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
  • B41M5/368—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 involving the creation of a soluble/insoluble or hydrophilic/hydrophobic permeability pattern; Peel development
• • 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
  • B41C2210/20—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation characterised by inorganic additives, e.g. pigments, salts
• • 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
  • B41C2210/22—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation characterised by organic non-macromolecular additives, e.g. dyes, UV-absorbers, plasticisers
• • 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
• • 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
  • B41C2210/26—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 not involving carbon-to-carbon unsaturated bonds
  • B41C2210/264—Polyesters; Polycarbonates
• • 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
  • B41C2210/26—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 not involving carbon-to-carbon unsaturated bonds
  • B41C2210/266—Polyurethanes; Polyureas

Definitions

• the invention pertains to the field of lithography and in particular to imaging materials for digital-on-press technology.
• lithographic printing is planographic and is based on the immiscibility of oil and water wherein the oily material or ink is preferentially retained in the image area of a printing plate and the water or fountain solution retained by the non-image area.
• a widely used type of lithographic printing plate has a light sensitive coating applied to a hydrophilic base support, typically made from anodized aluminum. The coating may respond to the light by having the portion that is exposed becoming soluble so that it may be removed by a subsequent development process. Such a plate is said to be positive working.
• the plate is referred to as a negative working plate.
• a hydrophilic support is coated with a thin layer of a negative-working photosensitive composition.
• Typical coatings for this purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids, and a large variety of synthetic photopolymers. Diazo-sensitized systems in particular are widely used.
• Imagewise exposure of such imagable light-sensitive layers renders the exposed image insoluble while the unexposed areas remain soluble in a developer liquid.
• the plate is then developed wth a suitable developer [quid to remove the imagable layer in the unexposed areas.
• a particular disadvantage of photosensitive imaging elements such as those described above for making a printing plate is that they work with visible light and have to be shielded from normal room lighting. Furthermore, they can have the problem of instability upon storage.
• thermo plates or “heat mode plates” therefore refer to the conversion mechanism by which the hydrophilicity of the surface of the plate is changed, and does not refer to the wavelength of the light being employed. Products that function on the basis of this principle are today on the market. One example is the Thermolite product from the company Agfa of Mortsel in Belgium.
• thermoplastic polymer particles By image-wise exposure to an infrared laser, the thermoplastic polymer particles are image-wise coagulated, thereby rendering the surface of the imaging element at these areas ink accepting without any further development.
• a disadvantage of this method is that the printing plate so obtained is easily damaged since the non-printing areas may become ink-accepting when some pressure is applied thereto. Moreover, under critical conditions, the lithographic performance of such a printing plate may be poor and accordingly such printing plate has little lithographic printing latitude.
• the printing surfaces produced by these materials provide run-lengths (number of printing impressions per plate) of the order of 20,000 to 30,000 impressions per prepared printing surface on good quality paper. This is rather shorter than the run-lengths achievable with some other kinds of media used in industry. This cause of this may be traced directly to the developability versus durability trade-off raised earlier.
• the commercially available thermal media also does not function well with lower quality uncoated paper or in the presence of some commonly used press-room chemicals such as set-off powder, reducing the run-length often to less than one third of that achieved under ideal conditions. This is unfortunate in that these materials and lower quality paper are both inherent realities of the commercial printing industry.
• the polymer emulsion coating is not light sensitive but the substrate used therein converts laser radiation so as to fuse the polymer particles in the image area.
• the glass transition temperature (Tg) of the polymer is exceeded in the imaged areas thereby fusing the image in place onto the substrate.
• the background can be removed using a suitable developerto remove the non-laser illuminated portions of the coating. Since the fused polymer is ink loving, a laser-imaged plate results without using a light sensitive coating such as diazo. However, there is a propensity for the background area to retain a thin layer of coating in such formulations. This results in toning of the background areas during printing.
• On-press imaging is a newer method of generating the required image directly on the plate or printing cylinder.
• Existing on-press imaging systems can be divided into two types.
• the mounting cylinder is split so that clamping of the ends of the plate can be effected by a clamping means that passes through a gap in the cylinder and a slit between the juxtaposed ends of the plate.
• the gap in the mounting cylinder causes the cylinder to become susceptible to deformation and vibration. The vibration causes noise and wears out the bearings.
