Compounding of Glycidyl Azide Polymer with Nitrocellulose and itsIn¯uence on the Properties of Propellants

Michael Niehaus*

Fraunhofer Institut fuÈr Chemische Technologie (ICT), D-76327 P®nztal (Germany)

Die Compoundierung von Glycidylazidpolymer mit Nitrocelluloseund deren Ein¯uû auf die Eigenschaften von Treibmitteln

Vielfach setzen sich Treibstoffformulierungen aus RDX undpolymeren Bindemitteln zusammen, wie hydroxyterminiertemPolybutadien (HTPB) und Celluloseacetobutyrat (CAB), ebenso wieGlycidylazidpolymer (GAP) und Nitrocellulose (NC) als energe-tischen Komponenten. Treibstoffe auf der Basis von RDX und GAPsind auf Grund der ungenuÈgenden mechanischen Eigenschaften desAzidpolymeren oft bruÈchig, wenn sie einen hohen Gehalt an RDXenthalten. Andererseits koÈnnen Formulierungen aus RDX und NC sehrleicht die tolerierbare Abbrandtemperatur mit ansteigender RDX-Konzentration uÈberschreiten. In dieser Studie werden deshalb Treib-stoffe mit einer hohen spezi®schen Energie und relativ niedrigerAbbrandtemperatur mit einem Compound aus NC und GAP alsenergetischem Binder formuliert. GemaÈû den thermodynamischenBerechnungen koÈnnen GAP=NC-Komposittreibstoffe formuliertwerden, die bei gleicher Abbrandtemperatur eine bis zu 15% hoÈherespezi®sche Energie haben wie die Seminitramine. Durch die Wahlgeeigneter Polymerisationsbedingungen koÈnnen chemisch stabileKompositionen hergestellt werden. ARC-Experimente zeigen, daû sichbei Temperaturen von 120 ± 160�C der Binder aÈhnlich wie NCzersetzt. Bei hoÈheren Temperaturen wechselt das Verhalten von derNC-artigen zur GAP-artigen Zersetzung. Im Vergleich zu GAP-gebundenen Treibstoffen kann die Druckfestigkeit von GAP=NC-gebundenen Treibstoffen betraÈchtlich um bis zu 420% bei Raum-temperatur erhoÈht werden. Obwohl die mit NC gebundenenSeminitramin-Treibstoffe bei Raumtemperatur noch eine um 10%hoÈhere Druckfestigkeit zeigen, sind die GAP=NC-Kompositionen beihoÈheren Temperaturen deutlich uÈberlegen.

Compoundage de polyglycidylazide avec de la nitrocellulose et sonin¯uence sur les proprieÂteÂs des propergols

Les formulations d'explosifs sont souvent composeÂes de hexogeÁneet de liants polymeÁres tels que les polybutadieÁnes hydroxyteÂleÂcheÂlique(PBHT) et l'aceÂtobutyrate de cellulose (CAB), ainsi que des compo-sants eÂnergeÂtiques polyglycidylazide (GAP) et nitrocellulose. Lespropergols aÁ base d'hexogeÁne et de GAP sont souvent fragiles du faitdes proprieÂteÂs meÂcaniques insuf®santes de l'azidopolymeÁre lorsqu'ilsont une teneur eÂleveÂe en hexogeÁne. D'autre part, des formulations aÁbase d'hexogeÁne et de NC peuvent treÁs facilement deÂpasser la vitessede combustion toleÂrable lorsque la concentration d'hexogeÁne aug-mente. Dans la preÂsente eÂtude, on formule donc des propergols aÁeÂnergie speÂci®que eÂleveÂe et tempeÂrature de combustion relativementfaible avec un compound aÁ base de NC et du liant eÂneÂrgeÂtique GAP.Selon les calculs thermodynamiques, on peut formuler des propergolscomposites GAP/NC qui, pour une meÃme tempeÂrature de combustion,posseÁdent une eÂnergie speÂci®que jusqu'aÁ 15 % supeÂrieure aÁ celle desseminitramines. Le choix de conditions de polymeÂrisation adapteÂespermet de syntheÂtiser des compositions stables chimiquement. DesexpeÂriences ARC montrent qu'aÁ des tempeÂratures de 120 ± 160�C, leliant se deÂcompose de la meÃme manieÁre que la NC. A des tempeÂraturesplus eÂleveÂes, le comportement passe d'une deÂcomposition de type NCaÁ une deÂcomposition de type GAP. Par rapport aÁ des propergols aÁ liantGAP, la reÂsistance aÁ la compression de propergols aÁ liant GAP/NCpeut eÃtre augmenteÂe consideÂrablement jusqu'aÁ 420% aÁ tempeÂratureambiante. Bien que les propergols seminitramine aÁ liant NC posseÁdentencore une reÂsistance aÁ la compression supeÂrieure de 10 % aÁ tem-peÂrature ambiante, les compositions GAP/NC sont nettement supeÂ-rieures aÁ des tempeÂratures plus eÂleveÂes.

