Electrophoretic Display and Driving Method Thereof

Abstract

The present invention provides an electrophoretic medium comprising a liquid dispersion medium and two or more groups of particles dispersed in the dispersion medium, a display device comprising the same, and a method for driving the same. The particles of the present invention have a Core-Shell-Corona structure comprising a core, a shell, and a corona swollen by a solvent, and have a polydisperse particle size distribution (polydisperse index PDIw ≥ 1.2) intentionally designed to be wide. The present invention defines the effective radius of the particles as the Radius of Gyration (Rg) and defines the state of the system by a novel parameter called the Effective Volume Fraction (Φeff), which includes the swelling of the corona layer. Furthermore, the present invention drives a display device comprising the medium using a hybrid voltage comprising a variable frequency AC electric field. Through this, the limitations of geometric models of conventional technology can be overcome, low viscosity and excellent dispersion stability can be secured even at high particle concentrations, and precise particle control can be enabled through frequency-selective driving, thereby enabling the realization of a high-speed response and high-reliability electrophoretic display device.

Description

Technology Field

[0001] The present invention relates to the field of electronic display technology, and more specifically, to an electrophoretic material comprising a plurality of charged particles, an electrophoretic display device using said material, and a method for driving said display device. Background Technology

[0002] Although technology for realizing color using multiple particles in electrophoretic displays (EPDs) has been developed, conventional technologies have the following technical characteristics and limitations.

[0003] First, there is a technique that relies on particle size and geometric arrangement within the system (U.S. Patents No. 9,494,842 B2 and 8,947,763 B2). This technique introduces a parameter called 'free volume' and aims to improve response speed by strictly defining the relationship between particle sizes (R - a₁ > a₂) so that small particles can pass through the gaps between large particles. However, this geometric model has limitations in that it does not adequately reflect the irregular shape and size distribution of actual particles and the complex physical interactions between particles, and precise particle size control required to satisfy strict 'passage conditions' hinders design flexibility.

[0004] Second, there is a technology that uses particles without a stable protective coating (U.S. Patent No. 7,170,670 B2). A feature of this system is that the uncoated pigment particles become charged through dynamic physical adsorption equilibrium with a charge control agent (CCA). However, this charging method is highly sensitive to changes in the external environment (especially temperature), causing the charge characteristics of the particles to change easily, which leads to instability in color reproduction. Furthermore, particles without a protective coating are highly likely to come into direct contact with each other and form irreversible aggregates, which reduces reliability during long-term operation.

[0005] Third, there is a technique that utilizes the size difference between different particle groups (U.S. Patent No. 6,120,839 A). This technique is characterized by controlling the particle size distribution of each particle group to be as narrow as possible—that is, close to monodisperse—for performance optimization. However, the process of synthesizing and classifying highly monodisperse particles is technically very demanding and involves high costs, which reduces the economic feasibility of mass production. Furthermore, this approach has the problem of limiting design flexibility by preventing the active utilization of the shape or width of the particle size distribution itself as a variable for system control.

[0006] As mentioned above, conventional technologies share common limitations, such as inaccuracy due to dependence on ideal models, reduced reliability caused by unstable charging mechanisms, and manufacturing difficulties and design rigidity resulting from a monodispersity orientation. Therefore, there is a need to develop a new type of electrophoresis system capable of overcoming these problems. Prior art literature

[0007] 1. US 9,494,842 B22. US 7,170,670 B23. US 8,947,763 B24. US 6,120,839 A The problem to be solved

[0008] The present invention aims to solve the following fundamental problems of conventional electrophoretic display device technology.

[0009] First, the problem that the present invention aims to solve is to address the issue where conventional technology relies on geometric models (e.g., free volume models) that assume all particles are ideally spherical and monodisperse, which fail to reflect the irregular shape and size distribution of actual particles and consequently make it difficult to accurately predict or control the dynamic behavior of the system. To this end, the present invention provides a system characterized by intentional polydispersity and introduces new parameters (radius of gyration, effective volume fraction) that reflect the actual dynamic size of the particles.

[0010] Second, the problem that the present invention aims to solve is to address the issue of reduced reliability during long-term operation caused by unintentional aggregation between particles or unstable charge characteristics, which arises from the use of uncoated or simply coated particles in conventional technology. To this end, the present invention provides particles with a core-shell-corona structure that simultaneously impart an intrinsic charge and a strong steric stabilization effect.

[0011] Third, the problem that the present invention aims to solve is to address the limitations in precisely separating and controlling complex particles in multi-particle systems, as conventional technology relies primarily on the electrophoretic effect caused by DC voltage pulses for operation. To this end, the present invention provides a novel driving method utilizing a variable frequency AC electric field that induces different forces depending on the particle size. means of solving the problem

[0012] An electrophoretic display device according to one embodiment of the present invention comprises: an upper substrate; a lower substrate; a first electrode and a second electrode facing each other disposed on one surface of each of the upper substrate and the lower substrate; and a display layer disposed between the first electrode and the second electrode and comprising an electrophoretic fluid comprising electrophoretic particles, wherein the electrophoretic particles comprise a first particle group and a second particle group, and the first and second particle groups have charges of opposite polarity to each other, and the first and second particle groups have a core-shell-corona structure comprising a core, a shell on the surface of the core, and a corona on the surface of the shell, and the first and second particle groups may each have a polydispersity particle size distribution having a polydispersity index (PDIw) of 1.2 or higher.

[0013] The average radius of rotation (Rg1) of the first particle group may be at least twice the average radius of rotation (Rg2) of the second particle group.

[0014] The effective volume fraction (Φeff,1) of the first particle group is 0.4 or more and less than 0.6, and the effective volume fraction (Φeff,2) of the second particle group may be 0.05 or more and 0.2 or less.

[0015] The resonance frequencies (f_res) of the first particle group and the second particle group differ by more than 10 times, and the frequency response sensitivity (FRS) of each group may be 2.0 or higher.

[0016] The centerline average roughness (Ra) of the electrophoretic film including the above display layer may be 500 nm or less.

[0017] The first particle group may include white light-scattering particles, and the second particle group may include black light-absorbing particles.

[0018] The first particle group above may include white and black particles, and the second particle group may include cyan, magenta, and yellow particles.

[0019] The display layer may have a structure in which the electrophoretic fluid is enclosed within a plurality of microcapsules or microcups.

[0020] The electrophoretic particles included in the electrophoretic display device according to one embodiment of the present invention include a first particle group and a second particle group, and the first particle group and the second particle group have a core-shell-corona structure including a core; a shell covalently bonded to the surface of the core; and a corona covalently bonded to the surface of the shell and swollen by a solvent, and the polydispersity index (PDIw), defined by the molecular weight distribution of the corona, may be 1.2 or higher.

[0021] The above shell may be a cross-linked polymer layer containing a charge-carrying monomer.

[0022] The above corona may be a flexible polymer chain containing lauryl methacrylate or stearyl methacrylate.

[0023] The above core may be a pigment having a color selected from the group consisting of white, black, cyan, magenta, and yellow.

[0024] An electrophoretic fluid according to one embodiment of the present invention comprises a plurality of electrophoretic particles; and a liquid dispersion medium, wherein the electrophoretic fluid may substantially not contain a charge control agent (CCA).

[0025] A microcapsule according to one embodiment of the present invention comprises a polymer shell; and a core portion encapsulated by said polymer shell, wherein the core portion may comprise an electrophoretic fluid.

[0026] An electrophoretic film according to one embodiment of the present invention may include a plurality of microcapsules; and a polymer binder supporting the plurality of microcapsules.

[0027] A method for manufacturing electrophoretic particles according to one embodiment of the present invention comprises: (a) a step of fixing a first initiator on the surface of the core; (b) a step of forming the shell by surface-initiating polymerization of a first monomer from the core surface on which the first initiator is fixed; and (c) a step of fixing a second initiator on the surface of the shell or activating a remaining initiation site, and then surface-initiating polymerization of a second monomer to form the corona, wherein in step (c), the supply rate or reaction temperature of the second monomer can be controlled so that the polydispersity index (PDIw) of the corona becomes 1.2 or higher.

[0028] A method for manufacturing a microcapsule according to one embodiment of the present invention may include the step of dispersing an electrophoretic fluid in an aqueous phase to form an emulsion, and polymerizing a monomer at the interface of the emulsion to form the polymer shell.

[0029] A method for manufacturing an electrophoretic film according to one embodiment of the present invention may include: (a) a step of preparing a slurry by mixing a plurality of microcapsules with the polymer binder; and (b) a step of applying the slurry onto a substrate and curing it.

[0030] A method for driving an electrophoretic display device according to one embodiment of the present invention includes the step of applying a hybrid voltage comprising a DC voltage and a variable frequency AC voltage between the first electrode and the second electrode, and by adjusting the frequency of the AC voltage, the first particle group or the second particle group can be selectively driven or fixed at a specific position.

[0031] A method for manufacturing an electrophoretic display device according to one embodiment of the present invention may include the step of manufacturing an electrophoretic film; and the step of bonding the electrophoretic film to a lower substrate including a thin-film transistor (TFT) array. Effects of the invention

[0032] The electrophoretic-dynamic medium, display device, and driving method according to the present invention have the following effects.

[0033] First, a new mechanism is implemented in which small particles move between densely packed large particles through polydisperse particle size distribution and effective volume fraction control. This increases design freedom compared to conventional techniques that strictly rely on specific particle size ratios and geometric 'passing conditions,' and enables the securing of stable high-speed response characteristics even with various particle combinations.

[0034] Second, the core-shell-corona structure introduced into all particles, particularly the flexible corona layer, maximizes the steric stabilization effect that prevents direct collisions between particles. This effectively suppresses particle aggregation compared to the simple coating structure of conventional technology and maintains fluid dispersion stability for a long period even during repeated operation, thereby improving the lifespan and reliability of the display device.

[0035] Third, by defining the effective size of the particle as the radius of gyration (Rg) rather than the geometric radius, and defining the system state as the effective volume fraction (Φeff), the actual dynamic behavior and rheological properties of the particle are reflected more accurately. This overcomes the limitations of the ideal spherical particle and free-volume models of conventional technology and enables more precise and predictable system design and control.

[0036] Fourth, by utilizing a variable frequency AC electric field for driving, a new method of particle control that was impossible with DC pulses alone is realized. Frequency-selective driving using the dielectrophoretic effect makes it possible to independently move groups of particles of a specific size or fix them at a specific location, thereby increasing the accuracy of color separation and expanding the diversity of driving waveforms. This is an advanced control technology compared to the conventional simple DC electrophoretic driving method.

[0037] Fifth, the introduction of a core-shell-corona structure enables independent control of the particle's surface and electrical properties. By determining the particle's basic charge in the shell layer and controlling dispersion stability and dynamic properties in the corona layer, particles optimized for various types of pigment cores and dispersion fluids can be easily manufactured. This has the effect of broadening the range of material choices and providing development flexibility compared to conventional technology. Specific details for implementing the invention

[0038] The present invention will be described in detail below through specific manufacturing examples, but these are merely examples to aid in understanding the invention and the scope of the invention is not limited thereto.

