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Influence of Nanoclay on the Durability of WovenCarbon/Epoxy Composites Subjected to UltravioletRadiationAlfred Tcherbi-Narteh a , Mahesh Hosur a & Shaik Jeelani aa Center for Advanced Materials , Tuskegee University , Tuskegee , Alabama , USAPublished online: 09 Dec 2013.

To cite this article: Alfred Tcherbi-Narteh , Mahesh Hosur & Shaik Jeelani (2014) Influence of Nanoclay on the Durability ofWoven Carbon/Epoxy Composites Subjected to Ultraviolet Radiation, Mechanics of Advanced Materials and Structures, 21:3,222-236, DOI: 10.1080/15376494.2013.834097

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Mechanics of Advanced Materials and Structures (2014) 21, 222–236Copyright C© Taylor & Francis Group, LLCISSN: 1537-6494 print / 1537-6532 onlineDOI: 10.1080/15376494.2013.834097

Influence of Nanoclay on the Durability of WovenCarbon/Epoxy Composites Subjected to UltravioletRadiation

ALFRED TCHERBI-NARTEH, MAHESH HOSUR, and SHAIK JEELANI

Center for Advanced Materials, Tuskegee University, Tuskegee, Alabama, USA

Received 2 June 2013; accepted 12 June 2013

Effects of ultraviolet (UV) radiation on mechanical and thermal properties of carbon fiber-reinforced polymer composites (CFRC)were investigated in this study. Composites samples used for this study were fabricated using an SC-15 resin system modifiedwith 1, 2, and 3 wt% loading of montmorillonite nanoclay (MMT; Nanomer R© 1.28E) and 8 harness satin weave carbon fiberreinforcements. Fabricated samples were UV conditioned using QUV/SE, an accelerated weathering chamber. Mechanical propertieswere characterized through quasi-static and dynamic compression, and flexural tests. Viscoelastic properties were determined throughdynamic mechanical analysis (DMA). Results show an increase in mechanical properties up to 56% for 2 wt% samples at roomtemperature, and an overall decrease in mechanical properties after conditioning. Viscoelastic properties, such as glass transitiontemperature (Tg), increased with nanoclay content for room temperature. However, 2 and 3 wt% samples showed a decrease in Tgafter conditioning, while an increase was observed in the case of neat and 1 wt% samples. On the other hand, storage modulus showedan increase in both conditioned and unconditioned samples with increasing nanoclay content up to 2 wt% and a decrease for 3 wt%.

Keywords: montmorillonite nanoclay (MMT), nanocomposites, nanoclay, durability, UV radiation

1. Introduction

Carbon fiber-reinforced composite materials are increasinglyreplacing traditional isotropic engineering materials due totheir versatility; ease of fabrication of complex parts in a singleor fewer steps; cost effectiveness in the long run; light weight;and attractive properties, such as high stiffness to weight ratio,low coefficient of thermal expansion, and anti-corrosiveness.As a result, the scope of its usage has widened to include indoorand outdoor applications across industries [1–3], where theybecome susceptible to a variety of environmental factors alongwith stress due to service loading. With time, environmentalconditions, such as ultra violet radiation (UV) and moisture,can induce stresses and strain from expansion and contrac-tions increasing the free volume within the polymer systemand making moisture diffusion easier in the event of moistureexposure. Elevated temperatures associated with UV radiationcan also promote and accelerate reactions between side groupsto form new compounds or radicals. These radicals becomescavengers cleaving other bonds within the polymer chainsin a vicious cycle, which over time leads to the degradationin material property [4, 5]. The chemical changes that occur

Address correspondence to Mahesh Hosur, Center for AdvancedMaterials, 104 Chappie James Center, Tuskegee University,Tuskegee, AL 36088, USA. E-mail: hosur@mytu.tuskegee.edu

in polymers may be irreversible, including thermo-oxidative,thermal, and hydrolytic aging [6–9]. Hydrolytic aging due tomoisture absorption in polymers generally causes plasticiza-tion and hydrolysis that can be either reversible or irreversible,affecting the storage modulus and glass transition tempera-ture (Tg). Hence, temperature, time, and history of loadingbecome major concerns during their use in applications, pre-dominantly due to the viscoelastic nature of polymers.

UV radiation has been known to be one of the most detri-mental environmental factors affecting the properties of poly-mer composites during outdoor applications, as it creates con-ditions for initiation and propagation of different degradationmechanisms, which along with other environmental attacks,like temperature and moisture, lead to property deteriora-tion [4, 10–12]. The interactions between composite materi-als and its service environment can also degrade the materialproperties with respect to its original desirable properties rais-ing durability issues over time. Degradation and degradationmechanisms in polymeric composites vary widely, mostly de-pending on the type of polymer and reinforcements, applica-tions, and loading condition as well as service environment[13]. In most applications, several environmental factors canattack polymeric materials simultaneously or sequentially, re-sulting in loss of bulk material property.

