Composites Science and Technology 71 (2011) 1563–1568

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Composites Science and Technology

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Electromagnetic interference shielding behavior of poly(trimethyleneterephthalate)/multi-walled carbon nanotube composites

Anju Gupta, Veena Choudhary ⇑Centre for Polymer Science & Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India

a r t i c l e i n f o

Article history:Received 29 October 2010Received in revised form 7 June 2011Accepted 21 June 2011Available online 6 July 2011

Keywords:A. Carbon nanotubeA. PolymerB. Electrical propertiesElectromagnetic shielding interference

0266-3538/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compscitech.2011.06.014

⇑ Corresponding author. Tel.: +91 11 26591423; faxE-mail address: veenach@hotmail.com (V. Choudh

a b s t r a c t

Poly(trimethylene terephthalate) [PTT]/multiwalled carbon nanotube [MWCNT] composites having vary-ing amounts of MWCNTs were fabricated with an aim to investigate the potential of such composites asan effective light weight electromagnetic interference (EMI) shielding material in the frequency range of12.4–18 GHz (Ku-band). PTT/MWCNT composite with shielding effectiveness (SE) of 36–42 dB wasobtained at 10% (w/w) MWCNT loading. Shielding mechanism was studied by resolving the total SE intoabsorption (SEA) and reflection loss (SER). PTT/MWCNT composite showed absorption dominated shield-ing; thus it can be used as microwave, radar absorbing and stealth material. The effect of MWCNT load-ings on electrical conductivity (r) and dielectric properties of PTT and the correlation amongconductivity, tan d, absorption loss and reflection loss were also studied.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Electromagnetic interference (EMI) is one of the most undesir-able by-products of rapid proliferation of electronic products andtelecommunications. Mutual interference among devices such asTVs, computers, mobile phones and radios degrade device perfor-mance [1]. Electromagnetic radiation also adversely affects humanhealth. Hence, efforts were made to reduce its effect using EMIshielding materials. Traditionally, metals, in the form of thin sheetsor sheathing, were used as EMI shielding materials. However, met-als were expensive, heavy, prone to corrosion and difficult to pro-cess [2]. Hence, polymer composites containing conductive fillerswere developed as an alternative EMI shielding material. Thesecomposites are light weight, cheap, resistant to corrosion and eas-ily processable [3–5].

The EMI shielding effectiveness (SE) of filler reinforced compos-ites depends on many factors, including filler’s intrinsic conductiv-ity, real part of permittivity and aspect ratio [6–8]. The excellentproperties of carbon nanotubes (CNTs) like small diameter, highaspect ratio, high conductivity and high mechanical strength makeit the most promising filler for fabricating high-performance EMIshielding composites. Recently, the EMI shielding behavior of sin-gle-walled (SWCNTs) and multi-walled carbon nanotubes(MWCNTs) based composites have been explored with variouspolymer matrices [1,9–12]. Yang et al. [13] studied the EMI shield-ing behavior of MWCNT/polystyrene (PS) composites and achievedSE of 20 dB at 7 wt% MWCNT loading. They also showed that pres-

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: +91 11 26591421.ary).

ence of MWCNT enhances the shielding effectiveness of carbonnanofiber (CNF) [14]. Liu et al. [15] studied SWCNT based polyu-rathene (PU) composites and obtained an EMI SE of 17 dB at20 wt% SWCNT loading in the frequency range of 8.2–12.4 GHz.However, Haung et al. [16] achieved much higher EMI SE (20–30 dB) at comparatively lower loading (15 wt% SWCNT) in epoxymatrix.

EMI shielding material can be classified on the basis of its dom-inant shielding mechanism, understanding of which lead to itsoptimum use in term of its effectiveness and cost. Three types ofEMI shielding mechanisms have been proposed, namely: reflection,absorption and multiple reflections [17]. Reflection depends onnumber of charge carriers or conductivity of the material; absorp-tion depends on presence of electrical or magnetic dipoles whereasmultiple reflections are the total internal reflections within thematerial. In polymer composites, shielding mechanisms are morecomplicated than those of homogeneous conductive barriers be-cause of large surface area available for reflection and multiplereflections. In recent studies on MWCNT composites, it is observedthat absorption loss is the major contributor to the total EMI SE(SEtotal). As absorption based EMI shielding materials are in grow-ing demand due to development of radar, microwave communica-tion technology, stealth (self concealing) technology, microwavedarkroom and anti-EMI coating, more in-depth understanding ofEMI shielding property of CNT reinforced composites will broadenits application area.

