Production method for water-absorbing resin particles, absorber, and absorbent article

Abstract

Provided is a production method for water-absorbing resin particles containing a crosslinked polymer, the production method comprising: a preparation step for preparing a polymer solution containing a solvent and a polymer having a weight-average molecular weight of at least 300,000; and a crosslinking step for crosslinking the polymer. This absorber contains water-absorbing resin particles obtained by said production method. This absorbent article includes said absorber.

Description

Method for producing water-absorbent resin particles, absorbent body, and absorbent article

[0001] The present invention relates to a method for producing water-absorbent resin particles, an absorbent body, and an absorbent article.

[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] In many of the techniques disclosed as methods for regenerating water-absorbent resins recovered from used sanitary products, the water-absorbent capacity of the water-absorbent resin is compared before and after a predetermined treatment, and if the water-absorbent capacity is at least the same as before the treatment, it is considered to have been regenerated. However, since the crosslinked structure of the water-absorbent resin is destroyed by the predetermined treatment, making the polymer components more likely to leach out, it is necessary to suppress not only the water-absorbent capacity but also the increase in the soluble polymer. However, it is difficult to say that Patent Document 1 achieves both maintaining the water-absorbent capacity and suppressing the dissolution of the polymer.

[0005] One aspect of the present invention relates to a method for producing water-absorbent resin particles having high water absorption capacity and low soluble content, which can be applied to regenerate water-absorbent resin particles from a polymer formed by chemically decomposing a water-absorbent resin.

[0006] One aspect of the present disclosure includes, for example, the following means. [1] A method for producing water-absorbent resin particles containing a crosslinked polymer, comprising: a preparation step of preparing a polymer solution containing a polymer having a weight-average molecular weight of 300,000 or more and a solvent; and a crosslinking step of crosslinking the polymer. [2] The production method according to [1], wherein the preparation step includes a cleavage step of obtaining the polymer by cleaving a crosslinked structure of the crosslinked polymer, the cleavage step being carried out in a treatment tank containing a reaction solution containing the water-absorbent resin, and including an operation of adjusting the oxygen concentration in the gas phase of the treatment tank to 18% by volume or less. [3] The production method according to [1] or [2], wherein the cleavage step includes an operation of heating the reaction solution to 50°C or higher. [4] The production method according to any one of [1] to [3], wherein the reaction solution includes at least one of an acid component and an alkaline component. [5] The production method according to any one of [1] to [4], wherein the crosslinking step includes crosslinking the polymer via a covalent bond. [6] An absorbent body comprising the water-absorbent resin particles obtained by the manufacturing method according to any one of [1] to [5]. [7] An absorbent article comprising the absorbent body according to [6].

[0007] According to one aspect of the present invention, it is possible to provide a method for producing water-absorbent resin particles having high water absorption performance and a small amount of dissolved content, which can be applied to regenerate water-absorbent resin particles from a polymer formed by chemically decomposing a water-absorbent resin.

[0008] Fig. 2 is a cross-sectional view showing an example of an absorbent article having an absorbent body. Fig. 3 is a schematic view showing a treatment tank for cleaving the cross-linked structure of a water-absorbent resin.

[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.

[0011] One example of a method for producing water-absorbent resin particles includes a preparation step of preparing a polymer solution containing a polymer having a weight-average molecular weight of 300,000 or more and a solvent, and a crosslinking step of crosslinking the polymer. In this method, the water-absorbent resin particles contain a crosslinked polymer. According to this method, for example, water-absorbent resin particles can be regenerated from a polymer formed by chemically decomposing a water-absorbent resin (specifically, by cleaving the crosslinked structure of the water-absorbent resin), and water-absorbent resin particles having high water absorption performance and a low dissolved content can be produced. The water-absorbent resin subjected to chemical decomposition (hereinafter, sometimes referred to as "recycled water-absorbent resin") is preferably a used water-absorbent resin (i.e., a water-absorbent resin that has become gelatinous due to absorption), but may also be an unused water-absorbent resin (e.g., waste water-absorbent resin generated during the production process of the water-absorbent resin), or a mixture of both.

[0012] 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.

[0013] The recycled water absorbent resin contains, for example, a polymer (crosslinked polymer) containing structural units derived from 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.

[0014] The crosslinked polymer may contain structural units derived from monomers other than ethylenically unsaturated monomers. The proportion of structural units derived from ethylenically unsaturated monomers (particularly, (meth)acrylic acid and salts thereof) in the crosslinked polymer may be 70 to 100 mol % relative to the total amount of monomer units. The proportion of structural units derived from (meth)acrylic acid and salts thereof in the ethylenically unsaturated monomers may be 70 to 100 mol %.

[0015] The recycled water absorbent resin contains, for example, a crosslinked polymer having a poly(meth)acrylic acid structure. Examples of such crosslinked polymers include: a 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; a 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; a 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.