• the gap in the ends of the plate also leads to paper waste in some situations.
• the overall process has the same elements.
• the printing surface whether plate or cylinder or sleeve, is cleaned. It is then coated with the thermal medium. The coating is then cured or dried to form a hydrophilic layer or one that can be removed by fountain or other aqueous solutions.
• This layer is then imaged using data written directly, typically via a laser or laser array.
• the printing surface is then developed using an appropriate developer iquid. This includes the possibility of using fountain solution.
• the coating in the unexposed areas is thereby removed, leaving the imaged hydrophobic areas.
• the printing surface is then inked and the ink adheres only to the hydrophobic imaged and coalesced areas, but not to the exposed areas of the hydrophilic substrate where there is water from the fountain solution, thereby keeping the ink, which is typically oil-based, from adhering.
• Printing is now performed. At the end of the cycle, the imaged layer is removed by a solvent and the process is restarted.
• thermal lithographic media that can produce extended run lengths and function effectively in the presence of press-room chemicals. It should also function effectively on lower quality paper and be compatible with the rapidly developing on-press technologies, including the more recent spray-on technologies.
• a lithographic printing precursor for lithographic offset printing.
• the lithographic printing precursor comprises a layer of imageable medium on a hydrophilic base.
• the imageable medium comprises hydrophobic polymer particles in an aqueous medium, a substance for converting light into heat and a coalescence inhibitor.
• the lithographic printing precursor may be used to make lithographic printing surfaces that obtain long run lengths on lower quality paper and in the presence of press-room chemicals.
• the lithographic printing precursor can be imaged and developed on-press and the imageable medium can also be sprayed onto a hydrophilic surface to create a printing surface that may be processed wholly on-press. It can also be processed in the more conventional fully off-press fashion.
• the hydrophilic surface can be a printing plate substrate or the printing cylinder of a printing press or a sleeve around the printing cylinder of a printing press.
• the cylinder can be conventional or seamless.
• a method for making a lithographic printing surface using media with coalescence inhibitor is made on a hydrophilic surface using a radiation-sensitive medium that comprises hydrophobic polymer particles in an aqueous medium, a substance for converting light into heat, and a coalescence inhibitor.
• the lithographic printing surface made by the method of this invention may be used for printing long run lengths on lower quality paper and in the presence of press-room chemicals.
• the method allows for on-press imaging and development of the lithographic printing surface.
• the radiation-sensitive medium can also be sprayed onto a hydrophilic surface to create a lithographic printing surface that may be processed wholly on-press. It can also be processed in the more conventional fully off-press fashion.
• an imageable medium intended for use as a coating on a lithographic printing precursor for lithographic offset printing.
• the imageable medium comprises uncoalesed particles of a hydrophobic thermoplastic polymer, a coalesence inhibitor and a converter substance capable of converting radiation into heat
• the coalesence inhibitor is preferably one of an inorganic salt, an organic base, an organic acid and a metal complex.
• the present invention is embodied in a thermally convertible lithographic printing precursor comprising a lithographic base with an imagable coating on those of its surfaces that are to be used for printing.
• the imagable medium of the imagable coating comprises uncoalesced particles of one or more hydrophobic thermoplastic polymers, one or more converter substances capable of converting radiation into heat and one or more coalescence inhibitors.
• the individual components may be applied to the lithographic base as a coating comprising a single layer or as a coating comprising separate layers. Where the coating is in separate layers, one or more components of the coating may be present in each layer.
• the converter substance can be in one layer and a mixture of the polymer particles and the coalescence inhibitor can be present in a second layer.
• the medium is prepared without one of the key components, namely the coalescence inhibitor, it exhibits no developability, the entire coating resisting washing off in aqueous media.
• the coalescence inhibitor therefore plays a key role as a development-enhancing agent.
• lithographic printing precursor is used herein to describe any printing plate, printing cylinder or printing cylinder sleeve, or any other surface bearing a coating of imageable material that may be either converted or removed imagewise to create a surface that may be inked selectively and used for lithographic printing.
• the phrase "lithographic printing surface” is used herein to describe the selectively inkable surface so created.
• the specific term “lithographic base” is used herein to describe the base onto which the imageable material is coated.
