WO2014161920A1

WO2014161920A1 - Method of producing a piezoelectric and pyroelectric fiber

Info

Publication number: WO2014161920A1
Application number: PCT/EP2014/056658
Authority: WO
Inventors: Bengt Hagström; Anja LUND; Erik Nilsson
Original assignee: Swerea IVF AB
Current assignee: Swerea IVF AB
Priority date: 2013-04-03
Filing date: 2014-04-03
Publication date: 2014-10-09
Anticipated expiration: 2015-10-03

Abstract

The invention relates to method for producing a piezoelectric and pyroelectric fiber (100) comprising: providing a fiber (100) comprising a core having a core material (110) and a surrounding material (111) enclosing the core material, the core material (110) comprises an electrically conductive flexible thermoplastic composite comprising at least one polymer and at least one conductive filler, and the surrounding material (111) is a permanently polarizable polymer; stretching the fiber along an elongation direction (113) of the fiber with a draw ratio sufficient to introduce beta-crystallites in the fiber; electrically grounding the core material (110); while the core material (110) is grounded, passing the fiber through an annular poling element (220); and applying a voltage between the annular poling element (220) and the core material (110) sufficient to establish an electric field for corona poling, whereby the core material (110) is exposed to the electric field electrically charging the surrounding material (111).

Description

METHOD OF PRODUCING A PIEZOELECTRIC AND PYROELECTRIC

FIBER

Field of the Invention

The present invention relates to method of producing piezoelectric and pyroelectric fibers. The invention further relates to a method of producing a yarn from such fibers and to a fabric made from the fibers.

Background

A piezoelectric sensor has the ability to generate an electric field when deformed and similarly, a pyroelectric sensor generates an electric field in response to changes in the temperature. Such sensors are attractive for numerous applications. For example, positioned on or in the vicinity of a human body they may be used to register vital signs including heartbeat, blood pressure and chest expansion during breathing. Careful integration of such sensors into garments may be used for continuous wireless health monitoring, e.g. in the home of a patient. Similarly, periodic movements (walking, breathing) of the human body can be used for energy harvesting, recharging a battery or capacitor. Piezoelectric sensors may also be used for structural health monitoring, i.e. monitoring vibrations and impact on building elements or vehicles by integration into existing structures.

Polymer-based piezoelectric or pyroelectric sensors commonly employ poly(vinylidene fluoride) (PVDF) or one of its copolymers. In order to give

PVDF its piezoelectric effect, polar β-phase crystallites must be formed; this is commonly achieved by stretching the polymer at a temperature below its crystalline melting point (solid state drawing). Further, poling at very high electric fields is required to achieve a net permanent polarization of the material. Thus, the production of piezoelectric PVDF-based fibers should include the following processing steps: (i) solid state drawing, (ii) the introduction of electrically conductive electrodes for applying electric fields to the material and/or measuring the piezoelectric signal and (iii) the application of a high electric field. Until recently, research on PVDF in fiber form has mainly focused on fibers with diameters in the nanometer range produced by electro-spinning, as this process introduces β-phase crystal I in ity. However these fibers have no integrated electrodes.

PVDF-based sensors are commercially available in the form of films or coaxial cables. In such cables, a copper wire is surrounded by either a wound-up film of PVDF, or an extruded layer of its copolymer poly(vinylidene fluoride - trifluoroethylene) (P(VDF-TrFE)). The copper wire is intended to be used as inner electrode for an application. A copper braid constitutes the outer electrode. A drawback of PVDF-TrFE is its prohibitively high cost. For coaxial cables with piezoelectric properties, suggested materials include highly-plastic semiconducting polymers and fusible metals with a low melting point. However, the commercially available piezoelectric coaxial cables typically have a large outer diameter and an unacceptable weight which makes them inappropriate for most textile applications. Furthermore, the metal parts of some cables introduce additional complications for some applications.

Furthermore, piezoelectric polymer films may consist of a thin PVDF- layer which is metalized on both sides and further embedded in epoxy for mechanical shielding. However, the lamination of a film onto a textile makes production relatively complicated. Furthermore, the film also has a negative effect on the properties (e.g. drape, comfort, breathability) of the final textile product.

PVDF-based piezoelectric fibers are disclosed in WO/2012/035350. WO/2012/035350 describes the melt extrusion and poling of PVDF-fiber. The fibers are defined as having a solid cross-section and a substantially homogeneous composition, and poling is carried out by the placement of a pair of electrodes above and below the thread line. Also described is a piezoelectric construct where the fibers are interposed between two conductive layers such as two thin sheets of aluminum. The production method described introduces limitations to the use of the produced fibers; poling homogeneous PVDF-fibers by passing them between two electrodes will orient the inherent dipoles so that the fiber cross-section may be described as having one positive and one negative side. Thus, two separate outer electrodes must be placed on exact locations on the fiber's

circumference, and they must never be in electrical contact with one another. Thereby, such fibers will be difficult to handle and they will introduce the same limitations as the PVDF films previously described.

