HK1070665B - Preparation of multimodal polymer dispersions using polymeric stabilizers, and use thereof - Google Patents

Description

Process for preparing multimodal polymer dispersions with polymeric stabilizers and use of the dispersions

The present invention relates to a process for preparing multimodal aqueous polymer dispersions in a very simple polymerization process which allows safe operation by using polymerization stabilizers and corresponding comonomers.

Multimodal polymer dispersions are useful in a variety of applications. Of particular emphasis is the field of water-based adhesives, which require high solids contents and optimum rheological properties. The high solids content leads to, for example, rapid drying and curing of the bond site and thus to a high strength being obtained after a very short time. Optimal rheological properties are important, for example in the field of mechanically applied packaging adhesives. Monomodal dispersions with high solids content can become highly viscous or swollen due to their maximum bulk density. Bimodal dispersions do not have this disadvantage.

Bimodal dispersions are dispersions which differ in particle size but have a particle size distribution with at least two distinct and independent maxima. For a given solids content, they have a lower viscosity and exhibit better rheology.

The polymer dispersions are generally prepared by emulsion polymerization. This typically produces a suspension containing particles having a single size (monodisperse particles). Depending on the manner of polymerization, these particles have a broader or narrower particle size distribution. The solids content in such systems depends on the maximum bulk density of the spheres. Thus, for example, the cubic or hexagonal close packing of hard spheres has a packing density of 74% by volume. This means that a high solids content of monodisperse spheres with a volume fraction of 74 vol% or more is theoretically impossible and the viscosity will increase to infinity.

This maximum packing effect is not possible with colloidal systems, represented by aqueous polymer dispersions. Wherein the lower maximum bulk density is found experimentally due to the hydrodynamic shell. In most cases, what is achieved is what is known as "random close packing". The maximum solids content is further reduced by hydrodynamic effects.

Experimentally the viscosity of the monodisperse colloidal particles increases sharply starting from 60% solids content. However, high solids content is necessary for polymer dispersions to meet performance characteristics.

Two particularly important properties for treating adhesives are, for example, open time and curing. Open time is a measure of the possible processing time while the adhesive remains uncured and the workpieces can still be interchanged with one another. The open time should be sufficiently long. In contrast, the bond should be very strong after a short time, which is reflected in the rapid curing properties. Both properties are tightly controlled by the solids content and/or volume fraction and the particle size distribution. In order to obtain optimum curing properties and very low viscosities, for example to ensure mechanical handling, high solids contents can only be achieved by multimodal particle size distributions.

DE-A-3,036,969 describes a process for preparing bimodal dispersions by mixing monomodal polymer dispersions having different particle sizes. In this case, two different monomodal dispersions are prepared separately and then mixed with one another in the desired ratio. The disadvantage of this process is that an additional mixing operation has to be introduced, which lengthens the preparation process and means higher costs. Furthermore, the solids content thereof can only be as high as the solids content of the individual dispersions.

This problem can be overcome by preparing bimodal dispersions in situ by means of, for example, a seed process.

Lepizzera et al, Macromol. chem. Phys.195, 3103-3115(1994) describe seeded emulsion polymerization of vinyl acetate in the presence of a polyvinyl alcohol (hereinafter "PVA") stabilizer. The seed emulsion is fed as an initial charge. Subsequent emulsion polymerization in the presence of PVA produces a second set of particles and thus produces a bimodal dispersion. The larger the molecular weight of the polyvinyl alcohol, the more new particles of the second group are produced. However, in this case the polyvinyl alcohol dissolves at 20 ℃. Thus PVA in aggregated form (incompletely dissolved form) also leads to secondary nucleation, since molecular solutions of polyvinyl alcohol are known to be obtainable only when dissolved at temperatures not lower than 85 ℃ (see Mowiol ® handbook by Kuraray specialities Europe GmbH or KSE).

DE-A-4,213,696 and DE-A-3,147,008 describe the preparation of bimodal dispersions using mixtures as seeds. Two dispersions with different particle sizes were mixed and used as seeds for emulsion polymerization. In the case of using the seed process as described above, it is necessary to prepare seeds in advance in both cases, which means increased cost and inconvenience.

DD-a-209,837 discloses a process in which polymerization is continuously initiated in two or more reaction vessels juxtaposed at the upstream end of the polymerization reaction vessel until the end of the particle formation stage. The disadvantage of this process is the high number of reaction vessels.

U.T urk in Die Angewandte Makromolekulare Chemie 46(1975), 109-133 describes how a bimodal particle size distribution can be obtained by secondary nucleation using emulsifiers during the polymerization. However, in this case the emulsifier must be added at a defined point during the polymerization process.

US-A-4,254,004 discloses A process for producing A bimodal particle size distribution from A two-stage process, during which the rate of monomer metering must be varied.

EP-A-818,471 describes ｃA process for producing ｃA bimodal particle size distribution by using microemulsion seeds.

But none of these prior art methods for producing bimodal particle size distributions is a simple process.

The known methods either require multiple vessels/reactors or require intervention in the process, such as discontinuous addition of additional components, or involve expensive and inconvenient pre-preparation processes (e.g. seeds).

