阅读说明：本技术 吸水性树脂颗粒 (Water-absorbent resin particles ) 是由 河原彻 于 2020-03-05 设计创作，主要内容包括：本发明公开了一种吸水性树脂颗粒,其中,以下述方法测定的凝胶流出试验力为5～14N。凝胶流出试验力测定方法：使吸水性树脂颗粒在搅拌下吸收该吸水性树脂颗粒的29倍量的生理盐水来制作溶胀凝胶。向在底部具有直径5mm的孔的内径5cm的缸体内均等地放入上述溶胀凝胶20g。使用直径4.9cm的固定工具以10mm/分钟的速度压缩上述缸体内的上述溶胀凝胶,将从上述缸体底部的孔流出一部分上述溶胀凝胶的时刻的试验力记录为凝胶流出试验力。(Disclosed are water-absorbent resin particles having a gel flow-out test force of 5-14N as measured by the following method. Gel flow-out test force determination method: the water-absorbent resin particles were allowed to absorb 29 times the amount of physiological saline in the water-absorbent resin particles under stirring to prepare a swollen gel. 20g of the above-mentioned swollen gel was equally put into a cylinder having an inner diameter of 5cm and having a hole of 5mm in diameter at the bottom. The swollen gel in the cylinder was compressed at a rate of 10 mm/min using a jig having a diameter of 4.9cm, and the test force at the time when a part of the swollen gel flowed out from the hole in the bottom of the cylinder was recorded as a gel flow-out test force.)

1. Water-absorbent resin particles having a gel flow-out test force of 5 to 14N as measured by the following method,

gel flow-out test force determination method: making the water-absorbent resin particles absorb 29 times of physiological saline in the amount of the water-absorbent resin particles under stirring to prepare a swollen gel; placing 20g of the swollen gel equally into a cylinder having an inner diameter of 5cm and having a hole of 5mm in diameter at the bottom; the swollen gel in the cylinder was compressed at a rate of 10 mm/min using a holding tool having a diameter of 4.9cm, and the test force at the time when a part of the swollen gel flowed out from the hole in the bottom of the cylinder was recorded as a gel flow-out test force.

2. The water-absorbent resin particles according to claim 1,

the water retention capacity of the physiological saline is 30-60 g/g.

3. The water-absorbent resin particles according to claim 1 or 2, wherein,

the value of the water absorption capacity 2 hours of the physiological saline under the load of 4.14kPa is more than 15 ml/g.

4. An absorbent material comprising 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.

6. The absorbent article of claim 5, which is a diaper.

7. A method for determining gel efflux test force comprising:

a step of preparing a swollen gel by allowing the water-absorbent resin particles to absorb 29 times as much physiological saline as the water-absorbent resin particles with stirring;

a step of equally putting 20g of the swollen gel into a cylinder body having an inner diameter of 5cm and having a hole of 5mm in diameter at the bottom; and

a step of compressing the swollen gel in the cylinder at a speed of 10 mm/min using a fixing tool having a diameter of 4.9cm, and recording a test force at a time when a part of the swollen gel flows out from the hole in the bottom of the cylinder as a gel flow-out test force.

8. A method for producing water-absorbent resin particles, comprising the step of sorting water-absorbent resin particles having a gel flow-out test force of 5 to 14N as measured by the measurement method according to claim 7.

9. A method for reducing the rewet amount of an absorbent article, comprising the step of setting the gel flow-out test force of the water-absorbent resin particles measured by the measurement method according to claim 7 to 5 to 14N.

Technical Field

The present invention relates to a water-absorbent resin particle.

Background

Water-absorbent resins are used in the field of sanitary articles. Specifically, the material is used as a material for an absorber included in an absorbent article such as a diaper (for example, patent documents 1 and 2).

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2006-199805

Patent document 2: international publication No. 2006/123561

Disclosure of Invention

Technical problem to be solved by the invention

When a load is applied to the absorbent article by the weight of the wearer or the like after the absorbent article has absorbed liquid, the absorbed liquid may temporarily permeate back to the surface of the absorbent article. If the absorption performance of the absorbent article is insufficient, the rewet amount tends to increase.

An object of the present invention is to provide water-absorbent resin particles capable of reducing the amount of rewet in an absorbent article.

Means for solving the problems

The present inventors have unexpectedly found that the higher the running-out property of the swollen gel, the lower the rewet amount in the absorbent article.

The water-absorbent resin particles of the present invention have a gel flow-out test force of 5 to 14N as measured by the following method.

Gel flow-out test force determination method: the water-absorbent resin particles were allowed to absorb 29 times the amount of physiological saline in the water-absorbent resin particles under stirring to prepare a swollen gel. 20g of the above-mentioned swollen gel was equally put into a cylinder having an inner diameter of 5cm and having a hole of 5mm in diameter at the bottom. The swollen gel in the cylinder was compressed at a rate of 10 mm/min using a jig having a diameter of 4.9cm, and the test force at the time when a part of the swollen gel flowed out from the hole in the bottom of the cylinder was recorded as a gel flow-out test force.

In the water-absorbent resin particles, the water retention capacity of physiological saline is preferably 30 to 60 g/g.

In the water-absorbent resin particles, the water absorption capacity of physiological saline under a load of 4.14kPa for 2 hours is preferably 15ml/g or more.

The present invention also provides an absorbent material containing the water-absorbent resin particles.

The present invention also provides an absorbent article provided with the absorber.

The absorbent article may be a diaper.

The invention also provides a method for measuring the gel outflow test force, which comprises the following steps: a step of preparing a swollen gel by allowing water-absorbent resin particles to absorb 29 times as much physiological saline as the water-absorbent resin particles with stirring; a step of uniformly placing 20g of the swollen gel in a cylinder having a bottom with a hole of 5mm in diameter and an inner diameter of 5 cm; and a step of compressing the swollen gel in the cylinder at a rate of 10 mm/min by using a fixing tool having a diameter of 4.9cm, and recording a test force at a time when a part of the swollen gel flows out from the hole in the bottom of the cylinder as a gel flow-out test force.

The present invention also provides a method for producing water-absorbent resin particles, which comprises the step of sorting water-absorbent resin particles having a gel discharge test force of 5 to 14N as measured by the above-mentioned measuring method.

The present invention also provides a method for reducing the rewet amount of an absorbent article, comprising the step of setting the gel flow-out test force of the water-absorbent resin particles measured by the above-mentioned measuring method to 5 to 14N.

Effects of the invention

According to the present invention, there is provided a water-absorbent resin particle capable of reducing the rewet amount in an absorbent article.

Drawings

Fig. 1 is a cross-sectional view showing an example of an absorbent article.

Fig. 2 is a plan view showing the general shape of the stirring blade used in the example.

FIG. 3 is a schematic cross-sectional view of an apparatus for measuring a gel flow-out test force.

FIG. 4 is a graph showing an example of the results of measurement of the gel flow-out test force.

Fig. 5 is a schematic view showing a device for measuring the amount of water absorbed under load.

Detailed Description

Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

In the present specification, "acrylic acid" and "methacrylic acid" are collectively referred to as "(meth) acrylic acid". "acrylates" and "methacrylates" are also referred to as "(meth) acrylates". "(poly)" refers to both the case of having the prefix "poly" and the case of not having the prefix "poly". In the numerical ranges recited in the present specification, the upper limit or the lower limit of the numerical range in one stage may be arbitrarily combined with the upper limit or the lower limit of the numerical range in another stage. In the numerical ranges described in the present specification, the upper limit or the lower limit of the numerical range may be replaced with the values shown in the examples. "Water-soluble" means a solubility of 5% by mass or more in water at 25 ℃. The materials exemplified in this specification may be used alone, or 2 or more kinds may be used in combination. The content of each component in the composition indicates the total amount of a plurality of substances present in the composition, when a plurality of substances corresponding to each component are present in the composition, unless otherwise specified. In the present specification, the physiological saline refers to an aqueous solution of NaCl with a concentration of 0.9 mass%, unless otherwise specified.

The water-absorbent resin particles according to the present embodiment have a gel flow-out test force of 5 to 14N as measured by the following method.

Gel flow-out test force determination method: the water-absorbent resin particles were allowed to absorb 29 times the amount of physiological saline in the water-absorbent resin particles under stirring to prepare a swollen gel. 20g of the above-mentioned swollen gel was equally put into a cylinder having an inner diameter of 5cm and having a hole of 5mm in diameter at the bottom. The swollen gel in the cylinder was compressed at a rate of 10 mm/min using a jig having a diameter of 4.9cm, and the test force at the time when a part of the swollen gel flowed out from the hole in the bottom of the cylinder was recorded as a gel flow-out test force. More specific measurement methods are shown in examples described later.

In the water-absorbent resin particles according to the present embodiment, the gel flow-out test force may be 13.5N or less, 13N or less, or 12.5N or less. The gel flow-out test force of the water-absorbent resin particles may be, for example, 6N or more, 7N or more, 8N or more, or 10N or more.