The prior art on piezoelectric bicomponent fiber based on PVDF includes work on a force sensor described in "Piezoelectric polymetric bicomponent fibers produced by melt spinning" (Journal of Applied Polymer Science vol. 126, 490-500). A sensor was constructed from a number of aligned fibers embedded in a carbon black/polymer compound which constituted the outer electrode. The sensor was poled by connecting the outer electrode and the cores of the fibers, via a thin copper wire, to a high DC voltage supply (a method known as "contact poling"). The sensor was subsequently tested under dynamic compression and was shown to generate an output voltage of 2 mV when compressed under a force of 1 N. Although the piezoelectric effect was sufficiently proved by these tests, the design of the outer electrode caused the sensor to be relatively cumbersome. Although the fibers could very well be woven in industrial weaving machines, this introduces defects in single fibers. The fabric was then coated with a carbon black/polymer compound and contact-poled, but the presence of defects caused local electrical shortcuts which led to electrical breakdown when a high voltage was applied. Consequently only very small areas of the woven fabric could actually be poled.

Thereby, there is a need in the art for a less complicated and more reliable method for producing piezoelectric and pyroelectric fibers suitable for a yarn and/or for integration in a fabric or another composite.

Summary of the invention

In view of the above, it is a general object of the present invention to provide an in-line method for producing piezoelectric and pyroelectric fibers with improved electrical properties. According to a first aspect of the present invention it is therefore provided a method for producing a piezoelectric and pyroelectric fiber comprising:

providing a fiber comprising a core material and a surrounding material enclosing the core material, the core material comprises an electrically conductive flexible thermoplastic composite comprising at least one polymer and at least one conductive filler, and the surrounding material is a

permanently polarizable polymer;

stretching the fiber along an elongation direction of the fiber with a draw ratio sufficient to introduce beta-crystallites in the fiber;

electrically grounding the core material;

while the core material is grounded, passing the fiber through an annular poling element; and

applying a voltage between the annular poling element and the core material sufficient to establish an electric field for corona poling, whereby the core material is exposed to the electric field electrically charging the

surrounding material.

In a piezoelectric material electric charge is redistributed in the material as a response to a mechanical stress. Similarly, in a pyroelectric material electric charge is redistributed as a response to a change in temperature of the material.

In general, the thermoplastic composite has properties which enable the composite to be moldable when heated to above a specific temperature, and to return to a solid state when subsequently cooled. Furthermore, the conductive filler is at least electrically conductive.

Permanently polarizable in accordance with the invention means that a displacement of electrons caused by an applied electric field remains substantially displaced after the electric field is removed for at least an longer than an hour, preferably several days, weeks, months or years. Thereby, the permanently polarizable material shows a net dipole moment after the electrical field is removed for at least an hour, preferably several days, weeks, months, or years. The surrounding material is preferably not electrically conductive meaning that the surrounding material preferably has an electrical resistivity higher than approximately 10¹⁴ Ωαη.

The stretching step is part of a process of producing a fiber having the appropriate dimensions. A fiber made according to the invention is typically of diameter 10-100 micrometer and suitable for a yarn or a fabric. The process including the stretching of the fiber may be a melt-spinning process. The stretching process comprises that the fiber is heated to a temperature below a crystal melting point of the material of the fiber and then is subject to stretching, thus the fiber is stretched in a longitudinal direction of the fiber. The stretching of the fiber below the crystal melting point of the material of the fiber is known as solid state drawing (SSD). An attainable draw ratio during stretching may depend on the molecular weight of the material used and a melt draw ratio (MDR) applied prior to solid state drawing (thus, melt draw is a stretching step during a melted state of the fiber prior to SSD), if such prior stretching has occurred. In any case, the solid state draw ratio is preferably as high as possible not creating spin line breaks during production in order to maximize the fraction of polar β-phase crystallites. A high fraction of β-phase crystallites in the surrounding material enables an efficient poling of the surrounding material of the fiber.

A draw ratio according to the invention is a ratio between the length of a fiber before stretching and the length of the same fiber after stretching. For example, such draw ratio may be in the interval 2-6 during solid state drawing. However, the solid state draw ratio should be sufficient to introduce (polar) β-phase (beta-phase) crystallites in the fiber.