It is therefore an object of the present invention to provide a novel, simple process for producing bimodal or multimodal particle size distributions in aqueous polymer dispersions while minimizing the known disadvantages associated therewith, such as process inconvenience.

It is a further object of the present invention to find a process which is easy to carry out with conventional polymerization apparatus and which can produce bimodal or multimodal polymer dispersions having a high solids content.

It has surprisingly been found that these objects can be achieved by a process for preparing aqueous polymer dispersions having a multimodal (or at least bimodal) particle size distribution, in particular using a combination of specific polymerization stabilizers and ionic comonomers in an emulsion polymerization process.

The invention relates to a method for producing aqueous polymer dispersions having an at least bimodal particle size distribution by emulsion polymerization of at least two ethylenically unsaturated monomers in the presence of polyvinyl alcohol, comprising

a) From 0.1 to 12% by weight, based on the total weight of all monomers used for preparing the polymer dispersion, of a polymer which is soluble in water in molecular or dispersed form, preferably polyvinyl alcohol, the molecular weight of which is at least 1.5 times the molecular weight of the polymer of component b),

b) from 0.1 to 12% by weight, based on the total weight of all monomers used for preparing the polymer dispersion, of a further polymer which is soluble in water in molecular or dispersed form, preferably a further polyvinyl alcohol, the molecular weight of which is at least 10000g/mol,

c) initially feeding 0.01 to 2 wt.%, based on the total weight of all monomers used to prepare the polymer dispersion, of at least one ionic comonomer which is an alpha, beta-monoethylenically unsaturated compound containing at least one group derived from a weak acid, and

d) to the mixture comprising components a), b) and c) are added at least one free-radically polymerizable, ethylenically unsaturated monomer and an initiator for the free-radical emulsion polymerization.

The process of the invention corresponds to emulsion polymerization in which the monomers are metered in continuously or discontinuously.

The process makes it possible to prepare bimodal or multimodal aqueous polymer dispersions. Bimodal or multimodal, respectively, in the present specification, means a particle size distribution having two or more maxima which are quite distinct (determined by means of a Malvern Mastersizer Micro Plus and evaluated by means of the "Mie polydispersity" model). The process of the invention is characterized in that the remaining monomers are added after the initial feeding of the combination of the specific stabilizer and the specific comonomer.

As stabilizer mixture, use is made of various water-soluble or water-dispersible polymers, preferably polyvinyl alcohol and/or modifications thereof. They differ mainly in molecular weight.

Examples of polymeric stabilizers are water-soluble or water-dispersible natural polymers, such as starch; water-soluble or water-dispersible modified natural polymers, for example cellulose ethers (such as methyl-, ethyl-, hydroxyethyl-or carboxymethyl-cellulose) or starches modified with saturated acids or epoxides; water-soluble or water-dispersible synthetic polymers, for example polyethylene oxide and copolymers thereof (e.g. polyethylene oxide/polypropylene oxide), polyvinyl alcohol (with or without residual acetyl content), polyvinyl alcohol partially esterified or acetalized or etherified with saturated groups, and polypeptides (e.g. gelatin), and polyvinylpyrrolidone and copolymers thereof (e.g. polyvinylpyrrolidone/polyvinyl acetate), polyvinylmethylacetamide or poly (meth) acrylic acid).

Preferred polymeric stabilizers are cellulose ethers, polyethylene oxides, modified starches, in particular polyvinyl alcohol and/or modifications thereof.

The polymeric stabilizers used in step a) and step b) differ only in their molecular weight or in their molecular weight and chemical composition.

Steps a) and b) are such that these polymerization stabilizers are already present in the initial charge and may also be added during the polymerization. It is also possible to add further amounts of polymerization stabilizers after the polymerization.

The polymeric stabilizers used in steps a) and b) must be soluble or dispersible in water at 20 ℃ if appropriate after a preceding temperature treatment.

The average molecular weight in this specification means a weight average (g/mol) unless otherwise specified. The molecular weight was determined by aqueous Gel Permeation Chromatography (GPC) on a SunChrom apparatus. The sample concentration was 3.5mg/ml and the injection amount was 100. mu.l. The sample was filtered on Teflon (1 μm). Detection was by refractive index detector (35 ℃). Eluted with 80: 20 water/acetone (0.05 wt% sodium nitrate). The columns used were the Suprema100, 1000 and 3000 types of PSS. The column temperature was 45 ℃. The standard used was polyethylene oxide.

The molecular weight of the polymer used in step a) is at least 1.5 times, preferably at least 2 times, in particular 3 to 10 times that of the polymer used in step b).

The molecular weight of the component used in step a) is preferably 30000-300000 g/mol, more preferably more than 50000-300000 g/mol.

The molecular weight of the polymer used in step b) is at least 10000, preferably at least 15000g/mol and less than 50000 g/mol.

When polyvinyl alcohol is used as a component in step a) and/or b), the preferred parameter is not the molecular weight but the viscosity of a 4% strength aqueous solution at 25 ℃ (measured using a H ö ppler viscometer).

Preference is given to using polyvinyl alcohols having a viscosity of less than or equal to 60 mPas, at least 7.5 mPas, more preferably 8 to 50 mPas, very preferably 8 to 40 mPas in a 4% strength aqueous solution at 25 ℃ in step a).

