Materials and Design 52 (2013) 128–133

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Materials and Design

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Carbon nanofiber/polyethylene nanocomposite: Processing behavior,microstructure and electrical properties

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.05.038

⇑ Corresponding author. Tel.: +962 2 7201000x22415; fax: +962 2 7204074.E-mail address: mhsaleh@just.edu.jo (M.H. Al-Saleh).

Mohammed H. Al-Saleh a,⇑, Genaro A. Gelves b, Uttandaraman Sundararaj b

a Department of Chemical Engineering, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordanb Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada T2N 1N4

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 March 2013Accepted 13 May 2013Available online 28 May 2013

Keywords:Composite materialMicrostructurePolymersElectrical conductivity

Electrically conductive polymer nanocomposite of high density polyethylene (HDPE) filled with carbonnanofibers (CNFs) were prepared by melt compounding in a batch mixer. The nanocomposite processingbehavior was studied by monitoring the mixing torque vs. time as function of filler content. Scanningelectron microscopy and optical microscopy were used to investigate the nanocomposite dispersion ofnanofiller and the adhesion between the nanofiller and polymer matrix. The electrical and electromag-netic interference (EMI) shielding behaviors of the nanocomposite were reported as function of nanofi-bers concentration, and an empirical correlation related the EMI SE to the nanocomposite’s electricalresistivity was developed. Good level of CNF dispersion was evident despite the poor adhesion exhibitedbetween the nanofibers and the HDPE matrix. At 1.5 vol% CNF loading, the nanocomposite exhibited anelectrical volume resistivity of 105 O�cm. EMI shielding effectiveness was found to increase with increasein nanofiller concentration. In the 0.1–1.5 GHz frequency range, 2 mm thick plate made of 5 vol% CNF/HDPE nanocomposite exhibits an EMI shielding effectiveness of 20 dB.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction P1

Electrically conductive polymer composites are typically pro-duced by blending conductive fillers with insulating polymermatrices. A critical concentration of filler known as the electricalpercolation threshold is needed to create conductive networkswithin the polymer matrix. At this critical concentration, the com-posite conductivity increases by several orders of magnitude. Con-ductive composite materials have several applications such asprotection of electronics from electrostatic discharge (ESD) andpreventing electromagnetic interference (EMI) to maintain func-tionality and compatibility of electronics [1–3]. The light weight,cost and adaptability to application needs are some of the advanta-ges that conductive composites have over metals for EMI shieldingapplications.

ESD is a serious problem for electronic devices. Electronics aresusceptible to ESD damage during manufacturing, assemblingand transportation. In order to protect electronics from ESD, pack-aging materials should have a surface resistivity in range of 106–109 O/sq. In addition to protection from ESD, electronic devicesshould be shielded to prevent outgoing and incoming EMI. Theshielding effectiveness (SE) of a material is the ratio of the incidentpower to the transmitted power. It is usually expressed in decibels(dB) according to the following equation:

SE ¼ 10 logP2

ð1Þ

where P1 and P2 are the incident power and transmitted power,respectively. Shielding effectiveness of 20 and 30 dB means that99 and 99.9%, respectively, of the EM waves have been attenuated.For most applications a SE greater than 20 dB is required. For auto-motive and computer industries, 30 dB is considered an adequate SEfor 50% of the applications [4].

Many options are available for shielding of EMI, including: me-tal sheets, conductive paints, electro-less plating and CPCs. Forexample, film/foil laminate and metal liners provide good shieldingat lower cost but they have limited design flexibility [5]. Metalcoated or plated polymers are widely used for EMI shielding. How-ever, conductive coatings can delaminate, the coating requires sev-eral expensive steps of fabrication, and coated materials are verydifficult to recycle. On the other hand, conductive composites havegreater design flexibility than coatings, but they suffer from thevery high filler loadings required to bring the composite materialto competitive shielding level. Thus, a composite with low fillerloading is required to overcome this problem. Technically, this ispossible by using filler with high electrical conductivity and highaspect ratio.

