ARC And Energetic MAterial

57 - 1

Thermal Behaviour of Energetic Materials in Adiabatic Selfheating Determined by ARCTM Manfred A. Bohn, Heike Pontius Fraunhofer Institut fuer Chemische Technologie (Fraunhofer ICT) D-76318 Pfinztal, Germany

Abstract The adiabatic selfheating of a variety of energetic materials was determined by ARCTM (Accelerating Rate Calorimeter). In comparison to DSC and TGA, which uses mostly a forced linear heating up of the sample and to HFMC, which is mostly used isothermally, ARCTM determines the self heating of the sample in a pseudo adiabatic environment. By this the decomposition process is sample controlled in the so-named exotherm and not forced from an outside temperature program. The state of adiabaticity is obtained by a closed measurement system and a precisely controlled heating up, which follows the self heating of the sample. A typical result is the self heat rate of the sample as function of the adiabatically reached temperature, which is caused by the exothermal decomposition of the sample, not by the instrument, which serves only as device to maintain adiabaticity also with small sample amounts. Typical sample amounts with energetic materials are in ARCTM between 200 mg and 600 mg, which is much more than normal DSC and TGA instruments can handle.

After an introduction of the principles of operation and of data evaluation, the paper presents results on several types of energetic materials: Formulations as gun propellants (single, double, triple base), rocket propellants and high explosive charges; ingredients alone as ADN, HNF, AP, FOX-7, FOX-12, nitroguanidine, TAGN, AN, RDX, HMX, TNT, NTO, ε-CL-20, GAP, NC and others more Also substances in toluene solution have been measured, which allows a further control on the decomposition behaviour and the determination of decomposition parameters.

Keywords Instrumentation of ARCTM, adiabatic selfheating, thermal decomposition behaviour, data evaluation, energetic materials

1.

Introduction

The thermal behaviour of energetic materials is an important property of them. Several useful methods exist to determine it: DSC (dynamic scanning calorimetry), TGA (thermal gravimetric analysis), heat flow microcalorimetry (HFMC), mass loss by weighing, EGA (evolved gas analysis) coupled with other methods, temperature resolved X-ray scattering, measurement of thermal transport properties by transient source methods as HotDiskTM, and other specialized methods. ARCTM (Accelerating Rat Calorimetry) determines ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Paper 57, pages 57-1 to 57-40. CD-Proceedings of the 43rd International Annual Conference of ICT on ‘Energetic Materials – Synthesis, Characterisation, Processing’, June 26 to 29, 2012, Karlsruhe, Germany. ISSN 0722-4087. Fraunhofer-Institut fuer Chemische Technologie (ICT), D-76318 Pfinztal. Germany.

57 - 2

the self heating of the sample in a pseudo adiabatic environment. By this the decomposition process is sample controlled and not forced from outside. The heating up of the oven surrounding the measurement cell follows the self heating of the sample. A typical result is the self heat rate of the sample as function of the adiabatically reached temperature, which originates from the exothermal decomposition of the sample, not by the instrument, which serves only as device to maintain adiabaticity also with small sample amounts. Typical sample amounts with energetic materials are in ARCTM between 200 mg and 600 mg, which are much more than normal DSC and TGA instruments can handle.

This pseudo adiabatic state of a decomposing sample is also found in real situations: samples with bigger size and low heat conductivity (materials in barrels or shipping drums), samples heated up by forced heating and showing a temperature increase in their center by heat accumulation (as in slow cook-off), energetic materials in reaction vessels, NC-based gun propellants in unstabilized state and starting with the thermal runaway which ends in thermal explosion, NC-based gun propellants still stabilized but piled up to heaps or collected in big drums, and more situations. From this point of view the determination of selfheating has the intention to assess the safety against thermal explosions.

Another aspect is the determination of thermally induced decomposition behaviour and the assessment of thermal stability. The samples are in a defined environment (adiabatic situation) and therefore their decomposition behaviour is well comparable. Because of the closed measuring device, decomposition gases are contained and may act back on the sample. This means autocatalytically caused decomposition is well recognizable by a steep increase of the self heat rate. Selfheating determined by ARCTM is well suited to compare materials with respect to their slow cook-off behaviour. With appropriately conducted measurements one can obtain controlled deflagration of the sample inside the measurement cell (means the pressure pulse does not destroy the cell) and the associated temperatures are quite near to the cook-off temperatures determined with typical slow cook-off devices. Also isothermal ageing with a switch to adiabatic condition in case of exotherm is a very useful and revealing technique. Because of the closed measurement conditions also the pressure increase and the decomposition gas generation can be determined. Moreover, other atmospheric conditions can be provided, as vacuum, argon, helium or nitrogen atmospheres, or reactive atmospheres as synthetic air, NO2 or what is intended to investigate.

2.

Measurement method

The ‘Accelerating Rate Calorimeter’ (ARCTM) was invented in Dow Chemical Company by Townsend /1/ and then commercialized by Columbia Scientific Industries, Austin, Texas, USA. After liquidation of this company the Arthur D. Little Inc., Cambridge, Massachusetts, USA take this part over, which some time later was outsourced as TIAX, LLC. Again this company extinguished and now the company Netzsch, (NETZSCH-Gerätebau GmbH, Wittelsbacherstrasse 42, D-95100 Selb, Germany) is the provider of this original ARC line.

A second ARC line exists, established by THT (Thermal Hazard Technology) 1 North House, Bond Avenue, Bletchley, MK1 1SW, England which is named esARCTM (enhanced systems ARCTM). A further version named EV-ACRTM is offered for testing of larger batteries. A third type of ARC is provided by company HEL and named Phi-TEC. The main components of the ‘Accelerating Rate Calorimeter’ (ARCTM) can be seen in Fig. 1. The heating block contains three separate heaters H1, H2 and H3 which can be regulated individually using the heating block thermocouples T1, T2 and T3. The measuring

57 - 3

cell is positioned in the center of the heating block. The spherical measurement cells with a connecting stem can be made from titanium or stainless steel or tantalum or Hastelloy C 276 with a diameter of one inch or 0.5 inches, see Fig. 2.

Fig. 1:

Scheme of the ‘Accelerating Rate Calorimeter’ (ARCTM).

titanium 0.5 inch

Fig. 2:

titanium 1 inch

Hastelloy 1 inch

Example of measurement cells in ARCTM. Shown are from left to right: two titanium bombs and one stainless steel bomb made from the nickel basis allow Hastelloy C276.

57 - 4

The measuring cell thermocouple Tm is fixed to the measuring cell using a clip. All thermocouples are sheathed and of type N (Nisil/Nicrosil). The measuring cell has a connection to the pressure transducer PT by a thin (1/16 inch) high pressure capillary tube. The measurements are made on the closed system, which makes possible the adiabaticity of the measurement system. The measurement signals are transmitted to the ARCTM processor, which executes the measuring program and controls the counter heating for the sample cell during the self heating of the sample. This counter heating generates a quasiadiabatic environment. An electronic ice point device producing a zero centigrade bath is used as a temperature reference point for the thermocouples.

Fig. 3:

ARCTM measuring principle of the measuring mode ‘heat-wait-search’ (H-W-S).

The course of the ARCTM measurement in the mode ‘heat-wait-search’ (H-W-S) is shown schematically in Fig. 3. The wait period serves to equilibrate the sample and the measuring cell with the environment after heating to the start temperature or after heating by one heat step. During the search period the processor checks whether the sample shows a self heating. To do this, the changes in temperature of the measuring cell are compared with a preset sensitivity parameter. If the temperature increase per unit time exceeds this preset level over a period of time, determined by another parameter, this is recognized as the start of the self heating and the device switches to the so-named exotherm mode and the processor starts the counter heating to establish adiabaticity. If this preset level is not exceeded during the search period, the sample is heated by a heat step and this in turn is followed by another wait and search period. This process is repeated until an exotherm has been found or until a preset end temperature has been reached.

The ARC instrument can be operated also in the isothermal mode with watching for the selfsustained exotherm. Later an example is shown, which combines isothermal ageing and the ‘search‘ mode, named as I-S mode. Because of the higher sample amounts useable with the ARCTM compared to a DSC apparatus, the sensitivity of an ARCTM is higher by a factor of about 200 to 800.

57 - 5

3.

Data evaluation

3.1

Taking the inert masses into consideration

During the exothermal decomposition, the sample also heats up the measuring cell itself.

This means that the self heat rate is decreased by the so-called inert mass (means it shows no own heat generation) of the measuring cell and other existing inert masses compared to a measurement without inert masses. To set up the heat balance equation, the increase in temperature at time t of the entire system ‘measuring cell+sample’ is called ΔTMS(t)=TMS(t)-TMS(0) and that of the simulated measurement on the sample alone is called ΔT(t)=T(t)-T(0) with TMS(0)=T(0). During the self heating both temperature differences change with time. The heat balance equation given in Eq.(1) takes into consideration the dependency on temperature of the specific heats and the decomposition of the sample during the course of the reaction R(t).

