Method for manufacturing an absorbent article comprising a water-absorbing resin

Abstract

A method for manufacturing an absorbent article containing a water-absorbing resin having excellent initial water-absorption speed under load even substantially without adding a liquid permeability enhancer is provided. The method steps for manufacturing the water-absorbing resin include the steps of adding a certain polyalkylene glycol to a polymerization monomer at the same time as the polymerization of the monomer so as to generate a crosslinked hydrogel polymer containing a polyalkylene glycol of a certain molecular weight during or after the polymerization, wherein the crosslinked hydrogel polymer has a centrifuge retention capacity within a given range and the final water-absorbing resin to be obtained has various physical properties (CRC, AAP, SFC, FSR) within a given range.

Description

Technical Field

[0001] This invention relates to a method for manufacturing absorbent articles comprising a water-absorbing resin. Background Technology

[0002] Superabsorbent polymers (SAPs) are used as absorbent cores in absorbent products such as paper diapers, sanitary napkins, and so-called incontinence pads. SAPs are typically surface-crosslinked absorbent resins.

[0003] As used in any of the aforementioned absorbent articles, the superabsorbent resin absorbs and swells an aqueous liquid such as urine, thereby retaining the aqueous liquid within the resin. Therefore, the superabsorbent resin needs to have a high absorption capacity for aqueous liquids and also a high absorption rate, as it needs to absorb the aqueous liquid rapidly. The free swelling rate (FSR) can be used as a physical property value representing the absorption rate of the superabsorbent resin.

[0004] In addition to high absorption capacity and high absorption rate, absorbent resins also need to have favorable liquid permeability. For absorbent resins with low liquid permeability, the discharged liquid may not enter the interior of the absorbent article and may remain on or near the surface. This can lead to liquid leakage from the absorbent article. Saltwater flow conductivity (SFC) can be used as a physical property value to represent liquid permeability.

[0005] Patent documents 7 to 12 disclose superabsorbent resins with an improved T20 (i.e., the time to achieve an uptake of 20 g / g), where T20 is used as a parameter indicating the initial water absorption rate under load. As a method for improving the T20 of the superabsorbent resin, patent document 8 describes foaming polymerization, and patent documents 9 to 12 describe gel disruption under specific disruption conditions.

[0006] As a technique for improving FSR in superabsorbent polymers, Patent Documents 14 and 15 disclose a so-called wet milling technique, which uses a gel crusher with a small die diameter to apply strong shear force to the gel when crushing it.

[0007] Patent Document 1 discloses a technique in which a substance (e.g., a surfactant) is added during the polymerization of the absorbent resin or during gel breakup to inhibit adhesion between gel particles, thereby reducing adhesion between broken gel particles. Patent Document 4 discloses the use of polyethylene glycol (PEG) as a release agent during gel breakup.

[0008] Patent Document 3 discloses a water-absorbing resin containing a compound derived from polyalkylene glycols and a polyvalent metal salt. Patent Document 5 discloses the addition of a water-soluble polymer to a water-absorbing resin to improve its water absorption rate. Patent Document 2 discloses the addition of PEG to a water-absorbing resin to suppress initial coloring. Patent Document 6 discloses the use of a polyol such as PEG alone as an internal crosslinking agent. Patent Document 13 discloses the use of PEG as a thickener in foam polymerization.

[0009] Reference List

[0010] [Patent Literature]

[0011] [Patent Document 1]

[0012] International Publication Booklet No. 2016 / 204302

[0013] [Patent Document 2]

[0014] International Publication Booklet No. 2008 / 096713

[0015] [Patent Document 3]

[0016] International Publication Booklet No. 2009 / 075204

[0017] [Patent Document 4]

[0018] Japanese Patent Application Publication No. Tokukai 2001-342258

[0019] [Patent Document 5]

[0020] Japanese Patent Application Publication No. Tokukaishou 57-167307 (1982)

[0021] [Patent Document 6]

[0022] Japanese Patent Application Publication No. Tokukaishou 55-84304 (1980)

[0023] [Patent Document 7]

[0024] Japanese translation of PCT international application published (Tokuhyo 2014-515987)

[0025] [Patent Document 8]

[0026] International Publication Brochure No. 2010 / 095427

[0027] [Patent Document 9]

[0028] International Publication Booklet No. 2017 / 164452

[0029] [Patent Document 10]

[0030] International Publication Booklet No. 2018 / 117391

[0031] [Patent Document 11]

[0032] International Publication Booklet No. 2018 / 139768

[0033] [Patent Document 12]

[0034] International Publication Booklet No. 2016 / 085123

[0035] [Patent Document 13]

[0036] International Publication Booklet No. 2016 / 204390

[0037] [Patent Document 14]

[0038] International Publication Booklet No. 2016 / 126079

[0039] [Patent Document 15]

[0040] International Publication Booklet No. 2016 / 158975 Summary of the Invention

[0041] Technical issues

[0042] Absorbent articles containing the aforementioned known absorbent resins (such as paper diapers) still absorb aqueous liquids relatively slowly and allow the aqueous liquid to seep out (rewet), which is disadvantageous. In overcoming these drawbacks of absorbent resins, the inventors of this invention have found it important to improve the initial absorption rate under load. That is, it is required that the absorbent resin can rapidly diffuse and absorb the aqueous liquid during the initial contact phase between the absorbent article containing the absorbent resin and the aqueous liquid.

[0043] Incidentally, adding liquid permeability enhancers to absorbent resins can lead to dust-related issues. For example, dust generation and / or filter clogging can occur during the manufacturing of the absorbent resin and in paper diaper production lines. Therefore, it is necessary to improve the initial absorbency under load without substantially adding liquid permeability enhancers to the absorbent resin.

[0044] One aspect of the present invention is to provide a method for manufacturing an absorbent article comprising a water-absorbing resin that has an excellent initial water absorption rate under load, even without the addition of a liquid permeability enhancer.

[0045] Solution to the problem

[0046] To achieve this objective, according to one aspect of the present invention, a method for manufacturing an absorbent article comprising a water-absorbing resin is as follows.

[0047] In a method for manufacturing an absorbent article comprising a surface-crosslinked particulate poly(meth)acrylate (salt)-based absorbent resin, the method includes the following steps:

[0048] (i) Preparation of an aqueous solution of (meth)acrylate (salt) monomer;

[0049] (ii) Polymerize the aqueous solution of the (meth)acrylate (salt) monomer;

[0050] (iii) Gel disruption of the cross-linked hydrogel polymer generated during or after polymerization to obtain microparticle hydrogels;

[0051] (iv) Dry the microparticle hydrogel to obtain a dry polymer;

[0052] (v) Pulverize and / or classify the dried polymer to obtain particulate water-absorbing resin before surface crosslinking;

[0053] (vi) Crosslinking the surface of the microparticle-based water-absorbing resin before surface crosslinking; and

[0054] (vii) Incorporating surface-crosslinked microparticle water-absorbing resin into the absorbent product, wherein

[0055] In step (i) and / or step (ii), a water-soluble polyalkylene glycol with a weight-average molecular weight of not more than 2000 is added to the aqueous monomer solution, such that the total amount of the water-soluble polyalkylene glycol added in step (i) and / or step (ii) is 0.01% to 1% by mass relative to the total mass of the monomer contained in the aqueous monomer solution.

[0056] The cross-linked hydrogel polymer has a centrifugal retention capacity of not more than 31 g / g and not less than 20 g / g, and

[0057] The surface-crosslinked microparticle poly(meth)acrylate (salt)-based superabsorbent resin obtained by the method satisfies the following (1) to (4):

[0058] (1) The centrifugation retention capacity (CRC) is not less than 20 g / g and not more than 35 g / g;

[0059] (2) The pressure absorption capacity (AAP) measured under a load of 0.7 psi is not less than 25 g / g;

[0060] (3) The saline fluid conductivity (SFC) is not less than 15 (×10⁻⁷ cm³·s / g); and

[0061] (4) Free swelling rate (FSR) is not less than 0.33 g / (g·s).

[0062] Preferably, in the method according to this aspect of the invention, the polyalkylene glycol has a weight-average molecular weight of not less than 200.

[0063] Preferably, in the method according to this aspect of the invention, the content of polyalkylene glycol is adjusted to 0.01% by mass to 1% by mass relative to the microparticle poly(meth)acrylate (salt)-based absorbent resin crosslinked throughout the entire surface.

[0064] Preferably, in the method according to this aspect of the invention, the surface-crosslinked particulate poly(meth)acrylate (salt)-based absorbent resin to be incorporated into the absorbent article is a surface-crosslinked absorbent resin having a non-uniformly pulverized shape.

[0065] Preferably, in the method according to this aspect of the invention, the water content of the surface-crosslinked microparticle poly(meth)acrylate (salt)-based absorbent resin to be incorporated into the absorbent article is not greater than 5% by mass.

[0066] Preferably, in the method according to this aspect of the invention, the microparticle hydrogel obtained as a result of gel disruption of the hydrogel crosslinked polymer in step (iii) has a logarithmic standard deviation (σ) of 0.2 to 1.5, which indicates the particle size distribution of the microparticle hydrogel.

[0067] Beneficial effects of the present invention

[0068] According to one aspect of the invention, a method for manufacturing absorbent articles comprising a water-absorbing resin provides a water-absorbing resin with excellent initial absorption rate (T20) under load, even without substantially adding a liquid permeability enhancer. Therefore, for example, in the preparation of absorbent articles such as paper diapers comprising a water-absorbing resin manufactured by the method according to this aspect of the invention, the absorbent article can achieve excellent liquid permeability through a swollen gel, rapidly absorbing aqueous liquids and inhibiting or reducing leakage (rewetting) of aqueous liquids. Since this aspect of the invention substantially does not involve the addition of a liquid permeability enhancer, problems involving dust, such as dust and / or filter clogging that would otherwise occur during the manufacture of the water-absorbing resin and in the paper diaper production line, can be suppressed or reduced. Furthermore, a safer absorbent article for consumers can be provided.

[0069] Description of the implementation plan

[0070] The following provides a detailed explanation of embodiments of the present invention. The present invention is not limited to these embodiments, but can be modified by those skilled in the art within the scope of the description. The present invention also includes any embodiments and any examples obtained by combining the technical means disclosed in different embodiments and examples within its technical scope. In this document, unless otherwise stated, the range "A to B" means "not less than A and not greater than B". The term "(meth)acrylic acid" means "acrylic acid and / or methacrylic acid". Unless otherwise stated, the mass of the superabsorbent resin, etc., is a value based on solid content.

[0071] [1] Definition of terminology

[0072] (1-1) Water-absorbing resin

[0073] In this document, “water-absorbing resin” refers to a cross-linked polymer that is both water-swellable and water-insoluble. “Water-swellable” in this document refers to an absorption capacity (also known as centrifugation retention capacity (CRC)) of not less than 5 g / g under no-load conditions, as defined in NWSP 241.0.R2(15). “Water-insoluble” in this document refers to a water-soluble content (Ext) of not more than 50% by mass, as defined in NWSP 270.0.R2(15).

[0074] The "hygroscopic resin" is preferably a hydrophilic crosslinked polymer obtained by polymerizing and crosslinking (meth)acrylate (salt) monomers. It should be noted that the hygroscopic resin does not necessarily contain the total amount (i.e., 100% by mass) of the hydrophilic crosslinked polymer. The hygroscopic resin may contain additives, etc., in an amount sufficient to meet the performance requirements of materials such as CRC and Ext.

[0075] In this document, "hygroscopic resin" can refer to either "a polymer that is cross-linked only internally (i.e., a polymer having an internal and surface cross-linking density that is substantially equal to each other)" or "a polymer that is cross-linked both internally and on its surface (i.e., a polymer whose surface cross-linking density is relatively higher than that of its internal cross-linking density)." Essentially, "polymers that are cross-linked only internally" and "polymers that are cross-linked both internally and on their surface" are not distinguished from each other, and each is described as a "hygroscopic resin." However, there may be situations where it is necessary to distinguish between polymers whose surfaces are cross-linked and polymers whose surfaces are not cross-linked. In such cases, "polymers that are cross-linked only internally" are described as "hygroscopic resins before surface cross-linking" or "base polymers" because "polymers that are cross-linked only internally" have not yet undergone surface cross-linking. Meanwhile, "polymers that are cross-linked both internally and on their surface (i.e., a polymer whose surface cross-linking density is relatively higher than that of its internal cross-linking density)" are described as "hygroscopic resins after surface cross-linking" or "hygroscopic resins with surface cross-linking" because "polymers that are cross-linked both internally and on their surface" have undergone surface cross-linking. It should be noted that the phrase "before surface crosslinking" refers to either "before the addition of the surface crosslinking agent" or "after the addition of the surface crosslinking agent but before the surface crosslinking reaction begins through heat treatment".

[0076] "Water-absorbing resin" can be a resin containing only resin components, or it can be a resin containing resin components and other components such as additives.

[0077] (1-2)"NWSP"

[0078] The term "NWSP" is an acronym for Non-Woven Standard Procedures, 2015. NWSP is a standard published jointly by the European Disposables and Nonwovens Association (EDANA) and the Association of the Nonwovens Industry (INDA), and is widely used in Europe and America for the evaluation of nonwoven fabrics and products containing such nonwoven fabrics. NWSP also describes standard measurement methods for absorbent resins. In this document, the physical properties of absorbent resins are measured according to the first edition of NWSP (2015). It should be noted that, unless otherwise stated, the various physical properties of absorbent resins are measured using the measurement methods employed in the examples described below.

[0079] [2] Water-absorbing resin

[0080] The absorbent article manufactured by the manufacturing method according to an embodiment of the present invention comprises a microparticle poly(meth)acrylate (salt)-based absorbent resin, which contains a water-soluble polyalkylene glycol with a weight-average molecular weight of not more than 2000, which substantially does not contain a liquid permeability enhancer, and which satisfies the following (1) to (5):

[0081] (1) The centrifugation retention capacity (CRC) is not less than 20 g / g and not more than 35 g / g;

[0082] (2) The pressure absorption capacity (AAP) measured at a load of 0.7 psi is not less than 25 g / g;

[0083] (3) The saline fluid conductivity (SFC) is not less than 15 (×10-7 cm3·s / g);

[0084] (4) The free swelling rate (FSR) is not less than 0.33 g / (g·s); and

[0085] (5) The initial water absorption rate (T20) under load shall not exceed 145 seconds.