Preference is given to using polyvinyl alcohols in step b) which have a viscosity of at least 3.5 mPas, at least 4 mPas, more preferably from 4 to 10 mPas, very preferably from 4 to 8 mPas, in a 4% strength aqueous solution at 25 ℃ and have a molecular weight which is at least 1.5 times lower than the corresponding value of the components used in step a).

Polyvinyl alcohols are generally prepared by hydrolysis of polyvinyl acetate.

Suitable polyvinyl alcohols preferably have a degree of hydrolysis of from 50 to 100 mol%, more preferably from 70 to 100 mol%, at least 80%, preferably greater than or equal to 85%, more preferably greater than or equal to 87%, and an aqueous solution thereof has a viscosity of from 2 to 70 mPas at 25 ℃.

Particularly suitable are polyvinyl alcohols which have a degree of hydrolysis of 80 to 99 mol%, more preferably 87 to 99 mol%, and whose viscosity in a 4% strength aqueous solution at 25 ℃ is 3 to 50 mPas, preferably 3 to 40 mPas.

These explicitly indicated viscosities and those below refer in each case to the measurements obtained with an H ö PPler viscometer.

Other suitable and particularly preferred polyvinyl alcohols can be polyvinyl alcohols which have been modified in any way, hydrophilically or hydrophobically.

An example of a hydrophobically modified polyvinyl alcohol containing no water soluble monomer units in its backbone is the Exceval ® type ethylene containing polyvinyl alcohol of KSE. However, other comonomers may also be present in the polyvinyl alcohol. The distribution of the comonomers in the polyvinyl alcohol can be blocky and/or random.

Another preferred possibility is to modify the polyvinyl alcohol, preferably the alcohol groups, by any desired side chain reaction. For example, the alcohol group of the polyvinyl alcohol may be partially acetalized such that the polyvinyl alcohol has any desired group, which may be hydrophobic or hydrophilic, in particular C_1-12Alkyl, particularly preferably butyl-modified polyvinyl alcohols, as described in DE-A-19650831.

The acetalized hydroxy group is preferably a group having the structure:

wherein R is₁Is alkyl, cycloalkyl, aryl or aralkyl and R₂Is hydrogen, alkyl, cycloalkyl, aryl or aralkyl.

The alkyl group represents a linear or branched alkyl group having preferably 1 to 10, particularly 1 to 8 carbon atoms. Examples of alkyl are methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl and n-hexyl.

Aryl is preferably phenyl or naphthyl. If aryl is substituted phenyl, it preferably has two substituents. These substituents are present in particular in the 2-and/or 4-position.

The aralkyl group is preferably a benzyl group.

Cycloalkyl being in particular C₃-C₆Cycloalkyl groups, particularly cyclopentyl and cyclohexyl are preferred. R₁And R₂Cycloalkyl groups may also be formed together.

The modifying groups may be arranged in a block or random manner.

However, polyvinyl alcohols with other modifications may also be used.

The groups discussed herein are preferably those having the following structure:

wherein R is₁As defined above.

The distribution of the hydrophobic and/or hydrophilic groups can be arbitrary and wherein it is possible to control the particle size distribution.

The modifying groups may thus be present alongside one another (block distribution) or they may be distributed randomly.

The grafting reaction may be such that the hydroxyl groups in the polyvinyl alcohol are completely or only partially converted.

The mixture of polymeric stabilizers, preferably polyvinyl alcohol and/or modified derivatives thereof, used in the present invention is preferably dissolved at the beginning of the polymerization (usually in water) and fed as initial charge before the polymerization at a temperature of at least 85 ℃, preferably at least 90 ℃ for 2 to 3 hours.

The weight ratio of the higher and lower molecular weight stabilizers, particularly the polyvinyl alcohol (i.e. components a and b), may be from 1: 99 to 99: 1.

The components a) and b) are preferably used in a weight ratio of from 10: 90 to 90: 10, more preferably from 20: 80 to 80: 20.

Preferably, the H ö ppler viscosity of a 4% strength aqueous solution of the polyvinyl alcohol used in step b) is at least 3.5 mPas, more preferably 4 to 10 mPas, very preferably 4 to 8 mPas; the H ö ppler viscosity of a 4% strength aqueous solution of the polyvinyl alcohol used in step a) is at least 7.5 mPas, more preferably 8 to 50 mPas, and very preferably 8 to 40 mPas; and the molecular weight of the polyvinyl alcohol used in step a) is at least 1.5 times that of the polyvinyl alcohol used in step b).

The polymeric stabilizers used, in particular polyvinyl alcohol and/or modified derivatives thereof, are preferably used as initial charge, but can also be added in stages by metering, the total amount of said polymeric stabilizers generally being from 1 to 15% by weight, preferably from 3 to 11% by weight, more preferably from 4 to 11% by weight, based on the total weight of all monomers used for preparing the polymer dispersion.

It will be appreciated that other stabilizers may be used in addition to the polymeric stabilizers used according to the invention during the emulsion polymerization, for example low molecular weight emulsifiers, such as those based on sulfates, sulfonic acids, carboxylic acids or polyethylene oxides or copolymers thereof, or other polymeric stabilizers having a molecular weight different from that of components a) and b), such as cellulose ethers, polyethylene oxides, starch derivatives or additionally polyvinyl alcohols. These further stabilizers may even be present in the initial charge together with components a), b) and c) or may be added during the polymerization.

