190 Propellants, Explosives, Pyrotechnics 2 7, 190-195 (1992)

Thermal Expansion, Transitions, Sensitivities and Burning Rates of HMX

Michael Herrmann, Walter Engel, and Norbert Eisenreich

Fraunhofer-Institut fur Chemische Technologie (ICT), D-7507 Pfinztal-Berghausen (FRG)

Dedicated to Professor Dr. Hiltmar Schubert on the Occasion of his 65th Birthday

Thermische Ausdehnung, Phasenumwandlungen, Empfindlichkeit und Abbrandgeschwindigkeit von HMX

Die Phasen von HMX und ihre Umwandlungen wurden mit der thermischen Analyse mit Hilfe der Rontgenbeugung untersucht. Die Proben wurden schrittweise aufgeheizt und abgekuhlt, nach jedem Temperaturschritt wurde ein Beugungsdiagramm aufgenommen. Die thermischen Ausdehnungskoeffizienten und die Volumenanderungen bei den Phasenumwandlungen wurden aus den gemessenen Daten ermittelt. P-HMX verdichtet sich vor der Umwandlung in die Hochtemperaturphase 6-HMX. Diese Kontraktion erhoht den Volu- mensprung bei der Umwandlung. Beim Abkuhlen wandelt sich 6-HMX erst in Tagen in ein Gemisch aus a- und P-HMX um. Die Ruckurnwandlung wird durch Zersetzungsprodukte weiter ver- langsamt, welche sich beim Erhitzen auf uber 490 K bilden.

Die Reib- und Schlagempfindlichkeit und die Abbrandgeschwindig- keit der Phasen wurde bestimmt. Die hohe Schlagempfindlichkeit von 6-HMX erhoht das Handhabungsrisiko, wenn die Substanz uber die Umwandlungstemperatur hinaus erhitzt wird. Das erhohte Risiko bleibt dann, selbst nach Abkuhlung auf Raumtemperatur, durch die extrem langsame Ruckurnwandlung langfristig erhalten.

Expansion thermique, transitions de phases, sensibilitb et vitesse de combustion de HMX

Les phases de HMX et ses transitions ont CtC CtudiCes par analyse thermique au moyen de la diffraction des rayons X. Les tchantillons ont Cte chauffCs et refroidis progressivement. Un diagramme de dif- fraction a CtC e n r e g i d apres chaque variation de temptrature. Les coefficients dexpansion thermique et les variations de volume lors des transitions de phases ont CtC dCterminCs ?I partir des donnees mesurtes. P-HMX se comprime avant la transition en une phase haute tempCra- ture 6-HMX. Cette contraction augmente la diffkrence de volume lors de la transition. Lors du refroidissement, il faut plusieurs jours pour que 6-HMX se transforme en un mClange de a-HMX et P-HMX. La reconversion est ralentie encore davantage par les produits de dCcom- position qui se forment i des tempCratures supkrieures ?I 490 K.

La sensibilite a la friction et au choc et la vitesse de combustion des phases ont CtC dCterminCes. La haute sensibilitk au choc de 6-HMX augmente le risque de manipulation lorsque la substance est chauffke au-deli de la temptrature de transition. Le risque Clevt subsiste long- temps, mtme aprks refroidissement ?I tempkrature ambiante, en raison de l'extrtme lenteur de la reconversion.

Summary Table 1. Phases of HMX

The phases of HMX and their transitions were investigated by ther- mal analysis using X-ray diffraction. Series of diffraction pattern were measured, while the samples were heated and cooled. The thermal expansion coefficients and the volume changes at the transitions were extracted from the diffraction series. A contraction of P-HMX was found before changing into 6-HMX resulting in a high volume differ- ence during the transition. On cooling, the reconversion of the high temperature phase requires days. It is further slowed down by decom- position products, which are formed at temperatures beyond 490 K. The final reconversion results in mixtures of 01- and P-HMX.

The mechanical sensitivities and the burning rates of the HMX phases were determined. The high sensitivity of 6-HMX against impact together with its slow reconversion creates handling risks when the HMX is exposed to temperatures above 440 K.

