EP4298172A1

EP4298172A1 - Multilayer adhesive strip with foamed subsequently applied compounds for improving cold shock resistance

Info

Publication number: EP4298172A1
Application number: EP22709665.8A
Authority: EP
Inventors: Thilo Dollase; Anika PETERSEN; Thomas Schubert; Michael Mayer; Hans-Peter Graef
Original assignee: Tesa SE
Current assignee: Tesa SE
Priority date: 2021-02-23
Filing date: 2022-02-23
Publication date: 2024-01-03
Also published as: CN117043291A; KR20230147193A; DE102021201684A1; WO2022180134A1

Abstract

The invention relates to a multilayer adhesive strip comprising a carrier layer and at least one adhesive layer arranged on the carrier layer, wherein the carrier layer comprises a syntactically foamed plastic; the adhesive layer comprises a syntactically foamed adhesive compound; the syntactically foamed adhesive compound comprises one or more polymers selected from the group consisting of poly(meth)acrylates and synthetic rubbers; and the syntactically foamed plastic and the syntactically foamed adhesive compounds comprise a plurality of expanded microballoons.

Description

Multi-layer adhesive tape with foamed re-coating to improve cold impact resistance

The invention relates to a multi-layer adhesive tape, a process for producing a multi-layer adhesive tape, the use of a multi-layer adhesive tape to produce bonds on medium- and low-energy substrates and the use of expanded microballoons in the carrier layer and the adhesive layer of an adhesive tape to increase cold impact resistance.

The subject matter of the invention is defined in the appended claims.

The joining of separate elements is one of the central processes in manufacturing technology. In addition to other methods, such as welding and soldering, bonding, i.e. joining using an adhesive, is of particular importance today. An alternative to the use of shapeless adhesives, which are applied from a tube, for example, are so-called adhesive tapes.

Pressure-sensitive adhesive tapes in which the adhesive effect is provided by a pressure-sensitive adhesive which is permanently tacky and adhesive under normal ambient conditions are particularly relevant for numerous technical applications. Corresponding pressure-sensitive adhesive tapes can be applied to a substrate by pressure and stick there, but can later be removed again more or less without leaving any residue. In particular in the field of vehicle production, however, components that have so-called low-energy surfaces (LSE surfaces = Low Surface Energy surfaces) or medium-energy surfaces (MSE) also have to be joined regularly. On such MSE/LSE surfaces, many pressure-sensitive adhesive tapes exhibit only an inadequate adhesive effect. This is the case, for example, when attaching plastic parts made of PP or PP/EPDM, which are used in modern vehicles due to their positive processing properties and the low material costs are becoming more and more important, or of painted components that have a reduced surface energy due to the painting. This is all the more true if these components are also used on the exterior of automobiles and are to be bonded there to mostly medium (MSE) to low-energy (LSE) clear or automotive paints, such as those sold by various manufacturers under trade names such as TMAC9000, Uregloss FF79 or Progloss FF99 are offered.

In order to still ensure sufficient bonding on these MSE/LSE surfaces, it is possible to subject the surfaces to be bonded, for example plastic components or painted car parts, to a chemical and/or physical surface treatment before joining in order to reduce the surface energy and thus the To increase bondability, e.g. through the use of adhesion promoters. However, these additional work steps are regularly felt to be disadvantageous because of the additional processing effort, and there is a need to provide pressure-sensitive adhesive tapes which enable adequate bonding to MSE/LSE surfaces without additional surface treatment of the substrates to be bonded.

An improvement in the adhesion properties on untreated LSE surfaces can be achieved, for example, by using PSAs based on synthetic rubbers and/or poly(meth)acrylates. With PSAs of this type, it is often possible to achieve good to very good adhesion properties even on non-pretreated, low-energy surfaces.

However, there is a problem here in that tacky or intrinsically tacky properties are not typical per se, at least for pure synthetic rubbers and the adhesives obtained therefrom, or are not sufficiently pronounced in the case of some poly(meth)acrylates. The effect of the pressure-sensitive tack in these adhesives is therefore often produced or reinforced in a targeted manner by the addition of so-called tackifying resins. In particular, due to the sometimes high tackifying resin concentration, corresponding pressure-sensitive adhesives regularly have a comparatively high Tg, ie a high glass transition temperature. This limits the

Performance of corresponding pressure-sensitive adhesives and the pressure-sensitive adhesive tapes produced therefrom at low temperatures. In particular, it has been found that the resistance of corresponding pressure-sensitive adhesive tapes to shock loading at low temperatures is often insufficient.

The problem described above is known in principle from the prior art and various solutions have been proposed in order to counteract this problem. To increase the low-temperature impact strength of adhesive tapes, DE 19730854 A1 discloses, for example, the use of foamed backing layers which are coated on both sides with unfoamed PSAs. As an alternative strategy, the use of a foamed or partially foamed pressure-sensitive adhesive based on conventional unfoamed ones also comes into consideration in the prior art

Carrier materials or open-pored foamed carrier materials can be used, as disclosed in WO 2019/076652 A1.

Even if the solution concepts known from the prior art can lead to an improvement in cold impact resistance in many cases, it has been shown in practice that the results that can be achieved with these approaches are not sufficient for certain applications, especially at particularly low temperatures, and that there is a need to further increase the cold impact resistance of PSA tapes.

The object of the present invention was therefore to eliminate or at least reduce the disadvantages of the prior art and in particular to provide a multilayer adhesive tape which has improved cold impact resistance compared to the prior art, so that even between substrates with medium to low-energy surfaces particularly durable bonds can be created that do not fail even at low temperatures and heavy impact loads.

It was an additional object of the present invention to specify a method for producing corresponding multilayer adhesive tapes, which is desirable using substances and methods should be feasible, which are already used in the field of adhesive technology, in order to enable existing processes to be easily adapted to the method.

In addition, the method to be specified should be able to be carried out in a time- and cost-efficient manner and enable a high level of process reliability.

The inventors of the present invention have now found that, surprisingly, the above objects can be achieved if, unlike in the prior art, not only either the adhesive or the backing layer is foamed, but rather that both the pressure-sensitive adhesive and the problem are efficiently solved the carrier layer must be foamed.

