Polyvinyl alcohol-stabilized polyvinyl esters as dispersion adhesives

Abstract

The invention relates to polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions, characterized in that the polyvinyl esters are stabilized by at least two polyvinyl alcohols, all of the polyvinyl alcohols having a viscosity in the range of 8 to 30 mPas and at least two polyvinyl alcohols differing with respect to their viscosity, under the proviso that the polyvinyl alcohol-stabilized polyvinyl esters are not stabilized by an emulsifier.

Description

[0001] Polyvinyl alcohol-stabilized polyvinyl esters as dispersion adhesives

[0002] The invention relates to polyvinyl esters in the form of aqueous dispersions, processes for their preparation and use in dispersion adhesives as well as processes for applying the dispersion adhesives by means of mechanical application processes, in particular conveyor belt processes, such as nozzle or roller application processes.

[0003] Dispersion adhesives based on polyvinyl esters have a wide range of applications, for example for bonding paper or cardboard in the manufacture of folding boxes, envelopes, brochures, or cigarettes. Such products are typically manufactured on an industrial scale using assembly line production. The dispersion adhesives are generally applied to the substrate using mechanical application methods such as nozzle application systems or roller technologies. In these application methods, adhesive contamination caused by imprecise or uncontrolled adhesive application, also known as "splashing," leads to production problems. If adhesive gets onto the conveyor belt, this can cause the manufactured material to stick, leading to machine downtime and costly cleanup work.During nozzle application, cone-shaped deposits often form at the nozzle outlet, deflecting the adhesive jet emerging from the nozzle. This is detrimental to precise control of the adhesive application and can also lead to contamination and ultimately to system downtime. In nozzle application systems, the dispersion adhesives are fed by pumps through pipe systems to a nozzle with a rapidly opening and closing valve, for example with switching frequencies of up to 1000 per second. Such high switching frequencies of the nozzle valves expose the dispersion adhesives inside the nozzle to extremely high shear forces. Suitable dispersion adhesives must therefore be very shear-stable. In addition, the dispersion adhesives should also have advantageous wet bonding properties.

[0004] Dispersion adhesives for nozzle application systems are described, for example, in US2008044565. The polymer dispersions of US2008044565 necessarily contain emulsifiers and optionally protective colloids for stabilization, without US2008044565 attaching any importance to the design of the protective colloids. US2008044565 even disregards protective colloid stabilization. Stabilizing the polymer dispersions with emulsifiers is also essential for WO2022 / 055511.

[0005] WO2022 / 055511 adds polyvinyl alcohol to the emulsifier-stabilized polymer dispersions after their preparation by polymerization. US2008039572 also teaches emulsifier-stabilized vinyl acetate-ethylene polymer dispersions for mechanical application processes.

[0006] However, emulsifiers are not tolerated in all applications, for example, in food contact applications. Therefore, there is a need for polymer dispersions that are not emulsifier-stabilized but still meet the requirements for dispersion adhesives for machine application. For US20160280974, polymer dispersions containing both medium-viscosity polyvinyl alcohols and high-viscosity polyvinyl alcohols (36 to 60 mPas) are essential.

[0007] Against this background, the task was to provide dispersion adhesives for mechanical application processes which show very good nozzle running properties and advantageous wet bonding properties and which contain polymer dispersions as binders which are not emulsifier-stabilized.

[0008] The invention relates to polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions, characterized in that the polyvinyl esters are stabilized by at least two polyvinyl alcohols, wherein all polyvinyl alcohols have a viscosity in the range from 8 to 30 mPas and at least two polyvinyl alcohols differ in their viscosity, with the proviso that the polyvinyl alcohol-stabilized polyvinyl esters are not emulsifier-stabilized.

[0009] Such polyvinyl alcohol-stabilized polyvinyl esters according to the invention in the form of aqueous dispersions are obtainable, for example, by polymerizing a) one or more vinyl esters and optionally b) one or more further ethylenically unsaturated monomers by means of radically initiated emulsion polymerization in an aqueous medium in the presence of at least two polyvinyl alcohols in the absence of emulsifiers, wherein all polyvinyl alcohols have a viscosity in the range from 8 to 30 mPas and at least two polyvinyl alcohols differ in their viscosity.

[0010] The polyvinyl alcohol stabilization is reflected in a structural feature of the polymer dispersion that is not obtained when, for example, polyvinyl alcohol is subsequently added to an emulsifier-stabilized polymer dispersion, as is known to those skilled in the art. Polyvinyl alcohol-stabilized polyvinyl esters are generally obtained by emulsion polymerization of vinyl esters in the presence of polyvinyl alcohol. In this case, polyvinyl alcohol is generally at least partially grafted, which is not the case with polyvinyl alcohol added subsequently after polymerization.

[0011] The information on the viscosities of polyvinyl alcohols in this application refers to the Höppler viscosity, determined at 20°C according to DIN 53015 in 4% aqueous solution.

[0012] For the sake of clarity, it should be noted that the polyvinyl esters according to the invention are generally not stabilized by polyvinyl alcohols having a viscosity greater than 30 mPas or a viscosity less than 8 mPas.

[0013] Preferably, at least one polyvinyl alcohol (polyvinyl alcohol a) ) has a viscosity of 8 to 18 mPas, particularly preferably 9 to 17 mPas and most preferably 11 to 15 mPas .

[0014] The proportion of polyvinyl alcohols a) is preferably 30 to 70 wt.%, particularly preferably 40 to 60 wt.% and most preferably 45 to 55 wt.%, in each case based on the total weight of the polyvinyl alcohols contained in the polyvinyl ester dispersion, in particular based on the total weight of the polyvinyl alcohols a) and ß).

