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Field-Portable Propellant Stability Test Equipment

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.

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, New Jersey.

Testing the days-to-fume on permanent laboratory samples was not the only propellant surveillance performed. Propellant fires continued to occur occasionally, and it was known that the propellant in field or fleet storage might not age at the same rate as the master sample in the service lab. Both services developed programs to bring propellant samples from the field or fleet and test them for available stabilizer at the service lab. Still active today, those programs are known as the Stockpile Propellant Program in the Army and the Fleet Return Program in the Navy.

A variety of test methods were used over the years, and, by the early 1980s, both labs had settled upon high-performance liquid chromatography (HPLC) as the best method for determining remaining effective stabilizer (RES) for most propellants. The HPLC method is still favored today by service labs and test labs worldwide. [Chromatography is a range of physical methods used to separate and analyze mixtures.]

In addition to the goal of maintaining master samples for all nitrocellulose-based propellants in the Army stockpile, each year the Joint Munitions Command (JMC) orders hundreds of propellant samples from a variety of storing locations to be prepared and shipped to the Army Propellant Surveillance Laboratory (APSL) at Picatinny Arsenal. The APSL has been testing samples on an annual basis since the early 1970s.

Because of the time it takes to process, prepare, and ship samples to the APSL and the lab’s resulting workload, routine sample test results are usually not available until several months after JMC initiates the stability test. Propellant samples that are identified as priority for testing can be processed quickly on an exception basis. However, it is clear from the volume of material that needs testing that most of the propellant lot samples cannot be treated as priority propellants.

Unfortunately, as robust as it is, our propellant surveillance system has not put an end to autoignition accidents. Seven propellant autoignition incidents, some involving 100,000 pounds or more of powder, occurred at Army installations in the 1980s and 1990s. Although it has been 10 years since the last accident, constant vigilance is required.

Excess and Obsolete Propellant Stocks

With the retirement of some weapon systems and modernization of others, greater reliance on artillery rockets, and the reduction in the size of the force, DOD has found itself with an immense quantity of excess, obsolete, and otherwise unwanted propellant. Much of the propellant is in separate-loading bag charges for Army and Marine Corps artillery. Millions of pounds of Navy gun charges have no purpose since capital ships are no longer equipped with major caliber guns. DOD also has many millions of pounds of excess propellant locked inside cartridge cases of obsolete or unserviceable fixed rounds and mortar projectiles. Along with the active stockpile, the propellant in the demilitarization account (ownership code B5A) requires close surveillance for stability.

The Army, as the single manager for class V (ammunition) demilitarization assets, assumes ownership of all ammunition and explosives of each service transferred to the B5A account. Hence, the Army Propellant Stability Program becomes increasingly burdened with providing sample testing of these propellants.

Conducting stability surveillance for bulk-packaged propellant, separate-loading propelling charges, and small component charges, such as those for mortar ammunition, is relatively straightforward. These items are mostly identified on stock records by their propellant lot or index number. Therefore, they are automatically included in the stockpile test portion of the Propellant Stability Program. This is not the case with propellants in fixed or semi-fixed cartridges. The Army considers such propellants so unlikely to autoignite in an uploaded-round configuration that the propellants are not included in the stockpile test program, and the propellants are not closely tracked.

Complete rounds, including ammunition for small arms, mortar, and artillery, are identified on stock records by the complete round lot number. When component lot information is listed, it contains lot numbers of items such as the fuse or the ball and tracer but not the propellant the rounds contain. The ammunition data card must be viewed to find the lot number of the propellant that is loaded into these rounds. In a number of cases, especially for small arms cartridges, the loaded propellant lot is not represented in the master sample program at APSL. Thus, much of the propellant loaded into cartridges of all calibers has not been tested for stability since the day it was loaded. For some older cartridges, this can mean the propellant has not been monitored since the 1950s or even earlier. Although the Army considers propellant in fixed rounds as not hazardous, when these rounds are no longer needed and are processed for demilitarization, propellant stability becomes an immediate safety issue.

