PROTECTED IMAGE

Abstract: A binary image (10b) comprising a plurality of first areas, at which a porous or particulate imaging material is adhered to a substrate (20), and a plurality of second areas, at which the substrate (20) is free from the imaging material, is protected by laminating thereto a laminating sheet (30) comprising a durable layer (34) and a support layer (38) with the durable layer (34) facing the image (10b) and at least one portion of the laminating sheet (30) extending beyond the periphery of the substrate (20), so that the durable layer (34) adheres to both the first and second areas of the image (10b). The support layer (38) is then displaced away from the image (10b) such that the durable layer (34), remains attached to the image, but in the portion of the laminating sheet (30) extending beyond the substrate (20) the durable layer (34) remains attached to the support layer (38) so that the durable layer (34) breaks substantially along the periphery of the substrate (20). The image (10b) remains covered with a durable layer (34) which: a) is substantially transparent; b) has an abrasion resistance of at least 10 cycles of 10 Newton force as measured by an Erikson Abrasion Meter and a critical load value of at least 100 grams as measured by ANSI PH1.37-1983; and c) is not removed from the image by contact with adhesive tape having an adhesion to steel of 33 grams per millimeter as measured by ASTM D-3330.

Description: PROTECTED IMAGE This invention relates to a protected image and a process for the production of such an image. International Patent Application No. PCT/US87/03249 (Publication No. WO 88/04237) describes a thermal imaging medium and a process for forming an image in which a layer of a porous or particulate imaging material (preferably, a layer of carbon black) is deposited on a heat-activatable image-forming surface of a first sheet-like or web material (hereinafter the "first sheet element"), the layer having a cohesive strength greater than its adhesive strength to the first sheet-like element. Portions of this thermal imaging medium are then exposed to brief and intense radiation (for example, by laser scanning) , to firmly attach exposed portions of the imaging material to the first sheet element. Finally, those portions of the imaging material not exposed to the radiation (and thus not firmly attached to the first sheet element) are removed, thereby forming a binary image comprising a plurality of first areas where the imaging material is adhered to the first sheet-like element and a plurality of second areas where the first sheet-like element is free from the imaging material. In a preferred embodiment of the imaging medium described in the aforementioned International Application, the imaging material is covered with a second laminated sheet-like element so that the imaging material is confined between the first element and this second element. After imaging and separation of the second element (with the unexposed portions of the imaging material) from the first element, a pair of images is obtained. A first image comprises exposed portions of image-forming substance more firmly attached to the £irst element by heat activation of the heat- activatable image-forming surface. A second image comprises non-exposed portions of the image-forming substance carried or transferred to the second sheet element. The respective images obtained by separating the sheets of an exposed thermal imaging medium having an image-forming substance confined therebetween may exhibit substantially different characteristics. Apart from the imagewise complementary nature of these images and the relation that each may bear as a "positive" or "negative" of an original, the respective images may differ in character. Differences may depend upon the properties of the image-forming substance, on the presence of additional layer(s) in the medium, and upon the manner in which such layers fail adhesively or cohesively upon separation of the sheets. Either of the pair of images may, for reasons of informational content, aesthetics or otherwise, be desirably considered the principal image, and all of the following discussion is applicable to both types of image. The image-forming process described in the aforementioned International Application is capable of producing high quality, high resolution images. However, the images produced by this process may suffer from low durability because, in the finished image, the porous or particulate imaging material, which is typically carbon black admixed with a binder, lies exposed on the surface of the image, and may be smeared, damaged or removed by, for example, fingers or other skin surfaces (especially if moist) , solvents or friction during manual or other handling of the image. It is known to protect various types of images by laminating transparent films over the image. For example, U.S. Patent No. 4,921,776 describes a method of providing a lower gloss protective covering for a pre¬ press color proof. This method comprises laminating to the image surface a thin, substantially transparent integral polymeric film consisting essentially of a mixture of at least two slightly incompatible polymers, whereby the film exhibits a 20° specular gloss that is at least 5% lower than the gloss of a film prepared from any one of the polymer constituents. U.S. Patent No. 4,902,594 describes a photoimaged article having a protected image composed of a colored image on a support, and a thin, transparent, flexible, non-self supporting, protective layer on the surface of the image. The layer is substantially non-tacky at room temperature, and has at least a major amount based on the weight of the layer of one or more thermoplastic resins of a vinyl acetal, vinyl chloride, or acrylic polymer or copolymer having a T_ of from about 35°C. to about 110°C. The layer is capable of being adhesively transferred directly to the image when the layer is first applied on the release surface of a temporary support, and the image and protective layer are laminated together under pressure at temperatures of between about 60°C. to about 180°C. with subsequent removal of the temporary support. U.S. Patent No. 4,719,169 issued Jan. 12, 1988 describes a method for protecting an image which comprises providing a colored image on a substrate and either: A. applying an antiblocking layer to a release surface of a temporary support; bonding a thermoplastic adhesive layer to the antiblocking layer; laminating the applied support to the colored image via the adhesive; and peeling away the temporary support from the antiblocking layer; or B. applying a thermoplastic adhesive layer to a release surface of a first temporary support; applying an antiblocking layer onto a release surface of a second temporary support, laminating the adhesive onto the colored image and peeling away the first temporary support; and laminating the antiblocking layer onto the adhesive layer and peeling away the second temporary support; wherein the adhesive layer is substantially non-tacky at room temperature, is laminated at temperatures of about 60°C to 90°C, and comprises one or more thermoplastic polymers or copolymers; and the antiblocking layer comprises one or more organic polymers or copolymers, which layer does not cohesively block at about 50°C. or less. The intended use of this invention is to protect color proofs used in the graphic arts industry. The protection of an image produced by the process described in the aforementioned International Application presents peculiar difficulties. Unlike images produced by silver halide processes, a differential adhesion image is three-dimensional, with areas of the imaging material protruding above the sheet element to which it is attached (hereinafter referred to as the "substrate") , and the surface characteristics of the imaging material are typically very different from those of the substrate. (If the imaging medium contains a release layer, as described in the aforementioned International Application, in some cases the areas of the image which are not covered by imaging material may have a surface formed of the release layer. Typically, the surface characteristics of this release layer are very different from those of a carbon black imaging material.) Furthermore, the porous or particulate imaging material used is typically more friable than, for example, printing ink, and thus more susceptible to abrasion, smearing and other deformation. Also, a differential adhesion image typically extends close to the periphery of the substrate, so that it is necessary that any protective layer applied to protect the image also extend to the periphery of the substrate; on the other hand, both for aesthetic reasons and for ease of handling, surplus protective layer should not extend beyond the periphery of the substrate, and the process for applying the protective layer should not require elaborate procedures for registering the protective layer with the image. This invention provides a process for protecting an image which can enable such an image to withstand the handling to which it may be subjected in a variety of uses. Accordingly, this invention provides process for protecting an image, this process comprising providing a laminating sheet; placing the laminating sheet in contact with the image; and laminating the laminating sheet to the binary image. The process of the present