Energy of UV photons reaching the surface of the earth isbetween 290–460 KJ/mole, which is comparable to dissocia-tion energy of the covalent bonds in the polymer molecules.

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Influence of Nanoclay on Carbon Composites 223

Fig. 1. Typical chemical structure of MMT nanoclay [17].

During exposure of polymers to UV radiation, the UV pho-tons react with the polymer molecules to form alkoxy, alkyl,and peroxy radicals, which initiate secondary reactions re-sulting in matrix erosion over time. The erosion of matrixfrom the surface compromises matrix dominated proper-ties affecting the load bearing capacity of the material. Theabsorbed UV light with energy in the presence of oxygenalso causes photo-oxidative reactions. This reaction causesa change in the chemical structure, typically chain scissionand or crosslinking, leading to material degradation and sub-sequent weight loss. The existence of different elemental com-position in polymers allows different polymeric materials torespond to environmental factors in a different way. For ex-ample, properties of polymeric systems, such as polyvinyl andpolyacrylonitrile, deteriorate when exposed to UV radiationdue to chromophoric groups induced by UV radiation withoutcausing any scission of the backbone [9, 11]. Other polymericsystems, such as polyethylene and polystyrene in the presenceof UV radiation, tend to become brittle, which is caused byscission of the main chain due to photo-induced crystalliza-tion and crosslinking that reduces molecular mobility. Theeffects of UV radiation degradation can result in loss of phys-ical, mechanical, and thermal properties due to the formationof microcracks on the surface [14]. These cracks contribute tothe reduced load bearing capacity and non-uniform distribu-tion of thermal stresses across the thickness of a compositematerial.

Introduction of organo-nanoclays in epoxy-based compos-ites has gained attention in recent years, as they were found tonot only enhance mechanical and thermal properties [8–11],but also have the ability to minimize the effects of UV radia-tion [14–16]. The inherent chemical structure could also act asa barrier to prevent the flow or diffusion of moisture within thepolymeric material thereby minimizing moisture attacks and

overall damages caused by UV radiation and moisture [12]and ultimately retard degradation in polymers [5]. It is, there-fore, essential that polymer composites for outdoor applica-tion be highly resistive to harsh environmental conditioning,especially UV radiation. Hence, in this study, investigationswere carried out to determine the effect of UV radiation oncarbon/epoxy-nanoclay nanocomposites and their durabilityas compared to neat carbon/epoxy composites.

2. Experimental

2.1. Materials and Fabrication of Nanocomposites

Samples for this study were fabricated using commer-cially available diglycidyl ether of bisphenol A (DGEBA)epoxy resin, SC-15 modified with montmorillonite nanoclay(Nanomer R© 1.28E), and reinforced with 8 harness satin weavecarbon fabric using vacuum assisted resin transfer molding(VARTM). Pure MMT nanoclays are very hydrophilic anddifficult to disperse; hence, the surface of most commerciallyavailable nanoclays used as fillers has been modified to en-hance compatibility related issues with polymers. Figure 1shows the chemical structure of Nanomer R© I.28E, a 2:1 smec-tite clay modified with 25–30 wt% trimethyl stearyl ammonium[18] obtained from Sigma Aldrich. The chemical structure ofSC-15, shown in Figure 2, a two part resin system (part Aand part B—hardener) is a room temperature curing systemwith low viscosity and low shrinkage during curing obtainedfrom Applied Poleramic Inc. It also has excellent interfacialbonding with most commercially available fibers and is widelyused in structural applications. The 8 harness satin weave fibercloths with 3K tow size and a density of 1.81 g/cm3 was ob-tained from US Composites Inc.

Fig. 2. Chemical structure of SC-15 from Applied Poleramic, Inc.

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224 A. Tcherbi-Narteh et al.

Dispersion of the montmorillonite nanoclay into part A ofthe epoxy resin system was done using a magnetic stirrer. Astoichiometry quantity of SC-15 part B (hardener) was addedto the epoxy/clay mixture in a mass ratio of 100:30, and stirredusing a high speed mechanical stirrer at around 800 RPM forabout 5–10 min. The mixture was then degassed to removeany trapped air bubbles resulting from the mechanical stir-ring. A small amount of degassed mixture of epoxy/clay resinwas poured into molds for characterization, while the rest wasinfused into a prearranged carbon fiber cloth using vacuum-assisted resin transfer molding (VARTM). During the fabri-cation process, several layers of fiber cloth were arranged withthe emphasis of attaining a more perfectly aligned fabric un-der vacuum. This is critical, since the orientation of the fiberaffects the mechanical properties and the moisture absorptionbehavior of the final laminate [19, 20]. The setup was allowedto cure for 24 h at room temperature, followed by post curingat 100◦C for 2 h.

