Transactions of the Institute of Measurement and Control-2007-Devaux-355-76

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DOI: 10.1177/01423312070817262007 29: 355Transactions of the Institute of Measurement and Control

Saihi Devaux, Vladan Koncar, Bohwon Kim, Christine Campagne, Cline Roux, Maryline Rochery and Dh

textile applicationsocessing and characterization of conductive yarns by coating or bulk treatment for sm

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Processing and characterization of

conductive yarns by coating or bulktreatment for smart textile applicationsEric Devaux, Vladan Koncar, Bohwon Kim,Christine Campagne, Celine Roux, Maryline Rocheryand Dhouha SaihiLaboratoire de Genie et Materiaux Textiles (GEMTEX), Ecole Nationale Superieuredes Arts et Industries Textiles (ENSAIT), 9 rue de lErmitage, BP 30329,59056 Roubaix Cedex 01, France

The use of intelligent materials reacting to external stimuli is rapidly growing in the fieldof technical textiles. In this paper, the processing of conductive yarns for the development ofsmart textiles is discussed. Two different methods are exposed: the coating of textile yarns usingconductive polymers, and the bulk treatment of spinnable polymers by conductive nanofillers.

In the first part of this article, polyaniline (PANI)-coated ultra-high-molecular-weightpolyethylene (UHMWPE, Dyneema) yarns were prepared. Their electrical, morphologicaland electro-mechanical properties including the temperature influence were investigated.Power handling of PANI-coated conductive yarns as a function of the current was alsoevaluated. Three different prototypes of conductive multiple yarns have been proposed. In thesecond part, the use of multi-walled carbon nanotubes as reinforcing conductive nanofiller forspinnable polymers has been studied. The major influence of the homogeneous dispersionof the nanotubes in the host matrix is particularly pointed out, and the electrical behaviour ofthe nanocomposite yarns has been investigated. Different conductive yarns, developed in ourlaboratory, are expected to be used as fibrous sensors, connection elements in smart clothing,electro-mechanical or thermal data acquisition devices and conductive fabrics for electromag-netic shielding applications.

Key words: carbon nanotubes; conductive coatings; fibrous sensor; nanocomposites;

polyaniline; power handling.

Address for correspondence: Eric Devaux, Laboratoire de Genie et Materiaux Textiles (GEMTEX),Ecole Nationale Superieure des Arts et Industries Textiles (ENSAIT), 9 rue de lErmitage, BP 30329,59056 Roubaix Cedex 01, France. E-mail: [email protected] 1, 6, 11, 12 and 1518 appear in colour online: http://tim.sagepub.com

Transactions of the Institute of Measurement and Control 29, 3/4 (2007) pp. 355376

The Institute of Measurement and Control 2007 10.1177/0142331207081726

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1. Introduction

Demand for the development of electronic textiles (e-textiles) is rapidly growing forindustrial applications such as sensors, electrostatic discharge, steel corrosion,electromagnetic interference shielding, dust-free clothing, monitoring, military

applications and data transfer in clothing (Gregory et al., 1998; Han and Im,2001; Kraljic et al., 2003; Park et al., 2001; Robert, 2003; Sastry, 2004; Tan et al., 2001).Therefore, it is important to modify the properties of textile structures by creatingnovel materials, which are able to react to external and internal stimuli for theseapplications. Modification of fibres or yarns using organic conductive polymersseems to be an interesting approach to achieve electrical functionality. Intrinsicallyconductive polymers (ICPs) such as polyaniline (PANI), polypyrrole (Ppy),polythiophene (PT) exhibit electrical properties because of their -conjugatedchain structures, which derive both from the conducting or neutral (non-conducting) forms (Heeger, 2002; Kumar and Sharma, 1998). These materials may

show high conductivities when doped by guest species. The goal of the first part ofour research is to prepare conductive yarns, which are able to be used as textilesensors, preserving at the same time textile mechanical properties such aslightweight, elongation, bending, shearing and twisting. These textile properties arevery important because the conductive fibres should be transformed in textilestructures by weaving, knitting or other manufacturing processes. Two differentmethods can be applied to synthesize conductive polymer-based yarns. Thefirst method is based on the use of conductive polymer composites (CPCs)processed by a melt-mixing process, whereas the second method is mentioned asa coating process.

