EP1414647B1 - Method for the production of flexographic printing forms by means of electron beam cross-linking and laser engraving - Google Patents

Abstract

A method for the production of flexographic printing forms by means of laser engraving, wherein at least one elastomer relief layer is applied to a dimensionally-stable carrier. The relief layer comprises at least one elastomer binding agent and at least one absorber for laser radiation; the relief layer is entirely cross-linked by means of electron radiation at a minimum overall dose of 40 kGy; a printed relief is engraved into the cross-linked relief layer by means of a laser. The invention also relates to flexographic printing forms which can be obtained according to said method.

Description

The present invention relates to a process for the preparation of flexographic printing plates by laser engraving by applying at least one elastomeric relief layer on a dimensionally stable Carrier, wherein the relief layer at least one elastomeric Binder and at least one absorber for laser radiation comprises, full-surface crosslinking of the relief layer by means of electron radiation in a minimum total dose of 40 kGy and engrave a relief in the networked relief layer by means of a laser. The invention further relates to flexographic printing plates, which are obtainable by the method.

In the technique of direct laser engraving for the production of flexographic printing plates becomes a suitable for printing relief directly into a engraved with appropriate relief layer. The engraving of rubber pressure cylinders By means of lasers is in principle since the end of 60s known. Has wider economic interest this technique but only in recent years with the advent gained from improved laser systems. To the improvements in laser systems better focusability of the laser beam, higher performance and computer-controlled beam guidance.

The direct laser engraving has opposite the conventional production of flexographic printing plates several advantages. A row from time-consuming process steps, such as creating a photographic one Negativs or developing and drying the printing form, can be omitted. Furthermore, the flank shape of the individual can be Customize relief elements in laser engraving technology. While in photopolymer plates the flanks of a relief point continuous from the surface to the relief ground diverge, can also laser engraved one in the top Area perpendicular or almost vertical sloping flank, extending widened, engraved only in the lower area. Consequently It also comes with increasing wear of the plate during the Printing process to no or at most a slight dot gain. Further details on the technique of laser engraving are For example, shown in "Technique of flexographic printing", p. 173 ff., 4. Ed., 1999, Coating Verlag, St. Gallen, Switzerland.

For the production of flexographic printing plates by laser engraving can 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 by full-surface Photochemically crosslinked irradiation and in a second Step engraved by means of a laser a printing relief becomes.

EP-A 640 043 and EP-A 640 044 disclose single-layered or multilayered ones Elastomeric laser engravable recording elements for Production of flexographic printing plates. The elements consist of "reinforced" elastomeric layers. For the production of the layer become elastomeric binders, in particular thermoplastic elastomers such as SBS, SIS or SEBS block copolymers used. Due to the so-called reinforcement, the mechanical Strength of the layer increased to allow flexographic printing. The Reinforcement is achieved either by introducing suitable fillers, photochemical or thermochemical crosslinking or combinations reached from it.

It is a prerequisite for the production of flexographic printing plates by means of laser engraving, that the laser radiation first of the relief layer is absorbed. Below a certain threshold energy, which must be entered in the relief layer is usually no engraving possible. Above the threshold energy depends on the speed or efficiency of the engraving of the pro Time unit absorbed energy. The absorbance of the relief layer for the selected laser radiation should therefore as possible be high.

In the laser engraving of flexographic printing elements, large amounts of material must be removed. Therefore, powerful lasers are required. For laser engraving of flexographic printing plates, CO ₂ lasers with a wavelength of 10640 nm can be used. Very powerful CO ₂ lasers are commercially available. The elastomeric binders commonly used for flexographic printing plates typically absorb radiation having a wavelength in the range of about 10 μm. They can thus be engraved with CO ₂ lasers (wavelength of 10640 nm) in principle, as disclosed for example by US 5,259,311, even if the speed of the engraving is not always optimal. Furthermore, the achievable resolution and thus the quality of the printing plate when engraving with CO ₂ lasers is limited. In addition to existing physical boundaries, the beam becomes increasingly difficult to focus with increasing power.

For laser engraving of flexographic printing elements, solid-state lasers with wavelengths in the range of around 1 μm can also be used. For example, powerful Nd / YAG lasers (wavelength 1064 nm) can be used. Nd / YAG lasers have the advantage over CO ₂ lasers that significantly higher resolutions are possible due to the significantly shorter wavelength. In general, however, elastomeric binders of flexographic printing plates do not or only poorly absorb the wavelength of solid-state lasers.

It has already been proposed to mix the relief layer to increase the sensitivity of absorbing IR radiation absorbing substances. When using Nd / YAG lasers, the engraving is usually made possible by the use of IR absorbers. With CO ₂ lasers, the speed of the engraving can be increased. Suitable absorbers are disclosed in EP-A 640 043 and EP-A 640 044 and include highly colored pigments such as carbon black or IR-absorbing dyes which are also usually highly colored.

