Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2000-05-25
2002-09-24
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S303000, C430S348000, C430S944000, C430S945000, C430S964000, C101S463100, C101S465000
Reexamination Certificate
active
06455230
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a heat sensitive ablatable imaging element.
More specifically the invention is related to a heat sensitive ablatable imaging element for preparing a lithographic printing plate, which requires no dissolution processing.
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.
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.
U.S. Pat. No. 5,605,780 discloses a lithographic printing plate comprising an anodized aluminum support and an image-forming layer overlying said support; said image-forming layer comprising an IR absorbing agent dispersed in a film-forming polymeric binder; said film-forming polymeric binder being a cyanoacrylate polymer and said IR-absorbing agent being dispersed therein in an amount sufficient for said image-forming layer to be imaged by laser-induced thermal ablation which completely removes said image forming layer in exposed regions thereof to thereby reveal said underlying support. This method has the disadvantage that in said way it is very difficult to generate a fog-free aluminum support. Plates made by ablation can not be printed without scumming.
EP-A- 652 483 discloses a lithographic printing 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. This principle has as disadvantage that the lithographic latitude is very narrow whereby pretty fast toning occurs at the exposed areas.
OBJECTS OF THE INVENTION
It is an object of the invention to provide an ablative imaging element for preparing a lithographic printing plate requiring no dissolution processing which is positive working.
It is also an object of the invention to provide an ablative imaging element for preparing a lithographic printing plate requiring no dissolution processing which shows a good ink-uptake in the exposed areas and no toning in the non-exposed areas.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method for preparing a lithographic printing plate, said method comprising the steps of
(1) providing an imaging element comprising on a lithographic hydrophilic support as contiguous top layer a heat-sensitive coating comprising a compound capable of converting light to heat and a compound, which becomes more hydrophilic under the action of heat;
(2) imagewise exposing said imaging heat-sensitive composition with a high intensity laser emitting a laser beam with an intensity at the surface of said heat-sensitive composition of at least 2 mW/&mgr;m
2
thereby at least partially removing said heat-sensitive composition to expose at least partially the lithographic support.
DETAILED DESCRIPTION OF THE INVENTION
The top layer, being the heat-sensitive or image-forming layer which becomes more hydrophilic under the influence of heat comprises a switchable binder and a compound capable of transferring light into heat. A switchable binder is a polymer or copolymer which under-the influence of heat and/or acid undergoes a polarity transfer from hydrophilic to hydrophobic or vice versa. According to the present invention a switchable binder is used which becomes more hydrophilic by heating and/or under the influence of acid. By “more hydrophilic” is meant an increase in the wettability of the coating by aqueous solutions of substantially neutral pH (i. e. in the pH range of 4 to 10), the coating retaining its structural integrity. Increased wettability is evidenced by a decrease in the contact angle formed between the coating and the aqueous solution. This sur
Damme Marc Van
Hendrikx Peter
Agfa-Gevaert
Baxter Janet
Breiner & Breiner L.L.C.
Gilmore Barbara
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