Electromagnetic wave-absorbing adhesive tape

Abstract

An electromagnetic wave-absorbing pressure-sensitive adhesive tape comprising a layer of a pressure-sensitive adhesive compound, the pressure-sensitive adhesive compound containing from 0.1 to 10% by weight carbon nanotubes, reliably absorbs electromagnetic radiation, in particular in a frequency range as used for radar sensors (electromagnetic waves of a frequency of 60 to 100 GHz), and at the same time is easy to fasten in a sensor. It thus combines an absorption effect and adhesiveness and is therefore particularly suitable for fastening sensors, for example in motor vehicles, since it makes it possible to suppress within the sensors disturbing noises that frequently occur and impair the detection accuracy of the sensors.

Description

[0001] 01.09.2025 1

[0002] 2023016 / PCT tesa Societas Europaea Norderstedt

[0003] Electromagnetic wave absorbing adhesive tape

[0004] The present invention relates to an electromagnetic wave-absorbing adhesive tape comprising a layer of an adhesive compound. The invention further relates to an electromagnetic sensor comprising the adhesive tape and the use of the electromagnetic wave-absorbing adhesive tape.

[0005] Electromagnetic sensors are used in numerous fields. They are particularly useful for measuring distances and for the non-contact detection of objects. A significant application area for such electromagnetic sensors is the automotive sector. Vehicles are increasingly being equipped with sensors, such as those needed for environmental perception, for example, for parking or distance control. This includes radar-based adaptive cruise control (ACC) and adaptive distance control (ADC), distance warning systems, and automatic parking aids. Older systems operate in a frequency range of 24 GHz, while newer systems utilize the frequency range of 76 to 81 GHz.

[0006] Interference noise, which occurs frequently and weakens the detection accuracy of the sensors, is problematic. In particular, if sensors are to be used in areas requiring especially high accuracy and reliability, such as autonomous driving, they must operate with exceptional sensitivity.

[0007] To suppress interference, components that absorb electromagnetic interference signals are integrated into electromagnetic sensors. Numerous absorber materials are known for the frequency range of 100 MHz to 10 GHz that convert electromagnetic radiation into heat and absorb the electromagnetic signals of this wavelength, thus eliminating interference. Unfortunately, these known absorber materials do not provide sufficient absorption and interference suppression at higher frequencies. In particular, there is no satisfactory method for interference suppression in the frequency range of 76 to 81 GHz, which is relevant for radar sensors used in vehicles for distance measurement and object detection.

[0008] Currently, polypropylene or polyacrylate absorbers manufactured using injection molding are used for noise suppression. Due to the manufacturing process, these absorbers have a thickness of approximately 800 µm and must be manually sanded down to the required maximum thickness of 300 µm. Carbon black, for example, is used as the absorbing component. A carbon black content of 20-30 wt% provides acceptable absorption. The absorption efficiency is approximately 60%. This value is acceptable but could be improved. Furthermore, the injection-molded absorber must be attached to the sensor. This is usually done by adhesive bonding, meaning an additional adhesive solution is required that meets the specific requirements of the sensor. JP 2003133784 A describes such an absorber. This absorber uses a thermoplastic rubber material containing carbon black for absorption.This absorber has a loss factor (tan delta) of 0.35 at a dielectric constant (dk) of 6.

[0009] Adhesive tapes that absorb electromagnetic radiation are also known in the prior art. For example, CN 210030552 11 discloses an adhesive tape for absorbing electromagnetic radiation, comprising a radiation-absorbing layer, a first adhesive layer, a carrier layer, and a second adhesive layer. The radiation-absorbing layer is formed by dispersion of an absorbing material such as iron, silicon, or aluminum in a polyurethane matrix. It is therefore a product with a multilayer structure.

[0010] To achieve the desired absorption effect, the absorber's dielectric constant must have a specific value. On the other hand, the dielectric constant must not be too high, as this causes reflection. Furthermore, the dielectric constant (dk) and the loss factor (tan delta) must be compatible.