Summary

Currently formulated propellants comprise RDX and polymericbinders, such as hydroxy-terminated polybutadiene (HTPB) and cel-lulose-acetate butyrate (CAB) as well as the energetic substancesglycidyl azide polymer (GAP) and nitrocellulose (NC). Propellantsbased on GAP are often brittle if they are formulated with a highcontent of cyclotrimethylene trinitramine (RDX) and due to theusually insuf®cient mechanical properties of GAP. On the other handformulations based on RDX and NC may exceed the tolerable burningtemperature with increasing RDX concentration. Therefore, in thisstudy propellants with a high force and with relatively low burningtemperature has been formulated by using a compound of NC andGAP as energetic binder. According to thermodynamic calculationsGAP=NC composite propellants can be formulated with up to 15percent more speci®c energy than seminitramines at the same burningtemperature. By choosing appropriate polymerization conditions che-mical stable compositions can be produced. ARC experiments giveevidence that at temperatures from 120�C to 160�C the binder

decomposes similar to NC. At higher temperatures the behaviourswitches from NC type to GAP type decomposition. In comparison toGAP bound propellants the compressive strength of propellants boundby the GAP=NC compound can be signi®cantly increased by up to 420percent at room temperature. Although the examined seminitraminepropellants bound with NC show a compressive strength which isabout 10 percent higher at room temperature, the GAP=NC composi-tions are quite superior at elevated temperature.

1. Introduction

Current formulations of propellants comprise energetic

components like RDX and polymeric binders(1,2). Conven-

tionally used binders are hydroxy-terminated bolybutadiene

(HTPB) and cellulose-acetate butyrate (CAB) as well as the

energetic substances glycidyl azide polymer (GAP) and

nitrocellulose (NC). Propellants formulated with GAP tend

to be brittle due to the high content of cyclotrimethylene

trinitramine (RDX) (75 to 90 percent)(3), which is necessary* Corresponding author; e-mail: ni@ict.fhg.de

# WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0721-3115/00/0511 ± 0236 $17.50�:50=0

236 Propellants, Explosives, Pyrotechnics 25, 236±240 (2000)

to stabilize the cross sectional geometry after extrusion.

During the combustion of the propellant an unpredictable

increase in pressure may occur due to the break up of

propellant grains. On the other hand propellants based on

RDX and NC burn with high temperatures. Hence the

concentration of RDX and therefore the force of the propel-

lant must be limited in order to avoid gun erosion.

This study aims to overcome the brittleness of GAP bound

propellants by reducing the ®ller concentration. Usually the

processing of propellant grains with a RDX concentration

below 70 percent is ineffective when the original binder

composition of bifunctional GAP and hexamethylene-triiso-

cyanate (Desmodur N 100) is being used. Obviously the low

viscosity of GAP=RDX pastes with `̀ low'' RDX concentra-

tions results in the production of propellant grains with

unspeci®c geometry. Therefore, the viscosity of the paste

must be increased in order to produce grains with an exact

geometry. In this study the viscosity of GAP=Desmodur

N100=RDX pastes is regulated by the addition of NC

resulting in the production of geometrically well de®ned

propellant grains. However, NC may react with isocyanates

via free hydroxy-groups thus forming a GAP=NC=Desmodur

N100 graft polymer.

