Self-contained humidity stabilized imaging media comprising microencapsulated color formers

[0002] The present invention relates to an imaging assembly which comprises at least one imaging layer comprising photohardenable microencapsulated coloring agents. Development is accomplished by the application of uniform pressure to the imaging assembly. Improved performance is obtained with respect to the imaging assembly's response to pressure by preconditioning and sealing the imaging layer to a preselected relative humidity.

[0003] The present invention relates to an imaging medium in the form of a self-contained imaging assembly and, more particularly, to an improved self-contained imaging assembly containing a photosensitive imaging layer or layers comprising photohardenable microcapsules encapsulating a coloring material and, outside the microcapsules, a developer material disposed between a first transparent support and a second support which may be opaque or transparent. The imaging medium or assembly can also be referred to as a recording medium, and the imaging layer can be referred to as a recording layer, since the assembly can serve both to capture an image (either the original image or an electronic copy), as does film, and also to display the image, as does a print. Consistent with this fact, the imaging assembly can form a positive image.

[0004] The photosensitive imaging layer (including microcapsules) is colored by pressure development after exposure to radiation based on image information. The microcapsules, whose mechanical strength changes (increases) when exposed to light, are ruptured by means of pressure development, whereupon the coloring material and other substances encapsulated in the microcapsules flow out (to varying amounts based on the exposure) and development occurs. The coloring material, such as a substantially colorless color former, migrates to, and reacts with, the developer material and coloring occurs, whereupon a color image is developed. The “rupture” of the microcapsules are not an all-or-nothing event. Rather, the microcapsules exposed to light are differentially photocured to release varying amounts of color former in order to achieve tonal depth in the exposed area. The differential exposure to light proportionately increases the viscosity of the photocurable composition and thus immobilizes the color former proportionately to the desired tonal depth in the exposed area. The rupture of the microcapsules and the release of the color former is accomplished by the uniform application of pressure. Development of the photosensitive imaging layer can be accomplished, for example, by passing the imaging assembly between a pair of upper and lower nip rollers.

[0005] Photohardenable imaging systems employing microencapsulated photosensitive compositions are the subject of various patents, including U.S. Pat. Nos. 4,399,209, 4,416,966, 4,440,846, 4,766,050, 5,783,353, and 5,916,727. Image forming devices (also referred to as printers) are disclosed, for example, in U.S. Pat. No. 4,740,809, wherein exposure occurs by guiding a light from a light source for a plurality of colors across a photosensitive recording medium. U.S. Pat. No. 4,992,822 discloses an image forming device, capable of producing a plurality of colors via a polygonal mirror, for repeatedly exposing the same pixels in a photosensitive recording medium. U.S. Pat. No. 5,893,662 discloses a device for printing an image wherein the device can be incorporated into a computer bay. U.S. Pat. No. 4,648,699 describes a development technique which employs, instead of a pair of nip rollers, a point contact ball moving relative to the photosensitive recording medium.

[0006] In the most typical embodiments, the photohardenable composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated together with a color former. Exposure to actinic radiation hardens the internal phase of the microcapsules. Then, as mentioned above, following exposure, the imaging media in the form of a sheet can be subjected to a uniform rupturing force by passing the sheet through the nip between a pair of pressure rollers.

[0007] One of the problems in providing a self-contained imaging assembly that provides a high quality print is image stability or “keeping” which is affected by humidity sensitivity. It is known that print quality, and in particular sensitometric response to actinic radiation, can be significantly affected by sensitivity to humidity and the relative humidity of the environment. Even if the media is manufactured and packaged at a particular humidity, which is found optimum for print quality, variations after the media acclimates to a different later environment can adversely affect the sensitometric properties. This has been believed due to the materials employed in the imaging media, in particular the degree of hardening or curing of the internal phase of a microcapsule and the consequent increase in the viscosity varying with a change in humidity. As a result thereof, photographic characteristics such as speed, maximum density and fogging density (Dmin) are changed from the original optimum. Furthermore, a full color imaging is adversely affected.

[0008] In forming a full color image, color precursors which develop into yellow, magenta and cyan colors and photo-initiators corresponding to blue, green and red lights are encapsulated in an internal phase of the microcapsules, and the three sets of the microcapsules are mixed to prepare a full color imaging material containing a developer. The photographic characteristics of the respective microcapsules vary with a change in humidity to different degrees, resulting in muddy colors or incorrect or suboptimal colors. For example, when it is desired that a yellow color be developed, cyan and magenta capsules are cured by red and green lights, and only a yellow color former reacts with a color-developer to form an image. However, if the cyan or magenta capsules insufficiently cure due to a change in humidity, the result may be a muddy color in which cyan or magenta is blended with yellow to some extent. Such muddy colors or other sensitometric phenomena due to a change in humidity has been a significant problem.

[0009] One technique that has been used to address the humidity problem and to improve media stability resides is conditioning the layer containing the developer and microcapsules to a relative humidity (RH) of about 10 to 40% and preferably about 20%. For example, U.S. Pat. Nos. 5,916,727 and 5,783,353 disclose conditioning the layer at about 20% RH for about 2 to 12 hours or more, at ambient temperatures, and subsequently sealing the assembly at this low RH level to assure that the layer is relatively moisture-free during the normal shelf-life of the assembly. Furthermore, U.S. Pat. No. 5,783,353 describes that the peripheral edges of the self contained imaging assembly may be sealed additionally, and that sealing of the assembly at low RH levels after conditioning assures that the layer is relatively moisture free during the normal shelf life of the assembly.

[0010] U.S. Pat. No. 5,996,793 discloses storing the image-forming material together with a humidity-controlling material. Further, the patent discloses storing the image-forming material and the humidity-controlling material within a package made from a low-moisture permeable film. The low-moisture permeable film can be a plastic film on which is deposited a metal. Other low-moisture permeable films mentioned include fluorinated resins such as polytetrachloroethylene, polytrifluoroethylene, chlorinated rubber, polyvinylidene chloride, a copolymer of polyvinylidene chloride and acrylonitrile, polyethylene, polypropylene, polyesters, and films obtained by depositing a metal such as aluminum and a metal oxide such as silicon oxide.

[0011] Unfortunately, when the imaging media are not used right away, especially if a plurality of media are stored for some time in a printing device prior to forming an image, the media may have an opportunity to adjust to ambient humidity and, especially in very dry or very humid climates, the RH of the media may decrease or increase substantially in a short time. Once the imaging media is removed from a package, it does not take very long for the environmental humidity to affect the media. Ambient humidity can soon penetrate the outside surface support on each side of the media causing a change in the moisture content within the media.

