EP1613484B1 - Laser-engravable flexographic printing element containing a conductive carbon black, and method for the production of flexographic printing forms - Google Patents

Abstract

In a laser-engravable flexographic printing element, the relief-forming layer comprises a conductivity carbon black having a specific surface area of at least 150 m<SUP>2</SUP>/g and a DBP number of at least 150 ml/100 g. Flexographic printing plates are produced by a process in which a printing relief is engraved into said flexographic printing element by means of a laser system.

Description

The invention relates to a laser-engravable flexographic printing element in which at least one relief-forming layer contains a conductivity black having a specific surface area of at least 150 m ² / g and a DBP number of at least 150 ml / 100 g. The invention further relates to a process for the production of flexographic printing plates, in which engraved by means of a laser system, a printing relief in said flexographic printing element.

In direct laser engraving for the production of flexographic printing forms, a printing relief is engraved directly into a suitable relief-forming layer using a laser or a laser system. The layer is decomposed at the points where it is hit by the laser beam and removed substantially in the form of dusts, gases, vapors or aerosols. A development step as in the conventional method -thermic or by means of washing-out is not required.

Although the engraving of rubber cylinders by laser has been known in principle since the 1960s, laser engraving has gained wider economic interest only in recent years with the emergence of improved laser systems. The improvements in the laser systems include in particular better focusability of the laser beam, higher power and computer-controlled beam modulation.

With the introduction of new, more powerful laser systems, however, the question of particularly suitable materials for laser-engravable flexographic printing plates is becoming increasingly important. In particular, in the engraving of high-resolution printing plates or very fine relief elements, now occur problems that did not matter at all in the past, because the laser systems, the engraving of very fine structures anyway not allowed. Improved laser systems lead to new demands on the material.

In the case of direct laser engraving, it should be noted in particular that the relief-forming layer, which is engraved with the laser, also forms the later printing surface. All errors that occur during the engraving are therefore visible during printing. In the laser direct engraving, therefore, in particular the edges of the relief elements must be made particularly precise in order to obtain a clean print image. Frayed edges or beads of molten material around relief elements, so-called Enamel margins worsen the printed image considerably. Naturally, these factors are the more important the finer the desired relief elements are.

From EP-B 640 043 such as EP-B 640 044 It has been proposed to "reinforce" laser-engravable flexographic printing elements and optionally to add laser radiation absorbing materials to improve the sensitivity. The use of carbon black is also suggested without specifying it in more detail.

Carbon black is not a defined chemical compound, but there are a very large number of different carbon blacks, which differ in terms of manufacturing process, particle size, specific surface area or surface properties, and which accordingly also have a wide variety of chemical and physical properties. For further details, reference is made, for example, to H. Ferch, "Pigmentruße", ed. U. Zorll, Vincentz Verlag, Hannover, 1995. Carbon blacks are often characterized by the specific surface area, for example by the BET method, and the so-called "structure". By "structure" the carbon black expert understands the linking of the primary particles to aggregates. The structure is often determined by dibutyl phthalate (DBP) adsorption. The higher the DBP absorption, the higher the structure.

A special class of carbon blacks are called conductivity blacks. In general, carbon blacks having a DBP absorption greater than 110 ml / 100 g and a relatively high specific surface area are referred to as conductivity blacks (Ferch et al., P. 82). Conductivity blacks are usually used for the purpose of making nonconductive materials electrically conductive with the lowest possible addition amount.

The use of carbon black in laser-engravable flexographic printing elements is also of EP-A 1 080 883 . WO 02/16134 . WO 02/54154 or WO 02/083418 been described. In the cited documents, however, no conductivity carbon blacks are disclosed, but carbon blacks with a relatively small specific surface area and small DBP number.

EP-A 1 262 315 and EP-A 1 262 316 disclose a method and a laser system for the production of flexographic printing plates. The laser system described operates with a plurality of laser beams, which may have different power and / or wavelength, and with which the superficially located areas of the printing form and lower areas can each be processed separately. Attention is drawn to the possibility of designing the surface of the flexographic printing element used differently than the underlying areas. However, the documents contain no suggestions of a particular chemical composition for the surface or the underlying areas.

The object of the invention was to provide a single-layer or multi-layer laser-engravable flexographic printing element, which also allows the engraving of fine relief elements with high precision without the appearance of melt edges. It should be particularly suitable for engraving with modern multi-beam laser systems.

Surprisingly, it has been found that this object can be achieved by the use of conductivity soot of the type defined. The flexographic printing elements can be engraved with high resolution without observing melt edges and other negative effects. The result was particularly surprising because the mentioned carbon blacks are by no means the ones which have the highest sensitivity to laser radiation.

