EP1136254A2 - Use of graft copolymers for the manufacture of relief laser-markable elements - Google Patents

Abstract

Graft copolymers (I) produced by radical polymerization of vinyl esters in the presence of polyalkylene oxides followed by at least partial hydrolysis of the ester functions are used to produce laser-engravable relief printing plates. An Independent claim is also included for a process for producing flexographic printing plates, comprising laser engraving of a relief in a laser-engravable element comprising a crosslinked relief layer comprising (I) and optionally an infrared absorber on a dimensionally stable transparent or metal support.

Description

The invention relates to the use of graft copolymers Production of laser-engraved relief printing forms, the Graft copolymers by radical polymerization of vinyl esters in the presence of polyalkylene oxides and subsequent saponification the ester function can be obtained. It continues to affect a process for the production of transparent flexographic printing plates by means of laser engraving using said graft copolymers, and a method for producing flexographic printing plates on metallic Carriers using laser engraving using said Graft copolymer.

The conventional technique for the production of flexographic printing plates starting from photopolymerizable raw panels comprises several Process steps such as back exposure, imagewise exposure with actinic light, washing out, drying, post-treatment as well as drying at room temperature and is overall a relative time-consuming process. Depending on the thickness of the plate, pass usually up to 24 hours to get from a raw photopolymer plate to produce a print-ready flexographic printing plate.

There has been no shortage of attempts to replace this time-consuming technique with other techniques, for example by laser direct engraving, in particular using IR lasers such as CO ₂ lasers or Nd-YAG lasers. With the help of a sufficiently powerful laser, indentations are engraved directly into a suitable plate, which in principle forms a relief suitable for printing. In principle, direct laser engraving has a number of other advantages. The shape of the relief can be chosen freely. While the flanks of a relief point in photopolymer plates continuously diverge from the surface to the relief base, the flank shape can be freely selected for laser-engraved plates. For example, a flank that slopes vertically or almost vertically in the upper area and that only widens in the lower area is common. Thus, with increasing wear of the plate during the printing process, there is no or at most a slight increase in tone value. Another advantage is that the image information can be transmitted in digital form directly from the layout computer to the laser apparatus, so that the production of a photographic mask for imaging is unnecessary. Further details on the technique of laser engraving are shown, for example, in "Technique of Flexographic Printing", p. 173 ff., 4th ed., 1999, Coating Verlag, St. Gallen, Switzerland.

In practice, the specialist sees himself in the implementation of the concept laser direct engraving, however, has a number of problems across from.

With laser direct engraving, large amounts of the material must be made out where the printing relief exists, can be removed by the laser. A typical flexographic printing plate is between, for example 0.5 and 7 mm thick and the non-printing depressions in the The plate is between 300 µm and 3 mm deep. On the apparatus Sufficiently powerful lasers must therefore be available stand in order to be able to engrave as economically as possible. Moreover the lasers must be able to be focused as well as possible in order to to ensure high resolution.

For the economy of the process, it is still crucial that the sensitivity of the material from which the printing is made Relief exists, as high as possible compared to laser radiation is so that the material can be engraved quickly.

Typically for the production of flexographic Printing plates used elastomeric binders such as SIS or SBS block copolymers are in principle sensitive to them Laser radiation. Recording elements containing such binders for the production of flexographic printing plates by laser engraving are disclosed, for example, by EP-A 640 043 and EP-A 640 044. The sensitivity to laser radiation is only moderate. There is therefore still a need for higher binders To provide sensitivity to laser radiation.

It has therefore also been proposed that the relief layers Laser absorbing materials add to the sensitivity to increase compared to laser radiation, for example DE-A 196 25 749, EP-A 710 573 or EP-A 640 043. As absorbent Carbon black in particular has been proposed for materials. Here However, it should be noted that the laser-engravable layer also important application properties for relief printing plates such as For example, elasticity, hardness, roughness, color acceptance or low Must show swellability in printing inks due to fillers possibly could be adversely affected. Optimizing the Material with a view to optimal engraving by laser limits are set by adding absorbent materials. Common photopolymer flexographic printing plates also lose through these fillers their transparency, what the exact registration Assembly difficult because of assembly crosses or similar markings are no longer visible through the plate. For filled Plates must use special assembly equipment.

