Water-based adhesive for paper straws and paper straws

Abstract

The present invention provides a water-based adhesive for paper straws that has excellent initial adhesiveness and film strength. The present invention relates to a water-based adhesive for paper straws that contains a polyvinyl alcohol polymer (X), cellulose nanofibers, and water, wherein the cellulose nanofiber content is 0.1 to 100 parts by mass per 100 parts by mass of the polyvinyl alcohol polymer (X).

Description

[Technical Field]

[0001] The present disclosure relates to a water-based adhesive for paper straws and paper straws. [Background technology]

[0002] Traditionally, paper straws have been used for crafts and art, such as for making miscellaneous goods, interior decorations, and room decorations. However, in recent years, marine pollution caused by microplastics has become a serious problem, and environmentally friendly paper drinking straws have been attracting attention as an alternative to plastic drinking straws.

[0003] Currently, paper straws are manufactured by applying an adhesive to a base paper, layering it around a metal core, and then spirally winding it. Adhesives such as starch, polyvinyl alcohol (PVA), and vinyl acetate resin emulsion are used.

[0004] One conventional adhesive proposed is an emulsion in which vinyl acetate monomer and N-methylolacrylamide are copolymerized using PVA as a protective colloid (Patent Document 1). Furthermore, various modified PVAs have been developed to improve specific PVA performance. For example, Patent Document 2 proposes an aqueous emulsion that exhibits higher water resistance than conventional PVA. The emulsion uses PVA containing a specific proportion of α-olefin units having four or fewer carbon atoms as a dispersant, and a (co)polymer containing one or more ethylenically unsaturated monomers as a dispersoid. Furthermore, Patent Documents 3 and 4 propose a method of emulsion (co)polymerizing vinyl acetate or vinyl acetate and a (meth)acrylic acid ester using an ethylene-vinyl alcohol copolymer as a protective colloid. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 10-121017 [Patent Document 2] Japanese Patent Application Publication No. 11-106727 [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-119621 [Patent Document 4] Japanese Patent Application Laid-Open No. 2001-123138 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the emulsion described in Patent Document 1 had insufficient film strength, and the emulsions described in Patent Documents 2 to 4 had insufficient initial adhesiveness.

[0007] Therefore, an object of the present disclosure is to provide an aqueous adhesive for paper straws that has excellent initial adhesion and film strength. [Means for solving the problem]

[0008] As a result of intensive research, the present inventors have found that the above-mentioned problems can be solved by the present disclosure. [1] An aqueous adhesive for paper straws, comprising a vinyl alcohol-based polymer (X), cellulose nanofibers, and water, wherein the content of the cellulose nanofibers is 0.1 to 100 parts by mass per 100 parts by mass of the vinyl alcohol-based polymer (X); [2] The aqueous adhesive for paper straws according to [1], wherein the vinyl alcohol polymer (X) has a viscosity-average degree of polymerization of 200 to 5,000 and a degree of saponification of 80 to 99.9 mol%; [3] The aqueous adhesive for paper straws according to [1] or [2], wherein the vinyl alcohol polymer (X) is one or more polymers selected from the group consisting of an ethylene-vinyl alcohol copolymer (A) and a polyvinyl alcohol (B); [4] The aqueous adhesive for paper straws according to [3], wherein the ethylene unit content of the ethylene-vinyl alcohol copolymer (A) is 1 mol% or more and less than 20 mol%; [5] The aqueous adhesive for paper straws according to any one of [1] to [4], in which the vinyl alcohol polymer (X) is dissolved in water; [6] The aqueous adhesive for paper straws according to any one of [1] to [5], further comprising a polymer (C) containing an ethylenically unsaturated monomer unit, and comprising an aqueous emulsion containing the polymer (C) containing an ethylenically unsaturated monomer unit as a dispersoid; [7] The aqueous adhesive for paper straws according to [6], wherein the polymer (C) contains 70 mass% or more of 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, based on the total monomer units; [8] The aqueous adhesive for paper straws according to [6] or [7], wherein the dispersant in the aqueous emulsion is a vinyl alcohol polymer (X); [9] The aqueous adhesive for paper straws according to any one of [6] to [8], wherein the total content of the vinyl alcohol polymer (X) and the cellulose nanofibers is 2 to 60 parts by mass per 100 parts by mass of the polymer (C);

[10] The aqueous adhesive for paper straws according to any one of [6] to [9], wherein the content of the vinyl alcohol polymer (X), the polymer (C), and the cellulose nanofibers in the solid content of the aqueous adhesive for paper straws is 55% by mass or more;

[11] A paper straw obtained by bonding paper substrates together using the adhesive according to any one of [1] to

[10] ;

[12] A paper straw having a vinyl alcohol polymer (X) and cellulose nanofibers on the adhesive surface between paper substrates. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide an aqueous adhesive for paper straws that has excellent initial adhesion and film strength. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present disclosure will be described, but the present disclosure is not limited to the embodiment.

[0011] The aqueous adhesive for paper straws of the present disclosure contains a vinyl alcohol polymer (X), cellulose nanofibers (hereinafter sometimes referred to as "CNF"), and water, and the CNF content is 0.1 to 100 parts by mass per 100 parts by mass of the vinyl alcohol polymer (X).

[0012] [Vinyl alcohol polymer (X)] The viscosity-average degree of polymerization of the vinyl alcohol polymer (X) is preferably 200 or more, more preferably 250 or more, and even more preferably 300 or more, and may be 400 or more, 500 or more, 600 or more, 700 or more, 900 or more, 1100 or more, or 1300 or more. When the viscosity-average degree of polymerization is above the lower limit, the film strength is better. The viscosity-average degree of polymerization is preferably 5000 or less, more preferably 4500 or less, and even more preferably 4000 or less, and may be 3800 or less, 3500 or less, 3000 or less, 2700 or less, 2500 or less, 2300 or less, or 2000 or less. When the viscosity-average degree of polymerization is below the upper limit, the viscosity of the aqueous adhesive for paper straws does not become too high, and the coatability tends to be excellent.

[0013] The viscosity-average degree of polymerization of the vinyl alcohol polymer (X) is measured in accordance with JIS K 6726:1994. Specifically, the intrinsic viscosity [η] (liters / g) of the vinyl alcohol polymer (X) is measured in water at 30°C, and the viscosity-average degree of polymerization P is calculated using the intrinsic viscosity [η] value according to the following formula. When the degree of saponification of the vinyl alcohol polymer (X) is less than 99.5 mol%, the polymer is saponified to a degree of saponification of 99.5 mol% or more, and then the intrinsic viscosity [η] is measured. P = ([η] × 10 4 / 8.29) (1 / 0.62)

[0014] The saponification degree of the vinyl alcohol polymer (X) is preferably 80 mol% or more, more preferably 83 mol% or more, even more preferably 85 mol% or more, and in some cases 87 mol% or more or 90 mol% or more is preferred. When the saponification degree is equal to or greater than the lower limit, the vinyl alcohol polymer (X) has excellent water solubility, making it easier to produce the aqueous adhesive for paper straws of the present disclosure. The saponification degree of the vinyl alcohol polymer (X) is preferably 99.9 mol% or less, more preferably 99.5 mol% or less, even more preferably 99 mol% or less, and particularly preferably 98.5 mol% or less. When the saponification degree is equal to or less than the upper limit, the vinyl alcohol polymer (X) can be stably produced. The saponification degree of the vinyl alcohol polymer (X) is measured in accordance with JIS K 6726:1994.

[0015] The vinyl alcohol polymer (X) is a polymer containing a vinyl alcohol unit. Examples of the vinyl alcohol polymer (X) include an ethylene-vinyl alcohol copolymer (hereinafter sometimes referred to as "ethylene-vinyl alcohol copolymer (A)") and polyvinyl alcohol (hereinafter sometimes referred to as "PVA (B)"). Among them, one selected from the group consisting of the ethylene-vinyl alcohol copolymer (A) and the PVA (B) is preferred, and from the viewpoint of the water resistance strength of the film, the ethylene-vinyl alcohol copolymer (A) is more preferred. The vinyl alcohol polymer (X) may contain one type alone or two or more types.

[0016] The content of ethylene units in the ethylene-vinyl alcohol copolymer (A) is preferably 1 mol% or more, more preferably 1.5 mol% or more, even more preferably 2 mol% or more, and in some cases, 2.5 mol% or more, 3 mol% or more, or 3.5 mol% or more is preferred. When the content of the ethylene units is equal to or greater than the above-mentioned lower limit, the water resistance strength of the film tends to be excellent. The content of the ethylene units is preferably less than 20 mol%, more preferably less than 15 mol%, even more preferably less than 13 mol%, particularly preferably less than 10 mol%, and in some cases, less than 8 mol% or less than 5 mol% is preferred. When the content of the ethylene units is less than the above-mentioned upper limit, the ethylene-vinyl alcohol copolymer (A) has excellent water solubility, making it easier to produce the aqueous adhesive for paper straws of the present disclosure.

