Thermomechanical and hygroelastic properties of an epoxy system under humid and cold-warm cycling conditions

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Thermomechanical and hygroelastic properties of an epoxy system under humid and cold-warm cycling

conditions

Jalal El Yakoubi, Gilles Lubineau, Shahid Saghir, Jacques Verdu, Abe Askari

To cite this version:

Jalal El Yakoubi, Gilles Lubineau, Shahid Saghir, Jacques Verdu, Abe Askari. Thermo- mechanical and hygroelastic properties of an epoxy system under humid and cold-warm cy- cling conditions. Polymer Degradation and Stability, Elsevier, 2014, 99 (1), pp.146-155.

�10.1016/j.polymdegradstab.2013.11.011�. �hal-00980344�

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This is an author-deposited version published in: http://sam.ensam.eu Handle ID: .http://hdl.handle.net/10985/7976

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Jalal EL YAKOUBI, Gilles LUBINEAU, Shahid SAGHIR, Jacques VERDU, Abe ASKARI - Thermomechanical and hygroelastic properties of an epoxy system under humid and cold-warm cycling conditions - Polymer Degradation and Stability - Vol. 99, n°1, p.146-155 - 2014

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Thermomechan ica l and hygroe last ic propert ies o f an epoxy system under hum id and co ld-warm cyc l ing cond it ions

Ja la l E l Yagoub i ^{a , b , c , d , *} , G i l les Lub ineau ^a , Shah id Sagh ir ^a , Jacques Verdu ^e , Abe Askar i ^f

a

K ing Abdu l lah Un ivers i ty o f Sc ience and Techno logy (KAUST) , Phys ica l Sc ience and Eng ineer ing D iv is ion , COHMAS Labora tory , Thuwa l 23955-6900 , Saud i Arab ia

b

Un iv . Bordeaux , I2M , UMR 5295 , F-33400 Ta lence , France

c

CNRS , I2M , UMR 5295 , F-33400 Ta lence , France

d

Ar ts e t Me t iers Par isTech , I2M , UMR 5295 , F-33400 Ta lence , France

e

Ar ts e t Me t iers Par isTech , Labora to ire P IMM , 151 Bou levard de l’ Hop i ta l , 75013 Par is , France

f

The Boe ing Company , Propu ls ion Sys tem D iv is ion , Evere t t , WA , USA

Keywords : Epoxy

Hygro therma l ag ing Hydro lys is Sorpt ion

abstract

In th is paper , we study the hygro therma l ag ing o f an anhydr ide-cured epoxy under temperature and hygrometry cond it ions s imu lat ing those exper ienced by an a ircra ft in wet trop ica l or subtrop ica l reg ions . Grav imetr ic and d imens iona l measurements were per formed and they ind icate that there are three stages in th is ag ing process : thefirst one , correspond ing to the ear ly cyc les can be ca l led the“induct ion stage”. The second stage o f about 1000 cyc les durat ion , cou ld be named the“swe l l ing stage”, dur ing wh ich the vo lume increase is a lmost equa l to the vo lume o f the ( l iqu id) water absorbed . Both the first and second stages are accompan ied by mod ificat ions o f the mechan ica l propert ies and the g lass tran- s it ion temperature . Dur ing the th ird (“equ i l ibr ium”) stage , up to 3000 cyc les , there is no s ign ificant change in the phys ica l propert ies desp ite the cont inuous increase o f water uptake . Th is can be exp la ined by the fact that on ly phys ica l ly sorbed water can influence phys ica l propert ies .

1 . Introduct ion

Epoxy-based mater ia ls are ut i l ized in many eng ineered struc- tures , rang ing from large c iv i l in frastructures[1]to m icroe lectron ics [2]because they o f fer strong mechan ica l propert ies a long w ith therma l and chem ica l stab i l ity . An importantfie ld o f app l icat ion is the one o f carbone /epoxy compos ites that exh ib it adap tab i l ity dur- ing the des ign [3] and h igh- leve l integrat ion dur ing the manu factur ing process[4]. In part icu lar , recent deve lopments in the aeronaut ica l industry have h igh l ighted the important ro le o f com- pos ite mater ia ls in improv ing a ircra ft des ign[5]. These compos ites have been introduced in pr imary components such as the fuse lage . Under serv ice cond it ions , such components are sub jected to var ied env ironmenta l cond it ions that can lead to mater ia l degradat ion[6e 8]. Exposure to water vapor s imu ltaneous ly w ith cyc l ic temperature changes is the predom inant cause o f degradat ion re lated to the outdoor env ironment . A lthough the phenomeno logy ag ing o f the

lam inated compos ites can appear to be very comp lex[6 ,7 ,9e11 ],a mandatory start ing po int is to understand the evo lut ion o f the propert ies o f the const itut ive res in . Many stud ies cons idered tem- peratures above 25 C , but few addressed ag ing under representa- t ive co ld ( 40 C) and warm (70 C) cond it ions . An exper imenta l ana lys is o f the ag ing o f bu lk po lymer is there fore requ ired to ident i fy the key mechan isms and prov ide the necessary database for the s imu lat ion o f the who le ag ing process .

F irst o f a l l , a genera l understand ing o f the phenomeno logy o f ag ing is necessary . We rev iew , in the fo l low ing , the e f fects o f the water penetrat ion in the po lymer , wh ich can be d iv ided into phys ica l interact ions and chem ica l e f fects .

