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The Effect of Thermooxidative Aging on theDurability of Glass Fiber-Reinforced Epoxy

Article in Advances in Materials Science and Engineering · December 2015

Impact Factor: 0.74 · DOI: 10.1155/2015/372354

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ability_of_Glass_Fiber-Reinforced_Epoxy?enrichId=rgreq-82ffcf39-43de-4147-b211-a94bee037ae1&enrichSource=Y292ZXJQYWdlOzI4ODAwMzQ2NztBUzozMTA1OTczOTkzMTg1MjhAMTQ1MTA2MzU5ODg3MA%3D%3D&el=1_x_3https://www.researchgate.net/publication/288003467_The_Effect_of_Thermooxidative_Aging_on_the_Durability_of_Glass_Fiber-Reinforced_Epoxy?enrichId=rgreq-82ffcf39-43de-4147-b211-a94bee037ae1&enrichSource=Y292ZXJQYWdlOzI4ODAwMzQ2NztBUzozMTA1OTczOTkzMTg1MjhAMTQ1MTA2MzU5ODg3MA%3D%3D&el=1_x_2


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Research ArticleThe Effect of Thermooxidative Aging on the Durability of Glass Fiber-Reinforced Epoxy

Amin Khajeh,1 Faizal Mustapha,1,2 Mohamed Thariq Hameed Sultan,1,2

György Bánhegyi,3,4 Zsuzsanna Karácsony,4 and Viktor Baranyai4

Department of Aerospace Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Aerospace Manufacturing Research Center (AMRC), Level , ower Block, Faculty of Engineering, UPM,

Serdang, Selangor, Malaysia Medicontur Medical Engineering Ltd., Herceghalmi Road, Zsámbék , Hungary Bay Zoltan Nonprot Ltd. for Applied Research, Fehérv ́ari Út , Budapest , Hungary

Correspondence should be addressed to Amin Khajeh; [email protected]

Received September ; Accepted November

Academic Editor: Katsuyuki Kida

Copyright © Amin Khajeh et al. Tis is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Tin-skinned organic matrix composites within aeronautical structures are subjected to thermooxidative aging during their servicelie, leading to reductions in their durability. In this paper, a durability evaluation o berglass epoxy prepreg is perormed on

the original composite thickness beore and afer h isothermal aging at ∘

C. Te characterization o both aged and unagedcomposites comprised tensile tests,DMA, FIR, weightloss measurements, SEM, and DSC. Te tensile strength and modulus o thecomposites increased afer beingexposed to pronounced aging conditions, whereas a decrease was observed in the toughness. DMAresults revealed that the glass transition temperature and rubbery state modulus increased as a result o the thermooxidative aging.FIR spectroscopy demonstrated the ormation o carbonyl compounds due to oxidation o the chemical structure o the resin.SEM observations indicated the existence o minor supercial cracking and poor ber-matrix adhesion afer aging. In addition, aminor mass change was observed rom mass loss monitoring methods. Te overall ndings suggest that postcuring and physicalaging enhanced thebrittlenesso theresin, leading to a signicant decline in theuseulstructural lie o thethin-skinnedcomposite.

1. Introduction

Te importance o persistently improving civil aircraf per-ormance and improving uel consumption has resulted innovel improvement in the designs o aeronautical structures.Tis improvement has also led to a drastic growth in theapplication o advanced materials. Consequently, aeronauti-cal industries have begun to use thin-skinned polymer matrixcomposite materials in aircraf construction. Te Boeing and Airbus A aircrat pioneered this innovative structuraldesign. Such high speed civil aircraf offer considerablepromise o being able to withstand long-term (, h)exposure [–] to temperatures ranging rom −to∘C[]and being sufficiently ductile to absorb low velocity impactenergies [–]. However, during thermal aging at moderate

temperatures, or example, below the glass transition tem-perature, organic matrix composites deteriorate by matrix

embrittlement resulting rom thermooxidative degradation[]. Importantly, because oxidation is a diffusion-relatedphenomenon [], using thin-skinned composites may accel-erate the onset o observable changes in the mechanical,thermomechanical, and chemical properties that may resultin lowered damage tolerance and structural integrity. As aresult, investigating thin-skinned composite durability at theearly stages o thermooxidative degradation is a crucial issuethat needs to be understood to avoid premature in-serviceailures.

