Chemical-resistant pressure-sensitive adhesive

Abstract

The aim was to provide a pressure-sensitive adhesive having high chemical resistance and high resistance to sebum, and additionally also having good shock resistance and a good initial tack. This is achieved with a pressure-sensitive adhesive that comprises at least one poly(meth)acrylate which is derivable from a monomer composition comprising a) at least one acrylic ester of formula (I) CH2=CH-C(O)OR1 (I), in which R1 is a linear or branched alkyl group having 1 to 10 carbon atoms, where acrylic esters of formula (I) are present in the monomer composition to a total extent of 50% to 89% by weight; b) benzyl acrylate and / or benzyl methacrylate, where benzyl acrylate and / or benzyl methacrylate are present in the monomer composition to a total extent of 10% to 40% by weight; and c) 0.5% to 10% by weight of acrylic acid. Further subjects of the invention are a process for producing such a pressure-sensitive adhesive and the use thereof as adhesive in the production of chemical- and / or sebum-resistant adhesive bonds.

Description

[0001] Chemical-resistant pressure-sensitive adhesive

[0002] The invention relates to the technical field of pressure-sensitive adhesives, such as those frequently used in adhesive tapes for the temporary or permanent bonding of materials. More specifically, the invention relates to a pressure-sensitive adhesive comprising a poly(meth)acrylate with a specific monomer composition and combining good chemical resistance with high initial adhesion and shock resistance.

[0003] With the increasing proliferation of electronic devices, their areas of application are expanding. This also results in growing demands on the components installed. For example, with the development of wearable electronic devices such as smartwatches, it is becoming increasingly important that the adhesives used in these devices exhibit high resistance to various chemicals and secretions from human skin, and that they exhibit little to no loss of adhesive strength even after prolonged storage in various media. Similar requirements are increasingly being placed on other electronic devices such as smartphones, tablets, notebooks, cameras, video cameras, keyboards, and touchpads.

[0004] Another area where chemical-resistant bonding is important is the application of labels in environments where contact with chemicals is possible, such as engine compartments. Tamper-evident labels often require high resistance to various chemicals. The use of poly(meth)acrylate-based adhesives, especially pressure-sensitive adhesives, has been described in various ways in this context.

[0005] WO 2016 / 089687 A1 describes a pressure-sensitive adhesive derived from the reaction product of 20 to 60 wt.% methyl acrylate; 40 to 80 wt.% ethyl, propyl and / or butyl acrylate; 0.2 to 5 wt.% of a functionalized acrylate monomer and a crosslinker.

[0006] WO 2017 / 132058 A1 discloses a pressure-sensitive adhesive comprising a polymer formed by polymerizing i) 2-ethylhexyl acrylate, butyl acrylate, isooctyl acrylate, 2-propylheptyl acrylate, n-octyl acrylate, 2-ethylhexyl methacrylate, butyl methacrylate, isooctyl methacrylate, 2-propylheptyl methacrylate and / or n-octyl methacrylate; ii) acrylonitrile and / or methacrylonitrile; and ill) acrylic acid and / or methacrylic acid.

[0007] US 5,665,835 describes a pressure-sensitive adhesive copolymer with a high polar content, which is obtained by copolymerization of alkyl acrylate monomers with 4 to 8 C atoms in the alkyl group and 15 to 50 wt.% of polar acrylic monomers functionalized with carboxy or methoxy groups.

[0008] JP 2020196884 A describes a pressure-sensitive adhesive comprising a (meth)acrylate copolymer containing 45 wt.% or more of a structural unit derived from an aromatic ring-containing (meth)acrylic acid ester, and

[0009] 30% by weight or more of a structural unit derived from a (meth)acrylic acid ester containing an alkyl group having 1 to 4 C atoms.

[0010] EP 1 780 561 A1 relates to an adhesive composition for a polarizing plate, comprising: an acrylic copolymer (A) comprising as monomer components:

[0011] (a1) a (meth)acrylic ester in an amount of 10 to 79.9 wt%;

[0012] (a2) a compound containing a benzene ring in an amount of 20 to 80 wt%; and

[0013] (a3) a functional group-containing compound in an amount of 0.1 to 10 wt%; and which has a benzene ring content of not less than 10 wt% and a weight-average molecular weight of 800,000 to 2,000,000; a crosslinking agent (B) in an amount of 0.01 to 0.3 parts by weight; and a silane coupling agent (C) in an amount of 0.01 to 0.5 parts by weight; wherein the amounts of components (B) and (C) are each based on 100 parts by weight of the acrylic copolymer (A).

[0014] WO 2019 / 106194 A1 describes a polyacrylate which is characterized in that the polyacrylate is based on the following monomer composition:

[0015] - 30 to 75 wt.% of at least one acrylic acid ester according to formula (I)

[0016] CH2=CH-C(O)OR 1 (I), where R 1 represents a linear or branched alkyl group having 1 to 10 C atoms;

[0017] 20 to 65 wt.% of at least one acrylic acid ester according to formula (II)

[0018] CH2=CH-C(O)OR 2 (II), where R 2 represents a phenoxyalkyl radical;

[0019] 0 to 40 wt.% of at least one acrylic acid ester of formula (III)

[0020] CH2=C(O)OR 3 (III), wherein R 3 represents an alkyldiglycol radical or an alkoxyalkyl radical;

[0021] 0.5 to 10 wt.% of at least one acrylate monomer according to formula (IV)

[0022] CH2=CH-C(O)OR 4 (IV), where R 4 represents an H atom or a hydroxyalkyl radical having 1 to 4 C atoms; and a pressure-sensitive adhesive composition which contains at least one such polyacrylate to a total of at least 50% by weight.

[0023] The development of chemical-resistant pressure-sensitive adhesives is increasingly complicated by the fact that certain monomers are no longer tolerated by customers due to toxicity or environmental concerns. As a result, the range of available monomers is narrowing, without compromising the performance of the pressure-sensitive adhesives.

