WO2014033545A1

WO2014033545A1 - Polymer dispersions having multimodal particle size distribution

Info

Publication number: WO2014033545A1
Application number: PCT/IB2013/002486
Authority: WO
Inventors: Martin Jakob; Marcel RUPPERT
Original assignee: Celanese Emulsions Gmbh
Priority date: 2012-08-31
Filing date: 2013-08-28
Publication date: 2014-03-06
Also published as: CN104379610A; EP2890723A1

Abstract

An aqueous polymer dispersion having a multimodal particle size distribution IS produced by emulsion polymerization of a mixture comprising at least one free-radically polymerizable monomer and a stabilizer composition comprising at least two different stabilizers, wherein one of the stabilizers comprises a polyvinyl alcohol having acid groups attached to its polymer backbone.

Description

POLYMER DISPERSIONS HAVING MULTIMODAL PARTICLE SIZE

DISTRIBUTION

FIELD

[0001] This invention relates to polymer dispersions having multimodal, and especially bimodal, particle size distribution.

BACKGROUND

[0002] Polymer dispersions having multimodal, and especially bimodal, particle size distribution are employed in a variety of applications including, for example, as binders for adhesives, paints and paper coating compositions. Multimodal polymer dispersions offer advantages over their monomodal counterparts in that they generally allow higher solids contents without the dispersion becoming overly viscous or dilatant. For example, with monomodal polymer dispersions it is difficult to achieve a solids content of 60% by weight without viscosity problems, whereas stable multimodal dispersions having acceptable viscosity properties can readily be produced at solids levels above 70% by weight. This difference is important since, for example, in the case of adhesive formulations, a higher binder solids content generally leads to a shorter setting time, whereas excessive viscosity is detrimental to the brushability of the formulation

[0003] Currently, multimodal polymer dispersions are generally produced by adding one or more seed polymers, that is the products of earlier polymerization processes, to the monomer mixture being used to produce the polymer dispersion. An example of the use of seeds in the production of multimodal polymer dispersions is disclosed in U.S. Patent No. 6,852,81, in which the emulsion polymerization of at least one ethylenically unsaturated monomer is conducted in the presence of at least one surfactant and of a seed of polymer particles with a diameter of between 200 and 450 nm. The seed particles represent from 5- 25% by weight of the total weight of the monomer and seed and the polymerization is initiated by a mixed water-soluble/fat-soluble system. However, this and other seed-based processes suffer from the problem of requiring the costly and inconvenient pre-production of the seed polymer.

[0004] An alternative process for producing multimodal polymer dispersions, which avoids the use of seed polymers, is disclosed in U.S. Patent Application Publication No. 2005/0043463. This process involves the emulsion polymerization of at least two ethylenically unsaturated monomers in which the initial charge to the polymerization reactor comprises a mixture of (i) at least two polyvinyl acohols whose molecular weight differs by a factor of at least 1.5 and which preferably each have a molar degree of hydrolysis of at least 80% and (ii) at least one ionic comonomer, which is an α,β-monoethylenically unsaturated compound containing at least one group derived from a weak acid. However, in practice this process has proved difficult to control and so has found limited applicability.

[0005] According to the invention, it has now been found that multimodal polymer dispersions can be produced in a reproducible manner and without the use of seeds by an emulsion polymerization process employing a stabilizer system including at least two different stabilizers wherein at least one of the stabilizers comprises an acid-modified polyvinyl alcohol typically having a molar degree of hydrolysis of less than or equal to 86%, optionally together with a standard grade polyvinyl alcohol and/or a surfactant.

SUMMARY

[0006] In one aspect, the invention resides in an aqueous polymer dispersion having a multimodal particle size distribution produced by emulsion polymerization of a mixture comprising at least one free-radically polymerizable monomer and a stabilizer composition comprising at least two different stabilizers, wherein one of the stabilizers comprises a polyvinyl alcohol having acid groups attached to its polymer backbone.

[0007] Generally, the polyvinyl alcohol having acid groups attached to its polymer backbone has a molar degree of hydrolysis less than 86%, such as less than 80%, and is present in the mixture in an amount between 0.2 and 2.0 % by weight based on the total weight of monomer(s) in the mixture. The acid groups typically comprise carboxylic acid and/or sulfonic acid groups.

[0008] Generally, the stabilizer composition comprises at least one further stabilizer selected from at least one surfactant, at least one protective colloid and mixtures thereof. Conveniently, the at least one further stabilizer comprises at least one polyvinyl alcohol substantially free of acid groups and having a molar degree of hydrolysis of at least 86%. This is referred to herein as a standard grade polyvinyl alcohol.

[0009] In one embodiment, the at least one further stabilizer comprises a standard grade polyvinyl alcohol which can consist of one or more polyvinyl alcohols having viscosity(ies) selected to give the desired viscosity of the end product. A suitable standard grade polyvinyl alcohol would have a Hoppler viscosity value in a 4 weight % aqueous solution at 20°C of 4.8 to 5.8 mPa.s in combination with a further standard grade polyvinyl alcohol having a Hoppler viscosity value in a 4 weight % aqueous solution at 20°C of 27 to 33 mPa.s. [00010] Generally, the mixture also contains a redox initiator system comprising an oxidizing agent having a water solubility less than or equal to 15 weight %, such as an organic peroxide, preferably tert-butylhydroperoxide.

