EP1311393B1 - Method for producing laser-engravable flexographic printing elements on flexible metallic supports - Google Patents

Description

The invention relates to methods for producing laser-engravable Flexographic printing elements on flexible metallic supports, which is a cross-linked elastomeric layer with an absorber for laser radiation include. The invention further comprises a method for the production of flexographic printing plates by means of laser engraving using such flexographic printing elements and by a Flexographic printing plates produced by such a method.

The conventional technique for the production of flexographic printing plates by placing a photographic mask on a photopolymer Recording element, irradiation of the element with actinic Light through this mask as well as washing out the unpolymerized Areas of the exposed element with a developer liquid is increasingly being replaced by techniques in which Lasers are used.

In the case of direct laser engraving, depressions are engraved directly into a suitable elastomeric layer with the aid of a sufficiently powerful laser, in particular by means of an IR laser, whereby a relief suitable for printing is formed. To do this, large quantities of the material from which the printing relief is made must be removed. A typical flexographic printing plate today is, for example, between 0.5 and 7 mm thick and the non-printing depressions in the plate are between 300 µm and 3 mm deep. The technique of direct laser engraving for the production of flexographic printing plates has therefore only found economic interest in recent years with the advent of improved laser systems, although the laser engraving of rubber printing cylinders with CO ₂ lasers has been known in principle since the late 1960s. Thus, the need for suitable laser-engravable flexographic printing elements as a starting material for the production of flexographic printing elements by means of laser engraving has also increased significantly.

In principle, commercially available photopolymerizable flexographic printing elements can be used to produce flexographic printing plates by means of laser engraving. US Pat. No. 5,259,311 discloses a method in which in a first step the flexographic printing element is photochemically crosslinked by full-area irradiation and in a second step a printing relief is engraved by means of a laser. The sensitivity of such flexographic printing elements to CO ₂ lasers is low, however, and a post-washing step is also required to remove residues.

It has therefore been proposed to use the elastomeric layer to be engraved with lasers to increase sensitivity Add substances absorbing IR radiation, so for example disclosed by EP-A 640 043 and EP-A 640 044. Such Substances such as carbon black or certain dyes however, also absorb very strongly in the UV / VIS range. flexographic printing, which contain these absorbers cannot more or at most in a very thin layer photochemically cross-linked become. For example, EP-A 640 043 discloses the production of a soot-containing elastomeric layer by means of photo crosslinking. This layer but has only a thickness of 0.076 mm, while the typical The thickness of commercially available flexographic printing plates is 0.5 to 7 mm.

EP-A 640 044 discloses multilayer, laser-engravable flexographic printing elements as well as their use for the production of flexographic printing plates by means of laser engraving. The elements consist of at least an upper layer and an intermediate layer. The upper class can be laser-engraved in any case. The intermediate layer may or may not be laser-engravable. The laser-engravable Layers are reinforced, i.e. either photochemically or thermochemically cross-linked or filled with a filler. The upper class has a thickness of 2.5-130 µm and the laser-engravable Interlayer has a thickness of 51 to 460 microns.

Flexo printing plates are used, among other things, for finishing sheetfed offset printing products, for example by painting or Gold printing (see e.g. "Inline finishing via flexo coating units", German printer 29 (1999) w2 - w6) used. For this Purpose-provided flexographic printing plates are therefore also available as Called lacquer plates. In this area is of particular value placed on register accuracy. Modern flexo coating units in sheetfed offset presses are often with quick release rails or with fully automatic plate feeders equipped only for the insertion of printing plates with a metallic support are suitable. In order to be suitable for this purpose commercially available flexographic printing plates on PET carriers an additional aluminum support glued on. This requires one additional work step, which is time and personnel intensive is. It is therefore desirable to have laser-engravable printing elements to be produced directly on a metallic carrier.

The object of the invention was therefore to provide a simple and economical Process for the production of laser-engravable flexographic printing plates to be provided on metallic supports.

(a) Herstellen einer thermisch vernetzbaren Mischung durch inniges Mischen von mindestens einem elastomeren Bindemittel, 0,5 bis 20 Gew.-% mindestens eines Absorber für Laserstrahlung sowie mindestens einem Polymerisationsinitiator, dessen Temperatur der 10h-Halbwertszeit 10h-t_1/2 mindestens 60°C beträgt, in einem geeigneten Lösemittel,

(b) Aufbringen der Mischung auf einen temporären Träger,

(c) Verdampfen des Lösemittels bei einer Temperatur T₁,

(d) Kaschieren der getrockneten Schicht mit der vom Träger abgewandten Seite auf einen flexiblen metallischen Träger,

(e) optional Entfernen des temporären Trägers, sowie

(f) thermisches Vernetzen der polymerisierbaren Schicht durch Erwärmen auf eine Temperatur T₂, wobei T₂ mindestens 80° C beträgt und T₂ größer als T₁ ist,
   wobei die Dicke der vernetzten elastomeren Schicht
   0,5 bis 5 mm beträgt.