This is also surprising insofar as the foaming of pressure-sensitive adhesive layers leads to a reduction in the bond strength and thus in the adhesion to the substrate, in accordance with the expectation of the person skilled in the art, since the proportion of pressure-sensitively adhesive components is reduced. In particular, since the mode of failure of many PSAs in cold impact tests is generally adhesive, it was not obvious that the adhesive failure in cold impact tests could be reduced by foaming both layers.

It has turned out to be particularly surprising that not all types of foaming are equally suitable. The use of open-pore foams, for example polyethylene foam, and the use of syntactically foamed plastics is known from the prior art in combination with unfoamed PSAs for the backing layer. In contrast, when foamed PSAs are used on unfoamed backing layers, mostly syntactically foamed PSAs are disclosed.

A large number of methods are proposed in the prior art for the production of syntactically foamed layers, for example the use of hollow microspheres made of glass or ceramic. However, the inventors of the present invention have now recognized that expanded microballoons must be used in both layers in order to achieve a particularly advantageous Achieving cold impact resistance when foamed backing layers are combined with foamed PSAs.

In this respect, it was a surprising finding that in the systems identified by the inventors, hollow glass microbeads, for example, which have proven suitable in other systems for producing syntactic foams, do not provide satisfactory results in multilayer pressure-sensitive adhesive tapes of the invention.

Without wishing to be bound by this theory, it is assumed that when two foamed layers are used, i.e. a foamed carrier layer with a foamed pressure-sensitive adhesive, the interaction at the boundary layer, which is also referred to as interlaminate adhesion, plays a major role in the overall cold impact resistance and that the adhesive interaction at the interface is adversely affected if the foaming of one of the two layers is carried out by hollow microspheres or other methods that do not represent syntactic foaming by expanded microballoons.

The objects mentioned above are thus achieved by a multilayer adhesive tape, methods for producing a multilayer adhesive tape and uses, as defined in the claims. Preferred configurations according to the invention result from the dependent claims and the following statements.

Such features of objects according to the invention which are referred to below as preferred become particularly preferred

Embodiments combined with other features designated as preferred. Combinations of two or more of the items designated below as particularly preferred are therefore very particularly preferred. Features of preferred processes and uses according to the invention result from the features of preferred multilayer adhesive tapes according to the invention. The invention relates to a multilayer adhesive tape comprising a carrier layer and at least one adhesive layer arranged on the carrier layer, the carrier layer comprising a syntactically foamed plastic, the adhesive layer comprising a syntactically foamed pressure-sensitive adhesive, the syntactically foamed pressure-sensitive adhesive comprising one or more polymers selected are from the group consisting of poly(meth)acrylates and synthetic rubbers, and wherein the syntactically foamed plastic and the syntactically foamed PSA comprise a multiplicity of expanded microballoons.

The term adhesive tape is clear to those skilled in the field of adhesive technology. In the context of the present invention, the term tape designates all thin, flat structures, i.e. structures with a predominant extension in two dimensions, in particular foils, foil sections and labels, preferably tapes with extended length and limited width and corresponding tape sections. An adhesive tape can, for example, be offered in rolled-up form as an adhesive tape roll. Adhesive tapes regularly comprise a carrier layer on which an adhesive layer is arranged on one or both sides.

Preference is given to a multilayer adhesive tape of the invention, the adhesive tape comprising two adhesive layers composed of syntactically foamed pressure-sensitive adhesive which are arranged on opposite sides of the backing layer. Preference is also given to a multilayer adhesive tape according to the invention, in which case the adhesive layer essentially completely covers the backing layer.

According to the understanding of the art, a pressure-sensitive adhesive tape is an adhesive tape which has pressure-sensitive adhesive properties, ie the property of entering into a permanent connection to an adhesion substrate even under relatively weak pressure. After use, pressure-sensitive adhesive tapes can usually be removed from the substrate without leaving any residue and are usually permanently inherently sticky even at room temperature, which means that they have a certain viscosity and tack. so that they wet the surface of a substrate even with little pressure.

The pressure-sensitive tack 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 regarded as an extremely highly viscous liquid with an elastic portion, which consequently has characteristic viscoelastic properties that lead to the permanent inherent tack and pressure-sensitive adhesiveness described above. It is assumed that in the case of corresponding pressure-sensitive adhesives, mechanical deformation leads both to viscous flow processes and to the build-up of elastic restoring forces. The proportionate viscous flow serves to achieve adhesion, while the proportionate elastic restoring forces are necessary in particular to achieve cohesion. The relationships between rheology and pressure-sensitive tack are known in the prior art and are described, for example, in "Satas, Handbook of Pressure Sensitive Adhesives Technology", Third Edition, (1999), pages 153 to 203.

The storage modulus (G') and the loss modulus (G") are usually used to characterize the degree of elastic and viscous content, which can be determined by means of dynamic mechanical analysis (DMA), for example using a rheometer, as is described, for example, in WO 2015/189323 is disclosed.

In the context of the present invention, an adhesive is preferably understood as pressure-sensitive adhesive and thus as pressure-sensitive adhesive if at a temperature of 23 ° C in the deformation frequency range of 10 ° to 10 ¹ rad / sec G 'and G "are each at least partially in the range of 10 ³ to 10 ⁷ Pa, the storage modulus particularly preferably being in the range from 2 ^* 10 ⁵ to 4 ^* 10 ⁵ Pa at a frequency of 0.1 rad/sec (0.017 Hz).

A foamed material, be it the plastic or the pressure-sensitive adhesive, is understood as meaning a structure made up of gas-filled, three-dimensional cells which are filled with liquid, semi-liquid, relatively high-viscosity or solid Cell webs are limited and which are present in such a proportion that the density of the foamed layer is reduced compared to the density of the matrix material, ie the totality of the non-gaseous materials from which the material is composed. According to the invention, the backing layer and the adhesive layer comprise a plastic or a PSA which are each syntactically foamed, ie in which the foam cells are not delimited by the matrix material itself. In the case of such syntactic foams, hollow spheres, for example made of ceramic, polymer or glass, are in the matrix material

Membrane to be separated.