[0024] Not least from this perspective, there is a continuing need for pressure-sensitive adhesives with high resistance to everyday chemicals that are characterized by other advantageous application properties.

[0025] It was an object of the invention to provide a pressure-sensitive adhesive with high chemical resistance and high resistance to sebum.

[0026] It was a further object of the invention to design the pressure-sensitive adhesive in such a way that it also has good shock resistance, i.e. that it largely retains its adhesive properties during impact events.

[0027] A supplementary object of the invention was to design the pressure-sensitive adhesive in such a way that it is easy to handle, which is expressed in particular in good initial adhesion or a good initial finger tack. A first and general subject matter of the invention, with which these objects are achieved, is a pressure-sensitive adhesive comprising at least one poly(meth)acrylate derived from a monomer composition comprising a) at least one acrylic acid ester according to the formula (I)

[0028] CH2=CH-C(O)OR 1 (I), where R 1represents a linear or branched alkyl group having 1 to 10 C atoms, wherein acrylic acid esters according to formula (I) are present in a total of 50 to 89 wt.% in the monomer composition; b) benzyl acrylate and / or benzyl methacrylate, wherein benzyl acrylate and / or benzyl methacrylate are present in a total of 10 to 40 wt.% in the monomer composition; and c) comprises 0.5 to 10 wt.% acrylic acid.

[0029] As has been shown, the above-mentioned property profile can be achieved with such pressure-sensitive adhesives, combining the properties of chemical resistance, shock resistance and good processability.

[0030] Preferably, the at least one poly(meth)acrylate is based on a monomer composition which consists of a) at least one acrylic acid ester according to the formula (I)

[0031] CH2=CH-C(O)OR 1 (I), where R 1represents a linear or branched alkyl group having 1 to 10 C atoms, wherein acrylic acid esters according to formula (I) are present in a total of 50 to 89 wt.% in the monomer composition; b) benzyl acrylate and / or benzyl methacrylate, wherein benzyl acrylate and / or benzyl methacrylate are present in a total of 10 to 40 wt.% in the monomer composition; and c) 0.5 to 10 wt.% acrylic acid.

[0032] Within the scope of the invention, it has also been possible to identify particularly suitable pressure-sensitive adhesives and monomer compositions. These and further embodiments, which are designated as preferred below, are combined in particularly preferred embodiments with features of other embodiments designated as preferred. Very particular preference is therefore given to combinations of two or more of the embodiments designated as particularly preferred below. Likewise preferred are embodiments in which a feature of an embodiment designated as preferred to any extent is combined with one or more further features of other embodiments designated as preferred to any extent. Features of preferred pressure-sensitive adhesives and uses emerge from the features of preferred pressure-sensitive adhesives.

[0033] To the extent that both specific amounts or proportions of an element, for example, for a particular monomer, and preferred embodiments of the element are disclosed below, the specific amounts or proportions of the preferably configured elements are also disclosed. Furthermore, it is disclosed that, with the corresponding specific total amounts or total proportions of the elements, at least some of the elements can be preferably configured, and in particular, that preferably configured elements can in turn be present in the specific amounts or proportions within the specific total amounts or total proportions.

[0034] According to expert understanding, a pressure-sensitive adhesive is an adhesive that possesses pressure-sensitive adhesive properties, i.e., the ability to form a permanent bond to a substrate even under relatively light pressure. Such adhesives or pressure-sensitive adhesive tapes are generally permanently tacky even at room temperature, meaning they exhibit a certain viscosity and tackiness, allowing them to wet the surface of a substrate even under light pressure. Without wishing to be bound by this theory, it is often assumed that a pressure-sensitive adhesive can be considered an extremely viscous liquid with an elastic component, which consequently possesses characteristic viscoelastic properties that lead to the permanent tackiness and pressure-sensitive adhesive capacity described above.It is assumed that mechanical deformation of pressure-sensitive adhesives results in both viscous flow processes and the buildup of elastic restoring forces. The partial viscous flow serves to achieve adhesion, while the partial elastic restoring forces are particularly necessary to achieve cohesion. The relationships between rheology and pressure-sensitive tack are well known in the art and are described, for example, in "Satas, Handbook of Pressure Sensitive Adhesives Technology", Third Edition, (1999), pages 153 to 203. To characterize the degree of elastic and viscous components, the storage modulus (G') and the loss modulus (G") are usually used; these can be determined by means of dynamic mechanical analysis (DMA), for example using a rheometer.In the context of the present invention, an adhesive is preferably understood as pressure-sensitive adhesive and thus as a pressure-sensitive adhesive if, at a temperature of 23 °C in the deformation frequency range from 10° to 10. 1 rad / sec G' and G“ each lie at least partly in the range of 103 to 107 Pa.

[0035] A pressure-sensitive adhesive according to the invention comprises at least one poly(meth)acrylate; thus, it may comprise one or more poly(meth)acrylates. The terms "at least one" or "one or more" or even "exactly one" refer herein, in the industry-standard manner, to the chemical nature of the corresponding compounds and not to their amount. For example, the monomer composition may comprise exclusively n-butyl acrylate as the first monomer, which would mean that the monomer composition comprises a plurality of n-butyl acrylate molecules.

[0036] According to the invention, the at least one poly(meth)acrylate comprises at least one acrylic acid ester according to the formula (I)

[0037] CH2=CH-C(O)OR 1 (I), where R 1 represents a linear or branched alkyl group having 1 to 10 carbon atoms, wherein acrylic acid esters according to formula (I) are present in the monomer composition in a total amount of 50 to 89% by weight, preferably in a total amount of 55 to 80% by weight. The weight percentages of the monomer composition refer in each case to the total weight of the monomer composition.

