Process useful in providing transparencies or reproductions having a controlled gloss finish

Abstract: Material and processes useful in providing visible images through reaction with a colour precursor and useful in providing transparencies or in providing reproductions with a controlled degree of gloss are described. A developer sheet material useful in forming reproductions having a controllable gloss finish, or in forming transparencies has a support with a layer of a finely divided, thermoplastic, developer material on its surface. The developer material is capable of reacting with a colour precursor to produce a visible image and is also capable of coalescing into a thin film which imparts gloss to the image upon application of heat and/or pressure. Photosensitive material in which images are formed by reaction of one or more chomogenic materials and a developer, in addition to the support and the developer material has a layer of microcapsules over­ lying the layer of developer material. The microcapsules contain a photosensitive composition which undergoes a change in viscosity upon exposure to actinic radiation and a colour precursor.

Description: The present invention relates to processes providing visible images through reaction with a colour precursor and useful in providing transparencies or in providing reproductions having a glossy finish. As explained in detail below, processes in accordance with the present invention can be designed to provide a controlled degree of gloss ranging from matte to high gloss. Specific embodiments of our process involve the use of conventional pressure-sensitive copy paper or photo-sensitive imaging systems employing microcapsules to provide visible images upon contact of a colour precursor (which is image-wise released from the microcapsules and transferred to a developing sheet) with the developer thereon. Pressure-sensitive copy paper is well known in the art. It is described in U.S. Patents 2,550,446; 2,712,507; 2,703,456; 3,016,308; 3,170,809; 3,455,721; 3,466,184; 3,672,935; 3,955,025 and 3,981,523. Photosensitive imaging systems employing micro-encapsulated radiation sensitive compositions are described in our U.S. Patents Nos. 4,399,209 and 4,416,966 and in our British Patent Specifications Nos. 2,112,536 and 2,187,298. These imaging systems are characterised in that an imaging sheet including a layer of microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photosensitive composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a colour precursor. Exposure image-wise hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is subjected to an uniform rupturing force by passing the sheet through the nip between a pair of pressure rollers. U. S. Patent No. 4,399,209 discloses a transfer system in which the imaging sheet is assembled with a developer sheet prior to being subjected to the rupturing force. Upon passing through the pressure rollers in contact with the developer sheet, the microcapsules rupture and image-wise release the internal phase whereupon the colour former migrates to the developer sheet where it reacts with a dry developer and forms a colour image. The imaging system can be designed to reproduce monochromatic or polychromatic full colour images. Our U. S. Patent 4,578,339 discloses oil-containing microcapsules on the imaging sheet and/or the developer sheet. The function of the dyeless microcapsules is to release from the photosensitive microcapsules to reduce its viscosity and thereby increase the mobility of the chromogenic material. As a result, the interaction between the developer material and the chromogenic material on the developer sheet is improved. While the reference discloses numerous examples of colour developers, they are not all useful in practice of the present invention because they are not necessarily designed to film out upon the application of heat. In those applications in which the aforementioned pressure-sensitive and photosensitive imaging systems are used to reproduce graphic or picture images, a high degree of gloss is often desired in the reproduction. In particular, where a transparency is desired, the reproduction must transmit light effectively. These objectives are difficult to achieve using conventional approaches. Because the reaction between the developer and the colour former occurs principally at the surface of the developer, finely divided developers such as resin grinds are the developer of choice because they provide a large reactive surface area to react with the colour former and provide high optical density. However, these compositions usually contain fine dispersing agents or pigments which scatter light and make the image appear dull. One technique which has been used previously to produce high gloss images is described in U. S. Patent No. 4,554,235. There, the developer layer is overcoated with a discontinuous layer of a thermoplastic polymeric pigment. After exposure, the imaging sheet is assembled with the developer sheet and subjected to pressure. The colour former is released from the microcapsules, passes through the layer of polymeric pigment, and reacts with the developer. Thereafter, the thermoplastic pigment is subjected to heat and/or pressure which causes the pigment to coalesce into a transparent continuous thin film which imparts a glossy finish to the image. In accordance with a first aspect of the present invention we provide a process for forming images which comprises the steps of: image-wise exposing to actinic radiation photosensitive material comprising a support having a layer of a finely divided, thermoplastic, developer material on the surface thereof, and a layer of microcapsules overlying said layer of developer material, said microcapsules containing a photosensitive composition which undergoes a change in viscosity upon exposure to actinic radiation and a colour precursor, said