MASS LOSS OF POLYMERIC COMPOSITES IN VACUUM IN DEPENDENCE ON ELECTRON RADIATION INTENSITY

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1 MASS LOSS OF POLYMERIC COMPOSITES IN VACUUM IN DEPENDENCE ON ELECTRON RADIATION INTENSITY R.H. Khasanshn (1), A.N. Tmofv (1), M.F. Ivanov (2) (1) Jont-stock company Kompozt, 4, Ponrskaya str., 1417, Korolv, Moscow rgon, Russa tl: +7(495) , -mal: (2) Jont Insttut for Hgh Tmpraturs of Russan Acadmy of Scncs,13/19, Izhorskaya str., 155 Moscow, Russa, tl: +7(495) , ABSTRACT Exprmntal rsults concrnng ffct of lctron radaton on mass loss of a polymrc compost n subsqunt thrmostmulatd outgassng procss ar prsntd. To analyz and ntrprt xprmntal data th mathmatcal modl dscrbng th outgassng procss of polymrc composts s usd. 1. INTRODUCTION In srvc condtons, spaccraft xtrnal coatngs ar subjctd to varous factors of spac nvronmnt, ncludng solar and onzng radatons, rsdual Earth atmosphr (for low orbts), tc. Among th most ngatv ffcts of spac nvronmnt s th mass loss of polymrc composts (PC) bng th spaccraft thrmal control coatngs, du to th outgassng and sublmaton procsss. Rats of th procsss ar manly dfnd by solar and low-nrgy onzng radatons that produc prncpal contrbuton to nar-surfac absorbd doss. Gnraton of volatl products (VP) and thr mrgnc from PC s a complx physcal-chmcal procss dpndng on ts chmcal composton, physcal stat, tmpratur, and xtrnal factors. Thrfor, chmcal transformatons n PC bng th rsult of rradatons dfn n gnral gnraton of VP, xcpt for mrgncs of adsorbd and absorbd gass, monomrc rmnants and mpurts. Ths radatonsubstanc ntracton maks th molcular systm to bcom onzd or xctd. Thus, gnraton of actv cntrs (xctd molculs, ons and radcals) n matral prcds to chmcal ractons. It s known that concntraton of radaton-nducd lctron xctatons s dfnd by absorbd dos rat and wakly dpndnt on radaton typ [1-2]. To confrm rsstanc of spaccraft matrals to spac nvronmnt acclratd tsts ar carrd on. Pror to tstng th radaton rsstanc of spaccraft matrals, dstrbuton of th radaton xposur doss that thy would b subjctd to n srvc condtons ar calculatd (for xampl, by th Mont-Carlo mthod). Ths absorbd nrgy dpth profls ar smulatd n laboratory by lctron and proton radatons. Manwhl onzng radaton ntnsts n smulatng plants ar tms gratr than thos n srvc condtons. Whthr th choc of acclraton factor s rasonabl or not th ssu rmans opn. Thrfor, th am of th prsnt rsarch s th study of mass loss of PC n dpndnc on lctron radaton ntnsty. 2. EXPERIMENTAL RESULTS To study mass loss rats on radaton ntnsts, a PC was appld on 3-mm alumnum dsks. Each was wghd bfor and aftr xprmnts. Th frst stag consstd n rradatng fv sampl parts by lctrons wth Ф е cm -2 wth dffrnt dffrntal flux dnsts (ϕ е, cm -2 s -1 ). Intal lctron nrgy was 4 kv. Irradaton mods ar gvn n Tabl 1. Tabl 1. Irradaton mods Sampl party ϕ е 1-11 cm -2 s -1 Ф е 1-16 cm Tm dpndncs of mass loss M total ( of rradatd PC sampls for th tmpratur-nducd outgassng procss n vacuum hav bn found at th scond stag of xprmnts. To fnd kntcs of mass loss of PC w usd a quartz mcrobalanc - masurng dvc that s mant for convrtng chang of mass on surfac of quartz pzolctrc rsonator nto accrton of output frquncy. Output frquncy n our cas was changd du to dposton of outgassd products on a snsor surfac of th mcrobalanc. Th followng condtons mantand n xprmnts concrnng tmpratur-nducd outgassng and dposton of VP: vacuum Pa, sampl tmpraturs n sothrmal outgassng procss was 1±1 С, quartz pzolctrc rsonator tmpratur was mnus 187±.5 С. Exprmntal rsults ar gvn n 1

Fg. 1. Hr s sn that mass loss rat of rradatd sampls rss wth ncras of flux dnsty from to 1.123 1 12 cm -2 s -1. But th oppost bhavor s obsrvd for ϕ е 2.

