Explosives
Dr. Prashant Mehta
Assistant Professor
National Law University, Jodhpur India
Explosives
An explosive is a substance or a mixture, which when subjected to thermal or
mechanical shock, gets very rapidly oxidized exothermically into products of
greatly increased volume with sudden release of potential energy.
In explosive reactions the products get heated up to very high temperature and
exert very high temperature on surroundings which can be used for constructive or
destructive purposes. When an explosion occurs in a confined space, the high
pressure conditions developed within the system can be instrumental to shatter the
confining walls.
The quantity of power realizable from given weight(or volume) of an explosive is
called “power to weight(or volume) ratio”-which is quite small in the case of gases
but large in solids or liquids when used as explosives. Explosives are mainly used for
defense purposes, for blasting mines for obtaining ores, drilling holes in mountains
for constructing tunnels, quarrying purposes, excavating earth(for dams), seismic
prospecting etc.
Explosives Terminology
Velocity of Detonation (VOL) – is the velocity with which
the given explosive detonates. It value also depends upon
the density of powdered explosives.
Explosive Strength – is the energy liberated per unit mass
of the explosive (cal/gm). Blasting gelatin is the strongest
commercial dynamite).
Brisance – refers to the shattering power of the explosives.
Sensitivity – of an explosive to impact, friction or heat or
electric spark or detonator wave. Sensitivity plays a key
role in the selection of explosive for a particular purpose.
Classification of Explosive
Explosives may be in the form of solids TNT(trinitrotoluene), liquids or gases (H2 and
O2). They may exist as a single component (TNT), a mixture of two or more substances
none of which is an explosives by itself (gun powder), a mixture of one or more
explosives compounds (TNT + NH4NO3) or an explosives compound mixed with a nonexplosive material (dynamite).
A variety of inorganic or organic compounds can serve as explosives. They are
generally in the form of nitrates, chlorates, perchlorates, peroxides or compounds
containing nitro (-NO), nitroso (-NO), azide (-N3) or fulminate (-ONC) groups.
All known explosives can be categorized under three heads: Atomic, mechanical and
chemical explosives. Besides this Explosives are broadly classified into three broad
groups.
Primary Explosives or Detonators
These are highly sensitive explosives which can explode under slightest shock or
blow, by ignition, and have to be very carefully handled. They are used in
comparative small quantities in blasting caps and cartridges.
Lead Azide (PbN6) – It is low cost, excellent initiating action and stable in storage. It
reacts with brass and caps loaded with it are made of aluminum.
Mercury Fulminate (Hg(CNO)2) – More sensitive as well as more expensive lead
azide but it is slightly toxic.
Tetracene (C2H7N7O) – it is low initiating primary explosive. It is mainly used as
detonator.
Diazodinitro phenol (DDNP) – it is quite sensitive and has high brisance value. It is
used in blasting caps.
Low Explosives or Propellants
They do not explode suddenly but only burn and, their rate of combustion rarely exceed 0.25
ms-1. the chemical reaction taking place in such explosives are fairly slow.
Colloidal cellulose nitrate(prepared by treating cellulose with nitric acid and sulphuric acid)
and gun powder (mixture of 75% potassium nitrate, 15% charcoal, 10% sulphur) are examples
of this category.
Low explosives are used as propellants(to propel missiles) and in pyro-techniques (fireworks).
The first category is mainly used for military purposes. Eg. RDX (cyclotrimethylene
trinitroamine), TNT, Nitrocellulose, Picric acid (Trinitrophenol), PETN (Pentaerythritol
tetranitrate), DDNP, Leadazide etc. whereas industrial explosives are GTN and Dynamite
etc.
High or Secondary Explosives
This type is quite insensitive to mechanical shock as well as to flame i.e. they do
not explode on ignition. They explode with great violence when initiated with
an aid of detonators. They possess higher energy contents then primary
explosives and are stable.