[0016] 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.

[0017] The preparation step can include a cleavage step of cleaving the crosslinked structure of a crosslinked polymer contained in the recycled water absorbent resin. The cleavage step can be carried out, for example, in a treatment tank containing a reaction solution containing the recycled water absorbent resin, and can include an operation of adjusting the oxygen concentration in the gas phase of the treatment tank to 18% by volume or less. In other words, the polymer in the preparation step can be obtained by cleaving the crosslinked structure of the crosslinked polymer in a reaction solution containing the recycled water absorbent resin (including the crosslinked polymer) contained in the treatment tank while bringing the reaction solution into contact with the gas phase in the treatment tank, in which the oxygen concentration is 18% by volume or less.

[0018] According to this method, it becomes easy to make the weight average molecular weight of a polymer obtained by cleaving the crosslinked structure of the crosslinked polymer of the recycled water absorbent resin to be 300,000 or more, and water absorbent resin particles having high water absorption performance and a small dissolved content can be produced (regenerated) from the polymer.

[0019] From the viewpoint that the weight average molecular weight of the obtained polymer is likely to increase and the dissolved portion of the water-absorbent resin particles produced from the polymer is reduced, the oxygen concentration in the gas phase of the treatment tank may be 16% by volume or less, 14% by volume or less, 12% by volume or less, 10% by volume or less, 8% by volume or less, 6% by volume or less, 4% by volume or less, 2% by volume or less, 1% by volume or less, 0.5% by volume or less, 0.1% by volume or less, or 0.05% by volume or less relative to the total volume of the gas phase. The oxygen concentration in the gas phase may be substantially 0% by volume, or may be more than 0% by volume, or may be 0.01% by volume or more. The oxygen concentration in the gas phase of the treatment tank may be 0 to 16% by volume, 0 to 12% by volume, 0 to 8% by volume, 0 to 6% by volume, 0 to 4% by volume, 0 to 1% by volume, 0 to 0.5% by volume, 0 to 0.1% by volume, 0 to 0.05% by volume, 0.01 to 0.5% by volume or less, 0.01 to 0.1% by volume or less, or 0.01 to 0.05% by volume or less. To set the oxygen concentration in the gas phase of the treatment tank to 18% by volume or less, for example, the gas phase of the treatment tank may be substituted with an inert gas (e.g., nitrogen).

[0020] The reaction liquid contains a water-absorbing resin containing at least a crosslinked polymer. The crosslinked polymer is dissolved in, for example, a solvent. The solvent of the reaction liquid may be any solvent that can dissolve the crosslinked polymer, and examples of the solvent include water and organic solvents.

[0021] The content of the water absorbent resin in the reaction liquid may be 30% by mass or less, 20% by mass or less, 15% by mass or less, 12% by mass or less, or 10% by mass or less, or 0.1% by mass or more, 0.5% by mass or more, 1% by mass or more, or 1.5% by mass or more, based on the total amount of the reaction liquid. The content of the crosslinked polymer in the reaction liquid may be 0.1 to 30% by mass, 0.5 to 20% by mass, 1 to 15% by mass, or 1 to 10% by mass.

[0022] The reaction liquid may contain a component other than the water-absorbent resin. For example, the reaction liquid may contain at least one of an acid component and an alkaline component (base component). The reaction liquid may contain an alkaline component, from the viewpoint that the weight-average molecular weight of the obtained polymer is likely to increase and the dissolved portion of the water-absorbent resin particles produced from the polymer is reduced.

[0023] The acid component may be an inorganic acid, for example, at least one inorganic acid selected from the group consisting of acetic acid, nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, and boric acid, or at least one inorganic acid selected from sulfuric acid and hydrochloric acid.

[0024] The alkaline component (base component) may be an inorganic alkaline component (inorganic base component), for example, at least one inorganic alkaline component (inorganic base component) selected from the group consisting of sodium hydroxide, ammonia, potassium hydroxide, and calcium hydroxide, or at least one inorganic alkaline component (inorganic base component) selected from the group consisting of sodium hydroxide, potassium hydroxide, and calcium hydroxide.

[0025] The content of the acid component or alkaline component (base component) in the reaction solution may be 0.003 mol / kg or more, 0.01 mol / kg or more, 0.1 mol / kg or more, 0.2 mol / kg or more, 0.4 mol / kg or more, 0.5 mol / kg or more, 0.6 mol / kg or more, 0.7 mol / kg or more, or 0.8 mol / kg or more, relative to the total amount of the reaction solution, or 2 mol / kg or less, 1.5 mol / kg or less, 1 mol / kg or less, 0.9 mol / kg or less, or 0.8 mol / kg or less. The content of the acid component or alkaline component (base component) in the reaction solution may be 0.003 to 2 mol / kg, 0.01 to 1.5 mol / kg, 0.1 to 1 mol / kg, 0.2 to 0.9 mol / kg, or 0.5 to 0.8 mol / kg, relative to the total amount of the reaction solution.