[0039] 1. Structure and Preparation of Electrophoretic Particles

[0040] The electrophoretic particles according to the present invention are characterized by having a core-shell-corona triple structure, which is fundamentally different from conventional simple coated particles or uncoated particles. This unique structure enables the independent and precise control of the particle's color, charge, and dynamic behavior, thereby serving as the basis for realizing the superior performance of the present invention.

[0041] (1) Core It is a pigment particle located at the innermost part of the particle that determines the target color and primary optical properties (light scattering or light absorption).

[0042] White particles (light-scattering) The core consists of a white inorganic pigment that has a high refractive index and effectively scatters light. Preferably, titanium dioxide (TiO₂) is used, and the crystal structure may be rutile or anatase. In addition, zinc oxide (ZnO), barium sulfate (BaSO₄), zirconia (ZrO₂), etc., may be used.

[0043] Black particles (light-absorbing)The core is composed of a pigment that absorbs light of a specific wavelength in the visible light region. Carbon black was used in Preparation Example 3, and synthetic black iron oxide (Fe₃O₄) was used in Preparation Example 4. In addition, organic black pigments such as copper chromite black spinel or aniline black can also be used.

[0044] Colored particles (light-absorbing) The core is composed of high-saturation organic pigments to realize the three primary colors of subtractive color mixing.

[0045] Cyan: Pigment Blue 15:3, a copper phthalocyanine pigment, was used. In addition, Pigment Blue 15:4, Pigment Blue 16, etc. can also be used.

[0046] Magenta: Pigment Red 122, a quinacridone-based pigment, was used. In addition, Pigment Red 202, Pigment Violet 19, etc., can be used to achieve similar colors.

[0047] Yellow: Pigment Yellow 150, an azo-based pigment, was used. In addition to this, Pigment Yellow 74, Pigment Yellow 139, Pigment Yellow 155, or Pigment Yellow 110, an isoindolinone-based pigment, may be used.

[0048] In addition to the pigments specified above, pigments having various CI (Color Index) numbers may be used depending on the desired color and durability. The average particle size of the core particles is generally selected in the range of 100 nm to 500 nm.

[0049] (2) Shell It is a thin, rigid polymer layer firmly fixed to the core surface by covalent bonds. The shell plays the following important roles.

[0050] Electric charge assignment Depending on the type of comonomer constituting the shell, it imparts an inherent and permanent positive (+) or negative (-) charge to the particle.

[0051] stability It prevents the leaching of impurities from the pigment core and acts as a stable intermediate layer that chemically connects the core and the outer corona layer.

[0052] structure It is desirable for the shell to form a rigid network structure that does not swell with solvents, by including rigid monomers such as styrene and methyl methacrylate (MMA) and crosslinking agents such as divinylbenzene (DVB).

[0053] (3) Corona It is a layer of solvent-friendly and flexible polymer chains grafted onto the shell surface via covalent bonds. The corona performs the following roles.

[0054] Steric stabilization: It forms a swollen chain structure that extends outward within the solvent, providing strong steric repulsion that prevents direct contact between particles. This fundamentally prevents irreversible aggregation even at high particle concentrations, thereby maximizing the long-term dispersion stability of the fluid.

[0055] Dynamic characteristic control The length and density of the corona determine the effective hydrodynamic diameter and radius of gyration (Rg) of the particle, which influence the particle's dynamic behavior, such as dielectrophoretic properties under an AC electric field.

[0056] Multidispersity In the present invention, the length of the corona chain is intentionally non-uniform, that is, synthesized to have a wide molecular weight distribution (polydispersity index PDIw ≥ 1.5), thereby optimizing the rheological properties of the system at high particle concentrations.

[0057] The particles of the present invention are manufactured through a multi-stage surface-initiated polymerization method that sequentially undergoes a core-shell formation step and a corona formation step.

[0058] (4) (Step 1) Preparation of core-shell particles :

[0059] Core Surface Initiator Immobilization: Prepared pigment core particles are dispersed in an organic solvent such as toluene anhydride. To this, a compound having both a silane group capable of reacting with the hydroxyl groups (-OH) on the core surface and an initiator capable of initiating controlled radical polymerization (CRP) is added. For example, 3-(2-bromoisobutyryloxy)propyltriethoxysilane is used for surface-initiated atom transfer radical polymerization (SI-ATRP). The mixture is heated to 110°C under a nitrogen atmosphere and refluxed and stirred for 24 hours, thereby immobilizing the silane coupling agent on the core surface via covalent bonding. Unreacted coupling agents are completely removed by washing.

[0060] Surface-initiated polymerization of the shell: The core particles immobilized with the initiator are dispersed in a reaction solvent. Monomers to form the shell are added thereto. For example, to form a positively charged shell, a mixture of styrene, divinylbenzene (DVB), and the positively charged monomer 2-(dimethylamino)ethyl methacrylate (DMAEMA)* is used. After adding an ATRP catalyst (e.g., copper(I) bromide, CuBr) and a ligand (e.g., bipyridine, Bpy), dissolved oxygen is removed, and polymerization is carried out at 60-90°C for several hours. Through this process, a thin, strong, and cross-linked positively charged shell grows from the core surface to form core-shell particles.

[0061] (5) (Step 2) Control of corona formation and polydispersity :

[0062] Corona surface-initiated living radical polymerizationThe core-shell particles prepared above are purified and prepared. These particles are then dispersed in a reaction solvent, and a monomer to form a corona is added. For example, lauryl methacrylate (LMA) or stearyl methacrylate (SMA) is used to impart dispersibility to hydrocarbon solvents. When using reversible addition-segment chain transfer (RAFT) polymerization, a suitable RAFT chain transfer agent (e.g., CTPPA) and a radical initiator (e.g., AIBN) are added.

[0063] Ensure polydispersity (PDIw ≥ 1.5) : During the polymerization process, reaction conditions are intentionally controlled to be non-ideal to make the length of the corona chains non-uniform. For example, a 'step-by-step monomer injection method' is used, in which only a small amount of monomer is supplied at the beginning of the reaction to grow short chains first, and then a large amount of monomer is supplied all at once in the middle of the reaction to induce the growth of long chains. Alternatively, a mixture of two types of initiators with different reaction rates may be used, or a 'temperature gradient method' may be applied by rapidly changing the temperature during the reaction. Through this process, core-shell-corona structured electrophoretic particles with a polydispersity index (PDIw) of 1.5 or higher are finally completed, in which corona chains of various lengths are grafted onto the surface of the core-shell particles.

[0064] 2. Preparation of Electrophoretic Fluid

[0065] The electrophoretic fluid according to the present invention is prepared by stably dispersing particles having the core-shell-corona structure described above and intended polydispersity in a nonpolar dispersion fluid. The fluid of the present invention operates by a novel physical principle different from the prior art through the inherent structural characteristics of the particles and the rheological control of the system.

[0066] Electrophoretic fluids are broadly composed of three stages: dispersion preparation, particle mixing, and homogeneous dispersion.

[0067] Dispersion Fluid and Additives: As the dispersion fluid, a non-polar liquid capable of swelling the corona by forming sufficient solvent-polymer interactions with the corona layer of the particles is used. Preferably, a solvent with a dielectric constant of less than 5 and low viscosity is used. Hydrocarbon solvents such as dodecane and the Isopar™ series may be used, and hydrofluoroether (HFE) series solvents may be used as fluorinated organic solvents. Since the particles of the present invention possess a strong stereostabilization effect due to the corona layer, they are characterized by not adding separate dispersion stabilizers or charge control agents (CCA) that were essential in the prior art, or by using only a very small amount of 0.1 wt% or less if necessary. This simplifies the chemical composition of the system and fundamentally blocks side effects caused by CCA (such as increased leakage current and changes in charging characteristics).

[0068] Particle Mixing and Uniform Dispersion: The prepared first particle group (e.g., white) and second particle group (e.g., black) are each introduced into the dispersion fluid at a target concentration. The concentration of each particle group is precisely controlled to reach the target range of the 'effect volume fraction (Φeff)' described below. The suspension into which the particles have been introduced is processed using a probe-type ultrasonic disperser, a roll mill, or a high-pressure homogenizer, etc., until the particle agglomerates are completely broken down and the particles are uniformly dispersed into individual particles.

[0069] The present invention introduces a new parameter that defines the actual rheological state of the system, replacing the conventional geometric 'free volume' model.

[0070] Effective Volume Fraction (Φeff)

[0071] Definition: It is the total volume fraction of particles in a system calculated based on the hydrodynamic volume of the particles, including the corona layer swollen by the solvent, rather than the simple core volume. It represents the proportion of the space actually occupied by the particles within the fluid and is directly related to the viscosity and fluidity of the system. It is calculated as Φeff = (number concentration of particles) × (hydrodynamic volume of a single particle).

[0072] Numerical limitation: The present invention is characterized by limiting the effective volume fraction (Φeff,1) of the first particle group (large particles) to 0.4 or more and less than 0.6, and the effective volume fraction (Φeff,2) of the second particle group (small particles) to 0.05 or more and 0.2 or less.

[0073] Technical Significance and Critical Significance:

[0074] The effective volume fraction is a key parameter that determines the phase behavior of a particle suspension. The lower threshold of Φeff,1, 0.4, indicates that the first group of particles has moved beyond a simple dispersed state and entered a 'concentrated' region where interactions between them become important. The upper threshold of 0.6 is a value close to the random close packing limit of polydisperse spheres, indicating a metastable state just before reaching a 'glass transition' or 'jamming' state where the system loses fluidity and behaves like a solid.

[0075] This presents a physical picture that is completely different from the 'free volume' model of conventional patents. That is, the system of the present invention operates similarly to a 'percolation' or 'channel diffusion' model, in which large first particles are densely packed to form a structure resembling a 'matrix' with almost no movement, while small second particles with a relatively low concentration move along fluidic channels formed between this matrix. Φeff,1 ≥ 0.4 is the minimum concentration condition for forming this matrix structure, and Φeff,1 < 0.6 is the upper limit condition for maintaining operable fluidity without the system becoming completely stuck. Therefore, this numerical limit holds critical significance in defining the unique particle driving mechanism of the present invention. Additionally, Φeff,2 ≤ 0.2 serves as an upper limit condition that allows small second particles to move freely within the channels without disturbing the structure of the large particle matrix.

[0076] 3. Microcapsule manufacturing

[0077] The electrophoretic display device according to the present invention preferably employs a microcapsule structure to stably compartmentalize and protect the electrophoretic fluid described above. The microcapsule has a core-shell structure in which the electrophoretic fluid serves as the core and is surrounded by a polymer shell.

[0078] Microcapsules are generally manufactured through interfacial polymerization, in which an organic phase (oil) is dispersed in an aqueous solution to form an emulsion, and then a polymer reaction occurs at the oil / water interface to form a shell, or through suspension polymerization, in which monomers contained in the internal phase are polymerized.

[0079] Preparation of the internal phase (oil phase): The internal phase consists of the prepared electrophoretic fluid as the main component. Reactive monomers or prepolymers to form the capsule walls are dissolved in the electrophoretic fluid. When using interfacial polymerization, aliphatic or aromatic polyisocyanate prepolymers (e.g., Desmodur N3300, toluene diisocyanate (TDI) prepolymer) may be used. Additionally, aminoplast resins such as melamine-formaldehyde or urea-formaldehyde prepolymers may be dissolved. For suspension polymerization, vinyl monomers such as methyl methacrylate (MMA) and styrene, crosslinking agents such as divinylbenzene (DVB), and oil-soluble radical initiators (e.g., AIBN) are added. A small amount of oil-soluble surfactant (e.g., sorbitan fatty acid ester series) may be added to increase emulsion stability.