Polyester polymers like poly(ethylene terephthalate) [PET] andpoly(butylene terephthalate) [PBT], with metals and conductingpolymers have been investigated for EMI shielding applications[18–20]. But to the best of our knowledge no work has been re-

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ported so far on EMI shielding behavior of CNT reinforced polyestercomposites specifically with poly(trimethylene terephthalate)[PTT]. PTT is a newly commercialized aromatic polyester havingproperties in between those of PET and PBT. It has the outstandingphysical properties of PET and processing characteristics of PBT,which makes PTT highly suitable for engineering applications. Re-cent studies show that mechanical, thermal and electrical proper-ties of PTT can be improved by incorporation of CNTs, but noemphasis has been made on evaluating its EMI shielding properties[21–24]. Hence, through present study, we tried to introduce PTTin the family of EMI shielding materials by fabricating MWCNTreinforced PTT composites. PTT/MWCNT composites were pre-pared by melt compounding with varying MWCNT loading. And ef-fect of MWCNTs loading on electrical conductivity (r), dielectricproperties and EMI SE of PTT was measured. The EMI shieldingproperty of PTT/MWCNT composites was measured in the fre-quency range of 12.4–18 GHz (Ku-band). The shielding mechanismof PTT/MWCNT composites was studied by quantifying the contri-bution of absorption and reflection loss to the total EMI SE. The cor-relation among conductivity, tan d, absorption loss and reflectionloss was also investigated.

2. Experimental

2.1. Materials

MWCNTs were produced by thermal decomposition of toluenein presence of iron catalyst. Scanning electron microscope (SEM)and transmission electron microscope (TEM) were used to deter-mine the average length, inner and outer diameter of carbon nano-tubes, which were �20–30 lm, 10 nm and 20–40 nm, respectively.The purity of carbon nanotubes determined from thermogravimet-ric analysis (TGA) was around 88%. The virgin PTT was procuredfrom Futura Polymers (Chennai, India) having viscosity averagemolecular weight (Mv) of 44,000. Mv was determined from intrinsicviscosity using Mark–Houwink equation {[g] = K(Mv)a}. The valuesof K and a used were 5.36 � 10�4 g/dL and 0.69, respectively [25].

2.2. Sample preparation

PTT/MWCNT composites were prepared by melt mixing usingco-rotating Dutch State Mines (DSM) micro-compounder. Prior tocompounding, PTT and MWCNTs were vacuum dried for 12 h at100 �C. Compounding was done at 265 �C for 5 min at a screwspeed of 170 rpm. MWCNT loading was varied from 0.5% to 10%(w/w) and samples were designated as PTT, PTT-0.5, PTT-1, PTT-2, PTT-3, PTT-5 and PTT-10 having 0%, 0.5%, 1%, 2%, 3%, 5% and10% (w/w) MWCNT loading, respectively. For EMI shielding andconductivity measurements, rectangular pellets of dimensions22.86 � 10.14 � 2 mm3 and 13 � 7 � 1.5 mm3, respectively weremolded in a Carver compression molder at 260 �C for 5 min under10 kPa pressure.

Since PTT is prone to hydrolytic degradation in presence ofmoisture which can deteriorate properties of composites, intrinsicviscosity measurements were carried out before and after com-pounding using single point viscosity method [25]. Measurementswere performed at 30 ± 0.1 �C using phenol: 1,1,2,2-tetrachloro-ethane [3:2] as solvent at a concentration of 0.5% (w/v). No changein viscosity of PTT was observed before and after compounding andthe viscosity values were in the range of 0.89–0.87 dL/g, which waswithin the experimental error limit. Thus, we can say that com-pounding has no deteriorating effect on the molecular weightand change in the physical properties of PTT is due to the intrinsicproperties of filler.

The density of composites was also determined from mass/vol-ume method. The density of neat PTT was 1.28 g/cm3 and that ofcomposite ranged from 1.29 to 1.35 g/cm3. The density of compos-ites increased marginally with increasing amount of MWCNT;however it was much lower compared to metals.