Secondary high explosives are mainly Nitro compounds, Nitrate and Nitramines
used as such or with an initiators or as mixtures. Trinitrotoluene(TNT),
Ammonium
nitrate(AN),
RDX,
Picric
acid,
Dinitrotoluene,
Pentaerythritotetranitrate(PETN), ethylene dinitramine etc. come under this
category of explosives which are generally used in the main body of projectiles.
Primary and secondary high explosives are generally referred to as high
explosives.
Plastic Explosives
Plastic explosives are combination of explosives, which are in plastic state and can be
hand molded / press-loaded and made into various shapes without any serious risk.
There are combination high explosives because they contain a high explosive admixed
with wax or oil, to render them less sensitive and can be molded into sheets. They are
used for industrial and military purposes and are available as flexible-sheet explosives.
Usually RDX or PTEN is the high explosive involved. These sheets can be cut into
required shapes and can be easily fixed to a metal part by an adhesive. Special caps
are needed to trigger explosion in them.
Gun Cotton(cellulose nitrate)
It is obtained by treating purified, bleached and dried cotton linters (cellulose) with
a mixture of 12% HNO3, 63%H2SO4, 0.5% N2O4 and 5.5% water.
[C6H7O2(OH)3]n + 3HNO3 ————> [C6H7O2(ONO2)3]n + 3H2O
Cellulose also forms some sulfate which will lead to violent explosion. This is to be
overcome by removing sulphuric esters by adding NH4OH or Na2CO3. some
stabilizers in the form of diphenyl amine or diphenyl urea are added to cellulose
nitrate to arrest the formation of HNO2.
Gun cotton is cellulose nitrate containing 13.2% nitrogen. It is obtained when cotton
is steeped for 20 min in a cooled mixture of conc. HNO3 or conc. H2SO4. Dry gun
cotton explodes very rapidly when ignited. But, when it is wet, it is set of by a small
amount of another explosive like mercury fulminate. Dry gun cotton can also be
ignited by shock. In the fibre form it is used in torpedoes and submarine mines. It is
also used as a propellants in rifles or artillery shells.
Dynamites contains Nitro glycerine which is an oily liquid which detonates
spontaneously above 50oC is one of the principle components. The amount of inert
adsorbent such as wood pulp, starch meal, saw dust etc are added to dynamite.
Dynamites an breaks rocks into fragment of usable size and also pulverize it.
Straight Dynamite - this contains 15-16% of GTN in woodpulp coupled with NaNO3,
the latter acting as an oxidizing agent. It can be used for blasting hard rocks, coals
and other minerals and also in demolition works etc.
Blasting Dynamite(Gelatine dynamite) - in this formulation GTN is partly
gelatinized by nitro or colloidal cotton. It’s approximately composition is GTN 91.5%,
nitrocotton 8% and CaCO3 0.5%. It is gel like, water proof, does not flow and sticks
well into hole in which it has been loaded. It can be used under wet conditions and
thus demand for nonfreezing dynamites for use in cold weather is increasing. Non
freezing dynamites in the form of a mixture of GTN, ethylene glycol dinitrate, saw
dust and NaNO3 is as powerful as straight dynamite.
Blasting Fuses
Safety Fuse
Safety Fuse consists of a small
diameter core of black powder
enclosed in water-proof fabrics. It has
an approximate burning speed of 3040 seconds per foot. While blasting, a
sufficient length of the fuse is used so
as to allow sufficient time for the shot
firer to reach a point of safety
(analogous to the wick present in fire
crackers).
Detonating Fuse
Detonating Fuse consists of a charge
of high velocity explosives (6000
meters per second) e.g.
TNT
contained in a small-diameter bent
tube. The line of fuse is in contact with
the charge throughout its length and
this is responsible for instantaneous
detonation. These are mainly used for
exploding charges of explosives in
deep-holes.
Assessment of Explosives
Shattering power or drop heights - Shattering power can be evaluated by either the sand
bomb test or the Trauzal lead block test.
In the sand bomb test, an explosive sample is kept inside a steel bomb containing a known
weight of sand of uniform particle size (30 mesh). After the explosion, the pulverized sand is
sieved and the weight of sand that passes through the 30 mesh is taken as the shattering
power of the sample.