Termal-oxidative aging may irreversibly change thechemical structure o polymer matrix composites [, ].Alterations to the chemical structure during thermooxidative

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2015, Article ID 372354, 13 pageshttp://dx.doi.org/10.1155/2015/372354

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Advances in Materials Science and Engineering

degradation include postcuring [, , , –], loss o volatiles [, –], dehydration [, , ], chain scission[, –], additional cross-linking [, ], and carbonylgrowth [, ]. Te initial chemical changes are accompa-nied by dehydration o secondary alcohols and the releaseo low molecular weight gaseous species due to random

chain scission. However, at moderate heat exposures (

∘

C–∘C), chain cross-linking is the dominant chemical changein the matrix compared to chain scission []. Indeed,the increase in the cross-link density o the cured matrixprimarily occurs during the initial aging period and is causedby postcure reactions [, , , ], which result in theexcessive brittleness o the matrix []. As aging proceedsin the presence o oxygen, susceptible chemical structuresin the resin are oxidized to various carbonyl containinggroups. Tereore, matrix embrittlement increases with theoxygen concentration and aging time [, ]. In additionto thermooxidative aging, the effects o physical aging [–] should be considered. Physical aging is universal, isindependent o any chemical change, and is related to thegradual densication o nonequilibrium glassy structures.Tese properties result in a higher modulus and strength,lower toughness, and slower relaxation (creep and stressrelaxation). Te aging is also accompanied by enthalpy relaxation. Physical aging can be accelerated by annealing,such as a thermal treatment below but not too ar rom theglass transition temperature. Tese parameters are exactly the conditions used or mild oxidative aging. Te effects o postcuring and chemical and physical aging are inextricably combined in epoxy resins.

Te thermomechanical behavior in organic matrix com-posites can be changed as a result o postcure reactionsand physical aging induced by thermooxidative degradation.More precisely, the cross-linking density is increased by acontinued postcure reaction and physical aging processesrestrict the molecular movement in the main chain o thematrix [, , ]. Te reduction in the molecular mobility results in an increase in the glass transition temperature[, , –] and the storage modulus [, –]. In act,an evaluation o the viscoelastic behavior can detect changesin the state o the molecular motion as unctions o timeand temperature [, ], which are responsible or relativestiffness in polymer matrix composites [, , ]. Storagemodulus and the peak, shape, and the area under tan curveare typically used as indicators in studies on molecular cross-linking [].

Matrix densication affects the mechanical propertieso OMCs (organic matrix composites) in the early stageso oxidative aging and is induced by physical aging and/orchemical aging (postcuring). Research conducted by sotsiset al. suggests that the matrix dependent properties areenhanced during the postcure stage and matrix degradationresulted rom densication or increased chain scission inthe thermosetting matrix polymers, respectively []. Otherstudies revealed that postcure reactions enhance the ber-dominant properties, such as the tensile strength and tensilemodulus [, , , ], and decrease the matrix dominatedproperties, such as toughness [, ]. Similarly, an increasein the tensile strength [, ] and modulus [–] and a

reduction in the toughness [] caused by physical aging havebeen observed by several researchers.

Te thermooxidative degradation o organic matrix com-posites has been observed [, , , , , ] to beaccompanied by the oxidation o the topmost surace o thematrix and by the ormation o supercial microcracks. In

act, with continued aging, void ormation occurred [, ]and cracks developed rom crazing by increasing extensionalstress [].A considerable amount o literature concludedthatchemical shrinkage[, , , –]or notable variations inthe matrix and ber thermal expansion actors [, ] may provide dilatational stresses (internal stresses) or synergisticeffects o both. In addition, in morphological investiga-tions, ber-matrix interacial adhesion also degraded andweakened as a result o moderate heat exposure [, –],resulting in ber pullout ailures as part o the racture mode[–].

