WO2024179886A1

WO2024179886A1 - Uv-curable solvent-free hotmelt adhesive comprising a uv-crosslinkable poly(meth)acrylate and a radiation-curable, free-radically polymerizable compound

Info

Publication number: WO2024179886A1
Application number: PCT/EP2024/054332
Authority: WO
Inventors: Matthias Gerst; Karl-Heinz Schumacher; Klaus Menzel; Dirk Wulff
Original assignee: Basf Se
Priority date: 2023-02-27
Filing date: 2024-02-21
Publication date: 2024-09-06

Abstract

Described here is a UV-curable solvent-free hotmelt adhesive comprising a UV-crosslinkable poly(meth)acrylate and a radiation-curable, free-radically polymerizable compound. The UV- crosslinkable poly(meth)acrylate is formed from an alkyl (meth)acrylate having 8 carbon atoms in the alkyl group, a monomer comprising at least one acid group and a copolymerizable photoinitiator monomer, and the UV-crosslinkable poly(meth)acrylate before crosslinking has a specific zero viscosity at 130 °C and a specific weight average molecular weight. The UV-curable solvent-free hotmelt adhesive can be used for making pressure-sensitive self-adhesive article, such as for example pressure sensitive adhesive tapes, pressure sensitive adhesive films and pressure-sensitive adhesive labels.

Description

UV-curable solvent-free hotmelt adhesive comprising a UV-crosslinkable poly(meth)acrylate and a radiation-curable, free-radically polymerizable compound

The invention relates to a UV-curable solvent-free hotmelt adhesive comprising a UV-crosslink- able poly(meth)acrylate and a radiation-curable, free-radically polymerizable compound. The UV-crosslinkable poly(meth)acrylate is formed from an alkyl (meth)acrylate having 8 carbon atoms in the alkyl group, a monomer comprising at least one acid group and a copolymerizable photoinitiator monomer, and the UV-crosslinkable poly(meth)acrylate before crosslinking has a specific zero viscosity at 130 °C and a specific weight average molecular weight. The UV-cura- ble solvent-free hotmelt adhesive can be used for making pressure-sensitive self-adhesive articles.

UV-curable solvent-free hotmelt adhesives based on UV-crosslinkable poly(meth)acrylate adhesive polymers are known for making pressure-sensitive self-adhesive articles. It has been found that adhesion to metal surfaces can be improved by increasing the amount of acid functional groups in the poly(meth)acrylate. It has also been found that a high acid content of the UV- crosslinkable poly(meth)acrylate may result in an undesired increase in melt viscosity (measured as zero viscosity at 130 °C) which deteriorates coatability of the hotmelt adhesive on carrier substrates. A sufficiently low viscosity at typical coating temperatures is desired for good coatability.

Although a sufficiently low melt viscosity of UV-crosslinkable poly(meth)acrylates with high acid content may be achieved by sufficiently reducing the weight average molecular weight of the UV-crosslinkable poly(meth)acrylate, a reduction of molecular weight results in an undesired deterioration of adhesive performance, in particular a deterioration of cohesion at high temperatures. Although cohesion can be increased to a certain extent by increasing the UV-C dose when crosslinking the UV-crosslinkable poly(meth)acrylates, an increase in crosslinking may result in undesired decrease of adhesion.

Thus, the problem to be solved was to provide a solution for this multi-factorial optimization, namely to provide a pressure-sensitive hot-melt adhesive with good adhesion to metal substrates, good coatability and an optimized balance of cohesion and adhesion, in particular high adhesive strength at elevated temperatures such as 70 °C or more.

UV-crosslinkable hotmelt adhesives based on (meth)acrylate polymers are known for example from DE 102004058070, EP-A 246 848, EP-A 377 191 , EP-A 445 641 or WO 01/23488. WO 2012/168208 A1 describes radiation cross-linkable hot-melt adhesives containing a poly(meth)acrylate, which can be radiation cross-linked, and at least one oligo(meth)acrylate, which contains non-acrylic C-C double bonds. It has been found that the oligo(meth)acrylates with non-acrylic C-C double bonds react very slowly which could result in residue and odour problems. Therefore, an alternative, highly reactive system is of interest in order to ensure a fast, sufficiently high conversion of the oligomers, which results in improved toxicological safety and a lower odour level and lower VOC content, while simultaneously providing good adhesion to metal substrates, good coatability on carrier substrates, good balance of cohesion and adhesion and high adhesive strength at elevated temperatures.

The problem is solved by a UV-curable solvent-free hotmelt adhesive comprising

(a) at least one UV-crosslinkable poly(meth)acrylate formed from

(i) at least 80%, preferably 80 to 94.9% by weight, based on the amount of all monomers, of at least one alkyl (meth)acrylate having 8 carbon atoms in the alkyl group;

(ii) at least 5%, preferably 5 to 14.5% by weight, based on the amount of all monomers, of at least one ethylenically unsaturated monomer comprising at least one acid group;

(iii) at least 0.1%, preferably 0.1 to 3% by weight, based on the amount of all monomers, of at least one ethylenically unsaturated copolymerizable photoinitiator monomer;

(iv) optionally at least one further monomer distinct from the monomers (i) to (iii), and

(b) 5 to 30 parts by weight, preferably 5 to 20 parts by weight, based on 100 parts by weight of the UV-crosslinkable poly(meth)acrylate (a), of at least one radiation-curable, free-radically polymerizable compound with a hydroxyl number of less than 10, preferably from 0 to 5, more preferably from 1 to 5, and containing two or more copolymerizable, ethylenically unsaturated acrylic groups;

(c) optionally at least one tackifier, preferably in an amount of 0 to 30 parts, more preferably 2 to 30 parts by weight based on 100 parts by weight of the UV-crosslinkable poly(meth)acrylate, wherein before crosslinking the UV-crosslinkable poly(meth)acrylate has a zero viscosity at 130 °C of 200 Pa s or less, preferably from 10 to less than 200 Pa s; wherein before crosslinking the UV-crosslinkable poly(meth)acrylate has a weight average molecular mass of at least 350 000 g/mol, preferably 350 000 to 2 000 000 g/mol, more preferably from 350 000 to 1 500 000, measured by size-exclusion chromatography (SEC).