The water-retention capacity of physiological saline in the water-absorbent resin particles according to the present embodiment is preferably in the following range. From the viewpoint of easily obtaining an excellent permeation rate in an absorbent article, the water retention amount is preferably 30g/g or more, 35g/g or more, 40g/g or more, 43g/g or more, or 45g/g or more. From the viewpoint of easily obtaining an excellent permeation rate in an absorbent article, the water retention amount is preferably 80g/g or less, 75g/g or less, 70g/g or less, 65g/g or less, 60g/g or less, 55g/g or less, 52g/g or less, or 50g/g or less. From these viewpoints, the water retention is preferably 30 to 80g/g, more preferably 40 to 65 g/g. The water retention may be at room temperature (25 ℃. + -. 2 ℃). The water retention amount can be measured by the method described in the examples described later.

The water-absorbent resin particles according to the present embodiment may have a 2-hour value of the water absorption capacity (water absorption capacity under load) of physiological saline under a load of 4.14kPa of, for example, 15ml/g or more, or 18ml/g or more, or 20ml/g or more. The water absorption capacity of physiological saline under a load of 4.14kPa may be, for example, 35ml/g or less, 30ml/g or less, or 25ml/g or less. The water absorption capacity of physiological saline under a load of 4.14kPa was measured by the method described in the examples below.

Examples of the shape of the water-absorbent resin particles include substantially spherical, crushed, and granular shapes. The median particle diameter of the water-absorbent resin particles may be 250 to 850. mu.m, 300 to 700. mu.m, or 300 to 600. mu.m. The water-absorbent resin particles according to the present embodiment may have a desired particle size distribution at the time of obtaining polymer particles by the production method described later, but the particle size distribution may be adjusted by performing an operation such as particle size adjustment using classification with a sieve.

The water-absorbent resin particles according to the present embodiment can contain, for example, a crosslinked polymer formed by polymerization of a monomer containing an ethylenically unsaturated monomer. The crosslinked polymer has monomer units derived from an ethylenically unsaturated monomer. That is, the water-absorbent resin particles according to the present embodiment can have a structural unit derived from an ethylenically unsaturated monomer.

Examples of the method for polymerizing the monomer include reversed-phase suspension polymerization, aqueous solution polymerization, bulk polymerization, and precipitation polymerization. Among these, the reversed-phase suspension polymerization method or the aqueous solution polymerization method is preferable from the viewpoint of ensuring good water absorption characteristics of the water-absorbent resin particles to be obtained and easily controlling the polymerization reaction. Hereinafter, a method of polymerizing an ethylenically unsaturated monomer will be described by taking a reversed phase suspension polymerization method as an example.

The ethylenically unsaturated monomer is preferably water-soluble, and examples thereof include (meth) acrylic acid and salts thereof, (meth) acrylamide, N-dimethyl (meth) acrylamide, hydroxyethyl 2- (meth) acrylate, N-hydroxymethyl (meth) acrylamide, polyethylene glycol mono (meth) acrylate, N-diethylaminoethyl (meth) acrylate, N-diethylaminopropyl (meth) acrylate, and diethylaminopropyl (meth) acrylamide. When the ethylenically unsaturated monomer contains an amino group, the amino group may be quaternized. The functional group such as a carboxyl group and an amino group of the monomer can function as a crosslinkable functional group in a surface crosslinking step described later. These ethylenically unsaturated monomers may be used alone, or 2 or more kinds thereof may be used in combination.

Among these, the ethylenically unsaturated monomer preferably contains at least one compound selected from the group consisting of (meth) acrylic acid and salts thereof, acrylamide, methacrylamide, and N, N-dimethylacrylamide, and more preferably contains at least one compound selected from the group consisting of (meth) acrylic acid and salts thereof, and acrylamide, from the viewpoint of being industrially easily available. From the viewpoint of further improving the water absorption characteristics, the ethylenically unsaturated monomer further preferably contains at least one compound selected from the group consisting of (meth) acrylic acid and salts thereof.

As the monomer, a part of monomers other than the above ethylenically unsaturated monomer may be used. Such a monomer can be used by being mixed in an aqueous solution containing the above ethylenically unsaturated monomer, for example. The amount of the ethylenically unsaturated monomer used may be 70 to 100 mol%, or may be 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 100 mol% with respect to the total amount of the monomers (the total amount of the monomers for obtaining the water-absorbent resin particles, for example, the total amount of the monomers that provide the structural units of the crosslinked polymer). Wherein the amount of the (meth) acrylic acid and the salt thereof may be 70 to 100 mol%, or 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 100 mol% based on the total amount of the monomers. The term "ratio of (meth) acrylic acid and a salt thereof" means a ratio of the total amount of (meth) acrylic acid and a salt thereof.

According to the present embodiment, as an example of the water-absorbent resin particles, there can be provided water-absorbent resin particles comprising a crosslinked polymer having a structural unit derived from an ethylenically unsaturated monomer, wherein the ethylenically unsaturated monomer comprises at least one compound selected from the group consisting of (meth) acrylic acid and a salt thereof, and the proportion of the (meth) acrylic acid and the salt thereof is 70 to 100 mol% with respect to the total amount of monomers for obtaining the water-absorbent resin particles.

Generally, ethylenically unsaturated monomers are suitably used as the aqueous solution. In general, the concentration of the ethylenically unsaturated monomer in the aqueous solution containing the ethylenically unsaturated monomer (hereinafter referred to as an aqueous monomer solution) may be 20 mass% or more and not more than the saturated concentration, preferably 25 to 70 mass%, more preferably 30 to 55 mass%. Examples of the water used include tap water, distilled water, and ion-exchanged water.

When the ethylenically unsaturated monomer used contains an acid group, the aqueous monomer solution may be used in such a manner that the acid group is neutralized by a basic neutralizing agent. From the viewpoint of improving the osmotic pressure of the water-absorbent resin particles to be obtained and further improving the water absorption characteristics such as water retention capacity, the degree of neutralization by the basic neutralizing agent in the ethylenically unsaturated monomer is 10 to 100 mol%, preferably 50 to 90 mol%, more preferably 60 to 80 mol% of the acidic groups in the ethylenically unsaturated monomer. Examples of the basic neutralizing agent include: alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogencarbonate, potassium hydroxide and potassium carbonate; ammonia, and the like. These alkaline neutralizing agents can be used in the form of an aqueous solution for the purpose of simplifying the neutralization operation. The above-mentioned basic neutralizing agents may be used alone or in combination of 2 or more. For example, neutralization of the acid groups of the ethylenically unsaturated monomer can be carried out by adding an aqueous solution of sodium hydroxide, potassium hydroxide, or the like dropwise to the above aqueous monomer solution and mixing.

In the reversed-phase suspension polymerization method, an aqueous monomer solution is dispersed in a hydrocarbon dispersion medium in the presence of a surfactant, and an ethylenically unsaturated monomer is polymerized by a radical polymerization initiator or the like.

Examples of the surfactant include nonionic surfactants and anionic surfactants. Examples of the nonionic surfactant include sorbitan fatty acid esters, polyglycerol fatty acid esters, sucrose fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, fatty acid sorbitol esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallylformaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropylene alkyl ethers, polyethylene glycol fatty acid esters, and the like. Examples of the anionic surfactant include fatty acid salts, alkylbenzenesulfonates, alkylmethyltaurates, polyoxyethylene alkylphenyl ether sulfate salts, polyoxyethylene alkyl ether sulfonates, phosphoric acid esters of polyoxyethylene alkyl ethers, phosphoric acid esters of polyoxyethylene alkylallyl ethers, and the like. Among these, the surfactant preferably contains at least one compound selected from the group consisting of sorbitan fatty acid esters, polyglycerol fatty acid esters, and sucrose fatty acid esters, from the viewpoint that the W/O type reverse phase suspension state is good, the water-absorbent resin particles can be easily obtained with an appropriate particle diameter, and the surfactant can be easily obtained industrially. Further, the surfactant more preferably contains a sucrose fatty acid ester from the viewpoint of improving the water absorption characteristics of the water-absorbent resin particles obtained. These surfactants may be used alone, or 2 or more of them may be used in combination.

The amount of the surfactant is preferably 0.05 to 10 parts by mass, more preferably 0.08 to 5 parts by mass, and still more preferably 0.1 to 3 parts by mass, per 100 parts by mass of the aqueous solution of the ethylenically unsaturated monomer, from the viewpoint of sufficiently obtaining the effect with respect to the amount of the surfactant to be used and from the viewpoint of economy.

Further, a polymer dispersant may be used together with the surfactant. Examples of the polymer-based dispersant include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene-terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-polybutadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, ethyl hydroxyethyl cellulose, and the like. Among these polymeric dispersants, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene-propylene copolymer are preferably used, particularly from the viewpoint of dispersion stability of the monomers. These polymeric dispersants may be used alone, or 2 or more of them may be used in combination.

The amount of the polymeric dispersant is preferably 0.05 to 10 parts by mass, more preferably 0.08 to 5 parts by mass, and still more preferably 0.1 to 3 parts by mass, per 100 parts by mass of the aqueous solution of the ethylenically unsaturated monomer, from the viewpoint of sufficiently obtaining the effect with respect to the amount of the dispersant used and from the viewpoint of economy.

