Gail Y. Stine Propellant Analysis Branch Naval Ordnance Station Indian Head, MD 20640
The Naval Ordnance Station is responsible for testing and monitoring propellants used for military purposes. Stored in our magazines are master samples of every propellant lot manufactured over the past 50 years and currently available for use. Our laboratory was recently asked to study a particular lot of propellant because there was some concern about its stability. In this article we will describe the analytical methods used to evaluate this lot of propellant. Our objective was to discern whether this material could independently cause a n explosion.
energetic plasticizers and burningrate modifiers can complicate their aging behavior. Over a long period of time, singlebase propellants will decompose, producing nitrous and nitric oxidesNO, gases that are reddish brown. These gases are acidic and in the presence of moisture form HNO, and HNO,, which will in turn attack the nitrocellulose, causing further decomposition. This catalytic decomposition can be prevented by the stabilizer, diphenylamine. Fume data
Long-term tests to monitor stability are part of an ongoing program at the Naval Ordnance Station. During these tests, the aging of a propellant is accelerated by placing a sample in a 65.5 "C oven and measuring the
ANALYTICAL APPROACH The questioned lot was a "singlebase" propellant, manufactured using a single energetic substance. It consisted of a colloid of nitrocellulose and a stabilizer, diphenylamine. The propellant was manufactured in 1944 and used in World War I1 and the Korean War. It was originally manufactured with a stabilizer content of 0.45%. From an analytical perspective, we were fortunate because single-base propellants are relatively simple to evaluate. Their aging behavior depends on the stability of the polymer, nitrocellulose, and the effect of the stabilizer. Other nitrocellulose-based propellants are similar in composition, but the addition of This article not subject to US. copyright. Published 1991 American Chemical Society.
time elapsed until reddish brown fumes appear. These days-to-fume data indicate the point at which the stabilizer is depleted. Newly manufactured single-base propellants may take from one to several years (at 65.5 OC) to fume. At this point, testing of another sample from the original lot is begun. As this process continues over the 40 or more years the material is being tested, a decrease in the number of days-to-fume will normally occur. When the propellant fumes within 30 days, it is considered to be unstable. At the 30 days-tofume point, the propellant has approximately one year of stable life left under ambient conditions and
the lot will be taken out of service and destroyed. The decrease in days-to-fume is normally approximated by a straight line. A least-squares linear regression analysis is performed on each lot and a regression equation calculated. The information is then entered into a database. Fume data are available for all propellant lots manufactured during the past 40 years. These data, along with the regression equation, are used to predict the point at which individual propellant lots should be destroyed. Propellant days-to-fume data were available for the lot in question. Additional master samples of this lot, as well as samples from the same lot stored in other facilities, were subjected to the days-to-fume test. Data from these samples indicated that this lot would fume in seven to eight months at 65.5 "C. (This corresponds to a "safe life" of approximately 30 years at ambient temperatures.) We could not wait for seven to eight months to pass to see if the prediction was accurate, so we developed and applied new analytical methods t o gather additional data. As it turned out, the samples fumed 9-11 months later. Analytical methods The degree of decomposition of a propellant sample can be determined by evaluating the changes in the molecular weight of nitrocellulose. However, because data on the original molecular weight of the nitrocellulose were not available, this method was not applicable. Other methods are based on the stabilizer depletion rate and measurements such as the evolution of NO, using gas analysis. We decided to use the most straightforward approach and determine the amount of stabilizer and stabilizer
ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
