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FOR LIQUID PROPELL (U) ARMY BALLISTIC RESEARCH LADp ABERDEEN PROVING GROUND RD R A FIFER ET AL SEP 8?UNCLASSIFIED BRL-TR-2827 F/G 21/9 1 W

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TECHNICAL REPORT BRL-TR-2837

DSC STABILITY TEST FORLIQUID PROPELLANTS: A

PRELIMINARY REPORT

DTICROBERT A. FIFER ELECTElPAMELA J. DUFF DEC 1 0 98j3

SEPTEMBER, 1987

APPROVED FOR PUBLIC RELEASE, DISTRIBUTION UNLIMITED.

US ARMY BALLISTIC RESEARCH LABORATORYABERDEEN PROVING GROUND, MARYLAND

87 11,3 11

DESTRUCTI ON NOTICME

Destroy this report when it is no longer needed. DO NOT return it to theoriginator.

Additional copies of this report may be obtained from the National TechnicalInformation Service, U.S. Department of Commerce, Springfield, VA 22161.

The findings of this report are not to be construed as an official Departmentof the Army position, unless so designated by other authorized documents.

The use of trade names or manufacturers' names in this report does not con-stitute indorsefent of any conercial product.

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UN C! \S S 1'l I- LSCA C1AS-,CA--,_N O THIS PACT A bA4?

Form ApprovedREPORT DOCUMENTATION PAGE OM O07408

la REPORT SECURITY CLASSIF'CATiON lb RESTRICTIVE MARKINGS

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Aberdeen Proving Ground, MD 21005-5066

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DSC STABILITY TEST FOR LIQUID PROPELLANTS: A PRELIMINARY REPORT

12 PERSONAL AUTHOR(S)Robert A. Fifer and Pamela J. Duff

13a TYPE OF REPORT 13b TIME COVERED - 114 DATE OF REPORT (Year, Month, Day) 1.PAECUNT

Final IFROMar44 TO 4)4q, 84iI16 SUPPLEMENTARY NOTATION

17COSATI CODES 1"SJECT TERMS (Continue on reverse if necessary and identify by block number)FIELD GROUP SUB-GROUP Liquid Propellants, Differential Scanning Calorimetry,

(1 -7 Q)ignition Temperature, W,~, W Stability, Decompositionll _

\[9 ABSTRACT (Continue on reverse if necessary and identify by block number)The use of a Differential Scanning Calorimeter (DSC) for monitoring the stability of HAN-

S base~d liquid propellants has been investigated. Preliminary experiments have been completerwhich establish criteria for obtaining reproducible initiation temperatures (T1 ). Carefulcontrol of sample Size (volu Jme), inert sampling materials and operation at eleiva'ted pressurihav'i jir1- ],finitive Ts with the lowest standard deviation. Results for 1845 inalliminm sans at 6~.9 MPa 'yield-d an average Ii' of 120 C and standard deviation of 4*C,

rnwhil" an1 av-rslge T~'-01 of 124.8 and standard deviation of 2.4 was obtained for 13 M HAN in thiPr,,s7Ti.-ib1v more ineI* tan tal urn pans at a somiwhat lower pressure (5.2 MPa). Thus, it

appearb that with further refinement the T. (0 values obtained in such a DSC experiment can bosIIcceus,fll~y used to monitor the relative staityf HAN-based propellants. Thevalipes, obtaiined reflpct iniriaticun ind-:r the DSC conditions with small simples; noext r apol1at ian to the i ~T1it i otempe rat tire of hulk HAN-based propellant under otherconditions, is attempted. (~i~..