invention is characterized in that: the binary image comprises a plurality of first areas at which a porous or particulate imaging material is adhered to a substrate and a plurality of second areas at which the substrate is free from the imaging material; the laminating sheet comprises a durable layer and a support layer, and is placed in contact with the image such that the durable layer faces the image and at least one portion of the laminating sheet extends beyond the periphery of the substrate; the laminating sheet is laminated to the binary image so that the durable layer adheres to both the first and second areas of the image; and the support layer is separated from the image such that the durable layer remains attached to the image, but, in the at least one portion of the laminating sheet, the durable layer remains attached to the support layer so that the durable layer breaks substantially along the periphery of the substrate, thereby covering the image with a durable layer, the layer: a. being substantially transparent; b. having an abrasion resistance of at least 10 cycles of a 10 Newton force as measured by an Erikson Abrasion Meter and a critical load value of at least 100 grams as measured by ANSI PHI.37-1983; and c. not being removed from the image by contact with adhesive tape having an adhesion to steel of 33 grams per millimeter, as measured by ASTM D-3330. In a preferred form of the process of the present invention, the image is produced by a process comprising: providing a layer of a porous or particulate imaging material on a heat-activatable image-forming surface of a first sheet-like element, this layer of imaging material having a cohesive strength greater than the adhesive strength between this layer and the first element; providing a second sheet-like element on the opposed side of the layer of imaging material from the first element, the imaging material having an adhesion to the second element greater than its adhesion to the first element, thereby providing a thermal imaging medium; imagewise subjecting portions of the thermal imaging medium to exposure of brief and intense radiation, thereby firmly attaching exposed portions of the imaging material to the first element; separating the first and second elements, thereby leaving those portions of the image-forming substance not exposed to the radiation attached to the second element and those portions of the image-forming substance exposed to the radiation attached to the first element, and thereby forming a pair of images on the first and second elements, each of these images comprising a plurality of first areas at which the imaging material is adhered to the first or second element and a plurality of second areas at which the first or second element is free from the imaging material. Either or both of the resultant images may be protected by the present process. This invention also provides a binary image comprising a plurality of first areas at which a porous or particulate imaging material is adhered to a substrate and a plurality of second areas at which the substrate is free from the imaging material. The binary image of the present invention is characterized by a durable layer covering the image and adhered to both the first and second areas thereof, the durable layer: a. being substantially transparent; b. having an abrasion resistance of at least 10 cycles of a 10 Newton force as measured by an Erikson Abrasion Meter and a critical load value of at least 100 grams as measured by ANSI PHI.37-1983; and c. not being removed from the image (10b) by contact with adhesive tape having an adhesion to steel of 33 grams per millimeter, as measured by ASTM D-3330. Figure 1 of the accompanying drawings shows in section a thermal imaging medium of the type described in the aforementioned International Application; Figure 2 shows a section, similar to that of Figure 1 through the medium as the first and second elements thereof are being separated to form a pair of complementary binary images; Figure 3 shows a section through one of the binary images formed in Figure 2 and a laminating sheet useful in the process of the present invention; Figure 4 shows in section the image and laminating sheet shown in Figure 3 laminated together; Figure 5 shows in section the image and laminating sheet shown in Figures 3 and 4 as the support layer is being separated from the image; Figure 6 shows in section the protected image produced after complete removal of the support layer; and Figure 7 shows a schematic side elevation of an apparatus useful for carrying out the process of the invention. As already mentioned, in the process of the present invention, a binary image comprising a plurality of first areas, at which a porous or particulate imaging material is adhered to a substrate, and a plurality of second areas, at which the substrate is free from the imaging material, is protected using a laminating sheet comprising a durable layer and a support layer. The laminating sheet is placed in contact with the image with the durable layer facing the image, and with one or more portions of the laminating sheet extending beyond -9- the periphery of the substrate. The laminating sheet is laminated to the image, and then the support layer is separated from the image so that the durable layer remains attached to the image, but where a portion of 5 the laminating sheet extended beyond the substrate, the excess durable layer remains attached to the support layer so that the durable layer breaks substantially along the periphery of the substrate. The image is thus protected by the overlying durable layer, which has the 10 characteristics mentioned in paragraphs a) to c) above. These characteristics ensure that the durable layer does not interfere with viewing of the image, and that the image is rendered sufficiently durable for practical purposes. 15 The substrate of the image may be opaque or transparent, that is the image may be intended to be viewed by either reflection or transmission.The substrate may be any of the substrates conventionally used in the formation of images. Thus, for example, if 20 the image is intended to be viewed by reflection, the substrate may be formed from paper or a similar material. The substrate is typically a plastic web having a thickness of from about 1 to about 1000 μm, and preferably about 25 to about 250 μm. As is well-known 25 to those skilled in the imaging art, the substrate may carry one or more sub-coats or be subjected to surface treatment to improve the adhesion of the imaging material to the substrate. Materials suitable for use as the substrate include polystyrene, polyester, 30 polyethylene, polypropylene, copolymers of styrene and acrylonitrile, poly(vinyl chloride) , polycarbonate and poly(vinylidene chloride) . An especially preferred web material from the standpoints of durability, dimensional stability and handling characteristics is poly(ethylene terephthaiate) , commercially available, for example, under the tradename Mylar, of E. I. duPont de Nemours & Co., or under the tradename Kodel, of Eastman Kodak Company. The imaging material typically comprises a porous or particulate colorant material admixed with a binder, the preferred colorant material being carbon black, although other optically dense colorants, for example graphite, phthalocyanine pigments and other colored pigments may be used. The binder may be, for example, gelatin, poly(vinyl alcohol) , hydroxyethylcellulose, gum arabic, methylcellulose, polyvinylpyrrolidone or polyethyloxazoline. The durable layer of the laminating sheet may be formed from any material which confers the desired properties upon the durable layer formed on the image. In general, it is preferred that the durable layer on the image not have a thickness greater than about 30 μm, since thicker durable layers may, in some cases, cause optical problems in viewing the image due to internal reflections and/or refraction effects within the durable layer. Desirably the thickness of the durable layer does not exceed lOμm, and most desirable this thickness is in the range of 2 to 6 μm. The durable layer should of course be resistant to materials with which it is likely to come into contact, including materials which may be used to clean the image. Although the exact materials which may contact the image will vary with the intended use of the protected image, in general it is desirable that the material for the durable layer be substantially unchanged by contact with water, isopropanol and petroleum distillates. Preferably the durable layer should be resistant to any other materials with which it may come into contact, for example accidental spills of coffee, which have a very deleterious effect on some plastics. It has been found that the protection of the image conferred by the durable layer is increased when the durable layer has high lubricity. Preferably, at least one of a wax, a solid silicone and a silicone surfactant is included in the durable layer to increase the lubricity of this layer. In the present process, the laminating sheet extends beyond the periphery of the substrate at one or more points, and the "excess" durable layer extending beyond the periphery of the substrate remains attached to the support layer, so that the durable layer breaks substantially along the periphery of the substrate; in practice, one normally uses a laminating sheet larger in both dimensions than the