Planographic printing

Details Planographic printing Planographic printing

1999-06-29

2001-02-06

Funk, Stephen R. (Department: 2854)

Printing

Planographic

Lithographic plate making, and processes of making or using...

C101S455000, C101S456000

Reexamination Certificate

active

06182571

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to planographic printing and provides a method of preparing a planographic printing member, for example a plate, and a planographic printing member per se. The invention particularly, although not exclusively, relates to lithographic printing.
BACKGROUND OF THE INVENTION
Lithographic processes involve establishing image (printing) and non-image (non-printing) areas on a substrate, substantially on a common plane. When such processes are used in printing industries, non-image areas are generally hydrophilic and image areas are generally oleophillic. Consequently, oil based inks are repelled from the non-image areas after water has been applied to the substrate.
Image and non-image areas can be created by processes which include a step of exposing a layer of image material on the surface of the substrate to radiation. The exposure to radiation creates solubility differences in the image material corresponding to image and non-image areas. During development, the more soluble areas are removed, leaving a pattern on the substrate corresponding to the image.
Preparation of the substrate for receiving a layer of the image material must ensure that the image material bonds to the substrate. However, it must allow release of the soluble image material during development.
In addition, other problems need to be addressed when preparing a printing plate. For example, it is desirable for the image material to change color when it is exposed. This is generally achieved by including a pH sensitive dye in the material of the image layer, the dye often being arranged to change color by reaction with acid produced during a photochemical reaction of the light sensitive material of the image layer. However, it has been found that properties of the substrate itself can affect the ability of the dye to change color. In addition, it is desirable for the dye not to stain the substrate to any significant degree since, otherwise, on development, such staining will be visible. This would be aesthetically unacceptable and, furthermore, would be commercially unacceptable since plate processing personnel would be unable to determine whether a plate had been fully developed.
It is also desirable for it to be possible to remove unwanted image areas from a printing plate after exposure, for example to correct an error, in order to prevent such areas inking in and, therefore, being printed. The ability to remove selected areas will depend, to some extent, on the adhesive force between the substrate and the material of the image layer.
It is an object of the present invention to address the above-described problems.
DESCRIPTION OF THE INVENTION
According to a first aspect of the present invention, there is provided a method of preparing a planographic printing member comprising:
a) forming a hydrophilic layer on a support by contacting the support with a fluid containing a particulate material;
b) applying a modifying means over said hydrophilic layer for modifying interaction between said hydrophilic layer and a material subsequently applied; and
c) providing an image layer over said hydrophilic layer, wherein an indicator means is associated with said image layer for indicating whether said image layer is in a first or a second state.
Preferably, said planographic printing member is a printing plate.
Preferably, said indicator means is adapted for visually indicating whether said image layer is in a first or second state. Preferably, said indicator means is arranged to undergo a chemical reaction, for example an acid-base or oxidation/reduction reaction, when said image layer changes from said first state to said second state.
Preferably, said indicator means is adapted to indicate whether the image layer has been exposed. Preferably, said indicator means causes the printing member to display a different color after exposure compared to the color displayed before exposure.
Said indicator means may comprise a photo-oxidative system, suitably using a free radical generator, for example triazine, arranged to oxidize an indicator material, for example a leucobase dye. Alternatively, said indicator means may comprise a photo-acid generator.
Preferably, the image layer is adapted to have a different, for example a lower, pH after exposure compared to the pH before exposure. For example, the image layer may be adapted to produce H
+
(or other acidic species) upon exposure.
Said indicator means is preferably pH sensitive. Said indicator means is preferably a pH indicator.
Said indicator means is preferably incorporated in said image layer, for example by being dispersed therein.
A suitable modifying means can be selected by comparing the effect of a proposed modifying means on said indicator means when said proposed modifying means has been applied, with the effect (if any) when said proposed modifying means has not been applied. When no modifying means is applied, the indicator means may be incapable of indicating between said first and second states of said image layer.
Said modifying means is preferably applied at ambient temperature or above, more preferably in the range 20° C. to 90° C., especially 20° C. to 60° C.
Preferably, said modifying means is arranged to lower the pH of at least the outermost surface of the hydrophilic layer, so that its pH after application of said modifying means is less than its pH before application. Preferably, the pH (before application) of the fluid used in step (a) is greater than the pH (before application) of the modifying means used in step (b). The pH of the fluid may be greater than 9.0, preferably greater than 9.5 and, more preferably, greater than 10.0. The pH of the modifying means may be less than 10.0, preferably less than 9.0, more preferably less than 8.0. Especially preferred is the case wherein the pH is about 7 or below. In the most preferred embodiments, the pH is less than 5.0. The pH may be above 1, preferably above 2 and more preferably above 3.
Some modifying means suitable for use in said method, for example sulphuric acid, affect whether a deletion means, for example a deletion fluid, in particular a deletion gel or pen, can be used to satisfactorily remove ink receptive areas of said image layer. Preferably, said modifying means is arranged such that a selected deletion means can be used to make areas of said printing member to which it is applied less ink receptive (and preferably substantially non-ink receptive) compared to when said modifying means consists essentially of 0.1M sulphuric acid.
Said deletion means preferably includes a solvent, suitably an organic solvent, with N-methylpyrolidone and cyclohexanone being especially preferred; an etchant material which may be acidic or alkaline but is preferably acidic, and is suitably selected from phosphoric acid, hydrogen fluoride and ammonium bifluoride; and other minor components, for example surface active agents, thickeners and the like.
Preferably, said modifying means includes water as a solvent. Preferably, said modifying means comprises an aqueous solution or dispersion. Preferably, said modifying means includes one or more components selected from acids, buffer formulations, colloidal suspensions and salts.
Preferred acids are protic acids and include phosphoric acid, sulphuric acid, hydrochloric acid, nitric acid, acetic acid, citric acid, sodium hydrogen carbonate and boric acid, with boric acid being especially preferred.
Preferred buffer formulations include formulations including citric acid and/or a monohydrogen phosphate and/or a dihydrogen phosphate and/or boric acid in combination with, for example a strong acid and/or an alkali. Especially preferred buffer formulations include a phosphate.
Preferred colloidal suspensions include insoluble oxides and/or hydroxides, preferably of Group III or IV elements, with silica and aluminum hydroxide being especially preferred.
Cations of salts may be selected from group I, II and III, metals and transition metals, especially, first row transition metals. Preferred salts have cations selected fro

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