Condensed-phase processes during combustion of solid gun propellants. II. nitramine composite propellants

Condensed-Phase Processes during Combustion of Solid GunPropellants. II. Nitramine Composite Propellants

M. A. SCHROEDER,* R. A. FIFER, M. S. MILLER, R. A. PESCE–RODRIGUEZ,C. J. S. MCNESBY, G. SINGH, and J. M. WIDDER

U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5066, USA

Burning samples of nitramine composite solid gun propellants were quenched, and the burned surfacesexamined microscopically and by chemical analysis. Studies were carried out on XM39 and M43 propellants, onan HMX-polyester composition (HMX2) and on pure RDX, at pressures ranging from atmospheric (0.1 MPato 2.0 MPa). Scanning electron microscopy examination of quenched samples burned at these low pressuresindicates that a liquefied layer 100 to 300 m thick forms during combustion of these nitramine compositions.Bubbles are present, especially at the lower pressures. Gas chromatography mass spectrometry analysis suggeststhat in the case of XM39, ethyl centralite stabilizer is depleted in the surface layers relative to the plasticizeracetyl triethyl citrate. High-performance liquid chromatography studies indicate that for XM39 (and presum-ably for M43), HMX2, and RDX, the surface layers exhibit formation of the mechanistically significantnitrosoamines MRDX (also known as ONDNTA) and DRDX. Examination of the burned surfaces of XM39and of HMX2 by photoacoustic Fourier-transform infrared spectroscopy and by microreflectance Fourier-transform infrared spectroscopy indicates the presence of increased amounts of binder and its decompositionproducts. Examination of the burned surface of RDX by photoacoustic Fourier-transform infrared indicates thepresence of RDX decomposition products. These observations suggest the occurrence of a significant amountof condensed-phase decomposition. Depth profiling, by surface-abrasion and by cross-section examination,indicates that in the nitramine propellants the molten oxidizer layer is overlain by a layer (;20 m thick) ofbinder and its decomposition products. The roles of vaporization and of thermal decomposition in the liquidlayers of the nitramine propellants are also discussed; it appears that both play significant roles. © 2001 by TheCombustion Institute

INTRODUCTION

This paper describes work aimed at understand-ing the nature and importance of condensed-phase reactions in the combustion of solid nit-ramine and other gun propellants. Thisinformation is needed as input for modelingstudies, and could also be very important inunderstanding the relationship of chemicalstructure and of physical properties such asmelting point, phase-transition temperatures,etc., to explosive and propellant behavior.

The literature contains a number of papersdescribing microscopic examination of burnedsurfaces of propellant grains of HMX and com-positions derived therefrom, and of “hot spots”in explosives [1–16]. In general, it appears thatthere is a liquid layer during combustion ofthese materials, and that this liquid layer dimin-ishes and disappears with increasing pressure.

There are also a number of papers describingchemical analysis of hot spots, and of the

burned surface of nitrate ester propellants [4–6,8–10, 17]. However, as far as we are aware,chemical analysis of burned surfaces has notbeen applied to nitramines or nitramine propel-lants; although in one study [11], the surfacelayers of a quenched RDX-polyester composi-tion were extracted with benzene and acetone,and the presence or absence of a residue undervarious conditions was noted. It was suggestedthat the variations in burning surface with par-ticle size indicated an increase in surface tem-perature with decreasing particle size.

After much of the present work had beencompleted, extensive studies of laser-assistedcombustion of XM39 and M43, with quenchingby deradiation, were reported [18, 19]. Thesestudies have resulted in a number of interestingcorrelations between melt layer thickness, num-ber and size of bubbles on the one hand, andsuch factors as pressure and radiant flux on theother. For example, the bubble size distributionshifted toward smaller diameters at higher pres-sures, and the foam layer thickness tended todecrease at higher pressures. Very recently [20],* Corresponding author. E-mail: [email protected]

COMBUSTION AND FLAME 126:1577–1598 (2001)© 2001 by The Combustion Institute 0010-2180/01/$–see front matterPublished by Elsevier Science Inc. PII S0010-2180(01)00267-X

ultraviolet (UV) and fine-wire thermocouplestudies of the temperature profiles of self-defla-grating RDX in an N2 environment at pressuresup to 0.79 Mpa were described. Depending onpressure the temperature at the bottom (melt-ing) side of the liquid/foam zone was found tobe ;500 to 520 K (227–247°C), and to be ;590to 690 K (317–417°C) at the top (vapor-phase)side. Finally, combustion behavior (includingscanning electron microscope [SEM] examina-tion of burned surfaces of samples quenched bydepressurization) and thermochemical proper-ties of XM39 and M43 were reported [21].

In the present work, the samples examinedwere burned and quenched by several proce-dures; these are described below. Thesequenching procedures were used in order toavoid disruption of the burning surface by sud-den depressurization. Samples obtained in theseways were examined microscopically and ana-lyzed chemically. For microscopic examination,the samples were cleaved parallel to the grainaxis and the cleaved surfaces and the top(burned) surfaces were examined with a SEM.In addition, the surface layers were removedfrom the extinguished propellant grains byscraping with a small, sharp knife. The resultingscrapings were analyzed by spectroscopic andchromatographic methods such as gas chroma-tography mass spectrometry (GCMS) and high-performance liquid chromatography (HPLC).The top (burned) surfaces of the samples were

also examined under a Fourier-transform infra-red (FTIR) microscope.

The present paper describes the results ofmicroscopic, chromatographic, and spectro-scopic studies on burned/quenched samples ofXM39, for M43, and on their principal ingredi-ent (RDX), as well as on HMX2, an HMX-polyester composition. Studies were also carriedout on a series of nitrate ester propellantsincluding JA2, M9, and M30; this work is de-scribed in a companion paper [23]. A prelimi-nary description of this work, including somefurther information, is given in a series of BRLand ARL Technical Reports [24–28].

EXPERIMENTAL

Starting Materials

The compositions of the propellants and formu-lations used are summarized in Table 1. Propel-lant and ingredient samples used were standardcompositions. Lot numbers and grain descrip-tions were as follows: XM39, Cl0885–200–1 or1H-XM39–0988–100A1, or 1H-XM39–0988–100A3, cylindrical, 1/4 3 1/4 in, 19-Perf. orcylindrical, 1⁄2 3 1⁄2 in, unperforated; M43, IH-HELP-0988–131 B3, cylindrical, 1⁄2 3 1⁄2 in,unperforated. The HMX2 composition [29] wasa composition containing 80% HMX and 20%polyester binder. It was received as unperfo-

TABLE 1

Compositions of Propellants Studied

PropellantOxidizer particle

size (mm) ComponentPercentageof total (%)

XM39 20 RDXa 76.0Cellulose Acetate Butyrate (CAB) 12.0Acetyl Triethyl Citrate (ATEC) 7.6Nitrocellulose (NC) (12.6% N) 4.0Ethyl Centralite (EC) 0.4

M43 20 RDXa 76.0Cellulose Acetate Butyrate (CAB) 12.0Plasticizer 7.6Nitrocellulose (NC) (12.6% N) 4.0Ethyl Centralite (EC) 0.4

HMX2 100 HMX 80.0Polyester (PE) 20.0

a The RDX used in these materials generally contains 5 to 10% HMX.

1578 M. A. SCHROEDER ET AL.

rated sticks 4-in long and 1/4-in on a side, whichwere cut to lengths of ;1/4 in for our experi-ments. RDX was Class-A RDX and was pressedinto 1⁄2 3 1/2-in cylindrical pieces of 91% theo-retical maximum density; these pieces werefurther cut and shaped into cylindrical piecesmeasuring 1/4 3 1⁄4 in.

Sample Preparation

The samples were burned following one of anumber of procedures; these included the fol-lowing: a) The grain was attached to a massivecopper stub, ignited in a strand burner andallowed to burn down to the copper stub. As theburning surface approached the copper stub,quenching occurred as a result of conduction ofheat away from the burning grain by the copperstub, as described by Novikov and Ryzantsev[22]. b) Low-pressure, self-extinguished samplesof XM39 and M43 were burned with hot-wireignition in a strand burner under nitrogen, atpressures of 0.2 to 0.3 MPa; at these pressures,the samples burned for a short period of time,then extinguished spontaneously due to the lowpressure (Videotapes show that the sampleswere in fact burning, not just decomposing). Atthe time of self-extinguishing, the combustionhad left a depression in the end of the grain buthad not yet spread across the entire end of thegrain. c) One end of the grain was ignited in airby contact with a candle, the burning end wasallowed to burn for several seconds and thegrain was dropped into water. In addition, d)several samples of XM39 were obtained whichhad, for unknown reasons, extinguished sponta-neously while being burned in the strand burnerat a pressure of 1.0 MPa under nitrogen; thesesamples were studied also, and are included inthe tables. After being extinguished, the grainswere split as described previously [23]

Microscopy and Chemical Analysis

One half of the split grain was preserved intactfor microscopic examination and the surfacelayers of the other piece were removed byscraping with a small knife. SEM and micro-reflectance FTIR microscopy were carried outas described previously [23]. Depth profiling by

surface abrasion was carried out as describedpreviously [23].

