Secondary explosives sensitivity

It is a treacherously sensitive, very brisant primary explosive (while its cobalt analogue is a well behaved secondary explosive). 

Primary explosives differ from secondary explosives in that they undergo a rapid transition from burning to detonation and have the ability to transmit the detonation to less sensitive (but more powerful) secondary explosives. Primary explosives have high degrees of sensitivity to initiation through shock, friction, electric spark, or high temperature, and explode whether confined or unconfined. Some widely used primary explosives include lead azide, silver azide, tetrazene, lead styphnate, mercury fulminate, and diazodinitrophenol. Nuclear weapon applications normally limit the use of primary explosives to lead azide and lead styphnate. 

Lead azide (PbN6) is a colorless to white crystalline explosive. It is widely used in detonators because of its high capacity for initiating secondary explosives to detonation. However, since lead azide is not particularly susceptible to initiation by impact, it is not used alone in initiator components. It is used in combination with lead styphnate and aluminum for military detonators, and is used often in a mixture with tetrazene. It is compatible with most explosives and priming mixture ingredients. Contact with copper must be avoided because it leads to formation of extremely sensitive copper azide. 

Pentaerythritol tetranitrate (PETN) is a colorless crystalline solid that is very sensitive to initiation by a primary explosive. It is a powerful secondary explosive that has a great shattering effect. It is used in commercial blasting caps, detonation cords, and boosters. PETN is not used in its pure form because it is too sensitive to friction and impact. It is usually mixed with plasticized nitrocellulose or with synthetic rubbers to form PBXs. The most common form of explosive composition containing PETN is Pentolite, a mixture of 20 to 50% PETN and TNT. PETN can be incorporated into gelatinous industrial explosives. The military has in most cases replaced PETN with RDX because RDX is more thermally stable and has a longer shelf life. PETN is insoluble in water, sparingly soluble in alcohol, ether, and benzene, and soluble in acetone and methyl acetate. 

It should be noted that there are modem detonators that are designed to function without primary explosives. These usually rely on an electrically generated shock to produce detonation in a small charge of a specially prepared and sensitive charge of a secondary explosive. 

The above classifications are not strictly applicable to the sensitivity of an explosive. Although it is true that most primary explosives are more sensitive than most secondary explosives, certain low explosives, eg Black Powder, are more sensitive than many secondary or even primary explosives... 

For pure primary secondary explosives (except for border-line HE such as Ammonium Nitrate or Ammonium Perchlorate) dcr decreases as p0 increases until p0 approaches very close single crystal density when dcr may increase drastically. Thus if we limit ourselves to p0 < 0.9pcrySt, increases in p0 (according to Eq 8) should result in a greater sensitivity to impact. This is quite the opposite of what is found for shock initiation and will be examined more closely later on. At a fixed density, dcr increases as p. increases (see A B, p 90). This increase is fairly pronounced at small p (<0.2 mm) but levels off becomes almost asymptotic at large p (>0.4mm, except for cast TNT or TNT with 1% paraffin oil). Thus an increase in p, as expected, leads to a decrease... 

On the other hand, Dudek et al. found that the impact sensitivity (h50% ) of e CL-20 28 cm is comparable to that of [5-HMX (h50% = 30 cm) [124]. Agrawal et al. have not only given impact sensitivity of CL-20 but also the mechanism of initiation on impact leading to its detonation. This is in line with the mechanism of initiation by hot spot formation on impact followed by detonation similar to other secondary explosives [125]. 

Nitrotriazolone and TATB are explosives of relatively low sensitivity compared with conventional secondary explosives but their performance levels are not high. The VOD for NTO is 8510ms"1 and density is 1.91 gem 3 whereas for TATB, VOD is 8000 ms"1 and density is 1.94 gem 3. Further, NTO can easily be obtained in particle size much larger than TATB, that is, 300 pm to 500 nn (NTO) as against 9pm-3pm (TATB). 

Primary explosives differ considerably in their sensitivity to heat and in the amount of heat they produce on detonation. The heat and shock on detonation can vary but is comparable to that for secondary explosives. Their detonation velocities are in the range of 3500-5500 ms-1. 

Secondary explosives (also known as high explosives) differ from primary explosives in that they cannot be detonated readily by heat or shock and are generally more powerful than primary explosives. Secondary explosives are less sensitive than primary explosives and can only be initiated to detonation by the shock produced by the explosion of a primary explosive. On initiation, the secondary explosive compositions dissociate almost instantaneously into other more stable components. An example of this is shown in Reaction 2.4. 

