Method for producing an adhesive bond and adhesive element for the method

Abstract

The invention relates to a method for producing an adhesive bond (10), comprising the method steps of: producing a multilaminate adhesive arrangement (14), comprising: i) a first adhesive layer (16) which is at least partially transparent and which contacts the substrate (12), comprising a first radiation-crosslinkable pressure-sensitive adhesive, which is crosslinkable with electromagnetic radiation having a wavelength Ai, ii) a second adhesive layer (18) which is at least partially transparent, comprising a second radiation-crosslinkable pressure-sensitive adhesive, which is crosslinkable with a wavelength λ2, and optionally iii) an interlayer (20) which is at least partially transparent, wherein λ1 > (λ2 + 20 nm), wherein the multilaminate adhesive arrangement (14) comprises UV blocker, in the interlayer (20) and / or in the first adhesive layer (16), which has a higher absorption for radiation having a wavelength λ2 than for radiation having a wavelength λ1, b) irradiating the multilaminate adhesive arrangement (14) with radiation of the wavelength λ2 for crosslinking the second radiation-crosslinkable pressure-sensitive adhesive and for producing a second crosslinked adhesive lamina (22), and c) irradiating the multilaminate adhesive arrangement (14) with electromagnetic radiation of the wavelength λ1 for crosslinking the first radiation-crosslinkable pressure-sensitive adhesive and for producing a first crosslinked adhesive lamina (24).

Description

[0001] tesa SE

[0002] Norderstedt

[0003] Method for producing an adhesive bond and adhesive element for the method

[0004] The invention relates to a method for producing an adhesive bond, in particular in the field of the electronics industry, and to a multilaminate adhesive element for use in such a method. Also disclosed, moreover, is an adhesive kit for use in a corresponding method for producing a multilaminate adhesive arrangement.

[0005] The joining of separate elements is one of the central methods in manufacturing. In addition to other methods, such as welding and soldering, for example, adhesive bonding, i.e. joining using an adhesive, is of particular importance nowadays. An alternative here to the use of formless adhesives, which are applied from a tube, for example, are so-called adhesive tapes. From everyday life, pressure-sensitive adhesive tapes in particular are known, in which the adhesive effect is ensured by a pressure-sensitive adhesive, which is permanently tacky and adhesive under normal ambient conditions. Corresponding pressure-sensitive adhesive tapes can be applied to a substrate by means of pressure and remain adhering there, but can later be removed in a more or less residue-free manner.

[0006] A further type of adhesive tapes is also of great importance, especially for use in industrial manufacture. These adhesive tapes, which are sometimes also referred to as reactive adhesive tapes, use a curable adhesive. Appropriate curable adhesives have not yet reached their maximum degree of crosslinking in the state intended for application and can be cured by external influences, by initiating the polymerization in the curable adhesive and thereby increasing the degree of crosslinking. The mechanical properties of the now cured adhesive change here, with the viscosity, surface hardness and strength increasing in particular. Curable adhesives are known in the art and can have very different compositions from a chemical point of view. Common to these curable adhesives is that the crosslinking reaction can be triggered by external influencing factors, for example by energy supply, in particular by thermal, plasma or radiation curing, and / or by contact with a polymerization-promoting substance. Owing to the performance advantages, the radiation-curing reactive adhesives which can be cured by application of electromagnetic radiation of a certain wavelength are of particular importance here in many fields of application.

[0007] From performance standpoints, many industrial applications give particular preference especially to reactive adhesives which combine good curability with pressure-sensitive properties. Such reactive pressure-sensitive adhesives and pressure-sensitive adhesive tapes based on them can prior to curing be applied reliably and easily, before the reactive pressure-sensitive adhesive is cured. The pressure-sensitive adhesiveness of corresponding reactive pressure-sensitive adhesives also allows easy preliminary fixing of elements to be bonded.

[0008] One industry branch in which reactive pressure-sensitive adhesive tapes are often used is the electronics industry, in which they are used, for example, to bond displays, especially touch-sensitive displays. The reactive pressure-sensitive adhesive tapes used are regularly located during later operation at least partially in transparent areas of the manufactured devices and must therefore be taken into account from the point of view of light management. Accordingly, these reactive adhesive tapes are regularly subjected to particularly exacting requirements with regard to the optical and other properties. The pressure-sensitive adhesives used must regularly in particular be highly transparent in the cured state, which is why the adhesives used are also referred to as "optically clear adhesive" (OCA).

[0009] As a result of the use of increasingly more complex designs, especially of complex display setups, and in order to optimize existing manufacturing processes, adhesive arrangements are increasingly being used that comprise two or more radiation-crosslinkable adhesives in superimposed layers. For many manufacturing methods, it would be desirable to be able to crosslink these radiation-crosslinkable adhesives, which can regularly be cured in similar wavelength ranges, as independently of one another as possible, ideally in a defined sequence in succession. The problem here, however, is that the radiationbased crosslinking of the individual layers with the methods and products known from the prior art technically cannot - or not sufficiently - be decoupled from each other and, as a result, during the crosslinking of a first curable adhesive layer, in many cases an unwanted (pre-)crosslinking of another adhesive layer of the generated adhesive arrangement can occur. Such unwanted (pre-)crosslinking can result in undesirable defects in the product to be produced, which in many cases either necessitates an expensive rework or the product produced is indeed so irreparable that it can only be disposed of.

[0010] The primary object of the present invention was to eliminate or at least reduce the disadvantages of the prior art.

[0011] In particular, it was the object of the present invention to provide a method for producing an adhesive bond that allows an adhesive bond to be produced by crosslinking of two radiation-crosslinkable pressure-sensitive adhesives, in which the crosslinking of the individual radiation-crosslinkable pressure-sensitive adhesives can very extensively be decoupled from each other.

[0012] In this respect, it was an object of the present invention that it should be possible, with the method to be specified, to crosslink a first radiation-crosslinkable pressuresensitive adhesive of a multilaminate adhesive arrangement, where the extent of any unwanted (pre-)crosslinking of a second radiation-crosslinkable pressuresensitive adhesive of the same adhesive arrangement should be as low as possible, it having been particularly desirable that a corresponding unwanted (pre- jcrosslinking should essentially be completely avoidable.

[0013] Against this background, it was an object of the present invention that the method to be specified should enable sequential production of a desired adhesive bond in a manner that can be controlled as supervisedly and as precisely as possible.

[0014] It was an object of the present invention that the method to be specified should be particularly time- and cost-efficient to carry out. It was a further object of the present invention that the method to be specified should be suitable particularly for use in the electronics industry, in particular in the bonding of displays. In this respect, it was an important proviso of the present invention that the method to be specified should be able to produce optically very clear adhesive bonds that enable advantageous light management.

[0015] It was a desirable proviso of the present invention that the method to be specified should impose as low requirements as possible on the apparatuses to be used and also, desirably, on the degree of training of the workforce deployed in the method.

[0016] It was a further object of the present invention that powerful and durable adhesive bonds should be able to be produced with the method to be specified.

[0017] It was a further object of the present invention that the method to be specified should operate as far as possible without the use of additional substances harmful to the environment and / or health.

[0018] It was a further object of the present invention to provide an advantageous multilaminate adhesive element which is optimized for use in the method to be specified.

[0019] It was a secondary object of the present invention, moreover, to provide an adhesive kit for use in a corresponding method as an alternative to multilaminate adhesive elements.

[0020] The inventors of the present invention have now found that the objects described above can surprisingly be achieved when, in a method for producing an adhesive bond with an adhesive arrangement comprising two radiation-crosslinkable pressure-sensitive adhesives which can be crosslinked at different wavelengths, two different curing wavelengths are used, which can be advantageously generated, for example, via common LIV-LED systems, when in the lower adhesive layer and / or in an interlayer arranged between the adhesive layers, a UV blocker is used which has a higher absorption for one of the wavelengths than for the other, so that the intensity of electromagnetic radiation experienced by the lower adhesive layer during the curing of the upper adhesive layer can be reduced, as defined in the claims.

[0021] With the method according to the invention, it is advantageously possible to decouple the UV crosslinking steps very largely from each other, so making it possible advantageously to carry out targeted UV crosslinking of a single lamina without thereby adversely affecting an underlying, likewise UV-sensitive adhesive layer. This makes it possible in an advantageous way to implement even complex process control of the bonding and to produce even complex display setups reliably, supervisedly and with a high bonding quality. Only very low requirements are imposed on the required apparatuses, as only two UV radiation sources have to be provided, differing in the wavelength maximum of their emission. The resulting method according to the invention can be implemented in a particularly time- and cost-efficient manner and makes comparatively low demands on the degree of training of the workforce employed in the method.

[0022] The above-stated objects are therefore achieved by the subject matter of the invention as defined in the claims. Preferred configurations according to the invention are apparent from the dependent claims and the observations below.

[0023] 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 multilaminate adhesive elements and adhesive kits are apparent from the features of preferred methods.

[0024] Insofar as both specific amounts or fractions of an element and preferred configurations of the element are disclosed below for this element, for example for the radiation-crosslinking pressure-sensitive adhesives or the UV blockers, in particular the specific amounts or fractions of the preferably configured 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.