The stabilizer may be included in the initial charge in all amounts at the start of the emulsion polymerization or preferably in part at the start and the remainder added continuously or in one or more steps after the initiation of the polymerization. This addition can be carried out alone or together with other components, such as monomers and/or initiators, or in the form of a monomer emulsion.

Examples of suitable ionic comonomers include α, β -monoethylenically unsaturated compounds containing at least one group derived from a weak acid, such as α, β -monoethylenically unsaturated monocarboxylic and dicarboxylic acids, for example acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid and water-soluble salts thereof; other suitable ionic comonomers are phosphoric or phosphonic esters having ethylenically unsaturated groups, for example vinyl phosphate or methacryloyl ethyl phosphate.

According to the invention, the amount of ionic comonomer of component c) initially fed is 0.01 to 2% by weight.

The process of the invention is suitable for preparing bimodal or multimodal aqueous polymer dispersions by free-radical emulsion polymerization of monomers containing at least one ethylenically unsaturated group.

Monomers containing at least one monoethylenically unsaturated group suitable for the process of the present invention include free-radically polymerizable monomers known per se.

These monomers are, for example, aromatic or aliphatic α, β -unsaturated, optionally halogenated hydrocarbons, preferably ethylene, propylene, 1-butene, 2-butene, vinyl chloride, vinylidene chloride, styrene, α -methylstyrene and/or o-chlorostyrene, preferably ethylene; and/or esters of vinyl alcohol with monocarboxylic acids having 1 to 18 carbon atoms, such as vinyl acetate, vinyl propionate, vinyl n-butyrate, vinyl laurate, vinyl stearate and vinyl versatate (Versatic acid); and/or esters of α, β -monoethylenically unsaturated monocarboxylic and dicarboxylic acids, preferably having from 3 to 6 carbon atoms, such as, in particular, acrylic acid, methacrylic acid, maleic acid and itaconic acid, with alcohols having in general from 1 to 12, preferably from 1 to 8, in particular from 1 to 4 carbon atoms, such as, in particular, methanol, ethanol, n-butanol, isobutanol or 2-ethylhexanol, in particular, methyl, ethyl, n-butyl, isobutyl and 2-ethylhexyl acrylates and methacrylates, dimethyl maleate or di-n-butyl maleate; and/or nitriles of α, β -monoethylenically unsaturated carboxylic acids, such as acrylonitrile; and/or conjugated dienes having 4 to 8 carbon atoms, such as 1, 3-butadiene and isoprene.

The monomers generally constitute the principal monomers, the fraction of which is generally greater than 45% by weight, based on the total amount of monomers to be polymerized by the free radical aqueous emulsion polymerization process.

Typically these monomers have only moderate to low levels of water solubility under standard conditions (25 ℃,1 atm).

It will be appreciated that other comonomers may be added to purposefully alter their properties. Such monomers are usually copolymerized only as modifying monomers in amounts of less than 50% by weight, usually from 0.5 to 20% by weight, preferably from 1 to 10% by weight, based on the total amount of monomers to be polymerized.

Monomers which are commonly used to increase the internal strength of films formed from aqueous polymer dispersions typically contain at least one epoxy, hydroxyl, N-methylol or carbonyl group, or at least two nonconjugated ethylenically unsaturated double bonds.

Examples of such monomers are N-alkylolamides of alpha, beta-monoethylenically unsaturated carboxylic acids having from 3 to 10 carbon atoms, of which N-methylolacrylamide and N-methylolmethacrylamide are particularly preferred; and esters of the carboxylic acids with alkanols containing 1 to 4 carbon atoms. Also suitable are monomers containing two vinyl groups, monomers containing two vinylidene groups and monomers containing two alkenyl groups.

Particularly advantageous herein are diesters of dihydric alcohols with α, β -monoethylenically unsaturated monocarboxylic acids, preferably acrylic acid and methacrylic acid.

Examples of such monomers containing two nonconjugated ethylenically unsaturated double bonds are alkylene glycol diacrylates and dimethacrylates (for example ethylene glycol diacrylate, 1, 2-propylene glycol diacrylate, 1, 3-butylene glycol diacrylate, 1, 4-butylene glycol diacrylate and ethylene glycol dimethacrylate, 1, 2-propylene glycol dimethacrylate, 1, 3-butylene glycol dimethacrylate and 1, 4-butylene glycol dimethacrylate) and also divinylbenzene, vinyl methacrylate, vinyl acrylate, allyl methacrylate, allyl acrylate, diallyl maleate, diallyl fumarate, methylenebisacrylamide, cyclopentadienyl acrylate, or triallyl cyanurate.

Of particular importance here are also C of acrylic acid and methacrylic acid₁-C₉Hydroxyalkyl esters (e.g., n-hydroxyethyl, n-hydroxypropyl or n-hydroxybutyl acrylate and methacrylate) and compounds such as diacetoneacrylamide, acetoacetoxyethyl acrylate and acetoacetoxyethyl methacrylate.

