GB2044473A - Thermographic imaging sheet - Google Patents

Abstract

A multi-layer, dry, peel-apart imaging sheet consisting of backing member coated on one surface with a vaporizable dye source material. After placing the imaging sheet in contact with graphic original and exposing the combination in a thermographic copier, the vaporizable dye source material is hand peelable from the backing member to produce vaporizable dye images of the original on an opaque or transparent background.

Description

SPECIFICATION Dry, peel-apart imaging sheet This invention relates to the preparation of projection transparencies from graphic originals.

The preparation of projection transparencies has been described in Newman, U.S. Patent No. 3,147,377 (the '377 patent). The Newman '377 patent discloses the preferential transfer of dyestuff from a dye source sheet to a transparent dye receptor sheet, while the sheets are in mutual contact with each other, with the dye source sheet in contact with a graphic original having preferential radiation absorbing areas. Dyestuff is transferred from the dye source sheet to those areas of the dye receptor sheet corresponding to preferential radiation absorbing areas of the graphic orignal by brief, intense irradiation of the three sheet sandwich.

Newman et al., U.S. Patent No. 3,682,684 (the '684 patent) discloses the production of transparencies by the heat-induced reaction between silver and ferric soaps coated on a transparent film. The visible product of the silver-ferric soap reaction is formed on those portions of the transparent film corresponding to the preferential radiation absorptive areas of the graphic original.

The Newman '377 patent requires separate dye-donor sheets and dye receptor sheets, the alignment of which with the graphic original must be externally maintained during the thermographic transparency producing process.

The Newman et al. '684 patent is limited to the production of images corresponding to the color of the reaction product of the heat induced surface reaction, e.g. black, and does not employ the preferential transfer of heat vaporizable dyes.

Other commercially available systems that produce single color transparencies employ radiation induced chemical reactions to create imaged areas that are less soluble in certain solvent media. Treatment of these imaged transparencies with specific solvents leaves the less soluble imaged areas remaining on the backing material. Such solubility based systems require additional wet processing steps with loss of time and investment in additional processing equipment. The transparencies produced in some of these processes have imaged areas that can be physically removed from the transparent background material, e.g. by abrading with a fingernail.

Surprisingly in the present invention it has been found that a highly volatile, fugitive dye, may be directly coated onto a very dye receptive surface without significant dye transfer prior to thermographic imaging.

More particularly, the present invention, in one aspect is a dry, peel-apart, imaging sheet comprising a dye receptive backing member having on one surface a uniform, continuous, dry layer of vaporizable dye source material dispersed in a film forming binder, wherein said backing member and dye source material are transparent to infrared radiation and wherein said non-tacky layer is hand-peelable from said backing member. In another aspect the present invention comprises a method for producing colored visible images of a graphic original on a background from an unitary multile-layered imaging sheet.The novel multiple-layer imaging sheet of this invention typically is used by superimposing it over a graphic original having preferential infrared radiation absorbing areas, briefly exposing the imaging sheet and graphic original to intense infrared radiation, separating the imaging sheet and graphic original, and then peeling the dye source from the backing member to produce a positive image of the graphic original on the backing member. When a preferred visible light transparent backing member is employed, the transparencies produced in the practice of this invention exhibit a sharp, precise positive image of the graphic original because coating the dye source material continuously over the dye receptive backing member minimizes lateral migration of the vaporizable dye and therefore reduces blurring of images.Coating a dye source material directly onto a backing member also reduces the distance vaporized dyes must migrate thereby producing images with a high color density that are especially well suited for overhead projection transparencies. Additionally, the color of the images produced in the practice of this invention may be that of any heat vaporizable dye that is commercially available. Lastly, after exposure to radiation, e.g., in a common thermographic copier, no further processing is necessary (other than removal of the dye source layer) to produce a dry, permanent, precisely defined positive image of a graphic original on a suitable backing.

The invention will now be further described in connection with the accompanying drawings wherein like reference characters refer to the same parts throughout the several views wherein the layers are greatly exaggerated and in which: Figures 1, 2, 3 and 4 are partial crosssections of the imaging sheet of the present invention.

