Aqueous polymer dispersions for pressure sensitive adhesives with broad temperature properties

Abstract

Disclosed herein are aqueous polymer dispersions, methods for their preparation and their use in pressure sensitive adhesives.

Description

Technical Field

[0001] The present invention relates to pressure-sensitive adhesives (PSA), and more particularly to polymer aqueous dispersions, their use in PSA, and methods for their preparation. Background Art

[0002] Pressure-sensitive adhesives (PSA) based on polymer aqueous dispersions obtainable by emulsion polymerization are a technology that has been established for a long time. The polymers in question are in particular poly(meth)acrylates. These are usually copolymers in which at least one of the monomers is an acrylate forming a polymer with a relatively low glass transition temperature, such as, for example, n-butyl acrylate or 2-ethylhexyl acrylate. Known acrylate copolymers based on n-butyl acrylate have adhesive properties sufficient to produce self-adhesive labels at room temperature. However, the temperature dependence of the surface tack means that the surface tack drops significantly at lower temperatures. This can be particularly challenging when applying the adhesive to a hydrophobic surface such as, for example, polyethylene. Lowering the glass transition temperature of the adhesive improves the surface tack at low temperatures but usually impairs the peel strength of the adhesive in a thermal environment. Particular problems arise if the adhesive has to have a wide application and use temperature range. In such applications, PSA based on polyacrylates often exhibit insufficient adhesion.

[0003] There is therefore a need for pressure-sensitive adhesive polymers that maintain good surface tack, peel, and static shear over a wide temperature range. Summary of the Invention

[0004] The present invention is based on the discovery that polymer dispersions having a low gel content and high molecular weight can be used to form continuous adhesive films having excellent properties compared to existing dispersion technologies. These polymer aqueous dispersions and methods for their preparation are provided herein such that the resulting pressure-sensitive adhesives provide good tack, peel, and static shear over a wide temperature range.

[0005] In one form thereof, the present disclosure provides a pressure-sensitive adhesive composition in the form of a polymer aqueous dispersion, which comprises at least one copolymer, and the at least one copolymer comprises: (i) 50% to 95% by weight of at least one soft (meth)acrylate monomer having a glass transition temperature of less than 0 °C when polymerized as a homopolymer; (ii) 0% to 25% by weight of at least one monomer selected from C1-C20 alkyl (meth)acrylates; (iii) 0.5% to 20% by weight of styrene; (iv) 0.1% to 5% by weight of at least one ethylenically unsaturated acid or at least one ethylenically unsaturated acid anhydride; (v) 0% to 10% by weight of other ethylenically unsaturated compounds other than monomers (i) to (iv); wherein the amounts of the monomers are each relative to the total amount of the monomers; and wherein the copolymer has a gel content of less than 35% by weight based on the total weight of the composition and a weight-average molecular weight of greater than 200 kDa.

[0006] In a second form thereof, the present disclosure provides a method for preparing a polymer aqueous dispersion, the method comprising: polymerizing a monomer mixture comprising: (i) 50% to 95% by weight of at least one soft (meth)acrylate monomer having a glass transition temperature of less than 0 °C when polymerized as a homopolymer; (ii) 0% to 25% by weight of at least one monomer selected from C1-C20 alkyl (meth)acrylates; (iii) 0.5% to 20% by weight of styrene; (iv) 0.1% to 5% by weight of at least one ethylenically unsaturated acid or at least one ethylenically unsaturated acid anhydride; (v) 0% to 10% by weight of other ethylenically unsaturated compounds other than monomers (i) to (iv); wherein the amounts of the monomers are each relative to the total amount of the monomers; and producing a copolymer having a polymer gel content of less than 35% by weight based on the total weight of the composition and a weight-average molecular weight of greater than 200 kDa. Detailed Description

[0007] I. Definitions

[0008] As used herein and unless otherwise specified, the weight % values are in each case based on the sum of all monomers used for polymerization.

[0009] A pressure-sensitive adhesive is a viscoelastic adhesive that remains permanently sticky and adhesive in a dry state in a solidified film at room temperature (20 °C). Adhesion to a substrate is achieved immediately under gently applied pressure.

[0010] As used herein, the name “(meth)acryloyl...” and similar names are occasionally used as abbreviations for “acryloyl... or methacryloyl...”. In the notation (meth)acrylic acid Cx alkyl ester and similar names, x represents the number of carbon atoms in the alkyl group.

[0011] As used herein, when describing monomers, the term “soft” refers to a monomer that has a glass transition temperature of less than 0 °C when polymerized as a homopolymer. Similarly, a “hard” monomer is a monomer that has a glass transition temperature of greater than 0 °C when polymerized as a homopolymer.

[0012] A cold surface is a surface having a surface temperature that is, in particular, at least 10 °C lower than the ambient temperature. A hot surface is a surface having a surface temperature that is, in particular, at least 10 °C higher than the ambient temperature.

[0013] As used herein, the phrase “within any range encompassed by any two of the foregoing values as endpoints” literally means that any range can be selected from any two values listed prior to this phrase, regardless of whether these values are lower or higher in the list. For example, a pair of values can be selected from two lower values, two higher values, or one lower value and one higher value.

[0014] II. Aqueous Polymer Dispersions for Pressure - Sensitive Adhesives

[0015] The present disclosure provides a pressure-sensitive adhesive composition in the form of a polymer aqueous dispersion, the pressure-sensitive adhesive composition comprising at least one copolymer, the copolymer comprising:

[0016] (i) 50 wt% to 95 wt% of at least one soft (meth)acrylate monomer that has a glass transition temperature below 0 °C when polymerized as a homopolymer,

[0017] (ii) 0 wt% to 25 wt% of at least one monomer selected from (meth)acrylic acid C1 to C20 alkyl esters,

[0018] (iii) 0.5 wt% to 20 wt% of styrene,

[0019] (iv) 0.1 wt% to 5 wt% of at least one ethylenically unsaturated acid or at least one ethylenically unsaturated acid anhydride,

[0020] (v) 0 wt% to 10 wt% of other ethylenically unsaturated compounds other than monomers (i) to (iv),

[0021] wherein the amounts of the monomers are each relative to the total amount of the monomers, and

[0022] wherein the copolymer has a gel content of less than 35 wt% based on the total weight of the composition and a weight-average molecular weight of greater than 200 kDa.