substrate of the image to be protected, and arranges the laminating sheet so that it extends beyond the periphery of the substrate all around the substrate, since this avoids any need to achieve accurate registration of the laminating sheet with the image and also ensures that no part of the image goes unprotected. To ensure that the durable layer breaks accurately along the periphery of the substrate, thereby providing a sharp edge on the protected image, the durable layer may comprise a continuous phase and a particulate solid dispersed in the continuous phase, since the presence of such a solid provides failure nuclei and thus assists accurate breakage of the durable layer. A preferred particulate solid for this purpose is magnesium silicate. The support layer of the laminating sheet may be formed from any material which can withstand the conditions which are required to laminate the laminating sheet to the image and which is sufficiently coherent and adherent to the durable layer to permit displacement of the support layer away from the image after lamination, with removal of those portions of the durable layer which extend beyond the periphery of the substrate. Typically, the support layer is a plastic film, and polyester (preferably poly(ethylene terephthalate) ) films are preferred. If desired, the support layer may be treated with a subcoat or other surface treatment, such as will be well-known to those skilled in the coating art, to control its surface characteristics, for example to increase or decrease the adhesion of the durable layer or other layers (see below) to the support layer. The laminating sheet may comprise additional layers besides the durable layer and support layer. For example, the laminating sheet may comprise a release layer interposed between the durable layer and the support layer, this release layer being such that, in the areas where the durable layer remains attached to the image, separation of the durable layer from the support layer occurs by failure within or on one surface of the release layer. The release layer is preferably formed from a wax, or from a silicone. As will be apparent to those skilled in the art, in some cases part or all of the release layer may remain on the surface of the durable coating after the support layer has been removed therefrom, and if a radiation-sensitive material is to be exposed through the protected image (see below) , care must of course be taken to ensure that any remaining release layer on the protected image does not interfere with such exposure. The laminating sheet may also comprise an adhesive layer disposed on the surface of the durable layer remote from the support layer so that, during the lamination, the durable layer is adhered to the image by the adhesive layer. Some durable layers can be satisfactorily laminated to an image simply by application of heat and/or pressure during the lamination step. In other cases, however, the use of an adhesive layer is desirable to achieve strong adhesion between the durable layer and the image, and/or to lower the temperature needed for lamination. Various differing types of adhesive may be used to form the adhesive layer; for example, the adhesive layer might be formed from a thermoplastic adhesive having a glass transition temperature in the range of about 50° to about 120°C (in which case the lamination is effected by heating the adhesive layer above its glass transition temperature) , an ultraviolet curable adhesive (in which case the lamination is effected by exposing the adhesive layer to ultraviolet radiation, thereby curing the adhesive layer) , or a pressure sensitive adhesive having an adhesion to steel of about 22 to about 190 grams per millimeter (in which case the lamination is effected simply by pressure) . The durable layer formed on the image must adhere sufficiently to the image that it is not removed therefrom by contact with adhesive tape having an adhesion to steel of 33 grams per millimeter, as measured by ASTM D-3330. Desirably, the durable layer is not removed from the image by contact with adhesive tape having an adhesion to steel of 44 grams per millimeter, as measured by the same standard; specific preferred processes of the invention can produce durable layers which are not removed from their images by adhesive tape having an adhesion to steel of at least 78 grams per millimeter, as measured by this standard. The properties of the durable layer formed on the image are not necessarily the same as those of the durable layer in the laminating sheet, since the physical and/or chemical properties of the durable layer may be changed during the lamination step. For example, the durable layer of the laminating sheet may comprise a latex having a plurality of discrete particles which coalesce during the lamination, thereby forming a continuous durable layer on the image. The images protected by the process of the present invention may be of various types. For example, the present process is convenient for protecting radiographs, CAT scans, ultrasonograms and similar medical images. In many cases, the medical personnel using such images will desire to view them on conventional lightboxes, to which the images will be fixed with heavy metal clips. Accordingly, in this application it is important that the durable layer be able to withstand repeated affixation to a lightbox by means of such clips. Also, since medical personnel are accustomed to washing conventional X-ray films and similar images with isopropanol, the durable layer should be chosen to have good resistance to this solvent. Another important application of the present process is the production of printing plates, pre-press proofs and contact duplicates. In the printing industry, it is conventional practice to form images of originals on transparency imaging film (a single image for monochrome printing, or a series of color separations for color printing) and then to prepare a printing plate to exposing a radiation-sensitive material through the transparency imaging film. The process of the aforementioned International Application is highly suitable for forming the transparency film images, but tends to produce images which are insufficiently durable to meet the needs of the printing industry. Conventional practices in the printing industry make stringent demands upon transparency film images. The image must of course have high optical clarity so that exposure of a printing plate can be effected through the image. The need for exposure of the radiation-sensitive material through the film also requires that the thickness of the film be limited, since too thick a film reduces the resolution which can be achieved in the final printing plate. The transparency film image must have good abrasion resistance so that it can withstand being pressed against the radiation-sensitive material, removed therefrom, stored for an extended period and then reused for making another printing plate. The transparency film image must also have non-blocking properties. When the protected image is to be used for exposing a radiation-sensitive material, desirably the formation of the durable coating over the image does not increase the transmission density of the second (white) areas by more than about 0.03 in the visible range (and in the ultraviolet, if the exposure is to be effected with ultraviolet radiation) . The substrate material must of course be chosen to transmit the radiation used to expose the radiation-sensitive material; in particular, in many commercial applications it may be necessary that the substrate transmit near ultraviolet radiation in the wavelength range of about 300 to about 400 nm. When a protected image is used to expose a radiation-sensitive material, the durable layer is placed in contact with the radiation-sensitive material. Consequently, the thickness of the durable layer affects the resolution which can be achieved in the final image in the radiation-sensitive material. To prevent undesirable loss of resolution, it is in general desirable that the durable coating formed have a thickness not greater than about 10 μm, and preferably in the range of from about 2 to about 6 μm, since durable layers of these thicknesses normally do not cause optical problems in viewing the image, and permit exposure of radiation-sensitive materials through the protected image without adversely affecting the resolution of the image produced. It should be noted that some plastics normally regarded as durable when in thick layers are insufficiently durable in 2 to 6 μm layers, and acrylic polymers, for example poly(methyl methacrylate) , and polyurethanes are the preferred materials for forming the durable layer. To permit the protected image to be exposed using the vacuum frames conventional in the printing industry, desirably the durable layer provides a durable coating which can sustain a vacuum drawdown of 660 mm Hg for five minutes without the appearance of Newton rings. It is also desirable that the durable coating produced be able to survive intimate contact by vacuum drawdown for five minutes with other films. Mylar, silver films, goldenrod, rubylith, orange vinyl, cardboard, paper stock, contact films, diazo films, graphic arts proofing films and plates without blocking or other damage to the film or protected image. To avoid air being trapped between the protected image and the radiation-sensitive material, it is desirable that