[0015] The proportion of polyvinyl alcohols a) is preferably 0.5 to 5 wt.%, particularly preferably 1 to 3 wt.%, most preferably 1.5 to 2.5 wt.%, based in each case on the dry weight of the polyvinyl esters. Preferably, at least one polyvinyl alcohol (polyvinyl alcohol ß)) has a viscosity of 19 to 30 mPas, particularly preferably 20 to 27 mPas, and most preferably 21 to 25 mPas.

[0016] The proportion of polyvinyl alcohols ß) is preferably 30 to 70 wt.%, particularly preferably 40 to 60 wt.% and most preferably 45 to 55 wt.%, in each case based on the total weight of the polyvinyl alcohols contained in the polyvinyl ester dispersion, in particular based on the total weight of the polyvinyl alcohols a) and ß).

[0017] The proportion of polyvinyl alcohols ß) is preferably 0.5 to 5 wt.%, particularly preferably 1 to 3 wt.%, most preferably 1.5 to 2.5 wt.%, in each case based on the dry weight of the polyvinyl esters.

[0018] The total amount of polyvinyl alcohols, in particular the total amount of polyvinyl alcohols a) and ß), is preferably 1 to 10 wt.%, more preferably 2 to 6 wt.%, particularly preferably 3 to 5 wt.% and most preferably 3.5 to 4.5 wt.%, in each case based on the dry weight of the polyvinyl esters.

[0019] The weight ratio of the polyvinyl alcohols a) to the polyvinyl alcohols ß) is preferably in the range from 99:1 to 1:99, particularly preferably 70:30 to 30:70 and most preferably 1:1.5 to 1.5:1.

[0020] The polyvinyl esters are preferably stabilized with two polyvinyl alcohols, particularly preferably exclusively with one polyvinyl alcohol a) and one polyvinyl alcohol ß). For the sake of clarity, it should be noted that polyvinyl alcohols a) and polyvinyl alcohols ß) are also referred to jointly as polyvinyl alcohols in the present application.

[0021] The polyvinyl alcohols can be partially or fully saponified. Partially saponified polyvinyl alcohols are preferred. The degree of hydrolysis of the polyvinyl alcohols is preferably 80 to 94 mol%, particularly preferably 83 to 92 mol%, and most preferably 85 to 90 mol%.

[0022] The polyvinyl alcohols are preferably composed exclusively of vinyl alcohol units and vinyl acetate units. Partially saponified, hydrophobically modified polyvinyl alcohols can also be used, although preferably no hydrophobically modified polyvinyl alcohols are used. Examples of these are partially saponified copolymers of vinyl acetate with hydrophobic comonomers such as isopropenyl acetate, vinyl pivalate, vinyl ethyl hexanoate, vinyl esters of saturated alpha-branched monocarboxylic acids having 5 or 9 to 11 C atoms, dialkyl maleates and dialkyl fumarates such as diisopropyl maleate and diisopropyl fumarate, vinyl chloride, vinyl alkyl ethers such as vinyl butyl ether, and olefins such as ethene and decene. The proportion of hydrophobic units is preferably 0.1 to 10% by weight, based on the total weight of the partially saponified polyvinyl alcohol. Mixtures of the polyvinyl alcohols mentioned can also be used.Further preferred polyvinyl alcohols are partially saponified, hydrophobized polyvinyl alcohols obtained by polymer-analogous reactions, for example, acetalization of the vinyl alcohol units with C1- to C4-aldehydes such as butyraldehyde. The proportion of hydrophobic units is preferably 0.1 to 10 wt. %, based on the total weight of the partially saponified polyvinyl acetate. The polyvinyl alcohols mentioned are obtainable by methods known to those skilled in the art.

[0023] The polyvinyl esters are generally based on a) one or more vinyl esters and optionally b) one or more other ethylenically unsaturated monomers.

[0024] Suitable vinyl esters a) are, for example, those of carboxylic acids having 1 to 22 carbon atoms, in particular 1 to 12 carbon atoms. Preference is given to vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate, and vinyl esters of a-branched monocarboxylic acids having 9 to 11 carbon atoms, for example VeoVa9R or VeoValOR (trade names of Momentive). Vinyl acetate is particularly preferred.

[0025] The vinyl esters a) are used in an amount of preferably 50 to 100 wt.%, particularly preferably 60 to 95 wt.% and most preferably 65 to 80 wt.%, in each case based on the total weight of the monomers.

[0026] As further ethylenically unsaturated monomers bl), one or more olefins, such as propylene or preferably ethylene, are selected in particular.

[0027] The monomers b1) are copolymerized in an amount of preferably 5 to 40 wt.%, particularly preferably 10 to 30 wt.% and most preferably 20 to 35 wt.%, in each case based on the total weight of the monomers.

[0028] As further ethylenically unsaturated monomers b2), optionally in combination with one or more olefins, such as ethylene, one or more ethylenically unsaturated monomers can be selected from the group comprising (meth)acrylic acid esters, vinyl aromatics, 1,3-dienes and vinyl halides.

[0029] Suitable monomers from the group of esters of acrylic acid or methacrylic acid are, for example, esters of unbranched or branched alcohols having 1 to 15 carbon atoms. Preferred methacrylic acid esters or acrylic acid esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, and 2-ethylhexyl acrylate. Methyl acrylate, methyl methacrylate, n-butyl acrylate, and 2-ethylhexyl acrylate are particularly preferred.

[0030] Preferred vinylaromatics are styrene, methylstyrene, and vinyltoluene. The preferred vinyl halide is vinyl chloride. The preferred dienes are 1,3-butadiene and isoprene.

