GB1586124A - Light valves - Google Patents

Description

(54) LIGHT VALVES (71) We, RESEARCH FRONTIERS INCOR PORATED, of 31 Cain Drive, Plainview, New York, United States of America, a corpora tion organised under the Laws of the State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be per formed, to be particularly described in and by the following statement:- This invention relates to light valves; more particularly, it relates to suspending materials which are used in liquid suspen sions to prevent or substantially reduce the agglomeration of the particles in suspension.

This agglomeration phenomenon is particu larly marked when liquid suspensions are used in a light valve under the influence of an electric field.

In colloidal suspensions, especially liquid colloidal suspensions, the particles in sus pension have a tendency to group together to form large clusters of particles. This phenomenon, "agglomeration", destroys the uniform distribution of the particles in sus pension and in many cases renders the sus pension of little value. The problem is par ticularly pronounced in suspensions that are used in light valves. A light valve is described in more detail in U.S. Patent No.

3,708,219.

It usually consists of two transparent flat substantially parallel walls which are sep arated by a relatively small distance, gen erally of the order of from 0.5 to 50 mis, and is sealed around its periphery to form an enclosed cell. Thin, conductive; trans parent coatings are applied on the interior surafces of the walls and the cell is filled with a liquid suspension which may include light polarizing particles. The suspended particles are normally randomly dispersed in the suspension and in this random con dition, due to Brownian motion, the liquid suspension appears dark, because the particles absorb light and tend to extinguish visible light rays attempting to pass through the suspension.However, when a voltage is applied to the conductive coatings of the light valve, (i.e. across the suspension), the particles, which are preferably rod-like or acicular, align perpendicular to the walls of the cell. In this condition, light passes through the suspension and the suspension appears transparent. It is principally this application of the voltage to the suspension that causes agglomeration of the particles.

There has been substantial research in an effort to develop a light valve suspension where the particles in suspension would stay uniformly distributed and not agglomerate or group together when an electric field is applied. Conventional suspensions will hold particles in suspension for various periods of time with no electric field. Examples of such suspensions are given in U.S.

Patent Nos. 1,951,664; 1,955,923; 1,963,493; 3,512,876; and 3,773,684. However when an electric field is applied, the prior art suspending materials are unable to maintain the particles in suspension in a dispersed condition; and the particles group together to form agglomerates. Various methods have been developed over the years to attempt to overcome this problem. One such method is described in U.S. Patent No. 2,481,621 wherein supersonic waves are transmitted into a liquid suspension to agitate and disorient the suspended particles in order to break up and prevent agglomeration.

Another method is described in U.S.

Patent No. 3,655,267 whereby a high frequency alternating current voltage is used to prevent agglomeration. Other proposals use a pulsating voltage as a means of reducing agglomeration. Still others use a smooth generally laminar flow to cause the constant movement of the particles and thereby prevent the particles from agglomerating.

These prior art methods have required the use of electrical or sonic methods or fluid movement to either prevent or break up agglomerates. They each require the use of specialised equipment in conjunction with the light valve to accomplish the antiagglomeration function.

Thus, a method of maintaining the dispersed condition of the particles by using a suspending material which maintains such dispersion upon the application of a voltage across the suspension would be extremely useful and of great value in the operation of light valves.

The present invention provides a light valve which comprises a cell containing a suspension in a liquid suspending medium of particles responsive to an electrical field to change the transmission of radiation through the suspension, the liquid suspending medium comprising an ester of a phenol and a carboxylic acid.

The suspending materials have a marked tendency to reduce or eliminate agglomeration of the colloidal particles in suspension upon the application of a voltage to the suspension. The suspending materials comprise aromatic esters having a structure of the type that results from reacting a phenol substituted or unsubstituted with an acid substituted or unsubstituted. The suspending material may comprise phenyl acetate, ortho- or para-cresyl acetate or similar substances taken alone or mixed with suitable non-aqueous liquids. More particularly, the present invention relates to suspending materials which are used in conjunction with colloidal particles to form a liquid suspension. The liquid suspension is one which is useful in light valve suspensions where an electric field is applied to the suspension.

Essentially, the purpose of the suspending material is to maintain the uniform distribution of the colloidal particles in the suspension and prevent the grouping together thereof. In liquid suspensions used in light valves, the uniform distribution is especially important because, if the particles group together or agglomerate, the light valve, when activated, will not appear transparent, but will include blotches of these agglomerates.

The anti-agglomerating suspending materials include electrically resistive materials, such as phenyl acetate, and cresyl acetates, including ortho-cresyl acetate and paracresyl acetate. They are highly resistive so that they will not adversely affect the ability of the particles to align when a voltage is applied across the suspension, (usually of the order of 8 x 101 ohm-cm and above).

The structures of some of these materials are shown in accompanying Figures 1, 2 and 3. Figure 1 represents the structure of phenyl acetate which is the reaction product of phenol and acetic acid. Figure 2 represents the structure of ortho-cresyl acetate which is the reaction product of ortho-cresol and acetic acid. Figure 3 represents the structure of para-cresyl acetate which is the reaction product of para-cresol and acetic acid. In these examples, an oxygen atom, which is part of the ester, is attached directly and adjacent to the ring.

