Composites Science and Technology 100 (2014) 198–203

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Composites Science and Technology

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Effect of filler on the creep characteristics of epoxy and epoxy-basedCFRPs containing multi-walled carbon nanotubes

http://dx.doi.org/10.1016/j.compscitech.2014.06.0110266-3538/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +371 67543120; fax: +371 67820467.E-mail address: Tatjana.Glaskova@pmi.lv (T. Glaskova-Kuzmina).

Tatiana Glaskova-Kuzmina a,⇑, Andrey Aniskevich a, Mauro Zarrelli b, Alfonso Martone b,Michele Giordano b

a Institute of Polymer Mechanics, University of Latvia, 23 Aizkraukles St, Riga LV-1006, Latviab Institute for Composite and Biomedical Materials, National Research Council, Portici, NA, Italy

a r t i c l e i n f o

Article history:Received 14 April 2014Received in revised form 27 May 2014Accepted 6 June 2014Available online 18 June 2014

Keywords:A. Polymer–matrix compositesA. Carbon nanotubesA. Carbon fibresB. Mechanical propertiesB. Creep

a b s t r a c t

The aim of this work was to determine the effect of carbon nanotubes (CNTs) on the elastic and viscoelas-tic properties of an epoxy resin used in carbon fiber-reinforced plastics (CFRPs) in the matrix-dominatedflexural testing mode. Neat and CNTs-containing (1 wt.%) epoxy resin and CFRP specimens were preparedand investigated. Three-point bending tests were carried out on nanocomposite (NC) and CFRP specimensat room temperature in quasi-static and cyclic creep regimes. The main effect of CNTs was observed inthe reduction of creep compliance of epoxy (40%) and CFRP (30%), especially at higher stresses. Thereduction of creep characteristics especially on viscoelastic and plastic strains both for epoxy and CFRPCNTs-containing specimens was attributed to the pulling out of CNTs from the agglomerates upon cyclicand gradually increasing stress in creep.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

CNTs provide new possibilities in improving the functional andmechanical properties of advanced polymer materials due to theirsmall size, high aspect ratio, low mass, and excellent mechanical,electrical, and thermal properties [1–3]. According to numerouspublications and results of investigations, the electrical conductiv-ity of polymers can be improved in the case of the most optimalCNT dispersion by 109 times [1,4,5]. In order to obtain highlyelectrically conductive nanocomposite multi-walled CNTs can becoated with a thin layer of a conductive polymer which becomesthe main electrical conductive path and promotes the formationof a percolation network [7]. By addition of multi-walled CNTsthe thermal conductivity of polymers can be improved even by150% [6] due to the formation of a thermally conductive network.Nevertheless, in the case of functionalized multi-walled CNTs alower than expected improvement in thermal conductivity (by15%) was observed due to scattering processes arising from cova-lent interaction between the multi-walled CNTs and the epoxythrough the functional groups [26]. Moreover the addition of welldispersed multi-walled CNTs into the epoxy matrix can lead to aremarkable effect on the mechanical properties. The Young’smodulus and the yield strength of the 1 wt.% composite have been

increased by respectively 100 and 200% compared to the pureepoxy matrix [5]. Such an improvement in the functional proper-ties of polymeric matrices can be successfully applied for their fur-ther use as nanomodified polymer matrices in fiber-reinforcedpolymer (FRP) structures in lightweight applications, e.g.,aerospace and wind energy applications [6–10].

However, there are still many problems associated with themanufacturing process of a multiscale three-component compos-ites. One of them is the effective and homogeneous dispersion ofCNTs in a polymer matrix before their incorporation to a fibrousreinforcement. Many mechanical techniques have been used overthe last two decades to overcome the dispersion problems, suchas ultrasonication [9,11,12], calendering [13–15], and ball milling[16,17]. Additionally to the techniques mentioned the dispersionof CNTs within the polymer resins can be improved by the chemi-cal route [7,18–20]: functionalization of nanotubes’ walls by reac-tive groups enhancing compatibility with the surrounding mediumor by the physical adsorption of dispersants onto the surface of thenanotubes.

