Synthetic Metals

(1)

Contents lists available atScienceDirect

Synthetic Metals

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / s y n m e t

Electrically high-conductive textiles

Dierk Knittel

^∗

, Eckhard Schollmeyer

Deutsches Textilforschungszentrum Nord-West e.V., D-47798 Krefeld¹, Germany

a r t i c l e i n f o

Article history:

Received 17 January 2008 Received in revised form 27 November 2008 Accepted 25 March 2009 Available online 29 April 2009

Keywords:

Conductive polymers

Poly(3,4-ethylenedioxythiophene) Textile coating

Heating element Electromagnetic shielding

a b s t r a c t

The development of textiles with high electrical conductivity, which may be further processed to flexible heating elements is described. Conductivity was obtained by establishing thin layers by impregnation of the textile with thiophene derivative monomer followed by oxidative polymerization. The characteristic flexibility of the textiles could be maintained. Especially the use of multifilament synthetic samples was found to be advantageous. As mechanical properties of treated materials are concerned one finds almost no change in tensile strength (<1%) with PETP and polyamide. With cotton a decrease in strength of about 20% (best value) was observed. The question of ageing will be studied more intensively in future.

Layers obtained from poly(ethylene dioxythiophene) additionally offered protection against aggressive chemicals like concentrated sodium hydroxide.

1. Introduction

The term ‘conductive textiles’ is used for a broad range of prod- ucts with widely differing specific (surface) conductivity. Beginning with antistatic finishes with rather low conductivity or the mod- ification of fibres by means of the incorporation of conductive particles in the spinning process, textiles may be modified with conductive metal coatings or the interweaving or stitching of metallic fibres, the latter being used, e.g., for signal transport in so-called smart textiles[1–5].

A new approach to highly conductive textile materials is the use of intrinsically conductive organic polymers (cf.[6,7]). One common approach route is to apply dispersions or powder of fully prepared conductive polymers as coatings. Those approaches usually result in rather low conducting materials. An interesting alternative is to create the conductive polymers by polymerization of monomers on the textile.

Methods for in situ polymerization (by oxidation) are well known for polypyrrol (PPy, from pyrrole) and polyaniline (PANI, from aniline)[8]. Basically, the oxidative polymerization on a fabric may follow three procedures (cf. also[9–13]):

(a) application of the oxidant to the textile followed by addition of the monomer,

∗ Corresponding author. Tel.: +49 2151 843 0.

E-mail address:knittel@dtnw.de(D. Knittel).

1info@dtnw.de.

(b) application of the monomer followed by oxidizing agent, and (c) application of a polymerizable mixture of monomer and oxi-

dant.

PPy as well as PANI are common within the group of conductive organic polymers, but handling of precursors is cumbersome or hazardous. Unwanted side reactions are known to occur during polymerization, and little is known about long-term stability of end product.

Recently, polymers derived from alkoxythiophenes like 3,4- ethylenedioxythiophene (EDOT) yielding poly(3,4-ethylene dioxythiophene) (PEDOT) ﬁnd increasing interest. The chemical formula of PEDOT is sketched inFig. 1. The anions in the conducting PEDOT-polymer required for charge compensation results from the speciﬁc conditions of synthesis (i.e. toluene sulfonate, polystyrene sulfonate or others). The monomers are easy to handle, the polymer is highly conductive and very stable [14–16]. Especially recipes from company Bayer (Baytron^® etc., now from H.C. Starck) prove valuable. Mostly, PEDOT is used for the fabrication of displays for electronics. PEDOT-applications for textiles are rather unknown, the admixing of fully polymerized PEDOT-powder (i.e. Baytron^®P) as a pigment to PVC coatings has been reported by Hardtke and Fuchs[17]. Very low conductivities were achieved, however.

The research group of Kim [18,19]has reported the preparation of conductive textiles by polymerization of EDOT serving as a shielding of electromagnetic noise (EMI). An efﬁcient electrical conductivity of the resulting PEDOT composites was obtained mainly by co-application of polyvinyl-pyrrolidone as a binder. In the case of polyamide (PA) fabrics severe ﬁbre damage by acid-corrosion 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.synthmet.2009.03.021

(2)

Fig. 1. Chemical structure of poly-(3,4-ethylendioxythiophene) (PEDOT).

occurred; however, no further effects on textile properties were mentioned. Similar studies were made by Hong et al.[20]. But they did not use acid scavengers during oxidative polymerization (so severe ﬁbre damage on polyamide was observed) and did only use substoichiometric values for the oxidant. No gravimetric data on add-on achieved nor colour values or voltages applied are given therein.