[0017] The content of ethylene units in the ethylene-vinyl alcohol copolymer (A) is 1 For example, the ethylene unit of a vinyl ester copolymer, which is a precursor or reacetylated product of the ethylene-vinyl alcohol copolymer (A), can be determined by H-NMR measurement. 1 The chromatographic index is determined by H-NMR measurement. More specifically, the vinyl ester copolymer is purified by reprecipitation three or more times with a mixture of n-hexane and acetone, and then dried under reduced pressure at 80°C for three days to obtain a vinyl ester copolymer for analysis. This is then dissolved in DMSO-d6 and 1 Measurement is performed using a H-NMR measurement device (e.g., 500 MHz) at 80°C. The ethylene unit content is calculated using the peak (4.7 to 5.2 ppm) derived from the main chain methine of the vinyl ester and the peak (0.8 to 1.6 ppm) derived from the main chain methylene of ethylene, vinyl ester, and third component (monomer other than ethylene and vinyl ester).

[0018] The content of 1,2-glycol bond units in the ethylene-vinyl alcohol copolymer (A) is preferably 1.2 mol% or more, more preferably 1.3 mol% or more, and even more preferably 1.4 mol% or more. When the content of 1,2-glycol bond units is equal to or greater than the above lower limit, the productivity of the ethylene-vinyl alcohol copolymer (A) is excellent, and the viscosity stability of the aqueous adhesive for paper straws tends to be excellent. The content of 1,2-glycol bond units in the ethylene-vinyl alcohol copolymer (A) is preferably 2.0 mol% or less, more preferably 1.9 mol% or less, and even more preferably 1.8 mol% or less. When the content of 1,2-glycol bond units is equal to or less than the above upper limit, the productivity of the ethylene-vinyl alcohol copolymer (A) is excellent, and the hue of the ethylene-vinyl alcohol copolymer (A) tends to be excellent.

[0019] The content of 1,2-glycol bond units in the ethylene-vinyl alcohol copolymer (A) is 1 This can be determined by H-NMR measurement. Specifically, the ethylene-vinyl alcohol copolymer (A) is saponified to a degree of saponification of 99.9 mol% or more, thoroughly washed with methanol, and dried at 90°C under reduced pressure for 2 days. The obtained completely saponified ethylene-vinyl alcohol copolymer is dissolved in DMSO-d6, and a few drops of trifluoroacetic acid are added to the sample. 1 Measurement is performed using H-NMR at 80°C. The peak derived from the methine proton of the vinyl alcohol unit is at 3.2 to 4.0 ppm (integral value S), and the peak derived from one methine proton of the 1,2-glycol bond is at around 3.15 to 3.35 ppm (integral value T). The content of the 1,2-glycol bond unit can be calculated using the following formula. 1,2-glycol bond unit content (mol%) = T / S × 100

[0020] The block character of the ethylene unit of the ethylene-vinyl alcohol copolymer (A) is preferably 0.8 or higher, more preferably 0.9 or higher, and in some cases, 0.93 or higher or 0.95 or higher is preferred. When the block character is above the lower limit, the aqueous adhesive for paper straws of the present disclosure tends to have excellent viscosity stability and high-speed coatability. The block character of the ethylene unit of the ethylene-vinyl alcohol copolymer (A) is preferably 1.1 or lower, and in some cases, 1.05 or lower, 1.0 or lower, or 0.99 or lower is preferred. When the block character is below the upper limit, the aqueous adhesive for paper straws of the present disclosure tends to have better water resistance.

[0021] The block character is a numerical value that represents the distribution of ethylene units and vinyl alcohol units generated by saponification of vinyl ester units, and takes a value between 0 and 2. 0 indicates that ethylene units or vinyl alcohol units are distributed completely in blocks, and as the value increases, the alternation increases, with 1 indicating that ethylene units and vinyl alcohol units are completely randomly present and 2 indicating that ethylene units and vinyl alcohol units are completely alternately present. The block character is 13 The following can be determined by C-NMR. First, the ethylene-vinyl alcohol copolymer (A) is saponified to a degree of saponification of 99.9 mol% or more, then thoroughly washed with methanol and dried under reduced pressure at 90°C for 2 days. The obtained completely saponified ethylene-vinyl alcohol copolymer is dissolved in DMSO-d6, and the obtained sample is subjected to spectrometry using a 500 MHz NMR spectrometer. 13 Measurement is performed at 80°C using C-NMR (Nuclear Magnetic Resonance Spectrometer "GX-500" manufactured by JEOL Ltd.). From the obtained spectrum chart, the molar fraction of two vinyl alcohol and ethylene unit chains (VE), the molar fraction of vinyl alcohol units (V), and the molar fraction of ethylene units (E) are assigned and calculated using the method described in T. Moritani and H. Iwasaki, Vol. 11, No. 6, pp. 1251-1259, Macromolecules (1978), and the block character (η) of the ethylene units is calculated using the following formula. η=(VE) / {2×(V)×(E)}

[0022] The vinyl alcohol polymer (X) may contain a monomer unit other than a vinyl alcohol unit, an ethylene unit, and a vinyl ester unit, as long as the effect of the present disclosure is not impaired. Examples of such a monomer include α-olefins such as propylene, n-butene, and isobutylene; acrylic acid and its salts; acrylic acid esters; methacrylic acid and its salts; methacrylic acid esters; acrylamide; acrylamide derivatives such as N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetoneacrylamide, acrylamidopropanesulfonic acid and its salts, acrylamidopropyldimethylamine and its salts or quaternary salts thereof, and N-methylolacrylamide and its derivatives; methacrylamide; N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid and its salts, methacrylamidepropyldimethylamine and its salts or quaternary salts thereof, and N-methylolacrylamide. Examples of suitable monomers include methacrylamide derivatives such as dimethacrylamide and its derivatives; 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; vinylidene halides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; unsaturated dicarboxylic acids and their salts or esters such as maleic acid, itaconic acid, and fumaric acid; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate. The content of these monomers varies depending on the purpose and application, but is preferably 10 mol% or less, more preferably less than 5 mol%, even more preferably less than 1 mol%, and particularly preferably less than 0.5 mol%, and may even be 0 mol%.

[0023] [Method for producing vinyl alcohol polymer (X)] PVA (B) can be produced by a known method, or commercially available PVA may be used. The ethylene-vinyl alcohol copolymer (A) can be obtained, for example, by copolymerizing ethylene with a vinyl ester monomer to obtain an ethylene-vinyl ester copolymer, then saponifying the ethylene-vinyl ester copolymer with a saponification catalyst such as sodium hydroxide, and optionally pulverizing and drying the copolymer.

[0024] Methods for copolymerizing ethylene with a vinyl ester monomer include known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Among these, bulk polymerization and solution polymerization, in which polymerization is performed without a solvent or in a solvent such as alcohol, are commonly used. Examples of the alcohol include lower alcohols such as methanol, ethanol, and propanol. Examples of initiators used in the copolymerization include known azo initiators or peroxide initiators such as 2,2'-azobis(isobutyronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), benzoyl peroxide, and n-propyl peroxydicarbonate.

[0025] The polymerization temperature is not particularly limited, and is preferably 0 to 150°C, more preferably from room temperature to 150°C, even more preferably from room temperature to the boiling point of the solvent used, and particularly preferably 30 to 60°C.

[0026] Examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate, with vinyl acetate being preferred.

[0027] A chain transfer agent may be used when copolymerizing ethylene with a vinyl ester monomer. Examples of the chain transfer agent include aldehydes such as acetaldehyde, propionaldehyde, butylaldehyde, 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. When a chain transfer agent is used, its amount can be determined depending on the chain transfer coefficient of the chain transfer agent used and the degree of polymerization of the desired ethylene-vinyl alcohol copolymer (A). While not particularly limited, the amount is preferably 0.1 to 10 parts by mass per 100 parts by mass of the vinyl ester monomer.

