Heat sensitive copy sheet

Nov. 16, 1965

3,218,166

J. L. RE1TTER

HEAT SENSITIVE COPY SHEET

Filed Nov. 23, 1962

INTERMEDIATE SHEET.

—^^-TRANSPARENT CARRIER.

(COATING COMPRISING PHOTOSENSITIVE -JpXREDUCING AGENT.

-- [COATING COMPRISING SIL VER SOAP AND ~£jSMALL AMOUNT OF LIGHT-STABLE [reducing agent.

T—.SUPPORTING SUBSTRATE.

/MAGEsheet.

^.2

COATING COMPRISING SIL VER SOAP AND SMALL AMOUNTS OF EACHPHOTOSENSNIYt AND LIGHT-STABLE REDUCING AGENTS.

-SUPPORTING SUBSTRATE.

INVENTOR.

^JOHNL. /?£/TT£R

BY

Attorney^

Patented Nov. 16, 1965 (

United States Patent Office image sheet. The visible change occurring on uniformly heating the exposed intermediate film, with its remaining small amounts of photosensitive reducing agent at image areas, in contact with the image sheet containing the small amount of uniformly distributed non-photosensitive reducing agent, is of much greater intensity than could be anticipated from the combined concentration of the two reactants, particularly in view of the complete absence of visible change at the adjacent background areas of the image sheet.

In the drawing, FIGURE 1 represents a preferred embodiment of the invention wherein an intermediate sheet carrying a photosensitive reducing agent for silver ion is placed against a separate image sheet carrying a coating comprising silver soap and a small amount of a light stable reducing agent. FIGURE 2 represents another embodiment in which both of the reducing-agent components are incorporated with the silver soap in a single coating.

/ In the association indicated in FIGURE 1, the two under room-temperature storage but reactive to provide dense black image areas on the silver-coated sheet when locally heated, for example by momentary contact with heated metal type or a heated metal test bar, or by transfer of a heat pattern from a differentially radiation-absorptive original in heat-conductive contact with the composite and subjected briefly to intense irradiation in the thermographic copying procedure.

Paper is a preferrred substrate for the image sheet. It may be of any desired weight. The reactive coating may also be applied to other heat-resisting substrates, e.g. wood, glass, and fabric. The transparent carrier of the intermediate sheet will ordinarily be a thin plastic film, Mylar polyester film being a preferred example. Thin unfilled paper is also useful.

The reactive components will ordinarily be retained in place on the carrier or substrate by means of minimal amounts of film-forming binder materials. Suitable subcoats or prime coats may be used to insure proper bonding of the active coating, particularly on the transparent films. The reactants may alternatively be introduced into fibrous paper-like webs during the formation thereof and in the substantial absence of any film-forming binder components.

Silver soaps of long-chain fatty acids, e.g. silver behenate and silver stearate, are colorless, visibly stable toward light, insoluble in many volatile liquid vehicles, and moisture-resistant; and these materials either alone or blended with additional fatty acid are preferred for use in the invention, although many other readily reducible normally solid organic acid salts of noble metals are

Photosensitive reducing agents which have been found effective include l-hydroxy-4-methoxy naphthalene, 1hydroxy-2-methyl-4-methoxy naphthalene, l-hydroxy-4ethoxy naphthalene, 4,4'-dimethoxy-l,l'-dihydroxy-2,2'binaphthyl, l,l'-dihydroxy-2.2'-binaphthyl, 1,4-dihydroxy napthtalene, 1,3-dihydroxy naphthalene, l-hydroxy-4amino naphthalene, the bis-dihydroxynaphthalene-dimethylmeihane condensation product of 1,5-dihydroxy naphthalene and acetone, l-hydroxy-2-phenyl-4-methoxy naphthalene, the condensation product of two mols of 1,5-dihydroxynaphthalene and one mol of adipoyl chloride. More generally, the photosensitive reducing agent may be an alpha-naphthol having attached directly to the hydroxysubstituted aromatic ring a preponderance of electron donor groups which may be alkyl, aryl, alkoxy, aryloxy, ₇₀ hydroxy or amino. Readily volatile photosensitive reducing agents are employed in the intermediate sheet of the two-sheet composite; materials of lesser volatility are

