EP1343632B1 - Method for producing flexographic printing forms by means of laser gravure - Google Patents

Abstract

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 which only acts at the surface, and a printing relief is engraved into the crosslinked recording layer by means of a laser, and flexographic printing plates obtainable by the process.

Description

The present invention relates to a method for manufacturing of flexographic printing plates by means of laser engraving in which the recording layer a networkable, laser-engravable flexographic printing element by combining a full-surface crosslinking step with a superficial cross-linking step networked and a printing relief using a laser engraved in the cross-linked recording layer. The present The invention further relates to flexographic printing plates which the method can be produced.

In the technique of direct laser engraving for the production of relief printing plates such as flexographic printing forms becomes a Print a suitable relief directly into a suitable relief layer engraved. With the advent of improved laser systems this technology is increasingly gaining economic interest.

You can use laser engraving to produce flexographic printing plates in principle commercially available photopolymerizable flexographic printing elements be used. US 5,259,311 discloses a method in which, in a first step, the flexographic printing element is covered by full-surface Radiation cross-linked photochemically and in a second Step engraved a printing relief using a laser becomes.

EP-A 640 043 and EP-A 640 044 disclose single-layer and multi-layer, respectively elastomeric laser-engravable recording elements for Production of flexographic printing plates. The elements consist of "reinforced" elastomeric layers. To make the layer become elastomeric binders, especially thermoplastic elastomers such as SBS, SIS or SEBS block copolymers. In addition, the layer can absorb IR radiation, usually contain strongly colored substances. By the so-called reinforcement becomes the mechanical strength of the Layer increased. The reinforcement is either by fillers, photochemical or thermochemical crosslinking or combinations of which achieved.

EP-B 640 043 also discloses various on page 8, lines 52-59 Techniques for detackifying the surface of reinforced laser-engraved flexographic printing elements, including the exposure with UV-C light or treatment with bromine or chlorine solutions. Irradiation can take place before or after laser engraving printing reliefs. As in the above Scripture is shown, provides such treatment But no other photochemical or thermochemical detacking Networking of the relief layer.

The relief layers of laser-engravable flexographic printing elements ideally should not melt in the course of laser engraving, instead there should be a direct transition of the degradation products take place in the gas phase. By melting the layer Melting edges can form around printing elements and the Edges of the relief elements become blurred. With flexographic printing forms, which have such irregularities become prints poorer quality than with printing forms without such Disorders.

The comparatively soft relief layers of flexographic printing plates, especially those with thermoplastic elastomers as Binding agents tend to have melting edges in the course of laser engraving to build.

While this problem can usually be solved by using very high amounts of IR absorbers such as soot in the order of magnitude at least 30 to 50% by weight of all components of the layer greatly reduced and possibly even avoided. Excessively high levels of IR absorber are disadvantageous, however, because the laser-engravable Layer not only as sensitive to each other Laser radiation should be, but also the mechanical and printing performance features of conventionally produced Flexographic printing forms must achieve. If the absorber content is too high important properties such as elasticity, Flexibility, cliché hardness, and color transfer behavior of the finished flexographic printing plate deteriorated. They also tend Edges of the relief elements at high IR absorber contents to fray.

Furthermore, it is also attractive in certain cases to completely do without the addition of IR absorbers. The sensitivity of conventional thermoplastic elastomeric binders to the radiation from Nd-YAG lasers is poor, but the sensitivity to CO ₂ lasers is at least so good that commercially available photopolymer flexographic printing elements can in principle be engraved with CO ₂ lasers after full exposure to actinic light , also without additional IR absorbers having to be added, as disclosed, for example, by US Pat. No. 5,259,311. The speed of engraving with CO ₂ lasers is not always ideal without additional absorbers, but the absence of strongly colored absorbers has the advantage that laser-engravable flexographic printing elements can be produced in the usual way by means of photopolymerization, and the person skilled in the art has all his knowledge of can further use the formulation of photopolymerizable recording layers for flexographic printing, structure-property relationships and production technology.

