Aqueous emulsion and adhesive agent

Abstract

This aqueous emulsion comprises an ethylene-vinyl alcohol copolymer (A), a vinyl alcohol-based polymer (B), and a polymer (C) including an ethylenically unsaturated monomer unit. The ethylene-vinyl alcohol copolymer (A) includes ethylene units in an amount of not less than 0.1 mol% to less than 20 mol%, and has a viscosity average degree of polymerization of 200-800 and a saponification degree of 90.0-99.9 mol%. The vinyl alcohol-based polymer (B) has a viscosity average degree of polymerization of 900-4000. This adhesive agent comprises the aqueous emulsion. Such an aqueous emulsion exhibits excellent water-resistant adhesiveness, viscosity stability, polymerization stability, and low water absorption.

Description

Water-based emulsions and adhesives

[0001] This invention relates to aqueous emulsions and adhesives.

[0002] Conventionally, vinyl alcohol polymers have been widely used as protective colloids for the emulsion polymerization of ethylenically unsaturated monomers, particularly vinyl ester monomers represented by vinyl acetate. Aqueous vinyl ester emulsions obtained by emulsion polymerization using these as protective colloids are widely used in various fields such as adhesives for paper, woodworking, and plastics, binders for impregnated paper and nonwoven products, admixtures, jointing materials, paints, paper processing, and textile processing. For example, Patent Document 1 discloses an aqueous emulsion composition containing a vinyl alcohol polymer having a silyl group, but further improvements are needed in terms of water-resistant adhesion, viscosity stability, polymerization stability, and low water absorption.

[0003] Japanese Patent Publication No. 2007-23148

[0004] The present invention aims to provide aqueous emulsions and adhesives that exhibit excellent water resistance, viscosity stability, polymerization stability, and low water absorption.

[0005] The present invention is as follows: [1] An aqueous emulsion comprising an ethylene-vinyl alcohol copolymer (A), a vinyl alcohol polymer (B), and a polymer (C) containing ethylenically unsaturated monomer units, wherein the ethylene unit content of the ethylene-vinyl alcohol copolymer (A) is 0.1 mol% or more and less than 20 mol%, the viscosity-average degree of polymerization is 200 or more and 800 or less, the degree of saponification is 90.0 mol% or more and 99.9 mol or less, and the viscosity-average degree of polymerization of the vinyl alcohol polymer (B) is 900 or more and 4000 or less. [2] The aqueous emulsion according to [1], wherein the total content of carboxyl groups and lactone rings located at the ends of the molecular chain of the ethylene-vinyl alcohol copolymer (A) is 0.08 mol% or more. [3] The aqueous emulsion according to [1] or [2], wherein the vinyl alcohol polymer (B) does not have silyl groups. [4] An adhesive comprising the aqueous emulsion according to any one of [1] to [3].

[0006] The aqueous emulsion of the present invention exhibits excellent water-resistant adhesion, viscosity stability, polymerization stability, and low water absorption.

[0007] The aqueous emulsion of the present invention is an aqueous emulsion comprising an ethylene-vinyl alcohol copolymer (A), a vinyl alcohol polymer (B), and a polymer (C) containing ethylenically unsaturated monomer units, wherein the ethylene unit content of the ethylene-vinyl alcohol copolymer (A) is 0.1 mol% or more and less than 20 mol%, the viscosity-average degree of polymerization is 200 or more and 800 or less, and the degree of saponification is 90.0 mol% or more and 99.9 mol or less, and the viscosity-average degree of polymerization of the vinyl alcohol polymer (B) is 900 or more and 4000 or less.

[0008] In an aqueous emulsion, the ethylene-vinyl alcohol copolymer (A) and the vinyl alcohol polymer (B) may be dispersants, and the polymer (C) containing ethylenically unsaturated monomer units may be the dispersed phase. In a preferred embodiment of the present invention, both the ethylene-vinyl alcohol copolymer (A) and the vinyl alcohol polymer (B) are dispersants, and the polymer (C) containing ethylenically unsaturated monomer units is the dispersed phase.

[0009] (Ethylene-vinyl alcohol copolymer (A)) The aqueous emulsion of the present invention contains ethylene-vinyl alcohol copolymer (A), wherein the ethylene-vinyl alcohol copolymer (A) has an ethylene unit content of 0.1 mol% or more and less than 20 mol%, a viscosity-average degree of polymerization of 200 or more and 800 or less, and a degree of saponification of 90.0 mol% or more and 99.9 mol or less.

[0010] Vinyl alcohol polymers are polymers that have vinyl alcohol units as monomer units. Vinyl alcohol polymers are obtained by saponifying vinyl ester polymers, which are formed by polymerizing vinyl ester monomers, and the vinyl alcohol polymer after saponification may contain vinyl ester units in addition to vinyl alcohol units.

[0011] Furthermore, vinyl alcohol polymers can be modified by copolymerizing a vinyl ester monomer (the raw material monomer) with other monomers, saponifying the copolymer to obtain a vinyl alcohol polymer containing monomer units other than vinyl alcohol units and vinyl ester units (copolymerization modification), or by reacting a specific chemical species with the vinyl alcohol polymer during or after saponification to introduce a specific functional group (post-modification). In this disclosure, such vinyl alcohol polymers may be referred to as "modified vinyl alcohol polymers," and the other monomers and the specific chemical species may be referred to as "modified species." In addition, vinyl alcohol polymers that have not undergone copolymerization modification or post-modification as described above are polymers consisting substantially only of vinyl alcohol units and optionally only vinyl ester units, and in this specification, such vinyl alcohol polymers may be referred to as "unmodified vinyl alcohol polymers." Unmodified vinyl alcohol polymers may have structures derived from, for example, polymerization initiators or chain transfer agents at the ends of the polymer chains. In this specification, vinyl alcohol polymers may be referred to as "PVA," and for example, modified vinyl alcohol polymers may be referred to as "modified PVA," and unmodified vinyl alcohol polymers as "unmodified PVA."

[0012] Ethylene-vinyl alcohol copolymer is an ethylene-modified vinyl alcohol polymer obtained by saponifying an ethylene-vinyl ester copolymer, which is formed by copolymerizing ethylene with a vinyl ester monomer. Ethylene-vinyl alcohol copolymer is a copolymer having ethylene units, vinyl alcohol units, and optionally vinyl ester units. Vinyl acetate is a typical example of a vinyl ester monomer, but other fatty acid vinyl esters (such as vinyl formate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate) can also be used. The ethylene-vinyl alcohol copolymer (A) contained in the aqueous emulsion of the present invention may be denoted as "PVA(A)".

