EP1578605A1

EP1578605A1 - Method for producing flexoprinting forms by means of laser engraving using photopolymer flexoprinting elements and photopolymerisable flexoprinting element

Info

Publication number: EP1578605A1
Application number: EP03767749A
Authority: EP
Inventors: Margit Hiller; Jens Schadebrodt; Wolfgang Wenzl
Original assignee: XSYS Print Solutions Deutschland GmbH; BASF Drucksysteme GmbH
Current assignee: Flint Group Germany GmbH
Priority date: 2002-12-13
Filing date: 2003-12-05
Publication date: 2005-09-28
Also published as: US20060112844A1; DE10258668A1; DE10258668A8; WO2004054803A1; AU2003292192A1; JP2006511356A

Abstract

The invention relates to a method for producing flexoprinting forms by means of direct laser engraving using photopolymerisable flexoprinting elements as base material. The cross-linking of the photopolymerisable flexoprinting elements occurs by means of actinic light passing through a protective element which is essentially permeable to actinic light.

Description

Process for the production of flexographic printing plates by means of laser engraving using photopolymeric flexographic printing elements and photopolymerizable flexographic printing element

description

The invention relates to a method for producing flexographic printing plates by means of direct laser engraving using photopolymerizable flexographic printing elements as the starting material, the crosslinking of the photopolymerizable flexographic printing element with actinic light being effected by a protective element which is essentially transparent to actinic light.

In the case of direct laser engraving for the production of flexographic printing forms, a printing relief is engraved with a laser directly into the relief-forming layer of a flexographic printing element. A subsequent development step as in the conventional process for the production of flexographic printing plates 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 with 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. Direct laser engraving therefore differs very clearly from other techniques known from the printing plate sector, in which lasers are only used to write on a mask, but the actual production of the printing form still uses a washout or development process he follows.

Starting materials which have been specially adapted for producing flexographic printing plates by means of laser engraving and methods for laser engraving have already been proposed on various occasions, for example from US Pat. No. 3,549,733, EP-A 640043, EP-A 640 044, EP-A 710 573,

EP-A 1 080883 or EP-A 1 136 254.

US Pat. No. 5,259,311 has proposed the use of commercially available photopolymerizable flexographic printing elements as the starting material for the production of flexographic printing plates by means of laser engraving.

Commercial photopolymerizable flexographic printing elements comprise a dimensionally stable support, usually made of PET film, a relief-forming layer of an elastomeric binder, ethylenically unsaturated monomers and a photoinitiator or photoinitiator system, a substrate layer and a cover film made of PET. The substrate layer is also known as a release layer. to The protective film is removed in the conventional, photochemical production of a flexographic printing plate. The substrate layer adheres more firmly to the photopolymerizable layer than to the protective film and thus remains on the photopolymerizable layer. A photographic mask is then placed on the substrate layer and irradiated with actinic light through this mask. The irradiation is usually carried out by means of a so-called vacuum frame or a vacuum film. The vacuum ensures particularly intimate contact between the photographic mask and the flexographic printing element, and in addition the diffusion of oxygen into the photopolymerizable layer is prevented or at least made more difficult. The task of the substrate layer is to ensure that the protective film can be removed from the flexographic printing element and that the photographic mask for exposure can also be placed on the flexographic printing element and then removed again without the mask sticking to the photopolymerizable layer or so strongly adheres to the fact that the surface of the relief-forming layer is damaged when the mask is removed. Following the irradiation, the substrate layer and the unexposed areas of the photopolymerizable layer are removed by means of a washing-out agent.

In the method of laser engraving proposed by US Pat. No. 5,259,311, the PET protective film of the conventional flexographic printing element is first removed, the substrate layer remaining on the photopolymerizable layer. The relief-forming layer is then photochemically crosslinked in its entire volume by irradiation with actinic light — through the substrate layer. Finally, the substrate layer is removed with an organic flexo detergent and the plate is dried. In a further step, a printing relief is engraved into the relief-forming layer by means of a CO ₂ aser. The disclosed method is completed by a renewed cleaning step with a flexo detergent. The plate must then be dried again.

