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haracterization of electromagnetic shielding fabrics obtained fromarbon nanotube composite coatings
enata Redondo Bonaldi ∗, Elias Siores, Tahir Shahnstitute of Materials Research and Innovation, University of Bolton, Deane Road, Bolton BL3 5AB, United Kingdom
r t i c l e i n f o
rticle history:eceived 25 March 2013eceived in revised form 24 August 2013ccepted 6 October 2013vailable online 13 November 2013
a b s t r a c t
The present paper reports novel electromagnetic shielding (EM) fabrics produced by knife-over-roll coat-ing and using combinations of carbon nanotube (CNT), conductive polymer and metal nanoparticles.The materials are analyzed by EM shielding and surface resistivity methodologies, scanning electronmicroscopy and BET surface area. The synergy among the conductive materials, percolation threshold, EMshielding behaviors and theoretical predictions are also investigated. The coating thickness obtained was100–200 �m, and the EM range tested was 200–1000 MHz. EM shielding fabrics of 95–99.99% (15–40 dB)
were obtained, and CNT was found to be the most effective material. The reported methodology and mate-rials are suitable for the production of customized, flexible, lightweight and porous conductive fabrics foreither EM shielding or functional electronic applications, including high specific surface area conductivematerials.
Electromagnetic (EM) shielding is important to block electro-agnetic radiation that could be harmful to electronic devices,
nvironment and humans. Textiles have been highly consideredor EM shielding applications in the electrical & electronic indus-ries as well as for the production of protective garments due to thencreasing concern about health issues caused by human exposureo radiation. The emerging role of textiles as EM shielding is mainlyue to their desirable properties in terms of flexibility, versatility,
ow mass and low cost.Textiles are intrinsically non EMI shielding materials and are
ather insulating materials; however, they can successfully turn toe EMI shielding after raw-material changes, new production pro-ess or process adaptations that make them electrically conductive1]. Some of the methods to obtain conductive fabrics are the usef metallic fibers and yarns, such as stainless steel, aluminum oropper yarns; however, these types of yarns tend to have low flex-bility due to their large diameter, which produces a heavier, stiffernd uncomfortable fabric. To reduce this problem, researchers have
een studying the influence of yarn density, fabric constructions,ifferent patterns, yarn diameter, quantity of conductive yarns inhe structure, layers and yarn direction [2–14].
Conductive fabrics can also be produced by conductive yarnswith the use of conductive fillers or coatings incorporated in theyarn production. These processes are based on mixing the fiberpolymer with fillers during their production processes such asmelt or wet spinning; or by twisting and wrapping a syntheticfiber with metallic yarns using mechanical spinning processes.The yarn coating approaches are mainly in situ polymerizationor plating techniques [15–19]. By using conductive fillers orcoating during yarn production, yarns of small diameter can beobtained, and therefore, very flexible and light weight fabrics.These coating technologies are less often used due to their inherentcomplexities.
In general, the above techniques are time-consuming, complexand require the utilization and know-how of the whole textilesupply-chain. Moreover, the resulting fabrics normally do notpossess isotropic EM shielding behavior, due to the yarn directionobtained by weaving and knitting processes [20].
Other approaches to develop conductive fabrics include theapplication of conductive materials on the surface of the fabricitself using plating techniques [21–35]. Coating usually does notchange the flexibility of the fabrics and is applied in very thin layer,low mass and closed fabric structures. Most of the commerciallyavailable EM shielding fabrics are produced by coating technologiesand have very homogeneous and closed structures thus exhibiting
extremely high EM shielding capabilities and isotropic behavior.In the present study, knife-over-roll coating was used instead ofplating techniques. This approach had not been previously investi-gated with the use of CNT.
permeability. The shielding takes place through a combination ofreflection, absorption and multiple reflections of the radiation bythe material (Fig. 1).
Metals and carbon products are the most frequently used mate-ials for EM shielding applications. Carbon nanotubes have beenntensively tested for EM shielding applications; however, mainlyor the fabrication of composite films for higher frequency appli-ations (X-Band and Ku-Band) and not for producing EM shieldingabrics [36–52]. In the present study the standard ASTM methodASTM D4935-10) was used, which utilizes frequency in the rangef 200 MHz–1 GHz for assessing EM shielding fabrics.