In addition, compounds of the formula R³Si(CH₃)_0-2(OR⁴)_3-1The organosilicon monomer of (1), wherein R³Is defined as CH₂＝CR⁴-(CH₂)_0-1Or CH₂＝CR⁵CO₂-(CH₂)_1-3，R⁵Is a branched or unbranched, optionally substituted alkyl radical having 3 to 12 carbon atoms, which may optionally be interrupted by ether groups, and R⁴Is H or CH₃。

Examples thereof are vinylmethyldimethoxysilane, vinylmethyldiethoxysilane, vinylmethyldi-n-propoxysilane, vinylmethyldiisopropyloxysilane, vinylmethyldi-n-butoxysilane, vinylmethyldi-sec-butoxysilane, vinylmethyldi-tert-butoxysilane, vinylmethyldi (2-methoxyisopropoxy) silane and vinylmethyldioctyloxysilane.

In the case of free-radical heterophase polymerization, preferably free-radical aqueous emulsion polymerization, the abovementioned monomers are generally copolymerized in an amount of from 0.2 to 10% by weight, based on the total amount of monomers to be polymerized.

Processes for the preparation of aqueous Polymer dispersions have been described in large numbers and are known to the person skilled in the art [ see, for example, Encyclopedia of Polymer Science and engineering, Vol.8, p.659 ff (1987) ].

The preparation is preferably effected by emulsion polymerization of monomers containing at least one ethylenically unsaturated group in the presence of preferably water-soluble polymerization initiators and optionally stabilizers and optionally emulsifiers, optionally further customary additives. Alternatively, it can be carried out in other multiphase systems, provided that the steps a) to d) described above are carried out.

The monomer is generally added by continuous feeding; however, it is also possible to include a portion of the monomers in the initial charge, for example up to 25% by weight.

The polymerization of the ethylenically unsaturated monomers of the present invention is carried out in the presence of at least one initiator for the free-radical polymerization of the ethylenically unsaturated monomers.

Suitable initiators for free-radical polymerization for initiating and maintaining polymerization during the preparation of the dispersion include all known initiators capable of initiating free-radical aqueous polymerization in a multiphase system.

These initiators may be peroxides (for example alkali metal peroxodisulfates and/or ammonium peroxodisulfates) or azo compounds (in particular water-soluble azo compounds).

As the polymerization initiator, those called redox initiators can also be used. Examples thereof are t-butyl hydroperoxide and/or hydrogen peroxide in combination with reducing agents, for example with sulfur-containing compounds such as sodium hydroxymethylsulfinate, sodium sulfite, sodium metabisulfite, sodium thiosulfate and acetone-bisulfite adducts, or with ascorbic acid or reducing sugars.

The amount of initiator or combination of initiators used in the process of the present invention is within the amount typically used for aqueous phase polymerization in multiphase systems. In general, the amount of initiator used does not exceed 5% by weight, based on the total amount of monomers to be polymerized.

The amount of the initiator used is preferably 0.05 to 2.0% by weight based on the total amount of the monomers to be polymerized.

The initiator can be fed in all amounts at the beginning of the polymerization or, preferably, part of the initiator is fed in at the beginning and the remainder is added in one or more steps or continuously after the initiation of the polymerization. This addition can be carried out alone or together with other components such as emulsifiers.

The molecular weight of the polymer in the aqueous dispersion can be adjusted by adding small amounts of one or more molecular weight-adjusting substances. As is known, these "regulators" are generally used in amounts of up to 2% by weight, based on the monomers to be polymerized. Any substance known to those skilled in the art may be used as a modulator. Preference is given to, for example, organic thio compounds, silanes, allyl alcohols and aldehydes.

The aqueous dispersion may also contain a number of other substances, such as plasticizers, preservatives, pH adjusters and/or defoamers.

The polymerization temperature is usually 20 to 150 ℃ and preferably 60 to 120 ℃.

If desired, the polymerization can be carried out under superatmospheric pressure.

As additional emulsifiers, it is possible in particular to use anionic emulsifiers or nonionic dispersants in addition to the polyvinyl alcohol, in particular in amounts of from 0.05 to 4% by weight, based on the total amount of monomers.

After the actual polymerization, it may be desirable and/or virtually necessary to remove odorous substances such as residual monomers and other volatile organic components from the resulting aqueous polymer dispersion. This can be achieved by conventional physical methods, for example by removal by distillation, in particular by steam distillation, or by stripping with inert gases. The reduction in the amount of residual monomers can also be achieved by chemical methods of free-radical postpolymerization, in particular under the action of redox initiator systems, as described in DE-A-4,435,423. Preference is given to postpolymerization using a redox initiator system consisting of at least one organic peroxide and an organic and/or inorganic sulfite and/or sulfinic acid derivative.

It is particularly preferred to use a combination of physical and chemical processes, wherein after the residual monomer content has been reduced by chemical post-polymerization it is further reduced by physical means to preferably < 1000ppm, more preferably < 500ppm, in particular < 100.

The monomer components can be contained in the initial charge or they can be metered in carefully at a uniform rate or according to a metering profile. This very simple method is worth particular emphasis. This eliminates the need for seeds or the use of sophisticated equipment or combinations thereof. This method is a simple monomer metering method.

