Electrochemical processes Corrosion inhibition Soaps and detergents Insecticides and herbicidrs Fuels Electroplating Leveling agent3
Release coatings Lubrication Foams Evaporation inhibition Plasticizer migration Emulsions
As the number and variety of these compounds become larger, even further uses-not now met by conventional materials-will be feasible. literature Cited
(1) Bernett, M. K., Zisman. W. .4.,J . Phvs. Chem. 63, 1912 (1959). ( 2 ) Blake, G. B., Ahlbrecht, .4.H., Bryce, H. G., “Perfluoroalkyl Surface Active Agents for Hydrocarbon Systems,” 126th Meeting, ACS, New York. 1954. Abstracts, p. l l Q .
( 3 ) Brice, 7’. J., Trott, P. W., L. S. Patent 2,732,398 (1956). (4) Geen, H. V., Zbid., 2,937,098 (1960). (5) Hama, G. M., Frederick, W. G., Millage. D., Brown, H.: A m . Ind. Hyg. Assoc. Quart. 15, 3 (1954). (6) Klevens, H. B., Raison, M., J . Chem. Phys. 51, 1 (1954). (7) Ryan, J. P., Kunz, R . J., Shepard. J. W., J . Phys. Chem. 64, 52s (1960). (8) Scholberg, H. M., Guenthner, R. A , , Coon, R. I., J. A m . Chem. SOC.57, 923 (1953). (9) Shepard, J. W., Ryan, J . P., J . Phys. Chem., 60, 127 (1956). (10) Talbot, E. L., “Effect of pH and Ionic Strength upon the Surface Tension of Certain Perfluoroalkyl Acids and Their Salts,” 124th Meeting, ACS, Chicago: Ill., 1953, Abstracts. p. 41. (11) Talbot, E. L., J,Phys. Chem. 63, 1666 (1959).
RECEIVED for review Sovember 24, 1961 A C C E P T E D June 19, 1962 Division of Industrial and Engineering Chemistry, Fluorine Symposium. 140th Meeting, .4CS. Chicago. Ill.. Septrmber 1961.
INCOMPATIBILITY IN EXPLOSIVE MIXTURES Detection of Thermal& Hazardous Explosives Mixtures R A Y M O N D
N.
ROGERS
Uniuerstty of Caltfornta, Lor Alamos Sctentz’jc Laboratory. Los Alarnos, .V
id.
A thermal initiation test is proposed to detect instability in explosive systems. Materials are termed thermally incompatible with an explosive when their addition lowers the thermal stability of the explosive. The degree of hazard associated with the handling of a given system under conditions that may cause temperature excursions is estimated from its thermal initiation curves.
of explosive mixtures for specific purposes, two different but closely related requirements must be met. First, the explosive system must yield the desired result and retain its properties through various terms and conditions of storage. Second. the system must not be unduly hazardous to compound or to handle. When normally inert materials or other explosives are mixed with familiar explosives. the properties of the mixture cannot be inferred from the properties of the components. The various components of the mixture may be found to be “incompatible” with one another-i.e.. the system will not operate as desired. or the mixture is hazardous. Prediction and/or diagnosis of these tlvo different types of incompatibility has led to some confusion. Incompatibility of the first type may be caused b!- secondary- chemical reactions. or mobility of residual solvents. gases, or plasticizers, leading to unexpected modifications of mechanical: physical. or electrical properties. Incompatibility of the second type appears as an unexpected increase in sensitivity or decrease in thermal stability, and ma! be caused by any of the foregoing phenomena. It is, however. primarily a problem in chemical kinetics, caused by chemical interaction between explosive components, or between a n explosive and a material that is normallv considered to b? ineri.
I
N THE D E v E L o P M E m
Assuming ideal conditions and complete knowledge of the components of a mixture, incompatibility of the first type may be detected or predicted by use of tests designed to look at the vapor phase over test mixtures; e.g., the vacuum stability test ( 7 . 4>7 ) : pyrolysis ( 6 ) ,Taliani ( 9 ) .or gas chromatography (2). In addition, unexpected reactions and phase changes can be detected by use of the differential thermal analysis technique (DTA) (5: 8 ) . However, the interpretation of the results of such tests usually has to be done in a relatively subjective manner. and the correctness of the final decisions should be checked by long term: full scale surveillance tests. KO explosives work should be attempted without access to complete sensitivity testing equipment. and incompatibilities leading to sensitivity hazards should be the primary concern in the preliminary testing of any system. Sensitivity testing is. unfortunately. an extremely complex problem. and it is outside the scope of this paper to summarize tests and methods. Thermal instabilities in mixtures containing explosives, leading to hazards in handling and fabrication, can be detected in a reliable. sensitive manner. The unexpected appearance of phase transitions-e.g., the appearance of a liquid phase (eutectic, or minimum melting solid solution) with associated rapid liquid phase decomposition kinetics, and lor the appearance of exothermal secondary reactions can be detected with a VOL. 1
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Figure 2. A.
