Water-absorbent resin particles, absorbent body and absorbent article

Abstract

The present invention relates to water-absorbing resin particles containing an imidazoline compound represented by formula (1). [In formula (1), R is a C1-3 alkyl group.]

Description

[Technical Field]

[0001] The present invention relates to water-absorbent resin particles, an absorbent body, and an absorbent article. [Background technology]

[0002] Water-absorbent resin particles are used for hygiene materials such as disposable diapers and sanitary products, agricultural and horticultural materials such as water-retaining materials and soil conditioners, industrial materials such as waterproofing materials for cables and anti-condensation materials, etc. Water-absorbent resin particles generally contain a polymer formed by a polymerization reaction of a monomer having a polymerizable functional group (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3231881 Summary of the Invention [Problem to be solved by the invention]

[0004] When the liquid to be absorbed contains iron, the viscosity of the swollen gel formed by the water-absorbent resin particles after absorbing the liquid tends to decrease over time, and as a result, it may be difficult to maintain sufficient liquid absorption performance. In addition, it is desirable that the water-absorbent resin particles used in applications such as diapers have excellent water absorption ability even under no pressure.

[0005] An object of one aspect of the present invention is to provide water-absorbent resin particles that are excellent in moisture absorbency under no pressure and that are inhibited from decreasing in viscosity over time when swollen in an aqueous solution containing iron. [Means for solving the problem]

[0006] The water-absorbent resin particles according to one aspect of the present invention contain an imidazoline compound represented by the following formula (1). [ka] [In formula (1), R represents an alkyl group having 1 to 3 carbon atoms.]

[0007] One aspect of the present invention relates to an absorbent body containing the water-absorbent resin particles. Another aspect of the present invention relates to an absorbent article including the absorbent body. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide water-absorbent resin particles that are excellent in moisture absorption ability under no pressure and that are inhibited from decreasing in viscosity over time when swollen in an aqueous solution containing iron. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view showing one embodiment of an absorbent article. [Figure 2] FIG. 1 is a schematic diagram showing a device for measuring the pressureless DW of water-absorbent resin particles. DETAILED DESCRIPTION OF THE INVENTION

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the preferred embodiments. However, the present invention is not limited to the following embodiments and can be practiced in various modified forms within the scope of the present invention.

[0011] In this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic." "Acrylate" and "methacrylate" are similarly referred to as "(meth)acrylate." In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in the Examples. "Water-soluble" means a solubility of 5% by mass or more in water at 25°C. The materials exemplified in this specification may be used alone or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of the multiple substances present in the composition, unless otherwise specified. "Physiological saline" refers to a 0.9% by mass aqueous solution of sodium chloride. "Room temperature" means 25°C ± 2°C.

[0012] [Water-absorbing resin particles] The water-absorbent resin particles according to this embodiment contain an imidazoline compound represented by the following formula (1) (hereinafter, also simply referred to as "imidazoline compound"). [ka] [In formula (1), R represents an alkyl group having 1 to 3 carbon atoms.]

[0013] The water-absorbent resin particles according to the present embodiment contain an imidazoline compound, and therefore have excellent water absorption properties under no pressure, and are also inhibited from decreasing in viscosity over time when swollen with an iron-containing aqueous solution. Examples of the iron-containing aqueous solution include human urine, blood, and a solution containing one of these (e.g., medical waste liquid).

[0014] Absorbency of water without pressure can be evaluated by measuring the 5-minute value of the no-pressure DW. The 5-minute value of the no-pressure DW is the volume of saline solution [mL / g] (volume per 1.00 g of water-absorbent resin particles) absorbed by 1.00 g of water-absorbent resin particles within 5 minutes of the start of absorption when physiological saline solution is absorbed by the water-absorbent resin particles using the no-pressure DW method. The no-pressure DW method is a water absorption test in which water-absorbent resin particles are placed on a liquid-permeable sheet (mesh sheet) placed on a measurement table with through-holes, and physiological saline solution supplied without pressure through the through-holes is absorbed by the water-absorbent resin particles. The inner diameter of the through-holes is typically 2 mm. The amount of water-absorbent resin particles used in the test is 1.00 ± 0.01 g, and this amount of water-absorbent resin particles is uniformly distributed in a circular area with a diameter of 30 mm, centered directly above the through-holes. The static water absorption rate is measured by the pressureless DW method in an environment of 25° C. temperature and 60±10% humidity. Other details of the test conditions will be explained in the examples below.

[0015] The 5-minute value of the no-pressure DW of the water-absorbent resin particles according to the present embodiment may be 27 mL / g or more, 30 mL / g or more, or 33 mL / g or more, and may be 60 mL / g or less, 55 mL / g or less, 50 mL / g or less, 45 mL / g or less, or 40 mL / g or less.

[0016] The decrease in viscosity over time when swollen in an iron-containing aqueous solution can be evaluated by measuring the viscosity retention rate of the swollen gel formed by swelling water-absorbent resin particles with iron-containing physiological saline. The viscosity retention rate of the swollen gel is calculated as the ratio (Y / X × 100) of the viscosity of the swollen gel left to stand for 5 hours after its formation (hereinafter referred to as the "5-hour value of swollen gel viscosity (Y)") to the viscosity of the swollen gel left to stand for 1 hour after its formation (hereinafter referred to as the "1-hour value of swollen gel viscosity (X)"). The iron-containing physiological saline used to form the swollen gel contains 50 ppm by mass of iron ions based on the total mass of the iron-containing physiological saline. The swollen gel is formed using 34 times the amount of iron-containing physiological saline relative to the solid content of the water-absorbent resin particles. The swollen gel is left to stand in a hot air dryer at 40°C. The swollen gel viscosity is measured using a B-type viscometer at 25°C. Other details of the test conditions will be explained in the examples below.

[0017] The viscosity retention rate of the swollen gel obtained by swelling water-absorbent resin particles with iron-containing physiological saline may be 42% or more, 45% or more, 48% or more, 50% or more, or 55% or more, and may be 60% or less, 65% or less, 70% or less, 75% or less, 80% or less, or 100% or less. The 1-hour value of the swollen gel viscosity (X) may be 18,000 mPa·s or more, 19,000 mPa·s or more, or 20,000 mPa·s or more, and may be 25,000 mPa·s or less, or 22,500 mPa·s or less. The 5-hour value of the swollen gel viscosity (Y) may be 8,200 mPa·s or more, 9,000 mPa·s or more, 9,500 mPa·s or more, or 9,900 mPa·s or more, and may be 15,000 mPa·s or less, or 13,000 mPa·s or less.