[0013] The ethylene unit content in PVA(A) is 0.1 mol% or more and less than 20 mol%. Here, the ethylene unit content in PVA(A) refers to the ratio of ethylene units to the total monomer units constituting PVA(A). The lower limit of the ethylene unit content in PVA(A) is preferably 1 mol%, more preferably 3 mol%, and still preferably 5 mol%. When the ethylene unit content in PVA(A) is above the lower limit, the viscosity stability and polymerization stability of the aqueous emulsion tend to be better. The upper limit of the ethylene unit content in PVA(A) is preferably 15 mol%, more preferably 12 mol%, still preferably 10 mol%, and still preferably 9 mol%.

[0014] The ethylene unit content of PVA(A) is, for example, that of ethylene-vinyl ester copolymer, which is a precursor or re-acetic acid product of PVA(A). １ It can be determined from H-NMR measurements.

[0015] The degree of saponification of PVA(A) is between 90.0 mol% and 99.9 mol%. If the degree of saponification is less than 90.0 mol%, the water-resistant adhesion of the aqueous emulsion will be insufficient. Preferably, the degree of saponification is 95.0 mol% or higher, and more preferably 96.0 mol% or higher. On the other hand, if the degree of saponification exceeds 99.9 mol%, it becomes difficult to stably produce the ethylene-vinyl alcohol copolymer, and it becomes difficult to achieve both water-resistant adhesion and viscosity stability. Preferably, the degree of saponification is 99.0 mol% or lower, and more preferably less than 98.0 mol%. The degree of saponification can be measured in accordance with JIS K6726 (1994).

[0016] The preferred viscosity-average degree of polymerization for PVA(A) is 200 to 800. If the viscosity-average degree of polymerization is less than 200, the water-resistant adhesion may be insufficient. More preferably, the viscosity-average degree of polymerization is 250 or higher, and even more preferably 300 or higher. More preferably, the viscosity-average degree of polymerization is 700 or lower, even more preferably 600 or lower, and particularly preferably 500 or lower. When the viscosity-average degree of polymerization is within the above range, the viscosity of the resulting aqueous emulsion is within an appropriate range, and tends to be easier to handle. The viscosity-average degree of polymerization can be measured in accordance with JIS K6726 (1994).

[0017] In PVA(A), it is preferable that the total content of carboxyl groups and lactone rings located at the ends of the molecular chain is 0.08 mol% or more. Having carboxyl groups or lactone rings at the ends of the molecular chain tends to further suppress the decrease in viscosity stability of the aqueous emulsion. It is more preferable that the total content of terminal carboxyl groups and terminal lactone rings in PVA(A) is 0.09 mol% or more. On the other hand, it is preferable that the total content is less than 0.2 mol%, and more preferably 0.18 mol% or less. Note that terminal carboxyl groups and terminal lactone rings are interconvertible depending on the environment. The total content of terminal carboxyl groups and terminal lactone rings is measured at 600 MHz. １ This can be determined from measurements using an H-NMR (JEOL-600) instrument.

[0018] PVA(A) may have carboxyl groups or lactone rings located in the interior (side chains) of its molecular chain, rather than at the ends. Specifically, by including a monomer capable of generating carboxyl groups or lactone rings as a copolymer component, carboxyl groups or lactone rings are introduced into the interior of the molecular chain, rather than at the ends. In this case, it is preferable that the total content of carboxyl groups and lactone rings located at the ends or interior of the molecular chain is 0.08 mol% or more and less than 0.2 mol%.

[0019] Monomers capable of generating carboxyl groups or lactone rings in the side chains of PVA molecular chains include monomers having carboxyl groups such as fumaric acid, maleic acid, itaconic acid, maleic anhydride, or itaconic anhydride, or their anhydrides; acrylic acid and its salts; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, and i-propyl acrylate; methacrylic acid and its salts; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, and i-propyl methacrylate; acrylamide derivatives such as acrylamide, N-methylacrylamide, and N-ethylacrylamide; and methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, and N-ethylmethacrylamide.

[0020] PVA(A) may contain monomer units other than vinyl alcohol units, ethylene units, vinyl ester units, and monomer units capable of generating carboxyl groups or lactone rings, as long as the effects of the present invention are not impaired. Examples of such monomer units include α-olefins such as propylene, n-butene, and isobutylene; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; nitriles such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride and vinyl fluoride; vinylidenes such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; vinyl silyl compounds such as vinyltrimethoxysilane; and those derived from isopropenyl acetate, etc. The content of these other monomer units in PVA(A) varies depending on the purpose and use, but is preferably less than 10 mol%, more preferably less than 5 mol%, even more preferably less than 1 mol%, and particularly preferably less than 0.5 mol%.

[0021] A preferred method for producing PVA(A) is a method for producing an ethylene-vinyl alcohol copolymer, which involves copolymerizing ethylene and vinyl acetate in the presence of methanol solvent and a radical initiator to obtain an ethylene-vinyl acetate copolymer, and then saponifying it to obtain an ethylene-vinyl alcohol copolymer, wherein the copolymerization is carried out with a mass ratio of vinyl acetate to methanol of 1.5 or more. In this method, when copolymerizing ethylene and vinyl acetate in the presence of methanol solvent and a radical initiator, setting the mass ratio of vinyl acetate to methanol to 1.5 or more facilitates the introduction of carboxyl groups and lactone rings at the ends of the molecular chains. The steps of the production method will be described in detail below.

[0022] The copolymerization method of ethylene and vinyl ester is not particularly limited, but solution polymerization in an alcohol solution is preferred. Examples of the alcohol include lower alcohols such as methanol and ethanol, but methanol is preferred. That is, it is preferable to copolymerize ethylene and vinyl acetate in the presence of methanol solvent and a radical initiator. Here, copolymerizing with a mass ratio of vinyl acetate to methanol of 1.5 or more is preferable because it is possible to efficiently introduce carboxyl groups or lactone rings to the ends of the molecular chains. More preferably, this mass ratio is 2 or more. Also, this mass ratio is usually 10 or less, and preferably 5 or less.

