Process for producing a removable adhesive tape with a foamed pressure-sensitive adhesive layer and structuring on the surface

Abstract

Method for structuring a foamed adhesive tape comprising at least one adhesive layer, wherein at least one adhesive layer is foamed, and wherein in the method the adhesive tape is structured on the surface by subsequent embossing of a liner and the adhesive tape with the aid of a structured roller, characterized in that the resulting structure creates channels in the adhesive through which a solvent can penetrate into the adhesive joint in order to be able to remove the adhesive tape again.

Description

[0001] The invention relates to a process for producing adhesive tapes which have at least one foamed adhesive layer and are structured on the surface.

[0002] Nowadays, many small components, especially in consumer electronics, are bonded with adhesive tape. It is advantageous if these adhesive tapes can be removed easily and, if possible, without leaving residue. The reason for this requirement is, on the one hand, the desire to replace defective components as easily as possible. This replacement may be necessary immediately after or during production if there were manufacturing problems, but also after extended use if the consumer discovers a defect. In addition, there are increasing efforts and legal regulations that encourage or require the recycling of various components.

[0003] The term “mobile devices” includes, for example, devices of the consumer electronics industry, which include electronic, optical and precision mechanical devices, and in the sense of this application, in particular mobile phones, smartphones and tablets.

[0004] Various methods for removing pressure-sensitive adhesive tapes are described in the literature. One simple method is to heat the tape, thereby reducing the bond strength. The disadvantage of this is that the electronic components often cannot withstand the required high temperatures and can also be damaged. Furthermore, this process softens the adhesive, causing it to lose cohesion. When the bond breaks, a portion of the adhesive remains on both surfaces, resulting in a cohesive fracture. The result is often a complex cleaning process, which is necessary to reuse at least one of the two bonding partners.

[0005] For this process, different methods have been described to bring the heat into the bonding layer, e.g. by heating the entire device in an oven or by using a laser to bring the heat into the bonding joint.

[0006] Another option for removing adhesive tape is the use of so-called stretch-release tapes. These reduce their bond strength when stretched into the bond plane. The disadvantage of this method is that a pull tab is required to pull the tape out of the bond line. This pull tab sometimes interferes with other components and detracts from the appearance. Furthermore, there are problems if the tape tears during the removal process, as it then becomes impossible to remove.

[0007] When bonding at least one transparent joining partner, an adhesive tape that cures with UV light and thus loses bond strength can also be used, such as the adhesive tape used in the production of silicone wafers. The disadvantage is that this only works with transparent joining partners.

[0008] KR 10 2019 0 035 217 A relates to an adhesive tape with a pattern formed on an underside of an adhesive layer to improve removability with the aid of a solvent.

[0009] DE 10 2006 062 247 A1 discloses an adhesive layer for bubble-free bonding, wherein the adhesive layer is formed from an adhesive mass and wherein at least one channel is introduced superficially in the adhesive mass.

[0010] DE 10 2004 037 910 A1 describes a reversible closure system for closing objects such as bags, pouches, packaging or similar with two adhesive strips, each having an upper and a lower side, wherein the adhesive strips each have a carrier, the lower side of which is coated with an adhesive, the adhesive contains expanded microballoons in a proportion of 1 wt.% to 40 wt.%, the upper side of the adhesive strips is located on the object to be closed, to close the object the two adhesive strips are joined to each other with their lower side

[0011] WO 2017 / 040 748 A1 discloses a method for producing an adhesive article for bonding glass or polymer panels in structural glazing or architectural panel applications, the method comprising contacting at least one adhesive surface with a release film having a microstructured surface to emboss the adhesive surface and thereby form a plurality of channels extending across the adhesive surface.

[0012] US Pat. No. 6,103,152 A describes an article made of a polymer foam with a surface roughness of 75 µm. The foam comprises microspheres, at least one of which is an expandable polymer microsphere.

[0013] The task was to develop a pressure-sensitive adhesive product that offers high adhesive strength yet can be removed with as little residue as possible. The attempt was made to dissolve the adhesive bond using a solvent. The solvent is applied around the bonding surface and then slowly migrates between the adhesive tape and the substrate, thereby significantly reducing the holding power. With most adhesive tapes, this process is very slow, especially for larger bonding surfaces, and is hardly suitable for industrial use. To accelerate this process, adhesive tapes with a rough surface were used, which allows the solvent to migrate into the bonding surface much more quickly through the resulting channels.

[0014] Structuring the adhesive can be achieved using various processes. The focus here is on subsequent embossing of the liner and adhesive tape after foaming.

[0015] A liner (release paper, release film) is not a component of an adhesive tape, but merely an aid in its production, storage, or further processing by die-cutting. Furthermore, unlike an adhesive tape carrier, a liner is not permanently bonded to an adhesive layer.

[0016] In the process according to the invention, the adhesive tape is structured on its surface by embossing a structure through a liner using a roller. Typically, at least one outward-facing surface of an adhesive layer of the adhesive tape is structured.

[0017] Since electronic devices are intended to be carried around at all times, they are becoming increasingly smaller and lighter. Carrying these mobile devices exposes them to increased stress, particularly mechanical stress, such as bumping into edges, dropping them, contact with other hard objects in a pocket, and even the constant movement of carrying them around. Mobile devices are also more exposed to stress due to moisture, temperature influences, and the like than "immobile" devices, which are typically installed indoors and are rarely or never moved.

[0018] These mobile devices particularly require adhesive tapes with high holding power. In some cases, there is also a desire for subsequent removal. Many applications also require high strength, even at elevated temperatures.

[0019] Furthermore, it is particularly important that the adhesive tapes do not fail in their holding power if the mobile device, such as a cell phone, is dropped and impacts the surface. The adhesive strip or bonding compound must therefore exhibit very high shock resistance.

[0020] Foamed adhesives have proven suitable for increasing shock resistance. The entire adhesive can be foamed, or, if there are multiple layers, only one of the layers.

[0021] Polymer foams can generally be produced in two ways: firstly, through the action of a propellant gas, either added as such or resulting from a chemical reaction, and secondly, by incorporating hollow spheres into the material matrix. Foams produced by the latter method are called syntactic foams.

[0022] In a syntactic foam, hollow spheres such as glass or ceramic hollow spheres (microspheres) or microballoons are embedded in a polymer matrix. This separates the cavities in a syntactic foam, and the substances contained within them (gas, air) are separated from the surrounding matrix by a membrane.

[0023] Foamed materials with hollow microspheres are characterized by a defined cell structure with a uniform size distribution of the foam cells. Hollow microspheres produce closed-cell foams without cavities, which, compared to open-cell versions, offer, among other things, better sealing against dust and liquid media. Furthermore, chemically or physically foamed materials are more susceptible to irreversible collapse under pressure and temperature and often exhibit lower cohesive strength.

[0024] Particularly advantageous properties can be achieved when expandable microspheres (also known as "microballoons") are used as microspheres for foaming. Due to their flexible, thermoplastic polymer shell, such foams exhibit greater conformability than those filled with non-expandable, non-polymeric hollow microspheres (e.g., hollow glass spheres). They are better suited to compensating for manufacturing tolerances, such as those commonly found in injection-molded parts, and, due to their foam nature, can also better compensate for thermal stresses.

[0025] Furthermore, the selection of the thermoplastic resin for the polymer shell can further influence the mechanical properties of the foam. For example, even if the foam has a lower density than the matrix, it is possible to produce foams with higher cohesive strength than with the polymer matrix alone. This allows typical foam properties such as adaptability to rough substrates to be combined with high cohesive strength for self-adhesive foams.

