Method for producing solid polymer and method for producing water-absorbent resin particles

Abstract

A method for producing a solid polymer according to the present invention includes a step for preparing a polymer solution containing a polymer in which a crosslinked structure of a water-absorbent resin has been cleaved, and a step for obtaining a polymer dispersion, in which a solid polymer is dispersed, by bringing the polymer solution into contact with a solvent that poorly dissolves the polymer to cause precipitation of the polymer.

Description

Method for producing solid polymer and method for producing water-absorbent resin particles

[0001] The present invention relates to a method for producing a solid polymer and a method for producing water-absorbent resin particles.

[0002]

[0003] Generally, disposable sanitary products are constructed by enclosing pulp fibers and a water-absorbent resin between a water-impermeable cover sheet and a water-permeable nonwoven fabric, and the water-absorbent resin absorbs water and swells to absorb bodily waste. After use, such sanitary products are disposed of by incineration or landfilling, but recently, research has been conducted into the recovery and reuse of components from used sanitary products. For example, Patent Document 1 discloses a water-absorbent resin decomposition technology that focuses on cleavage of crosslinks within the water-absorbent resin, and a technology for recovering the water-soluble recycled polymer obtained by decomposing the water-absorbent resin as an aggregate by crosslinking with multivalent metal ions.

[0003] Japanese Patent Application Laid-Open No. 2020-49398

[0004] After obtaining a polymer solution from the recovered water-absorbent resin, impurities are removed from the polymer solution by ultrafiltration to obtain a high-purity polymer solution, and then a high-purity solid polymer is obtained. However, when the amount of the polymer solution is large or the viscosity of the polymer solution is high, the time required for ultrafiltration becomes long, and as a result, it takes a very long time to obtain a high-purity solid polymer from the polymer solution.

[0005] Therefore, one aspect of the present invention relates to a method for producing a solid polymer, which allows a high-purity solid polymer to be obtained from a polymer solution in a short period of time.

[0006] One aspect of the present invention includes, for example, the following [1] to [6]. [1] A method for producing a solid polymer, comprising: preparing a polymer solution containing a polymer in which the crosslinked structure of a water-absorbent resin has been cleaved; and obtaining a polymer dispersion in which the solid polymer is dispersed by bringing a poor solvent for the polymer into contact with the polymer solution to precipitate the polymer. [2] The method according to [1], wherein the degree of neutralization of the polymer is 80 mol% or more. [3] The method according to [1] or [2], wherein the viscosity of the polymer solution at 25°C is 10 mPa·s to 200,000 mPa·s. [4] The method according to any one of [1] to [3], wherein the content of the poor solvent in the polymer dispersion is 60 mass% or less. [5] The method according to any one of [1] to [4], wherein the poor solvent comprises at least one selected from the group consisting of alcohols, phenols, and ketones having 1 to 10 carbon atoms. [6] A method for producing water-absorbent resin particles, comprising a step of crosslinking the solid polymer obtained by the method for producing water-absorbent resin particles according to any one of [1] to [5] to obtain a water-absorbent resin.

[0007] According to one aspect of the present invention, there is provided a method for producing a solid polymer, which allows a high-purity solid polymer to be obtained from a polymer solution in a short period of time.

[0008] 1 is a cross-sectional view showing an example of an absorbent article having an absorbent body.

[0009] The present invention is not limited to the following examples.

[0010] In this specification, "(meth)acrylic" refers to both acrylic and methacrylic. "Acrylate" and "methacrylate" are also written as "(meth)acrylate." The same applies to other similar terms. "(Poly)" refers to both cases with and without the prefix "poly." In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In the numerical ranges described in this specification, the upper or lower limit of that numerical range may be replaced with a value shown in the examples. The materials exemplified in this specification may be used alone or in combination of two or more. "Physiological saline" refers to a 0.9% by mass aqueous sodium chloride solution. "Standard sieve" refers to a test sieve (metal mesh sieve) specified in JIS Z 8801-1:2019. Unless otherwise specified, the examples and comparative examples were carried out in an environment of 1 atmosphere, normal temperature and normal humidity, and the various parameters disclosed in this specification were also values ​​measured in the same environment, with the temperature of the various samples being normal temperature. "1 atmosphere" is 101,325 Pa, "normal temperature" is 25°C, and "normal humidity" is 50% RH.

[0011] An example of the method for producing a solid polymer of the present invention includes: a step of preparing a polymer solution containing a polymer in which a crosslinked structure of a water absorbent resin has been cleaved (preparation step); and a step of bringing a poor solvent for the polymer into contact with the polymer solution to precipitate the polymer, thereby obtaining a polymer dispersion in which the solid polymer is dispersed (dispersion step).

[0012] <Water-absorbent resin> The water-absorbent resin may be a recycled water-absorbent resin. The recycled water-absorbent resin may be a used water-absorbent resin (i.e., a water-absorbent resin that has become gelatinous due to absorption of liquid), an unused water-absorbent resin (e.g., waste water-absorbent resin generated during the manufacturing process of a water-absorbent resin), or a mixture of both.

[0013] Examples of absorbent articles containing recycled water-absorbent resins include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, portable toilet components, and animal waste treatment materials. The absorbent articles may be used. The recycled water-absorbent resin recovered from used absorbent articles may form a gel due to absorption of liquid during use.

[0014] The recycled water-absorbing resin contains water and a water-absorbing resin. The recycled water-absorbing resin may contain fibrous materials such as cellulosic fibers and synthetic fibers.

[0015] The water-absorbent resin contains, for example, a polymer (crosslinked polymer) of an ethylenically unsaturated monomer. Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and its salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. The ethylenically unsaturated monomer may contain at least one compound selected from the group consisting of acrylic acid and its salts, methacrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide.

[0016] The crosslinked polymer may contain monomer units other than ethylenically unsaturated monomer units. The proportion of ethylenically unsaturated monomer units (particularly, (meth)acrylic acid and salts thereof) in the crosslinked polymer may be 70 mol % to 100 mol % relative to the total amount of monomer units. The proportion of (meth)acrylic acid and salts thereof in the ethylenically unsaturated monomers may be 70 mol % to 100 mol %.

[0017] The water-absorbing resin contains, for example, a crosslinked polymer having a poly(meth)acrylic acid structure.As such crosslinked polymer, for example, the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid and a crosslinking agent capable of reacting with the carboxyl group of (meth)acrylic acid to form a covalent bond; the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid to obtain a polymer, and then treating the surface of the polymer with a crosslinking agent capable of reacting with the carboxyl group of the polymer to form a covalent bond; the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid and a crosslinking agent capable of reacting with the carboxyl group of (meth)acrylic acid to form a covalent bond, and then treating the surface of the polymer with a crosslinking agent capable of reacting with the carboxyl group of the polymer to form a covalent bond.

[0018] The shape of the recycled water absorbent resin is not particularly limited, and if it is a used gel, it may be in the form of an irregular lump, and if it is an unused one (generally a dry powder), it may be in the form of an irregular crushed shape, flakes, granules, etc.

