Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C1/00—Forme preparation
  • B41C1/10—Forme preparation for lithographic printing; Master sheets for transferring a lithographic image to the forme
  • B41C1/1008—Forme preparation for lithographic printing; Master sheets for transferring a lithographic image to the forme by removal or destruction of lithographic material on the lithographic support, e.g. by laser or spark ablation; by the use of materials rendered soluble or insoluble by heat exposure, e.g. by heat produced from a light to heat transforming system; by on-the-press exposure or on-the-press development, e.g. by the fountain of photolithographic materials
  • B41C1/1025—Forme preparation for lithographic printing; Master sheets for transferring a lithographic image to the forme by removal or destruction of lithographic material on the lithographic support, e.g. by laser or spark ablation; by the use of materials rendered soluble or insoluble by heat exposure, e.g. by heat produced from a light to heat transforming system; by on-the-press exposure or on-the-press development, e.g. by the fountain of photolithographic materials using materials comprising a polymeric matrix containing a polymeric particulate material, e.g. hydrophobic heat coalescing particles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
  • B41M5/00—Duplicating or marking methods; Sheet materials for use therein
  • B41M5/26—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used
  • B41M5/36—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used using a polymeric layer, which may be particulate and which is deformed or structurally changed with modification of its' properties, e.g. of its' optical hydrophobic-hydrophilic, solubility or permeability properties
  • B41M5/366—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used using a polymeric layer, which may be particulate and which is deformed or structurally changed with modification of its' properties, e.g. of its' optical hydrophobic-hydrophilic, solubility or permeability properties using materials comprising a polymeric matrix containing a polymeric particulate material, e.g. hydrophobic heat coalescing particles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C2210/00—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation
  • B41C2210/04—Negative working, i.e. the non-exposed (non-imaged) areas are removed
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C2210/00—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation
  • B41C2210/08—Developable by water or the fountain solution
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C2210/00—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation
  • B41C2210/24—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation characterised by a macromolecular compound or binder obtained by reactions involving carbon-to-carbon unsaturated bonds, e.g. acrylics, vinyl polymers

Definitions

• the present invention relates to a method for making a printing plate involving the use of a heat sensitive imaging element and that can be developed by means of plain water or an aqueous liquid.
• 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.
• the areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
• a photographic material is made imagewise receptive to oily or greasy inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophilic background.
• 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 are widely used.
• 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.
• diazo based printing plates most commonly use an anodized and roughened aluminium as a support having a hydrophilic surface since they offer the advantage of a high printing endurance.
• a particular disadvantage of such type of printing plates is that they require special developing liquids for development which is costly and inconvenient.
• EP-A 601240 discloses a diazo based printing plate that uses a polyester film provided with a cross-linked hydrophilic layer as a lithographic base on which a photosensitive diazo layer is provided. Such a diazo based printing plate can be developed by rinsing it with plain water subsequent to image-wise exposure.
• Lithocraft 10008 FOTOPLATETM is a diazo based printing plate that comprises on a paper support a hydrophilic layer on top of which is provided a diazo based photosensitive layer.
• a plate can be prepared by image-wise exposure of the lithographic printing plate precursor or imaging element, mounting the exposed imaging element on the press and wiping its surface with Lithocraft® 10008 Developer Desensitizer.
• the plate instructions also contemplate a method wherein no developer desensitizer is used. However, such method most often results in poor lithographic preformance so that in practice a Developer Desensitizer is almost always needed.
• diazo-based printing plates carry in common irrespective of the type of lithographic base used, is that they have to be shielded from the light. Moreover, diazo's are insufficiently sensitive to be exposed by means of a commercial and economical laser.
• FR-A-1,561,957 describes a recording material comprising at least a recording layer containing a hydrophilic binder and an hydrophobic compound dispersed in said hydrophylic binder. Upon irradiation and development said recording material can be used as a planographic printing plate.
• US-A-3,476,937 describes also a recording material comprising at least a recording layer containing a hydrophilic binder and an hydrophobic compound dispersed in said hydrophylic binder. Upon irradiation and development said recording material can be used as a planographic printing plate.
• US-A- 3,580,719 describes an imaging element containing a recording layer containing at least about 80 % by weight of a normally water-soluble polymer which, when heated, undergoes a loss in its normal solubility in aqueous solvent.