The use of highly colored IR absorbers leads to the relief layers are also largely opaque in the UV / VIS range. Such layers can therefore no longer be photochemically strengthen or network, since the penetration depth of the actinic Radiation due to the very strong absorption extremely limited is. As a solution proposes EP-B 640 043 therefore, a thick Layer by pouring a variety of thin layers, each followed of photochemical crosslinking of each single layer. However, this procedure is cumbersome and expensive. In addition, the adhesion between the layers during pouring is a new layer on an already networked layer often unsatisfactory.

Laser-engravable flexographic printing elements that have an opaque relief layer can also be prepared by adding the layer pours and then thermally, e.g. using monomers and thermal polymerization initiators crosslinked. But also by casting only layers with limited thickness can be made be, because with increasing layer thickness at Evaporation of the solvent also increasingly causes coating defects. Flexographic printing plates have layer thicknesses of up to 7 mm on. Such layer thicknesses are usually only by means of multiple Achieve one another when high quality Layers are to be obtained, and the procedure is accordingly cumbersome and expensive. Many still have many Carrier films at the temperatures of thermal crosslinking no longer adequate dimensional stability.

The object of the invention was therefore to provide a process for the preparation of flexographic printing plates in which the printing relief by means of a laser in relief layers, the absorber for Laser radiation is included, engraved, and in which too thicker layers and other layers that may be present can be networked in a single operation.

Accordingly, the initially described method was found.

More specifically, the following is to be accomplished for the invention.

For the inventive method is initially an elastomeric Relief layer containing at least one elastomeric binder and includes at least one absorber for laser radiation, on one dimensioned stable support applied. As a rule, the relief layer opaque.

Examples of suitable dimensionally stable carriers include films polyethylene terephthalate (PET), polyethylene naphthalate (PEN), Polybutylene terephthalate, polyamide or polycarbonate, preferably PET or PEN films. As a carrier may also be conical or cylindrical Tubes made of said materials, so-called sleeves, be used. For sleeves also fiberglass fabrics are suitable or composites of glass fibers and suitable polymers Materials. Metallic supports are for carrying out the process generally unsuitable because they are under electron beam radiation too much heat, which is their use in special cases but not to exclude.

The dimensionally stable carrier can for better adhesion of the relief layer optionally coated with an adhesive layer.

The relief layer comprises at least one elastomeric binder. The selection of the binder is limited only insofar as For flexographic printing suitable relief layers must be obtained. Suitable binders are chosen by the skilled person depending on the desired Properties of the relief layer, for example, in terms of hardness, elasticity or color transfer behavior selected.

Examples of suitable elastomers include essentially 3 groups, without the invention being limited thereto.

The first group includes those elastomeric binders which are over ethylenically unsaturated groups. The ethylenically unsaturated Groups can be crosslinked by means of electron radiation. Such binders are, for example, those which are 1,3-diene monomers as isoprene or butadiene in copolymerized form. The ethylenically unsaturated group can be used as chain building block of the polymer (1,4-incorporation), or it may be considered Side group (1,2-incorporation) to be bound to the polymer chain. When Examples are natural rubber, polybutadiene, polyisoprene, styrene-butadiene rubber, Nitrile butadiene rubber, acrylate butadiene rubber, Acrylonitrile-isoprene rubber, butyl rubber, Styrene-isoprene rubber, polynorbornene rubber or ethylene-propylene-diene rubber Called (EPDM).

Other examples include thermoplastic elastomeric block copolymers from alkenylaromatics and 1,3-dienes. For the block copolymers It may be either linear block copolymers or um radial block copolymers act. Usually it is about It is also possible to use triblock copolymers of the A-B-A type Two-block polymers of the A-B type act, or to those with several alternating elastomeric and thermoplastic blocks, e.g. A-B-A-B-A. It can also be mixtures of two or more different Block copolymers are used. commercial Triblock copolymers often contain certain proportions of diblock copolymers. The diene units can be 1,2- and / or 1,4-linked. Both block copolymers of styrene and butadiene may be used as used by the styrene-isoprene type. you are for example, under the name Kraton® commercially available. Farther It is also possible to use thermoplastic elastomeric block copolymers with styrene endblocks and a random styrene-butadiene midblock, available under the name Styroflex® are.

Further examples of binder with ethylenically unsaturated Groups include modified binders in which crosslinkable Groups introduced by grafting reactions in the polymeric molecule become.

The second group includes those elastomeric binders having functional groups that are crosslinkable by electron beams. These are preferably lateral functional groups. But they can also be groups that are integrated into the polymer chain. Examples of suitable functional groups include -OH, -NH ₂ , -NHR, -NCO, -CN, -COOH, -COOR, -CONH ₂ , -CONHR, -CO-, -CHO or -SO ₃ H, where R is generally aliphatic and aromatic radicals. Particularly advantageous for the production of flexographic printing plates by means of electron beam crosslinking and laser engraving have protic functional groups, such as -OH, -NH ₂ , -NHR, -COOH or -SO ₃ H proved. Examples of binders include acrylate rubbers, ethylene-acrylate rubbers, ethylene-acrylic acid rubbers or ethylene-vinyl acetate rubbers and their partially hydrolyzed derivatives, thermoplastic elastomeric polyurethanes, sulfonated polyethylenes or thermoplastic elastomeric polyesters.

