WO2024107914A1

WO2024107914A1 - Aqueous polymer dispersion for pressure-sensitive adhesive with broad temperature performance

Info

Publication number: WO2024107914A1
Application number: PCT/US2023/079944
Authority: WO
Inventors: Sean Raymond GEORGE
Original assignee: Basf Corporation
Priority date: 2022-11-16
Filing date: 2023-11-16
Publication date: 2024-05-23

Abstract

Disclosed herein are aqueous polymer dispersions, their methods of making, and their applications in pressure sensitive adhesives.

Description

AQUEOUS POLYMER DISPERSION FOR PRESSURE-SENSITIVE ADHESIVE WITH BROAD TEMPERATURE PERFORMANCE

FIELD

[0001] The present disclosure is directed to pressure sensitive adhesives (PSAs), and more specifically, aqueous polymer dispersions, their applications in PSAs, and their methods of making.

BACKGROUND

[0002] Pressure-sensitive adhesives (PSAs) based on aqueous polymer dispersions obtainable by emulsion polymerization are a long-established art. The polymers in question are, in particular, poly(meth)acrylates. These are generally copolymers, in which at least one of the monomers is an acrylic ester that forms polymers with a relatively low glass transition temperature, such as n-butyl acrylate or 2- ethylhexyl acrylate, for example. Known acrylate copolymers based on n-butyl acrylate do have adhesive properties sufficient at room temperature for the production of self- adhesive labels. The temperature dependence of the surface tack, however, means that the surface tack goes down significantly at lower temperatures. This can be particularly challenging when the adhesive is applied to a hydrophobic surface, such as polyethylene, for example. Lowering the adhesive glass transition temperature can improve the surface tack at low temperatures, but will generally compromise the adhesive peel strength in hot environments. A particular problem arises if the adhesive must have a broad application and service temperature range. In such applications, polyacrylate-based PSAs frequently exhibit inadequate adhesion.

[0003] There is therefore a desire for pressure-sensitive adhesive polymers which retain good surface tack, peel, and static shear across a broad range of temperatures.

SUMMARY

[0004] The present disclosure is based upon the discovery that a polymer dispersion with a low gel content and high molecular weight can be used to form a continuous adhesive film with superior properties compared to existing dispersion technology. Provided herein are these aqueous polymer dispersions and their methods of preparation such that the resulting pressure sensitive adhesive provides good tack, peel, and static shear across a broad range of temperatures.

[0005] In one form thereof, the present disclosure provides a pressure-sensitive adhesive composition in the form of an aqueous polymer dispersion comprising at least one copolymer comprising: (i) 50 to 95% by weight of at least one soft (meth) acrylic acid ester monomer which, when polymerized as homopolymer, has a glass transition temperature of less than 0°C, (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) 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 the monomers (i) to (iv), wherein the amounts of the monomers each relate to the total amount of monomers, and 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 greater than 200 kDa.

[0006] In a second form thereof, the present disclosure provides a method for preparing an aqueous polymer dispersion, comprising: polymerizing a mixture of monomers comprising: (i) 50 to 95% by weight of at least one soft (meth) acrylic acid ester monomer which, when polymerized as homopolymer, has a glass transition temperature of less than 0°C, (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) 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 the monomers (i) to (iv), wherein the amounts of the monomers each relate to the total amount of monomers, and producing a copolymer having a polymer gel content less than 35 wt.%, based on the total weight of the composition, and a weight-average molecular weight greater than 200 kDa.

DETAILED DESCRIPTION

I. Definitions [0007] As used herein and unless otherwise noted, the wt.% figures are based in each case on the sum of all monomers used in the polymerization.

[0008] A pressure-sensitive adhesive is a viscoelastic adhesive whose set film at room temperature (20°C.) remains permanently tacky and adhesive in the dry state. Adhesion to substrates is accomplished immediately under gentle applied pressure.

[0009] As used herein, the designation “(meth)acryl . . . ” and similar designations are occasionally used as an abbreviated notation for “acryl . . . or methacryl . . . ”. In the notation Cx alkyl (meth)acrylate and similar designations, x denotes the number of C atoms in the alkyl group.

[0010] As used herein, the term "soft” when describing monomers means a monomer which, when polymerized as homopolymer, has a glass transition temperature less than 0°C. Likewise, a “hard” monomer is a monomer in which, when polymerized as homopolymer, has a glass transition temperature greater than 0°C.

[0011] Cold surfaces are surfaces having a surface temperature of in particular at least 10°C. below the ambient temperature. Hot surfaces are surfaces having a surface temperature of in particular at least 10°C. above the ambient temperature.

[0012] As used herein, the phrase “within any range encompassed by any two of the foregoing values as endpoints” literally means that any range may be selected from any two of the values listed prior to such phrase regardless of whether the values are in the lower part of the listing or in the higher part of the listing. For example, a pair of values may be selected from two lower values, two higher values, or a lower value and a higher value.

II. Aqueous Polymer Dispersions for Pressure Sensitive Adhesives [0013] The present disclosure provides a pressure-sensitive adhesive composition in the form of an aqueous polymer dispersion comprising at least one copolymer comprising:

(i) 50 to 95% by weight of at least one soft (meth) acrylic acid ester monomer which, when polymerized as homopolymer, has a glass transition temperature of less than 0°C, (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) 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 the monomers (i) to (iv), wherein the amounts of the monomers each relate to the total amount of monomers, and 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 greater than 200 kDa.

