INORGANIC PIGMENTS FOR USE IN LIQUID CRYSTAL DEVICES

Abstract

A method of preparing non-conductive coated pigment particles for use in liquid crystal applications. A dispersion is prepared of a pigment such as carbon black in a solution comprising a first solvent and a surfactant. The dispersion is disrupted in order to separate agglomerates. A non-conductive coating material is added. In some embodiments of the invention, the non-conductive coating comprises a polymer soluble in the first solvent, and the coating is prepared by addition of a second solvent in which the polymer is insoluble. In other embodiments, the non-conductive coating comprises a metal oxide, and the coating is prepared by addition of a metal alkoxide that hydrolyzes to form the coating. The non-conductive pigment particles are then separated from the supernatant liquid, dried, and reduced to a powder. Liquid crystal devices comprising the particles typically have a haze of less than 7% and a total transmittance of >55%.

Claims

1-54. (canceled)

55. A method for producing a coated pigment suitable for use in a liquid crystal device, wherein said method comprises: preparing a first solution comprising a surfactant in a first solvent; adding a coating precursor to said first solution; adding a pigment comprising particles to said first solution; mixing said pigment and said solution until a suspension or dispersion of said pigment in said first solution is obtained; disrupting said suspension or dispersion, thereby separating agglomerates of said pigment into particles and producing a dispersion of said particles of pigment; adding a second solvent miscible with said first solvent to said dispersion, thereby causing the deposition of a coating of material derived from said coating precursor onto a surface of at least a portion of said particles and producing coated particles; separating at least a portion of said coated particles from said dispersion; and reducing said coated particles separated from said dispersion to a powder.

56. The method according to claim 55, wherein said step of separating said coated particles from said suspension is followed by: separating coated particles above a predetermined size from said coated particles; discarding said coated particles above a predetermined size; and retaining coated particles at or below said predetermined size.

57. The method according to claim 55, wherein said step of separating at least a portion of said coated particles from said dispersion comprises separating at least a portion of said coated particles from said dispersion to yield coated particles characterized by a diameter of ≤100 nm as measured by dynamic light scattering.

58. The method according to claim 55, wherein said step of separating at least a portion of said coated particles is followed by a step of washing said coated particles separated from said suspension.

59. The method according to claim 58, wherein said step of washing comprises washing with said second solvent.

60. The method according to claim 55, wherein said step of separating at least a portion of said coated particles from said suspension is followed by drying said coated particles separated from said suspension.

61. The method according to claim 55, wherein said step of adding particles of a pigment comprises adding particles of a conductive pigment.

62. The method according to claim 55, wherein said step of adding particles of a pigment comprises adding particles of at least one pigment selected from the group consisting of carbon black, silver, boron carbide, titanium nitride, zirconium carbide, zirconium boride, tungsten carbide, and tungsten disulfide.

63. The method according to claim 55, wherein said step of adding a coating precursor comprises adding a polymer that is less soluble in said second solvent than it is in said first solvent.

64. The method according to claim 63, wherein one of the following is true: said first solvent is water; said first solvent is water and said second solvent is acetone.

65. The method according to claim 63, wherein said polymer is selected from the group consisting of: polymers comprising a hydrophobic chain and hydrophilic side groups; polymers that are soluble in, and selected to match a refractive index of, a predetermined liquid crystal material; and polymers characterized by a molecular weight of between 10 and 1300 kD; and, any combination thereof.

66. The method according to claim 63, wherein said polymer is polyvinylpyrrolidine.

67. The method according to claim 63, wherein: said steps of preparing a first solution comprising a surfactant in a first solvent and adding a coating precursor to said first solution are performed by preparing a first solution comprising a surfactant and a polymer; and said step of adding a pigment comprises adding a pigment to said first solution comprising said surfactant and said polymer.

68. The method according to claim 55, wherein: said step of adding a coating precursor comprises adding a sol-gel reagent that upon hydrolysis will yield a non-conductive oxide; and said method comprises adding a reagent that will initiate hydrolysis of said sol-gel reagent.

69. The method according to claim 68, wherein a condition from the group consisting of: said step of adding pigment precedes said step of adding a coating precursor; and said steps of adding pigment and disrupting said suspension or dispersion precede said step of adding a coating precursor; is true.

70. The method according to claim 68, wherein said steps of preparing a second solution and adding said second solution to said first solution precede said step of adding pigment.

71. A method for making a polymer dispersed liquid crystal (PDLC) composition comprising polymer-coated pigment particles, comprising: preparing a homogeneous mixture comprising prepolymer, a liquid crystal, and coated pigment particles prepared according to the method according to claim 55; and polymerizing said prepolymer to yield said PDLC composition.

