FLEXOGRAPHIC PRINTING FORM PRECURSOR AND A METHOD FOR MAKING THE PRECURSOR

Abstract

The invention pertains to a photosensitive element, particularly a photopolymerizable printing form precursor; and, a process of making the photosensitive element. The printing form precursor includes a cover sheet, a layer of a photosensitive composition, and a digital layer, or infrared ablation layer, that is adjacent to a side of the photosensitive layer. A microcell patterned is embossed onto the infrared ablation layer or an overcoat/barrier layer on the infrared ablation layer. Since the microcell pattern layer is integral with the printing form precursor, digital imaging can occur rapidly with relatively low resolution optics to form a mask without needing to also form a microcell pattern of the digital layer. The printing form precursor having the integrated microcell pattern layer facilitates the preparation of relief printing forms to have a print surface suitable for printing solids with uniform, dense coverage of ink.

Claims

1. A printing form precursor comprising: (a) a photopolymerizable layer comprising a first binder, a monomer, and a photoinitiator, wherein said photopolymerizable layer is supported by a support layer; (b) an infrared ablation layer that is ablatable by infrared radiation and opaque to non-infrared actinic radiation, the infrared ablation layer comprising: (i) at least one infrared absorbing material; (ii) a radiation opaque material, wherein (i) and (ii) can be the same or different; and (iii) at least one second binder; wherein said infrared ablation layer is embossed with a microcell pattern on the side facing the photopolymerizable layer; and (c) a coversheet.

2. The printing form precursor of claim 1, wherein said infrared ablation layer is embossed with a microcell pattern, and is applied by lamination to a surface of the photopolymerizable layer that is opposite the support.

3. The printing form precursor of claim 1, wherein said infrared ablation layer has a transmission optical density of greater than 2.0.

4. The printing form precursor of claim 1, wherein said microcell pattern comprises a plurality of features wherein each feature has an area of between 5 to 750 square microns.

5. The printing form precursor of claim 1, wherein said first binder is different from said second binder.

6. A printing form precursor comprising: (a) a photopolymerizable layer comprising a first binder, a monomer, and a photoinitiator, wherein said photopolymerizable layer is supported by a support layer; (b) an infrared ablation layer that is ablatable by infrared radiation and opaque to non-infrared actinic radiation, the infrared ablation layer comprising: (i) at least one infrared absorbing material; and (ii) a radiation opaque material, wherein (i) and (ii) can be the same or different; and (iii) at least one second binder; wherein said an infrared ablation layer contains an overcoat/barrier layer which is thermally embossable on the side facing the photopolymerizable layer and is embossed with a microcell pattern; and (c) a coversheet.

7. A printing form precursor comprising: (a) a coversheet: (b) a release layer, wherein said release layer is embossed with a microcell pattern on the side opposite the coversheet; and (c) a photopolymerizable layer comprising a binder, a monomer, and a photoinitiator, wherein said photopolymerizable layer is between the release layer and a support layer.

8. A method of making a printing form precursor comprising: a) providing a clear thermal polymer cover sheet; b) providing an infrared ablation composition forming an infrared ablation layer on the cover sheet, the infrared ablation composition comprising (i) at least one infrared absorbing material; (ii) a radiation opaque material, wherein (i) and (ii) can be the same or different; and (iii) at least one second binder; c) embossing the infrared ablation layer with a microcell pattern on the side opposite the cover sheet; and d) applying a photopolymerizable composition forming a photopolymerizable layer between the infrared ablation layer and a support layer, the photopolymerizable composition comprising a first binder, a monomer, and a photoinitiator.

9. A method of making a printing form precursor comprising: a) providing a clear thermal polymer cover sheet; b) applying an infrared ablation composition onto the cover sheet to form an infrared ablation layer, the infrared ablation composition comprising (i) at least one infrared absorbing material; (ii) a radiation opaque material, wherein (i) and (ii) can be the same or different; and (iii) at least one second binder; c) applying an overcoat/barrier layer which is thermally embossable onto the infrared ablation layer; d) embossing the overcoat/barrier layer on the side opposite the cover sheet with a microcell pattern; and e) applying a photopolymerizable composition forming a photopolymerizable layer between the overcoat/barrier layer and a support layer, the photopolymerizable composition comprising a first binder, a monomer, and a photoinitiator.

10. A method of making a printing form precursor comprising the steps of: a) providing a clear thermal polymer cover sheet; b) applying a release layer that is thermally embossable; c) embossing the release layer on the side opposite the cover sheet with a microcell pattern; and d) applying a photopolymerizable composition forming a photopolymerizable layer between the release layer and a support layer, the photopolymerizable composition comprising a binder, a monomer, and a photoinitiator.

11. The method of claim 9, further comprising the steps of: e) removing the coversheet; and f) applying a mask on top of the release layer.

