Thermal decomposition of energetic materials 73: the identity and temperature dependence of “minor” products from flash-heated RDX

Thermal Decomposition of Energetic Materials 73: The Identity and Temperature Dependence of “Minor” Products From Flash-Heated RDX P. E. GONGWER and T. B. BRILL*

Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716 The approximate identity and temperature dependence of volatile “minor” products (defined as ,4% mole fraction) from thermal decomposition of hexahydro-1,3,5-trinitro-s-triazine, RDX, were determined by heating a film at 800°C/sec under 4 atm Ar. The IR spectra from pyrolysis at specific temperatures in the 265–325°C range were resolved by multivariate regression, which enabled the major products to be removed and the minor products to be uncovered. The gaseous phase contained hexahydro-1-nitroso-3,5-dinitro-s-triazine (MRDX), a triazine modeled as s-triazine (TAZN), C-hydroxyl-N-methylformamide (HMFA), and both RDX vapor (RDXv) and aerosol (RDXs). The behaviors of HONO and HNCO are also discussed because they have mole fractions below 4%. The concentrations of MRDX, HMFA, RDXv, and RDXs decrease with increasing temperature. HONO and TAZN maintain relatively constant concentrations. HMFA and HNCO are oppositely correlated, suggesting that HNCO comes from HMFA. The relation between this work and previous studies of slower decomposition of RDX and on quenched burning of RDX-containing propellants is discussed in an attempt to unify the description of amides and nitrosoamines in the RDX decomposition scheme over a wide range of heating rates. © 1998 by The Combustion Institute

INTRODUCTION When hexahydro-1,3,5-trinitro-s-triazine (RDX) thermally decomposes, a variety of gaseous products including NO2, NO, N2O, N2, CO, CO2, CH2O, HCN, H2O, and H2 are liberated in quantities that depend on the reaction time, temperature, and pressure. For a number of years it has also been known that, in addition to these nominally gaseous products, some higher molecular weight products are also produced [1–21], especially when RDX is heated slowly. In most cases these products include a variety of amides and nitrosoamines along with a few other types of compounds related to the RDX structure. More recently, nitrosoamines have been identified in the condensed phase of quenched samples that had been subjected to much faster heating conditions, such as in dropweight impact of pure RDX [9] and from burning of RDX-containing propellants [15–17]. Thus, some of these larger molecules appear to be relevant to the combustion regime. The heating rates and reaction temperatures used in most thermal decomposition experiments, such as the detailed studies of Behrens and Bulusu [11, 12] are markedly lower than *Corresponding author. COMBUSTION AND FLAME 115:417– 423 (1998) © 1998 by The Combustion Institute Published by Elsevier Science Inc.

those expected on the surface of burning propellants, such as in the work of Schroeder et al. [15–17]. Thus, a bridging experiment might be T-jump/FTIR spectroscopy [22] in which controlled flash-heating conditions can be conducted while identifying and quantifying the volatile products. In previous studies of RDX decomposition by this method, however, we have not described finding any of these larger molecules [23, 24]. On the other hand, weak IR absorbances not attributable to the gaseous products listed in the foregoing paragraph were detected in the gas phase and were attributed mostly to an RDX aerosol. Some attention was also focused on the fact that the gaseous products do not appear simultaneously in the timeresolved IR spectrum, from which it can be concluded that intermediate “residue” compounds must be present [23, 24]. Unfortunately, it was not possible to identify these compounds because their absorbances mostly overlapped the more intense absorbances of the major gaseous products. Multivariate regression methods have recently been developed to resolve the IR spectra quantitatively in this experiment [25, 26]. The major products can be mathematically removed, which makes it possible to uncover the minor “buried” products. Amides, nitrosoamines, and 0010-2180/98/$19.00 PII S0010-2180(98)00011-X