The text below occasionally uses the designation “(meth)acrylic” or "(meth)acrylate” or similar as an abbreviating notation for “acrylic or methacrylic” or "acrylate or methacrylate". In the designation Cx-alkyl (meth)acrylate and analogous designations, x denotes the number of carbons (carbon atoms) in the alkyl group. A poly(meth)acrylate is a polymer which is predominantly, i.e. to an extent of more than 50% by weight, made from (meth)acrylic acid and acid derivatives thereof, in particular the salts, esters or amides of (meth)acrylic acid.

Reported quantities for monomers of a polymer are based, unless otherwise explicitly stated, on 100 parts by weight of the sum of all monomers.

A pressure-sensitive adhesive is a viscoelastic adhesive which sets to form a film that at room temperature (20°C) remains permanently tacky and adhesive in the dry state. Adhesion to substrates is effected immediately by gentle pressure.

The term UV-curable describes a substance or a composition which cures upon irradiation with UV light. The term UV-crosslinkable is to be understood as meaning that the hotmelt adhesive comprises at least one compound having at least one radiation-sensitive group and a crosslinking reaction is induced upon irradiation. Irradiation for crosslinking is carried out using UV light, in particular UV-C radiation.

Hotmelt adhesives are solvent-free products, i.e. they are not dissolved or dispersed in water or organic solvents. They are products that are solid at room temperature (20 °C), and they are in the hot, molten state sufficiently fluid to be applicable to a substrate surface due to the accompanying viscosity reduction and upon cooling they produce the adhesive bond, with UV-cross- linkable hotmelt adhesives additionally being irradiated. In radiation-crosslinkable pressure-sensitive adhesives the cohesion, i.e. the inner strength of the adhesive layer, is achieved after coating the adhesive onto a carrier by subsequent photochemically induced crosslinking.

The UV-crosslinkable poly(meth)acrylate is formed to an extent of at least 80%, preferably 80 to 94.9%, more preferably from 85 to 94.9% by weight, based on the amount of all monomers, of monomers (i) which are alkyl (meth)acrylates having 8 carbon atoms in the alkyl group. The alkyl (meth)acrylate monomer (i) having 8 carbon atoms in the alkyl group is preferably selected from 1 -octyl acrylate, 2-octyl acrylate and 2-ethyl-hexyl acrylate. More preferably, the alkyl (meth)acry- late having 8 carbon atoms in the alkyl group is 2-octyl acrylate. Preferably, 2-octyl acrylate is used in amounts of from 85 to 94.9% by weight, based on the amount of all monomers. Preferably, the carbon atoms of the 2-octyl group of 2-octyl acrylate are of biological origin.

The UV-crosslinkable poly(meth)acrylate is formed to an extent of at least 5%, preferably 5 to 14.5% by weight, based on the amount of all monomers, of at least one ethylenically unsaturated monomer comprising at least one acid group (monomers (ii)). Monomers (ii) include in particular monomers having carboxylic acid groups, sulfonic acid groups or phosphonic acid groups. Carboxylic acid groups are preferred. Monomers (ii) are preferably selected from acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid, vinyllactic acid, vinylsulfonic acid, styrene sulfonic acid, acrylamidomethylpropanesulfonic acid, sulfopropyl acrylate and sulfopropyl methacrylate. Most preferred are acrylic acid, methacrylic acid or mixtures thereof.

The UV-crosslinkable poly(meth)acrylate is formed to an extent of at least 0.1% by weight, preferably not less than 0.2% by weight, preferably 0.1 to 3%, more preferably 0.5 to 2% by weight, based on the amount of all monomers, of at least one ethylenically unsaturated copolymerizable photoinitiator monomer (monomers (iii)). The poly(meth)acrylate is UV-crosslinkable through irradiation with UV light. A photoinitiator monomer is an ethylenically unsaturated copolymerizable compound having at least one photoinitiator group. The hotmelt adhesive preferably contains 0.0001 to 0.5 mol, particularly preferably 0.0002 to 0.1 mol, very particularly preferably 0.003 to 0.01 mol of the molecular group active as a photoinitiator and bound to the polymer per 100 g of poly(meth)acrylate.

The photoinitiator may be selected for example from so-called a-splitters in which a chemical bond is split to form 2 radicals which initiate the further crosslinking or polymerization reactions. These include for example acylphosphine oxides, hydroxyalkylphenones, benzoin derivatives, benzil derivatives, dialkyloxyacetophenones. They may especially be so-called H-abstractors which detach a hydrogen atom from the polymer chain, for example photoinitiators having a carbonyl group. This carbonyl group inserts itself into a C-H bond to form a C-C-O-H moiety. Examples include in particular acetophenone, benzophenone and derivatives thereof. Both classes of photoinitiators may be used alone or in admixture. The photoinitiator monomer is incorporated into the polymer chain, for example by free radical copolymerization. The photoinitiator monomer preferably comprises an acryloyl or methacryloyl group to this end.

By irradiation with high-energy light, in particular UV light, the photoinitiator or the photoinitiator group brings about crosslinking of the polymer, preferably by means of a chemical grafting reaction of the photoinitiator group with a spatially adjacent polymer or oligomer chain. The crosslinking may in particular be effected by insertion of a carbonyl group of the photoinitiator into an adjacent C-H bond to form a -C-C-O-H moiety. The wavelength range in which the photoinitiator group may be activated, i.e. which comprises the primary absorption band of the photoinitiator group, is preferably 200 to 380 nm, particularly preferably 250 to 350 nm, very particularly preferably 250 to 280 nm. The UV-crosslinkable poly(meth)acrylate may be produced by free radical polymerization of ethylenically unsaturated monomers with co-polymerization of at least one radiation-sensitive, free- radically polymerizable organic compound. Radiation-sensitive, free-radically polymerizable organic compounds are hereinbelow referred to as polymerizable photoinitiator for short. The polymerizable photoinitiator may be incorporated into the polymer chain of copolymers by free radical copolymerization. Polymerizable photoinitiators preferably have the following structure:

A-X-B wherein A is a monovalent organic radical that preferably comprises a phenone group as the radiation-sensitive group,

X is an ester group selected from -O-C(=O)-, -(C=O)-O- and -O-(C=O)-O- and

B is a monovalent organic radical comprising an ethylenically unsaturated free-radically polymerizable group. Preferred radicals A are radicals comprising at least one structural element derived from phenones, in particular from acetophenones or benzophenones. Preferred radicals B comprise at least one, preferably precisely one, acrylic or methacrylic group. The ethylenically unsaturated group may be directly bonded to the group X. The radiation-sensitive group may likewise be directly bonded to the group X. However, there may also be a spacer group between the ethylenically unsaturated group and the group X and between the radiation-sensitive group and group X respectively. The spacer group may have a molecular weight of up to 500, in particular up to 300 or 200, g/mol for example.