In this study, the inuence o thermooxidative degrada-tion on the durability o glass ber-reinorced epoxy compos-ite is evaluated rom mechanical, chemical, and physical per-spectives. Tis work seeks to relate the premature alterationsin the thin-skinned composite panel to the deterioration o itsdurability. Examining thermal-oxidative degradation using aprinciples-based approach and by considering the originalcomposite thickness aids in assessing the actual toughnessbehavior available or the use condition by mechanical test-ing. As a result, measuring the toughness value by calculatingthe area under the stress-strain curve method was usedinstead o the Charpy impact test due to the subsize thicknesso the material available. Furthermore, the methodology introduced here is based on industrial interest because thethermooxidative aging o such thin-skinned composites hasnot received a great deal o attention. Tereore, this study is anticipated to contribute to the body o literature onthe materials used or the manuacturing o aircraf andespecially to the durability o the materials in terms o thepronounced short-term thermal-oxidative degradation.

Tis study uses aging o plain GFRPs (glass ber-reinorced plastics) laminates exposed to air or h at atemperature o ∘C. Various tests are conducted to char-acterize the deterioration o the material, including ten-sile testing, dynamic mechanical analysis (DMA), Fourier-transormed inrared spectroscopy (FIR), weight loss mea-surements, Scanning Electron Microscopy (SEM), and differ-ential scanning calorimetry (DSC).

2. Material

Te primary material used in the manuacturing o theplain laminates is the EHG-- prepreg. EHG-- prepreg is comprised o an E-glass abric impregnatedwith theDGEBAbased epoxy resinEH andmanuacturedusing the US standard. Te epoxy resin is a product o the Gurit Company. Based on the recommendation o themanuacturer, the laminates were cured in an autoclave [].Te length o the panel coincided with the plain weave onthe wef direction. Tis coincidence stems rom the properstacking orientation o the our prepreg plies. Beore cuttingthe mm thick panels into appropriate test pieces using a


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CNC machine, the material was inspected using an ultrasonicC-scan to identiy any deects.

3. Experimental

.. Isothermal Aging. In accordance with the ASM

D/DM standard, the samples used in this study were dried to a constant weight under vacuum conditionsat a temperature o ∘C beore aging []. Tis preliminary condition is essential because the matrix composite ishydrophilic in nature and variation in weight during agingserves as an oxidation index. o begin, the specimens wereisothermally aged in an air circulating oven or a period o h at ∘C. Te chosen temperature is slightly above thestandard use temperature (∘C) and is below (

∘C inMA). For the sake o preventing damage to the specimenedges during pronounced thermooxidative aging, theedges were sealed using aluminum oil bonded with Dow Corning silicon rubber, which is stable at temperatures

in excess o

∘

C. In addition, the relative humidity inthe oven was reduced using phosphorus pentoxide. Tisthermal condition acts as an articial thermooxidative agingcondition to reproduce the use condition or the short-termapplications.

.. ensile est. Using an Instron- universal testingmachine set at a mm/min crosshead speed, the statictensile tests were perormed in accordance with ASMD/DM []. Strain gauges were attached in the lon-gitudinal direction (the wef direction axis o the specimen)to measure the tensile strain. Te nal values were taken roman average o the ve specimens.

.. Dynamic Mechanical Analysis (DMA). A dynamic ther-mal analyzer (A instrument model Q) was used toconduct the thermal mechanical tests on both the aged andthe unaged samples according to the ASM D-estandard []. A three-point bending geometry was used toapply deormation. Te experiment was also conducted inscanning temperature mode, ranging rom 25 ± 2 to ∘C.Other conditions include a ∘C/min heating rate and a . Hzoscillating requency.

.. FIR Spectroscopy. From the aged and unaged samples,approximately mg o the previously dried composite was

used or the FIR. Te milled sample was pressed into a discafer being mixed with approximately mg o anhydrousPotassium Bromide (KBr). Spectra were acquired in thetransmission mode using a Termo Nicolet Nexus FIR spectrometer. Te disc analysis was perormed by treating theKBr as the background reerence. Acquisition o the spectra

was perormed over a range rom to cm−1. Afer-ward, three separate analyses were conducted on each palletat different locations to ensure the results were consistent.