• einen dimensionsstabilen Träger, sowie
• mindestens eine reliefbildende, vernetzte, elastomere Schicht (A) mit einer Dicke von 0,05 bis 7 mm, erhältlich durch Vernetzen einer Schicht, welche mindestens ein elastomeres Bindemittel (a1), eine Laserstrahlung absorbierende Substanz (a2) sowie Komponenten zum Vemetzen (a3) umfasst, aufweisen,

wobei es sich bei der Laserstrahlung absorbierenden Substanz um einen Leitfähigkeitsruß mit einer spezifischen Oberfläche von mindestens 150 m²/g und einer DBP-Zahl von mindestens 150 ml/100g handelt.Accordingly, flexographic printing elements for the production of flexographic printing plates by laser engraving were found, which are arranged one above the other at least

• a dimensionally stable carrier, as well
• at least one relief-forming, crosslinked, elastomeric layer (A) having a thickness of 0.05 to 7 mm, obtainable by crosslinking a layer comprising at least one elastomeric binder (a1), a laser radiation absorbing substance (a2) and components for crosslinking (a3 ),

wherein the laser radiation absorbing substance is a conductivity black having a specific surface area of at least 150 m ² / g and a DBP number of at least 150 ml / 100 g.

In a special. In another embodiment, the flexographic printing element further comprises at least one further relief-forming crosslinked elastomeric layer (B) between the support and layer (A), obtainable by crosslinking a layer comprising at least one elastomeric binder (b1) and components for crosslinking.

Furthermore, a process for the production of flexographic printing plates was found, in which one uses a flexographic printing element of the type mentioned above and engraved a relief with the aid of a laser system in the layer (A) and optionally layer (B), wherein the depth of the laser engraved with the relief elements at least 0.03 mm.

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

Examples of suitable dimensionally stable carriers for the flexographic printing elements according to the invention are plates, films and conical and cylindrical tubes (sleeves) Metals such as steel, aluminum, copper or nickel or of plastics such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate, polyamide, polycarbonate, if appropriate also fabrics and nonwovens, such as glass fiber fabric and composite materials, for example of glass fibers and plastics. Dimensionally stable substrates are, above all, dimensionally stable carrier films such as polyester films, in particular PET or PEN films or flexible metallic substrates, such as thin sheets or metal foils of steel, preferably of stainless steel, magnetizable spring steel, aluminum, zinc, magnesium, nickel, chromium or Copper into consideration.

The flexographic printing element further comprises at least one relief-forming, crosslinked, elastomeric layer (A). The relief-forming layer can be applied directly on the support. However, other layers may optionally also be present between the support and the relief layer, for example adhesion layers and / or elastic underlayers and / or at least one further relief-forming, crosslinked, elastomeric layer (B).

The crosslinked elastomeric layer (A) is obtainable by crosslinking a layer comprising at least a binder (a1), a laser radiation absorbing substance (a2), and crosslinking components (a3). The layer (A) itself thus comprises the binder (a1), the laser radiation absorbing substance (a2) and the network generated by the reaction of the components (a3), which may or may not include the binder.

Suitable binders (a1) for layer (A) are in particular elastomeric binders. However, it is also possible in principle to use non-elastomeric binders. The decisive factor is that the crosslinked layer (A) has elastomeric properties. The recording layer may, for example, adopt elastomeric properties by the addition of plasticizers to a per se non-elastomeric binder, or it may be used crosslinkable oligomers which form an elastomeric network only by the reaction with each other.

Suitable elastomeric binders (a1) for layer (A) are, in particular, those polymers which contain polymerized 1,3-diene monomers, such as isoprene or butadiene. Depending on the nature of the incorporation of the monomers such binders have crosslinkable olefin groups as part of the main chain and / or as a side group. Examples include natural rubber, polybutadiene, polyisoprene, styrene-butadiene rubber, nitrile-butadiene rubber, butyl rubber, styrene-isoprene rubber, polynorbornene rubber or ethylene-propylene-diene rubber (EPDM).

The binders (a1) may also be thermoplastic elastomeric block copolymers of alkenylaromatics and 1,3-dienes. The block copolymers may be either linear block copolymers or radial block copolymers. Usually they are triblock copolymers of the ABA type, but they may also be AB-type diblock polymers, or those having a plurality of alternating elastomeric and thermoplastic blocks, eg AB-AB-A. It is also possible to use mixtures of two or more different block copolymers. Commercially available triblock copolymers often contain certain proportions of diblock copolymers. The diene units can be linked differently. They can also be completely or partially hydrogenated. Both block copolymers of styrene-butadiene and of styrene-isoprene type can be used. They are available, for example under the name Kraton ^® commercially. Furthermore possible to employ thermoplastic-elastomeric block copolymers having end blocks of styrene and a random styrene-butadiene middle block, which are available under the name Styroflex ^®.

For the layer (A) but in principle ethylene-propylene, ethylene-acrylic ester, ethylene-vinyl acetate or acrylate rubbers can be used. Also suitable are hydrogenated rubbers or elastomeric polyurethanes, as well as modified binders in which crosslinkable groups are introduced into the polymeric molecule by grafting reactions.

The type and amount of binder (a1) are selected by the skilled person depending on the desired properties of the printing relief of the flexographic printing element. As a rule, an amount of 40 to 95% by weight of the binder with respect to the amount of all constituents of layer (A) has proven successful. Of course, mixtures of different binders can be used.