[0011] The object of the present invention was therefore to provide an absorber that eliminates the disadvantages described above, reliably absorbs electromagnetic radiation, particularly in a frequency range such as that used for radar sensors, and is simultaneously easy to attach to a sensor, ideally combining absorption and adhesive properties. This object is achieved by an electromagnetic wave-absorbing adhesive tape as described in the independent claim. The dependent claims relate to advantageous embodiments of the invention. Furthermore, the invention comprises an electromagnetic sensor including the adhesive tape and the use of the electromagnetic wave-absorbing adhesive tape.

[0012] Accordingly, the invention relates to an adhesive tape of the type mentioned above, in which the adhesive mass contains 0.1 to 10 wt.% carbon nanotubes and absorbs electromagnetic waves with a frequency of 60 to 100 GHz. Such an adhesive tape according to the invention, which can also be called an absorber tape, fulfills both requirements and provides both functions: On the one hand, it absorbs electromagnetic waves that interfere with radar signals very effectively, and on the other hand, due to its adhesive properties, it can be easily attached to a sensor. If the adhesive tape is double-sided, it even allows for the connection of further components or the fastening of the components to one another.

[0013] Carbon nanotubes have been described in many contexts and for numerous applications. Surprisingly, it has now been found within the scope of the present invention that they also have an electromagnetic radiation absorption effect and can therefore be used for noise suppression in sensors, particularly radar sensors, to increase their detection accuracy.

[0014] The wt.% figure refers to the composition of the entire adhesive compound, including the base polymers and any other components.

[0015] The adhesive compound according to the invention is particularly suitable when it contains 0.1 to 5 wt.%, preferably 0.1 to 4 wt.%, and most preferably 0.5 to 3 wt.% carbon nanotubes. It has been found that even a small proportion of 3 wt.% or less is sufficient to achieve a strong absorption effect for interference signals. Attenuation of 10–15 dB is achievable with the adhesive tape according to the invention.

[0016] Carbon nanotubes are an allotropic form of carbon. A distinction is made between single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT). The basic structure of single-walled carbon nanotubes is that of a regular hexagonal lattice drawn on an infinite cylindrical surface, with the vertices of which represent the positions of the carbon atoms. Multi-walled carbon nanotubes consist of nested single-walled carbon nanotubes in a nested tube-within-a-tube structure. Both types of nanotubes can be used for the present invention, either alternatively or in a mixture of single- and multi-walled nanotubes. Single-walled carbon nanotubes are particularly preferred for the purposes of the present invention.

[0017] Preferably, the adhesive compound of the adhesive tape according to the invention has a dielectric constant of 2 to 8.

[0018] In a particularly preferred embodiment, the adhesive is a curable adhesive. By using carbon nanotubes as absorbers of electromagnetic radiation, coloration of the adhesive can be avoided – unlike in the case of using carbon black. The penetration of radiation required for crosslinking, especially UV radiation, is therefore easily possible, allowing the use of curable adhesives, which in turn enable particularly good adhesion of the adhesive tape.

[0019] The adhesive compound according to the invention already exhibits sufficiently good strength without crosslinking. It is possible to further increase this strength by crosslinking.

[0020] The base polymer can be functionalized with further functional groups such as silanes, acrylates, or maleic anhydride, for example by reactive grafting. These functional groups enable subsequent crosslinking.

[0021] Cross-linking occurs with high-energy radiation, UV light, or chemicals such as peroxides, phenolic resins, or sulfur compounds.

[0022] Crosslinkers and promoters can therefore be added to the adhesive compound to facilitate crosslinking. Suitable crosslinkers for electron beam crosslinking and peroxide crosslinking include, for example, bi- or multifunctional acrylates, maleimides, quinones, cyanurates, bi- or multifunctional isocyanates (also in blocked form), or bi- or multifunctional epoxides.