You can also use carbon black or similarly strongly absorbent materials filled, opaque plates no longer, or at most only crosslinked by means of photopolymerization for very thin layers become. However, this has two serious disadvantages linked: On the one hand, the specialist owns just for the production a wide range of flexographic printing plates using photopolymerization Knowledge of the relationship between manufacturing parameters and properties of the received printing plates, which are no longer used can be. On the other hand, photopolymer plates can Use of thermoplastic elastomers in an elegant way Way by extrusion and calendering at elevated temperatures made using thermally stable photoinitiators become. This manufacturing route is with thermal crosslinking at least more difficult.

It is therefore entirely desirable to manufacture flexographic printing plates Suitable elements without fillers thanks to laser engraving to use.

Essential for the quality of what is obtained by laser engraving Relief is mainly that the material is laser irradiated if possible without melting directly into the Gas phase passes. If this is not the case, melted edges can appear around the recesses in the plate. Such Melting edges lead to a considerable deterioration of the Print image and reduce the resolution of the printing plate and Printed image. Especially flexographic recording elements with typical elastomeric binders such as SIS or SBS block copolymers tend - with or without the addition of laser-absorbing Materials - strong for the formation of melting edges.

To solve this problem is proposed by US 5,259,311 after laser engraving the plate obtained with solvents to clean and then dry again. there Apparatus and detergents are used, which are usually intended for the development of exposed flexographic printing plates are. Although through the described aftertreatment enamel edges are removable and get improved flexographic printing plates the above-mentioned time advantage of laser engraving goes compared to the conventional production of the plate again largely lost.

In addition to block copolymers such as SIS or SBS rubbers in organically developable photopolymerizable flexographic printing plates, the use of polyvinyl alcohols or polyvinyl alcohol derivatives for the production of aqueous developable photopolymer relief printing plates is also known. Laser engraving of relief printing plates with 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 with a low ceiling temperature, ie fillers which can be depolymerized at comparatively low temperatures, in order to improve the sensitivity to lasers. Although polyvinyl alcohols can be engraved using CO ₂ lasers without the addition of fillers, the speed of laser engraving is only slow.

The object of the invention was to provide laser-engravable marking elements to provide a very high sensitivity to Have laser radiation, and which are without melting edges have it engraved with lasers.

Surprisingly, it has been found that certain graft copolymers very good for the production of laser-engravable recording elements can be used. Such recording elements both have a sensitivity above average Laser radiation on and are without creating melting edges laser-engravable.

Accordingly, the use of the above was Graft copolymers obtained by radical polymerization of Vinyl esters in the presence of polyalkylene oxides and subsequent, obtained at least partial saponification of the ester function can be used for the production of laser-engraved relief printing plates found, as well as a process for the production of transparent Flexographic printing plates using laser engraving using such graft copolymers.

In the production of the graft copolymers used according to the invention the polyalkylene oxides are preferably grafted. However, mechanisms other than grafting are also conceivable. Among the graft copolymers used in the present invention are both pure graft copolymers and mixtures of Graft copolymers with residues of ungrafted polyalkylene oxides and to understand at least partially saponified polyvinyl esters.

The graft copolymers used according to the invention are in one first reaction stage by polymerization of vinyl esters in Presence of polyalkylene oxides and an initiator for radical Polymerization produced. In a second reaction stage can at least the ester groups in the graft copolymer obtained partially saponified to form vinyl alcohol structural units. Such graft copolymers, their production and the properties are disclosed, for example, in EP-A 224 164 We explicitly refer to their statements at this point.

