ANISOMETRICALLY SHAPED CARBON AND/OR GRAPHITE PARTICLES, LIQUID SUSPENSIONS AND FILMS THEREOF AND LIGHT VALVES COMPRISING SAME

Abstract: The invention contemplates anisometrically shaped carbon-containing particles for use as the particles of a liquid light valve suspension, films and light valves. The anisometric carbon-containing particles are crystalline or amorphous particles and can have various geometrical shapes including, for example, fibrils. The above object is achieved by carefully controlling the size and shape of such particles.

Description: ANISOMETRICALLY SHAPED CARBON AND/OR GRAPHITE PARTICLES, LIQUID SUSPENSIONS AND FILMS THEREOF AND LIGHT VALVES COMPRISING SAME FIELD OF INVENTION The present invention relates to particles of improved stability comprising anisometπcally shaped carbon and/or graphite particles for use in light valves and in light valve suspensions BACKGROUND Light valves have been known for over sixty years for modulation of light As used herein, a light valve may be described as a cell formed of two walls that are spaced apart by a small distance, at least one wall being transparent, the walls having electrodes thereon usually in the form of transparent conductive coatings The cell contains a light-modulating element, which may be either a liquid suspension of particles or a plastic film in which droplets of a liquid suspension of particles are distπbuted and encapsulated The liquid suspension (sometimes herein referred to as "a liquid light valve suspension") comprises small particles suspended in a liquid suspending medium In the absence of an applied electπcal field, the particles in the liquid suspension exhibit random Browman movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles and the energy content of the light The light valve is thus relatively dark in the OFF state However, when an electπc field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell The light valve is thus relatively transparent in the ON state Use of light valves have been proposed for numerous applications including e g , alphanumeric displays, television displays, windows, sunroofs, sunvisors, mirrors, eyeglasses and the like to control the amount of light passing therethrough Light valves of the type described herein are also known as "suspended particle devices" or SPDs" For many applications, it is preferable for the activatable mateπal, 1 e the light modulating element, to be a plastic film rather than a liquid suspension For example, in a light valve used as a variable light transmission window , a plastic film, in which droplets of liquid suspension are distributed, is preferable to a liquid suspension alone because hydiostatic pressure effects e g , bulging associated with a high column of liquid suspension can be avoided through use of a film, and the πsk of possible leakage can also be avoided Another advantage of using a plastic film is that, in a plastic film, the particles are generally present only within very small droplets and, hence, do not noticeably agglomerate when the film is repeatedly activated with a voltage A "light valve film" as used herein refers to a film having droplets of a liquid suspension of particles distπbuted in the film U S Patent No 5,409,734 exemplifies a type of light valve film that is made by phase separation from a homogeneous solution Light valve films made by cross-linking emulsions are also known See U S Patent Nos 5,463,491 and 5,463,492 both of which are assigned to the assignee of the present invention All of the above patents including any other patents and references cited therein or elsewhere herein are incorporated into this application by reference thereto For use in set suspensions such as hght-polaπzing sheets, sometimes called "sheet polarizers", which can be cut up and formed into polarized sunglass lenses or used as filters, hght-polaπzing particles can be dispersed or distπbuted throughout a sheet of suitable film- forming material, such as cellulose acetate or polyvmyl alcohol or the like Methods of making set suspensions for use in sheet polarizers are well known in the pnor art It is important to note, however, that the light polarizing particles mentioned above are immovable, 1 e , fixed See e g , U S Patent Nos 2,178,996 and 2,041,138 THE LIQUID LIGHT VALVE SUSPENSION 1 Liquid Suspending Media and Stabilizers A liquid light valve suspension may be any liquid light valve suspension known in the art and may be formulated according to techniques known to one skilled m the art The term "liquid light valve suspension" as used herein means a "liquid suspending medium" in which a plurality of small particles are dispersed The "liquid suspending medium" comprises one or more non- aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeπc stabilizer