Furthermore, the fibers are exposed to a strong electric field in order to polarize the surrounding material of the fiber - a process commonly known as poling - which is necessary to introduce pyroelectric and piezoelectric properties. The electric field is applied radially, which may be realized by i) connecting the fiber core (inner electrode) to ground via e.g. a grounding of the melt spinning machine or part of the melt spinning machine and ii) subjecting the outer surface (thus the surrounding material) to the high DC voltage corona poling electric field. Applying a voltage between the annular poling element and the core of the fiber is herein referred to as corona poling. Contrary to contact poling where a conductor is physically connected to the surface of the fiber, in corona poling the electric field is exposed to the fiber without physically contacting the outer surface of the electrode. This way, without the physical connection enabled by corona poling, a risk of electrical breakdown in the fiber is reduced.

The present invention is based on the realization that a bicomponent fiber having a core and an outer layer (the surrounding material) may be made piezoelectric and pyroelectric during production of the fiber. Moreover, it is realized that the fiber may be of a dimension and mechanical structure making the fiber suitable for e.g. textiles and/or fabrics. The fiber made according to the invention enables a sensor that is easily incorporated into a range of products from wearable garments to thin composites. The inventor realized that despite the short residence times in the corona field due to the high speeds encountered during melt-spinning, a sufficiently high electrical field can be generated in the fibers' radial direction due to the corona electric field, provided the fibers' cores are electrically grounded (for example via an extruder and gear pump in upstream contact with molten core material).

The present invention provides means for producing piezoelectric and pyroelectric sensors substantially smaller in diameter and of higher mechanical flexibility than those of the prior art sensors, and are more convenient to produce. In addition, a poling process which polarizes the fibers radially with an inner electrode as electrical reference, points towards a highly versatile sensor that is easily incorporated into a range of products from wearable garments to thin composites.

According to an embodiment of the invention, the annular poling element may comprise at least one needle element arranged on an inside surface of the annular poling element, wherein the at least one needle element is pointed in an inwards direction towards the fiber, wherein the voltage is applied between the at least one needle element and the core material. With the needle element, having a longitudinal extension pointing in the direction of the fiber in the annular poling element, preferably in the radial direction of the annular poling element, an improved corona poling may be achieved. The needle elements provides for the corona field generated by the annular poling element to have higher amplitude (i.e. stronger field strength) than without the needle elements. The needle elements may further be in the form of spikes. With the needle elements the efficiency of providing the fiber with piezoelectric and pyroelectric properties is improved, thereby, less time is needed for poling of the fibers providing a more efficient process.

According to an embodiment of the invention, the annular poling element is may be a hollow cylinder comprising the needle elements. In this way, the needle elements are arranged in an inner surface of the cylinder and the fiber is passed through the hollow cylinder but spaced apart from the needle elements. The cylinder provides for a possibility to have a higher density of needle elements, thereby further improving the efficiency of providing the fiber with piezoelectric and pyroelectric properties.

According to an embodiment of the invention, the method may further comprise heating the fiber to an elevated temperature prior to or while being exposed to the corona field. Heating the fiber in this way advantageously increases the degree of permanent polarization in the surrounding material of the fiber improving the piezoelectric and pyroelectric properties of the fiber obtained in the corona poling. The elevated temperature is preferably in the range of 50°C-150°C.

According to an embodiment of the invention, the method may further comprise providing a plurality of fibers, wherein the method further comprises: guiding the plurality of fibers through the annular poling element through an annular guiding element arranged inside the annular poling element, wherein the guiding element is arranged spaced apart from the annular poling element. In other words, more than one fiber may simultaneously be guided through the annular poling element. An annular guiding element ensures that the fibers are maintained a distance from the annular poling element (and/or the needle elements). For example, as the surrounding material of the plurality of fibers is charged by the corona poling process, the fibers repel each other, thus they approach the annular poling element (and/or the needle elements). Hereby, the guiding elements prevent the fibers from touching the annular poling element (and/or the needle elements). In this way, the fibers do not screen each other substantially which further improves corona poling.

Generally, a yarn (or a filament yarn) is a plurality of fibers arranged longitudinally with respect to each other. Thus it is equally applicable that a filament yarn may be formed from the plurality of fibers before the corona poling as after the corona poling. In case where the filament yarn is formed before the corona poling, the filament yarn enters the guiding elements for corona poling.

According to an embodiment of the present invention, the step of guiding may comprise guiding the fibers through the annular guiding element being arranged coaxially with said annular poling element. Thus, the annular guiding element is arranged coaxially with the annular poling element.

Furthermore, the annular guiding element may have a diameter smaller than a diameter than the annular poling element.

According to an embodiment of the present invention, wherein the surrounding material is PVDF or a vinylidene fluoride based copolymer comprising crystalline beta-form. Poly(vinylidene fluoride) (PVDF), enables improved pyroelectric and piezoelectric properties of the fiber as well as both the strength and flexibility required in a yarn for textile and composite applications made from the fiber. PVDF is chemically inert and UV- (ultraviolet) and fire resistant providing additional advantages. Furthermore, PVDF is advantageously not electrically conductive. Thereby, an electrode may be connected to the surrounding material without causing an electrical short circuit to the core material of the fiber. In other words, a short circuit between the electrode connected to the surrounding material (outer layer) and the core is prevented, as the surrounding material is not electrically conductive.