20IABLr OF ARSTRACT 21ABSTRACT SECURITY CLASSIFICATION

~/J~4LASS(f'P PJL It T11 1)M AT (P AF a u PS I -,_,q .-22as '.AV 0- PESPINSIRI.E NDVIDIIA: 2?n I, E PI-ONE (include Area Code) 22c OFFICE SYMBOL

DR. P'AkMEA Ir. PUFF 30t-278-6163 SLCRR-TR-T

DO FORM 1473, 84 %.AR 8 j t,-I- -. q/ tw i , u , oxhausted _SEri_'ITY CLASSIFICATION OF THIS PAGE

UINCLASSIFIED

TABLE OF CONTENTS

LIST EXPE IGUTAL...................*................ .............7

II. INRESUT............................................... * ......... 7

A. Atmospheric Pressure in Metal Containers ....................... 8B. Confined (Self-Pressurizing) Sap ............ ............ 15C. Samples in Pressurized DSC.. ............ . .. .. .. . . .. .. .. 17

ACKNOWLEDGEMENT .. . .. o .. .. ................ . ... . ... .. .. .. .. .. ... .22

REFERENCES. . . ... .. . .... .. .. .. . .. ......................... 23

DISTRIBUTION LIST.............. .. .. .. . ... .. .. .. . ........ .. ... .25

Pon)

1-6

V 3 3

LIST OF FIGURES

Figure Page

I Thermogram for 1845 in Pierced Aluminum Pans at I Atmosphere,Showing Erratic and Non-Reproducible Behavior Under StandardDSC Conditions .......................... ....... ........ ..... . 9

2 Representative Thermograms of HAN in Au Pans, Showing theAbsence of Pre-Endotherms and Improved Peak Shape, as Well asthe Range of Temperatures over which the Major Exotherm(Tin) Occurs ...................................................... 12

3 Thermograms Showing the Absence of the Pre-Endotherms forSamples in Aluminum Pans with Substrates .......................... 14

4 Representative Thermogram Showing Jagged Exotherms Obtainedwith HAN Confined to Narrow-Bore Stainless Steel Tubing ........... 16

5 Thermogram for Low Stability Sample of 1845 in PiercedAluminum Pan at 6.9 MPa (1000 psi), Showing Absence ofPre-Endotherms, and Reproducible Peak Shapes forPrepressurized Samples ........................................... 18

6 Thermograms for Three Samples of 13 M HAN at 6.9 MPa (1000 psi),Showing Second Exotherm Due to Formation of Ignition Residue ...... 19

i5

1 O

I. INTRODUCTION

The relative stability of HAN-based liquid propellants under varyingconditions of purity and age has previously been indeterminable. Theinitiation temperature of HAN as a function of concentration has been studied 1

and the conclusion was that the initiation temperature varies inversely withconcentration and that the initiation temperature for ii Molar nitrate is thesame as for 11 Molar HAN. Several studies on the thermal decomposition ofHAN-based LPs have also been reported, however, these have focused ondecomposition temperatures of selected HAN mixtures (e.g., HAN spiked withmetals i or HAN with 4-carbon amines 2 ). In both of these cases a 50 cc AMINCObomb equipped with thermocouples was used to measure initiation temperatures.The bomb had also been used earlier3 to determine decomposition gases ofvarious HAN solutions in an attempt to derive rate equations. However, notest has yet been developed which can determine the relative stabilities ofliquid propellants. This type of information could be quite useful, forexample, in monitoring the shelf life of the propellant, or any contamination,

degradation etc.

The paper describes the initial attempts to use a DSC to develop astability test for liquid propellant materials. A second paper has beenwritten in which results from this preliminary work are incorporated into the

*further development of a DSC test in which the effect of trace metals onliquid propellant stability is determined.

