EP2376279A2

EP2376279A2 - Ferroelectret double and multilayer composite and method for production thereof

Info

Publication number: EP2376279A2
Application number: EP09764187A
Authority: EP
Inventors: Werner Jenninger; Joachim Wagner; Günther WALZE; Dirk Schapeler; Heinz Pudleiner; Gunther Stollwerck; Reimund Gerhard; Werner Wirges; Ruy Alberto Altafim
Original assignee: Bayer MaterialScience AG
Current assignee: Bayer Intellectual Property GmbH
Priority date: 2008-12-13
Filing date: 2009-11-28
Publication date: 2011-10-19
Also published as: KR20110095943A; WO2010066348A3; TW201033008A; CN102317066B; WO2010066348A2; EP2286988A1; US20110309716A1; KR101515261B1; CA2746482A1; CN102317066A

Abstract

The invention relates to a method for producing double or multilayer ferroelectret with defined cavities by: structuring at least one first surface of a first polymer film (1) forming a height profile, applying at least one second polymer film (5, 1') to the structured surface on the first polymer film formed in step a), joining the polymer films (1, 1', 5) to give a polymer film composite with the formation of cavities (4, 4') and electrically charging of the inner surfaces of the cavities (4, 4') formed in step c) with opposing electrical charges. The invention further relates to ferroelectret multilayer composites, optionally produced by said method, comprising at least two polymer films, arranged one over the other and connected to each other, wherein cavities are formed between the polymer films. A piezoelectric element comprising a said ferroelectret multilayer composite is also disclosed.

Description

Ferroelectret two- and multi-layer composite and process for its preparation

The present invention relates to processes for the production of two- and multi-layer ferroelectrets with defined cavities and ferroelectret multi-layer composites prepared by these processes.

Because of their advantageous and specifically adjustable properties, such as low weight, thermal conductivity, mechanical deformability, electrical properties and barrier properties, polymers and polymer composites are used in a variety of commercial applications. They are used for example as packaging material for food or other goods, as construction or insulation materials, for example in construction or in vehicle construction. Functional polymers are increasingly gaining in importance as active components in sensor or actuator applications. An important application concept relates to the use of the polymers as electromechanical or piezoelectric transducers. Piezoelectric materials are capable of linearly converting a mechanical pressure into an electrical voltage signal. Conversely, an electric field applied to the piezoelectric material can be transformed into a change in transducer geometry. Piezoelectric materials are already being integrated as active components in a variety of applications. These include, for example, structured pressure sensors for keyboards or touchpads, acceleration sensors, microphones, loudspeakers, ultrasonic transducers for applications in medical technology, marine technology or materials testing. In WO 2006/053528 Al, for example, an electroacoustic transducer based on a piezoelectric element of polymer films is described.

In recent years, a new class of piezoelectric polymers, the so-called ferroelectrets, is increasingly in the interest of research. The ferroelectrets are also called piezoelectrets. Ferroelectrics are made of polymeric materials with a cavity structure that can store electrical charges over long periods of time. The previously known ferroelectrets have a cellular cavity structure and are formed either as foamed polymer films or as multilayer systems of polymer films or polymer fabrics. If electric charges are distributed on the different surfaces of the cavities according to their polarity, each charged cavity represents an electric dipole. If the cavities are now deformed, this causes a change in the dipole size and leads to a current flow between external electrodes. The ferroelectrets can exhibit a piezoelectric activity comparable to that of other piezoelectrics. No. 4,654,546 describes a process for producing polypropylene foam films as a precursor to a ferroelectret film. In this case, the polymer films are mixed with filler particles. The filler used is, for example, titanium dioxide. The polypropylene films are biaxially stretched after extrusion so that small voids are formed in the film around the filler particles. This method has since been applied to other polymers. For example, M. Wegener, M. Paajanen, O. Voronina, R. Schulze, W. Wirges, and R. Gerhard-Multhaupt "Voided cyclo-olefin polymer films: Ferroelectrets with high thermal stability", Proceedings, 12th International Symposium on Electrets (IEEE Service Center, Piscataway, New Jersey, USA 2005), 47-50 (2005) and Eetta Saarimäki, Mika Paajanen, Ann-Mari Savijärvi, and Hannu Minkkinen, Michael Wegener, Olena Voronina, Robert Schulze, Werner Wirges and Reimund Gerhard-Multhaupt "Novel Heat Durable Electromechanical Film: Processing for Electromechanical and Electret Applications", EEEE Transactions on Dielectrics and Electrical Insulation 13, 963-972 (October 2006), the Preparation of Ferroelectret Films from Cyclo Olefin Copolymers (COC) and cyclo-olefin polymers (COP). The foamed polymer films have the disadvantage that a broad distribution of the bubble size can result. As a result, not all bubbles can be charged equally well in the subsequent charging step.

Another method for producing foamed ferroelectret polymer films is the direct physical foaming of a homogenous film with supercritical fluids, for example with carbon dioxide. Advanced Functional Materials 17, 324-329 (2007), Werner Weges, Michael Wegener, Olena Voronina, Larissa Zirkel, and Reimund Gerhard-Multhaupt "Optimized preparation of elastically soft, highly piezoelectric, cellular ferroelectrets from nonvoided poly (ethylene terephthalate) films "and in Applied Physics Letters 90, 192908 (2007), P. Fang, M. Wegener, W. Wirges, and R. Gerhard L. Zirkel" Cellular polyethylene-naphthalate ferroelectrets: Foaming in supercritical carbon dioxide, structural and electrical preparation , and resulting piezoelectricity "is this process with polyester materials as well as in Applied Physics A: Materials Science & Processing 90, 615-618 (2008), O. Voronina, M. Wegener, W. Wirges, R. Gerhard, L. Zirkel, and H. Münstedt, "Physical foaming of fluorinated ethylene-propylene (FEP) copolymer in supercritical carbon dioxide: single film fluoropolymer piezoelectrets" for a fluoropolymer FEP (Fluorinated Etbylene-Propylene Copolymer).

In the ferroelectret multilayer systems, arrangements of hard and soft layers and charges introduced therebetween are known. In "Double-layer electret transducer", Vol. 39, pp. 33-40, 1997, R. Kacprzyk, A. Dobrucki, and JB Gajewski, multilayers of solid materials having widely different moduli of elasticity are described the disadvantage that these layer systems show only a relatively small piezoelectric effect. Several recent publications have described multilayer systems of closed outer layers and a porous or perforated middle layer. These include the articles by Z. Hu and H. von Seggern, Air-breakdown charging mechanism of fibrous polytetrafluoroethylene films, Journal of Applied Physics, Vol. 98, paper 014108, 2005, and Breakdown-induced polarization buildup in porous fluoropolymer sandwiches A thermally stable piezoelectret, Journal of Applied Physics, Vol. 99, paper 024102, 2006, and the publications of HC Basso, RAP Altafm, RAC Altafim, A. Mellinger, Peng Fang, W. Wirges, and R. Gerhard. Three-layer ferroelectrets from perforated Teflon-PTFE films between two homogeneous Teflon® FEP films "IEEE, 2007 Annual Report on Electrical Insulation and Dielectric Phenomena, 1-4244-1482-2 / 07, 453-456 (2007) and the Article by Jinfeng Huang, Xiaoqing Zhang, Zhongfu Xia, and Xuewen Wang "Piezoelectrets of laminated sandwiches of porous polytetrafluoroethylene films and nonporous fluoroethylenepropylene films" Journal of Applied Physics, Vol. 103, paper 084111, 2008. The coating systems m a porous or perforated middle layer often has larger piezo constants than the systems described above. However, sometimes the middle layers can not be reliably laminated with the solid outer layers. In addition, the perforation of the middle layer is usually very time consuming.