[0028] The ethylene-vinyl ester copolymer obtained in the polymerization step is saponified to obtain an ethylene-vinyl alcohol copolymer (A). The ethylene-vinyl ester copolymer is preferably saponified in an organic solvent by alcoholysis or hydrolysis in the presence of a catalyst. Examples of catalysts used in the saponification step include basic catalysts such as sodium hydroxide, potassium hydroxide, and sodium methoxide; and 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 alone or in combination. Among these, it is 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 as a basic catalyst, as this is simple and convenient. The amount of the saponification catalyst used is preferably 0.001 to 0.5 in terms of molar ratio to the vinyl ester units in the ethylene-vinyl ester copolymer. This molar ratio is more preferably 0.002 or greater. On the other hand, the molar ratio is more preferably 0.4 or less, and even more preferably 0.3 or less.

[0029] After the saponification step, a pulverization step and a drying step may be carried out. The pulverization step may further be divided into a preliminary pulverization step and a main pulverization step. After the saponification step, a washing step may be carried out as necessary to remove impurities such as sodium acetate.

[0030] [Cellulose nanofiber (CNF)] The water-based adhesive for paper straws of the present disclosure contains CNF. The average fiber diameter of the CNF contained in the aqueous adhesive for paper straws of the present disclosure is preferably 1 nm or more, more preferably 3 nm or more, and even more preferably 4 nm or more. When the average fiber diameter is equal to or greater than the above-mentioned lower limit, production is facilitated. The average fiber diameter is preferably 1000 nm or less, more preferably 500 nm or less, and even more preferably 400 nm or less, and in some cases, 300 nm or less, 200 nm or less, or 100 nm or less is preferred. When the average fiber diameter is equal to or less than the above-mentioned upper limit, the aqueous adhesive for paper straws of the present disclosure tends to have better water resistance. Note that the average fiber diameter of the CNF in the present disclosure is the number-average fiber diameter calculated by microscopic observation. The number-average fiber diameter can be calculated, for example, by observing 10 random CNFs with a scanning electron microscope (SEM), obtaining the fiber diameters of the CNFs from the observed image (SEM image) using analysis software, and then averaging the fiber diameters of the 10 CNFs as the number-average fiber diameter.

[0031] The average fiber length of the CNF is preferably 0.01 μm or more, more preferably 0.1 μm or more, and even more preferably 0.2 μm or more. When the average fiber length of the CNF is above the above lower limit, the aqueous adhesive for paper straws of the present disclosure tends to have excellent initial adhesion. The average fiber length of the CNF is preferably 1000 μm or less, more preferably 500 μm or less, even more preferably 100 μm or less, and particularly preferably 50 μm or less, and in some cases 5 μm or less or 2 μm or less is preferred. When the average fiber length of the CNF is below the above upper limit, the aqueous adhesive for paper straws of the present disclosure tends to have excellent initial adhesion and film strength. Note that the average fiber length of the CNF in the present disclosure is the number-average fiber length calculated by microscopic observation. The number average fiber length can be determined, for example, by observing 10 random CNFs with a scanning electron microscope (SEM), obtaining the fiber lengths of the CNFs from the observed image (SEM image) using analysis software, and then calculating the average fiber length of the 10 CNFs as the number average fiber length.

[0032] The aspect ratio of the CNF, i.e., the ratio of the average fiber length to the average fiber diameter (average fiber length / average fiber diameter), is preferably at least 3, more preferably at least 10, and even more preferably at least 50. When the aspect ratio of the CNF is at least the above-mentioned lower limit, the aqueous adhesive for paper straws of the present disclosure tends to have excellent initial adhesion and film strength.

[0033] As the CNF, for example, fibrillated cellulose fibers can be suitably used. Examples of raw materials for fibrillated cellulose fibers include wood, straw, bamboo, bagasse, bamboo grass, reeds, and rice husks. Fibrillation can be achieved by applying mechanical shear force to cellulose fibers using a beater, homogenizer, or the like. Cellulose fibers can also be fibrillated by chemical treatment. CNF may contain lignin. Lignin is a component contained in the raw materials for fibrillated cellulose fibers. The lignin content can be adjusted by adjusting the lignin removal rate during fibrillation. CNF may be modified with a carboxyl group or the like, or may be unmodified.

[0034] [Water-based adhesive for paper straws] The aqueous adhesive for paper straws of the present disclosure contains a vinyl alcohol polymer (X), CNF, and water, with the CNF content being 0.1 to 100 parts by mass per 100 parts by mass of the vinyl alcohol polymer (X). If the CNF content is less than 0.1 parts by mass per 100 parts by mass of the vinyl alcohol polymer (X), the initial adhesiveness and water-resistant strength of the film will be poor. On the other hand, if the CNF content is more than 100 parts by mass per 100 parts by mass of the vinyl alcohol polymer (X), the initial adhesiveness, film strength, and water-resistant strength of the film will be poor. The CNF content is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, even more preferably 0.3 parts by mass or more, and particularly preferably 0.5 parts by mass or more, per 100 parts by mass of the vinyl alcohol polymer (X). In some cases, 1 part by mass or more, 2 parts by mass or more, 3 parts by mass or more, 5 parts by mass or more, or 10 parts by mass or more is preferred. When the aqueous adhesive for paper straws of the present disclosure is composed of an aqueous emulsion as described below, the CNF content may be 15 parts by mass or more, 20 parts by mass or more, 30 parts by mass or more, or 40 parts by mass or more per 100 parts by mass of the vinyl alcohol-based polymer (X). The CNF content is preferably 90 parts by mass or less, more preferably 80 parts by mass or less, and even more preferably 70 parts by mass or less per 100 parts by mass of the vinyl alcohol-based polymer (X). When the aqueous adhesive for paper straws of the present disclosure is composed of an aqueous dispersion in which CNF is dispersed in an aqueous solution of the vinyl alcohol-based polymer (X) as described below, the CNF content is preferably 60 parts by mass or less, and in some cases 50 parts by mass or less, 40 parts by mass or less, or 30 parts by mass or less per 100 parts by mass of the vinyl alcohol-based polymer (X).

[0035] The content of the vinyl alcohol polymer (X) in the aqueous adhesive for paper straws of the present disclosure is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more. When the content of the vinyl alcohol polymer (X) is at least the above-mentioned lower limit, the initial adhesiveness and film strength are better. The content of the vinyl alcohol polymer (X) is preferably 30% by mass or less, more preferably 27% by mass or less, even more preferably 23% by mass or less, and particularly preferably 20% by mass or less, and in some cases 17% by mass or less, 15% by mass or less, 13% by mass or less, 10% by mass or less, or 8% by mass or less is preferred. When the content of the vinyl alcohol polymer (X) is at or below the above-mentioned upper limit, the aqueous adhesive for paper straws can maintain an appropriate viscosity, making it easy to handle.

[0036] The total content of the vinyl alcohol polymer (X) and CNF in the aqueous adhesive for paper straws of the present disclosure is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, and particularly preferably 1.5% by mass or more. When the total content of the vinyl alcohol polymer (X) and CNF is above the above-mentioned lower limit, the initial adhesion and film strength are better. The total content of the vinyl alcohol polymer (X) and CNF in the aqueous adhesive for paper straws of the present disclosure is preferably 30% by mass or less, more preferably 27% by mass or less, even more preferably 23% by mass or less, and particularly preferably 20% by mass or less, and in some cases 17% by mass or less, 15% by mass or less, 13% by mass or less, 10% by mass or less, or 8% by mass or less is preferred. When the total content of the vinyl alcohol polymer (X) and CNF is below the above-mentioned upper limit, the aqueous adhesive for paper straws can maintain an appropriate viscosity, making it easy to handle.

[0037] The aqueous adhesive for paper straws of the present disclosure contains water. The water content of the medium in the aqueous adhesive for paper straws of the present disclosure is preferably 40% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and may even be 100% by mass. The medium may contain an organic solvent, but the content is preferably 60% by mass or less, more preferably 30% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less. Examples of the organic solvent include water-soluble organic solvents (alcohols, ketones, etc.) that are soluble in water at any ratio.

[0038] In the aqueous adhesive for paper straws of the present disclosure, the vinyl alcohol polymer (X) is preferably dissolved in water. "The vinyl alcohol polymer (X) is dissolved in water" means that a part or all of the vinyl alcohol polymer (X) is dissolved in water, and it is preferable that substantially all of the vinyl alcohol polymer (X) is dissolved in water.

[0039] The aqueous adhesive for paper straws of the present disclosure may be composed of an aqueous dispersion in which CNF is dispersed in an aqueous solution of a vinyl alcohol-based polymer (X), or may be composed of an aqueous emulsion containing a polymer (C) containing an ethylenically unsaturated monomer unit (hereinafter sometimes referred to as "polymer (C)") as a dispersoid.