[0025] The invention relates in particular to a method for producing an adhesive bond, comprising the method steps of: a) producing a multilaminate adhesive arrangement arranged on a substrate, comprising: i) a first adhesive layer which is at least partially transparent for visible light and which contacts the substrate, comprising a first radiation-crosslinkable pressuresensitive adhesive, wherein the first radiation-crosslinkable pressure-sensitive adhesive is crosslinkable by irradiation with electromagnetic radiation having a wavelength i, ii) a second adhesive layer which is at least partially transparent for visible light and which is arranged on the side of the multilaminate adhesive arrangement that faces away from the substrate, comprising a second radiation-crosslinkable adhesive, wherein the second radiation-crosslinkable pressure-sensitive adhesive is crosslinkable by irradiation with electromagnetic radiation having a wavelength A2, and optionally iii) an interlayer which is at least partially transparent for visible light and which is arranged between the first adhesive layer and the second adhesive layer, wherein Ai > (A2+ 20 nm), wherein the multilaminate adhesive arrangement comprises one or more LIV blockers in the interlayer and / or in the first adhesive layer, wherein the one or more UV blockers have a higher absorption for electromagnetic radiation having a wavelength A2than for electromagnetic radiation having a wavelength Ai, wherein the multilaminate adhesive arrangement is designed such that for electromagnetic radiation of the wavelength A2impinging orthogonally on the side facing away from the first adhesive layer, in the first adhesive layer, a transmission of less than 20% is obtained at a penetration depth of 50% relative to its total thickness, and wherein the multilaminate adhesive arrangement is designed such that for electromagnetic radiation of the wavelength Ai impinging orthogonally on the side facing away from the first adhesive layer, in the first adhesive layer, a transmission of 20% or more is obtained at a penetration depth of 50% relative to its total thickness, the transmission being determined by means of UV-VIS spectroscopy with correction of the interfacial reflection losses, b) irradiating the multilaminate adhesive arrangement with electromagnetic radiation of the wavelength A2for crosslinking the second radiation-crosslinkable pressure-sensitive adhesive and for producing a second crosslinked adhesive lamina, and c) irradiating the multilaminate adhesive arrangement with electromagnetic radiation of the wavelength Ai for crosslinking the first radiation-crosslinkable pressure-sensitive adhesive and for producing a first crosslinked adhesive lamina.

[0026] The method according to the invention is used for the production of an adhesive bond and thus, in other words, for the production of a bonded substrate. The method according to the invention is particularly suitable for use in the electronics industry, where its advantages are particularly pronounced in the realization of complex setups. Preferable accordingly is a method according to the invention wherein the bonding takes place in an electronic device, preferably for securing a display. Preferable, in addition or alternatively, is a method according to the invention wherein the substrate is an electronic device, in particular a housing component, or a display.

[0027] In method step a), a multilaminate adhesive arrangement is first produced on the substrate to be bonded, which can take place advantageously by the use of multilaminate adhesive elements according to the invention or of the disclosed adhesive kits, as are further disclosed below. The adhesive elements used can be adhesive tapes, for example, where adhesive films or prefabricated adhesive elements, for example, can also be used. Preferable accordingly is a method according to the invention wherein the production of the multilaminate adhesive arrangement arranged on the substrate takes place using a multilaminate adhesive element according to the invention or a disclosed adhesive kit, preferably a multilaminate adhesive element according to the invention.

[0028] The multilaminate adhesive arrangement produced on the substrate initially comprises a first adhesive layer. This first adhesive layer contacts the substrate, according to the invention, and thus forms the lowest layer of the multilaminate adhesive arrangement. In addition, the adhesive arrangement comprises a second adhesive layer, which is present on the side of the multilaminate adhesive arrangement that faces away from the substrate. In an illustrative method according to the invention, the first adhesive layer and / or the second adhesive layer have an average thickness in the range from 10 to 200 pm, preferably in the range from 15 to 150 pm, particularly preferably in the range from 25 to 125 pm.

[0029] The first adhesive layer and the second adhesive layer each comprise a radiation- crosslinkable pressure-sensitive adhesive, where, in addition to the radiation- crosslinkable pressure-sensitive adhesives, in principle, further constituents may also be present, for example macroscopic fillers, which are dispersed in the radiation-crosslinkable pressure-sensitive adhesive. However, it is preferred for substantially all embodiments if the first adhesive layer and / or the second adhesive layer, preferably the first adhesive layer and the second adhesive layer, consist to the extent of a mass fraction of 90% or more, particularly preferably of 95% or more, very preferably substantially to an extent of 100%, of the respective radiation-crosslinkable pressure-sensitive adhesive.

[0030] In the simplest configurations, the first adhesive layer and the second adhesive layer may be in contact directly, so that the first adhesive layer is arranged between the second adhesive layer and the substrate. Such adhesive arrangements can be produced efficiently by using carrierless adhesive elements. However, as an alternative to this configuration, a further layer may also be provided between the first adhesive layer and the second adhesive layer, which is referred to as an interlayer in the context of the present invention and the use of which is preferred in the opinion of the inventors for the vast majority of cases, as further disclosed below. Preferable accordingly is a method according to the invention wherein the adhesive arrangement comprises the interlayer.

[0031] In the course of the method according to the invention, the first radiation- crosslinkable pressure-sensitive adhesive present in the first adhesive layer and the second radiation-crosslinkable pressure-sensitive adhesive present in the second adhesive layer are cured, with the resulting crosslinked adhesive arrangement being then joined more robustly to the substrate, beyond the previously existing pressure-sensitive adhesiveness. In agreement with the expert understanding, there may theoretically be applications in which the crosslinking of a multilaminate adhesive arrangement on the surface of a substrate is already desirable in itself, for example to specifically adjust the surface properties on the surface of the substrate. The skilled person hence automatically understands, however, that for the vast majority of cases it is desirable to bond a further component, in particular a component complementary to the substrate, over the multilaminate adhesive arrangement, preferably a display, a glass cover, or housing, by means of the adhesive arrangement on the substrate. Accordingly, for the vast majority of applications, it is preferred to place one or more elements to be bonded on the side of the adhesive arrangement that faces away from the substrate. For the purpose of clearly distinguishing these elements from the starting substrate, this is referred to in the context of the present invention as a top substrate, wherein the top substrate may be, for example, a display. The skilled person understands to this extent that the radiation-based curing of the radiationcrosslinking pressure-sensitive adhesives in the adhesive layers in the presence of a top substrate will take place in the vast majority of cases through the top substrate, so that it is expedient for the top substrate to be sufficiently transparent for the corresponding electromagnetic radiation, so that, for example, there is no need to resort to lateral curing. Preferable, therefore, for the vast majority of cases, is a method according to the invention wherein, prior to the crosslinking of the second pressure-sensitive adhesive, a top substrate is arranged which is preferably at least partially transparent for visible light and which contacts the second adhesive layer. Particularly preferred is a method according to the invention wherein the top substrate is a component of an electronic device, preferably a component of a display for an electronic device. Particularly preferred, additionally or alternatively, is a method according to the invention wherein the top substrate is at least partially transparent for electromagnetic radiation of the wavelength Ai and A2.

[0032] Particularly preferred is a method according to the invention further comprising the step of arranging a top substrate to be bonded on a side of the multilaminate adhesive arrangement that faces away from the substrate, wherein the top substrate is at least partially transparent for visible light and contacts the second adhesive layer, preferably wherein the top substrate is at least partially transparent for electromagnetic radiations of the wavelengths A1 and A2.

[0033] The first adhesive layer and the second adhesive layer each comprise a radiation- crosslinkable pressure-sensitive adhesive which is crosslinkable with electromagnetic radiation having a wavelength A1 or A2, respectively. Radiation- crosslinkable adhesives in general and radiation-crosslinkable pressure-sensitive adhesives in particular are comprehensively known to the skilled person, as described above, from the prior art, and also the individual components used for them from a chemical standpoint are known to the skilled person in isolation and are commercially available in different variations from numerous providers. In this context, the chemical nature of the radiation-crosslinking pressure-sensitive adhesives used does not matter for the implementation of the present invention, since only their functional suitability for crosslinking at the specified wavelengths is required. In this respect, the skilled person can freely resort to basic systems known to them for radiation-crosslinking pressure-sensitive adhesives and can select them in the light of the rest of the profile of requirements, in particular with regard to the optical properties.

[0034] The pressure-sensitive adhesives to be used according to the invention are radiation-crosslinking, i.e. can be crosslinked or cured by the use of suitable electromagnetic radiation. Owing to the possibility for crosslinking or curing, the curable adhesive can in some cases act preferably as a structural adhesive after curing. According to DIN EN 923:2016-03, structural adhesives are demonstrably suitable for the production of load-bearing constructions in which the adhesive bond can be stressed over long periods with a high percentage of the maximum breaking force without failure (according to the ASTM D907-15 (2023) definition: "bonding agents used for transferring required loads between adherends exposed to service environments typical for the structure involved"). They are therefore adhesives for highly chemically and physically stressable bonds that contribute to the solidification of the adhesive tapes in the cured state. Alternatively, however, the crosslinking can also only increase the degree of crosslinking in such a way that the pressure-sensitive adhesive property is retained and only the internal strength (shear strength, hardness) is increased by the crosslinking. In other words, it is a method according to the invention wherein the first radiation- crosslinkable pressure-sensitive adhesive can be converted into a first crosslinked adhesive lamina by crosslinking with electromagnetic radiation of the wavelength Ai, and / or wherein the second radiation-crosslinkable pressure-sensitive adhesive can be converted into a second crosslinked adhesive lamina by crosslinking with electromagnetic radiation of the wavelength A2.