The acetone-soluble portions of the scrapingswere analyzed by GCMS and by HPLC. TheHPLC apparatus was a Perkin—Elmer Series 4(Perkin—Elmer, Norwalk, CT, USA) fitted witha C-18 column and interfaced to an LC-85spectrophotometric ultraviolet detector operat-ing at 254 nm. The injection solvent was acetoneand the mobile phase was 3:1 water-methanol.The GCMS apparatus consisted of a Hewlett–Packard 5970 mass selective detector coupled toa Hewlett—Packard 5890 gas chromatograph(Hewlett—Packard, Palo Alto, CA, USA) con-taining an Alltech column (Alltech, Deerfield,IL, USA) of the following description: 30 mlong, 0.25-mm inner diameter, Heliflex, BondedFSOT, RSL-150, Stock No. 13639 . The carriergas was helium. The oven program was asfollows: initial hold time, 3 min at 50°C; Heat to225°C at 35°C/min; hold 15 min at 225°C.

Depth-Profiling by FTIR-PAS

In some cases burned surfaces were examinedby photoacoustic Fourier-transform infraredspectroscopy (FTIR-PAS). FTIR-PAS spectrawere obtained on a Mattson Sirius 100 Spectrom-eter (Thermo Mattson, Madison, WI) using anMTEC 100 photoacoustic cell . The velocity of theinterferometer moving mirror was 0.084, 0.115,0.316, or 0.633 cm/s; the corresponding Fourierfrequencies were determined from these values.Where stated, sampling depths were then esti-mated from an existing table [30] of samplingdepths vs. thermal diffusivities and frequencies,using the fact that thermal diffusivities for thesepropellants have recently been measured [32, 33]and found to be in the range of 0.001 to 0.003cm2/s. All spectra were obtained after thoroughlypurging the photoacoustic cell with helium. Spec-tra were measured at either 4-cm21 or 2-cm21

resolution and are the result of 64 co-added scans.

RESULTS AND DISCUSSION

SEM

Typical SEM photographs of the surfaces ofquenched samples of XM39 and M43 are shown

1579CONDENSED-PHASE COMBUSTION PROCESSES II

in Figs. 1 to 4. A summary of observations basedon photographs such as these is presented inTable 2.

Chemical Analysis

HPLC peak area ratios for RDX, its mononi-trosoamine derivative (hexahydro-1,3-dinitro-5-nitrosotriazine [MRDX][ONDNTA]) and its di-nitrosoamine derivative (hexahydro-1,3-dinitroso-5-nitro-triazine [DRDX]) are given inTable 3 for burned and unburned samples ofXM39 propellant, pure RDX, and HMX2. Thistable also includes peak areas ratios for anunknown peak referred to as NHMX(?), which,based on its retention time relative to HMX(present as impurity in the RDX), could possi-bly be a nitrosoamine arising from replacement

of one or more nitro groups of HMX by anitroso group.

Tables of GCMS peak areas for stabilizer(diethyl centralite) and plasticizer (ATEC) fromXM39 burned-layer scrapings and of unburnedXM39 are given in Table 4; this table alsoincludes stabilizer-plasticizer area ratios.

Several weak unknown peaks were also ob-served in the HPLC and GCMS chromato-grams. Retention times and masses for thesepeaks are summarized in Table 5. Typical chro-matograms for XM39 propellant are shown in apreliminary report on this work [24].

Typical FTIR-PAS spectra of unscraped,burned surfaces, and of unburned samples ofHMX2 and of RDX are given in Figs. 5 to 7.Because of the manner in which FTIR-PASspectra are obtained, these figures show theactual spectra of the top few microns of thesurface layers of the samples, with the largestcontribution coming from the layers closest tothe surface [28, 30].

Depth Profiling by Surface Abrasion

Results are exemplified in Fig. 8, which showstypical microreflectance FTIR spectra as a func-tion of depth removed for XM39, and aresummarized in Table 6.

Depth Profiling by Cross-Section Examination

Microreflectance FTIR spectra were obtainedas a function of depth for cleaved samples ofHMX2; spectra were taken on the surface ofunburned material, and on the burned surfaceand at depths of 0 to 40 and 40 to 80 mm on thecleaved surface of the extinguished sample.Results are summarized in Table 5.

Column 2 of Table 1 shows the particle sizesof the oxidizers in the propellants under study;this information was obtained from the propel-lant description sheets or by SEM examinationof the propellant in question. This informationis furnished so that, in combination with thearea examined (see Figure captions and Exper-imental section), the reliability of the microre-flectance FTIR spectra as average representa-tions of the sample can be evaluated.

Fig. 1. SEM photograph (3150) of cross-section of burnedsurface of XM39 (burned in air, water-quenched)

Fig. 2. SEM photograph (3370) of burned surface of XM39(2.0 MPa, copper-quenched)


Nature of Condensed-Phase Surface Layers

This section summarizes the results of examina-tion of the surface layers of the quenchedsamples, followed by a brief discussion of thechemical and physical nature of the condensed-phase surface layers in nitramine compositepropellant combustion.

SEM Results

Typical SEM photographs of the extinguishedsurfaces of XM39 and M43 are shown in Figs. 1to 4 and are summarized in Table 2.

Figure 1 is a view of the cross-section of theburned surfaces of a grain of XM39. This sam-ple was burned at atmospheric pressure in air,then quenched by dropping it into water. Thissample shows considerable evidence for thepresence during combustion of a liquid layerwith a thickness of ;100 to 300 mm; this layerappears to have solidified with recrystallizationof RDX after quenching. Evidence of the liquid

layer includes numerous bubbles and the forma-tion of what appears to be crystalline material,especially in the area immediately adjacent tothe unburned propellant, suggesting that crys-tallization may have been seeded by the RDXcrystals in the unburned propellant. The liquidlayer seems to be overlain in places by anotherlayer, possibly of molten binder.

Note also that there appears to be no evi-dence in these photographs for any change inthe structure of the unburned propellant beforemelting, as the structure below the liquid layerappears to remain constant right up to bottomof the liquid layer.

SEM photographs were taken (Table 2) ofsamples of XM39 that had been burned in astrand burner at pressures up to 2.0 MPa, withquenching by a massive block of copper. Cross-sections of the melt layer appeared to be notice-ably thinner than when burned at atmosphericpressure. Photographs looking down onto theburned surface of the same grain showed whatappears to be crystallized RDX overlain by

TABLE 2

Summary of Observations from Scanning Electron Microscope (SEM) Examination ofBurned/Quenched Propellant Samples

Sample Conditions Melt-layer thickness (mm) Description of surface

XM39 Water-quench ambient (air) 100–300 Yellow/orange, bubbles, crystallization; moltenbinder (Fig. 1)

Low-pressure self-quench 100–300 Yellow/orange, Generally smooth, few signs ofcrystallization, overlain by patches of darkermaterial. Many bubbles, crystallization atbottom of bubbles. (Fig. 4)

Copper-quench1.0 MPa ,100 Fewer bubbles, crystallization, decomposed

binder (?) overlying RDX2.0 MPa 30–100 Similar to 1.0 Mpa (Fig. 2) (For cross-section,

see Fig. 10 of Ref. [25])M43 Low-pressure self-quench 100–300 Generally smooth, few signs of crystallization;

overlain with patches of darker material;many bubbles, crystallization at bottomsof bubbles. (Figure 3)

HMX2 Water-quench ambient (air) 25–100 Bubbles, crystallization; solidified binder(?);melt layer poorly defined due to largeparticle size

Copper-quench0.5 MPa 10–75 Same as ambient1.0 MPa 10–75 Similar to 0.5 MPa2.0 MPa 10–75 Similar to 1.0 MPa

RDX Copper-quench0.25 MPa Present; otherwise unobservable. Considerable crystallization0.5 MPa Present; otherwise unobservable. Considerable crystallization


pieces of material that are presumably celluloseacetate butyrate (CAB) binder or decomposi-tion products thereof.