Description

DETAIL DESCRIPTION

[0066] Throughout the following detailed description, similar reference characters refer to similar elements in all figures of the drawings.

[0067] Unless otherwise indicated, the following terms as used herein have the meaning as defined below.

[0068] “Actinic radiation” refers to radiation capable of initiating reaction or reactions to change the physical or chemical characteristics of a photosensitive composition.

[0069] “Lines per inch” (LPI) is a measurement of printing resolution in systems which use a halftone screen. It is a measure of how close together lines in a halftone grid are. Higher LPI generally indicates greater detail and sharpness to an image.

[0070] “Halftone” is used for the reproduction of continuous-tone images, by a screening process that converts the image into dots of various sizes and equal spacing between centers. A halftone screen enables the creation of shaded (or grey) areas in images that are printed by transferring (or non-transferring) of a printing medium, such as ink.

[0071] “Continuous tone” refers to an image that has a virtually unlimited range of color or shades of grays, that contains unbroken gradient tones having not been screened.

[0072] “Dots per inch” (DPI) is a frequency of dot structures in a tonal image, and is a measure of spatial printing dot density, and in particular the number of individual dots that can be placed within the span of one linear inch (2.54 cm). The DPI value tends to correlate with image resolution. Typical DPI range for graphics applications: 75 to 150, but can be as high as 300.

[0073] “Line screen resolution”, which may sometimes be referred to as “screen ruling” is the number of lines or dots per inch on a halftone screen.

[0074] “Optical Density” or simply “Density” is the degree of darkness (light absorption or opacity) of an image, and can be determined from the following relationship:

Density=log.sub.10{1/reflectance}

where reflectance is {intensity of reflected light/intensity of incident light}. Density is commonly calculated in conformance with ISO 5/3:2009 International Standard for Photography and graphic technology—Density measurements—Part 3: Spectral conditions.

[0075] “Solid Ink Density” is a measure of the density of a printed area meant to display the maximum amount of print color.

[0076] “Graininess” refers to the variation in density of print areas. The ISO-13660 International Print Quality Standard defines it as, “Aperiodic fluctuations of density at a spatial frequency greater than 0.4 cycles per millimeter in all directions.” The ISO-13660 metric of graininess is the standard deviation of density of a number of small areas that are 42 um square.

[0077] “Embossed microcell pattern” refers to a composite of features that together form a pattern for inclusion at some stage of production of the photosensitive element of the present invention. An embossed microcell pattern in which a plurality of features is incorporated into a photosensitive element is distinguished from a microcell pattern that is conventionally formed in a digital layer of a photosensitive element with infrared laser radiation by a digital imager device.

[0078] “Microcells” refer to image elements or microcells that alter a print surface, which can appear as dimples and/or very tiny reverses, and that are each smaller in at least one dimension than the spacing between smallest periodic structures on the printing form that results from the photosensitive element of the present invention. The microcells are irregularities on a print surface of the relief printing form that are designed to improve the uniformity and apparent density of ink printed on a substrate by the relief printing form. In some embodiments, microcells of the relief printing form can correspond with features of the printed microcell pattern that is integrated into the present photosensitive element.

[0079] “Microcell pattern” refers to a composite of image elements or microcells that together form a pattern that alters a print surface of a relief printing form which results from the photosensitive element of the present invention.

[0080] The term “pattern” is not limited in reference to “microcell pattern”, and “printed microcell pattern”; and, refers to placement of the individual features relative to one another, to include as a composite of the individual feature patterns that are random, pseudo-random, or regular, in one or two directions.

[0081] “Visible radiation or light” refers to a range of electromagnetic radiation that can be detected by the human eye, in which the range of wavelengths of radiation is between about 390 and about 770 nm.

[0082] “Infrared radiation or light” refers to wavelengths of radiation between about 770 and 10.sup.6 nm.

[0083] “Ultraviolet radiation or light” refers to wavelengths of radiation between about 10 and 390 nm.

[0084] Note that the provided ranges of wavelengths for infrared, visible, and ultraviolet are general guides and that there may be some overlap of radiation wavelengths between what is generally considered ultraviolet radiation and visible radiation, and between what is generally considered visible radiation and infrared radiation.

[0085] “White light” refers to light that contains all the wavelengths of visible light at approximately equal intensities, as in sunlight.

[0086] “Room light” refers to light that provides general illumination for a room. Room light may or may not contain all the wavelengths of visible light.