[0018] The centrifuge retention capacity (CRC) of the water-absorbent resin particles may be 25 g / g or more, 30 g / g or more, and may be 60 g / g or less, 55 g / g or less, 50 g / g or less, 45 g / g or less, 40 g / g or less, or 35 g / g or less. The CRC is measured by the method described in the Examples below, with reference to the EDANA method (NWSP 241.0.R2(15), pages 769 to 778).

[0019] The moisture content of the water-absorbent resin particles may be, for example, 15% by mass or less, 10% by mass or less, 5% by mass or less, 3% by mass or less, or 2.5% by mass or less, based on the total mass of the water-absorbent resin particles. The moisture content based on the total mass of the water-absorbent resin particles is measured by the method described in the examples below.

[0020] The water-absorbent resin particles may have a size that allows them to pass through a JIS standard sieve with an opening size of 850 μm.

[0021] The median particle diameter of the water-absorbent resin particles (or polymer particles) may be 150 to 800 μm, 150 to 700 μm, 200 to 600 μm, or 250 to 500 μm. The median particle diameter can be measured by the following method. JIS standard sieves are combined in the following order from top to bottom: a sieve with an 850 μm mesh size, a 600 μm mesh size, a 500 μm mesh size, a 425 μm mesh size, a 300 μm mesh size, a 250 μm mesh size, a 150 μm mesh size, and a tray. 50 g of water-absorbent resin particles are placed on the top sieve of the combination, and classified in accordance with JIS Z 8815 (1994) using a Rotap type shaker (manufactured by Iida Seisakusho Co., Ltd.). After classification, the mass of the particles remaining on each sieve is calculated as a mass percentage of the total mass to determine the particle size distribution. The particles remaining on the sieves are integrated in order of particle size from largest to smallest, and the relationship between the sieve opening and the integrated mass percentage of the particles remaining on the sieve is plotted on logarithmic probability paper. The particle size corresponding to an integrated mass percentage of 50% by mass is determined as the median particle size by connecting the plots on the probability paper with a straight line.

[0022] The shape of the water-absorbent resin particles (or polymer particles) is not particularly limited, and may be, for example, approximately spherical, crushed, or granular, or may be formed as particles formed by agglomeration of primary particles having these shapes.

[0023] <Polymer particles> The water-absorbent resin particles usually contain polymer particles. The polymer particles may be water-absorbent particles containing a polymer containing an ethylenically unsaturated monomer as a monomer unit. The ethylenically unsaturated monomer may be a water-soluble monomer, examples of which include (meth)acrylic acid and its salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. The ethylenically unsaturated monomer may be used alone or in combination of two or more. The proportion of the polymer containing an ethylenically unsaturated monomer as a monomer unit in the polymer particles may be 50 to 100 mol%, 60 to 100 mol%, 70 to 100 mol%, or 80 to 100 mol%, based on the total molar amount of the monomers constituting the polymer particles. The polymer particles may be particles containing a (meth)acrylic acid-based polymer containing at least one of (meth)acrylic acid or a (meth)acrylate salt as a monomer unit. The proportion of the monomer unit derived from (meth)acrylic acid or a (meth)acrylate salt in the (meth)acrylic acid-based polymer may be 90 to 100 mol%, based on the total molar amount of the monomers constituting the polymer.

[0024] The polymer particles may be surface-crosslinked with a surface crosslinking agent. At least the polymer in the surface layer portion of the polymer particles may be crosslinked by reaction with the surface crosslinking agent. The surface crosslinking agent may be, for example, a compound having two or more functional groups (reactive functional groups) reactive with functional groups derived from ethylenically unsaturated monomers. Examples of the surface crosslinking agent include alkylene carbonate compounds such as ethylene carbonate and propylene carbonate; polyols such as 1,4-butanediol, diethylene glycol, triethylene glycol, propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin, epi Examples of the surface cross-linking agent include haloepoxy compounds such as bromohydrin and α-methylepichlorohydrin; compounds having two or more reactive functional groups, such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The surface cross-linking agent may contain an alkylene carbonate compound. When two types of surface cross-linking agents are used in combination, the ratio of the alkylene carbonate compound in the surface cross-linking agent may be 15 to 95 mass%, 20 to 80 mass%, 25 to 60 mass%, or 30 to 45 mass% based on the total mass of the surface cross-linking agent. The ratio of the polyol compound in the surface cross-linking agent may be 30 to 100 mass%, 40 to 90 mass%, 50 to 80 mass%, or 55 to 70 mass% based on the total mass of the surface cross-linking agent.

[0025] Inside the polymer particles, the polymer may also be internally crosslinked by self-crosslinking, crosslinking through a reaction with an internal crosslinking agent, or both.

[0026] The internal crosslinking agent can include, for example, one or more compounds including a compound having two or more polymerizable unsaturated groups, a compound having two or more reactive functional groups that are reactive with the functional group of the ethylenically unsaturated monomer, or a combination thereof.

[0027] Examples of compounds having two or more polymerizable unsaturated groups include di- or tri(meth)acrylic acid esters of polyols such as (poly)ethylene glycol (for example, in this specification, "polyethylene glycol" and "ethylene glycol" are collectively referred to as "(poly)ethylene glycol", the same applies hereinafter), (poly)propylene glycol, trimethylolpropane, glycerin polyoxyethylene glycol, polyoxypropylene glycol, and (poly)glycerin; and esters obtained by reacting the above polyols with unsaturated acids such as maleic acid and fumaric acid. Examples of suitable acrylic acid esters include unsaturated polyesters, bisacrylamides such as N,N'-methylenebis(meth)acrylamide, di- or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid, carbamyl di(meth)acrylic acid esters obtained by reacting polyisocyanates such as tolylene diisocyanate and hexamethylene diisocyanate with hydroxyethyl (meth)acrylate, allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, and divinylbenzene.

[0028] Examples of compounds having two or more reactive functional groups include glycidyl group-containing compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; (poly)ethylene glycol, (poly)propylene glycol, (poly)glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, and glycidyl (meth)acrylate.

[0029] The polymer particles may contain a certain amount of water in addition to the polymer of the ethylenically unsaturated monomer.

[0030] <Imidazoline compounds> The imidazoline compound is a compound comprising an imidazoline ring and an alkyl group having 1 to 3 carbon atoms bonded to the 2-position of the imidazoline ring. Since the imidazoline compound does not have a strong odor, the water-absorbent resin particles containing the imidazoline compound can be suitably used for sanitary material applications. One embodiment of the water-absorbent resin particles containing the imidazoline compound can achieve the above-mentioned effects of the present invention without generating an odor, for example.