which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension The liquid light valve suspension useful in the present invention may include any of the liquid suspending media previously proposed for use in light valves for suspending the particles Liquid suspending media known in the art which are useful herein, include but are not limited to the liquid suspending media disclosed in U S Pat Nos 4,247,175 and 4,407,565 In general one or both of the liquid suspending medium or the polymeπc stabilizer dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium The polymeric stabilizer when employed, can be a single type of solid polymer that bonds to the surface of the particles but also dissolves in the non-aqueous liquid or liquids of the liquid suspending medium Alternatively, there may be two or more solid polymeπc stabilizers serving as a polymeπc stabilizer system For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, provides a plain surface coating for the particles and one or more additional types of solid polymeric stabilizer that bond to or associate with the first type of solid polymeric stabilizer and also dissolve in the liquid suspending medium to provide dispersion and steπc protection for the particles Also, liquid polymeπc stabilizers may be used to advantage, especially in SPD light valve films, as descπbed in U S Patent No 5,463,492 2 Particles Inorganic and organic particles may be used in a light valve suspension, and such particles may be either light-absorbing or light-reflecting Conventional SPD light valves have generally employed polyhahde particles of colloidal size, that is the particles generally have a largest dimension averaging about 1 micron or less Preferably, most polyhahde particles have their largest dimension less than one-half of the wavelength of blue light l e , 2000 Angstroms or less to keep light scatter extremely low A detailed review of pnor art polyhahde particles can be found in "The Optical Properties and Structure of Polyiodides" by D A Godina and G P Faerman published in The Journal of General Chemistry, U S S R Vol 20, pp 1005-1016, (1950) Herapathite, for example, is defined as a quinine bisulfate polyiodide, and its formula is given under the heading "quinine lodosulfate" as 4C2oH 2 N 2 O 2 3H 2 SO 2HI I 4 6H 2 O in The Merck Index, 10 th Ed (Merck & Co , Inc , Rahway, N J ) In polyiodide compounds, the iodide anion is thought to form chains and the compounds are strong light polaπzers See U S Patent No 4,877,313 and Teitelbaum et al JACS 100 (1978), pp 3215-3217 The term "polyhahde" is used herein to mean a compound such as a polyiodide, but wherein at least some of the iodide anion may be replaced by another hahde anion More recently, improved polyhahde particles for use in light valves have been proposed in U S Patent Nos 4,877,313, 5,002,701, 5,093,041 and 5,516,463 These "polyhahde particles" are formed by reacting organic compounds, usually containing nitrogen, with elemental iodine and a hydrohahde acid or an ammonium or alkali metal hahde or alkaline earth metal hahde Such organic compounds are referred to herein as a "Precursor " However, for the particles to be commercially useful, it is necessary that the particles used in a liquid light valve suspension, whether or not incorporated into a film, have great chemical and environmental stability To obtain particles of great environmental stability, in turn, it may be desirable to use non-polyhahde particles in light valve suspensions and films, especially where the stability of the material composing the particles is known to be excellent The use of graphite particles in light valves was first proposed in U S Patent No 1,963,496 However, an attending disadvantage of employing conventional graphite particles in light valve suspensions or films is that graphite particles are prone to rapidly agglomerate when a light valve suspension of them is activated In addition, prior art graphite particles are also known to scatter an unacceptably large amount of light as well as settling due to the gravitational pull Many kinds of known pigments are used as colorants in paints, varnishes and inks Generally, such pigments have average particle sizes in the range of 0 1 to 10 microns, but usuall 1 micron or greater To obtain these small sized particles, mechanical devices are often used to comminute solid particles into smaller ones Ball mills, attritors, and bead mills and roll mills are commonly used for such purposes The use in a light valve suspension of particles produced by such mechanical methods is not practical, however, for several important reasons First, the particles are usually too large, generally having an average size (diameter) of 1 micron or greater Secondly, even if sub- micron sized particles are produced, the grinding and comminution process tends to make such