Of course, elastomeric binders can also be used which are both ethylenically unsaturated over like functional groups. Examples include copolymers of Butadiene with (meth) acrylates, (meth) acrylic acid or acrylonitrile, and also copolymers or block copolymers of butadiene or Isoprene with functionalized styrene derivatives, For example, block copolymers of butadiene and 4-hydroxystyrene. Unsaturated thermoplastic elastomeric polyester and unsaturated Thermoplastic elastomeric polyurethanes are also suitable.

The third group of elastomeric binders includes those neither ethylenically unsaturated groups nor functional Groups. To name here are, for example Ethylene / propylene elastomers, ethylene / 1-alkylene elastomers or products obtained by hydrogenation of diene units, such as SEBS rubbers.

Of course, mixtures of two or more elastomeric binder can be used, which it is both binders from each one of the described Groups can act or even to mixtures of binders two or all three groups. The combination options are limited only insofar as the suitability of the relief layer for the flexographic printing is not adversely affected by the binder combination may be. Advantageously, for example, a mixture of at least one elastomeric binder which does not having functional groups, with at least one other Binder having functional groups used become.

The amount of elastomeric binder (s) in the relief layer is usually 40 wt.% To 99 wt.% Regarding the sum of all components, preferably 50 to 95 wt.%, and most preferably 60 to 90 wt.%.

The relief layer further comprises at least one absorber for Laser radiation. It can also be mixtures of different absorbers be used for laser radiation. Suitable absorber for laser radiation have a high absorption in the range of the laser wavelength on. In particular, absorbers are suitable, the high Absorption in the near infrared as well as in the longer wavelength VIS range of the electromagnetic spectrum. Such absorbers are particularly suitable for absorbing the radiation of powerful Nd-YAG lasers (1064 nm) and IR diode lasers, the typically wavelengths between 700 and 900 nm as well as between 1200 and 1600 nm.

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

Further suitable absorbers are inorganic pigments, in particular intensively colored inorganic pigments such as Chromium oxides, iron oxides, iron oxide hydrates or carbon black.

Particularly suitable as absorber for laser radiation are finely divided Carbon blacks with a particle size between 10 and 50 nm.

Most of the above-mentioned laser absorbers also exhibit in the UV and in the VIS range of the electromagnetic spectrum high absorption and are accordingly intensely colored. The relief layers, which contain these absorbers are therefore, as a rule opaque or at least largely opaque and therefore not more fully photochemically crosslinkable. It will be at least 0.1 Wt .-% absorber with respect to the sum of all components of the laser engravable Relief layer used. The amount of added absorber will be determined by the person skilled in the art according to the respective desired properties the relief layer selected. In this context will the expert continues to take into account that the added absorber not only the speed and efficiency of the engraving of the influence elastomeric layer by laser, but also others Properties of the flexographic printing element, such as its Hardness, elasticity, thermal conductivity or ink acceptance. As a rule are therefore more than 40 wt.% Absorbers for laser radiation with respect to the sum of all components of the laser-engravable elastomer Layer unsuitable. Preferably, the amount of the absorber for laser radiation 1 to 30 wt .-% and particularly preferably 5 to 20% by weight.

Optionally, the elastomeric relief layer also by means of Electron radiation crosslinkable low molecular weight or oligomeric Compounds include. Oligomeric compounds generally have a molecular weight of not more than 20,000 g / mol. low molecular weight and oligomeric compounds are the following For the sake of simplicity, they are referred to as monomers.

On the one hand, monomers can be added to increase the speed increase the networking, if desired by the skilled person becomes. When using elastomeric binders from the Groups 1 and 2 is the addition of monomers for acceleration generally not mandatory. For elastomeric binders from Group 3 is the addition of monomers as a rule recommended, without this being mandatory in any case necessary would.

Regardless of the issue of networking speed can Monomers also for controlling the crosslinking density in the course of Electron beam hardening and to set the desired Hardness of the crosslinked material can be used. Depending on the type and Amount of added low molecular weight compounds are more or get closer networks.

As monomers, on the one hand, the known ethylenically unsaturated Monomers are used, which are also used for the production of conventional photopolymer flexographic printing plates are used can. The monomers should be compatible with the binders be and have at least one ethylenically unsaturated group. They should not be volatile. Preferably, the Boiling point of suitable monomers not lower than 150 ° C. Especially suitable are amides and esters of acrylic acid or methacrylic acid with mono- or polyfunctional alcohols, amines, amino alcohols or hydroxy ethers and esters, styrene or substituted ones Styrenes, esters of fumaric or maleic acid or allyl compounds prove. Examples include 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-dodecylmaleimide.

Furthermore, it is also possible to use monomers which have at least one functional group which can be crosslinked under the influence of electron beam curing. Preferably, the functional group is a protic group. Examples include -OH, -NH ₂ , -NHR, -COOH or -SO ₃ H. With particular preference it is also possible to use di- or polyfunctional monomers in which terminal functional groups are connected to one another via a spacer. Examples of such monomers include dialcohols such as 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, 1,9-nonanediol, diamines such as 1,6-hexanediamine, 1,8-hexanediamine, dicarboxylic acids such as oxalic acid, malonic acid, Adipic acid, 1,6-hexanedicarboxylic acid, 1,8-octanedicarboxylic acid, 1,10-decanedicarboxylic acid, phthalic acid, terephthalic acid, maleic acid or fumaric acid.

