- Email: [email protected]
Electrical and morphological properties of PP and PET conductive polymer fibers
Synthetic Metals 146 (2004) 167–174
Electrical and morphological properties of PP and PET conductive polymer fibers Bohwon Kima,∗ , Vladan Koncara , Eric Devauxa , Claude Dufourb , Pierre Viallierc a
Laboratoire de G´enie et Mat´eriaux Textiles (GEMTEX), Ecole Nationale Sup´erieure des Arts et Industries Textiles (ENSAIT), 9 rue de l’Ermitage BP 30329, 59056 Roubaix cedex 01, France b Institut d’Electronique ´ de Micro´electronique et de Nanotechnologie (IEMN), Cit´e Scientifique, Avenue Poincar´e, BP 69, 59652 Villeneuve d’Ascq cedex, France c Ecole Nationale Sup´ erieure des Industries Textiles de Mulhouse (ENSITM), 11 rue Alfred Werner, 68093 Mulhouse, France Received 22 November 2003; received in revised form 23 March 2004; accepted 23 June 2004 Available online 25 August 2004
Abstract Conductive fibers were obtained using two experimental processes (melt spinning and coating process). In melt spinning process, polyaniline (PANI), polypyrrole (PPy) and graphite were used in order to obtain conductive polypropylene (PP) based fibers with specific electrical and mechanical properties. PANI was treated using dodecylbenzene sulfonic acid (DBSA) to improve the solubility and the dispersion of PANI in xylene. PANI coating on PET yarns were performed by absorption of yarns through PANI solution. The electrical resistance and morphological characteristics of conductive yarns were investigated. These yarns are supposed to be used to create smart clothing, corrosion protection or conductive fabrics for electromagnetic shielding applications. © 2004 Elsevier B.V. All rights reserved. Keywords: Conductive fiber; Polyaniline (PANI); DBSA; Coating process
1. Introduction Recently, the textile industry has made considerable advances in the field of high value added textiles, mainly in the sectors of high performance textiles and yarns. The use of new materials with specific properties and the development of new structures and integration processes make it possible to develop supports able to convey information while being mostly based on properties of electric conduction. These new achievements of the textile industry enable electronic devices to be directly integrated into the structure of textile, therefore modifying the functionality of the apparel. Besides the main functions of apparel, which are protection and passive communication, the clothes become an interface with specific new functions between the individual and his environment. ∗
Corresponding author. E-mail addresses: [email protected] (B. Kim), [email protected] (V. Koncar). 0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.06.023
The term “intelligent apparel” describes a class of apparel that has active functions in addition to traditional properties of clothing. These novel functions or properties are obtained by utilizing special textiles or electronic devices integrated into the textile structures. Everyone wears clothing and most people are concerned with the appearance of communication apparel. However, the needs will be different within any given group of people. The broad principle topics are: 1. 2. 3. 4. 5.
Professionals [1–3] (the need for “free hands function”), Health care [4] (monitoring, training result diagrams), Every-day life [5] (telephone, wellness), Sports [6,7] (training, performances measurement), Leisure [3] (aesthetic customization network games).
The demand for electrically conductive fibers and textiles structures used for industrial materials like sensors, electrostatic discharge, welding of plastics, electromagnetic interference shielding, dust and germ-free clothing [8], data transfer in clothing but also for military applications like camouflage
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Fig. 1. Polyaniniline (a) emeraldine base and (b) emeraldine salt forms.
and stealth technology is growing up. For all the applications previously mentioned the basic element is of course the textile fiber. Therefore, it is important to modify the fiber properties (electrical and other) in order to modify the general properties of the textile structure and to add previously mentioned new functions. Modification of fibers based on conductive polymers seems to be the interesting approach enabling these new functionalities. Conductive polymers, used to fill or coat fibers, show electrical properties due to their conjugated double bond chain structures, which derive both their conducting or neutral (non-conducting) forms. However, they are inherently insoluble and infusible due to their strong intermolecular interactions. High quality conducting blends with conventional polymers by melt mixing [9–13] or by solution casting [14–17] are still in a development stage. Among the conjugated conducting polymers, polyaniline (PANI) has attracted much interest worldwide because of its excellent environmental, thermal and chemical stability. Polyaniline exists in a variety of oxidation states ranging from leucoemeraldine (completely reduced) to emeraldine (partially oxidized) to pernignaniline (completely oxidized). Most broadly studied forms of polyaniline are the emeraldine base form and the emeraldine salt form (Fig. 1). With simple doping process by charge transfer chemistry or by acid–base (protonation) chemistry, polyaniline converts the semiconducting emeraldine base form (10−5 S/cm) to the metallic emeraldine salt form (102 S/cm) [18–20]. In this paper, a doped polyaniline has been mainly used in order to create conductive fibers. The originality of our approach is to create sufficiently conductive yarns able to transport information or to be used as textile sensors, but with all the mechanical properties as elongation, bending, shearing and twisting corresponding to a real textile yarn. These textile properties are very important because the conductive fibers should be transformed in textile structures by weaving, knitting or other manufacturing processes. Two different methods have been developed to synthesize conductive yarns with textile mechanical properties.