This is a result of a phenol that was reacted with an acid to form the aromatic ester. The effect of such materials in preventing agglomeration is unexpectedly different from an aromatic ester which is the reaction product of an aromatic acid and an alcohol. An example of an aromatic ester of the lastmentioned type is dibutyl phthalate; see U.S. Patent Nos. 1,951,664; 1,955,923; and 3,625,869. However, dibutyl phthalate is ineffective in retarding or preventing agglomeration compared to the materials used according to the present invention. A reason for this difference in behaviour between aromatic esters derivable from phenols on the one hand, and aromatic esters derivable from aromatic acids on the other hand, is that esters derived from aromatic acids have a carbon atom, instead of an oxygen atom, which is part of the ester, attached directly to and adjacent to the ring.Accompanying Figure 4 represents the structure of dibutyl phthalate which is distinctly different from the structures in Figures 1, 2 and 3. It is unexpected that this difference in chemical structure should make so marked a difference in the effect upon agglomeration.

The materials used according to the present invention may be used alone or may be combined with non-aqueous liquids, such as aliphatic esters including branched esters, e.g. isopentyl acetate, and certain non-polar aromatic or aliphatic liquids, such as toluene or trifluorotrichloroethane, respectively. The presently used materials, if liquids, should be liquids which are preferably not detrimental to the light valve and to other suspending materials and the suspended particles, and also be liquids which preferably are relatively stable to heat and ultra-violet radiation. It has been found useful, but not essential, that about 55% of the final suspending material (by weight) be composed of other liquids about 45%, by weight, will be composed of the present suspending materials, i.e. of phenol ester.

The present invention may be used with light valves that are used as windows, windshields, displays, mirrors and other similar devices. In the present context and in that of the related cases mentioned below, the terms "perhalide" and "polyhalide" may be used intechangeably.

U.K. Patent No. 1 586 122 (Application No. 21213/77) relates, inter alia, to a light valve which comprises a cell containing a liquid suspension comprising; an electricallv resistive liquid suspending medium; suspended therein, a plurality of anisometric, polarizing, halogen -- containing particles; and, substantially dissolved therein, a copolymer comprising at least one monomer having a sterically unhindered functional group, which is a hydroxyl group or an acidic group, and at least one monomer having a branched group, the distance from the backbone of the copolymer to the most dis tant sterically unhindered functional group being less than the distance to the terminal carbon atom of the branched group.

U.K. Patent No. 1 586 123 (Application No. 21214/77) relates inter alia, to a light valve which comprises a cell containing a liquid suspending medium, suspended therein, particles of one or more halogenated alkaloid acid salts (as therein defined) or metal halides and, dissolved therein, a polymeric stabilizer to prevent agglomeration of the particles, the liquid suspending medium comprising a liquid halogenated hydrocarbon having a ratio of halogen atoms to all other atoms therein of greater than 1:1, the halogen atoms of the particles being iodine and/or bromine and the halogen atoms of the halogenated hydrocarbon being of lower atomic weight than the halogen atoms of the particles and at least 60% of the halogen atoms of the halogenated hydrocarbon being fluorine and/or chlorine.

U.K. Patent No. 1 586 125 (Application No. 21216/77) relates, inter alia, to a process for the preparation of a light-polarizing material which comprises; hydrogenating an unsaturated bond of a branch chain of an unsaturated alkaloid; forming an acid salt of the resulting saturated compound; and forming a polyhalide of the resulting alkaloid acid salt (as therein defined).

U.K. Patent No. 1 586 127 (Application No. 21218/77) relates, inter alia, to a light polarizing perhalide of an alkaloid salt having incorporated in its molecular structure at least one halide of calcium, rubidium or cesium, the perhalide being the reaction product of an alkaloid acid salt (as therein defined), elemental iodine and the halide.

U.K. Patent No. 1 586 126 (Application No. 21217/77) relates, inter alia, to a light valve which comprises a cell containing a suspension in a liquid (as therein defined) suspending medium of particles responsive to an electrical field to charge the transmission of radiation through the suspension, the liquid suspending medium comprising an ester of a phenol and a carboxylic acid, the liquid suspending medium comprising, dissolved therein, a polymeric material for preventing agglomeration of the particles, and we make no claim herein to such a light valve or the production thereof.

SUBJECT TO THE FOREGOING DIS CLAIMER, WHAT WE CLAIM IS: 1. A light valve which comprises a cell containing a suspension in a liquid suspending medium of particles responsive to an electrical field to charge the transmission of radiation through the suspension, the liquid suspending medium comprising an ester of a phenol and a carboxylic acid.

2. A light valve as claimed in claim 1 wherein the phenol ester is an acetate of a phenol.

3. A light valve as claimed in claim 2 wherein the phenol ester is phenyl acetate, para-cresyl acetate or ortho-cresyl acetate.

4. A light valve as claimed in claim 2 wherein the phenol moiety is substituted.

5. A light valve as claimed in any of claims 1 to 4 wherein the liquid suspending medium comprises substantially 45 %, by weight, of the phenol ester.

6. A light valve as claimed in any of claims 1 to 5 wherein the liquid suspending material comprises an aliphatic ester, an aromatic compound or a halogenated organic liquid.

7. A light valve as claimed in claim 6 wherein the halogenated liquid is a fluorocarbon.

8. A light valve as claimed in any of claims 1 to 7 wherein at least a majority of the suspended particles are colloidal.

9. A light valve as claimed in claim 8 wherein the suspended particles are light polarizing particles.

10. A light valve as claimed in claim 8 wherein the suspended particles are polyhalide particles.

11. A light valve as claimed in claim 1 substantially as herein described.

12. A process for the production of a light valve as claimed in claim 1 substantially as herein described.

13. A light valve as claimed in claim 1 when produced by a process as claimed in

Claims (1)

1. claim 12.