[0023] The initiator used in copolymerization is not particularly limited, but examples include peroxide-based initiators such as n-propyl peroxydicarbonate and benzoyl peroxide, and azo-based initiators such as 2,2'-azobis(isobutyronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and 2,2'-azobis(2,4-dimethylvaleronitrile). Among these, peroxide-based initiators are preferred from the viewpoint of efficiently introducing carboxyl groups or lactone rings to the ends of molecular chains, and n-propyl peroxydicarbonate is particularly preferred.

[0024] In polymerization operations, batch, semi-batch, and continuous polymerization methods can be employed. Examples of polymerization reactors include batch reactors, tubular reactors, and continuous tank reactors. There are no particular limitations on the polymerization temperature, but 0 to 180°C is preferred, room temperature to 160°C is more preferred, and 30 to 150°C is even more preferred. When polymerization is performed below the boiling point of the solvent used, either vacuum boiling polymerization or atmospheric pressure non-boiling polymerization can be selected. When polymerization is performed above the boiling point of the solvent used, either pressurized non-boiling polymerization or pressurized boiling polymerization can be selected.

[0025] The ethylene pressure in the polymerization reactor during polymerization is preferably 0.1 to 2 MPa, more preferably 0.2 to 1.5 MPa, and even more preferably 0.3 to 1.0 MPa. The polymerization rate at the outlet of the polymerization reactor is not particularly limited, but is preferably 20 to 90%, and more preferably 30 to 80%.

[0026] In the polymerization process, a chain transfer agent may be added to adjust the viscosity-average degree of polymerization of the resulting ethylene-vinyl ester copolymer. Examples of chain transfer agents include aldehydes such as acetaldehyde, propionaldehyde, butyraldehyde, and benzaldehyde; ketones such as acetone, methyl ethyl ketone, hexanone, and cyclohexanone; mercaptans such as 2-hydroxyethanethiol; thiocarboxylic acids such as thioacetic acid; and halogenated hydrocarbons such as trichloroethylene and perchloroethylene. Among these, aldehydes and ketones are preferably used. The amount of chain transfer agent added is determined according to the chain transfer constant of the added chain transfer agent and the viscosity-average degree of polymerization of the desired ethylene-vinyl ester copolymer, but is usually 0.1 to 10 parts by mass per 100 parts by mass of vinyl ester used.

[0027] The ethylene-vinyl ester copolymer obtained in the polymerization step is saponified in an organic solvent by alcohol decomposition or hydrolysis in the presence of a catalyst. Catalysts used in the saponification step include basic catalysts such as sodium hydroxide, potassium hydroxide, and sodium methoxide; or acidic catalysts such as sulfuric acid, hydrochloric acid, and p-toluenesulfonic acid. The organic solvent used in the saponification step is not particularly limited, but examples include alcohols such as methanol and ethanol; esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone; and aromatic hydrocarbons such as benzene and toluene. These can be used individually or in combination of two or more. Among these, it is convenient and preferable to use methanol, or a mixed solution of methanol and methyl acetate, as the solvent and carry out the saponification reaction in the presence of sodium hydroxide, which is a basic catalyst. The amount of saponification catalyst used is preferably 0.01 to 1 in molar ratio to vinyl ester monomer units in the ethylene-vinyl ester copolymer. More preferably, this molar ratio is 0.1 or higher, and even more preferably 0.2 or higher. On the other hand, the molar ratio is more preferably 0.7 or less, and even more preferably 0.5 or less.

[0028] A preferred embodiment of the saponification process is as follows. First, a saponification catalyst such as sodium hydroxide is added to the ethylene-vinyl ester copolymer solution obtained in the polymerization process and mixed. The solvent at this time is preferably methanol. Initially, the mixture is a homogeneous liquid, but as the saponification reaction proceeds and the vinyl ester units in the polymer are saponified and converted into vinyl alcohol units, the solubility in the solvent decreases and the polymer precipitates in the solution. At this time, the solution contains methyl acetate produced by alcoholesis with methanol. As the saponification reaction proceeds, the amount of precipitated polymer gradually increases and becomes a slurry, after which a solid block containing the ethylene-vinyl alcohol copolymer and the solvent is obtained.

[0029] The saponification reaction may be carried out in the solid block described above before pulverization, or the saponification reaction may be carried out after pulverization. The saponification temperature is preferably 20 to 80°C. If the saponification temperature is too low, the reaction rate will decrease. The saponification temperature is more preferably 30°C or higher, and even more preferably 40°C or higher. On the other hand, if the saponification temperature is too high, a large amount of solvent will evaporate, reducing the solvent content in the resulting solid block and worsening the solubility of the resulting ethylene-vinyl alcohol copolymer. The saponification temperature is more preferably 70°C or lower, and even more preferably 65°C or lower. The saponification time is preferably 5 minutes to 3 hours. The saponification time is more preferably 10 minutes or more, and even more preferably 15 minutes or more. Furthermore, the saponification time is more preferably 2 hours or less, and even more preferably 90 minutes or less.

[0030] It is preferable to have a neutralization step after the saponification and grinding steps. Neutralization can be done using methyl acetate or acetic acid.

[0031] After the saponification and grinding steps, a neutralization step may be provided as needed, followed by a washing step to remove impurities such as sodium acetate, if necessary. Examples of washing solutions include lower alcohols such as methanol and ethanol, water, and mixtures thereof.

[0032] The neutralization step and / or washing step described above can reduce the amount of saponification catalyst remaining in the ethylene-vinyl alcohol copolymer. Specifically, it is preferable that the sodium salt content in the ethylene-vinyl alcohol copolymer is 0.15% by mass (metal basis) or less. It is more preferable that the sodium salt content is 0.1% by mass or less.

[0033] After the saponification and grinding steps, the material is subjected to a neutralization and / or washing step as needed, and then to a drying step. The drying method is not particularly limited, but hot air drying is preferred, and the particle temperature during drying is preferably 60 to 120°C. If the temperature is too low, the production efficiency will decrease. A temperature of 70°C or higher is more preferable. On the other hand, if the temperature is too high, some particles will undergo excessive crystallization, and the solubility will deteriorate. A temperature of 100°C or lower is more preferable. The drying time is preferably 2 to 20 hours and can be adjusted as appropriate. In this way, a dried ethylene-vinyl alcohol copolymer can be obtained. The methanol content in the dried ethylene-vinyl alcohol copolymer is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1.5% by mass or less.

