Piezoelectric Composite Fiber

Abstract

The present invention relates to multicomponent fibers with a piezoelectric effect, comprising an electroactive fluoropolymer shell bonded to a metal core. The invention also relates to solvent-based fluoropolymer formulations that allow for optimal adhesion to bare conductive metal filaments. The invention also relates to a method for producing these composite fibers, and their use in industrial textiles, various sectors of filtration, and electronics.

Description

[Technical field]

[0001] The present invention relates to the field of composite piezoelectric fibers. More specifically, the present invention relates to multicomponent fibers with piezoelectric effect, comprising an electroactive fluoropolymer shell bonded to a metal core. The present invention also relates to a solvent-based fluoropolymer formulation that allows obtaining optimal adhesion to bare conductive metal filaments. Furthermore, the present invention relates to a method for producing these composite fibers, as well as their use in various sectors of industrial textiles, composites, filtration and electronics. [Background technology]

[0002] Ferroelectric and relaxor ferroelectric materials that generate mechanical action induced by an external electric field have attracted much attention and have found applications in various transducers, actuators and sensors.

[0003] Among piezoelectric materials, ceramics are the most commonly used materials due to their good operating properties and very wide bandwidth, but they have weaknesses that prevent them from being applied to curved or complex surfaces.

[0004] Other conductive devices use a polymer film sandwiched between two electrodes. Among the polymers that can be used, fluoropolymers, especially those based on vinylidene fluoride (VDF), represent a class of compounds with outstanding properties for many applications. Polyvinylidene fluoride (PVDF) and copolymers including VDF and trifluoroethylene (TrFE) are of particular interest due to their piezoelectric properties.

[0005] These flexible piezoelectric structures are only commercially available in the form of films, but some applications require the availability of polymeric piezoelectric fibers, which can be directly embedded into certain materials to form "smart" materials.

[0006] Such fibers are produced in the laboratory in monocomponent or multicomponent form.

[0007] Multicomponent spinning of conductive and piezoelectrically active polymers simultaneously allows the production of 100% polymeric multilayer fibers. Such fibers are for example described by the applicant in patent application WO2018 / 224583 for piezoelectric polymer fibers consisting of three different polymer layers: a layer consisting of at least one fluoropolymer, a layer consisting of at least one polyolefin and a layer consisting of a polyamide.

[0008] In general, conductive polymers used as electrodes must meet the following criteria: they can be used by extrusion, they are sufficiently fluid to be used in the spinning process, they have a spinning temperature close to that of the component piezoelectric material of the fiber, and they are sufficiently conductive to ensure efficient recovery of charge generated during biasing of the fiber and deformation of the piezoelectric material.

[0009] To meet all these criteria, the use of conductive polymer compounds (abbreviated as CPC) is generally advantageous. However, polymers that are inherently conductive, such as polyaniline, a mixture of PEDOT:PSS (PEDOT stands for poly(3,4-ethylenedioxythiophene) and PSS stands for poly(styrenesulfonate)), or polypyrrole, cannot be used in the melt route. Therefore, those skilled in the art manufacture CPCs using spinning grade polymers such as polyamide (PA), polyethylene (PE), poly(ethylene oxide) (PEO) or poly(ethylene terephthalate) (PET), and combine them with conductive fillers such as carbon black, carbon nanotubes, graphene, silver or metal nanowires, gold nanoparticles, etc.

[0010] The electrical conductivity of CPC granules is generally acceptable (e.g., up to 1000 S / m), but this value may be reduced by up to 10 3 1 / 10 or in some cases 6This phenomenon is mainly due to the alignment of the conductive fibers during the spinning process, which induces a decrease in the electrical permeability of these fibers and thus a decrease in the overall conductivity of the CPC filaments.

[0011] To solve this problem, the use of a metal core (for example made of stainless steel) coated with a piezoelectrically active polymer provided by extrusion (this technique is known as melt coating) has been proposed. This technique has proven difficult to implement in practice; it requires a piezoelectrically active polymer with a low melt viscosity, leading to considerable coating thicknesses of more than 100 μm. Another problem lies in the centralization of the metal core in the polymer coat, all the more difficult to control at smaller desired thicknesses.

[0012] For piezoelectric fibers, a polymer layer thickness of less than 50 μm is desired to overcome the viscoelastic behavior of PVDF-type polymers and thereby obtain a linear piezoelectric response in deformation. Also, excessive polymer thickness can be problematic as it requires the application of high bias voltages, which can cause air breakdown or require sophisticated equipment.