2.3. Characterization

Morphology of PTT/MWCNT composites was investigated usingSEM (EVO-50) and TEM (Technai G20-stwin). For TEM studies, ul-tra-thin sections (30–80 nm) of composites were prepared usingLeica Ultamicrotome. The dc electrical conductivity of PTT/MWCNTcomposites was determined using standard four-point contactmethod on Keithley SCS 4200. The EMI SE and dielectric propertiesof PTT/MWCNT composites were measured using a two port VectorNetwork Analyzer (VNA E8236B Agilent Technologies) in the fre-quency range of 12.4–18 GHz (Ku-band). For each sample 201 datapoints were taken within the specified frequency range.

3. Results and discussion

3.1. Morphological characterization

SEM and TEM images of sample PTT-1 and PTT-5 are shown inFig. 1a–d. The images show that MWCNTs are homogeneously dis-persed and distributed in the polymer matrix. TEM (c and d)images also show that MWCNT are hollow, cylindrical and ran-domly distributed in polymer matrix. MWCNTs exhibit a highlycurved and random coiled feature that is due to intrinsic van derWaals attractions between individual nanotubes in combinationwith high aspect ratio and large surface area. Fig. 1d shows over-lapping of nanotubes, indicating the formation of interconnectednetwork structure of MWCNTs in PTT matrix.

3.2. Electrical conductivity of PTT/MWCNT composites

The dc conductivity of PTT/MWCNT composites as a function ofMWCNT content (% w/w) is shown in Fig. 2. The conductivity ofPTT/MWCNT composites increased with increasing MWCNT con-tent and displayed a sharp increase (�five orders of magnitude)at 1% (w/w) MWCNT loading, indicating the formation of percolat-ing network or a continuous path for electrical conduction. Thebehavior of electrical conductivity with MWCNT loading can be de-scribed by power law relation as represented by Eq. (1) [26]:

r / ðv � vcÞb ð1Þ

where r is the composite conductivity; m is volume fraction ofMWCNT; mc is the volume fraction of MWCNTs at percolationthreshold and b is the critical exponent, which is related to thesystem dimension.

Since, the density of nanotubes can only be approximately esti-mated therefore, mass fraction (q) of MWCNT is used instead ofvolume fraction (m). The value of b was obtained from the slopeof log (r) vs. log (q � qc)/qc plot, which is shown as inset inFig. 2. Theoretical values of critical exponent (b) for a three dimen-sional (3D) percolating systems varies from 1.6 to 2.0 [27], whileexperimental values for carbon nanotube composites vary from0.7 to 3.1 [28–30]. In the present work, least square fit line of theconductivity data in the log–log plot gave b = 1.8 at qc = 1% (w/w)thus, indicating the formation of well dispersed 3D percolatingnetwork at �1% (w/w) MWCNT loading. Such a low percolationthreshold concentration could be due to the combined effect ofefficient dispersion and large aspect ratio of MWCNTs [31].

Fig. 1. SEM and TEM images of PTT composites (a and c) PTT-1 and (b and d) PTT-5.

Fig. 2. Plot of electrical conductivity (r) vs. MWCNT (wt%) in PTT/MWCNTcomposites [inset shows the log–log plot of r vs. (q � qc)/qc].

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3.3. Dielectric properties of PTT/MWCNT composites

According to the theory of permittivity, when a material comesunder the influence of an electromagnetic field, the electric field in-duces two types of electrical currents within the material, i.e., con-duction and displacement current. The former arises due to thepresence of free electrons for conduction and gives imaginary partof permittivity (ei). The latter arises due to the bound charges, i.e.,polarization and gives real part of permittivity (er). CNTs alwayscontain lattice defects like: vacancies, interstitial bonding and COor OH attachments [32], which act as active centers for the interac-tion of polymeric chains on the surface of CNTs and enhance realpart of permittivity. Similarly, imaginary part of permittivity in

CNT/polymer composites increases with increasing CNT contentdue to high conduction current.