2.2. Nanocomposite Characterization

2.2.1. X-Ray Diffraction (XRD)XRD analyses were performed on SC-15/MMT nanoclaystructures in the nanocomposites using Rigaku-DMAX-2000,equipped with Cu K� radiation source. The intergallery dis-tance between nanoclay platelets (d-spacing) of the pureorgano-nanoclay was obtained and compared with XRD stud-ies from nano-infused samples to establish the influence of theclay in a composite microstructure according to Bragg’s Law(n� = 2d sin �).

2.2.2. Transmission Electron Microscopy (TEM)Bright field TEM analyses were performed on SC-15/MMTnanocomposites to establish dispersion of nanoclay into theepoxy system. Nanocomposite samples were microtomed witha diamond cutter at room temperature with a nominal thick-ness of about 50 nm. The sections were collected on the sur-face of a solution of dimetyl sulfoxide and water, 60 and 40parts respectively, and then transferred on 200 mesh Cu grids.TEM micrographs were obtained to determine the extent ofnanoclay dispersion throughout the resin system using a mag-netic stirring method. Micrographs obtained from the TEMalong with data from XRD studies were used to completemicrostructural studies of the modified epoxy resin.

2.3. Conditioning of Samples

Samples were machined from the post-cured laminates, andthe edges were polished to remove any roughness and un-wanted cured epoxy, which might promote moisture absorp-tion through the edges. The sample dimensions were 100 ×60 mm, to fit the sample holders in the weathering chamber.Samples were subjected to continuous UV radiation using anaccelerated weathering chamber (QUV/SE) for a period of15 days at a temperature of 60◦C and radiance of 0.68 W/m2

using 340 nm UV fluorescent lamps. Weight and surfacemorphology along with the mechanical properties were stud-ied to determine the effects of UV radiation on the condi-

tioned samples with respect to the unconditioned. Weightchanges were monitored every 24 h of exposure time, whilemechanical properties were characterized every 120 h andrecorded.

2.4. Mechanical Characterization

2.4.1. Compression TestMaterial response to compression loading at different strainrates was investigated using quasi-static and dynamic tests.Samples were machined to a 12.70 ± 0.05 mm cube accordingto ASTM 695-02 standards, and polished to remove any roughedges due to cutting. Quasi-static tests were performed usinga 10 KN Servo-hydraulic MTS testing machine in a displace-ment mode with a constant cross head speed of 1.27 mm/minalong the fiber direction, while material behavior under dy-namic loading was conducted using a Split Hopkinson pres-sure bar apparatus (SHPB) equipment. A set of five samplesfrom each conditioned and unconditioned sample were tested.During dynamic loading testing, each sample was sandwichedbetween two identical maragingsteel bars of the SHPB knownas incident and transmission bars. Length and diameters ofthese two bars were 1.5 m and 38 mm, respectively. The sur-faces making contact with the incident and transmission barswere lubricated to reduce friction and also to ensure smoothpropagation of waves. A striker bar of 30 cm length and thesame cross section area and material as the incident bar im-pacted the incident bar. Impact of the striker bar generatesa longitudinal compressive incident stress pulse, whose dura-tion is equal to twice the length of the striker bar divided bywave speed. This pulse traveled down the incident bar and wasrecorded by a strain gauge mounted on the incident bar. Partof the pulse reaching the incident bar was reflected back intothe incident bar in the form of a tensile stress, while the remain-der traveled through the specimen into the transmission barand a typical signal obtained during testing is shown in Fig-ure 3. Stress-strain relation was derived from these three pulses,namely, incident, reflected, and transmitted pulses. Strain rate(ε), strain (ε), and stress (�) of the specimen were then evalu-ated [21]. The stress and strain of each specimen is governedby the following equations:

ε(t) = −2Cb

Lsεr (t), (1)

ε(t) = 2Cb

Ls

∫ t

0εt(t), (2)

�(t) = Eb Ab

Asεt(t), (3)

where Eb = modulus of the steel bar on the SHPB; Ab =cross-sectional area of the bar; As = cross-sectional area ofthe specimen; Ls = Length of the specimen; C0 = √

Eb/�b, thelongitudinal wave velocity in the incident bar; and εr(t), εI(t),and εt(t) are time-dependent reflected, incident, and transmit-ted strains, respectively.