the durable coating produced have a matte, slightly roughened surface, since such a matte surface allows for escape of air from between the durable coating and the radiation-sensitive material with which it is in contact, thereby preventing the formation of Newton's rings and other undesirable interference phenomena caused by trapped air. It has been found that the texture of the surface of the support layer in contact with the durable layer affects the texture of the durable coating produced, and accordingly it is desirable that this surface be matte. In the production of printing plates, it is highly desirable that the operator be able to distinguish visually between the two sides of the protected image in order to avoid accidental inversion of the protected image, with consequent lateral inversion of the image formed on the printing plate. Accordingly, it is preferred that the durable layer formed on the image have a gloss number in the range of from about 20 to about 80. A similar gloss number is desirable in the case of protected medical images in order to avoid images being stored with their imaging layers in contact, and to prevent unfortunate accidents caused by accidental lateral inversion of the image of a patient being treated. A preferred process of the invention will now be described, though by way of illustration only, with reference to the accompanying drawings. These drawings are not to scale, and in particular the thicknesses of the various layers shown in the drawings are greatly exaggerated relative to the widths of these layers. In Figure 1, there is shown a preferred thermal imaging laminar medium 10 suited to use in the production of a pair of images, shown as images 10a and 10b in a state of partial separation in Figure 2. Thermal imaging medium 10 includes a first sheet-like web material 12 having superposed thereon, and in order, porous or particulate image-forming layer 14, release layer 16, adhesive layer 18 and second sheet-like web material 20. Upon exposure of the thermal imaging medium 10 to radiation, exposed portions of image- forming layer 14 are attached firmly to sheet-like web material 12, so that, upon separation of the respective sheet-like web materials, as shown in Figure 2, a pair of images, 10a and 10b, is provided. The nature of the layers of thermal imaging medium 10 and their properties are importantly related to the manner in which the respective images are partitioned from the thermal imaging medium after exposure. The various layers of imaging medium 10 are described in detail hereinafter. Sheet-like web material 12 comprises a transparent material through which imaging medium 10 can be exposed to radiation. Web material 12 can comprise any of a variety of sheet-like materials, although polymeric sheet materials will be especially preferred. Among preferred web materials are polystyrene, polyester (desirably, poly(ethylene terephthalate)), acrylic polymers (for example poly(methyl methacrylate) , polyethylene, polypropylene, poly(vinyl chloride) , polycarbonate, poly(vinylidene chloride) , cellulose acetate, cellulose acetate butyrate and copolymeric materials such as the copolymers of styrene, butadiene and acrylonitrile, including poly(styrene- co-acrylonitrile) . The surface of web material 12 is important to the thermal imaging of medium 10. At least a surface zone or layer of web material 12 comprises a polymeric material which is heat activatable upon subjection of medium 10 to brief and intense radiation, so that, upon rapid cooling, exposed portions of the surface zone or layer are firmly attached to image-forming layer 14. According to a preferred embodiment, web material 12 comprises a portion 12a, of a web material such as polyethylene terephthalate, having a surface layer 12b of a polymeric material that can be heat activated at a temperature lower than the softening temperature of portion 12a. A suitable material for surface layer 12b comprises a polymeric material which tends readily to soften so that exposed portions of layer 12b and layer 14 can be firmly attached to web 12. A variety of polymeric materials can be used for this purpose, including polystyrene, poly(styrene-co-acrylonitrile) , poly(vinyl butyrate) , poly(methyl methacrylate) , polyethylene and poly(vinyl chloride) . The employment of a thin surface layer 12b on a substantially thicker and durable web material 12a permits desired handling of web material 12 and desired imaging efficiency. It will be appreciated, however, that web 12 can comprise a unitary sheet material (not shown) provided that, upon exposure of the medium to radiation and absorption of light and conversion to heat, the web material and particularly the surface portion or zone thereof adjacent layer 14 can be made to firmly attach to the image-forming material of layer 14. In general, the thickness of web material 12 will depend upon the desired handling characteristics of medium 10 during manufacture, on imaging and post- imaging separation steps and on the desired and intended use of the image to be carried thereon. Typically, web material 12 will vary in thickness from about 0.5 to 7 mils (13 to 178 μm) . Thickness may also be influenced by exposure conditions, such as the power of the exposing source of radiation. Good results can be obtained using a polymeric sheet 12 having a thickness of about 0.75 mil (0.019mm) to about two mils (0.051mm) although other thicknesses can be employed. Where surface zone 12b of web material 12 comprises a discrete layer of polymeric material, layer 12b will be very thin and typically in the range of about 0.1 to 5 μm. The use of a thin layer 12b facilitates the concentration of heat energy at or near the interface between layers 12b and 14 and permits optimal imaging effects and reduced energy requirements. It will be appreciated that the sensitivity of layer 12b to heat activation (or softening) and attachment or adhesion to layer 14 will depend upon the nature and thermal characteristics of layer 12b and upon the thickness thereof. Good results are obtained using, for example, a web material 12 having a thickness of about 1.5 to 1.75 mils (38 to 44 μm) carrying a surface layer 12b of poly(styrene-co-acrylonitrile) having a thickness of about 0.1 to 5 μm. Other web materials can, however, be employed. A discrete layer 12b of heat-activatable material can be provided on a web material 12a by resort to known coating methods. For example, a layer of poly(styrene-co-acrylonitrile) can be applied to a web 12a of polyethylene terephthalate by coating from an organic solvent such as methylene chloride. If desired, web material 12a. can contain additional subcoats (not shown) such as are known in the art to facilitate adhesion of coated materials. If desired, an additional compressible layer (not shown) having stress-absorbing properties can be included in medium 10 as an optional layer between web material 12a and surface layer 12b. Such optional and compressible layer serves to absorb physical stresses in medium 10 and to prevent undesired delamination at the interface of layer 12b and layer 14. Inclusion of a compressible layer facilitates the handling and slitting of medium 10 and permits the conduct of such manipulatory manufacturing operations as may otherwise result in stress-induced delamination. A thermal imaging medium incorporating a stress-absorbing layer is described and claimed in an International Patent Application of Polaroid Corporation claiming priority of U.S. Application Serial No. 07/616,854, filed November 21, 1990 by Neal F. Kelly, for Stress- Absorbing Thermal Imaging Laminar Medium. Image-forming layer 14 comprises an image- forming substance deposited onto layer 12b as a porous or particulate layer or coating. Layer 14, referred to as a colorant/binder layer, can be formed from a colorant material dispersed in a suitable binder, the colorant being a pigment or dye of any desired color, and preferably, being substantially inert to the elevated temperatures required for thermal imaging of medium 10. Carbon black is a particularly advantageous and preferred pigment material. Preferably, the carbon black material will comprise particles having an average diameter of about 0.01 to 10 μm. Although the description hereof will refer principally to carbon black, other optically dense substances, such as graphite, phthalocyanine pigments and other colored pigments can be used. If desired, substances which change their optical density upon subjection to temperatures as herein described can also be employed. The binder for the image-forming substance of layer 14 provides a matrix to form the porous or particulate substance thereof into a cohesive layer and serves to adhere layer 14 to layer 12b. Layer 14 can be conveniently deposited onto layer 12b using any of a number of known coating methods. According to a pre erred embodiment, and for ease in coating layer 14 onto layer 12b, carbon black particles are initially suspended in an inert liquid vehicle (typically, water) and the resulting suspension or dispersion is uniformly spread over layer 12b. On drying, layer 14 is adhered as a uniform image-forming layer onto the surface of layer 12b. It will be appreciated that the spreading characteristics of the suspension can be improved by including a surfactant, such as ammonium perfluoroalkyl sulfonate, nonionic ethoxylate or the like. Other substances, such as emulsifiers