Electrolytic Printing Using Polyvinyl Alcohol

Abstract

Hard copy prints are produced by an electrolytic scanning process. Free iodine is generated in the printing sheet at the writing electrode and reacts with colorless polyvinyl alcohol (PVA) in the 5,000 to 500,000 molecular weight range to produce optically dense colored spots. The color depends upon the molecular weights of the vinyl alcohol polymers present.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

Facsimile prints are produced from transmitted electrical signals.

2. Description of the Prior Art

There are, in the electrolytic facsimile art, several classes of writing processes. In one general class, metallic ions from one of the electrodes are introduced into the printing sheet and there either combine with colorless materials already present in the printing sheet in order to form colored complexes, or are precipitated as fine metallic particles. In another general class of writing processes the electrodes are not consumed, the writing being accomplished by the electrolytic modification of materials already in the printing sheet. A process of this latter type involves the starch-iodine reaction. In this process electrolysis of potassium iodide in the paper generates free iodine which reacts with the starch, also present in the paper, producing the widely known purple starch-iodine complex. The process is fairly sensitive requiring moderately low currents to produce a sufficiently dense image. However, it is not sufficiently stable when exposed to light or in file storage to be considered a permanent record.

SUMMARY OF THE INVENTION

For the electrolytic production of a print from transmitted electrical signals, it has been found that the use of polyvinyl alcohol (PVA) in the molecular weight range from 5,000 to 500,000 in place of starch, used in a prior art printing process involving the electrolysis of an iodide salt, yields a more sensitive printing sheet which produces prints of greatly increased permanence. In addition, the prints can be produced in any desired color by selecting the molecular weight or combination of molecular weights of the vinyl alcohol polymers incorporated in the printing sheet. The colors range from red at the lower end of this molecular weight range through the visible spectrum to blue at the upper end of this molecular weight range.

In the disclosed writing process PVA and an iodide salt are incorporated together with an electrolyte in the printing sheet. A writing head scans the printing sheet and selectively electrolizes portions of the sheet. This electrolysis generates free iodine from the iodide salt. This iodine complexes with the colorless PVA to form the colored image. The electrolyte is required to impart electrical conductivity to the printing sheet and ionize at least a portion of the iodide salt. Water can be used to form the electrolyte or such materials as polymers of ethylene glycol in the molecular weight range 300 to 2,500.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic view of an exemplary electrolytic printing apparatus.

DETAILED DESCRIPTION OF THE INVENTION

The disclosed printing process depends upon the presence in the printing sheet of three classes of constituents at the time printing is to take place. An iodide salt (usually an inorganic water soluble salt) is required for the formation of free iodine. Polyvinyl alcohol (PVA) is required as the complexing agent which forms the colored record. Finally, an electrolyte is required to impart electrical conductivity to the printing sheet and ionize at least a portion of the iodide salt. FIG. 1 shows a roll 10 dispensing a printing sheet 11 which passes around rollers 14 through a liquid 12 and then through the writing head 13. The writing head 13 is activated by the electrolytic signal source 15 which causes the writing head 13 to selectively electrolyze the printing sheet 11 in accordance with information derived from a source of information 16 which may be local or remote.

The three necessary classes of constituents described above can be present in the printing apparatus 1 in several different places. The three constituents can all be incorporated in a presensitized roll 10, in which case the liquid 12 is not necessary, or any of the three constituents can be introduced within the apparatus 1 as the printing sheet 11 passes through the liquid 12.

The writing head 13 can contain, for instance, a single electrode pair which scans mechanically back and forth the paper as the paper moves through the writing head 13 or it can contain an array of electrodes which are electronically scanned by logic circuitry contained in the electrolytic source 15. The voltage levels required for the disclosed printing process fall in the 2 to 10 volt range which is easily controllable by solid state circuitry.