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other products are indeed observed, although with rather low concentrations, in the gaseous phase from flash pyrolysis of RDX. By identifying these products (at least by general class) and determining their dependence on the filament temperature, the intention of this article is to help close the connection between the decomposition chemistry of RDX in the slow and fast heating regimes. BACKGROUND Amides [3, 4, 8, 11, 12, 14, 18, 19], nitrosoamines [1, 2, 5–7, 9 –18], and triazines [7, 11–13, 18, 20, 21], in addition to lower molecular-weight gases, are known to form when RDX is heated. Many of these higher molecular-weight products have been identified by mass spectrometry and chromatography, but IR, microwave, and photoelectron spectroscopy have also been employed. The product identities frequently differ among most previous studies, which reflects their probable dependence on the decomposition conditions and method of identification. The work of Behrens and Bulusu [12], however, reveals that amides and nitrosoamines can be major products when RDX is slowly heated. The work of Schroeder et al. [16] suggests that nitrosoamines occasionally occur in as much as 20% of the unreacted RDX on the surface of propellants containing RDX that have been quenched from the burning state. The dominant nitrosoamine found with both slow and fast heating is MRDX (Fig. 1). Other products previously suggested to form when RDX decomposes are also shown in Fig. 1, although this list is not a complete one. For example, fragment ions reported by mass spectrometry are not included. The dinitroso relative of MRDX (DRDX) is also not shown. DRDX has been found in low concentration after chromatographic separation of RDX decomposition products in a quenched propellant [16]. s-Triazine (TAZN) is shown in Fig. 1 and has been found by Schroeder [20] and suggested by Zhao et al. [21]. The work of Behrens and Bulusu [11, 12] indicates, however, that one or more isomers of oxy-s-triazine is present. No synthesis procedure for any of the putative oxy-derivatives of triazine was found in the

Fig. 1. Some previously identified volatile, higher-molecular-weight products from heating of RDX. Only a few of these compounds were indicated from experiments in this article.

literature and so the spectrum of TAZN was used in this article. EXPERIMENTAL T-Jump/FTIR Spectroscopy Approximately 0.2 mg of RDX was thinly spread on the center of a Pt filament and inserted into the spectroscopy cell [22]. The area covered by the sample is estimated to produce a film thickness of about 50 –75 mm. The cell was purged and then pressurized to 4 atm Ar. The filament was heated at 2000°C/s and stopped and held for 10 sec at set temperatures between 265 and 380°C by the use of a high-gain power supply. The 265–325°C range is discussed in this article. The actual heating rate of the RDX sample is approximately 800°C/s because of the heat capacity of the RDX and heat transfer limitations between the sample and filament. The true temperature of the Pt filament was determined for different applied voltages by calibrating the filament with compounds having standard melting points. Following pyrolysis of RDX no residue remained on the filament. The IR beam of the spectrometer was positioned about 3 mm above the surface of the filament. This enables the evolved reaction

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products to be quenched by the Ar atmosphere before they reach the IR beam. The IR spectra of the evolved pyrolysis products were recorded at 0.1 sec intervals and 4 cm21 resolution with a Nicolet 800 FTIR spectrometer. Further details of this experiment and physical models of the process appear elsewhere [22, 27, 28]. Analysis of the Volatile Pyrolysis Products A first-order spectral resolution method using quadratic-programming (QP) least-squares optimization [25, 26] was used to identify and quantify the IR-active pyrolysis products of RDX. Equation (1) was solved with the QP routine for the concentrations of each product, m 5 Ec~t! 2 r~t!

(1)

c(t), associated with a time series of IR spectra, r(t), using a separately calibrated absorptivity matrix containing each product, E, (vide infra) by minimizing the residual m. In some previous work [25], a non-negative least-squares (NNLS) routine was also utilized to solve Eq. 1 for the reaction products, since NNLS has a built-in constraint that prevents solutions with unrealistic negative product concentrations. The nonnegativity constraint, however, prevented incorporation of a baseline model into the NNLS routine, since negative coefficients of baseline equations were also forbidden. QP optimization [26] was used in this work so that the baseline portion of the solution could be unconstrained and thereby allow negative coefficients, while the concentration portion of the solution was manually constrained to be non-negative. The baseline correction model used a combination of zeroth-, first-, and second-order polynomials over the entire spectral range (600 – 4000 cm21). A great advantage is gained by the use of the entire spectrum as opposed to a single wavelength to determine concentrations of species in the spectrum. Multivariate regression in this manner enables overlapped absorbances to be resolved because the entire spectrum, as opposed to single absorptions, of individual components is used to obtain an optimum fit of the composite spectrum. The absorptivity matrix in Eq. 1 contains a calibrated IR spectrum for each identified IR-

