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
2000-08-23
2003-01-28
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
C430S273100
Reexamination Certificate
active
06511784
ABSTRACT:
The present invention relates to a laser-engravable recording material for producing relief printing plates, in particular for producing flexographic printing plates, comprising a dimensionally stable support and a recording layer comprising silicone rubbers and inorganic ferrous solids and/or carbon black as absorbers for laser radiation. It further relates to a process for producing relief printing plates by laser engraving of such recording materials, and to relief printing plates having a printing relief comprising silicone rubbers and inorganic ferrous solids and/or carbon black.
Increasingly, the conventional technique for producing photopolymeric relief printing plates, flexographic printing plates or gravure printing plates by placing a photographic mask onto a photopolymeric recording element, exposing the element to actinic light through this mask, and washing off the unpolymerized areas of the exposed element with a developer fluid is being replaced by techniques employing lasers. In this context a distinction should be made between essentially two different techniques:
First, it is known to provide photopolymeric relief printing plates with laser-writable layers. These layers consist, for example, of a binder containing dispersed carbon black. By irradiation with a IR laser it is possible to ablate this layer and mark an image into the layer. The image information is transferred directly from the layout computer system to the laser apparatus. From the laser-ablatable layer, therefore, a mask is produced which adheres directly to the photopolymeric printing plate. There is no longer a need for a photographic negative.
Subsequently, the printing plate is exposed and developed conventionally, in the course of which the residues of the laser-writable layer are removed as well.
Secondly, in the case of direct laser engraving, depressions are engraved directly into an appropriate plate using a sufficiently powerful laser, in particular an IR laser, to form a relief suitable for printing. Subsequent photopolymerization and development of the plate are not necessary.
A key difference between the techniques depicted lies in the amount of material that must be removed. Whereas the abovementioned laser-writable layers are usually just a few &mgr;m thick, so that only small amounts of the materials of which the IR ablative layer is composed must be removed, it is necessary in the case of direct laser engraving to remove large amounts of the material of which the printing relief is composed. A typical flexographic printing plate, for example, is between 0.5 and 7 mm thick and the nonprinting depressions in the plate are between 300 &mgr;m and 3 mm deep.
An essential factor for the quality of the printing relief obtained by laser engraving is in particular that under laser irradiation the material passes directly into the gas phase with as far as possible no melting beforehand, since otherwise melt edges are formed around the depressions in the plate. Melt edges of this kind result in a considerable deterioration in the printed image and reduce the resolution of the printing plate and of the printed image.
For the economics of the process it is critical that the sensitivity of the recording material to laser radiation is as high as possible in order that the material can be laser-engraved extremely rapidly. In this context, however, it must be borne in mind that the laser-engravable layer is also required to have the performance properties that are important for relief printing plates, such as elasticity, hardness, roughness, ink acceptance, or low swellability in printing inks, for example. Optimizing the material in terms of laser engravability must certainly not result in any impairment in said performance properties.
Materials for producing relief printing plates by means of direct laser engraving are known in principle.
U.S. Pat. No. 3,549,733 discloses a polyoxymethylene or polychloral recording material for producing printing plates by means of laser engraving. Additionally, glass fibers or rutile can be used as fillers.
DE-A 196 25 749 discloses a seamless printing form (sleeve) for rotary flexographic printing, in which the elastomer layer is formed by a cold-curing silicone polymer or a silicone fluoropolymer, along with aluminum hydroxide as filler.
The sensitivity of the two systems to laser radiation, however, leaves something to be desired, with the consequence that imagewise engraving of the printing plate takes a long time.
EP-A 710 573 discloses a laser-engravable printing plate made from a polyurethane elastomer, nitrocellulose, and carbon black. The high levels of nonelastomeric nitrocellulose (from 25 to 45% by weight of the laser-sensitive layer), however, cause difficulties in the production of flexographic printing plates.
EP-A 640 043 and EP-A 640 044 disclose, respectively, single-layer and multilayer elastomeric laser-engravable elements for producing flexographic printing plates. The elements disclosed consist of “reinforced” elastomeric layers. Binders used are thermoplastic elastomers typical for flexographic printing plates, such as SBS, SIS or SEBS block copolymers, for example. The so-called reinforcement is achieved alternatively by means of fillers, photochemical crosslinking or thermochemical crosslinking, or combinations thereof. In addition, the layer may optionally include substances which absorb IR radiation. A preferred IR-absorbent material is carbon black, which at the same time also acts as filler. The engraving of elements with thermoplastic elastomers as binders using IR lasers, however, is often accompanied by the formation of melt edges, leading to defects in the printed image.
It is an object of the present invention to find an improved material for producing relief printing plates by means of laser engraving, which possesses an increased level of sensitivity to laser radiation and with which relief printing plates without melt edges can be produced.
We have found that this object is achieved by a laser-engravable recording material for producing relief printing plates, in particular for producing flexographic printing plates, comprising a dimensionally stable support and a recording layer comprising silicone rubbers and inorganic ferrous solids and/or carbon black as absorbers for laser radiation. we have also found a process for producing relief printing plates by engraving such recording materials using a laser, and relief printing plates having a printing relief comprising silicone rubbers and inorganic ferrous solids and/or carbon black as absorbers for laser radiation.
The recording material of the invention comprises a laser-engravable layer applied with or without an adhesion layer to a dimensionally stable support. Examples of suitable dimensionally stable supports are plates, films, and conical and cylindrical sleeves made from metals such as steel, aluminum, copper and nickel or from plastics such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate, polyamide and polycarbonate, and, if desired, also woven and nonwoven materials, such as glass fiber fabrics, and also composite materials of glass fibers and plastics. Particularly suitable dimensionally stable supports are dimensionally stable support films, examples being polyester films, especially PET or PEN films.
The term “laser-engravable” means that the layer possesses the property of absorbing laser radiation, especially the radiation of an IR laser, so that at those points where it is exposed to a laser beam of sufficient intensity it is removed, or at least detached. Preferably, the layer is vaporized without melting beforehand or is decomposed thermally or oxidatively, so that its decomposition products are removed from the layer in the form of hot gases, vapors, smoke, or small particles. However, the invention also embraces the subsequent mechanical removal of the residues of the irradiated layer by means, for example, of a jet of liquid or of gas, or else, for example, by suction.
The laser-engravable layer comprise
Faulhaber Heinz
Hiller Margit
Roos Roland
BASF Drucksysteme GmbH
Hamilton Cynthia
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