[0080] Preparation of the outer phase (aqueous phase): The outer phase contains other reactants of the capsule shell (in the case of interfacial polymerization) and an emulsion stabilizer. In the case of interfacial polymerization, the outer phase consists mainly of deionized water and a protective colloid. Water-soluble polymers such as gum arabic, polyvinyl alcohol (PVA), gelatin, and carboxymethylcellulose (CMC) are used as protective colloids. This protective colloid serves to prevent oil phase droplets from merging together during emulsification. When using aminoplast resin, the pH of the aqueous phase is adjusted to acidic conditions (e.g., pH 4.0) to promote the reaction. The same protective colloid is also used to stabilize the emulsion in the case of suspension polymerization.

[0081] Emulsification and Encapsulation: An O / W (Oil-in-Water) emulsion is formed by slowly adding the oil phase to the aqueous phase while applying a strong shear force using a high-speed homogenizer or a high-shear mixer. The speed and time of the homogenizer are controlled so that the average diameter of the emulsion droplets becomes the target capsule size (e.g., 20-100 μm).

[0082] Encapsulation is carried out by slowly stirring the generated emulsion while raising the temperature to a range of 50-80°C and maintaining it for several hours. In the case of interfacial polymerization, an isocyanate in the oil phase reacts with water at the oil / water interface to form a polyurea shell, or an aminoplast prepolymer undergoes condensation polymerization under an acid catalyst to form a melamine resin or urea resin shell. In the case of suspension polymerization, monomers inside the oil phase droplets are polymerized by a radical initiator to form a polymethyl methacrylate (PMMA) or polystyrene (PS) shell.

[0083] After the reaction is finished, the generated microcapsule slurry is passed sequentially through mesh filters of different sizes to remove unaggregated capsules or impurities, and then purified and concentrated through centrifugation and washing to obtain the final product.

[0084] Size and shape: The manufactured microcapsules have a spherical shape, and the average diameter is controlled in the range of 20 to 100 μm, taking into account the resolution of the display device and the film thickness.

[0085] Shell thickness and mechanical strength: The thickness of the capsule shell is in the range of several hundred nm to several μm and must have sufficient mechanical strength to withstand the pressure of the display device manufacturing process (coating, bonding). If the shell is too thin, it may break easily and leak ink, and if it is too thick, optical transmittance may decrease or the packing rate between capsules may decrease.

[0086] Electrical characteristics: The capsule shell must have high electrical insulation. If the resistivity of the shell is high, such as 10^12 ohm·cm or higher, it prevents electrical interference (crosstalk) between adjacent pixels and effectively maintains the electric field within each pixel, contributing to the realization of a clear image. This, along with the high resistivity of the fluid inside the capsule, is an important factor in ensuring the bistability of the display device.

[0087] 4. Preparation of electrophoretic film

[0088] The electrophoretic display device according to the present invention is completed through a module assembly process that involves producing an electrophoretic film containing the microcapsules described above and combining it with a driving circuit and mechanical components.

[0089] The electrophoretic film is manufactured by mixing microcapsules with a polymer binder, applying them to a lower substrate, and bonding them to an upper electrode film.

[0090] Preparation of coating slurry: Prepare the manufactured microcapsule slurry. To this, a solution of water-dispersible polyurethane (PUD), water-dispersible acrylic emulsion, or polyvinyl alcohol (PVA) is added as a polymer binder to fix the capsules and form a film. To improve the mechanical strength of the binder, a water-dispersible isocyanate or a melamine-based crosslinking agent may be added. All components are stirred at a low speed to prepare a uniform coating slurry while ensuring that the capsules are not damaged.

[0091] Coating, Drying, and Lamination: A flexible PET film having a thin-film transistor (TFT) array formed thereon is prepared as a bottom substrate. The coating slurry is coated onto the bottom substrate to a uniform thickness (approximately 25-100 μm after drying) using a doctor blade or a slot die coater. The coated film is dried in a hot air dryer at 50-80°C to remove moisture and solvent and to perform primary curing of the binder.

[0092] As an upper electrode film, a transparent PET film with a transparent conductive material such as indium tin oxide (ITO) or graphene deposited thereon is prepared, and a thermoplastic adhesive layer is pre-formed thereon. The upper electrode film is aligned and placed on a first-dried capsule / binder layer, and the two films are strongly bonded by passing them through a roll laminator under conditions of 100-130°C and 0.3-0.7 MPa. Through this process, a final electrophoretic film is completed in which a microcapsule layer is uniformly formed between the TFT lower substrate and the upper transparent electrode film.

[0093] 5. Fabrication of an electrophoresis display device

[0094] The electrophoretic display device according to the present invention is completed through a module assembly process that combines a driving circuit and mechanical components based on an electrophoretic film.

[0095] Driving circuit connection: A flexible printed circuit board (FPCB) with a driving circuit mounted thereon is electrically connected to the connection pads exposed on the lower TFT substrate edge of the finished electrophoretic film by attaching them via a thermal compression method using an anisotropic conductive film (ACF).

[0096] Assembly of mechanical components and final process: An anti-glare protective film is attached to the front of the display device, and a back cover is installed on the back to protect the substrate. These are assembled into an integrated module using a frame. Finally, the assembled module undergoes an aging process in which it is operated in a high-temperature environment (e.g., 60°C) for tens of hours to eliminate initial defects and stabilize performance, after which the electrophoretic display device is completed through a final inspection.

[0097] The present invention introduces the following parameters to define the final performance of a display device.

[0098] Film Flatness (Ra): Definition: An indicator of the surface roughness of a finished electrophoretic film, which is the centerline average roughness (Ra) value measured using an atomic force microscope (AFM) or a 3D surface profiler.

[0099] Numerical limitation: The present invention is characterized by limiting the film flatness (Ra) to 500 nm or less, preferably 300 nm or less.

[0100] Technical Significance and Critical Significance: The flatness of a film is directly related to the optical uniformity of a display device. If the size distribution of microcapsules is wide (polydispersive) or compatibility with the binder is poor, the film surface becomes uneven, causing localized variations in light scattering characteristics, which manifest as fine mottles or uneven brightness on the screen. The upper threshold of Ra, 500 nm, represents a roughness level similar to that of visible light wavelengths (400-700 nm); exceeding this value results in visually perceptible image degradation due to light diffraction and diffuse reflection. Despite the use of intentionally polydispersive particles, the present invention achieves Ra ≤ 500 nm through a core-shell-corona structure and an optimized binder system. This implies that the manufacturing process of the present invention can control film uniformity at a fine level, possessing critical significance as an essential condition for realizing high-quality images.

[0101] Frequency Response Sensitivity (FRS): Definition: When driven by AC, it is an indicator of how sensitively the response of particles (e.g., velocity or concentration rate to a specific location) changes with frequency around the resonance frequency (f_res) where the dielectrophoretic force of a specific group of particles is maximum. It can be defined as FRS = (ΔResponse / Response) / (Δf / f_res).

[0102] Numerical Limitation: The present invention is characterized in that the resonance frequencies (f_res,1, f_res,2) of the first particle group (large particles) and the second particle group (small particles) differ from each other by more than 10 times, and the frequency response sensitivity (FRS) of each group is 2.0 or higher.

[0103] Technical Significance and Critical Significance: The variable frequency AC drive of the present invention utilizes the principle that particles of different sizes react differently at different frequencies. Only when the resonant frequencies of two particle groups are sufficiently separated can the desired particle group be selectively controlled without affecting the other particle group when an AC electric field of a specific frequency is applied. A tenfold difference in resonant frequencies is the minimum condition for clear separation, where the frequency response curves of the two groups barely overlap. Furthermore, FRS ≥ 2.0 implies that particle behavior can be effectively switched ON / OFF with only slight frequency changes near the resonant frequency, which is an essential condition for precise and rapid frequency-selective driving. Therefore, these parameters hold significant critical significance in defining the feasibility and effectiveness of the unique AC drive method of the present invention.

[0104] 6. Method of driving an electrophoretic display device

[0105] The driving method of the electrophoretic display device according to the present invention adopts a DC-AC hybrid driving method that is fundamentally different from the conventional simple DC pulse driving method by utilizing the unique physical characteristics of the polydisperse particle system with the core-shell-corona structure described above. This method precisely controls the particles by combining the electrophoresis effect caused by a DC electric field and the dielectrophoresis effect caused by a variable frequency AC electric field.

[0106] For driving, the upper transparent electrode (common electrode) of the display panel is set to a reference potential (e.g., 0V, ground), and a signal in which a DC voltage level and an AC voltage waveform of a specific frequency and amplitude are superimposed is applied to each pixel electrode of the lower substrate through a TFT switch.

[0107] Electrophoresis (EP): This is the force by which charged particles move toward electrodes under a DC electric field. This force is proportional to the charge of the particles and plays a fundamental role in expressing colors by moving the particles to the front (visible surface) or back of the screen.

[0108] Dielectrophoresis (DEP): This is the force applied to particles under a non-uniform AC electric field. The direction and magnitude of this force depend on the particle's size, shape, dielectric properties, and the frequency of the AC electric field. In this invention, the characteristic that two groups of particles of different sizes (first particle and second particle) react differently at different frequencies is actively utilized.

[0109] Low frequency response: In low frequency AC electric fields (e.g., 1-100 Hz), the first particle, which is mainly larger in size, receives a strong DEP force.

[0110] High-frequency response: In high-frequency AC electric fields (e.g., 1-100 kHz), mainly small secondary particles receive a strong DEP force.

[0111] Hereinafter, a driving method is described in detail using a black and white system including a first group of positively (+) charged particles (white, large particles) and a second group of negatively (-) charged particles (black, small particles) as an example.

[0112] White state implementation

[0113] Initialization: Particles are uniformly mixed by alternately applying short DC pulses and AC waveforms with a wide frequency range.

[0114] White driving: A negative (-) DC voltage and a low-frequency AC voltage are superimposed and applied to the pixel electrodes.

[0115] The negative DC voltage provides the primary electrophoretic force that strongly attracts positively charged large white particles to the visible surface (common electrode side).

[0116] Simultaneously, the low-frequency AC voltage applies additional DEP force to the large white particles, helping them to pack more densely toward the visibility surface. Conversely, the small black particles barely respond to this low-frequency AC field or are pushed backward due to a weak repulsive force.

[0117] Result: The poetic surface is densely covered with large white particles, realizing a white state with high reflectivity.

[0118] Implementation of Black state

[0119] Initialization: Proceed in the same way as above.

[0120] Black driving: A positive (+) DC voltage and a high-frequency AC voltage are superimposed and applied to the pixel electrodes.

[0121] Positive DC voltage provides an electrophoretic force that attracts small negatively charged black particles to the visible surface.

[0122] Simultaneously, the high-frequency AC voltage applies a strong DEP force to small black particles, accelerating their movement and focusing toward the visibility surface. Conversely, large white particles barely react to this high-frequency AC field or receive a weak repulsive force, causing them to remain near the back electrode.

[0123] Result: The poet's surface is covered with small black particles, realizing a black state with a high absorption rate.