Suitable copolymerizable photoinitiators are acetophenone or benzophenone derivatives which comprise at least one, preferably one, ethylenically unsaturated group as described for example in EP 377191 A or EP 1213306 A. The ethylenically unsaturated group is preferably an acryloyl or methacryloyl group. The ethylenically unsaturated group may be directly bonded to the phenyl ring of the acetophenone or benzophenone derivative. There is preferably a spacer group between the phenyl ring and the ethylenically unsaturated group. The spacer group may comprise for example up to 100 carbon atoms.

A preferred group X is the carbonate group -O-(C=O)-O-. Preferred polymerizable photoinitiators are compounds of formula F-1 :

F-1 wherein R1 represents an organic radical having up to 30 carbon atoms, R2 represents an H atom or a methyl group and R3 represents a substituted or unsubstituted phenyl group or a C1- C4-alkyl group. R1 particularly preferably represents an alkylene group, in particular a C2-C8-al- kylene group. R3 particularly preferably represents a methyl group or a phenyl group, very particularly preferably a phenyl group.

Further acetophenone and benzophenone derivatives suitable as copolymerizable photoinitiators are, for example, those of formula F-2

wherein R2 and R3 are as defined above and R4 may represent a single bond or (-CH2-CH2-O)n, wherein n represents an integer from 1 to 12.

The UV-crosslinkable poly(meth)acrylate may optionally be formed from at least one further monomer (iv) distinct from the monomers (i) to (iii). The amount of the further monomers (iv) is preferably 0 to 14.5%, more preferably 0 to 9.5% by weight, based on the amount of all monomers, for example from 0.1 % to 14.5% by weight or from 0.5% to 9.5% by weight. The further monomers (iv) are ethylenically unsaturated copolymerizable compounds, for example alkyl (meth)acrylates with 1 to 10 carbon atoms in the alkyl group (except monomers (i)), vinyl esters of carboxylic acids comprising up to 20 carbon atoms, vinylaromatics having up to 20 carbon atoms, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and 1 or 2 double bonds or mixtures of these monomers. Preferred alkyl (meth)acrylates are methyl methacrylate, methyl acrylate, ethyl acrylate, ethyl methacrylate, propyl (meth)acrylate, n-butyl acrylate and isobutyl acrylate. Vinyl esters of carboxylic acids having 1 to 20 carbon atoms are, for example, vinyl laurate, vinyl stearate, vinyl propionate, Versatic acid vinyl esters, and vinyl acetate. Suitable vinylaromatic compounds include for example vinyltoluene, alpha- and p-methylstyrene, alphabutylstyrene, 4-n-butylstyrene, 4-n-decylstyrene and preferably styrene. Examples of nitriles are acrylonitrile and methacrylonitrile. The vinyl halides are ethylenically unsaturated compounds substituted by chlorine, fluorine or bromine, preferably vinyl chloride and vinylidene chloride. Examples of vinyl ethers include vinyl methyl ether or vinyl isobutyl ether. Preference is given to vinyl ethers of alcohols comprising 1 to 4 carbon atoms. Hydrocarbons having 2 to 8 carbon atoms and two olefinic double bonds include butadiene, isoprene and chloroprene.

The further monomers (iv) may also be monomers having polar groups (polar monomers). These preferably have a water solubility at 21 °C of more than 5 g/liter or more than 10 g/liter. Polar monomers are for example (meth)acrylamide and hydroxyl-comprising monomers, in particular hydroxyalkyl (meth)acrylates with 1 to 10 carbon atoms in the alkyl group, preferably with 2 to 4 carbon atoms in the alkyl group. Preferred monomers having hydroxyl groups are said hydroxyalkyl (meth)acrylates, in particular hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate.

However, the further monomers (v) preferably comprise no N-containing monomers or essentially no N-containing monomers. The further monomers (v) preferably comprise no aromatic monomers or essentially no aromatic monomers. The term “essentially no” is to be understood as meaning usage amounts of less than 1 % by weight, preferably not more than 0.5% by weight.

Most preferred monomers (iv) are methyl acrylate, methyl methacrylate, ethyl acrylate, n-propyl acrylate, isobutyl acrylate, alkyl (meth)acrylates with 4 to 7 or 9 to 10 carbon atoms in the alkyl group and hydroxyalkyl (meth)acrylates with 2 to 4 carbon atoms in the alkyl group.

A preferred UV-crosslinkable poly(meth)acrylate (a) according to the invention is formed from

(i) 80 to 94.9% by weight, preferably 85 to 94.9% by weight, based on the amount of all monomers, of at least one alkyl (meth)acrylate having 8 carbon atoms in the alkyl group, selected from 1 -octyl acrylate, 2-octyl acrylate and 2-ethylhexyl acrylate, preferably 2-octyl acrylate;

(ii) 5 to 14.5% by weight, based on the amount of all monomers, of at least one ethylenically unsaturated monomer comprising at least one acid group selected from acrylic acid, methacrylic acid and itaconic acid;

(iii) 0.1 to 3%, preferably 0.5 to 2% by weight, based on the amount of all monomers, of at least one ethylenically unsaturated copolymerizable photoinitiator monomer;

(iv) optionally at least one further monomer distinct from the monomers (i) to (iii) in amounts of 0 to 14.5%, preferably 0 to 9.5% by weight, based on the amount of all monomers, and is selected from methyl acrylate, methyl methacrylate, ethyl acrylate, n-propyl acrylate, alkyl (meth)acrylates with 4 to 7 or 9 to 10 carbon atoms in the alkyl group and hydroxyalkyl (meth)acrylates with 2 to 4 carbon atoms in the alkyl group. The UV-crosslinkable poly(meth)acrylate before crosslinking has a glass transition temperature of preferably not more than 10°C, for example from -60°C to +10°C, in particular in the range from -60°C to 0°C, or from -55°C to -10°C, particularly preferably from -55°C to -15°C. The glass transition temperature is determined by differential scanning calorimetry (ASTM D 3418-08, midpoint temperature). The glass transition temperature of the polymer is the glass transition temperature obtained when evaluating the second heating curve at a heating rate of 20° C/min.