[0026] When using expandable microballoons, there is often the problem that the microballoons protrude from the adhesive mass after foaming and thus significantly reduce the bond strength of the adhesive tape.

[0027] WO 2009 / 090119 A1 describes a pressure-sensitive adhesive containing expanded microballoons, wherein the adhesive strength of the adhesive containing the expanded microballoons is reduced by a maximum of 30% compared to the adhesive strength of an adhesive of identical basis weight and formulation, which is defoamed by destroying the cavities created by the expanded microballoons. In this process, an at least partially foamed pressure-sensitive adhesive is formed between two liners using at least two rollers. The high pressure in the roller gap pushes the microballoons, which break through the surface, back into the polymer matrix to create a smooth surface without disruptive broken microballoons.

[0028] The adhesive layers used in adhesive tapes are typically pressure-sensitive adhesives. A pressure-sensitive adhesive is an adhesive that allows for a permanent bond to almost all substrates even under relatively light pressure and can be removed from the substrate after use, leaving essentially no residue. A pressure-sensitive adhesive is permanently tacky at room temperature, meaning it has a sufficiently low viscosity and high tackiness so that it wets the surface of the respective substrate even under light pressure. The bondability of the adhesive is based on its adhesive properties, and its removability on its cohesive properties. According to the invention, the terms "pressure-sensitive adhesive" and "self-adhesive" (or terms derived therefrom) are used synonymously.

[0029] The adhesives used according to the invention can be based on different polymers or polymer blends. An adhesive based on a specific polymer (or a specific polymer blend) typically means that the elastomer portion of the adhesive consists of at least 50 wt.% of this polymer or polymer blend, and preferably at least 90 wt.%. In a further preferred embodiment, the adhesive contains no other elastomers besides the base polymer in amounts that significantly influence the essential properties of the base polymer. Adhesives based on vinyl aromatic block copolymers

[0030] Synthetic rubber-based pressure-sensitive adhesives containing vinyl aromatic block copolymers are well-known and used in a wide variety of applications. The advantages of this type of pressure-sensitive adhesive are their high bond strength on substrates of varying surface energies, and especially on substrates with low surface energies. They also boast very high holding power under typical ambient conditions.

[0031] Pressure-sensitive adhesives based on vinylaromatic block copolymers that exhibit advantageous shock resistance are also described. DE 10 2016 202 018 A1 teaches that improved shock resistance can be achieved through the selection of suitable block copolymers. Furthermore, it is possible to improve shock resistance if the pressure-sensitive adhesive is foamed and contains, for example, expanded microballoons for this purpose.

[0032] To impart pressure-sensitive adhesive properties to polydienes, they must be mixed with adhesive resins. This also applies to vinylaromatic block copolymers containing polydiene blocks.

[0033] EP 3 075 775 A1 describes foamed block copolymer blends with adhesive resins (combinations). In addition to hydrocarbon and polyterpene resins, oxygen-containing adhesive resins can be used, but these are not further specified.

[0034] DE 10 2008 056 980 A1 and DE 10 2008 004 388 A1 teach formulations made tacky with adhesive resins and disclose a formulation containing, among other ingredients, a polystyrene-polyisoprene block copolymer and, for example, a rosin ester. The adhesives contain a relatively high proportion of microballoons, which correspondingly leads to very low densities. Elastomer component (a)

[0035] The elastomer component typically contains at least one synthetic rubber in the form of a block copolymer with a structure ABA, (AB) n , (AWAY) n X or (ABA) n X, where - the blocks A independently represent a polymer formed by polymerization of at least one vinyl aromatic compound; - the blocks B independently represent a polymer formed by polymerization of conjugated dienes having 4 to 18 C atoms; - X represents the residue of a coupling reagent or multifunctional initiator and - n stands for an integer ≥ 2.

[0036] In one embodiment, all synthetic rubbers of the pressure-sensitive adhesive (layer) can be block copolymers with a structure as described above. The pressure-sensitive adhesive (layer) can thus also contain mixtures of various block copolymers with a structure as described above.

[0037] The at least one suitable block copolymer typically comprises one or more rubber-like blocks B (soft blocks) and at least two glass-like blocks A (hard blocks). The elastomer component can additionally contain one or more diblock copolymers AB. In particular, the synthetic rubber of the pressure-sensitive adhesive (layer) is a mixture of block copolymers with a structure AB, ABA, (AB)3X, or (AB)4X, which preferably contains at least diblock copolymers AB and / or triblock copolymers ABA. Typically, pressure-sensitive adhesives or pressure-sensitive adhesive layers used are those based on block copolymers containing polymer blocks predominantly formed from vinylaromatics (A blocks), preferably styrene, and those predominantly formed by polymerization of 1,3-dienes (B blocks), such as butadiene and isoprene, or a copolymer thereof.

[0038] The block copolymers of the pressure-sensitive adhesives or pressure-sensitive adhesive layers preferably have polystyrene end blocks.

[0039] Instead of the preferred polystyrene blocks, polymer blocks based on other aromatic-containing homo- and copolymers (preferably C8 to C 12 -aromatics) with glass transition temperatures according to Test IV of greater than 75 °C, such as α-methylstyrene-containing aromatic blocks, can be used. Furthermore, identical or different A blocks can also be included.

[0040] Preferred conjugated dienes as monomers for soft block B are selected in particular from the group consisting of butadiene, isoprene, and mixtures of these monomers and their hydrogenated homologues. Block B can also be present as a homopolymer or as a copolymer.

[0041] The proportion of hard block (Block A) in the block copolymers is at least 12 wt.% and at most 40 wt.%, preferably at least 15 wt.% and at most 35 wt.%

[0042] In a preferred embodiment, the proportion of vinyl aromatic block copolymers, in particular styrene block copolymers, in total based on the entire pressure-sensitive adhesive (layer) is at least 35 wt.% and at most 65 wt.%, more preferably at least 45 wt.% and at most 55 wt.%. Adhesive resin component (b)

[0043] In addition to the at least one vinylaromatic block copolymer, the foamable pressure-sensitive adhesives or foamed pressure-sensitive adhesive layers comprise at least one adhesive resin to enhance adhesion as desired. The adhesive resin should be compatible with the elastomer block of the block copolymers.

[0044] According to the general understanding of those skilled in the art, an “adhesive resin” is understood to mean an oligomeric or polymeric resin that increases the adhesion (tack, inherent stickiness) of the pressure-sensitive adhesive compared to a pressure-sensitive adhesive that does not contain an adhesive resin but is otherwise identical.

[0045] Adhesive resins are special compounds with low molecular weight compared to elastomers, usually with a weight average molecular weight (Test V) M W < 5,000 g / mol. Typically, the weight-average molecular weight is from 400 to less than 5,000 g / mol, preferably from 500 to 2,000 g / mol.