[0019] <Preparation Step> In the preparation step, a polymer solution containing a polymer in which the crosslinked structure of the water-absorbent resin has been cleaved is prepared. In the preparation step, for example, the water-absorbent resin is brought into contact with a base component to obtain a polymer solution containing a polymer in which the crosslinked structure of the water-absorbent resin has been cleaved. The polymer solution may be a mixture of a polymer (a crosslinked polymer in which the crosslinked structure has been cleaved) and a base component. The polymer solution may or may not contain a water-absorbent resin (a crosslinked polymer in which the crosslinked structure has not been cleaved). When the crosslinked polymer contained in the water-absorbent resin is a crosslinked polymer having a poly(meth)acrylic acid structure, the polymer may be a polymer having a poly(meth)acrylic acid structure, and the polymer solution may be a mixture of a polymer having a poly(meth)acrylic acid structure and a base component.

[0020] The base component may be an inorganic base component, for example, at least one inorganic base component selected from the group consisting of sodium hydroxide, ammonia, potassium hydroxide, and calcium hydroxide, or at least one inorganic base component selected from the group consisting of sodium hydroxide, potassium hydroxide, and calcium hydroxide.

[0021] The degree of neutralization of the polymer solution may be adjusted by contacting the polymer solution with an acid component or the like. The degree of neutralization of the polymer refers to the ratio of neutralized acid groups (acid groups that have formed salts) to the total amount of acid groups in the polymer (including neutralized acid groups). For example, when the polymer is a partially neutralized product of polyacrylic acid, the degree of neutralization of the polymer refers to the ratio of carboxylate groups to the total amount of carboxyl groups and carboxylate groups contained in the partially neutralized product of polyacrylic acid.

[0022] The degree of neutralization of the polymer contained in the polymer solution may be 40 mol% or more, 50 mol% or more, or 75 mol% or more from the viewpoint of easily suppressing the amount of poor solvent used to precipitate a solid polymer, and may be 80 mol% or more or 90 mol% or more from the viewpoint of further improving the polymer yield, and may be 95 mol% or less, 85 mol% or less, 80 mol% or less, or 75 mol% or less from the viewpoint of suppressing the generation of salts by adjusting the degree of neutralization. From these viewpoints, the degree of neutralization of the polymer contained in the polymer solution may be 40 mol% to 100 mol%, 50 mol% to 100 mol%, 75 mol% to 100 mol%, 80 mol% to 100 mol%, or 90 mol% to 100 mol%.

[0023] The viscosity of the polymer solution at 25°C may be 10 mPa·s or more, 50 mPa·s or more, 100 mPa·s or more, 500 mPa·s or more, 1000 mPa·s or more, 1500 mPa·s or more, 3000 mPa·s or more, 5000 mPa·s or more, or 10000 mPa·s or more. The viscosity of the polymer solution at 25°C may be 200000 mPa·s or less, 150000 mPa·s or less, 100000 mPa·s or less, 80000 mPa·s or less, 50000 mPa·s or less, or 30000 mPa·s or less. The viscosity of the polymer solution at 25°C is 10mPa·s to 200,000mPa·s, 10mPa·s to 150,000mPa·s, 10mPa·s to 100,000mPa·s, 10mPa·s to 80,000mPa·s, 10mPa·s to 50,000mPa·s, 10mPa·s to 30,000mPa·s, 50mPa·s to 200,000mPa·s, 50mPa·s to 150,000mPa·s, 5 The viscosity may be 50 mPa·s to 100,000 mPa·s, 50 mPa·s to 80,000 mPa·s, 50 mPa·s to 50,000 mPa·s, or 50 mPa·s to 30,000 mPa·s, and from the viewpoint of being able to produce a solid polymer in a short time even from a highly viscous polymer solution that requires a long time for ultrafiltration, it is preferred to use a viscosity of 500 mPa·s to 200,000 mPa·s, 500 mPa·s to 150,000 mPa·s. s, 500mPa・s~100000mPa・s, 500mPa・s~80000mPa・s, 500mPa・s~50000mPa・s, 500mPa・s~30000mPa・s, 1000m Pa・s~200000mPa・s, 1000mPa・s~150000mPa・s, 1000mPa・s~100000mPa・s, 1000mPa・s~80000mPa・s, 1000mPa The viscosity may be from 1,000 mPa·s to 50,000 mPa·s, from 1,000 mPa·s to 30,000 mPa·s, from 10,000 mPa·s to 200,000 mPa·s, from 10,000 mPa·s to 150,000 mPa·s, from 10,000 mPa·s to 100,000 mPa·s, from 10,000 mPa·s to 80,000 mPa·s, from 10,000 mPa·s to 50,000 mPa·s, or from 10,000 mPa·s to 30,000 mPa·s.

[0024] The concentration of the polymer in the polymer solution may be, for example, 0.1% by mass or more, 0.5% by mass or more, or 1% by mass or more, and may be 25% by mass or less, 15% by mass or less, 10% by mass or less, 7% by mass or less, or 5% by mass or less. The concentration of the polymer in the polymer solution may be 0.1% by mass to 25% by mass or less, 0.1% by mass to 15% by mass or less, 0.1% by mass to 10% by mass, 0.5% by mass to 7% by mass, or 1% by mass to 5% by mass.

[0025] <Dispersion Step> In the dispersion step, a poor solvent for the polymer is brought into contact with the polymer solution to precipitate the polymer dissolved in the polymer solution, thereby obtaining a polymer dispersion in which a solid polymer is dispersed. The poor solvent refers to a solvent in which the solubility of the polymer in 100 g of the solvent at 25°C is less than 0.1 g. The poor solvent may be a water-soluble organic solvent.

[0026] Examples of methods for contacting a polymer solution with a poor solvent for the polymer include adding the poor solvent to the polymer solution and adding the polymer solution to the poor solvent. When adding a poor solvent to a polymer solution, the entire amount of poor solvent to be used may be added to the polymer solution at once, or may be added to the polymer solution over time. Similarly, when adding a poor polymer solution to a poor solvent, the entire polymer solution may be added to the poor solvent at once, or may be added to the poor solvent over time. Note that "addition over time" means adding the poor solvent (or polymer solution) to the polymer solution (or poor solvent) continuously or intermittently over a predetermined time. From the viewpoint of reducing the amount of poor solvent used, a method of adding the poor solvent to the polymer solution over time may be adopted.

[0027] Examples of the poor solvent include alcohols, phenols, ketones, ethers, nitriles, etc., each having 1 to 10 carbon atoms. From the viewpoint of facilitating the production of a solid polymer with higher purity, the poor solvent may be at least one selected from the group consisting of alcohols, phenols, and ketones having 1 to 10 carbon atoms.

[0028] Examples of alcohols having 1 to 10 carbon atoms include methanol, ethanol, propanol, butanol, pentanol, ethylene glycol, and glycerin. Examples of phenols include phenol, cresol, dibutylhydroxytoluene, and eugenol. Examples of ketones include acetone, methyl ethyl ketone, and diethyl ketone. Examples of ethers include dimethyl ether, ethyl methyl ether, diethyl ether, furan, and tetrahydrofuran. Examples of nitriles include acrylonitrile. The poor solvent may be at least one selected from the group consisting of methanol, ethanol, and acetone.

[0029] When the contact between the polymer solution and the poor solvent is performed by adding the poor solvent to the polymer solution over time, the concentration of the poor solvent in the finally obtained polymer dispersion may be 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, or 20% by mass or less, from the viewpoint of easily obtaining a solid polymer in a shorter time. The concentration of the poor solvent in the polymer dispersion may be 10% by mass or more or 20% by mass or more. The content of the poor solvent in the polymer dispersion may be 10% by mass to 60% by mass or 20% by mass to 50% by mass.

