THERMAL T-ABILITY OF SEALANTS FORMILITAtIRY AIRCRAFT: MODIFICATION OFPO-YSLLFIUF- SEALANTS WITH ETHER ANDTHIiLTtHtFR MONOMERS

AD-A264 079i mllnr111pll i7-N-X

PJ •N•LAN 9--) iA " ".(

KG U AFRY Iqqj uELECTF.

MAY1 3 1993 JJS C

Reproduced FromBest Available Copy

jL APPROVED commonwealth of Australia

FOR IUBLTC RELEASF,',

93-10335

MATERIALS RESEARCH LABORATORYDSTO

-9 • �,I L 101o

Thermal Stability of Sealants forMilitary Aircraft:- Modification of

Polysulfide Prepolymners with Ether andThzioether Monomers

P.j. Hanhela and W. Mazurek

NIRI. Technical ReportNMRI-TR-92-2

Ab1stract

The. dfi'cl o!f muod.ifytti: Thiiokol lecilt/6fed. prijiolveiters w'ithi .imeiercalptie dieethtt etherand dienercapto dieth~yl stilfide,' on flit, re~sislairce toe htit aga'iq: has l'tee.n ea'.i~iptedtogeth~er witlh a comerelle'cialhli indijlfe~d (PR-177() R-2) anrd an wittioedified~ ee.qu'ivlepitsi'atlant (PR-175/) B-2). Mechanical tests pt'rformed ont spxecimeils, sift er h~eat e.ige~inl sit182*C, indicated that the' ,ewificatioi:s did not intlnire~e' the' heat rt'sistanice. oif;volysilfld~e sealants.

4AGcesuow For

OTI ra 0

1)77C' QI7ALrTT WsINSICCED I

Availability Codea

Ave# a*nd i a

Dit SpecialMATE~RIALS IdiSIARCI I [AIIORA7 RY

l'ibl~l~i.'du by

DSTO Materials Research LabratoryCordite Avte'he. MarilnrnontiVictoria. .032 Australia

Tehleone: (03.) 246 8111Fax: (03) 246 8999©0 Cgnmgpugilpnalth ofAustralia 199.3AR No. (K06-864

AI'PROVED FOR PUBLIC RELEASE

Authors

P.1. Hanhela

Peter 1. Hanhela is a Senior Professional Officer Clauss B,in the Protective Chemistry Divisioin. fie jtoined MRLin 1970 and has worked int areas o~f tirg~anic chemristrY,.mlnitfinhq chgemipltinitesenice andt aprcrat! Sealant!.. Ileis cuorrentl v epkgag~ed ton studies III SubmnarineAtiowsly~jheres Research, in-udicidi:s, imi~sItu'rtok and carblpt'

__________________________ hox~leretnot'aI.

W. Mazurek

Dr Waldtemar Ma:,irek is a Proit it'l! Rfea'~rctr s~ut11tistin flte 'rotect im, Cheponstry Diision. lie Impintr AIRIopI t,7 pncwtpk tto the Se'alant. 'ectiew it op 1112, a-he~rehie b~ecame: Task hfiatnaer of flthe'c tdI01 /it' lito'. t'ivlPgictive IIn flte areas 4 chepprocaii dt-Sradattimr off.'aan'stritctire/propt'rti# relationshitip. anrd flth- urilkiitechantisusts of poh1,.ysu'fie ýea'aisunt4 I fe is curreithltmiasua%'ip. flthe Sai'n."ripe AtN-s 1'hueres Research Tas'k.with, research aci:ý'itfrs ino areas otichitpgdn' the pnu',:iortinandus the ;murificatimi o~f sud'marine atmuo'plhieres.

Contents

1. INTRODUCTION 7

2. EXPERIMENTAL. 82.1 Oxidation i( Dimercapto Viethyl Sulfide and Dimewrcalto I )'tlihwl Ether 82.2 Preparation of Modified 'repolyiners 82.3 Specimen Preparatim 92.4 Exposure at Elehated Temp~eratures 92.5 Hardness Measurements 92.6 Tensile. Ehlpgation and Mohdulus Measurements 92.7 "C NMR 92.8 Is•lation of PR-17711 B-2 and PR-M750 B-2 Prepolwiter, 102.9 Formulatrtn of Sedah:t Specimens 10

3. RESUI.TS AND D)ISCUSSION 10

4. CONCLUSIONS 26

5. REFERENCES 26

Thermal Stability of Sealants forMilitary Aircraft: Modification of

Polysulfide Prepolymers with Ether andThioether Monomers

1. Introduction

Since the introduction of polysulfide elastoiners in the l940sI ], polvsulfidebased sealants have found extensive application in the sealing of aircraftintegral fuel tanks [2, 31 where there is a requirement for fuel resistahce andflexibility at low and high temperatures. The polysulfide sealants can be us-edover the temperature range - 500C to + 180°C although they have only alimited capacity to withstand the maximum temperature Above thi,temperature, fluorosilicones" are the materials most commonl, used I-1. Forexample, in high performance military aircraft such as the F-I11, afluorosilicone sealant is used in the leading edge of the wing tank where theaerodynamic heating may exceed the temperature tolerance of a polysulfidebased sealant. The fluorosilicones are considerably more expensive and cannotreadily be applied to the required thickness in a single application 151 as thecuring process requires diffusion of moisture from the air into the polymer.