[0023] The main monomer (i) can be a soft (meth)acrylate monomer, which is present in an amount of 50% to 90% by weight, or more preferably 50% to 70% by weight. The soft monomer (i) is preferably selected from acrylates, particularly selected from C2 - C10 alkyl acrylates, or selected from C4 - C10 alkyl acrylates or selected from C4 - C8 alkyl acrylates. Suitable examples are ethyl acrylate, n-butyl acrylate, n-hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, and mixtures of these monomers. Ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate and their mixtures are preferred, and n-butyl acrylate and 2-ethylhexyl acrylate and their mixtures are particularly preferred.

[0024] The soft monomers can have certain characteristics that make them desirable for copolymer emulsions. When polymerized as homopolymers, the soft (meth)acrylate monomers can have, for example, a glass transition temperature of less than 10 °C, less than 5 °C, less than 0 °C, less than -5 °C, less than -10 °C, less than -15 °C, less than -20 °C, less than -25 °C, less than -30 °C, less than -35 °C, less than -40 °C, less than -45 °C, less than -50 °C, less than -55 °C, or less than -60 °C.

[0025] Monomer (ii) can be selected from C1 - C20 alkyl (meth)acrylates and is present in an amount of 0% to 25% by weight. C1 - C20 alkyl (meth)acrylates are, for example, methyl acrylate, methyl methacrylate, and hydroxypropyl acrylate, and mixtures of these monomers.

[0026] Monomer (iii) can be styrene, which is present in an amount of 0.5% to 20% by weight.

[0027] Monomer (iv) can be at least one ethylenically unsaturated acid or at least one ethylenically unsaturated acid anhydride. Suitable examples are monomers having a carboxylic acid group, a sulfonic acid group, or a phosphonic acid group. A carboxylic acid group is preferred. For example, acrylic acid, methacrylic acid, itaconic acid, maleic acid, or fumaric acid can be mentioned. The acid groups can be present in the form of their salts. Acrylic acid and methacrylic acid are particularly preferred.

[0028] Monomer (v) includes other ethylenically unsaturated compounds different from monomers (i) to (iv). Other monomers (v) different from monomers (i) to (iv) can be copolymerizable ethylenically unsaturated compounds. They can be used in an amount of 0 wt% to 10 wt%, preferably 0.1 wt% to 8 wt%. Other monomers (v) are preferably selected from C1-C20 alkyl (meth)acrylates, hydroxy-containing monomers, vinyl esters of carboxylic acids having up to 20 carbon atoms, vinyl aromatic compounds having up to 20 carbon atoms other than styrene, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols having 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds, monomers containing a hydroxy group, especially C1-C10 hydroxyalkyl (meth)acrylates, (meth)acrylamide or mixtures of these monomers. Additionally, other monomers that can be mentioned are phenoxyethyl ethylene glycol mono(meth)acrylate, glycidyl (meth)acrylate, aminoalkyl (meth)acrylates such as, for example, 2-aminoethyl (meth)acrylate. The alkyl group preferably has 1 to 20 C atoms. The C1-C20 alkyl (meth)acrylate has 1 to 20 C atoms in the alkyl group. The C1-C10 hydroxyalkyl (meth)acrylate has 1 to 10 C atoms in the hydroxyalkyl group. Other monomers that can be mentioned also include crosslinking monomers.

[0029] Suitable monomers are, for example, alkyl (meth)acrylates having a C3-C10 alkyl group. Particularly suitable are mixtures of alkyl (meth)acrylates. Vinyl esters of carboxylic acids having 1 to 20 C atoms are, for example, vinyl acetate, vinyl laurate, vinyl stearate, vinyl propionate and vinyl versatate. The contemplated vinyl aromatic compounds include vinyltoluene, α-methylstyrene and p-methylstyrene, α-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene. 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. Vinyl ethers include, for example, vinyl methyl ether or vinyl isobutyl ether. Preferred vinyl ethers are those of alcohols containing 1 to 4 carbon atoms. Suitable hydrocarbons having 4 to 8 C atoms and two olefinic double bonds are, for example, butadiene, isoprene and chloroprene.

[0030] Other monomers (v) can also contain crosslinking monomers. Preferred crosslinking monomers include diacetone (meth)acrylamide, acetylacetoxyethyl methacrylate, N-methylacryloyl (meth)acrylamide and glycidyl (meth)acrylate.

[0031] Other monomers (v) which are generally preferred are C3 to C10 alkyl acrylates and C3 to C10 alkyl methacrylates, more particularly C3 to C8 alkyl acrylates and C3 to C8 alkyl methacrylates; and vinyl esters, especially vinyl acetate; and mixtures thereof; and C2 to C10 hydroxyalkyl (meth)acrylates. Particularly preferred are n-hexyl acrylate, octyl acrylate, vinyl acetate and hydroxypropyl acrylate, and mixtures thereof.

[0032] Other monomers are generally used in small amounts; their total proportion is preferably less than 10% by weight, more particularly less than 8% by weight.

[0033] The copolymer of the aqueous polymer dispersion can be single-phase particles.

[0034] The copolymer of the polymer aqueous dispersion can have a gel content of less than 35% by weight, less than 30% by weight, less than 25% by weight, less than 20% by weight, less than 15% by weight, less than 10% by weight, less than 5% by weight, less than 1% by weight, less than 0.001% by weight or less than 0.00001% by weight based on the total weight of the composition.

[0035] The copolymer of the polymer aqueous dispersion can have a weight average molecular weight greater than 200 kDa, such as 300 kDa or greater, 400 kDa or greater, 500 kDa or greater, 600 kDa or greater, 700 kDa or greater, 800 kDa or greater, 900 kDa or greater, 1000 kDa or greater, 1100 kDa or greater, 1200 kDa or greater, 1300 kDa or greater, 1400 kDa or greater, 1500 kDa or greater, 1600 kDa or greater, 1700 kDa or greater, 1800 kDa or greater, 1900 kDa or greater, 2000 kDa or greater, 2500 kDa or greater, 3000 kDa or greater, 3500 kDa or greater, 4000 kDa or greater, 4500 kDa or greater, 5000 kDa or greater, 5500 kDa or greater, 6000 kDa or greater, 6500 kDa or greater, 7000 kDa or greater, 7500 kDa or greater, 8000 kDa or greater, 8500 kDa or greater, 9000 kDa or greater, 9500 kDa or greater, or 10,000 kDa or greater.

[0036] The copolymer of the polymer aqueous dispersion may have a glass transition temperature as low as -75 °C, -70 °C, -65 °C, -60 °C, -55 °C, -50 °C, -45 °C, -40 °C, -35 °C, -30 °C, -25 °C, -20 °C, -15 °C or as high as -10 °C, -5 °C, 0 °C, 5 °C, 10 °C, 15 °C, 20 °C or within any range covered by any two of the foregoing values as endpoints. For example, the copolymer of the aqueous dispersion may have a glass transition temperature of -65 °C to 0 °C. The glass transition temperature can be determined using differential scanning calorimetry (e.g., ASTM 3418 / 82, midpoint temperature).