[0086] The inventors of this invention conducted diligent research to achieve this objective and discovered that water-soluble polyalkylene glycols containing a specific molecular weight of absorbent resin can achieve an improved initial absorption rate (T20) under load even without substantially adding a liquid permeability enhancer. Based on this discovery, the inventors of this invention completed the present invention. The initial absorption rate under load (e.g., T20) affects the liquid absorbency or rewetting of absorbent articles. For example, absorbent articles (e.g., paper diapers) manufactured from absorbent resins having a rapid initial absorption rate under load (e.g., T20) can rapidly absorb excretions such as urine and can inhibit or reduce the leakage (rewetting) of excretions. Furthermore, no liquid permeability enhancer is added to the aforementioned absorbent resin. This allows for the suppression or reduction of problems involving dust, such as dust generation and / or filter clogging that may otherwise occur during the manufacture of the absorbent resin and in the paper diaper production line.

[0087] [2-1] Poly(meth)acrylate (salt)-based water-absorbing resin

[0088] In embodiments of the present invention, "poly(meth)acrylate (salt)-based hydrophilic superabsorbent resin" refers to a hydrophilic crosslinked polymer obtained by polymerizing and crosslinking a monomer composition containing (meth)acrylate (salt)-based monomers.

[0089] In this document, "(meth)acrylate (salt)" refers to (meth)acrylate and / or its salts. In this document, "monomer composition containing (meth)acrylate (salt) monomers" refers to a monomer composition containing not less than 50 mol% (meth)acrylate (salt) relative to all monomers except crosslinking agents.

[0090] In other words, a poly(meth)acrylate (salt)-based superabsorbent polymer is a crosslinked polymer containing not less than 50 mol% of structural units derived from (meth)acrylate (salt) relative to all structural units constituting the poly(meth)acrylate (salt)-based superabsorbent polymer, and may further contain grafted components as needed.

[0091] The poly(meth)acrylate (salt)-based superabsorbent resin is a crosslinked polymer made from a portion of the monomers involved in the polymerization reaction (which does not contain an internal crosslinking agent) containing not less than 50 mol%, preferably not less than 70 mol%, more preferably not less than 90 mol%, and more preferably not more than 100 mol%, more preferably substantially 100 mol% of (meth)acrylate (salt).

[0092] The absorbent article manufactured by the manufacturing method according to an embodiment of the present invention contains absorbent resin in particulate form. The absorbent resin in particulate form (particulate absorbent resin) may have, for example, a non-uniformly pulverized shape (non-uniform shape), a spherical shape, a fibrous shape, a strip shape, a substantially spherical shape, or a flat shape. Among these shapes, the particulate absorbent resin is preferably at least partially in a non-uniformly pulverized shape. Considering its use in absorbent articles such as baby diapers, from the viewpoint of low liquid (urine) diffusion and low possibility of shedding from the pulp, the absorbent resin is more preferably in a non-uniform shape among the above-mentioned particle shapes. The term "particulate absorbent resin" includes both individual particles of the particulate absorbent resin and aggregates of multiple particles of the absorbent resin. The expressions "in particulate form" and "particulate" each refer to particle form. "Particle" refers to a tiny solid or liquid particulate object having a measurable size (Japanese Industrial Standard Technical Terminology Glossary, 4th Edition, p. 2002).

[0093] <Single>

[0094] "Monomer" refers to the raw material component (monomer) that constitutes the absorbent resin (polymer). In addition to the (meth)acrylate (salt)-based monomer as the main component, examples of "monomer" also include (meth)acrylate (salt)-based monomers and monomers other than the internal crosslinking agent. That is, all monomers constituting the absorbent resin constitute a monomer composition. Examples of (meth)acrylate (salt)-based monomers include (meth)acrylic acid and its salts.

[0095] Among monomers having unsaturated double bonds (vinyl unsaturated monomers), monomers other than (meth)acrylate (salt) monomers are preferably monomers containing acid groups. Specific examples of such monomers include anionic unsaturated monomers such as maleic acid (maleic anhydride), fumaric acid, crotonic acid, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluenesulfonic acid, vinyl toluenesulfonic acid, styrene sulfonic acid, 2-(meth)acryloylamino-2-methyl propanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, and 2-hydroxyethyl(meth)acryloyl phosphate and / or their salts. One of these monomers can be used alone, or two or more can be used in combination as needed.

[0096] Examples of salts may include alkali metal salts, ammonium salts, amine salts, etc. Salts are more preferably sodium salts, potassium salts, lithium salts, or ammonium salts, and particularly preferably sodium salts.

[0097] Monomer compositions containing (meth)acrylate monomers are preferably neutralized to 10 mol% to 90 mol%, more preferably 40 mol% to 80 mol%, and particularly preferably 60 mol% to 75 mol%.

[0098] Therefore, it is preferable to neutralize monomer compositions containing (meth)acrylate monomers with a neutralizing solution containing: an alkaline compound, such as an alkali metal hydroxide (such as sodium hydroxide, potassium hydroxide, or lithium hydroxide); a carbonate, such as sodium carbonate or potassium carbonate; or ammonia. It is particularly preferred to neutralize the above monomer compositions with a neutralizing solution containing sodium hydroxide.

[0099] In addition to the monomers described above, such as "(meth)acrylate (salt) monomers" and "monomers other than (meth)acrylate (salt) monomers," the monomer composition may further contain, as needed, "hydrophilic or hydrophobic unsaturated monomers (hereinafter referred to as "additional monomers"). Examples of additional monomers include: unsaturated monomers containing thiol groups; unsaturated monomers containing phenolic hydroxyl groups; unsaturated monomers containing amide groups, such as N-vinyl-2-pyrrolidone, N-vinylacetamide, (meth)acrylamide, N-isopropyl(meth)acrylamide, N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; and unsaturated monomers containing amino groups, such as N,N-dimethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide. The amount of additional monomer to be used may be such that the physical properties of the resulting absorbent resin are not impaired. Specifically, the amount of additional monomer to be used relative to a portion of the monomer composition excluding the internal crosslinking agent is no more than 50 mol%, more preferably no more than 20 mol%.

[0100] <Internal cross-linking agent>

[0101] Water-absorbing resins undergo internal cross-linking through an internal cross-linking agent.

[0102] Examples of internal crosslinking agents include N,N'-methylenebis(meth)acrylamide, polyethylene (poly)di(meth)acrylate, propylene (poly)di(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerol tri(meth)acrylate, glycerol acrylate methacrylate, ethylene oxide modified trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkane, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, pentaerythritol, ethylenediamine, polyethyleneimine, and glycidyl methacrylate. At least one of these internal crosslinking agents is selected, taking into account reactivity, etc.

[0103] In embodiments of the present invention, from the viewpoint of the water absorption properties of the water-absorbing resin, an internal crosslinking agent having two or more polymerizable unsaturated groups is preferred as the internal crosslinking agent, and an internal crosslinking agent having two or more polymerizable unsaturated groups having a (poly)alkylene glycol structure is more preferred. Specific examples of polymerizable unsaturated groups include allyl and (meth)acrylate groups. Among them, (meth)acrylate groups are preferred. Examples of internal crosslinking agents having two or more polymerizable unsaturated groups having a (poly)alkylene glycol structure include polyethylene di(meth)acrylate. The number of alkylene glycol units (hereinafter indicated by "n") is preferably not less than one, more preferably not less than two, even more preferably not less than four, particularly preferably not less than six, and preferably not more than 100, more preferably not more than 50, even more preferably not more than 20, and particularly preferably not more than 10.

[0104] The amount of internal crosslinking agent to be used, relative to a portion of the monomer composition excluding the internal crosslinking agent, is preferably not less than 0.0001 mol%, more preferably not less than 0.001 mol%, even more preferably not less than 0.01 mol%, and preferably not more than 10 mol%, more preferably not more than 5 mol%, even more preferably not more than 1 mol%. By setting the amount of internal crosslinking agent used within the above range, a water-absorbing resin with the desired water absorption properties can be obtained. Meanwhile, setting the amount of internal crosslinking agent used outside the above range results in a decrease in gel strength, thereby leading to an increase in water-soluble content and / or a decrease in absorption capacity.

[0105] <Surface Crosslinking Agent>

[0106] The absorbent article manufactured by the manufacturing method according to an embodiment of the present invention comprises a surface-crosslinked water-absorbing resin. The surface crosslinking agent used for this purpose can be any surface crosslinking agent described in U.S. Patent No. 7,183,456. At least one of these surface crosslinking agents is selected, taking into account reactivity, etc. From the viewpoints of ease of disposal of the surface crosslinking agent and the water-absorbing properties of the water-absorbing resin, it is preferable to select a surface crosslinking agent having two or more functional groups that react with carboxyl groups and is an organic compound that forms a covalent bond.

[0107] Specific examples of surface crosslinking agents include: polyol compounds such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 2,3-pentanediol, 2,4-pentanediol, 1,2-hexanediol, 1, 3-Hexanediol, 1,4-Hexanediol, 1,5-Hexanediol, 1,6-Hexanediol, 2,3-Hexanediol, 2,4-Hexanediol, glycerol, polyglycerol, diethanolamine, and triethanolamine; polyamine compounds, such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyallylamine, and polyethyleneimine; halogenated epoxy compounds and condensates of polyamine compounds with halogenated epoxy compounds; oxazoline compounds, such as 1,2-ethylenediamine... Oxazolinones; oxazolidinone compounds; alkylene carbonate compounds, such as 1,3-dioxolane-2-one (ethylene carbonate), 4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxane-2-one, 4-methyl- 1,3-Dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one, and 1,3-dioxoheptane-2-one; polyfunctional glycidyl compounds, such as ethylene glycol diglycidyl ether, polyethylene diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, and glycidyl; oxetane compounds; vinyl ether compounds; and cyclic urea compounds. These compounds may be used alone or in combination of two or more.

[0108] More preferably, the absorbent article manufactured by the manufacturing method according to an embodiment of the invention comprises a surface-crosslinked absorbent resin having a non-uniformly pulverized shape. This is preferred because it provides absorbent performance under load and a low likelihood of shedding from the pulp. In particular, the absorbent article manufactured by the manufacturing method according to an embodiment of the invention comprises a surface-crosslinked absorbent resin having a non-uniformly pulverized shape relative to the total absorbent resin, preferably not less than 50% by mass, more preferably not less than 70% by mass, and even more preferably not less than 90% by mass.

[0109] [2-2] Polyalkylene glycol

[0110] The absorbent resin contained in the absorbent article manufactured by the manufacturing method according to an embodiment of the present invention contains a water-soluble polyalkylene glycol with a weight average molecular weight of not more than 2000. In this document, the statement "the absorbent resin contains a polyalkylene glycol" means that the polyalkylene glycol is present within the absorbent resin. In this state, the polyalkylene glycol only needs to be mostly present within the absorbent resin, and a portion of the polyalkylene glycol may also be present on the surface of the absorbent resin. That is, as long as the polyalkylene glycol is not only present on or near the surface, a portion of the polyalkylene glycol may be present on the surface.

[0111] Examples of polyalkylene glycols include polyalkylene glycols having a structure expressed by the following general formula (1).

[0112] H-(OR)n-OH…(1).

[0113] In general formula (1), R can be a C2-4 alkylene group, which can be either straight-chain or branched. In general formula (1), n ​​has an average value of 4 to 50, more preferably 6 to 15.

[0114] In general formula (1), the oxidized alkenyl groups (-OR-) in a molecule can be the same as each other or can be two or more different kinds. More specific examples of polyalkylene glycols include polyethylene glycol, polypropylene glycol, polyethylene glycol-polypropylene glycol copolymers, and polyethylene glycol-polypropylene glycol-polybutane glycol copolymers. One of these polyalkylene glycols can be used alone or in combination of two or more.

[0115] In one embodiment of the invention, the polyalkylene glycol is water-soluble. The term "water-soluble" as used herein means that at least 5 g, more preferably at least 10 g, of the substance can be dissolved in 100 g of water at 25°C. Water-soluble polyalkylene glycols are soluble in aqueous solutions of monomers and can therefore be suitably contained in water-absorbing resins.

[0116] In embodiments of the present invention, the polyalkylene glycols used have a weight-average molecular weight of no more than 2000. By selecting polyalkylene glycols with a weight-average molecular weight of no more than 2000, the water-absorbing resin containing such polyalkylene glycols can achieve an improved initial water absorption rate under load. The weight-average molecular weight of the polyalkylene glycols is preferably no less than 200, more preferably no less than 300, and even more preferably no less than 400. A weight-average molecular weight is more preferably no more than 1500, and even more preferably no more than 1000. It should be noted that the weight-average molecular weight of the polyalkylene glycols is a value measured by gel permeation chromatography.

[0117] The content of polyalkylene glycol in the superabsorbent resin is preferably 0.01% to 1% by mass relative to the entire superabsorbent resin. Setting the content of polyalkylene glycol to fall within the range of 0.01% to 1% by mass imparts excellent initial water absorption rate (e.g., T20) to the superabsorbent resin under load. The content of polyalkylene glycol is more preferably not less than 0.05% by mass, even more preferably not less than 0.10% by mass, further more preferably not less than 0.15% by mass, and even more preferably not less than 0.20% by mass. The content of polyalkylene glycol is more preferably not more than 0.80% by mass, even more preferably not more than 0.60% by mass, and particularly preferably not more than 0.40% by mass.