[0038] Preferably R 1 in formula (I) represents a linear alkyl group having 1 to 10 carbon atoms. Likewise preferably, the proportion of alkyl acrylates with branched alkyl groups in the monomer composition is a maximum of 10 wt.%, more preferably a maximum of 5 wt.%, particularly preferably a maximum of 2 wt.%; in particular, the monomer composition is free of alkyl acrylates with branched alkyl groups.

[0039] Preferably R 1in formula (I) represents a linear alkyl group having 1 to 6 C atoms, particularly preferably a linear alkyl group having 1 to 4 C atoms. The monomer composition likewise preferably comprises at least two monomers a). Particularly preferably, the monomer composition comprises as monomers a) methyl acrylate and / or n-butyl acrylate, in particular the monomer composition comprises as monomers a) methyl acrylate and n-butyl acrylate, very particularly preferably it comprises as monomers a) exclusively methyl acrylate and n-butyl acrylate. As monomers b), the monomer composition of the at least one poly(meth)acrylate of the pressure-sensitive adhesive of the invention comprises benzyl acrylate and / or benzyl methacrylate, wherein benzyl acrylate and / or benzyl methacrylate are present in the monomer composition in a total amount of 10 to 40 wt. %. Benzyl acrylate and / or benzyl methacrylate are preferably present in the monomer composition at 17 to 37 wt.%, particularly preferably at 20 to 35 wt.%.

[0040] Without wishing to be bound by this theory, the replacement of phenoxyethyl acrylate used in prior art pressure-sensitive adhesives with the benzyl acrylate and / or benzyl methacrylate used in the present invention appears to be particularly responsible for the fact that the pressure-sensitive adhesive exhibits not only good chemical resistance but also good shock resistance and good processability. The use of benzyl acrylate, especially over benzyl methacrylate, has proven particularly effective in this context. Therefore, the monomer composition comprises, in particular, benzyl acrylate as monomer b), and very particularly preferably, it comprises exclusively benzyl acrylate as monomer b).

[0041] The monomer composition further comprises 0.5 to 10 wt.% acrylic acid, preferably 1 to 5 wt.% acrylic acid, more preferably 2 to 4 wt.% acrylic acid. The monomer composition is preferably free of monomers with OH groups not bonded to carboxyl groups. In particular, with the exception of acrylic acid, the monomer composition does not comprise any monomers containing OH groups.

[0042] In one embodiment, the at least one poly(meth)acrylate of the pressure-sensitive adhesive according to the invention is based on a monomer composition consisting of

[0043] 10 to 30 wt% methyl acrylate;

[0044] 35 to 60 wt% n-butyl acrylate;

[0045] 15 to 40 wt.% benzyl acrylate and / or benzyl methacrylate; and

[0046] 0.5 to 6 wt.% acrylic acid.

[0047] A pressure-sensitive adhesive of the invention preferably comprises no other polyacrylates and polymethacrylates than those described so far. A pressure-sensitive adhesive of the invention particularly preferably comprises precisely one poly(meth)acrylate as described so far. A pressure-sensitive adhesive of the invention comprises, in addition to the at least one poly(meth)acrylate, at least one bond-strength-enhancing resin. A pressure-sensitive adhesive of the invention can thus comprise one or more bond-strength-enhancing resins; preferably, it comprises bond-strength-enhancing resins in a total of not more than 30 wt. Bond-strength-enhancing resins can also be referred to as "adhesive resins."

[0048] Preferably, the at least one adhesive strength-enhancing resin is a (meth)acrylate oligomer or polymer.

[0049] The skilled person understands that in pressure-sensitive adhesives according to the invention, both the poly(meth)acrylates and—where present—the tackifier resins are or can be prepared at least partially from (meth)acrylates. For the purpose of clearly distinguishing between the high-molecular-weight poly(meth)acrylates and the shorter-chain tackifier resins, the weight-average molecular weight can be used, which allows a clear differentiation between these components. Thus, the differences between the two components are expressed by precisely those parameters with which the skilled person also distinguishes in practice between the poly(meth)acrylates in the sense of the polymer base of the pressure-sensitive adhesive, on the one hand, and the tackifier resins in the sense of an additive, on the other.

[0050] The inventors have succeeded in identifying preferred weight-average molecular weights for both components of the pressure-sensitive adhesive—provided the pressure-sensitive adhesive is formulated accordingly—the adjustment of which results in particularly advantageous pressure-sensitive adhesives of the invention. It has been found, particularly for the tackifier resins, that particularly good chemical resistance combined with good pressure-sensitive adhesion can be achieved with comparatively short-chain tackifier resins. Preference is given to a pressure-sensitive adhesive of the invention wherein the one or more poly(meth)acrylates have a weight-average molecular weight Mw of 400,000 g / mol or more, preferably of 500,000 g / mol or more, more preferably of 600,000 g / mol or more, most preferably of 650,000 g / mol or more.Additionally or alternatively, preference is given to a pressure-sensitive adhesive according to the invention, wherein the one or more adhesive resins, if they are poly(meth)acrylates, have a weight-average molecular weight Mw in the range from 1000 to 15000 g / mol, preferably in the range from 1500 to 10000 g / mol, particularly preferably in the range from 2000 to 5000 g / mol.

[0051] The above-described distinction based on weight-average molecular weight is, of course, particularly important when the poly(meth)acrylate and the adhesive resin each contain a methacrylate component or no methacrylate component. If they differ in that the poly(meth)acrylate contains a methacrylate component but the adhesive resin does not, or vice versa, a distinction can easily be made based on the methacrylate components.

[0052] A pressure-sensitive adhesive composition according to the invention particularly preferably comprises at least one adhesive resin whose underlying monomer composition comprises at least 30 wt. %, more preferably at least 40 wt. %, of at least one monomer selected from the group consisting of methyl methacrylate (MMA), isobornyl methacrylate, cyclohexyl methacrylate, and aromatic methacrylates. The monomer composition underlying the adhesive resin particularly preferably comprises at least 30 wt. %, more preferably at least 40 wt. %, of methyl methacrylate.