[0087] The term “photosensitive” encompasses any system in which the photosensitive composition is capable of initiating a reaction or reactions, particularly photochemical reactions, upon response to actinic radiation. Upon exposure to actinic radiation, chain propagated polymerization of a monomer and/or oligomer is induced by either a condensation mechanism or by free radical addition polymerization. While all photopolymerizable mechanisms are contemplated, the compositions and processes of this invention will be described in the context of free-radical initiated addition polymerization of monomers and/or oligomers having one or more terminal ethylenically unsaturated groups. In this context, the photoinitiator system when exposed to actinic radiation can act as a source of free radicals needed to initiate polymerization of the monomer and/or oligomer. The monomer may have non-terminal ethylenically unsaturated groups, and/or the composition may contain one or more other components, such as a binder or oligomer, that promote crosslinking. As such, the term “photopolymerizable” is intended to encompass systems that are photopolymerizable, photocrosslinkable, or both. As used herein, photopolymerization may also be referred to as curing. The photosensitive element may also be referred to herein as a photosensitive precursor, photosensitive printing precursor, printing precursor, and precursor.

[0088] As used herein, the term “solid” refers to the physical state of the photosensitive layer that has a definite volume and shape and resists forces that tend to alter its volume or shape. The layer of the photopolymerizable composition is solid at room temperature, which is a temperature between about 5° C. and about 30° C. A solid layer of the photopolymerizable composition may be polymerized (photohardened), or unpolymerized, or both.

[0089] The term “digital layer” encompasses a layer that is responsive or alterable by laser radiation, particularly infrared laser radiation, and more particularly is ablatable by infrared laser radiation. The digital layer is also opaque to non-infrared actinic radiation. The digital layer may also be referred to herein as an infrared-sensitive layer, an infrared-sensitive ablation layer, a laser ablatable layer, or an actinic radiation opaque layer.

[0090] Unless otherwise indicated, the terms “photosensitive element”, “printing form precursor”, “printing precursor”, and “printing form” encompass elements or structures in any form suitable as precursors for printing, including, but not limited to, flat sheets, plates, seamless continuous forms, cylindrical forms, plates-on-sleeves, and plates-on-carriers.

Photosensitive Element

[0091] The photosensitive element is a photopolymerizable printing form precursor. The photosensitive element includes a layer of a composition sensitive to actinic radiation which in most embodiments is a composition that is photopolymerizable. The photosensitive element includes a layer of the photosensitive composition and a digital layer adjacent to the photosensitive layer. The digital layer is employed in digital direct-to-plate image technology in which laser radiation, typically infrared laser radiation, is used to form a mask of the image for the photosensitive element (instead of the conventional image transparency or phototool). The digital layer comprises an infrared ablation layer that is ablatable by infrared radiation and opaque to non-infrared actinic radiation. The infrared ablation layer comprises (i) at least one infrared absorbing material; and (ii) a radiation opaque material, wherein (i) and (ii) can be the same or different. In one embodiment, the infrared ablation layer is embossed with a microcell pattern on the side facing the photopolymerizable layer. In another embodiment where the infrared ablation layer contains an overcoat/barrier layer, the overcoat/barrier layer is embossed with a microcell pattern on the side facing the photopolymerizable layer. In yet another embodiment for an analog process, a photosensitive element comprises a coversheet and a release layer between the coversheet and a photopolymerizable layer. The release layer is embossed with a microcell pattern on the side opposite the coversheet. The coversheet is removed, and a mask is applied on top of the release layer before the photosensitive element is subject to UV exposure.

[0092] The microcell pattern includes a plurality of features in which each feature has an area between 5 to 750 square microns. The microcell pattern is introduced by thermal embossing.

[0093] Thermal embossing is a common graphic arts technique used to impart a raised surface onto substrates. It is commonly used for embossing paper, foils and plastic films. It is capable of submicron resolution, and is commonly used to reproduce surface holograms.

[0094] Thermal embossing begins by using a master image, which has a raised pattern matching the desired pattern in the final product. This master can be either flat or round, with the latter being used for high speed roll-to-roll applications. These masters can be anything with a raised surface. One common method of making a master involves either mechanical or laser etching methods. These masters can also be made by a photolithographic process as is commonly done in the case of holograms.

[0095] Advantages of the present photosensitive element having an embossed microcell pattern layer integral to the photosensitive element include that it saves the end-user time and can increase productivity in the preparation of a printing form from the photosensitive element. The presence of the embossed microcell pattern avoids the need for end-users to form a microcell pattern in the digital layer with a digital imager device, and can increase productivity in the preparation of the printing form, because the mask can be formed in the digital layer by a low resolution digital imager device that is operated at high speed. Since the microcell pattern is pre-embossed at manufacture, end-users can avoid the need for a costly high resolution digital imager device with a substantial increase in imaging time to create a plate cell pattern and a mask from the digital layer. Furthermore, the relief printing form that results from the present photosensitive precursor advantageously meets the increasing demands for print quality to improve the transfer of ink to printed substrate and to print, particularly solid areas, with uniform, dense coverage of ink, and capable of printing a full tonal range including printing of fine print elements and highlight dots.

