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Proceedings of the TRS 2012The 41 st Textile Research Symposium

12-14 September 2012Guimarães

Portugal

Textile MachinerySociety of Japan

University of MinhoSchool of Engineering


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Proceedings of the TRS 2012The 41 st Textile Research Symposium

12-14 September 2012Guimarães

Portugal

Textile MachinerySociety of Japan

University of MinhoSchool of Engineering


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Processing and electrical response of fully polymer piezoelectric filamentsfor e-textiles applications

Martins R. S. 1; Silva M. P. 2; Gonçalves R. 2, Rocha J.G. 3; Nóbrega J. M. 1; Carvalho H. 4; Souto A.P. 4; Lanceros-Mendez S. 2

1 IPC/I3N – Institute for Polymers and Composites, 2 Centro/Departamento de Física 3 Dep. Indust rial Electronics 4Centre for Textile Science and Technology, University of Minho, Portugal

[email protected]

AbstractThis paper describes the production and characterization of novel geometries piezoelectric products with alarge potential in the design and implementation of flexible sensors produced at low cost and high rates. In

particular, filament shaped geometry, appropriate for integration into textiles, is developed and described.

Keywords: piezoelectric, PVDF, e-textiles

1. IntroductionIn recent years, large efforts have been

developed for the incorporation of systems and

devices into textile products [1,2]. The need ofintegrating electronic products into textile materialsresults from the separate processing conditions andrequirements for the textile and the device. Ideally,new fibres with increased functionality, such assensing and actuating performance, should be used,

being them a more significant part or even thecomplete solution for a given application.

There are several approaches for the productionof textile products with transducing capabilities.One of the most interesting ones, from theindustrialization point of view, would be that thetextile material itself shows this functionality. This

integration can occur at the level of the fibre, of theyarns or of the fabric and can be based on

piezoelectric polymers.

2. Piezoelectric polymers and filamentsPoly(vinylidene fluoride), PVDF, is widely

studied material due to its piezoelectric properties,including in the context of integration into textiles.These properties can be used in applications such assensor and actuator devices. The electroactive

properties depend on the degree of crystallinity,structure and orientation of the crystalline fractionof the polymer, which in turn depend on the

processing conditions [3].PVDF presents at least 4 crystalline phases, the

non-polar -phase being the one typically obtained by crystallization from the melt [3]. The -phase isthe most interesting form the point of view ofelectrical activity and is most often obtained bystretching -PVDF at 80ºC using a stretch ratio (R)

between 3 and 5 [4,5].In order to optimize the piezoelectric response

and, therefore, the transduction capabilities, the

dipolar moments have to be further oriented by theapplication of an electric field, i.e. through a poling

process. The simplest method is the electrode

poling method, in which the material is subjected tofields of about 300 MV/m, depending on the polingtemperature. After poling, the piezoelectricresponse is optimised, i.e., a potential will be

produced upon mechanical excitation of the polymer, or a mechanical action is produced in the polymer when it is subjected to an electric field.

PVDF based sensors are available on the marketin the form of films. In this case, a thin PVDF layeris deposited with metallic electrode layers on bothsides which serve as electrodes for signalacquisition and/or material stimulation.

Some research work has been focused on the

development of PVDF sensors in the form offilaments. Walter et al [6,7] have extensivelystudied the phase transitions in extruded PVDFmonofilaments. A composite of simple PVDFfilaments and epoxy resin with the filaments in a

parallel arrangement was poled with a linearelectric field in a direction perpendicular to thefibres. The composite was shown to exhibit

piezoelectric activity.From the point of view of textile use, a

piezoelectric filament should ideally be arranged ina coaxial geometry, as shown in figure 1.

Fig.1: Structure of a coaxial piezoelectric filament


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The production processes and poling methodsfor this arrangement have also been previouslystudied, in particular with respect to the study of theinfluence of the conductive layers on the

piezoelectric properties of the PVDF.Comprehensive work by Lund et al and Ferreira etal have shown for two-layered filaments that theelectroactive phase content is not affected by theconductive inner core. The -phase content dependsonly on the processing temperature and stretch ratio,as for the single PVDF filaments.

