HYGROTHERMAL AGING OF POLYIMIDE MATRIX COMPOSITE LAMINATES

Transcription

1 HYGROTHERMAL AGING OF POLYIMIDE MATRIX COMPOSITE LAMINATES John A. Nairn and Man-Hee Han Maerials Science and Engineering, Universiy of Uah 122 S Cenral Campus Dr Rm 34, Sal Lake Ciy, UT, , USA SUMMARY: Two polyimide-marix sysems were subjeced o a series of hygrohermal aging experimens a various emperaures and relaive humidiies. For each aging condiion, we measured he microcracking fracure oughness as a funcion of aging ime. The oughness decreased wih ime. The rae of decrease increased wih emperaure and relaive humidiy. All experimenal resuls could be fi o a simple firs-order kineics analysis where he degradaion rae was assumed o be proporional o he oal exposure o waer above a hreshold level below which no degradaion occurs. The kineics analysis was used o consruc a hygrohermal aging maser plo ha can be used o make predicions of oughness degradaion due o arbirary hygrohermal environmens. The maser plo can be consruced from a few experimens on specimens immersed in waer and hus is a poenial acceleraed es mehod for characerizing hygrohermal sabiliy. KEYWORDS: Microcracking, hygrohermal aging, durabiliy, environmenal effecs, hydrolysis, acceleraed es mehods, polyimide marix, fracure oughness INTRODUCTION Various ypes of polyimide-marix composies are being considered for use in highemperaure applicaions such as for supersonic aircraf. I was previously observed, however, ha some polyimide laminaes can sponaneously microcrack when exposed o ho and we environmens [1]. For example, when quasi-isoropic, Avimid K polymer laminaes were immersed in waer a emperaures ranging from 35ºC o 95ºC, he waer upake iniially had Fickian behavior, bu laer, afer apparenly reaching an equilibrium level of waer sauraion, showed a non-fickian jump in upake rae. Microscopy resuls showed ha he secondary upake process was caused by sponaneous formaion of marix microcracks in he 9º plies despie he absence of any exernally applied load. The ime o iniiae waer-induced microcracks, m, (in minues), was log m = T (1)

2 which ranslaes o an apparen acivaion energy of 28 kj/mol. For example, when immersed in 8ºC, waer-induced microcracks iniiaed in abou 1 hours [1]. Sponaneous microcracking is caused by waer-induced degradaion of he oughness of he polyimide composie maerial [1]. Evenually, he oughness drops enough ha microcracks can form due o residual sresses alone. In his paper we repor resuls of monioring he degradaion in oughness properies prior o and afer he onse of sponaneous microcrack formaion. The experimens include resuls for wo differen polyimide maerials. For one maerial, experimens were done for waer immersion, for exposure o various levels of humidiy, and for various emperaures. All resuls can be inerpreed wih a simple kineics analysis in which degradaion rae depends on he inegraed exposure o waer. The kineics scheme was used o derive an effecive waer-exposure ime ha was hen used o generae a maser-plo for hygrohermal sabiliy. The maser plo can be used o predic oughness degradaion raes for any hygrohermal condiions. MATERIALS AND METHODS Laminae Maerials and Microcracking Experimens The wo laminae maerials used where DuPon Avimid K3B Polymer/IM7 carbon fiber and PETI5/IM7 carbon fiber composies. Each of hese maerials was laminaed ino [/9 2 ] s cross-ply laminaes and supplied o us by he Boeing Company. For hygrohermal aging experimens, a series of laminaes 12.7 mm wide, 15 mm long, and.76 mm hick were dried a 12ºC o remove moisure and hen placed in a es ube eiher immersed in waer or suspended over a sal soluion [2,3] o provide a consan relaive humidiy environmen. While aging, he moisure conen was periodically recorded. Afer seleced aging imes, specimens were removed, redried a 12ºC, and subjeced o microcracking experimens, described below, o measure he microcracking oughness as a funcion of aging ime. For Avimid K3B Polymer/IM7 laminaes, experimens a 8ºC were done for immersed samples and for sample a 76%, 5%, and 26% relaive humidiies. Addiional experimens were done for immersed samples a 6ºC and 43ºC. For PETI5/IM7, he only experimens were for samples immersed in waer a 8ºC. Microcracking Experimens and Analysis To deermine he oughness as a funcion of aging ime, we measured he densiy of microcracks as a funcion of applied load. For each aging condiion, hree separae specimens were end-abbed wih aluminum end abs and loaded a.1 mm/sec in a 25 kn MTS servohydraulic frame. The loading was periodically inerruped and he specimens were removed and observed on edge by opical microscopy o record he densiy of microcracks in he 9º plies. The resuling daa was analyzed using he mehods described in Ref. [4] o deermine microcracking fracure oughness denoed as G mc. In brief, he analysis assumes he nex microcrack forms when he energy released by ha crack exceeds he criical energy release rae or oughness, G mc, of he maerial [4,5]. A laer aging imes, many laminaes had sponaneously microcracked o some exen. These cracks were caused by residual sress and he analysis mehods [4] can accoun for such iniial cracks and sill deermine G mc.