• the lithographic bases used in accordance with the present invention are preferably formed of aluminum, zinc, steel, or copper.
• bi-metal and tri-metal plates such as aluminum plates having a copper or chromium layer; copper plates having a chromium layer and steel plates having copper or chromium layers.
• Other preferred substrates include metallized plastic sheets such as poly(ethylene terephthalate).
• Particularly preferred plates are grained, or grained and anodized, aluminum plates where the surface is roughened (grained) mechanically or chemically (e.g. electrochemically) or by a combination of roughening treatments.
• the anodizing treatment can be performed in an aqueous acid electrolytic solution such as sulphuric acid or a combination of acids such as sulphuric and phosphoric acid.
• the anodized aluminum surface of the lithographic base may be treated to improve the hydrophilic properties of its surface.
• a phosphate solution that may also contain an inorganic fluoride is applied to the surface of the anodized layer.
• the aluminum oxide layer may be also treated with sodium silicate solution at an elevated temperature, e.g.90° C.
• the aluminum oxide surface may be rinsed with a citric acid or citrate solution at room temperature or at slightly elevated temperatures of about 30 to 50° C.
• a further treatment can be made by rinsing the aluminum oxide surface with a bicarbonate solution.
• Another useful treatment to the aluminum oxide surface is with polyvinylphosphonic acid, polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinyl alcohol, polyvinylsulphonicacid, polyvinylbenzenesulphonicacid, sulphuric acid esters of polyvinyl alcohol, and acetals of polyvinyl alcohols formed by reaction with a sulphonated aliphatic aldehyde. It is evident that these post treatments may be carried out singly or as a combination of several treatments.
• the lithographic base having a hydrophilic surface comprises a flexible support, such as e.g. paper or plastic film, provided with a cross-linked hydrophilic layer.
• a suitable cross-linked hydrophilic layer may be obtained from a hydrophilic (co)polymer cured with a cross-linking agent such as a hydrolysed tetra-alkylorthosilicate, formaldehyde, glyoxal or polyisocyanate. Particularly preferred is the hydrolysed tetra- alkylorthosilicate.
• the hydrophilic (co-) polymers that may be used comprise for example, homopolymers and copolymers of vinyl alcohol, hydroxyethyl acrylate, hydroxyethyl methacrylate acrylic acid, methacrylic acid, acrylamide, methylol acrylamide or methylol methacrylamide.
• the hydrophilicity of the (co)polymer or (co)polymer mixture used is preferably higher than that 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 (co-) polymer, more preferably between 1.0 parts by weight and 3 parts by weight.
• a cross-linked hydrophilic layer of the lithographic base preferably also contains materials that increase the porosity and/or the mechanical strength of this layer.
• Colloidal silica employed for this purpose may be in the form of any commercially available water-dispersion of colloidal silica having an average particle size up to 40 nm. Additionally inert particles of a size larger than colloidal silica may be used e.g. alumina or titanium dioxide particles or particles having an average diameter of at least 100 nm but less than 1 ⁇ m which are particles of other heavy metal oxides. The incorporation of these particles causes a roughness, which acts as storage places for water in background areas.
• the thickness of a cross-linked hydrophilic layer of a lithographic base in accordance with this embodiment can vary between 0.5 to 20 ⁇ m and is preferably 1 to 10 ⁇ m.
• suitable cross-linked hydrophilic layers for use in accordance with the present invention are disclosed in EP 601240, GB-P-1419512, FR-P- 2300354, U.S. Pat. No. 3,971 ,660, and U.S. Pat. No.4,284,705.
• a particularly preferred substrate to use is a polyester film on which an adhesion- promoting layer has been added.
• Suitable adhesion-promoting layers for use in accordance with the present invention comprise a hydrophilic (co-) polymer and colloidal silica as disclosed in EP 619524, and EP 619525.
• the amount of silica in the adhesion-promoting layer is between 0.2 and 0.7 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.
• uncoalesced is used herein to describe a state of an assemblage of polymer particles that are not substantially fused together. This is to be contrasted with coalesced polymer particles where a plurality of particles has essentially fused together to form a contiguous whole.
• the hydrophobic thermoplastic polymer particles used in connection with the present invention preferably have a coalescence temperature above 35° C. and more preferably above 50° C.