.. Weight Loss Measurements. o monitor the weight lossrom the samples over the h oxidative aging at∘C, threespecimens were cut rom the original composite panel with a

0

50

100150

200

250

300

350

400

U l t i m a t e t e n s i l e s t r e n g t h ( M P a )

Unaged

Aged

F : Change in ultimate tensile strength o EHG--composite specimens subjected to h oxidative aging at ∘C.

width o approximately mm and a length o approximately

mm. In a periodic manner, the samples were removed romthe oven, placed in a desiccator, and allowed to cool to roomtemperature. Te samples were then weighed and returnedto the oven. Weighing was perormed using a microbalanceaccurate to within g. Each data point reported is anaverage o three separate specimens weighed independently.

.. Scanning Electron Microscopy (SEM). A Hitachi SUSEM was used to investigate the supercial matrix mor-phology. Additional SEM analyses were perormed on thecryoractured specimens using liquid nitrogen to study theracture behavior o the composites and possibly any matrix-

ber adhesion. In both morphological investigations, thesuraces o the specimens were sputter coated with gold.Additionally, observations were conducted on at least threedifferent locations on the samples to ensure that the reportedmicrophotograph represented the typical morphology.

.. Differential Scanning Calorimetry. Te DSC measure-ments were perormed using a DSC systemmanuacturedby Mettler oledo with an air ow o mL/min. A tempera-ture scan wasperormedon mg prepreg samples (EHG--) rom ∘C to ∘C at a heating rate o ∘C/min.

4. Results and Discussion

.. ensile est. Te comparison o both aged and unagedsamples based on changes in the tensile strength andmodulusis shown in Figures and , respectively. Te tensile strengthand Young’s modulus substantially increased under the pro-nounced aging conditions by % and %, respectively.Tese changes may be attributed to the increase in the cross-linking density o the epoxy matrix as a result o simultane-ous postcuring and physical aging during thermooxidativedegradation [, ]. o be more precise, by increasingcross-linking density, the cross-linked network within theepoxy matrix will bear the majority o the induced loadsin the composite material []. Consequently, the tensile


5/15


Unaged

Aged

0

5

10

15

20

25

30

Y o u n g ’ s

m o d u l u s ( G P a )

F : Change in Young’s modulus o EHG-- compositespecimens subjected to h oxidative aging at ∘C.

strength and modulus o elasticity rose signicantly over the

prolonged aging period.However, a .% drop in strain-to-ailure value was

observed afer pronounced oxidative aging process, as shownin Figure . Furthermore,the toughnessmeasuredby the areaunder the stress-strain curve decreased considerably by %afer h aging at ∘C compared to original sample, asshown in Figure . Tis remarkable loss in toughness andstrain to ailure may be explained by the embrittlement o theepoxy matrix induced by postcuring [], physical aging [],and the oxygen concentration [, ].

.. Dynamic Mechanical Analysis (DMA). Te inuence o thermooxidative aging over a period o h at ∘C onproperties, such as the glass transition temperature (),the onset temperature, mechanical damping (tan ), and thestorage modulus (), was studied by conducting dynamicmechanical thermal analyses. Te glass transition temper-ature () is taken as the temperature at which the lossactor is maximized. Additionally, the onset temperature isdetermined rom an inexion point in the modulus curve.Tereore, this measurement is less accurate than whenmeasured rom the tan maximum temperature. However,the onset temperature is used more by aircraf manuacturerscompared to the glass transition temperature () [–].

As shown in Figure , the glass transition temperature() increases moderately over the period o induced ther-

mooxidative aging. Additionally, tan peak in the aged com-posite is relatively narrow compared to the original sample.able shows a quantitative comparison o the area undertan peak between the aged and unaged samples. Te resultsshow a % reduction in the area under the damping peak afer the induced thermal aging. Moreover, there appearsto be doubling o the loss peaks in the unaged specimensat higher temperatures. Tis nding may be attributable toan inhomogeneous network structure or to the diffusioncontrolled oxidation process being stronger near the suraces.

