- Open Access
Textile piezoelectric sensors – melt spun bi-component poly(vinylidene fluoride) fibres with conductive cores and poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) coating as the outer electrode
© Åkerfeldt et al.; licensee springer 2014
- Received: 8 April 2014
- Accepted: 20 August 2014
- Published: 25 September 2014
The work presented here addresses the outer electroding of a fully textile piezoelectric strain sensor, consisting of bi-component fibre yarns of β-crystalline poly(vinylidene fluoride) (PVDF) sheath and conductive high density polyethylene (HDPE)/carbon black (CB) core as insertions in a woven textile, with conductive poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) coatings developed for textile applications. Two coatings, one with a polyurethane binder and one without, were compared for the application and evaluated as electrode material in piezoelectric testing, as well as tested for surface resistivity, tear strength, abrasion resistance and shear flexing. Both coatings served their function as the outer electrodes in the system and no difference in this regard was detected between them. Omission of the binder resulted in a surface resistivity one order of magnitude less, of 12.3 Ω/square, but the surface resistivity of these samples increased more upon abrasion than the samples coated with binder. The tear strength of the textile coated with binder decreased with one third compared to the uncoated substrate, whereas the tear strength of the coated textile without binder increased with the same amount. Surface resistivity measurements and scanning electron microscopy (SEM) images of the samples subjected to shear flexing showed that the coatings without the binder did not withstand this treatment, and that the samples with the binder managed this to a greater extent. In summary, both of the PEDOT:PSS coatings could be used as outer electrodes of the piezoelectric fibres, but inclusion of binder was found necessary for the durability of the coating.
- Piezoelectric sensor
- Textile coating
One of the most critical issues to address for the Smart Textiles concept is the integration of the smart components into textile structures (Kirstein 2013). Electroactive components generally lead to perceived bulkiness and loss of flexibility in the textiles. Furthermore, the lack of refined integration methods has been a serious pitfall for the possibilities for industrialization and the following commercialization of products deriving from the area. With more sophisticated methods and materials emerging, there is renewed hope of finding solutions that could offer electronics incorporated in the textile materials by industrially feasible methods. Sensing and actuating are vital to many smart textiles applications and have therefore been the focus of much research, but still require more optimal routes to textile integration (Schwarz et al. 2010). To achieve an active sensor, an interesting alternative for textile applications is a polymer that exhibits piezoelectric properties, i.e. generates an electric potential from deformation, such as poly(vinylidene fluoride) (PVDF).
PVDF is polymorphic and has four crystalline phases: α, β, γ and δ. The β-phase crystal structure is the most polar, which is required for piezoelectric properties in a polymeric material (Fukada and Takashita 1969; Kawai 1969). When PVDF solidifies from a melt or a solution, the normal scenario is that it crystallizes to form the non-polar α-phase. The α-phase can be transformed to β-phase crystallinity by mechanical deformation, such as drawing of films (S. H. Lee and Cho 2010). It was recently found that the same effect can be achieved with sufficient cold drawing, i.e. drawing in the temperature interval between Tg and Tm, during the melt spinning process of PVDF textile fibres (Lund and Hagström 2010; Steinmann et al. 2011).
Under certain conditions, β-crystalline PVDF fibres would make it possible for each fibre to act as a piezoelectric strain sensor. To obtain a voltage from stretching, the PVDF needs to be poled, meaning that the dipolar momentums of the PVDF-molecules are aligned, which is achieved by applying a high voltage through the material. In order to do this, as well as register (harvest) the voltage output, electrodes need to be attached. For PVDF films a sandwich-structure, with conductive phases on both sides, is generally used. PVDF-fibres can also be applied in a similar sandwich-structure, but this does not make use of the full potential of the fibre format. If the conductive phases (electrodes) were instead integrated parts of the fibre in the longitudinal direction, this would offer good opportunities both for the output of piezoelectric signals and the textile flexibility (Egusa et al. 2010; Pini et al. 2007).
