1. Field of the Invention
The present invention relates to an imageable element and a method of producing an imaged element that can be used in lithographic printing plates. More particularly, the present invention relates to an imageable element comprising a hydrophilic anodized aluminum base and coated thereon an image-forming layer comprising polymer particles and a method of producing the same.
2. Description of the Prior Art
Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. In the art of photolithography, a photographic material is made imagewise receptive to oily inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophilic background. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
In the production of common lithographic printing plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers, particularly diazo-sensitized systems, which are widely used. Upon image-wise exposure of the light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
Eurpean Patent Application No. 849,091 A1 and U.S. Pat. No. 6,001,536 disclose thermal coalescence of imageable compositions, including on-press developable compositions. These patent do not contain any disclosure regarding the oxide pore size on the surface of the substrate or the relationship of the oxide pore size to the particle size of the polymer in the image-forming layer.
U.S. Pat. No. 4,990,428 discloses an aluminum substrate having an oxide layer with 35-100 nm pore diameters, obtained by using phosphoric acid as the main electrolyte in the anodization process. When this substrate is overcoated with a free radical photo-polymerizable composition containing carboxylic acid groups and cured, the resulting lithographic plate exhibits superior press life. As above, this patent also does not contain any disclosure regarding the relationship of pore size to particle size of the polymer in the image-forming layer.
U.S. Pat. No. 4,865,951 discloses a bilayer anodic surface produced in a 2-stage process, which affords average pore size diameters of 10-75 nm in the upper layer and substantially greater diameters in the lower layer. A lithographic printing plate comprising an imageable layer on this support is shown to improve stain resistance. However, there is no disclosure regarding the relationship of pore size to particle size.
U.S. Pat. No. 5,922,507 discloses a photosensitive imaging element having a two-phase layer on a support. The two-phase layer has a hydrophilic continuous phase containing a hardened hydrophilic polymer and a dispersed hydrophobic photopolymerizable phase that has a multifunctionally polymerizable monomer and a photoinitiator. The hydrophobic photopolymerizable phase is formed of particles having an average particle size comprised between 0.1 and 10 xcexcm, i.e., 100-10,000 nm. Neither pore size on the support nor pore size/imaging layer particle size matching are mentioned.
The present invention provides average pore diameter to average particle diameter ratios that can enhance adhesion, which enhances the sensitivity and the press life of the printing plates prepared therefrom.
The present invention includes a radiation-imageable element for lithographic printing. The radiation-imageable ellement comprises a hydrophilic anodized aluminum base having a surface comprising pores, and coated thereon, an image-forming layer comprising polymer particles, the ratio of said average pore diameter to said average particle diameter being from about 0.4:1 to about 10:1.
The present invention also includes a method of producing an imaged element. The method comprises the steps of:
providing a radiation-imageable element for lithographic printing comprising: a hydrophilic anodized aluminum base having a surface comprising pores; and coated thereon, an image-forming layer comprising polymer particles, the ratio of the average pore diameter to the average particle diameter being from about 0.4:1 to about 10:1; and
imagewise exposing the radiation-imageable element to radiation to produce exposed and unexposed regions.
The present invention further includes a method of producing an imaged element having complementary ink receiving and ink rejecting regions. The method comprises the steps of:
providing a radiation-imageable element for lithographic printing comprising: a hydrophilic anodized aluminum base having a surface comprising pores; and coated thereon, an image-forming layer comprising polymer particles, the ratio of the average pore diameter to the average particle diameter being from about 0.4:1 to about 10:1;
imagewise exposing the radiation-imageable element to radiation to produce exposed and unexposed regions; and
contacting said imagewise exposed radiation-imageable element and a developer to selectively remove said exposed or said unexposed regions.
The present invention provides average pore diameter to average particle diameter ratios that can enhance the interaction of the image-forming layer with the substrate surface layer following thermal imaging by allowing the polymer particles to enter into the oxide pores of the substrate, thereby enhancing adhesion. The enhanced adhesion, in turn, will enhance the sensitivity and the press life of the printing plates.