[0026] When the reaction solution contains an acid component, the pH of the reaction solution may be 0.1 or more, 0.2 or more, 0.3 or more, 0.5 or more, 1.0 or more, 1.5 or more, or 2.0 or more, and may be 5.0 or less, 4.5 or less, 4.0 or less, 3.5 or less, 3.0 or less, or 2.5 or less. When the reaction solution contains an acid component, the pH of the reaction solution may be, for example, 0.1 to 5.0, 0.2 to 4.5, 0.3 to 4.0, 0.5 to 4.0, 1.0 to 3.5, 1.5 to 3.0, or 2.0 to 2.5.

[0027] When the reaction solution contains an alkaline component (base component), the pH of the reaction solution may be 7.0 or more, 7.5 or more, 8.0 or more, 8.5 or more, 9.0 or more, 9.5 or more, or 10.0 or more, and may be 14.0 or less, 13.5 or less, 13.0 or less, 12.5 or less, 12.0 or less, 11.5 or less, or 11.0 or less. When the reaction solution contains an alkaline component (base component), the pH of the reaction solution may be, for example, 7.0 to 14.0, 7.5 to 13.5, 8.0 to 13.0, 8.5 to 12.5, 9.0 to 12.0, 9.5 to 11.5, or 10.0 to 11.0.

[0028] The cleavage step may include an operation of adjusting the temperature of the reaction solution from the viewpoint that the weight average molecular weight of the resulting polymer is likely to increase and that the dissolved content of the water-absorbent resin particles produced from the polymer is reduced. For example, the cleavage step may include an operation of adjusting the reaction solution to 50°C or higher, 60°C or higher, 70°C or higher, 80°C or higher, or 90°C or higher, or an operation of adjusting the reaction solution to 150°C or lower, 120°C or lower, 110°C or lower, or 100°C or lower, or an operation of adjusting the reaction solution to 50 to 150°C, 60 to 120°C, or 70 to 100°C. The temperature adjustment of the reaction solution can be achieved, for example, by heating / cooling the treatment tank.

[0029] The temperature adjustment of the reaction liquid may be started after the reaction liquid and the gas phase in contact with the reaction liquid reach an equilibrium state in the treatment tank. For example, the temperature adjustment of the reaction liquid may be performed after the reaction liquid and the gas phase have been in contact with each other for 10 minutes or more. The reaction liquid may be stirred from the viewpoint of suppressing deposition of the water-absorbent resin and occurrence of temperature unevenness.

[0030] When the recycled water absorbent resin contains a crosslinked polymer having a poly(meth)acrylic acid structure, the polymer obtained by cleaving the crosslinked polymer has a poly(meth)acrylic acid structure. When the recycled water absorbent resin contains a crosslinked polymer having a poly(meth)acrylic acid structure, the crosslinked polymer may be cleaved by contacting the recycled water absorbent resin with an alkaline component (base component), from the viewpoint of facilitating the production of water absorbent resin particles having high water absorption performance and low soluble content by avoiding cleavage of the poly(meth)acrylic acid structure.

[0031] The weight average molecular weight of the polymer obtained in the cleavage step (the polymer in the preparation step) may be 300,000 or more, 400,000 or more, 500,000 or more, 700,000 or more, 1,000,000 or more, 1,300,000 or more, 1,500,000 or more, 1,700,000 or more, 1,900,000 or more, 2,000,000 or more, 2,100,000 or more, 2,200,000 or more, 2,300,000 or more, 2,400,000 or more, or 2,500,000 or more, from the viewpoint of reducing the dissolved content of the produced water absorbent resin particles. The weight average molecular weight of the polymer may be 10,000,000 or less, 8,000,000 or less, 6,000,000 or less, 4,000,000 or less, 3,500,000 or less, 3,000,000 or less, or 2,600,000 or less. The weight average molecular weight of the polymer may be 300,000 to 10,000,000, 400,000 to 8,000,000, 500,000 to 6,000,000, 700,000 to 4,000,000, 1,000,000 to 3,500,000, 1,300,000 to 3,000,000, 1,500,000 to 2,600,000, 2,000,000 to 3,500,000, or 2,000,000 to 3,000,000. The weight average molecular weight of the polymer can be measured by the method described in the examples below.

[0032] The solvent for the polymer solution may be any solvent capable of dissolving the polymer, and examples thereof include water and organic solvents.

[0033] After the crosslinked structure of the crosslinked polymer is cleaved to obtain a polymer, solid-liquid separation may be performed to remove impurities from the polymer solution before the crosslinking step is performed. For example, the polymer solution may be subjected to filtration or ultrafiltration. Before the filtration of the polymer solution, the pH of the polymer solution may be adjusted to approximately neutral.