[0031] The water-absorbent resin particles may contain an imidazoline compound inside the polymer particles, or may contain an imidazoline compound attached to the surface of the polymer particles. When the imidazoline compound is contained inside the polymer particles, the decrease in viscosity over time when the particles are swollen with an aqueous solution containing iron can be further suppressed.

[0032] In formula (1), R is an alkyl group having 1 to 3 carbon atoms. The number of carbon atoms in the alkyl group represented by R may be 2 to 3, or may be 3, from the viewpoint of more significantly achieving the effects of the present invention. Examples of R include a methyl group, an ethyl group, an n-propyl group, and an isopropyl group. R may be an alkyl group having 3 carbon atoms. Examples of imidazoline compounds include 2-(2-imidazolin-2-yl)propane and 2-propyl-2-imidazoline. The imidazoline compound may contain one type, or may contain two or more types in combination.

[0033]

[0113] From the viewpoint of suppressing a decrease in viscosity over time when swollen with an aqueous solution containing iron, which is more excellent due to the effect of the present invention, the content of the imidazoline compound may be 25 ppm by mass or more, 30 ppm by mass or more, 35 ppm by mass or more, 40 ppm by mass or more, 50 ppm by mass or more, 60 ppm by mass or more, 70 ppm by mass or more, 80 ppm by mass or more, 90 ppm by mass or more, 100 ppm by mass or more, 150 ppm by mass or more, or 200 ppm by mass or more, relative to the solid content of the water absorbent resin particle. The content of the imidazoline compound may be 11,000 ppm by mass or less, 9,000 ppm by mass or less, 7,000 ppm by mass or less, 5,000 ppm by mass or less, 3,000 ppm by mass or less, 1,000 ppm by mass or less, 800 ppm by mass or less, 600 ppm by mass or less, or 400 ppm by mass or less, or 300 ppm by mass or less, from the viewpoint of making gel blocking less likely to occur during liquid absorption and achieving even better moisture absorbency under no pressure. From the viewpoint of achieving even better moisture absorbency under no pressure, the content of the imidazoline compound may be 25 to 11,000 ppm by mass, 25 to 5,000 ppm by mass, 25 to 1,000 ppm by mass, 25 to 800 ppm by mass, 25 to 600 ppm by mass, 25 to 400 ppm by mass, or 25 to 200 ppm by mass, relative to the solid content of the water absorbent resin particles.

[0034] The content of the imidazoline compound in the water-absorbent resin particles relative to the solid content of the water-absorbent resin particles is measured by the method described in the examples below.

[0035] <Method of manufacturing water-absorbent resin particles> The water-absorbent resin particles according to this embodiment can be produced by a method including incorporating an imidazoline compound into water-absorbent resin particles.

[0036] (Method of producing polymer particles) The polymer particles can be obtained, for example, by a method including a step of polymerizing a monomer including an ethylenically unsaturated monomer. The monomer polymerization method can be selected from, for example, reverse phase suspension polymerization, aqueous solution polymerization, bulk polymerization, and precipitation polymerization. Internally crosslinked polymer particles may be obtained by polymerizing the ethylenically unsaturated monomer in the presence of an internal crosslinking agent. Polymer particles containing inorganic particles therein may be obtained by polymerizing the ethylenically unsaturated monomer in the presence of inorganic particles such as silica. A part or all of the ethylenically unsaturated monomer may form a salt such as an alkali metal salt.

[0037] In the case of aqueous solution polymerization, for example, polymer particles can be obtained by a method including polymerizing an ethylenically unsaturated monomer in an aqueous monomer solution containing the ethylenically unsaturated monomer and water to form a hydrogel polymer containing the polymer, and drying the hydrogel polymer. When a bulk hydrogel polymer is formed, it may be crushed, and the crushed hydrogel polymer may be dried. The hydrogel polymer or its crushed product may be dried and then crushed, and the particles obtained by crushing may be classified. The polymer particles may be a dried crushed product, or may be particles obtained by further crushing the crushed product. The polymer particles obtained by crushing the crushed product may be classified, and the particle size of the polymer particles may be adjusted as necessary.

[0038] The concentration of the ethylenically unsaturated monomer in the aqueous monomer solution may be 20% by mass or more and a saturated concentration or less, 25 to 70% by mass, or 30 to 50% by mass, based on the mass of the aqueous monomer solution.

[0039] The aqueous monomer solution may further contain a polymerization initiator. The polymerization initiator may be a photopolymerization initiator or a thermal radical polymerization initiator, or may be a water-soluble thermal radical polymerization initiator. The thermal radical polymerizable compound may be an azo compound, a peroxide, or a combination thereof. The amount of the polymerization initiator may be 0.01 to 15 millimoles per mole of the ethylenically unsaturated monomer.

[0040] Examples of persulfates used as polymerization initiators include potassium persulfate, ammonium persulfate, and sodium persulfate. Examples of azo compounds used as polymerization initiators include 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenylamidino)propane] dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidino]propane} dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidino]propane} dihydrochloride, 2,2'-azobis[2-(N-benzylamidino)propane] dihydrochloride, and 2,2'-azobis[2-(N-allylamidino)propane] dihydrochloride.

[0041] The aqueous monomer solution may further contain the above-mentioned internal crosslinking agent. The amount of the internal crosslinking agent may be 0 mmol or more, 0.001 mmol or more, 0.01 mmol or more, 0.015 mmol or more, or 0.020 mmol or more, and may be 2 mmol or less, 1 mmol or less, 0.5 mmol or less, or 0.1 mmol or less, per 1 mol of the ethylenically unsaturated monomer. The aqueous monomer solution may further contain other additives such as a chain transfer agent and a thickener, as necessary.

[0042] The polymerization temperature varies depending on the polymerization initiator used, but may be, for example, 0 to 130° C. or 10 to 110° C. The polymerization time may be 1 to 200 minutes, or 5 to 100 minutes.

[0043] The water content of the hydrogel polymer formed by polymerization (the water content based on the mass of the hydrogel polymer) may be 30 to 80 mass %, 40 to 75 mass %, or 50 to 70 mass %.