The so-called Fox equation allows the person skilled in the art to identify copolymers in the appropriate Tg range in advance and to produce them in a targeted manner by suitable variation of the type and amount of the monomers. According to Fox (T.G. Fox, Bull. Am. Phys. Soc. 1956 [Ser. II] 1 , page 123 and according to Ullmann's Encyclopedia of Industrial Chemistry, vol. 19, page 18, 4th edition, Verlag Chemie, Weinheim, 1980) the glass transition temperature of at most weakly crosslinked copolymers is given to a good approximation by:

1 /Tg = X¹/Tg¹ + X²/Tg² + .... XVTg", wherein x¹, x², .... xⁿ are the mass fractions of the monomers 1 , 2, .... n and T_g ¹, T_g ², .... T_g ⁿ are the glass transition temperatures in degrees kelvin of the polymers constructed from only one of the monomers 1 , 2, .... n at a time. The T_g values for the homopolymers of the majority of monomers are known and are listed for example in Ullmann's Encyclopedia of Industrial Chemistry, vol. A21 , page 169, 5th edition VCH Weinheim, 1992; further sources for glass transition temperatures of homopolymers are, for example, J. Brandrup, E.H. Immergut, Polymer Handbook, 1^st Ed., J. Wiley, New York 1966, 2^nd Ed., J. Wiley, New York 1975, and 3^rd Ed., J. Wiley, New York 1989.

The UV-crosslinkable poly(meth)acrylate before crosslinking has a zero viscosity at 130 °C of 200 Pa s or less, preferably from 10 to less than 200 Pa s, more preferably of 10 to 100 Pa s. Zero viscosity is measured as described in the examples section.

The UV-crosslinkable poly(meth)acrylate before crosslinking has a weight average molecular mass of at least 350 000 g/mol, preferably 350 000 to 2 000 000 g/mol, more preferably of 350 000 to 1 500 000 g/mol, of 400 000 to 1 500 000 g/mol, or from 400 000 to 1 200 000 g/mol. Weight average molecular mass is measured by size-exclusion chromatography (SEC) as described in the examples.

Preferably and for sustainability reasons, bio-based materials are used for producing the UV- crosslinkable poly(meth)acrylate, which preferably is partly or completely made of partly or fully bio-based monomers. Bio-based materials are materials which are made from a renewable source and have a smaller impact on the environment. They do not require all the refining steps required for petroleum-based products, which are very expensive in terms of energy. The production of CO2 is reduced such that bio-based materials contribute less to global warming. The term “bio-based” indicates that the material is of biological origin and comes from biomaterial and renewable resources. A material of renewable origin or biomaterial is an organic material wherein the carbon comes from the CO2 fixed recently (on a human scale) by photosynthesis from the atmosphere. A biomaterial (carbon of 100% natural origin) has an isotopic ratio ¹⁴C/¹²C greater than 10"¹², typically about 1.2x10^-12, while a fossil material has a zero ratio. Indeed, the isotopic ¹⁴C is formed in the atmosphere and is then integrated via photosynthesis, according to a time scale of a few tens of years at most. The half-life of the ¹⁴C is 5730 years. Thus, the materials coming from photosynthesis, namely plants in general, necessarily have a maximum content in isotope ¹⁴C. The determination of the content of biomaterial or of bio-carbon can be carried out in accordance with the standard ASTM D 6866-22. Preferably, the pressure-sensitive adhesive polymer has a content of bio-carbon of at least 10 mol-%, in particular of at least 15 mol-%, e.g. at least 20 mol-%, at least 30 mol-% or at least 40 mol-%, based on the total amount of carbon atoms in the pressure-sensitive adhesive polymer. A polymer which is partly made of fully or partly biobased monomers is a polymer where not all monomers used in the polymerization are partly or fully biobased monomers. A partly biobased monomer is a monomer where not all C-atoms are biobased, for example (meth)acrylic acid esters where only the acid part or only the alcohol part is biobased.

Suitable bio-based materials for producing the UV-crosslinkable poly(meth)acrylate, are for example (meth)acrylic esters, wherein the (meth)acrylic acid component or the alcohol component or both are bio-based. Various methods of producing bio-based acrylic acid from renewable plant materials are mentioned in EP 2626397 A1. Suitable bio-based alcohols are for example biobased iso-butanol, bio-based n-butanol, bio-based ethanol, bio-based iso-pentanol (3-methyl- butan-1-ol), bio-based 2-octanol, bio-based 1 -octanol and bio-based n-heptanol. Preferred partly biobased monomers are esters of (meth)acrylic acid and bio-based alcohols, preferably biobased 2-octanol, bio-based 1 -octanol, bio-based ethanol, bio-based iso-butanol, bio-based n-bu- tanol, bio-based iso-pentanol (3-methylbutan-1-ol), and bio-based n-heptanol. Preferred fully biobased monomers are esters of bio-based acrylic acid and bio-based alcohols as mentioned above.

Preferably at least the carbon atoms of the alkyl group of at least one of the alkyl (meth)acrylate monomers are of biological origin. More preferably, the UV-crosslinkable poly(meth)acrylate is completely or partly bio based and wherein the alkyl (meth)acrylate having 8 carbon atoms in the alkyl group is 2-octyl acrylate and at least the carbon atoms of the 2-octyl group are of biological origin. More preferably, at least 50 wt.%, more preferred 100 wt.% of 2-octyl acrylate monomer is made from bio-based 2-octanol and non-bio-based or bio-based acrylic acid, i.e. preferably at least the carbon atoms of the 2-octyl group of 2-octyl acrylate are of biological origin.

The UV-crosslinkable poly(meth)acrylates are producible by copolymerizing the monomeric components using the customary polymerization initiators and optionally chain transfer agents, wherein polymerization is carried out at the customary temperatures in bulk, in emulsion, for example in water or liquid hydrocarbons, or in solution. The poly(meth)acrylates are preferably produced by polymerization of the monomers in organic solvents, in particular in organic solvents having a boiling range of 50°C to 150°C, preferably of 60°C to 120°C, using the customary amounts of polymerization initiators which are generally 0.01% to 10% by weight, in particular 0.1 % to 4% by weight, based on the total weight of the monomers. The polymers may be produced at temperatures of 20°C to 150°C, preferably at temperatures in the range from 70°C to 120°C, and pressures of 0.1 to 100 bar (absolute), preferably at 0.3 to 10 bar, in the presence of 0.01% to 10% by weight of peroxides or azo starters as polymerization initiators based on the monomers and in the presence of 0% to 200% by weight of inert solvents, preferably 5% to 25% by weight based on the monomers, i.e. by solution or bulk polymerization. The reaction is preferably carried out under increasing vacuum, for example by reducing the pressure from standard pressure (1 bar) to 500 mbar (absolute). Solvents are, for example, hydrocarbons, alcohols such as methanol, ethanol, propanol, butanol, isobutanol, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, nitriles such as acetonitrile and benzonitrile or mixtures of the recited solvents. In a preferred embodiment the solvents for the polymerization are one or more ketones having a boiling point of below 150 °C at standard pressure (1 bar).