The radical polymerization initiator is preferably water-soluble, and examples thereof include: persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide; 2,2 ' -azobis (2-amidinopropane) dihydrochloride, 2 ' -azobis [2- (N-phenylamidino) propane ] dihydrochloride, 2 ' -azobis [2- (N-allylamidino) propane ] dihydrochloride, 2 ' -azobis [2- (2-imidazolin-2-yl) propane ] dihydrochloride, 2 ' -azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl ] propane } dihydrochloride, 2 ' -azobis { 2-methyl-N- [1, 1-bis (hydroxymethyl) -2-hydroxyethyl ] propionamide }, 2 ' -azobis [ 2-methyl-N- (2-hydroxyethyl) -propionamide ] (A), Azo compounds such as 4, 4' -azobis (4-cyanopentanoic acid). These radical polymerization initiators may be used alone or in combination of 2 or more. Of these, potassium persulfate, ammonium persulfate, sodium persulfate, 2 ' -azobis (2-amidinopropane) dihydrochloride, 2 ' -azobis [2- (2-imidazolin-2-yl) propane ] dihydrochloride, 2 ' -azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl ] propane } dihydrochloride, more preferably at least one azo-based compound selected from the group consisting of 2,2 ' -azobis (2-amidinopropane) dihydrochloride, 2 ' -azobis [2- (2-imidazolin-2-yl) propane ] dihydrochloride, and 2,2 ' -azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl ] propane } dihydrochloride. The proportion of the azo compound is preferably 20 to 100 mol%, more preferably 35 to 90 mol%, and still more preferably 50 to 85 mol% based on the total amount of the radical polymerization initiator.

The amount of the radical polymerization initiator used may be 0.00005 to 0.01 mol based on 1 mol of the ethylenically unsaturated monomer. When the amount of the radical polymerization initiator used is 0.00005 mol or more, the polymerization reaction does not need to be carried out for a long time, and the efficiency is high. When the amount is 0.01 mol or less, a rapid polymerization reaction tends not to occur.

The radical polymerization initiator can be used as a redox polymerization initiator together with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid.

In the polymerization reaction, a chain transfer agent may be included in the aqueous solution of the ethylenically unsaturated monomer used for polymerization. Examples of the chain transfer agent include hypophosphites, thiols, thiolates, secondary alcohols, and amines.

Further, in order to control the particle diameter of the water-absorbent resin particles, a thickener may be contained in the aqueous ethylenically unsaturated monomer solution used for polymerization. Examples of the thickener include hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, carboxymethylcellulose, polyacrylic acid (partially) neutralized product, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinyl pyrrolidone, and polyethylene oxide. In addition, if the stirring speed at the time of polymerization is the same, the higher the viscosity of the aqueous ethylenically unsaturated monomer solution is, the larger the median particle diameter of the obtained particles tends to be.

Examples of the hydrocarbon dispersion medium include: chain aliphatic hydrocarbons such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2, 3-dimethylpentane, 3-ethylpentane and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1, 2-dimethylcyclopentane, cis-1, 3-dimethylcyclopentane, and trans-1, 3-dimethylcyclopentane; aromatic hydrocarbons such as benzene, toluene, and xylene. These hydrocarbon dispersion media may be used alone, or 2 or more kinds may be used in combination. The hydrocarbon dispersion medium may contain at least one compound selected from the group consisting of a C6-8 chain aliphatic hydrocarbon and a C6-8 alicyclic hydrocarbon. The hydrocarbon dispersion medium may contain n-heptane, cyclohexane, or both of them from the viewpoint of easy industrial availability and stable quality. From the same viewpoint, a commercially available Exxsol Heptane (product of Exxon Mobil Corporation: 75 to 85% of a hydrocarbon containing n-Heptane and isomers) can be used as the mixture of the hydrocarbon dispersion medium.

The amount of the hydrocarbon dispersion medium to be used is preferably 30 to 1000 parts by mass, more preferably 40 to 500 parts by mass, and still more preferably 50 to 300 parts by mass per 100 parts by mass of the aqueous monomer solution, from the viewpoint of appropriately removing the heat of polymerization and easily controlling the polymerization temperature. When the amount of the hydrocarbon dispersion medium used is 30 parts by mass or more, the polymerization temperature tends to be easily controlled. By using the hydrocarbon dispersion medium in an amount of 1000 parts by mass or less, the productivity of polymerization tends to be improved and economical.

In general, internal crosslinking by self-crosslinking can occur during polymerization, but internal crosslinking can be performed by further using an internal crosslinking agent to control the water absorption characteristics of the water-absorbent resin particles. Examples of the internal crosslinking agent to be used include: di-or tri (meth) acrylates of polyhydric alcohols such as ethylene glycol, propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; unsaturated polyesters obtained by reacting the polyhydric alcohols with unsaturated acids such as maleic acid and fumaric acid; bis (meth) acrylamides such as N, N' -methylenebis (meth) acrylamide; di-or tri (meth) acrylates obtained by reacting polyepoxides with (meth) acrylic acid; carbamyl di (meth) acrylates obtained by reacting polyisocyanates such as tolylene diisocyanate and hexamethylene diisocyanate with hydroxyethyl (meth) acrylate; compounds having 2 or more polymerizable unsaturated groups such as allyl starch, allyl cellulose, diallyl phthalate, N', N ″ -triallyl isocyanurate, and divinylbenzene; polyglycidyl compounds such as (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerol triglycidyl ether, (poly) propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether; halogenated epoxy compounds such as epichlorohydrin, epibromohydrin, alpha-methylepichlorohydrin and the like; and compounds having 2 or more reactive functional groups such as isocyanate compounds such as 2, 4-tolylene diisocyanate and hexamethylene diisocyanate. Among these internal crosslinking agents, a polyglycidyl compound is preferably used, a diglycidyl ether compound is more preferably used, and (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerol diglycidyl ether are particularly preferably used. These crosslinking agents may be used alone, or 2 or more kinds may be used in combination.

The amount of the internal crosslinking agent is preferably 0 to 0.03 mol, more preferably 0.00001 to 0.01 mol, and still more preferably 0.00002 to 0.005 mol per 1 mol of the ethylenically unsaturated monomer, from the viewpoint of suppressing the water-soluble property by appropriately crosslinking the obtained polymer and showing a sufficient water absorption amount.

The aqueous phase containing the ethylenically unsaturated monomer, the radical polymerization initiator, and if necessary, the internal crosslinking agent, and the like can be mixed with the oil phase containing the components such as the hydrocarbon-based dispersion medium, the surfactant, and if necessary, the polymer-based dispersant, and the like, and heated under stirring to perform reversed-phase suspension polymerization in an aqueous-in-oil system.

In the reversed-phase suspension polymerization, an aqueous monomer solution containing an ethylenically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the presence of a surfactant and, if necessary, a polymer dispersant. In this case, the surfactant or the polymer dispersant may be added before or after the aqueous monomer solution is added, as long as it is before the polymerization reaction is started.

Among these, from the viewpoint of easily reducing the amount of the hydrocarbon dispersion medium remaining in the water-absorbent resin obtained, it is preferable to disperse the aqueous monomer solution in the hydrocarbon dispersion medium in which the polymer dispersant is dispersed, further disperse the surfactant, and then perform polymerization.

Such reversed-phase suspension polymerization can be carried out in a plurality of stages of 1 stage or 2 stages or more. From the viewpoint of improving productivity, the reaction is preferably carried out in 2 to 3 stages.

When the reversed-phase suspension polymerization is carried out in 2 or more stages, the reversed-phase suspension polymerization in the 1 st stage may be carried out, and then the ethylenically unsaturated monomer may be added to and mixed with the reaction mixture obtained in the polymerization reaction in the 1 st stage, and the reversed-phase suspension polymerization in the 2 nd and subsequent stages may be carried out in the same manner as in the 1 st stage. In the reversed-phase suspension polymerization in each stage after the 2 nd stage, it is preferable that the radical polymerization initiator or the internal crosslinking agent is added in the range of the molar ratio of each component to the ethylenically unsaturated monomer described above based on the amount of the ethylenically unsaturated monomer added in the reversed-phase suspension polymerization in each stage after the 2 nd stage, in addition to the ethylenically unsaturated monomer, and the reversed-phase suspension polymerization is performed. In the reversed-phase suspension polymerization in each stage after the 2 nd stage, an internal crosslinking agent may be used as necessary. When the internal crosslinking agent is used, it is preferable to add the components in the range of the molar ratio of the components to the ethylenically unsaturated monomer in each stage based on the amount of the ethylenically unsaturated monomer used in each stage, and perform reversed phase suspension polymerization.

The temperature of the polymerization reaction varies depending on the radical polymerization initiator used, but is preferably 20 to 150 ℃, more preferably 40 to 120 ℃ from the viewpoints of improving the economy by rapidly carrying out the polymerization and shortening the polymerization time, and easily removing the heat of polymerization to smoothly carry out the reaction. The reaction time is usually 0.5 to 4 hours. For example, the completion of the polymerization reaction can be confirmed by stopping the temperature rise in the reaction system. Thus, the polymer of ethylenically unsaturated monomers is generally obtained in the state of a hydrogel.

After polymerization, crosslinking after polymerization can be carried out by adding a crosslinking agent to the obtained water-containing gel-like polymer and heating. The degree of crosslinking of the hydrogel polymer is increased by crosslinking after polymerization, and the water absorption characteristics can be more preferably improved.