[00011] Generally, the at least one free-radically polymerizable monomer comprises a vinyl ester of a carboxylic acid with up to 20 carbons, preferably vinyl acetate, and in one embodiment comprises from 50 wt to 99 wt vinyl acetate, from 1 wt to 40 wt ethylene, and from 0 wt to about 10 wt of auxiliary co-monomers based on the total weight of monomers in the mixture.

[00012] Typically, the mixture is substantially free of seed polymer particles.

[00013] In one embodiment, the polymer dispersion has a polymer solids content of at least 55 weight % and comprises first polymer particles having an average diameter from 200 to 950 nm and second polymer particles larger than said first polymer particles and having an average diameter from 950 to 20,000 nm.

[00014] In further aspects, the invention resides in uses of the aqueous polymer dispersion described herein as a binder and in coating compositions and adhesives.

BRIEF DESCRIPTION OF THE DRAWINGS

[00015] Figure 1 is a photomicrograph of the bimodal particle size distribution of the polymer dispersion of Example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[00016] An aqueous polymer dispersion is described which has a multimodal particle size distribution and which is produced by emulsion polymerization of an aqueous mixture comprising at least one free-radically polymerizable monomer and a stabilizer composition comprising at least two different stabilizers, at least one of which comprises a polyvinyl alcohol having acid groups attached to its polymer backbone. The aqueous polymerization mixture can be substantially free of seed polymer particles, by which is meant that that the mixture contains less than 5 wt , preferably less than 1 wt and most preferably no added seed polymer particles.

Monomers

[00017] The aqueous polymerization mixture used to produce the present polymer dispersion comprises one or more free-radically polymerizable main monomers. Suitable main monomers are selected from Ci-C2o-alkyl (meth)acrylates, vinyl esters of carboxylic acids with up to 20 carbons, vinyl- aromatic compounds having up to 20 carbons, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of Ci-Cio alcohols, C₂-Cs aliphatic hydrocarbons with 1 or 2 double bonds, and mixtures of these monomers.

[00018] Preferred alkyl (meth)acrylates are Ci-Cio-alkyl (meth)acrylates, such as methyl methacrylate, methyl acrylate, n-butyl acrylate, ethyl acrylate and 2-ethylhexyl acrylate. Mixtures of alkyl (meth)acrylates can also be employed .

[00019] Examples of suitable vinyl esters of C1-C2₀ carboxylic acids include vinyl acetate, vinyl propionate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl 2-ethyl hexanoate and

Versatic acid vinyl esters, with vinyl acetate being particularly preferred.

[00020] Suitable vinyl-aromatic compounds include vinyltoluene, a- and p-methylstyrene, a-butylstyrene, 4-n-butylstyrene, 4-n-decyl-styrene and, preferably, styrene.

[00021] Examples of suitable nitriles include acrylonitrile and methacrylonitrile.

[00022] Suitable vinyl halides include chloro-, fluoro- or bromo-substituted ethylenically unsaturated compounds, such as vinyl chloride and vinylidene chloride.

[00023] Examples of vinyl ethers are vinyl methyl ether and vinyl iso-butyl ether, with preference being given to vinyl ethers of C1-C4 alcohols.

[00024] Examples of suitable C₂-C8 aliphatic hydrocarbons with one olefinic double bond include ethene and propene, whereas representative examples of C₂-Cs aliphatic hydrocarbons having two olefinic double bonds include butadiene, isoprene and chloroprene.

[00025] In one embodiment, the present polymer dispersion is produced from a mixture of free-radically polymerizable main monomers comprising from 50 wt to 99 wt vinyl acetate and from 1 wt to 40 wt ethylene.

[00026] In addition to the main monomers discussed above, the aqueous polymerization mixture used to produce the present polymer dispersion may comprise up to 10 wt of auxiliary co-monomer(s) based on the total weight of monomers in the mixture. Such auxiliary co-monomers can be those which promote better film or coating performance by the compositions herein or can provide films and coatings of desirable properties. Such desirable properties can include, for example, enhanced adhesion to surfaces or substrates, improved wet adhesion, better resistance to removal by scrubbing or other types of weathering or abrasion, and improved resistance to film or coating cracking. The optional co-monomers useful for incorporation into the emulsion copolymers of the compositions herein are those which contain at least one polymerizable double bond along with one or more additional functional moieties. Suitable auxiliary co-monomers include unsaturated organic acids, unsaturated silanes, glycidyl co-monomers, ureido co-monomers, co-monomers with crosslinkable functions, crosslinking co-monomers and combinations thereof.

[00027] Suitable auxiliary co-monomers including unsaturated organic acids comprise ethylenically unsaturated carboxylic acids and anhydrides and amides thereof, ethylenically unsaturated sulfonic acids, and ethylenically unsaturated phosphonic acids.

