EP4686564A1

EP4686564A1 - Adhesive printing form attachment device

Info

Publication number: EP4686564A1
Application number: EP24191380.5A
Authority: EP
Inventors: Philipp WEGENER; Silvano TAMAI; Matthias Koop; Sarita URBAN
Original assignee: Tesa SE
Current assignee: Tesa SE
Priority date: 2024-07-29
Filing date: 2024-07-29
Publication date: 2026-02-04
Also published as: WO2026027478A1

Abstract

The present invention relates to a printing form attachment device for mounting a printing plate onto a printing cylinder, the printing form attachment device comprising a support and a pressure-sensitive adhesive layer being provided on the support and having a surface being exposed to the printing plate,
wherein the surface of the pressure-sensitive adhesive layer being exposed to the printing plate comprises at least two areas having different adhesive forces towards the printing plate.

Description

The present invention relates to the technical field of flexographic printing. More precisely, the invention proposes a printing form attachment device (often simply referred to as sleeve) for mounting a printing plate, the printing form attachment device being equipped with a pressure-sensitive adhesive layer having two or more areas of different adhesive force towards the printing plate.
In a typical flexographic printing process, one or more flexible printing plates made from photopolymer or rubber are attached to a printing cylinder. The printing plates have a relief corresponding to the information to be applied by printing. During the actual printing process, the printing plates pass through an ink reservoir, where the relief takes up the printing ink. The printing cylinder then rotates the printing plate to a transfer station, where the surface containing printing ink comes into contact with the substrate to be printed. On removal of the printing plate from the substrate, the film of printing ink splits and leaves behind an imprint corresponding to the relief present on the printing plate surface. That ink transfer requires precise application of pressure when placing the printing plate onto the substrate, as this pressure substantially determines the uniformity of the printed image.
There are several known methods of attaching a printing plate to a printing cylinder. The most widely used method is the use of a double-sided adhesive tape. A second method involves the use of a sleeve or of a similar device being permanently equipped with an adhesive surface for fixing the printing plate.
The use of a double-sided adhesive tape is problematic in that there may occur difficulties in removing the tape from the printing cylinder and/or from the printing plate. Also, the double-sided adhesive tape frequently leaves residues behind which will later interfere with the reuse of the printing plate or will deteriorate the printing performance in subsequent printing operations. Attaching the double-sided adhesive tape uniformly and without causing surface irregularities that impair the printed image is also a cumbersome manual operation. In addition, the use of multiple pieces of double-sided adhesive tape, as is generally required, makes alignment of the printing plate on the printing cylinder difficult, especially since removal and repositioning is difficult.
WO 95/19267 A1 discloses an attachment means for attaching a flexographic printing form on a plate cylinder, characterized by a flexible supporting layer and an adherent photopolymeric layer supported by the supporting layer being completely exposed. Thus, WO 95/19267 A1 describes the use of an adhesively equipped plate cylinder to replace the double-sided adhesive tapes. The general term "adhesive" is used here in the meaning of "permanent tacky" or "permanent sticky". The document mentions that the adhesively equipped plate cylinder is able to maintain its adhesive properties even during continued use and re-use, and that residues can be easily removed, while no residual photopolymeric material remains on the printing form. There is however no specific teaching on the chemistry and the method for manufacturing the adhesively equipped plate cylinder, other than that it is photopolymeric.
WO 2010 090685 A1 describes a method for removably attaching a printing plate to a printing cylinder, said method comprising

a) attaching a photopolymer sheet to the printing cylinder which photopolymer sheet comprises first and second major faces which are opposed to each other wherein the first major face contacts and attaches to the printing cylinder and wherein the first and second major faces have a surface tack of at least 600 g as measured by ASTM standard D-2979-95;
b) contacting and attaching the printing plate to the second major face;

a) binder;
b) at least one monomer;
c) photoinitiator; and
d) microspheres with a diameter of less than 90 microns.