Nanoscale materials are reported to have the ability to fillp the vacancy of the conductive network formed by conductiveaterials of different shapes, resulting in a denser and more com-
lete conductive network. In addition, nanoscale materials haveigh specific surface area and low density. CNTs also have out-tanding structural, mechanical and electrical properties, and veryigh aspect ratio, which enhances the formation of conductiveetworks.
Conductive polymers such as polypyrrole and polyaniline haveeen used for the development of conductive fabrics and yarns,ostly via in situ polymerization [35]. In the present study,
anoparticles of polypyrrole were used instead of in situ polymer-zation.
. Experimental
Conductive fillers were applied on nonwoven and knitted fabricssing knife-over-roll coating technique. The coating formulationsere developed using aqueous dispersions of polyacrylate binders
nd thickeners, non-ionic surfactants and polyvinylpyrrolidonePVP). Homogenous and well dispersed recipes were obtained withhe use of stirrer and ultrasonication. The formulations were devel-ped using low concentration of conductive fillers in order to obtainn optimal dispersion and appropriate viscosity for the knife coat-ng applications. A multi-layer approach was adopted to graduallychieve the percolation threshold and maximum conductivity; asell as to maintain a good dispersion and homogeneity of the appli-
ation.The coating generates a thin layer of high density material on
he surface of the fabric; thus, it is represented by surface den-ity (g/m2) instead of weight percentage of the composite [wt%].he surface density (S) relates to the actual amount of the fillersed in the application, calculated by: mass of coated fabric (Cm)inus mass of pristine fabric (Pm) and minus solid mass of binders,
hickeners and surfactants (S). Represented by the equation: Sg/m2] = Cm [g/m2] − Pm [g/m2] − S [g/m2].
.1. Materials
The conductive materials used in the experiments are listedn Table 1; they were tested individually and in combinations toxplore their synergy.
The CNT was purchased from Nanostructured & Amorphousaterials Inc., and it has purity: 95%, outside diameter: 50–100 nm,
nside diameter: 5–10 nm, and length: 5–10 �m. This type of CNTsas chosen because they are supplied by the manufacturer as
eing a “highly conductive” type, due to their structural characteris-ics and purity. Furthermore, the MWCNTs used have considerablyower cost than their single-walled alternatives (SWCNTs) and
ould result in a commercially viable process for the production
PVP (g/L) 3 2 – –Surfactant (g/L) 2 – 1.5 –
of EM shielding fabrics. The properties of MWCNTs used were ana-lyzed via Raman spectroscopy, before and after the application, andno structural alterations were observed, suggesting that the CNTs’highly conductive properties were retained after the applicationonto the fabrics.
The nano Ag was purchased from Mknano, it has 99.9% purity,and particle size <90 nm. The Ni/CF was obtained from SulzerMetco (E-Fill 2901), it has density: 3.8 g/cm3 and composition:67Ni/33C wt%. The polypyrrole (Ppy), a conductive polymer, wasobtained from Eeonyx Corporation, under the name Eeonomer®
200 F (particle size avg. 40 nm, apparent density 0.03 g/cm3, andsurface area 570 m2/g) and Eeonomer WPpy (polypyrrole dispersedin water, 6% solids).
The fabrics used in the experiments were knitted (single jersey,cotton, 154 g/m2) and nonwoven (polyester, 75 g/m2) fabrics. Thechemical auxiliaries used and their quantities are summarized inthe Table 2. The binders used in these experiments have the purposeto adhere and bind the conductive fillers to the fabric, instead ofbeing used to form composites; therefore, small quantities wereused.
2.2. Coating methodology
A laboratory coating machine (Mathis Type SV) was used for theapplication of the developed recipes. This process allows a very thinlayer to be applied on one side of the fabric. In this process, a knife(blade) carries the coating paste along the fabric, and its height andvelocity determine the quantity and thickness applied on the fabric.A multi-layer approach was used, that is, the coating was appliedup to five consecutive layers, with increasing knife height, allowinga thicker layer to be obtained in every step. After the application ofeach layer, the fabric was dried in a stenter frame.