[0035] In accordance with the expert understanding, the radiation crosslinkability of a radiation-crosslinkable pressure-sensitive adhesive results in particular from the use of polymerizable chemical compounds, for example radically polymerizable compounds, in particular the use of crosslinkable polymers. Illustrative, because of the high industrial relevance in the OCA field, is a method according to the invention wherein the first radiation-crosslinkable pressure-sensitive adhesive and / or the second radiation-crosslinkable pressure-sensitive adhesive is a (meth)acrylate-based pressure-sensitive adhesive. These crosslinkable polymers are combined with suitable photoinitiators, as is known from the prior art. Corresponding photoinitiators are chemical compounds which, as a result of an irradiation with electromagnetic radiation in a defined wavelength range, can cause a polymerization reaction of the polymerizable compounds present in the pressuresensitive adhesive, for example by decomposing into radical initiators after activation. Other examples also include photolatent acids and bases. In the context of the present invention, the term “photoinitiator” also embraces photoinitiator systems, which comprise two or more components which interact in the radiation- induced initiation, for example in the form of co-initiators or so-called "sensitizers". Both individual photoinitiators and suitable components for photoinitiator systems are comprehensively known to the skilled person on the basis of their general knowledge and are available commercially from numerous providers. The skilled person automatically selects the photoinitiators used in the pressure-sensitive adhesives in the light of the application requirements and for the implementation of the above-defined properties with regard to the crosslinking wavelengths. Illustrative in this respect is a method according to the invention wherein the first radiation-crosslinkable pressure-sensitive adhesive comprises first photoinitiator in a combined mass fraction in the range from 0.05 to 5%, preferably in the range from 0.1 to 3.0%, particularly preferably in the range from 0.15 to 1.5%, based on the mass of the first radiation-crosslinkable pressure-sensitive adhesive, and / or wherein the second radiation-crosslinkable pressure-sensitive adhesive comprises second photoinitiator in a combined mass fraction in the range from 0.05 to 5%, preferably in the range from 0.1 to 3.0%, particularly preferably in the range from 0.15 to 1 .5%, based on the mass of the second radiation-crosslinkable pressuresensitive adhesive.

[0036] In the light of the above disclosure, the skilled person understands that the method according to the invention relies on the fact that the first radiation-crosslinkable pressure-sensitive adhesive and the second radiation-crosslinkable pressuresensitive adhesive are crosslinkable at different wavelengths. The skilled person is aware in this respect that the crosslinkability at a defined wavelength in many cases does not have a sharp dividing line, but that in many cases it is a question of the crosslinking efficiency that can be achieved at different wavelengths. Thus, it is sometimes found in practice that a radiation-crosslinkable pressure-sensitive adhesive which is optimized, for example, for crosslinking with a wavelength of 365 nm may also be cured with higher or lower wavelengths, with deterioration in the crosslinking efficiency, i.e. e.g. using more significantly higher radiation doses. The skilled person automatically understands that in practice the respective radiation- crosslinkable pressure-sensitive adhesive will regularly have an at least local maximum in the crosslinking efficiency at the respective wavelength or at least in the vicinity of the corresponding wavelength, e.g. at a distance of 5 nm or less, preferably 2 nm or less, wherein the crosslinking efficiency can be understood as a quotient of the degree of crosslinking divided by the applied radiation dose. In addition or alternatively, it is advantageously possible to define the differences between the respective pressure-sensitive adhesives by a relative description of the crosslinking rate which can be achieved with the respective wavelengths. Thus, for example, the first radiation-crosslinkable pressure-sensitive adhesive on irradiation with a wavelength Ai achieves the desired degree of final crosslinking preferably in a shorter time and / or as a result of a lower radiation dose applied. Therefore preferable for substantially all embodiments is a method according to the invention wherein the first radiation-crosslinkable pressure-sensitive adhesive by irradiation with electromagnetic radiation having a wavelength Ai with the same radiation dose can be crosslinked faster and / or more comprehensively than the second radiation-crosslinkable pressure-sensitive adhesive, and / or wherein the second radiation-crosslinkable pressure-sensitive adhesive can be crosslinked faster and / or more comprehensively than the first radiation-crosslinkable pressuresensitive adhesive by irradiation with electromagnetic radiation having a wavelength A2with the same radiation dose.

[0037] Preferable, in the same way, is a method according to the invention wherein the first radiation-crosslinkable pressure-sensitive adhesive comprises one or more than one first photoinitiator, wherein the first photoinitiator at the wavelength Ai shows a higher initiator effect than at the wavelength A2, and / or wherein the second radiation-crosslinkable pressure-sensitive adhesive comprises one or more than one second photoinitiator, wherein the second photoinitiator at the wavelength A2shows a higher initiator effect than at the wavelength Ai. The initiator effect here is defined by the amount of species relevant to the initiating effect that are formed on irradiation with the corresponding wavelength.

[0038] In addition or as an alternative to the above considerations on the different crosslinkability at the wavelengths Ai and A2, it can also be considered how well the respective pressure-sensitive adhesives can be radiation-crosslinked, i.e. in particular which radiation dose is required for a desired crosslinking. In this respect, the inventors of the present invention specifically propose that the first radiation- crosslinkable pressure-sensitive adhesive and the second radiation-crosslinkable pressure-sensitive adhesive should be sufficiently tuned to the wavelengths used for crosslinking in the method so that advantageous degrees of crosslinking can be achieved even with a moderate radiation dose. A corresponding design allows a significantly more time- and cost-efficient method regime and makes it possible, in particular in combination with significant wavelength differences between the activation wavelengths, to control the method very precisely and in a supervised manner, since the time required for the crosslinking can be reduced. Against this background, a method according to the invention is preferred wherein the first radiation-crosslinkable pressure-sensitive adhesive in the first adhesive layer can be crosslinked by irradiation with electromagnetic radiation of the wavelength Ai in a radiation dose of 6000 mJ / cm2or less, preferably 3000 mJ / cm2or less, particularly preferably 1000 mJ / cm2or less, to an extent of 70% or more, preferably to an extent of 80% or more, particularly preferably to an extent of 90% or more, based on the maximum attainable final crosslinking level, and / or wherein the second radiation-crosslinkable pressure-sensitive adhesive in the second adhesive layer can be crosslinked by irradiation with electromagnetic radiation of the wavelength Ai in a radiation dose of 6000 mJ / cm2or less, preferably 3000 mJ / cm2or less, particularly preferably 1000 mJ / cm2or less, to an extent of 70% or more, preferably to an extent of 80% or more, particularly preferably to an extent of 90% or more, based on the maximum attainable final crosslinking level.

[0039] The indication of the degree of crosslinking as a relative fraction of the maximum attainable final crosslinking level is a familiar indication for the skilled person and corresponds to the usual procedure in practice. The maximum attainable final crosslinking level for a radiation-crosslinkable pressure-sensitive adhesive can be ascertained experimentally by the skilled person by determining the crosslinking level as a function of the radiation dose applied until a plateau value is established which characterizes the maximum attainable final crosslinking level. Since the direct determination of the final crosslinking level is generally costly and inconvenient, it is common and sufficient for the skilled person to consider an experimental parameter which correlates with the crosslinking level; in this context, in accordance with the expert understanding, in particular the so-called gel value can be employed. The gel value refers to the proportion of insoluble constituents of the adhesive that remain after solvent extraction, for example with toluene or ethyl acetate, and so the gel value in other words denotes the weight fraction of the polymer components that is not soluble in toluene or ethyl acetate.

[0040] The radiation-crosslinkable adhesives used in the first adhesive layer and the second adhesive layer are both pressure-sensitive adhesives. A pressuresensitive adhesive, in accordance with the expert understanding, irrespective of any capacity for curing by crosslinking, is an adhesive that has pressure-sensitive adhesive properties, i.e. the property of entering into a permanent connection to an adhesion base even under relatively low contact pressure. Corresponding pressure-sensitive adhesive tapes are usually removable after use from the adhesion base essentially free of residues, and generally have permanent intrinsic adhesiveness at room temperature, which means that they have a certain viscosity and touch-stickiness, so that they wet the surface of a substrate even at low contact pressure. The pressure-sensitive adhesiveness of a pressure-sensitive adhesive tape is a result of the use of a pressure-sensitive adhesive as the adhesive. Without wanting to be bound 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 permanent intrinsic adhesiveness and pressuresensitive adhesive capability. It is assumed that with corresponding pressuresensitive adhesives, mechanical deformation results both in viscous flow processes and in the build-up of elastic restoring forces. The proportional viscous flow is used to achieve adhesion, while the proportional elastic restoring forces are necessary particularly for achieving cohesion. The relationships between rheology and pressure-sensitive adhesiveness are known in the prior 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, as disclosed, for example, in WO 2015 / 189323, 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 pressuresensitive adhesive when at a temperature of 23°C in the deformation frequency range from 10° to 101rad / sec, G' and G" are each at least in part in the range from 103to 107Pa.

