Cationically curable adhesive with holding-strength indication

Abstract

The invention relates to a curable adhesive comprising: a) one or more (co)polymers, b) one or more polymerizable epoxide compounds, c) one or more cationic initiators, and d) one or more pH colour indicators with at least one pH-dependent colour change, where the mass ratio of the combined mass of the cationic initiators to the combined mass of the pH colour indicators in the curable adhesive is 5:1 or more, and where the composition of the curable adhesive is such that a pH-dependent colour change of the one or more pH colour indicators takes place during the cationic curing of the curable adhesive at a moment at which the bond strength of the curing adhesive is in the range from 20% to 80% of the maximum bond strength achievable through the cationic curing.

Description

[0001] Description

[0002] Cationically curable adhesive with indication of holding strength

[0003] The invention relates to a curable adhesive and a reactive adhesive tape comprising a corresponding curable adhesive and an efficient method for bonding two or more components prior to further processing steps. Also disclosed are the use of corresponding curable adhesives and reactive adhesive tapes for bonding two or more components, as well as a method for producing corresponding curable adhesives.

[0004] Joining separate elements is one of the central processes in manufacturing technology. Alongside other methods such as welding and soldering, bonding, i.e. joining using an adhesive, is becoming increasingly important. Adhesive tapes offer an alternative to the use of formless adhesives, which are applied from a tube, for example. Pressure-sensitive adhesive tapes are particularly familiar from everyday use. These tapes use a pressure-sensitive adhesive that provides the adhesive effect and is permanently tacky and adhesive under normal ambient conditions. Such pressure-sensitive adhesive tapes can be applied to a substrate using pressure and remain there, but can later be removed more or less residue-free.

[0005] However, another type of adhesive tape is also of great importance, particularly for use in industrial manufacturing. These adhesive tapes, which are sometimes also referred to as reactive adhesive tapes, use a curable adhesive. Such 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 polymerization in the curable adhesive, thereby increasing the degree of crosslinking. This changes the mechanical properties of the now cured adhesive, with particular increases in viscosity, surface hardness, and strength.

[0006] Curable adhesives are known in the art and can have very different chemical compositions. These curable adhesives have in common that the crosslinking reaction can be triggered by external factors, for example, by the supply of energy, in particular by temperature, plasma, or radiation curing, and / or contact with a polymerization-promoting substance, as is the case, for example, with moisture-curing adhesives. Exemplary adhesives are disclosed, for example, in DE 102015222028 A1, EP 3091059 A1, EP 3126402 B1, EP 2768919 B1, DE 102018203894 A1, WO 2017174303 A1, JP 461 1463 B2, and US 4661542 A.

[0007] Despite the well-known advantages of curable adhesives, they also exhibit processing properties that are perceived as disadvantageous. Since curable adhesives typically only achieve the required bond strength during the curing process, it is often necessary to wait for the curable adhesive to cure sufficiently before further processing of the resulting component composites. This is particularly true in cases where the resulting component composite is subjected to severe mechanical stress during further processing, which can lead to bond failure if the curing is insufficient.

[0008] The problem described above is often aggravated in practice by the fact that the curing speed of curable adhesives is not always constant, but can depend on numerous factors. These factors include local variations in the chemical composition or environmental parameters, such as the

[0009] Ambient temperature and humidity, and in particular, inconsistent activation of the curing process, for example, due to varying energy input during radiation-based curing. However, since the bond strength already achieved is not easily determined even by experienced workers with conventional curable adhesives, these uncertainties must in many cases be taken into account by extending the period before further processing of a component composite joined with the curable adhesive within a standardized production process to ensure that bond failure can be reliably ruled out even in the event of slowed curing.

[0010] Due to the aspects described above, the bonding of components with a curable adhesive can often become a time-limiting step in corresponding manufacturing processes, which adversely affects time and cost efficiency.

[0011] Against this background, there is continued interest in the field of adhesive technology in improving curable adhesives and their use. In this respect, however, many development efforts are aimed at optimizing the basic curing speed and / or reducing its variance, which is usually achieved by changing the chemical composition of the curable adhesives. However, since the required physico-chemical properties of such adhesives are usually determined primarily by the respective application requirements, which essentially relate to the cured state, this approach has natural limits, since even the best curing behavior of a curable adhesive offers little advantage if the cured bond produced with it does not meet the specified requirements. The use of color indicators has been proposed in the prior art, for example in EP 3105276 B1.However, in the prior art, this use of color indicators was primarily intended to indicate the activation of the curable adhesives. Although this may offer application-related advantages, for example, for the easy identification of defective, non-activated adhesives, it does not alleviate the problem described above.

[0012] WO 20201 19898 A1 discloses curable adhesives to which a dye, in particular azo dyes or Sudan Blue, is added. As a result, they not only exhibit a color change after activation of the adhesive, but also a renewed color change after reaching the final bond strength, which in the case of WO 20201 19898 A1 is after 24 hours. This makes it possible to reduce the time buffer described above to ensure complete curing despite the variance in curing speed, since the achievement of the final bond strength can be visually detected by the workers involved.

[0013] However, in the opinion of the inventors of the present invention, the technology disclosed in WO 20201 19898 A1 is not optimally suited to the application-related conditions and requirements of modern manufacturing processes. In particular, the achievable increase in time and cost efficiency of the process is considered to be in need of improvement.

[0014] The primary object of the present invention was to eliminate or at least reduce the above-described disadvantages of the prior art.

[0015] In particular, it was the object of the present invention to provide a curable adhesive with which the bonding of components can be carried out with increased time and cost efficiency.

[0016] In addition, it was an object of the present invention that the curable adhesive should enable reliable bonding of the components and high process reliability during use, especially when using untrained workers.

[0017] It was a further object of the present invention that the curable adhesive to be specified should exhibit the greatest possible flexibility with regard to its chemical composition and thus with regard to the physicochemical properties achievable during curing, in particular with regard to the achievable adhesive strength. In particular, the curable adhesive to be specified should also be preparable predominantly from components used for conventional curable adhesives. Furthermore, the curable adhesive should desirably be capable of being implemented both as a liquid and as a solid, pressure-sensitive adhesive.

[0018] It was a supplementary object of the present invention to provide an advantageous reactive adhesive tape or

[0019] To provide pressure-sensitive adhesive tape. It was an additional object of the present invention to provide an efficient method for bonding two or more components prior to performing further processing steps.

[0020] In addition, it was a secondary object of the present invention to provide a use of the curable adhesives or reactive adhesive tapes to be specified for bonding two or more components and a process for producing corresponding curable adhesives.

[0021] The inventors of the present invention have now found that the objects described above can surprisingly be achieved if a color indicator is used in epoxy-based curable adhesives which cure via a cationic initiator, the selection of which is matched to the other composition of the curable adhesive such that the curable adhesive shows a color change when the bond strength is in the range of 20 to 80% of the maximum bond strength achievable by cationic curing, as defined in the claims.

[0022] The invention is based on the surprising finding that, in the opinion of the inventors, the solution found in WO 20201 19898 A1, which, in the opinion of the inventors, will certainly also have its fields of application, tends to place the wrong emphasis on the application-related framework conditions and needs of modern manufacturing processes, which ultimately prevents a particularly advantageous increase in the time and cost efficiency of the process control, as is possible within the scope of the present invention.

[0023] Without wishing to be bound by this theory, it is assumed that the concentration of the cationic initiator or the concentration of the curing-active species generated from it during activation, for example, strong acids, in the curable adhesive, as well as indirectly also the physicochemical parameters influenced by it, for example, the pH value, exhibit a non-linear progression after activation. Depending on the observed measured variable, we will at least pass through a minimum or maximum before the measured variable returns to the initial level with regard to most parameters upon reaching the maximum achievable curing and thus the maximum achievable bond strength.

[0024] Because WO 20201 19898 A1, in line with the previously known prior art, intends to indicate the immediate onset of activation, the color change of the dye used therein must occur at a time when the curable adhesive still corresponds almost to its initial state and the relevant parameter, e.g., the pH value, differs only slightly. However, this means, conversely, that the reverse color change back to the original color inevitably occurs when the relevant parameter is close to the initial value again, which, however, is only achieved when the maximum possible curing is reached at the earliest. This indication of complete curing, for example, when using Sudan Blue after 24 hours, is deliberately intended in WO 20201 19898 A1.

[0025] However, the inventors have found that the state of maximum achievable curing, as indicated in WO 20201 19898 A1, for example, by a color change from Sudan Blue II after 24 hours, is not the absolutely necessary state for most manufacturing processes, particularly in light of the often non-linear increase in bond strength, with a view to optimized time and cost efficiency. Rather, in most cases, reliable further processing is possible even when the curing adhesive is still significantly far from the state of maximum achievable bond strength. Surprisingly, this applies in many cases even when further processing involves noticeable mechanical stress on the component assembly.

