HK40048612B - Composite electrophoretic particles and variable transmission films containing the same - Google Patents

Description

Composite electrophoretic particles and variable transmission membranes containing composite electrophoretic particles

Reference to relevant applications

This application claims priority to U.S. Patent Application No. 62/809,978, filed February 25, 2019, which is incorporated herein by reference in its entirety.

Background of the Invention

This invention relates to variable transmission devices. More specifically, this invention relates to variable transmission devices containing an electrophoretic medium comprising composite particles that improve the optical performance of the variable transmission device.

Optical modulators represent a potentially significant market for electro-optic media. As energy performance becomes increasingly important in buildings and vehicles, electro-optic media can be used as coatings on windows (including rooftop and sunroof windows) to allow for electronic control of the proportion of incident radiation penetrating the window by altering the optical state of the electro-optic media. Effective implementation of this “variable transmissivity” (“VT”) technology in buildings is expected to provide (1) reduced undesirable heating effects during hot weather, thus reducing the amount of energy required for cooling, the size of air conditioning equipment, and peak power demand; (2) increased use of natural daylight, thus reducing energy used for lighting and peak power demand; and (3) enhanced user comfort by increasing thermal and visual comfort. Even greater benefits are expected to accumulate in automobiles, where the ratio of glazed surfaces to enclosed volumes is significantly greater than in typical buildings. Specifically, effective implementation of VT technology in automobiles is expected to provide not only the aforementioned benefits but also (1) improved driving safety, (2) reduced glare, (3) enhanced mirror performance (by using electro-optic coatings on mirrors), and (4) improved ability to use head-up displays. Other potential applications of VT technology include privacy glass and anti-glare sheets in electronic devices.

U.S. Patent No. 7,327,511 describes a variable transmission device comprising charged pigment particles distributed and encapsulated in a nonpolar solvent. These variable transmission devices can be driven to an open state using an AC driving voltage, thereby driving the charged pigment particles to the capsule wall. Therefore, such variable transmission devices can be used on viewing surfaces where translucency is desired to be arbitrarily altered, such as privacy glass, skylights, and windows in buildings.

U.S. Patent No. 7,327,511 also describes the adjustment of various important factors in the electrophoretic medium for optimal performance in optical modulators. One important factor is minimizing the haze value. In this application, "haze value" refers to the percentage of diffuse transmitted light (light scattered during transmission) relative to the total transmitted light. When designing an optical modulator that can be electronically switched from an open, transparent state to a closed, opaque state, it is desirable that the haze value in the open state be less than 10%, more preferably less than 2%.

Pigments used in VT devices, such as carbon black, attenuate transmitted light through a combination of scattering and absorption. Typically, the smallest grade of carbon black particles provides the most effective light attenuation. The nature of light scattering is also affected by the aggregate size of these particles. As the aggregate size increases, more and more light is scattered in the forward direction. This scattered light causes a hazy appearance in the window. The smallest particles tend to have the smallest aggregates and result in the smallest amount of haze. Since reducing haze is preferred in VT applications, it is desirable to use the smallest possible particle (or aggregate) size. However, the electrophoretic manipulation of particles improves with increasing size, making the smallest, most effective light-blocking agent very difficult to control. Since the switching speed and final dynamic range of the VT window are also important parameters, larger particles are desirable; therefore, in VT devices, the ability to reduce haze based on particle size is limited by the minimum particle size requirements needed to control the optical switching speed.

Variable transmission devices based on encapsulated particles can be bistable. The terms "bistable" and "bistable" are used herein in their conventional sense in the art to refer to a display comprising display elements having first and second display states that are different in at least one optical property, and such that after any given element is driven to present its first or second display state by an addressing pulse of finite duration, the state will persist for at least several times, for example, at least four times, the minimum duration of the addressing pulse required to change the state of the display element after the addressing pulse terminates. Bistableness can be enhanced by adding a flocculant (or depleting agent), which induces an osmotic pressure difference between pigment-pigment and pigment-depleting agent molecules. As a result, the internal phases within the microcapsule separate into a pigment-rich phase and a depleting agent-rich phase. However, when the capsule is open, large pigment aggregates in the pigment-rich phase can cause scattering and haze.

Therefore, there is a need to improve bistable electro-optic media that can be incorporated into variable transmission devices with acceptable switching rates and low fog values.

Invention Overview

In one aspect, the electro-optic medium comprises multiple microcapsules in a binder, each microcapsule containing a dispersion comprising multiple charged composite particles and a suspended fluid, the charged composite particles moving through the suspended fluid under the influence of an electric field. The composite particles include one or more pigment particles selected from manganese ferrite black spinel, copper per chromite black spinel, carbon black, and combinations thereof, and are at least partially coated with a polymeric material. Each of the binder, the charged composite particles, and the suspended fluid has a refractive index, and at 550 nm, the difference between the refractive index of the composite particles and at least one of the binder and the suspended fluid is less than or equal to 0.05.

On the other hand, the electro-optic medium comprises a polymer sheet containing a plurality of sealed micropores, each micropore containing a dispersion comprising a plurality of charged composite particles and a suspended fluid, wherein the charged composite particles move through the suspended fluid under the influence of an electric field. The composite particles comprise one or more pigment particles selected from manganese iron black spinel, copper chromium black spinel, carbon black, and combinations thereof, and are at least partially coated with a polymeric material. Each of the polymer sheet, the charged composite particles, and the suspended fluid has a refractive index, and at 550 nm, the difference between the refractive index of the composite particles and at least one of the polymer sheet and the suspended fluid is less than or equal to 0.05.

