Lithographic printing method using single-fluid ink

Details Lithographic printing method using single-fluid ink Lithographic printing method using single-fluid ink

2002-03-12

2003-07-22

Baxter, Janet (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Making printing plates

C430S019000, C430S138000, C430S270100, C430S348000, C430S401000, C430S494000, C430S944000, C101S450100, C101S465000, C101S466000, C101S467000, C101S478000

Reexamination Certificate

active

06596464

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to lithographic printing methods wherein an imaging material comprising a switchable image-recording layer is image-wise exposed and wherein single-fluid ink is then supplied to the exposed image-recording layer.
BACKGROUND OF THE INVENTION
Lithographic printing presses use a so-called printing master such as a printing plate which 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 which 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 which 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 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 200 488; EP 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 646 476; EP 949 088).
A major problem associated with the above non-ablative processless plates is the poor differentiation of the lithographic image, i.e. the small difference in hydrophilicity/oleophilicity of exposed and non-exposed areas resulting in a poor quality of the prints, which is the main reason why such materials have not found any practicle application so far. The poor differentiation is due to the fact that both image and non-image areas are defined by the same layer of which the image-wise conversion from one state to another is not as significant as in other plate materials wherein parts of an exposed layer are removed by wet processing or ablation, thereby revealing an underlying layer, e.g. the support, with an affinity towards ink or an ink-abhesive fluid that is highly different from the image-recording layer.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a printing method wherein a non-ablative processless imaging material is used for making a lithographic printing plate that provides high quality prints. This object is realized by the method of claim 1 and the preferred embodiments defined in the dependent claims. In the methods of the present invention, the known switchable materials are combined with so-called single-fluid ink. Printing with single-fluid ink is an alternative for the conventional offset methods wherein ink and an aqueous fountain solution is supplied to the plate. Single-fluid ink, which comprises an ink phase and a polar phase in a single liquid, enables printing in conventional ‘wet’ offset presses without supplying an aqueous fountain solution to the plate. It has surprisingly been found that the poor differentiation of the lithographic image of switchable plate materials provides high quality prints when single-fluid ink is used instead of the conventional ink/fountain.
Further advantages and embodiments of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
The imaging material used in the methods of the present invention comprise a switchable image-recording layer, i.e. a layer of which the affinity towards ink or an ink-abhesive fluid, such as an aqueous fountain solution, can be converted from one state to another by exposure to heat or light. The image-recording layer is not removable with the single-fluid ink that is supplied during the printing step, e.g. is not soluble in the single-fluid ink or does not form an emulsion therein.
Immediately after image-wise exposure, the image-recording layer comprises a lithographic image consisting of hydrophilic and oleophilic areas (‘wet’ offset plate) or of ink-accepting and ink-abhesive areas (driographic plate). In a negative-working embodiment the image (printing) areas, i.e. the oleophilic or ink-accepting areas, correspond to the exposed areas. In a positive-working embodiment the non-image (non-printing) areas, i.e. the hydrophilic or ink-abhesive areas, correspond to the exposed areas. In the preferred ‘wet’ offset embodiment, the image-recording layer can be hydrophilic before exposure and switched to an oleophilic state upon exposure. Alternatively, the image-recording layer can be oleophilic before exposure and switched to an hydrophilic state upon exposure. In preferred positive-working embodiments of ‘wet’ offset plates used in the present invention, the non-exposed ar

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