Examples of the crosslinking agent for crosslinking after polymerization include: polyhydric alcohols such as ethylene glycol, propylene glycol, 1, 4-butanediol, trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerol; compounds having 2 or more epoxy groups such as (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerol diglycidyl ether; halogenated epoxy compounds such as epichlorohydrin, epibromohydrin, and α -methylepichlorohydrin; compounds having 2 or more isocyanate groups such as 2, 4-tolylene diisocyanate and hexamethylene diisocyanate; oxazoline compounds such as 1, 2-vinylbisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis [ N, N-bis (. beta. -hydroxyethyl) ] hexanediamide. Among these, preferred are polyglycidyl compounds such as (poly) ethylene glycol diglycidyl ether, (poly) glycerol triglycidyl ether, (poly) propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether. These crosslinking agents may be used alone, or 2 or more kinds may be used in combination.

The amount of the crosslinking agent used for crosslinking after polymerization is preferably 0 to 0.03 mol, more preferably 0 to 0.01 mol, and still more preferably 0.00001 to 0.005 mol per 1 mol of the ethylenically unsaturated monomer, from the viewpoint of exhibiting suitable water absorption characteristics by appropriately crosslinking the obtained hydrogel polymer.

The addition timing of the post-polymerization crosslinking may be after polymerization of the ethylenically unsaturated monomer used for polymerization, and in the case of multistage polymerization, it is preferable to add the post-polymerization in multistage. In addition, in view of heat generation during and after polymerization, retention due to process delay, opening of a system when a crosslinking agent is added, and fluctuation of water due to addition of water or the like accompanying addition of the crosslinking agent, the crosslinking agent crosslinked after polymerization is preferably added in a region of [ water content immediately after polymerization ± 3 mass% ], from the viewpoint of water content (described later).

Subsequently, the obtained hydrogel polymer is dried to remove water therefrom. By drying, polymer particles of a polymer containing an ethylenically unsaturated monomer are obtained. Examples of the drying method include (a) a method of removing water by heating from the outside while the hydrogel polymer is dispersed in a hydrocarbon dispersion medium, and refluxing the hydrocarbon dispersion medium, (b) a method of extracting the hydrogel polymer by decantation and drying under reduced pressure, and (c) a method of filtering the hydrogel polymer through a filter and drying under reduced pressure. Among them, the method (a) is preferably used in view of simplicity in the production process.

For example, the particle diameter of the water-absorbent resin particles can be controlled by adjusting the rotation speed of a stirrer during the polymerization reaction or by adding a powdery inorganic coagulant to the inside of the system after the polymerization reaction or at the initial stage of drying. The particle diameter of the water-absorbent resin particles obtained can be increased by adding the coagulant. Examples of the powdery inorganic coagulant include silica, zeolite, bentonite, alumina, talc, titanium dioxide, kaolin, clay, hydrotalcite, and the like, and among them, silica, alumina, talc, and kaolin are preferable from the viewpoint of a coagulation effect.

In the reversed-phase suspension polymerization, as a method of adding the powdery inorganic coagulant, the following method is preferred: the powdery inorganic coagulant is dispersed in advance in the same kind of hydrocarbon dispersion medium or water as the one used for the polymerization, and then mixed in the hydrocarbon dispersion medium containing the water-containing gel under stirring.

In the production of the water-absorbent resin particles according to the present embodiment, it is preferable that the crosslinking agent is used to crosslink (surface crosslink) the surface portion of the hydrogel polymer in the drying step or in any of the subsequent steps. The surface crosslinking is preferably performed at a time point when the water content of the hydrogel polymer reaches a specific water content. The time of surface crosslinking is preferably when the water content of the hydrogel polymer is 5 to 50 mass%, more preferably 10 to 40 mass%, and still more preferably 15 to 35 mass%.

The water content (% by mass) of the hydrogel polymer was calculated by the following equation.

Water content [ Ww/(Ww + Ws) ]. times.100

And Ww: the water content of the hydrogel polymer is obtained by adding the water content, which is used as needed when a powdery inorganic coagulant or a surface crosslinking agent is mixed, to the water content obtained by subtracting the water content discharged to the outside of the system in the drying step from the water content contained in the aqueous liquid before polymerization in the entire polymerization step.

Ws: the amount of solid component is calculated from the amount of the ethylenically unsaturated monomer, crosslinking agent, initiator and other materials incorporated in the hydrogel polymer.

Examples of the surface crosslinking agent for surface crosslinking include compounds having 2 or more reactive functional groups. Examples thereof include: polyhydric alcohols such as ethylene glycol, propylene glycol, 1, 4-butanediol, trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerol; polyglycidyl compounds such as (poly) ethylene glycol diglycidyl ether, (poly) glycerol triglycidyl ether, trimethylolpropane triglycidyl ether, (poly) propylene glycol polyglycidyl ether, and (poly) glycerol polyglycidyl ether; halogenated epoxy compounds such as epichlorohydrin, epibromohydrin, alpha-methylepichlorohydrin and the like; 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-vinylbisoxazoline; carbonate compounds such as ethylene carbonate; hydroxyalkyl amide compounds such as bis [ N, N-bis (. beta. -hydroxyethyl) ] adipamide. Among these, polyglycidyl compounds such as (poly) ethylene glycol diglycidyl ether, (poly) glycerol triglycidyl ether, (poly) propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether are more preferable. These surface-crosslinking agents may be used alone, or 2 or more kinds may be used in combination.

The amount of the surface-crosslinking agent is usually 0.00001 to 0.02 mol, preferably 0.00005 to 0.01 mol, and more preferably 0.0001 to 0.005 mol based on 1 mol of the ethylenically unsaturated monomer used for polymerization, from the viewpoint of exhibiting suitable water absorption characteristics by appropriately crosslinking the obtained hydrogel polymer.

The amount of the surface-crosslinking agent used is preferably 0.00001 mol or more from the viewpoint of sufficiently increasing the crosslinking density at the surface portion of the polymer particles and improving the gel strength of the water-absorbent resin particles. In addition, from the viewpoint of increasing the water retention capacity of the water-absorbent resin particles, it is preferably 0.02 mol or less.

After the surface crosslinking reaction, water and the hydrocarbon dispersion medium are distilled off by a known method, whereby polymer particles as a surface-crosslinked dried product can be obtained.

The polymerization reaction can be carried out using various stirring machines having stirring blades. As the stirring blades, flat blades, grid blades, propeller blades, anchor blades, turbine blades, three-blade backward bent blades, ribbon blades, universal blades, Max blade (blend) blades, and the like can be used. The flat blade has a shaft (stirring shaft) and a flat plate portion (stirring portion) disposed around the shaft. The flat plate portion may have a slit or the like. When a flat blade is used as the stirring blade, the crosslinking reaction in the polymer particles is easily and uniformly performed, and the gel flow-out test force is easily adjusted to a desired range suitable for the present invention while maintaining the water absorption characteristics such as water retention amount.

The water-absorbent resin particles according to the present embodiment may be composed of only polymer particles, but may further contain various additional components selected from, for example, inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, antibacterial agents, deodorants, gel stabilizers, flowability improvers (lubricants), and the like. The additional component can be disposed inside the polymer particles, on the surface of the polymer particles, or both.

The water-absorbent resin particles according to the present embodiment preferably contain inorganic particles. Examples of the inorganic particles include silica particles such as amorphous silica. The amorphous silica may be a hydrophilic amorphous silica. For example, by mixing polymer particles with inorganic particles, the inorganic particles can be disposed on the surface of the polymer particles. The size of the inorganic particles here is generally small compared to the size of the polymer particles. For example, the inorganic particles may have an average particle diameter of 0.1 to 50 μm, 0.5 to 30 μm, or 1 to 20 μm. The average particle diameter herein is a value that can be measured by a dynamic light scattering method or a laser diffraction scattering method. When the amount of the inorganic particles added is within the above range, water-absorbent resin particles having good water absorption properties can be easily obtained.

For example, the fluidity of the water-absorbent resin particles can be improved by adding 0.05 to 5 parts by mass of amorphous silica as the inorganic particles to 100 parts by mass of the polymer particles. When the water-absorbent resin particles contain inorganic particles, the proportion of the inorganic particles relative to the mass of the polymer particles may be 0.05 mass% or more, 0.1 mass% or more, 0.2 mass% or more, 0.5 mass% or more, 1.0 mass% or more, or 1.5 mass% or more, or may be 5.0 mass% or less, 3.5 mass% or less, 1.5 mass% or less, 1.0 mass% or less, 0.8 mass% or less, 0.5 mass% or less, or 0.3 mass% or less.

The method for producing water-absorbent resin particles according to the present embodiment may include a step of sorting the water-absorbent resin particles having a gel flow-out test force of 5 to 14N measured by the above-described method. The above-mentioned production method may include a step of measuring the gel flow-out test force. The properties of the sorted water-absorbent resin particles may satisfy the above-mentioned forms of the water-absorbent resin particles (for example, the water retention capacity of physiological saline in a specific range, the water absorption capacity of physiological saline under a load of 4.14kPa in a specific range, the value of 2 hours, etc.).

The water-absorbent resin particles according to the present embodiment are excellent in the absorption of body fluids such as urine and blood, and can be used in the fields of sanitary products such as disposable diapers, sanitary napkins and tampons, and animal waste treatment materials such as pet pads and toilet blends for dogs and cats.

