CN118055989A - Pressure sensitive adhesive composition comprising a dispersion polymer formed by emulsion polymerization of isobutyl acrylate, 2-ethylhexyl acrylate and/or 1-octyl acrylate, styrene and a monomer having at least one acid group - Google Patents

Abstract

The present invention describes a pressure sensitive adhesive composition in the form of an aqueous polymer dispersion comprising a dispersed pressure sensitive adhesive polymer formed by emulsion polymerization of isobutyl acrylate, 2-ethylhexyl acrylate and/or 1-octyl acrylate and styrene, monomers having at least one acid group and optionally other monomers. The pressure sensitive adhesive composition can be used for the production of self-adhesive articles such as self-adhesive labels, self-adhesive tapes or self-adhesive films.

Description

Pressure sensitive adhesive composition comprising a dispersion polymer formed by emulsion polymerization of isobutyl acrylate, 2-ethylhexyl acrylate and/or 1-octyl acrylate, styrene and a monomer having at least one acid group

Description of the invention

The present invention relates to a pressure sensitive adhesive composition in the form of an aqueous polymer dispersion comprising a dispersed pressure sensitive adhesive polymer formed by emulsion polymerization of isobutyl acrylate, 2-ethylhexyl acrylate and/or 1-octyl acrylate, styrene and monomers having at least one acid group and optionally further monomers. The pressure sensitive adhesive composition can be used for the production of self-adhesive articles such as self-adhesive labels, self-adhesive tapes or self-adhesive films.

Aqueous pressure sensitive adhesives are characterized by their adhesion-cohesion balance. The improved adhesion/cohesion balance is applicable to most pressure sensitive adhesive applications, particularly for specialty tapes. Typically, one of these properties cannot be improved without degrading the other. However, there is a need to do so. For example, high performance tapes require aqueous pressure sensitive adhesives that have high adhesion to various substrates and at the same time high cohesion (shear resistance) at high temperatures. For label applications, high tack is required to ensure high labeling line speeds, while the adhesive needs to provide sufficient cohesion to achieve good convertibility. Another need for an aqueous pressure sensitive adhesive is to reduce the carbon footprint. Although this may be achieved by using a partially or fully bio-based monomer to produce the adhesive polymer, an adhesive polymer based at least in part on a bio-based monomer should also meet the adhesion/cohesion balance requirements.

US2020/0017725 A1 describes pressure sensitive adhesive compositions comprising emulsion polymerization products of a monomer mixture comprising alkyl esters of (meth) acrylic acid, vinyl aromatic monomers, hydroxy functional (meth) acrylate monomers and ureido substituted monomers, and optionally other monomers.

It is an object of the present invention to provide water-based (i.e., substantially organic solvent free) polymeric pressure sensitive adhesives having a good or improved adhesion/cohesion balance.

The present invention provides a pressure sensitive adhesive composition in the form of an aqueous polymer dispersion comprising at least one dispersed pressure sensitive adhesive polymer formed by emulsion polymerization of:

(i) 10 to 20 wt% of isobutyl acrylate based on the sum of all monomers;

(ii) 30 to 70% by weight, preferably 40 to 60% by weight, based on the sum of all monomers, of 2-ethylhexyl acrylate, 1-octyl acrylate, or a mixture of 2-ethylhexyl acrylate and 1-octyl acrylate;

(iii) 10 to 20% by weight of styrene, based on the sum of all monomers

(Iv) 0.1 to 10% by weight, based on the sum of all monomers, of at least one monomer having at least one acid group;

(v) Optionally at least one alkyl (meth) acrylate monomer which is different from monomers (i) and (ii) and which has a glass transition temperature of less than-20 ℃ when polymerized to a homopolymer;

(vi) Optionally at least one functional monomer selected from the group consisting of hydroxy-functional (meth) acrylate monomers and ureido-substituted ethylenically unsaturated monomers;

(vii) Optionally at least one alkyl (meth) acrylate monomer different from monomers (i), (ii) and (v);

(viii) Optionally further monomers other than (i) to (vii),

Wherein the weight ratio of isobutyl acrylate (i) to the sum of 2-ethylhexyl acrylate and 1-octyl acrylate (ii) is from 1:1.5 to 1:5;

Wherein the weight ratio of isobutyl acrylate (i) to styrene (iii) is from 1:5 to 5:1, preferably from 1:2 to 2:1;

Wherein the pressure sensitive adhesive polymer has a glass transition temperature below-20 ℃, preferably from-50 ℃ to-25 ℃, as determined by differential scanning calorimetry, as the midpoint temperature when evaluating the second heating curve at a heating rate of 20K/min.

It has been found that unexpected performance improvements can be achieved when specific, different amounts of isobutyl acrylate are used as a comonomer to prepare acrylic polymer dispersions suitable for pressure sensitive adhesives for tape and other applications. Even at high temperatures, adhesion to non-polar and polar surfaces remains constant while shear resistance increases significantly.

The reported weight% value of the monomer in each case relates to the sum of all monomers used in the polymerization, unless otherwise indicated.

The pressure sensitive adhesive is a viscoelastic adhesive that solidifies to form a film that remains permanently tacky and adhesive in a dry state at room temperature (20 ℃). Adhesion to the substrate is immediately achieved by gentle pressure.

The designation "(meth) acrylic acid" or "(meth) acrylate" and the like are occasionally used hereinafter as abbreviations for "acrylic acid or methacrylic acid" or "acrylate or methacrylate". In the name Cx alkyl (meth) acrylate and similar names, x represents the number of carbon atoms in the alkyl group.

The terms "aqueous polymer dispersion" and "aqueous solvent" refer to solvent systems based primarily on water, preferably containing no or less than 10 wt%, less than 5 wt%, or less than 1 wt% organic solvent based on the total composition.

The glass transition temperature was determined by differential scanning calorimetry as the midpoint temperature when evaluating the second heating curve at a heating rate of 20K/min (ASTM D3418-08).

The pressure sensitive adhesive polymer is prepared from isobutyl acrylate. The amount of isobutyl acrylate monomer (i) is from 10 to 20% by weight, preferably from 12 to 20% by weight, based on the sum of all monomers.

The pressure sensitive adhesive polymer is prepared from 2-ethylhexyl acrylate, 1-octyl acrylate or a mixture of 2-ethylhexyl acrylate and 1-octyl acrylate monomer (ii). The amount of monomer (ii) is from 30 to 70% by weight, preferably from 40 to 60% by weight, based on the sum of all monomers. The weight ratio of isobutyl acrylate (i) to monomer (ii) is from 1:1.5 to 1:5, preferably from 1:2 to 1:4.

