Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
1998-11-24
2003-03-04
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S270100, C430S303000, C430S944000, C430S945000
Reexamination Certificate
active
06528237
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a heat sensitive non-ablatable wasteless imaging element.
More specifically the invention is related to a heat sensitive non-ablatable wasteless imaging imaging element for preparing a lithographic printing plate with a difference in dye density between the image and non image areas.
BACKGROUND OF THE INVENTION
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 define the printing image areas and the ink-rejecting areas define the background areas.
In the art of photolithography, 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.
In the production of common 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.
Upon imagewise exposure of the light-sensitive layer 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.
Alternatively, printing plates are known that include a photosensitive coating that upon image-wise exposure is rendered soluble at the exposed areas. Subsequent development then removes the exposed areas. A typical example of such photosensitive coating is a quinone-diazide based coating.
Typically, the above described photographic materials from which the printing plates are made are exposed in contact through a photographic film that contains the image that is to be reproduced in a lithographic printing process. Such method of working is cumbersome and labor intensive. However, on the other hand, the printing plates thus obtained are of superior lithographic quality.
Attempts have thus been made to eliminate the need for a photographic film in the above process and in particular to obtain a printing plate directly from computer data representing the image to be reproduced. However the above mentioned photosensitive coatings are not sensitive enough to be directly exposed to a laser. Therefor it has been proposed to coat a silver halide layer on top of the photosensitive coating. The silver halide can then directly be exposed by means of a laser under the control of a computer. Subsequently, the silver halide layer is developed leaving a silver image on top of the photosensitive coating. That silver image then serves as a mask in an overall exposure of the photosensitive coating. After the overall exposure the silver image is removed and the photosensitive coating is developed. Such method is disclosed in for example JP-A-60-61 752 but has the disadvantage that a complex development and associated developing liquids are needed.
GB-1 492 070 discloses a method wherein a metal layer or a layer containing carbon black is provided on a photosensitive coating. This metal layer is then ablated by means of a laser so that an image mask on the photosensitive layer is obtained. The photosensitive layer is then overall exposed by UV-light through the image mask. After removal of the image mask, the photosensitive layer is developed to obtain a printing plate. This method however still has the disadvantage that the image mask has to be removed prior to development of the photosensitive layer by a cumbersome processing.
Furthermore methods are known for making printing plates involving the use of imaging elements that are heat-sensitive rather than photosensitive. A particular disadvantage of photosensitive imaging elements such as described above for making a printing plate is that they have to be shielded from the light. Furthermore they have a problem of sensitivity in view of the storage stability and they show a lower dot crispness. The trend towards heat mode printing plate precursors is clearly seen on the market.
For example, U.S. Pat. No. 4,708,925 discloses imaging elements including a photosensitive composition comprising an alkali-soluble novolac resin and an onium-salt. This composition can optionally contain an IR-sensitizer. After image-wise exposing said imaging element to UV—visible—or IR-radiation followed by a development step with an aqueous alkali liquid there is obtained a positive or negative working printing plate. A processing step is required and the printing results of a lithographic plate obtained by irradiating and developing said imaging element are poor.
EP-A-625 728 discloses an imaging element comprising a layer which is sensitive to UV- and IR-irradiation and which can be positive or negative working. This layer comprises a resole resin, a novolac resin, a latent Bronsted acid and an IR-absorbing substance. A processing step is required and the printing results of a lithographic plate obtained by irradiating and developing said imaging element are poor.
U.S. Pat. No. 5,340,699 is almost identical with EP-A-625 728 but discloses the method for obtaining a negative working IR-laser recording imaging element. The IR-sensitive layer comprises a resole resin, a novolac resin, a latent Bronsted acid and an IR-absorbing substance. A processing step is required and the printing results of a lithographic plate obtained by irradiating and developing said imaging element are poor.
Furthermore EP-A-678 380 discloses a method wherein a protective layer is provided on a grained metal support underlying a laser-ablatable surface layer. Upon image-wise exposure the surface layer is fully ablated as well as some parts of the protective layer. The printing plate is then treated with a cleaning solution to remove the residu of the protective layer and thereby exposing the hydrophilic surface layer. Here also a processing step is required.
EP-A-97 200 588.8 discloses a heat mode imaging element for making lithographic printing plates comprising on a lithographic base having a hydrophilic surface an intermediate layer comprising a polymer, soluble in an aqueous alkaline solution and a top layer that is sensitive to IR-radiation wherein said top layer upon exposure to IR-radiation has a decreased or increased capacity for being penetrated and/or solubilised by an aqueous alkaline solution. This material does not give a selective dissolution of the exposed or unexposed parts of the top and intermediate layer.
DD-217 645 discloses a method for providing lithographic plates by irradiation with laser with one or more dyes adapted for the wavelenght of the laser, comprising non light-sensitive hydrophilic material on a support and wherein the concentration gradient of the sensitizing dyes lies perpendicular on the surface of the support.
EP-A-652 483 discloses a lithographic plate requiring no dissolution processing which comprises a substrate bearing a heat-sensitive coating comprising a photothermal converter, which coating becomes relatively more hydrophilic under the action of heat.
DD-217 914 discloses the preparation of a lithographic plate by irradiation with a laser of a non-light sensitive hydrophilic material, coated on an anodic aluminuum support, which can comprises dyes or other additions, wherein the aluminumoxide layer is coloured with a dye, which absorbs at the wavelenght of the laser.
DD-213 530 discloses a method for the preparation of printing plates for flexographic and lithographic printing by irradiation with a laser of layers comprising spectral sensitizers wherein spectral sensitizers are used whose spectrum changes by irradiation.
EP-A-694 586 discloses indolenine cyanine disulphonic acid derivaztives as IR-
Leenders Luc
Rompuy Ludo Van
Agfa-Gevaert
Baxter Janet
Breiner & Breiner L.L.C.
Gilliam Barbara
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