Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2000-06-08
2001-12-04
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S269000, C430S306000, C430S300000
Reexamination Certificate
active
06326124
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to an edge-covering material and a process for preparing a flexographic printing form, whereby a photopolymerizable printing plate bearing a laser mask coating is premounted on a sleeve or cylinder, a laser mask is prepared, the photopolymerizable printing plate is exposed through the laser mask, and the exposed printing plate is washed off and dried.
2. Description of Related Art
Photopolymerizable printing plates are known to be used for preparing flexographic printing forms. The printing surface is produced by exposing imagewise, through a photographic original, a layer photopolymerizable by actinic radiation, and subsequently removing the unexposed, unphotopolymerized areas of the printing plate. Examples are found in the following patents: DE-C 22 15 090, EP-A 0 322 585 and U.S. Pat. No. 4,323,636.
Imagewise exposure of the photopolymerizable printing plates requires a photographic original, which is a mask having transparent and opaque areas in the form of an image to be reproduced. This photographic original is usually a photographic negative of the image to be reproduced. If corrections are required for the final image, a new original must be prepared. The same original can yield varying results, depending on processing conditions (temperature, humidity), so that registration problems can arise in the printing form montage. To simplify this time-consuming and costly process, photopolymerizable printing plates are provided with an additional layer that can be processed into an integrated original. Such materials having infrared (IR)-ablative, ultraviolet (UV)-absorbing layers and a process for their preparation and processing are described in WO 94/03838, WO 94/03839 and WO 96/16356. The integrated photomask is prepared by laser exposure.
The development of such materials is directed to distinctly simplifying the preparation of accurately registered cylinder shells, the so-called sleeves, and cylinders, in addition to direct processing of digital information and higher resolution in the resulting flexographic printing forms. This results from the montage of sections of laser-writeable, photopolymerizable, flexographic printing plates on the sleeve or cylinder. The sections can be positioned with a high degree of tolerance, because exposure with the precisely controlled laser head yields accurately registered alignment of the elements of the ultimate printed image. Sleeves or cylinders thus recorded are then exposed with UV in a circular exposure unit and washed off. Unfortunately, polymerized ridges remain on the edges of the plate sections and subsequently print in an undesirable manner.
During the preparation of ready-to-print cylinders or sleeves from sections of conventionally finished flexographic printing plates, it is possible to remove the unwanted ridges that result from exposure of the printing plate edges. Such plates can be cut before the montage is assembled. Even with laser-processed printing plates, there is the possibility of subsequent removal of undesirable ridges, for example, by cutting with sharp knives. However, this operation takes time, the danger of distorting the printing forms is very great, especially if the subject runs close to the edge of the printing form, and removing the severed ridge without leaving a residue is very expensive.
Therefore, the problem involved in the present invention is to avoid the disadvantages of the current state of the art and to reliably, rapidly and simply prevent the ridges from occurring in processing laser-processible, flexographic printing plates in a circular exposure unit.
SUMMARY OF THE INVENTION
This problem was solved surprisingly by an edge-covering material for photopolymerizable printing plates, containing at least one soluble, film-forming polymer, at least one UV absorber, and a solvent or solvent mixture, and by a process for preparing a flexographic printing form, in which a) a photo-polymerizable printing plate containing a laser mask coating is premounted on a sleeve, b) a laser mask is prepared, c) the photopolymerizable printing plate is exposed through the laser mask, and d) the resulting exposed printing plate is washed off and dried, characterized in that an edge-covering material containing at least one soluble film-forming polymer, at least one UV absorber, and a solvent or solvent mixture is applied on the edges of the photopolymerizable printing plate before the preparation of the laser mask in step b) or before the imagewise exposure of the photopolymerizable printing plate in step c).
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
Surprisingly, printing plate edges and also thicker flexographic printing plates are covered completely by the process and the edge-covering material of the present invention. Thus, edge exposure can be avoided and the unwanted ridges caused thereby can be prevented.
The polymer solutions of this invention to treat printing plates are prepared by dissolving a soluble, film-forming polymer and a UV absorber in an organic solvent. Useful polymers can be all known soluble, film-forming polymers, preferably those that dissolve well in the washoff solvent for the exposed printing plates. In particular, polymeric binders of the printing plate to be exposed are used. Particularly suitable are thermoplastic, elastomeric, block copolymers, as described in DE-C 22 15 090, EP-A 0 322 585 and U.S. Pat. No. 4,323,636. Particularly suitable binders are linear and radial block copolymers having polystyrene end blocks such as, for example, polystyrene-polybutadiene-polystyrene, (polystyrene-polybutadiene)
4
Si or the corresponding isoprene polymers. The average molecular weight M
n
of the block copolymers is usually between 80,000 and 300,000, preferably between 100,000 and 250,000. A polystyrene proportion of 10-40 percent by weight is advantageous, and particularly a proportion of 15-30 percent by weight. Selecting for the soluble, film-forming polymer the binders that are also used as flexographic printing plate components in the printing plate to be treated ensures that the edge-covering material of the invention is removed in the washoff process for the exposed printing plate without leaving a residue.
Useful UV absorbers are those absorbing in the range of 300-400 mm, preferably at 360 mn. Particularly suitable compounds are those compatible with the film-forming polymers. Typical examples are benzophenol derivatives such as, for example, 2,2′-dihydroxy-4,4′-dimethoxybenzophenol, benzotriazole, or benzotriazole derivatives. Particularly preferred are benzotriazole derivatives such as, for example, 2-(2-hydroxy-3,5-di-t-amylphenyl)-2H-benztriazole.
Useful solvents are the usual coating solvents for preparing printing plates. Particularly suitable are easily volatile solvents that have a high solvent capability for the film-forming polymer. Partially dissolving or loosening the laser mask layer can be avoided by suitable selection of the organic solvent. Toluene, xylene, methyl ethyl ketone, ethyl acetate, and their mixtures are preferred.
The edge-covering materials of the invention optionally also contain a thixotropic agent. This makes possible a reliable covering for the upper parts of the edges even with thick plates. All current thixotropic agents can be used such as, for example, bentonites, kaolins, and silicas. Pyrogenic silicas are particularly suitable.
The edge-covering materials can also contain dyes and/or pigments. All known dyes and pigments are suitable. Examples are inorganic and organic pigments, carbon, metal phthalocyanines, azo dyes, etc. Metal phthalocyanines are particularly preferred, especially Zapon Blue 807 (C.I. 74400).
A suitable viscosity for the edge-covering materials of the invention is regulated by the solids content and/or thixotropic agent content to permit even application on the lateral edges of the photopolymerizable printing plate and to yield the most uniformly possible thick film on these sites. This avoids drop formation and runs. The visco
Alince Wolfgang
Dudek Dietmar
Baxter Janet
E. I. Du Pont de Nemours and Company
Lee Sin J.
Magee Thomas H.
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