[0026] Accordingly, the inventors propose that, contrary to the teaching of WO 20201 19898 A1, the curable adhesive should be adjusted such that a color change occurs at a time when the adhesive is in a medium state of cure, which can be conveniently adapted by the person skilled in the art to the specific requirements of the subsequent processing steps, i.e., for example, the expected mechanical stress on the component assembly. This enables much faster cycle times in corresponding manufacturing processes, since the workers employed and / or sensors directed at the color change receive the visual information that the manufactured component assembly is ready for further processing at precisely the optimal time.

[0027] 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.

[0028] 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 tapes, uses, and methods emerge from the features of preferred curable adhesives.

[0029] To the extent that both specific amounts or proportions of an element and preferred embodiments of the element are disclosed below for an element, for example for the (co)polymers or the polymerizable epoxy compounds, the specific amounts or proportions of the preferably configured elements are also disclosed in particular. Furthermore, it is disclosed that, with the corresponding specific total amounts or total proportions of the elements, at least some of the elements can be preferably configured and, in particular, that preferably configured elements can in turn be present in the specific amounts or proportions within the specific total amounts or total proportions.

[0030] The invention relates to a curable adhesive comprising: a) one or more (co)polymers, b) one or more polymerizable epoxy compounds, c) one or more cationic initiators, and d) one or more pH color indicators with at least one pH-dependent color change, wherein the mass ratio of the combined mass of the cationic initiators to the combined mass of the pH color indicators in the curable adhesive is 5:1 or more, and wherein the curable adhesive is composed such that a pH-dependent color change of the one or more pH color indicators occurs during the cationic curing of the curable adhesive at a time at which the bond strength of the curing adhesive is in the range of 20 to 80% of the maximum bond strength achievable by cationic curing.

[0031] Curable adhesive compositions are, as described above, comprehensively known to the person skilled in the art from the prior art, wherein the individual components specified above are also known in isolation to the person skilled in the art and are commercially available in various variations from numerous different suppliers, wherein preferred and exemplary representatives for the individual components are also disclosed below.

[0032] These components defined above are used as "one or more" in accordance with the understanding of one skilled in the art. The term "one or more" refers, as is customary in the industry, to the chemical nature of the respective compounds and not to their quantity. For example, the curable adhesive may comprise exclusively epoxycyclohexylmethyl 3',4'-epoxycyclohexanecarboxylate as a polymerizable epoxy compound, which would mean that the curable adhesive comprises a plurality of the respective molecules.

[0033] The curable adhesive composition according to the invention is curable. Due to its ability to cure, the curable adhesive composition can function as a structural adhesive after curing. According to DIN EN 923:2016-03, structural adhesives are demonstrably suitable for the production of load-bearing structures in which the adhesive bond 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 bonds subject to high chemical and physical stresses, which, when cured, contribute to the strengthening of the adhesive tapes.

[0034] Central to the curable adhesive according to the invention is that the curable adhesive is composed such that a pH-dependent color change of the one or more pH color indicators occurs during the cationic curing of the curable adhesive at a time when the bond strength of the curing adhesive is in the range of 20 to 80% of the maximum bond strength achievable by the cationic curing, which expresses that the curable adhesive is designed or configured accordingly.

[0035] In practice, the correlation between the time of color change and the bond strength at that time is found to be very slightly dependent on the initiation conditions, even at low radiation intensities, so that, in the inventors' estimation, these do not necessarily need to be specified. However, the person skilled in the art will readily understand that the above design of the curable adhesive should expediently be tailored to a serious, practice-relevant activation. In other words, in the majority of cases, it will be a curable adhesive according to the invention, wherein the curable adhesive is composed such that, after activation with a radiation dose of 1000 mJ / cm 2 or more, preferably 2000 mJ / cm 2or more, a pH-dependent color change of the one or more pH color indicators occurs during the cationic curing of the curable adhesive at a time when the bond strength of the curing adhesive is in the range of 20 to 80% of the maximum bond strength achievable by cationic curing.

[0036] The inventors assume that, in case of doubt, fine-tuning will be carried out with a predetermined radiation dose corresponding to the radiation dose specified by the respective manufacturer of the curable adhesives for their subsequent use. Given the observed low dependence on the initiation conditions, however, in case of doubt, a typical radiation dose can also simply be specified. In this case, this is a curable adhesive according to the invention, wherein the curable adhesive is composed such that, after activation with a radiation dose of 3000 mJ / cm 2a pH-dependent color change of the one or more pH color indicators occurs during the cationic curing of the curable adhesive at a time when the bond strength of the curing adhesive is in the range of 20 to 80% of the maximum bond strength achievable by cationic curing. In other words, the person skilled in the art will understand that this is a curable adhesive comprising the constituents described above, the curable adhesive being composed such that a pH-dependent color change of the one or more pH color indicators occurs during the cationic curing of the curable adhesive at a time when the bond strength of the curing adhesive is in the range of 20 to 80% of the maximum bond strength achievable by cationic curing.

[0037] In other words, it is a curable adhesive which comprises the components described above, wherein the adhesive can be cured by cationic curing such that the adhesive shows a pH-dependent color change of the one or more pH color indicators when the curing adhesive has a bond strength which is in the range of 20 to 80% of the maximum bond strength achievable by cationic curing.

[0038] In other words, it is also a curable adhesive which comprises the components described above, wherein the adhesive, during cationic curing, at the time of a pH-dependent color change of the one or more pH color indicators, has a bond strength which is in the range of 20 to 80% of the maximum bond strength achievable by cationic curing.

[0039] The person skilled in the art in the field of chemistry, in particular in the field of adhesive technology, is able to prepare the curable adhesive in the light of the teachings imparted here by adapting the composition as defined above.

[0040] A person skilled in the art will understand that a pH-dependent color change must occur within the specified relative bond strength range, which is brought about by the pH color indicator, but also that not all pH-dependent color changes that the pH color indicator undergoes during curing need to lie within the relative bond strength range. This is due in particular to the fact that a pH color indicator with one change point will regularly experience two color changes during curing, namely when the change point is exceeded in each of the two possible directions, i.e. from high pH values ​​to low pH values ​​and vice versa. In the inventors' opinion, it will be the second color change in most cases that will indicate that the desired bond strength has been achieved. In practice, this second color change is in most cases the change from low to higher pH values.In most cases, this is therefore a curable adhesive according to the invention, wherein the curable adhesive is composed such that the pH-dependent color change of the one or more pH color indicators is caused by an increase in the pH. According to the expert's understanding, in particular the initial pH value in the non-activated curable adhesive, but also the temporal development of the pH value, depends significantly on the components used in the adhesive, for example the type and concentration of the cationic initiators. Therefore, the selection of the pH color indicators, which will significantly determine the setting of the curable adhesive in practice, is difficult to generalize.

[0041] In light of the above teaching, however, the person skilled in the art can produce the curable adhesive compositions within the scope of normal expert experiments and adapt them to the respective needs, in which case he can expediently start from the exemplary embodiments disclosed below.

[0042] In light of the teaching of the present invention, the person skilled in the art can, for example, proceed by first preparing a curable starting adhesive comprising: aal ) one or more (co)polymers, bb1 ) one or more polymerizable epoxy compounds, and cc1 ) one or more cationic initiators, in which the pH color indicators are thus still missing, but which is optimized with regard to the physicochemical properties, in particular after curing, to the respective requirements which arise for the person skilled in the art.

[0043] By means of tests, the person skilled in the art, who knows what he is aiming for in view of the invention, can determine the bond strength for this curable starting adhesive mass which is sufficient for the desired further processing of the component composite.

[0044] Especially for the initial rough adjustment, it is advisable for the expert to initially only record the time after which the desired bond strength is achieved, which, for example, allows further processing without damage to the adhesive bond. This advantageously eliminates the need to quantify the absolute values ​​of the desired bond strength or the maximum achievable bond strength at this point.

[0045] The skilled person can now identify suitable pH color indicators required for implementing the invention in various ways. For example, the skilled person can introduce various available pH color indicators into the starting adhesive and experimentally test which of these pH color indicators exhibit a color change near the time estimated by the skilled person, or ideally shortly after the estimated time. Alternatively, the skilled person can also determine the pH value of the curing adhesive, which they consider to be optimally cured, and select a suitable pH color indicator based on tabulated values ​​that tabulate the respective transition points for pH color indicators.

