Multilayer bonding element and method of parting a bond

Abstract

The invention relates to a multilayer bonding element, comprising: i) a first adhesive layer, ii) a second adhesive layer, and iii) a separation layer comprising a separation material, wherein the separation material has an absorption for laser radiation of at least one separation wavelength λT such that energy can be input into the separation material by means of laser radiation of the separation wavelength λT, wherein the separation wavelength λT is in the range of 100 to 11000 nm, wherein the separation layer is designed such that the bond between a first portion of the multilayer bonding element comprising the first adhesive layer and a second portion of the multilayer bonding element comprising the second adhesive layer can be weakened or eliminated at least in sections as a result of an energy input into the separation material by means of laser radiation of the separation wavelength λT, wherein the first adhesive layer and / or the second adhesive layer is a translucent adhesive layer, wherein the translucent adhesive layer is at least partly transparent to laser radiation of the separation wavelength λT, wherein the translucent adhesive layer has a haze value measured according to ASTM D 1003-21 (2021) in the range of 10% to 95%, and wherein the multilayer bonding element is designed such that laser radiation of the separation wavelength λT can be incident on the separation layer through a translucent adhesive layer.

Description

[0001] Multilayer adhesive element and method for releasing an adhesive bond

[0002] The invention relates to a multi-layer adhesive element, the use of such a multi-layer adhesive element for producing an adhesive bond, the corresponding adhesive bond and a method for dissolving such an adhesive bond.

[0003] Joining separate elements is one of the central processes in manufacturing technology. Along with other methods such as welding and soldering, bonding, i.e., joining using an adhesive, is becoming increasingly important. An alternative to the use of formless adhesives, such as those applied from a tube, is so-called adhesive tapes, whose adhesive effect is based on the adhesive masses used. The use of adhesive tapes is associated with many advantages that make this technology ideal for use in many industries, such as the electronics or automotive industries.

[0004] Advantageously, the connection of components achieved using adhesive tapes can, if necessary, be designed to be so durable and resilient that the corresponding bonds can only be removed with considerable effort. Such durable bonds are desired, for example, for touch panels such as those used in computer monitors or mobile electronic devices. Even if the durability of the bond is regularly considered essential during the use of such products, the strong bond also causes problems at the end of their service life. The bond, which is difficult to remove, often stands in the way of time- and cost-effective processing of the products and efficient recycling. For example, the bond complicates the separation of the different materials, e.g. the display from the plastic parts of the housing.

[0005] To address this problem, innovative multilayer adhesive elements have been proposed in the past, which at least partially circumvent the problem described above. The corresponding multilayer adhesive elements are equipped with an additional release layer adjacent to at least one of the adhesive layers. This release layer is designed to absorb electromagnetic radiation emitted by a laser. This leads to a local energy input into the release layer, which reduces the structural integrity of the release layer, for example, through decomposition of the processed release material, a melting process, or sublimation.Due to the loss of structural integrity of the release layer, the bond strength of the adhesive layer arranged on top of it to the remaining parts of the multilayer adhesive element is also weakened or even completely eliminated. Thus, the bond can be weakened or eliminated by treatment with a laser beam, allowing easy separation. This makes the reuse and recycling of corresponding products significantly more time- and cost-efficient, allowing even hard-to-reach bonds to be separated. In other words, this approach utilizes the concept of equipping the multilayer adhesive element with a deeper intermediate layer whose structural integrity can be selectively dissolved with a laser.Comprehensive information on corresponding multilayer adhesive elements and the technological background is disclosed, for example, in DE 102020209557 A1, DE 102021207161 A1, DE102021 134447 A1 and DE102022105185 A1.

[0006] Even though the multilayer adhesive elements known from the prior art are already considered very advantageous from a manufacturing perspective, there is still a need for improvements in other aspects. One disadvantage of the multilayer adhesive elements known from the prior art is that at least one of the adhesive layers must be permeable to the laser radiation used for treatment and, according to prevailing opinion, even largely transparent. Such transparency, which is usually also accompanied by transparency in the visible wavelength range, is often undesirable for the optical appearance of the bond, since in many applications the adhesive joint should not be visible from the outside.As a solution to this requirement, the prior art teaches the coloring of the adhesive provided in the adhesive layer with an infrared-transparent dye. This makes it possible to modify the layer's transmission in the visible light range while simultaneously maintaining its processability with an IR laser. Corresponding infrared-transparent dyes are currently available primarily in black and are used as dyes dissolved in the adhesive. In principle, however, non-black, infrared-transparent dyes are also conceivable, as disclosed, for example, in WO 2021200293 A1, EP 3805822 A1, US 10934443 B2, or JP 20171 16955 A.

[0007] However, this solution is sometimes considered in need of improvement. With regard to processability, the dyes absorbing visible light limit the usability of lasers with wavelengths below the infrared range, particularly when using the dominant black dyes. Furthermore, there is also a very high demand in the technical field for the color white for the adhesive layers. There is no satisfactory solution in the state of the art for implementing the corresponding multilayer adhesive elements with white adhesive layers, as the commonly used white pigments, in the usual quantities, adversely affect the processability of the underlying layers using lasers.

[0008] In addition, there is a constant interest in fundamentally further improving the weakenability of the release layer in corresponding multi-layer adhesive elements in order to reduce the required irradiation time and / or irradiation power for a satisfactory solution of the bond as much as possible.

[0009] The primary object of the present invention was to eliminate or at least reduce the disadvantages of the prior art.

[0010] In particular, it was the object of the present invention to provide a multi-layer adhesive element which, on the one hand, enables a permanent and secure bonding of two components to one another, but, on the other hand, if necessary, enables a clean and secure separation of the components by means of laser treatment, wherein the multi-layer adhesive element should offer a whitish optical appearance in the composite.

[0011] It was a further object of the present invention that the multilayer adhesive element to be specified should be attenuable with electromagnetic radiation in a broad wavelength range in order to thus enable a high degree of flexibility in the selection of the lasers to be used.

[0012] Furthermore, it was an object of the present invention to further improve the weakenability of the release layer in corresponding multi-layer adhesive elements in order to reduce as much as possible the required irradiation time and / or irradiation power for a satisfactory dissolution of the bond.

[0013] It was a further object of the present invention to specify a use of a corresponding multi-layer adhesive element in the production of an adhesive composite and to provide a corresponding adhesive composite.

[0014] Furthermore, it was an object of the present invention to provide a method for dissolving such an adhesive bond.

[0015] The inventors of the present invention have now found that the objects described above can be achieved if a multi-layer adhesive element is provided which comprises a release layer between the two adhesive layers, which has an absorption at at least one release wavelength AT, so that the energy input into the release material required for the attenuation is possible if at least one of the adhesive layers is designed as a translucent adhesive layer which is at least partially transparent to laser beams of the release wavelength AT, but furthermore has a specific "haze value", ie a scattering for visible light, as described in the claims.

