JP2022175090A - Poly(meth)acrylic acid (salt)-based water-absorbing resin, and absorber - Google Patents

Abstract

To provide a water-absorbing resin that can give an absorbent article having a reduced amount of returned liquid in liquid absorption for a second time.SOLUTION: A water-absorbing resin comprises a crosslinked polymer containing a constitutional unit derived from a poly(meth)acrylic acid (salt)-based monomer. The water-absorbing resin has a gel movement index of 0.20 or less and a water absorption rate (Vortex method) of 50 seconds or shorter.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a water absorbent resin and an absorbent body containing the absorbent resin. More specifically, the present invention relates to a poly(meth)acrylic acid (salt)-based water absorbent resin and an absorbent body.

Water absorbent resins are widely used in various applications (water absorbent articles) because of their excellent water absorption properties. In recent years, especially from the viewpoint of absorption of body fluids, water-absorbent resins have been used as components (absorbent bodies) of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

Examples of such water absorbent resins include hydrolysates of starch-acrylonitrile graft copolymers, neutralized starch-acrylic acid graft polymers, saponified vinyl acetate-acrylic acid ester copolymers, (meth ) partially neutralized acrylic acid crosslinked products of polymers are known, but from the viewpoint of water absorption performance, poly(meth)acrylic acid and/or its salt is used as the main component of the monomer Acrylic acid (salt)-based water absorbent resins are industrially produced in the largest amount.

By the way, in the absorbent article, liquid pooling or liquid return may occur during use. Since these substances cause discomfort when the absorbent article is worn, there is a demand for an absorbent article in which such liquid pools or liquid backflow is less likely to occur.

One of the causes of the above liquid pooling or liquid return in absorbent articles is known to be a decrease in water absorption performance due to gel blocking of the water absorbent resin. Patent Documents 1 to 3 disclose techniques for suppressing gel blocking and improving the water absorption performance of absorbent articles by using a water-absorbing resin having excellent elasticity (gel elasticity) after absorbing water. there is

WO2020/218160 WO2020/218161 WO2020/218162

However, in the absorbent article (absorbent body) using the above conventional water-absorbing resin, when a load (e.g., user's weight) is applied locally, even though the gel booking is suppressed, There is a problem that the liquid returns after the second liquid absorption, and from this point there is room for further improvement.

An object of one aspect of the present invention is to provide a water absorbent resin that can provide an absorbent article in which the amount of liquid returned after the second liquid absorption is reduced.

The present inventors have found that an absorbent article comprising a water absorbent resin having a small gel transfer index, which indicates variation in shape retention of the gel, when a load is applied to the absorbent resin under different conditions, The present inventors have found that the amount of liquid returned in the second liquid absorption can be suppressed when a load is applied, and have completed the present invention.

That is, one embodiment of the present invention is as follows.
<1> A particulate water absorbent resin containing a crosslinked polymer containing a structural unit derived from a poly(meth)acrylic acid (salt) monomer, wherein the gel transfer index obtained by the following procedure is 0. .20 or less and a water absorption rate (Vortex method) of 50 seconds or less:
(1) Into a 100 mL beaker with a barrel diameter of 55 mm and a height of 70 mm, 38 g of a 0.9% by mass sodium chloride aqueous solution at 25° C. and 2.00 g of a water absorbent resin are added to form a swollen gel.
(2) A compression jig comprising a disk portion and a rod-shaped portion having one end connected to the center of the disk portion (the disk portion has flat surfaces on the front and back, and has a diameter of 2.5 cm and a thickness of 1 cm. and the length of the rod-shaped portion is 5.5 cm) is positioned in the center of the beaker containing the swollen gel of (1), and the disk portion of the compression jig After bringing the lower surface of the swollen gel into contact with the surface of the swollen gel, the compression jig is positioned such that the lower surface of the disk portion is vertically raised by 0.05 mm from the surface of the swollen gel. Place it at the measurement start position.
(3) The compression jig arranged at the measurement start position is pushed into the swollen gel by 10.0 mm in the vertical direction at a speed of 10 cm/min.
(4) In (3), when the compression jig is pushed into a position 8.0 mm in the vertical direction from the measurement start position, the load (N) applied to the compression jig is observed, It is referred to as gel shape retention force A.
(5) The same operations as (1) to (4) are performed except that the speed of pushing the compression jig is changed to 1 mm/min, and the load (N) applied to the compression jig is observed. , gel shape retention force B.
(6) Gel transfer index for calculating the gel transfer index according to the following formula (1)=(gel shape retention force A/gel shape retention force B)−1 (1).
<2> The water-absorbent resin according to <1>, which is aggregate particles of spherical particles.
<3> The water absorbent resin according to <1> or <2>, which contains water-insoluble inorganic fine particles.
<4> The water absorbent resin according to any one of <1> to <3>, having a gel shape retention force A of 10.0 N or more.
<5> The water absorbent resin according to any one of <1> to <4>, having an AAP of 15 g/g or more.
<6> The water absorbent resin according to any one of <1> to <5>, which is obtained by reverse phase suspension polymerization in a hydrophobic organic solvent.
<7> The water absorbent resin according to any one of <1> to <6>, which is obtained by extruding the spherical particles with an extruder having a perforated plate.
<8> Contains the water absorbent resin according to any one of <1> to <7> and does not contain a hydrophilic fiber, or the weight of the water absorbent resin is the water absorbent resin and the hydrophilic fiber 50% by mass or more of the total mass of the absorbent.