The object of the invention was to provide a method for producing To provide flexographic printing plates by means of laser engraving, with which the appearance of enamel edges in a simple and convenient way can be avoided without mechanical or printing technology Features compared to those of conventional flexo plates be affected. In particular, the procedure should be: transparent flexographic printing elements that have no colored absorbers have for laser radiation, be applicable.

Accordingly, a method for making flexographic printing forms has been developed found by means of laser engraving, in which the recording layer a laser-engravable flexographic printing element by combining a full-surface crosslinking step cross-linked with a cross-linking step that only has a superficial effect and using a laser to print a relief into the network Engraved recording layer. In another aspect Flexographic printing forms found that can be produced by the process are.

In a special embodiment of the method according to the invention is the only superficial cross-linking step the effect of UV-C radiation in accordance with certain boundary conditions performed.

Surprisingly, it was found that the inventive Combination of two different networking steps Quality of the relief obtained compared to a relief which was only simply networked, significantly improved becomes. In particular, the print edges become annoying melting edges almost completely avoided without changing the mechanical properties the relief of pressure such as hardness, flexibility or resilience deteriorate. It is particularly positive this effect is noticeable with flexographic printing elements without absorber for Laser radiation.

The following is to be explained in detail regarding the invention:

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 it in places where they are more adequate to a laser beam Intensity is exposed, removed or at least replaced. The layer is preferably evaporated without melting beforehand or thermally or oxidatively decomposed so that their decomposition products in the form of hot gases, vapors, smoke or small particles are removed from the layer.

Examples of suitable dimensionally stable supports for the starting material Networked, laser-engravable flexographic printing element used are plates, foils as well as conical and cylindrical Tubes (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 fabrics and nonwovens, such as glass fiber fabrics as well as composite materials, e.g. made of glass fibers and Plastics. Above all, dimensionally stable carriers come as dimensionally stable Carrier films such as polyester films, especially 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 combinations of different metals coated steel sheets are used, or also such metal supports, which are produced by lamination identical or different types of metal sheets can be 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 used, particularly preferred is magnetizable spring steel.

The thickness of such flexible metallic supports is usually between 0.025 mm and 0.4 mm and depends on the desired degree of flexibility also depending on the type of used Metal. Steel beams are usually thick 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 starting material for the method further comprises at least a cross-linkable, laser-engravable recording layer, which directly or optionally via further layers on the Carrier is applied. The crosslinkable recording layer comprises at least one binder. You can support the Crosslinking include other components, such as polymerizable Monomers or oligomers, and / or compounds that Can trigger cross-linking reactions, such as initiators.

The recording layer is due to high energy radiation and / or thermally crosslinkable. Networking through high-energy Radiation can in particular be photochemically by means of short-wave visible or long-wave ultraviolet light. naturally but is also radiation of higher energy, such as short-wave UV light or X-rays, electron radiation or -be suitable sensitization - also longer-wave light in principle suitable. Thermal crosslinking takes place in particular through Heating, but can in principle also be carried out at room temperature become.

Elastomers are particularly suitable as binders for the layer Binder. In principle, however, it cannot be elastomeric Binders are used. The only decisive factor is that the crosslinkable recording layer after performing of the crosslinking step (a) has elastomeric properties. The recording layer can, for example, by adding Plasticizers assume or can have elastomeric properties crosslinkable oligomers are also used, which are only possible through the Reaction form an elastomeric network.

As an elastomeric binder for the laser-engravable layer particularly suitable polymers such as 1,3-diene monomers Polymerized isoprene or butadiene included. As examples be natural rubber, polyisoprene, styrene-butadiene rubber, nitrile-butadiene rubber, Butyl rubber, styrene-isoprene rubber, Polynorbornene rubber or ethylene-propylene-diene rubber (EPDM).

