Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2000-07-25
2003-05-06
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S270100, C430S271100, C430S273100, C430S281100, C430S286100, C430S287100, C430S309000, C430S348000, C430S401000, C430S432000, C430S944000, C430S945000, C430S964000
Reexamination Certificate
active
06558875
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for treating a photosensitive lithographic printing plate for exposure by laser.
As used herein, mass amount per unit area, when described in connection with a coating, is described in terms of “thickness”.
PRIOR ARTS
In recent years, a printing plate making system (CTP system) which comprises outputting digital data inputted into a computer and edited, directly on a photosensitive lithographic printing plate by laser scanning lines, has been developed to be practically usable. By this step, a large rationalization of a printing plate making step has become possible. As a photosensitive lithographic printing plate used for this system, a photopolymer type photosensitive lithographic printing plate using a photopolymerizable photosensitive material is useful from viewpoints of easy handling, a problem of a waste water, and the like. However, since a photopolymer type photosensitive material is poor in adhesiveness with a support, there has been a problem that a printing resistance becomes extremely poor depending on plate making and printing conditions. In order avoid these problems, a heat-treating step at about 100° C. is often carried out after subjecting a photosensitive lithographic printing plate to laser exposure as one step of printing plate making process. However, since this heat-treating step requires to maintain an apparatus at a high temperature, it is difficult to carry out unmanned operation even during night time and the apparatus must accompanied by an operator even after treatment until it is cooled to a certain level. This makes an obstacle to the above-mentioned rationalization which is one of objects of CTP system. On the other hand, it is known to carry out post-exposure after exposing a photopolymer type photosensitive lithographic printing plate to laser light and developing.
For example, JP-A-6-148885 and JP-A-6-289611 provide a photopolymerizable composition having a satisfactory developing property and a high resolving power by containing a specific photopolymerization initiator, and their examples illustrate steps of exposing a photopolymerizable photosensitive lithographic printing plate to laser light, developing with a sodium carbonate aqueous solution and then carrying out post-exposure by a metal halide lamp. More concretely, it is disclosed (1) to prepare a photosensitive lithographic printing plate by forming an overcoat layer of a dry thickness of 1.5 g/m
2
for shielding an enzyme after forming a photopolymerizable photosensitive layer of a dry weight of 3.5 g/m
2
on an aluminum plate of 0.6 &mgr;m having a surface roughness of Ra after surface-roughening treatment, (2) to describe a pattern on said photosensitive lithographic printing plate by scanning exposure with an argon ion laser of 1 mJ/cm
2
, and then (3) to carry out post-exposure with a metal halide lamp of 1 J/cm
2
to complete a printing plate.
However, according to the analysis by the present inventors, it has been found that even in the case of carrying out post-exposure, depending on a kind of photosensitive lithographic printing plate and a kind of a developer, a printing resistance and a developing property (stain property) before and after storing are unsatisfactory. Thus, the present invention provides a method for treating a photosensitive lithographic printing plate to provide a photopolymer type photosensitive lithographic printing plate having a stable printing resistance and excellent in other printing performances before and after storing.
On the other hand, an effect of improving an image strength achieved by carrying out post-exposure after development can be expected to some extent since a total light exposure amount imparted to a formed image becomes large. Heretofore, a total light exposure amount was seriously considered, but a light intensity (light exposure amount per unit time) on a surface to be exposed was not seriously considered. Also, in view of a general relation between a light intensity of a post-exposure light source and a distance from the light source to a surface to be exposed, the light intensity on the surface to be exposed was considered to be relatively low, usually at most about 15 mW/cm
2
, and such an image strength as to satisfy a commercial demand could not be achieved even by post-exposure treatment.
Thus, an object of the present invention is to provide a method for treating a photosensitive lithographic printing plate, which gives a satisfactory image strength to an image obtained by laser light scanning exposure and development and reduces a required light exposure amount at the time of laser light exposure.
DISCLOSURE OF THE INVENTION
The present inventors have conducted an extensive study and have found that the above problems can be solved by exposing a photosensitive lithographic printing plate having specific conditions to laser light, developing with a specific developer and then carrying out post-exposure treatment.
That is, the first essential feature of the present invention resides in a method for treating a photosensitive lithographic printing plate, which comprises exposing the photosensitive lithographic printing plate to laser light, developing with a developer containing an alkali metal silicate and then carrying out post-exposure treatment, said photosensitive lithographic printing plate being prepared by forming a photopolymerizable photosensitive layer having a film thickness of from 1.2 to 4 g/m
2
and further forming a protective layer having a film thickness of from 2 to 8 g/m
2
on a support having a centerline average height (Ra) of at least 0.35 &mgr;m.
Further, the present inventors have conducted an extensive study to solve the above problems, and have found that the above objects can be achieved by defining a light intensity on an image-forming surface in a specific range at the time of carrying out the post-exposure treatment. That is, the second essential feature of the present invention resides in a method for treating a photosensitive lithographic printing plate, which comprises exposing the photosensitive lithographic printing plate to laser light, developing and then carrying out post-exposure treatment at a light intensity of from 20 to 500 mW/cm
2
on a surface to be exposed at the time of the post-exposure treatment, said photosensitive lithographic printing plate being prepared by forming a photosensitive layer comprising a photopolymerizable composition containing an ethylenic compound, a photopolymerization initiator and a high molecular binder on the surface of a support.
Preferably, the post-exposure treatment of each of the above methods is carried out by a mercury lamp.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
Hereinafter, the present invention is described in details.
A photosensitive lithographic printing plate used in the method of the present invention is prepared by forming a photopolymerizable photosensitive layer and a protective layer as an oxygen-shielding layer in order on a support having a hydrophilic surface.
Examples of the support include metal, plastic, paper and the like, and an aluminum support is particularly preferably used. When an aluminum plate is used as the support, it is subjected to surface treatments including roughening treatment (sand blasting), anodization and optionally sealing treatment. These treatments can be carried out by well known methods.
Examples of the roughening treatment include a mechanical method, an etching method by electrolysis and the like. Examples of the mechanical method include ball abrading method, brush abrading method, hydro-honing abrading method, buff abrading method and the like. The hydrolysis etching is generally carried out in an acidic hydrolyte. The hydrolyte comprises preferably hydrochloric acid or nitric acid as the main component, and an acid concentration is usually from 0.5 to 5 wt %. Also, the hydrolyte may optionally contain further a chloride, amines, aldehydes, phosphoric acid, chromic acid, boric acid, acetic acid, oxalic acid and the like, if necessary
Okamoto Hideaki
Toshimitsu Eriko
Baxter Janet
Gilliam Barbara
Mitsubishi Chemical Corporation
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