The method disclosed has a number of disadvantages. The organic solvent used to remove the substrate layer does not dissolve the cross-linked relief-forming layer, but the layer nevertheless swells in it, the layer thickness increasing. However, solvent residues in the relief-forming layer disadvantageously reduce the quality of the printing relief obtained by laser engraving. The flexographic printing element must therefore be dried very thoroughly in order to remove solvent residues very completely. Very good drying is required for a second reason: In laser engraving, the focus of the laser should preferably be on the surface of the relief layer. If an incompletely dried plate is used, it will of course continue to evaporate over time. This means that their thickness is decreasing. Was the focus of the laser at the beginning of the If a flexographic printing element is still on the surface, it will lie above it with increasing engraving time. This leads to a different engraving result and, accordingly, the flexographic printing element is not engraved uniformly over the entire area, which leads to a poorer printed image. The double washing and drying step is therefore very time-consuming. As a result, the time advantage of direct laser engraving compared to the conventional process is lost again and in unfavorable cases the process takes even longer.

If the release layer is not removed in order to avoid treatment with solvents and the associated disadvantages, enamel edges form around the engraved layer elements. Such melting edges consist of remnants of the relief-forming layer and remnants of the substrate layer. The melting edges disturb the printed image. Naturally, this effect is all the more pronounced, the finer the elements of the relief layer to be engraved and the more material is ablated. This procedure is therefore not possible if you want to provide high-resolution plates using laser engraving.

The use of conventional photopolymerizable flexographic printing elements for the production of flexographic printing plates by means of laser engraving is therefore fraught with problems. This speaks for the use of special flexographic printing elements that are specially adapted to the requirements of laser engraving. On the other hand, the use of conventional photopolymerizable flexographic printing elements is in principle attractive because they can be produced particularly elegantly, with high precision and economically by extrusion. In addition, their important properties for the printing process, such as ink transfer, flexibility and mechanics, are guaranteed.

The object of the invention was therefore to provide an improved method for producing flexographic printing plates by means of laser engraving and also starting materials suitable for this purpose, which does not have the disadvantages of the prior art.

The starting material should be a largely conventional photosensitive flexographic printing element which should be adapted for use in the method according to the invention by means of measures that are as simple as possible.

Accordingly, a process for the production of flexographic printing plates by means of laser engraving has been found, in which a photopolymerizable flexographic printing element is arranged at least comprehensively one above the other as the starting material A dimensionally stable support,

A photopolymerizable, relief-forming layer with a thickness of at least 0.3 mm, at least comprising an elastomeric binder, an ethylenically unsaturated monomer and a photoinitiator, and

• uses a protective element which is essentially transparent to actinic light, the process comprising the following steps in this order

(a) crosslinking the relief-forming layer in the entire volume of the layer by irradiation with actinic light through the protective element,

(b) removing the protective element, as well

(c) engraving a print relief into the cross-linked relief-forming layer with the aid of a laser emitting between 3000 and 12000 nm, the depth of the relief elements to be engraved with the laser being at least 0.03 mm,

and the protective element is a film which has been treated or coated to remove adhesive on the side facing the relief-forming layer and which is applied directly to the relief-forming layer, the adhesion between the protective element and the relief-forming layer being set such that the protective element after process step (a) is peelable from the cross-linked relief-forming layer.

Furthermore, a photopolymerizable flexographic printing element was found, which at least arranged one above the other

A dimensionally stable support,

A protective element which is essentially permeable to actinic light,

comprises, wherein the protective element is a film which has been treated or coated to remove adhesive on the side facing the relief-forming layer and which is applied directly to the relief-forming layer, the adhesion between the protective element and the relief-forming layer being set such that the Protective element after exposure to actinic light through the protective element from the cross-linked relief-forming layer is removable. Surprisingly, it was found that by replacing the release layer and cover sheet of conventional flexographic printing elements with the protective element according to the invention and changing process steps compared to known methods for laser engraving, significantly better results can be achieved.

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

Polymer films, for example made of PET or PEN, or metal sheets, for example made of aluminum or steel, are particularly suitable as dimensionally stable supports for the starting material used according to the invention.