[0037] The copolymer of the polymer aqueous dispersion may have a unimodal, bimodal or multimodal particle size distribution of the dispersion. The number average size represents d50 of the particle size distribution, meaning that 50% by weight of the total mass of all particles has a particle diameter less than d50. The particle size distribution can be determined in a known manner using an analytical ultracentrifuge (W. Makromolekulare Chemie 185 (1984), pages 1025 - 1039). The number average particle size of the polymer aqueous dispersion may be 2000 nm or less, 1900 nm or less, 1800 nm or less, 1700 nm or less, 1600 nm or less, 1500 nm or less, 1400 nm or less, 1300 nm or less, 1200 nm or less, 1100 nm or less, 1000 nm or less, 900 nm or less, 800 nm or less, 700 nm or less, 600 nm or less, 500 nm or less, 400 nm or less, 300 nm or less, 200 nm or less, 100 nm or less, 50 nm or less, 25 nm or less, 10 nm or less, or 5 nm or less.

[0038] The polymer aqueous dispersion may have a pH of 4.5, more preferably a pH between 5 and 8.

[0039] III. Method for Preparing Polymer Aqueous Dispersions for Pressure - Sensitive Adhesives

[0040] The adhesive polymer of the present invention can be obtained by free radical polymerization of ethylenically unsaturated compounds (monomers). The polymer is preferably prepared by emulsion polymerization and is therefore preferably an emulsion polymer. Accordingly, the present invention also provides a pressure - sensitive adhesive dispersion comprising the pressure - sensitive adhesive polymer of the present invention prepared by emulsion polymerization dispersed in water.

[0041] In emulsion polymerization, ethylenically unsaturated monomers are polymerized in water, using ionic and / or non-ionic emulsifiers and / or protective colloids or stabilizers as surface-active compounds to stabilize the monomer droplets and the polymer particles formed subsequently from the monomers. The amount of the surface-active substance is customarily from 0.1 to 10 parts by weight, preferably from 0.2 to 5 parts by weight, based on 100 parts by weight of the monomers to be polymerized.

[0042] A comprehensive description of suitable protective colloids can be found in Houben-Weyl, Methoden der organischen Chemie, Volume XIV / 1, Makromolekulare Stoffe [Macromolecular Compounds], Georg-Thieme-Verlag, Stuttgart, 1961, pages 411 to 420. Emulsifiers contemplated include anionic emulsifiers, cationic emulsifiers, and non-ionic emulsifiers. The surface-active substance used is preferably an emulsifier, which generally has a molecular weight lower by 2000 g / mol compared to that of the protective colloid. When using a mixture of surface-active substances, the individual components must of course be compatible with one another, which can be checked in case of doubt by means of several preliminary tests. The surface-active substance used is preferably an anionic emulsifier and a non-ionic emulsifier. Commonly used co-emulsifiers are, for example, ethoxylated fatty alcohols (EO degree: 3 to 50, alkyl group: C8 to C36), ethoxylated monoalkylphenols, ethoxylated dialkylphenols, and ethoxylated trialkylphenols (EO degree: 3 to 50, alkyl group: C4 to C9), alkali metal salts of dialkyl esters of sulfosuccinic acid, and alkali metal salts and ammonium salts of alkyl sulfates (alkyl group: C8 to C12), alkali metal salts and ammonium salts of ethoxylated alkanols (EO degree: 4 to 30, alkyl group: C12 to C18), alkali metal salts and ammonium salts of ethoxylated alkylphenols (EO degree: 3 to 50, alkyl group: C4 to C9), alkali metal salts and ammonium salts of alkyl sulfonic acids (alkyl group: C12 to C18), and alkali metal salts and ammonium salts of alkyl aryl sulfonic acids (alkyl group: C9 to C18).

[0043] Other suitable emulsifiers are compounds of the general formula provided by the following formula 1.

[0044] Formula 1

[0045]

[0046] In formula 1, R5 and R6 are hydrogen or C4 to C14 alkyl groups and are not both hydrogen, and X and Y can be alkali metal ions and / or ammonium ions. Preferably, R5 and R6 are straight-chain or branched-chain alkyl groups having 6 to 18 carbon atoms or hydrogen, and in particular having 6, 12, and 16 carbon atoms, where R5 and R6 are not both hydrogen. X and Y are preferably sodium ions, potassium ions, or ammonium ions, and particularly preferably sodium. Particularly advantageous compounds are those in which X and Y are sodium, R5 is a branched-chain alkyl group having 12 carbon atoms, and R6 is hydrogen or R5. Industrial mixtures with a monoalkylation product fraction of 50% to 90% by weight are frequently used. Commercially available products of suitable emulsifiers are, for example A1, NP 50, OC 50, Emulgator 825, Emulgator825S, OG, NSO, 904S, I-RA, E 3065, FES 77, AT 18, VSL, NPS25. For the present invention, ionic emulsifiers or protective colloids are preferred. Particularly preferably, they are ionic emulsifiers, and more particularly salts and acids, such as carboxylic acids, sulfonic acids, and sulfates, sulfonates, or carboxylates. In particular, mixtures of ionic emulsifiers and non-ionic emulsifiers can also be used.

[0047] Emulsion polymerization can be initiated using water-soluble initiators. Water-soluble initiators are, for example, ammonium salts and alkali metal salts of persulfuric acid (e.g., sodium persulfate), hydrogen peroxide, or organic peroxides (e.g., tert-butyl hydroperoxide). Other suitable initiators include those known as redox (reduction-oxidation) initiator systems. Redox initiator systems consist of at least one, usually inorganic, reducing agent and an organic or inorganic oxidizing agent. The oxidizing component includes, for example, the initiators described above for emulsion polymerization. The reducing component includes, for example, alkali metal salts of sulfurous acid, such as sodium sulfite, sodium bisulfite; alkali metal salts of pyrosulfurous acid, such as sodium pyrosulfite; addition compounds of bisulfites with aliphatic aldehydes and ketones, such as acetone bisulfite; or reducing agents, such as hydroxymethanesulfinic acid and its salts, or ascorbic acid. Redox initiator systems can be used together with soluble metal compounds, the metal components of which can exist in multiple valence states. Examples of conventional redox initiator systems include ascorbic acid / ferrous sulfate (II) / sodium persulfate, tert-butyl hydroperoxide / sodium pyrosulfite, tert-butyl hydroperoxide / sodium hydroxymethanesulfinate. The individual components, such as the reducing component, can also be mixtures, one example being a mixture of the sodium salt of hydroxymethanesulfinic acid and sodium pyrosulfite.

