EP2678166A1 - Multicolor electrophoretic ink, associated display device, and production method - Google Patents

Abstract

The invention relates to a multicolor electrophoretic ink comprising at least four types of particles that are dispersed in a non-polar organic medium. Each type of particles contains a pigment of a color associated therewith and has a positive or negative electrostatic charge. The invention is characterized in that at least one of said types of particles has a magnetic property (magnetic core) such that each type of particles can migrate in a predefined manner under the combined effect of an electrostatic force and/or a magnetic restoring force.

Description

 POLYCHROME ELECTROPHORETIC INK, DISPLAY DEVICE THEREFOR, AND MANUFACTURING METHOD

The present invention relates to the field of inks for electrophoretic display devices, and more particularly to polychromatic inks.

More specifically, the invention relates to a polychromatic electrophoretic ink, to a process for manufacturing said ink, to a polychromatic electrophoretic display device comprising said ink and to a use of said polychromatic electrophoretic ink for producing a device. polychromic electrophoretic display.

There are currently essentially two information display modes. On the one hand, there are electronic displays of liquid crystal display (LCD) type or plasma type for example, and on the other hand displays by paper printing. The electronic displays are a great advantage because they are able to quickly update information displayed and therefore to change content, it is also said that they are rewritable. This type of display is however complex to achieve since its manufacture requires clean room work and advanced electronics. It is therefore relatively expensive. Displays made by print on paper, meanwhile, can be mass produced because very inexpensive, but do not re-register information over the old. This type of display is part of non-rewritable displays.

The idea of combining the benefits of both technologies was born a few years ago. A flexible display that can be manufactured at low cost and in large volume has been realized. This display is the analogue of the paper but in electronic version, that is to say that the information displayed on this medium can be erased to make room for other content quickly. In addition, unlike existing screens that need to be always powered in order to operate, the electronic paper consumes very little energy, only when changing the display. At a time when energy consumption is a major problem, having a reusable, flexible display device that mimics paper and consumes almost no energy is a great opportunity. On the other hand, the electronic paper is a reflective device, resulting in significantly improved reading comfort compared to backlit displays that tire the eye more importantly. This type of display is based on the EPIDS (ElectroPhoretic Image DisplayS) technology. This technology consists of dispersing charged particles in a nonconductive medium between two parallel electrodes. More specifically, the display comprises a conductive surface electrode, a cavity comprising pixels filled with electrophoretic ink, and a bottom electrode connected to transistors for controlling each pixel. The pixels can be made in different ways. They can for example be made by means of a grid which compartmentalizes the cavity in as many pixels as necessary for the display, or they can be in the form of microcapsules, each microcapsule defining a pixel and being filled with said ink . The electrophoretic ink has generally negatively charged white nanoparticles dipped in a black dye. When applying an electric field, the white nanoparticles of each pixel will migrate towards one or the other of the electrodes. Thus, when a negative electric field is applied, the white nanoparticles are placed on one end of the pixel revealing their white color or the color of the black dye according to their position relative to the surface of the display. Therefore, by placing millions of pixels in the display cavity and controlling them by electric fields, by means of an electronic circuit for managing the display of information, a two-color image can be generated. One of the advantages of this type of display is that the contrast obtained depends directly on the migration of the nanoparticles and the color thereof. In addition, the display obtained is bistable since the image remains in place even after the electric field is cut. Such displays based on the EPIDS technology are particularly envisioned for equipping mobile phones, electronic tablets, electronic books or on-board displays on smart cards for example. However, while having many advantages, screens based on EPIDS technology currently only display two-color information. In order to use this technology for mobile phone, tablet or e-book displays, it is important to improve this technology. display and propose a full-color display, in order to be competitive in the screens market.

Trials have been made to produce such screens capable of displaying information in color. Such a color display is based on 3 different principles: the establishment of a matrix of color filters over a two-color device, such as in particular described in US Pat. No. 7,289,101, the juxtaposition of two-color pixels displaying different colors, and finally the semi-selective migration of pigments based on a variation of the charge rate of the particles and thus on a variation of their electrophoretic mobility. However, such displays are quite complex to achieve and the display obtained does not offer a great contrast, so that the reading comfort is greatly reduced.