[0013] The use of solvent-based coating methods makes it possible to overcome these viscosity problems and obtain fluoropolymer layers with smaller thicknesses, typically between 1 and 30 μm. The main challenge of this technique is to produce an adherent polymer layer at the surface of the metal line, especially when using fluoropolymers.

[0014] The publication "Polymeric piezoelectric fiber with metal core produced by electrowetting-aided dry spinning method" by Liu, W., Chen, R., Ruan, X., and Fu, X., J. Appl. Polym. Sci. 133, n / an / a (2016), describes the electrowetting-assisted solvent-based coating of PVDF or P(VDF-TrFE) copolymers on a metal core. A 100 μm diameter copper wire enameled with polyester-imide (6 μm thick) is used as the central core. PVDF (Mw=5340000) and P(VDF-TrFE) (VDF / TrFE mass ratio: 70 / 30) copolymers in powder form are dissolved in a 1 / 1 dimethylformamide / acetone mixture at concentrations of 10, 15, and 20 wt.%. The enamelled wire passes through the polymer solution, through a copper needle with a diameter of 1 mm, and then through a series of concentric electrodes with diameters of 3.5 mm to 0.5 mm. The coating nozzle is biased with 1 kV. Its stripped copper core is grounded and the electrodes are connected to a continuous high voltage of 4 kV. The applied electric field is less than 20 MV / m. Polymer crystallization occurs due to evaporation of the solvent. The resulting fibre is metallized with gold by cathodic sputtering. The authors show that the use of the electrowetting method makes it possible to obtain, on copper, a more uniform, smoother and more adhesive polymer layer than without the use of high voltage. Layers with 15% P(VDF-TrFE) are more uniform than PVDF layers. The application of an electric field results in a beta phase. With most of the solutions used, a coat thickness between 3 and 8 μm is obtained. The use of high voltages improves the thickness uniformity. Nevertheless, the method remains complex to use and involves high voltages during production. Furthermore, no evidence of adhesion of the fluoropolymer to the enamelled wire, such as microscopic images of cross sections of the resulting filaments, is provided.

[0015] Thus, there is a need to develop a fluoropolymer-based coating formulation that makes it possible to obtain optimal adhesion to bare conductive metal filaments by a simple solvent-based method for continuous coating of the filaments with piezoelectric polymers without prior enameling or metal coating.The present invention is also directed to providing a multicomponent fiber with a piezoelectric effect, comprising an electroactive fluoropolymer shell that adheres to a metal core. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] International Publication No. 2018 / 224583 [Non-patent literature]

[0017] [Non-Patent Document 1] Liu, W., Chen, R., Ruan, X., and Fu, X., "Polymeric piezoelectric fiber with metal core produced by electrowetting-aided dry spinning method," J. Appl. Polym. Sci. 133, n / an / a (2016) Summary of the Invention

[0018] The present invention relates first to a polymer formulation for coating a metal core, comprising the following components: a) a 10-30% by weight solution of P(VDF-TrFE) copolymer in a volatile solvent; b) surfactants, and c) Adhesion promoters.

[0019] According to one embodiment, the P(VDF-TrFE) copolymer comprises units derived from VDF and units derived from TrFE, and the proportion of the units derived from TrFE is in the range of 5 mol % to 95 mol % based on the sum of the units derived from VDF and the units derived from TrFE.

[0020] According to one embodiment, the volatile solvent is selected from the following list: ketones, in particular methyl isobutyl ketone, methyl ethyl ketone (MEK), cyclopentanone and acetone, esters, in particular ethyl acetate, methyl acetate, propyl acetate, butyl acetate and propylene glycol methyl ether acetate, amides, such as dimethylformamide and dimethylacetamide, - dimethyl sulfoxide, - furans, especially tetrahydrofuran, - carbonates, in particular dimethyl carbonate, phosphates, in particular triethyl phosphate, - and mixtures thereof is selected from.

[0021] The surfactant ensures optimal wetting of the solution of fluorocopolymer on the metal core. According to one embodiment, said surfactant is of non-ionic type and is preferably based on an acrylic copolymer.

[0022] The adhesion promoter contributes to improving the wetting of the solution of the fluorocopolymer on the metal core and also makes it possible to obtain an adherent coating of the copolymer on the filament, which is a polymer functionalized with groups such as -NH2, -OH, -COOH or other associative groups.