The objective of this paper was to establish electrical conductivity on textiles made of poly(ethylene terephthalate) (PETP) and PA, and also on paper by oxidative polymerization of EDOT with stable and ﬂexible heating or EMI-shielding elements in mind. Besides the electrical properties, the effect on relevant textile parameters was studied accordingly.

2. Experiments and methods

Fabrics and chemicals used were as follows:

PETP-standard fabric, plain weave, 131 g/m², warp 25.4 threads/cm, weft 20.9 threads/cm, thickness 0.30 mm (Testex, D-53902 Bad Münstereifel, Germany).

PETP-Multiﬁlament fabric, plain weave, 281.6 g/m², warp 10.2 threads/cm, weft 14.5 threads/cm, thickness 0.59 mm (Verseidag, D-47803 Krefeld, Germany).

PA6.6-Multiﬁlament fabric, plain weave, 202 g/m² (Verseidag, Krefeld, Germany).

Polybutylene terephthalate non-woven (PBT), weight 51.8 g/m², thickness 0.39 mm (Freudenberg, D-69465 Weinheim, Germany).

Glass ﬁbre fabric, 200 g/m², thickness 0.2 mm, plain weave (Klevers, D-41199 Mönchengladbach).

PE-coatet paper (Polytrap^®296 PE, Schleicher & Schüll, D-37582 Dassel, Germany).

Baytron^®M (3,4-ethylenedioxythiophene) (H.C. Starck, D-51368 Leverkusen, Germany).

Baytron^®CB (Fe-III-toluene sulfonate in n-butanol) (H.C. Starck;

Aldrich resp.).

Imidazole (Sigma–Aldrich, D-82041 Deisenhof, Germany).

Ammonium peroxidisulfate, analytical grade (Fluka, CH-5033, Buchs, Switzerland).

Marlipal^® O13/80 (Sasol, D-45764 Marl, Germany) as non-ionic surfactant.

Silver containing solder E-Solder^®3021 (Epoxy Produkte, D-91720 Ansberg, Germany).

As top coats, silicones (Silicophene^®, Tego, D-45127 Essen, Ger- many) and polyvinylacetate (Sigma–Aldrich) were used.

2.1. Application of the monomers

The application of the monomers is followed by a standard recipe for spin-coating on ﬂat substrates by H.C. Starck. 2.5 g Baytron^®CB and 62.5 mg imidazole are mixed and cooled to about 5^◦C. After addition of 95␮l Baytron^® M the mixture is used for

2.2. Post-treatment

Treated fabrics were washed several times with oxalic acid solution for removal of ferric and ferrous ions, washed neutral with bi-distilled water and washed several times at 40–60^◦C in a Linitest laboratory washing apparatus (Nr. 7421, Heraeus, Kendro, D-30165 Hannover, Germany) using a solution of 1 g/l non-ionic surfactant.

After thorough rinsing with water the samples were kept under standard climate conditions.

2.3. Electrical connection, contacting equipment

The characterisation of the electrical properties of the samples was performed by measurement of current–voltage curves.

It should be noted here that standardised methods for conductivity measurements such as the test according to German standard DIN 54345/1, which employs ring electrodes, are not applicable to highly conductive samples. Therefore, measurements of the electrical current between contacting bars positioned at a distance of 30 mm from each other on the textile were performed as depicted inFig. 2. The bars had a width of 40 mm, and were pressed with a weight of 10 kg onto the samples for optimum contact. Lines of solder were applied to the samples, where the contact bars were positioned. The pressure on the textile exerted at the contacting points had a signiﬁcant role on contact resistance (depending on ﬁbre material and fabric construction). The stated 10 kg proved to

Fig. 2. Equipment for the measurement of current–voltage-curves. The fabric sam- ples are contacted by means of two contact bars of 4 cm length and distance of 3 cm, pressed onto the fabric with 10 kg. Two silver-solder contacting lines can be seen.

