:!!!Wiii :\ " " ,m . " ' h ihr , II:::::;::':;....,..,,

Die thermiSche Stabiljtit Von SHexanitrohexaazaisowm.tzDn ineiner EstaneFormulierung Stabilite thermique du ehexanitrohexaazaisOwmtzitane dans unehrm"on tne

> i

EineVerinde]rungde]rFarbeundDichtebeilmgererErwnmungUnemodificationdelacouleuretdeladensitelorsduchauffagepro-Ym!ioWum(IW)millong''efbnnulationdeS-heXah616i;/i;;i;;)r e Estane5702wurdeanalysiertmittelsl.llR-Spektroskopie.DieAus-avecdel'Estane5702aeteanalyseeaumoyendeIaspectroscople S e gangsmatelialien,unbehandelteundverfirbteProbenwurdenunter-infhrarougeAt]ransfOrmationdeFourier(FTIR).onaemdieIessucht.DieAnalyseoffenbartepolymolpheverunreinigungenmallenmateriauxdedepart,desechantillonsnonaitesetcolores.L'analysel dProduktions-undVerarbeitungsstufen,wobeisichdieA]rtundarev61edesimpuretespolymolphesdanstouteslesetapesdeproduc-BeschaffenheitmitderBehandlungdesMaterialsvernderte.AIs tionetdetraitement,Ieurnatureetleu]rconcentrationvariantenfonc-U]rSacheflirdieAbnahmede]rDichtewurdediethermischausgel6stetiondutraitementdumateriau.onadetemlineque1aconverslonpoIymomheUmwandlungemlittelt,ihrenddieFEIrbnderungals polymo]pheinduitethermiquementetaithl'o]iginedelareductiondeErgebnisdesBinderabbausposmliertwurde.Schlagfestigkeit,Rei- Iadensiteetonasupposequelechangementdecouleuretaitle]resul-bungundeleklrostatischeFunkenbildungwu]rdennichtvemnde]rt. tatdeladegradationduliant.LesresistancesAI'impact,aufrottement

etauxetincelles61ectrostatlquesn'ontpasetemodifiees.

8 u m m a r y

AchangemcoloranddensiC/duringprolongedheatingofafblmu-'- Ition containing s-hexanitrohexaazaisowurtzitane (HNIW) andEstane5702wasinvestigatedbyFouriertransfblminfra]red(FTm)spectroscopicanalysis.Startingmaterials,virginandfinalcoloredfOr-mulationwerestudied.Analysisrevealedpolymolphicimpu]mtyatall

E stagesofproductionandprocessing,thenamreandconcemrationofwhichchangedwithtreatmentofthematerial.Thermally-inducedpOIymo][phicconverslonwasdeterminedtobethecauseofthedecreaseindensic/,whereasthecolorchangewasspeculatedtobethe

resultofbinderdegradation.Sensitivitytoimpact,fiictionandeleco-staticspalkwereunchanged.

1.IntrOductin

behind the twm problems in HE fbrmulations of unwantedcrystal growth and phase conversion.

One polymomhic HE of recent interest, the polycyclicnitramine 2,4,6,8,10,12-hexanitrohexaazaisowurtzltane(mJIW)(1), has been the fbcus of research investigating itspolymorphs, their relative stability(2-5), and their thermalreaction chemistry(6). The basic molecular strucmre consistsof a ]jgid carbon isowurtzitane cage modified by the substi-mtion of six b]ridging nihogens, each with an attached nitl.ogroup (Fig. 1). Both changes in spatial onentation of thenitro groups about the five- and six-membered rings anddifferences 1n crystal 1attice packing distinguish the fourdifferent polymorphs that have been isolated so far. one ofthe first polymorphs discovered, beta phase (orthorhombicspace group Pb2,a), is the least stable with respect to poly-

H E XA N I T R O H EXAAZA I SOW U RTZ I TA N E

O 2 O 2 N

O 2 N

- N O 2

- N O 2

- N O 2

Figul.e 1. The hexaninPohexaazaisowurtzltane, (HNIW) moleculeshowing the rigid isowurtzitane cage with nill.o groups attached to thebridging nitrogens. Changes m spatial orientation of the nin.o groupsabout the cage, and differences in packing in the clystal 1attice, definethe four known polymolphs.