[0012] After extensive investigation, Applicants have found that humidity affects the mechanical properties of the imaging layers, as compared to the reaction properties of materials during photohardening of the imaging layer. The affect on mechanical properties cause undesirable variations in the degree of rupture of the microcapsules when the media is subjected to pressure during development. Although not wishing to be bound by theory, this may be due to the humidity changing the break strength of the capsules and/or it may be due to the change in pressure applied on the capsules due to the change in the stiffness of the image layer which in turn increases the stress applied to the microcapsules, more likely the latter.

[0013] An object of the present invention is, therefore, to provide a self-contained photohardenable imaging assembly that is resistant to the affect of humidity and which will print consistently in response to a means for applying pressure to the assembly.

[0014] It would be desirable to obtain an improved media that has no significant change in sensitometric properties with relative humidity, based on speed, Dmax, Dmin, tonal scale, and fall color correctness.

[0015] It would be particularly desirable if these objectives could be accomplished without requiring radical changes in conventional imaging chemistry, with respect to the microcapsules and the developer. It would be advantageous if these objectives could be attained in a product that was economical to manufacture and inexpensive for the customer to purchase.

[0016] In the self-contained imaging system of the present invention, an imaging layer containing developer and photohardenable microcapsules is placed between two support members to form an integral unit, wherein the imaging layer has been equilibriated within a preselected relative humidity prior to being enclosed between the two support members. In particular, the imaging layer should be equilibriated to a relative humidity of at least 50%. Preferably, the supports include a moisture barrier layer and the imaging assembly is sealed on the sides or edges.

[0017] The term “sealed,” as used herein, refers to a seal which is designed to be non-temporary. This seal is maintained during printing of the image and in the final imaged product, as compared to a temporary package.

[0018] In the imaging assembly of the invention, a first support is transparent and a second support may be transparent or opaque. Suitably, the transparent support has a percentage light transmission of at least about 80 percent at a wavelength of 550 nm. In the case of an opaque back support, an image is provided against a substantially white background as viewed through the transparent support and, in the former case, the image is viewed as a transparency preferably using an overhead or slide projector. Sometimes herein the first support may be referred to as the “front” support and the second support may be referred to as the “back” support.

[0019] FIG. 1 is a cross-sectional view of a prior-art imaging system.

[0020] FIG. 2 is a cross-sectional view of the prior-art imaging system of FIG. 1 after exposure and microcapsule rupture.

[0021] FIG. 3 is a cross-sectional view of an imaging system having a barrier layer according to the present invention.

[0022] FIG. 4 is a cross-sectional view of another embodiment of an imaging system having a barrier layer further comprising a polyester layer beneath the barrier layer.

[0023] FIG. 5 is a cross-sectional view of still another embodiment of an imaging system according to the present invention in which two barrier layers are present.

[0024] FIG. 6 is a cross-sectional view of still another embodiment of an imaging system according to the present invention in which a fluorinated polymer barrier layer is laminated onto a polyester layer.

[0025] FIG. 7 shows, based on the results of Example 2 below, the Dmin change for a barrier treated media for use in the present invention, at 40% relative humidity and 70° F. as a function of time, for a yellow dye.

[0026] FIG. 8 shows, based on the results of Example 2 below, the Dmin change for a barrier treated media for use in the present invention, at 40% relative humidity at 70° F. as a function of time, for a magenta dye.

[0027] FIG. 9 shows, based on the results of Example 2, below the Dmin change for a barrier treated media for use in the present invention, at 40% relative humidity/70° F. as a function of time, for a cyan dye.

[0028] The present invention is directed to a self-contained imaging assembly for both developing and printing an image, which assembly provides improved image quality by reducing the variation in the sensitometric response of the media caused by changes in the relative humidity of the environment.

[0029] As mentioned above, the self-contained imaging assembly comprises an imaging layer or series of layers in which a color developing material (also referred to as a color developer) reacts with a dye precursor (also referred to as a color former) inside microcapsules. Typically, the microcapsules encapsulate photohardenable compositions comprising a photosensitive initiator and hardenable material that undergoes a change of mechanical strength when irradiated with light of a predetermined wavelength, wherein the plurality of microcapsules encapsulates at least a dye precursor for coloring when brought into contact with the color developing material.

[0030] The plurality of microcapsules comprises three different types of microcapsules. The three types of microcapsules encapsulate the polymerization initiator, photocurable resin (each photocuring by irradiation with light of one of the three primary colors of light, respectively), and the colorless dye precursors for producing each of the colors of yellow, magenta and cyan. For example, when irradiating the self-contained imaging assembly with blue light (with a wavelength of about 470 nm), the photocurable resin of the microcapsules containing only yellow dye precursors is photocured, and these microcapsules (yellow) differentially rupture even when pressure developing the self-contained imaging assembly; however the microcapsules which were not photocured (magenta and cyan) rupture and the magenta and cyan dye precursors are forced out from the microcapsules and react with the color developing material, whereupon coloring occurs, and these colors mix to become a blue color, whereupon this blue color can be seen through the light-transmitting support.

[0031] Further, when irradiating the self-contained imaging assembly with green light (with a wavelength of about 525 nm), the photocurable resin of the microcapsules containing only magenta dye precursors is photocured, the yellow and cyan microcapsules are ruptured by pressure development, and as a result of the reaction of the color developing material with the yellow and cyan dye precursors the respective coloring occurs, whereupon these colors mix to become a green color. Moreover, when irradiating the self-contained imaging assembly with red light (with a wavelength of about 650 nm), the photocurable resin of the microcapsules containing only cyan dye precursors is photocured, the yellow and magenta microcapsules are ruptured by pressure development, and as a result of the reaction of the color developing material with the yellow and magenta dye precursors the respective coloring occurs, whereupon these colors mix to become a red color.

[0032] Furthermore, when all microcapsules are photocured to maximum hardness by exposure to light corresponding to the three types of microcapsules previously mentioned, they do not rupture even by pressure development. Therefore coloring does not occur, and the surface of the opaque support can be seen through the light-transmitting support, i.e. the surface color (white in the present embodiment) of the opaque support becomes the background color. In short, a color image is formed only in the areas where a coloring reaction occurred when the microcapsules ruptured. This coloring principal is sometimes called “self-coloring.”