1 .1 . Phys ica l in terac t ion

Thefirst step in any hydro lyt ic ag ing process is the penetrat ion o f water into the organ ic matr ix . Water is more or less so lub le in epox ies , depend ing on the po lar group content o f the po lymer , espec ia l ly groups ab le to estab l ish strong hydrogen bonds w ith water mo lecu les . Exper imenta l ev idence o f such hydrogen bonds , on the bas is o f NMR[12 ,13], from structure eso lub i l ity re lat ionsh ips [14]or from the ana lys is o f the heat o f d isso lut ion[15]is extens ive .

*Correspond ing author . K ing Abdu l lah Un ivers ity o f Sc ience and Techno logy (KAUST) , Phys ica l Sc ience and Eng ineer ing D iv is ion , COHMAS Laboratory , Thuwa l 23955-6900 , Saud i Arab ia . Te l . :þ966 (0) 565 560 239 .

E-ma i l address :ja la l .e lyagoub i@u-bordeaux1 . fr( J . E l Yagoub i) .

Thefirst important po int is that the predom inant part o f the absorbed water mo lecu les is e f fect ive ly d isso lved in the matr ix ( i .e . , form a s ing le phase w ith the po lymer) . The g lass trans it ion tem- perature (T

) o f such a m ixture is expected to be between the g lass trans it ion temperature o f the dry po lymer (T

) and the one o f water (T

) . Th is latter temperature (w120 K) is cons iderab ly lower than that o f the epoxy (genera l ly>300 K) . As a resu lt , water penetrat ion induces a not iceab le depress ion in theT

o f hydroph i l ic epox ies , a phenomenon ca l led“p last ic izat ion”. Accord ing to the free vo lume theory , the free vo lumes o f both po lymer and water are add it ive[16]. Us ing many s imp l i fy ing hypo theses , th is theory leads to the fo l low ing re lat ionsh ip[17 ,18]:

1 T g ¼ 1

T _gd þA _{p l} m w (1)

wherem

is the mass fract ion o f the water in the m ixture andA

is a so-ca l led“p last ic izat ion coe ffic ient”, wh ich can be expressed , in the s imp lest case , by :

A _{p l} ¼ 1 T gw

1

T _gd (2)

A

ranges from about 5 .510

K

forT

¼ 80 C to about 6 .5 10

K

forT

¼270 C . These re lat ionsh ips lead to :

dT g

dm w ¼ A _{p l} T _g ² (3)

thus , dT

=dm

ranges between 685 K forT

¼80 C to 1770 K for T

¼ 270 C . Th is leads to aT

depress ion per percent (mass) o f absorbed water that ranges from about 7 K forT

¼80 C to about 18 K forT

¼270 C .

Ana lys is o f NMR [13 ,19]or d ie lectr ic spectra[20 ,21]revea ls , however , the coex istence o f hydrogen-bonded and free water mo lecu les . These latter cou ld be present in c lusters ; in th is case , they wou ld not part ic ipate in po lymer /water interact ions . The know ledge o f the to ta l mass fract ion o f the water in the po lymer m ight there fore not be su ffic ient to character ize the system .

Another important consequence o f water d isso lut ion in po lymer matr ices is swe l l ing . A s imp le express ion o f vo lumetr ic e f fects resu lt ing from water d isso lut ion in a po lymer is :

V ¼ V p þV L þ c V p V L (4) where V

andV

are the respect ive vo lumes o f the po lymer and l iqu id and c is a so-ca l led“interact ion coe f fic ient”. In the case o f water , c ^{is negat ive .} In other words , water penetrat ion in the po lymer is accompan ied by a contract ion o f the m ixture but th is contract ion is not strong enough to inh ib it the swe l l ing to ta l ly[22].

Th is means that 0> c > V

.

In the trans ient reg ime o f d i f fus ion where water concentrat ion grad ients ex ist across the samp le’ s th ickness , d i f ferent ia l swe l l ing induces a stress state[23e25]. Schemat ica l ly , absorpt ion induces compress ive stresses in the superfic ia l layers wh i le desorpt ion in- duces tens i le stresses . These se l f-equ i l ibrated stress states can be severe enough to introduce m icro-damage , wh ich can , in turn , mod i fy the water sorpt ion character ist ics .

For a w ide fam i ly o f am ine-cured epoxy networks (w ith com- p lete cur ing) , there is no poss ib i l ity o f chem ica l react ions w ith the water and the on ly poss ib le degradat ion process is damage by swe l l ing stresses in the trans ient d i f fus ion reg ime .

1 .2 . Chem ica l reac t ion

Am ine-cured epox ies that conta in an excess o f epoxy groups and anhydr ide-cured epox ies conta in hydro lyzab le groups (epoxy , ester , anhydr ide) . In these cases , chem ica l po lymer-water in- teract ions comp l icate the ag ing behav ior .

In the presence o f unreacted epoxy or anhydr ide groups , hy- dro lys is can mod i fy the d i f fus ion behav ior , wh ich can turn from a F ick to a Langmu ir process[26 ,27]. Th is has no d irec t e f fects on the po lymer’ s mechan ica l behav ior because hydro lys is does no t a f fect the cross l ink dens ity .

In con tras t , es ter hydro lys is , in po lyesters or in anhydr ide-cured epox ies , induces cha in sc iss ion . Th is resu lts in a decrease in the cross l ink dens i ty and ne twork embr i t t lemen t . A lso , cha in sc iss ion genera tes sma l l mo lecu lar fragmen ts and can induce osmo t ic crack ing [28 ,29] .