[0046] Starting with the system that combines the starting adhesive with the pH color indicator identified as suitable, the skilled person can then proceed to fine-tuning, whereby the skilled person can optimize the concentrations of the individual components of the curable adhesive in the usual way through routine experiments. For example, in an iterative optimization process, the skilled person will primarily compensate for or take into account the addition of the pH color indicator, which can at least potentially change the properties of the starting adhesive, at least if it is to be used in larger quantities. In particular, some basic pH color indicators can also influence the pH value and influence cationic activation, so that the adhesive system can be gradually adjusted in a skilled manner until its properties are sufficient for the intended application.

[0047] The skilled person can then determine the bond strength at the time of the color change and compare it with the maximum bond strength achievable by cationic curing to verify that it lies within the interval to be set according to the invention. The skilled person can easily determine the maximum achievable bond strength as the bond strength that results as a resulting limit value after the initiation of cationic curing, within the measurement inaccuracy of the determination method. Since, in accordance with the skilled person's understanding, the bond strength generally essentially reaches a plateau value and the skilled person, based on their experience, will usually be able to reasonably estimate when curing is essentially complete, time-dependent measurements of the bond strength can usually be dispensed with in practice.In many cases, it is sufficient to determine the bond strength after a period of time during which the expert is certain that curing should be complete. The constancy of the value can be further confirmed by subsequent backup measurements after, for example, a few additional hours. A more precise determination of the maximum achievable bond strength is unnecessary for the vast majority of cases anyway and can be most useful if the relative bond strength during the color change is very close to the limit values ​​of the range defined above.

[0048] The bond strength to be determined within the scope of the present invention is an established parameter for those skilled in the field of adhesive technology. This parameter can in principle be measured in various test setups and under various conditions. Since within the scope of the present invention a relative bond strength is defined based on a final value, the person skilled in the art can in principle use any method to determine the bond strength, as long as both values ​​are determined using the same method, which will, however, be self-evident to the person skilled in the art. Within the scope of the present invention, in cases of doubt, particularly when the relative bond strength is close to the limits of the defined range, the bond strength is determined in the industry standard manner in a dynamic tensile shear test according to DIN EN 1465:2009-07 at 23 °C and 50% relative humidity for a test speed of 1 mm / min.

[0049] In the inventors' opinion, the color change should not occur too early, since below 20% of the maximum achievable bond strength, it is unlikely that the resulting component assemblies will exhibit sufficient strength in subsequent processing steps. However, if the color change only occurs at more than 80% of the maximum achievable bond strength, the inventors' opinion is that the achievable benefits of improved cycle timing will be too small. Thus, in curable adhesives according to the invention, there is a trade-off between sufficient bond strength and rapid further processing of the joined component assemblies. The inventors therefore propose that this trade-off is advantageously resolved, especially at medium relative bond strengths.Against this background, a curable adhesive according to the invention is preferred, wherein the curable adhesive is composed such that a pH-dependent color change of the one or more pH color indicators occurs during the cationic curing of the curable adhesive at a time at which the bond strength of the curing adhesive is in the range from 25 to 75%, preferably in the range from 30 to 70%, particularly preferably in the range from 35 to 65%, of the maximum bond strength achievable by the cationic curing.

[0050] In absolute values, in the opinion of the inventors, a curable adhesive according to the invention is preferred for most practical applications, the curable adhesive being composed such that a pH-dependent color change of the one or more pH color indicators occurs during the cationic curing of the curable adhesive at a time when the bond strength of the curing adhesive is in the range from 1.5 to 4.5 MPa, preferably in the range from 2.0 to 4.0 MPa, particularly preferably in the range from 2.5 to 3.5 MPa, measured according to DIN EN 1465:2009-07 at 23 °C and 50% relative humidity for a test speed of 1 mm / min.

[0051] The absolute value ranges specified above can be specified by the inventors based on their comprehensive knowledge of the requirements placed on curable adhesive compositions, particularly in the automotive industry. The person skilled in the art will understand that the invention also relates to a curable adhesive composition comprising: a) one or more (co)polymers, b) one or more polymerizable epoxy compounds, c) one or more cationic initiators, and d) one or more pH color indicators with at least one pH-dependent color change, wherein the mass ratio of the combined mass of the cationic initiators to the combined mass of the pH color indicators in the curable adhesive composition is 5:1 or more, and wherein the curable adhesive composition is composed such thatthat a pH-dependent color change of the one or more pH color indicators occurs during the cationic curing of the curable adhesive composition at a time when the bond strength of the curing adhesive composition is in the range of 1.5 to 4.5 MPa, preferably in the range of 2.0 to 4.0 MPa, particularly preferably in the range of 2.5 to 3.5 MPa, measured according to DIN EN 1465:2009-07 at 23 °C and 50% relative humidity for a test speed of 1 mm / min.,

[0052] Given the typical maximum achievable bond strengths, the disclosed curable adhesive composition will, in the vast majority of cases, be a curable adhesive according to the invention. Furthermore, in the inventors' estimation, the curable adhesive composition is also extremely advantageous in itself, with the above statements regarding the advantages and production, as well as the following statements regarding preferred curable adhesives according to the invention, each correspondingly applicable. The disclosed curable adhesive compositions can also be used, in particular, in corresponding adhesive tapes, processes, and applications.

[0053] The curable adhesives of the invention can, in principle, advantageously also be provided as liquid systems. However, for later use in the final application, it is advantageous for handling properties if the curable adhesive has intrinsic pressure-sensitive adhesive strength and can thus be classified as a pressure-sensitive adhesive. This pressure-sensitive adhesive strength allows reliable and secure application of the reactive adhesive tapes to the substrate prior to curing of the curable adhesive. Accordingly, preference is given to a curable adhesive of the invention, wherein the curable adhesive is a pressure-sensitive adhesive. To achieve pressure-sensitive adhesive strength, the inventors believe it is expedient to adjust the combined mass fraction of the (co)polymers in the curable adhesive to 25% or more, preferably 30% or more, particularly preferably 35% or more.

[0054] 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 can usually be removed from the substrate after use without leaving any residue and are generally permanently tacky even at room temperature. This means that they exhibit a certain viscosity and tackiness, allowing them to wet the surface of a substrate even under light pressure. The tackiness of a pressure-sensitive adhesive tape results from the fact that a pressure-sensitive adhesive is used as the adhesive.Without wishing to be bound by this theory, it is often assumed that a pressure-sensitive adhesive can be viewed as an extremely viscous liquid with an elastic component, which consequently exhibits characteristic viscoelastic properties that lead to the permanent inherent tack and pressure-sensitive adhesive capacity described above. It is assumed that with corresponding pressure-sensitive adhesives, mechanical deformation leads to both viscous flow processes and the build-up of elastic restoring forces. The partial viscous flow serves to achieve adhesion, while the partial elastic restoring forces are necessary in particular 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 characterise the degree of elastic and viscous components, the storage modulus (G') and the loss modulus (G") are usually used, which can be determined by means of dynamic mechanical analysis (DMA), for example using a rheometer, as disclosed, for example, in WO 2015 / 189323. In the context of the present invention, an adhesive is preferably understood to be pressure-sensitive and thus 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.

[0055] The components contained in the curable adhesive composition of the invention are explained in more detail below. In this respect, the inventors have succeeded in identifying particularly preferred configurations and mass fractions for the individual components, with which high-performance curable adhesive compositions of the invention can be obtained. In accordance with industry practice, the mass fractions are stated as the combined mass fractions of the one or more components, thereby expressing that the mass fraction of the correspondingly configured components taken together meets the corresponding criteria. In the absence of other information, the mass of the curable adhesive composition is the reference system.

[0056] Those skilled in the art will understand that the (co)polymers typically play the role of film formers, which becomes particularly important when, for example, pressure-sensitive adhesives are to be obtained. In other words, the use of the (co)polymers commonly used in the field of adhesive technology thus constitutes a curable adhesive, with the one or more (co)polymers being selected from the group consisting of film-forming (co)polymers.

[0057] Additionally or alternatively, preference is given to a curable adhesive composition according to the invention, wherein the one or more (co)polymers are selected from the group consisting of poly(meth)acrylates, polyurethanes, polyvinyl acetals, such as polyvinyl butyral, polysiloxanes, synthetic rubbers, polyesters, phenoxy polymers, polyvinyl alcohols, polyvinyl alcohol copolymers, and alkene-vinyl acetate copolymers, preferably selected from the group consisting of poly(meth)acrylates, phenoxy polymers, polyvinyl alcohols, polyvinyl alcohol copolymers, polyvinyl acetals, such as polyvinyl butyral, and ethylene-vinyl acetate copolymers (EVA or EVAC, poly(ethylene-co-vinyl acetate)), in particular selected from the group consisting of poly(meth)acrylates, phenoxy polymers and ethylene-vinyl acetate copolymers.