[0033] A prior-art image assembly is illustrated in FIG. 1 which imaging assembly 1 comprises in order: a first transparent support 2, a subbing layer 3, an imaging layer 4 comprising photohardenable microcapsules 5 and a developer material 6, a layer of adhesive 8, and a second support 10 which may or may not contain an opacifying agent 12. By image-wise exposing this unit to actinic radiation, the microcapsules are differentially hardened in the exposed areas and the exposed unit can then be subjected to pressure to rupture the microcapsules.

[0034] FIG. 2 illustrates the prior-art imaging assembly of FIG. 1 after exposure and rupture of the microcapsules 5. Ruptured microcapsules 16 release a color forming agent, whereupon the developer material 6 reacts with the color forming agent to form an image 14. The image formed is viewed through the transparent support 2 against the support 10 which can contain a white pigment. Typically, the microcapsules will consist of three sets of microcapsules sensitive respectively to red, green and blue light and containing cyan, magenta and yellow color formers, respectively, as described above and disclosed in U.S. Pat. No. 4,772,541 and U.S. Pat. No. 4,440,846.

[0035] Imaging layer 4 typically contains about 20 to 80% (dry weight) developer, about 80 to 20% (dry weight) microcapsules and 0 to 20% of a binder. The layer is typically applied in a dry coat weight of about 8 to 50 g/m². An example of such a coating formulation is illustrated in Example 1 of U.S. Pat. No. 5,916,727.

[0036] The prior art teaches that in the self-contained photohardenable imaging assembly as shown in FIG. 1, the first transparent support 2 through which the image is viewed can be formed from any transparent polymeric film, so long as a film is selected that provides good photographic quality when viewing the image and is preferably resistant to yellowing. The first support 2 is typically a transparent polyethylene terephthalate (PET) or polyvinylchloride as the light-transmitting or transparent support. PET provides a barrier to water vapor transmission to some extent and is transparent. Of course, thicker coatings will provide a greater barrier to vapor transmission but, at some point, thicker layers begin to be disadvantaged by decreasing transparency, processibility flexibility and the like.

[0037] The present invention is directed to a self-contained imaging assembly, preferably having a barrier layer in at least one, more preferably both, of the supports, and in which the relative humidity of the imaging layer has equilibrated, at the time of sealing, to at least 50%, preferably 50 percent to 90%, more preferably 60 to 85%, most preferably 65 to 85%.

[0038] In a typical embodiment of the invention, the transparent support is 5 to 250 microns thick, preferably 10 to 125 microns thick, and has a light transmission of at least about 80% at a wavelength of 550 nm, preferably a light transmission of at least 80% at a wavelength from 450 to 800 nm, more preferably a light transmission of at least 90% at 550 nm, most preferably from 450 to 800 nm.

[0039] Preferably, at least one of the two supports, preferably both, have a water vapor transmission rate of less than 0.77 g/m²/day (0.05 g/100 in²/day), preferably not more than about 0.47 g/m²/day (0.03 g/100 in²/day). The water vapor transmission rate is measured according to ASTM F-1249, hereby incorporated by reference. Although vapor transmission rate is decreased by increasing thickness, increasing thickness may begin to adversely affect the transparency of the support.

[0040] It is preferred (but optional) that the peripheral edges of the self-contained assembly are sealed, to prevent water vapor transmission through the side edges, although this is less a concern in view of the small surface area compared to the front and back of the imaging assembly. At least prior to sealing, therefore, the assembly is preconditioned to maintain said imaging layers at a relative humidity at least 50 percent, preferably within the range of from 50 to 90%, more preferably from 60 to 85%. The preconditioning may be accomplished prior to the attachment of the second of both supports to the imaging layer.

[0041] In a typical operation, the at least one imaging layer is applied over the transparent support which comprises an adhesion promotion subbing layer. The imaging layers coated over the transparent moisture barrier support are then conditioned at a preselected relative humidity for a period of time. The time duration can vary but preferably it is long enough to allow the moisture content of the imaging layer to reach its isotherm value. The next step in the operation is to laminate an opaque support to the above coated imaging layer. This can be accomplished using a pressure sensitive adhesive which is applied prior to lamination on the face side of the opaque support. The peripheral edges of the above laminated imaging assembly can then be sealed at the preselected relative humidity by any conventional means used to seal polymeric materials. Preferred sealing techniques for the practice of the present invention are described below.

[0042] By the term “support” herein is meant the material extending from the outside of the assembly to the imaging layer, whether from the bottom or from the top, except not including any adhesive layer for promoting adhesion to the imaging layer. As indicated above, at least one support comprises at least one barrier layer. This support may be a laminate comprising one or more barrier layers or the support may consist entirely of a barrier layer.

[0043] By the term “barrier layer” or “barrier material,” as used herein, is meant a material that has a water vapor transmission rate of less than 0.77 g/m2/day (0.05 g/100 in²/day) per 25 micron thickness of material by the ASTM F-1249 test. Since the barrier layer is part of a support, the support may have other layers that provide a higher water vapor transmission than the barrier layer, so long as the water vapor transmission rate of the entire “top” support, and preferably both supports, is less than 0.77 g/m2/day (0.05 g/100 in²/day). The support comprising a barrier layer may be referred to as a “barrier support.” A separable part of the barrier support containing a barrier layer may be referred to as a “barrier sheet,” for example, when referring to a material commercially available for use in the present invention.

[0044] Preferably, the top transparent support contains at least one layer that is a barrier material. This barrier material must have a preselected combination of properties, including thickness (if too thick, too hazy, if too thin not sufficient support) and optical properties. The barrier material must be highly transparent, colorless, practical and cost effective, manufacturable or commercially available, able to be applied via coating or lamination, and stable (non-yellowing). This combination of properties is difficult to find in a single material. Many materials previously used in forming barriers in packaging do not meet all the necessary criteria alone or at all, for example, nylon, PC, PET, polyolefins, and saran polymers. The latter materials do not provide sufficient barrier properties unless using thick layers that are impractical in the present invention. Some materials, while having good moisture barrier properties, have an unacceptable tint, for example silicon oxide coated polyester films. Some materials with exceptional moisture barrier properties are not transparent, for example, aluminum metallized film or paper.