72. The method according to claim 71, wherein said coated pigment particles are selected from the group consisting of: particles comprising carbon black coated with PVP; and particles comprising carbon black coated with a non-conductive metal oxide.

73. The method according to claim 71, wherein said step of preparing a homogeneous mixture comprises preparing a homogeneous mixture comprising between 0.3% and 1% by weight of polymer-coated pigment particles.

74. The method according to claim 71, wherein said polymer-coated pigment particles are characterized by a conductance sufficiently small so as to yield a PDLC composition characterized by a conductance of no greater than 10.sup.−12 (ohm-cm).sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0078] The invention will now be described with reference to the drawings, wherein:

[0079] FIG. 1 presents a flow chart showing schematically one embodiment of the method of preparation of inorganic dye for use in a liquid crystal device according to the present invention;

[0080] FIG. 2 presents a flow chart showing schematically a second embodiment of the method of preparation of inorganic dye for use in a liquid crystal device according to the present invention; and,

[0081] FIG. 3 discloses a schematic illustration of the process used to perform the method of preparation of inorganic dye for use in a liquid crystal device according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0082] The following description is provided so as to enable any person skilled in the art to make use of the invention and sets forth the best modes contemplated by the inventor of carrying out this invention. Various modifications, however, are adapted to remain apparent to those skilled in the art, since the generic principles of the present invention have been defined specifically to provide a novel method of manufacture of inorganic pigments suitable for use in liquid crystal devices, particularly devices that comprise liquid crystal films, and methods of manufacture of liquid crystal devices that incorporate the inorganic pigments made by the inventive method. In some cases, for clarity or conciseness, individual elements, individual method steps, specific combinations of elements, and specific combinations of method steps are described. Nonetheless, any combination of disclosed elements or of method steps that is not self-contradictory is considered by the inventors to be within the scope of the invention.

[0083] As used herein, the abbreviation “LC” stands for “liquid crystal” and the abbreviation “PDLC” for “polymer dispersed liquid crystal.”

[0084] As used herein, the term “coating precursor” refers to any substance that can provide a coating in situ for particles suspended or dispersed in a liquid, whether by direct deposition from solution or dispersion, or by chemical reaction to form the substance that coats the particles.

[0085] With respect to a transparent, translucent or semi-translucent object, the term “haze (H)” as used herein refers hereinafter to the quantity defined by eq (1)

[00001]$\begin{array}{cc}H=100.Math.\left(\frac{I_{s>2.5}}{I_0}\right)& \left(1\right)\end{array}$

where I.sub.s>2.5 is the intensity of light scattered through an angle greater than 2.5° with respect to the direction of the incident light, and I.sub.0 is the total intensity of the incident light.

[0086] As used herein, the term “total transmittance (T.sub.1)” refers to the quantity defined by eq (2)

[00002]$\begin{array}{cc}{T}_t=100.Math.\left(\frac{I_t}{I_0}\right)& \left(2\right)\end{array}$

where I.sub.t is the total intensity of transmitted light (i.e. integrated over all scattering angles).

[0087] As used herein, the term “direct transmittance (T.sub.d)” refers to the portion of the transmitted light that passes through an object with a scattering angle of less than 2.5° with respect to the direction of the incident light.

[0088] As used herein, the term “retrofit(ting)” refers to the modification of a conventional window or surface by combining the same in some manner with an enhancement, i.e., a switchable glazing, non-switchable light modulating device, etc.

[0089] As used herein, the term “dye” refers to a soluble substance that provides color to a substrate, and the term “pigment” to an insoluble substance that provides color to a substrate.

[0090] As used herein, the term “carbon black” refers to a black pigment comprising a particulate form of paracrystalline carbon. The term is used to refer to all such pigments, which are generally produced by incomplete combustion of petroleum products. Acetylene black, channel black, furnace black, lamp black, and thermal black are non-limiting examples of pigments that are described by the term “carbon black” as used herein.

[0091] As used herein, the term “nanoparticle” refers to a particle with a characteristic dimension (e.g. mean diameter) of 0.1-100 nm, and “nanoparticulate” to a substance made up of nanoparticles.

[0092] As used herein the term “microparticle” refers to a particle with a characteristic dimension of greater than 0.1 μm and less than 100 μm, and “microparticulate” to a substance made up of microparticles.