• the coalescence of the polymer particles may result from softening or melting of the thermoplastic polymer particles under the influence of heat.
• the specific upper limit to the coalescence temperature of the thermoplastic hydrophobic polymer should be below the decomposition temperature of the thermoplastic polymer.
• the coalescence temperature is at least 10° C below the decomposition temperature of the polymer particle.
• hydrophobic thermoplastic polymer particles for use in connection with the present invention with a Tg above 40° C. are preferably polyvinyl chloride, polyethylene, polyvinylidene chloride, polyacrylonitrile, poly(meth)acrylates etc., copolymers or mixtures thereof. More preferably used are polymethyl- methacrylate or copolymers thereof. Polystyrene itself or polymers of substituted styrene are particularly preferred, most particularly polystyrene copolymers or polyacrylates.
• the weight average molecular weight of the hydrophobic thermoplastic polymer in the dispersion may range from 5,000 to 1 ,000,000 g/mol.
• the hydrophobic thermoplastic polymer in the dispersion may have a particle size from 0.01 ⁇ m to 30 ⁇ m, more preferably between 0.01 ⁇ m and 3 ⁇ m and most preferably between 0.02 ⁇ m and 0.25 ⁇ m.
• the hydrophobic thermoplastic polymer particle is present in the liquid of the imagable coating.
• thermoplastic polymer A suitable method for preparing an aqueous dispersion of the thermoplastic polymer comprises the following steps:
• the amount of hydrophobic thermoplastic polymer dispersion contained in the image forming layer is preferably between 20% by weight and 95% by weight and more preferably between 40% by weight and 90% by weight and most preferably between 50% by weight and 85% by weight
• the imagable coating may be applied to the lithographic base while the latter resides on the press.
• the lithographic base may be an integral part of the press or it may be removably mounted on the press.
• the imagable coating may be cured by means of a curing unit integral with the press, as described by Gelbart in US Patent 5,713,287.
• the imagable coating may be applied to the lithographic base and cured before the complete thermally convertible lithographic printing precursor is loaded on the printing cylinder of a printing press. This situation would pertain in a case where a lithographic printing plate is made separate from the press or a press cylinder is provided with a lithographic printing surface without being mounted on the press.
• curing is here to be understood to include the hardening of the imagable medium, specifically including the drying thereof, either with or without cross-linking of the incorporated polymer.
• the lithographic base Before applying the imagable coating to the lithographic base, the lithographic base may be treated to enhance the developability or adhesion of the imagable coating.
• the imageable material of the coating is imagewise converted by means of the spatially corresponding imagewise generation of heat within the coating to form an area of coalesced hydrophobic polymer particles.
• the imaging process itself may be by means of scanned laser radiation as described by Gelbart in US Patent 5,713,287. The wavelength of the laser light and the absorption range of the converter substance are chosen to match each other. This process may be conducted off-press, as on a plate-setting machine, or on-press, as in digital-on-press technology.
• the heat to drive the process of coalescence of the polymer particles is produced by the converter substance, herewith defined as a substance that has the property of converting radiation into heat.
• the specific term "thermally convertible lithographic printing precursor” is used to describe the particular subset of lithographic printing precursors in which the imageable material of the coating is imagewise converted by means of the spatially corresponding imagewise generation of heat to form an area of coalesced hydrophobic polymer particles. This area of coalesced hydrophobic polymer particles will therefore be the area to which lithographic printing ink will adhere for the purposes of subsequent printing.
• the converter substances present in the composition have high absorbance at the wavelength of the laser.
• Such substances are disclosed in JOEM Handbook 2
• the representative examples include N-[4-[5-(4- dimethylamino-2-methylphenyl)-2,4-pentadienylidene]-3-methyl-2,5-cyclohexadiene-1- ylidene]-N,N-dimethylammonium acetate, N-[4-[5-(4-dimethylaminophenyl)-3-phenyi- 2-pentene-4-in-1-ylidene]-2,5-cyclohexadiene-1-ylidene]-N,N-dimethylammonium perchlorate, bis(dichlorobenzene-1 ,2-dithiol)nickel(2:1 )tetrabutylammonium and polyvinylcarbazol-2,3-dicyano-5-nitro1,4-naphthoquinonecompIex.