The formulation of the new binder system requires the

calculation of the thermodynamic properties of the system as

well as the investigation of the chemical stability. The

grafting of GAP and NC must in¯uence the mechanical

properties of the binder. Hence the compressive strength

and compressive failure of different binder RDX composi-

tions are investigated.

2. Experimental

The binders were kneaded in a horizontal mixer at 45�C.

Additionally NC and the GAP=NC mixtures were plasticized

with acetone. Then RDX, having an average particle dia-

meter of 19 mm, was added to the paste and the mixture was

kneaded for several hours. Afterwards the paste was ®lled

into a ram jet and pressed to seven-hole strands having an

overall diameter of 7.9 mm. The pressed strands were treated

at 50�C for several days and then cut into grains with a length

of 11 mm. The holes with a diameter of 0.85 mm were

arranged within the grain by having an exterior web of

1.7 mm and having an interior web of 1 mm.

The chemical stability of the propellants was examined by

performing the Dutch Test(4) at 105�C for 72 hours. An

Accelerating Rate Calorimeter (ARC, Columbia Scienti®c

Industries, Austin, TX) was applied to study the decomposi-

tion of the mixtures under adiabatic condition. A detailed

description and scheme of the ARC can be found else-

where(5). In this study 0.2 g of the samples were heated at a

heating rate of 10�C=min starting from 100�C and ending at

300�C. The impact and friction sensitivity tests were per-

formed according to the regulations of the Federal Bureau of

Materials Testing in Germany (BAM)(6,7). The compressive

strength and compressive modulus have been investigated

using an apparatus of Zwick (Zwick model No. 147670) at a

temperature of ÿ50�C, �23�C and �50�C. More detailed

information about the test method may be found elsewhere(8).

3. Thermodynamic Properties

Figure 1 shows the calculated speci®c energy and the

¯ame temperature of the GAP=NC=Desmodur N100=RDX

system (R: 1.0) as a function of the RDX and NC concentra-

tion. The speci®c energy ES as well as the ¯ame temperature

of the system increase hyperbolically by the increase of the

NC concentration. Also both parameters are enhanced by the

increasing amount of RDX in the system. Yet the surface of

the speci®c energy, or the surface of the ¯ame temperature in

Figure 1, are not symmetric. Hence, the quarternary system

may be optimized with regard to the speci®c energy and

¯ame temperature.

Tables 1 and 2 summarize the composition and thermo-

chemical properties of JA2, a conventional double base

Figure 1. Speci®c energy and burning temperature of energetic sys-tems containing RDX, NC and GAP=Desmodur N100.

Propellants, Explosives, Pyrotechnics 25, 236±240 (2000) Glycidyl Azide Polymer with Nitrocellulose 237

propellant, and of RDX=binder compositions comprising

GAP=N100 as well as NC and GAP=NC=N100. With the

exception of JA2 all propellants contain SOFT A, which acts

as an energetic plasticizer. TUGAP41 and TUGAP59 are

GAP bound systems with a composition exemplary for

typical GAP bound propellants(1). TGAPNC3 and

GAPNC54 comprise RDX, SOFT A and the graft polymer

as binder.

As indicated in Table 2, propellants with a NC concentra-

tion exceeding 45 percent (NC31, NC11 and JA2) exhibit

¯ame temperatures about 3400 K and more. Especially NC

11 may be regarded as a theoretical composition, because a

¯ame temperature above 3741 K is intolerable in most gun

systems. The formulations TUGAP41 and TUGAP59 clearly

demonstrate that the ¯ame temperature of propellants can be

reduced using GAP bound RDX systems. According to the

demand for the next generation of gun propellants the

speci®c energy should exceed 1400 J=g at a maximum

¯ame temperature of 3500 K(9). With conventional GAP

binders such propellants can only be accomplished with

RDX concentrations above 85 percent(1).