active product. Possible decomposition products of RDX were identified from the IR spectra by comparison with literature spectra, chemical intuition, and previous reports of product identities in Fig. 1. Of these, only RDX, MRDX, HMFA, and TAZN had finite concentrations. Inclusion of the other compounds in Fig. 1 did not improve the fit and therefore will not be discussed further. The spectra of the identified products were calibrated by determining their concentration-absorptivity relations based on the concept of the Lambert-Beer Law. Most products that are gaseous at standard temperature and pressure could be calibrated simply by using different partial pressures of the gas in Ar in the IR reaction cell. Some products required additional or different procedures as described individually below. The concentration-absorptivity relation for CH2O was determined by flash pyrolyzing paraformaldehyde and taking the ratio of the IR absorbance spectrum obtained to the published absorbance spectrum of a known concentration [29]. Similarly, hydrogen isocyanate (HNCO) was calibrated by flash pyrolyzing cyanuric acid and comparing the corresponding absorptivity to those of the isoelectronic CO2 and N2O molecules. The absorptivity of the asymmetric stretching frequency of N2O is 0.55 that of CO2, so HNCO is likely to have an absorptivity in the range between CO2 and N2O. A value of 0.8 that of CO2 was chosen. The concentration-absorptivity relation for H2O was determined using a heated spectroscopy cell fitted with ZnSe windows. An excess of water was placed in the cell, which was heated to set temperatures in the 25–50°C range. The vapor pressures of H2O at 25–50°C [30] were used to calculate the concentration associated with the IR spectra from the heated cell. Because of the propensity of water molecules to aggregate, the H2O spectrum in the heated cell had a broad, intense peak in the 2OH stretching region characteristic of liquid H2O. To eliminate this feature, the spectrum of H2O was taken in the IR reaction cell by placing a small amount of water on the Pt filament and gently heating it to vaporize the water. This spectrum had a narrower 2OH absorbance characteristic of H2O vapor and contained an amount of H2O that was representative of the quantities evolved

420 in the pyrolysis experiments. This spectrum was then ratioed against the spectra calibrated using the heated cell, utilizing areas of the peaks that were not broadened by the hydrogen bonding to scale the spectrum according to concentration. The IR spectrum of nitrous acid (HONO) was obtained with a reaction apparatus similar to that used by Febo et al. [31]. HCl gas was flowed through sodium nitrite powder, which was supported on a frit. The resulting gaseous product was HONO mixed with HCl. The HCl could be easily subtracted from the spectrum. The concentration-absorptivity relation for HONO was determined by scaling the spectrum obtained for the generated HONO by the appropriate absolute intensity factors given by Kagann and Maki [32]. Hydroxylmethylformamide (HMFA) was synthesized by mixing equimolar quantities of formamide and formaldehyde in an SS tube and heating in a fluidized sand bath at 140°C for 5 hours [8]. The resulting liquid HMFA was placed in an IR cell between two ZnSe windows with a known pathlength. Molar absorptivities of several absorbances were calculated assuming the Lambert-Beer Law, where the absorbance was determined using the peak area. The spectrum used in the absorptivity matrix, E, was a vapor phase spectrum generated in the IR reaction cell by placing a small amount of HMFA on the Pt filament and gently evaporating it. This spectrum was calibrated by comparing the molar absorptivity values from the liquid phase spectrum with the corresponding peak absorbance areas of the vapor phase spectrum taking into account the path length of the reaction cell. The IR spectrum of 1,3,5-triazine (TAZN) was obtained by placing a measured mass of TAZN on the Pt filament in the IR reaction cell and gently heating it to vaporize the TAZN. By using a known amount of sample, the concentration corresponding to the IR spectrum was calculated by dividing the molar amount of TAZN by the volume of the reaction cell. The solid phase spectrum of RDX (RDXs) was quantified by depositing a known amount of RDX in acetone solution onto a BaF2 window and then evaporating the acetone. The concentration-absorptivity relation was determined by using apertures of known diameter to control