The water-absorbent resin particles according to the present embodiment can be suitably used in an absorbent material. The absorbent material according to the present embodiment includes water-absorbent resin particles. From the viewpoint of obtaining sufficient liquid absorption performance when the absorbent body is used in an absorbent article, the content of the water-absorbent resin particles in the absorbent body is preferably 100 to 1000g (i.e., 100 to 1000 g/m) per 1 square meter of the absorbent body²) More preferably 150 to 800g/m²More preferably 200 to 700g/m². The content is preferably 100g/m from the viewpoint of exhibiting sufficient liquid absorption performance as an absorbent article, and particularly suppressing liquid leakage²The above. The content is preferably 1000g/m from the viewpoint of suppressing the occurrence of gel blocking, exhibiting liquid diffusing performance as an absorbent article, and further improving the liquid permeation rate²The following.

The absorbent body may further include fibrous materials in addition to the water-absorbent resin particles. The absorbent material may be, for example, a mixture containing water-absorbent resin particles and fibrous materials. The mass ratio of the water-absorbent resin particles in the absorbent body may be 2 to 100 mass%, preferably 10 to 80 mass%, and more preferably 20 to 70 mass% with respect to the total of the water-absorbent resin particles and the fibrous material. The absorbent body may be configured, for example, in a form in which the water-absorbent resin particles and fibrous materials are uniformly mixed, in a form in which the water-absorbent resin particles are sandwiched between fibrous materials formed in a sheet or layer, or in another form.

Examples of the fibrous material include finely pulverized cellulose fibers such as wood pulp, cotton linter, rayon and cellulose acetate, and synthetic fibers such as polyamide, polyester and polyolefin. The fibrous material may be a mixture of the above fibers.

In order to improve the shape retention of the absorbent body before and during use, the fibers may be bonded to each other by adding an adhesive (bonding) agent to the fibrous material. Examples of the adhesive agent include heat-fusible synthetic fibers, a heat-fusible adhesive, and an adhesive emulsion.

Examples of the heat-fusible synthetic fiber include a full melt adhesive such as polyethylene, polypropylene, and an ethylene-propylene copolymer, and a non-full melt adhesive formed of a side-by-side or core-sheath structure of polypropylene and polyethylene. In the above-mentioned non-full melt adhesive, only the polyethylene part is thermally fused. Examples of the hot-melt adhesive include blends of base polymers such as ethylene-vinyl acetate copolymer, styrene-isoprene-styrene block copolymer, styrene-butadiene-styrene block copolymer, styrene-ethylene-butylene-styrene block copolymer, styrene-ethylene-propylene-styrene block copolymer, and amorphous polypropylene with tackifiers, plasticizers, and antioxidants.

Examples of the adhesive emulsion include polymers of at least one monomer selected from the group consisting of methyl methacrylate, styrene, acrylonitrile, 2-ethylhexyl acrylate, butyl acrylate, butadiene, ethylene, and vinyl acetate. These adhesive adhesives may be used alone, or 2 or more kinds may be used in combination.

The absorber according to the present embodiment may further contain additives such as inorganic powder (for example, amorphous silica), deodorant, pigment, dye, antibacterial agent, perfume, pressure-sensitive adhesive, and the like. These additives can impart various functions to the absorbent. When the water-absorbent resin particles contain inorganic particles, the absorber may contain an inorganic powder different from the inorganic particles in the water-absorbent resin particles. Examples of the inorganic powder include silica, zeolite, kaolin, clay, and the like.

The shape of the absorbent body according to the present embodiment is not particularly limited, and may be, for example, a sheet shape. The thickness of the absorber (for example, the thickness of the sheet-like absorber) may be, for example, 0.1 to 20mm, 0.3 to 15 mm.

The absorbent article according to the present embodiment may include, for example, a core wrap sheet, a liquid-permeable top sheet, and a liquid-impermeable back sheet in addition to the absorbent body. The core wrap maintains the shape of the absorbent body. The liquid-permeable top sheet is disposed at the outermost portion of the side of the liquid-absorbing object into which liquid is to be impregnated. The liquid-impermeable back sheet is disposed at the outermost portion on the side opposite to the side into which the liquid of the liquid-absorbing object is to be introduced.

Examples of absorbent articles include diapers (e.g., paper diapers), toilet training pants, incontinence pads, sanitary products (sanitary napkins, tampons, and the like), sweat pads, pet pads, toilet components, and animal waste disposal materials.

Fig. 1 is a cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in fig. 1 includes an absorber 10, core sheets 20a and 20b, a liquid-permeable top sheet 30, and a liquid-impermeable back sheet 40. In the absorbent article 100, a liquid-impermeable back sheet 40, a core sheet 20b, an absorber 10, a core sheet 20a, and a liquid-permeable top sheet 30 are stacked in this order. In fig. 1, there is a portion illustrated as having a gap between the members, but the members may be closely attached without the gap.

The absorbent body 10 has water-absorbent resin particles 10a and a fibrous layer 10b containing a fibrous material. The water-absorbent resin particles 10a are dispersed in the fiber layer 10 b.

The core sheet 20a is disposed on one side of the absorbent body 10 (the upper side of the absorbent body 10 in fig. 1) in a state of being in contact with the absorbent body 10. The core sheet 20b is disposed on the other side of the absorbent body 10 (the lower side of the absorbent body 10 in fig. 1) in a state of being in contact with the absorbent body 10. The absorber 10 is disposed between the core wrap 20a and the core wrap 20 b.

The core sheet 20a and the core sheet 20b have, for example, main surfaces having the same size as the absorber 10. By using the core-covering sheet, the shape retention of the absorbent body is maintained, and the water-absorbent resin particles and the like constituting the absorbent body can be prevented from falling off and flowing. Examples of the core sheet include nonwoven fabric, woven fabric, paper towel, synthetic resin film having liquid permeable holes, mesh sheet having meshes, and the like, and paper towel obtained by wet-molding ground pulp is preferably used from the viewpoint of economy.

The liquid-permeable top sheet 30 is disposed at the outermost portion of the side into which the liquid of the absorption object enters. The liquid-permeable top sheet 30 is disposed on the core wrap sheet 20a in a state of being in contact with the core wrap sheet 20 a. The liquid-impermeable back sheet 40 is disposed at the outermost portion on the side opposite to the liquid-permeable top sheet 30 in the absorbent article 100. The liquid-impermeable back sheet 40 is disposed below the core wrap sheet 20b in contact with the core wrap sheet 20 b. The liquid-permeable top sheet 30 and the liquid-impermeable back sheet 40 have, for example, main surfaces wider than the main surfaces of the absorber 10, and outer edge portions of the liquid-permeable top sheet 30 and the liquid-impermeable back sheet 40 extend to the periphery of the absorber 10 and the core sheets 20a and 20 b.

Examples of the liquid-permeable top sheet 30 include nonwoven fabrics and porous sheets. Examples of the nonwoven fabric include a heat-bondable nonwoven fabric, a breathable nonwoven fabric, a resin-bonded nonwoven fabric, a spunbond nonwoven fabric, a meltblown nonwoven fabric, a spunbond/meltblown/spunbond nonwoven fabric, an air-laid nonwoven fabric, a spunlace nonwoven fabric, and a point-bonded nonwoven fabric. Among them, a heat-bonded nonwoven fabric, a breathable nonwoven fabric, a spunbonded nonwoven fabric, and a spunbonded/meltblown/spunbonded nonwoven fabric are preferably used.

As a material constituting the liquid-permeable topsheet 30, a resin or a fiber known in the art can be used, and from the viewpoint of liquid permeability, flexibility and strength when used in an absorbent article, there are mentioned polyolefin such as Polyethylene (PE) and polypropylene (PP), polyester such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT) and polyethylene naphthalate (PEN), polyamide such as nylon, rayon, other synthetic resin or synthetic fiber, cotton, silk, hemp, pulp (cellulose) fiber, and the like. As the constituent material, synthetic fibers are preferably used from the viewpoint of improving the strength of the liquid-permeable topsheet 30, and among them, polyolefin and polyester are preferable. These materials may be used alone, or 2 or more kinds of materials may be used in combination.

From the viewpoint of improving the liquid absorption performance of the absorbent article, the nonwoven fabric used for the liquid-permeable topsheet 30 is desired to have appropriate hydrophilicity. From this viewpoint, the "hydrophilicity of nonwoven fabric" described in International publication No. 2011/086843 (according to the pulp test method No.68(2000)) is preferably 5 to 200, more preferably 10 to 150. Such a hydrophilic nonwoven fabric may be one in which fibers having an appropriate degree of hydrophilicity are used as a material such as rayon fibers, or one in which an appropriate degree of hydrophilicity is imparted by hydrophilizing hydrophobic chemical fibers such as polyolefin fibers or polyester fibers by a known method.

Examples of the method of hydrophilizing the chemical fiber include a method of obtaining a nonwoven fabric by a spunbond method using a chemical fiber obtained by mixing a hydrophobic chemical fiber with a hydrophilizing agent in a spunbond nonwoven fabric, a method of producing a spunbond nonwoven fabric from a hydrophobic chemical fiber with a hydrophilizing agent, and a method of obtaining a spunbond nonwoven fabric from a hydrophobic chemical fiber and then immersing the spunbond nonwoven fabric in a hydrophilizing agent. As the hydrophilizing agent, anionic surfactants such as aliphatic sulfonates and higher alcohol sulfates, cationic surfactants such as quaternary ammonium salts, nonionic surfactants such as polyethylene glycol fatty acid esters, polyglycerol fatty acid esters and sorbitan fatty acid esters, silicone surfactants such as polyoxyalkylene-modified silicones, and antifouling agents composed of polyester, polyamide, acrylic and urethane resins can be used.