The laser-absorbing substance (a2) according to the invention is a conductive carbon black having a specific surface area of at least 150 m ² / g and a DBP number of at least 150 ml / 100 g.

The specific surface area is preferably at least 250 m ² / g and particularly preferably at least 500 m ² / g. The DBP number is preferably at least 200 ml / 100 g and more preferably at least 250 ml / 100 g. They may be acidic or basic carbon blacks. Preferably, the carbon blacks (a2) are basic carbon blacks. Of course, mixtures of different binders can be used.

Suitable conductive carbon blacks having specific surface areas of up to about 1500 m ² / g and DBP numbers of up to about 550 ml / 100 g are commercially available (for example under the name Ketjenblack ^® EC300 J, Ketjenblack ^® EC600 J Fa. Akzo ), Printex ^® XE (Messrs. Degussa) or Black Pearls® 2000 (Messrs. Cabot).

The type and amount of carbon black (a2) are chosen by the person skilled in the art according to the desired properties of the printing relief of the flexographic printing element. The amount also depends on whether the layer (A) is present as the sole relief-forming layer, or whether at least one further relief-forming layer (A) and / or (B) is present. If the flexographic printing element according to the invention comprises only a single layer (A) as a relief-forming layer, an amount of from 0.5 to 20% by weight of the carbon black with respect to the amount of all components of layer (A) has generally proven successful. Preferred is an amount of 3% to 18%, and most preferably 5 to 15%. If it is a multilayer flexographic printing element, which in addition to a layer (A) also includes other layers (A) and / or (B), then the carbon black content in the top layer (A) may be greater, for example up to 35 % By weight, and in special cases even higher. The thickness of such an uppermost layer (A) with a carbon black content greater than 20% by weight should as a rule not exceed 0.3 mm.

The type and amount of the components for crosslinking (a3) depend on the desired crosslinking technique and are selected accordingly by the person skilled in the art. The crosslinking is preferably carried out thermochemically by heating the layer or by irradiation by means of electron radiation. Since the layer is colored more or less black due to the carbon black contained, photochemical crosslinking is possible only in exceptional cases, namely when the carbon black content is only very low and / or the layer is only very thin.

Thermal crosslinking can be carried out by adding polymerizable compounds or monomers to the layer. The monomers have at least one polymerizable, olefinically unsaturated group. Suitable monomers in a manner known in principle are esters or amides of acrylic acid or methacrylic acid with monofunctional or polyfunctional alcohols, amines, aminoalcohols or hydroxyethers and esters, styrene or substituted styrenes, esters of fumaric or maleic acid or allyl compounds. The total amount of monomers possibly used is determined by the skilled person depending on the desired properties of the relief layer. As a rule, however, 30% by weight should not be exceeded with regard to the amount of all constituents of the layer.

Furthermore, a thermal polymerization initiator can be used. Commercially available thermal initiators for free-radical polymerization, such as, for example, suitable peroxides, hydroperoxides or azo compounds, can be used as polymerization initiators. For crosslinking and typical vulcanizers can be used.

The thermal crosslinking may also be carried out by adding a thermosetting resin such as an epoxy resin as the crosslinking component to the layer.

If the binder (a1) used has sufficiently crosslinkable groups, the addition of additional crosslinkable monomers or oligomers is not necessary, but the binder can be crosslinked directly by means of suitable crosslinkers. This is particularly possible with natural rubber or synthetic rubber, which can be crosslinked directly with conventional vulcanizers or peroxide crosslinkers.

Crosslinking by means of electron radiation can, on the one hand, be carried out in analogy to thermal crosslinking by crosslinking the layers comprising ethylenically unsaturated groups comprising monomers by means of electron radiation. The addition of initiators is not required. By means of electron radiation, binders which have crosslinking groups by means of electron radiation can also be crosslinked directly, without the addition of further monomers. Such groups include in particular olefinic groups, protic groups such as -OH, -NH ₂ , -NHR, -COOH or -SO ₃ H and groups which can form stabilized radicals and cations, eg -CR'R "-, -CH ( O-CO-CH ₃ ) -, -CH (O-CH ₃ ) -, -CH (NR'R ") - or -CH (CO-O-CH ₃ ). It is also possible to use compounds having protic groups. Examples include di- or polyfunctional monomers in which terminal functional groups are connected to one another via a spacer, such as dialcohols such as, for example, 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, 1,9-nonanediol, diamines, for example 1, 6-hexanediamine, 1,8-hexanediamine, dicarboxylic acids such as 1,6-hexanedicarboxylic acid, terephthalic acid, maleic acid or fumaric acid.

Photochemical crosslinking may be accomplished by employing the olefinic monomers described above in combination with at least one suitable photoinitiator or photoinitiator system. As initiators for the photopolymerization in a known manner benzoin or benzoin derivatives, such as α-methylbenzoin or benzoin ethers, benzil derivatives, such as benzil ketals, Acylarylphosphinoxide, Acylarylphosphinsäureester, Mehrkernchinone suitable without the listing should be limited thereto.