[0031] The monomers b2) are copolymerized in an amount of preferably 0 to 45 wt.% and particularly preferably 10 to 30 wt.%, based in each case on the total weight of the monomers. Most preferably, no monomers b2) are copolymerized.

[0032] Optionally, 0 to 10 wt. %, in particular 0.05 to 10 wt. %, based on the total weight of the monomer mixture, of auxiliary monomers may be copolymerized. Most preferably, however, no auxiliary monomers are copolymerized. Examples of auxiliary monomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid, and maleic acid; ethylenically unsaturated carboxamides and nitriles, preferably acrylamide and acrylonitrile; mono- and diesters of fumaric acid and maleic acid, such as the diethyl and diisopropyl esters, and maleic anhydride; ethylenically unsaturated sulfonic acids or their salts, preferably vinylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid.Further examples are pre-crosslinking comonomers such as polyethylenically unsaturated comonomers, for example divinyl adipate, diallyl maleate, allyl methacrylate, triallyl isocyanurate or triallyl cyanurate, or post-crosslinking comonomers, for example acrylamidoglycolic acid (AGA), methylacrylamidoglycolic acid methyl ester (MAGME), N-methylolacrylamide (NMA), N-methylolmethacrylamide, N-methylolallylcarbamate, alkyl ethers such as isobutoxy ether or esters of N-methylolacrylamide, N-methylolmethacrylamide and N-methylolallylcarbamate. Epoxy-functional comonomers such as glycidyl methacrylate and glycidyl acrylate are also suitable. Further examples are silicon-functional comonomers such as acryloxypropyltri(alkoxy)- and methacryloxypropyltri(alkoxy)silanes, vinyltrialkoxysilanes and vinylmethyldialkoxysilanes, where ethoxy and ethoxypropylene glycol ether residues may be present as alkoxy groups.Monomers with hydroxy or CO groups may also be mentioned, for example methacrylic acid and acrylic acid hydroxyalkyl esters such as hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate as well as compounds such as diacetone acrylamide and acetylacetoxyethyl acrylate or methacrylate.

[0033] Preferably, one or more polyvinyl esters are selected from the group comprising vinyl ester homopolymers, vinyl ester-ethylene copolymers, vinyl ester copolymers containing one or more vinyl ester units and one or more further monomer units from the group comprising vinyl aromatics, vinyl halides, acrylic acid esters, methacrylic acid esters and optionally ethylene.

[0034] Examples of preferred vinyl ester copolymers are based on 50 to 90 wt. % of one or more vinyl esters, 10 to 20 wt. % of ethylene and optionally 1 to 40 wt. % of one or more further monomers, based on the total weight of the monomers.

[0035] Also preferred are comonomer mixtures of vinyl acetate with 10 to 20 wt . -% ethylene; and comonomer mixtures of vinyl acetate with 10 to 20 wt . -% ethylene and 1 to 40 wt . -% of one or more further comonomers from the group vinyl esters with 1 to 12 C atoms in the carboxylic acid radical such as vinyl propionate, vinyl laurate, vinyl esters of alpha-branched carboxylic acids with 9 to 11 C atoms such as VeoVa9, VeoVal O, VeoVal l; and mixtures of vinyl acetate, 10 to 20 wt . -% ethylene and preferably 1 to 40 wt . -% acrylic acid esters of unbranched or branched alcohols with 1 to 15 C atoms, in particular n-butyl acrylate or 2-ethylhexyl acrylate; and mixtures with 30 to 75 wt . % vinyl acetate, 1 to 30 wt . % vinyl laurate or vinyl ester of an alpha-branched carboxylic acid with 9 to 11 C atoms, and 1 to 30 wt .-% acrylic acid esters of unbranched or branched alcohols having 1 to 15 C atoms, in particular n-butyl acrylate or 2-ethylhexyl acrylate, which also contain 10 to 20% by weight of ethylene; and mixtures with vinyl acetate, 10 to 20% by weight of ethylene and 1 to 60% by weight of vinyl chloride; where the mixtures may also contain the stated auxiliary monomers in the stated amounts, and the data in % by weight add up to 100% by weight in each case.

[0036] The polyvinyl esters are preferably bimodal or multimodal. The polyvinyl esters in the form of aqueous dispersions, with a solids content of 53% in water, have a viscosity of preferably 4,000 to 12,000 mPas, more preferably 5,000 to 10,000 mPas, and most preferably 7,000 to 8,000 mPas (determined using a Brookfield viscometer at 23°C and 20 rpm, using the spindle customarily used by those skilled in the art for the respective viscosity range).

[0037] The polyvinyl esters have weight-average particle diameters Dw of preferably 500 nm to 15 pm, particularly preferably 1 pm to 12 pm and most preferably 1 pm to 5 pm (determined by means of static light scattering with the measuring device LS 13320 from BeckmanCoulter).

[0038] The polyvinyl esters have a polydispersity PD of preferably > 2, more preferably 2 to 30, and particularly preferably 2.5 to 5. The polydispersity PD is known to be the ratio of the weight-average particle diameter Dw to the number-average particle diameter Dn, PD = Dw / Dn (determined by static light scattering using the BeckmanCoulter LS 13320 measuring device). The polyvinyl esters are preferably bimodal or multimodal.

[0039] The polyvinyl esters have glass transition temperatures Tg of preferably -30°C to +40°C, more preferably -20°C to +20°C, particularly preferably from -15°C to +10°C and most preferably from -10°C to 0°C. The monomers and the weight fractions of the comonomers are selected such that the aforementioned glass transition temperatures Tg result. The glass transition temperature Tg of the polymers is determined using a Mettler-Toledo DSC1 differential scanning calorimeter in a closed crucible at a heating rate of 10 K / min. The midpoint of the glass transition during the second heating cycle is evaluated. The Tg can also be approximately predicted using the Fox equation. According to Fox TG, Bull. Am. Physics Soc.