[0124] The driving method of the present invention achieves optimal performance by satisfying the parameters described above.

[0125] Applications of Frequency Response Sensitivity (FRS)The driving method of the present invention actively utilizes the characteristic that the resonance frequencies (f_res) of the first and second particles differ by more than 10 times, and the frequency response sensitivity (FRS) of each group is 2.0 or higher. For example, when driving white, a low-frequency AC close to the resonance frequency (f_res,1) of the first particle is applied, and when driving black, a high-frequency AC close to the resonance frequency (f_res,2) of the second particle is applied. Since the FRS is high at 2.0 or higher, the DEP power decreases rapidly even if it deviates only slightly from the target frequency. This enables 'frequency channel separation,' which turns only the target particle group 'ON' through a specific frequency and keeps other particle groups in an 'OFF' state, thereby dramatically improving the accuracy of color separation. If the FRS is less than 2.0, the frequency response is insensitive, making such selective control difficult. Therefore, FRS ≥ 2.0 is an essential critical condition for the AC hybrid driving of the present invention to operate effectively.

[0126] Importance of Film Flatness (Ra): The DEP force generated by an AC electric field is highly sensitive to the non-uniformity of the electric field. If the surface of the electrophoretic film is rough and the flatness (Ra) exceeds 500 nm, the distance between the electrode and the particle layer varies locally, forming a non-uniform AC electric field. This causes the DEP force to unintentionally become stronger or weaker in specific regions, leading to a 'DEP mottle' phenomenon where particles are distributed unevenly. Therefore, a film flatness (Ra) ≤ 500 nm holds critical significance as an important prerequisite for the AC driving method of the present invention to operate uniformly and stably to realize high-quality images.

[0127] 7. Preparation of Electrophoretic Particles

[0128] The core-shell-corona structured electrophoretic particles according to the invention are prepared through a multi-stage surface-initiated polymerization method in which an initiator is immobilized on the core surface, followed by the sequential growth of the shell and corona. In all preparation examples, the particles after the reaction were purified by centrifugation and solvent washing, and vacuum dried to obtain the final product.

[0129] Preparation Example 1: White particles (W-1)

[0130] (1) Preparation of core-shell particles: A rutile TiO₂ pigment with an average particle size of 250 nm was used as the core. First, 3-(2-bromoisobutyryloxy)propyltriethoxysilane, an ATRP initiator, was reacted on the surface of the core to fix the initiator via covalent bonding. Subsequently, the core particles with the fixed initiator were dispersed in a reaction solvent, and 15 g of styrene was added as a shell-forming monomer, 1 g of divinylbenzene (DVB) as a crosslinking agent, and 4 g of 2-(dimethylamino)ethyl methacrylate (DMAEMA) as a positive charge-giving monomer. The ATRP catalyst and ligand were added, and the mixture was polymerized at 80°C for 4 hours to produce core-shell particles with a robust positively charged shell approximately 15 nm thick.

[0131] (2) Corona formation: A corona was grown using the initiation site remaining on the surface of the core-shell particles. The core-shell particles were dispersed in a reaction solvent, and 30 g of lauryl methacrylate (LMA) was added as a corona-forming monomer, and ATRP was continued at 70°C for 6 hours. Through this process, white particles (W-1) with flexible LMA chains grafted onto the shell surface were obtained.

[0132] Preparation Example 2: White particles (W-2)

[0133] Core-shell particles were prepared in the same manner as in Preparation Example 1. In the corona formation step, a 'step-by-step monomer injection method' was used to impart intentional polydispersity. First, only 5g of LMA monomer was added and polymerized at 70°C for 1 hour to form short corona chains. Subsequently, the remaining 25g of LMA monomer was injected all at once, and polymerization was carried out for 5 more hours. Through this process, white particles (W-2) were prepared having a corona with a broad molecular weight distribution, in which short and long chains were mixed, and a polydispersity index (PDIw) of 1.6 was measured.

[0134] Preparation Example 3: Black particles (K-1, standard corona)

[0135] (1) Core-shell particle manufacturing:

[0136] 1. Fixation of core surface initiator: 100 g of carbon black pigment with an average particle size of 180 nm was placed in a 2 L reactor and dispersed in 1 L of anhydrous toluene. 10 g of 3-(2-bromoisobutyryloxy)propyltriethoxysilane, a surface initiator ATRP, was added to this, and the initiator was fixed to the core surface by covalent bonding by reflux stirring at 110°C under a nitrogen atmosphere for 24 hours.

[0137] 2. Surface-initiated polymerization of the shell: 100 g of a carbon black core immobilized with an initiator was dispersed in 1 L of anisole, a reaction solvent. To this, 150 g of styrene was added as a shell-forming monomer, 15 g of divinylbenzene (DVB) as a crosslinking agent, and 50 g of 2-(dimethylamino)ethyl methacrylate (DMAEMA) as a positive charge-giving monomer. 1.5 g of copper(I) bromide (CuBr) was added as an ATRP catalyst and 3.5 g of N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA) was added as a ligand, and dissolved oxygen was removed by repeating the freeze-pump-thaw cycle three times. Subsequently, polymerization was carried out at 80°C for 4 hours to produce core-shell particles with a robust positively charged shell approximately 15 nm thick.

[0138] (2) Corona formation: A corona was grown using the initiation site remaining on the surface of the core-shell particles. 100 g of purified core-shell particles were dispersed in 1 L of anisole, a reaction solvent, and 300 g of lauryl methacrylate (LMA) was added as a corona-forming monomer. After adding the ATRP catalyst and ligand, ATRP was carried out at 70°C for 6 hours. Through this process, a final black particle (K-1) with flexible LMA chains grafted onto the shell surface was obtained.

[0139] Preparation Example 4: Black particles (K-2, black iron oxide-based, polydisperse corona)

[0140] (1) Preparation of core-shell particles: 100 g of synthetic black iron oxide (Fe₃O₄) pigment with an average particle size of 200 nm was used as the core. Using the abundant hydroxyl groups on the surface of the black iron oxide, the ATRP initiator was immobilized in the same manner as in step (1)-1 of Preparation Example 3. Subsequently, black iron oxide core-shell particles were prepared by forming a positively charged shell with a thickness of about 15 nm using the same monomer composition and polymerization conditions as in step (1)-2 of Preparation Example 3.

[0141] (2) Formation of polydisperse corona: 100g of the above black iron oxide core-shell particles were dispersed in 1L of anisole. A 'step-by-step monomer injection method' was applied. First, 50g of LMA monomer and ATRP catalyst / ligand were added and polymerized at 70°C for 1 hour to form short corona chains. Then, the remaining 250g of LMA monomer was injected into the reactor all at once and polymerization was carried out for 5 more hours. Through this process, black particles (K-2) with a broad molecular weight distribution corona containing a mixture of short and long chains and a polydispersity index (PDIw) of 1.55 were produced.

[0142] Preparation Example 5: Cyanide particles (C-1, polydisperse corona)

[0143] (1) Preparation of core-shell particles: 100 g of cyan pigment (Pigment Blue 15:3) with an average particle size of 220 nm was used as the core, and the ATRP initiator was fixed in the same manner as in step (1)-1 of Preparation Example 3. In the shell formation step, 40 g of methacrylic acid (MAA), an acidic monomer, was used together with 150 g of styrene and 15 g of DVB instead of DMAEMA to impart a negative charge, and polymerization was carried out under the same conditions as in step (1)-2 of Preparation Example 3 to produce cyan core-shell particles with a strong shell that is negatively charged.

[0144] (2) Formation of polydisperse corona: Using 100g of the above cyanide core-shell particles, a 'step-by-step monomer injection method' (initial injection of 50g of LMA followed by additional injection of 250g) was applied, which is exactly the same as step (2) of Preparation Example 4, to form a polydisperse LMA corona, thereby producing a final negatively charged cyanide particle (C-1).

[0145] Preparation Example 6: Magenta particles (M-1, polydisperse corona)

[0146] Negatively charged magenta particles (M-1) were prepared through exactly the same procedure as Preparation Example 5 (initiator fixation, core-shell formation, polydisperse corona formation), except that 100 g of magenta pigment (Pigment Red 122) with an average particle size of 210 nm was used as the core particle and 40 g of MAA was used for shell formation.

[0147] Preparation Example 7: Yellow particles (Y-1, polydisperse corona)

[0148] Negatively charged yellow particles (Y-1) were prepared through exactly the same procedure as in Preparation Example 5, except that 100g of yellow pigment (Pigment Yellow 150) with an average particle size of 250 nm was used as the core particle and 40g of MAA was used for shell formation.

[0149] 8. Experimental Example of Electrophoretic Fluid Preparation

[0150] Experimental Example 1: Standard Performance Electrophoretic Fluid (Fluid-ST-H)

[0151] This experimental example was designed to evaluate the basic performance of standard polydisperse particles in hydrocarbon solvents.

[0152] (1) Preparation of dispersion: 82.5 g of dodecane, a non-polar hydrocarbon solvent, was accurately weighed into a 250 mL glass beaker. 1.5 g of a polyisobutylene succinimide-based dispersant (Lubrizol 2153) was added as a dispersion stabilizer, and a transparent dispersion was prepared by stirring with a magnetic stirrer at 200 rpm at room temperature (25°C) for 1 hour until the stabilizer was completely dissolved.

[0153] (2) Particle mixing: 10.0g of white particles (W-2) of Preparation Example 2, 2.0g of black iron oxide-based black particles (K-2) of Preparation Example 4, 1.5g of cyan particles (C-1) of Preparation Example 5, 1.5g of magenta particles (M-1) of Preparation Example 6, and 1.0g of yellow particles (Y-1) of Preparation Example 7 were added in order to a separate container to prepare a total of 16.0g of particle mixture.

[0154] (3) Uniform dispersion: The particle mixture was slowly added to the previously prepared dispersion while being lightly stirred with a glass rod. After all the particles were added, the beaker containing the mixture was placed under the probe of a **probe-type sonicator** and the probe was submerged in the center of the mixture. To prevent overheating, the beaker was placed in a cooling bath, and ultrasonic dispersion was performed for a total of 30 minutes under conditions of 400W output and 20kHz. After dispersion was completed, 100.0g of a standard performance electrophoretic fluid (Fluid-ST-H) in a uniform suspension state without visible particle clumping was obtained.

[0155] Experimental Example 2: Electrophoretic fluid for high-concentration systems (Fluid-HC-H)

[0156] This experimental example was designed to verify the characteristic of the polydisperse particle system of the present invention maintaining low viscosity even at high particle concentrations.

[0157] (1) Preparation of dispersion: A dispersion was prepared using 77.5 g of dodecane and 1.5 g of dispersion stabilizer (Lubrizol 2153) in the same manner as in Experimental Example 1.

[0158] (2) Particle Mixing: The total solid content of the particles was increased to 21.0 wt%. A total of 21.0g of particle mixture was prepared by weighing 12.0g of the white particles (W-2) of Preparation Example 2, 3.0g of the black particles (K-2) of Preparation Example 4, 2.0g of the cyan particles (C-1) of Preparation Example 5, 2.0g of the magenta particles (M-1) of Preparation Example 6, and 2.0g of the yellow particles (Y-1) of Preparation Example 7.