[0016] In this way, multilayer adhesive elements can advantageously be obtained which, when combined, offer a whitish optical appearance, while the underlying release layer advantageously remains treatable across a broad wavelength range, so that adhesive bonds created with it can be reliably released. Surprisingly, it was found that the haze of the adhesive layers—contrary to the actual expectations of the skilled person—actually improves the weakenability of the structural integrity. Without wishing to be bound by this theory, the inventors assume that scattering of the laser beam leads to improved surface treatment of the underlying release layer and not, as expected by the skilled person, a focusing of the irradiated laser energy.

[0017] The above-mentioned objects are thus achieved by the subject matter of the invention as defined in the claims. Preferred embodiments of the invention emerge from the subclaims and the following statements.

[0018] Such embodiments, which are designated as preferred below, are combined in particularly preferred embodiments with features of other embodiments designated as preferred. Combinations of two or more of the embodiments designated as particularly preferred below are thus very particularly preferred. Likewise preferred are embodiments in which a feature of an embodiment designated as preferred to any extent is combined with one or more further features of other embodiments designated as preferred to any extent. Features of preferred adhesive composites, methods, and uses emerge from the features of preferred multilayer adhesive elements.

[0019] Particularly preferred embodiments of the invention are disclosed in the exemplary embodiments. Particularly preferred embodiments of the invention accordingly comprise two or more, preferably three or more, and most preferably four or more, of the preferred features of the invention disclosed below, which are also implemented in the exemplary embodiments.

[0020] The invention particularly relates to a multi-layer adhesive element comprising: i) a first adhesive layer comprising a first adhesive, ii) a second adhesive layer comprising a second adhesive, and iii) a release layer arranged between the first adhesive layer and the second adhesive layer, comprising a release material, wherein the release material has an absorption for laser radiation of at least one release wavelength AT, so that an energy input into the release material by means of laser radiation of the release wavelength AT is possible, wherein the release wavelength AT is in the range from 100 to 11000 nm, wherein the release layer is designed such thatthat the bond between a first part of the multilayer adhesive element comprising the first adhesive layer and a second part of the multilayer adhesive element comprising the second adhesive layer can be weakened or eliminated at least in sections as a result of an energy input into the release material by means of laser radiation of the release wavelength AT, wherein the first adhesive layer and / or the second adhesive layer is a translucent adhesive layer, wherein the translucent adhesive layer is at least partially permeable to laser radiation of the release wavelength AT, wherein the translucent adhesive layer has a haze value measured according to ASTM D 1003-21 (2021) in the range of 10 to 95%, and wherein the multilayer adhesive element is designed such that laser radiation of the release wavelength AT can impinge on the release layer through a translucent adhesive layer.

[0021] The invention relates to a multilayer adhesive element, wherein the person skilled in the art understands that this multilayer adhesive element is designed to create a bond between two substrates, in particular a permanent bond, which can subsequently be weakened or completely eliminated by treating the release layer with a laser. Corresponding adhesive elements according to the invention can in principle take on a variety of forms and can also be used, for example, in the form of adhesive labels or die-cut lengths. For the vast majority of cases, however, a design as an adhesive tape is optimal for the respective application requirements. For most applications, a multilayer adhesive element according to the invention is preferred, wherein the multilayer adhesive element is an adhesive tape.

[0022] The multilayer adhesive element according to the invention comprises two adhesive layers, each of which has an adhesive composition, in accordance with the expert's expectations, so that it is a double-sided adhesive element. With regard to their adhesive properties, the adhesive layers can in principle be designed analogously to the adhesive compositions known from the prior art. According to the inventors, it is preferable to design the adhesive layers as pressure-sensitive adhesive layers, which advantageously makes it possible to achieve immediate adhesion during the production of adhesive bonds. The pressure-sensitive adhesive design is particularly preferred in cases where the adhesive compositions are also designed as reactive adhesives.Corresponding reactive pressure-sensitive adhesives are comprehensively familiar to those skilled in the art and can be cured by external energy supply, and in the cured state can function in particular as structural adhesives. The combination of curability with pressure-sensitive adhesive properties advantageously makes it possible to achieve immediate adhesion before the pressure-sensitive adhesive cures and, moreover, to ensure the positioning of the multilayer adhesive element during the curing process. Accordingly, a multilayer adhesive element according to the invention is preferred, wherein the first adhesive layer and / or the second adhesive layer, preferably the first adhesive layer and the second adhesive layer, is a pressure-sensitive adhesive layer, and / or wherein the first adhesive and / or the second adhesive, preferably the first adhesive and the second adhesive, is a pressure-sensitive adhesive.

[0023] According to expert understanding, a pressure-sensitive adhesive is an adhesive that possesses pressure-sensitive adhesive properties, i.e., the ability to form a permanent bond to a substrate even under relatively light pressure. Such pressure-sensitive adhesive tapes are generally permanently tacky even at room temperature, meaning they exhibit a certain viscosity and tackiness, allowing them to wet the surface of a substrate even under light pressure. Without wishing to be bound by this theory, it is often assumed that a pressure-sensitive adhesive can be considered an extremely viscous liquid with an elastic component, which consequently possesses characteristic viscoelastic properties that lead to the permanent tackiness and pressure-sensitive adhesive capacity described above.It is assumed that, in the case of corresponding pressure-sensitive adhesives, mechanical deformation results in both viscous flow processes and the build-up of elastic restoring forces. The partial viscous flow serves to achieve adhesion, while the partial elastic properties are particularly necessary to achieve cohesion. The relationships between rheology and pressure-sensitive tack are known in the art and are described, for example, in "Satas, Handbook of Pressure Sensitive Adhesives Technology", Third Edition, (1999), pages 153 to 203. To characterize the degree of elastic and viscous components, the storage modulus (G') and the loss modulus (G") are usually used. These can be determined by means of dynamic mechanical analysis (DMA), for example using a rheometer, as disclosed, for example, in WO 2015 / 189323 A1.In the context of the present invention, an adhesive is preferably understood as pressure-sensitive adhesive and thus as a pressure-sensitive adhesive if, at a temperature of 23 °C in the deformation frequency range from 10° to 10. 1 rad / sec G' and G“ each at least partly in the range of 10 3 up to 10 7 Pa lie.

[0024] Particularly preferred is a multilayer adhesive element according to the invention, wherein the first adhesive and / or the second adhesive, preferably the first adhesive and the second adhesive, is a reactive pressure-sensitive adhesive, preferably a radiation-curable reactive adhesive, particularly preferably based on epoxides.

[0025] Reactive adhesives are well known to those skilled in the art. Due to their ability to cure, the reactive adhesive can function as a structural adhesive after curing. According to DIN EN 923: 2006-01, structural adhesives are demonstrably suitable for the production of load-bearing structures in which the bonded joint can be subjected to a high percentage of the maximum breaking force over extended periods without failure (according to the ASTM definition: "bonding agents used for transferring required loads between adherends exposed to service environments typical for the structure involved"). These are therefore adhesives for highly stressable bonds that contribute to the strengthening of the adhesive tapes when cured.