1. Introduction

For many years there has been concern about the phase transitions of HMX in the field of propellants and explo- sives resulting in a number of publications. The substance crystallizes in 4 phases with different densities and stability ranges (Table 1). The crystal structures are reported in the literature (1-4).

The interest in the transitions stems from the observed differences in the sensitivities against friction and impact. It

Density Stability Range [s/cm31 [KI

Phase Gibbs ( 5 ) Cadv (6) ~

WHMX 1.84 376-435 P-HMX 1.90 RT-376 y-HMX 1.78 ? 6-HMX 1.79 435-m.p.

was found that especially the transitions into 6-HMX increase the ~ensitivity(~). This work suggests a careful investigation of the transitions for safety reasons.

The volume and enthalpy changes connected with the transitions of pure HMX raise the question if the burning characteristics of a propellant matrix with incorporated HMX doesn't change(*).

In spite of the general interest the details of the phase changes are far from being clear. Faced with this situation investigations were undertaken including - preparation of the different phases - thermal analysis by X-ray diffraction - determination of the sensitivities of the different phases - measurement of the burning rates of the different phases

0721-3 1 15/92/0408-0190 $3.50+.25/0 0 VCH Verlagsgesellschaft, D-6940 Weinheim, 1992

Propellants, Explosives, Pyrotechnics 17, 190-195 (1992) Thermal Expansion of HMX 191

2. Preparation of the Phases more than two hundred X-ray patterns can be measured a day.

P-HMX, grade B, class Dyno AIC, of Dyno Industrier was used as starting material.

a-HMX was recrystallized from 65% nitric acidc9). With 3 g HMX in 80 ml nitric acid and a cooling rate of 0.5 K/min starting from 388 K long needles of a-HMX were obtained. The X-ray diffraction pattern showed no peaks of the other phases.

y-HMX was recrystallized by cooling a hot saturated solution in acetic acid ('1. A factorial plan including the concentration of the acetic acid, HMX-concentration and cooling rate as parameters yielded y-HMX, which still con- tained p-HMX. The maximum content was obtained with 1 g HMX in 400 ml 60% acetic acid after cooling in an iceINaC1 mixture. Though y-HMX is formulated as a hydrate, no water was found with Karl-Fischer analysis or by heating on a thermobalance.

6-HMX was obtained after heating the other phases beyond 473 K.

The crystals of the different phases are easily recognized under the microscope. The double pyramids of p-HMX contrast to needles of a-HMX and the platelets of y-HMX.

3. Thermal Analysis by X-Ray Diffraction

3.1 Method

(1) Measuring System Phases transitions are mostly investigated by DTA and

DSC. However, these methods yield only indirect informa- tions on the lattice registering the thermal effects caused by the transition enthalpies. An identification of the phases is not possible in this way.

Direct informations on the lattice and the identification of the phases were obtained with a system that was develop- ed in ICT for the investigation of the phase transitions of ammonium nitrate (Fig. 1) (lo). It consists of an X-ray dif- fractometer combined with a heating and cooling device. The samples are heated stepwise followed by measuring a diffraction pattern after each step. Reducing the measuring times with a position sensitive proportional counter (PSPC)

S t o r a g e z Figure 1. Measuring system for thermal analysis by means of X-ray diffraction.

(2) Evaluation The measured series were evaluated by a least squares fit

procedure using a Gaussian profile yielding peak position, intensity and peak width. The peak positions were used for fitting the cell constants. The intensities, cell constants and volumes per molecule were plotted versus temperature. Thermal expansion, volume changes during the phase tran- sitions and transition temperatures were obtained from these curves.

(3) Measurements 2 series were measured with each phase using chromium

radiation. An angular range from 20 to 60 degrees 2 theta was scanned with 30 degrees per minute. The sample were heated and cooled in steps of 2 K with the temperature pro- grams 29311731493 K and 29314931173 K. Figure 2 con- tains selected patterns of a series starting with p-HMX.