Recently, it has been claimed that modification of existing commercialpolysulfide prepolymers has resulted in polysulfide sealants with improvedthermal resistance [61. The modification involved insertion of ethers andthioethers into the polymer backbone of Thiokol prepolymers osing thethiol-dilsulfide interchange reaction (eq 1).

H •3-C•H*-CH .f-C-LH, -O-L'H•-C.H. -• ,H , 'r1,.'- k-:;t' *-,-..,

. |It S-ch, ;-(i| -l -'l *' - O-s -ct.-: .H *' I,

Of the ethers testted, the incorporation oef dimercapto diethyl ether (1I)MI.') a.ddimercapto diethyl sulfide (DMDS) resulted in the best imprnsvenment o'f thrermalstability with Thiokol LP-12 polysulfide preplvymer. A molar raitio. of I 1 a.lle1:2 of prepolymer to dithiol gave optinmum physical prropertiv.'.

in order to verify the assertion that the incolrporation tit f tlhv et-hers- il ht thtlprepolymer could significantly improve the thermal prolperties oef the curedpolymer, the abovementioned ethers were inserted into three I'hiokolpolysulfide prepolymers. LP-2, LP-12 and L!.-32. This report di.-scribte theeffect of these manipulations upon the physical propertites (weight loss,hardness, elongation and tensile strength) of the sealants obtained after curingthe modified prep.lolymers with manganese dioxide and exposing the nrsullantelastomers at 182°C for various periods.

2. Experimental

Dimercapto diethyl ether and dimer.apto diethyl sulfide were obtained tromnAldrich Chemical Co. and the polysulfide prepnlymers wenr supplied I'Morton Thiokol. :All reagents were u.sed without further purificatiotnLiterature methods were used for the preparation of dibutvl trulfitfit 171 .ad1.2,5 trithiepane 181. -

2.1 Oxidation of Diinercapto Diethiyl Sulfide andDimercapto Diethyl Ether

Hydrogen peroxide (20 ml, 0.0h mol) was added dropwise to a stirred mixttureof dimercapto diethyl sulfide (6.1 g, 0.04 mol) and aqueous NaOl 1 (50 ml,0.08 mol). After I h the reaction mixture was filtered and the white residuewashed with water (2 x 50 ml) and dried in an oven at 600C (1.9 g). Thefiltrate was extracted with CIICI, (3 x 50 ml) and the combined extracts werewashed with water (3 x 50 ml) and dried over MgSO., filtered and the so••entwas removed to give a viscous colorless liquid (2.0 g).

Dimercapto diethyl ether was treated in a similar manner and g.Ive 1 VIScOU.Lscolorless liquid (3.5 g) without the formation of a solid.

2.2 Preparation of Modified Prepolymers

The dimercapto ethers were incorporated into the prepolymers at twoconcentralions, 4.0 g and 8.0 g pe-r 10( g of prepolymer for diniercapto dit-thylsulfide and 3.6 g and 7.2 g per 100 g of prelxivymer for dimerc.1hlp dit-tlvlether. Triethylamine (Ihiree drops) was added to each mixt•|r' and 0In1V W,'Lkwas allowed for equilibration of Ihe readionl mixkture, prior tit um-t'.

8

2.3 Spu'cinen Preparation

uasinig I spa to a or %% ith a ni--met nici~a li d ri ~en spindle I lit' rx-tiltiit nivtitiartwas poured ontot I stel fraime 1 41 -t 15 -t 2 mm) ret*,ing oni a Iii~tts~ ofreflon coated gla:ss mesh, Whent tihe mixture beganit)t harden .pr~ i~tl15 min at 40 C) it was preNsed between two rigid telplates. se-parattd I'%1'etlon sheets, with a load tf 3.3t) SAl'a at 1001V, released tour times lit, allow air

to be el -imiaed) and then finally pressed for I h. The sheets -re- pot iri

for five days at ;5WC. Specimens 094) x75 mmni were cut f romi the sheetst andvxposed in an oven for the required mime after which duini-ht*Il sAmpedlrvoniens were cut tout to'r tiabsequtnt mechanical testing

2.4 Expo~ii4re ait Eltez9atetI li'iipetraffires

1'.v 1.111 n pec itTit i i~ ereý tit i't % ttt I it~ ive Ia p-i .g ig I ITI a N~a tli t. 'r %~ t 1*1 1. I f 11il11a.inI .ir c irc:ulat11ing tan., qtapr.1Itett It 1 '2 IL anid tonitrolitd alt%[1 .i 'itIilistomamtic If teniperattire tomltrtallcr tto w-ithin t 2 C I hi. metilptr~amiatt.rt-presents, the upper operatting limit toar ptalysulfide stvalaiit. 121