[0040] [Water-based emulsion] In one preferred embodiment, the aqueous adhesive for paper straws of the present disclosure is composed of an aqueous emulsion containing polymer (C) as a dispersoid. Such an aqueous adhesive for paper straws has even better initial adhesion and water resistance of the film.

[0041] The polymer (C) is a polymer containing an ethylenically unsaturated monomer unit. Examples of the ethylenically unsaturated monomer include vinyl ester monomers, olefin monomers, (meth)acrylic acid and its salts, (meth)acrylic acid ester monomers (monofunctional (meth)acrylic acid ester monomers, polyfunctional (meth)acrylic acid ester monomers), (meth)acrylamide monomers, vinyl ether monomers, nitrile monomers, allyl monomers, α,β-unsaturated mono- or dicarboxylic acid monomers, diene monomers, aromatic vinyl monomers (e.g., styrene monomers, condensed polycyclic aromatic monomers), and heterocyclic vinyl monomers. Among these, at least one selected from the group consisting of vinyl ester monomers, (meth)acrylic acid ester monomers, styrene monomers, and diene monomers is preferred, and vinyl ester monomers are more preferred. These may be used alone or in combination of two or more. In the present disclosure, "(meth)acrylic" refers to one or more selected from the group consisting of acrylic and methacrylic.

[0042] Examples of vinyl ester monomers include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl versatate, vinyl cinnamate, vinyl crotonate, vinyl decanoate, vinyl hexanoate, vinyl octanoate, vinyl isononanoate, vinyl trimethylacetate, vinyl 4-tert-butylbenzoate, vinyl 2-ethylhexanoate, vinyl caproate, vinyl caprylate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, and vinyl benzoate, with vinyl acetate being preferred from an industrial viewpoint. Examples of the olefin monomer include ethylene and propylene. Examples of (meth)acrylic acid and salts thereof include acrylic acid, methacrylic acid, and sodium and potassium salts thereof. Examples of the monofunctional (meth)acrylic acid ester monomer include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate. Examples of the polyfunctional (meth)acrylic acid ester monomer include pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, pentaerythritol hexa(meth)acrylate, and trimethylolpropane tri(meth)acrylate. Examples of the (meth)acrylamide monomer include (meth)acrylamide, N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, diacetone(meth)acrylamide, (meth)acrylamidopropanesulfonic acid and its salts, (meth)acrylamidopropyldimethylamine and its salts or its quaternary salts, and N-methylol(meth)acrylamide and its derivatives. Examples of the vinyl ether monomer include 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. Examples of the nitrile monomer include acrylonitrile and methacrylonitrile. Examples of the allyl monomer include allyl acetate and allyl chloride. Examples of the α,β-unsaturated mono- or dicarboxylic acid monomer include aliphatic unsaturated dicarboxylic acids such as maleic acid, itaconic acid, and fumaric acid, as well as salts or esters thereof. Examples of the diene monomer include butadiene, isoprene, and chloroprene. Examples of aromatic vinyl monomers include styrene monomers such as styrene, α-methylstyrene, p-methylstyrene, chlorostyrene, α-butoxystyrene, 1-ethyl-2-vinylbenzene, and vinylbiphenyl; and condensed polycyclic aromatic monomers such as vinylnaphthalene, with styrene monomers being preferred. Examples of heterocyclic vinyl monomers include N-vinyl-2-pyrrolidone.

[0043] The content of the ethylenically unsaturated monomer units relative to all monomer units of the polymer (C) is preferably 70% by mass or more, and in some cases may be preferably 80% by mass or more, 90% by mass or more, or 95% by mass or more, and may even be 100% by mass. When the content of the ethylenically unsaturated monomer units is the above-mentioned lower limit or more, the emulsion polymerization stability of the vinyl alcohol polymer (X) is more excellent.

[0044] The dispersant contained in the aqueous emulsion is not particularly limited, and known dispersants used in emulsion polymerization of ethylenically unsaturated monomers can be used. Among these, from the viewpoint of further improving the initial adhesion of the resulting adhesive and the water resistance strength of the resulting film, the dispersant is preferably a vinyl alcohol polymer (X), more preferably an ethylene-vinyl alcohol copolymer (A) or PVA (B), and even more preferably an ethylene-vinyl alcohol copolymer (A). The ethylene-vinyl alcohol copolymer (A) and PVA (B) may be used together as the dispersant. PVA (B) may be either unmodified or modified. Examples of 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; polyoxyalkylene group-modified PVA; acetoacetyl group-modified PVA; and diacetone acrylamide-modified PVA. Ethylene-modified PVA is not included in PVA (B), but is included in ethylene-vinyl alcohol copolymer (A).

[0045] The total content of the vinyl alcohol polymer (X) and CNF in the aqueous adhesive for paper straws of the present disclosure is preferably 2 parts by mass or more, more preferably 2.5 parts by mass or more, and even more preferably 3 parts by mass or more, per 100 parts by mass of the polymer (C), and in some cases, 4 parts by mass or more or 5 parts by mass or more is preferred. When the total content of the vinyl alcohol polymer (X) and CNF is equal to or greater than the above-mentioned lower limit, the aqueous adhesive for paper straws of the present disclosure tends to have excellent initial adhesion. When the total content of the vinyl alcohol polymer (X) and CNF in the aqueous adhesive for paper straws of the present disclosure is equal to or less than 60 parts by mass, more preferably equal to or less than 50 parts by mass, per 100 parts by mass of the polymer (C), and in some cases, 45 parts by mass or less, 40 parts by mass or less, 35 parts by mass or less, 30 parts by mass or less, or 25 parts by mass or less is preferred. When the total content of the vinyl alcohol polymer (X) and CNF is equal to or less than the above-mentioned upper limit, the initial adhesion and the water resistance strength of the film tend to be better.

[0046] When the aqueous adhesive for paper straws of the present disclosure is composed of the above-mentioned aqueous emulsion, the solid content of the aqueous adhesive for paper straws is preferably 10 to 80% by mass. When the solid content is 10% by mass or higher, the viscosity of the aqueous adhesive for paper straws is not too low, making it difficult for particles to settle. The solid content is more preferably 15% by mass or higher, and in some cases, 20% by mass or higher, 25% by mass or higher, 30% by mass or higher, 35% by mass or higher, or 40% by mass or higher is preferred. On the other hand, when the solid content is 80% by mass or lower, aggregates are less likely to form during emulsion polymerization, making it easier to produce the aqueous emulsion. The solid content is more preferably 75% by mass or lower, and in some cases, 70% by mass or lower, 65% by mass or lower, or 60% by mass or lower is preferred. Here, "solid content" refers to the total amount of dry solids contained in the aqueous adhesive for paper straws, i.e., the total amount of solids remaining when the medium, such as water, is removed. In an embodiment in which the aqueous adhesive for paper straws of the present disclosure is composed of the above-mentioned aqueous emulsion, when the dispersant contains a vinyl alcohol-based polymer (X), the aqueous adhesive may contain a vinyl alcohol-based polymer (X) as a component other than the aqueous emulsion, in addition to what is contained in the aqueous emulsion.

[0047] The content of the vinyl alcohol polymer (X), polymer (C), and cellulose nanofibers in the solid content of the aqueous adhesive for paper straws of the present disclosure is preferably 55% by mass or more, more preferably 60% by mass or more, and even more preferably 80% by mass or more, and in some cases may be 90% by mass or more, 95% by mass or more, 99% by mass or more, or 100% by mass or more. When the total content of the vinyl alcohol polymer (X), polymer (C), and CNF is equal to or greater than the above lower limit, the film strength tends to be even better.

[0048] [Manufacturing method for water-based adhesive for paper straws] When the aqueous adhesive for paper straws of the present disclosure is composed of an aqueous dispersion in which CNF is dispersed in an aqueous solution of a vinyl alcohol-based polymer (X), its production method is not particularly limited, and examples include (i) a method in which CNF is added to an aqueous solution of a vinyl alcohol-based polymer (X) and dispersed therein, (ii) a method in which an aqueous dispersion in which CNF is dispersed in water is prepared in advance and the aqueous dispersion is mixed with an aqueous solution of a vinyl alcohol-based polymer (X), and (iii) a method in which a vinyl alcohol-based polymer (X) is added to an aqueous dispersion in which CNF is dispersed in water, etc. Among these, (i) or (ii) is preferred from the viewpoint of reducing the insoluble content of the vinyl alcohol-based polymer (X) and dispersing the CNF more uniformly.

[0049] In the above method (i) or (ii), examples of the apparatus used in the step of dispersing CNF or the step of mixing the aqueous dispersion of CNF and the aqueous solution of vinyl alcohol polymer (X) include a medium stirring type disperser, a high-pressure type disperser, and a rotary type disperser.