bon black, metal powders. Shacklette et al. [21] reported that a conductive form of polyaniline (VersiconTM, AlliedSignal, Inc.) is dispersible in polar thermoplastic matrix polymers such as polycaprolactone and poly(ethyleneterephtalate glycole). Ikkala et al. [10] reported on conducting polymer blends by blending thermoplastic bulk polymers such as polyolefins, polystyrene with Neste complex, polaniline salt complex developed by Neste Oy and UNIAX Corporation. In their report, 1–20 S/cm of conductivity has been achieved in the range of 1–30 wt.% of Neste complex. In addition, morphological study and reducing the sufficient amount of conductive filler (percolation threshold) into conductive polymer blends have been actively performed [11]. In this point, melt spinning process is our first approach for preparing conductive composites fibers. Polyaniline emeraldine salt form (PANI-ES), PPy and graphite were melt mixed with polypropylene (PP) or low-density polyethylene (LDPE) using a co-rotating twin-screw extruder (Fig. 2). Low PANI percolation concentrations (6–10 wt.%) were used for PANI particles dispersion in PP via melt blending. 1.2. Method 2 – coating process Our second approach for the synthesis of conductive fibers is based on a coating process. Although conducting polymers can be produced electrochemically in fibers or films forms, they are brittle to apply on large applications. Considering this difficulty, thin coating or polymerizing conductive poly-
1.1. Method 1 – melt spinning process One of the cost- and process-effective methods is blending common plastics with conductive fillers including car-
Fig. 2. Co-rotating twin-screw extruder, MiniLab (Haake mixer).
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Fig. 3. Overview of the coating process used in this study.
mers from solutions onto the surface of plastics or textiles should be a reasonable method to create conductive textiles structures. Since polypyrrole coated polyester textiles have been well developed by Milliken Research Corporation [22], many research groups are active in this field [23]. In addition, polyaniline coated or in situ polymerized on non-woven fabric, nylon 6, cotton, polyester fabric and nomex fabric are recently reported [23–26]. Water soluble conductive polymers are also in the phase of development by several research groups [27,28]. The soluble conductive form of PANI may be obtained by protonation with functionalised protonic acid, denoted as H+ (M− –R) [29]. H+ M− is protonic acid group and may be sulfonic acid, carboxylic acid, phosphonic acid, etc. The proton of the protonic acid reacts with imine groups of polyaniline and the M–R group serves as the counterion. R is an organic group that can be compatible with nonpolar or weakly polar organic solvents, e.g. N-methyl-2-pyrrolidione (NMP), Dimethylsulfoxide (DMSO), xylene and m-cresol. Efficient doping methods of PANI with dodecylbenzene sulfonic acid (DBSA) or camphor sulfonic acid (CSA) were reported [14–16,30]. The PANI chains tend to form extended chain conformation in presence of DBSA; electron transport between polymer chains should be enhanced. Moreover, long flexible alkyl chains of DBSA may enhance the crystallization rate of polyaniline. In the polyaniline/solvent system, the film-like agglomerates of polyaniline take place on the solution/solid interface when the solvent is removed. In our research work, this spontaneous molecular assembly has been used to coat conductive polymers on the material surface, and has been successfully applied to yarns and textiles. PANI coating was carried out during the impregnation of PET yarns in the PANI solutions (Fig. 3). Then, the electrical resistance and the morphology of the conductive yarns were investigated.
2. Experimental
The spinnable isotactic polypropylene (PP, Elf-Atochem) and the low-density polyethylene (LDPE) were used as matrix polymers. Dodecylbenzene sulfonic acid (DBSA, Fluka) was used as the dopant and surfactant to increase the conductivity and to obtain a homogeneous conducting solution of PANI in xylene. Fig. 4 shows the chemical structures of the materials used in this paper. For chemical coating process, PET spun yarn (20 Nm, Hyosung, Korea) and PET filament yarn (292 Dtex, Hyosung, Korea) were used. PET yarns were used after washing with the solution of 2 g/l of sodium carbonate and 1 g/l of Tinovetine at 70 ◦ C for 30 min. 2.2. Experimental process 2.2.1. Method 1 – melt spinning process Binary polymer blends consisting of PANI-ES, PPy or graphite in matrix polymer were melt mixed using a corotating twin-screw extruder (Mini Lab., Thermo Haake, Germany) at 50 rpm for 15 min. The blending temperature varied with the matrix polymers as follows. Blends of LDPE were melt mixed at 150 ◦ C, and PP at 200 ◦ C. All the materials were dried at 50 ◦ C for 24 h before use. The conductive materials concentrations range was from 1 to 40 wt.%. 2.2.2. Method 2 – coating process PANI-DBSA mixtures at a ratio PANI-ES:DBSA 1:2 (w/w) were dissolved in xylene to prepare 3, 5, 6 and 10 wt.% PANI/xylene solutions. These solutions were stirred vigorously at 120 ◦ C for 3 h. Then, they were treated in the ultrasonic bath at 80 ◦ C for 2 h. The coating of the yarn has been performed using an apparatus that can control the temperature and the taken-up speed. This process may contain several bathes filled with PANI solution. The temperature of these baths varies from 30 to 100 ◦ C. PET yarns were withdrawn through the baths in the presence of a dry airflow along the coating surfaces to
2.1. Materials Emeraldine salt form polyaniline (PANI-ES) and polypyrrole (PPy) were supplied by Sigma-Aldrich Chemical Company. Graphite spherical nanoparticles, from Carbon Nanotechnologies Inc. were also used as conductive fillers.
