Patent Application
20230273495
An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis of charged pigment particles dispersed in a solvent or solvent mixture. The display typically comprises two electrodes placed opposing each other which provide an electric field to drive the motion of the charged pigment particles. One of the electrodes is usually transparent. When a voltage difference is imposed between the two electrodes, the pigment particle(s) migrate to one side or the other causing either the color of the pigment particles or the color of the solvent (if colored) being seen from the viewing side. The electrophoretic fluid typically includes a non-polar solvent and one or more sets of charged particles. The particles may have different optical properties (colors), different charges (positive or negative), different charge magnitudes (zeta potentials), and/or different absorptive properties (broadly light-absorbing, broadly light-reflecting, or selectively-absorbing or selectively reflecting). In the instance where there are multiple particle sets with opposite charge polarities, application of an electric field may cause a particle of one set to appear at the viewing surface while the other particle is driven away from the viewing surface.
Many electrophoretic displays are bi-stable: their optical state persists even after the activating electric field is removed. Bistability is mostly due to induced dipole charge layers forming around the charged pigments due to complex interactions between the pigments, charge control agents, and free polymers dispersed in the solvent. A bistable display can last for years in the last-addressed optical state before being switched again with the application of a new driving field.
Driving an electrophoretic display requires a power source to provide the electric field between the electrodes. The power source is typically a battery, which provides power to the electrodes via driving circuitry. One or more electrodes may be incorporated into an active matrix backplane. The power supply could also be, e.g., a photovoltaic cell, a fuel cell, or a power supply that operates from wall current. The power supply could also be a piezo-electric element which creates charge through physical motion or thermal expansion, as described in U.S. Pat. No. 5,930,026, which is incorporated by reference in its entirety. In all of these examples, some type of driving circuitry is required to provide an electrical pathway between the power source and the electrodes and typically, the circuitry includes control elements such as switches, transistors, etc. In most instances, the circuitry is fairly routine, however it typically adds bulk and structural limitations (i.e., not flexible or twistable) to the final display. There is a need for very simple, flexible, durable, and thin electrophoretic displays for applications such as security markers, sensors, and indicators.
According to one aspect of the subject matter disclosed herein, an electro-optic display may include a layer of electrophoretic material; a first conductive layer; and a piezoelectric material positioned between the layer of electrophoretic material and the first conductive layer, the piezoelectric material overlaps with a portion of the layer of electrophoretic material, and a portion of the first conductive layer overlaps with the rest of the electrophoretic material.
In a first aspect, the invention includes an electrophoretic display film, less than 100 μm thick (top to bottom), comprising a first adhesive layer, an electrophoretic medium layer, a patterned piezo electric layer comprising zones of differential polarization, and a flexible, light-transmissive electrode layer. In some embodiments, the electrophoretic medium layer comprises a plurality of microcapsules containing a non-polar fluid and charged pigment particles that move toward or away from the piezo electric layer when the piezo electric layer is flexed, wherein the microcapsules are coupled to each other with a polymer binder. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles that move toward or away from the piezo electric layer when the piezo electric layer is flexed, wherein the non-polar fluid and charged pigment particles are sealed in the microcells with a sealing layer. In some embodiments, the film is less than 50 μm thick. In some embodiments, the patterned piezo electric layer comprises polyvinylidene fluoride (PVDF). In some embodiments, the PVDF is poled to create the zones of differential polarization. In some embodiments, the flexible, light-transmissive electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible, light-transmissive electrode layer comprises poly(3,4-ethylenedioxythiophene) (PEDOT). In some embodiments, the invention includes an electrophoretic display film assembly comprising a release sheet coupled to an electrophoretic display film as described above, wherein the release sheet is coupled to the first adhesive layer. In some embodiments, a second adhesive layer coupled to the flexible, light-transmissive electrode layer, and a second release sheet coupled to the second adhesive layer.