In another aspect, the electro-optic medium comprises a plurality of droplets in a continuous polymeric phase, each droplet containing a dispersion comprising a plurality of charged composite particles and a suspended fluid, the charged composite particles moving through the suspended fluid under the influence of an electric field. The composite particles include one or more pigment particles selected from copper chromium black spinel, carbon black, and combinations thereof, and are at least partially coated with a polymeric material. Each of the continuous polymeric phase, the charged composite particles, and the suspended fluid has a refractive index, and at 550 nm, the difference between the refractive index of the composite particles and at least one of the continuous polymeric phase and the suspended fluid is less than or equal to 0.05.

These and other aspects of the invention will become apparent from the following description.

Brief description of the attached diagram

The accompanying drawings illustrate one or more embodiments of the invention, and are by way of example only, not as a limitation.

Figure 1 is an illustration of a variable transmission device including first and second transparent electrode layers having an electro-optic medium disposed between the layers. When an electric field is applied, particles can move adjacent to the capsule wall, thus allowing light to pass through the medium, i.e., in an open state.

Figure 2A is a schematic diagram of a method for manufacturing composite particles according to an embodiment of the present invention.

Figure 2B is a schematic diagram of a second method for manufacturing composite particles according to another embodiment of the present invention.

Figures 3A and 3B are transmission electron micrographs of the microstructure of composite particles according to various embodiments of the present invention.

Figures 4A and 4B are graphs showing the zeta potential of carbon black relative to the composite pigments according to various embodiments of the present invention as a function of the concentration of the charge control agent.

Figure 5 is a graph showing the relative transmittance of fog values for encapsulated dispersions containing, or according to an embodiment of the present invention, composite particles or pigments in non-flocculating polymers and flocculating polymers.

Figures 6A and 6B are micrographs of encapsulated composite particles manufactured using an aqueous method according to another embodiment of the present invention.

Detailed Explanation

Generally, embodiments of the present invention provide an electro-optic medium comprising an encapsulated dispersion of charged composite particles and a suspended fluid, wherein the charged composite particles move through the suspended fluid under the influence of an electric field. The composite particles are preferably composed of pigment particles embedded in a polymeric material. The optical properties of the medium are controlled by the type and size of the pigment particles, and the type and thickness of the polymeric coating. The refractive index of the fluid is preferably matched to the total refractive index of the composite particles, such that the size of the composite particles does not affect light scattering and therefore does not promote the degree of fogging. Simultaneously, the composite particles are large enough to provide good system electrophoretic control. Furthermore, the polymeric coating provides separation between the pigment particles to suppress the formation of large aggregates that could cause fogging.

For variable transmission devices, the electrophoresis apparatus can be fabricated to operate in a so-called "shutter mode," as shown in Figure 1, where one operating state is substantially opaque and another is transparent. When this "shutter mode" electrophoresis apparatus is constructed on a transparent substrate, the transmittance of light through the apparatus can be adjusted.

The device 10 in Figure 1 is shown in an open state. Device 10 includes an electro-optic dielectric containing a capsule 16 in a polymeric binder 14. The capsule 16 contains a dispersion containing charged pigment particles 18 in a suspended fluid 19 that moves in response to an electric field. The capsule 16 is generally formed of a gelatinous material, as described in more detail below. The electro-optic dielectric layer is preferably adjacent to a layer of transparent conductive material, more preferably disposed between a first and a second layer of transparent conductive material (electrode layers 11a, 11b in Figure 1), which may be made of polyethylene terephthalate (PET) coated with a known material such as indium tin oxide (ITO). Alternatively, the electrode layer may contain metal electrodes that can be arranged as pixels. The pixels can be controlled as an active matrix, thus allowing switching of the separation areas of the device. An additional adhesive layer 12 is generally present between the electro-optic dielectric and one of the electrode layers 11a, 11b. This adhesive layer may be UV-cured and generally improves the planarity of the final device by filling the deviations caused by the capsule. Suitable adhesive formulations are described in U.S. 2017/0022403, which is incorporated herein by reference. The device 10 may further include at least one transparent substrate 20 on the electro-optical opposite side of one of the conductive material layers 11a, 11b; obviously, such a substrate may be provided for each electrode layer.

When a DC electric field is applied to the device 10 of Figure 1, the particles 18 move toward the viewing surface, thus changing the optical state from dark to bright. In Figure 1, when an AC electric field is applied to one of the electrodes 11a and 11b, the charged pigment particles 18 are driven to the wall of the capsule 16, causing light to pass through the aperture of the capsule 16, i.e., the open state. By using a nonpolar solvent, which also includes a charge control agent and/or stabilizer, as the suspending fluid 19, the optical state (open/closed) can be maintained for a long time (weeks) without maintaining the electric field. As a result, the device can be "switched" only twice a day and consumes very little power. In the case of the shutter mode device discussed below, the two extreme optical states can be referred to as "dark" and "bright" or "open" and "closed".

Numerous patents and applications transferred to or in the name of the Massachusetts Institute of Technology (MIT), E Ink Corporation, E Ink California, LLC, and related companies describe various techniques for encapsulating electrophoretic media and microporous electrophoretic media, as well as other electro-optic media. Encapsulating electrophoretic media comprises numerous small capsules, each containing an inner phase of electrophoretically moving particles in a fluid medium, and a capsule wall surrounding the inner phase. Typically, the capsule itself is held within a polymeric binder to form a coherent layer between two electrodes. In microporous electrophoretic displays, charged particles and fluid are not encapsulated within microcapsules but are retained within multiple cavities formed within a carrier medium (typically a polymeric film). The techniques described in these patents and applications include:

(a) Electrophoretic particles, fluids, and fluid additives; see, for example, 5,961,804; 6,017,584; 6,120,588; 6,120,839; 6,262,706; 6,262,833; 6,300,932; 6,323,989; 6,377,387; 6,515,649; 6,538,801; 6,580,545; 6,652,075; 6,693,620; 6,721,083 ; 6,727,881; 6,822,782; 6,831,771; 6,870,661; 6,927,892; 6,956,690; 6,958,849; 7,002,728; 7,038,655; 7,052,766; 7,110,162; 7,113,323; 7,141,688; 7,142,351; 7,170,670; 7,180,649; 7,226,550; 7 230,750；7,230,751；7,236,290；7,247,379；7,277,218；7,286,279；7,312,916；7,375,875；7,382,514；7,390,901；7,411,720；7,473,782；7,532,388；7,532,389；7,572,394；7,576,904；7,580,180；7,6 79,814；7,746,544；7,767,112；7,848,006；7,903,319；7,951,938；8,018,640；8,115,729；8,119,802；8,199,395；8,257,614；8,270,064；8,305,341；8,361,620；8,363,306；8,390,918；8,582,196；8,593 U.S. Patents 718; 8,654,436; 8,902,491; 8,961,831; 9,052,564; 9,114,663; 9,158,174; 9,341,915; 9,348,193; 9,361,836; 9,366,935; 9,372,380; 9,382,427; and 9,423,666; and U.S. Patents 2003/0048522; 2003/0151029; 2 003/0164480; 2003/0169227; 2003/0197916; 2004/0030125; 2005/0012980; 2005/0136347; 2006/0132896; 2006/0281924; 2007/0268567; 2009/0009852; 2009/0206499; 2009/0225398; 2010/0148385; 2011 /0217639; 2012/0049125; 2012/0112131; 2013/0161565; 2013/0193385; 2013/0244149; 2014/0011913; 2014/0078024; 2014/0078573; 2014/0078576; 2014/0078857; 2014/0104674; 2014/0231728; 2014/03 U.S. Patent Application Publications Nos. 39481; 2014/0347718; 2015/0015932; 2015/0177589; 2015/0177590; 2015/0185509; 2015/0218384; 2015/0241754; 2015/0248045; 2015/0301425; 2015/0378236; 2016/0139483; and 2016/0170106;

(b) Encapsulation, adhesives, and encapsulation methods; see, for example, sections 5,930,026; 6,067,185; 6,130,774; 6,172,798; 6,249,271; 6,327,072; 6,392,785; 6,392,786; 6,459,418; 6,839,158; 6,866,760; 6,922,276; 6,958,848; 6,987,603; 7,061,663; 7,071,913; 7,079,305; 7,109,968; 7,110,164; 7,184,197; 7,202,991; 7, U.S. Patents Nos. 242,513; 7,304,634; 7,339,715; 7,391,555; 7,411,719; 7,477,444; 7,561,324; 7,848,007; 7,910,175; 7,952,790; 7,955,532; 8,035,886; 8,129,655; 8,446,664; and 9,005,494; and U.S. Patent Application Publications Nos. 2005/0156340; 2007/0091417; 2008/0130092; 2009/0122389; and 2011/0286081;

(c) Microporous structures, wall materials, and methods for forming micropores; see, for example, U.S. Patents 7,072,095 and 9,279,906;

(d) Methods for filling and sealing micropores; see, for example, U.S. Patents 7,144,942 and 7,715,088;

(e) Films and subassemblies containing electro-optic materials; see, for example, U.S. Patents 6,982,178 and 7,839,564;

(f) Backplate, adhesive layer, and other auxiliary layers and methods for display; see, for example, U.S. Patents 7,116,318 and 7,535,624;

(g) Color formation and color adjustment; see, for example, U.S. Patents 7,075,502 and 7,839,564;

(h) A method for driving a display; see, for example, U.S. Patents 7,012,600 and 7,453,445; and

(i) Applications of displays; see, for example, U.S. Patents 7,312,784 and 8,009,348.

Many of the aforementioned patents and applications recognize that the walls surrounding the separated microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thus creating a so-called polymer-dispersed electrophoretic display, wherein the electrophoretic medium comprises a plurality of separated droplets of electrophoretic fluid and a continuous phase of polymeric material, and the separated droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered as capsules or microcapsules, even without a separating membrane connecting each individual droplet; see, for example, U.S. Patents 6,866,760 and 7,079,305 mentioned above. Therefore, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a subclass of encapsulated electrophoretic media.

Charged pigment particles can be of various colors and compositions. Furthermore, charged pigment particles can be functionalized with surface polymers to improve state stability. Such pigments are described in U.S. Patent No. 2016/0085132, which is incorporated herein by reference in its entirety. For example, if the charged particles are white, they can be formed from inorganic pigments such as _TiO₂ , _ZrO₂ , ZnO, _Al₂O₃ , _Sb₂O₃ , _BaSO₄ , _PbSO₄ , _etc. They can also be polymer particles with a high refractive index (>1.5 at 550 _nm ) and a specific size (>100 nm) to exhibit whiteness, or composite particles modified to have a desired refractive index. Black charged particles can be formed from CI Pigment Black 26 or 28, such as manganese iron black spinel or copper chromium black spinel, or carbon black. Other colors (neither white nor black) can be formed from organic pigments such as CI pigments PR 254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY83, PY138, PY150, PY155, or PY20. Other examples include Clariant Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV Fast Red D3G, Hostaperm Red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm Green GNX, BASF Irgazine Red L 3630, Cinquasia Red L 4100HD, and Irgazin Red L 3660HD; Sun Chemical Phthalocyanine Blue, Phthalocyanine Green, Diaryl Yellow, or Diaryl AAOT Yellow. Colored particles can also be formed from inorganic pigments such as CI Pigment Blue 28, CI Pigment Green 50, and CI Pigment Yellow 227. The surface of charged particles can be modified by known techniques based on the desired particle charge polarity and charge level, as described in U.S. Patents Nos. 6,822,782, 7,002,728, 9,366,935, and 9,372,380, and U.S. Patent No. 2014-0011913, the entire contents of which are incorporated herein by reference.