Fig. 4. Chemical structures of the materials.
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enhance solvent evaporation. The taken-up speed of yarn is adapted to the drying dynamics and for our experiments, it was set to 6 cm/min. PANI coated PET yarns obtained were then washed with distilled water and acetone for several times and dried at 25 ◦ C. PANI doped films have been made in our laboratory in order to evaluate their electrical properties. PANI films were also prepared by casting of PANI solutions onto the glass plates and by drying them at 50 ◦ C in an oven over 24 h to remove the solvent. After drying, dark green solid PANI films were obtained by peeling them off the glass substrates. 2.3. Morphological and electrical characterization The surface structure of the PP/PANI monofilament was observed simply using reflected light optical microscopy (Axioskop ZEISS, Germany). Detailed images of the PANI coated PET yarn surfaces were obtained using AFM (Nanoscope III Atomic Force Microscope from Digital Instruments) and SEM (JEOL 100 CX model ASID4-D Scanning Electron Microscope). The SEM samples were goldsputtered before observation. The electrical resistance tests were performed using the “two-probe test technique” with Keithley 617 electrometer used as an ammeter at 25 ◦ C, 55.56 HR%. Keithley 617 electrometer generates the variable voltage dc powers supply connected to the conductive fibers. In the same time, it measures the electric current with the pA maximal precision. The electrometer is connected to a PC computer, which controls the voltage of dc power supply and records the measured current. Surface electrical resistance of PANI film was also measured with same method to compare with that of PANI coated conductive yarns.
3. Results The Fig. 5 shows the electrical conductivity (σ) of conductive materials in PP or LDPE obtained by melt spin-
Fig. 5. Electrical conductivity of conductive materials/matrix polymer blends prepared using twin-screw extruder: (䊉) PANI-ES in PP, () PANIES in LDPE and graphite in PP.
Fig. 6. Morphology of 5 wt.% PANI-ES (black particles) in PP monofilament (grey region) observed using reflected light optical microscopy.
ning process. The average diameter of the conductive PP or LDPE monofilaments obtained by melt spinning process was 0.5 mm. It is obvious that the conductivity of these monofilaments is low. This can be explained by the non-homogeneous morphological structure of PP monofilament with 5% of PANI-ES presented in Fig. 6. Two phases exist; dark particles (PANI-ES) and surrounded grey PP region. The particles of PANI-ES were not completely dispersed and formed aggregates in PP. As Faez [31] and Hosier et al. [11] previously reported, a degree of connectivity between the conductive additives must exist in order to facilitate the electrical conduction. In our case the conductive particles appear to exist as distinct in the blend and the connections among them are missing. Moreover, insulating PP among particles and particle aggregates should explain the low conductivity. In the case of PP filled with PPy, the conductivity increases as the PPy content increases (Fig. 7). At high content, PPy phases should be more continuous thus these results are ex-
Fig. 7. Electrical conductivity of polypyrrole/PP monofilaments at different content.
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Table 1 Electrical resistance () of PANI coated PET spun yarns for different lengths (S.D.: standard deviation). 1 cm R PANI-spun 3% S.D. R PANI-spun 5% S.D. R PANI-spun 6% S.D. R PANI-spun10% S.D.
3 cm
1.70 × 7.29 × 104 1.35 × 104 3.59 × 103 1.36 × 102 3.08 × 101 6.64 × 101 1.21 × 101 105
5 cm
2.38 × 9.65 × 104 3.79 × 104 1.10 × 104 4.52 × 102 8.70 × 101 3.54 × 102 1.69 × 102 105
10 cm
5.29 × 1.82 × 105 6.96 × 104 1.19 × 104 6.31 × 102 5.22 × 101 3.64 × 102 7.7 × 101 105
15 cm
1.53 × 6.40 × 105 1.96 × 105 6.46 × 104 1.37 × 103 1.16 × 102 1.02 × 103 5.79 × 102 106
20 cm
2.54 × 4.22 × 105 3.47 × 105 6.35 × 104 2.19 × 103 1.46 × 102 1.71 × 103 3.05 × 102
50 cm
3.92 × 3.12 × 105 4.65 × 105 1.42 × 105 3.03 × 103 3.02 × 102 1.83 × 103 1.99 × 102
106
106
9.95 × 106 1.74 × 106 7.23 × 105 3.91 × 104 7.41 × 103 2.07 × 102 4.83 × 103 3.10 × 102
Table 2 Electrical resistance () of PANI coated PET filament yarns for different lengths.
R PANI-filament S.D. R PANI-filament S.D. R PANI-filament S.D.