Also known is the article entitled "Pigment-based tricolor ink particles via mini-emulsion polymerization for chromatic electrophoretic displays", published in 2010, including Mr. Ting Wen. This article describes the synthesis of charged colored particles by a mini emulsion polymerization technique but does not provide for the use of magnetic properties in the synthesized particles. It should also be noted that in this article, the polymer used is styrene, a non-functional polymer and the feed is fed by an additive. Finally, the technology envisaged with the synthesis of such particles is the obtaining of color by juxtaposition of pixels each containing only two types of positive and negative particles which consists of the juxtaposition of conventional electrophoretic cells.

In addition, there is the article entitled "Towards Multi-color Microencapsulated Electrophoretic Display", published in 2005, including Mr. Chul Am Kim. This article describes the synthesis of a single white particle negatively charged with methacrylic acid by dispersion polymerization in methanol and does not disclose or even suggest the use of magnetic properties in the synthesized particles. In this favorable context for the development of display devices based on the EPIDS technology, the development of new inks allowing a polychrome display becomes crucial to increase the performance of such devices and therefore to increase their competitiveness in the market. The invention therefore aims to remedy at least one of the disadvantages of the prior art. The invention aims in particular to synthesize an electrophoretic ink comprising several pigments of different colors. To this end, the subject of the invention is a polychromic electrophoretic ink, comprising at least four types of particles dispersed in an apolar organic medium, each type of particles containing a pigment of a color associated with it, having a positive electrostatic charge. or negative, characterized in that at least one of the above types of particles has a magnetic property (magnetic core) so that each type of particles can migrate in a predetermined manner under the combined action of an electrostatic force and a magnetic return force.

Thus, by mixing particles having pigments of different colors, each particle of a color having a magnetic and electrostatic characteristic of its own, it becomes possible to migrate one or more of these particles in the ink, depending on the force magnetic and the electrostatic force applied to them at each pixel. Depending on their migration, the color particles are superimposed so that they can display polychromic information. Preferably, the ink comprises at least two types of magnetic core particles and two types of non-magnetic particles. Both types of magnetic core particles are further electrostatically charged respectively positively and negatively. Likewise, the two types of non-magnetic particles are also electrostatically charged respectively positively and negatively.

Thus, to migrate a non-magnetic particle for example to one or the other of the electrodes of an electrophoretic display, it is necessary to apply a positive or negative voltage across the electrodes, according to its electrostatic charge. Note the applied voltage V + or V-. To migrate a magnetic particle it will also be necessary to apply a positive or negative voltage across the electrodes according to its charge, but this voltage must be greater than that applied to move a non-magnetic particle since it must, moreover, overcome a force. of magnetic reminder applied to the particle. Note the applied voltage, to migrate such a magnetic particle, V ++ or V ~.

Both types of magnetic core particles are each associated with a color. Each magnetic core is covered by the pigment associated with it, and then encapsulated in a functional polymer electrostatically chargeable respectively positively and negatively. Likewise, each type of non-magnetic particle is associated with a color. The pigment chosen for a non-magnetic particle type is encapsulated in a functional polymer electrostatically chargeable respectively positively and negatively. Preferably, three of the types of particles each contain a pigment such that, according to their migration, said three types of particles are capable of displaying the colors of the RGB system (acronym for the additive synthesis system "Red Green Blue" which is based on the three primary colors) or the colors of the CMY system (acronym for the "Cyan Magenta Yellow" subtractive synthesis system). The fourth type of particles preferably contains a pigment of white or black color.

Among the pigments used for the different colors, it is possible for example to use:

-for red, hematite or cadmium red, -for green, cobalt green or chromium oxide,

-for blue, copper silicate or cobalt blue, -for black, carbon black or magnetite.

This list of pigments is not exhaustive and any inorganic pigment (oxide, silicate, ...) can be used provided that finally all the pigments used to display the colors of the RGB system or the CMY system and black color. In addition, there are nuances for some pigments; for example cobalt blue can be declined in several shades, from dark blue to turquoise blue.

The process for producing this polychromic electrophoretic ink consists in synthesizing each type of particles separately in an organic medium apolar such as an oil or an organic solvent apolar or slightly polar, such as toluene or an alkane for example, and then to mix them. In this case, the apolar organic medium, in which the syntheses of the various particles have taken place, advantageously constitutes the dispersing medium of the ink or, at least, it is compatible with the latter.

With regard to magnetic core particles, their syntheses consist in covering a magnetic core with an inorganic pigment and then encapsulating it in a chargeable functional polymer.