Suitable polymerization initiators include for example azo compounds, ketone peroxides and alkyl peroxides, for example acyl peroxides such as benzoyl peroxide, dilauroyl peroxide, didecanoyl peroxide, isononanoyl peroxide, alkyl esters such as tert-butyl perpivalate, tert-butyl per-2- ethylhexanoate, tert-butyl permaleate, tert-butyl perisononanoate, tert-butyl perbenzoate, tertamyl per-2-ethylhexanoate, dialkyl peroxides such as dicumyl peroxide, tert-butyl cumyl peroxide, di-tert-butyl peroxide and peroxodicarbonates. Also employable as initiators are azo starters such as for example 2,2'-azobisisobutyronitrile, 2,2'-azobis(methylisobutyrate) or 2,2'-azobis(2,4-dime- thylvaleronitrile).

To perform the polymerization the reaction mixture may also be admixed with compounds which reduce the degree of polymerization, so-called chain transfer agents, for example in amounts of 0.1 to 0.8 parts by weight based on 100 parts by weight of the monomers to be polymerized. Suitable compounds include for example those having a thiol group, for example mercaptans such as mercaptoethanol, tert-butyl mercaptan, mercaptosuccinic acid, ethylhexyl thioglycolate, 3-mer- captopropyltrimethoxysilane or dodecyl mercaptan.

After the solution polymerization the solvents may be removed optionally under reduced pressure and at elevated temperatures, for example in the range from 100°C to 150°C. The polymers may then be employed in a solvent-free state (solvent content preferably less than 2% by weight based on the total composition), i.e. as melts.

The hotmelt adhesive according to the invention comprises at least one radiation-curable, free- radically polymerizable compound (b). Compound (b) contains two or more, preferably two, co- polymerizable, ethylenically unsaturated acrylic groups. Compound (b) preferably contains no ethylenically unsaturated non-acrylic groups. Compound (b) has a hydroxyl number of less than 10, preferably from 1 to less than 10 or from 0 to less than 5, more preferably from 1 to 5. The hydroxyl number can be measured according to DIN 53240-3:2016-03.

Compounds (b) with high OH-numbers (higher than 10), such as for example compounds prepared by epoxid ring opening reactions may lead to undesired gel formation via esterification reactions.

Radiation-curable, free-radically polymerizable compounds with non-acrylic double bonds have the disadvantage of reacting comparatively slowly with the risk of remaining unreacted residues and odour problems.

Compound (b) is used in an amount of 5 to 30 parts by weight, preferably 5 to 20 parts by weight based on 100 parts by weight of the UV-crosslinkable poly(meth)acrylate (a).

Suitable examples of radiation-curable, free-radically polymerizable compound (b) are elastic urethane acrylate oligomers. Such oligomers have higher molecular weight flexible chains to offset brittleness and impart elasticity. These oligomers are, for example, long chain diacrylate polyurethane oligomers. In one embodiment, the urethane acrylate oligomer is a urethane(meth)acrylate of formula (I)

In the above formula, R¹ is a divalent alkylene radical which has 2 to 12 carbon atoms, and which may optionally be substituted by Ci to C4 alkyl groups, hydroxyl groups, and/or interrupted by one or more oxygen atoms, said radical specifically having 2 to 10 carbon atoms, more specifically 2 to 8, and very specifically having 2 to 6 carbon atoms. R² in each case independently of any other is methyl or hydrogen, preferably hydrogen. R³ is a divalent alkylene radical which has 1 to 12 carbon atoms and which may optionally be substituted by Ci to C4 alkyl groups, hydroxyl groups, and/or interrupted by one or more oxygen atoms, said radical having specifically 2 to 10, more specifically 3 to 8, and very specifically 3 to 5 carbon atoms, and n and m independently of one another are positive numbers from 1 to 5, specifically 2 to 5, more specifically 2 to 4, very specifically 2 to 3, and more particularly 2 to 2.5. R⁴ is a divalent organic radical which is formed by abstraction of both isocyanate groups from an aliphatic, cycloaliphatic or aromatic diisocyanate. Methods of making such urethane acrylate oligomers may be found, for example, in WO 2014/191228.

A suitable urethane acrylate oligomer is at least one high strength and high flexibility urethane- (meth)acrylate having a weight average molar mass Mw of 800 to 5000 g/mol and two ethyleni- cally unsaturated acrylic double bonds per molecule, comprising as synthesis components (a1) at least one aromatic or cycloaliphatic, preferably cycloaliphatic diisocyanate, (a2) at least one polyesterdiol synthesized from (a21) optionally a diol having a molar weight below 250 g/mol, (a22) at least one oligomeric or polymeric diol having a molar mass Mn of more than 200 g/mol and selected from the group consisting of (a221) polytetrahydrofurandiol with a molar mass Mn of up to 2900 g/mol and (a222) at least one polycaprolactonediol with a molar mass Mn of up to 600 g/mol, (a23) at least one dicarboxylic acid selected from the group consisting of compounds of the for-

and/or compound of the formula (lb)

wherein R² is a single bond or a divalent alkylene radical comprising 1 to 3 carbon atoms, and R³ is hydrogen or an alkyl radical comprising 1 to 10 carbon atoms, and

(a3) a third compound comprising precisely one isocyanate-reactive group and precisely one free polymerizable acrylic group.

Preferred radiation-curable, free-radically polymerizable compounds (b) are selected from aliphatic urethane acrylate oligomers with an acrylate functionality of at least two, preferably with an acrylate functionality of two to three, i.e. aliphatic urethane diacrylates. A preferred aliphatic urethane diacrylate oligomer with an acrylate functionality of two is for example Laromer® UA 9089. An aliphatic urethane oligomer with an average functionality of 2.3 is Laromer® LR 8987.

