J. Phys. Chem. 1994,98, 3893-3900
3893
Compatibility of Ammonium Nitrate with Monomolecular Explosives. 11,* Jimmie C. Oxley,' James L. Smith, and Wen Wang Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 Received: September 30, 1993; In Final Form: December 28, 1993'
The thermal stability of mixtures of ammonium nitrate with a variety of organic explosives was examined over the temperature range 170 to 320 OC. Although the overall stability of the one-to-one mixtures was studied, the emphasis was comparison of the thermal stability (isothermal rate constants) of the individual components in the mixture with the neat species. It was found that nitrate esters, P E T N and nitrocellulose, nitramines, RDX and HMX, and nitroarenes, T N T and trinitroaniline, destabilized ammonium nitrate. In turn, ammonium nitrate, or its decomposition products, destabilized the organic explosives to varying degrees. For RDX and P E T N the destabilization was slight. The mechanistic bases for these observations are discussed; vapor-phase products are identified; and the effect of relative ratios of such mixtures are examined.
Background Ever since the Texas City disaster focused attention on the explosive power of ammonium nitrate, the use of ammonium nitrate based explosives has grown until they are the most widely used commercial explosive^.^ In recognition of their explosive potential, a number of studies have examined their thermal stability.& The thermal stability of single-molecule organic explosives of traditional military interest [2,4,6-trinitrotoluene (TNT)? 1,3,5-triaminotrinitrobenzene(TATB),8v9 1,3,5-trinitro1,3,5-triazocyclohexane (RDX),%ll octahydro-1,3,5,74etranitro1,3,5,7-tetrazocine(HMX),lOJ1and pentaerythritol tetranitrate (PETN)12J3]has also received much attention. However, despite the fact that many explosive formulations contain a combination of two or more energetic species, mechanistic thermal stability studies of such combinations are lacking. Ammonium nitrate was combined with nitrate esters as early as the 1870s when 'Nobel incorporated ammonium nitrate in dynamites.14 In World Wars I and 11,bothsides addedammonium nitrate to their conventional organic explosives when supplies ran low. Usually this involved ammonium nitrate being mixed in with TNT (Amatols), but this decreased the explosive potential. To improve the explosive power, RDX was sometimes added.15 Recently, in an effort to formulate more insensitive explosives, the military has begun adding ammonium nitrate to traditional organic explosives. At the same time, the commercial producers of ammonium nitrate blasting agents have begun to add surplused military explosives to their formulations to improve performance. With the increase in surplus military explosives this trend is likely to continue. Experimental Section Ammonium nitrate was used as received from Aldrich Chemical Co. 1,3,5-Trinitroanilinewas purchased (Aldrich) in a concentrated sulfuric acid solution, and the solid was precipitated by dropwise addition of 5 M aqueous NaOH. After neutralization and air and vacuum drying, the melting point of trinitroaniline matched that reported in the literature (188 "C). 1,3,5Trinitrotoluene (TNT) was obtained as production grade and recrystallized from benzene to a final melting point of 79-80 OC. 1,3,5-Trinitro- 1,3,5-triazocyclohexane (RDX) was manufactured by EastmanIKodak; no attempt was made to remove the contaminant 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). 1,35-Triaminotrinitrobenzene(TATB), pentaerythritol tetranitrate (PETN), HMX, and nitrocellulose (NC) were obtained from Los Alamos National Laboratory and used as received.
* Abstract published in Advance ACS Abstracts, February 15, 1994.
The energetic mixtures usually consisted of two ingredients in one-to-one weight ratio. For the solid ingredients and the solid mixtures, 100 mg of previously ground components was stirred together and ground for at least 45 min. The uniformity of the mixtures was checked by analyzing for each ingredient by chromatography, vide infra. Samples of 0.4 to 1.2 mg were thermolyzed in 40-50-pL melting point capillary tubes (0.9-1.1 mm i.d.) if the sampleswere to be analyzed for ammonium nitrate decomposition or in 200-rL glass tubes (2.0-2.2 mm i.d.) if the decomposition of the monomolecular explosive was to be quantified. The tubes were flame sealed. The thermal stability of the samples was probed by examining differential scanning calorimetric (DSC) thermograms and by analysis of the residue from isothermally heated samples. A Perkin-ElmerDSC-4 equipped with a TADS software data station was used in the DSC studies. Approximately 0.2 mg samples were sealed in glass capillaries which were held in an aluminum sleeve16 inside the DSC head. Unless otherwise stated, scans were performed at 20 OC/min and were calibrated against the melting endotherm of indium. DSC results were used primarily for qualitative thermal stability assessment,using the assumption that the lower the temperature of the exothermic maximum, the lower the thermalstability of thesample. Quantitation of thermal stability was derived from isothermal thermolyses. The glass capillary tubes used for isothermal decompositions were immersed in a temperature-controlledmolten metal (Wood's metal, m.p. 70 OC) bath for the desired length of time. The bath was maintained within 1 OC of the temperature desired (170320 "C), and the thermometer was calibrated before each run. A two-ply Plexiglas safety shield was placed in front of the bath apparatus as protection against explosions. For the mixtures, only the extent of decomposition for one ingredient, either ammonium or the organic explosive, could be assessed in a given sample; therefore, two sets of thermolyses had to be performed in order to obtain the rate constants for both ingredients. Two samples were run under each set of conditions-one for ammonium nitrate analysis and one for nitroarene analysis. First-order plots were constructed at each temperature, and the rate constants were taken from the linear regression describing the slope of the natural logarithm of the fraction remaining versus time (in seconds). The fraction remaining was calculated as the ratio of sample remaining after thermolysis to the amount of similarly treated but unheated sample. Sample amounts were expressed as peak height or peak area from chromatographic data. Ammonium nitrate decompositions were analyzed by breaking up sample tubes under distilled water. The water extract was analyzed by ion chromatography (IC), comparing the amount of
0022-365419412098-3893%04.50/0 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 14, 1994
Oxley et al.
