Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41N—PRINTING PLATES OR FOILS; MATERIALS FOR SURFACES USED IN PRINTING MACHINES FOR PRINTING, INKING, DAMPING, OR THE LIKE; PREPARING SUCH SURFACES FOR USE AND CONSERVING THEM
  • B41N3/00—Preparing for use and conserving printing surfaces
  • B41N3/08—Damping; Neutralising or similar differentiation treatments for lithographic printing formes; Gumming or finishing solutions, fountain solutions, correction or deletion fluids, or on-press development

Definitions

• This invention relates to a composition for prewetting a master carrying an image which has been prepared by electrophotographic reproduction, to produce a printing plate therefrom.
• Masters which are adapted by their form and dimensions for attachment to the cylinder of a lithographic printing press of either the office duplicating machine type or. the commercial printing type have heretofore been prepared by electrophotographic reproduction.
• a master may be either a paper sheet or a plastic sheet carrying on its surface a light-sensitizable coating which may consist of a photosensitive zinc oxide pigment dispersed in an insulating matrix of a synthetic resin.
• the master is made into either a positive or a negative print by a four-step procedure.
• the sheet is first made sensitive to light by giving it a blanket, negative, electrostatic charge on the coated side of the paper in the dark. A convenient way of doing this is by ion transfer from a corona discharge.
• the sheet while carrying the electrostatic charge, is exposed by any of the conventional photographic procedures, such as, by either contact or projection.
• the electrostatic charge is lost or reduced in the exposed areas and retained in the masked areas, to form a latent electrostatic image on the coated surface of the paper.
• the latent image on the paper is then developed by applying to the exposed surface a suitably pigmented resin powder, hereinafter referred to as a toner, carrying an electrostatic charge.
• the powder image is fixed by melting the resin powder so that it fuses to the coated paper surface to produce a durable image.
• the toner is retained by the image areas to form a direct image or by the background areas to form a reversed image depending upon its polarity with respect to the surface area to be developed.
• a direct image is obtained.
• a reversal image is obtained.
• the toner which is used in the production of lithographic printing plates should, after fusion, produce a surface which is wetted by lithographic ink.
• Resins such as copal, sandarac, Vinsol and rosin as well as hard waxes can be used as the toner for this purpose.
• the toner need not carry pigment, although pigment may be included to facilitate the inspection of the image during its development.
• the plate In lithographic printing, the plate is treated with water to make the background areas hydrophilic and the ink adheres to the hydrophobic letters or image on the plate, but not to the background areas. This ink is then transferred to another sheet of paper by the use of an off-set printing press. For this reason, the final step in the production of a printing plate of this type is to render the background areas of the plate hydrophilic, so that they are not receptive to a lithographic ink when wet with Water.
• Lithographic printing plates of this type can be prepared from an electrographic master by the foregoing procedure in a very short period of time. They may be prepared for either direct or off-set lithographic printing.
• the paperbase plates are satisfactory for the production of a limited number of copies, while the plasticbase plates are capable of reproducing a considerably greater number of copies.
• electrophotographic master is used herein to mean a sheet of paper or of a plastic base, carrying a coating which is adapted to receive an image by electrophotographic reproduction but which does not carry such an image, and which is adapted by form and dimensions for attachment to the roll of a lithographic printing press or of a lithographic ofiice duplicating machine.
• lithographic printing plate is used in the commonly accepted sense to mean a printing plate having hydrophobic image areas which are receptive to lithographic ink and hydrophilic background areas which, when wet with water, will not accept a lithographic ink. This term is used to designate plates which carry images adapted either for off-set lithography or direct lithograph ic printing.
• composition in accordance with this invention comprises essentially an aqueous solution of a water-soluble organic film-forming material having free carboxyl groups and of an alkali metal ferrocyanide or an alkali metal ferricyanide, which has a pH below about 7.25 and preferably within the range of about 3.0 to about 7.25.
• Organic film-forming materials which are suitable for use in the production of this composition are polyacrylic acid, alginic acid, sodium polymethacrylate, ammonium polymethacrylate, sodium carboxymethylcellulose, and sodium alginate.
• Alkali metal ferrocyanides which are suitable for use in this solution are, for example, sodium ferrocyanide, potassium ferrocyanide, sodium ferricyanide and potassium ferricyanide.
• the aqueous composition in accordance with this invention will desirably contain a buffering agent to adjust the pH to a value below about 7.25 and preferably within the range of about 3.0 to about 7 .25.
• the nature of the buffering agent included in this composition will depend upon whether or not the carboxyl group of the organic film-forming material used is initially in free or neutralized form. We have found that when using an organic film-forming material which has free carboxyl groups, such as, for example, polyacrylic acid or alginic acid, that trisodium phosphate, sodium carbonate, or sodium acetate is a suitable buffer for adjusting the pH of the solution.
• sodium polymethacrylate, ammonium polymethacrylate, sodium carboxymethylcellulose, or sodium alginate is used as the organic film-forming material
• an acid such as, for example, phosphoric acid or acetic acid to reduce the pH of the solution below 7.25.
• the material react chemically with the background areas of the electrophotographic master in converting it into a printing plate since they are retained thereon during the use of the plate in off-set printing with a tenacity which would not be expected in view of the original water-soluble character of the materials.
