US3240613A - Data processing media - Google Patents

Description

United States Patent 3,240,613 DATA PROCESSING MEDIA Herbert N. Schlein, Framingham, Mass., assignor to Itek Corporation, Lexington, Mass, a corporation of Delaware No Drawing. Filed Aug. 23, 1962, Ser. No. 218,829 6 Claims. (Cl. 106-19) The present invention relates generally to novel thermally responsive graphic data processing materials, and to novel sensitizing compositions therefor. While this invention is concerned broadly with thermally responsive materials, compositions and systems, it is particularly concerned with an improved heat-sensitive copying paper that is capable of reproducing copies of heat-absorbing graphic materials by thermographic copying.

In the thermographic copying process as it is usually practiced at the present time, a graphic original is placed in surface contact with a copy sheet. The original is then irradiated with infrared radiations that are directed through the copy sheet. The radiation is absorbed by the dark areas of the original, and is converted to heat. The heat causes a color change to occur on the copy sheet, to form a reproduction of the original.

Typically, the copy paper temperature is raised, during copying, to about 65 to 75 C. in the non-image areas, and to about 95 to 115 C. in the image areas. Thus, a basic requirement of a good image-forming system is that it forms a high density visible marking or discoloration at about 100 C. in a relatively short time period, while having no noticeable or material change at about 70 C. in this same period. Moreover, the system must be stable during a reasonable length of time at room temperatures, to permit normal handling and storage prior to use.

At present, the thermal copying papers that are widely used meet these temperature requirements by utilizing physical means to prevent any change except at copying temperatures. Usually, this involves the use of a carefully selected wax that melts and flows at copying temperatures, either to bring together chemicals that react to make a colored reaction product, or to flow to effect a change in transparency with a blushed layer of lacquer that is adjacent the wax. One serious problem with thermographic systems that utilize a wax is the bleeding of images and general smearing that may occur when the paper is overheated.

Finding a practical thermographic chemical system, that is compatible with the temperatures of thermography and that can operate without the use of a wax, or other physical means to keep the reactants apart, is a difficult task. Ordinarily, a chemical reaction that will produce a given image density at 100 C. in a particular time interval would be expected to produce an image having A; of the given density at 70 C., and would also be expected to produce an image of the same density at 20 C. if the time interval were increased by afactor of 256. Chemical reactions with rates that would satisfy the parameters of practical thermography would be expected to be explosions or autocatalytic processes.

As disclosed by Murray and Berman in the patent application previously identified, the chemical reaction between the ferrous, cuprous, and thiosulfate ions have been found to satisfy the thermal parameters for an image-forming system for thermographic processes as they are now ordinarily practiced. One preferred embodiment of the invention of Murray and Berman involves the use of a sensitizing solution for a thermographic paper that is prepared by dissolving in water ammonium thiosulfate, cuprous oxide, and a source of ferrous ion, such as, for example, ferrous ammonium sulfate. This 3,240,613 Patented Mar. 15, 1966 solution is applied to a suitable paper web, dried at room temperature, and then preferably is provided with a moisture barrier coating such as a coating of parafiin wax. The sensitized paper product that is thus obtained is a thermographic copying paper that produces sharp, clear, dense, greenish-black images when used in a thermographic copier in the ordinary way.

The thermographic reproduction, that is formed in the paper, is believed to be a metallic sulfide that is formed from the available cuprous and ferrous ions. The chemical reaction that occurs during thermographic reproduction depends upon the reaction temperature, the reaction time, the concentration of the reactive materials, and the pH. The reaction appears to be complicated when the oxidation products of thiosulfate are present.

One object of the present invention is to provide a practical, inexpensive, heat-sensitive copying paper that is capable of reproducing copies of heat-absorbing graphic materials by thermographic copying, with uniformly satisfactory and consistent results.

Another object of the invention is to provide a heatsensitive copying paper, for use in the thermographic copying of heat-absorbing graphic materials, that is consistent in performance, relatively inexpensive to produce, and that can produce high quality reproductions even after prolonged storage.

A related object of the invention is to provide a copying paper of the character described that is characterized by superior shelf life, and more particularly, to provide a copying paper that is characterized by a very low loss in sensitivity upon aging.