Polymers based on are particularly suitable as polyalkylene oxides Ethylene oxide, propylene oxide and butylene oxide as well as statistical Copolymers or block copolymers thereof in question. Preferably included the copolymers at least 50 mol% ethylene oxide. Especially polyethylene oxide is preferred. The OH end groups of the polyalkylene oxides can also be modified, for example esterified or be etherified. In addition to the straight-chain polyalkylene oxides branched can also be used. Branched polyalkylene oxides can be obtained by, for example, polyalcohols such as glycerol, ethylene oxide and / or other alkylene oxides attaches. Polyalkylene oxides can also be used still contain small amounts of other chain building blocks. Exemplary May be mentioned carbonate groups by the reaction of polyalkylene oxides are available with phosgene or urethane groups, which by reacting polyalkylene oxides with aliphatic or aromatic diisocyanates are available. The amount of such additional However, chain building blocks should usually 5 mol% with regard to the total amount of chain building blocks.

The molecular weights M _n (number average) of the polyalkylene oxides used are generally between 5000 and 100000 g / mol, preferably between 10000 g / mol and 50000 g / mol.

The vinyl esters of aliphatic C ₁ -C ₂₄ monocarboxylic acids may be mentioned in particular as the vinyl esters for the synthesis of the grafted-on side groups. Vinyl acetate and vinyl propionate are preferred, and vinyl acetate is particularly preferred.

In a special embodiment, in addition to the vinyl esters one or more additional ethylenically unsaturated monomers be used. In this way, the properties of the grafted-on side chains. The amount of these additional monomers should, however, 20 mol% with respect. do not exceed the total amount of monomers used. 0 to 5 mol% are preferred. Acidic monomers are exemplary such as acrylic acid or methacrylic acid and basic monomers such as Called vinylformamide or 1-vinylimidazole.

As initiators for radical polymerization, the the usual peroxo and / or azo compounds are used for this, for example dibenzoyl peroxide, t-butyl perbenzoate or azobisisobutyronitrile. The amounts of initiator or initiator mixtures used are between 0.01 and 10% by weight, preferred between 0.5 and 2% by weight, based on the vinyl ester or others Monomers.

The polymerization of the vinyl esters and optionally further monomers in the presence of polyalkylene oxides is advantageous at 50 to 150 ° C, preferably carried out at 80 to 120 ° C. You can after methods known to the person skilled in the art in solvents or in the absence be carried out by solvents. Particularly advantageous can polymerize in the absence of one Solvents are carried out in molten polyalkylene oxide. Suitable embodiments of the polymerization are in EP-A 224 164.

The amount of grafted vinyl ester and optionally more Monomers is generally 30 to 400 mol% based on the sum all monomeric units in the graft copolymer, preferably 30 to 80 mol%.

In the second reaction stage, the ester groups in the obtained graft copolymer in a known manner at least partially saponified to form vinyl alcohol structural units. For this reaction step can be, for example, sodium hydroxide solution or potassium hydroxide solution be used. It is also possible to use the carboxyl groups by transesterification, for example with a methanolic NaOH solution being vinyl alcohol groups and methyl acetate arise.

The degree of saponification is determined by the person skilled in the art depending on the desired properties of the polymer chosen. As a rule, however, at least 50 mol% of the vinyl ester structural units in the graft copolymer saponified, preferably at least 65 mol%. Especially the degree of saponification is preferably 80 to 98%.

Optionally, in a further process step by saponification the vinyl alcohol groups obtained with the ester function Compounds are implemented that contain olefinic groups. In this way, graft copolymers are created which contain additional contain pendant polymerizable groups. The implementation can in a known manner with esters, chlorides or preferably anhydrides of olefinically unsaturated carboxylic acids, such as, for example, acrylic acid, methacrylic acid or maleic acid become. For implementation, see, for example, EP-A 129 901 referenced. If present, there is an olefinic content Side groups of about 2 to 20 mol% based on the total amount the vinyl ester or vinyl alcohol units advantageous.

The properties of the graft copolymers used according to the invention can for example by selecting the type and Amount of additional ethylenically unsaturated monomers or changed by said additional functionalization, and on be adapted to the intended use in each case. So For example, graft copolymers can also be obtained have elastomeric properties. In the case of the invention Using the graft copolymers, the graft copolymers in laser-engravable elements for the production of relief printing forms, such as letterpress, flexographic or gravure forms, in particular of flexographic printing forms and especially of transparent ones Flexographic printing forms or flexographic printing forms on metallic supports used.

