Field of the Invention

[0001 ] The invention generally relates to the production of a piezoelectret film, especially, but not exclusively, for use in a smart floor covering. Background of the Invention

[0002] Piezoelectret (or ferroelectret) polymer films are foamed polymer films carrying permanent electric charges on the interface between the polymer and the gas inside the pores (voids) of the foam. The electric charges form macroscopic electric dipoles. Upon compression of the foam in the thickness direction thereof, the moments of the electric dipoles and/or the density of the electric dipoles change, which effect may be used to generate a voltage or a current between electrodes applied on the faces of the polymer films.

[0003] The polymer films used as the base material are typically foamed polypropylene (PP), cross-linked polypropylene (XPP) or biaxially oriented polypropylene (BOPP) films. For instance, the paper "Piezoelectricity of cross-linked polypropylene films treated by hot-stretching," D. Pan et al., Proceedings of the 9 ^th International Conference on Properties and Applications of Dielectric Materials, July 19-23, 2009, Harbin, China, discloses the use of commercial XPP (cross-linked polypropylene) foam sheets, which were hot-stretched, then electro-polarized by corona charging and finally metallized by aluminium evaporation. The paper "Piezoelectric properties of irradiation-cross-linked polypropylene ferroelectrets," X. Zhang et al., Applied Physics Letters 91 , 182901 (2007) discloses the use of irradiation-cross-linked PP (IXPP), which is hot-pressed and then cooled down before the corona charging in order to obtain disk-like voids. [0004] There are several techniques for producing foamed films which may be used in the production of piezoelectrets. Document WO 00/13879 A1 discloses a method for producing a biaxially oriented film from a foamed orientable thermoplastic polymer. A melt of thermoplastic polymer, e.g. polypropylene, containing a gas or a low-boiling liquid or a chemical foaming agent dissolved or finely dispersed therein is extruded and solidified by cooling. The solidified film is then simultaneously stretched in longitudinal and crosswise direction. [0005] Document US 2009/0008812 A1 discloses a process, wherein plastic material containing nucleating agent and at least one of chemical foaming agent, gas and foam- generating liquid is extruded through a cooled nozzle in order to prevent foaming when the plastic exits the extruder. The plastic film preform is stretched first in machine direction, then in cross-direction, whereby cavitation bubbles are formed. The formation of bubbles thus takes place concomitantly with stretching and relaxation, when the plastic is no longer in a molten state.

General Description

[0006] A first aspect of the invention relates to method of producing a piezoelectret film. The method comprises supercritical-fluid-foaming of a polymer material and continuously feeding the foamed polymer in the form of a foamed sheet into a continuous production line, in which the foamed sheet is subjected to a DC corona treatment creating a separation of electric charges in pores of the foamed sheet. Preferably, the supercritical-fluid-foaming is carried out the following way: the polymer material is fed into a plastic extruder and a blowing agent is admixed to the polymer material as a supercritical fluid by injection into the plastic extruder; the polymer material enriched with the blowing agent is then extruded through an extrusion die. Upon depressurization at the die, the blowing agent expands the polymer material to form a polymer scaffold in the form of foamed sheet. The foamed sheet is then introduced into the continuous sheet production line, in which the foamed sheet is subjected to corona charging treatment creating a separation of electric charges in pores of the foamed sheet and in which the foamed sheet may also be provided with a conductive layer on each face.

[0007] The production method according to the invention has the advantage that all steps of piezoelectret film manufacturing may be united in a continuous sheet production line. Previously explored production methods used commercially available foamed films or precursors of foamed films, which are then processed and charged individually or in batches. Batch production however results in variations of the properties of the piezoelectret films and to comparatively high reject rates. Furthermore, it may be difficult to identify the source(s) of any deficiencies of the final products. [0008] It will be appreciated that the blowing agent is a physical blowing agent, e.g. CO2, N2, pentane, isopentane, cyclopentane or a mixture thereof.

[0009] The polymer material could be a polyolefin (e.g. polypropylene), polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), cyclo olefin copolymer (COC) or any other suitable thermoplastic polymer material. According to an embodiment, the polymer material comprises or consists (essentially) of polypropylene, which has the advantages of easier processing, lower cost, and significantly higher melting temperature than polyethylene.

[0010] The polymer material may comprise a nucleating agent, preferably a solid- state nucleating agent, such as, e.g. CaCO3 particles. Solid-state nucleation agent is preferred as it is less prone to migrate in the foamed polymer film. Migration may be a cause of undesired decrease of the electric polarization of the foam.

[001 1 ] Downstream of the extruder, the foamed sheet may be subjected to stretching in the machine direction and/or in the cross-direction of the continuous sheet production line. If stretching is carried out in both directions, that may be carried out simultaneously or in a two-stage process

[0012] Preferably, the foamed sheet is stretched by a factor comprised in the range from 1 .05 and 5 (for each direction of stretching). More preferably, the stretching ratio is adjusted to optimize the aspect ratio of the voids to minimize the z direction (thickness direction) Young modulus of the foamed sheet.