It is also possible to use monomers which are both ethylenic have unsaturated groups such as functional groups. As examples its called ω-hydroxyalkyl acrylates, such as Ethylene glycol mono (meth) acrylate, 1,4-butanediol mono (meth) acrylate or 1,6-hexanediol mono (meth) acrylate.

Of course, mixtures of different monomers are used, provided the properties of the relief layer are not adversely affected by the mixture.

As a rule, the amount of monomer added is 0 to 30 % By weight with respect to the amount of all components of the relief layer, preferably 0 to 20 wt .-%.

The elastomeric relief layer may also contain additives and auxiliaries such as, for example, dyes, dispersing aids, Antistatic agents, plasticizers or abrasive particles include. The amount of such additives should, however, usually 20 wt .-% with respect to the amount of all components of the elastomeric Do not exceed the relief layer of the recording element.

The elastomeric relief layer may also consist of several relief layers being constructed. These elastomeric sublayers can be of same, approximately the same or of different material Be composition.

The thickness of the elastomeric relief layer or all relief layers together is generally between 0.1 and 7 mm, preferably 0.4 to 7 mm. The thickness is determined by the expert depending on the desired Purpose of the flexographic printing form chosen suitably.

The flexographic printing element used as the starting material may be optional still a top layer with a thickness of not have more than 100 microns. The composition of such a top layer may be in terms of optimal printing properties like For example, color transfer can be selected while the composition the underlying relief layer in terms of is selected for optimum hardness or elasticity. Prefers the thickness is 5 to 80 microns and more preferably 10 to 60 microns. The upper class must either be laser engravable, or at least in the course of the laser engraving along with the underneath lying relief layer be removable. It includes at least a polymeric binder which is not necessarily elastomeric have to be. It can also be an absorber for laser radiation or also comprise monomers or auxiliaries.

The starting material for the process can, for example, by Dissolve or disperse all components in a suitable solvent and pouring onto a carrier. at multilayer elements can in principle known type and Way several layers are poured on each other. Since wet-innass worked, the layers combine well with each other. Even an upper class can be infused. alternative For example, the individual layers can be on temporary supports poured and then the layers by laminating with each other get connected. After casting can still optional one Cover film for protection against damage to the starting material be applied.

Of very particular advantage for the inventive method but used thermoplastic elastomeric binders, and the preparation is carried out in a known manner by extrusion between a carrier film and a cover sheet or a Cover element followed by calendering, such as from EP-A-084 851. This way you can also make thick layers in a single operation. multilayer Elements can be made by coextrusion.

In method step (b), the relief layer is produced by means of electron radiation fully networked. If the flexographic printing element still has a protective film, this should be before crosslinking usually be deducted. But this is just in networking not necessarily compelling by means of electron beams.

Suitable devices for electron beam crosslinking are the person skilled in principle known. The irradiation with electrons can be both inline directly following the continuous Making the relief layer, e.g. immediately after to the calendering. The irradiation with electrons can but advantageously also be done in a separate process step.

In full-surface networking, this is used as the starting material used flexographic printing element as evenly as possible with electron radiation irradiated. Ideally, the entire area should be of the flexographic printing element are irradiated absolutely uniformly, although, of course, there are always certain fluctuations in practice will enter. Larger fluctuations should be avoided become. To achieve a uniform irradiation, that should Flexo printing as possible just placed on the pad.

In the method according to the invention, the flexographic printing elements in usually irradiated only from the top of the elements. The Of course, the invention also includes the procedure that you irradiated the element from the top and from the bottom.

The minimum total amount of crosslinking is 40 kGy (1 Gy = 1 J / kg). The maximum irradiation dose will vary according to the professional the desired properties such as hardness or restoring force the flexographic printing set. As a rule, it is recommended but not to use more than 200 kGy for networking and especially it is preferable not to crosslink more than 150 kGy use. Has proven to be a total dose for irradiation of 60 to 120 kGy.

The energy of the electron beam is depending on the expert Thickness and composition of the flexographic printing element determined. The Energy of electron radiation is decisive for the maximum Penetration depth of the electron beam in the relief layer. at the relief layers used in the invention, the absorber For laser radiation, it has but usually proven, electron beams with an energy of at least 2 Use MeV.

The irradiation with electrons can be made such that administered the entire dose in a single irradiation procedure becomes. The dose rate should be as high as possible to achieve the shortest possible irradiation times. On the other hand she is allowed not be so high that the flexographic printing element too Heavily heated because otherwise the dimensional stability of the flexographic printing element could be affected. A warm up above 80 ° C should be avoided. For an optimal result achieve, it is regularly advantageous, especially temperature-stable Carrier films, such as those from PEN use.

The irradiation is usually done in air, but the Irradiation can of course also in special cases under protective gases such as argon or nitrogen. if desired The plates to be irradiated can also be encapsulated to exclude air become.