[0045] Thus, one embodiment of the present invention is directed to a self-contained photohardenable imaging assembly packaged for commercial sale wherein the assembly comprises, in order, a first transparent support that is 5 to 250 microns in thickness and has a light transmission of at least about 80% at a wavelength of 550 nm and a water vapor transmission rate of less than 0.77 g/m2/day (0.05 g/100 in²/day); one or more imaging layers comprising a plurality of microcapsules encapsulating a photohardenable composition and a color precursor which can react with a developing material in the same or an adjacent imaging layer; and a second support which may be opaque or transparent that is 5 to 250 microns thick, wherein the relative humidity of the imaging layer has equilibrated, at the time of sealing the imaging layer between the supports, to at least 50%, preferably 50 percent to 90%, more preferably 60 to 85%, most preferably 65 to 85%.

[0046] In one embodiment, the imaging assembly further comprises, between the transparent support and the imaging layer, an intermediate resilient layer comprising a relatively resilient material (compared to first transparent support), wherein the Young's modulus of the resilient material is 0.02 to 10 ksi. This has been found beneficial for better distributing the pressure applied to the microcapsules during development.

[0047] Materials which can be used as a barrier sheet for a transparent support include, but are not limited to, fluorinated polymers, ceramic coated polymers, for example aluminum oxide, indium tin oxide, or silicon nitride coated on polyester or other transparent polymeric substrates, and other sheet materials meeting the above limitations. Especially preferred are Al₂O₃vacuum deposited coatings on a polyester film (for example, Toppan™ GL-AE, available from Toppan Printing Co.) and chlorotrifluoroethylene homopolymer and copolymer films (for example, ACLAR™ films available from Honeywell Corp.).

[0048] It is preferred that a barrier layer is on both sides of the imaging layer in order to better maintain the relative humidity within the assembly. In one embodiment of the invention, the relative humidity within the assembly, and particularly within the at least one imaging layer, is maintained at greater than 40%, preferably greater than 50%, by sealing the front and back supports on the sides, after the imaging layer has equilibrated to the desired relative humidity.

[0049] In a particularly preferred embodiment, the top or first support is a laminate comprising a polyester layer and a material having a lower water transmission rate, for example a polyester layer covered with a ceramic barrier layer or a halogenated polymeric barrier layer. The polyester layer may be on the outside or inside, or both the inside and the outside, of the barrier layer. Preferred polyester films include, but are not limited to, poly(ethylene terephthalate), poly(1,4-cyclohexanedimethylene terephthalate), poly(ethylene 1,2-diphenoxyethane-4,4′-dicarboxylate), poly(butylene terephthalate), poly(ethylene naphthalate) and the like; and blends or laminates thereof with other polymers. Particularly preferred embodiments are poly(ethylene terephthalate) and poly(ethylene naphthalate). The polymer film can contain an ultraviolet (UV) ray absorber.

[0050] FIG. 3 shows a first embodiment of a laminate structure for an imaging assembly according to the present invention in which the transparent support 19 has a polyester substrate coated with a barrier layer of a ceramic material such as aluminum oxide, indium tin oxide or silicon nitride. In the embodiment of FIG. 3, a polyester substrate 20, which here happens to be 12 microns thick, is coated with an aluminum oxide barrier layer 22. Aluminum oxide is a preferred barrier material of the invention. Aluminum oxide is an electrical insulator and is transparent to visible light. It is a strong, hard material and resists attack by most chemicals. The aluminum oxide barrier layer 22 can be deposited on the polymeric film substrate by vacuum deposition including chemical vapor deposition, electron beam evaporation, plasma-type sputtering process, ion assisted process such as ion plating, as well as others well known to those skilled in the art of vacuum coating or deposition. The aluminum oxide barrier layer 22 may be overcoated with a UV absorbing subbing layer 24. The transparent support 19 (including layers 20 through 24) forms the transparent side to the imaging layer 26. On the opaque side is a white support 30 with an aluminum barrier 32 attached to the imaging layer 26 by an adhesive layer 28.

[0051] FIG. 4 shows a second embodiment of a laminate structure for an imaging assembly according to the present invention that also has a transparent support having a layer of a ceramic barrier material as in FIG. 3, but also additional layers. A polyester substrate 36, for example, 12 microns thick for example, is coated with an aluminum oxide barrier layer 38 to form a barrier sheet or composite film 40. This barrier composite film 40 is laminated to a transparent polyester film 42 that may also have a primer layer 44 and a UV absorbing subbing layer 46. The barrier composite film 40 is laminated to the transparent polyester film 42 by an adhesive layer (not shown). The transparent support 35 (including layers 36 through 46) forms the transparent side to the imaging layer 48. On the opaque side is a white support 52 with an aluminum barrier 54 attached to the imaging layer 48 by an adhesive layer 50.

[0052] FIG. 5 shows yet another embodiment of a laminate structure for an imaging assembly according to the present invention in which a transparent support has a double layer of a ceramic barrier material. A polyester substrate 56, 12 microns thickness in this example, is coated with an aluminum oxide barrier layer 58 to form barrier composite film 60. This barrier composite film 60 is laminated, through adhesive layer 62, to a second barrier composite film 68 having a second polyester substrate 64 and a second aluminum barrier layer 66. This double barrier laminate may also have a primer layer 70 and a UV absorbing subbing layer 72. The transparent support 55 (including layers 56 through 72) forms the transparent side to the imaging layer 74. On the opaque side is a white support 78 with an aluminum barrier 80 attached to the imaging layer 74 by an adhesive layer 76.

[0053] FIG. 6 shows yet a further embodiment of a laminate structure for an imaging assembly according to the present invention in which a transparent support contains a barrier layer that is a halogenated polymer. As already mentioned, a preferred halogenated polymer is a chlorotrifluoroethylene homopolymer or copolymer film, available, for example, from Honeywell Corp as ACLAR™ film. Transparent support 81 has a polyester substrate 82 that is laminated with a chlorotrifluoroethylene polymer barrier layer 86 using an adhesive layer 84. The transparent support 81 may also have a primer layer 88 and UV absorbing subbing layer 90. The transparent support 81 (including layers 82 through 90) forms the transparent side to the imaging layer 92. On the opaque side is a white support 98 with an aluminum barrier 96 attached to the imaging layer 92 by an adhesive layer 94.

