An explosive is a material that decomposes rapidly and spontaneously
under influence of thermal or mechanical shock. The decomposition
reaction involves the release of large amounts of heat and gas.
One of the earliest explosives, black powder (a mixture of
charcoal, sulfur and saltpeter) was already known in China many
centuries ago for the manufacture of fireworks. Around 1300, black
powder was first utilized as a propellant for missiles.
There was little development in explosives until around 1850, with the
discovery of nitrated compounds such as nitroglycerine and
nitrocellulose, followed by the development of dynamite. World War
I and World War II triggered the mass production of cheap explosives,
while the space race required the development of more powerful, stable,
and uniform propellants. Explosives play an important role industrially,
especially for mining purposes.
Explosives are classified into three types: mechanical explosives function by means of rapid changes in the
thermodynamic state of a compound, atomic explosives function by means of nuclear fusion or
fission processes, and chemical
explosives function by irreversible chemical decomposition or reaction
of a compound.
Chemical explosives have the largest use industrially. These
compounds are further classified in:
Various applications require explosives with a wide range of
properties, such as brisance, heat value, rate of combustion and
detonation, shattering ability and sensitivity. The tests
are largely of an empirical nature. The brisance of an explosive is measured in a
standardized soft lead test cylinder. The test measures the expansion of
the cylinder wall caused by detonation. The sensitivity of an
explosive is simply measured by dropping explosive samples from
increasing heights until the point where the explosive detonates upon
impact. Explosives for industrial are designed to release minimal
amounts of toxic gases, and to have minimal environmental impact.
Several important explosives listed by field of application are listed
below.
Industrial Explosives
Traditionally, black powder has been used as blasting agent,
since it does not shatter as much. It consists of
saltpeter, sulfur, and charcoal in a 75:15:10 ratio.
Since the products contain about 50% solids, it is an undesirable
blasting agent when combustible solids or gases are present (e.g.
coal mines.)
Nitroglycerin has been an important industrial explosive. It is
made by slowly adding glycerol to a mixture of sulfuric acid, nitric
acid and water. However, nitroglycerin is a liquid that is not very
easy and safe to handle, and therefore it is usually processed into
dynamite. This is done by absorbing the nitroglycerin on a solid
support material.
Currently, the major industrial explosives are blasting agents
and slurry explosives. These are low-cost, safe explosives with high
energy content. They often consist of ammonium nitrate/fuel
mixtures.
Rocket Propellants
Typically, rocket propellants are low explosives that include their own
oxidant or reactant to induce the explosive reaction. Propellants are
used for launching missiles and spacecraft. The reaction needs to
proceed at high temperature and pressure, and the product gases must
be of low molecular weight. The efficiency of a rocket propellant is
defined by its specific impulse, which indicates the amount of
thrust on a weight basis.
Liquid propellants are usually bipropellant systems: the fuel and
the oxidizer are stored in two tanks and fed separately to the
combustion chamber. Typical liquid propellant systems are liquid
oxygen/liquid hydrogen, liquid fluorine/liquid hydrogen,
boroethane (diborane)/oxygen difluoride, liquid
oxygen/hydrazine.
Solid propellants are cheap, simple to handle, and easily stored.
Typical solid propellants are ammonium chlorate, and nitrocellulose.
These propellants are often used for solid
rocket boosters
Pyrotechnics
Fireworks and signal flares are made of relatively simple explosives, and
include many additives to obtain proper illuminating and coloring
effects. A typical explosive of a signal flare consists of barium
nitrate (oxidizer), magnesium (heat), aluminum powder
(illumination), sodium oxalate (yellow coloring), and calcium
stearate, castor oil and linseed oil as binders. Black
powder is used as igniter.
Matches
Strike-anywhere matches are made from
phosphorous sesquisulfide, and
an oxidizing agent such as
potassium chlorate or
barium chlorate.
Safety matches use
antimony sulfide instead of the phosphorous
sesquisulfide.
Military Explosives
The main focus of military explosives is on performance and good storage
life. Environmental and cost factors play a much smaller role in this
type of explosive. Some of the more commonly used military explosives
are:
- TNT (Trinitrotoluene) - This is an older type of explosive, but it
is still in common use in the military, especially in mixtures with
ammonium nitrate. TNT is made by nitration of toluene with nitric
acid and sulfuric acid.
- Tetryl (2,4,6-trinitrophenylmethylnitramine) - This explosive is
used as a charge for blasting caps, as a booster in high-
explosive shells, and as an ingredient in binary explosives. It is made
from dimethylaniline, nitric acid and sulfuric acid.
- Picric Acid (2,4,6-trinitrophenol) - This is often a compound for
other explosives. The metal salts of picric acid are very sensitive. It
is synthesized from phenolsulfonates and several
acids.
- Explosive D (ammonium picrate) - Often used as a bursting charge in armor-piercing
shells. It is synthesized from picric acid and ammonia.
- PETN (Pentaerythritol tetranitrate) - A compound that is very
brisant and sensitive. It is used as a bursting charge, booster
explosive or as plastic demolition charges. Because it is so sensitive,
it is often desensitized by mixing it with TNT or a wax. PETN is
synthesized from pentaerythritol and nitric acid.
- RDX (cyclonite, sym-trimethylenetrinitramine) - One of
the most powerful explosives available. It is often mixed with TNT and
aluminum (Torpex) for mines, depth charges and
torpedo warheads. It is also commonly used in bombs and shells. It is
synthesized by nitration of hexamethylenetetramine.
- Lead Azide (PbN3 - This compound is
generally used as an initiating explosive for blasting caps, since it is
very stable. Older type blasting caps required mercury, which is
poisonous and more expensive.
- Lead Styphnate - Synthesized from lead acetate or lead nitrate
with trinitroresorcinol. It is commonly used as a primer
material.
Source:
George Austin (ed.) - Shreve's Chemical Process Industries, 5th
edition, McGraw Hill.
Common Sense Disclaimer: content of this writeup are for
information purposes only. This is general knowledge from chemical text
books that are commonly available. Do not attempt to synthesize
explosives since this requires very careful control over processing
conditions, and is very dangerous.