[0093] As used herein, the terms “suspension” and “dispersion” are both used to refer to a system in which particles of a solid are dispersed in a liquid in which the solid is not soluble. While “suspension” generally refers to a system in which the solid particles are sufficiently large for sedimentation to take place (e.g., a system in which the insoluble solid comprises microparticles) “dispersion” to a system in which the solid particles are not sufficiently large to take place (e.g. a system such as a colloidal dispersion in which the insoluble solid comprises nanoparticles), unless specifically stated otherwise, as used herein, the terms are used interchangeably, and unless the particle size is specified, a the use of either of the terms with reference to a process or process step involving creation or treatment of a suspension or dispersion does not place any limitation on the sizes of individual particles in the system.

[0094] As used herein, with respect to a suspension or dispersion of insoluble particles in a liquid, the term “disrupting” is used to refer to any process that will reduce the size and/or number of agglomerates of particles in the suspension or dispersion.

[0095] As used herein, with reference to numerical quantities, the term “about” refers to a range of values ±25% of the nominal value.

[0096] In order for a pigment to be suitable for use in a liquid crystal device, particularly devices such as smart windows or displays based on PDLC films, it must have several important characteristics. First, the particles must be sufficiently small such that the final composition will have a low haze when the device is in its transparent state, preferably not more than 7%. Second, if the particles are to be incorporated into the liquid crystal layer, they must have a low conductivity in order that the device not short-circuit when a voltage is applied across the liquid crystal. On the other hand, the particles must have an albedo sufficiently low that the total transmittance of the device in its translucent state is low, preferably not more than 55%, more preferably not more than 30%, yet more preferably not more than 20%, even yet more preferably not more than 10%, and most preferably not more than 5%. Carbon black comprises small particles and has a low albedo, but commercially available carbon black is in general not appropriate for use in liquid crystal devices. First, although commercially available carbon black is generally nanoparticulate, off-the-shelf carbon black tends to have significant numbers of agglomerates of particles; these agglomerates are much too large for use as pigments in liquid crystal devices. Moreover, carbon black is conductive, and thus cannot be introduced as-is into liquid crystal devices.

[0097] Thus, any method of preparation of a pigment based on carbon black for use in liquid crystal devices must necessarily include a way of breaking up the agglomerates into individual nanoparticles and a way of reducing or eliminating its conductivity. As described below, in preferred embodiments of the invention disclosed herein, the agglomerates are broken up by physical disruption of a dispersion or suspension of the particles in a solvent, and the reduction of conductivity is accomplished by coating the pigment particles with a non-conductive coating such as a non-conductive polymer or metal oxide.

[0098] In order for a device incorporating pigment particles to have an acceptably low haze, not only must the pigment particles be small (preferably nanoparticles), but the optical properties of the polymer with which they are coated must be chosen to be appropriate for use with the particular liquid crystal being used in the device. In particular, the polymer should preferably be chosen such that the polymer-coated pigment particles will match as well as possible the refractive index of the liquid crystal when the liquid crystal composition is prepared.

[0099] The inventors have found a method for preparing inorganic pigments based on materials such as carbon black that yields pigment particles that meet all of the criteria for suitability given above. The particles produced by the inventive method are small, have low conductivity, and are refractive index matched to be usable in liquid crystal devices such as smart windows and display screens that are based on PDLC films. The haze of the devices that incorporate particles manufactured by the inventive method is typically less than 7%.

[0100] While these characteristics are suitable for some applications, using conductive pigments in liquid crystal devices is challenging, also because of their electric conductivity.

[0101] In order to use a conductive pigment in a LC device, there are two main options: either separate the pigment from the LC dispersion; or neutralize the electric conductive properties of the conductive pigments, therefore allowing the insertion of the dye inside the LC dispersion.

[0102] The present invention provides a novel method of manufacturing a conductive pigment dye that is suitable to be included in the LC dispersion.

[0103] PDLC films, which are composed of LC microdroplets dispersed in a polymer matrix, have been the subject of much academic and industrial research. These electro-optical systems can be switched by applying an electric field from a scattering field-off state to a transparent field-on state. This property can be used to construct devices with electrically modulated light and visual transmission for applications in large-area architectural glazing. A good product for these applications should have high opacity in the field-off state and high transparency over a wide viewing angle (low haze) in the field-on state. The phenomenon of haze in the field-on state of a PDLC arises from the residual refractive index difference between the polymer matrix and the aligned LC in the droplets. It is necessary to distinguish between “normal” haze measured in a direction perpendicular to the film plane and “off-axis” haze measured at other viewing angles. These values depend on various PDLC material and processing parameters.

[0104] The insertion of any material in the LC dispersion causes a natural increase in the haze due to an increase refractive index difference caused by the new material in the polymer matrix.

[0105] Micro-sized particles cause an increase in haze due to the scattering of the wavelength in the visible range.