In the lead bomb test, the increase in volume of the soft lead cylinder of specified dimensions
when a known weight of an explosive sample is exploded gives a measure of its shattering
power. A good explosive must have high brisance.
Assessment of Explosives
The sensitivity to impact particularly, in the case of
initiating explosives can be measured in terms of the
minimum height from which a standard weight (2Kg)
must be dropped to cause detonation of an explosive.
This is called drop height of an explosive.
TNT have a drop height of >100 while RDX has a
drop height of 33. mercury fulminate, lead azide,
DDNP and tetracene have low drop heights and are
used only as initiators.
The velocity of detonation (VOD) is assessed by
measuring electronically the time required for a
detonator to traverse a known distance between two
pairs of wires. The VOD of PETN and RDX is 8300ms-1
while, it is 6900 ms-1 for GTN, 7700 ms-1for TNT and
2700 ms-1for AN.
Application of Explosives
Objects at a smaller distance can be shattered with
high brisance explosives Eg. shells, bombs and mines
etc. Low brisance explosives can be used as
propellants.
Explosives are burst a few feet below the land so
that subsoil is brought up to the surface and gases
like SO2, CO2, etc are liberated, which can kill insects.
Nitrate based explosives incorporates certain
fertilizers into the soil.
Explosive are used to shape the metal to required
shapes and this process is of recent origin.
Explosives used for dislodging the coal seam in a coal
mines must have long-lasting flame. The addition of
NaCl and AN to GTN not only helps to lower the
flame temperature but also the duration of the
flame.
Classification of Explosives
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Explosives may be in the form of solids (TNT,trinitrotoluene), liquids(GTN) or gases (H2 and O2).
Explosives may exist as a single component(TNT), a mixture of two or more substances none of
which is an explosives by itself (gun powder), a mixture of one or more explosives compounds
(TNT + NH4NO3) or an explosives compound admixed with a non- explosive material
(dynamite - GTN).
Explosives may be inorganic or organic compounds like nitrates, chlorates, perchlorates,
peroxides or compounds containing nitro-, nitroso-(-NO), azide(-N3) or fulminate(-ONC)
groups.
Explosives can be Atomic, Mechanical and Chemical explosives or can be high and low or on the
basis of their application.
Requisites of Explosives
For any compound to serve as an explosive, it must have at least one chemical bond
that can be easily broken. The molecule should also have negative free energy of
formation. In order to facilitate cleavage of bonds under the influence of thermal or
mechanical shock, they should have a low energy of dissociation. Most explosives
contain N-N, N-O, N-Cl or O-Cl bonds. The electronegativity difference in each of
these cases is either nil or marginal.
It must be stable in thermal conditions of storage. It must decompose rapidly and
exothermically, giving rise to a large volume of gaseous products, so that the
consequence rise in volume due to heating will be very large.
Requisites of Explosives
It must be cheap.
It must have a positive oxygen balance, which is a
measure of the oxygen present in the molecule, that can
be used to oxidize C to CO and H2 to H2O. It is expressed
numerically as a percentage, surplus or deficient of
oxygen by weight.
For a molecule CXHYOZ, (X+Y/2) atoms of oxygen will be
required to oxidize x atoms of C to CO and y atoms of H
to H2O..
The surplus oxygen = z-(X+Y/2) atoms
Oxygen balance =
z-(X+Y/2) x 16
Mol. Wt. of compounds
x 100
Requisites of Explosives
The bonded nitrogen, if any , in the explosive is librated as N2 and the oxygen is not
used up in this process. If however, oxides of nitrogen are formed, the computation of
oxygen balance must take this also into account.
Materials having a negative oxygen balance must be mixed with those possessing
positive oxygen balance to realize successful combustion. Compounds like TNT, TNB
etc., must be admixed with AN. Nitrogen containing compound having a rather high
oxygen balance give rise to NO and NO2. During explosion whereas, those with low
oxygen balance tend to form CO. Lead azide does not have any oxygen balance but
is used as an explosive.