2.3 H-ardness Measuiremeneits

I iardnes% mnealuremevnt, %% ere ctonducteid with a ~Shore ( otanel-ailur I )LIT iirtetertype 2 A

2.6 Tensile, Elonkgationi and Modulus Mleasuremnents

Tensile strength. elong~atiton at breatk aind mOdulus 10t)(I..j terv deteritained III.accordance with a standaird test method 1~41 using an Amslter rt'tsilt rtste'rlAve~po-51'd [xecienici wvere 2 mian thick %% hile the thickness *a tmt- liaet *at

leut-imen'f was illealtiitid prior It, testing

2. 7 " CNjMR

NMR measurement-. were carried otiu using a Bruker model AMt "44,spectrometer. lnverse-gated decoupled spectra were recorded forapproximately 40Y%. (w/w) solutions in CDCI, using a 90' pulse, .I nt-covervdelay of 3 s, and a spevctral bandwidth of 15 kl-Iz with the accumulation tatlb OW4 data points. Between SO (W0 and 100) RV0 scans were, use4d tit obtainspectra. No line broadening was used. In all case-, the free iniductiton decay%were zcern filled to 32 k. Chemical shifts, in ppm. were referenced tit TIS

2.8 Isolation of PR-1770 B-2 and PR-1750 B1-2Prepolymers

The sealant is supplied in two packs: cure paste and compounded prepolymer.

The latter, for PR-1770 B-2 (422.7 g), was dispersed in methyl ethyl kethlne to .1total volume of 750 ml. The fillers were removed by centrifuging. theprepolymer solution wa.; decanted and thli .olvent was remowed on i rotary

evaporator to giwe .a vi -i's liquid 122ti I g. 53.51ý5 o til• e fornilaiton 'ý.imlarmanipulations with PR-175f B-2 01'4 g) aklo vielded a visctu% lihqitd 121" t'5Q.6' of the formulation)

2.9 Formulation of Sealant Specimens

With the exception of PR-17i 11-2 preptilv.mer. all prepolynmer- %%%t- . urtvd wlith

thile langanese dioxide cure paste supplieJd with and used to utire 17711 B- 2Permapol PA- sealant (l'rodtht R-,ardch L -r . USA) 1[I1 ] lh.mhi, m--&- -,fwas based on the ratio fit thiol content (meaIured by intrarett -. tri.-,l. oI Hllto cure paste formulated for this .ealant.

3. Results and Discussion

The "C NMR spectrum of the polysulfide prepolymer extracted from the

PR-1770 B-2 sealant (Fig. 1) is typical of a polysulfide polymer with tihe

dominant peaks arising from the idealized structure (1) 1121

Other components include small quantities of thioether (2) and thiol (3).

32.2 95.4Pol ym-O-CH:-O-CH -CH.SCH -CH•-0-CH; -O-Poiy

67.6

2

95.4 69.7Polym-O-CH -O-CH-Cl,:-SH

24.5

3

to

4 4 4 36 36 i; 3? 33 2 f,

I [i , '-,ic , It r im t t !V I% -I T 7 1 ,V 2 fit~ t v r .'. Ilo ~ill th a lt 4 t IP R • I ' " 1 ; OiT , • ( 2 .111,1i t h e ,I I ) ( h -k lt n dr i • " 1 1 - o i l~i, cl\ u n ,, b e aI,, , . , - .i t k I ,i l t .1 m .1 , ,' X .1 d tl I I , , ,1 1 .1 1 i ,4 -. l k .' I ; I I

IS8ti. 3," -,, I..8..ind 2-1 7 3I,, 2 1p1m)

At.i. tit it ot I 14 1)M I N I to II'- 12l t-ptl)I, ,.ier I Iig 3) re"-ulte'd 11., 1 trum ', % CI,itiilar to I tha t I Iit -I'R- 177t0 B-2 prt polvicr IFig I) I ,onimir-oi' M 11I , te -%,

spectrai with thait of I)MI)S (4) and I'-12 (lig+ 4) led tit the idtiittit dtiol otsmnall quantitiets of unreacted dithiol in the ,spectrumn of the todhitietd I I 12 iL•tPR-1770 13-2 prepolvmner•.

4

Sinmiarlv the ýjpectrumn ot the •orresponimng tdsultide polInvi 1,L preI•l.redi

bv the oItidation of the dithiol, resulted in the assignmntt ot ttln- '1o1poUli'mt ini

the spectra of the I)MlS nmodited [II'-12 and the PR- 177(1 -2 prepolk mner,

However the IV( NMR cannot be e•pected to distinguish the ditterenlebetweten the homopolymer of this disulfide and the copolvmer (6) formed fromthe disulfide and prepolymer (1) due to the relitive similarity ott the repeatingunits and the distance of the prepolvmer ether oxygen from I)D1T unit

The identity of the remaining two peaks 131 4, 38.4 ppm) couild not bedetermined. tlowever., ince these represent the most intens- ot the sa, peak!.their identity is important.

