Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2001-03-23
2003-09-30
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S273100, C430S275100, C430S286100, C430S287100, C430S288100, C430S348000, C430S944000, C430S945000
Reexamination Certificate
active
06627385
ABSTRACT:
The present invention relates to the use of graft copolymers for the production of laser-engravable relief printing plates, the graft copolymers being obtained by free radical polymerization of vinyl esters in the presence of polyalkylene oxides and subsequent hydrolysis of the ester function. It furthermore relates to a process for the production of transparent flexographic printing plates by means of laser engraving using said graft copolymers, and to a process for the production of flexographic printing plates on metallic substrates by means of laser engraving using said graft copolymers.
The conventional method for the production of flexographic printing plates starting from unexposed photopolymerizable plates comprises a plurality of process steps, such as exposure of the back, imagewise exposure to actinic light, washout, drying, aftertreatment and subsequent drying at room temperature, and is overall a relatively time-consuming process. Depending on the thickness of the plate, usually up to 24 hours are required for the production of a ready-to-print flexographic printing plate from an unexposed photopolymer plate.
There has therefore been no lack of attempts to replace this time-consuming method by other methods, for example by direct laser engraving, in particular using IR lasers, for example CO
2
lasers or Nd—YAG lasers. Indentations are engraved with the aid of a sufficiently powerful laser directly in a plate suitable for this purpose, with the result that in principle a relief suitable for printing is formed. Direct laser engraving has in principle a number of further advantages. For example, the shape of the relief can be freely chosen. Whereas in photopolymer plates the sidewalls of a relief dot divert continuously from the surface to the relief base, the sidewall shape can be freely chosen in the case of laser-engraved plates. For example, a sidewall which descends perpendicularly or virtually perpendicularly in the upper region and broadens only in the lower region is usual. Consequently, there is at most a small increase in tonal value, if any at all, even with increasing wear of plate during the printing process. A further advantage is that the image information can be transferred in digital form directly from the layout computer to the laser apparatus, so that the production of a photographic mask for image production is superfluous. Further details of laser engraving methods are given, for example, in Technik des Flexodrucks, page 173 et. seq., 4th Edition, 1999, Coating Verlag, St. Gallen, Switzerland.
In practice, however, those skilled in the art are confronted by a number of problems in implementing the concept of direct laser engraving.
In direct laser engraving, large amounts of the material of which the printing relief consists have to be removed by the laser. A typical flexographic printing plate is, for example, from 0.5 to 7 mm thick and the nonprinting indentations on the plate are from 300 &mgr;m to 3 mm deep. On the apparatus side, sufficiently powerful lasers must therefore be available in order to be able to engrave as economically as possible. Moreover, the lasers must be very accurately focusable in order to ensure high resolution.
Furthermore, it is decisive for the cost efficiency of the process that the sensitivity of the material of which the printing relief consists to laser radiation is very high so that the material can be engraved rapidly.
The elastomeric binders typically used for the production of flexographic printing plates, for example SIS or SBS block copolymers, are in principle sensitive to laser radiation. Such binder-containing recording elements for production of flexographic printing plates by laser engraving are disclosed, for example, in EP-A 640 043 and EP-A 640 044. However, the sensitivity to laser radiation is only moderate. There is therefore still a need to provide binders having higher sensitivity to laser radiation.
It has therefore also been proposed to add to the relief layers materials which absorb laser radiation, in order to increase the sensitivity to laser radiation, for example in DE-A 196 25 749, EP-A 710 573 or EP-A 640 043. In particular, carbon black has been proposed as an absorbing material. Here, however, it should be noted that the laser-engravable layer must also have the performance characteristics important for relief printing plates, for example resilience, hardness, roughness, ink acceptance or low swellability in printing inks, which might be adversely effected by fillers. The optimization of the material with respect to optimum engravability by lasers by the addition of absorbing materials is therefore subject to limits. Moreover, fillers cause conventional, photopolymer, flexographic printing plates to lose their transparency, which complicates mounting with accurate register, since register crosses or similar marks are no longer visible through the plate. Special mounting apparatuses have to be used for filler-containing plates.
Furthermore, opaque plates filled with carbon black or similarly highly absorbing material can no longer be crosslinked by means of photopolymerization, or at most only in the case of very small layer thicknesses. However, this is associated with two serious disadvantages: on the one hand, those skilled in the art have wide knowledge of the relationship between production parameters and properties of the resulting printing plates concerning precisely the production of flexographic printing plates by means of photopolymerization, which knowledge can now no longer be utilized. On the other hand, when thermoplastic elastomers are used, photopolymer plates can be produced in an elegant manner by extrusion and calendering at elevated temperatures using thermally stable photoinitiators. This production method is at least more difficult in the case of thermal crosslinking.
It is therefore entirely desirable to use suitable elements without fillers, for the production of flexographic printing plates by laser engraving.
Particularly important with respect to the quality of the printing relief obtained by laser engraving is that the material be converted directly into the gas phase, as far as possible without prior melting, on exposure to laser radiation. If this is not the case, fused edges form around the indentations on the plate. Such fused edges lead to a considerable deterioration in the printed image and reduce the resolution of the printing plate and of the printed image. It is precisely the flexographic recording element comprising typical elastomeric binders, for example SIS or SBS block copolymers, which have a strong tendency, with or without the addition of laser-absorbing materials, to form fused edges.
To solve this problem, U.S. Pat. No. 5,259,311 has proposed that, after the laser engraving, the plate obtained be subsequently cleaned with solvents and then dried again. This involves the use of apparatuses and washout media which are usually envisaged for the development of exposed flexographic printing plates. Although fused edges can be removed by the aftertreatment described and improved flexographic printing plates can be obtained, the abovementioned time advantage of laser engraving compared with the conventional production of the plate is substantially lost.
In addition to block copolymers, SIS or SBS rubbers, in photopolymerizable flexographic printing plates developable in organic media, the use of polyvinyl alcohols or polyvinyl alcohol derivatives for the production of photopolymer relief printing plates developable in aqueous media is also known. The laser engraving of the relief printing plates comprising such polymers is also known. DE-A 198 38 315 discloses a laser-engravable recording element which contains polyvinyl alcohol or polyvinyl alcohol derivatives in the relief layer. Furthermore, the recording elements disclosed therein contain particulate, polymeric fillers having a low ceiling temperature, i.e. fillers depolymerizable at comparatively low temperatures, for improving the sensitivity to lasers. Although polyvinyl alcohols can b
Hiller Margit
Leinenbach Alfred
Stebani Uwe
Wenzl Wolfgang
BASF Drucksysteme GmbH
Baxter Janet
Gilliam Barbara
Keil & Weinkauf
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