2.3. EM shielding and surface resistivity methodologies
EM shielding is the process of limiting the flow of EM fieldsbetween two locations by a barrier. The shielding barrier needsto have high conductivity/dielectric constant or high magnetic
In the Ag application on nonwoven fabric, the percolation wasachieved only after the application of the fourth layer, with 97 g/m2,resulting in 7 �/sq and 99.99% SE. The percolation was not obtained
Fig. 2. Relationship between filler loading and surface resistivity.
The standard method ASTM D4935 was used for the measure-ents [53]. The evaluation of the EM shielding fabrics was made
y measuring the Shielding Effectiveness (SE). SE is expressed inecibels (dB) and is a logarithmic representation of a ratio mea-urement. It is most commonly used for expressing power ratio atigh frequencies (Eq. (1)), where Pt is the transmitted EM powerhrough the fabric and the Pi is incident EM. The SE was deter-
ined by comparing the difference in attenuation of a referenceample to the test sample, taking into account the incident andransmitted radiations. The results were obtained in decibels andlso transformed into percentage:
E(+dB) = 10 logPi
Pt→ SE% =
(1 −
(1
10SE/10
))× 100 (1)
The standard test method used for the measurement of theurface resistivity was the AATCC Test Method 76-2005 “Electri-al Surface Resistivity of Fabrics” [54]. The surface resistivity waseasured using the concentric ring electrodes and is reported in
hms per square (�/sq). This method measures the resistance of material to the flow of current between two electrodes, and isndependent of its dimensional units.
There is no well-established textile standardization for EMhielding purposes, and the required level of shielding is highlyependent on the application; however, a standard has been devel-ped by the Taiwan’s functional and technical textiles industryor accreditation and certification of EM shielding fabrics. Thistandard classifies the shielding capability according to a scale from
to 5. For instance, the grade 3 (good) is assigned to values between0 and 20 dB (90–99%, Class 2 – for general use) [55].
. Results and discussion
.1. Percolation threshold and coating morphology
The conductive materials were tested separately in nonwovenNW) and knitted fabrics (K). The relationship between surfaceesistivity and filler loading is illustrated in Fig. 2. The applicationsith Ag and Ni/CF required much higher amounts of the product
o achieve percolation (>50 g/m2), the resistivity did not changeignificantly according to filler loading and these products did notchieve percolation at all when applied on the knitted fabric. On thether hand, CNT and Ppy achieved percolation threshold with veryow amounts (<10 g/m2) and the resistivity changed in proportiono the quantity of product used.
In the case of polypyrrole, the percolation threshold waschieved after the application of the first layer (0.3 g/m2 NW, 4 g/m2
), and the initial resistivity was very poor. After the application ofhe successive layers, the resistivity and EM shielding increased
Fig. 3. Polypyrrole coated nonwoven fabric.
according to the quantity of Ppy, achieving its maximum in thethird layer (3.2 k�/sq, ±12% SE, 10 g/m2 NW), (1.2 k�/sq, ±45%SE, 33 g/m2 K). The SEM image illustrates the surface of the fabriccoated with Ppy, which shows that a smother but cracked mor-phology was obtained. The cracking could have influenced the poorresistivity, due to more free space and less connectivity of the coat-ing (Fig. 3).
The percolation threshold of the Ni/CF was reached only afterthe application of the fourth layer, with 56 g/m2, however, the resis-tivity was very unstable. The fifth layer had the resistivity slightlyimproved, although still unstable, with ±90% SE at 83 g/m2. Thepercolation was not obtained in the knitted fabric at all. The SEMimage below shows the coated surface of the fabric, with a macro-scopic network-like structure due to the shape and size of the Ni/CFfiller (Fig. 4).
The percolation threshold of the fabrics coated with CNT wasachieved after the application of the second layer in nonwovenand first layer in knit, with very low amount of CNT (6 g/m2 NWand 5 g/m2 K). After the application of the successive layers, theresistivity and EM shielding increased according to the quantityof CNT, achieving its maximum in the fifth layer in the non-woven (100 �/sq, ±94% SE, 36 g/m2) and fourth layer in the knitted(70 �/sq, ±96% SE, 30 g/m2) fabric. The SEM image below illustratesthe surface of the fabric coated with CNT. A sponge-like structurewas obtained, with a very porous morphology (Fig. 5).
n the knitted fabric. The SEM image below shows the surface ofhe fabric, with a denser and more homogenous structure, whenompared to the other coatings (Fig. 6).
