Materials and Design 85 (2015) 76–81

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Materials and Design

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Electrically conductive carbon nanotube/polypropylene nanocompositewith improved mechanical properties

Mohammed H. Al-SalehDepartment of Chemical Engineering, Jordan University of Science and Technology, Irbid, Jordan

E-mail address: mhsaleh@just.edu.jo.

http://dx.doi.org/10.1016/j.matdes.2015.06.1620264-1275/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 June 2015Received in revised form 18 June 2015Accepted 28 June 2015Available online 4 July 2015

Keywords:Carbon nanotubePolypropyleneMelt mixingElectrical propertiesMechanical propertiesMicrostructure

Conductive polymer nanocomposites based on carbon nanotubes (CNTs) have wide range of applications in theelectronics and energy sectors. For many of these applications, such as the electromagnetic interference (EMI)shielding, high nanofiller loading is typically needed to achieve the desired properties. The high nanofiller con-centration deteriorates the composite's tensile strength due to the increase in nanofiller aggregation. In thiswork, highly conductive CNT/polypropylene (PP) nanocomposite with improved tensile strength was preparedbymelt mixing. The effects of CNT content on the processing behavior, microstructure, mechanical and electricalproperties of the nanocomposite were investigated. Scanning electron microscopy was used to investigate thecomposite microstructure. Good level of CNT dispersion with remarkable adhesion at the CNT/PP interface wasobserved. Based on a theoretical model, the interfacial strength was estimated to be in the range of36–58MPa. As a result of this microstructure, significant enhancement in ultimate tensile strength was reportedwith the increase of CNT content. The tensile strength of the 20wt.% CNT/PP nanocomposite was 80% higher thanthat of the unfilled PP.Moreover, and due to the good dispersion of CNTparticles, an electrical percolation thresh-old concentration of 0.93 wt.% (0.5 vol.%) was obtained.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Polymer nanocomposites based on carbon nanotubes (CNTs) havebeen one of the major research disciplines since the introduction ofthese amazingnanofillers in 1991. CNTs possess superlativemechanical,electrical and thermal characteristics owing to their unique struc-tures [1,2]. These combined characteristics, in addition to the highaspect ratio and high surface area to volume ratio, are essential prereq-uisites to formulate polymer nanocomposites with multifunctionalproperties [3–5]. For example, the high aspect ratio and conductivityof CNTs have enabled the researchers to formulate conductive nano-composites at extremely very low nanofiller content [6]. Electrical per-colation threshold concentrations lower than 0.1 vol.% CNT have beenreported by many research groups [7]. Conductive nanocompositeshavewide range of applications including static charge dissipation, elec-tromagnetic interference (EMI) and structure damage monitoring [8,9].

In the field of conductive nanocomposites, CNT loading should bekept at its lowest possible level to reduce the final product cost and toavoid the degradation in the nanocomposite mechanical propertiesdue to the agglomeration of CNT particles at high CNT concentrations.Theoretically and experimentally it is possible to induce conductivityin polymer nanocomposites at CNT concentration levels that do notalter the valuable properties of some polymer such as ductility,

toughness and tensile strength. However, for applications requiringhigh levels of electrical conductivity the use of moderate to high levelsof nanofillers is not avoidable yet. For example, to achieve an EMIshielding effectiveness of 30 dB in the X-band frequency range a1.0 mm plate made of nanocomposite with an electrical resistivity of~1.0 Ω·cm is required [10]. This level of conductivity requires, basedon the currently available types of CNT, a nanocomposite filled with atleast 5 wt.% CNT [10]. Thus, it is necessary to formulate nanocompositeswith high level of CNT concentration and enhanced mechanicalproperties.

