Thermal decomposition of energetic materials 5. High-rate, in situ, thermolysis of two nitrosamine derivatives of RDX by FTIR spectroscopy

C O M B U S T I O N A N D F L A M E 62:233-241

233

Thermal Decomposition of Energetic Materials 5.* High-Rate, In Situ, Thermolysis of Two Nitrosamine Derivatives of RDX By FTIR Spectroscopy Y. OYUMI and T. B. BRILL Department of Chemistry, University of Delaware, Newark, DE 19716

Thermolysis of hexahydro-l,3,5-trinitroso-s-triazine (TRDX), C3H6N3(NO)3, and hexahydro-l,3-dinitro-5nitroso-s-triazine (MRDX), C~H6N3(NO2)2(NO), was studied by rapid-scan FTIR spectroscopy as a function of heating rate (8-200K s i) and pressure (1-1000 psi, 0.0068-6.8 MPa). The results are compared with those for RDX in order to determine the effect of replacing -NO2 with -NO. N - N bond homolysis liberating NO is the most effective process with TRDX, but C - N bond cleavage becomes more pronounced as the pressure increases. Near its melting point the nitramine portion of MRDX degrades via C - N bond fission before the nitrosamine portion degrades. At higher heating rates, both C - N and N - N bond fission occur. As with RDX and HMX, increasing the pressure forces the decomposition to occur more by heterogeneous condensed phase reactions than by gas-phase reactions. The nitrosamine group appears to be at least as thermally stable if not more stable than the nitramine group. The ir spectra of TRDX, MRDX, and RDX are compared for the solid and melt phases. TRDX has a " p r e m e l t " solid-solid phase transition producing a new crystal lattice in which the molecular motion and structure of the molecule are very similar to that in the melt phase.

INTRODUCTION

Nitrosamines have been implicated as products of thermal decomposition of nitramines either by direct observation [1-4] or by inference [5, 6]. For RDX this includes both slow, controlled decomposition in solution [4] at 450-480K and decomposition in drop-weight impact tests on neat samples [3]. Nitrosamines may be recombination products of the amine radical produced upon N-N bond homolysis and NO. formed by the reduction of NO2 [1], or may simply form directly by O atom transfer. The solvent may become involved when the decomposition is carried out in solution [4]. The possible relevance of nitrosamine formation in nitramine decomposition has been presented by Schroeder [71. * Paper 4 in this series is Ref. [ 12]. Copyright © 1985 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbih Avenue, New York, NY 10017

In part because nitrosamines have low oxygen content and a relatively stable N-N bond [8], they are not particularly energetic materials. On the other hand, they apparently can form during nitramine decomposition under certain conditions, and they have structural and chemical features in common with nitramines. A study of a series of compounds in which the nitramine functionality is systematically replaced by a nitrosamine advances the objective of characterizing the influence various functional groups on the decomposition of nitramines and other energetic materials [9-12]. To this end, hexahydro-l,3,5-trinitroso-s-triazine (TRDX) (also called R-salt, TNOHX, and nitrosogen) and h e x a h y d r o - l, 3 - d i n i t r o - 5 - n i t r o s o - s - t r i a z i n e (MRDX) were studied. The thermolysis of these two molecules at various pressures and heating rates was compared with the results for the energetic molecule, hexahydro- 1,3,5-trinitro-s-

0010-2180/85/$03.30

234

Y. OYUMI and T. B. BRILL

triazine (RDX). In addition, the ir spectra of the solid and melt phases of these compounds are compared and the premelt solid-solid phase transition in TRDX is discussed. NO

NO

I

I

~H2 ~H2 N N NO/ ~CH/22 \ NO

CH2! CH2 N / 2 \ CH2 / W \ N02 NO

(TRDX)

(MRDX)

NO 2 I /N\ ~H2 ~H2 N N / \CH/2 x NO2 NO 2 (RDX)

EXPERIMENTAL TRDX was synthesized according to the method of Brockman et al. [13] and recrystallized from ethanol to a constant melting point of 378K. MRDX was prepared from procedures described by Simecek [14] and recrystallized six times from CH3NO2 to a constant melting point of 446-447K. Nitrosamines, such as MRDX and TRDX, containing s-hydrogen atoms are potent carcinogens and require special care in han~ dling. The spectra of the solid samples were recorded on a Nicolet 60SX FTIR spectrometer at room temperature by spreading a thin film of the polycrystalline material between two NaCI plates. Four cm-~ resolution and 32 summed interferograms were used. The spectra of TRDX(II) and the melt phases of TRDX and MRDX were obtained with a variable temperature cell in which the sample could be spread between two NaC1 windows. A heating rate of 5K min-J was used and the temperature was monitored with a thermocouple imbedded between the salt plates. The procedures and sample cell used to obtain the high heating rate pyrolysis data were described earlier [111. Tf is the final filament

temperature. The percent concentration by volume of the products, calculated from the infrared intensities, exclude homonuclear diatomic molecules, such as N2, that are undoubtedly present, but are ir inactive. H20 was also excluded in part because its concentration is low until late in the thermolysis process or unless combustion occurred.