[0118] The content of polyalkylene glycols in superabsorbent resins can be determined by extracting the water-soluble components from the superabsorbent resin and analyzing them using liquid chromatography. The extraction method is not limited to any particular method and can be selected from known methods. When superabsorbent resins are classified into various particle size ranges and their content is analyzed in each range, the presence of polyalkylene glycols in the superabsorbent resin can be demonstrated by confirming that the polyalkylene glycols exhibit equal content across all particle size ranges and no particle size dependence. This verification is supported by the fact that, in the state where polyalkylene glycols are present on the surface of the superabsorbent resin, the content of polyalkylene glycols in small-particle superabsorbent resins is greater than that in large-particle superabsorbent resins. Furthermore, in the state where polyalkylene glycols are contained within the superabsorbent resin, the content of polyalkylene glycols is uniform across all particle sizes.

[0119] [2-3] Liquid permeability enhancer

[0120] The absorbent articles manufactured by the manufacturing method according to embodiments of the present invention contain absorbent resins that substantially do not contain liquid permeability enhancers. The expression "substantially do not contain" herein means either a state where liquid permeability enhancers are not present or where liquid permeability enhancers are present but the content of the liquid permeability enhancers is less than 0.1% by mass (preferably less than 0.001% by mass) relative to the total absorbent resin. Absorbent resins containing not less than 0.1% by mass of liquid permeability enhancers tend to exhibit deteriorated physical properties, such as lower absorbency and / or lower absorption rate. In one embodiment of the invention, even without the addition of liquid permeability enhancers, the initial absorption rate of the absorbent resin under load (e.g., T20) can be increased. Furthermore, problems involving dust, such as dust generation and / or filter clogging that may otherwise occur during the manufacture of the absorbent resin and in paper diaper production lines, can be suppressed or reduced.

[0121] Liquid permeability enhancers are spacers (carriers) that maintain the space between particles of absorbent resin even under load. Examples of liquid permeability enhancers include those containing cationic polymers and those containing inorganic substances. Examples of liquid permeability enhancers containing inorganic substances include polyvalent metal salts and water-insoluble inorganic particles. Polyvalent metal salts contain divalent or higher-valent metal cations, preferably trivalent or higher-valent metal cations. Examples of trivalent or higher-valent metal cations include aluminum, zirconium, and titanium. Examples of polyvalent metal salts include polyvalent metal compounds, such as inorganic salts of polyvalent metals, such as aluminum sulfate, aluminum chloride, zirconium oxide chloride, ammonium zirconium carbonate, potassium zirconium carbonate, potassium zirconium carbonate, zirconium sulfate, zirconium hydroxide chloride, and zirconium nitrate; and organic salts of polyvalent metals, such as aluminum acetate, aluminum lactate, zirconium acetate, triethanolamine titanium, and titanium lactate.

[0122] Examples of water-insoluble inorganic particles include: water-insoluble inorganic powders in the form of microparticles such as: silica, titanium dioxide, alumina, magnesium oxide, zinc oxide, talc, metal phosphate salts (such as calcium phosphate, barium phosphate, and aluminum phosphate), metal borate salts (such as titanium borate, aluminum borate, iron borate, magnesium borate, manganese borate, and calcium borate), silicic acid or its salts, clay, diatomaceous earth, zeolite, bentonite, kaolin, hydrotalcite, and activated clay; and organic microparticles such as calcium lactate, aluminum lactate, and metal soaps (polyvalent metal salts of long-chain fatty acids). Typically, the inorganic fine particles have a volume average particle size of no more than 10 μm. In this embodiment, a water-absorbing resin with an improved initial water absorption rate (e.g., T20) under load can be obtained without substantially adding these polyvalent metal salts and water-insoluble inorganic particles.

[0123] [2-4] Physical properties of water-absorbing resins

[0124] The centrifugal retention capacity (CRC) of the absorbent resin contained in the absorbent article manufactured by the manufacturing method according to an embodiment of the present invention is preferably not less than 20 g / g, more preferably not less than 25 g / g, even more preferably not less than 28 g / g, and particularly preferably not less than 29 g / g. A higher upper limit for CRC is preferred. However, considering the balance between CRC and other physical properties, the CRC is preferably not greater than 35 g / g, more preferably not less than 32 g / g, even more preferably not greater than 31 g / g, and particularly preferably not greater than 30 g / g.

[0125] The absorbent resin contained in the absorbent article manufactured by the manufacturing method according to an embodiment of the present invention preferably has an absorbent pressure absorbency (AAP) of not less than 25 g / g, more preferably not less than 25.2 g / g, even more preferably not less than 25.4 g / g, particularly preferably not less than 25.6 g / g, and most preferably not less than 25.8 g / g, measured under a load of 0.7 psi. There is no particular upper limit to the AAP. However, considering the balance between AAP and other physical properties, the AAP is preferably not less than 30 g / g.

[0126] When the AAP is set to be not less than 25 g / g, the amount of liquid flowing out of the absorbent when pressure is applied (typically referred to as "rewetting") is not as large. Therefore, absorbents with an AAP of not less than 25 g / g are suitable for use in absorbent products such as paper diapers. It should be noted that AAP can be controlled by particle size, surface crosslinking agents, etc.

[0127] The saline flow conductivity (SFC) of the absorbent resin contained in the absorbent article manufactured by the manufacturing method according to an embodiment of the present invention is preferably not less than 15 × 10⁻⁷ cm³·s / g, more preferably not less than 18 × 10⁻⁷ cm³·s / g, even more preferably not less than 20 × 10⁻⁷ cm³·s / g, and particularly preferably not less than 30 × 10⁻⁷ cm³·s / g. A higher upper limit for SFC is preferred. There is no particular limitation on the upper limit of SFC.

[0128] The free swelling rate (FSR) of the absorbent resin contained in the absorbent article manufactured by the manufacturing method according to an embodiment of the invention is preferably not less than 0.33 g / (g·s), more preferably not less than 0.36 g / (g·s), even more preferably not less than 0.39 g / (g·s), and particularly preferably not less than 0.42 g / (g·s). FSR is related to the surface area of ​​the absorbent resin and the non-uniformly pulverized particle shape. It is assumed that absorbent resins with higher FSR have a larger surface area and / or a more non-uniformly pulverized particle shape (i.e., a particle shape more different from spherical).

[0129] The absorbent article manufactured by the manufacturing method according to an embodiment of the present invention comprises an absorbent resin having an initial absorption rate (T20) under load of no more than 145 seconds. T20 is preferably no more than 140 seconds, more preferably no more than 135 seconds, even more preferably no more than 130 seconds, no more than 120 seconds, no more than 110 seconds, and no more than 100 seconds. By setting the T20 of the absorbent resin suitable for absorbent articles such as paper diapers within the above range, the absorbent article can rapidly absorb excretions such as urine and can inhibit or reduce the seepage (rewetting) of excretions.

[0130] The manufacturing method according to embodiments of the present invention polymerizes monomers and produces a crosslinked hydrogel polymer containing a specific polyalkylene glycol during or after polymerization to prepare a water-absorbing resin containing a specific polyalkylene glycol. The CRC (gel CRC) of the crosslinked hydrogel is adjusted to a given range, and various physical properties (CRC, AAP, SFC, FSR) of the final water-absorbing resin are controlled within specified ranges. Therefore, a water-absorbing resin with a small T20 value and excellent initial water absorption rate under load can be manufactured. In other words, the water-absorbing resin obtained by the above manufacturing method and having various physical properties within the above range has a small T20 value and excellent initial water absorption rate under load. It should be noted that the various physical properties can be controlled, for example, by the particle size, particle shape, and / or surface crosslinking of the water-absorbing resin.

[0131] In the physical properties of the superabsorbent polymer, AAP(0.7) / SFC* is preferably not less than 0.2, not less than 0.3, not less than 0.4, not less than 0.5, not less than 0.6, not less than 0.7, and more preferably not greater than 5.0, not greater than 4.0, not greater than 3.0, not greater than 2.0, not greater than 1.5, and not greater than 1.3. Here, SFC* = SFC / (1×10-7).

[0132] The absorbent article manufactured by the manufacturing method according to an embodiment of the present invention comprises an absorbent resin having an initial absorbency rate (T15) under load of no more than 90 seconds. T15 is more preferably no more than 80 seconds, even more preferably no more than 75 seconds, and still more preferably no more than 70 seconds. By setting the T15 of the absorbent resin suitable for absorbent articles such as paper diapers within the above range, the absorbent article can rapidly absorb excretions such as urine, thereby inhibiting or reducing the seepage (rewetting) of excretions.

[0133] The absorbent article manufactured by the manufacturing method according to an embodiment of the present invention comprises an absorbent resin having an initial absorption rate (T10) under load of no more than 50 seconds. T10 is more preferably no more than 46 seconds, even more preferably no more than 43 seconds, and still more preferably no more than 40 seconds. By setting the T10 of the absorbent resin suitable for absorbent articles such as paper diapers within the above range, the absorbent article can rapidly absorb excretions such as urine, thereby inhibiting or reducing the seepage (rewetting) of excretions.

[0134] The absorbent article manufactured by the manufacturing method according to an embodiment of the present invention comprises an absorbent resin having an initial absorption rate (T5) under load of no more than 25 seconds. T5 is more preferably no more than 22 seconds, even more preferably no more than 21 seconds, and still more preferably no more than 19 seconds. By setting the T5 of the absorbent resin suitable for absorbent articles such as paper diapers within the above range, the absorbent article can rapidly absorb excretions such as urine, thereby inhibiting or reducing the seepage (rewetting) of excretions.

[0135] The sum of the initial water absorption rates (T5, T10, T15) of the absorbent resin contained in the absorbent article manufactured by the manufacturing method according to an embodiment of the present invention (T5+T10+T15) under load is preferably no more than 160 seconds, more preferably no more than 140 seconds, even more preferably no more than 135 seconds, and particularly preferably no more than 130 seconds. By setting the T5+T10+T15 value of the absorbent resin suitable for absorbent articles such as paper diapers within the above range, the article can rapidly absorb excretions such as urine, thereby inhibiting or reducing the seepage (rewetting) of excretions.

[0136] The absorbent article manufactured by the manufacturing method according to an embodiment of the present invention contains a water-absorbing resin having a moisture content of, for example, no more than 15% by mass, preferably no more than 10% by mass, more preferably no more than 5% by mass, even more preferably no more than 3% by mass, particularly preferably no more than 1% by mass, and particularly preferably substantially 0% by mass. For example, the moisture content is controlled by drying or surface crosslinking, and, if necessary, by adding water or further drying to achieve the aforementioned moisture content. Setting the moisture content to no more than 15% by mass can suppress or reduce the coloring and adhesion of the water-absorbing resin, as well as the reduction in its absorption capacity. Furthermore, setting the moisture content to no more than 15% by mass can suppress or reduce the reduction in the initial water absorption rate (T20, T5, T10, T15) under load in the water-absorbing resin.

[0137] [3] Method for manufacturing water-absorbing resin

[0138] A method for manufacturing an absorbent article containing a water-absorbing resin according to an embodiment of the present invention is a method comprising manufacturing a surface-crosslinked particulate poly(meth)acrylate (salt)-based water-absorbing resin, the method comprising the following steps:

[0139] (i) Preparation of an aqueous solution of (meth)acrylate (salt) monomer;

[0140] (ii) Aqueous solution of polymerized (meth)acrylate (salt) monomer;

[0141] (iii) Gel disruption of the cross-linked hydrogel polymer generated during or after polymerization to obtain microparticle hydrogels;

[0142] (iv) Drying the microparticle hydrogel to obtain a dried polymer;

[0143] (v) Pulverize and / or classify the dried polymer to obtain particulate water-absorbing resin before surface crosslinking;

[0144] (vi) Crosslinking the surface of the microparticle-based water-absorbing resin before surface crosslinking; and

[0145] (vii) Incorporating surface-crosslinked microparticle water-absorbing resin into the absorbent product, wherein

[0146] In step (i) and / or step (ii), a water-soluble polyalkylene glycol with a weight-average molecular weight of not more than 2000 is added to the aqueous monomer solution, such that the total amount of the water-soluble polyalkylene glycol added in step (i) and / or step (ii) is 0.01% to 1% by mass relative to the total mass of the monomer contained in the aqueous monomer solution.

[0147] The cross-linked hydrogel polymer has a centrifugal retention capacity of not more than 31 g / g and not less than 20 g / g, and

[0148] The surface-crosslinked microparticle poly(meth)acrylate (salt)-based superabsorbent resin obtained by the method satisfies the following (1) to (4):

[0149] (1) The centrifugation retention capacity (CRC) is not less than 20 g / g and not more than 35 g / g;

[0150] (2) The pressure absorption capacity (AAP) measured under a load of 0.7 psi is not less than 25 g / g;

[0151] (3) The saline fluid conductivity (SFC) is not less than 15 (×10⁻⁷ cm³·s / g); and

[0152] (4) Free swelling rate (FSR) is not less than 0.33 g / (g·s).

[0153] That is, in the method for manufacturing an absorbent article comprising a water-absorbing resin according to an embodiment of the present invention, a water-soluble polyalkylene glycol with a weight average molecular weight of not more than 2000 is added in step (i) of preparing an aqueous solution of a (meth)acrylate monomer or in step (ii) of polymerizing the aqueous solution of the (meth)acrylate monomer. Alternatively, the polyalkylene glycol may be added in both steps (i) and (ii). Therefore, an absorbent resin containing a polyalkylene glycol can be manufactured.

[0154] The method for adding polyalkylene glycols is not limited to any particular one. Polyalkylene glycols can be added directly to aqueous solutions or mixtures of monomers during polymerization. Alternatively, polyalkylene glycols can be added to solutions, particularly aqueous solutions.

[0155] The centrifugal retention capacity of the crosslinked hydrogel polymer generated during or after polymerization in step (ii) has an upper limit of not more than 31 g / g, preferably not more than 30 g / g, and a lower limit of not less than 20 g / g, preferably not less than 25 g / g. It is anticipated that setting the centrifugal retention capacity of the crosslinked hydrogel polymer containing polyalkylene glycol within a given range will result in the production of a water-absorbing resin having the form described later. Therefore, it is assumed that the water-absorbing resin has, for example, a lower T20 value and an improved initial water absorption rate under load. A method for measuring the centrifugal retention capacity of the crosslinked hydrogel polymer will be described later.