The laser-engravable elements include a laser-engravable one Layer with an adhesive layer on one if necessary Dimensionally stable carrier applied. Examples of more suitable Dimensionally stable supports are plates, foils as well as conical and cylindrical sleeves made of metals such as steel, aluminum, Copper or nickel or from plastics such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate, Polyamide, polycarbonate, optionally also fabric and nonwovens, such as glass fiber fabrics and composite materials Glass fibers and plastics.

As a dimensionally stable carrier - especially for transparent flexographic printing forms - come especially dimensionally stable carrier films such as for example polyester films, in particular PET or PEN films in question.

Flexible metallic supports are particularly advantageous. Under flexible in the sense of this invention should be understood that the carriers are so thin that they are bent around the impression cylinder can. On the other hand, they are also dimensionally stable and such thick that the carrier in the production of the laser-engravable Elementes or the assembly of the finished printing plate on the Printing cylinder is not kinked.

Above all, thin sheets come as flexible metallic supports or metal foils made of steel, preferably made of stainless steel, magnetizable spring steel, aluminum, zinc, magnesium, nickel, Chromium or copper into consideration, whereby the metals are also alloyed could be. Combined metallic supports can also be used such as with tin, zinc, chrome, aluminum, nickel or also combination of different metals coated steel sheets are used, or also such metal supports, which are produced by lamination metal sheets of the same or different types are obtained. Pre-treated sheets, such as, for example, can also be used phosphated or chromated steel sheets or anodized Aluminum sheets are used. As a rule, will degrease the sheets or foils before inserting them. Prefers Carriers made of steel or aluminum are particularly preferred is magnetizable spring steel.

The thickness of such flexible metallic supports is usually between 0.025 mm and 0.4 mm and aligns next to the desired one Degree of flexibility also depending on the type of used Metal. Steel beams usually have a thickness between 0.025 and 0.25 mm, in particular between 0.14 and 0.24 mm. Aluminum supports usually have a thickness between 0.25 and 0.4 mm.

The term "laser-engravable" means that the Relief layer has the property of laser radiation, in particular to absorb the radiation of an IR laser, so that them in places where they are more adequate to a laser beam Intensity is exposed, removed or at least detached becomes. The layer is preferably closed beforehand Melt evaporates or decomposes thermally or oxidatively, see above that their decomposition products in the form of hot gases, vapors, Smoke or small particles can be removed from the layer. The term "transparent" should be understood to mean that the relief layer of the laser-engravable element as well as conventional photopolymerizable Is largely transparent, i.e. that structures underneath with the bare Eye can be recognized. This does not rule out that Plate can be colored to a certain extent. It is on This point expressly points out that even a laser-engravable Element on metallic support in this sense can be transparent, i.e. have a transparent relief layer can, although such a laser-engravable element is of course not transparent as a whole.

The laser-engravable elements can also be several, one above the other arranged laser-engravable layers of different Have composition. Contains at least one of the layers at least one of said graft copolymers. It can too Mixtures of different graft copolymers are used. It however, it is preferred that each of the layers has at least one or contains more of said graft copolymers.

The laser-engravable layer can also be used for other different from the graft copolymers used according to the invention contain polymeric binders. Such additional Binding agents can be used, for example, to control the properties the layer can be used. Requirement for the Addition of other binders is that they are with the graft copolymer are tolerated. For example, other polyvinyl alcohols are suitable or polyvinyl alcohol derivatives or water-soluble Polyamides. The amount will be determined by the expert depending on the desired Properties of the layer selected. Here is particularly to note that with an additional binder the speed laser engraving not, at least not in excess should be reduced. As a rule, therefore, should not more than 20% by weight based on the total amount of binders used, preferably no more than 10% by weight of such additional ones Binders are used.

The laser-engravable layers are preferably cross-linked. The networking the laser-engravable layer can be replaced by a chemical Reaction, e.g. radical or ionic polymerization, by polycondensation or polyaddition, depending on Suitable crosslinkers are added to the crosslinking reaction. she can also be carried out using an ion beam. Prefers crosslinking takes place through photo-chemically initiated Polymerization.