[0041] As explained above, an interlayer can be provided between the first adhesive layer and the second adhesive layer. At least theoretically, this can be another component, i.e. an intermediate substrate to be bonded as well, which is to be secured on the substrate via the first adhesive layer and optionally to a top substrate via the second adhesive layer. In this case, the adhesive arrangement is produced, for example, by application of the first adhesive layer to the substrate, the subsequent application of the interlayer, and the later application of the second adhesive layer to the interlayer. However, for substantially all applications, an embodiment of the method according to the invention is preferred in which the interlayer is the carrier layer of a multilaminate adhesive element, in particular of a multilaminate adhesive element according to the invention. In other words, the interlayer in this case is the carrier layer of a multilaminate adhesive element as is also known from other double-sided adhesive tapes from the prior art. Hence a method according to the invention is conceivable wherein the interlayer is a component of an electronic device, preferably a component of a display for an electronic device. However, a method according to the invention is preferred wherein the interlayer is a carrier layer of a multilaminate adhesive element. Illustrative in this respect is a method according to the invention wherein the carrier layer of the multilaminate adhesive element is selected from the group consisting of plastics films, in particular PET-containing films, polycarbonate-containing films, poly(meth)acrylate-containing films, in particular PMMA-containing films, and polyolefin-containing films, for example cycloolefin copolymer-containing films.

[0042] As explained above, the method according to the invention relies on the fact that the radiation-crosslinkable pressure-sensitive adhesives used can be crosslinked as independently as possible from each other with different wavelengths, with the passage of the corresponding wavelengths through the adhesive arrangement being limited by the use of LIV blockers, as further disclosed below. According to the inventors' assessment, it is conducive to the performance of the method according to the invention, in particular for the efficient selection of the LIV blocker, for the wavelengths Ai and A2 to have a sufficient distance from each other. In order to achieve very targeted crosslinking of the respective adhesives and a UV block that is as reliable as possible, the inventors propose that the chosen wavelength difference between the crosslinking wavelengths should be relatively large. Against this background, a method according to the invention is preferred wherein A1 > (A2 + 25 nm), preferably A1 > (A2+ 30 nm), especially preferably A1 > (A2+ 35 nm).

[0043] The inventors of the present invention have succeeded in identifying particularly advantageous ranges for the wavelengths A1 and A2. These wavelength ranges not only allow an advantageous energy input into the radiation-crosslinkable pressuresensitive adhesives. In addition, a broad number of radiation sources are commercially available for the realization of corresponding wavelengths. In addition, the skilled person can resort to a wide range of suitable photoinitiators which enable crosslinking of radiation-crosslinkable pressure-sensitive adhesives after activation in the corresponding wavelength ranges. Indeed, a method according to the invention is preferred wherein A1 is in the range from 370 to 420 nm, preferably in the range from 380 to 415 nm, particularly preferably in the range from 390 to 410 nm, and / or wherein A2 is in the range from 350 to 400 nm, preferably in the range from 355 to 390 nm, particularly preferably in the range from 360 to 380 nm.

[0044] An essential feature of the method according to the invention is that a UV blocker is used in the adhesive arrangement. The UV blocker is used in the first adhesive layer, i.e. in the lower adhesive layer contacting the substrate, or - if one is present - additionally or alternatively in the interlayer. The function of the UV blocker is to prevent or at least significantly attenuate the passage of the electromagnetic radiation of the wavelength A2used for curing the overlying second adhesive layer into the underlying first adhesive layer. For this purpose, a UV blocker is used which absorbs electromagnetic radiation of the wavelength A2, i.e. the electromagnetic radiation used to cure the overlying second adhesive layer, more strongly than the electromagnetic radiation of the wavelength Ai used to cure the underlying first adhesive layer. As a result of this characteristic, it is possible to impede the passage of electromagnetic radiation having the wavelength A2by the dosage of the UV blocker, whereas electromagnetic radiation of the wavelength Ai can enter the underlying first adhesive layer.

[0045] In accordance with the expert understanding, the absorption behaviour of the UV blocker at different wavelengths can be determined in a transmission experiment, in particular by means of UV-VIS spectroscopy. The inventors have recognized that a particularly advantageous method regime according to the invention with very extensive decoupling of the two curing processes can be realized all the better the greater the differences in the absorption behaviour shown by the UV blocker for the two wavelengths. The difference in the absorption effect is all the more evident the more reliably a passage of the wavelength A2into the underlying first adhesive layer can be prevented without reducing too much the proportional passage of the first wavelength Ai required for curing the first adhesive layer. In light of the fact that many UV radiation sources do not emit a completely monochrome spectrum, it is particularly preferred that the higher absorption is also present in the region around the respective wavelengths. Accordingly, a method according to the invention is preferred wherein the one or more UV blockers have a higher absorption for electromagnetic radiation having a wavelength A2in absolute values of absorption by 25% or more, preferably 50% or more, particularly preferably 89% or more, than for electromagnetic radiation having a wavelength Ai. A method according to the invention is preferable, additionally or alternatively, wherein the one or more UV blockers have a higher absorption for all electromagnetic radiation in a wavelength range from A2- 5 nm to A2+ 5 nm, preferably from A2- 10 nm to A2+ 10 nm, than for all electromagnetic radiation in a wavelength range from Ai - 5 nm to Ai + 5 nm, preferably from Ai - 10 nm to Ai + 10 nm.

[0046] Suitable UV blockers, sometimes also referred to as UV absorbers in the field of the art, are known to the skilled person on the basis of their general knowledge and are available commercially from numerous providers. The absorption behaviour of the UV absorbers is usually readily apparent for the skilled person from the product information. Illustrative UV absorbers are disclosed, for example, in US 2013 / 0085215 A1 or WO 2015 / 032635 A1. In an illustrative method according to the invention, the UV blocker or blockers are selected from the group consisting of organic UV blockers. A method according to the invention is preferred wherein the UV blocker or blockers are selected from the group consisting of 2-(2- hydroxyphenyl)benzotriazoles (BTZ), 2-hydroxyphenyl-s-triazines (HPT) and 2- hydroxybenzophenones (BP), such compounds being commercially available for example under the trade name Tinuvin, for example for BTZ as Tinuvin 326, 384, 900 or 928, for HPT as Tinuvin 460 or 477 and for BP as Tinuvin 477.

[0047] In the context of the method according to the invention, the UV blocker or blockers are not arbitrarily distributed, but rather are used in the interlayer, in the first adhesive layer or both in the interlayer and in the first adhesive layer. This concept according to the invention is based on the idea that the passage of electromagnetic radiation having a wavelength A2in the second radiation-crosslinkable pressuresensitive adhesive is to be hindered as little as possible, but at the same time, by the use of the UV blocker in the lower parts of the adhesive arrangement, entry of this electromagnetic radiation into the first adhesive layer is to be prevented as best as possible. Against this background, the skilled person understands that for substantially all embodiments it is preferred to avoid the use of a UV blocker in the second radiation-crosslinkable pressure-sensitive adhesive as far as possible. Preferred accordingly is a method according to the invention wherein the second radiation-crosslinkable pressure-sensitive adhesive comprises UV blockers in a combined mass fraction of 0.1 % or less, preferably of 0.05% or less, particularly preferably of 0.01% or less, preferably substantially of 0%, based on the mass of the first radiation-crosslinkable pressure-sensitive adhesive.

[0048] The ability of the interlayer or the first radiation-crosslinkable pressure-sensitive adhesive to block the passage of electromagnetic radiation having the wavelength A2as a result of the UV blocker contained is determined significantly by the absorption behaviour of the UV blocker at the wavelength A2and by the concentration of the UV blocker in the respective layers; the skilled person adjusts these parameters on the condition that sufficient electromagnetic radiation of the wavelength Ai can still enter the first adhesive layer in order to cure it as well comprehensively. The exact concentration of the UV blocker depends on numerous parameters, in particular the wavelengths of electromagnetic radiation used, the absorption behaviour of the UV blocker at the corresponding wavelengths, and the position of the UV blocker in the adhesive arrangement. The optimization of the concentration is readily possible for the skilled person, in view of the invention and in the knowledge of the effect to be achieved, however, and can be adjusted through routine experiments. The skilled person will choose the concentration of the UV blocker sufficiently high that the intensity of electromagnetic radiation with wavelength A2measured in the first adhesive layer is sufficiently low for their application requirements, to prevent comprehensive (pre- jcrosslinking of the first adhesive layer, but at the same time, also enough electromagnetic radiation of wavelength Ai can pass through the UV-blocked parts of the adhesive arrangement to allow the first adhesive layer to cure. If the skilled person encounters problems during this activity, they can increase the distance between the activation lengths Ai and A2, optionally by adapting the photoinitiators, and / or choose a UV blocker that shows a greater difference in absorption behaviour at the corresponding wavelengths. Illustrative, additionally or alternatively, is a method according to the invention wherein the interlayer comprises UV blockers in a combined mass fraction in the range from 0.6 to 6.0%, preferably in the range from 0.8 to 4.5%, particularly preferably in the range from 1 .0 to 3.0%, based on the mass of the interlayer, and / or wherein the first radiation- crosslinkable pressure-sensitive adhesive comprises UV blockers in a combined mass fraction in the range from 0.5 to 8.0%, preferably in the range from 0.75 to 5.0%, particularly preferably in the range from 1 .0 to 4.0%, based on the mass of the first radiation-crosslinkable pressure-sensitive adhesive.