Another object of the invention is to provide a heatsensitive copying paper for use in thermography, that has improved thermal sensitivity. A related object of the invention is to provide a copying paper of the character described that has good physical properties.

A more specific object of the invention is to provide a thermographic paper that is thermally sensitized by an applied chemical composition that is impregnated in the paper while the composition is in solution, and that is visibly reactive in response to thermographic copying temperatures, to form a reproduction that is visible at the surface of the paper and that, as well, extends in situ into the paper fiber.

A related object of the invention is to provide a copying paper that is thermally sensitized by an applied chemical composition that is impregnated in the paper while the composition is in solution, and that is stable at normal room temperatures and, as well, at the background temperatures that are encountered in the thermographic copying process, by reason of the chemical constitution of the sensitizing composition.

A more general object of the invention is to provide improved data processing media for use in thermally responsive marking processes.

Another general object of the invention is to provide improved thermally responsive graphic data processing media providing improved thermal sensitivity.

A more specific object of the invention is to provide means for preserving the sensitivity of thermally sensitive compositions that include as the thermally sensitive components thereof sources of metallic ion and of thiosulfate ion that react chemically at elevated temperatures to form a colored sulfide reaction product and that are therefore useful in thermally responsive marking processes.

Other objects of the invention will be apparent hereinafter from the specification and from the recital of the appended claims.

It has now been discovered that, in a chemical system that is thermally responsive so as to be useful in marking processes, and that may evolve sulfite ion that has an adverse effect on the system, the stability and the sensitivity of the system are improved by incorporating in the system either a material or a source of a material, such as an aldehyde, that forms an inert bisulfite addition prod uct. Preferably, the source of material is one that is stable and that is reactive to the presence of sulfite ion to release the material that forms the inert bisulfite addition product. While such a stable source of material represents a preferred embodiment of the invention, improvement in stability and sensitivity is also obtained by incorporating in the system a material that itself reacts directly with sulfite ion to form an inert bisulfite addition product, such as, for example, an aldehyde or a methyl ketone.

In accordance with the preferred embodiment of the invention, the chemical system has incorporated therein a material that is a source of aldehyde in the presence of sulfite ion, such as, for example, methenamine (hexamethylenetetramine). Other sources of methenamine are the carboxylic acid reaction products with methenamine, such as, for example, the reaction products with methenamine of adipic, succinic, and citric acids.

It has also been discovered that the advantages of making available in a thermographie system a thermally responsive acid-producing or acidic material can be obtained most advantageously by incorporating the material in a moisture resistant barrier coating or, preferably, by selecting as the material a substance that has a waxy nature, and using it as the barrier coating.

Optimum thermographic paper characteristics are obtained when the paper base is sensitized with a solution of materials consisting of a source of cuprous ion, a source of a second, different heavy metal ion such as a ferrous ion, a source of sulfide ion such as a thiosulfate compound or complex, and methenamine or a functional equivalent of methenamine, and when both surfaces of the sensitized paper are protected by a moisture-resistant barrier coating to maintain the moisture level of the paper, and when the barrier coating comprises an acidic material such as, for example, alpha-sulfopalmitic acid. In such a paper, any sulfite ion that is formed is rapidly removed because the methenamine supplies formaldehyde that forms an inert bisulfite addition product that does not interfere with the thermographic characteristics of the system. The barrier coating retards oxidation of the system by diminishing the contact between air and the paper, helps maintain the moisture content of the paper, and improves sensitivity during the thermographic reproduction process by releasing acid, thereby to decrease the pH of the system during the sulfide-forming reaction. A paper of this type is characterized by improved initial sensitivity, and also by resistance to any cycling in sensitivity that might otherwise occur because of oxidation or high humidity.

Thermographic papers of the type with which this invention is concerned are fixable after use in the thermographic reproduction system to produce an image. By rinsing the thermographic paper with a suitable solvent, such as, for example, water, the background areas can be rendered incapable of forming a thermographic image by removal of the sensitizing material from the paper. Since the images in the paper are permanent because they are formed from insoluble metallic sulfides that are disposed in situ in the paper fibers, the copy may be stored indefinitely without deterioration or destruction except through aging of the paper. Moreover, if a transparent or translucent paper base is used, the thermographic copy may be used, whether fixed or not, for the reproduction of inexpensive diazo copies by using conventional diazo reproduction processes.

