EP1423280A1

EP1423280A1 - Method for producing flexo printing forms by means of laser-direct engraving

Info

Publication number: EP1423280A1
Application number: EP03759963A
Authority: EP
Inventors: Margit Hiller
Original assignee: BASF Drucksysteme GmbH
Current assignee: Flint Group Germany GmbH
Priority date: 2002-06-18
Filing date: 2003-06-16
Publication date: 2004-06-02
Anticipated expiration: 2023-06-16
Also published as: US20040231540A1; DE10227188A1; DE50301036D1; JP2005534524A; EP1423280B1; ATE302687T1; WO2003106172A1; US6880461B2; AU2003246440A1

Abstract

The invention relates to a method for producing flexo printing forms by means of laser direct engraving, whereby a flexo printing element is used as a starting material, comprising a relief-forming layer consisting of a combination of a styrene-butadiene-block copolymer and 20-40 wt. % of a softener. The invention also relates to flexo printing forms which are obtained according to said method and the use of thereof for flexo printing with water and alcohol based inks.

Description

Process for the production of flexographic printing plates by means of direct laser engraving

description

The invention relates to a method for producing flexographic printing plates by means of direct laser engraving, in which a flexographic printing element is used as the starting material, which has a relief layer with a combination of a styrene-butadiene block copolymer and 20-40% by weight of a plasticizer. The invention further relates to flexographic printing forms obtainable by this process and the use of the flexographic printing forms for flexographic printing with water-based or alcohol-based printing inks.

Lasers are now used in the field of offset printing plates as well as in the field of relief printing plates for various steps in the manufacturing process.

For example, it is known to describe the photosensitive layers of offset printing plates by means of suitable laser imagesetters. The light-sensitive layer is chemically changed by the laser, for example cross-linked. From the fine crude product (for example, Imaging Technology, chapter, see. 3.4.1.2., Ullmann's Encyclopedia of Industrial Chemistry ^•, 6 ^th Edt. 2000 Electronic release) the finished lithographic printing plate by means of a suitable development process obtained. The thickness of said light-sensitive layers of offset printing plates is usually 0.3 to 5 μm.

Furthermore, it is known to use IR-ablative masks for imaging flexographic printing plates instead of photographically created masks, as disclosed for example in EP-A 654 150. A thin, IR-sensitive, opaque layer is applied to the photopolymerizable layer. The thickness of such IR-ablative layers is usually only a few μm. The IR-ablative layer is imaged with an IR laser, ie removed at the points where it is struck by the laser beam. The actual print relief is produced in a conventional manner: through the mask created, exposure is made to actinic light and the relief layer is thus selectively cross-linked. Development is then carried out in a customary manner with a washing-out agent, both the photosensitive material being removed from the unexposed areas of the relief-forming layer and the remnants of the IR-ablative layer. Because the IR-ablative mask layer for the actual printing process irrelevant, the materials can only be selected for optimal use as a mask.

In the case of direct laser engraving for the production of flexographic printing plates, on the other hand, a printing relief is engraved by a laser directly into the relief layer of a flexographic printing element. A subsequent development step, as with the conventional or the mask process, is no longer necessary. Typical relief layer thicknesses of flexographic printing plates are between 0.5 and 7 mm, with special thin-layer plates under certain circumstances even only 0.2 mm. The non-printing depressions in the relief are at least 0.03 mm in the raster area, significantly more for other negative elements and can assume values of up to 3 mm for thick plates. Large amounts of material have to be removed with the laser.

EP-A 640 043 and EP-A 64-0 044 disclose single-layer or multi-layer elastic laser-engravable flexographic printing elements for the production of flexographic printing plates by means of laser engraving. The elements consist of "reinforced" elastic layers. Elastomeric binders are used to produce the layer. The so-called reinforcement increases the mechanical strength of the layer in order to enable flexographic printing. The reinforcement is achieved either by introducing suitable fillers, photochemical or thermochemical crosslinking or combinations thereof.

No. 5,259-, 311 discloses a method in which, in a first step, a commercially available flexographic element is photochemically crosslinked by full-area irradiation by means of UV / A, then the release layer is removed with a flexo detergent and in a second step a printing relief is engraved in by means of a laser becomes. A cleaning step is then carried out using a flexo detergent followed by a final drying of the plate.