Coup l ing o f phys icoechem ica l ag ing processes w ith mechan ica l load ing is , then , a key ob ject ive and has insp ired a re lat ive ly abundant l iterature[9e11 ,30]. As far as hydro lys is is concerned , react ioned i f fus ion coup l ing must be taken into account . A fter the p ioneer ing work o f Go l ike and Lasosk i in the ear ly 1960s[31], there was , to our know ledge , no s ign ificant work on d i f fus ion-contro l led hydro lys is dur ing the last ha l f century . Recent ly , our group pub- l ished a study propos ing a new formu lat ion for react ion ed i f fus ion coup l ing k inet ics and its app l icat ion to hygro therma l ag ing o f anhydr ide-epoxy matr ices[27].

It is worth not ing that a s im i lar framework ex ists for the ox idat ion o f epoxy networks conta in ing a l iphat ic structures . L iterature rev iews have been pub l ished[32 ,33 ]on th is top ic . Oxy- gen reacts w ith these groups , at re lat ive ly h igh temperatures , and th is contr ibutes to mater ia l ag ing v ia network degradat ion[34]and v ia shr inkage induced ox idat ion[6 ,35] .

Here , we study the behav ior o f an anhydr ide-cured epoxy under non-s teady cond it ions s im i lar to those o f aeronaut icfligh t cond i t ions . We cons ider two extreme c l imates , one s imu la t ing the ce i l ing fligh t , charac ter ized by low tempera ture ( 40 C ) , and the o ther s imu lat ing ground storage in a warm (70 C) , we t (90% RH) env ironmen t . In the fo l low ing sect ion , we descr ibe the mater ia l and we de ta i l the exper - imenta l procedures used to mon itor the po lymer ’ s behav ior . Then , exper imenta l resu l ts are repor ted for mass up take , the changes in the g lass trans i t ion temperature and the thermomechan ica l and hygroe- las t ic proper t ies . F ina l ly , we h igh l igh t the sorp t ion mechan isms and the re la t ionsh ips w ith the evo lu t ion o f the po lymer ’ s proper t ies . 2 . Exper imenta l

2 .1 . Ma ter ia l and samp les

The mater ia l used in th is study was a commerc ia l epoxy res in (EPOLAM 2063 , Axson Techno logy) based on a m ixture o f d ig lyc idy l ether o f b ispheno l A (DGEBA) w ith a cyc loa l iphat ic epox ide hard- ened by nad ic methy l anhydr ide (a lso from Axson Techno logy) . Both components were l iqu id . They were m ixed and st irred at 80 C for 15 m in . The m ixture was then cast in an a lum inum mo ld to prepare 1 mm th ick p lates . The p lates were then cured for 6 h at 80 C and post-cured for 6 h at 180 C to obta in as comp lete con- vers ion o f the epox ide-anhydr ide react ion as poss ib le .

Nea t po lymer samp les were cut us ing a d iamond saw . The samp les were cond it ioned in a dess icator at room temperature pr ior to ag ing .

2 .2 . Exposure cond i t ions

Spec imens for ag ing were p laced ins ide an env ironmenta l

chamber where both temperature and re lat ive hum id ity were

contro l led to 0 .3 C and 2% RH , respect ive ly .F ig . 1shows the arrangement o f the samp les ins ide the chamber . They were in- c l ined to avo id stagnat ion o f the condensed water on the ir sur face . The system was des igned to ensure good vent i lat ion o f the who le chamber vo lume . Thermocoup les (type T-316SS12-U-T4) were used to mon itor the temperature at each rack as the temperature may have var ied ins ide the c l imat ic chamber . The max imum var iat ion in the temperature was about 15 C when the programmed temper- ature reached 70 C .

Cyc l ing was preceded by a ho ld ing stage o f 12 h at 70 C and 90%

RH . Th is stage was immed iate ly fo l lowed by the cyc l ing as sche- mat ized in F ig . 2. H igh hum id ity (90% RH) was ma inta ined dur ing the port ion o f the cyc le in wh ich the temperature exceeded 55C . It rema ined h igher than 70% RH at temperatures above 25 C . It was a lmost zero dur ing the low temperature part o f the cyc le . It shou ld be noted that , for temperatures be low 25 C , and part icu lar ly for negat ive temperatures (<0 C) , the d i f fus ion and react ion processes are s ign ificant ly s lower than they are dur ing the ho t stage . The temperature ramps were 2 K /m in for coo l ing and 5 K /m in for heat ing . Exposures up to 3000 cyc les were run .

To mon itor the po lymer degradat ion , samp les were tested at pre-defined numbers o f cyc les between 0 and 3000 . The spec imens were tested at a h igh frequency up to 500 cyc les , because we ex- pected rap id changes in the po lymer’ s propert ies . Then , exper i- menta l data were co l lected at a lower frequency unt i l the end o f the campa ign ( i .e . , 3000 cyc les) . As dep icted inF ig . 2, samp les were co l lected dur ing the coo l ing phase when the temperature approached 25 C . Certa in samp les , una ltered by character izat ion methods , were re introduced in the chamber , a lso dur ing the coo l- ing phase at 25 C . In the fo l low ing sect ion , each exper imenta l method is descr ibed .

2 .3 . Charac ter iza t ion

2 .3 .1 . Grav ime tr ic and vo lume tr ic proper t ies

Two ident ica l spec imens o f 60 40 1 mm

were frequent ly taken out from the chamber for grav imetr ic and dens ity mea- surements . A fter be ing tested , these samp les were returned to the chamber .