The pressure sensitive adhesive polymer is prepared from styrene. The amount of styrene monomer (iii) is from 10 to 20% by weight, preferably from 12 to 19% by weight, based on the sum of all monomers. The weight ratio of isobutyl acrylate (i) to styrene (iii) is from 1:5 to 5:1, preferably from 1:2 to 2:1.

The pressure sensitive adhesive polymer is prepared from a monomer (iv) having at least one acid group. The amount of monomer (iv) having at least one acid group is 0.1 to 10 wt%, preferably 0.2 to 8 wt%, more preferably 0.5 to 5 wt%, based on the total of all monomers. The monomer (iv) is a monomer having at least one acid group (acid monomer), i.e., an ethylenically unsaturated acid or an ethylenically unsaturated acid anhydride, and can be polymerized by radical polymerization. Suitable acid monomers are, for example, ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulphonic acids and vinylphosphonic acids. Preferred for use as ethylenically unsaturated carboxylic acids are alpha, beta-monoethylenically unsaturated mono-and dicarboxylic acids comprising 3 to 6 carbon atoms in the molecule. Examples thereof are acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinyl acetic acid and vinyl lactic acid. Examples of suitable ethylenically unsaturated sulfonic acids include vinylsulfonic acid, styrenesulfonic acid, acrylamidomethylpropane sulfonic acid, sulfopropyl acrylate, and sulfopropyl methacrylate. Acrylic acid, methacrylic acid, itaconic acid and mixtures thereof are preferred, acrylic acid being particularly preferred.

The pressure sensitive adhesive polymer is optionally prepared from at least one monomer (v) which is an alkyl (meth) acrylate monomer different from monomers (i) and (ii) and has a glass transition temperature (so-called "soft" monomer) of less than-20 ℃ when polymerized as a homopolymer. The amount of soft (meth) acrylate monomer (v) is preferably from 0 to 30% by weight, preferably from 5 to 30% by weight or from 8 to 25% by weight, based on the sum of all monomers. The soft monomers (v) are preferably selected from acrylic esters, in particular from C ₂ -to C ₁₀ -alkyl acrylates, or from C ₃ -to C ₁₀ -alkyl acrylates or from C ₄ -to C ₈ -alkyl acrylates. Suitable examples include ethyl acrylate, n-propyl acrylate, n-butyl acrylate, n-hexyl acrylate, n-heptyl acrylate, 2-octyl acrylate and isooctyl acrylate, as well as mixtures of these monomers. Most preferred are n-butyl acrylate and 2-octyl acrylate.

The pressure sensitive adhesive polymer is optionally prepared from at least one monomer (vi) which is a functional monomer selected from the group consisting of hydroxy-functional (meth) acrylate monomers and ureido-substituted ethylenically unsaturated monomers. The functional monomer has at least one ethylenically unsaturated free radically polymerizable group and at least one functional group selected from the group consisting of hydroxyl groups and urea groups. The amount of functional monomer (vi) is preferably from 0 to 10% by weight, preferably from 0.1 to 10% by weight or from 0.6 to 7% by weight, based on the sum of all monomers. Suitable hydroxy-functional (meth) acrylate monomers are C ₁-C₁₀ -hydroxyalkyl (meth) acrylates having 1 to 10 carbon atoms in the hydroxyalkyl group. Most preferred is 2-hydroxy-ethyl acrylate. The preferred amount of hydroxy-functional (meth) acrylate monomer is from 0.5 to 5% by weight based on the total of all monomers.

Suitable ureido substituted ethylenically unsaturated monomers are monomers selected from the group consisting of (meth) acrylate monomers having substituents of the formula

Wherein X is NH or NR, and R is a C1 to C4 alkyl group, and the arrow at the N atom represents the point of attachment of the substituent to the (meth) acrylate monomer. Suitable ureido-substituted ethylenically unsaturated monomers are, for example, those of the formula

Wherein X is as defined above, R is hydrogen or methyl, and A is a divalent linking group, preferably a C1 to C10 alkyl group or a C2 to C4 alkyl group. Particular preference is given to using ureidoalkyl (meth) acrylates having from 1 to 10C atoms, preferably from 2 to 4C atoms, in the alkyl radical, most preferably ureidoethyl methacrylate (2-oxoimidazolidin-1-yl) ethyl methacrylate; also known as ureidomethacrylate or abbreviated UMA). The preferred amount of ureido substituted ethylenically unsaturated monomer is 0,05 to 2 wt%, more preferably 0.1 to 2 wt% or 0,1 to 1 wt%, based on the total of all monomers.

The pressure sensitive adhesive polymer is optionally prepared from at least one monomer (vii) which is an alkyl (meth) acrylate monomer different from monomers (i), (ii) and (v). The amount of monomer (vii) is preferably from 0 to 20% by weight or from 0.5 to 10% by weight, based on the sum of all monomers. The monomer (vii) is preferably selected from alkyl (meth) acrylates having 1 to 20 carbon atoms in the alkyl group. Most preferred are methyl acrylate and methyl methacrylate.

The pressure sensitive adhesive polymer is optionally prepared from at least one monomer (viii) different from monomers (i) to (vii). Monomer (viii) is a copolymerizable ethylenically unsaturated compound. The amount of monomer (viii) is preferably from 0 to 10 wt% or from 0 to 5 wt% based on the sum of all monomers. The monomers (viii) are selected, for example, from vinyl esters of carboxylic acids containing up to 20 carbon atoms, vinylaromatic compounds other than styrene and having up to 20 carbon atoms, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols containing from 1 to 10 carbon atoms, aliphatic hydrocarbons having from 2 to 8 carbon atoms and one or two double bonds, and (meth) acrylamides or mixtures of these monomers. Additional monomers also include phenoxyethyl ethylene glycol mono (meth) acrylate, glycidyl (meth) acrylate, aminoalkyl (meth) acrylate, such as 2-aminoethyl (meth) acrylate. The alkyl group preferably has 1 to 20 carbon atoms. Vinyl esters of carboxylic acids having 1 to 20 carbon atoms are, for example, vinyl acetate, vinyl laurate, vinyl stearate, vinyl propionate and vinyl versatate. Vinyl aromatic compounds include vinyl toluene, alpha-methyl styrene and para-methyl styrene, alpha-butyl styrene, 4-n-decyl styrene. Examples of nitriles are acrylonitrile and methacrylonitrile. Vinyl halides are ethylenically unsaturated compounds substituted with chlorine, fluorine or bromine, preferably vinyl chloride and vinylidene chloride. Examples of vinyl ethers include vinyl methyl ether or vinyl isobutyl ether. Vinyl ethers of alcohols containing 1 to 4 carbon atoms are preferred. Suitable hydrocarbons having 4 to 8 carbon atoms and two ethylenic double bonds are, for example, butadiene, isoprene and chloroprene.

