US3447944A - Thermographic copying system - Google Patents

Description

United States Patent 3,447,944 THERMOGRAPHIC COPYING SYSTEM Raymond E. Werner, Cincinnati, Ohio, assignor to Sterling Drug Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Dec. 6, 1965, Ser. No. 512,016 Int. Cl. 133% 29/06; D21h 1/22 U.S. Cl. 11736.2 3 Claims ABSTRACT OF THE DISCLOSURE A thermographic process and thermographic paper employing as the image-forming components a solid organic peroxide compound which decomposes in the temperature range 50-200 C. and a substantially colorless substance capable of reacting with the peroxide decomposition products to form a colored reaction product.

This invention relates to novel heat-sensitive duplicating systems adapted to produce exact copies directly from an imaged original under the efiects of intense radiation.

Copy systems are known which depend upon a chemical reaction initiated by localized heating to form a colored image. Such so-called thermographic copy systems employ the principle that intense radiant energy applied to an imaged original, that is, printed or graphic subject matter a copy of which is desired, is preferentially absorbed by the dark-colored image and is converted thereby into heat energy. A visible copy in a pattern corresponding to the imaged original is produced on a heat-sensitive, image-forming copy sheet placed in heat-conductive contact with the imaged original.

Although the heat-sensitive copy papers known to the art have been widely accepted, they are nevertheless noted for several disadvantages. For example, the copy papers now used in the art are relatively expensive, primarily because multiple treatments of the paper are usually required to effect a stable, smudge-proof coating of imageforming materials. A single treatment coating is desired. Moreover, the coatings of most copy papers in the art impart a waxy feel and high gloss to the paper. A copy paper with a natural paper feel and low gloss is desired. In addition, most coating treatments in the art require the use of relatively expensive non-aqueous media. A cheaper, aqueous-medium treatment is desired. The majority of copy papers in the art employ reactants which are colored or off-white; copies produced therewith have an undesirable colored background. A heat-sensitive coating consisting of colorless reactants, from which copies having a white background are prepared, is desired.

It is an object of this invention to provide a thermographic system for producing copies of an imaged original, which employs inexpensive and readily available heatsensitive, image-forming reactants.

Another object of this invention is to provide an effective thermographic copy system which employs copy paper produced by a single coating treatment.

Still another object of this invention is to provide a then'nographic copy system which yields heat-sensitive copy paper having a natural paper feel and low gloss.

Yet another object of this invention is to provide a thermographic copy system employing heat-sensitive, image-forming reactants which are substantially devoid of color.

Other objects will become obvious to one skilled in the art in the light of the following disclosure.

The particular objects set forth are realized by the invention described herein as follows.

In its first aspect the present invention is described as residing in the concept of the process for forming a latent image by the localized thermal decomposition of a solid 3,447,944 Patented June 3, 1969 organic peroxide compound having a decomposition temperature in the range 50-200" C. The latent image so formed is an undeveloped image which is capable of being developed in any of several ways. For example, a leuco dye or other substantially colorless material such as dichlorobenzidine, which becomes colored upon reaction with the decomposition products of the solid peroxide compound, when placed in intimate contact with the solid peroxide produces a positive image upon the localized decomposition of the peroxide compound. On the other hand, a negativeimage is produced by localized thermal decomposition of the peroxide compound in the absence of a leuco dye, followed by thermal decomposition of the remaining peroxide compound in the presence of a leuco dye, e.g., by placing a copy paper coated with the leuco dye in reactive-surface contact with a paper coated with the peroxide compound, which has previously been exposed to localized heating suflicient to decompose a portion of the peroxide compound in the form of an image.

In another of its aspects, the invention is described as residing in the concept of a process which comprises thermally decomposing a solid organic peroxide compound, having a decomposition temperature in the range 50-200 C., by subjecting said peroxide, placed in heat conductive contact with a differentially radiation-absorbing imaged original, to brief intense radiation.