The polymerization according to the invention is generally carried out at a pH of about 9 or less. For adjusting the pH of the polymer dispersion, it is in principle possible to use buffer systems such as sodium acetate.

A pH of 2 to 9 is preferred, and a pH of 3 to 8 is preferred.

The solids content of the dispersions prepared according to the invention is generally from 45 to 74% by weight, preferably from 49 to 70% and more preferably from 50 to 70%. The weight values in this case refer to the total amount of dispersion.

The bimodal or multimodal dispersions prepared according to the invention are particularly suitable for preparing coating materials, such as paints or food coatings, adhesives, for bonding wood, paper products and/or polymer films, and also for finishing textiles and paper products. The invention also provides the use of these dispersions for the stated purposes.

The dispersions prepared according to the invention can likewise advantageously be converted by spray drying into powders for building chemicals and adhesives.

The following examples are intended to illustrate the invention without limiting it.

Polyvinyl alcohol

The value quoted in the head of the model label of polyvinyl alcohol represents the viscosity of a 4% strength aqueous solution at 20 ℃ as a relative measure of the degree of polymerization of polyvinyl alcohol; the second number represents the degree of hydrolysis (degree of saponification) of the polyvinyl acetate on which the polyvinyl alcohols of the type in question (partially hydrolyzed polyvinyl alcohol and fully hydrolyzed polyvinyl alcohol) are based.

The data is a manufacturer-supplied routine deviation; that is, the variation in viscosity may be. + -. 0.5 mPas, and the variation in the degree of hydrolysis may be. + -. 1 mol%.

Determination of particle size distribution

Particle size distribution testing was performed using a Mastersizer Micro Plus laser diffraction device from Malvern. The scattering data was evaluated using the "polydisperse Mie" or "multimodal Mie" model supplied by Malvern.

Since diffraction experiments do not provide any information about morphology, optical micrographs were additionally taken using a differential interference contrast microscope (differential contrast microscope) from Leitz. The combination of these two methods allows conclusions to be drawn about the number and morphology of the particle size distributions.

Determination of the curing Properties on Wood

The curing properties were tested on beech sections (15.5X 2X 0.3cm) which had been stored beforehand under standard conditions (23 ℃, 50% relative humidity). One side of 15mm by 20mm area is coated with 150g/m of dispersion²。

The second test section was placed on the film and measured at 0.7N/mm²The two sections are pressed. The pressing times were 2.5 minutes and 5 minutes, respectively.

The breaking force was determined on 10 test specimens. Based on the specified area (unit N/mm)²) The average values of (a) and (b) correspond to the curing performance after 2.5 and 5 minutes, respectively.

Examples 1a to 5a (inventive examples, 60%, different PVA ratios)

The solution consisted of the following: 62 parts by weight (based on the amount of the main monomer) of deionized water, X parts by weight of polyvinyl alcohol 26-88 (where X is 1.5ppw in example 1a, 2.5ppw in 2a, 3.5ppw in 3a, 4.5ppw in 4a, 5.5ppw in 5 a), Y parts by weight of polyvinyl alcohol 4-88 (where Y is 8.5ppw in example 1a, 7.5ppw in 2a, 6.5ppw in 3a, 5.5ppw in 4a, 5.5ppw in 5 a), 0.1 part by weight of sodium acetate, 0.5 part by weight of methacrylic acid and 10 parts by weight of the main monomer vinyl acetate.

The polymerization was initiated by adding 0.05 part by weight of ammonium peroxodisulfate at 65 ℃. After the polymerization had started, the remaining main monomer, 0.02 part by weight of ammonium peroxodisulfate and 6 parts by weight of water were metered in over 5 hours. The polymerization is carried out at 70 to 85 ℃ with stirring using a compact-clearance (close-clearance) anchor stirrer.

Example 6 (inventive example, 50%)

The solution consisted of the following: 100 parts by weight (based on the amount of the main monomer) of deionized water, 3.5 parts by weight of polyvinyl alcohol 26 to 88, 6.5 parts by weight of polyvinyl alcohol 4 to 88, 0.1 part by weight of sodium acetate, 0.5 part by weight of methacrylic acid and 10 parts by weight of vinyl acetate, which is the main monomer. The polymerization was initiated by adding 0.05 part by weight of ammonium peroxodisulfate at 65 ℃. After the start of the polymerization, the remaining aqueous solution of vinyl acetate, 0.02 part by weight of ammonium peroxodisulfate and 6 parts by weight of water was metered in over a period of 5 hours. The polymerization is carried out at 70 to 85 ℃ with stirring using a compact-clearance (close-clearance) anchor stirrer.

Example 7 (inventive example, acrylic acid)

The preparation is carried out as in example 3a, but without using methacrylic acid. Instead, the same amount of acrylic acid was used.

Example 8 (inventive example, butyraldehyde-modified polyvinyl alcohol)

Preparation was carried out as in example 4a, but using butyraldehyde-modified polyvinyl alcohol instead of polyvinyl alcohol 26-88.

Example 9(Exceval ®, inventive example)

Preparation was carried out as in example 4a, but using Exceval ® AQ4105L instead of polyvinyl alcohol 4-88.

Comparative example C1 (methacrylic acid-free, monomodal)

The preparation is carried out as in example 3a, but without using methacrylic acid.