Of course, layer (A) may optionally also comprise further components such as, for example, plasticizers, dyes, dispersing aids, adhesion additives, antistatic agents, abrasive particles or processing aids. The amount of such additives serves to fine tune the properties and should generally not exceed 30% by weight relative to the amount of all the components of layer (A) of the recording element.

The flexographic printing element according to the invention may comprise only a single layer (A) as a relief-forming layer. It may also have two or more layers (A) on top of each other, which layers may have the same or different composition.

Optionally, the flexographic printing element according to the invention may also have at least one further, relief-forming, crosslinked elastomeric layer (B) between the support and the layer (A). It may also be two or more layers (B) of the same or different composition.

Layer (B) is obtainable by crosslinking a layer comprising at least one binder (b1) and components for crosslinking (b3). Suitable binders (b1) and components for crosslinking (b3) can be selected by the person skilled in the art from the same lists as listed for (a1) and (a3). Of course, layer (B) may optionally also comprise further components such as, for example, plasticizers, dyes, dispersing aids, adhesion additives, antistatic agents, processing aids or abrasive particles.

In a particularly preferred embodiment of (B), the binder (b1) is a thermoplastic elastomeric binder. Since an absorber for laser radiation is not absolutely necessary for the layer (B), transparent layers can also be produced in the UV / VIS range. In this case, the layer can also be particularly photochemically crosslinked.

However, the layer (b) may optionally contain a laser radiation absorbing substance (b2). It is also possible to use mixtures of different absorbers for laser radiation. Suitable absorbers for laser radiation have a high absorption in the range of the laser wavelength. In particular, absorbers are suitable which have a high absorption in the near infrared and in the longer wavelength VIS range of the electromagnetic spectrum. Such absorbers are particularly suitable for absorbing the radiation of high-performance Nd-YAG lasers (1064 nm) and of IR diode lasers typically having wavelengths between 700 and 900 nm and between 1200 and 1600 nm.

Examples of suitable absorbers for the laser radiation (b2) are dyes which absorb strongly in the infrared spectral range, for example phthalocyanines, naphthalocyanines, cyanines, quinones, metal complex dyes, for example dithiolenes or photochromic dyes.

Further suitable absorbers are inorganic pigments, in particular intensively colored inorganic pigments such as, for example, chromium oxides, iron oxides or iron oxide hydrates.

Particularly suitable as laser radiation absorbing substances are finely divided carbon blacks, the selection in (b2) is not limited to the above-mentioned conductivity soot. It is also possible to use carbon blacks with a lower specific surface area and lower DBP absorption. Examples of other suitable carbon blacks include Printex ^® U, Printex ^® A or Special Black 4 ^® (Messrs. Degussa).

The laser-engravable flexographic printing element may optionally include further layers.

Examples of such layers include elastomeric sub-layers of another formulation located between the support and the laser-engravable layer (s), which need not necessarily be laser engravable. With such sub-layers, the mechanical properties of the relief printing plates can be changed without affecting the properties of the actual printing relief layer.

The so-called elastic substructures, which are located under the dimensionally stable carrier of the laser-engravable flexographic printing element, ie on the side of the carrier facing away from the laser-engravable relief layer, serve the same purpose.

Other examples include adhesive layers that bond the backing to overlying layers or to different layers.

Furthermore, the laser-engravable flexographic printing element can be protected against mechanical damage by a, for example consisting of PET protective film, which is located on the topmost layer, and must be removed before engraving with lasers. The protective film may be surface-treated in a suitable manner to facilitate peeling, for example by Siliconization, provided that the surface treatment does not adversely affect the relief top layer in its printing properties.

The layer thickness of layer (A) and optionally layer (B) is suitably selected by the person skilled in the art, depending on the type and the intended use of the flexographic printing plate.

The thickness of layer (A) is usually 0.05 mm to 7 mm. If layer (A) is used as the only relief-forming layer, the thickness should not be less than 0.2 mm. In particular, a thickness of 0.3 to 7 mm, preferably 0.5 to 5 mm and particularly preferably 0.7 to 4 mm, has proven useful with a single-layer flexographic printing element.

If the layer (A) is used as a top layer in combination with a second relief-forming layer (B), a relatively thin layer (A) can also be used. Has proven useful in this case, in particular a thickness of 0.05 to 0.3 mm, preferably 0.07 to 0.2 mm and, for example, a thickness of about 0.1 mm. The total thickness of layer (A), layer (B) and optionally further layers together should as a rule be 0.3 to 7 mm, preferably 0.5 to 5 mm.

If the flexographic printing element according to the invention has two layers (A) and (B), it has proven particularly useful that the top layer (A) has the same or a greater Shore A hardness than the bottom layer (B), without the invention being thereon should be limited. This can be achieved for example by the choice of the degree of crosslinking and / or by a suitable choice of the binder. It has proven particularly useful to use a natural or synthetic rubber as the binder (a1) for the layer (A) in such a two-layer flexographic printing element. For layer (B), it has proven useful to use as binder (b1) a thermolastic elastomeric binder, preferably a styrene-isoprene or styrene-butadiene-type block copolymer, more preferably styrene-butadiene-type block copolymer. In the preferred embodiment of a two or more layered flexographic printing element, layer (B) has no additional absorber for laser radiation.

