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[14.0] Explosives

v1.3.0 / chapter 14 of 15 / 01 jun 16 / greg goebel

* "Pyrotechnics" are chemical technologies designed produce explosions, flames, smoke, or noise. Explosives are the most prominent pyrotechnic technology, not merely for useful for military purposes but for commercial ones, such as mining and construction, as well. This chapter discusses explosives; the next discusses other topics in pyrotechnics.

things go boom


[14.1] DEFINITIONS
[14.2] BASIC PRINCIPLES
[14.3] LOW EXPLOSIVES: BLACK POWDER
[14.4] COMMERCIAL HIGH EXPLOSIVES
[14.5] MILITARY HIGH EXPLOSIVES / EXPLOSIVES TAGGING
[14.6] DETONATORS & PROPELLANTS

[14.1] DEFINITIONS

* The term "explosives" is a specialized one, and some definitions are useful to sort them out from their relatives:

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[14.2] BASIC PRINCIPLES

* Most pyrotechnics and low explosives operate by combustion processes, burning a fuel along with in an oxidizer -- an oxidizer is required because reaction rates would be limited if the reaction had to rely on the oxygen in the atmosphere for combustion. For example, the fuel in black powder is charcoal and sulfur, while the oxidizer is saltpeter, or potassium nitrate (KNO3). The reaction is:

   2KNO3  +  S  -->  K2S  +  3CO2  + N2

The packaging of the mixture affects its behavior. Confinement greatly speeds up the combustion process by concentrating heat and hot gas in the reaction. In fact, black powder will generally simply burn instead of explode unless packed into an appropriate casing, such as the thick paper shell of a firecracker. Burning rate is also increased by the homogeneity of the mixture: fine powders burn faster than coarse grains.

Liquid explosives are unsafe because they are extremely homogeneous. Their mixing is at the molecular level, and so they can be set off by a mild physical shock. Liquid explosives also tend to settle and separate in storage, which changes their chemical properties, not generally for the better. Adding abrasives to an explosive material makes it more sensitive, while adding lubricants like wax makes it more stable. Materials that reduce explosive sensitivity are known as "stabilizers" or "moderators".

Most high explosives operate by a chemical breakdown in their molecular structure, not a combustion process between fuel and oxidizer. For example, nitroglycerine has the molecular formula C3N3H5O9. Any small disturbance, such as heat or physical shock, causes it to decompose into carbon dioxide (CO2), water (H2O), nitrogen (N2), and a little excess oxygen (O2). This process still involves oxidation reactions, but the oxygen is part of the molecule. During the breakdown of nitroglycerine, nitrogen-oxygen atomic bonds are replaced by far more stable carbon-oxygen, hydrogen-oxygen, and nitrogen-nitrogen bonds, with the process accompanied by a violent release of energy.

Explosives, incendiaries, pyrotechnic devices, and fireworks can be ignited by flame, friction, impact, electrical shock, high ambient temperatures, or even a laser beam. In general, high explosives are designed to be insensitive. They can't be set off by a flame or spark, and have to be set off by a shock from a detonator.

Certain metals, such as magnesium, aluminum, zirconium, and uranium, ignite at very high temperatures and burn very hot, releasing large amounts of energy. For this reason, aluminum powder is sometimes added to explosives to enhance blast effect, and magnesium is used to build bright flares.

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[14.3] LOW EXPLOSIVES: BLACK POWDER

* While people have devised fiery and smoke-making materials for most of recorded history, the first explosive material worthy of the name was black powder, developed by the Chinese sometime in the first millennium CE. The Chinese used it to make firecrackers and rockets for public entertainments, and also for a wide range of weapons. Traditionally it was believed that the Chinese didn't use it in warfare, but in fact they built devices such as arrow projectors; "fire arrows" and "fire spears"; bombs; mines; and crude cannon.