[0023] The pressure-sensitive adhesive is ideally based on acrylate, polyurethane, or rubber. Such adhesives are not only particularly effective but also well-suited to incorporating fillers like carbon nanotubes. The carbon nanotubes can be easily incorporated, ensuring a uniform distribution within the adhesive. The methods for producing such adhesives are well-known to experts. For example, planetary roller mixers, which are well-suited for incorporating fillers into highly viscous liquids, can be used.

[0024] The pressure-sensitive adhesive according to the invention is particularly preferably an acrylate-based adhesive having the following composition: a) 5 to 15 wt.%, preferably 7 to 12 wt.%, in particular 10 wt.% of at least one compound selected from the group consisting of 4-hydroxybutyl acrylate (HBA) and 2-hydroxyethyl acrylate (HEA); b) 50 to 90 wt.%, preferably 60 to 85 wt.%, in particular 80 wt.% of at least one compound selected from the group consisting of 2-ethylhexyl acrylate (EHA) and isobornyl acrylate (IBOA); c) 5 to 15 wt.%, preferably 5 to 11 wt.%, in particular 10 wt.% of at least one compound selected from the group consisting of 4-acryloylmorpholine (ACMO) and 1,6-hexanediol diacrylate (HDDA); d) 0 to 6 wt.%, preferably 0 to 4 wt.% of 2-(2-ethoxyethoxy)ethyl acrylate (EOEOEA)

[0025] The percentage by weight always refers to the composition of the entire adhesive compound.

[0026] The pressure-sensitive adhesive may contain additives such as fillers, colorants, or antioxidants (antiozonants, primary and secondary antioxidants, light stabilizers, etc.) to adjust its optical and adhesive properties. Typical additives used in the adhesive include: primary antioxidants such as sterically hindered phenols; and secondary antioxidants such as phosphites or thioethers.

[0027] • Light protectants such as UV absorbers or sterically hindered amines

[0028] The fillers can be reinforcing or non-reinforcing. Particularly noteworthy are silicon dioxides (spherical, needle-shaped, or irregular like pyrogenic silicas), layered silicates, calcium carbonates, zinc oxides, titanium dioxides, aluminum oxides, or aluminum oxide hydroxides.

[0029] The concentration of the additives influencing the optical and adhesive properties is preferably up to 20 wt.%, more preferably up to 15 wt.%, and more preferably up to 5 wt.%.

[0030] According to the invention, the total proportion of all added substances (besides base polymer, adhesive resin and plasticizer) such as fillers and / or dyes and / or antioxidants should not exceed 5 wt.%, preferably 2 wt.%.

[0031] The listed substances are not mandatory; the adhesive also works without them being added individually or in any combination, i.e., without fillers and / or dyes and / or antioxidants.

[0032] As mentioned at the outset, numerous absorbers exist for electromagnetic waves with frequencies from 100 MHz to 10 GHz. Absorbers for higher frequency ranges are significantly less common. The adhesive material particularly preferentially absorbs electromagnetic waves with wavelengths from 70 to 85 GHz, especially wavelengths from 76 to 81 GHz. This makes the adhesive tape according to the invention particularly suitable for applications involving radar waves, and especially for those used in the automotive sector, particularly for sensors for distance measurement or non-contact object detection. These sensors are used for radar-based adaptive cruise control (ACC) and adaptive distance control (ADC), distance warning systems, and automatic parking aids.

[0033] The adhesive tape according to the invention preferably has a thickness of 100 to 600 pm, particularly 200 to 500 pm, and most preferably 300 to 400 pm. Such a thickness provides good attenuation of interference signals on the one hand, while on the other hand being thin enough to take up as little space as possible when installed in a sensor.

[0034] The term "adhesive tape" as used in this invention encompasses all planar structures coated on one or both sides with adhesive material, such as structures extended in two dimensions (for example, films or film sections), tapes of extended length and limited width, tape sections, die-cuts, labels, and the like, including multilayer arrangements. Preferably, the adhesive tape is in the form of a continuous web in the form of a roll and not as a die-cut or label. The adhesive tape can, for example, be produced in the form of a roll, i.e., wound onto itself in the form of an Archimedean spiral. Within the scope of the present invention, a temporary carrier, in contrast to a permanent carrier, is not considered a component of the adhesive tape, but merely an aid in its production (process liner) or as a means of covering it.