[00028] For example, the auxiliary monomer may comprise an ethylenically unsaturated

C3-C8 monocarboxylic acid and/or an ethylenically unsaturated C4-C8 dicarboxylic acid, together with the anhydrides or amides thereof. Examples of suitable ethylenically unsaturated C3-C8 monocarboxylic acids include acrylic acid, methacrylic acid and crotonic acid. Examples of suitable ethylenically unsaturated C4-C8 dicarboxylic acids include maleic acid, fumaric acid, itaconic acid and citraconic acid.

[00029] Examples of suitable ethylenically unsaturated sulfonic acids include those having 2-8 carbon atoms, such as vinylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2- acryloyloxyethanesulfonic acid and 2-methacryloyloxyethanesulfonic acid, 2-acryloyloxy- and 3-methacryloyloxypropanesulfonic acid. Examples of suitable ethylenically unsaturated phosphonic acids also include those having 2-8 carbon atoms, such as vinylphosphonic acid and ethylenically unsaturated polyethoxyalkyletherphosphates.

[00030] In addition to or instead of said acids, it is also possible to use the salts thereof, preferably the alkali metal or ammonium salts thereof, particularly preferably the sodium salts thereof, such as, for example, the sodium salts of vinylsulfonic acid and of 2- acrylamidopropanesulfonic acid.

[00031] Unsaturated silanes usful as auxiliary co-monomers can generally correspond to the structural Formula I:

Formula I

in which R denotes an organic radical olefinically unsaturated in the ω-position and R¹ R² and R³ which may be identical or different, denote the group -OZ, Z denoting hydrogen or primary or secondary alkyl or acyl radicals optionally substituted by alkoxy groups. Suitable unsaturated silane compounds of the Formula I are preferably those in which the radical R in the formula represents an ω-unsaturated alkenyl of 2 to 10 carbon atoms, particularly of 2 to 4 carbon atoms, or an ω-unsaturated carboxylic acid ester formed from unsaturated carboxylic acids of up to 4 carbon atoms and alcohols carrying the Si group of up to 6 carbon atoms. Suitable radicals R¹, R², R³ are preferably the group -OZ, Z representing primary and/or secondary alkyl radicals of up to 10 carbon atoms, preferably up to 4 carbon atoms, or alkyl radicals substituted by alkoxy groups, preferably of up to 3 carbon atoms, or acyl radicals of up to 6 carbon atoms, preferably of up to 3 carbon atoms, or hydrogen. Most preferred unsaturated silane co-monomers are vinyl trialkoxy silanes.

[00032] Examples of preferred silane compounds of the Formula I include γ- methacryloxypropyltris(2-methoxyethoxy)silane, vinylmethoxysilane, vinyltriethoxysilane, vinyldiethoxysilanol, vinylethoxysilanediol, allyltriethoxysilane, vinyltripropoxysilane, vinyltriisopropoxysilane, vinyltributoxysilane, vinyltriacetoxysilane, trimethylglycolvinylsilane, γ-methacryloxypropyltrimethylglycolsilane, γ- acryloxypropyltriethoxysilane and γ-methacryloxypropyltrimethoxysilane.

[00033] Glycidyl compounds can also be used as optional auxiliary co-monomers to impart epoxy-functionality to the emulsion copolymer. Examples of suitable glycidyl optional co-monomers include glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether, and vinyl glycidyl ether.

[00034] Another type of optional co-monomer comprises cyclic ureido co-monomers. Cyclic ureido co-monomers are known to impart improved wet adhesion properties to films and coatings formed from copolymers containing these co-monomers. Cyclic ureido compounds and their use as wet adhesion promoting co-monomers are disclosed in U.S. Patent Nos. 4,104,220; 4,111,877; 4,219,454; 4,319,032; 4,599,417 and 5,208,285. The disclosures of all of these U.S. patents are incorporated herein by reference in their entirety.

[00035] Another type of optional co-monomer comprises co-monomers with crosslinkable functions such as N-methylolacrylamide, N-methylolmethacrylamide, N- methylolallylcarbamate, N-methylolmaleimide, N-methylolmaleamic acid, and the N- methylol amides of aromatic vinyl carboxylic acids, such as N-methylol-p-vinylbenzamide. N-ethanol(meth)acrylamide, N-propanol(meth)acrylamide, the N-methylol esters or N-alkyl ethers or Mannich bases of N-methylol(meth)acrylamide or N-methylolallylcarbamate, acrylamidoglycolic acid and/or its salts, methyl acrylamidomethoxyacetate or N-(2,2- dimethoxy- 1 -hydroxyethyl)acrylamide.

[00036] A further group of comonomers suitable for preparing the emulsion polymers used herein comprises crosslinking monomers, such as comonomers with polyethylenic unsaturation, and hence with a crosslinking action. Examples include diallyl phthalate, diallyl maleate, triallyl cyanurate, tetraallyloxyethane, divinylbenzene, butane- 1,4-diol dimethacrylate, triethylene glycol dimethacrylate, divinyl adipate, allyl (meth)acrylate, vinyl crotonate, methylenebisacrylamide, hexanediol diacrylate, pentaerythritol diacrylate and trimethylolpropane triacrylate.