Figure 5 is a partial cross-section of the imaging sheet of Fig. 3 during the irradiation process.

Figure 6 is a partial cross-section showing an irradiated partially peeled imaging sheet of this invention.

Figs. 1 and 2 illustrate embodiments of the imaging sheet 11 of the present invention wherein a dye-receptive backing member 1 2 is uniformly and continuously overcoated on one surface with a non-tacky layer of vaporiza ble dye source material 1 3. "Dye-receptive" as used herein means that the surface of the backing member is capable of adhering to surface deposits of vaporizable dyes as well as permitting dye to permeate into and be permanently retained thereby. In Fig. 2 the dye receptivity of the backing member 1 2 has been increased by means of a thin layer of dye receptive material 1 7 that strongly adheres to the backing member 1 2 and minimally adheres to the dye source material 1 3.

"Non-tacky layer" as used herein means a layer which is capable of releasing the graphic original without damage after being in surface-to-surface contact with the original during imaging in a thermographic copier. In these embodiments, a graphic original 1 9 (as in Fig.

5) having preferential radiation absorbing areas 21 is placed in contact with the non-tacky layer of dye-source material 13, the cqmbina- tion is irradiated or imaged through backing member 12, causing vaporizable dye to migrate from layer 1 3 into the dye-receptive backing member 1 2. After imaging, layer 1 3 is peeled from backing member 1 2 as shown by arrow 16, leaving a positive image of the graphic original on backing member 12.The backing member 1 2 for some applications may be opaque, i.e., not transparent to visible light It is required of the backing member material that it be transparent to infrared radiation to such an extent that only the infrared-absorbing image areas of the original become heated and thereby cause the vaporizable dyes to be preferentially transferred to backing 12.

Figs. 3 and 4 illustrate multi-layer embodiments of the present invention comprising dye-receptive backing members having on one surface a uniform continuous layer or overcoating of a vaporizable dye source material dispersed in a film forming binder, said dye source material being uniformly overcoated on its remaining surface with a non-tacky protective layer, wherein the protective layer and dye source layer are both hand-peelable from the backing member and all three layers, (i.e., backing member, dye source and protective) are infrared transparent. "Protective" as used herein means that the layer to inhibit the permeation of vaporized dye therethrough. A non-tacky, protective layer is included so that an image of a graphic original may be produced without damage to the original (i.e., without transfer of dye to the original).After imaging, the imaging sheet is removed from the original, the original being usable to produce more images. In applications where the graphic original is expendable a very thin protective layer may be employed, or the protective layer may be left off the imaging sheet completely.

Fig. 3 is a three layer embodiment of the imaging sheet 11 of the present invention wherein an infrared transparent dye receptive backing member 1 2 is overcoated on one surface dye receptive backing member 1 2 is overcoated on one surface with a preferred dye source material 1 3 comprising a rubber base material in which is dispersed particulate, vaporizable dye 14. The dye source material is overcoated on its remaining surface with a protective coating 1 8 which serves to protect the graphic original over which it is superimposed during the imaging process. After imaging the second and third layers are peeled from the first layer as shown by arrow at 16.

Fig. 4 is a four layer embodiment of the imaging sheet of the present invention where the dye receptivity of the backing member 1 2 has been increased with a coating of dye receptive materials 1 7 that strongly adheres to the backing member 12 and only minimally adheres to the dye source material 1 3.

In this embodiment, separation of the dye source material 13 and non-tacky layer 18 from the backing member 1 2 leaves the dye receptive coating 17 adhering to the backing member 12.

Fig. 5 depicts the imaging sheet of Fig. 3 in contact with a graphic original 1 9 having preferential radiation absorbing areas 21. The graphic original 19 may be opaque or transparent because the imaging sheet is exposed to irradiation from above the graphic original, indicated here by arrows 22. After irradiation the graphic original 1 9 and imaging sheet 11 are easily separated.