(3)

Fig. 3. SEM-micrograph of a PEDOT-treated PA fabric (3.6 wt.% add-on).

be a good compromise, higher loads having some, albeit low, dam- aging effect on the sample surface. Constant voltage was applied to the contacting bars with either a DC-voltage supply (TK3, Gossen) or a 30 V AC-voltage supply (Leyboldt Didaktik, Hürth). The resulting current was measured with a conventional multimeter with a range up to 400 mA (M-3270D, Metex).

Washings were performed in a Linitest (Heraeus) laboratory machine. Abrasion testing was done using a Martindale apparatus.

3. Results and discussion

The ﬁrst part of the study focused on the deposition of PEDOT layers on fabrics made of PETP. An exemplary treatment would be as follows.

1.6 g PETP-standard fabric is immersed in a mixture of 520 mg Baytron^®M, 10 g Baytron^®CB and 200 mg imidazole, squeezed to a pick up of 120–140 wt.% and dried at room temperature in air for several hours. Finally the sample is heated to 130^◦C for 30 min. One obtains after post-treatment a product having about 3% add-on.

In this example, current–voltage measurements indicated a surface resistance in the range of 100–110 k in the geometry of the described setup, i.e. with an electrode bar distance of 30 mm when using no contacting silver laquer for contacting.

The resulting PEDOT layers were almost black but did not sig- nificantly affect textile mechanical properties such as drapability, flexibility and strength. An exemplary SEM micrograph of the resulting PEDOT coating on PETP is shown inFig. 3. One observes a rather smooth covering with some interconnections between the filaments.

Taking about 1 m²(BET)-surface of 1 g PETP-multiﬁlament fabric and a 3% add-on one would arrive at a layer thickness of about 20 nm, which is not realistic (no semi-transparent coating as reported for plastic foils with some sub-␮m-layers of Baytron^® [16]). One would need an ‘accessible’ surface for the treatment. Percolation contact between adjoining PEDOTs (perhaps pigmentoidal-like islands) seems to be more essential than discussions on ﬁlm formation and thickness.

In general, PEDOT-layers of less than 2 wt.% led to surface resistances (according to measurements following DIN54345/1) in the range of k and M, whereas coatings with about 3–4 wt.% gave highly conducting textiles. The necessity to apply a certain amount of conducting polymer is assumed to be related to percolation requirements especially where gaps between ﬁlaments have to be bridged. Conductivity is aided by the take-up of PEDOT in the cap- illaries of, e.g., a multiﬁlament yarn or fabric. Accordingly, fabrics

Fig. 4. Comb-like conductive solder pattern on PEDOT-coated PETP fabric for the construction of heating elements.

Table 1

Current–voltage-characteristic of PEDOT-coated PETP- fabric contacted with a comb-like structure of conducting solder.

Control voltage (V) Current (mA)

2 63

4 126

6 192

8 196

12 196

made of monoﬁlaments such as sieving-fabrics without a capillary system of any signiﬁcance do not give usable PEDOT-layers.

To measure actual current–voltage curves, contacting structures of conductive solder were applied to samples treated as described before. The comb-like geometry of the structures is shown inFig. 4 (distance again between ﬁngers 3 cm). Currents measured at DC voltages between 2 and 12 V are presented inTable 1. Similar contacting structures can be elaborated for the construction of heating elements ﬁnding a compromise between resistance and Ohmic heating capability.

In order to study the abrasion resistance of the PEDOT layers, a sample was prepared and subjected to abrasion using the Martindale test. The sample was prepared as follows: 3.6 g of PETP- standard fabric was treated with 6 g Baytron^®CB, 200␮l Baytron^® M and 100 mg imidazole and squeezed to a pick up of 50–60 wt.%

and dried for 1 h at 105^◦C. After the post-treatment the conductivity of the sample was analysed using the described set-up with an electrode distance of 30 mm. The measured resistances were of the order of 1 k without conductive solder strips and between 40 and 50 with conductive solder strips, respectively. After the abrasion test (Martindale, 20,000 cycles) the fabric still shows a good connectivity between the ﬁlaments, but the surface resistance increased slightly. Photographs of the PEDOT-coated fabric before and after the abrasion test are given inFig. 5. It should be noted that the PEDOT-coated material can be protected by further coat-

Fig. 5. PEDOT-treated PETP fabric (2.6% add-on) as received (A) and after 20,000 abrasion cycles (B).