II. EXPERIMENTAL

All experiments were run on a Perkin Elmer (PE) DSC-4 equipped with aSystem 4 microprocessor controller, digital "interface" (Analog-to-Digitalconverter), and Model 3600 "data station" (microcomputer) operating with PE'sthermal analysis software. As recommended for decomposition studies, a "flow-through" cover plate was used in place of the standard cover plate. When itbecame clear during the course of this study that-pressure operation wasrequired, the DSC was modified for operation at up to 6.9 MPa (1000 psi).Since the chambers surrounding the sample and reference holders, and theelectrical feedthroughs into them, were already capable of withstanding thesepressures, the primary structural modification was the addition of a removablehold-down assembly for the flow-through cover plate, secured via connectingrods to a second plate mounted on the underside of the DSC head. Furthermodifications to this system have been subsequently made. 4 Flow rates weregenerally about 50 cc/min. Argon, commonly used as the purve gas in DScexperiments, could not be used at high pressure in this DSC since it resultedin very noisy, erratic baselines, presumably due to thermal fl|ictuations in

the high density gas flowing around the sample holdors. Switching to heliumfor high pressure operation completely eliminated tLi "noise" problem,presumably due to its much higher thermal c-nductivitv.

The samples investigated consisted of 13 Molnr hxdroxylnimmonikm nitrate(HAN), and the propellants NOS-365 and BRL-.i . 'Ianv lIiferOn: tvpos ofsimple containers were used, includine pir-. ! r w ,-d tl-ri alaminumPF "volatile" DC nans, PE stnin ,, ;,, t , ,,r ?O( psi)capsules, as well as P gold pans, c11t, -1 , , :1 iu, linumpans onto which was ;puttfrod ,- c,,nt in d . ,i, io,,

stainless steel HPLC tubing and glass capillary tubing were used. These and

other containers are discussed in more detail in the following section.

Typically, depending on the sample configuration, about 2 lI (-3 mg)propellant samples were used. In order to achieve reproducible (e.g., *5%)sample sizes, these were measured out with a 10 41 Hamilton #701 syringe, andweighed to *0.01 mg.

III. RESULTS

When we began this task, we thought it would be a relatively simple

matter to design a DSC stability test for liquid propellants, and then use itto study the effects of metal impurities, complexing agents, etc. This turned

out not to be the case. The HAN-based liquids behave very non-reproduciblyunder standard DSC conditions. A couple of hundred experiments have beenperformed, with many different configurations, conditions, sample holdermaterials, etc. The following is a roughly chronological account of some of

the preliminary techniques tried in an attempt to achieve reproducibleinitiation temperatures (Tin) in the DSC. The purpose of describing thisearlier work is not just to emphasize the difficulty we encountered indeveloping a usable DSC test, but to point out some of the many interestingbut as yet unexplained phenomena we discovered in the course of theinvestigations--phenomena that will undoubtedly have more meaning when furtherinformation becomes available about the kinetics and mechanisms of the

decomposition/initiation processes.

A. Atmospheric Pressure in Metal Containers

Under "standard" DSC conditions (programmed heating at 1-10 deg/min inpierced aluminum pans at atmospheric pressure), the propellants produce very

erratic results, as shown in Figure 1. The shape of the thermogram isdifferent for almost every experiment (Table 1). There are frequently

multiple exotherms, and the temperature of the first exotherm varies over alarge temperature range (e.g., 120-160°C, see Table A). The multipleexotherms are presumably due to boiling and splattering of the propellant.Endotherms often, but not always, precede the exotherms. In order to compare

the reproducibility of the Tin values within each treatment it is useful to

calculate an average Tio value and corresponding standard deviation. However,in this instance there is no single Tin for each run, so such a calculation is

meaningless.

HAN is a reactive acidic salt and its reactivity towards various metalsis well known. 3 However, at the time that this work was performed it was

suspected, although not definitively shown, that HAN reacts with Al. (Sincethen a report has been published which confirms the reactivity of HAN and

Al.) 5 In order to assure that this erratic behavior was not due to surface-related effects, experiments were carried out in aluminum pans coated withgold or platinum.