An ana lyt ica l ba lance (Ohaus DV214CD , prec is ion o f 0 .1 mg) a long w ith an integrated dens ity k it were used to mon itor the mass and the vo lume changes dur ing the ent ire ag ing per iod . The aged spec imens were removed from the chamber , care fu l ly w iped and immed iate ly tested . Exper iments were conducted at the amb ient temperature (22 .5 0 .5 C) . The measurement procedure cons isted o f the fo l low ing steps :

F irst , spec imens were we ighed to mon itor the evo lut ion o f the g loba l mass uptake (m

) , wh ich is der ived from the samp le mass (m

) accord ing to Eq .(5):

m w ðNÞ ¼ m s ðNÞ m ⁰ _s

m ⁰ _s (5)

where m

is the in it ia l mass o f the samp le andNis the number o f cyc les .

Second , the po lymer’ s dens ity was measured us ing Arch imede’ s method . Spec imens were we ighed in a pan that was immersed in d ist i l led water . The spec imen ’ s dens ity ( r

) was then ca lcu- lated accord ing to Eq .(6):

F ig . 1 .Schemat ic representat ion o f the env ironmenta l chamber where samp les were p laced and exposed to cyc l ic hygrotherma l ag ing .

F ig . 2 .Schemat ic representat ion o f the hygrotherma l cyc l ing ut i l ized for acce lerated

ag ing .

r _s ðNÞ ¼ r _{d m} ^{m s ðNÞ}

s ðNÞ m _i ðNÞ (6)

where r

was the dens ity o f the d ist i l led water and m

is the resu lt ing read ing o f the mass when the samp le is immersed in water . The re lat ive var iat ion o f the po lymer’ s dens ity , deno ted as r

,isde fined as fo l lows (Eq .(7)) :

r w ðNÞ ¼ r s ðNÞ r 0

r ₀ ⁽⁷⁾

The vo lume change (V

) is then der ived from the mass uptake and the dens ity change accord ing to the fo l low ing express ion (Eq .(8)) :

V w ðNÞ ¼ V s ðNÞ V ₀

V ₀ (8)

where V

is the vo lume o f the aged samp les andV

is the in it ia l vo lume o f the spec imens .

2 .3 .2 . Mechan ica l tes t ing

F irst , tens i le tests were per formed on the epoxy samp les that were no t subm itted to env ironmenta l cond it ions to determ ine the in it ia l tens i le propert ies (Young ’ s modu lus , Po isson’ s rat io) . The tests were conducted on an INSTRON mach ine under a constant extens ion rate o f 2 mm /m in . The dog-bone-shaped spec imens were prepared as per the ASTM standard D 638-03 . The respect ive in it ia l tens i le modu lus , Po isson’ s rat io , stress to fa i lure and stra in to fa i lure were : 3 .1 GPa ; 0 .3 ; 50 5 Mpa ; 2 0 .15% , respect ive ly .

Second , mon itor ing o f the degradat ion o f the mechan ica l propert ies dur ing ag ing was ach ieved by instrumented indentat ion tests (m icro indentat ion , Nanovea company) . Spec imens o f 40 40 1 mm

were removed from the chamber and care fu l ly w iped . Then , the po lymer p lates were g lued on top o f a 10 mm th ick sta in less-stee l substrate . The t ime that e lapsed be fore the inden- tat ion test was started was less than 15 m in . The tests were per- formed on n ine locat ions o f the samp les fo l low ing a 3 3 array w ith 0 .5 mm spac ing . We used an indentor w ith a 200 m m d iameter spher ica l t ip (d iamond) . The exper iments were per formed at a constant load rate (2 N /m in) dur ing load ing and un load ing . A fter reach ing the max imum leve l o f 4 N , the load was ma inta ined for 15 s and the creep d isp lacement was recorded .

The load-d isp lacement curves were ana lyzed to est imate the e last ic modu lus and the hardness o f the mater ia l (ASTM standard E2546) . F irst , the reduced modu lus (E

) was der ived from Eq .(9):

E r ðNÞ ¼ ffi ffi ffi ffi p p

2 S ffi c ffi ffi ðNÞ ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi A p ðNÞ

p (9)

where S

is the contact st i f fness ca lcu lated from the un load ing curve andA

is the pro jected contact area . The indentat ion modu lus (E

) and the hardness (H

) were then computed fo l low ing Eq .(10)and Eq .(11):

E _IT ðNÞ ¼ 1 n ² _s

"

1 E r ðNÞ

1 n ² _i E _i

# ₁

(10)

H IT ðNÞ ¼ F max

A p ðNÞ (11)

where n

and n

are the Po isson’ s rat ios o f the indentor t ip and the epoxy . As afirst approx imat ion , we assumed that n

was not a f fected by the ag ing .

2 .3 .3 . Dynam ic mechan ica l ana lys is

Dynam ic mechan ica l ana lys is (DMA) was conducted in the s ing le cant i lever mode at a frequency o f 1 Hz (Netzch DMA242) . DMA spec imens were 10 0 .07 mm in w idth , 1 0 .03 mm in th ickness and 35 0 .2 mm in length . V iscoe last ic propert ies were measured at temperatures rang ing from 25 C to 225 C w ith a heat ing rate o f 5 K /m in . These tests were used to determ ine the g lass trans it ion temperature .

