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Materials Science and Engineering B 187 (2014) 96–101

Contents lists available at ScienceDirect

Materials Science and Engineering B

jo ur nal home p age: www.elsev ier .com/ locate /mseb

haracterization of electro-conductive fabrics prepared by in situhemical and electrochemical polymerization of pyrrole ontoolyester fabric

yamal Maiti, Dipayan Das, Kushal Sen ∗

epartment of Textile Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

r t i c l e i n f o

rticle history:eceived 15 December 2013eceived in revised form 1 April 2014ccepted 12 May 2014

a b s t r a c t

This paper reports a study on electro-conductive fabrics prepared by a combined in situ chemical andelectrochemical polymerization of pyrrole. Specific observations are made to establish the roles ofadd-on and surface roughness on the surface resistivity of the electro-conductive fabrics. The perfor-mance characteristics of the fabrics are reported in terms of electrical conductivity, voltage–current and

vailable online 24 May 2014

eywords:lectro-conductive textilehemical polymerizationlectrochemical polymerization

voltage–temperature characteristics and electromagnetic interference (EMI) shielding capability. Thesurface resistivity of the fabric was found to be as low as 11.79 �. The voltage–current profile of thefabric is observed to be non-ohmic as well as the voltage–temperature curve is found to be exponential.The EMI shielding efficiency of the fabric was found to be about 98%.

© 2014 Published by Elsevier B.V.

olypyrrole

. Introduction

The award of the Nobel Prize in chemistry on the discov-ry of electro-conductive polyacetylene for the year 2000 [1]xcited many researchers all over the world that resulted in quickevelopment of many more electro-conductive polymers such asolyaniline, polypyrrole, polythiophene and their derivatives. Theonjugation in the polymers is responsible for the electrical con-uctivity which could come very close to that of metals; in fact,ometimes these are termed as synthetic metals. Subsequently itas realized that these polymers have some limitations, in that theyave poor processability and environmental stability [2,3]. Despitehese challenges, there have been notable successes in convertinghese electro-conductive polymers into fibres, yarns, fabrics andther flexible products.

Polypyrrole is established as one of the most promising electro-onductive polymers. It has received more attention due to its highonductivity, ease of preparation, and environmental stability andhown potential for a wide range of applications in sensors, actu-tors and electronic and electrical devices [4–7]. Pyrrole can be

olymerized by many techniques such as chemical polymerization,apor phase polymerization and electrochemical polymeriza-ion. Gregory et al. [8] demonstrated the process for preparing

∗ Corresponding author. Tel.: +91 1126591411; fax: +91 1126581936.E-mail address: kushal@textile.iitd.ernet.in (K. Sen).

ttp://dx.doi.org/10.1016/j.mseb.2014.05.003921-5107/© 2014 Published by Elsevier B.V.

electro-conductive textiles by in situ polymerization of either ani-line or pyrrole on the surface of polyester or nylon fabrics. Diazet al. [9] reported that polymerization of pyrrole by electrochemicalpolymerization on platinum electrode produced a stable polymerfilm with good electrical properties. Since then, Park et al. [10],Chen et al. [11], Hwang et al. [12], Bhadani et al. [13], Maiti et al.[14] and Sen et al. [15] made significant contributions to this field.Subianto et al. [16] prepared electro-conductive cotton fabric byelectrochemical polymerization of pyrrole. They reported that theconductivity decreased with an increase in current density, how-ever, at a fixed current density, the conductivity increased withan increase in the dopant concentration. Kim et al. [17] prepareda stretchable electro-conductive fabric by electrochemical poly-merization of pyrrole onto nylon/spandex stretchable fabric. Theyobserved that the electro-conductivity of the fabric first increasedand then decreased with the increase in the concentrations ofmonomer and dopant and the time of polymerization. This observa-tion, however, did not exactly match with those reported by Parket al. [10] and Hwang et al. [12]. Molina and co-researchers [18]developed electro-conductive textile by the combination of chem-ical and electrochemical polymerization and also found that changein counter ion during the process changed the conductivity of thefabric. Babu et al. [19] produced electro-conductive cotton fabric

by the combination of chemical and electrochemical polymeriza-tion at a constant current density (2 mA cm−2) at room temperaturefor 4 h. They reported that the conductivity and weight gainwere directly proportional to monomer concentration as well as

and Engineering B 187 (2014) 96–101 97

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Table 1Process factors and their levels according to Box–Behnken design.