[0034] After the crosslinked structure of the crosslinked polymer is cleaved to obtain a polymer, and before the crosslinking step is performed, dilution, heating, addition of a salt (a salt containing a monovalent cation), or the like may be performed in order to reduce the viscosity of the polymer solution.

[0035] After the crosslinked structure of the crosslinked polymer is cleaved to obtain the polymer, the polymer solution may be subjected to a sterilization treatment before the crosslinking step is carried out.

[0036] In the crosslinking step, for example, a solution containing a polymer and a crosslinking agent capable of forming a covalent bond with a functional group of the polymer is prepared, and the polymer is crosslinked via the covalent bond to obtain a crosslinked polymer. In the crosslinking step, the polymer solution may be gelled. In this case, the entire polymer solution loses fluidity, and a gel containing the crosslinked polymer and water is formed. When crosslinking progresses to the extent that a gel is formed, it is particularly easy to obtain water-absorbent resin particles having high water absorption performance and a small amount of dissolved matter.

[0037] Examples of functional groups possessed by the polymer include carboxyl groups. When the polymer has carboxyl groups, the 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 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.

[0038] 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.

[0039] The amount of the crosslinking agent may be 0.0001 parts by mass or more, 0.001 parts by mass or more, or 0.005 parts by mass or more, and may be 15 parts by mass or less, 10 parts by mass or less, or 5 parts by mass or less, relative to 100 parts by mass of the polymer, from the viewpoint of sufficiently crosslinking the polymer. The amount of the crosslinking agent may be 0.0001 to 15 parts by mass, 0.001 to 10 parts by mass, or 0.005 to 5 parts by mass, relative to 100 parts by mass of the polymer.

[0040] The reaction temperature when crosslinking the polymer is appropriately set depending on the type, amount, etc. of the crosslinking agent used, but from the viewpoint of sufficiently crosslinking the polymer, it 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 polymer may be 50 to 220° C., 80 to 200° C., or 100 to 180° C.

[0041] The reaction time for crosslinking the 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 from the viewpoint of sufficiently crosslinking the polymer.

[0042] 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.

[0043] The surface crosslinking agent may be a compound having two or more reactive functional groups reactive with the functional group (e.g., carboxyl group) of the crosslinked polymer. The surface crosslinking agent may be the same as or different from the crosslinking agent in the crosslinking step.

[0044] 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.

[0045] 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.

[0046] 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, from the viewpoint of sufficiently achieving surface crosslinking of the crosslinked polymer. The amount of the crosslinking agent may be 0.001 to 5 parts by mass, 0.005 to 3 parts by mass, or 0.01 to 1 part by mass, relative to 100 parts by mass of the polymer.

[0047] 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.

[0048] 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 to 220°C, 90 to 200°C, or 100 to 180°C from the viewpoint of efficiently removing water.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] The median particle size of the crosslinked polymer powder obtained through pulverization and, if necessary, classification may be, for example, 200 to 500 μm, or 300 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.

[0055] From the viewpoint of providing high water absorption performance to the water-absorbent resin particles, the water-retention capacity of the water-absorbent resin particles in physiological saline may be 70 g / g or less, 68 g / g or less, 66 g / g or less, 64 g / g or less, 62 g / g or less, 60 g / g or less, or 58 g / g or less. The water-retention capacity of the water-absorbent resin particles in physiological saline may be 40 g / g or more, 44 g / g or more, 48 g / g or more, 52 g / g or more, or 56 g / g or more. The water-retention capacity of the water-absorbent resin particles in physiological saline may be 40 to 70 g / g, 44 to 66 g / g, 48 to 62 g / g, 52 to 60 g / g, or 56 to 58 g / g. The water-retention capacity of the water-absorbent resin particles in physiological saline is a value measured by the method described in the Examples below.

[0056] The dissolved content of the water-absorbent resin particles may be 0.45 g / g or less, 0.4 g / g or less, 0.35 g / g or less, 0.3 g / g or less, 0.28 g / g or less, 0.26 g / g or less, 0.24 g / g or less, 0.22 g / g or less, 0.2 g / g or less, or less than 0.2 g / g. The smaller the dissolved content of the water-absorbent resin particles, the more the dissolution of the polymer can be suppressed, and if the dissolved content is 0.45 g / g or less, it can be said that the dissolution of the polymer is sufficiently suppressed. The lower limit of the dissolved content of the water-absorbent resin particles is not particularly limited, but may be, for example, 0.01 g / g or more, 0.02 g / g or more, or 0.03 g / g or more. The dissolved content of the water-absorbent resin particles may be 0.01 to 0.45 g / g, 0.02 to 0.4 g / g, or 0.03 to 0.35 g / g. The dissolved content of the water-absorbent resin particles is a value measured by the method described in the examples below.

[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 5 to 200 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 , 8 to 150 g / m ２ , or 10 to 100 g / m ２ The thickness of the liquid-permeable sheet 30 may be 20 to 1400 μm, 50 to 1200 μm, or 80 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 set to 10 to 50 g / m ２ may be.