The iodide salt used is usually an inorganic water soluble salt such as potassium iodide. When current passes between the writing electrodes some of the iodide ions present in the paper are iodized at the cathode to form iodine. This iodine rapidly complexes with PVA in the region with very little lateral migration. A resolution of 20 lines per millimeter has been observed. This resolution is more than adequate for facsimile reception since a resolution of 5 lines per millimeter is all that is required to resolve elite type. The iodide salt should be present in the printing sheet in a concentration of at least 10.sup. 15 molecules per square centimeter in order to produce an image. However, at least 10.sup. 17 is preferable. Any desired higher concentration can be used. However, it should not be necessary to use more than 10.sup. 20 molecules per square centimeter under ordinary circumstances.

The color of the PVA-iodine complex depends upon the molecular weight of the vinyl alcohol polymers present. Vinyl alcohol polymers with molecular weights of the order of 5,000 give a red complex. As the molecular weight of the polymer is increased, the dominant color of the complex moves through the spectrum until near a molecular weight of 500,000 the blue end of the spectrum is reached. Mixtures of vinyl alcohol polymers will yield the various subtractive mixture colors according to well-known principles. The colors produced include brown or grey which can be used to provide a conventional black and white printed appearance. PVA should be present in the printing sheet in a concentration of at least 0.001 gram per square centimeter in order to yield a sufficiently dark image. Any desired higher concentration can be used. However, concentrations greater than 0.03 gram per square centimeter give the sheet an overly gelatinous feel.

The electrolyte can be formed by simply including water in the printing sheet. The amount of water required depends upon the desired operating voltage. The drier the printing sheet, the higher the voltage required to print. However, an amount of water sufficient for printing at a 2 volt level will just give the paper a damp feel. As stated above, the water can be included in the presensitized printing sheet 10 or can be introduced at some point 12 within the printing apparatus 1.

Other electrolytes can be used to meet particular device requirements. For instance, if it is desired to store rolls 10 of presensitized printing sheet, polyethylene glycol in the molecular weight range of 300 to 2,500 can be used as an electrolyte. The glycol has a very low vapor pressure so that the presensitized rolls have a long shelf life. The useful range of ethylene glycol polymers is limited at the low end by increasing volatility and at the high end by increasing stiffness. Operation in the molecular weight range of 350 to 2,000 yields preferable results. Polyethylene glycol is sufficiently hygroscopic to retain a small amount of water originally introduced or to absorb a small amount of moisture from the atmosphere.

EXAMPLE

The preparation of a sensitized printing sheet, according to the disclosed invention, can be accomplished by the following procedure:

The preparation of a sensitized printing sheet, according to the disclosed invention, can be accomplished by

a. prepare a 10.sup.-.sup.3 molar to 1 molar aqueous iodide solution containing 1/2 percent to 10 percent PVA;

b. immerse in this solution a sheet of good quality bond paper;

c. remove the paper; and

d. allow the paper to dry in a room with relative humidity greater than 15 percent.

This sensitization process causes the deposition in the paper of 1 .times. 10.sup. 17 to 1 .times. 10.sup. 20 molecules of iodide salt per square centimeter and 0.001 to 0.03 gram of PVA per square centimeter.

The sheet is now in condition for writing. Writing can be done by placing the sheet on a conducting plate and applying 1 to 10 volts between a 10.sup..sup.-3 inch diameter metal pin and the conducting plate, then drawing the pin across the sheet.

Claims

What is claimed is:

1. Process for the electrolytic production of a print comprising the selective electrolyzing of a printing sheet characterized in that the said printing sheet contains an intimate physical combination of an iodide salt and at least one polymer of vinyl alcohol in the molecular weight range from 5,000 to 500,000.

2. A printing sheet comprising a sensitizing material characterized in that the said sensitizing material comprises an intimate physical combination of an iodide salt and at least one polymer of vinyl alcohol in the molecular weight range from 5,000 to 500,000.

3. A printing sheet of claim 2 in which the said sensitizing material comprises 1 .times. 10.sup. 17 to 1 .times. 10.sup. 20 molecules of the said iodide salt and 0.001 to 0.03 gram of the said at least one polymer of vinyl alcohol per square centimeter of the said sheet.

4. A printing sheet of claim 2 in which the said sensitizing material comprises sufficient water to make the said sheet electrically conductive.

5. A printing sheet of claim 3 in which the said sensitizing material comprises ethylene glycol.