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
1999-12-15
2001-02-13
Le, Hoa Van (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
C430S523000, C430S531000, C430S537000
Reexamination Certificate
active
06187507
ABSTRACT:
The present invention relates to a presensitised plate for use as a precursor in making a lithographic printing plate. More specifically it is concerned with a presensitised plate comprising a substrate having coated thereon a light sensitive layer including a silver halide emulsion, and also incorporating a further coating layer which is applied onto the back face of the substrate.
Silver halides are used for printing plate applications in a number of ways. Suitable coatings include, for example, the single sheet diffusion transfer type, as described in European Patents Nos. 131462 and 278766, in which a developer is applied to chemically develop exposed areas of a photosensitive coating, whilst at the same time physically developing the unexposed areas. In the unexposed areas, soluble silver complexes diffuse to a nucleation layer where metallic silver is formed by reduction and can be made oleophilic for use as the printing image of a lithographic printing plate.
Tanning development type coatings, such as described in British Patents Nos. 1547040, 1547350, 1567844, 1571155 and 1590058, and which comprise a silver halide in combination with gelatin or other matrix binder, are also suitable. Such systems involve oxidised developing agent, which is generated on development of exposed silver halide, crosslinking the gelatin or other matrix binder, so providing a tough, ink-receptive image.
Amongst other suitable coatings which can be employed are coatings which employ a silver halide layer as a mask on top of a conventional photopolymerisable layer, such as described in British Patents Nos. 1227603, 2052084, 2057154 and 2065318, coatings which involve crosslinking of polymers on development, such as described in British Patent No. 2040060 and European Patent No. 52942, and coatings which rely on silver halide initiation of polymerisation on development, such as described in U.S. Pat. Nos. 4,547,450, 4,859,568 and 4,879,201. In each case, the image area is oleophilic during printing operations.
The substrate used as the base material for the plate precursor forms the non-image areas of the final printing plate and it is necessary, therefore, that it should provide a hydrophilic surface in order that ink acceptance in non-image areas is prevented. Particularly favourable results are achieved with aluminium substrates in which the surface is grained and anodised. Optionally the aluminium may be laminated on one side to another material, preferably a plastic material.
Whilst being a particularly suitable base material for lithographic applications, however, it has to be borne in mind that aluminium is a reactive metal. Indeed, it is readily oxidised so that, in contact with air, a protective barrier is formed by reaction with atmospheric oxygen. However, if the protective barrier becomes damaged, the revealed aluminium surface is again reactive. In cases when such a damaged surface is in contact with silver halide emulsions, the resulting chemical interactions cause fogging of the emulsion such that it becomes of no use for its intended application.
Advantageously, lithographic printing plate precursors comprise an aluminium substrate which is mechanically or electrochemically grained to roughen the surface, and then electrochemically anodised such that an anodic film of aluminium oxide is formed on the roughened surface, one or more radiation sensitive layers then being coated on the anodised surface layer. Generally the coating and anodic layers provide a good degree of protection to the underlying aluminium, and it is more usual for damage to occur to the unprotected back surface of the aluminium substrate.
Mechanical damage to plates in the form of scratches can typically occur in the course of manufacture, or during operations such as stacking and transportation; indeed, the simple act of removing a plate from a pack of plates can result in damage to the plate. Most commonly, however, scratching of the plates occurs during the manufacturing process. Problems can then arise as a consequence of the continuous nature of this process, whereby plates can be stacked face-to-back within a few seconds of a scratch occurring; in such circumstances a revealed aluminium surface on the back of a plate will come into contact with a silver halide layer on the front of an adjacent plate, and there is the immediate potential for fogging of that layer to occur. Additionally, burred edges of plates and debris are often encountered together with scratches, and these are found to be equally as detrimental to the plates as the scratches themselves.
In order to overcome such difficulties, plate manufacturers have traditionally utilised tissue paper to provide interleaving between adjacent plates. Thus, contact between plates can be prevented, and scratches on the back surface of plates cannot cause damage to the silver halide layers of adjacent plates. Furthermore, scratching during transportation or when plates are removed from packs is eliminated by virtue of this interleaving.
Unfortunately, however, there are disadvantages associated with the use of interleaving paper. In particular, with the current trend towards Computer-to-Platesystems for pre-press work, which has resulted in increasing utilisation of automatic plate-feed devices, the presence of interleaving paper creates serious complications. Such devices are designed to mechanically remove a plate from a stack and load it onto an exposure device but, when tissue interleaving is present, additional machinery is required in order to mechanically de-interleave the plates and this, inevitably, leads to significant additional expense.
It is an objective of the present invention to eliminate the requirement for the use of tissue interleaving with lithographic printing plate precursors which incorporate a silver halide emulsion layer.
The use of the so-called back coatings on certain types of lithographic printing plate precursors has previously been disclosed. For example, European Patent Application No. 91310848.6 is concerned with a method for preparing a lithographic printing plate which involves the use of a developer comprising an aqueous solution of an alkali metal silicate, and wherein the back face of the substrate is coated with an organic polymeric compound in order to prevent attack by the developer on the anodic layer on this face, thereby preventing formation of insoluble material in the developer and extending developer life. European Patent Application No. 93111404.5, on the other hand, attempts to overcome the problem of developer contamination by providing a back surface coating comprising a layer of metal oxide. Difficulties associated with the handling of plates in which a mat layer is coated on top of the light sensitive layer are addressed in European Patent Application No. 92113982.0 which, like the earlier Application No. 91310848.6, also proposes the use of a back coating comprising an organic polymeric compound.
None of the prior art, however, considers the use of a back coat layer in conjunction with a silver halide emulsion layer, and the potential for chemical interactions which may cause fogging of the silver halide layer. The use of a metal oxide back coat layer, for example, would be totally inappropriate in such a situation. Consequently, it is a further objective of the present invention to provide an inert coating on the back surface of aluminium based lithographic printing plate precursors incorporating a silver halide emulsion layer such that chemical interactions involving the silver halide layer are prevented.
According to the present invention, there is provided a lithographic printing plate precursor comprising a grained and anodised aluminium substrate having coated thereon a silver halide emulsion layer and wherein the back surface of the substrate is coated with an inert coating.
The term “inert”, as applied to the coating, means that the coating does not chemically or physically interact with an adjacent silver halide layer when two plates are placed in front to back contact.
Said inert coating may
Gates Allen Peter
Goodin Jonathan William
Kitteridge John Michael
Mallison Malcolm James
Robinson Roland
Agfa-Gevaert, N.V
Breiner & Breiner
Le Hoa Van
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