A number of possibilities were considered. The possible formation oti cachidisulfide (7) was eliminated since this compound had similar chemical shifts tothe homop6ý'mer.

1 It\ 'I

7

12

i",gurt 2:

opp

Figure 3: "-C NMR tof LP-12 po~v-wIfide prepohjmer cvItatnrnng D)MS 19IS pro.

()denotes peaks aristingfrom DMPS.

Figure 4: "C NMR of LP-12 polysulfide prepolymer.

14

Ihe l pos•ibility of these peak,, being dut. to the formation of a trislmulhdt was,dvKLxirdehd when it was found that the tris.ulfide cxarbons (8) gave ris. torv'-,antcsv ,lightly down field from those of the disultides (9).

9

S)e..,pite not being abll. to unambiguou.t. ,!.,nign the tt.tw peaks at Al 4 andtIA4 ppm, the result., clearlv indicate that l'R-17T, 8-2 sealant IS mdiite-d "kh,.inter.acted withl I)IDSý. The data al,,, Indiate that thk dithiol ha%. wen largie,incorpratetd iw•v the prx.pollmer with only a small portion of the re' thiolbeing present ,I., shown by the low intensitit- of the |xeak% .t 24 7 ainl36.1 ppm. The amount of DMID incorporated into I.l'-12 appears to h,

aipproximnately 95".. at 8 parts per hundred rubber •phr) bas.ed on the frt'•dithiol peak ,at 36 i ppm and the combination of the dihulfide peak at 1; "; ppmand the unas.'igned peak at 11.4 ppm. This corresptmd•J, ito 2W•., more than inP'R--17"10 B-2 baSd kn the relative intensities tit the peak-. at 32 1 rm ppront piratewith the sumn of the intensities at 31.4 and 1; 5 ppm whin'h %ulg-,t=, I.•All,"content ot t1,8 phr

Unlike with U)NDS. the addition of DMDE to LI'-12 prepolymer rvesulted inthe appearance of only three additional peaks in the 'V NMR ,pettrum (:k34,69.2 and 72.4 ppm, Fig-5) compared with the LP-12. .Thouipeakscorresponded with those in the spectra of unreacted (10) and oxidizedDMDE (11). While free DMDE could barely be detected at 3.7 phr, at 7.2 phr it

was estimated that approximately 90% of the dithiol was incorporated into theprepolymer based on the ratios of the peak heights at 72.4 and 69.2 ppm

72.4

iS-(C" -Ch. -- c-. -CH. -Sr24.4

10

69.2[-S-CH.=-Cti2-O-CH.•-CH.-S-j•.

39.4

11

15

I~ T1~ I I I7 5 70 65, 60 5, so is to 35 30 is 70Dan

I ig it NA IR a , f I 12 lit tPr, tou lb, , I f - i tI/ i' ta- I t) 1) 'l

I hct-~ art, % mmr~is lta ttirs allut ting the thetrmial Ntabilittv tit tilt' cuired

Pitrpokvin.,r' midt w-.11anlt'. but theilt- dvilial cmil)(INtiItittn tit the pttlvinctr t.hain is'Iii ntanen~t a ( 1thu Ilr tactt 'r, int.u II the nii ttre tit Ihe ctii rng I),Vnlt% IadtillcrItt'~p 6.uni i It iý nvtcvs,ýarv to elitnitiate these cotrinbution'. hettire ctinchlti tm :.i1t't dr.tiri .111001 Ithe 'et, 1,I'tit pt)JVIiltitit' pruptolymer ~tmIlpt)Silfltlkv %I (lit'thtrinw) '.tabiiitv tit curted preptilymer otr ctimpiundt'd secalilat.

It triler ito tomipar the thermatl '.tabilitv is( the l'eiriaptil 111%'- 17711 B-2 M'aldilt

ct.ilmt, werkhemtti aged at 182 Ci for varitius perio~ds and thet phiv'ical prtbplritv%k,% tltijtd '-,imiilairlv, 'amplc'tit o I'-177t1 B-2 preptilyneritur. iire with

1-177(l B-2 t re r-ite. -,r, ~ttt under the s.ame ctintliuit'n inl tirdt-r litdLkrnurimi the' t4tvts t'. o tilh'r'. 4n the thermal !.tablilitv tit the '.talamNt

imiparimmtn' vere made with Vit- 17;() B-2 preptilvmer cuired with tIhvt weripxtv~ 'upplicki with the 'wcalant mid with the IT- 1 771) R-2 cure' .i'te ( lalit' I I ti,d'etrininu thte contribution o~f the cure paste Its the thermal stabilitv tit the cuiredlpit'ymlv~ufitd Detaish (if the ptilvsulfide foirmulations are given inI Fble I