The particles may exhibit a negative charge, or may be explicitly charged using a charge control agent, or may acquire a charge when suspended in a solvent or solvent mixture. Suitable charge control agents are well known in the art; they may be polymeric or nonpolymeric in nature, or ionic or nonionic. Examples of charge control agents include, but are not limited to, Solsperse 17000 (living polymeric dispersant), Solsperse 9000 (living polymeric dispersant), 11000 (succinimide ashless dispersant), Unithox 750 (ethoxylate), Span 85 (sorbitan trioleate), Petronate L (sodium sulfonate), Alcolec LV30 (soy lecithin), Petrostep B100 (petroleum sulfonate) or B70 (barium sulfonate), Aerosol OT, polyisobutylene derivatives, or poly(ethylene-co-butene) derivatives. In addition to suspended fluids and charged pigment particles, the internal phase may also contain stabilizers, surfactants, and charge control agents. When the stabilizing material is dispersed in a solvent, it can be adsorbed onto charged pigment particles. This stabilizing material keeps the particles separated from each other, such that the variable transmission medium is substantially non-transmissive when the particles are in their dispersed state.

According to one aspect of the invention, an electro-optic medium containing composite pigment particles is provided, which has an adjustable surface charge and low haze value in the presence of a flocculant, and a method for manufacturing the composite pigment particles. The composite particles can be synthesized by polymerization of a seed dispersion in a nonpolar solvent, such that an organic polymer is deposited on the surface of the seed particles, as schematically illustrated in FIG2A. The resulting composite particles have a size on the order of hundreds of nanometers, with effective electrophoretic and dielectrophoretic switching, and are stable by means of depletion flocculation. The pigment particles preferably have a diameter of about 0.01 to 0.2 μm, and the polymeric material coated on the surface of the pigment particles can have a thickness of about 0.5 to 2 μm, more preferably about 0.3 to 0.7 μm. The total diameter of the composite particles can be less than 5 μm, more preferably about 0.5 to 2 μm, and most preferably about 0.5 to 1 μm.

The composite pigment particles according to a first embodiment of the invention can be synthesized in a nonpolar solvent that can be readily mixed with other components to produce an inner phase suitable for encapsulation methods. Without wishing to be bound by theory, it is believed that by embedding seed pigments into a polymer matrix, the electro-optic properties of the pigments are decoupled from their surface characteristics and charge properties. As a result, the refractive index of the pigment particles can be modified in a manner that also improves the motility of the pigment particles.

In the first step, the dense suspension is prepared by milling pigment particles in a nonpolar solvent, or a mixture of two or more solvents. Examples of solvents include, but are not limited to, aliphatic hydrocarbons such as heptane, octane, and petroleum distillates such as Exxon Mobil or Total; terpenes such as limonene, for example 1-limonene; and aromatic hydrocarbons such as toluene. Optional surfactants may be incorporated into the dispersion to aid the milling process (e.g., 1.0 g surfactant/g pigment). The suspension may contain 10 to 50% by weight, more preferably 15 to 35% by weight, and most preferably about 25% by weight of pigment particles.

Second, the pigment suspension can be diluted with a solvent such as Isopar E or hexane, and the aforementioned optional charge control agent, so that the pigment concentration is reduced to less than 15% by weight, more preferably less than 10% by weight, and most preferably less than about 6.0% by weight. Preferably, the weight ratio of the nonpolar solvent to the diluent in the suspension is less than or equal to 20.0% by weight.

In the third step, the diluted suspension is stirred and heated before adding a mixture of one or more monomers and/or oligomers and one or more initiators. The volume ratio of the monomer solution to the pigment, V polymer/V seed, is preferably from about 1.0 to about 10.0. The monomer mixture preferably contains at least three different monomers, including but not limited to acrylates and methacrylates (e.g., methyl methacrylate, hexanediol dimethacrylate, trifluoroethyl methacrylate, trimethoxysilyl propyl methacrylate, tert-butyl methacrylate, isobutyl methacrylate, benzyl methacrylate, 2-fluoroethyl methacrylate, trifluoroethyl acrylate, heptafluorobutyl acrylate, heptafluoroisopropyl acrylate, 2-methoxyethyl acrylate) and their oligomers. Initiators that may be included in the monomer mixture include, but are not limited to, azobis(2,4-dimethyl)valerate, benzoyl peroxide, tert-butyl peroxyneodecanate, diisopropyl peroxydicarbonate, methyl ethyl ketone peroxide, tert-butyl peroxyneodecanate, and combinations thereof. The concentration of the initiator in the monomer mixture is preferably less than or equal to about 5% by weight of the total monomers, more preferably less than or equal to about 3%, and most preferably less than or equal to about 1.5% by weight. Referring to FIG. 2A, a mixture 20 containing pigment particles 26, nonpolar solvent 22, monomers, initiator, and charge control agent is heated and stirred to allow the monomers to polymerize, such that the pigment particles 26 are at least partially, preferably completely, coated with the resulting polymer 24 to form composite particles. The composite particles can be washed and dried after polymerization.