[0048] The initiator is generally used in the form of an aqueous solution, where the lower concentration is determined by the amount of water acceptable in the dispersion, and the higher concentration is determined by the solubility of the corresponding compound in water. Generally, the concentration of the initiator is from 0.1 wt% to 30 wt%, preferably 0.2 wt% to 20 wt%, more preferably 0.4 wt% to 2 wt% based on the monomer to be polymerized. A variety of different initiators can also be used in emulsion polymerization.

[0049] In the polymerization, the amount of the chain transfer agent is at least 0.01 parts by weight of the chain transfer agent per 100 parts by weight of the monomer, for example, 0.01 parts by weight to 0.8 parts by weight, or 0.01 parts by weight to 0.1 parts by weight of the monomer to be polymerized per 100 parts by weight. Using these reagents, the molar mass of the emulsion polymer can be controlled or reduced through chain termination reactions. These reagents bond to the polymer in this process, usually at the end of the chain.

[0050] Suitable chain transfer agents are, for example, organic compounds containing sulfur in bonded form (such as compounds having mercapto groups), aliphatic and / or araliphatic halogen compounds, aliphatic and / or aromatic aldehydes, unsaturated fatty acids (such as oleic acid), dienes having non-conjugated double bonds (such as, for example, divinylmethane, terpineol or vinylcyclohexene), hydrocarbons having hydrogen atoms that can be easily abstracted (such as, for example, toluene), organic acids and / or their salts (such as, for example, formic acid, sodium formate, ammonium formate), alcohols (such as, for example, isopropanol) and phosphorus compounds (such as, for example, sodium hypophosphite). However, mixtures of the above-described mutually non-interfering chain transfer agents can also be used. Chain transfer agents are generally low molecular weight compounds having a molecular weight of less than 2000 g / mol, more particularly less than 1000 g / mol. It is advantageous to provide part or all of the chain transfer agent to the aqueous reaction medium before initiating the free radical polymerization. In addition, during the polymerization, it can also be advantageous to provide part or all of the free radical chain transfer compound together with the monomer to the aqueous reaction medium.

[0051] Organic compounds having a thiol group are, for example, primary aliphatic thiols, secondary aliphatic thiols or tertiary aliphatic thiols, such as, for example, ethanethiol, n-propanethiol, 2-propanethiol, n-butanethiol, 2-butanethiol, 2-methyl-2-propanethiol, n-pentanethiol, 2-pentanethiol, 3-pentanethiol, 2-methyl-2-butanethiol, 3-methyl-2-butanethiol, n-hexanethiol, 2-hexanethiol, 3-hexanethiol, 2-methyl-2-pentanethiol, 3-methyl-2-pentanethiol, 4-methyl-2-pentanethiol, 2-methyl-3-pentanethiol, 3-methyl-3-pentanethiol, 2-ethylbutanethiol, 2-ethyl-2-butanethiol, n-heptanethiol and its isomeric compounds, n-octanethiol and its isomeric compounds, n-nonanethiol and its isomeric compounds, n-decanethiol and its isomeric compounds, n-undecanethiol and its isomeric compounds, n-dodecanethiol and its isomeric compounds, n-tridecanethiol and its isomeric compounds; substituted thiols, such as, for example, 2-hydroxyethanethiol; aromatic thiols, such as, for example, benzenethiol, o-methylbenzenethiol, m-methylbenzenethiol or p-methylbenzenethiol; mercaptoalkyl esters of, for example, C2 to C4 carboxylic acids, having 1 to 18 C atoms on the alkyl, such as, for example, 2-mercaptoethyl propionate; and all other sulfur compounds described in Polymer Handbook, 3rd Edition, 1989, J. Brandrup and E. H. Immergut, John Wiley & Sons, Part II, pp. 133 - 141. Preferred organic compounds containing sulfur in bonded form are in particular tert-butyl mercaptan, ethyl thioglycolate, mercaptoethanol, mercaptopropyltrimethoxysilane, tert-dodecyl mercaptan, thiodiglycol, ethylthioethanol, di-n-butyl sulfide, di-n-octyl sulfide, diphenyl sulfide, diisopropyl disulfide, 2-mercaptoethanol, 1,3-mercaptopropanol, 3-mercapto-1,2-propanediol, 1,4-mercaptobutanol, mercaptoacetic acid, 3-mercaptopropionic acid, mercaptosuccinic acid, thioacetic acid and thiourea. Particularly preferred sulfur compounds are tert-butyl mercaptan, ethyl mercaptoacetate, mercaptoethanol, mercaptopropyltrimethoxysilane or tert-dodecyl mercaptan.

[0052] Aliphatic and / or araliphatic halogen compounds are, for example, n-butyl chloride, n-butyl bromide, n-butyl iodide, dichloromethane, dichloroethane, chloroform, bromoform, bromotrichloromethane, dibromodichloromethane, carbon tetrachloride, carbon tetrabromide, benzyl chloride, benzyl bromide. Aliphatic and / or aromatic aldehydes are, for example, formaldehyde, acetaldehyde, propionaldehyde and / or benzaldehyde.

[0053] In the polymerization, an oxidizing agent can be added to help promote a low gel content and high molecular weight of the resulting copolymer. The oxidizing agent can be an inorganic oxidizing agent. Particularly preferred examples are sodium persulfate, ammonium persulfate and potassium persulfate. In a particularly preferred embodiment, the oxidizing agent does not contain tert-butyl hydroperoxide.

[0054] The selected oxidizing agent may be present in an amount as low as 0.00001 wt%, 0.001 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt% or as high as 0.4 wt%, 0.5 wt%, 1 wt%, 2 wt%, 3 wt% or 5 wt% based on the total weight of the monomers.

[0055] Emulsion polymerization is usually carried out at 50 °C to 110 °C, preferably 50 °C to 90 °C. The polymerization medium may consist only of water or additionally of a mixture of water and a water-miscible liquid such as methanol. Preferably only water is used. Emulsion polymerization can be carried out as a batch operation or in the form of a feeding process, including a staging procedure or a gradient scheme. Preferred is the feeding process, in which a part of the polymerization batch is introduced as an initial charge, heated to the polymerization temperature and its polymerization is started, and then the remaining polymerization batch is usually supplied to the polymerization zone through a plurality of spatially separated feeds, where one or more feeds contain monomers in pure form or in emulsified form, and this supply is carried out continuously, in stages or subject to a concentration gradient, and the polymerization reaction is maintained. In the polymerization, polymer seeds can also be included in the initial charge, for example, in order to more effectively set the particle size.