According to an embodiment of the present invention, the core material is a composite of at least one thermoplastic polymer and at least one carbonaceous filler. This provides the piezoelectric and pyroelectric fiber with an inner electrode with high flexibility (low stiffness, low resistance to stretching) and sufficiently high electrical conductivity for suitable electrical function, for example grounding.

According to a second aspect of the present invention there is provided a method of producing a yarn comprising: commingling a plurality of piezoelectric and pyroelectric fibers each produced according to the previous aspect with a plurality of electrically conductive fibers different from the piezoelectric and pyroelectric fibers, wherein the electrically conductive fibers electrically connect with the surrounding material of the piezoelectric and pyroelectric fibers, such that an electrode for the plurality of piezoelectric and pyroelectric fibers is formed by the electrically conductive fibers. In other words, a yarn may be obtained having an easily accessible outer electrode in form of the electrically conductive fibers. Examples of the electrically conductive fibers are metal ized filaments or carbon suffused filaments or even metallic filaments. The electrically conductive fibers may form a yarn.

A yarn may be a plurality of fibers arranged longitudinally substantially in parallel with each other, or in some examples, the fibers are twisted.

Further effects and features of this second aspect of the present invention are largely analogous to those described above in connection with the first aspect of the invention.

According to a third aspect of the present invention, there is provided a method of producing a yarn comprising: providing a plurality of piezoelectric and pyroelectric fibers made according to the previous aspects and embodiments; and embedding the plurality of fibers with a embedding the plurality of fibers with a second composite material being electrically conductive, wherein the second composite material forms an outer electrode of the yarn. Examples of such outer electrodes being carbon fiber reinforced conductive epoxy composites, carbon filled polymer, electrically conductive paint, moist soil and sea water.

Further effects and features of this third aspect of the present invention are largely analogous to those described above in connection with the first and second aspects of the invention. According to a fourth aspect of the present invention, there is provided a fabric comprising a piezoelectric and pyroelectric fiber made using the method according to the first aspect, wherein a plurality of piezoelectric and pyroelectric fibers are arranged in parallel in a warp direction and a plurality of a second type of fiber are arranged in parallel in a weft direction, or vice versa. Thus, the fabric may be woven using yarn made from the piezoelectric and pyroelectric fibers. This type of fabric enables integration of sensor capabilities in garments and clothing which may be used for monitoring of e.g. the health of a patient. Moreover, periodic movements (e.g. walking, breathing, etc) may be used for energy harvesting, for example recharging a battery or capacitor. The fabric could further be used for sensing e.g. vibration of building and structures. Furthermore, the fabric may be integrated in the material of a building or a structure for careful monitoring of e.g. movements or effects of impacts on e.g. the building.

According to an embodiment of the present invention, wherein the second fibers are electrically conductive and configured to be in electrical contact with the surrounding material of the piezoelectric and pyroelectric fibers, such that the second fibers form an electrode.

According to an embodiment of the present invention, an electrically conductive coating, applied to the fabric and electrically connected with the surrounding material of the piezoelectric and pyroelectric fibers, forms an electrode.

According to an embodiment of the present invention, a contact electrode may be arranged to electrically contact the core material of the piezoelectric and pyroelectric fibers at an outer edge of the fabric essentially parallel with the electrically conductive fibers arranged in parallel as warps or wefts.

According to an embodiment of the present invention, wherein the electrical contact between the contact electrode and the core material of the piezoelectric and pyroelectric fibers is obtained by fusion welding. For fusion welding, a conductive polymer based filler material compatible with the core material of the fibers is used. By compatible is here meant that the filler material forms a bond to the fiber core material upon cooling from the melting due to fusion welding. The welding process comprises that both the fiber core material and the filler material are brought above their melting point causing them to fuse together. The intimate contact between the two materials may be facilitated by the fact that the fiber core material may protrude out from the fiber end upon melting due to thermal expansion. The filler material preferably has a melting point below the melting point of the surrounding material of the fibers to avoid deforming the surrounding material during fusion welding. Suitable filler materials are carbon black filled ethylene or propylene based copolymers with melting points below about 150°. By applying such filler materials and fusion welding, the fiber cores can be electrically and mechanically connected to surrounding devices needed to realize a targeted functionality, such as for example as a sensor device.