[0014] The principal monomer (i) may be a soft (meth) acrylic acid ester monomer which is present in an amount of 50 to 90% by weight, or more preferably 50- 70% by weight. The soft monomers (i) are preferably selected from acrylates, in particular from C2 to C10 alkyl acrylates, or from C4 to C10 alkyl acrylates or from C4 to 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. Preference is given to ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate and mixtures thereof, particularly preferably n-butyl acrylate and 2-ethylhexyl acrylate and mixtures thereof.

[0015] The soft monomers may have certain characteristics that make them desirable for the copolymer emulsion. The soft (meth) acrylic acid ester monomers may, for example, have 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 - 0°C, less than -45°C, less than - 50°C, less than -55°C, or less than -60°C, when polymerized as a homopolymer.

[0016] Monomers (ii) may be selected from C1 to C20 alkyl (meth) acrylates and present in an amount of 0-25% by weight. C1 to C20 alkyl (meth) acrylates are, for example, methyl acrylate, methyl methacrylate, and hydroxypropyl acrylate and mixtures of these monomers.

[0017] Monomer (iii) may be styrene which is present in an amount of 0.5 to 20% by weight

[0018] Monomers (iv) may be at least one ethylenically unsaturated acid or at least one ethylenically unsaturated acid anhydride. Suitable examples, are monomers with carboxylic, sulfonic or phosphonic acid groups. Preferred are carboxylic acid groups. Mention may be made of acrylic acid, methacrylic acid, itaconic acid, maleic acid or fumaric acid, for example. The acid groups may be present in the form of their salts. Particularly preferred are acrylic acid and methacrylic acid.

[0019] Monomers (v) comprise other ethylenically unsaturated compounds other than the monomers (i) to (iv). The further monomers (v), different from the monomers (i) to (iv), may be copolymerizable, ethylenically unsaturated compounds. They can be used in amounts of 0 to 10 wt.%, preferably of 0.1 -8 wt.%. The further monomers (v) are preferably selected from the group consisting of C1 to C20 alkyl (meth) acrylates, hydroxyl-containing monomers, vinyl esters of carboxylic acids containing 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 from 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds, monomers comprising hydroxyl groups, especially C1 -C10 hydroxyalkyl (meth)acrylates, (meth)acrylamide, or mixtures of these monomers.

Further monomers that may be mentioned are, additionally, phenyloxyethyl glycol mono(meth)acrylate, glycidyl (meth)acrylate, aminoalkyl (meth)acrylates such as 2- aminoethyl (meth)acrylate, for example. Alkyl groups have preferably from 1 to 20 C atoms. C1-C20 Alkyl (meth)acrylates have 1 -20 C atoms in the alkyl groups. C1-C10 Hydroxyalkyl (meth)acrylates have 1 -10 C atoms in the hydroxyalkyl groups. Further monomers that may be mentioned also include crosslinking monomers.

[0020] Suitable monomers are, for example, (meth)acrylic acid alkyl esters with a C3-C10 alkyl radical. Also suitable in particular are mixtures of the (meth)acrylic acid alkyl esters. Vinyl esters of carboxylic acids having 1 to 20 C atoms are, for example, vinyl acetate, vinyl laurate, vinyl stearate, vinyl propionate, and vinyl esters of Versatic acid. Vinyl aromatic compounds contemplated include vinyltoluene, alpha- and p- methylstyrene, alpha-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene. Examples of nitriles are acrylonitrile and methacrylonitrile. The vinyl halides are ethylenically unsaturated compounds substituted by chlorine, fluorine or bromine, preferably vinyl chloride and vinylidene chloride. Vinyl ethers include, for example, vinyl methyl ether or vinyl isobutyl ether. Preferred vinyl ethers are those of alcohols comprising 1 to 4 C atoms. Suitable hydrocarbons having 4 to 8 C atoms and two olefinic double bonds are butadiene, isoprene, and chloroprene, for example.

[0021] The further monomers (v) may also comprise a crosslinkable monomer. Preferred crosslinkable monomers include diacetone (meth) acrylamide, acetoacetoxyethyl methacrylate, N-methoyl (meth) acrylamide, and glycidyl methacrylate.

[0022] Generally preferred further monomers (v) 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 also C2 to C10 hydroxyalkyl (meth)acrylates. Especially preferred are n-hexyl acrylate, octyl acrylate, vinyl acetate, and hydroxypropyl acrylate, and mixtures thereof. [0023] The further monomers are used in general in minor amounts; their proportion overall is preferably below 10 wt.%, more particularly below 8 wt.%.

[0024] The copolymer of the aqueous polymer dispersion may be a single-phase particle.

[0025] The copolymer of the aqueous polymer dispersion may have a gel content of less than 35 wt.%, less than 30 wt.%, less than 25 wt.%, less than 20 wt.%, less than 15 wt.%, less than 10 wt.%, less than 5 wt.%, less than 1 wt.%, less than 0.001 wt.% or less than 0.00001 wt.% based on the total weight of the composition.

[0026] The copolymer of the aqueous polymer dispersion may have a weightaverage 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.

[0027] The copolymer of the aqueous polymer 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 encompassed 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 to 0°C. The glass transition temperature may be determined by means of differential scanning calorimetry (e.g., ASTM 3418/82, midpoint temperature).