Stabilizers

[00037] In addition to the monomers described above, the aqueous polymerization mixture used to produce the present polymer dispersion comprises two or more different stabilizers. At least one of these stabilizers comprises a polyvinyl alcohol having acid groups, such as carboxylic acid and/or sulfonic acid groups, attached to its polymer backbone. Generally, this acid group-containing polyvinyl alcohol has a molar degree of hydrolysis less than 86%, such as less than 80%, and is present in the mixture in an amount between 0.2 and 2.0 % by weight based on the total weight of monomer(s) in the mixture. An example of a commercially available acid group-containing polyvinyl alcohol is Kuraray Poval™ KL-506, which is carboxylic acid group containing polyvinyl alcohol with 74-80 mol % degree of hydrolysis and a Hoppler viscosity in a 4 % aqueous solution at 20 °C according to DIN 53015 of 5.2-6.2 mPa*s.

[00038] Generally, the stabilizer composition comprises one or more further stabilizers selected from protective colloids, anionic and/or non-ionic surfactants and mixtures thereof. Generally, the further stabilizer(s) are present in the aqueous polymerization mixture in an amount between 0.5 and 15 % by weight based on the total weight of monomer(s) in said mixture.

[00039] Suitable protective colloids for use as one or more of the further stabilizer(s) include polyvinyl alcohols, polyvinyl pyrrolidone, methylcelluloses, hydroxyethyl- and propylcelluloses, and also sodium carboxymethylcellulose, gelatin, casein, starch, gum arabic, hydroxy ethyl starch and sodium alginate. A preferred protective colloid is polyvinyl alcohol. Suitable polyvinyl alcohols for use as a further stabilizer are substantially free of acid groups and have a molar degree of hydrolysis of at least 86. These standard grade polyvinyl alcohols have Hoppler viscosities which are chosen to match the desired end viscosity of the dispersion. Conveniently these standard grades comprise a mixture of at least two polyvinyl alcohols to adjust the viscosity of the polymer dispersion. In one embodiment, the at least one further stabilizer comprises a polyvinyl alcohol having a lower Hoppler viscosity value for example in a 4 weight % aqueous solution at 20°C of 4.8 to 5.8 mPa.s and a further polyvinyl alcohol having a higher Hoppler viscosity value for example in a 4 weight % aqueous solution at 20°C of 27 to 33 mPa.s. [00040] Suitable anionic surfactants include sodium, potassium and ammonium salts of straight-chain aliphatic carboxylic acids of chain length C12-C20, sodium hydroxyoctadecanesulfonate, sodium, potassium and ammonium salts of hydroxy fatty acids of chain length C12-C20 and their sulfation and/or acetylation products thereof, alkyl sulfates, also in the form of triethanolamine salts, alkyl-(Cio-C2o)-sulfonates, alkyl(Cio-C2o)- arylsulfonates, dimethyldialkyl-(C8-Ci8)-ammonium chloride, and sulfation products thereof, alkali metal salts of sulfosuccinic esters with aliphatic saturated monohydric alcohols of chain length C₄ -C16, sulfosuccinic 4-esters with polyethylene glycol ethers of monohydric aliphatic alcohols of chain length C₁₀-C₁₂ (disodium salt), sulfosuccinic 4-esters with polyethylene glycol nonylphenyl ether (disodium salt), sulfosuccinic acid biscyclohexyl ester (sodium salt), lignosulfonic acid and the calcium, magnesium, sodium and ammonium salts thereof, resin acids, hydrogenated and dehydrogenated resin acids and alkali metal salts thereof, sodium (dodecylated diphenyl ether) disulfonate and sodium laurylsulfate, or ethoxylated sodium lauryl ether sulfate. It is also possible to use mixtures of ionic emulsifiers.

[00041] Suitable nonionic surfactants include acyl, alkyl, oleyl and alkylaryl ethoxylates. Examples include ethoxylated mono-, di- and trialkylphenols (EO: from 3 to 50, alkyl substituted radical: C₄ to C₁₂) and ethoxylated fatty alcohols (EO: from 3 to 80; alkyl radical: C8 to C36), especially Ci2-Ci₄-fatty alcohol ethoxylates, C13-C15-OXO alcohol ethoxylates, C₁₆- Ci8-fatty alcohol ethoxylates, Cno-oxo alcohol ethoxylates, Ci-3-oxo alcohol ethoxylates, poly oxye thy lene sorbitanmonooleate with ethylene oxide groups, copolymers of ethylene oxide and propylene oxide with a minimum content of 10% by weight of ethylene oxide, the polyethylene oxide ethers of oleyl alcohol and the polyethylene oxide ethers of nonylphenol. Particularly suitable are the polyethylene oxide ethers of fatty alcohols, especially those of C₁₂-C₁₄-fatty alcohols.

Polymerization Process

[00042] The desired polymer dispersion is produced by free radical emulsion polymerization of the aqueous polymerization mixture described above in the presence of one or more free radical initiators.

