GB1586126A - Light valves - Google Patents

Description

(54) LIGHT VALVES (71) We, RESEARCH FRONTIERS IN CORPORATED, of 31 Cain Drive, Plainview, New York, United States of America, a corporation 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 performed, 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 suspensions to prevent or substantially reduce the agglomeration of the particles in suspension. This agglomeration phenomenon is particularly 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 suspension have a tendency to group together to form large clusters of particles. This phenomenon, "agglomeration", destroys the uniform distribution of the particles in suspension and in many cases renders the suspension of little value. The problem is particularly 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 separated by a relatively small distance, generally of the order of from 0.5 to 50 mils, and is sealed around its periphery to form an enclosed cell. Thin, conductive, transparent coatings are applied on the interior surfaces 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 condition, 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, acicular, or otherwise anisometric, 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 applied 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 in an 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. One proposal uses a pulsating voltage as a means of reducing agglomeration. Other proposals 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 of 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 dis persed 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 relates to a light valve which comprises a cell containing a suspension in a liquid (as hereinafter defined) 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, and the liquid suspending medium comprising, dissolved therein, a polymeric material for preventing agglomeration of the particles.

The suspending materials have a marked tendency to reduce or elminate agglomeration of the colloidal particles in suspension upon the application of a voltage to the suspension. The suspending materials are 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, such as p-butyphenyl acetate or pnonylphenyl acetate, taken alone or mixed with suitable non-aqueous solids and/or liquids. The suspension using the aforesaid suspending material may also include nitrocellulose or more preferably a branched copolymer which may be used to coat the particles to inhibit the grouping together thereof. Such branched polymeric coating materials comprise those discussed in U.S. Patent No. 4,164,365.

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 electrical 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 material, such as phenyl acetate, cresyl acetate, including ortho-cresyl acetate and para-cresyl acetate, and aromatic esters having aliphatic chain components, straightor branched-chain, pendant from the aromatic ring, such as para-butylphenyl acetate and para-nonylphenyl 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 on the order of 8 x 107 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 orthocresol 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 is 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, 45%, by weight, will be composed of the present suspending materials, i.e. the phenol ester.

The above suspending materials may be combined in the final suspension with the suspended particles that are coated with a polymeric compound, such as nitrocellulose, or, more preferably, the materials which are described in U.S. Patent No.

4,164,365. The latter are certain copolymers. Each such copolymer comprises at least one monomer having a sterically unhindered functional group which is a hydroxyl group or an acidic group and at least one of the monomers having a branched group, the distance from the backbone of the copolymer to the most distant sterically unhindered functional group being less than the distance to the terminal group of the branched group, the copolymer being substantially soluble in the suspending medium.

In one aspect, the polymer is one which has branch units which contain at least one functional radical selected from CO- and COO-.

These materials include the following compounds: copolymers of 3,5,5 trimethyl hexyl acrylate/2-hydroxy-propyl acrylate/ bis-2-ethylhexyl fumarate/fumaric acid; and bis-2-ethylhexyl fuma rate/3,5,5 trimethyl hexyl acrylate/vinylidene chloride/ mesaconic acid. Because of the properties thereof, including the 'size thereof, the OH or acid functional groups thereof, the solubility thereof, and the branched nature thereof, these materials tend to prevent the particles from agglomerating together. They are thought to bond to the surface of the particles. The use of these coating materials as additional suspending materials in conjunction with the aromatic ester suspending materials, acts to significantly retard agglomeration and under some conditions may retard agglomeration to the point where it apparently does not occur at all.

It is not preferred to use DC fields with the present invention.

It is to be understood that for the purposes of the present invention, the term "liquid" is to include gels and similar substances. Liquids of relatively high electrical resistivity are preferred in order to permit an applied electric field to orient the particles readily with relatively low voltages.

In order for a suspension to remain viable over a long period of time, the liquid, the suspended particles and the suspending materials including the polymeric coating materials must not deleteriously react with or degrade one another.

The suspended particles may be of many types, but are preferably colloidal in size and light polarizing in character. The term "light polarizing particles" includes in particular light polarizing perhalide particles, but is also meant to include other types of light polarizing particles.

The following Examples illustrate the present invention.

EXAMPLE I A test cell was constructed with three separate compartments alongside each other with common walls and common electrodes to which an activating alternating voltage was applied at 10 kilohertz and 300 volts peak-to-peak. The internal thickness of the cell, i.e. the thickness of the suspension, was 9 mils (0.009 inch). The first compartment was filled with a liquid suspension of particles of quinine bisulphate periodide, the particles having been made with a combination of calcium iodide and hydrogen iodine, (i.e. hydriodic acid) together with iodine, suspended in the liquid, isopentyl acetate. The suspension also included about 1 % nitrocellulose. The second compartment was filled with a suspension of the same particles and about 1 % nitrocellulose in diisodecyl adipate.The third compartment was filled with the same particles suspended in phenyl acetate having a resistivity of 8 x 101 ohm-cm. This suspension also included about 1%! nitrocellulose. All three compartments were activated simultaneously with the same voltage continuously applied for 2 hours. At the end of that time a great deal of agglomeration was observed in compartment 1, while only slight agglomeration was observed in compartment 2 and there was no observable agglomeration in compartment 3.