developer material being capable of reacting with said colour precursor to produce a visible image; subjecting said layer of microcapsules to a uniform rupturing force such that said colour precursor is image-wise released from said microcapsules to said developer material, whereby to form an image; and coalescing said developer material into a thin film which imparts gloss to said image. In a second and alternative aspect of this invention, we provide a process for forming images which comprises the steps of: image-wise transferring a colour precursor to the reactive surface of developer sheet material comprising a support having a layer of a finely divided, thermoplastic, developer material on the surface thereof, said developer material being capable of reacting with the colour precursor to produce a visible image; and coalescing said developer material into a thin film which imparts gloss to said image. We have found that certain thermoplastic resins and other developer materials are capable both of reacting with a colour precursor to produce a visible image and subsequently of being coalesced into a continuous film. This film is essentially transparent. It imparts a gloss finish when the image is formed on an opaque background and it transmits light efficiently when the image is formed on a transparent background to provide a transparency. Furthermore, the degree of gloss can be adjusted by controlling the coalescence conditions (e.g. heat treatment). Thus, by limited coalescence, finishes ranging from matte to high gloss (20% to 90% gloss) can also be provided. Processes in accordance with the second aspect of the invention are carried out with two sheets, namely a donor sheet and a receiver sheet. In one embodiment the developer materials are film forming, thermoplastic, phenolic resins (e.g., phenol-formaldehyde resins). These resins are provided on a support as finely divided particles which usually range from about 0.1 to 25 micrometers in particle size. The developer sheet is used in a conventional manner except, after or simultaneously with development, the developer particles are subjected to heat and/or pressure which causes them to coalesce into a film. Our process can be adapted to be carried out with self-contained systems in which the image is viewed through a transparent support against a white background. Our process can also be adapted to be used to record microfiche or microfilm images. The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which:- Fig. 1 is a cross-sectional schematic view of an embodiment of developer sheet material for use in an example of a process in accordance with the present invention; Fig. 2 is a view of the developer sheet of Fig. 1 after image-wise transfer of a colour precursor thereto; Fig. 3 is a view of the developer sheet of Fig. 2 after coalescence of the developer resin to provide a high gloss finish; Fig. 4 is a view of the developer sheet of Fig. 2 after coalescence to provide a matte finish; Fig. 5 is a cross-sectional schematic view of another embodiment of developer sheet material which may also be used in an example of a method in accordance with the present invention; Fig. 6 is a cross-sectional schematic view of yet another embodiment of developer sheet material which may be employed in an example of a process in accordance with the present invention; Fig. 7 is a cross-sectional view of an embodiment of self-contained imaging sheet useful in another example of a method in accordance with the present invention; and Figs. 8-10 are cross-sectional views illustrating exposure and development processing of the self-contained imaging sheet of Fig. 7. The developer sheet schematically shown in Fig. 1 is designated generally by the reference numeral 10. The sheet 10 includes a support 12 which is overcoated by a layer 14 of a finely divided thermoplastic developer material. The layer 14 may consist of developer material or be an admixture of the developer material with a binder which adheres the developer material to the support. Preferably, the developer material adheres to the support without a binder. The support 12 may be paper or a transparent film such as polyethylene terephthalate. Upon image-wise transfer of a precursor to the surface of sheet 10, a visible image is formed in layer 14 as shown by the cross-hatching 16. The visible image is usually the product of an acid-base reaction between the precursor, which is usually an electron donor, and the developer, which is usually an electron acceptor. After developing the image 16, the developer sheet is subjected to heat and/or pressure to coalesce the developer material. Depending on the processing conditions which are used (more particularly the amount of heat and pressure applied), degrees of gloss ranging from matte to low gloss to high gloss can be obtained. Fig. 3 schematically illustrates a high gloss finish. The developer 14 is essentially completely coalesced into a thin, continuous, translucent film. An intermediate degree of gloss is schematically shown in Fig. 4 in which partial coalescence of the layer 14 gives a finish which is glossier than the uncoalesced intermediate of Fig. 2 but less glossy than the finish of Fig 3. Where the developer sheet is to be used as a transparency, the support film is transparent and complete coalescence of the developer resin is used. In Fig. 5, the developer sheet 10 includes a support 12 which is overcoated by a layer of oil-containing microcapsules 18 and a layer of a finely divided thermoplastic developer material 14. In Fig. 6, the developer sheet 10 includes a support 12 which is overcoated by a dispersion of the oil-containing microcapsules 18 and the finely divided thermoplastic developer material 14. Although not illustrated, it should be understood that when the developer sheet 10 is used with an imaging sheet, the oil-containing microcapsules 18 can be present