Black powder migrated to the West in the Middle Ages. The English monk Roger Bacon (1214:1294) was one of the first Westerners to mention the material, in a letter written in 1267. Tradition has it that Bacon wrote a formula for black powder in 1242, using a code because of the deadly nature of the material, but this story appears to be a myth, as does the story that it was introduced by a German monk named Berthold Schwartz. Nobody really knows how black powder got to Europe, only that it was in use there around the year 1300. The fact that the formula for it was obtained from China is very difficult to dispute: early European formulations closely match those of mature Chinese formulations, which had been evolved over many years of trial-and-error tinkering and actually included some minor useless components that were also copied by the Europeans.

In the 14th century, black powder led to the development of new weapons. In weapons, black powder was used as a bursting agent and a propellant. Black powder charges were used as "petards", or mines, to break down the walls of fortifications, and later as filler in explosive shells and hand grenades. As a propellant explosive, black powder was used to fire balls from muskets, as well as stones from primitive cannon called "bombards", which eventually evolved into muzzle-loading artillery. Black powder became god of war, and remained so until the last decades of the 19th century.

Engineers and miners were slow to adopt black powder. The earliest mentions of its use in engineering projects go back to the middle of the 16th century and it didn't really become widespread in commercial uses until the middle of the 17th century. Bofors Industries, established in Sweden in 1646, was a pioneer in the manufacture of commercial black powder. The reason that black powder was not in commercial use for so long was because it had generally been an expensive material, mostly due to the cost of refining saltpeter, and miners in particular were unsurprisingly skittish about setting off explosions underground. It was also not trivial to figure out how to properly drill holes and pack explosives. One of the first major engineering projects performed with black powder was a 156-meter (512-foot) canal tunnel blasted through a hill near Beziers, France, in the 1690s. From that time, black powder remained a standard tool for miners until the late 19th century.

* Early Chinese mixtures of black powder consisted of equal weights of charcoal, sulfur, and saltpeter. Saltpeter is a shiny white crystalline material that could be found on the walls of bat caves or in well-aged manure piles; today it is known to be potassium nitrate (KNO3) and calcium nitrate (CaNO3). It is produced by bacteria that feed off organic waste. Eventually, the formula for black powder was refined to a mix of charcoal, sulfur, and saltpeter in the proportions 15:10:75 by weight. Oddly, other forms of carbon cannot be substituted for charcoal; charcoal inherits a latticelike structure from the wood from which it was created, and is also laced with by-products of its partial combustion.

Charcoal and sulfur were relatively easy to obtain. Saltpeter was not too hard to obtain in southeast China, where the hot and alternating wet / dry climate encouraged its creation on manure piles, but it was much harder to find in cool and damp Northern Europe. Producing saltpeter was an obnoxious business. Manure was collected and stored in a covered pit that was periodically "watered" with urine and turned over. The urine of wine-drinkers was favored, since it helped encourage the growth of the bacteria that generated the saltpeter, though at the time of course nobody knew exactly why it helped produce more saltpeter. After a year or so, the waste was "refined" to extract the saltpeter. It seems plausible that most people who worked in the business, the "salpeters", didn't have much social contact with those who didn't.

At first, there was no way to separate the calcium nitrate from the potassium nitrate. Both worked about as well as an oxidizer, but calcium nitrate absorbs water more easily than potassium nitrate, or in other terms calcium nitrate is more "hygroscopic". The ground-up charcoal in the powder was also hygroscopic, but calcium nitrate made matters much worse, and powder made with it tended to go damp and "spoil" much more easily. Eventually, schemes were developed by trial and error to refine out the calcium nitrate.