In multilayer adhesive tapes according to the invention, however, not all variants of syntactic foaming are relevant; instead, it is explicitly provided that the syntactically foamed plastic and the syntactically foamed pressure-sensitive adhesive comprise a multiplicity of expanded microballoons. This means that the syntactic foaming is at least partially achieved through the use of expanded microballoons, with multilayer adhesive tapes being preferred, the pressure-sensitive adhesive and/or the plastic being syntactically foamed exclusively by expanded microballoons, so that the backing layer is a plastic syntactically foamed with microballoons and the adhesive layer comprises a microballoon syntactically foamed pressure sensitive adhesive.

“Microballoons” are understood to mean hollow microspheres which are elastic and therefore expandable in their basic state and have a thermoplastic polymer shell. These spheres are usually filled with low-boiling liquids or liquefied gas. In particular, polyacrylonitrile, PVDC, PVC or poly(meth)acrylates are used as the shell material. In particular, short-chain hydrocarbons, for example isobutane or isopentane, are customary as the low-boiling liquid, which are enclosed, for example, as a liquefied gas under pressure in the polymer shell. As the microballoons heat up, the outer polymeric shell softens. At the same time, the propellant arranged inside expands. In doing so, the microballoons expand essentially irreversibly and expand three-dimensionally. Expansion is complete when the internal and external pressures equalize. Since the polymeric shell is retained, a closed-cell, syntactically foamed foam is achieved.

Microballoons which have not yet been thermally activated and which accordingly still have their original expansion are referred to in the context of the present invention as unexpanded microballoons and are not regarded as expanded microballoons in accordance with expert understanding.

Microballoons are available in a large number of designs, which can essentially be characterized by their size (usually 6 to 45 μm in diameter d50 in the unexpanded state) and their starting temperatures (75 to 220° C.) required for expansion. Unexpanded microballoons are available, for example, as an aqueous dispersion with a microballoon mass fraction of about 40 to 45% or as polymer-bound products, for example in ethylene vinyl acetate, with a microballoon mass fraction of about 65%. In the context of the present invention, however, it is preferred to use the unexpanded microballoons in powder form, with the powder preferably consisting essentially of the unexpanded microballoons.

The multilayer adhesive tape according to the invention can in principle be produced by adding either already expanded microballoons to the plastic of the backing layer and the pressure-sensitive adhesive, or by first adding unexpanded microballoons to both materials, which are then converted into expanded microballoons by thermal stress. the latter approach being explicitly preferred.

Due to the inherent dependence of the properties and the morphology of microballoons on the temperature used for expansion and the ambient pressure, or the deformability of the matrix material, im Within the scope of the present invention, expanded microballoons are understood to mean those microballoons which are at least partially expanded microballoons. This is to be understood in such a way that the microballoons compared to the unexpanded microballoons were treated at a temperature greater than or equal to the respective starting temperature for at least so long that a

Volume expansion occurs, preferably a volume expansion of more than 25%, preferably more than 50%, more preferably more than 100%, most preferably more than 150%. This means that the expanded microballoons do not necessarily have to be fully expanded. The expert knows that the foaming of the plastic or the

Pressure-sensitive adhesive chosen temperature depends not only on the type of microballoons but also on the desired foaming rate. The absolute density of the respective material gradually decreases as foaming continues. The lowest density state that can be achieved at a given temperature for a given material by foaming with expanding microballoons is referred to as full expansion, full foaming, 100% expansion, and 100% foaming, respectively.

Even if the precise definition of the expanded microballoons can be challenging, as with all amorphous materials whose properties are largely determined by the process used for production, the distinction between expanded and unexpanded microballoons is fortunately just as easy in practice for the person skilled in the art as the determination of full foaming, which can be determined reliably and easily in practice, at least with an error tolerance that is acceptable in the industry. According to the invention, the pressure-sensitive adhesive comprises one or more polymers selected from the group consisting of poly(meth)acrylates and synthetic rubbers. In the context of the present invention, the term poly(meth)acrylates includes polyacrylates and polymethacrylates in accordance with the understanding of the art. It can be considered an advantage of the multilayer adhesive tape according to the invention that this is relatively flexible with regard to the pressure-sensitive adhesive and two types of particularly relevant pressure-sensitive adhesives can be used.

The PSA preferably comprises synthetic rubbers with a mass fraction of 40% or more, preferably 60% or more. The synthetic rubbers are preferably block copolymers, in particular block copolymers which can be prepared by polymerizing vinylaromatics, in particular styrene, and conjugated dienes having 4 to 18 carbon atoms, in particular butadiene.

The pressure-sensitive adhesive preferably comprises poly(meth)acrylate with a mass fraction of 40% or more, preferably 60% or more, particularly preferably 90% or more, the pressure-sensitive adhesive particularly preferably consisting essentially of poly(meth)acrylate apart from the expanded microballoons . The poly(meth)acrylates can preferably be prepared by free-radical polymerization of acrylic acid and/or acrylic esters and/or methacrylic acid and/or methacrylic esters and, if appropriate, other copolymerizable monomers. The poly(meth)acrylates are particularly preferably preparable by free-radical polymerization of (meth)acrylic acid and/or butyl (meth)acrylate and/or ethylhexyl (meth)acrylate.

Preference is given to a multilayer adhesive tape of the invention in which the adhesive layer consists entirely of the syntactically foamed PSA. Even if it is possible in principle to also provide other adhesives in the adhesive layer in addition to the syntactically foamed PSA, it is advisable, with a view to optimized cold impact resistance, to form the entire adhesive layer from the syntactically foamed PSA. Preference is given to a multilayer adhesive tape of the invention in which the expanded microballoons are distributed essentially homogeneously in the pressure-sensitive adhesive, with expanded microballoons particularly preferably having a diameter d50 in the range from 20 to 120 μm, preferably 15 to 80 μm, particularly preferably 20 to 40 μm . A multilayer adhesive tape according to the invention is also preferred, the expanded microballoons being distributed essentially homogeneously in the plastic, the expanded microballoons particularly preferably have a diameter d50 in the range from 20 to 120 μm, preferably 15 to 80 μm, particularly preferably 20 to 40 μm. Corresponding multilayer according to the invention

Adhesive tapes have the advantage that a particularly uniform cold impact resistance can be obtained for the entire adhesive tape, especially when the expanded microballoons are distributed essentially homogeneously both in the PSA and in the plastic of the backing layer. The specified diameters of the microballoons have proven to be particularly suitable in numerous own experiments for producing syntactically foamed materials. In particular, the adhesive strength of the pressure-sensitive adhesive layer and the stability of the backing layer are impaired to a particularly small extent when corresponding microballoons are used.

