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
  • B41C1/1041—Forme preparation for lithographic printing; Master sheets for transferring a lithographic image to the forme by modification of the lithographic properties without removal or addition of material, e.g. by the mere generation of a lithographic pattern

Definitions

• the present invention relates to a method for making a negative-working, heat-sensitive printing plate precursor which is suitable for making a lithographic printing plate by direct-to-plate recording.
• Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press.
• the master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper.
• ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas.
• driographic printing the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
• Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor.
• plate precursor an imaging material
• heat-sensitive printing plate precursors have become very popular in the late 1990s.
• thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask.
• the material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross linking of a polymer, heat-induced solubilization, or by particle coagulation of a thermoplastic polymer latex.
• a (physico-)chemical process such as ablation, polymerization, insolubilization by cross linking of a polymer, heat-induced solubilization, or by particle coagulation of a thermoplastic polymer latex.
• the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating.
• the coating typically comprises an oleophilic binder, e.g. a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure.
• the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support.
• Typical examples of such plates are described in e.g.
• Some of these thermal processes enable plate making without wet processing and are for example based on ablation of one or more layers of the coating. At the exposed areas the surface of an underlying layer is revealed which has a different affinity towards ink or fountain than the surface of the unexposed coating; the image (printing) and non-image or background (non-printing) areas are obtained.
• US 5,605,780 discloses a lithographic printing plate comprising an anodized aluminum support and provided thereon an image-forming layer comprising an IR absorbing agent and a cyanoacrylate polymer binder.
• the image-forming layer is removed by laser-induced thermal ablation whereby the underlying hydrophilic support is revealed.
• EP-A 580,393 discloses a lithographic printing plate directly imageable by laser discharge, the plate comprising a topmost first layer and a second layer underlying the first layer wherein the first layer is characterized by efficient absorption of infrared radiation and the first and second layer exhibit different affinities for at least one printing liquid.
• EP 1,065,051 discloses a negative-working heat-sensitive material for making lithographic plates comprising in the order given a lithographic base having a hydrophilic surface, an oleophilic imaging layer and a cross-linked hydrophilic upper layer. The heat generated during exposure in the light-sensitive layer removes the hydrophilic upper layer by ablation.
• ablative plates generate ablation debris which may contaminate the electronics and optics of the exposure device and which needs to be removed from the plate by wiping it with a cleaning solvent, so that ablative plates are often not truly processless. Ablation debris which is deposited onto the plate's surface may also interfere during the printing process and result in for example scumming.
• thermal processes which enable plate making without wet processing are for example processes based on a heat-induced hydrophilic/ oleophilic conversion of one or more layers of the coating so that at exposed areas a different affinity towards ink or fountain is created than at the surface of the unexposed coating.
• US 5,855,173 , US 5,839,369 and 5,839,370 describe a method relying on the image-wise hydrophilic-hydrophobic transition of a ceramic such as a zirconia ceramic and the subsequent reverse transition in an image erasure step.
• This image-wise transition is obtained by exposure to infrared laser irradiation at a wavelength of 1064 nm at high power which induces local ablation and formation of substoichiometric zirconia.
• US 5,893,328 , US 5,836,248 and US 5,836,249 disclose a printing material comprising a composite of zirconia alloy and a-alumina which can be imaged using similar exposure means to cause localized "melting" of the alloy in the exposed areas and thereby creating hydrophobic/oleophilic surfaces.
• a similar printing material containing an alloy of zirconium oxide and Yttrium oxide is described in US 5,870,956 .
• the high laser power output required in these prior art methods implies the use of expensive exposure devices.
• EP 1,002,643 discloses a printing plate comprising an anodized titanium metal sheet which becomes hydrophilic or ink-repellent upon image-wise exposure to actinic light. Said printing plate can be regenerated after printing by first cleaning the plate and subsequently subjecting the plate to a heat-treatment step whereby the plate surface becomes evenly oleophilic or ink-accepting.
• EP 958,941 discloses a plate precursor having a surface layer comprising Si 3 N 4 and/or BN on which an image can be formed after image-wise exposure of the layer to light.
• US 6,240,091 discloses a method of producing a lithographic printing plate by image-wise irradiation of a printing plate precursor which comprises a support having a hydrophilic, metallic compound layer with photo-catalytic properties and light-heat convertible minute particles onto said layer, whereby the polarity of the metallic layer is converted and a hydrophobic area is obtained.
• EP 903,223 discloses a lithographic printing method using a printing plate precursor comprising a surface having a thin layer of TiO 2 , ZnO or a compound selected from the group consisting of RTiO 3 wherein R represents an alkaline earth metal atom, AB 2-x C x D 3-x E x O 10 wherein A represents a hydrogen atom or an alkali metal atom; B represents an alkaline earth metal atom or a lead atom; C represents a rare earth atom; D represents a metal atom of the group 5A of the Periodic Table; E represents a metal atom of the group 4A of the Periodic Table; and x represents a number from 0 to 2, Sn02, Bi 2 O 3 and Fe 2 O 3 .