[0106] It is a scope of the present invention to provide a novel method for producing an inorganic pigment that is suitable to be included in the LC dispersion, particularly in that inclusion of the pigment in the dispersion leads to a device with a haze level that is not perceivably different to the naked eye from a LC film without the pigment.

[0107] In one embodiment of the present invention, the methodology used is a process of isolation and precipitation of the conductive pigments or in other words, on solvent/antisolvent precipitation techniques.

[0108] Reference is now made to FIG. 1, which presents a schematic flowchart of one non-limiting embodiment of the method 100 disclosed herein for producing coated pigment particles, in which particles of a pigment are coated with a polymer. The general description of the method 100 given here is followed by a more detailed description of specific method steps. In one non-limiting exemplary embodiment, the method begins by preparing a solution of a surfactant and a polymer in a first solvent and then adding particles of a pigment, typically an electrically conductive pigment such as carbon black, to the solution (step 10 in the flowchart). In these embodiments, the polymer serves as the coating precursor. Any solvent in which the polymer and surfactant are soluble may be used; in typical applications, the first solvent is water. In some embodiments of the invention, rather than adding pigment particles to a surfactant/polymer solution, the pigment particles, surfactant, and polymer are added to the solvent together in a single step. The solution and pigment particles (or, in embodiments in which the surfactant and polymer are added with the pigment particles, all of the components) are then mixed (step 11) until a homogeneous suspension or dispersion of the pigment is obtained. In general, suspension or dispersion will include agglomerates that are too large to be suitable for use in a LC application because their incorporation “as is” into the device would lead to a haze level too high to be useful. Therefore, a step of disruption (12) is performed in order to reduce the size of the agglomerates in the suspension, preferably breaking them up into isolated single particles in dispersion; methods of performing this step are described in detail below.

[0109] The step of disruption is followed by addition of a second solvent that can act as an antisolvent for the polymer in the solution (step 13). The second solvent must be one that is miscible with the first solvent, but in which the polymer is significantly less soluble than it is in the first solvent. In preferred embodiments, the second solvent is chosen to be one in which the polymer is insoluble; in embodiments of the invention in which the first solvent is water, the second solvent is typically acetone. Upon addition of the second solvent, the polymer precipitates from the solution onto the surface of the pigment particles, thereby coating them, significantly lowering their electrical conductivity, and preferably rendering them non-conductive.

[0110] The step of adding a second solvent is followed by a step (14) of separating the coated particles from the dispersion. The separation is typically performed by centrifugation. In preferred embodiments of the invention, any particles or agglomerates remaining that are too large for use in the LC device are removed following the separation. In some embodiments of the invention, the coated particles are washed (typically with the second solvent) in order to remove first solvent from the wet particles.

[0111] In typical embodiments of the method, the coated particles that have been separated from the suspension or dispersion are then dried (step 15) in order to evaporate remaining solvent, leaving only the isolated non-conductive pigment. In typical embodiments, the particles tend to adhere to each other during the drying process, leading to formation of chunks and blocks of material. In order to put it in a form that is usable for introduction into LC devices, the dried material is then reduced to a powder (step 16), typically by pulverization, yielding freely flowing particles (typically comprising nanoparticles) that are suitable in all respects for, and easily usable in, LC applications.

[0112] In typical embodiments of the invention, in the first step of the process, surfactant and polymer are added to a suitable first solvent. In some preferred embodiments of the invention, the first solvent is water. Since, as explained above, the second solvent added at a later stage should be miscible with the first solvent but one in which the polymer is insoluble or sparingly soluble, the choice of first solvent will limit the choice of possible second solvent.

[0113] Non-limiting examples of solvents that can be used in the method herein disclosed include hexane, benzene, toluene, diethyl ether, chloroform, 1,4-dioxane, ethyl acetate, tetrahydrofuran, dichloromethane, acetone, acetonitrile, dimethylformamide, dimethyl sulfoxide, acetic acid, n-butanol, isopropanol, n-propanol, ethanol, methanol, formic acid, and water, with the limitation that the two solvents must be miscible, and the polymer coating must be significantly less soluble in the second solvent than it is in the first solvent.

[0114] Non-limiting examples of surfactants that can be used in the method herein disclosed include SDS, CTAB, Triton X-100, X-114, CHAPS, DOC, NP-40, octyl thioglucoside, octyl glucoside, dodecyl maltoside, nonoxynol-9, polysorbate, span, poloxamers, Tergitol, Antarox, PENTEX 99, PFOS, Calsoft, Texapon, Darvan, and sodium stearate.