0721-3 1 1 5/94/0204-063 $5 .00.25/0

Itisimportantto]mowthesolubi]ityofhighexplosives(HE)infblmulationbindersandsolventsinordertocontrolthestabilityandsensitivityoffb][mulatedsystemsasthey

e age.Withsolubilitycomescrystalsolvationandgrowth:thegreaterthesolubility,thegreaterthechangeintheparticle

lss sizedistributionwithtime.Inaddition,fOmmlationsbased0napolymo][phicHEcanwimessphaseconversiontome-tastablepolymolphswhenstoredatatemperatureabove

0f thatofconversion.Temperaturecyclingexacerbatesrecrys-tallizationof1heHEintometastablepolymorphs.Thiscon-

'versiontoahightemperaturestable,lowerdensitypoly-l l;:metastablephasewi]1causeundesirablevolumeexpansionll :;:ri I::::;:::".

l: rI'PEPYD5,PropellantsExplosPyrotech.PEPYD5 , Propellants Explos Pyrotech. 1 9. 1 994 No 3 1 13-158 . ISSN 0721-31 15

~

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mphic interconversion(2). This phase is confbrmational-1y/strucmrally different from the alpha and gamma phases,

which have the Same molecular confonnation but pack differently into the unit cell (orthorhombic Pbca, and mono-clinic P21/C, respectively). The alpha crystal lattice is stabi-1ized by the addition of H20 to the unit cell, with X-ray

wk confinning cavity occupancy of 17%-50% by watermolecules(1a,7). The most recently discovered polymorph,the epsilon phase (monoclinic space group P21/c), is yetanother strucmrally different fbrm of HNIW. The epsilonphase has been shown to be the themlodynamically stableone at ambient conditions(2,5). In addition to the threeknown alpha/gamma, beta and epsilon confOmlers, a highpressure zeta phase has been discovered arising from areversible transition fi.om gamma phase(3a). The polymorphs

have relative densities of the order E > > 'Y% AII poly-mmphs can be easily distinguished by their FTIR Spectrum

in the midIR fingerprint', region of 1200-700 cm-1.There have been several smdies investigating the solu-

bility(1a,8) and polymo]Fhic conversion(9) of HNIW. Thesolvents and binders studied have included those commonlyused in fOmmlation processing. Not included are materialswhose high viscosity defy standard solubi]ity testing meth-ods involving solute filtration. one such viscous materialused m a number of fOnnulations is the polyester co-poly-mer Estane [poly(urethane-ester-MDI), trade name Estane5702, Goodrich; Fig. 2] . This polymer is known to undergohydrolytic degradation. This decomposition is evidenced byreduction in intrinsic viscosity due to hydrolysis of the esterlinkage and fmgmentation into 1ow molecular weight seg-ments(lo). The end-chain isocyanate 8oup, because of itsreactivity, is also susceptible to the effects of water. Somedeterioration will invariably occur during storage, thedegree of which will depend on precautions taken to limitexposure to atmospheric moisture. Since it is impractical toobtain fresh binder before each formulation process, theEStane,, that actually goes into a particular formulationwill have a somewhat different fOnnula than that shown inFig. 2.

ESTANE5702

. l( I; )l;. ; & . : M I,Figul.e 2. The polyester co-polymer Estane, or poly(urethane-estel.-MDI) (1rade name Estane 5702) used m the RX-39-AC fOnnulation.The highly reactlve end-chain isocyanate group is susceptible to theeffects of water.

Thennal degradation is a second problem with EstanefO]rmulations. Progressive yellowing and some loss of bind-er resilience from prolonged heating have been seen inother HE/EStane fOrmulations(11). The decomposition mech-anism probably involves both the reaction of the isocyanateand hydl.oxyl ends of neighboring chains and the cleavageof these nascent segments fiFom the 1onger repeat umt. Theminimal amount of thelmal degradation seen in HE/EStaneformulations during processing has thus far not been con-sidered detrimental to the binding properties of the Estane.

L? ~r'n' F 3 J

Hf:i;;,:': }

l';}MM !l:;':(::" Mlcause of the grey coloring of the PBX. There were threel smdJ'batches of PBX made from this E-HNIW shipment, witl]l Half

i\i!lrll,:

r ~ ..~~.~7JE7~

1 I'

:; l %:?;:: h #I I I ;:,5l :\)uring water. The standard slurry coating formulation processven at included heatmg the HNIW/Estane mixture to 60C, above;haped the lowest measured temperature for HNIW polymolphiccorre_ Phase transition(5). The polymo:[phic purity of the as-period received IEllW was therefbre detemlined fbr baseline com-ty, the parison with as-made (b]riefly heated) fblmulation and fbr-1 ther- mulation heated for an extended period of time. Because ofd the the beaded nature of the three PBX batches, and the batch-,dy the to-batch color valiation, pressed parts of the blended RX-

phase 39AC formulation presented a light-dark mottled appear-,anges ance. After prolonged heating, the appearance was that of,mples lightgreen-dalkgreenmottling.ipuri_ Fouriertransfbnninfmred(FTIR)spech.oscopywasthe