[0028] The copolymer of the aqueous polymer dispersion may have a size distribution of dispersion particles which is monomodal, bimodal, or multimodal. The number average size indicates the d50 of the particle size distribution, meaning that 50 wt.% of the total mass of all particles have a particle diameter smaller than the d50. The particle size distribution can be determined in a known way using an analytical ultracentrifuge (W. Machtle, Makromolekulare Chemie 185 (1984), pp. 1025-1039). The number average particle size of the aqueous polymer 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.

[0029] The aqueous polymer dispersion may have a pH of 4.5, more preferably a pH of between 5 and 8.

III. Methods of Making Aqueous Polymer Dispersions for Pressure Sensitive Adhesives [0030] The adhesive polymers of the invention are obtainable by radical polymerization of ethylenically unsaturated compounds (monomers). The polymers are prepared preferably by emulsion polymerization and are therefore preferably emulsion polymers. The invention, accordingly, also provides pressure-sensitive adhesive dispersions comprising a pressure-sensitive adhesive polymer of the invention, prepared by emulsion polymerization, in dispersion in water.

[0031] In the emulsion polymerization, ethylenically unsaturated monomers are polymerized in water, with use of ionic and/or nonionic emulsifiers and/or protective colloids or stabilizers as surface-active compounds for stabilizing the monomer droplets and the polymer particles subsequently formed from the monomers. The surface-active substances are used customarily in amounts of 0.1 to 10 parts by weight, preferably 0.2 to 5 parts by weight, based on 100 parts by weight of the monomers to be polymerized. [0032] A comprehensive description of suitable protective colloids is found in Houben-Weyl, Methoden der organischen Chemie, volume XIV/1 , Makromolekulare Stoffe [macromolecular compounds], Georg-Thieme-Verlag, Stuttgart, 1961 , pp. 411 to 420. Emulsifiers contemplated include anionic, cationic, and nonionic emulsifiers.

Surface-active substances used are preferably emulsifiers, whose molecular weights, in contrast to those of the protective colloids, are customarily below 2000 g/mol. Where mixtures of surface-active substances are used, the individual components must of course be compatible with one another, something which in case of doubt can be checked using a few preliminary tests. Surface-active substances used are preferably anionic and nonionic emulsifiers. Common accompanying emulsifiers are, for example, ethoxylated fatty alcohols (EO degree: 3 to 50, alkyl radical: C8 to C36), ethoxylated mono-, di-, and trialkylphenols (EO degree: 3 to 50, alkyl radical: C4 to C9), alkali metal salts of dialkyl esters of sulfosuccinic acid, and also alkali metal salts and ammonium salts of alkyl sulfates (alkyl radical: C8 to C12), of ethoxylated alkanols (EO degree: 4 to 30, alkyl radical: C12 to C18), of ethoxylated alkylphenols (EO degree: 3 to 50, alkyl radical: C4 to C9), of alkylsulfonic acids (alkyl radical: C12 to C18), and of alkylarylsulfonic acids (alkyl radical: C9 to C18).

[0033] Further suitable emulsifiers are compounds of the general formula provided below in Formula 1.

Formula 1

[0034] In Formula 1 , R5 and R6 are hydrogen or C4 to C14 alkyl and not simultaneously hydrogen, and X and Y may be alkali metal ions and/or ammonium ions. Preferably, R5 and R6 are linear or branched alkyl radicals having 6 to 18 C atoms or hydrogen, and in particular having 6, 12, and 16 C atoms, with R5 and R6 not both simultaneously being hydrogen. X and Y are preferably sodium, potassium, or ammonium ions, with sodium being particularly preferred. Particularly advantageous compounds are those in which X and Y are sodium, R5 is a branched alkyl radical having 12 C atoms, and R6 is hydrogen or R5. Use is frequently made of technical mixtures having a fraction of 50 to 90 wt.% of the monoalkylated product. Commercial products of suitable emulsifiers are, for example, Dowfax®2 A1 , Emulan® NP 50, Dextrol® OC 50, Emulgator 825, Emulgator 825 S, Emulan® OG, Texapon® NSO, Nekanil® 904 S, Lumiten® l-RA, Lumiten® E 3065, Disponil® FES 77, Lutensol® AT 18, Steinapol® VSL, Emulphor® NPS 25. For the present invention, ionic emulsifiers or protective colloids are preferred. With particular preference they are ionic emulsifiers, more particularly salts and acids, such as carboxylic acids, sulfonic acids, and sulfates, sulfonates or carboxylates. In particular, use may also be made of mixtures of ionic and nonionic emulsifiers.

[0035] The emulsion polymerization may be started using water-soluble initiators. Water-soluble initiators are, for example, ammonium salts and alkali metal salts of peroxo-disulfuric acid, sodium peroxodisulfate for example, hydrogen peroxide, or organic peroxides, tert-butyl hydroperoxide for example. Other suitable initiators include those called reduction-oxidation (redox) initiator systems. The redox initiator systems consist of at least one, usually inorganic reducing agent and an organic or inorganic oxidizing agent. The oxidizing component comprises, for example, the initiators already stated above for the emulsion polymerization. The reducing components comprise, for example, alkali metal salts of sulfurous acid, such as sodium sulfite, sodium hydrogensulfite, alkali metal salts of disulfurous acid such as sodium disulfite, bisulfite addition compounds of aliphatic aldehydes and ketones, such as acetone bisulfite, or reducing agents such as hydroxymethanesulfinic acid and its salts, or ascorbic acid. The redox initiator systems may be used along with soluble metal compounds whose metallic component is able to exist in a plurality of valence states. Examples of customary redox initiator systems include ascorbic acid/iron(l I) sulfate/sodium peroxidisulfate, tert-butyl hydroperoxide/sodium disulfite, tert-butyl hydroperoxide/Na hydroxymethanesulfinic acid. The individual components, the reducing component for example, may also be mixtures, an example being a mixture of the sodium salt of hydroxymethanesulfinic acid and sodium disulfite.