T'hiltl oqtitt-n|" (.'llD' PMIS~" Vim ohms|V'Mt,, kIhr I I a a neI

r0

Il~i '-.' 1.2 15 I ll 440

IM 7

VN-I-:o It) 28 1 7 277

I1'A I1 1117

I AMI)I t' 3.2 21 0 130

t)I, )I 7 2 417 ,04

I1I1 t2 20M 130

Y)MI)S .81 4.h 30.5 47

I '- .S 14.h hwo

DMDIE 3• .I 39 226 112

DMIW : 72 h.2 325 40

l)MID$" 4 u 40 22 1 121

I)M1 I )L1 t 3 12 2 4

* 1 715

)MIDFl 3.h 3 1 20.3 189

DMIE 72 46 102 40

I)MDS 4 ) 3.1 20.1 107

I)IDS 8 ) 45 298 44

j Additil , ill phr (w/w), dimere'apto dethvl ether (I)MDE), dimercapto diethyl,,Llhtde (IN DS)

t' Determind by IR [18." Cure paste from PR-1770 B-2 was used maintaining the same ratio of cure paste

to thiol content as in PR-1770 B-2.

d Measured at 25*C using a Brookfield RVF viscometer, spindle 6 at 10 r/min

e IR-1750 8-2 cure paste was used as supplied by the manufacturer using therecommended ratio but allowing for the absence of fillers.

Weight loss determinations indicate little difference between PR-1770 B-2 and

fR-1750 B-2 sealants (- 11% after 10 h) with the latter showing slightly lowerweight losses Ihan the former (Fig. 6a). However, the PR-1770 B-2 cured

prepilymer shows substantially higher weight lIsses than the PIR-1750 D-2cured prempolymers. There is little difference between the PR-1750 D-2

17

prepolymer cured with PR-17t50 1-2 and I'R-1770 11-2 curt pastes indicating thatthere is no significant contribution of the curt- pastes to thl, thermal stabilitv ofthe polysulfidc±*. The prepolymer content of I'R-1770 11-2 (54".,) is similar tothat of PR-1750 B-2 (60%) and on the basis of the weight Itisse. shown by thesealants the maximum weight lo.sses of these prepolymers should beapproximately 22% as in the case of the PR-1770 B-2 prepolymer. Ihowever,the PR-1750 B-2 cured prepolymers have unexpectedly low maximum weightlosses (- 15%), suggesting the presence of volatile or less thermally stablecomponents in the PR-1770 B-2 prepolymer compared with that of P'R-I,7N) 11-2.

Hardne.ss measurements show an initial softening within 3 Ih ) lof-at ageitig ofall specimens except PR-1770 B-2 sealant, followed by a gradual increase(Fig. 6b). As expected both compounded ,ealants are signiticantlv harder thanthe cured prepolymers. The cured PR-1770 R-2 prepolymer appears to hN.softer than the cured PR-1750 11-2 prepolymers and this relationship . •.evidentfor approximately the first 5 hours of he-at ageing after which thle hardn,%,increases to the level of the cured I'R-?.750 B-2 prepolymner lht,, behatviour i,consistent with the high weight loss of the curld PR-177, 1)-2 rrin'p,,me'r(Fig. 6a).

The modulus at I(Ml". elongation is. highest for both the compound.d se'.lant-as exp.chted (Fig. tx'). After 5 h oi heat ageing thet . s.-o ni,. brittltland break before reaching IM")'.. lon1g,ltion lw' ILUred p.repol, mlr-. li.i' v .1modiulus approximately 10 timies hIower than the stealant1. %% Ith 1i 1.1.1 ilt-PR-1770 B-2 prepolyrn'r slightly lowe.r than the. IT-1754 11-2 imalogtic \ttih,6.5 h the cured prepolymers become too brittle to reach an elnhgation of I%',,

The greatest elongation at breaking point is shown by the cured l'R-1770 11-.prepolymer (Fig. 6d). The PR-175)0 I-2 prepolymer cured with PR-177t) 11-2cure paste has slightly greater elongation properties than both the compoiundedsealants and the PR-1 750 B-2 prepolymer cured with PR- 1750 11-2 curt, pastewhich show very similar properties. All samples show diminished elongationproperties after I h of heat ageing.

The tensile strength of the compounded sealants was lound ht li,approximately five times greater than that t1 the cured prepolvuner-, 0leg tie)While the tensile strength of the cured prepolyniers is largtelh, 1ll.aih-t %td |'heat ageing up to a period of 10 hours, the compounded ,valants. ,tlftfr .a -higlhtreduction in tensile strength after heat ageing

The general trend appears to be, a gradual hardening of the el,.ttomer', onheat ageing with the thiol modified elastomers showing nit significant n,%istl.t.to this process compared with the unmodified analogues.