It is also advantageous to use the flexographic printing element during the Irradiation to cool, for example by an air flow, the is transferred, or by placing on a chilled surface.

In a particularly advantageous embodiment of the invention Procedure is the total dose of electron radiation distributed over two or more subdoses. The partial doses can be the same size or different sizes, the electron beams can the same energy or different energy or the have the same or a different dose rate.

The individual sub-cans can follow each other directly. But they can also be advantageous for the same length or different long breaks have been interrupted. The Irradiation can be interrupted only briefly or even longer become. Irradiation breaks of more than 60 min between the individual Cans should be avoided, however. Have proven Irradiation breaks between 1 and 30 min.

The following are some embodiments for the step of Crosslinking by electron beams described in more detail have proven particularly useful.

In an embodiment for the electron beam crosslinking step is the energy of electron radiation administered at all Partial doses equal or approximately equal. After every partial dose a radiation break is taken. It is preferred with irradiated a relatively high dose rate, causing the relief layer strongly heated. Temperatures of more than 100 ° C should but be avoided. During irradiation breaks, the relief layer can abreact and cool off again.

In another embodiment, the energy is the electron beam at least one of the administered partial doses of that of the other sub-cans different. For example, the Energy of the electron beams of the first administered partial doses be chosen so that the flexographic printing element throughout the Depth of the relief is networked while the energy of the electron beams the last administered partial dose is measured that continues only in a thin layer on the surface is networked. Thus, a flexographic printing plate can be obtained which a relatively soft underlayer and harder by comparison Upper layer has.

The energy of the electron beams can also be applied to all partial doses be different. There are also other types of cross-linking profiles possible. For example, you can with the partial dose start, where the electron beams have the highest energy, and then the electron energy at each additional partial dose reduce. In this way can be a flexographic printing obtained at the crosslinking density of the relief layer stepped from the carrier foil to the printing surface increases.

It has been proven in all embodiments, at least one the steps electron beams with an energy of at least Use 2 MeV.

In another embodiment, one can increase the efficiency also stack several flexographic printing elements on top of each other. Around To achieve a uniform networking, it is recommended, too here in several subdoses to irradiate and the order of Cyclic flexographic printing elements in the stack at each irradiation exchange. You can also start with a whole stack or irradiate several times and in a final step at the Elements individually targeted the surface with electron radiation cure low penetration.

In process step (c), a printing relief by means of a Laser engraved in the crosslinked by electron beam layer. Advantageously, pixels are engraved in which the flanks of the picture elements initially fall vertically and down widen only in the lower part of the picture element. Thereby However, a good picture of the pixels will be lower Tonwertzunahme reached. But it can also be designed differently Flanks of the pixels are engraved.

Laser engraving is especially suitable for IR lasers. It can but also lasers with shorter wavelengths can be used, provided the laser has sufficient intensity. For example can also be a frequency doubled (532 nm) or frequency tripled (355 nm) Nd-YAG laser can be used or also excimer lasers (e.g., 248 nm). If needed for material removal, must each be adapted to the laser wavelength accordingly Absorber can be used for laser radiation.

For laser engraving, for example, a CO ₂ laser with a wavelength of 10640 nm can be used. Particularly advantageous lasers are used with a wavelength between 600 and 2000 nm. For example, Nd-YAG lasers (1064 nm), IR diode lasers or solid-state lasers can be used. Particularly preferred for carrying out the method according to the invention are Nd / YAG lasers. The image information to be engraved is transmitted directly from the lay-out computer system to the laser apparatus. The lasers can be operated either continuously or pulsed.

As a rule, the flexographic printing plate obtained can be used directly become. If desired, however, the resulting flexographic printing plate can still to be cleaned. By such a cleaning step will be detached, but may not be complete yet Plate surface removed removed layer components. As a rule is easy to handle with water, water / surfactant or Alcohol sufficient.

The process according to the invention can be carried out in a single production cycle be performed, in which all process steps in succession be executed. Advantageously, the method but also be interrupted after process step (b). The networked, Laser-engraved recording element can be assembled and stored at a later date by means of Laser engraved to a flexographic printing plate or flexo sleeve be further processed. It is advantageous, the flexographic printing element e.g. with a temporary cover sheet, for example to protect from PET, which of course deducted again before the laser engraving must become.

The inventive method has over the prior Technique offers a number of significant advantages:

It allows the production of flexographic printing plates, their relief layers Absorbers for laser radiation include even at high Layer thickness with high quality. Networking is just one operation required.

In the course of electron beam crosslinking, the adhesion between the carrier film and the relief layer significantly improved. The same applies to the liability between an optional existing Upper layer and the relief layer.

The distribution of the total radiation dose into several subdoses, whose electron beams have different energies, makes networking profiles accessible in a very simple way. In this way, for example, flexographic printing elements with hardened surface. hardened Surfaces have the advantage that when engraving by means of lasers no melt edges formed around the engraved relief elements around become. Enamel margins cause distortions of the printed image Print out. Furthermore, such plates have an increased Abrasion resistance on.