[0054] The bottom support may also be a barrier support or may be a conventional support. One common and conventional support, for example, is polyethylene terephthalate (PET) containing an opacifying agent, paper or paper lined with film such as polyethylene, polypropylene, polyester, etc. Opaque films can be composites or admixtures of the polymer and the opacifying agent or pigment in a single layer. Alternatively, the opacifying agent can be provided in a separate layer underlying or overlying a polymer film such as PET. The opacifying agent employed in these materials is an inert, light-reflecting material which exhibits a white opaque background. Materials useful as the opacifying agent include inert, light-scattering white pigments such as titanium dioxide, magnesium carbonate or barium sulfate. In a preferred embodiment the opacifying agent is titanium dioxide. The opaque support, for example, may be a polyethylene terephthalate support containing about 10% titanium dioxide which provides a bright white. Such a support is commercially available from DuPont Co. under the product designation Melinex™.

[0055] In one embodiment of the invention, the second support is an opaque barrier support that is 12 to 250 microns in thickness and has a water vapor transmission rate of less than 0.77 g/m2/day (0.05 g/100 in²/day), said second support comprising in order starting from the outside of the assembly: a backing layer, an aluminum metallized barrier layer, a tie (i.e., adhesive) layer, a paper layer, a tie layer, and a biaxially oriented microvoided polyolefin or resin coated pigmented polyolefin layer. The metallic barrier layer may be laminated or coated with a pigmented layer to reduce the metallic color, which may be aesthetically unpleasing or unfamiliar. Suitably, the face side of the opaque support, which is the side closest to the imaging layer, has a light reflectance of greater than 90 percent, preferably greater than 95%.

[0056] In still another embodiment of the invention, the second support is opaque, has a thickness of 12 to 250 microns and water vapor transmission rate of less than 0.77 g/m2/day (0.05 g/100 in²/day), and comprises in order starting from the outside surface of the assembly: a backing layer, an electrically non-conductive barrier layer (which may, for example, be the same barrier material as in the transparent support), a tie layer, a paper layer, a tie layer, and a biaxially oriented microvoided polyolefin or resin coated pigmented polyolefin layer. Again, the face side of the opaque support preferably has a light reflectance of greater than 90 percent, preferably greater than 95%.

[0057] The tint of the opaque support may be modified to compensate for the yellowish color typical of phenolic resin developers.

[0058] In the event that the second support is transparent, it preferably has a light transmission which is the same as that described above for the first support, which light transmission is at least about 80% at a wavelength of 500 nm. In this case, the same material may be used for both the top and bottom supports. Using the same material for both the top and bottom supports has the advantage of reducing curl of the self-contained assembly. On the other hand, if the second support is opaque, it may be more economical to use a laminate that includes aluminum or other metal foil to achieve the desired low vapor transmission rate of the present invention.

[0059] Adhesive materials useful for adhering the second support to the emulsion or imaging layer can be selected from the general class of “modified acrylics” that have good adhesion, which may be formulated with improved “tack” by addition of tackifying resins or other chemical additives. A useful adhesive must be designed for high initial adhesion and for adhesion to plastic substrates like polyester. It must have the ability to flow quickly for laminating to porous material (the imaging layer) and yet be inert with respect to the imaging layer. High strength adhesives useful in this invention, for example, are the film label stock adhesives of the 3M Company; including 3M's #300 and #310 adhesive formulas which exhibit “inertness” to the imaging layer. Other examples of adhesives useful in this invention are aqueous-based adhesives such as Aeroset™ 2177 or Aeroset™t 2550, 3240, and 3250 which are commercially available from Ashland Chemical Co., PD 0681, AP 6903, and W 3320 available from H. B. Fuller, or solvent-based pressure sensitive adhesives such as PS 508 sold by Ashland Chemical Co. The adhesives may be used separately or in combination. Preferably, the adhesive is transparent or translucent and most preferably it is a transparent adhesive which remains transparent even after subjecting the assembly to actinic radiation and pressure necessary to image-wise expose and rupture the microcapsules. The amount of the adhesive will vary depending on the nature of the adhesive and the support. The adhesive is generally applied in an amount of about 0.5 to 20 g/m².

[0060] A subbing layer for promoting adhesion between the transparent support and the imaging layer must have good compatibility with the imaging layer, must be transparent, must not effect the sensitometric response of the imaging layer, and must be chemically stable. Amorphous polyesters, which may be applied as an aqueous dispersion, have been found to work well as the subbing layer material. Polymers with molecular weights of 5,000-15,000, with a low hydroxyl number and low acid number, can be employed, for example, the AQ polymers from Eastman Chemical Co. and, more particularly, AQ38 and AQ55. The subbing layer is coated onto the transparent support at a dried coating weight of from about 0.1 to about 5.0 g/m², with a preferred dried coating weight of from about 0.5 to 2.0 g/m².

[0061] Preferably the subbing layer also includes an ultraviolet (UV) ray absorber. Many types of UV absorbing materials have been described in the prior art, including U.S. Pat. Nos. 3,215,530, 3,707,375, 3,705,805, 3,352,681, 3,278,448, 3,253,921, 3,738,837, 4,045,229, 4,790,959, 4,853,471, 4,865,957, and 4,752,298, 5,977,219, 5,538,840 and United Kingdom Patent 1,338,265. Most preferred UV absorbers are polymeric UV absorbers prepared by the method described in U.S. Pat. Nos. 4,496,650, 4,431,726, 4,464,462 and 4,645,735, 5,620,838, EP 0 190 003, U.S. Pat. Nos. 3,761,272, 3,813,255, 4,431,726, 4,455,368, and 4,645,735.

[0062] Suitable photohardenable compositions, photoinitiators, chromogenic materials, carrier oils and encapsulation techniques for the layer of microcapsules are disclosed in U.S. Pat. No. 4,440,846; 4,772,541; and 5,230,982. Although the latter photohardenable compositions are non-silver systems, silver-based photohardenable microencapsulated system such as that described in U.S. Pat. Nos. 4,912,011; 5,091,280 and 5,118,590 and other patents assigned to Fuji Photo Film Co are also suitable for use in the present invention.

[0063] In accordance with the preferred embodiments of the invention, a full color imaging system is provided in which the microcapsules are sensitive to red, green, and blue light, respectively. The photohardenable composition in at least one and preferably all three sets of microcapsules may be sensitized by a cationic dye-borate anion complex, e.g., a cyanine dye/borate complex as described in U.S. Pat. No. 4,772,541. For optimum color balance, the microcapsules are sensitive (lambda max) at about 450 nm, 540 nm, and 650 mn, respectively. Such a system is useful with visible light sources in direct transmission or reflection imaging. Such a material is useful in making contact prints or projected prints of color photographic slides. They are also useful in electronic imaging using lasers, light emitting diodes, liquid crystal displays or pencil light sources of appropriate wavelengths.