Conductive nanostructured polymeric materials are promisingalternatives for the currently used technologies for ESD and EMI.Because of their high aspect ratio and intrinsic conductivity, con-ductive nanofillers can create nanocomposites for ESD and EMIapplications at very low filler loading. The focus in this paper is

M.H. Al-Saleh et al. / Materials and Design 52 (2013) 128–133 129

on carbon nanofiber (CNF) based nanocomposites. Previous workshowed that CNF can enhance electrical, mechanical, and thermalproperties of polymer matrix nanocomposites at relatively lowloadings [6–8]. For example, only 0.5 vol% CNF was required toachieve electrical percolation threshold in polypropylene nano-composite [9]. Adding 1 wt% CNF enhanced the thermal conductiv-ity of CNF/epoxy nanocomposite by 45% [10]. Thermal stability ofmany polymers including PE [11], epoxy [12], PP [13,14], PS [15],PC [16,17], and PMMA [18,19] were improved by CNF addition.Addition of 8 vol% CNF was reported to enhance Young’s modulusand tensile strength of neat PP by 100% and 80%, respectively [20].In this work, CNF/high density polyethylene (HDPE) nanocompos-ites were prepared by melt compounding in a laboratory batchmixer. The study aims to investigate the processing behavior,microstructure, electrical percolation and EMI shielding propertiesof the nanocomposites as function of CNF concentration. The pro-cessing behavior was studied by analyzing the mixer torque vs.time curves as function of CNF addition and concentration. Scan-ning electron microscopy and optical microscopy were used tostudy the microstructure of the obtained nanocomposites in termsof adhesion between the nanofiller and polymer matrix and level ofnanofiller dispersion. The electrical percolation and EMI SE behav-iors were discussed and the relation between EMI SE and electricalconductivity was investigated.

2. Experimental details

2.1. Materials and procedure

The carbon nanofiller used in this work is low heat treated(LHT)-vapor grown carbon nanofiber (VGCNF) (PyrografIII™Applied Sciences, Inc., OH-USA). LHT-VGCNF is about 4.2 lmin length and 110 nm in diameter [21]. The polyethylene (HDPE-19G, MFI of 1.2 g/10 min (190 �C/2.16 kg), specific gravity of0.962) was kindly provided by Nova Chemicals. The CNF/HDPEnanocomposites were prepared by melt compounding in a Haakeinternal batch mixer (Haake Rheomix series 600 batch mixer, Ther-mo Scientific, Germany). Prior to compounding, the HDPE pelletsand CNF powder were placed in a vacuum oven at 70 �C and130 �C, respectively for about 16 h. The melt mixing conditionswere: mixing speed 50 rpm, mixing temperature 180 �C and mix-ing time 9 min (3 min pure polymer melting, 2 min CNF feedingand 4 min additional mixing). Circular disks for electrical resistiv-ity and EMI SE measurements, were prepared by compressionmolding using Carver compression molder (Carver Inc., Wabash-IN, USA) and had dimensions of 2.0 mm thickness and 133 mmdiameter. The compression molding conditions were as follows:temperature 200 �C, time 4 min and pressure 18.5 MPa. For eachformulation, at least two specimens were prepared andcharacterized.

Fig. 1. Processing behavior curves of CNF/HDPE nanocomposites.

2.2. Characterization

2.2.1. MorphologyScanning electron microscopy (SEM) and optical microscopy

(OM) were used to investigate the structure of the nanocompos-ites. For SEM characterization, the nanocomposites were examinedusing JEOL 6301F (Field Emission Scanning Electron Microscope).SEM specimens were fractured in liquid nitrogen and coated withgold prior to imaging. For the optical microscopy characterization,ZEISS AXIO Scope A1 microscope was used. The OM specimenswere thin sections of about 1 lm cut at �80 �C using Leica cryo-ultramicrotome (EM UC6/FC6).