In the publications of X. Zhang, J. Hillenbrand and G. M. Sessler, "Thermally stable fluorocarbon ferroelectrets with high piezoelectric coefficient". Applied Physics A, Vol. 84, pp. 139-142, 2006 and "Ferroelectrets with improved thermal stability made from fused fluorocarbon layers", Journal of Applied Physics, Vol. 101, paper 054114, 2007, and in Xiaoqing Zhang, Jinfeng Huang and Zhongfu Xia "Piezoelectric activity and thermal stability of Cellular fluorocarbon films "PHYSICA SCRIPTA Vol. T129 pp 274-277, 2007 describes the structuring of the polymer layers by pressing a metal lattice onto a polymer layer stack comprising at least three FEP and PTFE layers superimposed in alternating succession. By compressing the layers through the grid at a temperature which is above the melting point of FEP and below that of PTFE, the polymer layers are interconnected according to the grid structure such that dome-shaped or bubble-shaped cavities with a rectangular base area form between the bars. However, this method leads to ferroelectrets of different quality, since the formation of uniform cavities is difficult to control, especially with increasing number of layers.

Another method of producing bubble-shaped cavities using a grid is by RAC Altafim, HC Basso, RAP Altafim, L. Lima, CV De Aquino, L. Gonalves Neto and R. Gerhard-Multhaupt, in "Piezoelectrets from thermo-formed bubble structures of fluoropolymer-electret films, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 13, No.5, pp. 979-985, 2006. Here, two superimposed Teflon-FEP films between a metal grid and an upper cylindrical metal part are arranged. This structure is pressed with the metal grid on a lower cylindrical metal part, which has openings for applying a vacuum. The FEP films are heated by the upper metal part and by a vacuum applied to the lower metal part the lower film is pulled into the openings of the grid and corresponding cavities are formed. The described methods using a lattice to form voids in the polymer multilayer composites are expensive and difficult to scale-up.

A favorably simple production method for ferroelectrets with tubular cavities of homogeneous size and structure has been described by R.A. P. Altafim, X. Qiu, W. Wirges, R. Gerhard, R.A.C. Altafim, H.C. Basso, W. Jenninger and J. Wagner in the article "Template-based fluoroethylenepropylene piezoelectrets with tubular channels for transducer applications", accepted for publication in the Journal of Applied Physics. In the method described there, a sandwich arrangement of two FEP films and a PTFE mask film inserted between them is first provided. The formed film stack is laminated, the FEP films are joined together, and subsequently the mask film is removed to release the cavities.

Ferroelectrets continue to be of increasing interest to commercial applications such as sensor, actuator and generator systems. For economy, an applicability of a manufacturing process on an industrial scale is essential.

The invention is therefore based on the object to provide alternative ferroelectret multi-layer composites and alternative methods for the production of FeπOelektret multi-layer composites with which defined Ferroelektret cavity structures can be produced and which can be carried out easily and inexpensively on a large scale and industrial scale.

According to the invention, this object is achieved by the process for producing ferroelectret multilayer composites according to claim 1 and a ferroelectret multilayer composite produced by this process according to claim 12 or 13.

According to the invention, a method is proposed for producing a ferroelectret two- or multi-layer composite with defined cavities, comprising the following steps: a) structuring at least one first surface of at least one first polymer film to form a height profile; b) applying at least one second polymer film to the structured surface of the first polymer film formed in step a), c) bonding the polymer films to form a polymer film composite to form closed and / or open cavities and d) electrically charging the inner surfaces of step c) formed

Cavities with opposite electrical charges.

In other words, the two-layer and multi-layer composites produced according to the invention have stacked polymer films and at least voids formed between in each case two polymer films. The polymer films are connected to each other between the cavities. Advantageously, the shape and dimensioning of the cavities can be produced according to the invention very precisely predetermined and defined. The structuring in step a) and the formation of the height profile on at least one surface of at least the first polymer film in the process according to the invention are decisive for the formation of the defined cavities in the resulting polymer film composite.

It has been found that the method according to the invention makes it possible to produce in a simple manner ferroelectret multilayer systems with defined cavity structures. With the erfϊndungsgemäßen approach, it is also possible to variably set the resonant frequency and piezoactivity, and in particular the piezoelectric constant d33, to the respective application. Advantageously, high and uniform piezoelectric coefficients can also be achieved for larger areas with the ferroelectret multi-layer composite systems produced according to the invention. This basically opens up numerous applications for these ferroelectret multilayer composites. An additional advantage is that the processes proposed according to the invention are largely independent of material and can be automated.

In principle, the polymer films used can be made of any plastic which permits the formation of a height profile, the connection between the polymer films and the formation of voids between the films. According to the invention, the polymer films used may consist of identical or different polymer materials, for example from the group of polycarbonates, perfluorinated or partially fluorinated polymers and copolymers such as PTFE, fluorinated ethylene propylene (FEP), perfluoroalkoxyethylenes (PFA), polyesters such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), Cyclo-olefin polymers, cyclo-olefin copolymers, polyimides, in particular polyetherimide, polyethers, polymethyl methacrylate. and polypropylene or polymer blends thereof. With these materials, good to very good piezo activities can be achieved. The inventively wide selection of materials can advantageously also allow adaptation to certain applications. The polymer films may preferably have a thickness of> 10 μm to <500 μm, more preferably of> 15 μm to <300 μm. The thickness of the various polymer films in a ferroelectret multi-layer composite according to the invention can be chosen to be the same or different. A particularly suitable thickness of the polymer films can advantageously be selected in each case depending on the polymer material and with regard to the desired application. It is fundamentally important that the cavities formed in step c) of the method do not collapse. Thus, stiffer materials can be made thinner than comparatively more elastic polymer materials.

The polymer films can be advantageously configured as film sheets or, in particular with regard to large-scale production, also as film webs which can be arranged one above the other in step b) and joined together to form the cavities in step c). The foil sheets may have, for example, a rectangular, a regular or irregular polygonal shape or a round, for example circular, elliptical or oval base surface, wherein the films arranged one above the other expediently have the same base surface. Basically, the base can also be attached to a special

Application to be customized.

In step b) of the method according to the invention, in other words, a layered polymer film stack is provided. In this case, the overall height of the polymer film composite and the number of cavities and the number of cavities can be determined via the selected total number of polymer films and the selected sequence of structured and unstructured polymer films. As a layer of cavities, the voids between two identical polymer films are understood. In the inventive ferroelectret multi-layer composites, two, three or more polymer films with intervening cavities can be arranged one above the other and joined together. According to the invention, it is possible in each case to use structured and non-structured polymer films. These can for example be arranged alternately in the film stack one above the other. Alternatively, all the polymer films used can also have a height profile, that is to say structuring. Equally preferably it is also possible to use polymer films which are structured only on one side or only on both sides, or both types of foil in the same or different number. In principle, it is preferred in all variants that the outwardly directed surfaces are compact or not structured. This may optionally facilitate the application of electrodes on these outer surfaces of the polymer film composite. When using double-sidedly structured polymer films, for example, a non-structured or unilaterally structured polymer film can additionally be arranged in the top and bottom of the polymer film stack as a cover film. These cover films form the outer surface of the subsequently formed polymer film composite as a cover with its non-structured surface. Advantageously, the variants according to the invention, in which three or more polymer films and correspondingly also several layers of cavities in the ferroelectret multilayer composite are made softer compared to those with only two polymer films and the additional cavities present the sensitivity of the composite and thus the piezoelectric constant d33 increase.