(1)

⎛ ⎜ ⎜ ⎝

⎞

∑ m (R(t)) ⋅ C (T(t))⎟⎟⎠ ⋅ ΔT(t) = i

i

i

⎛ ⎜ ⎜ ⎝

∑ m (R(t)) ⋅ C (T(t)) + m i

i

i

M

⋅ C M (T(t)) +

∑m j

j

⎞ ⋅ C j (T(t ))⎟ ⋅ ΔTMS (t ) ⎟ ⎠

mi mass of reactant (i=1) and product i>1 Ci specific heat of reactant (i=1) and product i>1 ΔT(t) temperature raise caused by the decomposition of reactant as function of t without any container or inert mass to be heated R(t) reaction course, reaction coordinate, conversion with time t mM mass of measuring cell specific heat of measuring cell CM mJ additional inert masses j specific heats of inert masses j CJ ΔTMS(t) temperature raise caused by the decomposition of reactant as function of t of the measurement arrangement, includes the heating of inert masses The specific heats used are the ones at constant pressure, CP, as the volume of the sample is not actually kept constant. This applies particularly to solids and liquids, because their volume changes are mostly negligible. The pressure is not constant either. However, any changes in pressure are small, often below 10 bar, so the effect of such low pressures on CP of solid and liquids are negligible. However, CV has to be used for gas reactions. R(t) is the reaction coordinate or equivalently named the degree of reaction conversion, which is used to determine the sample mass mS (= mi with i=1) and the masses of the reaction products mi with i>1. The values with index M apply to the measuring cell; the sum expression over j includes additional thermally inert masses. Eq.(1) must be applied also before the onset of the measured exotherm, if there is already an appreciable decomposition, but not detected by the ARCTM. In the case of an ARCTM measurement without sample and decomposition product analysis, the course of the reaction with regard to R(t) is unknown. But it is the point of interest in thermal analysis to get the information from the thermal data without the knowledge of the real chemical data on the composition of the sample. It is pointed out that this is not possible always. One has to check carefully the system under consideration, if thermal analysis is applicable in an unambiguous way. To proceed with this method of thermal analysis, approximations are introduced. Only the sample mass mS and the aver-

57 - 6

age specific heat CS are used and similar approximations are made for the other terms.

The dependency on temperature is also not taken into consideration, as ΔTMS(t) is usually between 30°C and 70°C. The average value of CS for organic substances is taken as 2.092 J/g/K, if no actual value is available. This value is already increased to take into account the increase of CP with temperature. With inorganic substances as AP the actual values should be taken, they are mostly smaller than for organic materials. Eq.(2) is the simplified form of Eq.(1).

(2)

(

m S ⋅ C S ⋅ ΔT ( t ) = m S ⋅ C S + m M ⋅ C M +

∑m

j

)

⋅ C j ⋅ ΔTMS (t )

Eq.(1) can be written as Eq.(3) with the abbreviation φ given in Eq.(4).

(3)

ΔT(t) = φ ⋅ ΔTMS (t )

(4)

φ = 1+

mM C M +

∑m C j

j

mS C S

mS CS mM CM mJ CJ φ ΔT(t)

sample mass specific heat of sample mass of measuring cell specific heat of measuring cell additional inert masses specific heat of inert mass J thermal inertia temperature raise caused by the decomposition of reactant as function of t without any container or inert mass to be heated also ΔTMS(t) temperature raise caused by the decomposition of reactant as function of t of the measurement arrangement, includes the heating of inert masses

The quantity φ is called the thermal inertia of the system ‘sample+measuring cell’, in short named φ-factor. The φ-factor is used as a correction quantity, which means that the influence of the inert masses can be approximately eliminated /1/. A precise correction is possible using Eq.(1) and all the necessary experimental information.

3.2

Determination of Arrhenius parameters from adiabatic self heating

Fig. 4 shows typical adiabatic self heat rate curves h(T) for different reactions. The adiabatic self heat rates h(T) shown in Fig. 4 and in the following figures are presented as Ig(h[°C/min]) against 1/T [1/K] but also against T [°C] in linear scaling. On the abscissa the temperature values are indicated always in °C. The Arrhenius parameters used to calculate the curves in Fig. 4 without the autocatalytic curve are the ones given not in brackets in Fig. 4. The onset temperature T0 of the self heating is 150°C and its final temperature Tf = 250°C. Because Tf is the same for all reaction types considered the same amount of heat is released by all reaction types. However, the self heat rates are quite distinct and therefore the times to reach Tf are different. The curves have been calculated for φ = 1, that means for a simulated measurement without measuring cell or for a measurement with negligible influence by inert masses.

In the case of a reaction of zero order, h(T) results in a straight line, which ends at Tf because of the consumption of the substance. For reactions of first, second and third order, the h(T)-curves pass a maximum and then return to zero when they have reached Tf. The higher the reaction order n, the wider the range of the maximum of the curve. The h(T)-

57 - 7

curves with n ≠ 0 lie below the curve with n = 0. The curves clearly show that the order of reaction can be determined unambiguously from the shape of the curves.

Fig. 4:

Types of adiabatic self heat rate curves originating from decomposition reactions with different order n and of an autocatalytic reaction.

Fig. 4 also shows the course of h(T) for an autocatalytic reaction according to reaction scheme Eq.(5) and to the kinetic expression Eq.(6), with ΔHR,i as reaction enthalpies /2/.

(5)

k

1→B + Z A ⎯⎯A⎯ k

2 →2B + Z A + B ⎯⎯A⎯

(6)

ΔHR,1 ΔHR,2

dA (T(t)) = −k A1 (T(t)) ⋅ A (T(t)) − k A 2 (T(t)) ⋅ A (T(t )) ⋅ B(T(t )) dt

The primary decomposition of A with the reaction rate constant kA1 includes the formation of the autocatalytically effective product B by a reaction of first order. The second decomposition reaction is the autocatalytic reaction. The reaction rate constant kA2 has the Arrhenius parameters given in brackets in Fig. 4, kA1 has the ones used for the other curves. Here B is not a catalyst in the common definition, where it is not consumed and effects the reaction between two reactants by changing the activation parameters. Autocatalysis stands for the decomposition of the start substance in further reactions with its reactive decomposition products.

To determine the Arrhenius parameters of the reaction rate constant k(T) of the decomposition reaction, the consumption of the substance as a function of temperature and time is the necessary information. If the decomposition of the substance A follows a defined reaction, the reaction rate constant can be determined from the self heat rate using the appropriate kinetic expression and balancing the substance consumption (= degree of reaction conversion) and the temperature increase, Eq.(7). The increase in temperature is equivalent to the released heat QA by the decomposition reaction. The same

57 - 8

approximations that are used in Eq.(2) are used here too. In Eq.(7) the terms ‘mK⋅CK’ (sonamed thermal masses) have been eliminated already. The onset quantity QA(T(t0)) is normally set to zero, but the conversion up to onset can be considered by this quantity. In Eq.(7) the ΔT and the ΔTMS are no functions of time.

Q (T(t f )) − Q A (T(t )) Tf − T (t ) TfMS − TMS (t ) A (t, T ) A (T(t)) = = A r (T(t )) = A = = Q A (T(t f )) − Q A (T(t 0 )) A (0 ) A (T(0)) ΔT ΔTMS

(7)

A(T(t)) A(T(0)) T(t) TMS(t) ΔT

ΔTMS Tf TfMS QA(T(tf) QA(T(t) QA(T(t0)

amount of sample substance A (concentration or mass) at time t at temperature T = f(t) amount of substance A at T(0), the onset temperature of the adiabatic self heating. T(0) is also named T0 or T(t0). temperature of the sample alone at time t at condition φ = 1 temperature of the system ’sample+measuring cell’ at time t at φ ≠ 1 adiabatic temperature increase ΔT = Tf - T(0) of the sample alone at condition φ = 1 adiabatic temperature increase ΔTMS = TfMS - TMS(0) of the system ‘sample+measuring’ cell at φ ≠ 1 final temperature of the adiabatic self heating of the sample alone at condition φ = 1 final temperature of the adiabatic self heating of the system ‘sample+ measuring cell’ at condition φ ≠ 1 total heat of reaction in decomposing substance A heat of reaction in decomposing substance A up to time t heat of reaction in decomposing substance A up to onset at time t0

The differentiating of Eq.(7) gives the Eqn(8), where Eq.(8-2) is the normalized form with Ar(T(t)) = A(T(t)) / A(T(0)).

dA (T(t )) A (T(0)) dT (t ) A (T(0)) A (T(0)) =− ⋅ =− ⋅ h(T (t )) = − ⋅ h(TMS (t )) ΔT ΔT ΔTMS dt dt (8)

dA r (T(t)) 1 1 =− ⋅ h(T (t )) = − ⋅ h(TMS (t )) ΔT ΔTMS dt

The combination of Eqn(8) with a reaction kinetic expression, here a reaction of order n shown in Eq.(9) and in normalized form in Eq.(10), gives Eqn(11) for the adiabatic self heat rate dT(t)/dt = h(T(t)).