3.2 Thermal expansion and densities of the phases

The thermal expansions are listed in Table 2. With p- HMX and y-HMX a strong anisotropic expansion was found together with a simultaneous change of the angle p of the monoclinic lattice. The expansion is most pronounced within y-HMX followed by 6, p-, and least a-HMX. The calculated densities at room temperature amount to 1.89, 1.82, 1.76 and 1.78 [g/cm3] for p, a , yand 6-HMX, respec- tively. The values agree with those from the literature (Tab. 1).

Table 2. Thermal Expansion of the Cell Constants and the Volumes

Linear Molec. Volume Volume Phase Cell expans. vol.* expans. change**

constants 1 0 - ~ r % ~ 1 rA31 1 0 - ~ r % ~ 1 r%i

P a b C

P ci a

b C

Y a b C

P 8 (P)+ a

8 (a)+ a

6 (Y)+ a

C

C

C

-0.29 11.6 259.2 13.1 6.7 2.30 (7.7)*** 2.58 3.65 4.86 268.8 9.6 4.6 1.21

-0.75 3.5 282.4 15.4 -3.3

13.5 34.1 ++

-4.9 4.1 277.4 13.5 - 2.6 6.6 278.3 14.7 - 2.0 5.6 274.8 13.0 - 1.7

* at room temperature + starting phase ** at transition to 6-HMX ++ beyond 350 K

maximum volume difference ***

192 M. Henmann, W. Engel, and N. Eisenreich Propellants, Explosives, Pyrotechnics 17, 190-195 (1992)

0

0 0 5-- - 4

00 -.? EF--

0 0

YI

‘T [ 970.00

467.00

temp. [K]

1 I1 0 0

P - HMX 458. 00

P-HMX [ 455.00

41.00 45.00 49.00 53.00 57.00 61.00 2 Theta [degrees]

Figure 2. Selected diffraction patterns from a series measured with P-HMX using chromium radiation.

The experiments with y-HMX revealed interesting details. On heating from 330 K to 450 K the (200)-peak shifts to higher angles before a second peak appears at 390 K nearby. The shifted peak disappears at 450 K (Fig. 3). The corresponding difference of the lattice plane distance d(200) amounts to approximately 0.04 A. The reversed effect was not observed, when the sample was cooled. The volume curve showed a bend in this tempera- ture range. Measurements with the DSC showed a slight bent in the curve at about 390 K with y-HMX.

temp. [K]

457.00 U56.00 455.00 U5U. 00 1153.00 UU3.00 U33.00 423.00 U13.00 403.00 393.00 383.00 373.00 363.00 353.00 343.00 333.00 323.00 313.00 303.00

70.60 20.70 20. BD 20.90 21.00 2 theta (degrees)

Figure 3. Change of the (200) y peak in the diffraction patterns.

3.3 Phase transitions

On heating, p-, a- and y-HMX changed into 6-HMX. On cooling no reconversion was observed, as can be seen from the intensity curves of 2 representative peaks of p- and 6- HMX in Fig. 4.

On heating, p-HMX contracts before the transition to the high temperature phase (Fig. 5). The contraction found by X-ray diffraction and in TMA measurements was not rever- sible, when the sample was cooled immediately before changing into 6-HMX.

The volume changes obtained from the diffraction series amounts to 6.7%, 4.6% and -3.3% for the p + 6, a + 6 and y + 6 transitions, respectively (Table 2). The value for the p + 6 transition was calculated with a linearly extrapo-

170.00 250.00 330.00 U10.00 1190.00

3

c 0)

a 40 c o .- 0

0 0

1;O.OO 2k0.00 3 iD .00 t e m p . [KI

Figure 4. Intensity curves of the peaks (131)p and (1 13)6.

Propellants, Explosives, Pyrotechnics 17, 190-195 (1992) Thermal Expansion of HMX 193

J

transition c S-HMX

r - I

T lin. extrapolated

the transition 6 + P was to be determined. It was observed that the storage at 223 K freezes the transition. Therefore sample C consisted after 20 h at 223 K completely of 6-HMX. A and B contained approximately the same mixtu- re of 6- and P-HMX. It is concluded that cooling to 223 K doesn't enhance nucleation and reconversion into P-HMX.