In a second aspect, the invention includes a method of making an electrophoretic display film. The method includes the steps of coupling a film of polyvinylidene fluoride (PVDF) to a polymer film comprising acrylates, vinyl ethers, or epoxides to create a piezo-microcell precursor film, coupling the piezo-microcell precursor film to a flexible, light-transmissive electrode layer, coupling the light-transmissive electrode layer to a first release film with a first adhesive layer, embossing the piezo-microcell precursor film to create an array of microcells, wherein the microcells have a bottom, walls, and a top opening, filling the microcells with an electrophoretic medium through the top opening, and sealing off the top opening of the filled microcells with a water-soluble polymer. In some embodiments, the method further comprises applying a primer to the polymer film comprising acrylates, vinyl ethers, or epoxides before coupling the polymer film to the film of polyvinylidene fluoride (PVDF). In some embodiments, the method further comprises coupling the water-soluble polymer to a second release film with a second adhesive layer. In some embodiments, the method further comprises removing the first release film to produce an electrophoretic display film that is less than 100 μm thick. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles that move toward or away from the piezo electric layer when the piezo electric layer is flexed, wherein the non-polar fluid and charged pigment particles are sealed in the microcells with a sealing layer. In some embodiments, the PVDF is poled to create differential zones of polarization. In some embodiments, the flexible, light-transmissive electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible, light-transmissive electrode layer comprises poly(3,4-ethylenedioxythiophene) (PEDOT). In some embodiments, the film of polyvinylidene fluoride is patterned with an electric field to create areas of differing polarization. In some embodiments, the method further comprises patterning the completed electrophoretic display film with an electric field to create areas of differing polarization in the film of polyvinylidene fluoride.
In a third aspect, the invention includes a method of making an electrophoretic display film. The method comprises dispersing a polyvinylidene fluoride (PVDF) solution on a first release to produce a PVDF film less than 10 μm in thickness, coupling the PVDF film to a second release with a conductive adhesive, removing the first release, coupling a polymer film comprising acrylates, vinyl ethers, or epoxides to create a piezo-microcell precursor film, coupling the piezo-microcell precursor film to a flexible, light-transmissive electrode layer, coupling the light-transmissive electrode layer to a first release film with a first adhesive layer, embossing the polymer film comprising acrylates, vinyl ethers, or epoxides to create an array of microcells, wherein the microcells have a bottom, walls, and a top opening, filling the microcells with an electrophoretic medium through the top opening, and sealing off the top opening of the filled microcells with a water-soluble polymer. In some embodiments, the method further comprises applying a primer to the polymer film comprising acrylates, vinyl ethers, or epoxides before coupling the polymer film to the PVDF film. In some embodiments, the method further comprises coupling the water-soluble polymer to a second release film with a second adhesive layer. In some embodiments, the method further comprises removing the first release film to produce an electrophoretic display film that is less than 100 μm thick. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles that move toward or away from the piezo electric layer when the piezo electric layer is flexed, wherein the non-polar fluid and charged pigment particles are sealed in the microcells with a sealing layer. In some embodiments, the PVDF is poled to create zones of differential polarization. In some embodiments, the flexible, light-transmissive electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible, light-transmissive electrode layer comprises poly(3,4-ethylenedioxythiophene) (PEDOT). In some embodiments, the PVDF film is patterned with an electric field to create areas of differential polarization. In some embodiments, the method further comprises patterning the completed electrophoretic display film with an electric field to create areas of differential polarization in the PVDF film.
In a fourth aspect, an electrophoretic display film, less than 100 μm thick (top to bottom), comprising, a first adhesive layer, a patterned piezo electric layer comprising zones of differential polarization, an electrophoretic medium layer, and a flexible, light-transmissive electrode layer. In some embodiments, the electrophoretic medium layer comprises a plurality of microcapsules containing a non-polar fluid and charged pigment particles that move toward or away from the piezo electric layer when the piezo electric layer is flexed, wherein the microcapsules are coupled to each other with a polymer binder. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles that move toward or away from the piezo electric layer when the piezo electric layer is flexed, wherein the non-polar fluid and charged pigment particles are sealed in the microcells with a sealing layer. In some embodiments, the sealing layer is conductive. In some embodiments, the film is less than 50 μm thick. In some embodiments, the patterned piezo electric layer comprises polyvinylidene fluoride (PVDF). In some embodiments, the PVDF is poled to create differential zones of polarization. In some embodiments, the flexible, light-transmissive electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible, light-transmissive electrode layer comprises poly(3,4-ethylenedioxythiophene) (PEDOT). In some embodiments, the invention includes an electrophoretic display film assembly comprising a release sheet coupled to an electrophoretic display film as described above, wherein the release sheet is coupled to the first adhesive layer. In some embodiments, the electrophoretic display film additionally includes a second adhesive layer coupled to the flexible, light-transmissive electrode layer, and a second release sheet coupled to the second adhesive layer.