[0036] The stated initiators are used usually in the form of aqueous solutions, with the lower concentration being determined by the amount of water that is acceptable in the dispersion, and the upper concentration by the solubility of the respective compound in water. Generally speaking, the concentration of the initiators is 0.1 to 30 wt.%, preferably 0.2 to 20 wt.%, more preferably 0.4 to 2 wt.%, based on the monomers to be polymerized. It is also possible for a plurality of different initiators to be used in the emulsion polymerization.

[0037] In the polymerization, chain transfer agents are used in amounts of at least 0.01 part by weight of chain transfer agent per 100 parts by weight of monomers, e.g., from 0.01 to 0.8 part by weight, or from 0.01 to 0.1 part by weight, per 100 parts by weight of the monomers to be polymerized. Using these agents, through a chain termination reaction, the molar mass of the emulsion polymer can be controlled or reduced. These agents are bonded to the polymer in the process, generally to the chain end.

[0038] Suitable chain transfer agents are, for example, organic compounds containing sulfur in bonded form (e.g., compounds with a thiol group), aliphatic and/or araliphatic halogen compounds, aliphatic and/or aromatic aldehydes, unsaturated fatty acids (e.g., oleic acid), dienes having nonconjugated double bonds (such as divinylmethane, terpinolene, or vinylcyclohexene, for example), hydrocarbons having readily abstractable hydrogen atoms (such as toluene, for example), organic acids and/or their salts (such as formic acid, sodium formate, ammonium formate, for example), alcohols (such as isopropanol, for example), and phosphorus compounds (such as sodium hypophosphite, for example). Also possible, however, is the use of mixtures of mutually nondisrupting chain transfer agents as stated above. The chain transfer agents are generally low molecular mass compounds with a molar weight of less than 2000, more particularly of less than 1000 g/mol. It is advantageous to supply a portion or the entirety of the chain transfer agents to the aqueous reaction medium before the radical polymerization is initiated. Furthermore, a portion or the entirety of the radical chain transfer compound may also be advantageously supplied to the aqueous reaction medium together with the monomers, during the polymerization.

[0039] Organic compounds having a thiol group are, for example, primary, secondary 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-hydroxy-ethanethiol, aromatic thiols, such as benzenethiol, ortho-, meta-, or para-methyl-benzenethiol, mercaptoalkyl esters of, for example, C2 to C4 carboxylic acids, having 1 to 18 C atoms in the alkyl group, as for example 2-mercaptoethyl propionate, and also all further sulfur compounds described in Polymer Handbook, 3rd edition, 1989, J. Brandrup and E. H. Immergut, John Wiley & Sons, Section II, pages 133 to 141. Preferred organic compounds comprising 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-mercapto-propanol, 3- mercaptopropane-1 ,2-diol, 1 ,4-mercaptobutanol, thioglycolic acid, 3-mercaptopropionic acid, mercaptosuccinic acid, thioacetic acid, and thiourea. Particularly preferred thio compounds are tert.-butyl mercaptan, ethyl thioglycolate, mercaptoethanol, mercaptopropyltrimethoxysilane or tert-dodecyl mercaptan.

[0040] Aliphatic and/or araliphatic halogen compounds are, for example, n-butyl chloride, n-butyl bromide, n-butyl iodide, methylene chloride, ethylene dichloride, 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.

[0041] In the polymerization, an oxidizer may be added to help promote the low gel content and high molecular weight of the resulting copolymers. The oxidizer may be an inorganic oxidizing agent. Particularly preferred examples are sodium persulfate, ammonium persulfate, and potassium persulfate. In a particularly preferred embodiment, the oxidizer is free of tert-butyl hydroperoxide.

[0042] The selected oxidizer 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.

[0043] The emulsion polymerization takes place in general at 50 to 110°C, preferably at 50 to 90°C. The polymerization medium may consist either of water alone or else of mixtures of water and liquids miscible therewith such as methanol. Preference is given to using water alone. The emulsion polymerization may be carried out either as a batch operation or in the form of a feed process, including staged or gradient regimes. The feed process is preferred, in which a portion of the polymerization batch is introduced as an initial charge, and is heated to the polymerization temperature and its polymerization commenced, and then the remainder of the polymerization batch is supplied to the polymerization zone, customarily via a plurality of spatially separate feeds, of which one or more comprise the monomers in pure form or in emulsified form, this supply taking place continuously, in stages, or subject to a concentration gradient, with the polymerization being maintained. In the polymerization it is also possible for a polymer seed to be included in the initial charge, for the purpose of more effective setting of the particle size, for example. [0044] The manner in which the initiator is added to the polymerization vessel in the course of the radical aqueous emulsion polymerization is known to a person of ordinary skill in the art. It may both be included in its entirety in the initial charge to the polymerization vessel or introduced continuously or in stages at the rate at which it is consumed in the course of the radical aqueous emulsion polymerization. Individually, this is dependent on the chemical nature of the initiator system and also on the polymerization temperature. Preference is given to including part in the initial charge and supplying the remainder to the polymerization zone at the rate at which it is consumed. In order to remove the residual monomers, it is customary to add initiator after the end of the actual emulsion polymerization as well, i.e. , after a monomer conversion of at least 95%. In the case of the feed process, the individual components may be added to the reactor from above, at the side, or from below, through the reactor bottom.