[0023] According to one embodiment, the adhesion promoter comprises a functional copolymer of hydroxyl type with acid groups or a binder of ethoxysilane type.

[0024] According to one embodiment, the viscosity of the coating formulation is at a shear rate of 10 s -1 The viscosity of the coating solution is preferably between 10 and 100 Pa.s. Finally, solutions that exhibit shear thickening behavior are advantageous.

[0025] The present invention also relates to a solvent-based method for the continuous manufacture of composite piezoelectric fibers comprising a metal filament coated with a layer of a piezoelectric fluoropolymer, the method comprising the following steps: i) providing a bare metal wire; ii) coating said metal wire with a coating formulation according to the invention to obtain a coated metal wire, said step comprising passing said metal wire through a coating nozzle through which said coating formulation is injected, iii) drying the coated metal wire to evaporate the solvent; iv) heating the dried, coated metal film to achieve thermal annealing. The method includes:

[0026] The method also includes a step v) of winding the coated metal wire, which may be performed either before or after the thermal annealing step.

[0027] The metal wire acts as the internal electrode. Various uses of this composite piezoelectric wire require the deposition of an external electrode onto the polymer coating.

[0028] According to one embodiment, the method according to the invention comprises an additional step vi) of successively covering the coated metal wire with a counterelectrode. This operation for depositing the external electrodes can advantageously be carried out inline, following the previously described drying and annealing steps, before the winding step.

[0029] The method according to the invention also comprises a step vii) of biasing the piezoelectric polymer deposited on the surface of the metal core. This biasing can be carried out in several ways, either before or after the deposition of the external electrodes.

[0030] The invention also relates to a composite piezoelectric fiber comprising an electroactive fluoropolymer shell bonded to a metal core. Advantageously, the fiber according to the invention has an elastic deformation between 1% and 5%.

[0031] Another subject of the invention relates to the use of composite piezoelectric fibers in a variety of applications such as device composites, industrial fabric manufacture, textile manufacturing, fluid filtration, and in electronics.

[0032] The present invention makes it possible to overcome the drawbacks of the prior art. In particular, it provides a fluoropolymer-based coating formulation that makes it possible to obtain optimal adhesion on bare conductive metal filaments without the need for a prior enameling or metallization for the connection. This coating formulation is used by a simple solvent-based method for the continuous production of filament coatings with piezoelectric polymers. This method is easily industrializable and does not require modification of the properties for wetting (electrowetting) of the surface of the bare metal wire by application of a voltage. [Brief description of the drawings]

[0033] [Figure 1] Scanning electron microscope images on a copper filament with a diameter of 700 μm coated with a layer of a formulation based on 20% P(VDF-TrFE) copolymer in MEK. Images b and d relate to a fiber according to the invention, the coating layer of which comprises a surfactant (1%) and an adhesion promoter (0.25%). Images a and c relate to a comparative fiber, the coating layer of which is devoid of surfactant and adhesion promoter. Image 1c is an enlargement of image 1a. Image 1d is an enlargement of image 1b. [Diagram 2] Six bias curves obtained by gradually increasing the electric field at the end of the piezoelectric fiber are shown. The ferroelectric effect is clearly demonstrated by the hysteresis phenomenon and the increase in the zero-field permanent bias. The curves were obtained by varying the electric field at a frequency of 1 Hz. [Diagram 3] The bias curves, expressed in μC / cm2, obtained for maximum field values ​​of +86 MV / m and 86 MV / m, scanned at 1 Hz, are shown. The figure also shows the curve of the current (in μA) obtained during this bias. The current peak corresponds to the value of the coercivity, which is the field required for the alignment of the electric dipoles, further proof of the obtained ferroelectric and piezoelectric behavior. [Figure 4]1 is a scheme showing one-step DC biasing with a coating of a piezoelectric active polymer and a counter electrode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] The invention will now be explained in more detail and in a non-limiting manner in the following description.

[0035] According to a first aspect, the present invention relates to a coating formulation comprising the following components: a) 10-30% P(VDF-TrFE) copolymer solution in a volatile solvent; b) surfactants, and c) Adhesion promoters.