2 Fg. 1. Hr s sn that mass loss rat of rradatd sampls rss wth ncras of flux dnsty from to cm -2 s -1. But th oppost bhavor s obsrvd for ϕ е cm -2 s -1 whr th outgassng rat of th rradatd sampl bcam lss than that of th nonrradatd on (from party No 6). C R const, x [, h] t, (2) C D + ( a x C x x, + k ) C < t T, x h, < t T, (3) (4) whr σ j wghtng coffcnt of th radatonnducd dcomposton of -componnt through j- channl, s -1 ; M numbr of dstructon channls of - componnt; χ chmcal racton rats wth nvolvmnt of -componnt, s -1 ; k and D ffctv dsorpton and dffuson coffcnts of typ VP rspctvly; R concntraton of typ VP n matral at ntal momnt; S (x) sourc functon of -typ VP dpndnt on lctron nrgy, composton and thcknss of matral; a paramtr dfnng nflunc of radaton on dsorpton kntcs of typ VP, µm s T radaton tm, s. Th corrctd functon of -typ VP sourc was dtrmnd by calculatng dstrbuton of absorbd doss n matral usng th Mont-Carlo mthod. In th cas of monochromatc lctrons, f dsrgardng th brakng radaton, th sourc functon s th followng S ( x) Axp[ B( x C) ], (5) 2-1 ; Fg. 1. Kntcs of mass loss of non-rradatd and radatd PC sampls for dffrnt dffrntal lctron flux dnsts a: 1 ϕ е ; 2 ϕ е ; 3 ϕ е ; 6 ϕ е, b: 4 ϕ е ; 5 ϕ е ; 6 ϕ е whr А s dfnd by dffrntal lctron flux dnsty, В and С dpnd on matral composton, thcknss and lctron nrgy. 3. NUMERICAL ANALYSIS OF EXPERIMENTAL DATA Thr ar compttv procsss n PC undr lctron rradaton n vacuum, frst, VP gnraton du to radolytc dcomposton of organc componnts and, scond, dcras of VP concntratons С (x, (1, 2,, N) at th cost of outgassng procss, radaton-nducd dcomposton and chmcal ractons. Thrfor, numrcal analyss of th xprmntal data gav solutons of corrctd dffrntal quaton systm dscrbng th outgassng procss: C C M D ( x) C σ j (1) t x x j 1 χ C + S ( x), x [, h], < t T, Fg. 2. Dpth dstrbuton of radolyss products Fg. 2 dmonstrats accumulaton of radolyss products n PC that s supprssd by outgassng procss undr rradaton n vacuum. Fg. 4 shows spac-tm dstrbutons of VP concntratons n PC sampls that wr obtand by solvng (1)-(4) for dffrnt radaton ntnsts (b, d, f, h). It also dmonstrats changs of VP concntratons n 2

rfrnc sampls (a, c,, g) that wr n vacuum chambr, but not undr

sampls that hav bn subjctd to lctron radaton of th sam flunc (Ф е 1.

3 rfrnc sampls (a, c,, g) that wr n vacuum chambr, but not undr rradaton, durng th xposur tms of Т 25, 12, 3 and 1.5 hours rspctvly. Fg. 3. Spac-tm dstrbuton of VP concntratons n rfrnc sampls (a, c,, g) and n sampls that hav bn subjctd to lctron radaton of th sam flunc (Ф е cm -2 ) but dffrnt dffrntal lctron flux dnsts (ϕ е, cm -2 s -1 ): b ϕ е ; d ϕ е ; f ϕ е ; h ϕ е

On can s from Fg. 1 (b) that ncras of ϕ е from 1.42 1 11 to 1.23 1 12 cm -2 s -1 rsults n growth of mass loss of rradatd sampl.

Th gratr radaton ntnsty, th lssr tm th sampls ar nsd th vacuum chambr whr smultanously tak plac accumulaton of radolyss products n sampls and th outgassng procss (S plots of rfrnc sampls a, c,, g)