[0050] When the aqueous adhesive for paper straws of the present disclosure is composed of the above-mentioned aqueous emulsion, the method for producing the aqueous emulsion is not particularly limited, and examples include a production method comprising step (1) of emulsion-polymerizing an ethylenically unsaturated monomer in the presence of a vinyl alcohol-based polymer (X) using a dispersion medium to obtain an aqueous emulsion, and step (2) of blending CNF with the aqueous emulsion, or a production method comprising step (3) of obtaining an aqueous dispersion containing the vinyl alcohol-based polymer (X) and CNF, and step (4) of emulsion-polymerizing the aqueous dispersion with the ethylenically unsaturated monomer. An aqueous solution of the vinyl alcohol-based polymer (X) may be further added to the aqueous emulsion obtained by these production methods.

[0051] Step (2) is preferably carried out by adding an aqueous dispersion of CNF to the aqueous emulsion obtained in step (1) and mixing them. Step (3) is preferably carried out by adding CNF to an aqueous solution of vinyl alcohol polymer (X) and dispersing it, mixing an aqueous solution of vinyl alcohol polymer (X) and an aqueous dispersion of CNF, dissolving vinyl alcohol polymer (X) in an aqueous dispersion of CNF, or adding vinyl alcohol polymer (X) and CNF to water all at once to simultaneously dissolve vinyl alcohol polymer (X) and disperse CNF. In these methods, an aqueous dispersion containing CNF can be obtained by, for example, mixing water and CNF and dispersing them using a medium stirring disperser, a high-pressure disperser, or a rotary disperser.

[0052] The ethylenically unsaturated monomer used in step (1) and step (4) may be any of the ethylenically unsaturated monomers described above as examples of the ethylenically unsaturated monomer contained in polymer (C).

[0053] An example of a method for obtaining an aqueous emulsion in step (1) is a method in which a dispersant and an ethylenically unsaturated monomer are charged, and then an appropriately selected polymerization initiator is added to emulsion-polymerize the monomer. The method for charging or adding the dispersant is not particularly limited, and examples include a method in which the dispersant is charged all at once at the initial stage and a method in which the dispersant is added continuously during polymerization. Among these, a method in which the dispersant is charged all at once at the initial stage into the polymerization system is preferred from the viewpoint of increasing the grafting rate of the vinyl alcohol polymer (X) to the dispersoid. In these methods, the polymerization reaction can be controlled by appropriately adjusting the amount of dispersant, the amount of ethylenically unsaturated monomer, and the amount of solvent.

[0054] The emulsion polymerization method in step (4) may include a method in which an ethylenically unsaturated monomer and an appropriately selected polymerization initiator are added to the aqueous dispersion obtained in step (3) and the monomer is emulsion-polymerized. In such a method, the polymerization reaction can be controlled by appropriately adjusting the amount of the dispersant, the amount of the ethylenically unsaturated monomer, and the amount of the solvent.

[0055] The amount of dispersant used per 100 parts by mass of polymer (C) is not particularly limited, but is preferably 2 parts by mass or more and 60 parts by mass or less. The amount used is more preferably 2.5 parts by mass or more, and in some cases, 3 parts by mass or more, 4 parts by mass or more, or 5 parts by mass or more is preferred. When the amount of dispersant used is above the above-mentioned lower limit, the emulsion polymerization reaction is likely to be stable. The amount of dispersant used is more preferably 60 parts by mass or less, even more preferably 50 parts by mass or less, and in some cases, 45 parts by mass or less, 40 parts by mass or less, 35 parts by mass or less, or 25 parts by mass or less is preferred. When the amount of dispersant used is below the above-mentioned upper limit, the water-resistant adhesive properties of the aqueous adhesive for paper straws tend to be excellent. Here, in the present disclosure, when an aqueous emulsion is produced by emulsion polymerization of a monomer in the presence of a dispersant, the monomer is almost entirely polymerized to form polymer (C). Therefore, the amount of monomer remaining in the aqueous emulsion is very small and can be ignored.

[0056] In the emulsion polymerization, the polymerization initiator may be a water-soluble single initiator or a water-soluble redox initiator that is commonly used in emulsion polymerization. These polymerization initiators may be used alone or in combination of two or more. Among these, redox initiators are preferred.

[0057] Examples of the water-soluble single initiator include azo initiators, hydrogen peroxide, peroxides such as persulfates (potassium, sodium, or ammonium salts), etc. Examples of the azo initiator include 2,2'-azobis(isobutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile).

[0058] As the redox initiator, a combination of an oxidizing agent and a reducing agent can be used. As the oxidizing agent, a peroxide is preferred. As the reducing agent, a metal ion, a reducing compound, etc. can be mentioned. As the combination of an oxidizing agent and a reducing agent, a combination of a peroxide and a metal ion, a combination of a peroxide and a reducing compound, and a combination of a peroxide, a metal ion, and a reducing compound can be mentioned. As the peroxide, a hydroperoxide such as hydrogen peroxide, cumene hydroperoxide, t-butyl hydroperoxide, persulfates (potassium, sodium, or ammonium salts), t-butyl peracetate, peresters (t-butyl perbenzoate), etc. can be mentioned. As the metal ion, Fe 2+ , Cr 2+ , V 2+ , Co 2+ , Ti 3+ , Cu +Examples of reducing compounds include metal ions capable of undergoing one-electron transfer, such as sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, fructose, dextrose, sorbose, inositol, rongalite, 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 with one or more reducing agents selected from the group consisting of sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, rongalite, and ascorbic acid is preferred, and a combination of hydrogen peroxide with one or more reducing agents selected from the group consisting of sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, rongalite, and ascorbic acid is more preferred.

[0059] In emulsion polymerization, alkali metal compounds, surfactants, buffers, polymerization degree regulators, and the like may be used as appropriate within the scope that does not impair the effects of the present disclosure.

[0060] The alkali metal compound is not particularly limited as long as it contains an alkali metal (sodium, potassium, rubidium, cesium), and may be an alkali metal ion itself or a compound containing an alkali metal. Examples of the alkali metal compound include weakly basic alkali metal salts such as alkali metal carbonates, alkali metal acetates, alkali metal bicarbonates, alkali metal phosphates, alkali metal sulfates, alkali metal halide salts, and alkali metal nitrates; and strongly basic alkali metal compounds such as alkali metal hydroxides and alkali metal alkoxides. These alkali metal compounds may be used alone or in combination of two or more.

[0061] Examples of weakly basic alkali metal salts include alkali metal carbonates such as sodium carbonate, potassium carbonate, rubidium carbonate, and cesium carbonate; alkali metal bicarbonates such as sodium hydrogencarbonate and potassium hydrogencarbonate; alkali metal phosphates such as sodium phosphate and potassium phosphate; alkali metal carboxylates such as sodium acetate, potassium acetate, and cesium acetate; alkali metal sulfates such as sodium sulfate, potassium sulfate, and cesium sulfate; alkali metal halide salts such as cesium chloride, cesium iodide, potassium chloride, and sodium chloride; and alkali metal nitrates such as sodium nitrate, potassium nitrate, and cesium nitrate. Among these, alkali metal carboxylates, alkali metal carbonates, and alkali metal bicarbonates that behave as salts of weak acids and strong bases upon dissociation are preferred, and alkali metal carboxylates are more preferred, from the viewpoint of imparting basicity to the emulsion. By using these weakly basic alkali metal salts, the weakly basic alkali metal salts act as pH buffers during emulsion polymerization, thereby enabling stable emulsion polymerization.

[0062] The content of the alkali metal compound (in terms of alkali metal) can be appropriately selected depending on the type of alkali metal compound used. The content of the alkali metal compound (in terms of alkali metal) is preferably 100 to 15,000 ppm, more preferably 120 to 12,000 ppm, and even more preferably 150 to 8,000 ppm, based on the total mass of the aqueous emulsion (in terms of solids). When the content of the alkali metal compound is 100 ppm or more, the stability of the emulsion polymerization of the aqueous emulsion tends to be excellent, while when it is 15,000 ppm or less, discoloration of the coating composed of the aqueous emulsion is easily suppressed. The content of the alkali metal compound can be measured using an ICP emission spectrometer or the like. Here, "ppm" means "ppm by mass."