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ANALYTICAL APPROACH reaction products present. This would give us a quick snapshot of the propellant’s stability. The diphenylamine used in the propellant reacts with nitrogen oxide as it is formed. Diphenylamine nitrosation and nitration can produce as many as 21 derivative products (i.e., N-nitroso, mono-, and dinitrated products). Some trinitrated products, trinitrodiphenylamines, and mononitro-N-nitrosodiphenylaminesa r e also formed but in smaller amounts. Some of the early stabilizer reaction products act a s stabilizers themselves and react with additional NO, until complete nitration is achieved. By determining the identity and concentration of these stabilizer products, we hoped to be able to predict the aging status of the propellant. HPLC was selected because many of the reaction products (specifically the N-nitroso compounds) are thermally unstable, and GC or GCMS could not be used. We felt t h a t enough information could be obtained by positively identifying and quantitating diphenylamine and the first four degradation products eluted from the column. The first three degradation products contribute to the stabilization of the propellant. The appearance of the fourth product (2,4‘-dinitrodiphenylamine) indicates depletion of effective stabilizer. Samples of propellant lots from various naval facilities a s well a s master samples from our own station were delivered to our laboratory. Similar propellants made by different manufacturers were used for comparison studies, and partially burned propellant samples recovered from routine testing were also scrutinized. The samples consisted of large grains that required slicing by a guillotine cutter. Five propellant grains were cut in half, and one-half of each grain was cut into smaller (?4in.-sq.) pieces and composited into one sample. Five grams of this composite was extracted with 200 mL of acetonitrile. The organic solution was filtered through a 0.45-pm filter directly into HPLC autosampler vials. (The nitrocellulose does not dissolve in the acetonitrile but the stabilizer and stabilizer reaction products do.) Because the degree of homogeneity of the nitrocellulose and diphenylamine mixture in the grains was unknown, a large sample size was selected to obtain a more accurate picture of the average amount of stabilizer available. Additional analyses were performed on single grains ver476 A
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Figure 1. Chromatogramsof propellant lot samples. (a) Propellant lot sample under investigation. Components: A, 4-nitrodiphenylamine;B, N-nitrosodiphenylamine; C, diphenylamine; and D, 2-nitrodiphenylamine. (b) Second propellant lot sample of similar grain size and composition. Components: A, 4-nitrodiphenylamine; B, 2,4‘-dinitrodiphenylamine; C, N-nitrosodiphenylamine; D, diphenylamine; and E, 2-nitrodiphenylamine.
sus composites and on sections of grains to determine if there were grain-to-grain variations within the lot. By the time we finished this investigation, we had performed more than 4000 analyses. These included the lot in question and every lot of the same type and size in the Navy’s inventory. Three automated LC systems were used 24 h a day for three months. The LC system consisted of a C,, reversed-phase column in isocratic mode with an acetonitrile/water mobile phase (6535) and UV detection a t 254 nm. Quantitation was performed for diphenylamine and the N-nitroso, 2-nitro, 4-nitro, and 2,4’dinitro derivatives of diphenylamine. All these compounds except the 2,4’dinitro derivative act as stabilizers, and their weighted total is considered to be the stabilizer content of the propellant. The appearance of 2,4’-dinitrodiphenylamine indicates the onset of rapid depletion of available effective stabilizer (diphenylamine, N-nitrosodiphenylamine, 2nitrodiphenylamine, a n d 4-nitrodiphenylamine). Increased values of 2,4’-dinitrodiphenylaminealso indicate a rapid increase of other degradation products; thus, we used the
ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
amount of 2,4’-dinitrodiphenylamine present to indicate the shift from “effective stabilizer present” to “no effective stabilizer present.” There are many proposed schemes for the degradation mechanism of diphenylamine; the exact path, however, is still under debate. Eventually all of the stabilizer and stabilizer reaction products will progress to the trinitrated state. Environmental and storage factors influence the aging of the propellant. Nonetheless, our goal was to determine the decomposition status of the propellant at a particular point in time. Chromatograms of the lot in question indicated the presence of diphenylamine, N-nitrosodiphenylamine, and 2- and 4-nitrodiphenylamine in all samples. The average total stabilizer content was 0.28% with a standard deviation of 0.04. This is equivalent to 62% of the original stabilizer content. We found t h a t 2,4’-dinitrodiphenylamine was present in each sample at very low levels0.02% or less-indicating that the stabilizer had not started to decompose rapidly. Other stabilizer decomposition products were observed a t levels below quantification limits (Figure la). None of the 580 samples
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Figure 2. Chromatogramof a partially burned propellant sample. Components: A, 4-nitrodiphenylamine; B, 2,4’dinitrodiphenylamine; C, N-nitrosodiphenylamine; D, diphenylamine; and E, 2-nitrodiphenylamine.