[0058] Additionally or alternatively, block copolymers, e.g., (meth)acrylate block copolymers, can also be used as (co)polymers. Examples of such copolymers are disclosed, for example, in the documents US 201 1003947 A1, US 20080200589 A1, US 2007078236 A1, US 2007078236 A1, US 2012196952 A1, US 2016032157 A1, US 2008146747 A1, and US 2016230054 A1.

[0059] The number average molecular masses M n The molecular weights of the (co)polymers are preferably in a range from 50,000 to 10,000,000 g / mol, particularly preferably in a range from 100,000 to 5,000,000 g / mol, very particularly preferably in a range from 150,000 to 2,000,000 g / mol. The data for the number-average molar mass M nrefer to the determination by gel permeation chromatography (GPC). The determination is carried out on 100 μl of a clear-filtered sample (sample concentration 4 g / l). Tetrahydrofuran with 0.1 vol.% trifluoroacetic acid is used as the eluent. The measurement is carried out at 25 °C. A PSS-SDV column, 5 μm, 10 3 Ä, 8.0 mm * 50 mm (information here and below in the order: type, particle size, porosity, inner diameter * length; 1 Ä = 10 -10 m). For separation, a combination of columns of type PSS-SDV, 5 pm, 10 3 Ä and 10 5 Ä and 10 6 Ä with a diameter of 8.0 mm x 300 mm each (columns from Polymer Standards Service; detection using a Shodex RI71 differential refractometer). The flow rate is 1.0 ml per minute. Calibration is performed for polyacrylates against PMMA standards (polymethyl methacrylate calibration) and for other materials (resins, elastomers) against PS standards (polystyrene calibration).

[0060] Regardless of the specific selection of the (co)polymers, preference is given to a curable adhesive according to the invention, wherein the combined mass fraction of the (co)polymers in the curable adhesive is in the range from 1 to 70%, preferably in the range from 2 to 60%, particularly preferably in the range from 5 to 50%, very particularly preferably in the range from 10 to 40%, based on the mass of the curable adhesive. As explained above, to obtain a pressure-sensitive, curable adhesive, it is expedient to set the combined mass fraction of the (co)polymers in the curable adhesive to 25% or more, preferably 30% or more, particularly preferably 35% or more, very particularly preferably in a range from 25 to 40%.

[0061] The curable adhesive composition of the invention comprises, in addition to the (co)polymers, at least one polymerizable epoxy compound and optionally further polymerizable compounds. These compounds together form the part of the curable adhesive composition frequently referred to by those skilled in the art as the reactive resin. The term "polymerizable" refers, in accordance with the expert's understanding, to the ability of these compounds to enter into a polymerization reaction, optionally after suitable activation. In the case of polymerizable epoxy compounds, the polymerizability is enabled, for example, by the epoxy groups.

[0062] According to the understanding of those skilled in the art, epoxy compounds are those compounds that carry at least one oxirane group. They can be aromatic or aliphatic, especially cycloaliphatic, in nature. Polymerizable epoxy compounds can include both monomeric and oligomeric or polymeric epoxy compounds. Polymerizable epoxy compounds frequently have, on average, at least two epoxy groups per molecule, preferably more than two epoxy groups per molecule. In this respect, a curable adhesive composition according to the invention is preferred, wherein the one or more polymerizable epoxy compounds are selected from the group consisting of epoxy compounds with two or more epoxy groups, preferably two epoxy groups.

[0063] The oligomeric or polymeric epoxy compounds mostly include linear polymers with terminal epoxy groups (e.g., a diglycidyl ether of a polyoxyalkylene glycol), polymers with backbone oxirane units (e.g., polybutadiene polyepoxide), and polymers with pendant epoxy groups (e.g., a glycidyl methacrylate polymer or copolymer). The molecular weight of such epoxy compounds can vary from 58 to approximately 100,000 g / mol or more, with the molecular weight being an important variable for adjusting the dynamic viscosity. Exemplary polymerizable epoxy compounds include epoxycyclohexanecarboxylates, such as 4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate, 3,4-epoxy-2-methylcyclohexylmethyl-3,4-epoxy-2-methylcyclohexanecarboxylate, and bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate. Further examples of polymerizable epoxy compounds are disclosed, for example, in US Pat. No. 3,117,099.Other polymerizable epoxy compounds particularly useful in the practice of this invention include glycidyl ether monomers, such as those disclosed in US Pat. No. 3,018,262. Examples are the glycidyl ethers of polyhydric phenols obtained by reacting a polyhydric phenol with an excess of chlorohydrin, such as epichlorohydrin (e.g., the diglycidyl ether of 2,2-bis(2,3-epoxypropoxyphenol)propane). In particular, diglycidyl ethers of bisphenols, such as bisphenol A (4,4'-(propane-2,2-diyl)diphenol) and bisphenol F (bis(4-hydroxyphenyl)methane). Such reaction products are commercially available in various molecular weights and aggregate states (e.g., so-called Type 1 to Type 10 BADGE resins). Typical examples of liquid bisphenol A diglycidyl ethers are Epikote 828, DER331 and Epon 828. Typical solid BADGE resins are Araldite GT6071, GT7072, Epon 1001 and DER 662.Further reaction products of phenols with epichlorohydrin are the phenol and cresol novolac resins, such as the Epiclon types or Araldite EPN and ECN types (e.g., ECN1273). According to the inventors' assessment, preference is given to a curable adhesive according to the invention, wherein the one or more polymerizable epoxy compounds are selected from the group consisting of epoxy compounds having at least one cycloaliphatic group, in particular a cyclohexyl group or dicyclopentadienyl group. Additionally or alternatively, preference is given to a curable adhesive according to the invention, wherein the one or more polymerizable epoxy compounds are selected from the group consisting of bisphenol A diglycidyl ethers and bisphenol F diglycidyl ethers, preferably bisphenol A diglycidyl ethers.

[0064] According to the inventors, particularly advantageous curable adhesive compositions can be obtained if two or more different polymerizable epoxy compounds are used, in particular if they differ in their aggregate state at room temperature.Preference is given to a curable adhesive according to the invention, wherein the curable adhesive comprises one or more polymerizable epoxy compounds selected from the group of epoxy compounds which, at 25°C, are solids or highly viscous substances having a dynamic viscosity of 50 Pa s or more, preferably 100 Pa s or more, particularly preferably 150 Pa s or more, and / or wherein the curable adhesive comprises one or more polymerizable epoxy compounds selected from the group of epoxy compounds which, at 25°C, are a liquid having a dynamic viscosity of 40 Pa s or less, preferably 20 Pa s or less, very particularly preferably 10 Pa s or less. In the context of the present invention, the dynamic viscosity is determined according to DIN 53019-1 from 2008; at 25°C, with a shear rate of 1 s. -1 certainly.

[0065] Regardless of the specific selection of the polymerizable epoxy compounds, preference is given to a curable adhesive according to the invention, wherein the combined mass fraction of the polymerizable epoxy compounds in the curable adhesive is in the range from 35 to 95%, preferably in the range from 40 to 90%, particularly preferably in the range from 45 to 85%, very particularly preferably in the range from 50 to 80%, based on the mass of the curable adhesive.

[0066] The curable adhesives of the invention comprise at least one cationic initiator. Such cationic initiators are known to the skilled person based on their general technical knowledge and are frequently used, particularly in the field of epoxy-based reactive adhesives. The skilled person essentially adapts the catalyst system used for curing to the application requirements and the polymerizable epoxy compounds used.

[0067] With regard to subsequent handling properties, the inventors believe it is particularly advantageous to use radiation-crosslinking and / or thermally crosslinking systems, with radiation activation in particular providing significant handling advantages. Preference is given to a curable adhesive according to the invention, wherein the one or more cationic initiators are selected from the group consisting of radiation-activated initiators and thermally activated initiators, preferably selected from the group consisting of radiation-activated initiators, or wherein the curable adhesive is a radiation-curing and / or thermally curing, preferably radiation-curing, adhesive.

[0068] Preferred thermally activated cationic initiators include, for example, latent, thermally activatable cationic initiators (TAGs; so-called "thermal acid generators"). Suitable TAGs are known to those skilled in the art and are commercially available from numerous suppliers. Preference is given to TAGs that comprise a cation selected from the group consisting of 4-substituted benzylanilinium, 4-substituted benzylpyridinium, 4-substituted benzylphosphonium, S-substituted diphenylsulfonium, and substituted arylbenzylsulfonium, preferably selected from the group consisting of 4-substituted benzylanilinium. In principle, the TAG can contain any anion, with weakly coordinating anions such as tetrafluoroborate (BF4), hexafluorophosphate (PFej), hexafluoroantimonate (SbFej), tetrakis(pentafluorophenyl)borate (B(CeFs)4j) and trifluoromethylsulfonate (F3CSO3) being preferred. Trifluoromethylsulfonate (F3CSO3) and hexafluoroantimonate (SbFej) are particularly preferred.