can be used or added to improve the uniformity of distribution of the carbon black in its suspended state and, thereafter, in its spread and dry state. Layer 14 can range in thickness and typically will have a thickness of about 0.1 to about 10 μm. In general, it will be preferred, from the standpoint of image resolution, that a thin layer be employed. Layer 14 should, however, be of sufficient thickness to provide desired and predetermined optical density in the images prepared from imaging medium 10. Suitable binder materials for image-forming layer 14 include gelatin, poly(vinyl alcohol) , hydroxyethyl cellulose, gum arabic, methyl cellulose, polyvinylpyrrolidone, polyethyloxazoline, and poly(styrene-co-maleic anhydride) . The ratio of pigment (e.g., carbon black) to binder can be in the range of from 40:1 to about 1:2 on a weight basis. Preferably, the ratio of pigment to binder will be in the range of from about 4:1 to about 10:1. A preferred binder material for a carbon black pigment material is polyvinyl alcohol. If desired, additional additives or agents can be incorporated into image-forming layer 14. Thus, submicroscopic particles, such as chitin, polytetrafluoroethylene particles and/or polyamide and/or polystyrene latex can be added to colorant/binder layer 14 to improve abrasion resistance. Such particles can be present, for example, in amounts of from about 1:2 to about 1:20, particles to layer solids, by weight. As can be seen from Figure 2, the relationships of adhesivity and cohesivity among the several layers of imaging medium 10 are such that separation occurs between layer 14 and surface zone or layer 12b in non-exposed regions. Thus, imaging medium 10, if it were to be separated without exposure, would separate between surface zone or layer 12b and layer 14 to provide a D max on sheet 20. The nature of layer 14 is such, however, that its relatively weak adhesion to surface zone or layer 12b can be substantially increased upon exposure. Thus, as shown in Figure 2, exposure of medium 10 to brief and intense radiation in the direction of the arrows and in the areas defined by the respective pairs of arrows, serves in the areas of exposure to substantially lock or attach layer 14, as portions 14a, to surface zone or layer 12b. Attachment of weakly adherent layer 14 to surface zone or layer 12b in areas of exposure is accomplished by absorption of radiation within the imaging medium and conversion to heat sufficient in intensity to heat activate surface zone or layer 12b and on cooling to more firmly join exposed regions or portions of layer 14 and surface zone or layer 12b. Thermal imaging medium 10 is capable of absorbing radiation at or near the interface of surface zone or layer 12b of heat-activatable polymeric material and layer 14. This is accomplished by using layers in medium 10 which by their nature absorb radiation and generate the requisite heat for desired thermal imaging, or by including in at least one of the layers, an agent capable of absorbing radiation of the wavelength of the exposing source. Infrared-absorbing dyes can, for example, be suitably employed for this purpose. Porous or particulate image-forming layer 14 can comprise a pigment or other colorant material such as carbon black which is absorptive of exposing radiation and which is known in the thermographic imaging field as a radiation-absorbing pigment. While a radiation-absorbing pigment in layer 14 may be essentially the only absorber of radiation in medium 10, inasmuch as a secure bonding or joining is desired at the interface of layer 14 and surface zone or layer 12b, it is preferred that a light-absorbing substance be incorporated into either or both of layer 14 and surface zone or layer 12b. Suitable light-absorbing substances in layers 12b and/or 14, for converting light into heat, include carbon black, graphite or finely divided pigments such as the sulfides or oxides of silver, bismuth or nickel. Dyes such as the azo dyes, xanthene dyes, phthalocyanine dyes or the anthraquinone dyes can also be employed for this purpose. Especially preferred are materials which absorb efficiently at the particular wavelength of the exposing radiation. In this connection, infrared- absorbing dyes which absorb in the infrared-emitting regions of lasers which are desirably used for thermal imaging are especially preferred. Suitable examples of infrared-absorbing dyes for this purpose include the alkylpyrylium-squarylium dyes, disclosed in U.S. Patent No. 4,508,811, and including l,3-bis[(2,6-di-t-butyl-4H- thiopyran-4-ylidene)methyl]-2,4-dihydroxy-dihydroxide- cyclobutene diylium-bis{inner salt}. Other suitable IR- absorbing dyes include 4-[7-(4H-pyran-4-ylide)hepta- l,3,5-trienyl]pyrylium tetraphenylborate and 4-[[3-[7- diethylamino-2-(1,1-dimethylethyl)-(benz[b]-4H-pyran-4- ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-l- ylidene] ethyl]-7-diethylamino-2-(1,1-dimethylethyl)- benz[b]pyrylium hydroxide inner salt. These and other IR-absorbing dyes are disclosed in an International Patent Application of Polaroid Corporation claiming priority of U.S. Application Serial No. 07/616,639 filed November 21, 1990 by S. J. Telfer et. al. and in U.S. Application Serial No. 07/616,651 filed November 21, 1990 by Z.J. Hinz et al. As shown in Figure 2, exposed regions or portions of layer 14 separate sharply from non-exposed regions. Layer 14 is an imagewise disruptible layer owing to the porous or particulate nature thereof and the capacity for the layer to fracture or break sharply at particle interfaces. From the standpoint of image resolution or sharpness, it is essential that layer 14 be disruptible, such that a sharp separation can occur between exposed and unexposed regions of the thermally imaged medium, through the thickness of the layer 14 and along a direction substantially orthogonal to the interface of the layers 14 and 12b, i.e., substantially along the direction of the arrows in Figure 2. Shown in imaging medium 10 is a second sheet¬ like web material 20 covering image-forming layer 14 through adhesive layer 18 and release layer 16. Web material 20 is laminated over image-forming layer 14 and serves as the means by which non-exposed areas of layer 14 can be carried from web material 12 in the form of image 10b, as shown in Figure 2. Preferably, web material 20 will be provided with a layer of adhesive to facilitate lamination. Adhesives of the pressure- sensitive and heat-activatable types can be used for this purpose. Typically, web material 22 carrying adhesive layer 20 will be laminated onto web 12 using pressure (or heat and pressure) to provide a unitary lamination. Suitable adhesives include poly(ethylene- co-vinyl acetate) , poly(vinyl acetate) , poly(ethylene- co-ethyl acrylate) , poly(ethylene-co-methacrylic acid) and polyesters of aliphatic or aromatic dicarboxylic acids (or their lower alkyl esters) with polyols such as ethylene glycol, and mixtures of such adhesives. The properties of adhesive layer 20 can vary in softness or hardness to suit particular requirements of handling of the imaging medium during manufacture and use and image durability. A soft adhesive material of suitable thickness to provide the capability of absorbing stresses that may cause an undesired delamination can be used, as is disclosed and claimed in the aforementioned International Patent Application claiming priority of U.S. Application Serial No. 07/616,854. If desired, a hardenable adhesive layer can be used and cutting or other manufacturing operations can be performed prior to hardening of the layer, as is described in an International Patent Application of Polaroid Corporation claiming priority of U.S. Application Serial No. 07/616,853 filed November 21, 1990 by Neal F. Kelly, et al., for Hardenable Adhesive for Thermal Imaging Medium. According to a preferred embodiment, and as shown in Figure l, release layer 18 is included in thermal imaging medium 10 to facilitate separation of images 10a and 10b according to the mode shown in Figure 2. As described hereinbefore, regions of medium 10 subjected to radiation become more firmly secured to surface zone or layer 12b by reason of the heat activation of layer 12 by the exposing radiation. Non- exposed regions of layer 14 remain only weakly adhered to surface zone or area 12b and are carried along with web 20 on separation of web materials 12 and 20. This is accomplished by the adhesion of layer 14 to surface zone or layer 12b, in non-exposed regions, being less than: (a) the adhesion between layers 14 and 16; (b) the adhesion between layers 16 and 18; (c) the adhesion between layers 18 and 20; and (d) the cohesivity of layers 14, 16 and 18. The adhesion of web material 20 to porous or particulate layer 14, while sufficient to remove non-exposed regions of layer 14 from web surface zone or layer 12b, is controlled, in exposed areas, by release layer 18 so as to prevent removal of firmly attached exposed portions of layers 14a (attached to surface zone or layer 12b by exposure and by heat activation thereof) . Release layer 16 is designed such that its cohesivity or its adhesion to either adhesive 18 or porous or particulate layer 14 is less, in exposed regions, than the adhesion of layer 14 to surface zone or layer 12b. The result of these relationships is that release layer 16 undergoes an adhesive failure in exposed areas at the interface between layers 14 and 18, or at the interface