It is worth not ing that the DMA tests were started about 15 m in a fter the aged samp les were taken out from the env ironmenta l chamber . In add it ion , we eva luated the amount o f water that wou ld be desorbed dur ing a DMA exper iment . For th is purpose , we con- ducted thermograv imetr ic ana lys is under s im i lar cond it ions as those for the DMA tests . We found that more than 90% o f the sorbed water rema ined w ith in the po lymer when the temperature reached the g lass trans it ion temperature . Th is was a good ind icat ion that the g lass trans it ion temperature , der ived from the DMA curves , was s ign ificant to the ag ing process .

2 .3 .4 . S tra in measuremen t

A data acqu is it ion system (System 7000 from V ishay Company) was used to co l lect the stra in and the temperature s igna ls dur ing hygro therma l cyc l ing . For th is purpose , two epoxy spec imens were instrumented w ith stra in gauges . The temperature in the v ic in ity o f each samp le was measured by thermocoup les (TC-3 and TC-4) . The re ference temperature wasT

¼23 C . Regu lar ly , stra ins and tem- peratures were acqu ired dur ing a per iod o f two to three cyc les . The ded icated samp les were 15 0 .5 mm in w idth , 1 0 .03 mm in th ickness and 65 0 .5 mm in length .

3 . Resu lts 3 .1 . Wa ter up take

The evo lut ion o f the mass accounts for the overa l l amount o f water sorbed by the po lymer . We prev ious ly showed in Re f . [27]

that the stud ied epoxy system d isp lays non-F ick ian behav ior . The short-term isotherma l sorpt ion exper iments revea led two stages in the mass uptake[27].F ig . 3shows that the mass uptake under hygro therma l cyc l ing corroborates the resu lts o f the sorp t ion ex- per iments under isotherma l cond it ions . In fact , the mass uptake d id not stab i l ize even a fter 3000 cyc les . We ident ified three stages :

A rap id mass increase o f about 1% was observed fo l low ing the soak t ime (12 h at 70 C and 90% RH) .

From thefirst cyc le to 250 cyc les , the mass uptake increased at a decreas ing rate .

A fter a trans it ion at 250 cyc les , the mass o f the samp les increased l inear ly w ith respect to the number o f cyc les . 3 .2 . D imens iona l changes

Dur ing the hygro therma l cyc l ing , both the temperature and the hum id ity contr ibuted to the d imens iona l changes .

The evo lut ion o f the to ta l in-p lane l inear (L) hygro therma l stra in

(ε

ðT;NÞ) versus the temperature is p lotted inF ig . 4. From a

g loba l v iewpo int , th is de format ion can be part it ionned into a

therma l one (ε

) and a hydr ic one (ε

) that can be both a funct ion

o f the temperature (T) and the number o f cyc les (N) .F ig . 4shows

that the tota l in-p lane stra in evo lves l inear ly w ith the temper-

ature and the curves sh i ft up w ith the number o f cyc les . As afirst

approx imat ion , we can reasonab ly assume that the to ta l stra in

cou ld be part it ioned as fo l lows (Eq .(12)) :

ε ^L _ht ðT;NÞ ¼ε ^L _t ðT;NÞþε ^L _h ðNÞ (12) There fore , we ca lcu lated the coe ffic ient o f therma l expans ion (CTE , a ) as the der ivat ive o f the in-p lane stra in w ith respect to the temperature (Eq .(13)) .

a ðT;NÞ ¼ vε ^L _ht ðT;NÞ

vT (13)

Interest ing ly , we found that the CTE d id not exper ience any detectab le change w ith the number o f cyc les . A lso , it appears that the coe ffic ient has a tendency to increase w ith the temperature . It ranges from 61 10

C

at 30 Cto75 10

C

at 70 C . Then , the hydr ic in-p lane stra in (ε

) was obta ined a fter subtract ing the therma l stra in from the tota l stra in .

We measured the evo lut ion o f the vo lume change at the amb ient temperature (V

) as a funct ion o f the number o f cyc les (F ig . 5) . Th is on ly accounts for the mo isture- induced swe l l ing because it was measured at the re ference temperature chosen for the therma l stra in ,T

.

The var iat ion in the vo lume a lso exh ib its severa l stages : e A rap id vo lume increase o f about 0 .5% was subsequent to the

soak t ime (12 h at 70 C and 90% RH) .

e From thefirst cyc le to 250 cyc les , the mater ia l swe l led l ine- ar ly w ith the number o f cyc les .

e Between 250 cyc les and 1000 cyc les , the vo lume o f the samp les cont inued to increase l inear ly but at a rate that was reduced .

e F ina l ly , a fter 1000 cyc les , it appears that the vo lume rema ined most ly constant (about 1 .5%) .

Assum ing that the hygroe last ic behav ior was isotrop ic , we can est imate an equ iva lent hydr ic stra in from the vo lume uptake ac- cord ing to Eq .(14).F ig . 6shows that it is comparab le to the in-p lane stra in ,ε

(measured us ing stra in gauges) .

ε ^V _h ðNÞ ¼ V w

3 (14)

3 .3 . G lass trans i t ion tempera ture

We eva luated the e f fect o f ag ing on the g lass trans it ion tem- perature by means o f DMA . The reta ined defin it ion o f the g lass trans it ion temperature (T

) was based on the tanð d Þpeak . Th is peak revea led ne ither sp l itt ing nor s ign ificant broaden ing , support ing an e ffic ient d isso lut ion o f water in the po lymer .