In yet another of its aspects, the invention is described as residing in a thermographic copy system employing, as the essential heat-sensitive, image-forming reactants, a solid organic peroxide compound which produces decomposition products in the temperature range 50-200 C. and, in intimate contact therewith, a colorless substance capable of reacting with said decomposition products to form a colored product.

As used herein, the term solid organic peroxide compound means a compound of the formula which is a solid at ordinary temperatures, i.e., below about 50 C. and wherein R and R are H or organic radicals which can be the same or different. Solid, water-insoluble, organic peroxide compounds which decompose in the temperature range 50-200" C. to produce free radicals, oxidizing agents, or acids, or a combination thereof, are in general suitable for the purposes of my invention. Such organic peroxides include solid hydroperoxides, for example cyclohexyl hydroperoxide, triphenylmethyl hydroperoxide, tetralyl hydroperoxide and the like; solid dialkyl peroxides, for example bis(triphenylmethyl)peroxide, triphenylmethyl tert.-buty1 peroxide, B-naphthyldiphenylmethyl tert.-butyl peroxide, and the like; alkanoyl peroxides, for example, dilauroyl peroxide and the like; aroyl peroxides, for example, dibenzoyl peroxide (socalled benzoyl peroxide), di-p-toluoyl peroxide, bis(mchlorobenzoyl)peroxide, bis(p chlorobenzoyl)peroxide, bis (p-bromobenzoyl) peroxide, bis (p-methoxybenzoyl peroxide, bis(2-furoyl) peroxide, bis(fl-Z-furoylacryloyhperoxide, dinicotinyl peroxide, or,a'-dinaphthyl peroxide, 5, 6- dinaphthyl peroxide, and the like; monacyl peroxides of dibasic acids, for example, succinic acid monoperoxide, camphoric acid monoperoxide, and the like; and peroxy acids, for example, peroxy-Z-furoic acid, peroxycinnamic acid, monoperoxysuccinic acid, monoperoxyphthalic acid, monoperoxycamphoric acid, and the like. I prefer to employ diacyl peroxides, and, in particular, benzoyl peroxide, because of its excellent stability to moisture, air, and moderate light. Benzoyl peroxide decomposes at about C.

The colorless substance employed to develop the latent image into a visible image is selected from a number of compounds which are known to react with free radicals, oxidizing agents or acids, or a combination thereof,

to produce a colored reactant. The color-forming reaction may involve for example, oxidation, acid-catalyzed bond fission, or salt formation. The preferred colorless colorforming substances are leuco dyes which can be oxidized to the colored form by said decomposition products. However, the decomposition products of diacyl peroxides are not only oxidizing, but also acidic in nature. Therefore, leuco dyes which are transformed to their colored form either by oxidizing or acid or by both are operable in this invention. Examples of leuco dyes which are converted to colored form by oxidation and/ or acidification are known to the art and include leucomalachite green, pararosaniline, leucoindigo, crystal violet lactone, and N-acyl leucomethylene blues, for example, acetyl leucomethylene blue, isovaleryl leucomethylene blue, benzoyl leucomethylene blue, p-chlorobenzoyl leucomethylene blue, and 3,4-dichlorobenzoyl leucomethylene blue. I prefer to use benzoyl leucomethylene blue, having the structural formula which is readily available, stable to air oxidation, water insoluble, and is colorless in the leuco form and a pleasing shade of blue when oxidized to methylene blue.

In yet another of its aspects, this invention is described as residing in the concept of a heat-sensitive copying paper adapted to provide visible reproduction of an imaged original on being placed in heat-conductive contact therewith and on irradiation as herein described, said copying paper comprising a backing member carrying a visibly heat-sensitive coating comprising benzoyl peroxide, benzoyl leucomethylene blue and an alkaline stabilizer in admixture with a binding agent.