In step d) can be used for the electrical charging and polarity of the inner surfaces of the cavities advantageously on known and established methods. A polarity of the opposite sides of the cavities can be realized for example by a corona discharge or by plasma processes. A corona treatment is advantageously also suitable for large-scale use.

By structuring and forming the height profile in a polymer film carried out in step a) and the connection with at least one second polymer film in step b), precisely predetermined cavities can be produced and defined with the method provided. Another advantage is therefore that with the erfϊndungsgemäßen approach different resonance frequencies, as uncontrollably foamed ferroelectret films occur by non-uniform bubbles can be avoided. In contrast, according to the invention, it is even possible to produce differently configured cavities even in partial regions of the resulting polymer film composite and thus to set different properties, for example piezoactivities.

In one embodiment, the structuring of the at least one surface of the first polymer film in step a) can take place by means of an embossing process. Equally preferably, the embossing process can be carried out using a structured roller or by means of a stamping die. Both with the use of a structured roller, as well as with a structured embossing stamp each formed on the surface of the embossing tool structure can be transferred to form a height profile on a polymer film. It is possible to apply positive or negative forms on the surface of the embossing tool, that is to say the roller or the embossing punch. The structuring can be carried out directly after the extrusion of the films or as a single process, for example in a hot press. It is also included according to the invention that the respective polymer films can be processed from both surface sides with an embossing tool. For example, a polymer film can be embossed from its top and bottom sides, each with a structured roller, and thus structured.

In another alternative embodiment of the method, the structuring of the at least one surface of the first polymer film in step a) by deformation of an optionally heated polymer film under pressurization, for example with compressed air or another gas, in a mold with an optionally pre-tempered contour insert respectively. For example, a polymer film can be heated to a temperature below its softening temperature (glass transition temperature) and then suddenly deformed by application of compressed air of> 20 to <300 bar. For example, polycarbonate films can be heated (for example Macrofol Bayer MaterialScience AG) just below the glass transition temperature to 130-140 ⁰ C. Thereafter, the films can be subjected to an air pressure of 250 bar and pressed onto a mold and can adapt to the tool contour and be permanently deformed. Here, the polymer films used, for example, a thickness of> 10 microns to <500 microns and the depressions and / or elevations formed have a height of> 10 microns to <500 microns, and a width of> 10 microns to <5000 microns. A height of> 10 μm to <250 μm and a width of> 50 μm to <3000 μm is preferred for the cavities. Particularly preferably, the cavities have a width of> 100 microns to <2000 microns.

An analogous method is already established, in particular in the repetitive deformation of printed plastic films and described for example in the published patent application DE 39 05 177 A1. A polymer film can be positioned on a pallet system, heated if necessary and deformed in a corresponding mold over a pre-tempered contour insert by applying pressure. This high pressure forming process is also called High Pressure Forming (HPF); or HPF process. Advantageously, according to the invention, an apparatus of analogous construction to that described in DE 39 05 177 A1 can also be used for structuring the polymer films in step a).

All the structuring variants mentioned have the advantage that position-accurate transmission of the respective desired profile to the polymer films is made possible. Both shape and dimensioning of the cavities then formed in the next step b) can advantageously be selected almost arbitrarily with the methods mentioned above and can be adapted to the desired mechanical and electrical requirements of the respective application depending on the film materials and their properties and the respective film thickness , The combination of the film properties and the shape and dimensioning of the formed cavities is chosen so that the distance to be held foil sections can touch in any case of use. The said structuring methods also have the advantage that they can be automated and, if appropriate, can be carried out as a continuous process.

According to the invention, the structuring of the at least one surface of the first polymer film in step a) can also be effected by slot extrusion of the polymer film with a correspondingly shaped nozzle. For example, by using this structuring method, tube-like or channel-like structures and subsequently cavities corresponding in one step can be formed become. The slot extrusion is advantageously an already established process, which can also be carried out continuously and automatically beyond.

In a ferroelectret multi-layer composite produced according to the invention, the cavities may, for example, have a height of> 10 μm to <500 μm at a polymer film thickness of> 10 μm to <500 μm. By height is meant in particular the height of the cavities in cross section. Particularly preferably, the cavities can have a height of> 10 μm to <250 μm.

The cavities can be formed by the method according to the invention in many different forms. The shape of the cavities is therefore not limited to a cylindrical, tubular or channel-like shape having a circular or rectangular cross-sectional area perpendicular to the layer course of the polymer films. In addition, the inventive method offers the possibility to combine formed in different shapes cavities. In this way, on the one hand advantageously, the total void volume of the resulting cavities can be maximized. On the other hand, the electromechanical, in particular piezoelectric, properties of the ferroelectret multilayer composites and electromechanical transducers produced by the method according to the invention can be adapted by selecting the shape, size and shape of the cavities, their number, arrangement and / or distribution.

The cavities may be in shapes with a rather small area, such as lines, such as curved or straight, single or crossed lines or perimeter lines of geometric figures, such as a circle or perimeter of a cross, or as structures with a larger area, such as rectangles, circles , Crosses, et cetera, be trained. The shape and dimensioning of the cavities is preferably set in such a way that the polymer films can not touch perpendicularly to their layer course within the cavity and / or that the total void volume resulting after completion is as large as possible. In other words, in particular, the positive and negative charges applied by polarity to the inner surfaces of the cavities should not be able to touch.

The cavities may be formed into shapes having a cross-sectional area selected from the group consisting of substantially circular, for example, circular, elliptical or oval, polygonal, for example triangular, rectangular, trapezoidal, diamond-shaped, pentagonal, hexagonal, in particular honeycomb, cruciform, star-shaped and partially round and partially polygonal, for example S-shaped, cross-sectional areas. The cavities in different layers between the different polymer films in the Film stacks can be configured the same or different. This includes both their shape, size and shape as well as the number of cavities, their arrangement and / or distribution.

The voids within the formed polymer film composite can advantageously soften the ferroelectret multilayer composite to be made along its thickness, thereby lowering its modulus of elasticity, as well as allowing a poling process in the resulting voids.

In the context of the method according to the invention, the cavities in the polymer film composite formed may be formed both homogeneously and heterogeneously distributed. In particular, depending on the field of application of the ferroelectret multi-layer composite to be produced, it may also be advantageous to form the cavities in a targeted manner distributed in a spatially resolved, heterogeneously distributed manner.

The bonding of the polymer films to a polymer film composite in step c) can be carried out according to the invention, for example by lamination, gluing, stapling, clamping, screwing, riveting or welding (eg laser welding, ultrasonic welding, vibration welding).

The bonding of the polymer films by lamination may in particular be carried out thermally, under elevated pressure and / or by means of ultrasound and / or by irradiation with ultraviolet light or infrared light. In this way, advantageously, the choice of material for the polymer films can be increased. The conditions for the lamination are expediently chosen so that the film layers combine, but the structuring of the first polymer film and its height profile are retained as far as possible, thereby ensuring dimensional stability and defined formation of the cavities. Before lamination, the material of the first structured polymer film and / or the material of the second polymer film, which in other words forms a cover of the first film, may be completely solidified, for example completely dried and / or completely crosslinked, and / or completely solidified and / or or completely crystallized. As a result, the dimensional stability of the method according to the resulting, cavities comprehensive polymer film composite can be improved.