In a fifth aspect, the invention includes a method of patterning a piezo-electrophoretic medium film. The method includes coupling a film of polyvinylidene fluoride (PVDF) to a layer of electrophoretic media to create a piezo-electrophoretic medium film, and patterning the piezo-electrophoretic medium film with an electric field. In some embodiments, the electric field is provided by a corona discharge. In some embodiments, the method additionally includes disposing a conductive mask adjacent the piezo-electrophoretic medium film before patterning the piezo-electrophoretic medium film with the corona discharge. In some embodiments, the electric field is provided by a high-voltage write head. In some embodiments, the patterning includes forming regions of differing polarities within the PVDF. In some embodiments, the patterning creates a security marker. In some embodiments, the layer of electrophoretic media comprises a plurality of microcapsules containing a non-polar fluid and charged pigment particles that move toward or away from the piezo electric layer when the piezo electric layer is flexed, wherein the microcapsules are coupled to each other with a polymer binder. In some embodiments, the layer of electrophoretic media comprises a plurality of microcells containing a non-polar fluid and charged pigment particles that move toward or away from the piezo electric layer when the piezo electric layer is flexed, wherein the non-polar fluid and charged pigment particles are sealed in the microcells with a sealing layer.
In a sixth aspect, the invention includes an electrophoretic display film, less than 100 μm thick (top to bottom), including an adhesive layer, an electrophoretic medium layer, a patterned piezo electric layer comprising zones of differential polarization, and a conductive adhesive layer. In some embodiments, the electrophoretic medium layer comprises a plurality of microcapsules containing a non-polar fluid and charged pigment particles that move toward or away from the piezo electric layer when the piezo electric layer is flexed, wherein the microcapsules are coupled to each other with a polymer binder. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles that move toward or away from the piezo electric layer when the piezo electric layer is flexed, wherein the non-polar fluid and charged pigment particles are sealed in the microcells with a sealing layer. In some embodiments, the sealing layer is conductive. In some embodiments, the film is less than 50 μm thick. In some embodiments, the patterned piezo electric layer comprises polyvinylidene fluoride (PVDF). In some embodiments, the PVDF is poled to create the zones of differential polarization. In some embodiments, the invention includes an electrophoretic display film assembly comprising a release sheet coupled to an electrophoretic display film as described above, wherein the release sheet is coupled to the first adhesive layer. In some embodiments, the invention includes an electrophoretic display film assembly comprising a release sheet coupled to an electrophoretic display film including a conductive adhesive layer, wherein the release sheet is coupled to the conductive adhesive layer.
In a seventh aspect, the invention includes an electrophoretic display film, less than 100 μm thick (top to bottom), comprising an adhesive layer, a patterned piezo electric layer comprising zones of differential polarization, an electrophoretic medium layer, and a conductive adhesive layer.
Low-profile piezo-electrophoretic films and display films including low profile piezo-electrophoretic films are disclosed herein. In some embodiments, the piezoelectric material of the piezo-electrophoretic films can be patterned with high-voltage electric fields after fabrication of the piezo-electrophoretic films. This feature allows a final user to address the piezoelectric materials with, e.g., a corona discharge at the point of production, which may include, e.g., a bar code or a serial number that is only viewable when the piezo-electrophoretic film is manipulated. Such films are useful as security markers, authentication films, or sensors. The films are generally flexible. Some films are less than 100 μm in thickness. In some embodiments, the piezo-electrophoretic films are less than 50 μm and foldable without breaking. Displays formed with the films do not require an external power source.