3,218,166

HEAT SENSITIVE COPY SHEET

John L. Reitter, St. Paul, Minn., assignor to Minnesota Mining & Manufacturing Company, St. Paul, Minn., a corporation of Delaware

Filed Nov. 23, 1962, Ser. No. 239,807 14 Claims. (CI. 96—67)

This invention relates to the copying of graphic originals, including those printed in various colors as well as black and white, by processes whereby the visible copy is produced through the application of heat. The invention has particular reference to improved copy-sheet materials with which copies of superior image density and contrast are made possible. 15

Methods have recently been devised for producing copies of printed or other graphic originals .by first preferentially desensitizing a sensitive intermediate film at reflective background areas of the original through reflex light exposure, and then heating the film in contact with, 20 ____ _______________λ-ιμόλο ± m- _lwo a suitable receptor or image sheet which is thereby visibly ' . sheets form a heat-sensitive copy sheet product ^taWe changed at areas corresponding to the still sensitive image ”~^J----“ ------ ' - ’ areas of the intermediate film. Typically, the film may contain a photosensitive aromatic hydroxy reducing agent, the image sheet then containing a water-insoluble silver soap. The reducing agent is reactive with the silver soap at moderately elevated temperatures with liberation of free silver, accompanied by a visible change. Exposure to actinic radiation desensitizes the reducing agent, i.e. renders it nonreactive with the silver ion, and thus prevents occurrence of the image forming reaction at the exposed areas when the two sheets are subsequently heated in contacting face-to-face relationship.

The amount of reducing agent present in the intermediate film must be held to a minimum if exposure time is to be reasonably short. The extent of the reduction occurring in the image sheet during image formation is therefore limited. Accordingly it has been found difficult to obtain image areas having’sufficiently high optical density to provide fully satisfactory contrast in the final copy. Toners and other such adjuvants are useful in providing darker image areas, materials such as phthalazinone being particularly effective, but such materials have not completely solved the problem of obtaining fully defined, dense, high contrast images in copies prepared by the methods hereinabove outlined.

As previously noted,, the reducing agents employed in the intermediate films are photosensitive, being rendered incapable of causing reduction of the silver ion when sufficiently exposed to actinic radiation. Other known reducing agents for silver ions,do not possess this property of photosensitivity, or are so slightly photosensitive as to be essentially, permanently stable under long continued · also useful, exposure under ordinary storage and use conditions. The incorporation of such materials into the silver soap coating of the image sheet causes pronounced darkening of the sheet when heated. Excellent heat-sensitive copy sheets have been made containing these light stable reducing agents in intimate juxtaposition with the particles of silver soap, and such sheet materials have been widely used in thermographic copying processes involving brief intense irradiation of a differentially radiation-absorptive original while in heat-conductive contact with the heatsensitive copy sheet.

Surprisingly, it has now been found that the incorporation in the image or receptor sheet coating of certain essentially light-stable reducing agents for silver ion, and in significantly lesser amounts than normally required to produce desired image density on momentary heating, is effective in providing fully defined, high density images while avoiding any darkening of background areas of the

3.218.166 useful in unitary copy-sheets in which all of the reactants are contained in a single layer.

A particularly effective light stable silver ion reducing agent which has given excellent results in the practice of the invention is 2,6-ditertiarybutyl-p-cresol, also known as “Ionol” or “CAO-3.” Combined in small proportions with the silver soap and phthalazinone toner in the image sheet coating, e.g. within the approximate limits of 0.01 to 0.4 mol per mol of silver soap, this material reduces the silver ion and produces a visible change when the sheet is heated for a somewhat longer time and at somewhat higher temperature than is normally the case in thermographic copying processes. However the sheet fails to provide the required density of image when so heated, and remains virtually unchanged when heated for a time and at a temperature such as are ordinarily encountered in thermographic copying processes. At considerably higher concentrations of the Ionol, for example at equal molar concentrations. with the silver soap, an excellent light stable heat-sensitive copy sheet is produced. The : transfer of extremely small amounts of a photosensitive silver ion reducing agent such as 1-hydroxy4-methoxy naphthalene to the Ionol-deficient coated sheet followed by brief heating at thermographic temperatures, i.e. within the approximate range of 90 to 150° C., results in the formation of visible images of high density. The amount of l-hydroxy-4-methoxy naphthalene required is far less than the additional amount of Ionol which must be employed to provide comparable image density, and the time and temperature requirements are also reduced.

