US3539457A - Electrolytic recording medium - Google Patents

Description

United States Patent Office 3,539,457 ELECTROLYTIC RECORDING MEDIUM Robert Doughty Richards, Summit, N.J., and Alan Howard Vincent, Anerley, London, England, assignors to Muirhead Instruments, Inc., Mountainside, N.J., a corporation of New Jersey No Drawing. Filed Apr. 23, 1968, Ser. No. 723,568 Int. Cl. B21h 1/20 U.S. Cl. 2042 8 Claims ABSTRACT OF THE DISCLOSURE An electrolytic recording medium for marking with a silver anode, the medium impregnated with calcium or strontium formaldehyde sulfoxylate.

This invention relates to the art of electrolytic recording, and in particular to an improved electrolytic recording medium or paper.

Recording paper of the electrolytic type is generally marked by passing the paper between a positive eroding metal anode electrode and a negative non-eroding cathode electrode. The paper is impregnated with an electrolytically conducting solution containing a marking agent. When a voltage is applied between the electrodes and current is passed through the recording paper, metal ions are introduced into the paper from the anode and reacted with one or more of the ingredients of the paper impregnant to form a colored mark on the recording paper.

In order to be acceptable for facsimile recording purposes, the recording paper preferably should be white initially and should have the ability to produce marks having a density which is at least close in linear relationship to the current passed through the paper. The density of the marked area should be variable uniformly in a range from white for zero current flow through grays for currents of intermediate magnitude to black for currents of maximum value. The mark preferably should be black so as to provide a good contrast with the white background.

There should also be a minimum of bleeding or fringing of the mark so as to provide good resolution. The unmarked paper while stored in moist condition in sealed containers for extended periods of time prior to use should remain stable without chemical decomposition of its ingredients which would cause discoloration of the paper or affect the reproducibility of results without recorder adjustment. Further, the marked recording paper should not become discolored, give off unpleasant or other odors, or be subject to the transfer of the mark to adjacent materials. The marked recording paper should be insensitive to light even after being repeatedly run through any of the conventional duplicating machines involving exposure to ultraviolet or other strong light sources.

Heretofore successful recording papers have been provided employing pyrocatechin, also called catechol, as the marking compound. While generally satisfactory some difficulty has been had with the catechol papers because of the tendency of catechol to diffuse or transfer to and to discolor adjacent sheets of paper in contact with or in close proximity thereto. Catechol vapors also may cause discoloration of the walls and other surfaces near the recorder during the recording operation.

3,539,457 Patented Nov. 10, 1970 Prior to the development of the catechol papers, attempts were made to provide papers utilizing a silver electrode to supply the metal ions which were reacted with a reducing agent to form a mark. Such papers are disclosed in Elsey Pat. 2,063,992. Elsey teaches the use of formaldehyde to reduce silver ions in an electrolytic recording paper so as to produce a black mark. It is believed that the mark formed on the Elsey marking medium diffuses somewhat even though reagents are included to reduce this tendency. Further, because the formaldehyde used by Elsey is volatile the impregnated paper is unstable and thus is commercially impractical.

The next development in the art is the recording medium disclosed in Lieblich Pat. 3,332,857. In this patent Lieblich discloses the use of formaldehyde in the form of sulfoxylates and also discloses other aldehyde addition salts. The disclosures of this patent are limited to the use of the aldehyde salts of the metals of Group 1A of the Periodic Table of Elements, together with zinc or cadmium. In his disclosures the inventor makes much of the concept of combining formaldehyde in a sulfoxylate or bisulphite compound.

The next development in the prior art is that shown in Barnes et al. Pat. 3,349,013 in which the aldehyde addition salts of Leiblich are improved by the addition of certain stabilizers.

In marking the recording media of this art silver ions are released from a silver metal anode by the passage of an electric current. The silver ions enter the paper and are rapidly reduced by formaldehyde to form a black colloidal precipitate marking the paper.

During storage of these papers the recording characteristics deteriorate. The response as measured by the intensity of a mark produced by a particular applied voltage is not constant but varies according to the age of the paper. This disadvantage limits extensive use of the paper in the important area of newspaper photograph reception and even in the less critical field of data reception the papers are considered unusable after storage for a few months.

It would be expected that the prior art paper deterioration is caused by oxidation of the formaldehyde. However, unexpectedly, we have found that the deterioration is due to the production of sulfites by decomposition of the addition salts.

The effect of sulfite is to cause polarization of the silver anode which prevents silver ions from entering the paper. This effect can be demonstrated by the addition of sodium sulfite to an otherwise effective paper which causes it to immediately behave as a paper in which the formaldehyde compound has suffered decomposition. Normal depolarizing agents are not effective to overcome this deleterious effect.

In accordance with the invention we find that the use of calcium or strontium addition salts or mixtures thereof overcomes this disadvantage. Decomposition of the formaldehyde addition compound is not prevented but the sulfite compound which is formed removes itself by precipitation. The solubility of the sulfites of these metals is sufficiently low so as to prevent appreciable sulfite polarization of the anode.

Other objects and advantages of the invention will be apparent from the following description.