Fig. 6 depicts the dye source layer 1 3 and protective coating 18 partially removed from the backing member. Irradiation of the imaging sheet 11 in contact with a graphic original 1 9 causes the vaporizable dye to migrate from those areas 1 5 of the dye source layer 13 corresponding to the radiation absorbing areas of the graphic original 21 and to be deposited in the dye receptive backing member 12 in those areas 23 corresponding to the radiation absorbing areas 21, of the graphic original 23. Separation of layers 13 and 12 is preferably conducted by peeling layers 1 3 and 1 8 from backing 1 2 in the manner shown in Fig.

6.

The imaging sheet 11 of this invention comprises a first layer backing member of a transparent material 1 2. The backing member may bary in thickness from about .002 cm to about .03 cm, with a thickness of about .006 to .01 cm being preferred.

Typically, backing 1 2 is a flexible film such as polystyrene, polyester, cellulose acetate propionate, polyvinyl chloride or polycarbonate. It is required of backing member films that they be transparent to infrared radiation, although, as noted above, backing members may be opaque to visible light.

A preferred backing member for use in the practice of this invention consists of a heat resistance transparent film base such as Eastman Kodak Company's TA 401 triacetate, commercially available under the trade name "Kodacel." Another preferred backing member comprises a conventional polyester film carrying a thin continuous surface coating of a dye-receptive material 1 7 e.g., a vinyl resin "Vinylite VYNW" containing a very small proportion, e.g. two to three percent, of nickel octoate, or analogous coatings described, e.g.

in Evensen U.S. Patent No. 3,906,138, incorporated herein by reference. Such a dyereceptive coating preferably strongly adheres to the backing member, is receptive to and permanently retains heat-vaporizable dyes, and does not excessively adhere to the vaporizable dye source material so as to inhibit easy removal of the dye source material after thermographic imaging of the sheet with an original.

The dye source material or layer 1 3 of the imaging sheet of this invention may contain any heat vaporizable dye in a film forming binder or medium which readily liberates dye to the dye receptive layer upon being heated in areas corresponding to the preferential radiation absorption areas of a graphic original placed thereagainst in accordance with the manner of this invention. Rubber binders or media are found particularly effective in the practice of this invention and are preferred.

Useful types of rubber in this preferred embodiment of the invention include all natural rubbers, acrylic polymers, butadiene styrene copolymers, polybutene rubbers and other well known rubbers which are soluble in those aliphatic hydrocarbon solvents have a boiling point below about 80"C.

The tackiness of the preferred rubber-based dye source material of this invention may be controlled by the inclusion of varying amounts of tactifying resins, e.g., a polyterpene resin.

Tactifying resins present in an amount in the range .15 weight percent to 1.5 weight percent, with .2 weight percent to .6 weight percent being preferred, (weight percent of the wet dye source layer as coated) have been found to produce a desirable tack. Useful tactifying resins include resin esters commercially available from Reichhold Chemicals under the trademark "Ester-Gum" and ethylene glycol ester of rosin commercially available from the Hercules Powder Company, under the trade name "Flexalyn." Plasticizers may also be mixed with the rubbers of the preferred embodiment of this invention. To be efficacious, such plasticizers should only sparingly dissolve the vaporizable dye and preferably do not dissolve the dye at all.Representataive plasticizers include castor oil, lanolin and paraffin oil, and representative amounts of plasticizers fall in the range of about .2 weight percent to 3.0 weight percent with .4 weight percent to 1.5 weight percent being preferred (weight percent of the wet dye source layer as coatedd).

A filler is preferably added to the rubberbased dye material to improve dispersing of the dye and to separate the dye particles in the dye source layer and thereby produce a more uniform color distribution in the transparency. Cellulosic wood fiber, e.g., "SoKafloc BW-200," having an average major dimension of about 1 5 to 35 ym (commercially available from the Brown Company), has been found particularly useful in that it is lighter than other fillers, e.g., clay or minerals, and has the desirable quality of not absorbing-the dyestuff. In place of the cellulosic wood fiber, it is possible to use a filler comprising quartz particles having an average size no greater than about 35 jum (preferably less than 10.

zbm). Penn. Glass Company's quartz particles sold under the trademark "Min-U-Sil" is representative of such materials.

Dyes which are found useful in the practice of this invention are those which are heatvolatilizable (i.e. vaporizable in the temperature range of about 70"C to about 100"C).

