Processless thermal printing plate with cover layer...

Details Processless thermal printing plate with cover layer... Processless thermal printing plate with cover layer...

2000-06-01

2003-01-07

Baxter, Janet (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

C430S273100, C430S303000, C430S348000, C430S944000, C430S945000, C101S453000, C101S463100, C101S467000

Reexamination Certificate

active

06503684

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 that yields lithographic printing plates with high lithographic latitude.
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 image-wise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on an 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 image-wise 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 lithographic latitude, more particularly they need a lot of prints before the background area becomes free of printing ink.
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 improved lithographic latitude.
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 an image-forming layer comprising a hydrophilic binder, a cross-linking agent for a hydrophilic binder and dispersed hydrophobic thermoplastic polymer particles, characterized in that said image-forming layer is covered with a layer comprising at least one organic compound comprising cationic groups.
DETAILED DESCRIPTION OF THE INVENTION
The organic compounds having cationic groups for use in connection with the present invention are preferably hydrophilic and may be low molecular weight compounds but are preferably polymers. Preferred compounds are those having one or more ammonium groups or amino groups that can be converted to ammonium groups in an acidic medium. An especially preferred type of cationic compounds are polysaccharides modified with one or more groups containing an ammonium or amino group.
Most preferred organic compounds having cationic groups are dextrans or pullulan wherein at least some of the hydroxy groups have been modified into one or more of the following groups:
 —O—CO—R
2
wherein R
1
represents an organic residue containing an amino or ammonium group, e.g. an amine substituted alkyl, an amine substituted alkylaryl etc.
R
2
has one of the significances given for R
1
or stands for —OR
3
or —N(R
4
)R
5
, wherein R
3
has one of the significances given for R
1
and each of R
4
and R
5
which may be the same or different and have one of the significances given for R
1
.
Pullulan is a polysaccharide that is produced by micro-organism of the
Aureobasidium pullulans
type (
Pullularia pullulans
) and that contains maltotriose repeating units connected by a a-1,6 glycosidic bond. Pullulan is generally produced on industrial scale by fermentation of partially hydrolyzed starch or by bacterial fermentation of sucrose. Pullulan is commercially available from e.g. Shodex, Pharmacosmos.
Examples of dextrans or pullulan suitable for use in accordance with the present invention are dextrans or pullulan wherein some of the hydroxyl groups have been modified in one of the groups shown in table 1.
TABLE 1
no.
modified group
1
-O-CH
2
-CH
2
-NH
2
2
-O-CO-NH-CH
2
-CH
2
-NH
2
3
-O-CO-NH-CH
2
-CH
2
-N(CH
2
-CH
2
-NH
2
)
2
4
-O-CH
2
-CH
2
-NH-CH
2
-CH
2
-NH
2
5
-O-CH
2
-CH
2
-NH-CH
2
-CHOH-CH
2
-N
+
(CH
3
) Cl
−
6
-O-(CH
2
-CH
2
-O)
n
-CH
2
-CH
2
-NH
2
wherein n represents an integer from 1 to 50
7
-O-CO-NH-CH
2
-CH
2
-NH-CH
2
-CHOH-CH
2
-N
+
(CH
3
)
3
Cl
−
8
-O-CH
2
-CH
2
-N(CH
2
-CH
3
)
2
.HCl
9
-O-CH
2
-CH
2
-N(CH
2
-CH
2
-NH
2
)
2
10 
-O-CONH-CH
2
-CH
2
-N(CH
2
-CH
2
-NH
2
)
2
11 
-O-CONH-(CH
2
-CH
2
-O)
n
-CH
2
-CH
2
-NH
2
The modified dextrans or pullulan can be prepared by a reaction of a dextran with e.g. alkylating agents, chloroformates, acid halides, carboxylic acids etc.
The organic compound having one or more cationic groups according to the invention is preferably provided in an amount of 10 to 5000 mg/m
2
and more preferably in an amount of 20 to 1000 mg/m
2
.
According to the present invention to improve sensitivity and throughput and to avoid scumming an imaging element is provided comprising preferably hydrophobic thermoplastic polymer particles with an average particle size between 40 nm and 2000 nm. More preferably the hydrophobic thermoplastic polymer particles are used with an average particle size of 40 nm to 200 nm. Furthermore the hydrophobic thermoplastic polymer particles used in connection with the present invention preferably have a coagulation temperature above 50° C. and more preferably above 70° C. Coagulation may result from softening or melting of the thermoplastic polymer particles under the influence of heat. There is no specific upper limit to the coagulation temperature of the thermoplastic hydrophobic polymer particles, however the temperat

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