In further experiments samples of a- and P-HMX were

-

390.00 409.75 429.75 449.75 469.75 409.75[1(1

Figure 5. Contraction of P-HMX before the transition was measured by TMA.

P-HMX expansion cu rve 3

lated expansion curve. Because of the contraction of this phase, the maximum volume difference at the transition is about 1 % higher.

In the TMA measurements (Fig. 5 ) the recorded shrink- age is considerably higher, suggesting additional effects, such as sintering of the powder.

Table 3 contains the transition temperatures found with DSC at various heating rates. The transition intervals extend from 454 K to 466 K, 461 K to 467 K and 444 K to 455 K for P-, a- and y-HMX, respectively, with heating rates from 0.5 K/min to 10 K/min compared to 438 K, 461 K and 443 K given in literature @).

Table 3. Transition Temperatures at Various Heating Rates Measured with DSC

heated to 463 K-503 K for 0.5 h and 4 h and cooled to room temperature. Diffraction patterns were measured after 1, 4, 11 days and partially after 25 and 38 days. The phases were identified, their concentrations were estimated. The results are found in Table 4.

Rate [K/min]

50 20 10 5 2 1 0.5

Transition Temperature [K] P + S a+6

412 469 461 468 466 461 451 463 451 462 442 46 1 454 46 1

Y-16 463 458 455 45 2 450 441 444

3.4 Reconversion experiments

Having observed a strongly delayed reconversion of 6- HMX further samples were heated in ovens and stored at room temperature or in cryostates to avoid time consuming experiments with the diffractometer. Afterwards the diffrac- tion patterns were measured at room temperature, the pha- ses were identified and their concentrations were estimated.

Three samples A, B, C of P-HMX were heated for 1/2 h at 463 K and cooled by different temperature cycles:

A: 20 h at 293 K B: 5 h at 223 K, 15 h at 293 K C: 20 h at 223 K.