The thermal load of the flexographic printing element in the course of crosslinking can be compared to thermal crosslinking significantly be reduced or even avoided altogether. this leads to Flexographic printing plates with significantly improved dimensional stability and thus significantly better print quality.

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

Example 1:

A relief layer with a binder having ethylenically unsaturated groups was prepared. For the relief layer, the following components were used. components feedstocks Amount [wt%] binder Polybutadiene rubber (high vinyl content) 68.5 Absorber for laser radiation finely divided carbon black 10.0 monomers lauryl 10.0 additives Polybutadiene oil (plasticizer) 10.0 therm. stabilizer 1.5

Binders, additives and absorbers for laser radiation were used in a laboratory kneader at a melt temperature of 150 ° C mixed. After 15 minutes, the absorber for laser radiation was homogeneously dispersed. The compound thus obtained was together with the monomer dissolved in toluene at 80 ° C, cooled to 60 ° C and to a uncoated, 125μm thick PET film poured. After 24 hours Flash off at room temperature and dry for 3 hours 60 ° C, the resulting relief layer (layer thickness 900 microns) on a second, adhesive-coated, 125μm thick PET film laminated. Before further treatment, the item was 1 week stored at room temperature.

Example 2:

A relief layer was prepared with a binder mixture having ethylenically unsaturated groups. For the relief layer, the following components were used. components feedstocks Amount [wt%] binder EPDM rubber with 5% by weight of ethylidene norbornene as a termonomer 75.5 Polybutadiene rubber (high vinyl content 4.0 Absorber for laser radiation finely divided carbon black 10.0 monomers lauryl 7.5 trimethacrylate 1.5 additives therm. stabilizer, dispersing agent 1.5

Binders, additives and absorbers for laser radiation were used in a laboratory kneader at a melt temperature of 170 ° C mixed. After 15 minutes, the absorber for laser radiation was homogeneously dispersed. The compound thus obtained was together with the monomers dissolved in toluene at 80 ° C, cooled to 60 ° C and to a uncoated, 125μm thick PET film poured. After 24 hours Flash off at room temperature and dry for 3 hours 60 ° C, the resulting relief layer (layer thickness 800 microns) on a second, adhesive-coated, 175μm thick PET film laminated. Before further treatment, the item was 1 week stored at room temperature.

Example 3:

It was a relief pushes with a binder with ethylenically unsaturated groups by extrusion and subsequent calendering between a cover sheet and a carrier film produced. For the relief layer, the following components were used. components feedstocks Amount [wt%] binder SIS triblock copolymer with 15% by weight of styrene (Kraton D-1161, Kraton Polymers) 80.0 Absorber for laser radiation finely divided carbon black 6.0 monomers hexanediol 6.0 hexanediol 6.0 additives therm. stabilizer, ozone protection wax 2.0

The components were in a twin-screw extruder at a Melt temperature of 140 - 160 ° C mixed intensively, extruded through a slot die and then between a cover sheet and a carrier sheet calendered. The thickness of the Relief layer was 860 microns. Before further treatment the element was stored for 1 week at room temperature.

Example 4 (comparative example)

A relief layer with a binder having ethylenically unsaturated groups was prepared by extrusion and subsequent calendering between a cover film and a carrier film. For the relief layer, the following components were used. components feedstocks Amount [wt%] binder SIS triblock copolymer with 15% by weight of styrene (Kraton D-1161, Kraton Polymers) 79.0 Absorber for laser radiation finely divided carbon black 6.0 photoinitiator benzildimethylketal 1.0 monomers hexanediol 6.0 hexanediol 6.0 additives therm. stabilizer, ozone protection wax 2.0

The components were in a twin-screw extruder at a Melt temperature of 140 - 160 ° C mixed intensively, extruded through a slot die and then between a cover sheet and a carrier sheet calendered. The thickness of the Relief layer was 850 microns. Before further treatment the element was stored for 1 week at room temperature.

electron beam crosslinking

For crosslinking, an electron irradiation apparatus (rated power about 150 kW), which electron beams with Can generate electron energies of 2.5 - 4.5 MeV. The transport the electron-radiating elements through the zone of electron irradiation carried out by means of vertically freely suspended Aluminum pallets, which have a movable suspension with a guided conveyor belt were connected, so by the Control of conveyor belt speed uniform transport aluminum pallets through the zone of electron irradiation could be done.

Crosslinking by irradiation with UV-A light

For crosslinking by irradiation with UV-A light were the networking elements a specific, predetermined time in one F III platesetter of BASF Drucksysteme GmbH exposed under vacuum.

For this purpose, first the Schuzfolie of the relevant elements and then removes a transparent, UV-transparent Entklebungsfolie placed on the element to be irradiated to a To prevent sticking of the element surface with the vacuum foil. After covering the element to be irradiated with the vacuum film and switching on the vacuum was the element for the specified period of time irradiated over the entire surface with UV light.

Example 5:

A total of 6 elements according to Example 1 were used, of which 1 element was retained as reference (Sample No. 0). The Energy of electron radiation was about 3.0 MeV. there has been a successive irradiation series with 5 equal sub-doses to each 20 kGy performed. The waiting time between 2 partial doses was every 20 minutes. After each partial dose, one element was taken out taken from the radiation circuit, the rest were taken before administration the next partial dose turned by 180 °.