[0053] In addition to or as an alternative to bond-enhancing resins based on (meth)acrylate oligomers or polymers, a pressure-sensitive adhesive of the invention may comprise further adhesive resins. Generally speaking, and regardless of whether (meth)acrylate oligomers or polymers are used or other adhesive resins, a pressure-sensitive adhesive of the invention preferably comprises adhesive resins with a glass transition temperature (Tg) of 50°C to 100°C, more preferably of 60°C to 90°C.

[0054] A pressure-sensitive adhesive according to the invention preferably comprises one or more adhesive strength-enhancing resins in a total of 1 to 30 wt.%, more preferably in a total of 3 to 20 wt.%, particularly preferably in a total of 5 to 15 wt.%.

[0055] The production of the (mandatory) poly(meth)acrylates and also the (optional) (meth)acrylate-based adhesive resins from the respective monomers can generally be carried out using conventional processes, in particular by conventional free radical polymerizations or controlled radical polymerizations, or by combinations of different polymerization processes. Preferred polymerization processes are conventional free polymerization, ATRP, nitroxide / TEMPO-controlled polymerization, and the RAFT process.

[0056] The polymers or oligomers can therefore be prepared by copolymerizing the monomeric components using the customary polymerization initiators and, if appropriate, regulators, wherein polymerization can be carried out at customary temperatures, for example in bulk, in emulsion, e.g., in water or liquid hydrocarbons, or in solution. The poly(meth)acrylates and / or the adhesive resins are preferably prepared by polymerization in solvents, particularly preferably in solvents having a boiling point in the range from 50 to 150°C, particularly preferably in the range from 60 to 120°C, using the customary amounts of polymerization initiators, wherein the polymerization initiators are generally added to the monomer composition in a proportion of about 0.01 to 5%, in particular from 0.1 to 2%, based on the mass of the respective monomer composition.

[0057] Suitable polymerization initiators are, for example, radical sources such as peroxides, hydroperoxides and azo compounds, e.g. dibenzoyl peroxide, cumene hydroperoxide, cyclohexanone peroxide, di-t-butyl peroxide, cyclohexylsulfonylacetyl peroxide,

[0058] Diisopropyl percarbonate, t-butyl peroctoate, or benzpinacol. 2,2'-Azobis(2-methylbutyronitrile) or 2,2'-Azobis(2-methylpropionitrile) is particularly preferred as the radical initiator. Suitable solvents include, in particular, alcohols such as methanol, ethanol, n- and isopropanol, n- and isobutanol, preferably isopropanol and / or isobutanol, as well as hydrocarbons such as toluene and, in particular, gasolines with a boiling point in the range of 60 to 120 °C. Ketones, such as acetone, methyl ethyl ketone, and methyl isobutyl ketone, and esters, such as ethyl acetate, and mixtures of these solvents, can be used.

[0059] In this context, the invention further provides a process for producing a pressure-sensitive adhesive according to the invention, which is characterized in that the process comprises the polymerization of the at least one poly(meth)acrylate, wherein a first part of the monomer composition is initially introduced into a reaction vessel and the polymerization of this part is initiated; and the remaining part of the monomer composition, after the initiation of the polymerization of the first part, is metered into the reaction vessel such that the entire monomer composition to be polymerized has been added to the reaction vessel only after a time of at least 5%, preferably of at least 8%, in particular of at least 10% of the total polymerization time has elapsed.

[0060] The remaining portion can be added in portions or continuously; what is important is that it is added "gradually," i.e., by means of a metered addition over the relevant period. The polymerization according to the process of the invention can thus be characterized by a "semi-batch" procedure. Unlike the tackifier resins, with regard to the adhesive properties of pressure-sensitive adhesives of the invention, in particular to achieve high cohesion, it is preferred for the (mandatory) poly(meth)acrylate(s) to be at least partially crosslinked with one another, so that the pressure-sensitive adhesive preferably comprises at least one crosslinked poly(meth)acrylate.Preference is therefore given to a pressure-sensitive adhesive according to the invention wherein the at least one poly(meth)acrylate is obtainable by polymerizing the monomer composition and subsequently at least partially crosslinking the polymer molecules thus formed, crosslinking preferably taking place with a chemical crosslinker and / or a physical crosslinker. Particular preference is given to the poly(meth)acrylate(s) of the pressure-sensitive adhesive according to the invention being thermally crosslinked, in particular with at least one covalent crosslinker, at least one coordinative crosslinker, or with a combination of at least one covalent crosslinker with at least one coordinative crosslinker. Thermal crosslinking generally results in particularly homogeneous crosslinking, whereas, for example, a crosslinking profile with varying crosslinking density is frequently observed in radiation-crosslinked compositions.

[0061] In one embodiment, the poly(meth)acrylate(s) of the pressure-sensitive adhesive of the invention is or are crosslinked with one or more covalent crosslinkers. Without wishing to be bound by this theory, it is assumed that this is advantageous in that the network formed thereby is comparatively rigid and thus impedes the penetration of everyday chemicals into the pressure-sensitive adhesive. Preferred covalent crosslinkers are epoxy compounds, in particular epoxycyclohexyl derivatives and epoxy-amine compounds, for example (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate, bis(3,4-epoxycyclohexylmethyl)adipate, N,N,N',N'-tetrakis(2,3-epoxypropyl)-m-xylene-a,a'-diamine and N,N,N',N'-tetrakis(2,3-epoxypropyl)cyclohexane-1,3-dimethylamine. Covalent crosslinkers are preferably used in a proportion of 0.01 to 0.1 wt.%, based on the solids content of the poly(meth)acrylate or poly(meth)acrylates to be crosslinked.