(4)

Fig. 6. Current–voltage response of a PEDOT-treated PETP fabric (∼10 wt.% add-on) in warp and weft direction.

ings between the contacting lines. Accordingly protection layers could be made of, e.g., silicones or polyvinyl acetate without inﬂu- encing electrical properties, provided the coating procedure does not signiﬁcantly exceed 160^◦C. (Fig. 6)

An interesting alternative is to perform the PEDOT formation on the textile in two steps. This allows the use of more simple water-soluble oxidants like ammonium peroxidisulfate instead of Fe-III-toluene sulfonate, which is only stable in organic solvents.

The actual process would be as follows: 1.5 g PETP-standard fabric is ﬁrst treated with 120␮l Baytron^®M (in 5 ml n-propanol). After drying, the sample is covered with a solution of 1 g ammonium peroxidisulfate (in 6 ml water, surfactant added) and air dried again.

This treatment is followed by 3 impregnations with each 120␮l Baytron^®M-solution. After >24 h the cleaning procedure is applied.

A sample prepared accordingly was analysed with regard to its electrical properties. Using the set-up shown inFig. 2, i.e. with an electrode distance of 30 mm, and a constant DC voltage of 12 V, currents of up to 260–270 mA were measured in warp direction and about 150–160 mA in weft direction. Multilayers of treated fabrics give high current ﬂow over proportional to layer number illustrat- ing again the problems with contact formation.

Further examples refer to conductive coatings on fabrics made of PA 6.6 and glass ﬁbres, a non-woven made of PBT, and PE-coated paper.

Samples of the polyamide multiﬁlament fabric (3 g each) were treated with mixtures of 6 g Baytron^® CB, 120 mg imidazole and 400␮l Baytron^®M at room temperature and squeezed to 70 wt.

% pick-up. Reaction was done at room temperature. Following the post-treatment the fabric was almost black but still ﬂexible and showed a surface resistance of 4.6–10 k (measured according to the method described in DIN 54345/1). The untreated fabric had a resistance in excess of 200 T.

For ensuing measurements of AC- and DC-current–voltage curves, PA6.6 samples were treated with a mixture of 150␮l Baytron^®M, 2.3 g Baytron^®CB and 35 mg imidazole and dried in air.

One obtained a deep black sample of 3.6 wt.% add-on which showed no damages caused by acid attack. Results on fabric obtained for AC- and DC-conductivity are shown inFig. 7. The electrical properties of the samples were measured using the standard setup, i.e. with an electrode distance of 30 mm, but without conductive solder strips.

High conductivities could be readily achieved on the studied non-woven made of PBT. 0.6–0.7 g of the non-woven was placed in a mixture of 200 mg imidazole as catalyst, 600␮l Baytron^®M, and 12 ml Baytron^® CB40. Treated at room temperature, the samples had surface resistances as low as 70–80 (measured at 12 V). Again, no contacting solder was employed.

Fig. 7. AC- and DC- current–voltage response of a PEDOT-treated PA6.6 fabric (∼3.6 wt.-% add-on) measured in warp direction.

An oxidative polymerization of EDOT on glass fabrics was performed as follows: 3 g of the fabric was placed in a treating mixture of 9 g Baytron^® CB40, containing 150 mg imidazole and 600␮l Baytron^®M and squeezed to a pick up of 60–70 wt.%. After con- densation at room temperature for 24 h, a layer of 0.8 wt.% was obtained. According to DIN 54345/1 the surface resistance of the sample was 8–9 k, which can be regarded as an extremely low value regarding the low add-on. Abrasion resistance was found to be rather poor, however.

In addition to the discussed textile applications, experiments were done on one-side PE-coated paper. After a treatment of the paper side, i.e. not the PE, following a standard procedure like with polyamide, a current of 145 mA was measured at 12 V. The PEDOT coating had good adhesion to the paper.

4. Resume

Electrically conductive layers of poly(ethylenedioxythiophen)es (PEDOT) were established on textiles made of cotton and synthetic ﬁbres by an on-substrate polymerization.

As far as electrical properties are regarded there still exist problems in reproducibility during preparation (best results given in this paper) and some problems with ageing (a two years old PEDOT-on- PETP sample stored in the lab without protection dropped in current to about half of its original value). Ageing ﬁeld will be studied more intensively in future. As mechanical properties of treated materials are concerned one ﬁnds almost no change in tensile strength (<1%) with PETP and polyamide. With cotton a decrease in strength of about 20% (best value) was observed.