Factors Levels

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S. Maiti et al. / Materials Science

olymerization time. Maiti et al. [20] reported that the polymeriza-ion time and temperature played a significant role in determininghe electrical conductivity of cotton yarn. Apart from these studies,

few researchers also characterized the electro-conductive fabricsn terms of voltage–current and voltage–temperature characteris-ics. Acqua et al. [21] reported linear voltage–current characteristicsf viscose and lyocell fabrics treated with polypyrrole however,hat [22] and Cucchi [23] observed non-linear voltage–currentharacteristics of polyaniline treated cotton fabric and polypyrrolereated silk fabric, respectively. Das et al. [24], however, reportedon-linear voltage temperature behaviour of electro-conductive

abrics.While from the above, it is established that in situ chemical

r electrochemical polymerization can produces flexible electro-onducting textiles, many questions still need clear answers, suchs, (a) is the add-on related to conductivity [17–19], (b) doeshe surface morphology of deposited polymer plays any role, orc) are the voltage–current or voltage–temperature characteristics20–24] of these products linear or non linear. Clarity on these cannhance their potential for some interesting applications in future.

In the present study, an attempt has been made to get answerso the above questions and also to examine the electromagnetichielding behaviour of the polyester electroconductive fabrics pre-ared by in situ chemical and electrochemical polymerizationrocess.

. Experimental

.1. Materials and chemicals

Polyester woven fabric of 58 g/m2 weight, 0.19 mm thickness,8 ends per cm, and 32 picks per cm was chosen as a substrateor this study. The chemicals used were sodium carbonate, sodiumydroxide, non-ionic detergent lissapol N (HR Chemical, India), pyr-ole (Spectrochem, India), ferric chloride and p-toluene sulphoniccid (Lobal Chemie, India). All these chemicals were of laboratoryrade and they were used as received.

.2. Preparation of electro-conductive fabric

The electro-conductive fabrics were prepared by a combined initu chemical and electrochemical polymerization of pyrrole ontohe polyester fabric. The fabric was first saponified with 100 g/Lodium hydroxide at 90 ◦C for 20 min, keeping the material-to-iquor ratio as 1:30. A two-step polymerization of pyrrole wasarried out. In the first step, the in situ chemical polymerizationf pyrrole was carried out. The hydrolyzed fabric was immersednto pyrrole solution containing 0.5 M pyrrole at 25 ◦C for 20 min.he pyrrole-enriched fabric was then immersed into 0.75 M ferrichloride solution so as to initiate polymerization onto the fabric at◦C for 10 min. After that, the fabric was washed thoroughly witheionised water and dried in an oven at 60 ◦C for 40 min. These con-itions were chosen based on our previous studies [25,26]. In theecond step, the in situ electrochemical polymerization of pyrroleas carried out onto the fabric in a potentiostat using a regu-

ated DC power supply (ESCORP). The stainless steel electrodesf grade AISI 304 were kept vertically parallel to each other. Theabric was suitably fixed on the anode surface during the elec-rochemical polymerization The electrolyte solution was preparedn aqueous medium containing 0.3 M pyrrole along with 0.05 M-toluenesulfonic acid. The voltage, time and temperature of poly-

erization were varied to produce a wide range of conducting

abrics.Three process factors (applied voltage, polymerization time, and

olymerization temperature) were further optimized by means of

Polymerization time (min) 30 60 90Polymerization temperature (◦C) 15 25 35Applied voltage (V) 2.0 2.5 3.0

a 33 Box–Behnken design of experiments along with response sur-face methodology of analysis. Table 1 displays the levels of theprocess factors chosen for Box–Behnken design.