[0062] In a further embodiment, the poly(meth)acrylate(s) of the pressure-sensitive adhesive of the invention is / are crosslinked with one or more coordinative crosslinkers. Without wishing to be bound by this theory, it is assumed that this is advantageous in that the resulting network is comparatively dynamic and thus enables good flow of the pressure-sensitive adhesive onto the adhesive substrate. This, in turn, makes it more difficult for everyday chemicals to penetrate the pressure-sensitive adhesive via the interface between the pressure-sensitive adhesive and the substrate surface. Preferred coordinative crosslinkers are chelate compounds, especially polyvalent metal chelate compounds.The polyvalent metal atom is preferably selected from the group consisting of Al(III), Zr(IV), Co(II), Cu(I), Cu(II), Fe(II), Fe(III), Ni(II), V(II), V(III), V(IV), V(V), Zn(II), In(III), Ca(II), Mg(II), Mn(II), Y(III), Ce(II), Ce(IV), St(II), Ba(II), Mo(II), Mo(IV), Mo(VI), La(III), Sn(II), Sn(IV) and Ti(IV), more preferably selected from Al(III), Zr(IV) and Ti(IV), in particular it is Al(III). Alkyl esters, alcohol compounds, carboxylic acid compounds, ether compounds and ketone compounds are preferably used as ligands. The coordinative crosslinker is preferably selected from the group consisting of titanium dipropoxide bis(acetylacetonate).

[0063] Titanium dibutoxide bis(octylene glycolate), titanium dipropoxide bis(ethyl acetoacetate),

[0064] Titanium dipropoxide bis(lactate), titanium dipropoxide bis(triethanolaminate), titanium di-n-butoxide bis(triethanolaminate), titanium tri-n-butoxide monostearate, butyl titanate dimer,

[0065] poly(titanium acetylacetonate); Aluminum diisopropoxide monoethyl acetate, aluminum di-n-butoxide monomethyl acetoacetate, aluminum di-i-butoxide monomethyl acetoacetate, aluminum di-n-butoxide monoethyl acetoacetate, aluminum disec-butoxide monoethyl acetoacetate,

[0066] aluminum triacetylacetonate, aluminum triacetylacetonate,

[0067] Aluminum monoacetylacetonate bis(ethylacetoacetonate), tris(2,4-pentanedionato)aluminum(III), and zirconium tetraacetylacetonate; in particular, the coordinative crosslinker is tris(2,4-pentanedionato)aluminum(III). Coordinative crosslinkers are preferably used in a proportion of 0.03 to 0.5 wt. %, more preferably 0.1 to 0.3 wt. %, based in each case on the solids content of the poly(meth)acrylate(s) to be crosslinked.

[0068] Particularly preferably, the poly(meth)acrylate(s) of the pressure-sensitive adhesive according to the invention is or are crosslinked exclusively with one or more coordinative crosslinkers according to the above.

[0069] The gel value of the uncrosslinked poly(meth)acrylates of the pressure-sensitive adhesive according to the invention is preferably < 7%, more preferably < 5% and particularly preferably < 3%.

[0070] In one embodiment, the pressure-sensitive adhesive of the invention is foamed. A "foamed pressure-sensitive adhesive" is understood to mean a pressure-sensitive adhesive comprising a pressure-sensitively adhesive matrix material and a plurality of gas-filled cavities, such that the density of this pressure-sensitive adhesive is reduced compared to the mere matrix material without cavities. The foaming of the matrix material of the foamed pressure-sensitive adhesive can in principle be achieved in any desired manner. For example, the pressure-sensitive adhesive can be foamed by means of a propellant gas introduced into or released into it. The foamed pressure-sensitive adhesive preferably contains at least partially expanded hollow microspheres. These are understood to mean at least partially expanded microspheres that are elastic and expandable in their basic state and have a thermoplastic polymer shell. These spheres are - in their basic state - 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 common; they are enclosed in the polymer shell as a liquefied gas under pressure. Such microspheres are also referred to as "microballoons." When exposed to heat, the outer polymer shell of these microballoons 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 is retained, a closed-cell foam is produced, which is also referred to as syntactic foaming.

[0071] A pressure-sensitive adhesive according to the invention preferably comprises microballoons with an activation temperature of at least 120°C. A pressure-sensitive adhesive according to the invention also preferably comprises microballoons, the average diameter of the microballoons after expansion being 10 to 50 μm. The average diameter is determined by means of laser diffractometry (volume-averaged distribution) in accordance with ISO 13320. Less preferred, but also alternatively possible, is the determination of the average diameter via light microscopy, subsequent measurement of the diameters of individual particles in a sufficiently representative section and subsequent averaging. A pressure-sensitive adhesive according to the invention preferably comprises microballoons in a total of 0.25 to 2 wt. %, more preferably in a total of 0.5 to 1.25 wt. %, based in each case on the total weight of the pressure-sensitive adhesive.

[0072] A pressure-sensitive adhesive of the invention preferably comprises at least one pigment, more preferably at least one black pigment. The pigment is preferably predispersed in a solvent or a plasticizer. A particularly preferred pigment is the black pigment Hostatint® Black AN 100. A pressure-sensitive adhesive of the invention preferably comprises one or more pigments in a total amount of 0.5 to 5 wt. %, more preferably in a total amount of 1 to 3 wt. %, based in each case on the total weight of the pressure-sensitive adhesive.

[0073] To optimize the properties of the pressure-sensitive adhesive according to the invention, it may also contain other conventional additives such as fillers, for example electrically conductive fillers, thermally conductive fillers and the like, flame retardants, for example ammonium polyphosphate and its derivatives, ageing inhibitors, plasticizers and the like.

[0074] The preparation of a pressure-sensitive adhesive according to the invention is preferably carried out by first preparing the poly(meth)acrylate(s) in a solvent from the parent monomer mixtures by free-radical polymerization as described above. If multiple poly(meth)acrylates are present, these are then mixed. If required, crosslinkers are added during or, preferably, after the polymerization. If further additives are to be added, these are also mixed.

[0075] A pressure-sensitive adhesive of the invention is preferably coated as a solution—optionally after adjusting a specific solids content—onto a carrier or a release liner. Coating is preferably carried out using conventional coating processes such as anilox roll application, comma coating, multi-roll coating, or a printing process. The solvent can subsequently be removed in a drying tunnel or oven.