2.2.2. Electrical resistivity measurementsTwo different set-ups were used to measure the electrical resis-

tivity of the CNF/HDPE nanocomposites. For samples with electri-cal resistivity higher than 106 O�cm, the characterization wasconducted according to ASTM: D257 standard using a Keithley6517A electrometer connected to Keithley 8009 test fixture (Keith-ley Instruments, USA). For samples with lower electrical resistivity,measurements were conducted using a Loresta GP resistivity meter(MCP-T610 model, Mitsubishi Chemical Co., Japan) connected witha four-pin probe (MCP-TP08P model, Mitsubishi Chemical Co., Ja-pan). The inter-pin spacing is 5 mm and the pin diameter is 2 mm.

2.2.3. Shielding effectivenessEMI SE of the CNF/HDPE nanocomposites was conducted

according to the ASTM: D4935-99 standard for planar materials.The set-up consists of RF network analyzer (HP 8752C, Hewlett–Packard, USA), EMI shielding test fixture (EM-2017A, Electro-Met-rics Crop, Johnstown-NY, USA), two coaxial cables and two 10 dBattenuators. The set-up dynamic range is 80 dB. More informationabout the set-up can be found elsewhere [22]. For each formula-tion, a minimum of two load specimens were tested in the fre-quency range of 0.1–1.5 GHz. The input power used for all testswas 1 mW.

3. Results and discussion

3.1. Processing and microstructure

Fig. 1 depicts the mixing torque vs. time curves of CNF/HDPEnanocomposites as function of filler content. Three major process-ing zones can be observed in the figure. The first zone, fromt = 0 min to t = 3.0 min, is the pure polymer melting zone. The sec-ond zone, from t = 3.0 min to t = 5.0 min, is the CNF feeding zone. Inthis operation window, CNF was gradually fed to the mixture in or-der to avoid any sudden increase in torque and to avoid overflowproblems especially for nanocomposites with high nanofiller con-tent. The last zone, from t = 5.0 to t = 9.0 min, is the nanocompositecompounding zone. The increase in mixing torque with the in-crease in CNF loading is evident. However, this level of increaseis insignificant. For example, at the end of compounding, the mix-ing torque of the 5 vol% and 10 vol% nanocomposites were10.7 N�m and 13.4 N�m, respectively. Moreover, for the 1 vol%nanocomposite, the addition of nanofiller has negligible effect onthe mixing torque. For this nanocomposite the recorded mixingtorque was 9.5 N�m before the addition of CNF and 9.4 N�m atthe end of the compounding process.

130 M.H. Al-Saleh et al. / Materials and Design 52 (2013) 128–133

The processing behavior of CNF/HDPE is linked to the nanocom-posite’s microstructure, i.e. adhesion between filler and polymermatrix and the filler dispersion, distribution and aspect ratio. The-oretically, it is expected that the increase in filler volume fractionduring mixing leads to degradation of the filler aspect ratio dueto the increase in shear stress and filler/filler collisions. On theother hand, the increase in mixing torque that is associated withthe increase in filler concentration might enhance the dispersionand distribution of filler or it might enhances the filler’s tendencyfor agglomeration because of the space limitations. Fig. 2 showsSEM micrographs of 7.5 vol% CNF nanocomposite at two differentmagnifications. In general, the micrographs show good dispersionof the nanofiller in the HDPE matrix. In Fig. 2b, many well dis-persed nanofibers (several microns in length) can be observed. Inaddition, the micrograph shows voids between nanofibers and

CNF

(a)

(b)Fig. 2. SEM micrographs of 7.5 vol% CNF/HDPE nanocomposite at 2 differentmagnifications. The scale bars are: 10 lm in image (a) and 1 lm in image (b).

the polymer matrix indicating poor state of adhesion betweenthe nanocomposite constituents. This observation explains themarginal increase in mixing torque with the addition of nanofiller.