Suitable formulations for impregnation into square 3 feet of recording paper stock or other porous support are as follows:

EXAMPLE 1 Grams Calcium formaldehyde sulfoxylate 75 Sodium nitrate 110 Calcium hydroxide 2 Water 1000 EXAMPLE 2 Grams Strontium formaldehyde sulfoxylate 69 Strontium nitrate 95 Strontium hydroxide 1 Water 1000 With both of the formulations of the examples satisfactory marks were obtained using silver anodes with currents and potentials of commercial practicability such as is customarily used with the Lieblich and Barnes et a]. recording media.

In the examples the concentration of the reducing agent has been set as approximately 7 percent by weight. However, it is to be understood that the amount of reducing agent may be varied from a minimal amount up to the solubility limit thereof. Further, various combinations of the aldehyde addition salts may be used rather than a single salt.

Typical concentrations of reducing agent may vary between 0.1 and 2 percent by weight of available formaldehyde. These concentrations are selected as being commensurate with conventional recording speeds and marking apparatus sensitivity.

In Example 1 sodium nitrate is used as electrolyte and in Example 2 strontium nitrate is used as electrolyte. Other suitable electrolytes which may be used are the alkali metal nitrates, calcium nitrate, and ammonium nitrate. The concentration of the electrolyte may be varied as desired for purposes of varying the electrical conductivity as is known in the art. Such variations may be from minimal amounts to the solubility limits of the respective salts. Combinations of various salts may be made as desired.

It is desirable to provide a pH in the alkaline range because of the greater stability of these reducing agents in a basic medium. The pH may range between 7 and 14. The preferred range of pH is about 7.0 to 11.0 inasmuch as the paper or other support is generally more resistane to deterioration in this range. Alkali hydroxides, buffers or both may be used to obtain the desired pH value in this range. A carbonate-bicarbonate buffer is preferred because it provides a good medium for the reducing action of the aldehyde addition salts. The advantage of using a buffer is that it will tend to prevent a drift in the pH of the impregnant over a period of time. Suitable alternative buffering agents are monohydrogen phosphate-db hydrogen phosphate and other buffers known in the art.

The eroding anode electrode may be made of silver, silver alloyed with copper and/ or zinc, or any other silver alloy, copper, or any other metal which provides an ion which may be reduced to give a colored mark. It will be noted, for example, that silver produces a good black mark, copper a green mark, and iron a light yellow mark. Silver or silver alloys are preferred anode electrodes because of the high optical density of the mark produced and because of its stable image. Silver, in the production of univalent silver ions, requires less energy than common metal electrodes which produce multivalent ions. The use of silver thus generally results in the production of good density marks with a small expenditure of power. Silver alloy electrodes are available which provide increased 4 hardness, resistance to wear, and resistance to deforma tion without an appreciable effect upon their marking qualities.

The cathode electrode may be made of any suitable conductive metal able to withstand the abrasive effects of the moving recording paper. Metals found acceptable are platinum, platinum alloys, steel, and others as known in the art.

Recording paper may be prepared by impregnating porous white paper or other porous support of high wet strength with an impregnant prepared in accordance with the teaching set out herein. The impregnant is permitted to distribute itself evenly throughout the paper. Excess impregnant may be removed by passing the paper between pressure rollers, so that when ready for use in a recorder, the impregnated paper will have about 25 to 50 percent moisture by weight. In order to retard evaporation, the impregnated paper is stored in suitable sealed containers until needed.

While the invention has been described and illustrated with reference to specific embodiments thereof, it is to be understood that other embodiments may be resorted to without departing from the invention. Therefore, the form of the invention set out above should be considered as illustrative and not as limiting the scope of the following claims.

We claim:

1. An electrolytic recording medium comprising an impregnated sheet containing in an electrolytically conducting solution a formaldehyde sulfoxylate selected from the class consisting of calcium and strontium formaldehyde sulfoxylate.

2. An electrolytic recording medium according to claim 1 in which the impregnant contains an electrolyte selected from the group consisting of alkali metal nitrates, ammonium nitrate, calcium nitrate, and strontium nitrate.

3. An electrolytic recording medium according to claim 1 in which the pH of the impregnant ranges from 7 to 14.

4. An electrolytic recording medium according to claim 1 in which the impregnant consists essentially in approximate parts by relative weight of reducing agent 70, electrolyte 100, and water 1000.

5. An electrolytic recording medium according to claim 1 in which the impregnant is an aqueous solution containing an electrolyte, and calcium formaldehyde sulfoxylate.

6. An electrolytic recording medium according to claim 1 in which the impregnant is an aqueous solution containing an electrolyte, and strontium formaldehyde sulfoxylate.

7. An electrolytic recording medium according to claim 1 in which the impregnant is a solution including in approximate parts by relative weight water 1000, sodium nitrate 110, calcium hydroxide 2, and calcium formaldehyde sulfoxylate 75.

8. An electrolytic recording medium according to claim 1 in which the impregnant is a solution including in approximate parts by relative weight water 1000, strontium nitrate 95, strontium hydroxide 1, and strontium form aldehyde sulfoxylate 69.

References Cited UNITED STATES PATENTS 12/1936 Elsey 204-2 10/1967 Barnes 2042