Preferred pressure sensitive adhesive compositions comprise at least one pressure sensitive adhesive polymer formed by emulsion polymerization of:

(i) 10 to 20 wt% of isobutyl acrylate based on the sum of all monomers;

(ii) 40 to 60% by weight, based on the sum of all monomers, of 2-ethylhexyl acrylate, 1-octyl acrylate, or a mixture of 2-ethylhexyl acrylate and 1-octyl acrylate;

(iii) 10 to 20% by weight of styrene, based on the sum of all monomers

(Iv) 0.5 to 5% by weight, based on the sum of all monomers, of at least one monomer having at least one acid group selected from acrylic acid, methacrylic acid and itaconic acid;

(v) 5 to 30 wt%, based on the sum of all monomers, of at least one monomer selected from n-butyl acrylate and 2-octyl acrylate;

(vi) 0.5 to 5% by weight, based on the sum of all monomers, of 2-hydroxyethyl acrylate and 0.1 to 2% by weight, based on the sum of all monomers, of 2- (2-oxoimidazolidin-1-yl) ethyl methacrylate;

(vii) 0 to 20 wt%, based on the sum of all monomers, of at least one alkyl (meth) acrylate monomer which is different from monomers (i), (ii) and (v) and is selected from alkyl (meth) acrylates having 1 to 20 carbon atoms in the alkyl group;

(viii) From 0 to 10% by weight of other monomers than monomers (i) to (vii) selected from vinyl esters of carboxylic acids containing up to 20 carbon atoms, vinylaromatic compounds containing up to 20 carbon atoms other than styrene, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols containing from 1 to 10 carbon atoms, aliphatic hydrocarbons containing from 2 to 8 carbon atoms and one or two double bonds, and (meth) acrylamides.

Preferably and for sustainability reasons, bio-based materials are used to produce pressure sensitive adhesive polymers, which are preferably made partially or completely from part or all of the bio-based monomers. The terms "biobased" and "biological origin" are used synonymously herein. Bio-based materials are materials made from renewable sources and have little impact on the environment. They do not require all of the refining steps required for petroleum-based products, which are very expensive in terms of energy. The production of CO ₂ is reduced, thereby reducing the impact of biobased materials on global warming. The term "biobased" means that the material is of biological origin and is derived from biological materials and renewable resources. A material of biological origin (biomaterial) is an organic material in which carbon comes from CO ₂ that was recently (on the human scale) immobilized from the atmosphere by photosynthesis. Biological materials (100% natural source carbon) have a ¹⁴C/¹² C isotope ratio of greater than 10 ^-12, typically about 1.2 x 10 ^-12, whereas fossil materials have a ratio of 0. In fact, according to a timescale of up to several decades, the isotope ¹⁴ C is formed in the atmosphere and subsequently integrated via photosynthesis. ¹⁴ The half-life of C was 5730 years. Thus, the material from photosynthesis, i.e. typically a plant, must have the maximum content of isotope ¹⁴ C. The determination of the content of biological material or biological carbon can be carried out according to the standards ASTM D6866-12, method B (ASTM D6866-06) and ASTM D7026 (ASTM D7026-04). Preferably, the pressure sensitive adhesive polymer has a biocarbon content of at least 5 mole%, in particular at least 10 mole% or at least 15 mole% or more, for example 20 mole%, 30 mole% or 40 mole% or more, based on the total amount of carbon atoms in the pressure sensitive adhesive polymer. A polymer made partially from all or part of a bio-based monomer is a polymer in which not all of the monomers used for polymerization are part or all of the bio-based monomer. Part of the biobased monomers are monomers in which not all C atoms are biobased, for example (meth) acrylates in which only the acid moiety or only the alcohol moiety is biobased.

At least a portion of the material used to prepare the pressure sensitive adhesive polymer may also be a renewable raw material, according to a mass balancing method. Thus, in addition to fossil feedstocks, renewable feedstocks such as bio-naphtha (as described for example in EP 2 290 045 A1 or EP 2 290 034 A1), bio-methane or palm oil also enter chemical production systems such as steam crackers. Renewable raw materials are converted along a chemical value chain to products such as acrylic acid, isobutanol, isobutyl acrylate, styrene or 2-ethylhexyl acrylate. The content of renewable materials of these products is defined by the mass balance method and can be assigned to these products, preferably by certification according to REDcert ² certification.

Suitable biobased materials for producing the pressure sensitive adhesive polymer are, for example, (meth) acrylates, wherein the (meth) acrylic component or the alcohol component or both are biobased. Various processes for the production of bio-based acrylic acid from renewable plant materials are mentioned in EP 2626977 A1. Suitable biobased alcohols are, for example, biobased isobutanol, biobased n-butanol, biobased isoamyl alcohol (3-methylbutan-1-ol), biobased 2-octanol, biobased 1-octanol and biobased n-heptanol. Preferred partial biobased monomers are esters of (meth) -acrylic acid and biobased alcohols, preferably biobased isobutanol, biobased n-butanol, biobased isoamyl alcohol (3-methylbutan-1-ol), biobased 2-octanol, biobased 1-octanol and biobased n-heptanol. Preferred complete biobased monomers are esters of biobased acrylic acid and biobased alcohols as mentioned above. Preferably, at least 50 wt%, more preferably 100 wt% of the isobutyl acrylate monomer (i) is made from biobased isobutanol instead of biobased or biobased acrylic acid, i.e. preferably, at least the carbon atoms of the isobutyl group of isobutyl acrylate (i) are of biological origin. Preferably, monomer (v) comprises 2-octyl acrylate, wherein at least the carbon atoms of the 2-octyl group of the 2-octyl acrylate are of biological origin. Preferably, monomer (ii) comprises 1-octyl acrylate, wherein at least the carbon atoms of the 1-octyl group of the 1-octyl acrylate are of biological origin.

The glass transition temperature (Tg) of the pressure sensitive adhesive polymer is less than-20deg.C, preferably-50deg.C to-25deg.C. By targeted variation of the monomer type and amount, one skilled in the art can prepare aqueous polymer compositions according to the present invention, the polymers of which have glass transition temperatures within the desired range. Orientation may be performed using the Fox equation. The glass transition temperature of the copolymers according to Fox (T.G.Fox, bull.Am.Phys.Soc.1956[ Ser.II ]1, page 123 and according to Ullmann's Encyclo-pedia of Industrial Chemistry, volume 19, page 18, 4 th edition, VERLAG CHEMIE, weinheim, 1980) gives good approximations by the following formula:

1/T_g＝x¹/T_g ¹+x²/T_g ²+....xⁿ/T_g ⁿ,

where x ¹,x²,...xⁿ is the mass fraction of monomer 1,2,..n, and T _g ¹,T_g ²,...T_g ⁿ is the glass transition temperature in degrees kelvin of a polymer composed of only one of monomers 1,2,..n at a time. The T _g values of homopolymers of most monomers are known and are listed, for example, in Ullmann's En-cyclopedia of Industrial Chemistry, vol.5, vol.A 21, page 169, VCH WEINHEIM, 1992; other sources of glass transition temperatures for homopolymers are, for example, J.Brandrup, E.H.Immergut, polymer Handbook, 1 st edition, J.Wiley, new York 1966, 2 nd edition J.Wiley, new York 1975 and 3 rd edition J.Wiley, new York 1989.