Comparative example C2 (only one polyvinyl alcohol (high molecular weight), monomodal)

The preparation is carried out as in example 3a, but without using low molecular weight polyvinyl alcohol. Instead, 10ppw of high molecular weight PVA (polyvinyl alcohol 26-88) was used.

Results

The results are shown in the figure. Specifically, the method comprises the following steps:

FIG. 1(1a and 1b) shows the bimodal particle size distribution obtained by the process of the invention.

FIG. 2 shows different particle size distributions with different ratios of size particles prepared by varying the ratio of polyvinyl alcohol.

Figures 3 to 8 show the particle size distribution of different dispersions prepared by the process of the invention or by a comparative process.

FIG. 1a shows the particle morphology of the bimodal dispersion prepared according to the invention of example 3a, observed in a differential interference phase contrast optical microscope.

FIG. 1b shows the particle size distribution of the bimodal dispersion. The test was performed with a Mastersizer Micro Plus (polydisperse Mie evaluation).

It is evident from FIG. 2 that by varying the proportions of polyvinyl alcohol, virtually any desired particle size distribution can be produced with almost any desired particle size ratio. The graph shows the variation in the particle size distribution of the dispersions prepared according to the invention in examples 1a to 5a by varying the proportion of PVA (the total amount of PVA is kept constant). The test was performed with a Malvern Mastersizer Micro Plus and evaluated with the "polydisperse Mie" model.

In FIG. 3, a bimodal dispersion prepared according to the present invention (example 3a) is compared to a comparative dispersion prepared without ionic comonomer (comparative example C1). The test was performed with a Malvern Mastersizer Micro Plus and evaluated with the "polydisperse Mie" model.

In FIG. 4, a bimodal dispersion prepared according to the present invention (example 3a) is compared to a comparative dispersion prepared without the use of low molecular weight polyvinyl alcohol (polyvinyl alcohol 4-88) (comparative example C2). The test was performed with a Malvern Mastersizer Micro Plus and evaluated with the "polydisperse Mie" model.

Bimodal dispersions prepared according to the invention (example 3a (60%) and example 6 (50%)) are shown in FIG. 5. The test was performed with a Malvern Mastersizer Micro Plus and evaluated with the "polydisperse Mie" model.

A bimodal dispersion prepared according to the present invention (example 7 with different carboxylic acids as the ionic comonomer (acrylic acid)) is shown in figure 6. The test was performed with a Malvern Mastersizer Micro Plus and evaluated with the "polydisperse Mie" model.

A bimodal dispersion of example 8 polyvinyl alcohol modified with butyraldehyde prepared in accordance with the present invention is shown in fig. 7. The test was performed with a Malvern Mastersizer Micro Plus and evaluated with the "polydisperse Mie" model.

The bimodal dispersion of example 8 prepared according to the invention with an Exceval ® type hydrophobically modified polyvinyl alcohol is shown in figure 8. The test was performed with a Malvern Mastersizer Micro Plus and evaluated with the "polydisperse Mie" model.

The results described in fig. 2 to 8 were confirmed by morphological analysis obtained with a differential interference phase contrast optical microscope.

Table 1 below shows the viscosity comparison of bimodal dispersions prepared according to the invention with monodisperse dispersions.

TABLE 1

Dispersion product	Inventive example 3a	Comparative example C1
				Bimodal type	Unimodal type
Brookfield viscosity (7 ℃ rotor, 21 ℃), mpas	23000	78000
			Volume fraction of small particles (about < 1 μm)%	73	100
Volume fraction of large particles (about 5 μm)%	27	0

Table 1 shows that the viscosity of the dispersions of the invention is reduced by about two thirds compared to the viscosity of the corresponding monomodal comparative dispersions. This corresponds to the desired effect of bimodal dispersions in terms of improved packing.

Table 2 below shows the curing properties of three different dispersions with different solids contents and particle size distributions, which dispersions are comparable in viscosity and composition.

TABLE 2

Dispersion product	Example C3 comparative unimodal	Example 1b bimodal form of the invention	Example 2b inventive bisPeak type
				Solids content%	50	50	60
Curing Performance N/mm after 2.5 minutes2	1.9	2.8	3.2
				Curing Property N/mm after 5 minutes2	3.2	4.5	5.0

Table 2 shows the curing properties of three different dispersions. Dispersion C3 was a unimodal 50% comparative dispersion. Example 1b shows that the multimodal systems prepared according to the invention have a significantly improved curing behavior at the same solids content. Better curing properties are observed for bimodal 60% dispersions prepared according to the invention, which can only be prepared by the process of the invention: otherwise the resulting viscosity is too high and cannot be handled (see C1 in table 1).