[0115] The polymer with which the particles are to be coated is preferably chosen such that it will be soluble in the LC medium to be used in the LC device, and that the polymer-coated particles will match the index of refraction of the LC medium. In particularly preferred embodiments, especially those in which the LC device includes a PDLC layer, the polymer comprises a hydrophobic chain with hydrophilic side groups.

[0116] Any polymer known in the art that meets the physical, optical, and chemical criteria for use as a coating for particles to be incorporated in a LC device may be used. Non-limiting examples of suitable polymers include poly(ethylene glycol) (PEG); polyvinylpyrrolidone (PVP); hyaluronic acid; polyvinyl alcohol (PVA); polyacrylic acid (PAA); polyacrylamide; N-(2-Hydroxypropyl) methacrylamide (HPMA); divinyl ether-maleic anhydride (DIVEMA); polyoxazoline; polyphosphoesters (PPE); polyphosphazenes; xanthan gum; pectin; chitosan; chitosan derivatives; dextran; carrageenan; guar gum; and cellulose ethers such as hydroxypropylmethyl cellulose (HPMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), and sodium carboxy methyl cellulose (Na-CMC). In preferred embodiments of the invention, PVP is used.

[0117] The inventors have found that the molecular weight of the polymer can in some affect the usefulness of the final product. For example, while short-chain PVP dissolves in PDLCs, long-chain PVP does not, which means that for PDLC applications, the chain length of the PVP is a parameter of interest. Thus, in some embodiments of the present invention the polymer is characterized by a molecular weight of from about 10 kD to about 1300 kD.

[0118] In the second step of the process herein disclosed, particles of pigment are added to the solvent. In some embodiments of the invention, the particles of pigment are added to the solvent along with the surfactant and polymer. In preferred embodiments of the invention, the pigment particles are added only after a homogeneous solution of surfactant and polymer in the solvent has been prepared. Non-limiting examples of pigments that can be used in the process herein disclosed include carbon black, silver, boron carbide, titanium nitride, zirconium carbide, zirconium boride, tungsten carbide, and tungsten disulfide.

[0119] A variety of dyes can be used in order to confer a specific color to a LC film. While the scope of this invention is the manufacture of suitable black pigments, other colors can be prepared using the methods disclosed herein.

[0120] Dyes are classified according to their solubility and chemical properties, whether they are organic or inorganic, natural or synthetic.

[0121] In several embodiments of the present invention the organic dyes are selected from the group consisting of: Alizarin, Anthoxanthin, Arylide yellow, Azo compound, Bilin (biochemistry), Bistre, Bone char, Caput mortuum, Carmine, Crimson, Diarylide pigment, Dibromoanthanthrone, Dragon's blood, Gamboge, Indian yellow, Indigo dye, Naphthol AS, Naphthol Red, Ommochrome, Perinone, Phthalocyanine Blue BN, Phthalocyanine Green G, Pigment violet 23, Pigment Yellow 10, Pigment Yellow 16, Pigment Yellow 81, Pigment yellow 83, Pigment yellow 139, Pigment yellow 185, Quinacridone, Rose madder, Rylene dye, Sepia (color), Tyrian purple, and any combination thereof.