Lithographic printing is based on the immiscibility of oil and water. Ink receptive areas are generated on the surface of a hydrophilic surface. When the surface is moistened with water and then ink is applied, the hydrophilic background areas retain the water and repel the ink. The ink receptive areas accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is thereafter reproduced.
Lithographic printing plate precursors, i.e., imageable elements, typically include an imageable coating applied over the hydrophilic surface of a support material. If after exposure to radiation, the exposed regions of the coating become the ink-receptive image regions, the plate is called a negative-working printing plate. Conversely, if the unexposed regions of the coating become the ink-receptive image regions, the plate is called a positive-working plate. In the present invention, the imagewise exposed regions are rendered less soluble or dispersible in a developer and become the ink-receptive image areas. The unexposed regions, being more readily soluble or dispersible in the developer, are removed in the development process, thereby revealing a hydriphilic surface, which readily accepts water and becomes the ink-repellant image area.
The term xe2x80x9cgraftxe2x80x9d polymer or copolymer in the context of the present invention refers to a polymer which has as a side chain a group having a molecular weight of at least 200. Such graft copolymers can be obtained, for example, by anionic, cationic, non-ionic, or free radical grafting methods, or they can be obtained by polymerizing or co-polymerizing monomers, which contain such groups.
The term xe2x80x9cpolymerxe2x80x9d in the context of the present invention refers to high and low molecular weight polymers, including oligomers, and includes homopolymers and copolymers. The term xe2x80x9ccopolymerxe2x80x9d refers to polymers that are derived from two or more different monomers.
The term xe2x80x9cbackbonexe2x80x9d in the context of the present invention refers to the chain of atoms in a polymer to which a plurality of pendant groups are attached. An example of such a backbone is an xe2x80x9call carbonxe2x80x9d backbone obtained from the polymerization of an olefinically unsaturated monomer.
The term xe2x80x9chydrocarbylxe2x80x9d in the context of the present invention refers to a linear, branched or cyclic alkyl, alkenyl, aryl, aralkyl or alkaryl of 1 to 120 carbon atoms, and substituted derivatives thereof. The substituent group can be halogen, hydroxy, hydrocarbyloxy, carboxyl, ester, ketone, cyano, amino, amido and nitro groups. Hydrocarbyl groups in which the carbon chain is interrupted by oxygen, nitrogen or sulfur are also included in the term xe2x80x9chydrocarbylxe2x80x9d.
The term xe2x80x9chydrocarbylenexe2x80x9d in the context of the present invention refers to a linear, branched or cyclic alkylene, vinylene, arylene, aralkylene or alkarylene of 1 to 120 carbon atoms, and substituted derivatives thereof. The substituent group can be halogen, hydroxy, hydrocarbyloxy, carboxyl, ester, ketone, cyano, amino, amido and nitro groups. Hydrocarbylene groups in which the carbon chain is interrupted by oxygen, nitrogen or sulfur are also included in the term xe2x80x9chydrocarbylenexe2x80x9d.
The present invention includes a radiation imageable element comprising a hydrophilic, porous oxide base, which is overcoated with an image-forming layer comprising polymer particles. The ratio of the average surface oxide pore diameter of the hydrophilic base to the average particle diameter of the polymer particles is from about 0.4:1 to about 10:1, preferably 0.4:1 to 10:1, more preferably, the ratio is from about 0.5:1 to about 5:1. Radiation can be a photo, thermal or electron beam radiation.
The term xe2x80x9cparticlexe2x80x9d in the context of the present invention refers to a solid, which is dispersed in a continuous phase.
Preferably, the pores have an average pore diameter from about 10 to about 100 nm, more preferably, from about 10 to about 75 nm.
Preferably, the polymer particles have an average particle diameter from about 1 to about 250 nm, more preferably, from about 10 to about 200 nm.
The support material comprises an aluminum or aluminum alloy plate. Suitable aluminum alloys include alloys with zinc, silicon, chromium, copper, manganese, magnesium, chromium, zinc, lead, bismuth, nickel, iron or titanium which may contain negligible amounts of impurities. Preferred plates have a thickness of about 0.06 to about 0.6 millimeters.