[0056] The manner of adding the initiator to the polymerization vessel during the free-radical aqueous emulsion polymerization process is known to those of ordinary skill in the art. It can be entirely included in the initial charge of the polymerization vessel, or it can be introduced continuously or in stages at a rate at which it is consumed during the free-radical aqueous emulsion polymerization process. This depends individually on the chemical nature of the initiator system and also on the polymerization temperature. Preferably, a part of it is included in the initial charge and the remainder is supplied to the polymerization zone at the rate at which it is consumed. In order to remove residual monomers, an initiator is usually added after the actual emulsion polymerization is completed, i.e., after at least 95% monomer conversion. In the case of the feeding process, the individual components can be added to the reactor from above, from the side or from below through the bottom of the reactor.

[0057] After the emulsion polymerization, a drying step may optionally be carried out to remove moisture. This drying step can also be combined with an additional optional crosslinking step, which can be carried out during or after the drying step. The crosslinking step can include activating the crosslinking agent by the drying process. Suitable crosslinking agents include diacylhydrazides, diamines, polyisocyanates, melamine-formaldehyde resins and metal ion salts. The crosslinking step can also include using ultraviolet (UV) light, increasing the temperature or adding a catalyst.

[0058] In emulsion polymerization, a polymer aqueous dispersion having a solids content of usually 15% to 75% by weight, preferably 40% to 75% by weight, is obtained. For a high space-time yield of the reactor, a dispersion with an extremely high solids content is preferred. In order to achieve a solids content of >60% by weight, a bimodal or multimodal particle size should be established, because otherwise the viscosity becomes too high and the dispersion can no longer be controlled. Generation of a new generation of particles can be achieved, for example, by adding seeds (EP 81083), by adding an excess of emulsifier, or by adding a miniemulsion. Another advantage associated with the combination of low viscosity and high solids content is the improved coating characteristics at high solids content. Generation of one or more generations of new particles is something that can be done at any point in time. This time is guided by the desired particle size distribution for low viscosity.

[0059] IV. Pressure - Sensitive Adhesive Compositions

[0060] The pressure-sensitive adhesive composition of the present invention comprises a pressure-sensitive adhesive polymer, preferably in the form of a polymer aqueous dispersion, obtainable by or by the above emulsion polymerization. The pressure-sensitive adhesive composition can consist only of the polymer or of an aqueous dispersion of the polymer. Alternatively, the PSA can also contain other auxiliaries, such as fillers, dyes, flow control agents, thickeners (preferably associative thickeners), defoamers, crosslinking agents, plasticizers, pigments, wetting agents or tackifiers (tackifying resins). Tackifiers are known, for example, from Adhesive Age, July 1987, pages 19 - 23, or Polym. Mater. Sci. Eng. 61 (1989), pages 588 - 592. In order to wet the surface more effectively, the PSA can particularly contain wetting aids (wetting agents), such as fatty alcohol ethoxylates, alkylphenol ethoxylates, nonylphenol ethoxylates, polyoxyethylene / propylene or sodium dodecylsulfonate. The amount of the auxiliaries is usually from 0.05 parts by weight to 5 parts by weight, more particularly from 0.1 parts by weight to 3 parts by weight (solids) per 100 parts by weight of the polymer.

[0061] A tackifier is a polymer or oligomer additive used in adhesive polymers or generally in elastomers, which can improve their self - adhesiveness (tack, intrinsic adhesiveness, self - adhesiveness), meaning that after a short, gentle application of pressure, they adhere firmly to a surface. Tackifiers are, for example, natural resins such as rosin and its derivatives or terpene resins formed by disproportionation or isomerization, polymerization, dimerization or hydrogenation. These can exist in their salt form (with, for example, monovalent or polyvalent counterions (cations)), or preferably in their esterified form. The alcohol used for esterification can be a monohydric alcohol or a polyhydric alcohol. Examples are methanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,2,3 - propanetriol, and pentaerythritol. Also usable are hydrocarbon resins, examples being coumarone - indene resins, polyterpene resins, hydrocarbon resins based on unsaturated CH compounds such as butadiene, pentene, methylbutene, isoprene, piperylene, divinylmethane, pentadiene, cyclopentene, cyclopentadiene, cyclohexadiene, styrene, α - methylstyrene and vinyltoluene.

[0062] Polyacrylates with a low molar weight are also increasingly used as tackifiers. These polyacrylates preferably have a weight - average molecular weight Mw of less than 50,000, more particularly less than 30,000. The polyacrylates preferably consist of at least 60% by weight, especially at least 80% by weight of C1 - C8 alkyl (meth)acrylates. For example, the low - molecular - mass polymers and oligomers described in WO 2013 / 117428 are suitable, having a weight - average molecular weight of less than 50,000 and a glass transition temperature greater than or equal to - 40 °C to less than or equal to 0 °C, preferably greater than or equal to - 35 °C to less than or equal to 0 °C, which can be prepared by emulsion polymerization in the presence of at least one chain - transfer agent and can be prepared from a monomer mixture containing at least 40% by weight of at least one C1 - C20 alkyl (meth)acrylate.

[0063] Preferred tackifiers are natural or chemically modified rosins. Rosin mainly consists of abietic acid or derivatives of abietic acid. The tackifier can simply be added to the polymer dispersion. In this case, the tackifier itself is preferably in the form of an aqueous dispersion. The amount of the tackifier, based on 100 parts by weight of the polymer, is preferably 5 to 100 parts by weight, more preferably 10 to 50 parts by weight (solid / solid).

[0064] Pressure-sensitive adhesive polymers and pressure-sensitive adhesive dispersions can be used to prepare self-adhesive articles. The article is at least partially coated with PSA. After bonding, the self-adhesive article is preferably removable. The self-adhesive article can be, for example, a sheet, a tape or a label. Suitable backing materials are, for example, paper, polymer films and metal foils. The self-adhesive tape of the present invention can be a single-sided coated or double-sided coated tape containing the above substances. Particularly preferred are self-adhesive labels. The self-adhesive labels of the present invention can be labels on paper or thermoplastic films. The thermoplastic films contemplated include, for example, films of polyolefins (e.g., polyethylene, polypropylene), films of polyolefin copolymers; films of polyesters (e.g., polyethylene terephthalate) or polyacetates. The surface of the thermoplastic polymer film is preferably corona-treated. The label is coated with an adhesive on one side.

[0065] Preferred substrates for self-adhesive articles are paper and polymer films. Particularly preferred self-adhesive articles are paper labels.