Although the engraving of rubber printing cylinders by means of lasers has been known in principle since the 1960s and the property rights cited were also filed 10 years ago, laser engraving has only gained broader economic interest in recent years with the advent of improved laser systems. The improvements in the laser systems include better focusability of the laser beam, higher power and computer-controlled beam modulation. With the introduction of new, more powerful laser systems, the question of particularly suitable materials for laser-engravable flexographic printing plates is becoming increasingly important. Problems that did not play a role in the past because the laser systems did not allow the engraving of very fine structures are now becoming significant and are leading to new demands on the material.

The relief layers of flexographic printing plates are naturally soft and have relatively low melting or softening points. In laser engraving, they therefore have a strong tendency to form melting edges around the engraved elements. At the edge of the engraved elements, the layer melts under the influence of the laser beam, but is no longer or not completely decomposed. Such enamel edges can usually not be removed or at least not completely removed even after washing and lead to unclean pressure. Unwanted melting of the layer also results in a reduced resolution of the print motif compared to the digital data set.

To solve this problem, EP-A 1 136 254 suggests using relief layers with polyoxyalkylene / polyethylene glycol graft copolymers as binders. However, since these copolymers are water-soluble, relief printing plates of this type can only be used to a limited extent. The relief relief swells too much in water-based flexographic printing inks, so that undesired effects occur during printing, such as an increase in the toning which is no longer tolerable. Such printing forms can therefore essentially only be used for printing with UV inks. There is an urgent need to provide laser-engravable relief printing elements which are also suitable for printing with watercolors, but which are nevertheless undesirable. Have the layer melted with lasers.

Furthermore, the degradation products that arise in the course of laser engraving often cause problems. In addition to gaseous components, aerosols are also generated. As a rule, these are extremely sticky and can partially or completely re-deposit on the surface of the relief, in unfavorable cases even react again with the surface. This leads to unclean surfaces and thus to poor printing behavior.

No. 5, 259, 311 proposes to solve this problem by cleaning the surface of the relief printing form after laser engraving with the aid of an organic solvent. However, the sticky decomposition products have essentially the same solubility behavior as the relief layer. For relief layers of hydrophobic polymers must therefore be used to remove the decomposition products, an organic solvent. The cross-linked relief-forming layer is no longer soluble in it, but it can still swell. Following such a post-washing step, the layer must therefore be dried again in a further process step. As a result, the time and handling advantages achieved by laser engraving in the process are nullified, since the drying process takes up most of the time during processing. Decomposition products that have reacted with the surface can no longer be removed at all and can therefore also be seen in the print. It would be extremely desirable to be able to have a flexographic printing element in which any deposits can simply be removed with water or aqueous cleaning agents without the plate swelling.

Furthermore, the fastest possible engraving is required for the economical production of flexographic printing plates by means of laser engraving. The speed of the engraving depends on the laser system chosen. On the other hand, the sensitivity of the relief-forming layer to the laser radiation chosen should be as high as possible. When it comes to sensitivity, however, it must be taken into account that the relief layer of the flexographic printing plate imparts both the elastomeric properties and the printing-typical properties. Measures to improve sensitivity must therefore not impair the properties mentioned.

The object of the invention was therefore to provide a process for the production of flexographic printing plates by means of laser direct engraving, in which the occurrence of melting edges is ^" significantly reduced, as little aerosols as possible, any deposits of decomposition products are removed by simply treating the plate with water or aqueous cleaning agents which enables the fastest possible engraving in high resolution and the flexographic printing plates obtained are also suitable for printing with water-based flexographic printing inks.

Accordingly, a process for the production of flexographic printing by means of laser engraving has been found, in which as

Starting material uses a cross-linkable, laser-engravable flexographic printing element, which at least comprises one above the other A dimensionally stable support,

• at least one crosslinkable, laser-engravable relief layer with a thickness of at least 0.2 mm, at least comprising an elastomeric binder, a plasticizer and crosslinkable components

and the method comprises at least the following steps:

(a) cross-linking of the relief-forming layer,

(b) engraving a relief in the cross-linked relief-forming layer with the aid of a laser, the depth of the relief elements engraved with the laser being at least 0.03 mm,

the binder being a styrene-butadiene block copolymer with an average molecular weight M _w of 100,000 to 250,000 g / mol, a Shore A hardness of 55 to 85 and a styrene content of 20-40% by weight regarding the

Binder, and the amount of plasticizer is more than 20 to 40 wt.% Based on the sum of all components of the layer.

Furthermore, flexographic printing plates were found which are obtainable by the described process, and the use of these flexographic printing plates for flexographic printing with water-based and / or alcohol-based printing inks.

