Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-07-13
2002-05-07
Schilling, Richard L. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S273100, C430S306000, C430S327000, C430S330000, C430S502000
Reexamination Certificate
active
06383692
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a material that is suitable for making a flexographic printing plate. More specifically the invention is related to a material which comprises a photopolymer layer and a (photo)thermographic recording layer which can be exposed image-wise so as to obtain a mask for subsequent flood exposure of the photopolymer layer.
BACKGROUND OF THE INVENTION
Flexographic printing plates typically comprise a base and a photopolymer layer which is cured by image-wise exposure to (UV) light. During a subsequent processing step, the unexposed areas are removed by a developing liquid and a relief plate is obtained wherein the printing areas are raised above the surrounding non-printing areas. The relief image distinguishes flexography from other printing techniques such as lithography (planographic process wherein printing and non-printing areas are part of the same surface) and gravure (printing areas are recessed below the level of the non-printing areas).
Image-wise exposure of a flexographic plate is generally obtained by flood exposure of the photopolymer layer through a contact mask which is typically a film negative that is transparent in printing areas and opaque in the non-printing areas. Film production can be avoided by direct imaging of computer data onto flexographic plates by means of a digitally controlled imaging unit. Such computer-to-plate methods are becoming a major trend in the art. The associated plates are often called digital plates
WO 97/00777, U.S. Pat. No. 4,555,471, EP-A 487 260 and EP-A 491 457 describe digital plates comprising in the order given a base, a photopolymer layer, a peelable support and an image recording layer. After exposure to heat or light, e.g. by means of a laser or thermal head, and optional processing, an image is formed in the recording layer which can be used as a mask for exposure of the photopolymer layer. After flood UV-exposure through the mask, the support and the recording layer are peeled off and the photopolymer layer can be processed. For easy removal, the support can be silicon-coated or adhered to the photopolymer layer by a peelable adhesive as described in WO 97/00777. The image recording layer, carried by said peelable support is preferably a system which does not require wet processing to obtain the photomask after image-wise exposure. Suitable examples are e.g. an ablative layer which comprises a heat- or light-sensitive compound, such as a metal, a dye or a pigment such as carbon black. Typical ablative systems are described in WO 94/03838, WO 96/016356, EP-A 720 057, EP-A 741 330, EP-A 767 407 and U.S. Pat. No. 5,654,125. Other suitable image recording layers are thermochromic systems (U.S. Pat. No. 4,555,471), photothermographic compositions containing an organic silver salt, a silver halide and a reducing agent (EP-A 509 740, EP-A 530 674, U.S. Pat. No. 5,258,282, EP-A 491 457), or can be based on heat-induced coalescence of metal particles (EP-A 875 889 and references therein).
A disadvantage of the digital plates which are described above is the low pliancy due to the presence of two supports: the base of the photopolymer layer and the peelable support of the image recording layer. As a solution an image recording layer which dissolves in the processing liquid can be applied directly on the photopolymer layer as described in U.S. Pat. No. 4,288,528, EP-A 767 407, EP-A 741 330 and EP-A 720 057. The image recording layer of these plates are all ablative layers which can be selectively removed by a laser beam, so that a photomask is obtained without wet processing of the image recording layer. A problem associated with ablative systems is the generation of debris which may contaminate the optical and mechanical devices of the laser exposure unit used for direct exposure of the digital plate. In addition, ablative systems are characterised by a low speed, and therefore require high power lasers for exposure. Another disadvantage associated with a digital plate comprising a support between the photopolymer layer and the image recording layer, is the lower sharpness of the relief image due to the fact that the mask and the photopolymer layer are not in close proximity but separated by a distance equal to the thickness of the support.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved material which is suitable for making a flexographic printing plate by direct exposure to heat or light and is characterised by a high pliancy, a high speed and a high image quality. In addition, it is an object of the present invention to provide a convenient material which does not require wet processing to produce a photomask and does not generate debris upon said direct exposure. This object is realised by the material having the specific features defined in claim
1
. 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
The material of the present invention is a flexographic printing plate precursor and comprises in the order given a base, a photopolymer layer and a (photo)thermographic recording layer. Upon image-wise exposure to heat or light, an image is produced in the (photo)thermographic recording layer which then can be used as a photomask for the subsequent flood exposure of the photopolymer layer. Using a suitable solvent, the (photo)thermographic recording layer can be removed and the unexposed areas of the photopolymer layer can be washed out in a single development step. As a result, a relief image is developed which can be used as a print master for flexographic printing. Since there is no intermediate support present between the recording layer and the underlying photopolymer layer, the relief image is very sharp. The term intermediate “support” is to be understood in the meaning of an intermediate self-supporting foil, e.g. a plastic foil, distinguished from an intermediate “layer” which is not self-supporting, e.g. a coated layer such as the barrier layer referred to below.
The feature “(photo)thermographic” embraces photothermographic systems which contain a photosensitive element of which the colour or optical density can be changed by image-wise exposure to light and subsequent development by heat as well as so-called direct thermographic systems which contain a thermosensitive element of which the colour or optical density can be changed by image-wise heating without requiring a further heat treatment or other processing step. The recording layer used in the present invention is a photothermographic or thermographic layer comprising an organic silver salt. The photothermographic recording layer comprises a silver halide in catalytic association with a substantially light-insensitive organic silver salt or a component which is capable of forming photosensitive silver halide with the substantially light-insensitive organic silver salt. The thermographic recording layer does not require the presence of a photosensitive component and can be handled in daylight. Accordingly, thermographic systems provide higher convenience than photothermographic materials and, therefore, a thermographic recording layer is preferred in the present invention.
The major components of suitable (photo)thermographic recording layers which are based on organic silver salts will now be discussed. Preferred substantially light-insensitive organic silver salts for use in the present invention are silver salts of aliphatic carboxylic acids known as fatty acids, wherein the aliphatic carbon chain has preferably at least 12 C-atoms, e.g. silver laurate, silver palmitate, silver stearate, silver hydroxystearate, silver oleate and silver behenate, which silver salts are also called “silver soaps”. Further useful substantially light-insensitive organic silver salts are described in U.S. Pat. No. 4,504,575, EP-A 227 141, GB-P 1,111,492, GB-P 1,439,478 and U.S. Pat. No. 4,260
Andriessen Ronn
Daems Eddie
Leenders Luc
Agfa-Gevaert
Breiner & Breiner L.L.C.
Schilling Richard L.
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