Of the mechanical dispersion techniques mentioned, the high-energy ultrasonication has been widely used to disperse CNTs inlow-viscosity polymers prior to fabrication of a composite [13].However, the duration, power, and temperature need to be con-trolled and adjusted to each certain type of polymer in order toget homogeneous dispersion of CNTs and to prevent prematurecuring of resin [21,22].

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Besides the high stiffness and strength, the matrix materialshould be viscoelastic and/or viscoplastic to eliminate brittle frac-ture of the composite. These properties determine the long-termdeformability and strength of the matrix and consequently of thecomposite. An optimal addition and good dispersion of CNTs withtheir high aspect ratio and interfacial area with an epoxy matrixcan improve its elastic and strength characteristics, as well as todecrease the rate of creep strain at both room and elevatedtemperatures [23]. Much effort has been dedicated to improvingthe performance of laminates in the transverse direction[9,11–14,22]. One of the main factors responsible for the transversebrittleness of laminates is insufficient adhesion at the fiber/matrixinterface, leading to premature debonding [12]. In order to improvethe adhesion between carbon fibers and a polymeric matrix, the lat-ter can be modified with CNTs. The advantage of nanoscale fillers,compared with microscale ones, is their enormous surface area,which can act as an interface for the stress transfer. The efficiencyof interfacial stress transfer in FRPs filled with CNTs has been ana-lyzed in the literature [13,15,24,25], and it was concluded that inter-laminar shear strength could be increased �20% upon addition ofCNTs due to their more effective interfacial interaction with apolymer matrix. Nevertheless the addition of randomly dispersedmulti-walled CNTs with poor quality of adhesion can speed up thedamage process of the bulk resin, especially under shear loading,due to the increase in the dissipation sites in the polymer atnanoreinforcement/resin interfaces [26].

However, an analysis of literature data revealed a lack of infor-mation on the investigation of time-dependent mechanical proper-ties of FRPs filled with CNTs. Since FRPs are mostly used asstructural components and usually have a long-term period ofapplication, the creep characteristics of such materials, especiallyin the matrix-dominated flexure mode, should be investigatedand analyzed.

To determine the effect of CNTs on the elastic and viscoelasticproperties of an epoxy resin and CFRPs the materials were pre-pared and tested as described in Section 2. The results obtainedand their discussion for the optical analysis of CNT dispersion inthe epoxy resin, three-point bending tests in quasi-static and cycliccreep regimes for four different materials – pure epoxy resin,epoxy resin with 1 wt.% of CNTs, CFRP, and CFRP with 1 wt.% ofCNTs are provided in Section 3. The conclusions stating that theaddition of CNTs noticeably suppresses flexural creep of bothepoxy and CFRP specimens made in terms of mechanical testingare given in Section 4.

2. Material and methods

2.1. Preparation of NC specimens

Epoxy composites with a CNT content of 0 and 1 wt.% were pro-duced. Multi-walled CNTs (length 5–9 lm, diameter 110–170 nm,and aspect ratio 30–80, and average aspect ratio 55) were usedas received, without any purification treatment, from Sigma–Aldrich. The nanotubes were dispersed in a RTM6 monocomponentaeronautical-grade epoxy matrix from Hexcel Composites. Thisepoxy system is generally used for liquid infusion processes andis characterized by a high ultimate glass-transition temperature(200 �C) and low viscosity (50 mPa s) within the range of 100–120 �C. The manufacturing procedure of NC consisted of threesteps: (a) ultrasonication of nanotubes and matrix by using a dip-ping tip sonicator (Misonix S3000) for 60 min at a constant temper-ature (120 �C); (b) degassing for 30 min at 80 �C; (c) cure for 1 h at160 �C and post-cure for 2 h at 180 �C. The production method wasproperly designed for a good dispersion of pristine nanotubesaggregates [27]. The content of CNTs was chosen to be of 1 wt.%,

because this filler concentration was found to be effective bothfor the bending modulus in DMA experiments and of the systemviscosity [27]. An optical analysis for the microstructuralcharacterization of CNT dispersion within the epoxy matrix wascarried out by an optical microscope Olympus BX51.

