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Field-Portable Propellant Stability
Test Equipment
by eLena M. GraveS
The safety of ammunition stocks has been improved
with the development of field-portable propellant stability testing
equipment, which allows more ammunition samples to be tested.
The U.S. military has stockpiles of ammunition,
new and old, that can present safety hazards.
The primary ingredient of the propellant used
in these rounds, nitrocellulose, can deteriorate with
age and become prone to autoignition. To avoid the
destruction that could occur from the self-ignition of
this propellant, the Department of Defense (DOD) has
established a program for testing ammunition stocks
to determine the thermal stability of the nitrocellulose
propellants they contain.
History of Nitrocellulose
Shortly after French chemist Theophile Jule Pelouze
nitrated cotton in 1838 and created the world’s first
batch of nitrocellulose, potential users recognized that
it could be a dangerously unreliable explosive. Practical
use of nitrocellulose began in the mid-1840s with the
advent of Christian Shönbein’s improved manufacturing
process. However, its use was short-lived because
of frequent explosions of the impurely processed batches.
It was another 20 years before Frederick Abel of
Britain produced a good quality, commercially viable
nitrocellulose known as guncotton.
Unlike black and brown powders, the new nitrocellulose
powders had the desirable characteristics of
being relatively smokeless, powerful, and nonhygroscopic.
[Hygroscopic items readily absorb moisture
from the air.] However, they still decomposed at an
unreliably fast rate, causing so many accidental explosions
in storage and among gun crews that black and
brown powders remained the favored gun propellants
on land and sea through the end of the 19th century.
Nitrocellulose-based powders finally replaced black
and brown powders in the early 1900s, first at sea in the
world’s navies and then on land. Since reliable means
of stabilizing the nitrocellulose propellants had not yet
been developed, these powders were still in danger of
decomposition and, thus, instability. Devastating accidents,
like those aboard the French battleships Liberté
and Iena and the Russian Imperatritsa Mariya, lent
urgency to the search for an effective stabilizer.
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Propellant Stabilizers
As nitrocellulose-based propellants decompose,
they release nitrogen oxides. If the nitrogen oxides
are left free to react in the propellant, they can react
with the nitrate ester, causing further decomposition
and additional release of nitrogen oxides. The reaction
between the nitrate ester and the nitrogen oxides
is exothermic. (It produces heat.) Heat increases the
rate of propellant decomposition, and the exothermic
nature of the reaction may generate sufficient heat to
initiate combustion.
Stabilizers are chemical ingredients added to propellants
at the time of manufacture to decrease the rate
of propellant degradation and reduce the probability of
autoignition during its expected useful life. Stabilizers
that are added to propellant formulations react with
free nitrogen oxides to prevent their ability to react
with the nitrate ester. The stabilizers are scavengers
that act like sponges, but once they become “saturated,”
they are no longer able to remove nitrogen oxides
from the propellant. At this point, self-heating of the
propellant can occur unabated and may reach the point
of spontaneous combustion.
Propellant Stability Testing
Propellant autoignition accidents continued to occur
after the introduction of modern stabilizers during and
after World War I, but at a vastly reduced frequency.
Most early propellant powders were stabilized with
diphenylamine or ethyl centralite. Later 2-nitrodiphenylamine
and Akardite II also became common stabilizers
in the United States. The type of stabilizer used
depended on propellant formulation.
Shortly after the end of World War I, the Navy and
the Army each established permanent propellant surveillance
laboratories to monitor the safe status of their
propellants throughout their entire life cycles. Both
services adopted the 65.5 degrees Celsius surveillance
test as their primary tool. This test is a type of accelerated
aging test and is known as the fume test. It is
designed to preempt the autoignition of propellant in
JULY–AUGUST 2008