the Transition Temperature

Heating Temp. Time Storage at 293 K in days

Phase iK1 Ihl 1 4 11 25 38

P 463 0.5 p+S P P 4 Pts P P

413 0.5 p 6 P P 4 a+P+6 a+@ a+P+

483 0.5 p+6 p P 4 6 a+p a+p+

493 0.5 &(+a) a+P a+P+ 4 s a+P+6 a+p+

503 0.5 6 a+P a+p* brown 4 6 6 6(+a+B) a+D+6 a+B+6 +

~~~

a 463 0.5 a+p a+p a+P* 4 a+P a+P a+P-

413 0.5 a a a 4 a a a

483 0.5 a a a 4 a a a

493 0.5 a a a 4 a+6 a a

503 0.5 a a a brown 4 6 S 6(+a+P) a+wS a+p+6++

++: P-HMX dominant, +:more p than a, *: equal parts, -:more a than P,

It can be seen that the observed phase depends from the maximum temperature, the heating time, the storage time at room temperature and the starting phase.

Samples of P-HMX with a maximum temperature of 463 K reconverted slowly from 6- to P-HMX. With increasing maximum temperature and longer storage time a-HMX appeared and the reconversion slowed down. In the samples heated 4 h at 503 K the reconversion was not yet complete after 38 days, when the samples consisted of a mixture of

Samples of a-HMX with a maximum temperature of 463 K changed from 6- into a mixture of a- and P-HMX. With increasing maximum temperature the reconversion

a-, P- and 6-HMX.

194 M. Henmann, W. Engel, and N. Eisenreich

into a-HMX became predominant until with a maximum temperature of 503 K the same behavior was observed as with samples of P-HMX.

Starting with a - and P-HMX the samples became brown after heating to 503 K. Therefore the decomposition was investigated with a thermobalance heating P-HMX with a rate of 1 K/min.

The kinetic parameters were determined by a least squa- res fit procedure resulting in an activation energy of 236 kJ/mol, a reaction order of 0.47 and a pre-exponential factor In k, = 45.6.

Based on these data the decomposition of HMX was cal- culated for the investigated maximum temperatures and storage times (see Table 5). Comparing the data in Tables 4 and 5 it can be concluded that starting with a decomposition of 0.3% a-HMX and beyond a decomposition of 2%, 6-HMX is stabilized.

Table 5. Decomposition of HMX Calculated from TG Curves [weight %]

Temp. Heating Time [h] K I 0.5 4

463 0.0008 0.061 473 0.028 0.322 483 0.096 0.77 493 0.316 2.56 503 1 .oo 8.23 513 3.01 27.18

4. Sensitivities

The sensitivities against friction and impact according to BAM were determined with ground and unground samples. The grinding was done in a Pulverisette of Fritsch in an agate mortar.

In the case of 6-HMX, samples of P-HMX were ground before they were transformed into 6-HMX by heating to 483 K for 1/2 h. The sensitivities were determined imme- diately after cooling. The results are found in Table 6.

Table 6. Friction and Impact Tests of the HMX-Phases

Phases Friction Sensitivity Impact Energy [knl [kpml

p cryst. 10.8 12.0 0.25 0.20 ground 10.8 12.0 0.15 0.30

a cryst. 9.6 10.8 0.15 0.15 ground - 10.8 - 0.15

6 cryst. - 10.8 - 0.04 ground 9.6 10.8 0.10 0.08

Y crvst. 10.8 14.4 8.00 >10

The friction values are similar. a - and 6-HMX are slight- ly more sensitive than p- and y-HMX. No difference can be

Propellants, Explosives, Pyrotechnics 17, 190-195 (1 992)

seen between ground and unground samples. The values agree with the value of 12 kg given by Meyer(I2).

High impact energies were found with P-HMX. a-HMX is slightly more sensitive. A considerable decrease of the impact energies was observed with 6-HMX. Amongst the two 6-HMX samples ground HMX was less sensitive.

The values are considerably lower than the impact ener- gy of 0.75 kpm given by Meyer(12) reaching the sensitivities of primary explosives.

5. Burning Rates

Since the phase transitions are connected with different transition enthalpies and activation energies, it can be expected that the burning rates of HMX depend on the phases. The lattice energy of 0-HMX ist the lowest, follow- ed by the a-, y- and 6-HMX as can be seen from the endo- thermic enthalpies of the transitions to 6-HMX (Table 7). Therefore the enthalpy of the exothermic decomposition must be lowest for B-HMX, followed by a-, y- and 6-HMX. Consequently the lowest burning rate for the decomposition of HMX is assumed with 0-HMX as starting material.

Table 7. Activation Energies and Enthalpies of the Transitions of HMX [kJ/mol] __ ~. __ ~ __ ~ __

Transition Eq Enthalpie

P + S 204 9.3 a+& 208 6.7 Y+6 219 2.7

Karpowicz(*), Rylance(")

For the measurement of the burning rate, strands of the different phases were prepared by pressing fine ground material at 70 bar with a few drops of acetone. The strands were covered with solprene for symmetrical burning. For the measurement of 6-HMX strands of P-HMX were heated for 30 min at 463 K and measured the same day.