The following table shows the properties of the resulting flexographic printing element as a function of the irradiation dose. No. partial dose
[KGy] total dose
[KGy] Swelling in toluene
[Wt .-%] gel
[Wt .-%] Mech. Hardness (DIN 53505)
[Shore A] 0 --- --- ∞ 0 1 20 20 447 77 72 2 20 40 266 86 74 3 20 60 205 91 78 4 20 80 180 93 80 5 20 100 180 94 81

Example 6:

A total of 9 elements according to Example 2 were used, of which 1 element was retained as reference (Sample No. 0). The Energy of electron radiation was about 3.0 MeV. there has been a successive irradiation series with 8 z.T. different partial doses carried out. The partial doses were in detail successively 23, 22, 22, 35, 42, 30, 30 and 29 kGy. The waiting time between 2 partial doses was 20 minutes each. After every partial dose an element was taken from the radiation circuit, the remainder were reversed before administration of the next partial dose 180 ° turned.

The following table shows the properties of the resulting flexographic printing element as a function of the irradiation dose. No. partial dose
[KGy] total dose
[KGy] Swelling in toluene
[Wt .-%] gel
[Wt .-%] Mech. Hardness (DIN 53505)
[Shore A] 0 --- --- ∞ 0 1 23 23 444 90 72 2 22 45 274 94 72 3 22 67 199 96 72 4 35 102 167 98 73 5 42 144 157 97 74 6 30 174 162 97 74 7 30 204 129 98 74 8th 29 233 121 98 74

Example 7:

A total of 9 elements according to Example 3 were used, of which 1 element was retained as reference (Sample No. 0). The Energy of electron radiation was about 3.0 MeV. there has been a successive irradiation series with 8 z.T. different partial doses carried out. The partial doses were in detail successively 23, 22, 22, 35, 42, 30, 30 and 29 kGy. The waiting time between 2 partial doses was 20 minutes each. After every partial dose an element was taken from the radiation circuit, the remainder were reversed before administration of the next partial dose 180 ° turned.

The following table shows the properties of the resulting flexographic printing element as a function of the irradiation dose. No. partial dose
[KGy] total dose
[KGy] Swelling in toluene
[Wt .-%] gel
[Wt .-%] Mech. Hardness (DIN 53505)
[Shore A] 0 --- --- ∞ 0 1 23 23 ∞ 0 39 2 22 45 828 77 52 3 22 67 430 87 58 4 35 102 431 89 63 5 42 144 331 92 65 6 30 174 322 93 67 7 30 204 260 94 68 8th 29 233 260 94 68

Example 8 (Comparative Example)

A total of 6 elements according to Example 4 were used, of which 1 element was retained as reference (Sample No. 0). It was a series of irradiation with UVA light as described above with the following single irradiation times: 1, 5, 15, 30, 60 min.

The following table shows the properties of the flexographic printing element obtained as a function of the UVA irradiation time. No. Duration of UVA irradiation [min] Swelling in toluene [Wt .-%] gel [Wt .-%] Mech. Hardness (DIN 53505) [Shore A] 0 0 ∞ 0 1 1 ∞ 0 32 2 5 ∞ 0 33 3 15 ∞ 1 35 4 30 ∞ 3 36 5 60 ∞ 2 34

Laser engraving of the irradiated flexographic printing elements:

The resulting irradiated flexographic printing elements were coated with a CO ₂ -Lase (ALE, Meridian Finesse, 250 W, engraving speed = 200 cm / s) and an Nd-YAG laser (ALE, Meridian Finesse, 100 W, engraving speed = 100 cm / s) engraved. A test motif consisting of solid surfaces and various line elements was engraved into the respective flexographic printing element. Each 1 cm x 1 cm line elements consisted of parallel, individual negative lines with per line element of the same line width and the same line spacing. A list of the engraved line elements is given in the following table. Line element no. Width of the negative lines [μm] Distance of the negative lines [μm] 1 20 20 2 40 40 3 60 60 4 80 80 5 100 100 6 200 200 7 500 500 8th 1000 1000

The quality of the laser-engraved flexographic printing elements was with Assessed the help of a light microscope, which has a device for measuring distances or heights and depths.

For this purpose, the engraving depth was based on the entire area engraved area measured. Furthermore, each of the finest line element determined, in which the engraved individual lines under the microscope were still completely separated from each other. The individual lines were considered completely separate from each other assessed resolved when the surface of the between the negative lines remaining positive line elements have a width of had at least 5 microns and this surface except for a difference of 20 microns the same height possessed as the non-engraved areas the positive full surface. In this type of assessment means a low number of the number of the finest still pictured Line element therefore a good engraving quality while a high number of lower resolution and thus one worse engraving quality corresponds.