Fig. 3 shows typical optical micrographs of CNF/HDPE at twodifferent nanofiller loadings. This type of images is useful to detectlarge agglomerates within the polymer matrix that are more diffi-cult to observe using SEM and TEM micrographs. It is apparent thatboth nanocomposites contain agglomerates of several microns indiameter. As received CNF as shown in Fig. 4 contains both individ-ual nanofibers and agglomerates. Thus, the agglomerates seen inthe optical micrographs might have been formed due to the colli-sion between CNF during mixing or it might be that the some/allof the agglomerates in the as-received CNF are not dispersible un-der normal mixing conditions. Further investigation is required toknow the origin of nanofiller agglomerates within the polymermatrix.

3.2. Percolation behavior

Fig. 5 illustrates the electrical percolation behavior of the meltmixed CNF/HDPE nanocomposite at room temperature. It is appar-

Fig. 3. Optical micrographs of CNF/HDPE nanocomposites containing (a) 0.75 vol%CNF (b) 5 vol% CNF. The black areas are the CNF agglomerates.

CNF aggregate

Fig. 4. SEM of as-received CNF [34].

M.H. Al-Saleh et al. / Materials and Design 52 (2013) 128–133 131

ent that the electrical percolation threshold is between 1.25 vol%and 1.5 vol%; in this range the electrical resistivity (q) suddenly de-creased from 1015 O�cm to 105 O�cm (almost ten orders of magni-tude). Below 1.25 vol% CNF the nanocomposite was insulativeand has electrical resistivity close to that of unfilled polymer. At1.5 vol% CNF, the nanocomposite is conductive and suitable forESD protection applications and electrostatic painting applications.Increasing filler concentration from 1.5 vol% to 5 vol% further re-duced the electrical resistivity by another 5 orders of magnitude.For the nanocomposites with filler loading higher than 5 vol%,the increase of filler concentration had little effect on the nano-composite’s electrical resistivity because the 3D network has beenbuilt and the nanocomposite’s electrical resistivity become veryclose to its ultimate value.

The electrical percolation threshold can be estimated using thepercolation power law equation q ¼ qoðm� mcÞ�t� �

. In this equa-tion, the electrical resistivity vs. volume concentration data ofthe nanocomposite above the electrical percolation concentrationis fitted to find the percolation threshold [23–25]. In the equation,q is the nanocomposite’s electrical resistivity, qo is scaling factor, mis the volume fraction of filler, mc is the electrical percolationthreshold volume fraction and t is a critical exponent. The best fitof the resistivity data using the power law equation is depictedin the inset of Fig. 5. The fitting results showed that the percolationconcentration (mc) and critical exponent (t) of the CNF/HDPE nano-composite are 1.4 vol% and 2.7, respectively. This percolationthreshold is very low compared to what has been reported forpolyethylene nanocomposites. For example, a percolation thresh-old of 7.5 wt% (�4 vol%) was reported for multiwall carbon nano-tubes (MWCNTs)/linear medium density PE nanocompositeprepared in HAAKE mini-screw extruder [26]. Likewise, using

Fig. 5. Percolation behavior of CNF/HDPE nanocomposite.

DACA micro compounder 4 wt% (�4 vol%) filler was required topercolate single wall carbon nanotubes (SWCNTs)/HDPE nanocom-posite [27]. In addition, for a range of melt mixed carbon black/polyethylene nanocomposites, percolation threshold in the rangeof 2.5–5.0 vol% was reported [28–31]. The lower percolationthreshold of CNF/HDPE compared to those of CB/polyethylenenanocomposites is expected and can be mainly attributed to thehigher aspect ratio of CNF compared to CB. However, in compari-son with MWCNT and SWCNT nanocomposites where the fillershave higher aspect ratio than CNF, it seems that the dispersion ofCNF in polyethylene is better than what have been obtained forthe dispersion of CNTs. Thus, CNF/HDPE nanocomposite exhibitslower percolation threshold than of CNT/HDPE nanocomposites.