Conductive layers can, theoretically, be added to the fibre structure by multi-component fibre spinning. Since the piezoelectric material needs electrodes on both sides, a tri-component system, with one outer and one inner conductive phase, would be optimal. In reality, this has proven difficult to obtain, partly because it requires rare equipment, and partly because the spinning process becomes increasingly complicated to optimize with each added layer. Lund et al. (2012) produced bi-component fibres with β-phase PVDF as sheath and a conductive polymer composite (CPC) consisting of dispersed carbon black (CB) in polyethylene (PE) as core material. The fibres were inserted in a heat-pressed Co-PE/CB matrix that functioned as outer electrode. This outer electrode was mainly chosen because of its simplicity and to show that the system could be used for sensor applications, but was not refined enough to distinguish it from previously mentioned sandwich-structure, in terms of flexibility.
Egusa et al. (2010) proposed an alternate route to produce multi-component piezoelectric fibres using poly(vinylidene-flouride-triflouroethylene) copolymer (P(VDF-TrFE)) that spontaneously forms the β-phase upon solidification from the melt. They made preforms of P(VDF-TrFE) and CPC/indium electrodes that were thermally drawn into fibres of up to tens of metres of length. Although this method offers a possible route to small-scale production of piezoelectric fibres, it would not be preferred if quantities of several kilograms were demanded. P(VDF-TrFE) is also a much more expensive material than common PVDF.
The application of the outer electrode to woven substrates of the fibres with textile coating methods would allow for industrial-scale production, if a suitable coating system was found. Common routes to conductive textile surfaces are: metallization by plating (by for example Jiang et al. 2006) or sputtering (Depla et al. 2011); in situ polymerization of intrinsically conductive polymers (ICP) (Gregory et al. 1989; Knittel and Schollmeyer 2009; Oh et al. 1999); or CPC coating (Cristian et al. 2011; Zhang et al. 2012). Metallization offers the advantage of high conductivity, but metallic surfaces are poorly adapted to the demands of a flexible sensor as cracks are easily formed by mechanical forces (Jiang and Guo 2009). In situ polymerization of ICP is difficult to perform on PVDF because of its low surface energy, especially since a high amount of ICP is necessary to obtain the required conductivity, the method has however been studied for piezoelectric ceramic fibres (Pini et al. 2007). CPC coating would be a plausible alternative, but requires optimization to maintain the drapability of the textile.
Similarly to a CPC coating, the potential of using poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate), PEDOT:PSS, as the conductive material in a textile coating formulation was investigated (Åkerfeldt et al. 2013a). The formulation consisted of a water-based polyurethane (PU) coating binder, the PEDOT:PSS dispersion, a PU-based rheology modifier and ethylene glycol (EG) as conductivity enhancer. The coatings showed good abrasion resistance when applied on a plain weave of spun polyethylene terephthalate (PET) staple fibres (Åkerfeldt et al. 2013b). Thus, depending on the coating composition, thin and flexible textile coatings were achieved with comparably low surface resistivity.
PEDOT:PSS was also studied as electrode material for piezoelectric PVDF films, both with (Lee et al. 2005; Sielmann et al. 2013) and without (Schmidt et al. 2006) the addition of a high-boiling solvent as a conductivity enhancer. Although the obtained conductivities were found to be significantly lower than for metallic coatings, the flexibility was superior and as such, the films could be stretched repeatedly without any loss of signal.
It is, admittedly, difficult to quantify the perception of a textile, but if some of it is not retained the purpose of smart textiles would inevitably be lost. Some textile testing standards relevant for the application in this study were chosen: For coated textiles, it is particularly interesting to study the change in tear strength with the coating since this is the property that is most likely to differ (Bulut and Sülar 2011). The abrasion resistance can tell something of how well the coating remains on the textile during wear, but is not demanding enough to truly challenge most coatings. Instead, shear flexing can be used, which will subject the sample both to folding and high shearing forces in a harsh manner.