According to the water-absorbing resin of the present invention, it is possible to provide a water-absorbing resin capable of providing an absorbent article in which the amount of liquid returning after the second liquid absorption is reduced.

1 is a schematic diagram of a compression jig used for measuring gel transfer index in Examples. FIG. FIG. 10 is a plan view showing the process of manufacturing an absorbent sheet for evaluation manufactured in Examples. FIG. 2 is a cross-sectional view of an absorbent sheet for evaluation produced in Examples.

Hereinafter, the present invention will be described while showing the best mode. It should be understood that the terms used herein have the meanings commonly used in the art unless otherwise specified. Thus, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification (including definitions) will control. The present invention is not limited to the following embodiments, and can be modified in various ways within the scope of the claims.

[1. Definition of terms〕
[1-1. Water Absorbent Resin]
The "water-absorbent resin" in the present invention refers to a water-swellable water-insoluble polymeric gelling agent, and means one that satisfies the following physical properties. That is, as "water swellability", CRC defined by ERT441.2-02 is 5 g / g or more, and as "water insoluble", Ext defined by ERT470.2-02 is a physical property of 50% by weight or less. It refers to a polymeric gelling agent that satisfies

The water-absorbent resin can be appropriately designed according to its application, and is not particularly limited, but is preferably a hydrophilic crosslinked polymer obtained by crosslink-polymerizing an unsaturated monomer having a carboxyl group. Further, the total amount (100% by weight) of the water-absorbing resin is not limited to a form in which it is a polymer, and may be a water-absorbing resin composition containing additives within the range satisfying the above physical properties (CRC, Ext). may

Furthermore, the water-absorbing resin in the present invention is not limited to the final product, and intermediates in the manufacturing process of the water-absorbing resin (for example, a hydrous gel-like crosslinked polymer after polymerization, a dry polymer after drying, water absorption before surface cross-linking resin powder, etc.), and together with the water-absorbing resin composition, all of these are collectively referred to as "water-absorbing resin". Examples of the shape of the water absorbent resin include sheet-like, fibrous, film-like, particulate, gel-like, and the like. In the present invention, the particulate water absorbent resin is preferred.

[1-2. “EDANA” and “ERT”]
"EDANA" is an abbreviation for the European Disposables and Nonwovens Association, and "ERT" is an abbreviation for the European standard (almost a global standard) water-absorbing resin measurement method (EDANA Recommended Test Methods). . In the present invention, the physical properties of the water absorbent resin are measured according to the ERT original (2002 revision/publicly known document) unless otherwise specified.

[1-3. others]
In this specification, "X to Y" indicating a range means "X or more and Y or less".

In this specification, "ppm" means "mass ppm" unless otherwise specified.