The hotmelt adhesive according to the invention optionally comprises at least one tackifier (c). The amount of tackifier preferably is 0 to 30 parts by weight, more preferably 2 to 30 parts by weight, more preferably 2 to 20 parts by weight based on 100 parts by weight of the UV-cross- linkable poly(meth)acrylate (a). A tackifier is a polymeric or oligomeric additive for adhesive polymers or generally for elastomers which increases their autoadhesion (tack, inherent tack, selfadhesion) so that after short, light contact pressure they adhere to surfaces firmly. Tackifiers include for example natural resins, such as colophony resins and the derivatives thereof formed by disproportionation or isomerization, polymerization, dimerization or hydrogenation or terpene resins. These may be in their salt form (with for example monovalent or polyvalent counterions (cations)) or preferably in their esterified form. Alcohols used for esterification may be monohydric or polyhydric. Examples include methanol, ethanediol, diethylene glycol, triethylene glycol, 1 ,2,3-propanethiol, pentaerythritol. Also employable are hydrocarbon resins, for example cou- marone-indene resins, polyterpene resins, hydrocarbon resins based on unsaturated CH compounds, such as butadiene, pentene, methylbutene, isoprene, piperylene, divinylmethane, pentadiene, cyclopentene, cyclopentadiene, cyclohexadiene, styrene, alpha-methylstyrene, vinyltol- uene. Tackifiers are known for example from Adhesive Age, July 1987, pages 19-23 or Polym. Mater. Sci. Eng. 61 (1989), pages 588-592.

Polyacrylates having a low molar weight can also be used as tackifiers. These polyacrylates preferably have a weight-average molecular weight M_w less than 50 000, in particular less than 30 000. The polyacrylates preferably consist to an extent of at least 60% by weight, in particular at least 80% by weight, of Ci-Cs alkyl (meth)acrylates. Suitable tackifiers include for example the low molecular weight polymers and oligomers described in WO 2013/117428 having a weightaverage molecular weight of less than 50 000 and a glass transition temperature of not less than -40°C to not more than 0°C, preferably of not less than -35°C to not more than 0°C, producible by emulsion polymerization in the presence of at least one molecular weight regulator and producible from a monomer mixture comprising at least 40% by weight of at least one C1- to C20-alkyl (meth)acrylate. Preferred tackifiers include natural or chemically modified colophony resins. Colophony resins consist predominantly of abietic acid or abietic acid derivatives.

Most preferred tackifiers are esters of hydrogenated rosin. Examples are pentaerythritol ester of hydrogenated rosin (e.g. Foral® 105 with a softening point of 95-103 °C) or glycerol ester of hydrogenated wood rosin (e.g. Foral® 85 with a softening point of 80-88 °C). The softening point is preferably from 80 to 110 °C, more preferably from 90 to 110 °C, measured by ring and ball method (according to ISO 4625-1:2020).

The hotmelt adhesive according to the invention optionally comprises at least one photoinitiator additive which is not copolymerized in the UV-crosslinkable poly(meth)acrylate (a). An example is 1-Hydroxycyclohexyl-phenyl ketone of formula

An example is Omnirad® 184. The photoinitiator additive is preferably used in amounts of 0 to 10% by weight, or 5 to 10% by weight, based on the radiation-curable, free-radically polymerizable compound (b).

The UV-curable hotmelt adhesive according to the invention preferably has a viscosity at zero shear of less than 100 Pa s at 130°C. It is employed in solvent-free, meltable form. Solvent present as a consequence of manufacture may be removed beforehand by suitable processes, preferably to a residual content of less than 0.5% by weight based on the solids content.

The UV-curable hotmelt adhesive may further comprise additives different from compounds (a), (b) and (c) such as for example resins, plasticizers, antioxidants, crosslinkers, fillers, dyes, flow control agents etc.

To produce adhesive coatings the UV-curable pressure-sensitive hotmelt adhesives are applied as a melt to a carrier substrate, for example at temperatures of 50°C to 160°C, preferably 80°C to 150°C or more than 100°C. The application rate of the pressure-sensitive hotmelt adhesive is preferably from 5 to 50 g/m², particularly preferably from 10 to 30 g/m².

Preferred carrier substrates include paper and polymer films, for example films made of polyethylene (PE), polypropylene (PP), oriented polypropylene (OPP), polyethylene terephthalate (PET) and polyvinyl chloride (PVC). The film thickness is preferably 30 pm to 150 pm. However, it is also possible to employ in certain applications (for example sun protection films for car windows, PVC labels) thinner films having a thickness of from 20 pm and in the field of surface protection of automobile bodies very much larger thicknesses of up to 350 pm.

After application to the carrier substrate the UV-curable solvent-free hotmelt adhesive according to the invention is irradiated with high-energy radiation, preferably UV light, in particular UV-C radiation (200-280 nm), to effect crosslinking. To this end the coated substrates are generally placed on a conveyor belt and the conveyor belt is run past a radiation source, for example a UV lamp. The degree of crosslinking of the polymers depends on the duration and intensity of the irradiation. Employable UV sources include the customary sources, for example intermediate pressure mercury lamps having a radiative power of 80 to 240 W/cm.

The invention also provides a pressure sensitive self-adhesive article having an UV-cured adhesive layer formed from the hotmelt adhesive as described above. Pressure sensitive self-adhesive article are for example self-adhesive polymer films (e.g. self-adhesive graphics films), self- adhesive polymer sheets, self-adhesive labels or self-adhesive tapes.

The invention also provides a method of making a pressure-sensitive self-adhesive article, the method comprising the steps of coating an UV-curable solvent-free hotmelt adhesive according to the invention on a carrier substrate and crosslinking the adhesive by irradiation with UV-light with a UV-C dose of preferably 20 to 200 mJ/cm², more preferably 30 to 200 mJ/cm², more preferably 60 to 200 mJ/cm² more preferably 80 to 180 mJ/cm². UV-C radiation typically has a wavelength of 100 to 280 nm.

The invention also provides the use of an UV-curable solvent-free hotmelt adhesive as described above for making a pressure-sensitive self-adhesive article, preferably selected from pressure sensitive adhesive tapes, pressure sensitive adhesive films and pressure-sensitive adhesive labels.

UV-curable solvent-free hotmelt adhesive as described above has the benefits of

• low VOC content, low odour

• good adhesion to metal substrates

• good coatability on carrier substrates

• good balance of cohesion and adhesion, including high adhesive strength at elevated temperatures

Examples

Measurement of zero viscosity

Zero viscosity is the limit value of the viscosity function at infinitely low shear rates. It is measured with an Anton Paar Rheometer MCR 100 (US 200 evaluation software) in plate/plate geometry. The samples are measured in oscillatory shear at a small shear amplitude of 10%. Temperature 130°C (or as specified), angular frequency ramp log 100-0.1 1/s, measuring gap 0.5 mm, evaluation according to Carreau-Gahleitner I, piston diameter 25 mm.