[0046] Suitable adhesive resins include, among others, preferably non-hydrogenated, partially, or fully hydrogenated resins based on rosin or rosin derivatives, hydrogenated polymers of dicyclopentadiene, non-hydrogenated, partially, selectively, or fully hydrogenated hydrocarbon resins based on C-5, C-5 / C-9, or C-9 monomer streams, or polyterpene resins based on α-pinene and / or β-pinene and / or δ-limonene. The aforementioned adhesive resins can be used alone or in mixtures. In addition, the adhesive formulation can also contain adhesive resins that are liquid at room temperature. Soft resin component (c)

[0047] The optionally usable soft resin or soft resin mixture typically has a softening temperature of < 30 °C (*Ring & Ball, Test VI). The soft resin can be a rosin-based or, very preferably, a hydrocarbon- or polyterpene-based soft resin. The soft resin or soft resin mixture is used in a proportion of 0 wt.% to 15 wt.%, and preferably of at least 2 wt.% and at most 10 wt.%, of the total adhesive (layer). An excessively high proportion of soft resin leads to a reduction in cohesion, which negatively affects the thermal shear strength. Optional additional components (d)

[0048] Further additives, primarily protective agents, can be added to the foamable pressure-sensitive adhesive (PSA) or foamed PSA layer, as well as to any non-foamed adhesive layer. These include primary and secondary anti-aging agents, light and UV stabilizers, and flame retardants, as well as fillers, dyes, and pigments. The adhesive (or layer) can thus be colored as desired and be white, gray, or black.

[0049] As such or other additional additives can typically be used: • Plasticizers such as low molecular weight liquid polymers, such as low molecular weight polybutenes, preferably in a proportion of 0.2 to less than 5 wt.% based on the total weight of the pressure-sensitive adhesive (layer) • primary antioxidants such as sterically hindered phenols, preferably in a proportion of 0.2 to 1 wt.% based on the total weight of the pressure-sensitive adhesive (layer), • secondary antioxidants, such as phosphites or thioethers, preferably in a proportion of 0.2 to 1 wt.% based on the total weight of the pressure-sensitive adhesive (layer), • Process stabilizers such as C radical scavengers, preferably in a proportion of 0.2 to 1 wt.% based on the total weight of the pressure-sensitive adhesive (layer), • Light stabilizers such as UV absorbers or sterically hindered amines, preferably in a proportion of 0.2 to 1 wt.% based on the total weight of the pressure-sensitive adhesive (layer), • processing aids, preferably in a proportion of 0.2 to 1 wt.% based on the total weight of the pressure-sensitive adhesive (layer), • Endblock reinforcing resins, if desired, preferably in a proportion of 0.2 to 10 wt.% based on the total weight of the pressure-sensitive adhesive (layer) and • optionally further polymers, preferably of an elastomeric nature; correspondingly usable elastomers include, inter alia, those based on pure hydrocarbons, for example unsaturated polydienes such as natural or synthetically produced polyisoprene or polybutadiene, chemically substantially saturated elastomers such as saturated ethylene-propylene copolymers, α-olefin copolymers, polyisobutylene, butyl rubber, ethylene-propylene rubber, and chemically functionalized hydrocarbons such as halogen-containing, acrylate-containing, allyl- or vinyl ether-containing polyolefins, preferably in a proportion of 0.2 to 10 wt.% based on the total weight of the pressure-sensitive adhesive (layer).

[0050] The type and quantity of the blending components can be selected as needed, and the latter can also be higher than the preferred upper limits. It is also in accordance with the invention if the adhesive layer does not contain some or even all of the additives mentioned. Nitrile rubber adhesives

[0051] In addition to vinylaromatic block copolymers, other elastomers can also serve as the basis for the pressure-sensitive adhesive. Another well-suited elastomer group is nitrile rubber.

[0052] Adhesives based on nitrile rubbers contain, as a base polymer, at least one or more solid acrylonitrile-butadiene rubbers and tackifier resins, with the proportion of tackifier resins being 20-55 wt.%, and the acrylonitrile content in the solid acrylonitrile-butadiene rubber(s) being between 10 and 30 wt.%. Preferably, the acrylonitrile content in the solid acrylonitrile-butadiene rubber(s) is between 10 and 25 wt.%.

[0053] Inert release agents such as talc, silicates (talc, clay, mica), zinc stearate, and PVC powder can be added to the acrylonitrile-butadiene rubbers, particularly in a concentration of 3% by weight. The release agents are preferably selected from the group consisting of talc, silicates (talc, clay, mica), zinc stearate, and PVC powder.

[0054] Furthermore, thermoplastic elastomers such as synthetic rubbers can preferably be added to the acrylonitrile butadiene rubber in a proportion of up to 5 wt.% to improve processability. Representative examples include the particularly compatible styrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS) grades.

[0055] These pressure-sensitive adhesives may contain, in addition to one or more solid acrylonitrile-butadiene rubber(s), at least one liquid acrylonitrile-butadiene rubber, wherein the acrylonitrile content in the liquid acrylonitrile-butadiene rubber(s) is also between 10 and 30 wt.%.

[0056] The proportion of liquid acrylonitrile butadiene rubber is preferably up to 20 wt.%.

[0057] Liquid rubbers are distinguished from solid rubbers by their softening point of < 40 °C.

[0058] The data on the softening point TE of oligomeric and polymeric compounds, such as resins, refer to the ring-and-ball method according to DIN EN 1427:2007 with appropriate application of the provisions (examination of the oligomer or polymer sample instead of bitumen with otherwise unchanged procedure); the measurements are carried out in a glycerol bath.

[0059] Preferably, no other polymer is present in the pressure-sensitive adhesive besides the acrylonitrile-butadiene rubber. In this case, the pressure-sensitive adhesive is a composition of solid and liquid acrylonitrile-butadiene rubber, one or more tackifier resins, preferably anti-aging agents, and optionally release agents, which represents a preferred embodiment. Furthermore, the plasticizers, fillers, and / or dyes explained below may optionally be present in small amounts.

[0060] Alternatively, the base polymer contains more than 90 wt.%, preferably more than 95 wt.%, of solid and liquid acrylonitrile butadiene rubber.

[0061] As adhesive resins, hydrogenated and non-hydrogenated hydrocarbon resins and polyterpene resins can be used as the main component in the self-adhesive composition. Particularly suitable adhesive resins include hydrogenated polymers of dicyclopentadiene and hydrogenated polymers of preferably C8 and C9 aromatics. These can be obtained by hydrogenating polymers from pure aromatic streams or by hydrogenating polymers based on mixtures of different aromatics. Also suitable are partially hydrogenated polymers of C8 and C9 aromatics, hydrogenated polyterpene resins, hydrogenated C5 / C9 polymers, and aromatic-modified selectively hydrogenated dicyclopentadiene derivatives. The aforementioned adhesive resins can be used alone or in mixtures.

[0062] Hydrogenated hydrocarbon resins are particularly suitable as blending components, as described for example in EP 0 447 855 A1, US 4,133,731 A and US 4,820,746 A, since the absence of double bonds means that crosslinking cannot be disturbed.

[0063] In addition, non-hydrogenated resins can also be used if crosslinking promoters such as polyfunctional acrylates are used. Other non-hydrogenated hydrocarbon resins, non-hydrogenated analogues of the hydrogenated resins described above, can also be used.

[0064] Rosin-based resins can also be used.

[0065] The use of polyterpene resins based on α-pinene and / or β-pinene and / or δ-limonene or terpene phenolic resins is particularly preferred.

[0066] To stabilize the pressure-sensitive adhesive, primary antioxidants such as sterically hindered phenols, secondary antioxidants such as phosphites or thioethers and / or C radical scavengers are usually added.

[0067] Any combination of these can be used to tailor the properties of the resulting pressure-sensitive adhesive as desired. Reference is made to the current state of knowledge in the "Handbook of Pressure Sensitive Adhesive Technology" by Donatas Satas (van Nostrand, 1989).

[0068] The weight amount of the resins is 20 to 55 wt.%, preferably 30 to 55 wt.%, more preferably 35 to 50 wt.%.