Further effects and features of this fourth aspect of the present invention are largely analogous to those described above in connection with the first, second, and third aspects of the invention.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

Brief description of the drawings

These and other aspects will now be described in more detail with reference to the appended drawings, in which exemplary embodiments of the present invention are shown, wherein:

Fig. 1 illustrates a cross-section of a fiber having a core and a surrounding material;

Fig. 2 shows is a schematic representation of a melt spinning process comprising inline-poling; Fig. 3 shows a schematic of electric charge distribution in the cross- section of a fiber at corona poling;

Fig. 4 shows a representative piezoelectric signal and strain plotted as function of time;

Fig. 5A-B depict (photo and schematic) a multi-component piezoelectric yarn comprising a melt-spun yarn made according to an embodiment of the invention twisted together with an electrically conductive yarn;

Fig. 6 is a schematic illustration of a cross-section of a piezoelectric yarn comprising fibers embedded in a conductive polymer-based coating;

Fig. 7A-B shows a schematic cross-section and a schematic view of a filament yarn according to an embodiment of the invention;

Fig. 8 shows results from tensile testing of coated and uncoated yarn; Fig. 9 shows a piezoelectric signal as a function of time from an inline corona poled bi-component yarn twisted together with an electrically conductive yarn, as shown in Fig. 5;

Fig. 10 schematically illustrates a fusion welding process to connect fiber cores to auxiliary devices; and

Fig. 1 1 shows a woven fabric with a piezoelectric filament yarn incorporated in the warp direction.

Detailed description

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which a currently preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled addressee. Like reference characters refer to like elements throughout.

Fig. 1 is a cross-sectional view (1 a) and a schematic view (1 b) of a fiber 100 made according to an embodiment of the invention. The fiber 100 comprises a core having a core material 1 10 and a surrounding material 1 1 1 enclosing the core material 1 10. The core material 1 10 comprises an electrically conductive flexible thermoplastic composite comprising at least one polymer and at least one conductive filler. For example, the electrically conductive flexible thermoplastic may comprise 1 , 2, 3, 4, 5, or more polymers and/or 1 , 2, 3, 4, 5, or more conductive fillers. Furthermore, the surrounding material 1 1 1 is a permanently polarizable polymer and is arranged as a sheath material 1 1 1 . The fiber 100 has a longitudinal extension 1 13, or has a longitudinal axis 1 13.

Fig. 2 illustrates a method in the form of a manufacturing process according to an embodiment of the invention. The method is for producing a piezoelectric and pyroelectric fiber 100, using a melt spinning system 200. The melt spinning system 200 consists of a screw extruder 21 1 for a core material 1 10 and one screw extruder 210 for a surrounding material 1 1 1 being a sheath material 1 1 1 . Each extruder feeds one gear pump, 212 and 213 respectively. The gear pumps 212, 213 regulate the amount of material coming in to a spinneret 214. The spinneret is built so that one material will come out in the centre of a hole and the other material will form around the core material 1 10 like a sheath 1 1 1 , producing a bi-component fiber 100 having a core 1 10 and a sheath material 1 1 1 . A control system (not shown) controls both the extruder speed and the gear pump speed to ensure that a set amount of each material is fed to the spinneret 214 appropriately. The spinneret 214 may produce a plurality of fibers 224 simultaneously, forming a multifilament yarn 224. However, the spinneret 214 may also produce only one fiber which is later exposed to in-line poling. Between the spinneret 214 and a take-off roller 215 there is a melt drawing zone where the fibers are formed while solidifying. From the take-off roller 215 the fibers 224 (the filament yarn) continue to a first pair of drawing rollers 216. The rotational speed of the rollers 216 can be varied and the surface temperature of the rollers 216 can be changed to allow for solid state drawing at a specific temperature. Thus, the drawing zone for solid state drawing is between rollers 216 and the second pair of drawing rollers 217. Stretching of the fibers 224, along an elongated direction 1 13 of the fibers, in this drawing zone (between rollers 216 and rollers 217) is achieved by setting a higher rotational speed on rollers 217 than on rollers 216. After drawing, the fibers 224 (filament yarn) continue to rollers 218 and towards an annular poling element 220. A poling process of the fibers for providing the fibers with piezoelectric and pyroelectric properties is integrated in the manufacturing process (thus spinning/extrusion) by feeding the fibers 224 through an annular poling element 220 where a DC corona field is applied. This concept is referred to as in-line poling and the poling process is corona poling. Contrary to contact poling where a conductor is physically connected to the surface of the fiber, in corona poling the electric field is exposed to the fiber without contacting the outer surface of the fiber.