2.2. Preparation of CFRP specimens

The manufacturing was divided in two separated processes.First, the amount of needed carbon nanotubes was dispersed inthe epoxy resin according to the process described. Then, a com-posite plate was fabricated by the vacuum assisted RTM process.For this purpose, the preform was stacked on a preheated toolincluding a distribution package; then the vacuum bag was tight-ened by application of vacuum. Thereafter the preheated epoxyresin system was infused at temperature of 90 �C, impregnatingthe fabric stack. The plate was subjected to 1 bar of vacuum pres-sure and cured for 1 h at 160 �C. Then a post-curing cycle lasting2 h at 180 �C was applied. The stacking sequence of laminateswas [0/45/90/�45]2.

2.3. Three-point bending tests in quasi-static and cyclic creep regimes

NC and CFRP specimens of dimensions 2 � 10 � 155 mm3, withdifferent CNT content (0 and 1 wt.%), were tested using a universalZwick 2.5 testing machine at a crosshead speed of 2 mm/min andspan of 60 mm at room temperature according to ASTM D0790.At least five specimens per each filler mass fraction were tested,and the values given correspond to their arithmetic means.

In order to study the viscoelastic properties of NC and CFRPspecimens and to reveal the effect of CNT on the viscoelastic prop-erties, three-point bending tests were carried out at room temper-ature in cyclic creep regimes. Four cycles of gradually increasingstress, equal to �25%, 50%, 75%, and 90% of the flexural strength,were used for loading during 30 min, followed by 30 min ofunloading.

3. Results and discussion

3.1. Optical analysis of CNT dispersion in the epoxy resin

As it is obvious from the optical micrographs shown in Fig. 1(aand b), the dispersion of CNTs in the epoxy resin was rather homo-geneous at both magnifications. Since the CNTs were quite thick(the average diameter was about 140 nm), it was possible to ana-lyze their dispersion using an optical microscope. The random dis-persion of individual CNTs was observed at a larger magnification(Fig. 1b). Nevertheless, as seen from both micrographs, agglomer-ates of CNTs can be found in the epoxy system too. Their formationcan negatively influence the infusion process of the fibrous rein-forcement of CFRP and may result in a reduced improvement ofmechanical properties of the multiscale CM obtained.

3.2. Three-point bending tests

Bending tests were carried out for four different materials –pure epoxy resin, epoxy resin with 1 wt.% of CNTs, CFRP, and CFRPwith 1 wt.% of CNTs. The representative stress–strain curves aregiven in Fig. 2.

The flexural stress in the outer surface of test specimens wascalculated using the standard equation for homogeneous elasticmaterial tested in bending as a simple beam supported at twopoints and loaded at the midpoint

r ¼ 3PL

2bd2 ; ð1Þ

Fig. 1. Optical micrographs of an epoxy sample with 1 wt.% of CNTs at different magnifications: 200 (a) and 500 (b).

Fig. 2. Representative stress–strain curves for different materials (neat epoxy andCFRP – solid lines, epoxy and CFRP with CNTs – dotted lines).

200 T. Glaskova-Kuzmina et al. / Composites Science and Technology 100 (2014) 198–203

where P is the applied load, L is the support span, and b and d arethe width and thickness of the specimen tested.

The flexural strain is the nominal fractional change in the lengthof an element of the outer surface of a test specimen at themidspan, where the maximum strain occurs. It was calculatedusing the formula

e ¼ 6Dd

L2 ; ð2Þ

where D is the deflection at the center of the specimen in the mid-dle of the support span.

The flexural strength was defined as the maximal achievedvalue of stress in the specimens, and the flexural (secant) moduluswas calculated from the slope of a secant line between 0.05% and0.25% strains on the stress–strain plot. Results for the flexural mod-ulus and strength, for all the materials tested, are given in Fig. 3.

