Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2000-10-30
2002-05-14
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S303000, C430S273100
Reexamination Certificate
active
06387595
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lithographic printing plates. More particularly, it relates to on-press developable lithographic plates having on a photosensitive layer an ultrathin overcoat.
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, 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. Conventionally, the actinic radiation is from a lamp (usually an ultraviolet lamp) and the image pattern is generally determined by a photomask which 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 which is sensitized to a corresponding laser wavelength; photomask is unnecessary in this case.
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 or disperse, and clean off the non-exposed areas (for negative plate) or the exposed areas (for positive plates).
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. 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.
An on-press developable lithographic plate generally comprises, at least, a substrate and a photosensitive layer. In order for an on-press developable plate to be useful, the non-hardened (for negative working plate) or the solubilized (for positive working plate) areas of the photosensitive layer should be able to be cleaned off completely on press with ink (for waterless plate) or with ink and/or fountain solution (for wet plate) during the initial press operation. Acceptable printed sheets should be achieved within limited impressions, preferably less than 5 impressions. The non-hardened or solubilized areas of the photosensitive layer should be able to be penetrated, softened, and dispersed or dissolved by ink and/or fountain solution within seconds; the softened, and dispersed or dissolved areas of the photosensitive layer will be absorbed by ink and/or fountain solution, and/or removed by the press offset roller and printing papers. Considering the limited amount of ink or fountain solution on a printing press and the high viscosity of the ink, it is very difficult to obtain a plate which is non-tacky and can be developed on press quickly and cleanly. Therefore, on-press developable lithographic plates have more stringent design criteria than conventional lithographic plates developed with liquid developers.
While an on-press developable lithographic plate consisting of a substrate and a photosensitive layer is useful, it often suffers from slow roll-up and ink scumming during on-press development process.
On-press developable lithographic plates having an overcoat on a photosensitive layer has been disclosed in the patent literature, including U.S. Pat. Nos. 5,677,110, 5,599,650, 5,677,108, and 5,997,993. Here, one purpose of the overcoat was to reduce tackiness and/or protect the photosensitive layer and substrate from humidity attack. The overcoat was coated at a coverage of 0.25 to 0.32 g/m
2
. Such a high coating thickness is required to provide sufficient tackiness reduction and humidity impermeability.
While the overcoat is useful in reducing tackiness and/or humidity attack, the loss of oxygen inhibition can cause certain undesirable performance, such as higher white light sensitivity, ghosting or poor contrast (measured as longer Stouffer tail steps), and ink blinding (poor ink receptivity in imaging areas). The ink binding is often caused by free radical crosslinking reaction in the photosensitive layer and overcoat interface where the hydrophilic polymers in the overcoat are physically trapped or chemically bonded to the hardened photosensitive layer.
In order to address the ink blinding, ghosting, and white light stability issues, polymer containing free radical quenching groups has been formulated into the overcoat to retard polymerization on the photosensitive layer and overcoat interface (U.S. Pat. Nos. 5,599,650 and 5,677,108). However, the polymer containing free radical quenching groups is very expensive to make. It would be desirable if such an expensive polymer can be avoided in preparing a lithographic plate.
I have found that an on-press ink and/or fountain solution developable lithographic plate having on a substrate a photosensitive layer and a top ink and/or fountain solution soluble or dispersible ultrathin overcoat with a coverage of 0.001 to 0.150 g/m
2
can give excellent white light stability (for ultraviolet sensitive plate), high contrast (or short Stouffer tail steps), and excellent ink receptivity, while allowing fast on-press development without having initial ink-scumming.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an on-press ink and/or fountain solution developable lithographic printing plate with excellent white light stability (for ultraviolet sensitive plate), high contrast, and excellent ink receptivity, while allowing fast on-press development without having initial ink-scumming.
It is another object of the present invention to provide an on-press ink and/or fountain solution developable lithographic plate having on a photosensitive layer an ultrathin photo-insensitive overcoat with a coverage of from 0.001 to 0.150 g/m
2
.
Further objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments.
According to the present invention, there has been provided a lithographic printing plate, comprising:
(a) a substrate;
(b) a photosensitive layer capable of hardening or solubilization upon exposure to an actinic radiation, the non-hardened or solubilized areas of said photosensitive layer being soluble or dispersible in ink (for waterless plate) or in ink and/or fountain solution (for wet plate), and said photosensitive layer exhibiting an affinity or aversion substantially opposite to the affinity or aversion of said substrate to at least one printing liquid selected from the group consisting of ink and an adhesive fluid for ink; and
(c) an overcoat with a coverage of from 0.001 to 0.150 g/m
2
which is soluble or dispersible in ink and/or fountain solution.
According to another aspect of the present invention, there has been provided a method of lithographically printing images on receiving media, comprising in order:
(a) providing a lithographic printing plate comprising (i) a s
LandOfFree
On-press developable lithographic printing plate having an... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with On-press developable lithographic printing plate having an..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and On-press developable lithographic printing plate having an... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2873534