Te behavior o the dynamic storage modulus () duringthe pronounced thermooxidative degradation is shown inFigure . A minor increase is observed at the glassy state

0

50

100

150

200

250

300

350

400

T e n s i l e s t r e s s ( M P a )

Unaged

Aged

10.5 1.50 2

Strain (%)

F : Stress-strain curves on EHG-- laminates beoreand afer h oxidative aging at ∘C.

Unaged

Aged

0

5

10

15

20

25

30

35

40

45

T o u g h n e s s ( J )

F : Change in toughness value o EHG-- compositespecimens subjected to h oxidative aging at ∘C.

: Changes o tan curve o EHG-- compositesamples induced by h thermooxidative degradation.

Sample tan peak (∘C) tan peak areaa (min)

Unaged . .

Aged . .

aArea value indicates the area under tan peak.

compared with the original sample. Moreover, a trend in theincreased rubbery state with induced thermal aging is seen(Figure ).

Figure shows the changes in the onset temperature aferthe h thermal-oxidative process at ∘C compared tothe original composite. Te onset temperature drops rom.∘C to .∘C, which is close to the maximum servicetemperature recommended by the manuacturer [].

Te observations above may be explained as ollows. Tealteration in , tan peak, and the storage modulus implies


6/15


0 100 200 300

Temperature (∘C)

129.72∘C

115.09∘C

Unaged

Aged

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

T a n

F : Alteration o tan peak o EHG-- compositesamples a h thermooxidative aging at ∘C.

0 100 15050 200 250 300

Temperature (∘C)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

S t o r a g e m o d u l u s ( M P a )

Unaged

Aged

F : Viscoelastic trend o storage modulus beore and afer h oxidative aging at ∘C.

that the cross-link density o the matrix network’s structure isbeing enhanced by the increasing temperature (as a result o postcuring and physical aging). Indeed, matrix densicationresults in the restriction o the molecular mobility, leadingto a rise in the storage modulus and the glass transitiontemperature. However, the area under tan peak decreasesafer the induced aging because o the increased cross-linking

and steric hindrance in the matrix, which leads to a decreasein the damping capability (dened as the ability to absorb anddissipate energy or as the toughness) o the composite duringpronounced aging conditions. Tis may be in accordancewith the decrease in toughness observed in the tensile tests.Furthermore, the results rom the onset temperature revealthat the thermal durability o the composite drops by approx-imately ∘C. Tis, in turn, leads to premature mechanicalailures.

.. FIR Spectroscopy. In the hydroxyl regions (≈–

cm−1), we observe that or the pronounced aging con-

dition the broad band o the spectra is centered at cm−1

0 100 15050 200 250

Temperature (∘ C)

6000

8000

10000

12000

14000

16000

18000

20000

S t o r a g e m o d u l u s ( M P a )

Unaged

Aged

83.58∘C

80.58∘C

Un-aged

Aged

Universal V4.5A TA instruments

F : Changes in the onset temperature in EHG--composite samples (initially and afer h thermooxidative agingin air at ∘C).

A b s o r b a n c e

Unaged

Aged

3425

2964

2926

2870

3500 3300 3100 2900 2700 25003700

Wavenumber (cm−1)

F : FIR spectra o EHG-- composite sample inthe hydroxyl and CH stretching regions (initially and afer hthermooxidative aging).

(corresponding to O-H stretching o the hydroxyl group)

and the peak at cm−1 (corresponding to C-H stretchingo the methylene group) decreases in intensity, as shown inFigure . Additionally, a decrease in the absorption peak at

cm−1 is noted as a result o the aging process (Figure ).