• the three last imaging elements all have the disadvantage that the corresponding printing plates have low printing endurances.
• EP 514.145 describes a method for making a printing plate wherein a heat-sensitive imaging element is used that comprises on a lithographic base such as an anodised aluminium an image forming layer comprising core-shell particles and a light to heat converting substance.
• the shell of these particles is hydrophilic in nature and renders the particles developable.
• the core is hydrophobic in nature and flows out when heated.
• the image-forming layer can be rendered insoluble at the exposed areas.
• the image forming layer can be removed by means of an aqueous developer containing ethanolamine. Subsequent the material is baked.
• the present invention provides an imaging element comprising (i) on a hydrophilic surface of a lithographic base an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light to heat, said compound being comprised in said image forming layer or a layer adjacent thereto, characterised in that said image forming layer further comprises a cross-linking agent capable of cross-linking said hydrophilic binder upon heating in a ratio between 1 :100 and 200:1 by weight versus the hydrophilic binder.
• this invention provides a method for making a lithographic printing plate comprising the steps of:
• An imaging element for use in accordance with the present invention comprises on a hydrophilic surface of a lithographic base an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a cross-linking agent capable of cross-linking the hydrophilic binder upon heating.
• the hydrophilic binder used in connection with the present invention is perferably not cross-linked or only slightly cross-linked.
• the imaging element further includes a compound capable of converting light to heat. This compound is preferably comprised in the image forming layer but can also be provided in a layer adjacent to the image forming layer.
• the lithographic base can be an anodised aluminium.
• a particularly preferred lithographic base is an electrochemically grained and anodised aluminium support.
• an anodised aluminium support may be treated to improve the hydrophilic properties of its surface.
• the aluminium support may be silicated by treating its surface with sodium silicate solution at elevated temperature, e.g. 95°C.
• a phosphate treatment may be applied which involves treating the aluminium oxide surface with a phosphate solution that may further contain an inorganic fluoride.
• the aluminium 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 50°C.
• a further interesting treatment involves rinsing the aluminium oxide surface with a bicarbonate solution. It is further evident that one or more of these post treatments may be carried out alone or in combination.
• the lithographic base comprises a flexible support, such as e.g. paper or plastic film, provided with a cross-linked hydrophilic layer.
• a particularly suitable cross-linked hydrophilic layer may be obtained from a hydrophilic binder cross-linked with a cross-linking agent such as formaldehyde, glyoxal, polyisocyanate or a hydrolysed tetra-alkylorthosilicate. The latter is particularly preferred.
• hydrophilic binder there may be used hydrophilic (co)polymers such as for example, homopolymers and copolymers of vinyl alcohol, acrylamide, methylol acrylamide, methylol methacrylamide, acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate or maleic anhydride/vinylmethylether copolymers.
• the hydrophilicity of the (co)polymer or (co)polymer mixture used is preferably the same as or higher than the hydrophilicity of polyvinyl acetate hydrolyzed to at least an extent of 60 percent by weight, preferably 80 percent by weight.
• the amount of crosslinking agent, in particular of tetraalkyl orthosilicate, is preferably at least 0.2 parts by weight per part by weight of hydrophilic binder, preferably between 0.5 and 5 parts by weight, more preferably between 1.0 parts by weight and 3 parts by weight.
• a cross-linked hydrophilic layer in a lithographic base used in accordance with the present embodiment preferably also contains substances that increase the mechanical strength and the porosity of the layer.
• colloidal silica may be used.
• the colloidal silica employed may be in the form of any commercially available water-dispersion of colloidal silica for example having an average particle size up to 40 nm, e.g. 20 nm.
• inert particles of larger size than the colloidal silica can be added e.g. silica prepared according to Stöber as described in J. Colloid and Interface Sci., Vol.
• alumina particles or particles having an average diameter of at least 100 nm which are particles of titanium dioxide or other heavy metal oxides.
• the thickness of a cross-linked hydrophilic layer in a lithographic base in accordance with this embodiment may vary in the range of 0.2 to 25 ⁇ m and is preferably 1 to 10 ⁇ m.
• plastic film e.g. a polyester such as a substrated polyethylene terephthalate film or polyethylene naphthalate film, cellulose acetate film, polystyrene film, polycarbonate film etc.
• the plastic film support may be opaque or transparent.
• the amount of silica in the adhesion improving layer is 200 mg per m 2 and 750 mg per m 2 .