[0096] In some embodiments, the photosensitive element initially includes the digital layer disposed above and covers or substantially covers the entire surface of the photopolymerizable layer. In some embodiments, the infrared laser radiation imagewise removes, i.e., ablates or vaporizes, the digital layer to form the in-situ mask. Suitable materials and structures for this actinic radiation opaque layer are disclosed by Fan in U.S. Pat. No. 5,262,275; Fan in U.S. Pat. No. 5,719,009; Fan in U.S. Pat. No. 6,558,876; Fan in EP 0 741 330 A1; and Van Zoeren in U.S. Pat. Nos. 5,506,086 and 5,705,310. A material capture sheet adjacent the digital layer may be present during laser exposure to capture the material of the digital layer as it is removed from the photosensitive element as disclosed by Van Zoeren in U.S. Pat. No. 5,705,310. Only the portions of the digital layer that were not removed from the photosensitive element will remain on the element forming the in-situ mask.

[0097] Materials constituting the digital layer and structures incorporating the digital layer are not particularly limited, provided that the digital layer can be imagewise exposed to form the in-situ mask on or adjacent the photopolymerizable layer of the photosensitive element. The digital layer may substantially cover the surface or only cover an imageable portion of the photopolymerizable layer. The digital layer can be used with or without a barrier layer. If used with the barrier layer, the barrier layer is disposed between the photopolymerizable layer and the digital layer to minimize migration of materials between the photopolymerizable layer and the digital layer. Monomers and plasticizers can migrate over time if they are compatible with the materials in an adjacent layer, which can alter the laser radiation sensitivity of the digital layer or can cause smearing and tackifying of the digital layer after imaging. The digital layer is also sensitive to laser radiation that can selectively remove or transfer digital layer.

[0098] In some embodiments, the digital layer comprises a radiation-opaque material, an infrared-absorbing material, and an optional binder. Dark inorganic pigments, such as carbon black and graphite, mixtures of pigments, metals, and metal alloys generally function as both infrared-sensitive material and radiation-opaque material. The optional binder is a polymeric material which includes, but is not limited to, self-oxidizing polymers, non-self-oxidizing polymers, thermochemically decomposable polymers, polymers and copolymers of butadiene and isoprene with styrene and/or olefins, pyrolyzable polymers, amphoteric interpolymers, polyethylene wax, materials conventionally used as the release layer described above, and combinations thereof. The thickness of the digital layer should be in a range to optimize both sensitivity and opacity, which is generally from about 20 Angstroms to about 50 micrometers. The digital layer should have a transmission optical density of greater than 2.0 in order to effectively block actinic radiation and the polymerization of the underlying photopolymerizable layer.

[0099] The digital layer includes (i) at least one infrared absorbing material, (ii) a radiation opaque material, wherein (i) and (ii) can be the same or different, and at least one binder. The following materials are suitable as the binder for the digital layer and include, but not limited to, polyamides, polyethylene oxide, polypropylene oxide, ethylcellulose, hydroxyethyl cellulose, cellulose acetate butyrate, ethylene-propylene-diene terpolymers, copolymers of ethylene and vinyl acetate, copolymers of vinyl acetate and vinyl alcohol, copolymers of vinyl acetate and pyrrolidone, polyvinyl acetate, polyethylene wax, polyacetal, polybutyral, polyalkylene, polycarbonates, polyester elastomer, copolymers of vinyl chloride and vinyl acetate, copolymers of styrene and butadiene, copolymers of styrene and isoprene, thermoplastic block copolymers of styrene and butadiene, thermoplastic block copolymers of styrene and isoprene, polyisobutylene, polybutadiene, polycholorprene, butyl rubber, nitrile rubber, thermoplastic polyurethane elastomer, cyclic rubbers, copolymers of vinylacetate and (meth)acrylate, acrylonitrile-butadiene-styrene terpolymer, methacrylate-butadiene-styrene terpolymer, alkyl methacrylate polymer or copolymer, copolymers of styrene and maleic anhydride, copolymers of styrene and maleic anhydride partially esterified with alcohols, and combinations thereof. Preferred binders include polyamides, polyethylene oxide, polypropylene oxide, ethylcellulose, hydroxyethyl cellulose, cellulose acetate butyrate, ethylene-propylene-diene terpolymers, copolymers of ethylene and vinyl acetate, copolymers of vinyl acetate and vinyl alcohol, copolymers of vinyl acetate and pyrrolidone, polyvinyl acetate, polyethylene wax, polyacetal, polybutyral, polyalkylene, polycarbonates, cyclic rubber, copolymer of styrene and maleic anhydride, copolymer of styrene and maleic anhydride partially esterified with alcohol, polyester elastomers, and combinations thereof.