[0069] Advantageous combinations of a TAG consisting of cation and anion are those in which the resulting initiator has an activation temperature in the range from 50 °C to 150 °C, with the person skilled in the art preferably adjusting the activation energy to the respective application requirements, with a range from 80 °C to 120 °C being particularly preferred in many cases in order to be able to carry out the polymerization at a comparatively moderate activation temperature and nevertheless enable clearly defined initiation. The activation temperature of reactive or chemically activatable adhesives in general or of the TAG used is determined calorimetrically using differential scanning calorimetry (DSC) in accordance with DIN EN ISO 1 1357-3:2013-04. For this purpose, approximately 20 mg of the sample is weighed into an aluminum crucible and introduced into the measuring device (device: DSC 204 F1, Netzsch). Two heating curves are then recorded at a heating rate of 10 K / min.The samples are measured in aluminum crucibles with perforated lids and a nitrogen atmosphere. A chemical reaction such as the activation of the TAG is evident as an exothermic peak in the thermogram. The onset temperature is recorded as the activation temperature. This is determined for a peak as the intersection point of the virtual interpolated baseline and the tangent drawn at the inflection point of the peak beginning (according to DIN EN ISO 1 1357-1:2010-03). The reaction enthalpy in J / g is obtained by integrating the curing peak.

[0070] Sulfonium, iodonium, and metallocene-based systems are particularly suitable as initiators for cationic radiation-based, i.e., frequently UV-induced, curing of epoxy compounds. For examples of sulfonium-based cations, see US Pat. No. 6,908,722 B1. Examples of anions that serve as counterions for the above-mentioned cations include tetrafluoroborate, tetraphenylborate, hexafluorophosphate, perchlorate, tetrachloroferrate, hexafluoroarsenate, hexafluoroantimonate, pentafluorohydroxyantimonate, hexachloroantimonate, tetrakispentafluorophenylborate, tetrakis(pentafluoromethylphenyl)borate, bi(trifluoromethylsulfonyl)amide, and tris(trifluoromethylsulfonyl)methide. In addition, especially for iodonium-based initiators, chloride, bromide or iodide are also conceivable as anions, although initiators that are essentially free of chlorine and bromine are preferred.A powerful example of such a system is triphenylsulfonium hexafluoroantimonate. Other suitable initiators are disclosed, for example, in US 3,729,313 A, US 3,741,769 A, US 4,250,053 A, US 4,394,403 A, US 4,231,951 A, US 4,256,828 A, US 4,058,401 A, US 4,138,255 A, and US 2010 / 063221 A1.

[0071] Specific examples of usable sulfonium salts are in particular triarylsulfonium salts, for example triphenylsulfonium hexafluoroarsenate, triphenylsulfonium hexafluoroborate, triphenylsulfonium tetrafluoroborate, triphenylsulfonium tetrakis-(pentafluorobenzyl)borate,

[0072] Methyldiphenylsulfonium tetrafluoroborate, methyldiphenylsulfonium tetrakis (pentafluorobenzyl) borate,

[0073] Dimethylphenylsulfoniumhexafluorophosphat, Triphenylsulfoniumhexafluorophosphat, Triphenyl- sulfoniumhexafluoroantimonat, Diphenylnaphthylsulfoniumhexafluoroarsenat, Tritolylsulfoniumhexa- fluorophosphat, Anisyldiphenylsulfoniumhexafluoroantimonat, 4-Butoxyphenyldiphenylsulfoniumtetraflu- oroborat, 4-Chlorophenyldiphenylsulfoniumhexafluoroantimonat, Tris-(4-phenoxyphenyl)-sulfoniumhexa- fluorophosphat, Di-(4-ethoxyphenyl)-methylsulfoniumhexafluoroarsenat, 4-Acetylphenyldiphenylsulfonium- tetrafluoroborat, 4-Acetylphenyldiphenylsulfoniumtetrakis-(pentafluorobenzyl)-borat, Tris-(4- thiomethoxyphenyl)-sulfoniumhexafluorophosphat, Di-(methoxysulfonylphenyl)-methylsulfonium- hexafluoroantimonat, Di-(methoxynaphthyl)-methylsulfoniumtetrafluoroborat, Di-(methoxynaphthyl)- methylsulfoniumetrakis-(penta-fluorobenzyl)-borat, Di-(carbomethoxyphenyl)-methylsulfoniumhexa- fluorophosphat, (4-Octyloxyphenyl)-diphenylsulfoniumtetrakis-(3,5-bis-trifluoromethylphenyl)-borat,Tris- [4-(4-acetylphenyl)-thiophenyl]-sulfoniumtetrakis-(pentafluorophenyl)-borat, Tris-(dodecyl-phenyl)- sulfoniumtetrakis- (3,5-bis-trifluoromethylphenyl)-borat, 4-Acetamidphenyldiphe-nylsulfonium- tetrafluoroborat, 4-Acetamidphenyldiphenylsulfoniumtetrakis-(pentafluoro-benzyl)-borat, Dimethyl- naphthylsulfoniumhexafluorophosphat, Trifluoromethyldiphenyl-sulfoniumtetrafluoroborat, Trifluoro- methyldiphenylsulfoniumtetrakis-(pentafluorobenzyl)-borat, Phenylmethylbenzylsulfoniumhexafluoro- phosphat, 5-Methylthianthreniumhexa-fluorophosphat, 10-Phenyl-9,9-dimethylthioxanthenium- hexafluorophosphat, 10-Phenyl-9-oxothioxantheniumtetrafluoroborat, 10-Phenyl-9- oxothioxantheniumtetrakis-(pentafluoro-benzyl)-borat, 5-Methyl-10-oxothianthreniumtetrafluoroborat, 5- Methyl-10-oxothianthreniumtetrakis-(pentafluorobenzyi)-borat und 5-Methyl-10,10-dioxothianthrenium- hexafluorophosphat.,

[0074] Specific examples of usable iodonium salts are diphenyliodonium tetrafluoroborate, di-(4-methylphenyl)-iodonium tetrafluoroborate, phenyl-4-methylphenyliodonium tetrafluoroborate, di-(4-chlorophenyl)-iodonium hexafluorophosphate, dinaphthyliodonium tetrafluoroborate, di-(4-trifluoromethyl-phenyl)-iodonium tetrafluoroborate, diphenyliodonium hexafluorophosphate, di-(4-methylphenyl)-iodonium hexafluorophosphate, diphenyliodonium hexafluoroarsenate, di-(4-phenoxyphenyl)-iodonium tetrafluoroborate, phenyl-2-thienyliodonium hexafluorophosphate, 3,5-dimethylpyrazolyl-4-phenyliodonium hexafluorophosphate, diphenyliodonium hexafluoroantimonate, 2,2'-diphenyliodonium tetra- fluoroborate, di-(2,4-dichlorophenyl)-iodonium hexafluorophosphate, di-(4-bromophenyl)-iodonium hexafluorophosphate, di-(4-methoxyphenyl)-iodonium hexafluorophosphate, di-(3-carboxyphenyl)-iodonium hexafluorophosphate, di-(3-methoxycarbonylphenyl)-iodonium hexafluorophosphate, Di-(3-methoxysulfonylphenyl)iodonium hexafluorophosphate,Di-(4-acetamidophenyl)-iodoniumhexafluoro- phosphat, Di-(2-benzothienyl)-iodoniumhexafluorophosphat, Diaryl- iodoniumtristrifluormethylsulfonylmethid wie Diphenyliodoniumhexafluoroantimonat,

[0075] Diaryliodoniumtetrakis-(pentafluorophenyl)-borat wie Diphenyliodoniumtetrakis-(pentafluorophenyl)-borat, [4-(2-Hydroxy-n-tetradesiloxy)-phenyl]-phenyliodoniumhexafluoroantimonat, [4-(2-Hydroxy-n- tetradesiloxy)-phenyl]-phenyliodoniumtrifluorosulfonat, [4-(2-Hydroxy-n-tetradesiloxy)-phenyl]-phenyl- iodoniumhexafluorophosphat, [4-(2-Hydroxy-n-tetradesiloxy)-phenyl]-phenyliodoniumtetrakis-