The invention will now be described in greater detail by reference to specific examples that disclose procedures for preparing compositions in accordance with this invention and for coating the compositions on suitable carrier materials. Reference to parts and to percentages are to parts and percentages by weight.

Example I A sensitizing solution was prepared by dissolving 50 parts of ammonium thiosulfate in 42 parts of demineralized water, then adding 4 parts by weight of cuprous oxide to the thiosulfate solution while stirring the solution rapidly and maintaining the pH in the range 2.5 to 10.0 with adipic acid. After the cuprous oxide was completely dissolved, the pH was adjusted to 7.5 with adipic acid, and then 4.2 parts of methenamine were slowly added. The final solution was colorless.

This sensitizing solution was then applied to white paper sheets by dipping the sheets in the solution. The wet sheets were then dried at room temperature.

The sensitized sheets made good copies in the thermographic copying process when used with a Thermofax Secretary thermographic machine at a low setting. The sheets also developed a green-black coloration when heated to 120 C. for 0.25 second. The sensitivity remained constant during storage.

Example 2 A sensitizing solution was prepared by dissolving 44 parts of ammonium thiosulfate and 9.8 parts of glycerol in 43 parts of demineralized water. The solution was then agitated while 3.2 parts of cuprous oxide were slowly added until it was completely dissolved. The pH was then adjusted to 5.8 with adipic acid, and the solution was then filtered.

This sensitizing solution was then applied to a backing of 27 pound opaque white greaseproof paper. After drying at room temperature, the paper web was converted to sheet fonm. These sheets developed a green-black coloration upon heating to 110 C. for one second. Aging had little effect on sensitivity.

Example 3 A sensitizing solution was prepared by dissolving in 43 parts of demineralized water, 44 parts of ammonium thiosulfate and 5 parts of glycerol. This solution was then agitated while 3.2 parts of cuprous oxide were slowly added and dissolved. The pH was maintained throughout in the range between 2.5 and 10.0 by appropriate additions of adipic acid. After the cuprous oxide had completely dissolved, 4.8 parts of methenamine were added and dissolved. The colorless solution was then filtered.

This sensitizing solution was then applied to thin translucent paper stock. The treated paper was then dried by a blast of warmed air whose temperature was controlled so as not to exceed C.

The sensitized paper produced a green-black image on a white background when used for thermogra-phic copying. It was characterized by excellent stability during storage without any substantial loss in sensitivity.

Example 4 A sensitized solution was prepared using the following materials:

Parts Water 38 Ammonium thiosulfate 45 Glycerol 9.4 Cuprous oxide 3.0 Ferrous ammonium sulfate 1.0 Methenamine 3.6

justed to 5.2. The solution was then filtered. After filtration, the ferrous ammonium sulfate and methenamine were added in that order, and the solution was agitated for an extended period to insure that all the materials were completely dissolved and to make the solution uniform throughout.

White paper sheets were then dipped in this sensitizing solution and dried at room temperature. These sensitized sheets were used to make thermographic copies and produced good blue-black images in excellent contrast against the white background of the paper. This copying paper was stable during storage.

Example 5 A sensitizing solution was prepared substantially in accordance with Example 4 but with the substitution of cupric sulfate for the cuprous oxide of Example 4. This solution was then used to prepare heat-sensitive copying papers. Copying papers prepared from the freshly made solution exhibited good thermal sensitivity and made good thermographic copies of documents of many different kinds. Both the solution and the copying papers were stable upon aging.

Example 6 A thermally sensitive solution was prepared from the following materials:

Parts Water 400 Ammonium thiosulfate 350 Glycerol 80 Cuprous oxide Ferrous ammonium sulfate 8 Methenamine Adipic acid as required.

These materials were combined, following the same general procedure as employed in Example 4. The pH was adjusted to a final value of 5.4.

This sensitizing solution was then applied to a paper web by roll coating, at a rate sufficient to increase the weight of the paper after drying by 5 pounds per ream (3,000 sq. ft.). This increase in weight included glycerol as well as solids.