In the second part of our research, multi-walled carbon nanotubes (MWNTs) areused for the reinforcement of melt-spinnable polymers. Discovered in 1991 by SumioIijima, carbon nanotubes (CNT) are today considered one of the most importantmaterials in the nanotechnology world, because of the versatility of their potentialapplications. They have been proposed as promising materials for a variety ofapplications including polymer reinforcement (Ausman et al., 2000). They haveindeed unique electronic and structural properties that can be exploited to improveconductivity and mechanical resistance of polymer matrices (Landi et al., 2004).Applications could be smooth mechanical sensors or thermally self-regulated textilestructures. Numerous studies have already been carried out on nanotubes and their

dispersibility in various matrix polymers (Liao et al., 2004; Sandler et al., 2003),elastomeric materials (Flandlin et al., 2000, 2001) or in different solvents (Ausmanet al., 2000). However, only a few projects are concerning nanocharged yarns (Poulinet al., 2002). In this study, the spinnability of CNT-based polyamide 6 (PA-6)nanocomposites and the effect of different parameters on conductivity areinvestigated.

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2. Conductive polymer-based fibres

2.1 Processing of conductive fibres based on conductive polymers

2.1.1 Melt-mixing process: One of the most efficient methods of processing of

conductive polymeric materials is by dispersing conductive fillers in a thermoplasticpolymer and blending by a mechanical mixing process. High-quality conducting

blends of PANI, Ppy with polycaprolactone, polyolefins, polystyrene and poly(ethyleneterephtalate) polymers have been extensively studied. Morphologicalproperties and percolation threshold of conductive fillers on conductive polymer

blends have been also reported (Hosier et al., 2001; Ikkala et al., 1995; Yang et al.,1998; Zilberman et al., 1997, 1998). In our previous study, a melt-spinning processwas applied for preparing conductive composite fibres (Kim et al., 2004). PANIemeraldine salt form (PANI-ES), PPy and graphite were melt-mixed withpolypropylene (PP) or with low-density polyethylene (LDPE) using a co-rotating

twin-screw extruder. The conductivity of these blends was not satisfactory evenwhen 40% of fillers were added to PP. This can be explained by the non-homogeneous morphological structure of conductive fillers in PP. The particles ofconductive polymers formed aggregates because of their strong intermolecularinteractions. The melt-mixing process could be improved by changing the meltingtemperature, mixing time and/or mixing speed. Alternatively, solution blendingmay be proposed in order to obtain highly conductive blends with better dispersionmorphology.

2.1.2 Coating process: Though conductive polymers can be electrochemically

produced in the fibre or film form, they show weak mechanical properties to besuitable for traditional textile applications. Therefore, coating conductive solutions onthe surface of textile fibres should be an interesting approach in order to realizeconductive yarns. Since Ppy-coated polyester textiles were developed by the MillikenResearch Corporation, many research groups are actively involved in this field (Kuhnet al., 1998; Malinauskas, 2001; Shacklette et al., 1993). In addition, in situpolymerization of PANI on non-woven fabrics including nylon 6, cotton and polyester(Nomex) has been recently reported (Gregory, 1993; Kim et al., 2002; Oh andHong, 2001). PANI was used in our study because of its environmental and chemicalstability.

The dispersible conductive form of PANI may be processed in a number of organicsolvents such as n-methyl-2-pyrrolidone (NMP), toluene, dimethylsulphoxide(DMSO), xylene and m-cresol (Davies et al., 1995; Gettinger et al., 1995; Haba et al.,1999; Heeger, 1993; Ikkala et al., 1995; Riul et al., 2003). In the PANI/solvent system, thefilm-like aggregates of PANI can be observed on the substrate surface when thesolvent is removed. This spontaneous molecular assembly has been successfully

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applied to coat PANI on the textile yarn surface. The coating process involves thetransition system from a PANI solution into a solid gel phase on the PET fibresurface. PANI coating was carried out during impregnation of polyethylene textileyarns in the PANI solution. Ultra-high-molecular-weight polyethylene (UHMW-PE,Dyneema) fibres are used in this study because of its good solvent resistance,

and outstanding mechanical properties. The electrical, morphological and electro-mechanical properties of the conductive yarns were characterized.

2.2 Experimental

2.2.1 Materials: The polyaniline emeraldine salt form (PANI-ES) was supplied bythe SigmaAldrich Chemical Company. Dodecylbenzene sulphonic acid (DBSA,Fluka) was used to obtain a homogeneous conducting PANI solution (dispersion)in xylene. Ultra-high-molecular-weight polyethylene (UHMW-PE, Dyneema,220 dtex) yarns were supplied by the DSM Company.