When the three ingredients were obtained, they were then finely ground together with a mortar and pestle. This was a risky business, since it generated a fine, highly combustible dust that could easily lead to explosions. Eventually urine or watered-down wine was mixed in during grinding to help reduce the chance of explosions, though there was no way to eliminate it completely. Dampening the mix also helped in other ways. Early on, gunners using black powder had to be very careful how they fluffed the fine powder into their weapons. If it was packed in tightly, it would burn at the surface, just like a rocket, but not explode. To explode, it had to be spooned in loosely and with an airspace left over. Since the saltpeter provided oxygen to support the explosion, the airspace wasn't to admit oxygen, but to allow fire to propagate quickly through the charge. Dampened black powder was pressed into balls or loaves and dried, with the blocks of powder smashed down into grains before combat. Such "corned" powder was introduced early in the 15th century; it could be packed in with much less care than fine powder and still explode easily, since the grain structure gave plenty of space for the propagation of fire.

By the 19th century, the production of black powder had become an efficient industrial process. Elements of a powder mill were separated from one another and set up in buildings with three sturdy walls, plus a fourth thin wall and a thin roof that would direct the blast of an accident away from the other buildings. Production of charcoal was performed according to careful methods to generate the best possible product, and industrial chemistry came up with less noxious and more industrially efficient means of producing potassium nitrate.

Powder was ground under heavy mill wheel stones, and dampened with distilled water to inhibit explosions. The final product was squeezed under heavy pressure into a block with the hardness of slate, known as "presscake"; the denser powder packed more of a punch. The presscake was broken up into grains, with the grains sifted into grades by size, and then tumbled in barrels to give them a graphite "glaze" that kept the powder from agglomerating in storage. A special form of low-cost industrial blasting powder was invented that used cheap calcium nitrate instead of expensive potassium nitrate, with the grains heavily glazed to prevent them from soaking up water.

* Black powder is an excellent explosive in many respects. Its raw materials are cheap, abundant, nontoxic, and environmentally safe. It is stable, being indifferent to shock, and has an indefinite shelf life if kept dry. It can be easily ignited with a spark or fuze, though this is by no means entirely a virtue. Black powder also has a number of limitations:

Better explosives were needed and were developed, beginning in the middle of the 19th century. However, the low cost and good properties of black powder make it still the material of choice for fireworks, as discussed later.

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[14.4] COMMERCIAL HIGH EXPLOSIVES

* In 1846, an Italian chemist named Ascanio Sobrero (1812:1888) added glycerol to a mixture of nitric and sulfuric acids, the result being an explosion that nearly killed him. He had discovered the first high explosive. Sobrero unsurprisingly decided that the liquid he called "nitroglycerine", now known to have the chemical formula C3H5O3(NO2)3, was dangerous. He gave up his work on the material and did not publicize it.

A Swedish chemical manufacturer named Immanuel Nobel (1801:1872) began to produce nitroglycerine for rock blasting in 1863. The process involved mixing glycerol with nitric and sulfuric acids in several steps, using ice water as a coolant. Nitroglycerine can't be detonated by a simple cord fuze -- a match can be thrown into a batch of it and it won't go off -- but it is sensitive to shock, exactly the reverse of black powder. In 1865 Immanuel's son Alfred Nobel (1833:1896) devised the first detonator, a blasting cap consisting of a small charge of fulminate of mercury (discussed below) with a cord fuze, to set it off. Nobel's detonator was a significant step forward in the development of modern explosives technology.

nitroglycerine

Nitroglycerine is a liquid explosive and dangerously touchy. In fact, it was so unsafe that it is astounding that anybody wanted to use it, and Nobel's brother Emile was killed in 1864 while working with it. It was often carelessly shipped as normal freight, without markings to indicate special handling, and terrible accidents occurred. Nitroglycerine was banned in several nations. Late in the 1860s, workers found that nitroglycerine that had been frozen was almost impossible to detonate, and so manufacturers began to freeze it for shipment. However, that was clearly a stopgap solution.

Alfred Nobel was already working on ways to make a safer explosive. He determined that nitroglycerine was much less sensitive if it was absorbed in "diatomaceous earth", a porous clay that consisted of the deposits of the skeletons of tiny sea creatures laid down aeons before. This material could be packed into cardboard tubes to allow it to be reliably transported, handled, and detonated. It could not be set off by a spark or a flame. It was not only safer than nitroglycerine, it was even safer than black powder. Nobel named it "dynamite", and it quickly became the industrial explosive of choice.

