Processless printing plate with cover layer containing...

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

2000-06-23

2002-06-04

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

C430S302000, C430S944000, C430S945000

Reexamination Certificate

active

06399276

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a heat mode recording material for making a lithographic plate for use in lithographic printing. The present invention further relates to a method for imaging said heat mode recording material e.g. by means of a laser.
BACKGROUND OF THE INVENTION
Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink (oleophilic areas) whereas other areas will not accept ink (hydrophilic areas). According to the so called conventional or wet printing plates, both water or an aqueous dampening liquid and ink are applied to the plate surface that contains hydrophilic and oleophilic areas. The hydrophilic areas will be soaked with water or the dampening liquid and are thereby rendered oleophobic while the oleophilic areas will accept the ink.
When a laser heat mode recording material is to be used as a direct offset master for printing with greasy inks, it is necessary to have oleophilic-hydrophilic mapping of the image and non-image areas. In the case of heat mode laser ablation it is also necessary to completely image wise remove a hydrophilic or oleophilic topcoat to expose the underlying oleophilic respectively hydrophilic surface of the laser sensitive recording material in order to obtain the necessary difference in ink-acceptance between the image and non-image areas.
For example DE-A-2 448 325 discloses a laser heat mode “direct negative” printing plate comprising e.g. a polyester film support provided with a hydrophilic surface layer. The disclosed heat mode recording material is imaged using an Argon laser thereby rendering the exposed areas oleophilic. An offset printing plate is thus obtained which can be used on a printing press without further processing. The plate is called a “direct negative” plate because the areas of the recording material that have been exposed are rendered ink accepting.
Other disclosures in DE-A-2 448 325 concern “direct negative” printing plates comprising e.g. hydrophilic aluminum support coated with a water soluble laser light (Argon-488 nm) absorbing dye or with a coating based on a mixture of hydrophilic polymer and laser light absorbing dye (Argon-488 nm). Further examples about heat mode recording materials for preparing “direct negative” printing plates include e.g. U.S. Pat. No. 4,341,183, DE-A-2 607 207, DD-A-213 530, DD-A-217 645 and DD-A-217 914. These documents disclose heat mode recording materials that have on an anodized aluminum support a hydrophilic layer. The disclosed heat mode recording materials are image-wise exposed using a laser. Laser exposure renders the exposed areas insoluble and ink receptive, whereas the non exposed image portions remain hydrophilic and water soluble allowing to be removed by the dampening liquid during printing exposing the hydrophilic support. Such plates can be used directly on the press without processing.
DD-A-155 407 discloses a laser heat mode “direct negative” printing plate where a hydrophilic aluminum oxide layer is rendered oleophilic by direct laser heat mode imaging. These printing plates may also be used on the press without further processing.
From the above it can be seen that a number of proposals have been made for making a ‘direct negative’ offset printing plate by laser heat mode recording. They have such disadvantages as low recording speed and/or the obtained plates are of poor quality.
Another way of making direct lithographic plates is by laser ablation.
EP-A-580 393 discloses a lithographic printing plate directly imageable by laser discharge, the plate comprising a topmost first layer and a second layer underlying the first layer wherein the first layer is characterized by efficient absorption of infrared radiation and the first and second layer exhibit different affinities for at least one printing liquid.
EP-A-683 728 discloses a heat mode recording material comprising on a support having an ink receptive surface or being coated with an ink receptive layer a substance capable of converting light into heat and a hardened hydrophilic surface layer having a thickness not more than 3 &mgr;m. The lithographic properties of said material are not very good.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a material for a heat mode recording material of high sensitivity and high lithographic quality, especially in regard to little toning at start-up and a low dot gain.
SUMMARY OF THE INVENTION
According to the present invention there is provided a heat-sensitive material for making lithographic plates comprising in the order given on a support an IR-sensitive oleophilic layer and a cross-linked hydrophilic layer, characterized in that said heat-sensitive material is covered with a layer comprising at least an 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—R
1
—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
)
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 2000 mg/m
2
and more preferably in an amount of 20 to 1000 mg/m
2
.
Different kinds of hardened hydrophilic surface layers are suitable in connection with the present invention. The hydrophilic coatings are preferably cast from aqueous compositions containing hydrophilic binders having free reactive groups including e.g. hydroxyl, carboxyl, hydroxyethyl, hydroxypropyl, amino, aminoethyl, aminopropyl, carboxymethyl, etc. along with suitable crosslinking or modifying agents including e.g. hydrophilic organotitanium reagents, aluminoformyl acetate, dimethylol urea, melamines, aldehydes, hydrolyzed tetraalkyl orthosilicate, etc.

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