The connection of the polymer films in step c) by means of an adhesive bond can be carried out, for example, with acrylate adhesive. Alternatively, it is also possible to obtain the compound by applying a good solvent or a solvent composition for the respective polymer material to one or both films, subsequent compression of the films and evaporation of the solvent, in particular in the connection of polymer films of the same material. In other words, the polymer material becomes in place and / or Regions where the solvent has been applied dissolved and cured again by the evaporation of the solvent and can serve in this way as an adhesive between the polymer films. It is possible, for example, to bond polycarbonate films with methylene chloride. Advantage in the connection by this solvent method is that no thermal stress occurs and improves the dimensional stability especially in thermally deformable polymer materials and collapse of the formed cavities can be avoided.

In a further embodiment, the polymer films can also be bonded to one another by lamination in addition to the lamination. This bond can be produced for example by means of acrylate adhesive. As a result, the mechanical connection of the polymer films can be supported and improved.

In another embodiment of the method, before and / or after the electrical charging of the inner surfaces of the cavities in step d), the application of electrodes to the outer surfaces of the polymer film composite. The application of electrodes to the outer surfaces is understood to mean the provision of a conductive surface coating in at least one partial region, in particular on the outwardly directed surfaces of the polymer composite. The electrodes are preferably arranged on compact or non-structured surfaces of the polymer films used.

According to the invention, after the application of electrodes on the outer surfaces of the ferroelectret multi-layer composite direct charging by applying an electrical

Tension. Prior to the application of electrodes, a polarity of the opposite sides of the cavities can be realized for example by a corona discharge. A corona treatment is advantageously also suitable for large-scale use.

According to the invention, it is also possible first to provide a conductive surface coating on one surface, then to charge the polymer composite and finally to apply a second electrode on the opposite outer surface.

In other words, the ferroelectret multi-layer composites produced according to the invention can have at least partially a conductive coating on the outwardly directed surfaces of the polymer films. These conductive areas can be used as electrodes. The conductive coating, that is to say the electrodes, can be applied in a planar and / or structured manner. For example, a patterned conductive coating may be configured as an application in stripes or in lattice form. In this way, in addition, the sensitivity of the ferroelectret multi-layer composite can be influenced and adapted to specific applications. The selected electrode materials may be conductive materials known to those skilled in the art. According to the invention, for example metals, metal alloys, conductive oligo- or polymers, such as polythiophenes, polyanilines, polypyrroles, conductive oxides, such as mixed oxides such as ITO, or polymers filled with conductive fillers come into question. Suitable fillers for polymers filled with conductive fillers are, for example, metals, conductive carbon-based materials such as carbon black, carbon nanotubes (CNTs) or, in turn, conductive oligomers or polymers. The filler content of the polymers is above the percolation threshold, so that the conductive fillers form continuous electrically conductive paths.

The electrodes can be prepared by methods known per se, for example by metallization of the surfaces, by sputtering, vapor deposition, chemical vapor deposition (CVD), printing, knife coating, spin coating, gluing or pressing a conductive layer in prefabricated form or by a spray electrode made of a conductive plastic can be realized. The electrodes can be structured, for example in stripes or in lattice form, be configured. For example, according to the invention, the electrodes can also be structured in such a way that the ferroelectret multilayer composite has active and passive regions as an electromechanical transducer. In particular, the electrodes can be structured in such a way that, in particular in a sensor mode, the signals are detected spatially resolved and / or, in particular in an actuator mode, the active regions can be specifically controlled. This can be achieved, for example, by providing the active regions with electrodes, whereas the passive regions have no electrodes.

It is also included according to the invention that two or more ferroelectret multi-layer composites can be connected to the same polarized conductive layer, that is to say electrode. In other words, between two ferroelectrolyte multilayer composites according to the invention, an intermediate electrode can be formed, which can be switched against the two electrodes on the then outer surfaces. As a result, the ferroelectret multi-layer composites can be connected in series and the achievable piezoelectric effect can be doubled or multiplied.

The ferroelectret multi-layer composites according to the invention preferably contain two electrodes. The electromechanical transducers according to the invention having more than two electrodes can be, for example, stack constructions of a plurality of preferably prepared according to the invention, ferroelethricate multilayer composite systems.

In another embodiment of the process according to the invention, the steps a), b), c) and / or d) can be carried out as a continuous roll-to-roll process. Advantageously, the production of the multilayer composites can therefore be at least partially continuous Process, preferably as a roll-to-roll process, to be performed. This is particularly advantageous for the application of the processes on an industrial and industrial scale. The automation of at least part of the manufacturing processes simplifies the processes and enables the low-cost production of the multi-layered ferroelectret composite with cavities. According to the invention, all steps of the method are advantageously accessible to automation.

In one embodiment of the invention, prior to step b), the second polymer film can also be structured to form a height profile. As a result, the variability of the producible ferroelectret multi-layer composites can be increased. The selected total number of polymer films and the selected sequence of structured and unstructured polymer films can be used to determine the overall height and the number of cavities or number of cavities. In the ferroelectret multi-layer composites according to the invention, two, three or more polymer films with intervening cavities can thus be arranged one above the other and joined together. For example, structured and non-structured polymer films can be arranged alternately one above the other in the film stack. Alternatively, all the polymer films used can also have a height profile, wherein the films can have the same or a different structuring with respect to one another.

In another embodiment, the sealing of the edges of the polymer film composite formed in step c) is included in a further step e) before or after the charging in step d). The multilayer composites of the invention can thus be advantageously sealed at the edges to hermetically protect them from environmental influences, for. For example, in applications in an aggressive environment, for example, in atmospheres with high humidity or under water.

In a further preferred embodiment of the invention, a gas can be introduced into the cavities. The gas may be, for example, pure nitrogen (N ₂ ), nitrogen oxide (N ₂ O) or sulfur hexafluoride (SF ₆ ). As a result of the gas filling, significantly higher piezo constants can advantageously be achieved by the poling in the case of the ferroelectret multilayer composites produced according to the invention.

An immense advantage of the method according to the invention provided also in its various embodiments described above is that they are material-independent in a wide range and, as a result, a wide range of applications is given.

Another object of the present invention is a multi-layered ferroelectret composite comprising a layer stack of at least one first polymer film, and one with this connected second polymer film, wherein at least the first polymer film at least on its side facing the second polymer film surface has a structure with elevations and depressions and the first polymer film is formed with its formed by structuring height profile with the second polymer film so that between the polymer films one or more Hollows are formed and further provided the inner surfaces of the cavities with opposite electrical charges.

In the context of the present invention, at least a part of the cavities may be formed in shapes having a cross-sectional area in the direction of the layer course of the polymer films selected from the group consisting of substantially round, for example circular, elliptical or oval, polygonal, for example triangular, rectangular, trapezoidal, diamond-shaped, pentagonal, hexagonal, in particular honeycomb-shaped, cross-shaped, star-shaped and partially round and partially polygonal, for example S-shaped, have cross-sectional surfaces, as well as be completely formed in deviating forms. The geometric shapes can continue to be configured regularly and irregularly.

Irrespective of this, the cavities perpendicular to the layer course of the polymer films in the film stack may be formed partially or completely in forms which have a cross-sectional area selected from the group consisting of essentially round, for example circular, elliptical or oval, polygonal, for example triangular, rectangular, trapezoidal, diamond-shaped, pentagonal, hexagonal, in particular honeycomb-shaped, cruciform, star-shaped and partially round and partially polygonal, for example, S-shaped, cross-sectional surfaces, have, as well as be completely formed in deviating forms. The geometric shapes can continue to be configured regularly and irregularly.

In particular, the ferroelectret multi-layer composites according to the invention can have cavities which, partially or completely, have no purely bubble-shaped or dome-shaped form, in particular with a rectangular base. The deviating erfϊndungsgemäß possible shapes of the cavities allow variable adjustment of the essential properties of the resulting multilayer composites, such as the piezoelectric constant or the elasticity and softness of the multi-layer composite along its thickness and thus a wide range of applications. By selecting, in particular the forms and dimensions according to the invention, the cavities and their distribution, advantageously, the total void volume of the ferroelectret multi-layer composite can be optimized.