Dynamite offered much of the power of nitroglycerine with greatly improved safety. However, it wasn't perfect. The nitroglycerine in dynamite tended to "sweat out" in storage, and even form puddles in crates. Cold weather also tended to crystallize the nitroglycerine inside a stick of dynamite, making it more sensitive. Another problem is that nitroglycerine caused dilation of blood vessels. Since it could be absorbed through the skin, people who handled dynamite often had pounding headaches. Incidentally, because of its ability to dilate blood vessels, nitroglycerine is also used in small doses as a medicine for people with heart conditions.

The tendency of dynamite to become sensitive in storage made it dangerous to stockpile, and so military forces were not enthusiastic about it. In 1875, Nobel figured out how to mix nitroglycerine and nitrocellulose to produce a "blasting gelatin" that was more stable than dynamite, and could also be detonated underwater. Nitrocellulose explosives are discussed below.

* Despite its limitations, dynamite remained the predominant commercial explosive until the 1950s, when "ammonium nitrate" explosives were began to predominate. Ammonium nitrate (AN, with the chemical formula NH4NO3) is useful as an explosive when mixed with other combustible or explosive substances, and in fact Alfred Nobel adopted it as a dynamite enhancer as far back as 1867. A mixture of ammonium nitrate and diesel fuel known as "ammonium nitrate fuel oil (ANFO)" is now commonly used as an industrial explosive. It is also well known to makers of home-brewed explosives. The fuel oil in ANFO provides a source of energy, while the ammonium nitrate provides oxygen for the fuel's combustion. However, the breakdown of ammonium nitrate itself produces energy, giving ANFO a hefty explosive kick. ANFO is much cheaper and less sensitive than dynamite, and does not give users headaches, at least not by simply laying hands on it.

In principle, home-brewed ANFO is synthesized by mixing diesel fuel and ammonium nitrate fertilizer together until the mix has he consistency of toothpaste. In practice, although that works well enough for bomb-quality ammonium nitrate -- for which access is carefully controlled -- a simple mix of commercial fertilizer and diesel just burns and melts, rather than explodes. To actually make an explosive out of fertilizer, aluminum, zinc, or potassium sulfate (K2SO4) have to be added to the mix as boosters. If too little is added, the mix won't explode, and if too much is added, it's liable to go off unpredictably. It's not too hard to get it right; the proportions of additives can vary over a wide range and still result in a useful product, though the explosive yield is dependent on the mix. Bomb-quality ammonium nitrate does not require such fussing.

There are a considerable number of variations on ANFO -- for example, it can be mixed with aluminum powder to increase brisage; with high explosives ("heavy ANFO") to increase explosive yield; or with polystyrene beads (in graded proportions) to reduce yield. There are also "gelled slurry explosives (GSX)", which consist of ammonium nitrate with a fuel (sometimes sugar or aluminum powder) and a gelling agent such as polystyrene powder, mixed with light oil or water for use. Mixing with water seems somewhat counterintuitive, since ordinary ANFO has to be kept dry. Once mixed, GSX has a mudlike appearance and can be pumped into holes drilled in solid rock.

* As a footnote to the topic of commercial explosives, coal miners work in an environment full of flammable coal dust, making the use of any kind of true explosive dangerous. As a substitute, they use a "blasting charge" that consists of a cylinder full of liquefied carbon dioxide and containing a heating element. The carbon dioxide expands rapidly when the heating element is activated, and bursts the container, producing a blast but no flames.

