This invention relates to a process for making a lithographic printing plate by silver complex diffusion transfer process wherein laser beams, especially semiconductor laser beam are used as a light source.
A lithographic printing plate which utilizes as ink receptive areas a transferred silver image as produced by the silver complex diffusion transfer process has already been disclosed and well known in Japanese Patent Examined Publication (Kokoku) No. 30562/73 and Japanese Patent Unexamined Publication (Kokai) Nos. 21602/78, 103104/79, 9750/81, etc.
According to a typical silver complex diffusion transfer process suitable for making a lithographic printing plate, a light sensitive material comprising a support and provided successively thereon, a subbing layer which also serves as an antihalation layer, a silver halide emulsion layer and a physical development nuclei layer is subjected to an imagewise exposure and then development treatment to result in conversion of the silver halide of a latent image portion to a black silver in the emulsion layer. Simultaneously, silver halide of the area other than the latent image dissolves by the action of a silver halide complexing agent contained in the developer and diffuses toward the surface of the light sensitive material. The dissolved and diffused silver complex is precipitated by the reducing action of the developing agent as a silver image on the physical development nuclei in the surface layer. After the development, if necessary, the silver image thus obtained is subjected to sensitization treatment to enhance its ink receptivity. Then, the plate is set in an offset printing machine and inked image is transferred to a material to be printed.
In the conventional process, the silver halide emulsion layer is spectrally sensitized with a merocyanine dye, cyanine dye, etc. so as to have a sensitivity maximum at around 550 nm in the green region and this is exposed in a process camera to an ordinary light source such as tungsten bulb for several seconds to several tens of seconds. However, said conventional process has still limitations in sharpness and resolving power, even if said lithographic printing plate which is inherently excellent in sharpness and resolving power is used. Besides, in reproducing a color print from a color original, the conventional process still has problems in making the light sensitive material and lithographic printing plates in addition to the insufficient resolving power or the like.
At present, there has been proposed a process of plate making with laser beams in an attempt to solve the above problems. For example, U.S. Pat. No. 4,501,811, Japanese Patent Unexamined Publication (Kokai) Nos. 71055/84, 75838/85, 100148/85, etc. disclose use of helium-neon laser or light-emitting diodes for making lithographic printing plates. As also disclosed in these patent publications, in the lithographic printing plates made by the silver complex diffusion transfer process, the surface physical development nuclei layer greatly affects the spectral sensitization and, as a result, there is a decrease in the sensitivity to a laser beam employed or deterioration in the shelf stability. Further, since the silver complex diffusion transfer process produces a transferred and precipitated silver image of soft gradation, there occur a decrease in sharpness and resolution of the image, scumming, and wearing-off of the silver image during printing. This results in insufficient printing endurance and the desired lithographic printing plates of good quality are difficult to make.
Japanese Patent Unexamined Publication (Kokai) No. 75838/35 discloses that in making lithographic printing plates with helium-neon laser beam of 632.8 nm, resolving power and printing endurance can be improved by providing an antihalation layer of 10 or less in reflectance.
Methods which employ laser sources such as helium-neon, argon, etc. as light sources for scanning exposure have the defects that although high output can be obtained, devices are of large size and expensive and besides consumption power efficiency is low. On the other hand, semiconductor laser is small in size, inexpensive, easy in modulation and long in shelf life. As the semiconductor lasers, there are used semiconductors of such systems as Ga/As/P; Ga/Al/As; Ga/As; In/P; In/As, etc. Wavelengths of these laser beams are generally longer than 700 nm and mostly longer than 750 nm. Light sensitive materials for lithographic printing plates which use semiconductor laser beams of such longer wavelength as light sources for imagewise exposure are disclosed, for example, in Japanese Patent Unexamined Publication (Kokai) No. 61752/85. This patent publication discloses, as an especially preferable embodiment, a light sensitive material for lithographic printing plates which comprises a support and, successively provided thereon, a non-silver light sensitive layer (e.g., a light sensitive layer comprising o-naphthoquinone diazide) capable of forming an oleophilic image and a light sensitive silver halide emulsion layer and also discloses sensitizing dyes represented by the general formula (I)-IV) having sensitivity maxima in the range of wavelength longer than 700 nm which are used in said emulsion layer.
The light sensitive materials for lithographic printing plates having a silver halide emulsion layer containing such sensitizing dyes, an antihalation layer of 10% or less in reflectance for semiconductor laser diode beam of near infrared region and a surface physical development nuclei layer have sufficient printing endurance, but are very low in sensitivity to laser diode radiation and increase of reflectance of the antihalation layer in an attempt to increase the sensitivity to some extent results in deterioration of printing endurance. Thus, it has been very difficult to satisfy all of high sensitivity, high resolving power and high printing endurance.