Another method for preparing composite particles according to a second embodiment of the invention utilizes an aqueous method. In a first step, if desired, pigment particles are dispersed in an aqueous solution to form a nanodispersed phase (NDP) containing an optional dispersant, and polyethyleneimine (“PEI”) is added to the NDP. In a second step, the NDP is emulsified into a nonpolar continuous phase of propyl benzoate and/or methyl benzoate, containing a dissolved surfactant poly(hydroxystearic acid) (“PHSA”). In a third step, a solution containing a styrene/maleic anhydride copolymer (“SMA”) in propyl benzoate and/or methyl benzoate is added to the emulsion, and the SMA copolymer is allowed to react with the PEI, such that the interfacial reaction product forms a colloidal network or shell around the droplets of the aqueous pigment dispersion. A schematic diagram of this interfacial reaction is illustrated in Figure 2B. In an optional fourth step, the surface of the composite (i.e., the aqueous pigment dispersion with the PEI-SMA shell) is functionalized by the addition of lauroyl chloride. Lauroyl chloride will react with any unreacted amine groups in PEI and make the complex surface more hydrophobic.

The above-described aqueous method can be modified in many ways. For example, PEI can be replaced by another water-soluble amine compound having a refractive index of about 1.5 at 550 nm and sufficient amine functionality to generate a colloidal network as a result of the interfacial reaction. Similarly, SMA can be replaced by another polymer having a refractive index of about 1.5 at 550 nm and sufficient anhydride functionality to promote the colloidal network. Furthermore, the refractive index or hydrophobicity of SMA can be adjusted by forming custom copolymers, which are synthesized by modifying the amount of styrene and/or maleic anhydride, and/or incorporating other monomer units into copolymers such as ethylene, lauryl methacrylate, or trifluoroethyl methacrylate. In one exemplary embodiment, pigment particles can be coated with a polymeric material having a refractive index at 550 nm greater than or equal to 1.5, although a larger refractive index range is also intended, such as greater than or equal to 1.43, 1.44, 1.45, 1.46, 1.47, 1.48, or 1.49. Unless otherwise stated, the refractive index range reported herein is measured at temperatures from 20°C to 30°C.

The SMA may also need modification prior to the interfacial reaction, depending on the final choice of solvent for dispersing the composite particles. For example, if the composite particles are ultimately to be dispersed in an alkane such as Isopar, alkane groups can be added to the SMA to improve its compatibility in the solvent. Finally, the pigment particles can be dispersed in glycerol or other polyols to form NDPs, rather than an aqueous dispersion.

As is known in the art, the dispersion of charged particles (typically carbon black, as described above) in a low dielectric constant solvent can be aided by surfactants. Such surfactants typically contain a polar “head group” and a nonpolar “tail group” compatible with or soluble in the solvent. In this invention, it is preferred that the nonpolar tail group is a saturated or unsaturated hydrocarbon moiety, or other group soluble in hydrocarbon solvents, such as poly(dialkylsiloxane). The polar group can be of any polar organic functionality, including ionic materials such as ammonium salts, sulfonates, or phosphonates, or acidic or basic groups. Particularly preferred head groups are carboxylic acid or carboxylate groups. Suitable stabilizers for use in this invention include polyisobutylene and polystyrene. In some embodiments, dispersants such as polyisobutylene succinimide and/or sorbitan trioleate and/or 2-hexyldecanoic acid are added.

The fluid used in the variable transmission medium of this invention is generally of low dielectric constant (preferably less than 10 and ideally less than 3). This fluid is preferably a solvent with low viscosity, relatively high refractive index, low cost, low reactivity, and low vapor pressure/high boiling point. The suspension fluid is preferably a liquid, but the electrophoretic medium can be manufactured using a gaseous fluid; see, for example, Kitamura, T. et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y. et al., “Toner display using impulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Patents 7,321,459 and 7,236,291.

Examples of solvents that can be incorporated into the suspension fluid of the electro-optic medium according to various embodiments of the invention include, but are not limited to, aliphatic hydrocarbons such as heptane, octane, and petroleum distillates such as (Exxon Mobil) or (Total); terpenes such as limonene, for example 1-limonene; and aromatic hydrocarbons such as toluene. Limonene is a particularly preferred solvent because it possesses both a low dielectric constant (2.3) and a relatively high refractive index (1.47). The refractive index of the inner phase can be altered by adding a refractive index matching agent. For example, U.S. Patent No. 7,679,814 describes an electrophoretic medium suitable for use in a variable transmission device, wherein the fluid surrounding the electrophoretic particles comprises a mixture of partially hydrogenated aromatic hydrocarbons and terpenes, preferably d-limonene and partially hydrogenated terphenyl, commercially available from Cargille-Sacher Laboratories, 55 Commerce Rd, Cedar Grove N.J. 07009 under license number 5040. In the encapsulation media manufactured according to various embodiments of the present invention, it is preferable that the refractive index of the encapsulation dispersion is as close as possible to the refractive index of the encapsulation material to reduce haze. In most cases, the refractive index of the internal phase at 550 nm is at least 1.50, more preferably 1.51 to 1.57 at 550 nm, and most preferably about 1.54 at 550 nm.

In a preferred embodiment of the invention, the encapsulating fluid may comprise one or more non-conjugated olefins, preferably cyclic hydrocarbons. Examples of non-conjugated olefins include, but are not limited to, terpenes such as limonene; phenylcyclohexane; hexyl benzoate; cyclododecanetriene; 1,5-dimethyltetrahydronaphthalene; partially hydrogenated terphenyl, such as 5040; phenylmethylsiloxane oligomers; and combinations thereof. The most preferred composition of the encapsulating fluid according to one embodiment of the invention comprises cyclododecanetriene and partially hydrogenated terphenyl.

The gelatin-based capsule wall used in this variable transmission device has been described in many of the aforementioned E Ink and MIT patents and applications. Gelatin is available from various commercial suppliers such as Sigma Aldrich or Gelitia USA. Depending on the application requirements, it can be obtained in many grades and purities. Gelatin primarily comprises collagen collected and hydrolyzed from animal products (cattle, pigs, poultry, fish). It contains a mixture of peptides and proteins. In many embodiments described herein, gelatin is combined with acacia (gum arabic), which is derived from the cured sap of the acacia tree. Acacia is a complex mixture of glycoproteins and polysaccharides and is frequently used as a food stabilizer. As described below, the pH of the aqueous solution of acacia and gelatin can be adjusted to form a polymer-rich condensed phase capable of encapsulating nonpolar internal droplets.