(9)

dA (T(t)) = −k n (T(t)) ⋅ A n (T(t)) dt

(10)

dA r (T(t )) = −k n,r (T(t)) ⋅ A nr (T(t)) dt

⎛ T − T (t ) ⎞ h(T (t )) = k n,r (T(t )) ⋅ ΔT ⋅ ⎜ f ⎟ ⎝ ΔT ⎠

with kn,r(T(t)) = kn(T(t))/ An-1(T(0)) n

with φ = 1

(11)

⎛ T − TMS (t ) ⎞ ⎟⎟ h(TMS (t )) = k n,r (T(t)) ⋅ ΔTMS ⋅ ⎜⎜ fMS ΔTMS ⎝ ⎠

n

with φ ≠ 1

57 - 9

With Eq.(11-2) the reaction rate constants kn,r(T) can be determined just by arithmetic operations on the measured adiabatic self heat rate h(T). Eq.(11-1) was used to calculate the curves in Fig. 4 with n = 1, 2, 3. Eq.(11-1) results in the following equations for the reactions of order n with n equal 0, 1 and 2, respectively.

• reaction of zero order (12)

h(T (t )) = k 0,r (T(t )) ⋅ ΔT =

k 0 (T(t)) ⋅ ΔT A (T(0))

• reaction of first order (13)

h(T (t )) = k 1,r (T(t)) ⋅ (Tf − T (t )) = k 1 (T(t)) ⋅ (Tf − T (t ))

• reaction of second order (14)

h(T (t )) = k 2,r (T(t)) ⋅

(Tf − T(t ))2 ΔT

= k 2 (T(t )) ⋅

A (T(0)) 2 ⋅ (Tf − T (t )) ΔT

In a reaction of first order, h(T) is explicitly independent of A(T(0)) but implicitly dependent on A(T(0)), because Tf increases with increasing amount A(T(0)). To calculate the autocatalytic self heating in Fig. 4, the reaction kinetic scheme according to Eq.(5) was used as formulated in Eq.(15).

(15)

dA r (T(t)) = −k 1 (T(t)) ⋅ A r (T(t)) − k 2 (T(t)) ⋅ A r (T(t)) ⋅ (1 − A r (T(t))) dt with k1(T(t)) = k A1(T(t )) and k 2 (T(t)) = k A 2 (T(t)) ⋅ A(T(0))

Eq.(16) is gained by combining Eq.(15) with Eq.(8-2) and using Eq.(7) to substitute Ar(T(t)).

With Eq.(16) the autocatalytic self heat rate curve in Fig. 3 was calculated.

(16)

T − T (t ) ⎞ ⎛ h(T (t )) = k 1 (T(t )) ⋅ (Tf − T (t )) + k 2 (T(t )) ⋅ (Tf − T (t )) ⋅ ⎜1 − f ⎟ ΔT ⎠ ⎝

In /3/ further data evaluation is presented: • scaling of self heat rate curves to other phi-factors; • retrieving the corresponding heat generations and heat generation rates from the self heat rate curves. Strictly, this needs the knowledge of the specific heats of all involved substances. With some approximations one can get reliable data, as it was shown by comparison with heat flow calorimetry.

With autocatalytic reactions as well as with normal reactions one has already conversion in going from the start temperature TS to the onset temperature T(0) of the exotherm.

Strictly, also the conversion during heating to the start temperature has to be included in the considerations. To have a first correction for this effect in Eq.(16), the Eq.(17) can be applied, with T(0) as onset temperature and TS as start temperature, hereby neglecting the pre-conversion during the heat-up phase from ambient temperature to TS, see /2/.

The additionally included expression (T(0)-TS)/(Tf-TS) considers the quantity F=B(0)/A(0) given in the autocatalytic expressions in /2/. If this expression is not zero, the autocatalytic reaction is more intense at the onset of the exotherm.

(17)

⎛ ⎛ T(0) − TS ⎞ T − T (t ) ⎞ ⎟ ⎟⎟ + 1 − f h(T (t )) = k 1 (T(t)) ⋅ (Tf − T (t )) + k 2 (T(t)) ⋅ (Tf − T (t )) ⋅ ⎜⎜ ⎜⎜ ⎟ T T − Δ T S ⎠ ⎠ ⎝⎝ f

57 - 10

With F≠0 the autocatalytic course shifts towards the one of a reaction of 2nd order, this increasingly with increasing F, see /2/.

4.

Experimental data

4.1

General features of ARCTM results

In the following a series of experimental results are compiled and will be described and discussed in short. In part they are already given in earlier publications /4 to 9/ The Fig. 5 shows a typical ARCTM result: self heating of a sample by its decomposition under adiabatic condition, in short named adiabatic self heating, here shown with a lot of FOX-12 or GUDN (guanylurea dinitramide).

10 h [°C/min]

adiabatic self heat rate of GUDN / FOX-12

deflagration 1

sample amount sample cell phi factor start temperature sample residue onset temperature deflagration temp.

0.1

onset

= 215 mg = titanium sphere, 1’’ = 8.3 = 130°C = 42% (after deflagration) = 164.5°C = 178.8°C

T [°C] (linear) 0.01 160

165

170

175

180

185

Fig. 5: Adiabatic selfheating of guanylurea-dinitramide (GUDN) also named FOX-12. This figure shows the principles of the measurement on pure substances. The sample amount is adjusted in such a way that deflagration occurs after some selfheating without destroying the measurement cell.

The temperature on the abscissa is the one the sample has reached by its adiabatic self heating originating from exothermal decomposition. It is not an externally imposed temperature, means the sample was not forced by an external temperature programme.

The curve gives further information: detected onset of exotherm and the deflagration point of the sample. The measurements are normally conducted in such a way that the sample amount is so high that deflagration occurs. This ensures a low enough phi-factor.

In this way the thermal inertia of the measurement cell has the smallest influence on the course of the self heating of the sample. In Fig. 6 the typical reproducibility of an ARC measurement is shown with a GP formulation based on binder GAP and RDX as energetic filler. The series of figures 7 to 10 show in condensed form the experimental measurement outcome of an ARC run. First the temperature and the pressure evolution are recorded of the decomposing substance under adiabatic condition, Fig. 7, whereby the ARC is in the exotherm mode, means it follows the temperature increase caused by the decomposing sample. With these data one can make a check if reaction heat generation

57 - 11

and pressure evolution are in close correlation, which is shown in Fig. 8. Next, the time derivatives are formed from the temperature-time and pressure-time curves shown in Fig. 9. For the assessment of safety, stability, compatibility and ignition these data are taken. Further this type of data is suitable for kinetic evaluation of the decomposition behaviour. Fig. 10 shows another check of the correlation of reaction heat generation rate and pressure rate evolution in the decomposition course of a sample.

Fig. 6:

Example of reproducibility of ARC measurements, demonstrated with the research GP type KHP281 based on GAP-N100 bonded RDX.

240

7 T [째C]

p [bar]

adiabatic self heating FOX-7 lot 2007-M1

235 230

6

T p

225 the exotherm of the sample: adiabatically reached temperature raise by decomposition and self heating

220

5

215 210

pressure generation during adiabatic selfheating and decomposition of sample

205

4

time [min] 200 720

Fig. 7:

730

740

750

760

770

780

790

800

810

820

830

3 840

Temperature and pressure evolution during the adiabatic self heating, here shown for FOX-7, lot 2007-M1. The temperature is the one which the sample reaches by its decomposition under adiabatic condition.

57 - 12

6.5 p [bar] 6

adiabatic self heating FOX-7 lot 2007-M1

pressure generation during decomposition of sample under adiabatic condition

5.5 5 4.5 4 3.5 3

temperature raise by decomposition and self heating of the sample under adiabatic conditions

2.5

T [째C] 2 205

Fig. 8:

210

215

220

225

230

235

240

Pressure generation versus temperature increase of the sample FOX-7 lot 2007-M1 during its decomposition under adiabatic condition. The linear relationship indicates that both types of probing, reaction heat generation and gas evolution, are congruent and suitable for assessment.

10

10 h [째C/min]

dp/dt [bar/min]

adiabatic self heating FOX-7 lot 2007-M1

h dp/dt 1

1 self heat rate h of adiabatic selfheating 0.1 pressure rate dp/dt of pressure generation during adiabatic selfheating

0.1

0.01 phi=8.66 252mg T [째C] (linear) 0.01 200

Fig. 9:

205

210

215

220

225

230

235

0.001 240

Self heat rate of the sample FOX-7 lot 2007-M1 and pressure increase rate caused by decomposition of the sample under adiabatic condition.

57 - 13

1 dp/dt [bar/min]

adiabatic self heating FOX-7 lot 2007-M1 pressure rate versus self heat rate

0.1

0.01

h [째C/min] 0.001 0.01

0.1

1

10

Fig. 10:

Pressure rate versus self heat rate of sample FOX-7 lot 2007-M1 during its decomposition under adiabatic condition. The linear relationship indicates that both types of probing, reaction heat generation rate and gas evolution rate are congruent and suitable for assessment.

Fig. 11:

Adiabatic self heating of a series of high explosives in comparison. The phifactors have been adjusted to be comparable.

57 - 14

A general overview of the adiabatic self heating of a series of common high explosives can be seen in Fig. 11 /4/. The lower the self heat rate curve the less thermally stable is the material. The steeper the slope of the self heat rate is the greater the part of autocatalytic decomposition. Ammonium nitrate shows a very tardy decomposition, whereas Nigu, TAGN and RDX decompose with high autocatalytic acceleration.