[0064] Because cationic dye-borate anion complexes absorb at wavelengths greater than 400 nm, they are colored. The unreacted dye complex present in the microcapsules in the low density image areas can cause undesired coloration in the background area of the final picture, for example, the mixture of microcapsules tends to be green which may give the low density image areas a slight greenish tint. Approaches to reducing undesired coloration in the low density image area as well as the developed image include reducing the amount of photoinitiator used, adjusting the relative amounts of cyan, magenta and yellow microcapsules, or providing a compensating tint in the white opaque support.

[0065] The photohardenable compositions used in the microcapsules may also contain a disulfide coinitiator. Examples of useful disulfides are described in U.S. Pat. No. 5,230,982. By means of the optional use of such disulfides, the amount of the photoinitiators used in the microcapsules can be reduced to levels such that the background coloration or residual stain is less than 0.3 and preferably less than 0.25 density units.

[0066] The photohardenable compositions of the present invention can be encapsulated in various wall formers using conventional techniques, including coacervation, interfacial polymerization, polymerization of one or more monomers in an oil, as well as various melting, dispersing, and cooling methods. To achieve maximum sensitivities, it is important that an encapsulation technique be used that provides high quality capsules which are responsive to changes in the internal phase viscosity in terms of their ability to rupture. Because the borate tends to be acid sensitive, encapsulation procedures conducted at higher pH (e.g., greater than about 6) are preferred. Melamine-formaldehyde capsules are particularly useful. U.S. Pat. No. 4,962,010 discloses a conventional encapsulation useful in the present invention in which the microcapsules are formed in the presence of pectin and sulfonated polystyrene as system modifiers. A capsule size should be selected which minimizes light attenuation. The mean diameter of the capsules used in this invention typically ranges from approximately 1 to 25 microns. As a general rule, image resolution improves as the capsule size decreases. Technically, however, the capsules can range in size from one or more microns up to the point where they become visible to the human eye.

[0067] The developer materials employed in carbonless paper technology are useful in the present invention. Illustrative examples are clay minerals such as acid clay, active clay, attapulgite, etc.; organic acids such as tannic acid, gallic acid, propyl gallate, etc.; acid polymers such as phenol-formaldehyde resins, phenol acetylene condensation resins, condensates between an organic carboxylic acid having at least one hydroxy group and formaldehyde, etc.; metal salts of aromatic carboxylic acids or derivatives thereof such as zinc salicylate, tin salicylate, zinc 2-hydroxy napththoate, zinc 3,5 di-tert butyl salicylate, zinc 3,5-di-(a-methylbenzyl)salicylate., oil soluble metals salts or phenol-formaldehyde novolak resins (e.g., see U.S. Pat. Nos. 3,672,935 and 3,732,120) such as zinc modified oil soluble phenol-formaldehyde resin as disclosed in U.S. Pat. No. 3,732,120, zinc carbonate etc. and mixtures thereof. The particle size of the developer material can affect the quality of the image. In one embodiment, the developer particles are selected to be in the range of about 0.2 to 3 microns, preferably in the range of about 0.5 to 1.5 microns. One or more suitable binders selected from polyethylene oxide, polyvinyl alcohol, polyacrylamide, acrylic latices, neoprene emulsions, polystyrene emulsions, and nitrile emulsions, etc. may be mixed with the developer and the microcapsules, typically in an amount of about 1 to 8% by weight, to prepare a coating composition. A preferred developer material is one which provides good compatibility with the microcapsule slurry solution, for example Schenectady International resin HRJ-4250 solution.

[0068] The self-contained imaging assembly used as photosensitive recording medium is not limited to the embodiments that have been described before, but different variations or modifications thereof are possible. For example, instead of encapsulating the photocurable resin and the polymerization initiator inside the microcapsules of the self-contained imaging assembly, the photocurable resin and the polymerization initiator can also be included in the material constituting the microcapsules. Further, instead of photocurable microcapsules, the self-contained imaging assembly can contain photo-softening microcapsules, for example, microcapsules which have sufficient strength in the unexposed state, and which soften when exposed to light of a predetermined wavelength. In this case it is desirable to perform thermal-curing by heat-fixing.

[0069] There is no need to use red, green and blue light to capture the image in the imaging layer; depending on the characteristics of the photosensitive recording medium, light with various wavelengths can be selected. For example, light emitting elements producing infrared light, red, and green, or light emitting elements producing far infrared light, near infrared light, and red can also be selected. Ultraviolet and far ultraviolet are also advantageous examples of valid color choices for light emitting elements. Moreover, the number of colors of the light emitting elements is not limited to the three colors red, green, and blue; it is equally possible to use only one or two colors, or to select four colors, as in a typical color printer using yellow, magenta, cyan, and black, or even more colors. Furthermore, the choice of light emitting elements includes, but is not limited to LEDs, electroluminescent lamps (EL), light emitting plasma and laser devices, and other light emitting elements.

[0070] The manufacture of an assembly according to the present invention will now be described by way of example. A clear support can be manufactured by lamination or coextrusion of separate materials, for example a barrier sheet comprising a ceramic barrier layer and a polyester layer. These materials can be formed into a clear support with the addition of a UV absorbing subbing layer. The opaque support can be separately manufactured by lamination of separate materials, for example, a polyolefinic material, paper stock, and aluminum coated barrier sheet. These three materials can be formed into an opaque support with the addition of a backing/matte sheet.

[0071] An emulsion or image layer can then be applied to the clear support to form the first of two final components of the assembly, an emulsion-containing component which is then combined with the opaque support to form a final laminate. Preferably the imaging layers are preconditioned prior to making the final laminate, especially when using barrier supports since it would be difficult to introduce moisture into the imaging layers once the first and second supports are laminated together. The final laminate may then be cut and edge sealed, followed by packaging and bar coding.

[0072] In order to insure that the imaging system is effectively sealed between the supports, a subbing layer is provided to attach the transparent support to the imaging layer and an adhesive is provided between the back support and the imaging layer. For optical clarity, the subbing layer will typically be located between the first support and the imaging layer. However, which support receives the subbing layer and which support receives the adhesive is a function of which support is coated with the wet imaging layer composition and which is assembled with the coated and dried imaging layer. The support which is coated with the imaging layer composition (which is typically the front support) will be provided with the subbing layer and the support which is assembled will receive the adhesive. In accordance with the preferred embodiment of the invention, the subbing layer is formed from a compound having chemical moieties such as hydroxy groups which will react with and bind to the microcapsules.