The photopolymerizable flexographic printing element further comprises at least one photopolymerizable relief-forming layer, at least comprising an elastomeric binder, an ethylenically unsaturated monomer, a photoinitiator and optionally further additives. The relief-forming layer can be applied directly to the carrier. However, there may also be other layers between the support and the relief-forming layer, such as adhesive layers and / or elastic sub-layers.

The components of the relief-forming layer are the components usually used for the production of conventional flexographic printing plates. The person skilled in the art makes a suitable choice depending on the desired properties of the layer. Examples of suitable elatomeric binders include natural rubber, polybutadiene, polyisoprene, styrene-butadiene rubber, nitrile-butadiene rubber, butyl rubber, styrene-isoprene rubber, polynorbornene rubber or ethylene-propylene-diene rubber (EPDM). Other examples include thermoplastic elastomeric block copolymers of the styrene-butadiene or styrene-isoprene type.

Particularly suitable ethylenically unsaturated monomers are esters or amides of (meth) acrylic acid with mono- or polyfunctional alcohols, amines, amino alcohols or hydroxy ethers and esters. Examples include butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, 1,4-butanediol diacrylate or 1,6-hexanediol diacrylate.

In a known manner, suitable initiators for the photopolymerization are benzoin or benzoin derivatives, benzene derivatives, acylphosphine oxides or acylarylphosphinic acid esters, without the list being intended to be restricted thereto.

Mixtures of several binders, several monomers or several photoinitiators can of course also be used, provided that the properties of the relief-forming layer are not adversely affected thereby. The relief-forming layer can furthermore optionally comprise additives and auxiliaries, such as plasticizers, dyes, dispersing aids or antistatic agents, in a manner known in principle. They are chosen by the person skilled in the art depending on the desired properties of the layer. When making the selection, the person skilled in the art is aware that the term “photopolymerizable” presupposes that actinic light can penetrate the photopolymerizable layer with sufficient intensity and that there are limits to the addition of absorbing and / or scattering additives.

The photopolymerizable, relief-forming layer can also be built up from several partial layers. These crosslinkable partial layers can be of the same, approximately the same or different material composition.

The thickness of the relief-forming layer or of all the relief-forming partial layers together is at least 0.3 mm and usually not more than 7 mm. The thickness is preferably 0.5 to 3.5 mm and particularly preferably 0.7 to 2.9 mm.

According to the invention, a protective element is applied directly to the relief-forming layer. The protective element is essentially transparent to actinic light, i.e. it should be permeable to actinic light to such an extent that photopolymerization of the relief-forming layer is possible without loss of quality. The term "permeable" does not rule out that the protective element can absorb or scatter actinic light to a limited extent, i.e. without the crosslinking being adversely affected. For example, it can be quite cloudy.

The protective element is a film which has been treated or coated to remove adhesive on the side facing the relief-forming layer. It is applied directly to the relief-forming layer.

The film is usually polymeric films, for example films made of polyethylene or polypropylene, PET, PEN or polyamide. It can also be composite films made of several different polymeric materials. It is preferably a PET film. The film is treated for detackification or coated with a detackifying layer.

“Removable” is to be understood to mean that the entire protective element can be removed from the cross-linked, relief-forming layer so easily that the surface of the relief-forming layer is not damaged thereby, and that no residues of the protective element remain on the relief-forming layer on the other hand, be so high both before and after the irradiation that the protective element rather connected to the relief-forming layer to serve the purpose of protection.

The adhesion between the relief-forming layer and the protective element is set such that the protective element can be removed completely from the now cross-linked relief-forming layer after irradiation with actinic light in process step (a). A protective element which, although it can be removed from the non-crosslinked, relief-forming layer before irradiation, but not or at least not completely after the irradiation, is not suitable for carrying out the present invention. Conversely, a protective element, which is not removable before irradiation, but only after irradiation, is suitable for carrying out the present invention.

