Simulation and moderation of the thermal response of confined pressed explosive compositions

Simulation and Moderation of the Thermal Response of Confined Pressed Explosive Compositions IAN J. DAGLEY, ROBERT P. PARKER,* DAVID A. JONES, and LUCIANA MONTELLI Aeronautical and Maritime Research Laboratory (AMRL-DSTO), P.O. Box 4331, Melbourne, Vic. 3001,Australia The effects on the thermal response of pressed polymer bonded explosives caused by varying their components have been assessed at two extreme heating rates using the Super Small-scale Cookoff Bomb. Tests were primarily conducted on RDX-based compositions containing 5% ethylene-vinyl acetate binder with varying amounts of PETN or TATB. Some experiments were numerically simulated using a onedimensional finite difference code. The simulations are not able to predict the violence of the thermal response, but do accurately reproduce radial heat flow in the test assembly and satisfactorily predict both the time to thermal response and the surface temperature at response for the mixed explosive compositions. The influence of the ratio of the mixed explosives on the type of thermal response observed is discussed and several compositions which give very mild thermal responses have been identified.

INTRODUCTION A major cause of damage in accidents involving confined explosives has been their violent, typically detonative, response when heated in fires. In these events the nature of heat transfer determines the sites of ignition and two extreme external heating rates, commonly referred to as fast and slow cookoff conditions, are usually considered. A fast heating rate, which corresponds to direct exposure in a fuel-fire, results in a large radial temperature gradient across the explosive and ignition at or close to the surface of the explosive [1, 2]. In contrast, a very slow heating rate, corresponding to prolonged indirect exposure to a heat source, leads to thermal equilibration across the explosive and in this case more central ignition occurs [1, 3]. The nature of the thermal response depends on the extent of the passage from burning through deflagration (and the formation of compressive shock waves) to detonation at the time that overpressure caused by the gaseous products relieves the confinement [2]. Moderation of cookoff response requires controlled or inhibited burning reactions at increasing pressures, improving resistance to fracture under shock compression to reduce explosiveness, reduction of the shock sensitivity of the explosive

* Author to whom correspondence should be addressed.

or combinations of these (and other) approaches designed to avoid the occurrence of deflagration-to-detonation transition (DDT) reactions. Because of the complex nature of these processes, and the experimental difficulties in, studying these events, there is a poor understanding of the mechanisms involved in cookoff reactions and of how to achieve moderate responses. The effects of composition, pressure, and temperature on the burn rates of propellants have been studied extensively [4], but there is a paucity of published information of this type that is useful for relating the burning" behavior of various explosive compositions to cookoff behavior [3, 5]. The processes occurring in DDT reactions in porous beds of some explosive are now well characterized [6, 7]; however, this is not the case for polymerbonded explosive (PBX) compositions pressed to low (10% or less) voidage, that are widely used as explosive charges, and where factors determining susceptibility to DDT are not well understood. Several small-scale tests have been developed to predict, or rank, the cookoff response. of explosive compositions. They are typically used to identify explosive compositions that are likely to respond mildly. One important factor is the degree of confinement of the explosive under test, and in one small scale test the confinement is varied to determine the confinement pressure necessary to cause the COMBUSTIONAND FLAME 106:428-441 (1996)

0010-2180/96 SSDI 0010-2180(95)00260-X

Copyright © 1996 by Department of Defence, Australia Published by Elsevier Science Inc.

T H E R M A L RESPONSE OF PRESSED COMPOSITIONS explosive to undergo a transition from a burning reaction to a more violent response [8]. Most other small scale tests have a fixed confinement and varying methods for heating the test assembly [2, 9-11]. The most reproducible employ a controlled heat source (electric band heaters), and one of these, the Small-scale Cookoff Bomb (SCB) test [10, 11], has been adopted by the UN as a suitable test for classifying energetic materials with regard to their thermal response [12]. We chose to use a smaller version of this test, the Super Smallscale Cookoff Bomb (SSCB) test [11], to more thoroughly examine the influence of composition on the thermal response of confined pressed PBXs. PBX compositions for pressed charges are prepared by coating an explosive, or mixture of explosives, with a polymeric binder. These compositions typically contain the powerful nitramine explosives R D X (hexahydro-l,3,5trinitro-l,3,5-triazine) or HMX (octahydro1,3,5,7-tetranitro-l,3,5,7-tetrazocine), which alone respond violently under cookoff conditions. Their fast cookoff response is influenced by the extent to which the polymeric binder coats the explosive crystals [13], and can be moderated by blending R D X with the more thermally stable (and less sensitive) explosive TATB (1,3,5-triamino-2,4,6-trinitrobenzene) [14]; blending R D X with a less thermally explosive (such as pentaerythritol tetranitrate, PETN) would be expected to produce an earlier, and possibly less violent, thermal response. Certain polymeric binders that moderate cookoff response also have been identified [15]; however, the reasons for this moderation have not been established. This paper reports a systematic examination of the influence of the nature of the explosive component(s) on the cookoff response of pressed explosives; it was undertaken to obtain a better understanding of the processes that determine the response. As an initial step towards prediction of the cookoff response of these explosives, a mathematical model describing the heat flow, decomposition and the onset of ignition in the SSCB test has been developed and used to predict the temperature and time at which cookoff of mixed explo-

429

sives occurs. Previously, mathematical modeling has been similarly used in simulations of One Dimensional Time to eXplosion (ODTX) experiments [16, 17], but there are important differences. The ODTX experiments impose a constant boundary temperature at the surface of a heavily confined, spherical explosive sample, whereas the SSCB test applies a steadily increasing temperature to the surface of a more lightly confined explosive, and the tests are typically conducted at both fast and slow heating rates. EXPERIMENTS Materials