In view of the fact that both the adhesive strength of the pressure-sensitive adhesive layer and the stability of the backing layer can be adversely affected by syntactic foaming, it has proven preferable for some applications to limit the content of expanded microballoons. Correspondingly, preference is given to a multilayer adhesive tape according to the invention, the PSA and/or the plastic comprising expanded microballoons in a mass fraction of 0.1 to 10%, preferably 0.2 to 5%, particularly preferably 0.5 to 2.5%, based on the total mass of the PSA and/or the plastic.

In practice, for many applications it is efficient to define the multilayer adhesive tapes of the invention via the density of the pressure-sensitive adhesive layer or of the plastic or via its degree of foaming, since both variables can be determined particularly easily by the person skilled in the art. The multilayer adhesive tapes according to the invention, which the inventors were able to identify as particularly powerful in the course of their own tests, can also be characterized by these quantities.

Preference is given to a multilayer adhesive tape according to the invention, in which the adhesive layer and/or the PSA has a density in the range from 300 to 980 kg/m ³ , preferably from 500 to 900 kg/m ³ , particularly preferably from 650 to 800 kg/m ³ . Preference is analogously given to a multilayer adhesive tape according to the invention, the backing layer having a density in the range from 300 to 980 kg/m ³ , preferably 400 to 800 kg/m ³ , particularly preferably 500 to 600 kg/m ³ . Preference is given to a multilayer adhesive tape of the invention where the degree of foaming of the PSA is in the range from 70 to 100%, preferably 80 to 100%, particularly preferably 90 to 100%, based on the full foaming at the starting temperature of the microballoons. A multilayer adhesive tape according to the invention is correspondingly preferred, the degree of foaming of the syntactically foamed plastic being in the range from 70 to 100%, preferably 80 to 100%, particularly preferably 90 to 100%, based on the full foaming at the starting temperature of the microballoon. Particular preference is given to multilayer adhesive tapes of the invention, where the PSA and the plastic have essentially the same degree of foaming.

The PSA can contain one or more tackifier resins. In principle, the multilayer adhesive tapes according to the invention are very flexible with regard to the use of adhesive resin. In our own tests, however, it has been shown that too high a proportion of adhesive resins and an associated increase in the glass transition temperature can adversely affect cold impact resistance, so that even with multilayer adhesive tapes according to the invention, cold impact resistance is obtained at very low temperatures, which is not sufficient for certain applications be considered. Preference is given to a multilayer adhesive tape according to the invention, in which the pressure-sensitive adhesive comprises at least one adhesive resin, preferably in a proportion by mass of from 1 to 55%, particularly preferably from 5 to 50%, very particularly preferably from 10 to 35%, if the pressure-sensitive adhesive is a poly(meth)acrylate Main component includes, and preferably with a mass fraction of 10 to 70%, particularly preferably 15 to 60%, very particularly preferably 20 to 50%, if the pressure-sensitive adhesive comprises a synthetic rubber as the main component.

The adhesive resin is preferably a non-polar adhesive resin when the PSA comprises a synthetic rubber as the main component. The adhesive resin is preferably a non-polar or polar adhesive resin when the PSA comprises a poly(meth)acrylate as the main component. The adhesive resin is preferably selected from the group consisting of non-hydrogenated, partially or fully hydrogenated resins based on rosin or rosin derivatives, terpene-phenolic resins, resins based on methacrylates, hydrogenated polymers of dicyclopentadiene, non-hydrogenated, partially, selectively or fully hydrogenated hydrocarbon resins of C ₅ -, C5/C9- and Cg-monomer streams, and polyterpene resins based on a-pinene and/or ß-pinene and/or d-limonene.

Preference is given to a multilayer adhesive tape according to the invention, comprising an adhesive resin with a polydispersity of less than 2.1, preferably less than 1.8, particularly preferably less than 1.6 and/or comprising an adhesive resin with a softening point in the range from 90° C. to 120°C, more preferably in the range of 95°C to 115°C.

According to the inventors, in order to obtain a multilayer adhesive tape according to the invention, which can be processed particularly well, it is desirable to limit the thickness of the adhesive layer and/or the backing layer, but not to make both layers too thin, since this leads to problems with the syntactic Foaming can result, which complicates production. It is advantageous here if the carrier layer is thicker than the adhesive layer.

Preference is given to a multilayer adhesive tape according to the invention, the adhesive layer having a thickness in the range from 20 to 250 μm, preferably from 30 to 200 μm, particularly preferably from 50 to 150 μm. Preference is given to a multilayer adhesive tape according to the invention, the backing layer having a thickness in the range from 150 to 3000 μm, preferably from 500 to 1500 μm, particularly preferably from 700 to 1200 μm. One according to the invention is also preferred multilayer adhesive tape, the adhesive tape having a thickness in the range from 200 gm to 3000 gm, preferably from 250 to 2000 gm, particularly preferably from 500 to 1500 gm.

The multilayer adhesive tape according to the invention has proven to be very tolerant of the presence of additives in the adhesive layer. This is advantageous because it allows the physico-chemical properties of the adhesive layer to be optimized without reducing the positive effect of the invention on cold impact resistance.

Preference is given to a multilayer adhesive tape according to the invention, in which the PSA additionally comprises one or more additives selected from the list consisting of antioxidants (e.g. sterically hindered phenols, phosphites or thioethers), light stabilizers (e.g. sterically hindered amines), process stabilizers, UV absorbers, antioxidants, rheological additives and fillers. Advantageously, the multilayer adhesive tapes according to the invention are particularly flexible with regard to the plastic used in the backing layer, so that in this respect those plastics can be used which are also used in conventional backing materials. From the definition of the subject matter of the invention, it is self-evident for the person skilled in the art that the multilayer adhesive tape according to the invention must inevitably comprise at least one plastic which can be syntactically foamed, which, however, probably applies to all industrially relevant plastics.