Surprisingly, it was found that the combination according to the invention of the styrene-butadiene block copolymer with 20 to 40% by weight of plasticizers gives flexographic learning which has excellent sensitivity to lasers. The relief-forming layer hardly melts under the influence of the laser radiation, and there are hardly any melting edges around the negative elements. The flexographic printing plates obtained also allow printing with water and or alcohol inks without the relief layer swelling excessively with these inks.

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

Examples of suitable dimensionally stable supports for the flexographic printing elements used as the starting material for the process are plates, foils and conical and cylindrical tubes (sleeves) made of metals such as steel, aluminum, copper or nickel or of plastics such as polyethylene terephthalate (PET), poly ethylene naphthalate (PEN), polybutylene terephthalate, polyamide, polycarbonate, optionally also fabrics and nonwovens, such as glass fiber fabrics and composite materials, for example made of glass fibers and plastics. Dimensionally stable supports include, above all, dimensionally stable support films such as polyester films, in particular PET or PEN films, or flexible metallic supports, such as thin sheets or metal films made of steel, preferably made of stainless steel, magnetizable spring steel, aluminum, zinc, magnesium, nickel, chromium or Consider copper.

The flexographic printing element further comprises at least one laser-engravable, cross-linkable relief-forming layer. The . crosslinkable relief layer can be applied directly to the carrier. However, there may also be other layers between the carrier and the relief layer, such as, for example, adhesive layers and / or elastic sublayers.

The crosslinkable relief-forming layer comprises at least one elastomeric binder, crosslinkable components and 20 to 40% by weight of a plasticizer. As a rule, the crosslinkable relief layer as a whole already has elastomeric properties; for the invention, however, it is sufficient if only the crosslinked relief layer has the elastomeric properties typical of a flexographic printing form.

According to the invention, the elastomeric binder is a styrene-butadiene block copolymer. These can be two-block copolymers, three-block copolymers or multiblock copolymers, in which several styrene and butadiene blocks alternate in succession. It can be linear, branched or star-shaped block copolymers. The block copolymers used according to the invention are preferably styrene-butadiene-styrene three-block copolymers. SBS block copolymers of this type are commercially available, for example under the name Kraton®, it having to be taken into account that commercially available three-block copolymers usually have a certain proportion of two-block copolymers. Mixtures of different SBS block copolymers can of course also be used.

The elastomeric styrene-butadiene block copolymers used according to the invention in the starting material have an average molecular weight M _w (weight average) of 100,000 to 250,000 g / mol. M _w is preferably 150,000 to 250,000 g / mol and very particularly preferably 150,000 to 200,000 g / mol. The styrene content of the styrene-butadiene block copolymer used is 20 to 40% by weight, based on the binder, preferably 25 to 35% by weight.

The Shore A hardness of the binder is determined in accordance with ISO 868. According to the invention, the elastomeric styrene-butadiene block copolymer used has a hardness of 55 to 85 Shore A. The hardness of the binder is preferably 60 to 80 Shore A and very particularly preferably 65 to 75 Shore A.

In addition to the at least one styrene-butadiene block copolymer, the relief layer can optionally also have one or more secondary binders. Such secondary binders can be used by a person skilled in the art to fine-tune the properties of the relief layer. In principle, the selection of secondary binders is not restricted as long as the properties of the relief layer are not impaired thereby. Secondary binders are preferably styrene-butadiene block copolymers which merely do not meet the requirements described above with regard to molecular weight, hardness and styrene content. Of course, they can also be chemically different types of polymers. The amount of secondary binder should generally not exceed 20% by weight, preferably 10% by weight, based on the total amount of all binders used. If the secondary binder is a styrene-butadiene block copolymer, up to about 30% by weight can be used , ^'the total amount of all binders used can be used.% in special cases up to about 40 wt.% with respect to.

The total amount of binders, ie styrene-butadiene Blockcopo- lymeren and possibly secondary binders present together, is usually 40 to 80 weight ^'with respect to the sum% -. Of all components of the relief-forming layer, preferably 40 to 70% by weight and particularly preferably 45. up to 65% by weight.

For the process according to the invention, the binder is used in a mixture with at least one plasticizer. The amount of plasticizer is 20% by weight to 40% by weight with respect to all components of the relief-forming layer, preferably 25 to 40% by weight! % and particularly preferably 30 to 40% by weight.