[0034] The PVA (A) content in the aqueous emulsion is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, and even more preferably 0.5 to 10 parts by mass, per 100 parts by mass of polymer (C).

[0035] The total PVA content in the aqueous emulsion is preferably 1 to 50 parts by mass, more preferably 5 to 20 parts by mass, and even more preferably 6 to 12 parts by mass, per 100 parts by mass of polymer (C).

[0036] (Vinyl Alcohol Polymer (B)) The aqueous emulsion of the present invention contains a vinyl alcohol polymer (B) (hereinafter sometimes abbreviated as "PVA (B)"). The degree of saponification of PVA (B) is preferably 80.0 mol% or more, more preferably 85.0 mol% or more, and still more preferably 90.0 mol% or more. The degree of saponification is preferably 99.9 mol% or less, more preferably 99.5 mol% or less. PVA (B) may be unmodified PVA or modified PVA. Examples of the modified PVA include anion-modified PVA such as sulfonic acid group-modified PVA and carboxylic acid group-modified PVA; cation-modified PVA such as quaternary amine group-modified PVA; amide-modified PVA; acetoacetyl group-modified PVA; diacetone acrylamide-modified PVA; and ethylene-modified PVA. One of these may be used alone, or two or more thereof may be used in combination. Among these, ethylene-modified PVA is preferable from the viewpoint of water-resistant adhesive properties of the resulting aqueous emulsion. When PVA (B) contains a modifying group, the content thereof is preferably 0.5 to 10 mol%, more preferably 1 to 8 mol%, and still more preferably 1.5 to 5 mol%. In one preferred embodiment, PVA (A) is unmodified PVA or ethylene-modified PVA. The content of ethylene units in the ethylene-modified PVA is preferably 0.5 to 10 mol%, more preferably 1 to 8 mol%, and still more preferably 1.5 to 5 mol%.

[0037] The viscosity-average degree of polymerization (hereinafter sometimes simply referred to as "degree of polymerization") of PVA (B) is 900 or more and 4000 or less. When the degree of polymerization falls within the above range, an aqueous emulsion excellent in water-resistant adhesive properties can be obtained. When the degree of polymerization of PVA (B) exceeds 4000, the viscosity of the aqueous emulsion becomes excessively high, leading to degraded handleability. The upper limit of the degree of polymerization is preferably 3000, more preferably 2000, and still more preferably 1800. The lower limit of the degree of polymerization is preferably 1000, more preferably 1300, and still more preferably 1500.

[0038] PVA(B) may contain monomer units other than vinyl alcohol units and vinyl ester units. Examples of monomers (modified species) that provide these other monomer units include α-olefins such as ethylene, propylene, 1-butene, isobutene, and 1-hexene; acrylic acid and methacrylic acid; acrylic acid esters such as methyl acrylate and ethyl acrylate; methacrylic acid esters such as methyl methacrylate and ethyl methacrylate; maleic acid and its derivatives such as maleic acid, monomethyl maleic acid, and dimethyl maleic acid; acrylamide derivatives such as N-methacrylacrylamide and N-ethylacrylamide; methacrylamide derivatives such as N-methylmethacrylamide and N-ethylmethacrylamide; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, and n-butyl vinyl ether; ethylene glycol vinyl ether, 1,3-propanediol vinyl ether, 1, Examples include hydroxyl group-containing vinyl ethers such as 4-butanediol vinyl ether; allyl acetate; allyl ethers such as propyl allyl ether, butyl allyl ether, and hexyl allyl ether; monomers having an oxyalkylene group; isopropenyl acetate; hydroxyl group-containing α-olefins such as 3-buten-1-ol, 4-penten-1-ol, 5-hexen-1-ol, 7-octen-1-ol, 9-decen-1-ol, and 3-methyl-3-buten-1-ol; and monomers having a silyl group such as vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, vinyldimethylethoxysilane, 3-(meth)acrylamidopropyltrimethoxysilane, and 3-(meth)acrylamidopropyltriethoxysilane. Among these, ethylene is preferred as the modified species. In one embodiment, it is preferable that PVA(B) does not have a silyl group.

[0039] The PVA(B) content in the aqueous emulsion is preferably 0.1 to 30 parts by mass, more preferably 0.5 to 20 parts by mass, and even more preferably 1 to 10 parts by mass, per 100 parts by mass of polymer(C).

[0040] The mass ratio (A) / (B) of PVA (A) to PVA (B) in the aqueous emulsion of the present invention is not particularly limited, but is preferably 1 / 100 to 100 / 1, more preferably 1 / 50 to 50 / 1, still more preferably 1 / 30 to 30 / 1, even more preferably 1 / 20 to 20 / 1, and particularly preferably 1 / 10 to 10 / 1.

[0041] The method for producing PVA (B) is not particularly limited, but PVA (B) may be produced by saponifying a vinyl ester polymer obtained by polymerizing a vinyl ester monomer. In one embodiment, after copolymerizing a vinyl ester monomer with another monomer unit, saponification is performed, whereby a modified vinyl alcohol-based polymer having units derived from a monomer other than the vinyl ester monomer introduced thereinto can be obtained.

[0042] In the polymerization step, for example, a vinyl ester monomer may be polymerized to obtain a vinyl ester-based polymer. Examples of methods for polymerizing a vinyl ester monomer include known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Among these methods, bulk polymerization performed without a solvent and solution polymerization performed using a solvent such as an alcohol are preferred, and solution polymerization performed in the presence of a lower alcohol is more preferred. As the lower alcohol, alcohols having 3 or less carbon atoms are preferred, methanol, ethanol, n-propanol, and isopropanol are more preferred, and methanol is even more preferred. When carrying out the polymerization reaction by bulk polymerization or solution polymerization, either a batch system or a continuous system can be employed as the reaction method. The above method can also be applied when copolymerizing a monomer other than the vinyl ester monomer.

[0043] Examples of initiators used in the polymerization reaction include azo initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile); and known initiators such as organic peroxide initiators such as benzoyl peroxide and n-propyl peroxycarbonate. There is no particular limitation on the polymerization temperature when carrying out the polymerization reaction, but a range of 5°C to 200°C is appropriate.