5% 6% 10%
1 cm
3 cm
5 cm
10 cm
20 cm
2.76 × 104 6.03 × 103 7.83 × 102 3.89 × 102 9.86 × 102 2.31 × 103
2.05 × 105 4.51 × 104 1.02 × 103 3.29 × 102 1.47 × 103 1.77 × 102
3.27 × 105 7.36 × 104 1.28 × 103 2.85 × 102 2.90 × 103 3.06 × 102
7.81 × 105 1.42 × 105 1.51 × 103 2.22 × 102 7.70 × 103 1.38 × 103
4.36 × 106 4.28 × 105 2.88 × 103 1.89 × 102 2.04 × 104 7.89 × 103
pectable. However, with 40 wt.% of PPy content in PP, the conductivity is only 2.88 × 10−7 S/cm. The dispersion of PPy in PP is not satisfactory, connections are missing and this result is similar to the previous one. All the electrical resistances of PANI-PET yarns obtained by coating from PANI solutions are presented in Tables 1 and 2. The electrical resistance was calculated from intensity (I) versus applied voltage (V) measurements when V varies from −5 to 5 V. All the curves of I versus V characteristics are linear. The experiments have been repeated 30 times for different fiber lengths from 1 to 50 cm and average values have been computed. All the coated yarns were indicated using an abbreviation. For example, R PANI-spun 3% means the electrical resistance of PET spun yarn coated with 3% of PANI solution. The electrical resistance (R) of PANI film measured by same method is 3.46 × 102 /cm2 . However, as we can see in Tables 1 and 2, the resistances of PANI coated yarns raging from 1.02 × 103 (PANI-spun 10%) to 1.53 × 106 (PANI-
spun 3%) for a 10 cm long fiber. In the case of yarns, PANI is not coated uniformly (see Fig. 11). We think that contacts among PANI chains are less important on yarns than in the film on a glass substrate because of a circular form of the
Fig. 8. Electrical resistance of PANI coated PET spun yarns at 6 and 10% of PANI solutions.
Fig. 10. Electrical resistance of PANI coated PET filament yarns at 6 and 10% of PANI solutions.
Fig. 9. Electrical resistance of PANI coated PET spun yarn and filament yarn at 6% of PANI solution.
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Fig. 11. SEM micrographs of PANI coated PET spun yarn (a) ×200, (b) ×2000 and PET filament yarn (c) ×200, (d) ×2000.
yarns. Besides, the thickness of the PANI layer on the fiber is not uniform and there are some areas where the thickness is very low disabling contacts among PANI chains and making the conductivity inferior. R PANI-spun 3% and R PANI-spun 5% show higher resistance values than those of 6 and 10% solution coated ones (Table 1). Indeed, with the relatively low solution concentrations (1–5%), it is difficult to obtain good conductive yarns. On the other side R PANI-spun 10% has lower resistance values than R PANI-spun 6% as we can see in Fig. 8. PET filament yarns coated with 6% of PANI solution show similar electrical resistance with PANI coated spun yarns (Fig. 9). However, the 10% solution coated filament yarns show higher resistance than that coated with 6% solution (Fig. 10). This astonishing phenomenon can be explained by the fact that the gelation of PANI solution at high concentrations occurs very quickly. Therefore, the 10% PANI solution is less “liquid” than the 6% one. As the surface of filament yarn is smoother than the surface of the spun one, the thick coating is more difficult for the 10% solution and there are more non-homogeneous conductive areas. This phenomenon
should explain higher electrical resistances of PANI-filament 10%. Nevertheless, the taken-up speed of yarns has not been modified and it may be interesting to adapt this speed to PANI concentration in solution in order to obtain an optimal homogeneity of conductive areas. The geometrical structures of spun and filament yarns are quite different (Fig. 11). In the case of spun yarns, the structure is bulky (fibers are not geometrically organized). In the case of filament yarns, the structure is more uniform (fibers are often parallel). Free volumes are more important in spun yarns and PANI has more possibilities to fill the inner yarn structure than in the case of filament ones during the coating process. This phenomenon is supposed to improve contacts among PANI chains and therefore to improve the fiber conductivity. The weight for PANI coated spun yarns increases more than that of the PANI coated filament yarns at the same solution concentration. With the AFM observation results (Fig. 12), we can see more exactly the difference of PANI coating on the spun and filament yarns. As we explained above, spun yarns have more free volume on the fiber surface. This fiber formation allows the deep PANI coating on spun yarns even on the
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Fig. 12. AFM images of PANI coated PET (a) spun yarn and (b) filament yarn.
regions of fiber-to-fiber while thin coating was performed on the filament surface.
4. Conclusion This paper describes conductive fibers (yarns) prepared by two different methods, melt spinning and coating processes. The electrical conductivity of conductive materials/PP monofilament obtained by melt spinning process is not satisfactory due to the structure homogeneity problems and the aggregation of conductive materials. The melt mixing process should be improved by addition of several extended mixing steps in order to improve PANI homogeneity into the fibers. PANI coated PET yarns obtained by coating process have much better electrical properties than those of conductive fibers obtained by melt spinning method, preserving their original strength and flexibility. The electrical resistance of PANI coated spun yarns decreases as the concentration of PANI solution increases. In addition, PANI coated spun yarns show lower resistance than that of filament yarns at high concentration of solution. PANI molecules have more possibilities to penetrate into the spun yarns because of their inherent bulky surface formation and free volumes. Several parameters as the bath temperature, the taken-up speed of yarns, the duration of the treatment and the surface characteristic of the yarns influence the final result. Currently, we investigate the influence of all these parameters and we try to optimize the whole process.