Useful representative dyes include DuPont Oil Blue or Oil Blue A, Calco Oil Yellow EM (available from American Cyanamid), Amaplast Red AAP (C.l. 60505), Intratherm Red (available from Crompton and Knowles) 4 tricyanovinyl-N, N-diethylaniline, and

Each of these dyes sublimes at a temperature in the range of about 70"C to 100"C. Mixtures of dyes may be used where individual heat-volatilizable dyes of desired color are not available. Other conventional vaporizable dyes may also be used, of course.

The dye component in the preferred rubber binder remains a largely, if not wholly, undissolved heterogeneous mixture and is present in the rubber based dye source material in the form of finely divided discrete porticles or crystals. As a result, the color of the rubber binder dye source material when coated onto the dye receptive backing member does not necessarily represent the color of the image formed therefrom; for example, a sheet prepared with DuPont Oil Blue has a dull greenish tinge, but produces a clear intense bright blue image by thermographic transfer of the dye to the dye-receptive backing material.

Inclusion of a small amount, e.g., 4 weight percent of the coat with which it is applied, of TiO2 in either the dye source material, or the dye restrictive, protective, layer, tends to reduce the apparent color difference between imaging sheet and the image produced.

The dye restrictive protective layer 1 8 of the imaging sheet of this invention prevents the dye source material and the graphic original from adhering to each other, especially in those areas of the graphic original that are preferentially radiation absorbing. This also inhibits migration of the heat vaporizable dye from the dye source material into the graphic original causing discoloration of the original.

Representative materials found useful in the protective layer of this invention include a styrene-ethylene-butylene-styrene block polymer commercially available from Shell Chemical Co., under the trademark "Kraton G-1 652." Such a dye restrictive material may be mixed with about 2-10% by weight (of the wet protective layer coating material) of a filler, e.g., "Min-U-Sil," to aid the release of the imaging sheet from paper printed graphic originals and to reduce self-adhesion while being peeled from the backing member.

The graphic original which is to be imaged in the practice of this invention must have infrared radiation absorbing areas, e.g. letters, numbers or figures, that are more absorptive of infrared radiation than the material upon which they appear, e.g., lightly colored paper.

The greater the contrast between the preferentially absorptive areas and the background on which they appear, the more efficiently the vaporizable dye is transferred from the dye source material to the dye receptive backing member upon exposure to infrared radiation.

Transfer of the heat vaporizable dye from the dye source layer to the dye receptive layer is induced by brief, intense iradiation of the imaging sheet which in contact with the graphic original. The radiation time and intensity should be sufficient to maximally transfer the hear vaporizable dye from the dye source layer to the dye receptive layer in those areas of the receptor layer corresponding to the preferentially radiation absorbing areas of the graphic original, with little or no dye transfer in those areas corresponding to non-preferential radiation absorbing areas of the graphic original. Representative of a preferable radiation time and intensity is the exposure obtained in a conventional office thermographic copier (e.g., "Secretary," available from Minnesota Mining and Manufacture Company).

In the preferred embodiment, irradiation occurs from above the plane of the imaging sheet and passes through the imaging sheet from the uncoated side of the transparent backing member, is absorbed by the preferential absorbing areas of the original and causes vaporization and migration of the dye from the dye source layer to the dye receptive layer. In this embodiment the graphic original may be opaque or transparent to visible light there being no necessity for the graphic original to be transparent or translucent.

The following specific Examples with proportions given in parts by weight, unless otherwise indicated, will further illustrate the practice of this invention which, however, should not be construed as limited thereto.

EXAMPLE 1 A conventional transparent polyester film .004 in. (.0102 cm) thick is uniformly coated with a material of the following composition: tetrahydrofuran 84.7 butyl alcohol 2.3 nickel oleate 1.0 vinyl resin 12.0 ("Vinylite VYNW") This vinyl resin coat increases the dye receptivity of the polyester film. The vinyl resin is coated by means of a reverse roll coater, a knife coater, or rotogravure, to a dry weight of 0.55 g/ft.2 (5.9 g/m2) to .75 g/ft.2 (8.07 g/m2).