[0076] (pentafluorophenyl)-borat, Bis-(4-tert-butylphenyl)-iodoniumhexafluoroantimonat, Bis-(4-tert-butylphenyl)- iodoniumhexafluorophosphat, Bis-(4-tert-butylphenyl)-iodoniumtrifluorosulfonat, Bis-(4-tert-butylphenyl)- iodoniumtetrafluoroborat, Bis-(dodecylphenyl)-iodoniumhexafluoroantimonat, Bis-(dodecylphenyl)- iodoniumtetrafluoroborat, Bis-(dodecylphenyl)-iodoniumhexafluorophosphat, Bis-(dodecylphenyl)- iodoniumtrifluoromethylsulfonat, Di-(dodecylphenyl)-iodoniumhexafluoroantimonat, Di-(dodecylphenyl)- iodoniumtriflat, Diphenyliodoniumbisulfat, 4,4'-Dichlorodiphenyliodoniumbisulfat, 4,4'- Dibromodiphenyliodoniumbisulfat, 3,3'-Dinitrodiphenyliodoniumbisulfat, 4,4'- Dimethyldiphenyliodoniumbisulfat, 4,4'-Bis-succinimidodiphenyliodoniumbisulfat, 3- Nitrodiphenyliodoniumbisulfat, 4,4'-Dimethoxydiphenyliodoniumbisulffat, Bis-(dodecylphenyl)- iodoniumtetrakis-(pentafluorophenyl)-borat, (4-Octyloxyphenyl)-phenyliodoniumtetrakis-(3,5-bis-trifluoro-methylphenyl)-borate and (tolylcumyl)-iodonium tetrakis-(pentafluorophenyl)-borate, and ferrocenium salts (see for example EP 0 542 716 B1 ) such as r|5-(2,4-cyclopentadien-1 -yl)-[(1,2,3,4,5,6,9)-(1-methylethyl)-benzene]-iron.,

[0077] Photoinitiators are typically used individually or in combinations of two or more photoinitiators. When using photoinitiators, combinations with so-called sensitizers are very helpful for adapting the activation wavelength of the photoinitiation system to the selected emission spectrum. For this purpose, reference is made to literature familiar to those skilled in the art, such as "Industrial Photoinitiators: A Technical Guide" 2010 by AW Green. Typically, in these cases, the mass fraction of photoinitiators in the curable adhesive is not more than 4% but at least 0.1%, and is preferably in the range of 0.5 to 2%. The mass fraction of sensitizers is usually not more than 3% and is preferably in the range of 0.5 to 2%.

[0078] Largely independent of the selection of the specific cationic initiator, preference is given to a curable adhesive according to the invention, wherein the combined mass fraction of the cationic initiators in the curable adhesive is in the range from 0.1 to 7%, preferably in the range from 0.3 to 5%, particularly preferably in the range from 0.5 to 4%, based on the mass of the curable adhesive.

[0079] The concept of pH color indicators is already well-known to those skilled in the art from school, and the relationships for the color change of pH color indicators depending on the pH value are comprehensively understood. In accordance with the expert's understanding, the term "color change" here means that the absorption behavior of the pH color indicators for electromagnetic radiation of a wavelength in the visible light range, i.e., in the range from 380 to 780 nm, changes. This results in a change in transmission or remission for an adhesive compound mixed with the pH color indicator as a result of the color change.

[0080] Although it is conceivable in principle to use pH color indicators with multiple pH-dependent color changes, such as thymol blue, as color indicators, the inventors believe that for the clearest and most precise design of the curable adhesive, it is advantageous in most cases to use pH color indicators with only one color change. Accordingly, a curable adhesive according to the invention is preferred, wherein the one or more pH color indicators exhibit precisely one pH-dependent color change.

[0081] The inventors have succeeded in identifying pH color indicators with which the above-identified relative bond strengths can be set particularly reliably in most curable adhesives. According to the inventors' assessment, the pH of the transition point should be sufficiently low so that the color change only occurs when the pH of the curable adhesive has already noticeably decreased due to cationic curing and, accordingly, advanced curing has been achieved. For essentially all embodiments, therefore, very particular preference is given to a curable adhesive according to the invention, wherein the one or more pH color indicators exhibit a pH-dependent color change, preferably, in the case of multiple transition points, the highest color change in terms of pH, at a pH of 4 or less, preferably 3 or less, particularly preferably 2.5 or less.

[0082] Particularly preferred, based on the experiments of the inventors, is additionally or alternatively a curable adhesive composition according to the invention, wherein the one or more pH color indicators are selected from the group consisting of cresol red (CAS: 1733-12-6), methyl violet (CAS: 8004-87-3), crystal violet (CAS: 548-62-9), ethyl violet (CAS: 2390-59-2), malachite green (acetate - CAS: 1272-40-6; hydrochloride - CAS: 569-64-2; oxalate - CAS: 2437-29-8), methyl green (CAS: 71 14-03-6), ethyl green (CAS: 14855-76-6), 2-(p-dimethylaminophenylazo)pyridine (CAS: 13103-75-8), metanil yellow (CAS: 587-98-4), 4-phenylazodiphenylamine (CAS: 101-75-7), thymol blue (CAS: 62625-21-2), metacresol purple (CAS: 2303-01-7), 4-[(4-anilinophenyl)-azo]-benzenesulfonic acid sodium salt (CAS: 554-73-4), 4-o-tolylazo-o-toluidine (CAS: 97-56-3), 2,2',2",4,4'-pentamethoxytriphenylmethanol (CAS: 80202-77-3), quinaldine red (CAS: 1 17- 92-0), erythrosine disodium salt (CAS: 16423-68-0), 4,4'-Bis(2-Amino-1 -Naphthylazo)-2,2'-stilbenedisulfonic acid (CAS: 5437-12-7) and methyl yellow (CAS: 60-11-7), preferably selected from the group consisting of cresol red, methyl violet, crystal violet, ethyl violet, malachite green, methyl green, ethyl green, 2-(p-dimethylaminophenylazo)pyridine, metanil yellow, 4-phenylazodiphenylamine, thymol blue, metacresol purple, 4-[(4-anilinophenyl)-azo]-benzenesulfonic acid sodium salt, 4-o-tolylazo-o-toluidine and 2,2',2",4,4'-pentamethoxytriphenylmethanol.

[0083] The inventors have found that, particularly with a view to the targeted adjustment of the color change to the desired relative bond strength, but also with a view to the ranges of relative bond strength accessible via the pH color indicator, it is advantageous not to use azo dyes as pH color indicators and instead to design the curable adhesive as free as possible from azo dyes or with at least only a small proportion of azo dyes. Accordingly, preference is given to a curable adhesive according to the invention wherein the one or more pH color indicators are selected from the group consisting of azo bridge-free pH color indicators. Preference is given to a curable adhesive according to the invention wherein the combined mass fraction of azo dyes in the curable adhesive is 0.005% or less, preferably 0.0001% or less, based on the mass of the curable adhesive.The inventors have found that, particularly with regard to the targeted adjustment of the color change to the desired relative bond strength, but also with regard to the ranges of relative bond strength accessible via the pH color indicator, it is advantageous not to use Sudan Blue II, which is an anthraquinone dye from the group of Sudan dyes, and instead to make the curable adhesive as free from Sudan Blue as possible. Accordingly, a curable adhesive according to the invention is preferred, wherein the curable adhesive is essentially free from Sudan Blue II, wherein the mass fraction of Sudan Blue II in the curable adhesive is preferably 0.0001% or less, based on the mass of the curable adhesive.

[0084] From the finding that Sudan Blue II is less preferred as an anthraquinone dye from the group of Sudan dyes for curable adhesives according to the invention, the inventors deduce that this finding can presumably be transferred, at least in part, to other anthraquinone dyes and / or other Sudan dyes. Thus, a curable adhesive according to the invention tends to be preferred, wherein the combined mass fraction of Sudan dyes in the curable adhesive is 0.005% or less, preferably 0.0001% or less, based on the mass of the curable adhesive. Additionally or alternatively, a curable adhesive according to the invention tends to be preferred, wherein the combined mass fraction of anthraquinone dyes in the curable adhesive is 0.005% or less, preferably 0.0001% or less, based on the mass of the curable adhesive.

[0085] Largely independent of the selection of the specific pH color indicator, preference is given to a curable adhesive according to the invention wherein the combined mass fraction of the pH color indicators in the curable adhesive is in the range from 0.01 to 0.4%, preferably in the range from 0.015 to 0.3%, based on the mass of the curable adhesive. Additionally or alternatively, preference is also given to a curable adhesive according to the invention wherein the combined mass fraction of the pH color indicators in the curable adhesive is 0.25% or less, preferably 0.2% or less, based on the mass of the curable adhesive, and / or wherein the combined mass fraction of the pH color indicators in the curable adhesive is 0.1% or more, preferably 0.15% or more, based on the mass of the curable adhesive.

[0086] Even though the mass fractions of the cationic initiators and the pH color indicators can be selected by the skilled person, as explained above, largely independently of the chemical nature of the respective components, the inventors have recognized that there is a certain correlation between the mass fractions of the components relative to one another. In practice, the inventors believe that fine-tuning of the curable adhesives according to the invention will frequently be achieved by adjusting the mass ratio of these two components. In this respect, the inventors have recognized that larger proportions of pH color indicators can, in some cases, disrupt cationic curing, so that curing does not occur or occurs only incompletely. The inventors have recognized that this problem can be solved by increasing the relative mass fraction of cationic initiator.