The results under these conditions were only slightly improved. In most.cases only a single exotherm was generated, although in several instances

multiple peaks were still obtained (Figure 2). The endotherms which often

preceded the major exotherms in the case of Al pans were eliminated. Theinitiation temperature (Tin) was not uniform however, and varied over atemperature range of 70-180*C, with an average value of 129*C and standard

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A. Atmospheric Pressure

Sample/Size* Endotherms Exotherms(mg) (C) (C) Comments

Aluminum Pans3.3 134 151,155,158 Multiple exotherms; no3.9 84,122 97,108,136 average Tin or std. dev.7.6 138,140,157 obtained

14.71 149 174

Coated PansAu 4 .2a 86 Endotherms eliminated

1.0 63 Single exotherm obtained5.8 178 in most cases3.8 177,197 Large scatter in Tin,

Pt 3.2 140 T inf128.8, u=52.52.4 6.7 minb

Stainless Steel Tubing* 1.0 117,126,143 146,153 Multiple peaks, probably

1.0 153,157 due to degassing of sample1.2 119,125 126,142,153 Multiple exotherms; no1.0 98,102 103,109 average Tin or std. dev.1.0 76 82,88 estimated

Substrates in Al PansMolecular sieve, 3.2 141 Single exotherm obtained,Glass wool, 2.9 186 probably due to eliminationPorapak, 7.4 154 of splatteringSilica, 2.2 116 Std. dev. is high;Silica Gel, 3.3 130 Tinf144 .5, a=23.9

Zirconium powder, 2.9 109 Tin significantly lowerAluminum powder, 7.4 85

Fiberglass, 2.6 120-185 Broad exotherm0

10

V.%

Table 1. Major Endo- and Exotherms for Various HAN-Based Samples (Cont'd)

B. Elevated Pressure

Sample/Size* Endotherms Exotherms(mg) (C) (C) Comments

0.76 MPa

3.3 169 Single exotherms obtained3.1 1483.3 30 minb

2.4 8 minb

NOS 2.0 96 Single exotherm obtainedNOS 3.0 124 Std. dev. is high;NOS 3.6 132 Average Tin= 1 38 .3, a=2.4NOS 3.9 187NOS 2.9 142

5.2 MPa/Ta pans

3.1 123 Single exotherms obtained3.2 125 Std. dev. is greatly3.2 123 improved; Average3.2 128 Tin=1 24 .8 , a=2.4

5.2 MPa/AI pans3.2 119 Single exotherms obtained3.2 157 Std. dev. is high;3.2 104 Average Tinml 26 .6, o=22.3

6.9 MPa1845 2.9 120 Single exotherms obtained1845 2.9 116 Std. dev. is greatly1845 3.0 118 improved; Average1845 3.0 127 Tinmf20, a=4.01845 3.0 1171845 3.1 122

HAN 3.0 154 Single exotherms obtainedHAN 3.0 126 Std. dev. is high;HAN 3.0 124 Tin=133.2, a=20.2HAN 3.0 153HAN 3.1 125HAN 3.1 100

HAN 3.1 124HAN 3.2 114HAN 3.2 150HAN 3.2 162

*Unless stated otherwise, all samples were HAN, run at 10 C/min

a - I C/minb - Isothermal

11

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Figuire 2 . Rppresentative Thermograms of HAN in Au Pans, Showing theAbsence of Pre-Endotherms and Improved Peak Shape as Well as the Range of

Temperatures over which the Major Exotherm (Tin ) Occurs

12

deviation of 52.5-C for the Au coated pans (Table IA. A''

pans appeared completely and uniformly coated, aft,-r ha,.coating usually looked a bit distorted and occasionallvit is quite possible that HAN reacts with the Al durin4 tk- t,experiments (typically about 15 minutes). The blistering of tn et

p~ating also suggests reaction between the Al and HAN. Thus, t,,

exists that Au or Pt might be suitable materials and mor,- ,t*.- :1obtaining reproducible results if an improved ciatin' pr ,. ,

constructed of Au or Pt were used.

A number of experiments were carried out with th, pr,,*, ' - -

a powdered (or fibrous) substrate. The rationalf' was that th. ? r.substrate would prevent the sample from splattering during htatin , 1.possibly act as a "thermal sink" for the exothermic energy roltaso .s .

inducing slower, more controlled decomposition.