2.2.2 Sample preparation: PANI-ES and DBSA mixtures of 1 : 0.5 in weight ratiowere dissolved in xylene in order to prepare PANI dispersions (quasi solutions)ranging from 5 to 9 wt%. These solutions were vigorously stirred for 3 h at 908C, andthen treated in an ultrasonic bath for 2 h. The continuous coating process has beenperformed using an experimental device which may be equipped with several bathfilled with PANI solution. Dyneema yarns were dipped into the solution andthen taken up in the presence of a dry airflow around yarn surfaces to enhancesolvent evaporation. The take-up speed for wrapping PANI-coated Dyneema yarn

on a bobbin was controlled at 6 cm/min. The coated yarn was washed with acetonecarefully to solidify the PANI layer as well as to remove the residual solvent on thesurface.

2.2.3 Characterization: The electrical resistance of conductive yarns was measuredwith an Agilent 34401A multimeter at 20258C, 6065 RH%. The measurements were

repeated 20 times for different yarn lengths from 1 to 20 cm and mean values were

computed. Surface images of the PANI/Dyneema yarn were obtained using FIB

(Focus Ion Beam lithography, StrataTM DB 235, FEI Company) and SEM (Scanning

Electro Microscopy, JEOL 100 CX model ASID4-D). Tensile tests were performed using

a tensile tester (MTS, Mechanical Testing and Simulation) with its softwareTestworks at 20258C, 65 RH%. The sample length for tensile test was fixed

at 50 mm and the speed of traction was 12 mm/min. Figure 1 shows the

electro-mechanical test set-up with two electrodes (20 20 mm) connecting the

fibres to the multimeter and the tensile test machine. These measuring devices are

developed in our laboratory.




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2.3 Results and discussion

2.3.1 Electrical and morphological properties: The electrical resistance ofPANI/Dyneema yarns coated at different concentrations of PANI solutions wasmeasured for various yarn lengths. Figure 2 shows that the electrical resistance of

PANI/Dyneema

yarns decreases as the concentration of PANI solution increases.It is obvious that the concentration of PANI solution is important to obtain highlyconductive yarns. As has already been reported in our previous paper (Kim et al.,2004), the threshold of concentration of PANI solution for coating process is between5% and 9%. Low conductivity was obtained when less than 5% in weight of PANIsolution was used, and the PANI solution tends to form a gel at higher concentrations(410 wt%). The gelation of PANI solution is an irreversible reaction when the solvent

0

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0 5 10 15 20

Yarn length (cm)

Electricalresistance(ohm)

5%

7%

9%

Figure 2 Electrical resistance of Dyneema conductive yarnscoated with PANI solution at different concentration

(a) (b)

Figure 1 Electro-mechanical test set-up: (a) electrodes in copperand (b) head of electro-mechanical test

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was evaporated and is related to strong intra- and inter-chain hydrogen bondingbetween secondary amine and tertiary imine groups of PANI. This gelation of PANIsolution should be considered as an obstacle to perform the homogeneous coating onthe fibre surface (Mattes et al., 1999). It is also observed in Figure 3 that theelectrical resistance of multiple PANI-coated Dyneema conductive yarns decreases.The utilization of a bundle of conductive yarns affords potential possibilities for theelectrical applications.

The microstructure of PANI/Dyneema yarns were observed using bothFIB lithography and SEM photograph. Figure 4 shows the surface images and the

cross-section images of Dyneema conductive yarns coated with 7% of PANI solution.We can observe that PANI forms a compact layer on the Dyneema surface.This continuous PANI interaction may increase the degree of effectiveness in electrondelocalization, thus enhance the conductivity. A thin PANI conductive layer (about1m) on the fibre surface is clearly observed in Figure 4.