Generating Bulk Propellant From Download

With the exception of most small arms ammunition, when fixed rounds are demilitarized, the projectiles are usually pulled apart from the cartridge cases and the propellant inside is emptied into a large container—usually a fiber drum—that will hold from 50 to 100 pounds of propellant. A single demilitarization project may generate hundreds of drums of propellant, usually of many different lot numbers. Suddenly, propellant that has spent its entire life in a configuration that was considered inherently safe from the risk of autoignition is now bulk packaged and stored in a concentrated mass that may be sufficient to allow autoignition to occur. If unstable propellant is unknowingly packed into a bulk container, autoignition could occur within weeks or even days. Once a demilitarization job begins, there may not be time to prepare and ship a sample to the APSL and wait for the test result to know if unsafe material is being retained.

Military propellants are becoming increasingly valued as a commercially viable product. Unwanted propellants can be used as an ingredient in the manufacture of industrial blasting gels and slurries, remanufactured as smokeless powder for small arms, or processed into agricultural fertilizers. Even if the installation that is demilitarizing munitions has the capability and necessary environmental permits to burn the propellant (and many do not have them), propellants today have become a marketable commodity that can and should be recycled.

To retain the propellant or transfer it to a third party for recycling, the stability of the propellant needs to be determined. Shipping it to APSL is slow and expensive. Establishing a small propellant surveillance laboratory with HPLC capability at the installation level is not considered economically feasible or sustainable. We needed to find a better way.

Field-Portable Stability Test Capability

In the mid-1990s James Wheeler, then Associate Director for Demilitarization Technology and chair of the demilitarization subgroup of the Joint Ordnance Commanders Group, requested proposals for designing and building a field-portable propellant stability tester. He envisioned an easy-to-use device or kit that could be carried by one person, moved to wherever propellant is located, operated by existing ammunition logistics or surveillance personnel with minimum training, and, most importantly, produce real-time results considered safe and accurate. Jim Wheeler’s vision resulted in the development of two propellant stability field test kits: the ammunition peculiar equipment (APE) 1995 near infrared (NIR) propellant stability analyzer and the thin-layer chromatography (TLC) propellant stability test kit.

Both of these test sets are capable of providing qualitative data to determine safe stability levels for the storage, transport, or ownership transfer of nitrocellulose-based propellants. Although RES levels are identified by each test and may be expressed quantitatively in terms of percentage of RES by weight, the test results are used in more of a “go/no-go” fashion. The “no-go” RES level for both test sets is considerably higher than the level that we use to identify propellants as stability category D (less than 0.20 percent RES) and is higher than the level for minimum stability category C (less than 0.30 percent RES). Using a higher level of stability (between 0.35 percent and 0.45 percent RES) as a cutoff for our field test sets gives us a greater margin of safety. Propellants that test at or below the cutoff levels will be either demilitarized or sent to the APSL for a HPLC test.

APE 1995 NIR Propellant Stability Analyzer

The APE 1995 is a model of simple operation. Once assembled on a workbench or small tabletop, individual propellant samples can be sequentially analyzed for stability at a rate of no more than 5 to 10 minutes per sample. Since the test is completely nondestructive and requires nothing more than electricity to conduct an analysis, it generates no hazardous chemical or energetic wastes. The APE 1995 was developed for the Army Defense Ammunition Center (DAC) by a division of Science Applications International Corporation, formerly known as Geo-Centers, Inc., at Picatinny Arsenal.

The APE 1995 is made up of three major components: a FOSS NIRSystems Spectrometer, Model 5000II; a laptop computer; and an uninterruptible power supply. The operator loads propellant into a removable cell and places the cell into the unit’s transport module. The optical window-side of the cell faces a tungsten-halogen light source as the cell moves through the light. Any differences in the sample, such as color, size, shape, or grain orientation, are averaged. The light is reflected onto detector elements of silicon and lead sulfide. Differences in the reflected light patterns (spectra) indicate varying stabilizer levels. These spectra are compared to predictive chemometric models of the same propellant type that are stored in the computer. The results of these comparisons indicate if the sample’s stabilizer level is at or below the cutoff level that requires more extensive analytical testing.