Thus, the stickiness of the photopolymer is used to attach itself to the printing cylinder and to attach the printing plate to it. Microspheres are incorporated into the photopolymer sheet to provide a cushioning effect when in use on the printing press. The photopolymer layer referred to in this document is prepared by mixing the components of the photopolymer and curing the composition after it is cast or extruded into a sheet, followed by irradiation with UV. The document states that the amount of radiation necessary varies based upon the composition and thickness of the photopolymer, and that the amount of radiation used, and therefore the extent of curing, can be used to control the stickiness of the photopolymer. However, the curing must be sufficient to achieve sufficient integrity and strength. This is in particular required because the photopolymer layer of this document does not contain a substrate on which the photopolymeric layer is provided; instead, the extruded photopolymeric layer is attached directly to the printing cylinder.
With a layer of this kind, due to the adhesive attachment of the photopolymer sheet to both the printing cylinder and the printing form (printing plate) on opposing sides of the photopolymer sheet, the layer may come off the printing cylinder when the printing form is removed, as the adhesion may be equally strong on both sides. Also, damages to the surface carrying the printing form will likely occur upon removal or replacement of the printing form in case the adhesion is strong, thereby reducing the lifetime of the photopolymer sheet. Further, if the adhesion is not strong enough, the printing form may come off the printing cylinder during operation. In addition, the presence of microspheres may deteriorate the adhesive properties of the layer and may lead to irregularities on the surface, which will transpose through the printing form, thereby potentially reducing print quality and/or causing offset.
US 2021/0214589 A1 discloses an adhesive printing form attachment layer comprising a support and a permanently sticky layer disposed on the support, the permanently sticky layer comprising a crosslinked polyurethane-based material.
Thus, the use of printing form attachment devices, e. g., sleeves, being permanently equipped with a pressure-sensitive adhesive layer for attaching a printing plate to a printing cylinder is generally known in the art. In the flexographic printing industry, campaigns of different lengths are printed. As the printing quality requirements and the printing plates differ depending on the material to be printed on, usually foamed adhesive tapes secure a safe connection between printing plate and printing cylinder. However, tapes provide a one-time solution only. In contrast, permanently sticky devices may provide significant advantages regarding costs and sustainability, as the printing plates can be removed after printing and be replaced by another printing plate. However, some basic requirements must still be fulfilled.
Mounting of the printing plates must be simple and safe. Furthermore, it is often observed that printing plates peel off from the printing cylinder starting at their edges, which is known as "edge-lifting". Such behavior of the printing plates must not occur as it might jeopardize proper printing.
Furthermore, the printing plates shall be easily removable from the sleeve without any damages. A low removal force enables a pleasant removal process for the operator and lowers the risk of irreversibly damaging the printing plate.
However, high holding power during the printing process and easy removability afterwards represent opposing requirements which cannot each be fully served.
It was an object of the present invention to provide a device for attaching a printing plate to a printing cylinder featuring well-balanced and optimized holding power during printing and easy removability after printing.
It was a further object of the invention to provide a device which reduces the edge-lifting tendency of the printing plate.
It was an additional object of the invention to enable improved printing quality over many prints with one printing form and/or after re-use with many printing forms.
The solution to these problems is based on the fundamental idea underlying the present invention according to which the printing form attachment device shall be equipped with at least two areas of pressure-sensitive adhesive layer featuring different adhesive performance characteristics.
The objects stated above are achieved accordingly by the subject matter of the invention as defined in the claims. Preferred embodiments according to the invention result from the dependent claims and the observations below.
Embodiments which are hereinafter designated as preferred are combined in particularly preferred embodiments with features of other embodiments designated as preferred. Very particularly preferred, therefore, are combinations of two or more of the embodiments designated below as particularly preferred. Also preferred are embodiments in which a feature of one embodiment that is designated in any degree as preferred is combined with one or more further features of other embodiments that are designated in any degree as preferred. Features of preferred pressure-sensitive adhesive tapes and uses result from the features of preferred adhesives.
In so far as both specific amounts or fractions of an element and preferred embodiments of the element are disclosed subsequently for this element, it is the case in particular that the specific amounts or fractions of the preferably embodied elements are also disclosed. In addition, it is disclosed that with the corresponding specific total amounts or total fractions of the elements, at least a part of the elements can be preferably configured and in particular also that preferably configured elements within the specific total amounts or total fractions may in turn be present in the specific amounts or fractions.
A first and common subject matter of the present invention is a printing form attachment device for mounting a printing plate onto a printing cylinder, the printing form attachment device comprising a support, and a pressure-sensitive adhesive layer being provided on the support and having a surface being exposed to the printing plate,
wherein the surface of the pressure-sensitive adhesive layer being exposed to the printing plate comprises at least two areas having different adhesive force towards the printing plate.
Thus, the invention solves the problems mentioned above by segmenting the pressure-sensitive adhesive layer of the printing form attachment device into at least two zones having a different adhesive force towards the printing plate. These zones may be regarded as corresponding to geometrically defined areas in the surface of the pressure-sensitive adhesive layer being exposed to the printing plate. By adjusting the geometry of these zones, the invention allows for providing one or more predefined "mounting" and "separating" or "demounting" areas within the sleeve. The "mounting" areas will feature higher adhesive power and will thus support tight adhering of the printing plate to the printing cylinder, whereas the "separating" zones will feature lower adhesive power and will thus support ease of removal of the printing plate after the printing process.
The printing form attachment device usually has a cylindrical shape and an inner diameter that is adjusted to the diameter of the printing cylinder to tightly surround it. The printing form attachment device comprises a support and a pressure-sensitive adhesive layer. In many embodiments, the printing form attachment device according to the invention will be commonly referred to as a "sleeve"; therefore, preferably, the printing form attachment device is a sleeve for mounting a printing plate onto a printing cylinder.
The support - alternatively referred to as the substrate - may generally be single layered but preferably comprises two or more layers each providing specific functionalities.
Preferably, the support comprises a base. The base preferably is a cylindrical carrier made from metals like aluminum, polymers or, more preferably, from aa combination of different materials. E. g., the base may be made from glass fibre or carbon fibre fabrics which are encapsulated with epoxy resins and may further be coated with a polyurethane coating. Thus, the base very preferably consists of a composite material. Turning to its function, the base is the connecting element between the printing machine and any further functional layers of the printing form attachment device; furthermore, the base preferably provides stability and dimensional accuracy to the device.