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

[0075] <Evaluation Method> The weight average molecular weight Mw, water retention capacity, and soluble content were measured according to the following evaluation methods. The measurement results are shown in Tables 1 and 2.

[0076] [Weight-average molecular weight Mw] Measurement method: light scattering GPC. Autosampler: Autosampler AS-11 (manufactured by FLOM Corporation). Degasser: Gastrr 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 Resonac Corporation). Detector: Triple Detector TDA 302 (manufactured by Viscotec). Eluent: NaNO ３(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. A sample solution adjusted to contain 0.01 g of polymer was placed in a 300 mL beaker, and the above eluent was added to bring the total volume to 100 mL. 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 1 mol / L hydrochloric acid (Nacalai Tesque) or 1 mol / L aqueous sodium hydroxide solution (Nacalai Tesque). After stirring, the sample solution was passed through a 0.8 μm filter syringe and the filtrate was analyzed by GPC.

[0077] [Water Retention Capacity] The water retention capacity of the recycled water-absorbent resin particles in saline solution (room temperature, 25±2°C) was measured using the following procedure. First, a cotton bag (membrane broadcloth No. 60, 100 mm wide x 200 mm long) containing 2.0 g of recycled water-absorbent resin particles was placed in a 500 mL beaker. 500 g of saline solution was poured into the cotton bag containing the recycled water-absorbent resin particles all at once, taking care not to allow the bag to become lumpy. The top of the cotton bag was then tied with a rubber band and allowed to stand for 30 minutes to allow the recycled water-absorbent resin particles to swell. After 30 minutes, the cotton bag was dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., product number: H-122) set to a centrifugal force of 167 G, and the mass Wa [g] of the cotton bag containing the swollen gel after dehydration was measured. The same procedure was carried out without adding the recycled water-absorbent resin particles, and the wet empty mass Wb [g] of the cotton bag was measured. The water retention capacity of the recycled water-absorbent resin particles in physiological saline was calculated using the following formula: Water retention capacity [g / g] = (Wa - Wb) / 2.0

[0078] [Soluble Content] The soluble content of the recycled water-absorbent resin was measured under an environment of 25°C ± 2°C and 50% ± 10% humidity. 500 g of saline solution in a 500 mL beaker was stirred with a stirring bar (cylindrical, 8 mm diameter x 30 mm length, no ring) rotating at 600 rpm. The temperature of the saline solution was 25°C. 2.000 g of recycled water-absorbent resin particles were added, and the dispersion containing the recycled water-absorbent resin particles was stirred for 3 hours. The dispersion was filtered through a 75 μm standard sieve, and the filtrate was recovered. 80 g of the resulting filtrate was weighed into a weighed 100 mL beaker that had been pre-weighed at 140°C. The filtrate in the beaker was heated in a 140°C air-blowing dryer (FV-320, manufactured by ADVANTEC) for 15 hours to remove moisture, and the mass Wc (g) of the remaining solid component was measured. A blank test was carried out in the same manner as above, without adding the recycled water-absorbent resin particles to physiological saline, and the mass Wd (g) of the solid component remaining in the beaker was measured. The dissolved content was calculated according to the following formula: Dissolved content [g / g] = ((Wc - Wd) / 80) x 500 / 2.000

[0079] Example 1 A commercially available disposable diaper (Merrys Pants, Smooth Air Through, L size, manufactured by Kao Corporation) was prepared, and a water-absorbent resin containing a crosslinked polymer containing acrylic acid and an acrylate salt as monomer units was collected from inside the disposable diaper.

[0080] As shown in FIG. 2, a reflux condenser, a nitrogen gas inlet pipe, and a stirrer (a 150 mL round-bottom cylindrical separable flask equipped with a stirring blade having four inclined paddle blades with a blade diameter of 40 mm, manufactured by EYELA) were prepared. 1 g of water-absorbent resin collected from disposable diapers was weighed into the separable flask, and 50 g of ion-exchanged water was added and allowed to stand for 5 minutes to swell the water-absorbent resin. Next, 50 g of 1.6 mol / kg sulfuric acid was added to the separable flask, and stirring was started at 300 rpm. Nitrogen gas with an oxygen concentration of 0.02% by volume was blown into the separable flask at 200 mL / min for 15 minutes to replace the atmosphere in the separable flask with nitrogen. Thereafter, a temperature increase was initiated using a personal organic synthesis reaction apparatus (manufactured by EYELA), and the internal temperature was maintained at 100 ° C. for 24 hours, while the crosslinked structure of the crosslinked polymer of the water-absorbent resin was cleaved. A decomposition liquid (aqueous solution of water-soluble recycled polymer, pH 0.3) containing a polymer that is a water-absorbent resin solubilized by cleavage was obtained. The weight average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and was found to be 2,292,000.