TABLE 1: Rate Constants (s-l) for Neat Samples and 1/1 Mixtures temp, O
C
200 216 240 256 270 280 290 301 312 320 E,, kcal/mol
z,s-I R2 b
temp, O
C
200 216 240 256 270 280 Ea,kcal/mol
z,s-I R2
AN 8.57E-06 3.00E-05 1.06E-04 2.60E-04 6.25E-04 1.00E-03
AN/TNT'
TNT/ANa
TNT
6.85E-05 4.45E-04 1.14E-03 2.67E-03 3.61E-03
4.38E-05 8.09E-05 4.88E-04 1.43E-03 2.15E-03 3.84E-03
4.40E-06 1.27E-05 9.1 1E-05 2.83E-04 5.25E-04 1.04E-03 1.84E-03
3.43E-03 7.42E-03 1.07E-02 30.35 1.07E+09 1.00
33.80 9.79E+10 0.99
30.26 3.63E+09 0.99
AN
AN/TNA'
TNA/AN'
TNA
1.43E-04 5.08E-04 1.12E-03 3.26E-03 6.10E-03 3 1.38 1.32E+10 0.99
1-04E-05 2.04E-05 7.51E-05 2.85E-04 7.34E-04 1.19E-03 3 1.94 4.51E+09 0.98
9.70E-06 2.35E-05 5.45E-05 1.29E-04 2.41E-04 4.54E-04 24.09 1.25E+06 0.99
8.57E-06 3.00E-05 1.06E-04 2.60E-04 6.25E-04 1.00E-03 30.35 1.07E+09 1.oo
temp, OC
AN
AN/TATB'
216 240 256 270 280 301 320 E., kcal/mol z2 s-I
3.00E-05 1.06E-04 2.60E-04 6.25E-04 1.00E-03 3.43E-03 1.07E-02 30.35 1.07E+09 1.oo
2.43E-05 5.77E-05 2.67E-04 8.05E-04 1.92E-03 7.43E-03 1.50E-02 38.56 2.7E+12 0.98
R2 b temp, O
C
190 200 216 230 240 256 270 280 E,, kcal/mol
z2,s-I R2 b
temp,
O C
170 180 200 216 240 256 270 280
E., kcal/mol
ZZ,s-' R2 b
AN 8.57E-06 3.00E-05 1.06E-04 2.60E-04 6.25E-04 1.00E-03 30.35 1.07E+09 1.oo AN
8.57E-06 3.00E-05 1.06E-04 2.60E-04 6.25E-04 1.OOE-03 30.35 1.07E+09 1.oo
AN/RDX' 3.80E-04 1.31E-03 3.32E-03 7.68E-03 1.94E-02 8.06E-02 1.39E-01 38.57 2.21E+14 0.99
TATB/AN
TATB
RDX/ANa
RDX
5.75E-05 6.30E-04 2.67E-03 2.48E-02 6.04E-02
1.02E-04 5.85E-04 2.31E-03 7.48E-03 1.98E-02 4.29E-02
48.94 4.89E+04 0.97
39.22 8.12E+14 0.99
AN/PETN'
PETN/AN'
2.02E-03 6.26E-03 1.29E-02
3.78E-03 6.19E-03 3.64E-02 1.30E-01
22.71 2.05E+08 1.00
36.31 2.47E+11 1.00
33.89 1.9E+14 0.99
PETN 2.25E-03 5.83E-03 2.66E-02 1.07E-01
35.42 7.84E+14 1.oo
a Rateconstantsof mixturesarefor thespecies written first. Goodness of linear regression fit.
nitrate ion remaining in the mixture to that in a neat ammonium nitrate sample. The IC was a Dionex 2000 i/sp equipped with a conductivity detector, an ionpack AG4A guard column, and an ionpack GS4A separation column. The eluent was a 0.01 M NaHC03/NazCOs buffer. With a flow rate of 1.0 mL/min, nitrate ion appeared at about 7 min. For the monomolecular explosives, the analytical procedure was as follows: after thermolysis, the tip of the tube was broken,
TEMPERATURE (C)
DSC
Figure 1. DSC thermograms of AN with TATB.