The nonwoven fabric used for the liquid-permeable topsheet 30 is preferably appropriate in volume size and large in weight per unit area from the viewpoint of imparting good liquid permeability, flexibility, strength, and cushioning properties to the absorbent article and improving the liquid permeation rate of the absorbent article. The weight per unit area of the nonwoven fabric is preferably 5 to 200g/m²More preferably 8 to 150g/m²More preferably 10 to 100g/m². The thickness of the nonwoven fabric is preferably 20 to 1400 μm, more preferably 50 to 1200 μm, and still more preferably 80 to 1000 μm.

The liquid-impermeable back sheet 40 prevents the liquid absorbed in the absorbent body 10 from leaking out from the back sheet 40 side. As the liquid-impermeable back sheet 40, a liquid-impermeable film mainly composed of a polyolefin resin such as Polyethylene (PE) or polypropylene (PP), an air-permeable resin film, a composite film in which an air-permeable resin film is joined to a nonwoven fabric such as spunbond or spunlace, a spunbond/meltblown/spunbond (SMS) nonwoven fabric in which a water-resistant meltblown nonwoven fabric is sandwiched by high-strength spunbond nonwoven fabrics, or the like can be used. From the viewpoint of ensuring flexibility and not impairing the wearing feeling of the absorbent article, the back sheet 40 can be made of a Low Density Polyethylene (LDPE) resin having a basis weight of 10 to 50g/m²The resin film of (1). In addition, when the breathable material is used, stuffiness during wearing can be reduced, and discomfort to the wearer can also be reduced.

The size relationship among the absorber 10, the core sheets 20a, 20b, the liquid-permeable top sheet 30, and the liquid-impermeable back sheet 40 is not particularly limited, and can be appropriately adjusted depending on the use of the absorbent article and the like. The method of holding the shape of the absorber 10 using the core wrap sheets 20a and 20b is not particularly limited, and as shown in fig. 1, the absorber may be sandwiched by a plurality of core wrap sheets, or may be covered by one core wrap sheet.

The absorption body 10 can be bonded to a liquid-permeable top sheet 30. By bonding the absorbent body 10 and the liquid-permeable top sheet 30, the liquid is introduced into the absorbent body more smoothly, and therefore an absorbent article having a more excellent leakage prevention effect can be easily obtained. When the absorbent body 10 is sandwiched or covered by the core sheet, at least the core sheet and the liquid-permeable topsheet 30 are preferably bonded, and more preferably the core sheet and the absorbent body 10 are further bonded. Examples of the bonding method include a method in which a hot-melt adhesive is applied to the liquid-permeable top sheet 30 in a shape of a longitudinal stripe, a spiral, or the like at a predetermined interval in the width direction thereof, and a method in which a water-soluble adhesive selected from starch, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl pyrrolidone, and other water-soluble polymers is used for bonding. When the absorbent body 10 includes heat-fusible synthetic fibers, a method of bonding by the heat fusion can be employed.

The present invention also provides a method for measuring the gel-out test force of the water-absorbent resin particles. The determination method comprises the following steps: a step of preparing a swollen gel by allowing water-absorbent resin particles to absorb 29 times as much physiological saline as the water-absorbent resin particles under stirring; a step of uniformly placing 20g of the swollen gel in a cylinder having a bottom with a hole of 5mm in diameter and an inner diameter of 5 cm; and a step of compressing the swollen gel in the cylinder at a rate of 10 mm/min by using a fixing tool having a diameter of 4.9cm, and recording a test force at a time when a part of the swollen gel flows out from the hole in the bottom of the cylinder as a gel flow-out test force. More specific measurement methods are shown in examples described later.

When water-absorbent resin particles having a gel flow-out test force of 5 to 14N are used in an absorbent article, the amount of rewet of the absorbent article can be reduced. Accordingly, the present invention also provides a method for reducing the rewet amount of an absorbent article, comprising the step of setting the gel flow-out test force of the water-absorbent resin particles measured by the above-mentioned measuring method to 5 to 14N. More specific measurement methods of the gel flow-out test force are shown in the examples described later. The method for reducing the rewet amount of the absorbent article may further include, for example, a step of setting the water retention amount of physiological saline to 30 to 60 g/g; and a step of setting the water absorption capacity of the water-absorbent resin particles under a load of 4.14kPa for 2 hours to a value of 15ml/g or more. Specific examples of the production method of the water-absorbent resin particles having these predetermined properties are as described above. In order to set the gel flow-out test force of the water-absorbent resin particles to 5 to 14N, for example, the production conditions of the water-absorbent resin particles can be selected so that the uniformity of the crosslinking in the water-absorbent resin particles is high.

Examples

The present invention will be described in further detail with reference to the following examples, but the present invention is not limited to these examples.

< production of Water-absorbent resin particles >

[ example 1]

A round-bottomed, cylindrical, removable flask having an inner volume of 2L and an inner diameter of 11cm equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer was prepared. A stirring blade (flat blade) 200 having a general shape shown in fig. 2 is attached to the stirrer. The stirring blade 200 includes a shaft 200a and a flat plate portion 200 b. The flat plate portion 200b is welded to the shaft 200a, and has a curved front end. The flat plate portion 200b is formed with 4 slits S extending in the axial direction of the shaft 200 a. The 4 slits S are arranged in the width direction of the flat plate portion 200b, and the width of the inner 2 slits S is 1cm, and the width of the outer 2 slits S is 0.5 cm. The length of the flat plate portion 200b is about 10cm, and the width of the flat plate portion 200b is about 6 cm. Subsequently, 293g of n-heptane was added as a hydrocarbon dispersion medium and 0.736g of a maleic anhydride-modified ethylene-propylene copolymer (manufactured by Mitsui Chemicals, Inc., Hi-WAX1105A) was added as a polymer dispersant to the separable flask, thereby obtaining a mixture. After dissolving the dispersant by warming to 80 ℃ while stirring the mixture, the mixture was cooled to 50 ℃.

On the other hand, 92.0g (1.03 mol) of an aqueous 80.5 mass% acrylic acid solution was placed in a beaker having an internal volume of 300ml as an ethylenically unsaturated monomer, and 75 mol% neutralization was carried out by dropping 147.7g of a 20.9 mass% aqueous sodium hydroxide solution while cooling from the outside. Thereafter, 0.092g of hydroxyethylcellulose (Sumitomo Seika Chemicals Company, Limited, HEC AW-15F) as a thickener, 0.092g (0.339 mmol) of 2, 2' -azobis (2-amidinopropane) dihydrochloride as a radical polymerization initiator, 0.018g (0.067 mmol) of potassium persulfate, and 0.0046g (0.026 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a stage 1 aqueous liquid.

The prepared aqueous solution was added to the reaction solution in the separable flask and stirred for 10 minutes. Next, 0.736g of sucrose stearate (Mitsubishi-Chemical Foods Corporation, RYOTO SUGAR ESTER S-370, HLB: 3) was dissolved by heating in 6.62g of n-heptane as a surfactant to prepare a surfactant solution. The surfactant solution was further added to the reaction solution, and the inside of the system was sufficiently replaced with nitrogen gas while stirring at 425rpm of the stirrer. Thereafter, the temperature was raised by immersing the flask in a water bath at 70 ℃ and polymerization was carried out for 60 minutes, thereby obtaining a polymerization slurry of stage 1.

On the other hand, in a separate beaker having an internal volume of 500mL, 128.8g (1.44 mol) of an aqueous 80.5 mass% acrylic acid solution was added as an ethylenically unsaturated monomer, and 75 mol% neutralization was carried out by dropwise addition of 159.0g of a 27 mass% aqueous sodium hydroxide solution while cooling from the outside. Then, 0.129g (0.476 mmol) of 2, 2' -azobis (2-amidinopropane) dihydrochloride and 0.026g (0.096 mmol) of potassium persulfate as a radical polymerization initiator, and 0.0117g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare an aqueous liquid of stage 2.

The inside of the separable flask system was cooled to 25 ℃ while stirring at 650rpm as the stirrer. Subsequently, the total amount of the aqueous liquid in the 2 nd stage was added to the polymerization slurry in the 1 st stage, and the inside of the system was replaced with nitrogen gas for 30 minutes. Thereafter, the flask was again immersed in a water bath at 70 ℃ to raise the temperature, and polymerization was carried out for 60 minutes to obtain a water-containing gel-like polymer.

0.589g of a 45 mass% aqueous solution of diethylenetriaminepentaacetic acid pentasodium salt was added to the hydrous gel-like polymer after polymerization in stage 2 under stirring. Thereafter, the flask was immersed in an oil bath set at 125 ℃ and 234.6g of water was withdrawn to the outside of the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane. Thereafter, 4.42g (0.507 mmol) of a 2 mass% ethylene glycol diglycidyl ether aqueous solution was added as a surface cross-linking agent to the flask, and the mixture was held at 83 ℃ for 2 hours.

Thereafter, polymer particles (dried product) were obtained by evaporating and drying n-heptane at 125 ℃. The polymer particles were passed through a sieve having a pore diameter of 850 μm, and amorphous silica (Oriental silica Corporation, Tokusil NP-S) in an amount of 0.2 mass% relative to the mass of the polymer particles was mixed with the polymer particles, thereby obtaining 232.1g of water-absorbent resin particles containing amorphous silica. The median diameter of the water-absorbent resin particles was 355. mu.m.