Preferably, the support comprises a foam layer. The foam layer has a defined hardness and density and may thus enable cushioning of forces that occur during the printing process, thereby securing the desired compressibility of the printing plate and thus the printing quality. Very preferably, the foam layer is an open cell polyurethane foam having a thickness of 1.200 µm - 1.800 µm. Preferably, the foam layer is equipped on one or both of its main surfaces with an adhesive layer to provide tight contact with the support and/or any further functional layer following in the upper direction of the printing form attachment device. Particularly preferably, the foam layer is equipped on one or both of its main surfaces with a pressure-sensitive adhesive layer. Therefore, preferably, the foam layer may be regarded as a central layer of a single-sided or double-sided adhesive tape, and the support of the printing form attachment device preferably comprises a double-sided adhesive foam tape. The pressure-sensitive adhesive layers may provide tight contact of the foam layer to the base on its lower side and to further parts of the device, e. g., to a reinforcement film, on its upper side.
Preferably, the support comprises one or more reinforcement films. Further preferably, the reinforcement films have a thickness of 20 - 150 µm. Further preferably, the reinforcement films are polyester films, more preferably, poly(ethylene terephthalate) (PET) films. A reinforcement film may strengthen the stability of the multilayer laminate and may furthermore protect the foam layer from being damaged when the printing plate is removed from the sleeve.
Very preferably, the support comprises one or more functionalities as introduced hereinbefore and thus preferably comprises a base, a foam layer and one or more reinforcement films. More preferably, the support comprises a base, a double-sided adhesive foam tape and one or more reinforcement films. It is understood by a person skilled in the art that the support may comprise more functional layers than the one mentioned hereinbefore; e. g., the support may comprise more interlaminate adhesive layers.
The printing form attachment device according to the invention further comprises a pressure-sensitive adhesive layer that is provided on the support and has a surface that is exposed to the printing plate.
A pressure-sensitive adhesive is, in accordance with the expert understanding, an adhesive which has pressure-sensitive adhesive properties, i.e., the property of making a durable connection to an adhesion base even under relatively low applied pressure. Without wanting to be tied to this theory, it is often assumed that a pressure-sensitive adhesive can be considered as an extremely high-viscosity fluid with an elastic component, which consequently has characteristic viscoelastic properties, which lead to the above-described durable self-adhesiveness and pressure-sensitive adhesive capability. It is assumed that with corresponding pressure-sensitive adhesives, mechanical deformation results in both viscous flow processes and the build-up of elastic restoring forces. The proportional viscous flow is used to achieve adhesion, while the proportional elastic restoring forces are particularly necessary for achieving cohesion. The relationships between rheology and pressure-sensitive adhesiveness are known in the state of the art and are described, for example, in Satas, "Handbook of Pressure Sensitive Adhesive Technology", third edition (1999), pages 153 to 203. The storage modulus (G') and the loss modulus (G"), which can be determined by means of dynamic mechanical analysis (DMA), for example using a rheometer, are usually used to characterize the extent of elastic and viscous components. In the context of the present invention, an adhesive is preferably understood as being pressure-sensitively adhesive and thus as a pressure-sensitive adhesive when at a temperature of 23°C in the deformation frequency range from 10° to 10¹ rad/sec, G' and G" are each at least in part in the range from 10³ to 10⁷ Pa.
Generally, the exposed adhesive layer of the device for mounting a printing plate onto a printing cylinder shall provide adhesion towards the printing plate and shall enable ease of fixing and removing the printing plate without the need for additional adhesives or tape. Furthermore, the adhesive layer may provide additional damping which might further improve the printing quality.
In accordance with the invention, the surface of the pressure-sensitive adhesive layer being exposed to the printing plate comprises at least two areas having different adhesive force towards the printing plate. In other words, the sleeve according to the invention does not comprise one single continuous pressure-sensitive adhesive layer but a segmented pressure-sensitive adhesive layer having at least two different zones providing different adhesive properties instead. This concept advantageously allows for a design of the device providing at least one zone which can be regarded as a "mounting zone" exhibiting higher adhesive force, thereby enabling holding power and easy mounting of the printing plate, and at least one further zone which can be regarded as "demounting zone" exhibiting lower adhesive force, thereby enabling easy removal of the printing plate after the printing process.
An "area" is regarded as a part of the surface of the pressure-sensitive adhesive layer being exposed to the printing plate which is geometrically separated from other parts of that surface. Even if the adhesive force is described herein in relation to the surface of the pressure-sensitive adhesive layer it is understood by the skilled person that the adhesive force results from the nature of the entire pressure-sensitive adhesive layer on which the respective surface area is based.
Generally, there can be two or more areas each having a different adhesive force towards the printing plate. Furthermore, the surface of the pressure-sensitive adhesive layer being exposed to the printing plate may comprise a first area having a specific adhesive force towards the printing plate and several separated areas each having the same adhesive force towards the printing plate which is, however, different from the adhesive force towards the printing plate of the first area.
Preferably, one or more areas having a lower adhesive force towards the printing plate than one or more further areas amount to at least 70 % of the entire surface of the pressure-sensitive adhesive layer being exposed to the printing plate. That is, advantageously there is more low adhesive force area than high adhesive force area or, in other words, there is more "demounting zone" than "mounting zone". More preferably, one or more areas having a lower adhesive force towards the printing plate than one or more further areas amount to at least 75 %, particularly preferably to at least 80 %, e.g., to at least 85 % of the entire surface of the pressure-sensitive adhesive layer being exposed to the printing plate.
More preferably, one or more areas having a higher adhesive force towards the printing plate than one or more further areas amount to at least 2 %, more preferably to at least 5 % of the entire surface of the pressure-sensitive adhesive layer being exposed to the printing plate.
Preferably, each of the areas having different adhesive force towards the printing plate amounts to 5 to 95 % of the entire surface of the pressure-sensitive adhesive layer being exposed to the printing plate. That is, an area having a specific adhesive force towards the printing plate advantageously represents a significant part of the adhesive surface of the sleeve.
In an embodiment, the outline of the area having higher adhesive force forms a replication of the outline of the printing plate ends, i. e. of both edges of the printing plate which are parallel with the rotation axis of the printing cylinder or of the printing form attachment device, respectively. The area having higher adhesive force forms a strip having a width of 1 to 5 cm, preferably of 1.3 to 4.5 cm, more preferably of 1.5 to 4 cm, like of 1.7 to 3.5 cm, most preferably of 2 to 3 cm, the strip extending from each of the two plate end outlines in the direction towards the other plate end outline. This advantageously allows to allocate the higher adhesion to the end edges of the printing plate to avoid edge lifting, whereas the majority of the contact area is covered by the adhesive having lower adhesive force which promotes easy demounting.
Preferably, the surface of the pressure-sensitive adhesive layer being exposed to the printing plate comprises at least two areas having a difference in their adhesive force towards the printing plate of minimum 0.5 N/cm, more preferably, of 0.5 to 2.5 N/cm, particularly preferably of 0.7 - 2.3 N/cm and most preferably of 0.9 to 2.1 N/cm. Preferably, the adhesive force difference between any two areas of the pressure-sensitive adhesive layer being exposed to the printing plate having different adhesive force towards the printing plate is 0.5 to 2.5 N/cm, more preferably 0.7 - 2.3 N/cm and most preferably 0.9 to 2.1 N/cm.
The different adhesive forces towards the printing plate of the two or more respective areas within the surface of the pressure-sensitive adhesive layer being exposed to the printing plate can be realized by various means.
In one embodiment, the different adhesive force towards the printing plate is achieved by using different pressure-sensitive adhesives. "Different pressure-sensitive adhesives" is understood to refer to pressure-sensitive adhesives being of different chemical nature, e.g.,