[0066] The self-adhesive article is at least partially coated with the PSA of the present invention on at least one surface. The adhesive dispersion can be coated by a variety of common coating methods used in various markets, including roll labels, tapes and specialty tapes, graphic and flexible packaging. These methods include Mayer rod, slot die, direct gravure printing, reverse gravure printing, single-chamber reverse gravure printing and double-chamber reverse gravure printing, disk-fed or nip-fed roll coating, doctor blade coating on rolls, curtain coating and slide curtain coating. The choice of coating method will generally depend on the desired line speed, coating thickness, and the precision required in the transverse and longitudinal directions of the web during coating.

[0067] The adhesive dispersion is usually coated directly onto the face stock material (which can be paper, film, fabric), or more commonly onto a release-coated film or paper substrate (release liner) pre-coated with a specially formulated silicone chemistry (solvent-based, water-based, 100% solid technology), which is cured by thermal drying methods, UV light or electron beam (EB). The release coating can be coated online during the adhesive dispersion coating application, or the method can use a pre-release-coated liner.

[0068] The coated adhesive dispersion requires a heat source to remove water from the dispersion and cause the dispersion to coalesce into a continuous strong film. This is typically achieved by passing the coated adhesive dispersion through a gas furnace that can have multiple zones set at different temperatures (e.g., 50 °C to 150 °C) and air velocities to slowly but effectively drive off the moisture. In some cases, the drying step can also use an infrared (IR) heat source to assist in drying. The moisture content is reduced to a level that gives an optimal balance of adhesive flow, adhesion, and cohesion; this is typically at a moisture content of <3% relative to the weight of the dry dispersion, but depends on the desired product performance. More preferably, the moisture content is 2% or lower.

[0069] The coating weight is preferably from 0.1 g to 175 g, more preferably 2 g to 20 g of solids per m 2 0.1 g to 175 g, more preferably 2 g to 20 g of solids. The dry adhesive coating will be laminated to a suitable substrate by passing through a nip laminator. For example, if coated onto a release liner, the coated release liner will be laminated to a face stock, which can be of paper or film type. In the case of a self-winding tape, the release liner can have release coatings on both sides with different release forces for unwinding and in this case does not include a face stock.

[0070] The coated substrate thus obtained is used, for example, as a self-adhesive article such as a label, a linerless label, a tape, or a sheet. For this purpose, the backing can be cut before or after applying the adhesive to form a tape, a label, or a sheet. For subsequent use, the PSA-coated face of the substrate can be lined with release paper, such as, for example, silicone-coated paper.

[0071] The substrates on which the self-adhesive article can be advantageously applied can include, for example, metal, wood, glass, paper, or plastic. The self-adhesive article is particularly suitable for bonding to packaging surfaces, cartons, plastic packaging, books, windows, motor vehicle bodies or body parts. Preferred substrates are self-adhesive labels, more particularly self-adhesive paper labels and self-adhesive film labels. The backing material is paper or a polymer film and has a first surface and a second surface, where the first surface is self-adhesive and at least partially coated with the PSA of the present invention, while the second surface can be printed, or the second surface or the label can be at least partially colored. The coloring can be produced, for example, by coloring the coating with a pigment or a dye, by color printing, or in the case of thermal paper by exposure to heat.

[0072] Examples

[0073] Example 1

[0074] Example 1 was prepared according to the following procedure. 168 g of deionized (“DI”) water and 4.5 g of polystyrene seeds (32%) were fed into a two-liter reactor equipped with a condenser, a mechanical stirrer, a temperature-controlled thermocouple, and inlets for initiator and monomer, and heated to 85 °C. In a separate container, a monomer emulsion was prepared by mixing 184 g of DI water, 31 g of Disponil FES 77, 7 g of Calfax DB-45, and 660 g of a monomer mixture containing 95.3 wt% of 2-ethylhexyl acrylate (“2-EHA”), 4 wt% of styrene (“STY”), 0.5 wt% of methacrylic acid (“MAA”), and 0.2 wt% of diacetone acrylamide (“DAAM”). Then, a mixture solution of 2.3 g of sodium persulfate (“NaPS”) in 73 g of DI water was added to the reactor. Immediately after adding the NaPS solution, the monomer emulsion was fed into the reactor. The feeding was carried out for 200 minutes. After the addition of the monomer emulsion was completed, the reaction mixture was cooled to 80 °C, and then a solution of 0.65 g of NaPS in 8.6 g of DI water and a solution of 1 g of sodium metabisulfite in 8.2 g of DI water were gradually added via two separate feedings within 60 minutes. After the feeding was completed, the reaction was cooled to room temperature, and then 4.7 g of Aerosol OT 70PG and 12 g of DI water were added. Then the obtained dispersion was filtered through a 150 μm mesh filter cloth and then subjected to subsequent evaluation. The obtained dispersion had a glass transition temperature of -58 °C, a gel content of 2 wt%, and a weight-average molecular weight of 603 kDa. The pH of the polymer dispersion was adjusted to 7 or greater by adding ammonia water, and then a solution of 0.46 g of adipic dihydrazide (ADH) and 46 g of DI water was added to the dispersion.

[0075] Example 2

[0076] Example 2 was prepared according to the same procedure as Example 1, but using 2.0 g of sodium persulfate as the initiator. The obtained dispersion had a gel content of 4 wt% and a weight-average molecular weight of 548 kDa. The pH of the polymer dispersion was adjusted to 7 or greater by adding ammonia water, and then a solution of 0.46 g of adipic dihydrazide (ADH) and 46 g of DI water was added to the dispersion.

[0077] Example 3

[0078] Example 3 was prepared according to the same procedure as Example 1, but with 3 wt% styrene ("STY") and 1.5 wt% methacrylic acid ("MAA") in the monomer feed. The gel content of the obtained dispersion was 33 wt%, and the weight-average molecular weight was 354 kDa. The pH of the polymer dispersion was adjusted to 7 or greater by adding ammonia water, and then a solution of 0.46 g of adipic dihydrazide (ADH) and 46 g of DI water was added to the dispersion.

[0079] Example 4

[0080] Example 4 was prepared according to the same procedure as Example 1, but with 3 wt% styrene ("STY") and 1.5 wt% 2-hydroxypropyl acrylate ("HPA") in the monomer feed. The gel content of the obtained dispersion was 1 wt%, and the weight-average molecular weight was 317 kDa. The pH of the polymer dispersion was adjusted to 7 or greater by adding ammonia water, and then a solution of 0.46 g of adipic dihydrazide (ADH) and 46 g of DI water was added to the dispersion.

