Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2003-03-24
2004-04-20
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S292000, C430S945000, C430S271100, C430S273100, C430S303000, C430S309000
Reexamination Certificate
active
06723493
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lithographic printing plates. More particularly, it relates to on-press ink and/or fountain solution developable or non-alkaline aqueous developable negative lithographic plates comprising 4-piperonyl-2,6-bis(trichloromethyl)-s-triazine as free radical initiator and/or leucomalachite green as exposure indicating dye.
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.
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. 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. Conventionally, the actinic radiation is from a lamp (usually an ultraviolet lamp) and the image pattern is generally determined by a photomask that is placed between the light source and the plate. With the advance of laser and computer technologies, laser sources have been increasingly used to directly expose a printing plate according to digital imaging information.
Currently, most commercial lithographic plates require a development process after the plates being exposed and before put on press. A liquid developer is used to dissolve and clean off the non-exposed areas (for negative plate) or the exposed areas (for positive plate). 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 printing operation and then to print out regular printed sheets. No separate development process before mounting on press is needed, allowing savings on labor, material, and developer waste disposal costs. Among the on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,407,764, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, and 6,014,929.
Negative lithographic printing plates having on a substrate a photopolymerizable layer comprising a polymer, an ethylenically unsaturated monomer, and an initiator have been described in the literature, such as U.S. Pat. Nos. 3,905,815, 3,954,475, 3,987,037, and 4,997,745. Various additives such as surfactant and exposure indicating dye can be added. Among the initiator, trichloromethyl substituted triazine compounds are preferred because of their high efficiency. The inventor found that in order to be able to develop on press with ink and/or fountain solution or develop with a non-alkaline aqueous developer, the monomer to polymer weight ratio should be preferably larger than 1.0, more preferably larger than 1.5, and most preferably larger than 2.0. However, the inventor further found that, at such higher monomer to polymer ratios, the plate tends to show fingerprints of non-hardening in the finger pressed areas with a typical trichloromethyl substituted triazine initiator such as 4-(4-methoxynaphthyl)-2,6-bis(trichloromethyl)-s-triazine, and tends to show non-printout in the finger pressed areas with a typical exposure indicating dye such as leuco crystal violet, apparently due to insolubilization or crystalization of the initiator or the exposure indicating dye in the finger pressed areas.
However, applicant found, surprisingly, that 4-piperonyl-2,6-bis(trichloromethyl)-s-triazine when formulated into the above plate with a monomer to polymer ratio of larger than 1.0 does not cause fingerprints of non-hardening in the finger pressed areas. Applicant also found that leucomalachite green when formulated into the above plate with a monomer to polymer ratio of lager than 1.0 does not cause fingerprints of non-printout in the finger pressed areas.
Also, 4-piperonyl-2,6-bis(trichloromethyl)-s-triazine has an absorbency maximum of 356 nm that is below the absorbency maximum range of 360-400 nm for typical ultraviolet sensitive plates, and is an unlikely candidate for ultraviolet sensitive plates. However, applicant found, surprisingly, photopolymer plate comprising 4-piperonyl-2,6-bis(trichloromethyl)-s-triazine has excellent photosensitivity when exposed to a typical ultraviolet radiation.
SUMMARY OF THE INVENTION
This invention provides a negative lithographic printing plate comprising (i) a hydrophilic substrate and (ii) an oleophilic photosensitive layer comprising a polymer, a free radical polymerizable ethylenically unsaturated monomer, and 4-piperonyl-2,6-bis(trichloromethyl)-s-triazine; wherein the weight ratio of the monomer to the polymer is larger than 1.0. Here, 4-piperonyl-2,6-bis(trichloromethyl)-s-triazine can be the primary radiation absorber, or a photosensitizer can be added as the main radiation absorber. The chemical structure of 4-piperonyl-2,6-bis(trichloromethyl)-s-triazine is shown below.
This invention further provides a negative lithographic printing plate comprising (i) a hydrophilic substrate and (ii) an oleophilic photosensitive layer comprising a polymer, a free radical polymerizable ethylenically unsaturated monomer, an initiator, and leucomalachite green; wherein the weight ratio of the monomer to the polymer is larger than 1.0. Here, the initiator can be the primary radiation absorber, or a photosensitizer can be added as the main radiation absorber. The chemical structure of leucomalachite green is shown below.
This invention further provides a method of imagewise exposing the above lithographic printing plate (any of the above plates) with a radiation to cause hardening of the photosensitive layer in the exposed areas and then developing the plate with ink and/or fountain solution on a lithographic printing press during the initial printing operation, wherein the photosensitive layer is soluble or dispersible in ink and/or fountain solution.
This invention further provides a method of imagewise exposing the above lithographic printing plate (any of the above plates) with a radiation to cause hardening of the photosensitive layer in the exposed areas and then developing the plate with a non-alkaline aqueous developer comprising 60-99% by weight of water and 0.5 to 40% by weight of an alcohol solvent and having a pH of 3.0 to 10.0.
The plate can be exposed with an actinic radiation through a photomask or by direct exposure according to digital imaging information. For direct exposure, a laser is a preferred radiation source.
For on-press developable plate, the plate can be imaged off press and then mounted on a lithographic printing press for on-press development with ink and/or fountain solution and lithographic printing. Alternatively, the plate can be imagewise exposed with a laser while mounted on a plate cylinder of a lithographic press, on-press developed on the same plate cylinder with ink and/or fountain solution, and then directly print images to the receiving sheets.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The support employed in the lithographic plates of this invention can be any support that provides a hydrophilic surface. Such a support may be a metal sheet, a polymer film, or a coated paper. Aluminum (including aluminum alloys) sheet is a preferred support. Particularly prefer
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