[0036] According to various embodiments, the formulation comprises the following features, combined where appropriate:

[0037] <Fluorocopolymer> The fluorocopolymer of formula P(VDF-TrFE) consists of units derived from vinylidene fluoride and units derived from trifluoroethylene. The proportion of units derived from TrFE is preferably 5 mol% to 95 mol%, in particular 5 mol% to 10 mol%, or 10-15 mol%, or 15-20 mol%, or 20-25 mol%, or 25-30 mol%, or 30-35 mol%, or 35-40 mol%, or 40-45 mol%, or 45-50 mol%, or 50-55 mol%, or 55-60 mol%, 10 or 60-65 mol%, or 65-70 mol%, or 70-75 mol%, or 75-80 mol%, or 80-85 mol%, or 85-90 mol%, or 90-95 mol%, based on the sum of units derived from VDF and units derived from TrFE.

[0038] A range of 15 to 55 mol % is particularly preferred.

[0039] The molar composition of said units in the P(VDF-TrFE) copolymer can be determined by various means such as infrared or RAMAN spectroscopy. Conventional methods of elemental analysis of the carbon, fluorine and chlorine or bromine or iodine elements such as X-ray fluorescence spectroscopy make it possible to calculate the mass composition of the polymer from which the molar composition is estimated. By analyzing solutions of the polymer in suitable deuterated solvents, it is possible to determine the presence of polynuclear, especially proton ( 1 H) and fluorine ( 19 F) NMR techniques can also be used.

[0040] The P(VDF-TrFE) copolymer is preferably random and linear. It may be homogeneous or heterogeneous. A homogeneous polymer has a uniform chain structure, with a random distribution of units derived from the various monomers that varies little between chains. In heterogeneous polymers, the chains have a multimodal or spread type distribution of units derived from the various monomers. Thus, heterogeneous polymers contain chains rich in one unit and chains poor in this unit.

[0041] The Curie temperature of the copolymer is between 50 and 145° C. The Curie temperature of the polymer of the present invention can be measured by differential scanning calorimetry or dielectric spectroscopy.

[0042] The copolymers of the present invention can be prepared by using any known method such as emulsion polymerization, microemulsion polymerization, suspension polymerization, and solution polymerization.

[0043] According to one embodiment, the weight-average molar mass, also called "molecular weight" (Mw) in the context of the present patent application, of the copolymer ranges from 200000 g / mol to 1500000 g / mol, preferably from 250000 g / mol to 1000000 g / mol and more particularly from 300000 g / mol to 700000 g / mol.

[0044] The latter can be adjusted by modifying certain parameters of the process, such as the temperature in the reactor, or by adding a transport agent.

[0045] The molecular weight distribution can be estimated by SEC (size exclusion chromatography) using a series of three columns of increasing porosity with dimethylformamide (DMF) as eluent. The stationary phase is a styrene-DVB gel. The detection method is based on refractive index measurements and calibration is performed with polystyrene standards. The sample is dissolved in DMF at 0.5 g / l and filtered through a 0.45 μm nylon filter.

[0046] The molecular weight can also be evaluated by measuring the melt flow index at 230° C. under a load of 5 kg according to ASTM D1238 (ISO 1133).

[0047] The molecular weight can also be characterized by measuring the viscosity in solution according to standard ISO 1628. Methyl ethyl ketone (MEK) is the preferred solvent for determining the viscosity index.

[0048] In the coating formulation, the P(VDF-TrFE) copolymer is dissolved in a volatile solvent at 10-30% by mass.

[0049] According to one embodiment, the volatile solvent is selected from the following list: ketones, in particular methyl isobutyl ketone, methyl ethyl ketone (MEK), cyclopentanone and acetone, esters, in particular ethyl acetate, methyl acetate, propyl acetate, butyl acetate and propylene glycol methyl ether acetate, amides, such as dimethylformamide and dimethylacetamide, - dimethyl sulfoxide, - furans, especially tetrahydrofuran, - carbonates, in particular dimethyl carbonate, phosphates, in particular triethyl phosphate, - and mixtures thereof is selected from.

[0050] According to one embodiment, the volatile solvent is selected from methyl ethyl ketone, cyclopentanone, acetone, ethyl acetate.

[0051] <Surfactant> The surfactant ensures optimal wetting of the solution of the fluorocopolymer on the metal core. According to one embodiment, said surfactant is of the non-ionic type.

[0052] According to one embodiment, the surfactant is based on an acrylic copolymer (e.g., BYK 3440) or a high molecular weight block acrylic copolymer (e.g., Disperbyk 163).