[0035] Furthermore, the term "adhesive tape" also includes so-called "transfer tapes," meaning adhesive tapes without a backing. With a transfer tape, the adhesive is applied between flexible, temporary carriers, also known as liners, before application. These liners are provided with a release layer and / or have anti-adhesive properties. For application, one liner is typically removed first, the tape is applied, and then the second liner is removed.

[0036] The mass application (coating thickness) of the adhesive compound is preferably between 10 and 200 g / m². 2 , preferably between 15 and 100 g / m² 2 , especially preferably between 20 and 70 g / m² 2 .

[0037] The backing or liner materials used for the adhesive tape are those familiar and common to those skilled in the art, such as paper, fabrics, nonwovens, or films made of, for example, polyesters like polyethylene terephthalate (PET), polyethylene, polypropylene, expanded polypropylene, or polyvinyl chloride. Materials made from renewable resources, such as paper, fabrics made of, for example, cotton, hemp, jute, or nettle fibers, or films made of, for example, polylactic acid, cellulose, modified starch, or polyhydroxyalkanoate, can also be used. This list is not exhaustive; the use of other films is also possible within the scope of the invention. Films made of PET and expanded polypropylene are particularly preferred, with PET films being especially preferred as backing or liner materials.

[0038] The adhesive tape is formed by applying the adhesive compound partially or completely to the backing. The coating can also be applied in the form of one or more strips in the longitudinal direction (machine direction), or optionally in the transverse direction; however, it is most commonly applied across the entire surface. Furthermore, the adhesive compound can be applied in a dot pattern using screen printing, whereby the adhesive dots can vary in size and / or distribution, by intaglio printing in longitudinal and transverse directions, by halftone printing, or by flexographic printing. The adhesive compound can be in dome form (produced by screen printing) or in other patterns such as grids, stripes, or zigzag lines. It can also be sprayed on, resulting in a more or less irregular application pattern.

[0039] When using a temporary or permanent carrier to manufacture an adhesive tape, the carrier is coated with the pressure-sensitive adhesive. Optionally, cross-linking is also performed, but this is not required in the present invention. The carrier can be either permanent or temporary. In the context of this application, "coating a carrier with a pressure-sensitive adhesive" specifically means that the finished carrier is coated with the pressure-sensitive adhesive. However, it can also mean that the pressure-sensitive adhesive is co-extruded with the carrier. Furthermore, in this application, "coating a carrier with a pressure-sensitive adhesive" can also mean that the pressure-sensitive adhesive is brought into direct contact with a surface of the carrier, i.e., applied directly to a surface of the carrier.Alternatively, this can also mean that the adhesive is not brought into direct contact with a surface of the substrate, but rather that at least one further layer is arranged between the substrate and the adhesive when the substrate is coated with the adhesive. Preferably, when "coating a substrate with an adhesive," the adhesive is brought into direct contact with a surface of the substrate. The substrate can optionally be coated with an adhesive on one or both sides. If the substrate is coated on both sides with an adhesive according to the invention, the two sides of the substrate can either be coated with adhesives according to the invention that are identical in composition or with adhesives according to the invention that differ in composition; preferably, the adhesives according to the invention are identical in composition.

[0040] Particularly preferred in the context of the present invention is a single-layer adhesive tape without a temporary carrier. This allows for the provision of an adhesive tape with the lowest possible thickness, which, when used in, for example, a sensor, can simultaneously and permanently bond two other components together.

[0041] Furthermore, it is preferred that the adhesive layer comprises a liner on at least one side, and particularly preferably on both sides. Such a temporary carrier makes transport, storage, and application of the adhesive tape particularly easy and convenient.