The term “electro-optic”, as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
The term “gray state” is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the E Ink patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate “gray state” would actually be pale blue. Indeed, as already mentioned, the change in optical state may not be a color change at all. The terms “black” and “white” may be used hereinafter to refer to the two extreme optical states of a display, and should be understood as normally including extreme optical states which are not strictly black and white, for example, the aforementioned white and dark blue states. The term “monochrome” may be used hereinafter to denote a display or drive scheme which only drives pixels to their two extreme optical states with no intervening gray states.
The term “pixel” is used herein in its conventional meaning in the display art to mean the smallest unit of a display capable of generating all the colors which the display itself can show. In a full color display, typically each pixel is composed of a plurality of sub-pixels each of which can display less than all the colors which the display itself can show. For example, in most conventional full color displays, each pixel is composed of a red sub-pixel, a green sub-pixel, a blue sub-pixel, and optionally a white sub-pixel, with each of the sub-pixels being capable of displaying a range of colors from black to the brightest version of its specified color.
Several types of electro-optic displays are known. One type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium is also typically bistable.
Another type of electro-optic display is an electro-wetting display developed by Philips and described in Hayes, R. A., et al., “Video-Speed Electronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003). It is shown in U.S. Pat. No. 7,420,549 that such electro-wetting displays can be made bistable.
One type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays.
An electrophoretic display normally comprises a layer of electrophoretic material and at least two other layers disposed on opposed sides of the electrophoretic material, one of these two layers being an electrode layer. In most such displays both the layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongate row electrodes and the other into elongate column electrodes running at right angles to the row electrodes, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electrophoretic display, which is intended for use with a stylus, print head or similar movable electrode separate from the display, only one of the layers adjacent the electrophoretic layer comprises an electrode, the layer on the opposed side of the electrophoretic layer typically being a protective layer intended to prevent the movable electrode damaging the electrophoretic layer.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in these patents and applications include:
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
A related type of electrophoretic display is a microcell electrophoretic display, also known as MICROCUP®. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449, both of which are incorporated by reference in their entireties.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Pat. Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode. Electro-optic media operating in shutter mode may be useful in multi-layer structures for full color displays; in such structures, at least one layer adjacent the viewing surface of the display operates in shutter mode to expose or conceal a second layer more distant from the viewing surface.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See U.S. Pat. No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed, using a variety of methods, the display itself can be made inexpensively.
The aforementioned U.S. Pat. No. 6,982,178 describes a method of assembling a solid electro-optic display (including an encapsulated electrophoretic display) which is well adapted for mass production. Essentially, this patent describes a so-called “front plane laminate” (“FPL”) which comprises, in order, a light-transmissive electrically-conductive layer; a layer of a solid electro-optic medium in electrical contact with the electrically-conductive layer; an adhesive layer; and a release sheet. Typically, the light-transmissive electrically-conductive layer will be carried on a light-transmissive substrate, which is preferably flexible, in the sense that the substrate can be manually wrapped around a drum (say) 10 inches (254 mm) in diameter without permanent deformation. The term “light-transmissive” is used in this patent and herein to mean that the layer thus designated transmits sufficient light to enable an observer, looking through that layer, to observe the change in display states of the electro-optic medium, which will normally be viewed through the electrically-conductive layer and adjacent substrate (if present); in cases where the electro-optic medium displays a change in reflectivity at non-visible wavelengths, the term “light-transmissive” should of course be interpreted to refer to transmission of the relevant non-visible wavelengths. The substrate will typically be a polymeric film, and will normally have a thickness in the range of about 1 to about 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to 254 μm). The electrically-conductive layer is conveniently a thin metal or metal oxide layer of, for example, aluminum or ITO, or may be a conductive polymer. Poly (ethylene terephthalate) (PET) films coated with aluminum or ITO are available commercially, for example as “aluminized Mylar” (“Mylar” is a Registered Trade Mark) from E.I. du Pont de Nemours & Company, Wilmington Del., and such commercial materials may be used with good results in the front plane laminate.