and acetone (100 pL) was injected so that it ran down the inner walls of the tubes in order to dissolve all the products and unreacted sample. The nitroarene extracts were analyzed by gas chromatograph (GC) using a Varian 3600 G C equipped with an Alltech fused silica 5-BP capillary column and a flame-ionization detector (FID). The retention time of TNT was about 3 min a t a helium flow of 40 mL/min, and that of TNA was about 5 min at a helium flow of 30 mL/min. Quantifications of RDX and PETN were accomplished with a Beckman Model LC-332 high performance liquid chromatograph (HPLC) equipped with an Alltech Econsphere C18 reversed-phase column and a Waters486 tunable ultraviolet detector. For RDX, the mobile phase was CH30H/HzO (1/1) and the detection wavelength 216 nm. For PETN, the isocratic eluent was CH&N/H20 (1/ 1) and the detection wavelength 229 nm. With a flow rate of 1.0 mL/min, RDX appeared at about 8 min and PETN at about 14 min. Since TATB is insoluble in most organic solvents, no good method was available for analyzing its isothermal decomposition. Thus far, its compatibility with ammonium nitrate has been assessed only by differential scanning calorimetry (DSC). This is unfortunate, since DSC thermograms of some mixtures do not always suggest the same degree of incompatibility indicated by isothermal methods. At selected temperatures the overall thermal stability of the one-to-one mixtures was analyzed. The fraction reacted with time was determined by the amount of gas evolved to time with the amount of gas evolved at infinite time (10 half-lives or,more). A calibrated mercury manometer was used to analyze the amount of gas evolved. For identification of the decomposition gases, a Varian 3600 GC was used, configured with a Supelco Porpack Q 80/100 SS (12 ft X '/e in.) column in series across a thermal conductivity detector with a molecular sieve 13X 45/60 SS (9 ft X ' / g in.) column. The carrier gas (helium) flow rate was 15 mL/min. Gas composition was quantified by calibration with authentic samples.
Results Kinetics. Surprisingly, over the temperature range examined (216280 "C) ammonium nitrate and TNT had similar thermal stabilities although at the lower temperatures T N T appeared to decompose slightly more slowly than ammonium nitrate. (Firstorder rate constants as well as activation energies and frequency factors are listed in Table 1). When ammonium nitrate and T N T were mixed in a one-to-one ratio, the stability of both was adversely affected. The DSC thermograms of ammonium nitrate, TNT, and their 1/1 mixture confirmed this loss of stability. In fact, with the exception of TATB (Figure l), the presence of any of the monomolecular explosives, TNT, trinitroaniline (TNA), RDX, and PETN, increased the rate of ammonium nitrate decomposition (Table 1, Figures 2-5). DSC thermograms
Compatibility of Ammonium Nitrate with Explosives
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3895
/--
I 9
1
1.3
I
R,C
1 s"w.2o"clan
TEMPERATURE
(CJ
DSC
~. Ruo
1 ~ 1
2ooc/Inh
I*.I
lam
y
2 1 a m
inm
IUI
'J il~n
mm
zmm
1 m
32s )YI
TEMPERATURE (C)
3 m
mm
1
lam
nsc
Figure 5. DSC thermograms of AN with PETN.
Figure 2. DSC thermograms of AN with TNT.
ab
211
!!
a m
328, an
TEMFERATURE (CJ
*m
u m
d DSC
Figure 3. DSC thermograms of AN with TNA.
I
TEMPERATURE (0
DSC
TEMPERATURE CCJ
0%
Figure 4. DSC thermograms of AN with RDX.
Figure 7. DSC thermograms of AN with NC.
conveyed the same information since when ammonium nitrate was mixed one-to-one with either TNA, RDX, or PETN there was a single exothermic maximum at a temperature lower than that of neat ammonium nitrate. Theone-to-one mixtures of HMX and of nitrocellulose with ammonium nitrate were only cursorily examined; they also indicated destabilization of ammonium nitrate (Figures 6 and 7). The mixture of ammonium nitrate with TNT exhibited two exothermic maxima. The first, with an exothermic maximum a t a lower temperature than that of neat ammonium nitrate, was assumed to represent the ammonium nitrate decomposition, while the higher temperature exotherm was assumed to result primarily from TNT decomposition. Only when ammonium nitrate was mixed with TATB was the exotherm representing ammonium nitrate decomposition observed at a
temperature higher than neat ammonium nitrate (Figure 1). Qualitatively, this would be interpreted as stabilization of ammonium nitrate. However, the rate constants of ammonium nitrate determined in the ammonium nitrate/TATB mixtureover a 100 O C range (216-320 " C ) showed only slight stabilization a t the lowest temperature (Table 1). Comparison of the DSC thermograms of the mixtures of RDX or PETN with ammonium nitrate to those of neat RDX or PETN, respectively, indicates there is little effect on the thermal stability of the organic species (Figures 4 and 5 ) . The rate constants determined isothermally also show little change (Table 1). In contrast, when mixed one-to-one with ammonium nitrate, all three nitroarenes, TNT, TNA, and TATB, show significant destabilization (Table 1, Figures 1-3). The ammonium nitrate mixtures
8 1 Oxley et al.