In a typical low-pressure self-extinguishedXM39 or M43 propellant sample, the burnedarea appears as a hole or crater in one end ofthe cylindrical propellant grain. The burnedsurface generally appears yellow or orange;dark brown or black areas are also often appar-ent, especially at the center of the crater and as

a ring around the edge of the crater. Thequenched surfaces of these samples were rela-tively free from disruption and ablation and itwas relatively easy to draw conclusions frommicroscopic examination. SEM examination re-veals that the surfaces of these samples containnumerous bubbles, which appear as holes in thesurface. The very top layer of the surface oftenappears relatively smooth with only occasionalsigns of crystallization (Figs. 3 and 4), but the

TABLE 3

HPLC Chromatographic Area Ratios for RDX, HMX, and Nitrosoamine Peaks

Sample

Area percentages (Area ratios 3 100)

NHMX/HMX DRDX/RDX MRDX/RDX HMX/RDX

XM39 WQa 17.4 0.6 3.70 7.5HMX2 WQa 0.5HMX2 WQa 0.6XM39 Unburned 0.0 0.0 0.00 7.0XM39 WQa 21.6 1.1 5.80 8.6HMX2 WQa 1.4HMX2 Unburned 0.0RDX WQa 1.6 0.0 0.05 4.1RDX WQa 1.4 0.0 0.30 9.4RDX WQa 0.0 0.0 0.30 10.5RDX Unburned 0.0 0.0 0.00 5.3RDX Unburned 0.0RDX WQa,FMb 4.3 0.2 3.00 12.7RDX WQa,FMb 3.9 0.6 6.80 27.2XM39 WQa 13.8 0.5 3.50 13.5XM39 SE, 1.0MPac 0.0 0.2 1.70 —XM39 SE, 1.0MPac 0.0 0.9 6.20 9.7XM39 SE, 1.0MPac 3.2 0.8 2.30 10.7XM39 Unburned 0.0 0.0 0.00 6.1XM39 Unburned 0.0 0.0 0.00 7.7XM39 1.0 MPa,CQd 5.8 1.3 4.40 10.8XM39 2.0 MPa,CQd 0.0 0.0 0.80 9.6XM39 2.0 MPa,CQd 0.0 0.0 1.10 8.3XM39 Unburned 0.0 0.0 0.00 7.6XM39 1.0 MPa,CQd 5.9 0.5 4.90 11.7RDX 0.5 MPa,CQd 2.1 0.0 0.90 8.9RDX 0.25 MPa,CQd 0.7 0.0 1.30 7.5RDX Unburned 0.0RDX Unburned 0.0 0.0 0.00 6.1HMX2 0.5 MPa,CQd 3.6HMX2 1.0 MPa, CQd 0.0HMX2 2.0 MPa, CQd 0.0HMX2 Unburned 0.0HMX2 Unburned 0.0HMX2 Unburned 0.0

a WQ, burned-layer scrapings from a grain burned in air at atmospheric pressure and quenched in water.b FM, foamy material thrown off from burning RDX (atmospheric pressure, water quenching).c SE, burned layer scrapings from grain that self-extinguished in a strand burner at 1.0 MPa.d CQ, burned layer scrapings from a grain burned in a strand burner at the indicated pressure with quenching by the copper

mounting block.


layer beneath it, which is 100 to 300-mm thick(Fig. 3) and is visible through the bubble holes,shows much obvious crystallization, presumablydue to the preponderance of RDX in the meltlayer. An especially good illustration of this isapparent in Fig. 4 (XM39), which shows thesmooth surface, dotted with bubble-holes, at the

bottom of which crystallization is apparent. Thisis interpreted in terms of the top few microns ofthe melt layer showing relatively few signs ofcrystallization due to the fact that they contain alarge proportion of CAB and nitrocellulosebinders as well as their polymeric decomposi-tion products. This thin layer appears to be

TABLE 4

Stabilizer and Plasticizer Peak Areas (Arbitrary Units) and Ratios for Burned-SurfaceScrapings and From Unburned XM39

Sample

Area

Ratio (3100)Plasticizer Stabilizer

Unburned 219.0 8.580 3.9Unburned 65.20 2.550 3.9WQa 77.00 0.000 0.0WQa 186.00 0.000 0.0WQa 133.00 0.000 0.0WQa 147.00 2.400 1.6WQa 225.00 0.881 0.4Unburned 90.00 3.890 4.31.0 MPa, SEb 125.00 2.930 2.31.0 MPa, SEb 46.40 0.000 0.01.0 MPa, SEb 117.00 2.340 2.01.0 MPa, SEb 146.00 1.310 0.9Unburned 121.00 5.590 4.6Unburned 206.00 9.390 4.6Unburned 65.90 2.110 3.21.0 MPa, CQc 27.20 0.436 1.61.0 MPa, CQc 95.00 2.220 2.32.0 MPa, CQc 498.30 23.00 4.62.0 MPa, CQc 280.00 11.600 4.12.0 MPa, CQc 195.00 8.030 4.12.0 MPa, CQc 105.00 3.380 3.22.0 MPa, CQc 204.00 7.940 3.92.0 MPa, CQc 92.30 2.760 3.02.0 MPa, CQc 115.00 3.800 3.31.0 MPa, SEb 85.10 1.960 2.32.0 MPa, CQc 69.10 2.040 3.01.0 MPa, SEb 45.60 0.640 1.42.0 MPa, CQc 171.71 5.820 3.42.0 MPa, CQc 85.80 2.590 3.01.0 MPa, SEb 69.90 1.990 2.82.0 MPa, CQc 98.80 2.740 2.81.0 MPa, SEb 95.20 3.370 3.51.0 MPa, SEb 104.00 2.140 2.02.0 MPa, CQc 99.40 4.340 4.4Unburned 241.00 11.900 4.9Unburned 152.00 6.820 4.5Unburned 63.6 2.12 3.32.0 MPa, CQc 47.90 1.310 2.72.0 MPa, CQc 75.00 2.330 3.1

a WQ 5 burned-layer scrapings from a grain burned in air at atmospheric pressure and quenched in water.b SE 5 burned-layer scrapings from a grain that self-extinguished while burning in a strand burner at 1.0 MPa.c CQ 5 burned-layer scrapings from a grain burned in a strand burner at the indicated pressure, with conductive quenching

by the copper mounting block.


floating on top of a molten layer that is severalhundred microns thick and contains enoughRDX to form crystals readily after self-extin-guishing. In the above discussion, the speciespresent were assigned on the basis of (a) theknown presence of RDX and of CAB in thestarting materials, as well as (b) the IR spectra(Fig. 8) and the HPLC results (Table 3). Also,(c) the crystallized areas seem much more likelyto be made up of the small, polar molecules ofRDX than of the polymeric CAB and its (prob-ably mostly polymeric) decomposition products.

On many samples, there are patches of whatappears to be slag or char on the surface of theliquid layer; these patches often seem to corre-spond to the above-mentioned dark-brown orblack areas.

The above SEM results are consistent withthe idea that, at least at low pressures, theburned surfaces of nitramine-binder propellantssuch as M43 and XM39 consist of a thin (;20-m-thick) layer of binder and decompositionproducts, with a thicker (up to several hundredmicrons thick) layer composed primarily of mol-

TABLE 5

Unknown Peaks in HPLC and GCMS Chromatograms of Burned Samples of XM39and HMX2

Sample Pressure QuenchRetention time (min) ofunknown HPLC peaks

Retention time ofunknown GC

XM39 (Unburned) — — —ambient Water 1.0, 1.2 5.3a, 6.1b, 11.0c

0.5–2.0 MPa Copper 1.2, 2.0 4.1d, 5.3e, 6.1b,8.1f,g, 11.0c, 16.6g,h

HMX2 (Unburned) — — —ambient Water 1.0, 1.1, 1.8 —

0.5–2.0 MPa Copper 1.0, 1.4, 2.2 —

RDX (Unburned) — — —ambient Water 1.2 —

0.5–2.0 MPa Copper Not examined —

a The mass spectrum of this peak included m/e 60, 27, 73, 42, 41, 43, 45, 38, 29, 26, 15, 31, 38, 40, 44, and 55.b The mass spectrum of this peak included m/e 43, 30, 15, 88, and 58.c This peak appears to be due to the following two different components: 1) m/e 57, 43, 71, 85, 29, 41, 42, 55, and 56; and

2) m/e 29, 112, 139, 212, 27, 84, 213, 167, 39, 140, 138, 214, 185, 184, 157, 156, 128, 113, 85, 83, 69, 67, 66, 57, 55, 53, 45, 44,43, 42, 41, 38, 31, 30, 26, and 15.

d The mass spectrum of this peak included m/e 43, 87, 42, 15, 29, 41, 39, 72, 59, 58, and 61.e The mass spectrum of this peak included m/e 55, 43, 83, 98, 60, 39, 29, 15, 41, 42, and 53.f The mass spectrum of this peak included m/e 95, 81, 41, 55, 39, 152, 67, 69, 83, 43, 109, 108, 93, 82, 79, 77, 68, 53, 51, 44,

42, 24, 12, and 11.g Present only in self-extinguished samples.h The mass spectrum of this peak included m/e 149 (by far the most intense), 29, 41, 76, 104, 223, 150, 39, 205, 56, 151, 122,

121, 105, 93, 77, 75, 65, 57, 55, 51, 50, 44, 43, 42, 40, 39, 30, 28, 19, 18, 16, 15, and 11.

TABLE 6

Summary of Depth-Profiling Conclusions

PropellantMelt thickness

(mm) Conclusions

XM39 100–300 Abraded-surface examination: Molten CAB and its decomposition products in the top10–20 mm; remainder appears to be mostly RDX.