For example, the multilayer composite according to the invention can also contain more than two polymer films and correspondingly also multiple layers of cavities contain the same or different ones Shape, dimensioning, number and distribution of cavities may have. Furthermore, the multilayer composite according to the invention can be provided with electrodes. With regard to further features of a ferroelectret multi-layer composite according to the invention, reference is hereby explicitly made to the explanations in connection with the inventive method.

The invention further relates to a ferroelectret two- or multi-layer composite with cavities produced by a process according to the invention as described above. In this case, the various variants of the production process provided and the resulting ferroelectret multilayer composites can also be carried out, if appropriate, in combination with one another. Such two- and multi-layer composites according to the invention have stacked polymer films and at least cavities formed between in each case two polymer films. The polymer films are connected to each other between the cavities. Advantageously, the shape and dimensioning of the cavities can be produced according to the invention very precisely predetermined and defined.

The invention further relates to a piezoelectric element containing at least one inventive ferroelectret multilayer composite and / or at least one ferroelectret multilayer composite prepared by the process according to the invention. This piezoelectric element may for example be a sensor, actuator or generator element. Advantageously, the invention in a variety of different applications in the electro-mechanical and electro-acoustic field, in particular in the field of energy from mechanical vibrations (energy harvesting), acoustics, ultrasound, medical diagnostics, acoustic microscopy, mechanical sensors, in particular pressure - Force and / or strain sensors, robotics and / or communication technology can be realized. Typical examples include pressure sensors, electroacoustic transducers, microphones, loudspeakers, vibration transducers, light deflectors, diaphragms, optical fiber modulators, pyroelectric detectors, capacitors and control systems, and "smart" floors.

In addition, the invention further comprises an apparatus for producing ferroelectret multi-layer composites according to the invention. In other words, the invention further relates to an apparatus for carrying out the method according to the invention, wherein the apparatus comprises means for structuring at least one surface of a first polymer film. These means may be, for example, an embossing roll, an embossing punch or a device for deforming by means of pressurization. In summary, the invention provides processes for the production of ferroelectret multi-layer composites with cavities, which can be carried out simply and cost-effectively, even on an industrial scale. The ferroelectret multi-layer structures produced by the methods according to the invention can also be produced with a larger number of layers with a precisely defined cavity structure. Due to the variable adjustability of the cross-sectional geometry and the dimensions, the shape and size of the cavities, the sequence of layers and number of layers and the large selection of materials for the polymer films used, the ferroelectrons produced according to the invention can be adjusted particularly well to corresponding fields of application.

The figures described below are intended to further explain the invention in detail, without being limited to the embodiments shown and described.

Show it

1 schematically shows the structuring of a first polymer film having a groove-like structure on a surface by means of an embossing roller.

2 shows a first polymer film with a groove-shaped structuring introduced on both sides.

Fig. 3a in an oblique plan view schematically the preparation of a polymer film composite of a structured film with a second smooth film.

Fig. 3b in an oblique plan view schematically the preparation of a polymer film composite of a two-sided textured film with two non-structured films.

Fig. 3c in an oblique plan view schematically the preparation of a polymer film composite of a first structured film with a second equally structured film.

Fig. 3d in an oblique plan view schematically the preparation of a polymer film composite of two unilaterally structured film with a third unstructured film.

4a to 4g different forms of a formed by structuring height profile in a polymer film

Fig. 5 is an enlarged micrograph of a ferroelectret multilayer composite of two polycarbonate films according to the invention.

FIG. 1 schematically shows the structuring of a first polymer film 1 with a groove-like structure on a surface by means of an embossing roll 10. Under embossing roll 10 is understood to mean a roller which, as an embossing tool, can transfer its structure to a polymer film. The polymer film 1 can be passed, for example, directly after the extrusion between the embossing roller 10 and an unstructured guide roller 11. Alternatively, in the apparatus used as a counterpart to the embossing roller 10 instead of the guide roller 11, an unstructured plate could be used. Through the recesses 12 on the embossing roller 10, the corresponding height profile can be formed on the polymer film 1. Through the recesses, a channel-like structure can be formed on the polymer film, wherein the height profile can be formed by parallel spaced apart webs 2 on a straight base 3. The form of structuring shown could, according to a variant of the invention, also be obtained by slot die extrusion with a correspondingly shaped nozzle. The embossing roller used can advantageously also have other embossed structures, which can be matched to the desired shape of the cavities to be formed. In this embodiment, the base area 3 of the polymer film 1 forms, on its surface opposite the height profile, the unstructured second surface of the polymer film 1. The webs 2 are configured in the embodiment shown with vertical sides and straight edges. If such a structured polymer film 1 is connected according to the invention, for example, with a non-structured polymer film 5, channel-like cavities 4 with a rectangular cross-section can be formed as shown in FIG. 3a. The groove-like structure is not limited to the embodiment shown, and the depressions may also be formed, for example, with a semicircular cross-section. In principle, the invention provides that the outwardly directed surfaces of the finally formed polymer film composite are not structured. Electrodes may then be applied to these non-structured surfaces before and / or after the poling.

FIG. 2 shows a first polymer film 1 with a groove-shaped three-dimensional structure formed on both sides, for example by two embossing rolls arranged one above the other

10 (not shown here) can be introduced into the polymer film 1 guided therebetween. The embossing rollers 10 could each be arranged interlocking with a cylinder-like designed structure. Alternatively, the production of such a double-sided structured polymer film 1, for example, by deformation of an optionally heated polymer film under pressurization in a mold with an optionally pre-tempered contour insert done. In this version of the

Polymer film 1 does not set the height profile as shown in Figure 1 on a base 3 of the

Polymer film 1, but the polymer film 1 is deformed three-dimensional overall.

Subsequently, cavities 4 can be formed by connecting the first polymer film 1 on both sides, each with a non-structured film, to both surface sides of the polymer film 1, as shown in FIG. 3b. According to the invention it is also possible, the two-sided Structure of the polymer film 1 to be designed such that the height profile is formed on both surfaces starting from a base 3.

FIG. 3a schematically shows the production of a polymer film composite according to the invention from a structured polymer film 1 produced analogously to the structured polymer film 1 produced in FIG. 1 with a second non-structured polymer film 5. The second polymer film 5 can be applied to the surface of the polymer film

1, on which the height profile, for example in the form of webs 2, is formed. The cavities 4 formed therefrom may have a rectangular cross-section in the illustrated embodiment. The connection of the two polymer films 1 and 5 can be effected by lamination, gluing, stapling, clamping, screwing, riveting or welding (eg laser welding, ultrasonic welding, vibration welding).

FIG. 3b schematically shows the production of a polymer film composite according to the invention from the polymer film 1, structured on both sides and illustrated in FIG. 2, with two non-structured polymer films 5 and 5 '. The non-structured polymer films 5 and 5 'can each be connected in the direction of the arrow on one surface side with the structured polymer film 1 and in each case form a layer of cavities 4 and 4' through the compound. The cavities 4 and 4 'may each have a rectangular cross-section in the embodiment shown. The cavities 4 and 4 'according to the invention can in principle be configured independently of each other in variable shapes and sizes. This also applies to the cavities 4 or 4 'in one layer of the resulting polymer film composite. As a layer of cavities according to the invention are understood and referred to, which are formed between two identical polymer films. The cavities within the polymer film composite formed can advantageously soften the ferroelectret multilayer composite to be produced along its thickness, ie perpendicular to the layer course of the polymer films 1, 5, 5 ', in order to reduce its modulus of elasticity and enable a poling process in the resulting cavities. The connection of the two polymer films 1 and 5 can be effected by lamination, gluing, stapling, clamping, screwing, riveting or welding (eg laser welding, ultrasonic welding, vibration welding). The poling can in principle be carried out after the connection of the polymer films, for example by direct charging by application of an electrical voltage to already applied electrodes. Prior to the application of electrodes, a polarity of the opposite sides of the cavities can be realized for example by a corona discharge or a plasma process.