[0013] An AC corona treatment can be applied to one or both sides of the foamed sheet to allow further processing and integrating it into a final sensor structure. AC corona treatment intensity is preferably in the range from 10 to 50 W min/m ² . The unit W min/m ² is handy because it indicates the power (W) applied per web area passing by the corona treatment station in a minute (power in W / (line speed in m/min times the web width in m).

[0014] The DC corona treatment applied on the material to create a separation of electric charges in pores of the foamed sheet is preferably applied after the AC corona treatment. [0015] The conductive layer optionally applied on each face of the sheet may be a metallization, preferably of aluminum, silver, copper. The metallization may be deposited by any suitable technique compatible with continuous sheet production. The conductive layers can be provided by laminating a conductive sheet or a sheet carrying a conductive coating or comprising a conductive layer on the faces of the foamed sheet. Alternatively, the conductive layers may be applied by printing of conductive ink.

[0016] The extrusion die may have an annular opening through which the polymer material enriched with the blowing agent is extruded as a foamed tube. The foamed tube is then longitudinally slit to form the foamed sheet. Alternatively, the extrusion die could have a linear opening so that the extrudate is a sheet preform.

[0017] Preferably, an electrically insulating protective layer is provided on each of the conductive layers. According to an embodiment of the method, each protective layer carries a conductive layer (or vice versa) and is laminated on the foamed sheet. Conductive layer and protective layer are in this case applied on the foamed sheet at the same time. Alternatively, each protective layer is applied after the respective conductive layer is in place on the foamed sheet.

[0018] Optionally, the method comprises cross-linking the polymer material in the foamed sheet, e.g. using the e-beam technique.

[0019] Optionally, the method can comprise coextrusion of multiple layers, symmetrically or not, encapsulating the foamed core structure.

[0020] A second aspect of the invention relates to a piezoelectret film comprising a supercritical-foam produced polymer scaffold. Such a piezoelectret film may be obtained by the method according to the first aspect of the invention. According to an embodiment, the piezoelectret film has a piezoelectric d33 coefficient comprised in the range from 20 to 1000 pC/N.

[0021 ] A third aspect of the invention relates to a method of producing a smart floor covering. This method comprises providing a piezoelectret film, preferably produced in accordance with the method described hereinabove, providing a resilient floor covering comprising a walk-on-able (decorative) top surface and a bottom surface, and applying the piezoelectret film against the bottom surface of the resilient floor covering.

Brief Description of the Drawings

[0022] By way of example, preferred, non-limiting embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which: Fig. 1 : is an illustration of a first piezoelectret film production system configured for carrying out a method in accordance with an embodiment of the invention;

Fig. 2: is an illustration of a second piezoelectret film production system configured for carrying out a method in accordance with a further embodiment of the invention. Detailed Description Preferred Embodiments

[0023] Fig. 1 schematically illustrates a system for the production of piezoelectret films in accordance with an embodiment of the inventive method. Piezoelectret film production system 10 comprises an extruder 12 fed with flakes, pellets, granules, beads, or the like, of polymer material 14 via a hopper 16. A screw 18, driven by a motor 19, rotates in the barrel 20 of the extruder 12 and pushes the plastic material downstream.

[0024] While travelling through the barrel 20, the polymer material melts due to friction forces exerted on it by the screw 18 and the walls of the barrel 20. Temperature control elements (heating and/or cooling elements, not shown) positioned along the barrel 20 may introduce additional heat so as to facilitate the melting and/or remove excess heat so as to keep the polymer melt within a desired temperature range or to achieve a desired temperature profile in the extruder.

[0025] The extruder 12 further comprises a blowing agent inlet 22 for injecting blowing agent 23 (e.g. CO2 and/or N2) in the state of a supercritical fluid. Inside at least a portion of the barrel, the polymer melt is subjected to high temperature and pressure (e.g. in the range from 200 C to 300 C at a pressure in the range from 5 MPa to 10 MPa), which allows the blowing agent to be homogeneously mixed with the polymer melt. Depending on the polymer and blowing agent species, as well as the conditions of pressure and temperature inside the barrel, the mixture of polymer material and blowing agent may form a single-phase solution or a dispersion.

[0026] The mixture of polymer material and blowing agent is extruded through a die 24. The passageway of the die 24 defines the shape of the extrudate 26 and the pressure gradient at the die 24. The geometry of the die influences nucleation of the mixture or polymer material and blowing agent. Upon depressurization at the die 24, the blowing agent turns into the gas phase and forms bubbles with a matrix of polymer material. Preferably, the passageway of the die 24 has a substantially linear opening so that the foamed extrudate takes the form of a sheet preform. If the passageway of the die 24 is of annular cross-section, the foamed extrudate has the shape of a tube, which is slit longitudinally before being taken up by a continuous sheet production line 28.