[ example 2]

233.0g of water-absorbent resin particles were obtained in the same manner as in example 1, except that the amount of water withdrawn to the outside of the system by azeotropic distillation was changed to 229.2 g. The median particle diameter of the water-absorbent resin particles was 368. mu.m.

[ example 3]

231.0g of water-absorbent resin particles were obtained in the same manner as in example 1, except that the amount of water withdrawn to the outside of the system by azeotropic distillation was changed to 224.3 g. The median diameter of the water-absorbent resin particles was 342 μm.

[ example 4]

232.3g of water-absorbent resin particles were obtained in the same manner as in example 1, except that the amount of water withdrawn to the outside of the system by azeotropic distillation was changed to 207.9 g. The median diameter of the water-absorbent resin particles was 361 μm.

Comparative example 1

A round-bottomed cylindrical separable flask having a volume of 11cm and 2L in inner diameter and equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirring blade having 2-stage 4-piece inclined blades with a blade diameter of 5cm as a stirrer was prepared. In the separable flask, 293g of n-heptane was added as a hydrocarbon dispersion medium and 0.736g of a maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc, Hi-WAX1105A) was added as a polymer dispersant to obtain a mixture. After dissolving the dispersant by warming to 80 ℃ while stirring the mixture, the mixture was cooled to 50 ℃.

On the other hand, 92.0g (1.03 mol) of an aqueous 80.5 mass% acrylic acid solution was placed in a beaker having an internal volume of 300ml as an ethylenically unsaturated monomer, and 75 mol% neutralization was carried out by dropping 147.7g of a 20.9 mass% aqueous sodium hydroxide solution while cooling from the outside. Thereafter, 0.092g of hydroxyethylcellulose (Sumitomo Seika Chemicals Company, Limited, HEC AW-15F) was added as a thickener, 0.0736g (0.272 mmol) of potassium persulfate was added as a radical polymerization initiator, and 0.010g (0.057 mmol) of ethylene glycol diglycidyl ether was added as an internal crosslinking agent and dissolved to prepare an aqueous liquid of stage 1.

The prepared aqueous solution was added to the reaction solution in the separable flask and stirred for 10 minutes. Next, 0.736g of sucrose stearate (Mitsubishi-Chemical Foods Corporation, RYOTO SUGAR ESTER S-370, HLB: 3) was dissolved by heating in 6.62g of n-heptane as a surfactant to prepare a surfactant solution. The surfactant solution was further added to the reaction solution, and the inside of the system was sufficiently replaced with nitrogen while stirring at 550rpm as the rotation speed of the stirrer. Thereafter, the temperature was raised by immersing the flask in a water bath at 70 ℃ and polymerization was carried out for 60 minutes, thereby obtaining a polymerization slurry of stage 1.

On the other hand, in a separate beaker having an internal volume of 500mL, 128.8g (1.44 mol) of an aqueous 80.5 mass% acrylic acid solution was added as an ethylenically unsaturated monomer, and 75 mol% neutralization was carried out by dropwise addition of 159.0g of a 27 mass% aqueous sodium hydroxide solution while cooling from the outside. Thereafter, 0.090g (0.333 mmol) of potassium persulfate was added as a radical polymerization initiator and 0.012g (0.067 mmol) of ethylene glycol diglycidyl ether was added as an internal crosslinking agent, and the mixture was dissolved to prepare an aqueous solution of stage 2.

After the inside of the separable flask system was cooled to 25 ℃ while stirring with the stirrer rotating at 1000rpm, the total amount of the aqueous liquid in the 2 nd stage was added to the polymerization slurry in the 1 st stage, and the inside of the system was replaced with nitrogen gas for 30 minutes. Thereafter, the flask was again immersed in a water bath at 70 ℃ to raise the temperature, and polymerization was carried out for 60 minutes to obtain a water-containing gel-like polymer.

0.265g of 45 mass% aqueous solution of diethylenetriaminepentaacetic acid pentasodium salt was added to the hydrous gel-like polymer after polymerization in stage 2 under stirring. Thereafter, the flask was immersed in an oil bath set at 125 ℃ and 271.4g of water was withdrawn to the outside of the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. Thereafter, 6.40g (0.735 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added as a surface cross-linking agent to the flask, and the mixture was held at 83 ℃ for 2 hours.

Thereafter, polymer particles (dried product) were obtained by evaporating and drying n-heptane at 125 ℃. The polymer particles were passed through a sieve having a pore diameter of 850 μm, and amorphous silica (Oriental silica Corporation, Tokusil NP-S) in an amount of 0.5 mass% relative to the mass of the polymer particles was mixed with the polymer particles, thereby obtaining 230.6g of water-absorbent resin particles containing amorphous silica. The median diameter of the water-absorbent resin particles was 355. mu.m.

Comparative example 2

230.8g of water-absorbent resin particles were obtained in the same manner as in comparative example 1, except that the amount of water withdrawn to the outside of the system by azeotropic distillation was changed to 256.1g, the amount of ethylene glycol diglycidyl ether aqueous solution added as a surface-crosslinking agent was changed to 4.42g (0.507 mmol), and the amount of amorphous silica mixed with the polymer particles was changed to 0.1 mass. The median diameter of the water-absorbent resin particles was 349 μm.

Comparative example 3

231.0g of water-absorbent resin particles were obtained in the same manner as in comparative example 1, except that the amount of water withdrawn to the outside of the system by azeotropic distillation was changed to 247.9g, and the amount of ethylene glycol diglycidyl ether aqueous solution added as a surface-crosslinking agent was changed to 4.42g (0.507 mmol). The median diameter of the water-absorbent resin particles was 355. mu.m.

The water-absorbent resin particles thus obtained were evaluated for gel-out test force, physiological saline absorption under load, physiological saline retention, median particle diameter, and rewet of the absorbent article by the following methods.

< determination of gel flow-out test force >

[ preparation of swollen gel ]

In a 100ml glass beaker containing 58g of physiological saline at 25 ℃, 2g of water-absorbent resin particles were put under stirring at 600rpm (stirrer chip: 3cm in length, 8mm in diameter, acyclic). After it was confirmed that the water-absorbent resin particles were swollen and the vortex of the liquid surface converged, the stirring was stopped. After that, by standing for 10 minutes, a swollen gel in which the water-absorbent resin particles were swollen 30 times was obtained.

[ test force measurement ]

For the measurement of the gel flow-out test force, a small bench test machine (EZTest, model: EZ-SX) manufactured by Shimadzu Corporation and a measuring instrument shown in FIG. 3 were used. The measurement was carried out at 25. + -. 2 ℃ and a humidity of 50. + -. 10% (RH). The transparent acrylic cylinder 51 had a hole of 3.0cm in diameter at the center of the bottom surface, 5.0cm in inner diameter and 6.0cm in outer diameter. A metal disk 52 having a thickness of 3.0mm and a diameter of 4.9cm is mounted in the cylinder 51. A tapered hole having a diameter expanding toward both surfaces is provided in the central portion of the disc 52, and the hole has a minimum diameter of 5.0mm and a maximum diameter of 8.0 mm. 20.0g of the gel (swollen gel) of the water-absorbent resin particles 10a swollen as described above was put on a tray 52 in a cylinder 51, and the swollen gel was uniformly arranged in the cylinder 51 while being loosened by a spatula.

The compression of the swollen gel was performed as follows. A fixing jig 53 having a disk with a diameter of 4.9cm and a thickness of 1.2cm at the tip of a handle with a length of 14cm was attached to a load cell (load cell capacity: 50N manufactured by Shimadzu Corporation) of a small bench tester. A type having a hole with a diameter of 3.0cm in the center (manufactured by Shimadzu Corporation) was used as a measuring table of the testing machine. The cylinder 51 having the swollen gel and the disk 52 fitted in the hole on the measuring table is disposed directly below the fixing tool 53 so that the bottom of the cylinder 51 is horizontal. The fixing tool 53 is installed in a disc parallel to the bottom of the cylinder 51. The fixing tool 53 was manually lowered in the vertical direction until the tip of the fixing tool 53 was brought into contact with the surface of the swollen gel and the test force slightly exceeded 0.01N. The subsequent operation of the fixing tool 53 mounted on the testing machine was performed by a SHIMADZU stereograph machine using the software TRAPEZIUM X (manufactured by SHIMADZU Corporation). The fixing tool 53 is further lowered in the vertical direction, and a load is applied while linearly increasing the load with respect to the swollen gel. The gradient of the load increase was set to 4.9N/min. The fixing tool 53 is stopped at the moment when the load cell senses 4.9N. In order to reduce the gap, 4.9N was maintained for 30 seconds from the stop of the fixing tool 53. Thereafter, the test force was measured by pressing down the fixing tool at a speed of 10 mm/min to compress the swollen gel toward the bottom of the cylinder 51. The gel flow-out test force was defined as a test force at which the time when the swollen gel started to flow out from the hole of the disc 52 was visually confirmed.