The person skilled in the art selects suitable plasticizers depending on the desired properties of the relief layer. Examples of suitable plasticizers include modified and unmodified natural oils and resins, such as high-boiling paraffinic, naphthenic or aromatic mineral oils, synthetic oligomers or resins such as oligostyrene, oligomeric styrene-butadiene copolymers, oligomeric α-methyl oligomeric-methylstyrene / p-methylstyrene copolymers, liquid oligobutadienes, in particular those with a molecular weight between 500 and 5000 g / mol, or liquid oligomeric acrylonitrile-butadiene copolymers or oligomeric ethylene-propylene-diene rubber.

So-called inert plasticizers are particularly suitable for the process according to the invention. "Inert" in the sense of this invention. means that the plasticizer include at least chen essential in no or no polymerizable groups, the radical in the course of crosslinking the relief-forming layer can respond ^'such that the plasticizer in the polymeric network of the relief layer with using one of these. Inert plasticizers in particular have essentially no ethylenically unsaturated double bonds.

Examples of inert plasticizers include high boiling paraffinic, naphthenic and aromatic mineral oils. Essentially paraffinic and / or naphthenic mineral oils are preferred. Mineral oils of this type are also referred to as white oils, the person skilled in the art distinguishing between technical white oils, which may still have a low aromatic content, and medical white oils, which are essentially free of aromatics.

Mixtures of various plasticizers can of course also be used, provided that the properties of the relief layer are not adversely affected thereby. Preferred mixtures are those which comprise at least one inert plasticizer. A mixture of liquid oligobutadienes and white oil can be mentioned as an example.

The type and amount of the components for crosslinking the layer depend on the desired crosslinking technique and are selected accordingly by a person skilled in the art. The full-area crosslinking of the crosslinkable relief layer is preferably carried out photochemically, therochemically or by means of electron radiation.

In the case of photochemical crosslinking, the relief layer comprises at least one photoinitiator or a photoinitiator system and suitable monomers or oligomers.

In a known manner, suitable initiators for the photopolymerization are benzoin or benzoin derivatives, such as .alpha.-methylbenzoin or benzoin ether, benzene derivatives, such as, for example, benzil ketals, acylarylphosphine oxides, acylarylphosphinic acid esters, multinuclear quinones, without the enumeration being restricted to this. The monomers have at least one polymerizable, olefinically unsaturated group. Esters or amides of acrylic acid or methacrylic acid with ono- or polyfunctional alcohols, amines, amino alcohols or hydroxy ethers and esters, styrene or substituted styrenes, esters of fumaric or maleic acid or allyl compounds have proven to be particularly advantageous. Examples of suitable monomers include butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, 1, 4-butanediol diacrylate, 1, 6-hexanediol diacrylate, 1, 6-hexanediol dimethacrylate, 1, 9-nonanediol diacrylate, trimethylolpropanetriacrylate, dioctyl dylctyl dylctyl dylcrylate, dioctyl dylcrylate, dioctyl dylcrylate - mid. Suitable oligomers with olefinic groups can also be used. Mixtures of different monomers or oligomers can of course also be used, provided that there are no undesirable effects. The total amount of monomers is determined by the person skilled in the art depending on the desired properties of the relief layer. As a rule, however, 20% by weight should not be exceeded with regard to the amount of all components of the laser-engravable relief-forming layer.

Thermal crosslinking is preferably carried out in analogy to photochemical crosslinking, in that a thermal polymerization initiator is used instead of a photoinitiator. Commercially available thermal initiators for radical polymerization, such as peroxides, hydroperoxides or azo compounds, are suitable in principle. The thermal crosslinking can also be carried out by adding a thermosetting resin such as an epoxy resin as the crosslinking component to the layer.

The crosslinking by means of electron radiation is preferably carried out in analogy to the photochemical crosslinking, in that the already described photochemically crosslinkable relief layers are used and the UV radiation is replaced by electron radiation. 'The addition of initiators is not absolutely necessary.

Optionally, the crosslinkable relief layer can further comprise an absorber for laser radiation. Mixtures of different absorbers for laser radiation can also be used. 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-wave VIS range of the electromagnetic spectrum. Such absorbers are particularly suitable for absorbing the radiation from Nd-YAG lasers (1064 nm) and from IR diode lasers. stars that typically have wavelengths between 700 and 900 nm and between 1200 and 1600 nm.