It is interesting to note, that in contrast to TUGAP41 and

JA2 the composition TGAPNC3 has a ¯ame temperature of

3306 K, which is 7 percent less than the ¯ame temperature of

TUGAP41 and 3 percent less then the ¯ame temperature of

JA2. Again TUGAP41 must be considered as a theoretical

composition due to the high ¯ame temperature and due to the

high concentration of RDX. In comparison to JA2 having a

speci®c energy of 1144 J=g TGAPNC3 yields a speci®c

energy of 1286 J=g (�12 percent). Also the velocity of

sound of TGAPNC3 is 1445 m=s which is 10 percent more

then the velocity of sound of JA2 (1315 m=s). This suggests

that with TGAPNC3 higher muzzle velocities might be

achieved than with JA2. In comparison to NC31, which

might be regarded as a typical seminitramine, TGAPNC3

yields a somewhat higher speci®c energy at a lower ¯ame

temperature. Also the mole number of produced gas and the

chemical energy ECh are identical, so that the performance of

the two propellants might be rather identical.

4. Chemical Stability and Sensitivity to Impact and

Friction

The copolymerization of GAP with NC via Desmodur

N100 without using a catalyst results in a chemical destabi-

lized product. As it can be seen in Figure 2 the weight loss of

the GAP=NC system by performing the Dutch Test is a linear

function of the NC concentration in the matrix. In contrast to

the product cured without a catalyst a chemical stable

GAP=NC=Desmodur N100 polymer can be achieved by

choosing appropriate curing conditions. Figure 3 suggests

that in the grafted polymer NC decomposes without affecting

the stability of the GAP backbone.

Table 1. Composition of Propellants

RDX,wt %

NC (13.1% N),wt %

GAP=N100,wt %

R: 1.0

SOFT A,wt %

JA2 0 59.5 NGl: 14.9 DEGN: 24.8NC31 45 31.35 0 23.65NC11 75 13.75 0 11.25TUGAP41 85 0 9.31 3.75TUGAP59 75 0 18.75 6.25TGAPNC3 67.5 7.5 18.75 6.25GAPNC54 75 4.84 16.13 4.03

Table 2. Thermochemical Data of Propellants

Mole number ofproduced gas,mole=kg

T ,K

ES,J=g

QEx,J=g

ECh,J=g

JA2 41 3399 1144 4637 181NC31 47 3372 1252 4649 183NC11 43 3741 1332 5168 181TUGAP41 43 3558 1339 4852 174TUGAP59 48 3185 1274 4424 183TGAPNC3 47 3306 1286 4525 182GAPNC54 47 3348 1296 4581 182

Figure 2. Weight loss of GAP=NC systems by performing the Hol-land Test at 105 �C versus the concentration of NC.

Figure 3. Self-heating of GAP as well as of NC and of a GAP=NCsystem with 30 percent NC versus temperature.

238 Michael Niehaus Propellants, Explosives, Pyrotechnics 25, 236 ± 240 (2000)

Yet a somewhat higher self-heating of the grafted polymer

in comparison to pure NC indicates that at least a certain

amount of NC and GAP is bound chemically as a urethane via

the Desmodur N100.

Probably due to the high content of the very sensitive

plasticizers NGl and DEGN the propellant JA2 is more

sensitive to the Drop Hammer test and to the Friction test

than all other propellant formulations (Table 3). Yet it is

interesting to note that the reduction of the RDX concentra-

tion from 75 percent to 45 percent does not affect the impact

sensitivity of propellants bound by NC. In contrast to NC31

and NC11 the composition TGAPNC3 is less sensitive to the

Drop Hammer Test. This is surprising because the concen-

tration of RDX in TGAPNC is 68 percent and therefore the

material might be expected to be more brittle and hence more

sensitive to impact than NC31. On the other hand, as Figures

4 and 5 illustrate, the GAP=NC graft-polymer is more elastic

than the NC binder in NC31 and consequently might over-

come the brittleness of the composite. In comparison with

NC31 and NC11 the composition TGAPNC3 also is less

sensitive with respect to friction. The data show that graft-

polymers of NC and GAP are an interesting alternative as a

binder for gun propellants.