The flexographic printing element according to the invention can be produced for example by dissolving or dispersing all components in a suitable solvent and pouring onto a support. In the case of multilayer elements, several layers can be cast on one another in a manner known in principle. After casting, if desired, the cover sheet can be applied to protect it from damage to the starting material. It is also possible, conversely, to pour on the cover film and finally laminate the carrier.

It has been proven on a regular basis, first vorzumichen the conductivity soot with the binder or a part of the binder intensively, for example in a kneader, and only add to this mixture, the other components. As a result, a very homogeneous distribution of the conductivity soot in the layer (A) is achieved. The crosslinking can then take place in a manner known in principle, depending on the selected crosslinking technique, by irradiation with electron beams or with actinic light or by heating.

Thermoplastic elastomeric binder-containing layers can also be prepared in a manner known in principle by extrusion and calendering between a cover and a carrier film. This technique is particularly recommended when photochemical or electron beam radiation is to be crosslinked. In principle, it can also be used in thermal crosslinking. However, care must be taken to use a thermal initiator, which does not decompose at the temperature of extrusion and calendering and the layer does not polymerize prematurely.

Of course, it is also possible to use combinations of different production techniques. For example, the layer (A) may be cast on a peelable PET film. Layer (B) can be prepared by extrusion and calendering between a Trägerfolie- and a cover element, wherein as a cover element in analogy to that of EP-B 084 851 described procedure used with the layer (A) coated PET film. In this way, an intensive bond between the two layers is achieved. Subsequently, one can crosslink the whole composite by means of electron radiation. It is also possible to crosslink layer (A) already after casting, for example thermally. Layer (B) can be crosslinked after joining the composite, for example photochemically by irradiation through the carrier film.

The flexographic printing element according to the invention is preferably used for the production of flexographic printing plates by means of direct laser engraving. Of course, a printing relief but also in other ways, for example, be engraved mechanically.

In direct laser engraving, the relief layer absorbs laser radiation to such an extent that it is removed or at least detached at those locations where it is exposed to a laser beam of sufficient intensity. Preferably, the layer is thereby vaporized without premelting or thermally or oxidatively decomposed, so that its decomposition products in the form of hot gases, vapors, smoke or small particles are removed from the layer.

Due to the content of conductivity soot layer (A) has a good absorption especially in the entire infrared spectral range between 750 nm and 12000 nm. Therefore, it can equally well be engraved by means of CO ₂ lasers having a wavelength of 10.6 microns or using Nd-YAG lasers (1064 nm), IR diode lasers or solid state lasers.

For layer (B), the selection of the optimum laser depends on the structure of the layer, and in particular on whether an absorber for laser radiation (b2) is present or not. The flexotypic binders used for layer (B) usually absorb sufficiently in the range between 9000 nm and 12000 nm, so that the engraving of the layer is possible with the aid of CO ₂ lasers without having to add additional IR absorbers. The same applies to UV lasers, such as excimer lasers. When using Nd-YAG lasers and IR diode lasers, the addition of a laser absorber is usually required. The lasers can be operated either continuously or pulsed.

The depth of the elements to be engraved depends on the total thickness of the relief and the type of elements to be engraved and is determined by the person skilled in the art according to the desired properties of the printing form. The depth of the engraved relief elements is at least 0.03 mm, preferably 0.05 mm - is called here the minimum depth between individual grid points. Printing plates with too low relief depths are generally unsuitable for printing by means of flexographic printing technology because the negative elements are filled with printing ink. Individual negative points should usually have greater depths; for those of 0.2 mm diameter, a depth of at least 0.07 to 0.08 mm is usually recommended. For weggravierten surfaces is recommended a depth of more than 0.15 mm, preferably more than 0.3 mm. The latter is of course only possible with a correspondingly thick relief.

For engraving, a laser system can be used, which has only a single laser beam. Preferably, however, laser systems are used which have two or more laser beams. The laser beams can all have the same wavelength or laser beams of different wavelengths can be used. Further preferably, at least one of the beams is specially adapted for generating coarse structures, and at least one of the beams is specially adapted for writing fine structures. With such systems can be particularly elegant produce high-quality printing forms.

For example, it may be in the Laser exclusively CO ₂ laser, the beam for generating the fine structures having a lower power than the beams for producing coarse structures. Thus, for example, the combination of a beam with a power of 50 to 100 W in combination with two beams of 200 W each has proved to be particularly advantageous. It can also be an Nd / YAG laser for writing fine structures, in combination with one or more powerful CO ₂ lasers. For laser engraving particularly suitable multi-beam laser systems and suitable engraving methods are known in principle and, for example, in EP-A 1 262 315 and EP-A 1 262 316 disclosed. The described apparatus has a rotatable drum on which the flexographic printing element is mounted and the drum is rotated. The laser beams move slowly parallel to the drum axis from one end to the other end of the drum and are thereby modulated in a suitable manner. In this way, the entire surface of the flexographic printing element can be engraved gradually. The relative movement between the drum and laser beams can be done by movement of the laser and / or the drum.