[0081] (Example 2) The same operation as in Example 1 was carried out except that 50 g of 1.6 mol / kg hydrochloric acid was used instead of 50 g of 1.6 mol / kg sulfuric acid, to obtain a decomposition liquid (aqueous solution of water-soluble recycled polymer) containing a polymer that is a water-absorbent resin solubilized by cleavage. The weight-average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and the weight-average molecular weight Mw was 2,378,000.

[0082] (Example 3) The same operation as in Example 1 was carried out, except that 50 g of a 1.6 mol / kg aqueous solution of sodium hydroxide was used instead of 50 g of 1.6 mol / kg sulfuric acid, to obtain a decomposition liquid (aqueous solution of a water-soluble recycled polymer, pH 13.3) containing a polymer that is a water-absorbent resin solubilized by cleavage. The weight-average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and the weight-average molecular weight Mw was 2,594,000.

[0083] (Example 4) A reflux condenser, a nitrogen gas inlet pipe, and a stirrer (a 4L round-bottom cylindrical separable flask equipped with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 50 mm, manufactured by DURAN Co., Ltd.) were prepared. 90 g of water-absorbent resin collected from disposable diapers was weighed into the separable flask, and 1710 g of ion-exchanged water was added and left to stand for 5 minutes to swell the water-absorbent resin. Next, 1200 g of a 1.5 mol / kg aqueous sodium hydroxide solution was added to the separable flask, and stirring was started at 160 rpm. Nitrogen gas was blown into the separable flask at 200 mL / min for 15 minutes, and the atmosphere in the separable flask was replaced with nitrogen. Thereafter, the internal temperature was maintained at 80 ° C. for 24 hours, and the crosslinked structure of the crosslinked polymer of the water-absorbent resin was cleaved. A decomposition liquid (aqueous solution of water-soluble recycled polymer, pH 13.1) containing a polymer that is a water-absorbent resin solubilized by cleavage was obtained. The weight average molecular weight Mw of the resulting polymer was measured by light scattering GPC and found to be 2,490,000.

[0084] Example 5: 100 g of the aqueous solution of the water-soluble recycled polymer obtained in Example 4 was weighed into a 500 mL plastic beaker, and 200 g of ion-exchanged water was added. Stirring was initiated at 300 rpm using a mixer equipped with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 50 mm. 25 g of cation exchange resin (DOWEX 50Wx8 Strong Acid Ion Exchange Resin (H-type), manufactured by DuPont) was added, stirred for 30 minutes, and then filtered through a nylon mesh with 25 μm openings to remove the cation exchange resin. The resulting aqueous solution of the water-soluble recycled polymer had a pH of 6.8.

[0085] A reflux condenser, a nitrogen gas inlet tube, and a stirrer (a 150 mL round-bottom cylindrical separable flask equipped with a stirring blade having four inclined paddle blades with a blade diameter of 40 mm, manufactured by EYELA) were prepared. 100 g of the resulting aqueous solution of water-soluble recycled polymer (weight average molecular weight Mw 2,490,000, pH 6.8) was weighed into the separable flask and stirred at 300 rpm. Nitrogen gas was blown into the separable flask at 200 mL / min for 15 minutes to replace the atmosphere in the separable flask with nitrogen. Then, a personal organic synthesis reaction apparatus (manufactured by EYELA) was used to start heating, and the internal temperature was maintained at 70 °C for 24 hours. The weight average molecular weight Mw of the polymer in the aqueous solution of water-soluble recycled polymer after heating was measured by light scattering GPC, and the weight average molecular weight Mw was 2,530,000.

[0086] (Example 6) The same operation as in Example 1 was carried out, except that air having an oxygen concentration of 3.1% by volume was blown in instead of nitrogen gas when the atmosphere in the separable flask was replaced with nitrogen, to obtain a decomposition liquid (aqueous solution of a water-soluble recycled polymer) containing a polymer that is a water-absorbent resin solubilized by cleavage. The weight-average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and the weight-average molecular weight Mw was 1,939,000.

[0087] (Example 7) The same operation as in Example 1 was carried out, except that air having an oxygen concentration of 5.3% by volume was blown in instead of nitrogen gas when the atmosphere in the separable flask was replaced with nitrogen, to obtain a decomposition liquid (aqueous solution of water-soluble recycled polymer, pH 0.2) containing a polymer that is a water-absorbent resin solubilized by cleavage. The weight-average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and was found to be 1,153,000.

[0088] (Example 8) The same operation as in Example 1 was carried out, except that when the atmosphere in the separable flask was replaced with nitrogen, air having an oxygen concentration of 10.3% by volume was blown in instead of nitrogen gas, to obtain a decomposition liquid (aqueous solution of water-soluble recycled polymer, pH 0.2) containing a polymer that is a water-absorbent resin solubilized by cleavage. The weight-average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and was found to be 1,036,000.

