Rates of Explosive Decomposition
of Explosives EXPERIMENTAL AND THEORETICAL KINETIC STUDY AS A FUNCTION OF TEMPERATURE HYMAN HENKIN‘ AND RUSSELL McGILL2 U . S . Bureau
T
of
Mines, Pittsburgh, Pa.
HE determination of “explosion temperatures’’ is of considerable importance to the explosives industry, from the viewpoint of both safety in handling and storage and the information which such studies may give as to the mechanism of explosive decomposition. The explosion temperature is not constant under all conditions, but depends on a number of variables, including the thermal insulation of the sample and the length of time the sample is subjected to the given temperature. The time lag prior to explosion will depend on the initial temperature to which the explosive is subjected. The expression “explosion temperature” in this paper means temperature a t which decomposition becomes explosive in character within relatively short time intervals. It is believed that this meaning is sanctioned by long established usage In the explosive industry. The temperature reached by an explosive on explosion may also be Properly described as explosion temperaThis temperature has not been and it no doubt varies widely with conditions, particularly charge density. The procedures that have been used for the determination of explosion temperature have varied from the earlier simple techniques of Marshall ( 4 ) and Kostevitch ( 8 ) to the recent method of Robertson (6). The earlier methods suffered from the disadvantages of having poor thermal contact between explosive and bath and, consequently, of giving irreproducible results and allowing. a too subjective interpretation of results. The more recent methods are too complicated for use as a routine testing tool. The procedure described below is a modification of the earlier m e t h o d of M a r s h a l l (4), whirh peimits a mole accurate and objective determination of the time lag prior to explosion.
of steel pipe. Thirty feet of Nichrome wire, No. 18 B. & S. gage, prepared in the form of a coil on an '/cinch mandrel, was wound over a layer of asbestos paper on the curved outer surface of the cylinder, Asbestos cord was placed between the turns of the resistance wire to prevent short circuits. The container w?s mounted inside a box of Transite sheeting, 12 cm. high and 17.8 cm. square. The resistance of the heating coil was 12.3 ohms and at 115 volts allowed 9.3 amperes to flow. This large current caused rapid heating of container and contents. The steel container was filled to within about 1 cm. from the top with Wood’s metal. A Varitran variable transformer (not shown) was included in the heating circuit, that the heating current could be regulated to give different temperatures. A mercury thermometer, C, partially immersed in the molten Wood’s metal, was used to measure the bath temperature. During the course of a determination the temperature was not allowed to fluctuate more than & l o C. A small sample-25 mg. for materials that ignited or deflagrated and 3 mg. for detonating materials-was placed inside a hollow copper cylinder, E (8.25 cm. long and 0.635 cm. Of No. in diameter), such as those used for
detonators* A copper Or brass cap was placed On top Of the copper cylinder. A fine wire was soldered to the cap and this wire in turn connected to the impulse counter and interval timer, A (Central Scientific co., Catalog No. 73506h which M’as connected through an 18-volt step-down transformer, B , t o another wire, F , leading back into the Wood’s metal. The impulse counter moves once for every electrical impulse received
APPARATUS AND PROCEDURE
The apparatus used in this research is shown diagramniatically in Figure 1. The Wood’s metal bath, U , was made in the following manner: A cylindrical steel container, 6.3 em. deep and 10 cm. in diameter, was made by welding a sheet of steel onto the bottom of a section 1 Present address, C o 1 g a t e Palmolive-Peet Co , Jersey City N. J 9 Present address, U. S. Kava1 Ordnance Laboratory, White Oak, hld.