A multilayer adhesive tape according to the invention is preferred, in which the syntactically foamed plastic comprises a polymer selected from the group consisting of polyolefins, polyesters, poly(meth)acrylates, polyvinyl chloride and polyethylene terephthalate, preferably selected from the group consisting of polyethylene, polypropylene, Poly(meth)acrylates and mixtures of these polymers, particularly preferably selected from the group consisting of poly(meth)acrylates, the syntactically foamed plastic preferably consisting of the polymer. Preference is given to a multilayer adhesive tape according to the invention, the carrier layer having a viscoelastic carrier layer is, wherein the carrier layer preferably consists entirely of the syntactically foamed plastic.

The PSA and/or the carrier layer preferably comprises poly(meth)acrylates which have been crosslinked by means of a crosslinker, preferably a difunctional crosslinker. “Bivalent crosslinker” means that the crosslinker has exactly two points of contact, via which the connection to the polymer molecules to be crosslinked can be established. Suitable crosslinkers are known to those skilled in the art on the basis of their specialist knowledge. The crosslinker can be, for example, a thermal and/or a condensing crosslinker, it also being possible for both types of crosslinkers to be involved in the crosslinking. The crosslinker is particularly preferably a thermal crosslinker. The poly(meth)acrylates of the PSA were preferably crosslinked by means of epoxide(s) or by means of one or more substance(s) containing epoxide groups. The pressure-sensitive adhesive and/or the backing layer particularly preferably comprises poly(meth)acrylates which have been crosslinked by means of a crosslinker-accelerator system (“crosslinking system”) in order to obtain better control over the processing time, the crosslinking kinetics and the degree of crosslinking. Suitable crosslinking systems are known to those skilled in the art on the basis of their specialist knowledge. The crosslinker-accelerator system preferably comprises at least one substance containing epoxide groups as crosslinking agent and at least one substance as accelerator which acts to accelerate crosslinking reactions by means of compounds containing epoxide groups at a temperature below the melting temperature of the polymer to be crosslinked. Amines are particularly preferably used as accelerators.

With a view to the processing properties in subsequent use, it has proven to be advantageous if a release material is provided on the adhesive layers of the multilayer adhesive tapes of the invention. Such separating materials, which are also referred to as release liners, are used for this purpose in the case of the adhesive tapes which are usually wound up to form Archimedean spirals Used to prevent the different layers of tape from sticking together, especially with double-sided tapes, and to ensure easy unwinding. These release liners, which are removed from the adhesive prior to application of the adhesive tape, also ensure that the adhesive is not contaminated prior to application by contaminants that could adversely affect adhesive performance. In addition, by adjusting the material properties of the release materials, it is possible to adapt the unrolling behavior of adhesive tape rolls to the specifications provided for specific applications. Preference is given to a multilayer adhesive tape according to the invention, the adhesive tape additionally comprising at least one release liner which is arranged on the adhesive layer, the release liner preferably consisting of one or more materials selected from the list consisting of thin glass, paper, plastic, metal and Laminates of these materials are preferably selected from the list consisting of

Polyethylene vinyl alcohol, polyethylene naphthalate, polyethylene, polypropylene, cycloolefin copolymers, polyvinylidene chloride and papers or metal foils coated with these plastics.

In order to ensure efficient removal of the release materials from the underlying adhesives, the release materials used are frequently those materials which have a release coating on their surface, crosslinkable silicone systems frequently being used as the release coating. In this context, the person skilled in the art also speaks of a siliconization of the separating materials or of a silicon release layer. A multilayer adhesive tape according to the invention is preferred, the release liner on the surface facing the adhesive layer preferably being at least partially siliconized, the siliconization preferably having a dry weight of 0.1 to 5 g/m ² , particularly preferably 0.2 to 2.5 g / m ² is applied, preferably in a thickness in the range from 0.2 to 4 pm, particularly preferably in the range from 0.3 to 3 pm. Accordingly, preference is given to a multilayer adhesive tape according to the invention, in which the release liner comprises a silicone release layer, in which case the silicone release layer can be produced by crosslinking a crosslinkable silicone system, in which case the crosslinkable silicone system comprises one or more polysiloxanes, in which case the crosslinkable silicone system is preferably thermally curable in particular condensation- or addition-curable, the crosslinkable silicone system preferably comprising a crosslinking catalyst, the crosslinkable silicone system preferably being applied from solution or from emulsion or as a solvent-free silicone system, preferably as a solvent-free silicone system. The crosslinking catalysts described above include, for example, ruthenium, rhodium, palladium, osmium, indium or, in particular, platinum, their complexes and compounds and/or catalyst systems composed of several of these catalysts. It is expedient in the production of multilayer according to the invention

Adhesive tapes select the material components, in particular the composition of the pressure-sensitive adhesive and the selection of the expanded microballoons, so specifically that certain values are achieved when the cold impact resistance is determined. In this respect, preference is given to a multilayer adhesive tape according to the invention, the adhesive tape being used in determining the

Cold impact resistance according to Ford Specification FLTM BU 109-02 has a value of 7 or more, preferably 8 or more, more preferably 9 or more, most preferably a value of 10.

The Ford specification FLTM BU 109-02 is known to those skilled in the field of adhesive technology and represents an established method for evaluating cold impact resistance. In this method, 4 prepared test bars made of polyvinyl chloride are attached to a painted or varnished panel with the adhesive tapes to be tested and then conditioned for at least 4 hours at -29 °C. The panels are then subjected to a total of ten controlled impact loads. It is noted how many impact loads the bonded system withstands without one of the test bar falls. This number is recorded as the result, so scores of 10 equate to a complete pass on the test.