[0081] Comparative Example 1

[0082] Comparative Example 1 was prepared according to the same procedure as Example 1, but with 3 wt% styrene ("STY") and 1.5 wt% acrylamide ("AM") in the monomer feed. The gel content of the obtained dispersion was 77 wt%, and the weight-average molecular weight was 314 kDa.

[0083] Comparative Example 2

[0084] The acrylic emulsion used as Comparative Example 1 was commercially available from BASF Corporation under the trade name NX 2160. The glass transition temperature of Comparative Example Emulsion 1 was -58 °C, the gel content was 54 wt%, and the weight-average molecular weight was 250 kDa.

[0085] Characterization of the Dispersions

[0086] Emulsion polymerization is characterized by the non-volatile content (NV%) and Brookfield viscosity. The NV% was measured gravimetrically using a CEM Smart System 5 microwave moisture analyzer. The viscosity was determined using a Brookfield RV viscometer at 60 RPM (spindle 63). The particle size was determined using a DLS Microtrac with a 180° backscatter angle.

[0087] The molecular weight distribution was characterized by gel permeation chromatography (GPC) relative to a polystyrene calibration curve. 30 mg of the sample was dissolved in 10 mL of THF, and 100 μl of this solution was filtered and injected into a column running at a flow rate of 1.0 mL / min. The column set consisted of a guard column (Agilent PL1110 - 1120 PLgel 10 μm guard, 50×7.5 mm) and two analytical columns (Agilent PL1110 - 6100 PLgel Mixed - B, 300×7.5 mm) in series.

[0088] The gel percentage is a measure of the amount of insoluble gel remaining after soaking. A weighed dry polymer film was placed inside a 100 - mesh (149 μm) gold “Harris” cage and left in excess tetrahydrofuran (THF) for 48 hours. After 48 hours of extraction, the cage was removed from the solvent, and the polymer film was dried and re - weighed. This weight was divided by the original weight to obtain the gel percentage. Samples were run in duplicate and the results averaged.

[0089] Characterization of Adhesive Properties (A - F)

[0090] (A) Formulation of the Adhesive :

[0091] The pH of the polymer dispersion was adjusted to 7 or greater by adding ammonia water. In some cases, cross - linking of the binder after coating and drying was achieved by adding diacylhydrazide adipate (ADH) to the pH - adjusted dispersion such that ADH could react with the polymerized DAAM monomer after evaporation of water and ammonia.

[0092] (B) Laboratory Coating of the Adhesive :

[0093] The formulated binder was coated on a 1.5 - mil PET film at 18 g / m2 to 22 g / m2 based on dry weight, dried at 115 °C for 4 minutes, and then laminated with a release liner. Binder performance tests were carried out after conditioning the binder laminate overnight in a controlled temperature and humidity (CTH) environment (23 ± 2 °C and 50 ± 5% relative humidity) test laboratory.

[0094] (C) Adhesion / Peel Test :

[0095] The samples were tested on stainless steel, high-density polyethylene (“HDPE”), and B-flute corrugated board test panels according to the Fédération Internationale des fabricants et transformateurs d'Adhésifs et Thermocollants (“FINAT”) Test Method 1 (“FTM 1”). Testing under freezing (-20 °C) and heat (50 °C) conditions was achieved by conditioning the adhesive strips on the test panels under CTH and then separately in a freezer or oven; the peel test was carried out immediately after removal from the temperature conditioning such that the test panels remained within 3 °C of the conditioning temperature. The test panels could be insulated with foam sleeves to further reduce their temperature drift during testing.

[0096] (D) Adhesion / Tack Test :

[0097] FINAT Test Method 9 (“FTM 9”) was used for the “loop tack” initial adhesion testing on stainless steel, high-density polyethylene (“HDPE”), and B-flute corrugated board test panels.

[0098] (E) Cohesion / Shear Test :

[0099] The shear resistance test was carried out on a stainless steel plate using FINAT Test Method 8 (“FTM 8”). For each test, the qualitative failure mode was recorded: “AF” indicates adhesive failure. “AT” indicates adhesive failure from the facestock, i.e., the PET film. “CF” indicates cohesive failure, i.e., adhesive residue on the test panel and facestock. “SF” indicates substrate failure, i.e., tearing of the substrate panel.

[0100] (F) Mandrel Hold Test :

[0101] FINAT Test Method 24 (“FTM 24”) was applicable to the “mandrel hold” adhesion test on a 0.5” diameter HDPE rod. Here, we reported the “lift” as the shortest distance between the lifted edge of the label and the rod surface, rather than the length at which the label no longer contacted the rod. The results of this test were highly dependent on the facestock stiffness; a 50# (3300 ft 2 wet strength) paper was laminated to the back of each sample to provide high and uniform stiffness.

[0102] Table 1

[0103] Peel Force, lb / in, Average of 5 Tests

[0104]

[0105] Table 2

[0106] Loop Tack, lb / in, Average of 5 Tests

[0107] Samples Steel Plate HDPE Panel Corrugated Cardboard Panel E1 2.79AT 1.41AF 1.99AT E2 2.54AT 1.47AF 1.77AT E3 3.06AT 1.26AF 1.83AF E4 4.15AF 2.13AF 2.09AF CE1 2.12AT 1.35AF 1.88AT CE2 2.12AF 1.59AF 1.90AF

[0108] Table 3

[0109] Static Shear, hours, Average of 5 Tests

[0110] Samples Steel Plate E1 9CF E2 5.5CF E3 >120 E4 7.5CF CE1 >120 CE2 81CF

[0111] Table 4

[0112] Mandrel Lift, mm, Average of 4 Corners

[0113]

Claims

1. A pressure-sensitive adhesive composition in the form of a polymer aqueous dispersion, comprising at least one copolymer, said at least one copolymer comprising: (i) 50% to 95% by weight of at least one soft (meth)acrylate monomer which, when polymerized as a homopolymer, has a glass transition temperature below 0 °C, (ii) 0% to 25% by weight of at least one monomer selected from C1-C20 alkyl (meth)acrylates, (iii) 0.5% to 20% by weight of styrene, (iv) 0.1% to 5% by weight of at least one ethylenically unsaturated acid or at least one ethylenically unsaturated acid anhydride, (v) 0% to 10% by weight of other ethylenically unsaturated compounds other than monomers (i) to (iv), wherein the amounts of the monomers are each relative to the total amount of the monomers, and wherein the copolymer has a gel content of less than 35% by weight based on the total weight of the composition and a weight-average molecular weight of more than 200 kDa.