Many researches have been devoted for the investigation of theme-chanical properties of polymers reinforced with unmodified-CNT andfunctionalized-CNT [2,4,11]. Most of the studies, especially with theunmodified-CNT, ended up with different levels of enhancement inthe Young's Modulus and degradation in the tensile strength. For CNT/polypropylene (PP) nanocomposite, which is the nanocomposite of in-terest in this study, Andrews et al. [12] reported that compared withthe unfilled PP, a 12.5 vol.% CNT/PP nanocomposite has a 100% higherYoung's modulus and 60% lower tensile strength. Liu and Gao [13] re-ported that a 3 wt.% CNT/PP nanocomposite has improved tensilestrength, compact strength and Young's modulus compared to that ofthe unfilled PP. Dondero andGorga [14] found that the profiles of tensilestrength and tensilemodulus of CNT/PP filled with up to 3 wt.% CNT ex-hibitedmaximum peak at 0.25 wt.%. Identical behavior was reported byZhou et al. [15]with a tensile strength peak at 1.0 wt.% CNT. At the peak,the tensile strength was 16% higher than that of the unfilled polymer.

Fig. 1. SEM micrograph of CNT dispersion in melt mixed 1 wt.% CNT/PP nanocomposite.

77M.H. Al-Saleh / Materials and Design 85 (2015) 76–81

Micusik et al. [16] reported an increase in Young's modulus and inde-pendence of yield strength on CNT content for nanocomposites filledwith up to 8% CNT. Similarly, an independence of tensile strength onthe CNT content was also reported for PP filled with up to 12.5 wt.%CNT [17]. On the other hand, a significant improvement in tensilestrength was reported by Zhou et al. [3] for CNT/PP nanocompositesfilled with up to 5 wt.%.

It is widely accepted that poor interfacial adhesion and bad disper-sion of the nanofiller are responsible for the degradation of compositematerials' mechanical properties. Interfacial adhesion is responsible forload transfer, which can be accomplished by mechanical interlocking,chemical bonding and physical intermolecular forces [18]. While chem-icalmodification of nanotubes is expected to enhance thedispersion andload transfer at the polymer–nanofiller interface [11,19], it is known todegrade the electrical properties of the nanotube by creating insulatinglayer at the surface of nanotube [20]. Thus, for nanocompositewithmul-tifunctional characteristics, chemical modifications of nanofillers shouldbe avoided. This means that proper dispersion and good adhesion be-tween the polymermatrix and nanofiller should be achieved by physicalmeans.

In this study, CNT/PP nanocomposites filled with up to 20 wt.% CNTwere prepared by melt compounding followed by compression mold-ing. PP is a high-volume commodity polymer that has a wide-range ofapplications [21]. PP has low density, good resistance for many solventsand can be easily processed by melt compounding machines [22]. Thisstudy investigates the influence of a wide-range of CNT concentrationon the microstructure, electrical and mechanical properties of thenanocomposite.

2. Experimental details

2.1. Materials

The PP (PP 504P, Sabic, Saudi Arabia) has a density of 905 kg/m3 anda melt flow index of 3.2 kg/10 min (measured at 230 °C under 2.16 kgload). The nanotubes were multi-walled CNT (NC7000, Nanocyl,Belgium) produced by catalytic chemical vapor deposition. Accordingto the manufacturer, NC7000 nanotubes have an average diameter of9.5 nm, length of 1.5 μm, surface area of 250–300 m2/g and carbon pu-rity of 90%.

2.2. Nanocomposite compounding

The CNT/PP nanocomposites were prepared by melt compoundingin a batch mixer (Type W 50 EHT, Brabender, Germany) connectedto torque rheometer (Plastograph EC, Brabender, Germany). Beforemixing, the PP pellets and CNT powder were dried overnight in vacuumoven at 80 °C and 130 °C, respectively. In a typical experiment, 28.3 g ofPP was fed into a preheated (to 180 °C) mixing chamber and mixed for3.0 min to melt down the polymer. After that, a predetermined amountof CNT powder was fed to the mixing chamber and the whole mixturewas compounded for 10min at 100 rpm.At the endof the compoundingprocess, the nanocomposite was collected from the mixing chamberand left to cool at room temperature. A compression molding machine(Carver Inc., Wabash-IN, USA) was used to prepare 1.0 mm thick plates.The compression molding was conducted at 200 °C for 10 min under27.5 MPa pressure. For the electrical resistivity characterization, themolded plates were (40 × 20 × 1mm3) rectangles. For the tensile prop-erties characterization, initially (65 × 65 × 1 mm3) plates were pro-duced; then an ASTM D628-5-IMP die was used to cut dog-bone-shaped specimens.