Modifications of LP-2, LP-32 and LP-12 polysulfide prepolyners, haxt alobeen reported 161. All three prepolymers have similar chlmital composit.on%

with the major difference being the ev.tent of croslinking (2 0. i 1 and 11.2"..respectively 1131) In order ito evaluah, the effect ot Iht.. .n41hf• le4lttn IothDMD) and DMLI were intorlporated, in %imilar molar roncentratiol%. intolthese prepolymers and the amount of cutiring agent (I'R-I,') 11.21 cure- 11.t1%)required was based on the thiol content of the motdified prepolvnier tiing the.same ratio of curt, pasth to thiol content a% in P'R-1770 11.2 i tablh I Ir

I lit

A 60

2 20

01 i 1 1 _ 0 I I t I I I11 2 4, 6 5 1o 2 1. 6 0 10

Exposure Time. HOurs Exposure Time. Hours

3.0 Soo

(CI] (d)2.5h 400

IL 2.0

*.300

CO 2001

IL

0.5 tOO

0 1 a O 0 2 & 6 a 10Exposure Time. Hours Exposure Time. Hours

4.0

2 3.0

oa

C

O0 2 4, 6 tO 0

Exposure ~ Epour Time. HorsEpoueuirsHor-' 2.0

*119

: 1.0

0 2 & 6 O tO

Exposure Timt. Houri

figure 6: Mechanical properties of sealants after heat ageing at 7820 C.(e• ) PR.i1750 8-2 sealant, ( 0) PR-i7770 8-2 sealant, ( A) PR.1770 8-2 prepolymer curedwith, PR-I 770 8-2 cure paste, ( • ) PR-4750 B-2 pre polymer cured with PR-i1750 8-2 curepaste, ( 10 ) PR-1750 8-2 prepolvnwr cured willh PR-i 770 B-? cure paste.

19

Heat ageing of cured Thiokol 1.1-21 prepolymer and the preptilymers incorporatingDM05 and DMDE result in higher weight losses for the modified prepolymercompared with the unmodified material (Fig. 7a). The weight loss increases with theproportion of added dithiol. After 10 hours of ageing a maximum weight loss oft 21",.was evident for the specimens with 7.2 phr DMDE and 8.0 phr [)M1DS which v.comparable with that of the PR-1770 B1-2 prepolymer (Fig. Win.

Unlike the elastomers derived from VR-1770 11.2 and 1'1-175t) 11-2. the hardness ofthe modified L-P-32 prepoly mers generall% decreases rapidly during exposujrereaching a minimum after 2 to 5 hours of heat ageing after which it inCrealsks againindicating embrittlement (Fig. 77b). The specimens conta-ining the highestconcentrations of added dithiol were most affected. These changes are moredramatic than those shown bv the P'R- 17770 B-2 prepolvmer.

The modulus me~asurements at 100%,. elongation follow the hardness trends with anoverall decrease in modulus reaching a minimum between 2 and 5 hours. nd asubsequent increase after 5 hours (Fig. 770. There is little difference between thevarious samples.

Similar trends are evident in the elongation characteristics (Fig, 7d). Maximumelongation is reached after approximately two hours after which there is a steadyv

derese to rre-exposure values. Of these specimens,unoiedL32rplmris least affected with a four fold increase in elongation after two hours of heat ageingcompared with an eight fold increase for the 7.2 phr 1)MDS modified IY-32.

Tensile strength results are consistent with the other trends but there is only apartial recovery in this property after reaching a minimum during three to five hoursof heat ageing (Fig. 7e). The subsequent increase in tensile strength is an ind icatlion o fincreasing embrittlement. The results show no apparent improvement in resista nceto heat ageing on addition of DIVDE and DMD5 to LP-32 prepolymer.

Heat ageing of the modified and unmodified cured Thiokol LI'-2 prepoly mers (Fig..8a-) resulted in weight losses similar to those of the Lr-32- analogues (Fig. 7.0 whereas

the reductions in hardness of the cured 1_1-2 prepolymers (Fig. 8b) are not as m.Areas those of the I.P-32 analoigues (Fig. 77b) and the minimum laardness- levels are only

slighly les thn those of the PR-1754) 11-2 prepolymers (F:ig. fib) As with theote

modified prepolymers, the extent of weight loss is dependent tin the concentration otithe added modifying dithiol as this increas es the low molecular weight component oitthe polymer.

Reductions in the modulus of the cured LP-2 prepolymers. after heat ageing (Fig.8c), are not as substantial as those of the LP-32 series (Fig. 7c) even though the trends.are similar with the elastomers showing softening with increase in exposure timefollowed by an increase in embrittlement after six hours. Specimens containing thehighest concentrations of added thiol show a greater increase in embrittlementcompared with the unmodified 1.11-24 with the effect oif DMDS being slightly greaterthan that of DMDE. The P'R-1750 11.2and PR.1770 11-2 cured prepolymers bycomparison initially have a lower modulus which gradually decreases oin exposuireand shows no indication of embrittlement (Fig. 6c).

As expected, the changes in elongation with heat ageing are, substantially lesis in thecase- of the L11-2 prepolymers (Fig. kid) than the L1P-32 analogues (Fig. 7d). In bothcases the maximum elongation prior to fracture of the elastomers is significantlyincreased on heat ageing compared with the cured 11R1-1770 B-.2 and l'R-1.1750 11-ipriepolymerio (Fig, 6d). Specimens with the highest concentration of added thiol arteaffected to at greater extent with no %ubistantial difference being apparent bietweeti th~etwo thiols.