Sheets taken from this sensitized web produced very good blue-black images, affording excellent contrast against the White background of the paper, when subjected to a temperature 120 C. for one second in the thermographic reproduction process. The sensitized paper had excellent storage characteristics.

Example 7 One portion of the sensitized paper web, that was produced as described in Example 6, was treated further by applying to both of its surfaces a 1 /z% solution of parafiin wax in toluene. The rate of application was suflicient to apply to the paper about 0.2 pound per ream.

This thin wax barrier coating extended the shelf life of the sensitized paper under high humidity conditions, without adversely affecting the sensitivity.

Many other film forming materials, that will resist change in the moisture content of the thermographic paper, can be used instead of wax. Suitable film formers include, for example, common rubber base coatings and synthetic plastic coatings such as polyethylene and polyvinylidene chloride coatings.

Example 8 Another portion of the sensitized web, that was produced in accordance with Example 6, was treated by applying to both of its surfaces a 1 /2 solution of alphasulfostearic acid in toluene, at a rate sufiicient to apply about 0.2 pound per ream to the sensitized paper. The solvent was then permitted to evaporate at room temperature.

The barrier coating formed on the paper in this manner Q was useful in extending the shelf life of the sensitized paper, particularly under high humidity conditions. It was also found to be particularly beneficial in permitting the production of thermographic copies in which the images had increased density, greater contrast, and improved sharpness.

Although the barrier coat was applied to both surfaces of the thermographic paper in this example, equally beneficial and advantageous results have been obtained in other instances where some indicia or other signal was used to indicate the surface of the paper to which the sensitizing solution was applied, and the acid coating was applied only to the sensitized surface, while the other surface was coated with paraffin wax. However, since the present invention can be employed advantageously to produce thermographic copying papers in which either surface of the paper can be employed, through the use of appropriate sensitizing techniques, it is ordinarily preferable to apply the thermally responsive acidic or acid-producing material to both surfaces of the copying paper.

Example 9 A sensitizing solution was prepared by combining, in 50 parts of demineralized water, 30 parts of ammonium thiosulfate, 5 parts of glycerol, and 2 parts of cuprous chloride. The pH was adjusted to 4.5, and then 5 parts of ferrous ammonium sulfate hexahydrate and 8 parts of methenamine were added to the solution, with agitation to obtain complete solution and uniform mixing.

This sensitizing solution was then applied to a base paper having a weight of 23 pounds per ream; a brightness reading on the Photovolt scale of 83.0; single sheet opacity, as measured on the Bausch & Lomb scale, of 62.0; and high infrared transmissivity. The paper was dried by a'blast of air having a temperature of about 70 C.

Specimen portions of the sensitized paper web were tested and were found to exhibit excellent characteristics for use in thermographic copying, and good stability.

The sensitized paper was then coated with an acidic barrier coat by applying to it a 1 /2% solution of alphapalmitic acid in toluene. The solvent was removed from the coated paper by a blast of warm air having a temperature of about 70 C. The barrier coating was applied to both surfaces of the paper web.

The sensitized, barrier-coated web was then cut into sheets. The sheets exhibited excellent characteristics in thermographic copying and had good shelf life.

Some of the sheets were further treated by applying an edge coating of the solution of acid to all edges of the sheets while the sheets were stacked one on the other. A small improvement in shelf life was noted.

While the foregoing examples describe the invention with reference to a thermographic system in which either copper ion has been used alone or in combination with ferrous ion, the present invention applies to any thermally sensitive system that may evolve sulfite ion and wherein the sulfite ion is productive of an adverse effect. Thus, the metal ion in the thermally sensitive system, in which thiosulfate ion is the source of sulfide ion, may be supplied frorn compounds of lead, mercury, antimony, manganese, tin, bismuth, arsenic, and cadmium, by way of example, and these compounds may be employed either alone or in combination, depending on the parameters of the particular thermally sensitive system and the purpose for which it is intended.