[0053] The surfactant content ranges from 0.05 to 1% by weight, preferably from 0.1 to 0.5% by weight, limits included, based on the weight of the coating formulation.

[0054] <Adhesion promoter> The adhesion promoter contributes to improving the wetting of the solution of the fluorocopolymer on the metal core and also makes it possible to obtain an adherent coating of the copolymer on the filament, which is a polymer functionalized with groups such as -NH2, -OH, -COOH or other associative groups.

[0055] According to one embodiment, the adhesion promoter comprises a functional copolymer of hydroxyl type having acid groups (e.g. BYK 4510) or a binder of ethoxysilane type, e.g. 3-aminopropyltriethoxysilane (Dynasylan AMEO).

[0056] The mass content of the adhesion promoter in the coating formulation is between 0.05 and 3%, preferably between 0.2 and 0.5%, inclusive, relative to the weight of the coating formulation.

[0057] According to one embodiment, the viscosity of the coating formulation is at a shear rate of 10 s -1The viscosity is between 1 and 500 Pa.s, preferably between 10 and 100 Pa.s. It is advantageous to use coating solutions that exhibit shear-thickening behavior. The viscosity is measured at an ambient temperature of 20-25° C. using a Brookfield viscometer according to standard ASTM D2983.

[0058] According to one embodiment, the coating formulation according to the invention consists of these three components a, b and c.

[0059] The present invention also relates to a solvent-based method for the continuous manufacture of composite piezoelectric fibers comprising a metal filament coated with a layer of a piezoelectric fluoropolymer, the method comprising the following steps: i) providing a bare metal wire; ii) coating said metal wire with a coating formulation according to the invention to obtain a coated metal wire, said step comprising passing said metal wire through a coating nozzle through which said coating formulation is injected, iii) drying the coated metal wire to evaporate the solvent; iv) heating the dried, coated metal film to achieve thermal annealing. The method includes:

[0060] The method begins by unwinding a bare metal wire. The metal preferably has a conductivity greater than 0.5 MS / m and is selected from the following list: copper, platinum, stainless steel, molybdenum. The metal wire has a diameter ranging from 50 to 900 μm, preferably 80 to 300 μm, a Young's modulus greater than 1 GPa, and an elastic elongation between 1 and 10%.

[0061] Advantageously, the metal wire is degreased in a bath using an alkaline solution, an acidic solution, a cleaning liquid or an alcohol. As mentioned above, the degreased wire is subsequently dried and then passed through a coating nozzle where it is covered with a thin layer of the coating formulation according to the invention.

[0062] Preferentially, the method is performed vertically to allow optimal centering of the coating formulation on the filament. The injection speed is adapted depending on the diameter of the filament to be coated, the diameter of the hollow tube, the exit diameter of the coating nozzle, the viscosity of the injected solution and the throughput speed of the filament.

[0063] Several successive passes of the coating head make it possible to control the thickness of the coating formulation relative to the surface of the filaments.

[0064] The thickness of the piezoelectric polymer obtained by the solvent-based coating method described herein is between 5 and 100 μm, preferably between 10 and 30 μm.

[0065] The viscosity of the coating solution that allows this thickness to be obtained is 10s -1 The viscosity of the coating solution is ideally between 1 and 500 Pa.s for a shear rate of 100 Pa.s. The use of coating solutions with viscosities between 10 and 100 Pa.s, or that exhibit shear thickening behavior, is advantageous to obtain a centered, controlled thickness adherent polymer coat on the metal core.

[0066] By adapting the rheology of the coating formulation solution it is possible to optimize the thickness of the polymer layer on the metal filament and the uniformity of this layer, preventing the formation of pores, the thickness of which ranges from 5 to 100 μm, preferably between 10 and 30 μm.

[0067] <Drying step> The drying method consists of passing the coated filament through a tunnel of blown hot air at a temperature between 50 and 150°C, allowing the solvent used in the coating to evaporate.

[0068] <Annealing step> Once the metal wires are coated with the piezoelectric polymer layer, a thermal annealing step makes it possible to homogenize the structure and promote adhesion and piezoelectric behavior properties, which is preferably carried out at temperatures between 80 and 200°C for 10 minutes, and advantageously between 130 and 180°C for 5 minutes.

[0069] This annealing can also be carried out in-line during a one-step procedure after drying the polymer layer by evaporating the solvent and before the winding step.