The annular poling element 220 may be constructed from a hollow cylinder with internal protruding needles 222 or spikes 222, each connected to a high DC voltage supply 221 . The voltage may be positive or negative. With the needle elements 222, having a longitudinal extension pointing in the direction towards the fiber(s) 224 in the annular poling element 220, preferable in the radial direction of the annular poling element, an improved corona poling may be achieved. The needle elements 222 provides for the generated corona field to be higher than without the needle elements 222. Thus a stronger corona discharge is enabled. The needle 222 tips are preferably placed at a distance of 5-100 mm from the fibers 224 centre axis. Furthermore, there may optionally be a guiding element 226 made from an electrically isolative material arranged in the annular poling element 220 that maintains the fibers 224 (filament yarn) positioned in the centre of the annular poling element 220. The guiding element 226 may be annular and arranged coaxially with the annular poling element 220. The annular poling element 220 may be made from two halves of a hollow cylinder that can be opened and closed to facilitate threading the fibers (filament yarn) during start up of production. The conductive cores 1 10 of the fibers constitute an opposite electrode and must therefore be connected to ground which is achieved by attaching part of or the whole spinning system to ground 228. In Fig. 2, poling is carried out as a final stage before the fibers continue to the winder 219 where it is accumulated on a bobbin. However, corona poling may be carried out in one of several stages in the melt-spinning process.

Fig. 3 illustrates a similar cross-section as Fig. 1 , but with charges indicating the distribution of charge in the cross-section of a fiber 100 during corona poling. The previously described corona poling method will align the intrinsic dipoles 302 radially. Consequently, the highest piezoelectric effect will be seen when the fiber 100, 224 is deformed homogeneously around its circumference. This is analogues to stretching of the piezoelectric and pyroelectric fiber (or filament yarn) or to apply a hydrostatic pressure to the piezoelectric and pyroelectric fiber (or filament yarn), thus the piezoelectric fibers will be highly sensitive to stretching along the fiber axis along a longitudinal extension 1 13 of the fiber.

Fig. 4 shows an example of measurement results achieved by stretching a filament yarn comprising a plurality of fibers 224 along the longitudinal axis of a fiber. As can be seen in Fig. 4 a substantial piezosignal was achieved by a rather limited stress on the fibers. For example, at 0.14 % strain amplitude an 8.2 V output is generated from the dynamically stretched fibres 100.

After melt spinning including in-line corona poling, an outer electrode may be added to the fibers. Fig. 5A-B (Fig. 5A being a photograph and Fig. 5B a schematic illustration) show an example where the outer electrode 510 consists of an electrically conductive filament yarn 510 that is twisted around the piezoelectric and pyroelectric yarn 51 1 (made from piezoelectric and pyroelectric fibers). The tooling used here for twisting the yarns 510, 51 1 together is known from the textile industry in which twisting is common practice. Characteristic yarn features like stretchability may be achieved by combining the piezoelectric and pyroelectric yarn with conductive yarns and rubber-like yarns like Lycra and Spandex. In further variants of the

piezoelectric and pyroelectric yarn embodiment, an outer electrode in the form of a conductive coating, conductive yarn, fiber or film may be provided in one or more forms, including comingling, braiding, lamination, etc. Figure 6 shows a schematic cross-section of a further yarn 600 where the outer electrode 610 is constituted by an electrically conductive flexible embedding 610 such as a polymer compound with carbon filler, or a conductive compound with an intrinsically conductive polymer as the active component. Such a conductive embedding material 610 may be applied by pressing, dipping, spraying, painting, etc, to embed the fibers having a core material 1 10 and a surrounding material 1 10 (only one fiber is numbered in order to avoid cluttering in the drawing). The fibers 100 have for example been produced as explained above.

Fig. 7 shows a schematic cross-section and a schematic view of a filament yarn 700 according to an embodiment of the invention. In fig.7, a plurality of piezoelectric and pyroelectric fibers 702 is commingled with a plurality of second fibers 704 being electrically conductive. Thereby, a filamanent yarn is formed. In this way, a larger contact surface between an outer electrode constituting the second plurality of fibers and the surrounding material (the outer layer) of the piezoelectric and pyroelectric fibers is advantageously achieved. By commingling is meant that the different types of fibers are substantially randomly collected and aligned in a longitudinal direction. Thus, the illustrated cross-section may only be present at one location along the final filament and as understood may differ at different locations along the longitudinal direction.

Examples

The following examples are only for exemplary purposes of how to carry out the invention and shall not be limiting to the scope of the invention as defined by the appended claims.

Example 1 : In-line poling

An off-the-reel piezoelectric filament yarn was produced in a melt spinning equipment from Extrusion Systems Limited (ESL, Leeds, England) equipped with two single screw extruders, one for the core material, and one for the sheath material, as described with reference to Fig. 2. The core material was a (carbon black, CB) CB/polymer compound where the polymer component was high density polyethylene (HDPE) ASPUN 6835A from Dow (Midland, Ml) with melt flow index (MFI) = 17 g/10 min at 190°C for 2.16 g, density = 950 kg/m³ and melting point 129°C. The CB was Ketjenblack EC-600JD from AkzoNobel (Amersfoort, the Netherlands). HDPE was compounded with 10 wt% CB using a Coperion (Stuttgart, Germany) ZSK 26 K 10.6 twin screw extruder.