The system HMX-RDX
HMX (octahydro-l,3,5,7-tetranitro-s-tetrazine), 99.7% 2 2 4 ' C. 6. HMX, 94.4% pure [remainder RDX), T, = 21 7' C.
DTA test and hot stage microscope; however, the practical significance of the observations should be checked by a direct thermal initiation study. S o t e also that the degree of hazard may not be in direct proportion to the amount or type of gas evolved by a sample under the conditions of the above mentioned gas analytical tests. The present article deals with a test that is proposed as a method for the detection of thermal incompatibility. The Henkin test (3) has been used for many years to determine the thermal initiation characteristics of explosives. The test, useful though it was, was largely abandoned when it did not give correct numbers for the activation energy of pure explosives. Zinn's work on the theory of thermal initiation (70) has shown why the Henkin test failed to give activation energies, and hfader's experimental data, using a scaled-up Henkin-type test (70)>were actually used to verify the theory. The Henkin test has been modified, and is proposed as a test to detect thermal incompatibility. Experimental
Apparatus. A metal bath, control, and timing circuit similar to that specified by Henkin ( 3 ) . The metal should be confined with a sheet of steel above its surface, inserting the cell through a small hole drilled in the cover. The bath should be well insulated, and temperature control should be within 1 0 . 5 ' C. 170
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
pure, T,
=
Empty No. 8 blasting cap shells, 6.5 mm. gaschecks (small metal cups used to protect the bases of lead alloy bullets in reloading ammunition), and 00-size corks. Suitable gaschecks can be obtained from Hornady, Grand Island, Neb. A cap crimping tool, or tubing cutter ground to a rounded edge. A loose fitting die, ram, and punch set to hold the shells for assembly. The walls of the die should be heavy enough to confine explosions during the shell filling operation. A small hydraulic press or drill press with load cell. A Carver press with dial graduations of 0 to 1000 pounds applied force is adequate. A suitable powder balance or analytical balance, and measuring scoops. Blast shields of laminated glass. Procedure. Weigh the desired sample, usually 40 mg. of explosive (neglect inert), into a new KO.8 blasting cap shell. Routine or standard samples may be measured by volume once weights are known. Insert the shell, containing sample, into the die. R a m one 6.5-cm. gascheck part way into the shell, place one 00 cork in the shell, and ram both to the level of the sample. Place a second gascheck over the cork. Press the assembled shell to an applied force of 300 pounds (approximately 6000 p.s.i.). Eject the shell from the die, and using the cap crimper or dull tubing cutter, crimp the walls of the shell just above the upper gascheck and just above the lower gas-
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The system TNT-HMX
(Octol)
A.
Production grade RDX 6. RDX with rosin, 50% b y weight C. RDX with urea, 20% b y weight
check. Confinement obtained in this manner was quite positive. Rather than a metallic stopper, as suggested by Henkin, use a fine wire (No. 28 or smaller) wrapped around a 00 cork and inserted into the shell as a timing circuit contact. Recoil of the shell on firing, with chance of damage by flying fragments, is lessened by use of as light a timing contact as possible. Preheat the metal bath to the desired temperature, allowing time for equilibration before starting a test. Place the cell in the cell holder, and drop it into the metal bath. Make certain that blast shields are placed such that no molten metal or fragments can injure the operator. As in the original Henkin method, an explosion will stop the timer, registering the time to explosion automaticallv. It is usual procedure to plot log explosion time us. reciprocal temperature (absolute) for evaluation. Discussion and Results
A critical temperature (T,) is observed for any specific pure explosive in any given Teometry below which the self-heating of the sample is insufficient to cause explosion in any length of time. The critical temperature for any geometrical charge of many explosives can be calculated from experimental thermal initiation data by the method of Zinn [( 70) Equation 61. When new explosive mixtures are compounded, however, it is often observed that addition of a supposed inert will lower