Non-metal and typically inert powders such as molecular siev, s5gel, Porapak Q, amorphous silica and silica powder all gave a single, shar;exotherm, indicating that the splattering observed in Al pans with nosubstrates had been diminished (Figure 3). This was true even thouc, )n.vfirst two materials were wetted by the propellant, and is probablyattributable to the matrix breaking up the dissolved gases in a manner -

0 to boiling chips. When deposited over relatively large quantities )fzirconium or aluminum powders, the HAN trickled freely between trfeparticles. In these samples, lower reaction temperatures were observ"(-IO0°C), possibly due to catalytic effects and/or greater thermalconductivity of the powdered metals (Figure 3). However, in these instanot-,the sample pans were filled to a greater extent than in the precediny, 2xa'. ,and perhaps the volatiles trapped above the sample promoted furtherdecomposition. The aluminum-substrate sample, run with solid lids andtherefore having the potential to self-pressurize, did, in fact, exhibit th,lowest decomposition temperature in this series. After the experiment withthe aluminum substrate, it was observed that the top was blown off and thl.sample flipped over; thus, the shape of the exotherm is not entirelyrepresentative of HAN decomposition. Little evidence was observed that thematrices provided a heat sink and promoted slow reaction; in fact, rathersharp, regularly shaped peaks were obtained. It may be that the matrices

% (fairly good insulating materials) are heated thoroughly in the DSC an,,this heat is absorbed, act as a source of heat. Thus, once the rea':i .,ithe HAN begins, thermal energy is available in the surrounding matrixparticles to sustain it and the reaction proceeds rather quickly.

Slow reaction did occur (several minute duration) in samples drawn bv

capillary action into tightly-packed fiberglass fibers (Figure 3) . In t.samples individual HAN molecules are more likely to be adjacent to thi,'r:than in previous samples in which the HAN was deposited over tho now r±. iin some cases did not even wet them. It might be possible for HAN :i, on,

z in the fiberglass bundles to react independently of HAN at a distant .it

resulting in decomposition over a longer period of time.

These experiments showed that it was possible to roduce the ,ttc,to spreading and splattering. However, reproducible decompositioMtemperatures were not achieved, and results ranged from about l -O ' ",(

Jb, .1. the non-metal powder substrates, with an average teip- rtt,i r ,

13

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TEM4PERATUJRE (C) 0SC

Figure 3. Thermograms Showing the Absence of the Pre-Endotherms forSamples in Aluminum Pans with Substrates. The Most Definitive Tin isObtained Using an Inert Substrate (e.g., Class Wool) in Which the

Propellant is not Totally Absorbed by the Substrate

14

(Table IA). The variance was an unacceptably high 21.8"C, indicating that thereactions resulting in the release of gas from the samples is not

reproducible. This may, in part, be dut to the fact that these samples wererun in Al pans and the reactivity of the Al with HAN would not be expected tobe reproducible. Also, it is assumed that the substrates, mostly molecularsieve chromatographic packings and inert to most compounds, are also inert toHAN. If this were not true, such substrate-HAN interactions could add to theirreproducibility. Additional factors may be variations in the loading ofboth the HAN and substrates, but the major factor was believed to be watervaporization which was not sufficiently reduced in these treatments. Itshould be noted that the rather fast reactions under these conditions yieldpeaks with regular shapes, not unlike results obtained for solid propellantmaterials such as the nitramines.6 Since the purpose of this work isultimately to correlate changes in decomposition between samples of differentcomposition (e.g., variations in formulation or impurities and/ordegradation), sharper peaks allow smaller changes in decomposition temperatureto be observed.