3 2. 8 and 1 hrs Crack Densiy (1/mm) hrs 2 hrs 1 hrs Unaged Sress (MPa) Fig. 1: Microcrack densiy as a funcion of applied load for Avimid K polymer laminaes afer immersion in waer a 8ºC for various amouns of ime. RESULTS Some raw microcracking resuls from Avimid K polymer laminaes for microcrack densiy as a funcion of applied load following immersion in waer a 8ºC for various amouns of ime are ploed in Fig. 1. The raw daa clearly shifs o he lef wih aging ime indicaing a significan drop in oughness wih exposure ime. Afer 2 hours of aging here were some iniial microcracks, bu addiional microcracks formed during subsequen loading. Samples wih 5, 8, and 1 were nearly sauraed in microcracks before mechanical esing. Subsequen loading caused very few addiional microcracks. The smooh lines in Fig. 1 are he fracure mechanics analyses used o deermine G mc as a funcion of aging ime [4]. The unaged laminaes had a oughness of G mc = 96 J/m 2 ; his oughness rapidly dropped wih aging ime. The specimens wih aging imes greaer han 5 hours have no smooh lines. I is difficul o unambiguously fi hese fla resuls o find G mc, bu i is possible o deermine ha he oughness mus be very low (< 5 J/m 2 ); for ploing purposes, he oughness of laminaes sauraed wih microcracks prior o esing was assumed o be zero. Figure 2 plos G mc as a funcion of aging ime for he waer-immersion resuls in Fig. 1 and for similar experimens where he laminaes were exposed o various levels of relaive humidiy. The drop in oughness ges lower as he humidiy drops. A he lowes humidiy of 26%, here was no sign of oughness drop in he firs 1 hours. Figure 3 plos G mc as a funcion of aging ime for Avimid K polymer laminaes immersed in waer, bu a various emperaures. The degradaion rae ges slower a lower emperaure.

4 G (J/m 2 mc ) % 76% 5% 26% Time (hrs) Fig. 2: Microcracking fracure oughness, G mc, as a funcion of ime for Avimid K polymer laminaes aged a 8 C while immersed in waer (1%) or exposed o relaive humidiies of 76%, 5%, and 26%. 1 G (J/m 2 mc ) C 6 C 43 C Time (hrs) Fig. 3: Microcracking fracure oughness, G mc, as a funcion of ime for Avimid K polymer laminaes immersed in waer bu aged a 8ºC, 6 C, and 43 C. Some similar experimens were done for PETI5 laminaes, bu he degradaion rae was much slower. Figure 4 plos G mc as a funcion of aging ime for he waer-immersion resuls a 8ºC. The oughness did no sar o drop unil afer 2 hours. By 35 hours, i had only dropped 3%. We did no do experimens for PETI5 a oher humidiies or oher emperaures.