[0069] The acrylonitrile butadiene rubber-based pressure-sensitive adhesive may contain additives such as fillers, dyes or age-protection agents (antiozonants, antioxidants (primary and secondary), light stabilizers, etc.) to adjust optical and adhesive properties.

[0070] The following additives are typically used for the adhesive: • primary antioxidants such as sterically hindered phenols • secondary antioxidants such as phosphites or thioethers • Light stabilizers such as UV absorbers or sterically hindered amines

[0071] The fillers can be reinforcing or non-reinforcing. The most common fillers are silicon dioxide (spherical, acicular, or irregular, such as fumed silica), phyllosilicates, calcium carbonates, zinc oxides, titanium dioxides, aluminum oxides, or aluminum oxide hydroxides.

[0072] The concentration of the additives influencing the optical and adhesive properties is preferably up to 20 wt.%, more preferably up to 15 wt.%, more preferably up to 5 wt.%.

[0073] According to the invention, the proportions of all added substances (besides acrylonitrile butadiene rubber and adhesive resin) such as synthetic rubbers and / or thermoplastic elastomers and / or fillers and / or dyes and / or anti-aging agents should not exceed 5 wt.%, preferably 2 wt.%.

[0074] In order to maintain sufficient cohesion and suppress the cold flow of the adhesives, it is advantageous to crosslink the nitrile rubber compounds so that the structures remain even after removal of the liner.

[0075] There are basically two ways to crosslink. Chemical crosslinking can occur via the remaining double bonds, e.g., through addition reactions or radical crosslinking. Crosslinking through radiation, such as UV or, preferably, electron beams, is also possible. Polyacrylates

[0076] In addition to the adhesives described, those based on polyacrylates can also be used.

[0077] "Polyacrylates" are polymers whose molar monomer base consists of at least 30% acrylic acid, methacrylic acid, acrylic acid esters, and / or methacrylic acid esters, with acrylic acid esters and / or methacrylic acid esters generally being present at least proportionally, and preferably at least 30%. In particular, a "polyacrylate" is understood to mean a polymer obtainable by radical polymerization of acrylic and / or methylacrylic monomers and, optionally, other copolymerizable monomers.

[0078] Preferably, a polyacrylate is used that can be traced back to the following monomer composition: a) Acrylic acid esters and / or methacrylic acid esters of the following formula CH2 = C(R I )(COO II ) where R I = H or CH3 and R II is an alkyl radical with 4 to 14 C atoms, b) olefinically unsaturated monomers with functional groups that are reactive, for example, towards epoxy groups, c) optionally further acrylates and / or methacrylates and / or olefinically unsaturated monomers which are copolymerizable with component (a).

[0079] For the application of the polyacrylate as a pressure-sensitive adhesive, the proportions of the corresponding components (a), (b), and (c) are selected such that the polymerization product has, in particular, a glass transition temperature ≤ 15 °C (DMA at low frequencies).

[0080] It is particularly advantageous for the production of pressure-sensitive adhesives to select the monomers of component (a) in a proportion of 45 to 99 wt.%, the monomers of component (b) in a proportion of 1 to 15 wt.% and the monomers of component (c) in a proportion of 0 to 40 wt.% (the figures are based on the monomer mixture for the "base polymer", i.e. without the addition of any additives to the finished polymer, such as resins, etc.).

[0081] The monomers of component (a) are, in particular, plasticizing and / or nonpolar monomers. Preferably, acrylic and methacrylic acid esters with alkyl groups consisting of 4 to 14 carbon atoms, particularly preferably 4 to 9 carbon atoms, are used as monomers (a). Examples of such monomers are n-butyl acrylate, n-butyl methacrylate, n-pentyl acrylate, n-pentyl methacrylate, n-amyl acrylate, n-hexyl acrylate, hexyl methacrylate, n-heptyl acrylate, n-octyl acrylate, n-octyl methacrylate, n-nonyl acrylate, isobutyl acrylate, isooctyl acrylate, isooctyl methacrylate, and their branched isomers, such as, for example, 2-ethylhexyl acrylate or 2-ethylhexyl methacrylate.

[0082] The monomers of component (b) are in particular olefinically unsaturated monomers with functional groups, in particular with functional groups that can react with epoxy groups.

[0083] For component (b), monomers with functional groups selected from the group consisting of hydroxyl, carboxy, sulfonic, or phosphonic acid groups, acid anhydrides, epoxides, and amines are preferably used. Particularly preferred examples of monomers of component (b) are acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, aconitic acid, dimethylacrylic acid, β-acryloyloxypropionic acid, trichloroacrylic acid, vinylacetic acid, vinylphosphonic acid, itasconic acid, maleic anhydride, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, 6-hydroxyhexyl methacrylate, allyl alcohol, glycidyl acrylate, and glycidyl methacrylate.

[0084] In principle, any vinyl-functionalized compounds that are copolymerizable with component (a) and / or component (b) can be used as component (c). The monomers of component (c) can be used to adjust the properties of the resulting pressure-sensitive adhesive.

[0085] The polyacrylates can be prepared by processes familiar to those skilled in the art, particularly advantageously by conventional radical polymerizations or controlled radical polymerizations. The polyacrylates can be prepared by copolymerization of the monomeric components using conventional polymerization initiators and, if appropriate, regulators, with polymerization taking place at conventional temperatures in bulk, in emulsion, for example, in water or liquid hydrocarbons, or in solution.

[0086] The weight-average molecular weights M Wof the polyacrylates are preferably in a range from 20,000 to 2,000,000 g / mol; very preferably in a range from 100,000 to 1,000,000 g / mol, extremely preferably in a range from 150,000 to 500,000 g / mol [the data for the average molecular weight M W and the polydispersity PD in this document refer to the determination by gel permeation chromatography (see Test V; experimental section). For this purpose, it may be advantageous to carry out the polymerization in the presence of suitable polymerization regulators such as thiols, halogen compounds and / or alcohols in order to adjust the desired average molecular weight.

[0087] The poly(meth)acrylates are preferably crosslinked by linking reactions - particularly in the sense of addition or substitution reactions - of functional groups contained in them with crosslinking agents.

[0088] Preferably, crosslinkers are used in an amount of 0.1 to 5% by weight, in particular 0.2 to 1% by weight, based on the total amount of the polymer to be crosslinked.

[0089] Crosslinking via complexing agents, also known as chelates, is also possible. A preferred complexing agent, for example, is aluminum acetylacetonate.

[0090] The acrylates can also be crosslinked using radiation such as UV or electron beams.

[0091] The decisive factor here is that the adhesives are so strongly cross-linked that they show no or only very little cold flow and the structure in the adhesive is retained when the liner is removed.

[0092] To increase adhesion, adhesive resins can also be added to the polyacrylates.

[0093] These are preferably rosin resins as described above or terpene-phenolic resins. The proportion of adhesive resins is between 5 and 30 wt.%, preferably between 10 and 25 wt.%. Microballoons

[0094] The present invention relates to a foamable pressure-sensitive adhesive containing expandable, ie, unexpanded, microballoons. It also relates to a foamed pressure-sensitive adhesive layer containing at least partially expanded microballoons.

[0095] The term “at least partially expanded microballoons” is typically understood according to the invention to mean that the microballoons as a whole are expanded at least to such an extent that a reduction in the density of the adhesive is thereby effected to a technically reasonable extent compared to the same adhesive with the unexpanded microballoons. This means that the microballoons do not necessarily have to be fully expanded. Preferably, the individual microballoons, considered individually, are each expanded to at least twice their maximum expansion in the unexpanded state. Furthermore, the term “at least partially expanded microballoons” can also mean that only a portion of the microballoons in question are (partially) expanded. In a preferred embodiment of the foamed pressure-sensitive adhesive layer, the microballoons are fully expanded, i.e.the layer has been foamed in such a way that a minimum density of the layer is achieved for a given microballoon content.