The surface of the aluminum plate is preferably subjected to chemical cleaning such as degreasing with solvents or alkaline agents for the purpose of exposing a clean surface free of grease, rust or dust which is usually present on the aluminum surface. Preferably, the surface is grained. Suitable graining methods include glass bead graining, quartz slurry graining, ball graining, the blasting, brush graining and electrolytic graining. Following the graining operation, the support can be treated with an aluminum etching agent and/or a desmutting acid bath.
In a preferred embodiment, the ratio of the average surface oxide pore diameter of the hydrophilic base to the average particle diameter of the polymer particles is from about 0.8:1 to about 10:1 and the incident exposure dose is not more than about 340 mJ/cm2. More preferably, the pore size/particle size ratio is from about 1.0:1 to about 10:1 and the incident exposure dose is not more than about 300 mJ/cm2.
Preferably, the porous oxide base comprises anodized aluminum; the element is free of an interlayer between the porous anodized aluminum base and the image forming layer; the image forming layer also comprises a photothermal conversion material; the heat sensitive polymer particles have a glass transition temperature of at least 50xc2x0 C., preferably 60xc2x0 C.; and the image forming layer is negative working.
In another preferred embodiment, the present invention includes a radiation imageable element the ratio of the average surface oxide pore diameter of the hydrophilic base to the average particle diameter of the polymer particles is from about 0.6:1 to about 10:1 and the radiation imageable element is free of an interlayer between the porous anodized aluminum base and the image forming layer.
The porous oxide base is preferably an aluminum sheet comprising at least one anodically oxidized surface. In general, any known method of anodic oxidation, followed by etching, if necessary, that can provide an appropriate pore diameter corresponding to the polymer particles, may be used to prepare the aluminum base.
Anodic pore size for sulfuric acid anodization is typically less than 20 nm whereas anodic pore size for phosphoric acid anodization is typically greater than 30 nm. Typically, lithographic printing plates utilize an aluminum base, which is anodized in sulfuric acid, wherein the average oxide pore size is about 15 nm in diameter. However, phosphoric acid can be used instead of sulfuric acid. Phosphoric acid provides larger anodic pore size and enhances adhesion of photopolymer compositions. The use of large anodic pore substrates that are phosphoric acid anodized is preferred over sulfuric acid-anodized substrates. Other conventional anodization methods can also be used in the preparation of the anodized substrate of the present invention, including particularly those that produce an anodic pore size larger than anodic pore size produced by sulfuric acid anodization.
Thus, preparation of the anodically oxidized surface can be accomplished by anodically oxidizing the aluminum sheet in an aqueous phosphoric or sulfuric acid solution to produce an oxide layer. The anodic oxidation is optionally followed by etching of the the oxide layer to a fraction of its original thickness, such as, for example, to about xc2xd of its original thickness. Alternatively, a lithographic printing plate precursor can be prepared by the above method.
The anodised aluminum support may be treated to improve the hydrophilic properties of its surface. For example, the aluminum support may be silicated by treating its surface with sodium silicate solution at elevated temperature, e.g., 95xc2x0 C. Alternatively, a phosphate treatment may be applied which involves treating the aluminum oxide surface with a phosphate solution that may further contain an inorganic fluoride. Further, the aluminum oxide surface may be rinsed with a citric acid or citrate solution. This treatment may be carried out at room temperature or can be carried out at a slightly elevated temperature of about 30 to 50xc2x0 C. A further treatment can include rinsing the aluminum oxide surface with a bicarbonate solution. It is evident that one or more of these post treatments may be carried out alone or in combination.
Examples of the aluminum or aluminum alloy plate of the invention include a plate of pure aluminum and a plate of aluminum alloy with other metal such as silicon, copper, manganese, magnesium, chromium, zinc, lead, bismuth and nickel. The plate in the form of a sheet is preferably used. The aluminum or aluminum alloy plate is preferably grained before the anodic oxidation treatment by the conventional manner, such as brush (mechanical) graining, chemical graining, electrolytic graining and the like. Furthermore, after the anodic oxidation treatment, it may be optionally hydrophilized.
The oxide base comprises oxides and phosphates of aluminum and is present in a coverage of greater than 100 milligrams per square meter of the hydrophilic anodized aluminum base, preferably, greater than 500 milligrams per square meter of the hydrophilic anodized aluminum base. Preferably, the oxide base has a average thickness of at least 0.40 micrometers.