[0026] According to the inventors' assessment, poly(meth)acrylate- and polyurethane-based adhesives are particularly suitable for use in corresponding multilayer adhesive elements, since they allow the desired optical properties to be achieved particularly well, with particular preference being given to using the same types of adhesives in both adhesive layers. Consequently, a multilayer adhesive element according to the invention is preferred, wherein the first adhesive and / or the second adhesive, preferably the first adhesive and the second adhesive, comprise one or more adhesive polymers selected from the group consisting of poly(meth)acrylates and polyurethanes.

[0027] In accordance with the understanding of those skilled in the art, the adhesive layers can in principle also comprise other components in addition to the relevant adhesive. In this respect, however, the inventors consider it preferable for essentially all embodiments if the adhesive layer consists as largely as possible only of the respective adhesive. Accordingly, a multilayer adhesive element according to the invention is preferred, wherein the first adhesive layer consists of a mass fraction of 80% or more, preferably of 90% or more, particularly preferably of 95% or more, very particularly preferably of essentially 100% of the first adhesive, based on the mass of the first adhesive layer, and / or wherein the second adhesive layer consists of a mass fraction of 80% or more, preferably of 90% or more, particularly preferably of 95% or more, very particularly preferably of essentially 100% of the second adhesive, based on the mass of the second adhesive layer.Even if, in the inventors' estimation, this will not be preferred for many users due to the desired white coloration, colorants can, at least in principle, also be used in the adhesive layers to further influence the optical absorption behavior. For certain applications, a multilayer adhesive element according to the invention is therefore preferred, wherein the first adhesive layer and / or the second adhesive layer, preferably the first adhesive layer and the second adhesive layer, comprise one or more colorants, in particular dyes, wherein the colorants exhibit essentially no absorption for electromagnetic radiation of the separation wavelength AT.

[0028] The inventors have succeeded in specifying advantageous thicknesses for the adhesive layers, which ensure advantageous weakenability, particularly in the form of translucent adhesive layers. A multilayer adhesive element according to the invention is preferred, wherein the first adhesive layer and / or the second adhesive layer, preferably the first adhesive layer and the second adhesive layer, has an average thickness in the range of 4 to 400 μm, preferably in the range of 6 to 200 μm, particularly preferably in the range of 8 to 100 μm.

[0029] An essential component of the adhesive elements according to the invention, which enables the advantageous resolvability of the bond created by laser treatment, is the layer intended for laser weakening, which is referred to in the present invention as the release layer, which expresses that this release layer enables or promotes the separation of a portion of the multilayer adhesive element. This release layer comprises a release material that has sufficient absorption at least at a release wavelength XT, so that energy can be introduced into the release material by means of laser radiation of this release wavelength XT.A multilayer adhesive element according to the invention is preferred, wherein the release layer has a transmission of 30% or less, preferably 20% or less, particularly preferably 10% or less, very particularly preferably 5% or less, for laser radiation of the release wavelength AT impinging orthogonally on the surface of the release layer and passing through the release layer along the direction of the smallest thickness extension, measured according to ASTM D 1003-21 (2021).

[0030] The release material is selected such that its structural integrity can be reduced or completely eliminated by the energy input of the laser, for example, through liquefaction, evaporation, sublimation, or thermal decomposition. Within the scope of the present invention, the reduction in structural integrity, regardless of the actual mechanism, is also simply referred to as laser weakening. The release layer comprising the release material is designed such that, as a result of energy input into the release material and the resulting decrease in the structural integrity of the release material, the bond between the parts of the multilayer adhesive element comprising the first adhesive layer and the second adhesive layer, respectively, is weakened.

[0031] The formulation chosen within the scope of the present invention, that the separation layer as a whole—and not just the separation material—is designed to enable this separation, takes into account the fact that it would be at least theoretically conceivable to form the separation layer only partially from the separation material and to combine this, for example, with other materials in the form of support structures. In the inventors' estimation, a particularly advantageous embodiment, which is particularly efficient to manufacture, results when the separation layer is formed as largely as possible from the separation material, which advantageously also means that the separation can take place not only in sections, but essentially across the entire surface of the separation layer.Accordingly, a multi-layer adhesive element according to the invention is preferred, wherein the release layer consists of the release material to a mass fraction of 80% or more, preferably of 90% or more, particularly preferably of 95% or more, very particularly preferably of substantially 100%, based on the mass of the release layer.

[0032] Additionally or alternatively, a multi-layer adhesive element according to the invention is preferred, wherein the release layer is designed such that the connection between the first part of the multi-layer adhesive element comprising the first adhesive layer and the second part of the multi-layer adhesive element comprising the second adhesive layer can be weakened or eliminated, preferably essentially completely, over the entire surface of the release layer as a result of an energy input into the release material by means of laser radiation of the release wavelength AT.

[0033] Those skilled in the art will understand that, for functionality during laser treatment, it is necessary for the separation material to exhibit absorption at least at the separation wavelength XT used for separation, which allows energy to be introduced into the separation material. In practice, however, in the vast majority of cases, the separation material will exhibit absorption across a full range of wavelengths, for example, because the separation material is designed as a black separation material.

[0034] With regard to the selection of materials for the release material, materials known from the prior art and comprehensively discussed therein can be used within the scope of the present invention. In addition to metallic and oxide release layers, for example made of aluminum or aluminum oxide, the use of plastic layers is of particular importance, which can advantageously be applied and cured in the form of varnishes. Preference is given to a method according to the invention, wherein the release material is selected from the group consisting of metallic materials and plastics. Firstly, an example is a multi-layer adhesive element according to the invention, wherein the release layer is a metallic or oxide release layer, and / or wherein the release material is selected from the group consisting of metals, preferably aluminum, and metal oxides, preferably aluminum oxide.Additionally or alternatively, an example is a multi-layer adhesive element according to the invention, wherein the release material is selected from the group consisting of cured lacquers, preferably radiation-cured lacquers.

[0035] When using plastic-based separation materials, the desired absorption property can be adjusted by using suitable and, in principle, known laser absorbers. The laser absorber is selected by the person skilled in the art, particularly in light of the separation wavelengths XT that are particularly interesting for subsequent use in laser treatment. An additional or alternative example is a multilayer adhesive element according to the invention, wherein the one or more laser absorbers are selected from the group consisting of black laser absorbers, in particular carbon black, and non-black laser absorbers, in particular oxide laser absorbers, for example titanium dioxide.A multilayer adhesive element according to the invention is preferred, wherein the separation material comprises one or more laser absorbers, preferably in a mass fraction in the range from 1 to 40%, particularly preferably in the range from 2 to 7% in the case of black laser absorbers, in particular in the case of soot, or in the range from 15 to 40% in the case of non-black laser absorbers, in particular oxidic laser absorbers, for example titanium dioxide, wherein the laser absorber(s) has(have) an absorption value at the separation wavelength AT or in a wavelength range from (AT - 30 nm) to (AT + 30 nm), preferably from (AT - 20 nm) to (AT + 20 nm) which corresponds to 50% or more, preferably 70% or more, of the maximum absorption value in the wavelength range between 100 and 11000 nm.