In principle, however, ethylene-propylene, ethylene-acrylic ester, Ethylene vinyl acetate or acrylate rubbers are used become. Hydrogenated rubbers or are also suitable elastomeric polyurethanes.

Modified binders can also be used which crosslinkable groups by grafting reactions in the polymer Molecule are introduced.

Particularly suitable as elastomeric binders are thermoplastic elastomeric block copolymers of alkenyl aromatics and 1,3-dienes. The block copolymers can be either linear Block copolymers or radial block copolymers. Usually they are three-block copolymers of the A-B-A type, but it can also be a two-block polymer of the A-B type act, or those with several alternating elastomers and thermoplastic blocks, e.g. A-B-A-B-A. Mixtures can also be used two or more different block copolymers be used. Commercially available three-block copolymers contain often certain proportions of two-block copolymers. The diene units can be 1,2- or 1,4-linked. You can also whole or be partially hydrogenated. Both block copolymers can be used styrene-butadiene and styrene-isoprene type. They are commercially available, for example, under the name Kraton®. Thermoplastic elastomers can also be used Block copolymers with styrene end blocks and a statistical Styrene-butadiene middle block, available under the name Styroflex® are.

The type and amount of binder used are determined by Specialist depending on the desired characteristics of the printing relief of the flexographic printing element selected. As a rule, has an amount of 50 to 95% by weight of the binder with respect to the Proven quantity of all components of the laser-engravable layer. Mixtures of different binders can also be used.

The crosslinkable, laser-engravable layer has crosslinkable groups on that thermally, photochemically or under the influence high-energy radiation, be it directly or by means of suitable Initiators, polymer networks can form. Networkable groups can be constituents of the elastomeric binder itself. There can be networkable groups in the main chain to terminal Act groups and / or side groups. Of course, an elastomeric binder can be crosslinked Groups both as a side group as well as terminal or in the Have main chain.

Furthermore, the laser-engravable recording layer can be monomeric or oligomeric compounds are added, each have crosslinkable groups.

The number and type of other components for networking the Layer depend on the desired networking technique and are selected accordingly by a specialist.

In the case of photochemical crosslinking, the recording layer comprises at least one photo initiator or photo initiator system. As initiators for photopolymerization are known in Way benzoin or benzoin derivatives, such as α-methylbenzoin or benzoin ethers, benzene derivatives, such as e.g. benzil, Acylarylphosphine oxides, acylarylphosphinic esters, multinuclear quinones suitable, but the list is not limited to this should. Those photoinitiators are preferably used which have a high absorption between 300 and 450 nm.

The polymeric binder has sufficient crosslinkable Groups, so is the addition of additional cross-linkable Monomers or oligomers are not required. As a rule, will for photochemical crosslinking but other polymerizable ones Compounds or monomers added. The monomers should be compatible with the binders and at least one have polymerizable, olefinically unsaturated group. As esters or amides of acrylic acid have been found to be particularly advantageous or methacrylic acid with mono- or polyfunctional alcohols, Amines, amino alcohols or hydroxy ethers 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, N-dodecyl maleimide. Suitable oligomers can also be used be used with olefinic groups. Of course can also be mixtures of different monomers or oligomers can be used, provided they are compatible with each other. The total amount of any monomers used is from Specialist depending on the desired properties of the recording layer established. As a rule, however, should refer to 30% by weight the amount of all components of the laser-engravable layer not be exceeded.

Thermal crosslinking can be analogous to photochemical Networking can be done by instead of a photo initiator a thermal polymerization initiator used becomes. In principle, commercially available polymerization initiators thermal initiators for radical polymerization are used, such as suitable peroxides, Hydroperoxides or azo compounds. As with the photochemical Depending on the type of binder, crosslinking can include additional monomers or oligomers are used.

The thermal crosslinking can also be carried out by a thermosetting resin such as the layer an epoxy resin, or by using binders that themselves have sufficient quantities of polymerizable groups, thermally crosslinked directly using suitable crosslinkers.