2.3. Measurement of surface resistivity of electro-conductivefabric

The surface resistivity of the electro-conductive fabric was mea-sured by concentric ring-disc electrode configuration with 10-mmouter radius of the inner disc and 25-mm inner radius of theouter ring in a controlled atmosphere at 27 ± 2 ◦C temperature and65 ± 2% relative humidity. The testing was carried out as per ASTMstandard D-257. The surface resistivity was determined as follows:

�s = 2� ln(

r1

r2

)Rs (1)

where �s denotes surface resistivity, Rs indicates surface resistance,r1 stands for the radius of the disc and r2 refers to the inner radiusof the outer ring electrode.

2.4. Measurement of surface roughness of electro-conductivefabric

The surface roughness of the fabric was measured with aninstrument Form Talysurf Intra supplied by Taylor Hobson Preci-sion, UK. The stylus, attached to the device, was moved on thesubstrate from one end to the other and the vibrations wererecorded. The rougher is the surface the more is the vibration ofthe stylus.

2.5. Scanning electron microscopy of electro-conductive fabric

The surface morphology of the surface was studied using scan-ning electron microscope, Zeiss EVO 50. As the treated fabricsamples had enough electrical conductivity, coating of the samplewas not carried out.

2.6. Measurement of voltage–current characteristic ofelectro-conductive fabric

The voltage–current behaviour of the electro-conductive fab-rics was characterized using regulated DC power supply equipment(ESCORP). The conducting fabric sample (50 mm × 25 mm) wasfixed between two clamps and direct voltage, ranging from 0 V to8 V, was applied to the fabric and the current in the circuit wasrecorded.

2.7. Measurement of voltage–temperature characteristic ofelectro-conductive fabric

The voltage–temperature characteristics of the electro-conductive fabric were characterized by applying direct voltage toa strip of electro-conductive fabric (50 mm × 25 mm), which was

placed between two clamps. The temperature rise in the fabricsample was recorded (using IR thermometer) at different appliedvoltages (2 V, 4 V, 6 V and 8 V) for different time periods (2 min,4 min, 6 min and 8 min).

98 S. Maiti et al. / Materials Science and En

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Fig. 1. Block diagram for electromagnetic shielding measurement system.

.8. Measurement of electromagnetic shielding oflectro-conductive fabric

The electromagnetic shielding measurement on the electro-onductive fabric was carried out in an anechoic chamber. The blockiagram of the system used is displayed in Fig. 1. The transmit-ing antenna was connected to an electromagnetic wave source andhe receiving antenna was connected to a spectrum analyzer. Theistance between these antennas was kept constant at 1.2 m andhe operating frequencies were chosen as 2.4 GHz, 4 GHz, 6 GHz,nd 8 GHz. The electromagnetic shielding effectiveness (EMSE) wasalculated as follows:

MSE = 10 log(

P1

P2

)(2)

here P1 denotes the electromagnetic power received by thentenna in absence of the electro-conductive fabric and P2 indicateshe electromagnetic power received by the antenna in presence ofhe electro-conductive fabric. This measurement was performed inwo modes. In the first mode, the fabric was grounded and in theecond mode, the fabric was not grounded.

. Results and discussion

.1. Effect of add-on on surface resistivity of electro-conductiveabric

Using the Box–Behnken design of experiments an optimization

tudy was conducted to understand the effects of polymerizationemperature and voltage on the add-on and resistivity of the pre-ared electro-conductive fabric samples. The response surface andontour plots representing the add-on and the resistivity of the

Fig. 2. Plots of temperature and vo

gineering B 187 (2014) 96–101

fabric for the temperature ranging from 15 ◦C to 35 ◦C and theapplied voltage ranging from 2 V to 3 V, at a constant polymer-ization time of 60 min, are shown in Fig. 2. It may be seen thatadd-on increases with an increase in the applied voltage while theresistivity initially decreases and then increases at irrespective ofthe polymerization temperature. Similarly, add-on increases withtemperature but the resistivity goes through a minimum. Theseresults defy normal logic. While the decrease may be attributed tomore and uniform deposition of polymer, the increase in resisti-vity is intriguing. Degradation of polymer at such low voltages andtemperatures is questionable.