The presented results show the promising potential of this tech- nique, which may be used to create flexible heating elements, shielding materials for EMI-protection, or signal transducers, and even enhance further galvanic metal deposition on the modified fabrics. In addition, the results indicate a certain protection against some chemical attack by the PEDOT-layer. In summary, it may be said that the in situ polymerization of PEDOT opens an innova- tive perspective for textile finishing. More systematic research is needed to optimise or adapt the polymerizing process as well as the textile construction with regard to the homogeneity of the layers.

Acknowledgements

We wish to thank the Ministerium für Innovation, Wissenschaft, Forschung und Technologie des Landes Nordrhein-Westfalen (Dept.

of Innovation, Science, Research and Technology of the state of Nordrhein-Westfalen) for ﬁnancial support. This support was granted within the project ‘DTNW/Support for attainment of further funds’.

(5)

The authors express their gratitude for the supply of research chemicals and for the valuable discussions of Dr. Elschner, Dr. Jonas and Dr. Kirchmeyer from the company H.C. Starck, Leverkusen, and to colleague Dr. Bahners for linguistic help.

References

[1] M. Lewin, J. Preston, High Technology Fibers, Vol. III, Part C; Handbook of Fiber Science and Technology Series, Marcel Dekker, New York, 1993.

[2] M. Okoniewski, Intern. Techtextil Sympos. 2 (18) (1994) 1–9.

[3] F. Marchini, Chemiefasern/Textilindustrie 40 (92) (1990) T164–T170.

[4] A. Neudeck, U. Möhring, H. Müller, S. Gimpel, W. Scheibner, Proceedings of the 4th International Conference on Textile Coating and Laminating, ENSITM, Mulhouse, 19–20 October, 2004 (Frankreich, Lecture 14).

[5] T. Stegmaier, G. Schmeer-Lioe, H.-P. Vogel, H. Planck, Techn. Text. 49 (2006) 57–60.

[6] P. Chandrasekhar, Conductive Polymers, Fundamentals and Applications, A Practical Approach, Kluwer Acad. Press, Boston, USA, 1999.

[7] J. Gilbert, R.G. Steijskal, Pure Appl. Chem. 74 (2002) 857–867.

[8] G.G. Wallace, G.M. Spinks, L.A.P. Kane-Maguire, P.R. Teasdale, Conductive Elec- troactive Polymers, Intelligent Materials Systems, 2nd ed., CRC Press, Boca Raton, USA, 2003.

[9] Show-An Chen, Y.Ch. Tsai, Angew. Makromol. Chem. 169 (1989) 153–157.

[10] R. Jolly, J.C. Marechal, F.D. Menneteau, C. Petrescu, J.C. Thieblemont, Techtextil Sympos. 6 (1994) 1–8 (Band 4.4, Lecture No. 445).

[11] P. Xue, X.M. Tao, K.W.Y. Kwok, M.Y. Leung, Text. Res. J. 74 (2004) 929–936.

[12] R.V. Gregory, W.C. Kimbrell, H.H. Kuhn, Synth. Met. 28 (1989) C823–C835.

[13] R.V. Gregory, W.C. Kimbrell, H.H. Kuhn, J. Coated Fabr. 20 (1991) 167–175.

[14] G. Schopf, G. Koßmehl, Adv. Polym. Sci. (Springer, Berlin) 129 (1997).

[15] L.B. Groenendaal, J. Jonas, D. Freitag, H. Pielartznik, J.R. Reynolds, Adv. Mater. 12 (2000) 481–494.

[16] S. Kirchmeyer, K. Reuter, J. Mater. Chem. 15 (2005) 2077–2088.

[17] G. Hardtke, H. Fuchs, Techn. Text. 43 (2000) 221–223.

[18] H.K. Kim, M.S. Kim, S.Y. Chun, Y.H. Park, B.S. Jeon, J.Y. Lee, Y.K. Hong, J. Joo, S.H.

Kim, Mol. Cryst. Liquid Cryst. 405 (2003) 161–169.

[19] H.K. Kim, M.S. Kim, K. Song, Y.H. Park, S.H. Kim, J. Joo, J.Y. Lee, Synth. Met.

105–106 (2003) 135–136.

[20] K.H. Hong, K.W. Oh, T.J. Kang, J. Appl. Polym. Sci. 97 (2005) 1326–1332.