The explosives, and their median particle sizes, were: RDX, 180/~m; RDX(fine), 20/xm; HMX, 190 /zm; HMX(fine), 9 /xm; TATB, 40 /zm; and PETN, 27/xm. Polymers used as binders were Elvax 210 (DuPont) and the acrylic dispersion Rhoplex HA-24 (Rohm and Haas). The compositions (typically explosive/polymer 95:5 w/w) were prepared by adapting previously described methods [15, 18]. Measurements

The cookoff behavior of the compositions was assessed using the Super Small-scale Cookoff Bomb [10, 19]. The SSCB test samples consisted of four pellets 16 mm diameter x 16 mm long, pressed to 90% theoretical maximum density (TMD), with a total mass of approximately 20 g. Tests were performed at fast (approximately 1° C / s ) and slow (approximately 0.1 ° C / s ) heating rates. Although the heating rates used for this test do not correspond with those specified for full-scale munitions fast and slow cookoff tests, the SSCB test does provide similar responses to those obtained in the full-scale tests [10]. Generally, duplicate tests were performed at the fast heating rate, and those compositions which gave non-detonative responses were then also tested at the slow heating rate. This testing strategy was used since slow cookoff conditions generally produce more violent responses [19]. The SSCB test assembly is shown in Fig. 1.

430

I . J . DAGLEY ET AL.

I "~-"------~

Thetlocouple Sealing plug Top IMate Outer cyUnder

f-

Liner 8and healers Explosive samples Base plale

Washer

The shock sensitivity of the pressed compositions was determined using the MRL smallscale gap test [20]. The donor was a UK Mk 3' exploding bridge wire detonator attenuated by brass shim, and the acceptor was two 12.7 mm diameter × 12.7 mm high cold-pressed cylinders of the explosive under study. A detonation was confirmed using a mild steel witness block. Results were from 20-30 firings and represent' the thickness of brass shim, in millimeters, required to attenuate a standard shock to give a 50% probability of detonation, together with the standard deviation. NUMERICAL MODEL We assume that the heat flow within the SSCB is cylindrically symmetric and can be described by the following one-dimensional radial heat flow equation [21]:

Fig. l. SSCB test assembly,

~T pC(T) c~t The results obtained from the SSCB test are the type of response, the explosive surface temperature at response (obtained from the thermocouple positioned in a slot in the liner, via calibration graphs obtained with inert-filled SSCBs) and the time to response. The SSCB test is normally used in a qualitative manner to assess the cookoff response of compositions, and caution must be exercised in comparing the results from this test. Replicate tests on a given composition may produce different responses, and so an individual test result should be taken as an indicative, but not unequivocal, definition of that material's cookoff behavior under the test conditions used. Measurements to verify radial temperature-time profiles from some computer simulations were made using a modified test assembly, with two fine thermocouples (0.2 mm diameter wire, in Teflon insulation) inserted 16 mm deep into the upper explosive pellet (three pellets, each 21 mm long, were used for these tests) via small holes (1.5 mm diameter) drilled through the top plate and into the explosive pellet. The thermocouples were positioned at the half-radius point and on the axis of the pellet.

O(A(T)~T ) Or -~r 2A(T) c~T + -

-

r

--

Or

+ s,

(1)

where the thermal conductivity A and specific heat C are functions of the temperature T,' and p denotes the density of the explosive mixture. Jones and Parker [22] have recently shown that the temperature distribution within the related Small-scale Cookhoff Bomb (SCB) can be accurately described by a one-dimensional heat flow model, and McGuire a n d Tarver [17] have shown the importance of including the temperature dependence of the specific heat and thermal conductivity when modeling heat flow in ODTX experiments. The source term S in Eq. 1 describes the rate of heat generation per unit volume at temperature T, and for an explosive mixture consisting of N species has the form [23]: N

S = ~ pcoimiQiA i e x p [ - E i / R T ] ,

(2)

i=I

where oJi is the fraction of undercomposed explosive for species i, m i the mass, ai the heat of reaction per unit mass, A i the preexponential factor, E i the activation energy, and R

T H E R M A L RESPONSE OF PRESSED COMPOSITIONS the gas constant. Equation 1 is solved by an operator splitting approach and standard finite difference techniques [24], and the method of Farnia and Beck [25] is used to form finite difference expressions for terms containing temperature-dependent thermal properties. The melting of individual explosive components is modeled by holding the temperature at any given node constant at the melting point value of that component until an energy equivalent to the latent heat of fusion of the explosive has been absorbed. To calculate the thermal conductivity of the explosive mixtures we used an expression originally derived by Maxwell [26], and more recently by Jeffrey [27], who derived an expression for the thermal conductivity to a higher order in the species concentration. Jeffrey studied the conduction of heat through a stationary, random and statistically homogeneous suspension of spherical particles in a matrix of uniform conductivity under the condition that the volume fraction of the particles is small. If the matrix has a thermal conductivity hi, and the spheres a thermal conductivity of h 2, then to first order in the concentration c the effective conductivity A is given by A = Al(1 + 3/3c},

(3)

where

(4)

13 ~ ( '~2 - hj ) / ( h2 + h~ ).

