Patent ID: 7288361

Claim:
A lithographic printing process which comprises the steps of: imagewise exposing to infrared light a presensitized lithographic plate which comprises a hydrophilic support and a removable image-forming layer containing an infrared absorbing agent having the absorption maximum within an infrared region and a dye precursor having a p-phenylenediamine structure and a releaseable group attached to nitrogen atom of the p-phenylenediamine structure, and having substantially no absorption within a visible region that decomposes on heating or light exposure to form a visible dye having an absorption within a visible region within the exposed area by releasing the releaseable group from p-phenylenediamine structure, and to make the image-forming layer irremovable within the exposed area; removing the image-forming layer within the unexposed area of the lithographic plate mounted on a cylinder of a printing press; and then printing an image with the lithographic plate mounted on the cylinder of the printing press, wherein the dye precursor is represented by the formula (I), and the visible dye is represented by the formula (II): R 2 N—(p-Ph)—N(—Y)—L—Z (I) R 2 N—(p-Ph)—N=L (II) in which each of the two groups represented by R independently is hydrogen or an alkyl group having 1 to 20 carbon atoms; p-Ph is p-phenylene, which can be substituted with an alkyl group having 1 to 20 carbon atoms; L is a divalent linking group; and each of Y and Z independently is a releasable group.