• Carbon black, other black body absorbers and other infra red absorbing materials, dyes or pigments may also be used as the converter substance, particularly with higher levels of infra-red absorption/conversion at 800-1100nm and particularly between 800 and 850nm.
• Light to heat converter substances Some specific commercial products that may be employed as light to heat converter substances include Pro-jet 830NP, a modified copper phthabcyanine from Avecia of Blackley, Lancashire in the U.K., and ADS 830A, an infra-red absorbing dye from American Dye Source Inc. of Montreal, Quebec, Canada.
• the light to heat converter substance has a preferred concentration of 0.25 to 10% of the dry polymer weight and preferably this concentration is between 0.5% and 6%.
• Embodiments of the present invention provide a coalescence inhibitor for use in the lithographic printing precursor.
• the coalescence inhibitors are chosen for their miscibility with or solubility in water, aqueous solution or press fountain solution.
• the concentration of coalescence inhibitor used is sufficient to make the unexposed dispersion more permeable to water or fountain solution whilst at the same time can be extracted by the fountain solution from the coalesced areas.
• the non- coalesced areas are easily developed because of the presence of the coalescence inhibitor.
• the coalescence inhibitor is slowly extracted out of the coalesced areas of the coating due to its solubility in fountain solution. The result is that the coalesced area becomes more hydrophobic.
• the leaching out of the coalescence inhibitor enhances the long-term durability of the plate throughout its run.
• the function of the coalescence inhibitor is such that it should be substantially soluble in the dispersion that is to be coated.
• the coalescence inhibitors should also be capable of facilitating the removal of the unexposed portions of the image coat by fountain solution, thus enhancing the developability of the un-irradiated portion of the imaging element.
• the coalescence inhibitor must be capable of being extracted from the coalesced image, thus maintaining the durability of the image area during the print run and increasing the resistance of the image to wear by offset powder or other press-room chemicals.
• a further enhancing feature of the incorporation of the coalescence inhibitor is that it permits polymers to be used that have lower coalescence temperatures than could be used hitherto. This has the beneficial effect of increasing the conversion sensitivity of the system to the laser light.
• prior art lithographic precursors of this generic type prepared without a coalescence inhibitor, there were therefore fewer degrees of freedom in the design of the media in that the performance of the media was fundamentally constrained by the thermal properties of the polymer used.
• the addition of the coalescence inhibitor allows the mutually independent optimization of the durability, and therefore run length, on the one hand, and the sensitivity of the media in terms of thermal conversion, on the other.
• the preferred concentration of such coalescence inhibitors is between 0.1 %ww of the hydrophobic thermoplastic polymer particles and 500%ww of the hydrophobic thermoplastic polymer particles.
• the more preferred concentration of coalescence inhibitor is dependent on the particular class of inhibitor chosen, as exemplified below. However, the concentration of specific coalescence inhibitors should not be so high as to cause attack and dissolution of the anodic layer.
• suitable coalescence inhibitors include, but are not limited to:
• inorganic salts such as sodium acetate, potassium carbonate, lithium acetate, sodium metasilicate etc,
• organic bases such as piperazine, 2-methylpiperazine and 4- dimethylaminobenzaldehydein,
• organic acids such as malonic acid, D,L lactic acid and citric acid
• metal complexes such as zinc acetate , copper (II) phthalocyaninetetrasulphonic acid, tetra sodium salt, aluminium acetylacetonate, copper acetylacetonate, cobalt acetylacetonate and zinc acetylacetonate
• Preferred concentrations (in %w/w of hydrophobic thermoplastic polymer particles) of the above four categories of coalescence inhibitors are respectively:
• Inorganic salts 2%w/w to 50%w/w, most preferably 10%w/w to 40%w/w
• Organic bases 50%w/w to 500%w/w, most preferably 80%w ⁇ /v to 200%w/w
• Organic acids 0.1 %w/w to 100%w/w, more preferably 10%w/w to 80%w/w and most preferably 20%w/w to 50%w/w.
• Metal complexes 0.1 %w/w to 100%w/w, more preferably 10%w/wto 80%w/w and most preferably 20%w/w to 50%w/w
• coalescence inhibitor could in fact be a mixture of two or more coalescence inhibitors and such a mixture could perform synergistically in a more improved way than any one coalescence inhibitorwould suggest. Similarly, coalescence inhibitors that form part of a mixture may not necessarily perform in the desired way when used alone.