[0044] When crushing a hydrogel polymer block, the crushed product obtained by crushing may be in the form of particles or may have an elongated shape such that the particles are connected together. The minimum width of the crushed product may be, for example, about 0.1 to 15 mm, or about 1.0 to 10 mm. The maximum width of the crushed product may be about 0.1 to 200 mm, or about 1.0 to 150 mm. Examples of equipment for crushing include kneaders (e.g., pressure kneaders, double-arm kneaders, etc.), meat choppers, cutter mills, and farmer mills. The hydrogel polymer block may be cut into pieces before crushing, if necessary.

[0045] The hydrogel polymer or its crushed product is dried mainly to remove water. The drying method may be a common method such as natural drying, heat drying, or vacuum drying. The dried hydrogel polymer or its crushed product is further pulverized, and the resulting particles are classified, if necessary, to obtain polymer particles having an appropriate particle size. The pulverization method is not particularly limited, and examples of suitable methods include a roller mill (roll mill), stamp mill, jet mill, high-speed rotary pulverizer (hammer mill, pin mill, rotor beater mill, etc.), or a container-driven mill (rotary mill, vibration mill, planetary mill, etc.). The classification method is also not particularly limited, and examples of suitable methods include a vibrating sieve, rotary sifter, cylindrical stirring sieve, blower sifter, or rotor-tap shaker.

[0046] The polymer particles produced by the above-mentioned method may be surface-crosslinked with a surface crosslinking agent. For example, the surface-crosslinked polymer particles can be produced by a method comprising heating a reaction mixture containing a crosslinking agent solution containing water and a surface crosslinking agent, and polymer particles, thereby surface-crosslinking the polymer particles.

[0047] The crosslinker solution may be a solution containing water and a surface crosslinker dissolved in water. The solvent contained in the crosslinker solution may be substantially water alone. The proportion of the solvent other than water may be 25% by mass or less, 10% by mass or less, 5% by mass or less, or 1% by mass or less based on the mass of the crosslinker solution.

[0048] The amount of the surface cross-linking agent may be 0.01 to 40 millimoles, 0.1 to 30, or 1 to 20 millimoles per mole of the monomer units constituting the polymer in the polymer particles.

[0049] The heating temperature and heating time for surface cross-linking are adjusted so that the cross-linking reaction proceeds appropriately, taking into consideration the type of surface cross-linking agent, etc. For example, the heating temperature for surface cross-linking may be 80°C or higher, 100°C or higher, 120°C or higher, 150°C or higher, or even higher than 180°C, or may be 190°C or higher. The heating temperature for surface cross-linking may be 250°C or lower. The heating time for surface cross-linking may be, for example, 5 to 90 minutes.

[0050] The surface-crosslinked polymer particles may be further dried and classified as necessary.

[0051] The method for incorporating an imidazoline compound into water-absorbent resin particles is not particularly limited. For example, the imidazoline compound may be incorporated into water-absorbent resin particles by a method including heating a mixture containing a compound solution containing an imidazoline compound and water and polymer particles (for example, surface-crosslinked polymer particles). The obtained water-absorbent resin particles may be further dried or classified as necessary.

[0052] The compound solution may be a solution containing water and an imidazoline compound dissolved in water. The solvent contained in the compound solution may be substantially water alone. The proportion of the solvent other than water may be 25% by mass or less, 10% by mass or less, 5% by mass or less, or 1% by mass or less, based on the mass of the compound solution.

[0053] The heating temperature for incorporating the imidazoline compound may be, for example, 80° C. or higher, 100° C. or higher, 120° C. or higher, 150° C. or higher, or 180° C. or higher, or may be 250° C. or lower. The heating time for incorporating the imidazoline compound may be, for example, 5 to 90 minutes.

[0054] The imidazoline compound may be contained in the water-absorbent resin particles by a method including mixing the imidazoline compound with a raw material for producing the polymer particles (e.g., a monomer aqueous solution). When the imidazoline compound is mixed with the raw material for the polymer particles, water-absorbent resin particles containing the imidazoline compound inside the polymer particles tend to be easily obtained. The mixing of the imidazoline compound may be performed, for example, before, during, or after the polymerization reaction of the monomers.

[0055] The imidazoline compound can also be contained in the water-absorbent resin particles by a method in which a precursor compound of the imidazoline compound is contained in the raw material of the polymer particles. As the precursor compound, a compound capable of forming the imidazoline compound after reaction can be used.

[0056] (Absorbent articles) Fig. 1 is a cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in Fig. 1 comprises a sheet-like absorbent body 10, core wraps 20a and 20b, a liquid-permeable sheet 30, and a liquid-impermeable sheet 40. In the absorbent article 100, the liquid-impermeable sheet 40, the core wrap 20b, the absorbent body 10, the core wrap 20a, and the liquid-permeable sheet 30 are layered in this order. In Fig. 1, some parts are shown as having gaps between the members, but the members may be in close contact with each other without any gaps.

[0057] The absorbent body 10 includes the water-absorbent resin particles 10a according to the above-described embodiment and a fiber layer 10b containing fibrous material. The water-absorbent resin particles 10a are dispersed in the fiber layer 10b.

[0058] The core wrap 20a is arranged on one side of the absorbent body 10 (the upper side of the absorbent body 10 in FIG. 1) while in contact with the absorbent body 10. The core wrap 20b is arranged on the other side of the absorbent body 10 (the lower side of the absorbent body 10 in FIG. 1) while in contact with the absorbent body 10. The absorbent body 10 is arranged between the core wrap 20a and the core wrap 20b. Examples of the core wraps 20a and 20b include tissue and nonwoven fabric. The core wrap 20a and the core wrap 20b have, for example, a main surface of the same size as the absorbent body 10.

[0059] The liquid-permeable sheet 30 is disposed on the outermost side of the absorbent article 100, on the side into which the liquid to be absorbed penetrates. The liquid-permeable sheet 30 is disposed on the core wrap 20a in a state of contact with the core wrap 20a. Examples of the liquid-permeable sheet 30 include nonwoven fabrics and porous sheets made of synthetic resins such as polyethylene, polypropylene, polyester, and polyamide. The liquid-impermeable sheet 40 is disposed on the outermost side of the absorbent article 100, on the side opposite the liquid-permeable sheet 30. The liquid-impermeable sheet 40 is disposed below the core wrap 20b in a state of contact with the core wrap 20b. Examples of the liquid-impermeable sheet 40 include sheets made of synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride, and sheets made of composite materials of these synthetic resins and nonwoven fabric. The liquid-permeable sheet 30 and the liquid-impermeable sheet 40 have, for example, a main surface that is wider than the main surface of the absorbent body 10, and the outer edges of the liquid-permeable sheet 30 and the liquid-impermeable sheet 40 extend around the absorbent body 10 and the core wraps 20a, 20b.