Accordingly, the present invention relates to a method for producing laser-engravable flexographic printing elements which comprise a crosslinked elastomeric layer with at least one absorber for laser radiation on a flexible metallic carrier, characterized by the method steps:

(a) producing a thermally crosslinkable mixture by intimately mixing at least one elastomeric binder, 0.5 to 20% by weight of at least one absorber for laser radiation and at least one polymerization initiator, the temperature of the 10h half-life 10h-t _1/2 at least 60 ° C, in a suitable solvent,

(b) applying the mixture to a temporary support,

(c) evaporating the solvent at a temperature T ₁ ,

(d) laminating the dried layer with the side facing away from the carrier onto a flexible metallic carrier,

(e) optionally removing the temporary carrier, as well

(f) thermal crosslinking of the polymerizable layer by heating to a temperature T ₂ , where T _{2 is} at least 80 ° C. and T _{2 is} greater than T ₁ ,
the thickness of the cross-linked elastomeric layer
0.5 to 5 mm.

(a) Herstellen einer thermisch vernetzbaren Mischung durch inniges Mischen von mindestens einem elastomeren Bindemittel, 0,5 bis 20 Gew.-% mindestens eines Absorber für Laserstrahlung sowie mindestens einem Polymerisationsinitiator, dessen Temperatur der 10h-Halbwertszeit 10h-t_1/2 mindestens 60°C beträgt, in einem geeigneten Lösemittel,

(b) Aufbringen der Mischung auf einen flexiblen, metallischen Träger,

(c) Verdampfen des Lösemittels bei einer Temperatur T₁,

(d) thermisches Vernetzen der getrockneten, polymerisierbaren Schicht durch Erwärmen auf eine Temperatur T₂, wobei T₂ mindestens 80° C beträgt und T₂ größer als T₁ ist.
   wobei die Dicke der vernetzten elastomeren Schicht
   0,5 bis 5 mm beträgt.

In a further embodiment, the invention relates to a further method for producing such laser-engravable flexographic printing elements, characterized by the following method steps:

(a) producing a thermally crosslinkable mixture by intimately mixing at least one elastomeric binder, 0.5 to 20% by weight of at least one absorber for laser radiation and at least one polymerization initiator, the temperature of the 10h half-life 10h-t _1/2 at least 60 ° C, in a suitable solvent,

(b) applying the mixture to a flexible, metallic support,

(c) evaporating the solvent at a temperature T ₁ ,

(d) thermal crosslinking of the dried, polymerizable layer by heating to a temperature T ₂ , where T _{2 is} at least 80 ° C. and T _{2 is} greater than T ₁ .
the thickness of the cross-linked elastomeric layer
0.5 to 5 mm.

The invention further relates to a method for manufacturing of flexographic printing plates by engraving a printing relief by means of a laser in the according to the inventive method Flexographic printing elements obtained and obtained by the method Flexographic printing plates.

The details of the method according to the invention are as follows perform.

The flexographic printing elements obtained by the method according to the invention include a laser-engravable, cross-linked elastomer Layer on a flexible metallic support.

The term "laser-engravable" means that the Layer possessing the property of laser radiation, in particular the To absorb radiation from an IR laser, so that it adheres to such Places where they have a laser beam of sufficient intensity exposed, removed or at least detached. Preferably the layer is evaporated without melting beforehand or thermally or oxidatively decomposed so that their decomposition products in the form of hot gases, vapors, smoke or small ones Particles are removed from the layer. The invention encompasses but also, the residues of the irradiated layer afterwards mechanically removed, e.g. by blasting with a liquid or a gas or for example by suction or wipe with a cloth.

The metallic used for the inventive method Carriers for the flexographic printing elements are flexible. Under flexible For the purposes of this invention, it should be understood that the carrier are so thin that they can be bent around impression cylinders. On the other hand, they are also dimensionally stable and so thick that the carrier during the production of the flexographic printing element or the assembly of the finished printing plate on the printing cylinder is not is kinked.

