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
2000-07-13
2002-03-19
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
C430S281100, C430S306000
Reexamination Certificate
active
06358668
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 an ink receiving layer whereon ink can be jetted 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, e.g. a laser scanning apparatus. 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 of such digital flexographic plates can be an ablative layer comprising a heat- or light-sensitive compound, e.g. 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), heat-developable photosensitive 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. Examples of such plates are described in U.S. Pat. No. 4,288,528, EP-A 767 407, EP-A 741 330 and EP-A 720 057. An additional barrier layer can be provided between the photopolymer layer to prevent diffusion of monomers from the photopolymer layer to the image recording layer, as described in WO 94/03838, WO 94/03839 and U.S. Pat. No. 5,654,125.
EP-A 465 949 describes methods wherein toner, pigment or ink is image-wise transferred by electrophotography, thermal transfer or ink-jet directly onto the photopolymer layer of a flexographic plate so as to obtain a mask that is suitable for flood (UV-)exposure. DE-A 41 17 127 describes a similar process wherein an additional barrier layer, which simultaneously acts as an ink receiving layer, is present on top of the photopolymer layer. Said layer has two functions : preventing the diffusion of monomers from the photopolymer layer to the image recording layer and accepting the ink which is applied image-wise to obtain a mask. Especially when water-based inks are used, it is necessary that monomer diffusion is reduced effectively because the presence of such monomers in the ink receiving layer decreases the hydrophilicity of said layer and, as a result, reduces the ink receiving capability. A number of polymers are indicated in DE-A 41 17 127 for use as a binder of such a barrier/ink-receiving layer. The polymers that provide the best ink receiving properties, such as gelatin, actually have inferior barrier properties. Vice-versa, the polymers that provide good barrier capability, such as polyamides, have poor ink receptivity, i.e. the optical density of the jetted water-based ink is too low. This problem is not recognised in DE-A 41 17 127 and a fortiori not solved.
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 ink-jet methods in a convenient way. More specifically, it is an object of the present invention to provide a material which is characterised by a high receptivity of water-based ink so that an exposure mask can be applied having a high optical density with regard to the light that is used for flood exposure of the photopolymer layer. It is a further object of the present invention to provide a material which retains its high receptivity of water-based ink during storage. 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
In ink-jet printing tiny drops of ink fluid are deposited directly onto an ink receptor surface without physical contact between the printing device and the receptor by means of an image-wise controllable drop-ejection device called print head. In the method of the present invention, the known continuous ink-jet printing methods as weld as drop-on-demand methods such as piezo-electric ink-jet or the so-called “bubble-jet” and “thermal jet” methods can be used for jetting a water-based ink image-wise on the surface of the material of the present invention. Said material is a flexographic printing plate precursor and comprises in the order given a base, a photopolymer layer, a barrier layer and an ink receiving layer. That image thus obtained can then be used as a photomask for the subsequent flood exposure of the photopolymer layer. After washing out the unexposed areas with a suitable solvent, a relief image is developed which can be used as a print master for flexographic printing.
The specific advantages of the present invention are obtained by using a polymer different from gelatin as main component of the barrier layer and by using gelatin as main component of the ink receiving layer. The feature “main component” is generally understood as a component present in an amount which is not less than 50% of the total weight of the layer. In a preferred embodiment the ink receiving layer contains not less than 75% and even more preferably not less than 90% of gelatin. Gelatin forms a clear, hydrophilic coating which is characterised by a high absorption of water-based inks thereby providing rapid drying characteristics of the ink. The drying mechanism typically involves absorption by the ink receiving layer, penetration into said layer and evaporation.
The optimal composition of the water-based ink used in the method of the present invention is dependent on the specific ink-jet method used. In addition, the spectral absorption of the ink is preferably adjusted to the wavelength of the light used during exposure of the photopolymer layer through the jetted photomask. Ink compositions that are suitabl
Andriessen Ronn
Daems Eddie
Leenders Luc
Oelbrandt Leo
Agfa-Gevaert
Baxter Janet
Breiner & Breiner L.L.C.
Gilmore Barbara
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