P. E. GONGWER AND T. B. BRILL

Fig. 2. A: An IR spectrum of the gaseous phase 10 sec. after a film of RDX was heated at about 800°C/s to 325°C under 4 atm Ar. B: The calculated (best fit) spectrum including only NO2, N2O, NO, CO, CO2, CH2O, H2O, HCN, HONO, and HNCO. C: The residual (spectrum A minus spectrum B) showing the existence of other products.

the IR beam diameter impinging on the deposited RDX film and then dividing the resulting IR spectrum by the volume of the IR reaction cell. The vapor phase spectrum of RDX (RDXv) was obtained by placing an excess of RDX in a heated IR spectroscopy cell and slowly heating the cell to 175°C [33]. The concentration-absorptivity relation for the resulting IR absorbance spectrum was then determined using the vapor pressure of RDX and the van’t Hoff equation [34 –36]. The spectrum of 1-mononitroso-3,5-dinitrohexahydro-1,3,5-triazine (MRDX) was obtained by depositing an acetone solution of MRDX onto a BaF2 window and then evaporating the acetone. The concentration-absorptivity relation was determined by the same procedure used above to calibrate RDXs. RESULTS AND DISCUSSION Figure 2 shows the IR spectrum of the gas phase over RDX that was heated at 800°C/s to 325°C and held for 10 seconds. Similar spectra were obtained at 265–325°C. In no case did any residue remain on the filament at these temperatures. Spectrum C in Fig. 2 is the residual after subtraction of spectrum B, which contains the optimized concentrations of the previously identified and discussed [23, 24, 37] gases NO2, N2O, NO, CO2, CO, CH2O, H2O, HCN, HONO, and HNCO. Of these products only

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Fig. 3. The equivalent of Fig. 2 in which MRDX, HMFA, TAZN, RDXv, and RDXs are included in the calculated spectrum. The residual spectrum contains mostly noise.

HNCO and HONO will be discussed further in this article. It is apparent from spectrum C that additional volatile products are present. We previously attributed these mostly to RDX vapor (RDXv) [23]. Figure 3 shows the calculated spectrum when all of the products used in Fig. 2B, as well as RDXv, RDXs, MRDX, HMFA, and TAZN (Fig. 1), are included in the optimization. The residual spectrum C in Fig. 3 is distinctly improved over that in Fig. 2. Ideally, spectrum C in Fig. 3 would entirely consist of noise, but we were not able to improve it by the inclusion of any of the other previously identified compounds in Fig. 1. In fact, the addition of other compounds in Fig. 1 did not contribute to the fit and thus are statistically excluded under flash pyrolysis conditions. The existence of absorbance intensity above the noise level in Fig. 3C has several potential sources. The calibration procedures for TAZN and RDXs used a film of material as opposed to an aerosol, which is more probably present in the gas phase during flash pyrolysis. Moreover, an oxy-derivative of TAZN may be present as indicated by Behrens and Bulusu [11, 12]. It is noteworthy that residual intensity exists in spectrum C of Fig. 3 at 1250 –1300 cm21. This is the range expected of the N-O stretch of a nitroxide [38]. In addition the 1620 –1680 cm21 range is imperfectly fit. This is the range where the intense amide I mode of aliphatic amides absorbs. Hence, either a small calibration error or a slightly different amide than HMFA (although apparently not one of those in Fig. 1) could account for the residual intensity. With the above cautionary comments in mind, it is appar-

Fig. 4. The mole fractions vs the filament set temperature showing the temperature dependence of the “minor” pyrolysis products of RDX.