While the examples mention the use of ammonium thiosulfate as the source of thiosulfate ion, virtually any equivalent source of thiosulfate will suffice. For example, rather than using ammonium thiosulfate as the source of sulfide ion, any of the alkali metal thiosulfates can be employed. Moreover, the type of metallic compound, that is employed in conjunction with the source of thiosulfate ion, can be selected with a broad range of discrimination. Readily soluble salts such as the nitrates and sulfates ordinarily are preferred, but halides can also be used, and in some cases, oxides can also be employed, such as, for example, the cuprous oxide mentioned in several of the examples. Moreover, copper thiosulfate complexes can also be employed, as disclosed, for example, by Murray and Berman.

The solvent that is used for the sensitizing solution may be any suitable solvent material. Water is ordinarily preferred because of its ready availability and other desirable properties. However, many of the alcohols are suitable solvents, and may be used, usually together with water. For example, ethanol and water can be employed together at the vehicle for the sensitizing solution for application to a paper web, for greater ease in application of the solution to the web, and more rapid drying, as com pared to an all-water vehicle.

Methenamine is the material of choice for incorporation in the sensitizing solution, to release formaldehyde in the presence of sulfite ion, to react with the sulfite ion to form an inert bisulfite addition product. However, many other materials can be used, although they are not considered as satisfactory as methenamine for a variety of reasons. For example, formaldehyde can be used, but it evaporates at a relatively rapid rate, so that its effect is not of sufliciently long duration for some thermal marking applications. Paraformaldehyde can also be used but it has relatively less desirable reactivity with sulfite ion, as compared to methenamine. Trioxane and paraldehyde may also be used.

The carrier material, to which the sensitizing compositions of this invention may be applied, may be any material that is satisfactory for use in a thermal marking process for some desired application. Suitable carrier materials include, for example, paper, cloth, and sheets or films of natural and synthetic materials or combinations thereof. Any carrier to which the sensitizing compositions can be applied satisfactorily, and that is suitable for the thermally sensitive application, can be employed.

In applying the sensitizing compositions of this invention to a web to make a thermographic paper, roll coating is preferred, but the sensitizing solution can be applied in any convenient manner, such as, for example, brushing, spraying, dipping, and the like. The concentration of the solution that is applied to the web can be varied over a wide range, from a heavy dispersion to a dilute solution. Ordinarily, for the production of thermally sensitive copying paper, a solution at a concentration of about 30% solids is convenient. The sensitizing composition may be applied in such form that it will form a surface coating primarily, but preferably, it is applied in an aqueous solution directly on an absorbent paper web so that impregnation occurs and the solution is applied not only to the surface of the web but also throughout at least a substantial part of its thickness.

When applying the sensitizing solution to a paper Web to make a heat-sensitive copying paper, the quantity' of active materials that is applied to the paper is important in connection with the quality of the copies to be produced. T light an application of active materials will result in a thermographic paper that makes light copies; whereas too heavy an application of active materials may result in undesirable physical characteristics of the copying paper. The proper amounts to apply ordinarily can be determined by those skilled in the art and will depend upon the particular sensitizing composition involved. For the copper-thiosulfate and the copper-iron-thiosulfate compositions described above, ordinarily the preferred weight of application is an amount sufficient to increase the weight of the base paper stock between about 3.5 pounds and about 6 pounds per ream, dry basis.

For the production of a thermographic paper of desirable charateristics, the base paper that is selected for coating with the sensitizing solution preferably should have a weight in the range between about 21.0 and 24.0

pounds per ream, and preferably, 22.0 to 23.0 pounds per ream, with a brightness reading on the Photovolt scale in the range from about 79.0 to about 86.0 and preferably in the range from 80.0 to 84.0, with high infrared transmissivity, and with single sheet opacity, as measured on the Bausch & Lomb scale, in the range from about 60.0 to about 66.0, and preferably in the range from 61.0 to 65.0.

The thermally acid-producing or acidic material, that is incorporated in or employed as the barrier coating, can be selected from a broad range of materials. While some of these acidic or acid-producing materials can be employed themselves as the barrier coating material, in other cases it is advantageous to mix the active material with a wax, a rubber or a plastic material such as polyethylene, that is to function as the barrier coating. Among the thermal-acid-providing materials that can be applied there can be mentioned, by way of example, materials such as ammonium bisulfate, semicarbazide hydrochloride, malonic acid, zinc fluoroborate, 4-amino-1-naphthol hydrochloride, 2-3oimethylaniline hydrochloride, oxalic acid, ammonium formate, malic acid, ammonium sulfate, and ammonium nitrate. In addition, a wide selection of materials is available among the sulfo-fatty acids that can function physically, per se, as barrier coating materials, to maintain the moisture content of the paper.