[0063] The surfactant may be any of nonionic surfactants, anionic surfactants, and cationic surfactants. Examples of nonionic surfactants 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, and glycerin fatty acid esters. Examples of anionic surfactants include alkyl sulfates, alkylaryl sulfates, alkyl sulfonates, sulfates of hydroxyalkanols, dialkyl sulfosuccinate salts, and sulfates and phosphates of alkyl or alkylaryl polyethoxyalkanols. Examples of cationic surfactants include alkylamine salts, quaternary ammonium salts, and polyoxyethylene alkylamines. The amount of surfactant used is preferably 2% by mass or less based on the total amount of ethylenically unsaturated monomers. Using an amount of surfactant of 2% by mass or less improves the water resistance of the aqueous adhesive for paper straws of the present disclosure.

[0064] Examples of the buffer include acids such as acetic acid, hydrochloric acid, and sulfuric acid; bases such as ammonia, amines, sodium hydroxide, potassium hydroxide, and calcium hydroxide; and alkali carbonates, phosphates, and acetates. Examples of the polymerization degree regulator include mercaptans and alcohols.

[0065] The dispersion medium in the emulsion polymerization is preferably an aqueous medium containing water as a main component. The aqueous medium containing water as a main component may contain a water-soluble organic solvent (such as an alcohol or a ketone) that is soluble in water at any ratio. Here, "aqueous medium containing water as a main component" refers to a dispersion medium containing 50% by mass or more of water. From the viewpoints 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 dissolve the dispersant in the dispersion medium and heat it before starting the emulsion polymerization, followed by cooling and nitrogen substitution. In this case, the heating temperature is preferably 80 to 100°C. The emulsion polymerization temperature is preferably about 20 to 95°C, more preferably about 40 to 90°C.

[0066] The aqueous adhesive for paper straws of the present disclosure may contain other components in addition to the vinyl alcohol polymer (X), polymer (C), CNF, and water, as long as the effects of the present disclosure are not impaired. These other components include resins other than the vinyl alcohol polymer (X) and polymer (C), organic solvents, plasticizers, crosslinking agents, suspending agents, thickeners, flow improvers, preservatives, adhesion improvers, antioxidants, penetrating agents, antifoaming agents, fillers, wetting agents, colorants, binders, water retention agents, bulking agents, sugars such as starch and its derivatives, and latex.

[0067] Other examples of the other components include inorganic dispersants such as metal salts of phosphate compounds (e.g., sodium polyphosphate and sodium hexametaphosphate) and water glass; polyacrylic acid and its salts; sodium alginate; anionic polymeric compounds and their metal salts (e.g., α-olefin-maleic anhydride copolymers); and surfactants such as nonionic surfactants (e.g., ethylene oxide adducts of higher alcohols and copolymers of ethylene oxide and propylene oxide). Addition of these components improves the fluidity of the adhesive. To further improve water resistance, one or more crosslinkers selected from the group consisting of water-soluble metal compounds, colloidal inorganic substances, polyamidoamine epichlorohydrin adducts, and glyoxal resins may also be added. Examples of water-soluble metal compounds include aluminum chloride, aluminum nitrate, ammonium zirconium carbonate, and titanium lactate. Examples of colloidal inorganic substances include colloidal silica and alumina sol. Examples of polyamidoamine epichlorohydrin adducts include those obtained by adding epichlorohydrin to various polyamidoamines. Examples of glyoxal resins include urea-glyoxal resins. Furthermore, water-resistant strength can be further improved by using a water-soluble metal salt or colloidal inorganic material in combination with a polyamidoamine epichlorohydrin adduct or a glyoxal resin. Methylol group-containing compounds (resins), epoxy compounds (resins), aziridine group-containing compounds (resins), oxazoline group-containing compounds (resins), carbodiimide compounds, aldehyde compounds (resins), and the like, can also be used in combination with the above-mentioned crosslinking agents, provided that performance is not impaired. To further improve adhesive strength, water-soluble boron compounds such as boric acid, borax, and borate esters of polyhydric alcohols such as glycerin and ethylene glycol, as well as sodium naphthalenesulfonate-formaldehyde condensates, can also be added. Natural adhesives such as starch, casein, gelatin, guar gum, gum arabic, and sodium alginates, as well as processed natural adhesives such as carboxymethyl cellulose, oxidized starch, and methyl cellulose can also be added. These may be used alone or in combination of two or more.The content of the above other components in the aqueous adhesive for paper straws of the present disclosure is preferably 100% by mass or less, and in some cases may be preferably 50% by mass or less, 20% by mass or less, 10% by mass or less, 5% by mass or less, 2% by mass or less, 1% by mass or less, or 0.5% by mass or less.

[0068] Another preferred embodiment of the present disclosure is a paper straw having paper substrates bonded together using the aqueous adhesive for paper straws of the present disclosure. The paper straws have excellent strength and water resistance. By using the aqueous adhesive for paper straws of the present disclosure, which has excellent initial adhesion and film strength, such paper straws can be efficiently manufactured.

[0069] The present disclosure also provides a paper straw having a vinyl alcohol-based polymer (X) and CNF on the adhesive surface between paper substrates. The paper straw has excellent strength and water resistance. The preferred embodiments of the vinyl alcohol-based polymer (X) and CNF, and the preferred content of CNF relative to the vinyl alcohol-based polymer (X), are the same as those described above.

[0070] These paper straws can be obtained by common manufacturing methods, such as applying the aqueous adhesive for paper straws of the present disclosure to a tape-shaped paper substrate, spirally winding the paper substrate around a metal core rod while stacking it, and then drying it. [Example]

[0071] The present disclosure will be described in more detail with reference to examples. The present disclosure is not limited to these examples, and many modifications can be made by those skilled in the art within the technical spirit of the present disclosure. In the examples and comparative examples, "%" and "parts" represent "% by mass" and "parts by mass," respectively, unless otherwise specified.

[0072] [Viscosity average degree of polymerization and degree of saponification] The viscosity average degree of polymerization and the degree of saponification of the vinyl alcohol polymer were determined by the method described in JIS K 6726:1994.

[0073] [Initial adhesion] The initial adhesiveness of the aqueous adhesives obtained in the Examples and Comparative Examples was evaluated by the following method. In an atmosphere of 20°C and 65% RH, the water-based adhesive was applied to kraft paper using a No. 6 wire bar (bar coater), and after lamination, the paper was pressed three times with a hand roll. The time from completion of pressing until the kraft paper completely tore without peeling when peeled off by hand was measured. Evaluation was based on the following criteria. A: The kraft paper was completely torn in less than 3 seconds. B: The kraft paper was completely torn in 3 to 5 seconds. C: The kraft paper was completely torn in 5 to 10 seconds. D: The kraft paper was completely torn in 10 seconds or more.

[0074] [Coating strength (Young's modulus measurement)] The film strength of the aqueous adhesives obtained in the examples and comparative examples was evaluated by the method shown below. The aqueous adhesive was cast onto a PET film at 20°C and 65% RH and dried at room temperature for 7 days to obtain a coated product with a dry film. The dried film was peeled off from the coated product to obtain a 50 μm thick film. The film was cut into 10 mm widths and conditioned for one week at 20°C and 65% RH. A tensile test (chuck spacing: 20 mm, tensile speed: 100 mm / min) was then performed using a Shimadzu Corporation "Autograph AG-IS" precision universal testing machine to measure the Young's modulus. Based on the Young's modulus, the film strength was evaluated according to the following criteria. A: Young's modulus 50 kgf / mm 2 End B: Young's modulus 40 kgf / mm 2 More than 50kgf / mm 2 less than C: Young's modulus 30 kgf / mm 2 Over 40kgf / mm 2 less than D: Young's modulus 30 kgf / mm 2 less than

[0075] [Water resistance strength of film (measurement of Young's modulus after immersion in water)] Using the same method as for the evaluation of film strength, a film 50 μm thick and 10 mm wide was prepared. The film was immersed in water at 20°C and removed after 24 hours. Immediately afterwards, a tensile test (chuck spacing: 20 mm, tensile speed: 100 mm / min) was performed using a precision universal testing machine "Autograph AG-IS" manufactured by Shimadzu Corporation to measure Young's modulus. Based on the Young's modulus, the water resistance strength of the film was evaluated according to the following criteria. A: Young's modulus is 5 kgf / mm 2 End B: Young's modulus is 1 kgf / mm 2 More than 5kgf / mm 2 less than C: Young's modulus 0.5 kgf / mm 2 More than 1kgf / mm 2 less than D: Young's modulus 0.5 kgf / mm 2 less than