Both the surface properties of the relief-forming layer and the side of the protective element facing the relief-forming layer are important for adjusting the adhesion. According to the invention, the surface properties of both layers are coordinated with one another in such a way that the desired peelability is obtained after the irradiation in process step (a). It is self-evident for the person skilled in the art that not every combination of protective elements with relief-forming layers leads to the desired result. A protective element which can be peeled off from a relief-forming layer of a certain composition does not necessarily have to be peelable from one with a different composition.

The surface of the film of the protective element is treated to be detackifying on the side facing the relief-forming layer or is coated with a detackifying layer.

A detackifying treatment can be, for example, siliconization or teflonization of the film.

Polymer materials are particularly suitable for producing a detackification layer. They can be produced, for example, by dissolving the polymer and pouring it onto the film, followed by evaporation of the solvent. For example, it can be polyamide.

When using stripping layers, there must be a reliably reproducible difference in adhesion between the stripping layer and the film on the one hand and the stripping layer and the relief-forming layer on the other, so that the detackifying layer adheres more strongly to the film than to the relief-forming layer, and that reliable peelability of the protective film is guaranteed without the surface quality of the flexographic printing element being impaired by the peeling. Therefore, an adhesive layer can optionally be located between the stripping layer and the protective film, which increases the adhesion between the stripping layer and the film. In a further embodiment, the surface of the film can be modified in order to achieve greater adhesion, for example by introducing inorganic particles into the surface. In a third embodiment, the film can be subjected to a corona treatment prior to the application of the stripping layer, which improves the adhesion of the stripping layer to the film. Details of a corona treatment are disclosed, for example, in DE-A 197 11 696.

The surface properties of the relief-forming layer can be influenced by the selection of the components of the relief-forming layer and their quantity.

In order to obtain the desired removability after the irradiation, it has proven useful to use elastomeric block copolymers of the styrene-butadiene type in the relief-forming layer as the elastomeric binder.

The block copolymers 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 particularly preferably styrene-butadiene-styrene three-block copolymers. The styrene content of the styrene-butadiene block copolymer used is usually 20 to 40% by weight, based on the binder, preferably 25 to 35% by weight. Such SBS block copolymers 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.

A particularly advantageous combination for carrying out the invention results from the use of styrene-butadiene block copolymers in the relief-forming layer and from the use of a protective element which has a detackification layer which comprises polyamide.

The flexographic printing element can be produced, for example, by dissolving or dispersing all components in a suitable solvent and pouring them onto the dimensionally stable support. In the case of multilayer elements, several layers can be cast onto one another in a manner known in principle. After casting, the protective element is applied. Conversely, it is also possible to pour onto the protective element and finally to laminate the carrier.

If thermoplastic elastomeric binders are used, the relief layer can be produced particularly advantageously in a manner known in principle by melt extrusion between a dimensionally stable carrier film and the protective element and calendering of the composite obtained, as disclosed, for example, by EP-A 084 851. Multi-layer elements can be produced 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 is also possible to pour onto the protective element and then to laminate on the metallic support.

The described photochemically cross-linkable flexographic printing element is used as the starting material for the method according to the invention.

In step (a) of the process according to the invention, the relief-forming layer is photochemically crosslinked in the entire volume of the layer by irradiation with actinic light. The irradiation takes place from the top of the flexographic printing element through the protective element which is essentially transparent to actinic radiation.

Optionally, in addition to the exposure through the protective element, a back-side pre-exposure can also be carried out. The latter naturally presupposes that the dimensionally stable carrier is transparent to actinic radiation. Back-side pre-exposure is therefore naturally not possible with metallic supports. If a back is made with the exposure, it can be done before, after or simultaneously with the exposure from the front of the plate. A back exposure is preferably carried out beforehand.

Process step (a) can be carried out in the presence or absence of atmospheric oxygen. The use of vacuum as with conventional flexographic printing elements is not necessary. The protective element protects the relief-forming layer from oxygen so effectively that inhibiting oxygen cannot diffuse into it to a significant degree, and also the uppermost sections of the relief-forming layer

Sufficient layer polymerized to obtain a print relief of sufficient quality. UV-A radiation with a wavelength between approximately 320 and 400 nm and / or UV-AΛIS radiation with a wavelength of approximately 320 to approximately 700 nm are particularly suitable as actinic light. After method step (a), the protective element in its entirety is removed or removed in method step (b).