2.3. Characterization tools

The nanocomposite microstructure was characterized using Quanta450 FEG Environmental Scanning Electron Microscope (ESEM). Prior to

imaging the samples were fractured in liquid nitrogen and coated witha thin layer of gold using sputtering machine (Q150R ES, QuorumTechnologies Ltd., UK). The electrical properties were characterizedusing two different setups depending on the nanocomposite resistivity.For samples with electrical resistivity greater than 106 Ω·cm, thecharacterization was conducted using Keithley 6517B electrometer(Keithley, Ohio, USA) connected to Keithley 8009 test fixture. Formore conductive materials, a set-up consisting of Keithley 2010 digitalmultimeter (DMM) connected to a 4-wire probe test fixture was used.The reported results represent an average of at least six specimens.Tensile testing was conducted according to the ASTM standard D638-03 using a tensile testing machine (WDW-20, Jinan Testing EquipmentIE Corporation, China). For each formulation, at least six dog-bone-shaped specimens (Type V ASTM D638-03) were tested at a crossheadspeed of 1 mm/min.

3. Results and discussion

3.1. Microstructure

The properties of a nanocomposite depend on its microstructure.SEM analysis was conducted to investigate the CNT particles dispersionstate and the adhesion between the CNT particles and PP matrix. Figs. 1and 2 show, respectively, representative SEM micrographs for 1 wt.%CNT/PP and 20 wt.% CNT/PP nanocomposites. At low CNT content(Fig. 1), it is apparent that there is a good level of CNT dispersion; how-ever perfect dispersion cannot be claimed since aggregates of few mi-crons in size can be observed. At high CNT content (Fig. 2), almost noCNT-aggregates can be seen. This apparent enchantment in CNT disper-sion can be ascribed to the increase in the shear stress with the increaseof CNT content, as shown in Fig. 4. The shear stress applies hydrodynam-ic forces on the nanofiller and enhances its dispersion. In addition, it isalso possible that at high CNT concentration, the well-dispersed nano-tubes and CNT-aggregates are undistinguishable because they areclose to each other.

Regarding the adhesion between CNT and PP, Fig. 3 depicts highmagnification SEM micrograph for one micron CNT aggregate. The mi-crograph clearly shows that the nanotubes at the external surface ofthe aggregate have good level of adhesionwith the PPmatrix. The aver-age diameter of nanotubes is much larger than the average diameter ofthe CNT particles before mixing, which is 9.5 nm. This means that thereis good adhesion between the CNT and the PP and/or it means thatthe CNT particles have acted as a nucleation agent and this layer is

Fig. 2. SEM micrograph of 20 wt.% CNT/PP nanocomposite.

0 2 4 6 8 10 12 14

0

10

20

30

40

50

60

70

80

90

Tor

que

(N·m

)

Time (min)

1 wt%3 wt%10 wt%15 wt%20 wt%

Fig. 4. Processing curves of CNT/PP nanocomposites as function of CNT content.

78 M.H. Al-Saleh / Materials and Design 85 (2015) 76–81

transcrystalline PP [23]. However, it is also apparent that there are voidsinside the CNT aggregate. This observationmeans that in some cases thePP chains were not able to infiltrate the CNT-aggregates.

3.2. Processing behavior

Fig. 4 depicts the effect of CNT concentration on the processing be-havior, i.e. mixing torque vs. time curve, of the CNT/PP nanocomposites.Two processing zones can be observed in thefigure; thefirst is the poly-mer pelletsmelting zone from t=0.0min to t=3.0min and the secondis the CNT/PP nanocomposite compounding zone from t = 3.0 min tot = 13.0 min. It is evident that the processing torque increases withthe increase in CNT concentration. For instance, the steady-state mixingtorque increased from ~10 N·m to ~14 N·m with the increase in CNTconcentration from 0.25 wt.% to 10 wt.%. This 40% increase in mixingtorque looks significant; however it is much lower than our previouslyreported increase in mixing torque for a CNT/ABS nanocompositewhere the mixing torque over the same concentration range was in-creased by more than 180% [24]. This difference in the level of increaseinmixing torquemay indicate that CNT has better dispersionwithin the

Fig. 3. SEM micrograph of CNT aggregate in 1 wt.% CNT/PP nanocomposite.