E,de E_ diagnostic tool used to dete][mine the nature ofpolymphic)losive impurity in the e-mJIW. An attempt was also made to iden-abora_ tify the source of the green color by FTIR spectroscopy.,1ation Spectra were taken of standard 1 % by weight KBr pellets ated and 4.0, 2.0, and 1.0 cm-l resolution with 10 collection scanssd and using a benchtop Nicolet instrument (model 730). Since39_AC Sample resolution was best matched with an insmlment res-d (g) a 01ution of 2 cm-1 in identifying HNIW polymorphic Con-3apons t[lminants, most of the analysis used this intemlediate set-

ht %, ting. The infrared spectrum of the 4% by weight Estane)drich, Component of the RX-39-AC mixture was intense, masking

nining the absption Specbrum of the InW when the fO][mulationlensity was analysed intact. The HNIW was therefore recovered;e dur_ fi0m the folmulation by first dissolving the Estane in chlo-

roform,methylenechlorideorethylenedichloride,thenrinsingtheInVIWresidueinmethanol.Thesesolventswere

chosenbecauseoftheirlowsolubi]itytowardmIW(1,8,9).TheEstanebinderinsamplesthathadbeenpressedwasvelyslowtodissolve.AItematively,attemptstosmdythe0riginofthegreencolormthebinderinvolveddissolving

themIWinacetone(highHNIWsolubility)(1a,8,9),thenIinsingthebinderresidueinmethanoLThecleanedEstanewasdissolvedinchlorofbrmandappliedasasyruptoKBr

l in the platesforFTmanalysis.Tostudytheeffectofextendedheatingaloneonthefbr-

mlllation,someoftheC.J.S.91-104batchofRX-39-ACmaterialwasheatedforvaryingperiodsoftimepriorto

,estion FTIR analysis. Samples of as-made RX-39-AC fOrmulationsigna- were heated in glass and in stainless steel pans (~5 g each)

. "elevatedtemperamreinsealedovens(FonnaScientificc t l v at -HNIW 521). Initial heating at 100oC was in a dry oven, but foras the h6ating longer than 24 hrs a pan of water was added to: three study the affect of humidity on themlally-mduced changes.

, with Halfgram samples of the RX-39-AC were taken from the}rs 91_ 0v6ns once every hour for up to eight hours, and the

ion in IBmaining InIW left to heat for a total of 24 hrs. After a)1_102 ninth sample was taken, the remaining InJIW was 1eft for) were rln extended period of time, heating an additional 72 hrs atskim- lO5oC in the presence of moisture. AII twemy samples (ten

sta'}e l

.tion in9 1 - 1 02

2) were/ skim-

D u r i n g)ven at

shapedl corre-

; periodlity, the:d ther-

d theudy thel phaseh an g e s, amp l e sld puri-lade e-

p 1 o s iv e ab o r a -u 1 a t i o n3ed andted and3 9 - A C

ld (g) ae ap o n sght %,, o dri ch ,m i n i n gd e n s i t yge dur-

ll e S tl o n

s s i g n a -

d in the

l c t l v at -

H N I W'as the

threet, with

rs 91-

F E @ D y A EIIuXrLE5oWIuTIL UJ

each from glass and steel pans) were checked by FTIRanalysis for HNIW polymorphic pu]rity. In addition, both anRX-39-AC formulation made with a different 1ot of e-

mJIW, and a non-Estane fbrmulation (S-IIW/EVA 95/5weight %) were heated in a wet oven at 105C for 96 hrsfor visual observation of color change.

To study the effect of pressure coupled with heating,samples of the C.J.S. 91-104 batch of RX-39-AC fOmmla-tion were pressed at different temperatures using a heatablepress (BIackhawk Mfg. Co., Porto-power). Four pellets of~500 mg each of RX-39-AC were pressed in a 0.5 inchdiameter die set. Pressing conditions dup]icated those of theproblematic green RX-39-AC fbnnulation. The sample anddie Set were preheated to allow for equilibration at the spec-ified elevated temperamre (75oC, 85oc, 95oC, and 105oC)prior to pressing. The samples were held under vacuum(less than 400 Mm Hg) during three pressure cycles at30,000 psi (208.6 MPa). The pressu]re dwells were 5 min-utes per cycle with a relaxation period of one minutebetween cycles. The pellets were allowed to cool to roomtemperature for at 1east two hours before the binder wasdissolved in methylene chloride for polymorphic FTIRanalysis of the HNIW. A similar RX-39-AC pellet was alsopressed from material that had been heated 96 hrs (24 hrs at100oC plus an additional 72 hrs at 105C).

Safetytestingincludedtheelechnstaticspalktest(1.0J,500 Q resistance, with pigmented mylar film over sam-ple)(14), the 50% drop-weight impact sensitivity test (Dh50,type 12 tool, 2.5 kg weight)(15), and the friction test(BAM)(16). Both original virgin fOrmulation and some thathad tumed green after 96 hrs heating were tested and theirresults compared.