[0036] The inventors of the present invention have succeeded in identifying preferred ranges for the thickness of the release layers, with which multilayer adhesive elements can be obtained in practice, in which laser treatment of the release layer is reliably possible, wherein these ranges are particularly preferably combined with preferred thicknesses of the adhesive layers. A multilayer adhesive element according to the invention is preferred, wherein the release layer has an average thickness in the range of 30 nm to 100 pm, preferably in the range of 40 nm to 50 pm, particularly preferably in the range of 50 nm to 20 pm.

[0037] At least theoretically, it would be conceivable to construct the multilayer adhesive element exclusively from the two adhesive layers and the release layer. In this case, the two adhesive layers would be at least partially—or in practice, probably almost completely—separated from each other by the release layer, so that the release layer could function as a carrier, as is known from double-sided adhesive elements. However, in the inventors' estimation, such a structure is less preferred. Rather, in the inventors' estimation, it is clearly preferred for essentially all embodiments if the multilayer adhesive element also comprises a conventional carrier layer in addition to the release layer between the first adhesive layer and the second adhesive layer.This makes it possible to adjust the release layer primarily to achieve advantageous weakening properties without having to also adjust the mechanical properties of the multilayer adhesive element through the release layer, since this adjustment can be made through the carrier layer. Accordingly, a multilayer adhesive element according to the invention is preferred, wherein the multilayer adhesive element additionally comprises: iv) one or more, preferably exactly one, carrier layers arranged between the release layer and the first adhesive layer or between the release layer and the second adhesive layer, wherein the release layer is preferably integrally bonded to the carrier layer.

[0038] A multilayer adhesive element according to the invention is preferred, wherein the carrier layer is formed from a carrier material, wherein the carrier material is preferably selected from the group consisting of plastics, preferably polyethylene terephthalate, polyethylene, or polypropylene. Additionally or alternatively, a multilayer adhesive element according to the invention is preferred, wherein the carrier layer is a carrier film.

[0039] Those skilled in the art will understand that, for the usability of the multilayer adhesive elements according to the invention, it is expedient that any carrier layer present does not prevent laser radiation of the separation wavelength XT from impinging on the separation layer through one of the translucent adhesive layers. To ensure this particularly efficiently, the inventors propose that the carrier layer itself be designed such that it is at least partially permeable to electromagnetic radiation of the separation wavelength XT and, additionally or alternatively, be arranged only on one side of the separation layer such that the separation layer can be irradiated through a translucent adhesive layer without the carrier layer blocking the beam path.Thus, a multilayer adhesive element according to the invention is preferred, wherein the carrier layer is a carrier layer that is at least partially, preferably substantially completely, permeable to laser radiation of the separation wavelength AT. Additionally or alternatively, a multilayer adhesive element according to the invention is preferred, wherein the separation layer is integrally bonded to the first adhesive layer and / or the second adhesive layer, wherein the separation layer is preferably integrally bonded to one or two translucent adhesive layers.

[0040] A person skilled in the art will approximately assume that translucency in standard illuminant D65, as used in haze or transmission measurements, is also present in the near infrared range, i.e., approximately 800 nm to 2000 nm, in which a large number of lasers operate. For the present invention, it is therefore assumed that measured translucency under standard illuminant D65 is also present for this range. Lasers in the infrared range are preferably used for the subsequent laser treatment. Even if it can be seen as an advantage of the present invention that it can in principle also be used for lower separation wavelengths XT, the inventors consider a design of the multilayer adhesive elements according to the invention for longer wavelengths to be advantageous. Thus, a multilayer adhesive element according to the invention is preferred, wherein the separation wavelength ΔT is in the range from 400 to 5000 nm, preferably in the range from 800 to 2000 nm.

[0041] In order to ensure that the release layer arranged between the adhesive layers can be irradiated with the necessary laser radiation of the release wavelength XT in order to weaken or break a created bond in this way, according to the invention at least one of the adhesive layers is designed in a special form, which in the context of the present invention is referred to as a "translucent adhesive layer". These translucent adhesive layers are at least partially permeable to laser radiation of the release wavelength XT and, moreover, also have the "haze value" to be set according to the invention. Even if it would in principle be sufficient, at least with regard to ensuring laser treatability, to design only one of the adhesive layers as a translucent adhesive layer, the inventors believe that it is preferable, particularly from a manufacturing perspective, to design both adhesive layers as translucent adhesive layers.Accordingly, a multi-layer adhesive element according to the invention is preferred, wherein the first adhesive layer and the second adhesive layer are each a translucent adhesive layer.

[0042] To ensure energy-efficient laser treatment of the release layer, the inventors recommend that the transmission at the release wavelength XT, i.e. the permeability, should be as high as possible. The transmission can be determined according to ASTM D1003-21 (2021), which is known to those skilled in the art. Accordingly, a multilayer adhesive element according to the invention is preferred, wherein one or all, preferably all, of the translucent adhesive layers each have a transmission of 60% or more, preferably 70% or more, more preferably 80% or more, most preferably 90%, for radiation of the release wavelength AT incident orthogonally on the surface of the translucent adhesive layer and passing through the translucent adhesive layer along the direction of the smallest thickness extension, measured according to ASTM D1003-21 (2021) without correction for interfacial reflection losses.

[0043] Additionally or alternatively, a multilayer adhesive element according to the invention is preferred, wherein one or all, preferably all, of the translucent adhesive layers each have a transmission of 60% or more, preferably 70% or more, particularly preferably 80% or more, very particularly preferably 90%, for light radiation of all wavelengths in the range from 400 to 700 nm impinging orthogonally on the surface of the translucent adhesive layer and passing through the translucent adhesive layer along the direction of the smallest thickness extension, measured according to ASTM D1003-21 (2021) with standard illuminant D65, without correction of interfacial reflection losses, for example with a Hazemeter from Byk of the Haze-Gard Dual type.

[0044] As the adjective "translucent" of the translucent adhesive layers suggests, although they are at least partially permeable to electromagnetic radiation of the corresponding wavelength ranges, they are not completely transparent. Rather, a specific "haze value" is set as defined above. The haze value is a parameter well known to those skilled in the art, which describes the proportion of transmitted light that is scattered forward at a large angle by the translucent adhesive layer. In other words, the haze value quantifies those structures in the surface or volume of the adhesive layers that disrupt clear visibility. The method for measuring the haze value is described in ASTM D1003-21 (2021). Light type D65 is used. The standardized procedure according to "Procedure A" requires the measurement of four transmission measurements. The light transmittance is calculated for each transmission measurement.The four transmittances are calculated to determine the percentage haze value. For the purposes of this invention, the haze value is measured using a BYK "Haze-Gard Dual" spectrophotometer. Light scattering in the translucent adhesive layers results in a white color impression.

[0045] Surprisingly, light scattering does not impair the effectiveness of the laser beam for the separation process. Surprisingly, it was even found that scattering can remove the separation layer more effectively, thus improving the separation process.