[0089] (Example 9) The same operation as in Example 1 was carried out, except that when the atmosphere in the separable flask was replaced with nitrogen, air having an oxygen concentration of 15.1% by volume was blown in instead of nitrogen gas, to obtain a decomposition liquid (aqueous solution of water-soluble recycled polymer, pH 0.3) containing a polymer that is a water-absorbent resin solubilized by cleavage. The weight-average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and the weight-average molecular weight Mw was 860,510.

[0090] (Example 10) The same operation as in Example 1 was carried out, except that when the atmosphere in the separable flask was replaced with nitrogen, air having an oxygen concentration of 17.3% by volume was blown in instead of nitrogen gas, to obtain a decomposition liquid (aqueous solution of water-soluble recycled polymer, pH 0.2) containing a polymer that is a water-absorbent resin solubilized by cleavage. The weight-average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and the weight-average molecular weight Mw was 538,236.

[0091] (Example 11) The same operation as in Example 1 was carried out, except that when the atmosphere in the separable flask was replaced with nitrogen, air having an oxygen concentration of 20.1% by volume was blown in instead of nitrogen gas, to obtain a decomposition liquid (aqueous solution of a water-soluble recycled polymer) containing a polymer that is a water-absorbent resin solubilized by cleavage. The weight-average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and the weight-average molecular weight Mw was 480,296.

[0092] (Comparative Example 1) The same operation as in Example 2 was carried out, except that air having an oxygen concentration of 20.1% by volume was blown in instead of nitrogen gas when the atmosphere in the separable flask was replaced with nitrogen, to obtain a decomposition liquid (aqueous solution of a water-soluble recycled polymer) containing a polymer that is a water-absorbent resin solubilized by cleavage. The weight-average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and the weight-average molecular weight Mw was 144,006.

[0093] (Comparative Example 2) The same operation as in Example 5 was carried out, except that air having an oxygen concentration of 20.1% was blown in instead of nitrogen gas when the atmosphere in the separable flask was replaced with nitrogen, to obtain a decomposition liquid (aqueous solution of water-soluble recycled polymer) containing a polymer that is a water-absorbent resin solubilized by cleavage. The weight-average molecular weight Mw of the obtained polymer was measured by light scattering GPC, and the weight-average molecular weight Mw was 285,786.

[0094] (Example 12) 20.0 g of commercially available partially neutralized polyacrylic acid (Akupana AP-70, manufactured by Sumitomo Seika Chemicals Co., Ltd.) was weighed into a 500 mL polybeaker and dissolved in 180.0 g of ion-exchanged water to prepare a 10% by weight aqueous solution. When the weight average molecular weight Mw was measured by light scattering GPC, the weight average molecular weight Mw was 3,136,000. To this aqueous solution, 1.00 g of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether was added, and the mixture was stirred at 650 rpm for 5 minutes using a hand mixer (manufactured by Panasonic Corporation, MK-H4). After stirring, the aqueous solution was transferred to a PTFE-coated SUS tray (135 x 170 mm) and dried at 115 ° C. for 1 hour in a hot air dryer (manufactured by ADVANTEC Corporation, FV-320) to obtain a gel. The obtained gel was cut into pieces of approximately 3 cm and dried in a hot air dryer at 115°C for 2 hours. The dried product was then pulverized using a centrifugal pulverizer (Verder Scientific, ZM200, screen diameter 1 mm, 6000 rpm). The pulverized product was classified using a sieve, and recycled water-absorbent resin particles with particle sizes of 180 to 850 μm were collected. The water retention capacity and soluble content of these recycled water-absorbent resin particles were measured, and the water retention capacity was 57.2 g / g and the soluble content was 0.199 g / g.

[0095] (Example 13) 20.0 g of commercially available partially neutralized polyacrylic acid (Akupana AP-70, manufactured by Sumitomo Seika Chemicals Co., Ltd.) was weighed into a 500 mL polybeaker and dissolved in 180.0 g of ion-exchanged water to prepare a 10% by mass aqueous solution. This solution was uniformly spread on an SUS tray (250 × 185 mm) and irradiated with ultraviolet light for 60 minutes using a UV lamp (Spectro-UV Fluorescent Bench & Display Lamps: xx-15NF, wavelength 254 nm) at an irradiation distance of 5 cm. The weight average molecular weight Mw was measured by light scattering GPC and found to be 535,228. This aqueous solution was returned to a 500 mL polybeaker, and 4.48 g of a 2% by weight ethylene glycol diglycidyl ether aqueous solution was added. The mixture was stirred for 5 minutes at 650 rpm using a hand mixer (Panasonic, MK-H4). The stirred aqueous solution was transferred to a PTFE-coated SUS tray (135 x 170 mm) and dried at 115 °C for 1 hour in a hot air dryer (ADVANTEC, FV-320) to obtain a gel. The resulting gel was cut into pieces of approximately 3 cm and dried at 115 °C for 2 hours in a hot air dryer. The dried product was then pulverized using a centrifugal pulverizer (Verder Scientific, ZM200, screen diameter 1 mm, 6000 rpm). The pulverized product was classified using a sieve, and recycled water-absorbent resin particles with particle sizes of 180 to 850 μm were collected. The water retention capacity and soluble matter of the recycled water-absorbent resin particles were measured, and the water retention capacity was 57.9 g / g and the soluble matter was 0.251 g / g.