[0040] 1, 3, page 123 (1956) applies: 1 / Tg = X]_ / Tgj_ + xg / Tgg + ... + x n / day n , where x n is the mass fraction (wt% / 100) of the monomer n, and Tg nis the glass transition temperature in Kelvin of the homopolymer of the monomer n. Tg values ​​for homopolymers are listed in Polymer Handbook 2nd Edition, J. Wiley & Sons, New York (1975).

[0041] The polyvinyl esters preferably exhibit only one glass transition temperature Tg. The polyvinyl esters are preferably homogeneous and particularly preferably not heterophasic.

[0042] The invention further relates to processes for the preparation of polyvinyl esters in the form of aqueous dispersions by means of radically initiated emulsion polymerization of a) one or more vinyl esters and optionally b) one or more further ethylenically unsaturated monomers in an aqueous medium in the presence of at least two polyvinyl alcohols in the absence of emulsifiers, characterized in that all polyvinyl alcohols have a viscosity in the range from 8 to 30 mPas and at least two polyvinyl alcohols differ in their viscosity.

[0043] Emulsion polymerization is usually carried out in an aqueous medium, i.e. usually in the absence of organic solvents. In the copolymerization of gaseous comonomers such as ethylene, 1,3-butadiene or vinyl chloride, it is also possible to work under pressure, generally between 5 bar and 120 bar, preferably between 65 and 80 bar. The polymerization temperature is generally 40°C to 120°C, preferably 50°C to 80°C and particularly preferably 70 to 80°C. The polymerization is preferably initiated using the redox initiator combinations customary for emulsion polymerization. Examples of suitable oxidation initiators are the sodium, potassium and ammonium salts of peroxodisulfuric acid, hydrogen peroxide, t-butyl peroxide, t-butyl hydroperoxide, potassium peroxodiphosphate, tert. -Butyl peroxopivalate, cumene hydroperoxide, isopropylbenzene monohydroperoxide, azobisisobutyronitrile.Particularly preferred are the sodium, potassium, and ammonium salts of peroxodisulfuric acid and hydrogen peroxide. These initiators are generally used in an amount of 0.01 to 2.0 wt.%, based on the total weight of the monomers. The oxidizing agents mentioned, in particular the salts of peroxodisulfuric acid, can also be used alone as thermal initiators.

[0044] Suitable reducing agents are, for example, the sulfites and bisulfites of alkali metals and ammonium, such as sodium sulfite, the derivatives of sulfoxylic acid such as zinc or alkali formaldehyde sulfoxylates, for example sodium hydroxymethanesulfinate (Brüggolit), (iso)ascorbic acid or salts thereof, and mixtures of the salts of 2-hydroxy-2-sulfinatoacetic acid and 2-hydroxy-2-sulfonatoacetic acid with sodium sulfite (FF6). Preference is given to sodium sulfite, sodium bisulfite, and in particular (iso)ascorbic acid or its (alkali) alkali salts and FF6. The amount of reducing agent is preferably 0.015 to 3% by weight, based on the total weight of the monomers.

[0045] The polymerization is usually carried out at pH values ​​of < 9, preferably 2 to 9 and particularly preferably 3 to 8. The pH can be adjusted using conventional methods, such as acids, bases or, in particular, buffers such as sodium acetate or phosphates.

[0046] Regulators can be used during polymerization to control the molecular weight. If regulators are used, they are typically employed in amounts between 0.01 and 5.0 wt. %, based on the total weight of the monomers to be polymerized, and are added separately or premixed with the reaction components. Examples of such substances are n-dodecyl mercaptan, tert-dodecyl mercaptan, mercaptopropionic acid, methyl mercaptopropionate, isopropanol, and acetaldehyde. Preferably, no regulators are used.

[0047] The polymerization takes place in the presence of the polyvinyl alcohols mentioned above and, if appropriate, one or more other protective colloids. However, further protective colloids are preferably omitted. The dispersion adhesives or the polyvinyl esters in the form of aqueous dispersions therefore preferably contain no other protective colloids besides polyvinyl alcohols.Examples of other protective colloids are polyvinylpyrrolidones; polysaccharides in water-soluble form such as starches (amylose and amylopectin), celluloses and their carboxymethyl, methyl, hydroxyethyl, hydroxypropyl derivatives; proteins such as casein or caseinate, soy protein, gelatin; lignin sulfonates; synthetic polymers such as poly (meth) acrylic acid, copolymers of (meth) acrylates with carboxyl-functional comonomer units, poly (meth) acrylamide, polyvinylsulfonic acids and their water-soluble copolymers; melamine formaldehyde sulfonates, naphthalene formaldehyde sulfonates, styrene-maleic acid and vinyl ether-maleic acid copolymers.

[0048] The polyvinyl alcohols and any other protective colloids used are generally added in a total amount of 0.5 to 20 wt . % , based on the total weight of the monomers , during the emulsion polymerization .

[0049] Emulsion polymerization takes place in the absence of emulsifiers. Examples of emulsifiers are listed below.

[0050] The polymerization can be carried out in conventional polymerization reactors, for example in pressure reactors and / or pressureless reactors. Conventional, appropriately dimensioned steel reactors with stirring devices, heating / cooling systems and lines for supplying the reactants and removing the products can be used as pressure reactors or pressureless reactors. When using gaseous monomers, such as ethylene, a pressure reactor and, if appropriate, an unpressurized reactor are preferably used. The preferred working pressure in the pressure reactor is 3 to 120 bar, particularly preferably 10 to 80 bar. The preferred working pressure in the unpressurized reactor is 100 mbar to 5 bar, particularly preferably 200 mbar to 1 bar.