3. Results

At each step in the process of producing, heating, andpressing the RX-39-AC formulation, some polymorphicimpurity was identified by FTIR spectroscopy in theE-HNIW. The amount of impurity varied from on the orderof 10% to almost 100%. The concentration was seen tochange from one bead of RX-39-AC to another, and (aver-age of Several beads) from short heating time to 1ong. TheFTIR spectra revealed the presence of small amounts ofpolymomhic impurity in E-HNIW by (1) the appearance ofnew, weaker spectral features, and (2) the broadening ofexisting spectral feamres where there was insufficient spec-

hal resolution in these peaks from one phase to the next(8a).Table 1 lists the characteristic peaks in the 1200-700 cm-1range used to distinguish the HNIW polymorphs. The

nature of the identifiable impurity (, , or 'Y) changed ateach step, depending on the treatment of the material. Theas-received C-128 E-HNIW was found to contain primarilybeta polymorph contaminant (on the order of 10%) asdescribed in Ref. 8a. Figure 4 shows the 2 cm-1 resolutionmid-IR fingerprint, region FTIR spectra of C-128

e-HNIW, cleaned e-HNIW, and pure -mIW. The cleanede-InHW was prepared by stirring the contaminated HNIWfor 26 hrs at room temperature (R.T.) in a mixture ofxylenes plus 30% by volume acetophenone. This solvent

7

YD5.PropellantsExplos.Pyrotech.19.1994,No3113-158.lSSNO721-3115

> 1

OO /lrorUILZ

Table 1. Characteristic Peaks of HNIW Polymolphs m the 1200-700cm Range

Characteristic Peaks for HNIW Polymorphs from 1200-700 [cm-1]Ep s il o n AIpha Beta Gamma

705.1723.9738.2744.5751.4758.2820.2831.6855.4883.8900.3913.2938.3944.4980.9998.7

1 022 . 11 05 1 . 61 0 8 7 . 21 1 25. 11 139.21 1 8 2 . 51 1 9 1 .7

7 1 8.0746.575 1 . 1757.7764.9825.3835.4860.2881.7904.3945.2947.7949. 1951.7953.0989.2990.7

1 0 5 2 . 41 0 7 2 . 81 0 7 7 . 01 0 7 8 31 0 8 0 . 31 0 8 2 . 01 0 9 4 . 91 1 18.31 1 1981 1 2 1 . 11 1 68 . 11 1 69.6

718.97464766.8

8354882.9907.7944.99592991.8

l 0 5 2 . 31 0 9 4 . 71 1 54.41 1 7 1 . 81 1 7 8 . 6

7 1 9 .574 1 . 0755.8764. 1831.78344858.3879.28920909.4938.1958.8970.6

l 043 . 81 0 8 0 . 41 1 06 . 1l 1 5 3 .01 1 80 .4

mixture was found in an ear]ier thelmal study(5) to increase1he rate of solvation for 1he -polymomh, allowing relative-ly rapid recrystallization even at R.T. to the thennodynami-ca]1y stable epsilon polymo]ph. There is modest broadeningand shoulder-smoothing (938.3 and 998.7 cm-1) of somefeamres in the as-received E-HNIW due to beta impurity,but most pronounced are relative intensity changes in thelow frequency quartet (738.2-758.2 cm-1) and doublet(83 1 .6- 820 .2 cm- 1) .

l D D 0 u U U D O 0 D D 0 o o c H F 0 g H o D D o 0 D . G 0 o D o . - . H1200 1150 110D 1050 10DO 950 900 850 BOO 750 700

wavBnumbGrm']

Figure 4. The fingerprmt region (2 cm-1) FTIR spectra of (a) as-received lot C-128 E-HNIW, (b) C-128 E-HNIW cleaned of beta

impurity, and (c) pure -ImlW. Beta impuriO/ is most easily resolvedby peak intensiD/ changes around in the 1ow fTequency quartet (738.2-758.2 cm-1) and doublet (83 1 .6-820.2 cm-1).

r~~ ~r / ~ =~

Figure 5 shows the FTIR Speclra of as-received C-128E-HNIW, HNIW recovered from two samples of RX-39-AC

formulation, and pure -InnW. The recovered InJIW sam-ples no 1onger have impurity spectral features typical ofbeta phase, but do have those indicative of alpha phase.Again the quartet of 738.2-758.2 cm-1 are affected; in Figs.