So far, no papers have been found addressing the outer electrode of PVDF bi-component fibres in a woven construction with conductive PEDOT:PSS coatings. The purpose of this study was to achieve this and to study the textile behaviour of these sensor systems in terms of tear strength, abrasion resistance and resistance to shear flexing. In contrast to previous studies of knife coating with PEDOT:PSS on textiles, the aim was here to add the conductive layer so that it enfolded individual fibres to as great extent as possible to maximize the possible output signal from each fibre. To achieve this configuration, coatings were applied by dip coating of the substrate followed by passing through nip rollers, also known as the pad-mangle method.
2.1.1 Melt spun bicomponent fibres
The PVDF homopolymer was grade Kynar 705 (Arkema, France). According to the supplier, its melting point was 172°C, its melt flowindex (MFI) is 56 g/10 min at 230°C (for 2.16 kg), and its density was 1780 kg m−3. The polymer used for the fibre core material was high density polyethylene (HDPE) ASPUN 6835A from Dow (Midland, MI) with a density of 950 kg/m3, Tm = 129°C and MFI of 17 g/10 min. HDPE was compounded with 10 wt-% of carbon black (CB) of grade Ketjenblack EC-600 JD (Akzo-Nobel, Netherlands), density 1800 kg/m3 and BET surface area of 1400 m2/g (all data according to suppliers) in a ZSK 26 K 10.6 twin screw extruder (Coperion, Germany) as described in a previous paper (Lund et al. 2012).
Fibre production parameters
Flow rate (cm3 min−1)
Godet roll temp (°C)
2.1.2 The woven substrate
The multifilament fibres were woven into a plain construction with a PE monofilament warp yarn (Nm 36). The density of the woven substrate was 150 g/m2, with 8 picks/cm in the weft and 16 ends/cm in the warp direction.
2.1.3 The conductive coatings
Components and solids content in weight-% of the coating formulations
Coating formulation A:
60% PEDOT:PSS dispersion
2% HEUR rheology modifier
Solids content (w/o EG): 11.4%
Concentration PEDOT:PSS in coating: 6.20%
Coating formulation B:
80% PEDOT:PSS dispersion
7.5% HEUR rheology modifier
Solids content (w/o EG): 4.5%
Concentration PEDOT:PSS in coating: 19.6%
The components of the coating formulations, see Table 2, were mixed with a stirrer (RW20, IKA®, Germany) for two minutes at 600 rpm, after which the formulations macroscopically appeared homogenous and stable.
2.2 Sample preparation
Samples sized 25 * 50 cm were prepared from the woven substrate. The coating formulations were applied to the substrate via pad-mangle (Roaches Ltd, UK). The nip pressure was 1 kPa and the speed 1.5 m/min and the resulting wet pick-up was approximately 50% for all samples. The samples were dried at 80°C for 4 minutes (labdryer LTE-S(M), Werner Mathis AG, Switzerland).
PVDF is generally known to have a very low surface energy due to the flourine incorporated to its structure. Therefore, hydro- and even oleophobicity are inherent properties of PVDF and as a consequence, the adhesion to other materials can be a difficulty. The formulations were thickened to a higher viscosity than what is commonly suggested for pad mangle coating; the purpose of this being to obtain as much pick-up of the coating on the substrate as possible, thus resulting in better contacting of the PVDF. With the subsequent passing through nip rollers, the excess coating was squeezed out, but enough coating formulation was constrained to obtain a macroscopically coherent coating after drying.
The woven substrate was cut into strips in the weft (PVDF-bicomponent yarn) direction. The conductive cores of the yarns were contacted with a CB-PE matrix, heat-pressed onto the end of each strip. The cores were connected to ground and the strip put into a construction with needles pointing towards the fibre surfaces. The specimen, with the needle construction, was put into an oven at 75°C. A voltage of −10 kV was applied through the needles during 5 minutes before both heat and voltage were turned off and the sample was allowed to cool to room temperature before removal from the oven. This procedure was shown to be sufficient to orient the dipoles by Nilsson et al. 2013.