2.3.2 Thermal influence on the electrical properties: The thermal influence on theelectrical resistance of PANI/Dyneema 7% conductive yarns is studied andthe results are shown in Figure 5. The electrical resistance of PANI/Dyneema

conductive yarns increases slowly up to 558C. After passing this criticaltemperature, the electrical resistance increases more quickly. It is possible tonotice that our PANI-coated Dyneema conductive yarns may be used below 558Cwithout modification of the electrical properties and as a temperature sensor

beyond 558C.Figure 6 shows the results from thermogravimetric analysis of the PANI-coated

Dyneema fibres, neat Dyneema fibre, PANI powder and PANI/DBSA


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Time (s)

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(a) 1 yarn

(b) 2 yarns

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Figure 3 Electrical resistance of multiplied PANI-coatedDyneema conductive yarns




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(dopant) film. PANI powder is thermally stable until almost 2708C under nitrogen,and the decomposition of PE fibre is accelerated from 3008C. PANI-coatedDyneema conductive fibre is also thermally stable to up to 2208C. However,PANI/DBSA film loses a larger amount of its initial weight. This result suggeststhat DBSA used as a processing aid influences strongly the thermal stability ofPANI-coated Dyneema fibre. From the thermal gravimetric analysis (TGA)studies, it was found that the mass loss of PANI/Dyneema conductive fibresfrom 25 to 2508C is about 8%.

Figure 4 Scanning electron micrographs of PANI/Dyneema

conductive fibres

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00 600500400300200100

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(b)

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Weightloss(%)

(a)

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(a) Neat Dyneema fibre(b) PANI powder(c) PANI/Dyneema9 %(d) PANI/DBSA film

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Temperature (C)

Weightloss(%)

(a)

(b)

(c)

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(a) Neat Dyneema fibre(b) PANI powder(c) PANI/Dyneema 9 %(d) PANI/DBSA film

Figure 6 (a) The TGA curves of neat Dyneema, PANI-ES,PANI/Dyneema 9% yarns and PANI-ES/DBSA film under airand (b) enlarged graph


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Figure 5 Thermal influence on the electrical resistance for 5 cm ofPANI/Dyneema 7% conductive yarn




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2.3.3 Electro-mechanical properties: The dependence of the electrical resistance ofconductive yarn on different elongations was also studied. The tensile testof conductive yarn was continued up to 90% of its breaking load (N) and theelectrical resistance () was measured at the same time. The electrical resistance ofPANI/Dyneema conductive yarn increases about two times at 20 N of the load,

as can be seen in Figure 7. However, PANI-coated Dyneema yarns were strongenough and did not break. Increases of resistance at different elongations can beexplained by micro-cracks in conductive PANI layers on the fibre surface under strain.We suppose that with increase of the distances among PANI molecular chains(micro-scracks in conductive layers), the number of connections between polymerchains decreases. In fact, the conductivity mechanism could not be explained onlywith a single chain theory. Conductivity of polymers is rather a supramolecularphenomenon. The morphological structure, interaction of chains, defects at themolecular level, the direction and the arrangement of the chains and the chaindistances are regarded as important parameters for conductivity. Therefore, models of

solitons and polarons have been more popular to explain the conducting mechanismin conductive polymers (Heeger et al., 1988).

The oxidation of conductive materials is normally a limiting factor in the long-termstability of the conductive yarns. The ageing effect with time on the electricalproperties of PANI-coated conductive yarns was studied under standard conditions at258C, 5556 RH%. The electrical resistance for 5 cm of Dyneema conductive yarnscoated with 7% of PANI solution increases highly until 12 weeks as shown in Figure 8.The electrical stability of PANI-coated yarns could be controlled by isolating themwith an external protective layer.

2.3.4 Power handling and destruction: It is important to study the specific electricalproperties of conductive yarns such as power handling and tension threshold


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Load (N)

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Figure 7 Electro-mechanical property for 5 cm of PANI/Dyneema 7% conductive yarn at different loads

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of conduction (Kim, 2005). Figure 9 shows the intensity (I) according to the appliedvoltage (V) for 5 cm of Dyneema conductive yarns coated with 9% of PANI solution.The threshold tension of conduction ranging between 1.5 and 2 V and the intensity ofone and two of PANI-coated Dyneema yarns increases linearly until 15 V of appliedvoltage. At the rupture of 15 V, the intensity for one yarn and two yarns are 0.027 A

and 0.05 A, respectively. The maximum power handling for 5 cm of one and two yarnscould be 0.42 and 0.75 W. The electrical behaviour at maximum power handling ofconductive yarns is not yet explained, but the local deformations of conducting layerat rupture are supposed to play an important role. The morphological modification ofthe PANI conducting layers on fibre surface can be observed in Figure 10. We supposethat the temperature increases at maximum voltage and the PANI layers are locallydeformed and disconnected, hence, the conductivity decreases.