[0156] A method for manufacturing an absorbent article comprising a water-absorbing resin according to an embodiment of the present invention may include the following steps: checking whether the cross-linked hydrogel polymer to be gelled in step (iii) is a cross-linked hydrogel polymer having a centrifugal retention capacity within the aforementioned range. The method for checking whether the centrifugal retention capacity of the cross-linked hydrogel polymer is within the aforementioned range may, for example, involve cutting off a portion of the cross-linked hydrogel polymer generated during or after polymerization in step (ii), using this portion of the cross-linked hydrogel polymer as a test sample, measuring the centrifugal retention capacity of the test sample, and checking whether the measured value of the centrifugal retention capacity is within the aforementioned range. It should be noted that after confirming that the centrifugal retention capacity value of the test sample is within the aforementioned range, the remaining portion of the cross-linked hydrogel polymer not used for measuring the centrifugal retention capacity is gelled, thereby performing the gelling in step (iii). Alternatively, if steps (i) and (ii) are performed under the condition that it is previously confirmed that a cross-linked hydrogel polymer with a centrifugal retention capacity value within the aforementioned range can be manufactured, the gelling in step (iii) may be performed without checking the centrifugal retention capacity of the test sample.

[0157] Gathering

[0158] By setting the centrifugal retention capacity of the crosslinked hydrogel polymer within a given range, a certain degree of high strength can be achieved. During the gel-breaking step, the crosslinked hydrogel polymer forms secondary aggregates through the aggregation of primary particles. In this regard, considering the high strength and therefore resistance to deformation of the crosslinked hydrogel polymer as primary particles, and the fact that the crosslinked hydrogel polymer contains specific polyalkylene glycols, the secondary aggregates are assumed to have a number of portions in which the primary particles are loosely bound together. Similar to the aforementioned secondary aggregates, the water-absorbing resin shown in the embodiments of the present invention, obtainable through gel breaking, drying, pulverizing and / or grading, and surface crosslinking of the crosslinked hydrogel polymer, is also assumed to have a number of portions in which the primary particles are loosely bound together. Here, it is assumed that the loosely bound portions have voids. Even if such voids cannot be confirmed, these portions are considered to have the ability to absorb liquids such as water. It is assumed that the water-absorbing resin shown in the embodiments of the present invention has such a form that it has a number of portions with the ability to absorb liquids such as water, thereby having a smaller T20 value and an improved initial water absorption rate under load.

[0159] In addition to the above, it is necessary to set the various physical properties (CRC, AAP, SFC, FSR) of the final absorbent resin within the specified range. As a result, the initial water absorption rate of the absorbent resin under load in the manufactured absorbent article can be improved, and the T20 value of the absorbent resin can be reduced to, for example, no more than 145 seconds.

[0160] The amount of polyalkylene glycol to be added is 0.01% to 1% by mass relative to the total mass of monomers contained in the aqueous monomer solution. Setting the polyalkylene glycol content to fall within the range of 0.01% to 1% by mass yields a water-absorbing resin with excellent initial water absorption rate under load (e.g., T20). The amount of polyalkylene glycol added is preferably not less than 0.05% by mass, more preferably not less than 0.10% by mass, even more preferably not less than 0.15% by mass or more, and particularly preferably not less than 0.20% by mass. The amount of polyalkylene glycol added is preferably not more than 0.80% by mass, more preferably not more than 0.60% by mass, and even more preferably not more than 0.40% by mass. The amount of polyalkylene glycol added is preferably adjusted such that the content of polyalkylene glycol is 0.01% to 1% by mass relative to the microparticle poly(meth)acrylate (salt)-based water-absorbing resin crosslinked throughout the entire surface.

[0161] The physical properties of the resulting poly(meth)acrylate (salt)-based water-absorbing resin, polyalkylene glycol, liquid permeability enhancer, and water-absorbing resin are as described in "[2] Water-absorbing resin".

[0162] [3-1] Preparation steps of aqueous solution of (meth)acrylate (salt) monomer (step (i))

[0163] This step involves preparing an aqueous solution of the monomer composition (particularly a monomer composition containing a (meth)acrylate (salt) monomer as a major component and at least one internal crosslinking agent) as a raw material for preparing the superabsorbent resin (polymer). It should be noted that a slurry of the monomer composition can also be used. However, for convenience, this description pertains to the use of an aqueous solution of the monomer composition.

[0164] (monomer)

[0165] As described above in "[Poly(meth)acrylate (salt)-based superabsorbent resin]", the monomers used in this step are the raw material components (monomers) that form the superabsorbent resin (polymer), and include (meth)acrylate (salt)-based monomers, monomers other than (meth)acrylate (salt)-based monomers, and internal crosslinking agents. That is, all the monomers that form the superabsorbent resin are equivalent to a monomer composition.

[0166] (Neutralize with an alkaline compound)

[0167] As described above, in embodiments of the invention, it is preferable to use an alkaline compound to neutralize the (meth)acrylic acid (salt) portion herein. That is, in embodiments of the invention, it is preferable to include a water-absorbing resin comprising poly(meth)acrylic acid having partially neutralized acid groups.

[0168] Examples of basic compounds include alkali metal carbonates and bicarbonates, alkali metal hydroxides, ammonia, and organic amines. From the viewpoint of the absorbent properties of the absorbent resin, compounds with strong basicity are selected. It should be noted that, from the viewpoint of ease of handling, basic compounds are preferably in the form of aqueous solutions.

[0169] Neutralization can be performed before, during, or after polymerization. It can be done in multiple stages or at various times. From the viewpoint of manufacturing efficiency of the superabsorbent resin, continuous neutralization is preferred.

[0170] As described above, in embodiments of the present invention, the neutralization rate of (meth)acrylic acid (salt) relative to the acid groups of the monomer composition is preferably not less than 10 mol%, more preferably not less than 40 mol%, even more preferably not less than 50 mol%, particularly preferably not less than 60 mol%, and preferably not more than 90 mol%, more preferably not more than 85 mol%, even more preferably not more than 80 mol%, particularly preferably not more than 75 mol%. Setting the neutralization rate within the above range can suppress or reduce the deterioration of the water absorption properties of the water-absorbing resin. It should be noted that the neutralization rate applies to any of the following: pre-polymerization neutralization, polymerization neutralization, and post-polymerization neutralization. The neutralization rate also applies to water-absorbing resins.

[0171] (Internal cross-linking agent)

[0172] The internal crosslinking agent used in this step and the amount of internal crosslinking agent are as described in the above "[Poly(meth)acrylate (salt)-based superabsorbent resin]".

[0173] In one embodiment of the invention, the internal crosslinking agent is added whenever it enables the polymer to crosslink uniformly. For example, the internal crosslinking agent can be added to the aqueous solution of the monomer composition before polymerization, or it can be added to the hydrogel during or after polymerization. A method of pre-adding a given amount of internal crosslinking agent to the aqueous solution of the monomer composition is particularly preferred.

[0174] (Polyalkylene glycol)

[0175] In a preferred embodiment of the invention, a polyalkylene glycol can be added to an aqueous solution of the monomer composition. The addition of the polyalkylene glycol can be carried out in the polymerization step described later. The amount of polyalkylene glycol added is as described above. In particular, it is preferable to add the polyalkylene glycol during the step of preparing the aqueous solution of the monomer composition.

[0176] (Additional substances added to aqueous solutions of monomer compositions)

[0177] In one embodiment of the invention, in order to improve the physical properties of the water-absorbing resin, the additional substance described later may be added to the aqueous solution of the monomer composition, the solution during the reaction, or the solution after the reaction at at least one of the following times: when preparing the aqueous solution of the monomer composition, during the polymerization and crosslinking reactions, and after the polymerization and crosslinking reactions.

[0178] Examples of additives include: hydrophilic polymers such as starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol (PVA), polyacrylic acid (salt), existing cross-linked polyacrylic acid (salt); and compounds such as carbonates, azo compounds, foaming agents that generate various bubbles, surfactants, chelating agents, and chain transfer agents.

[0179] When a hydrophilic polymer is used, a water-absorbing resin grafted with the hydrophilic polymer can be obtained. For example, a polyacrylate (salt)-based water-absorbing resin grafted with starch and a polyacrylate (salt)-based water-absorbing resin grafted with PVA can be obtained. These grafted water-absorbing resins are also included in the scope of polyacrylate (salt)-based water-absorbing resins.

[0180] The amount of additives added is adjusted so as not to impair the effects of the invention. For example, relative to an aqueous solution of the monomer composition, the total amount of additives added is preferably no more than 50% by mass, more preferably no more than 20% by mass, even more preferably no more than 10% by mass, particularly preferably no more than 5% by mass, and preferably no less than 0% by mass, more preferably greater than 0% by mass.

[0181] (Monomer component concentration)

[0182] An aqueous solution containing all monomer components (i.e., a mixture of monomer compositions, polyalkylene glycols, substances, and components) can be prepared by selecting some of the monomer compositions, polyalkylene glycols, and the aforementioned substances and components (referred to as "monomer components" in this section) according to the intended purpose and mixing them in amounts within the aforementioned range. Note that in one embodiment of the invention, the aqueous solution containing all monomer components can be replaced by a solution prepared by mixing water containing all monomer components with a hydrophilic solvent.

[0183] From the viewpoint of the physical properties of the superabsorbent resin, the total concentration of the monomer components is preferably not less than 10% by mass, more preferably not less than 20% by mass, even more preferably not less than 30% by mass, and preferably not more than 80% by mass, more preferably not more than 75% by mass, even more preferably not more than 70% by mass. The total concentration of the monomer components can be obtained by adding the concentrations of the monomer components together. The concentration of each monomer component can be calculated according to the following formula (A):

[0184] Monomer concentration (mass%) = [(mass of monomer) / (mass of aqueous solution containing all monomers)] × 100 ... Equation (A).

[0185] In formula (A), "the mass of the aqueous solution containing all monomer components" does not include the mass of the hydrophobic organic solvent used in the reverse suspension polymerization.

[0186] [3-2] Aggregation steps (step (ii))

[0187] This step involves polymerizing a monomer aqueous solution to obtain a crosslinked hydrogel polymer (hereinafter referred to as "hydrogel"). Preferably, this step involves polymerizing an aqueous monomer solution obtained in the monomer aqueous solution preparation step, which includes a monomer containing acrylic acid (salt) as a major component and at least one internal crosslinking agent to obtain the hydrogel.

[0188] (Polymerization initiator)

[0189] The polymerization initiator used in embodiments of the present invention may be one or more selected from polymerization initiators commonly used to prepare superabsorbent resins, depending on the type of monomer to be polymerized, polymerization conditions, etc. Examples of polymerization initiators include thermal polymerization initiators and photopolymerization initiators.

[0190] Examples of thermal polymerization initiators include: persulfates, such as sodium persulfate, potassium persulfate, and ammonium persulfate; peroxides, such as hydrogen peroxide, tert-butyl peroxide, and methyl ethyl ketone peroxide; and azo compounds, such as azonitrile compounds, azomididine compounds, cyclic azomididine compounds, azoamide compounds, alkyl azo compounds, 2,2'-azobis(2-amidinylpropane) dihydrochloride, and 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride.

[0191] Examples of photopolymerization initiators include benzoin derivatives, benzoyl derivatives, acetophenone derivatives, benzophenone derivatives, and azo compounds.

[0192] Persulfates are preferred, considering cost and residual monomer reduction capabilities. Alternatively, redox polymerization initiators obtained by combining oxidizable polymerization initiators such as the aforementioned persulfates or peroxides with a reducing agent for promoting the decomposition of the oxidizable polymerization initiator can be used. Examples of reducing agents include: (ii) sulfites, such as sodium sulfite and sodium bisulfite; L-ascorbic acid; reducing metals, such as ferrous salts; and amines.

[0193] The amount of polymerization initiator used relative to monomers without internal crosslinking agents is preferably not less than 0.001 mol%, more preferably not less than 0.010 mol%, and more preferably not more than 1.000 mol%, more preferably not more than 0.500 mol%, and even more preferably not more than 0.100 mol%. The amount of reducing agent used relative to monomers without internal crosslinking agents is preferably not less than 0.0001 mol%, more preferably not less than 0.0005 mol%, and more preferably not more than 0.0200 mol%, more preferably not more than 0.0150 mol%. By setting the amount of each reagent within the above ranges, the desired water absorption properties can be imparted to the water-absorbing resin.

[0194] Alternatively, in one embodiment of the invention, the polymerization reaction can be initiated by irradiation with activation energy rays such as radiation, electron beams, or ultraviolet light. Alternatively, irradiation with activation energy rays and the aforementioned polymerization initiator can also be combined to initiate the polymerization reaction.

[0195] (Aggregational form)

[0196] The polymerization forms applicable to embodiments of the present invention can be, for example, aqueous solution polymerization, reverse suspension polymerization, spray polymerization, droplet polymerization, bulk polymerization, or precipitation polymerization. From the viewpoint of ease of polymerization control and the water absorption properties of the water-absorbing resin, aqueous solution polymerization or reverse suspension polymerization is preferred, and aqueous solution polymerization is more preferred. For example, aqueous solution polymerization is explained in Japanese Patent Application Publication Tokukaihei 4-255701 (1992). For example, reverse suspension polymerization is explained in International Publications WO 2007 / 004529 and WO 2012 / 023433.

