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
2003-07-29
2004-09-21
Gilliam, Barbara (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
C430S271100, C430S281100, C430S286100, C430S306000, C430S309000, C430S348000, C430S494000, C430S945000
Reexamination Certificate
active
06794115
ABSTRACT:
The present invention relates to a process for the production of thermally crosslinked, laser-engravable flexographic printing elements, to the production of relief printing plates from the laser-engravable flexographic printing elements, and the thermally uncrosslinked flexographic printing elements.
The conventional method for the production of flexographic printing plates by laying a photographic mask on a photopolymeric recording element, irradiating the element with actinic light through this mask, and washing the unpolymerized areas of the exposed element out using a developer liquid is increasingly being replaced by methods in which lasers are used.
In laser direct engraving, pits are engraved directly into a suitable elastomeric layer with the aid of a laser of sufficiently high power, in particular by means of an IR laser, forming a relief which is suitable for printing. To this end, large amounts of the material of which the printing relief consists have to be removed. A typical flexographic printing plate is nowadays, for example, between 0.5 and 7 mm in thickness and the non-printing pits in the plate are between 0.3 and 3 mm in depth. The method of laser direct engraving for the production of flexographic printing plates has therefore only attracted commercial interest in recent years with the appearance of improved laser systems, although laser engraving of rubber printing cylinders using CO2 lasers has basically been known since the late 1960s. The demand for suitable laser-engravable flexographic printing elements as starting material for the production of flexographic printing plates by means of laser engraving has thus also increased significantly.
In principle, commercially available photopolymerizable flexographic printing elements can be employed for the production of flexographic printing plates by means of laser engraving. U.S. Pat. No. 5,259,311 discloses a process in which, in a first step, the flexographic printing element is photochemically crosslinked by irradiation over the full area and, in a second step, a printing relief is engraved by means of a laser. However, the sensitivity of flexographic printing elements of this type to CO2 lasers is low.
It has therefore been proposed, for example in EP-A 0 640 043 and EP-A 0 640 044, to admix substances which absorb IR radiation with the elastomeric layer to be laser-engraved in order to increase the sensitivity. However, substances of this type, such as carbon black or certain dyes, also absorb very strongly in the UV/VIS region. Flexographic printing elements which comprise these absorbers therefore can at best be photochemically crosslinked in a very thin layer, or not at all. Thus, EP-A 0 640 043 discloses the production of a carbon black-containing, elastomeric layer by photocrosslinking. However, this layer only has a thickness of 0.076 mm, while the typical thickness of commercially available flexographic printing plates is from 0.5 to 7 mm.
It has therefore likewise been proposed, for example in EP-A 0 640 044, to add thermally decomposing polymerization initiators to the elastomeric layer which is to be laser-engraved and to crosslink this layer thermally. Photocrosslinkable, flexible printing plates based on thermoplastic elastomers are produced in an elegant manner by extrusion and calendering at elevated temperatures using thermally stable photoinitiators. However, this production method is difficult on use of thermally decomposing initiators since, owing to the high working temperatures and owing to the high shear during production of the thermally crosslinkable mixture in the extruder, premature crosslinking may occur. Owing to the temperature sensitivity of the crosslinkable mixture, low working temperatures of significantly below 100° C. are necessary, and consequently processing in a twin-screw extruder, for example, is not possible.
It is an object of the present invention to provide a process for the production of laser-engravable flexographic printing plates having a thermally crosslinked, elastomeric, relief-forming layer.
We have found that this object is achieved by a process for the production of a laser-engravable flexographic printing element comprising a thermally crosslinked, elastomeric, relief-forming layer E, having the following steps:
(i) production of a multilayer composite at least comprising a two-layer composite consisting of a depot layer D and an uncrosslinked precursor layer V for the relief-forming layer E which is directly adjacent to the depot layer D, and optionally further layers, support foils or films and/or protective films,
where the precursor layer V comprises
(a) at least one elastomeric binder,
(b) at least one ethylenically unsaturated monomer,
(c) optionally an absorber for laser radiation, and
(d) optionally further additives,
and the depot layer D comprises
(e) at least one elastomeric binder,
(f) at least one thermally decomposing polymerization initiator,
(g) optionally an absorber for laser radiation, and
(h) optionally further additives,
(ii) allowing the thermally decomposing polymerization initiators to diffuse out of the depot layer D into the precursor layer V,
(iii) if desired removal of the depot layer D, and
(iv) thermal crosslinking of the precursor layer V to give the elastomeric, relief-forming layer E.
In a first step (i), a multilayer composite at least comprising a two-layer composite consisting of the depot layer D and the uncrosslinked precursor layer V for the relief-forming layer E which is adjacent to the depot layer D is produced.
The precursor layer V comprises at least one elastomeric binder as component (a).
The elastomeric binders employed can be all known binders also used for the production of photopolymerizable flexographic printing plates. In principle, both elastomeric binders and thermoplastic elastomeric binders are suitable. Examples of suitable binders are the known three-block copolymers of the SIS or SBS type, which may also be fully or partially hydrogenated. It is also possible to employ elastomeric polymers of the ethylene-propylene-diene type, ethylene-acrylic acid rubbers or elastomeric polymers based on acrylates or acrylate copolymers. Further examples of suitable polymers are disclosed in DE-A 22 15 090, EP-A 084 851, EP-A 819 984 or EP-A 553 662. It is also possible to employ mixtures of two or more different binders.
The type and amount of the binder employed are selected by the person skilled in the art depending on the desired properties of the printing relief. In general, the amount of binder is from 50 to 90% by weight, preferably from 60 to 90% by weight, based on the sum of all constituents of the precursor layer, i.e. the sum of components (a) to (d).
The precursor layer comprises at least one ethylenically unsaturated monomer as component (b).
Ethylenically unsaturated monomers which can be employed are basically those which are usually also employed for the production of photopolymerizable flexographic printing elements. The monomers should be compatible with the binders and have at least one polymerizable, ethylenically unsaturated double bond. Suitable monomers generally have a boiling point of above 100° C. at atmospheric pressure and a molecular weight of up to 3000 g/mol, preferably up to 2000 g/mol. Monomers which have proven particularly advantageous are esters or amides of acrylic acid or methacrylic acid with monofunctional or polyfunctional alcohols, amines, aminoalcohols or hydroxyethers and -esters, styrene or substituted styrenes, esters of fumaric or maleic acid or allyl compounds. Examples of suitable monomers are butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, 1,9-nonanediol diacrylate, trimethylolpropane triacrylate, dioctyl fumarate and N-dodecylmaleimide. It is also possible to employ mixtures of different monomers. In general, the total amount of the monomers is from 5 to 30% by weight, preferably from 5 to 20% by weight, based on the sum of components (a) to (d).
The precursor la
Hiller Margit
Schadebrodt Jens
Telser Thomas
Wenzl Wolfgang
BASF Drucksysteme GmbH
Gilliam Barbara
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