[0045] The emulsion polymerization may also be optionally followed by a drying step to remove moisture. This drying step may also be combined with a further optional crosslinking step which may occur either during or after the drying step. The crosslinking step may involve activation of a crosslinking agent by the drying process. Suitable crosslinkers include dihydrazides, diamines, polyisocyanates, melamineformaldehyde resins, and metal ion salts. The crosslinking step may also comprise the use of ultraviolet (UV) light, increased temperature, or addition of a catalyst.

[0046] In the emulsion polymerization, aqueous dispersions of the polymer with solids contents generally of 15 to 75 wt.%, preferably of 40 to 75 wt.%, are obtained. For a high space/time yield of the reactor, dispersions with an extremely high solids content are preferred. In order to be able to achieve solids contents >60 wt.%, a bimodal or polymodal particle size ought to be established, since otherwise the viscosity becomes too high and the dispersion can no longer be managed. Producing a new generation of particles can be accomplished, for example, by adding seed (EP 81083), by adding excess amounts of emulsifier, or by adding miniemulsions. Another advantage associated with the combination of low viscosity and high solids content is the improved coating characteristics at high solids contents. Producing one or more new generations of particles is something which can be done at any point in time. This time is guided by the particle size distribution that is desired for a low viscosity.

IV. Pressure Sensitive Adhesive Compositions

[0047] A pressure-sensitive adhesive composition of the invention comprises the pressure-sensitive adhesive polymers preferably in the form of the aqueous polymer dispersion as obtainable or obtained by the emulsion polymerization described above. The pressure-sensitive adhesive compositions may consist solely of the polymers or of the aqueous dispersion of the polymers. Alternatively, the PSA may include further adjuvants as well, examples being fillers, dyes, flow control agents, thickeners, preferably associative thickeners, defoamers, crosslinkers, 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. For more effective wetting of surfaces, the PSAs may comprise, in particular, wetting assistants (wetting agents), examples being fatty alcohol ethoxylates, alkylphenol ethoxylates, nonylphenol ethoxylates, polyoxyethylenes/propylenes, or sodium dodecylsulfonates. The amount of adjuvants is generally 0.05 to 5 parts by weight, more particularly 0.1 to 3 parts by weight, per 100 parts by weight of polymer (solid).

[0048] A tackifier is a polymeric or oligomeric adjuvant for adhesive polymers or, generally, for elastomers, which increases their autoadhesion (tack, inherent stickiness, self-adhesion), meaning that they adhere firmly to surfaces after brief, gentle applied pressure. Tackifiers are, for example, natural resins, such as rosins, and their derivatives formed by disproportionation or isomerization, polymerization, dimerization and/or hydrogenation, or terpene resins. They may be present in their salt form (with, for example, monovalent or polyvalent counterions (cations)) or, preferably, in their esterified form. Alcohols used for the esterification may be monohydric or polyhydric. Examples are methanol, ethanediol, diethylene glycol, triethylene glycol, 1 ,2,3- propanetriol, and pentaerythritol. Also used, furthermore, 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, alpha-methylstyrene, and vinyltoluene.

[0049] Also being used increasingly as tackifiers are polyacrylates which have a low molar weight. These polyacrylates preferably have a weight-average molecular weight Mw below 50,000, more particularly below 30,000. The polyacrylates consist preferably to an extent of at least 60 wt.%, more particularly at least 80 wt.%, of C1-C8 alkyl (meth)acrylates. Suitability is possessed, for example, by the low molecular mass 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 greater than or equal to -40°C. to less than or equal to 0°C., preferably of greater than or equal to -35°C. to less than or equal to 0°C., preparable by emulsion polymerization in the presence of at least one chain transfer agent and preparable from a monomer mixture comprising at least 40 wt.% of at least one C1 to C20 alkyl (meth)acrylate.

[0050] Preferred tackifiers are natural or chemically modified rosins. Rosins consist predominantly of abietic acid or derivatives of abietic acid. The tackifiers can be added simply to the polymer dispersion. In this case the tackifiers themselves are preferably in the form of an aqueous dispersion. The amount by weight of the tackifiers is preferably 5 to 100 parts by weight, more preferably 10 to 50 parts by weight, based on 100 parts by weight of polymer (solid/solid).

[0051] The pressure-sensitive adhesive polymer and the pressure-sensitive adhesive dispersion may be used for producing self-adhesive articles. The articles are coated at least partly with the PSA. After they have been bonded, the self-adhesive articles are preferably removable. The self-adhesive articles may be, for example, sheets, tapes or labels. Suitable backing materials are paper, polymeric films, and metal foils, for example. Self-adhesive tapes of the invention may be single- or double-sided coated tapes comprising the above substances. Particularly preferred are self-adhesive labels. Self-adhesive labels of the invention may be labels of paper or of a thermoplastic film. Thermoplastic films contemplated include, for example, films of polyolefins (e.g., polyethylene, polypropylene), polyolefin copolymers, films of polyesters (e.g., polyethylene terephthalate) or polyacetate. The surfaces of the thermoplastic polymer films are preferably corona treated. The labels are coated on a single side with adhesive.

[0052] Preferred substrates for the self-adhesive articles are paper and polymer films. Particularly preferred self-adhesive articles are paper labels.