Though the addition of CNTs should increase the interfacialstrength of CNT/epoxy composites and should produce an increasein the shear component of the flexural modulus [13], an evidenteffect of CNTs on the flexural modulus of epoxy and epoxy-based

Fig. 3. Flexural modulus and strength of different materials tested.

CFRP was not observed. Also, no positive effect on the flexuralstrength and the strain at break of NC in relation to CNT contentwas found. However, the flexural strength of CFRP was more sen-sible to the addition of CNTs. A CFRP with 1 wt.% of CNT showeda slight improvement in the flexural modulus (by 9%) and strength(by 16%) in comparison with those of the unfilled CFRP. This can beexplained by the dominance of fiber breaking over the interlaminarshear during the failure on the surface of specimens.

Similar slight improvements in the flexural properties of CFRPsdue to the addition of CNTs were reported in [9,14,22]. As stated inthe literature, the reason for the insignificant improvement in theflexural strength is the aggregation of CNTs, causing weak CNT–matrix–fiber interfacial interactions at a relatively high contentof CNTs owing to the formation of agglomerates of CNTs. As seenfrom Fig. 1, this can be a reasonable explanation, because sparseagglomerates of CNTs were observed even by an optical micro-scope. Surely, the effectiveness of CNT dispersion in a polymermatrix could benefit from optimization of the dispersion processand/or chemical functionalization of CNTs, which is necessary toimprove the interfacial strength between CNTs and a polymerresin.

3.3. Cyclic three-point bending creep tests

Keeping in mind that the creep of polymer material is acceler-ated by an increase of stress, one can suppose that the effect ofnanofiller will be more noticeable at larger values of stress. Thescheme of loading–unloading cycles and the corresponding repre-sentative creep curve is shown in Fig. 4 for the epoxy resin. A cer-tain type of loading–unloading procedure was designed based onan experimental investigation of the flexural properties of epoxyand epoxy-based CFRP in the quasi-static three-point bendingmode, as well as on preliminary experiments on the creep andrecovery of these materials. The rather rigid epoxy resin and CFRPshowed low creep before the failure. In order to analyze the effectof cycling on unfilled and CNT-filled epoxy resin and epoxy-basedCFRP specimens and to reveal the time-dependent properties at agradually increasing load, four relatively short time steps of load-ing, followed by unloading, were chosen.

The creep strains used for characterization of the effect of CNTson the long-term deformability of NC and CFRP were described inthe previous publication [28]. The most significant components ofstrain developed during four cycles � elastic, viscoelastic and plas-tic ones � of unfilled and CNT-filled epoxy and CFRP are given inTable 1.

As seen from Table 1 upon addition of CNTs to the epoxy matrix,the strains decreased: the elastic ones by 19%, the viscoelastic by44% and the plastic by 44%. A similar effect of decreasing deforma-bility in creep was also observed for CFRP upon addition of CNTs:the elastic strain decreased by 7%, the viscoelastic by 19%, andthe plastic by 32%. The most prominent effect of CNTs (more than

Fig. 4. Scheme of four loading–unloading cycles (a) and the representative creep curve of the epoxy matrix (b).

Table 1Elastic (ee), viscoelastic (eve), and plastic (ep) strains of unfilled and CNT-filled epoxy, and CFRP.