B. Confined (Self-Pressurizing) Samples

Since the suppression of water vaporization/boiling is probably necessaryto achieve reproducible decomposition or initiation, several techniques were

* tried that involved self-pressurizing sample containers. The disposable PE"volatile" aluminum pans that were used (pierced) in the experiments describedabove have a diameter of about 3 mm, a volume of about 20 microliters, and arated pressure of a few atmospheres. Samples of propellant (-3 mg) heated insealed (unpierced) pans give somewhat less erratic results than with the panspierced, with fewer exotherms and perhaps a somewhat higher decompositiontemperature, but the results are still too non-reproducible to be of use. Theresults were similar with the stainless steel or gold-plated stainless steelPE 45 microliter reusable screw-top "high pressure capsules", which have apressure rating of 15 MPa (2175 psi). Such pressures should be quiteeffective in suppressing vaporization and boiling; the problem is that

vaporization and boiling are not sufficiently suppressed at the low loadingdensities that must be used to prevent damage to the capsule or DSC. (On alarger scale, this applies to previous heatable closed bomb work as well.)

One technique was tried that did involve "total confinement" (no initialfree volume). Propellant was drawn upward through a length of cleaned and

propellant-treated small bore (0.38 mm, or 0.015 in i.d.) stainless steel HPLCtubing. A number of short (4-5 mm) sections of tubing were then crimped offwith heavy-duty wire-cutting pliers. Experiments with the HPLC pump (using asolvent, not propellant) showed that the ends crimped-off in this manner had arupture pressure greater than the maximum pressure of the HPLC pump (41 MPa,or 6000 psi). The sections of tubing were run directly in the DSC sampleholders (no sample pan). Although difficult to see visually, post-runweighing confirmed that the tubing did, in fact, rupture upon initiation.Despite the confinement, the results for several samples were no morereproducible than for the sample pan experiments. Often the curves showedrather jagged endotherms and exotherms, possibly due to degassing which mightoccur at the high pressures generated in the stainless steel tubing (seeFigure 4, Table A). This precluded the calculation nf a meaningful averagevalue of the Tin and viriance. However, several samples run a day after beingloaded in the tuihinR gave different results from those of the day before,

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suggesting that the propellant was not compatible with the tubing. Thesimplicity of this technique, and the fact that several samples could be runsimultaneously, suggest that it perhaps deserves further evaluation, usingmicrobore tubing of some other material.

C. Samples in Pressurized DSC

An alternative to confinement for suppression of water vaporization and-/ boiling is instrument prepressurization. Although confinement techniques like

that described above can produce higher pressures than are practical usingprepressurization, prepressurization has some potential advantages: any kindof sample container can be used that is normally used at atmospheric pressure

(metal pans with swaged on lids need to be pierced to prevent collapse duringinstrument pressurization), and there are no stringent requirements as far asprecise control of loading density, avoidance of bubbles, removal of dissolvedgases, etc., as for highly-confined samples. Makeshift techniques forpressurizing the DSC were used at first: simple valves were used to restrictthe purge gas flow exiting the sample and reference chambers, and a large pileof lead bricks was placed on the cover plate to assist the standard hold-downmechanism. A number of experiments were carried out at 0.76 MPa (110 psi),using pierced aluminum pans. The results were very encouraging; although theinitiation temperatures (Tin) still varied from run to run, multiple exotherms

* were eliminated, and in almost every case no pre-endotherms were observed (seeTable iB). In addition, there seemed to be a correlation between sampleloading and temperature (Table IB). A regression analysis of the results forNOS indicated that the temperature increased linearly with sample loading.This relation did not hold for samples run under any of the conditionsdescribed previously. These preliminary experiments were sufficientlypromising that the DSC was subsequently modified for pressure operation at upto 6.9 MPa (1000 psi), as described in the experimental section above and indetail in Reference 4.