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994
3896
porcont T A T .
ao
100
porcont
40
60
20
3 5 0100
0
RDX
80
310
270
..-"
150
0
20
40
:0
60
100
'
I 20
0
percont AN 0
AN
POrCOnt TATB
0
I3
Figure 8. Exothermic maximum of AN-TATB mixtures. porcont
1
80
3 5 0100
f w ~
t1
-
100
AN 0
RDX
100
3101
0
R
B
270 0
-
308
80
pofoont PBTN
1
60
0 322
AN
60
Figure 11. Exothermic maximum of AN-RDX mixtures.
-
336
40
B 0
---
0
-
294
20
0
40
percent 0
AN
60
80
100
TNT
0
0
porcont TNA
360
1
I
200
'
I 20
40
60
ao
100
prcont AN 0
AN
20
0
40
60
80
100
AN
0
PETN
Figure 12. Exothermic maximum of AN-PETN mixtures.
100
0
0
porcont A N
Figure 9. Exothermic maximum of AN-TNT mixtures.
9
150
AN
TNA
Figure 10. Exothermic maximum of AN-TNA mixtures.
of HMX and of nitrocellulose also exhibit lower DSC exotherms than the neat explosives (Figures 6 and 7). DSC scans were used to assess quickly whether the basic stability trends observed in the one-to-one mixtures were applicable to other ratios of the two ingredients. In each case a DSC scan from 50 to 450 OC was performed at 20 OC/min, and the temperature of the exothermic maximum was noted. Three scans were performed per formulation. If one scan deviated in appearance significantly from the other two, it was ignored. Figures 8-12 were prepared by plotting the average of the exothermic maximum versus sample composition. In several cases some ratios exhibited two peaks; the maxima of both are shown in the figures.
TABLE 2: Rate Constants (s-l) for 1/1 Mixtures AN mixed with TNT TNA PETN TATB RDX (270 "C) (270 "C) (270 "C) (216 "C) (216 "C) Rate Constants from Chromatography of Each Material in Mix AN 2.67E-03 3.26E-03 8.05E-04 1.31E-03 1.29E-02 organic 2.15E-03 7.34E-04 2.67E-03 1.30E-01 average 2.41E-03 2.00E-03 1.99E-03 7.15E-02 Rate Constants from Total Gas Evolved total 1.99E-03 1.53E-03 4.60E-04 2.00E-03 3.12E-02 Because the rate of product formation, specifically gas evolution, is frequently used to assess the overall thermal stability of formulations, the rate of gas evolution was followed at one temperature for each mixture. Overall rate constants, thus determined, are shown in Table 2. Decomposition Products. Ammonium nitrate decomposition yielded exclusively dinitrogen and nitrous oxide gases along with water. All other energetics produced some residue, although for PETN and RDX the amount of residue was small. Thermolysis of trinitroaniline or T N T produced large quantities of a black polymeric film, while TATB produced a particulate black material. No attempt was made to identify the condensed-phase decomposition products of TATB; the condensed-phase decomposition products of PETN,12 RDX,9 TNT? and TNA2 are discussed elsewhere. In all cases, when the organic explosive was mixed in a one-to-one ratio with ammonium nitrate, the amount of residue formed during thermolysis decreased. Presumably this can be attributed to the oxidizing power of ammonium nitrate aiding in the breakdown to gaseous products of the oxygen-deficient explosives. To study the effect on gaseous decomposition products, samples of neat ammonium nitrate, neat organic explosive, and
Compatibility of Ammonium Nitrate with Explosives
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3897
TABLE 3: Decomwsition Gases from Complete Reaction. sample time(h) Nzb Cob C0zb NzOb totalb oxygen balance residue Temperature = 216 OC 55.0 156 0 0 148 AN 304 20 water only PETN 0.33 17 307 151 15 489 -10 T, black AN + 50% PETN 0.33 176 65 132 8 38 1 -4 water only 2.0 132 70 106 124 RDX 433 -22 S,black AN + 50% RDX 2.0 200 46 101 96 443 -1 1 T, black Temperature = 270 OC AN 4.0 81 0 0 236 317 20 water only 3.8 90 18 162 TNT 0 270 -74 L, black film AN + 50% TNT 1 .o 197 18 146 18 379 -50 M, black film 24 137 TNA 5 .O 105 0 266 -57 L, black film AN + 50% TNA 2.8 196 19 157 49 422 -37 S, black film TATB 68.0 123 33 200 3 358 -56 L, black particles 16 130 AN + 50% TATB 68.0 197 19 421 -38 M, black particles a Amount of residue: T = tiny; S = small; M = medium; L = large. Corrected to 1 atm and 25 "C. * In pL/mg, for milligrams of mixture. TABLE 4: Comparative Stabilities PETN RDX TNA TNT meltingpoint 143 204 193 81 ("C) DSCexomax 210 257 388 330 ("C) k (SI)
TATB 448 397
1.1E-1 2.3E-3 2.OE-5 1.3E-5
1E-6'
(216 "C)"
k (SI)
7.3E-4 5.3E-4
(270 "C)"
210 ANDSC exotherm rate constants
252
254
305, 325
356
neatAN, 328 OC
(s-L)C
at 216 O c a 1.3E-2 1.3E-3 1.4E-4 6.9E-5 2.4E-5 3.OE-5 at 270 Oca 8.1E-2 3.3E-3 2.7E-3 8.1E-4 6.3E-4 a Selected isothermal rate constants from Table 1. Estimated from DSC data in ref 17. Rate constants of AN with 50% monomolecular explosive. their one-to-one mixtures were thermolyzed to complete decomposition. Results are shown in Table 3. With the exception of PETN which decomposed almost completely to gases when neat, all the mixtures formed more gas than the organic explosives alone. In all cases the dinitrogen to nitrous oxide ratio increased, and usually the carbon dioxide to carbon monoxide ratio increased as well.