[0122] In several embodiments of the present invention the inorganic dye or pigment is selected from the group consisting of: Ultramarine violet: (PV15) Silicate of sodium and aluminum containing sulfur, Han Purple: BaCuSi.sub.2O.sub.6, Cobalt Violet: (PV14) cobaltous orthophosphate, Manganese violet: NH.sub.4MnP.sub.2O.sub.7 (PV16) Manganic ammonium pyrophosphate, Ultramarine (Na.sub.8-10 Al.sub.6Si.sub.6O.sub.24S.sub.2-4), Persian blue, (Na,Ca).sub.8(AlSiO.sub.4).sub.6(S,SO.sub.4,Cl).sub.1-2, Cobalt Blue (PB28), Cerulean Blue (PB35), Egyptian Blue (CaCuSi.sub.4O.sub.10), Han Blue: BaCuSi.sub.4O.sub.10, cupric carbonate hydroxide (Cu.sub.3(CO.sub.3).sub.2(OH).sub.2), Prussian Blue (PB27) (Fe7(CN)18), YInMn Blue (YIn.sub.1-xMn.sub.xO.sub.3), Cadmium Green, Chrome green (PG17 (Cr.sub.2O.sub.3), Viridian (PG18) (Cr.sub.2O.sub.3.H.sub.2O), Cobalt green (CoZnO.sub.2), Malachite (Cu.sub.2CO.sub.3(OH).sub.2), Paris Green (Cu(C.sub.2H.sub.3O.sub.2).sub.2.3Cu(AsO.sub.2).sub.2), Scheele's Green (CuHAsO.sub.3), Verdigris (Cu(CH.sub.3CO.sub.2).sub.2), malachite (Cu.sub.2CO.sub.3(OH).sub.2), Green earth (K[Al,Fe.sup.III),(Fe.sup.II,Mg](AlSi.sub.3,Si.sub.4)O.sub.10(OH).sub.2), Orpiment (As.sub.2S.sub.3), Cadmium Yellow (PY37), Chrome Yellow (PY34) (PbCrO4), Aureolin (PY40): Potassium cobaltinitrite (K.sub.3Co(NO.sub.2).sub.6), Yellow Ochre (PY43) (Fe.sub.2O.sub.3.H.sub.2O), Naples Yellow (PY41), Lead-tin-yellow (PbSnO.sub.4 or Pb(Sn,Si)O.sub.3), Titanium Yellow (PY53), Mosaic gold (SnS.sub.2), Zinc Yellow (PY36) (ZnCrO.sub.4), Cadmium Orange (PO20), Chrome Orange(PbCrO.sub.4+PbO), Realgar (As.sub.4S.sub.4), Cadmium Red (PR108) (CdSe), Sanguine, Caput Mortuum, Indian Red, Venetian Red, Oxide Red (PR102), Red Ochre (PR102) (Fe.sub.2O.sub.3), Burnt Sienna (PBr7), Minium, Pb.sub.3O.sub.4, Vermilion (PR106), Mercuric sulfide (HgS), Clay earth pigments (naturally formed iron oxides), Raw Umber (PBr7) (Fe.sub.2O.sub.3+MnO.sub.2+nH.sub.2O+Si+AlO.sub.3), Raw Sienna (PBr7), Carbon Black (PBk7), Ivory Black (PBk9), Vine Black (PBk8)k, Lamp Black (PBk6), Mars Black (Iron black) (PBk11), Manganese dioxide (MnO.sub.2), Titanium Black (Ti.sub.2O.sub.3), Antimony White (Sb.sub.2O.sub.3), Barium sulfate (PW5) (BaSO.sub.4), Lithopone (BaSO.sub.4*ZnS), Cremnitz White (PW1) ((PbCO.sub.3).sub.2.Pb(OH).sub.2), Titanium White (PW6) (TiO.sub.2), Zinc White (PW4) (ZnO), and any combination thereof.

[0123] After the components are added to the solvent, they are mixed until a homogeneous suspension or dispersion of the pigment particles in the surfactant/polymer solution is obtained.

[0124] Any suitable method of mixing known in the art may be used. Non-limiting examples of types of mixers that can be used include Ribbon Blender, V Blender, Continuous Processor, Cone Screw Blender, Screw Blender, Double Cone Blender, Double Planetary, High Viscosity Mixer, Counter-rotating, Double & Triple Shaft, Vacuum Mixer, High Shear Rotor Stator, Impinging mixer, Dispersion Mixers, Paddle, Jet Mixer, Mobile Mixers, Drum Blenders, Intermix mixer, Horizontal Mixer, Hot/Cold mixing combination, Vertical mixer, Turbomixer, Planetary mixer, and Banbury mixer.

[0125] As was explained above, in general, commercially available pigments such as carbon black comprise particles that are either too large to be useful in LC applications, or are present in the form of agglomerates that cannot be used until and unless the particles are separated. Thus, a step of reducing the size of agglomerates and separating the particles that form them is necessary to obtain suitable pigment particles. In preferred embodiments of the invention, this step is performed after the suspension or dispersion of pigment particles in the surfactant/polymer solution has been prepared. The particles are reduced by “disrupting” the solution via an energy input sufficiently strong to break up the agglomerates into particles that are small enough to be suitable; in preferred embodiments of the invention, the agglomerate size is reduced at this stage to yield a nanoparticulate material with particles characterized by sizes of ≤100 nm as measured by dynamic light scattering. Typical particle sizes to which the particles are reduced in the cases of the pigments listed above are silver, 80 nm; boron carbide, 60 nm; titanium nitride, 40 nm; zirconium carbide, 30 nm; zirconium boride, 50 nm; tungsten carbide, 80 nm; and tungsten disulfide, 60 nm.

[0126] Any appropriate method of disrupting the solution that will reduce the particle size sufficiently may be used. In preferred embodiments of the invention, the step of reducing the particle size is performed by sonication or ultrasonication of the suspension, typically at frequencies of >20 kHz.