FIG. 3c shows schematically the production of a polymer film composite according to the invention from a structured polymer film 1 produced analogously to the structured polymer film 1 produced in FIG. 1 with a second similarly structured polymer film 1 '. Both polymer films 1 and 1 'have as height profile webs 2 on a base 3. The polymer films 1 and 1 'can each with their textured surface sides are connected to the formed webs. In this case, the webs 2 can be set accurately to one another in the direction of the arrow, with channel-like cavities 4 having a rectangular cross-section perpendicular to the layer course of the polymer films 1 and 1 '. The connection of the two polymer films 1 and 1 'can be effected by lamination, gluing, stapling, clamping, screwing, riveting or welding (eg laser welding, ultrasonic welding, vibration welding).

FIG. 3d schematically shows the production of a polymer film composite according to the invention from a structured polymer film 1 produced similarly to the structured polymer film 1 produced in FIG. 1 with a second similarly structured polymer film 1 'and a further non-structured polymer film 5. According to the invention, it is possible to use the second structured polymer film 1 'to arrange with their non-structured surface in the arrow direction on the structured surface side of the polymer film 1 and to connect with this. By connecting the polymer film 1 'with a further polymer film 5, a second layer of cavities can then be formed. In the embodiment shown, the structured foils 1 and 1 'are arranged on top of each other with the same orientation of the structure and subsequently joined together. Likewise, the structures can also be aligned differently. For example, the structures - in this case the webs - could be arranged at an angle of 45 ° or of 90 ° to each other, wherein according to the invention all arrangements in different angles or orientations of the structures to each other are possible. The layer sequence of the polymer films 1 and 1 'can be variably continued and varied with one or more structured and / or unstructured polymer films. Advantageously, the production of a multi-layered multilayer composite with voids is thus possible in different ways and can optionally be adapted to existing polymer films as precursors or to a planned application and desired properties, such as modulus of elasticity and piezoelectric constants.

4a-4g show schematic plan views of various embodiments of embossed structures in polymer films 1 and thus the possible embodiment of the bases of the corresponding cavities transverse to the layer course of the polymer films 1. The structures can, for example, by embossing basically as positive or negative forms, ie as wells or elevations are introduced into a polymer film 1. The illustrated embodiments and configurations of the structuring are only examples and are not intended to limit the invention in any way. For reasons of clarity, in FIGS. 4a to 4g, only one recess of a mold is identified by way of example by a reference numeral. FIG. 4 a shows a structured polymer film 1 comprising depressions 6, the depressions having a circular base area. The recesses 6 can, as illustrated in FIG. 4 a, moreover be formed as a multiplicity of small depressions 6.

FIG. 4b shows a structured polymer film 1 comprising depressions 6, the depressions 6 having an elongated, rectangular base area.

FIG. 4c shows a structured polymer film 1 comprising depressions 6, the depressions 6 of which have a cross-shaped base surface.

FIG. 4 d shows a structured polymer film comprising different depressions 6, 6 ', the depressions of which partially have a circular base surface 6 and partially a diamond-shaped base surface 6'. FIG. 4 d illustrates that in the case of a homogeneously distributed arrangement of depressions with circular 6 and diamond-shaped 6 'cross-sectional areas, a particularly large total void volume can advantageously be achieved.

FIG. 4e shows a polymer film 1 comprising depressions 6, the depressions 6 of which have a honeycomb-shaped base surface. FIG. 4 e illustrates that an arrangement which is based exclusively on depressions 6 with honeycomb-shaped cross-sectional areas can likewise achieve an advantageously large total void volume.

FIG. 4f shows a structured polymer film 1 comprising recesses 6, 6 ', 6 "whose structure is formed in different shapes and sizes and which have cruciform 6', 6" and substantially honeycomb surfaces 6. FIG. 4f furthermore shows that the depressions 6, 6 ', 6 "can be distributed in an inhomogeneous manner and can be partially connected to one another.

FIG. 4g shows a polymer film 1 comprising depressions 6, the depressions 6 of which are formed by applying a combination of different structures, in particular of hexagons / honeycombs, crosses and points of different point and line thicknesses. FIG. 4g further shows that at least the edge regions of the continuous polymer layer can be formed with a closed structure in order to obtain one or more closed cavities in contact with the continuous polymer layers after completion of the inventive production process. In this way, a continuous cavity can be formed. FIG. 4g further illustrates that in the context of the present invention a structured polymer film 1 with height profile can also be understood as meaning a polymer film which has only one depression 6, which can also be understood as a combination or connection of several depressions. 5 shows an enlarged micrograph of a ferroelectret multi-layer composite according to the invention of two polycarbonate films in cross-section. The structured polymer film 1, a polycarbonate film (Makrofol Bayer MaterialScience AG) with a thickness of 75 microns, was heated just below the glass transition temperature to 130-140 ⁰ C. Thereafter, the polycarbonate film 1 was pressed with an air pressure of 250 bar on the mold with a groove profile. By the mold, the polycarbonate film 1 was deformed such that formed semi-cylindrical depressions. In this case, on the opposite surface of the polymer film 1, the structure was formed as a half-cylinder-shaped height profile. On the structured polycarbonate sheet 1, on the surface side having the recesses as a structure, a smooth polycarbonate sheet 5 of 75 μm in thickness was overlaid and bonded to the first by lamination. This resulted in cavities 4 with a semicircular cross section perpendicular to the layer course of the polymer films 1 and 5. The cavities 4 had a height of 100 microns in cross section. The compact outer surface of the polymer film 1, which has the elevation, and the outwardly directed non-structured surface of the polymer film 5 were each subsequently provided with an aluminum electrode of 50 nm thickness. The polarity of the inner cavities 4 was carried out by directly applied electrical voltage. The composite produced showed a good piezoactivity which was comparable to the piezoactivity of the samples obtained according to Example 5.

By the examples listed below, the invention will be further explained without being limited to these.

Examples

Example 1 :

Preparation of a lubricant additive masterbatch

Production of the lubricant additive compound with conventional twin-screw compounding extruders (eg ZSK 32) at processing temperatures customary for polycarbonate of 250 to 330 ^° C.

A masterbatch with the following composition was prepared:

Polycarbonate Makrolon 2600 000000 from Bayer MaterialScience AG with a proportion of 98% by weight

Di-isopropyl-dimethylammonium perfluorobutanesulfonate as a colorless powder in a proportion of 2% by weight. Example 2:

Film extrusion:

A compound of the following composition was mixed:

Lubricant additive masterbatch according to Example 1 with a content of 20% by weight and polycarbonate Makrolon 2600 from Bayer MaterialScience AG with a content of 80.0% by weight

The equipment used for the production of the films consists of

a main extruder with a screw of 105 mm diameter (D) and a length of 4IxD; the screw has a degassing zone;

- An extrusion slot die with 1500 mm width;

a three-roll smoothing calender with horizontal roll arrangement, wherein the third roll is pivotable by +/- 45 ° relative to the horizontal;

- a roller conveyor;

- a device for two-sided application of protective films;

- a trigger device;

- Winding station.

