Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified... – With viewing layer of specified composition
Reexamination Certificate
2000-10-27
2002-07-09
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
With viewing layer of specified composition
C428S001500, C428S448000, C252S582000, C252S583000, C252S585000, C359S296000, C522S071000, C522S099000, C522S148000, C522S172000
Reexamination Certificate
active
06416827
ABSTRACT:
FIELD OF INVENTION
The present invention relates to improved suspended particle device (“SPD”) films, and to light valves comprising such films.
BACKGROUND OF THE INVENTION
Light valves have been known for over sixty years for the modulation of light. As used herein, the term “light valve” is defined as a cell formed of two walls that are spaced apart by a small distance, with at least one wall being transparent. The walls are provided with electrodes thereon, usually in the form of transparent, electrically conductive coatings. The cell contains a light-modulating element (sometimes herein referred to as an “activatable material”), which may be either a liquid suspension of particles or a plastic film in which droplets of a liquid suspension of particles are distributed.
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 electrical field, the particles in the liquid suspension assume random positions due to Brownian 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 electric 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. Light valves of the type described herein are also known as “suspended particle devices” or “SPDs”.
Light valves have been proposed for use in numerous applications including, e.g., alphanumeric displays and television displays; filters for lamps, cameras, optical fibers and for displays; and windows, sunroofs, sunvisors, eyeglasses, goggles and mirrors and the like, to control the amount of light passing therethrough or reflected therefrom as the case may be. Examples of windows, without limitation, include architectural windows for commercial buildings, greenhouses and residences, windows for automotive vehicles, boats, trains, planes and spacecraft, windows for doors including peepholes, and windows for appliances such as ovens and refrigerators, including compartments thereof.
For many applications, it is preferable for the activatable material, i.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 hydrostatic pressure effects, e.g., bulging, associated with a high column of liquid suspension can be avoided through use of a film, and the risk 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 that term is used herein, refers to a film having droplets of a liquid suspension of particles distributed in the film or in part of the film.
Light valve films made by cross-linking emulsions are known. See, for example, U.S. Pat. Nos. 5,463,491, 5,463,492 and 5,728,251, and 6,114,405, all of which are assigned to the assignee of the present invention. All of the above patents and any other patents or patent applications and references cited herein are incorporated into this application by reference thereto.
The following is a brief, non-limiting description of liquid light valve suspensions as known in the prior art, which suspensions may be modified in accordance with the present invention as taught herein to obtain improved SPD films and light valves comprising the same.
1. Liquid Suspending Media and Stabilizers.
The 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 in 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 polymeric 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 polymeric stabilizer dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The electrically resistive liquids chosen for use in prior art suspending media in SPD films are typically either “high” molecular weight polymeric liquids or mixtures of “high” and “medium” molecular weight polymeric liquids, as those terms (i.e., high and medium molecular weight) are defined herein, in order to obtain suspensions having optimal viscosity for a variety of coating applications. The prior art, in fact, teaches that by increasing the viscosity of the liquid suspending medium, e.g., with the use of high molecular weight (or mixtures of high and medium molecular weight) suspending liquids, the stability of the emulsion used in forming the light valve film is improved, with a resultant improvement in the properties and performance of the film produced therefrom.
The polymeric stabilizer, when employed, can be a single type of solid polymer that bonds to the surface of the particles, but which also dissolves in the non-aqueous liquid or liquids of the liquid suspending medium. Alternatively, two or more solid polymeric stabilizers may serve as a polymeric stabilizer system. For example, the particles may be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, provides a plain surface coating for the particles, after which the coated particles are recoated with one or more additional types of solid polymeric stabilizer that bond to or associate with the first solid polymeric stabilizer and which also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. Liquid polymeric stabilizers may also be used to advantage, especially in SPD light valve films, as described in U.S. Pat. No. 5,463,492.
2. Particles.
Inorganic and organic particles may be used in a light valve suspension, and such particles may be light-absorbing or light-reflecting in all or part of the visible portion of the electromagnetic spectrum.
Conventional SPD light valves have generally employed polyhalide particles of colloidal size. As used herein, the term “colloidal”, when referring to particle size, means that a particle has a largest dimension averaging 1 micron or less. Preferably, polyhalide or other types of particles used or intended for use in an SPD light valve suspension will have a largest dimension which is less than one-half of the wavelength of blue light, i.e., 2000 Angstroms or less, to keep light scatter extremely low.
3. Deficiencies of Prior Art Films
Prior art cross-linkable SPD films, incorporating suspensions of the type described above, suffer from a variety of deficiencies as described herein which have prevented such films from being mass produced and from thus reaching their full commercial potential.
For example, U.S. Pat. Nos. 5,463,491 and 5,463,492 describe cross-linked SPD films usually cured with heat. However, the length of time necessary to cure such a film with heat, i.e., often about 1
Chakrapani Srinivasan
Fanning Barry
Saxe Robert L.
Slovak Steven M.
Dawson Robert
Farley Mark A.
Research Frontiers Incorporated
Robertson Jeffrey B.
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