5b and 5c, the epsilon 744.5 cm1 peak is made moreintense by overlap with the alpha 746.5 cm1 peak. Extrafeamres characterislic of alpha phase at, fOr example 835.4,904.3, 990.7, 1094.9, and 1169.6 cm-1, vary in scale ini]1ustration of the range in alpha phase impunty concentra-tion in as-made RX-39-AC. The simultaneous disappeal-ance of beta phase and appearance of alpha phase duringthe fbrmulation process indicates a phase conversion in themWIW impurity. The reason fbr this conversion 1ies in thefommlation procedure itself, which included jacket heatingto 60oC dming air sweep removal of the ethylene dichlolidefiom the Estane. This temperature is well above a measured

conversion temperature of 37 1oC for -hydrate(5).

l o o o 0 d d 0 D D D 0 D D D 0 D g U o c P 0 c o c 0 0 U D 0 . H1200 1150 1100 105D 1000 950 900 B50 800 750 700

wavonumber[cm.5

Figm.e 5. The fingerprint region FTIR spectra of (a) as-receivedC-128 e-HNIW, (b) and (c) HNIW recovered from two as-made

RX-39-AC fOnnulation samples, and (d) pure -mIW. Evidence ofalpha not beta impurity in (b) and (c) can be seen m the appearance ofalpha peaks at 835.4, 904.3, 990.6, 1094.9, and 1 169.6 [cm-1].

The FTIR spectra shown in Fig. 6 are those taken ofHNIW recovered from RX-39-AC heated in stainless steelpans in a sealed oven fbr periods of one to 96 hrs. Includedfor comparison are the FTIR spectra of HNIW recoveredfTom as-made RX-39-AC and pure gamma phase InJIW,The amount of polymolphic impurity does not increasemonotonically from one scan to the next, i.e. with heatingtlme. This non-monotomc growth is probably due to sam-pling error, since there is varying impurlty concentrationbetween individual beads of formulation. There is neverthe-less a trend with longer heating time of decreasing intensityof alpha features together with increasing intensity ofgamma peaks. Most pronounced is the rising prominence ofthe gamma peaks at 755.8-764.1, 938.1-970.6, 1106.1, and1153.0 [cm-1]. Spectra corresponding to longer heatingtlmes show greater conversion of InnW to gamma phase

due to both 'Y and E 'Y transitions, with a]most all the

r "

( L )

/ (( K )

/ (( J )

A(

( H )

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( F )

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steel

l u d e dN/ er e dNI W re a s e

L2UI 4JLJoAv0' JLULL'L'0' 1> UJ~U> 1L>U

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I . . . . H . . . R K . D U 0 o o U o o 8 B D 0 o D o H1200 1150 11@0 105o 10CO 950 9Oo 88o 800 75o 700

wavenumber[cm-11

Figul.e 6. The finge]print FTm spec of HNIW from (a) as-madeRX-39AC; (b) through (j) firom RX -39-AC samples heated 1-8, 24hrs at 100oc; 0f) from RX-39-AVC heated a total of 96 hrs at 100oC-105oC; and (l) pure gamma phase. There is a trend in increasing8(lmma impuno/ concemratlon with 1onger heating time.

:atlng 6psilon mHW converted to gamma phase by 96 hrs heat-sam- ing. Material heated in both glass and stainless steel pansation show 1he same trend in polymolphic conversion. Both Sets,rthe 0fsamples also underwent progressive color change withl ; " \ ) ] @ \ :Ce of which had been heated 24 hrs were as green as the initial,, and problematic RX-39-AC fOmmlation. The affect of humidityating 0n the samples during additional 72 hrs heating was not)hase clear. AIIhough it is unlikely that the rate of polymPhic11 the conversion is modified by exposure to moismre, the rate of

l- j

PYD5 . Propellants Explos. Pyrotech. 19.

mErmHI~E5mUmn/br e-IiexanlmohealsOWUrtzItane

l . . . . n . . , . 0 o . C ' l D 0 o c 0 D u 0 u~U~U~U~U . ====--ll2DO 1150 11Do 1050 1000 950 900 850 800 750 700

wavenumber[cm-1]

Figure 7. The fingerplint FTm speclra of mIW from (a) as-madeRX-39-AC; (b) through (e) from RX-39-AC samples heated at 75oc,85oC, 95oc and 105 C; (f) ffom RX=39-AC heated a total of 96 hrs al100.c-105oC; and (g) pure gamma phase. There is no hend in HNIWpolymmphic conversion.

EStane bmder degradation could be affected. AIthough bothdegree of coloration and amount of polymophic conversionwere obviously greatest for material heated 96 hrs, neitherobservable could be considered extreme compared to ther-mal ends.

Figure7showstheFIn%spectraofRX-39-ACfOmmla-tion samples that were heated (75oC, 85C, 95oc, and105oC) du]ring cycle-pressing. For comparison, also shownare the specha of InJIW recovered from as=made and heat-ed (24 hrs at 100oC) RX-39-AC formulation and puregamma phase HNIW. Unlike Fig. 6, there is no clear-cutend m polymphic change in the impurity from one scanto the next, i.e. with pressing temperature. The only pro-nounced change from alpha to gamma HNIW impurity isseen m Fig. 7f, which is identical to Fig. 6k. This conver-sion is pnmarily due to extended heating at elevated tem-peramre, not elevated pressure.