The irradiation required for the use of the copy paper of this invention is conveniently provided by commercial thermographic copying machines. Either highintensity flash or continuous-glow sources of infra-red radiation known to the art are suitable for the purposes of this invention. While it is difiicult to measure the heat developed under working conditions of such sources, an infra-red intensity which develops an effective temperature of 50200 C. produces excellent results with papers of this invention, the preferred temperature being dependent upon the reactants employed. When the imageforming reactants are benzoyl peroxide and benzoyl leucomethylene blue, the preferred temperature range is about 100-150 C.

The backing member is preferably light-weight paper, but it may also be a polymeric film or other fibrous or non-fibrous we'b. Suitable backing members for the copy paper are sufiiciently transparent to transmit the major part of the irradiation which strikes it. For this purpose, white paper of approximately 9 pounds per ream has been found particularly valuable.

Although benzoyl peroxide is relatively stable under the ordinary conditions of temperature and moisture, I have found that the copy papers described herein are improved by the incorporation of an alkaline stabilizer in the coating. For example, any of the common bases or buffer mixtures of pH approximately 9-12 serve to deter autocatalytic decomposition of the peroxide. I generally prefer to employ a sodium hydroxide-disodium phosphate buffer mixture which has a pH of about 11.

In order to impart smudge resistance and storage stability to the copy papers of my invention, I incorporate in the paper treatment mix a film-forming agent or binder. The binder can be one or more of a number of filmforming agents known to the art. As examples of such film-forming agents, I mention, without limiting the invention thereto, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, oxidized starch, and various chemically modified starch products. I prefer a low viscosity, oxidized starch powder, because it imparts superior stability and smudge resistance to my copy papers.

The quantity of organic peroxide and color-forming reagent employed depends upon the nature of the individual reactants. When benzoyl peroxide and benzoyl leucomethylene blue are used, I prefer that they be incorporated into the paper-treating mix in the ratio 1 part benzoyl peroxide to 1.0 to 2.0 parts of benzoyl leucomethylene blue, the parts being by weight.

The foregoing discussion is offered to illustrate methods suitable for the practice of my invention and not to limit its scope. My invention is further illustrated by the following examples without, however, being limited thereto. In these examples, the term parts in each instance is used to designate parts by weight.

EXAMPLE 1 The paper-treating mix is formulated from the following stock dispersions:

A. Binder paste (20 percent solids) Add a mixture of 760 parts low viscosity oxidized starch (Stayco M, A. E. Staley Mfg. Co.) to 3000 parts of distilled water, stirring at room temperature until the mixture is uniform. With continued stirring, heat the mixture at about 93 C. for twenty minutes. Upon cooling to room temperature, the resulting starch paste gels; it may be stored, but it is recommended that the paste be warmed to about 50 C. and allowed to cool to room temperature to insure uniformity. Should a surface skin form, it should be skimmed off prior to using the paste.

B. Buffer solution To a solution containing parts of disodium phosphate (anhydrous basis) in 500 parts of distilled water, add 28 parts of sodium hydroxide. Stir the mixture until solution of the reagents is complete.

C. Benzoyl leucomethylene blue dispersion Prepare 274 parts of leuco dye dispersion by thoroughly mixing 40 parts of benzoyl leucomethylene blue and 33 parts of binder paste A with parts of distilled water. Adjust the pH of the resulting paste to pH 11 with approximately 41 parts of buffer solution B, and mill the paste to a fine dispersion in a ball mill for a period not exceeding 30 minutes.

D. Benzoyl peroxide dispersion Prepare 495 parts of a peroxide paste by thoroughly mixing 82 parts of benzoyl peroxide-starch mixture containing 32 percent by weight of benzoyl peroxide, 163 parts of binder paste A, 142 parts of buffer solution B, and 108 parts of distilled water, Adjust the pH to 11.0 with buffer solution B and homogenize the resulting paste in an Eppenbach (high-speed shear-type) mixer for about fifteen minutes.

E. paper coating mix Thoroughly blend 274 parts of the benzoyl leucomethylene blue dispersion C and 495 parts of the benzoyl peroxide dispersion D. The coating mix thus prepared should be applied to paper immediately, according to the following procedure.

