US2949409A

US2949409A - Electrolytic recording media

Info

Publication number: US2949409A
Application number: US672123A
Authority: US
Inventors: Richards Robert Doughty
Original assignee: Muirhead and Co Ltd
Current assignee: Muirhead and Co Ltd
Priority date: 1956-12-20
Filing date: 1957-07-16
Publication date: 1960-08-16
Anticipated expiration: 1977-08-16
Also published as: GB818937A; FR1135353A; FR72350E; DE1010821B; GB786496A

Description

ELECTROLYTIC RECORDlNG MEDIA Robert Doughty Richards, Beckenham, England, aslsigner to Muirhead & Co. Limited, Beckenham, Engand No Drawing. Filed July 1'6, 1957, Ser. No. 672,123 Claims priority, application Great Britain Dec. 20, 1956 7 Claims. (Cl. 204-2) This invention relates to electro-sensitive media of the kind used in electrolyte recorders and picture telegraph receivers.

A well known medium of this kind consists of a paper base impregnated with a solution containing chemical compounds which produce a dark-coloured stain on the passage of an electric current through the paper, which is used in a moist condition.

A known type of impregnating solution for the paper comprises a polyhydroxyphenolic compound, such as catechol, together with a strong electrolyte and possibly other elements, which may be added to produce specific recording characteristics.

In the recorder two electrodes are arranged in contact with opposite surfaces of the paper and means are provided to move at least one of the electrodes in such a manner as to provide a series of scanning lines adjacent to each other so that the whole area of the paper is progressively scanned. One of the electrodes (the anodic electrode) contains a metal, such as iron, which will go into solution in the impregnant under the infiuence of an electric current and the dissolved metal reacts with the chemical compounds in the impregnant to form a dark-coloured, water-insoluble dye.

A well known type of recording paper containing catechol includes a strong electrolyte such as potassium chloride or nitrate. In such a paper the marking is of a dark purple colour and does not provide the full degree of contrast whihc is desirable when recording pictorial matter. In our copending patent application Serial No. 517,659 a recording medium of this type is de scribed which, in addition to the dye-forming compound and an electrolyte, contains an oxidizing agent, such as potassium or sodium chlorate. The addition of the chlorate produces a blacker mark, which lacks the undesired purple tinge, and produces a higher degree of contrast; furthermore, the paper may be intensified by heating to a low temperature after recording in order further to increase the contrast in the recording.

Where potassium chloride or nitrate, or a mixture of the two, is used as electrolyte only a small proportion of chlorate may be introduced because of the relatively low solubility of potassium salts. With some formulae, the addition of more than about 6% of a chlorate causes. precipitation so that crystals and wooly growths form on the paper during storage. As an alternative, the patent specification referred to describes an impregnating medium in which the electrolyte consists entirely of a so dium compound or compounds which, as is well known, are highly soluble. By using sodium chlorate as the oxidizing agent in such a formula it is possible to include a very much larger amount, with the result that a deep black mark and a high contrast range between white and black are produced. Furthermore, the marking may be intensified very readily at a low temperature. Both sodium nitrate and sodium chlorate are hygroscopic and this property is undesirable as the impregnated paper dries slowly. In moist conditions, i.e., during periods of damp weather, a paper containing a high proportion of these elements never dries completely but remains limp and does not acquire the crispness which is a desirable feature. Furthermore, the dye which forms the marking is able to spread through the fibres of the paper so that the marking loses its sharpness and clarity.

The invention consists of an electrolytic recording medium impregnated with an aqueous solution containing a polyhydroxyphenolic compound which is water-soluble, substantially colourless in weak solution and capable of forming a dark-coloured water-insoluble dye with an appropriate metal, together with an electrolyte, an oxidizing compound and a compound which is efilorescent to assist rapid drying of the paper.

The invention further consists of an electrolytic recording medium of the kind described in which the electrolyte consists of a compound which is efilorescent, together with an oxidizing compound.

Various compounds may be added to the impregnating solution for the purpose of the invention. Di-sodium hydrogen phosphate is efilorescent but it increases the initial pH value of the solution and also suppresses the pH value during recording, thus inhibiting lake formation; furthermore, it reduces the storage life of the paper and alters the recording characteristics.

Ammonium di hydrogen phosphate might also be used but it is found that the records deteriorate quite rapidly after recording.

It is preferred to use sodium sulphate, or a sodium sulphate hydrate, which does not appreciably alter the recording characteristics or the storage life of the paper. Sodium sulphate decahydrate, Na SO .l0H O, for example, will lose water under almost any atmospheric conditions.

It is found that a recording medium containing sodium sulphate dries very rapidly after recording and acquires a crispness which is very desirable in handling. Spreading of the dye which forms the markings is prevented.

A suitable formula for an impregnating solution containing sodium sulphate is as follows:

Parts by weight Sodium sulphate decahydrate 10 Sodium chlorate 7.5 Sodium nitrate 4.5 Pyrocatechol 3.5 Citric acid 0.22. T hiourea 0.22 Water to If a potassium compound is used as electrolyte in a formula such as that given above, then the amount of sodium chlorate must be reduced, preferably to less than 6 percent.