The networkable, laser-engravable flexographic printing element can continue comprise an absorber for laser radiation. It can too Mixtures of different absorbers used for laser radiation become. Suitable absorbers for laser radiation have a high Absorption in the range of the laser wavelength. In particular are Suitable absorbers that have a high absorption in the near infrared, as well as in the longer-wave VIS range of the electromagnetic spectrum exhibit. Such absorbers are particularly suitable for absorption the radiation from Nd-YAG lasers (1064 nm) and from IR diode lasers, which typically have wavelengths between 700 and 900 nm and between 1200 and 1600 nm.

Examples of suitable absorbers for the laser radiation are in infrared spectral range highly absorbent dyes like for example phthalocyanines, naphthalocyanines, cyanines, quinones, Metal complex dyes such as dithiolenes or photochromic dyes.

Other suitable absorbers are inorganic pigments, in particular intensely colored inorganic pigments such as Chromium oxides, iron oxides, soot or metallic particles.

Fine particles are particularly suitable as absorbers for laser radiation Types of carbon black with a particle size between 10 and 50 nm.

The amount of the optionally added absorber depends on the person skilled in the art according to the desired properties of the laser-engravable Recorded element selected. In this context, the Specialist take into account that the added absorbers are not only Speed and efficiency of the engraving of the elastomeric layer influenced by laser, but also other properties of the relief printing element obtained as the end product of the process, such as its hardness, elasticity, thermal conductivity or color transfer behavior. As a rule, it is recommended therefore, no more than 20% by weight, preferably no more than 10 % By weight and very particularly preferably not more than a maximum of 5% by weight. % of absorber for the laser radiation. For the procedure can, of course, also be laser-engraved in individual cases Elements with higher levels of absorber are used become.

As a rule, it is not recommended to use recording layers, that are to be photochemically crosslinked, absorbers for laser radiation add, which also absorb in the UV range, as a result the photopolymerization is severely impaired.

The laser-engravable layers according to the invention can furthermore also additives and auxiliaries such as Dyes, dispersing agents, antistatic agents, plasticizers or include abrasive particles. The amount of such additives should be as a rule, however, 10% by weight with respect to the amount of all components the crosslinkable, laser-engravable layer of the recording element do not exceed.

The crosslinkable, laser-engravable recording layer can also can be built up from several recording layers. These laser-engravable, cross-linkable sublayers can have the same, in approximately the same or different material composition his. Such a multilayer structure, especially a two-layer structure is sometimes advantageous because of it Surface properties and layer properties independent can be changed from one another to achieve an optimal printing result to reach. The laser-engravable recording element can have, for example, a thin laser-engravable top layer, their composition with a view to optimal color transfer was selected while the composition of the below lying layer with regard to optimal hardness or elasticity was selected.

The thickness of the crosslinkable, laser-engravable recording layer or all of the recording layers together is usually between 0.1 and 7 mm. The thickness is depending on the expert suitable for the intended use of the printing plate selected.

The cross-linkable, laser-engravable material used as the starting material Flexographic printing element can optionally comprise further layers.

Examples of such layers include an elastomeric underlayer from another wording that is between the Carrier and the laser-engravable layer (s) is located and which does not necessarily have to be laser-engravable. With Such sub-layers can have mechanical properties of the relief printing plates are changed without the properties to influence the actual printing relief layer.

So-called elastic substructures serve the same purpose itself under the dimensionally stable support of the laser-engravable Are recording element, so on the opposite Laser-engravable layer page. Elastic substructures or elastomeric lower layers can be crosslinkable and also in the In the course of the crosslinking step (a). You can but also be networked and with the other layers for example, be joined together by lamination.

Other examples include adhesive layers that the carrier with over it lying layers or different layers one below the other connect.