[0100] Materials suitable for use as the radiation opaque material and the infrared absorbing material include, but is not limited to, metals, metal alloys, pigments, carbon black, graphite and combinations thereof. Mixtures of pigments in which each pigment functions as the infrared absorbing material, or the radiation opaque material (or both) can be used with the binder. Dyes are also suitable as infrared absorbing agents. Examples of suitable dyes include poly(substituted)phthalocyanine compounds; cyanine dyes; squarylium dyes; chalcogenopyrloarylidene dyes; bis(chalcogenopyrylo)-polymethine dyes; oxyindolizine dyes; bis(aminoaryl)-polymethine dyes; merocyanine dyes; croconium dyes; metal thiolate dyes; and quinoid dyes. Preferred is carbon black, graphite, metal, and metal alloys that functions as both the infrared absorbing material and the radiation opaque material. The radiation opaque material and the infrared absorbing material may be in dispersion to facilitate handling and uniform distribution of the material.

[0101] The photopolymerizable layer is a solid layer formed of the composition comprising a binder, at least one ethylenically unsaturated compound, and a photoinitiator. The photoinitiator is sensitive to actinic radiation. Throughout this specification actinic radiation will include ultraviolet radiation and/or visible light. The solid layer of the photopolymerizable composition is treated with one or more solutions and/or heat to form a relief suitable for relief printing. As used herein, the term “solid” refers to the physical state of the layer which has a definite volume and shape and resists forces that tend to alter its volume or shape. A solid layer of the photopolymerizable composition may be polymerized (photohardened), or unpolymerized, or both. In some embodiments, the layer of the photopolymerizable composition is elastomeric. In one embodiment, the photosensitive element includes a layer of photopolymerizable composition composed at least of a binder, at least one ethylenically unsaturated compound, and a photoinitiator. In another embodiment, the layer of the photopolymerizable composition includes an elastomeric binder, at least one ethylenically unsaturated compound, and a photoinitiator. In some embodiments, the relief printing form is an elastomeric printing form (i.e., the photopolymerizable layer is an elastomeric layer).

[0102] The binder can be a single polymer or mixture of polymers. In some embodiments, the binder is an elastomeric binder. In other embodiments, the layer of the photopolymerizable composition is elastomeric. Binders include natural or synthetic polymers of conjugated diolefin hydrocarbons, including polyisoprene, 1,2-polybutadiene, 1,4-polybutadiene, butadiene/acrylonitrile, and diene/styrene thermoplastic-elastomeric block copolymers. Preferably, the elastomeric block copolymer of an A-B-A type block copolymer, where A represents a non-elastomeric block, preferably a vinyl polymer and most preferably polystyrene, and B represents an elastomeric block, preferably polybutadiene or polyisoprene. In some embodiments, the elastomeric A-B-A block copolymer binders can be poly(styrene/isoprene/styrene) block copolymers, poly(styrene/butadiene/styrene) block copolymers, and combinations thereof. The binder is present in an amount of about 10% to 90% by weight of the photosensitive composition. In some embodiments, the binder is present at about 40% to 85% by weight of the photosensitive composition.

[0103] Other suitable binders include acrylics; polyvinyl alcohol; polyvinyl cinnamate; polyamides; epoxies; polyimides; styrenic block copolymers; nitrile rubbers; nitrile elastomers; non-crosslinked polybutadiene; non-crosslinked polyisoprene; polyisobutylene and other butyl elastomers; polyalkyleneoxides; polyphosphazenes; elastomeric polymers and copolymers of acrylates and methacrylate; elastomeric polyurethanes and polyesters; elastomeric polymers and copolymers of olefins such as ethylene-propylene copolymers and non-crosslinked EPDM; elastomeric copolymers of vinyl acetate and its partially hydrogenated derivatives.

[0104] The photopolymerizable composition contains at least one compound capable of addition polymerization that is compatible with the binder to the extent that a clear, non-cloudy photosensitive layer is produced. The at least one compound capable of addition polymerization may also be referred to as a monomer and can be a single monomer or mixture of monomers. Monomers that can be used in the photopolymerizable composition are well known in the art and include, but are not limited to, addition-polymerization ethylenically unsaturated compounds with at least one terminal ethylenic group. Monomers can be appropriately selected by one skilled in the art to provide elastomeric property to the photopolymerizable composition. The at least one compound capable of addition polymerization (i.e., monomer) is present in at least an amount of 5%, typically 10 to 20%, by weight of the photopolymerizable composition.

[0105] The photoinitiator can be any single compound or combination of compounds which is sensitive to actinic radiation, generating free radicals which initiate the polymerization of the monomer or monomers without excessive termination. Any of the known classes of photoinitiators, particularly free radical photoinitiators may be used. Alternatively, the photoinitiator may be a mixture of compounds in which one of the compounds provides the free radicals when caused to do so by a sensitizer activated by radiation. In most embodiments, the photoinitiator for the main exposure (as well as post-exposure and backflash) is sensitive to visible or ultraviolet radiation, between 310 to 400 nm, and preferably 345 to 365 nm. Photoinitiators are generally present in amounts from 0.001% to 10.0% based on the weight of the photopolymerizable composition.