Fig. 3a and b shows frequency dependent spectra of real andimaginary part of permittivity of composites containing varyingamounts of MWCNT. It is observed that both real and imaginarypart of permittivity increase with increasing MWCNT contentand decrease with increasing frequency from 12.4 to 18 GHz. Atlower loadings of MWCNT, frequency has little effect on both realand imaginary part of permittivity, but at higher loadings, signifi-cant decrease is observed, e.g., in sample PTT-5 and PTT-10. The ef-fect of frequency on permittivity is directly related to polarization,i.e., as the frequency of the field is raised, the periodic reversal ofelectric field occurs so quickly that there is no excess ion diffusionin the field direction. Hence, polarization due to charge accumula-tion decreases, leading to decrease in permittivity value withincreasing frequency [33]. However, with increasing MWCNT load-ing, both real and imaginary part of permittivity increase and at10% (w/w) MWCNT loading, both increase by 7 and 170 times at12.4 GHz, respectively. Such a significant increase in both realand imaginary part of permittivity is due to increase in conductiv-ity and dipole moment of PTT/MWCNT composites. It is also inter-esting to note that at higher MWCNT loadings, the real andimaginary part of permittivity show a complex fluctuation in themeasured frequency range, like; a broad peak can be seen between15 and 17.5 GHz. This result suggests the existence of resonancephenomenon, which is expected in case of highly conductive com-posites as skin effect becomes significant [34].

Fig. 4 shows the plots of tan d vs. frequency [12.4–18 GHz] ofPTT/MWCNT composites as a function of MWCNT loading. Since,tan d value indicates the absorptive property of a material, i.e.,the ability of a material to convert applied energy into heat; mate-rials with high tan d value are used as microwave absorbing mate-rials and in stealth technology [35]. The tan d values of PTT/MWCNT composites increased with increasing MWCNT loadingand frequency. The effect of frequency is more pronounced at high-

Fig. 3. Plot of real (er) and imaginary (ei) part of permittivity vs. frequency in PTT/MWCNT composites.

Fig. 4. Plot of tan d vs. frequency in PTT/MWCNT composites.

Fig. 5. (a) EMI SE of PTT/MWCNT composites in frequency range from 12.4 to 18 GHz and

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er MWCNT loading, e.g., in case of samples PTT-10 [10% (w/w)MWCNT loading] tan d increased from 2.73 to 3.78 as the fre-quency increases from 12.4 to 18 GHz. The increase in tan d valueof PTT upon incorporation of MWCNT can be attributed to thephase transition of material, i.e., conversion from insulator to con-ducting material [2].

3.4. EMI SE and shielding mechanism of PTT/MWCNT composites

EMI SE of a material is defined as the attenuation of propagatingelectromagnetic waves produced by the shielding materials. Thus,the total EMI SE (SEtotal) can be represented by sum of contribu-tions from absorption loss (SEA), reflection loss (SER) and multiplereflections (SEM) i.e. SEtotal = SEA + SER + SEM. The EMI SE of a mate-rial can also be defined as the ratio of transmitted power to inci-dent power [36] and can be represented by the following equation

SEðdBÞ ¼ �10 logðPt=P0Þ ð2Þ

where Pt and P0 are the transmitted and incident electromagneticpowers, respectively.

Eq. (2) was used to experimentally determine the contributionof absorption and reflection loss. As we know, when electromag-

(b) contribution of reflection and absorption to EMI SE in PTT/MWCNT composites.

Fig. 7. Effect of MWCNT content on the tan d and SEA of PTT.

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netic radiation is incident on a shield, basically three phenomenaoccur namely, absorption (A), reflection (R) and transmission (T)whose sum (A + R + T) is equal to one. The value of transmittancecan be measured from the ratio of Pt to P0 i.e., T = (Pt/P0). Thus, SE-total of shielding material can be written as SEtotal = �10 log T, andso the effective absorbance (Aeff) can be defined as

Aeff ¼ ð1� R� TÞ=ð1� RÞ ð3Þ

The effective absorbance represents loss due to absorption andmultiple reflections. Hence, SE due to reflectance and effectiveabsorbance can be described as SER = �10 log(1 � R) andSEA = �10 log[T/(1 � R)]. In the present study, these equations wereused to determine the contribution of absorption and reflectionloss.