[0070] The method also includes a step v) of winding the coated metal wire, which may be performed either before or after the thermal annealing step.

[0071] In a piezoelectric composite fiber comprising a metal wire and a fluoropolymer layer, the metal wire acts as the internal electrode. Various uses of this composite piezoelectric fiber require the deposition of an external electrode onto the polymer coating.

[0072] According to one embodiment, the method according to the invention comprises an additional step vi) of successively covering the coated metal wire with a counter electrode.

[0073] <External electrode deposition> Filaments of metal and P(VDF-TrFE) copolymer prepared as above can be fabricated by the following route: - solvent-based coating in a bath (dipping) or coating nozzle, for example with a solution of PEDOT:PSS (PEDOT stands for poly(3,4-ethylenedioxythiophene) and PSS stands for poly(styrenesulfonate)), or - liquid-based coating with conductive inks of silver lacquer or carbon varnish type (carbon black or nano-carbon black) by immersion or in a coating nozzle, or - Covering of the melt path with CPC polymer, where there is no orientation or alignment of the fillers and the electrical conductivity is greater than 100 S / m, or - Metallization by evaporation of gold or other conductive metals, or - wrapping a conductive metal wire around the piezoelectric active coating, or - Braiding metal wires around a piezoelectric active coating (cords, cables, etc.) The electrode can be covered continuously with a counter electrode according to one of the methods described above.

[0074] A particularly advantageous method for obtaining conductive external electrodes deposited successively on the piezoelectric polymer consists in using a film-forming aqueous PEDOT:PSS solution, obtained for example by passing the wire to be coated through the mentioned conductive polymer solution and then evaporating said solution to dryness before winding.

[0075] This operation of depositing the external electrodes can be advantageously carried out in-line following the drying and annealing steps mentioned above, prior to the winding step.

[0076] The method according to the invention also includes a step vii) of biasing the piezoelectric polymer deposited on the surface of the metal core.

[0077] Once the piezoelectric polymer is deposited on the surface of the metal core, the fluorinated material needs to be biased under an electric field to obtain a macroscopic piezoelectric response from the device.

[0078] This biasing can be accomplished in a number of ways, either before or after deposition of the external electrodes.

[0079] <DC bias without counter electrodes> This operation consists of applying an electric field between the surface of the adhered polymer layer and the central metallic core. Two methods are routinely employed by those skilled in the art to accomplish this: direct contact bias and non-contact bias.

[0080] According to one embodiment, the DC bias involves direct contact and includes the following steps: - connecting the bare conductive wire to zero potential (earth terminal - earth) either by earthing the metal wire reel or by a contact roller system which is itself earthed; - coating the wire with a piezoelectrically active polymer by coating the wire with a polymer solution followed by evaporation of the solvent as previously described; - passing the coated wire over a system of conductive rollers maintained at a high potential so that the electric field at the ends of the polymer metal coating is between 20 and 200 MV / m; Includes.

[0081] According to another embodiment, the DC bias is non-contact and uses the corona effect. Efficient biasing of the fluoropolymers of the present invention is achieved at high field values ​​of the order of 100 V / μm. However, the maximum field that can be applied in air before breakdown is about 3 V / μm. Above this value, discharges are observed due to ionization of the air between two electrodes subjected to a high potential difference, generating atmospheric plasma and generating ozone. This is the mechanism used in the corona bias method.

[0082] An apparatus for DC biasing of fibers is described in WO2014 / 161920, in which a corona tunnel serves as the external electrode. This corona tunnel consists of a number of conductive needles subjected to a high potential. The conductive core of the piezoelectric fiber is itself grounded. The distance between the needle electrodes and the fluorinated surface of the fiber is kept constant and an electric field of more than 3 V / μm is applied between the two electrodes of the system. An electric discharge is obtained due to ionization of the air between the needles and the fluorinated surface. The fluorinated surface then becomes highly charged under the effect of this discharge, which contributes towards locally biasing the polymer to an electric field significantly exceeding 3 V / μm.

[0083] According to one embodiment, an electric field / plasma diffuser is inserted between the corona needle and the surface to be biased, which is a conductive metal grid. The grid acts as a diffuser of the excessively localized electric field generated by the corona needle. This acts as a bias current source, making it possible to obtain a more uniform bias on the surface facing the electrode.

[0084] <DC bias using counter electrodes> The counter electrode was deposited in DC mode as described above.