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[14.5] MILITARY HIGH EXPLOSIVES / EXPLOSIVES TAGGING

* As mentioned in the previous section, explosives like nitroglycerine, dynamite, and ANFO are not very well suited to combat use, though they all have been used in battle to an extent. Different explosives have been developed for the battlefield and are widely used by military forces. Of course, military explosives are also used to an extent in commercial applications, but they are relatively expensive, and ANFO remains the bulk explosive of choice for civilian uses.

The ideal military explosive is powerful; easy to handle; can be stockpiled for long periods of time in any climate; and hard to detonate, except under precisely specified conditions. It also has to be loaded into shells, bombs, and the like, and so has to be meltable to allow it to be poured into shells; or plastic, allowing it to be "extruded" into shells like caulk from a tube; or insensitive enough to allow it to be packed safely into the shell in bulk form.

Military explosives have been improved for over a century and are now thoroughly refined. The first military high explosive to be put into service was "trinitrophenol" or "picric acid", a yellow crystalline substance with the chemical formula C6H3O7N3, which was first demonstrated by the French in 1885. The British used in under the trade name of "Liddite". However, picric acid has a high melting point, making the process of filling shells with it difficult; reacts with heavy metals to form toxic compounds; tends to be corrosive; and is also inclined to be sensitive.

The Germans developed a second military explosive in this time frame, known as "nitroguanidine", chemical formula CH4N4O2. It was used in World War I and to an extent in World War II in mortar shells and the like, usually mixed with other explosives. Another military high explosive, "trinitrophenylmethlnitramine" or "tetryl" was introduced at nearly the same time. It was powerful but not all that stable, and was often used in detonators and as a booster. All three of these explosives are now generally out of service.

The first modern military explosive was "trinitrotoluene (TNT)", with the formula C7H5N3O6. TNT was first discovered in the 1860s, but was not adopted for military use until the early 20th century. It was widely used by most of the combatants in World War I. It is relatively insensitive, and can be melted at low temperature to allow it to be poured into bombs and shells. The British also used TNT during World War I, but after the war adopted a more powerful explosive named "Research Department Explosive (RDX)". RDX, more precisely known as "cyclotrimethylenetrinitramine" and sometimes called "cyclonite" or "hexogen", was originally formulated in 1899. It has the formula C3H6N6O6. It has the insensitivity of TNT but greater explosive yield.

high explosives

TNT and RDX are still the most important military explosives. Other military explosives include:

HMX (octogen)

* In practice, most military explosives are mixtures of these explosives and other materials. For example:

In the early 1950s the notion of plastic explosives was extended by the introduction of "polymer bonded explosives (PBX)", in which crystals of explosive materials are intimately bonded into a polymer matrix. A wide range of PBX materials have been fabricated, with explosives such as HMX, PETN, and RDX incorporated into polymers such as nylon, polyurethane, and teflon. More recent work focused on developing matrix plastics that actually have explosive properties themselves, with such amusing names as "polyAMMO" and "polyBAMO".

One particularly interesting new explosive is "octaninitrocubane". This experimental material is derived from "cubane", a hydrocarbon built around a cubical arrangement of carbon atoms that was synthesized in the 1960s. The cubical core of cubane makes it very dense, almost twice as dense as gasoline. In the early 1980s, US Army researchers realized if cubane could be modified into a high explosive, its high density would permit faster propagation of breakdown reaction, leading to a more powerful explosive, as well as a more compact one. Octaninitrocubane consists of a cubic core of eight carbon atoms, with an N2O group attached to each corner of the cube. Although octaninitrocubane is still being evaluated, researchers believe that it may be twice as powerful as TNT and very stable, and that its breakdown products will be non-toxic carbon and nitrogen compounds.

octaninitrocubane

The US Army has now adopted a new high explosive named "HMX-101" to replace TNT. It has the same general characteristics as TNT, except it is much more insensitive -- a shell loaded with HMX-101 will simply burn if caught up in an external explosion, instead of detonating. An even newer explosive developed by the US Navy and known as "High Density Reactive Materials" is described only as a "mix of metals and polymers" that not only provides enhanced explosive yield, but also is physically rugged enough to not need a casing. Details of these new explosives are otherwise unclear.

