Radiation imagery chemistry: process – composition – or product th – Diazo reproduction – process – composition – or product – Composition or product which contains radiation sensitive...
Reexamination Certificate
2001-06-25
2002-08-20
Chu, John S. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Diazo reproduction, process, composition, or product
Composition or product which contains radiation sensitive...
C430S191000, C430S192000, C430S193000, C430S326000
Reexamination Certificate
active
06436601
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to thermally sensitive coating compositions useful for the preparation of lithographic printing plates, color proofing films and the like.
2. Description of Related Art
The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A very widely used type of lithographic printing plate has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion which is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive-working. Conversely, when that portion of the coating which is exposed becomes hardened, the plate is referred to as negative-working. In both instances the image area remaining is ink-receptive or oleophilic and the non-image area or background is water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then becomes more soluble and is removed. In the case of a negative plate the converse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating under the clear area of film is hardened by the action of light while the area not struck by light is removed. The light-hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image area which has had the coating removed through he action of a developer is desensitized and is therefore hydrophilic.
Direct digital imaging of offset printing plates has become increasingly important in the printing industry. Advances in solid-state laser technology have made medium to high-powered diode lasers attractive energy sources for platesetters, particularly lasers emitting energy in the near infrared (800-850 mm) regions. The use of controlled laser exposure obviates the need to use a film or mask when making image exposures, thereby facilitating a platemaking operation.
There are a number of United States patents relating to imaging compositions which are sensitive to infrared energy and which contain one or a mixture of phenolic resins and at least one infra-red absorbing dye or pigment. Positive acting plates based on a mixture of a novolak or resole or polyhydroxy-styrene resin and an IR absorbing dye are disclosed in U.S. Pat. No. 6,063,544. Printing plates based on a mixture of a novolak resin, a resole resin, an infrared absorbing dye or pigment and a latent Bronstead acid are disclosed in U.S. Pat. Nos. 5,372,907, 5,372,915, 5,466,577, and 5,491,046. Exposure of these plates to infrared radiation decomposes the latent Bronstead acid to yield species which will serve to crosslink the resole and novolak resins, thereby hardening the mixture in the exposed areas. Further heating of the exposed plate tends to further harden the exposed coating which becomes insoluble in aqueous alkaline developer, while the non-exposed areas remain soluble in developer solution.
In addition, U.S. Pat. Nos. 5,705,322 and 5,858,626 disclose laser-imagable photosensitive elements based on one or a mixture of a phenolic resin and an o-diazonaphthoquinone derivative or the esterification product thereof with a phenolic resin and an infrared absorbing compound. Elements of the '322 patent are negative working and require image exposure first followed by floodlight exposure prior to development. Elements of the '626 patent are positive working and require no floodlighting prior to or after development.
One of the problems associated with these and similar systems is that there is often insufficient integrity of the image areas remaining after development of the printing plate to effectively perform the printing process over long printing runs, resulting in print images having less than desired resolution and print quality.
SUMMARY OF THE INVENTION
The invention provides a radiation sensitive composition useful for the preparation of an imaging layer on a support comprising a mixture of:
a) a first novolak resin which is the esterification product of a novolak resin and of a diazo compound selected from the group consisting of 2-diazo-1-naphthol-4-sulfonic acid, 2-diazo-1-naphthol-5-sulfonic acid and ester-reactive derivatives thereof, said esterification product containing from about 0.5 to 50 mole % of said diazo compound;
b) a second novolak resin which is the esterification product of a novolak resin and a diazo compound selected from the group consisting of 2-diazo-1-naphthol-4-sulfonic acid, 2-diazo-1-naphthol-5-sulfonic acid and ester-reactive derivative thereof, said esterification product containing from about 0.5% to 50 mole % of said diazo compound, said second esterification product having a content of said diazo compound which differs by at least about 3 mole % from the diazo compound content of said first esterification product; and
c) an infrared radiation absorbing compound.
The invention also provides a process for preparing an image comprising
i) providing an imaging layer coated on a support material, and imaging layer comprising the esterification product mixture of (a), (b) and (c) above;
ii) imagewise exposing said imaging layer to energy emitting an infrared laser beam of sufficient energy to at least partially decompose the diazo compounds present in said esterification products; and
iii) contacting said imaging layer with a developer material wherein the exposed areas of said imaging layer are selectively removed from said support.
The radiation sensitive compositions of this invention may be applied to various substrates to form photosensitive elements. If applied to a textured and anodized aluminum plate, the coated plate may be used as a planographic (lithographic) printing plate capable of printing thousands of high quality, high resolution images. If the composition is applied to a transparent film support, e.g., a polyester film, it may be advantageously used as a film for color proofing. The composition may also be used as a photoresist for making printed circuits.
DETAILED DESCRIPTION OF THE INVENTION
The novolak resins used in the present invention are the condensation product of a phenolic or an aliphatic substituted hydroxy aromatic compound and an aldehyde. Preferred novolak resins include a condensation product of phenol, o-chlorophenol, o, m or p-cresol, p-hydroxy benzoic acid, 2-naphthol or other hydroxy aromatic monomers with an aldehyde such as formaldehyde, acetaldehyde, fural, benzaldehyde, or any other aliphatic or aromatic aldehyde. This polymer is preferred to have a molecular weight in the range of 1000 to 70,000, more preferably in the range of 2,000 to 40,000, and most preferably in the range of 3,000 to 12,000. Novolaks are common materials readily available commercially. Due to how they are prepared, there is a variability that will exist from lot-to-lot that makes the coatin
Cusumano Charles S.
Seth Prakash
Chu John S.
Citiplate, Inc.
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