[0044] As described above, when polymerizing vinyl ester monomers, other copolymerizable monomers can be copolymerized within the limits that do not impair the spirit of the present invention. Examples of such other monomers include monomers (modified species) that provide the other monomer units mentioned above. The upper limit of the amount of these other monomers used varies depending on the purpose and application of use, but is preferably 20 mol%, and more preferably 10 mol%, relative to the total monomers. On the other hand, the lower limit of the amount of these other monomers used may be, for example, 0.1 mol%, or 1 mol%.

[0045] When polymerizing vinyl ester monomers, a chain transfer agent may be added to adjust the degree of polymerization of the resulting vinyl alcohol polymer. Examples of chain transfer agents include aldehydes such as acetaldehyde, propionaldehyde, butyraldehyde, and benzaldehyde; ketones such as acetone, methyl ethyl ketone, hexanone, and cyclohexanone; mercaptans such as 2-hydroxyethanethiol; thiocarboxylic acids such as thioacetic acid; and halogenated hydrocarbons such as trichloroethylene and perchloroethylene. Among these, aldehydes and ketones are preferred. The amount of chain transfer agent added is determined according to the chain transfer constant of the added chain transfer agent and the desired degree of polymerization of the vinyl alcohol polymer, but generally, 0.1 to 10 parts by mass per 100 parts by mass of the vinyl ester monomer used is preferred.

[0046] In the saponification step, the vinyl ester polymer may be saponified in an alcohol solution using an alkaline catalyst to obtain PVA(B). For the saponification reaction of the vinyl ester polymer, it is preferable to apply an alcohol decomposition or hydrolysis reaction using a conventionally known basic catalyst such as sodium hydroxide, potassium hydroxide, or sodium methoxide. Solvents used in the saponification reaction include alcohols such as methanol and ethanol; esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone; and aromatic hydrocarbons such as benzene and toluene. These can be used individually or in combination of two or more. Among these, it is convenient and preferable to use methanol or a mixed solution of methanol and methyl acetate as the solvent and carry out the saponification reaction in the presence of sodium hydroxide.

[0047] The solid material containing PVA(B) obtained through the saponification process can be washed, dried, and / or pulverized to obtain PVA(B) powder. The washing can be carried out using an alcohol such as methanol. The drying conditions are not particularly limited, but the drying temperature can be, for example, 50°C to 120°C, or 60°C to 100°C. The drying time can be, for example, 1 hour to 24 hours, or 2 hours to 20 hours, or 16 hours or less, 12 hours or less, or 6 hours or less.

[0048] (Polymer (C) containing ethylenically unsaturated monomer units) The aqueous emulsion of the present invention contains polymer (C) containing ethylenically unsaturated monomer units (hereinafter sometimes simply referred to as "polymer (C)"). Polymer (C) may be a dispersed phase in the aqueous emulsion.

[0049] Ethylene-unsaturated monomers include vinyl ester monomers such as vinyl formate, vinyl acetate, vinyl propionate, and vinyl versatate; (meth)acrylic acid monomers such as acrylic acid and methacrylic acid; and (meth)acrylic acid monomers such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, 2-hydroxyethyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, and quaternaries thereof. Examples include acrylic acid ester monomers; styrene monomers such as styrene, α-methylstyrene, p-styrenesulfonic acid and their sodium and potassium salts; diene monomers such as butadiene, isoprene, and chloroprene; olefin monomers such as ethylene, propylene, and isobutylene; acrylamide monomers such as acrylamide, methacrylamide, N-methylolacrylamide, N,N-dimethylacrylamide, acrylamide-2-methylpropanesulfonic acid and its sodium salt; halogenated olefins such as vinyl chloride, vinyl fluoride, vinylidene chloride, and vinylidene fluoride; and N-vinylpyrrolidone. In particular, polymer (C) is preferably a polymer containing monomer units derived from at least one selected from the group consisting of vinyl ester monomers, (meth)acrylic acid ester monomers, styrene monomers, and diene monomers. Furthermore, it is preferable that the total content of vinyl ester monomers, (meth)acrylic acid ester monomers, styrene monomers, and diene monomers relative to the total monomer units of polymer (C) be 70% by mass or more, and more preferably 75% by mass or more. In particular, it is especially preferable that polymer (C) contains vinyl ester monomer units at a concentration of 75% by mass or more relative to the total monomer units.

[0050] The solid content of the aqueous emulsion of the present invention is preferably 30% by mass or more, and more preferably 35% by mass or more. On the other hand, the solid content is preferably 60% by mass or less, and more preferably 55% by mass or less. When the solid content is 30% by mass or more, the viscosity stability of the aqueous emulsion tends to be better. On the other hand, when the solid content is 60% by mass or less, the open time tends to be longer and the handling properties tend to be better.

[0051] In the aqueous emulsion of the present invention, the ethylenically unsaturated monomer may be graft polymerized in at least a portion of PVA(A) and PVA(B). That is, PVA(A) and PVA(B) may have graft chains consisting of the ethylenically unsaturated monomer units.

[0052] (Method for producing aqueous emulsion) In one embodiment, one example of a method for producing aqueous emulsion is to emulsion polymerize ethylenically unsaturated monomers using a polymerization initiator in the presence of a dispersant containing PVA(A) and PVA(B). The aqueous emulsion obtained in this way has excellent stability at low temperatures and water-resistant adhesion. The dispersant may consist only of PVA(A) and PVA(B).

[0053] In the above method, there are no particular restrictions on the method of preparing or adding the dispersant to the polymerization tank. Methods include adding the dispersant to the polymerization tank all at once initially, or adding it continuously during polymerization. Among these, the method of adding the dispersant to the polymerization tank all at once initially is preferred. In this case, it is preferable to add the dispersant to cold water or preheated warm water, and then heat and stir the mixture to 80-90°C to uniformly disperse the dispersant.

[0054] During emulsion polymerization, the amount of dispersant used is preferably 0.2 to 40 parts by mass, more preferably 0.3 to 20 parts by mass, and even more preferably 0.5 to 15 parts by mass, per 100 parts by mass of ethylenically unsaturated monomer. When the amount of dispersant used is above the lower limit, aggregation of dispersed particles in the aqueous emulsion is less likely to occur, and the polymerization stability when preparing the aqueous emulsion tends to be excellent. Furthermore, when the amount of dispersant used is below the upper limit, the viscosity of the polymerization solution does not become too high, polymerization tends to proceed more uniformly, and the heat of polymerization tends to be efficiently removed.