ABS matrix compared to the PP and/or the adhesion between CNT andABS is better than that between CNT and PP.

3.3. Mechanical properties

The exceptional mechanical characteristics, tensile modulus of 1 TPaand tensile strength of 63 GPa, and high aspect ratio of nanotubes maketheman ideal reinforcing agents formany compositematerials [4]. Fig. 5depicts the effect of CNT content on the tensile strength of CNT/PP nano-composite. It is evident that the incorporation of CNT in the PP matrixhas significantly enhanced the nanocomposite tensile strength. For ex-ample the 1 wt.% CNT/PP nanocomposite shows 18% improvement inthe tensile strength. Similar level of improvement in tensile strengthwas reported by Bao and Tjong [21] for 1 wt.% mutli-walled CNT/PPnanocomposite prepared by melt mixing in twin-screw extruderfollowed by injection molding and by Razavi-Nouri et al. [25] for1 wt.% single-walled CNT/PP nanocomposite prepared by melt mixingin batchmixer followed by compressionmolding. At high CNT contents,the 20% CNT/PP nanocomposite exhibited a tensile strength that is 1.8times that of the unfilled polymer. This finding proves that high concen-trations of CNT can be incorporated in PP without degrading the tensile

20

25

30

35

40

45

50

55

0 1 3 10 15 20

Ten

sile

Str

eng

th (

MP

a)

CNT wt%

Fig. 5. Tensile strength of CNT/PP nanocomposites as function of CNT content.

0%

2%

4%

6%

8%

10%

12%

14%

1 3 10 15 20

Elo

nga

tio

n a

t b

reak

CNT wt%

Fig. 6. Elongation at break of CNT/PP nanocomposites as function of CNT content.

79M.H. Al-Saleh / Materials and Design 85 (2015) 76–81

strength. However this finding contradicts with most of the publishedstudies that shows degradation of tensile strength of CNT/PP nanocom-posites at high CNT contents due to the increase in size and number ofCNT aggregates [22,26]. In most of the previous studies, the ultimatetensile strength was obtained at CNT content below 2.5 wt.% [22].

Then, what are the reasons for the enhancement in the tensilestrength of the CNT/PP nanocomposite with addition of up to 20 wt.%CNT? Typically, the enhancement in tensile strength can be related toenhancement in CNT dispersion, good compatibility between thenanofiller and polymer matrix and/or change in the polymer matrixproperties (such as crystallinity) as a result of CNT addition. Based onthe SEM analysis, perfect dispersion cannot be claimed for the CNT/PPnanocomposite since some aggregates were observed. In addition, it isunlikely that significant enhancement of tensile strength is only due tothe proper dispersion of the nanofiller particles. Thus, this factor is notexpected to be the controlling factor for the enhancement in the tensilestrength. Regarding the compatibility, the second possible factor, basedon the processing behavior curves (Fig. 4), it may be concluded that atthe processing temperature CNT and PP are not compatible since the in-crease in mixing torque with the addition of up to 5 wt.% CNT was notvery significant. Thus, it ismost likely that the development of the nano-composite microstructure during the nanocomposite cooling is respon-sible for the enhancement in the tensile strength.