Capsules incorporating gelatin/acacia can be prepared as follows; see, for example, U.S. Patent No. 7,170,670, which is incorporated herein by reference in its entirety. In this method, an aqueous mixture of gelatin and/or acacia is emulsified with a hydrocarbon inner phase (or another water-immiscible phase to be encapsulated) to encapsulate the inner phase. The solution may be heated to 40°C prior to emulsification to dissolve the gelatin. After obtaining the desired droplet distribution, the pH is generally lowered to form aggregates. The capsules are formed by controlled cooling and mixing of the emulsion—generally to room temperature or lower. If the wetting and dispersion conditions are correct, which are primarily determined by the inner phase composition, appropriate mixing and specific encapsulation formulations (e.g., gelatin & acacia concentration & pH) are achieved to gelatinize the aggregates around the inner phase droplets in a homogeneous manner. This method produces capsules in the 20-100 μm range and often incorporates more than 50% of the starting material into the available capsules. The resulting capsules are then separated by size using screening or other size-screening methods. Capsules larger than 100 μm are generally rejected because they are visible to the naked eye, and larger capsules increase the electrode gap, which in turn increases the required driving voltage.

After size screening, the capsule can be mixed with a binder to prepare a slurry for coating, for example using slit coating, knife coating, spin coating, etc. The binder preferably has a refractive index of at least 1.5 at 550 nm. Examples of binder materials include, but are not limited to, water-soluble polymers (e.g., polysaccharides, polyvinyl alcohol, N-methylpyrrolidone, N-vinylpyrrolidone, various series (Union Carbide, Danbury, Conn.), and poly(2-hydroxyethyl acrylate)), water-based polymers (e.g., polyurethane latex, optionally compounded with one or more acrylic, polyester, polycarbonate, or polysiloxane), oil-soluble polymers, thermosetting and thermoplastic polymers, radiation-cured polymers, and combinations thereof. In particular, mixtures of fish gelatin and polyanionic polymers such as acacia have been found to be excellent binders for capsules formed from aggregates of (pig) gelatin and acacia. Polyanionic polymers that may be included in the binder containing fish gelatin include, but are not limited to, carbohydrate polymers such as starch and cellulose derivatives, plant extracts (e.g., acacia), and polysaccharides (e.g., alginate); proteins such as gelatin or whey protein; lipids such as waxes or phospholipids; and combinations thereof. In some embodiments of the invention, the electrophoretic medium may contain a capsule-to-binder weight ratio of about 15:1 to about 50:1. In other embodiments of the invention, the electrophoretic medium may contain a higher proportion of binder, such as at least 1 part by weight of binder per 15 parts by weight of capsules, up to 1 part by weight of binder per 4 parts by weight of capsules.

The refractive index of the composite particles incorporated into the dispersion can be adjusted to match the refractive index of at least one of the suspending fluid and the binder. Preferably, each of the binder, the charged composite particles, and the suspending fluid has a refractive index, and at 550 nm, the difference between the refractive index of the composite particles and at least one of the binder and the suspending fluid is less than or equal to 0.05. More preferably, at 550 nm, the difference between the refractive index of the composite particles and the refractive indices of both the binder and the suspending fluid is less than or equal to 0.05.

As shown above, instead of microencapsulation, embodiments of the present invention may incorporate an electrophoretic dispersion into a polymer-dispersed layer, wherein multiple electrophoretic fluid-separated droplets are dispersed within a continuous polymeric phase. In the polymer-dispersed layer, the charged composite particles, the suspended fluid, and the continuous phase preferably have a refractive index, and at 550 nm, the difference between the refractive index of the composite particles and at least one, preferably both, of the refractive index of the continuous polymeric phase and the suspended fluid is less than or equal to 0.05. Alternatively, the electrophoretic fluid may be sealed within multiple micropores formed of polymer sheets, wherein at 550 nm, the difference between the refractive index of the composite particles and at least one, preferably both, of the refractive index of the suspended fluid and the polymer sheets is less than or equal to 0.05.

Example

Examples are now given, but only as illustrations, to show details of the preferred composite particles of the invention.

Example 1

First, a suspension of carbon black (Raven 3500, manufactured by Birla Carbon U.S.A. Inc. of Marietta, GA) in limonene and OLOA11000 was prepared. The suspension, containing 25 wt% carbon black and 12.5 wt% OLOA11000, was dispersed and milled until the carbon black particle size was approximately 120 nm. Two sets of composite particles were then prepared using this suspension.

The first group of composite particles was prepared initially by mixing 150 g of suspension with 570.0 g of Isopar E at a V polymer/V seed ratio of 1.5. The diluted suspension was then heated and mixed with a monomer mixture of 44.99 g methyl methacrylate (MMA), 5.62 g hexanediol dimethacrylate (HDDM), 5.62 g trifluoroethyl methacrylate (TFEM), and 0.84 g azobisisobutyronitrile (AIBN). The composite particles were washed and dried after polymerization.

The second group of composite particles was prepared by mixing 75 g of suspension with 570.0 g of Isopar E at a V polymer/V seed ratio of 2.0. The diluted suspension was then heated and mixed with a monomer mixture of 37.49 g methyl methacrylate, 4.69 g hexanediol dimethacrylate, 4.69 g trifluoroethyl methacrylate, and 0.70 g azobisisobutyronitrile. After polymerization, the composite particles were washed and dried.

