The energetic materials used by the military as propellants and explosives are mostly organic compounds containing nitro (NO2) groups. The three major classes of these energetic materials are nitroaromatics (e.g., tri-nitrotoluene or TNT), nitramines (e.g., hexahydro-1,3,5 trinitroazine or RDX), and nitrate esters (e.g., nitrocellulose and nitroglycerine).

Since the invention of the cannon, the explosive fills used to drive lethal mechanisms have been the subject of ever increasing interest and study. Traditionally, munitions designers have used such ex-plosives as Comp-B, TNT, or LX-14, depending upon the particular application.

During the 1920s and into the 1940s, the Army's Picatinny Arsenal was instrumental in designing, modeling and evaluating such high explosive material as TNT, RDX, and Haleite. This work greatly influenced battlefield lethality during WWII where explosives exhibiting a higher brisance, or shattering effect, than TNT were in great demand.

The 1960s brought new explosives such as HMX that was chemically analogous to RDX, but even more powerful to give soldiers greater lethality capability. Picatinny laboratories also developed precision warheads for several missile systems, including the DRAGON-MAW, a Medium Antiarmor Weapon.

The Army uses Research Department Explosive (RDX) and High Melt Explosive (HMX) as basic explosives for munitions and tactical missiles as well as propellants for strategic missiles rather than TNT because of their superior energy.

Most modern explosives are reasonably stable and require percussive shock or other triggering devices for detonation. Energetic materials are especially vulnerable to elevated temperature, with possible consequences ranging from mild decomposition to vigorous deflagration or detonation. Energetic materials can also be initiated by mechanical work through friction, impact, or electricity (e.g., current flow, spark, electrostatic discharge, or electromagnetic radiation). Other stimuli (e.g., focused laser light or chemical incompatibility) can have consequences ranging from mild decomposition to detonation.

Explosives may be toxic, with exposure pathways being inhalation of dust or vapor, ingestion, or skin contact. Most explosives are not highly toxic, but improper handling can result in systemic poisoning, usually affecting the bone marrow (i.e., the blood cell-producing system) and the liver. Some explosives are vasodilators, which cause headaches, low blood pressure, chest pains, and possible heart attacks. Some explosives may irritate the skin.

Some detonation or combustion products from explosives are toxic. Such products can be respiratory and skin irritants and lead to systemic effects following short-term exposure to high levels. Soot from detonated explosives is not mutagenic; however, soot from burned gun propellants may be mutagenic and is therefore treated as a mutagen.

Fortunately, contamination usually occurs in dilute, aqueous solutions or in relatively low concentrations in the soil and present no explosion hazard. Masses of pure crystalline explosive material have, however, been encountered in soils associated with wastewater lagoons, leach pits, burn pits, and firing ranges. These materials remain hazardous for long periods of time and great care must be used during the investigation and remediation process.

Molecular weights are moderate, of the order of a few hundreds of grams per mole. The molecular structure, particularly the types and positions of subsidiary functional groups, controls environmental behavior.

All of the common explosives are solid at normal environmental temperatures and pressures. Melting point temperatures for explosives solids are moderate (50-205 0C). Melting points are of little direct value in predicting environmental fate and transport, but several parameter estimation relations for solids incorporate the influence of molecular crystal bonding by including a term dependent on the melting point. Melting points are not available for many of the breakdown products. Most of the explosives and associated contaminants have very low volatility, with vapor pressures estimated to be less than 6 x 10-4 torr. Henry's law constants (KH) range from 10-4 to 10-11 atmm-2mole-1. Only those with KH greater than 10-5 volatilize significantly from aqueous solution. Though explosives compounds may not be volatile, some of the transformation products, other key reactants, or products may be volatile to semivolatile.



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