Summarizing the preparation of the preferred dye source material of this invention, a binder composition is prepared (designated "Binder Composition A") which is then intimately mixed with a dye concentrate, (designated "Dye Concentrate B"). Preferably, the Binder Composition A and Dye Concentrate B are mixed together with volatile carriers and then uniformly coated onto the backing member previously coated with the above-described dye-receptive material.

The composition of Binder Composition A is as follows: Parts Crepe rubber 6.72 Polyterpene resin (Piccolyte S-11 5") .74 Antioxidant (Plastinox 2246") .22 Mineral Oil American White Oil #31) -plasticizer .34 Volatile solvent (heptane) 90.72 Liquifier (ethyl alcohol) 1.56 The rubber is milled sufficiently to form a smooth bank on the rolls and is then dissolved, together with the resin, antioxidant and oil, in the heptane. The alcohol is subsequently added to reduce the viscosity of the solution.

The composition of Dye Concentrate B is as follows: Parts Binder Composition A 32.67 Dye (e.g., blue, indoaniline) 8.67 Ground glass ("Min-U-Sil") 4.33 Volatile Solvent (Heptane) 54.33 The several components are mixed together in a ball mill until complete mixing is attained.

The dye source layer coating composition is a mixture of the following: Parts Dye Concentrate B 43.76 Binder Composition A 24.35 Lanolin .77 Volatile solvent (heptane) 28.1 2 The coating composition is applied by spreading a uniform layer on the polyester backing member and rapidly drying the coated composite in a current of warm air The coating weight after drying was .65 g/sq.ft. (6.99 g/m2). The coating weight of the dye source material may vary between about .25 and .85 g/sq.ft. (2.79 g/sq.mito 9.1 g/sq.m).

Other useful volatile solvents which do not solubilize the dye include perchloroethylene, hexane and VMP naphtha.

One embodiment of the non-tacky protective layer of the invention has a composition as follows: Parts "Kraton G-1652" 14.38 Ground Glass ("Min-U-Sil") 4.16 Volatile Solvent (heptane) 81.46 "Kraton G-1652" is a ene-styrene block copolymer commercially available from the Shell Chemical Company.

To make the color of the imaging sheet co re- spond more closely to the color of the ima es produced, about 4 parts of TiO2, ZnO or CaCO2 may be added to the composition for the protective layer above. A small amount of the vaporizable dye attaches to these materials and conveys a more accurate reflection color of the images produced by the imaging sheet.

The dry coating weight of this protective layer may vary from about .5 g/ft2 to 1.0 g/ft2 (5 g/m2 to 10.8 g/m2).

The multiple layer imaging sheet is then placed over and in contact with an opaque or transparent graphic original (having infrared absorptive image areas) with the non-tacky protective layer in contact with the original as shown in Fig. 3. The imaging sheet and graphic original is put through a thermographic copier, e.g. "Secretary," at a belt speed of 1.2 in/sec. and a temperature of 235"F (110"C). The dye source layer and non-tacky layer are then peeled from the polyester film backing leaving a positive, colored image of the graphic original on a transparent background.

EXAMPLE 2 A polyester film is coated with a vinyl resin, as in Example 1 to increase its dye receptivity.

The resulting vinyl resin coated polyester film is coated with an intimate mixture of the following composition: Parts "Kraton G-1652" 12.10 "Min-U-Sil" 3.83 Dye (blue, indoaniline) 1.50 Lanolin .30 Heptane 82.27 The resulting two layer imaging sheet is capable of producing an image of a graphic original on the backing members with minimum color density. The ground glass "Min-U Sil" acts to reduce the sticking of the original to the imaging sheet during imaging.

Various modifications and alterations of the invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be limited to the illustrative embodiments and examples set forth herein.

Claims (20)

1. A dry, peel-apart, imaging sheet comprising a dye receptive backing member having on one surface a uniform, continuous, non-tacky layer of vaporizable dye source material dispersed in a film forming binder, wherein said backing member and dye source material are transparent to infrared radiation and wherein said non-tacking layer is handpeelable from said backing member.