Many researchers reported an increase in the percentage of crystal-linity and crystallization temperature of semi-crystalline polymers, in-cluding PP, as a result of incorporation of CNT particles [27] revealingthat CNT can act as heterogeneous nucleation agents. In addition, athigh CNT loadings, another crystallization peak was observed in thecrystallization thermogram of PP [27–29]. The appearance of this newpeak was attributed to the formation of transcrystalline layer (TCL) ofPP around the CNT particles. The TCL growth proceeds perpendicularto the fiber axes until it reaches the edge of spherulites nucleated inthe PP bulk [30]. The formation of TCL is expected to enhance the me-chanical properties of the nanocomposites providing that good adhe-sion between this layer and the bulk polymer can be achieved. Forcarbon nanofiber (CNF)/polyethylene (PE) composite [31,32], it was ob-served that layers of polyethylene were coating the external surface ofCNF; however, clear gapswere observed between the coated nanofibersand the PE bulk. Thus, no improvement in the mechanical properties ofCNF/PE nanocomposite was obtained. However, in the case of CNT/PPnanocomposite it is expected that there are entanglements betweenthe TCL and the PP bulk since no voids were observed between thewrapped nanotubes and the PP matrix. Thus an enhancement in me-chanical properties was obtained with the increase in CNT content.However, it is worth mentioning here that the role played by TCL atthe interfaces and its effect the mechanical properties of compositesmaterials is notwell understood and agreed upon yet.Many researchersbelieve that the TCL can improve the adhesion at the filler/polymer in-terface leading to improvement in the mechanical properties of com-posites. However, there is a second group of researchers whoconsiders the TCL of either no or even negative influence on the me-chanical properties [30]. Thus, more fundamental studies are neededto identify the magnitude and clarify the mechanism of stress transferbetween semicrystalline polymers and nanofillers [22].

In order to have a quantitative indication about the adhesion andtype of bonding at the CNT/PP interface, the interfacial shear stress (τ)was calculated based on the following model [33]:

σ c ¼ ηoτλ2

−σm

� �V f þ σm: ð1Þ

In themodel,σc andσm are the tensile strength of the composite andpolymer matrix, respectively, λ is the nanofiller aspect ratio, ηo is theorientation factor (ηo = 0.6 for randomly ordinated nanotubes) [33]and Vf is the nanofiller volume fraction. By substituting the measuredvalues and assuming that the aspect ratio is in the range of 50–80

(30%–50% of the initial aspect ratio before melt mixing [34]), it can befound that the interfacial shear stress is in the range of 36–58 MPa.This interfacial shear stressmeans that there is an excellent physical ad-hesion at the CNT/PP interface. It is worth mentioning that for systemswith chemical bonding at the interface, the interfacial shear stress willbe several times higher than the above reported values [4,31].

Figs. 6 and 7 show, respectively, the effect of CNT concentration onthe elongation at break and toughness of the CNT/PP nanocomposite.Remarkable reduction in both elongation at break and toughness withthe addition of CNT can be observed. For example, at 1 wt.% CNT, theelongation at break is 10.4%, where it is only 4.2% for the 20 wt.% CNTnanocomposite. For the nanocomposite toughness, an almost linear re-duction in toughnesswas obtainedwith the increase in CNTweight per-cent. Even though that at high CNT loadings, a significant increase intensile strength was obtained, the reduction in the nanocompositeductility was more pronounced on the nanocomposite toughness.

3.4. Electrical percolation behavior

A critical concentration of conductive nanofiller is required to createa network within an insulating polymer matrix. At this critical concen-tration, which is known as the electrical percolation threshold, thenanocomposite electrical resistivity is sharply decreased by several or-ders of magnitude. The electrical percolation threshold depends on sev-eral factors, of which the conductive nanofiller aspect ratio and itsdispersion state are of the most significance. Fig. 8 shows the electricalpercolation curve of CNT/PP nanocomposite. The nanocomposite ap-pears to transfer from the insulating state into the conductive state atCNT loading around 1 wt.%. At this concentration, the nanocompositeelectrical resistivity is about 10 orders of magnitude lower than that ofunfilled PP. Above the electrical percolation point, the increase in CNTconcentration resulted in remarkable reduction in the electrical resistiv-ity. For example, four orders of magnitude reduction in the electrical re-sistivity can be observed by increasing the CNT loading from 1 wt.% to5 wt.%. In order to estimate the electrical percolation threshold, the sta-tistical percolation theory power law, Eq. (1), was used.