Equation 3 is valid when the concentration is small enough to make all interactions between

431

the spheres negligible. Jeffrey has extended the expression for A to second order in c by allowing for interactions between pairs of spheres, but we have calculated the thermal conductivity using the lower order correction only. Use of Eq. 3 gives results which are very close to those calculated by assuming that the thermal conductivity of the mixture to be simply the mass average of the values for the individual explosives. This is the method used by McGuire and Tarver in their calculations [17]. The decomposition of each of the explosives is described by a first-order kinetic scheme, and the thermochemical data for RDX, PETN, and TATB are listed in Table 1. The thermal conductivity and specific heat data for RDX and TATB are from McGuire and Tarver [17], while the remaining data are from earlier work of Rogers [28], Zinn and Rogers [29], and Zinn and Mader [30]. Our models for the two explosive mixtures ( R D X / P E T N and R D X / T A T B ) considered in this section are necessarily different. The R D X / P E T N model employs a first-order reaction scheme for both explosives, and allows for the melting of both materials. It does not include the temperature dependence of the thermal conductivity and specific heat of PETN, since this data are unavailable. The temperature dependence of the thermal conductivity and specific heat of TATB has been reported by McGuire and Tarver [17] and Drake [16], and was included in the model for the mixtures of RDX and TATB containing

TABLE 1

Thermochernical Constants for RDX, PETN and TATB

Density (Mg m -3 ) Thermal Conductivity, A (W m - 1 K - l ) Specific Heat, C (kJ kg- ~ K i ) Preexponential factor, A

(s-1)

Heat of reaction, Q (MJ kg- 1) Activation energy, E (kJ m o l - i ) Latent heat of fusion (kJ kg 1) Melting point (° C)

RDX

PETN

TATB

1.80 0.26 (293K) 0.21 (433K) 1.0 (293K) 1.8 (623K) 3.162 x 10 TM

1.74 0.25 (293K)

6.300 X 1019

1.84 0.80 (293K) 0.59 (433K) l. 1 (293K) 1.8 (623 K) 3.180 >~ 1019

2.09

1.25

2.51

200

197

251

160

41.9

--

200

141

320

1.14 (293K)

432

I . J . DAGLEY ET AL.

Elvax 210 (5% w/w). First order reaction schemes were employed for both RDX and TATB, although the TATB does not react in the temperature range considered here. No allowance for TATB melting was necessary, since its melting point (320 ° C [31]) is above the temperature at which response occurs in any of the present experiments or calculations. The choice of chemical model to describe the decomposition of the RDX is particularly important. In addition to the first-order reaction schemes, we also implemented the more recent three-term model of McGuire and Tarver [17] which was obtained from data on ODTX experiments. It was expected that this would provide better agreement with experiment. Preliminary calculations on the reference composition RDX/Elvax 210 (95:5), however, showed that the first-order decomposition scheme gave better agreement. Table 2 compares the simulated time to response, and surface temperature at response, with experiment for both decomposition schemes when applied to RDX/Elvax 210 (95:5) in the SSCB. These results indicate that the simple firstorder scheme provides better agreement with experiment than the more sophisticated threeterm scheme, especially for the slow cookoff results. The reason for this is believed to be related to the differing amounts of confinement in the SSCB and ODTX tests. Tarver et al. [32] and Catalano et al. [33] have shown that the time to explosion is strongly influenced by the void volume of the containment vessel. Even the addition of a relatively small void space in the ODTX experiments resulted in almost a doubling of the time to explosion

TABLE 2

Effect of Different RDX Decomposition Schemes on Simulated Time to Response and Surface Temperature at Response First-Order Multiterm Kinetics Kinetics Experiment

Fast Cookoff Time to Event (s) Surface Temp. (° C)

263 245

235 226

246 240

1580 214

1315 201

1654 218

Slow Cookoff

Time to Event(s) Surface Temp.(° C)

for LX-04 (HMX/binder 85:15). In the earlier experimental work of Rogers [28] the explosive samples were less well confined and the explo-~" sive decomposition products were easily vented, whereas in the ODTX experiments great care was taken to ensure that none of the gaseous ' products was allowed to escape from contact with the high explosive sample. Calculations of critical temperatures for the explosives TNT ~ (2,4,6-trinitrotoluene), TATB and LX-10 (HMX/binder 95:5) using a multiterm decomposition scheme similar to the one above gave good agreement with results from ODTX tests, but were appreciably lower than values calculated using the earlier single step schemes [32].~ Similar reductions in surface temperature at response are observed here for the RDX/Elvax 210 composition shown above. The SSCB contains a relatively large void space at the top of the cylinder, and the top confining plate contains a small hole which allows access for thethermocouple. This remains unsealed during the test (Fig. 1). Hence the gaseous decomposition products are relatively easily vented, and the SSCB is more aptly described by the single step reaction schemes constructed to model the earlier experiments. When the multiterm kinetic scheme is used the effect is to reduce both the time to explosion and the surface r temperature at explosion, for both fast and slow cookoff. The calculations use a reference composition consisting of 100% RDX rather than the actual composition of RDX/Elvax 210 (95:5). Approximate calculations show this to have ~ little effect on the predicted cookoff temperature and time to response, even though the binder has a significant effect on the violence of the reaction. Data on the thermophysical constants of Elvax 210 are difficult to obtain, but the thermal conductivity of Elvax 265 (a" related polymer which should have very similar thermophysical properties to Elvax 210) is 0.30 W m-1 K-1 at 30° C and 0.22 W m-~ K-a at 200° C [18]. These are very close to the thermal conductivity values of 0.26 W m -1 K -~ at 20° C and 0.21 W m -t K -1 at 160° C for RDX' used by McGuire and Tarver [17]. An estimate of the effective thermal conductivity of the RDX/Elvax 210 (95:5) mixture can be made by assuming that the composition consists of