[0051] The polymerization is preferably carried out in a batch or semi-batch process, but can also be carried out in a continuous process.

[0052] In a batch or semi-batch process, for example, the monomers can be initially charged or metered in. The preferred procedure is to initially charge 20 to 100 wt. %, in particular 30 to 60 wt. %, based on the total weight, of the monomers, and to meter in the remaining amount of monomers at a later point in time during the emulsion polymerization. The metering can be carried out separately (spatially and temporally), or the components to be metered can be all or partially pre-emulsified.

[0053] The polyvinyl alcohols and any additional protective colloids used can, for example, be introduced in their entirety or partially. Preferably, at least 25% by weight, particularly preferably at least 70% by weight, of the polyvinyl alcohols and any additional protective colloids are introduced in their entirety, based in each case on the total amount of polyvinyl alcohols and, if present, additional protective colloids used. Most preferably, the polyvinyl alcohols and any additional protective colloids are introduced in their entirety.

[0054] For example, the initiators can be added either in full or in part. Preferably, the initiators are added in full.

[0055] Preferably, a post-polymerization is carried out after completion of the polymerization. During the post-polymerization, any remaining amounts of residual monomer are polymerized. The post-polymerization is carried out using known methods, generally with redox catalyst-initiated post-polymerization.

[0056] Volatile compounds, such as residual monomer or impurities from initiator components or other raw materials, can also be removed from the aqueous dispersion by distillation or stripping. During stripping, volatile compounds are removed from the dispersions, optionally under reduced pressure, by passing or overflowing inert carrier gases such as air, nitrogen, or steam.

[0057] The polyvinyl esters in the form of aqueous dispersions have a solids content of preferably 30 to 75 wt . -% , particularly preferably 50 to 60 wt . -% .

[0058] In one embodiment, the aqueous dispersions of the polyvinyl alcohol-stabilized polyvinyl esters do not contain any emulsifiers.

[0059] In an alternative process for producing aqueous polyvinyl ester dispersions, one or more emulsifiers are added after the emulsion polymerization. The invention further relates to processes for producing polyvinyl esters in the form of aqueous dispersions by means of free-radical-initiated emulsion polymerization of a) one or more vinyl esters and optionally b) one or more further ethylenically unsaturated monomers in an aqueous medium in the presence of at least two polyvinyl alcohols in the absence of emulsifiers, characterized in that all polyvinyl alcohols have a viscosity in the range from 8 to 30 mPas and at least two polyvinyl alcohols differ in their viscosity and one or more emulsifiers are added after the emulsion polymerization has been carried out.

[0060] The invention further relates to polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions obtainable by means of radically initiated emulsion polymerization of a) one or more vinyl esters and optionally b) one or more further ethylenically unsaturated monomers in an aqueous medium in the presence of at least two polyvinyl alcohols in the absence of emulsifiers, characterized in that all polyvinyl alcohols have a viscosity in the range from 8 to 30 mPas and at least two polyvinyl alcohols differ in their viscosity and one or more emulsifiers are added after the emulsion polymerization has been carried out (post-addition).

[0061] The emulsifiers are therefore added after the emulsion polymerization has been carried out, i.e., for example, at a conversion of the total monomers used, in particular of the total vinyl esters used, of > 95%, in particular > 97%. The conversion of the monomers can be determined, for example, by means of 1H NMR spectroscopy, preferably using the vinyl esters, particularly preferably using vinyl acetate. The post-addition of the emulsifiers takes place, for example, after the addition of all the initiator quantities for the emulsion polymerization. The post-addition of the emulsifiers is preferably carried out after the post-polymerization.

[0062] Examples of emulsifiers are anionic, cationic, non-ionic or amphoteric emulsifiers.

[0063] Examples of anionic emulsifiers are alkyl sulfates with a chain length of 8 to 18 C atoms, alkyl or alkylaryl ether sulfates with 8 to 18 C atoms in the hydrophobic radical and up to 40 ethylene or propylene oxide units, alkyl or alkylaryl sulfonates with 8 to 18 C atoms, esters and half esters of sulfosuccinic acid with monohydric alcohols or alkylphenols.

[0064] Examples of non-ionic emulsifiers are alkyl polyglycol ethers with 8 to 40 ethylene oxide units or preferably gemini surfactants.

[0065] Preferred gemini surfactants are alkyne derivatives containing two alcohol groups. Particularly preferred gemini surfactants are alkynediol derivatives in which one or, in particular, both of the alcohol groups are substituted with polyethylene glycol radicals, for example, with polyethylene glycol chains having 1 to 50 ethylene glycol units. Also particularly preferred as gemini surfactants are reaction products of epoxides with alkynediol derivatives, where one or both of the alcohol groups of the alkynediol derivatives may be transformed with epoxides.

[0066] Examples of amphoteric emulsifiers are betaines such as coco-

[0067] Dipropionate and its salts, 2-ethylhexyl dipropionate and its salts, cocoamphodipropionate and its salts, sultaines such as cocamidopropyl hydroxysultaine and its salts, and amino acids and their salts. Disodium 2-ethylhexyl dipropionate is particularly preferred as an amphoteric emulsifier.

[0068] By means of post-addition, preferably up to 5 wt.%, particularly preferably 0.05 to 2 wt.% and most preferably 0.1 to 1 wt.% of emulsifiers are introduced, based on the dry weight of the polyvinyl esters.