[0030] When the contact between the polymer solution and the poor solvent is carried out by adding the poor solvent to the polymer solution over time, the rate of addition of the poor solvent may be 30% by mass / min or less, or 25% by mass / min or less, relative to the polymer solution, from the viewpoint that the solid polymer is more likely to precipitate in the form of small particles, thereby making it less likely that impurities will remain inside the precipitated solid polymer (i.e., the purity of the solid polymer will be higher), and from the viewpoint of reducing the time required for dropwise addition, it may be 0.1% by mass / min or more, 1% by mass / min or more, or 5% by mass / min or more. The dropwise addition rate of the poor solvent may be 0.1% by mass / min to 30% by mass / min, 0.1% by mass / min to 25% by mass / min, 1% by mass / min to 30% by mass / min, 1% by mass / min to 25% by mass / min, 5% by mass / min to 30% by mass / min, or 5% by mass / min to 25% by mass / min.

[0031] The impurity removal rate in the dispersion step may be 75% by mass or more, 80% by mass or more, 85% by mass or more, or 90% by mass or more. The impurity removal rate can be calculated by the method described in the examples below.

[0032] The polymer yield in the dispersion step may be 75% by mass or more, 80% by mass or more, 85% by mass or more, or 90% by mass or more. The polymer yield can be calculated by the method described in the examples below.

[0033] The solid polymer dispersed in the polymer dispersion may be in the form of particles or lumps.

[0034] The weight average molecular weight of the solid polymer in the polymer dispersion may be 1.7 million or more, 1.8 million or more, or 1.9 million or more, and may be 2.5 million or less, or 2.4 million or less. The weight average molecular weight of the polymer in the polymer dispersion may be 1.7 million to 2.5 million, or 1.8 million to 2.4 million.

[0035] <Crosslinking step> After obtaining a polymer dispersion, a solid polymer in the polymer dispersion obtained by removing a supernatant may be crosslinked to obtain a water-absorbent resin. That is, another embodiment of the present invention is a method for producing water-absorbent resin particles, comprising a step (crosslinking step) of crosslinking a solid polymer in the polymer dispersion obtained by the above-mentioned production method to obtain a water-absorbent resin.

[0036] After obtaining the solid polymer, and before carrying out the crosslinking step, the polymer dispersion may be diluted, heated, or a salt (a salt containing a monovalent cation) may be added to the polymer dispersion in order to reduce the viscosity of the polymer dispersion.

[0037] After obtaining the solid polymer, a step of adjusting the degree of neutralization to a predetermined value may be carried out before crosslinking the solid polymer. After obtaining the solid polymer, the solid polymer may be subjected to a sterilization treatment before crosslinking the solid polymer.

[0038] In the crosslinking step, for example, a solution containing a crosslinking agent capable of forming a covalent bond with a functional group of the solid polymer is prepared, and the solid polymer is crosslinked via the covalent bond to obtain a crosslinked polymer. In the crosslinking step, the polymer dispersion may be gelled. In this case, the entire polymer dispersion loses fluidity, and a gel containing the crosslinked polymer and water is formed. When crosslinking progresses to the extent that a gel is formed, water-absorbent resin particles having high water absorption capacity and a small amount of dissolved matter are particularly likely to be obtained.

[0039] Examples of functional groups possessed by the solid polymer include carboxyl groups. When the solid polymer has carboxyl groups, the solid polymer is crosslinked by a reaction between the carboxyl groups and a crosslinking agent and / or a reaction between the carboxyl groups themselves. The covalent bond may be at least one selected from the group consisting of an ester bond, a thioester bond, an amide bond, an ether bond, and a carbon-carbon bond. The solid polymer may be crosslinked, for example, via at least one group selected from the group consisting of a carboxylic acid ester group, a thioester group, an amide group, an acid anhydride group, an oxyalkylene group, and an oxyarylene group.

[0040] Examples of the crosslinking agent include aliphatic polyhydric alcohols such as (poly)ethylene glycol, (poly)propylene glycol, (poly)glycerin, and pentaerythritol; glycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; bisacrylamide compounds such as N,N'-methylenebis(meth)acrylamide; allylated starch; diallyl phthalate; N,N',N"-triallyl isocyanurate; divinylbenzene; ethylenediamine, polyethyleneimine, and glycidyl (meth)acrylate.

[0041] The amount of the crosslinking agent may be 0.0001 parts by weight or more, 0.001 parts by weight or more, or 0.005 parts by weight or more, and may be 10 parts by weight or less, 10 parts by weight or less, or 5 parts by weight or less, per 100 parts by weight of the solid polymer. The amount of the crosslinking agent may be 0.0001 parts by weight to 15 parts by weight, 0.001 parts by weight to 10 parts by weight, or 0.005 parts by weight to 5 parts by weight, per 100 parts by weight of the solid polymer.

[0042] The reaction temperature when crosslinking the solid polymer is appropriately set depending on the type, amount, etc. of the crosslinking agent used, but may be 50° C. or higher, 80° C. or higher, or 100° C. or higher, and may be 220° C. or lower, 200° C. or lower, or 180° C. or lower. The reaction temperature when crosslinking the solid polymer may be 50° C. to 220° C., 80° C. to 200° C., or 100° C. to 180° C.

[0043] The reaction time for crosslinking the solid polymer is appropriately set depending on the type and amount of the crosslinking agent used, the reaction temperature, etc., but may be 1 to 200 minutes, or 5 to 150 minutes.

[0044] After obtaining a crosslinked polymer through the crosslinking step, a surface crosslinking step of surface crosslinking the crosslinked polymer may be carried out. The surface crosslinking can be carried out, for example, by adding a crosslinking agent for surface crosslinking (surface crosslinking agent) to the crosslinked polymer and allowing it to react.

[0045] The surface cross-linking agent may be a compound having two or more reactive functional groups that are reactive with the cross-linked polymer. The surface cross-linking agent may be the same as or different from the cross-linking agent in the cross-linking step.

[0046] The reactive functional groups of the surface cross-linking agent may be carbonate groups, alcoholic hydroxyl groups, epoxy groups, halogeno groups in haloepoxy compounds, isocyanate groups, oxetanyl groups, oxazoline groups, or combinations thereof. Carbonate groups are considered two reactive functional groups because they can react with two other molecules.

[0047] Examples of surface cross-linking agents having a carbonate group include alkylene carbonates (ethylene carbonate, etc.). Examples of surface cross-linking agents having an alcoholic hydroxyl group include polyol compounds such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin, and hydroxyalkylamide compounds (bis[N,N-di(β-hydroxyethyl)]adipamide, etc.). Examples of surface cross-linking agents having two or more epoxy groups include (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether. Examples of haloepoxy compounds having an epoxy group and a halogeno group include epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin. Examples of surface crosslinking agents having an isocyanate group include 2,4-tolylene diisocyanate and hexamethylene diisocyanate. Examples of surface crosslinking agents having an oxetanyl group include 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol.