The granules were fed to the hopper of the extruder. In the plasticizing cylinder / screw of the extruder, the melting and conveying of the material took place. The molten material was fed to the smoothing calender whose rolls had the temperature mentioned in Table 1. On the smoothing calender (consisting of three rolls) was the final shaping and cooling of the film.

Table 1

process parameters

Temperature of the main extruder 275 ° C +/- 5 ⁰ C

Temperature of the coextruder 260 ⁰ C +/- 5 ⁰ C

Temperature of the deflection head 285 ° C +/- 5 ⁰ C

Temperature of the nozzle 300 ⁰ C +/- 5 ⁰ C

Speed of the main extruder 45 min ^'1

Speed of the coextruder 12 min ¹

For one-sided structuring of the film surface while a rubber roller was used at a first position in the system used. The rubber roller used for structuring the film surface is disclosed in US Pat. No. 4,368,240 to Nauta Roll Corporation.

On a second position in the plant, a metal roller structured with an Advanced Compound Parabolic Concentrator (ACPC) structure was used. The ACPC structure was used with the following parameters: acceptance angle: 8 °, reduction factor: 0.05.

The Compound Parabolic Concentrator (CPC) region of the structure could be determined by:

a) calculating the aperture angles in the medium θi and Θ ₂ from the Fresnel equations by means of the defined acceptance angles;

b) Construction of the two parabolic branches Pi and P ₂ with the opening angles in the medium θi or Θ ₂ according to the equation:

(x + cos θ _{λ 2} ) ² 1 ± sin θ _{x 2}

-Vw = 2 (l + SUi ^ _{1 2} ) 2

where θ _{1, 2 is} the aperture angle in the medium of the left (G ₁ ) and right (G ₂ ) parabola, x is the x coordinate, and y _{1 2 is} the y coordinate of the left (yi) and right (y ₂ ) parabolas ;

c) calculation of the endpoints of the parabola branches F), F ₂ and Ei, E ₂ ;

d) rotation of the parabolas around the opening angle in the medium -θi or θ ₂ and translation of the parabola P ₂ along the x-axis;

e) optionally in the case of an asymmetrical variant with θi ≠ θ _2, the determination of the inclination of the inclined surface determined by the points Ei and E ₂ ; f) determining the effective acceptance angles in air from the geometry constructed in steps a) to e);

g) Comparison of the effective acceptance angles with the defined acceptance angles, and if greater than 0.001% repetition of steps a) to f) with corrected acceptance angles instead of the defined acceptance angles in step a), wherein the corrected acceptance angles are unequal to the defined acceptance angles, and wherein corrected acceptance angle be chosen so that the effective acceptance angle from step f) match the defined acceptance angles; and

h) upon reaching a deviation, the effective acceptance angle of the defined acceptance angles of 0.001% or less shortening of the parabolas in the y-direction to the extent determined by the shortening factor.

The construction description given above is so general that the roll supporting the ACPC structure may in principle also be made of different materials (medium 1: for example PMMA or polycarbonate). In addition, the ACPC area can be used in various environments (medium 2: for example, air or water). That is, medium 1 and medium 2 then enter their refractive indices into the said Fresnel equation.

Subsequently, the embossed film was transported through a trigger. Thereafter, a protective film made of polyethylene could be applied on both sides and carried out a winding of the film. A film of 180 μm thickness of the base layer was obtained, in which on one side the ACPC structure was impressed and on the other side a texturing with a surface roughness R ₃ Z of 8 μm. The height of the ACPC structure starting from the base layer was 73 μm and the distance was 135 μm. In other words, there is a distance from valley to valley of 135 microns and in the vertical distance from valley to peak of the mountain of 73 microns.

Example 3:

Production of a Ferroelectret Multilayer Composite from a First Roll-Structured Film and a Smooth Polycarbonate Film of 20 μm Thickness

A smooth 20 μm thick polycarbonate film was placed on the structured side of a 285 μm thick polycarbonate film provided with the ACPC roll profile as described in Example 1. This film composite was then laminated at 205 ^° C. After lamination, the film composite has a layer thickness of 285 μm. Following the depth profile of the film with roll profile, voids are formed in the polymer film composite of the two polycarbonate films. These cavities have in cross section a height of 40 microns and a width of 25 microns. The distance of the cavities is determined by the embossed roll profile. In which Lamination process, however, the flattened profile slightly flattened so that the cavities are smaller than the height of the original roll profile pretends. As a result, the total thickness of the layer stack is smaller than the sum of the layer thicknesses of the individual films before the lamination process. The film composite was subsequently provided on both surfaces with aluminum electrodes of 50 nm thickness. The polarity of the internal cavities was carried out by directly applied electrical voltage of 17 to 19 kV. The piezoelectric effect was measured directly after polarity.

The measurements for the 17 kV polarity gave a d33 coefficient of 4 pC / N directly after poling, and for the 19 kV polarity a d33 coefficient of 5 pC / N resulted. Five measurements were taken for each sample and the mean was formed.

Example 4:

Production of a Ferroelectret Multilayer Composite from a Polycarbonate Film with Roll Profile and a Smooth Polycarbonate Film with 50 μm Thickness: A smooth 50 μm thick polycarbonate film was placed on the profiled side of a film provided with the roll profile with a total thickness of 285 μm analogously to Example 1. This film composite was then laminated at 205 ^° C. After lamination, the film composite has a thickness of 320 μm. Following the depth profile of the film with roll profile triangular cavities form in the polymer film stack. These cavities have a depth of about 40 microns and a width of 60 microns. The distance of the cavities is determined by the embossed roll profile. In the lamination process, the 50 μm thick polycarbonate layer is pressed into the roll profile so that the cavities are smaller than the height of the original roll profile. As a result, the total thickness of the polymer film composite is smaller than the sum of the layer thicknesses of the individual films before the lamination process. The film composite was subsequently provided on both surfaces with aluminum electrodes of 50 nm thickness. The polarity of the internal cavities was carried out by direct applied electrical voltage of 20 kV. The piezoelectric effect was measured directly after polarity. According to this example, four 4 cm x 4 cm samples were prepared, each measured five times. For measurements directly after polarity, the mean values shown in Table 2 were obtained.

Table 2 Example 5:

Production of a Ferroelectret Multilayer Composite from Two Polycarbontate Films Structured by Embossing Rolls: The profiled sides of two films provided with the roll profile, each with a total thickness (base area and structure) of 285 μm, were placed on top of each other in such a way that the embossed structures intersect. This film composite was then laminated at 205 ^° C. After lamination, the film composite has a layer thickness of 550 μm. Following the depth profile of the films with roll profile, cavities are formed in the layer stack. The cavities were measured 45 ° to the crossed structure and have a height of about 50 microns in cross section and a width of 100 microns. The distance of the cavities in the measurement below 45 ° is 190 microns. In the lamination process, the roll profile was flattened so that the dimensions of the cavities were smaller than the height of the original roll profile specified. As a result, the total thickness of the layer stack was smaller than the sum of the layer thicknesses of the individual films before the lamination process. The film composite was subsequently provided on both outwardly facing surfaces with aluminum electrodes of 50 nm thickness. The polarity of the internal cavities was carried out by direct applied electrical voltage of 20 kV. The piezoelectric effect was measured directly after polarity. According to this example, nine 4 cm x 4 cm samples were prepared and measured. In each case, five measurements of the piezoelectric constants were carried out and the average value formed therefrom.

For measurements directly after polarity, the values shown in Table 3 were obtained.