.2. EM shielding results of the factorial experiment 24
A factorial experiment was performed with the four productsCNT, Ni/CF, Ag, Ppy), on the nonwoven fabric, in order to investi-ate the synergy and effects of these products on the EM shielding.he combined recipes were prepared mixing the individual recipesroportionally. The factor effects of those four products were calcu-
ated, in relation to the average SE (from 200 to 1000 MHz), and theesults demonstrated that the CNT effect was the strongest amongsthe four products (46.6), followed by Ni/CF (7.4), Ag (0.6) and Ppy−1.1). It means the EM shielding results were highly influenced byhe quantity of CNT used. In fact, by considering the graph, the high-st values were achieved with combinations containing CNT, suchs the CNT + Ni/CF, CNT + Ppy and CNT + Ag. The fabric coated withg achieved the highest EM shielding; however, the filler loadingas significantly high (Fig. 7).
The application with the highest SE% and the lowest filler load-ng was obtained with CNT + Ppy (97% SE, 28 g/m2), likely to beelated to the fact that both products have nanoscale properties,hus achieving a denser and more complete conductive networkith very low loading, which is due to the high aspect ratio, surface
rea and low density.CNT was important in anticipating the percolation threshold
f Ag and Ni/CF, while increasing the overall EM shielding. CNT
Fig. 6. Ag coated nonwoven fabric.
Fig. 7. EM shielding results of the factorial experiment.
enhanced the coating stability and durability that Ag and Ni/CF indi-vidual applications did not have, by providing a more consistent andreliable conductive path.
Combination of Ni/CF, Ppy and Ag did not produce relevant EMshielding results, and Ni/CF and Ag synergy did not achieve perco-lation at all. However, Ppy enhanced the shielding behavior of Ni/CFwhen both were used, as relatively higher shielding was obtainedwith very low filler loading. This may be attributed to the fact thatPpy provided a more complete, stable and reliable connectivity forthe network formed by the rod-shape Ni/CF particles.
3.3. Absorption and reflection behaviors of EM shielding
During the SE measurements, the reflected radiation (ReturnLoss or S11 parameter) and the transmitted radiation (InsertionLoss or S21 parameter) were measured and calculated according toEqs. (2) and (3).
IL(−dB) = 10 logPt
Pi→ T% = (10IL/10) × 100 (2)
RL(−dB) = 10 logPr
Pi→ R% = (10RL/10) × 100 (3)
The total SE is expressed by Eq. (4); it is a sum of reflection (R),absorption (A) and multiple reflection (MR). The absorption term(%) is obtained by Eq. (5), whereas the absorption term in decibelsis calculated by Eq. (6), in the later case, the relative value is calcu-lated, that is, the reflected radiation is subtracted from the incidentradiation, according to the EM shielding theory [56–58].
SE = R + A + MR (4)
A% = 100 − T% − R% (5)
AdB = 10 logPi − Pr
Pt(6)
The results of the most relevant applications are represented inFig. 8. Conductive materials have different absorption and reflec-tion behaviors: Metals provide high reflectivity; whereas carbonmaterials and conductive polymers are associated with higher
% Absorption % Reflection % Transmission Se total dB Reflection dB Absorption dB
arshp
CtsTa
thi
3
ttMca
tcct
TB
Resistivity ( /sq)
Fig. 8. EM shielding behavior – average from 200 MHz to 1 GHz.
bsorption behavior. This characteristic was also evident from theesults. CNT showed high absorption behavior (avg. 42%), and Aghowed very high reflection behavior (avg. 96%). Ni/CF showedigh absorption (avg. 51%), likely to be related to the ferromagneticroperties of nickel.
A reduction in absorption was observed for the combinations ofNT with Ag, Ppy and Ni/CF, which may be related to the fact thathe coating morphology of these combinations was denser and theurface flatter, when comparing to the sponge-like CNT coating.his observation suggests that the absorption and reflection maylso be related to coating morphology.