The crystallinity in the fibres was evaluated with attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR), using Bruker Tensor 27 and software Opus 7.2 (Bruker Optik GmbH, Germany).
2.3.2 Piezoelectric characterization
The coated strips were subjected to dynamic strain using a servo-hydraulic tensile testing machine (Model 66-21B-01, MTS systems, USA). Each sample was clamped between two rubber sheets, to prevent sliding and for electrical isolation. The starting distance between the clamps was set on 100 mm. After the sample was secured between the clamps a pre-tension force of 15.4 N was applied in order to prevent slack in the sample during measurement.
All samples were exposed to a sinusoidal strain with amplitude of 1%. The sensor electrodes were connected to a data acquisition device (with an input impedance of 100 GΩ in parallel with 100 pF) (NI DAQPad-6016, National Instruments, USA) connected to a computer running a LabVIEW Software, which controlled the measurement. The piezoelectric output voltage from the fibres was recorded at 3 Hz, which gives the intrinsic piezoelectric voltage. In addition, an analog signal from the MTS machine proportional to the strain was recorded.
2.3.3 Surface resistivity
Surface resistivity measurements were performed using a multimeter (Fluke 8846A, USA) in a four-wire resistance mode and an in-house designed and produced four-point probe, details published elsewhere (Åkerfeldt et al. 2013b). A weight of 2.2 kg was placed on the probe, and the resistivity values were read after one minute according to standard CEI/IEC 93:1980.
2.3.4 Scanning electron microscopy (SEM)
The appearance of the samples was studied using field emission scanning electron microscopy (FE-SEM) (JEOL JSM-7800 F, Japan). The SEM was equipped with energy dispersive spectroscopy (EDS) (Quantax X-ray mapping system, Bruker Nano GmbH, Germany), allowing elemental analysis of the samples. The samples with destroyed conductive coatings due to the flexing treatment were sputtered with a layer of 2 nm platinum. Cross-sections of the samples were embedded in epoxy, frozen to - 60°C and polished with a broad ion beam (BIB). The specimens were also sputtered with carbon by means of resistance vaporization to a thickness of 5 nm. The cross-sections were imaged with a back scatter detector and EDS-mapping.
2.3.5 Stress viscometry
The shear viscosity of the coating formulations was evaluated with a stress-controlled rheometer (Bohlin CS Melt, Sweden) and a cone-and-plate set-up. Samples were subjected to stress sweeps, for coating A 1.62-55.5 Pa and for coating B 0.31-367 Pa, corresponding to a similar range of shear rates for the two samples, of approximately 0.015-180 s−1.
2.3.6 Tear strength
Tear strength in the warp direction (the bi-component weft yarns torn) was determined according to standard EN ISO 4674-1B in a tensile tester (Instron 4502, UK). A minimum of three replicas of each sample was tested.
2.3.7 Abrasion resistance
Abrasion was studied using a Martindale (Nu-Martindale model 403, James Heal & Co. Ltd, UK) and wool abradant fabric, according to standard EN ISO 5470–2. The sample holders were in accordance with standard EN ISO 12945–2 (diameter 90 mm). The effect of the abrasion was evaluated based on resistivity measurements and surface appearance as described above. The total weight on each sample during abrasion was 563 g. Three replicas of each sample were abraded, and the mean values with standard deviations calculated.
2.3.8 Resistance to shear flexing
Testing for resistance to shear flexing was performed in accordance with ISO standard 5981:2007 (method B, without pressure foot) in an apparatus specifically constructed for this test method (Meadowbank Innovations Ltd, UK). Samples were in the size of 100 mm in the weft direction and 50 mm in the warp direction, allowing them to be clamped into the adjacent holders and leaving an area of 2250 mm2 of fabric between that was folded and subsequently subjected to shearing when the holders moved juxtaposed each other. The samples were subjected to 1000 cycles each, where after their surface resistivity was measured in accordance with previous description.