The microstructure of the resulting composite particles can be observed in the transmission electron microscopy images in Figures 3A and 3B. In Figure 3A, the composite particles fabricated using a lower volume ratio of 1.5 are only partially coated, while in Figure 3B, the particles fabricated using a higher volume ratio of 2.0 are completely coated with the polymer.

Example 2

A second suspension was prepared using carbon black (Raven 1200, manufactured by Birla Carbon U.S.A. Inc. of Marietta, GA) in limonene and OLOA11000. The suspension containing 25% by weight carbon black and 12.5% by weight OLOA11000 was dispersed and milled until the carbon black particle size was approximately 115 nm.

To examine the effect of charge control agent (CCA) concentration on the composite particles, each composite particle sample was prepared using the same procedures as the first group of composite particles in Example 1, except that the type and amount of pigment suspension and monomer were slightly modified for some samples. The type and amount (g) of pigment suspension and monomer used to prepare each sample are provided in the table below:

*Trimethoxysilylpropyl methacrylate

Four separate 5 wt% dispersions were prepared, each containing one set of composite particles in limonene prepared according to the steps described in Table 1. Two control samples were also prepared: a 5 wt% Raven 3500 suspension in limonene and a 5 wt% Raven 1200 suspension in limonene. The charge capacity of each composite particle was determined by measuring the velocity of sound, density, and viscosity of the suspension using Acoustosizer II X and M (Colloidal Dynamics). The zeta potential of the pigment particles was calculated using these data. To monitor how the zeta potential changed with CCA loading, the dispersions were titrated by adding additional CCA, starting at a dose of 0.02 mL. The zeta potential graphs of the control suspensions containing Raven 3500 in Figures 4A and 4B are indicated by the symbol “□”, and the zeta potential graph of the control suspension containing Raven 1200 in Figure 4B is indicated by the symbol “○”.

Referring to the results in Figures 4A and 4B, it was observed that the zeta potential of the control suspension containing Raven 3500 showed a very small change with increasing CCA content. However, a sharp increase in charge was observed when copolymerized with functional monomers such as trifluoroethyl methacrylate or trimethoxysilyl propyl methacrylate. For example, as shown in Figure 4A, at a CCA to composite particle mass ratio of approximately 20% (10 mg/g), the zeta potential of the composite particles containing poly(MMA-copolymer-TMSPM) decreased to approximately -130 mV. When two different types of carbon black pigments with different charges were coated with the same copolymer composition, the resulting particles showed similar charges, as seen in Figure 4B with Raven 3500 and Raven 1200 pigments encapsulated in poly(MMA-copolymer-TFEM). From these measurements, we conclude that the surface potential of the composite particles is adjustable and may be determined by the type of monomers constituting the shell and matrix of the composite particles.

Example 3

To determine the effect of pigment particles on haze value, a 1.0 wt% pigment dispersion in limonene was prepared using pigment particles containing Raven 3500 encapsulated in an MMA-TFEM copolymer, prepared according to Example 2. A second control sample of a 1.0 wt% dispersion of carbon black (Raven 3500) was also prepared. Each sample of both dispersions was then diluted with a mixture of solvents (hydrogenated terphenyl/limonene 1/1, wt/wt) with or without a flocculating polymer (polystyrene, Mw ~ 35,000) to a pigment concentration varying from 0.5 to 0.01 wt%. The samples were mixed and poured into a transparent liquid container, and after flocculation for 10 minutes, the transmittance of light through the container and the haze value were measured using a spectrophotometer (CM-3600A, Konica Minolta). Figure 5 shows the relative transmittance data of the haze value for the control sample (square symbol) and the composite pigment particles (circle symbol) in the presence (solid symbol) and absence (hollow symbol) of flocculating polymers. As illustrated in Figure 5, the composite pigment particles behave more closely to a dispersion without flocculating polymers in the presence of flocculating polymers than the control sample.

Example 4

Aqueous Method 1: First, an NDP is formed using pre-dispersed pigments: Cab-o-Jet 465M magenta or 450C cyan (received as a 15 wt% dispersion in water) or dried Emperor 2000 carbon black (received as a dry powder). Carbon black is dispersed in water at 20 wt% pigment and 10 wt% dispersant Kolliphor P188 by blending the pigment into a dispersant solution, followed by ultrasonication of the mixture. The particle size in this feedstock is approximately 80 nm, as determined by dynamic light scattering. These pigment dispersions are then blended separately with a 50 wt% aqueous solution of PEI (branched, 1200 MW, from Sigma) and additional water to obtain a final NDP comprising 5 wt% pigment, 5 wt% PEI, 2.5 wt% dispersant (if present), and the remainder being water.

Next, 10 g of NDP was emulsified into 100 g of propyl benzoate in a 500 mL glass reactor. Initially, a propeller was used for the coarse emulsification, followed by final titration using a rotor-stator homogenizer (IKA 1 cm rotor-stator running at 7,000-10,000 RPM for 3 minutes). After rotor-stator mixing, stirring was resumed in the remaining steps using a propeller at 300 RPM.

Finally, within 5-10 minutes, inject 25g of SMA at 1% by weight dissolved in propyl benzoate at a concentration of 1% by weight into the emulsion through a pipette below the emulsion surface, and continue stirring for at least 60 minutes.

In the final step, the composite particles are hydrophobized by adding 10g of 5% by weight lauroyl chloride in propyl benzoate to react with the remaining amine groups in PEI, and the mixture is stirred continuously for more than 60 minutes. The mixture is then transferred to a polypropylene bottle and rolled overnight on a roller mill.