[0096] Foaming occurs primarily through the introduction and subsequent expansion of microballoons.

[0097] "Microballoons" are elastic, and thus expandable in their ground state, hollow microspheres with a thermoplastic polymer shell. These spheres are filled with low-boiling liquids or liquefied gas. Polyacrylonitrile, PVDC, PVC, or polyacrylates are particularly used as shell materials. Low-boiling liquids, such as isobutane or isopentane, are particularly suitable as low-boiling liquids, which are enclosed in the polymer shell as a liquefied gas under pressure.

[0098] When exposed to external influences, particularly heat, the outer polymer shell softens. At the same time, the liquid propellant gas contained within the shell transforms into a gaseous state. The microballoons expand irreversibly and expand three-dimensionally. The expansion is complete when the internal and external pressures equalize. Since the polymer shell remains intact, a closed-cell foam is created.

[0099] A variety of microballoon types are commercially available, which differ essentially in their size (6 to 45 µm diameter in the unexpanded state) and the starting temperatures required for expansion (75 to 220°C). One example of commercially available microballoons is the Expancel ® DU types (DU = dry unexpanded) from Nouryon, another Matsumoto Microsphere ® F / FN from Matsumoto Yushi Seiyaku company.

[0100] Unexpanded microballoon grades are also available as aqueous dispersions with a solids or microballoon content of approximately 40 to 45 wt.%, as well as polymer-bound microballoons (masterbatches), for example, in ethylene-vinyl acetate with a microballoon concentration of approximately 65 wt.%. Both the microballoon dispersions and the masterbatches, like the DU grades, are conceivable for the production of a foamed pressure-sensitive adhesive.

[0101] A foamed pressure-sensitive adhesive layer can also be created using so-called pre-expanded microballoons. In this group, the expansion takes place before mixing into the polymer matrix. Pre-expanded microballoons are available, for example, under the name Dualite. ® from Chase Corp. or with the type designation Expancel DE (Dry Expanded) from Nouryon.

[0102] According to the invention, at least 90% of all cavities formed by microballoons in the foamed pressure-sensitive adhesive layer preferably have a maximum diameter of 20 to 75 µm, more preferably 25 to 65 µm. The "maximum diameter" is understood to mean the maximum extension of a microballoon in any spatial direction of the cryogenic fracture edge in the SEM.

[0103] The diameters are determined using a cryogenic fracture edge in a scanning electron microscope (SEM) at 500x magnification. The diameter of each individual microballoon is determined graphically.

[0104] When foaming is performed using microballoons, the microballoons can be added to the formulation as a batch, paste, or as an undiluted or blended powder. They can also be suspended in solvent.

[0105] According to the invention, the proportion of microballoons in the adhesive (layer) is typically between 0.3 wt.% and 2.5 wt.%, preferably between 0.5 wt.% and 2.0 wt.%, and very particularly between 0.7 wt.% and 1.7 wt.%, in each case based on the total composition of the adhesive (layer). With regard to the foamable adhesive, the data typically refer to unexpanded microballoons, and with regard to the foamed adhesive layer, typically to the unexpanded or preexpanded microballoons used.

[0106] A pressure-sensitive adhesive containing expandable hollow microspheres used according to the invention may also contain non-expandable hollow microspheres. The only crucial requirement is that almost all gas-containing cavities are sealed by a permanently sealed membrane, regardless of whether this membrane consists of an elastic and thermoplastically expandable polymer mixture or, for example, of elastic and—within the range of temperatures possible in plastics processing—non-thermoplastic glass.

[0107] More important than the quantity of microballoons used in determining the performance of the pressure-sensitive adhesive layer is its density. The density of the foamed pressure-sensitive adhesive layer, as determined according to Test VII, is at least 600 kg / m 3 and a maximum of 920 kg / m 3 , preferably at least 650 kg / m 3 and a maximum of 870 kg / m 3 , very preferably at least 700 kg / m 3 and a maximum of 820 kg / m 3. Lower densities can be achieved with larger microballoons for the same amount of material used. To achieve the desired performance for the present task, correspondingly fewer larger microballoons are used than smaller ones. The typical application range is particularly advantageous for microballoons with a maximum diameter of less than 40 µm. For microballoons with an expanded diameter of 40 µm, less than 2.0% is used.

[0108] The invention also relates to self-adhesive products, especially double-sided self-adhesive products, i.e., in particular, double-sided adhesive tapes containing at least one pressure-sensitive adhesive layer. Transfer adhesive tapes are particularly advantageous. Alternatively, the self-adhesive product can also contain a (permanent) intermediate carrier.

[0109] Self-adhesive tapes produced using at least one pressure-sensitive adhesive layer can accordingly be designed in particular as • single-layer, double-sided self-adhesive tapes, so-called “transfer tapes” made from a single layer of pressure-sensitive adhesive; • multilayer, double-sided self-adhesive tapes, in which the layers each consist of the disclosed pressure-sensitive adhesive layers or a disclosed pressure-sensitive adhesive layer and a non-disclosed pressure-sensitive adhesive layer; • adhesive tapes that are self-adhesive on both sides and have an intermediate carrier (a so-called permanent carrier) that is arranged either in an adhesive layer or between two adhesive layers.

[0110] Furthermore, an embodiment of the self-adhesive product is preferred in which the intermediate carrier consists of only a single layer, in particular a polymer film. It is also preferred if the intermediate carrier contains at least one layer of a formulation containing at least one type of vinylaromatic block copolymer and at least one type of adhesive resin. Regardless of the type of intermediate carrier, the double-sided products can have a symmetrical or asymmetrical product structure with regard to the type of pressure-sensitive adhesive layers, such as the composition and / or thickness of the pressure-sensitive adhesive layers.

[0111] Typical packaging forms for the pressure-sensitive adhesive layer are adhesive tape rolls and adhesive strips, such as those available in die-cut form. Preferably, all layers are fully bonded to one another.

[0112] The general term “adhesive tape” in the sense of this invention includes all flat structures such as films or film sections extended in two dimensions, tapes with an extended length and a limited width, tape sections and the like, and ultimately also die-cuts or labels.

[0113] The adhesive tape is, in particular, in web form. A web is defined as an object whose length is many times greater than its width, and whose width is approximately constant, preferably exactly the same, along its entire length.

[0114] The adhesive tape can be produced in the form of a roll, i.e. rolled up on itself in the form of an Archimedean spiral.

[0115] Foamed pressure-sensitive adhesive layers are used in self-adhesive products as described above. These self-adhesive products can be designed as adhesive films, adhesive tape, or adhesive die-cuts. The self-adhesive products contain at least one foamed pressure-sensitive adhesive layer. This layer can have a layer thickness of 10 to 2000 µm. The thickness is preferably between 15 µm and 250 µm, more preferably between 25 µm and 150 µm, and in particular, it is at most 100 µm. The self-adhesive products are typically designed to be double-sided adhesive. The advantages of the formulations in double-sided self-adhesive products can be particularly well utilized when two components, particularly in a mobile device, are to be bonded together.