[0069] Any emulsifiers are preferably introduced into the polyvinyl ester dispersions exclusively by post-addition.

[0070] Preferably, no protective colloids, more preferably no polyvinyl alcohols, and most preferably no polyvinyl alcohols with the viscosity according to the invention are introduced into the polyvinyl ester dispersions by means of post-addition. Alternatively, although less preferably, protective colloids, in particular polyvinyl alcohols, can also be introduced into the polyvinyl ester dispersions by means of post-addition. Preferably, <5% by weight, more preferably <0.9% by weight, and even more preferably <0.4% by weight of protective colloids, in particular polyvinyl alcohol, are introduced by means of post-addition, based on the dry weight of the polyvinyl esters. Preferably, <5% by weight, more preferably <0.9% by weight, and even more preferably <0.4% by weight of protective colloids, in particular polyvinyl alcohol, are introduced by means of post-addition, based on the total weight of the polyvinyl alcohols present in the polyvinyl ester dispersions.

[0071] The invention further relates to dispersion adhesives comprising one or more polyvinyl alcohol-stabilized polyvinyl esters, one or more additives and water, characterized in that the polyvinyl alcohol-stabilized polyvinyl esters are obtainable by means of radically initiated emulsion polymerization of a) one or more vinyl esters and optionally b) one or more further ethylenically unsaturated monomers in an aqueous medium in the presence of at least two polyvinyl alcohols in the absence of emulsifiers, wherein all polyvinyl alcohols of the polyvinyl alcohol-stabilized polyvinyl esters have a viscosity in the range from 8 to 30 mPas and at least two polyvinyl alcohols differ in their viscosity and optionally one or more emulsifiers are added after the emulsion polymerization has been carried out (post-addition).

[0072] The dispersion adhesives preferably contain at least 40 wt.%, particularly preferably at least 50 wt.%, and most preferably at least 60 wt.% polyvinyl ester. The dispersion adhesives preferably contain at most 99 wt.%, and particularly preferably at most 95 wt.% polyvinyl ester. The wt.% values ​​refer to the dry weight of the dispersion adhesives.

[0073] Optionally, the dispersion adhesives also contain one or more additives, for example plasticizers such as phthalates, benzoates or adipates, film-forming agents such as triacetin or glycols, in particular butyl glycol, butyl diglycol, butyldipropylene glycol and butyltripropylene glycol, wetting agents, generally surfactants, thickeners such as polyacrylates, polyurethanes, cellulose ethers or polyvinyl alcohols, defoamers, tackifiers or other additives customary for formulating adhesives. The proportion of these additives can be, for example, up to 40 wt.%, preferably 0 to 25 wt.%, more preferably 1 to 15 wt.%, particularly preferably 1 to 10 wt.% and most preferably 1 to 5 wt.%, each based on the dry weight of the dispersion adhesives.

[0074] The dispersion adhesives have a solids content of preferably 30 to 75 wt.%, particularly preferably 50 to 60 wt.%. The remaining portion is preferably water. The total amounts of solids and water add up to 100 wt.%.

[0075] The dispersion adhesives can be produced using conventional methods, generally by mixing the aforementioned components. Mixing can be carried out in conventional mixers, such as stirrers or dissolvers. Mixing is preferably carried out at temperatures of 5 to 50°C, particularly preferably 15 to 40°C, and most preferably 20 to 30°C.

[0076] A further subject of the invention are methods for applying dispersion adhesives according to the invention by means of mechanical application methods.

[0077] The dispersion adhesives according to the invention can be used in conventional mechanical application processes for dispersion adhesives, such as nozzle or roller application processes. The dispersion adhesives are applied to substrates. Application can be continuous, line-like, or spot-like. The dispersion adhesives according to the invention are suitable for bonding various substrates, preferably paper, cardboard, wood, fiber materials, coated cartons, as well as for bonding cellulosic materials to plastics, such as polymer films, for example, polyethylene, polyvinyl chloride, polyamide, polyester, or polystyrene films. The dispersion adhesives are used in particular as paper adhesives, packaging adhesives, wood adhesives, and adhesives for woven and nonwoven fiber materials.The dispersion adhesives are particularly suitable for bonding cellulosic substrates, in particular paper, cardboard or cotton fabric, with plastic films, or for bonding plastic films to one another (film / film bonding).

[0078] The dispersion adhesives of the invention are ideally suited for application by mechanical application methods. Thus, the occurrence of undesirable adhesive deposits at the application nozzle or uncontrolled "splashes" can be avoided to the desired extent with the dispersion adhesives of the invention. The dispersion adhesives exhibit advantageous rheological properties, such as low shear thinning. The dispersion adhesives of the invention also enable the rapid setting speed required for mechanical processes to be achieved. The dispersion adhesives are also storage-stable and have advantageous wet bonding properties.

[0079] It was also surprising that the inventive procedure eliminates the need for emulsifiers to the dispersion adhesives or polyvinyl ester dispersions, yet the dispersion adhesives of the invention can still be applied mechanically and achieve the desired performance properties. This also enables the use of the polyvinyl ester dispersions in food contact applications where emulsifier-containing dispersion adhesives are critical or not approved.

[0080] By subsequently adding emulsifiers, in particular amphoteric emulsifiers, to the polymer dispersions, i.e. by adding emulsifiers after polymerization, deposits at the application nozzle during mechanical application of the dispersion adhesives can be further reduced.

[0081] The following examples serve to further illustrate the invention. The Höppler viscosities given below for polyvinyl alcohols were determined at 20°C in a 4% aqueous solution according to DIN 53015. The Brookfield viscosities (BF20) of the aqueous polyvinyl ester dispersions were determined at the specified solids content at 23°C using a Brookfield viscometer at 20 rpm.