[0048] The amount of the surface crosslinking agent may be 0.001 parts by mass or more, 0.005 parts by mass or more, or 0.01 parts by mass or more, and may be 5 parts by mass or less, 3 parts by mass or less, or 1 part by mass or less, relative to 100 parts by mass of the crosslinked polymer. The amount of the crosslinking agent may be 0.001 parts by mass to 5 parts by mass, 0.005 parts by mass to 3 parts by mass, or 0.01 parts by mass to 1 part by mass, relative to 100 parts by mass of the crosslinked polymer.

[0049] A regenerated water-absorbent resin containing a crosslinked polymer is formed by a method including removing water from a crosslinked polymer obtained through an optional surface crosslinking step. When the reaction liquid to be subjected to the crosslinking step is an aqueous solution, the water-absorbent resin particles are formed by a method including a drying step of removing water from a block of crosslinked polymer formed by gelling the reaction liquid itself, drying the crosslinked polymer to form a dried product, and a pulverizing step of pulverizing the dried product.

[0050] The method for drying the crosslinked polymer may be a common method such as a squeezing method such as centrifugation, dehydration using an organic solvent, natural drying, heat drying, air drying, freeze drying, or a combination thereof. The heating temperature for drying may be 80°C to 220°C, 90°C to 200°C, or 100°C to 180°C from the viewpoint of efficiently removing water.

[0051] The moisture content of the dried product may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less. The moisture content of the dried product means the proportion of moisture in the dried product based on the total amount of the dried product. When a crosslinked polymer containing water is heated at 200°C for 2 hours, the difference in mass of the crosslinked polymer before and after heating can be considered to be the moisture content of the dried product.

[0052] Before drying the crosslinked polymer, the crosslinked polymer may be crushed to form a crushed product containing structures of a certain size. By forming a crushed product, water can be efficiently removed. The structures constituting the crushed product can be, for example, elongated structures, granular structures (particles), or a combination thereof. The crushed product may include a plurality of structures having a shape that can pass through a circular hole with a diameter of 10 mm or 7 mm. The elongated structures may be curved, and as long as their maximum width is 10 mm or less, they can be said to have a shape that can pass through a circular hole with a diameter of 10 mm. The granular structures (particles) may be irregular in shape and may have a shape that can pass through a circular hole with a diameter of 10 mm while changing direction. Examples of crushing devices for crushing the crosslinked polymer include kneaders (e.g., pressure kneaders, double-arm kneaders), meat choppers, cutter mills, and farm mills.

[0053] The dried product is pulverized to form water-absorbent resin particles. The pulverization method is not particularly limited. For example, the dried product can be pulverized using a pulverizer such as a centrifugal pulverizer, a roller mill, a stamp mill, a jet mill, a high-speed rotary pulverizer, or a container-driven mill.

[0054] The water-absorbent resin particles obtained by pulverization may be classified. Classification refers to an operation of dividing a particle group (powder) into two or more particle groups having different particle size distributions. A part of the water-absorbent resin particles after classification may be pulverized and classified again.

[0055] The classification method is not particularly limited, and may be, for example, screen classification or air classification. Screen classification is a method of classifying particles on a screen into particles that pass through the meshes of the screen and particles that do not pass through by vibrating the screen. Screen classification can be performed using, for example, a vibrating sieve, a rotary sifter, a cylindrical stirring sieve, a blower sifter, or a rotary shaker. Air classification is a method of classifying particles by utilizing an air flow.

[0056] The median particle size of the crosslinked polymer powder obtained through pulverization and, if necessary, classification may be, for example, 200 μm to 500 μm, or 300 μm to 500 μm. The particle size distribution may be adjusted by mixing two or more powders obtained by classification and having different median particle sizes.

[0057] Fig. 1 is a cross-sectional view showing an example of an absorbent article having an absorbent body containing water-absorbent resin particles. The absorbent article 100 shown in Fig. 1 comprises a water-absorbent sheet 50 having a film-like absorbent body 10, a liquid-permeable sheet 30, and a liquid-impermeable sheet 40.

[0058] The water-absorbent sheet 50 includes an absorbent body 10 containing a powder of water-absorbent resin particles 1, and two core wrap sheets 20a and 20b. The absorbent body 10 is disposed inside the core wrap sheets 20a and 20b. The absorbent body 10 maintains its shape by being sandwiched between the two core wrap sheets 20a and 20b. The core wrap sheets 20a and 20b may consist of two sheets, a single folded sheet, or a single bag. A sheet member that does not have any other components on the outside of the core wrap sheets 20a and 20b that wrap the absorbent body 10 is sometimes referred to as a water-absorbent sheet.

[0059] The absorbent body 10 is a component that mainly contains a powder of water-absorbent resin particles 1 and is retained to have a certain shape. The absorbent body 10 may contain fibrous material 3 in addition to the powder of water-absorbent resin particles 1, or may not contain fibrous material 3. The content of the water-absorbent resin particles 1 in the absorbent body 10 may be 50% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, or 90% by mass or more and 100% by mass or less, based on the mass of the absorbent body 10.

[0060] The thickness of the absorbent body 10 may be, for example, 20 mm or less, 15 mm or less, 10 mm or less, 5 mm or less, 4 mm or less, or 3 mm or less, or may be 0.1 mm or more, or 0.3 mm or more. The thickness of the absorbent body 10 may be 0.1 mm or more and 20 mm or less. The mass per unit area of ​​the absorbent body 10 is 1000 g / m ２ Below, 800g / m ２ or less than 600 g / m ２ or less, and ２ It may be more than that.

[0061] The fibrous material 3 can be, for example, a cellulosic fiber, a synthetic fiber, or a combination thereof. Examples of cellulosic fibers include comminuted wood pulp, cotton, cotton linters, rayon, and cellulose acetate. Examples of synthetic fibers include polyamide fibers, polyester fibers, and polyolefin fibers. The fibrous material may also be a hydrophilic fiber (e.g., pulp).

[0062] The absorbent 10 may further contain inorganic powder (e.g., amorphous silica), a deodorant, an antibacterial agent, a fragrance, etc. When the water-absorbent resin particles 1 contain inorganic particles, the absorbent 10 may contain inorganic powder in addition to the inorganic particles in the water-absorbent resin particles 1.

[0063] The water-absorbent sheet 50 may further have an adhesive 21 interposed between the core wrap sheet 20a and the absorbent body 10. An adhesive layer may be interposed between the core wrap sheets 20a, 20b on both sides and the absorbent body 10. The adhesive 21 is not particularly limited, and may be, for example, a hot-melt adhesive.

[0064] The core wrap sheets 20a, 20b may be, for example, nonwoven fabrics. The two core wrap sheets 20a, 20b may be the same or different nonwoven fabrics. The nonwoven fabric may be a nonwoven fabric made of short fibers (i.e., staple) (short fiber nonwoven fabric) or a nonwoven fabric made of long fibers (i.e., filaments) (long fiber nonwoven fabric). The staple may have a fiber length of, but is not limited to, typically several hundred millimeters or less.

[0065] The core wrap sheets 20a, 20b may be a thermal bonded nonwoven fabric, an air-through nonwoven fabric, a resin bonded nonwoven fabric, a spunbonded nonwoven fabric, a meltblown nonwoven fabric, an airlaid nonwoven fabric, a spunlace nonwoven fabric, a point bonded nonwoven fabric, or a laminate containing two or more types of nonwoven fabric selected from these.