In accordance with the present invention, on top of a hydrophilic surface there is provided a radiation-sensitive image forming layer. Various materials suitable for forming images for use in the lithographic printing process can be used. Any suitable radiation imageable layer, which after exposure and subsequent development, if necessary, can provide an area in imagewise distribution suitable for printing can be used.
Thus, the image forming layer according to the present invention comprises polymer particles, and can further comprise pigments. The polymer particles can be a thermoplastic polymer or thermoset polymer. The thermoplastic polymer can be a hydrophobic polymer or a polymer that has both hydrophobic and hydrophilic segments thereon, such as a graft polymer or copolymer. The thermoset polymer can be a latex particle.
Examples of the polymer particles include:
(1) a thermoplastic homopolymer or copolymer formed from polymerization of one or more monomers selected from: acrylic acid, methacrylic acid, acrylamide, methacrylamide, ester of acrylic acid, ester of methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, acrylamide, methacrylamide, N-hydroxyethyl acrylamide, N-hydroxyethyl methacrylamide, styrene, p-hydroxystyrene, xcex1-methylstyrene, p-methylstyrene, vinyl acetate, methyl vinyl ether, ethyl vinyl ether, hydroxyethyl vinyl ether, vinylphosphonic acid, vinyl chloride, vinylidene chloride, acrylonitrile, N-vinyl pyrrolidone and N-vinyl carbazole;
(2) a thermoset polymer, such as, a phenol-formaldehyde resin, a cresol-formaldehyde resin, melamine-formaldehyde resin, a polyurethane resin and a combination thereof;
(3) a graft polymer having hydrophilic and hydrophobic segments, such as, a graft polymer or copolymer having a hydrophobic polymer backbone and a plurality of pendant groups represented by the formula:
-Q-W-Y 
wherein Q is a difunctional connecting group; W is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment; Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment; with the proviso that when W is a hydrophilic segment, Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment.
Specific examples of polymer particles for use in connection with the present invention include polystyrene, polyvinyl chloride, polyvinyl acetate, polymethyl methacrylate, polyvinylidene chloride, polyvinyl carbazole, polyacrylonitrile, graft polymer and copolymer particles and mixtures thereof.
The graft copolymer is a thermally sensitive polymer having a hydrophobic polymer backbone and a plurality of pendant groups represented by the formula:
-Q-W-Y 
wherein Q is a difunctional connecting group; W is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment; Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment; with the proviso that when W is a hydrophilic segment, Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment.
Preferably, the thermally sensitive graft copolymer comprises repeating units represented by the formula: 
wherein each of R1 and R2 can independently be H, alkyl, aryl, aralkyl, alkaryl, COOR5, R6CO, halogen or cyano.
Q can be one of: 
wherein R3 can be H or alkyl; R4 can independently be H, alkyl, halogen, cyano, nitro, alkoxy, alkoxycarbonyl, acyl or a combination thereof.
The segment W can be a hydrophilic segment or a hydrophobic segment, wherein the hydrophilic segment can be a segment represented by the formula: 
wherein each of R7, R8, R9 and R1 can independently be H or methyl; R3 can be H and alkyl; and wherein the hydrophobic segment can be xe2x80x94R12xe2x80x94, xe2x80x94Oxe2x80x94R12xe2x80x94Oxe2x80x94, xe2x80x94R3Nxe2x80x94R12xe2x80x94NR3xe2x80x94, xe2x80x94OOCxe2x80x94R12xe2x80x94Oxe2x80x94 or xe2x80x94OOCxe2x80x94R12xe2x80x94Oxe2x80x94, wherein each R12 can independently be a linear, branched or cyclic alkylene of 6-120 carbon atoms, a haloalkylene of 6-120 carbon atoms, an arylene of 6-120 carbon atoms, an alkarylene of 6-120 carbon atoms or an aralkylene of 6-120 carbon atoms; R3 can be H or alkyl.