On the one hand, the crosslinking can be polymerized without the addition of further Connections are made when the graft copolymers described above used with olefinically polymerizable groups become.

However, the graft copolymers are preferred in a mixture with polymerizable, ethylenically unsaturated, compatible with the binder Connections used. There can only be one Monomer can be used, or several can be mixed with one another be used. Suitable compatible monomers are for example mono- and di (meth) acrylates of di- or polyalcohols, such as ethylene glycol, di-, tri-, tetra- or polyethylene glycols. Examples include ethylene glycol monoacrylate, ethylene glycol dimethacrylate or called methyl polyethylene glycol monoacrylate. The amount of monomers admixed can depend on the person skilled in the art according to the desired application properties such as for example hardness and elasticity of the layer can be selected. If graft copolymers with olefinic side groups are used, as a rule, no more than 15% by weight of additional Monomers required. Are graft copolymers without olefinic side groups are used, so larger quantities, in general, however, not more than 50% by weight and preferably 15 to 45% by weight.

Can be used as initiators for thermal polymerization for example, typical peroxides or hydroperoxides are used become. The thermal crosslinking is usually done by heating of the laser-engravable element.

Can be used as initiators for the photochemical polymerization for example in a known manner acyloins and their derivatives such as For example, benzoin or vicinal diketones such as, for example, benzil become. The photopolymerization can be carried out in a known manner be triggered by actinic light.

Furthermore, the laser-engravable recording layer can also still include auxiliaries and additives. Examples of such Additives are dyes, color pigments, plasticizers, Dispersing agents or adhesion promoters. As a plasticizer for Use with the graft copolymers used according to the invention Glycerol or polyethylene glycols, for example, are particularly suitable.

Although the use of the graft copolymers according to the invention gives transparent, laser-engravable recording elements which have excellent sensitivity to laser radiation and which can also be used for the production of relief printing plates without the addition of laser radiation-absorbing additives, and furthermore no such additives are the preferred embodiment of the Invention is, the present invention also includes the use of such additives. For example, aluminum oxide or hydrated oxide, iron oxides or soot can be used. As a result, the plate loses its transparency and becomes opaque. The easily depolymerizable polymer particles described above, for example made of polymethyl methacrylate (for example Agfaperl®), can also be used. In addition, fillers that serve other purposes can also be used. Fine SiO ₂ particles (for example Aerosil®, from Degussa) to influence the relief properties could be mentioned here as examples. The latter have a particle size that is smaller than the wavelength of visible light, so that the plate remains transparent when the filler is adequately dispersed.

The thickness of the laser-engravable recording layer or all Recording layers together is usually between 0.1 and 7 mm. The thickness is determined by the expert depending on the desired Appropriately chosen for the intended use of the printing plate.

Optionally, the recording element according to the invention can also be a thin top layer on the laser-engravable recording layer include. Such an upper layer can affect the printing behavior and color transfer essential parameters such as roughness, Abrasiveness, surface tension, surface stickiness or Solvent resistance on the surface can be changed, without the relief-typical properties of the printing form such as for example, to influence hardness or elasticity. Surface properties and layer properties can therefore be independent be changed from one another to achieve an optimal printing result to reach. The composition of the upper class is only insofar limited than the laser engraving of those below laser-engravable layer must not be impaired and the upper layer must be removable together with this. The Top layer should be thin compared to the laser-engravable layer his. As a rule, the thickness of the top layer does not exceed 100 µm, preferably the thickness is between 1 and 80 µm, particularly preferably between 3 and 10 microns. The upper class should be preferred be easily laser-engravable, and includes as a polymeric binder therefore preferably also one used according to the invention Graft copolymer. In particular, such can be advantageous here Graft copolymers are used whose side chains through Copolymerization of vinyl esters with other monomers targeted were modified, for example by the color acceptance of the plate to improve. In addition, other polymeric binders as well Auxiliaries used to set the desired properties become.