Assembly of an electro-optic display using such a front plane laminate may be effected by removing the release sheet from the front plane laminate and contacting the adhesive layer with the backplane under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, layer of electro-optic medium and electrically-conductive layer to the backplane. This process is well-adapted to mass production since the front plane laminate may be mass produced, typically using roll-to-roll coating techniques, and then cut into pieces of any size needed for use with specific backplanes.
U.S. Pat. No. 7,561,324 describes a so-called “double release sheet” which is essentially a simplified version of the front plane laminate of the aforementioned U.S. Pat. No. 6,982,178. One form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of the adhesive layers being covered by a release sheet. Another form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of the double release film are intended for use in a process generally similar to the process for assembling an electro-optic display from a front plane laminate already described, but involving two separate laminations; typically, in a first lamination the double release sheet is laminated to a front electrode to form a front sub-assembly, and then in a second lamination the front sub-assembly is laminated to a backplane to form the final display, although the order of these two laminations could be reversed if desired.
The subject matter presented herein, in particular, relates to piezo-electrophoretic display structural designs which do not need a power supply (e.g., battery or wired power supply, photovoltaic source, etc.,) in order for the electrophoretic display to operate. The assembly of such an electrophoretic display is therefore simplified. In some embodiments, the piezoelectric material and the electrophoretic media are directly laminated together. The electrophoretic medium may be contained in microcells, microcapsules, or the electrophoretic medium may be dispersed in a polymer matrix, as described above. In some embodiments the piezoelectric material is polarized (i.e., written) with a high-voltage electric field after the piezo-electrophoretic film or piezo-electrophoretic display has been created, as discussed below.
Piezoelectricity is the charge which accumulates in a solid material in response to applied mechanical stress. Suitable materials for the subject matter disclosed herein may include polyvinylidene fluoride (PVDF), quartz (SiO2), berlinite (AlPO4), gallium orthophosphate (GaPO4), tourmaline, barium titanate (BaTiO3), lead zirconate titanate (PZT), zinc oxide (ZnO), aluminum nitride (AlN), lithium tantalite, lanthanum gallium silicate, potassium sodium tartrate and any other known piezo materials. In piezoelectric materials,
Piezo-electrophoretic films and piezo-electrophoretic displays described herein use piezoelectricity to drive the charged pigments of an electrophoretic medium Thus, when the piezoelectric material coupled to an electrophoretic media layer is manipulated, the color of the electrophoretic material at the viewing surface changes. For example, by bending or introduce stress to a piece of piezo material, voltage may be generated and this voltage can be utilized to cause movement of the color pigments of the electrophoretic material. If segments of piezoelectric material with different polarizations are used, or if areas of differential polarization are created in a piezoelectric film, an electrophoretic medium having two types of oppositely-charged pigments can be used to create patterns with high contrast ratios, as shown in
In principle, a piezoelectric material (optionally adjacent an electrophoretic material) can be polarized with a localized strong electric field, as shown in
While many piezoelectric materials are crystalline, a number of flexible piezo-active polymers are known, such as polyvinylidene fluoride (PVDF) and its copolymers, polyamides, and parylene-C. Non-crystalline polymers, such as polyimide and polyvinylidene chloride (PVDC), fall under amorphous bulk polymers. The standard procedure to make piezo active films, such as polyvinylidene fluoride (PVDF), is to create the polymer film and stretch it to create stress and align the dipoles. Stretching transforms unpolarized alpha phase regions of PVDF to polarized beta phase. A subsequent stimulus is added to pole regions of beta phase, for example, using strong electric fields. Other methods of aligning beta phases have been described in the literature, such as laser irradiation and intense magnetic fields. See, e.g., U.S. Pat. No. 9,831,417. If the stimulus is can be done with sufficiently high resolution, the poles can be used to create visible patterns, e.g., as illustrated in
An exemplary method for poling a thin film of piezoelectric material is illustrated in
As shown in
Using the techniques shown in
Once the thin piezo-electrophoretic medium film 405 has been created, thin film of piezoelectric material 410 can be addressed as described above with respect to
The techniques described above can be used to achieve a great variety of thin piezo-electrophoretic films as described in the following figures.