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Electrical and morphological properties of PP and PET conductive polymer fibers Bohwon Kima,∗ , Vladan Koncara , Eric Devauxa , Claude Dufourb , Pierre Viallierc a
Laboratoire de G´enie et Mat´eriaux Textiles (GEMTEX), Ecole Nationale Sup´erieure des Arts et Industries Textiles (ENSAIT), 9 rue de l’Ermitage BP 30329, 59056 Roubaix cedex 01, France b Institut d’Electronique ´ de Micro´electronique et de Nanotechnologie (IEMN), Cit´e Scientifique, Avenue Poincar´e, BP 69, 59652 Villeneuve d’Ascq cedex, France c Ecole Nationale Sup´ erieure des Industries Textiles de Mulhouse (ENSITM), 11 rue Alfred Werner, 68093 Mulhouse, France Received 22 November 2003; received in revised form 23 March 2004; accepted 23 June 2004 Available online 25 August 2004
Abstract Conductive fibers were obtained using two experimental processes (melt spinning and coating process). In melt spinning process, polyaniline (PANI), polypyrrole (PPy) and graphite were used in order to obtain conductive polypropylene (PP) based fibers with specific electrical and mechanical properties. PANI was treated using dodecylbenzene sulfonic acid (DBSA) to improve the solubility and the dispersion of PANI in xylene. PANI coating on PET yarns were performed by absorption of yarns through PANI solution. The electrical resistance and morphological characteristics of conductive yarns were investigated. These yarns are supposed to be used to create smart clothing, corrosion protection or conductive fabrics for electromagnetic shielding applications. © 2004 Elsevier B.V. All rights reserved. Keywords: Conductive fiber; Polyaniline (PANI); DBSA; Coating process
1. Introduction Recently, the textile industry has made considerable advances in the field of high value added textiles, mainly in the sectors of high performance textiles and yarns. The use of new materials with specific properties and the development of new structures and integration processes make it possible to develop supports able to convey information while being mostly based on properties of electric conduction. These new achievements of the textile industry enable electronic devices to be directly integrated into the structure of textile, therefore modifying the functionality of the apparel. Besides the main functions of apparel, which are protection and passive communication, the clothes become an interface with specific new functions between the individual and his environment. ∗
Corresponding author. E-mail addresses: [email protected] (B. Kim), [email protected] (V. Koncar). 0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.06.023
The term “intelligent apparel” describes a class of apparel that has active functions in addition to traditional properties of clothing. These novel functions or properties are obtained by utilizing special textiles or electronic devices integrated into the textile structures. Everyone wears clothing and most people are concerned with the appearance of communication apparel. However, the needs will be different within any given group of people. The broad principle topics are: 1. 2. 3. 4. 5.
Professionals [1–3] (the need for “free hands function”), Health care [4] (monitoring, training result diagrams), Every-day life [5] (telephone, wellness), Sports [6,7] (training, performances measurement), Leisure [3] (aesthetic customization network games).
The demand for electrically conductive fibers and textiles structures used for industrial materials like sensors, electrostatic discharge, welding of plastics, electromagnetic interference shielding, dust and germ-free clothing [8], data transfer in clothing but also for military applications like camouflage
168
B. Kim et al. / Synthetic Metals 146 (2004) 167–174
Fig. 1. Polyaniniline (a) emeraldine base and (b) emeraldine salt forms.
and stealth technology is growing up. For all the applications previously mentioned the basic element is of course the textile fiber. Therefore, it is important to modify the fiber properties (electrical and other) in order to modify the general properties of the textile structure and to add previously mentioned new functions. Modification of fibers based on conductive polymers seems to be the interesting approach enabling these new functionalities. Conductive polymers, used to fill or coat fibers, show electrical properties due to their conjugated double bond chain structures, which derive both their conducting or neutral (non-conducting) forms. However, they are inherently insoluble and infusible due to their strong intermolecular interactions. High quality conducting blends with conventional polymers by melt mixing [9–13] or by solution casting [14–17] are still in a development stage. Among the conjugated conducting polymers, polyaniline (PANI) has attracted much interest worldwide because of its excellent environmental, thermal and chemical stability. Polyaniline exists in a variety of oxidation states ranging from leucoemeraldine (completely reduced) to emeraldine (partially oxidized) to pernignaniline (completely oxidized). Most broadly studied forms of polyaniline are the emeraldine base form and the emeraldine salt form (Fig. 1). With simple doping process by charge transfer chemistry or by acid–base (protonation) chemistry, polyaniline converts the semiconducting emeraldine base form (10−5 S/cm) to the metallic emeraldine salt form (102 S/cm) [18–20]. In this paper, a doped polyaniline has been mainly used in order to create conductive fibers. The originality of our approach is to create sufficiently conductive yarns able to transport information or to be used as textile sensors, but with all the mechanical properties as elongation, bending, shearing and twisting corresponding to a real textile yarn. These textile properties are very important because the conductive fibers should be transformed in textile structures by weaving, knitting or other manufacturing processes. Two different methods have been developed to synthesize conductive yarns with textile mechanical properties.
