History will record an underwear bomber and a shoe bomber. Luckily for the passengers of one transatlantic flight, the anonymous martyr on board was incompetent. Like the shoe bomber before him, this murderous buffoon failed to plan for a reliable means of triggering his bomb.

PETN, or pentaerythritoltetranitrate, was found to be the explosive agent used in the attempted inflight bombing of  Northwest Flight 253. This is a relatively common and powerful explosive in the category of aliphatic nitrate esters. It is a colorless powder that can be used in mixed and cast explosives or as the pure material. Like many detonable materials, it does not need to be placed in confinement to produce an explosion. PETN becomes unstable above 71 C, a fact that limits its suitability for some applications. My references do not clarify what is meant by unstable, but the material could be prone to chemical degradation above this temperature which would adversely affect its quality.

Other aliphatic nitrate esters include nitroglycerin, BTTN or 1,2,4-butanetriol trinitrate, EGDN or ethylene glycol dinitrate, and PETRIN, the trinitrate analog of PETN. A nitrate ester has a C-O-NO2 linkage and differs from aliphatic or aromatic nitro compounds which have C-NO2 linkages instead.

Nitrate esters are made from an alcohol or polyol and nitric acid. Nitro aromatics like TNT are made by acid catalyzed nitration of reasonably electron rich aromatic compounds like toluene or phenolics. The oxygen in the C-O-NO2 ester linkage confers some extra measure of instability to the molecule.

PETN is commonly used in Primacord, an explosive cord comprised of a PETN core inside a thin fabric or plastic sleeve. Primacord can be used as a blasting agent itself or it can be used as a fuse or delay line to trigger other explosives from a central point.

PETN is an explosive with a high brisance value. That is, it produces a shock that has a shattering effect on materials. In fact, brisance is quantified by the “sand test” which measures the production of fines from the shattering of 200 g of 30 mesh Ottawa sand. After the test, the sand is re-screened and the finer material that later passes through the screen is weighed. The greater the mass of fines, the greater the brisance.

Explosive         Sand Crush (g)   Heat of Explosion (cal/g) 
Black powder         8                                    684
Lead Azide            19                                  367
Comp C-4             55.7                            1590
TNT                      48                                1080
RDX                  60.2                        1280
Nitroglycerin         51.5                           1600
AN                               nil                                346
Picric Acid              48.5                           1000
PETN                         62.7                            1385
Source:  Cooper & Kurowski, Introduction to the Technology of Explosives, 1996, Wiley-VCH, p76-77. ISBN 0-471-18635-X

Pentolite is a composition prepared from a 50/50 blend of trinitrotoluene (TNT) and PETN with wax as a bonding agent and plasticizer. There are many blends of explosive materials. The composition is adjusted for the application.

The job of an explosive is to do PV work on objects. It does this by generating an abrupt pulse of heat and a large number of small gas molecules like N2 and CO2. The detonation velocity of PETN is ~ 8 km/s, so that a relatively small number of PETN molecules in a small volume are converted rapidly into a larger number of  gas phase molecules, all seeking to occupy the molar volume of 22.4 L/mol. 

The prompt generation of many moles of hot, small molecules results in the expansion of decomposition gases which forcefully press against the surroundings. The gases resulting from the 8 km/s detonation wave in the bulk solid explosive expand and compress the nearby air into a shock front that expands approximately spherically. As it does this the gases cool and the shock dissipates.

Explosive Power is a measure of an explosives ability to do work. Explosive power = Q x V,  Q = heat of explosion and V = volume of gas generated. The Power Index of a material is the ratio of explosive power to that of picric acid times 100 %. The power index of PETN is 167, TNT is 119, and RDX is 169.

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