With the beam for producing fine structures, only the edges of the relief elements and the uppermost layer section of the relief-forming layer are preferably engraved. The more powerful beams are preferably used to deepen the structures produced and to dig larger non-printing depressions. Of course, the details also depend on the motif to be engraved.

Such multi-beam systems can be used for engraving the flexographic printing elements according to the invention with only one layer (A). Of particular advantage they are used in combination with a multilayer flexographic printing element with a layer (A) and one or more layers (B). In this case, the thickness of the upper layer (A) and the power of the lower-power laser beam and the other laser parameters are matched to one another in such a way that the lower-power beam substantially engraves layer (A), while the more powerful rays essentially layer (B) or also engrave (A) and (B) together. As a rule, a layer thickness of 0.05 mm to 0.3 mm, preferably 0.07 mm to 0.2 mm is sufficient for the top layer (A).

Advantageously, the resulting flexographic printing plate can be cleaned after the laser engraving in a further process step. In some cases, this can be done by simply blowing off with compressed air or brushing.

However, it is preferred to use a liquid cleaning agent for subsequent cleaning in order to be able to remove polymer fragments completely. This is particularly recommended, for example, when using the flexographic printing form food packaging which are subject to particularly stringent requirements with respect to volatile constituents.

The post-purification can be carried out very advantageously by means of water or an aqueous cleaning agent. Aqueous cleaning agents consist essentially of water and optionally small amounts of alcohols and can aid in supporting the cleaning process, such as surfactants, emulsifiers, dispersants or bases. Mixtures commonly used to develop conventional water-developable flexographic printing plates can also be used.

For post-cleaning, mixtures of organic solvents may in principle also be used, in particular those mixtures which usually serve as washout agents for conventionally produced flexographic printing plates. Examples include washout agents based on high boiling, dearomatized petroleum fractions, such as EP-A 332,070 or also "water-in-oil" emulsions, such as from EP-A 463 016 disclosed.

The post-cleaning can be done for example by simply dipping or spraying the relief printing form or in addition by mechanical means, such as brushes or plushes are supported. It is also possible to use conventional flexo washers.

During the post-washing step, any deposits as well as the residues of the additional polymer layer are removed. Advantageously, this layer prevents, or at least hampers, the fact that polymer droplets formed in the course of the laser engraving again firmly connect to the surface of the relief layer. Deposits can therefore be removed particularly easily. It is regularly recommended to carry out the post-wash step immediately after the laser engraving step.

By the use of conductivity carbon blacks in the flexographic printing elements according to the invention, very high quality flexographic printing plates are obtained. While the conductivity carbon black is not as sensitive as conventional carbon blacks, flexographic printing forms can be obtained whose relief elements have very sharp edges and the appearance of melt edges is almost completely suppressed.

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

Example 1: Single-layer flexographic printing element with conductivity black

For the elastomeric relief-forming layer (A), the following doughing materials are used: SBS oil compound consisting of: 67 parts of SBS triblock copolymer (30% styrene, M _w = 170,000 g / mol) 53% 33 parts paraffinic mineral oil SB diblock copolymer (9% styrene, M _w = 230,000 g / mol) 9% Polybutadiene plasticizer 18% 1,6-hexanediol diacrylate 9% Kerobit TBK (thermal stabilizer) 1 % Ketjenblack EC 300 J (conductivity black, BET = 800 m ² / g, DBP adsorption = 310-345 ml / 100 g) 10% total 100%

By means of extrusion (ZSK 53 twin-screw extruder, Werner & Pfleiderer) and subsequent calendering of the melt between an adhesion-lacquer-coated PET carrier film (125 μm) and a silicone-coated protective film, flexographic printing elements of the composition according to the invention described above are produced. The carbon black is metered by means of a flanged side extruder, so that a homogeneous metering and mixing of the carbon black is ensured in the polymer melt. The thickness of layer (A) is 1.02 mm.

After production, the carbon black-filled flexographic printing elements are stored for 4 days at room temperature and then with the aid of electron beams in accordance with the in WO 03/11596 crosslinked processes described. For this purpose, 5 flexographic printing elements each with intermediate layers are packed in a cardboard box and crosslinked by irradiation with electron beams (electron energy 3.5 MeV) into 4 partial doses of 25 kGy each.

After peeling off the protective film is in the crosslinked, carbon black-filled flexographic printing element by means of a laser system with three CO ₂ laser beams (STK BDE 4131, stencil technique Kufstein, 1st beam 100 Watt, 2 and 3, beam 250 Watt) a test pattern with a resolution of Engraved 1270 dpi. The test motif contains various typical elements such as grids, solid areas, non-printing areas to evaluate the quality of the engraving Areas, fine positive and negative points and lines. After engraving, the surface is manually cleaned and dried using a brush with a water-surfactant mixture. The experimental conditions and results are summarized in Table 1.

