AN INVESTIGATION INTO
PROPELLANT STABILITY
Gail Y. Stine Propellent 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 t h e r e 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 an 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 oxides— NO, gases that are reddish brown. These gases are acidic and in the presence of moisture form H N 0 2 and H N 0 3 , 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 t h e
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 II 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 U.S. 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 °C) 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 to g a t h e r additional d a t a . As i t 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 N 0 2 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 · 475 A
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., TV-nitroso, mono-, and d i n i t r a t e d products). Some trinitrated products, trinitrodiphenylamines, and mononitro-N-nitrosodiphenylamines are also formed but in smaller amounts. Some of the early stabilizer reaction products act as 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 ./V-nitroso compounds) are thermally unstable, and GC or GC/MS 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 as well as master samples from our own station were delivered to our laboratory. Similar propellants made by different m a n u f a c t u r e r s 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 (î4-in.-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-μπι filter direct ly 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 diphenyl amine mixture in the grains was un known, a large sample size was se lected to obtain a more accurate picture of the average amount of sta bilizer available. Additional analyses were performed on single grains ver-
(a)
- AJ W 12 Time (min)
Figure 1. Chromatograms of 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, W-nitrosodiphenylamine; D, diphenylamine; and E, 2-nitrodiphenylamine.
sus composites and on sections of grains to determine if t h e r e were grain-to-grain variations within the lot. By the time we finished this in vestigation, 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 sys tems were used 24 h a day for three months. The LC system consisted of a C 1 8 reversed-phase column in isocratic mode with an acetonitrile/water mo bile phase (65:35) and UV detection at 254 nm. Quantitation was per formed 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 consid ered to be the stabilizer content of the propellant. The appearance of 2,4'-dinitrodiphenylamine indicates the onset of rapid depletion of avail able effective stabilizer (diphenyl amine, N-nitrosodiphenylamine, 2n i t r o d i p h e n y l a m i n e , and 4-ni trodiphenylamine). Increased values of 2,4'-dinitrodiphenylamine also in dicate a rapid increase of other deg radation products; thus, we used the
476 A · ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
amount of 2,4'-dinitrodiphenylamine present to indicate the shift from "ef fective stabilizer present" to "no ef fective stabilizer present." There are many proposed schemes for the degradation mechanism of diphenylamine; the exact path, how ever, is still under debate. Eventual ly 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 particu lar point in time. Chromatograms of the lot in ques tion indicated the presence of diphen ylamine, iV-nitrosodiphenylamine, and 2- and 4-nitrodiphenylamine in all samples. The average total stabi lizer content was 0.28% with a stan dard deviation of 0.04. This is equiv alent to 62% of the original stabilizer content. We found t h a t 2,4'-dinit r o d i p h e n y l a m i n e was p r e s e n t in each sample a t very low levels— 0.02% or less—indicating t h a t the stabilizer had not started to decom pose rapidly. Other stabilizer decom position products were observed at levels below quantification limits (Figure la). None of the 580 samples
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Figure 2. Chromatogram of 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 t h e low levels of 2,^-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%, w h e n compared w i t h t h e stored lots. Although additional degr a d a t i o n products were observed with t h e b u r n e d propellant (Figure 2), rapid decomposition of t h e stabilizer was not expected because diphenylamine and TV-nitrosodiphenylamine were still present in the sample. The total effective stabilizer was at a level acceptable to warrant use
Figure 3. Chromatogram of 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- a n d 4-nitrodiphenylamine. The partially burned propellant analysis indicated that the stabilizer remained at 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 t h e accelerated fumed samples. Once t h e s e p r o p e l l a n t s a m p l e s 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, 2and 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 an abundance of
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ANALYTICAL APPROACH peaks in the samples, indicating the presence of multiple reaction products (Figure 3). These products were not identified or q u a n t i t a t e d a n d 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 t h e 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 a t low levels, especially when combined with higher levels of diphenylamine and yV-nitrosodiphenylamine, implies stability. The LC data from the lot in question were in a g r e e m e n t with t h e days-to-fume d a t a from t h e oven tests, t h u s corroborating t h a t t h e propellant was stable. Safe life predictions based on data from the last 30 years indicated that this propellant had a t least 30 years of useful life remaining. Stabilizer data indicated that there was an effective stabilizer concentration of 0.05% for this
propellant before the stabilizer could no longer prevent rapid fuming. 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 t h e nitration/nitrosation of the diphenylamine during t h e 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 propellents. This testing has spawned analytical projects that will lead to additional studies, including t h e characterization of aging propellents and kinetic studies and measurements of gas diffused from the propellant grains.
Gail Y. Stine is an analytical chemist for the Test and Evaluation Department of the Naval Ordnance Station (NOS) where she manages the Propellant AnalySuggested reading sis Branch. She received an M.S. degree The Analysis of Explosives; Yinon, J.; Zitrin, in analytical chemistry from Georgetown University and has been employed by NOS S., Eds.; Pergamon: New York, 1981. Apatoff, J.; Norwitz, G. "Analytical Chemsince 1985. She is currently developing istry of Diphenylamine in Propellants, A new chromatographic methods and techSurvey Report"; 1973; Frankford Arseniques to further characterize propellants nal, Philadelphia, PA. Chemistry and Technology of Explosives; Un- and explosives.
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