In a preferred embodiment, the crosslinked relief-forming layer is crosslinked from the surface up to a limited depth of penetration beyond the extent of the crosslinking density brought about by step (a), optionally in a step (b) following process step (b ¹ ).

If step (b ') is provided, not all of the ethylenically unsaturated groups in the layer are converted in the course of the crosslinking in process step (a) to form a polymeric network, but the crosslinking is carried out in such a way that unreacted groups remain. The incomplete implementation can be achieved, for example, by limiting the exposure time.

Only parts of the relief-forming layer are affected by the crosslinking step (b '), which has only a superficial effect. There is no further crosslinking in the entire volume of the layer, but only in a partial volume of the layer. The effectiveness of the crosslinking step (b ') has a penetration depth that is limited from the surface of the relief-forming layer, so that the uppermost zone of the layer is crosslinked to a greater extent than would be the case if method step (a) were used exclusively. Here, crosslinkable groups that are not implemented in process step (a) are implemented in whole or in part.

The width of the zone within which the crosslinking density is increased by step (b ¹ ) or the effective penetration depth of the measure taken for crosslinking is generally at least 5 μm and not more than 200 μm from the surface of the recording layer seen from without the width should necessarily be limited to this. The penetration depth is preferably 5-150 μm and particularly preferably 5-100 μm. The transition from the zone whose crosslink density is increased in the course of step (b ') beyond the extent of process step (a) to the zone which is no longer covered by process step (b ¹ ) can be abrupt, comparatively steep or be gradual. The inflection point crosslinking density depending on the penetration depth is used to determine the penetration depth.

An embodiment in which the cross-linked flexographic printing element is irradiated with UV light of the wavelength 200 nm to 300 nm, so-called UV-C light, has proven particularly useful for carrying out step (b '). Due to the comparatively strong scattering of the short-wave light in the layer, the intensity of UV-C radiation decreases significantly with increasing depth of penetration, so that only the top zone of the flexographic printing element is effectively networked. Further details on method step (b ') are disclosed in WO 02/49842, to which we expressly refer at this point.

In process step (c), a printing relief is engraved into the cross-linked, relief-forming layer by means of a laser emitting between 3000 and 12000 nm. In this wavelength range, the elastomeric binders generally have sufficient absorption, so that additional absorbers for laser radiation do not have to be used. CO ₂ lasers (wavelength 10.6 μm) are particularly suitable. In principle, it is possible to use other laser types of comparable wavelength for engraving. The lasers can either be operated continuously or pulsed.

The relief-forming layer is removed or at least detached at those points at which it is exposed to a laser beam of sufficient intensity. The layer is preferably vaporized or thermally or oxidatively decomposed without melting beforehand, so that its decomposition products in the form of hot gases, vapors, smoke or small particles are removed from the layer.

The image information to be engraved with the laser can be transferred directly from the lay-out computer system to the laser apparatus.

Relief elements are advantageously engraved in which the flanks of the elements initially drop vertically and only widen in the lower region. As a result, the base points of the relief points are well capped, but the dot gain is still slight when printing with the printing plate obtained. Flanks with a different design can also be engraved.

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 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 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. For engraved surfaces, a depth of more than 0.15 mm, preferably more than 0.4 mm, is recommended. The latter is of course only possible with a correspondingly thick relief. It is regularly advantageous to keep decomposition products formed away from the surface of the flexographic printing element or the relief layer in the course of laser engraving and to remove them irreversibly as far as possible. This measure completely or at least largely prevents degradation products on the relief surface from being able to reconnect to the relief surface. For example, a suitable suction device can be used to suction the decomposition products formed, in particular aerosols, from the plate surface. In a further embodiment, a gas or a gas mixture can be blown over the surface of the plate, the gas stream carrying the decomposition products with it. It is preferably an air or nitrogen stream.