[0106] The photopolymerizable composition can contain other additives depending on the final properties desired. Additional additives to the photopolymerizable composition include sensitizers, plasticizers, rheology modifiers, thermal polymerization inhibitors, colorants, processing aids, antioxidants, antiozonants, dyes, and fillers.

[0107] The thickness of the photopolymerizable layer can vary over a wide range depending upon the type of printing plate desired, for example, from about 0.005 inches to about 0.250 inches or greater (about 0.013 cm to about 0.64 cm or greater). In some embodiments, the photopolymerizable layer has a thickness from about 0.005 inch to 0.0450 inch (0.013 cm to 0.114 cm). In some other embodiments, the photopolymerization layer has a thickness from about 0.020 inches to about 0.112 inches (about 0.05 cm to about 0.28 cm). In other embodiments, the photopolymerizable layer has a thickness from about 0.112 inches to about 0.250 inches or greater (0.28 cm to about 0.64 cm or greater). As is conventional in the art, manufacturers typically identify the printing precursors relative to the total thickness of the printing form on press, which includes the thickness of the support and the photopolymerizable layer. The thickness of the photopolymerizable layer for the printing form is typically less than the manufacturer's designated thickness since the thickness of the support is not included.

[0108] The photosensitive element can include one or more additional layers on or adjacent the photosensitive layer. In most embodiments the one or more additional layers are on a side of the photosensitive layer opposite the support. Examples of additional layers include, but are not limited to, a protective layer, a capping layer, an elastomeric layer, a barrier layer, and combinations thereof. The one or more additional layers can be removable, in whole or in part, during one of the steps to convert the element into a printing form, such as treating.

[0109] Optionally, the photosensitive element may include an elastomeric capping layer on the at least one photopolymerizable layer. The elastomeric capping layer is typically part of a multilayer cover element that becomes part of the photosensitive printing element during calendering of the photopolymerizable layer. Multilayer cover elements and compositions suitable as the elastomeric capping layer are disclosed in Gruetzmacher et al., U.S. Pat. Nos. 4,427,759 and 4,460,675. In some embodiments, the composition of the elastomeric capping layer includes an elastomeric binder, and optionally a monomer and photoinitiator and other additives, all of which can be the same or different than those used in the bulk photopolymerizable layer. Although the elastomeric capping layer may not necessarily contain photoreactive components, the layer ultimately becomes photosensitive when in contact with the underlying bulk photopolymerizable layer. As such, upon imagewise exposure to actinic radiation, the elastomeric capping layer has cured portions in which polymerization or crosslinking have occurred and uncured portions which remain unpolymerized, i.e., uncrosslinked. Treating causes the unpolymerized portions of the elastomeric capping layer to be removed along with the photopolymerizable layer in order to form the relief surface. The elastomeric capping layer that has been exposed to actinic radiation remains on the surface of the polymerized areas of the photopolymerizable layer and becomes the actual printing surface of the printing plate. In embodiments of the photosensitive element that include the elastomeric capping layer, the cell pattern layer is disposed between the elastomeric capping layer and the digital layer.

[0110] For some embodiments of photosensitive elements useful as relief printing forms, the surface of the photopolymerizable layer may be tacky and a release layer having a substantially non-tacky surface can be applied to the surface of the photopolymerizable layer. Such release layer can protect the surface of the photopolymerizable layer from being damaged during removal of an optional temporary coversheet or other digital mask element and can ensure that the photopolymerizable layer does not stick to the coversheet or other digital mask element. During image exposure, the release layer can prevent the digital element with the mask from binding with the photopolymerizable layer. The release layer is insensitive to actinic radiation. The release layer is also suitable as a first embodiment of the barrier layer which is optionally interposed between the photopolymerizable layer and the digital layer. The elastomeric capping layer may also function as a second embodiment of the barrier layer. Examples of suitable materials for the release layer are well known in the art, and include polyamides, polyvinyl alcohol, hydroxyalkyl cellulose, copolymers of ethylene and vinyl acetate, amphoteric interpolymers, and combinations thereof.

[0111] The photosensitive printing element may also include a temporary coversheet on top of an uppermost layer of the element, which may be removed prior to preparation of the printing form. One purpose of the coversheet is to protect the uppermost layer of the photosensitive printing element during storage and handling. Examples of suitable materials for the coversheet include thin films of polystyrene, polyethylene, polypropylene, polycarbonate, fluoropolymers, polyamide or polyesters, which can be subbed with release layers. The coversheet is preferably prepared from polyester, such as Mylar® polyethylene terephthalate film.