THERMAL RESPONSE OF PRESSED COMPOSITIONS spheres of RDX uniformly coated with Elvax 210. Helsing and Grimvall [34] quote the following expression for the effective thermal conductivity of such a composite system:

(

'~eff = )ka "~ f/3 1//(,~/3

_

,

/~a) "}- fe//(3~)

) (5)

where phase /3 represents the RDX, phase a represents the Elvax 210, and f,, and f~ represent the respective volume fractions. With this expression to calculate the thermal conductivity at the lower temperature, a value of 0.26 W m - ' K-1 is obtained. This is the same as the value for RDX at this temperature, indicating that use of the thermal conductivity of RDX for the effective thermal conductivity of the composite system is a valid approximation. The specific heat of Elvax 210 is 2.39 ld kg-1 K-1 at 150° C and 2.63 kJ kg -1 K -1 at 220 ° C [18], which is considerably higher than the values of 1.0 kJ kg-1 K-1 at 20° C and 1.8 kJ kg-1 K-1 at 350° C for RDX used by McGuire and Tarver [17]. The effective specific heat of the composite material will still be very close to the RDX value, however, because the contribution from Elvax 210 (which is directly proportional to its mass fraction) will be small compared with the contribution from RDX. The above discussion is based on the use of partly known and partly inferred thermophysical constants for the binder. Given that these approximate expressions result in estimates for the effective thermal conductivity and specific heat for the composite material which are very close to those for RDX, we feel justified in neglecting the effect of the binder on the heat flow within the SSCB, and considering the reference composition to consist entirely of RDX. Using the above model, the time to response and surface temperature at response have been simulated for explosive/Elvax 210 (95:5) compositions, containing mixtures of RDX with both PETN and TATB. The temperature at the surface node was obtained from a reference table which lists the experimentally measured temperature as a function of time at the surface of the SSCB for both fast and slow cookoff. These values were determined during

433

calibration runs using an SSCB filled with an inert material whose thermal properties closely matched those of the explosive fills. Thermal explosion was considered to have occurred in the simulation when the temperature at any point in the explosive reached a predetermined high value. For the calculations reported here this value was 555 ° C, although the actual value used had negligible effect on the results.

ASSESSMENT OF THE MODEL Radial Temperature Profile During Heating in SSCB Tests The simulation can be used to predict the radial temperature profiles within the SSCB test at selected times during heating under both fast and slow cookoff conditions. Some of these predictions along a radius for R D X / Elvax 210 (95:5) are shown in Figs. 2 and 3. Under the fast cookoff conditions (Fig. 2), there is a considerable thermal gradient within the SSCB, the temperature is highest at the edge of the cookoff bomb, and thermal runaway commences very close to the surface of the explosive. The simulations predict that only a

280

A

240 " o0~®~_200160,-

256.8 s ..... 209.1 s - - - 139.4 s

[

- ..... 6 9 ~ , / ,

"

~_ 120 --

// /

,.

"

/

,-

/

/

80 40 0

/ / ..f"

/

I I I J I I I I 0.0 0.1 0.2 0.3 0.4 0.5 0•6 0.7 0.8 Radial distance, cm

Fig. 2. Simulated radial temperature profiles in R D X / Elvax 210 (95:5) in the SSCB; fast heating rate.

434

I . J . D A G L E Y ET AL. 280

-

240

1,645.8 s - - -

1,602.6s

.....

1,393.5 s

......

1,045.2 s

-

.....

/

q

--

200 180

-

696.8 s

&& •/

-

200 140

P

• "

O o

_~ 16o e CL

.........................

--''-'"

E

~

- . . . . . Axis - experimental v Axis - simulation

160

............ 348.4 s

Half-radius - experimental Half-radius - simulation

•

v

•

•

•

120

•

loo

,,~.

v' v

/

v ,'

,

120 ®

80

,,~

1-

,v'jv~

80 60

40

,~

40 20

0 0.0

I

I

I

I

I

I

I

I

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Radial

0

, 0

I

,

50

I

,

100

distance, cm

Fig. 3. Simulated radial temperature profiles in R D X / Elvax 210 (95:5) in the SSCB; slow heating rate.

very thin layer of R D X on the surface of the sample will have melted prior to ignition. Figure 3 shows radial temperature profiles at selected times under slow cookoff conditions. There is now a much more uniform temperature distribution within the SSCB and the thermal runaway commences closer to the center of the bomb. The model predicts that in this case all of the R D X along the radius will have melted approximately two minutes prior to the response. To assess the accuracy of the model, thermocouples were placed at the edge and at a depth of 16 mm on both the axis and a midradius point in the pellets of the pressed R D X / E l v a x 210 (95:5), and the temperature-time profiles were measured under both fast and slow conditions in the SSCB test. The experimental edge temperature-time record was then used to provide input values to the model to simulate temperature values at the axis and mid-radius point in the SSCB. The measured values are compared to those predicted by the mathematical model in Figs. 4 and 5. The lines denote the experimental measurements, while the triangles denote the simulated results calculated from the experimental

I

,

150

I

,

200

I

,

250

I

,

300

Time, s

Fig. 4. Temperature-time plots for two positions in pressed RDX/Elvax 210 (95:5) pellets in the SSCB test assembly; fast heating rate.