on the surface of the imaging sheet instead of on the surface of the developer sheet 10. The principal function of the microencapsulated oil is to reduce the amount of the heat and time required during the coalescing step to cause the developer resin 14 to melt and to provide a more uniform gloss. In the past, high density image regions have tended to gloss more quickly due to the oil present than low density image regions. By incorporating the microencapsulated oil either in the developer layer of the developer sheet 10 or on the surface of the imaging sheet, the oil serves to plasticize the developer resin and accelerate film formation. Coalescable thermoplastic developer resins are also useful in self-contained imaging systems of the type described in U.S. Patent 4,440,846. An example is provided in Fig. 7. Therein a self-containing imaging sheet 20 is shown. The imaging sheet 20 includes a substrate 22 coated with a layer of microcapsules 24. The microcapsules are filled with an internal phase 26 containing a chromogenic material and a photosensitive composition. Interposed between the layer of microcapsules 24 and the substrate 22 is a layer of the coalescable developer resin 30. The imaging sheet 20 may be image-wise exposed from side 42 carrying the layer of microcapsules 34. It is particularly desirable to form substrate 22 from a transparent film. In this case, the imaging material may be image-wise exposed to actinic radiation from either side 42 or side 44. Typically, the imaging material is exposed from side 42 and viewed from side 44. Image-wise exposure of the imaging material 20 by transmission imaging is shown in Fig. 8 wherein a source of radiation-L is positioned above the surface of the imaging sheet 20 with a Mask M therebetween. In this illustration, the photosensitive material is a radiation curable material and hence, the imaging material is positive working. Exposure solidifies the internal phase 26′ in the exposed areas 36 whereas the internal phase 26 remains liquid in the unexposed areas 38. Fig. 9 illustrates the imaging sheet 20 after the microcapsules have been ruptured. For simplification, the microcapsules are shown as being ruptured in the unexposed areas 38 and unruptured in the exposed areas 36. In actuality, all or a portion of the microcapsules may also rupture in area 36 and there will be areas of intermediate exposure in which all or a portion of the capsules may also be ruptured. In unexposed areas 38, the chromogenic material and the developer react to form a visible image I. Upon heating the imaging material 20, the developer layer 30 becomes essentially transparent as shown in Fig. 9. After exposure, development and coalescence of the developer 30, the image can be viewed. If the substrate 22 is opaque, the image is viewed from side 42. On the other hand, if substrate 22 is transparent, the image can be viewed from side 44. When substrate 22 is transparent, it is particularly desirable to laminate the self-contained material 20 to a sheet of plain paper or a similar material providing a white background. This is illustrated in Fig. 10 wherein the developed imaging material from Fig. 9 is shown laminated to a sheet of plain paper 50 by means of a transparent adhesive 52. In this manner, the paper or similar material provides a white background against which the image I can be viewed. Typically, the microcapsules are ruptured by passing the imaging sheet alone (in the case of a self-contained material) or in combination with a developer sheet (in the case of a transfer material), through a pair of pressure rollers. In addition to pressure, however, a magnetic brush as described in U.S. Patent 4,592,986 or free particles as described in U.S. Patent 4,578,340 may be used. The developer materials used in practice of the present invention are finely divided thermoplastics. Their softening points typically range from about 100 to 200°C, but those skilled in the art will appreciate that materials with higher and lower softening points may also be useful. The particle size is preferably in the range of about 0.5 to 25 microns. A typical example of a thermoplastic developer material useful in the practice of the present invention is phenolic resins. Phenolic resins have been conventionally used as developer materials in pressure-sensitive recording materials. These resins may be the condensation product of phenols (including substituted phenols) and formaldehyde. The resins may be further modified to include amounts of salicylic acids or substituted salicylic acids in a manner known in the art. Examples of phenolic resins useful in practice of the present invention are described in U. S. Patent Nos 3,455,721; 3,466,184; 3,672,935; 4,025,490; and 4,226,962. Another class of phenolic resin useful in practice of the present invention is the products of oxidative coupling of substituted or unsubstituted phenols or biphenols. Oxidative coupling may be catalyzed by various catalysts but a particularly desirable catalyst is the enzyme, horseradish peroxidase. A particularly desirable developer is the class of resins described in our U. S. Patent 4,647,952, the disclosure of which is to be regarded as incorporated herein by reference, and more particularly the product of oxidative coupling of bisphenol A. Phenolic developers used in practice of the present invention may be metallated to improve their developing characteristics. They may be metallated by reaction with a salt selected from copper, zinc, aluminium, tin, cobalt and nickel salts. Most typically, the resins are zincated to improve development. The metal content of the resins generally is about 1 to 5% by weight but may range up to 15%. Preferably developer materials such as phenol-formaldehyde condensation products are used. More particularly, alkylphenolic resins and, still more particularly, metallated products of alkylphenolic resins