Above all, thin sheets come as flexible metallic supports or metal foils made of steel, preferably made of stainless steel, magnetizable Spring steel, aluminum, zinc, magnesium, nickel, Chromium or copper into consideration, whereby the metals are also alloyed could be. Combined metallic supports can also be used such as with tin, zinc, chrome, aluminum, nickel or also combinations of different metals coated steel sheets are used, or also such metal supports, which are produced by lamination identical or different types of metal sheets can be obtained. Pre-treated sheets, such as, for example, can also be used phosphated or chromated steel sheets or anodized Aluminum sheets are used. As a rule, will degrease the sheets or foils before inserting them. Prefers Carriers made of steel or aluminum are used. Particularly preferred is magnetizable spring steel. Flexographic printing plates on such carriers can be used without adhesive tapes or the like. directly on magnetic Printing cylinders are clamped.

The thickness of such flexible metallic supports is usually between 0.025 mm and 0.4 mm and aligns next to the desired one Degree of flexibility also depending on the type of used Metal. Steel beams are usually thick between 0.025 and 0.30 mm, in particular between 0.14 and 0.24 mm. Aluminum supports usually have a thickness between 0.25 and 0.4 mm.

The flexible metallic carrier is advantageous with an in Provide printing inks insoluble and swell-resistant adhesive layer. The adhesive layer mediates good adhesion between the flexible, metallic carrier and the laser-engravable to be applied later Layer so that the latter is bent around the Laser drum or around the printing cylinder does not come off.

In principle, any adhesive layer can be used to carry out the present Process are used, provided the adhesive layer is in the usual organic or organic components containing solvents of flexographic printing inks, such as Ethanol or isopropanol insoluble and resistant to swelling.

Has been suitable for carrying out the method according to the invention For example, an adhesive layer has proven to be a binder comprises, which is embedded in a suitable polymeric matrix is. As a rule, discrete can be seen under the microscope Recognize domains of elastomeric binder and the matrix.

Examples of suitable binders for the adhesive layer include elastomeric or thermoplastic elastomeric polymers, usually also used for the production of relief printing plates such as polymers or copolymers of 1,3-dienes or SIS or SBS block copolymers. Mixtures of two or several different elastomeric binders used become.

The amount of elastomeric binder in the adhesive layer is determined by the Expert determined depending on the desired properties. It is usually 10 to 70% by weight based on the sum of all components the adhesive layer, in particular 10 to 45% by weight and whole especially 15 to 35% by weight.

The polymeric matrix is usually one crosslinked polymeric matrix, which is formed using a suitable crosslinking Systems is available. The cross-linked polymer matrix can thermally by polycondensation or polyaddition of suitable monomers or oligomers are obtained, for example by reaction of polyisocyanates and suitable hydroxyl groups Compounds such as polyurethane resins containing hydroxy groups or Polyester resins to form cross-linked polyurethanes.

Optionally, the adhesive layer can contain other components and auxiliary substances such as additional binders to influence the Properties, dyes, pigments or plasticizers include.

To produce the adhesive layer, the binder and the further components of the adhesive layer are usually dissolved in suitable solvents such as, for example, THF, toluene or ethyl acetate, mixed intensively with one another, the solution optionally filtered and applied to the flexible metallic support. The application can take place, for example, by means of a roller or by means of a caster. After application, the solvent is evaporated off and then the system is crosslinked.
The residual solvent content in the layer should be less than 5% by weight with respect to all components of the layer.

The thickness of the adhesive layer is usually 2 to 100 μm, preferably 10 to 50 µm and particularly preferably 15 to 30 µm. It can also several adhesive layers of the same, approximately the same or different compositions are used one above the other.

The described adhesive layer on the one hand provides good adhesion between the laser-engravable layer and the flexible metallic Carrier and is not soluble and not swellable in organic Solvents commonly used for flexographic inks be used. On the other hand, it also has a special one good non-stickiness. This is especially true of Advantage if the metallic carrier is not immediately after Coating to be processed further. Metallic coated in this way Carriers can be used during production without additional Measures such as loading paper as Interlayer, stacked or rolled without sticking together. The invention of course also includes an adhesive layer to be applied in-line.

For the production of the laser-engravable elastomer layer in one process step an intimate mixture of at least one elastomeric binders, at least one polymerization initiator and at least one absorber for laser radiation in one suitable solvent. The mixture can also also ethylenically unsaturated monomers and others Aids and / or additives include.