Results at 6.9 MPa (1000 psi) were found to be even more reproducible;peak shapes at this pressure are very reproducible, although there is still

considerable variation in the Tin of the HAN samples. This occurred in spiteof the fact that all samples were kept at the same loading density (3.1 *0.1 ug). However, the reproducibility for 1845 significantly improved with anaverage temperature of 120C and standard deviation of only 4.0C (Table1B). Unfortunately, a rather small number of samples were treated in thismanner and thus more work needs to be done with inert sample pans. Figures 5and 6 show typical thermograms for 1845 and 1.3 M HAN, respectively, in piercedaluminum pans. No endotherms are observed prior to initiation; in fact, thereis no sign in the baseline of any kind of reaction prior to initiation. For

4.,' 1845 in metal sample pans at 6.9 MPa, there is a single exotherm. (The weakendotherm immediately following the exotherm in Figure 5 is probably not real,but due to re-equilibration of the instrument following initiation.) For 13 MHAN, the initiation exotherm is followed by a broader secondary exotherm atsomewhat higher temperatures, as shown in Figure 6. Apparently, initiation ofHAN leads to a residue, which then subsequently undergoes further reaction.It may be that this product is simply aluminum nitrate, which decomposes to150°C, however, further work is necessary to determine this. If this were thecase, its absence when 1845 is analyzed could be explained by the fact that1345 is stochiometric and the nitrate is completely consumed in the

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decomposition. If the HAN ignites at an abnormally low temperature (Figure

6), the separation between the primary and secondary exotherms increases;

conversely, for more stable samples of HAN with higher Tin, the secondaryexotherm is observed as an unresolved shoulder on the high temperature side of

the main initiation exotherm. Even after heating aqueous HAN to 200C andcooling, a residue remains that corresponds to about 15% of the initial weightof HAN; for the propellant 1845, on the other hand, complete reaction occurs

at initiation, and no residue is found after the experiment.

In the same time frame in which the 6.2 MPa (1000 psi) experiments were

performed another series was also run at 5.2 MPa (750 psi) using either Al orTa pans. (Tantalum is believed to be inert to HAN-based materials.) The Tins

obtained using Al pans still show significant scatter, however the

reproducibility for the Ta samples is the best obtained yet (see Table 1B),

with a variance of only 2.4*C. Although a relati vely small number of samples

were analyzed in the Ta pans, the results are promising enough to pursue theconcept of using pre-pressurization and containers which are totally inert.

IV. SUMMARY

Basic physical phenomena and technical problems involved in using a DSCto measure the relative decomposition temperature of liquid propellants havebeen outlined. Although the procedures have been obtained for liquid

propellants, they may be useful in other areas as well. The main problems tobe overcome in the liquid propellant system include the following which

manifest themselves as numerous and variable Tins in the DSC traces:

a. Reactivity of the propellant with the instrumental/samplingmaterials.

b. Spreading and splattering of the propellant prior to reaction.

c. Irreproducible vaporization as the experiment progresses.

The first problem can be minimized by selecting materials which are compatiblewith the propellant. However, in the case of the LP materials this is not a

simple matter and care must be taken so that no metal from the equipment orsupplies contaminates the samples, including sample pans as well as syringe

needles, etc. Partial success was obtained by using sample pans coated with

Au and Pt (atmospheric conditions); however, Tins are still widely varying.More work has been done in both selecting sample containers and pre-treating

sampling materials.

The second problem, sample splattering, has also been addressed in thiswork. This also results in numerous endotherms and exotherms being obtained

and in the case of LPs, can be minimized by dispersing powders and fibers into

propellants. In most cases, this seemed to be effective in eliminating allbut the major exotherm, probably due in part to minimizing splatter. However,

occasionally runs were obtained with multiple peaks and, in general, the Tins

are not reproducible.