[0055] In the emulsion polymerization described above, a water-soluble single initiator or a water-soluble redox initiator commonly used in emulsion polymerization can be used as the polymerization initiator. These initiators may be used individually or in combination of two or more. Among these, redox initiators are preferred.

[0056] Examples of water-soluble solitary initiators include azo initiators, hydrogen peroxide, and peroxides such as persulfates (potassium, sodium, or ammonium salts). Examples of azo initiators include 2,2'-azobis(isobutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile).

[0057] As a redox initiator, a combination of an oxidizing agent and a reducing agent can be used. Peroxides are preferred as the oxidizing agent. Examples of reducing agents include metal ions and reducing compounds. Combinations of oxidizing and reducing agents include peroxides and metal ions, peroxides and reducing compounds, and peroxides, metal ions, and reducing compounds. Examples of peroxides include hydrogen peroxide, hydroxyperoxides such as cumene hydroxyperoxide and t-butyl hydroxyperoxide, persulfates (potassium, sodium, or ammonium salts), t-butyl peracetate, and peracid esters (t-butyl perbenzoate). Examples of metal ions include Fe. ２＋ , Cr ２＋ , V ２＋ Co ２＋ Ti３＋ ,Cd ＋ Examples of metal ions capable of undergoing one-electron transfer include sodium bisulfite, sodium bicarbonate, tartaric acid, fructose, dextrose, sorbose, inositol, rongalit, and ascorbic acid. Among these, a combination of one or more oxidizing agents selected from the group consisting of hydrogen peroxide, potassium persulfate, sodium persulfate, and ammonium persulfate, and one or more reducing agents selected from the group consisting of sodium bisulfite, sodium bicarbonate, tartaric acid, rongalit, and ascorbic acid is preferred, and a combination of hydrogen peroxide and one or more reducing agents selected from the group consisting of sodium bisulfite, sodium bicarbonate, tartaric acid, rongalit, and ascorbic acid is more preferred.

[0058] Furthermore, during emulsion polymerization, alkali metal compounds, surfactants, buffers, polymerization degree regulators, etc., may be used as appropriate, as long as they do not impair the effects of the present invention.

[0059] The alkali metal compound is not particularly limited, but examples include compounds containing sodium, potassium, rubidium, cesium, etc. The alkali metal compound may be the alkali metal ion itself, or it may be a compound containing an alkali metal.

[0060] When using the alkali metal compound, its content (in alkali metal equivalent) can be appropriately selected depending on the type of alkali metal compound used, but is preferably 100 to 15,000 ppm, more preferably 120 to 12,000 ppm, and even more preferably 150 to 8,000 ppm relative to the total solid content of the aqueous emulsion. When the alkali metal compound content is 100 ppm or more, the stability of emulsion polymerization when preparing the aqueous emulsion tends to be excellent, and when it is 15,000 ppm or less, the film made of the aqueous emulsion is less likely to be discolored. The alkali metal compound content can be measured by an ICP emission spectrometer. In this specification, "ppm" means "mass ppm".

[0061] Examples of alkali metal compounds include weakly basic alkali metal salts (e.g., alkali metal carbonates, alkali metal acetates, alkali metal bicarbonates, alkali metal phosphates, alkali metal sulfates, alkali metal halides, alkali metal nitrates) and strongly basic alkali metal compounds (e.g., alkali metal hydroxides, alkali metal alkoxides). These alkali metal compounds may be used individually or in combination of two or more.

[0062] Examples of weakly basic alkali metal salts include alkali metal carbonates (e.g., sodium carbonate, potassium carbonate, rubidium carbonate, cesium carbonate), alkali metal bicarbonates (e.g., sodium bicarbonate, potassium bicarbonate, etc.), alkali metal phosphates (e.g., sodium phosphate, potassium phosphate, etc.), alkali metal carboxylates (e.g., sodium acetate, potassium acetate, cesium acetate, etc.), alkali metal sulfates (e.g., sodium sulfate, potassium sulfate, cesium sulfate, etc.), alkali metal halide salts (e.g., cesium chloride, cesium iodide, potassium chloride, sodium chloride, etc.), and alkali metal nitrates (e.g., sodium nitrate, potassium nitrate, cesium nitrate, etc.). Of these, alkali metal carboxylates, alkali metal carbonates, and alkali metal bicarbonates, which can behave as salts of weak acids and strong bases upon dissociation, are preferred, with alkali metal carboxylates being more preferred.

[0063] When these weakly basic alkali metal salts are used, they act as pH buffers during emulsion polymerization, allowing the emulsion polymerization to proceed stably.

[0064] As the surfactant, any of nonionic surfactants, anionic surfactants, or cationic surfactants may be used. Examples of nonionic surfactants are not particularly limited, but include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene fatty acid esters, polyoxyalkylene alkyl ethers, polyoxyethylene derivatives, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene sorbitol fatty acid esters, glycerin fatty acid esters, and the like. Examples of anionic surfactants are not particularly limited, but include alkyl sulfates, alkylaryl sulfates, alkyl sulfonates, sulfates of hydroxyalkanols, sulfosuccinates, sulfates and phosphates of alkyl or alkylaryl polyethoxyalkanols, and the like. Examples of cationic surfactants are not particularly limited, but include alkylamine salts, quaternary ammonium salts, polyoxyethylene alkylamines, and the like. The amount of surfactant used is preferably 2 parts by mass or less per 100 parts by mass of ethylenically unsaturated monomer (e.g., vinyl acetate).

[0065] Examples of buffering agents include acids such as acetic acid, hydrochloric acid, and sulfuric acid; bases such as ammonia, amines, sodium hydroxide, potassium hydroxide, and calcium hydroxide; or alkali carbonates, phosphates, and acetates. Examples of polymerization degree regulators include mercaptans and alcohols.

[0066] The dispersion medium used in the above emulsion polymerization is preferably an aqueous medium mainly composed of water. The aqueous medium mainly composed of water may contain a water-soluble organic solvent (alcohols, ketones, etc.) that is soluble in water in any proportion. Here, "aqueous medium mainly composed of water" refers to a dispersion medium containing 50% by mass or more of water. From the viewpoint of cost and environmental impact, the dispersion medium is preferably an aqueous medium containing 90% by mass or more of water, and more preferably water. In the method for producing the aqueous emulsion, it is preferable to heat the dispersion medium containing PVA(A) and PVA(B) before starting the emulsion polymerization to dissolve PVA(A) and PVA(B), then cool it and purge it with nitrogen. The heating temperature is preferably 80°C or higher. The temperature of the emulsion polymerization is not particularly limited, but is preferably around 20 to 85°C, and more preferably around 40 to 85°C.