• Aqueous solutions of organic film-forming materials which have free carboxyl groups, which do not contain a complex inorganic salt are effective in rendering the background areas of an electrophotographic master wettable by water.
• we prefer to include a complex inorganic salt in the solution since these inorganic materials enhance the wetting ability of the composition in a synergistic manner.
• the combination of a complex inorganic salt and of a film-forming material carrying free carboxyl groups is more effective than either alone in aqueous solution.
• the aqueous compositions in accordance with this invention may contain about 0.5%, by weight, to about 2.5%, by weight, of an organic film-forming material carrying free carboxyl groups and about 0.5% to about 2.5%, by weight, of a water-soluble complex inorganic salt. They are desirably buffered at a pH below 7.25 and preferably to a pH within the range of about 3.0 to about 7.25 by the addition of an effective amount of a suitable buffer. When using an organic film-forming material which initially has free carboxy groups, an amount of trisodium phosphate, sodium carbonate, or sodium acetate, Within the range of about 0.5%, by weight, to about 2.5%, by weight, is suitable for this purpose. The remainder of the composition is water.
• the preferred aqueous composition in accordance with this invention may contain about 0.5%, by weight, to about 2.5%, by weight, of polyacrylic acid and about 0.5 by weight, to about 2.5 by weight, of potassium ferrocyanide with an amount of trisodium phosphate which adjusts the pH of the solution to a value within the range of about 3.0 to about 7.25.
• Example 1 Parts by weight Polyacrylic acid sold in the form of an aqueous solution under the tradename Acrysol A-3.
• the aqueous composition in accordance with this invention may be used in the final step in the preparation of a printing plate from any of the electrophotographic masters which have heretofore been developed.
• An electrophotographic image is imposed on the master in the usual manner, the image is developed by the application of a toner, and the toner on the image areas is fused to produce a resinous film on those areas.
• the resulting image areas are inherently hydrophobic in nature.
• the background areas are merely exposed areas of the electrophotographic coating of the original master. These background areas are then rendered hydrophilic and freely wettable by water by spreading the composition in accordance with this invention over the surface of the master as a uniform, thin film by the use of, for example, a cotton swab moistened with the composition.
• the image areas on the surface of the master are not wet by the composition due to their hydrophobic nature.
• aqueous composition in accordance with this invention is also quite suitable, when diluted by the addition of water, for use as a fountain solution in offset printing with a plate produced by electrophotographic reproduction.
• a solution is used during the printing operation to keep the background areas of the plate wet With water and non-receptive to the printing ink.
• one part by volume of the composition may, for example, be diluted with 25 parts by volume of water.
• the printing plates which have heretofore been prepared by electrophotographic methods have offered definite advantages over printing plates prepared by other methods in the rapidity and ease with which they may be prepared.
• the background areas of these prior art printing plates tend to lose their hydrophilic character and to pick-up traces of printing ink which causes the background areas of the printed copies to have a dirty, unsightly appearance which is highly undesirable.
• the printing plates produced by the use of the aqueous composition in accordance with this invention have all the advantageous features of the prior art printing plates produced by electrophotographic methods. In addition to these inherent advantages, they are definitely superior to the printing plates which have heretofore been produced by electrophotographic methods due to the fact that their background areas tenaciously retain their hydrophilic character of being non-wettable by printing ink and even after prolonged use produce printed copies with clean, ink-free background areas.
• a liquid prewetting composition capable of producing a lithographic printing plate from an electrophotographic master by rendering the background areas of the said master hydrophilic in nature without affecting the hydrophobic nature of its image areas, When it is applied to the image-bearing surface of the said master as a moistening film, and
• a water-soluble, inorganic salt selected from the group consisting of sodium ferrocyanide, potassium ferrocyanide, sodium ferricyanide and potassium'ferricyanide, I
• a buffer which adjusts the pH of the solution to a value within the range of about 3.0 to about 7.25.
• a liquid prewetting composition capable of producing a lithographic printing plate from an electrophoto- 5 graphic master, by rendering the background areas of the said master hydrophilic in nature without afiecting the hydrophobic nature of its image areas, when it is applied to the image-bearing surface of the said master as a moistening film, and
• trisodium phosphate in an amount which adjusts the pH of the solution to a value within the range of about 3.0 to about 7.25,
• a liquid prewetting composition capable of producing a lithographic printing plate from an electrophotographic master, by rendering the background areas of the said master hydrophilic in nature without affecting the hydrophobic nature of its image areas, when it is applied to the image-bearing surface of the said master as a moistening film, and

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Citations (17)

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• 0
  • NL NL252749D patent/NL252749A/xx unknown
  • BE BE591935D patent/BE591935A/xx unknown
  • NL NL128878D patent/NL128878C/xx active
• 1959
  • 1959-06-18 US US821106A patent/US3211686A/en not_active Expired - Lifetime
• 1960
  • 1960-05-23 CH CH603860A patent/CH384364A/de unknown
  • 1960-05-24 GB GB18201/60A patent/GB922753A/en not_active Expired
  • 1960-06-11 DE DEP25155A patent/DE1190005B/de active Pending
  • 1960-06-14 LU LU38820D patent/LU38820A1/xx unknown
  • 1960-06-14 FR FR829940A patent/FR1264127A/fr not_active Expired

Patent Citations (17)

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