[0159] (3) Uniform dispersion: After adding the above high-concentration particle mixture to the prepared dispersion, the particles were completely dispersed by applying the same ultrasonic dispersion conditions as in Experimental Example 1 (probe type, 400W, 20kHz, 30 min). As a result, 100.0g of high-concentration electrophoretic fluid (Fluid-HC-H) with good fluidity despite the high solid content was successfully prepared.

[0160] Experimental Example 3: Fluorine-based solvent-based fast response electrophoretic fluid (Fluid-FR-F)

[0161] This experimental example was designed to maximize high-speed response characteristics using a low-viscosity fluorine-based solvent.

[0162] (1) Preparation of dispersion: 84.5 g of hydrofluoroether (HFE-7200), a fluorinated organic solvent, was prepared as the dispersion fluid. To this, 1.5 g of perfluoropolyether (Krytox™ 157 FSH), a dispersion stabilizer specialized for fluorinated solvents, with modified functional groups was added and stirred at room temperature for 1 hour to completely dissolve it.

[0163] (2) Particle Mixing: 8.0g of white particles (W-2) from Preparation Example 2, 2.0g of black particles (K-2) from Preparation Example 4, 1.5g of cyan particles (C-1) from Preparation Example 5, 1.5g of magenta particles (M-1) from Preparation Example 6, and 1.0g of yellow particles (Y-1) from Preparation Example 7 were weighed to prepare a total of 14.0g of particle mixture.

[0164] (3) Uniform dispersion: After adding the above particle mixture to a fluorine-based dispersion, a bath-type ultrasonic disperser was used to minimize solvent evaporation. The beaker containing the mixture was sealed and placed in the center of a cooling bath at 20°C, and dispersion was carried out at a frequency of 40 kHz for 1 hour. Finally, 100.0 g of uniformly dispersed fluorine-based solvent-based fast response electrophoretic fluid (Fluid-FR-F) was prepared.

[0165] Experimental Example 4: Monodisperse Particle Comparative Fluid (Fluid-Mono-H)

[0166] This experimental example was prepared using particles having a monodisperse corona to compare and verify the 'intentional polydispersity' effect of the present invention.

[0167] (1) Preparation of dispersion: A dispersion was prepared with 82.5 g of dodecane and 1.5 g of dispersion stabilizer, in the same manner as in Experimental Example 1.

[0168] (2) Particle mixing: The white particles of Preparation Example 1 (W-1, standard corona), the black particles of Preparation Example 3 (K-1, standard corona), and monodisperse colored particles that formed a standard corona on core-shell particles identical to those of Preparation Examples 5, 6, and 7 were mixed in the same composition as Experimental Example 1 (total 16.0 g). The PDIw of this particle group was all less than 1.1.

[0169] (3) Uniform dispersion: The above particle mixture was added to the dispersion, and 100.0 g of monodisperse particle comparison fluid (Fluid-Mono-H) was prepared through the same ultrasonic dispersion process as in Experimental Example 1.

[0170] Experimental Example 5: Minimal Constituent Black and White Fluid (Fluid-BW-H)

[0171] This experimental example was designed to evaluate the characteristics of the most basic two-particle (black / white) system of the present invention.

[0172] (1) Preparation of dispersion: A dispersion was prepared with 86.0 g of dodecane and 1.5 g of dispersion stabilizer, in the same manner as in Experimental Example 1.

[0173] (2) Particle mixing: A total of 12.5g of particle mixture was prepared by weighing only 10.0g of the white particles (W-2) of Preparation Example 2 and 2.5g of the black particles (K-2) of Preparation Example 4.

[0174] (3) Uniform dispersion: The above particle mixture was added to the dispersion, and 100.0 g of the minimum constituent black and white fluid (Fluid-BW-H) was prepared through the same ultrasonic dispersion process as in Experimental Example 1. This fluid is used to analyze the difference in behavior of black and white particles according to AC frequency.

[0175] 9. Experimental example of microcapsule preparation

[0176] Experimental Example 6: Preparation of Standard Performance Microcapsules (Capsule-ST-H)

[0177] This experimental example was designed to encapsulate the standard performance electrophoretic fluid (Fluid-ST-H) prepared in Experimental Example 1.

[0178] (1) Preparation of internal phase (oil phase): 50.0 g of Fluid-ST-H from Experimental Example 1 was added to a 250 mL beaker. To this, 2.5 g of Desmodur N3300, an aliphatic polyisocyanate prepolymer, was added as a reactive monomer for capsule wall formation, and 0.5 g of sorbitan trioleate, an oil-soluble surfactant for emulsion stability, was added. This mixture was stirred using a magnetic stirrer at 200 rpm for 30 minutes at room temperature (25°C) to prepare an oil phase in which all components were uniformly dissolved.

[0179] (2) Preparation of external phase (aqueous phase): 200.0 g of deionized water and 10.0 g of gum arabic, which acts as a protective colloid, were placed in a 1 L capacity reaction vessel and stirred at 60°C for 1 hour to completely dissolve the mixture. After cooling the solution to room temperature, the pH was precisely adjusted to 4.0 using a 0.1 M aqueous hydrochloric acid solution while monitoring with a pH meter.

[0180] (3) Emulsification: While slowly adding the oil phase prepared above to the aqueous phase, a rotor-stator type high-speed homogenizer was operated at 3000 rpm. Homogenization was carried out for 10 minutes to form an O / W (Oil-in-Water) emulsion in which droplets of the oil phase were uniformly dispersed in the aqueous phase. At this time, a portion of the emulsion was sampled and observed using an optical microscope, and it was confirmed that the average diameter of the droplets was about 40 μm.

[0181] (4) Encapsulation: The temperature of the reaction vessel was raised to 55°C while slowly stirring the generated emulsion at 150 rpm using a mechanical stirrer. The temperature was maintained for 3 hours to allow the formation of polyurea capsule walls through interfacial polymerization of isocyanate and water at the oil / water interface. After the reaction was finished, the mixture was cooled to room temperature and passed through 200 μm and 20 μm mesh filters sequentially to remove unaggregated capsules and impurities. Finally, the capsules were recovered by centrifugation (2000 rpm, 10 min) and washed three times with deionized water to obtain a standard performance microcapsule (Capsule-ST-H) slurry containing Fluid-ST-H inside with an average particle size of about 40 μm.

[0182] Experimental Example 7: Preparation of Microcapsules (Capsule-HC-H) for High Concentration Systems

[0183] This experimental example was designed to encapsulate the highly concentrated electrophoretic fluid (Fluid-HC-H) of Experimental Example 2.

[0184] 50.0 g of Fluid-HC-H from Experimental Example 2 was used as the internal phase. Considering the high viscosity (approx. 15 cP) of the highly concentrated fluid, the amount of sorbitan trioleate was increased to 0.8 g and the stirring time was extended to 1 hour during the oil phase preparation to ensure uniformity. In the emulsification step, the speed of the high-speed homogenizer was increased to 3500 rpm and operated for 15 minutes to obtain the target 40 μm emulsion droplets. The remaining processes of aqueous phase preparation, encapsulation reaction (55°C, 3 hours), and purification were carried out under the same conditions as in Experimental Example 6. As a result, a microcapsule (Capsule-HC-H) slurry for a highly concentrated system with an average particle size of approximately 40 μm, containing the highly concentrated fluid stably enclosed inside, was obtained.

[0185] Experimental Example 8: Preparation of Fluorine-Based Fast Response Microcapsules (Capsule-FR-F)

[0186] This experimental example was designed to encapsulate the fluorine-based electrophoretic fluid (Fluid-FR-F) of Experimental Example 4.

[0187] (1) Preparation of internal phase (oil phase): 50.0 g of Fluid-FR-F from Experimental Example 4 was added. Considering the interfacial characteristics of the fluorinated oil, 0.5 g of hexamethylene diisocyanate (HDI) was added as a comonomer along with 2.5 g of Desmodur N3300 as a monomer for forming capsule walls. 0.6 g of sorbitan sesquioleate, which is suitable for fluorinated oil, was used as a surfactant.

[0188] (2) Encapsulation: The water phase preparation, emulsification (3000 rpm, 10 min), encapsulation reaction (55°C, 3 hours), and purification processes were carried out under the same conditions as in Experimental Example 6 above. Since the density difference between the fluorine-based solvent and water is large (>1.5 g / cm³), the stirring speed was precisely controlled to maintain the stability of the emulsion during emulsification. After all processes were completed, a fluorine-based high-speed response microcapsule (Capsule-FR-F) slurry with an average particle size of about 40 μm, in which Fluid-FR-F was stably encapsulated inside, was successfully prepared.

[0189] Experimental Example 9: Preparation of Comparative Microcapsules of Monodisperse Particles (Capsule-Mono-H)

[0190] This experimental example was designed to encapsulate the monodisperse particle comparison fluid (Fluid-Mono-H) of Experimental Example 4.

[0191] Microencapsulation was carried out using exactly the same chemical substances, composition, and process conditions as in Experimental Example 6, except that 50.0 g of Fluid-Mono-H from Experimental Example 4 was used internally. After all processes were completed, a comparative microcapsule (Capsule-Mono-H) slurry of monodisperse particles with an average particle size of about 40 μm, containing monodisperse particle fluid inside, was successfully prepared.

[0192] Experimental Example 10: Preparation of Minimal Constituent Black and White Microcapsules (Capsule-BW-H)

[0193] This experimental example was designed to encapsulate the minimum constituent black and white fluid (Fluid-BW-H) of Experimental Example 5.

[0194] Microencapsulation was carried out using exactly the same chemical substances, composition, and process conditions as in Experimental Example 6, except that 50.0 g of Fluid-BW-H from Experimental Example 5 was used internally. After the completion of all processes, a minimally composed black-and-white microcapsule (Capsule-BW-H) slurry containing only black-and-white particles inside and having an average particle size of approximately 40 μm was successfully prepared. This capsule is used to verify the AC driving principle of the present invention.

[0195] 10. Experimental example of preparing an electrophoretic film containing microcapsules

[0196] Experimental Example 9: Preparation of Standard Performance Electrophoretic Film (Film-ST)

[0197] Experimental Example 11: Preparation of Standard Performance Electrophoretic Film (Film-ST-H)

[0198] (1) Preparation of coating slurry: 50.0 g (solid content approximately 60 wt%) of the standard performance microcapsule (Capsule-ST-H) slurry prepared in Experimental Example 6 was added to a 100 mL stirring vessel. To this, 10.0 g (solid content 30 wt%) of a waterborne polyurethane (PUD) solution was added as a polymer binder for film formation, 0.5 g of a waterborne isocyanate was added as a crosslinking agent for curing the binder, and 0.1 g of a nonionic surfactant BYK-348 was added sequentially to help uniform coating by lowering surface tension during coating. To minimize mechanical damage to the capsules, this mixture was gently stirred at a low speed (100 rpm) for 30 minutes using a planetary mixer to prepare a coating slurry in which all components were uniformly mixed.

[0199] (2) Coating and Drying: A 0.5 mm thick PET (Polyethylene terephthalate) film with a thin-film transistor (TFT) array formed on its surface was prepared as the lower substrate of the film. The prepared coating slurry was coated onto the TFT surface of this lower substrate using a doctor blade. The gap of the doctor blade was precisely adjusted to 80 μm so that the film thickness after drying would be approximately 45 μm. The coated film was dried in a hot air dryer at 60°C for 10 minutes to primarily remove moisture and bring the binder to a semi-cured state.