According to a possibility offered by the invention, the synthesis of the magnetic core consists in synthesizing stable magnetic particles in apolar organic medium, then in synthesizing a latex containing the magnetic core, by polymerization techniques in a heterogeneous medium in aqueous or organic media. polar or apolar, from a monomer of styrene or methyl methacrylate. In the context of the present invention, the term "latex" means a dispersion in a solvent of particles formed partially or entirely of polymer.

Advantageously, the magnetic particles synthesized or used are metal oxides.

In other words, a magnetic latex is first synthesized and then covered with a pigment and finally encapsulated in an electrostatically chargeable polymer shell.

The polymers forming the outer shell have acid units (for the negative particles), or basic (for the positive particles). Therefore, a simple acid-base reaction allows these patterns to tear or capture a proton and thus to acquire the respective negative or positive charge desired. For positive particles, instead of capturing a proton, they can also capture any chemical group that can bind to a nitrogen atom of the basic units.

Magnetic latex, still called magnetic core in the following description, is manufactured in several steps. A first step is to prepare an organic ferrofluid, according to a process known as "process Massart ". This process consists of co-precipitating ferric chloride (FeCl ₃ ) and ferrous chloride (FeCl ₂ ) in an aqueous medium to form magnetite (Fe ₃ O ₄ ). This co-precipitation takes place in a basic medium, in the presence of concentrated ammonia. Oleic acid then makes it possible to pass from an aqueous ferrofluid to a ferrofluid in the organic phase, by grafting, on the surface of the magnetite nanoparticles, carbon chains.

A second step then consists in synthesizing a magnetic latex intended to encapsulate the magnetite obtained and thus to form the core of the magnetic particle. For this, the magnetite synthesized in the first step is dispersed in hexadecane, which is a very hydrophobic agent, with styrene which is the monomer used to encapsulate magnetite. Sodium dodecasulphate (SDS) for example is used as a surfactant, and potassium persulfate is used as the initiator of the polymerization. According to one embodiment variant, nonionic surfactants such as Tween 80 (Polysorbate 80) or Span 80 (sorbitan monooleate) may also be used.

A pigment is then precipitated on the surface of this magnetic core, by hydrolysis of a precursor.

The encapsulation of this colored magnetic core in a chargeable polymer is then carried out. This step of encapsulation of a colored magnetic particle consists in dispersing said colored magnetic particle in said apolar organic medium, then in synthesizing at least one stable polymer latex in said organic medium, said latex precipitating around said particle to form a shell protective, said synthesis of the latex being carried out by polymerization, in said organic medium, of an electrostatically chargeable functional monomer, from a combined use of a macro-initiator and a co-initiator.

Similarly, with regard to the non-magnetic particles, an associated pigment is encapsulated directly in a polymer that can be loaded according to the encapsulation process that has just been described.

The different types of particles having been synthesized separately, they are then mixed to obtain a polychromatic electrophoretic ink. The ink thus manufactured is then used in particular for producing a polychromic electrophoretic display device. The invention furthermore relates to a polychromatic electrophoretic display device comprising the ink which has just been described. This device comprises a conductive surface electrode, a cavity comprising cells filled with polychrome ink, each cell being in fluid communication with its neighbor and defining a pixel, a bottom electrode comprising a contact pad under each pixel, each pad being connected to a transistor of an integrated circuit for controlling the application of an electrostatic force on each pixel, and finally a magnetic means adapted to apply a magnetic restoring force on the magnetic core particles. The magnetic means may advantageously be chosen from the following elements: a magnetic tape, or an electromagnet, for example.

Other advantages and characteristics of the invention will become apparent on reading the following examples given by way of illustrative and nonlimiting example, with reference to the appended FIG. 1, which represents a very simplified diagram of 4 pixels juxtaposed with a display in FIG. which are diagrammatically the four different types of particles composing the polychromatic electrophoretic ink. Each pixel is controlled on the one hand by a magnetic force and on the other hand by a different electrostatic force, so that one or more different types of particles migrate to the surface electrode in each of the pixels, so that get a full color display.

Example 1 Synthesis of a white particle with a magnetic core

Electrophoretic ink is made by mixing all types of particles separately. The synthesis of a magnetic or amagnetic particle is based on the same process with more or fewer steps. In this example is described the synthesis of a white particle with a magnetic core. Of course, this synthesis can be performed with any pigment to obtain the desired color particle. Similarly, for non-magnetic type particles, the first steps of the synthesis, consisting of preparing a magnetic core (steps 1 and 2), then covering it with a pigment (step 3) will not be reproduced.