Comparative Examples A, B and C: Single-layer flexographic printing elements with non-conductive carbon black types

Analogously to Example 1, flexographic printing elements were produced by means of extrusion and calendering of the melt between an adhesion-lacquer-coated PET carrier film and a silicone-coated protective film. The composition of the elastomeric layer was similar to that of Example 1, but various non-conductive types of carbon black were used. The particular carbon black used can be found in the following table: Comparative Example A Printex® U (gas black, BET = 100 m ² / g, DBP adsorption = 115 ml / 100 g) Comparative Example B Printex® A (furnace carbon, BET = 45 m ² / g, DBP adsorption = 118 ml / 100 g) Comparative Example C Special black 4 (gas black, BET = 180 m ² / g, DBP adsorption = 88 ml / 100 g)

Analogously to Example 1, the soot-containing flexographic printing elements are crosslinked by irradiation with electron beams (electron energy 3.5 MeV) in 4 sub-doses of 25 kGy each. After peeling off the protective film, the test motif of Example 1 is engraved into the crosslinked flexographic printing element by means of a laser. The experimental conditions and results are summarized in Table 1. Table 1: example 1 A B C Carbon black product Arne Ketjenblack EC 300 J Printex U Printex A Special black 4 soot [%] 10 10 10 10 DBP [Ml / 100g] 310-345 115 118 88 BET [m ² / g] 800 100 45 180 average primary particle size [Nm] - 25 41 25 type of carbon black highly conductive gas black Fumaceruß gas black pH 8 to 10 4.5 9 3 crosslinking conditions electron irradiation [KGy} 100 100 100 100 Laser engraving parameters laser STK BDE 4131 STK BDE 4131 STK BDE 4131 STK BDE 4131 Setting laser 1 38 38 38 38 Setting laser 2 80 80 80 80 Setting laser 3 80 80 80 80 rotation speed m / sec 7 7 7 7 Laser engraving result engraving depth [.Mu.m] 555 565 585 545 Negative point [400μm] diameter [.Mu.m] 432 461 460 446 Negative point [400μm] depth [.Mu.m] 249 290 250 275 Negative point [200μm] diameter [.Mu.m] 228 254 258 248 Negative point [200μm] depth [.Mu.m] 113 110 108 108

Experiment conditions and results of Example 1 and the Comparative Examples A, B and C.

The engraving results illustrate that the use of conductivity black instead of non-conductive blacks results in improved resolution. This is particularly evident from the fact that negative elements have a smaller diameter at comparable engraving depths. The reason for this is the less melting of the edges.

Furthermore, no pronounced deposits and eruptions occur, as a result of which even fine elements print with sharp edges.

The smoothness of the surface is particularly evident on fine positive elements. Figures 1 and 2 show light micrographs of a 50 micron positive point of a flexographic printing plate according to Example 1 and according to Comparative Examples A, B and C.

The figures clearly show that the use of conductivity carbon black (Example 1), in contrast to other carbon blacks (Comparative Examples A, B and C), results in a much smoother surface, less surface contamination and fewer bursts of printing elements.

Example 2: Two-layer flexographic printing element consisting of one layer (A) and one layer (B)

First, a 100 μm thick, elastomeric layer (A) according to Example 1 was prepared by extrusion between 2 siliconized protective films. After crosslinking the layer by means of electron beams in analogy to Example 1, one of the siliconized films was peeled off in order to obtain a cover element.

Among the components of the photochemically crosslinkable layer (B) it was the components of a nyloflex ^® FAH printing plate (BASF Drucksysteme GmbH).

According to the in EP-A 084 851 The two-layer flexographic printing element was prepared in the usual manner by melt extrusion of the components of layer (B) and calendering between a transparent support film and a cover element, said composite of layer (A) and siliconized film was used as a cover element. As a result, a layer composite of a photochemically crosslinkable, elastomeric layer (B) and a conductivity soot-containing upper layer (A) is produced. The thickness of layer (B) was 0.92 mm.

Layer (B) was irradiated for 20 minutes with UV / A light for photochemical crosslinking through the transparent carrier film (nyloflex F III platesetter, 80-watt tubes). Subsequently, the siliconized cover sheet was peeled off.

The described flexographic printing element can alternatively be obtained by laminating the above-described composite of layer (A) and foil on a finished FAH plate.

The two-layer flexographic printing element from layers (A) and (B) is engraved with a two-beam laser device (100 W Nd-YAG, 250 W CO ₂ ) with different resolutions (1270 dpi, 1778 dpi, 2540 dpi).

By means of the Nd-YAG laser, the fine elements in engraved layer (A) were engraved, the CO ₂ laser was used for engraving the lower areas and, where appropriate, for excavating coarse areas. The achievable resolution was 2540 dpi with sharp edge imaging of fine printing elements.