bon black, metal powders. Shacklette et al. [21] reported that a conductive form of polyaniline (VersiconTM, AlliedSignal, Inc.) is dispersible in polar thermoplastic matrix polymers such as polycaprolactone and poly(ethyleneterephtalate glycole). Ikkala et al. [10] reported on conducting polymer blends by blending thermoplastic bulk polymers such as polyolefins, polystyrene with Neste complex, polaniline salt complex developed by Neste Oy and UNIAX Corporation. In their report, 1–20 S/cm of conductivity has been achieved in the range of 1–30 wt.% of Neste complex. In addition, morphological study and reducing the sufficient amount of conductive filler (percolation threshold) into conductive polymer blends have been actively performed [11]. In this point, melt spinning process is our first approach for preparing conductive composites fibers. Polyaniline emeraldine salt form (PANI-ES), PPy and graphite were melt mixed with polypropylene (PP) or low-density polyethylene (LDPE) using a co-rotating twin-screw extruder (Fig. 2). Low PANI percolation concentrations (6–10 wt.%) were used for PANI particles dispersion in PP via melt blending. 1.2. Method 2 – coating process Our second approach for the synthesis of conductive fibers is based on a coating process. Although conducting polymers can be produced electrochemically in fibers or films forms, they are brittle to apply on large applications. Considering this difficulty, thin coating or polymerizing conductive poly-
1.1. Method 1 – melt spinning process One of the cost- and process-effective methods is blending common plastics with conductive fillers including car-
Fig. 2. Co-rotating twin-screw extruder, MiniLab (Haake mixer).
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Fig. 3. Overview of the coating process used in this study.
mers from solutions onto the surface of plastics or textiles should be a reasonable method to create conductive textiles structures. Since polypyrrole coated polyester textiles have been well developed by Milliken Research Corporation [22], many research groups are active in this field [23]. In addition, polyaniline coated or in situ polymerized on non-woven fabric, nylon 6, cotton, polyester fabric and nomex fabric are recently reported [23–26]. Water soluble conductive polymers are also in the phase of development by several research groups [27,28]. The soluble conductive form of PANI may be obtained by protonation with functionalised protonic acid, denoted as H+ (M− –R) [29]. H+ M− is protonic acid group and may be sulfonic acid, carboxylic acid, phosphonic acid, etc. The proton of the protonic acid reacts with imine groups of polyaniline and the M–R group serves as the counterion. R is an organic group that can be compatible with nonpolar or weakly polar organic solvents, e.g. N-methyl-2-pyrrolidione (NMP), Dimethylsulfoxide (DMSO), xylene and m-cresol. Efficient doping methods of PANI with dodecylbenzene sulfonic acid (DBSA) or camphor sulfonic acid (CSA) were reported [14–16,30]. The PANI chains tend to form extended chain conformation in presence of DBSA; electron transport between polymer chains should be enhanced. Moreover, long flexible alkyl chains of DBSA may enhance the crystallization rate of polyaniline. In the polyaniline/solvent system, the film-like agglomerates of polyaniline take place on the solution/solid interface when the solvent is removed. In our research work, this spontaneous molecular assembly has been used to coat conductive polymers on the material surface, and has been successfully applied to yarns and textiles. PANI coating was carried out during the impregnation of PET yarns in the PANI solutions (Fig. 3). Then, the electrical resistance and the morphology of the conductive yarns were investigated.
2. Experimental
The spinnable isotactic polypropylene (PP, Elf-Atochem) and the low-density polyethylene (LDPE) were used as matrix polymers. Dodecylbenzene sulfonic acid (DBSA, Fluka) was used as the dopant and surfactant to increase the conductivity and to obtain a homogeneous conducting solution of PANI in xylene. Fig. 4 shows the chemical structures of the materials used in this paper. For chemical coating process, PET spun yarn (20 Nm, Hyosung, Korea) and PET filament yarn (292 Dtex, Hyosung, Korea) were used. PET yarns were used after washing with the solution of 2 g/l of sodium carbonate and 1 g/l of Tinovetine at 70 ◦ C for 30 min. 2.2. Experimental process 2.2.1. Method 1 – melt spinning process Binary polymer blends consisting of PANI-ES, PPy or graphite in matrix polymer were melt mixed using a corotating twin-screw extruder (Mini Lab., Thermo Haake, Germany) at 50 rpm for 15 min. The blending temperature varied with the matrix polymers as follows. Blends of LDPE were melt mixed at 150 ◦ C, and PP at 200 ◦ C. All the materials were dried at 50 ◦ C for 24 h before use. The conductive materials concentrations range was from 1 to 40 wt.%. 2.2.2. Method 2 – coating process PANI-DBSA mixtures at a ratio PANI-ES:DBSA 1:2 (w/w) were dissolved in xylene to prepare 3, 5, 6 and 10 wt.% PANI/xylene solutions. These solutions were stirred vigorously at 120 ◦ C for 3 h. Then, they were treated in the ultrasonic bath at 80 ◦ C for 2 h. The coating of the yarn has been performed using an apparatus that can control the temperature and the taken-up speed. This process may contain several bathes filled with PANI solution. The temperature of these baths varies from 30 to 100 ◦ C. PET yarns were withdrawn through the baths in the presence of a dry airflow along the coating surfaces to
2.1. Materials Emeraldine salt form polyaniline (PANI-ES) and polypyrrole (PPy) were supplied by Sigma-Aldrich Chemical Company. Graphite spherical nanoparticles, from Carbon Nanotechnologies Inc. were also used as conductive fillers.