[0053] The self-adhesive articles are coated on at least one surface, at least partly, with a PSA of the invention. The adhesive dispersion may be coated via a multitude of common coating methods utilized in a variety of markets including roll label, tapes and specialty tape, graphics, and flexible packaging. These methods include Mayer rod, slot die, direct gravure, reverse gravure, single chamber reverse gravure and dual chamber reverse gravure, pan fed or nip fed roll coating, knife over roll coating, curtain coating and slide curtain coating. The choice of coating method will generally depend on desired line speed, coating thickness, and degree of accuracy needed in cross and machine direction of web in coating.

[0054] Adhesive dispersions are generally coated either directly to a face stock material (this may be paper, film, fabric), or more commonly, coated to a release coated film or paper substrate (release liner) that is previously coated with specially formulated silicone chemistry material (solvent based, water based, 100% solids technology) that is cured via thermal drying methods, UV light or electron beam (EB). The release coating may be coated in line during adhesive dispersion coating application, or the process may use a pre-release coated liner.

[0055] The coated adhesive dispersion requires a heat source to remove the water from the dispersion and allow coalescence of the dispersion into a continuous strong film. This is typically accomplished in passing the coated adhesive dispersion through gas fired ovens that may employ multiple zones, set at differing temperatures (50 to 150°C, for example), and air velocities to drive off the moisture slowly, but effectively. In some cases, the drying step may also employ the usage of infrared (IR) heat sources to assist with the drying. Moisture content is reduced to a level such that an optimum balance of adhesive flow, adhesion and cohesion is obtained; this is generally at a moisture level < 3% with respect to the dried dispersion weight, but dependent on product performance needed. More preferable are moisture levels at 2% or less. [0056] The coat weight is preferably 0.1 to 175 g, more preferably 2 to 20 g, of solids per m². The dried adhesive coating will be laminated to an appropriate substrate by passing through a pressure nip laminator. For example, if coated to a release liner, the coated release liner will be laminated to a face stock, which may be either paper or film type. In the case of a self-wound tape, the release liner may have release coating on both sides, having a differential release force for unwind, and in this case no face stock is involved.

[0057] The coated substrates obtained accordingly are used, for example, as self-adhesive articles, such as labels, linerless labels, adhesive tapes or sheets. For this purpose, before or after the adhesive has been applied, the backings may be slit to form adhesive tapes, labels or sheets. For subsequent use, the PSA-coated side of the substrates may be lined with a release paper, such as with a siliconized paper, for example.

The substrates to which the self-adhesive articles may be advantageously applied may comprise, for example, metal, wood, glass, paper or plastic. The self-adhesive articles are suitable in particular for bonding on packaging surfaces, cartons, plastic packaging, books, windows, motor vehicle bodies or bodywork parts. Preferred substrates are self- adhesive labels, more particularly self-adhesive paper labels and self-adhesive film labels. The backing material is paper or polymer film and has a first surface and a second surface, with the first surface being self-adhesive and being at least partly coated with a PSA of the invention, while the second surface may be printed, or the second surface or the label may be at least partly colored. Coloring may have been brought about, for example, by a colored coating with pigments or dyes, by colored printing or, in thermal papers, by exposure to heat.

EXAMPLES

Example 1

[0058] Example 1 is prepared according to the following procedure. A two-liter reactor equipped with a condenser, a mechanical stirrer, a temperature-controlled thermocouple and inlets for initiators and monomers, is fed with 168 g of deionized ("DI") water and 4.5 g of a polystyrene seed (32%), and heated to 85°C. In a separate container, a monomer emulsion is 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 comprising 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"). Next, a solution of a mixture of 2.3 g of sodium persulfate ("NaPS") in 73 g DI water is added into the reactor. Immediately after addition of the solution of NaPS, the monomer emulsion is fed into the reactor. The feeding proceeds for 200 minutes. Upon completion of the monomer emulsion addition, the reaction mixture is cooled to 80°C before gradual addition of a solution of 0.65 g NaPS in 8.6 g DI water, and a solution of 1 g sodium metabisulfite in 8.2 g DI water, via two separate feeds over 60 minutes. Upon completion of the feeds, the reaction is cooled to room temperature prior to adding 4.7 g of Aerosol OT 70 PG and 12 g of DI water. The obtained dispersion is then filtered through a 150 pm mesh filter cloth prior to subsequent evaluation. The obtained dispersion has a glass transition temperature of -58°C, a gel content of 2 wt.%, and a weight-average molecular weight of 603 kDa. The polymer dispersion is adjusted to a pH of 7 or greater by addition of aqueous ammonia, and a solution of 0.46 g adipic acid dihydrazide (ADH) and 46 g of DI water is then added to the dispersion.

Example 2

[0059] Example 2 is prepared according to the same procedure as Example 1 , but with 2.0 g of sodium persulfate as initiator. The obtained dispersion has a gel content of 4 wt.%, and a weight-average molecular weight of 548 kDa. The polymer dispersion is adjusted to a pH of 7 or greater by addition of aqueous ammonia, and a solution of 0.46 g adipic acid dihydrazide (ADH) and 46 g of DI water is then added to the dispersion.

Example 3

[0060] Example 3 is prepared according to the same procedure as Example 1 , but with 3 wt.% of styrene ("STY"), and 1 .5 wt.% of methacrylic acid ("MAA") in the monomer feed. The obtained dispersion has a gel content of 33 wt.%, and a weightaverage molecular weight of 354 kDa. The polymer dispersion is adjusted to a pH of 7 or greater by addition of aqueous ammonia, and a solution of 0.46 g adipic acid dihydrazide (ADH) and 46 g of DI water is then added to the dispersion.