[0042] The present invention also relates to the use of the electromagnetic wave-absorbing adhesive tape according to the invention for the absorption of electromagnetic radiation in electronic devices, particularly in electromagnetic sensors. Unlike conventional polypropylene or polyacrylate absorbers used for noise reduction, which are manufactured by injection molding, the adhesive tape according to the invention already has the desired thickness of 300 pm. Significantly higher absorption efficiencies (up to 95%) can be achieved than with conventional absorbers. Attenuation of 12 to 14 dB is possible. Because the absorber is in the form of an adhesive tape, it can be easily attached to a sensor. The absorber essentially provides its own adhesive. Furthermore, it can even be used to attach other components of a sensor.

[0043] The present invention further relates to an electromagnetic sensor, wherein the sensor comprises at least one layer of an electromagnetic wave-absorbing adhesive tape according to the invention. Particularly preferred are sensors used in motor vehicles for environmental detection, for example, for parking or distance control. They can be used as radar-based distance sensors or distance warning systems and automatic parking aids. The sensors particularly preferably utilize the frequency range of 76 to 81 GHz. In a typical vehicle sensor, in which a circuit board is mounted in a housing with a heat sink using adhesive tape, the electromagnetic wave-absorbing adhesive tape is advantageously arranged between the antenna and the cover. A separate adhesive bond, which is required in conventional sensors to attach the antenna and the cover to each other, is not necessary.This function is also performed by the absorber adhesive tape.

[0044] The absorber adhesive tape according to the invention is particularly suitable for sensors in the automotive sector. Of course, it can also be used in sensors in other application areas or in other components where absorption of electromagnetic waves is required.

[0045] Examples

[0046] Testing methods

[0047] Unless otherwise specified, measurements are carried out under a test climate of 23 ± 1 °C and 50 ± 5 % relative humidity.

[0048] Adhesive strength

[0049] The peel strength (adhesive strength) test is performed according to ASTM D3330.

[0050] A 2 cm wide strip of single-sided adhesive tape, consisting of a 23 µm thick PET film etched with trichloroacetic acid and a 30 µm thick adhesive coating applied to it, is adhered to the test substrate, an ASTM steel plate, by double-rolling it five times with a 4 kg roller. The surface of the steel plate is cleaned with acetone beforehand. The plate is clamped, and the adhesive strip is peeled off at its free end on a tensile testing machine at a peel angle of 180° and a speed of 300 mm / min (unless otherwise specified), and the force required is determined. The measurement results are given in N / cm and are averaged over three measurements and normalized to the width of the strip in N / cm.A 2 cm wide and 15 cm long strip of the 250 µm thick adhesive tape sample is covered on one side with a 25 µm thick PET film and bonded to a polished steel plate with the other side of the tape. Five double passes with a 4 kg roller ensure a defined bond. The plate is clamped, and the adhesive strip is peeled off at its free end on a tensile testing machine at a peel angle of 180° and a speed of 300 mm / min. The test climate is 23°C / 50% RH. The results are reported in N / cm.

[0051] The initial adhesive strength (adhesive strength ASTM steel) was measured immediately after bonding and for a maximum of 10 minutes after bonding.

[0052] Absorption effect

[0053] The absorption effect is determined using the so-called "free space" method. In this method, a signal with a selected frequency range, preferably at 76 GHz, is emitted from an antenna onto a metal plate as a reference, and the reflected signal is detected.

[0054] For comparison, the adhesive tape is glued onto the metal plate and the change in the reflected signal is expressed in -db, which is caused by the absorption effect of the adhesive tape.

[0055] The loss factor and the dk are also determined using this method.

[0056] The test was performed using a QAR-50 from Rhode & Schwartz.

[0057] The invention will now be explained in more detail using several examples, without unnecessarily limiting its scope. Production of the pressure-sensitive adhesives

[0058] For the pressure-sensitive adhesive compound listed in the first comparative example, the EPDM granules were homogenized in a kneader and mixed with 30 wt% carbon black. The homogenized mixture was then pressed to the required thickness between two release films in a standard hot press.

[0059] The test specimens produced in this way are characterized using the test methods described above for adhesion to steel and absorption effect.