20

'0______

(a) 1b)

2 0,

00o 2 4 6 a 0 2 6

Exposure Time. Hours Exposure Time. Hours

1.0 600

(c1 (d)

0.6 o

"+ ~4000.6

0.0.?

S0.2 1O00

0 -- 00 2 4 6 I0 2 6

Exposure Time. Hours Expouure Tiiee Houro

).32 ((el1.0

• 0.8 -

r. 0.4

0.2

000 2 4 6 a

Exposure Time. Routs

Figisre 7: Mechanical properties of sealants after heat ageing at 1820C, in air.

) LP-32 prepolymer, (0) LP-32 + DMDE (3.6 phr), (0) LP.32 + DMDE (7.2 phr), •...(A) LP-32 + DMD$ (4.0 phr), (10) LP-32 + DMDS (8.0 phr).

21

,0 1b)

R 30

20t - 2

c10

S '0

6 • 1. 6 C 0 2 4 6 a 10Exposure TIme. Hours Exposure Time. Hours

1.0 1.600

"- (1.1 (d)0.8

1200

o a-0.6 Cý

o ,000

~, 0.~o t400

~~0.2

0 00 2 I 6 a 10 0 2 4 6 a 10

Exposure Time. Hours Exposure Time. Hours

1.2

1.0 Ce)

* 0.8

, 0.6

- 0.2

00 2 4 6 8 I0

Exposure Time. Hours

Figure 8: Mechanical properties of sealants after heat ageing at 182°C, in air.Ai LP-2 prepolynter, (0) LP-2 + DMDE (3.6 phr), (e) LP-2 + DMDE (7.2 phr),(A) LP-2 + DMDS (4.0 phr), (13) LP.2 + DMDS (8.0 phr).

22

Similarly, changes in tensile strength of the cured LP-2 prepolyrnets (Fig. Me) are notaffected to the same extent as those of the LP-32 group (Fig. 70) and are of similarmagnitude to those of the cured PR-173 B-2 and PR-1770 B-2 prepolymers (Fig, e).After 7.5 hours heat exposure the tensile strengths of the modified LP-2 prepolymersare slightly higher than the unmodified prepolymer with those containing the highestconcentrations of modifying thiol showing slightly higher tensile strengths over thelower concentrations. Them is little difference between the two thiols in terms oftheir effect on the tensile strengths after six hours of exposure.

As the only difference between LP-2 and LP-32 is in the concentration of thecrossltnking agent, the difference in properties between the two preply1m1rs may beattributed to this factor.

While the results indicate that the thermal stability of thiol modifted Thikolprepolymers are not improved compared with the unmodified prepolyvmr*- weightlos measurements published on DMI . and l)MDE modified Thiok4l a.12 suI ITt

the reverse I() Comparisons of weight losses of cured prepolyrners at 1142C altereight hours have s;uggteted that a DMI•) modified lP. 12 prepolvimne was. k-astaff-cted, followed by a DMI)E rnodified LP-12 prepolymer with the unmodifkedprepolymer having the lowest thermal stability Similar experiment, in ourlaboratorv with Thiokol LP-12 have indicated that the modification, reult inincrea.ed wei.ht loses of the cured prepolvrner compared with that of theunmodified cured LP-12 (Fig. 9) This i% consistent with the data for I2-" and IfT-1thiol modified sysvterms previously discuwed AlthiiKh the wtinght lt for theD[II)S modified prepolymer appear to be higher than for the I)1D analou1, i the.case, the behaviour of the other rnodified pn'rt vrm-r %how- that there i% httlk-difference in tlh effect of the twso thiobl (Fig% 7 and 14)

44'

0 Lp 1:1 I110 til AtI

I~

The introsduction of a monomeric dithiol into a polysulfide prepillvmer ml eaw thlecontent of low molecular weight species and thc thiols in the system (T.ble I)Consequently, more curing agent is required and there is a greater probability of lowmolecular weight prepolymer remaining after the curing prwsess. Oinbeat ageing.the latter would be expected to contribute to the weight loss of the cured rrerd't%, meror sealant through volatilization. Under these circumstances it is not surprising; thatthe thiol modified prepolymers show greater weight losses on heat ageing than the~unmosdified analogues. The presence of the low molecular weight componerNtwould also be expected to affect thle mechanical properties of the curedi polv16SUlfide 1Inacting as plasticizers. Thus the contributions of the thiols introduced into thlepolvsulfide prepolymers. may be masked by the behaviour of the low molecularPolymer fragments produced by the thiol-disulfide interchange reaction (ejl 1).