[0027] The foamed sheet 26 obtained downstream of the die 24 (possibly after the slitting operation) is introduced into the continuous sheet production line 28, which comprises a stretching station 30. In the illustrated piezoelectret film production system 10, the stretching station 30 comprises a longitudinal stretching device 30a and a cross-direction stretching device 30b arranged one after the other. In other embodiments of the inventive method, stretching may be omitted or be carried out only in the longitudinal or in the transversal direction. It will be appreciated that stretching may have a major impact on the shape of the pores. Mono-directional stretching produces oblong (e.g. ovoid, oval or prolate ellipsoid) pores oriented in the direction of the stretching. Bi-directional stretching produces roughly lens-like (or disk-shaped or oblate ellipsoid) pores. It should be noted, however, that the extrusion process itself also influences the size and shape of the pores.

[0028] After stretching, the foamed sheet 26 passes an AC corona treatment machine 32, wherein the faces of the foamed sheet are subjected to an electrical surface treatment. The potential difference between the electrodes is preferably in the range from 20 to 80 kV at a frequency in the range from 15 to 25 kHz. The corona treatment forms reactive free radicals on the surface of the foamed sheet, which react with oxygen and yield functional groups that increase the surface energy and improve the wettability of the surface. The functional groups formed may include, e.g., carbonyl, carboxyl, hydroperoxide and hydroxyl moieties.

[0029] The foamed sheet 26 then passes a DC corona treatment machine 34. The DC corona treatment machine 34 creates a strong electric field across the foamed sheet. The electric field ionises gas inside pores of the foamed sheet and causes the negative and positive ions to move into opposite directions and deposit on opposite sides of the pores, thereby giving rise to a macroscopic electric dipole.

[0030] After corona charging, conductive sheets 36, 38 are applied on the opposite faces of the foamed sheet 26. In the illustrative piezoelectret film production system 10, the conductive sheets 36, 38 are laminated on the foamed sheet 26. Other processes, such as, e.g. thin film deposition, printing, etc., could be used as alternatives to lamination. [0031 ] Fig. 2 shows a schematic illustration of a system 1 10 for the production of piezoelectret films in accordance with a further embodiment of the inventive method. Piezoelectret film production system 1 10 comprises an extruder 1 12 substantially identical to the one detailed with respect to Fig. 1 . Extruder 1 12 comprises a barrel 120 accommodating a screw 1 18. Polymer material 1 14 is introduced via a hopper 1 16. Blowing agent 123 is introduced through blowing agent inlet 122. The mixture of polymer material and blowing agent is extruded through a die 124 so that a foamed sheet 126 is obtained. For further details on the extrusion process, the reader is invited to refer to the description of Fig. 1 . [0032] The foamed sheet 126 obtained downstream of the die 124 is introduced into a continuous sheet production line 128, which comprises a stretching station 130. The stretching station 130 is configured to stretch the foamed sheet 126 in the machine direction and, optionally, in the cross-direction.

[0033] After stretching, the foamed sheet 126 passes an AC corona treatment machine 132, wherein the faces of the foamed sheet are subjected to an electrical surface treatment, and a DC corona treatment machine 134, wherein the electrostatic charges inside the foamed sheet 126 are created.

[0034] After corona charging, conductive layers are applied on the opposite faces of the foamed sheet 126 by printing. A printing press is schematically shown at reference numeral 140.

[0035] After application of the conductive layers, protective insulating films 142 are applied thereon, e.g., by lamination.

Example 1

[0036] A stream of PP polymeric melt was created in a single-screw extruder purchased from Barmag using PP homopolymer flakes in combination with calcium carbonate as nucleating agent. The concentration of CaCO3 amounted to 1 phr (1 phr = part by weight per 100 parts of polymer resin). The blowing agent was a mixture of CO2 and N2 (4 parts by weight of CO2 for 1 part of N2). Injection of blowing agent was controlled so as to yield a concentration of between 0.75 and 1 .25 phr in the polymer melt. The temperature of the extruder barrel was between 240°C and 250°C in the zone from the blowing agent inlet to the breaker plate. The pressure in this region was between 7.5 and 8 MPa. The die was of the coat-hanger type. The foamed sheet was taken up by the calender rolls of the continuous sheet production line. No stretching was carried out. The foamed sheet was subjected to a corona surface treatment at 10 Wmin/m ² using a Softal corona generator to increase the surface energy. After the surface treatment, the foamed sheet was corona charged by DC corona treatment. Electrodes (conductive layers) were applied by printing on both faces of the foamed sheet.

[0037] While specific embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.