Process to Make Photosensitive Element

[0112] The process of making the photosensitive element includes a step in which a microcell pattern is created by embossing a microcell pattern onto a layer of the photosensitive element; or, onto a layer of a separate element or film that forms an assemblage with the photopolymerizable layer to form the photosensitive element. The microcell pattern is integrated into the photosensitive element at time of manufacture. In most embodiments, the microcell pattern is embossed and incorporated as part of the digital layer and facing the photopolymerizable layer of the photosensitive element. In some embodiments, the microcell pattern is embossed onto an overcoat/barrier layer adjacent the digital layer. The overcoat/barrier is thermally embossable and is embossed on the side facing the photopolymerizable layer. In some other embodiments, the microcell pattern is embossed onto a release layer between a coversheet and a photopolymerizable layer with the microcell pattern on the side opposite the coversheet.

[0113] It is well within the skill of the practitioner in the art to make or manufacture a photosensitive element printing form precursor that includes a layer of the photopolymerizable composition formed by admixing the binder, monomer, photoinitiator, and other optional additives. Since in most embodiments, the cell pattern layer is applied by printing onto a surface of the digital layer that will be adjacent the photopolymerizable layer, the cell pattern layer should withstand and not be disturbed or destroyed by the elevated temperature/s that is typically used to manufacture the photopolymerizable printing form precursor. In most embodiments, the photopolymerizable mixture is formed into a hot melt, extruded, calendered at temperatures above room temperature to the desired thickness between two sheets, such as the support and the temporary coversheet having the digital layer, or between one flat sheet and a release roll. Alternately, the photopolymerizable material can be extruded and/or calendered to form a layer onto a temporary support and later laminated to the desired final support or to a digital coversheet. The printing form precursor can also be prepared by compounding the components in a suitable mixing device and then pressing the material into the desired shape in a suitable mold. The material is generally pressed between the support and the coversheet. The molding step can involve pressure and/or heat.

[0114] The photosensitive element includes at least one photopolymerizable layer that can be of a bi- or multi-layer construction. Further, the photosensitive element may include an elastomeric capping layer on the at least one photopolymerizable layer. Multilayer cover elements and compositions suitable as the elastomeric capping layer are disclosed in Gruetzmacher et al., U.S. Pat. Nos. 4,427,759 and 4,460,675.

[0115] Cylindrically shaped photopolymerizable elements may be prepared by any suitable method. In one embodiment, the cylindrically shaped elements can be formed from a photopolymerizable printing plate that is wrapped on a carrier or cylindrical support, i.e., sleeve, and the ends of the plate mated to form the cylinder shape. The cylindrically shaped photopolymerizable element can also be prepared extrusion and calendering in-the-round according to the method and apparatus disclosed by Cushner et al. in U.S. Pat. No. 5,798,019.

[0116] The photosensitive element can be manufactured in several ways, and sold in one embodiment as a printing form precursor having all requisite layers, i.e., the photopolymerizable layer, and the digital layer with embossed microcell pattern; or the photopolymerizable layer, the digital layer, and an overcoat/barrier layer between the digital layer and the photopolymerizable layer that is embossed with a microcell pattern on the side facing the polymerizable layer. Alternatively, the photosensitive element can sold as separate components, e.g., a coversheet, a release layer that is embossed with a microcell pattern on the side opposite the coversheet, and a photopolymerizable that is between the release layer and a support. These separate components are manipulated separately, but are assembled to form a photosensitive element prior to imagewise exposure of the photopolymerizable layer. [0117] 1. In one embodiment, a digital composition is coated onto a web of a polymeric film, such as polyester film, to form the digital layer on the film. A microcell pattern is embossed continuously onto a side of the digital layer that is opposite the film, to form a digital coversheet. The features of the microcell pattern are embossed continuously so that the embossed microcell pattern does not include seams, breaks, or segmentation of the pattern on the digital layer web. The photopolymerizable composition is extruded to form the photopolymerizable layer between a base support, e.g., polyester film, and the digital coversheet, wherein the side of the digital coversheet having the microcell pattern layer is contacted to the photopolymerizable layer opposite the support. Printing form precursors that include the base support, the photopolymerizable layer, the digital layer with an embossed microcell pattern, and the polymeric film as an optional coversheet can be cut to any finished size for sale to end-users. [0118] 2. In one other embodiment, a digital composition is coated onto a web of a polymeric film, such as polyester film, to form the digital layer on the film, which is then cut to specific size/s of sheets. A microcell pattern is embossed onto the sheets on a side of the digital layer that is opposite the film, to form a digital coversheet. The digital coversheet can be laminated to a photopolymerizable layer to form a printing form precursor with the embossed microcell pattern contacting the photopolymerizable layer. [0119] 3. In one other embodiment, a polymeric film, such as polyester film, is embossed with a microcell pattern to form a microcell pattern on the film; and a layer of a thermally imageable composition is applied on the film, on the side opposite the microcell pattern, to create a digital coversheet. The digital coversheet is laminated to a surface of a photopolymerizable layer to form a printing form precursor with the embossed microcell pattern contacting the photopolymerizable layer. [0120] 4. In yet another embodiment, a photopolymerizable layer is placed on a support that is a polymeric film, such as polyester film. A release layer is embossed with a microcell pattern, and placed on the photopolymerizable layer with the side having the microcell pattern contacting the photopolymerizable layer. A coversheet is placed on top of the release layer to form a printing form precursor. The coversheet can be removed and replaced with a mask on top of the release layer during conversion of the printing form precursor to a relief printing form.

