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.
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. 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. 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 prints 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. No. 5,258,263 (Cheema, et al), U.S. Pat. No. 5,516,620 (Cheng, et al), U.S. Pat. No. 5,561,029 (Fitzgerald, et al), U.S. Pat. No. 5,616,449 (Cheng, et al), U.S. Pat. No. 5,677,110 (Chia, et al), and U.S. Pat. No. 5,811,220 (Cheng, et al). Most of these patents are aimed to address the inherent problems which often encounter on-press developable plates, such as poor developability, tackiness, poor durability, and poor shelf-life stability.
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 which is sensitized to a corresponding laser wavelength. This would allow the elimination of the photomask film, reducing material, equipment and labor cost.
U.S. Pat. No. 4,132,168 (Peterson) discloses a lithographic plate consisting of on a substrate an ultraviolet light (UV) sensitive layer and a top mask layer which is opaque to UV light and is capable of being removed or rendered transparent to UV light by a non-actinic laser radiation. While this plate is capable of digital imaging, it requires two cumbersome chemical processes after exposure, namely a mask layer removal process and a development process.
U.S. Pat. No. 5,858,604 (Takeda, et al) discloses a lithographic plate having on a substrate a photosensitive layer containing an o-quinonediazide or diazo compound, and a water or aqueous alkaline soluble laser sensitive mask layer. While this plate is capable of digital imaging and one-step process, a cumbersome, liquid waste-generating wet process using a liquid developer is required.
U.S. Pat. No. 5,922,502 (Damme, et al) discloses a lithographic plate having on the substrate a photosensitive layer, an aqueous medium soluble or swellable interlayer, and an infrared laser sensitive mask layer. The introduction of the interlayer allows simultaneous removal of the mask layer and development of the photosensitive layer, as well as the removal of the interlayer. However, the removal of the mask layer in large non-imaging areas is difficult, since the mask layer is not soluble or dispersible in the developer and it is hard for the developer to reach these areas of interlayer which are fully covered by the mask layer.
Despite the progress in conventional on-press developable plates and digital laser imagable plates, there is still a desire for a laser imagable plate which can be processed on-press with ink and/or fountain solution, has excellent press performance, and can be manufactured conveniently and inexpensively.