The polymer dispersions used according to the invention can be obtained by free-radical emulsion polymerization of ethylenically unsaturated free-radically polymerizable compounds (monomers). Emulsion polymerization involves polymerizing ethylenically unsaturated compounds (monomers) in water, using ionic and/or nonionic emulsifiers and/or protective colloids or stabilizers as surface-active compounds to stabilize the monomer droplets and subsequently the polymer particles formed from the monomers. The amount of the surface-active substance used is usually 0.1 to 10 parts by weight, preferably 0.2 to 5 parts by weight, based on 100 parts by weight of the monomer to be polymerized.

A detailed description of suitable protective colloids can be found in Houben-Weyl, methoden der organ-ISCHEN CHEMIE [ Methods of Organic Chemistry ], volume XIV/1, makromoleku-lare Stoffe [ Macromolecular Materials ], georg-Thieme-Verlag, stuttgart,1961, pages 411 to 420. Useful emulsifiers include anionic emulsifiers, cationic emulsifiers and nonionic emulsifiers. As surface-active substances, emulsifiers having a molecular weight generally below 2000g/mol compared with the protective colloid are preferably used. When mixtures of surface-active substances are used, the components must of course be compatible with one another; in case of doubt, this can be checked based on several preliminary experiments. It is preferable to use anionic emulsifiers and nonionic emulsifiers as surface-active substances. Common accompanying emulsifiers are, for example, ethoxylated fatty alcohols (EO degree: 3 to 50, alkyl radical: C ₈ to C ₃₆), ethoxylated monoalkylphenols, dialkylphenols and trialkylphenols (EO degree: 3 to 50, alkyl radical: C ₄ to C ₉), alkali metal and ammonium salts of dialkyl esters of sulfosuccinic acid, and alkali metal and ammonium salts of alkyl sulfates (alkyl radical: C ₈ to C ₁₂), ethoxylated alkanols (EO degree: 4 to 30, alkyl radical: C ₁₂ to C ₁₈), ethoxylated alkylphenols (EO degree: 3 to 50, alkyl radical: C ₄ to C ₉), alkali metal and ammonium salts of alkylsulfonic acids (alkyl radical: C ₁₂ to C ₁₈), and alkali metal and ammonium salts of alkylarylsulfonic acids (alkyl radical: C ₉ to C ₁₈).

Other suitable emulsifiers are compounds of the general formula:

Wherein R5 and R6 are hydrogen or C4-to C14-alkyl and are not simultaneously hydrogen, and X and Y may be alkali metal ions and/or ammonium ions. R5, R6 are preferably straight-chain or branched alkyl groups having 6 to 18 carbon atoms or hydrogen, in particular having 6, 12 and 16 carbon atoms, wherein R5 and R6 are not simultaneously hydrogen. X and Y are preferably sodium ions, potassium ions or ammonium ions, sodium ions being particularly preferred. Compounds in which X and Y are sodium, R5 is a branched alkyl radical having 12 carbon atoms and R6 is hydrogen or R5 are particularly advantageous. Generally, an industrial mixture is used which comprises the monoalkylated product in a proportion of 50 to 90% by weight. Commercially available products of suitable emulsifiers are, for example 2A1、NP 50、OC 50、Emulgator 825、Emulgator 825S、OG、NSO、904S、I-RA、E 3065、FES 77、AT 18、VSL、NPS25、TSP-16N (tristyrylphenol ether sulfate, ammonium salt) and2535 (Alkylphenol ethoxylates). The invention is preferably an ionic emulsifier or protective colloid. Particular preference is given to ionic emulsifiers, in particular salts and acids, such as carboxylic acids, sulphonic acids and sulphates, sulphonates or carboxylates. Mixtures of ionic and nonionic emulsifiers can also be used in particular.

The polymerization can also be carried out in the presence of protective colloids. Protective colloids are polymeric compounds that bind large amounts of water upon solvation and are capable of stabilizing dispersions of water insoluble polymers. In contrast to emulsifiers, they generally do not reduce the interfacial surface tension between the polymer particles and water. The number average molecular weight of the protective colloid is, for example, above 1000g/mol.

Emulsion polymerization may be initiated using a water-soluble initiator. The water-soluble initiator is, for example, an ammonium salt and an alkali metal salt of peroxodisulfuric acid (for example, sodium peroxodisulfate), hydrogen peroxide or an organic peroxide, for example, tert-butyl hydroperoxide. So-called reduction-oxidation (redox) initiator systems are also suitable as initiators. The redox initiator system consists of at least one inorganic or organic reducing agent and an inorganic or organic oxidizing agent. The oxidizing component is, for example, the emulsion polymerization initiator already mentioned above. The reducing agent component is, for example, an alkali metal salt of sulfurous acid, for example sodium sulfite, sodium bisulfite, an alkali metal salt of disulfonic acid, for example sodium bisulfite, a bisulfite addition compound of fatty aldehydes and ketones, for example acetone bisulfite, or a reducing agent, such as hydroxymethanesulfinic acid and salts thereof, or ascorbic acid. The redox initiator system may be used with soluble metal compounds whose metal component may occur in a variety of valence states. Typical redox initiator systems are, for example, ascorbic acid/iron (II) sulfate/sodium peroxodisulfate, tert-butyl hydroperoxide/sodium metabisulfite, tert-butyl hydroperoxide/sodium hydroxymethanesulfinate. The individual components, for example the reducing agent component, may also be mixtures, for example mixtures of sodium salts of hydroxymethanesulfinic acid and sodium metabisulfite.

The initiators listed are generally used in the form of aqueous solutions, the lower concentration being determined by the amount of water which is acceptable in the dispersion and the upper concentration being determined by the solubility of the particular compound in water. The concentration of the initiator is generally from 0.1% to 30% by weight, preferably from 0.5% to 20% by weight, more preferably from 1.0% to 10% by weight, based on the monomers to be polymerized. Two or more different initiators may also be used in the emulsion polymerization.