[0066] The nonwoven fabric used as the core wrap sheets 20a, 20b can be made of synthetic fibers, natural fibers, or a combination thereof. Examples of synthetic fibers include fibers containing synthetic resins selected from polyolefins such as polyethylene (PE) and polypropylene (PP), polyesters such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT) and polyethylene naphthalate (PEN), polyamides such as nylon, and rayon. Examples of natural fibers include fibers containing cotton, silk, hemp, or pulp (cellulose). The fibers forming the nonwoven fabric may be polyolefin fibers, polyester fibers, or a combination thereof. The core wrap sheets 20a, 20b may also be tissue paper.

[0067] The water-absorbent sheet 50 may be used to manufacture various other absorbent articles. Examples of absorbent articles include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, portable toilet components, and animal waste disposal materials. The absorbent bodies that make up these absorbent articles often move or deform due to the movements of the user of the absorbent article, etc.

[0068] The liquid-permeable sheet 30 is positioned as the outermost layer on the side into which the liquid to be absorbed penetrates. The liquid-permeable sheet 30 is positioned outside the core wrap sheet 20b while in contact with the core wrap sheet 20b. The liquid-impermeable sheet 40 is positioned as the outermost layer on the opposite side of the absorbent article 100 from the liquid-permeable sheet 30. The liquid-impermeable sheet 40 is positioned outside the core wrap sheet 20a while in contact with the core wrap sheet 20a. The liquid-permeable sheet 30 and the liquid-impermeable sheet 40 have main surfaces that are wider than the main surface of the water-absorbent sheet 50, and the outer edges of the liquid-permeable sheet 30 and the liquid-impermeable sheet 40 extend around the absorbent body 10 and the core wrap sheets 20a, 20b. However, the size relationships among the absorbent body 10, the core wrap sheets 20a, 20b, the liquid-permeable sheet 30, and the liquid-impermeable sheet 40 are not particularly limited and may be appropriately adjusted depending on the intended use of the absorbent article, etc.

[0069] The liquid-permeable sheet 30 may be a nonwoven fabric. The nonwoven fabric used as the liquid-permeable sheet 30 may have appropriate hydrophilicity from the viewpoint of the liquid absorption performance of the absorbent article. From that viewpoint, the liquid-permeable sheet 30 may be a nonwoven fabric having a hydrophilicity of 5 to 200 as measured in accordance with the Pulp and Paper Testing Method No. 68 (2000) of the Paper and Pulp Technology Association. The hydrophilicity of the nonwoven fabric may also be 10 to 150. For details of Pulp and Paper Testing Method No. 68, see, for example, WO2011 / 086843.

[0070] Hydrophilic nonwoven fabrics may be made from fibers with moderate hydrophilicity, such as rayon, or from fibers obtained by hydrophilizing hydrophobic chemical fibers, such as polyolefin and polyester fibers. Methods for obtaining nonwoven fabrics containing hydrophilized hydrophobic chemical fibers include spunbonding a mixture of hydrophobic chemical fibers and a hydrophilizing agent, adding a hydrophilizing agent to the hydrophobic chemical fibers, and impregnating a spunbond nonwoven fabric obtained from hydrophobic chemical fibers with a hydrophilizing agent. Examples of hydrophilizing agents include anionic surfactants such as aliphatic sulfonates and higher alcohol sulfates, cationic surfactants such as quaternary ammonium salts, nonionic surfactants such as polyethylene glycol fatty acid esters, polyglycerin fatty acid esters, and sorbitan fatty acid esters, silicone surfactants such as polyoxyalkylene-modified silicones, and stain release agents made from polyester, polyamide, acrylic, or urethane resins.

[0071] The basis weight (mass per unit area) of the nonwoven fabric used as the liquid-permeable sheet 30 is set to 5 g / m from the viewpoint of imparting good liquid permeability, flexibility, strength, and cushioning properties to the absorbent article, and from the viewpoint of increasing the liquid permeation rate of the absorbent article. ２ ~200g / m ２ , 8 g / m ２ ~150g / m ２ , or 10 g / m ２~100g / m ２ The thickness of the liquid-permeable sheet 30 may be 20 μm to 1400 μm, 50 μm to 1200 μm, or 80 μm to 1000 μm.

[0072] The liquid-impermeable sheet 40 prevents liquid absorbed by the absorbent body 10 from leaking out through the liquid-impermeable sheet 40. The liquid-impermeable sheet 40 may be a resin sheet or a nonwoven fabric. The resin sheet may be a sheet made of a synthetic resin such as polyethylene, polypropylene, or polyvinyl chloride. The nonwoven fabric may be a spunbond / meltblown / spunbond (SMS) nonwoven fabric in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics. The liquid-impermeable sheet 40 may be a composite sheet of a resin sheet and a nonwoven fabric (e.g., a spunbond nonwoven fabric or a spunlace nonwoven fabric). The liquid-impermeable sheet 40 may be breathable to reduce stuffiness during wear and to alleviate discomfort to the wearer. For example, a low-density polyethylene (LDPE) resin sheet can be used as the breathable liquid-impermeable sheet 40.

[0073] In order to ensure flexibility and not impair the wearing comfort of the absorbent article, the basis weight (mass per unit area) of the liquid impermeable sheet 40 is 10 g / m ２ ~50g / m ２ may be.

[0074] The present invention will be described in more detail below with reference to examples.

[0075] <Evaluation Methods> Viscosity, weight average molecular weight, solid content, impurity removal rate, polymer yield, and required time were measured according to the following evaluation methods. The measurement results are shown in Table 1.

[0076] [Viscosity] The viscosity of the polymer solution was measured using a rheometer (TA Instruments, model number: AR 2000ex) under the following conditions: shear rate: 4.6 s －１ The steady flow viscosity is: Jig: 2°40 mm cone plate Measurement temperature: 25°C

[0077] [Weight-Average Molecular Weight Mw] The weight-average molecular weight of a solid polymer was measured by light-scattering GPC under the following conditions. Autosampler: Autosampler AS-11 (manufactured by FLOM Corporation) Degasser: Gastr AG-16 (manufactured by FLOM Corporation) Liquid delivery unit: LC-10AD (manufactured by Shimadzu Corporation) Column: OHpak SB-807HQ, SB-806HQ, SB-804HQ (Shodex series, manufactured by Resonaq Corporation) Detector: Triple Detector TDA 302 (manufactured by Viscotec) Carrier solvent: NaNO3 (0.2 mol / L) / methylparaben (2 mmol / L) / distilled water Measurement conditions: injection volume 500 μL, flow rate 0.5 mL / min, column / detector temperature 40°C, dn / dC 0.2270 0.8 μm membrane filter (manufactured by Advantec) <Sample preparation procedure and measurement procedure> A solid polymer containing 0.01 g of solid content was placed in a 300 mL beaker, and 0.2 M NaNO as an eluent was added to the beaker so that the total amount became 100 g. ３ An aqueous solution was added and the mixture was stirred at 250 rpm for 1 hour. If the pH of the sample solution was not neutral, it was adjusted to pH 7 using hydrochloric acid (1 mol / L hydrochloric acid, manufactured by Nacalai Tesque, Inc.). After stirring, the sample solution was passed through a 0.8 μm filter syringe and the filtrate was measured by GPC.

