Radiation imagery chemistry: process – composition – or product th – Diazo reproduction – process – composition – or product – Composition or product which contains radiation sensitive...
Reexamination Certificate
2000-02-01
2001-10-09
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Diazo reproduction, process, composition, or product
Composition or product which contains radiation sensitive...
C430S270100, C430S302000, C430S944000
Reexamination Certificate
active
06300032
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a heat-sensitive material for preparing lithographic printing plates.
More specifically the invention is related to a processless heat-sensitive material which yields lithographic printing plates with a high sensitivity.
BACKGROUND OF THE INVENTION
Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink, whereas other areas will not accept ink.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on a ink-repelling background.
In the production of common lithographic plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon imagewise exposure of such light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
On the other hand, methods are known for making printing plates involving the use of imaging elements that are heat-sensitive rather than photosensitive. A particular disadvantage of photosensitive imaging elements such as described above for making a printing plate is that they have to be shielded from daylight. Furthermore they have a problem of unstable sensitivity with regard to the storage time and they show a lower resolution. The trend towards heat-sensitive printing plate precursors is clearly seen on the market.
For example, Research Disclosure no. 33303 of January 1992 discloses a heat-sensitive imaging element comprising on a support a cross-linked hydrophilic layer containing thermoplastic polymer particles and an infrared absorbing pigment such as e.g. carbon black. By image-wise exposure to an infrared laser, the thermoplastic polymer particles are image-wise coagulated thereby rendering the surface of the imaging element at these areas ink accepting without any further development. A disadvantage of this method is that the printing plate obtained is easily damaged since the non-printing areas may become ink-accepting when some pressure is applied thereto. Moreover, under critical conditions, the lithographic performance of such a printing plate may be poor and accordingly such printing plate has little lithographic printing latitude.
Furthermore EP-A-770 494, 770 495, 770 496 and 770 497 disclose a method for making a lithographic printing plate comprising the steps of (1) image-wise exposing to light a heat-sensitive imaging element comprising (i) on a hydrophilic surface of a lithographic base an image-forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light to heat, said compound being comprised in said image-forming layer or a layer adjacent thereto; (2) and developing a thus obtained image-wise exposed element by rinsing it with plain water.
The above mentioned heat-sensitive imaging elements for making lithographic printing plates are not optimal regarding sensitivity.
EP-A-771 645 discloses a method for making a lithographic printing plate comprising the steps of:
(1) image-wise exposing an imaging element having on a hydrophilic surface of a lithographic base a photosensitive layer comprising a photosensitive polymer containing aryl-diazosulfonate units;
(2) and developing a thus obtained image-wise exposed imaging element by mounting it on a print cylinder of a printing press and supplying an aqueous dampening liquid and/or ink to said photosensitive layer while rotating said print cylinder.
The sensitivity of such imaging element could be improved.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a processless heat-sensitive imaging material for making lithographic printing plates having excellent printing properties.
It is a further object of the invention to provide a heat sensitive imaging material for making lithographic printing plates with an improved sensitivity of the imaging material.
Further objects of the present invention will become clear from the description hereinafter.
SUMMARY OF THE INVENTION
According to the present invention there is provided a heat-sensitive material for making lithographic printing plates comprising on a lithographic support a first image-forming layer comprising a hydrophilic binder, a cross-linking agent for said hydrophilic binder and dispersed hydrophobic thermoplastic polymer particles, and as top image-forming layer a heat switchable image forming layer comprising a heat switchable polymer wherein said top image-forming layer or a layer adjacent to said top image-forming layer comprises a compound capable of converting light into heat, characterized in that said heat switchable polymer is a polymer containing aryldiazosulphonate units.
DETAILED DESCRIPTION OF THE INVENTION
A higher sensitivity of an imaging element can be used to an advantage in several ways. First of all, the exposure time of the imaging may be shorter, increasing the output of the imaging apparatus. In a second way, the power of the exposing source may be lower, resulting in an increase of life-time of the exposing source.
A layer which becomes on the one hand more hydrophilic or soluble in water or on the other hand more oleophilic or less soluble in water under the influence of heat is called a heat switchable layer. The compound which is responsible for said switching of polarity is called a heat switchable compound.
The top image-forming layer which becomes more oleophilic under the influence of heat comprises a polymer or copolymer which contains aryldiazosulphonate units.
A heat switchable polymer having aryldiazosulphonate units, also called aryldiazosulphonate resin, preferably is a polymer having aryldiazosulphonate units corresponding to the following formula:
wherein R
0,1,2
each independently represent hydrogen, an alkyl group, a nitrile or a halogen, e.g. Cl, L represents a divalent linking group, n represents 0 or 1, A represents an aryl group and M represents a cation;
L preferably represents divalent linking group selected from the group consisting of:
—(X)
t
—CONR
3
—, —(X)
t
—COO—, —X—and —(X)
t
—CO—, wherein t represents 0 or 1, R
3
represents hydrogen, an alkyl group or an aryl group, X represents an alkylene group, an arylene group, an alkylenoxy group, an arylenoxy group, an alkylenethio group, an arylenethio group, an alkylenamino group, an arylenamino group, oxygen,sulfur or an aminogroup;
A preferably represents an unsubstituted aryl group, e.g. an unsubstituted phenyl group or more preferably an aryl group, e.g. phenyl, substituted with one or more alkyl group, aryl group, alkoxr group, aryloxy group or amino group;
M preferably represents a cation such as NH4
+
or a metal ion such as a cation of Al, Cu, Zn, an alkaline earth metal or alkali metal.
A polymer having aryldiazosulphonate units is preferably obtained by radical polymerisation of a corresponding monomer. Suitable monomers for use in accordance with the present invention are disclosed in EP-A-339 393 and EP-A-507 008. Specific examples are:
Aryldiazosulphonate monomers, e.g. as disclosed above, can be homopolymerised or copolymerised with other aryldiazosulphonate monomers and/or with vinyl monomers such as (meth)acrylic acid or esters thereof, (meth)acrylamide, acrylonitrile, vinylacetate, vinylchloride, vinylidene chloride, styrene, alpha-methyl styrene etc. In case of copolymers however, care should be taken not to impair the water solubility of the polymer. Preferably, the amount of aryldiazo
Damme Marc Van
Vermeersch Joan
AGFA-GEVAERT
Baxter Janet
Breiner & Breiner L.L.C.
Gilmore Barbara
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