Fig. 4 shows an example of the measurement result of the gel flow-out test force. When the test force generated by pressing in at a speed of 10 mm/min was measured, the displacement (moving distance) of the fixing tool 53 was increased, and the test force was smoothly increased during the time when the swollen gel was compressed. When the swollen gel starts to flow out from the hole of the disc 52, the smoothly increasing test force increases while repeatedly increasing and decreasing. The time at which the decrease in the test force was initially observed coincides with the time at which it was visually confirmed that the swollen gel began to flow out of the hole of the disc 52. That is, in the graph of fig. 4, the gel flow-out test force is a peak test force before a decrease in the test force is initially observed, which is indicated by an arrow in the graph. The results are shown in Table 1.

Measurement of Water absorption of physiological saline under load of < 4.14kPa >

The water absorption capacity of physiological saline under a load of 4.14kPa (water absorption capacity under load) was measured by a measuring apparatus shown schematically in FIG. 5. The measurement was performed twice for 1 type of water-absorbent resin particles, and the average value was determined. The measuring apparatus includes a dripping unit 1, a jig 3, a catheter 5, a stand 11, a measuring table 13, and a measuring unit 4 placed on the measuring table 13. The dropping unit 1 includes: a burette 21 with graduations; a rubber stopper 23 sealing the upper opening of the burette 21; a cock 22 connected to the front end of the lower part of the burette 21; an air inlet pipe 25 connected to a lower portion of the burette 21; and a cock 24. The dropping unit 1 is fixed by a jig 3. The flat plate-like measuring table 13 has a through hole 13a having a diameter of 2mm formed in the central portion thereof, and is supported by the height-variable table 11. The through hole 13a of the measuring table 13 and the cock 22 of the titration part 1 are connected by the catheter 5. The inner diameter of the conduit 5 is 6 mm.

The measuring section 4 has a cylinder 31 made of acrylic resin, a polyamide mesh 32 bonded to one opening of the cylinder 31, and a weight 33 vertically movable in the cylinder 31. The cylinder 31 is placed on the measuring table 13 through a polyamide net 32. The cylinder 31 has an inner diameter of 20 mm. The pore size of the polyamide mesh 32 was 75 μm (200 mesh). The weight 33 had a diameter of 19mm and a mass of 119.6g, and as described later, a load of 4.14kPa was applied to the water-absorbent resin particles 10a uniformly arranged on the polyamide net 32.

The measurement of the water absorption capacity of physiological saline under a load of 4.14kPa by the measuring apparatus shown in FIG. 5 was carried out at 25 ℃ in a room having a humidity of 50. + -. 10% (RH). First, the cock 22 and the cock 24 of the titration section 1 are closed, and 0.9 mass% physiological saline adjusted to 25 ℃ is put into the burette 21 from the opening at the upper part of the burette 21. Subsequently, the upper opening of the burette 21 is sealed with the rubber stopper 23, and then the cock 22 and the cock 24 are opened. The inside of the duct 5 is filled with 0.9 mass% saline 50 to prevent air bubbles from entering. The height of the measuring table 13 was adjusted so that the height of the water surface of the 0.9 mass% brine reaching the inside of the through-hole 13a was the same as the height of the upper surface of the measuring table 13. After the adjustment, the height of the water surface of the 0.9 mass% brine 50 in the burette 21 is read by the scale of the burette 21, and the position thereof is set to zero (read value at 0 second).

In the measurement section 4, 0.10g of the water-absorbent resin particles 10a were uniformly arranged on the polyamide mesh 32 in the cylinder 31, the weight 33 was arranged on the water-absorbent resin particles 10a, and the cylinder 31 was set so that the center portion thereof coincides with the inlet of the pipe at the center portion of the measurement table 13. The amount of decrease (i.e., the amount of physiological saline absorbed by the water-absorbent resin particles 10 a) Wa (ml) of the physiological saline in the burette 21 2 hours after the water-absorbent resin particles 10a started to absorb the physiological saline from the catheter 5 was read, and the water absorption capacity of the water-absorbent resin particles 10a under a load of 4.14kPa was calculated by the following equation. The results are shown in Table 1.

Water absorption capacity (ml/g) of physiological saline under a load of 4.14kPa Wa (ml)/mass (g) of water-absorbent resin particles

< measurement of Water-holding Capacity of physiological saline >

A cotton bag (broadloom cotton No. 60, 100mm in width. times.200 mm in length) weighing 2.0g of water-absorbent resin particles was placed in a beaker having a volume of 500 ml. 500g of a 0.9 mass% aqueous sodium chloride solution (physiological saline) was once injected into a cotton bag containing water-absorbent resin particles so as not to cause blocking, and the upper part of the cotton bag was bound with a rubber band and left to stand for 30 minutes, thereby swelling the water-absorbent resin particles. The cotton bag after the lapse of 30 minutes was dehydrated by a dehydrator (manufactured by KOKUSN USAN Co. Ltd., product No. H-122) set to a centrifugal force of 167G for 1 minute, and the mass Wb (G) of the cotton bag containing the dehydrated swollen gel was measured. The same procedure was carried out without adding the water-absorbent resin particles, and the empty mass wc (g) of the cotton bag when it was wet was measured, and the water retention of physiological saline was calculated from the following equation. The measurement of the water retention of physiological saline was carried out at 25 ℃ plus or minus 2 ℃ under a humidity of 50 plus or minus 10% (RH). The results are shown in Table 1.

The water retention capacity (g/g) of the physiological saline is [ Wb-Wc ]/2.0

< determination of median particle diameter (particle size distribution) >

50g of the water-absorbent resin particles were used for measuring the median particle diameter (particle size distribution). A JIS standard sieve was combined with a sieve having an aperture of 850. mu.m, a sieve having an aperture of 500. mu.m, a sieve having an aperture of 425. mu.m, a sieve having an aperture of 300. mu.m, a sieve having an aperture of 250. mu.m, a sieve having an aperture of 180. mu.m, a sieve having an aperture of 150. mu.m, and a tray in this order from the top.

Water-absorbent resin particles were added to the uppermost screen of the combination, and classified by means of a rotary shaker (manufactured by iida-sesakasho Japan Corporation) in accordance with JIS Z8815 (1994). After classification, the mass of the water-absorbent resin particles remaining on each sieve was calculated as a mass percentage with respect to the total amount, and the particle size distribution was determined. The particle size distribution was plotted on a logarithmic probability paper by integrating the particle diameters on the sieve in descending order of the particle diameter and the relationship between the pore diameter of the sieve and the integrated value of the mass percentage of the water-absorbent resin particles remaining on the sieve. The plotted points on the probability paper were connected by a straight line, and the particle diameter corresponding to 50 mass% of the cumulative mass percentage was defined as the median particle diameter.

< measurement of Return volume of absorbent article >

[ production of absorbent article ]

A sheet-like absorbent body having a size of 40cm × 12cm was produced by uniformly mixing 10g of water-absorbent resin particles and 6.8g of ground pulp by air-laid paper making using an air-jet mixing device (Otec co., Ltd, Pad former). Then, the resultant was measured with a basis weight of 16g/m which was the same as that of the absorbent body²The laminate was obtained by applying a load of 196kPa for 30 seconds to the whole body and pressing the same in a state where 2 sheets of paper towels were sandwiched between the upper and lower sides of the absorbent body. Further, the basis weight was set to 22g/m by setting the same size as the above-mentioned absorbent body²The polyethylene-polypropylene air-permeable porous liquid-permeable sheet was disposed on the upper surface of the laminate, and an absorbent article for evaluation was obtained.

The absorbent article for evaluation was placed on a horizontal table so that the surface provided with the liquid-permeable sheet became the upper surface, and a cylinder for liquid administration having a volume of 100mL and having an inlet port with an inner diameter of 3cm was placed in the center of the absorbent article for evaluation. 80ml of physiological saline was put into the cylinder at one time. The cylinder is removed from the absorbent article, and the absorbent article is left to stand in an intact state. After 30 minutes (2 nd time) and 60 minutes (3 rd time) from the start of the 1 st test solution input, the same operation was performed using the measuring instrument at the same position as in the 1 st time. After 60 minutes from the 3 rd input, 40 pieces of filter paper having a mass (wd (g)) measured in advance and having a width of 10cm square was placed near the test solution input position on the absorbent article, and a weight having a mass of 5kg and a bottom surface of 10cm × 10cm was placed thereon. After a load of 5 minutes, the mass of the filter paper (We) (g)) was measured, and the amount of added mass was defined as the rewet amount (g). The test of the amount of the rewet was carried out in a room adjusted to 25 ℃ and 50% (RH) humidity. It can be said that the smaller the rewet amount, the more preferable it is for use as an absorbent article. The results are shown in Table 1. The absorbent article using the water-absorbent resin particles of the examples showed a reduced rewet amount.

Return seepage (g) ═ We-Wd

[ Table 1]

Description of the reference numerals

1-dropping part, 3-clamp, 4-measuring part, 5-catheter, 10-absorber, 10 a-water-absorbent resin particles, 10 b-fiber layer, 11-bench, 13-measuring bench, 13 a-through hole, 20a, 20 b-core wrapping sheet, 21-burette, 22-cock, 23-rubber plug, 24-cock, 25-air inlet tube, 30-liquid-permeable top sheet, 31-cylinder, 32-polyamide net, 33-weight, 40-liquid-impermeable back sheet, 51-cylinder, 52-disc, 53-fixing tool, 100-absorbent article, 200-stirring wing, 200 a-shaft, 200 b-flat plate part, S-slit.