[0116] Polyester films, and particularly preferably films based on polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), are particularly suitable as film materials for at least one layer of a possible carrier for this design. Polyester films are preferably biaxially oriented. Furthermore, films made of polyolefins, in particular polybutene, cycloolefin copolymer, polymethylpentene, polypropylene, or polyethylene, for example, monoaxially oriented polypropylene, biaxially oriented polypropylene, or biaxially oriented polyethylene, are conceivable. This list is intended to illustrate examples; other systems corresponding to the concept of the present invention are known to those skilled in the art.

[0117] Stretchable carriers can also be used, such as carriers based on polyurethane, polyamide or different rubbers.

[0118] Metal foils, fabrics, fleeces or other knitted or woven carriers or paper carriers are also conceivable.

[0119] Furthermore, sheet-like foams (e.g. made of polyethylene and polyurethane) are suitable.

[0120] The intermediate supports can be designed in multiple layers.

[0121] Furthermore, the intermediate carriers can have cover layers, for example, barrier layers that prevent components from the adhesive from penetrating the intermediate carrier or vice versa. These cover layers can also have barrier properties to prevent the diffusion of water vapor and / or oxygen.

[0122] To better anchor the pressure-sensitive adhesives to the intermediate carrier, the intermediate carriers can be pretreated using conventional methods such as corona, plasma, or flame treatment. The use of a primer is also possible. Ideally, however, pretreatment can be omitted.

[0123] The thickness of the intermediate carrier layer is typically in the range of 2 µm to 200 µm, preferably between 5 and 100 µm and in particular between 10 and 80 µm.

[0124] Preferably, the adhesive tape does not contain a film carrier and consists only of one or more layers of the adhesive.

[0125] The adhesive tape is covered on one or both sides with a liner, i.e. a temporary carrier that is coated with an anti-adhesive layer on one or both sides.

[0126] If expandable microballoons are used in the adhesive layers, it is advantageous if the liner can withstand a foaming process at high temperatures, e.g. 150 - 200°C, depending on the microballoons used, without damage.

[0127] Many structured liners have a polyethylene coating that provides the structure. However, this PE coating melts at high temperatures, which can lead to transfer of the silicone or polyethylene into the adhesive. This problem can be avoided by spreading the adhesive onto a smooth liner, preferably made of polyethylene terephthalate (PET), and covering it with a similar liner. Structuring then occurs.

[0128] When using expandable microballoons, there is often the problem that the microballoons protrude from the adhesive mass after foaming and thus significantly reduce the bond strength of the adhesive tape.

[0129] One way to circumvent this is to form the at least partially foamed adhesive between two liners using at least two rollers. The high roller pressure pushes the microballoons back into the adhesive, creating a smooth surface without any disruptive broken microballoons.

[0130] Structuring then takes place, ideally while the compound is still warm. The still-warm composite of foamed adhesive between two liners is passed over a structured roller with a corresponding counter roller. If the pressure is sufficient, the roller's structure is pressed into the PET liner and thus also into the compound. The counter roller is advantageously smooth so that only one side of the adhesive is structured. The roller is ideally heated. It has proven advantageous if the liner has a temperature of 100 to 200°C during structuring, preferably between 100 and 170°C. At higher temperatures, structuring can be achieved at lower pressure.

[0131] Instead of a structured roller, a printing plate applied to a roller can also be used for structuring.

[0132] Advantageously, the depth to which the structure is pressed into the product is between 5 and 40µm, preferably between 10 and 30µm.

[0133] The type of structure is crucial for removability. Continuous structures that create interconnected channels in the adhesive and, after bonding, in the bond joint have proven particularly advantageous. In this case, the solvent can migrate more easily and quickly into the bond joint. Grid patterns such as honeycombs, squares, or diamonds are preferred, but irregular structures such as irregular polygons are also conceivable.

[0134] The formulations, i.e. pressure-sensitive adhesives, and the coatings or self-adhesive products produced therefrom can be produced using organic solvents or solvent-free.

[0135] In the process according to the invention, the substrate is preferably a planar element, in particular a carrier material, a film, a (release) liner, a transfer material, and / or a cover material. Substrates can also be the surfaces of the production line in the manufacturing process. A pressure-sensitive adhesive is processed into at least one pressure-sensitive adhesive layer with a thickness of greater than or equal to 15 µm or even greater than or equal to 10 µm, and the applied surface-mounted pressure-sensitive adhesive layer is optionally dried or the solvents are removed.

[0136] For applying the pressure-sensitive adhesive to the surface elements used according to the invention, coating methods that can be used include, among others, doctor blade coating, nozzle coating, roller bar coating, extrusion nozzle coating, pouring nozzle coating, and pouring nozzle coating. Also within the scope of the invention are application methods such as roller coating, printing, screen printing, anilox roller coating, inkjet coating, and spraying. Hotmelt coating (extrusion, nozzle coating) is preferred.

[0137] If necessary, additional layers or material plies are then laminated or coated inline or offline so that multi-layer product structures can also be created.

[0138] In a further embodiment of the process according to the invention, the resulting combination of surface element and pressure-sensitive adhesive is cut to size and / or punched out into strips sold by the meter, and optionally the strips are rolled up into a banderol.

[0139] Finally, the invention also extends to adhesive composites obtained by using self-adhesive products containing at least one pressure-sensitive adhesive layer. This means that the invention relates to a composite comprising a typically double-sided adhesive tape and two substrates, such as, in particular, components of a mobile device, which are bonded to the adhesive tape. Accordingly, the invention also relates to the use of double-sided adhesive variants of the adhesive tapes in a composite comprising two substrates.

[0140] The surface-structured adhesive tapes are designed to be easily removable without leaving residue after bonding by applying a solvent around the bonding partners. This solvent then flows into the channels created by the structuring between the adhesive and the substrate, thereby reducing the bond strength.

[0141] In principle, almost any solvent can be used as a solvent, but it is preferable that the solvents are not absorbed very well by the adhesive masses so that they are not absorbed before they can penetrate into the channels.

[0142] Solvents that do not react with other components in the electronic device and that are neither hazardous to employees nor to the environment are particularly preferred. Therefore, ethanol and water are particularly preferred, as are water containing various detergents, such as soapy water, to reduce the surface tension of the water and facilitate its flow into the channels. Test methods

[0143] Unless otherwise stated, all measurements to determine adhesive properties were performed at 23 °C and 50% relative humidity. Test I - Adhesive strength

[0144] The bond strength is determined (according to AFERA ​​5001) as follows. A polished steel plate with a thickness of 2 mm is used as the defined substrate. The bondable surface element to be tested (which has a 36 µm etched PET film on the back as a support film) is cut to a width of 20 mm and a length of approximately 25 cm, provided with a handling section, and immediately afterwards pressed five times onto the selected substrate using a 4 kg steel roller at a feed rate of 10 m / min. Immediately afterwards, the bondable surface element is peeled off the substrate at an angle of 180° using a tensile testing device (Zwick) at a speed of v = 300 mm / min, and the force required for this purpose is measured at room temperature. The measured value (in N / cm) is the average of three individual measurements.

[0145] To measure the bond strength after exposure to ethanol, the test is modified as follows. After applying the primer and rolling it over with a steel roller, the bonded steel plate is briefly immersed in ethanol for 2 seconds. The bonded steel plate is then removed from the ethanol and placed on a horizontal table for 1 minute. Measurements are then taken as described above. Test II - Breakthrough strength; z-plane (DuPont test)

[0146] A square, frame-shaped sample is cut from the adhesive tape to be tested (external dimensions: 33 mm × 33 mm; web width: 2.0 mm; internal dimensions (window cutout): 29 mm × 29 mm). This sample is bonded to a polycarbonate (PC) frame (external dimensions: 45 mm × 45 mm; web width: 10 mm; internal dimensions (window cutout): 25 mm × 25 mm; thickness: 3 mm). A 35 mm × 35 mm PC window is bonded to the other side of the double-sided adhesive tape. The PC frame, adhesive tape frame, and PC window are bonded such that the geometric centers and diagonals overlap (corner to corner). The bonded area is 248 mm. 2 The bond is pressed for 5 s with 248 N and stored for 24 hours at 23 °C / 50 % relative humidity.