The relief printing plate obtained can optionally be cleaned in a process step (d). The cleaning can be done mechanically, for example, simply by brushing or rubbing off the printing form obtained. However, the surface of the printing form can also be blasted using a gas jet, such as compressed air. The higher the pressure or the speed of the gas jet, the better the cleaning effect. If the pressure is too high, however, the surface of the pressure plate can be damaged. Accordingly, the specialist will choose a compromise between the best possible cleaning and process reliability.

For cleaning, a liquid cleaning agent which does not substantially swell the relief layer is preferably used in order to also be able to completely remove polymer fragments. 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 choice of a suitable cleaning agent depends on the composition of the relief layer. For the - frequently occurring - case that the relief layer has components soluble in organic solvents, for example SBS or SIS block copolymers and monomers compatible therewith, water or predominantly aqueous cleaning agents are used. Aqueous cleaning agents consist essentially of water and optionally small amounts of alcohols and can contain auxiliaries such as surfactants, emulsifiers, dispersing agents or bases to support the cleaning process. Emulsions of water, organic solvents and suitable auxiliaries for post-cleaning can also advantageously be used. The microemulsion detergents comprising water, alkyl esters of saturated or unsaturated fatty acids and surfactants disclosed by WO 99/62723 have proven to be particularly advantageous. Mixtures that are usually used for development can also be used Mixtures are used, which are usually used to develop conventional, water-developable flexographic printing plates.

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, for example, brushes or plushes. Conventional flexo washers can also be used. Due to the use of non-swelling cleaning agents, time-consuming drying of the printing form after the subsequent cleaning is not necessary.

Naturally, the photopolymerizable flexographic printing element used as the starting material for the process is usually produced on an industrial scale by a printing plate manufacturer, while the laser engraving (c) and, if appropriate, a post-cleaning step are usually carried out by a plate maker or a printing house.

There are several possibilities with regard to steps (a), (b) and optionally (b '). This can be done by the cliché or printing house itself. The photochemical crosslinking can be carried out, for example, in commercially available flexo illuminators. UVC imagesetters are also usually available in clichés or printing plants.

Steps (a), (b) and optionally (b ') can of course also be carried out by the printing plate manufacturer itself, so that a customer receives a material prepared for the laser engraving.

The continuous process shown in Figure 2 has proven to be the preferred embodiment for this: a thermoplastic elastomeric binder is used. The components of the relief-forming layer are melted in a known manner in an extruder (1) and mixed intensively with one another. The hot, photopolymerizable mass is discharged through a slot die (2) into the nip of a calender (3). The carrier film (5) is guided over a calender roll (4) and the protective element (7) is guided over the second calender roll (6) and the hot photopolymeric mass is calendered between the two films. The photopolymerizable flexographic printing element is allowed to cool after passing through the calender and is then irradiated with actinic light (UV-A) from the front by means of an exposure station (8) and optionally also from the rear by means of a further exposure station (9) and thus crosslinked photochemically , The protective element can be removed after the networking station. For example, as shown in Figure 1, this can be done by winding onto a roll (10). optional the irradiation can then be carried out using UV-C (11). If no UV-C exposure is provided, the protective element can of course remain on the flexographic printing element.

The foil position can also be interchanged, i.e. the carrier film can also be fed via the upper calender roll (6) and the protective element via the lower calender roll (4). The positions of the exposure stations and, if applicable, the pull-off device (10) then change accordingly.

The method according to the invention gives flexographic printing plates of significantly higher quality than the method described by US Pat. No. 5,259,311. In the method described by US 5,259,311, problems occur especially in the fine raster area. In the course of laser engraving, a lot of molten material is created, which reconnects to the surface and cannot be washed off, even with organic solvents. A lot of time is saved by avoiding drying twice. The exposure through the cover film leads to a particularly smooth layer surface and good color transfer during printing. The UV-C exposure produces particularly sharp edges.

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

Experimental:

A three-beam CO ₂ laser system of the type BDE 4131 (from STK) was used for the engraving tests. The three laser beams had a power of 77, 166 and 151 W on the plate surface. The device has a rotating drum. For engraving, the flexographic printing element is mounted on the drum and the drum is set in rotation. The speed on the surface of the drum was 7 m / s in all tests, the advance of the laser beams transverse to the direction of rotation was 20 μm per revolution.