Aqueous Method 2: Repeat Aqueous Method 1, except that 3.3 g of 10 wt% oleylamine in propyl benzoate was added to 25 g of 1 wt% SMA in propyl benzoate while stirring to modify SMA. This solution (28.3 g) was used as a substitute for the 25 g unmodified SMA solution in Aqueous Method 1.

Figure 6A (pigment composite prepared by oleylamine-functionalized SMA) and Figure 6B (unfunctionalized SMA) provide micrographs of the resulting composite particles. Comparing the micrographs, the microencapsulation success rate of particles prepared with functionalized SMA is slightly higher than that of particles prepared using unfunctionalized SMA.

Many variations and modifications can be made to the specified embodiments of the present invention without departing from the scope of the invention, as will be apparent to those skilled in the art. Therefore, the entire foregoing description should be interpreted in an illustrative rather than restrictive sense.

All of the aforementioned patents and applications are incorporated herein by reference in their entirety.

Claims (18)

1. An electro-optic medium comprising a binder and a plurality of microcapsules, each microcapsule containing a dispersion comprising a plurality of charged composite particles and a suspended fluid, wherein the charged composite particles move through the suspended fluid under the influence of an electric field. The charged composite particles comprise one or more pigment particles at least partially coated with a polymeric material, the pigment particles having a diameter of 0.01 to 0.2 micrometers, and the pigment particles being selected from manganese iron black spinel, copper chromium black spinel, carbon black, or combinations thereof. Each of the binder, charged composite particles, and suspending fluid has a refractive index, and at 550 nm, the difference between the refractive index of the charged composite particles and the refractive index of the suspending fluid, and the difference between the refractive index of the charged composite particles and the refractive index of the binder, are less than or equal to 0.05. 2. The electro-optic medium according to claim 1, wherein at 550 nm, the refractive index of each of the binder, charged composite particles and suspending fluid is greater than or equal to 1.5. 3. The electro-optic medium according to claim 1, wherein the pigment particles comprise carbon black. 4. The electro-optic medium according to claim 1, wherein the polymeric material is derived from monomers selected from the group consisting of methyl methacrylate, hexanediol dimethacrylate, trifluoroethyl methacrylate, trimethoxysilyl propyl methacrylate, tert-butyl methacrylate, isobutyl methacrylate, benzyl methacrylate, 2-fluoroethyl methacrylate, trifluoroethyl acrylate, heptafluorobutyl acrylate, heptafluoroisopropyl acrylate, 2-methoxyethyl acrylate, or combinations thereof. 5. The electro-optic medium according to claim 1, wherein the polymeric material comprises polymethyl methacrylate. 6. The electro-optic medium according to claim 1, wherein the polymeric material comprises a reaction product of polyethyleneimine and a copolymer comprising styrene and maleic anhydride. 7. The electro-optic medium according to claim 1, wherein the thickness of the polymeric material is 0.3 to 0.7 micrometers. 8. The electro-optic medium according to claim 1, wherein the suspended fluid comprises a solvent selected from aliphatic hydrocarbons, terpenes, aromatic hydrocarbons, or combinations thereof. 9. The electro-optic medium according to claim 1, wherein the binder is selected from water-soluble polymers, aqueous polymers, oil-soluble polymers, thermosetting polymers and thermoplastic polymers, radiation-cured polymers or combinations thereof. 10. The electro-optic medium according to claim 1, wherein the binder comprises fish gelatin and polyanionic adhesive. 11. The electro-optic medium according to claim 1, wherein the weight ratio of microcapsules to binder is 15:1 to 50:1. 12. The electro-optic medium according to claim 1, wherein the refractive index of the polymeric material is greater than or equal to 1.5 at 550 nm. 13. A variable transmission film comprising a layer of electro-optic medium according to claim 1 and a layer of light-transmitting conductive material adjacent to the layer of electro-optic medium. 14. The variable transmission film according to claim 13, wherein the electro-optic dielectric layer is disposed between the first and second transparent conductive material layers. 15. An electro-optic medium comprising a polymer sheet having a plurality of sealed micropores, each micropore containing a dispersion, the dispersion comprising a plurality of charged composite particles and a suspended fluid, wherein the charged composite particles move through the suspended fluid under the influence of an electric field. The charged composite particles comprise one or more pigment particles at least partially coated with a polymeric material, the pigment particles having a diameter of 0.01 to 0.2 micrometers, and the pigment particles being selected from manganese iron black spinel, copper chromium black spinel, carbon black, or combinations thereof. Each of the polymer sheet, the charged composite particle, and the suspended fluid has a refractive index, and at 550 nm, the difference between the refractive index of the charged composite particle and the refractive index of the suspended fluid, and the difference between the refractive index of the charged composite particle and the refractive index of the polymer sheet are less than or equal to 0.05. 16. The electro-optic medium according to claim 15, wherein the refractive index of the polymeric material is greater than or equal to 1.5 at 550 nm. 17. An electro-optic medium comprising a plurality of droplets in a continuous polymeric phase, each droplet containing a dispersion comprising a plurality of charged composite particles and a suspended fluid, wherein the charged composite particles move through the suspended fluid under the influence of an electric field. The charged composite particles comprise one or more pigment particles at least partially coated with a polymeric material, the pigment particles having a diameter of 0.01 to 0.2 micrometers, and the pigment particles being selected from manganese iron black spinel, copper chromium black spinel, carbon black, or combinations thereof. Each of the continuous polymeric phase, the charged composite particles, and the suspended fluid has a refractive index, and at 550 nm, the difference between the refractive index of the charged composite particles and the refractive index of the suspended fluid, and the difference between the refractive index of the charged composite particles and the refractive index of the continuous polymeric phase, are less than or equal to 0.05. 18. The electro-optic medium according to claim 17, wherein the refractive index of the polymeric material is greater than or equal to 1.5 at 550 nm.