The invention also relates to a method for producing a multilayer adhesive tape, preferably a multilayer adhesive tape according to the invention, comprising the steps: a) producing or providing a starting plastic comprising a multiplicity of expandable microballoons, preferably unexpanded microballoons, b) producing or providing a starting pressure-sensitive adhesive comprising one or multiple polymers selected from the group consisting of

Poly(meth)acrylates and synthetic rubbers, and a large number of expandable microballoons, preferably unexpanded microballoons, or a corresponding starting pressure-sensitive adhesive mass that is at least partially dissolved in solvent, c) shaping the starting plastic with expansion of the expandable microballoons to form a backing layer which has a syntactically foamed, preferably by means Extrusion-shaped plastic comprises, d) producing at least one adhesive layer comprising a syntactically foamed pressure-sensitive adhesive comprising one of the steps i) shaping the starting pressure-sensitive adhesive with expansion of the expandable microballoons, or ii) applying the at least partially dissolved in solvent

Starting pressure-sensitive adhesive on a backing, in particular a liner, drying and optionally crosslinking the starting pressure-sensitive adhesive dissolved in solvent and subsequent temperature control to expand the expandable microballoons, e) connecting the backing layer to the at least one adhesive layer, preferably connecting the backing layer to two adhesive layers on both sides. Preference is given to methods according to the invention in which the layers produced are in contact on both sides with a carrier and/or another layer at the moment of foaming in order to obtain surfaces that are as smooth as possible despite the foaming of the expandable microballoons. The starting plastic is preferably already formed in step c) between two of the adhesive layers to be produced according to step d). The carrier layer and/or the at least one adhesive layer are preferably subjected to a corona treatment before the connection in step e).

In this respect, it is also preferred that the shaping of the backing layer in step c) and/or the bonding in step d) takes place using a calender with two or more rolls, with the foamed backing layer preferably being shaped in the calender between the already foamed layer of adhesive in order to bind it thereby to connect.

As defined above, the starting pressure-sensitive adhesive, which serves as the starting point for the syntactically foamed pressure-sensitive adhesive, can be processed either directly from the melt (option i; also referred to as hotmelt) or from the solution (option ii), in the latter case a corresponding starting PSA which is at least partially dissolved in solvent and which can be produced easily by the person skilled in the art is used. In a typical solvent-based process, for example, all the constituents of the starting pressure-sensitive adhesive are dissolved in a solvent mixture such as, for example, petrol/toluene/acetone. The expandable microballoons are, for example, suspended in petrol and stirred into the dissolved PSA. In principle, the known compounding and stirring units can be used for this purpose, care being taken to ensure that the expandable microballoons do not yet, or at least not fully, expand during mixing. As soon as the expandable microballoons are distributed homogeneously in the solution, the starting PSA dissolved in solvent can be applied to the backing, in particular a liner, using customary coating systems. For example, the coating by a doctor blade on a conventional PET liner. In the next step, the coating is dried at 100° C. for 15 minutes, for example, in order to remove the solvent, and crosslinked if necessary. In none of the above steps is there a complete expansion of the expandable microballoons. After drying, the adhesive layer produced in this way is lined with a second layer of PET liner, for example, and foamed in the oven within a suitable temperature window, for example at 130 to 180 °C, in order to produce a particularly smooth surface.

In the production of the adhesive layer from the melt, the expansion of the expandable microballoons is typically achieved through the temperature and residence time of the starting pressure-sensitive adhesive in the extruder. It is preferred here if the constituents of the starting pressure-sensitive adhesive, in particular the polymers and the expandable microballoons and optionally further additives, are mixed in a mixing unit and heated to expansion temperature, so that the expandable microballoons already at least partially expand during mixing. The starting pressure-sensitive adhesive, together with the expanded microballoons, is then shaped in a roller applicator to give an adhesive layer, which can be applied, for example, first to a release material in sheet form. Alternatively or additionally, it can be advantageous if an overpressure is set during the mixing, so that the expansion of the expandable microballoons is made more difficult during the mixing. In this case, the expandable microballoons expand, especially when they exit the mixing unit. When processing microballoons that have already expanded, it can happen that the microballoons float due to their low density in the pressure-sensitive adhesive or the plastic into which they are to be incorporated, making it difficult to achieve a homogeneous distribution of the expanded microballoons that is fundamentally desirable later on achieve multilayer tape. In these cases, the concentration is higher in the upper area of the respective layer, so that a density gradient occurs over the layer thickness. For this reason, the method according to the invention propose that still expandable microballoons, ie not yet fully expanded microballoons, preferably even completely unexpanded microballoons, should be used in both materials to be syntactically foamed. Advantageously, particularly homogeneously foamed layers can be obtained with this method. In addition, it has proven to be particularly advantageous with a view to interlaminate adhesion if the

The coating process, i.e. the production of the layer system, takes place before the microballoons expand.

A process according to the invention is preferred, where the process is a continuous or a semi-continuous process, preferably a continuous process. It can be regarded as a great advantage of the method according to the invention that it is thereby possible to produce multilayer adhesive tapes according to the invention in a continuous method. In the light of the above statements, it is obvious to the person skilled in the art that the invention also relates to the simultaneous use of expanded microballoons in the backing layer and the adhesive layer of a multilayer adhesive tape, preferably a multilayer adhesive tape according to the invention, to increase cold impact resistance. Finally, the invention relates to the use of a multilayer adhesive tape according to the invention for the production of cold-resistant bonds on medium- and low-energy substrates.

In the context of the present invention, medium- and low-energy substrates are understood to mean substrates with a surface energy of <100 mJ/m ² , preferably <50 mJ/m ² . Preferred low-energy substrates are selected from the group consisting of painted surfaces, varnished surfaces, plastic surfaces, preferably selected from the group consisting of polyolefin surfaces (e.g. PE, PP or PE/PP copolymer surfaces), ethylene-propylene Diene terpolymer (EPDM) surfaces, ABS (acrylonitrile butadiene styrene) surfaces, polycarbonate surfaces and surfaces painted with clear coats.