[0096] (Comparative Example 3) 20.0 g of commercially available partially neutralized polyacrylic acid (Akupana AP-70, manufactured by Sumitomo Seika Chemicals Co., Ltd.) was weighed into a 500 mL polybeaker and dissolved in 180.0 g of ion-exchanged water to prepare a 10% by mass aqueous solution. To reduce the molecular weight, this solution was spread evenly on an SUS tray (250 × 185 mm) and irradiated with ultraviolet light for 230 hours using a UV lamp (Spectro-UV Fluorescent Bench & Display Lamps: xx-15NF, wavelength 254 nm) at an irradiation distance of 5 cm. The weight average molecular weight Mw was measured by light scattering GPC and found to be 260,968. This aqueous solution was returned to a 500 mL polybeaker, and 8.40 g of a 2% by weight ethylene glycol diglycidyl ether aqueous solution was added. The mixture was stirred for 5 minutes at 650 rpm using a hand mixer (Panasonic, MK-H4). The stirred aqueous solution was transferred to a PTFE-coated SUS tray (135 x 170 mm) and dried at 115 °C for 1 hour in a hot air dryer (ADVANTEC, FV-320) to obtain a gel. The resulting gel was cut into pieces of approximately 3 cm and dried at 115 °C for 2 hours in a hot air dryer. The dried product was then pulverized using a centrifugal pulverizer (Verder Scientific, ZM200, screen diameter 1 mm, 6000 rpm). The pulverized product was classified using a sieve, and recycled water-absorbent resin particles with particle sizes of 180 to 850 μm were collected. The water retention capacity and soluble matter of the recycled water-absorbent resin particles were measured, and the water retention capacity was 78.9 g / g and the soluble matter was 0.492 g / g.

[0097]

[0098]

[0099] The results of Examples 12 and 13 and Comparative Example 3 show that recycled water-absorbent resin particles obtained using a polymer with a weight-average molecular weight of 300,000 or more have a moderately large water retention capacity and a low soluble content, and therefore are water-absorbent resin particles with high water absorption performance and a low soluble content. Furthermore, the results of Examples 1 to 10 and Comparative Examples 1 and 2 show that polymers with a large weight-average molecular weight can be obtained by lowering the oxygen concentration in the step of cleaving the crosslinked structure of the crosslinked polymer, and in particular, it can be seen that polymers with a weight-average molecular weight of 500,000 or more can be obtained by keeping the oxygen concentration at 18% by volume or less.

[0100] DESCRIPTION OF SYMBOLS 1...water-absorbent resin particles, 10...absorbent body, 20a, 20b...core wrap sheet, 30...liquid-permeable sheet, 40...liquid-impermeable sheet, 50...water-absorbent sheet, 100...absorbent article, 110...round-bottom cylindrical separable flask, 120...four-inclined paddle blade, 130...nitrogen gas inlet pipe, 140...reflux condenser, 150...oxygen concentration meter, 160...gas outlet pipe, 170...stirring motor, 180...shaft holder, 185...stirring shaft, 190, 195...three-way cock, 200...treatment tank.

Claims

1. A method for producing water-absorbing resin particles containing a crosslinked polymer, A preparation step to prepare a polymer solution containing a polymer having a weight-average molecular weight of 300,000 or more and a solvent, A crosslinking step for crosslinking the aforementioned polymer, A manufacturing method that includes the following features.

2. The preparation step includes a cleavage step to obtain the polymer by cleaving the crosslinked structure of the crosslinked polymer contained in the water-absorbent resin to be subjected to chemical decomposition, The manufacturing method according to claim 1, wherein the cracking step is carried out in a treatment tank containing a reaction solution containing a water-absorbent resin to be subjected to the chemical decomposition, and further includes an operation to reduce the oxygen concentration in the gas phase of the treatment tank to 18% by volume or less.

3. The manufacturing method according to claim 2, wherein the cleavage step includes raising the reaction solution to 50°C or higher.

4. The manufacturing method according to claim 2 or 3, wherein the reaction solution contains at least one of an acidic component and an alkaline component.

5. The manufacturing method according to any one of claims 1 to 3, wherein the crosslinking step includes crosslinking the polymer via covalent bonds.

6. An absorbent body comprising water-absorbent resin particles obtained by the manufacturing method described in any one of claims 1 to 3.

7. An absorbent article comprising the absorbent material described in claim 6.