1st stage: preparation of an organic ferrofluid: The synthesis of the ferrofluid is carried out according to a process known as the "Massart process". This process consists in co-precipitating ferric chloride (FeCl ₃ ) and ferrous chloride (FeCl ₂ ) in an aqueous medium to obtain magnetite (Fe ₃ O ₄ ). For this, 180 g of FeCl ₂ , 100 ml of HCl and 500 ml of water are mixed in a beaker. Chloridic acid (HCI) is added at the beginning of the synthesis essentially to facilitate the dissolution of FeCl ₂ . While stirring rapidly, 370 ml of FeCl ₃ and then 2 L of water are then added and the mixture is still stirred vigorously. However, this co-precipitation can be carried out only in basic medium. For this reason, 1 L of concentrated ammonia is added rapidly at once and the mixture is left stirring for 30 minutes. After this time, an aqueous ferrofluid is obtained.

136 g of oleic acid are then added to the resulting ferrofluid and the mixture is then stirred at 70 ° C. for 30 minutes. The oleic acid makes it possible to pass from a ferrofluid in aqueous phase to a ferrofluid in organic phase by grafting carbon chains on the surface of the nanoparticles of magnetite. The ferrofluid is then decanted, washed and then re-dispersed in the organic phase in an alkane, such as octane or cyclohexane, for example.

2nd step: Preparation of a magnetic latex

The magnetite obtained in the first step is then encapsulated in a polymer in order to produce the magnetic core of the magnetic core-type particles within the meaning of the invention. For this purpose, 2 g of this magnetite obtained in 6 g of styrene and 0.25 g of hexadecane are dispersed. The whole is subjected to ultrasound to disperse the magnetite and create a mini-emulsion. Styrene is the monomer used to encapsulate magnetite. Hexadecane is a very hydrophobic agent allowing the realization of the mini-emulsion. In a beaker, 0.2 g of SDS (sodium dodecyl sulphate) is then dissolved in 25 g of water, then the miniemulsion is added and the mixture is stirred for 20 minutes. SDS is a surfactant that allows the particles to be well dispersed in the mini-emulsion. The assembly is then sonicated for 5 minutes to maintain good dispersion of the particles, then 0.10 g KPS (potassium persulfate) diluted in water is added. KPS is here the initiator of the polymerization. The whole is then heated for 12 hours at 70 ° C. All the while, a polymer precipitates and covers each particle of magnetite. Magnetic latex particles, also called magnetic cores, are then obtained.

3rd step: coloring of the magnetic heart by the pigment

At this stage, the magnetic latex obtained in the preceding step is first dispersed in an alcoholic solvent, such as ethanol for example. A solution of water / ammonia is then added to this mixture, and tetrabutyl titanate is then added dropwise for about 1 hour and the mixture is then left stirring for a further 2 hours. The water / ammonia solution in this case allows the precursor (tetrabutyl titanate) to condense into titanium oxide (TiO ₂ ) around the magnetic latex. The resulting assembly is then washed by centrifugation / redispersion cycles. At the end of these cycles, a magnetic latex coated with a white layer of titanium oxide is obtained.

Of course, this example is only illustrative and we can color the magnetic latex in any color through the use of suitable pigments. Thus, for example, if it is desired to coat a magnetic latex with a yellow-colored layer, for example with cadmium sulphide, this chromium oxide is precipitated on the magnetic core by hydrolysis of its precursor, for example. The CdS precursor is a solution of Cd ²⁺ ion obtained from cadmium acetate in water to which thioacetamide is added to. The precipitation of the yellow pigment is then done over time. In this case, there is no need to have a water / ammonia solution, the two reactants spontaneously react together. Whatever the case may be, the coloration of a magnetic latex by any one of the pigments may be carried out according to methods already known to those skilled in the art by mixing the compounds making it possible to precipitate the pigment on the surface of the magnetic latex. When the magnetic core is colored, a final step in the method of manufacturing the magnetic-type particle is to encapsulate it in an electrostatically chargeable polymer.

Similarly, for non-magnetic type particles, it is necessary to encapsulate the pigment chosen for such a particle, in an electrostatically chargeable polymer shell. For this, an intermediate step (the fourth step described below) consists in synthesizing a macro-initiator. This macro-initiator, used in combination with a co-initiator, will allow not only the polymerization of the polymer shell around the pigment, or the magnetic core colored according to the type of particle, but also the stabilization of the particles thus synthesized in the organic medium. apolar and control of their sizes so that they are all homogeneous.