[0046] In the inventors' opinion, a haze value of 10% is the lower limit, as at a lower haze value the whitish impression is barely present. 95%, on the other hand, is the upper limit, as the laser beam is widened so much that removal of the separation layer material becomes less reliable. Accordingly, there is a conflict of objectives between achieving a desired whitish impression and reliable removal of the separation layers, particularly for thicker separation layers. At high haze values, particularly advantageous degrees of whiteness are achieved while the desired laser treatment is fundamentally functional, while in the lower range particularly reliable removal of the separation layers is achieved. In the inventors' opinion, a particularly advantageous solution to this conflict of objectives arises in the medium value range.Thus, firstly, a multilayer adhesive element according to the invention is preferred, wherein one or all, preferably all, of the translucent adhesive layers each have a haze value measured according to ASTM D 1003-21 (2021) in the range from 15 to 80%, preferably in the range from 20 to 70%, particularly preferably in the range from 30 to 60%. Particularly preferred is a multilayer adhesive element according to the invention, wherein one or all, preferably all, of the translucent adhesive layers each have a haze value measured according to ASTM D 1003-21 (2021) in the range from 10 to 50%, preferably in the range from 15 to 45%, particularly preferably in the range from 20 to 40%.Alternatively, a particularly preferred multilayer adhesive element according to the invention is one in which one or all, preferably all, of the translucent adhesive layers each have a haze value measured according to ASTM D 1003-21 (2021) in the range from 50 to 80%, preferably in the range from 55 to 75%, particularly preferably in the range from 60 to 70%.

[0047] In a preferred embodiment, the haze of the adhesive layer is adjusted by a particulate filler that does not itself absorb the laser beam. This includes reflective or transparent materials. According to DIN ISO 18451-1-2019-09, "particles" refers to primary particles, aggregates, and agglomerates of a material in the solid state. "Particle size" refers to the maximum dimension of a particle. The particle size is preferably determined by laser diffraction according to ISO 13320-2020-01, but other methods known to those skilled in the art are also suitable. The shape of the particles can be diverse, e.g., spherical, rod-shaped, fibrous, or platelet-shaped.

[0048] Thus, a multilayer adhesive element according to the invention is preferred, wherein the adhesive compositions of the translucent adhesive layers comprise one or more particulate fillers, preferably in a combined mass fraction in the range from 1 to 15%, preferably in the range from 1 to 10%, particularly preferably in the range from 5 to 10%, based on the mass of the respective adhesive compositions.

[0049] Particularly preferred is a multilayer adhesive element according to the invention, wherein the one or more particulate fillers are selected from the group consisting of fillers with an average particle size according to ISO 13320 - 2020-01 D50 in the range from 0.1 pm to 100 pm, preferably in the range from 1 pm to 80 pm, particularly preferably in the range from 10 pm to 50 pm. Additionally or alternatively, particularly preferred is a multilayer adhesive element according to the invention, wherein the one or more particulate fillers are selected from the group consisting of fillers with an average particle size according to ISO 13320-2020-01 D50 in the range from 0.5 to 20 pm, preferably in the range from 1 pm to 10 pm.

[0050] At least theoretically, it would be conceivable to generate the required haze values ​​through the use of conventional inorganic pigments, for example titanium dioxide, barium sulfate, zinc oxide, or chalk. However, this has the disadvantage that even with a very low filler content, a very high haze value is achieved with low transmittance, whereby the balance between haze and transmittance can only be controlled to a very limited extent. The use of inorganic white fillers is therefore explicitly not preferred. Additionally or alternatively, a multilayer adhesive element according to the invention is preferred, wherein the one or more particulate fillers are selected from the group consisting of fillers which are essentially non-absorbent for laser radiation of the separation wavelength AT, preferably at least partially transparent.

[0051] Rather, in the inventors' opinion, it is particularly preferable to achieve the desired haze value while maintaining the transparency of the adhesive layers by using differences in the refractive index within the adhesive layer, as is particularly possible with transparent materials. Light scattering is essentially generated by the difference in refractive index between the adhesive matrix and the filler.

[0052] In the present invention, the refractive index is determined based on ISO 489:2022-03, Method A (measurement wavelength 589 nm), at a temperature of 20 °C and a relative humidity of 50%. For this purpose, molded bodies are produced from meltable polymer particles using pressure and temperature. Cinnamon oil was used as the contact fluid for the measurement. For non-meltable particles, the refractive index is determined based on ISO 489:2022-03, Method B (measurement wavelength 589 nm).

[0053] Thus, a multilayer adhesive element according to the invention is preferred, wherein the one or more particulate fillers are selected from the group consisting of fillers which are transparent or translucent to visible light, preferably transparent, whose refractive index, measured according to ISO 489:2022-03, differs from the refractive index of the surrounding adhesive matrix, preferably by 0.03 or more, particularly preferably by 0.08 or more, very particularly preferably by 0.12 or more.

[0054] The inventors have succeeded in identifying particularly suitable fillers with which the desired haze values ​​can be achieved particularly effectively while maintaining the desired advantageous transmission. A multilayer adhesive element according to the invention is generally preferred, wherein at least one of the particulate fillers is selected from the group consisting of polymeric filler particles, microcrystalline wax filler particles, solid polymer spheres, hollow polymer spheres, and amorphous silicon dioxide.

[0055] Particularly preferred is a multilayer adhesive element according to the invention, wherein at least one of the particulate fillers is selected from the group consisting of polymeric filler particles, preferably selected from the group consisting of filler particles made of polymethyl methacrylate (PMMA), silicone, polyethylene, polyurethane and fluoropolymer. The fluoropolymer is preferably selected from the group consisting of poly(hexafluoropropylene oxide) (PHFPO), fluorinated ethylene propylene polymer (FEP), poly(tetrafluoroethylene) (PTFE), poly(tetrafluoroethylene hexafluoropropylene vinylidene fluoride) (THV), perfluoroalkoxy polymer (PFA), poly(ethylene tetrafluoroethylene) (ETFE), poly(chlorotrifluoroethylene) (PCTFE), poly(vinylidene fluoride) (PVDF) and poly(ethylene chlorotrifluorotheylene) (ECTFE) as well as their copolymers and terpolymers.

[0056] Additionally or alternatively, a multilayer adhesive element according to the invention is particularly preferred, wherein at least one of the particulate fillers is selected from the group consisting of filler particles made of microcrystalline wax. Such waxes are offered, for example, under the trade name Ceridust F by Clariant, or Ceraflour by Byk. According to Habenicht (see Habenicht: Kleben, 6th edition, 2009, pages 71 and 156 ff.), the term "wax" refers to various products of plant, animal, or geological origin, which, in terms of their chemical basis, essentially consist of hydrocarbons, higher alcohols, acids, and esters. Regardless of the chemical composition, the classification of a wax is determined by its physical properties such as melting point, solubility, inertness, etc. The essential criteria are:

[0057] - a melting point of at least 40 °C,

[0058] - a relatively low melt viscosity and not stringy in contrast to many resins,

[0059] - increasing solubility and no chemical decomposition at elevated temperatures.