between layers 16 and 14; or, as shown in Figure 2, a cohesive failure of layer 16 occurs, such that portions (16b) are present in image 10b and portions (16a) are adhered in exposed regions to porous or particulate layer 14. Portions 16a of release layer 16 may serve to provide some surface protection for the image areas of image lOa, against abrasion and wear; however, the degree of protection provided by portions 16a is limited, and if image lOa is to be retained and used, in most cases it is advantageous to protect image 10a by the present process, as discussed in more detail below. Release layer 16 can comprise a wax, wax-like or resinous material. Microcrystalline waxes, for example, high-density polyethylene waxes available as aqueous dispersions, can be used for this purpose. Other suitable materials include carnauba, beeswax, paraffin wax and wax-like materials such as pol (vinyl stearate) , poly(ethylene sebacate) , sucrose polyesters, polyalkylene oxides and dimethylglycol phthalate. Polymeric or resinous materials such as polystyrene, poly(methyl methacrylate) and copolymers of methyl methacrylate and monomers copolymerizable therewith can be employed. If desired, hydrophilic colloid materials, such as poly(vinyl alcohol) , gelatin or hydroxyethyl cellulose can be included as polymer binding agents. Resinous materials, typically coated as latices, can be used and latices of poly(methyl methacrylate) are especially useful. Cohesivity of layer 16 can be controlled so as to provide the desired and predetermined fracturing. Waxy or resinous layers which are disruptible and which can be fractured sharply at the interfaces of particles thereof can be used to advantage. If desired, particulate materials can be added to the layer to reduce cohesivity. Examples of such particulate materials include, silica, clay particles and particles of poly(tetrafluoroethylene) . Thermal imaging laminar medium 10 can be imaged by creating (in medium 10) a thermal pattern according to the information imaged. Exposure sources capable of providing radiation which can be imaged onto medium 10, and which can be converted by absorption into a predetermined pattern, can be used. Gas discharge lamps, xenon lamps and lasers are examples of such sources. The exposure of medium 10 to radiation can be progressive or intermittent. For example, a two-sheet laminar medium, as shown in Figure 1, can be fastened onto a rotating drum for exposure of the medium through web material 12. A light spot of high intensity, such as is emitted by a laser, can be used to expose the medium 10 in the direction of rotation of the drum, while the laser is moved slowly in a transverse direction across the web, thereby to trace out a helical path. Laser drivers, designed to fire corresponding lasers, can be used to intermittently fire one or more lasers in a predetermined manner to thereby record information according to an original to be imaged. As is shown in Figure 1, a pattern of intense radiation can be directed onto medium 10 by exposure to a laser from the direction of the arrows, the areas between the pairs of arrows defining regions of exposure. If desired, a thermal imaging laminar medium of the invention can be imaged using a moving slit or stencils or masks, and by using a tube or other source which emits radiation continuously and which can be directed progressively or intermittently onto medium 10. Thermographic copying methods can be used, if desired. Preferably, a laser or combination of lasers will be used to scan the medium and record information in the form of very fine dots or pels. Semiconductor diode lasers and YAG lasers having power outputs sufficient to stay within upper and lower exposure threshold values of medium 10 will be preferred. Useful lasers may have power outputs in the range of from about 40 milliwatts to about 1000 milliwatts. An exposure threshold value, as used herein, refers to a minimal power required to effect an exposure, while a maximum power output refers to a power level tolerable by the medium before "burn out" occurs. Lasers are particularly preferred as exposing sources inasmuch as medium 10 may be regarded as a threshold-type of film; i.e. , it possesses high contrast and, if exposed beyond a certain threshold value, will yield maximum density, whereas no density will be recorded below the threshold value. Especially preferred are lasers which are capable of providing a beam sufficiently fine to provide images having resolution as fine as 1,000 (e.g., 4,000 to 10,000) dots per centimeter. Locally applied heat, developed at or near the interface of layer 14 and surface zone or layer 12b can be intense (about 400°C) and serves to effect imaging in the manner described above. Typically, the heat will be applied for an extremely short period, preferably of the order of <0.5 microsecond, and exposure time span may be less than one millisecond. For instance, the exposure time span can be less than one millisecond and the temperature span in exposed regions can be between about 100°C and about 1000°C. Apparatus and methodology for forming images from thermally actuable media such as the medium of the present invention are described in International Patent Application No. PCT/US91/06880 of Polaroid Corporation. The imagewise exposure of medium 10 to radiation creates in the medium latent images which are viewable upon separation of the sheets thereof (12 and 20) as shown in Figure 2. Sheet 20 can comprise any of a variety of plastic, paper or other materials, depending upon the particular application for image 10b. Thus, a paper sheet material 20 can be used to provide a reflective image. In many instances, a transparency will be preferred, in which case, a transparent sheet material 20 will be employed. A polyester (e.g., polyethylene terephthalate) sheet material is a preferred material for this purpose. As already mentioned, separation of the sheets 12 and 20 produces a pair of complementary binary images, each of which comprises a plurality of first areas at which the imaging layer 14 is adhered to the underlying sheet 12 or 20 and a plurality of second areas at which the sheet 12 or 20 is free from the imaging layer 14. The first areas of the image on the sheet 12 comprise the areas covered by the portions 14a of the imaging layer 14, while the second areas of the same image comprise the areas from which the portions 14b of the imaging layer 14 have been removed. On the other hand, the first areas of the image on the sheet 20 comprise the areas covered by the portions 14b of the imaging layer 14, while the second areas of the same image comprise the gaps left by the portions 14a of the imaging layer 14 which remain on the sheet 12. The images on the sheets 12 and 20 are thus complementary, a white area in one image corresponding to a black area in the other. Either or both of these images may be protected by the process of this invention. In Figures 3 to 6, the image on sheet 20 is shown being protected, but it will be seen from the following description that no significant changes in the procedure are required to use the same process for the protection of the image on sheet 12. Figure 3 of the accompanying drawings shows in section a laminating sheet (generally designated 30) disposed over the binary image 10b formed on sheet 20, as described above. The laminating sheet 30 comprises an adhesive layer 32, a durable layer 34, a release layer 36 and a support layer 38. The laminating sheet 3 0 is larger in both footprint dimensions (i.e., length and width) than the sheet 20. Either or both of the adhesive layer 32 and the release layer 36 can be omitted from the laminating sheet in some cases. Some durable layers can function as their own adhesives without the need for a separate adhesive layer, and some durable layers will release cleanly from the support layer without the need for a separate release layer. As shown in Figure 4, the laminating sheet 30 is laminated to the image 10b so that the adhesive layer 32 adheres to both the first and second areas thereof, and so that the laminating sheet 30 protrudes beyond the periphery of the sheet 20 all around the sheet. Next, the laminating sheet 30 is separated from the image lOb, as shown in Figure 5; conveniently, one edge of the laminating sheet is gripped, manually by an operator or mechanically, and the laminating sheet 30 simply peeled away from the image 10b. As seen in Figure 5, in peripheral portions of the laminating sheet where the adhesive layer 32 is not attached to the image 10b, the peripheral portions 32a and 34a of the adhesive layer 32 and the durable layer 34 respectively remain attached to the release layer 36 and the support layer 38, while the central portions 32b and 34b of the adhesive layer 32 and the durable layer 34 respectively remain attached to the image 10b, so that the adhesive layer 32 and the durable layer 34 break substantially along the periphery of the sheet 20, thereby providing clean edges to the protected image 10b. Depending upon the nature of the release layer 36, none, part or all of the release layer 36 may remain with the central portions 32b and 34b of the adhesive layer 32 and the durable layer 34 on the image 10b. The central portions 32b and 34b of the adhesive layer 32 and the durable layer 34 respectively (together with any release layer 36 remaining therewith) form a durable coating over the image 10b, as