Example 4

[0061] Example 4 is prepared according to the same procedure as Example 1 , but with 3 wt.% of styrene ("STY"), and 1 .5 wt.% of 2-hydroxypropyl acrylate ("HPA") in the monomer feed. The obtained dispersion has a gel content of 1 wt.%, and a weightaverage molecular weight of 317 kDa. The polymer dispersion is adjusted to a pH of 7 or greater by addition of aqueous ammonia, and a solution of 0.46 g adipic acid dihydrazide (ADH) and 46 g of DI water is then added to the dispersion.

Comparative Example 1

[0062] Comparative Example 1 is prepared according to the same procedure as Example 1 , but with 3 wt.% of styrene ("STY"), and 1 .5 wt.% of acrylamide ("AM") in the monomer feed. The obtained dispersion has a gel content of 77 wt.%, and a weightaverage molecular weight of 314 kDa.

Comparative Example 2

[0063] The acrylic emulsion used as Comparative Example 1 is commercially available from BASF Corporation under the trade name Acronal® NX 2160.

Comparative Example Emulsion 1 has a glass transition temperature of -58°C, a gel content of 54 wt.%, and a weight-average molecular weight of 250 kDa.

Characterization of Dispersions

[0064] Emulsion polymerizations were characterized in terms of non-volatile content (NV%) and Brookfield viscosity. NV% was measured gravimetrically using a CEM Smart System 5 Microwave Moisture Analyzer. Viscosity was determined with a Brookfield RV viscometer at 60 RPM (spindle 63). Particle size was determined using a DLS Microtrac with 180° back-scatter angle.

[0065] Molecular weight distributions were characterized by gel permeation chromatography (GPC) relative to a polystyrene calibration curve. 30 mg of sample was dissolved into 10 mL of THF, and 100 l of this solution is filtered and injected to the columns operating at a flow rate of 1 .0 mL/min. The column set consisted of a guard column (Agilent PL1110-1120 PLgel 10 pm guard, 50 x 7.5mm) and two analytical columns (Agilent PL1110-6100 PLgel Mixed-B, 300 x 7.5mm) in series.

[0066] Percent gel is a measure of the insoluble gel remaining after soaking. A weighed dried polymer film is placed inside a 100-mesh (149 pm) metal “Harris” cage and allowed to stand for 48 hours in an excess of tetrahydrofuran (THF). After extraction for 48 hours, the cage is removed from the solvent, and the polymer film is dried and reweighed. This weight is divided by the original weight to arrive at a gel percentage.

Samples are run in duplicate with the results averaged.

Characterization of Adhesive Properties (A-F)

[0067] (A) Formulation of Adhesive:

[0068] The polymer dispersion is adjusted to a pH of 7 or greater by addition of aqueous ammonia. In some cases, crosslinking of the adhesive after coating and drying is achieved by addition of adipic acid dihydrazide (ADH) to the pH-adjusted dispersion, such that the ADH may react with the polymerized DAAM monomer after evaporation of water and ammonia.

[0069] (B) Lab Coating of Adhesive:

[0070] The formulated adhesive is coated on a 1.5 mil PET film at 18-22 g/m2 on a dry weight basis_and dried at 115°C for 4 minutes before laminating with a release liner. Adhesive performance testing is conducted after the adhesive laminate is conditioned in a controlled temperature and humidity (CTH) environment (23 ± 2°C and 50 ± 5% relative humidity) testing laboratory overnight.

[0071] (C) Adhesion/Peel Test:

[0072] Samples are tested on stainless steel, high density polyethylene

("HDPE"), and B-fluted corrugated board test panels according to Federation INtemationale des fabricants et transformateurs dAdhesifs et Thermocollants ("FINAT") Test Method 1 (“FTM 1”). Testing at freezing (-20°C) and hot (50°C) conditions is achieved by conditioning, after application of adhesive strip to the test panel at CTH, in a freezer or oven, respectively; the peel test is performed immediately after removal from the temperature conditioning, such that the test panel remains within 3°C of the conditioning temperature. Test panels can be insulated with a foam sleeve to further reduce their temperature drift during testing.

[0073] (D) Adhesion/Tack Test:

[0074] FINAT Test Method 9 (“FTM 9”) is used for the "Loop-tack" Initial Adhesion test on stainless steel, high density polyethylene ("HDPE"), and B-fluted corrugated board test panels.

[0075] (E) Cohesion/Shear Test:

[0076] FINAT Test Method 8 (“FTM 8”) is used for the shear resistance test on stainless steel panels._A qualitative failure mode is recorded for each test: "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 both test panel and facestock. "SF" indicates substrate failure, i.e. tearing of the substrate panel.

[0077] (F) Mandrel Hold Test:

[0078] FINAT Test Method 24 (“FTM 24”) is adapted for the “Mandrel Hold” adhesion test on HDPE rods with diameter 0.5”. Here we report “lift” as the shortest distance between the lifting label edge and the rod surface, rather than the length of label no longer in contact with the rod. The results of this test are highly dependent on the facestock stiffness; a 50# (3300 ft² wet strength) paper is laminated to the backside of each sample to provide a high uniform stiffness.