Material r/rmax (%) ee ± Dee (%) eve ± Deve (%) ep ± Dep (%)

Epoxy 25 0.986 ± 0.099 0.042 ± 0.008 ; 0.012 ± 0.002 ;Epoxy with 1 wt.% of CNTs 0.825 ± 0.094 0.032 ± 0.002 0.006 ± 0.002CFRP 0.270 ± 0.005 0.001 ± 0.001 0.016 ± 0.002 ;CFRP with 1 wt.% of CNTs 0.244 ± 0.007 0.001 ± 0.001 0.009 ± 0.004Epoxy 50 1.957 ± 0.180 0.135 ± 0.024 ; 0.016 ± 0.005 ;Epoxy with 1 wt.% of CNTs 1.611 ± 0.169 0.079 ± 0.011 0.010 ± 0.004CFRP 0.495 ± 0.004 0.003 ± 0.000 0.003 ± 0.001 ;CFRP with 1 wt.% of CNTs 0.459 ± 0.010 0.002 ± 0.001 0.002 ± 0.001Epoxy 75 3.045 ± 0.298 ; 0.339 ± 0.038 ; 0.030 ± 0.006 ;Epoxy with 1 wt.% of CNTs 2.419 ± 0.256 0.155 ± 0.029 0.019 ± 0.006CFRP 0.717 ± 0.004 0.005 ± 0.000 0.002 ± 0.001 ;CFRP with 1 wt.% of CNTs 0.671 ± 0.012 0.004 ± 0.002 0.001 ± 0.000Epoxy 90 3.705 ± 0.358 ; 0.500 ± 0.136 ; 0.032 ± 0.009 ;Epoxy with 1 wt.% of CNTs 2.853 ± 0.302 0.217 ± 0.054 0.014 ± 0.006CFRP 0.849 ± 0.002 0.005 ± 0.001 0.001 ± 0.000CFRP with 1 wt.% of CNTs 0.798 ± 0.016 0.006 ± 0.001 0.001 ± 0.000

Fig. 5. Maximal creep compliance achieved at the end of each cycle, regardless theinstantaneous creep compliance, for different materials (as indicated in the legend)in relation to the stress level applied.

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20%) on the creep strains of epoxy and CFRP is denoted by arrowsin Table 1. This means that the presence of a nanofiller in the epoxymatrix and CFRP slows down the relaxation processes underlyingthe creep. Thus, CNTs having high aspect ratio act as restrictionsites for the relatively slow regrouping of polymer macromoleculesupon creep, and the addition of CNTs significantly raises the creepresistance of epoxy matrix and CFRP.

It should be noted that according to the results obtained theelastic deformation of epoxy filled with CNTs for the first cycle isalmost insensitive to the addition of CNTs and decreases only byapp. 0.2%. The result is similar as for quasistatic bending testswhere elastic modulus of epoxy and epoxy filled with CNTs coin-cided. The higher the stress level is, the more pronounced is theeffect of CNTs causing a more significant decrease of elastic defor-mation. Such result can be attributed by the pulling out of CNTsfrom the agglomerates and subsequent destruction of agglomer-ates [29] which occur upon cyclic and increasing loading. Duringthis process the size of agglomerates decreases, while the densityof the agglomerates increases, leading to the improved matrix–filler interfacial interactions.

The growing creep resistance can be also attributed to theincreased tendency of epoxy to exhibit its elastic nature ratherthan its viscous one with addition of CNTs [30]. However, due tothe agglomeration of CNTs at a high filler content, this effect isinsignificant when taking into account the potential influence ofhighly elastic multi-walled CNTs on the improvement of creepresistance of the epoxy matrix.

Furthermore according to recently published data [31] CNTsreduced the physical aging of the polymer so the presence of CNTsled to a strong suppression of the physical aging phenomenonwhen the system is deep in the glassy state. Above considerationslead to state that the physical aging could not be considered aspotential explanation of the data presented especially for speci-mens post cured at high temperature and taking into accountrelatively short duration of active loading during the creep test.

The creep performance is commonly characterized by the creepcompliance

JðtÞ ¼ eðtÞr ; ð3Þ

where e(t) is the creep strain and r is the applied stress.In order tofocus on the maximal value of creep compliance achieved at the endof each cycle, regardless of the instantaneous creep complianceJ0 ¼ ee

r , the corresponding results were obtained for the unfilledand CNT-filled epoxy and CFRP (Fig. 5).