It was decided to prepare a large number of electro-conductivefabric samples at variedly different process conditions to achievea very broad range of add-ons and examine the generalized trendof resistivity. The generalized effect of deposition of conductingpolymer, irrespective of process parameters, on the surface resis-tivity of the electro-conductive fabric is shown in Fig. 3a. It canbe seen that the surface resistivity of the fabric decreased rapidlywith an increase in add-on, but levelled off after about 10 g/m2. Asthe textile fabric has no electrical conductivity and the depositedpolypyrrole imparts the conductivity in the fabric, the resistivitydecreased rapidly at initial stage. It may be hypothesized that at thislevel of deposition, the entire available surface of the fabric wouldbe covered by the polymer; therefore, any additional deposition ofpolymer might not further lower the resistivity. This appears sensi-ble but is at variance with the optimization experiment as describedabove. A closer look on the stabilized region, however, revealedthat the add-on between 50 g/m2 and 150 g/m2 although showedvery low resistivity but at the same time showed an irregular trendbetween surface resistivity and add-on within a band of resistivityof 0–200 � (Fig. 3b). It may be observed that some fabrics with ahigher add-on showed more resistivity, while some other fabricswith higher add-on showed less resistivity. It may be concludedhere that below a value of about 200 �, the resistivity and add-on did not correlate well. Fig. 4 shows the electron micrographsof some typical samples. Clearly, the surface morphology of thesesamples is very different and so is the add-on value. In order tounderstand this behaviour, it was decided to measure the surfaceroughness of the fabrics and examine if roughness and resistivitywere related.

3.2. Effect of surface morphology on surface resistivity of

electro-conductive fabrics

The data of surface roughness and electrical resistivity of typicalfabric samples are presented in Table 1. It may be observed that the

ltage on electrical resistivity.

S. Maiti et al. / Materials Science and Engineering B 187 (2014) 96–101 99

Table 2Add-on, surface roughness and surface resistivity of typical films and fabrics.

Material Add-on/weight per unit area (g/m2) Surface roughness (�m) Surface resistivity (�)

Fabric34.90 14.5 46.26

132.73 12.4 14.19254.63 31.0 181.21

Film surface not facing electrode83 6.0 32.35

135 24.6 45.03164 37.5 63.40

Film surface facing electrode83

135

164

Fig. 3. Effect of add-on on surface resistivity of electro-conductive fabric.

Fig. 4. Scanning electron micrograph

0.70 19.740.42 18.441.30 16.86

fabric that had higher add-on but a lower surface roughness showedlower surface resistivity and vice versa. To get more insight into this,polypyrrole films were prepared directly on the anode plate, i.e.,without the fabric, and were tested for roughness and resistivity.It was obvious that the film surface facing the electrode plate wassmoother; morphology was dependent on the smoothness of theelectrode surface, while the surface not facing the electrode wasrougher as the polymer kept on depositing with time. It may be seen(Table 2) that the surface resistivity of the film is dependent on theroughness. The higher is the roughness, the higher is the resistivity.It may be because (a) the electrons have to travel a longer path withthe increase in surface roughness, thus leading to higher surfaceresistivity and (b) the rougher is the surface, the poorer would bethe contact with the measuring electrode under a constant pressureexerted by the electrode onto the fabric/film samples. The fabricwith higher roughness, therefore would have less contact points,hence may register higher resistivity.