are used. The alkyl phenols are monosubstituted by an alkyl group which may contain 1 to 12 carbon atoms. Examples of alkyl phenols are ortho - or para -substituted ethylphenol, propylphenol, butylphenol, amylphenol, hexylphenol, heptylphenol, octylphenol, nonylphenol, t-butylphenol, t-octylphenol, etc. Another class of thermoplastic developer material which may be used in practice of the present invention is a resin-like condensation product of a polyvalent metal salt, such as a zinc salt, and a phenol, a phenol-formaldehyde condensation product, or a phenol-salicylic acid-formaldehyde condensation product. Such developer materials are available from Schenectady Chemical Co., under the designation HRJ 4250 and HRJ 4252. These products are reported to be a metallated condensation product of an ortho - or para -substituted alkylphenol, a substituted salicylic acid, and formaldehyde. It is critical that the developer materials employed in practice of the present invention be present on the surface of the developer sheet as finely divided particles or microspheres. In the past, finely ground developer resins have been obtained by grinding a mixture of a developer such as a phenolic resin or a zinc salicylate with another resin and clay to produce a resin grind which is applied to the surface of the developer sheet. See, for example, U. S. Patent 3,924,027. This process is not desirable because fine particles of uncoalesced material remain in the coating after processing which detract from the transparency and glossiness of the layer. Finely divided thermoplastic developer material useful in practice of the present invention may be obtained by several processes. A developer material can be prepared in a conventional manner and a melt of the material can be atomized. Alternatively a melt of the developer material can be injected into a rapidly agitated aqueous medium whereupon the melt is solidified as droplets which are recovered. The developer material can also be dissolved in a solvent/non-solvent system and the solvent removed. Other materials such as Schenectady HRJ 4250 and HRJ 4252 resins are obtained in a dispersed form. The developer layer may consist of thermoplastic developer material or an admixture of a thermoplastic developer material and a thermoplastic latex, or an admixture of a thermoplastic developer material and a binder having a refractive index which closely approximates or is equal to that of the melted thermoplastic developer material. (The terms "coalescable" and "thermoplastic" as used herein refer to materials which will form a film upon processing the developer layer as previously described.) Useful oils for the oil-containing microcapsules 18 include oils conventionally used as "carrier oils" in the carbonless pressure-sensitive paper art. Useful examples include alkylated biphenyls (e.g., monoisophylbiphenyl), polychlorinated biphenyls (environmentally hazardous), castor oil, mineral oil, deodourized kerosene, naphthenic mineral oils, dibutyl phthalate, dibutyl fumerate, brominated paraffins etc. As indicated above, the support for the developer sheet may be transparent or opaque. In the most typical embodiments, it is paper or a synthetic film such as polyethylene terephthalate. The developer sheet may be prepared by coating a support with a coating composition of the finely divided developer material using conventional coating techniques. The coating composition may be an aqueous suspension or emulsion of the developer. The developer material is usually applied to the surface of the support in an amount of about 8 to 15g/m². Depending upon the nature of the developer material and how it is obtained, a binder may be necessary to adhere the developer resin to the support. In most cases, heat or a combination of heat and pressure is used to coalesce the developer resin. In a typical example of a process in accordance with the present invention, the colour precursor is image-wise transferred to the developer sheet by typing, in pressure-sensitive recording, or by passing an exposed photosensitive imaging sheet through the nip between a pair of pressure rollers in photosensitive systems. The developer resin is subsequently coalesced, for example, by heating the developer sheet by contact with heated roller or a pair of heated rollers or by passage of the developer sheet over a heated platen. Alternatively, the developer sheet can be placed in a hot oven. A photosensitive imaging sheet can be assembled with the developer sheet and the colour precursor can be transferred and the developer resin coalesced simultaneously by passage through a pair of heated pressure rolls. This practice is, however, sometimes undesirable because the developer resin can coalesce before there is adequate reaction of the colour precursor. Developer layers, particularly developer layers prepared by coating aqueous suspensions or dispersions of finely divided thermoplastic developer particles on a support, are particularly useful in applications such as copying microfilm or microfiche images where very high resolution is required. We have shown that internal phase transfer to the surface of a developer sheet can be described in terms of a capillary model (Washburn equation) in which the depth of liquid penetration is proportional to the square root of both the average pore radius and the contact time between the internal phase and the capillaries defined by the spaces among the developer particles on the surface of the developer sheet. We have also shown that resolution improves as the void volume of the developer layer increases. By controlling the particle size of the developers, we have provided developers which provide both good capillary action and higher void volume. These developers provide the high resolution images required in microfiche or microfilm imaging or copying. Resolutions of 100 line pairs/mm have been achieved.