Fig. 4. Chemical structures of the materials.
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enhance solvent evaporation. The taken-up speed of yarn is adapted to the drying dynamics and for our experiments, it was set to 6 cm/min. PANI coated PET yarns obtained were then washed with distilled water and acetone for several times and dried at 25 ◦ C. PANI doped films have been made in our laboratory in order to evaluate their electrical properties. PANI films were also prepared by casting of PANI solutions onto the glass plates and by drying them at 50 ◦ C in an oven over 24 h to remove the solvent. After drying, dark green solid PANI films were obtained by peeling them off the glass substrates. 2.3. Morphological and electrical characterization The surface structure of the PP/PANI monofilament was observed simply using reflected light optical microscopy (Axioskop ZEISS, Germany). Detailed images of the PANI coated PET yarn surfaces were obtained using AFM (Nanoscope III Atomic Force Microscope from Digital Instruments) and SEM (JEOL 100 CX model ASID4-D Scanning Electron Microscope). The SEM samples were goldsputtered before observation. The electrical resistance tests were performed using the “two-probe test technique” with Keithley 617 electrometer used as an ammeter at 25 ◦ C, 55.56 HR%. Keithley 617 electrometer generates the variable voltage dc powers supply connected to the conductive fibers. In the same time, it measures the electric current with the pA maximal precision. The electrometer is connected to a PC computer, which controls the voltage of dc power supply and records the measured current. Surface electrical resistance of PANI film was also measured with same method to compare with that of PANI coated conductive yarns.
3. Results The Fig. 5 shows the electrical conductivity (σ) of conductive materials in PP or LDPE obtained by melt spin-
Fig. 5. Electrical conductivity of conductive materials/matrix polymer blends prepared using twin-screw extruder: (䊉) PANI-ES in PP, () PANIES in LDPE and graphite in PP.
Fig. 6. Morphology of 5 wt.% PANI-ES (black particles) in PP monofilament (grey region) observed using reflected light optical microscopy.
ning process. The average diameter of the conductive PP or LDPE monofilaments obtained by melt spinning process was 0.5 mm. It is obvious that the conductivity of these monofilaments is low. This can be explained by the non-homogeneous morphological structure of PP monofilament with 5% of PANI-ES presented in Fig. 6. Two phases exist; dark particles (PANI-ES) and surrounded grey PP region. The particles of PANI-ES were not completely dispersed and formed aggregates in PP. As Faez [31] and Hosier et al. [11] previously reported, a degree of connectivity between the conductive additives must exist in order to facilitate the electrical conduction. In our case the conductive particles appear to exist as distinct in the blend and the connections among them are missing. Moreover, insulating PP among particles and particle aggregates should explain the low conductivity. In the case of PP filled with PPy, the conductivity increases as the PPy content increases (Fig. 7). At high content, PPy phases should be more continuous thus these results are ex-
Fig. 7. Electrical conductivity of polypyrrole/PP monofilaments at different content.
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Table 1 Electrical resistance () of PANI coated PET spun yarns for different lengths (S.D.: standard deviation). 1 cm R PANI-spun 3% S.D. R PANI-spun 5% S.D. R PANI-spun 6% S.D. R PANI-spun10% S.D.
3 cm
1.70 × 7.29 × 104 1.35 × 104 3.59 × 103 1.36 × 102 3.08 × 101 6.64 × 101 1.21 × 101 105
5 cm
2.38 × 9.65 × 104 3.79 × 104 1.10 × 104 4.52 × 102 8.70 × 101 3.54 × 102 1.69 × 102 105
10 cm
5.29 × 1.82 × 105 6.96 × 104 1.19 × 104 6.31 × 102 5.22 × 101 3.64 × 102 7.7 × 101 105
15 cm
1.53 × 6.40 × 105 1.96 × 105 6.46 × 104 1.37 × 103 1.16 × 102 1.02 × 103 5.79 × 102 106
20 cm
2.54 × 4.22 × 105 3.47 × 105 6.35 × 104 2.19 × 103 1.46 × 102 1.71 × 103 3.05 × 102
50 cm
3.92 × 3.12 × 105 4.65 × 105 1.42 × 105 3.03 × 103 3.02 × 102 1.83 × 103 1.99 × 102
106
106
9.95 × 106 1.74 × 106 7.23 × 105 3.91 × 104 7.41 × 103 2.07 × 102 4.83 × 103 3.10 × 102
Table 2 Electrical resistance () of PANI coated PET filament yarns for different lengths.
R PANI-filament S.D. R PANI-filament S.D. R PANI-filament S.D.