[00043] A preferred free radical initiator which is used in the beginning of the polymerization is a redox initiator system comprising an oxidizing agent having a water solubility less than or equal to 15 weight %, since these are found to produce polymer dispersions with less agglomeration of the polymer particles and hence lower viscosity of the final polymer dispersion, as well as particles of more spherical shape. Examples of such oxidizing agents include organic peroxides, such as benzoyl peroxide, lauryl peroxide, i-butyl peroxide, azoisobutyronitrile and i-butyl hydroperoxide. The preferred oxidizing agent is t- butyl hydroperoxide. Alkali metal salts of oxymethanesulfinic acid, hydroxylamine salts, sodium dialkyldithiocarbamate, sodium bisulfite, ammonium bisulfite, sodium dithionite, diisopropyl xanthogen disulfide, ascorbic acid, tartaric acid, and isoascorbic acid can be used as reducing agents.

[00044] Typically, each of the oxidizing agent and the reducing agent is charged to the reaction mixture prior to polymerization in an amount from about 0.01% to about 1.0%, preferably from about 0.02% to about 0.5%, more preferably from about 0.025% to about 0.2%, by weight based on total weight of co-monomers. Generally, the molar ratio of oxidizing agent to reducing agent in the redox initiator system is from 10:1 to 1 :10.

[00045] The redox initiator system used herein can also optionally comprise catalyzing metal salts of iron, copper, manganese, silver, platinum, vanadium, nickel, chromium, palladium, or cobalt. These catalyzing salts may be used at levels of from about 0.1 to about 100 ppm, with or without metal complexing agents. Preferably iron or cobalt are used.

[00046] The polymerization may be carried out in one, two or more stages using any known polymerization reactor system, such as a batch, loop, continuous, or cascade reactor system.

[00047] The polymerization temperature generally ranges from about 20 °C to about 150 °C, more preferably from about 50 °C to about 120 °C. The polymerization generally takes place under pressure if appropriate, preferably from about 2 to about 150 bar, more preferably from about 5 to about 100 bar.

[00048] In a typical polymerization procedure involving, for example, vinyl acetate copolymer dispersions, the vinyl acetate, stabilizing system and any other co-monomers can be polymerized in an aqueous medium under pressures up to about 120 bar in the presence the specified stabilizers and initiators. The aqueous reaction mixture in the polymerization vessel can be maintained by a suitable buffering agent at a pH of about 2 to about 7.

[00049] The manner of combining the several polymerization ingredients, i.e., stabilizing system, co-monomers, initiator system components, etc., can vary widely. Generally an aqueous medium containing at least part of the stabilizing system can be initially formed in a polymerization vessel with the various other polymerization ingredients being added to the vessel thereafter.

[00050] Co-monomers can be added to the polymerization vessel continuously, incrementally or as a single charge addition of the entire amounts of co-monomers to be used. Co-monomers can be employed as pure monomers or can be used in the form of a pre-mixed emulsion. When present, ethylene as a co-monomer can be pumped into the polymerization vessel and maintained under appropriate pressure therein.

[00051] It is possible for the total amount of redox initiator system to be included in the initial charge to the reactor at the beginning of the polymerization. Preferably, however, a portion of the initiator is included in the initial charge at the beginning, and the remainder is added after the polymerization has been initiated, in one or more steps or continuously. In a preferred embodiment a first redox initiator system is used in the initial charge at the beginning of the polymerization comprising an oxidizing agent having a water solubility less than or equal to 15 weight %, The reaction is then continued by a second initator system which can be the same or different from the first redox initiator system. The second initiator system can be a redox initiator system or a thermal initiator system. In a preferred embodiment the reaction is continued by means of a different initiator system which can be a thermal initiator system comprising an oxidizing agent having a water solublity of greater than 15 weight %. Examples for such oxidizing agents are hydrogen peroxide, ammonium- sodium- or potassium persulfates, peroxidisulfates or water soluble azoamidines.

[00052] As mentioned previously, the present polymerization process is normally conducted without the use of a seed latex.

[00053] On completion of the polymerization, a further, preferably chemical aftertreatment, especially with redox catalysts, for example combinations of the abovementioned oxidizing agents and reducing agents, may follow to reduce the level of residual unreacted monomer on the product. In addition, residual monomer can be removed in known manner, for example by physical demonomerization, i.e. distillative removal, especially by means of steam distillation, or by stripping with an inert gas. A particularly efficient combination is one of physical and chemical methods, which permits lowering of the residual monomers to very low contents (<1000 ppm, preferably <100 ppm).

Characteristics of the Polymer Dispersion

[00054] The polymer dispersion produced by the present process comprises polymer particles having a multimodal particle size distribution. In one particular embodiment, the polymer dispersion comprises first polymer particles having an average diameter from 200 to 950 nm, typically from 350 to 800 nm, and second polymer particles larger than said first polymer particles and having an average diameter from 950 to 20,000 nm, typically from 1,500 to 10,000 nm. The first polymer particles are generally present in an amount from 10 to 90 wt % of the polymer solids, whereas the second polymer particles are generally present in an amount from 90 to 10 wt % of the polymer solids.