[0197] Preferred forms of continuous aqueous solution polymerization include, for example, high-temperature initiation polymerization, high-concentration polymerization, or foaming polymerization. "High-temperature initiation polymerization" refers to a polymerization form in which the temperature of the monomer aqueous solution at the start of polymerization is preferably set at no less than 35°C, more preferably at no less than 40°C, even more preferably at no less than 45°C, particularly preferably at no less than 50°C, and preferably no higher than the boiling point of the monomer aqueous solution. "High-concentration polymerization" refers to a polymerization form in which the monomer concentration at the start of polymerization is preferably set at no less than 30% by mass, more preferably at no less than 35% by mass, even more preferably at no less than 40% by mass, particularly preferably at no less than 45% by mass, and preferably no higher than the saturation concentration of the monomer aqueous solution. "Foaming polymerization" refers to a polymerization form in which an aqueous solution of monomer containing a foaming agent or bubbles is polymerized. One of these polymerization forms can be carried out alone, or two or more can be combined. Aqueous solution polymerization can be carried out in batch or continuous forms. From the viewpoint of manufacturing efficiency, continuous polymerization is preferred. As a polymerization form suitable for embodiments of the present invention, a polymerization form in which the polymerization reaction is initiated immediately after the addition of the polymerization initiator is preferred. For example, a polymerization form in which the polymerization reaction is initiated within one minute after the addition of the polymerization initiator is preferred. In this polymerization form, a polymerization form in which the polymerization reaction is terminated rapidly is also preferred. For example, a polymerization form in which the polymerization reaction is terminated within one minute after the start of polymerization is preferred.

[0198] Examples of continuous aqueous solution polymerization include, for example, continuous belt polymerization described in U.S. Patent Nos. 4,893,999, 6,906,159, 7,091,253, 7,741,400, and 8,519,212, as well as Japanese Patent Application Publication Tokukai 2005-36,100, and continuous kneader polymerization described, for example, in U.S. Patent No. 6,987,151.

[0199] In foam polymerization, bubbles can be dispersed using any of the following methods. For example, the solubility of a gas dissolved in an aqueous monomer solution is reduced, causing the gas to disperse as bubbles. In another example, gas is introduced from an external source, causing it to disperse as bubbles. Yet another example is the addition of a foaming agent to the aqueous monomer solution to induce foaming. Depending on the physical properties of the target superabsorbent resin, two or more dispersion methods can be selected and combined.

[0200] In methods of introducing gas from the outside, the gas can be, for example, oxygen, air, nitrogen, carbon dioxide, ozone, or a mixture thereof. From the viewpoint of polymerizability and cost, inert gases, such as nitrogen or carbon dioxide, are preferred, with nitrogen being more preferred.

[0201] Examples of suitable foaming agents include solutions, dispersions, and powders of azo compounds and organic or inorganic carbonates with a particle size of not less than 0.1 μm and not greater than 1000.0 μm. Inorganic carbonates are preferred. Specifically, carbonates such as sodium carbonate, ammonium carbonate, magnesium carbonate, or bicarbonate can be used.

[0202] Gel breakage of the foamed hydrogel obtained by foam polymerization facilitates drying. Obtaining the superabsorbent resin in foam form increases the water absorption rate and also facilitates the fixation of the superabsorbent resin onto the absorbent article. Whether the superabsorbent resin is in foam form can be determined by confirming the presence or absence of pores on the surface of the superabsorbent resin using an electron microscope, for example, confirming the presence or absence of pores with a diameter of not less than 1 μm and not more than 100 μm. The number of pores in each superabsorbent resin particle is preferably not less than one, more preferably not less than 10, and preferably not more than 10,000, more preferably not more than 1,000. The number of pores in each superabsorbent resin particle can be controlled by adjusting the conditions during foam polymerization.

[0203] [3-3] Gel disruption steps (step (iii))

[0204] This step is a process of breaking down the hydrogel to obtain a hydrogel in particulate form, and is performed during and / or after the polymerization step. Specifically, the hydrogel may be broken down during or after the polymerization step. That is, this step is a process of gel breaking down the hydrogel to obtain a hydrogel in particulate form (hereinafter referred to as "particulate hydrogel"). It should be noted that this step is described as "gel breaking down" to distinguish it from "crushing" in the pulverization step described later. The goal of gel breaking down is not limited to the hydrogel obtained in the polymerization step. Unless otherwise stated, the goal of gel breaking down may include a particulate gel obtained by mixing an aqueous liquid with a fine powder collected in the classification step described later. Unless otherwise stated, this also applies to other steps.

[0205] Gel crushing refers to the process of adjusting the size of hydrogels to a specified size using kneaders, screw extruders (such as meat grinders), and gel crushers (such as knife grinders).

[0206] The gel disruption of the hydrogel is preferably carried out simultaneously with the addition of hot water and / or steam in a gel disruptor. Adding hot water and / or steam yields microparticle hydrogels with low adhesion and excellent air permeability, i.e., easily dried microparticle hydrogels. Therefore, adding hot water and / or steam is preferred. The temperature of the hot water is preferably not lower than 40°C, more preferably not lower than 50°C, even more preferably not lower than 60°C, and preferably not higher than 100°C.

[0207] Regarding the implementation scheme and / or operating conditions for gel disruption, the methods for aqueous solution polymerization described in the literature describing continuous aqueous solution polymerization are adopted. The content of International Publication WO 2011 / 126079 is also preferably applicable to embodiments of the present invention. Note that when kneading polymerization is used as the polymerization form, the polymerization step and the gel disruption step are performed simultaneously. By performing the gel disruption step in the embodiments of the present invention, a water-absorbing resin with a non-uniformly pulverized shape can be obtained.

[0208] A method for manufacturing an absorbent article containing a water-absorbing resin according to an embodiment of the present invention may further include: a granulation step of mixing an aqueous liquid with fine powder collected in a classification step described later to obtain a particulate gel; and a particulate gel addition step of adding the particulate gel to the hydrogel, the particulate gel addition step being performed in at least one step between the end of drying in the gel crushing step and the drying step and / or at an opportune time between these steps. Additionally, in the gel crushing step according to an embodiment of the present invention, it is preferable to appropriately control the gel milling energy. Particulate hydrogels obtained by performing gel crushing at the gel milling energy given below result in the production of a water-absorbing resin exhibiting improved physical properties, specifically, absorption rate, such as the FSR described in International Publication WO 2009 / 016055 and / or the vortex as evaluated according to the "Testing method for water absorption rate of super absorbent polymers" as defined in JIS K7224 (1996).

[0209] According to embodiments of the present invention, "gel grinding energy" refers to the unit energy required for a gel crusher to break down hydrogels, i.e., the mechanical energy per unit mass of hydrogel, excluding the energy used for heating or cooling the jacket or the energy of water and steam to be introduced. It should be noted that "gel grinding energy" is abbreviated as "GGE".

[0210] When the gel crusher is driven by three-phase AC power, GGE can be calculated according to the following formula (I):

[0211] GGE[J / g]={√3×voltage×current×power factor×motor efficiency} / {mass of hydrogel introduced into the gel crusher per second}……Equation (I).

[0212] "Power factor" and "motor efficiency" are values ​​specific to gel crushers, varying depending on the crusher's operating conditions, and ranging from 0 to 1 (inclusive). These values ​​can be obtained from the gel crusher manufacturer. In the case of a gel crusher driven by single-phase AC power, GGE can be calculated according to equation (I), where "√3" is replaced by "1". Note that the unit of voltage is [V], the unit of current is [A], and the unit of hydrogel mass is [g / s].

[0213] The "power factor" and "motor efficiency" in GGE are values ​​obtained during gel crushing. The power factor and motor efficiency values ​​during idling are roughly defined as in equation (I) above due to the low current during idling. For example, the "mass of hydrogel introduced into the gel crusher per second" [g / s] in equation (I) is a value converted to [g / s] when hydrogel is continuously supplied by a metering feeder. However, it should be noted that, as discussed later, the hydrogel may sometimes contain recycled particulate gel.

[0214] According to embodiments of the present invention, the gel grinding energy (GGE) for gel disruption is preferably no greater than 100 J / g, more preferably no greater than 80 J / g, even more preferably no greater than 60 J / g, and preferably no less than 20 J / g, more preferably no less than 25 J / g, even more preferably no less than 30 J / g. By controlling the gel grinding energy within the above range, gel disruption can be achieved while applying suitable shear force and suitable compressive force to the hydrogel.

[0215] There may be situations where multiple devices are used for gel disruption, such as using a screw extruder after kneading polymerization and / or using multiple screw extruders. In this case, the total energy consumed by the devices is considered to be gel milling energy (GGE).

[0216] Gel milling can be controlled by combining the above-described method with the addition of hot water at the above-described temperature. This can provide more effective results. Furthermore, gel disruption can be performed based on gel milling technology after conventional gel disruption.

[0217] From the viewpoint of ease of drying and the physical properties of the absorbent resin to be obtained, the particle size of the microparticle hydrogel obtained in the gel breaking step is preferably not less than 0.1 mm and not more than 10 mm. The mass-average particle size (D50) of the microparticle hydrogel is preferably from 0.1 mm to 5.0 mm, more preferably from 0.1 mm to 2.0 mm. By ensuring that the mass-average particle size (D50) of the microparticle hydrogel falls within the above range, sufficient drying can be achieved. In embodiments of the present invention, the mass-average particle size of the hydrogel to be subjected to the drying step is preferably within the above range. More preferably, both the particle size and the mass-average particle size of the hydrogel to be subjected to the drying step satisfy the above range.

[0218] The logarithmic standard deviation (σζ) representing the narrowness of the particle size distribution of the microparticle hydrogel is preferably 0.2 to 1.5, more preferably 0.2 to 1.3, and even more preferably 0.2 to 1.2. The logarithmic standard deviation (σζ) represents the narrowness of the particle size distribution. A smaller logarithmic standard deviation indicates a more uniform particle size, which advantageously allows for uniform drying. However, to set the logarithmic standard deviation (σζ) of the particle size distribution to less than 0.2, special operations, such as grading, are required on the microparticle hydrogel after gel fragmentation. Therefore, from a productivity and cost perspective, it is difficult to substantially set the logarithmic standard deviation (σζ) to less than 0.2. It should be noted that the mass-average particle size (D50) and logarithmic standard deviation (σζ) of the microparticle hydrogel are measured, for example, by the method described in WO 2021 / 140905.

[0219] For the purpose of uniform and efficient drying, the water content of the microparticle hydrogel is preferably not less than 30% by mass, more preferably not less than 45% by mass, and more preferably not more than 70% by mass, more preferably not more than 55% by mass.

[0220] [3-4] Drying steps (step (iv))

[0221] This step involves drying the hydrogel, which has already been broken down by gelation, to obtain a dried polymer. Specifically, this step involves drying the particulate hydrogel or (if particulate gel is added) both particulate hydrogel and particulate gel until a desired solids content is achieved. The solids content, i.e., the value of subtracting the water content from 100% by mass of the gel, is preferably not less than 80% by mass, more preferably not less than 85% by mass, even more preferably not less than 90% by mass, particularly preferably not less than 92% by mass, and preferably not more than 99% by mass, more preferably not more than 98% by mass, particularly preferably not more than 97% by mass. Setting the solids content of the dried polymer to fall within the above range allows for efficient crushing, grading, and surface crosslinking. It should be noted that the expression "drying complete" herein refers to a state where the solids content has reached 80% by mass. In this step, the dried polymer can be in the form of a block. The block may have different water contents in its upper, lower, middle, and edge portions. In this case, sheets can be appropriately removed from the various portions of the dried polymer and can be crushed as needed. The water content can then be measured and an average water content can be calculated.

[0222] In this document, a dried polymer having a solids content lower than expected may sometimes be referred to as "undried material". The "material to be dried" or "particulate hydrogel" in the drying step may sometimes comprise both particulate hydrogel and particulate gel. The drying steps according to embodiments of the invention can provide more efficient conditions, particularly when the "material to be dried" or "particulate hydrogel" comprises both particulate hydrogel and particulate gel. It should be noted that, also in other steps, the hydrogel and the substance obtained by treating the hydrogel may sometimes include particulate gel and the substance obtained by treating the particulate gel.

[0223] Examples of drying methods used in the drying step include thermal drying, hot air drying, vacuum drying, fluidized bed drying, infrared drying, microwave drying, drying by azeotropic dehydration with hydrophobic organic solvents, high-humidity drying using high-temperature steam, and stirred drying. From the viewpoint of drying efficiency, stirred drying or hot air drying is preferred. Stirred drying is preferably performed using stirred dryers such as paddle dryers or rotary drum dryers. Hot air drying is preferably performed using a straight-flow belt dryer that performs hot air drying on a straight flow belt. Using a straight-flow belt dryer allows for efficient drying while preventing physical damage to the polymer being dried and / or the material to be dried (e.g., particulate hydrogels during drying) and / or problems such as the generation of fine powder due to friction.

[0224] Considering drying efficiency, the drying temperature in hot air drying, i.e., the hot air temperature, is preferably not lower than 120°C, more preferably not lower than 130°C, even more preferably not lower than 150°C, and preferably not higher than 250°C, more preferably not higher than 230°C, even more preferably not higher than 200°C. The drying time is preferably 10 to 120 minutes, more preferably 20 to 90 minutes, even more preferably 30 to 60 minutes. Setting the drying temperature and drying time within the above range yields a water-absorbing resin with the desired physical properties. It should be noted that other drying conditions can also be appropriately set according to the water content, total mass, and target solids content of the particulate hydrogel and / or granular gel to be dried. In the case of belt drying, various conditions disclosed in, for example, international publications WO 2006 / 100300, WO 2011 / 025012, WO 2011 / 025013, and WO 2011 / 111657 can be appropriately applied.

[0225] [3-5] Crushing step, grading step (step (v))

[0226] The pulverizing step is the step of pulverizing the dried polymer, and the grading step is the step of removing fine powder from the pulverized dried polymer. Specifically, the pulverizing step pulverizes the dried polymer obtained through the drying step, and the grading step adjusts the particle size of the thus pulverized dried polymer to a desired range, thereby obtaining a water-absorbing resin. By subjecting the dried polymer to the pulverizing step, a water-absorbing resin with a non-uniform pulverized shape can be obtained.