A. B.
TNT alpha isomer (2,4,6-trinitrotoluene), production grade 75/25-HMX/TNT (Octol)
the thermal stability of the major components. I t is absolutely essential to determine the significance of chemical or physical interactions to the ignition characteristics of a system before undertaking large scale production and/or fabrication. For the practical detection of incompatible systems, it is necessary only to demonstrate that explosions are obtained a t temperatures significantly below the T , of the pure explosive and more quickly at temperatures above T , after addition of the material in question. An estimation of the degree of hazard associated with any new system may then be made by comparing the thermal initiation curve of the new system with standard curves of known explosives and/or mixtures. A considerable fund of practical stability information is available on standard military explosives-e.g., T N T (2,4,6-trinitrotoluene), R D X (hexahydro-1,3,5-trinitro-s-triazine):PETN (pentaerythritol tetranitrate), and compositions containing these explosives. Categories of stability can be established, and new materials can be classified on the basis of their thermal initiation curve. The effect of incompatible materials on the thermal initiation characteristics of explosives, whether causing a lowered melting point or contributing heat of reaction to the decomposition, becomes more evident at lower temperatures. The low temperature, long explosion time region of the thermal initiation curve? largely ignored by Henkin in his investigations of pure VOL 1
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explosives, should be of primary interest. Thermal lag becomes negligible when explosion times become large, making agreement with theoretical initiation curves best in the relatively low temperature range. Explosion times as long as 7000 seconds have been measured, and times to 10,000 seconds should be monitored. The shape of the initiation curve makes it possible to tolerate large variations in time to explosion in the region of T , without causing a n appreciable shift in T,. Variations in confinement may or may not cause a variation in explosion time as determined by the test, depending upon the mechanism of the specific decomposition in question. Most incompatible and/or autocatalytic systems, however, reach a minimum explosion time at absolute confinement; therefore, confinement should be as positive as possible, and multiple samples should be run in each temperature range to check for consistent confinement. The sample weight mentioned has not been found to be critical, agreeing with Henkin's observations. Explosion times do not vary to a n appreciable extent with sample weights in the 20- to 60-mg. range; however, small samples may not give a positive event, whereas large samples tend to make the test into a firing site operation. Considerable modification appears to be necessary if the explosive were the minor component of a mixture, causing major chanyes in dimension us. weight of explosive. A large amount of a true inert (on the order of 50%), mixed with a test explosive, will cause an appreciable increase in the time to explosion at temperatures above T,. The additional mass involved will cause a considerable increase in the recoil of the cell a t explosion, making additional precaution necessary to prevent metal from erupting from the metal bath. A small amount of true inert has no significant effect on time to explosion. An obvious example of the thermal initiation curves of an incompatible system, one that is extremely hazardous, is shown in Figure 1. Semiconfined, as in a small-scale DTA cell, the system shows a very large exotherm at 58' C. The appearance of gas varies extensively with experimental conditionsgas analytical pressure, pyrolysis gas flow rate, etc.-making methods appear to be somewhat unreliable for the estimation of the degree of hazard associated with this system. The mix-
172
I & E C P R O D U C T R E S E A R C H A N D DEVELOPMENT
ture, giving explosions a t the freezing point of Wood's alloy, would be quite hazardous to handle. Indeed, explosions can be obtained from a confined mass of the material when insulated a t room temperature. Another example of incompatibility, believed to be due to a lowered melting point with associated rapid liquid phase decomposition. is shown in Figure 2. Examples of incompatible mixtures with R D X are shown in Figure 3. Both materials produce a liquid phase below the melting point of pure R D X and, in addition, react exothermally with R D X . Thermal intiation curves for T N T (2,4,6-trinitrotoluene) and a 75/25-HMX,/TNT mixture--the military explosive octolare shown in Figure 4. The H M X used in the preparation of the octol is approximately of the same degree of purity as that shown in Figure 2B. Even though pure T X T is extremely stable, it does not improve the stability of the mixture. The octol system, however, would not be considered to be hazardous on the basis of thermal stabilitv. since the mixture is a t least as stable as R D X (Figure 3 A ) .
literature Cited (1) Clear: A. J.. Picatinny Arsenal. Dover, N. J., PA-TR-FRLTR-25, January 1961. (2) Frazer, J. W., University of California, Lawrence Radiation Laboratory, Livermore, Calif.. UCRL-6244, February 1961. (3) Henkin, H., McGill. R.. Ind. Eng. Chcm. 44, 1391 (1952). (4) Joint Army-Navy Specifications for 50/50 Pentolite, JAN-P408, October 17, 1946. ( 5 ) Rogers, R. N.. Microchem. J . V,91 (1961). \6) Rogers. R. N.. Yasuda. S. K.. Zinn. J . . Anal. Chcm. 32, 672 (1960). (7) Rosen. 3. M.. U. S. Naval Ordnance Laboratory, White Oak, Silver Spring, Md.. NOLM 10287, December I , 1949. (8) Smothers. \V. J., Chiang, Y.."Differential Thermal Analysis: Theory and Practice." pp. 21-40. Chemical Publishing Co., New York. 1958. (9) iViggam, D. R.. Goodyear. E. S.. Ind. En€. Chem., Anal. Ed. 4, 7 3 (1932). (10) Zinn. .I.. Mader. C. L.. ,J, ;Ippi. Phvs. 31, 323 (1960).
RECEIVED for review April 16, 1962 ACCEPTEDJune 26. 1962 Work performed under the duspices of the C . S. Atomic Energy Commission.