particles spheπcal in shape or amorphous, reducing or virtually eliminating their aspect ratio For use in a light valve suspension anisometπcally shaped particles are important, hence, particles shaped like needles, rods or plates and the like are preferred and generally necessary because their anisometric shape facilitates oπentation in an electπc or magnetic field Thirdly, an inherent disadvantage of mechanical comminution is that there is a large distπbution of sizes, generally resulting in the presence of particles having a size of 1 micron or more even if the average particle size is less than 1 micron Particles larger than 0 2 micron (one-half the wavelength of blue light) tend to scatter light, and such scatteπng increases exponentially with particle size This fact and the fact that relatively large particles promote agglomeration are additional reasons why such comminuted particles are not desirable for use m a light valve suspension There is thus a need m the art for light valves containing a light valve suspension of ultrafine particles Moreover, while vanous types of particles have been suggested in the pnor art for use in light valves, heretofore it has not been practical to obtain particles of a submicron size and amsometπc shape and good optical properties, except for polyiodide particles However, in some cases polyiodide particles may not be sufficiently stable to ultraviolet radiation, and light valve suspensions of such polyiodide particles may degrade in terms of color and performance if exposed to intense ultraviolet radiation for a prolonged peπod of time unless special measures are taken to protect against UV radiation such as, including, UV absorbers in the light valve suspension or film or using a UV filter to intercept UV radiation before it stnkes the SPD Also, nearly all polyiodide particles are limited to a blue color, whereas it is also desirable to have light valve suspensions which have off-state colors which are not blue Accordingly, new types of particles for light valve suspensions are needed which will be suitably small and anisometπcally shaped, which will tolerate high levels of ultraviolet radiation for long peπods of time without significant degradation and/or which have a vaπety of off-state colors US Patent No 5,650,872 provides an electro-optical device, such as a light valve or electrophoretic display, compπsing a cell formed of opposed cell walls, a hght-modulatmg unit compπsmg a suspension containing anisometπc particles suspended in a liquid suspending medium between said cell walls, and opposed electrode means operatively associated with said cell walls for applying an electπcal field across said suspension, the anisometnc particles having an average particle size of about 0 2 microns or less, preferably about 0 1 micron or less In another embodiment disclosed in US Patent No 5,650,872, a light valve is provided compnsmg a cell formed of opposed cell walls, a hght-modulatmg unit compπsing a liquid light valve suspension containing ansometπc particles suspended in a liquid suspending medium between said cell walls, and opposed electrode means operatively associated with said cell walls for applying an electrical field across said suspension, said anisometπc particles having been prepared by the Evaporative Dispersion Process, and said liquid light valve suspension being capable of decreasing the transmission of light therethrough when an electπcal field is applied across said suspension The liquid light-modulating suspensions used in US Patent No 5,650,872 have a wide array of off-state colors and can include mateπals superior to polyiodide particles in terms of their ability to tolerate high levels of ultraviolet radiation for long peπods of time without serious degradation However, all of the particles made in accordance with U S Patent No 5,650,872 must be made with the evaporative dispersion, and this process has not been shown to form sufficiently small anisometπcally shaped carbon or graphite The present invention attempts to overcome the disadvantages of using conventional carbon and/or graphite particles in light valve suspensions, the achievement of which becomes apparent from the discussion appeaπng below DESCRIPTION OF THE INVENTION The present invention is based on the discovery that suitably sized anisometnc carbon and/or graphite particles can be produced and used as the particles of a liquid light valve suspension, films and light valves The particles used in the instant light valves can be in vanous geometπc forms The particular geometric form is not cπtical as long as the particles are anisometπc A non-limiting example of anisometic particles includes fibπls Carbon fibrils are essentially cylindrical discrete carbon particles characterized by a substantially constant average diameter ranging from