comprising base polymers which belong to different polymer classes or comprising base polymers which belong to the same polymer class but are based on different monomer compositions; or
comprising different additives which accordingly impart different adhesive properties, e.g., different adhesive resins or the same adhesive resins but in different concentration; or
differing in the nature and/or amount of a chemical crosslinker used.

Likewise, any combination of the aforementioned methods can be applied.
Accordingly, layers of different pressure-sensitive adhesives can be attached to the support. This can be achieved by, e.g., laminating layers of different pressure-sensitive adhesives onto temporary carrier materials, laminating the layers on the support in the desired geometry, and filling the seams with non-crosslinked pressure-sensitive adhesive which is subsequently crosslinked, thereby joining the layers.
In a further embodiment, the different adhesive force towards the printing plate is achieved by different surface structures of the otherwise same pressure-sensitive adhesive. E.g., a smooth surface may provide comparatively high adhesive force, whereas a rough, structured surface of the otherwise same pressure-sensitive adhesive layer may provide a reduced adhesive force. A structured surface of the pressure-sensitive adhesive layer can be achieved by contacting a microembossed pattern to the pressure-sensitive adhesive layer and forming a microreplicated surface therein. This can be achieved by at least any of casting using a tool having a microembossed pattern coating the pressure-sensitive adhesive onto a release liner having that microembossed pattern or passing through a nip roll to compress the pressure-sensitive adhesive layer against a release liner having that microembossed pattern. Desired embossing topography can be formed in tools via any of a number of generally known techniques, selected depending in part upon the tool material and features of the desired topography. Illustrative techniques include etching (e.g., via chemical etching, mechanical etching, or other ablative means such as laser ablation or reactive ion etching, etc.), photolithography, stereolithography, micromachining, knurling (e.g., cutting knurling or acid enhanced knurling), scoring or cutting, etc.
Structuring the surface can be achieved by providing grooves, thereby reducing the contact area between the pressure-sensitive adhesive and the printing plate.
Different surface structures may be provided by, e.g., different release liners which will be attached to the not fully cured pressure-sensitive adhesive. One of these release liners may have a smooth surface, the other may have a structured surface comprising a plurality of ridges which provide the microreplicated surface of the pressure-sensitive adhesive layer.
In a further embodiment, different adhesive force towards the printing plate is achieved by different physical and/or chemical treatment, such as radiation-crosslinking the adhesive via UV radiation or electron beam, of various segments of the otherwise same pressure-sensitive adhesive. Preferably, the different adhesive force towards the printing plate is achieved by different UV-radiation of the otherwise same pressure-sensitive adhesive. Thus, this embodiment may require the pressure-sensitive adhesive to be radiation-crosslinkable, particularly preferably UV-crosslinkable. Different adhesive forces may then be achieved by using different radiation profiles for various segments of the pressure-sensitive adhesive layer. This, in turn, may be achieved by selective activation of the radiation sources. This method advantageously allows for covering the entire sleeve surface with the same non-crosslinked pressure-sensitive adhesive composition and requires only different treatment of the segments.
Alternatively, and particularly preferably, the different adhesive force towards the printing plate is realized by exposing at least one area of the surface of the pressure-sensitive adhesive layer being exposed to the printing plate to a dosage of UV radiation that causes loss of adhesive force, the loss exceeding any loss of adhesive force caused by crosslinking the pressure-sensitive adhesive, while avoiding such exposure in at least one further area. Thereby, the area which as been treated will become lower adhesive whereas one or more other areas are protected from such violent treatment by, e.g., masking these areas by means of a UV-absorbing film. Alternatively, the different treatment of various areas as described above can be achieved by selective activation or shielding of the radiation source.
The pressure-sensitive adhesive layer of the device according to the invention may be based on one or more poly(meth)acrylates, vinyl aromatic block copolymers, natural rubbers, polyolefins, hydrogenated polyolefins, polyurethanes, and mixtures of two or more of the aforementioned polymers. Furthermore, the pressure-sensitive adhesive layer may comprise one or more additives like adhesive resins, plasticizers, fillers, antioxidant etc. as generally known in the art.
Independent from the various methods to achieve different adhesive force as set forth above, the pressure-sensitive adhesive layer of the printing form attachment device according to the invention preferably comprises, more preferably is based on one or more polyurethanes. In their uncrosslinked state, the polyurethanes preferably each have two or more crosslinkable groups, which are more preferably ethylenically unsaturated groups. Polyurethanes generally comprise two or more -NHC(O)O- linkages (urethane linkages) as obtainable by reaction of a hydroxyl group and an isocyanate group. Usually, polyurethanes are formed by reaction of a polyol and a polyisocyanate, e.g. by reaction of a diol and a diisocyanate. The reaction between a hydroxyl group and an isocyanate group forms an - NHC(O)O- linkage, and least two NHC(O)O- linkages are formed if a polyol is reacted with a polyisocyanate. This reaction is generally known in the art to produce polyurethanes (PU).
The monomer composition on which a polyurethane of the pressure-sensitive adhesive of the sleeve according to the invention is based may additionally comprise one or more (meth)acrylic monomers, a part of which may form additional crosslinking compounds. Furthermore, the precursor composition on which the pressure-sensitive adhesive layer is based, i.e., the uncrosslinked composition for forming the pressure-sensitive adhesive layer after crosslinking (in the following also referred to as the "PSA precursor composition"), may also contain a thermal or photosensitive polymerization initiator for initiating a crosslinking reaction. Furthermore, the PSA precursor composition may also contain a solvent, a polymerization inhibitor to avoid premature crosslinking, a plasticizer, a rheology modifier, a desiccant or similar additives, as generally known to the person skilled in the art.