[0227] Examples of pulverizing devices used in the pulverizing process include high-speed rotary mills (such as roller mills, hammer mills, auger mills, or pin mills), vibratory mills, knuckle-type mills, and cylindrical mixers. From the viewpoint of pulverizing efficiency, roller mills are preferred. Two or more of these pulverizing devices can be used in combination.

[0228] Examples of methods for adjusting particle size used in the classification step include sieve classification and air classification involving the use of JIS standard sieves (JIS Z 8801-1(2000)). From the viewpoint of classification efficiency, sieve classification is preferred. It should be noted that from the viewpoint of ease of pulverization, a classification step may also be performed prior to the pulverization step.

[0229] For the particle size distribution of the superabsorbent resin, the mass-average particle size (D50) is preferably not less than 300 μm and not more than 600 μm, and the proportion of particles smaller than 150 μm is preferably not more than 5% by mass. The upper limit of the mass-average particle size (D50) is more preferably not more than 500 μm, and even more preferably not more than 450 μm. The proportion of particles smaller than 150 μm is more preferably not more than 4% by mass, even more preferably not more than 3% by mass, and particularly preferably not more than 2% by mass. The logarithmic standard deviation (σζ) representing the narrowness of the particle size distribution is preferably not less than 0.20, more preferably not less than 0.25, even more preferably not less than 0.27, and preferably not more than 0.50, more preferably not more than 0.45, even more preferably not more than 0.43, particularly preferably not more than 0.40, and most preferably not more than 0.35. The logarithmic standard deviation (σζ) of the particle size distribution represents the narrowness of the particle size distribution. Setting a smaller logarithmic standard deviation advantageously provides more uniform particle size and less particle segregation. Preferably, the mass-average particle size (D50) and the ratio of particles smaller than 150 μm are set to fall within the aforementioned range. More preferably, the mass-average particle size (D50), the ratio of particles smaller than 150 μm, and the logarithmic standard deviation are set to fall within the aforementioned range. The mass-average particle size, particles smaller than 150 μm, and logarithmic standard deviation selected within the aforementioned range can be appropriately combined.

[0230] It should be noted that the mass-average particle size (D50) and the logarithmic standard deviation (σζ) can be measured by the measurement method described in “(3) Mass-average particle size (D50) and logarithmic standard deviation (σζ) of particle size distribution” in U.S. Patent No. 7,638,570.

[0231] The aforementioned particle size also applies to the absorbent resin after the pulverizing and grading steps. Therefore, surface crosslinking is preferably performed in the surface crosslinking step to maintain the particle size of the absorbent resin before surface crosslinking adjusted to fall within the aforementioned range. More preferably, a sieving step is provided after the surface crosslinking step to adjust the particle size. That is, for the absorbent resin contained in the absorbent article manufactured by the manufacturing method according to an embodiment of the present invention, it is also preferable to set the mass average particle size (D50) and the proportion of particles smaller than 150 μm within the aforementioned range, and more preferably, the mass average particle size (D50), the proportion of particles smaller than 150 μm, and the logarithmic standard deviation (σζ) of the particle size distribution within the aforementioned range. Even more preferably, the absorbent resin contained in the absorbent article manufactured by the manufacturing method according to an embodiment of the present invention has a mass average particle size (D50) of 300 μm to 600 μm, a proportion of particles smaller than 150 μm of no more than 5% by mass, and a logarithmic standard deviation (σζ) of the particle size distribution of 0.20 to 0.50.

[0232] [3-6] Surface crosslinking steps (step (vi))

[0233] This step involves providing a portion with a high crosslinking density to the surface layer of the pre-crosslinked absorbent resin obtained through the above steps, and includes mixing, heat treatment, and cooling steps. In the surface crosslinking step, reactions occur on the surface of the pre-crosslinked absorbent resin, such as free radical crosslinking, surface polymerization, and crosslinking reaction with a surface crosslinking agent. Therefore, a surface-crosslinked absorbent resin is obtained.

[0234] The maximum temperature (powder temperature) of the water-absorbing resin in the surface crosslinking step, that is, the maximum temperature (powder temperature) of the water-absorbing resin in the heat treatment step, is preferably not less than 180°C, and more preferably not less than 190°C.

[0235] [3-6-1] Mixing Steps

[0236] This step involves mixing a solution containing a surface crosslinking agent (hereinafter referred to as "surface crosslinking agent solution") with a water-absorbing resin before surface crosslinking in a mixing device to obtain a mixture.

[0237] (Surface crosslinking agent)

[0238] In one embodiment of the invention, a surface crosslinking agent is used in the surface crosslinking. The surface crosslinking agent is as described above in "[Polyacrylate (salt)-based water-absorbing resin]".

[0239] The amount of surface crosslinking agent (the total amount when using multiple surface crosslinking agents) is preferably 0.01 to 10.00 parts by weight, more preferably 0.01 to 5.00 parts by weight, and even more preferably 0.01 to 2.00 parts by weight, relative to 100 parts by weight of the crosslinked polymer. Setting the amount of surface crosslinking agent to fall within the above range allows an optimal crosslinking structure to be formed in the surface layer of the crosslinked polymer. Therefore, a water-absorbing resin with excellent physical properties can be obtained.

[0240] The surface crosslinking agent added to the absorbent resin before surface crosslinking is preferably in the form of an aqueous solution. In this case, the amount of water to be used is preferably 0.1 to 20.0 parts by weight, more preferably 0.3 to 15.0 parts by weight, and even more preferably 0.5 to 10 parts by weight, relative to 100 parts by weight of the absorbent resin before surface crosslinking. Setting the amount of water within the above range improves the ease of handling the surface crosslinking agent solution, thereby allowing the surface crosslinking agent to be uniformly mixed with the absorbent resin before surface crosslinking.

[0241] Furthermore, the hydrophilic organic solvent can be used in combination with water as needed to obtain a surface crosslinking agent solution. In this case, the amount of hydrophilic organic solvent to be used is preferably no more than 5 parts by mass, more preferably no more than 3 parts by mass, and even more preferably no more than 1 part by mass, relative to 100 parts by mass of the absorbent resin before surface crosslinking. Specific examples of hydrophilic organic solvents include: lower alcohols, such as methanol; ketones, such as acetone; ethers, such as dioxane; amides, such as N,N-dimethylformamide; sulfoxides, such as dimethyl sulfoxide; and polyols, such as ethylene glycol. However, the amount of hydrophilic organic solvent to be used is preferably as small as possible.

[0242] In addition, various additives can be added to the surface crosslinking agent solution in amounts not exceeding 5 parts by weight or can be added during the mixing step.

[0243] (Mixing methods and mixing conditions)

[0244] The mixing of the water-absorbing resin and the surface crosslinking agent solution can be carried out by spraying or dripping the pre-prepared surface crosslinking agent solution into the crosslinking polymer, preferably by spraying the pre-prepared surface crosslinking agent solution into the crosslinking polymer.

[0245] The mixing apparatus used for mixing preferably has the torque required to uniformly and reliably mix the water-absorbing resin and the surface crosslinking agent. The mixing apparatus is preferably a high-speed stirring mixer, more preferably a high-speed continuous stirring mixer. The rotational speed of the high-speed stirring mixer is preferably not less than 100 rpm, more preferably not less than 300 rpm, and preferably not more than 10,000 rpm, more preferably not more than 2,000 rpm.

[0246] From the viewpoint of miscibility with the surface treatment agent solution and / or the cohesiveness of the humidifying mixture, the temperature of the water-absorbing resin supplied in this step is preferably 35°C to 80°C, more preferably 35°C to 70°C, and even more preferably 35°C to 60°C. The mixing time is preferably not less than one second, more preferably not less than five seconds, and preferably not more than one hour, more preferably not more than 10 minutes.

[0247] [3-6-2] Heat treatment steps

[0248] This step involves heating the mixture obtained in the mixing step to cause a cross-linking reaction on the surface of the absorbent resin. The heat treatment of the absorbent resin can be performed by heating a stationary absorbent resin, or by heating the absorbent resin while it is flowing using stirring or similar methods. Preferably, the absorbent resin is heated while being stirred, as this ensures uniform heating of the entire wetting mixture. From this perspective, heat treatment apparatuses such as paddle dryers, multi-blade dryers, and tower dryers can be used for this purpose.

[0249] The temperature control of the so-called heat treatment apparatus only needs to be set to a temperature sufficient to heat the absorbent resin to the temperature described later, and it does not need to be constant from the beginning to the end of this step. However, to prevent problems such as localized overheating, the control temperature is preferably between 50°C and 300°C. In cases where the physical properties of the obtained absorbent resin, particularly its damage resistance, are of particular importance, the control temperature is more preferably not higher than 250°C, more preferably between 70°C and 230°C, and even more preferably between 90°C and 220°C.

[0250] Meanwhile, where water absorption performance is particularly important, the temperature control is more preferably 120°C to 280°C, even more preferably 150°C to 250°C, and particularly preferably 170°C to 230°C.

[0251] The heating time is preferably from 1 minute to 180 minutes, more preferably from 5 minutes to 120 minutes, even more preferably from 10 minutes to 120 minutes, and even more preferably from 15 minutes to 60 minutes. Setting the heating time to less than 1 minute can result in insufficient surface crosslinking treatment, thereby reducing absorbency against pressure (AAP). At the same time, setting a long heating time can cause coloration of the absorbent resin and / or excessively reduce the centrifugal retention capacity (CRC) of the absorbent resin.

[0252] [3-6-3] Cooling Steps

[0253] This step is an optional step that is added as needed after the heat treatment and / or drying steps. This step is to force the high-temperature water-absorbing resin after the heat treatment step to a given temperature to quickly stop the surface crosslinking reaction.

[0254] Cooling can be achieved by cooling the stationary absorbent resin, or by flowing the absorbent resin using stirring and / or similar methods. Preferably, the absorbent resin is cooled while being stirred, as this ensures uniform cooling of the entire resin. From this perspective, cooling devices such as paddle dryers, multi-blade dryers, and tower dryers can be used. Alternatively, the cooling device can be of the same specifications as the heat treatment device used in the heat treatment step. Alternatively, a heat treatment device that replaces the heat medium with a refrigerant can be used as the cooling device.

[0255] The cooling temperature in this step can be appropriately set according to the heating temperature in the heat treatment step, the water absorption performance of the water-absorbing resin, etc., and is preferably 40°C to 100°C, more preferably 50°C to 90°C, and even more preferably 50°C to 70°C.

[0256] [4] Absorbent products

[0257] The absorbent resin manufactured by the manufacturing method according to an embodiment of the invention is incorporated into the absorbent article. "Absorbent article" refers to a product placed close to or adjacent to the wearer's body to absorb and contain various bodily fluids (such as urine, feces, blood, etc.). Absorbent articles include paper diapers and trousers (worn by infants, babies, and / or adults), absorbent inserts for paper diapers or trousers, and feminine hygiene absorbent articles such as sanitary napkins and panty liners.

[0258] Absorbent articles may each include a top sheet, a bottom sheet, an absorbent core, and an optional collection-dispensing system. The absorbent core is placed between the bottom sheet and the top sheet, and the optional collection-dispensing system is typically placed between the absorbent core and the top sheet.

[0259] The absorbent resin manufactured by the manufacturing method according to embodiments of the present invention can be incorporated into the absorbent core of an absorbent article. The absorbent core may or may not contain other absorbent materials, such as non-crosslinked cellulose fibers (pulp fibers). The absorbent core may contain at least 60% by mass, at least 75% by mass, at least 85% by mass, at least 95% by mass, or at least 98% by mass, or 100% by mass of absorbent resin, such as the absorbent resin disclosed herein.

[0260] "Paper diapers" and "pants" respectively refer to absorbent articles worn around the lower body by infants, babies, and incontinent individuals (adults) to encircle the wearer's waist and legs and are particularly suitable for receiving and containing urine and feces. In pants, as used herein, the longitudinal edges of the first and second waistbands are attached to each other to pre-form waist openings and leg openings. The pants are put on by the wearer by inserting their legs into the leg openings and pulling the absorbent article of the pants to a position near the wearer's lower body. The pants can be pre-shaped using any suitable method, including but not limited to joining the portions of the absorbent article together using repeatable and / or non-repeatable adhesives (e.g., stitching, welding, adhesives, glue bonding, fasteners, etc.). The pants can be pre-shaped at any location along the periphery of the article (e.g., side fastening, front waistband fastening). In paper diapers, the waist opening and leg opening are formed only when the paper diaper is worn on the wearer in such a way that the longitudinal edges of the first and second waist zones of the paper diaper are releasably attached to each other on both sides of the wearer using a suitable fastening system. A suitable fastening system may include, for example, a band protrusion comprising hook material and a landing area cooperating therewith (e.g., a nonwoven fiber web providing loops in a hook-and-loop fastening system).

[0261] Paper diapers or pants may also include elastic leg cuffs and barrier leg cuffs, which improve restraint of liquids and other bodily fluids, especially in the leg opening area. Typically, the leg cuffs and barrier cuffs each consist of one or more elastic threads.

[0262] "Feminine hygiene absorbent products" are personal care products used by women during menstruation to absorb and retain menstrual blood, vaginal secretions, and other substances from bodily functions related to the vulva. Feminine hygiene absorbent products include panty liners and sanitary napkins.

[0263] The present invention may include the following [1] to [8]:

[0264] [1] An absorbent article comprising a poly(meth)acrylate (salt)-based absorbent resin in particulate form, the absorbent resin comprising:

[0265] A water-soluble polyalkylene glycol with a weight average molecular weight of not more than 2000, wherein the polyalkylene glycol is contained in the water-absorbing resin.

[0266] This absorbent resin does not substantially contain liquid permeability enhancers containing polyvalent metal salts and / or water-insoluble inorganic particles.

[0267] The water-absorbing resin satisfies the following (1) to (5):

[0268] (1) The centrifugation retention capacity (CRC) is not less than 20 g / g and not more than 35 g / g;

[0269] (2) The pressure absorption capacity (AAP) measured under a load of 0.7 psi is not less than 25 g / g;

[0270] (3) The saline fluid conductivity (SFC) is not less than 15 (×10-7 cm3·s / g);

[0271] (4) The free swelling rate (FSR) is not less than 0.33 g / (g·s); and

[0272] (5) The initial water absorption rate (T20) under load shall not exceed 145 seconds.