HMX2 25–100 Cross-section examination: Relative amounts of polyester and its decompositionproducts decrease with increasing distance from the surface; most change isin the top 40 mm.


ten RDX beneath it. At higher pressures, themelt layer becomes thinner (Table 2; see alsoFig. 10 of Ref. [25])

SEM photographs were taken of burned sam-ples of HMX2. The presence of a liquid layercan be inferred from the smooth appearance ofthe top of the burned surface; its thickness isdifficult to evaluate but appears to be ;100 mm,noticeably thinner than the XM39 melt layerunder the same conditions.

HPLC Results

Chromatograms from solutions of the burned-layer scrapings from XM39 and from RDXindicated the presence of the nitrosoaminesderived from RDX by replacement of one(MRDX or ONDNTA) or two (DRDX) nitro

groups by nitroso groups (Table 3). (The trini-troso derivative hexahydro-1,3,5-trinitroso-1,3,5-triazine [TRDX] was apparently notpresent in amounts detectable by our methods.)In agreement with this, these nitrosoamineshave been detected in residues from thermaldecomposition [34, 35] and drop-weight impacttesting [17], although as far as we are aware ourprevious reports [24, 25] were the first time theyhave been detected from propellant combus-tion. Because the response factors for thesecompounds are similar [17, 35] the relativeintensities in Table 3 should provide roughestimates of the amounts of nitrosoaminesformed, relative to HMX and RDX. It is thusestimated that the nitrosoamines may bepresent in amounts of up to 5 or 10% of theunreacted HMX and RDX in some cases.

In addition, the compositions containingHMX exhibited a rather weak peak with aretention time slightly lower than that of HMX.Possibly this peak is due to a nitrosoaminederivative of HMX.

Several trends were observed in the HPLCdata (Table 3). First, there seems to be atendency for the experiments at 2.0 MPa (thehighest pressure used) to show less nitrosoam-ine formation than the experiments at lowerpressures. This may well result from a fasterburning rate, and hence a lower residence timein the liquid layer at higher pressures. However,although an attempt was made to scrape awayonly the liquid layer, there is still a possibilitythat the thinner liquid layers on the higherpressure samples resulted in a higher propor-tion of unburned material in the scrapings.Second, the samples with highest nitrosoamineconcentrations also have the highest HMX/RDX ratio; presumably, this enrichment resultsfrom a higher decomposition rate for RDX thanfor HMX under these conditions.

GCMS Results

The main features of a typical GCMS chro-matogram for XM39 propellant were: a) a large,broad peak at ;8 to 9 min, which on the basis ofchromatograms on solutions of pure RDX, isbelieved to contain primarily gaseous productsfrom the inside-injector and on-column decom-position of RDX; b) a very sharp, intense peak

Fig. 3. SEM photograph (3110) of cross-section of burnedsurface of M43 (0.25 MPa, self-extinguished)

Fig. 4. SEM photograph (3110) of cross-section of burnedsurface of XM39 (0.29 MPa, self-extinguished)


at 12.5 min, which was identified by its massspectrum and retention time as being due to theplasticizer ATEC; and c) a weak, sharp peak at15.9 min which was identified by its mass spec-trum and retention time as being due to thestabilizer diethyl centralite.

Table 4 shows the relative areas of peaks (b)and (c) for a number of burned and unburnedsamples. Note that the intensity of the stabilizerpeak, relative to the plasticizer, is considerablyless for the burned-layer chromatograms thanfor the chromatograms from unburned XM39.This indicates that the relative amount of stabi-lizer present in the surface/liquid layers is lessthan in the unburned propellant. This is consis-tent with the following mechanisms: a) stabilizeris removed in the liquid layer by reactions withnitrogen oxides formed by decomposition ofRDX and NC (possibly this removal occurs bymechanisms similar to those involved in stabili-zation of the propellant by removal of traceamounts of nitrogen oxides and acids); b) thestabilizer vaporizes from the surface morereadily than does the plasticizer; c) the stabilizerdecomposes by a unimolecular mechanismfaster than the plasticizer disappears (this seemsless likely because the stabilizer is originallyadded with the expectation that at storage tem-peratures it will react with nitrogen oxides fasterthan it will decompose alone.).

Note that there seems to be less stabilizerdepletion in the higher pressure (2.0 MPa) runs(Table 4). Note also that a similar trend withregard to nitrosoamine formation was describedin the previous section. These trends may wellresult from more rapid combustion at higherpressures, and a resulting lower residence timein the liquid layer. However, it should be re-membered that although an attempt was madeto scrape away only the resolidified liquid layer,it is possible that since the liquid layers on thehigher pressure samples are thin, a higher pro-portion of unburned material was included inthe scrapings.

It has been reported [4], in a JANNAF paperdescribing HPLC analysis of burned layer sam-ples from nitrate ester propellants, that if thepropellant specimens were not sampled soonafter quenching, the nitroglycerine (NG) tendedto diffuse into the NG-depleted zones near thesurface from the deeper layers. It seems unlikely

that such an effect is entirely responsible for theapparent stabilizer depletion seen in Table 4.This follows from the fact that the burnedsamples corresponding to lines 3 to 5 of Table 4,which showed no detectable stabilizer remain-ing, were scraped on the same day that theexperiments were carried out. The remainingburned samples, which were scraped up to 1week or more after burning, did show somestabilizer, although less than in the case ofunburned XM39.

FTIR Results

Figure 8 shows photoacoustic FTIR spectra ofunburned XM39 propellant and of the abradedand unabraded burned surfaces of a typicalsample of XM39 propellant. Comparison withan authentic spectrum [28] of CAB indicatesthat the extinguished surface of XM39 includesa far higher proportion of CAB and/or itspolymeric decomposition product than does theunburned XM39.

Figures 5 and 6 show FTIR-PAS spectra ofextinguished and of unburned samples ofHMX2 (80% HMX and 20% polyester binder).The only signs of the polyester in the spectrumof the unburned sample (Fig. 5) are the car-bonyl band at 1720 cm21 and three othersmaller bands, one at 1025 cm21 and two near700 cm21; the remaining polyester bands areobscured by HMX bands. These bands arelarger in the spectrum of the extinguished

Fig. 5. FTIR-PAS spectrum, surface of unburned HMX2


HMX2 (Fig. 6) than in the spectrum of un-burned HMX2 (Fig. 5).

Taken together, these results suggest that forXM39 and HMX2, and possibly for nitramine-binder propellants generally, the surface layersduring combustion contain enhanced amountsof the binder and/or its condensed-phase de-composition products.

Because propellants such as XM39 and M43contain ;70 to 80% RDX, it seems appropriateto study the changes that occur on burning/quenching RDX as a monopropellant. Figure 7shows the FTIR-PAS spectra of burned RDX(at two different mirror velocities/samplingdepths) and of unburned RDX. The most obvi-ous change caused by burning/quenching is theappearance of absorption bands at 1750 cm21

and at 1690 cm21; these are in regions charac-teristic [36] of carbonyl-containing functionalgroups, such as carboxylic acids or esters, and ofamides, respectively. Note that amides havepreviously been identified among the decompo-sition products of RDX [37–39]. Furthermore,infrared spectra of fresh melts of RDX and ofHMX [40] indicated that they consisted largelyof intact RDX and HMX molecules, togetherwith small amounts of formamide-like species,possibly N-(hydroxymethyl)formamide (HC(5O)-NH-CH2OH). Our burned/quenchedRDX samples also showed additional absorp-tion in the regions around 1450, 1020, and 710cm21. Also observed was a weak absorption inthe region around 1500 cm21; absorption in thisregion is characteristic of nitrosoamines [36].The appearance of this absorption is consistentwith our detection (Table 3 and HPLC Resultssection) of nitrosoamines related to RDX inburned samples of RDX and XM39. Note alsothe weak band at 1150 cm21 in the unburnedRDX; the source of this band, which disap-peared on burning, is unknown.

Depth-Profile Analysis

Depth profiling was carried out by the methodof cross-section examination and by the methodof surface abrasion (see Results section).

Figure 8 shows microreflectance FTIR spec-tra of quenched and virgin surfaces of an extin-guished grain of XM39, and of the quenched

Fig. 6. FTIR-PAS spectrum, surface of HMX2 burned at1.0 MPa

Fig. 7. FTIR-PAS spectrum, surface of RDX: (a) burned,estimated sampling depth 15 to 25 mm; (b) burned, esti-mated sampling depth 5 to 12 mm; (c) unburned (burned inair, water-quenched).