Examples of suitable absorbers for laser radiation are highly absorbing dyes such as phthalocyanines, naphthalocyanines, cyanines, quinones, metal complex dyes such as dithiolenes or photochromic dyes in the infrared spectral range. Other suitable absorbers are inorganic pigments, in particular intensely colored inorganic pigments such as chromium oxides, iron oxides, soot or metallic particles. Finely divided soot types with a primary particle size between 10 and 50 nm are particularly suitable as absorbers for laser radiation.

The amount of the absorber optionally added is chosen by the person skilled in the art depending on the properties of the laser-engravable flexographic printing element that are desired in each case. In this context, the person skilled in the art will take into account that the absorbers added not only influence the engraving of the elastomeric layer by laser, but also the properties of the relief printing plate obtained as the end product of the process, such as, for example, its hardness, elasticity, thermal conductivity or ink transfer behavior. As a rule, it is therefore recommended that no more than 20% by weight, preferably no more than 10% by weight, of absorber for the laser radiation with respect to use of all ^components' of the layer of the sum.

As a rule, it is not advisable to add relief layers that are to be photochemically crosslinked, absorbers for laser radiation that also absorb in the UV range, since this at least severely impairs the photopolymerization and may make it completely impossible. It is regularly advisable to crosslink such layers containing laser absorbers thermally or by means of electron beams.

The relief-forming layer can furthermore also comprise additives and auxiliary substances such as, for example, dyes, dispersing agents or antistatic agents. However, the amount of such additives should generally not exceed 5% by weight, based on the amount of all components of the crosslinkable, laser-engravable layer in the learning of the record.

The crosslinkable relief-forming layer can also be built up from several partial layers. These crosslinkable sublayers can be of the same, approximately the same or different material composition. The thickness of the laser-engravable, elastomeric layer is at least 0.2 mm. The thickness is preferably 0.3 to 7 mm, particularly preferably 0.5 to 5 mm and very particularly preferably 0.7 to 4 mm. The thickness is suitably chosen by the person skilled in the art depending on the intended use of the flexographic printing plate.

In a preferred embodiment the starting material comprises an additional, water-soluble or at least swellable ^• laser-engravable polymer layer which is arranged on the laser-engravable relief-forming layer, and the. comprises at least one polymer which is soluble, swellable or dispersible in aqueous solvents. Such a layer serves to facilitate an optional cleaning step. Solid decomposition products formed in the course of laser engraving can deposit on this auxiliary layer and be detached more easily.

Examples of the polymer which is soluble or at least swellable in aqueous solvents include polyvinyl alcohol, polyvinyl alcohol / polyethylene glycol graft copolymers, polyvinyl pyrrolidone and its derivatives or cellulose derivatives, in particular cellulose esters and cellulose ethers such as, for example, methyl cellulose, ethyl cellulose, benzyl cellulose, hydroxyalkyl cellulose or , Mixtures of several polymers can of course also be used.

The additional laser-engravable polymer layer ^■ can also contain additives and auxiliaries, for example plasticizers or laser absorbers. If it is intended to crosslink the laser-engravable relief layer photochemically, then the additional polymer layer should be as transparent as possible in the UV range. This is not absolutely necessary for other networking techniques.

The thickness of the additional polymer layer should be as small as possible. It essentially depends on the depth of focus of the laser used for engraving in the process. It is limited so that there is no significant broadening of the focus on the surface of the relief layer.

The thickness of such an additional polymer layer ^'should as a rule 100 μ not exceed. As a rule, satisfactory results are no longer achieved with larger thicknesses. Preferably, the thickness should not exceed 50 ^microns'. The thickness is particularly preferably 1-40 μm and very particularly preferably 2-25 μm. The laser-engravable flexographic printing element can optionally comprise further layers.

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

So-called elastic substructures, which are located under the dimensionally stable support of the laser-engravable flexographic printing element, that is to say on the side of the support facing away from the laser-engravable relief layer, serve the same purpose.

Other examples include adhesive layers that connect the support to layers above or different layers to one another.

Furthermore, the laser-engravable flexographic printing element can be protected against mechanical damage by a protective film, for example made of PET, also known as a cover film, which is located on the top layer in each case and which has to be removed before laser engraving. The protective film can be surface-treated in a suitable manner to facilitate removal, for example by siliconization, provided that the relief properties of the relief top layer are not negatively influenced by the surface treatment.