[0049] At least in principle, it is possible to use the UV blocker exclusively in the first adhesive layer or the first radiation-crosslinkable pressure-sensitive adhesive. In particular in those configurations in which the use of the interlayer is dispensed with, for example when using a carrier-free double-sided adhesive tape or in cases in which the interlayer is formed by a component of the substrate to be bonded, for example a display constituent, the use of the UV blocker in the first radiation- crosslinkable pressure-sensitive adhesive is the only practicable solution. Conceivable therefore is a method according to the invention wherein the interlayer is a component of an electronic device, wherein the multilaminate adhesive arrangement comprises the one or more UV blockers in the first adhesive layer, in some cases preferably exclusively.

[0050] The skilled person understands to this extent that it would be at least theoretically desirable to provide a strong concentration gradient of the UV blocker in the first adhesive layer, so that it is present in a particularly high concentration in the side of the first adhesive layer that faces away from the substrate. With a view to practical reality and efficient manufacture of corresponding adhesive layers, the inventors estimate that in practice the UV blocker will however be present at least largely homogeneously in the first radiation-crosslinkable pressure-sensitive adhesive. The skilled person understands that in this case, entry of electromagnetic radiation of the wavelength A2, which can cause at least an unwanted (pre-)crosslinking in the first radiation-crosslinkable adhesive, cannot be completely prevented. Rather, the inventors propose that the concentration and the absorption behaviour of the UV blocker in this case should be configured such that the intensity of electromagnetic radiation of wavelength A2in the first adhesive layer drops at least so rapidly that when the upper second adhesive layer is cured, only the upper parts of the first adhesive layer are pre-crosslinked and the great majority of the first adhesive layer can be crosslinked in the downstream crosslinking process with electromagnetic radiation of wavelength Ai.

[0051] Against the background of the observations above, the inventors consider it particularly advantageous if the UV blocker is used additionally or even exclusively in the interlayer. This makes it possible in an advantageous manner, by tuning the concentration of the UV blocker in the interlayer, to prevent passage of the electromagnetic radiation of the wavelength A2into the underlying first adhesive layer particularly efficiently, so that the corresponding configurations are characterized in that they enable a particularly precise method regime and a particularly pronounced decoupling of the two crosslinking processes. A method according to the invention is therefore preferred wherein the multilaminate adhesive arrangement comprises the one or more LIV blockers in the interlayer, preferably based on the total mass of the UV blocker, to an extent of more than 95%, particularly preferably to an extent of more than 99%, especially preferably to an extent of substantially 100%. Preferred, in addition or alternatively, is a method according to the invention wherein the interlayer is a carrier layer of a multilaminate adhesive element, wherein the multilaminate adhesive arrangement comprises the one or more UV blockers in the interlayer, in some cases preferably exclusively.

[0052] The skilled person understands that when the UV blocker is used simultaneously in the interlayer and in the first adhesive layer, it is very difficult to provide meaningful content information, owing to the different reference systems and the non-linearity of the transmission in the resulting structure. At the same time, adjusting the contents of UV blocker does not pose any major problems for the skilled person in this case, as they can identify suitable distributions of the UV blocker in routine experiments. The skilled person may, for example, start out from an arrangement in which the UV blocker is initially present exclusively in one component, in order to reduce the content in that component to the desired degree, in order to then gradually increase the content in the respectively other component until the desired overall attenuation is regained.

[0053] In the above definition of the method according to the invention, the suitability of the multilaminate adhesive arrangement produced for the method according to the invention is ensured in that the suitability of the multilaminate adhesive arrangement for attenuating the electromagnetic radiation of wavelength A2and for transmissibility for the electromagnetic radiation of wavelength Ai is additionally defined. This definition is usefully made by way of the transmission measured for the respective electromagnetic radiation at a certain point of the first adhesive layer, wherein in accordance with the expert understanding, the radiation considered in each case is that which impinges orthogonally on the adhesive arrangement. Even if it is quite possible in the later method during the actual curing for the electromagnetic radiation to impinge at least partially obliquely on the surface, it is expedient to define the measurement direction for characterizing the transmission behaviour.

[0054] According to the invention, the multilaminate adhesive arrangement is designed such that at a penetration depth of 50%, i.e. in the middle of the first adhesive layer, a residual transmission of at least 20% is obtained for the electromagnetic radiation of the wavelengths Ai required for curing the first adhesive layer. Using the example of a 100 pm thick first adhesive layer, this would mean, for example, that at a penetration depth of 50 pm, the transmission should still have 20% or more of the initial value, so that the corresponding electromagnetic radiation was attenuated by less than 80% when passing through the multilaminate adhesive tape arrangement up to this measuring point, i.e. in particular after passing through the second adhesive layer and any interlayer present. Similarly, it is defined that the transmission for electromagnetic radiation of the wavelength A2that is determined in the first adhesive layer at the specified penetration depth is to be less than 20%.

[0055] According to the invention and in accordance with the expert expectation, the transmission is determined within the scope of the present invention by means of UV-VIS spectroscopy, wherein a correction of the interfacial reflection losses, known fundamentally to the skilled person, is carried out. The transmission can be determined by the skilled person in a customary manner with a suitable spectrometer, in particular a commercially available UV-VIS spectrometer, wherein the relevant determination parameters, in particular the wavelength and the sample thickness, are obtained from the above definitions. A UV-VIS spectrometer of the type SPECORD 250 Plus (two-beam spectrophotometer) from Analytik Jena is preferably used. In order to correct the interfacial reflection losses, the UV-VIS measurement is performed on multilaminate adhesive arrangements arranged on a glass substrate in comparison to a reference measurement on the isolated glass substrate, with the reference measurement being defined as the maximum attainable transmission. A glass substrate made of borosilicate (trade name: Borofloat 33, company: Schott Technical Glass Solutions) with a thickness of 1 .1 mm is preferably used. The background is that interfacial reflection losses occur especially at the two phase boundaries between the air and the transparent solid. Compared to a transmission measurement on an isolated multilaminate adhesive arrangement, the measurement of the reference makes it possible to take into account interfacial reflection losses at the phase boundary between the air and a transparent solid, since the differences in the refractive indices between the glass substrate and the multilaminate adhesive arrangement are significantly lower than between the respective components and the air; this also results from the intended application in the context of optically clear bonding.

[0056] The transmission at the corresponding penetration depth can be readily determined by the skilled person even without the use of sophisticated experimental setups, by reducing the thickness of the first adhesive layer for the UV-VIS measurement in such a way that the remaining total thickness corresponds to the corresponding penetration depth in the starting material. This thickness reduction can be reduced, for example, by removal of a part of the first adhesive layer. In most cases, however, it is preferable to realize the thickness reduction by producing the measured test samples directly with the corresponding reduced thickness of the adhesive layer. The transmission at a penetration depth of 50% can be determined for a multilaminate adhesive element having a 100 pm thick first adhesive layer, for example, on a comparison sample which is produced under otherwise identical conditions with only a 50 pm thick first adhesive layer.

[0057] The inventors consider it particularly advantageous for the method if the intensity of the electromagnetic radiation of the wavelength A2is already as low as possible at an extremely low penetration depth. At the same time, it is advantageous for reliable and fast curing of the first radiation-crosslinkable adhesive, conversely, if a transmission as high as possible is measured even at a large penetration depth, so that the continuous multilaminate adhesive arrangement affects the corresponding electromagnetic radiation of the wavelength Ai as little as possible in its intensity.

[0058] Preferred, accordingly, is a method according to the invention wherein the multilaminate adhesive arrangement is designed such that for electromagnetic radiation of the wavelength A2impinging orthogonally on the side facing away from the first adhesive layer, in the first adhesive layer, a transmission of less than 20% is obtained at a penetration depth of 25% relative to its total thickness, preferably of 10%, particularly preferably of 5%, very preferably of 1%, and / or wherein the multilaminate adhesive arrangement is designed in such a way that for electromagnetic radiation of the wavelength A2impinging orthogonally on the side facing away from the first adhesive layer, in the first adhesive layer, at the specified penetration depth, a transmission of less than 15% is obtained, preferably of less than 10%, particularly preferably of less than 5%, very preferably less than 1%. A method according to the invention is preferred, additionally or alternatively, wherein the multilaminate adhesive arrangement is designed such that for electromagnetic radiation of the wavelength Ai impinging orthogonally on the side facing away from the first adhesive layer, in the first adhesive layer, a transmission of 20% or more is obtained at a penetration depth of 75% relative to its total thickness, preferably of 90%, particularly preferably of 95%, very preferably of 99%, and / or wherein the multilaminate adhesive arrangement is designed in such a way that for electromagnetic radiation of the wavelength Ai impinging orthogonally on the side facing away from the first adhesive layer, in the first adhesive layer, at the specified penetration depth, a transmission of 50% or more is obtained, preferably of 70% or more, particularly preferably of 80% or more, very preferably of 90% or more.