• the ratio of silica to hydrophilic binder is preferably more than 1 and the surface area of the colloidal silica is preferably at least 300 m 2 per gram, more preferably a surface area of 500 m 2 per gram.
• an image forming layer on top of a hydrophilic surface there is provided an image forming layer.
• an image forming layer in connection with the present invention comprises thermoplastic polymer particles dispersed in a hydrophilic binder and a cross-linking agent that can be activated by heat.
• Suitable hydrophilic binders for use in an image forming layer in connection with this invention are preferably those that contain reactive groups e.g. hydroxy, amine or carboxyl groups.
• hydrophilic binders are synthetic homo or copolymers such as a polyvinylalcohol, dimethylhydantoine-formaldehyde resin, a poly(meth)acrylic acid, a poly(meth)acrylamide, a polyhydroxyethyl(meth)acrylate, a polyvinylmethylether or natural binders such as gelatin, a polysacharide such as e.g. dextran, pullulan, cellulose, arabic gum, alginic acid.
• Hydrophobic thermoplastic polymer particles used in connection with the present invention have a preferably a glass transition temperature of at least 90 °C, more preferably of at least 100 °C.
• Hydrophobic thermoplastic polymer particles used in connection with the present invention preferably have a coagulation temperature above 50°C and more preferably above 70°C. Coagulation may result from softening or melting of the thermoplastic polymer particles under the influence of heat. There is no specific upper limit to the coagulation temperature of the thermoplastic hydrophobic polymer particles, however the temperature should be sufficiently below the decomposition of the polymer particles. Preferably the coagulation temperature is at least 10°C below the temperature at which the decomposition of the polymer particles occurs.
• said polymer particles When said polymer particles are subjected to a temperature above coagulation temperature they coagulate to form a hydrophobic agglomerate in the hydrophilic layer so that at these parts the hydrophilic layer becomes insoluble in plain water or an aqueous liquid.
• hydrophobic polymer particles for use in connection with the present invention are e.g. polystyrene, polyvinyl chloride, polymethyl methacrylate, polyvinylidene chloride, polyacrylonitrile, polyvinyl carbazole etc. or copolymers and/or mixtures thereof. Most preferably used is polymethyl methacrylate.
• the weight average molecular weight of the polymers may range from 5,000 to 1,000,000g/mol.
• the hydrophobic particles may have a particle size from 0.01 ⁇ m to 50 ⁇ m, more preferably between 0.05 ⁇ m and 10 ⁇ m and most preferably between 0.05 ⁇ m and 0.5 ⁇ m.
• the polymer particles are present as a dispersion in the aqueous coating liquid of the image forming layer and may be prepared by the methods disclosed in US-P-3.476.937. Another method especially suitable for preparing an aqueous dispersion of the thermoplastic polymer particles comprises:
• the amount of hydrophobic thermoplastic polymer particles contained in the image forming layer is preferably above 35% by weight and more preferably above 50% by weight and most preferably above 65% by weight.
• 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, carbon black, metal carbides, borides, nitrides, carbonitrides, bronze-structured oxides and oxides structurally related to the bronze family but lacking the A component e.g. WO 2.9 .
• conductive polymer dispersion such as polypyrrole or polyaniline-based conductive polymer dispersions.
• 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 very good and favorable results.
• a light to heat converting compound in connection with the present invention is most preferably added to the image forming layer but at least part of the light to heat converting compound may also be comprised in a neighbouring layer.
• Such layer can be for example the cross-linked hydrophilic layer of a lithographic base according to the second embodiment of lithographic bases explained above.
• Suitable heat-activatable cross-linking agents for use in the image forming layer in connection with the present invention are used in a ratio between 1:100 to 200:1 by weight versus the hydrophilic binder, more preferably in a ratio between 1:50 to 50:1, most preferably in a ratio betxeen 1:10 to 10:1.
• Suitable heat-activatable cross-linking agents for use in the image forming layer in connection with the present invention are preferably compounds that have two or more groups that can react with the hydrophilic binder, e.g. with one of the reactive groups listed above.
• a cross-linking agent in connection with the present invention may be a low molecular weight compound or may be an oligomer or polymer.
• suitable cross-linking agents for use in an image forming layer in accordance with the present invention are e.g. aldehydes such as formaldehyde, hexamethoxymethyl melamine, amine-formaldehyde resins such as e.g.