As used herein, "acid (salt)" means "acid and/or its salt". "(Meth)acryl" means "acryl and/or methacryl". "Poly(meth)acrylic acid (salt)-based water absorbent resin" means a water absorbent resin mainly composed of (meth)acrylic acid (salt) as a repeating unit. Among the monomers (excluding the cross-linking agent), the (meth)acrylic acid (salt) is preferably 50 to 100 mol%, more preferably 70 to 100 mol%, still more preferably 90 to 100 mol%, particularly preferably substantially It refers to a water absorbent resin containing 100 mol %.

In this specification, the unit of volume "liter" may be written as "l" or "L".

In this specification, "weight" and "mass", "wt%" and "mass%", and "parts by weight" and "parts by mass" are treated as synonyms.

[2. Physical properties of water absorbent resin]
(Shape of water absorbent resin)
In the present invention, the shape of the water absorbent resin is preferably particulate. More specific examples of the shape of the particles include crushed irregular shapes, spherical shapes, football shapes, aggregate shapes, and the like. Among them, because the particle shape is spherical, or particularly because it is in the form of aggregates, the absorption rate is high and the water-absorbing resin takes in the liquid and the amount of liquid return is suppressed. It is particularly preferred that the particles are shaped like particles. In this specification, the term “spherical” includes not only true spheres but also substantially spherical ones having an aspect ratio of 1.0 to 1.2.

(Additives contained in water absorbent resin)
In the present invention, the water absorbent resin may contain additives for expressing various functions. Specific examples of such additives include surfactants, compounds having phosphorus atoms, oxidizing agents, organic reducing agents, water-insoluble inorganic fine particles, organic powders such as metal soaps, deodorants, antibacterial agents, pulp and thermoplastics. A fiber etc. are mentioned. Among these, water-insoluble inorganic fine particles are preferably included. These additives may be used singly or in combination of two or more. As the water-insoluble inorganic fine particles, compounds disclosed in "[5] Water-insoluble inorganic fine particles" of International Patent Publication No. 2011/040530 are applied to the present invention. Among these water-insoluble inorganic fine particles, especially hydrophilic fine particles (hydrophilicity described in European Patent No. 0629411 (water/methanol = 70/30 mixed liquid expressed by the proportion of fine particles suspended in a colloidal state) (e.g., 70% or more) and/or those having a low contact angle with water (e.g., 10° or less) described in Japanese Patent No. 6837139), such as silica (silicon dioxide ) and hydrotalcite, the swollen water-absorbing resin (swelling gel) is moderately caught on each other, and even if a load is applied locally, the swelling gel is difficult to move in the absorbent body (i.e., the swelling gel is biased It is preferable because it is difficult to absorb) and exhibits excellent absorption performance.

The content of the water-insoluble inorganic fine particles in the water-absorbent resin of the present invention is 0.01 to 5 with respect to 100 parts by mass of the water-absorbent resin, from the viewpoint of improving the liquid compatibility of the water-absorbent resin (for example, water-absorbent resin particles). parts by mass, more preferably 0.05 to 3 parts by mass, still more preferably 0.1 to 1 part by mass, and particularly preferably 0.2 to 0.5 parts by mass.

Here, the water-insoluble inorganic fine particles usually have a microscopic size compared to the size of the water absorbent resin. For example, the water-insoluble inorganic fine particles may have an average particle size of 0.01 to 50 μm, 0.1 to 30 μm, or 1 to 20 μm. The average particle size of the water-insoluble inorganic fine particles can be measured by a pore electrical resistance method or a laser diffraction/scattering method depending on the properties of the particles.

(CRC)
"CRC" is an abbreviation for Centrifuge Retention Capacity, and means the water absorption capacity of a water absorbent resin under no pressure.

The CRC of the water absorbent resin of the present invention is preferably 30 g/g or more, more preferably 32 g/g or more. The upper limit is not particularly limited, and a higher CRC is preferable, but from the viewpoint of balance with other physical properties, it is preferably 50 g/g or less, more preferably 48 g/g or less, 46 g/g or less, and 44 g/g. 42 g/g or less, 40 g/g or less, 38 g/g or less, and 36 g/g or less.