[0060] The size relationships among the absorbent body 10, core wraps 20a, 20b, liquid-permeable sheet 30, and liquid-impermeable sheet 40 are not particularly limited and are adjusted as appropriate depending on the intended use of the absorbent article, etc. Furthermore, the method for maintaining the shape of the absorbent body 10 using the core wraps 20a, 20b is not particularly limited, and the absorbent body may be wrapped with multiple core wraps as shown in Fig. 1, or may be wrapped with a single core wrap.

[0061] The water-absorbent resin particles according to this embodiment contain an imidazoline compound, which provides excellent water absorption without pressure, and inhibits a decrease in viscosity over time when the particles are swollen in an iron-containing aqueous solution. As one embodiment of the present invention, an agent for improving water absorption without pressure for water-absorbent resin particles is provided, which contains an imidazoline compound as an active ingredient. As another embodiment of the present invention, an agent for inhibiting a decrease in viscosity over time when the particles are swollen in an iron-containing aqueous solution is provided, which contains an imidazoline compound as an active ingredient. As another embodiment of the present invention, a method for improving water absorption without pressure for water-absorbent resin particles, a method for inhibiting a decrease in viscosity over time when the particles are swollen in an iron-containing physiological saline solution, or a method for improving water absorption without pressure for water-absorbent resin particles and inhibiting a decrease in viscosity over time when the particles are swollen in an iron-containing aqueous solution is provided, which includes incorporating an imidazoline compound into the water-absorbent resin particles. [Example]

[0062] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[0063] <Production Example 1: Preparation of surface-crosslinked polymer particles> [Preparation of partially neutralized acrylic acid solution] 475.65 g (6.60 mol) of acrylic acid was placed in a round-bottomed cylindrical separable flask equipped with a stirrer, having an inner diameter of 11 cm and an internal volume of 2 L. While stirring the acrylic acid, 395.70 g of ion-exchanged water was added to the separable flask, and then 415.65 g of 48% by mass sodium hydroxide was added dropwise in an ice bath to prepare 1287.00 g of a sodium-partially neutralized solution of acrylic acid with a monomer concentration of 45% by mass (neutralization rate: 76 mol%).

[0064] [Polymerization process] To 1253.30 g of the sodium-neutralized solution of acrylic acid with a monomer concentration of 45% by weight prepared above, 231.91 g of ion-exchanged water and 1.32 g of polyethylene glycol diacrylate (NOF Corporation, Blenmer ADE-400A) were added to obtain a reaction solution (monomer aqueous solution). The reaction solution was then fed into a 3-L jacketed stainless steel double-arm kneader equipped with a thermometer, a nitrogen inlet tube, two sigma-type blades, an openable lid, and nitrogen gas replacement for 1 hour while maintaining the reaction solution at 25°C. Subsequently, while stirring the reaction solution, 17.26 g (3.19 mmol) of a 5.0% by weight aqueous potassium persulfate solution and 4.83 g of a 0.5% by weight aqueous L-ascorbic acid solution were added. After approximately 1 minute, the temperature began to rise and polymerization began. After 8 minutes, the maximum temperature during polymerization reached 75°C. Stirring was continued while maintaining the jacket temperature at 60°C. The hydrogel was removed 60 minutes after the start of polymerization. The resulting hydrogel was gradually placed in a meat chopper (12VR-750SDX) manufactured by Kiryu Royal Co., Ltd., and chopped into small pieces. The diameter of the holes in the plate located at the outlet of the meat chopper was 6.4 mm.

[0065] [Drying and grinding process] The obtained coarsely crushed material was spread on a wire mesh with openings of 0.8 cm x 0.8 cm and dried by hot air drying at 180°C for 30 minutes to obtain a dried material. The dried material was pulverized using a centrifugal pulverizer (Retsch, ZM200, screen diameter 1 mm, 6000 rpm). The powder obtained by pulverization was sieved using sieves with openings of 850 μm and 180 μm by shaking for 1 minute. By classification, polymer particles (A) were recovered as the fraction that passed through the 850 μm sieve but not through the 180 μm sieve.

[0066] [Surface crosslinking process] 30 g of polymer particles (A) were weighed into a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. Next, while stirring at 300 rpm, a surface cross-linking agent solution obtained by mixing 0.094 g of ethylene carbonate, 0.150 g of propylene glycol, and 0.600 g of deionized water was added dropwise to the separable flask using a Pasteur pipette and stirred for 5 minutes to obtain a mixture. This mixture was heated at 200°C for 35 minutes. After cooling to room temperature, the mixture was classified using a sieve with 850 μm openings. The particles that passed through the 850 μm openings were obtained as surface-cross-linked polymer particles (hereinafter also referred to as "material particles (A)"). This procedure was repeated 10 times to obtain the required amount of material particles (A).

[0067] Example 1 20 g of material particles (A) were weighed into a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. Next, while stirring at 300 rpm, 0.197 g of 2-propyl-2-imidazoline (Tokyo Chemical Industry Co., Ltd.) and 0.400 g of deionized water were mixed to obtain a 2-propyl-2-imidazoline aqueous solution, which was added dropwise to the separable flask using a Pasteur pipette and stirred for 5 minutes to obtain a mixture. This mixture was heated at 180°C for 30 minutes. After cooling to room temperature, the mixture was classified using a sieve with an opening of 850 μm. By classification, water-absorbent resin particles that passed through the 850 μm sieve were obtained as the water-absorbent resin particles of Example 1. At this time, quantitative analysis by GC-MS revealed that 2-propyl-2-imidazoline was contained in an amount of 860 ppm by mass relative to the solid content of the water-absorbent resin particles.

[0068] <Example 2> 20 g of material particles (A) were weighed into a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. Next, while stirring at 300 rpm, 0.029 g of 2-propyl-2-imidazoline (Tokyo Chemical Industry Co., Ltd.) and 0.400 g of deionized water were mixed to obtain a 2-propyl-2-imidazoline aqueous solution, which was added dropwise to the separable flask using a Pasteur pipette and stirred for 5 minutes to obtain a mixture. This mixture was heated at 180°C for 30 minutes. After cooling to room temperature, the mixture was classified using a sieve with an opening of 850 μm. By classification, water-absorbent resin particles that passed through the 850 μm sieve were obtained as the water-absorbent resin particles of Example 2. At this time, quantitative analysis by GC-MS revealed that 2-propyl-2-imidazoline was contained in an amount of 174 ppm by mass relative to the solid content of the water-absorbent resin particles.