Fig. 8. FTIR microscope spectrum of XM39 propellantsurfaces: (a) virgin; (b) virgin, blasted once with abrasive; (c)burned/quenched, 367 mm removed; (d) burned/quenched,17 mm removed; (e) burned/quenched, unblasted (burned inair, water-quenched; area examined 40 3 40 mm).


sample after removal of portions of the surfacematerial. Even after removal of only 17 mm(estimated) by a single abrasive blasting, thespectrum resembles the virgin material muchmore than the quenched, unabraded sample.SEM photographs indicate a melt layer ;100-to 300-mm thick for samples burned/quenchedunder the conditions used here. Thus, it appearsthat only the top part of the quenched layerconsists of CAB and its decomposition prod-ucts. The remainder seems to be mostly RDX;this is consistent with its apparent crystallinenature (Figs. 1–4) and with the low meltingpoint (204°C) of RDX.

Microreflectance FTIR spectra were ob-tained on a burned/quenched sample of HMX2.The quenched surface showed an intense car-bonyl peak (1730–1750 cm21) and aliphatic CHstretch peaks (2800–3000 cm21) assignable tothe polyester binder. As the distance below thesurface increased to 0–40 mm and then to 40–80 mm, the relative intensities of these peaksdecreased and the relative intensities of theHMX peaks increased. The melt layer of thissample was ;80- to 100-mm thick; this is inagreement with Table 2, which summarizesSEM examinations indicating a melt-layer thick-ness of 25 to 100 mm for HMX2 under theseconditions. These results are consistent with thetheory that the concentration of binder in themelt layer is greatest near the surface anddecreases with increasing distance from thesurface, with most of the changes in the first 40mm below the surface.

SEM examination of the low-pressure-self-extinguished samples (quenching method (b) inthe Experimental section) reveals that thequenched surfaces of XM39 and M43 propel-lants contain numerous bubbles, which appearas holes in the surface. The very top layer of thesurface usually appears relatively smooth withonly occasional signs of crystallization (Fig. 2),but the layer beneath it, which is ;100- to300-mm thick (Figs. 3 and 4), shows muchobvious crystallization, presumably due to thepreponderance of RDX in the melt layer. Anespecially good illustration of this is apparent inFig. 4 (XM39), which shows the smooth surface,dotted with bubble-holes, at the bottom ofwhich crystallization is apparent. This is inter-preted in terms of the top few micrometers of

the melt layer showing relatively few signs ofcrystallization because they contain a large pro-portion of CAB and NC binders as well as theirpolymeric decomposition products. This thinlayer appears to be floating on top of a moltenlayer that is several hundred micrometers thickand contains enough RDX to form crystals afterself-extinguishment. These SEM results provideadditional qualitative evidence suggesting that,at least at low pressures, the burned surfaces ofnitramine-binder propellants such as M43 andXM39 consist of a thin layer of binder anddecomposition products, with a thicker (up toseveral hundred micrometers thick) layer com-posed primarily of molten RDX beneath it.

Thus, for most of the nitramine-binder com-positions (XM39, M43, and HMX2) studiedhere, and quite possibly for nitramine-bindercompositions generally, the surface layers ap-pear to consist of a layer of molten oxidizer(RDX or HMX) tens to hundreds of microme-ters thick, overlaid by a much thinner, 10- to20-mm-thick layer of binder and/or its decom-position products. Because HMX2 was studiedby the method of cross-section examination andXM39 was studied by the method of abraded-surface examination, it can be said that thesetwo methods have produced results in qualita-tive agreement with each other.

As mentioned above, Fig. 7 shows the FTIR-PAS spectra of burned RDX (at two differentmirror velocities/sampling depths) and of un-burned RDX; this is an example of depthprofiling by FTIR-PAS. As mentioned at theend of the FTIR Results section, several addi-tional bands appear in the surface layers when asample of pure RDX is partially burned, thenquenched. The intensities of these bands in-crease with increasing sampling depth; possiblythis can be explained by one of the following: a)the bands are associated with a thin layer ofdecomposition residue which lies on the surfaceof the burning RDX and which partially decom-poses or vaporizes at near-surface depths due toheat from the flame front above the surface; orb) water-soluble combustion products are dis-solved out of the surface layers of the sample bythe water used to quench combustion. In eithercase, it would appear that some degree of depthprofiling has been effected by the FTIR-PASapproach.


With regard to nitrosoamines, it may be pos-sible to get a degree of depth profiling byexamining lines 13 and 14 of Table 3. Theselines arose as follows: When the ambient-pres-sure RDX samples were burned and put inwater, material apparently from foam or liquidthrown off during combustion was noticed float-ing on the surface of the water. This was gath-ered and analyzed; it gave higher nitrosoaminelevels than any other RDX sample. Thisthrown-off material presumably came from theouter edges of the liquid layer, and it seemsunlikely that water alone could lead to forma-tion of nitrosoamines from RDX. Therefore, itshigher nitrosoamine concentration suggests thatnitrosoamine concentrations may increase ongoing from the bottom to the top of the liquidlayer.

Chemical and Physical Nature ofCondensed-Phase Surface Layers

It seems appropriate to briefly discuss the aboveresults and conclusions, and to their applicabil-ity to combustion-modeling efforts. First, in thecase of nitramine composite propellants such asXM39 and M43 (Fig. 9), there appears to be aliquid layer on the burning surface of the pro-pellant The liquid layer can be seen clearly inthe SEM photographs (Figs. 1–4). Bubbles,apparently due to decomposition of the oxidizerRDX are plainly visible. Signs of recrystalliza-tion are evident; the crystallized material ispresumably due to the presence of liquid oxi-dizer (RDX) during combustion, followed bycrystallization of this liquid RDX on coolingafter quenching. The thickness of the liquidvaries over the surface; it is ;100- to 300-mmthick for samples of XM39 and M43 that have

been burned in air at 1 atm, then water-quenched, and somewhat thinner, ;100 mm orless, for HMX2 or when combustion takes placeat pressures of 1.0 and 2.0 MPa under nitrogen(Figs. 1–4). Very similar results were obtained[27] from studies on samples resulting fromquenching by deradiation of the low-pressurelaser-assisted combustion of samples of variouspropellants. Where comparison is possible,these results agree with recent results fromstudies on deradiation-quenched samples re-sulting from low-pressure, laser-assisted com-bustion of XM39 and M43 [18]. As mentionedin the introduction to the present paper, theseauthors [18] also report a number of very inter-esting relationships between number and size ofbubbles, and factors such as pressure and heatflux. For example, the bubble size distributionshifted toward smaller diameters at higher pres-sures, and the foam layer thickness tended todecrease at higher pressures. Another recent,paper [20] contains observations that suggest asomewhat thicker (up to 1.2 mm) melt layer onRDX samples burned under nitrogen at low(0.1–0.17 Mpa; 1.0–1.7 atm) pressures; as pres-sure increases to 0.79 Mpa this thickness de-creased until it was comparable to the size (;25mm) of the thermocouple junctions used. An-other, very recent paper [21] describes SEMexamination of quenched samples of XM39 andM43 propellants; where comparison is possible,their observations seem to be in reasonableagreement with ours.

In any case, the molten-oxidizer layer forRDX at low pressures appears to be severalhundred microns thick and to be topped by athinner layer comprised mostly of moltenbinder and decomposition products. This fol-lows from several pieces of evidence. First, theFTIR-PAS spectra of the quenched surfaces ofXM39 (Fig. 8) and HMX2 (Figs. 5 and 6) showa considerable increase, over the correspondingspectra of unburned propellant, in the IR ab-sorptions due to the binder; presumably thisintensity also includes some binder decomposi-tion products. Second, when the burned sur-faces of quenched samples are abraded with anabrasive-blaster (Fig. 8) and the amount recov-ered is measured, FTIR spectra indicate thatsurfaces abraded to estimated depths greaterthan 10 to 20 mm below the burned surface

Fig. 9. Diagram of cross-section of burning nitraminepropellant.


contain considerably more RDX than thefreshly quenched, unabraded surface; in facttheir spectra suggest about the same composi-tion as the unburned propellant. Third (Table6), microreflectance FTIR spectra of the splitsurfaces of extinguished grains of HMX2 indi-cated that, at levels close to the burned surface,the spectra of the split samples showed rela-tively intense absorption bands apparently dueto the presence of binder/decomposition prod-ucts. However at greater depths, the spectrashowed mostly nitramine oxidizer, plus smalleramounts of binder. Finally (Figs. 3 and 4), SEMexamination of the burned surfaces and of thecross-sections after splitting of the low-pressure,self-extinguished samples (present work) re-veals that much of the surfaces show only verylimited crystallization, whereas the portions be-low the burned surface show extensive crystalli-zation. This is attributed to the presence ofpolymeric binder and decomposition productsin the layers closer to the surface, whereas thematerial at greater depths into the liquid layer iscomposed mostly of RDX and so exhibits amuch greater degree of recrystallization.