[0147] Immediately after storage, the adhesive bond consisting of the PC frame, adhesive tape, and PC window is clamped into a specimen holder with the protruding edges of the PC frame aligned horizontally. The PC frame rests flat on the specimen holder along its protruding edges, allowing the PC window to float freely beneath the PC frame (held in place by the adhesive tape pattern). The specimen holder is then centrally inserted into the designated holder of the DuPont Impact Tester. The 150 g impact head is inserted so that the circular impact geometry with a diameter of 24 mm rests centrally and flush on the surface of the PC window, which is freely accessible from above.

[0148] A 150 g weight, guided by two guide rods, is dropped vertically onto the assembled sample holder, sample, and impact head from a height of 5 cm (measurement conditions: 23 °C, 50% relative humidity). The height of the drop weight is increased in 5 cm increments until the impact energy destroys the sample due to the impact load and the PC window detaches from the PC frame.

[0149] To compare experiments with different samples, the energy is calculated as follows: E[J]=Height [m]*Mass Weight [kg]*9.81 kg / m*s2

[0150] Five samples per product are tested and the average energy value is given as an indicator of the breakdown strength. Test III - Push Out

[0151] A rectangular sample measuring 10 cm × 6 cm is cut from the adhesive tape to be tested. This sample is bonded to a rectangle of the same size made of 2 mm thick polyvinyl chloride. The other bonding partner is an aluminum body, which has a recess that is 0.5 cm larger in both directions than the PVC sheet. The aluminum body has a 3 × 3 cm hole in the center of the recess. The PVC sheet is then bonded centrally into the recess.

[0152] The two bonding partners are pressed together with 100 kg for 5 s and the test setup is stored for 24 h or 336 h (or 14 d) at room temperature (23°C) and 50% RH.

[0153] The PVC plate is then pressed out of the metal body through the hole using a stamp and the force is measured.

[0154] In the ethanol exposure test, 1 ml of ethanol is dripped around the PVC plate into the recess of the aluminum body immediately before measuring. Then wait 1 minute and then perform the test as described above. Test IV - Glass Transition Temperature (DSC)

[0155] The glass transition temperature of polymer blocks in block copolymers is determined using dynamic scanning calorimetry (DSC). Approximately 5 mg of the untreated block copolymer samples are weighed into an aluminum crucible (volume 25 µL) and sealed with a perforated lid. A Netzsch DSC 204 F1 is used for the measurement, and the sample is inertized under nitrogen. The sample is first cooled to -150 °C, heated at a heating rate of 10 K / min to +150 °C, and cooled again to -150 °C. The subsequent second heating curve is run again at 10 K / min, and the change in heat capacity is recorded. Glass transitions are recognized as steps in the thermogram. The glass transition temperature is evaluated as follows (see Fig.1). A tangent is drawn to the baseline of the thermogram before 1 and after 2 of the step. In the step area, a best-fit line 3 is drawn parallel to the ordinate so that it intersects the two tangents, creating two areas 4 and 5 (between the tangent, the best-fit line, and the measurement curve) of equal area. The intersection point of the best-fit line thus positioned with the measurement curve gives the glass transition temperature. Test V - Molar mass (GPC)(i) Peak molecular mass of individual block copolymer modes

[0156] GPC is a suitable measurement technique for determining the molar mass of individual polymer modes in mixtures of different polymers. For the block copolymers produced by living anionic polymerization that can be used in this invention, the molar mass distributions are typically sufficiently narrow so that polymer modes that can be assigned to triblock copolymers, diblock copolymers, or multiblock copolymers appear sufficiently resolved from one another in the elugram. The peak molar mass for the individual polymer modes can then be read from the elugrams.

[0157] Peak molecular masses M P are determined by gel permeation chromatography (GPC). THF is used as the eluent. The measurement is carried out at 23 °C. The pre-column used is PSS-SDV, 5 µ, 10 3 Å, ID 8.0 mm × 50 mm. For separation, columns PSS-SDV, 5 µ, 10 3 Å and 10 4 Å and 10 6Å, each with an ID of 8.0 mm × 300 mm. The sample concentration is 4 g / l, and the flow rate is 1.0 ml per minute. Measurements are made against PS standards (µ = µm; 1 Å = 10 -10 m). (ii) weight-average molecular weight, particularly of adhesive resins

[0158] The weight-average molecular weight M w (MW) is determined by gel permeation chromatography (GPC). THF is used as eluent. The measurement is carried out at 23 °C. PSS-SDV, 5 µ, 10 3 Å, ID 8.0 mm × 50 mm. For separation, columns PSS-SDV, 5 µ, 10 3 Å and 10 4 Å and 10 6 Å, each with an ID of 8.0 mm × 300 mm. The sample concentration is 4 g / l, and the flow rate is 1.0 ml per minute. Measurements are made against PS standards (µ = µm; 1 Å = 10 -10 m). Test VI - (Adhesive) resin softening temperature

[0159] The (adhesive) resin softening temperature, also called (adhesive) resin softening point, is carried out according to the relevant methodology known as Ring & Ball and standardized according to ASTM E28. Test VII - Density

[0160] The density, ie absolute density, of an adhesive or adhesive layer is determined by forming the quotient of the mass application and the thickness of the adhesive layer applied to a carrier or liner.

[0161] The mass application can be determined by determining the mass of a section of such an adhesive mass (layer) applied to a carrier or liner, defined in terms of its length and width, less the (known or separately determinable) mass of a section of the same dimensions of the carrier or liner used.

[0162] The thickness of an adhesive (layer) can be determined by determining the thickness of a section of such an adhesive layer applied to a carrier or liner, defined in terms of its length and width, minus the (known or separately determinable) thickness of a section of the same dimensions of the carrier or liner used. The thickness of the adhesive (layer) can be determined using commercially available thickness gauges (probe testing devices) with accuracies of less than 1 µm. In this application, the precision thickness gauge Mod. 2000 F from Wolf Messtechnik GmbH is used, which has a circular probe with a diameter of 10 mm (flat). The measuring force is 4 N. The value is read 1 s after loading.If variations in thickness are detected, the mean value of measurements taken at at least three representative locations is given, i.e. in particular not measured at creases, folds, spots and the like. Examples

[0163] The pressure-sensitive adhesive tapes are described below in a preferred embodiment using several examples, without intending to limit the invention in any way.

[0164] Furthermore, comparative examples are listed in which unsuitable adhesive tapes are shown.

[0165] The components of the pressure-sensitive adhesives were dissolved in 40% special-boiling-point spirit / toluene / acetone for the vinylaromatic block copolymer compositions and 30% in butanone for the nitrile rubber compositions. Both were optionally mixed with the microballoons suspended in spirit or butanone. The mixture was then spread with a spreader bar onto a PET film treated with a silicone release agent, with or without structuring, in the desired layer thickness. The solvent was then evaporated at 100°C for 15 minutes, thus drying the composition layer. This is possible in the examples listed because microballoons with an expansion temperature above 100°C are used. When using other microballoons, the skilled person will select appropriate production temperatures without departing from the scope of the present invention.