A number of other nonphotosensitive or light stable reducing agents for silver ion have also been found useful in the practice of the invention. It has been found impossible adequately to define these materials in terms of common structure or formula. However it has been found that those active-hydrogen aromatic organic reducing agents are useful which are characterized by requiring an induction period of at least about six seconds at 100° C. before reacting rapidly with silver behenate and phthalazinone to a dense black mass.

The test is run as follows: a blend of 0.25 g. of silver behenate, 0.25 g. of the reducing agent, 0.05 g. of phthalazinone, and 3 ml. of methylethyl ketone is prepared by thorough mixing, and a single drop of the mixture is placed on a small thin cover glass and allowed to dry. The glass is placed on the flat surface of a metal plate heated to 100° C. and the time required to cause rapid darkening of the dry film is noted. It is assumed that the sample reaches the plate temperature within the first second, and accordingly no correction is applied for this period.

A convenient means for obtaining the desired thorough mixing is an ordinary electrically driven high speed dental mixer. A few small glass beads may be added to assist in the mixing if desired. In any event the reactants must be thoroughly mixed so that the color change occurs uniformly throughout the entire film on the glass slide.

With some materials a gradual darkening is observed over an extended heating period. As an example, the test with p,p'-methylenebis(N,N'-dimethyl aniline) requires some 70 seconds to develop full color change. With these materials the test mixture does not undergo the required rapid darkening, in which the full color change takes place within not more than one or two seconds, and usually even more rapidly.

When tested as just described, many of the reducing agents ordinarily suggested for use with silver soap in thermographic heat-sensitive copy sheet formulations cause complete blackening of the : test sample within the first second or at most within two to four seconds; and these materials have been found ineffective for the purposes of the invention. Thus, methyl gallate requires one to two seconds; catechol and pyrogallol require less than one second; protocatechuic acid and spiroindane require ⁴ four seconds. Image sheets coated with mixtures of these compounds and silver soap show pronounced darkening at background areas, or almost no improvement in image area density, when heated in contact with a reflex-ex5 posed intermediate film as hereinbefore described.

On the contrary, Ionol, requiring six to eight seconds ’ for the color change in the described test, provides intense black image and clean white background areas when similarly employed. Other useful light stable reducing agents IQ having analogous image amplifying properties, and the time required for them to produce rapid blackening in the test described, include: 2,6-ditertiarybutyl-4-methoxymethyl phenol, 8-9 seconds; 2,6-ditertiarybutyl-4-ethyl penol, 13-14 seconds; 2,6-ditertiarybutyl-4-octyl phenol, 15 17-20 seconds; p-cresol, 10 seconds; 2,4,6-tritertiaryamyl phenol, 25 seconds; and 4,4'-methylenebis(2,6-ditertiarybutylphenol), 12 seconds.

The use of a photosensitive volatilizable reducing agent in the intermediate sheet permits the preparation of re20 productions of graphic originals capable of providing differential light images and is ordinarily preferred. Other reducing agents may be substituted where heat images are to be employed, as in direct thermographic copying or in printing with heated metal type or stylus. As an ex25 ample, the coating on the intermediate film may contain pyrogallol or catechol in place of l-hydroxy-4-methoxy naphthalene, and images may be produced on an image sheet containing silver soap and small amounts of Ionol by locally heating the intermediate film at image areas 30 in contact with the image sheet. It is also possible to combine other classes of reducing agents with Ionol or the like and with the silver soap in a one-sheet heat-sensitive copy-paper which may at the same time be photosensitive if desired. For example, a combination: of silver 35 behenate, phthalazinone, Ionol and l-hydroxy-4-methoxy naphthalene provides a coating which converts rapidly to an intense black on brief heating but which after exposure to actinic light does not darken beyond a weak brownish black even on prolonged heating.

The following examples will serve further to illustrate but not to limit the invention.