shown in Figure 6. Figure 7 shows an apparatus 40 which may be used to carry out the lamination process of Figures 3 to 6. This apparatus 40 comprises a feed roll 42 on which is wrapped a supply of laminating sheet 30 (which is shown for simplicity in Figure 7 as comprising only the durable layer 34 and the support layer 38, although it may of course include other layers as described above) , a first guide bar 44 and a pair of electrically heated rollers 46 and 48 having a nip 50 therebetween. The rollers 46 and 48 are provided with control means (not shown) for controlling the temperature of the rollers and the force with which they are driven towards one another, and thus the pressure exerted in the nip 50. The apparatus 40 further comprises a series of second guide bars 52 and a take-up roll 54. Laminating sheet 30 is fed from the feed roll 42, around the guide bar 44 and into the nip 50 under a tension which is controllable by tension control means (not shown) provided on the feed roll 42 and/or the take-up roll 54. The image 56 to be protected is fed (manually or mechanically) , image side up, into the nip 50 below the laminating sheet 30; the laminating sheet is made wider than the image so that excess laminating sheet extends beyond both side edges of the image 56. The heat and pressure within the nip 50 laminate the image 56 to the laminating sheet 30 and the two travel together beneath the guide bars 52, until the laminating sheet is bent sharply around the last of the guide bars 52. Because the thin laminating sheet 30 is more flexible than the image 56, this sharp bending of the laminating sheet causes, in the area where the laminating sheet 30 overlies the image 56, separation of the durable layer 34 from the support layer 38 with the durable layer 34 remaining attached to the image 56, whereas in areas where the laminating sheet 30 does not overlie the image 56, the durable layer 34 remains attached to the support layer 38. The support layer 38, and the areas of the durable layer 34 remaining attached thereto are wound onto the take-up roll 54. The following Examples are now given, though by way of illustration only, to show details of preferred materials, conditions and techniques for use in the present invention. Example 1 This Example illustrates a process of the invention in which the laminating sheet comprises both a thermoplastic adhesive layer and a release layer. Onto a first sheet of poly(ethylene terephthalate) of 1.75-mil (44 μm) thickness were deposited the following layers, in succession: a 0.5 μm thick heat-activatable layer of poly(styrene-co-acrylonitrile) ; a 0.8 μm thick layer of carbon black pigment and polyvinyl alcohol, at a weight ratio of 5:1 (this carbon black layer had an optical density in excess of 4.0); and a 0.4 μm thick release layer comprising: ten parts high-density polyethylene wax (from Michael an- 32535 neutral wax dispersion) ; ten parts silica; and one part poly(styrene-co-maleic anhydride) . Onto a second sheet of poly(ethylene terephthalate) of 4 mil (102 μm) thickness was deposited a layer of heat-activatable adhesive, coated from a solution of a thermoplastic resin, available as Vitel PE-200 dispersion from Goodyear Chemicals Division of The Goodyear Tire and Rubber Company and having a sealing temperature of about 205°F (90.6°C), dissolved in methyl ethyl ketone and toluene. The layer on drying was a 10 μm thick adhesive layer. The aforementioned poly(ethylene terephthalate) sheet materials were brought into face- to-face superposition and passed through a pair of heated rolls at a temperature of about 190°F (87.8°C) to provide a laminar thermally actuable imaging element of the invention, having the structure shown in Figure 1. This laminar imaging element was imaged by laser exposure (through the thinner of the polyester sheets thereof) using high intensity semiconductor lasers. The laminar imaging element was fixed (clamped) to a rotary drum with the 4 mil polyester component thereof facing the drum. The radiation of semiconductor lasers was directed through the 1.75 mil polyester component thereof in an imagewise manner in response to a digital representation of an original image to be recorded in the thermally actuable imaging element. After exposure to the high-intensity radiation (by scanning of the imaging element orthogonally to the direction of drum rotation) and removal of the thus- exposed imaging element from the drum, the respective sheets of the imaging elements were separated to provide a first image on the first 1.75 mil polyester sheet and a second and complementary image on the second 4 mil polyester sheet. This second image was a black half-tone image having a visible optical density of 3.5-4.5. A laminating sheet was prepared having a support layer of 0.92 mil (23 μm) smooth polyethylene terephthalate, a release layer of polymeric wax, a durable layer of cured acrylic polymer and an adhesive layer of thermoplastic acrylic adhesive. This laminating sheet was laminated on a Consultants International Laboratory Bench Top Hot Laminator at a hot roller temperature of 95°C, a piston air pressure of 75 psig and a speed setting of 5 feet/minute to the black halftone image. After the lamination, the laminating sheet was peeled from the image, causing a failure to occur in the wax release layer and leaving a glossy surface of wax, durable layer and adhesive layer on the image. The durable layer on the image did not fail after 100 scratches at 10 Newtons force on an Erikson Abrasion Meter, and only failed after 18 scratches at 20 Newtons force on the same meter. The durable layer showed a critical load value of over 150 g y and was found to be resistant to 90% isopropanol, coffee, water and Anchor Lithkemko and Varn graphic arts film cleaners. The durable layer was not removed from the image by contact with adhesive tape having an adhesion to steel of 44 grams per millimeter. Example 2 Example 1 was repeated except that the support layer of the laminating sheet was formed from matte 1 mil (25 μm) poly(ethylene terephthalate) , to give a protected image having a matte finish. Example 3 This Example illustrates a process of the invention in which the laminating sheet comprises an ultraviolet curable adhesive layer. A laminating sheet was prepared having a support layer of 1 mil (25 μm) smooth poly(ethylene terephthalate) , a release layer of polymeric wax, a durable layer of cured acrylic polymer and an adhesive layer of ultraviolet curable pressure sensitive adhesive. This laminating sheet was laminated at room temperature to a black halftone image having a visible optical density of 3.5-4.5 formed on a 4 mil (102 μm) poly(ethylene terephthalate) film in the manner described in Example 1 above. After the lamination, the adhesive was cured by exposing it to 380 nm ultraviolet light through the support layer for 40 seconds, after which the laminating sheet was peeled from the image leaving a glossy durable coating on the image. Example 4 Example 3 was repeated except that the support layer of the laminating sheet was formed from matte 1 mil (25 μm) poly(ethylene terephthalate) , to give a protected image having a matte finish. Example 5 This Example illustrates a process of the invention in which the laminating sheet comprises a pressure sensitive adhesive layer. A laminating sheet was prepared having a support layer of 1 mil (25 μm) smooth poly(ethylene terephthalate) , a release layer of polymeric wax, a durable layer of cured acrylic polymer and an adhesive layer of silicone pressure sensitive adhesive with an adhesion to steel of 40 oz/inch (44 g/mm) . This laminating sheet was laminated at room temperature to a black halftone image having a visible optical density of 3.5-4.5 formed on a 4 mil (102 μm) poly(ethylene terephthalate) film in the manner described in Example l above. After the lamination, the laminating sheet was peeled from the image leaving a glossy durable coating on the image. Example 6 Example 5 was repeated except that the support layer of the laminating sheet was formed from matte 1 mil (25 μm) poly(ethylene terephthalate) , to give a protected image having a matte finish. Example 7 This Example illustrates a process of the invention in which the laminating sheet comprises only a durable layer and a support layer. A laminating sheet was prepared having a support layer of 1 mil (25 μm) smooth poly(ethylene terephthalate) , and a 2.5 μm thick durable layer comprising 96 per cent by weight poly(methyl methacrylate) , 2 per cent by weight of a silicone surfactant, 1 per cent by weight of magnesium silicate and 1 per cent by weight of polypropylene wax. This laminating sheet was laminated in the manner described in Example 1 above to a black halftone image having a visible optical density of 3.5-4.5 formed on a 4 mil (102 μm) poly(ethylene terephthalate) film in the manner described in Example 1 above. After the lamination, the laminating sheet was peeled from the image leaving a glossy durable coating on the image. Example 8 Example 7 was repeated except that the support layer of the laminating sheet was formed from matte 1 mil (25 μm) poly(ethylene terephthalate) , to give a protected image having a matte finish. Example 9 This Example illustrates a process of the invention in which the durable layer of the laminating sheet is coated as a discontinuous layer from a latex and clears during lamination to produce a clear durable layer. A