The known ones, usually, can be used as elastomeric binders used for the production of photopolymerizable flexographic printing plates Binders are used. In principle, both elastomeric binders as well as thermoplastic elastomeric binders suitable. Examples of suitable binders are the known ones Three-block copolymers of the SIS or SBS type, which are also whole or can be partially hydrogenated. It can also be elastomeric Ethylene / propylene / diene type polymers, ethylene / acrylic acid rubbers or elastomeric polymers based on acrylates or Acrylate copolymers are used. More examples of suitable ones Polymers are described in DE-A 22 15 090, EP-A 084 851, EP-A 819 984 or EP-A 553 662. The polymeric binders can be crosslinkable groups, for example ethylenically unsaturated Have groups in the main chain of the polymer. It can also be used binders, the crosslinkable side groups exhibit.

Mixtures of two or more different binders can also be used be used.

The type and amount of binder used are determined by Specialist depending on the desired characteristics of the printing relief selected. As a rule, the amount of binder is 50 to 95% by weight based on the amount of all components of the dried, laser-engravable layer, i.e. without consideration of the solvent. The amount is preferably 60 to 90% by weight.

The recording layer according to the invention further comprises 0.5 to 20% by weight of at least one absorber for laser radiation. Mixtures of different absorbers for laser radiation can also be used be used. Suitable absorbers for laser radiation a high absorption in the range of the laser wavelength. In particular absorbers are suitable which have a high absorption in the near Infrared, as well as in the longer-wave VIS range of the electromagnetic Show spectrum. Such absorbers are particularly suitable for the absorption of radiation from powerful Nd-YAG lasers (1064 nm) as well as IR diode lasers, which are typically wavelengths between 700 and 900 nm and between 1200 and 1600 nm exhibit.

Examples of suitable absorbers for the laser radiation are in infrared spectral range highly absorbent dyes like for example phthalocyanines, naphthalocyanines, cyanines, quinones, Metal complex dyes such as dithiolenes or photochromic dyes. Other suitable absorbers are inorganic pigments, in particular intensely colored inorganic pigments such as Chromium oxides, iron oxides, soot or metallic particles.

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

Further particularly suitable absorbers for laser radiation are iron-containing solids, in particular intensely colored iron oxides. Such iron oxides are commercially available and are usually used as color pigments or as pigments for magnetic recording. Suitable absorbers for laser radiation are, for example, FeO, goethite α-FeOOH, Akaganeit β-FeOOH, lepidocrocite γ-FeOOH, hematite α-Fe ₂ O ₃ , maghemite γ-Fe ₂ O ₃ , magnetite Fe ₃ O ₄ or Berthollide. Furthermore, doped iron oxides or mixed oxides of iron with other metals can be used. Examples of mixed oxides are Umbra Fe ₂ O ₃ xn MnO ₂ or Fe _x Al _(1-x) OOH, in particular various spinel black pigments such as, for example, Cu (Cr, Fe) ₂ O ₄ , Co (Cr, Fe) ₂ O ₄ or Cu (Cr, Fe, Mn) ₂ O ₄ . Examples of dopants are, for example, P, Si, Al, Mg, Zn or Cr. Such dopants are generally added in small amounts in the course of the synthesis of the oxides in order to control particle size and particle shape. The iron oxides can also be coated. Such coatings can be applied, for example, in order to improve the dispersibility of the particles. These coatings can consist, for example, of inorganic compounds such as SiO ₂ and / or AlOOH. However, organic coatings, for example organic adhesion promoters such as aminopropyl (trimethoxy) silane, can also be applied. Particularly suitable absorbers for laser radiation are FeOOH, Fe ₂ O ₃ and Fe ₃ O _4, most preferably Fe ₃ O _4.

The size of the iron-containing, inorganic solids used, in particular the iron oxides, is selected by the person skilled in the art depending on the desired properties of the recording material. Solids with an average particle size of more than 10 µm are usually unsuitable. Since iron oxides in particular are anisometric, this information relates to the longest axis. The particle size is preferably less than 1 μm. So-called transparent iron oxides can also be used, which have a particle size of less than 0.1 μm and a specific surface area of up to 150 m ² / g.

Also suitable as an absorber for laser radiation containing iron Compounds are ferrous metal pigments. Are particularly suitable acicular or rice grain pigments with a length between 0.1 and 1 µm. Such pigments are as magnetic pigments known for magnetic recording. In addition to the iron can also other dopants such as Al, Si, Mg, P, Co, Ni, Nd or Y may be present, or the iron metal pigments may be used with it be coated. Ferrous metal pigments are used to protect against corrosion superficially anoxidized and consist of a possibly doped Iron core and a possibly doped iron oxide shell.