Furthermore, the laser-engravable flexographic printing element can counteract mechanical damage caused by, for example, PET Protective film can be protected on the respective top layer, and each before engraving with Lasers must be removed. The protective film can help of the stripping also siliconized or with a suitable Removing the adhesive layer.

The laser-engravable flexographic printing element can, for example, by Dissolve or disperse all components in a suitable solvent and pouring onto a carrier. at multilayer elements can in a known manner and Several layers are poured on top of each other. alternative For example, the individual layers can be placed on temporary supports poured and then the layers by lamination with each other get connected. In particular, photochemically crosslinkable systems can be made by extrusion and / or calendering become. In principle, this technology can also be used for thermally cross-linkable Systems are used, provided only such components are used that do not crosslink at the process temperature.

The cross-linkable, laser-engravable material used Flexographic printing element is in the first step (a) of the Process according to the invention cross-linked over the entire surface. Through this Crosslinking step, the entire volume of the layer is recorded.

Depending on the type of networking system chosen, the recording element do this with high-energy radiation, for example irradiated with UV-A radiation or with electron beams or the recording element is heated. The radiation or heating should be done as evenly as possible to avoid inhomogeneities to avoid in the degree of crosslinking of the layer if possible. Uniform irradiation can also be achieved in this way, for example by placing the layer on the one hand from the top and also from the bottom through the dimensionally stable support is irradiated through. Of course, this presupposes that the carrier is transparent to the respective radiation. Of course can also use both networking methods be combined. Although homogeneity is desirable, so the present invention does not exclude that crosslink density May have inhomogeneities. For example the crosslink density has a gradient.

It is essential for the method according to the invention that in In the course of said full-area networking in the course of the process step (a) not all groups which can in principle be crosslinked in implemented the layer to form a polymer network but that are not yet implemented, networkable groups remain in the crosslinked recording layer.

This incomplete implementation can be achieved, for example be by looking at the irradiation time or the duration of the warming so that the implementation is not yet complete is when the heating or radiation of the flexographic printing element is ended. It can also be limited, for example the amount of initiator done so that it is reached before complete sales of networkable groups are used up is.

The incomplete implementation can also be achieved by: uses a laser-engravable flexographic printing element, the layer of which has crosslinkable groups of different reactivity, and the reaction conditions are chosen so that in the course of the crosslinking reaction preferably only one type of crosslinkable group reacts, while the other type is not yet being implemented. The Recording layer can also, for example, both thermally as have photochemically crosslinkable groups and only thermally or just be photochemically cross-linked, making some kind of groups remains.

Of course, the methods can also be combined become. The degree of sales in the course of networking is from Expert depending on the desired properties of the crosslinked Layer set.

Of the only superficial crosslinking step (b) only parts of the laser-engravable layer are affected. It takes place no further networking in the entire volume of the laser-engravable Layer, but only in a partial volume of the layer. The effectiveness of the crosslinking step (b) has one of the Surface of the laser-engravable recording layer seen from limited penetration depth so that the top zone of the laser-engravable Layer is networked to a greater extent than this with the exclusive application of process step (a) Would be the case. Here, crosslinkable groups are used in the process step (a) not implemented will be implemented in whole or in part.

Process step (b) is carried out after process step (a).

The width of the zone within which the crosslink density is step (b) is raised, or the effective penetration depth the measure taken for networking is usually at least 5 µm and not more than 200 µm from the surface of the Seen from the recording layer without necessarily the width should be limited to this. The penetration depth is preferably 5 - 150 µm and particularly preferably 5 - 100 µm.

Are used as the starting material for the inventive method multilayered laser-engravable recording elements are used, depending on the thickness of the layer, several can be used Layers affected by process step (b). It understands by itself that the crosslink density of recording layers different composition can. The crosslinking density is achieved by the method according to the invention in each of these layers - up to the maximum penetration depth - increased beyond what was achieved in process step (a).

