Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-05-24
2001-10-09
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S452000, C359S253000, C345S107000, C356S441000, C525S176000
Reexamination Certificate
active
06301040
ABSTRACT:
FIELD OF INVENTION
The present invention relates to improved SPD films, and in particular to improvements in the matrix polymer used or useable for such films, and to light valves comprising such films.
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 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 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 U.S. Pat. Nos. 5,463,491, 5,463,492 and 5,728,251 and U.S. patent application Ser. No. 08/941,599, now U.S. Pat No. 6,081,272, all of which are assigned to the assignee of the present invention. All of the above patents and patent applications and any other patents and references cited therein or elsewhere herein are incorporated into this application by reference thereto.
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 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 polimeric stabilizer 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 polymeric stabilizers serving as a polymeric 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 steric protection for the particles. Also, liquid polymeric stabilizers may 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 shall mean 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.
PRIOR ART FILM PROBLEMS
Prior art cross-linkable SPD films, described below, have suffered from a variety of problems which have prevented the films from being mass produced commercially.
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, often about 1 hour, is inconveniently long for industrial film-coating processes. By comparison, UV curing of coated films is often achieved industrially in several seconds, enabling a web carrying coated film to move at high speed. Also, in the case of the aforesaid two patents, in order to achieve a relatively uniform distribution of droplets in the cured matrix polymer, it was generally necessary either to use a separate emulsifier or to incorporate pendant ester groups on the matrix polymer to serve as an emulsifier, a so-called “cross-linkable emulsifier”. Although effective, the cross-linkable emulsifier was difficult to synthesize, and did not have a long shelf life.
Ultraviolet coating of films is a well-established industrial technique. The first attempt to use ultraviolet radiation to cure an SPD film (see U.S. Pat. No. 5,463,491, Example 13) resulted in suspension being encapsulated within the matrix, but was not commercially viable because the liquid suspension in the droplets (capsules) had been severely degraded as evidenced by a color change from blue to red. Moreover, the time needed to cure the film with UV radiati
Chakrapani Srinivasan
Slovak Steven M.
Ben Loha
Greenberg & Traurig, LLP
Research Frontiers Incorporated
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