F. Reflex office copy paper Coat 9 lb./ream paper by drawing a pool of the mix over the paper and drying in a stream of cool air. The weight of the dry coating is preferably about 5 .5-5 .7 mg./ sq. in., but the coating weight can be varied if desired. The paper so treated is white and is quite stable under ordinary conditions of heat, light and moisture. The paper is smudge proof, and it has the normal feel of paper, with no artificial Waxy or slick feeling and very low gloss. The paper is virtually transparent when placed in surface contact with a printed page.

For use in reflex copying, the paper prepared as described above is placed on top of an original having infrared absorbing characters printed thereon, and with the coated side of the copy paper facing upward. High intensity radiation, such as provided by a thermographic oflice copy machine, is passed through the copy paper, and is preferentially absorbed by the infrared absorbing characters on the printed original, The absorbed radiation is converted into heat, which causes the formation of a blue image on the copy paper corresponding to the pattern of the original.

If the original has infrared absorbing characters printed only on one side thereof, and if it is sufficiently transparent to the exposing radiation, the imaged original can be placed above the copy paper during exposure to the radiation. This method generally produces poorer quality copies than the reflex method unless the original is virtually transparent.

EXAMPLE 2 Master office copy paper Coat paper of light to moderate weight (e.g., 9 to 18 pounds per ream) with benzoyl peroxide dispersion D. A preferred Weight of the coating is approximately 2 mg./ sq. in. The resulting paper is the master sheet; the original image is imprinted on the untreated side. Other papers of any weight are coated on one side thereof with a 2 mg./sq. in. layer of benzoyl leucomethylene blue dispersion C. These papers are the copy sheets, and are placed under the imprinted master sheet with the reactive surfaces of the two sheets in contact. Localized decomposition of benzoyl peroxide in the master sheet in response to localized heating produced by preferential absorption of infrared radiation in the imprinted characters causes the development of corresponding images on the copy sheet. Because of the minute quantity of benzoyl peroxide required to develop a substantial blue image, a number of copy sheets can be so developed from one master sheet.

I have prepared copy papers as described in the above examples and have found them to be well adapted for office-copying procedures, Moreover, the copy papers so prepared eminently fulfill the particular objects of the invention set forth above.

It will be understood that equivalent specific copy papers employing the reactant systems described or their equivalents in various combinations are also within the purview of my invention. Selection of the specific leuco dye and solid peroxide compound will depend not only upon the temperature at which the color forming reaction is to occur, but also upon the color of the copy image to be formed.

I claim:

1. A thermographic copy system including at least one infrared transparent web and employing, as the essential heat-sensitive, image-forming coating reactants, a solid organic peroxide compounds which produces decomposition products in the temperature range -200 C. and, in intimate contact therewith, a leuco dye capable of reacting with said decomposition products to form a colored product, said reactants being supported on the same or separate webs.

2. A thermographic copy system as described in claim 1, wherein the solid organic peroxide compound is benzoyl peroxide and the leuco dye is benzoyl leucomethylene blue.

3. A heat-sensitive copying paper adapted to provide a visible reproduction of an imaged original on being placed in heat-conductive contact therewith and on irradiation as herein described, said copying paper comprising a backing member carrying a visibly heat sensitive coating comprising benzoyl peroxide, benzoyl lcucomethylene blue and an alkaline stabilizer in admixture with a bonding agent.

References Cited UNITED STATES PATENTS 2,755,203 7/1956 Stallman 117--36.2 2,783,228 2/1957 Adams et a1. 11736.2 2,915,415 12/1959 Francis et al. 117-36.2 2,935,938 5/1960 OSullivan 117-362 3,076,721 2/1963 Coles et a1, 11736.8 3,129,109 4/1964 Workman 117-36.8

MURRAY KATZ, Primary Examiner.

US. Cl. X.R.