There will be 0.5 to 20 wt .-% absorber based on the sum of all components the laser-engravable elastomer layer. The amount of absorber added is determined by the person skilled in the art desired properties of the laser-engravable flexographic printing element selected. In this context, the expert take into account that the absorbers added are not just speed and efficiency of engraving the elastomeric layer influenced by laser, but also other properties of the Flexographic printing element, such as its hardness, elasticity, Thermal conductivity, abrasion resistance or ink acceptance. The amount of the absorber for laser radiation is preferably 0.5 up to 15% by weight and particularly preferably 0.5 to 10% by weight.

In principle, commercially available polymerization initiators thermal initiators used for radical polymerization such as peroxides, hydroperoxides or Azo compounds.

The selection of suitable initiators comes for the implementation of the The method according to the invention is of particular importance. suitable thermal initiators only disintegrate in the final Step of the method according to the invention, thermal crosslinking, with high reaction speed in radicals. They are in the preceding process steps of mixing and dispersing, Pouring, evaporation of the solvent and thermal lamination largely stable. The term "largely stable thermally" means in this context that the initiators in the course of Execution of these steps of the inventive method at most decay so slowly that the layer crosslinks and / or the mixture by polymerization only in minor Dimensions can take place and the proper implementation of the Procedure not affected.

The thermal stability of an initiator is usually indicated by the temperature of the 10h half-life 10h-t _1/2 , that is to say the temperature at which 50% of the original amount of initiator has broken down into radicals after 10 h. Further details can be found in "Encylopedia of Polymer Science and Engineering", vol. 11, pages 1ff., John Wiley & Sons, New York, 1988.

Initiators particularly suitable for carrying out the process according to the invention have a 10h-t _1/2 of at least 60 ° C., preferably of at least 70 ° C. Particularly suitable initiators have a 10h-t _1/2 of at least 80 ° C.

Examples of suitable initiators include certain peroxyesters, such as t-butyl peroctoate, t-amyl peroctoate, t-butyl peroxyisobutyrate, t-butylperoxymaleic acid, t-amylperbenzoate, di-t-butyldiperoxyphthalate, t-butyl perbenzoate, t-butyl peracetate or 2,5-di (benzoylperoxy) -2,5-dimethylhexane, certain diperoxyketals such as 1,1-di (t-amylperoxy) cyclohexane, 1,1-di (t-butylperoxy) cyclohexane, 2,2-di (t-butylperoxy) butane or ethyl 3,3-di (t-butylperoxy) butyrate, certain dialkyl peroxides such as di-t-butyl peroxide, t-butyl cumene peroxide, dicumol peroxide or 2,5-di (t-butyl peroxy) 2,5-dimethylhexane, certain diacyl peroxides such as dibenzoyl peroxide or diacetyl peroxide, certain t-alkyl hydroperoxides such as t-butyl hydroperoxide, t-amyl hydroperoxide, pinane hydroperoxide or Cumene hydroperoxide. Certain azo compounds are also suitable such as 1- (t-butylazo) formamide, 2- (t-butylazo) isobutyronitrile, 1- (t-butylazo) cyclohexanecarbonitrile, 2- (t-butylazo) -2-methylbutanitrile, 2,2'-azobis (2-actoxypropane), 1,1'-azobis (cyclohexane carbonitrile), 2,2'-azobis (isobutyronitrile) or 2,2'-azobis (2-methylbutanenitrile).

Usually 1 to 15% by weight of initiator with respect to the Quantity of all components of the laser-engravable layer used, preferably 1 to 10% by weight.

The method according to the invention can be carried out by for crosslinking only those in the binder as side groups or in the main chain existing ethylenically unsaturated groups become. But it can also be ethylenically unsaturated Monomers are used. As ethylenically unsaturated Basically, monomers can be used those which usually also for the production of photopolymerizable flexographic printing elements be used. The monomers should with the Binders be compatible and at least one polymerizable, have ethylenically unsaturated double bond. As special esters or amides of acrylic acid have been found to be advantageous or methacrylic acid with mono- or polyfunctional alcohols, Amines, amino alcohols or hydroxy ethers and esters, styrene or substituted styrenes, esters of fumaric or maleic acid or allyl compounds. Examples of suitable monomers are butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, 1,9-nonanediol diacrylate, trimethylolpropane triacrylate, dioctyl fumarate, N-dodecyl maleimide. Mixtures of different types can also be used Monomers are used. The type and amount of the monomer is the specialist depending on the desired properties and the binder used. Usually is the total amount of monomers but not more than 30% by weight the amount of all components of the laser-engravable Layer and preferably not more than 20% by weight.