In the remainder of the description, the term "co-initiator" or "initiator" denotes an additive for starting a polymerization reaction. After the initiation of the polymerization reaction, the initiator forms a homopolymer which, by its precipitation will be at the origin of the particles and responsible for their magnification. Throughout the remainder of the description, the co-initiator used is an initiator manufactured and marketed by Arkema under the trademark "Blockbuilder".

A macro-initiator is an additive composed of a hydrophobic polymer chain for stabilizing the particles, and an initiator portion which serves to initiate the polymerization reaction and ultimately results in the formation of a copolymer. In the remainder of the description, in order to differentiate the hydrophobic polymer chain used for the stabilization of the particles, it is designated by the term "steric repulsion hair". The macro-initiator is advantageously synthesized from the co-initiator. Therefore, the initiator portion of the macro-initiator is identical to the co-initiator. The macro-initiator and co-initiator both initiate in parallel the polymerization reaction of a functional monomer. At the end of the polymerization reaction, a copolymer comprising a newly formed polymer chain at the end of the steric repulsion hair is formed and which is anchored in the particle. Thus, the steric repulsion hair remains attached to the particle and can thus stabilize it in the apolar organic medium.

The co-initiator serves just to initiate the reaction and only makes a homopolymer. The combination of these two initiators in adequate proportions, allows to precisely control the size of the latex particles that will be obtained in the end. Indeed, the proportion between the two types of initiators is influence the ratio of homopolymer to copolymer and thus the size of the particles obtained.

4th step: Synthesis of a macro-initiator for the final stage of polymerization in organic dispersion In a 100 ml flask, 1.33 g of co-initiator and 26.10 g of 2-ethylhexylacrylate are mixed in 30 ml of toluene. The solution is stirred until it is homogeneous. Vacuum / nitrogen cycles are then performed with stirring to remove all dissolved gases. The flask is then heated at 120 ° C. for 2 h with stirring and then cooled in a cold water bath. The macro-initiator thus formed is precipitated in methanol to purify the remaining monomer. The viscous liquid obtained is then dried under vacuum at 50 ° C. to remove the solvent residues. The macro-initiator thus synthesized is ready to be used for the subsequent step of encapsulation of the colored pigment or magnetic core according to the type of particle to be encapsulated. Step 5: Synthesis of the final particle

In a 250 ml beaker, 3 g of the previously synthesized particles are mixed, that is to say, according to the types of final particles to be synthesized either the colored magnetic cores, or the inorganic pigments, and 4 g of SPAN 80 (Sorbitan monooleate). in 200ml of toluene. SPAN80 is the surfactant that allows a better dispersion of colored magnetic particles or inorganic pigments in the apolar organic solvent used (here toluene). Stirring is carried out for 5 minutes until the SPAN 80 is completely dissolved, and the mixture is then sonicated to disperse the particles to be encapsulated. For this, we use an ultrasound probe whose power is set at about 420W for 8 min, alternating 2s pulse and 2s of rest. During this sonication, the beaker containing the suspension is placed in a bath of cold water to prevent a rise in temperature of the organic medium.

At the same time, 0.2 g of macro-initiator and 0.5 mg of co-initiator are dissolved in 5 ml of toluene. 5 ml of 4vinyl pyridine are also prepared. 4vinylpyridine is one of the monomers that makes it possible to form the polymer shell around inorganic pigments or colored magnetic cores. This hull can then be positively charged (in the case of the 4 vinylpyridine) or negatively (if one takes an acidic monomer type acrylic acid, methacrylic or their derivatives, copolymerized or not). At the end of the sonication, the dispersion of particles is immediately poured into a 250 ml reactor with mechanical stirring at 300 rpm. The mixture of macro-initiator and co-initiator dissolved in toluene and then 4-vinylpyridine are then added to the reactor and the mixture is heated at 120 ° C. for 12 hours under a nitrogen sweep. The white magnetic particles thus synthesized are then recovered and then purified by centrifugation / redispersion at 3000 rpm in toluene. This centrifugation step allows to keep only particles of uniform size. Another way to recover particles of uniform size is to perform dialysis.

The functional monomers for forming the electrostatically chargeable polymer shell are selected based on the final charge that the particle will have to carry. Thus, to have positively charged particles for example, the functional polymer covering the pigments is formed from 4-vinylpyridine monomers, or dimethylaminomethacrylate-co-styrene for example. In order to have negatively charged particles, the functional polymer covering the pigments is formed from an acrylic acid, or methacrylic acid and its derivatives, copolymerized or otherwise, with another neutral monomer such as styrene or MMA (methyl methacrylate).