5% 6% 10%
1 cm
3 cm
5 cm
10 cm
20 cm
2.76 × 104 6.03 × 103 7.83 × 102 3.89 × 102 9.86 × 102 2.31 × 103
2.05 × 105 4.51 × 104 1.02 × 103 3.29 × 102 1.47 × 103 1.77 × 102
3.27 × 105 7.36 × 104 1.28 × 103 2.85 × 102 2.90 × 103 3.06 × 102
7.81 × 105 1.42 × 105 1.51 × 103 2.22 × 102 7.70 × 103 1.38 × 103
4.36 × 106 4.28 × 105 2.88 × 103 1.89 × 102 2.04 × 104 7.89 × 103
pectable. However, with 40 wt.% of PPy content in PP, the conductivity is only 2.88 × 10−7 S/cm. The dispersion of PPy in PP is not satisfactory, connections are missing and this result is similar to the previous one. All the electrical resistances of PANI-PET yarns obtained by coating from PANI solutions are presented in Tables 1 and 2. The electrical resistance was calculated from intensity (I) versus applied voltage (V) measurements when V varies from −5 to 5 V. All the curves of I versus V characteristics are linear. The experiments have been repeated 30 times for different fiber lengths from 1 to 50 cm and average values have been computed. All the coated yarns were indicated using an abbreviation. For example, R PANI-spun 3% means the electrical resistance of PET spun yarn coated with 3% of PANI solution. The electrical resistance (R) of PANI film measured by same method is 3.46 × 102 /cm2 . However, as we can see in Tables 1 and 2, the resistances of PANI coated yarns raging from 1.02 × 103 (PANI-spun 10%) to 1.53 × 106 (PANI-
spun 3%) for a 10 cm long fiber. In the case of yarns, PANI is not coated uniformly (see Fig. 11). We think that contacts among PANI chains are less important on yarns than in the film on a glass substrate because of a circular form of the
Fig. 8. Electrical resistance of PANI coated PET spun yarns at 6 and 10% of PANI solutions.
Fig. 10. Electrical resistance of PANI coated PET filament yarns at 6 and 10% of PANI solutions.
Fig. 9. Electrical resistance of PANI coated PET spun yarn and filament yarn at 6% of PANI solution.
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Fig. 11. SEM micrographs of PANI coated PET spun yarn (a) ×200, (b) ×2000 and PET filament yarn (c) ×200, (d) ×2000.
yarns. Besides, the thickness of the PANI layer on the fiber is not uniform and there are some areas where the thickness is very low disabling contacts among PANI chains and making the conductivity inferior. R PANI-spun 3% and R PANI-spun 5% show higher resistance values than those of 6 and 10% solution coated ones (Table 1). Indeed, with the relatively low solution concentrations (1–5%), it is difficult to obtain good conductive yarns. On the other side R PANI-spun 10% has lower resistance values than R PANI-spun 6% as we can see in Fig. 8. PET filament yarns coated with 6% of PANI solution show similar electrical resistance with PANI coated spun yarns (Fig. 9). However, the 10% solution coated filament yarns show higher resistance than that coated with 6% solution (Fig. 10). This astonishing phenomenon can be explained by the fact that the gelation of PANI solution at high concentrations occurs very quickly. Therefore, the 10% PANI solution is less “liquid” than the 6% one. As the surface of filament yarn is smoother than the surface of the spun one, the thick coating is more difficult for the 10% solution and there are more non-homogeneous conductive areas. This phenomenon
should explain higher electrical resistances of PANI-filament 10%. Nevertheless, the taken-up speed of yarns has not been modified and it may be interesting to adapt this speed to PANI concentration in solution in order to obtain an optimal homogeneity of conductive areas. The geometrical structures of spun and filament yarns are quite different (Fig. 11). In the case of spun yarns, the structure is bulky (fibers are not geometrically organized). In the case of filament yarns, the structure is more uniform (fibers are often parallel). Free volumes are more important in spun yarns and PANI has more possibilities to fill the inner yarn structure than in the case of filament ones during the coating process. This phenomenon is supposed to improve contacts among PANI chains and therefore to improve the fiber conductivity. The weight for PANI coated spun yarns increases more than that of the PANI coated filament yarns at the same solution concentration. With the AFM observation results (Fig. 12), we can see more exactly the difference of PANI coating on the spun and filament yarns. As we explained above, spun yarns have more free volume on the fiber surface. This fiber formation allows the deep PANI coating on spun yarns even on the
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Fig. 12. AFM images of PANI coated PET (a) spun yarn and (b) filament yarn.
regions of fiber-to-fiber while thin coating was performed on the filament surface.
4. Conclusion This paper describes conductive fibers (yarns) prepared by two different methods, melt spinning and coating processes. The electrical conductivity of conductive materials/PP monofilament obtained by melt spinning process is not satisfactory due to the structure homogeneity problems and the aggregation of conductive materials. The melt mixing process should be improved by addition of several extended mixing steps in order to improve PANI homogeneity into the fibers. PANI coated PET yarns obtained by coating process have much better electrical properties than those of conductive fibers obtained by melt spinning method, preserving their original strength and flexibility. The electrical resistance of PANI coated spun yarns decreases as the concentration of PANI solution increases. In addition, PANI coated spun yarns show lower resistance than that of filament yarns at high concentration of solution. PANI molecules have more possibilities to penetrate into the spun yarns because of their inherent bulky surface formation and free volumes. Several parameters as the bath temperature, the taken-up speed of yarns, the duration of the treatment and the surface characteristic of the yarns influence the final result. Currently, we investigate the influence of all these parameters and we try to optimize the whole process.
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