Optionally, the laser-engravable element can also have an underlayer include that between the carrier and the laser-engravable Layer. The lower layer can be laser-engravable; she but can also not be laser-engravable. With such lower layers can change the mechanical properties of the relief printing plates be changed without the characteristics typical of the relief to influence the printing form.

Furthermore, the laser-engravable recording element can be optional against mechanical damage from, for example Protective film made of PET, which is located on the each top layer, each before engraving must be removed with lasers.

The laser-engravable elements can be removed by loosening the components in suitable solvents and pouring onto the carrier followed by evaporating the solvent. Several Layers can be poured on top of each other.

You can continue, for example, by mixing in suitable ones Kneading or extruding, followed by extrusion and calendering, are produced at elevated temperatures. The latter method is used particularly advantageously in photopolymerizable systems.

Especially when using metallic supports proven, the laser-engravable layer on a temporary Pouring and drying supports, for example onto a PET film, and then in a second step the dried, laser-engravable layer with the layer facing away from the temporary support Laminated side on the flexible metallic carrier.

An optionally available upper layer can either be known per se Way to be infused or laminated, or it can be co-extruded simultaneously with the laser-engravable one Layer.

The photochemical crosslinking can advantageously be carried out immediately after shaping the laser-engravable printing form by irradiation done with actinic light. But it is also possible that Only carry out networking at a later point in time. The Exposure can only be from one side or from both sides.

The thermal crosslinking takes place by heating the laser-engravable Element.

Those produced using graft copolymers according to the invention Laser-engravable elements serve as the starting material for the production of relief printing plates. The process includes that the cover film - if present - is first removed. in the following process step is a printing relief in the Recording material engraved with a laser. Advantageous picture elements are engraved in which the flanks of the Image elements initially drop vertically and only in the lower one Widen the area of the picture element. This will be a good one Socking of the pixels with a slight increase in tone value reached. However, flanks of different design can also be used Pixels are engraved.

Particularly suitable for laser engraving are CO ₂ lasers with a wavelength of 10640 nm, but also Nd-YAG lasers (1064 nm) and IR diode lasers or solid-state lasers, which typically have wavelengths between 700 and 900 nm and between 1200 and 1600 nm . However, lasers with shorter wavelengths can also be used, provided the laser is of sufficient intensity. For example, a frequency-doubled (532 nm) or frequency-tripled (355 nm) Nd-YAG laser can also be used, or excimer laser (eg 248 nm). The image information to be engraved is transferred directly from the lay-out computer system to the laser apparatus. The lasers can be operated either continuously or in pulsed mode.

The method according to the invention has the great advantage that the Relief layer is removed very completely by the laser, so that intensive post-cleaning is usually not necessary is. If desired, the printing plate obtained can still be cleaned. Through such a cleaning step detached, but possibly not completely from the Removed layer components removed from the plate surface. As a rule simply hosing down with water is sufficient.

Those produced using graft copolymers according to the invention Recording elements are characterized by an extraordinary high sensitivity to laser radiation. she can be considerably faster than conventional, SIS or Engrave flexographic printing plates containing SBS block copolymers with lasers. Alternatively, consider the same speed the engravings get deeper reliefs.

The following examples are intended to explain the invention in more detail, without restricting their scope.

example 1

A mixture of the following components was prepared in water / n-propanol (volume ratio 6: 4): Feedstock source Part by weight [%] Graft copolymer approx. 70000 g / mol, based on polyethylene glycol 35000 g / mol, 42 mol% of vinyl alcohol / ester groups, degree of saponification 97% Alcotex 975 (Harco Chemical) 36 Graft copolymer approx. 62000 g / mol, based on polyethylene glycol approx. 25000 g / mol, 75 mol% proportion of vinyl alcohol / ester groups, degree of saponification 86% PVAL 486 (BASF AG) 9 Phenylglycidyl ether acrylate (monomer) Laromer LR 8830 (BASF AG) 43.25 Glycerin (plasticizer) 10 Thermal polymerization inhibitor Kerobit TBK (BASF AG) 0.5 Photoinitiator Irgacure 651 (Ciba) 1.2 dye Brilliant Blue R 0.05