The final effect, vaporization suppression, may be the most critical in

obtaining reproducible Tin s ' Preliminary attempts to achieve this have

20

included self-pressurization (e.g., small samples in containers with littledead volume) and external pressurization (instrumental pressurization). DSCcurves from the former method have quite jagged exotherms, possibly due tovibrations of the sample container and/or degassing under the extremely highpressures which arise from strict confinement of the sample in such a smallvolume. Thus, iastrument pressurization has shown the most promise andinitial results yielded single exotherms with very regular shape. However,Tills are still variable and lie in the range of 110 to 190"C. Further workhas alo been done using high pressure and a variety of sampling techniques

for which Tins are statistically reproducible. Preliminary results with Tapans resulted in the most reproducible results yet, with an average Tin of124.7C and variance of 2.4*C. Thus, the control of vaporization andsplattering, combined with the use of inert materials has shown promise forthe 'ise of the DSC as a stability test for liquid propellants alone as well asspiked with metals.

The control of vaporization is critical for any liquid sample run in theDSC and it is seen that instrument pressurization is useful in reducingvaporization and therefore improving reproducibility. Sample size and shapeis important also and care should be taken to accurately measure the sampleand to reproducibly deposit the sample each time to ensure that the drop sizeis constant. For samples that tend to splatter, the inert substrates such as

• those previously mentioned might be used to minimize this effect. However, aswas pointed out above, if the splattering is due to the escape ofdecomposition gases (and not simply to deaeration), then reproducibility might

be limited by such decomposition and the inert substrates may not be sohelpful.

One final point should be noted. The purpose of this work is to obtainconditions for which reproducible Tins for HAN-based samples are obtained.This work has shown that slight variations in sampling or DSC conditions canresult in widely varying Tin values. Thus, it is essential to select one setof conditions as the reference from which the relative stability of thepropellants can be determined. The Tins should reflect the relative stabilityof samples of differing composition (formulation, impurity and degradationlevel, etc.). No attempt is made to extrapolate the initiation temperature ofbulk quantities of HAN under storage, handling or gun conditions from the Tinobtained with milligram quantities of propellants decomposed under the uniqueconditions of the DSC. The DSC Tins are potentially a reliable indicator ofthe stability of HAN based liquid propellants and the authors propose nothingmore .

2!

ACKNOWLEDGEMENT

The authors wish to thank Mr. William McBratney for assistance inpreparing the gold and platinum coated DSC pans.

2

E2

REFERENCES

1. N. Klein, R.A. Sasse', R.L. Scott, amd K.E. Travis, "Thermal Decompositionof Liquid Monopropellants," BRL Report No. 1970, March 1977; N. Klein andR.A. Sasse', "Ignition Studies of Aqueous Monopropellants," ARBRL-TR-

02232, April 1980; N. Klein, "Preparation and Characterization of SeveralLiquid Propellants," ARBRL-TR-02471, February 1983.

2. N.A. Messina and N. Klein, "Thermal Decomposition Studies of HAN-BasedLiquid Monopropellants," Proceedings, 22nd JANNAF Combustion Meeting, CPIAPub. 432, Vol. 1, pp. 185-192, October 1985.

3. N. Klein and C.R. Wellman, "Reactions Involving the Thermal Stability ofAqueous Monopropellants," US Army Ballistic Research Laboratory, BRLReport No. 1876, May 1976. Also described in N. Klein, "Liquid PropellantStability Studies," Proceedings, ICT Jahrestagung, pp. 167-180, 1984.

4. a. R.A. Fifer, L.J. Decker, and P.J. Duff, "DSC Stability Test for Liquid

Propellants," Proceedings, 22nd JANNAF Combustion Meeting, CPIA Pub. No.432, Vol. II, pp. 203-211, October 1985.b. R.A. Fifer and L.J. Decker, "DSC Stability Test for Liquid

* Propellants," BRL Report in progress.

5. M.M. Decker, J.R. Ward, P.A. Tarantino, and P.M. Davis, "Presence andRemoval of Trace Transition Metal Ions in Hydroxylammonium Nitrate (HAN)Solutions," CRDC Technical Report No. 84062, 1984.

6. P.J. Duff and L.J. Decker, "Studies of the Effect of Hivelite and OtherBoron Compounds on Nitramine Decomposition by Pyrolysis GC-FTIR," BRLReport in progress.

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