Measurement of weight average molecular weight Mw

Weight average molecular weight is measured by size-exclusion chromatography (SEC) with the following parameters:

Instrument DRI Agilent 1100 App_R [254 nm]

Column designation PLgel MIXED-B

Separation material PLgel 10 pm

Eluent THF + 0.1% trifluoro acetic acid

Column temperature 40 °C

Flow rate 1 ml/min

Injection amount 100 pl

Concentration 2 mg/ml

Sample solutions are dissolved in the eluent.

The elution curves are converted into the actual molecular weight distribution curves with the aid of a polystyrene calibration curve. The calibration is carried out with narrowly distributed polystyrene standards from Polymer Laboratories with molecular weights from M ~ 580 to M = 2,476,000. Values outside this elution range are extrapolated.

Hydroxyl numbers can be measured according to DIN 53240-3:2016-03.

Abbreviations nBA n-butyl acrylate

EHA 2-ethylhexyl acrylate

2-OA 2-octyl acrylate

MA methyl acrylate iBA iso-butyl acrylate

AA acrylic acid

Fl photoinitiator monomer; polymerizable photoinitiator (35% solution in MEK) of formula F-1

MEK solvent; methyl ethyl ketone t-BPPiv initiator; tert-butyl perpivalate (75% solution in mineral oil)

Foral® 105 tackifier; pentaerythritol ester of hydrogenated rosin, softening point 95-

103 °C Laromer® UA 9089 aliphatic urethane acrylate with an acrylate functionality of two; hydroxyl number of 3

Mw weight average molecular weight Production of polymers - General procedure

In a polymerization apparatus consisting of a glass reactor, a reflux condenser, a stirrer and a nitrogen inlet 120 g of MEK are initially charged under a light nitrogen stream and heated to 80°C. 40 g of altogether 800 g of a monomer mixture (as described in table 1) are added. Upon reattaining 80°C 3.27 g of a starter solution of 5.33 g of tert-butyl perpivalate (75% in mineral oil) and 60 g of MEK are added and the mixture is polymerized for 3 min. Then the remaining 760 g of monomer mixture and 62.07 g of starter solution are added over 3 h. The temperature is then increased to 90°C and a solution of 3.2 g of tert-butyl perpivalate (75% in mineral oil) in 20 g of MEK is added over 30 min. A vacuum is then applied and the solvent is distilled off at not more than 135°C and less than 50 mbar. The mixture is then degassed with slow stirring for 1 h at 135°C and the maximum achievable vacuum. The melt is drained into a metal or polypropylene cup.

Table 1 : Monomer compositions; amounts in parts by weight; photoinitiator Fl amount 1 part by weight

¹⁾ comparative example ²⁾ photoinitiator Fl amount 0.27 parts by weight

The data in table 1 show that high acid content in comparative adhesive polymers based on BA leads to undesirable high zero viscosities (above 200 Pa s) which is undesired for good coatability of hotmelt adhesives made with these adhesive polymers. Viscosity of high acid content adhesive polymers can be reduced to an acceptable level by lowering the weight average molecular weight of these polymers, for example by using higher amounts of peroxide initiator for polymerization. But such low molecular weights are undesired for good adhesive performance. High weight average molecular weights are desired good cohesion.

The data show that polymers based on C8-alkyl acrylates according to the invention lead to polymers with sufficiently low zero viscosity (below 200 Pa s, preferably even below 100 Pa s) even with high acid content such as 7.5 wt.% or even 10 wt.%, although having high weight average molecular weight above 350.000 g/mol and even up to 940 000 g/mol.

Formulation examples

UV-curable solvent-free hotmelt adhesive compositions are prepared by heating and melting an UV-crosslinkable poly(meth)acrylate (Polymers P1 to P16) and adding 10 wt.% Laromer® UA 9089 and 10 wt.% Foral® 105 with stirring.

Performance tests

The measurements are carried out at room temperature (20°C) unless explicitly stated otherwise. Preparation of test strips, adhesive articles

The resins are applied directly on polyester film (Hostaphan® RN 36) as the carrier substrate in an amount of 60 g/m² using a coating table heated to 110°C. Then the coated papers are irradiated with UV light (H spectrum; Hg medium pressure, 120 W/cm) with IIV-C doses from 25 to 140 mJ/cm². The irradiation dose is given in table 2.

The carrier coated with pressure-sensitive adhesive was cut into 25 mm wide test strips. The tests were either continued immediately or the test strips were stored for 3 days at 70°C and 50% rel. humidity before testing to determine plasticizer resistance.

Shear strength measurement

To determine shear strength the test strips are bonded to steel sheet (AFERA steel) with a bonded area of 25 x 25 mm, rolled on once with a 1 kg roller, stored for 24 h and then loaded with a 2 kg hanging weight. The shear strength (cohesion) is determined at 70°C; 50% relative atmospheric humidity. The measure for shear strength is the time in hours until the weight drops off. An average is formed from three measurements in each case.

S.A.F.T test (heat resistance; Shear Adhesion Failure Test)

The test strips are bonded to AFERA steel with a bonded area of 25 x 25 mm, rolled 4 times with a 2 kg roller and, after a contact time of 16 hours, subjected to hanging stress with a 1 kg weight. During the subjection to hanging stress the test strips are continuously heated at a rate of 0.5 °C/min starting from 23°C and up to 180 °C. The heating temperature achieved when the weight falls off is a measure of the heat resistance of the adhesive. The average of 3 measurements is calculated in each case. Results for samples which do not fall of at 180 °C are noted as “> 180 °C”.

The adhesive test results are summarized in table 2.

Table 2: Adhesive test results

¹⁾ comparative example

Preferred is a S.A.F.T value of > 180 °C and/or a shear strength of > 600 min.

Most preferred is a S.A.F.T value of > 180 °C in combination with a shear strength of > 600 min.

Test results show that comparative examples E1, E1a, E4 and E4a with adhesive polymers based on butyl acrylate with 5 wt.% or 10 wt.% of acrylic acid and molecular weights below 350 000 g/mol lead to S.A.F.T test temperatures below 180 °C and to comparatively low shear strength values.