I N F R A R E D S P E C T R A OF THE SOLID, MELT, A N D V A P O R P H A S E S RDX Much vibrational spectroscopy has been conducted on RDX in the condensed phase [ 15-19]. Two polymorphs are known [20]. The ir spectrum suggests that the unstable ~-RDX polymorph has higher molecular symmetry (approximately C3v) than the much more stable form, c~-RDX(C0 [18]. The molecular structure of RDX in the decomposing melt phase resembles that of the ~-polymorph [19], of RDX in CD3CN and DMSO solutions [21], and of RDX aerosol [21]. The spectrum of solid c~-RDX at room temperature is shown in Fig. 1 and the assignments [16] are given in Table I.

TRDX Solid TRDX at room temperature (Fig. 1) has been studied by ir and Raman spectroscopy and tentative assignments of the modes have been made (Table I) [22]. Upon heating, a solid-solid phase transition, discovered originally by DSC [23] at 365-369K, is clearly evident in the ir spectrum of TRDX. The phase transformation by infrared occurs reversibly in the 366.5368.5K range. The spectrum of the low temperature solid, TRDX(I), the high temperature solid, TRDX(II), and the melt phase (MP = 375K) are assembled in Fig. 2. The spectrum of TRDX(II) and the melt phase are essentially identical, showing that the molecular structure and motion in the TRDX(II) lattice and the melt phase closely resemble one another. TRDX(II) is, therefore, a " p r e m e l t " solid in which rapid molecular rotation about the centroid axis of the

THERMAL DECOMPOSITION OF TRDX AND MRDX

RDX

!

,

MRDX

i"

r

3=50

r

(r

j'

=i~,' I ~' ,j ~' ,j'

J

3000

2850

3000

2850

! ' "

3150

f 'i: I' j,~ f,,, i

~,

I

' f

'J

J :' i'

r

'lI I 1660

•

1320 980 WAVENUMBER

J

/

235

Unlike TRDX, MRDX has no premonitory phase transformation before melting. The spectrum of the melt phase is merely a line-broadened version of the solid-phase spectrum, suggesting that the molecular structures in the solid and melt phases are similar. Most of the modes shift to slightly higher frequency upon melting. MRDX will decompose if the melt phase remains for any length of time. Evaporation of MRDX also occurs, producing molecules that are structurally similar to MRDX in the melt phase according to the superimposition of their ir spectra.

_

I!L ,I'L 640

Fig. 1. The mid-ir spectrum of neat solid T R D X , M R D X , and RDX shown in the absorbance mode.

s-triazine ring is occurring. A solid-state NMR study of TRDX will be reported separately [24]. TRDX immediately above its melting point exhibits no decomposition after 0.5 h. However, decomposition becomes apparent if the melt phase is heated beyond 420K for any length of time. The oily yellow residues resulting from decomposition of TRDX, RDX, and MRDX produce similar ir spectra [12]. TRDX readily evaporates from the melt as intact molecules. The spectrum of the gas-phase species could be recorded before deposition occurred on the cell windows provided that rapid-scan FTIR and high heating rates were used. Vapor phase TRDX has all of the spectral features of the solid and melt phases. The NO stretching mode shifts to slightly higher frequency while several of the ring modes shift to lower frequencies upon vaporization.

Assignments

Few of the vibrational frequencies listed in Table I for RDX, TRDX, and MRDX can be assigned to group frequencies. Complex coupling occurs that creates frequency shifts and intensity changes that are difficult to predict from molecule to molecule. The NO2, NO, and CH2 stretching motions are reasonably pure, while the other modes are complex combinations of various motions [16]. The ir spectrum of TRDX is simpler than that of MRDX and RDX mostly because it has fewer atoms. As expected, MRDX has spectral features in the NO and NO2 group frequency region that are intermediate between those of TRDX and RDX. The remainder of the spectrum in each of these compounds appears to contain mostly coupled modes complicated by overlapping band envelopes. Having examined in detail the spectra of the solid and melt phases for each of these compounds, we concluded that further study of the condensed phase would not advance the objective of understanding their behavior upon thermolysis.