If an impregnating solution containing relatively large amounts of sodium sulphate and sodium chlorate is used a further feature results. Papers to be used for recording must be thin and of fine texture in order to have the desired low electrical resistance and uniform recording characteristics. Such papers shrink considerably in drying but if the solid content of the solution is high then the fibres of the paper are supported by these solids whilst drying, with the result that shrinkage is reduced to a fraction of what it would otherwise be and a smoother surface results, which is free from wrinkling and cockling.

A formula containing larger amounts of sodium sulphate, sodium chlorate and electrolyte is:

Parts by weight Sodium sulphate decahydrate 15 Sodium chlorate 15 Sodium nitrate l Pyrocatechol 5 Citric acid 0.2 Thiourea 0.2 Water to 100 8.6 Sodium chlorate 8.6 Pyrocatechol 3.5 Citric acid 0.22 Thiourea 0.22 Water to 100 The quantities of sodium sulphate and sodium chlorate may safely be increased up to 20 percent in a formula. of this type.

. It will be understood that the formulae are only given by way of example. Many variations may be made Without departing from the scope of the invention.

We claim:

'1; An electrolytic recording medium for facsimile or photo-telegraphic receivers comprising a base impregnated with an aqueous solution containing a polyhydroxyphenolic compound which is Water-soluble, substantially colourless in weak solution and capable of forming a dark-coloured water-insoluble lake with an appropriate metal, together with an electrolyte, sodium chlorate to deepen the colour of the recording and more than 1% of solution weight of sodium sulphate compound to promote rapid drying of the medium after recording.

. 2. An electrolytic recording medium, as claimed in claim l, in which the electrolyte comprises a potassium compound, the oxidizing compound consists of up to 6 percent of solution weight of sodium chlorate, and up to 10 percent of solution weight of sodium sulphate decahydrate.

3. An electrolytic recording medium, as claimed in claim 1, in which the electrolyte consists of a sodium compound, the oxidizing agent consists of up to 15 percent of solution weight of Sodium chlorate and up to 15 percent of solution weight of sodium sulphate decahydrate.

4. An electrolytic recording medium for facsimile or photo-telegraphic receivers comprising a base impregnated with an aqueous solution containing a polyhydroxyphenolic compound selected from the class consisting of catechol and its derivatives which is water-soluble, substantially colourless in weak solution and capable of forming a dark-coloured water-insoluble lake with an appropriate metal, together with sodium chlorate to deepen the colour of the recording, and an electrolyte which is water-soluble, substantially colourless and more than 1% of solution weight of efllorescent sodium sulphate.

5. An electrolytic recording medium, as claimed in claim 4, comprising up to 10 percent of solution weight of pyrocatechol, up to 20 percent of solution weight of a sodium chlorate, and up to 20 percent of solution weight of sodium sulphate decahydrate.

6. A recording medium electrolytically markable on passage of electric current therethrough between a pair of metal electrodes, comprising a porous support impregnated with an aqueous solution of an electrolyte, a polyhydroxyphenolic compound forming a coloured lake with the metal of one said electrode, an oxidant selected from the class consisting of chlorate and perchlorate metal salts to deepen the colour of said lake, and more than 1% of solution weight efflorescent sodium sulphate to promote rapid drying of the medium after marking.

7. A recording medium electrolytically markable on passage of electric current therethrough between a pair of metal electrodes, comprising a porous support impregnated with an aqueous solution containing a polyhydroxyphenolic com-pound forming a coloured lake with the metal of one said electrode, an oxidant selected from the class consisting of chlorate and perchlorate metal salts to deepen the colour of said lake, and at least 4% of solution weight of an electrolyte which is efliorescent sodium sulphate to promote rapid drying of the medium after marking.

References Cited in the tile of this patent UNITED STATES PATENTS 2,485,678 Trib-ble Oct. 25, 1949 2,681,309 Kline June 15, 1954 2,686,858 Kohn Aug. 17, 1954

Claims

1. AN ELECTROLYTIC RECORDING MEDIUM FOR FACSIMILE OR PHOTO-TELEGRAPHIC RECEIVERS COMPRISING A BASE IMPREGNATED WITH AN AQUEOUS SOLUTION CONTAINING A POLYHYDROXYPHENOLIC COMPOUND WHICH IS WATER-SOLUBLE, SUBSTANTIALLY COLOURLESS IN WEAK SOLUTION AND CAPABLE OF FORMING A DARK-COLOURED WATER-INSOLUBLE LAKE WITH AN APPROPRIATE METAL, TOGETHER WITH AN ELECTROLYTE, SODIUM CHLORATE TO DEEPED THE COLOUR OF THE RECORDING AND MORE THAN 1% OF SOLUTION WEIGHT OF SODIUM SULPHATE COMPOUND TO PROMOTE RAPID DRYING OF THE MEDIUM AFTER RECORDING.