[0273] [2] The absorbent article as described in [1], wherein the polyalkylene glycol has a weight-average molecular weight of not less than 200.

[0274] [3] The absorbent article as described in [1] or [2], wherein the content of polyalkylene glycol is from 0.01% by mass to 1% by mass relative to the whole absorbent resin.

[0275] [4] The absorbent article as described in any one of [1] to [3], wherein the absorbent resin is a surface-crosslinked absorbent resin having a non-uniformly pulverized shape.

[0276] [5] The absorbent article as described in any one of [1] to [4], wherein the absorbent resin has an initial water absorption rate (T5) under load of not more than 25 seconds.

[0277] [6] The absorbent article as described in any one of [1] to [5], wherein, for the absorbent resin, the sum of T5, T10 and T15 is not greater than 160 seconds, and T5, T10 and T15 are each the initial water absorption rate under load.

[0278] [7] The absorbent article as described in any one of [1] to [6], wherein the absorbent resin has a water content of not more than 5% by mass.

[0279] [8] A method for manufacturing an absorbent article comprising a poly(meth)acrylate (salt)-based absorbent resin in particulate form, the method comprising the following steps:

[0280] (i) Preparation of an aqueous solution of (meth)acrylate (salt) monomer;

[0281] (ii) Polymerize the aqueous solution of the (meth)acrylate (salt) monomer;

[0282] (iii) Gel disruption of the cross-linked hydrogel polymer generated during or after polymerization to obtain a hydrogel in the form of microparticles;

[0283] (iv) Dry the hydrogel in particulate form to obtain a dried polymer;

[0284] (v) Pulverizing and / or grading the dried polymer to obtain a water-absorbing resin in particulate form;

[0285] (vi) Crosslinking the surface of the particulate absorbent resin; and

[0286] (vii) The surface-crosslinked microparticle water-absorbing resin is incorporated into the absorbent article, wherein

[0287] In any step prior to step (iii), an amount of water-soluble polyalkylene glycol with a weight average molecular weight not exceeding 2000 is added in a proportion of 0.01% to 1% by mass relative to the total mass of the monomer contained in the aqueous monomer solution.

[0288] No liquid permeability enhancers comprising polyvalent metal salts and / or water-insoluble inorganic particles are substantially added in any step, and

[0289] The obtained water-absorbing resin satisfies the following (1) to (5):

[0290] (1) The centrifugation retention capacity (CRC) is not less than 20 g / g and not more than 35 g / g;

[0291] (2) The pressure absorption capacity (AAP) measured under a load of 0.7 psi is not less than 25 g / g;

[0292] (3) The saline fluid conductivity (SFC) is not less than 15 (×10-7 cm3·s / g);

[0293] (4) The free swelling rate (FSR) is not less than 0.33 g / (g·s); and

[0294] (5) The initial water absorption rate (T20) under load shall not exceed 145 seconds. Example

[0295] The following description will discuss the absorbent resins contained in the absorbent articles of the present invention according to the examples and comparative examples. However, it should be understood that the present invention is not limited to these examples and comparative examples. The physical properties described in the claims and examples of the present invention were calculated under conditions of room temperature (23°C ± 2°C) and humidity of 50% ± 10% RH.

[0296] [Measurement and Evaluation of the Physical Properties of Water-Absorbent Resins]

[0297] The physical properties of the absorbent resins obtained in the following examples and comparative examples were measured and evaluated in the following manner.

[0298] (No-pressure absorption capacity (CRC))

[0299] The CRC of the absorbent resin was measured according to NWSP 241.0.R2(15). Specifically, 0.2 g of absorbent resin was added to a nonwoven bag. The bag was then immersed in a large excess of 0.9% by mass aqueous sodium chloride solution for 30 minutes to allow free swelling. The bag was then drained using a centrifuge (250 G) for three minutes. Thereafter, its centrifugal retention capacity (CRC) (in g / g) was measured.

[0300] (Average Pressure Absorption (AAP))

[0301] The AAP (Absorbent Aptitude) of the superabsorbent resin was measured according to NWSP 242.0.R2(15). It should be noted that the measurements were performed under modified pressure conditions, specifically at 4.83 kPa (49 g / cm², 0.7 psi). More specifically, 0.9 g of superabsorbent resin was swollen for one hour in a large excess of 0.9% sodium chloride aqueous solution at 4.83 kPa (49 g / cm², 0.7 psi). Subsequently, its AAP (Absorbent Aptitude) (in g / g) was measured. That is, all AAP (Absorbent Aptitude) values ​​in this paper are measured at 4.83 kPa.

[0302] (Centrifugation retention capacity of cross-linked hydrogel polymers: gel CRC)

[0303] The same procedures were performed as in the CRC measurement of the superabsorbent polymer, except that 0.6 g of cross-linked hydrogel polymer was used as the sample and the free swelling period was set to 24 hours. Additionally, the resin solids content in the cross-linked hydrogel polymer was measured using an additional method described later. The weight of the superabsorbent resin contained in 0.6 g of the cross-linked hydrogel polymer was obtained, and the gel CRC was calculated according to the following formula (2). For a single sample, five measurements were performed, and the average value was taken.

[0304] Gel CRC(g / g)=[{(mwi-mb) / msi}-1]×(100 / Wn)……Equation (2).

[0305] In equation (2),

[0306] MSI: Weight (g) of the cross-linked hydrogel polymer before measurement;

[0307] mb: Weight (g) of blank (non-woven fabric only) that has been freely swollen and drained;

[0308] mwi: Total weight (g) of the free-swelling and drained cross-linked hydrogel polymer and nonwoven fabric; and

[0309] Wn: Solid content (by weight) of the cross-linked hydrogel polymer.

[0310] (Resin solids content in cross-linked hydrogel polymers)

[0311] The weight of an aluminum cup with a bottom surface of approximately 50 mm in diameter was measured beforehand, and the resulting weight was taken as “Wa(g)”. 1.00 g of crosslinked hydrogel polymer was weighed and placed in the aluminum cup, and the total weight of the crosslinked hydrogel polymer and the aluminum cup was measured as Wb(g). The aluminum cup containing the absorbent resin was then placed in an oven at an ambient temperature of 180°C for three hours to dry. After three hours, the absorbent resin and the aluminum cup were removed from the oven and cooled to room temperature in a desiccator. The total weight of the dried absorbent resin and the aluminum cup was then measured as Wc(g). Using Wa, Wb, and Wc, the resin solids content of the crosslinked hydrogel polymer was obtained according to the following formula (3):

[0312] Resin solids content (wt%) in cross-linked hydrogel polymers = {(Wc-Wa) / (Wb-

[0313] Wa)}×100... Formula (3).

[0314] (Salt water conductivity (SFC))

[0315] The salt flow conductivity (SFC) of the absorbent resin (unit: ×10-7 cm3·s / g) was measured according to the measurement method described in U.S. Patent No. 5669894.

[0316] Specifically, 1,500 g of absorbent resin was evenly placed in a container. The absorbent resin was then immersed in artificial urine to swell under a pressure of 2.07 kPa. Artificial urine was prepared by mixing 0.25 g of calcium chloride dihydrate, 2.0 g of potassium chloride, 0.50 g of magnesium chloride hexahydrate, 2.0 g of sodium sulfate, 0.85 g of ammonium dihydrogen phosphate, 0.15 g of diammonium hydrogen phosphate, and 994.25 g of pure water.

[0317] Sixty minutes after pressurization, the height (cm) of the gel layer, which is the swollen absorbent resin, is recorded. Next, while pressurizing the gel layer at 2.07 kPa, a 0.69% by mass brine solution is passed through it. The room temperature during this process is adjusted to 20°C to 25°C. Then, using a proportional balance and a computer, the amount of brine solution passing through the gel layer is recorded at 20-second intervals, thus measuring the flow rate Fs(T). The flow rate Fs(T) can be obtained by dividing the increase in the mass (g) of brine solution passing through it every 20 seconds by the elapsed time (s). Here, it is assumed that Ts represents the time when the hydrostatic pressure of the brine solution becomes constant and a stable flow rate can be obtained. Then, based on the data measured for 10 minutes starting from Ts, the flow rate Fs(T=0) is calculated. Specifically, Fs(T=0) is calculated based on the result obtained by plotting Fs(T) against time and applying the least squares method to it. Fs (T = 0) is the flow rate (g / s) of the first flow of the brine solution through the gel layer. Then, the brine flow conductivity (SFC) is calculated according to the following equation (4).

[0318] SFC={Fs(T=0)×L0} / (ρ×A×ΔP)……Equation (4).

[0319] In equation (4), L0 is the height of the gel layer (cm), ρ is the density of the salt solution (g / cm3), A is the cross-sectional area A of the gel layer (cm2), and ΔP is the hydrostatic pressure applied to the gel layer (dynes / cm2).

[0320] (Free swelling rate (FSR))

[0321] The free swelling rate (FSR) is the rate at which 1 g of absorbent resin absorbs 20 g of 0.9% sodium chloride aqueous solution (g / (g·s)), and is measured according to the method described in International Publication No. 2009 / 016055.

[0322] (Moisture content)

[0323] Weigh 1.00 g of absorbent resin and place it in an aluminum cup with a bottom surface diameter of approximately 50 mm. Then, measure the total mass W1 g of the absorbent resin and the aluminum cup. Next, place the aluminum cup containing the absorbent resin in an oven at an ambient temperature of 180°C for three hours to dry it. After three hours, remove the absorbent resin and the aluminum cup from the oven and cool them to room temperature in a desiccator. Then, measure the total mass W2 g of the dried absorbent resin and the aluminum cup, and obtain the moisture content according to the following formula.

[0324] Moisture content (mass%) = (W1-W2) / (mass of water-absorbing resin (g)) × 100.

[0325] (T20, T5, T10, T15)

[0326] The time (in seconds) required for 20g of an aqueous solution prepared by dissolving 9g of sodium chloride and 0.1g of Lorodac (major component: C12-14 straight-chain alcohol ethoxylate, CAS No. 68439-50-9) in 1L of distilled water to be absorbed by 1g of superabsorbent resin is defined as T20. Specifically, T20 is measured according to the measurement method described in the Japanese translation of PCT International Application Publication No. 2014-515987. Similarly, T5, T10, and T15 are the time periods required for 1g of superabsorbent resin to absorb 5g, 10g, and 15g of the aqueous solution, respectively. Similar to T20, T5, T10, and T15 are measured according to the measurement method described in the Japanese translation of PCT International Application Publication No. 2014-515987.

[0327] (EFFC)

[0328] EFFC is the average of centrifugal retention capacity (CRC) and pressure absorbance (AAP).

[0329] [Example 1]

[0330] (Preparation of aqueous solutions of (meth)acrylate monomers)

[0331] An aqueous solution (1) was prepared by introducing 400 parts by weight of acrylic acid, 185 parts by weight of 48% sodium hydroxide aqueous solution, 2.5 parts by weight of polyethylene diacrylate (PEGDA, average molar addition of ethylene oxide: 9), 1.3 parts by weight of 2% trisodium diethylenetriaminepentaacetate aqueous solution, 373 parts by weight of deionized water, and 0.9 parts by weight (0.225% by weight relative to acrylic acid) of polyethylene glycol (PEG) 600 (weight average molecular weight 600, purchased from FUJIFILM Wako PureChemical Corporation) into a 2L propylene container and mixing them. The aqueous solution (1) was heated to 40°C beforehand.

[0332] Next, while stirring the heated aqueous solution (1), 185 parts by mass of a 48% sodium hydroxide aqueous solution were introduced into the aqueous solution (1) over approximately 30 seconds under open atmosphere, and then the solutions were mixed. This prepared the monomeric aqueous solution. Note that the temperature of the monomeric aqueous solution was raised to approximately 84°C by the heat of neutralization and the heat of solution generated during mixing.

[0333] Table 1 below shows the weight-average molecular weight of PEG, the amount of PEG added (relative to the mass percentage of acrylic acid), and the amount of PEGDA added (relative to the mass percentage of acrylic acid).

[0334] (Polymerization of (meth)acrylate monomers)

[0335] When the temperature of the monomer aqueous solution reaches 83°C, 13 parts by mass of 5% sodium persulfate aqueous solution are added to the monomer aqueous solution as a polymerization initiator, and the resulting mixture is stirred for about five seconds. Thus, the reaction liquid (1) is obtained.

[0336] Next, the reaction liquid (1) is injected into a stainless steel tray-shaped container (340mm × 340mm at the bottom, 25mm in height, and with Teflon (registered trademark) on the inner surface) in an open atmosphere. Note that the tray-shaped container should be preheated with a hot plate to bring its surface temperature to 40°C.

[0337] After the reaction liquid (1) was injected into the tray-shaped container, the polymerization reaction began within one minute. As a result of the polymerization reaction, the reaction liquid (1) expanded and foamed upwards in all directions, while generating water vapor. As a result of the polymerization reaction proceeding in this way, the reactants shrank to a size slightly larger than the bottom surface of the tray-shaped container. The polymerization reaction ended in approximately one minute. As a result of the polymerization reaction, a cross-linked hydrogel polymer was obtained. A portion of the cross-linked hydrogel polymer was cut off and used as a measurement sample. The measurement sample was used for the measurement of the gel CRC. The gel CRC values ​​of the obtained cross-linked hydrogel polymer are shown in Table 1.

[0338] (Gel breakage)

[0339] Next, the remaining portion of the cross-linked hydrogel polymer not used in the gel CRC measurement was cut into appropriately sized pieces, and the gel was crushed using a meat grinder (model "HL-G22SN", purchased from Remacom Co. Ltd.) with a die containing 33 pores, each with a diameter of 8 mm, to obtain microparticle hydrogels. The mass-average particle size of the microparticle hydrogels was 400 μm.