Fig. 8 shows that the lower the surface resistivity, the higher theotal SE and reflection behaviors (%). Whereas the absorption (%) isigher in applications of higher surface resistivity. This character-
stic is in agreement with the EM shielding theory.
.4. BET surface area characterization
BET surface area analysis was undertaken in order to investigatehe three-dimensional (3D) surface area of the microporous struc-ures. This test was performed in a Micromeritics TriStar 3000 and
icromeritics ASAP 2020. The gas used was nitrogen (molecularross-sectional area 0.1620 nm2) and the samples were degassedt room temperature for two hours.
The 3D surface areas of the CNT coated fabrics and combina-
ions were found to be considerably high, whereas the Ag and Ni/CFoated fabrics was found to be very low. Table 3 reveals the 3D spe-ific surface area (m2/g) measured by the BET technique, whereashe 2D surface area (m2/g) was calculated from the dimensions of
Fig. 9. Coating morphology of CNT coated fabrics, showing CNTs homogenouslydispersed, individually separated, exposed and partially embedded by the binder.
the fabrics. The ratio of 3D/2D surface area was considered in orderto compare both pristine and coated fabrics, as the 2D area remainsthe same before and after the coatings and between the fabrics,whereas the fabric mass is different.
It can be observed that the 3D/2D ratio was highly increased dueto the presence of CNT in relation to the pristine fabrics. The resultsshow that values higher than 1000 m2 were measured for CNT + PpyNW and CNT + Ag Knit. This means that 1 m2 of those fabrics have>1000 m2 of three-dimensional surface area, which is immenselyhigher than the pristine fabric.
The specific surface area of pristine CNT powder (∼48 m2/g) wascompared to the quantity of CNTs used per m2 of the knitted fabric(30 g/m2) and its measured surface area ratio (925 m2). It was foundthat ∼65% of the pristine CNT surface area was exposed, which con-firms that the CNTs were indeed only partially embedded withinthe coating. This is also illustrated in the SEM images presented inFig. 9.
An investigation was undertaken in order to study the corre-lations among specific surface area, EM shielding behavior andcoating filler density, on the most relevant applications. The resultsdepicted in Fig. 10 show that the surface area is correlated with theabsorption (Pearson’s correlation of 0.6), and also with the fillerdensity (inverse correlation of −0.7). It was also observed that thehigher the absorption the lower the total shielding, as an excellentcorrelation was obtained (−0.9).
Correlation analysis measures how strong is the relationshipbetween two variables, the range is from −1 to +1, in which ‘0’
means no correlation and ‘+1/−1’ means high correlation.
It can also be observed from Fig. 10 that the combinationof CNT + Ppy achieved the highest surface area (1473 m2), which
supposed to be negligible due to the electrically thin character ofthese coatings.
The CNT coated nonwoven sample was further investigated andthe regression curves obtained (Fig. 11) show that a very high
R = 4E+06x-3.084
R = 0.67
SE = 42ln(x) - 53 R = 0.92
0
20
40
60
80
100
1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
0 10 20 30 40
SE (%
)
Log-
Sur
face
Res
istiv
ity (
/sq)
MWCNT (g/m2)
Fig. 10. Relationship among surface area, EM shielding and filler density.
ay explain the relatively high EM shielding obtained using only8 g/m2 of the conductive filler.
.5. Theoretical EM shielding prediction
In order to theoretically calculate EM shielding, several method-logies can be adopted, which depend on the shield geometry,eld being attenuated, electrical properties and characteristics ofhe shield. In the present study, ‘Plane wave theory’ was consid-red, and ‘Single layer shield’ modeling was used to analyze thexperimental results and to investigate the EM shielding behavior.