The last issue for discussion about the percolation behavior ofCNF/HDPE nanocomposite is the critical exponent of the powerlaw. This exponent is related to the filler geometry and filler dis-persion within the matrix. For CNT/polymer nanocomposites, crit-ical exponent values in the range of 1.3–4.0 have been reported.For our CNF/HDPE nanocomposite, the critical exponent was foundto be 2.7. According to Balberg [32] the high critical exponent value(t > 2.0) is due to the presence of barriers between filler particles.Many crystalline polymer, such as polypropylene and polyamide,were reported to form crystalline layer around the nanofiller andby this they create tunneling barriers between the fibers [33]. Thisanalysis agrees with our previously reported SEM observations andcharacterizations in which layers of polyethylene were found coat-ing the surface of CNF [34].

3.3. EMI shielding effectiveness

Preventing electronic devices from emitting or being affected byEMI is essential to preserve their electromagnetic compatibility.EMI SE of a material depends on several factors including the con-ductivity, magnetic properties, and thickness of the shieldingmaterial and the frequency of the EMI. Fig. 6 shows EMI attenua-tion characteristics of 2 mm thick CNF/HDPE nanocomposites asfunction of filler content and radiation frequency. The EMI SE ofCNF/HDPE nanocomposites with filler loading of 10 vol% or lessare clearly seen to be independent on the frequency. However,the 15 vol% CNF/HDPE nanocomposite shows an increase of EMISE by 7 dB from 37 dB to 44 dB with the increase in frequency from100 MHz to 1500 MHz.

For the effect of CNF loading, the increase in EMI SE with in-crease in CNF loading is evident and this increase is not linear asshown in Fig. 7. The increase in overall shielding with increasingfiller concentration is due to the increase in nanocomposite’s con-ductivity. The increase in conductivity enhances the shielding byreflection and the shielding by absorption which are added to-gether to give better overall SE. At 1 GHz, the EMI SE increased

Fig. 6. EMI SE of CNF/HDPE nanocomposites as function of frequency and fillerconcentration.

Fig. 7. EMI SE in the 0.1–1.5 GHz frequency range of 2 mm thick plate made of CNF/HDPE nanocomposite as function of CNF content.

Fig. 8. Relation between volume resistivity and EMI SE of 2 mm thick CNF/HDPEplates.

132 M.H. Al-Saleh et al. / Materials and Design 52 (2013) 128–133

from 21 dB to 35 dB by increasing CNF from 5 vol% to 10 vol%. The5 vol% CNF/HDPE nanocomposite is suitable for many applicationsincluding laptops and desktops shielding.

A limited number of studies concerning the EMI SE of CNF/poly-mer nanocomposites in the 0.1–1.5 GHz frequency range is avail-able. EMI SE of 26 dB was reported for shielding plate 1.45 mmin thickness made of 15 wt% VGCNF/liquid crystal polymer nano-composite [4]. For 7.5 vol% CNF/polystyrene (PS) nanocompositeprepared by melt mixing, 24 dB was the EMI SE of a 2 mm thickplate [35]. In this study, the EMI SE of a similar plate (2 mm inthickness made of 7.5 vol% CNF/HDPE nanocomposites) is around

Table 1Experimental and estimated EMI SE of 2 mm thick nanocomposite plates.

Nanocomposite q (O�cm) EMI SE (dB) estimate

CNF/PS 1.5 27.8CNF/PS 1.4 28.7CNF/PS 2.1 25.5CNF/PS 1.8 26.6CNF/HDPE 1.2 29.8CNF/HDPE 2.7 23.6CNF/HDPE 4.8 19.3CNF/HDPE 12.8 12.1CB/PP/PS 9.2 14.6CB/PP/PS 6.5 17.1CB/PP/PS 5.4 18.4CB/PP/PS 2.6 24.0CB/PP/PS 5.5 18.3

30 dB. The higher EMI SE of the CNF/HDPE compared to the CNF/PS nanocomposite reveals that the dispersion of CNF within theHDPE matrix is better than that in the PS matrix.