The evo lut ion o f the g lass trans it ion temperature in re lat ion to the number o f cyc les is shown inF ig . 7. The water uptake resu lted in a decrease in the g lass trans it ion temperature . Contrary to the mass up take and the vo lume change , the evo lut ion o f th is quant ity w ith the number o f cyc les was no t monoton ic .F ig . 7revea ls three stages :

A rap id decrease was observed dur ing thefirst 200 cyc les . Then ,T

re increased s low ly between 250 cyc les and 1000 cyc les . Th is part icu lar observat ion is in agreement w ith the resu lts re- ported by Zhou and Lucas[13].

F ina l ly ,T

rema ined most ly constant unt i l the end o f the ag ing campa ign .

3 .4 . Mechan ica l proper t ies

The e f fect o f ag ing on the po lymer’ s mechan ica l behav ior was mon itored by means o f instrumented indentat ion tests . The in it ia l va lue o f the indentat ion modu lus (3 .02 GPa) was very c lose to the tens i le modu lus obta ined on dog-bone-shaped samp les (3 .1 GPa) . F ig . 8shows the modu lus changes , wh ich were c lear ly corre lated to T

changes . It can be reasonab ly supposed that both evo lut ions resu lted from the same processes .

Var iat ion in the hardness is shown in re lat ion to the number o f cyc les inF ig . 9. S im i lar ly to theT

and the modu lus , the hardness F ig . 3 .Mass uptake in re lat ion to the number o f cyc les .

F ig . 4 .Tota l in-p lane hygrotherma l stra in (ε

ðT;NÞ) versus temperature .

decreased rap id ly dur ing thefirst stage o f water up take . A s low increase can be surm ised in the second stage , but it was part ly masked by the scatter . The var iat ion in the max imum creep d isp lacement a fter 15 s is shown inF ig . 10. S im i lar ly to the other propert ies , it var ies ( increases) rap id ly dur ing thefirst stage but rema ins a lmost constant dur ing the rest o f ag ing campa ign . 4 . D iscuss ion

In the prev ious sect ion , we reported exper imenta l resu lts on the sorp t ion k inet ics , the thermomechan ica l and hygroe last ic behav- iors , and the g lass trans it ion temperature . In the fo l low ing , we propose a degradat ion scenar io that is cons istent w ith the above- ment ioned resu lts .

4 .1 . Sorp t ion mechan isms and k ine t ics

Let usfirst cons ider the mass uptake k inet ics (F ig . 3) . The absence o f equ i l ibr ium suggests the ex istence o f a chem ica l reac- t ion in wh ich water mo lecu les are incorporated in the po lymer structure , i .e . a hydro lys is phenomenon . At low convers ions , hy- dro lys is behaves as a zero-order process , i .e . , at an a lmost constant rate , wh ich exp la ins the l inear shape o f the curve in its second

stage . As afirst approx imat ion , the who le mass uptake ,m

, can be d iv ided into two parts (m

¼ m

þm

) . The water uptake ,m

, dur ing the second stage , wh ich accounts for the hydro lys is react ion and the hydrophy l ic ity increase[27], can be approx imated by the fo l low ing equat ion :

m ² _w ¼ 4: 17 10 ⁶ N (15)

By subtract ingm

from the who le mass uptake ,m

, we obta in a curve ,m

¼ m

m

¼ fðNÞ, w ith an hor izonta l asymptote o f wh ich the ord inate (m

¼0 .0175) wou ld correspond to the in it ia l equ i l ibr ium concentrat ion o f the phys ica l ly sorbed water . Thefirst stage thus corresponds to the phys ica l absorpt ion o f water . The k inet ics presumab ly obey F ick’ s law but th is is d i fficu lt to confirm ow ing to the lack o f data in the ear ly per iod o f exposure for mass uptake va lues lower than 60% o f the equ i l ibr ium va lue . Neverthe- less , F ick ian behav ior was con firmed by the resu lts o f grav imetr ic exper iments reported in a prev ious paper [27]on the same mater ia l .

D i f fus ion and react ion processes occur at d i f ferent t ime sca les , wh ich we d iscuss in the fo l low ing by cons ider ing the character ist ic t ime o f the temperature var iat ions .

Accord ing to our prev ious work , the coe ffic ient o f d i f fus ion o f water at 50 Cis5 10

m

s

. The character ist ic t ime o f d i f fus ion ,t

(h

=D) , for samp les o f 1 mm th ickness (2h) wou ld be o f the order o f a few tens o f hours at 70C . It wou ld be 6e20 t imes longer at amb ient temperature depend ing on the act ivat ion energy o f d i f fus ion (presumab ly 30e50 k J /mo l) . These data suggest that most o f the water uptake resu lts from water absorp t ion dur ing the in it ia l ho ld ing stage wh ich is presumab ly not very d i f ferent from the character ist ic t ime o f d i f fus ion . In contrast , th is is s ign ificant ly longer than the character ist ic t ime ,t

(Eq .(16)) , o f coo l ing , wh ich is defined as :

t c ¼ 1 T dT

dt

1 (16)

At 70 C , thet

is o f the order o f 10

s compared w ith 10

s for d i f fus ion . It thus appears that temperature changes are presumab ly too fast to induce important changes in the water concentrat ion . Indeed , the d i f ference betweent

andt

increases when the tem- perature decreases . In other words , we can suppose that most o f the water transport occurs in the hot /wet stage o f the cyc le . The water d istr ibut ion in the samp le th ickness is rap id ly“frozen ”when the temperature beg ins to decrease .