Discussion Ammonium Nitrate. It is generally agreed that the first step in ammonium nitratedecomposition is dissociation into ammonia and nitric acid.,,s We have shown that at low temperature the subsequent decomposition steps involve an ionic mechanism, the slow step being the protonation of nitric acid.4 As a result, added acidic species, such as ammonium salts, nitric acid, nitrogen dioxide, accelerate ammonium nitrate decomposition. Conversely, added basic species, such as the salts of weak bases, retard ammonium nitrate decomposition (Scheme 1).
SCHEME 1 NH,NO,
-
-
NH,
slow
HNO,
NO:
+ H+
+ NH,
overall:
-
+ HNO,
-
H20N02+ NO,'
-
NH3NO;
NH,NO,
+ H,O
N 2 0 + H,O+
+
N 2 0 2H20
Judging from the rate constants observed in the one-to-one mixtures, the destabilizing effect of the monomolecular explosives on ammonium nitrate is roughly in line with their relative thermal
stability. PETN, a nitrate ester, most dramatically destabilizes ammonium nitrate; the rate constant of ammonium nitrate a t 216 OC in its one-to-one mixture with PETN is over 300 times faster than that of neat ammonium nitrate (Table 4). RDX, a nitramine, is second in its ability to destabilize ammonium nitrate; the 216 OC rate constant of ammonium nitrate is 30 times faster in their admixture than in the neat ammonium nitrate melt. The two nitroarenes, trinitroaniline and trinitrotoluene, accelerated the decomposition of ammonium nitrate by roughly a factor of 3, while the nitroarene TATB had little effect. Since all the organic explosives examined destabilized ammonium nitrate and the extent of destabilization was roughly in line with their own thermal stability, we looked for a common trend in their decomposition. One of the mechanisms by which each of these classes of organic explosives is postulated to decompose is by loss of NO2.'-I3 The order of their thermal stability is consistent with the X-NO2 bond energy: nitrate ester O-NO2, 40 kcal/mol; nitramine N-NOz, 47 kcal/mol; and nitroarene C-NO2, 70 kcal/mol.'I Since NO2 is known to accelerate ammonium nitrate decomposition,4,5 we suggest that production of nitrogen dioxide by decomposing organic explosives results in the destabilization of ammonium nitrate. The relative ordering of the effect would be related to the amount of nitrogen dioxide produced. PETN and nitrocellulose would have the largest effect because with their low thermal stability they decompose below the temperature wheredecomposition of ammonium nitrate is normally significant. For RDX and HMX, the rationale is similar. For RDX and PETN the DSC thermograms of the oneto-one mixtures show a single exothermic maximum at the same temperature as the neat organic explosive; this temperature is substantially below that of neat ammonium nitrate (Figures 4 and 5). The effects of HMX and of nitrocellulose on ammonium nitrate decomposition are similar, except that the single exotherm observed for each mixture is also below the temperature of the normal exotherm for the neat organic explosive (Figures 6 and 7). When these organic explosivesdecompose, generating nitrogen dioxide, they cause ammonium nitrate to decompose along with them. The DSC exothermic maxima of the three neat nitroarenes appear at temperatures higher than that of ammonium nitrate (Figures 1-3); therefore, it is not surprising that they destabilize ammonium nitrate to a lesser extent than the nitrate esters or nitramines. It should be noted that from examination of the DSC thermogram one would expect T N T to be less thermally stable than trinitroaniline and, thus, exhibit the greater destabilizing effect upon ammonium nitrate, whereas the observed isothermal rate constants for ammonium nitrate in mixtures show it decomposing slightly faster in trinitroaniline than in T N T (Table 4). This is an example of a case where programmed DSC thermograms can be misleading. Under the conditions of a programmed DSC scan a t 20 OC/minute, T N T appears less thermally stable, but the isothermal rate constants obtained at
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The Journal of Physical Chemistry, Vol. 98, No. 14, 1994
216 O C show that it is more thermally stable than trinitroaniline; therefore, at that temperature ammonium nitrate decomposition is more accelerated by the presence of trinitroaniline than by TNT. At 270 "C, where the decomposition of TNT is slightly faster than trinitroaniline, the ammonium nitrate rate constants are almost identical in both mixtures (Table 4). TATB decomposes quite slowly a t temperatures where ammonium nitrate decomposition is significant. The rate constant estimated from DSC results17 for TATB at 216 OC, 1 X 1 W s-1, is at least a factor of 20 slower than the other nitroarenes. Therefore, it is not surprising that it has no acceleratory effect on ammonium nitrate decomposition. However, the DSC thermogram (Figure 1) suggests it should have a stabilizing effect. Consistent with this is the observation that the rate constants below 270 "C of ammonium nitrate in the one-to-one TATB mixture were somewhat lower than those of neat ammonium nitrate (Table 1). Above 270 "C, the rate constants for ammonium nitrate decomposition in mixture with TATB were slightly higher than those of neat ammonium nitrate, suggesting TATB generated enough nitrogen dioxide at higher temperatures to have a deleterious effect