Optionally, other additives and aids such as for example plasticizers, fillers, dyes, compatibilizers or dispersing agents to adjust the desired properties of the relief layer can be used. The amount of such other ingredients should, however, as a rule Do not exceed 20% by weight, preferably 10% by weight.

The components for the production of the laser-engravable layer are mixed intimately in a suitable solvent, so that a homogeneous solution or dispersion of the components is obtained. As a rule, it is recommended to everyone to dissolve organic components of the layer as completely as possible, and inorganic components such as carbon black or Iron oxide pigments as absorbers for the laser radiation evenly disperse in the organic matrix.

A suitable solvent is chosen by the person skilled in the art depending on the one used Components of the layer selected. Suitable solvents include in particular toluene, xylenes, cyclohexane or THF. It Mixtures of different solvents can also be used.

The ingredients can be mixed intimately at room temperature or at temperatures above room temperature. The person skilled in the art will ensure that he chooses a temperature for the dissolving process which is adapted to the boiling point of the solvent and the 10h-t _{1/2 of} the initiator. As a rule, mixing should not be carried out at temperatures above 60 ° C. Conventional stirring or dispersing units can be used for intimate mixing. If necessary, the solution can be filtered before use.

In the first embodiment of the method according to the invention, the mixture is applied to a temporary carrier. As a temporary carrier, PET films are particularly suitable, which can also be modified to make them easier to remove later, for example by siliconization. The application is generally carried out by means of a roller or a caster, the thickness of the layer being set by parameters known in principle to the person skilled in the art, such as the setting of the casting gap, draw-off speed and / or viscosity of the solution. After the application, the solvent is evaporated at a temperature T ₁ . The solvent can be evaporated, for example, in a drying tunnel. The temperature T ₁ can be chosen by the person skilled in the art depending on the desired conditions, such as the boiling point of the solvent, the desired drying rate or the desired residual solvent content. As a rule, T _{1 is} greater than 25 ° C. T ₁ is preferably between 30 ° C. and 80 ° C. and for example at 40 ° C. However, temperatures above 80 ° C. can also be selected in special cases. In order to avoid premature polymerization, the temperature T ₁ is in any case lower than the temperature T ₂ , at which thermal crosslinking takes place in a later process step. The residual solvent content in the layer after the drying process should be less than 5% by weight with respect to all components of the layer. The residual solvent content is preferably less than 3% by weight, based on the sum of all components of the layer.

Several laser-engravable layers of the same, approximately, can also be used same or different composition on top of each other be poured. In principle, both can be poured wet-on-wet or the lower layer can be dried first or be completely dried before the second layer is infused.

Additional layers can optionally be cast, who take on other tasks in the system and their composition therefore deviates from that of the laser-engravable layer (s).

For example, a thin top layer can be cast, the the printing surface of the finished flexographic printing plate forms. With such an upper class for Print behavior and ink transfer essential parameters such as roughness, Abrasiveness, surface tension, surface stickiness, Ink acceptance or solvent resistance on the printing surface be changed without the characteristics typical of the relief to influence the printing form such as hardness or elasticity. Surface properties and layer properties can so be changed independently of each other to achieve an optimal To achieve printing result. The upper layer can be an absorber for laser radiation included without this being absolutely necessary is. The composition of the upper class is only insofar limited than the laser engraving of the laser-engravable ones underneath Layer must not be affected and the Upper layer must be removable together with this. The upper class should be thin compared to the laser-engravable layer. As a rule, the thickness of such an upper layer exceeds not 100 µm, preferably the thickness is between 5 and 80 µm, particularly preferably between 10 and 50 microns.

Furthermore, a thermally polymerizable, but not laser-engravable underlayer can be cast, which are in the finished Flexographic printing element between the carrier and the laser-engravable Layer. With such lower layers, the mechanical properties of the relief printing plates are changed, without affecting the relief characteristics of the printing form.

The dried, thermally polymerizable layer or the composite corresponding layers with that of the temporary support opposite side on the flexible metallic support Laminated using a suitable solvent. As Laminating solvent is suitable, for example, tetrahydrofuran.

After lamination, it is recommended to use the temporary carrier deduct to avoid possible complications due to shrinkage or to firmly adhere the support to the laser-engravable Avoid layer in the course of networking without necessarily doing so is required in each individual case.

