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
2001-12-03
2003-07-29
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
C430S306000, C430S944000, C430S945000, C101S395000, C101S453000, C101S463100
Reexamination Certificate
active
06599679
ABSTRACT:
The present invention relates to a photosensitive flexographic printing element for the production of flexographic printing plates by digital imaging by means of lasers which has an IR-ablative layer comprising a polyether-polyurethane as binder. The invention furthermore relates to a process for the production of flexographic printing plates using an element of this type.
The conventional method for the production of flexographic printing plates by laying a photographic mask on a photopolymeric recording element, irradiating the recording element with actinic light through this mask and washing out the unpolymerized areas of the exposed element with a developer liquid is increasingly being replaced by CtP (computer-to-plate) technology, frequently also known as digital imaging. In CtP methods, the photographic mask in conventional systems is replaced by the masks integrated into the recording element.
Although a number of different methods have been proposed, only two have hitherto achieved significant importance in the market. In the first method, the photopolymerizable element is provided with a suitable ink receptor layer, and a mask is printed on by means of an ink-jet printer, as disclosed, for example, in WO 97/25206. The element can subsequently be exposed and developed in a known manner.
In the second method, the photopolymerizable element is coated with a substantially opaque, IR-ablative layer. Layers of this type usually comprise carbon black. Imagewise irradiation by means of a laser removes the IR-ablative layer at the points where it is hit by the laser beam, and the underlying photopolymerizable layer is uncovered. The image recording element can then be irradiated over its full area with actinic light through the ablatively formed mask in a known manner and washed out using a developer liquid. In the washing-out step, the nonablated residues of the IR-ablative layer and the underlying unpolymerized areas of the exposed element are removed.
Flexographic printing elements having IR-ablative layers are known in principle. EP-A 654 150 discloses a flexographic printing element having an IR-ablative layer. The IR-ablative layer comprises an IR-absorbent material. In addition, polymeric binders and a large number of different auxiliaries, for example dispersion aids or plasticizers, are disclosed as optional constituents. Furthermore, an additional barrier layer between the photopolymerizable layer and the IR-ablative layer is disclosed. This is intended to prevent diffusion of monomers from the photopolymerizable layer into the IR-ablative layer.
EP-A 741 330 discloses an IR-ablative flexographic printing element which has no barrier layer of this type. A multiplicity of widely differing polymers is disclosed as binders for the IR-ablative layer. Furthermore, the IR-ablative layer may also comprise a second binder in a smaller amount, for which a multiplicity of widely varying polymers is likewise disclosed.
EP-A 767 407 discloses a flexographic printing element having an IR-ablative layer which has an elastomeric, film-forming binder. Binders disclosed are polyamides and polyvinyl alcohol polyethylene glycol graft copolymers.
In the process for the production of flexographic printing plates by IR ablation, the quality of the IR-ablative layer is the crucial parameter for the quality of the flexographic printing plate and the economic efficiency of the process. The IR-ablative layer must satisfy a number of widely varying quality criteria:
It should have high sensitivity to lasers in order to ensure rapid and complete removal of the layer with the lowest possible laser power.
It should be soluble in conventional wash-out agents for the photopolymerizable layer so that it can be removed together with the unpolymerized constituents of the layer during the conventional development step. Otherwise, two wash-out steps would have to be carried out.
The laser apparatuses used nowadays are usually instruments with rotating drums (external or internal drums). The IR-ablative layer must therefore be elastic in order that it does not tear or wrinkle on clamping onto the drums.
It must be tack-free in order that no dust is attracted which could interfere with the IR ablation.
For storage and transport, flexographic printing elements are usually protected against damage by means of a protective film, which has to be removed before the IR ablation. The protective film must have only low adhesion to the IR-ablative layer in order that the IR-ablative layer is not damaged on removal.
Conversely, the IR-ablative layer must adhere strongly to the light-sensitive layer in order that it is not removed at the same time on removal of the protective film and in order that no air bubbles nullify the advantage of direct contact between the IR-ablative layer and the photopolymerizable layer.
The person skilled in the art who would like to produce a high-sensitivity, high-quality flexographic printing element having an IR-ablative layer sees himself confronted with a typical catch 22 situation. In order to obtain an IR-ablative layer with the highest possible sensitivity, the use of a self-oxidative binder, such as nitrocellulose, is advisable. However, nitrocellulose layers are very brittle, and consequently the elasticity of nitrocellulose layers is unsatisfactory and flexographic printing elements of this type wrinkle easily on clamping onto drum instruments. Although the brittleness can be reduced by addition of suitable plasticizers, the addition of plasticizers frequently has the consequence, however, of tacky layers with excessive cover film adhesion. However, typical tack-free binders, such as certain polyamides, have significantly lower sensitivity.
None of the cited documents from the prior art contains an indication of which components have to be selected from the multiplicity of possible components in order to obtain a flexographic printing element which meets all the outlined requirements.
It is an object of the present invention to provide a flexographic printing element having an IR-ablative layer which does not have the disadvantages of the prior art and satisfies the abovementioned requirements.
Accordingly, a photosensitive flexographic printing element for the production of flexographic printing plates by digital imaging by means of lasers has been found which comprises at least
a dimensionally stable support,
a photopolymerizable layer, at least comprising an elastomeric binder, a polymerizable compound and a photoinitiator or photoinitiator system,
a laser-ablatable layer having an optical density in the actinic spectral region of at least 2.5 and comprising at least one elastomeric binder and an IR absorber for laser radiation, and
optionally a removable, flexible protective film,
where the elastomeric binder of the IR-ablatable layer is an elastomeric polyether-polyurethane.
We have also found a process for the production of flexographic printing plates using an element of this type.
In detail, the following comments should be made regarding the invention.
In the photopolymerizable printing element according to the invention, a conventional photopolymerizable layer, if desired with an adhesion layer, is applied to a dimensionally stable support. Examples of suitably dimensionally stable supports are plates, films and conical and cylindrical tubes (sleeves) made from metals, such as steel or aluminum, or plastics, such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN).
The photopolymerizable layer consists of a negative-working photopolymerizable mixture, i.e. one which is cured by exposure. This can be carried out by photocrosslinking with previously prepared polymers, by photopolymerization of low-molecular-weight, photopolymerizable compounds or both. Photopolymerizable layers essentially consist of a polymeric, elastomeric binder which can be washed out in the developer, an ethylenically unsaturated, free-radical-polymerizable compound, a photoinitiator or a photoinitiator system, and optionally further additives and auxiliaries. The composition
Kaczun Jürgen
Philipp Sabine
BASF Drucksysteme GmbH
Baxter Janet
Gilliam Barbara
Keil & Weinkauf
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