Preferably, the pressure-sensitive adhesive layer comprises one or more polyurethanes to a total amount of at least 50 wt.-%, more preferably of at least 60 wt.-%, very preferably of at least 70 wt.-%, in particular at least 80 wt.-%, e.g., at least 85 wt.-%, and most preferably of at least 90 wt.-%, based on the total weight of the pressure-sensitive adhesive layer.
A polyurethane of the PSA precursor composition, i.e., a polyurethane of the pressure-sensitive adhesive in its uncrosslinked state, preferably comprises at least two crosslinkable groups, more preferably at least two ethylenically unsaturated groups. These groups can be introduced into the polyurethane by forming the polyurethane first, and then modifying the polyurethane by reacting the polyurethane with a compound (modifying compound) that is capable of introducing the crosslinkable groups into the polyurethane. Such a reaction can be performed by reacting the modifying compound with unconsumed hydroxy and/or isocyanate groups which are present at the chain ends of the polyurethane. For instance, if the polyurethane is derived from the reaction of a diol and a diisocyanate and is thus linear, the introduction of the crosslinkable groups, such as ethylenically unsaturated groups, can be achieved by reacting the polyurethane polymer with a modifying compound having a crosslinkable group and either one of an isocyanate group and a hydroxyl group, or both. For instance, a terminal isocyanate group may be reacted with a hydroxyl-containing (meth)acrylate compound to introduce an ethylenically unsaturated group. Such a modification is preferably effected to such an extent that at least two crosslinkable groups are introduced. This can be achieved by using the same type of reaction with the same modifying compound in case of a polyurethane having at least two groups that are reactive with the modifying compound in a molecule, and in this case there are used at least two equivalents of the modifying compound relative to one equivalent of the non-modified polymer. Of course, it is also possible to use two or more different modifying compounds that are reactive with two or more different groups present on the non-modified polymer.
Alternatively, the crosslinkable groups can be introduced into the polyurethane polymer by using a suitable starting compound for the polyurethane polymer synthesis, such as a polyol (e.g. a diol or triol) and/or a polyisocyanate (such as a diisocyanate or triisocyanate) having one or more crosslinkable groups. These groups then may remain unreacted during the formation of the polyurethane polymer and may subsequently undergo a crosslinking reaction upon proper initiation, e.g., radically. Preferably, the monomer composition on which a polyurethane of the pressure-sensitive adhesive facing towards the printing plates according to the invention is based comprises a (meth)acrylate diol, more preferably a dihydroxyalkyl (meth)acrylate, e.g., 2,3-dihydroxypropyl methacrylate.
Generally, the crosslinkable groups can be freely chosen, but are neither hydroxy groups nor isocyanate groups, and of course also no -NHC(O)O groups. Preferably, a polyurethane of the PSA precursor composition, i.e., a polyurethane of the pressure-sensitive adhesive in its uncrosslinked state, comprises at least two crosslinkable groups capable of undergoing a crosslinking reaction by a radical or ionic pathway, more preferably by a radical pathway. While thus the crosslinking groups can be chosen from those that are able to react with the same or another crosslinking group in another molecule of the PSA precursor composition, such as epoxy groups or alcohol/carboxylic ester combinations to form an ester bond, the crosslinkable groups are preferably ethylenically unsaturated groups. The reason for this is that upon proper initiation the ethylenically unsaturated groups react via a radical mechanism and are also able to react to some extent with the polymer backbone of the polyurethane in a radical chain reaction. This is believed to be preferable because in this way no exact alignment of reactive groups needs to be achieved, as the radical reaction is able to form a wide variety of crosslinks upon proper initiation.
The polyols and the polyisocyanates used for the polyurethane backbone are generally not particularly limited. In one embodiment, the polyurethane is linear and thus made from one or more diols and one or more diisocyanates. The polyurethane backbone may be derived from only one diol or a mixture of two or more diols and from only one diisocyanate or a mixture of two or more diisocyanates. Thus, the polyurethane backbone may be obtained from one diol and one diisocyanate, may be obtained from a combination of two or more diols with one diisocyanate, may be obtained from a combination of two or more diisocyanates with one diol, and may be obtained from a combination of two or more diisocyanates with two or more diols.
The diols include small molecule diols having a molecular weight of 500 or less, such as ethylene glycol and propylene glycol, but also large diols having a molecular weight of more than 500 or more or 600 or more or 1000 or more, but generally 10,000 or less, such as 8,000 or less or6,000 or less, e.g., 5,000 or less, in order to maintain the PU characteristics. Here, the molecular weight refers to the weight average molecular weight in case of polymeric compounds having a molecular weight distribution.
Examples thereof include homopolymers and copolymers of two or more ethylenically unsaturated compounds, preferably selected from the group consisting of styrenes, alkenes and polyenes having 2 to 12 carbon atoms and cycloalkenes having 3 to 12 carbon atoms, such as ethylene, propylene, n-butene, isobutene, 1-pentene, 2-pentene, 2-methyl-but-1-en, 1,3-butadiene, 1,3-, 1,4- and 1,5-hexadiene, styrene, and α-methyl styrene and copolymers of these, which have been functionalized by providing two (usually terminal) hydroxy groups. An example is a homopolymer of ethylene, propylene, 1-butene, isobutylene or 1,3 butadiene, or a copolymer of or two or more of these, into which two hydroxy groups have been terminally added. A polymer derived from a polyene, such as polybutadiene, is typically hydrogenated before functionalizing it to provide for e.g. two terminal hydroxy groups in order to make it less susceptible to decay, but in the context of the present invention also a non-hydrogenated polybutadiene can be used and subsequently functionalized to provide for e.g. two terminal hydroxy groups, as it provides already reactive groups for a subsequent crosslinking reaction.
Further examples of large diols include polyether polyols and polyester polyols, of which polyether polyols are preferred in view of polyester polyols generally being more susceptible to hydrolysis and polyether polyols providing improved stickiness. Specific examples include poly(tetramethylene) glycol (PTMO), polypropylene oxide) (PPO) glycol, and poly(ethylene) glycol (PEG). In both the polyester polyol and the polyether polyol, the number of repeating units is typically 50 or more, such as 100 or more or 200 or more, but generally 1000 or less, such as 800 or less.