[0069] Example 3 20 g of material particles (A) were weighed into a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. Next, while stirring at 300 rpm, 0.005 g of 2-propyl-2-imidazoline (Tokyo Chemical Industry Co., Ltd.) and 0.400 g of deionized water were mixed to obtain a 2-propyl-2-imidazoline aqueous solution, which was added dropwise to the separable flask using a Pasteur pipette and stirred for 5 minutes to obtain a mixture. This mixture was heated at 180°C for 30 minutes. After cooling to room temperature, the mixture was classified using a sieve with an opening of 850 μm. By classification, water-absorbent resin particles that passed through the 850 μm sieve were obtained as the water-absorbent resin particles of Example 3. At this time, quantitative analysis by GC-MS revealed that 2-propyl-2-imidazoline was contained in an amount of 31 ppm by mass relative to the solid content of the water-absorbent resin particles.

[0070] <Comparative Example 1> 20 g of material particles (A) was weighed into a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. Next, while stirring at 300 rpm, 0.400 g of deionized water was dropped into the separable flask using a Pasteur pipette, and the mixture was stirred for 5 minutes to obtain a mixture. This mixture was heated at 180°C for 30 minutes. After cooling to room temperature, the mixture was classified using a sieve with an opening of 850 μm. By classification, water-absorbent resin particles that passed through the 850 μm sieve were obtained as water-absorbent resin particles of Comparative Example 1.

[0071] Example 4 [Preparation of partially neutralized acrylic acid solution] 475.65 g (6.60 mol) of acrylic acid was placed in a round-bottomed cylindrical separable flask equipped with a stirrer, having an inner diameter of 11 cm and an internal volume of 2 L. While stirring the acrylic acid, 395.70 g of ion-exchanged water was added to the separable flask, and then 415.65 g of 48% by mass sodium hydroxide was added dropwise in an ice bath to prepare 1287.00 g of a partially neutralized acrylic acid solution (neutralization rate: 76 mol%) with a monomer concentration of 46% by mass.

[0072] [Polymerization process] 252.86 g of the prepared sodium partially neutralized solution of acrylic acid, 39.65 g of ion-exchanged water, 0.27 g of polyethylene glycol diacrylate (NOF Corporation, Blenmer ADE-400A) as an internal crosslinking agent, and 0.17 g of 2-propyl-2-imidazoline (Tokyo Chemical Industry Co., Ltd.) were placed in a fluororesin-coated stainless steel tray (internal dimensions of opening: 175 mm × 130 mm, internal dimensions of bottom: 155 mm × 110 mm, height: 30 mm). A single stir bar (8 mm diameter, 45 mm length, no ring) was placed in the center of the stainless steel tray, and a uniform liquid mixture (monomer aqueous solution) was formed by stirring. A thermometer was placed in the center of the stainless steel tray to measure the temperature of the mixture. The opening of the stainless steel tray was then covered with polyethylene film. After adjusting the temperature of the mixture to 25°C, nitrogen gas was bubbled through a tube inserted into the mixture to purge the reaction system with nitrogen until the dissolved oxygen content was 0.1 ppm or less. Next, while stirring the mixture at 300 rpm, the nitrogen purge tube was removed from the reaction system, and 8.71 g (0.644 mmol) of a 2% by weight aqueous potassium persulfate solution and 0.98 g of a 0.5% by weight aqueous L-ascorbic acid solution were added dropwise to the mixture in the stainless steel tray using a syringe (3 mL disposable syringe manufactured by Henke Sas Wolf, syringe needle manufactured by Terumo Corporation).

[0073] The polymerization reaction started immediately after the L-ascorbic acid aqueous solution was added dropwise. Stirring was stopped 3 minutes after the addition of the L-ascorbic acid aqueous solution was completed. As the polymerization reaction progressed, the viscosity of the reaction solution increased, and then the mixture gelled. Nine minutes after the addition of the L-ascorbic acid aqueous solution was completed, the thermometer measuring the temperature of the mixture reached a maximum value of 73°C. The stainless steel tray containing the hydrogel polymer containing water and polymer, which had been formed by the gelation of the reaction solution, was then immersed in a 75°C water bath, and the hydrogel polymer was allowed to age in that state for 20 minutes.

[0074] The gelled mixture, the hydrogel polymer, was removed from the stainless steel tray and immediately cut into pieces approximately 5 cm wide. The cut hydrogel polymer was then roughly crushed using a meat chopper (Kire Royal Co., Ltd., 12VR-750SDX). A crushed material containing elongated structures of the hydrogel polymer was discharged from multiple circular discharge holes in a plate attached to the end of the kneading chamber of the meat chopper. The diameter of the discharge holes was 6.4 mm. Crushing with the meat chopper was continued for 5 minutes, starting from the time the hydrogel polymer was added. The resulting crushed material was an aggregate formed by multiple elongated structures measuring 4 to 6 mm in width.

[0075] [Drying and grinding process] The obtained coarsely crushed material was spread on a wire mesh with 0.8 cm × 0.8 cm openings and dried with hot air at 180°C for 30 minutes to obtain a dried product. The dried product was pulverized using a centrifugal pulverizer (Retsch, ZM200, screen diameter 1 mm, 6000 rpm). The powder obtained by pulverization was sieved using 850 μm and 180 μm opening sieves by shaking for 1 minute. By classification, polymer particles (B) were recovered as the fraction that passed through the 850 μm sieve but not the 180 μm sieve.

[0076] [Surface crosslinking process] 30 g of polymer particles (B) was weighed into a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. Next, while stirring at 300 rpm, a surface crosslinking agent solution obtained by mixing 0.094 g of ethylene carbonate, 0.150 g of propylene glycol, and 0.600 g of deionized water was dropped into the separable flask using a Pasteur pipette and stirred for 5 minutes to obtain a mixture. This mixture was heated at 200°C for 35 minutes. After cooling to room temperature, the mixture was classified using a sieve with an opening of 850 μm. By classification, water-absorbent resin particles that passed through the sieve with an opening of 850 μm were obtained as water-absorbent resin particles of Example 4. At this time, quantitative analysis by GC-MS revealed that 2-propyl-2-imidazoline was contained in an amount of 37 ppm by mass relative to the solid content.