Eq .(15)suggests interest ing supp lementary in format ion . The mass uptake in the second stage is 4 .17 .10

g o f water per in it ia l gram o f po lymer per cyc le . Th is quant ity corresponds to about F ig . 5 .Re lat ive vo lume change in re lat ion to the number o f cyc les .

F ig . 6 .Compar ison between the in-p lane hydr ic stra in and the g loba l stra in der ived

from the vo lume change assum ing isotrop ic hygroe last ic behav ior .

2 .8 .10

mo l /L . cyc le . Accord ing to [27], the water so lub i l ity in- crease contr ibutes about a th ird to the mass uptake measured in the second stage at 50 C . The actua l durat ion ,t

, o f the ho t /wet p lateau o f the cyc le is 15 m in 5 m in . Assum ing that most o f the hydro lys is events occur dur ing th is p lateau and that on ly 2 /3 o f the mass increase resu lts from hydro lys is , we est imate the hydro lys is rate ,r:

r¼ 2

3 2: 8 10 ⁴

t _H w2: 1 10 ⁷ mo l: L ¹ s ¹ (17)

Th is va lue is s l ight ly h igh compared , for instance , w ith that for unsaturated po lyester (2e15) 10

mo l .L

s

at 100 C and approx imate ly 7 t imes lower at 70 C (w ith an act ivat ion energy o f about 70 k J /mo l[36]) . Th is means that the spec ies undergo ing hydro lys is is more react ive w ith water than is a common a l iphat ic or aromat ic ester .

F ina l ly , property changes can be interpreted , at least atfirst g lance , on the bas is o f the fo l low ing s imp le scenar io : thefirst stage , o f a durat ion presumab ly shorter than 200 cyc les , is dom inated by F ig . 8 .The indentat ion modu lus in re lat ion to the number o f cyc les .

F ig . 9 .Hardness in re lat ion to the number o f cyc les .

F ig . 7 .Var iat ion o f the g lass trans it ion temperature in re lat ion to the number o f cyc les .

F ig . 10 .Max imum creep d isp lacement dur ing the indentat ion test in re lat ion to the

number o f cyc les .

the phys ica l sorp t ion o f water . Dur ing th is stage , the to ta l water concentrat ion in the po lymer increases , hydro lys is beg ins but its convers ion rema ins low and its e f fects on propert ies are neg l ig ib le . The ma in phys ica l processes a f fect ing po lymer propert ies are p last ic izat ion and swe l l ing . At the end o f thefirst stage , sorp t ion has reached its equ i l ibr ium and the to ta l water concentrat ion w i l l undergo changes l inked , for instance , to the bu i ld-up o f h igh ly hydroph i l ic groups (a lcoho ls and ac ids) resu lt ing from hydro lys is . 4 .2 . P las t ic iza t ion

Cons ider ing that the equ i l ibr iumT

va lue is 147 C and the mass uptake at equ i l ibr ium is 1 .75% , theT

depress ion per percent is : D T

= D mw 12 .6 K /% . Th is va lue is about doub le the theoret ica l va lue (7 K /percent) accord ing to Eq .(3). Th is is not surpr is ing , g iven that water is a pecu l iar so lvent because its m isc ib i l ity w ith other sub- stances is ma in ly governed by the poss ib i l ity o f estab l ish ing very strong secondary (hydrogen) bonds w ith the po lar groups o f the partner . What we see here is that water seems to be a more e ffic ient p last ic izer than wou ld be a non-po lar or moderate ly po lar so lvent hav ing the sameT

.

I fT

¼ fðNÞe f fect ive ly d isp lays a secondary max imum dur ing the second stage , we can re ject the hypo thes is o f secondary cross l ink ing proposed by Zhou and Lucas[13]in a s im i lar case because o f the occurrence o f“decross l ink ing”. The decrease a fter 1000 cyc les wou ld be extreme ly d i fficu lt to just i fy . It seems to us better to env isage an e f fect o f the swe l l ing process occurr ing in th is stage .

4 .3 . Mechan ica l proper t ies

F ig . 11a shows that the indentat ion modu lus is not corre lated w ith the mass uptake . Indeed , a fter 250 cyc les , the mass o f the samp le cont inues to increase wh i le the modu lus s l ight ly recovers . On the contrary ,F ig . 11b po ints out a remarkab le corre lat ion be- tween the g lass trans it ion temperature and the e last ic modu lus . It can be reasonab ly postu lated that both evo lut ions resu lt from the same processes .

The rap id modu lus decrease dur ing thefirst stage (F ig . 8)is usua l ly interpreted in terms o f p last ic izat ion but th is is not obv ious . As a matter o f fact , in severa l cases , as we l l in l inear po lymers as in epoxy networks , p last ic izat ion is accompan ied by a modu lus in- crease in the temperature interva l between thefirst sub-g lass trans it ion (T _b ) and T

[37]. Th is phenomenon , ca l led ant i- p last ic izat ion , cou ld however be spec ific to po lymers d isp lay ing an intense d iss ipat ion band at ( T _b ) , wh ich is not the case for anhydr ide-cured epox ies and more genera l ly tr id imens iona l po ly- esters . It is d i fficu lt to interpret th is sma l l (max imum 7%) change in terms o f mater ia l structura l changes on ly , because dur ing th is per iod , the water is not homogeneous ly d istr ibuted across the samp le th ickness , wh ich generates a stress state and can mod i fy the mater ia l’ s mechan ica l response .