A test pattern consisting of various elements including lines, positive points, negative points, letters (capital "A"), numbers ("3%") and various grids was engraved.

The quality of the relief was assessed using the following parameters:

the visual appearance of the 3% grid (ideal = round halftone dots) - the general appearance the appearance of melting edges, the engraving depth (measured in the base of the large "A") as the height difference between a completely removed area and the plate surface. Diameter of a 200 μm positive point - diameter of a 200 μm negative point. Intermediate depth in the 200 μm negative point

Width of a 20 μm negative line that runs parallel to the laser direction Width of a 20 μm negative line that runs transverse to the laser direction

Materials used:

For comparative experiments 3, 4 and 5, a commercially available photopolymerizable flexographic printing element was used with a conventional release layer and a cover sheet made of PET ^(® nyloflex ART, BASF Drucksysteme GmbH). With this element, the adhesion between the release layer and the photopolymerizable layer is greater than between the cover film and the release layer. The binder in the photopolymerizable layer is of the styrene-butadine type. The cover element is produced in a conventional manner by extrusion and calendering of the hot, photopolymerizable composition between the carrier film.

For experiments 1 and 2 and comparative experiments 1 and 2, a flexographic printing element was used, the photopolymerizable layer of which corresponded to the nyloflex® ART. The flexographic printing element only had a protective element according to the invention instead of the conventional release layer and the conventional cover film. The protective element consisted of a PET film (Lumirror X 43) coated with the polyamide Macromelt® 6900 (from Henkel). The adhesion between the film and the detackifying coating was greater than the adhesion between the additional coating and the PET film, so that the protective element as a whole, i.e. including the coating was peelable from the relief-forming layer after exposure.

Example 1 :

The flexographic printing element according to the invention described above was used.

The flexographic printing element was crosslinked through the protective element for 15 min with UV-A radiation (filler imagesetter). The crosslinking was not complete and unreacted ethylenically unsaturated monomers remained in the layer. After exposure to UV-A, the protective element was removed. No residues of the protective element remained on the photopolymerizable layer. The stripping layer of the protective element remained completely adhered to the film.

The relief-forming layer was then irradiated from the top for 15 minutes using UV-C light. This increased the crosslinking in the uppermost part of the layer and the relief-forming layer was thus hardened on the surface.

The test pattern described above was then engraved into the crosslinked layer using the laser system described above.

The engraved relief was evaluated. The results are summarized in Table 1. An image of the relief is shown in Figure 1.

Example 2:

The procedure was as in Example 1, only the additional exposure step with UV-C was omitted.

Comparative experiment 1:

The flexographic printing element according to the invention was used and proceeded as in experiment 1, but the protective element was removed before exposure to UV-A radiation. The results are summarized in Table 1. An image of the relief is shown in Figure 1.

Comparative Experiment 2: The flexographic printing element according to the invention was used and proceeded as in Experiment 1, but the protective element was removed before exposure to UV-A radiation and no post-exposure to UV-C was carried out. The results are summarized in Table 1. An image of the relief is shown in Figure 1.

Comparative experiment 3:

The commercially available nyloflex® ART flexographic printing element was used. The cover film was removed from the flexographic printing element, the release layer remaining on the photopolymerizable layer. The flexographic printing element was crosslinked through the release layer for 15 min with UV-A radiation.

Comparative experiment 4:

Procedure according to US 5,259,311.

It procedure was as in comparative experiment 3, only the release layer by means of the Flexoplattenwaschmittels nylosolv ll ^® (BASF Drucksysteme GmbH) was prepared by the crosslinking with UV-A radiation is removed. The cross-linked flexographic printing element was dried for 15 min at 65 ° C. and then engraved with the laser system.

Comparative experiment 5:

Procedure as US 5,259,311.

The procedure was as in Comparative Experiment 4, except that drying was carried out at 65 ^° C. for 120 minutes.

Table 1: Results of examples and comparative examples

The experiments show that high quality flexographic printing plates are obtained with the method according to the invention.