Finally, in particular, melt edges and deposits in the peripheral zones of the negative elements and solid surfaces were assessed visually. Example no. Cure type crosslinking conditions laser type Melt edges (visual) Engraving depth [μm] Finest line element [No.] 5 IT 60 kGy CO ₂ Little 760 3 5 IT 80 kGy CO ₂ None 830 1 5 IT 60 kGy Nd-YAG Little 810 2 5 IT 80 kGy Nd-YAG None 830 1 6 IT 67 kGy CO ₂ medium 640 3 6 IT 102 kGy CO ₂ Little 700 2 6 IT 67 kGy Nd-YAG medium 660 3 6 IT 102 kGy Nd-YAG Little 690 2 7 IT 102 kGy CO ₂ medium 650 2 7 IT 144 kGy CO ₂ None 710 2 7 IT 102 kGy Nd-YAG medium 660 2 7 IT 144 kGy Nd-YAG None 680 1 8th UVA 15 minutes CO ₂ Very strong 390 7 8th UVA 60 min CO ₂ strongly 480 5 8th UVA 15 minutes Nd-YAG Very strong 430 6 8th UVA 60 min Nd-YAG Very strong 450 5

By means of examples no. 5 to 7 it can be seen that with the laser-engravable flexographic printing elements according to the invention in contrast to Comparative Example No. 8 fine relief elements in good quality and can be mapped without strong melting phenomena. In addition, with the flexographic printing elements according to the invention surprisingly achieved a higher engraving depth than with a laser engravable Flexographic printing element according to the prior art (Comparative Example No. 8).

Surprisingly, all electron-beam crosslinked compounds were also found Flexographic printing elements according to Example No. 7 a much higher Adhesion to the carrier as the UV-crosslinked flexographic printing elements according to Comparative Example No. 8.

Claims (27)

1. A process for the production of flexographic printing plates by means of laser engraving, comprising the following steps:

   a) application of at least one elastomeric relief layer to a dimensionally stable substrate, the relief layer comprising at least one elastomeric binder and at least one absorber for laser radiation,

   b) uniform crosslinking of the relief layer,

   c) engraving of a printing relief into the crosslinked relief layer by means of a laser,

   wherein the uniform crosslinking is carried out by means of electron beams in a minimum total dose of 40 kGy.
2. A process as claimed in claim 1, wherein, in a step (a'), an upper layer having a thickness of not more than 100 µm is furthermore applied, the upper layer comprising at least one polymeric binder.
3. A process as claimed in claim 1 or 2, wherein the electron beams have an energy of at least 2 MeV.
4. A process as claimed in claim 1 or 2, wherein the total dose of electron beams is distributed over two or more part-doses.
5. A process as claimed in claim 4, wherein the irradiation is stopped for an irradiation pause after the administration of any part-dose.
6. A process as claimed in claim 4 or 5, wherein the energy of the electron beam is identical for each of the administered part-doses.
7. A process as claimed in claim 4 or 5, wherein the energy of the electron beam for at least one of the administered part-doses differs from that of the other part-doses.
8. A process as claimed in claim 4 or 5, wherein the energy of the electron beam differs for all administered part-doses.
9. A process as claimed in claim 8, wherein the initial part-dose is the one in which the electron beam has the highest energy, and the energy for each further part-dose decreases stepwise.
10. A process as claimed in any of claims 4 to 8, wherein at least one of the part-doses has an energy of at least 2 MeV.
11. A process as claimed in any of claims 1 to 10, wherein a total dose of 200 kGy is not exceeded.
12. A process as claimed in any of claims 1 to 10, wherein a total dose of 150 kGy is not exceeded.
13. A process as claimed in any of claims 1 to 12, wherein the irradiation is carried out using electrons in air.
14. A process as claimed in any of claims 1 to 13, wherein the elastomeric binder has ethylenically unsaturated groups.
15. A process as claimed in any of claims 1 to 13, wherein the elastomeric binder has functional groups crosslinkable under the action of electron beams.
16. A process as claimed in claim 15, wherein the functional groups are protic groups.
17. A process as claimed in any of claims 1 to 13, wherein the elastomeric binder has ethylenically unsaturated groups and functional groups crosslinkable under the action of electron beams.
18. A process as claimed in any of claims 1 to 13, wherein a mixture of at least one elastomeric binder which has no functional groups with at least one further binder which has functional groups is used.
19. A process as claimed in any of claims 1 to 18, wherein the relief layer furthermore comprises at least one low molecular weight or oligomeric compound crosslinkable by means of electron beams.
20. A process as claimed in claim 19, wherein the low molecular weight compound is an ethylenically unsaturated monomer.
21. A process as claimed in claim 19, wherein the low molecular weight or oligomeric compound is a compound having functional groups.
22. A process as claimed in claim 21, wherein the functional groups are protic groups.
23. A process as claimed in any of claims 1 to 22, wherein the elastomeric binder is a thermoplastic elastomeric binder and the relief layer is produced by extrusion followed by calendering.
24. A process as claimed in any of claims 1 to 23, wherein the relief layer is opaque.
25. A process as claimed in any of claims 1 to 24, wherein the laser engraving (c) is carried out using a laser having a wavelength of 600 - 2000 nm.
26. A process as claimed in claim 25, wherein the laser engraving (c) is carried out using an Nd-YAG laser.
27. A flexographic printing plate obtainable as claimed in any of claims 1 to 26.