[0078] [Solid content] An aluminum pan was placed in a thermogravimetric differential thermal analyzer (Hitachi High-Tech Corporation, TG / DTA7220), and the weight of the pan was measured at room temperature to perform zero point correction. An arbitrary amount of sample (approximately 5 mg to 40 mg) was then placed on the pan, and the weight (C [g]) was measured. The pan was heated at 5°C / min in a room temperature measurement atmosphere purged with nitrogen at 200 mL / min. The weight (D [g]) of the sample was then measured when the pan reached 200°C, and the solid content was calculated using the following formula: Solid content (mass%) = (D / C) x 100

[0079] [Impurity Removal Rate] The polymer solution before purification (before polymer precipitation) was diluted with ion-exchanged water until it had a viscosity that allowed ultrafiltration. The polymer concentration in the diluted polymer solution was a １(mass%), and the concentration of substances other than polymers (impurity concentration) is b １ (mass%), and the solid content (a １ +b １ The diluted polymer solution was then placed in a centrifugal concentrator with a molecular weight cutoff of 3,000, and centrifuged at 2,000 rpm for 30 minutes. The resulting filtrate was used as a sample to measure its solid content ({b １ / (100-a １ )}×100 (mass%)) was measured using TG-DTA. １ , b １ Since the simultaneous equations are obtained, a １ and b １ Calculate the impurity ratio b of the polymer before purification １ / a １ The same procedure was carried out for the polymer dispersion after purification (after polymer precipitation) (polymer solution after ultrafiltration in the comparative example), and the polymer concentration a ２ (mass%) and the concentration of substances other than polymer b ２ (mass%), and the impurity ratio b of the purified polymer ２ / a ２ Then, the impurity removal rate was calculated using the following formula: Impurity removal rate (mass%) = 100 - {(b ２ / a ２ ) / (b １ / a １ ) x 100

[0080] [Polymer Yield] The polymer dispersion (polymer solution after ultrafiltration in the comparative example) was left standing on a standard sieve with a mesh size of 75 μm for 30 minutes, and the weight after draining was taken as the mass c (g) of the precipitate (polymer in the comparative example). Subsequently, a portion of the precipitate after draining was sampled, and a 30-fold dilution was made using ion-exchanged water in an amount 29 times the weight of the obtained sample, and the solid content thereof was measured using a TG / DTA7220. The solid content d (mass%) of the precipitate was calculated by multiplying the obtained solid content by 30, and the polymer yield was calculated using the following formula: Polymer Yield (mol%) = 100 × (B / A) In the above formula, A is the total amount (mol) of monomer units constituting the water-absorbent resin particles whose crosslinked structures were to be cleaved. In the examples and comparative examples of the present application, water-absorbent resin particles (AQUAKEEP, sodium polyacrylate with a neutralization degree of 75 mol%) manufactured by Sumitomo Seika Chemicals Co., Ltd. were used, as described below. Therefore, A (mol) corresponds to the total amount (mol) of monomer units constituting sodium polyacrylate with a neutralization degree of 75 mol%. Note that, since the monomer units constituting sodium polyacrylate with a neutralization degree of X mol% are composed of two types of monomers, sodium acrylate (molecular weight: 94.04) and acrylic acid (molecular weight: 72.06), the molecular weight of the monomer units constituting sodium polyacrylate with a neutralization degree of X mol% is considered to be the weighted average of the molecular weights of these two types of constituent units. Specifically, the molecular weight of the monomer units constituting sodium polyacrylate with a neutralization degree of X mol% is calculated as {94.04 × X / 100} + {72.06 × (100 - X) / 100}. Therefore, the molecular weight of the monomer unit constituting sodium polyacrylate having a degree of neutralization of 75 mol% is 88.5. Then, A (mol) is calculated by dividing the amount (g) of the water absorbent resin particles to be cleaved by 88.5.In the above formula, B is the total amount (mol) of monomer units constituting the precipitated solid polymer. In the Examples and Comparative Examples of the present application, as described below, the crosslinked structure of water-absorbent resin particles (AQUAKEEP, sodium polyacrylate with a neutralization degree of 75 mol%) manufactured by Sumitomo Seika Chemicals Co., Ltd. is cleaved using an aqueous NaOH solution, and the neutralization degree is adjusted to 100 mol%, 75 mol%, or 50 mol%, so B (mol) corresponds to the total amount (mol) of monomer units constituting the precipitated solid polymer, sodium polyacrylate. Therefore, B (mol) is calculated by dividing the total weight (g) of the precipitated solid polymer (sodium polyacrylate) by the molecular weight of the monomer units constituting sodium polyacrylate (94.04 for a neutralization degree of 100 mol%, 88.55 for a neutralization degree of 75 mol%, and 83.05 for a neutralization degree of 50 mol%). The total weight (g) of the precipitated solid polymer (sodium polyacrylate) was calculated based on the total solid content (c×d / 100) (g) of the precipitated solid polymer at a mass ratio of 1:(b. ２ / a ２ ) because polymer (sodium polyacrylate) and impurities exist, (c × d / 100) × ({1 / {1 + (b ２ / a ２ ))))

[0081] [Time Required] When a poor solvent was added dropwise to a polymer solution, the time required was the time it took to drop the poor solvent, and when ultrafiltration was performed, the time required was the time it took to perform the ultrafiltration.

[0082] Example 1 A 4 L round-bottom cylindrical separable flask (manufactured by DURAN Co., Ltd.) equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer having a stirring blade with two stages of four inclined paddle blades with a blade diameter of 100 mm was prepared, and 90 g of water-absorbent resin particles (manufactured by Sumitomo Seika Chemicals Co., Ltd., AQUAKEEP HP600, sodium polyacrylate with a neutralization degree of 75 mol%, unused product) was weighed out therein, and 900 g of artificial urine (a mixture of 100 parts by mass of sodium chloride, 3 parts by mass of calcium chloride dihydrate, 6 parts by mass of magnesium chloride hexahydrate, 25 parts by mass of 1% aqueous Triton solution, and 9866 parts by mass of ion-exchanged water), 810 g of ion-exchanged water, and 1200 g of a 1.5 mol / kg aqueous NaOH solution were added thereto, and the internal temperature was maintained at 80° C. while stirring at 160 rpm, and the mixture was reacted for 24 hours to cleave the crosslinked structure of the water-absorbent resin, thereby obtaining a polymer solution. The amount of the aqueous NaOH solution used was determined so that it was greater than the amount of sodium polyacrylate with a neutralization degree of 75 mol % contained in the polymer solution, which would cause the neutralization degree to exceed 100 mol %.

[0083] Next, 40 g of the polymer solution was placed in a 300 mL polybeaker and stirred at 1000 rpm using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel 4-blade inclined paddle blade with a blade diameter of 50 mm). While stirring, 50.7 mL (40 g) of ethanol (manufactured by Hayashi Pure Chemical Industries Co., Ltd., special grade ethanol (99.5)) as a poor solvent was added dropwise at 10 mL / min (19.8 mass% / min with respect to the polymer solution) using a pump (manufactured by Yamato Scientific Co., Ltd., Masterflex, easy load 7518-16), to precipitate a small particle-like solid polymer to obtain a polymer dispersion. After the ethanol was added dropwise, stirring was stopped and the shape of the precipitate was confirmed visually, and it was small particle-like. The polymer dispersion was filtered through a standard sieve with 75 μm openings, the supernatant was discarded, and the precipitate remaining on the sieve was left to stand on a standard sieve with 75 μm openings for 30 minutes to drain off the water, yielding a solid polymer.

