Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2000-01-11
2001-11-06
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S138000, C430S271100, C430S273100, C430S302000
Reexamination Certificate
active
06312866
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to image formation and is concerned with the formation of images directly from electronically composed digital sources.
For many years it has been a long term aim in the printing industry to form printing images directly from an electronically composed digital database, i.e. 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 the 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; imagewise 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-A-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 insolubilisation of the image. In this way, good quality images of high durability are obtained.
Such insolubilisation 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-A- 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.
Therefore, the present invention seeks to overcome the difficulties associated with surface overhearing which have been experienced with prior art thermally imageable printing plates.
According to one aspect of the present invention, there is provided a radiation sensitive plate, imageable by exposure to thermal radiation, which comprises a substrate coated with:
(i) an imaging layer which comprises (1) a disperse phase comprising a water insoluble heat softenable component (A) and (2) a binder or continuous phase consisting of a component (B) which is soluble or swellable in aqueous, preferably aqueous alkaline, medium;
(ii) a substance (C) capable of strongly absorbing radiation and transferring the energy thus obtained as heat to the disperse phase so that at least partial coalescence of the coating occurs, said substance being contained either within the imaging layer (i) or in a separate layer; and
(iii) a topmost covering layer having, at the chosen wavelength of exposure, an optical density which is lower than that of the imaging layer (i), said covering layer comprising at least one of the following.
(1) a disperse phase comprising a water-insoluble heat softenable component (D) and a binder or continuous phase consisting of a component (E) which is soluble or swellable in aqueous, preferably aqueous alkaline, medium;
(2) a polymer resin (F) which is soluble in aqueous medium; or
(3) a polymer resin (G) which is dispersible in aqueous or alcoholic medium, but insoluble in aqueous alkaline medium.
Optionally, the topmost covering layer may also contain a substance (H) capable of strongly absorbing radiation and transferring the energy thus obtained as heat to the disperse phase.
Preferably, the topmost covering layer comprises (iii) (1), containing (D), (E) and the optional component (H) these components optionally being the same as (A), (B) and (C) respectively,
References below to components A, B and C also apply to components D, E and H respectively.
The components A and E are preferably polymers and/or oligomers, at least one of which contains reactive groupings or precursors, thus providing a system in which at least one of the following conditions is fulfilled:
a) Component A is crosslinkable;
b) Component B is crosslinkable;
c) Component A reacts with component B to form a crosslinked structure:
d) Component A is a mixture of two or more materials A
1
, A
2
, A
3
, etc. which are either mutually reactive and/or react with component B;
e) Component B is a mixture of two or more materials B
1
, B
2
, B
3
etc. which are either mutually reactive and/or react with component A.
The imaging layer contains discrete domains of components A and B. The disperse or discontinuous phase A is encapsulated by the continuous phase B. The two phases A and B may form a core-shell system, as described in EP-A-514,145, in which case the core and shell components may be linked together via chemical bonding. Under ambient conditions, both components are preferably solid and immobile.
Component B may, for example, be incorporated in the composition of the coating through its use as a binder in predispersed pigmentary material added to the composition as the radiation-absorbing substance.
In practice, it is desirable to select components such that the components of the coating will not react sufficiently under normal storage conditions to hinder the imaging and development processes, but will react sufficiently rapidly at elevated temperatures to give a durable, solvent resistant image. This lack of reactivity at ambient temperature may result from the mutually reactive groups being present each in a d
Obuchowicz Jacek Paul
Zumsteg Fredrick Claus
AGFA-GEVAERT
Baxter Janet
Breiner & Breiner L.L.C.
Gilmore Barbara
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