[0073] To maximally maintain the relative humidity within the assembly, the assembly is sealed on the sides (edges).

[0074] As indicated above, the assembly is preconditioned before the final lamination to maintain said imaging layers at a relative humidity of at least 50 percent, preferably within the range 55 to 90%, more preferably from 60 to 85%. In the typical operation, the imaging layers are applied over the transparent support which comprises an adhesion promotion subbing layer. The subbing layer is located between the imaging layers and the transparent barrier support. The imaging layers coated over the transparent support are then conditioned at the preselected relative humidity for a period of time. The time duration can vary but preferably it is long enough to allow the moisture content of the imaging layer to reach its isotherm value, thereafter laminating the opaque support on the above coated imaging layer. This can be accomplished, as mentioned above, by using a pressure sensitive adhesive which is applied prior to lamination on the face side of the opaque support.

[0075] The peripheral edges of the above laminated imaging assembly can be sealed at the preselected relative humidity by any conventional means used to seal polymeric materials. Preferred sealing techniques for the practice of the present invention include impulse heating seal, ultrasonic seal and induction seal.

[0076] Impulse heating seal method follows a very simple resistance heating principle. In this method a heating tool is a metal band made of a special alloy. The band becomes hot when a current flows through it. By measuring the current and the voltage on the band a control system can precisely control the band temperature, usually to within 1° C. The hot surface melts the moisture barrier supports and the imaging layers, and a bond is formed among the supports and imaging layer under pressure. In a typical process, a rectangular ship band will be made to seal four edges of the self-contained imaging assembly. The heating time is typically about 1.5 seconds, but another 1.5 seconds cooling period is usually needed to complete the full seal.

[0077] The ultrasonic seal method uses a “horn” to couple mechanical vibrations from a transducer connected with an ultrasonic power supply to the peripheral edges of the imaging assembly. The high frequency (about 20 KHz) mechanical vibrations generate heat inside the edges of the imaging assembly because of its visco-plastic property. The heat can be controlled by the vibration amplitude of the horn. There are two setups for ultrasonic thin film sealing. The first one is called plunge operation in which a single horn with a rectangular bottom surface seals the four edges of the imaging assembly. The sealing usually takes about 1 second, and there is no need of cooling time. The second one is called a sewing operation. In operation, the horn is stationary and a guiding wheel is under the horn. Guided by the wheel, the imaging assembly continuously passes through in between the horn and the wheel and gets sealed. The speed of the process will depend on the power level, materials and pressure.

[0078] The edge seal also can be done on an induction edge knurling system. The basic working tools of the system are a upper knurling wheel and a bottom pressure roller. The upper knurling wheel is heated by an induction coil connected to a RF power generator. The imaging assembly is sealed when it passes through between the wheel and the roller. A control system controls the speed, the pressure and the temperature of the process. With an IR sensor the accuracy of the knurling temperature can be controlled within 2° F.

[0079] The imaging assembly of the present invention can be exposed in any suitable camera or other exposure device to provide an image. The imaging assembly of this invention is especially suitable for exposure using a liquid crystal array or light emitting diodes driven by a computer generated signal or a video signal for the reproduction of images from a video cassette recorder, a camcorder, or the like. It is possible to utilize, for example, with the current state of technology, a very economical compact printer, weighing under 500 g and having a size less than 100,000 mm³that prints directly from a digital camera utilizing a CompactFlash™ card interface and provides a resolution of 150 ppi or more with continuous tone and over 250 gradation levels.

[0080] The print is “developed,” based on the “latent image” formed by the selectively photohardened microencapsulated color formers, by the application of pressure or by the application of both heat and pressure. See, for example, the image forming device described in U.S. Pat. No. 5,884,114 to Iwasaki, in which a photo and pressure sensitive printer provides the feeding and discharging of a photosensitive imaging medium at the front of the printer housing, which device can have the added advantage of being easily integrated into other equipment such as a personal computer. In this particular device, the latent image is formed by a movement in the main scanning direction of an LED-type exposure head. Thereafter, an upper nip roller of a developing mechanism is moved from a separated position to a pressing position. The capsules that have not been photohardened are ruptured by pressure and a full color image is formed on the sheet, heat-fixing (which is optional to the present invention) is performed by a film heater, and the imaged assembly is discharged from the front of the housing for the device or printer.

[0081] A typical pressure-type image-forming device (which can be referred to as a printer) typically comprises a printer housing with a lightproof cartridge for accommodating photosensitive imaging media (alternately referred to as recording media) mounted to the front of the printer housing so as to be easily detachable. In some devices, a preheater is employed for preheating the photosensitive imaging medium. A typical exposure mechanism may include an exposure head for exposing while scanning in a direction perpendicular to the surface of the drawing and a developing mechanism for pressure development by means of a pair of an upper and a lower nip roller. The roller may be maintained under pressure by a spring. An optional fixing heater for heat-fixing the developed photosensitive imaging medium may be used. A discharge tray may be provided at the rear end of the printer housing. The pressure sensitive printer may be designed so that sheets are both fed and discharged at the front side of the printer housing.

[0082] An image forming device for treatment of the imaging media can, for example, comprise exposure means for forming a latent image on the imaging medium upon exposure based on image information, developing means for developing the latent image by means of the coloring material coming out of the microcapsules when pressure is applied to the photosensitive imaging medium on which the latent image was formed by the exposure means, wherein the developing means comprise a pair of an upper and a lower nip roller facing each other and sandwiching the transport path of the photosensitive imaging medium, pressing means for pressing one nip roller against the other nip roller, roller switching means for alternately switching between a pressing position in which the one nip roller is brought into pressure contact with the other nip roller and a separated position in which the one nip roller is separated from the other nip roller, and a transport path for transporting the photosensitive imaging medium comprises a feed path for feeding the photosensitive imaging medium on the inlet side, a discharge path for discharging the recorded photosensitive imaging medium.

[0083] In one embodiment, the developing mechanism may comprise a pair of an upper and a lower nip roller, a rectangular frame fixed inside the printer housing for supporting the nip rollers, a pair of compression springs for pressing both ends of the roller axis of the upper nip roller toward the lower nip roller, and a roller switching mechanism for alternately switching between a pressing position in which the upper nip roller is brought into pressure contact with the lower nip roller and a separated position in which it is separated from the lower nip roller. If the pressing force of each of the springs is 150 kgf, the upper nip roller presses with a total force of 300 kgf on the lower nip roller. However, other means for applying pressure can be employed, for example, a pressure stylus.