4 On can s from Fg. 1 (b) that ncras of ϕ е from to cm -2 s -1 rsults n growth of mass loss of rradatd sampl. Estmatons of dstrbuton of VP concntratons n PC undr rradaton ndcatd that С(x, grows along wth th radaton ntnsty (S plots b, d, f). Ths can b xpland as follows. Th gratr radaton ntnsty, th lssr tm th sampls ar nsd th vacuum chambr whr smultanously tak plac accumulaton of radolyss products n sampls and th outgassng procss (S plots of rfrnc sampls a, c,, g) rstrctng growth of VP concntratons. Furthr ncras of dffrntal flux dnsty lads to acclraton both th chmcal ractons n sampls undr rradaton and thr outgassng. Thrfor, as soon as ϕ е cm -2 s -1 s achvd, VP concntraton n rradatd sampl (Fg. 3, h) bcam lss than n corrspondng rfrnc sampls (Fg. 3, g). As a rsult, mass loss of rradatd sampl undr tmpratur-nducd outgassng bcam lss than that for ntal on (S Fg.1 b). Obtand rsults can b xpland by th fact that n addton to radolyss products, xtra vbratonal xctatons appar n PC undr rradaton. Thr gnraton s du to nonradatv transtons of lctron xctatons nto thrmal vbratons. Bcaus numbr of lctron xctatons s proportonal to radaton ntnsty, concntraton of vbratonal xctatons should also b proportonal to t. So, apparanc of addtonal xctatons wth ncras of ϕ е acclrats dffuson (ncludng th surfac dffuson) and dsorpton of VP that s th outgassng procss. Local warmng of VP sourc s also th rason of growth of outgassng rat undr grat ntnsts [4]. Havng obtand nough dtald nformaton concrnng ffct of th radaton ntnsty on spactm dstrbuton of VP concntratons n PC, our nxt stp was studyng nflunc of modl paramtrs on outgassng kntcs through th matral-vacuum boundary. To fnd th outgassng rat on should know VP concntratons n th nar-surfac layr at any momnt that s th functon С (h,. It can b found by solvng (1)-(4). Usng ths solutons and boundary condtons (3) of propr dffrntal quatons (1) on may fnd functons of outgassng rats of ndvdual componnts from VP sourc surfac unt df ( dt k C ( h,. (6) Its tm ntgraton gv valus of VP fluxs F ( through th matral-vacuum boundary wthn th tm ntrval t ( ) t (7) F t k C ( h, τ ) dτ. Whn makng analyss of xprmntal data, anothr fatur of outgassng procss was rvald. In fact, sampls of PC n ach party had varous thcknss (h 8; 9; 1; 12 µm). It was found that whn all othr thngs bng qual th procss rat dpnds on sampl thcknss and molcular composton of outgassd products. Fg. 4 shows rsults of numrcal analyss of two typs of VP fluxs from surfac unt of a sampl n dpndnc on ts thcknss. Fg. 4. Dpndnc of -typ (a) and j-typ (b) outgassd product fluxs on thcknss of PC sampl at k k j, D 5D j To analyz nflunc of kntcs paramtrs of a mathmatcal modl on bhavor of С (x, and M total (, th non-dmnsonal paramtr Nu k h / D (analog of th Nusslt numbr) was usd. Fg. 5 (a) shows dpndnc of VP concntraton profl aftr 25 hours of vacuum outgassng on Nu and (b) mass loss from unt surfac on th matral-vacuum boundary n dpndnc on Nu and tm. Hr s sn that rstrcton of th outgassng procss s dsorptv for Nu.1 and dffusv for Nu 5. 4

It s sn from th outgassng kntcs of PC rradatd by lctrons Ф е 1.

5 It s sn from th outgassng kntcs of PC rradatd by lctrons Ф е cm -2 that ncras of dffrntal flux dnsty from 1 11 to 1 12 cm -2 s -1 rsults n subsqunt growth of mass loss for tmpraturnducd outgassng n vacuum. But f ϕ е> cm -2 s -1 mass loss of rradatd sampl bcam lss that that of orgnal on. Ths can b xpland by th followng: 1. Startng from a thrshold valu of dffrntal flux dnsty dcras of VP concntraton n PC du to th outgassng procss undr rradaton compard wth thr accumulaton n radolyss bcam prvalnt; 2. If part of lctron xctaton nrgy proportonal to radaton ntnsty transfrs to thrmal vbraton nrgy, addtonal vbratonal xctatons appar n matral. Thrfor, ncras of ϕ е acclrats kntc procsss lk dffuson and dsorpton of VP that s acclrats th outgassng procss undr rradaton; 3. Bhavor of ntramolcular ractons changs undr hgh radaton ntnsts whn possblty of multpl xctatons ncrass. Thrfor, charactr and aggrgat yld of outgassd products ar proportonal to absorbd nrgy and may chang undr grat absorbd dos. 5. ACKNOWLEDGEMENTS Th work was don undr th sponsorshp of th Russan Foundaton for Basc Rsarch (Projct No ). Fg. 5. Effct of Nu on outgassng procss: a for 25- hour profl dstrbuton of VP concntraton; b for mass loss kntcs of PC sampl To fnd tm dpndnc of mass loss of a PC w usd (7) that dfnd th functon F (. Thus, total mass loss of a PC wthn th prod t can b calculatd as follows I M ( S m F ( S m k C ( h, τ ) dτ, total 1 1 (8) whr m mass of typ molcul, S surfac ara of a sampl. Agan Fg. 5 confrms valdty of such numrcal analyss of th functon С(x, bcaus dsorptv and dffusv rstrctons of th outgassng procss ar obvously sn on th plot (а). I t 6. REFERENCES 1. Kasha M., Rawls H.R. Photochm. and Photobolog, Vol. 7, 561, Wllams F.- In collcton: Radaton chmstry of macromolculs. M. Atomzdat, 1972, Ovchnnkov A.А. Optcs and spctrum, Vol. 27, 748, Khasanshn R.Ch., Kostuk V.I., Blakov N.A., Bajdav D.V. On nflunc of complx of factors of spac nvronmnt on outgassng of nonmtal matrals, Problms of atomc scnc and tchnqu, Vol. 1-2, 44-48, CONCLUSIONS Rsults of ths work ar ssntal to undrstand th physcs of radaton-nducd procsss n PC and fnd ntnsty of rradaton n tsts of spaccraft matrals and coatngs. Whn carryng on radaton rsstanc tsts of PC th dffrntal lctron flux dnsty should b lmtd by valu of ϕ е 1 12 cm -2 s -1. 5

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