The invention is explained in more detail below using exemplary embodiments. Production of the carrier layers

A 100 l glass reactor conventional for free-radical polymerizations was filled with 1.5 kg of acrylic acid, 25.5 kg of butyl acrylate, 13.0 kg of 2-ethylhexyl acrylate and 26.7 kg of acetone/isopropanol (94:6). After nitrogen gas had been passed through it with stirring for 45 minutes, the reactor was heated to 58° C. and 30 g of AIBN were added. The external heating bath was then heated to 75° C. and the reaction was carried out constantly at this external temperature. After a reaction time of 1 h, another 30 g of AIBN (2,2'- ^azobis (isobutyronitrile)) were added. After 4 and 8 hours, the mixture was diluted with 10.0 kg of acetone/isopropanol (94:6) each time. To reduce the residual initiators, 90 g each of bis(4-tert-butylcyclohexyl)peroxydicarbonate were added after 8 hours and after 10 hours. The reaction was terminated after a reaction time of 24 h and cooled to room temperature. The polyacrylate (AC) has a K value of 77.8, a solids content of 55.9%, an average molecular weight of Mw=1,040,000 g/mol, polydispersity D (Mw/Mn)=13.3 and a static one Glass transition temperature of Tg = - 45.1 °C.

The polyacrylate (AC) produced in this way was largely freed from solvent (residual solvent content <0.3% by weight) using a single-screw extruder (concentration extruder, Berstorff GmbH, Germany). The parameters for the concentration of the polyacrylate (AC) were as follows: the screw speed was 150 rpm, the motor current was 15 A and a liquid throughput of 58.0 kg/h was realized. A vacuum was applied to three different domes for concentration. The negative pressures were in each case between 20 mbar and 300 mbar. The outlet temperature of the concentrated hotmelt was around 115 °C. After this concentration step, the solids content was 99.8%. The concentrated polyacrylate (AC) was melted in a feed extruder and conveyed with it as a polymer melt via a heatable hose into a planetary roller extruder (PWE) from ENTEX (Bochum) (in particular, a PWE with four independently heatable modules was used).

Pentaerythritol tetraglycidyl ether (Polypox R16) was added as a crosslinking agent to the concentrated polyacrylate (AC). If intended for an experiment, additional additives such as microballoons or hollow glass spheres were added via the dosing opening. All components were mixed to form a homogeneous polymer melt.

The respective polymer melt was transferred into a twin-screw extruder (from BERSTORFF) by means of a melt pump. Triethylenetetramine (Epikure 925) was added as an accelerator component. The entire mixture was freed from all gas inclusions in a vacuum dome at a pressure of 175 mbar. Following the vacuum zone, there was a blister on the screw, which allowed pressure to build up in the subsequent segment. By suitably controlling the extruder speed and the melt pump, a pressure of more than 8 bar was built up in the segment between the blister and the melt pump and, if necessary, microballoons or hollow glass balls were added and incorporated homogeneously into the premix using a mixing element.

The resulting melt mixture was transferred to a die. After leaving the nozzle, ie after a drop in pressure, the microballoons that may have been incorporated expanded, and the pressure drop resulted in low-shear, in particular shear-free, cooling of the polymer mass. In this way, a (possibly syntactically foamed) carrier layer was obtained, which was then either coated between two release materials, which can be reused after removal (process liner), and formed into a web using a roller calender (offline) or directly using a roller calender between the foamed pressure-sensitive adhesive layers to be applied thereto in web form (inline).

Hollow glass spheres (GHK) with a diameter d50 of 21 gm (12-38 gm), which are available under the trade name Sphericel 45P25 from Potters Industries, were used as hollow glass spheres. Microballoons (MB) used were microballoons available under the trade name Expancel 920DU40 (dry, unexpanded microballoons; diameter 10-16 gm (d50) expansion start temperature 123-133° C.; TMA density <17 kg/m ³ ).

In further tests, instead of the carrier layer produced above, a polyethylene foam with a density of 160 kg/m ³ was used, which is available under the trade name SCHAUM WEISS D1100 TEE SRZ 0701.1 (PE foam).

Production of the adhesive layers:

Five different pressure-sensitive adhesives were produced by mixing the individual components in a manner customary in the industry. Figures in % are mass percentages based on the total mass of the pressure-sensitive adhesive. The trade names and CAS numbers of the compounds used are given in brackets where appropriate.

A 25% SBS (78% by weight diblock, block polystyrene content: 32% by weight; Kraton D1118, 9003-55-8), 25% SBS (16% by weight diblock,

Block polystyrene content: 31% by weight; Kraton D1101, 9003-55-8), 447% a-pinene resin (softening point: 115 °C; Dercolyte A115; 25766-18-1), 2% pioneer 2070 P antioxidant 1% (Irganox 1010);

B 25% SBS (78% by weight diblock, block polystyrene content: 31% by weight; Kraton D1118, 9003-55-8), 25% SBS (radial, Calprene 411; 30%

polystyrene content), 50% a-pinene resin (softening point: 115 °C; Dercolyte A115; 25766-18-1), antioxidant 1% (Irganox 1010);

C 80% acrylate component (5% acrylic acid, 47.5% butyl acrylate, 47.5% 2-ethylhexyl acrylate), 20% SBS (Calprene 7318; 31% block polystyrene content; D 40% SBS (Calprene 4202, block polystyrene content 30%), 7% SBS (Calprene 7318; 31% block polystyrene content), 49% (Dercolyte A115), 3% soft hydrocarbon resin (Wingtack 10), 1% antioxidant (Irganox 1010); E 100% acrylate component (43% 2-ethylhexyl acrylate, 45% butyl acrylate, 12% acrylic acid).

This pressure-sensitive adhesive was either used unchanged in the experiments or previously mixed with microballoons available under the trade name Expancel 920DU20 (dry unexpanded microballoons; diameter 5-9 pm (d50) expansion start temperature 120-145 ° C; TMA density <25 kg / m ³ ). The content of microballoons (MB) is given below in percent by mass, based on the mass of the adhesive.

The adhesive layers were formed from the starting PSA dissolved in solvent, which was applied to liners, by drying the solvent. If necessary, the microballoons were expanded by tempering.

Before the microballoons are foamed, the person skilled in the art does not speak of a syntactically foamed adhesive. In the present experiments, the microballoons were foamed only in selected pressure-sensitive adhesives in order to obtain syntactically foamed pressure-sensitive adhesives in accordance with the present invention.

As explained above, the structure of the three-layer systems examined can take place inline or offline.