About ha l f o f the in it ia l decrease is recovered dur ing the second stage . Here a lso , it seems reasonab le to estab l ish a re lat ionsh ip between th is phenomenon and the swe l l ing that occurs dur ing th is stage . It shou ld be noted that the evo lut ion o f the modu lus is ano ther e lement to re ject the hypo thes is o f secondary cross l ink ing [13]because it wou ld not be sens it ive to cross l ink ing at room temperature . It is temp t ing to suppose that a l l the changes that occur between about 250 and 1000 cyc les are l inked to trans ient processes and that th is va lue o f 5% corresponds to an equ i l ibr ium state correspond ing to an equ i l ibr ium concentrat ion (1 .75%) o f phys ica l ly sorbed water . Dur ing the th ird stage (beyond 1000 cy- c les) , the modu lus rema ins a lmost constant , d i f fer ing from the in it ia l va lue by about (5 0 .6)% .

In add it ion , the hardness is re lat ive ly we l l corre lated to the modu lus : HwE=10 . The max imum creep d isp lacement dur ing the indentat ion test is more sens it ive than the modu lus or the hardness because it increases by about 20% dur ing thefirst stage . Then , it rema ins a lmost constant for the rema inder o f the ag ing campa ign . 4 .4 . Vo lume tr ic proper t ies

The curve o f vo lume change (F ig . 5) has the same shape as the mass uptake one , at least unt i l 1000 cyc les . The vo lume increases rap id ly in thefirst stage at a cont inuous ly decreas ing rate . It con- t inues to increase in the second stage at a reduced but a lmost constant rate . Dur ing thefirst stage , water penetrat ion in the samp le is accompan ied by a contract ion because the vo lume in- crease is sma l ler than the vo lume o f ( l iqu id) water absorbed (F ig . 12) .

The curve inF ig . 12and the evo lut ion o f dens ity (F ig . 13) can be

schemat ized into a three-stage process . Dur ing thefirst stage ,

wh ich corresponds to the first cyc les , swe l l ing is low . Th is type o f

behav ior has been observed in severa l po lymers among wh ich are

F ig . 11 .The emp ir ica l re lat ionsh ip between the indentat ion modu lus and (a) the mass

uptake and (b) the g lass trans it ion temperature .

epox ides[38 ,39]and po lysu l fones[40]. Thefirst stage ( few cyc les) is dom inated by samp le“fil l ing ”w ith no or l itt le swe l l ing . In fact , desp ite the observed contract ion (accord ing to Eq .(4), the inter- act ion coe ffic ient c wou ld be negat ive) , there is g loba l ly a vo lume increase ( i .e . 0> c >V

) and thus swe l l ing stress e f fects . Dur ing the second stage , the vo lume increase is a lmost equa l to the vo lume o f the ( l iqu id) water absorbed . Th is stage stops a fter about 1000 cyc les . In th is second stage , we can est imate the coe ffic ient o f hy- dr ic expans ion ( b ^{) , as dep icted inF ig} . 14. Hence , the second stage is dom inated by swe l l ing dur ing wh ich the po lymer-water system approaches phys ica l equ i l ibr ium .

W ith longer exposures (th ird stage) , there is no more samp le expans ion , wh ich is cons istent w ith the hypothes is that a l l the supp lementary water absorbed in th is per iod is chem ica l ly bound to the po lymer and does not part ic ipate in swe l l ing . Hence , the th ird stage is dom inated by chem ica l incorporat ion o f water into the macromo lecu les w ith no swe l l ing .

5 . Conc lus ion

In th is paper , we descr ibed in deta i l the ag ing behav ior o f an anhydr ide-cured epoxy res in under hygrotherma l cond it ions . We reported important resu lts on the sorpt ion k inet ics , the thermo- mechan ica l and hygroe last ic behav iors , and the g lass trans it ion temperature . The d i f ferent stages observed in the evo lut ion o f the

targeted quant it ies (m

,T

,E

, etc .) ind icate the comp lex ity o f the ag ing process . We hence proposed a degradat ion scenar io that is cons istent w ith our exper imenta l observat ions .

Our l ine o f reason ing is ma in ly based on d imens iona l mea- surements , part icu lar ly on the re lat ionsh ip between the vo lume change and the water uptake . We cons idered th is resu lt as the most re levant to descr ib ing the comp lex evo lut ion o f the po lymer’ s propert ies dur ing cyc l ic hygro therma l ag ing . Three ma in stages were ident ified :

The“induct ion stage ”dur ing wh ich the water that penetrates into the po lymer has a sma l l swe l l ing e f fect . Th is resu lts from the compet it ion between water- induced swe l l ing and the contract ion e f fect character ized by the“interact ion coe f fi- c ient”, c .

The“swe l l ing stage”dur ing wh ich the stra in increases a lmost l inear ly w ith water concentrat ion , w ith the coe f fic ient o f hydr ic expans ion be ing : b ¼ 5 .23 10

m

/mo l (the water concen- tra t ion be ing expressed in mo le per m

) .

The“equ i l ibr ium stage”dur ing wh ich the supp lementary mass up take is due to water incorporat ion to macromo lecu les as a resu lt o f hydro lys is . Th is water does not contr ibute to swe l l ing . References

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0 0 .005 0 .01 0 .015 0 .02 0 .025 0 .03 0

0 .005 0 .01 0 .015 0 .02 0 .025 0 .03

Vo lume of wa ter absorbed

Vo lu me c ha ng e

0 Cyc le

250 Cyc les

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0 500 1000 1500 2000 2500 3000 1 .225

1 .23 1 .235 1 .24 1 .245

Number of cyc les

De nsi ty ( g/ c m3 )

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