The transition from the zone, the network density of which in the course of the Step (b) increased beyond the extent of process step (a) becomes to the zone which is no longer from process step (b) recorded, can be abrupt, comparatively steep or gradual his. The inflection point becomes crosslink density to determine the penetration depth used depending on the depth of penetration.

The person skilled in the art is responsible for carrying out process step (b) several methods are available. The choice of method is only limited insofar as the method does not have other properties of the flexographic printing element can be adversely affected allowed to.

For example, the flexographic printing element can be superficial high-energy radiation irradiated or superficially heated become. The element can also be used with polymerization initiators or Networking agents, optionally followed by radiation or warming, be treated.

For laser-engravable flexographic printing elements that are still photochemically networkable groups, there has been one Proven embodiment, in which the cross-linked laser-engravable Flexographic printing element with UV light of the wavelength 200 nm to 300 nm, so-called UV-C light is irradiated. The method is special suitable if the layer as olefinic crosslinkable groups Has double bonds. By comparative strong scattering of the short-wave light in the layer, takes the intensity of UV-C radiation with increasing depth of penetration significantly, so that only the top zone of the flexographic printing element is effective is networked.

The necessary exposure time depends on performance and arrangement the UV-C light source and the type of flexographic printing element, especially based on its IR absorber content. The radiation with UV-C also leads to more filled panels to the effect of the invention.

At this point it is expressly pointed out that the superficial crosslinking with UV-C light does not require that the layer is therefore photochemical in the preceding process step (a) must have been networked. It can also be thermally cross-linked Recording elements are used, provided they still have crosslinkable olefinic double bonds.

Networking using UV-C light is without an additional photo initiator possible. A particularly advantageous embodiment of the However, the invention is a laser-engravable recording element use, the recording layer a photo initiator comprises activated by light of the wavelength 200 to 300 nm becomes. Such an initiator becomes part of the manufacturing process added to the laser-engravable layer and is put together processed with all other components into a layer or one treats the layer with the initiator shortly before step (b). In the case of multilayer recording elements, it is also advantageous not all, but only the top layer or layers to admit said photo initiator.

Examples of suitable initiators which absorb in the UV-C range include aryl ketones of the general formula R-CO-aryl, wherein R is in particular alkyl groups such as, for example Methyl, ethyl or propyl, or substituted alkyl groups such as a benzyl group. The aryl residue can also be further substituted.

If process step (a) is carried out photochemically, the full-surface crosslinking should not normally be done with UV-C light be carried out, although such an embodiment should not be excluded for special cases.

The additional networking in the top zone can also be done by superficial heating of the layer can be made, whereby further crosslink existing thermally crosslinkable groups. Superficial warming can be caused, for example, by briefly Irradiation is carried out. This is particularly true powerful heat radiators suitable with which the surface of the element can be heated briefly but vigorously, e.g. by placing the recording elements on a conveyor belt an IR lamp passes slowly. It is important that one uniform heating of the element as a whole is avoided. The surface heating can also be done, for example, by the Treatment with microwaves. It is still possible the recording element an additional thermal polymerization initiator admit that only at the temperatures of superficial warming, but not at the manufacturing temperatures the layer disintegrates. With multilayer flexographic printing elements it is still advantageous not to all, but only the or add the initiator to the top layers.

It is also possible to use polymerization initiators not laser-engravable Add recording layer, but the surface the laser-engravable flexographic printing element with a suitable one To treat polymerization initiator. The surface can for example brought into contact with a solution of the initiator become. Solvents can be used here slightly swell the surface of the recording element, to facilitate the penetration of the polymerization initiator. Excessive swelling should be avoided, otherwise the Printing properties of the finished flexographic printing plate impaired could become. Examples of polymerization initiators include thermally labile organic peroxides or peresters, for example those containing t-butyloxy, cumyloxy, methyl or phenyl radicals can form hydrogen peroxide or inorganic peroxides. Furthermore, thermally labile azo compounds such as for example azo-bis-isobutyronitrile or similar compounds be used. Other examples include pure halogens or in dissolved form, sulfur-halogen compounds or redox initiator systems.