[0085] According to one embodiment, the in-line biasing process comprises the following steps: - earthing the metal wire reel or connecting the bare conductive wire to zero potential (earth terminal - earth) by means of a contact roller system that is itself earthed; - coating the wire with a piezoelectrically active polymer by coating the wire with a polymer solution and then evaporating the solvent as previously described; - depositing a counter electrode, and - passing the coated wire over a system of conductive rollers maintained at a high potential so that the electric field at the ends of the polymer metal coating is between 20 and 200 MV / m; Includes.

[0086] According to another embodiment, the biasing is performed in one last step of the in-line deposition of the counter electrode.

[0087] This embodiment is illustrated in the scheme of FIG.

[0088] The invention also relates to a composite piezoelectric fiber comprising an electroactive fluoropolymer shell bonded to a metal core. Advantageously, the fiber according to the invention has an elastic deformation between 1% and 5%. The fluoropolymer layer is biased and can be coated with an external electrode made of metal or conductive polymer.

[0089] Another subject of the invention relates to the use of composite piezoelectric fibers in various applications such as industrial fabric manufacture, fluid filtration, and in electronics.

[0090] Thus, a fiber of the invention, made of fluoropolymer and having a conductive core, constituting a first electrode, and a conductive outer metal coating constituting a second electrode, can constitute an actuator. By applying an electric field between these electrodes, it is possible to modify the mechanical properties of the fiber. When this fiber is incorporated into a textile structure, the application of this electric field makes it possible to modify the mechanical properties of this textile structure.

[0091] A subject of the invention is also a piezoelectric device comprising a piezoelectric fibre according to the invention.

[0092] The present invention is also directed to a textile material comprising the described piezoelectric fibers. EXAMPLES

[0093] The following examples illustrate the invention without limiting it.

[0094] [Example 1. Production of fiber shown in SEM image in Figure 1] A concentrated solution containing 20 wt % VDF-TrFE (80 / 20) copolymer in MEK was obtained by dissolving without heating, to which 1% Disperbyk 3440 and 0.2% DynaSylan Ameo adhesion promoter (triethoxysilane) were added without heating.

[0095] This solution is used for DC coating of a 700 μm diameter copper filament, followed by drying at 80° C. and then in-line annealing at 135° C. in two separate ovens. Finally, the fiber is wound.

[0096] The optimized formulation and solvent-based coating method make it possible to obtain bare copper filaments adhesively coated with the copolymer without the use of insulating enamelling or wetting methods by applying an electric field.

[0097] Example 2: Fabrication of a piezoelectric fiber device with a metal core A piezoelectric fiber is manufactured according to the invention, consisting of a copper filament with a diameter of 200 μm constituting the internal electrode, a layer of 20 μm of P(VDF-TrFE) copolymer containing 20 mol % TrFE attached to said filament, and a layer of conductive carbon varnish manually deposited on the surface of the fluoropolymer constituting the external electrode.

[0098] The device is electrically connected and electrical contact is provided by a compatible high voltage pin or a flat alley clip. One pin is connected to the metal core and grounded, and the other pin is placed in contact with an external electrode receiving a variable potential.

[0099] The area facing the two electrodes is expressed by the following formula: S = πφL where f is the diameter of the metal core and L is the distance between the electrodes.

[0100] When L=5cm, this is S=π×200×10 -6 x5x10 -2 =31.42mm 2 Gives.

[0101] The device is then biased "off-line" using application of an increasing sinusoidal voltage of 1 Hz frequency up to 100 MV / m, i.e. up to 2000 V for a sample of thickness 20 μm. During biasing, the electric field and current values ​​are recorded.

[0102] The ferroelectric effect is clearly demonstrated by the hysteresis phenomenon and the increase in the zero-field permanent bias (see Fig. 2). The measured current peak corresponds to the value of the coercive field, which is the electric field required for the alignment of the electric dipoles, indicating that ferroelectric and piezoelectric behavior is obtained (Fig. 3).

[0103] These devices may also be subjected to cyclic mechanical stresses (DMA, traction, bending, etc.).

Claims

1. A polymer formulation for coating metal wires, comprising the following components: a) a 10-30% by weight solution of a P(VDF-TrFE) copolymer in a volatile solvent; b) a surfactant, and c) Adhesion promoter The viscosity of the blend is at a shear rate of 10 s －１ between 1 and 500 Pa.s, the surfactant is based on an acrylic copolymer or on a high molecular weight block copolymer and its content ranges from 0.05 to 1% by weight relative to the weight of the formulation, compound.