[0200] (3) Lamination: As an upper electrode film, a layer of thermoplastic adhesive (hot-melt adhesive) was pre-coated on the ITO surface of a 0.1 mm thick transparent PET film, on which indium tin oxide (ITO) was deposited to a thickness of 150 nm. The upper electrode film was placed on top of the first dried capsule / binder layer with the adhesive layer facing it, and passed through a roll laminator at 110°C and 0.4 MPa. The adhesive was activated by heat and pressure to strongly bond the capsule layer and the upper electrode, while the final cross-linking reaction of the binder was completed. After cooling, a standard performance electrophoretic film (Film-ST-H) was completed in which a standard performance microcapsule layer was uniformly formed between the TFT substrate and the upper transparent electrode.

[0201] Experimental Example 12: Preparation of an electrophoretic film (Film-HC-H) for a high-concentration system

[0202] When preparing the coating slurry, 50.0 g of the microcapsule (Capsule-HC-H) slurry for the high-concentration system prepared in Experimental Example 7 was used. Since the viscosity of the slurry was measured to be approximately 20% higher than the standard due to the high-concentration capsule, the coating speed of the doctor blade was adjusted to be 20% slower than the standard to ensure coating uniformity, and the amount of surfactant BYK-348 was increased to 0.15 g. The composition of the remaining binder (PUD 10.0 g) and crosslinking agent (0.5 g) was the same as in Experimental Example 11. Subsequently, the hot air drying at 60°C and the roll lamination process under conditions of 110°C and 0.4 MPa were also carried out in the same manner as in Experimental Example 11. After the completion of all processes, an electrophoretic film (Film-HC-H) for the high-concentration system, which is expected to have improved optical performance due to higher particle density, was successfully manufactured.

[0203] Experimental Example 13: Preparation of Fluorine-Based High-Speed ​​Response Electrophoretic Film (Film-FR-F)

[0204] When preparing the coating slurry, the composition and mixing conditions of the binder (PUD 10.0g), crosslinking agent (0.5g), and surfactant (0.1g) were exactly the same as those in Experimental Example 11, except that 50.0g of the fluorine-based solvent-based fast response microcapsule (Capsule-FR-F) slurry prepared in Experimental Example 8 was used. Subsequently, coating using a doctor blade (gap 80μm), hot air drying at 60°C, and roll lamination processes at 110°C and 0.4 MPa were carried out sequentially. After the completion of all processes, a fluorine-based solvent-based fast response electrophoretic film (Film-FR-F), which is expected to exhibit a fast response speed by containing a low-viscosity fluorine-based fluid inside, was successfully prepared.

[0205] Experimental Example 14: Preparation of a Comparative Electrophoretic Film of Monodisperse Particles (Film-Mono-H)

[0206] When preparing the coating slurry, exactly the same binder, crosslinking agent, and surfactant compositions and mixing conditions as in Experimental Example 11 were applied, except that 50.0 g of the monodisperse particle comparison microcapsule (Capsule-Mono-H) slurry prepared in Experimental Example 9 was used. Subsequently, the coating, drying, and bonding processes using a doctor blade were also carried out in the same manner as in Experimental Example 11, thereby successfully preparing a monodisperse particle comparison electrophoretic film (Film-Mono-H) to compare and evaluate the 'intentional polydispersity' effect of the present invention.

[0207] Experimental Example 15: Preparation of Minimal Construct Black and White Electrophoretic Film (Film-BW-H)

[0208] When preparing the coating slurry, the composition of the binder, crosslinking agent, and surfactant, as well as the mixing conditions, were exactly the same as those in Experimental Example 11, except that 50.0 g of the minimum configuration black and white microcapsule (Capsule-BW-H) slurry prepared in Experimental Example 10 was used. Subsequently, the coating, drying, and bonding processes using a doctor blade were also carried out in the same manner as in Experimental Example 11, thereby successfully producing a minimum configuration black and white electrophoretic film (Film-BW-H) to verify the AC driving principle of the present invention.

[0209] 11. Experimental example of manufacturing an electrophoretic display device including an electrophoretic film

[0210] Experimental Example 16: Preparation of a Standard Performance Electrophoretic Display Device (Display-ST-H)

[0211] (1) Driving circuit connection: A standard performance electrophoretic film (Film-ST-H) prepared in Experimental Example 11 was prepared. An anisotropic conductive film (ACF) was temporarily attached to the driver IC connection pad area exposed on the edge of the lower TFT substrate of the film using a hot press. After aligning the output terminals of a flexible printed circuit board (FPCB) on which a gate driver IC and a source driver IC were mounted onto the ACF, the FPCB and the TFT substrate were electrically permanently connected by applying pressure and heating at 180°C and 3 MPa for 5 seconds using a final bonding device. In addition, the common electrode lead line of the upper transparent electrode film was connected to the ground (GND) or common voltage (Vcom) terminal of the FPCB using silver (Ag) paste and UV cured.

[0212] (2) Module Assembly: On the front (visibility surface) of the display device, a PET protective film with anti-glare and a hard coating of 3H was attached using a roller to prevent air bubbles from forming. On the rear of the display device, a 0.5mm thick aluminum back cover was installed to protect the TFT substrate from mechanical shock and to aid in heat dissipation. The front protective film, electrophoretic film, and back cover were firmly assembled into an integrated module using a plastic frame with double-sided adhesive tape attached.

[0213] (3) Aging and Inspection: The assembled display module was placed in an aging chamber and various test patterns (full white, full black, color pattern, grayscale, etc.) were repeatedly run once every minute for 24 hours in an environment of 60°C and 50% relative humidity to detect initial defects and stabilize the driving characteristics of the electrophoretic particles. After the aging process was completed, the product that passed the appearance inspection (scratches, foreign matter, etc.) and optical characteristic inspection (brightness, contrast ratio, color coordinates, etc.) was completed as the final standard performance electrophoretic display (Display-ST-H).

[0214] Experimental Example 17: Preparation of an electrophoretic display device (Display-HC-H) for a high-concentration system

[0215] Except for using the electrophoretic film (Film-HC-H) for a high-concentration system manufactured in Experimental Example 12 as the core component of the display device, the manufacturing process of the display device was carried out under exactly the same procedure and process conditions as in Experimental Example 16. Specifically, an FPCB of the same specifications was attached to the TFT substrate of Film-HC-H using ACF, and a common electrode was connected. The module was assembled using the same anti-glare protective film on the front and the same aluminum back cover on the back. Finally, after undergoing an aging and inspection process at 60°C for 24 hours, an electrophoretic display device (Display-HC-H) for a high-concentration system, which is expected to have excellent optical performance due to its high particle density, was successfully completed.

[0216] Experimental Example 18: Preparation of a Fluorine-Based High-Speed ​​Response Electrophoretic Display (Display-FR-F)

[0217] The fluorine-based solvent-based high-speed response electrophoretic film (Film-FR-F) prepared in Experimental Example 13 was used as the core component of the display device. All other processes, namely FPCB attachment (ACF bonding), common electrode connection, module assembly using a protective film and back cover, and the final aging and inspection processes, were performed under conditions exactly identical to those of Experimental Example 16. During the aging and inspection stage, since this display device contains a low-viscosity fluorine-based fluid, testing was conducted using a dedicated driving waveform employing a lower driving voltage (±12V), shorter DC pulses, and AC waveforms with a higher frequency range. Finally, a fluorine-based solvent-based high-speed response electrophoretic display device (Display-FR-F) characterized by a fast screen switching speed was successfully completed.

[0218] Experimental Example 19: Preparation of a Monodisperse Particle Comparative Electrophoresis Display Device (Display-Mono-H)

[0219] Except for using the monodisperse particle comparative electrophoresis film (Film-Mono-H) prepared in Experimental Example 14 as the core component of the display device, the manufacturing process of the display device was carried out under exactly the same procedure and process conditions as in Experimental Example 16. This display device was used as a control group to evaluate the 'intentional polydispersity' effect of the present invention, and was used to focus on analyzing stability and rheological properties, particularly during high-concentration operation. Finally, the monodisperse particle comparative electrophoresis display device (Display-Mono-H) was completed.

[0220] Experimental Example 20: Preparation of a Minimal Configuration Monochromatic Electrophoretic Display (Display-BW-H)

[0221] Except for using the minimal configuration monochrome electrophoretic film (Film-BW-H) manufactured in Experimental Example 15 as the core component of the display device, the manufacturing process of the display device was carried out under exactly the same procedure and process conditions as in Experimental Example 16. This display device was fabricated to clearly verify the frequency-dependent size-selective particle control (dielectrophoretic effect) among the AC driving principles of the present invention. During the aging and inspection stages, the behavior of black and white particles was intensively analyzed when low-frequency AC and high-frequency AC were applied, respectively. Finally, the minimal configuration monochrome electrophoretic display device (Display-BW-H) was completed.

[0222] 12. Comparative example for comparative testing

[0223] Comparative Example 1: Manufacture of a free-volume model-based display device

[0224] This comparative example was created to reproduce the technical concepts of 'free volume' and 'passage conditions' of the prior art.

[0225] (1) Preparation of particles and fluid: Two types of particles were prepared, namely the first particle (white) and the second particle (black). During particle synthesis, the target size was precisely set so that the final particle size would satisfy the 'pass condition (R - a₁ > a₂)'. Specifically, a TiO₂ core with an average particle size of 450 nm and a carbon black core with an average particle size of 200 nm were used. The surface of each particle was thinly coated with polystyrene for simple dispersion stability. The particle size distribution was subjected to a classification process using a centrifuge several times to be as close as possible to monodispersity (PDI < 1.1). Dodecane was used as the dispersion fluid, and the particle concentration was optimized according to the free volume calculation described in the patent above.

[0226] (2) Manufacture of a display device: Microcapsules were manufactured using an interfacial polymerization method similar to Experimental Example 6 of the present invention, using a fluid containing the two types of particles above. These microcapsules were mixed with a binder and coated onto a TFT substrate, and then bonded with an upper transparent electrode film to complete an electrophoretic film. Afterward, a comparison display device was manufactured through the same final module assembly process as Experimental Example 16 of the present invention.

[0227] Comparative Example 2: Manufacture of 'coating-free' particle and free polymer-based display device

[0228] (1) Preparation of particles and fluid: Two types of particles, namely white (TiO₂) and black (carbon black) pigments, were prepared, but no covalently bonded coating was intentionally formed on the surface of the pigments. These uncoated particles were dispersed in a dodecane solvent, and an electrophoretic fluid was prepared by adding Solsperse 17000 at a concentration of 2.0 wt% as a charge control agent (CCA) and polyisobutylene (molecular weight 20,000) at a concentration of 5.0 wt% as a free polymer to improve bistability. The charge of the particles was made to depend on dynamic adsorption between the pigment surface and the CCA.

[0229] (2) Manufacturing of a display device: Using the above fluid, a comparative display device was completed through the same procedure as Comparative Example 1 (microencapsulation, film coating and bonding, final module assembly).

[0230] Comparative Example 3: Manufacture of a display device based on a monodisperse particle system

[0231] This comparative example was created to reproduce the monodispersity-oriented technical concept of the prior art.