[0059] In principle, the method according to the invention is suitable for numerous applications in which complex structures are to be produced by sequential curing of the adhesive layers. The above definition of the remanent transmission, which is measured at the specified penetration depth in the first adhesive layer, ensures that the curability of the underlying adhesive layer is guaranteed, since the adhesive layers and the interlayer have sufficient transparency for the corresponding electromagnetic radiation. It follows from the high relevance of the method according to the invention for use in the electronics industry, in particular in the bonding of displays, that it is particularly preferred for the vast majority of applications if the corresponding laminas are as largely transparent as possible and accordingly allow visible light to pass through with as little hindrance as possible. Even if it is in principle desirable, at least for reasons of customer acceptance, to implement the adhesive layers in the uncured state transparently as well, the transparency of the crosslinked adhesive laminas is particularly crucial with regard to the later purpose of application. Preferred accordingly is a method according to the invention wherein the first adhesive layer and / or the second adhesive layer and / or the interlayer and / or the first crosslinked adhesive lamina and / or the second crosslinked adhesive lamina, preferably the interlayer, the first crosslinked adhesive lamina and the second crosslinked adhesive lamina, particularly preferably the entire crosslinked adhesive arrangement, are predominantly transparent, preferably substantially completely transparent, for visible light. In other words, additionally or alternatively, a method according to the invention is preferred wherein the first adhesive layer and / or the second adhesive layer and / or the interlayer and / or the first crosslinked adhesive lamina and / or the second crosslinked adhesive lamina, preferably the interlayer, the first crosslinked adhesive lamina and the second crosslinked adhesive lamina, particularly preferably the entire crosslinked adhesive arrangement, exhibits a transmission of 80% or more, preferably of 90% or more, particularly preferably of 95% or more, very preferably of 99% or more in a wavelength range from 450 to 750 nm, preferably in a wavelength range from 430 to 750 nm, particularly preferably in a wavelength range from 410 to 750 nm, for all wavelengths.

[0060] In order to obtain particularly advantageous bonds which are particularly suitable for use in the electronics industry, the inventors propose that the layers of the adhesive arrangement should preferably be adjusted to a low yellow value and to minimal light scattering. In this case, the skilled person, for setting the yellow value, adjusts the selection of the UV blockers and of the photoinitiators used, in particular, whereas the light scattering, which is also referred to as "haze", can be adjusted in particular via the base polymers used in the radiation-crosslinkable pressure-sensitive adhesives and via the additives employed.

[0061] A method according to the invention is particularly preferred wherein the first adhesive layer and / or the second adhesive layer and / or the interlayer and / or the first crosslinked adhesive lamina and / or the second crosslinked adhesive lamina, preferably the interlayer, the first crosslinked adhesive lamina and the second crosslinked adhesive lamina, particularly preferably the cured adhesive arrangement as a whole, has a yellow value b* measured according to DIN EN ISO 11664-4:2019 (L*a*b* (CIE), D / 65, 10°) of 2.5 or less, preferably of 1 .5 or less, more preferably of 0.75 or less. Particularly preferred additionally or alternatively is a method according to the invention wherein the first adhesive layer and / or the second adhesive layer and / or the interlayer and / or the first crosslinked adhesive lamina and / or the second crosslinked adhesive lamina, preferably the interlayer, the first crosslinked adhesive lamina and the second crosslinked adhesive lamina, particularly preferably the entire crosslinked adhesive arrangement, has a haze value measured according to ASTM D1003-21 , CIE-C of 1.0 or less, preferably of 0.6 or less, particularly preferably of 0.3 or less.

[0062] In the method steps b) and c) of the method according to the invention, the radiation-based crosslinking of the radiation-crosslinkable pressure-sensitive adhesives in the respective adhesive layers is then carried out in order, from these layers, to produce crosslinked adhesive laminas. The skilled person understands that it can be seen as a great advantage of the method according to the invention that the first radiation-crosslinkable pressure-sensitive adhesive can be crosslinked sequentially after the second radiation-crosslinkable pressuresensitive adhesive, so that it is accordingly preferred to carry out the method in this manner, even if it is conceivable in individual cases, if necessary and / or in sections, first to cure the underlying first adhesive layer, wherein in most cases the second adhesive layer will (pre-)crosslink, in order to then completely cure the second adhesive layer. Preferred accordingly is a method according to the invention wherein the irradiation of the multilaminate adhesive arrangement with electromagnetic radiation of the wavelength A2is carried out before the irradiation with electromagnetic radiation of the wavelength Ai.

[0063] The inventors propose that the irradiation used for crosslinking should advantageously be carried out with electromagnetic radiation which has a very narrow spectral width, where in particular the use of LEDs is advantageous because of the usually realizable narrow wavelength distribution. Accordingly, a method according to the invention is preferred wherein the irradiation in method step b) is carried out with electromagnetic radiation whose emission wavelength maximum lies at A2± 5 nm, preferably at A2± 2 nm, particularly preferably at A2± 1 nm, very preferably substantially at A2, and / or wherein the irradiation in method step b) is carried out with a radiation device, preferably an LED, in particular a UV- LED. A method according to the invention is preferred, in addition or alternatively, wherein the irradiation in method step c) is carried out with electromagnetic radiation whose emission wavelength maximum lies at A1 ± 5 nm, preferably at A1 ± 2 nm, particularly preferably at A1 ± 1 nm, very preferably substantially at A1, and / or wherein the irradiation in method step c) is carried out with a radiation device, preferably an LED, in particular a UV-LED.

[0064] The inventors have succeeded in identifying particularly favourable radiation doses for the curing, with which the method according to the invention can be operated particularly efficiently. A method according to the invention is preferred, indeed, wherein the irradiation of the multilaminate adhesive arrangement takes place with electromagnetic radiation of the wavelength A2with a radiation dose in the range from 0.5 to 12 J / cm2, preferably in the range from 1 to 10 J / cm2, particularly preferably in the range from 2 to 8 J / cm2, and / or wherein the irradiation of the multilaminate adhesive arrangement takes place with electromagnetic radiation of the wavelength A1 with a radiation dose in the range from 1 to 12 J / cm2, preferably in the range from 2 to 11 J / cm2, particularly preferably in the range from 3 to 10 J / cm2.

[0065] In order to obtain a particularly advantageous bond, the inventors propose that the respective adhesive layers are to be crosslinked as extensively as possible in the method steps b) and c). A method according to the invention is preferred in this respect wherein the irradiation of the multilaminate adhesive arrangement with electromagnetic radiation of the wavelength A2is carried out such that the second radiation-crosslinkable pressure-sensitive adhesive in the second adhesive layer is crosslinked to an extent of 70% or more, preferably to an extent of 80% or more, particularly preferably to an extent of 90% or more, based on the maximum attainable final crosslinking level, and / or wherein the irradiation of the multilaminate adhesive arrangement with electromagnetic radiation of the wavelength A1 is carried out such that the first radiation-crosslinkable pressure-sensitive adhesive in the first adhesive layer is crosslinked to an extent of 70% or more, preferably to an extent of 80% or more, particularly preferably to an extent of 90% or more, based on the maximum attainable final crosslinking level.

[0066] Particularly preferred in the synopsis is a method according to the invention wherein the irradiation in method step b) is carried out with a radiation dose of 6000 mJ / cm2or less, preferably 3000 mJ / cm2or less, particularly preferably 1000 mJ / cm2or less, wherein the second radiation-crosslinkable pressure-sensitive adhesive is crosslinked to an extent of 70% or more, preferably to an extent of 80% or more, particularly preferably to an extent of 90% or more, based on the maximum attainable final crosslinking level, and / or wherein the irradiation in method step c) is carried out with a radiation dose of 6000 mJ / cm2or less, preferably 3000 mJ / cm2or less, particularly preferably 1000 mJ / cm2or less, wherein the first radiation-crosslinkable pressure-sensitive adhesive is crosslinked to an extent of 70% or more, preferably to an extent of 80% or more, particularly preferably to an extent of 90% or more, based on the maximum attainable final crosslinking level.

[0067] The invention also relates to a multilaminate adhesive element for use in a method according to the invention, comprising: i) a first adhesive layer which is at least partially transparent for visible light and which is intended for contact with the substrate, comprising a first radiation- crosslinkable pressure-sensitive adhesive, wherein the first radiation-crosslinkable pressure-sensitive adhesive is crosslinkable by irradiation with electromagnetic radiation having a wavelength i, ii) a second adhesive layer which is at least partially transparent for visible light and which is arranged on the side of the multilaminate adhesive element that faces away from the first adhesive layer, comprising a second radiation-crosslinkable pressure-sensitive adhesive, wherein the second radiation-crosslinkable pressuresensitive adhesive is crosslinkable by irradiation with electromagnetic radiation having a wavelength A2, and optionally ill) an interlayer which is at least partially transparent for visible light and which is arranged between the first adhesive layer and the second adhesive layer, wherein Ai > (A2+ 20 nm), wherein the multilaminate adhesive element comprises one or more LIV blockers in the interlayer and / or in the first adhesive layer, wherein the one or more LIV blockers have a higher absorption for electromagnetic radiation having a wavelength A2than for electromagnetic radiation having a wavelength A1, wherein the multilaminate adhesive element is designed such that for electromagnetic radiation of the wavelength A2impinging orthogonally on the side facing away from the first adhesive layer, in the first adhesive layer, a transmission of less than 20% is obtained at a penetration depth of 50% relative to its total thickness, and wherein the multilaminate adhesive element is designed such that for electromagnetic radiation of the wavelength A1 impinging orthogonally on the side facing away from the first adhesive layer, in the first adhesive layer, a transmission of 20% or more is obtained at a penetration depth of 50% relative to its total thickness, the transmission being determined by means of UV-VIS spectroscopy with correction of the interfacial reflection losses,

[0068] The multilaminate adhesive element according to the invention is advantageous because it is particularly suitable for use in the method according to the invention. In particular, it is particularly easy to produce the necessary adhesive arrangement in the method according to the invention with the multilaminate adhesive element according to the invention, which can be implemented, for example, as adhesive tape. The multilaminate adhesive element according to the invention can be implemented as a double-sided adhesive element with and without a carrier; in the latter case, the adhesive layers can be directly materially connected to each other. In practice, a corresponding multilaminate adhesive element according to the invention will usually have a release layer on both adhesive layers for handling reasons before application, for example, a release liner provided optionally with a silicone release layer. For use in the method according to the invention, for example, the release liner can be removed from the first adhesive layer and the latter placed on the substrate to be bonded, utilizing the pressure-sensitive adhesive property. Subsequently, the release liner can be removed from the second adhesive layer, for application of a top substrate to be connected to the substrate, which may be, for example, a display. The adhesive arrangement produced in this way can then be advantageously cured in the method according to the invention.