The diisocyanates can equally be chosen from known diisocyanates for the production of PU, such as aromatic diisocyanates like methylene diphenyl diisocyanate (MDI) and hydrogenated MDI in all stereoisomeric forms such as 2,2'-, 2,4' and 4,4'-; and toluene diisocyanate (TDI), of which all stereoisomers such as 2,4; and 2,6 can be used. Alternatively, and preferably, aliphatic diisocyanates are used, as they can reduce environmental burden and are less likely to cause health hazards. Examples thereof include 1,4-butanediisocyanate (BDI), 1,6-hexamethylenediisocyanate (HDI), 2,2,4-trimethyl hexamethylene diisocyanate (TMDI), ethyl-2,6-diisocyanatohexanoate (ELDI) and methyl-2,6-diisocyanatohexanoate (MLDI), isophorone diisocyanate (IPDI), and 1,4-cyclohexane diisocyanate, lysine diisocyanate etc.
Similar to the diols, the diisocyanates can also be compounds having a small (500 or less) or large molecular weight, such as having a molecular weight of more than 500, 600 or more or 1000 or more, but generally 10,000 or less, such as 8,000 or less or 6,000 or less, e.g. 5,000 or less, expressed as weight average molecular weight in case of a polymeric compound having a molecular weight distribution, in order to maintain the PU characteristics. In principle, these can be prepared by functionalizing a corresponding compound with two isocyanate groups or by introducing at two positions a group carrying an isocyanate group.
While thus both the diols and the diisocyanates can each be selected from small molecules and large molecules as defined above, in one embodiment one of the diols and the isocyanates is a small molecule (M_w 500 or less), and the other one is a large molecule (M_w more than 500, preferably 1,000 or more). In view of availability, it is preferably the polyol that is a large molecule. As mentioned hereinbefore, it is also possible to use more than one diol and/or more than one diisocyanate. Also in this case, preferably at least one of the two or more diols or at least one of the two or more diisocyanates is a large compound having a M_w of 500 or more, such as 1,000 or more.
In each case, the polyols and the polyisocyanates can be of synthetic origin or can be of natural origin. Examples of polyols of natural origin include sugars and other carbohydrates having two or more hydroxy groups, or hydrogenated castor oil or a palm-oil-based polyester polyol.
The PSA precursor composition preferably comprises a crosslinking initiator. "Crosslinking initiator" refers to a compound capable of initiating a crosslinking reaction by generating an ion or a radical upon heating or irradiation. In a preferred embodiment, the crosslinking initiator is a UV initiator. The UV initiator can be selected from known UV initiators, such as benzyl dimethyl ketal (IRGACURE^® 651), benzoin isobutyl ether (BIBE), benzophenone and associated derivatives, 2,2-diethoxyacetophenone, cyclohexyl phenyl ketone and such derivatives (IRGACURE^® 184), mono and di-acylphosphine oxide derivatives (IRGACURE^® 819), and similar substances. The most preferred UV initiators are BIBE, IRGACURE^® 184, and IRGACURE^® 651, among which BIBE is most preferred. The concentration of UV initiator can range from 0.1 to 3 wt.-% but is preferably from 1 to 2 wt.-%, relative to the total weight of the PSA precursor composition excluding the optional solvent. The UV initiator can be one compound, but also a combination of UV initiators can be used, e.g., with sensitivity towards different wavelengths.
The PSA precursor composition optionally includes a solvent. Preferably, the PSA precursor composition is free of solvent, as the evaporation of the solvent reduces the volume of the layer formed from the PSA precursor composition and makes a proper thickness adjustment more difficult. Evaporation of the solvent may also lead to a porous structure, which is not desired. If present, amount of solvent is typically 50 wt.-% or less, preferably 25 wt.-% or less, such as 15 wt.-% or less or 10 wt.-% or less. The solvent can be chosen from known organic solvents, such as protic and aprotic solvents and mixtures thereof. The solvent preferably has a boiling point of 75°C or less at 10⁵ Pa. In one embodiment, the solvent is selected from ethers; alcohols; ketones and esters, such as acetone, methyl ethyl ketone, acetic acid ethyl ester and acetic acid methyl ester; ethanol; methanol; isopropanol; tetrahydrofuran, or diethyl ether.
The pressure-sensitive adhesive layer preferably comprises one or more crosslinked polyurethanes as obtained after crosslinking the PSA precursor composition described hereinbefore, typically by initiation using heat or radiation. While the pressure-sensitive adhesive layer may contain other components besides the crosslinked polyurethane(s), the one or more crosslinked polyurethanes in their entirety preferably form 80 wt.-% or more, such as 90 wt.-% or more or 95 wt.-% or more of the pressure-sensitive adhesive layer. The pressure-sensitive adhesive layer may also consist of the crosslinked polyurethane(s).
The pressure-sensitive adhesive layer is preferably formed by providing the PSA precursor composition on a substrate and then initiating the crosslinking reaction. The formation may also include heating the PSA precursor composition in order to evaporate any solvent and/or unreacted monomer, if desired.
The thickness of the pressure-sensitive adhesive layer is not particularly limited as long as the object of the present invention is achieved, but is typically 1.00 mm or less, preferably 0.90 mm or less, 0.80 mm or less, 0.70 mm or less, 0.60 mm or less, 0.50 mm or less, or 0.45 mm or less. The lower limit is not particularly limited, but can be 0.05 mm or more, such as 0.10 mm or more, 0.15 mm or more, or 0.20 mm or more, such as 0.25 mm or more.
The pressure-sensitive adhesive layer preferably does not contain microspheres.
In one embodiment, at least one of the areas having different adhesive force towards the printing plate is optically identifiable. Thus, the printing form attachment device according to the invention preferably comprises an identification marking providing distinguishability of at least two areas of the pressure-sensitive adhesive layer being exposed to the printing plate having different adhesive force towards the printing plate. Such identification marking allows for easy identification of segments having higher adhesive force or lower adhesive force and therefore may support the operator or machine who or which performs the mounting or demounting of the printing plate in identifying "mounting" and "demounting" zones. Identification marking may be provided, e.g., by color coding. Preferably, optical identifiability in accordance with the present embodiment is provided by color marking and/or by writing. For example, the reinforcement film of the support may be printed with specific colors, shapes or writing. The print may be visible through the translucent outer pressure-sensitive adhesive layer. Thus, a high or low adhesion zone may be visibly highlighted by color code or writing matching the outline of the respective zone.
The invention may be further explained by means of figures 1 - 3.