Claims

1 . A UV-curable solvent-free hotmelt adhesive comprising

(a) at least one UV-crosslinkable poly(meth)acrylate formed from

(b) 5 to 30 parts by weight, preferably 5 to 20 parts by weight, based on 100 parts by weight of the UV-crosslinkable poly(meth)acrylate (a), of at least one radiation-curable, free- radically polymerizable compound with a hydroxyl number of less than 10, preferably from 1 to 5, and containing two or more copolymerizable, ethylenically unsaturated acrylic groups;

(c) optionally at least one tackifier, preferably in an amount of 0 to 30 parts, more preferably

2 to 30 parts by weight based on 100 parts by weight of the UV-crosslinkable poly(meth)acrylate, wherein before crosslinking the UV-crosslinkable poly(meth)acrylate has a zero viscosity at 130 °C of 200 Pa s or less, preferably from 10 to less than 200 Pa s; wherein before crosslinking the UV-crosslinkable poly(meth)acrylate has a weight average molecular mass of at least 350 000 g/mol, preferably 350 000 to 2 000 000 g/mol, measured by size-exclusion chromatography.

2. The hotmelt adhesive according to the preceding claim, wherein before crosslinking the UV- crosslinkable poly(meth)acrylate has a glass transition temperature of not more than 10°C, preferably of -60°C to +10°C.

3. The hotmelt adhesive according to claim 1 or 2, wherein the alkyl (meth)acrylate monomer (i) having 8 carbon atoms in the alkyl group is selected from 1 -octyl acrylate, 2-octyl acrylate and 2-ethylhexyl acrylate.

4. The hotmelt adhesive according to the preceding claim, wherein the UV-crosslinkable poly(meth)acrylate is completely or partly bio based and wherein the alkyl (meth)acrylate having 8 carbon atoms in the alkyl group is 2-octyl acrylate and at least the carbon atoms of the 2-octyl group are of biological origin.

5. The hotmelt adhesive according to any one of claims 1 to 4, wherein monomer (i) is 2-octyl acrylate and used in amounts of from 85 to 94.9% by weight, based on the amount of all monomers.

6. The hotmelt adhesive according to any one of claims 1 to 5, wherein monomer (ii) is selected from acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid, vinyllactic acid, vinylsulfonic acid, styrene sulfonic acid, acrylami- domethylpropanesulfonic acid, sulfopropyl acrylate and sulfopropyl methacrylate, preferably acrylic acid or methacrylic acid; and is used in amounts of 5 to 14.5% by weight, based on the amount of all monomers.

7. The hotmelt adhesive according to any one of claims 1 to 6, wherein monomer (iii) is used in amounts of 0.1 to 3%, preferably 0.5 to 2% by weight by weight, based on the amount of all monomers.

8. The hotmelt adhesive according to any one of claims 1 to 7, wherein monomer (iv) is used in amounts of 0 to 14.5%, preferably 0 to 9.5% by weight, based on the amount of all monomer and is selected from methyl acrylate, methyl methacrylate, ethyl acrylate, n-propyl acrylate, alkyl (meth)acrylates with 4 to 7 or 9 to 10 carbon atoms in the alkyl group and hydroxyalkyl (meth)acrylates with 2 to 4 carbon atoms in the alkyl group.

9. The hotmelt adhesive according to any one of claims 1 to 8, wherein the radiation-curable, free-radically polymerizable compound (b) is employed in an amount of 5 to 20 parts by weight based on 100 parts by weight of the UV-crosslinkable poly(meth)acrylate, and is selected from aliphatic urethane acrylate with an acrylate functionality of at least two.

10. The hotmelt adhesive according to any one of claims 1 to 9, wherein the adhesive further comprises at least one not copolymerized photoinitiator in an amount of 5 to 10% by weight based on the radiation-curable, free-radically polymerizable compound (b).

11 . The hotmelt adhesive according to any one of claims 1 to 10, wherein the adhesive comprises the at least one tackifier (c) in an amount of 2 to 20 parts by weight based on 100 parts by weight of the UV-crosslinkable poly(meth)acrylate, and the tackifier is an ester of hydrogenated rosin with a softening point of preferably from 80 to 110 °C, more preferably from 90 to 110 °C.

12. The UV-curable solvent-free hotmelt adhesive according to any one of claims 1 to 11 , wherein the UV-crosslinkable poly(meth)acrylate (a) is formed from

(i) 80 to 94,9% by weight, preferably 85 to 94,9% by weight, based on the amount of all monomers, of at least one alkyl (meth)acrylate having 8 carbon atoms in the alkyl group, selected from 1 -octyl acrylate, 2-octyl acrylate and 2-ethyl-hexyl acrylate, preferably 2-octyl acrylate;

(ii) 5 to 14,5% by weight, based on the amount of all monomers, of at least one ethylenically unsaturated monomer comprising at least one acid group selected from acrylic acid, methacrylic acid and itaconic acid;

(iv) optionally at least one further monomer distinct from the monomers (i) to (iii) in amounts of 0 to 14.5%, preferably 0 to 9.5% by weight, based on the amount of all monomers, and is selected from methyl acrylate, methyl methacrylate, ethyl acrylate, n-propyl acrylate, alkyl (meth)acrylates with 4 to 7 or 9 to 10 carbon atoms in the alkyl group and hydroxyalkyl (meth)acrylates with 1 to 4 carbon atoms in the alkyl group.

13. The UV-curable solvent-free hotmelt adhesive according to any one of claims 1 to 12, wherein before crosslinking the UV-crosslinkable poly(meth)acrylate has a zero viscosity at 130 °C of from 10 to 100 Pa s.

14. The UV-curable solvent-free hotmelt adhesive according to any one of claims 1 to 13, wherein before crosslinking the UV-crosslinkable poly(meth)acrylate has a weight average molecular mass of 400 000 to 1 500 000 g/mol.

15. A pressure sensitive self-adhesive article having an UV-cured adhesive layer formed from the hotmelt adhesive according to any one of claims 1 to 14.

16. A method of making a pressure-sensitive self-adhesive article, the method comprising the steps of coating an UV-curable solvent-free hotmelt adhesive according to any one of claims 1 to 14 on a substrate and crosslinking the adhesive by irradiation with UV-light with a UV-C dose of 20 to 200 mJ/cm², preferably 100 to 180 mJ/cm² .

17. Use of a hotmelt adhesive according to any one of claims 1 to 14 for making a pressuresensitive self-adhesive article, preferably selected from pressure sensitive adhesive tapes, pressure sensitive adhesive films and pressure-sensitive adhesive labels.