Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Finishing or perfecting composition or product
Reexamination Certificate
1999-02-12
2002-04-09
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Finishing or perfecting composition or product
C430S302000
Reexamination Certificate
active
06368777
ABSTRACT:
This invention relates to printing plates and is concerned with the improvement of plate performance and with a treatment solution for achieving said improvement.
Lithographic printing is a process of printing from surfaces which have been prepared in such a way that certain areas are capable of accepting ink (oleophilic areas), whereas other areas will not accept ink (oleophobic areas). The oleophilic areas form the printing areas while the oleophobic areas form the background areas.
Plates for use in lithographic printing processes may be prepared using a photographic material that is made imagewise receptive or repellent to ink upon photo-exposure of the photographic material and subsequent chemical treatment. However, this method of preparation, which is based on photographic processing techniques, involves several steps, and therefore requires a considerable amount of time, effort and expense.
Consequently it has, for many years, been a long term aim in the printing industry to form images directly from an electronically composed digital database, ie by a so-called “computer-to-plate” system. The advantages of such a system over the traditional methods of making printing plates are:
(i) the elimination of costly intermediate silver film and processing chemicals;
(ii) a saving of time; and
iii) the ability to automate the system with consequent reduction in labour costs.
The introduction of laser technology provided the first opportunity to form an image directly on a printing plate precursor by directing a laser beam at sequential areas of the plate precursor and modulating the beam so as to vary its intensity. In this way, radiation sensitive plates comprising a high sensitivity photocrosslinkable polymer have been exposed with water-cooled UV argon-ion lasers and electrophotographic plates having sensitivity stretching from the visible spectral region into the near infra-red region have been successfully exposed using low-powered air-cooled argon-ion and semiconductor laser devices.
Imaging systems are also available which involve a sandwich structure which, on exposure to a heat generating infra-red laser beam, undergoes selective (imagewise) delamination and a subsequent transfer of materials. Such so-called peel-apart systems are generally used as replacements for silver halide films.
The present applicants have previously disclosed, in EP-A-514,145 a method of image formation which comprises: providing a radiation sensitive plate comprising a substrate and a coating containing a heat softenable disperse phase, an aqueous soluble or swellable continuous phase and a radiation absorbing substance; Eimagewise exposing the plate to at least partially coalesce the particles of the disperse phase in the image areas; and developing the imagewise exposed plate to remove the coating in the unexposed areas. The directly imaged plates thus obtained may then be used to provide printed images in the normal way using a conventional printing press.
The plates obtained in this way, however, were found to have rather poor durability in printing operations; in particular, they suffered from poor run length on the press. This drawback was believed to be associated with the fact that the at least partial coalescence of the particles of the disperse phase which occurred during imagewise exposure involved a purely physical mixing process. Consequently, it was concluded that more satisfactory performance would be achieved by the use of a system in which new chemical bond formation could be induced in image areas of the plates prior to their use on a printing press, thus providing a greater image toughness and durability.
Accordingly, EP-B-599,510 teaches a method of image formation as previously disclosed in EP-A-514,145, but which additionally comprises the step of heating the developed plate or subjecting it to irradiation to effect insolubilization of the image. In this way, good quality images of high durability are obtained.
Such insolubilization is brought about by chemical reaction between one or more of the components of the coating, which occurs as a result of the heating or irradiation treatment. In order to facilitate such chemical interactions, it is necessary that at least one of the heat softenable disperse phase and the aqueous soluble or swellable continuous phase should include a chemically reactive grouping or precursor therefor.
Despite the improvements which have been effected in this way, however, some further difficulties have been experienced with plates of the type disclosed in EP-B-599,510. In particular, the very short exposure times associated with laser imaging techniques inevitably mean that it is extremely difficult to achieve uniform heating throughout the coating, since the film surface is heated substantially more than those regions well below the surface. As a consequence, surface overheating can occur, causing damage to, or ablation of, the surface material. As well as leading to poor image formation, weak images and potentially impaired press performance, such overheating may also give rise to a plume of ablated debris and pyrolysis products that can attenuate and deflect the imaging laser beam.
Consequently, a system has been disclosed in United Kingdom patent application No. 9709404.9 wherein radiation sensitive plates of this type are provided with an additional, topmost covering layer, said layer having, at the chosen wavelength of exposure, an optical density which is lower than that of the imaging layer. Plates incorporating such a topmost layer achieve more uniform heating through the coating and thereby overcome the difficulties associated with surface overheating; thus, it is possible to obtain improvements in terms of run length, solvent resistance, handleability and scratch resistance.
Surprisingly, however, it has now been found that yet further significant improvements in press life may be achieved by treatment of the imaged plates, prior to post-development baking, with a suitable finishing solution which further enhances coalescence of the particles in the coating.
According to the present invention, there is provided a finishing solution for application to a thermally sensitive printing plate having an imaging layer including particles which are required at least partially to coalesce to form an image, said finishing solution comprising a coalescing aid.
The coalescing aid for use in the finishing solution of the present invention, wherein said plate has a radiation sensitive coating including a disperse phase and a continuous phase, comprises a solvent or mixture of solvents having solubility characteristics which facilitate softening or insolubilization of both said phases, thereby allowing phase separation and reticulation to be avoided. The solubility characteristics may be conveniently expressed in terms of Hansen solubility parameters. Typically, suitable Hansen solubility parameters would fall in the ranges &dgr;d (dispersion)=7.0-9.8, &dgr;p (polar)=1.5-8.8 and &dgr;h (hydrogen bonding)=1.7-5.2 but, for any given solvent, one or more of the parameters may fall outside the specific ranges.
The coalescing aid should also have a boiling point in excess of 250° C., preferably in excess of 300° C., in order that its total evaporation during the baking of the plate should be avoided.
Preferably, the solvent or solvent mixture which is present in the coalescing aid comprises a ketone, e.g. &ggr;-butyrolactone or isophorone, an organic carbonate, for example ethylene carbonate or propylene carbonate, an alcohol such as glycerol or diethylene glycol, a hydrocarbon, e.g. 1,2,3,4-tetrahydronaphthalene (available commercially from E.I. du Pont de Nemours and Company as Tetralin®), or a dibasic ester of a dicarboxylic acid, most preferably an aliphatic dicarboxylic acid. Suitable aliphatic carboxylic acids are those containing lower alkyl—preferably C
2
6
alkyl—chains, for example succinic, glutaric and adipic acids. Particularly infavorable results are achieved with the dimethyl, diethyl and dipropyl esters of these aci
Agfa-Gevaert
Baxter Janet
Breiner & Breiner L.L.C.
Gilmore Barbara
LandOfFree
Performance of printing plates does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Performance of printing plates, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Performance of printing plates will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2819175