5. Mechanical Properties

Figures 4 and 5 show the compressive failure and the

compressive strength of TGAPNC3 as well as TUGAP59 and

NC31 as a function of temperature. Usually grafted polymers

have mechanical properties which are a mixture of the

mechanical properties of the pure components. For this

reason it is not surprising that the propellant bound by the

GAP=NC graft-polymer shows mechanical properties which

are a combination of NC31 and TUGAP59. It is interesting to

note that in the range of ÿ40�C to �20�C the compressive

failure of TGAPNC3 is signi®cantly enhanced in comparison

to TUGAP59. For example atÿ40�C the compressive failure

of TGAPNC3 reaches 86 percent of the value for NC31. In

contrast to this the compressive failure of TUGAP59 reaches

only 55 percent of the value for NC31. At�50�C TGAPNC3

even shows better mechanical properties than the NC bound

system.

6. Conclusions

By using a compound of NC, GAP and Desmodur N100 as

energetic binder, propellants with a high force and with a

relatively low burning temperature can be formulated. For

example thermodynamic calculations show that with this

new binder composition propellants with a speci®c energy of

1286 J=g and a ¯ame temperature of 3306 K can be formu-

lated. Also other thermodynamic data like for instance the

velocity of sound suggest an increased performance of the

new propellants in contrast to conventional systems. Accord-

ing to the Dutch Test and to Adiabatic Rate Calorimetry

propellants formulated with the grafted polymer binder show

appropriate chemical stability. Moreover, the new propellant

compositions have attractive properties with respect to the

Drop Hammer and the Friction tests and therefore might be

an interesting alternative for the production of LOVA

propellants. Compressive tests give evidence that in compar-

ison to propellants bound by GAP, the systems bound by

GAP=NC have superior mechanical properties. At�50�C the

quarternary formulation even shows better mechanical prop-

erties than propellants bound by NC.

Table 3. Impact and Friction Sensitivities

Impact,Nm

Friction,N

JA2 2.0 120NC31 5.0 144NC11 5.0 120TUGAP41 6.0 128TUGAP59 7.5 192TGAPNC3 6.0 160

Figure 4. Compressive failure of propellants bound by NC (NC 31)as well as GAP=N100 (TUGAP59) and GAP=NC=N100 versustemperature.

Figure 5. Compressive modulus of propellants bound by NC (NC 31)as well as GAP=N100 (TUGAP59) and GAP=NC=N100 as a functionof temperature.

Propellants, Explosives, Pyrotechnics 25, 236±240 (2000) Glycidyl Azide Polymer with Nitrocellulose 239

7. References

(1) F. Schedlbauer, `̀ GAP ± A Binder Material for LOVA-GUN Pro-pellants'', 14th Int. Symp. Ball., QueÂbec, Canada, 1993.

(2) D. Mueller, `̀ New Gun Propellant with CL-20'', PropellantsExplosives, Pyrotechnics 24, 176 ± 181, (1999).

(3) M. Niehaus, `̀ Grundsatzuntersuchungen zum Temperaturverhaltenvon Treibladungen'', FhG-Bericht 100897, (2000).

(4) R. Meyer, `̀ Explosives'', 3rd Ed., VCH Weinheim, New York,(1987).

(5) D. I. Townsend and J. C. Tou, `̀ Thermal Hazard Evaluation by anAccelerating Rate Calorimeter'', Thermochimica Acta 37, 1 ± 30,(1980).

(6) BAM PruÈfungsvorschrift, `̀ BAM-Fallhammer'', Abschnitt 21, 3a,Berlin, (1990).

(7) BAM PruÈfungsvorschrift, `̀ BAM-Reibeapparatur'', Abschnitt 25,3b, Berlin, (1990).

(8) Technische Lieferbedingungen, `̀ Untersuchung und PruÈfung vonFesttreibstoffen und FeststofftreibsaÈtzen ± Teil 2: Bestimmung dermechanischen Eigenschaften im einachsigen Zugversuch'', TL1376-701, BWB, Meppen, (1976).

(9) R. L. Simmons, `̀ Guidelines to Higher Energy Gun Propellants'',27th Int. Annual Conf. of ICT, Karlsruhe, Germany, June 25 ± 28,1996.

(Received May 24, 2000; Ms 2000/021)

240 Michael Niehaus Propellants, Explosives, Pyrotechnics 25, 236 ± 240 (2000)