Engineering Chemistry S Datta

EXPLOSIVES AND PROPELLANTS 339
Classification of Explosives
Explosives are classified under three broad groups, namely primary, low and high
explosives.
(A) Primary explosives. Primary explosives are the most sensitive to heat, friction, impact,
shock and electrostatic energy. They should be handled with great care. They are used to
initiate the explosion of high explosives which consist of explosives of increasing mass and
decreasing sensitivity. Only few compounds can act as primary explosives for military and
industrial requirements, they are dense, metallo-organic compounds. They are:
1. Mercury fulminate. Mercury fulminate or mercuric cyanate is a grey-white powder. It
is prepared by mixing of solution of one part of mercury with eleven parts of 57% nitric acid
and poured into ten parts of 95% ethyl alcohol when mercury fulminate forms as fine crystals
of 99% purity. It is the most sensitive of initiating agents to impact and friction. It decomposes
when stored at elevated temperature, reacts with metals in presence of water. It is slightly
toxic and its use is limited.
2. Lead azide. The azides are among the very few useful explosive compounds that do
not contain oxygen. It is extensively used as primary explosive in military detonators.
Lead azide is made in small batches buffered by the reaction solutions of lead nitrate or
acetate with highly toxic sodium azide.
Pb(NO 3
) 2
+ 2NaN 3
⎯⎯→ 2NaNO 3
+ PbN 6
Nucleating agents like polyvinyl alcohol (PVA), sodium carboxy methyl cellulose (CMC)
or dextrin may be added during precipitation to yield free flowing crystals or rounded
agglomerates. Lead azide is less sensitive to ignition than mercury fulminate. Of the four
polymorphic forms (α to δ) the α is the most common. Lead azide tends to hydrolyse at high
humidity and forms hydrazoic acid, which reacts with Cu and its alloys to produce very sensitive
cupric azide.
3. Lead styphnate. Lead 2, 4, 6-trinitro resorcinate is used to start ignition process in the
explosive sequence. It is stable and non-corrosive. Addition of graphite improves its electrical
conductivity in systems using electrical initiation.
4. Diazo dinitrophenol. It is an orange-yellow compound and prepared from picramic
acid [NH 2
(NO 2
)C 6
H 2
OH], with sodium nitrite and HCl. The product obtained is washed and
recrystallized from hot acetone. It is non-hygroscopic and sensitive to friction and impact. It is
more effective than lead azide and is used as an initiator in commercial blasting caps.
5. Tetrazene. It is pale-yellow crystalline compound. It is prepared by treating 1-aminoguanidine
hydrogen carbonate in dilute acetic acid with sodium nitrate. The compound is stable
upto 75°C. It has high explosion energy and is used as a detonator.
(B) Low explosives or propellants. Low explosives do not explode but burn and produce
large volume of gas at controlled, predetermined rates. The burning proceeds from the surface
inwards at a slow rate. They are applicable wherever a well controlled force is to be generated
for a short period. Typical examples are:
1. Black powder or gun powder. It is a mixture of 74% potassium nitrate, 15.6% carbon
and 10.4% sulphur. Black powder ignites and burns fast and has reproducibility and high heat
flux. It deteriorates at humidity. It can be stored many years if dry. The hygroscopicity of it is
due to the presence of carbon and impurities in KNO 3
. It is prepared by intimate mixing of the
thoroughly milled powdered ingredients. The decomposition reaction is
10KNO 3
+ 3S + 8C ⎯⎯→ 2K 2
SO 4
+ 2K 2
CO 3
+ 6CO 2
↑ + 5N 2
↑