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Influence of Nanoclay on Carbon Composites 225

-200

200

-0.00025 0 0.00025

Compressive Incident Pulse

Tensile Reflected Pulse

Compressive transmitted Pulse

Time, ms

Volta

ge, m

V

Fig. 3. Typical compressive and reflected signals obtained from HSR (color figure available online).

2.4.2. Flexure TestA flexure test was conducted to determine the flexural strengthand modulus before conditioning and corresponding residualbending strength and modulus after exposure to UV radiation.The tests were performed using a Zwick Roell Z25 machine ac-cording to ASTM D790-02 with a constant cross-head speedof 2.0 mm/min. Specimens were machined into a rectangu-lar shape conforming to the standards. The thickness of thespecimen was 3.5 ± 0.25 mm, and the ratio of span lengthto thickness was maintained as 16:1, according to the ASTMstandards. The tests were conducted in a displacement con-trol mode and data at the end of each test was analyzed andreported in the form maximum stress, flexural modulus, andpercentage elongation and a stress-strain curve.

2.5. Thermal Analysis

2.5.1. Dynamic Mechanical AnalysisDynamic mechanical analysis (DMA) was performed usingTA Instruments Q800 operating in the three-point bendingdual cantilever mode, oscillating at a frequency of 1 Hz andamplitude of 15 �m. The temperature was equilibrated at30◦C, and subsequently ramped to 160◦C at a rate of 5◦C/min,while data was being collected. Effects of UV radiation on vis-coelastic properties, such as storage modulus and glass transi-tion temperatures, were characterized through this techniqueas a function of temperature. Three rectangular shaped sam-ples with a length of 54.00 ± 1.00 mm, width of 12.50 ±

1.00 mm, and thickness of 3.50 ± 0.10 mm were machinedfrom unconditioned and conditioned samples. The glass tran-sition temperature was determined from the peak of the tandelta curve according to ASTM D4065-06 standards.

2.5.2. Thermogravimetric AnalysisThermogravimetric analysis (TGA) was performed on thespecimens using TA Instruments Q500 to determine the ther-mal stability of the composites and its fraction of volatile com-ponents by monitoring the percentage weight change while thespecimen was being heated. The equipment and sample werepurged with dry nitrogen gas during the test at a flow rate of 40and 60 mL/min for equipment and sample, respectively. Thechange in weight was recorded as a function of increasing tem-perature. Platinum pans were used with three specimens fromeach set measuring 12–15 mg to ensure repeatability and datacomparison. The specimens were heated from 30 to 850◦C ata rate of 10◦C/min, where it was believed that all the chemicalreactions would have been completed, i.e., polymeric compo-nents would have been burned out leaving only carbon char.

3. Results

3.1. Characterization

3.1.1. MicroscopicThe key objective in working with nanoparticles is to havea uniformly dispersed system as enhancements in material

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Fig. 4. TEM micrographs of 2 wt% and 3 wt% nanocomposites.

properties have been directly linked to degree of exfolia-tion [22–24]; hence, the goal in dispersing nanoclay is toobtain a well exfoliated system. However, there have beenreports of improvements with intercalated structures andnot so great reports on other exfoliated systems [1]. Mi-crographs obtained from TEM studies are shown in Fig-ure 4, which depicts the dispersion state of 2 and 3 wt%of MMT clay in the nanocomposites, respectively. The mi-crographs showed good dispersion of MMT particle with

no visible agglomeration; however, highly organized silicatelayers of MMT clay were observed showing typical interca-lated structure in ordered stacked. XRD patterns obtainedfor pure MMT nanoclay revealed an intense diffraction peakat 2� = 19.64◦, followed by peaks with relatively lower in-tensities at 26.46◦, 34.98◦, and 61.82◦. XRD patterns ob-tained for SC-15/MMT clay nanocomposites, as shown inFigure 5, lack the peaks observed in the diffraction patternsof MMT I.28E, which indicates a disordered stack of the

20 40 60 80

2 wt. % Epoxy/clay

1 wt. % Epoxy/clay

MMT- I.28E

3 wt. % Epoxy/clay

2θ (Degree)

Re

lativ

eIn

ten

sity

,a

u

Fig. 5. X-ray diffraction patterns of MMT and 1–3 wt% nanocomposites (color figure available online).

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Influence of Nanoclay on Carbon Composites 227

-0.8

-0.6

-0.4

-0.2

0

0 120 240 360

0 wt. % Nanoclay (Control)1 wt. % Nanoclay2 wt. % Nanoclay3 wt. % Nanoclay

Exposure Time, hr

%w

eig

ht

ga

in/lo

st

Fig. 6. Weight gained/loss during UV exposure (color figure available online).

clay platelets, an indication of an exfoliated composition. Theabsence of the peaks observed from XRD together with mi-crographs (exfoliation and intercalation) obtained from TEMstudies points to a uniform dispersion of MMT into epoxyused for this study.

