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
1999-05-11
2001-04-03
Le, Hoa Van (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
C430S272100, C430S302000
Reexamination Certificate
active
06210857
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a heat sensitive imaging element. More specifically the invention is related to a heat sensitive imaging imaging element for preparing a lithographic printing plate which can be imaged by rub-off or on the press.
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. they show a lower dot crispness. The trend towards heat mode printing plate precursors is clearly seen on the market.
EP-A-444 786, JP-63-208036,and JP-63-274592 disclose photopolymer resists that are sensitized to the near IR. So far, none has proved commercially viable and all require wet development to wash off the unexposed regions. EP-A-514 145 describes a laser addressed plate in which heat generated by the laser exposure causes particles in the plate coating to melt and coalescence and hence change their solubility characteristics. Once again, wet development is required.
EP-A-580 393 disclose a lithographic printing plate directly imageable by laser discharge, the plate comprising: (a) a topmost first layer; and (b) a second layer underlying the first layer; wherein the first layer is characterized by efficient absorption of infrared radiation; and (d) the first and second layer exhibit different affinities for at least one printing liquid selected from the group consisting of ink and an abhesive fluid for ink. According to this invention printing plates for wet offset and printing plates for waterless offset can be prepared. However the plates for wet offset have a poor endurance.
WO 94/18005 discloses a heat mode recording material having a high recording speed comprising on a support having an ink receptive surfaceor being coated with an ink receptive layer a substance capable of converting light into heat and a hardened hydrophilic surface layer having a thickness not more than 3 &mgr;m. The substance capable of converting light into heat is present in the support or in a separate recording layer. This is detrimental for a high sharpness of the image due to lateral diffusion of the heat and the light diffraction.
OBJECTS OF THE INVENTION
It is an object of the invention to provide an ablative imaging element for preparing a lithographic printing plate which is negative working.
It is also an object of the invention to provide an ablative imaging element for preparing a lithographic printing plate which
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 shows a good ink-uptake in the exposed areas and no scumming in the non-exposed areas.
It is also an object of the invention to provide an ablative imaging element for preparing a lithographic printing plate which shows a good printing endurance.
It is also an object of the invention to provide an ablative imaging element for preparing a lithographic printing plate which can be exposed and developed on the printing press.
It is also an object of the invention to provide an ablative imaging element for preparing a lithographic plate with a high sharpness.
Further objects of the invention will become clear from the description hereafter.
SUMMARY OF THE INVENTION
According to the present invention there is provided a heat-sensitive imaging element for providing a lithographic printing plate, comprising a lithographic base with a hydrophobic oleophilic surface and a top layer layer comprising a compound capable of converting light into heat and a hydrophilic polymer, characterized in that said hydrophilic polymer is crosslinked.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention the lithographic base comprises a hydrophobic oleophilic surface. Preferred bases are plastic films such as polyethylene film, polypropylene film, polyvinylchloride film, polycarbonate film, polystyrene film, polyethylene terephthalate film and polyethylene naphthalate film. The plastic film support may be opaque or transparent
It is particularly preferred to use a polyester film support to which an adhesion improving layer has been provided. Particularly suitable adhesion improving layers for use in accordance with the present invention comprise a hydrophilic binder and colloidal silica as disclosed in EP-A-619 524, EP-A-620 502 and EP-A-619 525. Preferably, the amount of silica in the adhesion improving layer is b
Damme Marc Van
Kokkelenberg Dirk
Vermeersch Joan
AGFA-GEVAERT
Breiner & Breiner
Gilmore Barbara
Le Hoa Van
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