Chain transfer agents may be used for the polymerization. Preferably no chain transfer agent is used. When chain transfer agents are used, they are preferably used in an amount of at least 0.01 parts by weight, for example, from 0.01 to 5 parts by weight, or from 0.01 to 3 parts by weight, preferably from 0.01 to 0.75 parts by weight, of chain transfer agent per 100 parts by weight of monomer to be polymerized. This allows the control/reduction of the molar mass of the emulsion polymer by chain termination reactions. In this procedure, chain transfer agents are bonded to the polymer, typically to the chain ends. The addition may be carried out continuously or in stages during the polymerization. Suitable chain transfer agents are, for example, organic compounds containing sulfur in bonded form (e.g. compounds having a thiol group), aliphatic and/or araliphatic halogen compounds, aliphatic and/or aromatic aldehydes, unsaturated fatty acids (e.g. oleic acid), dienes having non-conjugated double bonds (e.g. divinyl methane, terpineol or vinylcyclohexene), hydrocarbons having readily extractable hydrogen atoms (e.g. toluene), organic acids or salts thereof (e.g. formic acid, sodium formate, ammonium formate), alcohols (e.g. isopropanol) and phosphorus compounds (e.g. sodium hypophosphite). Alternatively, compatible mixtures of the above chain transfer agents may be used. Chain transfer agents are generally low molecular weight compounds having a molecular weight of less than 2000g/mol, in particular less than 1000 g/mol. Advantageously, a small or full amount of chain transfer agent is supplied to the aqueous reaction medium prior to initiating the free radical polymerization. In addition, small or total amounts of the radical chain transfer compound may advantageously be supplied to the aqueous reaction medium together with the monomers during the polymerization. Preferred organic compounds comprising sulfur in bonded form are in particular tert-butylmercaptan, ethyl thioglycolate, mercaptoethanol, mercaptopropyl trimethoxysilane, tert-dodecyl mercaptan, thiodiglycol, ethylmercaptoethanol, di-n-butylsulfide, 2-isopropylsulfide, di-n-octylsulfide, diphenylsulfide, diisopropyldisulfide, 2-mercaptoethanol, 1, 3-mercaptopropanol, 3-mercaptopropane-1, 2-diol, 1, 4-mercaptobutanol, mercaptoacetic acid, 3-mercaptopropionic acid, mercaptosuccinic acid, thioacetic acid and thiourea. Particularly preferred thio compounds are tert-butylmercaptan, ethyl thioglycolate, mercaptoethanol, mercaptopropyl trimethoxysilane, 2-ethylhexyl thioglycolate (EHTG), isooctyl 3-mercaptopropionate (IOMPA) or tert-dodecyl mercaptan (tDMK).

The emulsion polymerization is generally carried out at 30℃to 130℃and preferably at 50℃to 95 ℃. The polymerization medium may consist of water alone or a mixture of water and a liquid miscible therewith, such as methanol. Preferably, only water is used.

Emulsion polymerization may be carried out as a batch process or as a feed process, including staged and gradient modes of operation of the feed process. Preference is given to a feed process in which a portion of the polymerization batch is initially charged, heated to the polymerization temperature and polymerization is started before the remainder of the polymerization batch is supplied, usually via a plurality of spatially separated feeds, in which one or more comprise the monomers in pure or emulsified form, and the polymerization is maintained in the polymerization zone continuously, in stages or superimposed in concentration gradients.

For example, polymer seeds may also be charged first in the polymerization to more effectively adjust particle size. The polymerization is preferably carried out under seed control, i.e.in the presence of polymer seeds (seed latex). The seed latex is an aqueous dispersion of polymer particles having an average particle diameter preferably from 20nm to 40 nm. The seed latex is preferably used in an amount of 0.01 to 0.5 parts by weight, particularly preferably 0.03 to 0.3 parts by weight, based on 100 parts by weight of the monomer. Latex based on polystyrene or on polymethyl methacrylate, for example, is suitable. One preferred seed latex is polystyrene seed.

The manner in which the initiator is added to the polymerization vessel during the free radical aqueous emulsion polymerization is known to those of ordinary skill in the art. All of which may be added first to the polymerization vessel or used continuously or in stages at their consumption rate during the free radical aqueous emulsion polymerization. Depending on the chemical nature of the initiator system and the polymerization temperature. Preferably, a portion is first charged and the remainder is supplied to the polymerization zone at its consumption rate. In order to remove residual monomers, the initiator is generally also added after the actual emulsion polymerization has ended, i.e. after a monomer conversion of at least 95%. In the feed process, the individual components can be added to the reactor from above, from the side or from below through the bottom of the reactor.

Emulsion polymerization generally provides aqueous polymer dispersions having solids contents of from 15% to 75% by weight, preferably from 40% to 75% by weight, or from 40% to 60% by weight, particularly preferably not less than 50% by weight. In order to obtain a high reactor space/time yield, dispersions with a very high solids content are preferred. In order to be able to achieve a solids content of > 60% by weight, bimodal or multimodal particle sizes should be established, otherwise the viscosity becomes too high and the dispersion is difficult to handle. For example, the next generation of particles can be produced by adding seeds (EP 81083), by adding an excess amount of emulsifier dose or by adding a miniemulsion. Another advantage associated with low viscosity at high solids content is improved coating properties at high solids content. The generation of new particle generation may be performed at any point in time. It is guided by the search for a particle size distribution of low viscosity. The polymers thus produced are preferably used in the form of their aqueous dispersions. The size distribution of the dispersion particles may be unimodal, bimodal or multimodal.

Neutralization of the acid groups of the polymer is preferably carried out by adding a neutralizing agent during or after the polymerization, wherein the acid groups are completely or partially neutralized by adding a base. The neutralizing agent may be added, for example, in a separate feed at the same time as the feed of the monomer mixture. After all monomers are fed, the amount of neutralizing agent required to neutralize at least 10%, preferably from 10% to 100% or from 25% to 90% of the acid equivalents is preferably present in the polymerization vessel. A particularly preferred neutralizing agent is ammonia. The pH of the polymer dispersion is preferably adjusted to a pH of greater than 4.5, more particularly to a pH between 5 and 8.

The pressure sensitive adhesive composition preferably comprises at least one tackifier (tackifying resin). The amount of tackifier is preferably 5 to 40 parts by weight based on 100 parts by weight of the adhesive polymer. Tackifiers are polymeric or oligomeric additives for adhesive polymers or generally for elastomers which increase their self-adhesion (tack, intrinsic tack, self-adhesion) so that they adhere firmly to surfaces after a short, slight contact pressure. Tackifiers include, for example, natural resins such as rosin resins and their derivatives or terpene resins formed by disproportionation or isomerization, polymerization, dimerization or hydrogenation. These may be in their salt form (with, for example, monovalent or multivalent counterions (cations)), or preferably in their esterified form. The alcohol used for esterification may be a monohydric or polyhydric alcohol. Examples include methanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,2, 3-glycerol, pentaerythritol. Other useful tackifying resins are hydrocarbon resins such as coumarone-indene resins, polyterpene resins, hydrocarbon resins based on unsaturated CH compounds such as butadiene, pentene, methylbutene, isoprene, piperylene, divinyl methane, pentadiene, cyclopentene, cyclopentadiene, cyclohexadiene, styrene, alpha-methylstyrene, vinyltoluene. Tackifiers are known, for example, from ADHESIVE AGE, month 7 of 1987, pages 19 to 23 or from Polym. Mater. Sci. Eng.61 (1989), pages 588 to 592.