[0166] After drying, if the adhesive contained microballoons, the adhesive layer was covered with a second layer of PET liner as defined above, free of any air inclusions, and foamed for 30 s at 170 °C while suspended between the two liners in a circulating drying cabinet.

[0167] Table 1 shows the raw materials used. Table 1: Raw materials used. Elastomer component (a) Kraton D1102(Kraton) Polystyrene-polybutadiene block copolymer Linear SBS*PS content 31%* Kraton D1118(Kraton) Polystyrene-polybutadiene block copolymer Linear SBS*PS content 32%* Nipol DN 2850 (Nippon Zeon) Nitrile rubber ACN content 28%, Mooney viscosity 45-55 Adhesive resin component (b) Foral 105-E (Eastman Chemical) hydrogenated rosin pentaerythritol ester Softening point 101 °C Foral 85-E (Eastman Chemical) hydrogenated rosin glycerol ester Softening point 85 °C Dertophene T105 (DRT) Terpene phenol resin Softening point 105 °C Dercolyte A115(DRT) alpha-pinene resin Softening point 115 °C ACP = +35 °C Soft resin component (c) Wingtack 10 (Cray Valley) Aliphatic C5 liquid resin Softening point 10 °C; melt viscosity 22 Pa s Additives (d) Irganox 1010 (BASF) Primary anti-aging agent (sterically hindered phenol derivative) Irgafos 168 (BASF) Secondary anti-aging agent (phosphoric acid ester) Microballoons Expancel 920DU20 (Nouryon)

[0168] Tables 2 and 3 show the recipes used and their characteristics. Table 2: Recipes and their characteristics. Raw materials Adhesive 1 Adhesive 2 (a) Elastomer Kraton D1102 29,0 29,0 [%] Kraton D1118 20,0 20,0 (b) Adhesive resin K1 [%] DercolyteA115 45,0 30,0 (b) Adhesive resin K2 [%] Forum 85 20,0 (c) Soft resin [%] Wingtack 10 5,0 - (d) Additive [%] Irganox 1010 0,5 0,5 Irgafos168 0,5 0,5 (e) Microballoons Expancel 920DU20 X X Table 3: further recipes and their characteristics. Raw materials Adhesive 3 (a) Elastomer [%] Nipol DN2850 55,0 (b) Adhesive resin K1 [%] Forum 85 44,0 (d) Additive [%] Irganox1010 0,5 Irgafos168 0,5 (e)Microballoons Expancel920 DU20 X

[0169] The following adhesive tapes, examples 1 - 3, were produced from these adhesives by coating onto a liner, foaming and possibly subsequent structuring.

[0170] The liners used were a 50 µm liner with a higher release force and a 36 µm liner with a lower release force.

[0171] The adhesive masses were coated from solution onto the 50µm liner and dried, with the dried masses having an application weight of approx. 38 g / m 2 The 36µm liner was then laminated, and the adhesive tape was foamed at 160°C, resulting in a thickness of approximately 50µm. Immediately after foaming, the warm adhesive tape was embossed between two rollers, one of which had a printing plate glued to it. The printing plate had a diamond pattern with diamonds measuring 5mm lengthwise and 3mm crosswise. The web width was approximately 150µm. The pressure was adjusted so that the embossing depth was approximately 25µm.

[0172] In Example 3, the adhesive tape was then cross-linked with electron beams at 250kV with a dose of 30kGy. Counterexample 1:

[0173] The same compound was used as in Example 1 (Adhesive Compound 1), but the microballoons were omitted, which meant that the compound could not be foamed. Therefore, the compound was immediately mixed at approximately 50 g / m 2 (after drying) spread out. Counterexample 2:

[0174] Like counterexample 1 but with adhesive 2. Counterexample 3:

[0175] Like example 1, except that this time the structuring was omitted and the adhesive tape had a smooth surface after production.

[0176] Counterexample 4: As example 3, but the pattern was not cross-linked with electron beams.

[0177] Tables 4 and 5 show the test results of the inventive examples and the counterexamples (comparative examples). Table 4: Test results of the examples according to the invention. Example 1 Example 2 Example 3 Dielectric strength [mJ] 351 329 482 Adhesive strength without EtOH [N / cm] 9,8 10,5 7,9 Adhesive strength with EtOH [N / cm] 0,5 0,8 0,9 Decrease in adhesive strength [%] 95 92 89 Push-out without EtOH [mJ] 376 407 411 Push-out with EtOH 24 h[mJ] 20 32 42 Drop PushOut 24 h[%] 95 92 90 Push-out with EtOH 14 d[mJ] 23 44 65 Waste PushOut 14 d[%] 94 89 84 Table 5: Test results of the counterexamples (i.e. comparison examples). Counterexample 1 Counterexample 2 Counterexample 3 Counterexample 4 Dielectric strength [mJ] 104 126 371 504 Adhesive strength without EtOH [N / cm] 8,5 9,8 10,4 8,6 Adhesive strength with EtOH [N / cm] 0,5 1,2 7,6 2,3 Decrease in adhesive strength [%] 94 88 27 73 Push-out without EtOH[J] 346 380 365 379 Push-out with EtOH 24 h[J] 18 34 257 82 Drop PushOut 24 h[%] 95 91 30 78 Push-out with EtOH 14 d[J] 18 46 231 193 Waste PushOut 14d [%] 95 88 37 49

[0178] It turns out that the shock performance can be significantly improved by foaming the adhesives.

[0179] Structuring the surface of the adhesive tape aids removability; without structuring, no drop in adhesive strength or push-out values ​​of more than 40% can be achieved. Without structuring, detachment also occurs, not only on the metal side but also partially from the PVC.

[0180] It is preferred if the removability is maintained even after prolonged storage; very soft adhesives such as counterexample 6 lose their structure and thus their easy removability over time.

Claims

[1] Method for structuring a foamed adhesive tape comprising at least one layer of adhesive, wherein at least one layer of adhesive is foamed, and wherein in the method the adhesive tape is structured on the surface by subsequent embossing of a liner and the adhesive tape with the aid of a structured roller, characterized by that the resulting structure creates channels in the adhesive through which a solvent can penetrate into the adhesive joint in order to remove the adhesive tape again. [2] Method according to claim 1, characterized by that the embossing takes place at temperatures of at least 100°C. [3] Method according to one of the preceding claims, characterized by that the depth of the structures is 10 to 50 µm deep, particularly preferably 10 to 35 µm deep. [4] Method according to one of the preceding claims, characterized bythat the structure is still visible at least 2 weeks after application and storage at room temperature (23°C). [5] Method according to one of the preceding claims, characterized by that the adhesive tape consists of at least one layer of adhesive. [6] Method according to one of the preceding claims, characterized by that the adhesive layers are pressure-sensitive adhesive layers. [7] Method according to one of the preceding claims, characterized by that the adhesive tape is double-sided. [8] Method according to one of the preceding claims, characterized by that the foaming of at least one adhesive layer is carried out with the aid of microballoons. [9] Method according to one of the preceding claims, characterized by that at least one adhesive layer is based on vinyl aromatic block copolymers. [10] Method according to one of claims 1 to 9, characterized bythat at least one adhesive layer is based on nitrile rubber. [11] Method according to one of the preceding claims, characterized by that the adhesive tape contains a film or fabric carrier. [12] Method according to one of claims 1 to 10, characterized by that the adhesive tape does not contain a film or fabric carrier.

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