Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
1999-12-22
2002-03-19
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
Reexamination Certificate
active
06358669
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to multilayer thermally imageable elements useful in lithographic printing. More particularly, this invention relates to lithographic printing plates that can be thermally imaged by imagewise exposure with a laser or a thermal printing head.
BACKGROUND OF THE INVENTION
The art of lithographic printing is based on the immiscibility of oil and water. Ink receptive areas are generated on the surface of a hydrophilic surface. When the surface is moistened with water and then ink is applied, the hydrophilic background areas retain the water and repel the ink and the ink receptive areas accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Lithographic printing plates typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a support. If after exposure to radiation, the exposed regions of the coating become soluble and are removed in the developing process, revealing the underlying hydrophilic surface of the support, the plate is called as a positive-working printing plate. Conversely, if exposed regions of the plate become insoluble in the developer and the unexposed regions are removed by the developing process, the plate is called a negative-working plate. In each instance, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive and the regions of the hydrophilic surface revealed by developing process accept water, a typically fountain solution, and repel ink.
Direct digital imaging of offset printing plates, which obviates the need for exposure through a negative, is becoming increasingly important in the printing industry. Heat-sensitive imaging elements for the preparation of positive-working lithographic printing plates have been disclosed comprising a substrate, an aqueous alkali soluble underlayer, and an infrared sensitive top layer. On infrared exposure, the exposed regions of the top layer become soluble or permeable in aqueous alkali so that the developer can penetrate the top layer and remove the underlayer, revealing the underlying substrate. Imaging can be carried out with an infrared laser.
Thermally imageable systems have also been developed in which it is believed that a thermally frangible complex is formed between a polymeric material and a solubility-suppressing component, which reversibly suppresses the solubility of the polymeric material in the developer. The solubility-suppressing component can be a separate dissolution inhibitor compound and/or the polymeric material can act as a solubility-suppressing component. After imagewise thermal exposure, the rate of dissolution of the exposed regions is greater that the rate of dissolution of the unexposed regions. The exposed regions may be removed by the developer to form an image. Such systems are disclosed in, for example, Parsons, WO 97/39894, and U.S. application Ser. No. 08/981,620; Nagasaka, EP 0 823 327; Miyake, EP 0 909 627; West, WO 98/42507 and U.S. application Ser. No. 08/821,844; and Nguyen, WO 99/11458, and U.S. application Ser. No. 08/922,190, all of which are incorporated herein by reference.
These systems have limited shelf life and can only be used with a restricted group of developers. They have only a narrow working latitude compared to conventional positive-working printing plates. These systems also have limited resistance to aggressive blanket washes, such as a UV wash.
Systems have been produced in which a developer-insoluble top layer is coated over a developer-soluble underlayer. After exposure the top layer becomes permeable to developer, which penetrates the top layer and dissolves the underlayer, revealing the hydrophilic substrate. However, the developer insoluble material in the top layer, which is removed when the underlayer is removed, causes sludging in the developer. Insoluble particles, which can adhere to the imaged and developed printing plate and adversely affect the quality of the printed image, are produced. The processor must be modified so that the insoluble particles are not circulated in the developer. Thus, a need exists for an improved thermally imageable element, useful as a lithographic printing member, that does not suffer from these disadvantages.
SUMMARY OF THE INVENTION
The invention is a multilayer thermally imageable element, useful as a precursor for a lithographic printing member. The element comprises, in order:
a) a substrate, the substrate having a hydrophilic surface;
b) an underlayer over the hydrophilic surface, the underlayer comprising a first polymeric material; and
c) a top layer over the underlayer, the top layer comprising a second polymeric material; in which:
the top layer is ink receptive;
the top layer is insoluble in aqueous alkaline developer;
the underlayer comprises a photothermal conversion material;
the underlayer is soluble in aqueous alkaline developer; and
the second polymeric material is soluble in aqueous alkaline developer.
The second polymeric material is soluble in aqueous alkaline developer, but the top layer is insoluble in aqueous alkaline developer. However, the top layer becomes soluble in aqueous alkaline developer following thermal exposure.
In a preferred embodiment, the top layer comprises a solubility-suppressing component and the second polymeric material contains phenolic hydroxyl groups. The solubility-suppressing component may be a dissolution inhibitor compound and/or the second polymeric material may act as the solubility-suppressing component. Novolac resins functionalized with sulfonate ester groups are preferred polymeric materials that function as solubility-suppressing components. Quinonediazides (i.e., compounds that contain the o-diazonaphthoquinone moiety) are preferred dissolution inhibitor compounds.
In another preferred embodiment, the underlayer comprises an absorber than absorbs radiation in the range of the imaging wavelength, typically about 800 nm to 1200 nm.
In another embodiment, the invention is an exposed and developed element, which can be used as a lithographic printing member. In another embodiment, the invention is a process for forming the lithographic printing member. In still another embodiment, the invention is a method of printing using the lithographic printing member.
DETAILED DESCRIPTION OF THE INVENTION
The thermally imageable element (sometimes referred to as a printing plate precursor) comprises a hydrophilic substrate, typically comprising an aluminum or polyester support; a radiation absorbing underlayer comprising a first polymeric material; and an ink-receptive top layer comprising a second polymeric material. Although other layers, such as radiation absorbing layers may be present, typically no other layers are present.
The top layer preferably comprises a compound that functions as a solubility-suppressing component. The solubility-suppressing component may be a separate dissolution inhibitor compound and/or the second polymeric material may also function as a solubility-suppressing component. However, the top layer becomes more readily soluble in aqueous alkaline developer after the element has been exposed to radiation, allowing the developer to remove the top layer and reveal the hydrophilic surface of the substrate in the exposed regions. The lithographic printing member thus formed has excellent properties, including the absence of sludging of the developer.
Hydrophilic Substrate
The hydrophilic substrate, i.e., the substrate that comprises at least one hydrophilic surface, comprises a support, which may be any material conventionally used to prepare lithographic printing plates. The support is preferably strong, stable and flexible. It should resist dimensional change under conditions of use so that color records will register in a full-color image. Typically, it can be any self-supporting material, inc
Huang Jianbing
Patel Jayanti
Savariar-Hauck Celin
Shimazu Ken-ichi
Timpe Hans-Joachim
Baxter Janet
Gilmore Barbara
Kodak Polychrome Graphics LLC
Ratner & Prestia
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