In the last process step for producing the flexographic printing element according to the invention, the polymerizable layer is thermally crosslinked by heating to the temperature T ₂ . The temperature T ₂ is at least 80 ° C and is greater than T ₁ . The difference between T ₁ and T ₂ is determined by the person skilled in the art depending on the specific circumstances. As a rule, a difference of at least 10 ° C is recommended, preferably a difference of at least 20 ° C and particularly preferred is a difference of at least 30 ° C. Larger differences, for example those of 50 ° C., can also be selected. As a rule, T _{2 is} between 80 ° C. and 180 ° C., preferably between 80 ° C. and 150 ° C. and particularly preferably between 90 ° C. and 130 ° C. For example, T ₂ is 100 ° C.

The thickness of the cross-linked, elastomeric layer or the layer composite is between 0.5 and 5 mm. The thickness is determined by the expert depending on the intended use of the printing plate suitably chosen.

If the laser-engravable flexographic printing element is not a temporary one Carrier has more, it can optionally by a protective film, For example, a PET film can be protected on the surface is applied or laminated.

In a further embodiment of the method according to the invention the laser-engravable layer will not be temporary Cast carrier, but directly on the flexible metallic Carrier that can optionally be coated with an adhesive layer. The step of laminating can thus be omitted.

The laser-engravable obtained by the method according to the invention Flexographic printing elements serve as the starting material for production of flexographic printing plates.

The method includes firstly the protective film, if present, is subtracted. In the following process step, a printing relief into the flexographic printing element by means of a laser engraved. Image elements are advantageously engraved in which the flanks of the picture elements initially drop vertically and widen only in the lower area of the picture element. Thereby a good socketing of the pixels is still low Dot gain achieved. But it can also be different designed flanks of the pixels are engraved.

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

Laser engraving can advantageously be carried out in the presence of an oxygen-containing one Gases, especially air. The oxygen-containing gas can be engraved on the recording element be blown. A comparatively gentle one Gas flow can be generated, for example, with the aid of a fan. But it can also be a stronger one with the help of a suitable nozzle Be blown over the recording material. This Embodiment has the advantage that detached solid components the layer can be removed effectively.

Optionally, the flexographic printing plate obtained can still be cleaned become. With such a cleaning step, detached, but not yet completely removed from the plate surface Layer components removed. The printing plate can, for example can be cleaned with a brush. This cleaning process can with a suitable aqueous and / or organic solvent get supported. A suitable solvent is known to the person skilled in the art chosen under the proviso that it is not the relief layer loosen or swell strongly. The cleaning can, for example also with compressed air or by suction.

The method according to the invention provides laser-engraved ones Flexographic printing plates on metallic supports that are are characterized by excellent dimensional stability. she are particularly suitable for use in flexo coating units from Sheet-fed presses.

The following examples are intended to explain the invention in more detail, without the invention being limited thereto.

Experimental:

For the experiments on laser engraving, the laser-engravable Flexographic printing elements by means of an adhesive tape on the cylinder ALE laser machine (type Meridian Finesse) glued on. This Machine is equipped with a Nd-YAG laser with the power of 130 W. After adjusting the focus to the plate thickness, the plate at a speed of 160 cm / s and a feed exposed to 20 µm of laser radiation.

Example 1:

A mixture of the following components was prepared in toluene at a temperature of 30 ° C: component feedstock Weight fraction [%] (without toluene) binder Kraton 1161, SIS block copolymer, Shell 77 α-methylstyrene / vinyl toluene, copolymer Piccotex 100, Hercules 8th monomers hexanediol 7 hexanediol 4 soot Printex U, Degussa 1 initiator tert-butyl peroctoate (10h-t _1/2 735C) 3

The components were dissolved and the carbon black was dispersed therein. The The homogeneous dispersion obtained was degassed and by means of a Kammergiessers on a PET film as a temporary carrier (Lumirror X43, 150 µm) spread. After drying (2 hours at 40 ° C, circulating air), the dry layer thickness was 950 µm. This Layer was coated with an adhesive varnish metallic carrier (steel, thickness 0.14 mm) connected. Subsequently the film was removed. The dried layer was crosslinked thermochemically by heating to 100 ° C. for 45 min.

Laser engraving:

The laser-engravable flexographic printing element obtained was as above described engraved with lasers. It became a deep relief obtained from 460 µm. The resolution was 60 lines / cm.

Example 2:

The mixture obtained in Example 1 was by means of a chamber caster directly onto a metallic one coated with an adhesive varnish Cast beam (steel, thickness 0.05 mm). The layer was Dried at 40 ° C for 2 h. The dried layer was heated thermochemically crosslinked to 100 ° C for 45 min.

Laser engraving:

The laser-engravable flexographic printing element obtained was as above described engraved with lasers. It became a deep relief obtained from 460 µm. The resolution was 60 lines / cm.