[00055] The aqueous copolymer dispersions as prepared herein will generally have a solids content of at least 55% by weight and a viscosity ofless than 40,000, preferably less than 5,000 mPa.s at 25 °C, as measured with a Brookfield viscometer at 20 rpm.

Uses of the Polymer Dispersion

[00056] The copolymer dispersion described herein can be formulated into coatings for a wide variety of materials such as paper, wood, concrete, metal, glass, ceramics, plastics, plaster, roofing substrates such as asphaltic coatings, roofing felts, foamed polyurethane insulation, polymer roof membranes, and masonry substrates such as brick, cinderblock, and cementitious layers, including exterior building cladding systems such as EIFS (Exterior Insulating Finishing Systems). The substrates include previously painted, primed, undercoated, worn, or weathered substrates. The method comprises contacting a surface of the substrate with the latex coating composition to form a coating; and drying the coating to harden the coating. The term "coating" as used herein broadly encompasses a thin film (e.g., a layer having a thickness of 0.02 to 5 millimeters, as would be formed using a paint or ink formulation) and thicker layers, for example thick bead of material (e.g., a bead having a thickness of 5 to 50 millimeters or more, as would be formed using a caulking or material). The term "coating" further includes continuous as well as patterned layers. The aqueous coating composition can be applied to the materials by a variety of techniques well known in the art such as, for example, curtain coating, sponge, brush, roller, mop, air-assisted or airless spray, electrostatic spray, caulking gun, ink jet, and the like. In one particlar embodiment, the copolymer dispersion can be formulated into an interior or exterior, wood or facade paint.

[00057] The copolymer dispersion described herein can also be used as a binder material for inorganic material like glass wool, mineral wool, glass fibers, textile and non-woven materials made from natural fiber, such as cellulose fiber, or synthetic fiber including but not limited to one or more of polyester, polyethylene, polypropylene and polyvinyl alcohol, or viscose fiber, or a combination of any of these. Further uses of the polymer dispersion as a binder include in the manufacture of building products, such as of chip board, particle board and paper board, and of insulating materials comprising paper fibers or plastic fibers. Another field of use is as a binder for glass fibers which are used, for example, for strengthening plastic tiles, moldings and as insulating material, or as binder for ceramic. [00058] Another field of application of the present copolymer dispersion is in formulating adhesives for use in bonding porous, non-porous and semiporous substrates. Specific examples include adhesive for bonding cellulosic substrates, such as wood, in particular solid wood or materials derived from wood, and engineering materials, for example veneers, plywood, laminated wood, glued layered wood, densified compressed wood, composite boards or wood fiber materials, such as porous, hard or medium density wood fiber boards, or plastic-coated decorative wood fiber boards. Further examples include the formulation of adhesives for bonding of paper, board, corrugated board, foam, cement, leather, textiles and impregnated laminates. Other applications are in adhesives for the construction sector, as floor, wall or ceiling adhesive or as furniture film or carpet backing adhesive.

[00059] The invention will now be more particularly described with reference to the accompanying drawing and the following non-limiting Examples, in which the following test methods and polymerization procedures were used.

Measurement of particle size distribution

[00060] The particle size distribution (PSD) measurements were conducted using a Mastersizer Microplus laser diffraction instrument from Malvern. The scatter data were evaluated using the "Polydisperse Mie" model provided by Malvern. The model was used under following assumptions: the refractive index of the dispersed polymer was assumed to be 1.456. All particle sizes refer to peak maxima of the volume (mass) average curve. Particle sizes are given in μιη. Particle fraction proportions were calculated from peak areas of the PSD curves.

[00061] In addition to the diffraction measurements, microscope pictures were taken by means of a differential interference contrast microscope from Leitz in order to evaluate the morphologies of the particles.

Viscosity measurements

[00062] Viscosities were measured using a Brookfield RVT viscosimeter at 23 °C temperature at 20 rpm. The test spindles used for these measurements are given for each measurement.

Polymerization Process

[00063] To carry out the polymerizations, a cylindrical glass vessel was used equipped with anchor stirrer, heating/cooling bath, feed inlets and electronic temperature control. [00064] An aqueous phase was prepared using 37 parts of deioinized water in which the following polyvinyl alcohols were dissolved at 90 °C for 1 hour:

• Kuraray Poval™ KL-506 - as described above;

• S-PVOH-1 - a standard grade polyvinyl alcohol having 86.5-89 mol % degree of hydrolysis and a Hoppler viscosity in a 4 % aqueous solution at 20 °C according to DIN 53015 of 4.8 - 5.8 mPa*s; and

• S-PVOH-2 - a standard grade polyvinyl alcohol having 86.5 -89 mol % degree of hydrolysis and a Hoppler viscosity in a 4 % aqueous solution at 20 °C according to DIN 53015 of 27 - 33 mPa*s.

[00065] After dissolution of the polyvinyl alcohols, the solutions were allowed to cool to room temperature. Prior to polymerization a second aqueous solution containing 0.95 parts (active) of a commercial nonionic surfactant (a mixture of ethoxylated linear fatty alcohols with average degree of ethoxylation of 30) in 25.3 parts deionized water, 0.8 parts formic acid (90 ) and 30 ppm (active) iron (III) chloride hexahydrate in water were added at room temperature. The pH of the combined aqueous phase was around 3.4 +/- 0.1.