Experiments 1 and 2 delivers a flexographic printing plate that provides a clean print relief with sharp edges and has no melting edges (see Figure 1).

If, however, the protective element is removed from the flexographic printing element according to the invention and otherwise processed the same way, the flexographic printing plates obtained have clear melting edges (comparative tests 1 and 2 in FIG. 1). In comparison test 2 it can also be seen that the 3% grid is no longer as well developed as in the examples according to the invention. The shape of the halftone dots is no longer exactly round, but oval.

When engraving a conventional flexographic printing element, i.e. a flexographic printing plate with enamel edges is obtained through a release layer remaining on the relief-forming layer (comparative experiment 3, Figure 1). If the release layer, as described by US Pat. No. 5,259,311, is removed with a flexo detergent prior to laser engraving, the occurrence of melting edges can be avoided (comparative tests 4 and 5, Figure 1). The sensitivity even increases somewhat as a result of washing out. Disadvantageously, however, the edges of the image motifs become much less clear due to the treatment with solvents. This can be clearly seen from the lettering "3%", which appears "washed out" after treatment with solvent. This effect cannot be avoided even by drying for a long time. The flexographic printing element is therefore irreversibly changed negatively by treatment with solvent before laser engraving.

The method according to the invention and the flexographic printing element according to the invention thus lead to a significant improvement compared to the method disclosed by US Pat. No. 5,259,311.

Claims

claims

1. Process for the production of flexographic printing plates by means of laser engraving, in which a photopolymerizable flexographic printing element is arranged at least comprehensively one above the other as the starting material

A dimensionally stable support,

• a photopolymerizable, relief-forming layer with a thickness of at least 0.3 mm, at least comprising an elastomeric binder, an ethylenically unsaturated monomer and a photoinitiator, and «a protective element which is essentially permeable to actinic light, characterized in that the process -in this order- includes the following steps:

(a) crosslinking the relief-forming layer in the entire volume of the layer by irradiation with actinic light through the protective element, (b) removing the protective element, and

(c) Engraving a print relief into the cross-linked relief-forming layer with the aid of a laser emitting between 3000 and 12000 nm, the depth of the relief elements to be engraved with the laser being at least 0.03 mm, and the protective element being one on that of the relief-forming layer Layer facing side that has been treated with adhesive or coated, which is applied directly to the relief-forming layer, the adhesion between the protective element and the relief-forming layer being set such that the protective element can be peeled off from the cross-linked, relief-forming layer after process step (a) is.

2. The method according to claim 1, characterized in that the protective element comprises a detackifying coating.

3. The method according to claim 2, characterized in that the stripping layer consists essentially of a polyamide and the elastomeric binder in the relief-forming layer is a thermoplastic elastomeric block copolymer of the styrene-butadiene type.

4. The method according to any one of claims 1 to 3, characterized in that the method additionally comprises a post-cleaning step (d).

5. The method according to any one of claims 1 to 4, characterized in that the decomposition products formed in step (c) are suctioned off.

6. The method according to any one of claims 1 to 5, characterized in that after removing the protective film (b) in a subsequent process step (b ") the cross-linked relief-forming layer seen from the surface to a limited depth of penetration over the Extent of the crosslink density brought about by step (a).

7. The method according to claim 6, characterized in that the penetration depth, up to which in step (b ') is additionally crosslinked, is 5 to 200 μm.

8. The method according to claim 6 or 7, characterized in that the superficial crosslinking step (b ') is carried out with UV light of 200 to 300 nm wavelength.

9. Photopolymerizable flexographic printing element arranged at least comprehensively one above the other

A dimensionally stable support,

A protective element which is essentially permeable to actinic light,

characterized in that the protective element is a film which has been treated or coated to remove adhesive on the side facing the relief-forming layer and is applied directly to the relief-forming layer, the adhesion between the protective element and the relief-forming layer being set in such a way that that after exposure to actinic light, the protective element can be removed from the cross-linked relief-forming layer through the protective element.

10. flexographic printing element according to claim 9, characterized in that the protective element comprises a detackifying coating.

11. Flexographic printing element according to claim 10, characterized in that the stripping layer consists essentially of polyamide.