[0082] Example 1 (Ex.l):

[0083] The following components were placed in a pressure reactor with a volume of 600 liters: 122 kg water,

[0084] 58 kg of a 10% aqueous solution of polyvinyl alcohol with a degree of hydrolysis of 88% and a viscosity according to Höppler of a 4% aqueous solution of 23 mPas (523),

[0085] 58 kg of a 10% aqueous solution of polyvinyl alcohol with a degree of hydrolysis of 88% and a viscosity according to Höppler of a 4% aqueous solution of 13 mPas (513), 200 g of formic acid 98%, 142 g of iron (II) ammonium sulfate solution (10% in water).

[0086] The pressure reactor was evacuated, and 91 kg of vinyl acetate were added to the initial charge. The reactor was then heated to 50°C and pressurized to 45 bar of ethylene (corresponding to 34 kg of ethylene).

[0087] The polymerization was initiated by metering in a 3% aqueous hydrogen peroxide solution at a rate of 443 g / h and a 10% aqueous Brüggolit FF6 solution at a rate of 443 g / h. At the start of polymerization, the temperature was increased from 50°C to 75°C. Ten minutes after the start of polymerization, vinyl acetate was metered in at a rate of 60 kg / h for 2 hours, followed by ethylene at a rate of 18 kg / h for 2 hours.

[0088] After the vinyl acetate addition was complete, the addition of the hydrogen peroxide solution and the Brüggolit FF6 solution was continued for another 60 minutes. The total polymerization time was 3.5 hours.

[0089] The resulting polymer dispersion was then transferred to a pressureless reactor. A pressure of 0.7 mbar was applied to the pressureless reactor. 2.2 kg of a 10% aqueous tert-butyl hydroperoxide solution and 1.6 kg of a 10% aqueous Brüggolit FF6 solution were added to the pressureless reactor, and polymerization was continued. The pH was adjusted to 4.5 by adding 10% aqueous sodium hydroxide solution. Finally, the mixture was filtered through a 150 μm mesh sieve.

[0090] The properties of the polymer dispersion are listed in Table 1 .

[0091] Comparative example 1 (VBsp.l):

[0092] The polymerization was carried out analogously to Example 1, with the difference that instead of the polyvinyl alcohol with a viscosity of 13 mPas (513), 58 kg of a 10% aqueous solution of a polyvinyl alcohol (05 / 88) with a degree of hydrolysis of 88% and a viscosity according to Höppler of a 4% aqueous solution of 5 mPas was used.

[0093] The properties of the polymer dispersion are listed in Table 1 .

[0094] Comparative example 2 (VBsp.2):

[0095] The polymerization was carried out analogously to Example 1, with the difference that 12 kg of a 10% aqueous solution of a third polyvinyl alcohol with a Höppler viscosity (4% in aqueous solution) of 5 mPas (05 / 88) were additionally used.

[0096] The properties of the polymer dispersion are listed in Table 1. Comparative Example 3 (VBsp. 3):

[0097] Aqueous dispersion of a vinyl acetate-ethylene copolymer prepared according to Example 2 of DE102006037318 .

[0098] The properties of the polymer dispersion are listed in Table 1 .

[0099] Comparative example 4 (VBsp . 4 ):

[0100] Aqueous dispersion of a polyvinyl alcohol-stabilized vinyl acetate-ethylene copolymer prepared according to Example 2 of DE102013226114, i.e. the polyvinyl alcohol stabilization was carried out by three different polyvinyl alcohols with a viscosity of 5 mPas, 23 mPas and 40 mPas, respectively.

[0101] The properties of the polymer dispersion are listed in Table 1 .

[0102] Comparative example 5 (VBsp . 5 ):

[0103] Aqueous dispersion of a purely emulsifier-stabilized vinyl acetate-ethylene copolymer prepared according to example 4b of WO2022 / 055511 with post-addition of polyvinyl alcohol.

[0104] The properties of the polymer dispersion are listed in Table 1 .

[0105] Comparative example 6 (VBsp . 6 )

[0106] Commercially available vinyl acetate-ethylene dispersion stabilized with polyvinyl alcohol and emulsifier. The dispersion had a solids content of 53.5%, a Brookfield viscosity of 7600 mPas, and a glass transition temperature Tg of -7 °C.

[0107] Example 2 (Ex. 2):

[0108] After polymerization, 0.25%, based on the total dispersion, of the gemini surfactant Surfynol 465 (trade name of Evonik Industries AG) was added to the polymer dispersion from Example 1 by post-addition.

[0109] The properties of the polymer dispersion are listed in Table 1 .

[0110] Table 1: Properties of the polymer dispersions: a) Polyvinyl alcohol with a degree of hydrolysis of 88 mol% and a Höppler viscosity of 5 mPas; b) Polyvinyl alcohol with a degree of hydrolysis of 88 mol% and a Höppler viscosity of 13 mPas; c) Polyvinyl alcohol with a degree of hydrolysis of 88 mol% and a Höppler viscosity of 23 mPas; d) Polyvinyl alcohol with a degree of hydrolysis of 88 mol% and a Höppler viscosity of 50 mPas; e) Data in wt.% refer to the total dispersion; f) EG = solids content; g) BF20 = Brookfield viscosity; h) Tg = glass transition temperature

[0111] Example 3 (Ex.3):

[0112] After polymerization, 0.20%, based on the total dispersion, of the amphoteric emulsifier sodium N-(2-carboxyethyl)-N-(2-ethylhexyl)-ß-alaninate (Librateric BA-40, trade name of Libra Specialty Chemicals LTD) was added to the polymer dispersion from Example 1 by post-addition. The properties of the polymer dispersion are listed in Table 1.