2. The pressure-sensitive adhesive composition according to claim 1, wherein the copolymer comprises 50% to 70% by weight of at least one soft (meth)acrylate monomer which, when polymerized as a homopolymer, has a glass transition temperature below 0 °C, based on the total weight of the monomers.

3. The pressure-sensitive adhesive composition according to claim 1, wherein the soft (meth)acrylate monomer is selected from n-butyl acrylate, 2-ethylhexyl acrylate and ethyl acrylate.

4. The pressure-sensitive adhesive composition according to any one of the preceding claims, wherein the additional ethylenically unsaturated compound (v) is selected from C1-C20 alkyl (meth)acrylates, hydroxy-containing monomers, vinyl esters of carboxylic acids having up to 20 carbon atoms, vinyl aromatic compounds having up to 20 carbon atoms other than styrene, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols having 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds.

5. The pressure-sensitive adhesive according to any one of the preceding claims, wherein the copolymer has a gel content of 0.0001% to 20% by weight based on the total weight of the composition.

6. The pressure-sensitive adhesive according to any one of the preceding claims, wherein the copolymer has a gel content of 0.0001% to 10% by weight based on the total weight of the composition.

7. The pressure-sensitive adhesive according to any one of the preceding claims, wherein the copolymer has a weight-average molecular weight of 400 kDa to 10,000 kDa.

8. The pressure-sensitive adhesive according to any one of the preceding claims, wherein the copolymer has a weight-average molecular weight of 1,000 kDa to 10,000 kDa.

9. The pressure-sensitive adhesive according to any one of the preceding claims, wherein the glass transition temperature of the copolymer is -65 °C to 0 °C.

10. The pressure-sensitive adhesive according to any one of the preceding claims, wherein the copolymer is an emulsion polymer.

11. The pressure-sensitive adhesive according to any one of the preceding claims, wherein the copolymer further comprises a crosslinkable monomer.

12. The pressure-sensitive adhesive according to claim 11, wherein the crosslinkable monomer is selected from diacetone (meth)acrylamide, acetoacetoxyethyl methacrylate, N-methylacryloyl(meth)acrylamide, and glycidyl methacrylate.

13. The pressure-sensitive adhesive according to any one of the preceding claims, wherein the copolymer is a single-phase particle.

14. The pressure-sensitive adhesive according to any one of the preceding claims, wherein the copolymer has a number-average particle size of 1000 nm or less.

15. A self-adhesive article comprising the pressure-sensitive adhesive according to any one of the preceding claims.

16. The self-adhesive article according to claim 15, wherein the self-adhesive article is a label, a linerless label, a tape, or an adhesive sheet.

17. The self-adhesive article according to claim 15 or 16, wherein according to the FTM 1 test method, the self-adhesive article has a peel force of 0.5 lb / in to 10 lb / in on a steel plate at 50 °C.

18. The self-adhesive article according to claim 15 or 16, wherein according to the FTM 1 test method, the self-adhesive article has a peel force of 1.5 lb / in to 10 lb / in on an HDPE plate at 23 °C.

19. The self-adhesive article according to claim 15 or 16, wherein according to the FTM 1 test method, the self-adhesive article has a peel force of 0.25 lb / in to 10 lb / in on an HDPE plate at 50 °C.

20. The self-adhesive article according to claim 15 or 16, wherein according to the FTM 1 test method, the self-adhesive article has a peel force of 0.8 lb / in to 10 lb / in on a corrugated cardboard at 50 °C.

21. The self-adhesive article according to claim 15 or 16, wherein according to the FTM 9 test method, the self-adhesive article has a tack adhesion force of 2.0 lb / in to 10 lb / in on a steel plate.

22. The self-adhesive article according to claim 15 or 16, wherein according to the FTM 24 test method, after 1 day, it has a mandrel lift of 0.01 mm to 0.7 mm on a 0.5" HDPE rod.

23. The self-adhesive article according to claim 15 or 16, wherein according to the FTM 24 test method, after 7 days, it has a mandrel lift of 0.01 mm to 0.95 mm on a 0.5" HDPE rod.

24. A method for preparing a polymer aqueous dispersion, the method comprising: polymerizing a monomer mixture comprising: (i) 50% to 95% by weight of at least one soft (meth)acrylate monomer, which has a glass transition temperature below 0 °C when polymerized as a homopolymer, (ii) 0% to 25% by weight of at least one monomer selected from C1 to C20 alkyl (meth)acrylates, (iii) 0.5% to 20% by weight of styrene, (iv) At least one ethylenically unsaturated acid or at least one ethylenically unsaturated acid anhydride in an amount of 0.1% to 5% by weight, (v) Other ethylenically unsaturated compounds other than monomers (i) to (iv) in an amount of 0% to 10% by weight, wherein the amounts of the monomers are each relative to the total amount of the monomers, to prepare a copolymer having a polymer gel content of less than 35% by weight and a weight average molecular weight of greater than 200 kDa based on the total weight of the composition.

25. The method according to claim 24, wherein the copolymer comprises at least one soft (meth)acrylate monomer in an amount of 50% to 70% by weight based on the total weight of the monomers, and the soft (meth)acrylate monomer has a glass transition temperature of less than 0 °C when polymerized as a homopolymer.

26. The method according to claim 24 or claim 25, further comprising adding an oxidizing agent during the polymerization.

27. The method according to claim 26, wherein the oxidizing agent is an inorganic oxidizing agent.

28. The method according to claim 26, wherein the oxidizing agent is selected from sodium persulfate, ammonium persulfate, and potassium persulfate.

29. The method according to claim 27, wherein the oxidizing agent does not contain tert-butyl hydroperoxide.

30. The method according to any one of claims 26 to 29, wherein the oxidizing agent is present in an amount of 0.001% to 0.5% by weight based on the total weight of the monomers.

31. The method according to any one of claims 24 to 30, wherein the polymerization step is carried out at a temperature of 50 °C to 110 °C.

32. The method according to any one of claims 24 to 31, wherein a drying step is carried out after the polymerization step.

33. The method according to any one of claims 24 to 32, wherein the drying step further comprises a crosslinking step.

34. The method according to claim 33, wherein the crosslinking step comprises adding a crosslinking agent selected from diacylhydrazides, diamines, polyisocyanates, melamine-formaldehyde resins, and metal ion salts.

35. The method according to claim 33, wherein the crosslinking step comprises using UV light, increasing the temperature, or adding a catalyst.

36. The method according to any one of claims 33 to 35, wherein the drying step and the crosslinking step are carried out simultaneously.

37. The method according to any one of claims 33 to 36, wherein the drying step is carried out before the crosslinking step.

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