Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
1998-03-17
2001-12-04
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S271100, C430S273100, C430S256000, C430S937000
Reexamination Certificate
active
06326128
ABSTRACT:
The present invention relates to a process for the production of a photosensitive, in particular photopolymerizable, recording material, especially for the production of relief printing plates, for example of flexographic printing plates, which is composed of a dimensionally stable substrate or of a substrate film, if required an adhesion-promoting layer, at least one photosensitive, in particular photopolymerizable, layer, a nonphotosensitive release layer and a removable protective cover sheet. The various layers are applied to the substrate or the protective cover sheet in a suitable manner, for example by coating with a solution of the components and drying, by application of a melt or by lamination under pressure and, if required, heat.
In the production of letterpress printing plates, it is usual to dissolve the components of the photosensitive layer in a suitable solvent and to pour the solution onto a preproduced laminate of protective cover sheet and release layer. After the photosensitive layer has been dried, the laminate is finally laminated with a substrate film coated with a mixture of adhesive-forming components.
In the production of flexographic printing plates, it is usual to melt the components of the photosensitive layer in an extruder and to mix them thoroughly. The melt is then discharged through a slot die and introduced into the gap of a calender. The protective cover sheet laminated with the release layer is fed in over one roll of the calender while the substrate film coated with the mixture of adhesion-promoting components is fed in over the second roll of the calender. In the calender gap, the photosensitive melt is laminated with the composite elements comprising protective cover sheet/release layer and mixture of adhesion-promoting components/substrate film.
In processing the printing plate, the protective cover sheet must be removed without the layers underneath being detached from their laminate. In particular, the release layer must remain completely on the photosensitive layer. If, in the preferred embodiment of the production process, the release layer is first applied to the protective cover sheet from solution and dried and the laminate is then combined with the molten, photosensitive layer by calendering, removal of the protective cover sheet from the release layer frequently gives rise to adhesion problems since in some cases the release layer adheres better to the protective cover sheet than to the photo-sensitive layer.
For good processibility of the printing plate, however, there must be a reliably reproducible difference in adhesion between release layer and photosensitive layer on the one hand and release layer and protective cover sheet on the other hand. The same applies to laminates which contain, for example, additional layers. Such materials are described, for example, in EP-A 84 851 and 456 336. Here, the additional layers are produced separately and then combined with the rest of the laminate by lamination or calendering. If the adhesion between the photosensitive layers is insufficient here, this leads during washout, in particular in parts with fine relief structures, to image defects which give poor or even non-printing plates.
It is an object of the present invention to provide a process for the production of a photosensitive recording material, in particular for the production of relief printing plates, which gives multilayer printing plates whose layers have exactly and reproducibly adjustable adhesion to one another and to substrates and to protective cover sheets so that it is possible reliably to remove parts of the multilayer material, in particular temporary protective cover sheets, without unintentional separation in other parts, ie. between other layers.
We have found that this object is achieved, according to the invention, by a process for the production of a photopolymerizable recording material for the production of relief printing plates, in which a photosensitive layer is laminated with a composite element comprising a removable protective cover sheet, a release layer and, if required, a further layer. In the novel process, the surface of the uppermost layer of the composite element or the surface of the photosensitive layer is subjected to a corona treatment immediately before contact.
Preferably, the photosensitive layer is applied as a melt or from a solution to the composite element comprising protective cover sheet, release layer and any further layer. In this case, the uppermost layer of the composite element is usually subjected to a corona treatment.
The release layer applied to the protective cover sheet, as the uppermost layer, is preferably subjected to the corona treatment.
As a result of this treatment, the adhesion between the release layer and the photosensitive layer is increased substantially and so uniformly that, on removal of the protective cover sheet from the release layer, the latter remains bonded completely on the layer underneath, without any defects.
If it is intended to produce a printing plate, in particular a flexographic printing plate, which contains more than one photosensitive layer, the abovementioned composite element may also carry a photosensitive layer on the nonphotosensitive release layer. Said photosensitive layer need not be photosensitive from the beginning; it is sufficient if a layer which contains only a polymeric binder but no polymerizable compound and no photo-initiator is initially applied. During storage of the plate, when this layer is in contact with the other photosensitive layer, its low molecular weight components, such as polymerizable compounds and photoinitiators, can diffuse into the polymer layer and render the latter photosensitive too.
If such a layer is intended, it is generally applied as a solution to the release layer and is dried. Of course, the components of the release layer and the solvent of the additional layer should be chosen so that the release layer is dissolved only to a slight extent, if at all. It is also possible to apply the additional layer from a dispersion in a solvent, for example water, in which the release layer is insoluble. The additional layer generally has a thickness of from about 20 to 200 &mgr;m. If it has a certain minimum thickness, for example about 50 &mgr;m, the additional layer too may be applied from the melt by extrusion and calendering. Before the application of the additional layer, the surface of the release layer can also be corona-treated according to the invention, particularly if the additional layer is applied by calendering.
Very generally, in the production of a multilayer laminate, each layer surface with which another layer is to be firmly bonded can be corona-treated by the novel process. It is important that the corona treatment be carried out immediately before the application of the layer, ie. within from about 1 to 3000, preferably from about 60 to 600, seconds before this step. This can be achieved on a large industrial scale particularly advantageously by a continuous production process in which, before the coating station or before entering the calender gap, the strip to be coated passes a corona station in which the corona treatment is thus carried out in-line.
A thin adhesion-promoting layer may furthermore be provided below the photosensitive layer, on the substrate which usually remains permanently bonded to the layers present thereon.
The photosensitive layer or the photosensitive layers is or are usually negative-working, ie. it or they are cured by exposure to light. This can be effected by photocrosslinking of preproduced polymers or by photopolymerization of low molecular weight polymerizable compounds. Photopolymerizable layers essentially contain a polymeric binder which can be washed out in a developer, an ethylenically unsaturated compound capable of free radical polymerization and a polymerization initiator which can be activated by actinic radiation, in particular long-wave ultraviolet or short-wave visible light. The composition of such layers is known and is described,
Philipp Sabine
Telser Thomas
Wegener Stefan
Zurger Manfred
BASF Drucksysteme GmbH
Baxter Janet
Gilmore Barbara
Keil & Weinkauf
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