[0084] Example 2 A 4L round-bottomed cylindrical separable flask (manufactured by DURAN Co., Ltd.) equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a two-stage stirring blade having four inclined paddle blades with a blade diameter of 100 mm was prepared. 30 g of water-absorbent resin particles (manufactured by Sumitomo Seika Chemicals Co., Ltd., AQUAKEEP HP600, neutralization degree 75%, unused product) was weighed out, 1200 g of artificial urine, 1370 g of ion-exchanged water, and 400 g of 1.5 mol / kg NaOH aqueous solution were added, and the internal temperature was maintained at 80°C while stirring at 160 rpm. The reaction was allowed to proceed for 24 hours to cleave the crosslinked structure of the water-absorbent resin, thereby obtaining a polymer solution. The amount of NaOH aqueous solution used was determined so that the amount of sodium polyacrylate with a neutralization degree of 75 mol% contained in the polymer solution exceeded the amount in which the neutralization degree exceeded 100 mol%.

[0085] Next, 40 g of the polymer solution was placed in a 300 mL polybeaker and stirred at 1000 rpm using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel 4-blade inclined paddle blade with a blade diameter of 50 mm). While stirring, 50.7 mL (40 g) of ethanol (manufactured by Hayashi Pure Chemical Industries Co., Ltd., special grade ethanol (99.5)) as a poor solvent was added dropwise at 10 mL / min (19.8 mass% / min with respect to the polymer solution) using a pump (manufactured by Yamato Scientific Co., Ltd., Masterflex, easy load 7518-16), to precipitate a small particle-like solid polymer to obtain a polymer dispersion. After the ethanol was added dropwise, stirring was stopped and the shape of the precipitate was confirmed visually, and it was small particle-like. The supernatant was discarded, and the precipitate was left to stand on a standard sieve with a mesh size of 75 μm for 30 minutes to drain off the water, thereby obtaining a solid polymer.

[0086] Example 3 A 4L round-bottomed cylindrical separable flask (manufactured by DURAN Co., Ltd.) equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a two-stage stirring blade having four inclined paddle blades with a blade diameter of 100 mm was prepared. 150 g of water-absorbent resin particles (Sumitomo Seika Chemicals Co., Ltd., AQUAKEEP HP600, sodium polyacrylate with a neutralization degree of 75 mol%, unused product) was weighed out, 750 g of artificial urine, 100 g of ion-exchanged water, and 2000 g of a 1.5 mol / kg NaOH aqueous solution were added, and the internal temperature was maintained at 80°C while stirring at 160 rpm. The reaction was allowed to proceed for 24 hours to cleave the crosslinked structure of the water-absorbent resin, thereby obtaining a polymer solution. The amount of NaOH aqueous solution used was determined so that the amount of sodium polyacrylate with a neutralization degree of 75 mol% contained in the polymer solution exceeded 100 mol%.

[0087] Next, 40 g of the polymer solution was placed in a 300 mL polybeaker and stirred at 1000 rpm using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel 4-blade inclined paddle blade with a blade diameter of 50 mm). While stirring, 50.7 mL (40 g) of ethanol (manufactured by Hayashi Pure Chemical Industries Co., Ltd., special grade ethanol (99.5)) as a poor solvent was added dropwise at 10 mL / min (19.8 mass% / min with respect to the polymer solution) using a pump (manufactured by Yamato Scientific Co., Ltd., Masterflex, easy load 7518-16), to precipitate a small particle-like solid polymer to obtain a polymer dispersion. After the ethanol was added dropwise, stirring was stopped and the shape of the precipitate was confirmed visually, and it was small particle-like. The supernatant was discarded, and the precipitate was left to stand on a standard sieve with a mesh size of 75 μm for 30 minutes to drain off the water, thereby obtaining a solid polymer.

[0088] Example 4 A polymer dispersion was obtained by precipitating a solid polymer in the same manner as in Example 1, except that the amount of ethanol added was changed to 12.7 ml (10 g). The shape of the precipitate was confirmed to be small particles. The supernatant was discarded, and the precipitate was placed on a standard sieve with 75 μm openings for 30 minutes to drain, yielding a solid polymer.

[0089] Example 5 A polymer dispersion was obtained by precipitating a solid polymer in the same manner as in Example 1, except that methanol (special grade reagent, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the poor solvent instead of ethanol and the amount of methanol added was changed to 50.6 ml (40 g). The shape of the precipitate was confirmed to be small particles. The supernatant was discarded, and the precipitate was left to stand on a standard sieve with 75 μm openings for 30 minutes to drain, yielding a solid polymer.

[0090] Example 6 A polymer dispersion was obtained by precipitating a solid polymer in the same manner as in Example 1, except that acetone (special grade reagent, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the poor solvent instead of ethanol and the amount of acetone dropped was changed to 21.6 ml (17.1 g). The shape of the precipitate was confirmed to be small particles. The supernatant was discarded, and the precipitate was left to stand on a standard sieve with 75 μm openings for 30 minutes to drain, thereby obtaining a solid polymer.

[0091] Example 7 A polymer solution was obtained in the same manner as in Example 1. 40 g of the polymer solution was placed in a 300 mL plastic beaker, and 1.24 g of 95% sulfuric acid was added while stirring at 100 rpm using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel four-blade inclined paddle blade with a blade diameter of 50 mm). Next, the rotation speed of the stirrer was changed to 100 rpm, and while stirring, 53.2 ml (42 g) of ethanol (manufactured by Hayashi Pure Chemical Industries Co., Ltd., special grade ethanol (99.5)) as a poor solvent was added dropwise at 10 mL / min (19.8 mass% / min with respect to the polymer solution) using a pump (manufactured by Yamato Scientific Co., Ltd., Masterflex, easy load 7518-16), to precipitate a small particle-like solid polymer, thereby obtaining a polymer dispersion. After the ethanol was added dropwise, stirring was stopped and the shape of the precipitate was confirmed visually, and it was found to be lumpy. The supernatant was discarded, and the precipitate was left to stand on a standard sieve with a mesh size of 75 μm for 30 minutes to remove water, thereby obtaining a solid polymer. The amount of sulfuric acid used was determined so that the degree of neutralization of sodium polyacrylate with a neutralization degree of 100 mol% contained in the polymer solution would be 75 mol%.

[0092] Example 8 A polymer solution was obtained in the same manner as in Example 1. 40 g of the polymer solution was placed in a 300 mL polybeaker, and 1.41 g of 95% sulfuric acid was added while stirring at 100 rpm using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel four-blade inclined paddle blade with a blade diameter of 50 mm). Next, the rotation speed of the stirrer was changed to 100 rpm, and while stirring, 54.0 ml (43 g) of ethanol (manufactured by Hayashi Pure Chemical Industries Co., Ltd., special grade ethanol (99.5)) as a poor solvent was added dropwise at 10 mL / min (19.8 mass% / min with respect to the polymer solution) using a pump (manufactured by Yamato Scientific Co., Ltd., Masterflex, easy load 7518-16), to precipitate a small particle-like solid polymer, thereby obtaining a polymer dispersion. After the ethanol was added dropwise, stirring was stopped and the shape of the precipitate was confirmed visually, and it was found to be lumpy. The supernatant was discarded, and the precipitate was left to stand on a standard sieve with a mesh size of 75 μm for 30 minutes to remove water, thereby obtaining a solid polymer. The amount of sulfuric acid used was determined so that the degree of neutralization of sodium polyacrylate with a neutralization degree of 100 mol% contained in the polymer solution would be 50 mol%.