[0060] Furthermore, additional pressure-sensitive adhesives based on UV syrup formulations were produced and blended with single-wall carbonanotubes. First, a pre-polymer was synthesized in a laboratory reactor with the composition specified in the table, including the photoinitiators (PI-184) (F1). Subsequently, to a fraction of this composition (F1), also specified in the table, further components, including the photoinitiators PI-184 and P 981, were added (80 wt% F1 plus the other specified components, collectively designated as F2). The final SWCN concentration was adjusted (1.5–2 wt% Tubal 204 from OCSiAl), and the mixture was homogenized and degassed using a planetary roller mixer.

[0061] The coating was then applied on a two-roll calender to adjust the appropriate layer thickness of the adhesive tape.

[0062] The results for the respective test samples are shown in Table 1.

[0063]

Claims

Patent claims 1. Electromagnetic wave absorbing adhesive tape comprising a layer of an adhesive compound, characterized in that the adhesive compound contains 0.1 to 10 wt.% carbon nanotubes and absorbs electromagnetic waves of a frequency of 60 to 100 GHz.

2. Electromagnetic wave absorbing adhesive tape according to claim 1, characterized in that the adhesive composition contains 0.1 to 5 wt.%, preferably 0.1 to 4 wt.%, particularly preferably 0.5 to 3 wt.% carbon nanotubes.

3. Electromagnetic wave absorbing adhesive tape according to claim 1 or 2, characterized in that the adhesive compound is a curable adhesive compound.

4. Electromagnetic wave absorbing adhesive tape according to one of claims 1 to 3, characterized in that the adhesive compound is an acrylate-based, polyurethane-based or rubber-based adhesive compound.

5. Electromagnetic wave absorbing adhesive tape according to one of claims 1 to 4, characterized in that the adhesive mass absorbs electromagnetic waves of a wavelength of 70 to 85 GHz, in particular a wavelength of 76-81 GHz.

6. Electromagnetic wave absorbing adhesive tape according to one of claims 1 to 5, characterized in that the adhesive tape has a thickness of 100 to 600 pm, in particular of 200 to 500 pm, particularly preferably of 300 to 400 pm.

7. Electromagnetic wave absorbing adhesive tape according to one of claims 1 to 6, characterized in that the adhesive compound is acrylate-based and has the following composition: a) 5 to 15 wt.%, preferably 7 to 12 wt.%, in particular 10 wt.% of at least one compound selected from the group consisting of 4-hydroxybutyl acrylate (HBA) and 2-hydroxyethyl acrylate (HEA) b) 50 to 90 wt.%, preferably 60 to 85 wt.%, in particular 80 wt.% of at least one compound selected from the group consisting of 2-ethylhexyl acrylate (EHA), isobornyl acrylate (IBOA) c) 5 to 15 wt.%, preferably 5 to 11 wt.%, in particular 10 wt.% of at least one compound selected from the group consisting of 4-acryloylmorpholine (ACMO) and 1,6-hexanediol diacrylate (HDDA) d) 0 to 6 wt.%, preferably 0 to 4 wt.% of 2-(2-ethoxyethoxy)ethyl acrylate (EOEOEA).

8. Electromagnetic wave absorbing adhesive tape according to one of claims 1 to 7, characterized in that the adhesive tape is formed in a single layer.

9. Electromagnetic wave absorbing adhesive tape according to one of claims 1 to 8, characterized in that the adhesive layer comprises a liner on at least one side.

10. Electromagnetic wave absorbing adhesive tape according to one of claims 1 to 9, characterized in that the carbon nanotubes are single-walled carbon nanotubes. 1 1. Electromagnetic sensor, characterized in that the sensor has at least one layer of an electromagnetic wave absorbing adhesive tape according to one of claims 1 to 10.

12. Electromagnetic sensor according to claim 11, characterized in that the electromagnetic wave absorbing adhesive tape is arranged between the antenna and the cover.

13. Use of the electromagnetic wave absorbing adhesive tape according to any one of claims 1 to 10 for the absorption of electromagnetic radiation in electronic devices, in particular in electromagnetic sensors.

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