The thin, surface layer of molten binder andits decomposition products presumably arisesbecause, when binder is added to the alreadyslightly under-oxidized RDX, the binder tendsto burn/decompose more slowly than the oxi-dizer RDX, and so tends to accumulate in theregions just under the burning surface. Mixingdue to the passage of the bubbles is apparentlynot sufficient to completely homogenize thesurface layers. In this connection, it would beuseful to have information on the solubility ofthe molten binder and decomposition productsin the molten RDX oxidizer. It is difficult to seehow this layer could be an artifact of any of thequenching processes used on this work. Thisthin, surface layer of molten binder and itsdecomposition products could be important,due to the possibility that it could be affectingthe relative amounts of RDX evaporation vs.condensed-phase decomposition.

Evidence for Chemical Reactionin Condensed Phase

There is also evidence for a significant amountof chemical reaction in the liquid phase during

the combustion of nitramine propellants. Thisevidence includes the following observations:

(a) When grains of XM39 were burned in airand quenched by dropping them into water orburned at low pressure with conductive quench-ing by a block of copper, and the surface layerswere scraped off and analyzed by HPLC (Table3), it was found that significant amounts (insome cases up to 5–10%, relative to unreactedRDX) of the nitrosoamines MRDX (also re-ferred to as ONDNTA [37, 38] and DRDXwere present in the liquid layer. These nitro-soamines were also found in similar experi-ments carried out on samples of pure RDX.

(b) The GCMS and HPLC chromatogramsfor the burned-surface scrapings indicated thepresence of small amounts of a number ofunidentified compounds; the mass spectra andretention times of these are tabulated in Table5. These compounds were presumably formedduring combustion.

(c) It was found in the present work (Fig. 7)that burning of RDX monopropellant in air atatmospheric pressure, followed by quenching inwater, led to detection, by FTIR-PAS, of severalIR absorptions on the burned surface that arenot present in the spectrum of unburned RDX;these included absorptions typical of amidesand other products. This constitutes furtherevidence for occurrence of chemical reactionsinvolving RDX in the condensed phase.

Taken together, these results would appear toindicate that a significant amount (more than;1 or 2%) of chemical reaction takes place inthe foam/liquid layer.

In this connection, it seems worth mentioningthat Gongwer and Brill [41] have employedcomputer techniques to remove the major gas-eous products from the infrared spectrum of thedecomposition products of RDX; this enabled anumber of minor products to be uncovered andstudied. These included the mononitrosoamineMRDX; a triazine derivative modeled as 1,3,5-triazine; one or more amide derivatives mod-eled as N-(hydroxymethyl)-formamide; andRDX present as both vapor and aerosol. Thenitrosoamine and amide derivatives seem inagreement with the present work (Table 3 andaccompanying discussion; Fig. 7 and accompa-nying discussion), whereas the triazine and/orderivative seems in agreement with reports


[37–40, 42, 43] of detection of 1,3,5-triazine or ofa 1,3,5-triazine oxide from thermal decomposi-tion of RDX. There have also been a number ofrecent reports of mass spectrometric detectionof species with m/e 81 (1,3,5-triazine) and/or 97(1,3,5-triazine oxide) in the subsurface [44] andnear gas phase [45–48] from studies of laser-assisted combustion of RDX, RDX propellants,and HMX. The species with m/e 81 has beenattributed [44] to fragmentation of RDX in themass spectrometer. However, ionization-energyeffects and daughter-ion spectra suggest [45, 46]that m/e 81 and 97 represent actual decompo-sition products, as do species of m/e43(HNCO?), 45(formamide?), 47(HONO?),54, and 70 that were also observed. The triazinederivatives would most likely not have beendetected in our work, due to the tendency of1,3,5-triazine [49], and possibly of its oxide aswell, to undergo rapid hydrolysis in the presenceof the water used in quenching and in HPLCanalysis. The gas-phase concentrations of thesespecies are highest at the burning surface [46,47], suggesting that they are also present in theliquid layer. Also, 1,3,5-triazine is reported [47,48] to be formed in HMX combustion; possiblythis is explainable in terms of its formation bythermolysis of 1,3,5,7-tetraaza-cyclooctatet-raene initially formed by loss of four moleculesof HNO2 from HMX; this is in agreement withthe observations [50–52] that cyclooctatetraeneand its aza derivatives decompose thermally tobenzene and azabenzenes.

Chemical and Physical Behavior inMelt/Foam Layer

This section discusses the relative importance ofa) decomposition in the condensed-phase; andb) vaporization followed by gas-phase decom-position. We will attempt to estimate the tem-perature of the foam/liquid layer in burningnitramine composite propellants, and to com-pare it to the boiling point of the moltenoxidizer, usually RDX. The results described inthe previous sections indicate that (a) decom-position in the condensed-phase does occur.However, there is evidence that (b) vaporizationfollowed by gas-phase decomposition is alsoimportant. Zenin [53] studied the combustion ofHMX and RDX at pressures of 1, 5, 20, and 70

atm by a thermocouple technique and deter-mined a number of properties including (but notlimited to) surface temperature, flame temper-ature, heat release in solid and heat feedbackfrom gas to solid. He found that, for both HMXand RDX, heat release in the solid (or con-densed) phase was negative (corresponding toendothermic behavior or heat absorption) atlow (1 and 5 atm) pressures, but became posi-tive at higher pressures (20 and 70 atm). Thiswas interpreted as meaning that, at low pres-sures, evaporation (an endothermic process)made a larger contribution to heat release thandid chemical reaction (an exothermic process),but that at higher pressures the balance shiftsand more heat is released in the condensedphase by chemical reaction than is taken up byevaporation of RDX/HMX. Heat release in thesolid phase was defined by the equation:

Q 5 c(Ts 2 T0) 2 q 2 qr 1 qm (1)

where Q is total heat release in the solid, c isspecific heat of the solid, Ts is the surfacetemperature of the burning propellant, T0 is theinitial temperature, q and qr are respectively theheat feedback from the gas phase to the burningsurface by conduction and radiation, and qm isthe heat of melting. Thus, it appears that Q infact refers to the entire condensed (solid andliquid) phase, not just the solid phase. Thesurface temperatures for RDX at 1 and 5 atmwere found to be 320 and 360°C, respectively.

Another estimate of the temperature of theliquid phase can be obtained by an argumentinvolving the following steps:

(a) An approximate rate constant for RDXdecomposition under combustion conditions isestimated from the burning rate and the meltlayer thickness.

(b) An existing plot [55] of rate constant vs.inverse temperature for liquid-phase decompo-sition of RDX is used to relate the approximaterate constant from (a) to an average tempera-ture at which RDX is disappearing in the liquidlayer during combustion.

(c) The temperature obtained in (b) is thenconsidered to be an approximate estimate of theaverage temperature in the liquid phase.

The estimation begins with the fact that whenthe copper-quenched RDX and XM39 grains


were burned, burning rates were recorded [59](private communication, M. S. Miller, U.S.Army Research Laboratory, Aberdeen ProvingGround, 1990). These burn rates varied from;0.05 to ;0.2 cm/s, depending on pressure andon the nature of the sample. For this estimation,an average value of 0.1 cm/s is used. Pressureswere between 0.25 and 2.0 MPa. For the meltlayer thicknesses of these samples, an averagevalue of ;70 mm (present work) is used. Anapproximate value for the rate constant, k, ins21, for disappearance of liquid-phase RDXunder combustion conditions, is obtained bydividing the burning rate (in cm/s) by the thick-ness of the melt layer (in centimeters); thisapproximate rate constant is 14 s21 (character-istic time ;70 ms). Examination of an existing[55] plot of log10 k vs. 1000/T indicates that avalue of 14 s21 for k corresponds to a value of;1.7 for 1000/T, which yields T 5 588 K or315°C. Variations in melt-layer thickness or inburning rate by a factor of 2 cause T to varybetween ;300 and 330°C.

This range seems to agree satisfactorily withthe reported surface temperature of 320°C forRDX burning as a monopropellant at a pressureof 1 atm [53], with a calculated surface temper-ature of 560 K (287°C) [56] for RDX burning at1 atm, with an estimated surface temperature of;620 K (347°C) [57] for HMX or RDX burningat 1 atm, and with temperatures [20], for the top(vapor-phase) side of the liquid/foam zone,varying from ;590 K (317°C) for RDX burningat 0.17 Mpa (1.7 atm), to 690 K (417°C) forRDX burning at 0.79 Mpa. Brill [57] also sum-marizes several additional calculated surfacetemperatures from the literature for HMX andRDX burning at pressures from 0.5 to 100 atm;for RDX at 1 atm, these include values of 593 K(320°C), 549 K (276°C), 560 K (287°C), and 690K (417°C). Note that with the single exceptionof the last, all of these are well below the 391°Cboiling point of RDX. Calculated surface tem-peratures for RDX at higher pressures are alsoquoted [57].

For HMX at one atmosphere, Brill [57]quotes a predicted surface temperature of 680K (407°C), whereas Zenin [53] reports an ex-perimental surface temperature of 360°C. Be-cause Maksimov [61] estimates the boiling pointof HMX to be 471 6 37°C, it appears that, for

HMX as well as for RDX, the surface temper-atures are well below the boiling point.