Figure 1 shows a printing form attachment device (1) as generally known in the art comprising a base (4) and a foam layer (3), which together form the support of the device. The entire surface being exposed to the printing plate is covered by pressure-sensitive adhesive (2).
Figure 2 shows an exemplary printing form attachment device (1) according to the invention, wherein the surface of the pressure-sensitive adhesive being exposed to the printing plate comprises
- the area (2) which may be the area having a higher adhesive force towards the printing plate, and
- the area (5) which may be the area having a lower adhesive force towards the printing plate, area (5) extending over the whole length of the printing form attachment device.
Figure 3 as well shows an exemplary printing form attachment device (1) according to the invention, wherein the surface of the pressure-sensitive adhesive being exposed to the printing plate comprises
- the area (2) which may be the area having a higher adhesive force towards the printing plate, and
- the area (5) which may be the area having a lower adhesive force towards the printing plate, area (5) being provided in a geometrical arrangement that is different from that of figure 2.

Thus, figure 3 represents an arrangement where the surface of the pressure-sensitive adhesive layer being exposed to the printing plate comprises a first area (2) having a specific adhesive force towards the printing plate and several separated areas (5) each having the same adhesive force towards the printing plate which is, however, different from the adhesive force towards the printing plate of the first area (2).

Experimental part

Test methods

Method A - peel adhesion

For the determination of peel adhesion, layers of the adhesives having a thickness of 250 µm were subject to an indirect measurement.
A reinforcement steel plate was equipped with a double-sided adhesive tape. The layer of pressure-sensitive adhesive to be tested was coated on an etched PET film and covered with a release liner. The layered body thus received was applied with the PET film side onto the double-sided adhesive tape. The liner was removed to expose the free surface of the pressure-sensitive adhesive and a non-etched, standard PET test film having a width of 20 mm was applied and fixed by five times rolling back and forth using a 4 kg roll. The test assembly was directly fixed and the PET test film was peeled off from the pressure-sensitive adhesive with a Zwick machine (Zwick Roell Z2.5) at an angel of 90 ° and at a speed of 300 mm/min. The required force was determined with a tensile tester. The results were determined as an average of 3 specimens, standardized to the width of the strip and are given in N/cm.
For peel adhesion measurement directly on the sleeve, the sleeve was first fixed in an auxiliary device to hold it in place during measurement. Then, the PET test film was applied directly on the sleeve and firmly fixed using a plastic applicator. After that, it was peeled off and the required force was measured as set forth above.

Method B - edge lifting test

A printing plate (Cyrel type, Du Pont, thickness 1.7 mm, dimension 200 mm by 150 mm) was applied on the self-adhesive sleeve to be tested (repeat of 500 mm) and tightly pressed on using a plastic applicator to exclude any trapped air between printing plate and adhesive. It was secured that the edges of the printing plate are completely flush with the adhesive.
The assembly thus prepared was stored in a climate chamber at conditions given in table 3 for 3 d. Thereafter, the assembly was reconditioned to normal climate (23 °C, 50% r. H.). Finally, it was measured with a lineal how far the edges of the printing plate have been lifted. The results are given in mm.

Manufacturing of pressure-sensitive adhesive

Polyurethane prepolymers have been manufactured solvent-based starting from the components given in table 1.

Table 1: Chemicals used

Tradename	Chemical name/function	Manufacturer/Provider
Krasol^® LBH 2000	polybutadiene diol	Cray Valley
GMMA	2,3-dihydroxypropyl methacrylate (CAS: 5919-74-4); diol	Allnex
IPDI	isophorone diisocyanate (IPDI) (CAS: 4098-71-9)	Sigma Aldrich
Coscat^® 83	bismuth trisneodecanoate; catalyst	Vertellus
2-Butanone	2-butanone; solvent (CAS: 78-93-3)	Sigma Aldrich
DMPA	2,2-dimethoxy-2-phenyl acetophenone; (CAS: 24650-42-8); photoinitiator	Sigma Aldrich

High adhesion PSA precursor composition 1

124.79 g Krasol^® LBH 2000, 60.00 g 2-butanone and 1.30 g GMMA are added to a 500 mL glass reaction vessel. The obtained mixture is stirred until a homogenous solution is obtained. Then 13.70 g of IPDI is added and the mixture is stirred until it becomes a homogenous solution again. Then 0.21 g of the catalyst is added, and the mixture is stirred while being cooled to room temperature for 24 h. Afterwards, the reaction mixture is discharged to another vessel and stored for 48 h at 40 °C to allow completion of the reaction.