ent by Fig. 3 that the inclusion of several minor products into Eq. 1 clearly improves the spectral fit of the pyrolysis gases of RDX, although it cannot be said with certainty that a quantitative accounting has been achieved. The temperature-dependence details of the products having mole fractions below 4% in the fit of Fig. 3 are shown in Fig. 4. The data points shown are the average of three replicate determinations and, because of the low concentrations involved, have uncertainties in the 10 – 50% range. As a result, the trends are worthy of discussion, but the “sawtooth” behavior of several of the products is not statistically significant. Perhaps the most prominent trend in Fig. 4 is the fact that most of the highest molecular weight volatiles (HMFA, RDXv, RDXs, and MRDX) decrease in concentration with increasing temperature. Further understanding of this pattern benefits from consideration of the main controlling factors in this experiment. The tendency for products to be detected in the gaseous phase using T-jump/FTIR spectroscopy is determined mostly by the competition among three factors: their formation rate, their decomposition rate, and their evaporation rate (vapor pressure). Since the vapor pressure of these materials increases with temperature, the observed decrease in the concentration of HMFA,

422 MRDX, RDXs, and RDXv in Fig. 4 argues against the relative evaporation rates as the controlling parameter in the observation of these products. Instead, their appearance is more likely to be a function of the relative rates of formation and decomposition in the condensed phase. The fact that the overall concentrations of these higher molecular-weight products decrease with temperature implies that their formation rate is slower than the other channels of decomposition of RDX and/or their destruction rate increases faster than their formation rate at higher temperatures. Either way less of the higher molecular-weight products will appear in the gaseous phase at higher temperature. It is worth mentioning that most of the decomposition products of these higher molecular-weight products resemble the decomposition products of RDX (for example, see studies of MRDX [39, 40]). Consequently, it is difficult to apportion the roles of the individual parent compounds in contributing in the final, lower molecular-weight, gaseous products. The trends in the HMFA, RDXv, RDXs, and MRDX concentrations in Fig. 4 are consistent with the observation that relatively slow pyrolysis at lower temperatures produces much larger concentrations of MRDX and triazine-like molecules [12]. On the other hand, MRDX appears in concentrations of 1–20% of that of unreacted RDX on the surface of RDX-containing LOVA propellants that have been quenched from combustion [15–17]. Figure 4 suggests that only a few percent of MRDX might be expected. Three explanations for this difference seem plausible. First, it may be that the use of vaporphase IR spectroscopy to estimate the quantity of MRDX in near real time is simply not comparable to solvent extraction of the surface of a quenched composite propellant. Second, the other components of LOVA propellant may affect the decomposition characteristics of RDX. Third, some MRDX may be formed in the “cool-down” stage of the quenched propellant, thereby increasing the concentration to higher than the steady-state value during combustion. There is no ambiguity, however, in these two experiments that at least some MRDX is present in the RDX decomposition scheme at temperatures representative of the surface during burning.

P. E. GONGWER AND T. B. BRILL A second notable trend in Fig. 4 is the fact that HMFA and HNCO are oppositely correlated. This suggests that HNCO mainly comes from the pyrolysis of amides like HMFA as the temperature is increased. The NCO linkage already exists in these amides making it relatively easy to liberate HNCO. No other isomers of the NCO linkage are detected in any of our work. The decomposition of amides as the source of HNCO was previously suggested from experimental data at both fast [23, 41] and slow heating rates [42]. Neither HONO nor TAZN exhibit much temperature dependence in the range studied in Fig. 4, although HONO is moderately less prevalent at higher temperatures. A third indicative feature of Fig. 4 is the fact that RDXv and RDXs decrease more or less in parallel with increasing temperature. This trend suggests that RDXs is an aerosol that most likely results from condensation of RDXv in the cooler Ar atmosphere above the filament. RDX is expected to have a significant vapor pressure [34 –36] in this temperature range.

CONCLUSIONS Volatile amides, nitrosoamines, and triazinelike molecules resulting from partial decomposition of RDX are detected in low concentrations in the vapor phase upon flash-heating of an RDX film. The concentrations of these products generally decrease with increasing temperature, which is consistent with the view that they either do not have time to form in large concentration or they have lower steady-state concentration as a result of decomposition as the temperature is increased. Although inconsistencies still exist in the details when trying to connect these results to slow decomposition on one side and the combustion regime on the other, the general pattern in terms of their relevance and concentration behavior as a function of temperature seems to be in hand. We are grateful for support of this work by the Army Research Office on DAAL03-92-G-0118 (R. W. Shaw, program manager) through a subcontract with Pennsylvania State University.

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Received 23 October 1997; revised 12 January 1998; accepted 14 January 1998