Y can be a hydrophilic segment or a hydrophobic segment, wherein the hydrophilic segment can be H, R15, OH, OR16, COOH, COOR16, O2CR16, a segment represented by the formula: 
wherein each of R7, R8, R9 and R10 can independently be H or methyl; R3 can be H and alkyl; wherein each R13, R14, R15 and R16 can be H or alkyl of 1-5 carbon atoms and wherein the hydrophobic segment can be a linear, branched or cyclic alkyl of 6-120 carbon atoms, a haloalkyl of 6-120 carbon atoms, an aryl of 6-120 carbon atoms, an alkaryl of 6-120 carbon atoms, an aralkyl of 6-120 carbon atoms, OR17, COOR17 or O2CR17, wherein R17 can be an alkyl of 6-20 carbon atoms.
Z can be H, alkyl, halogen, cyano, hydroxy, alkoxy, alkoxycarbonyl, hydroxyalkyloxycarbonyl, acyl, aminocarbonyl, aryl and substituted aryl;
j is at least 1;
k is at least 1;
m is at least 2; and
n is from 1 to about 500; with the proviso that when W is a hydrophilic segment, Y is a hydrophilic segment or a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment. The substituent in the above substituted aryl can be alkyl, halogen, cyano, alkoxy or alkoxycarbonyl. Preferably, the alkyl group is an alkyl of 1 to 22 carbon atoms.
In another preferred embodiment, the segment W-Y can be represented by the formula:
xe2x80x94(OCH2CH2)nxe2x80x94OCH3 
wherein n is from 25 to about 75. In this preferred embodiment, the thermally sensitive graft copolymer has, for example, repeating units represented by the formula: 
wherein j and k are each at least 1; m is at least 5; and n is from 25 to about 75. More preferably, n has an average value of about 45.
In another preferred embodiment, the thermally sensitive graft copolymer comprises repeating units represented by the formula: 
wherein j and k are each at least 1; m is at least 5; and n is from 25 to about 75, more preferably, n has an average value of about 45.
The thermally sensitive graft copolymer having hydrophobic and/or hydrophilic segments can be prepared by known methods.
Other materials that can be useful in this invention include systems that are well known in the art, and include silver halide emulsions, as described in Research Disclosure, publication 17643, paragraph XXV, December, 1978, and references cited therein; polymeric and nonpolymeric quinone diazides as described in U.S. Pat. No. 4,141,733 and references cited therein; light sensitive polycarbonates, as described in U.S. Pat. No. 3,511,611 and references cited therein; diazonium salts, diazo resins, cinnamal-malonic acids and functional equivalents thereof and others described in U.S. Pat. No. 3,342,601 and reference cited therein; light sensitive polyesters, polycarbonates and polysulfonates, as described in U.S. Pat. No. 4,139,390 and references cited therein; and the materials described in the commonly owned U.S. Pat. No. 4,865,951. The contents of these patents are incorporated by reference as fully set forth herein.
Although a negative image formed by thermal coalescence of a heat sensitive polymer is described in the examples that follow, any photo or thermal process, either positive working or negative working, in which polymer particles are involved in the formation of an image is expected to benefit from the present invention. Such processes can include negative working systems wherein, for example, polymer particles are thermally ruptured to produce a crosslinking agent or a reactant. They can also include positive working systems wherein, for example, thermally ruptured polymer particles release a reactant or catalyst which solubilizes a polymer by converting hydrophobic groups into hydrophilic groups, as is the case in the acid-catalyzed unblocking of acid labile esters to produce carboxylic or sulfonic acids. Thus, the present invention can be used in any photo or thermal imaging application.
The polymer particles used in connection with the present invention have a glass transition temperature of at least 40xc2x0 C., more preferably of at least 50xc2x0 C. and preferably have a coagulation temperature above 40xc2x0 C., more preferably of at least 60xc2x0 C. Coagulation may result from softening or melting of the thermoplastic polymer or graft copolymer particles under the influence of heat. There is no specific upper limit to the coagulation temperature of the polymer particles, however the temperature should be sufficiently below the decomposition of the polymer particles. Preferably, the coagulation temperature is at least 10xc2x0 C. below the temperature at which the decomposition of the polymer particles occurs. When the polymer particles are subjected to a temperature above coagulation temperature they coagulate to form an agglomerate, which becomes insoluble in aqueous developer.