Tis decrease is responsible or the vibration o C-C-O asso-ciated with the secondary alcohol. From the changes in thepeaks, we can deduce that the degradation o the epoxy resinoriginated in the -C-C(OH)-C- linkages. A theory was alsoormulated that the degradation o the epoxy resin secondary alcohol structure might be associated with the dehydrationreaction that leads to the ormation o C=C bands [–].A similar idea was also put orth where the degradation o the epoxy secondary alcohol structure may be attributed tothe thermal-oxidative reaction that may lead to the ormationo peroxides [, ]. From the FIR observations, oxygendid not attack the material during the oxidative aging processto amine groups to orm amide compounds. However, at


7/15


A b s o r b a n c e

1737

16431607

1580

1558

1509

1182

10951038

830

1800 1600 1400 1200 1000 800 600 4002000

Wavenumber (cm−1 )

Unaged

Aged

F : FIR spectra o EHG-- composite sample inthe ngerprint (– cm−1) domain (initially and afer hthermooxidative aging).

A b s o r b a n c e

Unaged

Aged

1600 1580 1560 1540 15201620

Wavenumber (cm−1)

F : FIR spectra o EHG-- composite sample rom to cm−1 (initially and afer h thermooxidative aging).

approximately cm−1, moderate growth was observed inthe intensity o the peroxide group.

Te IR spectra o theEepoxysystem in the ngerprintregion are shown in Figure . Although the absorption band

at cm−1 diminished or disappeared during the curingprocess, absorption o the N-H group in the curing agent

at cm−1 and cm−1 was observed in both the agedand virgin samples. However, as the aging time increased,

the absorption band at cm−1 and cm−1 decreased,

as shown in Figure . Te reduction in these bands may be attributed to the moderate temperatures associated withthe thermooxidative aging advancing the mobility o themolecular chains, which increases the reaction probability between the resin and the curing agent (known as theprobability o densication) [].

In the carbonyl region (≈– cm−1), we observe

that the absorbance o the band at cm−1, which cor-responds to the characteristic stretching o C=O in phenyl,slightly increased in intensity (Figure ). Te ormation o carbonyl groups may result rom the oxidation o aromaticether groups in the resin [, ]. Tis oxidation process isaccompanied by a slight reduction in the intensity o the

A b s o r b a n c e

1580

1509

1737

1607

1558

1643

Unaged

Aged

1850 1800 1750 1700 1650 1600 1550 15001900

Wavenumber (cm−1 )

F : FIR spectra o EHG-- composite sample in thecarbonyl region (initially and afer h thermooxidative aging).

200 400 600 8000

Aging time (hours)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

W e i g h t l o s s ( % )

F : Weight loss o EHG-- composite sample during hours o aging at ∘C.

aromatic ether group, approximately cm−1 []. Teoverall oxidation rate is very slow, signicantly more so thanunder the high temperature oxidation conditions [], asshown by the very moderate increase in the carbonyl band,which is a sensitive measure o the degree o oxidation in allorganic polymers.

.. Weight Loss Measurements. Te weight loss percentageexperienced by the composite specimens during the h o thermooxidative aging at ∘C is depicted in Figure . Te

weight loss rises sharply during the initial h o aging. Tismay be attributed to desorption o the residual moisture and

volatiles and is not related to polymer degradation [, ].A aster weight loss rate is evident over the to hrange. Over this period, the mass loss rate reaches a peak o .% at h beore staying constant over the remaindero the aging. Tis accelerated weight loss may occur due tothe appearance o supercial cracks in the samples, whichprovide an increased surace area available to react withoxygen [].

.. Scanning Electron Microscopy (SEM). Afer h o ther-mooxidative degradation at ∘C, notable changes occurred


8/15


F : SEM micrograph o an unpolished surace afer agingtreatment.

on the sample surace. First, a SEM micrograph showingthe microstructure o an unpolished sample is shown inFigure in the backscattered electron mode. Due to di-

erent phase contrasts in the scattered electron distribution,oxidized carbonaceous layers appear as white spots on thematrix surace in contrast to the original resin surace.Next, a signicant increase in the size and concentrationo voids (indicated by arrows) in the aged sample becomesevident (Figure (b)), which may be explained as a resulto the thermooxidative degradation process. In addition, asupercial crack is observed in Figure (b), which is a resulto crazing rom local stress concentrations and/or regionso microstructural inhomogeneity. Indeed, crazing ormsrom interlocked voids with extremely tightly drawn brils[]. By increasing the extensional local stresses and matrixembrittlement induced by the elevated temperatures or aperiod o time, the brils are unable to absorb the dilatationalthermal stress, which leads to the ailure o brillated crazingstructures and crack propagation.