[0113] Nozzle application process: Determination of the web structure:

[0114] The dispersion adhesives were applied to a rotating stainless steel roller using a nozzle. The stainless steel roller had a circumference of 80 cm and rotated around its own axis at a speed of 120 or 140 revolutions per minute (RPM). The dispersion adhesives were applied using an HHS application system with GKD4-114-2m valves and LVK-4 nozzles. The nozzles were mounted vertically above the roller surface at a distance of 4 mm.

[0115] The dispersion adhesives were diluted with water to a viscosity of 800 mPas and fed to the nozzles via hose lines using a piston pump at a pressure of 9 bar. The dispersion adhesives were applied to the stainless steel roller through the nozzles in a pulsed manner, opening and closing the nozzles at a consistent rhythm. A cycle of single nozzle opening and closing is referred to as a pulse. 18 nozzle pulses were set per revolution of the stainless steel roller. The dispersion adhesives were immediately scraped off the stainless steel roller using a plastic squeegee. The testing was conducted under standard conditions at 23°C and a relative humidity of 50%.

[0116] 120 minutes after the start of nozzle application, the size of the cone-shaped buildup (web buildup) was measured at the nozzle. The test results are listed in Table 2. If the cone-shaped buildup reached almost to the roller surface before the 120 minutes had elapsed, the test was aborted and the measured value was reported as > 4 mm. Determination of the manual setting speed:

[0117] A cardboard (for example Zenith ZENP235 (235 g / m 2 ) ) was cut to 45 cm long and 10 cm wide. The coated side of the cardboard was marked with a cm scale. The adhesive (50 pm wet film thickness) was applied using a squeegee.

[0118] Immediately afterwards, a paper strip (for example Vari- tess 290.150 (150 g / m 2) Co. Lahnstein; length 55 cm and width 5 cm) was placed on the adhesive film and adhered using a hand roller. The paper strip was immediately peeled off (by hand at a speed of 1 cm / s). When a significant fiber tear occurred, the distance traveled until the fiber tear was correlated with time (1 cm travel distance corresponds to 1 s). This value indicated the setting time of the adhesive and is shown in Table 2 for the respective example.

[0119] Table 2: Test results:

Claims

Patent claims:

1. Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions, characterized in that the polyvinyl esters are stabilized by at least two polyvinyl alcohols, wherein all polyvinyl alcohols have a viscosity in the range from 8 to 30 mPas and at least two polyvinyl alcohols differ in their viscosity, with the proviso that the polyvinyl alcohol-stabilized polyvinyl esters are not emulsifier-stabilized.

2. Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions according to claim 1, characterized in that at least one polyvinyl alcohol (polyvinyl alcohol a)) has a viscosity of 8 to 18 mPas. 3 . Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions according to claim 1 or 2, characterized in that the proportion of polyvinyl alcohols a) is 30 to 70 wt . -%, based on the total weight of the polyvinyl alcohols contained in the polyvinyl ester dispersion.

4. Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions according to claims 1 to 3, characterized in that the proportion of polyvinyl alcohols a) is 0.5 to 5 wt.%, based on the dry weight of the polyvinyl esters. 5 . Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions according to claims 1 to 4, characterized characterized in that at least one polyvinyl alcohol (polyvinyl alcohol ß) ) has a viscosity of 19 to 30 mPas.

6. Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions according to claims 1 to 5, characterized in that the proportion of polyvinyl alcohols ß) is 30 to 70 wt.%, based on the total weight of the polyvinyl alcohols contained in the polyvinyl ester dispersion.

7. Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions according to claims 1 to 6, characterized in that the proportion of polyvinyl alcohols ß) is 0.5 to 5 wt.%, based on the dry weight of the polyvinyl esters.

8. Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions according to claims 1 to 7, characterized in that one or more polyvinyl esters are selected from the group comprising vinyl ester homopolymers, vinyl ester-ethylene copolymers, vinyl ester copolymers containing one or more vinyl ester units and one or more further monomer units from the group comprising vinyl aromatics, vinyl halides, acrylic acid esters, methacrylic acid esters and optionally ethylene.

9. Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions according to claims 1 to 8, characterized in that the polyvinyl esters in the form of aqueous dispersions have a viscosity of 4,000 to 12,000 mPas (determined with a Brookfield viscometer, at 23°C and 20 rpm) at a solids content of 53%.

10. Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions according to claims 1 to 9, characterized in that the aqueous dispersions of the polyvinyl alcohol-stabilized polyvinyl esters do not contain any emulsifiers.

11. A process for the preparation of polyvinyl esters in the form of aqueous dispersions by means of radically initiated emulsion polymerization of a) one or more vinyl esters and optionally b) one or more further ethylenically unsaturated monomers in an aqueous medium in the presence of at least two polyvinyl alcohols in the absence of emulsifiers, characterized in that all polyvinyl alcohols have a viscosity in the range from 8 to 30 mPas and at least two polyvinyl alcohols differ in their viscosity. 12 . A process for the preparation of polyvinyl esters in the form of aqueous dispersions according to claim 11 , characterized in that one or more emulsifiers are added after the emulsion polymerization has been carried out .

13. Polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions obtainable according to claim 12.

14. Dispersion adhesives containing one or more polyvinyl alcohol-stabilized polyvinyl esters in the form of aqueous dispersions according to claims 1 to 10 or 13, one or more additives and water.

15. A method for applying the dispersion adhesives of claim 14 by means of a mechanical application process.

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