EXAMPLE II Two of the compartments of the test cell of Example I were filled as follows: The first compartment was filled with a suspension of cinchonidine sulphate periodide, made with hydrogen iodide and iodine, in a liquid comprising 99 % isopentyl acetate and approximately 1% nitrocellulose. The second compartment was filled with a suspension of the same particles in a liquid comprised of 49.5% isopentyl acetate, 49.5% ortho-cresyl acetate (having a resistivity of 6.5 x 108 ohm-cm) and 1% nitrocellulose. An activating alternating voltage of 100 volts R.M.S. at 60 hertz was applied to the electrodes of both compartments simultaneously for 2 hours 35 minutes. The thickness of the suspension in the cell was 9 mils (0.009 inch).At the end of the test period, there was noticeable agglomeration of the particles in compartment 1, while there was no significant agglomeration in compartment 2.

EXAMPLE III A test was made with the same cell operating under the same conditions as in Ex ample II, using the same suspended particles, except that the suspending liquids in the three compartments were as follows: In compartment 1, the suspending liquid was 99% isopentyl acetate with 1% nitrocellulose - therein. In compartment 2, the suspending liquid was 49.5% isopentyl acetate, 49.5% ortho-cresyl acetate with 1% nitrocellulose therein. In comportment 3, the suspending liquid was 49.5% isopentyl acetate, 49.5% para-cresyl acetate with 1% nitrocellulose therein. The cell and all three compartments were activated simultaneously and continuously for 17.5 hours, at the end of which time the following observations were made. There was heavy agglomeration in compartment 1.In compartment 2, there was very slight agglomeration at the top of the suspension, i.e. at the air-suspension interface, but this agglomeration is attributed to initial interaction with the air at the surface because this agglomeration started almost immediately after the cell was activated and then did not increase any further throughout the 17.5 hours. Only slight agglomeration was observable in compartment 3 at the end of the 17.5 hours.

EXAMPLE IV A suspension of dihydrocinchonidine sulphate periodide was suspended in a liquid consisting of 5% /O isopentyl acetate and 41% phenyl acetate, with 8% nitrocellulose therein. This suspension was placed in a display cell in which a symbol D was delineated by shaping the conductive coating on one wall of the cell in the shape of the character, as described in Canadian Patent No. 972,857. The suspension in the display cell was 4.5 mils thick. The cell was activated with an alternating voltage of 170 volts peak-to-peak at 60 hertz applied continuously for 7 weeks. At the end of this test no agglomeration was visible to the naked eye, not even around the edges of the display symbol.The fact that there was no agglomeration visible at the edge of the symbol is significant because the gradient of electric potential at the edges of the symbol is greater than the gradient would be if the electrodes on both walls of the cell were continuously covered with conductive coatings in the areas where the display symbol was located. Without the benefits of the present invention, high potential gradients produce more agglomeration than low potential gradients.

EXAMPLE V A liquid suspension of particles of quinine bisulphate periodide, made from calcium iodide, hydrogen iodide and iodine, was placed in a 2 mil cell, and suspended in a liquid of the following composition: Trichlorotrifiuoroethane 66.0% Isopentyl acetate 7.5% Chloroform 7.5% Polymer 11.0% Para-cresyl acetate 7.5% Particles 0.5% 100.0% The polymer was a tetrapolymer consisting of the following monomers in the indicated weight percent: 3,5,5 trimethyl hexyl acrylate 37.5% bis-2-ethylhexyl fumarate 37.5% 2-hydroxypropyl acrylate 22.0% fumaric acid 3.0% 100.0% A 60 hertz alternating potential of 4.5 volts peak-to-peak (a gradient of 2.25 volts peakto-peak per mil) was applied continuously for 24 hours.At the end of this time, no agglomeration was visible to the naked eye.

Ortho-cresyl acetate or phenyl acetate may be substituted for para-cresyl acetate, except that somewhat higher voltage gradients will be required.

The Examples given above demonstrate that agglomeration is greatly reduced by the use of aromatic esters of the type that may be produced from phenols. These aromatic esters include phenyl acetate, orthocresyl acetate and para-cresyl acetate. In addition, many aromatic esters having- aliphatic groups pendant from the ring may be used including: methyl phenyl acetate, ethyl phenyl acetate, propyl phenyl acetate, butyl phenyl acetate, amyl phenyl acetate, hexyl phenyl acetate, heptyl phenyl acetate, octyl phenyl acetate, nonyl phenyl acetate and decyl phenyl acetate, in particular when the aliphatic group is located in the para position.