Preferably, the Number Average Molecular Weight of the polymers, including the graft copolymers, is from about 2,000 to about 2,000,000 and a glass transition temperature of at least 40xc2x0 C., more preferably, from about 50xc2x0 C. to about 150xc2x0 C.
The amount of polymer particles contained in the image forming layer is preferably between 20% by weight and 65% by weight and more preferably between 25% by weight and 55% by weight and most preferably between 30% by weight and 45% by weight.
The polymer particles can be present as a dispersion in the aqueous coating liquid of the image forming layer. An aqueous dispersion of the thermoplastic polymer particles can be prepared by dissolving the thermoplastic polymer in an organic, water immiscible solvent, dispersing the thus obtained solution in water or in an aqueous medium, and removing the organic solvent by evaporation.
Examples of the pigments include: carbon blacks, metal carbides, borides, nitrides, carbonitrides and bronze-structured oxides. Such pigments may absorb radiation in the ultraviolet, visible or infrared spectral regions and may also function as light to heat converting compounds in the present invention.
A light to heat converting compound in connection with the present invention can be preferably added to the image forming layer but at least part of the light to heat converting compound may also be included in a neighbouring layer, if such a layer is present.
Suitable compounds capable of converting light into heat are preferably infrared absorbing components although the wavelength of absorption is not of particular importance as long as the absorption of the compound used is in the wavelength range of the light source used for image-wise exposure. Particularly useful compounds are for example dyes and in particular infrared dyes and carbon black. The lithographic performance and in particular the print endurance obtained depends on the heat-sensitivity of the imaging element. In this respect it has been found that carbon black yields favorable results.
Classes of materials that are useful as photothermal converters include, but are not limited to, squarylium, croconate, cyanine (including phthalocyanine), merocyanine, chalcogenopyryloarylidene, bis (chalcogenopyrylo) polymethine, oxyindolizine, quinoid, indolizine, pyrylium and metal thiolene dyes or pigments. Other useful classes include thiazine, azulenium and xanthene dyes. Still other useful classes are carbon blacks, metal carbides, borides, nitrides, carbonitrides and bronze-structured oxides. Particularly useful as photothermal converters are infrared absorbing dyes of the cyanine class.
The amount of infrared absorbing compound in the image forming layer is generally sufficient to provide an optical density of at least 0.5 in the layer and, preferably, an optical density of from about 1 to about 3. This range would accommodate a wide variety of compounds having vastly different extinction coefficients. Generally, this is at least 1 weight percent and, preferably, from about 5 to about 30 weight percent.
An imaged element according to the present invention can be produced with or without a development step.
In the first instance, the method of producing an imaged element of the present invention comprises the steps of:
providing a radiation-imageable element for lithographic printing comprising: a hydrophilic anodized aluminum base having a surface comprising pores; and coated thereon an image-forming layer comprising polymer particles, the ratio of the average pore diameter to the average particle diameter being from about 0.4:1 to about 10:1;
imagewise exposing the radiation-imageable element to radiation to produce exposed and unexposed regions; and
contacting the imagewise exposed radiation-imageable element and a developer to selectively remove said exposed or said unexposed regions.
In the second instance, the method of producing an imaged element of the present invention comprises the steps of:
providing a radiation-imageable element for lithographic printing comprising: a hydrophilic anodized aluminum base having a surface comprising pores; and coated thereon an image-forming layer comprising polymer particles, the ratio of the average pore diameter to the average particle diameter being from about 0.4:1 to about 10:1; and
imagewise exposing the radiation-imageable element to radiation to produce exposed and unexposed regions.
The lithographic printing plate of the present invention can be exposed by conventional methods, for example through a transparency or a stencil, to an imagewise pattern of actinic radiation. Suitable radiation sources include sources rich in visible radiation and sources rich in ultraviolet radiation. Carbon arc lamps, mercury vapor lamps, fluorescent lamps, tungsten filament lamps, photoflood lamps, lasers and the like are useful herein. The exposure can be by contact printing techniques, by lens projection, by reflex, by bireflex, from an image-bearing original or by any other known technique.