Intensity(A)

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Figure 9 Intensity (I) as function of the applied voltage (V) for5 cm of PANI/PE 9% conductive yarns


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Figure 8 Time effect on the electrical resistance for 5 cm of PANI/Dyneema 7% conductive yarns




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2.3.5 Applications prototype: Since the prepared conductive fibres are developedfor textile applications, their surfaces have to be protected with an external layer inorder to increase their stability and insulate them electrically. We have developedthree different types of protecting layers for PANI-coated conductive fibres(Kim, 2005). The first one is covering with a plastic film to protect fibres fromabrasion. This prototype is flexible and lightweight as shown in Figure 11(a).

These yarns can be used to transfer the electricity of approximately 36 V to light upan LED (light emitting diode) of 20 mA (Figure 11b). This prototype may also be usedfor data transfer elements. A second prototype is prepared by covering the conductiveyarns with a thermoretractable polyolefin tube as presented in Figure 12(a). A thirdprototype is realized using a weaving machine. PANI-coated conductive multipleyarns have been protected by weaving a layer of Kevlar fibres (Figure 12b).These prototypes show various possibilities for potential applications preservingelectrical and mechanical properties of textiles.

2.4 Conclusion

We have developed two methods of producing conductive textile yarns: melt-mixingand coating processes. In this paper, PANI-coated polyethylene (Dyneema) conduc-tive textile yarns were obtained by coating, and their electro-mechanical properties,time and temperature effects on the electrical properties were studied. Our conductiveyarns showed interesting properties for electrical applications preserving the originalstrength and flexibility of used textile. From SEM photographs, we observe

Figure 10 Local modifications of PANI conductive layers on theDyneema conductive yarns surface at maximum power apply

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that approximately 1 mm of the PANI layer was deposited on the fibre surface.The thickness and morphology of coating layer are important parameters influencingthe electrical response to stimuli. The coating layer properties can be controlled by theconcentration of conducting materials in solution, the applied tension, take-up speedof yarns (fabric) and drying conditions during the coating process. The power

handling and destruction of PANI-coated conductive yarns was also studied. It couldbe improved by using multiple yarns. Local deformation of PANI layers occurred atmaximum current caused by increasing of the temperature and the electricalproperties have been modified. The electrical stability to environmental damages

(b)(a)

Figure 12 (a) Prototype prepared using thermoretractable tubeand (b) conductive yarns protected by Kevlar fibres

(b)(a)

Figure 11 (a) Prototype of conductive yarns covered with plasticfilms and (b) LED lighting test




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(thermal, abrasion and humidity, etc.) can be controlled by preserving the coatedyarns with external protecting layer. Three different prototypes of conductive multipleyarns have been developed in our laboratory using external protecting layers. Furtherimprovement of processing is expected to enable the utilization of conductive yarnsin a wide range of potential applications such as fibrous sensors, connection devices in

smart clothing or for electromagnetic shielding (EMI) applications.

3. Carbon nanotubes-based nanocomposite fibres

3.1 Materials and method

Thin MWNTs are provided by Nanocyl. Their main characteristics are inner diameterof 37 nm, outer diameter of 525 nm and length up to 50 mm (Figure 13). They aresynthesized by a catalytic chemical vapour decomposition (CVD) system, whichaffords larger-scale production and thus a lower cost for carbon nanotubes. In the

CVD technique, CNTs are grown from nucleation sites on a catalyst (iron, cobalt orother metals) in carbon-based gas environments (ethylene, methane . . .) at elevatedtemperatures (around 10008C) (Decossas, 2001). The polymer used in this study is aspinnable thermoplastic PA-6 provided by Nylstar. Pellets of PA-6 were held at 808Cfor 48 h, in order to remove every trace of water before extrusion or spinning.

The chemical vapour deposition process requires the use of a catalyst that createsimpurities such as cobalt or iron particles. Thus, before the use, nanotubes need to bepurified. In this study, the carbon nanotubes were immersed in aqueous acid(HCl 6 mol/l) followed by sonication and heating at 608C for 30 min. They were heldin this medium for 24 h at room temperature. The nanotubes were then recovered by

filtration and finally washed with deionized water and acetone.