4.2

Retrieving decomposition parameters from ARC data

The following part is based on the kinetic description of ARC data given in section 3.2.

More details about this can be found in /3/.The substances used for this part are listed below.

• mixture (58.8 / 41.2 in mass-%) N-methyl-/N-ethyl-NENA (Me/Et-NENA) from ICI / USA; NENA means N-(2-nitratoethyl)-nitramine • DANPE, 1,5-diazido-3-nitraza-pentane, from Rocketdyne, USA • GAPA, short chain GAP with N3-end groups instead of OH, from Rocketdyne • EGBAA (technical name A17), ethylene glycol-bis-(α-azidoacetate), from ICT • Octylazide, from ICT, as model substance, not intended for use in explosives • GAP-AA-2000, GAP with α-azidoacetic acid esterified OH-end groups, from ICT • GAP-AA-500, as GAP-AA-2000, however short chains, from ICT • uncured PolyGLYN, (also named PG in USA), poly-glycidylnitrate, from ICI / UK • uncured GAP, poly-glycidylazide, from SNPE, France • uncured GAP, poly-glycidylazide, from ICT

Fig. 12:

Adiabatic self heating of uncured GAP diol resin and of uncured GAP diol dissolved in toluene and dioxane.

Fig. 12 shows three measurements: one with uncured GAP diol directly as it is filled into the measurement cell. The other two measurements have been solutions of GAP in the solvents toluene and dioxane each with 10 mass-% of GAP. The appearance of the self heat rate curves is quite different. The curve of pure GAP starts with the onset at 145°C

57 - 15

and ends in deflagration at 211°C, phi was 6.2. The GAP in solution has higher onset which is explained by the much higher phi-factor of 12.5 of GAP itself in the solution. But the essential feature is the course through a maximum in self heat rate. This occurs because of substance consumption without going into deflagration. Such type of curves could be obtained also with pure GAP, but then the sample amount must be much smaller as it has been in obtaining the curve 1 in Fig. 12. In Fig. 13 the kinetic evaluations of GAP in the two solution are shown, whereby Eq.(13) was used to transform the data to the Arrhenius plots /3/.

Fig. 13:

Reaction kinetic evaluation of the self heat rate data of GAP diol dissolved in toluene and in dioxane. A reaction of first order describes the decomposition in both cases.

Fig. 14:

Scaling of measured self heat rate data of dissolved GAP in dioxane to φ=1, means to a situation without thermal inertia and to situation with φ=2.

57 - 16

Fig. 15:

Scaling of measured self heat rate data of dissolved GAP in dioxane with φ=12.5 to φ=1, and of GAP diol with φ=6.2 to φ=1. The scaled GAP curve coincides with the one obtained from GAP dissolved in dioxane.

Fig. 16:

Adiabatic self heat rates of some energetic plasticizers and binders dissolved in toluene, solution of always around 10 mass-%. The different energy contents of the substances are clearly expressed in the extension of the self heat rate: in height of h and in the temperature span.

57 - 17

Fig. 17:

Adiabatic self heat rate as function of reduced and phi-normalized time of energetic plasticizers and binders, up to maximum heat rate. The reduced time is the one when subtracting from the total measurement time t the time t(h=0.05°C/min), means when h=0.05°C/min was reached. In this way one starts at a heat rate similar to the one in slow cook-off tests, namely 3.33°C/h = 0.056 °C/min. The value 3.33°C/h corresponds to 6°F/h.

In Fig. 14 and Fig. 15 the scaling to other phi-factors can be seen, starting from the one applied in the measurements. The scaling formulas can be found in /3/. Fig. 16 shows a survey on the results of adiabatic self heating for several energetic plasticizers and some binders, all in solution of toluene. The different energy contents are clearly expressed in the extension of the self heat rate: in height of h and in the temperature span. Finally, in Fig. 17 the self heat rates are shown as function of reduced and normalized time, as described in the legend of Fig. 17. See also /3/.

4.3

Data on adiabatic self heating from some substance groups

The behaviour of pure AN (ammonium nitrate), AN with 5 mass-% CuO, AN with 5 mass% NiO, TAGN (triaminoguanidinium nitrate) and a mixture of AN with TAGN can be seen in Fig. 18. AN itself is quite tardy in decomposition. But with addition of CuO or NiO the decomposition is much faster, but still starts at quite high temperatures. TAGN decomposes at much lower temperatures and this relatively fast with autocatalytic acceleration.

The mixture of AN with TAGN shows also a type of mixture in decomposition behaviour.

The self heating starts in the range of the onset of TAGN, but it reaches not high rate values and it extends to the temperature range of AN. A further feature can be seen in Fig. 18. There are two TAGN curves determined at different phi-factors. The lower the phi-factor, the lower is the onset of the self heating.

In Fig. 19 three nitrogen rich substances are compared, all with similar molecular features: nitroguanidine (Nigu, prills with diameter of 100 to 400μm), TAGN and FOX-12 or

57 - 18

GUDN (guanylurea dinitramide). In principle they decompose in the same temperature range from 150°C to 180°C. Nigu has at the beginning the steepest slope, indicating a strong autocatalytic feedback by reaction products. FOX-12 would have nearly the same onset temperature as Nigu, when a sample will be measured with similar phi-factor as Nigu has.

The Fig. 20 shows an example of a 1:1 mixture between TNT and TAGN and the pure substances in comparison. The onset temperature of the mixture is 35°C lower than the one of the lowest lying substance, here TAGN, indicating an already strong inter-reaction between TAGN and TNT. The whole behaviour leads to the assessment incompatible for mixtures of TAGN with TNT. With Fig. 21 the effect of different measurement conditions using Nigu is demonstrated.

First a series of phi-factors (5.82, 4.08, 2.72) have been applied in pure H-W-S mode searching for the exotherm. Because the measurement cell has always similar mass the weighed in sample amount is increased to attain lower phi-factors. The lower the phifactor is the lower lying the main exotherm. Further one can observe a type of prereaction with Nigu, which is more pronounced in lowering the phi-factor. The next type of measurement was made at first in the ‘isothermal-search’ mode (I-S mode) with an isothermal load of 120°C over 42 hours. After 42 hours the mode H-W-S was set automatically. In spite of the high phi-factor of 16.2 the exotherm is quite low lying in temperature and fits in shape to the one at phi=5.82. The last measurement type was again in H-W-S mode, but with an open measurement system in that the capillary connection to the pressure transducer was let open. This allows the decomposition gases to escape in part and the system is non-adiabatic also in the exotherm mode, because the loss of thermal energy by the escaping hot reaction products cannot be compensated by the control algorithm of the ARC oven. The result is a quite mild decomposition reaction of Nigu without running in deflagration. This experiment clearly demonstrates that the confined reaction products of Nigu have a strong autocatalytic effect.

100 comparison: AN types and TAGN

h [°C/min]

10

1

AN with CuO

AN with CuO phi=5.45 AN with NiO phi=5.47 AN pure phi=5.89 AN/TAGN (80/20) phi=4.8 TAGN pure phi=3.05 TAGN pure phi=7.19

TAGN pure, Φ=3.05 TAGN pure, Φ=7.19

AN with NiO AN/TAGN (80/20) 0.1

AN pure 0.01 150

Fig. 18:

170

190

210

230

250

270

T [°C] (linear) 290

310

Adiabatic self heat rate of AN types: pure AN, AN with 5 mass-% CuO, AN with 5 mass-% NiO, mixture of AN/TAGN and TAGN at different phi-factors.

57 - 19

100 h [°C/min]

10

comparison of N-rich compounds

Nigu, Φ=5.82 FOX-12, Φ=8.3

TAGN, Φ=3.09

1 TAGN pure phi=7.19 TAGN pure phi=3.05 FOX 12 phi=8.3 Nigu-1 phi=5.82

TAGN, Φ=7.19 0.1

TAGN, Φ=3.09 0.01 150

160

170

T [°C] (linear) 180

190

200

210

220

Fig. 19:

Comparison of adiabatic self heat rates of three nitrogen rich compounds: Nitroguanidine (Nigu) prilled, TAGN (triaminonitroguanidinium nitrate) and GUDN/ FOX-12. With TAGN the influence of phi-factor is demonstrated.

Fig. 20:

Comparison of TNT, TAGN and the 1:1 mixture TNT/ TAGN. The onset temperature of the mixture is lowered by about 35°C with respect to TAGN. This means one can assess the mixture of TNT and TAGN as incompatible.