Low adhesion PSA precursor composition 2

123.16 g Krasol^® LBH 2000, 60.00 g 2-butanone and 2.10 g GMMA are added to a 500 mL glass reaction vessel. The obtained mixture is stirred until a homogenous solution is obtained. Then 14.54 g of IPDI is added and the mixture is stirred until it becomes a homogenous solution again. Then 0.21 g of the catalyst is added, and the mixture is stirred while being cooled to room temperature for 24 h. Afterwards, the reaction mixture is discharged to another vessel and stored for 48 h at 40 °C to allow completion of the reaction.

Coating

A photoinitiator (0.2 wt% referenced to the solid precursor formulation) was added to the PSA precursor composition manufactured as set forth above. The mixture thus obtained was homogenized for 5 min and then coated onto an etched PET carrier film. The solvent was evaporated in an air convection oven at 80 °C for a duration of 10 minutes. The dried layers were covered with a siliconized PET film and were exposed to UV radiation the source of which was adjusted to the initiator at a dose of 2,000 mJ/cm². Pressure-sensitive adhesive layers 1 and 2 each having a thickness of 250 µm were obtained.

Manufacturing of sleeves

Method 1: Providing zones of different adhesive force via UV post treatment

On a tesa^® Twinlock 74201 sleeve having a repeat of 500 mm and a length of 1.400 mm a 100 mm broad strip of UV-impermeable film was applied over the whole length, thus covering 20 % of the adhesive surface and leaving 80 % of the adhesive surface uncovered. The surface of the sleeve thus prepared was exposed to UV radiation at a dose of 5,000 mJ/cm² in a UV curing chamber. After that, the covering film was removed, and the adhesive force was measured; test results are given in table 3.
As a comparative example, the tesa^® Twinlock 74201 sleeve was UV treated in the same way over the entire adhesive surface, i. e., without any covering.

Method 2: Manufacturing of a sleeve having zones of different adhesive force via use of different adhesives

A base cylinder having a repeat of 500 mm that is equipped with a double-sided adhesive polyurethane foam tape was provided. A 250 µm thick layer of pressure-sensitive adhesive layer 1 prepared as set forth above having a width of 100 mm was laminated onto the surface of the sleeve over its whole length; and a 250 µm thick layer of pressure-sensitive adhesive layer 2 prepared as set forth above having a width of 400 mm was laminated onto the surface of the sleeve over its whole length. Uncured PSA precursor composition 1 was added using a syringe to cover the seam between the different adhesive layers. By local irradiation using UV light, the photosensitive composition was cured to form a tight seal.
As comparative examples, the base cylinder was covered with pressure-sensitive adhesive layers 1 and 2, respectively, over its entire surface.
In the examples, easiness of plate demounting was assessed individually by 5 independent persons each being experienced in plate mounting operations. Results are given according to a scale from 1 to 4:

1 -: easy demounting
2 -: average demounting
3 -: hard demounting
4 -: demounting impossible without causing serious damage to printing plate

Test results are given in table 3.

table 3: test results

	tesa^® Twinlock 74201 (c. e.)	tesa^® Twinlock 74201 PostTreatment (entire surface, c. e.)	tesa^® Twinlock 74201 after partial PostTreatment (method 1)	High adhesion PSA - Composition 1 (entire surface, c. e.)	Low adhesion PSA - Composition 2 (entire surface, c. e.)	Combination of Comp. 1 and Comp. 2 (method 2)
Peel Adhesion [N/cm] on PET	1.8 N/cm	1.2 N/cm	1.8 N/cm (plate edge); 1.2 N/cm (plate body)	2.4 N/cm	0.9 N/cm	2.4 N/cm (plate edge); 0.9 N/cm (plate body)
Edge Lifting after 3d @ RT [mm]	5	12	5	0	15	0
Edge Lifting after 3d @ 35°C / 85% r.h. [mm]	7	23	7	2	25	2
Plate demounting [Scale 1 to 5]	2-average demounting	1 - easy demounting	1 - easy demounting	3 - hard demounting	1 - easy demounting	1 - easy demounting

c. e. - comparative example

Reference list

1 -: printing form attachment device
2 -: pressure-sensitive adhesive
3 -: foam layer
4 -: base
5 -: pressure-sensitive adhesive

Claims

Printing form attachment device for mounting a printing plate onto a printing cylinder, the printing form attachment device comprising a support and a pressure-sensitive adhesive layer being provided on the support and having a surface being exposed to the printing plate,
wherein the surface of the pressure-sensitive adhesive layer being exposed to the printing plate comprises at least two areas having different adhesive forces towards the printing plate.
Printing form attachment device according to claim 1, characterized in that one or more areas having a lower adhesive force towards the printing plate than one or more further areas amount to at least 70 % of the entire surface of the pressure-sensitive adhesive layer being exposed to the printing plate.
Printing form attachment device according to claim 1 or 2, characterized in that one or more areas having a higher adhesion force towards the printing plate than one or more further areas amount to at least 2 % of the entire surface of the pressure-sensitive adhesive layer being exposed to the printing plate.
Printing form attachment device according to any of the preceding claims, characterized in that the different adhesive force towards the printing plate is achieved by using different pressure-sensitive adhesives.
Printing form attachment device according to any of claims 1 to 3, characterized in that the different adhesive force towards the printing plate is achieved by different surface structures of the otherwise same pressure-sensitive adhesive.
Printing form attachment device according to any of claims 1 to 3, characterized in that the different adhesive force towards the printing plate is achieved by different physical and/or chemical treatment of the otherwise same pressure-sensitive adhesive.
Printing form attachment device according to claim 6 characterized in that the different adhesive force towards the printing plate is achieved by different UV-radiation of the otherwise same pressure-sensitive adhesive.
Printing form attachment device according to any of the preceding claims, characterized in that at least one of the areas having different adhesive force towards the printing plate is optically identifiable.
Printing form attachment device according to any of the preceding claims, characterized in that the surface of the pressure-sensitive adhesive layer being exposed to the printing plate comprises at least two areas having a difference in their adhesive force towards the printing plate of minimum 0.5 N/cm.