Lead azide has a good shelf life in dry conditions, but is unstable in the presence of moisture, oxidizing agents and ammonia. It is less sensitive to impact than mercury fulminate, but is more sensitive to friction. Lead azide is widely used in detonators because of its high capacity for initiating other secondary explosives to detonation. On a weight basis, it is superior to mercury fulminate in this role. However, since lead azide is... 

HMX (2.15), also known as Octogen and cyclotetramethylenetetra-nitramine (C4H8N808), is a white, crystalline substance which appears in four different crystalline forms differing from one another in their density and sensitiveness to impact. The /J-form, which is least sensitive to impact, is employed in secondary explosives. 

The values given here clearly show that primary explosives are much more sensitive to friction than secondary explosives. Therefore, primary explosives are more hazardous to handle and care must be taken. 

Table 4.2 Sensitivity of some primary and secondary explosives to impact and friction...

Primary explosives have low values for the activation energy and collision factor compared with secondary explosives. Therefore, it takes less energy to initiate primary explosives and makes them more sensitive to an external stimulus, i.e. impact, friction, etc., whereas secondary explosives have higher values for the activation energy and collision factor, and are therefore more difficult to initiate and less sensitive to external stimulus. 

Explosives are generally categorized as either primary or secondary, depending on their sensitivity to shock. Primary explosives are the most sensitive to heat and shock. They are generally used in detonators, blasting caps, and military fuses to initiate the explosion of the less-sensitive secondary explosives. Mercury(II) fulminate, Hg(ONC)2, was the first initiator to be used commercially, but it has been largely replaced by lead(II) azide, Pb(N3)2, which is more stable when stored under hot conditions. 

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. 

Primary ignition refers to the concept that a tiny bit of relatively unstable, high-intensity explosive can be used safely to detonate relatively stable explosive compounds. If a primary ignition system is used, waxes and clay can be added to sensitive secondary explosives to stabilize them for transportation and deployment. Then very fast, easily detonated mixtures can be used in very small amounts to make the whole batch go. In some cases, two booster stages are employed, leading to final detonation of the main explosive charge. This is, in fact, what occurs when commercial powder handlers use a cap to detonate primer cord which, in turn, is wrapped around a block or more of C-4. 

Primary explosives are substances which show a very rapid transition from deflagration to detonation and generate a shock-wave which makes transfer of the detonation to a (less sensitive) secondary explosive possible. Lead azide and lead styphnate are the most commonly used primary explosives today. However, the long-term use of these compounds (which contain the toxic heavy metal lead) has caused considerable lead contamination in military training grounds. Costly clean-up... 

Primary explosives are substances which unlike secondary explosives show a very rapid transition from combustion (or deflagration) to detonation and are considerably more sensitive towards heat, impact or friction than secondary explosives. Primary explosives generate either a large amount of heat or a shockwave which makes the transfer of the detonation to a less sensitive secondary explosive possible. They are therefore used as initiators for secondary booster charges (e.g. in detonators), main charges or propellants. Although primary explosives (e.g. Pb(N3)2) are considerably more sensitive than secondary explosives (e.g. RDX), their detonation velocities, detonation pressures and heat of explosions are as a rule, generally lower than those of secondary explosives (Tab. 2.1). 

Tab. 2.1 Typical sensitivity and performance data of primary and secondary explosives.

In attempts to strive for better performance, safety aspects (lower sensitivities) can not be ignored. For example, by using the formulations Composition B and octol (see Tab. 1.2) instead of TNT, the performance can be increased significantly (Fig. 2.1). However, the sensitivity increases as well which causes safety to decrease accordingly. The goal of current research is to develop considerably less sensitive secondary explosives, which offer maximum performance and a high safety standard (Fig. 2.9). 

Additionally, one must bear in mind that although primary explosives are much more dangerous to handle based on the probability of unexpected initiation due to their higher sensitivity secondary explosives exhibit a much higher performance and if initiated, result in much larger damage than a primary explosive. 

A famous secondary explosive of the nitrated alcohol group, commonly called nitroglycerine, is itself allot more stable then most people have been lead to believe. Hollywood movies and TV programs tend to over inflate the sensitivity of nitroglycerine. Other secondary explosives with similar properties have been over come with the same stereotypes. 

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