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13.8 What is the chemical structure of common explosives?




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This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.

13.8 What is the chemical structure of common explosives?

Exothermic oxidation-reduction reactions are the source of energy, and they
can be produced from mixtures of discrete fuels and oxidisers, or from
molecular decomposition - such as from nitroglycerine. Propellants and
explosives produce large volumes of gases, whereas pyrotechnics do not.

                                             Gas     Reaction  Ignition 
                                           Volume      Heat   Temperature 
                                           (cm3/g)   (MJ/KG)     (C)
Photoflash (30:40:30 Ba(NO3)2:Al:KClO4)      15       8.989      700  
TNT                                         710       4.560      310

Most explosives are organic compounds or mixtures that contain carbon,
hydrogen, oxygen and nitrogen. Metallic fuels ( eg aluminium ) may be
added to increase the heat of reaction. Industrial dynamites traditionally
used nitroglycerine, nitrocellulose, and inorganic salts as sources of
oxygen, but these have been replaced by formulations that use ammonium
nitrate as the primary oxygen source. Note that the specific energy is
usually lower than the combustion of common fuels in air because the fuels
obtain their oxygen from air.

Many explosive can either burn or detonate, usually depending on the
type of initiation, confinement, and physical properties of the fuel.
When initiated, burning first occurs at an increasing rate during the first
few microseconds as it creates a high velocity, high pressure shock wave
that exothermically decomposes the explosive as it passes. The wave is
sustained by the transfer of energy from the reacted explosive to the
unreacted explosive via shock compression. The reaction rate depends on
the rate of propagation of the shock wave, rather than the rate of heat
transfer - as occurs during burning.

Explosives are usually classified into:
Low Explosives or Propellants
eg colloidal cellulose nitrate ( smokeless powder ), black powders,
gun and rocket propellants.
- they are usually mixtures of chemical compounds that produce large
volumes of high temperatures gases at controllable rates, and do not
require atmospheric oxygen. Ammonium perchlorate and ammonium nitrate
are commonly used as oxidisers.

Initiating or Primary Explosives ( detonators )
eg lead azide, mercury fulminate, diazodinitrophenol (DDNP).
- they are used to initiate the next component of an explosive chain, and
are usually dense, organometallic compounds.
- these are sensitive materials and fairly dangerous to handle as they
can be ignited by heat, shock and electrostatic energy.

                                            Lead     Mercury     DDNP
                                            Azide    Fulminate
Density             (g/cm3)                 4.0        4.2       1.60
Heat of Combustion  (MJ/KG)                 2.64       3.93     13.58
Heat of Detonation  (MJ/KG)                 1.54       1.79      3.43 
Gas Volume          (cm3/g at STP)           308        316       876
Detonation Velocity (m/s)                   5100       5400      6900

High or Secondary Explosives
There is a distinction between secondary and high, however many of the
common explosives are considered "secondary high explosives".
eg cyclotrimethylenetrinitramine (RDX), 2,4,6-trinitrotoluene (TNT),
cyclotetramethylenetetranitramine (HMX), ammonium picrate (AP).
"Secondary explosives" include trinitrophenylmethylnitramine (Tetryl),
nitrocellulose (NC) nitroglycerine (NG), pentaerythritol tetranitrate
(PETN), and nitromethane. High and secondary explosives require explosive
shock to initiate their detonation, otherwise they would only burn if
unconfined or unshocked.

                               NG      TNT    AP    RDX    HMX   Tetryl
Density             (g/cm3)   1.59    1.65   1.72   1.85   1.90   1.70     
Heat of Combustion  (MJ/KG)   6.80   15.02  12.09   9.46   9.88  12.24
Heat of Detonation  (MJ/KG)   6.29    4.23   4.31   4.54   5.67   4.63  
Gas Volume          (cm3/g)    715     710    680    780    755    760       
Detonation Velocity (m/s)     7600    6940   7050   8570   9160   7920
Detonation Pressure (GPa)      -      18.9     -    33.8   39.3   26.2  