2. 2. The formulation of claim 1, wherein the P(VDF-TrFE) copolymer consists of units derived from VDF and units derived from TrFE, the proportion of the units derived from TrFE being in the range of 5 mol % to 95 mol % relative to the sum of the units derived from VDF and the units derived from TrFE.

3. The volatile solvent is selected from the following list: - a ketone selected from the group consisting of methyl isobutyl ketone, methyl ethyl ketone (MEK), cyclopentanone and acetone, esters selected from the group consisting of ethyl acetate, methyl acetate, propyl acetate, butyl acetate and propylene glycol methyl ether acetate, amides selected from the group consisting of dimethylformamide and dimethylacetamide, - dimethyl sulfoxide, - furan, which is tetrahydrofuran; - a carbonate which is dimethyl carbonate, a phosphate which is triethyl phosphate, - and mixtures thereof The formulation according to claim 1 or 2, wherein the formulation is selected from the group consisting of

4. A formulation according to any one of claims 1 to 3, wherein the surfactant content ranges from 0.1 to 0.5% by weight of the formulation.

5. 5. The formulation according to claim 1, wherein the adhesion promoter comprises a functional copolymer of hydroxyl type with acid groups or a binder of ethoxysilane type, the mass content of which is between 0.05 and 3% relative to the weight of the formulation.

6. A formulation according to any one of claims 1 to 5, consisting of components a), b) and c).

7. A volatile solvent-based method for the continuous manufacture of composite piezoelectric fibers comprising a metal wire coated with a layer of a piezoelectric fluoropolymer, the method comprising the steps of: i) providing a bare metal wire; ii) coating said metal wire with the formulation according to any one of claims 1 to 6 to obtain a coated metal wire, said step comprising passing said metal wire through a coating nozzle through which said formulation is injected, iii) drying the coated metal wire to evaporate the solvent; iv) A thermal annealing step in which the dried, coated metal wire is heated. The method includes:

8. The method of claim 7, wherein the bare metal wire is selected from the list of copper, platinum, stainless steel, molybdenum and has a diameter in the range of 50 to 900 μm.

9. 9. The method according to claim 7 or 8, wherein the thickness of the compound on the surface of the metal wire is adjusted by successive passes through a coating head, the thickness being between 5 and 100 μm.

10. The method according to any one of claims 7 to 9, comprising a step v) of winding the coated metal wire, which may be performed either before or after the thermal annealing step.

11. The method according to any one of claims 7 to 10, comprising a step vi) of successively covering the coated metal wire with an external electrode.

12. The formation of the external electrodes is performed by the following route: - solvent-based coating with conductive polymer solutions or in coating nozzles, - liquid-based coating with conductive inks of the silver lacquer or carbon varnish type by immersion or in a coating nozzle; a coating with a conductive polymer compound applied by extrusion, - metallization by evaporation of gold or other conductive metals, - winding a conductive metal wire around the composite piezoelectric fibre; - Braiding said metal wire around said composite piezoelectric fibres. The method of claim 11, wherein the method is carried out according to one of the following:

13. The method according to any one of claims 7 to 12, comprising a step vii) of DC biasing the layer of piezoelectric fluoropolymer deposited on the surface of the metal wire, this step being carried out either before or after the deposition of the external electrodes.

14. DC biasing without external electrodes involves direct contact and involves the following steps: - connecting the bare metal wire to zero potential (earth terminal - earth) either by earthing the wire reel or by a contact roller system which is itself earthed; - depositing a layer of piezoelectric fluoropolymer; and - passing the coated metal wire over a system of conductive rollers, which are themselves held at a high potential; The method of claim 13, comprising:

15. The DC bias is non-contact without using an external electrode, and includes the following steps: - creating a corona tunnel consisting of a conductive needle subjected to a high electric potential; - grounding the metal wire of the composite piezoelectric fiber; - maintaining a constant distance between the conductive needle and the layer of piezoelectric fluoropolymer of the composite piezoelectric fiber; - applying an electric field of more than 3 V / μm between the two electrodes of the system, leading to the formation of an electric discharge by ionization of the air between the conductive needle and the charged fluorinated surface; The method of claim 13, comprising:

16. The method of claim 13 , further comprising applying a DC bias after deposition of the outer electrodes.

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