[0084] A control unit for the image-forming device may comprise a CPU, a ROM and a RAM, an I/O interface, and drive circuits, wherein a stepping motor for paper transport, a solenoid actuator for driving a switching plate, a film heater, a motor for roller switching, a stepping motor for driving the carriage, the exposure head, etc., are respectively connected to the drive circuits. A connector and a control panel may also be connected to the control unit. In one embodiment, image data (RGB image data) from an external host computer may be fed via a connector to the control unit.

[0085] The ROM can store control programs for controlling all operations of the printer, a control program for calculating, from the input image data, the duration for which each LED of the exposure head is turned on and the timing thereof, a control program for controlling the transport of the self-contained imaging assembly by controlling the stepping motor for sheet transport synchronously with the exposure to green, red and blue light, a control program for controlling the scanning of the exposure head by controlling the stepping motor for driving the carriage synchronously with the exposure to green, red and blue light, etc. The different buffers and memory types necessary for running the control programs are in the RAM. The number of copies to be printed, the enlargement or reduction ratio of the image, the size of the image forming area of the imaging assembly, etc., input by an operator at the control panel, can be stored in the memory of the RAM. Exposure can take place upon calculation of the driving conditions for the stepping motor.

[0086] In one type of image-forming device, when image data of an image is sent to the control unit, the image data is divided into R image data, G image data, and B image data and stored in a buffer of the RAM. Each LED of an exposure heat can be electrically driven by a drive circuit via a cable.

[0087] In one embodiment of practicing the invention, imaging medium sheets may be packaged as a stack of sheets which go into the printer. The individual sheets may be picked from the stack of sheets and transported into the “printing path” of the printer. However, if two or more sheets at the same time are picked up and fed into the printing path the printer, the printer may become jammed, requiring disassembly by the user. To avoid this problem, the static in the sheets may be reduced or eliminated just prior to the final packaging; and a precision hinge on the printer film cassette or tray may be used. Also, a method to further aid the feeding of sheets into the printer is to add a “back coat” or backing layer to the imaging medium. This is preferably coated on the outer surface of the bottom or second support (which has the adhesive on the opposite side), typically the white support layer. This backing layer material may be formulated to have some perceived aesthetic value (such as a writable or printable coating), but must have some difference relative to the adjacent next sheet surface which causes these surfaces not to adhere together. The preferred backing layer material has a slight roughness to aid the “printer pickup roller” initiating the sheet movement into the printer, for example, a Sheffield Smoothness of 150-180 Sheffield units, such as a standard face matte coating on 3M label stock material. Other commercially available or known matte coatings can also be used. In general, these coatings may include a binder and a grit or abrasive such as silica. Preferably, the front side of the first support and the back side of the second support has a coefficient of friction of less than 0.4.

[0088] The present invention is illustrated in more detail by the following non-limiting example.

[0089] This example illustrates the effect of relative humidity on the sensitometric properties of the imaging assembly. Imaging media containing photohardenable microcapsules in an imaging layer, commercially available from Cycolor, Inc, were obtained packaged in a plastic cartridge. The media in the cartridge were preconditioned at various relative humidities and 30° C. for 24 hours. Prints were then made with these preconditioned media using a Vivitar VPP-150™ Photo Printer, and these prints provided the sensitometric response for the media. The respective developed minimum color densities (Dmin) of the prints were measured via a red filter, a green filter and a blue filter in order to obtain Dmin for the cyan dye, magenta dye, and yellow dye, respectively. Table 1 below shows the results for the percentage increase in Dmin values relative to the fresh media. The fresh Dmin values for the yellow dye, magenta dye, and cyan dye were, respectively 0.11, 0.12, and 0.17.

[0090] The results in Table 1 above show that the Dmin values increase significantly at and below 50% relative humidities.

[0091] This example illustrates the effect of a moisture barrier on the sensitometric response of an imaging assembly containing photohardenable microcapsules, wherein the media in the cartridge were, preconditioned and maintained at 80% relative humidity/70° F. for 3 days compared to a control equilibrated to 40% relative humidity/70° F. Imaging media containing photohardenable microcapsules in an imaging layer, commercially available from Cycolor, Inc, were obtained packaged in a plastic cartridge. Prints were then made with some of these preconditioned media using a Vivitar VPP-150™ Photo Printer. These prints provided the sensitometric response for fresh media. The remaining preconditioned media were then removed from the cartridge and separated in a darkroom into two groups, Group I and Group II. Individual samples of media in group I were packaged in separate bags made of a barrier material having a water vapor transmission rate of about 0.47 g/m²/day (0.03 g/100 in²day) in a darkroom at 80% relative humidity. The bag was made to essentially the same dimensions (length times width) as the media and was sealed on the edges by heat. The media in Group II were used as the control. Both groups of media were then kept in a darkroom at 40% relative humidity/70° F. and prints were made with a Vivitar VPP-150™ Photo Printer at given time intervals, 0, 2, 4, 7, 14, 28, 35, 42 and 49 days, to determine the effect of the lower humidity on sensitometric response for the two groups of media. The Group I media were removed from the sealed bag and loaded into an empty cartridge just prior to printing. The respective optical densities (Dmin) of prints from both groups were measured using an X-Rite 820TR™ Densitometer in order to obtain Dmin values for the cyan dye, magenta dye, and yellow dye, respectively. The results thereof are shown in FIGS. 7 to 9.

[0092] FIG. 8 shows the Dmin for the yellow dye for prints made from media from Group I and Group II (control) that were exposed to 40% relative humidity for various times.

[0093] FIG. 9 shows the Dmin for the magenta dye for prints made from media from Group I and Group II (control) that were exposed to 40% relative humidity for various times.

[0094] FIG. 10 shows the Dmin for the cyan dye for prints made from media from Group I and Group II (control) that were exposed to 40% relative humidity for various times.

[0095] These experiments show that the Dmin for the control (Group II media) increased significantly after only 2 days. On the other hand, the Group I media that had a moisture barrier showed no change in Dmin after exposure to 40% relative humidity for as much as 35 days. The results of this study demonstrate that laminating the media with a barrier material according to the present invention would minimize variations in sensitometric response of the imaging assembly due to changes in humidity.

[0096] Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.