Figure 1. Diagram of Apparatus 1391
INDUSTRIAL AND ENGINEERING CHEMISTRY
1392
and for 60-cycle current will move 120 scale divisions for each second that current flows. PROCEDURE. The explosive was placed in the copper cylinder which was fastened in the insulated lever (working on a ball bearing); the cap and attached wire were then placed on the cylinder and quickly lowered into the Wood's metal bath. The lever arm was so arranged that the cylinder was submerged to the same depth as the thermometer bulb and within a 0.5 inch of it. The instant that the cylinder touched the molten metal,
I
25
Vol. 44, No. 6
the sample be such that the entire sample would go through a 30-mesh screen. The explosion time was very nearly independent of the size of the sample in the range of 10 to 40 mg., so that the results' for all 25-mg. samples are comparable and the data for 3-mg. samples are probably equally comparable. MATERIALS
The explosives used in the experiments listed below were recrystallized grades of commercial high explosives for those materials which were commercially available. The remaining high explosives were laboratory preparations which were subjected to a number of careful recrystallizations. I t is hoped that analytical data on these samples may be presented in a future publication. The detonators, nitrocellulose, and black powder were tested as received, except for drying. EXPERIMENTAL DATA
2.0
1.5
I
d
/
GLYCERIN
1.0
Using the procedure described above, the lowest explosion temperatures were determined for the explosives listed below. The explosion times a t a number of temperatures were also obtained, No attempt was made to differentiate between ignition and explosion. Each point in the tables and graphs represents an average of 10 to 20 determinations. In Table I, there are given data for pentaerythritol tetranitrate (PETN) along with the standard deviation of the points a t eaah temperature: this latter gives an indication of the reproducibility of the data. While the impulse counter gives results in 1/120 second, all data have been recalculated and expressed in seconds. Similar data for other explosives are given in Tables I1 and 111. The standard deviations shown for pentaerythritol tetranitrate may be considered as t.ypical of the other explosives tested. The data obtained on the explosion t'emperatures for ammonium nitrate were not reproducible and are not represented in tabular form. The lowest temperature a t ivhich explosion was observed for this material was 300" C. The same was true of di-( P-nitroxyethyl) nitramine, for which the Ion-est observed explosion temperature was 235 C. The data given in Tables I to I11 were used to prepare Table IV, in which the compounds are listed in the order of decreasing lowest observed explosion temperatures. Data have been rounded off to the nearest 5". This table represents the lowest explosion temperatures using t,he above-described apparatus and not necessarily the lowest possible explosion temperature. For example, on more prolonged heating with bet,ter thermal insulation, nitrocellulose and nitroglycerin are known to explode a t considerably lower temperatures than those listed. A coinparieon of Ion-est observed explosion temperatures is only one means of comparing relative ease of explosion. Another way would be t o compare temperatures a t which explosion or ignition took place a t a definite time after exposure t o this temperabure. This has been done in the last column of Table IV, in which t,he exploaion temperatures after 1 second are tabulated. The time of 1 second was select.ed rather arbitrarily. I t is probable that the use of a longer time interval would give O
0.5 1.5
1.7
I.9
2.1
Figure 2. Data for Tetryl, Pentaerythrityl Tetranitrate, Nitroglycerin, and Lead Azide
electrical contact was made through the Wood's metal and the impulse counter hand moved until the explosion of the sample blew off the cap and broke the circuit. The counter was therefore self-operating. For long periods of time a stop watch was used in conjunction with the impulse counter. This was done at a series of temperatures and the time lag prior to explosion at each temperature was recorded. The temperature was lowered in intervals of 10" to 20" until a temperature was reached a t which explosion or ignition did not take place. To ensure electrical contact between cylinder and cap, it was found advisable to crimp the cap very slightly. To guard against ruining a determination because of absence of electrical contact a t the cylinder and cap junction, the completeness of the circuit was tested by seeing if the counter would respond when a wire immersed in the Wood's metal mas touched to the cylinder wall when the apparatus was in the position shown in Figure 1. Because the success of the method depended on the rapid conduction of heat from the Woodls metal to the explosive through the copper wall, it was essential that the copper be clean and free of oxide coating. This was ensured by sandpapering just prior to use. In order to obtain reproducible results, the samples had to be dried in the vacuum desiccator prior to use and the fineness of
TABLE
I. EXPERIMEKTAL DATAOK P E N T A I 3 R Y T H R I T O J ~ TETRANITRATE
Temperature, 0
c.
