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
1996-11-08
2002-05-21
Schilling, Richard L. (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
C430S302000, C430S309000, C430S330000, C430S348000, C430S964000, C430S955000, C430S927000
Reexamination Certificate
active
06391516
ABSTRACT:
DESCRIPTION
1. Field of the Invention.
The present invention relates to a method for making a printing plate involving the use of a heat sensitive imaging element and that can be developed by means of plain water or an aqueous liquid.
2. 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 form the printing image areas and the ink-rejecting areas form 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 image-wise 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.
Commercially available diazo based printing plates most commonly use an anodized and roughened aluminium as a support having a hydrophilic surface since they offer the advantage of a high printing endurance. A particular disadvantage of such type of printing plates is that they require special developing liquids for development which is costly and inconvenient.
EP-A 601240 discloses a diazo based printing plate that uses a polyester film provided with a cross-linked hydrophilic layer as a lithographic base on which a photosensitive diazo layer is provided. Such a diazo based printing plate can be developed by rinsing it with plain water subsequent to image-wise exposure.
Commercial plates are also available that use a flexible support such as paper provided with a hydrophilic layer. For example, Lithocraft 10008 FOTOPLATE™ is a diazo based printing plate that comprises on a paper support a hydrophilic layer on top of which is provided a diazo based photosensitive layer. According to plate instructions of the supplier, a plate can be prepared by image-wise exposure of the lithographic printing plate precursor or imaging element, mounting the exposed imaging element on the press and wiping its surface with Lithocraft® 10008 Developer Desensitizer. The plate instructions also contemplate a method wherein no developer desensitizer is used. However, such method most often results in poor lithographic preformance so that in practice a Developer Desensitizer is almost always needed.
A particular disadvantage that the above diazo-based printing plates carry in common irrespective of the type of lithographic base used, is that they have to be shielded from the light. Moreover, diazo's are insufficiently sensitive to be exposed by means of a commercial and economical laser.
On the other hand, methods are known for making printing plates involving the use of imaging elements that are heat sensitive rather than photosensitive. For example, Research Disclosure no. 33303 of January 1992 discloses a heat sensitive imaging element comprising on a support a cross-linked hydrophilic layer containing thermoplastic polymer particles and an infrared absorbing pigment such as e.g. carbon black. By image-wise exposure to an Infrared laser, the thermoplastic polymer particles are image-wise coagulated thereby rendering the surface of the imaging element and these areas ink acceptant without any further development. A disadvantage of this method is that the printing plate obtained is easily damaged since the non-printing areas may become ink accepting when some pressure is applied thereto. Moreover, under critical conditions, the lithographic performance of such a printing plate may be poor and accordingly such printing plate has little lithographic printing latitude.
FR-A-1,561,957 describes a recording material comprising at least a recording layer containing a hydrophilic binder and an hydrophobic compound dispersed in said hydrophylic binder. Upon irradiation and development said recording material can be used as a planographic printing plate.
U.S. Pat. No. 3,476,937 describes also a recording material comprising at least a recording layer containing a hydrophilic binder and an hydrophobic compound dispersed in said hydrophylic binder. Upon irradiation and development said recording material can be used as a planographic printing plate.
U.S. Pat. No. 3,580,719 describes an imaging element containing a recording layer containing at least about 80% by weight of a normally water-soluble polymer which, when heated, undergoes a loss in its normal solubility in aqueous solvent.
The three last imaging elements all have the disadvantage that the corresponding printing plates have low printing endurances.
EP 514.145 describes a method for making a printing plate wherein a heat-sensitive imaging element is used that comprises on a lithographic base such as an anodised aluminium an image forming layer comprising core-shell particles and a light to heat converting substance. The shell of these particles is hydrophilic in nature and renders the particles developable. The core is hydrophobic in nature and flows out when heated. Thus upon image-wise exposure with an infrared laser diode, the image-forming layer can be rendered insoluble at the exposed areas. At the non-exposed areas, the image forming layer can be removed by means of an aqueous developer containing ethanolamine. Subsequent the material is baked. Although such printing plates can yield a high printing endurance, their development puts a burden on the environment because of the use of an alkanol amine.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a heat-sensitive imaging element for making a printing plate that can be developed in a convenient and environmental friendly way and that preferably can be exposed by means of a commercially available laser.
It is a further object of the present invention to provide a heat-sensitive imaging element that can be used to obtain printing plates having a high printing endurance.
Further objects of the invention will become clear from the description hereinafter.
The present invention provides an imaging element comprising (i) on a hydrophilic surface of a lithographic base an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light to heat, said compound being comprised in said image forming layer or a layer adjacent thereto, characterised in that said image forming layer further comprises a cross-linking agent capable of cross-linking said hydrophilic binder upon heating in a ratio between 1:100 and 200:1 by weight versus the hydrophilic binder.
Further, this invention provides a method for making a lithographic printing plate comprising the steps of:
(1) image-wise exposing an imaging element as defined above to light;
(2) developing a thus obtained image-wise exposed imaging element it with plain water or an aqueous liquid;
(3) and optionally overall heating a thus obtained imaged imaging element.
DETAILED DESCRIPTION OF THE INVENTION
An imaging element for use in accordance with the present invention comprises on a hydrophilic surface of a lithographic base an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a cross-linking agent capable of cross-linking the hydrophilic binder upon heating. The hydrophilic binder used in connection with the present inve
Damme Marc Van
Vermeersch Joan
Agfa-Gevaert
Breiner & Breiner L.L.C.
Schilling Richard L.
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