• the exposure step with actinic light through a film mask renders the surface hydrophilic at the exposed areas and subsequent heating results in a hydrophilic/hydrophobic conversion.
• a major problem associated with the prior art materials based on metal oxides is that these materials require exposure with high power laser light and/or the use of expensive exposure devices.
• Other plates based on metal oxides have to undergo a photo-reduction reaction prior to their use to induce a hydrophobic/hydrophilic conversion.
• This photo-reduction reaction can be initiated by for example a pre-heat treatment step and/or a flood UV-exposure step which have to be performed by the end-user.
• pre-treatment steps make plate making a cumbersome process.
• a printing plate precursor having a support with a surface comprising titanium is converted from a hydrophobic state into a hydrophilic state by first etching said support followed by an anodizing step.
• a switch from a hydrophilic state into a hydrophobic state is obtained.
• the support of the printing plate precursor having a surface comprising titanium is preferably a titanium metal sheet.
• the support is a base onto which a thin layer of titanium metal is applied.
• the titanium metal sheet may be a commercially available titanium metal sheet having preferably a 99.5 %wt to 99.9 %wt purity. Also suitable is an alloy of titanium containing about 4 %wt to 5 %wt of for example aluminium, vanadium, manganese, iron, chromium and molybdenum.
• the thickness of the titanium metal sheet is not critical: it may be between 0.05 mm to 0.6 mm, preferably from 0.05 mm to 0.4 mm, more preferably from 0.1 mm to 0.3 mm.
• the base onto which a thin layer of titanium is applied may be a metal sheet including for example aluminum, stainless steel, nickel, and copper.
• a flexible plastic support such as polyester or cellulose ester, waterproof paper, polyethylene-laminated paper, or polyethylene-impregnated paper.
• the support can also be a laminate comprising an aluminum foil and a plastic layer, e.g. polyester film.
• the surface of the metal base may have been roughened by any of the known methods.
• the surface roughening may be conducted by mechanical means, electrochemical means and chemical etching means, or by combinations of these methods.
• a particularly preferred lithographic support is an electrochemically grained and anodized aluminum support.
• the grained and anodized aluminum support is preferably grained by electrochemical graining, and anodized by means of anodizing techniques employing phosphoric acid or a sulphuric acid/phosphoric acid mixture. Methods of both graining and anodization of aluminum are very well known in the art.
• the aluminium support By anodizing the aluminium support, its abrasion resistance and hydrophilic nature are improved.
• the microstructure as well as the thickness of the Al 2 O 3 layer are determined by the anodizing step, the anodic weight (g/m 2 Al 2 O 3 formed on the aluminium surface) varies between 1 and 8 g/m 2
• the thin layer of titanium present on the base may be applied by known methods such as for example vapor deposition, spray pyrolysis, sputtering, or electrodeposition.
• the thickness of the deposited titanium metal layer is preferably from 0.01 ⁇ m to 10 ⁇ m, more preferably from 0.05 ⁇ m to 1.0 ⁇ m, most preferably the thickness varies between 0.10 ⁇ m and 0.30 ⁇ m.
• the titanium sheet or the base provided with a titanium layer may be subjected to a surface roughening step treatment prior to the etching step.
• a degreasing step may be conducted with for example a surfactant, an organic solvent or an aqueous alkali solution.
• the surface roughening treatment of the titanium sheet or the base provided with a titanium layer can be conducted by various methods; examples thereof include mechanically roughening (e.g. grinding with balls, brushing, blasting, or buffing), electrochemical dissolution (e.g. surface roughening in an electrolytic solution with application of an AC or DC current) or chemical dissolution (e.g. immersing the metal in an aqueous solution of one or more alkaline salts selected from sodium hydroxide, sodium carbonate, sodium silicate or sodium pyrophosphate). These methods may be used alone or in combination.
• mechanically roughening e.g. grinding with balls, brushing, blasting, or buffing
• electrochemical dissolution e.g. surface roughening in an electrolytic solution with application of an AC or DC current
• chemical dissolution e.g. immersing the metal in an aqueous solution of one or more alkaline salts selected from sodium hydroxide, sodium carbonate, sodium silicate or sodium pyrophosphate.
• the anodization of titanium is performed by treatment of the surface comprising titanium with an aqueous electrolyte solution at a concentration of 0.001 mol/l to 5 mol/l, preferably from 0.005 mol/l to 3 mol/l, a liquid temperature of 5°C to 70°C, preferably from 15°C to 30°C, a DC voltage of 1 V to 100 V, preferably 5 V to 50 V, more preferably 10 V to 30 V, and an electrolysis period of 10 seconds to 10 minutes, preferably 1 minute to 8 minutes.