The burning rates were determined in an optical bomb with different pressures, including two measurements of each phase and pressure. The burning was observed with a diode array. Details of the instrumentation are reported elsewhere(I3).

The results in Figure 6 show that a-HMX has a slightly higher burning rate than P-HMX. With 6-HMX the burning rate is strongly increased.

6. Discussion

The investigation with thermal analysis using X-ray dif- fraction yielded reasonable results. The transition tempera- tures and the densities at room temperature agree with liter- ature.

The thermal expansion of a-HMX is lowest. The values are similar for the other phases. The volume changes during the transitions are consistent with the differences of the densities at room temperature. An anomaly was found with

Propellants, Explosives, Pyrotechnics 17, 190-195 (1992)

10 2 . 5 0 5 . 0 0 7 . 5 0 10 .00

6-HMX

I0 2 . 5 0 5 .00 7 . 50 10 .00

0 m

7

0 0

m

0 m

+

0 0

0

Pressure [ ba r ]

Figure 6. Burning rates of the phases.

y-HMX. It consists of a bend in the thermal expansion at 350 K as well as in the DSC curve and a peak shift de- scribed in Figure 3. This anomaly could correspond to the decomposition of the hydrate water, though the loss of water could not be detected with a thermobalance and the expected content of hydrate water could not be found with Karl-Fischer analysis.

As a further anomaly a contraction of P-HMX was observed immediately before the transition into 6-HMX. The p+6 transition was connected with the strongest expansion and the maximum volume change during the transition was increased by the contraction of b-HMX.

The reconversion experiments confirmed the results reported by cad^(^). The slow reconversion results in a mix- ture of p- and a-HMX. An interesting correlation between decomposition and phase behavior was found. With in- creasing decomposition first a-HMX then 6-HMX was stabilized. The reconversion slowed down with increasing decomposition.

The observed burning rates confirm qualitatively the expected relation between transition enthalpies and burning rates. With 6-HMX the burning rate is strongly increased.

The impact energies of 6-HMX are extremely low, reach- ing values of primary explosives. An interesting feature is the fact that the high impact sensitivities are still more expressed with larger crystals of the unground product.

Thermal Expansion of HMX 195

The high sensitivity of 6-HMX connected with the low reconversion rate increases considerably the handling risk, when HMX is exposed to temperatures above 440 K. Due to the slow reconversion the increased risk is left for long periods even after cooling HMX to room temperature.

7. References

(1) H.H. Cady, A.C. Larson, and D.T. Cromer, “The Crystal Struc- ture of a-HMX and a Refinement of the Structure of 0-HMX”, Acta Cryst. 16 (7), 617-23 (1963).

(2) C.S. Choi and H.P. Boutin, “A Study of the Crystal Structure of P-HMX by Neutron Diffraction”, Acta Cryst. B26, 1235-40 (1970).

(3) R.E. Cobbledick and R.W.H. Small, “Crystal Structure of the &-Form of HMX’,Acra Cryst. Sect.B, B30 (8), 1918-22 (1974).

(4) P. Main, R.E. Cobbledick, and R.W.H. Small, “ Structure of the Fourth Form of HMX (7-HMX)”, Acta Cryst. Secr.C,

(5) T.R. Gibbs and A. Populato, “LASL Explosive Properry Data”,

(6) H.H. Cady and L.C. Smith, “Studies on the Polymorphs of

(7) H.H. Cady, “Sensitivity and Handling Hazards of 6-HMX Obtained by Heating PBX 9404”, 17th International Annual Conference oflCT, 1986, [Proc.] pp. 17.1-17.12.

(8) R.J. Karpowicz and T.B. Brill, “Solid Phase Transition Kinetics. The Role of Intermolecular Forces in the Condensed-Phase Decomposition of HMX’, J.Phys. Chem. 86(21), 4260-5 (1982).

(9) M. Bedard, H. Huber, J.L.Myers, and G.F. Wright, “The Crystalline Form of HMX”, Can. J . Chem. 40,2278-99.

(10) V. Kolarik, W. Engel, and N. Eisenreich, “Beobachtungen des Phasenverhaltens von trockenem Ammoniumnitrat mit Rontgen- beugung unter zyklischer Temperaturbelastung”, 20th Znterna- tional Annual Conference of ICT, 1989: “Environmental Testing in the ~O’S”, [Proc.] pp. 92.1-92.14.

(1 1) J. Rylance and D. Stubley,”Heat Capacities and Phase Transitions of Octahydro-l,3,5,7-Tetranitro-l,3,5,7-Tetrazocine(HMX)”, Thermochimica Acta 13,253-259 (1975).

C41(9),1351-4 (1985).

University of California Press, Berkeley, (1980).

HMX’, LAMS-2652, (1962).

(12) R. Meyer, “Explosivstofse”, Weinheim: Verlag Chemie (1979). (13) H.P. Kugler and F. Sinn, “An Optical System for Measuring the

Burning Rate of Propellant Strands”, Propellants, Explos., Pyro- tech. 12,78-80 (1987).

Acknowledgement

We thank Professor Dr. H. Wondratschek for his guidance and fruitful discussion.

(Received March 25, 1992; Ms S2/92)