[0076] [Production Example 1: Production of ethylene-vinyl alcohol copolymer (A) (PVA-1)] A 250 L pressure reactor equipped with a stirrer, a nitrogen inlet, an ethylene inlet, a polymerization initiator inlet, and a delay solution inlet was charged with 106.1 kg of vinyl acetate (VAc) and 43.9 kg of methanol, and the temperature was raised to 60°C. After that, the system was purged with nitrogen by nitrogen bubbling for 30 minutes. Then, the reactor pressure was reduced to 1.4 kg / cm. 2 Ethylene was introduced so that the reaction pressure was 1.4 kg / cm. As a polymerization initiator, a 2.8 g / L solution of 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (AMV) was dissolved in methanol, and nitrogen was substituted by bubbling with nitrogen gas. After the internal temperature of the polymerization vessel was adjusted to 60°C, 53 ml of the polymerization initiator solution was injected to start polymerization. During polymerization, ethylene was introduced to maintain the reactor pressure at 1.4 kg / cm. 2The polymerization temperature was maintained at 60°C, and polymerization was carried out using the above polymerization initiator solution by continuously adding AMV at a rate of 168 ml / hr. After 4 hours, when the conversion reached 20%, the polymerization was terminated by cooling. The reactor was opened to remove ethylene, and nitrogen gas was then bubbled through. Unreacted vinyl acetate monomer was then removed under reduced pressure, and methanol was added to obtain a methanol solution of ethylene-vinyl acetate copolymer (concentration: 25% by mass). 46.5 g of a 10% methanol solution of NaOH (molar ratio [MR] of the amount of NaOH to the vinyl acetate units in the ethylene-vinyl acetate copolymer: 0.10) was added to 400 g of the methanol solution of ethylene-vinyl acetate copolymer (100 g of ethylene-vinyl acetate copolymer in the solution), and saponification was carried out at 40°C. After the addition of the NaOH methanol solution, the gel was pulverized in a grinder, and the saponification reaction was carried out for a total of 1 hour. After that, 1,000 g of methyl acetate was added to neutralize the remaining alkali. After confirming the completion of neutralization using a phenolphthalein indicator, the white solid obtained by filtration was washed with 1000 g of methanol at room temperature for 3 hours. After repeating the washing procedure three times, the solid obtained by centrifugal dewatering was left in a dryer at 70°C for 2 days to obtain ethylene-vinyl alcohol copolymer (PVA-1).

[0077] [Production Example 2: Production of ethylene-vinyl alcohol copolymer (A) (PVA-2)] An ethylene-vinyl alcohol copolymer (PVA-2) was produced in the same manner as in Production Example 1, except that the polymerization conditions, such as the amounts of ethylene, vinyl acetate, and methanol, polymerization initiator, and polymerization rate, as well as the concentration of the ethylene-vinyl acetate copolymer solution and the amount of NaOH solution during saponification, were changed as shown in Table 1.

[0078] [Production Example 3: Production of polyvinyl alcohol (B) (PVA-3)] A reactor equipped with a stirrer, nitrogen inlet, and polymerization initiator addition port was charged with 2.4 kg of vinyl acetate and 1.0 kg of methanol. The temperature was raised to 60°C, and the system was then purged with nitrogen by bubbling with nitrogen for 30 minutes. Next, a 10% by weight solution of 2,2'-azobis(isobutyronitrile) (AIBN) as a polymerization initiator was prepared in methanol, and nitrogen was purged by bubbling with nitrogen gas. The internal temperature of the polymerization vessel was adjusted to 60°C, and 10 ml of the polymerization initiator solution was added to initiate polymerization. After 1.4 hours, the polymerization was terminated by cooling when the conversion reached 30%. Unreacted vinyl acetate monomer was removed, and methanol was added to obtain a methanol solution of polyvinyl acetate (PVAc) (concentration: 25% by weight). To 400 g of the PVAc methanol solution (100 g of PVAc in solution), 32.6 g of a 10% NaOH methanol solution (molar ratio [MR] of the amount of NaOH to vinyl acetate units in PVAc: 0.07) was added, and saponification was carried out at 40°C. After the addition of the NaOH methanol solution, the gel was pulverized in a grinder, and the saponification reaction was carried out for a total of 1 hour. 1000 g of methyl acetate was then added to neutralize the remaining alkali. After confirming the completion of neutralization using a phenolphthalein indicator, 1000 g of methanol was added to the white solid obtained by filtration, and the solid was left to stand at room temperature for 3 hours for washing. The above washing procedure was repeated three times, and the solid obtained by centrifugal dewatering was left to stand in a dryer at 70°C for 2 days to obtain PVA (PVA-3).

[0079] [Table 1]

[0080] [Example 1] (Preparation of water-based adhesive) The resulting ethylene-vinyl alcohol copolymer (PVA-1) powder (15 parts) was added to stirred water (85 parts, 20°C) and heated to 95°C to dissolve the ethylene-vinyl alcohol copolymer, yielding an aqueous solution. To this solution, 100 parts by mass of a 2% by mass aqueous dispersion of unmodified CNF (aspect ratio approximately 200, average fiber diameter 50 nm, average fiber length 10 μm) was added, and the mixture was stirred with a magnetic stirrer for 1 hour to obtain an aqueous adhesive. The amount of CNF added was 2 parts by mass per 100 parts by mass of the ethylene-vinyl alcohol copolymer (A) aqueous solution. The initial adhesion, film strength, and water resistance of the resulting aqueous adhesive were evaluated according to the methods described above. The results are shown in Table 2.

[0081] [Examples 2 to 4] As shown in Table 2, aqueous adhesives were prepared in the same manner as in Example 1, except that a predetermined amount of PVA-2 or PVA-3 was used instead of PVA-1 and the amount of CNF added was changed. The aqueous adhesives were evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0082] [Production Example 4: Production of aqueous emulsion (Em-1)] A 1-liter glass polymerization vessel equipped with a reflux condenser, dropping funnel, thermometer, and nitrogen inlet was charged with 275 g of ion-exchanged water and heated to 95°C. 19.5 g of PVA-1 was added and stirred for 45 minutes to dissolve. 0.3 g of sodium acetate was then added and mixed to dissolve. The aqueous solution containing PVA-1 was cooled and purged with nitrogen, then heated to 60°C while stirring at 200 rpm. Next, 2.4 g of a 20% by weight aqueous solution of tartaric acid and 3.2 g of a 5% by weight aqueous solution of hydrogen peroxide were added in shots, followed by the addition of 27 g of vinyl acetate to initiate polymerization. The completion of the initial polymerization (remaining vinyl acetate content less than 1%) was confirmed 30 minutes after the start of polymerization. After adding 1 g of a 10% by mass aqueous solution of tartaric acid and 3.2 g of a 5% by mass aqueous solution of hydrogen peroxide in shots, 251 g of vinyl acetate was added continuously over 2 hours, and the polymerization temperature was maintained at 80°C to complete the polymerization, yielding an aqueous emulsion (Em-1) with a solid content of 50% by mass.

[0083] [Production Example 5: Production of aqueous emulsion (Em-2)] A 1-liter glass polymerization vessel equipped with a reflux condenser, dropping funnel, thermometer, and nitrogen inlet was charged with 275 g of ion-exchanged water and heated to 95°C. 19.5 g of PVA-3 was added and stirred for 45 minutes to dissolve. 0.3 g of sodium acetate was then added and mixed to dissolve. The aqueous solution containing PVA-3 was cooled and purged with nitrogen, then heated to 60°C while stirring at 200 rpm. Next, 2.4 g of a 20% by weight aqueous solution of tartaric acid and 3.2 g of a 5% by weight aqueous solution of hydrogen peroxide were added in shots, followed by the addition of 27 g of vinyl acetate to initiate polymerization. The completion of the initial polymerization (remaining vinyl acetate content less than 1%) was confirmed 30 minutes after the start of polymerization. After adding 1 g of a 10% by mass aqueous solution of tartaric acid and 3.2 g of a 5% by mass aqueous solution of hydrogen peroxide in shots, 251 g of vinyl acetate was added continuously over 2 hours, and the polymerization temperature was maintained at 80°C to complete the polymerization, yielding an aqueous emulsion (Em-2) with a solid content of 50% by mass.

[0084] [Example 5] (Preparation of water-based adhesive) To 100 parts by mass of aqueous emulsion (Em-1), 100 parts by mass of a 2% by mass aqueous dispersion of unmodified CNF (aspect ratio approximately 200, average fiber diameter 50 nm, average fiber length 10 μm) was added, and the mixture was stirred for 1 hour with a magnetic stirrer to obtain an aqueous adhesive. The amount of CNF added was 2 parts by mass per 100 parts by mass of aqueous emulsion (Em-1). The initial adhesion, film strength, and water resistance of the film were evaluated using the methods described above. The results are shown in Table 2.