To trigger or to complete the superficial networking can the laser-engravable flexographic printing element in the connection to the treatment with initiator as described above be irradiated or heated superficially.

In process step (c), a printing relief is created using a Laser engraved in the cross-linked, laser-engravable layer. Picture elements are advantageously engraved in which the flanks of the picture elements initially drop vertically and only widen the lower area of the picture element. This will make one good socketing of the pixels with a slight increase in tonal value reached. Flanks with a different design can also be used of the pixels are engraved.

CO ₂ lasers with a wavelength of 10640 nm are particularly suitable for laser engraving, but depending on the material design 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 that 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 an eximer 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 either be operated continuously or pulsed.

As a rule, the flexographic printing plate obtained can be used directly become. If desired, the flexographic printing plate obtained can, however still to be cleaned. Through such a cleaning step are detached, but may not yet be completely removed from the Removed layer components removed from the plate surface. As a rule is simple treatment with water or alcohol completely sufficient.

The method according to the invention can be carried out in a single production step be carried out in which all process steps in succession be carried out. However, the method can be advantageous be interrupted even after process step (b). The networked laser-engravable recording element can be assembled and stored and only at a later date by means of Laser engraving can be processed into a flexographic printing plate. It is advantageous to use the flexographic printing element e.g. with a temporary cover film, for example made of PET to protect the natural must be removed before laser engraving.

The advantages of the method according to the invention with a two-stage Networking is shown by the flexographic printing plate obtained. Through step (b), the surface of the laser-engravable Flexographic printing element hardened without the elastic properties of the layer are impaired. The such a cross-linked layer can be engraved using lasers, without melting edges caused by the process of engraving become.

Examples

The following examples are intended to explain the invention in more detail:

example 1

A commercially available flexographic printing element (type: nyloflex® FAH, thickness 1.14 mm) is used as the starting material. The cover sheet was removed and the substrate layer with alcohol washed. The flexographic printing element was then for 15 min completely irradiated with UVA light. It became incomplete networked relief layer preserved in the not yet implemented Double bonds were detectable. Then the exposed Plate divided into five pieces of approximately the same size. On One piece remained untreated for comparison purposes, another was subjected to a conventional detackifying treatment, and with three pieces the surface of the element was as below described further networked.

Example 2

A commercially available flexographic printing element (type: Cyrel® NOW, thickness 1.14 mm DuPont) is used as the starting material. The cover sheet was removed and the substrate layer with alcohol abgwaschen. The flexographic printing element was then for 15 min completely irradiated with UVA light. It became incomplete networked relief layer preserved in the not yet implemented Double bonds were detectable. Then the exposed Plate divided into two roughly large pieces. A bit remained untreated for comparison and the other was the surface of the element is further cross-linked as described below.

Example 3

A photosensitive mixture of the following components was obtained manufactured: 124 g Kraton D-1102, 16 g Lithene PH, 16 g Lauryl acrylate, 2.4 g Lucirin BDK and 1.6 g Kerobit TBK. The components were dissolved in 240 g of toloule at 110 ° C. The received homogeneous solution was cooled to 70 ° C and using a Doctor blade so applied to several transparent PET films, that get a homogeneous dry layer thickness of 1.2 mm each becomes. The layers produced in this way were initially used for 18 Dried at 25 ° C for hours and finally at 50 ° C for 3 hours. Then the dried layers were opened a piece of the same size of a second PET film coated with adhesive varnish concealed. After a storage period of one day, the Layers exposed to UV / A for 5 min after removing the cover film. An incompletely cross-linked relief layer was obtained in the double bonds not yet implemented were detectable. Subsequently the exposed plate was roughly the same size in three Pieces divided. One piece remained untreated for comparison purposes, another was a conventional detackifying treatment subjected, and in another piece the Surface of the element further cross-linked as described below.