Sequential processing methods for piezoelectriccables have been described by Mazurek et al [10,11]. In such a process, the influence of coolingtemperatures and the thermal impact of subsequentcoating procedures are important to consider.Mazurek reports on an increase of 30 to 40 % of

-phase content for cooling temperatures of -30 ºCafter stretching of the PVDF, when compared tocooling temperatures of 5ºC. Also, a reduction

between 20 to 50% of the -phase content resultsfrom the coating process necessary for the

placement of the outer electrode after the production of the inner layers.

In this work, multilayered fully polymerfilaments incorporating electrically conductivelayers used as electrodes, in a coaxial arrangement,are produced using a process based in conventionalextrusion, more specifically using co-extrusion, thatallow the filaments to be produced in a single step.This paper will focus on the production and

properties of a two layer filament, composed of aconductive inner core and an outer PVDF layer.

3. Experimental

3.1 Extrusion A prototype monofilament extrusion line is used

to produce the two-layered filament. Theco-extrusion die employed was designed to producemulti-layer filaments, but was not optimized for thematerials employed. However it is expected to

produce satisfactory geometries, for the required proof of concept. The production steps aresummarily depicted in figure 2, a cut-view of theextrusion die can be observed in figure 3.

Fig.2: Production steps of two-layered coaxial piezoelectricfilament

Fig.3: Cross-section of the extrusion die

The materials used were the Solvay Solef Ta-1010PVDF and Premix 1396 conductive PP. A stable

process at drawing ratios of about 4 could beachieved, with temperatures of 120 ºC duringdrawing [8].

3.2. PolingThe filaments preparation was finished by

creating an outer electrode using silver conductiveink and exposing the inner conductive layer. Thetwo layers were then connected directly to ahigh-voltage source and voltage was stepped up,seeking to maximize the electrical field applied.The voltage source’s protection automaticallystopped the voltage increase when current increased,which resulted either from material breakdown orlocalized defects. Voltages of about 7.7 kV werereached, which, considering the filament geometry

presented later, result in electrical fields of about40MV/m in the PVDF layer.

Three replicate samples were exposed to thisvoltage during different periods of time (15, 30, 45and 90 minutes) and at temperatures of 20 and 80ºC.The samples poled at 80ºC were cooled down toroom temperature under the applied field.

3.3.Electromechanical evaluationTwo different experimental set-ups were used to

evaluate the electromechanical response of thefilaments.

First, the filament was attached to a vibrationgenerator (figure 4) through a set of acrylic partsconstructed for this purpose.


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Fig.4: Electromechanical testing set-up based on a vibration

generator

The vibration generator was fed with a 0.5 Hzsquare wave which produced an oscillatingmovement of the generator’s axis with an amplitudeof ~ 3 mm, and a consequent bending deformationto the filament.

Second, the filaments were fixed to the grips ofa universal testing machine, working in the tensilemode, and submitted to 20 loading-unloadingcycles with an amplitude of 0.2 mm at a speed of100 mm/min. (figure 5)

Fig.5: Stretching in universal testing machine

3.3.Signal conditioning and acquisitionThe filaments were connected to a custom-built

charge amplifier. The output signal of the amplifierwas connected to a National Instruments NI-6259data acquisition board and custom-developedsoftware based on Labview was used for signal

acquisition, display and storage.The acquired signals showed to be affected bysome 50 Hz-noise and its harmonics, which blurredthe signals of smaller amplitude. Correct groundingand shielding of the conditioning electronics

provided a significant reduction of this noise. AButterworth 10th-order bandstop filter with astopband between 45 and 55 Hz was implementedin the software and in this configuration even thesmallest signals could be depicted.

4. Results

4.1 ExtrusionFigure 6 shows a cross-section image and Figure

7 a side photograph of the two-layer co-extrudedPVDF filament with conductive inner core.

Fig.6: Cross-section image of the filament: the inner layercorresponds to the conductive material and the external one to

the piezoelectric PVDF

Fig. 7: Longitudinal view of the filament

It is observed that the shape of the layers is not fullyaxisymmetric, which is to be ascribed to the lack ofoptimization of the extrusion die used to producethe filaments.