200 Half-radius - experimental 180

•

Half-radius - simulation

Jt

- . . . . . A~s - e x p e r i m e n t a l ~ , ~ v v Axis - simulation ~ "

160

/

140 120

100

"~

,4 ~,

Y

80 I--

60 40 20 ,

0

I

200

,

I

400

,

I

600

,

I

800

,

I

,

I

,

1000 1200

Time, s

Fig. 5. Temperature-time plots for two positions in pressed RDX/Elvax 210 (95:5) pellets in the SSCB test assembly, slow heating rate.

T H E R M A L RESPONSE OF PRESSED COMPOSITIONS edge temperatures. The deflections appearing at 100° C in the experimental traces are caused by residual water at the base of the thermocouple holes in the explosive pellets (the pellets were drilled under water for safety reasons), so the simulated temperature-time plots are not expected to agree with the experimental values above 100° C. Below this temperature there is quite good agreement between the experimental and simulated results at both heating rates. Data obtained at higher temperatures using these thermocouples have shown that, under slow cookoff conditions, melting of the RDX occurs from the edge and finally at the axis, approximately 100 seconds prior to response. The differences in the extent of melting of the R D X under fast and slow cookoff conditions have important implications for the interpretation of results presented later in this paper and for strategies designed to mediate the response of the composition under the different heating rates.

220

.0

@

o~

210

o

0

~

o o

2O0 •

0 •

•,9o

0

(a) I. 0

J

I 10

o

1:300 -

~

I 20

o ' ~ _

~

-

I 30

,

I 40

o_ _ . . ._. _ _ o o

1200

1100

Temperature and Time of the Response

,O O O.\

435

-(b) I 0

,

,I 10

l

I 20

,

I 30

i

I 40

%PEIN

To further assess the mathematical model, the experimental and simulated surface temperature at response (Figs. 6a-9a), and time to response (Figs. 6b-9b) were compared for explosive/Elvax 210 (5%) compositions containing RDX and up to 40% PETN or TATB. In all of the plots discussed here, the solid curves represent lines of best fit (second order) to the simulated results, while the dashed curves are lines of best fit (second order) to the experimental results. Time-to-explosion test data for PETN [28] indicate that this explosive is less thermally stable than R D X (critical temperature 200-203 ° C cf. 215-217 ° C). Incremental replacement of R D X by PETN might be expected to progressively lower the surface temperatures at response and decrease the time to response. This effect should be more pronounced in the slow cookoff case, where the radial thermal gradient is more uniform. For the R D X / P E T N / E l v a x 210 (5%) compositions, such behavior was observed experimentally and predicted by the simulation (Figs. 6 and 7). The large scatter in the experimental points makes a more detailed comparison dif-

Fig. 6. Pressed RDX/PETN/Elvax 210 (5%) at slow heating rate. (a) Surface temperature at response. (b) Time to response: ©, experimental; 0 , simulation.

ficult. The extents of temperature and time reduction in the slow cookoff case are apparently slightly greater for the simulated results, and this may be due to the neglect of the temperature dependence of the specific heat and thermal conductivity of the PETN. In the fast cookoff case, the simulated results are in quite good agreement with the line of best fit to the scattered experimental results for the time to response (Fig. 7b), and also for surface temperature at response but only up to about 20% PETN. Above this level the experimental temperatures decrease more rapidly than those simulated (Fig. 7a). If the RDX source term is omitted from Eq. 2 then the simulations produce almost identical results, indicating that the response is triggered by the PETN which undergoes thermal runaway before the RDX begins to react appreciably. Time-to-explosion test data for TATB [28] indicate that this explosive is much more thermally stable than RDX (critical temperature

436

I . J . DAGLEY ET AL.

250

255 0

0

0

O

O O

25O

24O 0 0

~J 245 ~---

O O

22O

i

\

.

210

O

O

\ O

-(a) I 0

,

I

l

10

I

i

20

236

O

I

l

I

230

40

30

I 0

,

I 10

A

%FERN

I 20

i

I 30

L

I 40

i

J 40

%TAIB 30O

O

27o~-

29O

o

260 ~ O

O s

28O

O

230 --

i=

O

o

2,40

o

=

230

J O

~

~

s O

O

~

o O

22o

."(b)

O

210

I 0

=

I 10

,

I 20

,

I 30

40

%FEIN

I 0

i

I 10

i

I 20

l

I 30

% TAIB

Fig. 7. Pressed R D X / P E T N / E l v a x 210 (5%) at fast heating rate. (a) Surface temperature at response. (b) Time to response; Q, experimental; O, simulation.

Fig. 8. Pressed RDX/TATB/Elvax 210 (5%) at fast heating rate. (a) Surface temperature at response. (b) Time to response: O, experimental; 0 , simulation.

331-332 ° C, cf. 215-217 ° C). With mixtures of these explosives, the TATB will contribute to the reaction driving the detonation when initiated by a shock wave. However, when thermal decomposition commences for the RDX, the TATB is expected to act essentially as an inert diluent. In contrast to the case for R D X / PETN, the R D X / T A T B simulations predict that the surface temperature and time to response for all compositions will be almost independent of the amount of explosive diluting the RDX (Figs. 8 and 9). The differences arise because in this case the RDX is both the predominant and also the least thermally stable component. The responses all occur under conditions similar to those observed for RDX/Elvax 210 (95:5) at the same heating rate, because of the high levels of the reacting component. Examination of the experimental data for these mixed explosive compositions containing