Conventional detackification with bromine solution

11.7 g of potassium bromide, 3.3 g of potassium bromate and 85 g of water became prepared a solution (solution 1). Subsequently, 10 g Solution 1, 500 g water and 5 g conc. HCl the post-treatment solution (Solution 2) prepared.

Solution 2 was placed in a bowl in which the corresponding, UV / A-exposed plate piece (air bubble free). After 5 minutes of one-sided immersion in solution 2, this becomes Plate piece rinsed with deionized water and dried. By measurement the pendulum stickiness became the surface detackification of the plate.

Additional superficial networking Variant A: crosslinking with peroxide solution

50 g of tert-butyl peroctoate were dissolved in 450 g of toluene. This 10% peroxide solution was placed in a bowl. The respective The UV / A-exposed plate piece became one-sided for a period of 15 min immersed (bubble-free). The plates were taken out dried and then 10 minutes at 160 ° C in a drying cabinet networked.

Variant B: crosslinking with peroxide solution

50 g of dicumyl peroxide were dissolved in 450 g. The 10% peroxide solution becomes superficial on the relevant UV / A-exposed piece of plate applied in a wet layer thickness of approx. 100 µm. After drying for 24 hours at room temperature, layer 10 min at 160 ° C in the drying cabinet. Then the Washed plate obtained and dried.

Variant C: crosslinking by UV / C

The relevant UV / A-exposed plate piece was from the top 20 min UV / C exposed. The intensity was chosen that the penetration depth of the UV / C radiation into the plate is 200 µm did not exceed.

Engraving of the plates

All plate pieces obtained (without and with further treatment) are engraved with a CO ₂ laser (ALE, Meridian Finesse, 250 W, engraving speed = 200 cm / s). A complete test motif consisting of solid surfaces and various raster elements was engraved into the respective flexographic printing element. The quality of the flexographic printing plate obtained was assessed under the microscope. Particular attention was paid to melting edges around negative elements. The results are summarized in Table 1.

Claims (8)

1. A process for the production of flexographic printing plates by laser engraving, in which the starting material employed for the process is a crosslinkable, laser-engravable flexographic printing element which comprises at least, arranged one on top of the other,

   a dimensionally stable support,

   at least one crosslinkable, laser-engravable recording layer comprising at least one binder,

   and the process comprises at least the following process steps:

   (a) full-area crosslinking of the entire volume of the recording layer in which not all groups in the layer which are crosslinkable in principle are reacted with formation of a polymeric network,

   (c) engraving of a print relief into the crosslinked recording layer by means of a laser,

   characterized in that
   the process comprises a further crosslinking step (b) which acts only at the surface and by means of which the recording layer, regarded from the surface, is crosslinked to a limited penetration depth beyond the extent of the crosslinking density effected by step (a), wherein process step (a) is carried out first, followed by process step (b).
2. A process as claimed in claim 1, characterized in that process step (a) is carried out photochemically or thermally.
3. A process as claimed in claim 1 or 2, characterized in that the penetration depth to which crosslinking is additionally carried out in step (b) is from 5 to 200 µm.
4. A process as claimed in one of claims 1 to 3, characterized in that the surface crosslinking step (b) is carried out with UV light having a wavelength of from 200 to 300 nm.
5. A process as claimed in one of claims 1 to 3, characterized in that the surface crosslinking step (b) is carried out by warming the surface of the laser-engravable recording layer.
6. A process as claimed in one of claims 1 to 3, characterized in that the surface crosslinking step (b) is carried out by treating the surface of the laser-engravable layer with a polymerization initiator or a crosslinking reagent.
7. A process as claimed in claim 6, characterized in that the treated surface is irradiated or warmed at the surface in a further process step.
8. A flexographic printing plate obtainable by a process as claimed in one of claims 1 to 7.