Claims: A process for forming images which comprises the steps of: image-wise exposing to actinic radiation photo-sensitive material comprising a support having a layer of a finely divided, thermoplastic, developer material on the surface thereof, and a layer of microcapsules overlying said layer of developer material, said microcapsules containing a photosensitive composition which undergoes a change in viscosity upon exposure to actinic radiation and a colour precursor, said developer material being capable of reacting with said colour precursor to produce a visible image; subjecting said layer of microcapsules to a uniform rupturing force such that said colour precursor is image-wise released from said microcapsules to said developer material, whereby to form an image; and coalescing said developer material into a thin film which imparts gloss to said image. A process according to Claim 1, comprising the additional step of assembling said photosensitive material with a material which provides a white background such that said layer of microcapsules is interposed between a transparent substrate and said material providing said white background. A process according to Claim 1, wherein said support is transparent and said images are microfiche or microfilm images requiring high resolution. A process for forming images which comprises the steps of: image-wise transferring a colour precursor to the reactive surface of developer sheet material comprising a support having a layer of a finely divided, thermoplastic, developer material on the surface thereof, said developer material being capable of reacting with the colour precursor to produce a visible image; and coalescing said developer material into a thin film which imparts gloss to said image. A process according to Claim 4, wherein said step of coalescing said developer material substantially completely coalesces said developer material. A process according to Claim 4, wherein said step of coalescing said developer material only partially coalesces said developer material. A process according to any of Claims 4, 5 or 6, wherein said step of image-wise transferring said colour precursor includes the steps of image-wise exposing to actinic radiation an imaging sheet including a support having a layer of microcapsules on the surface thereof, said microcapsules including a photohardenable or photo-softenable photosensitive composition as the internal phase and having a chromogenic material associated therewith; assembling said image-wise exposed imaging sheet with said developer sheet material; and subjecting said imaging sheet to a uniform rupturing force such that said microcapsules rupture and release said internal phase in accordance with said image-wise exposure. A process according to Claim 7, wherein said layer of finely divided, thermoplastic developer material additionally comprises oil-containing microcapsules, said microencapsulated oil being capable of plasticizing said developer material, and wherein the steps of assembling said image-wise exposed imaging sheet with said developer sheet material and subjecting said imaging sheet to a uniform rupturing force cause oil-containing microcapsules to be ruptured so that said oil is released and plasticizes said developer material. A process according to Claim 7, wherein said imaging sheet additionally comprises oil-containing microcapsules, and wherein the steps of assembling said image-wise exposed imaging sheet with said developer sheet material and subjecting said imaging sheet to a uniform rupturing force cause oil-containing microcapsules to be ruptured so that said oil is released and plasticizes said developer material. A process according to any preceding claim, wherein said developer material is a phenolic resin, preferably a phenol-formaldehyde resin. A process according to any preceding claim, wherein said developer material has a particle size of about 0.5 to 25 micrometers. A process according to any preceding claim, wherein said developer material has a softening point less than 200°C. A process according to Claim 12, wherein said developer material is an alkylphenol-formaldehyde resin, preferably metallated alkylphenol-formaldehyde resin. A process according to Claim 12, wherein said developer material is a metallated condensation product of a phenol and formaldehyde, or a condensation product of a phenol, formaldehyde and a salicylic acid. A process according to Claim 12, wherein said developer material is a product of oxidative coupling of a phenol or biphenol. A process according to Claim 15, further characterised in that said phenol or biphenol is bisphenol-A.

Inventors: ADAIR PAUL C, MCCRARY CARL, METZ GEORGE D, O'CONNOR JOSEPH G, ADAIR, PAUL C., MCCRARY, CARL, METZ, GEORGE D., O'CONNOR, JOSEPH G.
Applicants: MEAD CORP, THE MEAD CORPORATION
Application year: 1987
Main classification: G03C1
IPC codes: G03C1, G03F7