[0076] When using covalent crosslinkers, crosslinking generally occurs to a lesser extent at room temperature and to a greater extent upon application of heat, especially during solvent removal. Coordinative crosslinkers generally crosslink the poly(meth)acrylate regardless of temperature. For this reason, substances are occasionally added that initially block the crosslinker. These are then removed with the solvent, so that crosslinking begins immediately upon solvent removal.

[0077] Alternatively, the coating can also be applied using a solvent-free process. For this, the poly(meth)acrylate is heated and melted in an extruder. Further process steps such as mixing with additives, filtration, or degassing can be performed in the extruder. The melt is then applied as a layer to a carrier or release liner using a calender.

[0078] The invention further provides an adhesive tape comprising a pressure-sensitive adhesive according to the invention. In the simplest case, an adhesive tape according to the invention consists of only one layer of a pressure-sensitive adhesive according to the invention. To enable the adhesive tape to be applied rolled up into a plate reel or cross-wound into a spool without self-adherence, the pressure-sensitive adhesive layer is preferably covered with at least one release liner.

[0079] An adhesive tape according to the invention can, in addition to a pressure-sensitive adhesive according to the invention, also comprise at least one carrier and optionally also further layers, for example further pressure-sensitive adhesive layers, barrier layers, further reinforcing carrier layers, etc. In principle, there are no restrictions with regard to the design of the carrier and the further layers. Typical carrier materials are, for example, woven fabrics, scrims and plastic films, for example PET films and polyolefin films. In such embodiments too, at least one pressure-sensitive adhesive of the adhesive tape is preferably covered with a release liner in order to enable problem-free winding and unwinding and to protect the pressure-sensitive adhesive(s) from contamination. Release liners usually consist of a plastic film (e.g. PET or PP) siliconized on one or both sides or a siliconized paper carrier.They are not considered part of the adhesive tape, but are only temporarily connected to it as an aid.

[0080] The invention further provides for the use of a pressure-sensitive adhesive according to the invention as an adhesive in the production of chemical- and / or sebum-resistant bonds. In particular, pressure-sensitive adhesives according to the invention are used in the production of bonds in electronic, optical, or precision mechanical devices.

[0081] Electronic, optical and precision mechanical devices within the meaning of the invention are in particular devices as classified in Class 9 of the International Classification of Goods and Services for the Purposes of the Registration of Marks (Nice Classification), 10th edition (NCL(10-2013)), insofar as they are electronic, optical or precision mechanical devices, as well as clocks and chronometric devices according to Class 14 (NCL(10-2013)), such as in particular scientific, nautical, surveying, photographic, cinematographic, optical, weighing, measuring, signalling, checking (supervision), life-saving and teaching apparatus and instruments;

[0082] Apparatus and instruments for conducting, switching, transforming, accumulating, regulating or controlling electricity; image recording, processing, transmission and reproducing apparatus, such as televisions and the like;

[0083] Acoustic recording, processing, transmission and reproduction devices, such as radios and the like;

[0084] Computers, calculating and data processing equipment, mathematical devices and instruments, computer accessories, office equipment – ​​such as printers, fax machines, photocopiers, typewriters –, data storage devices;

[0085] Remote communication and multifunctional devices with remote communication functions, such as telephones, answering machines;

[0086] Chemical and physical measuring devices, control devices and instruments, such as battery chargers, multimeters, lamps, tachometers;

[0087] Nautical equipment and instruments;

[0088] Optical devices and instruments;

[0089] Medical devices and instruments and those for athletes;

[0090] Clocks and chronometers;

[0091] Solar cell modules, such as electrochemical dye solar cells, organic solar cells, thin-film cells; and

[0092] Fire extinguishers.

[0093] Technical development is increasingly focused on devices that are becoming smaller and lighter so that their owners can carry them with them at all times. This is usually achieved by realizing low weight and a suitable size for such devices. Such devices are referred to as mobile devices or portable devices. In this development trend, precision mechanical and optical devices are increasingly being equipped with electronic components, which increases the possibilities for minimization. Because mobile devices are carried around, they are increasingly exposed to stress - especially mechanical and chemical stress - such as bumping into edges, being dropped, coming into contact with other hard objects in a pocket, but also simply due to the constant movement caused by carrying them.However, mobile devices are also subject to greater stress due to moisture, temperature influences, and the like than "immobile" devices, which are usually installed indoors and are not or hardly moved. The adhesive used in the invention has proven particularly advantageous for withstanding such disruptive influences and, ideally, even mitigating or compensating for them.

[0094] A pressure-sensitive adhesive according to the invention is therefore preferably used for producing bonds in portable electronic, optical, or precision mechanical devices. Such portable devices include, in particular:

[0095] Still cameras, digital cameras, photographic accessories (such as light meters, flash units, apertures, camera housings, lenses, etc.), film cameras, video cameras, small computers (mobile computers, pocket computers, calculators), laptops, notebooks, netbooks, ultrabooks, tablet computers, handhelds, electronic diaries and organizers (so-called "electronic organizers" or "personal digital assistants", PDAs, palmtops), modems;

[0096] Computer accessories and control units for electronic devices, such as mice, drawing pads, graphics tablets, microphones, speakers, game consoles, gamepads, remote controls, remote controls, touchpads;

[0097] Monitors, displays, screens, touch-sensitive screens (sensor screens, "touchscreen devices"), projectors;

[0098] Readers for electronic books ("e-books");

[0099] Small televisions, pocket televisions, film players, video players, radios (including small and pocket radios), walkmen, discos, music players for example for CDs, DVDs, Blu-rays, cassettes, USB, MP3, headphones, cordless telephones, mobile phones, smartphones, walkie-talkies, hands-free devices, personal call devices (pagers, beepers);

[0100] Mobile defibrillators, blood glucose meters, blood pressure monitors, pedometers, heart rate monitors;

[0101] flashlights, laser pointers;

[0102] Mobile detectors, optical magnifiers, long-range vision devices, night vision devices, GPS devices, navigation devices, portable satellite communication interface devices;

[0103] Data storage devices (USB sticks, external hard drives, memory cards); and

[0104] Wristwatches, digital watches, pocket watches, chain watches, and stopwatches are increasingly attracting attention. Devices worn directly on the body and thus directly exposed to skin secretions (so-called "wearables") are also increasingly coming into focus.