350 324 272 256
244 229 220
215
(25 mg.) Explosion Time, Seconds
Standard Deviation 0.023 0,050 0.032 0 091 0.114 0.475 0.080 1.03 0.122 1.57 0,280 2.93 0.231 4 55 N o explosion a t this or lower temperatures
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June 1952
EXPLODING BELOW 360' TABLE 11. EXPLOSIVES (25-mg. sample) Temperature, 360 346 329 314 285 269 264
Explosion Time, Seconds 0,325 0.425 0.582 0.742 1.45 2.22 No explosion
356 322 300 270 243 229 213 208
0,080 0,183 0.316 0,650 1.69 2.70 4.75 No explosion
350 320 300 270 249 232 225
0.300 0.490 0.760 1.35 2.40 3.35 N o explosion
c.
Trinitrophenylmethylnitramine (tetryl)
N , N'-Dinitro-N, N'-di-( 6nitroxyethyl) oxamide
Tetramethylolcyclopentanone tetranitrate
d
c.
0,088 0,112 0.459 0.910 1.63 2.02 2.48 3.40 No explosion Ethylenedinitramine
314 286 266 251 232 221 203 192 180 176 169 162 158 149 142
Ethyltrimethylolmethane trinitrate
351 277 260 246 228 221 215
Black powder
359 356 350 341 325 320 315
Nitrocellulose (13.4% N)
358 331 292 264 242 234 225 210 198 185 180 174 170
Nitrocellulose (12.6% N)
325 287 246 243 227 218 210 192 188 180 170
Picric acid
350 330 312 294 286 277 273 267 260
0.166 0.242 0.333 0.450 0.554 0.750 1.18 2.08 4.88 6.80 13.5 37.1 120 793 No explosion 0.077 0.551
0,880 1.16 2 05 2.54 No explosion 12.0 13.5 15.4 21.1 29.0 34.1 No explosion 0.074 0.100 0 383 1.30 3 99 5 80 10 2 21 5 41 1 201 440 1200 No explosion 0.143 0.442 2.87 4.20 8.17 13.7 21.5 76.5 102 458 No explosion
4 0
3.0
2.0
10
14
18
2 6
2.2
Figure 3. Data for Nitrocellulose, Mercury Fulminate, Erythritol Tetranitrate, and Ethylenedinitramine
more valid results, but examination of the above data shows that in a number of cases the lowest explosion temperature occurred after a time lag of only a very few seconds, so that it was not feasible, for comparison purposes, to employ a longer time interval. The apparatus was not operable above 360" C. and at that temperature ammonium dichromate, ammonium picrate, cyclotetramethylenetetranitramine, cyclotrimethylenetrinltramine, N,N'-dinitro-N,N'-dimethyloxamide, nitroguanidine, potassium picrate, trinitrobenzene, and trinitrotoluene did not explode. The explosives were removed from the bath after 5 minutes (Table I ) if no explosion had occurred a t 360" C. DISCUSSION OF RESULTS
It is generally considered that an explosion is preceded by a relatively slow reaction which increases more or less rapidly to explosive violence. It is immaterial to this discussion whether the increase in velocity to give ultimate explosion is due to thermal self-heating because of inability to dissipate the heat of reaction, or to the operation of a chain reaction mechanism, as suggested by Semenov (8). Both mechanisms yield the same equation for the relationship governing the time lag, t , prior to explosion after heating to a temperature, T K.): ( O
log t = E/RT
+ constant
(1) where E is the activation energy for the reaction in question. This relationship has been shown to hold for the explosive decomposition of mercury fulminate by Garner and Hailes ( 2 ) and for lead azide by Garner and Gomm ( 1 ) ; it has been demonstrated also within the past few years to be valid for the decomposition of tetryl and ethylenedinitramine (6). I n Figure 2 the data for tetryl, pentaerythritol tetranitrate, nitroglycerin, and lead azide are given graphically. As will be aeen from the graph, the logarithmic relationship between the time lag prior to explosion and the reciprocal of the absolute temperature results in a straight line for all four explosives, as required by Equation 1. This same relationship is also shown by tetramethylolcyclopentanone tetranitrate, N,N'-dinitroN,N'-di( 6-nitroxyethyl) oxamide, lead styphnate, picrate acid, black powder. ethyltrimethylolmethane trinitrate, and tetra-
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
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TABLE 111.
