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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)
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