[0105] In particular, a pressure-sensitive adhesive according to the invention is used for producing bonds in wearables, smartphones (mobile phones), tablets, notebooks, cameras, video cameras, keyboards or touchpads.

[0106] Examples

[0107] Production of pressure-sensitive adhesives

[0108] Table 1 : Raw materials used polymerization

[0109] The monomers (400 g, composition see Table 2) were initially charged in ethyl acetate (309 g) and isopropanol (27.3 g) in a 2 l laboratory reactor inertized with nitrogen, equipped with a stirrer, reflux condenser, and a dosing device. The monomer solution was rendered inert for 30 min using a nitrogen stream. The solution was heated to 67 °C, and when it reached 58 °C, a 5% solution of Vazo® 67 in ethyl acetate (5 g) was added. This marked the start of the polymerization. At the same time, the dosing of a monomer solution (600 g with the same composition), dissolved in 471 g of ethyl acetate, was initiated over a period of 2 h. After 40 min, 8 g of the 5% solution of Vazo® 67 were added, followed by another 8 g after a further 50 min. 6 h and 7.5 h after the start of the reaction, 30 g of a 5% solution of Perkadox® 16 in ethyl acetate were added. After 5.5 h and 7 h, 40 g of ethyl acetate were added.18 h after the start of the reaction, the reaction solution was cooled to 30 °C and the polymerization was terminated. The solution was adjusted to a solids content of 37 wt.%.

[0110] formulation

[0111] Subsequently, a crosslinker / crosslinker system (see Table 2) was stirred into a solution in acetone. Microballoons and the pigment (see Table 2) were also stirred in after prior slurrying with ethyl acetate. The resulting composition was coated in solution onto a siliconized PET film using a comma coater. The solvent was removed in a drying oven (20 min, 100 °C), and the coating weight was 95 g / m². 2 The resulting layer was laminated to a second, identical layer, and the laminate was foamed at 172 °C for 25 s. After storage at 40 °C for 6 d, physical testing began.

[0112] Table 2: Pressure sensitive adhesives

[0113] Measurement and testing methods

[0114] Method A: Molecular weight

[0115] The weight-average molecular weight is determined by gel permeation chromatography (GPC) on 100 ml of filtered sample (sample concentration 0.5 g / l). Tetrahydrofuran with 0.1 vol.% trifluoroacetic acid is used as the eluent. The measurement is carried out at 25 °C. A PSS-SDV column, 10 pm, ID 8.0 mm x 50 mm, is used as the precolumn. PSS-SDV columns, 5 pm, 10 3 Ä (SN9090201 ) and 5 pm, 10 2Ä (SN9090200) with an ID of 8.0 mm x 300 mm each were used (detection using a PSS-SECurity 1260 RID differential refractometer). The flow rate was 0.5 ml per minute. Calibration was performed using the commercially available ReadyCal Poly(styrene) high kit from PSS Polymer Standard Service GmbH, Mainz. The results were universally converted to polymethyl methacrylate (PMMA) using the Mark-Houwink parameters K and alpha, so that the data were expressed in PMMA mass equivalents.

[0116] Method B: Glass transition temperature

[0117] The glass transition temperature of poly(meth)acrylates is determined using dynamic scanning calorimetry (DSC). For this purpose, approximately 5 mg of an untreated polymer sample is weighed into an aluminum crucible (volume 25 μl) and sealed with a perforated lid. A Netzsch DSC 204 F1 is used for the measurement. The measurement is carried out under nitrogen for inerting. The sample is first cooled to -150 °C, then 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 detected as steps in the thermogram.

[0118] The glass transition temperature is obtained as follows:

[0119] The linear portion of the measurement curve before and after the step is extended in the direction of increasing (area before the step) or decreasing (area after the step) temperatures. In the step region, a best-fit line is placed parallel to the ordinate so that it intersects the two extension lines, creating two areas of equal area (between the extension line, the best-fit line, and the measurement curve). The intersection point of the best-fit line positioned in this way with the measurement curve yields the glass transition temperature. Method C: Chemical Resistance Testing

[0120] After removing the siliconized PET film, the adhesive tapes obtained as described above (see "Formulation") were applied in a width of 10 mm each to a previously cleaned ASTM steel plate and rolled back and forth five times in each direction using a 4 kg roller. The resulting bonds were then stored for 24 hours under standard conditions (air, 23 °C, 50% relative humidity). The test samples were then stored for 72 hours in a sealed box filled with the respective test chemical in a water bath heated to 60 °C. Each sample was immersed in the test chemical and then completely immersed in it.

[0121] Test chemical 1 : Oleic acid with a purity of > 85 %

[0122] Test chemical 2: Isopropanol / water 70 / 30 (weight ratio)

[0123] Test chemical 3: Sebum.

[0124] After the boxes were removed from the water bath and the test samples were removed from the boxes, the samples were carefully cleaned with a cloth and the adhesive strength was determined after 2 hours of conditioning in standard climate.

[0125] The adhesive strength was determined at a test temperature of 23 °C + / - 1 °C and 50% + / - 5% relative humidity. The adhesive tape was peeled off the steel substrate at a speed of 300 mm / min and at an angle of 180°.

[0126] The measurement results are given in N / cm and averaged from three measurements.

[0127] The adhesive tapes are considered resistant to the test chemicals if they still show adhesive strength after storage.