3.1 The piezoelectric effect
3.2 The coated textiles
Two coating formulations were compared for the application, see Experimentals and Table 2. The purpose was to evaluate whether the binder had a relevant function for the coating, since it was known that the binder molecules could hinder the conductivity of the PEDOT:PSS. Macroscopically, the coated textiles appeared similar with regard to handle and aesthetic properties.
3.3 Surface resistivity
Surface resistivity of samples coated with formulation A and B
Initial surface resistivity (Ω/square)
Surface resistivity after Martindale (50 000 cycles) (Ω/square)
Surface resistivity after shear flexing (1000 cycles) (Ω/square)
3.4 Distribution of the conductive coatings
It is relevant to compare the top view mapping of the samples in Figures 9 and 10 with the distribution of the coatings showed in Figure 7. The top-view images showed coating on the top of the weft yarn for both coatings, so even coating A enfolded the weft yarns, though probably to a lesser extent than coating B. Due to the phase separation of coating A, the distribution of the PEDOT:PSS was however less homogeneous than in coating B. Considering the surface resistivity, it can be concluded that the relatively small amounts of the coatings and their distribution was still sufficient to form a reasonably conductive network.
To understand how the microstructure of the coatings affected the conductivity in greater detail, it is required to look at the approximated compositions of the dried coatings. The PEDOT:PSS concentration in the dried coatings was about 6 wt-% for coating A and 20 wt-% for coating B, not taking the liquid EG into account (see Table 2). Also, in coating A, up to 90 wt-% of the solids content is the high molecular weight polyurethane, whereas in coating B the only other solid is the HEUR, which is of more oligomeric size. The morphologies depicted in the low voltage SEM-images in Figure 10 shows that PEDOT:PSS and HEUR seem to blend with each other a lot better than PEDOT:PSS and PU, which may be a result of the more compatible sizes of the molecules. However, with the lower amount of solids in coating B and since both coatings were picked up by the substrate to the same extent, the actual amount of PEDOT:PSS per square meter (A: 0.53 g/m2 and B: 0.66 g/m2) is about the same for both coatings. Comparing with the initial resistivities of the coatings in Table 3, it appears as though the PU-binder in coating A does hinder the conductivity of the PEDOT:PSS clusters, with one order of magnitude.
3.6 Textile properties and durability
The tear strength is indicative of how the coating and the textile interact and as such a good tool to evaluate coated textiles, whereas the durability tests tell more about the coating, especially how well it adheres to the textile fibres (Sen 2008). From Table 3, the initial surface resistivity measurements of the coated textiles show a difference of about one order of magnitude, where coating B has the lower resistivity. After 50 000 abrasion cycles, the resistivity of the samples with coating B increased with five times (500%) of its original value, whereas the samples coated with A only increased with 28% of its original value. After the flexing test, also in Table 3, the coating without binder (coating B) has lost almost all of its conductivity while the coating with the binder (coating A) has maintained a reasonable conductivity, with an increase of two orders of magnitude in resistivity.
Coatings of PEDOT:PSS were successfully used as outer electrode material for a woven substrate of bi-component PVDF fibres, with the potential application of a piezoelectric strain sensor. The coatings were thin, macroscopically flexible and exhibited a surface resistivity in between 10–150 Ω/square. The coated textiles without binder exhibited an initially lower resistivity than those with the binder, but did not withstand flexing to the same extent. SEM showed that the coating without binder flaked off and the resistivity increased drastically after 1000 cycles whereas the coating with the binder was rubbed off with less increase in resistivity. Both coatings had reasonable tear strength, but the inclusion of binder polymers was necessary for their durability. A tougher system, but with retained conductivity, would be desired and other types of binder polymers will therefore be the subject of future study.
The authors thank Lars Eklund (Swerea IVF) and Professor Bengt Hagström (Swerea IVF) for their invaluable scientific guidance during the course of this work. Financial support is gratefully acknowledged to Swedish Foundation for Strategic Research (SSF), Sparbanksstiftelsen Sjuhärad and, through the Smart Textiles initiative, VINNOVA.
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