about 3 0 to about 66 nanometers, an average length of at least about three times the diameter and the lengths of the fibrils averaging less than abot 200 nanometers Preferably, the carbon fibπls include at least an outer region of multiple, essentially continuous, layers of ordered carbon atoms disposed substantially concentrically about the cylindrical axis of the fibnl As well, it is preferred that the entire fibril be substantially free of thermal carbon overcoat The term "carbon fibrils" as used herein means carbon and/or graphite filaments having average lengths of about 200 nanometers or less Although the present invention compnses both amorphous carbon and crystalline carbon (graphite) particles, graphite particles are preferred because of their generally higher electπcal conductivity for a given size particle Accordingly, the anisometnc particles useful m the present invention compπse anisometπc particles, which are composed entirely of amorphous carbon, or entirely of graphite, or may have a discrete core, which may be hollow or composed of amorphous carbon overlain by a layer or layers of graphite If the anisometπc particle, such as for example, a fibnl, has a distinct core, the wall thickness of the particle or fibril is about 0 1 to 0 4 times the external diameter of the particle or fibnl Methods of making carbon fibnls are known in the art For example, see U.S Patent No 4,663,230, which is incorporated in its entirety by reference herein Methods of dispersing carbon and/or graphite particles, such as fibnls in liquids are known in the art Generally, a polymer soluble in the liquid of the dispersion will bond to and disperse the carbon particles of the present invention if the polymer has in its structure a sufficient number of low polanty or non-polar groups Non-limiting examples of low or non- polar groups include alkyl and aryl hydrocarbon groups and high polarity groups include groups such as, lonomer, hydroxyl, carboxyl or alkylene oxide such as ethylene oxide Surfactants and dispersants for carbon particles are known in the art. See, also U.S. Patent Nos. 4,164,365, 4,273,422 and 5,279,773, which are assigned to the assignee of the present invention, and describe suitable copolymers and polymeric dispersion systems for light valve suspensions. As a practical matter, carbon fibrils cannot be easily made with diameters less than about 3.0 nanometers. In order to keep light scatter very low in a light valve suspension, the fibrils' lengths should average less than about 200 nanometers (one-half the wavelength of blue light). Preferably, no more than five percent of the particles in a suspension of the invention will have lengths of 200 nanometers or more; more preferably one percent or less of the particles will have lengths of 200 nanometers or more; and most preferably, 0.2% or less of the particles will have lengths of 200 nanometers or more. On the other hand, if the particles, such as fibrils, have too small a length, a different problem arises. A suspension of very small length particles requires a relatively great amount of voltage to orient, whereas longer particles require less voltage because of their greater torque. Hence, as a practical matter, the particles should preferably have an average length of about 50 nanometers or more. A suspension of anisometric carbon particles should preferably have five percent or less of its particles with lengths less than 50 nanometers; more preferably one percent or less of its particles will have lengths less than 50 nanometers; and most preferably 0.2% or fewer of its particles will have lengths less than 50 nanometers. Useful in the instant invention are carbon and/or graphite particles which are prepared in a manner such that anisometric materials having an aspect ratio, i.e., the ratio of length to width, of about 3:1 or greater, preferably about 10:1 or greater or more preferably of about 20: 1 or greater are obtained It is preferred that the particles have a thickness which is substantially smaller than the length or width of the particle The particles useful in the present invention should be dimensioned such that the largest possible dimension is an average of 200 nanometers or less Preferably, the largest average possible dimension should range from about 50 to 200 nanometers, more preferably an average of from about 75 to about 180 nanometers Although comminution is generally harmful because it reduces the aspect ratio of the particles being comminuted, a comminution process may be used if two of the three spatial dimensions of the particles are extremely small For example, if cyhndncally shaped fibnls have a very small average diameter e g , 10 nanometers and an average length of 1 micron or more, the fibnls could be comminuted by methods known in the pnor art to have average