The flexographic printing element used as the starting material for the process can be produced, for example, by dissolving or dispersing all components in a suitable solvent and pouring them onto a support. In the case of multilayer elements, several layers can be cast onto one another in a manner known in principle. After casting, the cover sheet of the ^'raw material when requested may be applied to protect against damage. Conversely, it is also possible to borrow the cover film and then laminate the carrier. The casting method is particularly recommended if thermal crosslinking is to be used.

If photochemical or electron beam crosslinking is provided, the relief layer is preferably produced in a manner known in principle by melt extrusion between a carrier film and a cover film or a cover element and capacitor. landieren of the composite obtained, as disclosed for example by EP-A 084 851. In this way, thick layers can also be produced in a single operation. Multi-layer elements can be produced, for example, by means of coextrusion. Flexographic printing elements with metallic supports can preferably be obtained by casting or extruding onto a temporary support and then laminating the layer onto the metallic support.

It has proven itself regularly, first of all the styrene-butadiene

To process block copolymers with part of the plasticizer in a suitable mixing unit to form a homogeneous mass. The mass obtained is then processed in a second step in the extruder together with the other components of the layer and the rest of the plasticizer. This also advantageously allows a large amount of plasticizer to be incorporated over a short extruder length, and the plasticizer to be incorporated particularly homogeneously. In addition, the residence times of the polymeric mass in the hot zone of the extruder can be reduced.

The additional polymer layer can be applied, for example, by dissolving the constituents in a suitable solvent and pouring them onto the relief-forming layer. However, the cover film is preferably coated with the additional polymer layer and laminated onto the relief-forming layer or used as a film for the extrusion process.

In the method according to the invention, the starting material is first crosslinked over the entire surface in the first method step (a).

The full-area crosslinking of the crosslinkable relief layer can take place photochemically, in particular by irradiation with UV-A radiation with a wavelength between 320 and 400 nm, or UV-A / VIS radiation with a wavelength of approximately 320 to approximately 700 nm men. Full-surface thermochemical crosslinking is achieved by heating the relief layer as uniformly as possible and at constant temperature. Furthermore, it can be crosslinked by uniform irradiation with electron beams. The radiation dose required for crosslinking can be divided particularly advantageously into several partial doses.

Photochemical crosslinking is particularly suitable for relief layers that do not contain any strongly colored absorbers for laser radiation and that are transparent or at least largely transparent in the UV / VIS range. Of course, transparent relief layers can also be crosslinked thermochemically or by means of electron beams. Colored laser absorbers containing Delivery layers can advantageously be crosslinked thermochemically or by means of electron radiation.

Naturally, the flexographic printing element used as the starting material for the process is usually produced by a printing plate manufacturer, while the laser engraving is carried out by a cliché or printing company. The full-surface networking (a) can be carried out by the cliché itself. For example, the photochemical crosslinking can be carried out in commercially available flexo imagesetters. Networking can of course also be done by or at the manufacturer of the flexographic printing element.

In method step (b), a printing relief is engraved into the cross-linked relief layer using a laser. If a protective film is present, it is removed before the engraving.

The term “laser-engravable” is to be understood to mean that the relief layer has the property of absorbing laser radiation, in particular the radiation from an IR laser, so that it is removed at those locations where it is exposed to a laser beam of sufficient intensity or at least replaced. The layer is preferably vaporized or thermally or oxidatively decomposed without melting beforehand, so that its decomposition products are removed from the layer in the form of hot gases, vapors, smoke or small particles.

IR lasers are particularly suitable for engraving. For example, a C0 laser with a wavelength of 10.6 μm can be used. Nd-YAG lasers (1064 nm), IR diode lasers or solid-state lasers can also be used. Lasers with shorter wavelengths can also be used, provided that the laser is of sufficient intensity. For example, a frequency-doubled (532 nm) or frequency-tripled (355 n) Nd-YAG laser or excimer laser (e.g. 248 nm) can also be used.

The addition of absorbers for laser radiation essentially depends on the type of laser that is to be used for engraving. The styrene-butadiene block copolymers used for the relief layer absorb the radiation from CO lasers to a sufficient extent, so that when using this type of laser, additional IR absorbers in the relief layer are generally not necessary. The same applies to UV lasers such as excimer lasers. With Nd-YAG lasers and IR diode lasers, the addition of a laser absorber is usually necessary. The image information to be engraved can be transferred directly from the lay-out computer system to the laser apparatus. The lasers can either be operated continuously or pulsed.

Relief elements are advantageously engraved, in which the

Flanks of the elements initially fall vertically and only widen in the lower area. This results in a good base of the relief points with a slight increase in tone value. Flanks with a different design can also be engraved.

