Heat-mode driographic printing plate precursor

Details Heat-mode driographic printing plate precursor Heat-mode driographic printing plate precursor

1999-09-08

2002-05-14

Baxter, Janet (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, C430S303000, C101S467000

Reexamination Certificate

active

06387591

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a heat-mode imaging material which is suitable for making a driographic printing master.
BACKGROUND OF THE INVENTION
Rotary printing presses use a so-called master such as a printing plate which is mounted on a cylinder of the printing press. The master carries an image which is defined by the ink accepting areas of the printing surface and a print is obtained by applying ink to said surface and then transferring the ink from the master onto a substrate, which is typically a paper substrate. In conventional lithographic printing, ink as well as an aqueous fountain solution are fed to the printing surface of the master, which consists of oleophilic (i.e. ink accepting) and hydrophilic (water accepting) areas. In driographic printing, only ink is applied to the printing surface of the master, which consists of ink accepting and ink repelling areas. These ink repelling areas are often called oleophobic or ink-abhesive areas. Driographic plates are sometimes simply called ‘dry’ plates as distinct from the conventional ‘wet’ plates.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, colour separation, screening, trapping, layout and imposition are accomplished digitally and each colour 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 may be called ‘computer-to-press’ and printing presses with an integrated image-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).
The computer-to-press methods referred to above preferably use so-called thermal or heat-mode imaging materials, i.e. plate precursors or on-press coatable compositions which comprise a compound that converts absorbed light into heat, which then triggers the imaging mechanism of the plate precursor. Thermal plates offer the potential advantages of daylight handling and elimination of processing after exposure. The best known heat-mode driographic materials are based on ablation such as the plates disclosed in e.g. EP-A 580 393; EP-A 684 133; U.S. Pat. No. 5,540,150; U.S. Pat. No. 5,551,341; and U.S. Pat. No. 5,379,698.
All these plates work according to a similar mechanism ablative absorption of a recording layer provokes the removal of an ink-abhesive surface layer to reveal an underlying ink accepting surface. The recording layer is typically a thin metal layer, which is melted or vaporised upon exposure. Silicone coatings are generally used as an ink-abhesive top layer. Several problems are associated with ablative dry plates as described above, especially when used in computer-to-press methods:
(i) The redeposited debris of the top layer is difficult to remove because the silicones are cross-linked to achieve a wear resistant surface layer enabling long press runs. A complete removal of silicone in printing areas is however necessary to obtain high quality prints. Actually, the requirements of wear resistance and easy removal are contradictive and therefore difficult to realise.
(ii) The debris generated upon exposure may disturb the printing process or may contaminate the exposure optics of the integrated image-setter. This problems is to some extent solved by “semi-ablative” plates wherein only the anchorage between the ink-abhesive top layer and the recording layer is disrupted upon exposure instead of complete ablation of the layers. However, such materials still contain a removable silicone top layer and require a mechanical processing step using a special cleaning liquid that comprises a silicone solvent, as described in EP-A 830 942.
(iii) The known driographic thermal materials are suitable for exposure with either an internal drum image-setter (i.e. typically a high-power short-time laser exposure) or an external drum image-setter (i.e. relatively low-power long-time laser exposure). Providing a universal material that can be exposed with satisfactory results on both these types of devices known in the graphic arts is a requirement difficult to fulfil.
Turning in particular to EP-A 580 393, this patent application claims a lithographic printing plate directly imageable by laser discharge, the plate comprising a topmost first layer and a second layer underlying the first layer, wherein the first layer is characterised by efficient absorption of infrared radiation and 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. In spite of the broad claim covering both wet and dry plates, this patent application only enables the making of a driographic plate wherein a silicon layer is removed by ablative absorption and cleaning. Accordingly, the driographic plates disclosed by EP-A 580 393 are characterised by the same disadvantages as described above.
WO99/16621 describes a method for making a driographic plate wherein an ink-abhesive support is coated with an ink-accepting formulation which preferably comprises less than 25 wt. % of light absorbing compound.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a heat-mode imaging material which is suitable for making a driographic printing master and is not based on removal of a silicone layer so that the problems associated therewith, as described above, do not occur. It is another object of the present invention to provide materials which require no processing or are processed by the printing process itself. It is still another object of the present invention to provide a universal material which can be exposed with internal as well as external drum image-setters. The above objects are realised by the material specified in claim
1
. Preferred embodiments of the material of the present invention are specified in the dependent claims.
It is another object of the present invention to provide a method for making a driographic printing master characterised by the above advantages. This object is realised by the method which is specified in claims
6
and
7
with preferred embodiments thereof being specified in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
The features of the present invention, as specified in the claims, are defined herein as follows. The term “image” is used in the context of driographic printing, i.e. a pattern consisting of ink accepting and ink-abhesive areas. The material of the present invention is negative working, meaning that the areas, which have been exposed to light, are ink accepting and that the non-exposed areas are ink-abhesive. The material is a “heat-mode” material, meaning that the imaging mechanism is triggered by heat, which is generated upon exposure to light by the presence of a light absorbing compound. The material of the present invention can be used as a driographic printing master directly after exposure without a processing step, because the material comprises a light absorbing layer, defined as the “first layer”, which can be removed at the non-exposed areas by starting a pressrun. The removal of said first layer reveals an underly

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