EXPLOSIVES EXPLODING BELOW (3-mg. sample) Temperature,
c.
hlercury fulminate
Lead azide
Diazodinitropheno
Lead st yphnat e
Nitroglycerin
Erythritol tetranitrate
3 04 280 257 238 216 200 194 184 176 170 360 349 341 329 319 314 247 215 207 200 190 186 177 168 160 328 315 301 294 288 280 275 261 245 230 2 19 211 205 275 257 248 239 222 214 209 200 205
360’ C.
Explosion Time, Seconds 0.108 0.225 0.492 1.03 2.84 7,55 10.9 34.4 105 No explosion 0.550 0.710 0.865 1.14 1.65 No explosion 0.133 0.333 0.460 0.725 1.45 2.29 5.00 16.0 No explosion 0.560 1.79 4.47 9.62 15.6
N1o. 3exploziun 4 0.217 0.358 0.675
1.15 3.75 No exploaiou 0.108 0.243 0.475 0.343 1.01 1.83 3.11 4.95 Fo
Vol. 44, No. 6
lished data. It is unlikely that we are dealing with two competing mechanisms, one predominant a t lower temperature and the other at higher temperature. If this were the case, it would be expected that the reaction at higher temperatures would have a greater slope, or activation energy, than the one at lower temperatures; otherwise it. would not predominate as the teniperature was raised. The fact that for every case examined the temperature coefficient is greater at lower temperatures is considered sufficient reason t.0 discard t,his explanation, I t is also considered unlikely that an explanat’ion based on a considerable time lag to get the explosive t o temperature is the true one. Examination of the data for picric acid, for example, shows no break even up to 50 seconds’ time lag. If there were a considerable time lag for heating up to temperature, then such a curve would show deviations from a straight line at high temperatures and short times. This is not observed. Furthermore, there is no reason to believe that an explanation based on the assumption that. rat,e of heat conduction is the rate-controlling step will give an exponential relationship between time lag and i reciprocal of temperature. In Table V are given the activation energies for all the explosives examined, obtained from the use of Equation 1. In this table El represents the activation energy a t the higher teniperature, while Ez is the activation energy in t,he lower temperature range for those explosives which do not follow a straight-line relationship over the entire temperature range studied. It will be seen from the dat,a in Table V that the activation energies, E,, are very much lower than those reported in the literature-e.g., by Roginsky (?)-for some of these explosives. M o s t previous work on activation energies has been done at lower temperatures than the work described in this report. In general, the activation energies previously obtained have been more in line with the values listed under Ez. The decomposition of trinitrotoluene a t high temperat’ure has been followed by Urbanski and Rychter (9) and has likewise shown a. low activation energy of 14 kg.-cal. in the range 390 to 450 C., whereas Robertson (6) working at somewhat lower temperatures reports a value of 32 kg.-cal. for this decomposition. Robertson attributes this discrepancy to heat, absorption by the vaporization of trinitrotoluene above its boiling point and concludes that this should be a general phenomenon above the boiling point for explosive decomposition. It is probable that the low activation energies shown in Table V are due to this phenomenon. While no accurate data are available for the relative boiling points of the explosives listed, it appears probable that lead styphnate, picric acid, and diazodinitrophenol are among the least volatile of these explosives (all being solids a t room temperature and having the possibility of hydrogen bonding). These three explosives have relatively high activation energies compared to some of the other explosives. An obvious result of some interest niay be readily seen from Figure 3. The data for. nitrocellulose of 12.6 and 13.4% nitrogen O