RDX and HMX are substantially desensitized when mixed with TNT or coating
with polymer/elastomer binders. Most RDX in the USA is converted into
"Composition B" (59.5:39.5:1 RDX:TNT:Wax)
"A5" (98.5:1.5 RDX:Stearic Acid)
"C4" (91:5.3:2.1:1.6 RDX:dioctyl sebacate:polyisobutylene:oil).
Amatol AN

                                B     80/20   C4    AN   ANFO  Slurry
Density             (g/cm3)   1.72     -     1.64  1.72  0.93   1.40   
Heat of Combustion  (MJ/KG)  11.67    4.19    -    2.62   -      -
Heat of Detonation  (MJ/KG)   5.28    4.10   6.61  2.63  3.76   3.05
Gas Volume          (cm3/g)    -       860    -     980   -      -
Detonation Velocity (m/s)     7900    5200   8340  2700  4560   6050
Detonation Pressure (GPa)     29.5     -     25.7   1.1   6.0   10.4

Note that explosives usually have less potential energy than gasoline, but
it is the high rate of energy release that produces the blast pressure.
TNT has a detonation velocity of 6,940 m/s compared to 1,680 m/s for the
detonation of pentane in air, and the 0.34 m/s stoichiometric flame speed
of gasoline combustion in air.

Other than ammonium nitrate/fuel oil (ANFO), most common explosives are
trinitrated organic compounds. Nitroglycerine (glyceryl nitrate),
trinitrotoluene (TNT), picric acid, C4 (plasticized RDX/Cyclonite),
and tetryl (2,4,6-trinitrophenylmethylnitramine), fall into this category.
Refer to Merck or Kirk Othmer for chemical structures of common explosives.

A range of Semtex plastic explosives were produced by the Semtin Glassworks
in Czechoslovakia ( now known as VCHZ Synthesia ). Semtex-H is commonly used
by terrorists and, although examples are of variable composition, it
typically contains approximately 8% oil, 9% rubber, and approximately equal
quantities of RDX and PETN, but with known composition ranges of >21.5% RDX
and <64.5% of PETN. [3,4].

ANFO was proposed in 1867, but it was the development of anti-caking agents
in the 1950s that made ANFO suitable for rock blasting. Typical commercial
ANFO blasting agents consist of 94% ammonium nitrate prills (coated with an
anti-caking agent) and 6% fuel oil. They are reclassified as blasting
explosives if the formulation is sensitised by the addition of high
explosive. ANFO explosives are usually initiated by a high-explosive booster
such as formulation B. Maximum sensitivity to initiation occurs around 2-4%
fuel oil, with the presence of water decreasing sensitivity. The recent bomb
in Oklahoma City (estimated 1800kg ANFO)[5], demonstrated the destructive
capacity of ANFO explosives.

There were solubility problems using ANFO in wet drill holes, so aqueous-
based slurries were developed. These are usually thickened suspensions
dispersed in a saturated salt solution that has been made water resistant
by the addition of hydrophilic colloids that inhibit water migration.
Ammonium nitrate-based explosives account for 97% of the US industrial
explosives.

The infamous nitrogen tri-iodide ( touch powder ) produced by the reaction
of ammonia with iodine, is not actually NI3, but a nitrogen iodide/ammonia
complex with the structure NI3(NH3)n where n = 1, 3, or 5 - depending on
conditions. NI3 has only recently been isolated, and is stable at -196C,
decomposes slowly at -78C, and decomposes spontaneously at 0C [6]. Refer to
an older inorganic chemistry text, such as "The Chemical Elements and their
Compounds"[7], for further details and references.

Recently, there has been great interest in the development of more energetic
materials, and several new compounds are expected to replace existing
materials - once manufacturing costs are reduced. Examples include;- ADN
(Ammonium Dinitramide - NH4N(NO2)2, used as a propellant by the Soviet Union),
CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexazaisowurtzitane, aka HNIW,
the most powerful single-component explosive known - which, when combined
with a polymer binder is also known as LX19), and TNAZ
(1,3,3-trinitroazetidine) [8,9,10].

 

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