Lithographic printing plate precursor

Details Lithographic printing plate precursor Lithographic printing plate precursor

2000-10-25

2002-11-05

Hamilton, Cynthia (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

C430S284100

Reexamination Certificate

active

06475700

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a lithographic printing plate precursor comprising a photopolymerizable composition. More particularly, it relates to a negatively working lithographic printing plate precursor having high sensitivity to visible light, high mechanical strength and, especially, excellent highlight characteristics.
BACKGROUND OF THE INVENTION
Negatively working lithographic printing plate precursors are widely known and include various types according to the photosensitive layer, such as a diazo resin-containing type, a photopolymerizable type, and photo-crosslinkable type. A lithographic printing plate is generally produced from these negatively working lithographic printing plate precursors by imagewise exposure to ultraviolet light with a transparent negative film original (lith film) in contact with the photosensitive layer, which is a laborious and time-consuming process.
The recent development of image formation techniques has enabled production of a printing plate without using a film original. For example, a printing plate precursor having a highly sensitive photopolymerizable layer fit for non-contact projection alignment or visible laser beam scanning is scanned with a laser beam of small diameter to directly form an image.
However, conventional printing plates of this type do not necessarily have strong adhesion between the photosensitive layer and the support and tend to produce such defects as missing of a solid image, a fine line or a highlight when used for high-speed large-volume printing
Further, when a printing plate as mounted on a printing machine is wiped with a plate cleaner to remove ink scum in the course of printing, the highlight tends to come off because of weak adhesion to the support. In large-volume printing, too, the weak adhesion to the support also causes fine lines and highlights to successively vanish as in the case of wiping with a plate cleaner.
There have recently been keen market demands that the exposure time be shortened for improvement of productivity and that the laser be used at a lower output for extension of its service life. It is a perennial subject in the art to increase the sensitivity of a lithographic printing plate precursor capable of direct platemaking. However, under the present situation it is very difficult to develop a photopolymerizable lithographic printing plate or a precursor thereof which has higher sensitivity and sufficient adhesion between the photosensitive layer and the support thereby retaining its initial tone reproducibility even when wiped with a plate cleaner in the course of printing or when used for producing hundred thousands of prints and also which is resistant to staining.
JP-B-2712564 proposes incorporating a polyurethane resin having an allyl group into a photosensitive layer as a binder for the purpose of improving abrasion resistance. However, application of the disclosed technique fails to obtain a lithographic printing plate precursor with sufficient characteristics.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a negatively working photopolymerizable lithographic printing plate precursor having high sensitivity and strong adhesion between the photosensitive layer and the support and capable of direct writing with a laser thereby providing. a lithographic printing plate which is excellent in press life and stain resistance and does not suffer from missing of fine lines or highlights.
As a result of extensive investigation, the present inventors have found that the above object of the invention is accomplished by incorporating an alkali-soluble urethane binder having at least one ethylenically unsaturated polymerizable group in the side chain thereof as a binder component of a photopolymerizable photosensitive layer.
The present invention provides a lithographic printing plate precursor comprising an aluminum support having thereon a photopolymerizable photosensitive layer which contains (1) an alkali-soluble urethane binder having at least one ethylenically unsaturated polymerizable group on the side chain thereof, (2) an addition polymerizable compound having an ethylenically unsaturated double bond, and (3) a photopolymerization initiator.
In a highly preferred embodiment of the invention, the urethane binder is a reaction product between (A) an ethylenically unsaturated group-containing diisocyanate compound obtained by allowing (A1) a compound having three or more isocyanate groups per molecule and (A2) an ethylenically unsaturated compound having an active hydrogen atom to react and (B) a diol compound having an alkali-soluble group.
While the mechanism of action of the urethane binder according to the invention is not necessarily clear, it is considered that the ethylenically unsaturated polymerizable group in the side chain causes the urethane binder itself to undergo radical polymerization to afford a denser crosslinked structure. According to JP-B-2712564, on the other hand, since the side chain possessed by the urethane binder is an allyl group, the urethane binder itself does not undergo such a radical polymerization reaction as to form a sufficiently dense crosslinked structure.
DETAILED DESCRIPTION OF THE INVENTION
[I] Alkali-soluble Urethane Binder Having at Last One Ethylenically Unsaturated Polymerizable Group in the Side Chain
The urethane binder, which is to be incorporated into the photopolymerizable photosensitive layer of the lithographic printing plate precursor according to the invention, is the most essential element of the present invention. While the urethane binder is not particularly limited as long as it is alkali-soluble and carries at least one ethylenically unsaturated polymerizable group in its side chain, it is preferably a polyurethane resin obtained by the reaction between (A) a diisocyanate compound containing an ethylenically unsaturated group and (B) a diol compound containing an alkali-soluble group.
(A) Ethylenically Unsaturated Group-containing Diisocyanate Compound
The method of obtaining the ethylenically unsaturated group-containing diisocyanate compound (A) is not particularly restricted. For example, it is obtainable by allowing (A1) a compound having three or more isocyanate groups (hereinafter simply referred to as a polyisocyanate compound) and (A2) an ethylenically unsaturated compound having an active hydrogen group to react in such a manner as to leave two isocyanate groups of the polyisocyanate compound unreacted.
Where the polyisocyanate compound (A1) is a triisocyanate compound having three isocyanate groups, it is allowed to addition react with an equivalent of an ethylenically unsaturated compound having one active hydrogen group. Where the polyisocyanate compound is a tetraisocyanate compound having four isocyanate groups, it is allowed to addition react with two equivalents of an ethylenically unsaturated compound having one active hydrogen group or an equivalent of an ethylenically unsaturated compound having two active hydrogen groups.
The polyisocyanate compound is not particularly limited, but a triisocyanate compound is preferred for its availability. Suitable triisocyanate compounds are shown below.
The ethylenically unsaturated compound having an active hydrogen group (A2) is not particularly limited. Useful ones include monofunctional alcohol or amine compounds having an ethylenically unsaturated group. Illustrative examples of suitable monofunctional alcohol or amine compounds having an ethylenically unsaturated group are listed below.
Illustrative examples of the ethylenically unsaturated group-containing diisocyanate compound (A) which is obtained by the reaction between (A1′) a triisocyanate compound and (A2′) an equivalent of a monofunctional alcohol or amine compound having an ethylenically unsaturated group are shown below.
(B) Alkali-soluble Group-containing Diol Compound
The alkali-soluble group of the diol compound (B) is not particularly limited and includes a carboxyl group, a sulfonamido group, and a phenolic hydroxyl

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