[0060] In addition or alternatively, a multilayer adhesive element according to the invention is particularly preferred, wherein at least one of the particulate fillers is selected from the group consisting of solid polymer spheres and hollow polymer spheres, in particular expanded microballoons, solid glass spheres, hollow glass spheres and glass fibers. Microballoons are elastic hollow spheres which have a thermoplastic polymer shell. These spheres are filled with low-boiling liquids or liquefied gas. Polyacrylonitrile, PVDC, PVC or polyacrylates are particularly used as shell material. Hydrocarbons of the lower alkanes, for example isobutane or isopentane, are particularly suitable as low-boiling liquids, which are enclosed in the polymer shell as liquefied gas under pressure. By acting on the microballoons, in particular by the application of heat, the outer polymer shell softens.At the same time, the liquid propellant gas contained in the shell transforms into a gaseous state. The microballoons expand irreversibly and expand three-dimensionally. The expansion is complete when the internal and external pressures equalize. Since the polymer shell remains intact, a closed-cell foam is created.

[0061] Particularly preferred, due to the advantageous overall properties which can be achieved thereby, is a multilayer adhesive element according to the invention, wherein at least one of the particulate fillers is selected from the group consisting of hollow polymer spheres, in particular expanded microballoons.

[0062] Preference is again given, additionally or alternatively, to a multilayer adhesive element according to the invention, wherein at least one of the particulate fillers is selected from the group consisting of amorphous silicon dioxide, preferably precipitated amorphous silicon dioxide and pyrogenic amorphous silicon dioxide, particularly preferably pyrogenic amorphous silicon dioxide.

[0063] Instead of using fillers, corresponding differences in the refractive index can also be achieved by introducing gas-filled cavities into the adhesive layers, thereby obtaining foamed adhesive layers. These gas-filled cavities differ in refractive index from the refractive index of the surrounding adhesive matrix and produce a similar effect to corresponding fillers. In this preferred case, the adhesive element according to the invention is a multilayer adhesive element, wherein one or all, preferably all, of the translucent adhesive layers is a foamed adhesive layer. The foam can be open-cell or closed-cell. It is preferably closed-cell. Gases or expanded fillers, for example, can be used as foaming agents. Expanded fillers, in particular hollow glass spheres or expanded microballoons, are preferred.The volume fraction of these foaming agents in the adhesive is preferably between 2 and 20%, particularly preferably between 5 and 12%.

[0064] A preferred embodiment, particularly with regard to an appealing white color impression, consists in the release layer located beneath a translucent adhesive layer being designed with an at least partially reflective surface, so that visible light passing through the translucent adhesive layer onto the reflective surface of the release layer is reflected back through the translucent adhesive layer, resulting in further scattering and a particularly advantageous white impression. This can be advantageously achieved, particularly when using metallic release materials.In this case, a multi-layer adhesive element according to the invention is preferred, wherein the release layer has a reflective surface at least on the surface facing in the direction of a translucent adhesive layer, which is designed to have a reflectometer value according to EN ISO 2813:1999 (standard illuminant C) at a measuring angle of 60° of more than 10, preferably of more than 30, particularly preferably of more than 50, very particularly preferably of more than 90.

[0065] Those skilled in the art will understand that the multilayer adhesive elements according to the invention are designed and intended to bond two substrates to one another. For this purpose, the respective adhesive layers are bonded to the respective substrates in a material-to-material manner, namely via their pressure-sensitive adhesive properties and / or through the curing of a reactive adhesive. The invention thus also relates to the use of a multilayer adhesive element according to the invention for producing an adhesive composite comprising a first substrate and a second substrate, wherein, to produce the adhesive composite, the multilayer adhesive element is bonded to the first substrate in a material-to-material manner via the first adhesive layer and to the second substrate in a material-to-material manner via the second adhesive layer.

[0066] At least theoretically, it would be conceivable to achieve the advantageous bonding solution using laser treatment even if the substrates themselves were opaque to the laser radiation. In this case, however, it would be necessary to strike the thin adhesive joint laterally and, so to speak, to separate and detach the release layer from the side. Accordingly, those skilled in the art will understand that, for essentially all embodiments, it is explicitly preferred if at least one of the substrates is so permeable to electromagnetic radiation of the separation wavelength XT that the laser treatment can be carried out through the substrate. In this case, it is at least theoretically possible to further adjust the corresponding substrate with regard to its other absorption properties using suitable colorants, for example IR-transparent colorants, and to adapt it to the respective application requirements.For essentially all embodiments, an adhesive bond according to the invention is thus preferred, wherein the first substrate and / or the second substrate is at least partially transparent to laser radiation of the separation wavelength AT, wherein the first substrate and / or the second substrate is preferably substantially completely transparent to visible light.

[0067] Additionally or alternatively, a multilayer adhesive element according to the invention is conceivable, wherein the first substrate and / or the second substrate comprise one or more colorants, in particular dyes, wherein the colorants preferably have essentially no absorption for electromagnetic radiation of the separation wavelength AT.

[0068] In light of the above statements, the invention also relates to an adhesive composite comprising:

[0069] I) a first substrate,

[0070] II) a second substrate, and

[0071] III) a multi-layer adhesive element according to the invention arranged between the first substrate and the second substrate, wherein the multi-layer adhesive element is materially bonded to the first substrate via the first adhesive layer and materially bonded to the second substrate via the second adhesive layer, and wherein the first and / or the second substrate, at least the substrate bonded to a translucent adhesive layer, is at least partially permeable to laser radiation of the separation wavelength AT.

[0072] Based on the above statements on the use of the multilayer adhesive elements according to the invention and the adhesive bonds that can be produced therewith, the method for dissolving such an adhesive bond according to the invention is of particular importance in the context of the present invention.

[0073] The invention thus also relates to a method for dissolving an adhesive bond according to the invention, comprising the method steps: a) producing or providing an adhesive bond according to the invention, b) irradiating the release layer of the multilayer adhesive element with laser radiation of the release wavelength AT through a substrate which is at least partially transparent to laser radiation of the release wavelength AT, and through a translucent adhesive layer, to introduce an energy input into the release material, so that the bond between the first part of the multilayer adhesive element comprising the first adhesive layer and the second part of the multilayer adhesive element comprising the second adhesive layer is at least partially weakened or eliminated as a result of the energy input, c) dissolving the bond created by the multilayer adhesive element between the first substrate and the second substrate of the adhesive bond,wherein the first part of the multilayer adhesive element remains on the first substrate and wherein the second part of the multilayer adhesive element remains on the second substrate.,

[0074] In the method disclosed above, an adhesive composite according to the invention is first provided, which can be provided in particular within the framework of a recycling process. Starting from this, the release layer of the multilayer adhesive element is irradiated with laser radiation of the release wavelength XT. Lasers suitable for this purpose and typical operating parameters of corresponding lasers are known to the person skilled in the art and are disclosed, for example, in the prior art cited above, to which reference is made here in addition to the following exemplary embodiments. An example of a method according to the invention is one in which the laser radiation of the release wavelength ΔT is pulsed laser radiation, preferably with pulse durations in the range of picoseconds or femtoseconds.

[0075] The laser used introduces energy into the release material, which maintains the structural integrity of the release material and thus of the release layer arranged between the adhesive layers. An example of a method according to the invention is one in which the release layer is at least partially liquefied and / or vaporized and / or decomposed by introducing energy into the release material.