[0232] (1) Preparation of particles and fluid: Two types of particles were prepared, namely, a first particle (white) and a second particle (black). After particle synthesis, the density gradient classification method using an ultracentrifuge was repeated several times to narrow the particle size distribution to an extreme degree. Finally, the particles were purified so that the polydispersity index (PDI) of each particle group was less than 1.05. The average particle size of the first particle was set to 400 nm and the average particle size of the second particle was set to 200 nm so that the particle size distributions of the two groups did not substantially overlap. Dodecane was used as the dispersion fluid.

[0233] (2) Manufacture of a display device: A comparative display device was completed using a fluid containing the highly monodisperse particles as described above, through the same procedure as Comparative Example 1.

[0234] Comparative Example 4: Manufacture of a 'core-shell' structure and a DC-driven based display device

[0235] This comparative example was designed to compare and analyze the effects of a simple core-shell structure without a 'corona' among the 'core-shell-corona' structures of the present invention.

[0236] (1) Preparation of particles and fluid: In the preparation example of the present invention, the corona formation step was omitted to prepare white and black particles formed only up to the core-shell structure. These particles have an inherent charge, but the steric stabilization effect is relatively weak because there is no flexible corona layer. Using these core-shell particles, an electrophoretic fluid was prepared with the same composition as in Experimental Example 1.

[0237] (2) Manufacturing of a display device: Using the above fluid, a comparative display device was completed through the same procedure as Comparative Example 1. This display device is evaluated by driving it with a conventional DC pulse waveform instead of AC driving.

[0238] Comparative Example 5: Manufacture of 'polydisperse' particles and a DC-driven based display device

[0239] This comparative example was designed to analyze the effect of applying the 'intentionally polydisperse' particles of the present invention to a conventional DC drive method.

[0240] (1) Preparation of particles and fluid: White and black particles having a 'polydisperse corona' prepared in Preparation Example 2 (W-2) and Preparation Example 4 (K-2) of the present invention were used. Using these particles, an electrophoretic fluid was prepared with the same composition as in Experimental Example 1.

[0241] (2) Manufacturing of a display device: Using the above fluid, a comparative display device was completed through the same procedure as Comparative Example 1. This display device is evaluated by driving only with a conventional DC pulse waveform, rather than the AC hybrid driving of the present invention.

[0242] 13. Performance Evaluation and Verification of Effectiveness

[0243] Test Example 1 (Advanced): Evaluation of Rheological Properties and Dispersion Stability at High Concentrations

[0244] Test Objective: Quantitatively compare the effect of the 'polydispersity' and 'core-shell-corona' structure of the present invention in maintaining low fluid viscosity and improving dispersion stability at high particle concentrations with that of the prior art, and confirm the critical significance of a polydispersity index (PDIw) ≥ 1.2.

[0245] Design Method:

[0246] Test group (10 samples each, n=10):

[0247] Experimental Example 2 (The Present Invention): Fluid-HC-H (polydisperse core-shell-corona particles)

[0248] Experimental Example 4: Fluid-Mono-H (Monodisperse core-shell-corona particles)

[0249] Comparative Example 2 Fluid: Uncoated particles + Free polymer

[0250] Comparative Example 4 Fluid: Simple core-shell particles

[0251] Measurement items: Viscosity was measured at a concentration of 21 wt% using a rotational rheometer, and sedimentation and aggregation were observed after storage at 60°C for 240 hours.

[0252] Rheological properties and stability of high-concentration (21 wt%) fluid (n=10, mean ± standard deviation)

[0253]

[0254] The polydisperse fluid of the present invention (Experimental Example 2) had a viscosity approximately four times lower than that of the monodisperse fluid (Experimental Example 4, Comparative Example 4) at the same high concentration. This is because particles with high PDIw more efficiently pack the space, thereby maintaining fluidity. The monodisperse fluid exhibited behavior close to a 'glass transition' in which viscosity increases rapidly at low shear rates, requiring a large driving force. The uncoated particles (Comparative Example 2) exhibited very low stability due to severe sedimentation and irreversible aggregation at high temperatures. The simple core-shell particles (Comparative Example 4) were more stable than the uncoated particles, but some aggregation was observed due to a lack of steric stabilization effect caused by the absence of a flexible corona layer.

[0255] Critical Significance (PDIw ≥ 1.2): Fluid viscosity directly affects response speed and driving voltage. At high concentrations, driving performance slows significantly if viscosity exceeds 50 cP. The results of this test demonstrate that PDIw ≥ 1.2 is a critical condition that enables high-speed / low-power driving by maintaining low viscosity even at high particle concentrations. If PDIw is lower than this value (approaching monodispersity), it is difficult to secure commercially significant performance due to the problem of a rapid viscosity spike when implementing high concentrations. Furthermore, the 'core-shell-corona' structure is shown to be a key element ensuring product reliability by providing excellent dispersion stability even at high temperatures.

[0256] Test Example 2: Evaluation of Response Speed ​​and Driving Voltage

[0257] Test Objective: To demonstrate that the particle structure and fluid design of the present invention improve response speed compared to conventional technology and enable low-power operation.

[0258] Design Method:

[0259] Test group (10 samples each, n=10): Experimental Example 16 (Display-ST-H), Experimental Example 18 (Display-FR-F), Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, Comparative Example 5.

[0260] Measurement Items: The time taken to switch between white and black states on each panel (response speed) and the minimum driving voltage required to achieve clear white (L*>80) / black (L*<10) were measured.

[0261] Comparison of Response Speed ​​and Minimum Driving Voltage (n=10, mean ± standard deviation)

[0262]

[0263] All of the display devices of the present invention exhibited a fast response speed of less than 300 ms, and in particular, Experimental Example 18, which used a low-viscosity fluorine-based solvent, achieved the highest level of speed of 150 ms. This is 24 times faster than Comparative Example 1, which relies on 'pass conditions', Comparative Example 5, which uses non-linear mobility, and Comparative Example 2, which has unstable charging characteristics. In addition, the system of the present invention, having stable charge and low viscosity, was efficiently operated even at lower voltages (±12V±15V).

[0264] Critical Significance: Generally, commercial electronic paper requires a response speed of 300ms or less; if it is slower than this, a delay perceptible to the user occurs when turning pages. The results of this test demonstrate that the comprehensive system design of the present invention (particle structure, polydispersity, fluid) can reliably satisfy the commercial requirement of a response speed of 300ms or less. This is the result of overcoming the limitations (geometric constraints, unstable charge, high viscosity, etc.) that conventional technologies individually possess.

[0265] Test Example 3: Evaluation of the Effects of AC-DC Hybrid Driving and Frequency Response Sensitivity (FRS)

[0266] Test Objective: To demonstrate the superiority of the variable frequency AC driving method of the present invention and to confirm the critical significance of frequency response sensitivity (FRS) ≥ 2.0.

[0267] Design Method:

[0268] Test group (10 samples each, n=10): Experimental Example 20 (Display-BW-H, Rg1 / Rg2 ≤ 2.2) and comparison sample (Rg1 / Rg2 ≤ 1.5).

[0269] Measurement Item: Calculate FRS by measuring the improvement rate of L* value during DC+AC driving compared to DC-only driving and the change in L* value according to frequency.

[0270] AC Driving Effect and FRS Measurement Results (n=10, Mean ± Standard Deviation)

[0271]

[0272] The present invention (Experimental Example 20) demonstrated that when low-frequency AC was superimposed on DC driving, the white L* value was improved by 7.8%, effectively packing white particles with a large DEP force of the AC electric field. The AC effect was negligible in the comparison sample with a low Rg ratio. The FRS was measured to be high at 2.5, confirming that the particle response is very sensitive near the resonance frequency.

[0273] Critical significance (FRS ≥ 2.0): This is a critical condition for effectively controlling particles with an AC electric field. If FRS is less than 2.0, the difference in response according to frequency is insensitive, making selective control difficult. Therefore, FRS ≥ 2.0 is an essential critical condition for the AC hybrid drive of the present invention to operate effectively, and this implies that the Rg1 / Rg2 ratio must be sufficiently large, at least 2.0.

[0274] Test Example 4: Evaluation of the Relationship Between Film Flatness (Ra) and Optical Uniformity

[0275] Test Objective: To demonstrate that film flatness (Ra) ≤ 500 nm is a critical condition for ensuring optical uniformity of a high-definition display device.

[0276] Design Method:

[0277] Test group (10 samples each, n=10): Experimental Example 11 (Film-ST-H, Ra–280 nm) and comparison film (Ra–750 nm).

[0278] Measurement items: Luminance uniformity and Mottle index measurements.

[0279] Comparison of Optical Uniformity According to Film Flatness (n=10, mean ± standard deviation)

[0280]

[0281] The film of the present invention, which has excellent flatness, showed a high luminance uniformity of 92% and a low Mottle index, making the entire screen very uniform. On the other hand, the comparison film, which has poor flatness, had a low luminance uniformity of 75% and a high Mottle index, so fine mottledness was observed even with the naked eye.

[0282] Critical Significance (Ra ≤ 500 nm): When Ra > 500 nm, which is the film surface roughness level approaching the visible light wavelength level, the fine scattering characteristics of light become non-uniform, resulting in a visually perceptible degradation of image quality. Therefore, Ra ≤ 500 nm holds critical significance as an important prerequisite for realizing high-quality images.

[0283] Test Example 5: Long-term reliability and durability evaluation

[0284] Test Objective: To demonstrate that the core-shell-corona structure of the present invention possesses superior long-term reliability compared to conventional technology.

[0285] Design Method:

[0286] Test group (10 samples each, n=10): Experimental Example 16, Comparative Example 2, Comparative Example 4.

[0287] Measurement items: After 100,000 repeated runs, measure the rate of change of the white L* value and the average ΔE*ab (amount of color change).

[0288] Comparison of Performance Changes After Repeated Run (n=10, Mean ± Standard Deviation)

[0289]

[0290] The display device of the present invention showed minimal performance degradation even after 100,000 cycles of operation. On the other hand, particles without coating (Comparative Example 2) showed severe brightness degradation and color change, and simple core-shell particles (Comparative Example 4) also showed significant performance degradation.

[0291] Critical Significance: If ΔEab exceeds 5.0, it is generally considered that there is a problem with product quality. This result clearly demonstrates that a stable core-shell-corona structure is a key technology that ensures excellent long-term reliability at the ΔEab < 3.0 level.

[0292] Test Example 6: Evaluation of System Design Degrees of Freedom

[0293] Test Objective: To demonstrate that the design principles of the present invention provide a wider degree of design freedom compared to conventional 'pass condition' models.

[0294] Design Method:

[0295] Test group: (1) A fluid in which Φeff,1 is fixed at 0.5 and the Rg1 / Rg2 ratio is varied according to the design principle of the present invention. (2) A fluid in which the a1 / a2 ratio is varied based on the 'pass condition' of Comparative Example 1. Ten samples (n=10) were prepared for each condition.

[0296] Measurement Item: Measure the response speed of each cell and compare the range of design parameters that maintain high performance (less than 300ms).

[0297] Performance Maintenance Range According to Changes in Design Parameters

[0298]

[0299] The present invention maintained excellent performance over a wide range of Rg1 / Rg2 ratios from 1.8 to 2.8. On the other hand, Comparative Example 1's performance deteriorated rapidly when the a1 / a2 ratio fell outside a very narrow 'optimal point' between 2.1 and 2.3.