[0087] The advantageous and reliable curability of curable adhesives according to the invention is ensured by the mass ratio of the combined mass of the cationic initiators to the combined mass of the pH color indicators in the curable adhesive being 5:1 or more.

[0088] Based on their own experiments, the inventors have succeeded in identifying particularly advantageous mass ratios for the two components. Preferred is a curable adhesive according to the invention wherein the mass ratio of the combined mass of the cationic initiators to the combined mass of the pH color indicators in the curable adhesive is 6:1 or more, preferably 8:1 or more, particularly preferably 10:1 or more. Additionally or alternatively, preferred is a curable adhesive according to the invention wherein the mass ratio of the combined mass of the cationic initiators to the combined mass of the pH color indicators in the curable adhesive is in the range from 5:1 to 30:1, preferably in the range from 5:1 to 20:1.

[0089] It can be seen as an advantage of the curable adhesives according to the invention that they are very flexible with regard to the presence of additional components, which advantageously makes it possible to adapt the physicochemical properties in a particularly targeted manner to the respective application requirements. For example, a curable adhesive according to the invention is preferred, wherein the curable adhesive comprises one or more polyols, preferably in a combined mass fraction in the range of 0.5 to 15%, particularly preferably in the range of 1 to 10%.Additionally or alternatively, a curable adhesive according to the invention is also preferred, wherein the curable adhesive comprises one or more further additives, preferably in a combined mass fraction in the range from 0.1 to 50%, particularly preferably in the range from 0.2 to 40%, based on the mass of the curable adhesive, and / or wherein the one or more further additives are preferably selected from the group consisting of adhesive resins, ageing inhibitors, light stabilizers, UV absorbers, rheological additives and additives for increasing opacity.

[0090] In special applications, an opacity (“haze”) may be desired for the curable adhesive according to the invention or the adhesive tape comprising the curable adhesive according to the invention. Such special applications include, for example, adhesive tapes for the electrical insulation of an object, such as battery cells. Here, a certain opacity is desirable to conceal visual defects such as scratches. Accordingly, preference is given to the curable adhesive according to the invention which further comprises one or more additives for increasing the opacity. Combinations of a combined mass fraction of not more than 0.3% pH color indicator and a combined mass fraction of not more than 5% of one or more additives for increasing the opacity, based on the mass of the curable adhesive, have proven particularly advantageous with regard to the curability of the adhesive, color strength and hiding power.Particularly preferred are combined mass fractions in the range of 0.1 to 0.25% pH color indicator and combined mass fractions in the range of 0.1 to 0.3% titanium dioxide.

[0091] A special case of the additional components that serve to adjust the properties of adhesives are insoluble fillers that can be added to the curable adhesive to obtain a filled curable adhesive. These are particulate fillers with an average particle diameter (D50) of 5 μm or more, preferably 10 μm or more, particularly preferably 20 μm or more, which are insoluble in the curable adhesive and are accordingly present therein as a dispersion, as well as macroscopic fillers such as, for example, fibers. The insoluble fillers are preferably selected from the group consisting of particulate fillers. The insoluble fillers are particularly preferably selected from the group consisting of expandable hollow polymer spheres, non-expandable hollow polymer spheres, solid polymer spheres, hollow glass spheres, solid glass spheres, hollow ceramic spheres, solid ceramic spheres and / or solid carbon spheres.However, fibers, scrims, platelets, and rods made of materials that are insoluble in the curable adhesive can also be considered as insoluble fillers. Due to their partially macroscopic dimensions and their lack of solubility, these essentially have no influence on the above-disclosed relationships in the compositional chemistry of the curable adhesives, but rather are present in a heterogeneous mixture with the curable adhesive. Accordingly, these insoluble fillers are not attributed to the curable adhesive in the context of the present invention and are accordingly not taken into account when calculating mass fractions relative to the mass of the curable adhesive. Rather, in the context of the present invention, it is defined that the addition of insoluble fillers to a curable adhesive according to the invention results in a filled curable adhesive, i.e.a filled curable adhesive composition comprising: i) a curable adhesive composition according to the invention, preferably as disclosed above as preferred, and ii) one or more insoluble fillers.

[0092] The combined mass fraction of the insoluble fillers is particularly preferably in the range from 1 to 50%, preferably in the range from 2 to 40%, particularly preferably in the range from 5 to 30%, based on the mass of the filled curable adhesive.

[0093] Curable adhesives according to the invention can, for example, be used directly as adhesives, and depending on the application method, they can be provided in particular in the form of tapes. The invention thus also relates to an adhesive tape, in particular a reactive adhesive tape, comprising a curable adhesive according to the invention as the adhesive layer, wherein the adhesive tape preferably comprises a carrier layer.

[0094] With a view to the most favorable handling properties possible, particularly advantageous results are regularly achieved when curable adhesive compositions according to the invention are used as the adhesive layer of a single-sided or double-sided adhesive tape which also comprises a carrier layer or when the adhesive layer is arranged on a release layer, for example a liner, from which the adhesive layer can be easily detached.

[0095] The term "adhesive tape" is familiar to those skilled in the field of adhesive technology. Within the context of the present invention, the term "tape" refers to all thin, flat structures, i.e., structures with a predominantly two-dimensional extension, in particular films, film sections, and labels, preferably tapes with an extended length and a limited width, as well as corresponding tape sections.

[0096] The carrier layer usually refers to the layer of such a multilayer adhesive tape that significantly determines the mechanical and physical properties of the adhesive tape, such as tear resistance, stretchability, insulation, or resilience. Typical materials for the carrier layer include fabrics, scrims, and plastic films, such as PET films and polyolefin films. However, the carrier layer itself can also be pressure-sensitively adhesive. In a preferred embodiment, the adhesive tape according to the invention can be a double-sided adhesive tape whose carrier layer is provided on both sides with a curable adhesive according to the invention.

[0097] The carrier layer can also have electrically insulating properties, so that the corresponding adhesive tape according to the invention has electrically insulating properties and can be used for the electrical insulation of an object. For this purpose, insulating carrier films with a specific volume resistance of >10 15 Dem, preferably >10 16 The, further preferred >10 17The adhesive tape according to DIN EN 62631-3-1 (VDE 0307-3-1): 2017-01 is used. Accordingly, in a preferred embodiment, the adhesive tape according to the invention can be a double-sided adhesive tape whose insulating carrier film is provided on both sides with a curable adhesive according to the invention. In another preferred variant, the adhesive tape according to the invention is a single-sided adhesive tape whose electrically insulating carrier film is provided on one side with a curable adhesive according to the invention. Such single-sided adhesive tapes are outstandingly suitable for encasing battery cells in hybrid vehicles and purely electric vehicles.

[0098] Preferably, the insulating carrier film comprises one or more materials selected from the group consisting of polyimide, polybenzimidazole, polyamideimide, polyetherimide, polyacetal, polyphenylene sulfide, polyetheretherketone, polytetrafluoroethylene, polyamide 6, ultra-high molecular weight polyethylene, polypropylene, vinyl chloride resin, polystyrene, polyethylene terephthalate, acrylonitrile-butadiene-styrene, polycarbonate, polyvinyl chloride, ethylene-vinyl acetate copolymer and polyester, more preferably from the group consisting of polypropylene, polyethylene terephthalate, polycarbonate and polyvinyl chloride, more preferably from the group consisting of polypropylene and polyethylene terephthalate.

[0099] There are generally no particular restrictions regarding the thickness of the support. The support preferably has a thickness in the range of 20 pm to 100 pm, more preferably in the range of 30 pm to 90 pm, and even more preferably in the range of 40 pm to 75 pm.

[0100] In adhesive tapes according to the invention, the adhesive layers can be covered with a release liner to enable smooth unwinding and protect the pressure-sensitive adhesive from contamination. Such release liners typically consist of a single- or double-sided siliconized plastic film (e.g., PET or PP) or a siliconized paper carrier.

[0101] In light of the above statements, the invention also relates to a method for connecting two or more components before further processing steps, with which advantageous timing and excellent time and cost efficiency can be achieved.The method according to the invention comprises the method steps: w) producing or providing an adhesive tape according to the invention; x) connecting two or more components to the adhesive tape and initiating the cationic curing of the curable adhesive by activating the one or more cationic initiators to obtain a component composite, y) curing the curable adhesive until the color change of the one or more pH color indicators to obtain a component composite ready for further processing with a still incompletely cured curable adhesive, and z) carrying out one or more further processing steps on the component composite ready for further processing.