57 - 20

100 h [°C/min]

comparison of Nigu at different phi-factors and loads Nigu prilled, d=100 to 400µm phi=5.82

phi=4.08

10 Nigu-1 phi=5.82

preloaded at 120°C, 42h, phi=16.19

Nigu-2 phi=4.08 1

Nigu-3 phi=2.72 Nigu-4 phi=16.19 42h, 120°C Nigu-5 phi=8.68, open phi=4.08

phi=2.72

0.1

phi=5.82

non-adiabatic, phi=8.68

0.01

T [°C] (linear) 0.001 100

Fig. 21:

110

120

130

140

150

160

170

180

190

200

Comparison of adiabatic self heat rates of nitroguanidine prills at several phi-factor (H-W-S mode), at one preloaded condition (starting in I-S mode and after 42 h switching to H-W-S mode) and in measurement cell with non sealed outlet capillary, means at non-adiabatic condition but in H-W-S mode. The lower the phi-factor the lower the onset temperature and the lower the main exotherm is positioned. In non-adiabatic condition the fast decomposition is not built-up. This means that some reaction products have autocatalytic effect on the Nigu decomposition. For all measurement the mass losses have been in the same range of 46 to 52%

A series of measurements have been done on NTO, RDX, TNT and mixtures of NTO/TNT and NTO/RDX /6/. The purpose was to find out possible incompatibilities between NTO TNT and NTO – RDX. NTO has an acid H atom which can cause problems with some ingredients. Further, NTO has reductive potential, which could interfere with the oxidizing ability of the NO2 groups of TNT and RDX, similar as between TAGN and TNT. Indeed, Fig. 22 shows some inter-reactive effects between NTO and RDX in that the exotherm of the mixture is positioned at lower temperatures than the one of RDX. The shift is not so large that already incompatibility results. Also the mixture NTO/TNT shows inter-reactive parts. A further question also was the ageing of the components. Fig. 23 gives some results on the ageing of NTO at 65.5°C over weeks. There is an effect of ageing, which expresses itself formally as an increased thermal stability of NTO. If this is a real stability increase could not be verified in the frame of this programme.

Another substance of interest was and is FOX-7 (DADNE). Some measurements are compiled in Fig. 24. The lot to lot variation is clearly demonstrated and could create problems in comparing formulations made with FOX-7.

The next figures 25 to 27 present a comparison of the adiabatic self heating of a series of high explosives and the oxidizers ADN and HNF. The relative thermal stability is very clearly expressed. The thermal stability of ε-CL-20 is even somewhat less than the one of RDX, especially if one compares with I-RDX, see later. The low stability of HNF /7,8/, its severe problems with compatibility and the not controllable burning behaviour has prompted to finish its use in developing new formulations.

57 - 21

Fig. 22:

Comparison of RDX, TNT and NTO and 1:1 mixtures NTO/RDX and NTO/TNT. The mixtures are somewhat less stable than the pure substances.

Fig. 23:

Ageing influence on the self heat rate of NTO. Ageing in weeks (week =Wo.) at 65.5째C.

57 - 22

Fig. 24:

Comparison of adiabatic self heat rate of three lots of FOX-7 (DADNE).

The sample 3 is FOX-7, lot 2007-M2. A significant lot to lot variation must be stated.

100

comparison of adiabatic self heat rates

h [째C/min]

10 ADN

RDX HNS FOX-12 FOX-7

1

0.1

220 mg phi = 9.1 215 mg phi = 8.3

257mg phi = 7.0

0.01 110

Fig. 25:

130

150

170

HMX

209 mg phi = 9.5

215 mg phi = 8.3

249 mg phi = 8.1

190

T [째C] (linear) 210

230

250

270

290

310

Comparison of the adiabatic self heat rates of some energetic substances including the oxidizer ADN. RDX is DYNO standard quality from 1992, the FOX-7 is lot 2007-M2.

57 - 23

100 adiabatic selfheating determined with ARCTM

h [°C/min]

RDX 10

PETN

1 CL20

TNT HMX

NC (N=12.6 m.-%)

NC

0.1

RDX T [°C] (linear) 0.01 110

Fig. 26:

120

130

140

150

160

170

180

190

200

210

220

230

240

Comparison of adiabatic self heat rates of some high explosives together with nitrocellulose (NC, N=12.6 mass-%).(RDX is standard quality of DYNO from 1992). Remarkable: ε-CL20 has a lower onset temperature than standard RDX. Probably CL20 is at its onset already in the γ crystal phase.

100 h [°C/min]

adiabatic self heating

10

HNF

ADN

CL20

1

0.1

NC (N =12.6 m.-%)

T [°C] (linear) 0.01 90

Fig: 27:

100

110

120

130

140

150

160

170

180

190

200

210

Comparison of adiabatic self heat rates of HNF, ADN, NC and ε-CL20.

The Figs 28 to 30 show the results of the measurement on an ammonium perchlorate (AP) sample. In the course of the adiabatic selfheating AP passed also the crystal transition from orthorhombic to cubic lattice, which is clearly recognizable in Fig. 28. The onset

57 - 24

temperature demonstrates its thermal stability. But one has to note that the decomposition starts with 205°C already far below the crystal phase change at 238°C. Moreover, it starts with strong acceleration. Often in DSC work this early decomposition is not noted and one sees the decomposition after the phase change. Fig. 28 contains also the pressure increase rate. It drops somewhat earlier then the self heat rate. This may be due to secondary reactions of the reactions products, after the AP decomposition has already stopped. The Fig. 29 shows the course of temperature and pressure increase during the exotherm. The pressure decreases at the end, may be due to reactions of the formed HCl with the measurement material, but other reactions could happen also. The cross correlation can be seen in Fig. 30. The generated pressure versus reached temperature gives the view on the not linearly correlated gas generation with the temperature increase.

10

1 h [°C/min]

dp/dt [bar/min]

adiabatic self heating of ammonium perchlorate (AP) 302 mg phi = 10.4

h dp/dt

dp/dt

1

0.1

0.1

0.01 crystal phase transition from orthorhombic to cubic, 238°C

T [°C] (linear) 0.01 190

Fig. 28:

200

210

220

230

240

250

260

270

280

290

0.001 300

Adiabatic selfheating of ammonium perchlorate (AP). Self heat rate caused by exothermal decomposition together with the pressure increase rate of the pressure generation.

57 - 25

300

30 T [째C]

p [bar]

adiabatic self heating of ammonium perchlorate (AP)

290

27

280

24 T p

270

21

pressure

260

18

temperature

250

15

240

12

230

9

220

6

210

3

time [min]

200 800

900

Fig. 29:

1000

1100

1200

1300

0 1500

1400

Adiabatic selfheating of ammonium perchlorate (AP): Adiabatically reached temperature by exothermal decomposition and pressure evolution of the reaction products.

30 p [bar]

25

adiabatic self heating of ammonium perchlorate (AP) pressure versus temperature

pressure generation during decomposition of sample under adiabatic condition

20

15

10

temperature raise by decomposition and self heating of the sample under adiabatic conditions

5

T [째C] 0 200

Fig. 30:

210

220

230

240

250

260

270

280

290

300

Adiabatic selfheating of ammonium perchlorate (AP). Pressure generation as function of temperature reached by adiabatic selfheating.

57 - 26

100 h [째C/min]

10

adiabatic selfheating determined with ARC db GP L 5460 PETN

sb GP A5020 tb GP KN6540

RDX

PETN L5460 (JA2) KN6540

1

A5020 0.1

TNT

CL20

HMX NC

RDX

NC (N=12.6 m.-%)

T [째C] (linear) 0.01 110

Fig. 31:

120

130

140

150

160

170

180

190

200

210

220

230

240

Comparison of adiabatic self heat rates of NC-based GP with the ones of some high explosives. (RDX is standard quality of DYNO from 1992).

A5020: single base GP, 20mm; L5460: double base GP, 120mm tank gun, same as JA2; KN6540: triple base GP, 110mm tank gun, M30 type; The exotherm of KN6540 is positioned at the lowest temperature range of the three GP, followed by L5460 and then A5020.

100 h [째C/min]

10

adiabatic selfheating of some gun propellents in comparison with NC and PETN

sb GP A5020 db GP L5460

L5460 (JA2) PETN

tb GP KN6540 PETN KN6540

1

A5020 0.1

NC (N=12.6 m.-%) NC

PETN A5020

0.01 110

Fig. 32:

120

130

140

T [째C] (linear) 150

160

170

Comparison of three GP with NC (N=12.6 mass-%) and PETN.

180

57 - 27

100 h [°C/min]

adiabatic selfheating of NC, PVN and PETN

NC PETN PVN, ICT Los 67-MS-1

10

PETN

PVN 1 PVN NC (N=12.6 m.-%)

0.1 NC (N=12.6 m.-%)

PETN T [°C] (linear)

0.01 110

Fig. 33:

120

130

140

150

160

170

180

Comparison of the three nitric acid ester compounds NC, PVN (polyvinyl nitrate) and PETN.

The Figs 31 und 32 compare the self heat rates of nitrate ester based substances with high explosives. Surely the NC-based GP and PETN must have lower lying self heat rate curves compared to RDX, TNT and HMX. The Fig. 32 focuses on the GP and NC. There is an order recognizable, which is always found: the most stable GP are single base GP, followed by double base GP and the less stable ones are the classical triple base propellants containing nitroguanidine. Nitroguanidine with its reducing NH-groups reacts with the oxidizing ONO2 groups of NC and nitrate ester based plasticizers. Three nitric acid ester compounds NC, PVN (polyvinyl nitrate) and PETN are compared in Fig. 33. As to be expected, they lie with their exotherms together in a narrow temperature range.