4. Conclusions

It has been claimed that the thermal stability osf Thiokol polysuilfide prepol.. mev. ina%lie enhanced by the incorpora tion of( d imercapto ethters (e g. I )MIW D adit IM )I )S Asimi lar motd ification was ufndertaken using Th iokol lJ'-2, L P-32 a nil IX- 12, utit11.l fingthe thiol-disulfide interchange reaction. The prolducts were shown tot bie of %imilahrchemical composition to the l'ermapol Il'R-177t1 13-2 prepolymer. I leat ageing at$2'C has .failedto ttshoiw a ignificaint adva.ntaigeof thes-,e modif:.:aliios in mprot~ing

the resistance of polysulfide prepoilymers. to exposure at elevated temperatureNSimilar conclusions Were drawn from the comparisson of PR1-7,0) 11-2 with l'R- 174 It-2 sealant and the cured prepolvmtrs extracted from the uncured sealailts %% len testeduinder the same conditions,

5. References

I. liertozi, E.R. (19611),Chemistry and technology of elastorneric polysulfide po~lymerss Rid-h-r Cherw:Techpiol.. 41. pp. 114-160.

2 US Department oif Lh-fense (19112)-sealih opiv intil ,uegral Net Orl cal iutirs. 1pilerniIte'nt uw t.~ 111'r) 1.Kill.-S-,1343A.

I US Depatrtment (if r~~fvnwA (19814).%eaipie ropt il~widi , i' i pi p el$'rtifiIa r4*5614flu itsf ha ml finll- faIA s oil& . 1 111 Iai'll -.#I -aw YINt,$

4. Pedcral Materialf; Specifikatikun (14775).FMVS, 1041A, Amendment Nis. I WUS).

24

5. fluang, R.H.E. and Mazurek, W. (190).Effects of sealant thickness on the cure time of a fluorosilicone .walanti Mat*krtatl.rorumi, 14, pp. 120-23.

6. Singh. H. (1987).A new class of high performance polvsulfide polymers. Rubber Wotrhl, 196 (r,).pp. 32-36.

7. Decker, Q.W. and Post, H.W. (1957).Preparation and ultraviolet absorption spectra of certain alkyl polysulfides.Journal of Organic Chemistry, 22, 145.

8. Field, L. and Foster, C.H. (1970).Biologically oriented sulfur chemistry. IV. Synthesis and properties of 1.2,5trithiepane, a model for study of sulfide and disulfide moietie% in pro~imilyJouralroac(f Organic Chelti'istrt, 35, 749

1. Test method for rubber properties in tension 019t87).ASTM d412-87, Method A, American 'ciety for Testing and Nrhateril%,Philadelphia, PA.

1I). lPrtxouct Research and Chemical Corp. (1989).Laboratory product report on PR-1770 B-2.

11. Davidson, R.G. and Mathys, G.I. (1984).The determination of thiol groups in polysulfide prepolymers by infraredspectrometry. Anal. Chii,. Ada, 160, pp. 197-204.

12. Mazurek, W. and Moritz, A.G. (1991).13C NMR of polysulfide prepolymers. Macromol., 24, pp. 3261-65-

13. Thiokol Corp. (1979).Technical Information Bulletin TD-1251 (6/79) 5M.

25

SECURITY CLA."FICATION OF T1IlS rAc.F UNCLASSIFIED

MRL-TR-92-2 AR.O%.?-M4b4LW,%I~

TITLEIbcrntal stability (if gicalants for militairy .airerait kIuudiicaioiaw tit

polysulifde prtephiymers with etIherand thitieth.r mo'tiomemr

r.j. Hanbela and W. Mazurck I)-.TO M.1teriau Rie&e.urch I .it-oraulai

RE1K)RT DATI tvk\ ~'Fobaruay 1993

FMY. NO IIHV1C6/4/8.418401

(*hil.f, It',tstNtit c hv'uimir% Ih )g. %tsm

Anno'uncement *if this retvitrt IN onliihlted

KLVVORt~

Thermal propertics IDimtrcapto ethers leal AgeittpThioI-di-milfide interchainge. reactiomn

AIISRACT

The effect (if modiifying Thickkol poIvoiutIfolfe preps'Ivmt'r'. with J ini.rei.;t,' Ii oirtI.th~l 0,iztd multillers-i;'t4%diethyl -sulfide, tin the rtmimt.1v tll heat agipcng ha.v Ittn emanioned toellietr with. a .. mriamodified (FIR.177flo I.2p and an iinmidft-lof twik-auv.lnil %alatnt (I'R.I17,N 11-21 -\t,% tumial h-IN ti %.1 It's Im'stin opreimemnific htfealw ag.eing at 19(2 C, insii iaIlt- tha~t the i' rqifivo.,in.it. il0ti lst uimorr'lili, 11%, 11

resitani' f p~lvsl~le salant..

414 RINIV t1A"f'Atl~AK)t4t ti illot PAM~

Thcrmil mat~ibityiit( Se'alant.; tor~ xititairl itjrcrjft:Ný rw~~1 :tkf I'.t~siidtI~ PrI2pbo~lvnr% w~ith ilivi, .ilhd *141111tjitef %IoNt'Otr

P1. IIjri~t~a.,nd IV \1 r,iýi~

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