TABLE IV. PkPi,osIox TEMPERATUHES Tell,peratllre f o r
Lowest Explosjori Explosive Temperature, C . Black powder 320 Lead azide 315 Ammonium nitrate 300 280 Lead styphnate 265 Tetryl 265 Picric acid Di-( @-nitroxyethyl)nitramine 235 Tetramethylolcyclopentanone 230 tetranitrate Ethyltrimethylolmethane tririitrate 220 Pentaerythritol tetranitrate 215 &-,N‘-Dinitro-N, N’-di-( p n i t r o x y 210 ethyl) oxamide 210 Nitroglycerin Erythritol tetranitrate 205 Tetramethylolcyclohexa~iol 200 pentanitrate Nitrocellulose 175 175 Mercury fulminate 165 Diaeodinitrophenol Ethylenedinitramine 150
Explosion after 1 Second >360 340 320 ... 300 360 280 285 255 255 270 220 220 250 270 240 195 205
TABLE V. methylolcyclohexanol pentanitrate. These eleven explosives follow the logarithmic relation down to the lowest temperatures -
at ITrhich explosions could be obtained. The graphs for erytliritol tetranitrate. nitrocellulose (12.6% nitrogen), nitrocellulose (13.4% nitrogen), ethylenedinit’ramine, and mercury fulminate are given in Figure 3. As will be seen, they follow a more complicated pattern than do those of Figure 2. I n the cases of nitrocellulose and erythritol tetranitrate there are sharp breaks in the graphs, giving essentially an intersection of two straight lines. For ethylenedinitramine the break is less sharp, while for mercury fulminate there is only moderate deviation from linearity. This change in slope in the graphs for these explosives is of considerable interest in interpreting these and previously pub-
O
. ~ C T I V A T I O N EXERGIES
Explosive Pentaerythritol tetranitrate tetranitrare Tetrarnethylolcyclopentanone N,N’-Dinitro-hr,N’-di-(B-nitroxyethyl) oxamide Tetryl Lead styphnate Picric acid Leadazide
Activation -~
Ei
22.0 13.5 17.2
~Tetramethylolcyclohexanol l t a ~ y : t ~ ~ ~ ~trinitrate ~pentanitrate ylol~~tha~ie Nitroglycerin Erythritol tetranitrate
~ ~ ~ ~ ~ $ $ ~ 1 3‘ , ~4 ~ ; ~ ~ n d Mercury fulminate Diaaodinitrophenol Di-(P-nitroxyethyl) nitranline
Energy, _ _Kg.-(’al. . ~
Et
.. ..
17 2
June 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
fall on one curve, showing that the decomposition properties of the two nitrocelluloses are practically identical. The data given in these tables are comparative only and are based on results obtained on one particular apparatus. Thus trinitrotoluene did not explode in the apparatus a t 360” C., whereas Robertson (8) under different conditions obtained explosions a t temperatures almost 100” lower. The authors believe, however, that the apparatus described is well suited for obtaining reproducible data on relative stability of explosives. LITERATURE CITED
(1) Garner, W. E., and Gomm, A. S., J. Chem. SOC.,1931, 2123. (2) Garner, W. .E.,and Hailes, H. R., Proc. Roy. SOC. (London), A139, 576 (1933).
1395
(3) Kostevitch, M., 2. ges. Schiess-Sprengstoffw., 23, 156 (1928). (4)Marshall, A., “Explosives,” Vol. 2, Philadelphia, P. Blakiston’s Son & Co., 1917. (5) Robertson, A. J. B., Trans. Faraday Soe., 44, 677 (1948). (6) Ibid., p. 977. (7) Roginsky, S., Physik. 2. Sowjetunion, 1, 640 (1932). (8) Semenov, N., “Chemical Kinetics and Chain Reactions,” Chap. 17, Oxford, Clarendon Press, 1935. (9) Urbanski, T., and Rychter, Compt. rend., 208, 900 (1939). RECEIVED for review April 30, 1951. ACCEPTEDFebruary 29, 1952. Work performed a t the Pittsburgh Station of the Bureau of Mines during World War 11, supported by a transfer of funds from the Office of Soientific Research and Development. I t was described in OSRD Report 1986, reoently deolassified by the Ordnance Department of the Army. The authors are grateful t o Colonel C. H. M. Roberts of the Ordnance Department for his assistance in having the report regraded.
