Light Valve, Light Valve Suspension Materials and Suspension Therefor

This invention relates to suspending materials and more particularly to suspending materials which are used in fluid suspensions to prevent or substantially reduce the agglomeration of the particles in suspension. This agglomeration phenomenon is particularly marked when fluid suspensions are used in a light valve under the influence of an electric field.

In colloidal suspensions and 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. Pat. No. 3,708,219, assigned to the assignee of the present invention. It usually consists of two transparent flat substantially parallel walls which are separated by a relatively small distance, generally on the order of 0.5 mil 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 fluid 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 fluid 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. Pat. Nos. 1,951,664 Land; 1,955,923 Land; 1,963,493 Land; 3,512,876 Marks; and 3,773,684 Marks. However, when an electric field is applied, the suspending materials in the art, prior to this invention, 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. Pat. No. 2,481,621 Rosenthal; 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. Pat. No. 3,655,267 whereby a high frequency alternating current voltage is used to prevent agglomeration. Other patents propose to 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 patents 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 special equipment in conjunction with the light valve to accomplish the anti-agglomeration 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.

Suspending materials and a fluid suspension using said suspending materials which 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 include an aromatic ester with the structure of the kind that results from reacting an aromatic alcohol substituted or unsubstituted with an acid substituted or unsubstituted. Said structure can also be obtained by other well-known chemical methods. The suspending material includes phenyl acetate, ortho or para-cresyl acetate or similar substances such as p-butylphenyl acetate or p-nonylphenyl acetate, taken alone or mixed with suitable non-aqueous solids and/or liquids. The suspension using the aforesaid suspending material also includes nitrocellulose or more preferably a branched copolymer which can be used to coat the particles to inhibit their grouping together. Such branched polymeric coating materials comprise those discussed in U.S. application Ser. No. 596,198, invented by one of the inventors of the present application and assigned to the assignee of the present application, which is included in this application by reference.

An object of this invention is suspending materials which prevent or substantially retard agglomeration of fluid suspensions which contain particles which are responsive to an electric field.

Another object of this invention is a light valve including a combination of suspending materials which prevent or substantially retard agglomeration of fluid suspensions which contain particles which are responsive to an electric field when an electric field is applied.

Another object of this invention is a first suspending material which is used in cooperation with a second suspending material, namely a polymeric coating material, to prevent or substantially retard agglomeration of the particles in suspension.

Another object of this invention is the use of aromatic esters, including aromatic esters having aliphatic components pendant from the aromatic ring, as suspending materials.

This invention relates to suspending materials which are used in conjunction with colloidal particles to form a fluid suspension. The fluid suspension is one which is particularly 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 their grouping together. In fluid 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 of this invention 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, straight chain or branched, 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 .times. 10.sup.7 ohm-cm and above).

The structures of some of these materials are diagrammed in FIGS. 1, 2 and 3. FIG. 1 is the structure of phenyl acetate which is the reaction product of phenol and acetic acid. FIG. 2 is the structure of ortho-cresyl acetate which is the reaction product of ortho-cresol and acetic acid. FIG. 3 is 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 an aromatic alcohol 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 last mentioned kind is dibutyl phthalate; see U.S. Pat. Nos. 1,951,664 Land; 1,955,923 Land; and 3,625,869 Marks. However, dibutyl phthalate is ineffective in retarding or preventing agglomeration compared to the materials of the present invention. A reason for this difference in behavior between aromatic esters derivable from aromatic alcohols 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. FIG. 4 is a diagram of the structure of dibutyl phthalate which is distinctly different from the structures in FIGS. 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 of this invention can be used alone or can 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 materials of this invention, 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 ultraviolet radiation. It has been found useful but not essential, that about 55% of the final suspending material (by weight) be composed of liquids not of this invention; 45% by weight will be composed of the suspending materials of this invention.

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 detail in U.S. application Ser. No. 596,198. The latter are copolymers. Each such copolymer includes at least two different monomers, at least one of which includes an unhindered functional group taken from the group consisting of an OH group and an acidic group, at least one of said monomers being soluble in the suspending medium, and at least one of said monomers being branched, and wherein the distance from the backbone to the available functional group most distant from the backbone in one monomer is less than the distance from the backbone to a non-functional group in another monomer.

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, among others, the following compounds: copolymers of 3,5,5 trimethyl hexyl acrylate/2-hydroxypropyl acrylate/bis-2-ethylhexyl fumarate/fumaric acid; and bis-2-ethylhexyl fumarate/3,5,5 trimethyl hexyl acrylate/vinylidene chloride/mesaconic acid. Because of their properties, including their size, their OH or acid functional groups, their solubility, and their branched nature, these materials as described in the said application, tend to prevent the particles from agglomerating together. They are thought to bond to the surface of the particles. When these coating materials are used as additional suspending materials in conjunction with the aromatic ester suspending materials, the combination acts to significantly retard agglomeration and under some conditions can retard agglomeration to the point where it does apparently not occur at all.

It is not preferred to use DC fields with this invention.

It is understood that gels and similar substances are intended to be included in this invention. 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 light polarizing perhalide particles, but is meant to include other kinds of light polarizing particles as well.

The following are examples of the use of the suspending materials of this invention, alone or in combinations with additional non-aqueous materials. Examples are also provided to illustrate use of the aromatic ester suspending materials aforementioned with the polymeric coating materials. These examples are only intended to be illustrative and not limiting in any manner.

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 bisulfate periodide said particles having been made with a combination of calcium iodide and hydrogen iodide, (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 .times. 10.sup.7 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, whereas only faint agglomeration was observed in compartment 2, and there was no observable agglomeration in compartment 3.

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 sulfate periodide, made with hydrogen iodide and iodine, in a liquid comprising 99% isopentyl acetate and approximately 1% nitrocellulose. The second compartment as 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 .times. 10.sup.8 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 said test period, there was noticeable agglomeration of the particles in compartment 1, whereas there was no significant agglomeration in compartment 2.

A test was made with the same cell operating under the same conditions as in Example 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. The compartment 2, the suspending liquid was 49.5% isopentyl acetate, 49.5% ortho-cresyl acetate with 1% nitrocellulose therein. In compartment 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.

A suspension of dihydrocinchonidine sulfate periodide was suspended in a liquid consisting of 51% 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 Pat. 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 this invention, high potential gradients produce more agglomeration than low potential gradients.

A liquid suspension of particles of quinine bisulfate 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:

The polymer was a tetrapolymer consisting of the following monomers in the indicated weight percent:

A 60 hertz alternating potential of 4.5 volts peak-to-peak (a gradient of 2.25 volts peak-to-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 can 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 kind that may be produced from aromatic alcohols. These aromatic esters include phenyl acetate, ortho-cresyl 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.

In one broad aspect the esters of the present invention are aromatic esters wherein the aromatic radical may be aryl, (e.g. benzyl phenyl) alkylaryl (methylbenzyl, methyl phenyl), naphthyl and the like.

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 previously described.

The aforesaid 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 or suspension of the present invention is capable, when an electric field is appropriately applied thereto, of transmitting at least two times 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 2 hours.

The present invention and the invention embodied in copending application Ser. No. 604,410 represent the first instances known to the inventors wherein a fluid suspension, such as a suspension usable in a light valve, has been stabilized from agglomeration, even when subjected to an electric field for a prolonged period of time, by solely chemical means.

The present invention can be used with light valves that are used as windows, windshields, displays, mirrors and other similar devices.

While specific embodiments of the invention have been described it will be appreciated that the invention is not limited thereto and many modifications thereof may be made by one skilled in the art which falls within the spirit and scope of the invention.