Figure 13 SEM photographs of MWNTs

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The main difficulty for the processing of nanotubespolymer composites is thestrong tendency for the nanotubes forming aggregates. This is related to the Van derWaals interactions between nanotubes, their weak interaction with the polymer matrixand their high aspect (length/diameter) ratio. Thus, it was necessary to graftfunctional groups on the surface of nanotubes to induce either a repulsive force

between particles or a good interaction with the polymer matrix and make a betterdispersion. In this study, the nanotubes were functionalized by immersion in a 10%NH2OH (HCl) (hydroxylamine chlorhydrate) solution followed by sonication for30min at 608C. The solution containing nanotubes was then stirred for 24 h at 258Cand filtered. The functionalized nanotubes were finally washed with deionized waterand acetone.

A twin-screw extruder (model Rheomex PTW-16/25p from ThermoPrism) wasused to mix the nanotubes with the PA-6 polymer and to obtain nanocompositepellets. The nanotubes were incorporated into the molten polymer and homo-geneously dispersed. The shear stress applied to the molten polymer had to be strong

enough, combined with long residence time within the barrel, to facilitate dispersion.The rotational screw speed is then fixed at 400 rpm. This extruder includesfive heating zones, in which the temperature is independently fixed from 170to 2508C. The material obtained is a rod used either directly for measurementsor pelletized and introduced in the spinning machine to obtain yarns formeasurements.

A melt-spinning machine called Spinboy I manufactured by BusschaertEngineering is used to obtain the multifilament yarns. The multifilament iscovered with a spin finish, rolled up on two heated rolls with varying speeds(S1 and S2) to ensure a good draw. The theoretical drawing of multifilament is

given by the ratio E

S2/S1. During the fibre spinning, the molten polymercontaining the carbon nanotubes is forced through a die head with a rather lowdiameter (approximately 400mm), and through a series of filters. The presence ofcarbon nanotube aggregates has to be prevented in order to obtain a satisfactorydispersion of nanotubes in the polymer. During the melt-spinning process, thesupercooled polymer has to be stretched to favour the orientation of theconstitutive macromolecules in the direction of the filament. For these reasons,we have fixed the temperature scale along the die head from 2358C to 2408C,the drawing rate at E 2.25 (S1 200 rpm and S2 550 rpm), and the temperaturesof the heating rolls T1 708C and T2 808C. Flow diagram for preparing CNT-PA6polymer composites containing purified and functionalized carbon nanotubes isshown in Scheme 1.

The polarity of nanotubes, after functionalization, was proved by dispersing0.5 mg of the material in 5 ml of formic acid. An observation of the stability of thedispersion was investigated as a function of time. A Nanoscope 3 Atomic ForceMicroscope (AFM) was employed to examine the topography of nanocompositeyarns. Electrical properties were measured using a Keithley 617 multimeter.




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Resistance was directly measured and the conductivity was calculated using thefollowing equations:

R l

S and C

1

where R is the electrical resistance of the material (), is the resistivity (.m), l is thelength (m), S is the cross-section of the material (m2) and C is the electricalconductivity (1. m1).

TGA was carried out using a TA 2950 thermal analyser; 10 mg of carbon nanotubeswere placed in an aluminium crucible and heated up in air from 258 to 10008C atheating rate of 108C/min. The residual weight at 10008C corresponds to the quantityof impurities in the material. Infra red spectroscopy (IR) was used to observe thepresence of grafted functional groups on nanotubes, after functionalization.

3.2 Experimental results and discussion

3.2.1 Atomic force microscopy (AFM): PA-6 yarns and raw MWNTs (2%)/PA-6yarns were observed using AFM. Figure 14 presents the topography of the yarnssurface. PA-6 yarn shows a smooth and homogeneous surface. However, the surface

PA-6 Raw Carbonnanotubes (MWNT)

Purified MWNTsFunctionalization Functionalized

MWNTs

ExtrusionExtrusionMelt

spinning

Melt spinning

Rod

Multifilaments

PA-6 yarnPA-6/MWNTs2 wt % yarn

PA-6/MWNTs2 wt %

PA-6/MWNTs4 wt %

PA-6/functionalisedMWNTs 4 wt %

Scheme 1 Flow diagram for preparing the PA-6 yarn and fourdifferent types of PA-6 nanocomposites yarns containing purifiedor functionalized nanotubes

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of the MWNTs (2%)/PA-6 yarn is rough and heterogeneous. The non-functionalizednanotubes aggregate in bundles on the surface of the yarn. The main problemsassociated with processing of conductive fibres include homogeneous dispersion,orientation and alignment of the nanotubes in the polymer. Because of their highlength and their polarity, the carbon nanotubes tend to show strong entanglements(especially for MWNT) and prevent their complete alignment. The functionalizationof nanotubes and chemical mixing process were developed to overcome thesedifficulties.