3.3. Voltage–current characteristic of electro-conductive fabric

The electro-conductive fabric was tested for its voltage–currentcharacteristics. It may be seen (Fig. 5) that as the applied voltagewas increased across the fabric sample strip, the current generatedalso increased and this behaviour is found to be non-ohmic. A sim-ilar observation was reported earlier with the electro-conductivefabrics prepared by in situ chemical polymerization of monomerslike pyrrole and thiophene [21,24]. It is an important observationfor some one who may design products based on this type of flex-ible electro-conducting textile. In order to further characterize thevoltage–current relationship, an exponential relation of the formgiven below was fitted to the experimental data:

I = a(ebV − 1) (3)

where I denotes current, V indicates voltage, and a & b refer to thecoefficient and exponent of the exponential relationship, respec-tively. The values of the coefficient and exponent were found to be0.04726 A and 0.4274 V−1, respectively. The behaviour of the fitted

s of electro-conductive fabrics.

100 S. Maiti et al. / Materials Science and Engineering B 187 (2014) 96–101

Fig. 5. Plot of voltage versus current.

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vvtItt

Fd

Fig. 6. The effect of time on heat generation at different voltages.

ine is also shown in Fig. 5. The coefficient of determination wasound to be 0.9999.

.4. Voltage–temperature characteristic of electro-conductiveabric

The electro-conductive fabric was also tested for itsoltage–temperature characteristic by applying a range of DColtages across the fabric for different time intervals. The effect of

ime on heat generation at different voltages is shown in Fig. 6.t may be observed that at a constant voltage, the temperature ofhe fabric showed a significant increase. It may also be noticedhat the higher was the voltage applied, the higher was the rise

ig. 7. Plot of increase in temperature against the voltage multiplied by current forifferent time intervals.

Fig. 8. Plot of electromagnetic shielding behaviour of the electro-conductive fabricvis-à-vis the control fabric.

in temperature. May be such material can find application inheating pads and bandages. In order to further characterize thevoltage–temperature relationship, an exponential relation of theform given below was fitted to the experimental data:

�T = c(1 − ed�) (4)

where �T indicates increase in temperature, � denotes time ofapplication of voltage, and c & d refer to coefficient and exponent ofthe relationship, respectively. The values of the coefficient c werefound vary from 1.07 ◦C to 63.16 ◦C and the exponent d was foundto be around −0.3250. Then Eq. (4) can be rewritten as follows:

�T = b(1 − e−0.3250t) (5)

It was interesting to note that the coefficient was dependent onthe applied voltage, but the exponent was dependent on the time.This kind of behaviour was due to the fact that the rate of heat losswas initially less than the heat generation but as time went on, heatloss would increase as the difference from the ambient temperaturebecomes high.

Further, it may be interesting to note that when the raised tem-perature was plotted against the product of voltage and current fordifferent time intervals, a straight-line relationship with positiveslope was found (Fig. 7). It is interesting to note that the rise intemperature (�T) is linearly proportional to the electrical powerbut not with supplied energy.

3.5. Electromagnetic shielding of electro-conductive fabric

The electromagnetic shielding behaviour of the electro-conductive fabric vis-à-vis that of the control fabric is shown inFig. 8. It may be noted that the electro-conductive fabric couldshield the electromagnetic waves remarkably better than the con-trol fabric at all frequencies studied, and further improvement inshielding behaviour was found when the fabrics were grounded.One may notice that this polypyrrole-treated-polyester fabric wasable to shield 98% electromagnetic waves radiating at a frequencyof 8 GHz. It promises to be a very effective flexible product for EMIshielding application.

4. Conclusions

The surface resistivity of the fabrics decreased rapidly with anincrease in add-on. But, after a certain level of add-on, the sur-face resistivity of the fabrics was stabilized more or less. Below a

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esistivity value of about 200 � the add-on and resistivity wereot correlated. This behaviour can be explained in terms of sur-

ace roughness of the fabrics. It was observed that higher add-onut lower surface roughness resulted in lower surface resistivity.

similar behaviour was observed the in case of films as well. Theoltage–current and voltage–temperature behaviours were foundo be non-linear. The electro-conductive fabric exhibited 98% elec-romagnetic shielding efficiency with and without grounding.

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