In general, in preparing sensitizing solutions in accordance with one preferred embodiment of this invention, for use in the manufacture of thermographic copying paper, and Where copper and thiosulfate ions are employed as the active ingredients, it is preferred that the ratio of copper to thiosulfate ion, in terms of moles, be in the range of from about 0.075-0.35 to 1, and preferably in the range from about 0.150.25 to 1. Similarly, where ferrous ion is incorporated in the composition, the ratio of copper ion to ferrous ion, in terms of moles, preferably should be in the range from about 15-9 to 1. Moreover, the pH of the composition, immediately prior to application of the carrier material, should be in the range between about 2.5 and 8.0, and preferably, close to about 5.5. The pH can be regulated by additions of ammonia, when necessary, or by additions of any one of many organic or inorganic acids. Among the most satisfactory acids are the water soluble dica'rboxylic acids,

such as, for example, adipic, glutaric, and the like.

Although the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinabove set forth, and as fall within the scope of the invention or the limits of the appended claims.

What is claimed is:

1. In a thermally responsive marking composition reacting in response to heat to form a colored metal sulfide reaction product and adaptable to application to a carrier for the thermal sensitization thereof, said composition comprising a thiosulfate canpound as a source of sulfide ion and a source of ions of at least one metal forming a colored sulfide, the improvement wherein said composition additionally comprises hexamethylene tetramine, as a source of formaldehyde, in quantities sufficient to scavenge sulfite ion produced by decomposition of said thiosulfate compound by reaction of said formaldehyde with said sulfite ion to form a bisulfite addition product.

2. In a thermally responsive data processing medium comprising a carrier sensitized with a thermally responsive marking composition reacting in response to heat to form a colored metal sulfide reaction product, said composition comprising a thiosulfate compound as a source of sulfide ion and a source of ions of at least one metal forming a colored sulfide, the improvement wherein said composition additionally comprises hexamethylene tetramine as a source of formaldehyde, in quantities sufiicient to scavenge sulfite ion produced by decomposition of said thiosulfite compound by reaction of said formaldehyde with said sulfite ion to form a bisulfite addition product.

3. A data processing medium as in claim 2 wherein said composition is applied to a porous web as a carrier.

4. A data processing medium as in claim 2 wherein said composition is applied to paper as a carrier.

5. A data processing medium as in claim 2 additionally comprising a moisture resistant barrier coating applied to said sensitized carrier, said barrier coating comprising a thermally responsive source of acid for providing a pH change in said composition at an elevated temperature.

6. A data processing medium as in claim 5 wherein said thermally responsive source of acid is a sulfo fatty acid.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Ellis: Printing Inks, Reinhold Pub. Corp., New York, 1940 (pages 402 to 406 relied on).

ALEXANDER H. BRODMERKEL, Primary Examiner. MORRIS LIEBMAN, Examiner.

Claims (1)

1. IN A THERMALLY RESPONSIVE MARKING COMPOSITION REACTING INRESPONSE TO HEAT TO FROM A COLORED METAL SULFIDE REACTION PRODUCT AND ADAPTABLE TO APPLICATION TO A CARRIER FOR THE THERMAL SENSITIZATION THEREOF, SAID COMPOSITION COMPRISING A THOSULFATE COMPOUND AS A SOURCE OF SULFIDE ION AND A SOURCE OF IONS OF AT LEAST ONE METAL FORMING A COLORED SULFIDE, THE IMPROVEMENT WHEREIN SAID COMPOSITION ADDITIONALLY COMPRISES HEXAMETHYLENE TETRAMINE, AS A SOURCE OF FORMALDEHYDE, IN QUANTITIES SUFFICIENT TOSCAVENGE SULFITE ION PRODUCED BY DECOMPOSITION OF SAID THIOSULFATE COMPOUND BY REACTION OF SAID FORMALDEHYDE WITH SAID SULFITE ION TO FORM A BISULFITE ADDITION PRODUCT.