[00066] 5 parts of a total 100 parts of vinyl acetate were added to the combined aqueous phase and the mixture heated to 60 °C, at which point starting initiator solutions were added according to the information given in respective Examples. The reaction temperature was then allowed to rise to 72 °C. After 30 minutes the remaining 95 parts of vinyl acetate were gradually added over a period of 4 hours and a solution of 0.375 active parts of hydrogen peroxide dissolved in 7 parts deionized water was added in parallel with the vinyl acetate addition over 4.25 hours. At the end of the monomer addition, the reaction temperature was raised to 78 °C and kept at this value for 1 hour. The batch was then cooled down and treated with tert-butyl hydroperoxide and sodium formaldehyde sufoxylate at 60 °C for demonomerization. Finally the emulsion was adjusted to pH 4 with sodium hydroxide solution to give a product with a solids contents of 57 +/- 1 %.

Example 1

[00067] To prepare the aqueous phase, 1.1 parts of S-PVOH-1, 0.47 parts of S-PVOH-2 and 0.95 parts of Poval KL-506 were used. The starting initiator solutions consisted of 0.05 parts sodium metabisulfite in 0.8 parts water and 0.05 active parts tert-butylhydroperoxide (70 ) in 1.2 parts water. [00068] The viscosity of the resulting emulsion was 2,520 mPa*s (spindle 3) at a solids content of 57.0 .

[00069] The PSD showed two distinct particle fractions with maxima at dw=0.58 μιη (40 ) and dw=3.60 μιη (60 ). The microscopic picture of this PSD is shown in Figure 1. The second particle fraction was found to consist of approximately spherical, largely unagglomerated particles.

Example 2

[00070] Example 1 was repeated, but the starting initiator solution consisted of 0.025 parts sodium metabisulfite in 0.8 parts water and 0.025 active parts tert-buty Hydroperoxide (70 ) in 1.2 parts water.

[00071] The viscosity of the resulting emulsion was 3,225 mPa*s (spindle 3) at a solids content of 57.2 .

[00072] The PSD showed two distinct particle fractions with maxima at dw=0.58 μιη (35 ) and. dw=6.63 μιη (65 ). The second particle fraction was found to consist of approximately spherical, largely unagglomerated particles.

Example 3

[00073] Example 1 was repeated, but the starting initiator solution consisted of 0.08 parts sodium metabisulfite in 0.8 parts water and 0.075 active parts tert-butylhydroperoxide (70 ) in 1.2 parts water.

[00074] The viscosity of the resulting emulsion was 3,100 mPa*s (spindle 3) at a solids content of 57.2 .

[00075] The PSD showed two distinct particle fractions with maxima at dw=0.31 μιη (48 ) and.dw=3.09 μιη (48 ). A smaller third fraction had a maximum at dw=16.57 (4%). The second particle fraction was found to consist of approximately spherical, largely unagglomerated particles.

Example 4

[00076] Example 1 was repeated, but the starting initiator solution consisted of 0.08 parts sodium metabisulfite in 0.8 parts water and 0.054 active parts of hydrogen peroxide (35 ) in 1.2 parts water.

[00077] The viscosity of the resulting emulsion was 12,300 mPa*s (spindle 5) at a solids content of 57.0 %. The product showed dilatant flow. [00078] The PSD showed two particle fractions with maxima at dw=0.27 μιη (25 ) and dw=1.95 μm (75 ). The microscopic photograph showed the second particle fraction contained a significant portion of non-sperical, agglomerated particles.

Comparative Example 1

[00079] Example 1 was repeated, but 2.05 parts of S-PVOH-1 and 0.47 parts of S-PVOH- 2 and no Poval KL-506 were used.

[00080] The viscosity of the resulting emulsion was 10,800 mPa*s (spindle 5) at a solids content of 56.1 %.

[00081] The PSD showed only one particle fraction with a maximum at dw=1.44 μιη.

Claims

CLAIMS:

1. An aqueous polymer dispersion having a multimodal particle size distribution produced by emulsion polymerization of a mixture comprising at least one free-radically polymerizable monomer and a stabilizer composition comprising at least two different stabilizers, wherein one of the stabilizers comprises a polyvinyl alcohol having acid groups attached to its polymer backbone.

2. The polymer dispersion of Claim 1, wherein said polyvinyl alcohol having acid groups attached to its polymer backbone has a molar degree of hydrolysis less than 86% preferably less than or equal to 80%.

3. The polymer dispersion of Claim 1 or Claim 2, wherein said polyvinyl alcohol having acid groups attached to its polymer backbone is present in said mixture in an amount between 0.2 and 2.0 % by weight based on the total weight of monomer(s) in said mixture.

4. The polymer dispersion of any one of the preceding Claims, wherein the stabilizer composition comprises at least one further stabilizer selected from at least one surfactant, at least one protective colloid and mixtures thereof.

5. The polymer dispersion of Claim 4, wherein the at least one further stabilizer comprises at least one polyvinyl alcohol substantially free of acid groups.