• melamine-formaldehyde resin or guanamine-formaldehyde resin dimethylolurea-formaldehyde resins, phenol-formaldehyde resins, compounds having two or more expoxy groups e.g. a polymer having epoxy groups etc...
• a catalyst to an image forming layer in connection with the present invention.
• Such catalyst will speed-up the cross-linking reaction and accordingly total plate making time can be reduced while maintaining a high level of cross-linking needed to obtain a high printing endurance.
• Particularly suitable catalysts for use in this context are acid catalysts. It may furthermore be advantageous to use a precursor of a catalyst so as to improve the selectivity of the process and to obtain the best lithographic performance. Such a precursor will convert to the actual catalyst upon heating i.e. the catalyst will be formed at least partially during the image-wise exposure.
• Suitable precursors of a catalyst are for example precursors that release an acid upon heating.
• suitable acid releasing catalyst precursors are sulfonium compounds, in particular benzylsulfonium compounds, as disclosed in e.g. EP 612065, EP 615233, and US 5.326.677, inorganic nitrates such as e.g. Mg(NO 3 ) 2 .6H 2 O or organic nitrates such as guanidinium nitrate, ammonium nitrate, pyridinium nitrate etc... as disclosed in EP 462763, WO 81/1755, US 4.370.401, compounds that release a sulfonic acid such as 3-sulfolenes, e.g.
• thermolytic compounds disclosed in GB 1.204.495 2,5-dihydrothio-thiophene-1,1-dioxides as disclosed in US 5.312.721, thermolytic compounds disclosed in GB 1.204.495, co-cristalin adducts of an amine and an volatile organic acid as disclosed in US 3.669.747, aralkylcyanoforms as disclosed in US 3.166.583, thermo-acids disclosed in EP 159725 and DE 3515176, squaric acid generating compounds as disclosed in US 5.278.031, acid generating compounds disclosed in US 5.225.314 and US 5.227.277 and RD 11511 of November 1973.
• an imaging element in accordance with the present invention is image-wise exposed to light and subsequently developed preferably by rinsing it with plain water.
• the obtained imaged imaging element is then preferably overall heated to obtain the highest printing endurance.
• the compound capable of converting light into heat absorbs the light used for image-exposure and converts this in heat so as to generate an image-wise pattern of heat in the image-forming layer.
• the hydrophobic thermoplastic polymer particles coagulate and render the image forming layer insoluble for plain water or an aqueous liquid while the non-exposed parts remain soluble in plain water or an aqueous liquid.
• the imaged imaging element is preferably overall heated which will cause substantial cross-linking of the image-forming layer and accordingly will improve the wear resistance of the printing areas during printing. Nevertheless, even without the additional overall heating, a printing endurance can be obtained that is suitable for a number of print jobs.
• a gum before subjecting the imaged imaging element to the overall heat treatment. This will secure the hydrophilic properties at the non-printing areas in particular when an anodised aluminium is used as a lithographic base.
• Suitable gums for this purpose are well-known and commercially available e.g. Polychrome PC965TM (Polychrome).
• Image-wise exposure in connection with the present invention is preferably an image-wise scanning exposure involving the use of a laser or L.E.D.. It is highly preferred in connection with the present invention to use a laser emitting in the infrared (IR) and/or near-infrared, i.e. emitting in the wavelength range 700-1500nm. Particularly preferred for use in connection with the present invention are laser diodes emitting in the near-infrared.
• IR infrared
• near-infrared i.e. emitting in the wavelength range 700-1500nm.
• laser diodes emitting in the near-infrared are particularly preferred for use in connection with the present invention.
• a preferred imaging apparatus suitable for image-wise scanning exposure in accordance with the present invention preferably includes a laser output that can be provided directly to the imaging elements surface via lenses or other beam-guiding components, or transmitted to the surface of a blank imaging element from a remotely sited laser using a fiber-optic cable.
• a controller and associated positioning hardware maintains the beam output at a precise orientation with respect to the imaging elements surface, scans the output over the surface, and activates the laser at positions adjacent selected points or areas of the imaging element.
• the controller responds to incoming image signals corresponding to the original document and/or picture being copied onto the imaging element to produce a precise negative or positive image of that original.
• the image signals are stored as a bitmap data file on a computer.
• Such files may be generated by a raster image processor (RIP) or other suitable means.