RDX (or RDX(fine), data not included) showed no clear trends in the variation of experimental data for surface temperature or time to response with the level of TATB. For the slow cookoff case, the ranges of both surface temperature and time for response at 0% and 30% TATB overlap (Fig. 9). They also overlap for surface temperature at response at the fast heating rate (Fig. 8a), and there is near overlap in the time-to-response experimental data (Fig. 8b). The lines of best fit to the fast cookoff experimental data indicate an increase in both surface temperature and time to response at higher TATB levels, but are misleading because of the limited data and the wide variance in results at a given level of TATB. These trends are not evident in the more extensive data obtained for the RDX(fine)/TATB/Elvax 210 (5%) compositions. This conclusion is similar to that drawn from fast cookoff experiments on a series of RDX(fine)/TATB/PTFE

T H E R M A L RESPONSE OF PRESSED COMPOSITIONS 225

0

220

0

0

-0

. . . . 0

215

=

0

~

I

I

J

0

10

0

~

I

,

210

2~

(a) 200

I

20

,

I

30

4o

% TATB

18130

1700 m

o

~-o o-

o -

--'-

_ _'-

_

--

O

-

o

N 1500 1400

(b) 1300

[ 0

o L

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=

I 20

,

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,

437

tion by replacement of up to 35% of the RDX (melting point 204 ° C, critical temperature 215-217 ° C) with the less thermally stable explosive, PETN (melting point 141-142 ° C, critical temperature 200-203 ° C) [28], are shown in graphically in Fig. 10 and some shock sensitivity data on mixed explosive compositions and reference explosives are included in Table 3. At the fast heating rate all the compositions containing PETN gave more violent responses than RDX/Elvax 210 (95:5); at least one detonation was observed at all levels of PETN. Introduction of PETN produces several changes that could enhance the violence of the response. Replacement of RDX (180/~m) with the finer particle size PETN (27/zm) will reduce the polymer coating efficiency. Because PETN has a higher burn rate than R D X [35], mixtures with progressively higher levels of PETN will burn more rapidly to detonation. Also, increased levels of PETN raise the shock sensitivity of the mixture (see Table 3).

I 40

%TATB

Fig. 9. Pressed R D X / T A T B / E l v a x 210 (5%) at slow heating rate. (a) Surface temperature at response. (b) Time to response: O, experimental; 0 , simulation.

(5%) compositions subjected to small-scale fuel fire cookoff tests; the cookoff temperature did not increase for TATB incorporated at levels of less than 60% of the composition [14]. Although the mathematical model provides a useful description of heat flow in the SSCB and valid predictions for the surface temperature at response or time to response for some of the explosive compositions, it is not capable of predicting the violence of the response. Before this can be achieved a much better understanding of the physical processes that occur during cookoff is required. The empirical studies described below were undertaken to gain an improved insight into factors important in moderation of cookoff response. TYPE OF THERMAL RESPONSE

II1'

~_ExpI. -6 ~ Oell.

Bum

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(a)

0

The range of cookoff responses that occurs on modifying the RDX/Elvax 210 (95:5) composi-

30

2O

40

PETN Content, % Deln,

~ ExpL

,

I

I |

I Bt~'n

(b) 0

Effects of Mixed Explosives

10

10

20

30

40

PETN Content, %

Fig. 10. Range of SSCB test responses for R D X / PETN/Elvax 210 (5%) compositions plotted against PETN content. (a) Fast cookoff. (b) Slow cookoff.

438

I . J . D A G L E Y E T AL. TABLE 3

Shock Sensitivity(Ms0~) of Pressed Explosive/Elvax210 (95:5) Compositionsand Other Reference Explosives Composition RDX/Elvax 210 95:5 RDX/PETN/Elvax 210 90:5:5 80:15:5 70:25:5 60:35:5 RDX/TATB/Elvax 210 85:10:5 75:20:5 65:30:5 RDX(fine)/TATB/Elvax 210 75:20:5 65:30:5 50:45:5 ReferenceExplosives RDX(fine)/TATB/Rhoplex HA-24/ZnSt 50:45:4:1 RDX(250-300/xm sieve cut) RDX(fme)

At the slow heating rate the addition of P E T N generally led to milder responses (deflagrations and mild explosions instead of detonations). Under these conditions the P E T N will be a liquid prior to ignition and much more of it may have decomposed to give higher dynamic internal pressures prior to ignition. This could contribute to the earlier release of confinement that must occur to prevent a transition to detonation. The changes in the ranges of cookoff response that occur on modifying the R D X / Elvax 210 (95:5) composition by replacement of up to 30% of the R D X (melting point 204 ° C, critical temperature 215°-217 ° C) with the more thermally stable explosive, T A T B (melting point 350 ° C, critical temperature 331°-332 ° C) [28], are shown graphically in Fig. 11. The fast cookoff response increases in violence to an explosion (violent) at 20% T A T B then decreases to deflagrations at 30% T A T B . The increase in violence is attributed to the reduction in coating efficiency alone (the T A T B is finer than the RDX). The reduction in violence observed at 30% T A T B may be caused by the reduction in the burn rate. T A T B bums

Shock Sensitivity Std Deviation

Density (% TMD)

M50~

90.00

2.23

0.054

89.87 89.99 89.99 90.02

2.35 2.45 2.84 2.99

0.012 0.022 0.024 0.039

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1.60 0.69 0.30

0.030 0.008 0.007

90.00 90.00 90.01

2.53 2.54 1.96

0.028 0.029 0.012

90.00 90.00 90.04

2.09 3.76 3.79

0.022 0.12 0.15

at a lower rate than R D X and the burn rate of the mixture would be expected to be an average rate weighted on a volume basis [3]. Another cause is the diminished shock sensitivity (Table 3). Both of these changes reduce the chance of D D T reactions occurring. The slow cookoff response is moderated by the incorporation of TATB. This is caused by reduction in the burn rate a n d / o r shock sensitivity as a result of the addition of TATB. The changes in SSCB test response that occur on modifying the RDX(fine)/Elvax 210 (95:5) composition by partial replacement of up to 60% RDX(fine) with TATB are shown in Fig. 12. If the most violent responses at the fast heating rate are considered, there is a clear moderation of the most extreme cookoff response (from detonation to burn) with an increased level of TATB. The results observed for the composition containing 30% T A T B are unexpectedly mild when compared with those for compositions containing 25% and 35% TATB. These are probably caused by unusually efficient polymer coating and, presumably, more violent responses would be obtained if additional batches of this composition were

