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
1998-10-28
2001-02-20
Hamilton, Cynthia (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
C430S307000, C430S944000
Reexamination Certificate
active
06190832
ABSTRACT:
The present invention relates to a process for the production of photopolymeric gravure printing plates which are to be developed with water or water/alcohol mixtures after imagewise exposure and have, arranged one on top of the other on a dimensionally stable substrate, a layer (A) which may be bonded to the dimensionally stable substrate by an adhesion-promoting layer and is crosslinkable by actinic radiation, a layer (B) sensitive to IR radiation, if required a parting layer arranged between layer (A) and layer (B), and a peelable protective film applied to the layer (B), and are suitable for digital information transfer, and gravure printing plates produced by this process.
Photopolymeric gravure printing plates which can be developed with water or with an aqueous alcoholic medium consist of a photosensitive photopolymerizable layer which is soluble or dispersible in the developer and is applied to a dimensionally stable substrate, for example polyethylene terephthalate film or steel or aluminum sheet. In order to provide a firm bond, an adhesion-promoting layer may be applied between the photosensitive, photopolymerizable layer which can be developed with water or an aqueous alcoholic medium and the substrate.
A release layer may also be present on the photosensitive, photopolymerizable layer which can be developed with water or an aqueous alcoholic medium, said release layer being required whenever said layer present underneath is so tacky that, when the photographic negative is placed on top, the air present in the vacuum printing frame cannot be uniformly removed and consequently vacuum errors occur during exposure.
The photosensitive, photopolymerizable layer which can be developed with water or an aqueous alcoholic medium can be protected from mechanical damage by a protective film which is present either on said layer or on the release layer present thereon (cf. for example Ullmannn's Encyclopedia of Industrial Chemistry, Vol. A 13, page 629).
The information contained in the photographic negative is transferred to this photosensitive, photopolymerizable layer which can be developed with water on an aqueous alcoholic medium, in general after removal of the protective film, if present, by application of the photographic negative, removal of the air by means of a vacuum frame and uniform exposure. The well depth of the gravure printing plate and hence the amount of ink transferred during the printing process are controlled by means of the exposure time.
In the subsequent development step, the unpolymerized parts, the wells, can be washed out with the aid of water or water/alcohol mixtures; the photopolymerized parts are insoluble or nondispersible and thus form the surface of the gravure printing plates. In order to remove the water or the water/alcohol mixture from the swollen relief layer, a subsequent drying step is effected. Depending on the photosensitive material, the plate may require further processing steps. In the case of some printing plates, for example, uniform post exposure is required in order completely to cure the printing plates.
In this process, the information is thus transferred via a photographic negative. According to the prior art, photographic negatives are produced by exposure of a photosensitive film by means of an exposure unit, for example an optomechanical exposure unit, a CRT (cathode ray tube) exposure unit or a laser exposure unit (He—Ne laser, &lgr;=632 nm), with subsequent development (cf. for example Armin Lautert, Allgemeine Fachkunde der Drucktechnik, pages 42-45, 11th Edition 1993, Baden-verlag CH-Baden).
The information transferred in this manner to the film originates from a digitized data set. In rotary gravure printing, the printing plate cannot be produced in seamless and continuous form by the use of a negative.
For these reasons, it would be desirable to be able to produce a gravure printing plate without the circuitous route via a photographic negative.
In addition to the shorter production times, the costs for the production of a photographic negative would not be incurred. In addition, the wet chemical processes in film development would no longer be required. This would be associated with the ecological advantage that effluents contaminated with chemicals and metal salts would no longer be produced.
Furthermore, the geometric dimensions of a photographic negative may change as a result of the temperature and atmospheric humidity, which may lead to register problems of the finished gravure printing plate since, when a corresponding photosensitive, photopolymerizable layer is exposed, the information of the photographic negative is transferred as a 1:1 projection. According to the prior art, UV exposure is carried out through a photographic negative with application of a vacuum, since intensive contact is required between negative and plate surface in order to avoid side lighting and copying errors. The pressure required for the intensive contact between plate surface and negative may press in the dust particles and hence lead to holes in the plate surface which subsequently transfer ink in an undesirable manner in the printing process.
However, UV exposure without reduced pressure would be desirable, in order to be able to utilize the inhibitory effect of the atmospheric oxygen, to be able to control the well depths with less scattering and to avoid the surface defects caused by dust particles.
Digital processes in which no photographic negative is required for information transfer have long been known.
For example, DE 4117127 A1 describes a process in which an image-bearing mask is produced on photosensitive recording elements by using an inkjet printer or an electrophotographic printer. The disadvantage here is that the resolution is insufficient for high-quality gravure printing.
EP 0001138-A1 describes a process for the production of lithographic printing plates with formation of a fugitive negative.
WO 94/03839 describes a process in which the photosensitive recording element is structured by means of an IR laser and an image-bearing mask is produced in this manner. Here, the photosensitive recording material consists of a substrate, a photosensitive layer, an IR-sensitive layer which is opaque to actinic light and a release layer. On exposure to the IR laser, the exposed parts of the IR-sensitive layer are fixed to the release layer on top and can be removed together with the release layer by peeling off the latter. An image-bearing mask on the photosensitive recording layer is thus obtained. The disadvantage here is that damage, for example scratches in the release layer, which are simultaneously to serve for mechanical protection, leads to defective information transfer. Furthermore, particularly in the case of fine image sections, there is the danger that surrounding parts that remain on the photosensitive recording material may be damaged when the release layer is peeled off, likewise leading to defective information transfer. U.S. Pat. No. 5,262,275 and WO 94/03838 describe a process in which flexographic printing plates are provided with a digital structure by means of an IR laser. The recording element consists of a substrate, a photosensitive flexographic layer, a barrier layer and an IR-sensitive layer which is opaque to actinic light. The IR-sensitive layer is recorded on directly by means of the IR laser, the IR-sensitive material being evaporated in the parts recorded on and these parts thus becoming transparent to actinic light. The barrier layer present under the IR-sensitive layer on the one hand protects the IR-sensitive layer against diffusion of monomers and other components from the UV-sensitive flexographic layer and prevents the entry of oxygen into the UV-sensitive flexographic layer during UV exposure. As described in WO 94/03838, the UV exposure is not reproducible in the presence of atmospheric oxygen. Moreover, longer exposure times are required in the presence of oxygen. Hence, barrier layers which are not permeable to oxygen are used.
It is an object of the present inventio
Goffing Friedrich
Loerzer Thomas
Sandig Hartmut
Tonnessen Markus
BASF - Aktiengesellschaft
Hamilton Cynthia
Keil & Weinkauf
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