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.
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 storage by forcing it to happen much earlier in the laboratory. When a tested propellant lot’s “days to fume” reach a defined minimum level, all quantities of that lot, wherever stored, are ordered destroyed. Until 1963, Navy ships had propellant labs on board to conduct this test. Although techniques have improved over the years, the accelerated aging test is still conducted by the Navy service lab at Indian Head, Maryland, and the Army lab at Picatinny Arsenal