5 G (J/m 2 mc ) Time (hrs) Fig. 4: Microcracking fracure oughness, G mc, as a funcion of ime for PETI5 laminaes aged a 8 C while immersed in waer. While samples were aged under various condiions, we periodically weighed he specimens and recorded he amoun of waer upake as a funcion of ime. Such daa can be used o calculae effecive diffusion consans for waer absorpion in hese laminaes. Table 1 summarizes he waer upake resuls for he sauraion level of waer absorpion and for he ime o reach 9% of he sauraion level. The oal weigh gain when immersed is nearly independen of emperaure, alhough he absorpion ime scales ge slower a lower emperaure as he waer diffusion rae ges lower. A 8ºC, he oal weigh gain scales approximaely wih he relaive humidiy. Noe ha he relaive humidiies lised in Fig. 2 where no measured bu raher were he documened humidiies for various sal soluions [2,3]. Because we did experimens a emperaures ouside he range of he documen sal-soluion humidiies, our acual humidiies may be differen. Perhaps ha raio of he sauraion waer level a humidiy o he sauraion waer level under reduced humidiy is a beer measure of he acual relaive humidiy. This raio is given in Table 1 as he Effecive RH. Also lised in he able is he ime o reach 9% of oughness degradaion; his ime is much longer han he ime o reach sauraion. In oher words, mos degradaion occurs afer reaching sauraion waer levels. Table 1: Summary or waer upake resuls for Avimid K polymer laminaes and a comparison o he rae of degradaion in oughness Condiions Weigh Gain (%) Effecive RH (%) Hrs o 9% Weigh Gain Hrs o 9% Decrease in G mc 8 C, Immersed C, 76% RH C, 5% RH >12 8 C, 26% RH >>12 6ºC, Immersed ºC, Immersed >14

6 DISCUSSION For all hygrohermal condiions and for boh polyimide sysems, he microcracking oughness decreased wih ime. The decrease was probably caused by hydrolysis of he polyimide resins [6]. There appears, however, o be a hreshold effec. The oughness remained consan a shor amoun of ime and hen decreased. In his secion, we sugges a simple firs order kineics analysis for he degradaion process. We assume he rae of degradaion is proporional o he waer conen. To accoun for he hreshold effec, we assume ha he firs waer goes ino sies ha do no cause damage and only he laer waer causes hydrolysis. Thus, he firs order degradaion is firs order in waer concenraion (% weigh gain) above some hreshold concenraion. By he firs order kineics wih a hreshold effec, he rae in decrease in oughness is which inegraes o dgmc = d k [ H2O( )] [ H2O] hres ( ) for < hres for > hres (2) Gmc Gmc = Gmc exp k ([ H2O( )] [ H2O] hres) d hres for < hres for > hres where k is a rae consan, [H 2 ()] is he waer weigh gain as a funcion of ime, [H 2 ] hres is he hreshold concenraion below which no degradaion occurs, hres is he ime for he waer weigh gain o reach [H 2 ] hres, and G mc is he oughness before aging. For a given se of hygrohermal aging condiions, he wo parameers for fiing experimenal resuls are k and [H 2 ] hres. We were able o fi all experimenal resuls o Eqn (4). Furhermore, he resuls for all humidiies a a single emperaure could be fi wih he same values for k and [H 2 ] hres. A 8 C, he hreshold waer weigh gain was deermined o be.23%. This resul predics ha for effecive RH below 53%, here should be no degradaion in oughness. This predicion agrees wih he resuls a RH 26% (which had an effecive RH of 28%); a leas for he firs 1 hours, he resuls a low humidiy showed no degradaion in oughness. By he form of Eqn (3), any wo samples ha have experienced he same inegraed exposure o waer should have he same degradaion in oughness. Thus, i should be possible o derive a maser plo for hygrohermal degradaion where resuls a any emperaure and humidiy are shifed o some reference emperaure and humidiy. The effecive ime under he sandard condiions is calculaed by solving he numerical equaion eff k (, Tref, RHref ) d = k (, T, RH) d (4) ref hres hres for eff or he effecive ime a he reference emperaure, T ref, and reference humidiy, RH ref. ref Here is he acual ime a emperaure T and relaive humidiy RH, hres and hres are he imes o reach he hreshold waer concenraion a T ref and T, respecively, and (3)