It is obvious from Fig. 5 that, with increasing stress, the creepcompliance of both epoxy and CFRP filled with CNTs decreased incomparison with that of unfilled materials. The higher the stress

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level, the more pronounced is the effect of CNTs causing a growthin the creep resistance. Such result can be again attributed by thepulling out of CNTs from the agglomerates and subsequentdestruction of agglomerates which occur upon cyclic andincreasing loading.

An application of the time-stress superposition principle allowsone to extend the time scale by using elevated stresses for theacceleration of relaxation processes and the description of a seriesof creep-recovery curves obtained at various stresses. The acceler-ation of creep of a polymer material by reducing relaxation timeswith a stress grow underlines the principle of time–stresssuperposition, which is widely used for the prediction of long-termdeformability.

For the description of creep curves at various values of stressthe Boltzmann–Volterra linear integral equation can be used[32,33]. Then taking into account time-stress superposition princi-ple the creep compliance of the composite material developing intime t is found by equation

JðtÞ ¼ J0 � 1þXn

i¼1

bi 1� e�t�arsi

� � !ð4Þ

where J0 is the instantaneous creep compliance, bi and si is the spec-trum of retardation times, ar is the time-stress shift factor.

Let us illustrate the superposition principle on the pure epoxyand epoxy filled with 1 wt.% of CNTs to reveal the effect of CNTson the time-stress shift factor ln ar, which can be determined fromshort-term creep tests at various levels of stress [33]. The resultingmaster curve obtained by shifting the representative creep compli-ance curves of unfilled and CNT-filled epoxy along the time axis is

Fig. 6. Master curves of unfilled (s) and CNT-filled epoxy (d) obtained using thetime-stress superposition principle.

Fig. 7. Time-stress shift factor vs. the difference in the applied stress starting fromthe stress of the first cycle for the unfilled (�s�) and CNT-filled epoxy (�d�).

shown in Fig. 6. It can be noted that the master curve of CNT-filledepoxy is located below the master curve of unfilled epoxy,especially at longer times, which confirms an improvement inthe long-term creep resistance of CNT-filled epoxy i.e. theoperation time of such materials will be higher.

The results obtained for the time-stress shift factor (Fig. 7) forepoxy with different content of CNTs (0 and 1 wt.%) lead to thesame conclusions as for the improved creep resistance. As it isobvious from Fig. 7, the time-stress shift factor is reduced almosttwice for the nanomodified epoxy specimens at all levels ofstresses applied during the research.

4. Conclusions

According to the experimental results the addition of CNTsnoticeably suppresses flexural creep of both epoxy and CFRP spec-imens but has nearly no effect on flexural static characteristics. Thereduction of creep especially on viscoelastic and plastic strainsboth for epoxy (by 44% and 19%, accordingly) and CFRP CNT-filledspecimens (by 44% and 32%, accordingly) can be attributed to thepulling out of CNTs from the agglomerates and subsequentdestruction of agglomerates which occur upon cyclic and increas-ing loading. During the pulling out of CNTs from the agglomeratesthe size of agglomerates decreases, while the density of theagglomerates increases, leading to the improved matrix–fillerinterfacial interactions.

The insensitivity of quasistatic flexural characteristics can beexplained by the formation of agglomerates of CNTs causing subse-quent weak CNT-matrix–filler interfacial interactions at a rela-tively high content of CNTs as well as by the dominance of fiberbreaking over the interlaminar shear during the failure on the sur-face of specimens.

Above considerations lead to state that the presence of CNTs inan epoxy matrix and CFRP slows down the relaxation processesunderlying the creep i.e. the operation time of such materialsapplied in composite structures under the same stress conditionswill be higher in comparison with unfilled CFRP. The resultsobtained during the investigation can be used in designing thecomposition of NC for the structural applications of CFRPs basedon the short-term creep and recovery tests of NC specimens.

Acknowledgments

T.G.-K. and A.A. are grateful to ERAF Project No. 2010/0201/2DP/2.1.1.2.0/10/APIA/VIAA/005 for the financial support.

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