The method makes it possible to obtain latex particles with a size of between 50 nm and 50 μm. Below 50 nm there is a risk of polymer chains being too short which will not precipitate and therefore do not form particles.

The particle size, for the intended application, is preferably between 0.5 and 2 μιη.

Advantageously, the choice of size is obtained by varying the percentage of co-initiator relative to the percentage of fixed-monomer macro-initiator. The macro-initiator / co-initiator molar ratio for the intended application is preferably between 2.5 and 30. In practice, by increasing the molar concentration of co-initiator relative to the molar concentration of macro-initiator, increases the particle size and vice versa. The polymer shell itself charges in the presence of a suitable compound. The polymers forming the outer shell have patterns that are either acidic (for the negative particles) or basic (for the positives). So a simple acid-base reaction allows these patterns to tear or capture a proton and thus acquire the charge. For positive particles, instead of capturing a proton, they can also capture any chemical group that can bind to a nitrogen atom of the basic units. This is for example what happens when one puts the white particles thus synthesized in the presence of iodomethane: the particles are charged positively. For the synthesis of the nonmagnetic particles, only steps 4 and 5 are carried out, that is to say the synthesis of the macro-initiator necessary for the step 5 of encapsulation of an inorganic pigment. The inorganic pigment is previously dispersed in the apolar organic medium by means of a surface treatment or a surfactant. The surface treatment may for example consist of a grafting of carbon chains on the hydroxyl groups of the pigment in order to increase its hydrophobicity. Once the surface modification is done, the ultrasound is used for 5 to 10 minutes to disperse the pigment.

According to one embodiment variant, a surfactant such as sorbitan monooleate (SPAN 80) is used, so as to modify the surface tension of the pigment. The inorganic pigment is then dispersed in the apolar organic medium by means of ultrasound for 5 to 10 minutes.

Once all types of particles are manufactured separately according to the method just described, they are mixed to form the polychrome ink which will be poured into the pixels of an electrophoretic display.

Example 2 Synthesis of a black particle with a magnetic core

The products used for this synthesis are the following: a black pigment of magnetite Fe ₃ O ₄ , Span 80 (sobitan monooleate) as a surfactant to allow a good dispersion of the pigment particles in the apolar solvent, the co-polymer initiator marketed by Arkema under the trademark "Blockbuilder", 2-ethylhexyl acrylate intended to be used for the synthesis of the macro-initiator, 4-vinylpyridine which is the monomer intended to form the positively charged polymer shell and encapsulating the black pigment, toluene as apolar solvent. The monomers of 2-ethylhexyl acrylate and 4-vinylpyridine are pre-purified on a desiccant, such as calcium hydride CaH ₂ , and distilled under reduced pressure in order to eliminate any residual inhibitor.

1 ^st step: Synthesis of Macro-initiator:

In a 100 ml flask, 1.33 g of co-initiator and 26.10 g of 2-ethylhexylacrylate are mixed in 30 ml of toluene. The solution is stirred until it is homogeneous. Vacuum / nitrogen cycles are then performed with stirring to remove all dissolved gases. The flask is then heated at 120 ° C. for 2 h with stirring and then cooled in a cold water bath. The macro-initiator thus formed is precipitated in methanol to purify the remaining monomer. The viscous liquid obtained is then dried under vacuum at 50 ° C. to remove the solvent residues. The macro-initiator thus synthesized is ready to be used for the subsequent step of encapsulating the pigment.

Step ^2: Encapsulation of pigment FeSO ₄ by dispersion polymerisation

In a 250 ml beaker, 3 g of Fe ₃ O ₄ and 4 g of SPAN 80 (Sorbitan monooleate) are mixed in 200 ml of toluene. SPAN 80 is the surfactant which allows a better dispersion of the pigment particles in the apolar organic solvent. The solution is stirred for about 5 minutes until all SPAN 80 has dissolved, and the mixture is sonicated to disperse the pigment particles. For this, we use an ultrasonic probe whose power is set to about 420 W (Watt) for 8 min, alternating 2 s (second) of pulse and 2 s of rest. During this sonication, the beaker containing the suspension is placed in a bath of cold water to prevent a rise in temperature of the organic medium.