4.2.Electrical test and signalsThe non-uniform thickness of the PVDF layer

will cause anisotropic poling of the piezoelectricmaterial. This should result in different electricalsignals of the filament under bending, according tothe direction of the bending action. Moreover, at 7.7kV, with a minimum layer thickness of 200µm, theelectrical field is roughly 40 MV/m, a relatively low

value, which may result in a weak electrical activityof the filament.

Figure 8 shows an example of a signal acquiredwhen compressing the filament manually with a

plastic part.


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Fig. 8: Electrical signal produced by manually compressingthe filament with a plastic tool

The response is typical for a piezoelectric sensorcoupled to a charge amplifier. Piezoelectric sensorsgenerate signals only upon force variations. Static

forces do not produce any signal. The chargeamplifier picks up the signal produced duringvariations in a capacitor present in the amplifier’sfeedback loop, but the capacitor discharges throughthe feedback resistor with an adjustable timeconstant. This time constant can be chosenaccording to the application.

In figure 9, the signals resulting from theexcitation of the filament with the vibrationgenerator are shown. Signal amplitude is in thiscase about 25 mV.

Fig. 9: Electrical signal produced by mechanical excitationthrough the shaker

The traction produced by the universal testingmachine produces the typical signal shown in figure10.

Fig. 10: Electrical signal produced by mechanical excitationthrough the universal testing machine

In this case, the amplitude of the signals is about150 mV, an order of amplitude higher than thosefound with the light bending action produced by the

vibration generator A measurement of the amplitudes of signals

produced by the mechanical deformations appliedwith the vibration generator at the samples poled at20 and 80 ºC is given in the graphs of figures 11and 12

It can be observed that no specific trend isobtained for the signal amplitude relating to polingtimes, but clearly, on average, the filaments poled atthe higher temperature produce a stronger signal.

4. Discussion

The sensitivity of the filament sensors producedare low when compared to commercial PVDF filmsensors, i.e., the measured signal amplitudes aresmall A DT series piezo element from MeasurementSpecialties produced a signal an order of magnitudehigher when subjected to the same mechanicalaction. This was expected considering the polingconditions, with a relatively low electrical fieldapplied. Still, the mechanical action applied to thefilaments is clearly depicted in the electricalresponse, such as expected from any other

piezoelectric sensor.


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Fig. 11: Electrical signal amplitudes produced by mechanical

excitation through the shaker on filaments poled at 20 ºC

Fig. 12: Electrical signal amplitudes produced by mechanicalexcitation through the shaker on filaments poled at 20 ºC

Higher poling temperatures produced betterresults, but it has not been possible to draw anyconclusion about the relation between poling timesand sensor sensitivity.

The improvement of the sensor sensitivityshould be achieved by first improving its geometry,in particular by obtaining more symmetricgeometries and/or thinner layers. The extrusion dieused for this preliminary experiment should beimproved. A new extrusion die, allowing theextrusion of three-layered filaments has already

been designed and is being tested by optimising processing conditions in order to produce aconsistent, defect-free filament that would allowhigher poling voltages with thinner PVDF layers.

5.Concluding remarks

A proof-of-concept has been developed for a

co-extruded filament sensor, which produces anelectrical signal when stimulated by mechanicaltraction, bending or impact, behaving in the sameway as standard piezoelectric film sensors.

Improvements in the production of the filamentare necessary nevertheless in order to optimise theobtained results. A new extrusion die for two andthree-layered filaments is currently under test forthe development of higher-level piezoelectricresponses and full implementation and integrationinto e-textiles.

Acknowledgements

The authors thank the Portuguese Foundation forScience and Technology (FCT) for financial supportunder POCI, PDTC (PTDC/CTM/108801/2008,PTDC/CTM/69316/2006 andPTDC/CTM-NAN/112574/2009) and Plurianual

programmes. They also wish to thank the IN2TECinitiative of the School of Engineering/Universityof Minho which supported some initial work on

piezoelectric filaments and the COST actionsMP1003 (The ‘European Scientific Network forArtificial Muscles’ (ESNAM)), and MP0902(Composites of Inorganic Nanotubes and Polymers(COINAPO)).

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