A new class of triple base GP was developed at Fraunhofer ICT, based on NC, RDX and the triple plasticizer DNDA, see section 6 and /9/. They show a so-named temperature independent effect in gun burning chamber. This means that the evolved gas pressure is nearly independent of the temperature of the charge in the range -40°C to 65°C. This has great advantages in operating a machine gun for example, see /9/ and literature given there. The autoignition temperature determined at 5°C/min heat rate is with the new formulations significantly above the ones of the conventional GP JA2 (120 mm tank gun) and the so-named MRCA GP, here type Q5560 (27 mm aircraft machine gun) /9/. Different cook-off behaviour can be expected. This is also documented by the adiabatic self heating, shown in Fig. 34. It shows also the data for a typical triple base GP of type M30 (KN6540) and for the double base propellant type JA2 (=L5460). For comparison the US XM39 GP is shown also. All conventional GP have a low lying transition temperature from controlled self heating to deflagration in the range 148°C to 158°C. Because of the RDX and DNDA content this transition is shifted to higher temperatures with the new type GP caused by partly endothermal decomposition reactions in the propellants, recognizable at the dip in the self heat rate curves of for example TLP 2N, 5W and 6. Also the CAB bonded propellant shows endothermal decomposition, here to see by a break in the exotherm.

57 - 28

100

Comparison of adiabatic self heat rate

h [°C/min]

TLP 1N

TLP 4G

10 M30

JA2

TLP 2N

TLP 6

TLP 5W single base

CAB-LOVA

1

0.1

CAB-LOVA T [°C]

0.01 125

Fig. 34:

4.4

135

145

155

165

175

185

195

205

Adiabatic self heating determined by an ARCTM for typical conventional and of new type GP, including the XM39 GP with a binder based on CAB /9/.

Adiabatic self heating of fast burning RP formulations containing ε-CL-20

To provide rocket propellants for high velocity tactical rockets (HFK = Hochgeschwindigkeitsflugkörper, high velocity (HV) rockets) special formulation have been manufactured with high burning rates and high gravitational specific impulses IS-G, see Table 1. They are based on GAP-N100 binder and ε-CL-20, AP and energetic plasticizers. Because of this special compositions investigations on the ageing behaviour have been conducted also /10, 11, 12/. In Fig. 35 the adiabatic self heating of six formulations can be seen, together with the uncured GAP and the used ε-CL-20 (ε means a stable crystal phase of ε-CL-20 at room temperature. The transition to the less dense γ-phase occurs at about 164°C). Four formulations show a transition in the slope of self heat rate, at about 160 to 170°C. This is the range where ε-CL-20 starts higher decomposition intensity. The GAP itself does not show any change in decomposition intensity over the whole temperature range from 150°C up to deflagration at about 210°C. Remarkable are the reduced onset temperatures of the formulations in comparison to the GAP. In part this is due to the presence of the nitric acid ester type energetic plasticizers TMETN and BTTN. But also an interreaction between GAP and ε-CL-20 has in part an influence, see section 4.7. Fig. 36 shows the pressure increase rate which goes in parallel to the self heat rate. Again both properties are congruent and mass loss and heat generation rate can be used to assess the ageing as it was done in /11, 12/. To assess the ‘speed’ from the start of essential self heating (h=0.05 °C/min) to deflagration the Fig. 37 is very useful. These graphs give the time ranges, when the cook-off event will take place after reaching the assigned temperatures in the center of the test body, when using a slow cook-off heat rate of 3.333°C/h = 6°F/h = 0.0556°C/min. Remarkable are the ‘outliers’ 182 and 184, which contain no GAP-A as second plasticizer. Their self heat rates deviate substantially from the other four ones in shape and in temperature range as well as in time to deflagration.

57 - 29

Table 1:

Composition in mass-% and some performance data of the investigated HFK type formulations /10/.

178

180

181

182

184

185

189

GAP-A TMETN BTTN GAP-N100 ε-CL-20 (HNIW) AP others

[m.-%] [m.-%] [m.-%] [m.-%] [m.-%] [m.-%] [m.-%]

12.25 12.25 10.5 42 20 3

14 14 7 42 20 3

9 9 12 47 20 3

18 12 47 20 3

21 14 42 20 3

10.5 10.5 14 42 20 3

9 2.25 6.75 12 47 20 3

O2 balance IS-G (70:1, eq. flow) r (100bar = 10 MPa) QEX, water condensed

[%] [Ns /N] [mm/s] [J/g]

-28.7 251 47.1 4840

-27.8 252 51.3 4940

-27.1 253 38.0 4960

-20.0 258 34.8 5370

-22.9 255 29.4 5200

-31.2 249 35.9 4750

-25.8 254 45.3 5020

10 h [°C/min]

1

adiabatic self heat rate of high burn rate RP formulations, h = f(T)

178 180 181 182 184 185 189 e-CL20-T-7µm e-CL20-T-4.5µm GAP diol

185 182

180 184

189 178

89°C, WER 189 2.42d 181 4.54d 185 6.51d 178 7.77d 180 12.98d

181 0.1

e-CL20-T-7µm e-CL20-T-4.5µm

GAP diol 0.01 120

Fig. 35:

130

140

150

160

T [°C] 170

180

190

200

210

Adiabatic self heat rates of six RP formulations for so-named HFK (HV) rockets together with the uncured binder GAP and the main energetic ingredient ε-CL-20 (ε-HNIW) /10/.

57 - 30

1 dp/dt [bar/min]

adiabatic selfheating of high burn rate RP formulations pressure evolution, dp/dt = f(T) 185

178 180 181 182 184 185 189 e-CL20-T-4.5µm GAP diol

0.1

0.01

180 182

189 184

178

181

e-CL20-T-4.5µm 0.001

GAP diol

T [°C] 0.0001 120

130

Fig. 36:

140

150

160

170

180

190

200

210

Pressure rate during adiabatic self heat rates of six HFK RP formulations together with the uncured binder GAP and ε-CL-20 (ε-HNIW). If both pressure rate and self heat rate go in parallel then two assessment quantities can be used to predict in-service times.

10 adiabatic self heat rate of high burn rate RP formulations as function of reduced and phi-normalized time

h [°C/min]

178 180 181 182 184 185 189 e-CL20-T-7µm

1 e-CL-20, Thiokol

185 189

178

180 184

182

181

0.1

(t - t(h=0.05))/phi [min] 0.01 0

Fig. 37:

3

6

9

12

15

18

21

24

27

30

33

Self heat rate versus reduced and phi-factor normalized times from time at h=0.05 °C/min to deflagration. ε-CL-20 (ε-HNIW) decomposes fastest, followed by the formulations 182 and 184, which contain no GAP-A plasticizer. This graph gives a time range, when the slow cook-off event will take place after reaching the temperatures at h=0.05°C/min. Slow cook-off heat rate is 3.33°C/h = 0.056°C/min.

57 - 31

4.5

Simultaneous application of isothermal load and search for self heating

The typical run in an ARC experiments follows the H-W-S mode. There is another useful mode which starts with an isothermal mode at a chosen temperature, whereby the system looks in search mode for selfheating. This mode is named ‘isothermal-search’ (I-S) mode. The Fig. 38 and Fig. 39 show such experiments, conducted in CERL, Ottawa, Canada /13/. The material used for these measurements was dry NC with N = 13.14 mass-%.

The interesting quantity to be determined was the time to event ta, means the times of the start of the measureable selfheating of the NC. After this time very quickly the autocatalytic runaway starts. The measurements were conducted in air and in argon. In argon the times to autocatalytic runaway are longer. Using the times ta given in Fig. 38 and 39 an Arrhenius parameterization was made according to Eq.(18).

(18)

Ea ⎛ 1⎞ ln⎜ ⎟ = ln(Z) − R⋅T ⎝ ta ⎠

The graphs of the Arrhenius plots are presented in Fig. 40, together with the Arrhenius parameters. The activation energy for the reaction during the so-named induction time ta is in air much smaller then in argon. The induction time is a time region of decomposition, where mainly the intrinsic decomposition is active, see Eq.(5), first reaction. The strong increase in temperature shortly after the start of the autocatalytic reaction is not only due to the autocatalysis but also due to the temperature rise in the sample, means one has two accelerating effects causing this steep increase in measured temperature as shown in Fig. 38 and 39.

NC in air

Fig. 38:

Isothermal ageing of NC in air using ARC up to autocatalytical decomposition. Data from /13/.

57 - 32

NC in argon

Fig. 39:

Isothermal ageing of NC in argon using ARC up to autocatalytical decomposition. Data from /13/.

-1.5 ln(1/ta [h])

isothermal ARC NC in argon and air time ta to event: start of autocatalytic decomposition data from R. Turcotte a.o., CERL, Ottawa, Canada

-1.7 -1.9

in argon in air

-2.1 -2.3 -2.5 -2.7 -2.9 -3.1

NC in air Ea = 79.8 ± 4 kJ/mol lg(Z [1/h]) = 10.145 ± 0.58 R2=0.9896

NC in argon Ea = 127.7 ± 9 kJ/mol lg(Z [1/h]) = 16.358 ± 1.21 R2=0.9904

-3.3 1/T [1/K] -3.5 0.00255

Fig. 40:

0.0026

0.00265

0.0027

0.00275

0.0028

Arrhenius plots of the times ta (taken from the above figures) to autocatalytic decomposition of NC (N=13.14 mass-%), aged isothermally using ARCTM. The decomposition rate of NC during the so-named induction period is in air clearly higher than in argon.