Two of the strongest features of the APE 1995 NIR analyzer are its simplicity of operation and speed of analysis. If samples are made available to the operator, propellants can be analyzed at a comfortable rate of 10 to 12 lots per hour.

In September 2002, the Joint Propellant Safety Surveillance Board (PSSB) validated the APE 1995 NIR Propellant Stability Analyzer as a screening tool for determining the general stability of solid propellants. The PSSB is the joint services technical advisory board of the Joint Ordnance Commanders Group quality assurance subgroup for policies and procedures pertaining to gun propellant stability. Since the PSSB endorsement, the APE 1995 units have been fielded to Arifjan, Kuwait, and to Tooele Army Depot, Utah, for use in demilitarization operations, and a unit has been sold to the Saudi Arabian National Guard. Indiana Ordnance Works, Inc., is also operating an APE 1995 in conjunction with an Army propellant recycling contract. APE 1995 units are slated for issue to Hawthorne Army Depot in Nevada, Crane Army Ammunition Activity in Indiana, and Anniston Munitions Center in Alabama.

At least a dozen different models of artillery and small arms propellant, including the most-used types, such as M1, M6, M8, M9, and WC-series, are currently within the APE 1995’s test capability. Work continues to expand the types and models of propellants that APE 1995 is capable of testing.

TLC Propellant Stability Test Kit

Eight years of research and development by the Lawrence Livermore National Laboratory’s Forensic Science Center led to the TLC propellant stability test kit, a field-portable set that gives trained quality assurance specialists (ammunition surveillance) (QASASs) and others the ability to produce lab-quality results in an onsite, real-time mode. A miniaturized wet laboratory with single-person portability, the kit is powered by either a redundantly designed, dually capable 110-volt wall current or self-contained, rechargeable batteries. The TLC test kit can be used almost anywhere to test for safe levels of RES in solid propellants that are stabilized with diphenlyamine, 2-nitrodiphenylamine, ethyl centralite, or Akardite II. The ability to analyze all four of the most-used stabilizers makes it possible to test almost all of the propellant powders in the DOD inventory.

The TLC method provides a go/no-go response within a critical range of RES levels; propellants with results that fall below the predetermined cutoff level are either demilitarized or subjected to a full chemical analysis by HPLC. The TLC method was developed for the Army to be used as a screening tool to determine the amount of stabilizer contained in solid propellant that is not in the active stockpile (third party assets or demilitarization assets). When directed by the JMC-managed Propellant Stability Program, both the NIR and TLC methods may also be used to test propellants in the active stockpile.

Conventional TLC analysis is routinely used in analytical laboratories worldwide for qualitative and semiquantitative characterization of unknown materials. Although TLC is ideal for rapid screening, is highly sensitive, and readily identifies the analytes sought in the complex propellant stabilizer samples, it has previously been considered a technique appropriate for use only in the laboratory, never for the field environment.

Unlike column chromatography approaches, such as HPLC or gas chromatography-mass spectrometry, that can only process single samples sequentially, a single TLC plate can accommodate and analyze multiple samples and standards. Samples are chromatographed simultaneously in a solvent tank, separating the stabilizer analytes from the sample matrix. Semi-quantitative assessments with nanogram detection limits are readily obtained by inspection of the plates. The kit is designed and equipped with sufficient supplies and equipment for the analysis of up to 30 individual samples by a single operator per day.