[0128] The results are shown in Table 6 under “CR” (chemical resistance) and are to be interpreted as follows:

[0129] Table 3: Evaluation of the measurement results for chemical resistance

[0130] Test method D: Droptower test (shock behavior - penetration strength)

[0131] A square, frame-shaped sample was cut out of the adhesive tape to be tested (area 180 mm 2 ; bridge width 2.0 mm).

[0132] Sample preparation:

[0133] The sample was bonded to a steel frame (ASTM) cleaned with acetone. A steel window cleaned with acetone was bonded to the other side of the adhesive tape. The steel frame, adhesive tape frame, and steel window were bonded in such a way that the geometric centers and diagonals overlapped each other (corner to corner). The bond was pressed for 10 s with 62 N and stored for 48 hours at 23 °C / 50% relative humidity.

[0134] Test execution:

[0135] To perform the measurement, the test specimen was inserted into the sample holder of the instrumented drop tester such that the composite was positioned horizontally with the steel window facing downwards. The measurement was performed instrumentally and automatically using a load weight of 5 kg and a drop height of 205 mm. The kinetic energy introduced by the load weight destroyed the bond by breaking the adhesive tape between the window and frame. The force was recorded by a piezoelectric sensor at ps intervals. Accordingly, the associated software displayed a graph for the force-time curve after the measurement, from which the maximum force F was calculated. max could be determined. Shortly before the rectangular impact geometry hit the window, the speed of the falling weight was measured using two light barriers. Assuming that the applied energy was large compared to the impact strength of the bond, the work performed by the bond until complete detachment, i.e., the detachment work, was determined from the force curve, the time required until detachment, and the speed of the falling weight. Five test specimens of each sample were tested; the final impact strength result consists of the average detachment work (energy in J) or the maximum force (Fmax in N) of these five samples.

[0136] The results are shown in Table 6 under “Shock Behavior / Droptower” and are to be interpreted as follows:

[0137] Table 4: Evaluation of the measurement results on shock behavior

[0138] Method E: Finger Tack

[0139] The tackiness of the adhesive was assessed by 5 identical persons using a simple finger test according to the scheme ++ / + / 0 / - / -.

[0140] Method F: Static shear strength at 40 °C

[0141] The shear strength at 40°C is a measure of the internal strength of the adhesive at slightly elevated temperatures and is tested in the so-called static shear test as follows:

[0142] The test is carried out in accordance with PSTC-7 at 40 °C using a weight of 1.0 kg. A 1.3 cm wide strip of the sample (95 g / m on both sides) 2A pressure-sensitive adhesive layer on 36 μm etched PET film (total material thickness 200 μm) is bonded to a polished steel plate over a length of 2 cm using a 2 kg roller by rolling over it twice. The plates are equilibrated for 30 minutes under test conditions (40 °C) but without load. A test weight (0.5 kg) is then suspended, creating a shear stress parallel to the bonding surface, and the time until the bond fails is measured. The measurement result is given in minutes, and in the event of failure, the failure type (cohesive failure or adhesive failure) is given. The median of three individual measurements is given.

[0143] The results are given in Table 6 under “Static shear strength” and are to be interpreted as follows: Table 5: Evaluation of the measurement results for static shear strength

[0144] Table 6: Test results

Claims

Patent claims 1. Pressure-sensitive adhesive comprising at least one poly(meth)acrylate which is applied to a monomer composition comprising a) at least one acrylic acid ester according to formula (I) CH2=CH-C(O)OR 1 (I), where R 1 represents a linear or branched alkyl group having 1 to 10 C atoms, wherein acrylic acid esters according to formula (I) are present in a total of 50 to 89 wt.% in the monomer composition; b) benzyl acrylate and / or benzyl methacrylate, wherein benzyl acrylate and / or benzyl methacrylate are present in a total of 10 to 40 wt.% in the monomer composition; and c) 0.5 to 10 wt.% is acrylic acid.

2. Pressure-sensitive adhesive according to claim 1, characterized in that R 1 in formula (I) represents a linear alkyl group having 1 to 10 C atoms.

3. Pressure-sensitive adhesive according to one of claims 1 and 2, characterized in that R 1in formula (I) represents a linear alkyl group having 1 to 4 C atoms.

4. Pressure-sensitive adhesive according to one of the preceding claims, characterized in that the monomer composition comprises at least two monomers a).

5. Pressure-sensitive adhesive according to one of the preceding claims, characterized in that the poly(meth)acrylate is based on a monomer composition consisting of 10 to 30 wt% methyl acrylate; 35 to 60 wt% n-butyl acrylate; 15 to 40 wt.% benzyl acrylate and / or benzyl methacrylate; and 0.5 to 6 wt.% acrylic acid.

6. Pressure-sensitive adhesive according to one of the preceding claims, characterized in that the pressure-sensitive adhesive comprises at least one adhesive strength-enhancing resin, wherein the pressure-sensitive adhesive comprises adhesive strength-enhancing resins in a total of not more than 30% by weight.

7. Pressure-sensitive adhesive according to claim 6, characterized in that the at least one adhesive strength-enhancing resin is a (meth)acrylate oligomer or polymer.

8. Pressure-sensitive adhesive according to one of the preceding claims, characterized in that the pressure-sensitive adhesive is foamed.

9. A process for producing a pressure-sensitive adhesive according to any one of the preceding claims, characterized in that the process comprises the polymerization of the at least one poly(meth)acrylate, wherein a first portion of the monomer composition is initially introduced into a reaction vessel and the polymerization of this portion is initiated; and the remaining portion of the monomer composition, after the initiation of the polymerization of the first portion, is metered into the reaction vessel such that the entire monomer composition to be polymerized is added to the reaction vessel only after a time of at least 20% of the total polymerization time has elapsed.

10. Use of a pressure-sensitive adhesive according to one of the preceding claims as an adhesive in the production of chemical- and / or sebum-resistant bonds.

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