Radiation imagery chemistry: process – composition – or product th – Transfer procedure between image and image layer – image... – Diffusion transfer process – element – or identified image...
Reexamination Certificate
1999-09-21
2001-05-08
Schilling, Richard L. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Transfer procedure between image and image layer, image...
Diffusion transfer process, element, or identified image...
C430S403000, C396S582000, C396S587000, C396S626000, C396S627000, C396S628000, C396S632000, C396S638000
Reexamination Certificate
active
06228557
ABSTRACT:
The present invention relates to a method and apparatus for processing a presensitised plate precursor in order to produce a lithographic printing plate. More specifically it is concerned with a method for the processing of a presensitised plate precursor comprising a substrate having coated thereon a light sensitive layer including a silver halide emulsion, whereby improved image quality and press durability may be achieved by the elimination of electrochemical reactions which can occur during processing. The invention also provides an apparatus for the performance of said method.
Silver halides are used for printing plate applications in a number of ways. Particularly suitable coatings include those based on the silver complex diffusion transfer- or DTR-process, the principles of which have been fully described in U.S. Pat. No. 2,352,014 and in the publication “Photographic Silver Halide Diffusion Processes” by Andre Rott and Edith Weyde, The Focal Press, London and New York, 1972.
In the DTR process, the unexposed silver halide in an information-wise exposed silver halide emulsion layer is transformed, by treatment with a so-called silver halide solvent, into soluble silver complex compounds which are allowed to diffuse into an image receiving element and are reduced therein by means of a developing agent, generally in the presence of physical development nuclei, to form a metallic silver image having reversed image density values (“DTR image”) with respect to the black silver image obtained in the exposed areas of the photographic material. A DTR image-bearing material produced in this way may be used as a planographic printing plate wherein the DTR silver image areas form water-repellent, ink-receptive image areas on a water-receptive, ink-repellent background.
Two different types of DTR system are available. A two-sheet DTR system includes a first element comprising a photographic silver halide emulsion material and a separate image-receiving second element, generally containing a physical development nuclei layer; after forming an image in the image-receiving element, the two elements are placed in contact in the presence of one or more developing agents and one or more silver halide solvents in the presence of an alkaline processing liquid, and subsequently peeled apart, to provide a metallic silver layer in the second element. A single sheet DTR system comprises a single element including a silver halide emulsion layer which is integral, and in water permeable relationship, with an image-receiving layer, which again generally includes a physical development nuclei layer. Following exposure, the element is treated with one or more developing agents and one or more silver halide solvents in the presence of an alkaline processing liquid in order to provide a metallic silver layer in the image-receiving layer. The present invention is concerned with the latter type of DTR system.
Single sheet DTR systems are themselves provided in two alternative versions. U.S. Pat. No. 4,722,535 and British Patent No. 1241661 describe a system wherein a support is provided on its surface with a silver halide emulsion layer, over which is provided an image-receiving layer including physical development nuclei; following information-wise exposure, and subsequent development, the imaged element may be used as a printing plate without removal of the spent emulsion layer. Alternatively, a DTR system may comprise a support provided on its surface with an image-receiving layer including physical development nuclei, over which is provided a silver halide emulsion layer; after information-wise exposure, and subsequent development, the imaged element is washed with water to remove the spent emulsion layer, to leave a support carrying a silver image which may then be utilised as a printing plate. Lithographic printing plates produced from the latter type of system are disclosed in European Patents Nos. 131462, 278766 and 410500.
The present invention is concerned with the means of processing printing plates based on the latter type of single sheet DTR system, in which the image-receiving layer, containing physical development nuclei, is in direct contact with the support. Typically, the support in such a system comprises aluminium which has preferably been grained and anodised on one surface in order to improve its lithographic characteristics. Whilst grained and anodised aluminium is a particularly suitable support material in terms of its printing properties, however, its use in conjunction with silver halide-based imaging systems can lead to difficulties being experienced during the processing of said systems, due to the electrochemical nature of such processing. Indeed, the method of processing of the said systems is critical to the subsequent performance of the printing plates.
During the processing stages, the areas of a plate exposed to actinic radiation undergo a process known as chemical development. Said process involves the conversion of exposed silver halide grains into filamentary metallic silver by the action of a reducing agent dissolved in the developer solution. Typically, the reducing agent-otherwise referred to as a developing agent—comprises hydroquinone which is provided in aqueous alkaline solution. The reduction process may be represented schematically by the following equation:
2AgX+H
2
Q+2OH
−
=2Ag+Q+2X
−
+2H
2
O [1]
where H
2
Q is hydroquinone and X is chloride, bromide or iodide, or any combination thereof.
By contrast, in the unexposed areas the silver halide crystals are complexed by a silver halide solvent, which is typically a simple inorganic salt such as sodium thiosulphate. The resulting complex then diffuses to the nucleation surface which, in the case of a lithographic single sheet DTR system in which the image-receiving layer is in direct contact with the support, comprises a grained and anodised aluminium surface onto which has been coated a nucleating layer; said nucleating layer typically comprises a colloidal metal or metal sulphide. Reduction then takes place by the action of a reducing agent which, again, is typically supplied in the form of an alkaline solution of hydroquinone. This process is generally referred to as physical development and results in the production of a form of silver which successfully accepts lithographic printing ink. The physical development process can be schematically represented by the following equations:
AgX+2S
2
O
3
2−
=[Ag(S
2
O
3
)
2
]
3−
+X
−
[2]
2[Ag(S
2
O
3
)
2
]
3−
+H
2
Q+2OH
−
=2Ag+Q+4S
2
O
3
2−
+2H
2
O [3]
where the symbols have the meanings previously ascribed.
The conditions of processing, including pH, temperature and concentration of reducing agent, are chosen such that the chemical development process is essentially complete within 2 or 3 seconds of the moment at which the exposed parts of the plate come into contact with the developer, whilst the time taken for completion of the physical development process can be in the region of 45 seconds. During the course of the development process, in view of the chemical reactivity of aluminium metal, it is possible for chemical attack on the aluminium support to take place. Whilst the surface of the aluminium which carries an anodic layer is protected from such attack, it is generally true that the back surface and the edges of the aluminium plate have very little anodising and, once any anodising has been removed, it is possible for corrosion of the aluminium to occur. The presence of this corroded aluminium surface, together with the silver image, and the developer acting as an electrolyte, results in the formation of an electrochemical cell. The principles of such electrochemical reactions are described in more detail in the publication “Electrochemistry” by J. Koryta, J. Dvorak and V. Bohackova, Methuen and Company, London, 1970. The reactions can be repres
Blanchard Simon John
Kitteridge John Michael
AGFA-GEVAERT
Breiner & Breiner
Schilling Richard L.
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