lengths below 200 nanometers, without significantly affecting the particles' diameters Particles of different sizes or size ranges may be separated from one another by known methods such as filtration and centπfugation The chira ty of deposition determines whether tubes are metallic or semi-conductive Carbon nanotubes can be metallic or semi-conductive depending on their chirahty A dispersion of the carbon particles of the present invention may be easily prepared by rapidly mixing the anisometnc particles such as fibnls into any suitable light valve liquid e g isopentyl acetate or tn-pentyl tπmel tate in the presence of any suitable polymer which acts as a dispersant, such as those descnbed in the prior art When a dispersion of the anisometnc carbon particles such as fibrils is placed in a light valve and activated with an AC voltage, light transmission through the cell is observed to increase readily It is to be understood that the present invention is not limited in scope by the exemplified embodiments which are intended as illustrations of single aspects of the invention, and embodiments and methods which are functionally equivalent are withm the scope of the invention Indeed, vanous modifications of the invention in addition to those descnbed herein will become apparent to those skilled in the art from the foregoing descnption

Claims: CLAIMS: 1 An electro-optical device, compnsmg a cell formed of opposed cell walls, a hght- modulating unit compnsmg a suspension containing anisometrically shaped carbon particles suspended in a liquid suspending medium between said cell walls, and opposed electrode means operatively associated with said cell walls for applying an electrical field across said suspension, said anisometrically shaped carbon particles having an average length of about 200 nanometers or less 2 The device according to claim 1, wherein said electro-optical device is a light valve and said suspension is a light valve suspension 3 The device according to claim 2, wherein said anisometπcally shaped carbon particles have an average length of between about 50 to about 200 nanometers 4 The device according to claim 2, wherein said anisometncally shaped carbon particles have an average length of between about 75 to about 180 nanometers 5 The device according to claim 2, wherein said light valve suspension is a liquid suspension or a film 6 The device according to claim 2, wherein the anisometπcally shaped carbon particles are amorphous or crystalline carbon 7 The device according to claim 6, wherein the anisometrically shaped carbon particles are carbon fibrils 8 The device according to claim 7, wherein the carbon fibnls have an average diameter of about 3 0 to about 60 nanometers 9 In a liquid suspension for use m a light valve, the improvement comprising anisometπcally shaped carbon-containmg particles suspended m a hquid-suspendmg medium, where the anisometncally shaped carbon-containing particles have an average length of about 200 nanometers or less 10. The liquid suspension of claim 9, wherein said anisometrically shaped carbon particles have an average length of between about 50 to about 200 nanometers. 11. The liquid suspension of claim 9, wherein said anisometrically shaped carbon particles have an average length of between about 75 to about 180 nanometers. 12. The liquid suspension of claim 9, wherein the anisometrically shaped carbon particles are amorphous or crystalline carbon. 13. The liquid suspension of claim 9, wherein the anisometrically shaped carbon particles are carbon fibrils. 14. The liquid suspension of claim 9, wherein the carbon fibrils have an average diameter of about 3.0 to about 60 nanometers. 15. Anisometrically shaped carbon particles useful in light valves and light valve suspensions wherein said particles have an average length of about 200 nanometers or less. 16. The anisometrically shaped carbon particles of claim 15, wherein said anisometrically shaped carbon particles have an average length of between about 50 to about 200 nanometers. 17. The anisometrically shaped carbon particles of claim 15, wherein said anisometrically shaped carbon particles have an average length of between about 75 to about 180 nanometers. 18. The anisometrically shaped carbon particles of claim 15, wherein the anisometrically shaped carbon particles are amorphous or crystalline carbon. 19. The anisometrically shaped carbon particles of claim 15, wherein the anisometrically shaped carbon particles are carbon fibrils. 20. The anisometrically shaped carbon particles of claim 15, wherein the carbon fibrils have an average diameter of about 3.0 to about 60 nanometers.

Inventors: SAXE ROBERT L, SAXE, ROBERT, L.
Applicants: RESEARCH FRONTIERS INC, RESEARCH FRONTIERS INCORPORATED
Application year: 2000
Main classification: G02F1
IPC codes: G02F1