2. A multi-layered dry, peel-apart, imaging sheet comprising a dye receptive backing member having on one surface a uniform, continuous non-tacky layer of a vaporizable dye source material dispersed in a film forming binder, said dye source material being uniformly overcoated with a protective layer wherein said backing, dye source and protective layers are infrared transparent and said non-tacky layer and said protective layer are hand-peelable from said backing member.

3. An imaging sheet in accordance with Claim 2 wherein said backing member comprises a visibly transparent flexible film.

4. An imaging sheet in accordance with Claim 1 wherein said backing member is dyereceptive by reason of a thin layer of dye receptive material coated on said backing member surface which is over coated with said non-tacky layer of dye source material.

5. An imaging sheet in accordance with Claim 4 wherein said dye receptive material comprises a copolymer of vinyl acetate and vinyl chloride.

6. An imaging sheet in accordance with Claim 2 wherein said backing member is dye receptive by reason of a thin layer of dye receptive material coated on said backing member surface which is overcoated with said dye source material.

7. An imaging sheet in accordance with Claim 6 wherein said dye receptive material comprises a copolymer of vinyl acetate and vinyl chloride.

8. An imaging sheet in accordance with Claim 2 wherein said dye source materal comprises a rubber binder in which is dis persed particulate volatilizable dye and a parti culate filler which is non-absorptive of said dye, wherein said particulate filler has a major dimension no greater than about 35 pm.

9. An imaging sheet in accordance with Claim 8 wherein said particulate filler is se lected from the group consisting of cellulosic wood fiber or quartz particles.

10. An imaging sheet in accordance with Claim 8 wherein said dye is present in an amount of about 10 to 35% by weight based on the weight of said dye source material.

11. An imaging sheet in accordance with Claim 8 wherein said rubber comprises natu ral rubber and there is included in said dye source material a polyterpene resin in an amount no greater than about 10% by weight based on the weight of said layer.

12. An imaging sheet in accordance with Claim 8 wherein said rubber binder is present in an amount of about 20% to 50% by weight, said dye is present in an amount of about 10% to 35% by weight, and said particulate filler is present in an amount of about 10% to 60% by weight, based on the weight of said nontacky layer.

1 3. An imaging sheet in accordance with ,,,Claim 2 wherein said protective layer com prises styrene-ethylene-butylene-styrene block polymer.

14. A peel-apart thermographic imaging sheet comprising a visibly transparent, vaporizable, dye receptive backing member, a dye source layer comprising a rubber binder in which is dispersed particulate, volatilizable dye, a particulate filler comprising quartz particles of major dimension no greater than about 35 jLtm said particulate filler being non-absorptive of said dye, wherein said dyesurce layer is coated continuously over one surface of said backing member, and a non-tacky, protective layer, said protective layer comprising a quartz particulate filler and a vinyl resin wherein said protective layer is coated continuously over said dye-source layer, wherein said dye-source layer and said protective layer are hand-peelable from said backing member and wherein said backing member, dye-source layer and protective layer are infrared transparent.

1 5. A method for producing positive, colored images of a graphic original having differentially radiation-absorptive areas comprising: superimposing the imaging sheet of Claim 1 over said graphic original to form a couplet; exposing said couplet to infrared radiation for a time sufficient to cause vaporizable dye to transfer to said dye-receptive backing member in areas corresponding to radiation absorptive areas in said original; removing said dye-source layer and said protective layer from said backing member of said imaging sheet.

1 6. A method for producing positive, colored images of a graphic original having differentially radiation-absorptive areas comprising: superimposing the imaging sheet of Claim 2 over said graphic original to form a couplet; exposing said couplet to infrared radiation for a time sufficient to cause vaporizable dye to transfer to said dye-receptive backing member in areas corresponding to radiation absorptive areas in said original; removing said dye-source layer and said protective layer from said backing member of said imaging sheet.

1 7. A dry, peel-apart, imaging sheet substantially as herein described with reference to the accompanying drawings.

1 8. A dry, peel-apart, imaging sheet substantially as herein described with reference to the Examples.

19. A method substantially as herein described for producing positive, coloured images of a graphic original.

20. Any novel element, or combination of elements, herein described and/or shown in the accompanying drawings, irrespective of whether the present claim is within the scope of, or relates to the same invention as, any of the preceding claims.