As shown in
A piezo-electrophoretic film or a piezo-electrophoretic display will often include at least one electrode layer, which may be light-transmissive, and which may be flexible. Suitable materials include commercial ITO-coated PET, which may be used as substrate for manufacturing. In some other embodiments, flexible and transparent conductive coatings including other transparent conductive oxides (TCOs) may be used, such as, zinc oxide, zinc tin oxide, indium zinc oxide, aluminum zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, or fluorinated variants of these oxides such as fluorine-doped tin oxide. In many of the embodiments described herein, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is used because it has excellent bending properties and is optically transparent. While the overall conductivity is not as high as, e.g., PET/ITO, PEDOT:PSS is sufficient to provide the necessary electric field to drive the electrophoretic particles in the electrophoretic medium. Other materials include polymers, typically light-transmissive polymers, that are doped with conductive materials such as carbon black, metal flakes, metal whiskers, carbon nanotubes, silicon nitride nanotubes, or graphene. In some instances, the electrode layer is a metal film, such as a copper, silver, gold, or aluminum film or foil. Metal-coated polymer films may also be suitable for use as an electrode layer. The resistance of the electrode layer may be at 500 Ohm-m or less, e.g., 100 Ohm-m or less, e.g., 1 Ohm-m or less, e.g., 0.1 Ohm-m or less, e.g., 0.01 Ohm-m or less. (For comparison the electrophoretic medium layer typically has a resistance of approximately 107 to 108 Ohm-m, and the piezoelectric material has a resistance of 1011 to 1014 Ohm-m.)
A piezo-electrophoretic film or a piezo-electrophoretic display will often include at least one adhesive layer, formed from a polymer such as an acrylic or a polyurethane. polyurethanes, polyureas, polycarbonates, polyamides, polyesters, polycaprolactones, polyvinyl alcohol, polyethers, polyvinyl acetate derivatives such as poly(ethylene-co-vinylacetate], polyvinyl fluoride, polyvinylidene fluoride, polyvinyl butyral, polyvinylpyrrolidone, poly(2-ethyl-2-oxazoline), acrylic or methacrylic copolymers, maleic anhydride copolymers, vinyl ether copolymers, styrene copolymers, diene copolymers, siloxane copolymers, cellulose derivatives, gum Arabic, alginate, lecithin, polymers derived from amino acids, and the like. The adhesives may additionally include one or more low dielectric polymers or oligomers, ionic liquids, or conductive fillers such as carbon black, metal flakes, metal whiskers, carbon nanotubes, silicon nitride nanotubes, or graphene. Adhesives including such charged and/or conducting materials are conductive adhesives. The polymers and oligomers used in the adhesive layer may have functional group(s) for chain extension or crosslinking during or after lamination. The adhesive layer may have a resistivity value of roughly 106 Ohm*cm to 108 Ohm*cm, preferably less than 1012 Ohm*cm.
Among the polymers and oligomers mentioned above, polyurethanes, polyureas, polycarbonates, polyesters and polyamides, especially those comprising a functional group, are particularly preferred because of their superior adhesion and optical properties and high environmental resistance. Examples for the functional groups may include, but are not limited to, —OH, —SH, —NCO, —NCS, —NHR, —NRCONHR, —NRCSNHR, vinyl or epoxide and derivatives thereof, including cyclic derivatives. The “R” in the functional groups mentioned above may be hydrogen or alkyl, aryl, alkylaryl or arylalkyl of up to 20 carbon atoms which alkyl, aryl, alkylaryl or arylalkyl may be optionally substituted or interrupted by N, S, O or a halogen. The “R” preferably is hydrogen, methyl, ethyl, phenyl, hydroxymethyl, hydroxyethyl, hydroxybutyl or the like. Functionalized polyurethanes, such as hydroxyl terminated polyester polyurethanes or polyether polyurethanes, isocyanate terminated polyester polyurethanes or polyether polyurethanes or acrylate terminated polyester polyurethanes or polyether polyurethanes are particularly preferred.
