US3692659A - Electrolytic reversible color display device - Google Patents

Abstract

A REVERSIBLE COLOR DISPLAY DEVICE UTILIZING A POROUS MEMBER IMPREGNATED WITH AN ELECTROLYTIC INDICATOR HAVING A PH RANGE VERY NEAR THE PH AT WHICH THE INDICATOR UNDERGOES A COLOR CHANGE. THE IMPREGNATED POROUS MEMBER IS PLACED BETWEEN A PAIR OF ELECTRODES ON OF WHICH MAY BE A WRITING ELECTRODE SUCH AS A STYLUS AND THE OTHER A BACKING ELECTRODE. UPON THE APPLICATION OF CURRENT TO THIS STRUCTURE COLOR MARKS ARE MADE ON THE POROUS MEMBER. THE COLOR CHANGE IS REVERSIBLE BY APPLYING A CURRENT OF THE OPPOSITE POLARITY TO THE DEVICE.

Description

p 9 H. D. LEES ETAL 3,692,659

ELECTROLYTIC REVERSIBLE COLOR DISPLAY DEVICE FileqjAu 28, 1970 2 Sheets-Sheet '1 VIEW FIG. I

A'H/////////////////////J FIG. 2a

INVENTORS HAROLD D. LEES PAUL F. EVANS MARTIN S. MALTS EDWARD W. VIPOND ATTORNEY Sept. 19, 1972 H. D. LEES ET AL 3,

ELECTROLYTIC REVERSIBLE COLOR DISPLAY DEVICE Filed Aug. 28, 1970 2 Sheets-Sheet 2 ADDRESS CKTS 5 FIG. 3

United States Patent 3,692,659 ELECTROLYTIC REVERSIBLE COLOR DISPLAY DEVICE Harold D. Lees, Henrietta, Paul F. Evans, Pittsford, Martin S. Maltz, Rochester, and Edward W. Vipond, gush, N.Y., assiguors to Xerox Corporation, Stamford,

oun.

Filed Aug. 28, 1970, Ser. No. 67,731 Int. Cl. C2311 5/68 US. Cl. 204-224 1 Claim ABSTRACT OF THE DISCLOSURE A reversible color display device utilizing a porous member impregnated with an electrolytic indicator having a pH range very near the pH at which the indicator undergoes a color change. The impregnated porous member is placed between a pair of electrodes one of which may be a writing electrode such as a stylus and the other a backing electrode. Upon the application of current to this structure color marks are made on the porous member. The color change is reversible by applying a current of the opposite polarity to the device.

This invention relates to a display device wherein a color change is effected by a change in pH of an electrolytic indicator when a current is passed through the device.

BACKGROUND OF THE INVENTION Along with the information explosion there has arisen a need to create devices to channel data incoming from a myriad of sources, such as computers, into some form of manageable order. Foremost among information handling devices are visual display devices which permit ahnost instantaneous viewing of the transmitted information. An electrolytic indicator display can satisfy the need for information handling devices.

An electrolytic indicator solution will undergo a color change when its acidity is altered. Indeed, the pH of a solution connotes the acidity of the solution. A pH value below 7 which is neutral indicates an acid condition while a pH above 7 indicates an alkaline state. The color changing property of indicators or electrolytics has been utilized in record mediums known in the art. However, in these prior art devices a solution is prepared that undergoes a chemical reaction resulting in a change in pH which in turn causes a color change characteristic of the compound in the solution. For example, US. Pat. No. 3,402, 109 to Berman et al. teaches that when a record medium or sheet is impregnated with an electrolytically conducting solution containing a compound selected from the group consisting of absorbic acid and erythorbic acid with a silver anode placed on one side of the sheet and a cathode on the other side of the sheet, a dense blue-black mark is formed where the paper is contacted by the anode. Silver ions are released into the paper during current flow between the anode and cathode by electrolytic erosion of the anode. These ions are reduced to metallic silver ions by the action of the absorbic or erythorbic acid leaving a blue mark on the paper. This process produces a color change on the impregnated paper by the results of two distinct processes; namely, a reduction process or chemical reaction and a pH change resulting from the reduction process.

We have discovered that we can produce a color change of an indicator in a solution having a pH range very nearly the value of pH at which it changes color by passing a current through the solution. Thus, it is not necessary to precede the change in pH with a chemical reaction. In our invention a change in current polarity also gives rise to a reversible change in color of the indicator.

Accordingly, it is an object of this invention to provide a display device which creates a visual effect brought about by a change in the pH gradient of the indicator.

It is a further object of the invention to provide a record medium impregnated with an electrolyte or indicator which alters its color state solely as a result of current flow therethrough.

A further object of this invention is to provide an electrolytic indicator which has enhanced storage time.

Yet another object of this invention is to provide a soft electrolytic display device which can reverse its color state and at the same time electrically control the color obtained.