57 - 33

4.6

Adiabatic self heating as differentiating tool for RDX qualities

The adiabatic self heating can be used as differentiating tool for production qualities of energetic materials as already shown with FOX-7 in Fig. 24. Here another interesting example is presented with a series of RDX qualities. Fig. 41 presents the adiabatic self heating of two I-RDX qualities, a coated RDX from Nexplo-Bofors (now Eurenco Nexplo) and two standard RDX qualities from Dyno, Norway. Eurenco produces the RDX by the Woolwich process, which gives nearly HMX free RDX. To obtain I-RDX special recrystalisation is performed to reduce further impurities and voids in the crystals. Dyno produces according to the Bachmann process, which always gives HMX as side product, up to 10 mass-%. Because of the high purity of the I-RDX qualities the decomposition starts at significantly higher temperatures than with the other RDX qualities. Small I-RDX particles are even lower in content of voids than large particle I-RDX so the onset temperature of small particle I-RDX is highest. The lowest onset temperature has the coated RDX from Nexplo, see also Fig. 42. Here very probably one has already a reaction between the plasticizer DOS and RDX. The RDX alone used in this product was not available to make a direct comparison. In Fig. 43 only the RDX qualities of Eurenco are compared. The spherical RDX without further purification has the lowest onset temperature, but already higher than the standard RDX of Dyno. The three I-RDX qualities have the highest onset temperatures and their sequence is as expected. The endothermic dip in the self heat rates of the I-RDX samples indicates the melting point of them. All three types have nearly the same melting point. The melting point temperature of the spherical RDX is somewhat lower.

10

adiabatic self heating of RDX lots

h [°C/min]

1

Dyno NSI-95-J

Eurenco I-RDX-3, 191µm Eurenco I-RDX-4, 5.7µm Nexplo RDX, 8.3µm, DOS coating RDX, Dyno, NSI-95-J RDX, Dyno, 1993 Nexplo with DOS coating RDX/DOS: 94.5/5.5

I-RDX-3, 191µm

0.1

Dyno 1993

I-RDX-4, 5.7µm

198.7°C

182°C 0.01 175

Fig. 41:

180

199.5°C

187°C

179.5°C

Dyno NSI-95-J (typical standard RDX) 185

190

195

T [°C] 200

205

Adiabatic self heating of several RDX lots, including so-named I-RDX (insensitive RDX produced by Eurenco).

57 - 34

10 h [°C/min]

1

0.1

adiabatic self heating of insensitive RDX lots

Eurenco spherical RDX, 183µm Eurenco I-RDX-2, 218µm Eurenco I-RDX-3, 191µm Eurenco I-RDX-4, 5.7µm Nexplo RDX, 8.3µm, DOS coating

Nexplo RDX with DOS coating RDX/DOS: 94.5/5.5

I-RDX-4, 5.7µm I-RDX-2, 218µm

spherical, 183µm

I-RDX-3, 191µm 0.01 175

Fig. 42:

180

185

190

195

T [°C]

200

205

Adiabatic self heating of several I-RDX lots, a standard Eurenco RDX treated to have increased sphericity (these RDX materials are produced by Woolwich process, inherently nearly HMX free) and a coated RDX from Nexplo, produced by Bachmann process, which always has HMX as side product.

10 h [°C/min]

1

adiabatic selfheating of Eurenco RDX lots I-RDX-3, 191µm

Eurenco spherical RDX, 183µm Eurenco I-RDX-2, 218µm Eurenco I-RDX-3, 191µm Eurenco I-RDX-4, 5.7µm

spherical RDX, 183µm 0.1 I-RDX-4, 5.7µm I-RDX-3, 191µm

I-RDX-2, 218µm T [°C]

0.01 194

Fig. 43:

195

196

197

198

199

200

201

202

203

Comparison of I-RDX lots with different particle size and one so-named spherical RDX which was not treated to be insensitive.

204

57 - 35

4.7

Adiabatic self heating in compatibility testing

The use of adiabatic self heating in compatibility testing was already shown in short with Fig. 20. Further examples are presented in Fig. 44 and Fig. 45. Strictly seen, the temperature is not a very suitable quantity to assess compatibility, because it is an intensive quantity. For quantitative assessment only extensive variables are suitable, as mass, gas amount or heat amount. This is shown and discussed also in /14/ and especially in /15/.

However, if the difference in onset temperature of the mixture and the lowest onset value of the two components exceeds some value gained by experience then incompatibility can be assigned. But in order to quantify the extent of the inter-reaction between the components an analysis must be made to determine one of the extensive quantities mentioned. Therefore the shifts in onset temperatures between GAP and ε-CL20/GAP as well as between GAP-N100 and ε-CL20/GAP-N100 have been substantiated by determination of the gas generation in standard vacuum stability test. The outcome is compiled in Table 2. ε-CL20 has a higher reactivity with GAP than β-HMX, also in uncured state. In uncured state no critical gas generation was observed. But ε-CL20 incorporated in cured GAP-N100 shows clearly incompatibility and the mixture was instable. The observed difference in onset temperatures between GAP-N100 and ε-CL20/GAP-N100 is with 49°C very high. So the incompatibility between ε-CL20 and GAP-N100 is easy to recognize, but not to quantify by this value.

Fig. 44:

Adiabatic self heating of the 1:1 mixtures and their components, determined with an ARCTM. The curve of ε-CL20-GAP diol is clearly shifted to lower temperatures compared to GAP alone, ΔT = 12°C, indicating a higher reactivity than in β-HMX-GAP diol. The adiabatic self heating of the β-HMXGAP mixture starts at about the same temperature as the one of GAP diol alone /14, 15/.

57 - 36

Fig. 45:

Adiabatic self heating of the formulations and their components. The great shift to lower onset temperature, ΔT = 49°C, of ε-CL20/GAP-N100 is indicative for a high reactivity between ε-CL20 and GAP-N100 /14, 15/.

Table 2:

Gas generations GG and reactivities RGG at 100°C over 40h of the 1:1 mixtures and their components as well as of the two formulations ε-CL20/GAPN100 and β-HMX/GAP-N100 and their components, determined with the standard vacuum stability test /14,15/.

1:1 mixtures by mass and components

GAP diol ε-CL20, 3.2μm β-HMX, 5 μm ε-CL20–GAP diol β-HMX–GAP diol

GG [ml/g] 0.25 0.06 0.02 0.42 0.20

ε-CL20–GAP diol β-HMX–GAP diol

RGG [ml/g] + 0.265 + 0.065

5.

45:55 formulations by mass and components

GAP-N100 ε-CL20, 25 μm β-HMX, 10 μm ε-CL20/GAP-N100 β-HMX/GAP-N100

GG [ml/g] 0.29 0.07 0.08 3.03 0.43

ε-CL20/GAP-N100 β-HMX/GAP-N100

RGG [ml/g] + 2.85 + 0.25

assessment stable stable stable stable stable

compatible compatible

assessment stable stable stable instable stable

incompat. compatible

Summary and conclusions

The adiabatic selfheating means the thermal tracking of exothermal decomposition of a substance with a rate determined by the decomposition rate of the sample itself. The sample is not forced externally by a temperature programmes as in DSC or TGA. The in-

57 - 37

strument provides just the adiabatic environment for the decomposition process. This gives the sample the time to react in equilibrium because it is not driven by an external heating rate, whereby always a lack between forcing temperature and sample temperature exists, as long as no high decomposition rates are reached.

Adiabatic self heating is determined in a closed measurement system to provide full adiabaticity. Escaping reaction products transport heat out of the reaction zone and the reaction course is influenced, normally retarded. The closed system allows tracking the pressure generation parallel to the temperature rise. This means the sample is probed with two quantities: reaction heat and gas generation or mass loss. If both correlate linearly the classical conversion relations between pressure rise, temperature rise and substance conversion are unambiguously fulfilled. In performing an ARC run with energetic materials one should carefully adjust the sample amount with respect to two demands: Use of sample amount as large as possible to reduce the effect of thermal inertia; Use of just so much sample amount that no measurement cell burst happens.

The typical sample amount with energetic materials is in the range of 200 to 600 mg. This far more than standard TGA and DSC can handle. From this larger sample amount the higher sensitivity of ARC measurements results. The ARC is about 200 to 800 times more sensitive than typical DSC. The most informative representation of the measured data temperature and pressure are the rate presentations of these primary measurement quantities. With the adiabatic self heat rate the thermal stability of energetic substances is well assessable and distinguishable. Because of the closed measurement situation any autocatalytic feedback of reaction products is sensitively detected. Also compatibility testing is enabled by this method in comparing the onset temperatures of the mixture and its two components.

The adiabatic selfheating allows for screening of substances with regard to their slow cook-off behaviour. The heat rate in this test is 3.33°C/h = 0.056 °C/min (= 6°F/h). In taking the reduced time t-t0.05 and this normalized by the phi-factor the times to reach the cook-off response can be predicted at least as comparative data. In slow cook-off the material reacts in the center. Reaching the self heat rate of 0.05°C/min in ARC run is near the forced heat rate in slow cook-off. But because of near adiabatic situation in the center of the test sample very soon the self heat rate of the material will be higher than the outside one and the decomposition course is comparable to that in ARC. Finally, the data allow also quantitative kinetic evaluation if the measurements are conducted in a special way or if the conversion analysis is achieved by mass loss determination and interruption of the selfheating before deflagration.

