Printing – Printing members – Blanks and processes
Reexamination Certificate
2002-12-04
2004-08-17
Funk, Stephen R. (Department: 2854)
Printing
Printing members
Blanks and processes
C101S395000, C101S401000, C430S306000, C430S327000
Reexamination Certificate
active
06776095
ABSTRACT:
The present invention relates to a process for the production of flexographic printing plates by laser engraving in which the recording layer of a crosslinkable, laser-engravable flexographic printing element is crosslinked by the combination of a full-area crosslinking step with a crosslinking step acting only at the surface, and a printing relief is engraved into the crosslinked recording layer by means of a laser. The present invention furthermore relates to flexographic printing plates which can be produced by the process.
TECHNICAL FIELD
In the technique of laser direct engraving for the production of relief printing plates, for example flexographic printing plates, a relief which is suitable for printing is engraved directly into a relief layer which is suitable for this purpose. With the appearance of improved laser systems, this technique is increasingly also attracting commercial interest.
BACKGROUND OF THE INVENTION
For the production of flexographic printing plates by laser engraving, it is in principle possible to employ commercially available photopolymerizable flexographic printing elements. U.S. Pat. No. 5,259,311 discloses a process in which, in a first step, the flexographic printing element is photochemically crosslinked by full-area irradiation and, in a second step, a printing relief is engraved by means of a laser.
EP-A 640 043 and EP-A 640 044 disclose single-layer or multilayer elastomeric laser-engravable recording elements for the production of flexographic printing plates. The elements consist of “reinforced” elastomeric layers. For the production of the layer, use is made of elastomeric binders, in particular thermoplastic elastomers, for example SBS, SIS or SEBS block copolymers. In addition, the layer may comprise IR radiation-absorbent, generally strongly colored substances. The so-called reinforcement increases the mechanical strength of the layer. The reinforcement is achieved either by means of fillers, photochemical or thermochemical crosslinking, or combinations thereof.
EP-B 640 043 also discloses, on page 8, lines 52-59, various techniques for removing surface tackiness of reinforced laser-engravable flexographic printing elements, including exposure to UV-C light or treatment with bromine or chlorine solutions. The irradiation can be carried out before or after the laser engraving of the printing relief. As shown in the cited specification, treatment of this type for removing surface tackiness does not, however, represent further photochemical or thermochemical crosslinking of the relief layer.
The relief layers of laser-engravable flexographic printing elements should in the ideal case not melt during the laser engraving, but instead a direct transition of the degradation products into the gas phase should if possible take place. Melting of the layer may result in formation of melt borders around the printing elements, and the edges of the relief elements become less sharp. Flexographic printing plates having irregularities of this type give prints of worse quality than with printing plates without such defects.
The comparatively soft relief layers of flexographic printing plates, in particular those having thermoplastic elastomers as binders, tend to form melt borders during laser engraving.
Although this problem can generally be at least greatly reduced and in some cases even avoided by using very large amounts of IR absorbers, such as carbon black, in the order of from 30 to 50% by weight of all constituents of the layer, excessively high contents of IR absorber are, however, disadvantageous since the laser-engravable layer should not only be as sensitive as possible to laser radiation, but must also achieve the mechanical and printing performance features of conventionally produced flexographic printing plates. Excessively high absorber contents result, for example, in an impairment in important properties, such as elasticity, flexibility, cliche hardness and ink transfer behavior of the finished flexographic printing plate. In addition, the edges of the relief elements tend to fray if the IR absorber contents are too high.
Furthermore, it is in certain cases also extremely attractive to omit the addition of IR absorbers completely. Although the sensitivity of conventional thermoplastic-elastomeric binders to the radiation of Nd:YAG lasers is poor, the sensitivity to CO
2
is at least sufficiently good that commercially available photopolymeric flexographic printing elements that have been exposed to actinic light over the entire area can in principle be engraved by means of CO
2
lasers even without the need to add additional IR absorbers, as disclosed, for example, in U.S. Pat. No. 5,259,311. Although the engraving rate by CO
2
lasers is not always ideal without additional absorbers, the omission of strongly colored absorbers has the advantage that laser-engravable flexographic printing elements can be produced in the conventional manner by photopolymerization, and the person skilled in the art can continue to utilize his entire knowledge on the formulation of photopolymerizable recording layers for flexographic printing, the structure-property relationships and production technology.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for the production of flexographic printing plates by laser engraving by means of which the occurrence of melt borders can be prevented in a simple and straightforward manner without mechanical or printing performance features being impaired compared with those of conventional flexographic printing plates. In particular, it should be possible to use the process for transparent flexographic printing elements which contain no colored absorbers for laser radiation.
We have found that this object is achieved by a process for the production of flexographic printing plates by laser engraving in which the recording layer of a laser-engravable flexographic printing element is crosslinked by the combination of a full-area crosslinking step with a crosslinking step acting only at the surface, and a printing relief is engraved into the crosslinked recording layer by means of a laser. In a further aspect, we have found flexographic printing plates which can be produced by the process.
In a particular embodiment of the process according to the invention, the crosslinking step acting only on the surface is carried out through the action of UV-C radiation according to certain boundary conditions.
Surprisingly, it has been found that the novel combination of two different crosslinking steps significantly improves the quality of the resultant print relief compared with a printing relief which has been crosslinked only once. In particular, melt borders which impair the print appearance are almost completely prevented without the mechanical properties of the print relief, such as hardness, flexibility or rebound resilience, being impaired. This effect is evident in a particularly positive manner in the case of flexographic printing elements without absorbers for laser radiation.
The following details apply to the invention:
The term “laser-engravable” is taken to mean that the relief layer has the property of absorbing laser radiation, in particular the radiation from an IR laser, so that it is removed or at least delaminated at the points at which it is exposed to a laser beam of sufficient intensity. The layer is preferably evaporated or decomposed thermally or oxidatively in advance without melting, so that its decomposition products in the form of hot gases, vapors, fumes or small particles, can be removed from the layer.
Examples of suitable dimensionally stable supports for the crosslinkable, laser-engravable flexographic printing element employed as starting material are plates, films and conical and cylindrical tubes (sleeves) made from metals such as steel, aluminum, copper or nickel or plastics, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate, polyamide, polycarbonate, optionally also woven and nonwoven fabrics, such as w
Hiller Margit
Kaczun Jürgen
Schadebrodt Jens
Telser Thomas
BASF Drucksysteme GmbH
Funk Stephen R.
Keil & Weinkauf
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