on it. However, a t low temperatures, TATB may have a stabilizing effect because it acts more as a base than as a source of nitrogen dioxide. The direct correlation of scanned DSC results to isothermal results has always been a matter of interest to us. In previous studies1*we observed that the gaseous decomposition products of ammonium nitrate formed during a 20 OC/min DSC scan matched most closely the distribution of gaseous products observed in isothermal decomposition a t 270 "C. Now we observe that the DSd thermogram obtained in a 20 OC/min scan of a mixture of ammonium nitrate and TATB suggests stabilization of ammonium nitrate, an observation supported only by the isothermal data below 270 "C. Therefore, when examining ammonium nitrate we use as a rule of thumb that a scan rate of 20 "C/min gives results similar to a sample heated isothermally a t about 270 OC. We would not expect this correlation to apply to materials which exhibit exothermic maximum a t substantially different temperatures than ammonium nitrate. Organic Explosives. The effect of added monomolecular explosives on the thermal stability of ammonium nitrate is more dramatic than the effect of ammonium nitrate on the organic explosives. Neither the nitrate ester PETN nor the nitramine RDX had its rate constant much changed by the addition of ammonium nitrate; both exhibited slightly higher rate constants in the presence of ammonium nitrate than they did neat (Table 1). The DSC thermograms of their one-to-one mixtures show single exothermic maxima a t the same temperature as neat RDX or PETN (Figures 4 and 5 ) . That addition of amqonium nitrate has little effect on the decomposition of PETN and RDX is not surprising if the rate-determining step in their decomposition is O-NO2 (nitrate ester) or N-NO2 (nitramine) homolysis. In fact, we have examined the effect of a variety of additives, acids, bases, N02, and radical scavengers, on nitramines and observed no effect on the rate of decomposition.9 The sensitivity of nitrocellulose to acid is well-known;lg therefore, it is not surprising that the DSC thermogram of its one-to-one mixture with ammonium nitrate indicates both it and ammonium nitrate were destabilized (Figure 7) since ammonium nitrate decomposes to nitric acid and ammonia. Unlike the RDX/ammonium nitrate mixture, the HMX/ ammonium nitrate mixture exhibited an exotherm a t a lower temperature than the neat nitramine (Figure 6). At first it was thought that this lowering of the HMX exotherm might be related to the phase of HMX during the thermolysis; but examination of HMX using a hot-stage microscope revealed that, in excess ammonium nitrate or neat, HMX underwent the usual beta to gamma phase change upon heating. However, the HMX/ ammonium nitrate mixture did show a slight lowering of the
Oxley et al. ammoniumnitratemelt (168versus 169 "C),indicatingaeutectic melt. A similar mixture of ammonium nitrate and RDX also indicated a eutectic melt. These observations indicate that both RDX and HMX are liquefied a t lower temperatures in ammonium nitrate than when they are neat. However, the acceleration in decomposition rate when HMX goes from solid to liquid phase is much greater than the corresponding acceleration when RDX melts. Some indication of the difference between the solid- and liquid-phase decomposition rate is indicated by the thermograms of the neat nitramines. For RDX the melt endotherm can usually be observed just before the decomposition exotherm. For HMX the liquid-phase decomposition is so rapid that the melt endotherm is not usually observed before the decomposition exotherm. Thus, HMX is more strongly affected by the presence of ammonium nitrate than RDX, since ammonium nitrate promotes its liquefaction with resulting rapid liquid-phase kinetics and enhanced decomposition. The rate of decomposition of all three nitroarenes examined was accelerated by the addition of ammonium nitrate. Initially this might seem surprising since a mechanism involving C-NO2 homolysis should be as insensitive to acid or base additives as ones involving 0-NO2 or N-NO2 homolysis. However, it is wellknown that nitroarenes are sensitive to bases.21 A detailed study of their response is presented in a subsequent paper.2 Suffice it to say that nitroarenedecomposition is affected by both acid and base addition; and, thus, they are susceptible to ammonium nitrate which generates both nitric acid and ammonia in its initial decomposition step (Scheme 1). DSC Examination of Explosives Mixtures in Various Ratios. The exothermic maxima observed in the DSC thermograms of ammonium nitrate/monomolecular explosive mixtures of various proportions are summarized in Figures 8-12. In some cases only one exothermic peak was observed, in others, two. When two exotherms were observed, we assume that the higher temperature exotherm represents mainly the decomposition of the material which in the neat state exhibits the higher thermal stability and that the lower temperature exotherm is mainly produced by the decomposition of the less stable component. In most cases there is no clear separation between the two exotherms. Since the observed separation of two exothermic events is highly dependent upon the DSC scan rate, we do not attach great significance to those cases in which only one exotherm was observed. Focusing on the