Polyacrylates with low molar weights can also be used as tackifiers. These polyacrylates preferably have a weight average molecular weight M _w of less than 50, in particular less than 30, 000. The tackifying polyacrylate preferably consists of at least 60% by weight, in particular at least 80% by weight, of C ₁-C₈ alkyl (meth) acrylate. Suitable tackifiers include, for example, low molecular weight polymers and oligomers described in WO 2013/117428 having a weight average molecular weight of less than 50 000 and a glass transition temperature of from no less than-40 ℃ to no more than 0 ℃, preferably from no less than-35 ℃ to no more than 0 ℃, can be prepared by emulsion polymerization in the presence of at least one molecular weight regulator, and can be prepared from a monomer mixture comprising at least 40 weight percent of at least one C1-to C20-alkyl (meth) acrylate.

Preferred tackifiers are selected from the group consisting of natural resins, hydrocarbon resins, tackifying polyacrylates, or mixtures thereof. Particularly preferred tackifiers include natural or chemically modified rosin resins. Rosin resins are mainly composed of rosin acids or rosin acid derivatives or hydrogenated derivatives thereof. The adhesion promoters can be added to the polymer dispersions in a simple manner. The adhesion promoter itself is preferably in the form of an aqueous dispersion. The amount of tackifier is preferably 5 to 100 parts by weight, particularly preferably 5 to 50 parts by weight, 5 to 40 parts by weight or 10 to 40 parts by weight based on 100 parts by weight of polymer (solid/solid).

The pressure sensitive adhesive composition may contain other additives such as fillers, dyes, leveling agents, flow control aids, thickeners (preferably associative thickeners), defoamers, crosslinkers, plasticizers, pigments, UV protectants or wetting agents. For better wetting of the surface, the pressure sensitive adhesive may in particular comprise wetting aids (wetting agents), such as fatty alcohol ethoxylates, alkylphenol ethoxylates, sulfosuccinates, nonylphenol ethoxylates, polyoxyethylene/polyoxypropylene copolymers or sodium dodecyl sulfonate. The amount of the additive is preferably 0.05 to 10 parts by weight, 0.1 to 5 parts by weight, particularly 0.1 to 3 parts by weight per 100 parts by weight of the adhesive polymer (solid).

The tackified pressure sensitive adhesive composition preferably comprises (on a solids basis)

60 To 95 parts by weight of a pressure sensitive adhesive polymer,

5 To 40 parts by weight of a tackifier, and

0 To 10 parts by weight of other additives.

The non-tackified pressure sensitive adhesive composition preferably comprises (on a solids basis)

90 To 100 parts by weight of a pressure sensitive adhesive polymer; and

0 To 10 parts by weight of other additives.

The pressure sensitive adhesive composition is preferably a one-component pressure sensitive adhesive. One-component adhesives are adhesives that do not add an external crosslinking agent (e.g., an isocyanate crosslinking agent) immediately prior to use.

After drying, the pressure sensitive adhesive composition forms a film having a tack, measured as loop tack relative to the steel surface (adhesive applied to a 36 μm thick polyethylene terephthalate film at an application rate of 60g/m ², measured at a tear rate of 300mm/min relative to the steel at 23 ℃ and 50% relative humidity), preferably not less than 10N/25mm, see examples for details.

After drying, the pressure sensitive adhesive composition forms a film having a peel strength of preferably greater than 2N/25mm relative to the polyethylene surface (measured at an application rate of 60g/m ², applied to a 36 μm thick polyethylene terephthalate film, measured at a tear rate of 300mm/min at 23℃and 50% relative humidity relative to the polyethylene, see examples for details).

After drying, the pressure sensitive adhesive composition forms a film having a shear resistance at 70 ℃ of preferably at least 200 minutes, as measured in the examples.

The pressure sensitive adhesive composition can be used to produce self-adhesive articles. The article is at least partially coated with a pressure sensitive adhesive. The self-adhesive article may be a self-adhesive label, a self-adhesive tape or a self-adhesive film, including graphic films and protective films. Suitable carrier materials are, for example, paper, plastic films and metal foils. The self-adhesive tape of the present invention may be a tape coated with the above-mentioned substances on one or both sides. The self-adhesive labels of the present invention may be labels made from paper or thermoplastic film. Particularly preferred are tapes made from thermoplastic films. Suitable thermoplastic films include, for example, films made from polyolefins (e.g., polyethylene or polypropylene), polyolefin copolymers, films made from polyesters (e.g., polyethylene terephthalate), polyvinyl chloride, or polyacetates. The surface of the thermoplastic polymer film is preferably corona treated. Foam carriers are also possible. The label is coated with adhesive on one side. Preferred substrates for self-adhesive articles are paper and polymeric films.

The self-adhesive article is at least partially coated on at least one surface with a pressure sensitive adhesive according to the invention. The adhesive may be applied to the article by conventional methods such as roll coating, knife coating or coating. The amount applied is preferably from 0.1g to 300g, more preferably from 2g to 150g solids/m ². The application is typically followed by a drying step to remove water. The water may be removed by drying, for example, at 50 ℃ to 150 ℃. The coated substrates thus obtained are used, for example, as self-adhesive articles, such as adhesive labels, adhesive tapes or adhesive films. For this purpose, the carrier may be cut into tapes, labels or films before or after the application of the adhesive. The side of the substrate coated with the pressure sensitive adhesive may be covered with a release paper, for example siliconized paper, for later use.

The substrate to which the self-adhesive article may advantageously be applied may be, for example, metal, wood, glass, paper or plastic. The self-adhesive articles are particularly suitable for adhering to packaging surfaces, cartons, plastic packaging, books, windows, moisture barriers, motor vehicle bodies, tires or body parts.

The invention also relates to the use of the above-described pressure-sensitive adhesive composition for producing self-adhesive labels, self-adhesive tapes or self-adhesive films. The invention also relates to self-adhesive articles coated with the above pressure-sensitive adhesive composition.

The invention also relates to a method for producing a self-adhesive article.

The method comprises the following steps:

(1) Providing a pressure sensitive adhesive composition as described above, and

(2) At least a portion of the surface of the substrate is coated with a pressure sensitive adhesive composition.