Table 1

Peel Force, Ib/in, Average of 5 Tests

Table 2

Loop Tack, Ib/in, Average of 5 Tests

Table 3

Static Shear, hours, Average of 5 Tests

Table 4

Mandrel Lift, mm, Average of 4 Corners

Claims

C LAI MS:

1 . A pressure-sensitive adhesive composition in the form of an aqueous polymer dispersion comprising at least one copolymer comprising:

(i) 50 to 95% by weight of at least one soft (meth) acrylic acid ester monomer which, when polymerized as homopolymer, has a glass transition temperature of less than 0°C,

(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,

2. The pressure-sensitive adhesive composition as claimed in claim 1 , wherein the copolymer comprises 50 to 70 wt.% of at least one soft (meth) acrylic acid ester monomer which, when polymerized as homopolymer, has a glass transition temperature of less than 0°C, based on the total weight of the monomers.

3. The pressure-sensitive adhesive composition according to claim 1 , wherein the soft (meth) acrylic ester 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 further ethylenically unsaturated compounds (v) are selected from the group consisting of C1 to C20 alkyl (meth) acrylates, hydroxyl- containing monomers, vinyl esters of carboxylic acids containing up to 20 carbon atoms, vinyl aromatic compounds having up to 20 carbon atoms other than styrene, ethy lenical ly unsaturated nitriles, vinyl halides, vinyl ethers of alcohols having from 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds.

5. The pressure sensitive adhesive as claimed in any one of the preceding claims, wherein the copolymer has a gel content of from 0.0001 to 20 wt.% based on the total weight of the composition.

6. The pressure sensitive adhesive as claimed in any one of the preceding claims, wherein the copolymer has gel content of from 0.0001 to 10 wt.% based on the total weight of the composition.

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

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

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

10. The pressure sensitive adhesive as claimed in any one of the preceding claims, wherein the copolymer is an emulsion polymer.

11 . The pressure sensitive adhesive as claimed in any one of the preceding claims, wherein the copolymer further comprises a crosslinkable monomer.

12. The pressure sensitive adhesive as claimed in claim 11 , wherein the crosslinkable monomer is selected from the group consisting of diacetone (meth) acrylamide, acetoacetoxyethyl methacrylate, N-methoyl (meth) acrylamide, and glycidyl methacrylate.

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

14. The pressure sensitive adhesive as claimed in 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 as claimed in any one of the preceding claims.

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

17. The self-adhesive article as claimed in claim 15 or 16, having a peel force on a steel panel of from 0.5 to 10 Ib/in at 50°C, according to the FTM 1 test method.

18. The self-adhesive article as claimed in claim 15 or 16, having a peel force on a

HDPE panel of from 1.5 to 10 Ib/in at 23°C, according to the FTM 1 test method.

19. The self-adhesive article as claimed in claim 15 or 16, having a peel force on a HDPE panel of from 0.25 to 10 Ib/in at 50°C, according to the FTM 1 test method.

20. The self-adhesive article as claimed in claim 15 or 16, having a peel force on a corrugate board panel of from 0.8-10 Ib/in at 50°C, according to the FTM 1 test method.

21. The self-adhesive article as claimed in claim 15 or 16, having a tack adhesion on a steel panel of 2.0-10 Ib/in, according to the FTM 9 test method.

22. The self-adhesive article as claimed in claim 15 or 16, wherein having a mandrel lift on a 0.5” HDPE rod is 0.01 -0.7 mm after 1 day, according to the FTM 24 test method.

23. The self-adhesive article as claimed in claim 15 or 16, wherein having a mandrel lift on a 0.5” HDPE rod is 0.01 -0.95 mm after 7 days, according to the FTM 24 test method.

24. A method for preparing an aqueous polymer dispersion, comprising: polymerizing a mixture of monomers comprising:

(iii) 0.5 to 20% by weight of styrene,

(v) 0 to 10% by weight of other ethylenically unsaturated compounds other than the monomers (i) to (iv), wherein the amounts of the monomers each relate to the total amount of monomers, producing a copolymer having a polymer gel content less than 35 wt.%, based on the total weight of the composition, and a weight-average molecular weight greater than 200 kDa.

25. The method of claim 24, wherein the copolymer comprises 50 to 70 wt.% of at least one soft (meth) acrylic acid ester monomer which, wherein the soft (meth) acrylic acid ester when polymerized as homopolymer, has a glass transition temperature of less than 0°C, based on the total weight of the monomers.

26. The method of claim 24 or claim 25, further comprising adding an oxidizer during the polymerization.

27. The method of claim 26, wherein the oxidizer is an inorganic oxidizing agent.

28. The method of claim 26, wherein the oxidizer is selected from the group consisting of sodium persulfate, ammonium persulfate, and potassium persulfate.

29. The method of claim 27, wherein the oxidizer is free of tert-butyl hydroperoxide.

30. The method of any of claims 26-29, wherein the oxidizer is present in an amount of 0.001 - 0.5 wt.%, based on the total weight of the monomers.

31 . The method of any of claims 24-30, wherein the polymerization step is conducted at a temperature of 50-110°C

32. The method of any of claims 24-31 , wherein the polymerization step is followed by a drying step.

33. The method of any of claims 24-32 wherein the drying step further comprises a crosslinking step.

34. The method of claim 33, wherein the crosslinking step comprises adding a crosslinking agent selected from the group consisting of dihydrazides, diamines, polyisocyanates, melamine-formaldehyde resins, and metal ion salts.

35. The method of claim 33, wherein the crosslinking step comprises use of UV light, increased temperature, or addition of a catalyst.

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

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