monomolecular explosives, we see that the DSC thermograms of RDX and of PETN, in all proportions, are little affected by the addition of any amount of ammonium nitrate, in line with the slight destabilization observed in the isothermal decompositions of their one-to-one mixtures (Figures 11 and 12). Furthermore, it is observed that all three nitroarenes are greatly destabilized by ammonium nitrate (Figures 8-10). Looking at the effect of just 10 wt % added ammonium nitrate, we see that trinitroaniline (TNA) is the most destabilized with a decrease in the exothermic maximum of about 100 OC (Figure 10). Considering the effect of the monomolecular explosives on ammonium nitrate, we see that at the 10 wt % level, only T N T and PETN destabilize ammonium nitrate (Figures 9 and 12). PETN, as the least thermally stable species, might be expected to have the greatest effect on ammonium nitrate, but the reason that even small amounts of T N T are enough to destabilize dramatically ammonium nitrate is not clear. Both T N A and RDX at 50 wt 56 have a large destabilizing effect, lowering the exotherm of ammonium nitrate from 328 OC to about 250 O C , but at the lOwt56levelthereislittleornoeffectontheammonium nitrateexotherm (Figures lOand 11). TATBat allconcentrations has the least effect on ammonium nitrate, in line with the trend observed in the rate constants of the one-to-one mixtures (Figure 8). As discussed above, all the monomolecular explosives produce NO2, but TATB produces little at low temperatures.
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3899
Compatibility of Ammonium Nitrate with Explosives
TotalMixtureKinetics. Normally, the typeof kinetics available for formulations containing two or more energetic ingredients is the overall kinetics of the mixture. This is because the kinetics are usually determined by total heat released, Le., a DSC study, or by total moles of gas evolved. The present study is unusual in that the kinetic data cited is for each ingredient in the mixture. To assess how well the normal overall mixture kinetics describe the true situation, the overall mixture kinetics were measured by gas evolution at one temperature for each formulation (Table 2). We found that the overall rate constant, as measured by gas evolution, was roughly an average of the rate constants of the two components, but in most cases it indicated a slightly lower rate of decomposition than the simple averageof the two rate constants. From a safety point of view, this is most unfortunate since it leads one to believe the decomposition is slower than it really is, and predictions of critical temperature22 based on such kinetics may be falsely reassuring. DecompositionProducts. When ammonium nitrate was added to the oxygen-deficientorganic explosives, their complete oxidation was promoted. Explosives are often rated in terms of their oxygen balance.23 Theconcept refers to the fact that to completely gasify an explosive, there needs to be sufficient oxygen to convert all the hydrogen to water and all the carbon to CO or C02. In a detonation, nitrogen atoms usually end up as the diatomic gas Nz. It is customary when referring to oxygen balance to state whether the balance is to CO or C02. The oxygen balance of each explosive to COZis listed in Table 3; the calculation is shown in the followingequation, where n is the number of oxygen, carbon, or hydrogen in the compound and MW is the molecular weight of the compound:
stabilizing effect on ammonium nitrate below 270 OC and a slight destabilizing effect above that temperature. It is postulated that the formation of NO2 produced by X-NO2 bond scission in the monomolecular explosives destabilizes ammonium nitrate. The degree to which a particular organic explosive destabilizes ammonium nitrate is directly proportional to the thermal stability of that organic material. Nitrate esters, being the least stable, have the most deleterious effect on ammonium nitrate thermal stability, while nitroarenes, being the most stable, have the least effect. The thermal stability of the nitrate ester PETN and the nitramine RDX is little affected by their admixture with ammonium nitrate. This is assumed to be due to their mechanism of decomposition, X-NO2 homolysis, which is little affected by intermolecular reactions. In contrast, the thermal stabilities of the nitroarenes, TNT and TNA, are adversely affected by the addition of ammonium nitrate. This is undoubtedly due to their ability to decompose by routes other than NO2 homolysis. This makes their decomposition vulnerable to catalysis by the nitric acid and/or ammonia formed in the initial dissociation of ammonium nitrate. Due to its acid sensitivity, nitrocellulose decomposition is also accelerated in the presence of ammonium nitrate. The sensitivity of HMX to added ammonium nitrate is attributed to the formation of a eutectic melt and the enhanced decomposition of HMX in the liquid phase. Measurement of the total gas evolved from the mixtures was found to be a slight underestimation of the extent of decomposition of the mixtures. Since the relative ratios of the ingredients in the mixtures were found to affect the thermal stability to varying degrees, for the sake of accurate safety assessment, the kinetics of each ingredient should be determined in the exact ratio as in the intended mixture.