The final materials tested included the heated andpressed green RX-39-AC fOmmlation in question, an RX-39-AC fonnulation made from a different 1ot of E-InJIW, a

non-Estane HNIW fbrmulation (S-mIW/EVA 95/5 weight%), and Estane recovered from heated RX-39-AC (rinsedclean of HNIW). The Spectra of mnW retrieved fTom theproblematic gleen RX-39-AC material is shown in Fig. 8,along with that of IEJIW from as-made RX-39-AC, puregamma phase ImJIW, and HNIW from RX-39-AC that had

1994 NO 3 113-158 . ISSN O721-3115

, i

b5 1Vl.1r.lllZ

( A )d

~TU o .U~U~~0~D~HU~U D p l g @ 0 d 0 G D l o o o 0 c . . . u . . . . H - ~ ~ ~ D1200 1150 110D 1050 1000 950 900 850 8DO 75D 700

waunumber[cm]

Figure 8. The finge]rprint FTIR spechPa of HNIW ffom (a) as-madeRX-39-AC; (b) from RX39-AC heated 24 hrs at 100C; (c) from 1heproblematic geen RX-39-AC fblmulation; and (d) pure gamma phase.The presence of gamma impurity in (C) is shown by new peaks at755.8-764. 1 . 938. 1-970.7, 1 106. 1, and 1 153.0 [cm-1].

been heated at 100oC for 24 hrs. The amount of gammaphase impurity in the problematic RX-39-AC is 1ess thanthat seen in the material heated 24 hrs, as typified by therelative intensity of the same gamma peaks noted in Fig. 6.Since the gamma phase has the 1owest density of the fourpolymorphs, the change in sample density from one pressedpart to the next is apparently due to growth of this phaseduring heating prior to pressing.

The RX-39AC formulation from a different 1ot ofe-HNIW did not change color appreciably with heating96 hrs at 105C (wet oven), other than slight yellowing thatcan probably be attributed to themlal EStane degradation.The new 1ot of HNIW, unlike the original batch, did nothave activated charcoal. In addition, the non-Estane fbrmu-lation, also without activated charcoal impurity, didnot change color during 96 hrs heating at 105oC in a wetoven.

Severalattemptsweremadetoidentifythesourceofthegreen color in heated RX-39-AC fOrmulation. The HNIWwas dissolved in acetone and filtered, and the binder resi-due rinsed in methanoL The mJIW/acetone solution wasonly faintly yellow. The cleaned binder residue was quiterubbery, and when extracted fiPom formulation that had

been heated till Peen, was a daJk forest green. This residuewas dissolved in chlorofonn and applied as a Syrup to KBrplates for FTIR analysis. Virgin Estane itself and Estanefrom both heated and as-made formulation was studied.There was no identifiable difference seen between the mid-

n FTm Spectra of these Estane binders. Either the concen-tration of species that caused the strong coloration in theheated binder was low, and/or the species exhibited a mid-IR FTIR Spectrum not substantially different from virginEstane.

Resultsfromthesafetytestsshowed1ittledifferencebetween as-made and heated (96 hrs heating at 105C,green) RX-39-AC fbnnulations. The results are shown inTable 2. Two values are 1isted fbr the drop-weight impact

rlUPALEIPLUVI,J/1ULI',-'~L''LA~T/

Tab]e 2. Safety Tests of the RX-39-AC FommlationsSafeb/ Test

nnpact Drop-weightTest Dh50(a)EIectrostatic SparkT e s t

Friction TestB AM ( b )

RX-39-AC

as-made heated24hrs(green)36.7, 39.7 cm

0/10(no go)

1/10at 1 1 .6 kg

376 cm

0/10(no go)

1/10at 1 1 .2 kg

(a)For comparison, LX-14-9 (HMX/Estane 5702-FI, 95.5/4.5weight%)hasa50%drop-weightheightDh50=50.0cm.

(b) The as-received e-HNIW had one reaction in ten at 7.2 kg.

test(15) fOr as-made RX-39-AC, both of which are close tothe value measured fbr the heated sample. The approach=

ingelectrode spa]k sensitivity test(14) for both heated andas-made samples gave no reaction m ten mes. The weightrequired to give one reaction in ten for friction sensitiv.ity(16) essentially did not change wilh heating. The amountof polymo]T)hic change in the InJIW or chemical change mthe EStane binder was therefore msufficient to affect notice~ably the sensitivity of the formulation with respect toimpact, Spark and fiiction.