• the thermally convertible lithographic printing precursor may be subsequently developed after exposure using an aqueous medium.
• an aqueous medium such as fountain solution.
• this process may be conducted on the press as part of the digital-on-press technological approach.
• the exposed areas of the imagable coating will be the areas to which the lithographic printing ink will adhere. This makes possible the subsequent use of the inked surface for the purposes of printing.
• the present invention pertains very directly to the manufacture of lithographic plates, it has particular significance in the on-press-processing environment.
• the thermally convertible lithographic printing precursor of the present invention meets these criteria.
• the imagable medium forming part of the thermally convertible lithographic printing precursor of the present invention is of such consistency as to be sprayable. This is required in some cases for on-press application of the medium to the lithographic base.
• the imagable medium contained within the present invention is also capable of being cured without cross-linking such that the unexposed imagable medium may be removed by an aqueous medium.
• the thermally convertible lithographic printing precursor of the present invention also exhibits good sensitivity to the light wavelength of interest; this being determined by the light-to-heat converting material that is added to the imagable medium. Upon being imagewise exposed to such radiation, there is good coalescence of the hydrophobic polymer particles in order to produce areas of hydrophobic polymer corresponding to the image. The illuminated and coalesced area is distinctly more hydrophobic than the lithographic base, adheres well to it, and does not wash off in aqueous media.
• the unexposed areas of the same imagable medium on the thermally convertible lithographic printing precursor are readily washed off by aqueous media.
• This difference in removability between exposed and unexposed areas of the imagable medium determines the basic contrast and, therefore, the effectiveness of the thermally convertible lithographic printing precursor of the present invention.
• the thermally convertible lithographic printing precursor of the present invention furthermore demonstrates, upon coalescence of the hydrophobic polymer particles, durability of such scope as to withstand the rigors of practical lithographic offset printing. This is a key factor wherein existing thermally convertible lithographic media do not excel.
• thermally convertible lithographic printing precursors made in accordance with the present invention.
• materials were supplied as follows:
• UCAR 471 from Union Carbide, Danbury, Connecticut, U.S.A. RhopIex HG-1630, WL-51 and WL-91 from Rohm & Haas, Philadelphia, Pennsylvania, U.S.A. Flexbond 289 and Vancryl 989, Air Products, Allentown, Pennsylvania, U.S.A. Xenacryl 2651 from Baxenden Chemicals, Baxenden, Lancashire, UK.
• Pro-jet 830NP a modified copper phthalocyanine, Avecia, Blackley, Lancashire, U.K.
• ADS 830A and 830WS are infra-red absorbing dyes from American Dye Source Inc. Montreal, Quebec, Canada.
• Ethanol was obtained from VWR Canlab of Mississauga, Ontario, Canada.
• Trendsetter® plate setting machine is a product of Creo Inc. of Burnaby, B.C., Canada
• a lithographic element was prepared with one of the key components intentionally omitted. 6g Texigel 13-800, 12g 1 wt% ADS 830A in ethanol, 44g deionized water were mixed and the resultant emulsion was coated onto grained anodized aluminum. The coating was dried in an oven at 60C for 1 minute. When the coating was dry, a coating weight of 0.9 g/m 2 was obtained. The plate was imaged using a Creo Inc. Trendsetter laser plate setting machine with 830nm light The exposure was carried out with 500 mJ/cm 2 at 12 Watts. Following exposure the plate was washed with town water the unexposed polymer did not wash off in the non-image areas. Clearly this approach leads to a result that does not obtain a usable thermally convertible lithographic printing precursor.
• a lithographic element was prepared with one of the key components intentionally omitted. 6g Rhoplex WL-91, 12g 1 wt% ADS 830A in ethanol, 44g deionized water were mixed and the resultant emulsion was coated onto grained anodized aluminum. The coating was dried in an oven at60C for 1 minute. When the coating was dry, a coating weight of 0.9 g/m 2 was obtained. The plate was imaged using a Creo Inc. Trendsetter laser plate setting machine with 830nm light The exposure was carried out with 500 mJ/cm 2 at 12 Watts. Following exposure, the plate was washed with town water. The unexposed polymer did not wash off in the non- image areas. Clearly this approach leads to a result that does not obtain a usable thermally convertible lithographic printing precursor.