OB (balanced to CO,) = [ 1600(n, - 2n, - O.Sn,)]/MW When the oxygen-deficientorganic explosives were mixed with the oxygen-rich ammonium nitrate, more gas and less residue were observed. PETN was the only species which did not form appreciably more gas when it was mixed with ammonium nitrate. However, PETN underwent almost complete decomposition to CO and CO2 without ammonium nitrate. It did, like the other organic explosives, show an increase in the COz to CO ratio, indicative of the increased oxidizing power of the mixture with ammonium nitrate. TNT was the only explosive which saw no increase in the COz to CO ratio. TNT also produced the most residue in mixture with ammonium nitrate; evidently even in that mixture there was insufficient oxidizing power available. To roughly estimate how the overall oxygen balance changed when ammonium nitrate was added, the following calculation was made for oxygen balance to C02. To represent the contribution of ammonium nitrate (AN), three was added to the number of oxygen available and four to the number of hydrogen, while 80 was added to the molecular weight of the explosive.
OB (AN mixture) = [1600((n+ 3),-2nC-0.5(n+4),)]/(MW
+ 80)
Ammonium nitrate alone decomposes to dinitrogen and nitrous oxide gases along with water. The relative ratios of the two nitrogen-containing gases depend on the decomposition temperature.4 When ammonium nitrate was mixed with the oxygendeficient explosives, the dinitrogen to nitrous oxide ratio increased since the oxygen normally found in N20 was used to oxidize the organic species.
Conclusions Over the temperature range examined (170-320 "C), ammonium nitrate is destabilized by the addition of nitrate esters PETN and nitrocellulose, nitramines RDX and HMX, and nitroarenes TNT and TNA. TATB, a nitroarene, exhibits a slight
Acknowledgment. The authors thank the Research Center of Energetic Materials at New Mexico Institute of Mining and Technology (NMIMT) for funding, Nancy Gilson for DSC analyses, and Dr. Hongtu Feng of NMIMT and Dr. Howard Cady of Los Alamos National Laboratory for their assistance in analysis and interpretation of the HMX results. References and Notes (1) Taken in part from the Master's thesis of Wen Wang, New Mexico Institute of Mining and Technology, July, 1993. (2) Part 2: Oxley, J. C.; Smith, J. L.; Wang, W., following paper in this issue. (3) Cook, M. A. The Science of Industrial Explosives; Graphic Service & Supply for Ireco: Salt Lake City, 1974; pp 12-17. Institute of Makers of Explosives 75th Annual Booklet, 1988. (4) Brower, K. R.; Oxley, J. C.; Tewari, M. P. J . Phys. Chem. 1989,93, 4029. (5) Rosser, W. A.; Inami, S. H.; Wise, H. Trans. Faraday Soc. 1964, 60, 1618. (6) Wood, B. J.; Wise, H. J. Chem. Phys. 1955,23(4), 693. Smith, R. D. Trans. Faraday. SOC.1957,53, 1341. Koper, J. H.; Jansen, 0. G.; Van den Berg, P. J. Explosiustoffe 1970, 8, 181. Feick, G. J . Am. Chem. Soc. 1954, 76, 5858. (7) Minier, L.; Brower, K.; Oxley, J. C. J . Org. Chem. 1991.56, 3306. Robertson,A. J. B.J. Trans.Faraday Soc. 1948,44,677. Tsang, W.;Robaugh, D.; Mallard, W. G. J. Phys. Chem. 1986,90, 5968. Lewis, K. E.; McMillen, D. F.; Golden, D. M. J . Phys. Chem. 1980,84,226. Gonzalez, A. C.; Larson, C. W.; McMillen, D. F.; Golden, D. M. J. Phys. Chem. 1985,89,4809. He, Y. Z.; Cui, J. P.; Mallard, W. G.; Tsang, W. J. Am. Chem. SOC.1988,110, 3754. Urbanski,T. ChemistryandTechnologyofExplosives;Pergamon Press, New York, 1985; Vol. 4, pp 162-166. (8) Sharma, J.; Hoffsommer, J. C.; Glover,D. J.; C0ffey.C. S.;Santiago, F.; Stolovy, A.; Yasuda, S.In Shock Waves in Condensed Matter; Amy, J. R., Graham, R.A., Straub, G. K., Eds.; Elsevier: New York, 1984; pp 543546. Catalano, E.; Rolon, C. E. Thermochim. Acta 1983, 61, 53. Sharma, J.;Garrett, W. L.;Owens, F. J.;Vogel, V. L. J. Phys. Chem. 1982,86, 1657. Farber, M.; Srivastava, R. D. Combust. Flame 1981,42, 165. Roger, R. N. Thermochim. Acta 1975, 11, 131. Zeman, S.Thermochim. Acta 1979, 31, 269. (9) Oxley, J. C.; Kooh, A. B.; Szekeres, R.; Zheng, W. Proc. ADPA Energetic Mater. Technol. New Orleans 1992,188. Oxley, J. C.; Hiskey, M. A., Naud, D.; Szekeres, R. J. Phys. Chem. 1992, 96, 2505-2509. (10) Melius, Carl F. Proc. 25th JANNAF Combust. Meetg., Huntsville, AL 1988.
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