Note the following approximations and pos-sible sources of error in the above argument:

(a) Unlike the pure RDX whose decomposi-tion was used to derive the points in the plot ofFig. 1 of Ref. [55], the liquid layer of burningnitramine propellants such as XM39 and M43presumably contains a significant amount ofbinder ingredients and their decompositionproducts.

(b) The temperature is presumably not con-stant throughout the melt layer, but is expectedto increase from the bottom to the top of themelt layer, and at the top of the melt layer, it ispresumably closer to the hypothetical boilingpoint of RDX.

(c) Under the rapid-heating conditions char-acteristic of propellant combustion, it is notinconceivable that, near the top of the meltlayer, temperatures may actually reach the boil-ing point of RDX before the RDX has alldisappeared, resulting in boiling or in very rapidvaporization of any remaining RDX.

(d) The RDX may be completely decom-posed on the upper surface (containing mainlyslag and condensed-phase decomposition prod-ucts) of the melt layer. As a result, the assumedmelt layer thickness may be an upper limit, andhence the estimated rate constant and temper-ature may be lower limits. Note, however, thatthis effect could be corrected by assuming anartificially small melt layer thickness, and that(preceding paragraph) the effect of variations inassumed melt layer thickness seems to be rela-tively small.

(e) The above argument assumes that RDXdisappears from the liquid layer entirely bythermal decomposition rather than by intactvaporization; thus the estimated decompositionrate in the liquid layer is a maximum value, as isthe estimated liquid-layer temperature. An at-tempt was made to estimate the effect, onmelt-layer temperature, of the relative amountsof decomposition and vaporization. This wasaccomplished by reducing the estimated rateconstants by the fraction of thermal decompo-sition (ignoring the effect of the heat of vapor-ization); this resulted in estimated surface tem-peratures of 308°C for 50/50 decomposition/


vaporization and 295°C for 6/94 decomposition/vaporization.

(f) The above procedure for estimating thetemperature of the melt layer is in generalhighly approximate.

Another question that arises is whether thebubbles observed in the liquid layer of burningRDX and RDX propellants are due to thermaldecomposition gases, or to the boiling of RDX.Because our results to date [24–28] (presentwork) indicate that there is some reaction in theliquid phase, this question arises regardless ofthe relative contributions of vaporization andthermal decomposition to the disappearance ofRDX from the condensed phase. The availableevidence indicates that the bubbles result pri-marily from thermal decomposition of RDX,rather than from boiling of RDX. This is basedon several lines of reasoning.

First, as discussed in the previous paragraphs,RDX is clearly decomposing to some extent inthe liquid phase, and this decomposition isexpected to evolve bubbles of product gasesformed in the decomposition.

Another line of reasoning is based on com-parison of the hypothetical boiling point of pureRDX with the temperatures at which decompo-sition appears to be taking place in the liquidlayer of burning RDX. The thermal decompo-sition of RDX has been studied by many au-thors (see reviews in Refs. [55] and [58]), includ-ing a group at American Cyanamid [59, 60].This group followed the decomposition by mea-suring the gases evolved and found that RDX,which melts at 204°C, begins to decompose athigher temperatures. At a temperature of227°C, increasing scatter in the data was attrib-uted to the rate of decomposition being toorapid (t1/2 75 s) to follow by the manometricprocedure employed. The boiling point of pureRDX has never been observed experimentallybecause of decomposition before boiling, butMaksimov [61] has estimated the hypotheticalboiling point of RDX from data on vaporpressure of RDX as a function of temperature;the estimated boiling point for RDX was foundto be 391 6 33°C. Maksimov [61] also cites anearlier estimate [62] of 340°C for the boilingpoint of RDX, but considers this value unreli-able due to the possibility of decomposition ofRDX during the measurements.

In the following discussion, we will use thevalue of Maksimov [61] (391 6 33°C) for theboiling point of RDX. Because decompositionseems to begin at the melting point of RDX andto be proceeding too fast to measure accuratelyat a temperature only 20 to 30°C above thistemperature, it seems most reasonable to sup-pose that, as RDX is heated, it will reach itsdecomposition point before it reaches the hypo-thetical boiling point (391°C) of RDX. Further-more, as was discussed in the preceding para-graphs, the average temperature in the meltlayer seems to be in the range of ;300 to 320°C;this is below the hypothetical boiling point(391°C) of RDX. Based on this information, itseems reasonable to suppose that the bubblesobserved in the liquid phase in the present andother [18, 19, 24, 25, 27] work on quenchedpropellant samples are not due primarily toboiling of liquid RDX, but rather to evolution ofproduct gases from thermal decomposition ofliquified RDX. However, it seems quite possiblethat, especially at temperatures within a fewtens of degrees of its melting point, consider-able RDX could be entrained in the decompo-sition gases and carried into the vapor phase.

Third, even if it should turn out that vapor-ization predominates over decomposition as apathway for disappearance of RDX from theliquid layer, a very small proportion of thermaldecomposition relative to vaporization couldstill account for the observed bubbles. This canbe seen by combining the following:

(a) One mole of an ideal gas occupies avolume of 22.4 L, and the molecular weight ofRDX is 222, and its thermal decomposition isexpected to yield ;6 moles of initial gaseousproducts (N2O, H2C5O, HCN, nitrogen oxides,acids, and water). If a mole of molten RDX isconsidered to occupy 222 mL (0.222 L), itsvolume is expected to increase on decomposi-tion by a factor of 6 3 (22.4/0.222), or ;600.

(b) Examination of cross-section photographsof the melt layers of low-pressure quenchedsamples of RDX monopropellant and of RDXcomposite propellants obtained in the course ofthis and other work [18, 19, 24, 25, 27, 28]suggests that about one half of the volume ofthe melt layer is taken up by the space inside thebubbles. This is consistent with the report fromthe modeling study by Liau and Yang [56] that


the calculated void fraction was 0.45 at thesurface of RDX burning at 1 atm.

(c) The combination of (a) and (b) suggeststhat decomposition of as little as 1/(600/0.5) 58.3 3 1024 of the total RDX present in the meltlayer could account for the bubbles present atany one time. Note that this argument is notbeing made in order to propose that the amountof decomposition is this small relative to vapor-ization, but rather in order to point out thateven with very small amounts of decompositionrelative to vaporization, the bubbles could stillbe due to the relatively large volumes of gasesgiven off in the decomposition, rather than toboiling action of the liquid RDX.

As the product gases from decompositionevolve, they presumably assist vaporization ofRDX by entraining RDX molecules in thevapor. This process is probably especially im-portant near the top of the melt layer, where theRDX presumably has a higher vapor pressure asa result of the higher temperature. Entrainmentof RDX vapor in this way might well causevaporization of intact molecules to become asignificant process in combustion of RDX andpropellants containing RDX. The temperaturemost likely varies throughout the melt layer,being near the melting point of RDX (;200°C)at the lower edge of the melt layer, where theremay be a thin, bubble-free layer of liquid RDX.In qualitative agreement with this, it was foundfrom fine-wire thermocouple studies [20] that,depending on pressure, the temperature at thebottom (melting) side of the liquid/foam zone ofburning RDX was ;500 to 520 K (227–247°C),somewhat higher than the 204°C melting pointof RDX. It seems likely that temperature thenincreases through the melt until the RDX hasreacted, just before the vapor phase is reached.Presumably, the hypothetical boiling point ofRDX (391 6 33°C) [61] is an upper limit fortemperature in the melt layer, but as discussedabove the actual temperature at the surface maywell be lower than that because RDX decom-poses below its hypothetical boiling point. Thetemperature may well not vary smoothly be-tween the top and the bottom of the melt layerdue to such factors as agitation by bubbles. Inthis connection, it seems worthwhile to mentiona paper by Brewster and Schroeder [63], whoused microthermocouples to study the temper-

ature history of laser-assisted burning of RDXat atmospheric pressure. On examination oftheir Fig. 2, the condensed-phase temperatureappeared to rise smoothly until the meltingpoint of RDX was marked by a small disconti-nuity at ;475 K (;200°C); the temperaturethen continued to rise smoothly until a temper-ature of ;600 K (;320–330°C) is reached, afterwhich the temperature oscillates irregularlyaround an average temperature of ;650 K(377°C). This is consistent with bubbling due todecomposition of RDX beginning at ;325°C. Inany case, the melting point (;204°C) and thehypothetical boiling point (;391°C) of RDXwould appear to be boundary values for thetemperature in the melt layer.