Plane wave (or transverse EM wave) is obtained when thelectric and magnetic fields are perpendicular to each other andransverse to the direction of propagation, it is related to far-fieldonditions. Far-field is generated when the distance between theadiation source and the shielding material is higher than (�/2�),here � is the free-space wavelength of the emitted radiation. This
s the characteristic of the field being attenuated in the presenttudy, due to the methodology utilized for EM shielding measure-ent. According to this theory, the exact equations to theoretically
alculate EM shielding due to absorption (A), reflection (R) andultiple reflections (MR) are given below [56].
dB = 20 log
∣∣∣∣ (�o + �)2
4�o�
∣∣∣∣ (7)
dB = 20 log et/ı (8)
RdB = 20 log
∣∣∣∣[
1 −(
�o − �
�o + �
)2e−2(t/ı)e−j2(t/ı)
]ej(t/ı)e−jˇ0t
∣∣∣∣ (9)
he intrinsic impedance (�) of the shield is the ratio of the electricnd magnetic field’s amplitudes. In the case of far-field radiation,he intrinsic impedance of the wave in free-space is constant andquals to �0 = 377 � [56]. The intrinsic impedance of the shield cane obtained by Eq. (10).
The phase constant of free-space is given by (2�f/c), and equalso ˇ0 = 20.93, where t represents the thickness of the shield (m) and
is the skin depth in the shield (m). Skin depth is the penetrationistance in which the electric field drops exponentially to 1/e of the
ncident radiation strength when passing through the shield. Skinepth is a function of electrical conductivity and magnetic perme-bility of the shield; and the frequency of radiation, according to Eq.11), where f (frequency in Hz), � (magnetic permeability) = �0�r,
= 4� × 10−7 H/m and � (electrical conductivity in �−1 m−1).
For the theoretical calculations, the magnetic permeability of thecoated fabrics was considered �r = �0 (non-magnetic), the fre-quency was 1 GHz and the electrical conductivity was derived fromthe surface resistivity (� sq) by obtaining the volume resistivity (Eq.(12)).
� = 1( sq) × t
(12)
According to the plane wave shielding theory [55–57], when theshield is electrically thin (t � ı) the absorption is neglected andthe multiple reflection is considered, this is the characteristic ofthe fabrics obtained in this study, as the skin depth was found tobe ∼2 mm and the coating thickness was 100–200 �m. This rangeof thicknesses obtained within the sample are due to the intrinsicirregular surface of the fabric and do not influence the results as theoverall thickness is much smaller than the skin depth. The theoret-ical results obtained at 1 GHz as well as the experimental resultsare given in Table 4.
The theoretical SET was found to be close to the experimental;however, the RdB value was much higher, the AdB was consider-ably lower and MRdB was negative, i.e. reducing SE. The negligibleabsorbance and negative multiple reflection term were theoret-ically expected due to the electrically thin characteristic of thecoatings.
It can be concluded that this theory predicts the ‘Total SE’ val-ues with reasonable accuracy. On the other hand, the reflectionand absorption experimental results differ significantly from thetheory. The absorption behavior also contradicts the theory, as it is
Surface Resistivity Shielding Effectiveness Pot (R) Log (SE)
Fig. 11. Relationship among filler loading, EM shielding and surface resistivity.
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orrelation (r2 = 0.92) exists between CNT loading and a logarithmicurve, which means SE can be directly predicted by the CNT loadingsing the equation shown in Fig. 11. In addition, a good correlationas also obtained for the surface resistivity in relation to a fittedower curve (r2 = 0.67). The percolation behavior, theoretical pre-ictions and regression formulas obtained by these results can leado potentially tailor-made and customized applications.
. Conclusion
Novel EM shielding fabrics are reported in this paper. Most of theabrics produced achieved EM shielding ≥95%. The factorial experi-
ents showed that CNT is the most influential synergistic material.he synergy between CNT and metals was positive in enhancing theercolation threshold and achieving higher conductivity by pro-iding a more reliable and consistent network and pathway withinhe metal filler. The synergy between CNT and Ppy was found toe very good, as a relatively high shielding was obtained (∼97%)ith very low filler loading (28 g/m2). This result may be attributed
o the nanoscale properties of both materials, which produced aore complete network and also very high specific surface area
1473 m2 three-dimensional area per 1 m2 of fabric), which mayave contributed to a more efficient EM shielding.
This study has demonstrated that CNT nanocomposite coatedabrics are potential materials for tailor-made and customizedlectronic and EM shielding applications, due to the percolationharacteristics and theoretically predictable results. The producedabrics utilize cost-efficient materials and environmentally friendly
ethodology and are lightweight, flexible, thin and porous. Thepproach for the development of EM shielding fabrics reportederein allows further optimization and characterization of the fab-ics for specific applications.
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