Radiation imagery chemistry: process – composition – or product th – Diazo reproduction – process – composition – or product – Composition or product which contains radiation sensitive...
Reexamination Certificate
1997-07-08
2002-12-03
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...
C430S171000, C430S270100, C430S278100, C430S302000, C430S348000, C430S944000, C430S945000, C430S964000
Reexamination Certificate
active
06489078
ABSTRACT:
1. FIELD OF THE INVENTION
The present invention relates to a radiation sensitive material for making a lithographic printing plate. The present invention further relates to a method for preparing a printing plate from said radiation sensitive material.
2. BACKGROUND OF THE INVENTION
Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
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 hydrophilic 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 the light. Furthermore they are less suited for computer-to-plate imaging. The trend towards heat sensitive printing plate precursors is clearly seen on the market.
For example, EP-A 952022871.0, 952022872.8, 952022873.6 and 952022874.4 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.
EP-A 625728 discloses a lithographic plate with an image forming layer which is UV- and IR-sensitive which can be as well positive as negative working. Said image forming layer comprises (1) a resole resin, (2) a novolac resin, (3) a latent Bronsted acid and (4) an IR absorber. By exposing with UV or IR (830 nm) light followed by a baking (60 seconds at 100° C.) before the development step a negative working printing plate is obtained. The method for obtaining a positive working plate is not disclosed.
U.S. Pat. No. 4,708,925 discloses a positive working printing plate comprising a light sensitive composition comprising (1) an alkalisoluble novolac resin and an onium-salt and optionally an IR spectral sensitizing dye. By exposing with UV, visible or IR light followed by a development step with an alkali solution there is obtained a positive working printing plate. Said plate is sensitive to room handling.
3. SUMMARY OF THE INVENTION
It is an object of the present invention to provide an imaging element being infrared light and heat sensitive to facilitate digital imaging by exposure to infrared radiation or a thermal printing head.
It is a further object of the present invention to provide an imaging element being insensitive to visible radiation so as to facilitate room-light handling.
It is another object of the present invention to provide a method for obtaining in a convenient way a positive working lithographic printing plate of a high quality using said imaging element.
Further objects of the present invention will become clear from the description hereinafter.
According to the present invention there is provided an IR radiation-sensitive imaging element comprising on a hydrophilic surface of a lithographic base an image forming layer comprising (1) a water insoluble, alkali soluble or swellable resin having a phenolic hydroxy group, (2) a latent Bronsted acid, and (3) a crosslinking agent capable of reacting with the water insoluble, alkali soluble or swellable resin under the influence of an acid, characterized in that said image forming layer comprises a carbon black pigment as infrared absorber.
According to the present invention there is also provided a method for obtaining a positive working lithographic printing plate comprising the steps of:
(a) image-wise or information-wise exposing to IR light or heat an imaging element as described above;
(b) heating said exposed imaging element to provide reduced solubility in the unexposed areas;
(c) developing said exposed and heated imaging element with an aqueous alkaline developing solution in order to remove the exposed areas and thereby form a lithographic printing plate.
4. DETAILED DESCRIPTION OF THE INVENTION
It has been found that imaging elements which are recordable with UV and IR radiation and heat and which yield lithographic printing plates of high quality can be obtained according to the method of the present invention using an imaging element as described above.
To utilize it as a positive working plate requires the steps of imagewise exposure to activating radiation, heating of the plate to provide reduced solubility in unexposed areas and contact with an aqueous alkaline processing solution to remove the exposed areas. The use of both a water insoluble, alkali soluble or swellable resin having a phenolic hydroxy group and a crosslinking agent is essential.
While applicants do not wish to be bound by any theoretical explanation for the manner in which their invention functions, it is believed that it is based upon an acid-catalyzed chemical amplification mechanism which occurs upon heating of the unexposed areas of the plate. This mechanism reduces the solubility of the unexposed areas by hardening the mixture of resins.
In the exposed areas the mechanism is not fully understood. An unknown interaction of the acid being formed as a result of the infrared light exposure with the carbon black is suggested.
The functioning of the plate as a positive working plate is critically dependent upon the use of a mixture of a crosslinking agent and a water insoluble, alkali soluble or swellable resin having a phenolic hydroxy group since the use of either compound alone does not provide a useful developed image.
The carbon black pigment renders the composition sensitive to infrared radiation and makes the printing plate useful as a direct laser addressable plate which can be imaged by exposure to a laser which emits in the infrared region. As carbon black pigment all common types can be used which absorb in the infrared wavelength region. The carbon pigment may be as well in the amorphous as in the cristalline state. Preferably the number average diameter of the carbon black pigment ranges from 0.01 to 1 &mgr;m, more preferably from 0.1 to 0.5 &mgr;m. Examples of commercially available carbon black pigments are CORAX L6, FARBRUSS FW 200, SPECIALSCHWARZ 4A, SPECIALSCHWARZ 250 and PRINTEX U.
Suitable crosslinking agents according to the invention are melamine-phenol-formaldehyde resins, phenol-formaldehyde resins and resole resins. These resins are commercially available from Occidental Chemical Corporation and Ashland-Sud Chemie, Gmbh.
More preferred are amino crosslinking agents. An amino crosslinking agent according to the invention is a compound obtainable by the condensation of an amino group containing substance and formaldehyde. Said amino crosslinking agent has paired functional
Damme Marc Van
Peter Hendrikx
Vermeersch Joan
Agfa-Gevaert
Baxter Janet
Breiner & Breiner L.L.C.
Gilliam Barbara
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