• the plate was washed with a commonly available fountain solution for 20 seconds. The plate was allowed to dry and the image examined. Dampening the plate for 2 revolutions before the ink form rollers were applied started the printing. Good printing quality on coated paper was obtained for the duration of the 2,000 impressions of the print-run.
• Example 6 5g of Rhoplex WL-91 , 20g of 10 wt% piperazine in deionised water, 10g of 1 wt% ADS 830A in ethanol and 20g of deionised water were mixed and the resultant emulsion was coated onto a grained, anodized aluminium plate. The coating was dried in an oven at 60°C for 1 minute. When the coating was dry a coating weight of 0.9 g/m 2 was obtained. The plate was mounted onto a single colour SM74 press and imaged with a Creo Inc. digital on-press laser exposure device using 830 nm light. The exposure was carried out with 500mJ/cm 2 at 15 Watts. Following exposure the plate was washed with fountain solution for 30 seconds. The plate was allowed to dry and the image examined. The plate was dampened for 2 revolutions before the ink form rollers were applied. 2,000 impressions were obtained when printed on uncoated recycled paper
• Rhoplex WL-91 20g of 10 wt% 2-methylpiperazine in deionised water, 10g of 1 wt% ADS 830A in ethanol and 20g of deionised water were mixed and the resultant emulsion was coated onto a grained, anodized aluminium plate.
• the coating was dried in an oven at 60°C for 1 minute. When the coating was dry a coating weight of 0.9 g/m 2 was obtained.
• the plate was mounted onto a single colour SM74 press and imaged with a Creo Inc. digital on-press laser exposure device using 830 nm light. The exposure was carried out with 500mJ/cm 2 at 15 Watts. Following exposure the plate was washed with fountain solution for 30 seconds. The plate was allowed to dry and the image examined. The plate was dampened for 2 revolutions before the ink form rollers were applied. 2,000 impressions were obtained when printed on uncoated recycled paper.
• Example 8 6g Flexbond 289, 12g 5wt% malonic acid in water, 12g 1wt% ADS 830A in ethanol, 36g deionized water were mixed. The pH value was measured at 2.06. The mixture was coated onto grained anodized aluminum. The coating was dried in an oven at 60C for 1 minute. The coating weight of emulsion on the plate was 0.9 g/m 2 . The plate was imaged using a Trendsetter® laser plate setting machine with output at 830nm. The exposure used was 500 mJ/cm 2 with 15 Watts power. The imaged sample was mounted onto a Ryobi single color printing press, dampened with fountain solution for 30 revolutbns and then the ink was applied to the plate.2,000 impressions were printed on coated paper.
• Rhoplex WL-51 12g 5wt% citric acid in water, 12g 1wt% ADS 830A in ethanol, 36g deionized water were mixed.
• the resultant emulsion had a pH value of 3.20, was coated onto grained anodized aluminum. The coating was dried in an oven at 60C for 1 minute the resultant plate had a coating weight of 0.9g/m 2 .
• the plate was imaged using a Trendsetter® laser plate setting machine with an output at 830nm. The exposure was carried out using 500 mJ/cm 2 at 15 Watts.
• the imaged sample was mounted onto a Ryobi single color printing press, dampened with fountain solution for 30 revolutions before the ink was applied to the plate. 2,000 impressions were printed on coated paper.
• the exposure was carried out at 500 mJ/cm 2 and 15 Watts. Following exposure the plate was washed with fountain solution for 20 seconds and subsequently allowed to dry. Once the image was examined, the plate was dampened for 2 revolutions before the ink rollers were applied. One thousand impressions were obtained when printed on uncoated recycled paper.
• the exposure was carried out at 500 mJ/cm 2 and 15 Watts. Following exposure the plate was washed with fountain solution for 20 seconds and subsequently allowed to dry. Once the image was examined, the plate was dampened for 2 revolutions before the ink rollers were applied. One thousand impressions were obtained when printed on uncoated recycled paper.

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• 2002
  • 2002-06-25 EP EP02740170A patent/EP1409250A1/de not_active Withdrawn
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