• a RIP can accept Input data in page-description language, which defines all of the features required to be transferred onto the imaging element, or as a combination of page-description language and one or more image data files.
• the bitmaps are constructed to define the hue of the color as well as screen frequencies and angles in case of amplitude modulation screening.
• the present invention is particularly suitable for use in combination with frequency modulation screening as disclosed in e.g. EP-A 571010, EP-A 620677 and EP-A 620674.
• the imaging apparatus can be configured as a flatbed recorder or as a drum recorder, with the imaging element mounted to the interior or exterior cylindrical surface of the drum.
• the requisite relative motion between the laser beam and the imaging element is achieved by rotating the drum(and the imaging element mounted thereon) about its axis and moving the beam parallel to the rotation axis, thereby scanning the imaging element circumferentially so the image "grows" in the axial direction.
• the beam can move parallel to the drum axis and, after each pass across the imaging element, increment angularly so that the image on the imaging element "grows" circumferentially.
• the beam is drawn across either axis of the imaging element, and is indexed along the other axis after each pass.
• the requisite relative motion between the beam and the imaging element may be produced by movement of the imaging element rather than (or in addition to) movement of the beam.
• the beam is scanned, it is generally preferable (for reasons of speed) to employ a plurality of lasers and guide their outputs to a single writing array.
• the writing array is then indexed, after completion of each pass across or along the imaging element, a distance determined by the number of beams emanating from the array, and by the desired resolution (i.e. the number of image points per unit length.
• a 0.2mm thick aluminium foil was degreased by immersing the foil in an aqueous solution containing 5g/l of sodium hydroxide at 50°C and rinsed with demineralised water.
• the foil was then electrochemically grained using an alternating current in an aqueous solution containing 4g/l of hydrochloric acid, 4 g/l of hydroboric acid and 0.5g/l of aluminium ions at a temperature of 35°C and a current density of 1200 A/m 2 to form a surface topography with an average center-line roughness R a of 0.5 ⁇ m.
• the aluminium foil was then etched with an aqueous solution containing 300g/l of sulfuric acid ate 60°C for 180 seconds and rinsed with demineralised water at 25°c for 30 seconds.
• the foil was subsequently subjected to anodic oxidation in an aqueous solution containing 200 g/l of sulfuric acid at at temperature of 45°c, a voltage of about 10V and a current density of 150 A/m 2 for about 300 seconds to form an anodic oxidation film of 3g/m 2 Al 2 O 3 , then washed with demineralised water, post treated with a solution containing 20 g/l of sodium bicarbonated at 40°C for 30s, subsequently rinsed with demineralised water of 20°C during 120s and dried.
• the obtained lithographic base was submersed in an aqueous solution containing 5% by weight of citric at 50°C for 60s, rinsed with demineralised water and dried at 40°C.
• An imaging element according to the invention was produced by preparing the following coating composition and coating it to the above described lithographic base in an amount of 30g/m 2 (wet coating amount) and drying it at 35°C.
• the obtained imaging element was subjected to an image-wise scanning exposure using an infrared laser diode emitting at 830 nm.
• the scanspeed was 1 m/s, spot size 10 ⁇ m and 120 mW power on the plate surface.
• the imaging element was subsequent developed in a Polychrome PC28ETM processor filled with water in the developing section and a gum (Polychrome PC965TM) in the gumming section.
• the obtained printing plate was then mounted on a Heidelberg GTO46 offset press equipped with K + E 125 ink and as dampening liquid Rotamatic. 15000 clear copies were obtained with no ink-acceptance in the non-image areas. Even after printing 15000 copies, no damage to the image areas could be seen.
• Printing plates were prepared as described in example 1 but with the modification that hexamethoxymethylmelamine (cross-linker) was not used in the coating composition. Printing was carried out as in example 1 and only 6000 copies could be printed due to damage to the image areas.

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• Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Thermal Sciences (AREA)
• Engineering & Computer Science (AREA)
• Manufacturing & Machinery (AREA)
• Photosensitive Polymer And Photoresist Processing (AREA)
• Manufacture Or Reproduction Of Printing Formes (AREA)
• Printing Plates And Materials Therefor (AREA)

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Citations (32)

• 1996
  • 1996-10-22 EP EP19960202950 patent/EP0773112B1/de not_active Expired - Lifetime

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