Once the chromatography is completed, the resolved propellant stabilizer components that appear as separated spots on the TLC plates are further enhanced by coloring with a unique reagent if the samples are diphenlyamine or 2-nitrodiphenylamine stabilized propellant types. If stabilized with ethyl centralite or Akardite II, the spots are viewed under the ultraviolet light that is fitted to the camera box. Quantitative analysis is performed using the digital imaging box, camera, and data acquisition equipment. The major advantages of the TLC method are simultaneous chromatography of multiple samples and standards, extremely low detection limits, the ability to calculate within a given range, and simplicity of operation.

TLC Endorsed by Joint Services Board

The Joint PSSB, which was chartered to “Establish criteria used to evaluate the safety of propellant inventories,” prepared a validation test plan that was used to evaluate the TLC test kit and its methodology. The results of the validation tests led to the endorsement by the board.

The NIR propellant analyzer was brought into the Army inventory as an item of APE. However, the TLC test kit was developed for Army use with the intention of transferring the technology to industry. The transfer began in 2005, when Lawrence Livermore began to work with Pelatron, Inc., of Honolulu, Hawaii, to initiate the commercialization process. In 2006, DAC issued a contract through the Army Corps of Engineers, Honolulu District, that brought the talents of Pika International, Inc., into the final equipment fielding effort.

The fielding of a TLC test kit involves a 2-week training course on kit operation and the handing-off of kit and supplies at the receiving installation. In 2006 and 2007, personnel from Pika International and Pelatron led a fielding team that included scientists from Lawrence Livermore as well as hands-on involvement and management oversight from DAC. Using personnel from Lawrence Livermore as a technical resource, the training and fielding events were planned, managed, and conducted by a mobile analysis team that trained QASASs and other users to become certified TLC operators.

Pelatron has assumed final and future commercial development, manufacture, distribution, maintenance, and management of the TLC test kit. The Army and other Government kit holders will use Pelatron for supplies and services.

Fielding of the TLC Kit

The first training and fielding event occurred at Tooele Army Depot, Utah, in August 2006. Four Tooele personnel were trained, and a TLC test kit was issued for their use. Tooele personnel began actual propellant stability screening tests during fiscal year 2007.

Hawthorne Army Depot was next to receive the TLC capability in January 2007. Anticipating the installation of a blasting agent manufacturing facility on the depot during 2008, Hawthorne needed the ability to conduct onsite TLC testing that would allow its personnel to screen propellant from the demilitarization stocks that would become a component of the blasting slurries. The TLC test kit is primarily used to test the propellants for which the APE 1995 is not calibrated.

Personnel from Aberdeen Proving Ground, Maryland, and Yuma Proving Ground, Arizona, were trained at Aberbeen Proving Ground in April and May 2007, with special emphasis placed on the wide variety of nonstandard propellants that require stability testing. The proving grounds maintain hundreds of lots of very small quantities of propellant that are frequently not represented in the master sample program. The Army Test and Evaluation Command plans to use the TLC test method to maintain effective safety surveillance of these assets.

Safety surveillance of artillery and small-arms propellants for safe stability levels will be required for as long as we continue to use nitrocellulose as a primary energetic material. The continuing work of the service propellant laboratories and effective field surveillance programs have prevented most autoignition accidents that might have occurred. The Army still has a large stockpile of active propellants. The decreased reliance on major caliber gun systems both on land and at sea has resulted in the retirement of many gun systems and the accumulated storage of tens of millions of pounds of unneeded propellants. These excess propellant powders, ranging in age from nearly new to over 60 years, will remain an additional burden on the propellant surveillance community until their final disposition through sale, reuse, ingredient recovery, or destruction. The growing availability and use of field-portable propellant stability test equipment, such as the NIR and TLC systems, will help APSL implement a more robust, responsive, and flexible propellant surveillance program to meet our current and future needs.

Elena M. Graves is a project manager in the Technology Directorate of the Army Defense Ammunition Center at McAlester, Oklahoma. She is an Army member and past chair of the Joint Propellant Safety Surveillance Board and is a quality assurance specialist (ammunition surveillance). She is a graduate of Ball State University.