UV-sensitive imaging element for making lithographic...

Radiation imagery chemistry: process – composition – or product th – Diazo reproduction – process – composition – or product – Composition or product which contains radiation sensitive...

Reexamination Certificate

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C430S188000, C430S302000

Reexamination Certificate

active

06641970

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an imaging element for making lithographic printing plates.
More specifically the invention is related to an UV sensitive imaging element that can be developed on or off press.
BACKGROUND OF THE INVENTION
Lithographic printing presses use a so-called printing master such as a printing plate that is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional (so-called ‘wet’) lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
In recent years the so-called computer-to-plate method has gained a lot of interest. This method, also called direct-to-plate method, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter. A special type of a computer-to-plate process involves the exposure of a plate precursor while being mounted on a plate cylinder of a printing press by means of an image-setter that is integrated in the press. This method is often called ‘computer-to-press’ and printing presses with an integrated plate-setter are sometimes called digital presses. A review of digital presses is given in the Proceedings of the Imaging Science & Technology's 1997 International Conference on Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods have been described in e.g. EP-A 770 495, EP-A 770 496, WO 94001280, EP-A 580 394 and EP-A 774 364. In most computer-to-press methods so-called thermal or heat-mode materials are used, i.e. plate precursors or on-press coatable compositions that comprise a compound that converts absorbed light into heat. The heat which is generated on image-wise exposure triggers a (physico-) chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex, and after optional processing, a lithographic image is obtained.
Typical plate materials used in computer-to-plate methods are based on ablation. A problem associated with ablative plates is the generation of debris that is difficult to remove and may disturb the printing process or may contaminate the exposure optics of the integrated image-setter. Other methods require wet processing with chemicals which may damage or contaminate the electronics and optics of the integrated image-setter and other devices of the press. Therefore computer-to-press methods normally require the use of plate materials which are not ablative and do not need wet processing. Known examples of such non-ablative processless plate materials contain a so-called ‘switchable’ image-recording layer, i.e. a layer of which the affinity towards ink or an ink-abhesive fluid can be converted upon image-wise exposure from one state to the opposite state, e.g. from hydrophilic to oleophilic or from ink-accepting to ink-abhesive. Such materials are based on
switchable polymers (e.g. EP-A 924 102) which can be image-wise converted from a hydrophobic state to a hydrophilic state (WO92/09934; EP 652 483) or vice-versa (U.S. Pat. No. 4,081,572; EP-A 200 488; EP-A 924 065).
thermally induced coalescence of thermoplastic polymer particles in a crosslinked binder (U.S. Pat. No. 3,476,937; EP-A 882 583; Research Disclosure no. 33303).
thermally induced rupture of microcapsules and the subsequent reaction of the microencapsulated oleophilic compounds with functional groups on cross-linked hydrophilic binders (U.S. Pat. No. 5,569,573; EP-A 646 476; EP-A 949 088).
A non-ablative plate which can be processed with fountain and ink is described in EP-B 770 494. The latter patent specification discloses a method wherein an imaging material comprising an image-recording layer of a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is image-wise exposed, thereby converting the exposed areas into an hydrophobic phase which define the printing areas of the printing master. The press run can be started immediately after exposure without any additional treatment because the layer is processed by interaction with the fountain and ink that are supplied to the cylinder during the press run. So the wet chemical processing of these materials is ‘hidden’ to the user and accomplished during the first runs of the printing press.
EP-A 339393 and EP-A 507008 disclose the use of aryldiazosulfonate resins in a photosensitive layer of an imaging element for making a lithographic printing plate. According to these disclosures, the imaging elements can be developed by rinsing them with plain water. However, the sensitivity of these systems to UV-light is low.
The use of UV-light for image-wise exposure of printing plates is advantageous in view of the widespread availability of UV contact frames (analogue exposure) and the emerging technologies for direct (digital) exposure with UV-light by means of digitally modulated UV sources using UV lasers or UV lamps with digital mirror devices, light valves etc.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide an imaging element for making a lithographic printing plate having an improved sensitivity to UV-light that can be developed with an aqueous solution on or off the press. The above mentioned objects are realized by the imaging element having the specific features defined in claim 1.
It is a further object of the present invention to provide a method for making a lithographic printing plate using the imaging element defined in claim 1. This object is realized by the method of claim 5.
The imaging element defined in claim 1 comprises a polymer having aryldiazosulfonate units or aryltriazenylsulfonate units. The addition of a compound capable of generating a radical and/or acid upon exposure to UV light increases the sensitivity of the imaging element towards UV-light.
Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention the imaging element for making a lithographic printing plate comprises an imaging layer comprising (i) a polymer having aryldiazosulfonate units and/or aryltriazenylsulfonate units and (ii) a compound capable of generating a radical and/or an acid upon exposure to UV light, which is typically the wavelength range between 100 nm and 400 nm.
According to the present invention, the addition of a compound capable of generating a radical and/or acid upon exposure to UV light increases the sensitivity of the imaging element. The radical and/or acid generating compounds that can be used in the present invention have a sensitivity in the range of UV light. These compounds are frequently used as photoinitiators for free radical and cationic polymerization. A detailed

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