3.1.2. PhysicalIntensity of photo-degradation mechanism in polymers canbe attributed to a variety of factors, such as the polymer type,intensity, and the angle of incidence of UV rays with respectto the surface of the samples. The QUV equipment used forthe conditioning has been calibrated such that the UV raygenerated from a 340-nm fluorescent lamp was at an angle of90◦ to the surfaces of the samples. This ensures a maximumincidence of rays on the surface of the samples. Visual obser-vation of samples prior to conditioning showed no distinctionbetween the surfaces of the nanocomposites and that of theneat. However, there were significant changes on the surfacemorphology at the end of conditioning compared to uncondi-tioned samples. Physical changes, such as weight loss, samplediscoloration (yellowish color), surface roughness, and pres-ence of matrix cracking, were observed throughout the sam-ples exposed to UV radiation. Scanning electron microscopic(SEM) micrographs at the end of the study period show mi-crocracks on all the samples. The intensity of discolorationincreased with exposure time and may have been different in-tensities but since that was not monitored through intensities

of the primary colors red, green, and blue (RGB), it was un-clear whether that was the case. On the other hand, weight ofsamples was monitored every 24 h over the duration of thestudy. Three samples from each set of laminates were removedfrom the chamber and immediately weighed and the averageweight change was determined and plotted against the squareroot of the exposure time as shown in Figure 6, using thefollowing expression:

Weight gain ratio =Wt − Wd

Wd×100, (5)

where Wt is the weight of the sample exposed to environmentalconditions for a period of time t and Wd is the weight of thedried sample at the beginning of the conditioning.

Interestingly, there was a sharp decrease in weight for allsamples in the first 120 h with 2 wt% samples losing the most.Beyond this point, there was no particular trend in weight losscharacteristics till the end of the study where 2 wt% sampleslost the most on average. Scanning electron micrographs ofexposure surfaces of samples are shown in Figure 7, wherethe presence of microcracks can be observed on all samples.Results from morphological studies of samples exposed to UVradiation showed a clear depiction of photo-degradation oc-currence caused by the presence of akyl and carbonyl radicals,which causes intense erosion of the matrix on the surface ofthese samples.

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Fig. 7. SEM micrographs of surface of samples exposed to 360 h of UV radiation.

3.2. Mechanical Characterization

3.2.1. Quasi-Static Compression TestsTypical compressive stress–strain curves of all samples dur-ing testing along the in-plane direction are shown in Figure 8,where they show a nearly linear behavior at lower (quasi-static)strain rates. At room temperature, there was an increase instrength and modulus with increasing nanoclay loading, with2 wt% loading giving the most enhancements in strength andmodulus, 14.25 and 17.86%, respectively, as summarized inTable 1. However, after 360 h of continuous UV radiation,strength and modulus of all samples decreased. The failuremode in all samples regardless of percentage loading and typeof conditioning was generally micro buckling, and kink band-ing and delamination. The failure was initiated in the shearmode of microbuckling, followed by kink banding in local-ized areas. The stress concentration at these localized areasinitiated longitudinal splitting resulting in interfacial debond-ing. However, in 2 wt% loading samples, propagation of the

delamination was somehow impeded as shown in Figure 9.Average data is shown in Figures 10a and 10b comparing thematerial properties and how they varied with exposure time.

3.2.2. Dynamic Compression TestFive samples from each laminate were tested under three dif-ferent sets of gas pressures: 137.89, 172.37, and 206.84 kPa,translating into average strain rates of 450, 540, and 640 s−1,respectively. Typical stress-strain behavior curves of samplesbefore conditioning at a strain rate of 640 s−1 are shown inFigure 11a and respective compressive strength as a functionof exposure time in Figure 11b. The dynamic stress–straincurves initially show a nearly linear behavior at small strainsand then became nonlinear as the strain increases until failure.The initial linear portions of the curves are indications thatthe composite was undergoing a linear elastic deformationwithout any damage at small strains. However, the nonlin-ear portions show the behavior of the composite undergoing

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Influence of Nanoclay on Carbon Composites 229

Table 1. Average compressive properties of unconditioned and conditioned samples (quasi static)

Compressive modulus, GPa

Exposure time, h Conditioning 0 wt% 1 wt% 2 wt% 3 wt%

0 Room temperature 11.20 ± 0.84 11.34 ± 0.71 13.20 ± 1.08 12.21 ± 0.83120 UV 8.07 ± 0.52 10.34 ± 0.89 13.80 ± 0.73 13.70 ± 1.04