[0085] [Example 6] Aqueous adhesives were prepared in the same manner as in Example 5, except that Em-2 was used instead of Em-1, as shown in Table 2. The initial adhesion, film strength, and water resistance of the resulting aqueous adhesives were evaluated according to the methods described above. The results are shown in Table 2.

[0086] [Example 7] (Preparation of water-based adhesive) Ethylene-vinyl alcohol copolymer (PVA-1) powder (15 parts) was added to stirred water (85 parts, 20°C) and heated to 95°C to dissolve the ethylene-vinyl alcohol copolymer, resulting in a 15% aqueous solution. 100 parts by mass of the resulting PVA-1 aqueous solution and 100 parts by mass of aqueous emulsion (Em-1) were blended at room temperature, followed by the addition of 200 parts by mass of a 2% by mass aqueous dispersion of unmodified CNF (aspect ratio approximately 200, average fiber diameter 50 nm, average fiber length 10 μm). The mixture was stirred for 1 hour with a magnetic stirrer to obtain an aqueous adhesive. The amount of CNF added was 2 parts by mass per 100 parts by mass of the mixed aqueous dispersion of PVA-1 aqueous solution and Em-1. The initial adhesion, film strength, and water resistance of the resulting aqueous adhesive were evaluated according to the methods described above. The results are shown in Table 2.

[0087] [Examples 8 and 9] As shown in Table 2, aqueous adhesives were obtained in the same manner as in Example 7, except that Em-2 was used instead of Em-1 (Examples 8 and 9), and that an aqueous solution of PVA-3 was used instead of an aqueous solution of PVA-1 (Example 9 only). The initial adhesion, film strength, and water resistance strength of the obtained aqueous adhesives were evaluated according to the methods described above. The results are summarized in Table 2.

[0088] [Comparative Example 1] Ethylene-vinyl alcohol copolymer (PVA-1) powder (15 parts) was added to stirred water (85 parts, 20°C) and heated to 95°C to dissolve the ethylene-vinyl alcohol copolymer, yielding an aqueous solution. To this aqueous solution, 100 parts by mass of water was added, and the mixture was stirred with a magnetic stirrer for 1 hour to yield an aqueous adhesive. The initial adhesion, film strength, and water resistance of the resulting aqueous adhesive were evaluated according to the methods described above. The results are shown in Table 2.

[0089] Comparative Example 2 Aqueous adhesives were obtained in the same manner as in Example 1, except that the amount of CNF added was changed as shown in Table 2. The initial adhesion, film strength, and water-resistant strength of the resulting aqueous adhesives were evaluated according to the methods described above. The results are shown in Table 2.

[0090] Comparative Example 3 As shown in Table 2, an aqueous adhesive was obtained in the same manner as in Comparative Example 1, except that PVA-3 was used instead of PVA-1. The initial adhesion, film strength, and water-resistant strength of the obtained aqueous adhesive were evaluated according to the methods described above. The results are shown in Table 2.

[0091] Comparative Example 4 100 parts by weight of the aqueous emulsion (Em-1) was added to 100 parts by weight of water and stirred for 1 hour with a magnetic stirrer to obtain an aqueous adhesive. The initial adhesion, film strength, and water resistance of the resulting aqueous adhesive were evaluated according to the methods described above. The results are shown in Table 2.

[0092] Comparative Example 5 Ethylene-vinyl alcohol copolymer (PVA-1) powder (15 parts) was added to stirred water (85 parts, 20°C) and heated to 95°C to dissolve the ethylene-vinyl alcohol copolymer, resulting in a 15% aqueous solution. 100 parts by mass of the resulting PVA-1 aqueous solution and 100 parts by mass of aqueous emulsion (Em-1) were blended at room temperature, followed by the addition of 200 parts by mass of water and stirring with a magnetic stirrer for 1 hour to obtain a water-based adhesive. The initial adhesion, film strength, and water-resistant strength of the resulting water-based adhesive were evaluated according to the methods described above. The results are shown in Table 2.

[0093] [Table 2]

[0094] When the aqueous adhesive was composed of an aqueous dispersion of CNF dispersed in an aqueous solution of a vinyl alcohol polymer (X), and the CNF content was 0.1 to 100 parts by mass per 100 parts by mass of the vinyl alcohol polymer (X), the initial adhesion and film strength were excellent (Examples 1 to 4). In particular, when the vinyl alcohol polymer (X) was an ethylene-vinyl alcohol copolymer (A), not only were the initial adhesion and film strength excellent, but the water-resistant strength of the film was also excellent compared to Comparative Examples 1 to 5 (Examples 1, 2, and 4). Furthermore, when the aqueous adhesive contained a vinyl alcohol polymer (X), CNF, and water, and the CNF content was 0.1 to 100 parts by mass per 100 parts by mass of the vinyl alcohol polymer (X), and was composed of an aqueous emulsion containing a polymer (C) as a dispersoid, the initial adhesion and film strength were even better, and the water-resistant strength of the film was even better (Examples 5 to 9). Thus, the aqueous adhesive for paper straws of the present disclosure has excellent initial adhesion and film strength, making it suitable for the production of paper straws. On the other hand, when the aqueous adhesive was an aqueous solution of vinyl alcohol-based polymer (X) and did not contain CNF, the initial adhesion and water-resistant strength of the film were significantly poor (Comparative Examples 1 and 3). Furthermore, when the aqueous adhesive was composed of an aqueous solution of vinyl alcohol-based polymer (X) and the CNF content was greater than 100 parts by mass per 100 parts by mass of vinyl alcohol-based polymer (X), the initial adhesion and water-resistant strength of the film were significantly poor, and the film strength was also insufficient (Comparative Example 2). When the aqueous adhesive was composed of an aqueous emulsion containing polymer (C) as a dispersoid and did not contain CNF, the water-resistant strength of the film was significantly poor, and the initial adhesion or film strength was insufficient (Comparative Examples 4 and 5).

Claims

1. An aqueous adhesive for paper straws comprising a vinyl alcohol polymer (X), cellulose nanofibers, and water, wherein the cellulose nanofiber content is 0.1 to 100 parts by mass relative to 100 parts by mass of the vinyl alcohol polymer (X), the cellulose nanofibers have an average fiber length of 10 μm or more and 1000 μm or less, the vinyl alcohol polymer (X) has a viscosity-average degree of polymerization of 200 to 5000, and a degree of saponification of 80 to 99.9 mol%, and the vinyl alcohol polymer (X) is one or more polymers selected from the group consisting of ethylene-vinyl alcohol copolymer (A) and polyvinyl alcohol (B).

2. The aqueous adhesive for paper straws according to claim 1, wherein the ethylene unit content of the ethylene-vinyl alcohol copolymer (A) is 1 mol% or more and less than 20 mol%.

3. The aqueous adhesive for paper straws according to claim 1 or 2, wherein the vinyl alcohol polymer (X) is dissolved in water.

4. The aqueous adhesive for paper straws according to any one of claims 1 to 3, further comprising a polymer (C) containing an ethylenically unsaturated monomer unit, and comprising an aqueous emulsion containing the polymer (C) containing an ethylenically unsaturated monomer unit as a dispersoid.

5. 5. The aqueous adhesive for paper straws according to claim 4, wherein the polymer (C) contains 70% by mass or more of 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, based on all monomer units.

6. The aqueous adhesive for paper straws according to claim 4 or 5, wherein the dispersant in the aqueous emulsion is a vinyl alcohol polymer (X).

7. The total content of the vinyl alcohol polymer (X) and the cellulose nanofibers is 2 to 60 parts by mass per 100 parts by mass of the polymer (C). The aqueous adhesive for paper straws according to any one of claims 4 to 6.

8. The aqueous adhesive for paper straws according to any one of claims 4 to 7, wherein the content of the vinyl alcohol polymer (X), the polymer (C), and the cellulose nanofiber in the solid content contained in the aqueous adhesive for paper straws is 55% by mass or more. The aqueous adhesive for paper straws.

9. A paper straw obtained by bonding paper substrates together using the adhesive according to any one of claims 1 to 8.

10. A paper straw having a vinyl alcohol polymer (X) and cellulose nanofibers on the adhesive surface between paper substrates, wherein the cellulose nanofibers have an average fiber length of 10 μm or more and 1000 μm or less, the vinyl alcohol polymer (X) has a viscosity-average degree of polymerization of 200 to 5000 and a degree of saponification of 80 to 99.9 mol%, and the vinyl alcohol polymer (X) is one or more polymers selected from the group consisting of ethylene-vinyl alcohol copolymer (A) and polyvinyl alcohol (B).

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