[0067] From the viewpoint of improving the applicability of the aqueous emulsion to the substrate, it is preferable that the aqueous emulsion further contains an aliphatic alcohol. There are no particular restrictions on the method of adding the aliphatic alcohol; examples include adding it all at once in the polymerization tank beforehand, adding it continuously during emulsion polymerization, or adding it all at once to the aqueous emulsion obtained by emulsion polymerization.

[0068] The aliphatic alcohol content is preferably 0.0001 to 1 part by mass, and more preferably 0.001 to 1 part by mass, per 100 parts by mass of the total of PVA(A) and PVA(B). This range of aliphatic alcohol content improves the applicability of the aqueous emulsion to the substrate. Furthermore, a content of 1 part by mass or less of aliphatic alcohol tends to result in superior water-resistant adhesion.

[0069] Aliphatic alcohols are aliphatic alcohols other than vinyl alcohol polymers, and specifically include aliphatic monoalcohols and aliphatic polyhydric alcohols.

[0070] Examples of aliphatic monoalcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 2-methyl-2-butanol, 3-methyl-2-butanol, 3-methoxy-3-methyl-1-butanol, 1-hexanol, 2-hexanol, 3-hexanol, 2-ethylhexanol, 2-methyl-1-pentanol, 3-methyl-1-pentanol, 4-methyl-1-pentanol, 2-methyl-2-pentanol, 3-methyl-2-pentanol, and 4-methyl-2-pentanol.

[0071] Examples of aliphatic polyhydric alcohols include diols (e.g., ethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,2-hexanediol, and 1,6-hexanediol), and trivalent to tetravalent polyhydric alcohols (e.g., glycerin, trimethylolpropane, pentaerythritol, 1,2,6-hexanetriol, and diglycerin). Divalent to tetravalent aliphatic alcohols are preferred, divalent to trivalent aliphatic alcohols are more preferred, and trivalent aliphatic alcohols are even more preferred.

[0072] Among these, glycerin and methanol are preferred as aliphatic alcohols, with glycerin being more preferred.

[0073] The aqueous emulsion may contain various additives as long as they do not impair the effects of the present invention. Examples of such additives include organic solvents (aromatic compounds such as toluene and xylene, alcohols, ketones, esters, halogenated solvents, etc.), crosslinking agents, plasticizers, preservatives, fluidity improvers, preservatives, defoaming agents, fillers, wetting agents, colorants, binders, and water-retaining agents. These may be used individually or in combination of two or more. The aqueous emulsion may contain a crosslinking agent, such as a polyfunctional isocyanate, or it may not. Furthermore, the aqueous emulsion may be an aqueous emulsion containing PVA(A) and PVA(B) as dispersants and a polymer (C) as a dispersed phase, to which an aqueous emulsion containing compounds other than PVA(A) and PVA(B) as dispersants is added. The total amount of PVA(A) and PVA(B), polymer(C), aliphatic alcohol, dispersion medium, alkali metal compound, and other additives other than surfactant in the aqueous emulsion is preferably 30% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, particularly preferably 3% by mass or less, and most preferably 1% by mass or less.

[0074] (Adhesive) The adhesive of the present invention is an adhesive comprising the aqueous emulsion of the present invention. Preferably, the adhesive consists only of the aqueous emulsion.

[0075] Suitable substrates for the adhesive of the present invention include paper, wood, and plastic, with wood being particularly preferred. The adhesive of the present invention is suitably used in applications such as laminated wood, plywood, decorative plywood, and fiberboard. Furthermore, the adhesive is also suitably used in woodworking and paper processing (for example, for forming paper tubes and paper straws).

[0076] The aqueous emulsion of the present invention is suitably used in paints and textile processing applications. Furthermore, this aqueous emulsion can be used in a wide range of applications, such as inorganic binders, cement admixtures, and mortar primers. Moreover, so-called powdered emulsions, obtained by powdering the aqueous emulsion through spray drying or other methods, are also effectively utilized.

[0077] Next, the present invention will be described in further detail with reference to working examples and comparative examples. In the following working examples and comparative examples, "parts" and "%" are based on mass unless otherwise specified.

[0078] [Ethylene Unit Content (Ethylene Modification Amount) of Ethylene-Vinyl Alcohol Copolymer] A part of a methanol solution of ethylene-vinyl acetate copolymer obtained in the production process of ethylene-vinyl alcohol copolymer is taken as a measurement sample. After removing unreacted vinyl acetate monomer from the measurement sample, reprecipitation purification consisting of precipitation with n-hexane and dissolution with acetone was performed three times, followed by vacuum drying at 80°C for three days to obtain a purified ethylene-vinyl acetate copolymer. The ethylene-vinyl acetate copolymer was dissolved in DMSO-d ６ and measured at 80°C using １ H-NMR (JEOL-500) at 500 MHz to determine the content (mol%) of ethylene units. The content of ethylene units in the ethylene-vinyl acetate copolymer is substantially the same as the content of ethylene units in the ethylene-vinyl alcohol copolymer obtained by saponifying the same.

[0079] [Degree of Saponification] The degree of saponification of the vinyl alcohol-based polymer was determined by the method described in JIS K6726 (1994).

[0080] [Viscosity-Average Degree of Polymerization] The viscosity-average degree of polymerization of the vinyl alcohol-based polymer was determined by the method described in JIS K6726 (1994).

[0081] [Water-Resistant Adhesion] Water-resistant adhesion was evaluated in accordance with EN204. The obtained aqueous emulsion was applied to two pieces of European beech wood (straight-grained) at 200 g / m ２ respectively, and the surfaces of the beech wood coated with the aqueous emulsion were bonded together to obtain a test piece. This test piece was pressed under a load of 0.7 N / mm ２ at 20°C for 2 hours. Thereafter, the test piece was immersed in water at 20°C for 4 days, taken out, and the adhesive strength (unit: N / mm ２ ) was measured while the test piece was still wet.