In this connection, it seems of interest tomention the case of combustion of frozen ozone[64], in which a liquid layer is involved but it isknown that there are no reactions taking placewithin the condensed phase. At 1 atm, thefreezing point of ozone is 80 K, and its boilingpoint is 161.3 K. The computed [64] surfacetemperature is 158.0 6 0.1 K, depending onwhether a possible gas-surface reaction is in-cluded in the model. Thus, the surface temper-ature is only 3 K (;4% of the differencebetween melting point and boiling point) belowthe boiling point. On the other hand, in the caseof RDX, where at least some subsurface reac-tion does appear to take place, the melting pointis ;477 K (204°C) and the hypothetical boilingpoint is 664 K (391°C), and the surface temper-ature (preceding discussion) seems to be ;583K (310°C) (;40%, 103 greater than for frozenozone) of the difference between melting pointand boiling point). This difference is in agree-ment with the idea that if the bubbles in thesurface layers of burning RDX propellants weredue to boiling of RDX, the surface temperaturewould be approaching the boiling point of RDX(391 6 30°C) rather than down in the region of300 to 320°C. Note however that if the situationwas actually as simple as that, the surface tem-perature in the frozen-ozone case would beexpected to be strictly equal to the boiling pointof ozone, not 3 K below it. Possible sources ofcomplexity include superheating, supercooling,and surface chemical reactions.

Melius [65] reported the results of a modelingstudy, which suggested that only 5 to 7% of the


decomposition occurs in the condensed phase.Li, Williams, and Margolis [66] report the re-sults of a modeling study which suggests that thepercentage of decomposition that occurs in theliquid phase during RDX deflagration is alwaysless than 0.1%. Also, Ermolin and Zarko [67]have reviewed modeling efforts on nitraminecombustion; they quote a number of results withregard to calculated relative amounts of liquid-and gas-phase decomposition. Most of the num-bers quoted suggest that the percentage ofliquid-phase decomposition is between 1 and10%, although one study [68] reported between25 and 35% calculated liquid-phase decomposi-tion for HMX. (A statement [67] that RDX at 1atm was calculated [56] to exhibit 50% liquid-phase decomposition, in fact referred to 90 atm;the authors stated elsewhere in the paper [56]that at 0.5 atm (and presumably also at 1 atm)“only limited RDX decomposition occurs in thesubsurface region.” Finally, Prasad et al. [69]and Davidson and Beckstead [70] have pre-dicted fractional amounts of RDX decomposi-tion in the liquid and gaseous phases. Theypredict 43% and “up to 25”% decomposition,respectively, of RDX in the liquid phase, theremainder of the RDX decomposing after va-porization.

Thus, it would appear that most of thesemodeling results suggest ;1 to 10% decompo-sition, with some estimates less than 1%, andnone higher than 40%. The estimates of around1 to 10%, or somewhat larger, seem qualita-tively in agreement with the percentages ofnitrosamine content reported in Table 3. Ifthese relatively small amounts of condensed-phase decomposition are a reasonable reflec-tion of reality, it would appear that, at least atlow pressures, most of the decomposition takesplace in the gas phase after vaporization ofintact molecules of RDX. If this is true, theabove discussion suggests the possibly of initialdecomposition, with bubble formation, of atleast a small amount of oxidizer (RDX orHMX). The bubbles, full of hot decompositiongases, then sweep through the oxidizer (which ismolten and has a high vapor pressure, beingwithin a few tens of degrees below its hypothet-ical boiling point) and carry along largeamounts of unreacted oxidizer, resulting in a

relatively large amount of vaporization relativeto decomposition.

Suggestions for Modelers

Based on our results, we can offer some sugges-tions for modelers working in the area of solid-propellant combustion.

First, all of the above results provide anindication of what physical attributes (e.g., meltlayer presence and thickness) of a burning sam-ple should be reproduced, at least for combus-tion at low pressures. In combustion of nitra-mine composite propellants, the results indicatethat, at least at low pressures, a liquid layer doesexist, and that significant bubble formation andchemical reaction take place in the liquid layer.

As far as we are aware, the most advancedmodeling studies currently being carried out onRDX monopropellant are exemplified by thosebeing done at a number of laboratories [56,69–73] (private communication, K. K. Kuo,Pennsylvania State University, 1995). The ob-jective of this work is development of a compre-hensive theoretical model to describe the de-tailed reaction zone structure of RDXmonopropellant burning under constant ambi-ent pressure. Some of these models include notonly chemical details of the gas-phase flamereactions, but also the solid-phase conductiveheating zone (i.e., the solid phase below theliquid or foam layer), and a two-phase (liquid/gas bubble) foam zone, with some degree ofdetailed chemical representation of gaseous re-actions within the bubbles and of reactionswithin the liquid phase.

Note that the thermal decomposition of RDXis a very complex process, and the detailedmechanisms of the reactions involved are inmany cases not at all well understood. Further-more, many of the physical properties of RDXare not well known. Therefore, it would help ifwe could improve our experience with detailedmodeling of three-phase combustion systems ofsimple, well-known chemistry. Accordingly,Miller [64] is carrying out modeling studies onthe combustion of frozen ozone. This combus-tion system, like that of RDX, involves gaseous,liquid, and solid phases but has a much simplerchemistry involving a total of only three revers-ible, elementary reactions for the homogeneous


gas phase as well as a single gas-surface reac-tion. Furthermore, the required physical prop-erties of ozone are better known than those ofRDX.

One implication of the present work is that, atleast in the case of nitramine propellant com-bustion, there does appear to be some decom-position taking place in the liquid phase; thisfollows from (a) the detection of nitrosoaminesin the surface (liquid) layers by HPLC; and from(b) the observed changes in surface IR spectraon combustion of RDX; (c) the reduction instabilizer/plasticizer ratio may also be due toliquid-phase reaction. In agreement with this,the modelers mentioned previously [56, 69, 70–73] (private communication, K. K. Kuo, Penn-sylvania State University, 1995) are consideringtwo-phase phenomena in the foam layer of theirmodel.

The detection of nitrosoamines in the liquidlayer in the present work suggests that nitro-soamine-forming reactions should be includedin the liquid-layer chemistry. The best candidatefor this is probably a bimolecular combinationof NO and the nitrogen-centered radical result-ing from loss of a nitro group from RDX:

This is based on the observed formation ofthe nitrosoamine MRDX (also known asONDNTA) with scrambling in “crossover”studies [37] of the decomposition of fully la-beled (with 15N in both ring and nitro-nitro-gens) RDX, and of unlabeled RDX. This reac-tion could possibly be represented as a typicaldiffusion-controlled radical recombination reac-tion; the follow-up decomposition reactions ofMRDX could be represented by analogs of thedecomposition reactions included for RDX, re-placing N2O by N2 and H2C 5 NNO2 by H2C 5NNO where appropriate. We also detectedsmaller amounts of DRDX (dinitroso analog ofRDX); this compound is most likely formed

from and sequentially to the mononitroso ana-log MRDX, by a nitro-nitroso exchange se-quence similar to the above. Thus, if the avail-able time and computer facilities allow, it wouldbe interesting to extend the previous approachto include formation of DRDX from MRDX,and possibly even of the trinitroso derivativeTRDX from the dinitroso-derivative DRDX, aswell as the decomposition of these nitrosoam-ines.

It should be remembered that the liquid/foamlayer in combustion of RDX is really a multi-component system involving a number of com-ponents (RDX and its decomposition products)that are or may be gaseous at the temperaturesinvolved, and that any or all of these compo-nents may be involved in nucleation processesleading to bubble formation. Therefore, an-other useful feature of a model would be toinclude the capability of taking account of si-multaneous formation or nucleation of bubblesby both (a) vaporization/boiling involving intactRDX molecules, and by (b) formation fromdecomposition gases dissolved in the liquidRDX. Possibly the vaporization/boiling compo-nent could be based on whatever models may

exist for the boiling of more conventional liq-uids such as water. The component involvingformation of bubbles from dissolved decompo-sition gases could possibly be based on whatevermodels may exist for processes of applied im-portance. It would also be helpful if the decom-position component of the model included pro-vision for entrainment of RDX vapor in thedecomposition-gas bubbles. If formation of bub-bles by both (a) and (b) simultaneously wereincluded in the model, the relative amounts ofbubbles predicted to form by (a) and by (b)could be compared, leading to improved under-standing of the mode(s) of bubble formation.

Finally, combustion modeling will without

Diagram


doubt eventually advance to the point wheretruly detailed models such as those previouslymentioned [56, 69–73] (private communication,K. K. Kuo, Pennsylvania State University, 1995)can be used to model actual RDX-polymeric-binder propellants rather than just neat RDX asis the case at present. At that time, the signifi-cance of phenomena unique to heterogeneoussystems (e.g., our finding of higher concentra-tion of molten binder in the upper portions ofthe surface layers) will become relevant to mod-elers.

The authors thank Robert J. Lieb for helpfuldiscussions and for the use of some of his equip-ment, including a drop-hammer apparatus forsplitting propellant grains. The work describedhere could not have been accomplished withoutequipment provided by the Productivity Enhance-ment Capital Investment Program. The authorsare also grateful to a Reviewer for calling ourattention to Ref. 63.

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Received 23 November 1999; revised 13 April 2001; ac-cepted 6 May 2001


Documents

Condensed-phase processes during combustion of solid gun propellants. II. nitramine composite propellants