[0127] In some embodiments of the invention, the particle size is reduced by use of the well-known technique of bead beating. In this method, small inert beads made of a sufficiently hard substance (e.g. glass, ceramic, or steel) are mixed with the suspension and then agitated, e.g. by stirring or shaking. The collisions between the beads and the suspended particles cause the particles to break up into particles of smaller size. Bead beating has several advantages. For example, it can be used to disrupt very small sample sizes, process many samples at a time with no cross-contamination concerns, does not release potentially harmful aerosols in the process, provides moderate mechanical shear during the process. Any appropriate bead beating apparatus known in the art may be used. In typical embodiments of the inventive method in which bead beating is used, a volume of beads equal to that of the suspension is added to and the sample is vigorously mixed on a common laboratory vortex mixer. Specialized shaking machines can be used to lower the process time. Such shaking machines can agitate the sample at about 2000 oscillations per minute, and material disruption is complete in 1-3 minutes of shaking.

[0128] After the disruption of the suspension or dispersion to produce separated particles, in some embodiments of the invention, the particles are then coated with polymer. The coating is preferably accomplished by addition of a second solvent. The technique is that of the use of antisolvent which is well-known as a method of crystallization. In the method herein disclosed, a second solvent is added that is miscible with the first solvent but in which the polymer is much less soluble; in preferred embodiments, the polymer is sparingly soluble or insoluble in the second solvent. As a non-limiting example, in embodiments in which the first solvent is water, the second solvent is typically acetone. The addition of the second solvent causes the polymer to precipitate from solution (not necessarily as crystals) and to coat the surface of the particles suspended therein. As explained above, the polymer coating provides the optical and electrical properties that make the pigment particles suitable for use in LC applications.

[0129] Following the step of adding the second solvent and thereby coating the particles, the coated particles are then separated from the liquid. Because of their small size, the natural settling time is far too long for separation by settling to be efficient, and so in preferred embodiments of the invention, the separation is performed by centrifugation. Any type of centrifuge known in the art that is appropriate for separating the coated particles from the liquid may be used. The supernatant liquid can then be poured off, leaving the separated particles behind.

[0130] In some embodiments of the invention, the separated particles are washed in order to remove excess solvent and polymer. As a non-limiting example, they can be washed with the second solvent to remove the first solvent. In cases in which the second solvent is significantly more volatile than the first solvent such as the example of water and acetone given above, the washing both removes the first solvent and makes subsequent drying easier.

[0131] Following the separation and optional washing, in preferred embodiments of the invention, the particles are then dried. Any technique that is known in the art that is appropriate for drying the particles may be used. Non-limiting examples include vacuum drying and hot air drying. In typical embodiments of the invention, the drying is performed in the vessel in which the separation took place (e.g. the centrifuge tube in embodiments in which the separation was effected by centrifugation).

[0132] The particles that are produced by the steps listed thus far typically dry into blocks or lumps in which the particles adhere to one another. Since the particles need to be dispersed when they are used in the LC composition, the step of drying the particles is typically followed by a step of reducing the dried particle composition to a powder. Any appropriate technique for reducing the particles to a nanoparticulate powder may be used. Non-limiting examples include pulverization (e.g. using a mortar and pestle or a mechanical pulverizer such as a pulverizer mill) and grinding.

[0133] The powder comprising polymer-coated pigment particles can then be stored for use in a LC device.

[0134] In some non-limiting embodiments of the invention, rather than a polymer, the non-conductive coating comprises a metal oxide. In these embodiments, the coated particles are prepared by a sol-gel process. Reference is now made to FIG. 2, which shows a schematic flow chart of a non-limiting embodiment 300 of the method of the present invention, in which the coating of the pigment particles is achieved via a process similar to sol-gel processes known in the art. The method 300 begins by adding the pigment particles and a surfactant to a first suitable solvent (step 30) to form a suspension or dispersion, which is then disrupted (step 31) to reduce the particle size and separate agglomerated particles. Next, a second suitable solvent and a base are added to the solution (step 32). As a non-limiting example, if the first solvent is water, then the second solvent can be ethanol and the base ammonium hydroxide. In the next step 33, suitable sol-gel reagent such as an alkoxide is added to the dispersion or suspension as the coating precursor, and the dispersion or suspension mixed. Non-limiting examples of suitable sol-gel reagents include tetraethoxysilane, titanium tetraisopropoxide, titanium tetrabutoxide, titanium ethoxide, and zirconium propoxide. The basic conditions cause hydrolysis of the sol-gel reagent to yield a metal oxide, which is deposited on the pigment particles, thereby producing a non-conductive oxide coating. Centrifugation is then performed (step 34) in order to separate the coated pigment particles from the supernatant liquid. The separated particles are then dried (step 35) and the material reduced to a powder (step 36) comprising non-conductive pigment particles that can be stored for use in LC applications.