[0121] Those skilled in the art, having benefit of the teachings of the present invention as hereinabove set forth, can affect numerous modifications thereto. These modifications are to be construed as being encompassed within the scope of the present invention as set forth in the appended claims.

EXAMPLES

[0122] In the following examples, all percentages are by weight unless otherwise noted. CYREL® photopolymerizable printing plates, CYREL® exposure unit, and CYREL® processor are all available from The DuPont Company (Wilmington, Del.).

[0123] Commercially available DPR® digital plate by DuPont™ were used to make printing plates in the following examples. The coversheet film of the digital plate can have special coatings on the PET as further described in the examples below. The original unembossed coversheet films were used, as is, to serve as a control. Embossment of the coversheet films was conducted using a random pattern obtained from a sheet of 3M 261X Lapping film with a 5-micron surface. The embossing was done on a PL238WF system, from Professional Laminating Systems, at 300° F. at a speed of 6.5 inches per minute. It was done with the rough surface of the lapping film in contact with the coated surface of the PET coversheet. The lapping film was removed immediately after lamination and discarded. Flexographic printing plate precursors were made by laminating this coversheet to a photopolymer using the same laminating system at the same temperature, but at a speed of 55 inches per minute. The laminated plate was then allowed to sit in an over at 60° C. overnight.

[0124] The printing plate precursors were converted to relief printing plates in accordance with the Cyrel® Process-of-Use Manual. Analog plates were imaged using a clear negative to produce a large solid area. Digital plates were imaged on an Esko CDI Spark 2530 using an image that produced a large solid area suitable for measuring solid ink density. The relief printing plates were tested for printing solids onto a substrate. A Mark Andy 830 Printing Press and an Aquaverse Pro Cyan ink from Sun Chemical were employed. The ink density of the solid printed areas was measured using a Techkon SpectroJet scanning spectrophotometer-densitometer (from Techkon USA (Danvers, Mass., U.S.A)).

Example 1

[0125] In this example, the coversheet film of a DPR® digital plate was used directly, without any additional coating, to produce a standard digital flexographic printing plate. The coversheet consists of a 2.5 micron thick ablatable layer coated on PET film. One sample of this coversheet film was used, as is, as a control. Another sample was embossed as described above. Flexographic printing plates incorporating these coversheet films were made and tested in printing. The one from unembossed sample had a solid ink density of 1.12, whereas the one from the embossed sample had a solid ink density of 1.23. This resulted in a noticeably visible improvement in the printed sample.

Example 2

[0126] In this example, a coversheet film of a DPR® digital plate was coated with an oxygen barrier of 10-micron in thickness. This barrier layer consisted of a polyamide resin. One sample of the modified coversheet film was used, as is, as a control to make a digital flexographic plate with a barrier layer to inhibit oxygen transfer to the plate during cure. This barrier layer helps produce flexographic plates with flat topped dots that are sometimes preferred during printing. Another sample of the modified coversheet film was embossed as described above. Flexographic printing plates incorporating these coversheet films were made and tested in printing. The one from the unem bossed sample had a solid ink density of 1.12, whereas the one from the embossed sample had a solid ink density of 1.25. This resulted in a noticeably visible improvement in the printed sample.

Example 3

[0127] In this example, a commercially available flexo plate HORB® digital plate by DuPont™ was used to produce a standard analog flexographic printing plate. The coversheet film of the flexo plate had a release layer of 4.0 micron in thickness coated on a PET film. One sample of this coversheet film was used, as is, as a control. Another sample of the coversheet film had its release layer embossed as described above. Flexographic printing plates incorporating these coversheet films were made and tested in printing. The one from the unem bossed sample had a solid ink density of 1.08, whereas the one from the embossed sample had a solid ink density of 1.20. This resulted in a noticeably visible improvement in the printed sample.

[0128] As shown in Example 1-3, a significant improvement in solid ink density was obtained by embossing the coating on a coversheet prior to making a flexographic printing plate. Further improvements in solid ink density can be made by optimizing the pattern used for embossing.