Example 3: Two-layer flexographic printing element consisting of one layer (A) and one layer (B)

First, a vulcanizable natural rubber-carbon black mixture of the following composition is prepared on a roll mill: Natural rubber (Norrub 340P) 84% Thermal stabilizer (Vulkanox 4010 NA / LG) 1 % stearic acid 1 % Zinc oxide (zinc oxide RS P5) 2% Sulfur crosslinking system 3% Ketjenblack EC 300 J
(Conductivity carbon black, BET = 800 m ² / g, DBP adsorption = 310-345 ml / 100 g) 9% total 100%

By pressing the layer between two siliconized protective films at 150 ° C. for 20 minutes, a 100 μm thick, crosslinked elastomeric layer (A) is obtained. Before further processing, a protective film is peeled off.

Among the components of the photochemically crosslinkable layer (B) it was the components of a nyloflex ^® FAH printing plate (BASF Drucksysteme GmbH). According to the in EP-A 084 851 The two-layer flexographic printing element was prepared in the usual manner by melt extrusion of the components of layer (B) and calendering between a transparent support film and a cover element, said composite of layer (A) and siliconized film was used as a cover element. As a result, a layer composite of a photochemically crosslinkable, elastomeric layer (B) and a conductivity soot-containing upper layer (A) is produced. The thickness of layer (B) was 0.92 mm.

Layer (B) was irradiated with UV / A light (nyloflex F III platesetter, 80 watt tubes) for photochemical crosslinking through the transparent support film for 20 minutes. Subsequently, the siliconized cover sheet was peeled off.

The described flexographic printing element can alternatively be obtained by laminating the above-described composite of layer (A) and foil on a finished FAH plate.

The two-layer flexographic printing element from layers (A) and (B) is engraved with a two-beam laser device (100 W Nd-YAG, 250 W CO ₂ ) with different resolutions (1270 dpi, 1778 dpi, 2540 dpi).

By means of the Nd-YAG laser, the fine elements in engraved layer (A) were engraved, the CO ₂ laser was used for engraving the lower areas and, where appropriate, for excavating coarse areas. The achievable resolution was 2540 dpi with sharp edge imaging of fine printing elements.

Comparative Example D:

For comparison, the two-layer flexographic printing element from Example 2 was engraved with only a 250 W CO ₂ Einstrahllasergerät.

The achievable resolution for mapping grids is max. 1270 dpi. Fine relief elements have coarsely structured flanks than in Example 2.

The fine elements can be engraved with the combination of ND / YAG laser and CO ₂ laser with a finer resolution than just with the CO ₂ laser. Fine halftone dots are much sharper.

Comparative Example E:

For comparison, the two-layer flexographic printing element of Example 3 was engraved with only a 250 W CO ₂ Einstrahllasergerät.

The achievable resolution for mapping grids is max. 1270 dpi. Fine relief elements have coarsely structured flanks than in Example 3.

The fine elements can be engraved with the combination of ND / YAG laser and CO ₂ laser with a finer resolution than just with the CO ₂ laser. Fine halftone dots are much sharper and the flanks of the elements show no eruptions

Claims (9)

1. A flexographic printing element for the production of flexographic printing plates by means of laser engraving, at least comprising, arranged one on top of the other,

   • a dimensionally stable substrate and

   • at least one relief-forming, crosslinked, elastomeric layer (A) having a thickness of from 0.05 to 7 mm, obtainable by crosslinking a layer which comprises at least one binder (a1), a substance (a2) absorbing laser radiation and components for crosslinking (a3),

   wherein the substance absorbing laser radiation is a conductivity carbon black having a specific surface area of at least 150 m²/g and a DBP number of at least 150 ml/100g.
2. A flexographic printing element as claimed in claim 1, wherein the substance is a conductivity carbon black having a specific surface area of at least 500 m²/g and a DBP number of at least 250 ml/100 g.
3. A flexographic printing element as claimed in claim 1 or 2, which comprises at least one further, relief-forming, crosslinked elastomeric layer (B) between the substrate and the layer (A), obtainable by crosslinking a layer which comprises at least one binder (b1) and components for crosslinking (b3).
4. A flexographic printing element as claimed in claim 3, wherein the binder (b1) is a thermoplastic elastomeric binder.
5. A flexographic printing element as claimed in claim 3 or 4, wherein layer (B) furthermore comprises a substance (b2) absorbing laser radiation.
6. A flexographic printing element as claimed in any of claims 3 to 5, wherein the binder (a1) in layer (A) is a natural or synthetic rubber.
7. A process for the production of flexographic printing plates, wherein a flexographic printing element as claimed in any of claims 1 to 6 is used, and a printing relief is engraved with the aid of a laser system into the layer (A) and, if required, layer (B), the depth of the relief elements to be engraved by means of the laser being at least 0.03 mm.
8. A process as claimed in claim 7, wherein the laser system is a laser system having two or more laser beams.
9. A process as claimed in claim 8, wherein at least one of the laser beams is used for producing coarse structures and at least one for producing fine structures.

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