THERMAL RESPONSE OF PRESSED COMPOSITIONS

439

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20

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TATB Content, %

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TATB Content, %

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(b)

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10

20

30

40

TATB Content, %

0

113

20

30

40

50

60

TATB Content, %

Fig. 12. Range of SSCB test responses for RDX

Fig. 11. Range of SSCB test responses for R D X / TATB/Elvax 210 (5%) compositions plotted against TATB content. (a) Fast cookoff. (b) Slow cookoff,

(fine)/TATB/Elvax 210 (5%) compositionsplotted against

prepared and tested. This trend of fast cookoff response moderation with increasing levels of TATB parallels the fast cookoff results obtained in a small-scale fuel fire cookoff test for RDX(fine)/TATB/PTFE (5%) compositions [14]. In those experiments mild burns were observed at TATB levels of 60% and higher, mild pressure ruptures for TATB levels of 25%-60%, and more violent responses, ranging up to detonation, were obtained at lower TATB levels. It appears that the moderation in response observed at high levels of TATB in these compositions is similar to that observed for the R D X / T A T B / E l v a x 210 (5%) compositions with the 30% level of TATB. This suggests that, in both cases, the common cause of the moderation is a reduction in the burn rate at higher levels of TATB. The effect most probably occurs at lower levels of TATB when RDX rather than RDX(fine) is used in the mixture, because the coarser RDX has the slower bum rate. The reduction in shock sensitivity is a possible but less likely cause of the

moderation since, at the TATB levels required to produce deflagrative responses, the shock sensitivity of RDX(fine)/TATB composition is much higher than that of the R D X / T A T B composition (approximately 1.96 mm cf. 0.30 mm, Table 3). The RDX(fine)/TATB/Elvax 210 (5%) compositions containing 20 and 30% TATB, and two batches prepared containing 25% TATB, showed relatively mild slow cookoff resuits, with the most severe responses across the range varying from a deflagration to mild explosions. The third batch of the RDX (fine)/TATB/Elvax 210 (70:25:5) composition detonated under these conditions, indicating that moderate slow cookoff response cannot consistently be expected for batches containing these low (20-30%) levels of TATB (Fig. 12). Duplicate slow cookoff tests on the RDX(fine)/TATB/Elvax 210 (5%) compositions with higher levels of TATB all yielded a detonation. One factor favoring more violent responses than in the fast cookoff experiments

TATB content. (a) Fast cookoff.(b) Slowcookoff.

440

I . J . DAGLEY ET AL. TABLE 4

Cookoff Test (SSCB) Response for Several RDX(fiue)/TATB/Rhoplex HA-24/Zinc Stearate Compositions Response Composition

Fast Cookoff

Slow Cookoff

50:45:4:1 49:5:44:5:4:2 49:44:4:3 49:5:4:5:3:3

Deflagration, Deflagration Deflagration Burn (mild), Burn Burn, Deflagration

Deflagration, Deflagration/Explosion Burn/Deflagration Deflagration, Deflagration Deflagration (mild), Deflagration

is the higher burn rates of these compositions at these higher temperatures [3].

Effect of the Binder

Mild overall cookoff responses were observed for a RDX(fine)/TATB/Rhoplex HA-24/zinc stearate (50:45:4:1) composition. Cookoff responses for this and other related compositions are listed in Table 4. The mild fast cookoff response (deflagrations) can be attributed to the high level of TATB (45%). The moderate slow cookoff behavior (deflagration and deflagration/explosion) is probably caused by the binder system. A R D X / R h o p l e x HA-24/zinc stearate (95:4:1) composition gave only explosions (mild and violent) in slow cookoff tests, compared to detonations which are usually observed when compositions with this high R D X content, but different binders, are subjected to this test [15]. To moderate further the overall cookoff response the binder proportion was raised to 6-7%, with the proportion of zinc stearate increased to improve the handling properties. None of those compositions gave responses more violent than deflagrations.

CONCLUSIONS The simulations have accurately reproduced the time to response and surface temperature at response for RDX-based compositions containing varying PETN or TATB concentrations at both fast and slow heating rates. The calculations clearly illustrate the need to include the temperature dependence of the thermal properties of the material, and a kinetic decomposition scheme appropriate to the degree of

confinement, before good agreement between simulated and experimental results for surface temperature at response and time to response can be obtained. However, a more detailed understanding of the fundamental mechanisms involved in the progress of burning through deflagration to detonation under cookoff conditions is required before numerical simulations to predict the nature of the response can be attempted. The cookoff response of pressed RDX-based PBXs is strongly influenced by the nature and level of a second explosive component, the particle size of the explosive crystals and the type of binder. The responses of a large range of compositions containing one ethylene-vinyl acetate binder were studied extensively. Substantial moderation of fast cookoff response was achieved by blending low levels of TATB with RDX, or higher levels of TATB with finer RDX. The violence of slow cookoff responses was decreased by the addition of PETN or TATB to RDX, but even high levels TATB did not reliably prevent compositions containing finer R D X from undergoing detonation reactions. A binder that moderates the slow cookoff response of fine RDX has been used in combination with high levels of TATB to give compositions that respond only mildly under both cookoff conditions.