[0076] As a result, the bond between the two parts of the multilayer adhesive element, which are bonded to the first substrate and the second substrate, respectively, is weakened or even completely eliminated depending on the extent of the laser treatment. Starting from this weakened bond, the final dissolution of the bond now takes place in process step c). This can be achieved, for example, by applying mechanical force, for example, by detaching a display from the housing using mechanical force after the adhesive bond has been broken. Advantageously, this can be achieved with comparatively little effort, for example, using conventional manual forces, thanks to the prior laser treatment.

[0077] Analogous to the above statements, a method according to the invention is preferred, wherein the first substrate and / or the second substrate has a transmission of 60% or more, preferably 70% or more, particularly preferably 80% or more, very particularly preferably 90%, for laser radiation of the separation wavelength AT incident orthogonally on the surface of the substrate and passing through the substrate along the direction of the smallest thickness extension, measured according to ASTM D1003-21 (2021), without correction of

[0078] Interface reflection losses.

[0079] In connection therewith, a method according to the invention is preferred, wherein the laser radiation of the separation wavelength AT is radiated through the first substrate and / or through the second substrate.

[0080] Additionally or alternatively, a method according to the invention is preferred, wherein the dissolution of the connection created by the multi-layer adhesive element between the first substrate and the second substrate of the adhesive composite takes place substantially over the entire surface of the multi-layer adhesive element.

[0081] Further preferred is, additionally or alternatively, a method according to the invention, wherein the release of the bond created by the multilayer adhesive element between the first substrate and the second substrate of the adhesive composite takes place over a portion of the surface of the multilayer adhesive element. This is particularly advantageous if, on at least one side of the multilayer adhesive element, only a portion of the surface of the adhesive element is covered with adhesive or only a portion of the surface of the adhesive element is bonded to a substrate. Thus, only the portion of the adhesive element that contributes to the bond is released.Further preferred is additionally or alternatively a method according to the invention, wherein the adhesive composite comprises three or more further substrates which are materially bonded to the second substrate via the translucent adhesive layer, wherein the bond created by the multi-layer adhesive element between the first substrate and the further substrates is also released.

[0082] In the following, the invention and preferred embodiments of the invention are further explained and described with reference to experiments.

[0083] A. Production of multi-layer adhesive elements:

[0084] A reactor conventional for radical polymerizations was charged with 54.4 kg of 2-ethylhexyl acrylate, 20.0 kg of methyl acrylate, 5.6 kg of acrylic acid, and 53.3 kg of acetone / isopropanol (94:6). After 45 minutes of nitrogen gas flow with stirring, the reactor was heated to 58 °C, and 40 g of azobis(isobutyronitrile) AIBN were added. The external heating bath was then heated to 75 °C, and the reaction was carried out at a constant external temperature. After 1 h, another 40 g of AIBN were added, and after 4 h, the reaction mixture was diluted with 10 kg of acetone / isopropanol mixture (94:6). After 5 h and after 7 h, the reaction was reinitiated with 120 g of bis(4-tert-butylcyclohexyl) peroxydicarbonate. After a reaction time of 22 hours, the polymerization was stopped and the mixture was cooled to room temperature. The polyacrylate had a K value of 58.8, a solids content of 55.9%, and an average molecular weight of Mw = 746.000 g / mol, a polydispersity of D (Mw / Mn) = 8.9, and a static glass transition temperature of Tg = -35.6°C. The solids content was adjusted to 50 wt. % by dilution with acetone. 1.5 parts of the crosslinker 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (commercial product: Uvacure 1500) were added per 100 parts of solids. The refractive index of the resulting acrylate matrix was 1.47.

[0085] To adjust the optical properties, the fillers listed in Table 1 were used in the working examples.

[0086] Table 1 - Fillers used

[0087] Filler: 2 polymethyl methacrylate spheres (PMMA); particle size D50 = 15 pm, refractive index (23 °C) = 1.49, trade name: Spheromers CA15-1; Microbeads AS

[0088] The adhesives summarized in Table 2 were prepared by mixing the polyacrylate with the fillers. The adhesive layer was spread in a thickness of 100 μm and dried for 15 minutes at 120 °C in a convection oven. For Example E6, a drying temperature of 140 °C was chosen to allow the added microballoons to expand during drying. Haze and transmittance were then determined as described above according to ASTM D 1003-21 (2021) using standard illuminant D65 without correction for interfacial reflection losses.

[0089] Table 2 - Composition of the adhesives

[0090] The whitish colour impression was rated as sufficient for samples E4, E5 and E6, whereas the whitish colour impression was rated as very favourable for E2 and especially E1 and E3 as well as V2.

[0091] To produce a film laminate according to the invention, a black-metallized PET film with a thickness of 12 μm was used. The release layer consists of aluminum oxide and has a thickness of 100 nm. The transmission of the layer is only approximately 20% compared to the laser wavelength used for lasering. To produce a double-sided adhesive element, a layer of the adhesive films of Examples E1 to E6 and Comparative Examples C1 and C2 was laminated to both sides of the film. The adhesive element produced in this way is used to construct a test specimen for the push-out test, as disclosed below.

[0092] B. Adhesive experiments - push-out test:

[0093] To evaluate bond strength, the push-out test, which is well-known in the technical field and is disclosed, for example, in DE 102016207550 A1, is carried out. This push-out test enables statements to be made about the bond strength of an adhesive product in the direction of the adhesive layer normal. For this purpose, a circular first substrate with a diameter of 21 mm was bonded to a second substrate with the adhesive element to be tested. The second substrate has a circular hole with a diameter of 9 mm. The adhesive element also has a diameter of 21 mm and was cut or punched accordingly. The first substrate, with the adhesive element to be tested, was applied centrally over this hole. A composite consisting of the first substrate (polycarbonate, trade name: Macrolon 099) and the second substrate (steel analogous to ISO 29862:2018) was tested. The first substrate had a thickness of 3.0 mm and the second substrate a thickness of 2.0 mm.When constructing the test specimens, the separation layer is oriented in the direction of the second substrate.

[0094] The adhesive element is pre-laminated to the first substrate at 23 °C, and the composite of the first substrate and the adhesive element is then laminated to the second substrate. The entire composite is then pressed for 1 minute at 1 MPa pressure and a temperature of 23 °C. After pressing, the test specimens are stored in the test environment for 24 hours.

[0095] Using a cylindrical punch (7 mm diameter) clamped into a tensile testing machine, the bond formed by the first substrate and the adhesive element is pressed through the hole in the second substrate, exerting a force on the bonded joint. The second substrate is fixed in the tensile testing machine in such a way that it is supported / fixed as evenly as possible on all sides, while the first substrate can be freely pressed out by the punch. The test speed is 10 mm / min. The force recorded is the force at which the bond fails and the first substrate is detached from the second substrate. The force is applied to the bonded area (282 mm 2 ), so that push-out strengths are given in units of N / mm 2 The test climate is 23 °C and 50% relative humidity, i.e. the results are averages from three individual tests.