[0093] (Example 9) A 1 L round-bottomed cylindrical separable flask (manufactured by Shibata Scientific Co., Ltd.) equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirring blade having two stages of four inclined paddle blades with a blade diameter of 100 mm was prepared, and 40 g of water-absorbent resin particles (manufactured by Sumitomo Seika Chemicals Co., Ltd., AQUAKEEP HP600, sodium polyacrylate with a neutralization degree of 75 mol%, unused product) was weighed out there, and 125 g of artificial urine, 68.25 g of ion-exchanged water, and 266.75 g of a 3 mol / kg NaOH aqueous solution were added, and the internal temperature was maintained at 80 ° C. while stirring at 50 rpm. The reaction was allowed to proceed for 24 hours to cleave the crosslinked structure of the water-absorbent resin, and a polymer solution was obtained. The amount of NaOH aqueous solution used was determined so that the neutralization degree of the sodium polyacrylate with a neutralization degree of 75 mol% contained in the polymer solution was greater than the amount exceeding 100 mol%.

[0094] Next, 40 g of the polymer solution was placed in a 300 mL polybeaker and stirred at 1000 rpm using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel 4-blade inclined paddle blade with a blade diameter of 50 mm). While stirring, 21.7 ml (17.1 g) of ethanol (manufactured by Hayashi Pure Chemical Industries Co., Ltd., special grade ethanol (99.5)) as a poor solvent was added dropwise at 3.3 mL / min (6.6 mass% / min with respect to the polymer solution) using a pump (manufactured by Yamato Scientific Co., Ltd., Masterflex, easy load 7518-16), to precipitate a small particle-like solid polymer to obtain a polymer dispersion. After the ethanol was added dropwise, stirring was stopped and the shape of the precipitate was confirmed visually, and it was small particle-like. The supernatant was discarded, and the precipitate was left to stand on a standard sieve with a mesh size of 75 μm for 30 minutes to drain off the water, thereby obtaining a solid polymer.

[0095] (Example 10) A 1L round-bottomed cylindrical separable flask (manufactured by Shibata Scientific Co., Ltd.) equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirring blade having two stages of four inclined paddle blades with a blade diameter of 100 mm was prepared, and 50 g of water-absorbent resin particles (manufactured by Sumitomo Seika Chemicals Co., Ltd., AQUAKEEP HP600, sodium polyacrylate with a neutralization degree of 75 mol%, unused product) was weighed out there, and 100 g of artificial urine, 16.75 g of ion-exchanged water, and 333.25 g of a 3 mol / kg NaOH aqueous solution were added, and the internal temperature was maintained at 80 ° C. while stirring at 50 rpm, and the reaction was allowed to proceed for 24 hours to cleave the crosslinked structure of the water-absorbent resin, thereby obtaining a polymer solution. The amount of NaOH aqueous solution used was determined so that the neutralization degree of the sodium polyacrylate with a neutralization degree of 75 mol% contained in the polymer solution was greater than the amount exceeding 100 mol%.

[0096] Next, 40 g of the polymer solution was placed in a 300 mL polybeaker and stirred at 1000 rpm using a stirrer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", stainless steel 4-blade inclined paddle blade with a blade diameter of 50 mm). While stirring, 21.7 ml (17.1 g) of ethanol (manufactured by Hayashi Pure Chemical Industries Co., Ltd., special grade ethanol (99.5)) as a poor solvent was added dropwise at 3.3 mL / min (6.6 mass% / min with respect to the polymer solution) using a pump (manufactured by Yamato Scientific Co., Ltd., Masterflex, easy load 7518-16), to precipitate a small particle-like solid polymer, thereby obtaining a polymer dispersion. After the ethanol was added dropwise, stirring was stopped and the shape of the precipitate was confirmed visually, which was found to be lumpy. The supernatant was discarded, and the precipitate was left to stand on a standard sieve with a mesh size of 75 μm for 30 minutes to drain, thereby obtaining a solid polymer.

[0097] Comparative Example 1 A polymer solution was obtained in the same manner as in Example 1. The polymer solution was diluted 30 times with pure water. The diluted polymer solution was placed in an ultrafiltration device, and the reservoir was filled with pure water. Filtration was initiated under the following conditions: 0.15 MPa, stirring at 100 rpm. Sampling was performed periodically, and filtration was continued until the impurity removal rate reached 75% or more. After filtration for 2,880 minutes, a polymer solution was obtained in which the impurity removal rate was 80.8% by mass, the polymer yield was 89% by mass, and the weight-average molecular weight was 1,949,000. Ultrafiltration device: UHP-150K (Advantec) Reservoir: RP-1, RP-2 (Advantec) Ultrafiltration membrane (Syndre, PX, 4000 MWCO, 0.0154) Filtration area: 0.0154 m ２ ・Ultrafiltration membrane cutoff molecular weight: 400,000

[0098]

[0099] Examples 1 to 10 and Comparative Example 1 are experimental systems that were constructed on the assumption that a solid polymer was obtained from water-absorbent resin particles used in diapers. For this purpose, in a state in which artificial urine was absorbed into the water-absorbent resin particles, a crosslinked structure of the water-absorbent resin particles was cleaved using an NaOH aqueous solution to prepare a polymer solution.

[0100] In Examples 1 to 10, a high-purity solid polymer was obtained in a shorter time than in Comparative Example 1. In addition, in Examples 7 and 8, after the solid polymer precipitated as small particles, the small particles of the solid polymer aggregated to form clumps. On the other hand, in Example 10, phase separation occurred between the polymer solution and the poor solvent, resulting in the precipitation of clumps of the solid polymer.

[0101] 1...water-absorbent resin particles, 10...absorbent body, 20a, 20b...core wrap sheets, 30...liquid-permeable sheet, 40...liquid-impermeable sheet, 50...water-absorbent sheet, 100...absorbent article.

Claims

1. A method for producing a solid polymer, comprising: a step of preparing a polymer solution containing a polymer obtained by cleaving a crosslinked structure of a water-absorbent resin; and a step of bringing the polymer solution into contact with a poor solvent for the polymer to precipitate the polymer, thereby obtaining a polymer dispersion in which the solid polymer is dispersed.

2. The method according to claim 1, wherein the degree of neutralization of the polymer is 80 mol % or more.

3. The manufacturing method according to claim 1, wherein the viscosity of the polymer solution at 25°C is 10 mPa·s to 200,000 mPa·s.

4. The method according to claim 1, wherein the content of the poor solvent in the polymer dispersion is 60% by mass or less.

5. The production method according to claim 1, wherein the poor solvent comprises at least one selected from the group consisting of alcohols, phenols, and ketones having 1 to 10 carbon atoms.

6. A method for producing water-absorbent resin particles, comprising a step of crosslinking the solid polymer obtained by the method according to any one of claims 1 to 5 to obtain a water-absorbent resin.

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