Examples

The following input materials and abbreviations are used:

EHA 2-ethylhexyl acrylate

BA n-butyl acrylate

MMA methyl methacrylate

S styrene

IBA acrylic acid isobutyl ester

Bio-IBA isobutyl acrylate with 57% biochar from biobased isobutanol (BCH Bruhl-CHEMIKALIEN HANDEL GmbH)

AA acrylic acid

HEA acrylic acid 2-hydroxyethyl ester

HPA acrylic acid 2-hydroxypropyl ester

UMA ureido methacrylate; 2- (2-oxoimidazolidin-1-yl) ethyl methacrylate

2-OA acrylic acid 2-octyl ester

Pphm parts by weight per 100 parts by weight of monomer (parts per hundred parts of monomer)

SC standard conditions, 50% relative humidity, 1 bar, 23 DEG C

Tg glass transition temperature

HDPE high density polyethylene

Anionic surfactant from Stepan, TSP-16N

2535. Nonionic surfactants from Solvay

C sodium hydroxymethanesulfinate from Brueggemann

OT-75 surfactant from Solvay/>

I-SC wetting agent; anionic surfactants

933E rosin ester based tackifier dispersions

Preparation of pressure sensitive adhesive polymer dispersions

According to Table 1, 233.3g of water, 57.0g of 20% by weight are mixedFeed 1 was prepared from aqueous TSP-16N, 6.1g Abex 2535, 760g monomer. Feed 2 was prepared by dissolving 2.3g of sodium persulfate in 54.8g of water. A reactor equipped with stirrer, temperature controller, nitrogen inlet and various injection possibilities was charged with 308.4g of water and 2.3g of 20 wt%Aqueous TSP-16N solution. The reaction mixture was purged with nitrogen and heated to 78 ℃. At 78 ℃, 31g of feed 1 was added to the reactor. 1.5g of sodium persulfate dissolved in 13.7g of water was then added to the reactor. The nitrogen purge was turned off. 15 minutes after the exothermic peak was reached, the remaining feed 1 and feed 2 were fed into the reactor over 3.5 hours and the reactor temperature was maintained at 85 ℃. The reaction mixture was then stirred at 87 ℃ for 45 minutes. To the reaction mixture was added 2.7g of t-butyl hydroperoxide in 20g of water (feed 3) and 1.2g of rongalite C in 16g of water over 45 minutes. After a further 15 minutes, the reactor was cooled to room temperature and the pH was adjusted to 6.0-8.0 with 25% ammonia solution. Then 7.6g was addedOT-75 and mixed for 1 hour.

Table 1: monomer composition of the pressure sensitive adhesive polymer dispersion;

The amount is in parts by weight

	E1	E2	E3	E4	C1	C2
							EHA	50	50	50	50	50	50
BA	13.51	13.51	6.75	-	28.5	25.5
							IBA	15.74	-	15.74	15.74	-	-
bio-IBA	-	15.74	-	-	-	-
							MMA	-	-	-	-	3.75	3.75
S	17	17	17	17	14	17
							AA	1.5	1.5	1.5	1.5	1.5	1.5
HEA	2	2	2	2	2	2
							UMA	0.25	0.25	0.25	0.25	0.25	0.25
2-OA	-	-	6.75	13.51	-	-
							Tg	-31℃	-32℃	-31℃	-31℃	-35℃	-32℃
Particle size	181nm	174nm	177nm	184nm	167nm	170nm

The examples labeled c.

In comparison with the comparative examples, BA and MMA were replaced by a sufficient amount of IBA or IBA and 2-OA, resulting in polymers according to the invention having almost the same glass transition temperature.

Preparation of pressure-sensitive adhesive compositions E5 and E6 for self-adhesive labels

Pressure sensitive adhesive compositions for self-adhesive labels were prepared by mixing the pressure sensitive adhesive polymer dispersion with additives and tackifiers according to table 2.

Table 2: pressure sensitive adhesive composition

¹⁾ Weight ratio of adhesive polymer to tackifier on a solids basis = 80:20

Performance testing

To test the adhesive properties used in the tape, the pressure sensitive adhesive of the examples was applied to a carrier at an application rate of about 60g/m ² On RN 36 (biaxially oriented film made of polyethylene terephthalate, 36 μm thick) and dried at 90℃for 5 minutes.

To test the adhesive properties used in paper labels, the pressure sensitive adhesives of examples E5 and E6 were coated onto siliconized release paper at an application rate of about 18g/m ² and dried. The dried adhesive polymer film is then transferred to a label paper facestock.

Unless otherwise indicated, the adhesive coated substrates were stored under standard conditions for 24 hours before the adhesive properties were measured under standard conditions (SC: 23 ℃,50% relative humidity).

Loop tack (quick adhesion, corresponding to FINAT test method FTM 9)

Measuring loop tack (also known as surface tack or quick stick) involves measuring the force required to remove adhesive applied to a carrier material from a substrate by pressureless adhesive bonding to the substrate at a defined tear speed at 23 ℃ and 50% relative humidity. The test substrate was stainless steel, HDPE or glass. Test strips 25mm wide and 250mm long were cut from the adhesive coated carrier and stored at SC for at least 16 hours. The test strip was folded to about 1cm long at both ends with the adhesive side facing inward. The adhesive strip was used to form a loop with the adhesive side facing outward and the two ends were brought together and clamped into the upper jaw of a tensile testing machine. The test substrate holder is clamped into the lower jaw. The adhesive side of the test strip was adhered to the substrate without additional pressure by moving the adhesive strip loop down through the tensile tester at a speed of 300 mm/min. The tensile tester was stopped and immediately moved upwards again when the bottom edge of the upper jaw was 40mm above the substrate. The test results are reported in N/25mm width. The maximum value (Fmax) on the display is read as a measure of the surface tackiness. The average of the two individual results was taken.

Stripping test (corresponding FINAT test method FTM 1)

Peel strength is a measure of adhesion measured at 23 ℃ and 50% relative humidity. In the determination of the peel strength, a test strip 25mm wide was in each case bonded to a test specimen made of HDPE, stainless steel or glass and rolled twice with a 2kg roller (diameter=85 mm, shore hardness=80A). One end is then clamped in the upper jaw of a tensile strain tester. The adhesive strip was removed from the test surface at an angle of 180 ° at 300mm/min, i.e. the adhesive strip was bent and removed parallel to the test sample, and the force required for this was measured. The peel strength measurement was taken as the average of five measurements, as measured in N/25mm force. Peel strength was measured 24 hours after bonding. After this time, the adhesive strength has developed sufficiently. The average of the two individual results was taken.