BRIEF DESCRIPTION OF THE INVENTION There is provided in one embodiment a display system comprising an indicator cell in which a porous member layer such as filter paper impregnated with an indicator or electrolyte overlays another porous member layer without an indicator or electrolyte. This structure is then placed between two electrodes that are connected to a voltage source by means of a reversible polarity switch. The electrode overlaying the impregnated layer may be made transparent so that a change of color thereon may be viewed. In another embodiment of the invention the transparent conductor may be replaced with a writing electrode such as a stylus matrix or character wheel. In yet a third embodiment the impregnated layer may be formed into a belt or drum and placed in a hermetically sealed liquid vapor filled housing having a viewing window. A stylus electrode is placed within the housing and forms characters on the belt as it rotates. In each of these embodiments when a current is passed through the electrodes and porous member, a mark is made on the impregnated porous layer due to a change in pH gradient resulting from the current flow.

In order to gain a better understanding of the invention, as well as other objects and further features thereof, reference is made to the following detailed description of the invention to be read in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a cross sectional view of an indicator ce 1;

FIG. 2a is a cross sectional view of an indicator cell showing a character wheel writing electrode;

FIG. 2b is a view similar to FIG. 2a with the addition of a second porous member;-

FIG. 3 is a cross sectional view similar to FIG. 2a showing a stylus matrix writing electrode; and

FIG. 4 is a side view showing a porous member in the form of a belt impregnated with an indicator.

Referring now to FIG. 1, there is shown a schematic cross sectional view of an electrolytic cell generally shown at 10 incorporating the principles of the invention. A layer 12 which may be an absorbent or porous sheet of filter paper impregnated with an indicator such as a compound of phenolphthalien overlays another absorbent layer of filter paper 13 without an indicator. Layers 12 and 13 are disposed in an electrically conductive relation between electrodes 11 and 14. Both electrodes 11 and 14 may comprise thin films of electrically conductive material such as tin foil or the like. In addition, electrode 11 may be made of a transparent glass with a conductive coating such as is known in the art. Where electrode 11 is made of a transparent material, the cell is viewed from that side. However, it is within the purview of the invention, as it will also occur to those skilled in the art, that electrode 14 may be made transparent as well. Furthermore, layer 13 may be omitted entirely. Electrodes 11 and 14 are connected by wire conductors 17 and 18 respectively to a reverse polarity switch S1. The reverse polarity switch S1 connects a source of potential 15 across electrodes 11 and 14.

Cell 10 is activated by wetting the porous layer 13 with a solution having a pH very near the value of pH at which the indicator in layer 12 changes color. Where phenolphthalein is the indicator, ordinary tap water is used as an activator solution. When switch S1 is closed a current of approximately 60 ma. at a voltage of approximately 20 volts is passed through cell 10 and an internal pH gradient is set up across the cell causing layer 12 impregnated with the indicator to revert to a red characteristic pH color state. If the direction of the current through cell 10 is now reversed, the pH gradient will change the indicator layer 12 to its neutral characteristic color. By utilizing pH indicators which go through a number of color changes at various pH ranges it is possible to set and control the color of the cell by altering cell current. Once the color has been set the current can be removed and the cell will remain in this color.

The following table lists some examples of indicators. However, the invention is not intended to be limited to the examples specified and it will occur to those skilled in the art that other indicators could be used in practice of the invention.

TABLE I Approximate pH range Color change Indicator Bromophenol blue 3. -4. 6 Yellow, red or blue images on white Thyrnolphthalin.. 9. 3-10.5 Blue-black image on white background.

Porous layer 12, if a red image is desired, may be impregnated with phenolphthalien by (l) preparing a 1% by weight solution of methyl alcohol and phenolphthalien; (2) spraying layer 12 with the solution and allowing the alcohol to evaporate; and (3) allowing the sensitized layer 12 to be activated by deionized or ordinary tap water.

If a blue image is desired a cell may be constructed in the manner described above but employing bromophenol blue as an indicator. A bromophenol blue indicator is yellow in the acid or low pH range of approximately 3.0 and is a deep blue in the neutral pH range of approximately 7.0. Bromophenol blue also has a narrow transition range wherein a slight change in pH will cause the color to shift reversibly from yellow to red to blue.

In preparing a display having a blue image a solution of dilute ammonium hydroxide and bromophenol blue is mixed and hydrogen chloride is added up to the point at which the solution begins to turn red. The solution is then poured onto a porous member or filter paper and allowed to dry. The dry filter paper is then cut and placed into a cell. When current is applied to the cell the indicator turns yellow. Reversing the polarity of the current causes the cell to turn blue. When the current through the cell is reduced, applying an opposite polarity pulse causes the cell to revert to its red neutral color.

Similarly, other sensitized sheets can be prepared in the manner described using cresol red and thymolphthalien.