[0077] <Moisture content> 2.0 g of water-absorbent resin particles are placed in an aluminum foil case (No. 8) previously adjusted to a constant weight (W1 (g)), the opening of the aluminum foil case is loosely closed, and the total mass W2 (g) of the aluminum foil case containing the sample is precisely weighed. The aluminum foil case containing the sample is dried for 2 hours in a hot air dryer (manufactured by ADVANTEC, model: FV-320) with the internal temperature set to 200°C. After drying, the aluminum foil case containing the sample is allowed to cool to room temperature in a desiccator. After cooling, the total mass W3 (g) of the aluminum foil case containing the sample is measured. The moisture content of the sample is calculated using the following formula. Moisture content [mass%]=[{(W2-W1)-(W3-W1)} / (W2-W1)]×100

[0078] <Centrifuge holding capacity> The centrifuge retention capacity (CRC) was measured using the following procedure, based on the EDANA method (NWSP 241.0.R2(15), pages 769-778). The measurement was performed in an environment with a temperature of 25°C ± 2°C and a humidity of 50% ± 10%.

[0079] A nonwoven fabric measuring 60 mm x 170 mm (product name: Heat Pack MWA-18, manufactured by Nippon Paper Papylia Co., Ltd.) was folded in half lengthwise to reduce the size to 60 mm x 85 mm. A 60 mm x 85 mm nonwoven fabric bag was produced by heat-sealing the nonwoven fabric together on both sides extending in the lengthwise direction (5 mm-wide crimped portions were formed on both sides along the lengthwise direction). 0.2 g of water-absorbent resin particles was precisely weighed and placed inside the nonwoven fabric bag. The remaining short-side edge was then heat-sealed to close the nonwoven fabric bag.

[0080] The nonwoven fabric bag was floated without being folded over on 1000 g of saline contained in a stainless steel tray (240 mm × 320 mm × 45 mm) to completely wet the entire nonwoven fabric bag. One minute after placing the nonwoven fabric bag in the saline, the nonwoven fabric bag was immersed in the saline with a spatula to obtain a nonwoven fabric bag containing the gel.

[0081] The nonwoven fabric bag was removed from the saline solution 30 minutes after it was placed in the saline solution (a total of 1 minute of floating time and 29 minutes of immersion time). The nonwoven fabric bag was then placed in a centrifuge (Kokusan Co., Ltd., model number: H-122). After the centrifugal force in the centrifuge reached 250 G, the nonwoven fabric bag was dehydrated for 3 minutes. After dehydration, the mass Ma [g] of the nonwoven fabric bag, including the mass of the gel, was weighed. The same operation as described above was performed on the nonwoven fabric bag without containing water-absorbent resin particles, and the mass Mb [g] of the nonwoven fabric bag after dehydration was measured. The CRC [g / g] was calculated based on the following formula: Mc [g] is the precisely weighed value of 0.2 g of the water-absorbent resin particles used in the measurement. CRC=[(Ma-Mb)-Mc] / Mc

[0082] <No pressure DW> The no-pressure DW of the water-absorbent resin particles was measured using a measuring device Z shown in Fig. 2. The particles to be measured are a fraction of the water-absorbent resin particles that pass through a sieve with a mesh size of 500 µm but do not pass through a sieve with a mesh size of 250 µm.

[0083] The measuring device Z includes a burette unit 71, a conduit 72, a flat measurement table 73, a nylon mesh 74, a stand 75, and a clamp 76. The burette unit 71 includes a burette 71a with a scale, a rubber stopper 71b that seals the upper opening of the burette 71a, a stopcock 71c connected to the lower tip of the burette 71a, and an air inlet tube 71d and a stopcock 71e connected to the lower part of the burette 71a. The burette unit 71 is fixed with a clamp 76. The measurement table 73 has a through-hole 73a with a diameter of 2 mm formed in its center and is supported by a height-adjustable stand 75. The through-hole 73a of the measurement table 73 and the stopcock 71c of the burette unit 71 are connected by a conduit 72. The inner diameter of the conduit 72 is 6 mm.

[0084] The measurements were performed in an environment with a temperature of 25°C and a humidity of 60±10%. First, stopcocks 71c and 71e of the burette 71 were closed, and saline solution 77 adjusted to 25°C was poured into the burette 71a through the opening at the top of the burette 71a. After sealing the opening of the burette 71a with rubber stopper 71b, stopcocks 71c and 71e were opened. The inside of the conduit 72 was filled with saline solution 77 while preventing air bubbles from entering. The height of the measurement platform 73 was adjusted so that the level of the saline solution 77 reaching the through-hole 73a was the same as the level of the upper surface of the measurement platform 73. At this time, it was confirmed that air of the same volume as the saline solution 77 sucked out of the through-hole 73a was rapidly supplied into the burette through the air inlet tube 71d. After adjustment, the level of the saline solution 77 in the burette 71a was read on the scale of the burette 71a, and this position was designated as the zero point (the reading at 0 seconds).

[0085] A nylon mesh 74 (100 mm x 100 mm, 250 mesh, thickness: approximately 50 μm) was laid near the through-hole 73a on the measurement table 73, and a cylinder with an inner diameter of 30 mm and a height of 20 mm was placed in the center of the mesh. 1.00 g of target particles 78 was uniformly dispersed in this cylinder. The cylinder was then carefully removed, and a sample in which the target particles 78 were dispersed in a circular pattern was obtained in the center of the nylon mesh 74. Next, the nylon mesh 74 with the target particles 78 placed on it was quickly moved so that its center was positioned at the through-hole 73a, but not so that the target particles 78 were scattered, and measurement was started. The point when air bubbles were first introduced into the burette 71a from the air inlet tube 71d was defined as the start of water absorption (0 seconds).

[0086] The amount of saline solution 77 lost in the burette 71a (i.e., the amount of saline solution 77 absorbed by the particles 78 to be measured) was read in 0.1 mL increments, and the amount of saline solution 77 lost Wc [g] 5 minutes after the particles 78 to be measured started to absorb water was read. The 5-minute value of the no-pressure DW was calculated from Wc using the following formula. The no-pressure DW is the amount of water absorbed per 1.00 g of particles 78 to be measured. No-pressure DW value [mL / g] = Wc / 1.00

[0087] [Viscosity evaluation of swollen gels prepared in aqueous solutions containing iron] The viscosity of the swollen gel in an aqueous solution containing iron was evaluated by measuring the viscosity of the swollen gel in a physiological saline solution containing iron. The viscosity of the swollen gel was evaluated for the water-absorbent resin particles obtained in the Examples and Comparative Examples by the following method.