[0102] The person skilled in the art understands that the further processing steps can be all typical processing steps such as may occur, for example, in a typical production line, for example in automobile production, wherein the further processing steps are, in particular, further processing steps in which the connection of the components produced in the component assembly is subjected to mechanical stress, wherein the mechanical stress is expediently lower than the bond strength at the time of the color change.In this process, a pH-dependent color change of the one or more pH color indicators during the cationic curing of the curable adhesive thus preferably occurs at a time at which the bond strength of the curing adhesive reaches a predetermined value which is greater than the mechanical stress experienced by the component composite ready for further processing in process step z).

[0103] Starting from the curable adhesive composition according to the invention and the adhesive tape according to the invention, the use of a curable adhesive composition according to the invention or an adhesive tape according to the invention for bonding two or more components by curing the curable adhesive composition, preferably in a process according to the invention, is also disclosed.

[0104] In the following, the invention and preferred embodiments of the invention are further explained and described with reference to experiments.

[0105] A. Preparation of the curable adhesives:

[0106] From the components summarized in Table 1, curable adhesives were obtained by mixing the components in the usual way (as laboratory smears from a 40% butanone solution). Table 1 - Composition of the curable adhesives, all data in parts by weight

[0107] The commercially available ethylene-vinyl acetate copolymer Levamelt® 700 (vinyl acetate content of 70 weight percent) from Arlanxeo was used as the (co)polymer. A commercially available solid bisphenol A diglycidyl ether (DER 662E) and a commercially available liquid bisphenol A diglycidyl ether (DER 331) from Olin were used as the epoxy compounds.

[0108] Triarylsulfonium hexafluorophosphate (50% in propylene carbonate; CAS: 109037-77-6; the weight in Table 1 refers to the solution) was used as the cationic initiator. A commercially available polyester polyol based on polycaprolactone (Capa 2054) from Ingevity was used as the polyol.

[0109] From the prepared curable adhesives, adhesive tapes with a thickness of approximately 100 μm were produced for the solid curable adhesives E2, E3, and E4 by spreading. B. Adhesive Experiments & Color Evaluation:

[0110] Adhesive strength:

[0111] The bond strengths were determined analogously to ISO 29862 (Method 3) at 23 °C and 50% relative humidity, at a peel speed of 300 mm / min and a peel angle of 180°. The thickness of the adhesive layer was 100 μm in each case. An etched PET film with a thickness of 50 μm, available from Coveme (Italy), was used as the reinforcing film. Steel plates in accordance with the standard were used as the substrate. The uncured test strip was bonded using a 4 kg rolling machine at a temperature of 23 °C. The adhesive tapes were removed immediately after application. The measured value (in N / cm) was the average of three individual measurements, and the failure pattern was documented as follows: adhesive failure (A) or cohesive failure (K).

[0112] Bonding strength:

[0113] The bond strength was quantitatively determined in a dynamic tensile shear test according to DIN EN 1465:2009-07 at 23 °C and 50 % relative humidity for a test speed of 1 mm / min (results in N / mm 2 = MPa). Steel test specimens were used, which were cleaned with acetone prior to bonding. The layer thicknesses of the adhesive tapes for the solid curable adhesives E2, E3, and E4 corresponded to the above specifications. Before joining the test specimens, the adhesive tapes were treated with suitable light (Hönle 365 nm LED lamp with a UV-A dose of 3000 mJ / cm 2) and the test specimens were joined immediately afterwards. The liquid curable adhesive E1 was applied as a formless adhesive with a layer thickness of 100 μm (as is usual when testing liquid adhesives, some glass beads with a particle size of 100 μm were scattered into the bond line as spacers). The curable adhesive was irradiated with suitable light (Hönle 365 nm LED lamp with a UV-A dose of 3000 mJ / cm 2 ) and the test specimens were immediately joined together. The bond strength was measured after a color change was observed. To determine the relative bond strength, the maximum bond strength achievable through cationic curing was also determined after 7 days on fully cured specimens. For example, for E2, this was 8.9 MPa.

[0114] The results are shown in Table 2. Table 2 - Summary of the bonding experiments

[0115] A: adhesive failure; K: cohesive failure

[0116] Based on the fracture pattern in the bond strength test, samples E2 to E4 demonstrate favorable cohesion and pressure-sensitive adhesive properties. As expected, the liquid adhesive E1 exhibits cohesive failure before curing, making it impossible to determine a meaningful bond strength.

[0117] The observed color change is a color change that lies within the expected interval of the relative bond strength, whereby in most cases a further color change is observed immediately after irradiation, which advantageously indicates the successful irradiation, but in which the relative bond strengths will in any case still be outside the interval.

[0118] The tensile shear tests after curing show that the adhesives of the invention cure, and during the color change, bond strengths between 45% and 78% of the respective maximum achievable bond strength can be achieved. This color change can reliably indicate that component assemblies bonded with corresponding curable adhesives can already be subjected to further processing without fear of bond failure. This can advantageously increase the cycle rate and thus the production speed in a manufacturing process that includes such bonding.

[0119] In contrast to these inventive examples, when unsuitable dyes are used, the color change only occurs at almost full cure (V1). If the ratio of dye to initiator is too high, as shown in the example in V2, i.e., if too much dye is used, the adhesive will no longer cure properly.

Claims

Patent claims 1 . A curable adhesive composition comprising: a) one or more (co)polymers, b) one or more polymerizable epoxy compounds, c) one or more cationic initiators, and d) one or more pH color indicators with at least one pH-dependent color change, wherein the mass ratio of the combined mass of the cationic initiators to the combined mass of the pH color indicators in the curable adhesive composition is 5:1 or more, and wherein the curable adhesive composition is composed such that a pH-dependent color change of the one or more pH color indicators occurs during the cationic curing of the curable adhesive composition at a time at which the bond strength of the curing adhesive composition is in the range of 20 to 80% of the maximum bond strength achievable by cationic curing.

2. Curable adhesive according to claim 1, wherein the one or more pH color indicators exhibit a pH-dependent color change at a pH of 4 or less.

3. Curable adhesive composition according to one of claims 1 or 2, wherein the one or more pH color indicators are selected from the group consisting of cresol red, methyl violet, crystal violet, ethyl violet, malachite green, methyl green, ethyl green, 2-(p-dimethylaminophenylazo)pyridine, metanil yellow, 4-phenylazodiphenylamine, thymol blue, metacresol purple, 4-[(4-anilinophenyl)azo]benzenesulfonic acid sodium salt, 4-o-tolylazo-o-toluidine, 2,2',2",4,4'-pentamethoxytriphenylmethanol, quinaldine red, erythrosine disodium salt, 4,4'-bis(2-amino-1-naphthylazo)-2,2'-stilbenedisulfonic acid and methyl yellow.

4. Curable adhesive composition according to one of claims 1 to 3, wherein the combined mass fraction of the pH color indicators in the curable adhesive composition is in the range of 0.01 to 0.4%, based on the mass of the curable adhesive composition.

5. Curable adhesive composition according to one of claims 1 to 4, wherein the combined mass fraction of the cationic initiators in the curable adhesive composition is in the range of 0.1 to 7%, based on the mass of the curable adhesive composition. . The curable adhesive according to any one of claims 1 to 5, wherein the mass ratio of the combined mass of the cationic initiators to the combined mass of the pH color indicators in the curable adhesive is 6:1 or more. . The curable adhesive according to any one of claims 1 to 6, wherein the combined mass fraction of the (co)polymers in the curable adhesive is in the range of 1 to 70%, based on the mass of the curable adhesive. . The curable adhesive according to any one of claims 1 to 7, wherein the combined mass fraction of the polymerizable epoxy compounds in the curable adhesive is in the range of 35 to 95%, based on the mass of the curable adhesive. . The curable adhesive according to any one of claims 1 to 8, wherein the curable adhesive is a pressure-sensitive adhesive. 0.The curable adhesive composition according to any one of claims 1 to 9, wherein the curable adhesive composition is composed such that a pH-dependent color change of the one or more pH color indicators occurs during cationic curing of the curable adhesive composition at a time when the bond strength of the curing adhesive composition is in the range of 25 to 75% of the maximum bond strength achievable by cationic curing.

1. An adhesive tape comprising, as an adhesive layer, a curable adhesive composition according to any one of claims 1 to 10. 2.A method for joining two or more components before carrying out further processing steps, comprising the method steps: w) producing or providing an adhesive tape according to claim 1 1 ; x) joining two or more components to the adhesive tape and initiating the cationic curing of the curable adhesive by activating the one or more cationic initiators to obtain a component composite, y) curing the curable adhesive until the color change of the one or more pH color indicators to obtain a component composite ready for further processing with a still incompletely cured curable adhesive, and z) carrying out one or more further processing steps on the component composite ready for further processing.

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