[0096] C. Conducting the adhesive experiments:

[0097] The manufactured adhesive elements were tested in the push-out test, both untreated and after laser treatment of the release layer. A FAYb (fiber laser) laser, emitting at a separation wavelength of 1.06 prn, was used for the release process. The laser is manufactured by SUNX / Panasonic Electric Works. The laser is marketed under the designation LP-V10. The laser is characterized by the parameters summarized in Table 3.

[0098] Table 3 - Laser parameters

[0099] The optimal separation of the release layer between the two adhesive layers was adjusted using a test matrix of laser power, frequency, and deflection speed (laser energy: 3.0% - corresponding to 0.36 W; scan speed: 3000 mm / sec; laser pulse duration: 500 gs; line width: 0.1 mm; line spacing: 0.1 mm). The laser treatment is performed from the side of the first substrate.

[0100] As a result of the energy input into the separation material by means of the laser radiation, the connection between a first part of the multi-layer adhesive element comprising the first adhesive layer and a second part of the multi-layer adhesive element comprising the second adhesive layer is weakened.

[0101] The results of the push-out test obtained before and after laser treatment are summarized in Table 4. Table 4 - Measured values

[0102] The examples show that, surprisingly, the push-out strength after laser activation does not increase due to the increased haze within the defined range, meaning that the process can be used successfully despite laser beam scattering. Surprisingly, it was even found that the push-out strength after laser activation in the examples according to the invention was even lower than that of the comparative examples.

[0103] In a further experiment, an attempt was made to find an explanation for this surprising finding. For this test, only the black metallized PET film was treated with the laser from the side of the PET film using the above-mentioned parameters, with only a portion of the PET film being covered with an adhesive layer of Example E2. Increased sublimation of the release layer was observed in the area covered with the adhesive layer according to the invention. This increased sublimation of the release layer evidently leads to a more pronounced reduction in strength than without scattering of the laser beam. The scattering effect therefore surprisingly even has a positive effect.

Claims

Claims 1. Multi-layer adhesive element, comprising: i) a first adhesive layer comprising a first adhesive, ii) a second adhesive layer comprising a second adhesive, and iii) a release layer arranged between the first adhesive layer and the second adhesive layer, comprising a release material, wherein the release material has an absorption for laser radiation of at least one release wavelength AT, so that an energy input into the release material by means of laser radiation of the release wavelength AT is possible, wherein the release wavelength AT is in the range from 100 to 11000 nm, wherein the release layer is designed such thatthat the bond between a first part of the multilayer adhesive element comprising the first adhesive layer and a second part of the multilayer adhesive element comprising the second adhesive layer can be weakened or eliminated at least in sections as a result of an energy input into the release material by means of laser radiation of the release wavelength AT, wherein the first adhesive layer and / or the second adhesive layer is a translucent adhesive layer, wherein the translucent adhesive layer is at least partially permeable to laser radiation of the release wavelength AT, wherein the translucent adhesive layer has a haze value measured according to ASTM D 1003-21 (2021) in the range of 10 to 95%, and wherein the multilayer adhesive element is designed such that laser radiation of the release wavelength AT can impinge on the release layer through a translucent adhesive layer.

2. Multi-layer adhesive element according to claim 1, wherein the first adhesive layer and / or the second adhesive layer is a pressure-sensitive adhesive layer.

3. Multi-layer adhesive element according to one of claims 1 or 2, wherein one or all of the translucent adhesive layers each have a haze value measured according to ASTM D 1003-21 (2021) in the range of 15 to 80%.

4. Multilayer adhesive element according to one of claims 1 to 3, wherein the adhesive compositions of the translucent adhesive layers comprise one or more particulate fillers in a combined mass fraction in the range of 1 to 15%, based on the mass of the respective adhesive compositions.

5. Multilayer adhesive element according to claim 4, wherein the one or more particulate fillers are selected from the group consisting of fillers transparent or translucent to visible light, whose refractive index, measured according to ISO 489:2022-3, differs from the refractive index of the surrounding adhesive matrix.

6. Multilayer adhesive element according to one of claims 4 or 5, wherein at least one of the particulate fillers is selected from the group consisting of polymeric filler particles, filler particles of microcrystalline wax, solid polymer spheres, hollow polymer spheres and amorphous silicon dioxide.

7. Multi-layer adhesive element according to one of claims 1 to 6, wherein one or all, preferably all, of the translucent adhesive layers is a foamed adhesive layer.

8. Multi-layer adhesive element according to one of claims 1 to 7, wherein the release layer has a transmission of 30% or less for laser radiation of the release wavelength AT incident orthogonally on the surface of the release layer and passing through the release layer along the direction of the smallest thickness extension, measured according to ASTM D 1003-21 (2021).

9. Multilayer adhesive element according to one of claims 1 to 8, wherein the release layer has an average thickness in the range of 30 nm to 100 pm.

10. Multilayer adhesive element according to one of claims 1 to 9, wherein the separation wavelength AT is in the range of 400 to 5000 nm.

11. Multi-layer adhesive element according to one of claims 1 to 10, wherein one or all of the translucent adhesive layers each have a transmission of 60% or more for radiation of the separation wavelength AT incident orthogonally on the surface of the translucent adhesive layer and passing through the translucent adhesive layer along the direction of the smallest thickness extension, measured according to ASTM D1003-21 (2021) without correction of Interface reflection losses.

12. Multi-layer adhesive element according to one of claims 1 to 11, wherein the release layer has a reflective surface at least on the surface facing in the direction of a translucent adhesive layer, which is designed to have a reflectometer value according to EN ISO 2813:1999 (standard illuminant C) at a measuring angle of 60° of more than 10.

13. Multi-layer adhesive element according to one of claims 1 to 12, wherein the multi-layer adhesive element additionally comprises: iv) one or more carrier layers arranged between the release layer and the first adhesive layer or between the release layer and the second adhesive layer.

14. Adhesive composite, comprising: I) a first substrate, II) a second substrate, and III) a multi-layer adhesive element according to the invention arranged between the first substrate and the second substrate, wherein the multi-layer adhesive element is integrally bonded to the first substrate via the first adhesive layer and to the second substrate via the second adhesive layer, and wherein the first and / or the second substrate, at least the substrate bonded to a translucent adhesive layer, is at least partially transparent to laser radiation of the separation wavelength AT.

15. A method for releasing an adhesive bond according to claim 14, comprising Method steps: a) producing or providing an adhesive composite according to the invention, b) irradiating the release layer of the multilayer adhesive element with laser radiation of the release wavelength AT through a substrate which is at least partially transparent to laser radiation of the release wavelength AT, and through a translucent adhesive layer, to introduce an energy input into the release material, so that the connection between the first part of the multilayer adhesive element comprising the first adhesive layer and the second part of the multilayer adhesive element comprising the second adhesive layer is at least partially weakened or eliminated as a result of the energy input, and c) releasing the connection created by the multilayer adhesive element between the first substrate and the second substrate of the adhesive composite,wherein the first part of the multilayer adhesive element remains on the first substrate and wherein the second part of the multilayer adhesive element remains on the second substrate.,

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