6. The polymer dispersion of Claim 4 or claim 5, wherein the at least one further stabilizer comprises at least one polyvinyl alcohol having a molar degree of hydrolysis of at least 86%.

7. The polymer dispersion of Claim 5 or Claim 6, wherein the at least one further stabilizer comprises at least two polyvinyl alcohols each having a different Hoppler viscosity value in a 4 weight % aqueous solution at 20°C.

8. The polymer dispersion of Claim 7, wherein the at least one further stabilizer comprises a polyvinyl alcohol having a Hoppler viscosity value in a 4 weight % aqueous solution at 20°C of 4.8 to 5.8 mPa.s and a further polyvinyl alcohol having a Hoppler viscosity value in a 4 weight % aqueous solution at 20°C of 27 to 33 mPa.s.

9. The polymer dispersion of any one of Claims 4 to 8, wherein the at least one further stabilizer also comprises an anionic surfactant.

10. The polymer dispersion of any one of Claims 4 to 9, wherein the at least one further stabilizer is present in said mixture in an amount between 0.5 and 15 % by weight based on the total weight of monomer(s) in said mixture.

11. The polymer dispersion of any one of the preceding Claims, wherein the mixture also contains a redox initiator system comprising an oxidizing agent having a water solubility less than or equal to 15 weight %.

12. The polymer dispersion of Claim 12, wherein said oxidizing agent comprises an organic peroxide, preferably tert-butylhydroperoxide.

13. The polymer dispersion of Claim 11 or Claim 12, wherein said redox initiator system is present in said mixture in an amount between 0.01 and 0.25 % by weight % weight based on the total weight of monomer(s) in said mixture.

14. The polymer dispersion of any one of the preceding Claims, wherein said at least one free-radically polymerizable monomer is selected from Ci-C2o-alkyl (meth)acrylates, vinyl esters of carboxylic acids with up to 20 carbons, vinyl- aromatic compounds having up to 20 carbons, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of Ci-Cio alcohols, C₂- C8 aliphatic hydrocarbons with 1 or 2 double bonds, or mixtures of these monomers.

15. The polymer dispersion of any one of the preceding Claims, wherein said at least one free-radically polymerizable monomer comprises a vinyl ester of a carboxylic acid with up to 20 carbons.

16. The polymer dispersion of Claim 15, wherein the vinyl ester comprises vinyl acetate.

17. The polymer dispersion of Claim 15 or claim 16, wherein said at least one free- radically polymerizable monomer further comprises ethylene.

18. The polymer dispersion of any one of the preceding Claims, wherein mixture comprises from 50 wt% to 99 wt% vinyl acetate, from 1 wt% to 40 wt% ethylene, and from 0 wt% to about 10 wt% of auxiliary co-monomers, based on the total weight of monomers in the mixture.

19. The polymer dispersion of any one of the preceding Claims and comprising first polymer particles having an average diameter from 200 to 950 nm and second polymer particles larger than said first polymer particles and having an average diameter from 950 to 20,000 nm.

20. The polymer dispersion of any one of the preceding Claims and having a polymer solids content of at least 55 weight %.

21. A process of producing an aqueous polymer dispersion having a multimodal particle size distribution, the process comprising emulsion polymerizing an aqueous mixture comprising at least one free-radically polymerizable monomer, at least one free radical initiator and a stabilizer composition comprising at least two different stabilizers, wherein one of the stabilizers comprises a polyvinyl alcohol having acid groups attached to its polymer backbone.

22. The process of Claim 21, wherein said polyvinyl alcohol having acid groups attached to its polymer backbone has a molar degree of hydrolysis less than 86%, preferably less than or equal to 80%.

23. The process of Claim 21 or Claim 22, wherein said polyvinyl alcohol having acid groups attached to its polymer backbone is present in said mixture in an amount between 0.2 and 2.0 % by weight based on the total weight of monomer(s) in said mixture.

24. The process of any one of Claims 21 to 23, wherein the stabilizer composition comprises at least one further stabilizer selected from at least one surfactant, at least one protective colloid and mixtures thereof.

25. The process of Claim 24, wherein the at least one further stabilizer comprises at least one polyvinyl alcohol substantially free of acid groups.

26. The process of any one of Claims 21 to 25, wherein said aqueous mixture comprises a redox initiator system comprising an oxidizing agent having a water solubility less than or equal to 15 weight %.

27. The process of Claim 26 and further comprising:

(a) providing in a polymerization reaction zone an initial charge comprising water, at least a portion of said redox initiator system, a portion of the at least one free- radically polymerizable monomer and said stabilizer composition; and

(b) after polymerization has been initiated, adding to polymerization reaction zone the remainder of the at least one free -radically polymerizable monomer and a further initiator system.

28. The process of any one of Claims 21 to 27, wherein said aqueous mixture is substantially free of seed polymer particles.

29. Use of the aqueous polymer dispersion of any one of Claims 1 to 20 as a binder

30. Use of the aqueous polymer dispersion of any one of Claims 1 to 20 in a coating composition.

31. Use of the aqueous polymer dispersion of any one of Claims 1 to 20 in an adhesive.