% Change wrt RT −27.95 −8.82 4.55 12.20% Change wrt neat −27.95 −7.68 23.21 22.32

240 UV 7.62 ± 0.45 10.60 ± 0.68 13.80 ± 0.88 13.50 ± 0.97% Change wrt RT −31.96 −6.53 4.55 10.57% Change wrt neat −31.96 −5.36 23.21 20.54

360 UV 7.95 ± 1.78 7.93 ± 1.11 13.12 ± 2.08 13.00 ± 3.11% Change wrt RT −29.02 −30.07 −0.60 6.47% Change wrt neat −29.02 −29.20 17.14 16.07

0

100

200

300

400

0 0.01 0.02 0.03 0.04

0 wt. % Nanoclay (controll)1 wt. % Nanoclay2 wt. % Nanoclay3 wt. % Nanoclay

Compressive Strain,mm/mm

Co

mpr

ess

ive

Str

ess,

MP

a

Fig. 8. Typical compressive stress-strain responses of samples at room temperature (color figure available online).

Fig. 9. Failure mode of samples tested under quasi static loading (color figure available online).

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230 A. Tcherbi-Narteh et al.

Table 2. Average dynamic compressive properties of unconditioned and conditioned samples

Dynamic compressive modulus, GPa

Exposure time, h Conditioning 0 wt% 1 wt% 2 wt% 3 wt%

0 Room temperature (RT) 26.36 ± 5.02 34.17 ± 4.13 40.74 ± 6.03 37.15 ± 7.43120 UV 35.47 ± 3.46 40.55 ± 6.03 45.21 ± 4.45 37.31 ± 5.58

% Change wrt RT 34.56 18.67 10.97 0.43% Change wrt baseline 34.56 53.83 71.51 41.54

240 UV 31.99 ± 3.90 32.16 ± 4.11 35.19 ± 4.08 27.25 ± 0.15% Change wrt RT 21.36 −5.88 −13.62 −26.65% Change wrt baseline 21.36 22.00 33.50 3.38

360 UV 23.32 ± 1.93 34.62 ± 4.27 37.52 ± 2.33 31.75 ± 4.52% Change wrt RT −11.53 1.32 −7.9 −14.54% Change wrt baseline −11.53 31.34 42.34 20.45

Fig. 10. (a) Compressive strength as a function of wt% MMT and (b) comparison of compressive as function of number of days ofexposure.

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Influence of Nanoclay on Carbon Composites 231

0

200

400

0 0.02 0.04 0.06 0.08

0 wt. % Nanoclay (Control)1 wt. % Nanoclay2 wt. % Nanoclay3 wt. % Nanoclay

C

(a)

(b)

ompressive Strain, mm/mm

Com

pres

sive

Str

ess,

MP

a

300

350

400

450

500

550

0 5 10 15

0wt% Nanoclay (Control)1wt% Nanoclay2wt% Nanoclay3wt% Nanoclay

Strain, mm/mm

Com

pres

sive

Str

engt

h,M

Pa

Fig. 11. (a) Typical compressive stress-strain responses of sample’s room temperature under dynamic loading at 640 s−1 and (b)respective dynamic compressive strength as a function of exposure time (color figure available online).

stress-induced damage and its cumulative effect leading tofailure. At lower strain rates, the failure mode varied fromsample to sample, with 2 and 3 wt% enduring the least dam-age. Conversely, as the strain rate increased, the distinctionbetween the failed samples seemed all but indifferent, regard-

less of the nanoclay loading. Debonding, fiber breakage, andfiber pulled-out and longitudinal cracking and fiber bucklingwere all observed as mode of failures (Figure 12). It was alsoobserved that most of the crushed specimens were ruptured,especially samples exposed to UV radiation at a relatively

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232 A. Tcherbi-Narteh et al.

Fig. 12. SEM micrograph of failed samples under dynamic loading.

lower strain rate compared to room temperature samples atthe same strain rate. This could be due to brittleness of thespecimen as a result of photo-oxidation. The strain valuesat peak stress for all samples exposed to UV radiation werenearly halved compared to room temperature samples, withthe exception of the neat sample. For example, strain at peakstress for 2 wt% samples at room temperature was 0.020 andthat of the conditioned one was found to be 0.014. Summaryof compressive property of samples tested at strain rate of640 s−1 are presented in Table 2. The compressive strengthsat the end of the study showed a decrease in 0, 1, 2, and

3 wt% in the order of 26.99, 23.11, 11.23, and 24.64%, re-spectively, noting that 2 wt% showed minimum reduction,hence, higher retention value. The decrease in compressivestrength was observed to be more linear in 1 wt% samples af-ter 5 days, compared to the others, which exhibited a nonlineartrend over the same period. From this observation, exposureto higher temperatures have a strong bearing upon the stiff-ness and strength of laminates due to their viscoelastic prop-erties. The peak loading and total energy absorbed duringimpact were substantially reduced with increasing exposuretime.