7 ( ) k (, T, RH ) = k ( T ) [ H2O (, T )] [ H2 O ( T )] hres (5) is an effecive rae consan. Wihin k (,T,RH), he consans k(t) and [H 2 (T)] hres depend only on emperaure while he experimenal daa for [H 2 (,T)] depend on emperaure and humidiy. Give a se of experimenal resuls for waer upake a various emperaures and humidiies, he resuls can subsiued ino Eqn (4) and numerically inegraed o consruc a maser plo for hygrohermal aging. By he resuls in Table 1, however, i is noed ha he ime for significan oughness degradaion is much greaer han he ime o reach sauraion waer upake. Wih his observaion, i is possible o approximaely solve Eqn (4) by ignoring he rising porion of he waer upake curves and assuming all degradaion occurs a sauraion waer conen. In oher words, we assume k (,T,RH) is a consan or independen of ime and equal o k (,T,RH) or is value a sauraion waer level. Eqn (4) hen inegraes o eff ref k (, T, RH) = hres + hres k (, Tref, RHref ) ( ) (6) Figure 5 gives a maser plo for all our resuls on Avimid K polymer laminaes using reference condiions of waer immersion a 8ºC. Shifing by Eqn (6) requires knowledge of he emperaure and humidiy dependence of k (,T,RH). The emperaure dependence for sauraion condiions from our limied experimens can be described by an apparen acivaion energy of 26 kj/mol which is very close o he apparen acivaion energy from sponaneous microcracking formaion experimens [1]. Because k(t) and [H 2 (T)] hres appear o be independen of humidiy and [H 2 (,T)] scales wih humidiy, k (,T,RH) a any humidiy can be calculaed from he immersed resul (RH=1%) using k (, T, RH) RH F = k (, T, 1%) 1 F (7) where F is he raio of he hreshold waer concenraion, [H 2 (T)] hres, o he sauraion waer weigh gain when immersed in waer. CONCLUSIONS Polyimide-marix composies show degradaion in oughness when exposed o waer and high emperaure. All experimenal resuls can be fi o simple firs-order kineics by assuming here is some hreshold waer concenraion below which no degradaion occurs. The rae consans calculaed from hese fis can be used o shif all resuls o a maser plo for hygrohermal aging. By exrapolaing rae consans o oher emperaures i is possible o use he maser plo o predic hygrohermal degradaion a any emperaure and humidiy. By calculaing he inegraed exposure o waer under any hygrohermal condiions, such as hygrohermal cycling condiions, i should similarly be possible o use he maser plo o predic he exen of oughness degradaion caused by hose condiions. Consrucion of hygrohermal aging maser plos for oher marix sysems is a poenial acceleraed es mehod for characerizing he hygrohermal sabiliy of marix resins. The

8 G (J/m 2 mc ) C, 1% RH 6 C, 1% RH 43 C, 1% RH 8 C, 76% RH 8 C, 5% RH Effecive Time (hrs) Fig. 5: Maser plo for hygrohermal aging of Avimid K polymer laminaes for reference condiions of immersion in waer a 8ºC. minimal experimens are o measure oughness degradaion a he reference emperaure while immersed in waer and a several oher emperaures while immersed in waer. Such resuls a several emperaures can be used o give he emperaure dependence of k (,T,1%). The humidiy dependence can be exrapolaed from k (,T,1%) using Eqn (7) wihou he need for addiional experimens a reduced levels of humidiy. Acknowledgmens This work was suppored, in par, by a gran from he Mechanics of Maerials program of he Naional Science Foundaion (CMS ), and, in par, under a Boeing conrac NAMP REFERENCES 1. Kim, H. W., Grayson, M. A. and Nairn, J. A., The Effec of Hygrohermal Aging on he Microcracking Properies of Some Carbon Fiber/Polyimide Laminaes, Advanced Composie Leers, Vol. 4, 1995, pp Greenspan, L., Humidiy Fixed Poins of Binary Sauraed Aqueous Soluions, Journal of Research of he Naional Bureau of Sandards - A. Physics and Chemisry, Vol. 81A, 1977, pp Wexler, A. and Hasegawa, S., Relaive Humidiy-Temperaure Relaionships of Some Sauraed Sal Soluions in he Temperaure Range o 5 C, Journal of Research of he Naional Bureau of Sandards, Vol. 53, 1954, pp

9 4. J. A. Nairn, S. Hu, and J. S. Bark, A Criical Evaluaion of Theories for Predicing Microcracking in Composie Laminaes, Journal of Maerial Science, Vol. 28, 1993, pp J. A. Nairn, Exac and Variaional Theorems for Fracure Mechanics of Composies wih Residual Sresses, Tracion-Loaded Cracks, and Imperfec Inerfaces, Inernaional Journal of Fracure, submied, R. DeIasi and J. Russell, Aqueous Degradaion of Polyimides, Journal of Applied Polymer Science, Vol. 15, 1971, pp