In FIGS. 2a and 2b, there is shown an indicator cell wherein one of the electrodes of FIG. 1 is replaced by a writing electrode. Referring to FIG. 2a, character wheel 20 which has alphanumeric symbols about its periphery contacts porous member or filter sheet 21 impregnated with an indicator solution of the type described. Porous member 21 in turn overlays electrode layer 22. The electrodes 20 and 22 are connected to the negative and positive terminals respectively of a DC. power source 23. Writing speeds up to 85 inches per second have been demonstrated using a writing electrode pulsed with a 30 volt pulsed oscillator.

The image retention or storage time of the display system is a function of several variables but mainly dependent upon the stylus or electrode current. Any variables which will affect the current supplied to the writing electrode will, in general affect the image density and storage time. For example, if the writing electrode pressure is increased or the writing electrode voltage increased, the image retention or storage time will be increased.

FIG. 2b is a view similar to FIG. 2a with the addition of a second porous layer. In addition, FIG. 2b illustrates a means of obtaining either a temporary or permanent copy. Specifically, there is shown in FIG. 2b a writing electrode or character wheel 20 and a first porous member 21 which may be a layer of filter paper having such high porosity that bleeding or running of the marks on the sheet is eliminated. Filter paper of this kind may be obtained from the Millipore Corporation, Bedford, Mass., under the name Millipore filter. A second porous member 21a is disposed between the first porous member 21 and an aluminum back electrode or platen 22. The second porous member may be soaked in a 10% solution of sodium chloride (NaCl). This process slows down image decay on the first porous layer 21 and provides a temporary copy thereof. However, if layer 21 is dried soon after imaging a permanent copy is produced. An alternate method of forming a temporary image is to roll the negatively biased character Wheel over the porous layer wet with ordinary tap water. Another alternate method of forming a permanent copy is to wet the porous member with a salt solution as described. In forming either the temporary or permanent image in this alternate method, the latent image is developed after the wetting step by spraying on a pH indicator. Phenolphthalien is used for a red image and thymolphthalien is used for a blue image. In the case of the permanent copy, the paper is then quickly dried.

The addition of a metallic salt such as sodium chloride to the activator solutions of the impregnated member extends the image storage time considerably. Without the salt the maximum storage time is approximately five minutes when the writing electrode current has been adjusted to provide maximum image density.

In FIG. 3 the character Wheel is replaced by stylus 30 which may comprise a seven pin matrix. Porous medium 32 impregnated with an indicator of the type already described overlaps backing electrode or platen 33 which may be composed of a conductive material such as aluminum. Address circuits 35 furnish pulses of the proper polarity to driver circuits 34. In driver circuits 34 the pulses are amplified and applied to the proper pin electrode of the stylus 30 predetermined by the signal from the address circuit. As a result of the current fiow, visible dots in the form of an image are made upon the porous member 32 because of the change in pH gradient of the impregnated porous member.

Turning now to FIG. 4, there is shown yet another embodiment of the invention. A belt 40 is comprised of a porous member impregnated with an indicator such as phenolphthalien or other indicator materials cited above and is enclosed in a hermetically sealed water vapor filled housing 44. The water vapor Within housing 44 provides a source of moisture for porous member 40, so that a solution may be formed with the indicator material within it. A window 43 permits viewing of the belt 40 as it is rotated in a clockwise direction about rollers 41 and 42. Also contained within housing 44 is a stylus 46 which contacts belt 40 at the periphery of roller 41. Roller 41 is grounded as shown to complete the electrical circuit between it and stylus 46. Suitable conductors connect stylus 46 to a source of potential 57 through switch S4. This embodiment of the display system uses the natural image decay mechanism for the erasure of images made by stylus 46. The formed characters are brought into view as the belt rotates about rollers 41 and 42 and image decay occurs on the opposite side of the belt out of view.

From the foregoing, a display device employing a color reversible indicator system has been disclosed.

What is claimed is:

1. A display device comprising:

a belt of a porous medium impregnated with an indicator having a pH range very near the value of pH at which the indicator undergoes a color change and capable of changing to a first characteristic color when current is permitted to flow through it in one direction and to another characteristic color when current is permitted to flow through it in the opposite direction,

a first roller and means for applying ground to said first roller,

a second roller spaced from said first roller,

said belt enwrapped around said first and second rollers,

a writing electrode touching the periphery of said first roller and touching the outer side of said belt and forming an electrical circuit with said belt and said first roller,

a housing containing a water vapor sealed therein and enclosing said writing electrode and said belt en- References Cited UNITED STATES PATENTS 1,970,539 8/1934 Bausch 204-2 1,068,774 7/1913 Hutchison 204-224 699,784 5/1902 Brooks 204-2 OTHER REFERENCES Handbook of Chemistry and Physics, 32nd ed., 1950, pp. 1482-1485, Chemical Rubber Publishing Co.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 204-2,

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