ρ ¼ ρo Ф−Фcð Þ−t ð2Þ

In the equation, ρ is the nanocomposite volume resistivity, ρo is ascaling factor related to the filler intrinsic electrical resistivity, Ф is thefiller volume fraction, Фc is the filler electrical percolation thresholdand t is a critical exponent related to the system dimensionality [7,35].

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1 3 10 15 20

Tou

gh

nes

s (M

J/m

3 )

CNT wt%

Fig. 7. Toughness of CNT/PP nanocomposites as function of CNT content.

Fig. 9. Statistical power law fit.

80 M.H. Al-Saleh / Materials and Design 85 (2015) 76–81

For most of the CNT based nanocomposites, a critical exponent in therange of 1.3–4.0was reported [7]. The experimental data at CNT concen-tration ≥1 wt.% were fitted by plotting log (σ) versus log (Ф − Фc). Фc

was incrementally changed until the best linear fit shown in Fig. 9 wasobtained. In the calculations of CNT volume fraction, it was assumedthat the specific gravity of CNT is 1.66 [36].

The fitting of the electrical resistivity data indicated that the percola-tion threshold is 0.51 vol.% (0.93 wt.%) and the critical exponent is 2.2.Similar fitting parameters and percolation behavior were recently re-ported by Ameli and coworkers [37] for CNT/PP nanocomposite pre-pared by melt mixing in a DSM twin screw compounder. The criticalexponent is within the range of the previously reported values forCNT-based nanocomposites [7]. The very low electrical percolationthreshold is related to the good state of CNT dispersion within the PPmatrix and the presence of crystalline phase within the PP matrixthat is free of CNT. The PP spherulites force the CNT to localize in theamorphousphase of the PPmatrix leading tohigher effective concentra-tion of CNT within the amorphous phase. In addition, the fitting resultsshowed that the scaling factor is 1.4 × 10−3 Ω ⋅ cm. This factor is veryclose to our previously estimated values for the same CNT in an acrylo-nitrile–butadiene–styrene (ABS) [24] and ultrahigh molecular weightpolyethylene (UHMWPE) [38]. In addition, this value is supposed

0 2 4 6 8 10 12 14 16 18 2010-1

100

101

102

103

104

105

106

107

108

109

1010

1011

1012

1013

1014

1015

CNT wt%

ρ (Ω

.cm

)

Fig. 8. Electrical percolation curve of CNT/PP nanocomposite.

to reflect the intrinsic conductivity of the individual MWCNT [39].However, because of the effect of contact resistance between thenanotubes, the actual intrinsic volume resistivity of a single MWCNT isexpected to be less than 10−3 Ω ⋅ cm.

4. Conclusions

CNT/PP nanocomposite with remarkably enhanced tensile strengthand very low electrical percolation threshold was fabricated by conven-tional melt mixing. The SEM analysis showed good level of CNT disper-sion within the PP matrix, enhancement in CNT dispersion with theincrease in CNT content due to the increase in shear stress, and good ad-hesion between the CNT particles and PP matrix. It was also observedthat the average diameter of nanotubes inside the polymer matrix ismuch larger than that of the pristine-CNT due to the formation of PPlayers around the nanotubes. The polymer layers around the nanotubesmight be of transcrystalline nature due to the nucleation effect of CNT.However, it should be noted that the nanocomposite exhibited verylow electrical percolation threshold and high electrical conductivity athigh CNT content. This observation means that the layers around thenanotubes were formed after the conductive network formation; other-wise a non-conductive nanocomposite is expected. At 20 wt.% CNT, thenanocomposite exhibited high electrical conductivity with enhancedmechanical characteristics enabling the use of such material in ad-vanced applications such as the electromagnetic interference shielding.

Acknowledgment

The author thanks the Scientific Research Support Fund, Ministry ofHigher Education and Scientific Research, Amman — Jordan, for the fi-nancial support of this research (Grant Number Bas/2/05/2010).

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