• an aqueous electrolyte solution at a concentration of 0.001 mol/l to 5 mol/l, preferably from 0.005 mol/l to 3 mol/l, a liquid temperature of 5°C to 70°C, preferably from 15°C to 30°C, a DC voltage of 1 V to 100 V, preferably 5 V to 50 V, more preferably 10 V to 30 V, and an electrolysis period of 10 seconds to 10 minutes, preferably 1 minute to 8
• the surface of the support is anodized in an aqueous electrolyte solution containing at least one of the following chemicals:
• aqueous electrolyte solutions may be used alone or in combination.
• concentration of the solutions depends on the kind of the electrolyte used for the anodization process.
• the electrolyte solution comprises oxalic acid at a concentration of 0.6 mol/l, and the anodizing reaction is carried out at room temperature using 20 V DC for a period of 5 minutes.
• Doping the anodized surface with a metal such as platinum, palladium, gold, silver, copper, nickel, iron, or cobalt or a mixture thereof may be advantageous.
• the support is etched.
• Etching of metal surfaces can be done in many ways.
• aqueous solutions comprising one or more alkaline salts can be used.
• a mixture comprising H 2 O 2 and NaOH is used wherein the concentration of H 2 O 2 varies between 0.05 mol/l and 1 mol/l, more preferably between 0.1 mol/l and 0.8 mol/l and the concentration of NaOH varies between 0.1 mol/l and 5 mol/l, more preferably between 0.5 mol/l and 3.5 mol/1.
• the etching temperature varies preferably between 50 °C and 100 °C, more preferably between 60 °C and 80 °C and the reaction time varies preferably between 0.5 minute and 10 minutes, more preferably between 0.5 minute and 5 minutes.
• the etching and anodizing step may be carried out in one step.
• the etched and anodized printing plate precursor may optionally undergo further post-treatments such as chemical reducing treatments e.g. annealing at reduced pressure as defined in the first method, or photolytic reduction e.g. UV-treatment.
• the lithographic printing plate precursor comprising an etched and anodized support thus obtained may be rinsed with water, with a liquid containing a surfactant or with a desensitizing liquid (so called gum solution) containing gum arabic or a starch derivative, or with combinations thereof.
• a liquid containing a surfactant or with a desensitizing liquid (so called gum solution) containing gum arabic or a starch derivative, or with combinations thereof.
• the surface of the printing plate precursor is hydrophilic and upon image-wise exposure to heat and/or light, the exposed areas become ink accepting.
• This conversion from a hydrophilic to a hydrophobic state can for example be characterized by an increase of the contact angle for water measured on the surface: the contact angle for water increases after the treatment of the support indicating a hydrophilic/hydrophobic conversion.
• the contact angle is defined as the angle between the tangent of the edge of the water droplet at the contact zone between the support and the droplet.
• a layer which comprises a compound capable of absorbing light and converting the absorbed energy into heat may optionally be coated onto the etched and anodized support.
• the compound capable of absorbing light and converting it into heat is preferably an infrared absorbing agent.
• Preferred IR absorbing compounds are dyes such as cyanine, merocyanine, indoaniline, oxonol, pyrilium and squarilium dyes or pigments such as carbon black. Examples of suitable IR absorbers are described in e.g. EP-As 823327 , 978376 , 1029667 , 1053868 , 1093934 ; WO 97/39894 and 00/29214 .
• a preferred compound is the following cyanine dye IR-A: wherein X - is a suitable counter ion such as tosylate.
• the coating may in addition to the layer comprising the infrared absorbing agent also contain one or more additional layer(s) such as i.e. a protective layer or an adhesion-improving layer between the layer comprising the infrared absorbing agent and the support.
• additional layer(s) such as i.e. a protective layer or an adhesion-improving layer between the layer comprising the infrared absorbing agent and the support.
• the layer comprising a compound capable of absorbing light or an optional other layer may further contain additional ingredients.
• additional ingredients for example binders, surfactants such as perfluoro surfactants, silicon or titanium dioxide particles or colorants may be present.
• the heat-sensitive printing plate precursor thus obtained is then image-wise exposed directly with heat or indirectly with infrared light, preferably near infrared light.
• infrared light is preferably converted into heat by an IR light absorbing compound as discussed above.
• the printing plate precursor is not sensitive to ambient light so that it can be handled without the need for a safe light environment.
• the printing plate precursor can be exposed to infrared light by means of e.g. LEDs or an infrared laser.
• the light used for the exposure is a laser emitting near infrared light having a wavelength in the range from about 700 nm to about 1500 nm, e.g. a semiconductor laser diode, a Nd:YAG or a Nd:YLF laser.