Table 2 p

Sample 1 2 3 4 5 6 7 8 9 d33 4 4 12 22 4 10 6 14 3 pC / N pC / N pC / N pC / N pC / N pC / N pC / N pC / N pC / N

Example 6:

Production of a ferroelectret multilayer composite from a first, by means of

Pressurizing in a mold structured foil and a smooth

Polycarbonate film.

A polycarbonate film (Makrofol Bayer MaterialScience AG) with a thickness of 75 microns was heated just below the glass transition temperature to 130-140 ⁰ C. Thereafter, the polycarbonate film was pressed with an air pressure of 250 bar on the mold with a groove profile. The

Polycarbonate film adapted to the tool contour and was permanently deformed groove-shaped.

The film was deformed in its entirety, so that on a surface a height profile and correspondingly formed on the other surface side of the polycarbonate sheet groillenformige depressions. Onto this structured polycarbonate film, a smooth polycarbonate film of 75 μm thickness was placed and bonded to the first by lamination. There were cavities with a semicircular cross-section, perpendicular to the layer course of the polymer films. The cavities had a height of 100 μm in cross section. The film composite was subsequently provided on both surfaces with aluminum electrodes of 50 nm thickness. The polarity of the internal cavities was carried out by directly applied electrical voltage. The composite produced showed a good piezoactivity which was comparable to the piezoactivity of the samples obtained according to Example 5. An enlarged section of the polymer film composite in the region of a cavity is shown in FIG.

Example 7:

Ferroelectret multi-layer composite of a polycarbonate film with die profile and a smooth polycarbonate film with a thickness of 125 μm An embossing stamp made of aluminum was provided with a groove structure. Here, the grooves have a distance of 1 mm, a depth of 80 microns. A polycarbonate film (Makrofol DE 1-1, 125 μm thick) was pressed into this embossing die in a hot press, so that the groove structure was raised as a height profile on the polycarbonate film. On a second polycarbonate film (Makrofol DE 1-1, 125 microns thick) was an Apec 1800 solution in mesitylene, 1 methoxy-2-propanol acetate, 1, 2,4-trimethylbenzene, ethyl 3-ethoxypropionate, cumene and solvent naphtha aufgerakelt. Subsequently, the two films were lightly pressed together until the solvents had evaporated, and thus joined together. The composite produced was provided on the outwardly facing surfaces with electrodes and the inner cavities 4 poled. The polymer film composite showed a good piezoactivity which was comparable to the piezoactivity of the samples obtained according to Example 5.

Experimental set-up for the mechanical measurement of the d33 piezo constants of the manufactured ferroelectret multi-layer composite systems and measurements

Basically, the following three main components are required for the measuring device: Force generator, force measuring device and charge measuring device. As power generator, an electric shaker type 4810 from Brüel & Kjaër was chosen. The vibration exciter makes it possible to exert a defined force as a function of the input voltage. This vibrator was mounted on a movable platform, the position of which is manually adjustable in the vertical direction. The height adjustment of the vibration exciter is necessary for clamping the samples. In addition, it can be used to set the static pre-pressure required for the measurement. To control the vibration exciter was a function generator DS 345 of Stanford Research Systems used in conjunction with a type 2718 power amplifier from Brüel & Kjaer. The force gauge used was a force sensor Type 8435 from Burster. The force sensor is designed for both pressure and tension measurements in the range of 0 to 200 N. However, the force may only be vertical, so that no lateral force components or torques act on the sensor. To ensure this, the force sensor was provided with a cylindrical pressure guide rail with a stainless steel bolt sliding almost smoothly therein. At the free end of the bolt was a two-centimeter-wide polished plate, which served as a support surface for the samples. The signals from the force sensor are detected by a Burster type 9243 module amplifier and passed on to a GOULD 4094 oscilloscope.

The charge meter used was a charge amplifier Type 2635 from Brüel & Kjaer. The charge amplifier makes it possible to detect charges up to 0.1 pC. To measure the surface charge, the two sides of the sample must be electrically connected to the charge amplifier. The electrical contact to the lower side of the sample is made possible by the support surface, which in turn is connected to the entire structure. The upper side of the sample was connected to the charge amplifier through the brass pressure-applying punch. The stamp is electrically insulated from the rest of the structure by an attachment made of Plexiglas on the oscillator and connected by a cable to the charge amplifier. The cable should be as thin and soft as possible to withstand mechanical stresses and thus

To avoid distortions of the measurement results. The measured signal is finally passed from the charge amplifier to the oscilloscope. By default, a pre-pressure of 3 N (static) was set and measured with an amplitude of 1 N (dynamic).

Claims

claims

1. A process for producing a ferroelectret two- or multi-layer composite with cavities characterized by the following steps: a) structuring at least a first surface of a first polymer film (1) to form a height profile, b) applying at least one second polymer film (5, 1 ' on the structured surface of the first polymer film formed in step a), c) bonding the polymer films (1, 1 ', 5) to a polymer film composite to form cavities (4, 4') and d) electrically charging the inner surfaces of the polymer films in step c) formed cavities (4, 4 ') with opposite electrical charges.

2. The method according to claim 1, characterized in that the structuring of the at least one surface of the first polymer film (1) takes place in step a) by embossing.

3. The method according to claim 2, characterized in that the embossing is carried out using a structured roller.

4. The method according to claim 2, characterized in that the embossing is carried out using a structured stamping die.

5. The method according to claim 1, characterized in that the structuring of the at least one surface of the first polymer film (1) in step a) by deformation of the optionally heated polymer film (1) under pressurization in a mold with an optionally pre-tempered contour insert.

6. The method according to claim 1, characterized in that the structuring of the at least one surface of the first polymer film (1) in step a)

Slit extrusion of the polymer film with a shaped nozzle takes place.

7. The method according to claim 1, characterized in that the bonding of the polymer films to form a polymer film composite in step c) takes place by lamination, gluing, clamping, clamping, screwing, riveting or welding (for example laser welding, ultrasonic welding, vibration welding).

8. The method according to claim 1, characterized in that before and / or after the electrical charging of the inner surfaces of the cavities in step d), the application of electrodes to the outer surfaces of the polymer film composite takes place.

9. The method according to claim 1, characterized in that the steps a), b), c) and / or d) are carried out as a continuous roll-to-roll process.

10. The method according to claim 1, characterized in that it comprises as a further step e) before or after the charging in step d) the sealing of the edges of the polymer film composite formed in step c).

11. The method according to claim 1, characterized in that it comprises, as a further step f) before the polarity in step d), the filling of a gas into the cavities in the polymer film composite.

12. ferroelectret two- or multi-layer composite comprising a layer stack of at least one first polymer film (1) and a second connected to this

Polymer film (1 ', 5), characterized in that at least the first polymer film (1) at least on its the second polymer film (V, 5) facing surface side structuring with elevations and depressions and the first polymer film (1) with the second polymer film ( 1 ', 5) is connected such that between the polymer films (1) (T, 5) one or more cavities (4) are formed and further the inner

Surfaces of the formed cavities (4) are provided with opposite electrical charges.

13. Ferroelectret two- or multi-layer composite according to claim 12, characterized in that the shape of the cross-sectional areas of the cavities parallel and perpendicular to the layer course of the polymer films, independently, partially or completely selected from regular and irregular, round, elliptical or oval, polygonal, honeycomb , cross-shaped, star-shaped and partially round and partially polygonal forms.

14. Piezoelectric element containing at least one ferroelectret Two- or multi-layer composite according to claim 12 and / or at least one ferroelectret Two or multi-layer composite produced by a process according to claim 1.

15. An apparatus for carrying out the method according to claim 1, characterized in that it comprises means for structuring at least one surface of a first polymer film.