REFERENCES 1. Campbell, A. W., Flaugh, H. L., Popalato, A., and Ramsay, J. B., Seventh Symposium (International) on Detonation, Naval Surface Warfare Center, NSWCMP 82-334, Maryland, 1981, p. 566. 2. Huchinson, C. D., Eighth Symposium (International) on Detonation, Naval Surface Warfare Center, NSWC-MP 86-194, Maryland, 1985, p. 1105.

THERMAL RESPONSE OF PRESSED COMPOSITIONS 3. Castille, C., Bainville, D., Reynier, P., and Belmas, R., Ninth Symposium (International) on Detonation, Office of the Chief of Naval Research, OCNR 113291-7, Virginia, 1989, p. 1070. 4. Nguyen, T. T., MRL Report MRL-TR-92-5, Materials Research Laboratory, Melbourne, 1992. 5. Isler, J., Propell. Explos. Pyrotech., 16:151-155. 6. McAfee, J. M., Asay, B. W., Campbell, A. W., and Ramsay, J. B., Ninth Symposium (International) on Detonation, Office of the Chief of Naval Research, OCNR 113291-7, Virginia, 1989, p. 265. 7. Bernecker, R. R., Sandusky, H. W., and Clairmont, Jr., A. R., Seventh Symposium (International) on Detonation, Naval Surface Warfare Center, NSWC-MP 82-334, Maryland, 1981, p. 115. 8. Collignon, S. L., Burgess, W. P., Wilson, W. H., and Gibson, K. D., Proceedings for the Insensitive Munitions Technology Symposium, American Defence Preparedness Association, Williamsburg, VA, 1992, p. •36. 9. Donaldson, A. B., and Lee, D. O., Sandia National Laboratories, SLA-74-0345, Albuquerque, New Mexico, 1974. 10. Pakulak, J. M., Jr., and Cragin, S., Naval Weapons Center, NWC TP 6414, China Lake, California, 1983. 11. Pakulak, J. M., Jr., Minutes of 21st Department of Defence Explosives Safety Board Explosives Safety Seminar, Houston, Texas, 1984. 12. Recommendations on the Transport of Dangerous Goods. Tests" and Criteria, 2nd ed., United Nations, New York, 1990, pp. 39-41, 67-68. 13. Anderson, C. M., and Pakulak, Jr., J. M., J. Hazard. Mater. 2:143 (1977/78). 14. Kabik, I., and Ringbloom, V. D., U.S. Patent 4,394,197 (1983). 15. Dagley, I. J., Montelli, L., Parker, R. P., and Silva, V. M., J. Energetic Mat. 10:127-149 (1992). 16. Drake, R. C., Report ETB No. 221, AWE Aldermaston, UK, 1990. 17. McGuire, R. R., and Tarver, C. M., Seventh Symposium (International) on Detonation, Naval Surface Warfare Center, NSWC-MP 82-334, Maryland, 1981, p. 56. 18. Dagley, I. J., Ho, S. Y., MonteUi, L., and Louey, C. N., Combust. Flame 89:271-285 (1992).

441

19. Parker, R. P., MRL Report, MRL-TR-89-9, Materials Research Laboratory, Melbourne, 1989. 20. Wolfson, M. G., MRL Report, MRL-R-897, Materials Research Laboratory, Melbourne, Victoria, 1983. 21. Ozisik, M. N., Boundary Value Problems of Heat Conduction, Dover, New York, 1968. 22. Jones, D. A., and Parker, R. P., MRL Report, MRLTR-91-12, Materials Research Laboratory, Melbourne, 1991. 23. Bowes, P. C., Self-Heating: Evaluating and Controlling the Hazards, Elsevier, Amsterdam, 1984. 24. Smith, G. D., Numerical Solution of Partial Differential Equations: Finite Difference Methods, 2nd ed., Oxford University Press, 1978. 25. Farnia, K., and Beck, J. V., J. Heat Transf., 99:471-478. 26. Maxwell, J. C., Electricity and Magnetism, Clarendon, 1873. 27. Jeffrey, D. J., Proc. R. Soc. Lond. A:355-367 (1973). 28. Rogers, R. N., Thermochim. Acta 11:131 (1975). 29. Zinn, J., and Rogers, R. N., J. Phys. Chem., 66:2646-2653 (1962). 30. Zinn, J., and Mader, C. L., J. Appl. Phys. 31:323-328 (1959). 31. Dobratz, B. M., and Crawford, P. C., LLNL Explosives Handbook, Properties of Chemical Explosives and Explosive Simulants. UCRL 52997 Change 2, Lawrence Livermore National Laboratory, Livermore, 1985. 32. Tarver, C. M., McGuire, R. R., Lee, E. L., Wrenn, E. W., and Brein, K. R., Seventeenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1978, pp. 1407-1413. 33. Catalano, E., McGuire, R., Lee, E., Wrenn, E., Ornellas, D., and Walton, J., Sixth Symposium (International) on Detonation, Office of Naval Research, ACR-221, 1976, pp. 214-222. 34. Helsing, J., and Grimvall, G , J. Appl. Phys. 70:1198-1206 (1991). 35. Fong, C. W., and Smith, R. F., Combust. Sci. Technol. 57:1-15 (1988).

Received 11 April 1995; revised 28 November 1995