Evaluation of Flame Speed at Burner Flame Tip L. C . LICHTY Yale University, New Haven, Conn.
T
H E Bunsen burner flame has long been used to determine the flame velocity of a combustible mixture of fuel and air. Usually, laminar flow conditions are maintained in order to eliminate the effect of turbulence on flame velocity. One of the common methods ( 4 )for evaluating flame velocity is to determine the velocity component of some lamina in the mixture stream perpendicular t o the flame surface the lamina enters. Usually, the velocity is determined for the lamina a t a radius of 0.707 of the inside radius of the burner tube as the velocity of this lamina is the mean flow velocity for the laminar burner stream. The assumption is made that in laminar flow the various stream laminas flow vertically out of the burner tube to the flame surface. The excellent work of Lewis and von Elbe ( I ) , in determining the direction of flow lines of the various laminas of mixture before entering the flame and of the products of combustion after leaving the flame, has shown that the customary angle method is inaccurate because of the bending of the flow lines as they approach the flame surface. Thus, the actual velocity with which the mixture stream enters the flame surface, except for the flame tip, is appreciably higher than usually computed on the assumption that all flow lines do not change direction before entering the flame surface. Only a t the flame tip does it appear that the unburned mixture actually enters the flame surface perpendicularly without changing its direction of flow. Consequently, Lewis and von Elbe concluded that the flame velocity at the burner tip is equal to the velocity of the unburned central lamina mixture velocity as it leaGes the burner tube. This indicates a very high velocity compared t o velocity determinations at various flame positions, even a t those very near the flame tip. Lewis and von Elbe found by their measurements that the flame velocity increased from 63 om. per second a t a distance of less than 0.02 cm. from the burner axis to 213 cm. per second a t the axis. It is this extremely large increase in flame velocity between two laminas only a short distance apart that ia under question and to which this study is directed. The relatively high flame velocity at the flame tip is usually accounted for by the assumption that the central mixture stream is heated appreciably before entering the flame surface and that this rise in mixture temperature increases the flame velocity very appreciably. The experimental evidence of Lewis and von Elbe ( I ) , for the flame of a mixture of 7.50% natural gas in air, shows an increase in stream velocity, except for the flame tip, as the unburned mix-
ture passes through the flame surface and emerges as products of reaction. This increase in velocity of products compared to mixture indicates a momentum effect. von Elbe and Mentser (6) have made use of this momentum effect to evaluate the mean flame velocity for an acetylene-oxygen burner flame from the pressure build-up beneath the flame as measured in the burner tube. The usual assumption, made by them and others, was that the flow area does not increase in passing through the flame. This is equivalent t o assuming that the flame thickness is very small, The same assumption is used in this paper to determine the limiting or minimum value for the tip flame speed. However, the increase in flow area that may be caused by heat transfer into the unburned lamina before it reaches the flame, the increase in flow area that may result from appreciable flame thickness, and the effect of each on the computed tip flame speed are indicated. The flow of the burner central mixture stream t o and through the burner flame tip should be subject to the usual momentum effects that were observed for other mixture laminas in passing through the flame cone at positions other than the flame tip. This would indicate a products velocity immediately above the flame tip appreciably higher than the c e n t r a l m i x t u r e velocity leaving the burner , depending on the amount of heat transfer to the unburned mixture before entering the flaine. This is not in accord with the experimental eviHEATING 1 -% dence mentioned. However, ---It* the equality of the velocities of the burner central stream RAM SECTION before and after the flame surface can be reconciled with the inherent momentum effects if the following proc-- 4, esses occur: CENTRAL BLRN,ER STREAM‘,, \
Figure 1. Central Burner Stream
1. The central mixture stream is rammed from pl and TIto p z and TI’(Figure 1) as it approaches the tip. This decreases the stream velocity from o1 to 2r2. This ram-