THERMAL MRDX

The ir spectrum of polycrystalline MRDX is shown in Fig. 1 and tentative assignments are summarized in Table I. Figure 3 shows the spectrum of MRDX in the melt phase at 443K.

DECOMPOSITION

TRDX and MRDX sublime at atmospheric pressure and elevated temperature. This material, along with trace amounts of CO, NO2, and HONO, has been omitted from the concentration-time profiles in most cases. The methods

236

Y. OYUMI

and T. B. BRILI.

TABLE ! Infrared Frequencies (3100-700 cm-~)~ and the Tentative Assignments for Polycrystalline Solid Samples at 295K

RDX

MRDX

TRDX

Assignments [16, 22]

3080 m 3070 m

3075 m, sh 3060 m

3020m 3010 sh

}va(CH2)

2997 w 2935 w

2985 w, br

2950 w, br

}u~(CH2)

1590 sh 1572 s 1533 s

1577 s 1561 sh

.va(NO2)

1493 s

1487 s

1458m 1423 m 1389 m 1350 m

1442 1421 1368 1346

1435 m

1312m

1299 s 1290 sh

1267 s

1265 s

1267 m

v(N-N) + NO2 def.

1233 s 1218 m

1241 s 1215 s

1250 m

CH2, N - N , and ring modes

1116 w 1021 s

1157 1076 s

Ring

1038 m 1018m

m s w s

1342 s 1308m

v(NO)

CH2 def.

} v~(NO2)

960 w

997 m

940 sh 918 s 881 m

946m 910 s 879 s

953 s

843m 782 m 753 m 738 w

841 m 781 s

841 m 767 rn 701 m

CH2 rock

Ring, N - N , and NO2 876 m

735 w

s = strong, m = medium, w = weak, sh = shoulder, b = broad.

NO, NO2, N - N , and ring deformation

THERMAL DECOMPOSITION OF TRDX AND MRDX

237

employed to obtain these results are the same as those developed for RDX and HMX [11]. 298 K

TRDX 3160

]00(]

2840

N - N bond homolysis is cited to be the most important initial step during the thermal decomposition of nitrosamines [reaction (1)] [8] : R2NNO A

368 K

1

378 K 316o

50

1250

3000

950

2840

650

WAVENUMBER

Fig. 2. The ir spectrum of neat solid TRDX(I) at room temperature, solid TRDX(II) at 368K in the "premelt" solid phase, and TRDX in the melt phase (378K).

t,

298 K

I

/r~, I

443 K

I

(1)

Several studies pertaining to the rate of gas evolution and thermochemistry (DTA) of TRDX have been conducted [25-27]. The main product was concluded to be NO because exposure of the trapped gas to the atmosphere resulted in the formation of a brown gas (NO2) [25]. According to Figs. 4 - 6 , TRDX at all heating rates tested (50-170K s - l ) indeed liberates NO as the predominant decomposition product. We were perplexed by the appearance of HONO among the product gases despite the fact that no NO2 is detected. The concentration of HONO is shown in Fig. 6 because it is not negligible with the heating conditions used. Although 99.9% N2 was used for the atmosphere, 02 is inevitably physisorbed on the cell surfaces and could react with the high concentration of NO to produce NO2 and eventually HONO. The equilibrium NO + NO2 + H20 ~ 2HONO can also produce HONO in the gas phase. N20 is the second most prevalent product at all heating rates used, implying that C - N , as well as N - N , bond fission occurs. At heating

. 2850

:~

80 'ij', i :

N20

•

NO

•

CO2

@

~aa

4

~ ,% / L/

/

. 3000

/

'

"J

/ 3150

/,

~

NO.

100 /

i ,~,~,

R2N'+"

~j

N4o

/ 20 1660

1320

980

640

WAVENUMBER

Fig. 3. The ir spectrum of MRDX at room temperature compared with MRDX in the fresh melt phase.

o~

2

4

6

8 "

,o

TIME, SEC

Fig. 4. The gas product concentrations from pyrolysis of TRDX at a heating rate o f 5 0 K s I (T~ = 640K) in an N2 atmosphere (15 psi). Gaseous TRDX is not included.