Example 3

A mixture of the following components is prepared in toluene at a temperature of 30 ° C: component feedstock Weight fraction [%] (without toluene) binder EPDM rubber Keltan 1446A, DSM 77 α-methylstyrene / vinyl toluene, copolymer Piccotex 100, Hercules 8th monomers hexanediol 7 hexanediol 4 soot Printex U, Degussa-Huls 1 initiator tert-butyl peroctoate (10h-t _1/2 73 ° C) 3

The components were dissolved and the carbon black was dispersed therein. The The homogeneous dispersion obtained was degassed and by means of a Kammergiessers on a PET film as a temporary carrier (Lumirror X43, 150 µm) spread. After drying (2 hours at 40 ° C, circulating air), the dry layer thickness was 950 µm. This Layer was coated with an adhesive varnish metallic carrier (steel; thickness 0.14 mm) connected. Subsequently the film was removed. The dried layer was crosslinked thermochemically by heating to 100 ° C. for 45 min.

Laser engraving:

The laser-engravable flexographic printing element obtained was as above described engraved with lasers. It became a relief obtained from 530 µm. The resolution was 60 lines / cm.

Example 4:

The mixture obtained in Example 3 was by means of a chamber caster directly onto a metallic one coated with an adhesive varnish Cast beam (steel, thickness 0.05 mm). The layer was Dried at 40 ° C for 2 h. The dried layer was heated thermochemically crosslinked to 100 ° C for 45 min.

Laser engraving:

The laser-engravable flexographic printing element obtained was as above described engraved with lasers. It became a relief obtained from 540 µm. The resolution was 60 lines / cm.

Claims (10)

1. A process for the production of laser-engravable flexographic printing elements, at least comprising a flexible metallic support and a crosslinked elastomeric layer which comprises at least one absorber for laser radiation, which comprises the following steps:

   (a) preparation of a thermally crosslinkable mixture by intimate mixing of at least one elastomeric binder, 0.5 to 20% by weight of at least one absorber for laser radiation and at least one polymerization initiator whose 10h-t_1/2 10 hour half-life temperature is at least 60°C, in a suitable solvent,

   (b) application of the mixture to a temporary support,

   (c) evaporation of the solvent at a temperature T₁,

   (d) lamination of the dried layer by means of the side facing away from the support onto a flexible metallic support,

   (e) optionally removal of the temporary support, and

   (f) thermal crosslinking of the polymerizable layer by warming to a temperature T₂, where T₂ is at least 80°C and T₂ is greater than T₁,

   the thickness of the crosslinked elastomeric layer being from 0.5 to 5mm.
2. A process for the production of laser-engravable flexographic printing elements, at least comprising a flexible metallic support and a crosslinked elastomeric layer which comprises at least one absorber for laser radiation, which comprises the following steps:

   (a) preparation of a thermally crosslinkable mixture by intimate mixing of at least one elastomeric binder, 0.5 to 20% by weight of at least one absorber for laser radiation and at least one polymerization initiator whose 10h-t_1/2 10 hour half-life temperature is at least 60°C, in a suitable solvent,

   (b) application of the mixture to a flexible, metallic support,

   (c) evaporation of the solvent at a temperature T₁,

   (d) thermal crosslinking of the dried, polymerizable layer by warming to a temperature T₂, where T₂ is at least 80°C and T₂ is greater than T₁,

   the thickness of the crosslinked elatomeric layer being from 0.5 to 5 mm.
3. A process as claimed in either of claims 1 and 2, wherein the thermally crosslinkable mixture furthermore comprises at least one ethylenically unsaturated monomer.
4. A process as claimed in any one of claims 1 to 3, wherein the thermally crosslinkable mixture comprises further additives and auxiliaries.
5. A process as claimed in any one of claims 1 to 4, wherein the flexible, metallic support is a support made from aluminum, steel or magnetizable spring steel.
6. A process as claimed in any one of claims 1 to 5, wherein the flexible metallic support is provided with an adhesive layer.
7. A process as claimed in any one of claims 1 to 6, wherein the amount of the absorber for laser radiation is from 0.5 to 10% by weight, based on the amount of all constituents of the crosslinked, elastomeric layer.
8. A process as claimed in any one of claims 1 to 7, wherein the absorber for laser radiation is carbon black and/or an iron-containing, inorganic solid.
9. A process for the production of flexographic printing plates, which comprises engraving a relief into a laser-engravable flexographic printing element obtainable by a process as claimed in claims 1 to 8, by means of a laser.
10. A flexographic printing plate obtainable by a process as claimed in claim 9.