4. Discussion

s.:l%: :change. The beta phase impurity initiaUy present in the as.received E-mUW converted, all or in part, to alpha phas61during the 60oC solvent stripping stage of formulation Thilconversion is consistent with a previously measured con-version(5) of beta to alpha at 37oC. Heating of the as-madeRX-39-AC at 100oC before pressing allowed not only th6alpha phase, but also some fraction of the bulk epsilo]nphase HNIW, to convert to the 1owest density gammaphase. These conversions are consistent with phase transi-tlons seen m earlier work, the E 'Y conversion(5) occuningat 64 1oC. Near complete conversion of the InJIW togamma was seen to occur after 96 hrs heating at 105ocThe most probable reason for the decrease in sample den.sity seen in the initial, problematic RX-39-AC formulation

is mIW conversion to 1ow density material.Solventswithcarbonylgroupshavebeenshowntohave

high solubility towards HNIW(8,9). As a nitrate ester withhigh carbonyl content, Estane would be expected to hav6moderate HNIW solubility. The relatively 1ow carbonylcontent of the EVA binder used in the non-Estane PBXsuggests a lower HNIW solubility for this material. Therole of solvation in polymo]Thic phase transitions is to1ower the bamer to conversion, thus increasing the rate ofconversion(8a). Therefbre, over the relatively short heatin8period used in this work (days instead of months/years) (lgreater degree of conversion would be expected to be seel]in the Estane PBX. It is not clear whether a 1ower solubilitybinder (e.g. EVA) could nevertheless still undergo signifi.cant E 'Y conversion during very 1ong telm storage at temperamres above 64oc. Without prolonged sample heating in

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whether ?oss phase conversion could also occur in this for-m u 1 a t i o n .

Color change in the RX-39-AC from greyish-white to01ive drab green during prolonged heating is not due to theImlW phase conversions, but most 1ikely originates withthe activated charcoal impurity. The role played by thisimpurity could 1ie in sulface catalyzed decomposition of thebinder. Altematively, themlally-driven reactive degradation0f the Estane binder could originate with impurities that arereleased fi?om the activated charcoal duling prolonged heat-ing. Washing of the fomlulation to isolate the InJIW andthe Estane components for analysis showed intense colora-tion remaining in the stiffer, rubbery Estane residue. The

loss in elasticity of the Estane fiom heatmg and pressing isalso indicative of long chain fi.agmentation. However, mid-m FTIR spectra of dissolved, strongly colored binderrevealed no new spectral features that could be used toidentify the source of coloration. UItraviolet olr far-IR spec-troscopic analysis might be more usefUl in such a determi-n a t i o n .

5. Conclusion

Pmlonged heating of a PBX fOnnulation (RX-39-AC)consisting of E-mIW/EStane 5702 (95.8/4.2 weight %) at00oC for up to 36 hrs prior to pressing caused a changem color and sample density(12). These changes were investi-gated by Fourier transfbml infmred (FTIR) Spectroscopicanalysis of starting mate]rials, as-made and final colored for-

mulation samples. Analysis revealed polymo?hic impurityat all stages of production and processing, the nature and

concennation of which changed with treatment of the mate-rial. The small amount (on the order of 10%) of beta phaseimpurity m the as~received HNIW converted to alpha phaseduring the fommlation process itself, which involved heat-mg to 60oC. Extended heating of RX-39-AC samples at

lOOoC105oC revealed not only conversion of the alphaimpurity to low density gamma, but also (after 96 hrs) con-ve]rsion of the bulk epsilon phase to gamma. The speed withwhich this phase conversion went to completion can beattributed to the high HNIW solubility of Estane, which intum is due to its high carbonyl content. Thennally-inducedpoIymorphic conversion was detennined to be the cause ofthe decrease in sample density. The color change of RX-39-AC was not identified, but is speculated to be the result ofbinde]r degradation arising fi?om the presence of activatedch@rcoal impulity. Either su]face catalysis or themlally-dri-ve]1 reactions of the Estane with impurities released fiFomthe activated charcoal during prolonged heating, could pro-duce colored, short chain decomposition products. Sensiti-vjty to impact, fiiction and electrostatic spa]k was unchan-8 e d ,

6, Refbrences

(l) (a) A.T. Nielsen, M.L. Chan, K.J. Kraeutle, C.K. Lowe-Ma, R.A.Hollins, M.P. Nadler, R.A. Nissan, W.P. Nonis, D.J. vanderah,

LIIlllliLhluIuLyuIj-examlronexaazalsowurlzIIane o

andR.Y.YeePOb""pyzCgedEXp/O.yjves,P"7,'ReportNWCTP7020(Nov.1989),Navalweaponscenter,ChinaLake,CA;(b)D.J.vanderah,M.P.Nadler,R.Y.Yee,andA.T.Nielsen,ReportNWCTP7058(Sept.1990),NavalWeaponsCenter,ChinaLake,CA.

(2) P. Politzer, Report ONR NOOOI4-85-K-0217 (May 1991), Depalt-mentofchemisdy,Universic/ofNewOrleans,NewOrleans,LA.