238

Y. OYUMI and T. B. BRILL 100

BO

i

N20

• co2

HCN

•

NO

~ 6o

.~ 4a 2O

4

6

TIME,

8

SEC

Fig. 5. The gas product concentrations from pyrolysis of TRDXataheatingrateof100Ks I(T,. = 805K) inanNz atmosphere (15 psi). rates above 50K s-1 HCN and CO2, which are fragments from the ring, are important products from TRDX (Figs. 5 and 6). Except to enhance the amount of sublimation of TRDX, the concentration-time profiles of the products show that reducing the pressure in the pyrolysis cell to 1 psi has little effect on the decomposition. On the other hand, pronounced effects are induced by increasing the pressure. Three applied pressures (200, 500, 1000 psi of N2 gas) were tested. Some of these results are compiled in Table II along with those for RDX using a comparable heating rate, final filament temperature, and time. The most striking feature for TRDX is the fact that CH20 is a significant product at elevated pressure, whereas it is not present at atmospheric pressure. [CH20] is also greatly increased from RDX when the thermoly-

100 a N20 HCN

• NO Z

c~

• CO 2 •

CO

• HON0

6O

40

2O

2

4

6

8

Io

T I M E . SEC

Fig. 6. The gas product concentrations from pyrolysis of TRDX at a heating rate of 170K s ~(Tf = 950K) in an N2 atmosphere (15 psi).

sis is conducted under elevated pressure. Considered along with the decrease in [NO2] from RDX and [NO] from TRDX, these results are consistent with pressure enhancing the extent of the heterogeneous reactions involving the condensed phase (probably liquid in this case) and causing a greater amount of C - N bond fission relative to N - N bond fission. The relative stability of TRDX versus RDX is of interest because it has been mentioned that TRDX decomposes about 90K below RDX [Ta] despite the fact that TRDX might be expected to be at least as stable as RDX. It is difficult to compare directly the decomposition temperatures of TRDX and RDX in bulk samples because TRDX melts at 377K whereas RDX melts at about 480K. Extremely slow gas evolution from TRDX occurs at 387K [1] but the melt phase must be heated to a much higher temperature (425-450K) before thermolysis occurs at a moderate rate. On the other hand, RDX decomposes in the solid phase below its melting point (450-480K) [28, 29] with a rate that is surely impeded by the matrix effect of the crystal lattice. From these observations the thermal stabilities of RDX and TRDX are probably not particularly different. MRDX

MRDX is a crossover compound from the trinitramine to the trinitrosamine. Studying its decomposition was helpful for weighing the balance between the particulars of nitramine and nitrosamine decomposition. The initial gas products formed during isothermal decomposition at the melting point (460K) are almost exclusively N20 and CH20. N20 could originate from the NNO fragment of MRDX or from the CH2NNO2 fragment. The fact that N20 and CH20 appear in essentially equal concentrations (Fig. 7) is evidence that they come from the CH2NNO2 fragment as a result of C - N bond cleavage. Thus, a preference toward fragmentation of the nitramine portion of the molecule over the nitrosamine portion appears to exist in this molecule. According to Fig. 8, the thermolysis mecha-

THERMAL DECOMPOSITION OF TRDX AND MRDX

239

TABLE I1 The PercentCompositionsof Products fromThermolysisof RDX. TRDX, and MRDXas a Functionof Pressure Using Comparable Times, Heating Rates. and Final Filament Temperatures

Concentration (%) Pressure

Time

Temp.

Compound

(psi)

(s)

(K)

CO.,

N20

NO

CH20

HONO

NO2

HCN

RDX

15 ~ 500

9.61 8.02

645 610

4 10

22 20

5 17

15 38

8 b

32 15

14 h

TRDX

15 ~ 500

9.84

645

8

26

72

b

,

,.

b

9.82

605

7

18

53

21

b

b

b

15 o

9.84 7.78

645 605

4 4

23 49

10 12

16 34

c b

31 2

16

MRDX

500

.

.

.

.

.

.

.

Heating rate is 50K s- t. b Not observed. " Trace.

100 :,

N20 CHZD

•

CO2

6O

g 40

20

%

2'o

~o

"~o

"

5b"

60

TIME. SEC

Fig. 7. The gas product concentrations from isothermal pyrolysis of MRDX slightly above its melting point (460K) using a heating rate of 20K s-~ and an N2 atmosphere (15 psi). MRDX aerosol is not included.