After reaching a homogeneous solution, it was degassed and using a chamber caster on a PET film (Lumirror X 43, 150 µm). The wet application was chosen so that after drying (2 hours at 80 ° C, circulating air) a dry layer thickness of 950 µm was present. The photopolymer layer was by lamination with a 190 µm thick, transparent PET carrier film provided which with an adhesive varnish as in DE 3045516 described. Through double-sided radiation with actinic light (λ = 360 nm, UVA lamps from Philipps, TL10 (60W) became the photoactive mixture within one minute polymerized. It was colored blue, but still clear, Obtained transparent laser-engravable element.

Engraving of the laser-engravable element using a CO ₂ laser

The manufactured, laser-engravable plate was glued to the cylinder of an ALE laser machine (type Meridian Finesse) using an adhesive tape and the PET protective film was removed. This machine is equipped with a CO ₂ laser with an output of 200 W. After adjusting the focus to the plate thickness, the plate was exposed to the laser radiation at a rotation speed of 266 rpm and a feed of 20 μm. A test motif consisting of solid areas and various grid elements the size of a DIN A4 page was engraved within 30 minutes. The depth of the relief obtained was 800 µm. The resolution was 60 lines / cm (determined by counting the number under a microscope).

Example 2

Production of the laser-engravable element by extrusion with a twin-screw extruder (ZSK 53). The following mixture was used for extrusion. Feedstock source Part by weight [%] Graft copolymer approx. 70000 g / mol, based on polyethylene glycol 35000 g / mol, 42 mol% of vinyl alcohol / ester groups, degree of saponification 97% Alcotex 975 (Harco Chemical) 36 Graft copolymer approx. 62000 g / mol, based on polyethylene glycol approx. 25000 g / mol, 75 mol% proportion of vinyl alcohol / ester groups, degree of saponification 86% Mowiol GE 4-86 (Clariant) 9 Phenylglycidyl ether acrylate (monomer) Laromer LR 8830 (BASF AG) 43.25 Glycerin (plasticizer) 10 Thermal polymerization inhibitor Kerobit TBK (BASF AG) 0.5 Photoinitiator Irgacure 651 (Ciba) 1.2 dye Basazol Red 71 P 0.05

The binder was pre-compounded with the glycerin. This Pre-compounding facilitates problem-free melting of the Binder already at temperatures from 120 to 150 ° and thus a gentle processing of the polymers. In the monomer photo initiator, inhibitor and dye was dissolved and added to the Melt incorporated. The homogeneous melt was broken down into one 100 ° C heated calender between cover and carrier film. The types described in Example 1 were used as films. The photopolymerization was carried out as described in Example 1 carried out. A plate with a total thickness of 2.84 mm received.

Engraving of the laser-engravable element using a CO ₂ laser

The plate thus produced was engraved in the same manner as described in Example 1 using a CO ₂ laser. The depth of the relief obtained was 800 µm. The resolution was 60 lines / cm.

Example 3

The photopolymeric layer on PET carrier obtained in Example 1 was by lamination with a flexible, with the adhesive varnish according Example 1 provided metallic carrier (aluminum, thickness, 0.25 mm). By irradiation with actinic light (λ = 360 nm, UVA lamps from Philipps, TL 10 (60 W)) from The photoactive mixture was polymerized on the top. It was a blue colored, but still clear, transparent Preserved laser-engravable element.

Engraving of the laser-engravable element using a CO ₂ laser

The PET film was removed and the laser-engravable element was engraved using a CO ₂ laser as described in Example 1.

It has a relief depth of 810 µm with a resolution of 60 lines / cm reached.

Comparative Example 1

A plate of a crosslinked, carbon black-filled natural rubber (rubber 85% by weight, carbon black 9.5% by weight, 5.5% by weight plasticizer and crosslinking agent) was engraved in the same manner as described in Example 1 using a CO ₂ laser. The depth of the relief obtained was 650 µm. The resolution was only 54 lines / cm. The engraved plate also had enamel edges around the depressions.

