Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2001-06-04
2003-04-01
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S302000, C430S944000, C430S327000, C430S273100
Reexamination Certificate
active
06541183
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lithographic printing plates. More specifically, it relates to lithographic plates comprising on a substrate a semisolid radiation-sensitive layer suitable for imaging with a laser according to digital imaging information.
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, and 6,071,675.
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 are plates sensitive to infrared laser (such as from infrared laser diode at about 830 nm and Nd/YAG laser at about 1064 nm), HeNe laser (about 633 nm), frequency-doubled Nd/YAG laser (about 532 nm), argon ion laser (about 488 nm), violet diode laser (about 410 nm), and ultraviolet gas laser (about 365 nm). Infrared laser sensitive plates are also called thermosensitive plates or thermal plates because the infrared laser is generally converted to heat to cause a certain chemical or physical change (such as hardening, solubilization, ablation, phase change, or thermal flow) needed for imaging formation on a plate, although sensitization of the initiator by the infrared dye through charge transfer may also play a role in some thermosensitive plates.
Various laser imagable plates have been disclosed in the patent literature. Examples of laser imagable 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 and 5,677,106 (negative thermosensitive plates), U.S. Pat. Nos. 5,491,046 and 6,117,610 (both positive and negative thermosensitive plates, depending on the process), U.S. Pat. No. 5,919,600 and 5,955,238 (thermosensitive positive waterless plate), U.S. Pat. No. 4,063,949 (negative plate imagable with a laser, such as argon ion laser and CO
2
laser), and U.S. Pat. No. 4,486,529 and 4,248,959 (visible laser sensitive negative plate comprising a para-aminobenzene diazonium compound).
Despite the progress in digital laser imagable plates, the current laser imagable photopolymer plates have relatively low photospeed which limits the imaging throughput. For examples, the current thermal plate imagers are often equipped with many (such as 15) laser diodes in order to meet the throughput requirement; such a multiple laser design is very expensive and complex. For violet and other visible laser imagable plates, there has been a challenge in achieving sufficient photospeed. Therefore, there is a need for a laser imagable photopolymer lithographic plate (wet or waterless plate) with significantly improved photospeed.
I have found that laser imagable negative lithographic plates having a semisolid radiation-sensitive layer can have significantly improved photospeed over plates having a solid radiation-sensitive layer of similar composition. Such plates allow laser imaging at more desirable photospeeds. The use of semisolid radiation-sensitive layer in laser imagable negative lithographic plates is unexpected because traditionally the radiation-sensitive layer of lithographic plates is formulated as a solid coating in order to avoid handling problems such as tackiness, finger printing, and blocking. However, I have also found that the tackiness, finger printing, and blocking problems associated with a semisolid radiation-sensitive layer can be eliminated or reduced by various plate designs such as coating the radiation-sensitive layer on a rough substrate in a conformal configuration as described in U.S. patent application Ser. No. 05/609,018, utilizing a thick overcoat, utilizing a transparent cover sheet, and utilizing a non-sticky interleaf.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a negative lithographic plate (wet or waterless plate) with fast photospeed suitable for imaging with an infrared laser, visible laser (including violet laser), or ultraviolet laser.
It is another object of this invention to provide a laser imagable negative lithographic plate comprising on a substrate a semisolid radiation-sensitive layer capable of hardening upon exposure to an infrared laser, visible laser, or ultraviolet laser.
It is another object of this invention to provide a laser imagable negative waterless lithographic plate comprising on an oleophilic substrate a semisolid radiation-sensitive layer and a top oleophobic layer, said radiation-sensitive layer being capable of hardening upon exposure to an infrared laser, visible laser, or ultraviolet laser.
It is another object of this invention to provide an on-press ink and/or fountain solution developable, laser imagable lithographic plate comprising on a substrate a semisolid radiation-sensitive layer, with or without a top oleophobic layer.
It is another object of this invention to provide a method of on-press developing an ink and/or fountain solution developable, la
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