[0340] (Drying, pulverizing, and grading)

[0341] Microparticle hydrogels were spread on a 50-mesh (mesh size: 300 μm) wire mesh and dried with hot air at 180°C for 30 minutes using a ventilated batch dryer (model "71-S6", Satake Chemical Equipment Mfg. Ltd.). This yielded a dried polymer. The dried polymer was then subjected to a pulverizing step involving a roller mill, and the resulting pulverized material was graded using wire meshes of 710 μm and 150 μm. This resulted in (microparticle) crosslinked polymer powder with a non-uniform pulverized shape and a particle size ranging from 150 μm to 710 μm. The resulting crosslinked polymer powder corresponds to the water-absorbing resin prior to surface crosslinking.

[0342] (Surface crosslinking)

[0343] A surface crosslinking agent aqueous solution comprising 0.4 parts by mass of ethylene carbonate, 0.7 parts by mass of propylene glycol, and 2.9 parts by mass of deionized water was spray-mixed (4.0 parts by mass) into 100 parts by mass of the crosslinked polymer powder thus obtained. The resulting mixture was then heat-treated for 40 minutes at a heating medium temperature of 210°C using a mixer. The mixture was then pulverized to a size that allows it to pass through a JIS standard sieve with a mesh size of 710 μm. This yields a surface-crosslinked water-absorbing resin. The physical properties of the resulting water-absorbing resin are shown in Table 1 below.

[0344] [Examples 2 to 7]

[0345] The procedure was the same as in Example 1, except that the weight-average molecular weight and amount of PEG added were changed to the values ​​shown in Table 1. Thus, the absorbent resins of Examples 2 to 7 were obtained. The physical properties of the thus obtained absorbent resins are shown in Table 1. In Table 1, the weight-average molecular weight of PEG200 (purchased from FUJIFILM Wako Pure Chemical Corporation) is 200, and the weight-average molecular weight of PEG2000 (purchased from FUJIFILM Wako Pure Chemical Corporation) is 2000.

[0346] [Example 8]

[0347] The procedure was the same as that in Example 1, except that (i) the weight-average molecular weight and amount of PEG were changed to the values ​​shown in Table 1; (ii) the die used in the gel breaking step was changed to a die with 52 holes, each 6 mm in diameter; and (iii) an aqueous solution of a surface crosslinking agent containing 0.18 parts by weight of 1,6-hexanediol, 0.4 parts by weight of triethylene glycol, and 2.5 parts by weight of deionized water was applied relative to 100 parts by weight of the crosslinked polymer powder obtained in the surface crosslinking step (3.08 parts by weight). Thus, the water-absorbing resin of Example 8 was obtained.

[0348] [Example 9]

[0349] The procedure was the same as that in Example 1, except that (i) the weight-average molecular weight and amount of PEG were changed to the values ​​shown in Table 1; (ii) the die used in the gel breaking step was changed to a die with 52 holes, each 6 mm in diameter; and (iii) an aqueous solution of a surface crosslinking agent (3.30 parts by mass) containing 0.67 parts by mass of ethylene carbonate and 2.63 parts by mass of deionized water was applied relative to 100 parts by mass of the crosslinked polymer powder obtained in the surface crosslinking step. Thus, the water-absorbing resin of Example 9 was obtained.

[0350] [Example 10]

[0351] The procedure was the same as that in Example 1, except that (i) the weight-average molecular weight and amount of PEG were changed to the values ​​shown in Table 1; (ii) the die used in the gel breaking step was changed to a die with 52 holes, each 6 mm in diameter; and (iii) an aqueous solution of a surface crosslinking agent (3.0 parts by mass) containing 1.0 parts by mass of triethylene glycol and 2.0 parts by mass of deionized water was applied relative to 100 parts by mass of the crosslinked polymer powder obtained in the surface crosslinking step. Thus, the water-absorbing resin of Example 10 was obtained.

[0352] [Comparative Example 1]

[0353] The procedure was the same as that in Example 1, except that PEG was not added in Comparative Example 1. Therefore, the water-absorbing resin of Comparative Example 1 was obtained. The physical properties of the water-absorbing resin thus obtained are shown in Table 1.

[0354] [Compare Examples 2 and 3]

[0355] The procedure was the same as in Example 1, except that the amount of PEG added was changed to the values ​​shown in Table 1 and the heating conditions were adjusted to obtain SFCs as shown in Table 1 during the surface crosslinking step. Thus, the water-absorbing resins of Comparative Examples 2 and 3 were obtained. The physical properties of the water-absorbing resins thus obtained are shown in Table 1.

[0356] [Comparative Example 4]

[0357] 170 g of acrylic acid, 1800 g of 37% sodium acrylate aqueous solution, 0.99 g of polyethylene diacrylate (weight average molecular weight 523), 6.688 g of PEG2000 (weight average molecular weight 2000, purchased from FUJIFILMWako Pure Chemical Corporation) (0.8% by mass relative to acrylic acid), and 216 g of deionized water were introduced into a reaction vessel consisting of a thermometer, a lid with a nitrogen inlet and an exhaust port, a bottom plate with a surface area of ​​300 mm × 220 mm and a depth of 60 mm, and mixed. The reaction vessel was then immersed in a water bath at 20°C. Nitrogen gas was introduced into the aqueous solution, and the aqueous solution was degassed for 20 minutes.

[0358] After confirming that the solution temperature reached 20°C, under a flowing nitrogen atmosphere, 6.61 g of 20% sodium persulfate aqueous solution and 6.33 g of 0.1% L ascorbic acid aqueous solution were added, and the mixture was stirred. The monomer concentration was 38% by mass.

[0359] After one minute, polymerization began. The temperature of the reaction system at this time was 20°C. After polymerization started, the reaction vessel was immersed in a 20°C water bath for cooling without stirring the polymerization system. After 17 minutes, the polymerization system showed a temperature of 89°C, which was the highest temperature. Then, the water bath temperature was raised to 70°C, and the polymerization reaction was carried out for 20 minutes to obtain a crosslinked hydrogel polymer. A portion of the crosslinked hydrogel polymer was cut off and used as a measurement sample. The measurement sample was used for gel CRC measurement. The gel CRC values ​​of the obtained crosslinked hydrogel polymer are shown in Table 1.

[0360] The remaining portion of the obtained cross-linked hydrogel polymer that was not used in the gel CRC measurement was cut into pieces, and the process involved using a meat grinder with a template having a diameter of 9.0 mm to gel break up the block-shaped cross-linked hydrogel polymer pieces obtained by cutting, so as to obtain particulate hydrogel polymer.

[0361] The resulting particulate hydrogel polymer was dried in a hot air dryer at 180°C for 30 minutes to obtain a dried polymer. The dried polymer was then pulverized using a roller mill and graded using metal meshes with a mesh size of 850 μm and 150 μm. This yielded a particulate crosslinked polymer powder. The resulting crosslinked polymer powder corresponds to the water-absorbing resin before surface crosslinking.

[0362] A surface crosslinking agent solution containing 0.025 parts by weight of ethylene glycol diglycidyl ether, 0.3 parts by weight of ethylene carbonate, 0.5 parts by weight of propylene glycol, and 2.0 parts by weight of deionized water was sprayed onto 100 parts by weight of crosslinked polymer powder, and the resulting mixture was mixed. The resulting mixture was heated at 200°C for 35 minutes to obtain a surface-crosslinked microparticle water-absorbing resin.

[0363] 0.6 parts by mass of pyrolytic silica (AEROSIL 200, purchased from Nippon Aerosil Co. Ltd.) were mixed into 100 parts by mass of surface-crosslinked absorbent resin. Thus, the absorbent resin of Comparative Example 4 was obtained. The physical properties of the thus obtained absorbent resin are shown in Table 1.

[0364] [Compare Examples 5 and 6]

[0365] The procedure was the same as that in Example 1, except that the weight-average molecular weight and amount of PEG added were changed to the values ​​shown in Table 1. Thus, the absorbent resins of Comparative Examples 5 and 6 were obtained. The physical properties of the thus obtained absorbent resins are shown in Table 1. In Table 1, the weight-average molecular weight of PEG6000 (available from Kishida Chemical Co., Ltd.) is 7300 to 9300, and the weight-average molecular weight of PEG20000 (available from Kishida Chemical Co., Ltd.) is 18000 to 25000.

[0366]

[0367]

[0368] According to Table 1, the superabsorbent resins of Examples 1 to 10 containing PEG have a smaller T20 compared to the superabsorbent resin of Comparative Example 1 which does not contain PEG. According to Table 1, the superabsorbent resins of Examples 1 and 5, which each contain PEG but have a low SFC, have a smaller T20 compared to the superabsorbent resins of Comparative Examples 2 and 3, where the amount of PEG is the same as that of Comparative Examples 2 and 3 but the SFC is not less than 15 (×10⁻⁷ cm³·s / g). In each of the superabsorbent resins of Examples 1 to 7, CRC, AAP, EFFC, and FSR are good, SFC is as high as not less than 15, and T20, T15, T10, and T5 are reduced. The superabsorbent resin of Comparative Example 4, which has PEG with the same weight-average molecular weight as the superabsorbent resin of Example 7, shows an FSR of less than 0.33 (g / (g·s)) and a higher T20.

[0369] As described above, the absorbent resin shown in this invention possesses excellent properties such as absorbency (CRC, AAP), absorption rate (FSR), and liquid permeability (SFC). Furthermore, the absorbent resin has, for example, a low T20 value, meaning it exhibits excellent initial absorption rate under load. Therefore, when the absorbent resin shown in this invention is incorporated into absorbent articles such as paper diapers, the absorbent article can suppress or reduce urine leakage (rewetting). Thus, it is expected that users wearing such absorbent articles can enjoy a comfortable feeling without feeling stuffy.

[0370] Absorbent articles containing the aforementioned absorbent resin can inhibit or reduce the leakage (rewetting) of exudate. Therefore, the absorbent resin described in this invention can be incorporated into various absorbent articles. Consequently, users wearing absorbent articles containing the absorbent resin described in this invention (e.g., paper diapers) can enjoy comfort without feeling stuffy.

Claims

1. A method for manufacturing an absorbent article comprising a surface-crosslinked microparticle poly(meth)acrylate (salt)-based absorbent resin, the method comprising the following steps: (i) Preparation of an aqueous solution of (meth)acrylate (salt) monomer; (ii) Polymerizing the aqueous solution of the (meth)acrylate (salt) monomer; (iii) Gel disruption of the cross-linked hydrogel polymer generated during or after the polymerization to obtain microparticle hydrogels; (iv) Dry the microparticle hydrogel to obtain a dry polymer; (v) The dried polymer is pulverized and / or graded to obtain particulate water-absorbing resin before surface crosslinking; (vi) Crosslinking the surface of the microparticle-based water-absorbing resin prior to surface crosslinking, including a controlled temperature of 50°C to 300°C and a heating time of 1 minute to 180 minutes; and (vii) The surface-crosslinked microparticle water-absorbing resin is incorporated into the absorbent article, wherein In step (i) and / or step (ii), a water-soluble polyalkylene glycol with a weight-average molecular weight of not more than 2000 is added to the aqueous monomer solution, such that the total amount of the water-soluble polyalkylene glycol added in step (i) and / or step (ii) is 0.01% to 1% by mass relative to the total mass of the monomer contained in the aqueous monomer solution. The cross-linked hydrogel polymer has a centrifugal retention capacity of not more than 31 g / g and not less than 20 g / g, and The surface-crosslinked microparticle poly(meth)acrylate (salt)-based water-absorbing resin obtained by the method satisfies the following (1) to (4): (1) The centrifugation retention capacity (CRC) is not less than 20 g / g and not more than 35 g / g; (2) The pressure absorption capacity (AAP) measured under a load of 0.7 psi is not less than 25 g / g; (3) The saline fluid conductivity (SFC) is not less than 15 (× 10⁻⁷ cm³·s / g); and (4) Free swelling rate (FSR) is not less than 0.33 g / (g·s).

2. The method according to claim 1, wherein the polyalkylene glycol has a weight-average molecular weight of not less than 200.

3. The method according to claim 1 or 2, wherein the content of the polyalkylene glycol is adjusted to 0.01% by mass to 1% by mass relative to the microparticle poly(meth)acrylate (salt)-based absorbent resin crosslinked throughout the surface.

4. The method according to claim 1 or 2, wherein the surface-crosslinked microparticle poly(meth)acrylate (salt)-based absorbent resin is a surface-crosslinked absorbent resin having a non-uniformly pulverized shape.

5. The method according to claim 1 or 2, wherein the water content of the surface-crosslinked microparticle poly(meth)acrylate (salt)-based water-absorbing resin is not greater than 5% by mass.

6. The method according to claim 1 or 2, wherein the microparticle hydrogel obtained as a result of gel disruption of the hydrogel crosslinked polymer in step (iii) has a logarithmic standard deviation (σ) of 0.2 to 1.5, the logarithmic standard deviation (σ) indicating the particle size distribution of the microparticle hydrogel.

7. The absorbent article according to claim 1 or 2, wherein the absorbent article is a paper diaper or trousers.

8. The method according to claim 1 or 2, wherein, In step (vii), the surface-crosslinked microparticle water-absorbing resin is incorporated into the absorbent core, and the absorbent core is included in the absorbent article.

9. The method of claim 8, wherein the absorbent core is prepared by providing two nonwoven fiber webs and incorporating the surface-crosslinked microparticle water-absorbing resin between the two nonwoven fiber webs.

10. The method according to claim 9, wherein the absorbent core between the two nonwoven fiber webs comprises less than 20% by weight of cellulose fibers, preferably less than 10% by weight of cellulose fibers, and more preferably less than 5% by weight of cellulose fibers.

11. The method of claim 10, wherein the method comprises the further step of adhering and fixing the surface-crosslinked microparticle water-absorbing resin between the two nonwoven fiber webs.

12. The method of claim 8, wherein the method comprises the following further steps: Provide top film Provide negatives The absorbent core is provided between the top sheet and the bottom sheet.

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