laminating sheet was prepared having a support layer of 1 mil (25 μm) smooth poly(ethylene terephthalate) , and a 3 μm thick durable layer comprising 90% by weight acrylic polymer latex and 10% by weight poly(vinyl alcohol) binder. The durable layer was prepared by adding the PVA binder to the latex and coating the resultant mixture (without the addition of any surfactant) onto the poly(ethylene terephthalate) support layer by means of a loop coater to produce a laminating sheet having a discontinuous, slightly hazy latex durable layer. The laminating sheet thus prepared was laminated in the manner described in Example 1 above to a black halftone image having a visible optical density of 3.0-3.2 formed on a 7 mil (178 μm) poly(ethylene terephthalate) film in the manner described in Example 1 above. After the lamination, the laminating sheet was peeled from the image leaving a glossy, clear durable coating on the image. Example 10 This Example illustrates a process of the invention in which the durable layer of the laminating sheet is coated as a discontinuous layer from a latex and clears during lamination to produce a clear durable layer. A laminating sheet was prepared having a support layer of 1 mil (25 μm) smooth poly(ethylene terephthalate) , and a 5 μm thick durable layer comprising 80% by weight acrylic polymer, 10% by weight polyethylene/paraffin wax and 10% by weight aqueous based nylon binder. The durable layer was prepared by mixing the polymer and wax latices, adding the binder and finally adding a silicone surfactant, and coating the resultant mixture onto the polyester support layer by means of a loop coater to produce a laminating sheet having a discontinuous, slightly hazy latex durable layer. The laminating sheet thus prepared was laminated in the manner described in Example 1 above to a black halftone image having a visible optical density of 3.0-3.2 formed on a 7 mil (178 μm) polyester film in the manner described in Example 1 above. After the lamination, the laminating sheet was peeled from the image leaving a glossy, clear durable coating on the image. Example 11 This Example illustrates a process of the invention in which the durable layer of the laminating sheet is coated as a discontinuous layer from a latex and clears during lamination to produce a clear durable layer. A laminating sheet was prepared having a support layer of 1 mil (25 μm) smooth poly(ethylene terephthalate) , and a 3 μm thick durable layer comprising 66.7% by weight styrene/ethyl aerylate copolymer latex, 16.7% by weight polyethylene wax and 16.6% by weight styrene/ aleic acid copolymer binder. The durable layer was prepared by mixing the latex and the wax, adding the binder and hand coating the resultant mixture (without the addition of any surfactant) onto the polyester support layer to produce a laminating sheet having a discontinuous, hazy latex durable layer. The laminating sheet thus prepared was laminated at 100°C in the manner described in Example 1 above to a black halftone image having a visible optical density of 3.0-3.2 formed on a 7 mil (178 μm) poly(ethylene terephthalate) film in the manner described in Example 1 above. After the lamination, the laminating sheet was peeled from the image leaving a glossy, clear durable coating on the image.

Claims: CIAIMS 1. A process for protecting an image (10b) , this process comprising: providing a laminating sheet (30) ; placing the laminating sheet (30) in contact with the image (10b) ; and laminating the laminating sheet (30) to the binary image (10b) , the process being characterized in that the binary image (10b) comprises a plurality of first areas at which a porous or particulate imaging material (14) is adhered to a substrate (20) and a plurality of second areas at which the substrate (20) is free from the imaging material (14) ; the laminating sheet (30) comprises a durable layer (34) and a support layer (38) , and is placed in contact with the image (10b) such that the durable layer (34) faces the image (10b) and at least one portion of the laminating sheet (30) extends beyond the periphery of the substrate (20) ; the laminating sheet (30) is laminated to the binary image (10b) so that the durable layer (34) adheres to both the first and second areas of the image (10b) ; and the support layer (38) is separated from the image (10b) such that the durable layer (34) remains attached to the image (10b) , but, in the at least one portion of the laminating sheet (30) , the durable layer (34) remains attached to the support layer (38) so that the durable layer (34) breaks substantially along the periphery of the substrate (20) , thereby covering the image (10b) with a durable layer (34) , the layer: a) being substantially transparent; b) having an abrasion resistance of at least 10 cycles of a 10 Newton force as measured by an Erikson Abrasion Meter and a critical load value of at least 100 grams as measured by ANSI PHI.37-1983; and c) not being removed from the image (10b) by contact with adhesive tape having an adhesion to steel of 33 grams per millimeter, as measured by ASTM D-3330. 2. A process according to claim 1 characterized in that the laminating sheet (30) further comprises a release layer (36) interposed between the durable layer (34) and the support layer (38) and in that, in the areas where the durable layer (34) remains attached to the image (10b) , separation of the durable layer (34) from the support layer (38) occurs by failure within or on one surface of the release layer (36) . 3. A process according to claim 1 or 2 characterized in that the laminating sheet (30) further comprises an adhesive layer (32) disposed on the surface of the durable layer (34) remote from the support layer (38) , such that, during the lamination, the durable layer (34) is adhered to the image (10b) by the adhesive layer (32) . 4. A process according to any one of the preceding claims characterized in that the durable layer (34) of the laminating sheet (30) comprises a latex having a plurality of discrete particles and the discrete particles are coalesced during the lamination, thereby forming a continuous durable layer (34) on the image (10b) . 5. A process according to any one of claims 1 to 3 characterized in that the durable layer (34) comprises a continuous phase and a particulate solid dispersed in the continuous phase. 6. A process according to any one of the preceding claims characterized in that the durable layer (34) formed on the image (10b) has a thickness not greater than 30 μm. 7. A process according to claim 6 characterized in that the durable layer (34) formed on the image (10b) has a thickness in the range of from 2 to 6 μm. 8. A process according to any one of the preceding claims characterized in that the substrate (20) is substantially transparent and the formation of the durable layer (34) over the image (10b) does not increase the transmission density of the second areas by more than 0.03. 9. A process according to any one of the preceding claims characterized in that the durable layer (34) can sustain a vacuum drawdown of 660 mm. Hg for five minutes without the appearance of Newton rings. 10. A process any one of the preceding claims characterized in that the binary image (10b) has been formed by: providing a layer of a porous or particulate imaging material (14) on a heat-activatable image-forming surface of a substrate (20) , the layer of the imaging material (14) having a cohesive strength greater than the adhesive strength between the layer (14) and the substrate (20) , thereby providing a thermal imaging medium (10) ; imagewise subjecting portions of the thermal imaging medium (10) to exposure to brief and intense radiation, thereby firmly attaching exposed portions of the imaging material (14) to the substrate (20) ; removing from the substrate (20) those portions of the image-forming substance not exposed to the radiation. 11. A binary image (10b) comprising a plurality of first areas at which a porous or particulate imaging material (14) is adhered to a substrate (20) and a plurality of second areas at which the substrate (20) is free from the imaging material (14), the binary image (10b) being characterized by a durable layer (34) covering the image (10b) and adhered to both the first and second areas thereof, the durable layer (34) : a) being substantially transparent; b) having an abrasion resistance of at least 10 cycles of a 10 Newton force as measured by an Erikson Abrasion Meter and a critical load value of at least 100 grams as measured by ANSI PHI.37-1983; and c) not being removed from the image (10b) by contact with adhesive tape having an adhesion to steel of 33 grams per millimeter, as measured by ASTM D-3330. 12. A binary image (10b) according to claim 11 further comprising an adhesive layer (32) disposed between the durable layer (34) and the substrate (20) . 13. A binary image (10b) according to claim 11 or 12 characterized in that the durable layer (34) has a thickness not greater than 30 μm. 14. A binary image (10b) according to claim 13 characterized in that the durable layer (34) has a thickness in the range of from 2 to 6 μm.

Inventors: BAKER DAVID B, FEHERVARI AGOTA F, GAUDIANA RUSSELL A, SCHOEPKE PHILIP J, BAKER, DAVID, B., FEHERVARI, AGOTA, F., GAUDIANA, RUSSELL, A., SCHOEPKE, PHILIP, J.
Applicants: POLAROID CORP, POLAROID CORPORATION
Application year: 1991
Main classification: G03F7
IPC codes: B41M5, B41M7, G03F3