• the exposure step may optionally be followed by a rinsing step and/or a gumming step.
• the gumming step involves post-treatment of the heat-sensitive printing plate with a gum solution.
• a gum solution is typically an aqueous liquid which comprises one or more surface protective compounds that are capable of protecting the lithographic image of a heat-sensitive material or printing plate against contamination or damaging. Suitable examples of such compounds are film-forming hydrophilic polymers or surfactants.
• the heat-sensitive printing plate is then ready for printing without an additional development step.
• the exposed plate can be mounted on a conventional, so-called wet offset printing press in which ink and an aqueous dampening liquid are supplied to the material.
• the non-image areas hold the dampening water and the image areas withhold the ink.
• the single-fluid ink comprises an ink phase, also called the hydrophobic or oleophilic phase, and a polyol phase as described in WO 00/32705 .
• the printing plate is first mounted on the printing cylinder of the printing press and then image-wise exposed directly on the press by means of an integrated image-recording device. Subsequent to exposure, the plate is ready for printing.
• the printing plate can be regenerated after printing. After printing, the printing plate is subjected to a flood exposure with UV light whereby hydrophobic areas are converted to a hydrophilic state and recover sensitivity to infrared light and/or heat irradiation.
• a cleaning step may be performed before the flood exposure step.
• Suitable solvents that can be used for cleaning include hydrophobic petroleum solvents such as aromatic hydrocarbons commercially available as printing ink solvents: kerosine, benzol, toluol, xylol, acetone, methyl ethyl ketone, and mixtures thereof.
• the regenerated printing plate precursor thus obtained can be used for a next printing operation involving image-wise exposure and printing.
• a Ti-foil (Goodfellow TI000380 99,6%, 125 ⁇ m foil) was cleaned by ultrasound treatment in isopropanol and was subsequently rinsed with water.
• the different printing plate precursors were anodized at room temperature utilizing a DC voltage of 20 V (Table 1). A titanium counter electrode was used and the distance between the two electrodes was 2.4 cm. Table 1: Etching and anodizing conditions. Etching conditions Anodizing conditions Printing plate precursor (PPP) nr. Conc. H 2 O 2 mol/l Conc. NaOH Mol/l Etching time min Conc.
• the printing plate precursors 4 - 10 were subsequently exposed with a single beam IR laser diode at 830 nm with a pitch of 7 ⁇ m at different powers and drum speeds.
• the resulting energy densities are given in Table 2.
• Table 2 Energy densities.
• Laser Setting Power MW Drumspeed m/s Energy density mJ/cm 2 6 280 4 1000 7 200 4 714 8 140 4 500 9 280 8 500 10 200 8 357
• Van SON 167 ink (trademark of Van Son) was used together with a fountain solution of 5% G671 (trademark of Agfa-Gevaert) in water.
• G671 trademark of Agfa-Gevaert
• a non-compressible rubber blanket was used and 250 copies were printed on 80 g offset paper.
• the ink density on the prints were measured using a Gretag Macbeth D19C densitometer (available from Gretag Macbeth AG) and are summarized Table 3 (ink density after 50 prints) and Table 4 (ink density after 250 prints). Table 3: Ink density after 50 prints. Printing plate (PP) nr.
• Tables 3 and 4 show that an etching step followed by an anodization step results in a support having a surface with hydrophilic properties. Upon exposure to infrared light, the ink density values increase. The best results are obtained by the "strongest" etching condition i.e. longest reaction time and highest concentration of H 2 O 2 and NaOH (see invention printing plate 5). When no etching step is carried out (only an anodization step), the support has a surface with hydrophobic properties (ink density value ⁇ 1; see comparative printing plates 1 and 2).
• printing plate 5 was cleaned by removing the ink from the plate; the plate cleaner Howson Normakleen RC910 (trademark of Howson Normakleen) was used.
• the plate was irradiated for 24 hours with a mercury lamp emitting at 254 nm at 0.5 mW/cm2.
• Van SON 167 ink was used together with a fountain solution of 5% G671 (trademark of Agfa Graphics NV) in water.
• G671 trademark of Agfa Graphics NV
• the part of the plate which was not irradiated with the IR-laser showed at print 50 ink density values varying between 0.05 and 0.15.
• the part of the plate which was irradiated with the IR-laser showed ink density values of 1.0.
• the ink density values were measured using a Gretag Macbeth D19C densitometer (available from Gretag Macbeth). This example shows that flood exposure of the printing plate with UV-light results in a precursor which can be reused in a next cycle of imaging and printing.

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• Engineering & Computer Science (AREA)
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• Materials For Photolithography (AREA)
• Photosensitive Polymer And Photoresist Processing (AREA)
• Printing Plates And Materials Therefor (AREA)

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