50 NO I t.

, N2 o

40

,-

CH20 HCN

• ~o • C02

•

HONO

z 30

g ~20

I0

TIME, SEC

Fig. 8. The gas product concentrations from pyrolysis of MRDX at a heating rate o f 7 0 K s ~ (T, = 710K) in an N2

atmosphere (15 psi).

nism of MRDX is markedly altered if the sample is heated at a higher rate (70K s -~) and to a higher final filament temperature (710K). In addition to C - N bond fission leading to CH20 and N20, N-NO2 bond homolysis apparently becomes involved, leading to NO2 and HCN. N NO2 bond fission reduces the number of nitramine groups that are available to form CH20 + N20. Thus, the amount of NzO that is able to form diminishes. Secondary reactions involving CH20 decrease its concentration with time. It is notable that [NO] extrapolates approximately to zero at the onset of decomposition under these thermal conditions. From this observation we concluded that N - N O bond homolysis still does not occur to any great extent despite the fact that this reaction is the principal thermal degradation pathway of a nitrosamine. Instead the nitramine portion of the molecule degrades. At a still higher heating rate (170K s-~) and filament temperature (950K), Fig. 9 reveals that C - N bond cleavage producing N20 and CH20 becomes less important than N-NO2 bond homolysis which leads, eventually, to NO2 and HCN. [NO] now does not extrapolate to zero at the onset of decomposition, suggesting that N NO bond homolysis may finally be occurring. However, all of these observations seem consistent with degradation occurring preferentially

Y. OYUMI and T. B. BRILL

240 5O 40

9

N02

•

NO

c

N2O

•

C02

•

HONO CO

~ CH2fl HCN

•

z31]

"-

TIME,SEC

Fig. 9. The gas product concentrations from p y r o l y s i s of

MRDXataheatingrateofl70Ks ~(Tf = 950K) inanN2 atmosphere (15 psi). in the nitramine portion of the molecule rather than in the nitrosamine portion. As is the case with RDX [11] HONO is an early thermolysis product from MRDX at all heating rates above 70K s -1 (Tf = 710K). Its concentration is well above that from TRDX, which implies that it forms from the MRDX molecule itself and not simply by reactions of NO with trace 02 in the cell. HNCO is present at 1-3% when MRDX is heated at high rate ( > 100K s - l ) . Significant sublimation of MRDX occurs at pressures below atmospheric. As shown in Fig. 10, the concentrations of products at N - N bond fission (NO2, HCN) are enhanced while those from C - N bond cleavage (N20, CH20) diminish at low pressure. This behavior, by analogy with HMX and RDX [11], suggests that a greater 5O t~o2

•

NO

N20 • C0:

40 ..

CH20 HCN

230 e.

10

4

6

TIME, SEC

Fig. 10. The gas product concentrations from low pressure pyrolysis at a heating rate of 110K s - ' (Tf = 995K) in an N2 a t m o s p h e r e (1 psi).

proportion of the decomposition occurs in the gas phase when the pressure is low. The balance is shifted in the opposite direction when MRDX is decomposed under elevated pressure. Pressures up to about 1000 psi were tested in this work. As shown in Table II, the concentrations of N20 and CH20 greatly increase while those of NO2 and HCN decrease. This observation is consistent with the interpretation that increasing .the pressure forces the decomposition to occur more by heterogeneous reactions involving the condensed phase rather than the gas phase because the diffusion rate of the gases is reduced. CONCLUSIONS a. A nitrosamine does not necessarily decompose like a nitrosamine when it is in the presence of a nitramine. The principal mode of thermal decomposition of TRDX is, as expected, NO elimination. However, N - N O homolysis does not occur in MRDX (until it is heated very vigorously) in the face of preferential degradation of the N-NO2 portion of the molecule. The observations in this work are consistent with the nitrosamine group having stability equal to to somewhat greater than the stability of the nitramine group when the molecular environments are comparable. b. At higher pressures and lower heating rates there appears to be a preference for decomposition in the heterogeneous condensed phase (gas/liquid) leading to products reminescent of C - N bond fission. At lower pressures and higher heating rates, the decomposition may occur more in the gas phase, leading to products that appear to be derived mostly from N - N bond fission. This crossover between largely condensed phase to largely gas-phase decomposition is similar to the situation found with RDX and HMX. c. The large quantity of NO emanating from the thermal decomposition of nitrosamines is believed to be pertinent to their combustion rate [30]. Most nitrosamines burn more slowly than nitramines [30] perhaps because

THERMAL DECOMPOSITION OF TRDX AND MRDX

the activation energy for reduction of NO is relatively large [31].

We are grateful to the Air Force Office o f Scientific Research, Aerospace Sciences (AFOSR-80-0258), f o r support o f this work.

15.

16. 17.

18.

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Received 8 February 1985; revised 28 May 1985