In many embodiments, a piezo-electrophoretic film or a piezo-electrophoretic display will often include a release sheet. The release may be use temporarily to facilitate processing piezo-electrophoretic film or a piezo-electrophoretic display, e.g., when embossing, filling, cutting, etc. In other embodiments the release may be used to deliver a final piezo-electrophoretic film or a piezo-electrophoretic display that will be adhered to a final product. In some instances the release will protect a functional adhesive layer that will be used to manipulate the piezo-electrophoretic film or a piezo-electrophoretic display prior to the piezo-electrophoretic film or a piezo-electrophoretic display being disposed in a final product. The release may be formed from a material selected from the group consisting of polyethylene terephthalate (PET), polycarbonate, polyethylene (PE), polypropylene (PP), paper and a laminated or cladding film thereof. The release may also be metalized to facilitate quality control measurements and/or to control static electricity during handling, shipping, and downstream incorporation into products. In some embodiments, a silicone release coating may be applied onto the release to improve the release properties.
While not shown in
In general, the layers of a piezo-electrophoretic film 501-504 can be arranged/laminated in the order that produces the best performance for an end application. For example, as shown in
Closely-related, but alternative stacks are shown in
Of course, it is also possible to build the stack such that the openings of the microcell layer 530 face toward the piezoelectric layer 560, as in piezo-electrophoretic film 504 illustrated in
The piezo-electrophoretic films (501, 502, 503, 504) shown in
A series of piezo-electrophoretic films of the type exemplified in
Table 1 suggests that multiple types of electrophoretic media will respond suitably to the small electric fields produced by flexing thin piezo films. In particular, it was found that a spin-coated polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) film of less than 3 μm had sufficient charge injection to cause DV electrophoretic media to switch. See Experiment number 7. Such a piezo-electrophoretic film 801 (see
Similar to
An alternative method of constructing piezo-electrophoretic films and piezo-electrophoretic displays is described with respect to the flow chart of
As shown in
Finally, it is to be appreciated that an electrode need not be coupled to the piezoelectric film 1260 prior to embossing the stack comprising the piezoelectric film 1260 and the microcell precursor material. Rather a stack including release 1210, adhesive 1220, piezoelectric film 1260, and microcell precursor can be prepared and the microcell precursor subsequently embossed, filled, and sealed as described above. Alternatively, a stack including release 1210, adhesive 1220, electrode 1285, piezoelectric film 1260, and microcell precursor can also be prepared and the microcell precursor subsequently embossed, filled, and sealed as described above, as shown in
It is to be appreciated that piezo-electrophoretic films and piezo-electrophoretic displays described herein can be combined with other known techniques for creating security markers or authenticity labels. For example, a piezo-electrophoretic film or piezo electrophoretic display may additionally include a semi-transparent overlay that does not change optical properties when the piezoelectric film is manipulated. For example, a smiley-face overlay may include eyes constructed from piezo-electrophoretic displays such that when the layered material is bent, the eyes appear to blink. In some embodiments, images or shapes may be printed or laminated onto a solid-colored (e.g., white) background, and must be viewed through the piezo-electrophoretic films to see a pre-arranged pattern. Thus, when not in use, a viewer only sees the solid color, i.e., the printed image or shape will be hidden. However, the printed image or shape will be displayed when the device is manipulated. It is also feasible to adhere a piezo-electrophoretic film or piezo-electrophoretic display to a separate light-transmissive polymer film included in the target product (e.g., bank note) such that the pattern in the piezoelectric layer is only viewable when the target product is held up to a light source and manipulated.
It will be apparent to those skilled in the art that numerous changes and modifications can be made to the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be interpreted in an illustrative and not in a limitative sense.
This application claims priority to U.S. Patent Application No. 63/314,584, filed Feb. 28, 2022. All patents and publications disclosed herein are incorporated by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| 63314584 | Feb 2022 | US |