3.2.2 Purity of carbon nanotubes: The thermal degradation behaviour of purifiedand non-purified MWNTs was observed by TGA analysis and represented inFigure 15. The results show that purified nanotubes have a residual weight afterheating lower than non-purified nanotubes (Table 1). Moreover, the degradationprocess appears to be faster in the case of purified nanotubes. The metal particles weresuccessfully removed during the purifying process. This purification method isefficient and easy to carry out.

Lossofweight(%)

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Figure 15 TGA curves for purified and non-purified nanotubes

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2H

H

Figure 14 AFM image of MWNTs (2%)/PA-6 yarn (left) and AFMimage of PA-6 yarn (right)




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3.2.3 Chemical grafting of carbon nanotubes: The main challenge relating to theproduction of yarns as pointed out earlier is to obtain a homogeneous dispersion ofcarbon nanotubes into the polymer matrix. So, that the nanotubes are dispersedindividually. To facilitate dispersion, the carbon nanotubes have been functionalizedin order to enhance the interaction between the filler and the polymer. Comparing

neat CNT and functionalized CNT using IR spectra is a good way to identify graftedgroups. This method was thus used to check the efficiency of the functionalization.The spectra (Figure 16) show that NH groups have been grafted corresponding tothe peak at 3129 cm1.

3.2.4 Dispersibility: The success of the functionalization of carbon nanotubeshas also been investigated by dispersing the carbon nanotubes in a polar solvent(formic acid). We observed that the dispersibility is enhanced considerably afterpurification and functionalization (Figure 17). In fact, the functionalized MWNTs arehydrophilic and well dispersed in formic acid. The non-functionalized MWNTsagglomerate at the bottom. The functionalization of MWNTs containing polar groupsmay be promising in the formulation of polymer CNT-PA6 polymer composites.

3.2.5 Electrical properties: Electrical conductivity is the phenomenon that describesthe transport of electric charge through materials. CPCs are obtained by the dispersionof conductive fillers in a thermoplastic polymer matrix. The electrical properties of

binary mixtures depend strongly on the microstructure. In particular, dispersion state,

Table 1 Purity results for purified and non-purifiednanotubes

Residual weight (%) Purity (%)

Purified nanotubes 8 92

Raw nanotubes 15 85

3129.6

2

0.06

0.04

Transmittance

0.02

0.00

2000Wavenumber (cm1)

Figure 16 IR spectra for functionalized CNT and raw CNT

Devaux et al. 371



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filler geometry and fillermatrix interactions strongly affect the electrical properties ofnanocomposite materials. Table 2 shows the electrical properties of different systemsof PA-6 with or without the incorporation of MWNTs in the polymer. The yarn lengthfor measurement was fixed at 10 cm, a tension was applied while the apparatusmeasured the intensity. The resistance was then calculated automatically. PA-6 ismerely insolent (3.8 106 S/m) while PA-6/nanotubes (2 wt%) composites show agreat increase in conductivity (7.57 101 S/m). Multifilament yarns are 10 timesmore conductive than that of the rods at the same filler content. This is related to thedraw spinning process, which helps the parallelizing of nanotubes along the length ofthe yarn. PA-6/functionalized nanotubes (4 wt%) rod is almost 10 times moreconductive than that of the PA-6/non-functionalized nanotubes (4 wt%). It confirmsthat the functionalization of nanotubes before mixing with the matrix polymer isefficient to enhance the electrical properties. Percolation threshold for the rod of PA-6is found to be around 3.5 wt%, corresponding to the concentration of nanotubes at theinsulating/conductive transition of composites (Figure 18). This value might bechanged, depending on the nanofiller geometry. Indeed, while increasing the aspect

ba

Figure 17 Dispersibility behaviour of MWNTs (a) and MWNTsfunctionalized (b) after 2 h in acid formic

Table 2 Electrical properties of PA-6 as a function of nanotubes content andfunctionalization

System Diameter (m) Resistivity ( m) Conductivity (S/m)

PA-6 yarn 4.8 104 2.63105 3.8106

PA-6/MWNTs 2 wt% yarn 4.4 104 1.32 7.57101

PA-6/MWNTs 2 wt% rod 9.41 104 13.85 7.22102

PA-6/MWNTs 4 wt% rod 1.18 103 2.03 4.93101

PA-6/functionalized MWNTs 4 wt% rod 1.33 103 3.31101 3.02




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(length/diameter of filler) ratio, the percolation threshold is reduced (Sandler et al.,2003). In our study, CNT-PA6 composite yarns show the same conductivity as thetraditional semi-conductive properties at relatively low charge content.