Comparative example 2:

In line with the teaching of DE-A 197 56 327, a laser-engravable element was produced from a high-temperature crosslinking two-component silicone rubber and engraved in the same manner as described in Example 1 using a CO ₂ laser. The depth of the relief obtained was 600 µm. The resolution was only 48 lines / cm. In addition, the edges of line elements were not sharp but frayed.

Engraving the laser-engravable elements using an excimer laser

Various laser-engravable elements were engraved with a UV laser at different energy densities. Laser parameters: 10 Hz = clock frequency, 100 pulses, variable energy density, λ = 248 nm. The results are shown in Table 3. The engraving depth for different materials is shown depending on the energy density of the excimer laser. material 3.5 J / cm ² *. 3.0 J (cm2 * 2.5 J / cm ² * 2.0 J / cm ² * example 1 185 190 180 165 Example 2 185 190 180 165 Ethylene / propylene / diene rubber + carbon black 105 103 102 100 Natural rubber and carbon black, material from Comparative Example 1 75 78 72 67 commercial photopolymerizable flexographic printing plate with styrene-diene block copolymer (nyloflex FAH) 82 78 75 65

The examples and comparative examples show that printing plates with excellent sensitivity to laser radiation are obtained using the graft copolymers according to the invention. The laser-engravable elements obtained can be easily engraved in the infrared using a CO ₂ laser or in the ultraviolet using an excimer laser.

At the same speed of the laser in the examples 1 and 2 in the engraving of the materials containing the graft copolymers receive higher relief depths than in the comparative examples. Compared to silicone rubber, they are also larger Preserve relief.

When engraving with a UV laser, they turn out to be Use of graft copolymers prepared according to the invention Elements as the easiest to engrave.

Claims (12)

Use of graft copolymers, obtainable by free radicals Polymerization of vinyl esters in the presence of polyalkylene oxides and subsequent, at least partial saponification the ester function of graft copolymers, for the preparation laser-engraved relief printing forms. Use of graft copolymers according to claim 1, characterized in that in addition to the vinyl esters, at least one further radically polymerizable monomer is used in the radical polymerization. Use of graft copolymers according to claim 1, characterized in that the hydroxyl groups obtained by saponification of the ester function are at least partially reacted with compounds containing olefinic groups. Use of graft copolymers according to one of claims 1 up to 3 for the production of laser-engravable flexographic printing plates. Method for producing transparent flexographic printing plates by engraving a printing relief into a laser-engravable element with the aid of a laser, the laser-engravable element comprising a cross-linked relief layer applied to a dimensionally stable support, characterized in that the relief layer comprises at least one graft copolymer according to one of Claims 1 to 3 includes. A process for the production of transparent flexographic printing plates according to claim 5, characterized in that the graft copolymer is an elastomeric graft copolymer. Method for producing transparent flexographic printing plates according to one of claims 5 or 6, characterized in that the cross-linked relief layer is obtained by photochemical cross-linking. Process for the production of transparent flexographic printing plates according to one of claims 5 or 6, characterized in that the cross-linked relief layer is obtained by thermochemical cross-linking. A method for producing transparent flexographic printing plates according to one of claims 5 to 8, characterized in that the laser-engravable element comprises an additional top layer on the cross-linked relief layer. A method for producing flexographic printing plates by engraving a printing relief into a laser-engravable element with the aid of a laser, the laser-engravable element comprising a cross-linked relief layer applied to a dimensionally stable support, characterized in that the relief layer comprises at least one graft copolymer according to one of Claims 1 to 3 and includes at least one IR absorber. A method for producing flexographic printing plates according to claim 10, characterized in that the laser-engravable element comprises an additional top layer on the cross-linked, elastomeric layer. A method for producing flexographic printing plates by engraving a printing relief into a laser-engravable element with the aid of a laser, the laser-engravable element comprising a cross-linked relief layer applied to a dimensionally stable support, characterized in that the relief layer comprises at least one graft copolymer according to one of Claims 1 to 3 comprises and that the dimensionally stable support is a metallic support.