6.

ARCTM DSC HFCM

Abbreviations

TGA

Accelerating Rate Calorimeter Differential Scanning Calorimetry heat flow microcalorimeter ‘heat-wait-search’ measurement mode in ARC to detect the selfsustained exotherm ‘isothermal-wait’ measurement mode in ARC to detect the selfsustained exotherm Thermal Gravimetric Analysis

GP

gun propellant

H-W-S I-S

57 - 38

RP HFK MRCA

rocket propellant Hochgeschwindigkeitsflugkörper (high speed rocket) multi role combat aircraft

A5020 JA2 KN6540 L5460 Q5560 M30 XM39

single base GP, 20mm machine gun as L5460 triple base GP, 110mm tank gun double base GP, 120mm tank gun Triple base GP, 27mm machine gun triple base GP, 110mm tank gun, as KN6540 CAB bonded GP for 155 mm (howitzer), classified as insensitive munition

ADN AN AP CAB CL-20 DADNE FOX-12 FOX-7 GUDN HMX HNF HNS NC Nigu NTO PETN PVN I-RDX RDX

ammonium dinitramide ammonium nitrate ammonium perchlorate cellulose acetate butyrate HNIW, hexanitro-hexa-aza-iso-wurtzitane; ε-CL-20: crystal phase of CL20 as FOX-7 GUDN, guanylurea dinitramide 1,1-diamino-2,2-dinitro ethylene (DADNE) guanylurea dinitramide octogen, cyclotetramethylentetranitramine hydrazinium nitroformate hexanitrostilbene nitrocellulose nitroguanidine (picrite, NQ) 3-nitro-1,2,4-triazol-5-one nitropenta, pentaerythritol tetranitrate polyvinyl nitrate insensitive RDX, produced by Eurenco hexogen, cyclotrimethylentrinitramine , 1,3,5-trinitro-1,3,5-triaazacyclohexane ( Research Development eXplosive or Royal Demolition eXplosive ) triaminonitroguanidinium nitrate trinitrotoluene

TAGN TNT A17 BTTN DANPE DNDA

azido plasticizer, ethylene glycol-bis-(α-azidoacetate), EGBAA 1,2,4-butanetriol trinitrate 1,5-diazido-3-nitraza-pentane DNDA energetic plasticizer based on the n,(n+2)-dinitro-n,(n+2)diaza group DNDA 57 mixture of DNDA5, DNDA6 and DNDA7 DNDA5 2,4-dinitro-2,4-diaza pentane DNDA6 2,4-dinitro-2,4-diaza hexane DNDA7 3,5-dinitro-3,5-diaza heptane

DOS EGBAA Et-NENA GAP GAP-N100 GAP-A GAP-AA-

dioctylsebacate, plasticizer ethylene glycol-bis-(α-azidoacetate), A17 N-ethyl-N-(2-nitratoethyl)-nitramine glycidyl azide polymer, binder diol cured GAP binder, cured with polyisocyanate N100 plasticizer, azide end capped GAP oligomer GAP with α-azidoacetic acid esterified OH-end group

57 - 39

2000 Me-NENA NENA PG TMETN

N-methyl-N-(2-nitratoethyl)-nitramine N-(2-nitratoethyl)-nitramine polyglyn, poly-glycidylnitrate trimethylolmethane trinitrate

7.

References

/1/

D.I. Townsend, J.C. Tou. Thermal hazard evaluation by an accelerating rate calorimeter.

Thermochimica Acta 37 (1980) 1-30.

/2/

M.A. Bohn Kinetic Description of Mass Loss Data for the Assessment of Stability, Compatibility and Aging of Energetic Components and Formulations Exemplified with ε-CL20.

Propellants Explosives Pyrotechnics 27 (2002) 125-135.

/3/

M.A. Bohn.

Determination of the kinetic data of the thermal decomposition of energetic plasticizers and binders by adiabatic self heating.

Thermochimica Acta 337 (1999), 121-139.

/4/

M.A. Bohn, F. Volk Adiabatische Selbstaufheizung bei Treib- und Explosivstoffen.

Paper 8, in Proceedings of the 24th International Annual Conference of ICT, 1993, Karlsruhe. Fraunhofer-Institut für Chemische Technologie (ICT), Postfach 1240, D-76318 Pfinztal-Berghausen, Germany.

/5/

H. Pontius, M. Dörich, M.A. Bohn.

Korrelation zwischen Gasentwicklung und Umsatz von Ammoniumdinitramid (ADN) bestimmt bei der adiabatischen Aufheizung.

Paper 174, pages 174-1 to 174-12 in Proceedings of the 34th International Annual Conference of ICT, pages 174-1 to 174-12, June 24 to 27, 2003, Karlsruhe, Germany. Fraunhofer-Institut für Chemische Technologie (ICT), Postfach 1240, D76318 Pfinztal-Berghausen, Germany.

/6/

M.A. Bohn, H. Pontius, St. Löbbecke, St. Wilker, G. Pantel.

Investigation on Stability and Reactivity of NTO and the mixtures NTO/RDX and NTO/TNT to Assess Their Safe Use. Paper147, pages 147-1 to 147-33. Proceedings of the 29th International Annual Conference of ICT, June 30 - July 3, 1998, Karlsruhe, Germany. Fraunhofer-Institut für Chemische Technologie (ICT), D-76318 Pfinztal-Berghausen, Germany.

/7/

M.A. Bohn.

Thermal Stability of HNF Investigated by Mass Loss and Heat Generation in the Temperature Range 50°C to 80°C and Lifetime Predictions.

Paper 167, pages 167-1 to 167-25. Proceedings of the combined 36th International Annual Conference of ICT and the 32nd International Pyrotechnics Seminar, June 28 to July 1, 2005, Karlsruhe, Germany. Fraunhofer-Institut für Chemische Technologie (ICT), D-76318 Pfinztal-Berghausen, Germany.

/8/

M.A. Bohn.

57 - 40

Thermal Stability of Hydrazinium Nitroformate (HNF) Assessed by Heat Generation Rate and Heat Generation and Mass Loss.

Journal of Pyrotechnics, Issue 26, 2007(8), 65-94. /9/

M.A. Bohn, D. Mueller.

Insensitivity aspects of NC bonded and DNDA plasticizer containing gun propellants.

Paper 47, pages 47-1 to 47-11 in Proceedings of the 37th International Annual Conference of ICT, June 27 to 30, 2006, Karlsruhe, Germany. Fraunhofer-Institut für Chemische Technologie (ICT), D-76318 Pfinztal, Germany.

/10/

Manfred A. Bohn, Manuela Dörich, Heike Pontius Adiabatic Self Heating of Solid Propellants Containing ε-CL20 (HNIW), Ammonium Perchlorate, GAP Binder and Energetic Plasticizers.

Paper 175 (p. 175-1 to 175-16) on the 34th International Annual Conference of ICT, June 24 to 27, 2003, Karlsruhe, Germany. Fraunhofer-Institut fuer Chemische Technologie (ICT), Postfach 1240, D-76318 Pfinztal-Berghausen, Germany.

/11/

M.A. Bohn.

Ageing and service time period assessment of novel solid rocket propellant formulations containing ε-CL20, AP and energetic plasticizers.

Paper in Proceedings of the 28th International Pyrotechnics Seminar, S. 781-795, Adelaide, South Australia, Australia, 2001.

/12/

M.A. Bohn.

Thermal ageing of rocket propellant formulations containing ε-HNIW (ε-CL-20) investigated by heat generation rate and mass loss.

Thermochimica Acta 401 (2003) 27-41.

/13/

R. Turcotte, B. Acheson, K. Armstrong, Q.S.M. Kwok, D.E.G. Jones, Mario Paquet Thermal decomposition properties of nitrocellulose and its mixtures with nitroglycerine.

CERL Report 2006-19 (OP-J), April 2006.

Paper in the Proceedings of the 33rd International Pyrotechnics Seminar, July 1621, 2006, Fort Collins, CO, USA.

/14/

M.A. Bohn, M. Dörich, J. Aniol, H. Pontius, P.B Kempa, V. Thome.

Reactivity between ε-CL20 and GAP in Comparison to β-HMX and GAP. Paper 4, pages 4-1 to 4-30. Proceedings of the 35th International Annual Conference of ICT, June 29 to July 2, 2004, Karlsruhe, Germany. Fraunhofer-Institut für Chemische Technologie (ICT), D-76318 Pfinztal-Berghausen, Germany.

/15/

M.A. Bohn Generic formulation of performance assessment quantities for stability, compatibility and ageing of energetic materials.

Paper 59, pages 59-1 to 59-34 in Proceedings of the 43rd International Annual Conference of ICT on ‘Energetic Materials – Synthesis, Characterisation, Processing’, June 26 to 29, 2012, Karlsruhe, Germany. ISSN 0722-4087. Fraunhofer-Institut fuer Chemische Technologie (ICT), D-76318 Pfinztal. Germany.