3.3 Conclusion

The main problem encountered in the processing of carbon nanotubes is obtaininga homogeneous dispersion of the nanotubes in the polymer. The non-functionalizedMWNT shows a low affinity with PA-6 and agglomerates leading to low conductivityin the final nanocomposite. Solution process was used to functionalize the carbon

nanotubes. The chemical functionalization of carbon nanotubes optimized thedispersion and the orientation, which enhanced the electrical properties of theCNT-PA6 composite yarns.

4. Conclusion and perspective

In this article, the production of conductive textile yarns using two methods, chemicalcoating and a melt-mixing process, has been reported. In the first part of the study,PANI-coated polyethylene conductive fibres have been prepared and analysed.The morphological, mechanical and thermal influences on the electrical propertiesof PANI-coated polyethylene yarns are discussed. The power handling test ofconductive yarns is also evaluated. Several prototypes of conducting yarns areproposed at the end for applications of fibrous sensors and data transfer devices.The chemical coating process is economical as well as easy to perform using differenttypes of textile materials (fibre, tissue and non-woven, etc.). However, the solutionsshould be carefully prepared with good viscosity to control the coating layers.

3.5

3

2.5

2

1.5

Conductivity(S/cm)

1

0.5

0

Charges load (%)

Percolation thresholdInsulating/conductive transition

4 3.5 3 2.5 2 1.5

Figure 18 Representation of the percolation threshold

Devaux et al. 373



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The relatively low storage capabilities (oxidation of PANI on exposure to theenvironment), the electro-mechanical deformation of coated yarns in service would beother limiting factors of the coating process. Moreover, the surfaces of PANI coatinghave to be protected with external layer in order to increase their stability and insulatethem electrically.

In the second part of this article, the functionalization of carbon nanotubes,the preparation and characterization of PA-6/carbon nanotube composite yarns isdescribed. This process consists in incorporating conductive fillers (carbon nanotubes)into synthetic fibres via a melting process. The melt-mixing process has manyadvantages compared with coating. The composites containing the nanotubes insideare less sensitive to oxidation and the electrical conductivity remains stable.In addition, the melt-spinning processing is much faster than the coating process.This method allows also a fast integration of the new fibres into the different types offabrics (such as non-woven, knitted or woven fabrics) without any demand onproduction line. In addition, nanocomposite fibres based on the integration of carbon

nanotubes show different functionalities without any surface treatments. On the otherhand, the main problem encountered is obtaining a homogeneous dispersion of thenanotubes in the polymer. In order to obtain CPCs with the desired electricalproperties, it is necessary to control precisely the dispersion of the carbon nanotubesin the polymer matrix. The presence of aggregates seems to decrease the mechanicalproperties and the electrical conductivity of the nanocomposites. To improveconductivity, several methods have been proposed to overcome those difficultiessuch as chemical functionalization of the naotubes before mixing and modifyingmechanical mixing conditions (temperature and/or extruder rotation speed). Anotherproblem encountered during the transformation of carbon nanotubes/polymer

composites to yarns is the spinnability of the polymer. The spinnability ofnanocomposites depends on the melting behaviour of matrix polymer. At present,the cost of nanotubes is also one of the limitations for using them as nanofillers. Workis underway to optimize the spinning process for producing high-quality CNT-PA6polymer composite yarns.

Future research and development is focused on the realization of (semi)conductive textile tissue by weaving or knitting for electronic sensing applications,electro-mechanical or thermal data acquisition, and even for data transfer elements.

Acknowledgements

The authors would like to thank Mrs Sabine Chlebicki for her contributions during theextrusion and spinning process. We would like to thank Nanocyl (Sambreville, Belgium)for the supply of raw carbon nanotube materials. The authors would also like to thankMr Francois Dassonville and Mr David Troadec (IEMN, Lille, France) for their technicalhelp in AFM, SEM and FIB analysis.




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Documents

Transactions of the Institute of Measurement and Control-2007-Devaux-355-76