The depth of the elements to be engraved depends on the overall thickness of the relief and the type of elements to be engraved and is determined by the person skilled in the art depending on the desired properties of the printing form. The depth of the relief elements to be engraved is at least 0.03 mm, preferably at least 0.05 mm - the minimum depth between individual grid points is mentioned here. Printing plates with too low relief depths are generally unsuitable for printing using flexographic printing technology because the negative elements fill up with printing ink. Individual negative points should usually have greater depths; for those with a diameter of 0.2 mm, a depth of at least 0.07 to 0.08 mm is usually recommended. At removed by engraving surfaces a depth of more than 0.15 mm is recommended, preferably more than 0.4 ^'mm. The latter is of course only possible with a correspondingly thick relief.

The flexographic printing plate obtained is advantageously cleaned after the laser engraving in a further process step (c). In some cases this can be done by simply blowing off with compressed air or brushing.

In a preferred embodiment, a liquid cleaning agent is used for subsequent cleaning in order to be able to completely remove polymer fragments as well. This is particularly recommended, for example, if the flexographic printing form is to be used to print on food packaging that has particularly strict requirements with regard to volatile components.

The post-cleaning can be carried out particularly advantageously using water or an aqueous cleaning agent. Aqueous cleaning agents consist essentially of water and, optionally, small amounts of alcohols and can contain auxiliaries such as surfactants, emulsifiers, dispersing aids or bases to support the cleaning process. Mixtures can also be used, which are usually used to develop conventional, water-developable flexographic printing plates be used. Since the relief layer cannot be swelled with styrene-butadiene block copolymers in water, the use of water or aqueous cleaning agents avoids time-consuming drying of the printing form.

The post-cleaning can be carried out, for example, by simply immersing or spraying the relief printing form, or it can also be supported by mechanical means, such as brushing or plushing. Conventional flexo washers can also be used.

In the post-washing step, any deposits and the remnants of the additional polymer layer are removed. This layer advantageously prevents, or at least makes it more difficult for polymer droplets formed in the course of the laser engraving to bond firmly to the surface of the relief layer again. Deposits can therefore be removed particularly easily. It is regularly recommended to carry out the post-washing step immediately after the laser engraving step.

Although not the preferred option can ^■ in principle for the subsequent cleaning, mixtures of organic solvents used, in particular those mixtures which typically serve as washout for conventionally produced flexographic printing plates. Examples include leaching agents based on high-boiling, de-aromatized petroleum fractions, as disclosed for example by EP-A 332 070 or "water-in-oil" emulsions as disclosed by EP-A 463 016. This variant can be used above all if there is no additional polymer layer. If an additional polymer layer is present but cannot be removed with the organic solvent used, it must also be cleaned with water or an aqueous cleaning agent.

The flexographic printing plates obtained are particularly suitable for printing with water colors and alcohol colors. However, they are of course also suitable for printing with UV inks or flexographic printing inks that contain small amounts of esters. ^'

The following examples are intended to illustrate the invention:

Example 1:

A photochemically crosslinkable laser-engravable relief-forming layer was produced with the following starting materials

The components were made with an extruder. (ZSK 53) processed at 140 ° C, dime ^{'nsionsstabile} carrier film of PET and a protective film made of PET introduced by means of a slot die between one and then calendered by means of a DERS Zweiwalzenkalan-. The layer thickness of the crosslinkable, laser-engravable layer obtained was 1.14 mm.

Example 2:

The components were processed as in Example 1. The layer thickness of the crosslinkable, laser-engravable layer obtained was 1.14 mm.

Example 3 The procedure was as in Example 1, except that an additional polymer layer composed of a water-soluble polymer was applied to the relief layer (polyvinyl alcohol, Alcotex 4-86, thickness: 3 μm). For this purpose, the protective film made of PET mentioned at the outset was coated with a solution of Alcotex 4-86 in a water / alcohol mixture and the solvent mixture was evaporated. The coated PET film was used for the extrusion process described. The layer thickness of the crosslinkable, laser-engravable layer obtained was 1-14 mm.

Comparative Example 1:

There was a photochemically vernetzba ^'re laser-engravable relief-forming layer with the following starting materials

Carrying out the process according to the invention:

The PET protective film was removed from the laser-engravable flexographic printing elements obtained in the examples and comparative examples. In a first process step, they were crosslinked over the entire surface by exposure to UVA light for 20 minutes. In Examples 1 and 2, an additional crosslinking of the uppermost region of the relief layer was carried out with UVC light.