[0135] The following non-limiting examples are provided in order to assist a person of ordinary skill in the art to make and use the instant invention.

Example 1

[0136] Reference is now made to FIG. 3, which shows a schematic illustration of the general principles of one non-limiting embodiment of the present invention (200).

[0137] In typical embodiments of the invention, from about 0.1 g to about 1 g of polymer and from about 0.1 ml to about 1.0 ml of surfactant are added to 100-500 ml of the first solvent. After complete dissolution of the polymer and surfactant, from about 0.1 g to about 1.0 g of conductive pigment are added. The suspension is stirred for from about 1 hour to about 6 hours at room temperature and then placed into ultrasound bath for from about 10 minutes to about 120 minutes, followed by stirred overnight on a stirring plate at room temperature. In typical embodiments, the particle size is then reduced by using a probe sonicator (e.g. SONICS, 750W, amplitude from about 10% to about 60%, from about 10 kHz to about 75 kHz) for about 10 minutes to about 60 min. From about 100 ml to about 600 ml of the second solvent is then added to a container holding about 50 ml to about 250 ml of the suspension with continuous stirring. The nanoparticulate suspension is then stirred for from about 10 min to about 90 minutes, and the particles thereby obtained purified via washing centrifugation cycles in a centrifuge. The particles are typically dried overnight in a vacuum oven (typically at a pressure of 60 mm Hg and a temperature of 35-60° C., and the dried particles pulverized to yield a powder.

Example 2

[0138] As an example of a second non-limiting embodiment of the invention, non-conductive pigment particles were prepared by a process of condensation of silicon compounds on the surface of the conductive pigments using silica/sol-gel modification techniques.

[0139] A surfactant (tryton X) was dissolved in water. Carbon black powder was added to the solution and the resulting suspension was disrupted by sonication. Next, ethanol and NH.sub.4OH were added. Finally, an alkoxide (in different samples, titanium or silicate) was added and the resulting mixture stirred for 24 hours, thereby producing oxide-coated carbon black particles. The particles were separated from the supernatant liquid by centrifugation and dried in a vacuum oven.

Example 3

[0140] PVP was completely dissolved in water, then particles of conductive pigment was added. The solution was stirred during 24 hours. The solution was then filtered and resuspended in ammonia in ethanol, then thoroughly mixed. Then the reagent was added under stirring. Finally, the solution was centrifugated and dried. The pellet was then crushed to create a powder.

Example 4

[0141] The following example demonstrates usefulness of polymer coated pigment particles prepared according to the method disclosed herein in a LC device.

[0142] Samples of carbon black were obtained from two different commercial suppliers, and for each of the two samples, a powder comprising non-conductive pigment particles coated with PVP was prepared according to the method disclosed herein.

[0143] LC devices incorporated the coated particles were then prepared according to literature methods. A mixture of LC, polymer-coated carbon black particles (0.5% by weight), and a prepolymer was prepared. The prepolymer was then polymerized under photolysis to produce a PDLC composition containing polymer-coated carbon black particles. Properties of the pigment particles and LC devices containing them are summarized in Table 1, where “A” and “B” refer to the respective powders made from the two samples of carbon black; T.sub.on=total transmittance when the device is in its transparent state; H.sub.on=haze when the device is in its transparent state; T.sub.min=direct transmittance when the device is in its translucent state; T.sub.max=direct transmittance when the device is in its transparent state; and V.sub.90=voltage required to obtain 90% of T.sub.on.

TABLE-US-00001 TABLE 1 Sample Size after sonication, nm T.sub.on H.sub.on T.sub.min T.sub.max V.sub.90 A 102 ± 28.3 (98.5%), 49.5 7.18 3.38 48.76 32.08 5077 ± 561 (1.5%) B 102 ± 28.3 (98.5%), 60.6 6.82 3.44 59.04 32.97 5077 ± 561 (1.5%)

[0144] In order to understand the source of haze formation, LC devices were prepared according to the same protocol but either without any pigment whatsoever or by incorporating 0.5% uncoated carbon black particles. The haze of the resulting devices was then measured. In the devices prepared without pigment, the haze was 2.5%. In the devices into which uncoated particles were incorporated, the haze was 3.4% (T.sub.on=46%). That is, of the ˜7% haze in the devices containing polymer-coated pigment particles, about 1% of addition to haze value is due to scattering from the dye particles themselves, and about 3% due to the polymer coating or the morphology of the coated particles. Further reduction of the particle size could lead to additional reduction in haze.