Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2001-09-14
2003-06-10
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S176000, C430S273100, C430S944000, C430S945000, C430S302000
Reexamination Certificate
active
06576401
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lithographic printing plates. More specifically, it relates to on-press developable thermosensitive lithographic plates comprising an onium or borate salt as free radical initiator in the thermosensitive layer.
BACKGROUND OF THE INVENTION
Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
Lithographic printing can be further divided into two general types: wet lithographic printing (conventional lithographic printing) and waterless lithographic printing. In wet lithographic printing plates, the ink-receptive areas consist of oleophilic materials and the ink-repelling areas consist of hydrophilic materials; fountain solution (consisting of primarily water) is required to continuously dampen the hydrophilic materials during printing operation to make the non-image areas oleophobic (ink-repelling). In waterless lithographic printing plates, the ink-receptive areas consist of oleophilic materials and the ink-repelling areas consist of oleophobic materials; no dampening with fountain solution is required.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties (such as hydrophilic vs. oleophilic, and oleophobic vs. oleophilic). The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. Here, hardening means becoming insoluble in a certain developer. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. The exposed plate is usually developed with a liquid developer to bare the substrate in the non-hardened or solubilized areas.
On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be directly mounted on press after exposure to develop with ink and/or fountain solution during the initial press operation and then to print out regular printed sheets. No separate development process before mounting on press is needed. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675, and 6,242,156.
Conventionally, the plate is exposed with an actinic light (usually an ultraviolet light from a lamp) through a separate photomask film having predetermined image pattern which is placed between the light source and the plate. While capable of providing plate with superior lithographic quality, such a method is cumbersome and labor intensive.
Laser sources have been increasingly used to imagewise expose a printing plate that is sensitized to a corresponding laser wavelength. This allows the elimination of the photomask film, reducing material, equipment and labor cost.
Among the laser imagable plates, infrared laser sensitive plates are the most attractive because they can be handled and processed under white light. Infrared laser sensitive plates are also called thermosensitive plates or thermal plates because the infrared laser is usually converted to heat to cause a certain chemical or physical change (such as hardening, solubilization, ablation, phase change, or thermal flow) needed for plate making (although in some systems certain electron or energy transfers from the infrared dye to the initiator may also take place).
Various thermosensitive plates have been disclosed in the patent literature. Examples of thermosensitive plates are described in U.S. Pat. Nos. 4,054,094 and 5,379,698 (laser ablation plates), U.S. Pat. Nos. 5,705,309, 5,674,658, 5,677,106, 6,153,356, 6,232,038, and 4,997,745 (negative thermosensitive plates), U.S. Pat. Nos. 5,491,046 and 6,117,610 (both positive and negative thermosensitive plates, depending on the process), and U.S. Pat. Nos. 5,919,600 and 5,955,238 (thermosensitive positive waterless plate).
Despite the progress in conventional on-press developable plates and digital laser imagable plates, there is a desire for a lithographic plate which can be imaged by thermal laser (infrared laser), does not produce ablation debris, and does not require a separate liquid development process. More specifically, there is a desire for a thermosensitive lithographic plate which is on-press developable with ink and/or fountain solution.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a thermosensitive lithographic plate which is imagable with an infrared laser and on-press developable with ink and/or fountain solution.
It is another object of this invention to provide a thermosensitive lithographic plate comprising on a substrate a thermosensitive layer comprising an ethylenically unsaturated monomer, an infrared absorbing dye, and a free radical initiator selected from the group consisting of onium salt and borate salt.
It is yet another object of this invention to provide a method of on-press development or on-press imaging and development of the above lithographic plate.
Further objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments.
According to the present invention, there has been provided a method of lithographically printing images on a receiving medium, comprising in order:
(a) providing a lithographic plate comprising (i) a substrate; and (ii) a thermosensitive layer comprising a free radical polymerizable ethylenically unsaturated monomer (or oligomer) having at least one terminal ethylenic group, an infrared absorbing dye, and a free-radical initiator selected from the group consisting of onium salt and borate salt; wherein said thermosensitive layer is capable of hardening upon exposure to an infrared laser radiation, is soluble or dispersible in ink (for waterless plate) or in ink and/or fountain solution (for wet plate), and exhibits an affinity or aversion substantially opposite to the affinity or aversion of said substrate to at least one printing liquid selected from the group consisting of ink and an abhesive fluid for ink;
(b) imagewise exposing the plate with the infrared laser radiation to cause hardening of the thermosensitive layer in the exposed areas; and
(c) contacting said exposed plate with ink and/or fountain solution on a lithographic press to remove the thermosensitive layer in the non-hardened areas, and to lithographically print images from said plate to the receiving medium.
The plate can be imagewise exposed with an infrared laser on a plate exposure device and then transferred to a lithographic press for on-press development with ink and/or fountain solution by rotating the plate cylinder and engaging ink and/or fountain solution roller. The developed plate can then directly print images to the receiving sheets (such as papers). Alternatively, the plate can be imagewise exposed with an infrared laser while mounted on a plate cylinder of a lithographic press, on-press developed on the same press cylinder with ink and/or fountain solution, and then directly print images to the receiving sheets.
The plate is usually coated on a manufacture line by coating the thermosensitive layer, and optionally the overcoat and/or the interlayer, o
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