[0082] [Viscosity Stability] The viscosity of the aqueous emulsions obtained in Examples 1-6, 8-9 and Comparative Examples 1-7 was measured using a B-type viscometer (30°C, 20 rpm) and the viscosity (η) was measured. ０ The viscosity (η) was measured. The aqueous emulsion was then left to stand at 10°C for 4 weeks. After standing, the viscosity (η) was measured again using a B-type viscometer (10°C, 20 rpm). ４Ｗ The viscosity was measured. The viscosity was measured as η ４Ｗ / η ０ The viscosity was calculated as follows. A lower thickening ratio indicates better viscosity stability. In addition, for Example 7 and Comparative Example 8, the aqueous emulsion was left for 3 days, and the viscosity stability was evaluated by visually checking for gelation.

[0083] [Polymerization Stability] 500 g of the obtained aqueous emulsion was filtered through a 60-mesh wire mesh, and the filtration residue was weighed to evaluate the ratio of the filtration residue to the mass of the aqueous emulsion. A smaller filtration residue indicates fewer aggregates and more stable emulsion polymerization.

[0084] [Water Absorption Test] The obtained aqueous emulsion was cast onto a PET film at 20°C and 65% RH and dried for 7 days to obtain a 500 μm dry film. This film was punched out into a 2.5 cm diameter circle, and when this was immersed in 20°C water for 24 hours as a sample, the water absorption rate of the film in % [{(mass after immersion - mass before immersion) / mass before immersion} × 100] was determined.

[0085] Table 1 summarizes the vinyl alcohol polymers used in the examples and comparative examples. PVA-1 to 3 and PVA-7 to 9 are ethylene-vinyl alcohol copolymers, while PVA-4 to 6 are unmodified vinyl alcohol polymers. PVA-1 was produced according to Production Example 1 below.

[0086]

[0087] [Manufacturing Example 1] (Manufacturing of PVA-1) A polymerization vessel (continuous polymerization tank) equipped with a reflux condenser, raw material supply line, thermometer, nitrogen inlet and stirring blades, and an apparatus equipped with a reflux condenser, raw material supply line, reaction solution removal line, thermometer, nitrogen inlet, ethylene inlet and stirring blades were used. Vinyl acetate (2400 kg / h), methanol (1050 kg / h), and a 2 mass% methanol solution of di-n-propyl peroxydicarbonate (NPP) (20 kg / h) were continuously supplied to the continuous polymerization tank under a nitrogen atmosphere using a metering pump. The ethylene pressure inside the tank was adjusted to 0.30 MPa. The polymerization solution was continuously removed from the polymerization tank so that the liquid level inside the polymerization tank remained constant. The polymerization rate at the polymerization tank outlet was adjusted to 75%. The residence time (polymerization time) inside the polymerization tank was 10.0 hours. The temperature at the polymerization tank outlet was 60°C. The polymerization solution was recovered from the polymerization tank, and unreacted vinyl acetate monomer was removed by introducing methanol vapor into the recovered solution to obtain a methanol solution of ethylene-vinyl acetate copolymer.

[0088] To a methanol solution (35% by mass) of the ethylene-vinyl acetate copolymer obtained in the polymerization step, a methanol solution (30% by mass) of sodium hydroxide, a saponification catalyst, was added so that the molar ratio of sodium hydroxide to vinyl acetate units in the ethylene-vinyl acetate copolymer was 0.33, and the mixture was saponified. The resulting solid block was crushed to obtain particles, which were then left at 60°C for 1 hour to further saponify. Subsequently, the obtained particles were washed with methyl acetate for neutralization, washed with methanol three more times, and then dried overnight at 80°C to obtain purified ethylene-vinyl alcohol copolymer (PVA-1). The obtained ethylene-vinyl alcohol copolymer had a degree of saponification of 97.4 mol%, a viscosity-average degree of polymerization of 350, an ethylene unit content of 6.5 mol%, and a total content of terminal carboxyl groups and terminal lactone rings of 0.12 mol%.

[0089] [Example 1] In a 1-liter glass polymerization vessel equipped with a reflux condenser, dropping funnel, thermometer, and nitrogen inlet, 270 g of deionized water, 7.0 g of PVA-1, and 21 g of PVA-2 were charged, and the mixture was heated to 90°C while stirring to obtain a homogeneous aqueous solution. This aqueous solution was cooled to 60°C, purged with nitrogen, and 28 g of vinyl acetate was charged while stirring, and polymerization was started with a hydrogen peroxide / tartaric acid-based polymerization initiator. 45 minutes after the start of polymerization, 252 g of vinyl acetate and the above polymerization initiator were added, and emulsion polymerization was carried out by stirring for 2 hours. A polyvinyl acetate emulsion with a solid content of 50% was obtained. The results are shown in Table 2.

[0090] [Examples 2-9, Comparative Examples 1-8] Polyvinyl acetate emulsions were obtained in the same manner as in Example 1, except that the conditions described in Tables 2 and 3 were changed. The results are shown in Tables 2 and 3.

[0091]

[0092]

[0093] Examples 1-6 and 8-9 showed good results in all tests, including water-resistant adhesion, viscosity stability, polymerization stability, and water absorption. In comparison with Comparative Example 8, Example 7, which contained PVA-1 and PVA-5 in a 1:1 ratio, showed superior results in water-resistant adhesion and water absorption compared to Comparative Example 8, which used PVA-5 alone. Furthermore, regarding viscosity stability, Comparative Example 8 gelled after one day, while Example 7 showed no gelling even after three days, indicating that Example 7 also showed good viscosity stability.

Claims

1. An aqueous emulsion comprising an ethylene-vinyl alcohol copolymer (A), a vinyl alcohol polymer (B), and a polymer (C) containing ethylenically unsaturated monomer units, wherein the ethylene unit content of the ethylene-vinyl alcohol copolymer (A) is 0.1 mol% or more and less than 20 mol%, the viscosity-average degree of polymerization is 200 or more and 800 or less, the degree of saponification is 90.0 mol% or more and 99.9 mol or less, and the viscosity-average degree of polymerization of the vinyl alcohol polymer (B) is 900 or more and 4000 or less.

2. The aqueous emulsion according to claim 1, wherein the total content of carboxyl groups and lactone rings located at the terminal ends of the molecular chain of the ethylene-vinyl alcohol copolymer (A) is 0.08 mol% or more.

3. The aqueous emulsion according to claim 1, wherein the vinyl alcohol polymer (B) does not have a silyl group.

4. An adhesive comprising the aqueous emulsion described in any one of claims 1 to 3.

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