[0300] Critical Significance: This demonstrates that the rheological approach of the present invention enables system design that is much more robust and flexible than conventional strict geometric constraints. This is highly advantageous for utilizing various materials and absorbing variations in the production process, which is an important effect that enhances the commercial value of the present invention.

[0301] 14. Physical property analysis data of manufactured electrophoretic particles

[0302] To demonstrate that the core-shell-corona structured particles manufactured above were successfully synthesized to possess the intended structure and physical properties, the following physical properties were measured and analyzed.

[0303] (1) Analysis of particle size, shape, and core-shell-corona structure

[0304] The particle shape, size, distribution, and multilayer structure were comprehensively analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS).

[0305] Results of Size and Structure Analysis of Core-Shell-Corona Particles

[0306]

[0307] Size and Shape: SEM analysis confirmed that the final particles are within the target size range and possess a relatively uniform spherical or quasi-spherical shape.

[0308] Core-Shell-Corona Structure: TEM image analysis revealed a distinct triple core-shell-corona structure in all particles, in which a rigid shell (medium brightness layer) with intermediate electron density surrounds an inorganic pigment core (darkest part) with high electron density, and a flexible corona (brightest layer) with the lowest electron density surrounds the outside. It was confirmed that the thickness of each layer was formed within the design range.

[0309] Dispersibility and Polydispersity: DLS analysis results showed that the polydispersity index (PDI) of all particles was measured to be less than 0.3, confirming that they are macroscopically stable and dispersed without clumping. In addition, the polydispersity index (PDIw) of the corona layer analyzed via molecular weight distribution measurement (GPC) was 1.5 or higher, proving that it has a broad molecular weight distribution as intended.

[0310] (2) Chemical composition and bonding analysis

[0311] Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analysis were performed to determine whether each layer has the intended chemical composition and is stably connected to each other by covalent bonds.

[0312] Results of Surface Chemical Analysis of Particles

[0313]

[0314] Final particle (C-1 et al.) XPS: No N 1s peak, S 2p or C 1s(COOH) peak detected, negatively charged groups are present in the shell.

[0315] Comparison of FT-IR spectra confirmed that each polymerization step proceeded successfully through spectral changes after shell formation and corona formation. In particular, the peak intensity corresponding to the ester group (C=O) and alkyl group (CH) of LMA increased significantly after corona formation, demonstrating that a substantial amount of corona was grafted. XPS analysis analyzed the elemental composition at a depth of several nanometers from the surface, clearly showing the presence of intended charge-carrying monomers (positive: nitrogen (N), negative: sulfur (S) or additional oxygen (O)) inside the shell of each particle.

[0316] (3) Analysis of electrical and dynamic characteristics

[0317] To confirm the surface charge characteristics and radius of gyration of the particles, analysis using laser Doppler electrophoresis and DLS was performed.

[0318] Results of Analysis of Electrical and Dynamic Properties of Particles

[0319]

[0320] Charge Characteristics: Zeta potential measurements confirmed that the white / black particle group exhibited high positive values ​​of +40mV or higher, while the colored particle group showed high negative values ​​of -35mV or lower, indicating that they were charged with clearly opposite polarities.

[0321] Radius of Gravity and Structure: The hydrodynamic radius (Rh) and radius of gravity (Rg) were obtained simultaneously from DLS measurements. The Rg / Rh ratio provides information about the internal structure and density distribution of the particles. The measured Rg / Rh value was found to be less than 1.0 (approximately 0.774 for a uniform sphere), supporting the fact that the particles have a non-uniform structure consisting of a high-density core and a low-density shell / corona. This is important data that indirectly proves that the core-shell-corona structure of the present invention was successfully formed.

[0322] The electrophoretic medium, display device, and driving method according to the present invention, as demonstrated through the above test examples, have the following effects.

[0323] First, core-shell-corona particles with polydispersity (PDIw ≥ 1.2) maintain excellent fluidity by reducing viscosity by more than four times compared to monodisperse particle systems, even at high particle concentrations (21 wt% or higher). This overcomes the limitations of conventional geometric 'pass-through condition' models and enables the production of high-concentration, high-performance electrophoretic fluids, thereby having the effect of simultaneously improving the optical density and response speed of display devices. (Refer to Test Example 1)

[0324] Second, the strong steric stabilization effect provided by the flexible corona layer significantly suppresses irreversible aggregation and sedimentation in high-temperature and long-term repetitive operation environments compared to uncoated or simply coated particle systems. This extends the lifespan of the display device and leads to high reliability that ensures consistent performance in various usage environments. (See Test Examples 1 and 5)

[0325] Third, through new parameters—the radius of gyration (Rg), which reflects the actual dynamic size of the particles, and the effective volume fraction (Φeff), which represents the rheological properties of the system—the complex physical behavior of the actual system can be predicted and controlled more accurately. This enhances design accuracy and degrees of freedom compared to conventional techniques that relied on ideal geometric models, thereby providing development flexibility for various material combinations. (See Test Example 6)

[0326] Fourth, the hybrid driving method utilizing a variable frequency AC electric field along with a DC electric field enables precise particle control that was impossible with DC driving alone, through the dielectrophoretic effect based on particle size. By designing particles to have a Frequency Response Sensitivity (FRS) of 2.0 or higher, the packing efficiency of large particles is maximized via the AC electric field, thereby further improving brightness in the white state by more than 7%, and selectively driving small particles enhances the accuracy of color separation. (Refer to Test Example 3)

[0327] Fifth, through the organic combination of the above components, a fast response speed of less than 300ms, specifically at the level of 150ms when using fluorine-based solvents, and a low driving voltage of ±15V or less are simultaneously achieved. This overcomes the limitations of conventional technologies, which had to sacrifice other performance aspects to improve specific performances such as response speed, driving voltage, and reliability, and has a comprehensive effect of improving overall performance indicators in a balanced manner. (Refer to Test Example 2)

[0328] The materials and manufacturing process required for the manufacturing of color particles, unit pixels, and microcapsules of the present invention, as well as for the manufacturing of a display device including the same, were referenced in a patent of the same applicant registered prior to this application.

[0329] KR 10-1984763 B1 (2019.05.27.)

[0330] KR 10-1913709 B1 (2018.10.25.)

[0331] KR 10-2102294 B1 (2020.04.13.)

[0332] KR 10-2255328 B1 (2021.05.17.)

[0333] KR 10-2156044 B1 (2020.09.09.)

[0334] KR 10-2156063 B1 (2020.09.09.)

[0335] KR 10-2340892 B1 (2021.12.14.)

Claims

Claim 1 An upper substrate; a lower substrate; a first electrode and a second electrode facing each other disposed on one surface of each of the upper substrate and the lower substrate; The electrophoretic display device comprises a display layer disposed between the first electrode and the second electrode and including an electrophoretic fluid containing electrophoretic particles, wherein the electrophoretic particles include a first particle group and a second particle group, the first and second particle groups have charges of opposite polarities to each other, the first and second particle groups have a core-shell-corona structure including a core, a shell on the surface of the core, and a corona on the surface of the shell, the average swivel radius (Rg1) of the first particle group is at least twice the average swivel radius (Rg2) of the second particle group, the resonance frequency (f_res) of the first particle group and the second particle group differs by at least 10 times, the frequency response sensitivity (FRS) of each group is 2.0 or higher, and the first and second particle groups each have a polydispersity particle size distribution with a polydispersity index (PDIw) of 1.2 or higher. Claim 2 delete Claim 3 An electrophoretic display device according to claim 1, characterized in that the effective volume fraction (Φeff,1) of the first particle group is 0.4 or more and less than 0.6, and the effective volume fraction (Φeff,2) of the second particle group is 0.05 or more and less than 0.

2. Claim 4 delete Claim 5 An electrophoretic display device according to claim 1, characterized in that the centerline average roughness (Ra) of the electrophoretic film including the display layer is 500 nm or less. Claim 6 An electrophoretic display device according to claim 1, characterized in that the first particle group comprises white light-scattering particles and the second particle group comprises black light-absorbing particles. Claim 7 An electrophoretic display device according to claim 1, characterized in that the first particle group comprises white and black particles, and the second particle group comprises cyan, magenta, and yellow particles. Claim 8 The electrophoretic display device according to claim 1, wherein the display layer has a structure in which the electrophoretic fluid is enclosed within a plurality of microcapsules or microcups. Claim 9 The electrophoretic particle included in the electrophoretic display device of claim 1 comprises a first particle group and a second particle group, wherein the first particle group and the second particle group have a core-shell-corona structure comprising a core; a shell covalently bonded to the surface of the core; and a corona covalently bonded to the surface of the shell and swollen by a solvent, and wherein the polydispersity index (PDIw), defined by the molecular weight distribution of the corona, is 1.2 or higher. Claim 10 In claim 9, the electrophoretic particle is characterized in that the shell is a cross-linked polymer layer containing a charge-giving monomer. Claim 11 An electrophoretic particle according to claim 9, characterized in that the corona is a flexible polymer chain comprising lauryl methacrylate or stearyl methacrylate. Claim 12 An electrophoretic particle according to claim 9, characterized in that the core is a pigment having a color selected from the group consisting of white, black, cyan, magenta, and yellow. Claim 13 An electrophoretic fluid comprising a plurality of electrophoretic particles according to claim 9; and a liquid dispersion medium, wherein the electrophoretic fluid substantially does not contain a charge control agent (CCA). Claim 14 A microcapsule comprising a polymer shell; and a core portion encapsulated by said polymer shell, wherein the core portion comprises the electrophoretic fluid of claim 13. Claim 15 An electrophoretic film characterized by comprising a plurality of microcapsules according to claim 14; and a polymer binder supporting the plurality of microcapsules. Claim 16 A method for manufacturing an electrophoretic particle according to claim 9, comprising: (a) a step of fixing a first initiator on the surface of the core; (b) a step of forming the shell by surface-initiating polymerization of a first monomer from the core surface on which the first initiator is fixed; and (c) a step of fixing a second initiator on the surface of the shell or activating a residual initiation site, and then surface-initiating polymerization of a second monomer to form the corona, wherein in step (c), the supply rate or reaction temperature of the second monomer is controlled so that the polydispersity index (PDIw) of the corona is 1.2 or higher. Claim 17 A method for manufacturing a microcapsule according to claim 14, characterized by comprising the step of dispersing the electrophoretic fluid of claim 13 in an aqueous phase to form an emulsion, and polymerizing a monomer at the interface of the emulsion to form the polymer shell. Claim 18 A method for manufacturing an electrophoretic film according to claim 15, comprising: (a) a step of preparing a slurry by mixing a plurality of microcapsules according to claim 14 with the polymer binder; and (b) a step of applying the slurry onto a substrate and curing it. Claim 19 A method for driving an electrophoretic display device according to claim 1, comprising the step of applying a hybrid voltage including a DC voltage and a variable frequency AC voltage between the first electrode and the second electrode, and by adjusting the frequency of the AC voltage, selectively driving the first particle group or the second particle group or fixing them at a specific position. Claim 20 A method for manufacturing an electrophoretic display device according to claim 1, comprising: a step of manufacturing an electrophoretic film according to claim 15; and a step of bonding the electrophoretic film to a lower substrate including a thin film transistor (TFT) array.

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