Primary explosives ignition

Heat from Primary Explosion Ignites Dust Cloud... 

Lead Azide. The azides belong to a class of very few useflil explosive compounds that do not contain oxygen. Lead azide is the primary explosive used in military detonators in the United States, and has been intensively studied (see also Lead compounds). However, lead azide is being phased out as an ignition compound in commercial detonators by substances such as diazodinitrophenol (DDNP) or PETN-based mixtures because of health concerns over the lead content in the fumes and the explosion risks and environmental impact of the manufacturing process. 

While this book does not cover shock-sensitive powders, such as primary explosives, UN-DOT Class 4.1 Flammable Solids are within its scope. These include thermally unstable powders that can both deflagrate in an oxidant and decompose in bulk. Examples include some nitrogen blowing agents. Should ignition occur at any point, a propagating decomposition... 

Dust explosions are relatively rare but ean involve an enormous energy release. A primary explosion, involving a limited quantity of material, ean distribute aeeumulations of dust in the atmosphere whieh, on ignition, produees a severe secondary explosion. 

Electrostatics enters into the problem of secondary explosions in several ways. First, an electrostatic spark may be the ignition source of the primary explosion. Second, an electrostatic spark can serve to ignite a dust cloud if particulate spews out at high velocity as a result of a sudden piping or vessel containment failure. Similar containment failures have occurred with insulating liquids and can not be ruled out for powders. For example,... 

Dust presents a different type of hazard, because while it has a lower explosive limit, it does not have an upper explosive limit. This can result in a primary explosion, followed by secondary explosions as new air is provided. Secondly, dust does not diffuse away from its point of release, but settles out of the air and accumulates into layers. Unlike vapor, the dust explosion is caused by the radiant heat from one particle igniting the next. Because of this, the lower explosive limits for dusts are greatly higher than for vapors. Also, the size and shape of the dust particles are important factors in effecting its lower explosive limit. 

Safety fuses are used for the direct ignition of propellants, pyrotechnics, and primary explosives. A safety fuse is composed of a black powder core, three layers of cotton or jute yarn wound around the core, bitumen impregnation, and plastic coating. The burn rate for a safety fuse is usually around 120 + 10 seconds/meter. 

Tetrazene (C2H8N10O) is a pale yellow crystalline explosive generally used in ignition caps, where a small amount is added to the explosive composition to improve its sensitivity to percussion and friction. Tetrazene is not suitable for filling detonators because its compaction properties make the transition from burning to detonation very difficult. This primary explosive is stable in ambient temperatures. Its ignition temperature is lower and it is slightly more sensitive to impact than mercury fulminate. 

Priming Composition. A physical mixture of materials that is very sensitive to impact or percussion and, when so exploded, undergoes very rapid autocombustion. The products of such an explosion are hot gases and incandescent solid particles. Priming compositions are used for the ignition of primary explosives, BkPdr, igniter charges and propellants in small arms ammunition (Ref 40a, p 112) (See also in Section 3, Part D and Section 4,... 

Primary explosives can be ignited and will burn without detonating if they are essentially unconfined (Refs 4 6). Lead Azide appears to be an exception because its ignition results in detonation or at most a very short-lived combustion which almost instantaneously goes over into detonation (Ref 6). However, dextri-nated Lead Azide can be made to flash without detonating (Ref 5)... 

D. T. Zebree, 53, 17514 (1959) Claims an improved delay fuse for igniting primary explosive compositions in detonators squibs. The preferred delay mixture consists of 60% Ba02,... 

K. Yamamoto, Primary Explosives. V. Ignition of Primary Explosives When Heated in an Electric Furnace , KKK 25 (5), 258-70... 

Operating on gas pressures of approx 40 psig, the initiator will ignite primary explosives or propellants in 5 ro 10 milliseconds. A predetermined time delay can also be built into the device. In testing for NASA Langley, ignitions were obtained... 

Some pyrotechnic formulations Ti/KDN/NC and Ti/CsDN/NC were formulated and tested for combustion velocity, sensitivity to impact, friction and electrostatic discharge etc. The data show that both formulations are extremely sensitive to impact (the sensitivity being in the range of pure HMX and PETN ). Further, it is also seen that the Ti/KDN/NC formulation is less sensitive [145] than Ti/ CsDN/NC. On the contrary, the formulations show a moderate sensitivity to friction and electrostatic discharge. The evaluation of such systems as components in ignition formulations appear to be very promising as this may allow replacement of the heavy metal primary explosives which are toxic in nature. 

Primary explosives are a group of substances which are highly sensitive to the action of mechanical shock and are readily ignited by direct contact with flame or electric sparks. 

The explosive properties of sodium, calcium, strontium and barium azides have been investigated at the Chemisch-Technische Reichsanstalt [135]. These azides differ markedly from lead, silver and cupric azides in that they show none of the properties of primary explosives. All three may be ignited by a spark, a glowing wire or the flame of blackpowder. Calcium azide bums most rapidly and has distinctly marked explosive properties. Larger quantities of it may explode when ignited in a closed tin, while strontium and barium merely bum violently. Calcium azide detonates under the influence of a detonating cap. The sodium azide does not decompose in these conditions. The other azides show weak decomposition under the influence of a standard (No. 3) detonator. Their most important properties are tabulated below. 

Explosives can therefore be classified by the ease with which they can be ignited and subsequently exploded. Primary explosives are readily ignited or detonated by a small mechanical or electrical stimulus. Secondary explosives are not so easily initiated they require a high velocity shockwave generally produced from the detonation of a primary explosive. Propellants are generally initiated by a flame, and they do not detonate, only deflagrate. 

Primary explosives explode from shock, from friction, and from heat. They are used in primers where it is desired by means of shock or friction to produce fire for the ignition of powder, and they arc used in detonators where it is desired to produce shock for the initiation of the explosion of high explosives. They are also used in toy caps, toy torpedoes, and similar devices for the making of noise. Indeed, certain primary explosives were used for this latter purpose long before the history of modern high explosives had yet commenced. 

Secondary explosives, or high explosives, are generally less sensitive to heat and shock than primary explosives and are therefore safer to manufacture, transport, and handle. Most secondary explosives will simply burn rather than explode when ignited in air, and most can be detonated only by the nearby explosion of a primary initiator. Among the most common secondary explosives are nitroglycerin, trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), and RDX. 

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