At the same time, 0.2 g of macro-initiator and 0.5 mg of co-initiator are dissolved in 5 ml of toluene. 5 ml of 4-vinylpyridine to be added are also prepared. At the end of the sonication, the dispersion of Fe ₃ O ₄ is immediately poured into a 250 ml reactor with mechanical stirring at 300 rpm. The mixture of macro-initiator and dissolved co-initiator is then added to the reactor. in toluene, then 4-vinylpyridine and the whole is heated to 120 ° C for 12h under a nitrogen sweep. 4-vinylpyridine is the monomer that will form the polymer shell around the pigment and that we will then be able to charge positively.

The black particles thus synthesized are then recovered and then purified by centrifugation / redispersion at 3000 rpm in toluene. This centrifugation step allows to keep only particles of uniform size. Another way to recover particles of uniform size is to perform dialysis.

The black particles synthesized as described in the exemplary embodiment are then positively charged in the presence of iodomethane for example or in contact with other particles having acidic groups. Black, magnetic, positively charged particles are thus obtained.

Example 3 Display Device Comprising Polychromic Electrophoretic Ink

In FIG. 1 are diagrammatically 4 pixels respectively referenced P1, P2, P3 and P4 of a display device. The display device comprises a transparent surface electrode referenced 10, covering all the pixels. It further comprises a bottom electrode referenced 20. Between the two electrodes, a cavity January 1 is formed and filled with polychrome electrophoretic ink. In fact, the cavity comprises cells that communicate with each other. These cells are delimited on the one hand by vertical walls 21 perpendicular to the bottom electrode 20 and on the other hand by the bottom electrode 20. These cells in fact define the pixels P1 to P4 of the display . They communicate with each other to let the ink flow freely and fill all the cells. The bottom electrode 20 comprises contact pads 22. There is in fact a contact pad under each cell or pixel, each pad 22 being connected to a transistor 32 of an integrated circuit 30 intended to control the application of a different electrostatic force on each pixel. Finally, a magnetic means referenced 40 is disposed under the bottom electrode 20. This magnetic means 40 may for example be in the form of a magnetic strip or an electromagnet, for example. The ink filling each of the pixels P1 to P4 is represented by four types of particles composing it, these particles being respectively referenced A, B, C, D. In this illustrative but in no way limiting example, the particle A is for example blue in color. non-magnetic and positively charged, the particle B is for example yellow, with a magnetic core and positively charged, the particle C is red, amagnetic and negatively charged, and finally the particle D is black, with a magnetic core and negatively charged.

Each of the magnetic core particles, i.e. the particles B and D in this example, undergo a magnetic resetting force induced by the magnetic tape or the electromagnet 40 disposed at the bottom of the display device. Therefore, to migrate the magnetic particles to the surface electrode 10, it is necessary to increase the voltage applied between the electrodes with respect to the voltage applied to move non-magnetic type particles, in order to surpass this magnetic restoring force. In the remainder of the description, therefore, V + (V-), the voltage threshold necessary to move the nonmagnetic particles and V ++ (V-) the voltage threshold necessary to move the magnetic particles.

Thus, on the pixel P1, a voltage V + is applied between the electrodes so that the amagnetic and negatively charged particle C moves to the positive surface electrode. Consequently, the pixel P1 displays the red color of the particle C. On the pixel P2, a voltage V ++ is applied between the electrodes, so that the amagnetic and negatively charged particle C, as well as the negatively charged magnetic particle D, migrate towards the pixel P2. the positive surface electrode. As a result, the red and black colors of the two particles C and D are superimposed on the surface of the pixel P2, so that the latter displays a black color. On the pixel P3, a voltage V- is applied between the electrodes so that the positively charged amagnetic particle A migrates to the negatively charged surface electrode. The pixel P3 therefore appears blue, of the color of the particle A. Finally, on the pixel P4, a voltage V- is applied so that the particle A, non-magnetic and positively charged, as well as the particle B, magnetic and positively charged. migrate to the negatively charged surface electrode. As a result, the blue and yellow colors of Part A and Part B are superimpose on the surface of the pixel P4 so that the latter displays a green color.

The case just described is only an illustrative example to explain the operation of a polychrome display containing such an ink. The colors displayed will depend on the choice of colored particles that will be magnetic or not and negatively or positively charged. In addition, the particles composing the ink are preferably chosen so that, according to their migration in the pixels, they can display the colors of the RGB system or of the CMY system and the black color. Of course, one could choose another system of color representation without departing from the scope of the invention.