Fig. 5a shows the effect of MWCNT loading and frequency onthe EMI SE of PTT/MWCNT composites. From Fig 5a, we can saythat the EMI SE of composites is almost independent of frequencyexcept at 10% MWCNT loading; however SE increased withincreasing MWCNT content. The EMI SE > 23 dB is obtained justat 5% (w/w) MWCNT loading which is higher than the required va-lue of EMI shielding effectiveness (20 dB) for commercial applica-tions. Such a high value of EMI SE at sufficiently low loading ofMWCNTs shows the efficiency of compounding technique (i.e.good dispersion) and excellent properties of MWCNTs like highconductivity and large aspect ratio. The maximum value of EMISE obtained at 10% (w/w) MWCNT loading [sample PTT-10] is36–42 dB in the measured frequency range. At 10 wt% MWCNTloading, the increase in EMI SE with increasing frequency is dueto decrease in skin depth (depth at which the field drops to 1/eof its original strength) of the material with increasing frequency[37]. The measured SE value of PTT/MWCNT composites with 5–10% (w/w) MWCNT loading meets the commercial SE applicationrequirement.

The effect of MWCNT loading on SER and SEA is shown in Fig. 5b.With increasing MWCNT loading, both SEA and SER increased, butrate of increase of SEA is much higher than SER. Results show thatabsorption becomes the major contributor to the total EMI SEabove percolation concentration. At percolation concentration,i.e., at 1% (w/w) MWCNT loading, total EMI SE is 5.6 dB havinghigher reflection loss (55.2%) contribution than absorption loss(44.6%) at 12.4 GHz. But at 2% (w/w) MWCNT loading, absorptioncontribution increased to 79.0% (at 12.4 GHz) which increased fur-ther with increasing MWCNT loading. This contribution shiftbehavior is the manifestation of change of intrinsic properties ofthe composites, i.e., conversion from insulator to conductor withsignificantly high value of tan d. The materials with tan d > 1 are

Fig. 6. Effect of MWCNT content on EMI SE and electrical conductivity (r) of PTT.

considered as lossy materials having strong absorption characteris-tics [38,39]. Since in the present study tan d values of the compos-ites increases from 0.38 to 2.7, PTT/MWCNT composites act as goodconductor and exhibits strong absorption properties.

The relation between the electrical conductivity and EMI SE isshown in Fig. 6. It can be seen that both conductivity and SE in-creased with increasing CNT loading, but conductivity did notchange significantly after percolation concentration while SE kepton increasing with increasing filler loading (Fig. 6). This shows thatthe number of percolating networks increased with increasingMWCNT loading which interact with incident radiation and leadto the higher SE without affecting the conductivity of the material.As the reflection loss is related to conductivity of the material, weobserved that the reflection loss follows same pattern as conduc-tivity, i.e., sharp increase at percolation concentration and onlymoderate increase after that. But, absorption loss showed no sig-nificant effect of percolation concentration and increased at thesame rate even after percolation concentration. This behavior canbe explained from the fact that absorption of electromagnetic waveoccurs by charge transfer and polarization which depends on thetotal amount of MWCNTs. The increasing absorptive behavior ofPTT/MWCNT composites can also be predicted from the tan d val-ues. The relation between tan d value and SE is shown in Fig 7. Thetan d value of pure PTT is almost zero, indicating that PTT canhardly attenuate or absorb electromagnetic waves. As MWCNTloading increased from 0.5% to 10% (w/w), tan d increased from0.15 to 2.7 at 12.4 GHz (Fig. 7), correspondingly SEA value increasedfrom 0.05 to 31 dB. Thus, we can say that the EMI SE of PTT/MWCNT composites depends on MWCNT loading and the EMI SEis strongly related to electrical conductivity and dielectric proper-ties of the material.

4. Conclusions

Well dispersed PTT/MWCNT composites with varying amountsof MWCNT (0.5–10% w/w) were successfully prepared by meltcompounding. The electrical conductivity, permittivity and EMISE of composites were found to be dependent on MWCNTs concen-tration and showed an increase with increasing MWCNT loading.Electrical percolation was observed at 1% (w/w) MWCNT loadingand SE of 36–42 dB was obtained at 10% (w/w) MWCNT loading[PTT-10] in the frequency range of 12.4–18 GHz. The study onshielding mechanism showed that absorption loss is the majorcontributor to the total EMI SE. We also observed that absorptionloss strongly depends on tan d and reflection loss on the conductiv-ity of the material. Finally it can be concluded that PTT/MWCNT

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composites having 5–10% (w/w) MWCNT can be used as an effec-tive, light weight EMI shielding material.

Acknowledgments

University Grants Commission for providing fellowship to oneof the authors and National Physical Laboratory (for providingfacility to measure electrical properties) is gratefullyacknowledged.

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