Laser engraving of the flexographic printing elements

A three-beam C0 ₂ laser (from STK, Kufstein, type BDE 4131) was used for laser engraving tests. After the flexographic printing element was clamped onto a cylinder, a test motif consisting of different, representative, positive and negative elements was engraved into the flexographic printing element. In addition to completely engraved open areas and 100% tonal values, the motif also contains various halftone areas with tonal values between 1% and 98% as well as 40 μm wide negative lines in the axial and transverse directions to the cylinder axis of rotation. The rotation speed of the cylinder was 7 m / s. The power setting of the steels was: 1st beam 40, 2nd and 3rd beam 90.

Following the laser engraving, the flexographic printing plates obtained were washed with water for two minutes while simultaneously brushing the surface. To this end, ^J is a nyloprint Washer (overall councils combination "CW 22 x 30", BASF Printing Systems GmbH).

The following characteristics are determined to assess the quality of the flexographic printing plates:

• The engraving depth T as a measure of the sensitivity, measured as the height difference between a completely removed area and the plate surface.

• The visual assessment of the formation of deposits, enamel edges and sticky droplets (“deposits”) and the visual assessment of the washability of superficial deposits (“washability”) when washing with water.

The results are summarized in Table 1.

Furthermore, Figures 1 and 2 each show an image of the flexographic printing plate obtained according to Comparative Example 1 and Example 1.

Table 1 Both the measurement results and the figures clearly show that the method according to the invention gives flexographic printing plates which have hardly any melt edges and significantly fewer deposits than in the comparative example. The depth of the engraved relief elements is significantly larger in the example than in the comparative example.

The flexographic printing plates obtained according to the invention are well suited for printing with alcohol and water colors.

Explanation of the pictures:

Fig.l: Flexographic printing plate according to comparative example 1 Fig.2: Flexographic printing plate according to example 1

The "A" is 6 mm wide and 7 mm high.

Claims

claims

1. Process for the production of flexographic printing plates by means of laser engraving, in which a crosslinkable, laser-engravable flexographic printing element is used as the starting material, which element comprises at least one above the other

A dimensionally stable support,

At least one crosslinkable, laser-engravable relief-forming layer with a thickness of at least 0.2 mm, at least comprising an elastomeric binder, a plasticizer and components for crosslinking,

and the method comprises at least the following steps:

(a) cross-linking of the relief-forming layer,

(b) engraving a print relief into the cross-linked relief layer with the aid of a laser, the depth of the relief elements to be engraved with the laser being at least 0.03 mm,

characterized in that the binder is a styrene-butadiene block copolymer with an average molecular weight M _w of 100,000 to 250,000 g / mol, a Shore A hardness of 55 to 85 and a styrene content of 20-40 % By weight based on the binder, and the amount of plasticizer is 20 to 40% by weight based on the sum of all components of the layer.

2. The method according to claim 1, characterized in that the average molecular weight of the binder M _{w is} 150,000 to 250,000 g / mol.

3. The method according to claim 1 or 2, characterized in that the styrene content of the binder is 25 to 35 wt.%.

4. The method according to any one of claims 1 to 3, characterized in that the plasticizer is an inert plasticizer.

5. The method according to claim 4, characterized in that the inert plasticizer is at least one selected from the group of aromatic, naphthenic and paraffinic mineral oils.

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6. The method according to any one of claims 1 to 5, characterized in that the full-surface crosslinking (a) is carried out photochemically, by means of electron beams or thermally.

10 7. The method according to any one of claims 1 to 6, characterized in that the relief layer additionally comprises an absorber for laser radiation.

8. The method according to any one of claims 1 to 7, characterized in that the flexographic printing element comprises an additional, water-soluble laser-engravable layer which is arranged on the laser-engravable relief-forming layer and at least one which is soluble, swellable or dispersible in aqueous solvents Includes polymer, and which is removed after 20 process step (b) in a further process step (c) by means of water or an aqueous cleaning agent.

9. The method according to claim 8, characterized in that the polymer is at least one selected from the group

Group of polyvinyl alcohol, polyvinyl alcohol / polyethylene glycol graft copolymers, polyvinyl pyrrolidone or cellulose derivatives.

30 10. Flexographic printing plate, obtainable by a process according to one of claims 1 to 9.

11. Use of a flexographic printing plate according to claim 10 for flexographic printing with water-based inks and / or alcohol-based. 35

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