[0069] As an alternative to the above-described use of a multilaminate adhesive element, it can be seen in principle that the adhesive arrangement for the method according to the invention can be produced from two or more components, namely at least two adhesive components. The system required for this purpose is referred to as an adhesive kit within the scope of the present invention. This adhesive kit comprises in the simplest case as separate constituents a first carrier-free adhesive tape, which consists of the first adhesive layer, and a second carrier-free adhesive tape, which consists of the second adhesive layer. This adhesive kit, in which the first adhesive component comprises the UV blocker, can be applied to the substrate in a method according to the invention before, optionally after affixing of an interlayer, the second adhesive component is applied, to produce the desired adhesive arrangement. In particularly preferred configurations, at least the first adhesive component comprises a carrier layer in which particularly preferably the UV blocker can be used, so that the carrier layer of the first adhesive component in the carrier arrangement can act as an interlayer to the second adhesive component applied thereto. For special applications, in particular for generating a certain surface functionality, the second adhesive component can also be provided with a carrier layer, which can act in the method according to the invention as a top substrate, for example by forming a protective layer over a bonded substrate.

[0070] Further disclosed, therefore, is an adhesive kit for use in a method according to the invention for producing a multilaminate adhesive arrangement, comprising as separate constituents: X) a first adhesive component which is at least partially transparent for visible light, comprising a first adhesive layer intended for contact with a substrate to be bonded, comprising a first radiation-crosslinkable pressure-sensitive adhesive, wherein the first radiation-crosslinkable pressure-sensitive adhesive is crosslinkable by irradiation with electromagnetic radiation having a wavelength i, and optionally a first carrier lamina, and

[0071] Y) a second adhesive component which is at least partially transparent for visible light, comprising a second adhesive layer intended for contact with the first carrier lamina, comprising a second radiation-crosslinkable pressure-sensitive adhesive, wherein the second radiation-crosslinkable pressure-sensitive adhesive is crosslinkable by irradiation with electromagnetic radiation having a wavelength A2, and optionally a second carrier lamina, wherein Ai > (A2+ 20 nm), wherein the first adhesive component comprises one or more LIV blockers in the first carrier lamina and / or in the first adhesive layer, wherein the one or more LIV blockers have a higher absorption for electromagnetic radiation having a wavelength A2than for electromagnetic radiation having a wavelength Ai, wherein the first adhesive component is designed such that for electromagnetic radiation of the wavelength A2impinging orthogonally on the first adhesive layer, or on the side facing away from the first adhesive layer if a first carrier lamina is present, in the first adhesive layer a transmission of less than 20% is obtained at a penetration depth of 50% relative to its total thickness, and wherein the first adhesive component is designed such that for electromagnetic radiation of the wavelength Ai impinging orthogonally on the first adhesive layer, or on the side facing away from the first adhesive layer if a first carrier lamina is present, in the first adhesive layer a transmission of 20% or more is obtained at a penetration depth of 50% relative to its total thickness, wherein the second adhesive component is designed such that for electromagnetic radiation of the wavelength Ai impinging orthogonally on the second adhesive layer, or on the side facing away from the first adhesive layer if a second carrier lamina is present, a transmission of 40% or more is obtained at the surface of the second adhesive layer.

[0072] Illustrative is an adhesive kit according to the invention wherein the carrier laminas of the adhesive components are selected from the group consisting of plastics films, in particular PET-containing films, polycarbonate-containing films, poly(meth)acrylate-containing films, in particular PMMA-containing films and polyolefin-containing films, for example cycloolefin copolymer-containing films.

[0073] In the use of such adhesive kits, preference is given to a method according to the invention wherein the interlayer is the first carrier lamina of the first adhesive component, and / or wherein the top substrate is the second carrier lamina of the second adhesive component.

[0074] In the following, the invention and preferred embodiments of the invention are explained and described in more detail with reference to the accompanying figures. In these figures,

[0075] Fig. 1 shows a schematic representation of the method steps of the method according to the invention using a multilaminate adhesive element according to the invention in a preferred embodiment;

[0076] Fig. 2 shows a schematic visualization of the wavelength-dependent transmission profile in the first adhesive layer for different penetration depths in the plot of the transmission T against the wavelength A; and

[0077] Fig. 3 shows a schematic visualization of the observed penetration depths in the first adhesive layer for different wavelengths.

[0078] Fig. 1 shows a schematic representation of the method steps of the method according to the invention using a multilaminate adhesive element 26 according to the invention in a preferred embodiment.

[0079] In Fig. 1 a), a multilaminate adhesive element 26 according to the invention is initially provided. This comprises a first adhesive layer 16, which in the example shown consists entirely of an acrylate-based first radiation-crosslinkable pressuresensitive adhesive (straight acrylate without resins; crosslinking via polyfunctional acrylates, e.g. TMPTA (trimethylolpropane triacrylate; CAS 15625-89-5), which is designed for crosslinking with electromagnetic radiation of a wavelength of 405 nm and can be crosslinked at this wavelength with a high crosslinking efficiency. Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (trade name: Omnirad 819, company: IGM Resin) is used here as photoinitiator in a mass fraction of 0.5%.

[0080] In addition, the multilaminate adhesive element 26 according to the invention comprises a second adhesive layer 18, which in the example shown consists entirely of an acrylate-based second radiation-crosslinkable pressure-sensitive adhesive, which is similar to the base adhesive and is designed for crosslinking with electromagnetic radiation of a wavelength of 365 nm and can be crosslinked at this wavelength with a high crosslinking efficiency. For example, 1 - hydroxycyclohexyl phenyl ketone (trade name: Omnirad 184, company: IGM Resin) or a mixture of oxy-phenylacetic acid 2-[2-oxo-2-phenyl-acetoxy-ethoxy]- ethyl ester and oxy-phenylacetic acid 2-[2-hydroxy-ethoxy]-ethyl ester (trade name: Omnirad 754, company: IGM Resin) is used here as photoinitiator in a mass fraction of 0.5%.

[0081] The radiation-crosslinkable pressure-sensitive adhesives are each designed by the choice of different photoinitiators in such a way that they show a significantly lower crosslinking efficiency at the respectively other wavelength.

[0082] On the first adhesive layer 16 and the second adhesive layer 18, a respective release liner (28a, 28b) is arranged prior to use.

[0083] The first adhesive layer 16 and the second adhesive layer 18 are separated from an interlayer 20, which is implemented as a plastics film, in the preferred multilaminate adhesive element 26 according to the invention shown, with this interlayer 20 comprising a UV blocker whose absorption and thus blocking effect at wavelengths of 365 nm and below is significantly higher than at 405 nm and above. For example, 2-(2H-benzotriazol-2-yl)-6-(1 -methyl-1 -phenylethyl)-4- (1 ,1 ,3,3-tetramethylbutyl)phenol (trade name: Tinuvin 928) can be used as UV blocker in a mass fraction of 3.2%. Alternatively possible, for example, are hydroxyphenyl-s-triazines containing 18-20% 2-methoxy-1 -propyl acetate (trade name: Tinuvin 477) or hydroxyphenyltriazines (trade name: Tinuvin 460), each in a mass fraction of 1 .0%.

[0084] In the preferred example shown, the LIV blocker is provided essentially exclusively in the interlayer 20. In addition, however, it is also possible for the first adhesive layer 16 in the first radiation-crosslinkable pressure-sensitive adhesive also to comprise additional LIV blocker, in particular when in the designing of the multilaminate adhesive element 26 it is apparent that the concentration of the UV blocker in the interlayer 20, given its thickness, is not yet sufficient to achieve the desired shielding of the first adhesive layer 16.

[0085] Fig. 2 visualizes the wavelength-dependent profile of the transmission T at different penetration depths in the first adhesive layer 16, when the multilaminate adhesive element 26 of Fig. 1 a) is irradiated with electromagnetic radiation of the corresponding wavelength after the removal of the release liners 28a, 28b from the side of the second adhesive layer 18.

[0086] Fig. 3 visualizes in addition to Fig. 2, for four selected wavelengths, which residual transmission is obtained in the first adhesive layer 16 at different penetration depths, where in Fig. 3 it is clearly visible that the radiation of the wavelength 405 nm is able to pass almost unhindered through the entire first adhesive layer 16 and to cure it, whereas the radiation of the wavelength 365 nm can only penetrate into the topmost layers of the first adhesive layer 16 and is rapidly absorbed there, which in the example shown in Fig. 3 is promoted by the provision of UV blocker in the first adhesive layer 16 as well.