“Inline” here means that the syntactically foamed PSA layers are produced separately from the foamed backing layer, but the structure of the multilayer product takes place directly together with the shaping of the syntactically foamed backing layer in such a way that the backing layer is formed into the web between the PSA layers to be applied to it . The pressure-sensitive adhesive layers are, if required, immediately before application to the foamed Backing layer corona-treated with a corona system from VITAPHONE, Denmark, at 70 W-min/m ² .

"Offline", on the other hand, means that the carrier layer and the adhesive layer are first produced in isolation, then corona-treated with a corona system from VITAPHONE, Denmark, at 70 W-min/m ² and then laminated to one another, whereby these Steps were taken at separate devices/stations.

The multilayer adhesive tapes, each consisting of two adhesive layers and the backing layer, were produced using the offline process described above.

The multilayer adhesive tapes produced within the scope of the present invention were subjected to the determination of the cold impact resistance according to the known Ford specification FLTM BU 109-02. In each case, 4 prepared test bars made of polyvinyl chloride are attached to a painted or varnished panel with the adhesive tapes to be tested and then conditioned at -29 °C for at least 4 hours. The panels are then subjected to controlled impact loads a total of ten times and it is recorded how many impact loads the bonded system can withstand without one of the test bars falling off. This number is recorded as the result, so that values of 10 correspond to a complete passing of the test.

The samples examined and the results obtained for determining the cold impact resistance (KSB, as an average over several measurements) are summarized in Table 1 below, where d in each case denotes the thickness of the respective adhesive layers or the backing layers.

In Table 1, the multilayer adhesive tapes Nos. A4 to A7, B2 to B4, C2, D1 to D3 and E2 are multilayer adhesive tapes according to the present invention.

The comparison of the results for the comparison adhesive tapes A1, A3 and A9 with the multilayer adhesive tapes according to the invention makes it clear that a better cold impact resistance can be achieved when constructing multilayer adhesive tapes in accordance with the present invention. It should be noted here that very good low-temperature impact resistance can be achieved in particular with the multilayer adhesive tapes numbers A6 and A7, although the adhesive composition A used has, on average, comparatively poor low-temperature impact resistance.

Consideration of Comparative Sample Numbers A2 and A8 makes it clear that the benefits of the present invention are not observed when glass bubbles are substituted for the expanded microballoons, although this also results in syntactic foaming.

In particular for the multilayer adhesive tapes of the invention, which use the adhesives of type B, C, D or E, excellent cold impact resistances are achieved in each case. It is consistently shown here that the cold impact resistance obtained for multilayer adhesive tapes of the invention is significantly improved compared to the adhesive tapes in which at least one layer is not foamed.

In this respect, the experimental results make it clear that particularly advantageous cold impact resistances can be achieved with multilayer adhesive tapes of the invention.

Table 1

Claims

patent claims

1. Multilayer adhesive tape comprising a backing layer and at least one adhesive layer arranged on the backing layer, the backing layer comprising a syntactically foamed plastic, the adhesive layer comprising a syntactically foamed pressure-sensitive adhesive, the syntactically foamed pressure-sensitive adhesive comprising one or more polymers which are selected from from the group consisting of poly(meth)acrylates and synthetic rubbers, and wherein the syntactically foamed plastic and the syntactically foamed PSA comprise a multiplicity of expanded microballoons.

2. Multilayer adhesive tape according to claim 1, wherein the pressure-sensitive adhesive comprises at least one adhesive resin, preferably with a mass fraction of 1 to 55% if the pressure-sensitive adhesive comprises a poly(meth)acrylate as the main component, and preferably with a mass fraction of 10 to 70%, if the pressure-sensitive adhesive comprises a synthetic rubber as the main component.

3. Multilayer adhesive tape according to claim 1 or 2, wherein the syntactically foamed plastic comprises a polymer selected from the group consisting of polyolefins, polyesters, poly(meth)acrylates, polyvinyl chloride and polyethylene terephthalate,

4. Multilayer adhesive tape according to one of claims 1 to 3, wherein the PSA comprises synthetic rubbers with a mass fraction of 40% or more and/or wherein the PSA comprises poly(meth)acrylate with a mass fraction of 40% or more.

5. Multilayer adhesive tape according to any one of claims 1 to 4, wherein the pressure-sensitive adhesive and/or the plastic comprise expanded microballoons in a mass fraction of 0.1 to 10%.

6. The multilayer adhesive tape as claimed in any of claims 1 to 5, where the degree of foaming of the pressure-sensitive adhesive and/or of the plastic is in the range from 70 to 100%, based on the full foaming at the starting temperature of the microballoons.

7. Multilayer adhesive tape according to any one of claims 1 to 6, wherein the

Adhesive tape scored 7 or more when determining cold impact resistance according to Ford specification FLTM BU 109-02

8. A process for producing a multilayer adhesive tape, preferably a multilayer adhesive tape according to the invention, comprising the steps: a) producing or providing a starting plastic comprising a multiplicity of expandable microballoons, preferably unexpanded microballoons, b) producing or providing a starting pressure-sensitive adhesive comprising one or more polymers, which are selected from the group consisting of poly(meth)acrylates and synthetic rubbers, and a large number of expandable microballoons, preferably unexpanded microballoons, or a corresponding starting pressure-sensitive adhesive mass which is at least partially dissolved in solvent, c) shaping the starting plastic with expansion of the expandable microballoons to form a backing layer , which comprises a syntactically foamed plastic, d) producing at least one adhesive layer comprising a syntactically foamed pressure-sensitive adhesive composition comprising one of the steps i) Ausfor men of the starting pressure-sensitive adhesive with expansion of the expandable microballoons, or ii) applying the starting pressure-sensitive adhesive, which is at least partially dissolved in solvent, to a carrier, drying and optionally crosslinking the starting pressure-sensitive adhesive dissolved in solvent and subsequent temperature control to expand the expandable microballoons, e) joining the backing layer to the at least one adhesive layer.

9. Use of expanded microballoons in the carrier layer and the

Adhesive layer of a multi-layer adhesive tape to increase cold impact resistance.

10. Use of a multilayer adhesive tape as claimed in any of claims 1 to 7 for the production of cold-resistant adhesive bonds on medium- and low-energy substrates.