3.4. EMI SE vs. electrical resistivity

Electrical resistivity and EMI SE are related properties, i.e. EMISE increases with decrease in electrical resistivity. However, forsome types of composites where depletion of fibers from the sur-face is possible, electrical resistivity measurement will not be accu-rate and consequently cannot be used as a proof for no relationbetween the conductivity of a polymer composite and its EMIshielding. For the CNF/HDPE nanocomposites, Fig. 8 clearly showsthat EMI SE increases with decrease in nanocomposite’s electricalresistivity. This increase occurs at two different rates dependingon the nanocomposite electrical resistivity. From the figure it isevident that the change in the rate of increase in EMI SE with thedecrease in electrical resistivity occurs at a critical electrical resis-tivity of about 20 O�cm. At this electrical resistivity, the EMI SE isabout 10 dB. It is worth noting that this electrical resistivity is dif-ferent from the electrical percolation threshold resistivity whichwas about 1 � 105 O�cm. This observation implies that connectivityis not enough to achieve good shielding of interferences. Both con-nectivity and high conductivity are required to achieve high levelsof EMI attenuation. For nanocomposites with electrical resistivityabove 20 O�cm, the increase in the EMI SE with the decrease inelectrical resistivity is marginal and insignificant. For nanocompos-ites with electrical resistivity below 20 O�cm, a perfect linear rela-tion (R2 = 0.9915) between the EMI SE and the logarithm of volumeresistivity is evident. In this region, EMI SE significantly increasedwith the decrease in the electrical resistivity. For example, bydecreasing the electrical resistivity from 3.5 O�cm to 1.1 O�cm,the EMI SE increased from 21 dB to 30 dB. According to the linearfit, the relation between the electrical resistivity (q) and EMI SEat 1 GHz of 2 mm thick plate made of CNF-nanocomposite is:

EMI SE ¼ �17:1 logðqÞ þ 31:0 ð2Þ

In order to verify the applicability of this correlation to othernanocomposite systems, we have estimated the EMI SE of differentelectrically conductive nanocomposites using the above correla-tion and compared the estimated values with the experimentalfindings as listed in Table 1. The experimental EMI SE values inthe table were obtained from Refs. [22,35,36]. Generally speakingthe correlation provided very close estimation for most of thenanocomposites characterized. This is clear for nanocompositesbased on the same type of filler (CNF in this case) where on averagethe difference between the estimated and the experimental was5%. For nanocomposites based on CB, on average the experimentalvalues were about 13% less than the estimated values.

d EMI SE (dB) experimental Difference (%)

27.7 �0.427.7 �3.924.4 �4.426.2 �1.429.5 �1.222.6 �4.618.6 �3.913.7 11.713.6 �7.115.1 �13.216.6 �11.122.1 �8.415.3 �19.5

M.H. Al-Saleh et al. / Materials and Design 52 (2013) 128–133 133

4. Conclusions

CNF/HDPE nanocomposites were prepared by melt mixing in abatch mixer. The processing behavior, structure, electrical and EMISE properties of the nanocomposites were studied as function ofnanofiller concentration. The mixing torque marginally increasedwith addition of CNF due to the poor adhesion between nanofibersand the HDPE matrix. The SEM and optical micrographs showedgood level of nanofibers dispersion where well dispersed individualnanofibers and some agglomerates were observed. It was found that1.4 vol% CNF was required to create a continuous conductivenetwork within the HDPE matrix. Increasing filler concentration be-yond the percolation point increased the nanocomposite’s electricalconductivity and EMI SE. Nanocomposites with CNF loading higherthan 5 vol% are suitable for shielding applications. The highest EMISE found in this work was 43 dB for a 2 mm thick plate made of15 vol% CNF/HDPE nanocomposite. For the 15 vol% nanocomposite,the EMI SE was found to increase from 37 dB to 43 dB with increas-ing frequency from 0.10 to 1.5 GHz. All other formulations showedalmost independence of EMI SE on the electromagnetic radiationfrequency in the 0.1–1.5 GHz frequency range. For nanocompositeswith electrical resistivity below 20 O�cm, EMI SE was found to line-arly increase with the logarithm of volume resistivity.

Acknowledgment

The authors would like to thank Dr. Bin Lin for collecting theoptical images. Part of this paper was presented in ANTEC 2012,Orlando-, FL, USA.

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