Typically, the step of exposing the imageable element to thermal radiation is carried out using an infrared laser. However, other methods such as visible or UV laser imaging may also be used, provided that a photoconverter, i.e., a photothermal converter, is present. Thus, for exposure with such visible or UV radiation sources, the imageable composition generally includes a photothermal converting material. Alternatively, the imageable element of the present invention can be imaged using a conventional apparatus containing a thermal printing head or any other means for imagewise conductively heating the imageable composition, such as, with a heated stylus or with a heated stamp.
The imagewise exposure of the imageable element to thermal radiation is carried out using an exposure dose sufficient for imaging. Typically, an incident exposure dose of from about 50 to about 1000 mJ/cm2 is used in thermal imaging. Preferably, the incident exposure dose is not more than 600 mJ/cm2, more preferably, the incident exposure dose is not more than 400 mJ/cm2 and most preferably, the incident exposure dose is not more than 300 mJ/cm2.
The step of exposure of the imageable element to thermal radiation is followed by a development step preferably using an aqueous developer. The aqueous developer composition is dependent on the nature of the composition of the polymer particles. Common components of aqueous developers include surfactants, chelating agents, such as salts of ethylenediamine tetraacetic acid, organic solvents, such as benzyl alcohol, and alkaline components, such as, inorganic metasilicates, organic metasilicates, hydroxides and bicarbonates. The pH of the aqueous developer is preferably within about 5 to about 14, depending on the nature of the composition of the polymer particles.
For the development step, a diluted alkaline solution optionally containing preferably up to 10% by volume of organic solvent may be used. Examples of alkaline compound include inorganic compound such as sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium silicate and sodium bicarbonate, and organic compound such as ammonia, monoethanolamine, diethanolamine and triethanolamine. Preferable examples of water-soluble organic solvent include isopropyl alcohol, benzyl alcohol, ethyl cellosolve, butyl cellosolve, diacetone alcohol and the like. The developing solution may contain a surfactant, dye, salt for inhibiting the swelling or salt for corroding the metal substrate.
Following development, a postbake may optionally be used to increase press life. In the practice of the present invention, a post-exposure, pre-development heat step may also be used. This pre-development heat step can further aid in increasing differentiation between exposed and unexposed areas.
In addition to the imageable layer, the imageable element can have additional layers, such as, an overlying layer. Possible functions of an overlying layer include:
(1) to prevent damage, such as scratching, of the surface layer during handling prior to imagewise exposure; and
(2) to prevent damage to the surface of the imagewise exposed areas, for example, by over-exposure which could result in partial ablation.
The overlying layer should be soluble, dispersible or at least permeable to the developer.
The present invention enhances interaction of the image-forming layer with the substrate surface layer, thereby enhancing press life. The results suggest that, if the heat sensitive particles are able to enter into the oxide pores of the substrate, an enhanced adhesion would result following imaging.
Lithographic plates prepared by photochemical processes in which photopolymerizable polymer particles are employed in image formation, such as the no-process plate described in U.S. Pat. No. 5,922,507, are expected to benefit from this substrate oxide pore-size/imaging layer particle size matching described in the present invention. In addition, photo and thermal imaging compositions, in which polymer particles are not involved in image formation, but rather are used to reinforce and enhance durability of the image, can also benefit from this approach.
The present invention provides average pore diameter to average particle diameter ratios that can enhance the interaction of the image-forming layer with the substrate surface layer following thermal imaging by allowing the polymer particles to enter into the oxide pores of the substrate, thereby enhancing adhesion. The enhanced adhesion, in turn, will enhance the sensitivity and the press life of the printing plates.
Without being bound by any theory, it is believed that the ability of the polymer particles to enter the oxide pores enhances adhesion of the imageable layer to the anodized aluminum base following thermal or photo imaging. Thus, including an interlayer between the imageable layer to the anodized aluminum base would reduce the ability of the particles to enter the pores and increasing the incident exposure dose would enhance the ability of the particles to enter the pores.
The present invention provides a radiation imageable composition that is useful in photo or thermal imaging of, for example, lithographic plates and printed circuit boards.
The invention is further described in the following examples, which are intended to be illustrative and not limiting.
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