[0088] (Preparation of iron-containing saline solution) An iron-containing physiological saline solution (iron ion concentration 50 ppm by mass) was prepared with the following composition. Sodium chloride: 9.0g Ferrous sulfate heptahydrate: 0.249g Ion-exchanged water: 990.75g

[0089] (Preparation of swollen gel in iron-containing saline) The swollen gel was prepared by using 34 times the amount of iron-containing physiological saline solution relative to the solid content of the water-absorbent resin particles. 97.14 g of the above iron-containing saline solution was weighed into a 100 mL tall beaker (Shibata Scientific Co., Ltd., 50 mm outer diameter, 88 mm height). A magnetic stirrer bar (6 mm diameter x 20 mm, no ring) was added and placed on a magnetic stirrer (Iuchi Co., Ltd., HS-30D). The magnetic stirrer bar was then adjusted to rotate at 600 rpm. Next, 2.93 g of water-absorbent resin particles (water content 2.3%, solids content 2.86 g) were added to the stirring beaker, and stirring was continued until the vortex disappeared and the liquid level became horizontal, preparing a swollen gel to be used as a measurement sample. Immediately after preparation of the swollen gel, the stirrer tip was gently removed from the beaker, and the beaker containing the swollen gel was covered with plastic wrap (Mitsubishi Chemical Corporation, DiaWrap).

[0090] (Evaluation of viscosity of swollen gel) The gel was then left to stand in a hot air dryer at 40°C for 1 hour, and the viscosity of the swollen gel was measured using a B-type viscometer (Shibaura Systems Co., Ltd., Vismetron Viscometer VDH2, rotation speed: 20 rpm, rotor: No. 6, timer: 60 sec.), and the value measured was taken as the 1-hour viscosity of the swollen gel [mPa s]. The measurement was carried out in an environment at a temperature of 25°C. Each gel viscosity was measured three times, and the average value was used. The gels were then left to stand for another 4 hours in a hot air dryer at 40°C, and the viscosity of the swollen gels was measured in the same manner as above. The value at this point was taken as the 5-hour viscosity value of the swollen gels [mPa s]. The measurements were carried out in an environment at a temperature of 25°C. Each gel viscosity was measured three times, and the average value was used.

[0091] [Calculation of viscosity maintenance rate of swollen gel] The viscosity retention rate [%] of the swollen gel was calculated using the following formula. Viscosity retention rate of swollen gel [%] = 5-hour viscosity of swollen gel [mPa·s] / 1-hour viscosity of swollen gel [mPa·s] × 100

[0092] [Quantitative analysis of 2-propyl-2-imidazoline contained in water-absorbent resin particles using GC / NPD and GC / MS analyzers] Quantitative analysis of 2-propyl-2-imidazoline contained in the water-absorbent resin particles was carried out by the following method, and the content [ppm by mass] of the imidazoline compound in 1 g of the water-absorbent resin particles was obtained.

[0093] 5 mL of distilled water was added to 1 g of water-absorbent resin particles to swell them, and then 25 mL of acetone was added to precipitate the polymer. The resulting acetone-soluble matter was collected in a recovery flask while filtering through filter paper and concentrated in an evaporator. The concentrated liquid was then transferred to a 10 mL measuring flask, and the volume was adjusted while rinsing the recovery flask with acetone. The liquid was then subjected to GC / NPD and GC / MS measurements.

[0094] The conditions for GC / NPD and GC / MS analysis were as follows: The content of imidazoline compounds was calculated using a calibration curve prepared using 2-propyl-2-imidazoline (Tokyo Chemical Industry Co., Ltd.).

[0095] Analytical equipment: GC / NPD and GC / MS Agilent Technologies, 7890A / 5975C Column: DB-HeavyWax 30 m x 0.25 mm ID x 0.25 μm Carrier gas: He (3.0 mL / min) Column temperature: 40°C (3 min) → 20°C / min → 250°C Inlet: Split (5:1) Inlet temperature: 250℃ Detector: MS, NPD (selective detector for nitrogen and phosphorus compounds) Temperature: 300℃ H2 flow rate: 3 mL / min Air flow rate: 60 mL / min Ionization method: EI Emission current: 35 μA Electron energy: 70 eV EM voltage: 1576V Source temperature: 230℃ Q pole temperature: 150℃ Interface temperature: 300℃

[0096] [Calculation of the content of imidazoline compounds relative to the solid content of water-absorbent resin particles] The content of the imidazoline compound [ppm by mass] relative to the solid content of the water-absorbent resin particles was calculated using the following formula.

[0097] Imidazoline compound relative to the solid content of water-absorbent resin particles [ppm by mass] = (Imidazoline compound content [ppm by mass] in 1 g of water-absorbent resin particles) / (Solid content rate of water-absorbent resin particles [% by mass]) × 100 Solid content of water-absorbent resin particles [mass%] = 100 - moisture content [mass%]

[0098] [Table 1]

[0099] The measurement results of the 5-minute value of the no-pressure DW and the viscosity maintenance rate of the swollen gel are shown in Table 1. As shown in Table 1, it was confirmed that by including 2-propyl-2-imidazoline in the water-absorbent resin particles, the water absorbency under no pressure is excellent, and the decrease in viscosity over time when swollen with an aqueous solution containing iron is suppressed. [Explanation of symbols]

[0100] 10...absorbent body, 10a...water-absorbent resin particles, 10b...fiber layer, 20a, 20b...core wrap, 30...liquid-permeable sheet, 40...liquid-impermeable sheet, 71...burette part, 71a...burette, 71b...rubber stopper, 71c, 71e...cock, 71d...air introduction tube, 72...conduit, 73...measurement table, 74...nylon mesh, 73a...through hole, 75...stand, 76...clamp, 77...physiological saline solution, 78...particles to be measured, 100...absorbent article, Z...measuring device.

Claims

1. It contains an imidazoline compound represented by the following formula (1): The 5-minute value of the pressureless DW is 27 mL / g or more; Water-absorbent resin particles, which have a viscosity retention rate of 42% or more when swollen with iron-containing physiological saline. 【Chemistry 1】 [In formula (1), R represents an alkyl group having 1 to 3 carbon atoms.]

2. 2. The water-absorbent resin particles according to claim 1, wherein the content of the imidazoline compound is 25 to 11,000 ppm by mass relative to the solid content of the water-absorbent resin particles.

3. The water-absorbent resin particles according to claim 1, wherein the imidazoline compound is 2-propyl-2-imidazoline.

4. An absorbent material containing the water-absorbent resin particles according to any one of claims 1 to 3.

5. An absorbent article comprising the absorbent body according to claim 4.

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