from this lot had stabilizer levels below 0.14%. The questionable lot had stabilizer levels comparable to those of other lots tested with the same composition and grain size. Figure l b shows a sample chromatogram for propellant from a different lot indicating the low levels of 2,4’-dinitrodiphenylamine present. A study was conducted to determine grain-to-grain variations within a propellant lot. Individual grains were cut into sections and analyzed for the percentage of stabilizer content. A statistical comparison of the means and standard deviations for a multiple of grains and grain sections indicated that there were no significant differences. In addition, partially burned propellant samples were analyzed for their percentage of stabilizer. This provided additional information concerning the stability of the propellant after thermal decomposition (burning) occurred. These samples contained equal or slightly lower amounts of stabilizer, 0.20%, when compared with t h e stored lots. Although additional degradation products were observed with the burned propellant (Figure 2), rapid decomposition of the stabilizer was not expected because diphenylamine and N-nitrosodiphenylamine were still present in the sample. The total effective stabilizer was at a level acceptable to warrant use
Figure 3. Chromatogramof a fumed propellant sample. Components: A, 4-nitrodiphenylamine; B, 2,4’dinitrodiphenylamine;and C, 2-nitrodiphenylamine.
but, although composed primarily of diphenylamine and N-nitrosodiphenylamine, it contained higher levels of 2- and 4-nitrodiphenylamine. The partially burned propellant analysis indicated t h a t t h e stabilizer remained a t an effective level even during the burning process. One additional question remained unanswered. Was there a point when stabilizer was present but not at a sufficient concentration to prevent rapid fuming of the propellant? In other words, what is the stabilizer content, if any, when a propellant fumes? We tried to answer this question by analyzing the accelerated fumed samples. Once these propellant samples were fumed in the oven, they were removed and on the same day placed directly into acetonitrile. Assuming that they were unstable, we did not attempt to slice the grains but rather extracted whole grains for analysis. Our results showed that the diphenylamine and N-nitrosodiphenylamine were almost depleted. In addition, 2a n d 4-nitrodiphenylamine were present at levels averaging 0.05% in most samples and slightly higher in others. The most noticeable difference was that the 2,4’-dinitrodiphenylamine concentration was 0.20% or more. There was a n abundance of
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propellant before the stabilizer could no longer prevent rapid fuming.
peaks in the samples, indicating the presence of multiple reaction products (Figure 3). These products were not identified or quantitated and were very different from the results found for the questionable lot samples, which had a total stabilizer level of 0.28%. A stabilizer content of 0.05% would be considered the extreme point at which a propellant can be prevented from fuming. Therefore, as we indicated before, the appearance of significant amounts of 2,4’-dinitrodiphenylamine can be used as a marker to indicate when a propellant lot is about to fume. Its absence or presence at low levels, especially when combined with higher levels of diphenylamine and N-nitrosodiphenylamine, implies stability. The LC data from the lot in question were in agreement with the days-to-fume data from t h e oven tests, thus corroborating t h a t the propellant was stable. Safe life predictions based on data from the last 30 years indicated that this propellant had at least 30 years of useful life remaining. Stabilizer data indicated that there was an effective stabilizer concentration of 0.05% for this
Future work
Our recently purchased liquid chromatograph/mass spectrometer will be used to analyze the saved sample extracts. We plan to characterize the products resulting from the nitratiodnitrosation of the diphenylamine during the propellant degradation. Characterization of these products a t various times during the accelerated aging process will allow us to more precisely determine safe life and storage times for these propellants. This t e s t i n g h a s spawned analytical projects that will lead to additional studies, including the characterization of aging propellants and kinetic studies and measurements of gas diffused from the propellant grains. Suggested reading The Analysis of Explosives; Yinon, J.; Zitrin, S., Eds.; Pergamon: New York, 1981. Apatoff, J.; Norwitz, G. “Analytical Chemistry of Diphenylamine in Propellants, A Survey Report”; 1973; Frankford Arsenal, Philadelphia, PA. Chemistry and Technology of Explosives; Un-
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banski, T., Ed.; Pergamon Press: Oxford, 1964-67; VOIS.1-3. Fxpplosives, 3rd ed.; Meyer, R., Ed.; VCH: Weinheim, Germany; New York, 1987. Haberman, J.; Tech. Report No. ARAEDTR-86017, 1986; ARDC, Dover, NJ. Stine, G.; Franklin, S.; Salama, J.; 1988 JANNAF Propellant Characterization Subcommittee Meeting, CPIA Publication 497, Nov. 1988; pp. 297-304.
!-I Gail Y. Stine is an analytical chemist for the Test and Evaluation Department of the Naval Ordnance Station (NOS) where she manages the Propellant Analysis Branch. She received an M.S. degree in analytical chemistry from Georgetown University and has been employed by NOS since 1985. She is currently developing new chromatographic methods and techniques to further characterize propellants and explosives.
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