Since the pixels are very small and very close together, the human eye does not have enough resolution to be able to distinguish them from each other, which is why the colors displayed by 3 or 4 juxtaposed pixels also appear to be superimposed on each other. the human eye. Thus, the eye reconstructs a whole palette of colors with many nuances. Thus, for example when looking at a set of pixels, each displaying the three primary colors of the RGB system, the human eye superimposing them, it will see a point of white color displayed on the screen.

The polychrome ink thus synthesized has many advantages. These include a unique ink, capable of displaying at least the 3 colors of the RGB system (red-green-blue) necessary for the production of full-color display devices. With this ink, there is no loss of contrast compared to displays by filters or by juxtaposition of two-color pixels, which can in some cases lose between 50 and 75% of the maximum contrast. This is made possible by the fact that each pixel can display all the colors. Another advantage lies in the method of producing the color display device itself. Indeed no control at the filling of the pixels by the ink is necessary because it is a single ink.

Claims

A polychromic electrophoretic ink, comprising at least four types of particles dispersed in an apolar organic medium, each type of particles containing a pigment of a color associated therewith, having a positive or negative electrostatic charge, characterized in that at least one of the above types of particles has a magnetic property (magnetic core) so that each type of particles can migrate in a predetermined manner under the combined action of an electrostatic force and a magnetic restoring force.

2. polychromatic electrophoretic ink according to claim 1, characterized in that it comprises two types of magnetic core particles, and in that, for each of them, said magnetic core is covered with a pigment of a color which is associated with it, then encapsulated in a functional polymer electrostatically chargeable, respectively positively and negatively.

3. polychromic electrophoretic ink according to claim 1 or 2, characterized in that it comprises two types of nonmagnetic particles each comprising a pigment of a color associated with it, encapsulated in a functional polymer electrostatically chargeable, respectively positively and negatively.

4. polychromic electrophoretic ink according to any one of the preceding claims, characterized in that three of the types of particles each contain a pigment such that, according to their migration, said types of particles are capable of allowing a color display of the RGB system or of the CMY system and that a fourth type of particles contains a white or black pigment.

5. A method of manufacturing said polychromic electrophoretic ink according to one of claims 1 to 4, characterized in that it consists in synthesizing each type of particles separately, in an organic medium apolar, and then mixing them, said apolar organic medium then constituting the dispersing medium of the ink obtained.

6. Process according to claim 5, characterized in that the synthesis of a magnetic core particle consists of synthesizing a magnetic core, covering it with an inorganic pigment, and then encapsulating it in a chargeable functional polymer.

7. Method according to claim 6, characterized in that the synthesis of the magnetic core consists in synthesizing magnetic particles stable in an apolar organic medium, and then in synthesizing a latex containing the magnetic core, by polymerization techniques in a heterogeneous medium in media. aqueous or organic polar or apolar, from a styrene monomer or methyl methacrylate.

8. Process according to claim 7, characterized in that the magnetic particles synthesized or used are metal oxides.

9. The method of claim 5, characterized in that the synthesis of a non-magnetic particle consists of encapsulating an inorganic pigment in a chargeable functional polymer.

10. The method of claim 6 or 9, characterized in that the encapsulation step of a colored magnetic core or an inorganic pigment comprises dispersing said colored magnetic core or said pigment in said apolar organic medium, then to synthesizing at least one stable polymer latex in said organic medium, said latex precipitating around said colored magnetic core or said pigment, to form a protective shell, said synthesis of the latex being carried out by polymerization, in said organic medium, of a functional monomer electrostatically chargeable, from a combined use of a macro-initiator and a co-initiator.

11. A polychromic electrophoretic display device comprising a polychromic electrophoretic ink according to one of claims 1 to 4, characterized in that it comprises:

a surface electrode (10), a cavity (1 1) comprising cells filled with said polychromic electrophoretic ink, each cell being in fluid communication with its neighbor and defining a pixel (P1, P2, P3, P4),

 - a bottom electrode (20) comprising a contact pad (22) under each pixel, each pad being connected to a transistor (32) of an integrated circuit (30) for controlling the application of an electrostatic force on every pixel,

- Magnetic means (40) adapted to apply a magnetic restoring force on magnetic core type particles contained in each pixel.

12. A polychromatic electrophoretic display device according to claim 1 1, characterized in that said magnetic means (40) is selected from the following elements: a magnetic tape or an electromagnet.

13. Use of the polychromatic electrophoretic ink according to one of claims 1 to 4, for the production of a polychromatic electrophoretic display device according to claims 1 1 to 12.