Table 1 Fibre production parameters, where MDR= Vi ie^iu and SSDR= V₂n V₂ i6

Flow rate

(cm³ min ^"1 )

Table 2 Settings for in-line poling

Corona Tempe rature setti ng I Average pie zoe lectri c

voltage (kV) Godet rol l (218) ( °C) output vol tage ( V)

-16 50 0.1

-15 70 0.18

-15 100 0.21

-18 100 0.62

-18.5 120 0.45

After solid state drawing the filament yarn passed through a pipe designed for corona poling and were collected for piezoelectric characterization. The surface temperature of the (godet) rollers (218) during solid state drawing and the corona poling voltage were varied according to Table 2. For piezoelectric characterization, silver paint was applied to a length of 25 mm of the yarn to serve as outer electrode. One yarn end was also covered with silver paint to contact the fibers' cores and serve as inner electrode. The yarn was positioned in servo-hydraulic tensile testing machine, Model 66-21 B-01 from MTS systems. The static bias force was 2N. All samples were exposed to a sinusoidal strain with an amplitude value of 0.07 %, corresponding to force amplitude of about 0.05 N. The electrodes were connected to a data acquisition unit sampling the output voltage during deformation. Several samples were tested and their average output voltages are given in Table 2. While no significant temperature dependence could be observed, it was clear that increasing the poling voltage yielded a higher piezoelectric response. Tensile testing of yarns was also carried out in order to verify that the silver- paint did not significantly affect their mechanical properties. Figure 8 shows the results; dashed lines represent a yarn with silver paint and the solid lines represent a yarn without silver paint. Clearly, the application of silver paint as an outer electrode had only a marginal effect on the force-elongation curve of the yarn as measured by tensile testing. The strain in the silver painted portion of the yarn is thus approximately the same as the strain outside this region.

Example 2: An in-line poled yarn twisted together with a conductive yarn as outer electrode

A piezoelectric yarn was produced as described in Example 1 . The yarn was subsequently twisted as described with reference to Fig. 5 together with a conductive carbon suffused polyamide yarn (Shakespeare F9416 M040 produced by Conductive Fibers, LLC, USA). The resistivity of this yarn is about 5000 Ohm per centimeter (/cm). Characterization was in dynamic axial tension. The output voltage produced from the yarn when stretched in a sinusoidal fashion is shown in Fig. 9. Curve 802 indicates the extension of the fiber and curve 804 indicates the corresponding output voltage. The

piezoelectric and pyroelectric fibers cores were interconnected by fusion welding at about 120°C, as schematically shown in Fig. 10. In Fig. 10, a metal wire 901 is used for enabling to connect the cores 1 10 of the piezoelectric and pyroelectric fibers to an external device 904. In order to interconnect the cores 1 10 of the piezoelectric and pyroelectric fibers a conductive filler 902 compatible with the core material is used. By compatible is here meant that the filler material 902 forms a bond to the fiber core 1 10 material upon cooling from the melt. The welding process is characterized by that both the fiber core 1 10 material and the filler material 902 is brought above their melting point fusing together. In this particular example, the filler material was a composite of an ethylene-octane copolymer elastomer (ENGAGE 8401 produced by Dow, USA) with 10 wt-% Carbon Black (Ketjenblack EC-600JD from

AkzoNobel). The melting point of the filler material was about 80°C. The signal was collected between the welded filler material and the outer conductive yarn.

Example 3: A textile with in-line poled yarn and a conductive cladding as outer electrode

The inline poled filament yarn according to Example 1 may be woven into a fabric with the piezoelectric yarn 1020 in the warp direction and cotton fibers 1030 in weft as shown in Fig. 1 1 . Fig. 1 1 shows the fibers ends before and after fusion welding of fibers ends with a filler material in the form of foil (ENGAGE 8401 + 10% CB). Fusion welding was performed at 130°C and by applying a slight pressure between two heated plates 1010, and trimming of superfluous filler material an electrode connecting all the fiber cores in the yarn was obtained . The fabric was coated with a conductive sil icone (ELASTOSIL® LR 3162 A/B from Wacker, Germany) constituting an outer electrode. A clear signal was recorded between the fiber ends and the outer electrode when the textile was stretched in a sinusoidal fashion in the warp direction. In the fibers of the fabric, the sheath material was PVDF, Solef 1006 from Solvay Solexis (Belgium). During fiber spinning, the temperature settings of both extruders were identical: extruder zones 1 , 2, and 3 were set to 190°C, 220°C, and 230°C. The temperature of the gear pumps and spinneret was set to 230°C. The spinneret used had 24 holes with a diameter of 0.6 mm and a land length of 1 .2 mm. The relative rate of metering of polymer to the spinneret controls the relative amounts of core and sheath material in the fiber. The diameter of the produced fibers is controlled by the draw ratio through the entire system. Draw ratios and flow rates are given in Table 1 . As an example, with reference to Fig. 2, a melt draw ratio (MDR) is the filament speed at rollers 216 (V₂16) divided by the exit speed of the molten filaments from the spinneret 214 (V₂i₄). Solid state draw ratio (SSDR) is the yarn speed at rollers 217 (V217) divided by the yarn speed at rollers 216 (V₂16). The speed of the take off roller 215 is the same as the bottom rollers 216, that is V215 =