These liquids are particularly effective in retarding agglomeration and maintaining the dispersion of a variety of different suspended particles even in the presence of an electric field when used in combination with a polymeric coating material as described above.

The above Examples illustrate that by using the present invention, and assuming that an adequate amount of either natural or artificial ambient light is available, a light valve is capable, when an electric field is appropriately applied thereto, of transmitting at least twice as much light as when the field is not applied, and is capable of maintaining its suspension in a well dispersed condition, if required, for a period of at least two hours.

The present invention is thought to be the first instance wherein a fluid suspension, such as a suspension usable in a light valve, has been stabilized from agglomera tion, even when subjected to an electric field for a prolonged period of time, by solely chemical means.

The present invention may be applied to 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 interchangeably.

U.K. Patent No. 1 586 122 (Application No. 21213/77) relates, inter alia, to a light valve which comprises à cell containing a liquid suspension comprising; an electrically 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 distant sterically unhindered functional group being less than the distance to the terminal carbon atom of the branched group.

U.K. Patent No. 1 568 123 (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, inta 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. 1 586 124 (Application No. 21215/77) relates, inter alia, to 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.

WHAT WE CLAIM IS: 1. A light valve which comprises a cell containing a suspension in a liquid (as hereinbefore defined) 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, and the liquid suspending medium comprising, dissolved therein, a polymeric material for preventing agglomeration of the particles.

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 claim 4 wherein the substituted phenol ester is methyl phenyl acetate, ethyl phenyl acetate, propyl phenyl acetate, butyl phenyl acetate, amyl phenyl acetate, hexyl phenyl acetate, heptyl phenyl acetate, octyl phenyl acetate, nonyl phenyl acetate or decyl phenyl acetate.

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

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

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

9. A light valve as claimed in any of claims 1 to 8 wherein the polymeric material is nitrocellulose.

10. A light valve as claimed in any of claims 1 to 8 wherein the polymeric material is 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 distant sterically unhindered functional group being less than the distance to the terminal carbon atom of the branched group, the branched groups in the copolymer being sufficiently

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (20)

**WARNING** start of CLMS field may overlap end of DESC **. tion, even when subjected to an electric field for a prolonged period of time, by solely chemical means. The present invention may be applied to 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 interchangeably. U.K. Patent No. 1 586 122 (Application No. 21213/77) relates, inter alia, to a light valve which comprises à cell containing a liquid suspension comprising; an electrically 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 distant sterically unhindered functional group being less than the distance to the terminal carbon atom of the branched group. U.K. Patent No. 1 568 123 (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, inta 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. 1 586 124 (Application No. 21215/77) relates, inter alia, to 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. WHAT WE CLAIM IS:

1. A light valve which comprises a cell containing a suspension in a liquid (as hereinbefore defined) 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, and the liquid suspending medium comprising, dissolved therein, a polymeric material for preventing agglomeration of the particles.

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 claim 4 wherein the substituted phenol ester is methyl phenyl acetate, ethyl phenyl acetate, propyl phenyl acetate, butyl phenyl acetate, amyl phenyl acetate, hexyl phenyl acetate, heptyl phenyl acetate, octyl phenyl acetate, nonyl phenyl acetate or decyl phenyl acetate.

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

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

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

9. A light valve as claimed in any of claims 1 to 8 wherein the polymeric material is nitrocellulose.

10. A light valve as claimed in any of claims 1 to 8 wherein the polymeric material is 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 distant sterically unhindered functional group being less than the distance to the terminal carbon atom of the branched group, the branched groups in the copolymer being sufficiently

soluble for the copolymer to be substantially dissolved in the liquid suspending medium.

11. A light valve as claimed in claim 10 wherein the polymer is a copolymer of 3, 5, 5 trimethylhexyl acrylate/bis-2ethylhexyl fumarate/2 - hydroxyl - propyl acrylate/fumaric acid or of bis-2-ethylhexyl fumarate/3, 5, 5 trimethyl hexyl acrylate/ vinylidene chloride/mesaconic acid.

12. A light valve as claimed in any of claims 1 to 11 wherein at least a majority of the suspended particles are colloidal.

13. A light valve as claimed in claim 12 wherein the suspended particles are light polarizing particles.

14. A light valve as claimed in claim 12 wherein the suspended particles are polyhalide particles.

15. A light valve as claimed in any of claims I to 14 which is capable, when an electric field is applied, of transmitting at least twice as much light as when the field is not applied and which is capable of maintaining such dispersion, if required, for a period of at least two hours.

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

17. A light valve as claimed in claim 1 substantially as herein described with reference to the Examples.

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

19. A process for the production of a light valve as claimed in claim 1 substantially as herein described with reference to the Examples.

20. A light valve as claimed in claim 1 when produced by a process as claimed in claim 18 or claim 19.