Shear strength (corresponding to FINAT test method FTM 8)

Shear strength is a measure of cohesion measured at SC or 70 ℃. The carrier coated with the pressure sensitive adhesive was cut into test strips 25mm or 12.5mm wide. To determine the shear strength, the test strips were bonded to stainless steel (tape) or glass (paper label) with a bonding area of 25×25mm (measured with tape at 70 ℃, or with paper label at SC), or 12.5×12.5mm (measured with tape at SC), and rolled twice with a 2kg roller (diameter=85 mm, shore hardness=80A), stored for 10 minutes or 24 hours at SC (measured with paper label or tape, respectively), and then subjected to a suspension stress of 1kg weight (measured with tape at SC, or with paper label at SC), or 2kg weight (measured with tape at 70 ℃). The measure of shear strength is the time (in minutes) required for the weight to fall off; in each case an average of 3 measurements was taken.

The results of the performance tests are shown in the table below.

Table 3: performance results tape

The examples show that examples with polymers comprising isobutyl acrylate units lead to high shear strength and at the same time sufficiently high peel strength (high cohesion and at the same time sufficiently high adhesion).

Table 4: performance results adhesive labels

Claims (13)

1. A pressure sensitive adhesive composition in the form of an aqueous polymer dispersion comprising at least one dispersed pressure sensitive adhesive polymer formed by emulsion polymerization of:

(i) 10 to 20 wt% of isobutyl acrylate based on the sum of all monomers;

(ii) 30 to 70 wt%, based on the sum of all monomers, of 2-ethylhexyl acrylate, 1-octyl acrylate, or a mixture of 2-ethylhexyl acrylate and 1-octyl acrylate;

(iii) 10 to 20 wt% styrene based on the sum of all monomers;

(iv) 0.1 to 10% by weight, based on the sum of all monomers, of at least one monomer having at least one acid group;

(v) Optionally at least one alkyl (meth) acrylate monomer which is different from monomers (i) and (ii) and which has a glass transition temperature of less than-20 ℃ when polymerized to a homopolymer;

(vi) Optionally at least one functional monomer selected from the group consisting of hydroxy-functional (meth) acrylate monomers and ureido-substituted ethylenically unsaturated monomers;

(vii) Optionally at least one alkyl (meth) acrylate monomer different from (i), (ii) and (v); (viii) Optionally further monomers other than (i) to (vii),

Wherein the weight ratio of isobutyl acrylate (i) to 2-ethylhexyl acrylate, 1-octyl acrylate, or a mixture (ii) of said 2-ethylhexyl acrylate and 1-octyl acrylate is from 1:1.5 to 1:5;

Wherein the weight ratio of isobutyl acrylate (i) to styrene (iii) is from 1:5 to 5:1, preferably from 1:2 to 2:1;

Wherein the pressure sensitive adhesive polymer has a glass transition temperature below-20 ℃, preferably from-45 ℃ to-25 ℃, as determined by differential scanning calorimetry, as the midpoint temperature when evaluating the second heating curve at a heating rate of 20K/min.

2. The pressure sensitive adhesive composition of claim 1, wherein the pressure sensitive adhesive composition comprises at least one tackifier in an amount of 5 to 40 parts by weight based on 100 parts by weight of the polymer.

3. The pressure sensitive adhesive composition of claim 1 or 2, wherein the tackifier is selected from the group consisting of natural resins, hydrocarbon resins, tackifying polyacrylates, or mixtures thereof.

4. A pressure sensitive adhesive composition according to any one of claims 1 to 3, wherein monomer (v) is used in an amount of 5 to 30 wt% based on the sum of all monomers.

5. The pressure-sensitive adhesive composition according to any one of claims 1 to 4, wherein monomer (v) is selected from n-butyl acrylate and 2-octyl acrylate.

6. The pressure sensitive adhesive composition according to the preceding claim, wherein at least the carbon atoms of the isobutyl group of isobutyl acrylate (i) and/or the carbon atoms of the 2-octyl group of 2-octyl acrylate and/or the carbon atoms of the 1-octyl group of 1-octyl acrylate are of biological origin.

7. The pressure sensitive adhesive composition according to any one of claims 1 to 6, wherein monomer (vi) comprises 2-hydroxyethyl acrylate in an amount of 0.5 to 5 wt% based on the sum of all monomers.

8. The pressure sensitive adhesive composition according to any one of claims 1 to 7, wherein monomer (vi) comprises 2- (2-oxoimidazolidin-1-yl) ethyl methacrylate in an amount of 0.1 to 2 wt.%, based on the sum of all monomers.

9. The pressure-sensitive adhesive composition according to any one of claims 1 to 8, wherein monomer (vii) is used in an amount of 0 to 20% by weight based on the sum of all monomers, and monomer (vii) is at least one monomer which is different from monomers (i), (ii) and (v) and is selected from alkyl (meth) acrylates having 1 to 20 carbon atoms in the alkyl group, preferably methyl acrylate or methyl methacrylate.

10. The pressure sensitive adhesive composition according to any one of claims 1 to 9, comprising at least one pressure sensitive adhesive polymer formed by emulsion polymerization of:

(i) 10 to 20 wt% of isobutyl acrylate based on the sum of all monomers;

(ii) 40 to 60% by weight, based on the sum of all monomers, of 2-ethylhexyl acrylate, 1-octyl acrylate, or a mixture of 2-ethylhexyl acrylate and 1-octyl acrylate;

(iii) 10 to 20 wt% styrene based on the sum of all monomers;

(iv) 0.5 to 5% by weight, based on the sum of all monomers, of at least one monomer having at least one acid group selected from acrylic acid, methacrylic acid and itaconic acid;

(v) 5 to 30 wt%, based on the sum of all monomers, of at least one monomer selected from n-butyl acrylate and 2-octyl acrylate;

(vi) 0.5 to 5% by weight, based on the sum of all monomers, of 2-hydroxyethyl acrylate and 0.1 to 2% by weight, based on the sum of all monomers, of 2- (2-oxoimidazolidin-1-yl) ethyl methacrylate;

(vii) 0 to 20 wt%, based on the sum of all monomers, of at least one alkyl (meth) acrylate monomer which is different from monomers (i), (ii) and (v) and is selected from alkyl (meth) acrylates having 1 to 20 carbon atoms in the alkyl group;

(viii) From 0 to 10% by weight of other monomers than monomers (i) to (vii) selected from vinyl esters of carboxylic acids containing up to 20 carbon atoms, vinylaromatic compounds containing up to 20 carbon atoms other than styrene, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols containing from 1 to 10 carbon atoms, aliphatic hydrocarbons containing from 2 to 8 carbon atoms and one or two double bonds, and (meth) acrylamides.

11. Use of the pressure sensitive adhesive composition according to any of the preceding claims for the production of self-adhesive labels, self-adhesive tapes or self-adhesive films.

12. A self-adhesive article coated with the pressure-sensitive adhesive composition according to any one of claims 1 to 10.

13. A method for producing a self-adhesive article comprising the step of coating at least a portion of a substrate surface with a pressure sensitive adhesive composition according to any one of claims 1 to 10.