Additionally, variations to the disclosed embodiments can be

understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

It should be understood that the inventive concept is not limited to the described exemplary embodiments; rather the scope being generally defined by the accompanying claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

Claims

1 . A method for producing a piezoelectric and pyroelectric fiber comprising:

providing a fiber comprising a core material and a surrounding material enclosing said core material, said core material comprises an electrically conductive flexible thermoplastic composite comprising at least one polymer and at least one conductive filler, and said surrounding material is a permanently polarizable polymer;

stretching said fiber along an elongation direction of said fiber with a draw ratio sufficient to introduce beta-crystallites in said fiber;

electrically grounding said core material;

while said core material is grounded, passing said fiber through an annular poling element; and

applying a voltage between said annular poling element and said core material sufficient to establish an electric field for corona poling, whereby said core material is exposed to said electric field electrically charging said surrounding material.

2. The method according to claim 1 , wherein said annular poling element comprises at least one needle element arranged on an inside surface of said annular poling element, wherein said at least one needle element is pointed in an inwards direction towards said fiber, wherein said voltage is applied between said at least one needle element and said core material.

3. The method according to claim 2, wherein said annular poling element is a hollow cylinder comprising said needle elements.

4. The method according to any one of claims 1 to 3, further comprising heating said fiber to an elevated temperature prior to or while being exposed to said corona field.

5. The method according to claim 4, wherein said elevated temperature is in the range of 50°C-150°C.

6. The method according to any one of claims 1 to 5, wherein said step of providing a fiber comprises providing a plurality of fibers, wherein said method further comprises:

guiding said plurality of fibers through said annular poling element through an annular guiding element arranged inside said annular poling element, wherein said guiding element is arranged spaced apart from said annular poling element.

7. The method according to claim 6, wherein said step of guiding comprises guiding said fibers through said annular guiding element being arranged coaxially with said annular poling element.

8. The method according to any of the preceding claims, wherein said surrounding material is PVDF or a vinylidene fluoride based copolymer comprising crystalline beta-form.

9. The method according to any of the preceding claims, wherein said core material is a composite of at least one thermoplastic polymer and at least one carbonaceous filler.

10. A method of producing a yarn comprising:

commingling a plurality of piezoelectric and pyroelectric fibers each produced according to claim 6 with a plurality of electrically conductive fibers different from said piezoelectric and pyroelectric fibers, wherein said electrically conductive fibers electrically connect with said surrounding material of said piezoelectric and pyroelectric fibers, such that an electrode for said plurality of piezoelectric and pyroelectric fibers is formed by said electrically conductive fibers.

1 1 . A method of producing a yarn comprising:

providing a plurality of piezoelectric and pyroelectric fiber according to claim 6; and

embedding said plurality of fibers with a second composite material being electrically conductive, wherein said second composite material forms an outer electrode of said yarn.

12. A fabric comprising a fiber made using the method according to any one of claims 1 to 9, wherein a plurality of piezoelectric and pyroelectric fibers are arranged in parallel in a warp direction and a plurality of a second type of fiber are arranged in parallel in a weft direction, or vice versa.

13. The fabric according to claim 12, wherein said second fibers are electrically conductive and configured to be in electrical contact with said surrounding material of said piezoelectric and pyroelectric fibers, such that said second fibers form an electrode.

14. The fabric according to claim 12, wherein an electrically conductive coating, applied to the fabric and electrically connected with said surrounding material of said piezoelectric and pyroelectric fibers, forms an electrode.

15. The fabric according to any of claims 12 to 14, wherein a contact electrode is arranged to electrically contact the core material of the

piezoelectric and pyroelectric fibers at an outer edge of said fabric essentially parallel with said electrically conductive fibers arranged in parallel as warps or wefts.

16. The fabric according to claim 15, wherein said electrical contact between said contact electrode and said core material of said piezoelectric and pyroelectric fibers is obtained by fusion welding.