[0087] Fig. 1 b) shows how the multilaminate adhesive element 26 according to the invention is arranged on a substrate 12 after the removal of the release liners 28a, 28b, utilizing the pressure-sensitive adhesive property, with a top substrate 30, which may be, for example, the transparent constituents of a display, being also applied. Starting from the structure shown in Fig. 1 b), Fig. 1c) visualizes how the second radiation-crosslinkable pressure-sensitive adhesive of the second adhesive layer 18 is crosslinked with electromagnetic radiation of the wavelength 365 nm to obtain the second crosslinked adhesive lamina 22. In this case, in the example shown in Fig. 1 c), a radiation dose of around 3000 mJ / cm2, which is generated by means of LIV LEDs whose maximum emission is at this wavelength, is used to attain crosslinking of more than 90% of the maximum attainable final crosslinking level. The UV blocker present in the interlayer 20 and / or the first adhesive layer 16 largely prevents the radiation of the wavelength 365 nm being able to penetrate too deep into the first adhesive layer 16.

[0088] After the crosslinking of the second adhesive layer 18, the underlying first adhesive layer 16 is selectively crosslinked with electromagnetic radiation of the wavelength 405 nm in order to obtain the first crosslinked adhesive lamina 24, as shown in Fig. 1d). In the example shown, the crosslinking takes place again with a LIV-LED whose maximum emission is at the corresponding wavelength, and with which a crosslinking of more than 90% of the maximum attainable final crosslinking level can be attained with a radiation dose of around 3000 mJ / cm2.

[0089] The resulting bond 10 has an advantageous durability and advantageous optical properties. In particular, the resulting bond 10 has a transmission of 80% or more in a wavelength range from 450 to 750 nm for all wavelengths and shows a yellow value b* measured according to DIN EN ISO 11664-4:2019 (L*a*b* (CIE), D / 65, 10°) of 0.5 or less and a haze value measured according to ASTM D1003-21 of less than 0.3.

[0090] Reference symbols

[0091] 10 adhesive bond

[0092] 12 substrate

[0093] 14 multilaminate adhesive arrangement 16 first adhesive layer

[0094] 18 second adhesive layer

[0095] 20 interlayer

[0096] 22 second crosslinked adhesive lamina

[0097] 24 first crosslinked adhesive lamina 26 multilaminate adhesive element

[0098] 28a, 28b release liners

[0099] 30 top substrate

Claims

Claims1 . Method for the production of an adhesive bond (10), comprising the method steps of: a) producing a multilaminate adhesive arrangement (14) arranged on a substrate (12), comprising: i) a first adhesive layer (16) which is at least partially transparent for visible light and which contacts the substrate (12), comprising a first radiation-crosslinkable pressure-sensitive adhesive, wherein the first radiation-crosslinkable pressure-sensitive adhesive is crosslinkable by irradiation with electromagnetic radiation having a wavelength i, ii) a second adhesive layer (18) which is at least partially transparent for visible light and which is arranged on the side of the multilaminate adhesive arrangement (14) that faces away from the substrate (12), comprising a second radiation-crosslinkable pressure-sensitive adhesive, wherein the second radiation-crosslinkable pressuresensitive adhesive is crosslinkable by irradiation with electromagnetic radiation having a wavelength A2, and optionally iii) an interlayer (20) which is at least partially transparent for visible light and which is arranged between the first adhesive layer (16) and the second adhesive layer (18), wherein Ai > (A2+ 20 nm), wherein the multilaminate adhesive arrangement (14) comprises one or more LIV blockers in the interlayer (20) and / or in the first adhesive layer (16), wherein the one or more LIV blockers have a higher absorption for electromagnetic radiation having a wavelength A2than for electromagnetic radiation having a wavelength Ai, wherein the multilaminate adhesive arrangement (14) is designed such that for electromagnetic radiation of the wavelength A2impinging orthogonally on the side facing away from the first adhesive layer (16), in the first adhesivelayer (16), a transmission of less than 20% is obtained at a penetration depth of 50% relative to its total thickness, and wherein the multilaminate adhesive arrangement (14) is designed such that for electromagnetic radiation of the wavelength Ai impinging orthogonally on the side facing away from the first adhesive layer (16), in the first adhesive layer (16), a transmission of 20% or more is obtained at a penetration depth of 50% relative to its total thickness, the transmission being determined by means of UV-VIS spectroscopy with correction of the interfacial reflection losses, b) irradiating the multilaminate adhesive arrangement (14) with electromagnetic radiation of the wavelength A2for crosslinking the second radiation- crosslinkable pressure-sensitive adhesive and for producing a second crosslinked adhesive lamina (22), and c) irradiating the multilaminate adhesive arrangement (14) with electromagnetic radiation of the wavelength Ai for crosslinking the first radiation-crosslinkable pressure-sensitive adhesive and for producing a first crosslinked adhesive lamina (24).

2. Method according to Claim 1 , wherein the multilaminate adhesive arrangement (14) comprises the interlayer (20).

3. Method according to one of Claims 1 or 2, wherein the interlayer (20), the first crosslinked adhesive lamina (24) and the second crosslinked adhesive lamina (22) show a transmission of 80% or more in a wavelength range from 450 to 750 nm for all wavelengths.

4. Method according to any of Claims 1 to 3, wherein the interlayer (20), the first crosslinked adhesive lamina (24) and the second crosslinked adhesive lamina (22) have a yellow value b* measured according to DIN EN ISO 11664-4:2019 (L*a*b* (CIE), D / 65, 10°) of 2.5 or less.

5. Method according to any of Claims 1 to 4, wherein the interlayer (20), the first crosslinked adhesive lamina (24) and the second crosslinked adhesive lamina (22) have a haze value measured according to ASTM D1003-21 , CIE- C of 1.0 or less.

6. Method according to any of Claims 1 to 5, wherein the first radiation- crosslinkable pressure-sensitive adhesive in the first adhesive layer (16) can be crosslinked by irradiation with electromagnetic radiation of the wavelength Ai in a radiation dose of 6000 mJ / cm2or less, to an extent of 70% or more, based on the maximum attainable final crosslinking level, and / or wherein the second radiation-crosslinkable pressure-sensitive adhesive in the second adhesive layer (18) can be crosslinked by irradiation with electromagnetic radiation of the wavelength A2in a radiation dose of 6000 mJ / cm2or less, to an extent of 70% or more, based on the maximum attainable final crosslinking level.

7. Method according to any of Claims 1 to 6, wherein Ai is in the range from 370 to 420 nm, and / or wherein A2is in the range from 350 to 400 nm.

8. Method according to any of Claims 1 to 7, wherein the multilaminate adhesive arrangement (14) comprises the one or more LIV blockers in the interlayer (20).

9. Method according to any of Claims 1 to 8, wherein the irradiation of the multilaminate adhesive arrangement (14) with electromagnetic radiation of the wavelength A2takes place before the irradiation with electromagnetic radiation of the wavelength Ai.

10. Method according to any of Claims 1 to 9, wherein the method further comprises the method step of: a’) arranging a top substrate (30) to be bonded on a side of the multilaminate adhesive arrangement (14) that faces away from the substrate (12), wherein the top substrate (30) is at least partially transparent for visible light and contacts the second adhesive layer (18), preferably wherein the top substrate (30) is at least partially transparent for electromagnetic radiations of the wavelengths Ai and A2.11 . Multilaminate adhesive element (26) for use in a method according to any of Claims 1 to 10, comprising: i) a first adhesive layer (16) which is at least partially transparent for visible light and which is intended for contact with the substrate (12), comprising a first radiation-crosslinkable pressure-sensitive adhesive, wherein the first radiation-crosslinkable pressure-sensitive adhesive is crosslinkable by irradiation with electromagnetic radiation having a wavelength A1, ii) a second adhesive layer (18) which is at least partially transparent for visible light and which is arranged on the side of the multilaminate adhesive element (14) that faces away from the first adhesive layer (16), comprising a second radiation-crosslinkable pressure-sensitive adhesive, wherein the second radiation-crosslinkable pressuresensitive adhesive is crosslinkable by irradiation with electromagnetic radiation having a wavelength A2, and optionally iii) an interlayer (20) which is at least partially transparent for visible light and which is arranged between the first adhesive layer (16) and the second adhesive layer (18), wherein A1 > (A2+ 20 nm), wherein the multilaminate adhesive element (26) comprises one or more LIV blockers in the interlayer (20) and / or in the first adhesive layer (16), wherein the one or more UV blockers have a higher absorption for electromagneticradiation having a wavelength A2than for electromagnetic radiation having a wavelength i, wherein the multilaminate adhesive element (26) is designed such that for electromagnetic radiation of the wavelength A2impinging orthogonally on the side facing away from the first adhesive layer (16), in the first adhesive layer (16), a transmission of less than 20% is obtained at a penetration depth of 50% relative to its total thickness, and wherein the multilaminate adhesive element (26) is designed such that for electromagnetic radiation of the wavelength Ai impinging orthogonally on the side facing away from the first adhesive layer (16), in the first adhesive layer (16), a transmission of 20% or more is obtained at a penetration depth of 50% relative to its total thickness, the transmission being determined by means of LIV-VIS spectroscopy with correction of the interfacial reflection losses.

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