450

500

550

600

650

700

0 5 10 15

0wt% Nanoclay (Control)1wt% Nanoclay2wt% Nanoclay3wt% Nanoclay

Exposure Time, Days

Fle

xura

lSt r

engt

h,M

Pa

Fig. 13. Flexural strength of samples subjected to UV radiation with respect to exposure time (color figure available online).

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Influence of Nanoclay on Carbon Composites 233

25

30

35

40

45

50

0 5 10 15

0wt% Nanoclay (Control)1wt% Nanoclay2wt% Nanoclay3wt% Nanoclay

Expoaure Time, Days

Flex

ural

Mod

ulus

,GP

a

Fig. 14. Flexural modulus of samples subjected to UV radiation with respect to exposure time (color figure available online).

3.2.3. Flexure TestSummaries of flexural stress-strain properties of samples atroom temperature and post-conditioning are shown in Fig-ures 13 and 14. Material strength and modulus increased lin-early with increasing nanoclay content, with 2 wt% sampleshowing the most remarkable improvement. Strain at maxi-mum peak stress decreased in all nanophased samples com-pared to neat, an indication of material brittleness due toaddition of nanoclay. Addition of nanoclay considerably in-creased flexural strength and modulus of nanophased com-posites when compared to neat counterpart. However, whensamples were exposed to UV radiation, a different trend wasobserved. The strength and stiffness increased for the first5 days of exposure, perhaps due to radiation induced polymer-ization (photo-polymerization) resulting in highly cross-linkedpolymer networks, before gradually decreasing in the case ofneat samples. Strength and stiffness of nanophased samplesincreased after 10 days, possibly due to residual crosslinking,or due to some reaction between the nanoclay particles andpolymer molecules while crosslinking. Further work is neededto ascertain the exact phenomenon taking place chemicallyafter 10 days of UV exposure. The average properties mea-sured (Figures 13 and 14) from the tests show that sampleswith 2 wt% nanoclay increased in modulus and strength by54.55 and 12.91%, respectively. Further, 1 wt% showed a com-paratively modest improvement of 29.63 and 2.04%, while3 wt% showed 40.93 and 10.79% for modulus and strength,respectively. Once again the 2 wt% samples outperformed oth-ers owing to higher clay content and better exfoliation of

nanoclay in the epoxy system. Mode of damage was foundto be similar in all samples regardless of the weight percentloading of nanoclay. Failure occurred around mid section ofthe samples, with fiber fracture, matrix cracks, and interlami-nar delamination found for all samples.

3.3. Thermal Analysis

3.3.1. Dynamic Mechanical AnalysisResults from DMA showed an improvement in storage mod-ulus and glass transition temperatures with the introductionof nanoclay and are summarized in Table 3. Improvement instorage modulus was observed with an increase in weight per-centage loading of the nanoclay up to 2 wt%, which decreasedfor samples with 3 wt% nanoclay loading. The decrease in 3wt% could be attributed to the fact that, given the same pro-cessing condition, lower weight percent loading might yieldbetter dispersion. Also, as nanoclay loading increases, prob-lems such as agglomeration arises due to considerable increasein the viscosity resulting in lower storage modulus. Strength ofpolymeric materials is generally related to the strength of thebond formed, otherwise known as crosslinking density, whichincreased at the onset of UV exposure. An increase in storagemodulus was observed in all samples for the first 5 days ofUV conditioning. Neat and 2 wt% samples improved by 11.38and 10.82%, respectively, while 1 and 3 wt% improved by theleast amount of 6.42%. This phenomenon showed that theUV radiation exposure acted as a much needed post curing

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234 A. Tcherbi-Narteh et al.

0.1

0.2

0.3

0.4

300 330 360 390 420

0 wt% Nanoclay (Control)1 wt% Nanoclay2 wt% Nanoclay3 wt% Nanoclay

332.54

336.20

332.27

333.05

Temperature, oC

Der

iv.W

eigh

t,%

/°C

(a)

(b)

0.10

0.15

0.20

0.25

0.30

0.35

300 325 350 375 400

0 wt% Nanoclay UV 15 Days (Control)1 wt% Nanoclay UV 15 Days2 wt% nanoclay UV 15 Days3 wt% Nanoclay UV 15 Days

334.38

333.13333.13

334.59

Temperature, °C

Der

iv.W

eig h

t,%

/°C

Fig. 15. Derivative weight loss curve obtained from TGA of (a) unconditioned samples and (b) samples exposed 15 days to UVradiation (color figure available online).