Te microscopic observations in the core cross-sectionalarea show the marginal effects o thermal degradation on thedamage mechanisms that result in matrix shrinkage com-pared to the virgin sample (Figure ). Te resin shrinkage isrelated to the physical aging (densication) and/or volatiliza-tion o small molecular compounds during the postcureprocess. However, no evidence o ber-matrix debondingcaused by matrix shrinkage was observed. Tereore, thelocal stresses induced by resin shrinkage are insufficientto overcome ber-matrix interacial bonding but may besufficient to weaken it.

Figure illustrates the cryoractured sections o boththe aged and unaged samples. An interesting observationrom Figures (a) and (b) on the unaged sample is thatthe suraces o the glass laments have delicate coating o resin. Tis implies there is good bond strength between thematrix and the ber. However, afer temperature-induceddegradation, the matrix completely detached rom the la-ments and ber bundle, as shown in Figures (c) and (d),respectively. In addition, ber pullout rom the matrix wasobserved, as depicted in Figures (e) and (). All theailure modes take the orm o interacial debonding, and theber pullout may be attributed to decreases in the interacialber-matrix adhesion resulting rom resin embrittlement as a

(a)

(b)

F : SEM observation on the polished matrix surace beore(a) and afer (b) h thermooxidative aging at ∘C.

result o thermal degradation. It is worth mentioning that thepoor interacial ber-matrix bonding causes a reduction in

interacial dominant properties, such as the toughness. Tisreduction, in turn, leads to reduced damage tolerance andlack o long-term durability.

.. Differential Scanning Calorimetry. In addition to theDMAand FIR analyses,DSC curves were also used to detectthe possible postcuring and physical aging effects.

Figures and show the DSC curves or theunaged andaged specimens, respectively.

Te differences are summarized as ollows:

(i) calculated rom the midpoint curved baseline

(Δ) shifs rom . to .∘C afer aging, whichmay indicate postcuring. Tese values shouldbe compared to tan maxima shown in Figure (corresponding to . and .∘C). It is wellknown that DSC usually determines the glass tran-sition to be at lower temperatures than the DMA.Te increase in is considerably larger in the DMA

(approximately ∘C) than in the DSC (approximately .∘C). However, it should be noted that the DMAtan curve o the aged specimen shown in Figure is doubled, and there is a subcomponent close to theoriginal one, which may correspond to the transitiondetected by the DSC.


9/15


(a) (b)

F : SEM micrographs o ber ends at the cross-sectional area o unaged (a) and aged (b) sample.

F : SEM observation on the cryoractured surace o EHG-- composite specimens; (a) ber-matrix adhesion o unagedsample; (b) zoom on ber-matrix adhesion corresponding to (a);(c) ber-matrix adhesion o aged sample; (d) zoom on ber-matrix adhesioncorresponding to (c); (e) matrix detached rom ber bundle o aged sample; () ber pullout o aged sample.


10/15


∧ exo Unaged composite

40 60 80 100 120 140 160 180 200 220 240

(∘C)

0 2 4 6 8 10 12 14 16 18 20

(min)

Glass transitionOnset

Onset

MidpointEndset

Endset74.60∘C

79.78∘C

89.37∘C

Integral

Normalized −1.08 Jg−1

117.79∘C

119.94∘C

121.29∘C

3.43 mJ

0.38 Jg−1

128.08∘C

136.27∘C

153.09∘C

Peak

−9.71 mJ

Onset

Endset

Integral

Normalized

Peak

−0.20

−0.15

−0.10

−0.05

0.00

0.05

0.10

0.15

( W g −

1 )

F : DSC curve o the unaged prepreg specimen.