Example 1

A mixture of equal mol percent of silver behenate and 45 behenic acid is prepared by reacting together one mol of silver nitrate and two mols of sodium behenate, made from commercial behenic acid, in acidic aqueous medium. The washed and dried precipitate is in the form of a fine powder fusing at about 135° C. and melting to a liquid 50 at about 175° C. A coating mixture is prepared by mixing together 12.6 parts by weight of the silver soap mixture, 50 parts of zinc oxide, 5.2 parts of phthalazinone, 10 parts of powdered “Piccolyte S—135” terpene resin, 12.5 parts of cellulose acetate, 7.3 parts of polyvinyl acetate, 55 0.2 part of tetrachlorophthalic anhydride, and 2.2 parts of 2,6-di-tertiary-butyl-4-methylphenol in sufficient acetone to provide a coatable slurry. The mixture is applied in a thin uniform layer to the surface of bond paper and the acetone removed by evaporation. The dry 60 coating weight is about one gram per square foot.

The resulting coated image sheet darkens on heating, the coating assuming a dull brownish black appearance when the sheet is heated, for example for 2 minutes at 160° C.

Increasing the amount of 2,6-di-t-butyl-4-methyl-phenol component to 12.6 parts in the above formula produces a sheet which permanently darkens to an intense black when briefly heated, for example for 6 seconds at 100° C. or on momentary contact with a metal test bar at 120° C., and 70 which therefore is effective as a heat-sensitive copy sheet in the thermographic copying of differentially radiationabsorptive graphic originals.

A light-sensitive film product is prepared by applying to one mil (.001 inch) Mylar transparent polyester film 75 a thin uniform coating of a blend of 50 parts by weight

3,218,166 ⁵ of ethyl cellulose, 1.4 parts of l-hydroxy-4-methoxynaphthalene, and 0.3 part of erythrosin B. The mixture is applied from solution in acetone. The weight of the dried coating is 0.2 gram per square foot.

The film is placed with its coated surface in contact with 5 a printed original having light-absorptive inked image areas on a reflective white paper background and which is then uniformly exposed through the film, i.e. in the reflex position, to intense illumination from a bank of tungsten filament lamps for a time just sufficient to desensitize the jq coating completely at the background areas. A typical exposure time may be 12-15 seconds. The image areas are also affected but to a significantly lesser extent due to absorption in the image areas of the graphic original of that portion of the actinic radiation not previously ab- 15 sorbed in the sensitive coating.

The exposed film is next placed with its coated surface against the coated sheet surface and the composite is heated, for example between rolls or platens, for 4-5 seconds at 125-140° C. A copy of the graphic original 20 is produced on the coated sheet. The image areas are a dense black; the background areas remain visibly unchanged.

Heating a similary exposed film in contact with a coated sheet from which the 2,6-di-t-butyl-4-methyl- 25 phenol has been omitted produces a faint and incomplete copy of the original, with brownish image areas. Films coated with much larger amounts of the l-hydroxy-4methoxynaphthalene and exposed to complete desensitization at background areas produce darker but still incom- 30 plete image areas when heated in contact with such silvercoated sheets, and require excessive exposure for desensitization.

Example 2

Essentially the same results are obtained when the phthalazinone of the foregoing example is replaced with phthalic anhydride as the toner for the silver image. More specifically, the silver soap mixture differs from that of Example 1 in substituting 2.6 parts of phthalic acid for 40 the 5.2 parts of phthalazinone, and by increasing the amount of 2,6-di-tertiary-butyl-4-methylphefflbl from 2.2 parts to 8 parts. On heating the reflex-exposed lightsensitive film in contact with the coated image paper, there is developed on the latter a blue-brown image. 45

Example 3

An image sheet prepared as in Examples 1 and 2 with a silver soap coating on a paper-like carrier may be converted into a unitary copy-sheet by the application of a 50 further coating over the silver soap layer. A mixture of 0.1 part by weight of 4-methoxy-l-naphthol, 5 parts of ethyl cellulose, and 95 parts of methanol is coated at a 3-mil orifice over the silver soap coating of an image sheet prepared as described in Example 1, and the sheet is dried, 55 forming a unitary heat-sensitive copy sheet with which copies of graphic originals may be prepared by thermographic back-print copying procedures. Such copies may be desensitized against further heat-induced visible change In the unprinted areas by exposure to ultraviolet radiation. 60 Alternatively, the sheet may first be exposed to an ultraviolet radiation image, as by exposure through a negative transparency or a stencil, and the unexposed areas then developed by heating to produce a visible pattern.