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Influence of Nanoclay on Carbon Composites 235

Table 3. Average viscoelastic properties of unconditioned and conditioned samples

Glass transition Percent Storage modulus, PercentSamples Conditioning temperature, Tg

◦C change GPa @ 30◦C change

0 wt% Room Temp. 95.13 ± 1.11 17.37 20.43 ± 1.13 30.01UV radiation 111.65 ± 0.58 26.56 ± 6.45

1 wt% Room Temp. 100.70 ± 1.89 7.99 22.89 ± 2.64 23.37UV radiation 108.75 ± 0.37 28.24 ± 1.66

2 wt% Room Temp. 112.20 ± 0.71 −4.51 30.06 ± 1.15 52.03UV radiation 107.13 ± 0.87 45.71 ± 5.65

3 wt% Room Temp. 111.73 ± 0.87 −2.35 23.09 ± 1.45 48.16UV radiation 109.10 ± 0.90 40.27 ± 10.50

condition thereby increasing the crosslinking density and sub-sequently, the desirable properties. However, with increasingexposure time, the destructive effect of UV became more ap-parent through property degradation. At the end of the study,there was a loss in storage modulus for all the samples, withthe neat losing the most, 10.82%, followed by 1 wt% sample,9.79%, and the least from the 2 wt%, at 2.02%. In the currentstudy, there was a decrease in glass transition temperatures inall samples, while storage modulus increased with increasingweight percent loading of MMT clay. A decrease in glass tran-sition could affect the long-term durability of composite ma-terials in the form of thermal retention of material properties.

3.3.2. Thermogravimetric AnalysisResults from TGA analysis was plotted in the form of weightloss as a function of temperature for all room temperature andconditioned samples. The mass of each sample was measuredas a function of temperature, while the material was subjectedto a controlled temperature program. The temperature cor-responding to the peak of the weight loss derivative curvewas determined to be the decomposition temperature and isshown in Figure 15 for unconditioned samples and those after15 days of conditioning. From the study, it was determinedthat the decomposition temperature was least affected, andthe temperature difference between the samples was ±4◦Cfor unconditioned and conditioned samples. The differencesbetween the absolute value of decomposition temperatures be-tween unconditioned and conditioned samples were less than1% for each weight percent sample and, therefore, effect ofUV radiation on samples was statistically insignificant. Theheights of the peaks of the derivative curves decreased withexposure time, indicating that less mass was decomposed, es-pecially with peaks of 2 wt% samples consistently lower com-pared to the other samples in both room and conditionedcurves. This could be attributed to stronger bonds in 2 wt%samples; hence, more energy was required to break the bondsduring decomposition.

4. Conclusion

UV radiation is one of the environmental destructive factorsaffecting polymeric composites and, hence, its effects on themechanical and thermal properties of neat and nanophased

composites were studied. Experiments show that UV degrada-tion mechanism on material properties are gradual and overtime can be detrimental. Based on the results obtained fromthis study, the following conclusions were drawn:

• Nanophased samples after UV exposure showed a decreasein properties with respect to their corresponding room tem-perature samples. However, the decrease constitutes an im-provement when compared to the control sample. An indi-cation that nanoclay infusion helped in mitigating the effectof UV degradation on the composites materials.

• Damage under quasi static compression loading varied be-tween nanophased and controlled samples. It was mainlyinitiated by microbuckling with kink band formations inlocalized areas, resulting in interfacial debonding. Thedebonding was more pronounced in the controlled and1 wt% samples compared to 2 and 3 wt%.

• Mode of failure during dynamic loading varied fromnanophased to the controlled samples at lower strain rate,while no differences were observed at higher strain rate re-gardless of the conditioning, mainly debonding and fiberbreakage for room temperature samples, and fiber pull-outsand rupture due to brittleness after UV exposure.

• Nanophased samples behaved differently from that of thecontrol sample after conditioning under flexural loading.

• Storage modulus increased after UV radiation conditioningfor all samples, while glass transition temperature increasedfor controlled and 1 wt%, and decreased for 2 and 3 wt%.

• Addition of nanoclay or the UV radiation did not have anyeffect on the decomposition temperature.

The duration of this study was not enough to predict mate-rial behavior over a long period of time, since most applica-tions are designed to last for decades and, hence, a longer pe-riod of conditioning and characterization after conditioning isunderway.

Acknowledgments

The authors gratefully acknowledge the support received bythe Office of Naval Research and the National Science Foun-dation through the EPSCoR program for carrying out thiswork.

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