40 60 80 100 120 140 160 180 200 220 240

(∘C)

Glass transitionOnset

OnsetMidpointEndset

Endset

IntegralNormalized

73.68∘C

81.29∘C

89.01∘C

−10.13 mJ−0.84 Jg−1

109.61∘C

120.44∘C

131.35∘C

1.01 mJ84.37e − 03 Jg−1

131.39∘C

133.60∘C

140.85∘C

Peak

Onset

Endset

IntegralNormalized

Peak

−0.14

−0.12

−0.10

−0.08

−0.06

−0.04

−0.02

0.00

0.02

0.04

( W g −

1 )

∧exo Aged composite

0 2 4 6 8 10 12 14

(min)

16 18 20

F : DSC curve o the aged prepreg specimen.


11/15


(ii) Te post- region o both DSC curves is compli-cated, and there are two broad, subsequent processes:one in the endothermal and one in the exothermaldirection. Additionally, the unaged specimen exhibitsa small but detectable melting peak, which disappearsafer aging. Under these conditions, the selection

o the proper dividing line and the value o theintegral obtained or the subprocesses are somewhatambiguous. Nevertheless, we establish that the sharpmelting disappears, the integral o the endothermremains practically unchanged, and the integral o the exotherm decreases aferaging. Te interpretation(in the absence o the exact chemical composition) istentative at best. Te melting in the unaged specimenmay be due to some unreacted component, whichdisappears afer the thermal aging. Te endothermmay be smeared overshoot attributable to physicalaging (although it is usually sharper and increaseswith annealing) andthe exotherm may be the residualreaction heat due to incomplete cross-linking, whichdiminishes afer thermal aging (postcuring).

5. Conclusions

Tis study was designed based on industrial interests toevaluate the effect o thermooxidative aging on the durability o thin-skinned EHG -- prepreg over a period o h o isothermal aging at ∘C. Te results obtained romthe mechanical, chemical, and physical experiments lead tothe ollowing conclusions:

(i) Te tensile strength andmodulus increase afer aging,whereas the toughness measured by the area under

the stress-strain curve decreases (due to lower elon-gation at break). Tese characteristics are probably attributable to the embrittlement o the matrix resinas a result o postcuring and oxygen concentration.

(ii) Te glass temperature and the rubbery plateau modu-lusincrease, which is clearly indicative o a postcuringprocess.

(iii) According to the FIR observations, postcuring reac-tions occurred as the aging proceeded. Additionally,dehydration reactions cause the ormation o sec-ondary alcohol andaromatic ether groupsin theresin,which are oxidized to carbonyl compounds.

(iv) From the electron microscopy observations, the pres-ence o minor cracks resulting rom postcuring is

veried. However, no skin-core cracking is observed.In addition, matrix embrittlement induced by agingleads to ber pullout and weakening o the ber-matrix adhesion.

(v) Te overlap o thebroadendothermal andexothermalprocess observed above in the DSC make an exactinterpretation difficult, but a diminishing exothermcould be identied and is compatible with postcuring.

Tese ndings suggest that, in general, matrix embrit-tlement resulted rom postcure reactions and physical aging

plays a signicant role in reducing the durability o thin-skinned composite by causing premature ailures. Tis study provides a better understanding o the nominal useul struc-tural lie or such composite materials. However, one sourceo weakness in this research which limited the thermaland FIR analyses was that the chemical structure and

compositional data o the matrix were not revealed by the manuacturer. Finally, it is recommended that urtherresearch investigate the effect that thermooxidative aging hason thechemical structureo thetoughening agent in theresin.

Conflict of Interests

Te authors declare that there is no conict o interestsregarding the publication o this paper.

Acknowledgment

Te authors would like to thank Ministry o Higher Educa-

tion Malaysia or providing the research grants, Fundamen-tal Research Grant Scheme (FRGS) FundamentalResearch Grant Scheme (FRGS) , or this researchwork.

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the effect of thermooxidative aging on the durability of glass fiber reinforced epoxy - 2015 - 15p

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