Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2000-10-17
2003-11-18
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S273100, C430S331000, C430S348000, C430S413000, C430S435000, C430S494000
Reexamination Certificate
active
06649324
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lithographic printing. In particular, this invention relates an aqueous developer for multi-layered lithographic printing plates.
BACKGROUND OF THE INVENTION
In lithographic printing, ink receptive areas, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions 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 plate precursors, often called printing plates or printing forms, 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 are removed in the developing process revealing the underlying hydrophilic surface of the support, the plate is called a positive-working printing plate. Conversely, if 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 the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging of offset printing plates, which obviates the need for exposure through a negative, is becoming more important in the printing industry. Multi-layer lithographic printing plates comprise a top layer and an underlayer on a substrate with a hydrophilic surface. After exposure to radiation, the exposed regions of the printing plate are removed during the developing process, revealing the underlying hydrophilic surface of the support.
Typically, the underlayer of these systems is soluble in the developer, but the top layer is only dispersible in the developer. The material generated by removal of the top layer in the exposed regions builds up in the developer. This material can coagulate and redeposit as sludge on the rollers of the processor and the on the walls and bottom of the developer tank. Sludge formation can severely limit developer throughput (i.e., the area of exposed printing plates developed, in m
2
, per liter of developer).
Thus, a need exists for a developer for multi-layered lithographic printing plates that develops multi-layer printing plates quickly, with high throughput, but with no sludge formation in the developing process.
SUMMARY OF THE INVENTION
In one embodiment, the invention is a method of developing an exposed multi-layer imageable element. The method comprises the steps of:
(A) providing an imagewise exposed imageable element comprising exposed and unexposed regions,
in which:
the imageable element comprises, in order:
a top layer;
an underlayer; and
a hydrophilic substrate;
the underlayer comprises a polymeric material that is soluble or dispersible in a developer;
the top layer comprises a polymeric material that is dispersible in the developer; and
the top layer is ink-receptive;
(B) applying the developer to the imagewise exposed imageable element, removing the exposed regions of the imageable element, and forming the image;
in which:
the developer comprises water and a dispersing agent or a mixture of dispersing agents; and
the developer has a pH of about 7 to about 11.
In another embodiment, the invention is a composition useful as a developer for multi-layered lithographic printing plates. The composition comprises:
water;
a dispersing agent or a mixture of dispersing agents;
a buffer; and
an organic solvent or a mixture of organic solvents;
in which the composition has a pH of about 7 to about 11.
DETAILED DESCRIPTION OF THE INVENTION
Multi-layer, heat-sensitive imageable elements for the preparation of positive-working lithographic printing plates comprise at least two layers, an underlayer and a top layer, on a substrate with a hydrophilic surface. The underlayer is over the hydrophilic surface of the substrate, and the top layer over the underlayer. These systems are disclosed in, for example, in U.S. patent application Ser. No. 09/301,866 [WO 99/67097] and EP 864,420, incorporated herein by reference. Other layers, such as radiation absorbing layers may also be present in the heat-sensitive imageable element. The back side of the substrate (i.e., the side opposite the underlayer and top layer) may be coated with an antistatic agent and/or a slipping layer or matte layer to improve handling and “feel” of the imageable element.
The substrate may consist only of a support, or it may additionally comprise one or more optional subbing and/or adhesion layers. The support is of sufficient thickness to sustain the wear from printing and is thin enough to wrap around a printing form. Polyethylene terephthalate or polyethylene naphthanate, typically has a thickness of from about 100 to about 310 &mgr;m, preferably about 175 &mgr;m. Aluminum sheet typically has a thickness of from about 100 to about 600 &mgr;m. Typically, polymeric films contain a sub-coating on one or both surfaces to modify the surface characteristics to enhance the hydrophilicity of the surface, to improve adhesion to subsequent layers, to improve planarity of paper substrates, and the like. The nature of this layer or layers depends upon the support and the composition of subsequent coated layers. Examples of subbing layer materials are adhesion promoting materials, such as alkoxysilanes, aminopropyltriethoxysilane, glycidoxypropyltriethoxysilane and epoxy functional polymers, as well as conventional subbing materials used on polyester bases in photographic films.
The underlayer comprises a polymeric material that is dispersible or, preferably, soluble in the developer, and insoluble in the solvent used to coat the top layer so that the top layer can be coated over the underlayer without dissolving the underlayer. These polymeric materials include those that contain an acid and/or phenolic functionality, and mixtures of such materials. Useful polymeric materials include carboxy functional acrylics, vinyl acetate/crotonate/vinyl neodecanoate copolymers, styrene maleic anhydride copolymers, phenolic resins, maleated wood rosin, and combinations thereof.
Solvent resistant underlayers have been developed to provide improved chemical resistance, i.e., resistance to both fountain solution and to aggressive washes. Polymeric materials useful in solvent resistant underlayers include: polyvinylacetals; copolymers that comprise N-substituted maleimides, especially N-phenylmaleimide, methacrylamides, especially methacylamide, and acrylic and/or methacrylic acid, especially methacrylic acid; aqueous alkaline developer soluble copolymers that comprise a monomer that has a urea bond in its side chain (i.e., a pendent urea group), such are disclosed in Ishizuka, U.S. Pat. No. 5,731,127; and alkaline developer soluble polymeric materials that comprise a pendent sulfonamide group, such as are disclosed in Aoshima, U.S. Pat. No. 5,141,838. Negative-working, base-soluble or dispersible photosensitive compositions, such as are disclosed in described in Baumann, U.S. Pat. No. 5,700,619, and in
Photopolymers: Radiation Curable Imaging Systems
, B. M. Monroe in
Radiation Curing: Science and Technology
, S. P. Pappas, Ed., Plenum, N.Y., 1992, pp. 399-440, may be used to produce a solvent resistant underlayer.
The top layer becomes soluble or dispersible in the developer following thermal exposure. It comprises a polymeric material that is ink-receptive and insoluble in the aqueous solution, such as acrylic and methacrylic polymers and copolymers, such as polymethyl methacrylate; polystyrene; styrene-acrylic copolymers; polyesters; polyamides; polyureas; polyurethanes; nitrocellulosic polymers; epoxy resins; or combination
Fiebag Ulrich
Savariar-Hauck Celin
Baxter Janet
Gilliam Barbara
Kodak Polychrome Graphics LLC
RatnerPrestia
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