(3) (a) T.P. Russell, P.J. Miller, G.J. Piemlarini, and S. BIock, HighPressurePhaseTransitionin'YHexanitrohexaazaisowurtzitane,',J.P/iyy.C'/hel7m.Q5,5509(1992);(b)T.P.Russe]l,P.J.Miller,G.J.Pielmarini,ands.BIock,Pressure/TemperamrePhaseDiagramofHexanitrohexaazaisowurtzitane,,J.P/iy,5'.C/he/".97,1993(1993).(4) R.Y. Yee, MP. Nadler, and A.T. Nielsen, Polymphs and Ther-

malPropertiesofHexanitrohexaazalsowurtzitane,',mPJ.oc.IQ90JAMV[AFP/.Op.Me"zg,Anaheim,CA,(Oct.1990),p.204.

(5) M.F. Foltz, C.L. Coon, F. Garcia, and A.L. Nichols m, ThelmalStabiliD/ofthePolymolphsofHexanitrohexaazaisowurtzitane,PartsIandnP/.ope//"y,E;p/oJjve.y,Pyro/ec/cJI(1994).

(6) (a) K.J. Kraeutle, The Themlal Decomposition of a New Propel-lantOxidizer,''25//i/AMVHAFCo/bMoMei/2g,Vo/m,CPIAPub.No.498,NASA-MarshalSpaceFIightcenter,Huntsville,Oct.1988;(b)R.J.Doyle,Jr.,TheGas-phaseDissociationofaNewPolyazapolycyclicNitramine:Hexanitrohexaazalsowurtzi-tane,O7g.A",5ipeC.26,723(1991);(c)D.G.PatilandT.B.Bri]l,ThennalDecompositionofEnergeticMate]rials.53.Kine-tlcsandMechanismofThelmolysisofHexaninohexaazaisowurt-

zitane,Co/"bw.F/"e87,145(1991);(d)R.A.Pesce-Rodri-guez,R.A.Fifer,K.L.McNesby,J.B.Morris,M.A.Schroeder,

C.SMiser,andS.A.LiebmanT/./?T/Dec"poq/.H/VIW"dH]VIW-BseFo/."mM/"os,TechnicalReportBRL-TR-3402(Sept.1992),Bal]isticResearchLaborato/,AberdeenProv-ingGround,MD.

(7) R. Gilardi, private commumcatlon.(8) (a) E. von Holtz, D. Omellas, M.F. Foltz, and J.E. CIalkson, The

Solubilio/ofE-mllwinSelectedMaterials,submittedtoPropel-Iants,Explosives,Pyrotechnics;(b)N.C.Johnson,Y.Tran,L.

Lee,S.Tumer,L.Nock,andD.Cichra,I992J:AMVEAFPmpe///Deve/O/"e/M"C/hJ.z/joMee"@g,CPIAPub.No.578,NASAKennedySpacecenter,April1992.

(9) C. Coon, prlvate communication.(10)(a) D.W. Brown, R.E. Lowry, and L.E. Smith, Kinetics of

HydrolyticAgingofPolyesterUrethaneEIastomers,M.o/"o/.I3,248(1980);(b)J.W.S.HearleandM.MiraftabTheFIexFatigueofPolyamideandPolyesterFibres,,'J.Mm,gcj.26,2861(1991).

( 1 1 )C. Walkup and G. Moody, Lawl.ence Livelmore National Labora-ty,prlvatecommunication.

(12)Physics hltemational Company, San Leandro, CA.(13)Gen Co]poration, AerOjet Propulsion Division, Sacramento, CA.

(14)M. Kirshenbaum, 'RepomJe q/PI." Exp/om/ ro GJ"eoDisc/iJ.ge5'"T/"provA"roc/m2g-E/erodeE/ero,Fr"c

Se"lO/A"7l''TechnicalReport4955(Oct.1976),Pica-tinnyArsenal,DOver,NewJersey.(15)B.M. Dobratz and P.C. Crawfbrd, LLIVL p/FjveJ! Ht"oo/t

P/.opeFiesqfC/zic/EXp/o5"e,ydEXp/o.yrveSM/"!y,ReportUCRL-52997Change2(Jan.1985),LawrenceLivelmoreNationalLaboratory,Livermore,CA.

(16)R. Meyer, EXp/oJjvey, 3rd ed., VCH Publishers, New York,1987,pp.147-9.

A C /( o w / e dg e /7 s.

The author would 1ike to gratefUlly acknowledge Chris Gatrousisand Tom Sugihara for their support, and Rich whipple, Rich Simpson,Marc Hoffinan, Mike MuIphy, and Randy Simpson fO]r their technicalasslstance and advice.

Work performed under the auspices of the U.S. Department ofEnergy by the Lawrence Livelmore National Laboratoly under con-hact No. W-7405-ENG-48.

(Received November 16, 1992; Ms 62/92)

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