* In the modern "age of terror", the ability to "tag" explosives to make them traceable has become increasingly important. One scheme involves mixing explosives with tiny plastic chips, about the size of a grain of pepper, with the chips consisting of up to 10 colored layers in a unique sequence for each batch of explosives. The tags are also used in shampoos and the like to track product counterfeiters.

A more traditional and subtler tagging scheme is based on creating tags in the form of molecules selectively modified with relatively rare atomic isotopes. For example, molecules can be synthesized to include atoms of deuterium (heavy hydrogen) to replace ordinary hydrogen in different patterns. This scheme has been used in applications such as identifying batches of ammonium nitrate.

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[14.6] DETONATORS & PROPELLANTS

* Detonators have traditionally been made from "fulminate of mercury". This is a salt of "fulminic acid (HCNO)", which is a dangerous and unstable liquid explosive, with the formula Hg(CNO)2. Apparently there are a series of metal fulminates, such as silver fulminate, that are more powerful but are generally too unstable to be safely handled.

Fulminate of mercury was first synthesized in the 17th century, but it not easy to handle and did not come into widespread use until the early 19th century. It is highly unstable, and mercury is a relatively expensive material as well as a toxic heavy metal. It was never suitable for general use as an explosive and found its niche as a detonator material, particularly for percussion firearms. Due to its drawbacks, it is now generally out of use.

In modern times, "lead azide", with the formula Pb(N3)2, is the preferred material for detonators, being more stable if less powerful. It is a salt of "hydrazoic acid", or HN3. There are a number of different azides, with "sodium azide" or NaN3 in widespread use for inflating automotive airbags.

* Black powder had been the only propellant available for firearms and artillery until the mid-19th century, when various chemists began to investigate treatments of paper, wood pulp, and particularly cotton with nitric acid (HNO3). These experiments resulted in "guncotton", which had promise as a propellant as it burned quickly and produced a large amount of gas. However, early formulations of guncotton were unsafe to produce and handle. It also burned too fast, and could cause firearms to blow up in the shooter's face.

In 1865, a British chemist named Sir Frederick Abel (1827:1902), working on the problem of manufacturing guncotton for the British government, came up with a manufacturing process that involved pulping the cellulose feedstock, boiling it, and washing it before treatment. Further effort led to nitrocellulose explosives could be used safely and effectively. The first form of smokeless powder to gain widespread acceptance was "Poudre B" or "B powder", synthesized in 1884 by a French chemist named Paul Vielle. In 1888, Alfred Nobel developed a smokeless powder name "ballistite", based on his formulation for blasting gelatin. This led to another smokeless powder, based on a mixture of guncotton, gelatinized nitroglycerine, and petroleum jelly, developed by Frederick Abel and James Dewar in 1889. The material was drawn out in a cord and so was named "cordite". Cordite was adopted by the British, who did not trust B powder, and by World War I cordite was the dominant propellant.

* All such "smokeless" powders were based on nitrocellulose, synthesized from plant cellulose by treatment with nitric acid to replace hydroxyl groups (OH) on the chain with nitrate groups (NO3). The higher the percentage of hydroxyl groups replaced, the more powerful and sensitive the powder became. Creating a reliable smokeless powder required manipulating the percentage of nitrate groups through processing, and adding the appropriate moderators and other useful elements.

Modern propellants are categorized as "single-base", "double-base", and "multi-base" or "composite" powders:

Firearms now generally use single-base or composite powders. Double-base powders were once used as a propellant for small solid-fuel rockets, but have been largely or completely replaced by modern solid fuels, discussed below. Smokeless powders are not truly smokeless, but they burn much more cleanly than black powder. Burning rate of smokeless powders, as with other explosives, can be controlled by varying the size of the powder grains, with large grains burning more slowly. Grains can also be perforated so they burn from the inside as well as outside.

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