Claims (24)

1. A process for preparing an aqueous polymer dispersion having an at least bimodal particle size distribution by emulsion polymerization of at least two ethylenically unsaturated monomers in the presence of polyvinyl alcohol, which comprises

a) From 0.1 to 12% by weight, based on the total weight of all monomers used for preparing the polymer dispersion, of a polymer which is soluble in water in molecular or dispersed form and has a molecular weight which is at least 1.5 times the molecular weight of the polymer of component b),

b) initially feeding 0.1 to 12% by weight, based on the total weight of all monomers used for preparing the polymer dispersion, of a further polymer soluble in water in molecular or dispersed form and having a molecular weight of at least 10000g/mol,

c) initially feeding 0.01 to 2 wt.%, based on the total weight of all monomers used to prepare the polymer dispersion, of at least one ionic comonomer which is an alpha, beta-monoethylenically unsaturated compound containing at least one group derived from a weak acid, and

d) to the mixture comprising components a), b) and c) are added at least one free-radically polymerizable, ethylenically unsaturated monomer and an initiator for the free-radical emulsion polymerization.

2. The process of claim 1 wherein the water-soluble polymer used in steps a) and b) which is molecularly or dispersedly soluble is selected from the group consisting of cellulose ethers, polyethylene oxide, modified starch, polyvinyl alcohol, modified polyvinyl alcohol and mixtures thereof.

3. The process according to claim 2, wherein the water-soluble polymers used in steps a) and b) are polyvinyl alcohol and/or backbone-modified or side-chain-modified polyvinyl alcohol, respectively.

4. The process according to claim 3, wherein the 4% strength aqueous solution of polyvinyl alcohol used in step b) has an H ö PPler viscosity of at least 3.5 mPas; wherein a 4% strength aqueous solution of the polyvinyl alcohol used in step a) has an H ö ppler viscosity of at least 7.5 mPas.

5. The process according to claim 4, wherein a 4% strength aqueous solution of polyvinyl alcohol used in step b) has an H ö ppler viscosity of 4 to 10 mPas.

6. The process according to claim 4, wherein a 4% strength aqueous solution of polyvinyl alcohol used in step b) has an H ö ppler viscosity of 4 to 8 mPas.

7. The process according to claim 4, wherein the 4% strength aqueous solution of polyvinyl alcohol used in step a) has an H ö ppler viscosity of 8 to 50 mPas.

8. The process according to claim 4, wherein the 4% strength aqueous solution of polyvinyl alcohol used in step a) has an H ö ppler viscosity of 8 to 40 mPas.

9. The process according to claim 1, wherein the molecular weight of the molecularly or dispersedly water-soluble polymer used in step a) is more than 50000g/mol to 300000g/mol and wherein the molecular weight of the molecularly or dispersedly water-soluble polymer used in step b) is at least 10000g/mol and less than 50000 g/mol.

10. The method of claim 1, wherein the water soluble or water dispersible polymer used in steps a) and b) is a polyvinyl alcohol having a molar hydrolysis of at least 80% each.

11. The method of claim 10, wherein the water soluble or water dispersible polymer used in steps a) and b) is a polyvinyl alcohol having a molar hydrolysis of greater than or equal to 85%, respectively.

12. The method of claim 10, wherein the water soluble or water dispersible polymer used in steps a) and b) is a polyvinyl alcohol having a molar hydrolysis of greater than or equal to 87%, respectively.

13. The method of claim 2, wherein a polyvinyl alcohol modified by any desired side chain reaction is used.

14. The method of claim 13, wherein a polyvinyl alcohol modified on alcohol groups by any desired side chain reaction is used.

15. The process according to claim 13 or 14, wherein a polyvinyl alcohol modified by partial acetalization of alcohol groups with a C1-12 alkyl group is used.

16. The process according to claim 1, wherein, in the case of free-radically polymerizable ethylenically unsaturated monomers, esters of vinyl alcohol with monocarboxylic acids having from 1 to 18 carbon atoms are used in step d); and/or aromatic or aliphatic alpha, beta-unsaturated, optionally halogenated hydrocarbons.

17. The process according to claim 16, wherein the ester of vinyl alcohol with a monocarboxylic acid having 1 to 18 carbon atoms used in step d) is selected from vinyl acetate, vinyl propionate, vinyl n-butyrate, vinyl laurate, vinyl stearate and vinyl versatate with respect to the free-radically polymerizable ethylenically unsaturated monomers.

18. The process according to claim 16, wherein the aromatic or aliphatic α, β -unsaturated, optionally halogenated hydrocarbons used in step d) are, for the free-radically polymerizable, ethylenically unsaturated monomers, ethylene, propylene, 1-butene, 2-butene, vinyl chloride, vinylidene chloride, styrene, α -methylstyrene and/or o-chlorostyrene.

19. The process according to any one of claims 16 to 18, wherein vinyl acetate and ethylene and/or vinyl versatate are used in step d) for the free-radically polymerizable ethylenically unsaturated monomers.

20. The process according to any one of claims 16 to 18, wherein vinyl acetate is used in step d) for the free-radically polymerizable, ethylenically unsaturated monomers.

21. The process of claim 1, wherein the ionic comonomer in step c) has at least one carboxylic acid group.

22. The process according to claim 21, wherein the ionic comonomer in step c) is acrylic acid and/or methacrylic acid.

23. The process of claim 1, wherein the polymerization is carried out at a pH of 2 to 9.

24. Use of the polymer dispersion containing polyvinyl alcohol prepared according to the process of claim 1 for the preparation of wood, paper products or binders for polymer materials, for the preparation of coating materials, in particular paints or food coatings, for the preparation of powders or chemicals for the building industry or for finishing textiles or paper products.