If the CRC of the water-absorbing resin is 30 g/g or more, the amount of liquid absorption is sufficient, and it is suitable as an absorbent for absorbent articles such as paper diapers. Also, if the CRC is 50 g/g or less, the rate of absorbing body fluids such as urine and blood is prevented from lowering, making it suitable for use in high water absorption rate type paper diapers and the like. The CRC value of the water absorbent resin can be controlled by changing the type and amount of the internal cross-linking agent, the surface cross-linking agent, and the like.

(ext)
“Ext” is an abbreviation for Extractables (water-soluble matter), and means the amount of soluble matter extracted from the water absorbent resin. Ext is measured according to the EDANA method (ERT470.2-02).

The Ext of the water absorbent resin of the present invention is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less. The lower limit is not particularly limited, and is, for example, 0% by mass or more, but from the viewpoint of balance with other physical properties, it is preferably 2% by mass or more, and more preferably 4% by mass or more.

If the Ext of the water-absorbing resin is 30% by mass or less, the rate of absorbing body fluids such as urine and blood is prevented from lowering, making it suitable for use in high water-absorbing speed type paper diapers and the like. It is also preferable because the swollen gel becomes less slippery and variation in shape retention of the swollen gel when different loads are applied is reduced. The Ext value of the water-absorbing resin can be controlled by changing the type and amount of the polymerization initiator, internal cross-linking agent, surface cross-linking agent, etc. In addition, a chain transfer agent can be used in the polymerization process. You can control it though.

(AAP)
"AAP" is an abbreviation for Absorption Against Pressure, and means the water absorption capacity of a water absorbent resin under pressure. As used herein, AAP (absorbent capacity under pressure) is measured according to the EDANA method (ERT442.2-02), except that the load condition is changed to 4.83 kPa (0.7 psi). Specifically, using a 0.9% by mass sodium chloride aqueous solution, 0.9 g of a water-absorbent resin is swollen for 1 hour under a pressure of 4.83 kPa, and then AAP (absorption capacity under pressure) (g / g) to measure.

The AAP of the water absorbent resin of the present invention is preferably 15 g/g or more, more preferably 16 g/g or more, and still more preferably 17 g/g or more from the viewpoint of excellent water absorption properties when used in sanitary materials. The upper limit of AAP of the water absorbent resin is not particularly limited, but is preferably 40 g/g or less.

(moisture content)
"Moisture content" is measured according to the EDANA method (ERT430.2-02) except that the sample amount is changed to 1.0 g and the drying temperature is changed to 180°C.

The water content of the water absorbent resin of the present invention is not particularly limited, but is preferably 1% by mass to 20% by mass, more preferably 1% by mass to 15% by mass, still more preferably 2% by mass to 13% by mass, and still more preferably 5% to 13% by mass, more preferably 8% to 13% by mass, particularly preferably 10% to 13% by mass. If the water content of the water-absorbing resin is 1% by mass to 20% by mass, the rate of absorption of body fluids such as urine and blood is prevented from decreasing, making it suitable for use in high water-absorbing speed type paper diapers and the like.

(Mass average particle size (D50))
"Mass-average particle diameter (D50)" is described in "(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution" described in columns 27 and 28 of US Pat. Measured in compliance.

The weight average particle diameter (D50) of the water absorbent resin of the present invention is preferably 200 μm to 700 μm, more preferably 250 μm to 600 μm, even more preferably 250 μm to 500 μm, particularly preferably 300 μm to 450 μm. Also, the proportion of particles having a particle diameter of less than 150 μm is preferably 20% by mass or less, more preferably 10% by mass or less. If the mass-average particle size of the water absorbent resin is 200 μm or more, the dust is small and the handleability is good. Also, if the mass average particle size of the water absorbent resin is 700 μm or less, the rate of absorbing body fluids such as urine and blood is prevented from decreasing, making it suitable for use in high water absorption rate type paper diapers and the like.

(Number average particle size)
When the water-absorbent resin has an aggregated particle shape in which primary particles are aggregated, the number average particle diameter of the primary particles constituting the aggregates is measured using an electron microscope. The number average particle diameter of the primary particles of the water absorbent resin is preferably 5 to 1000 μm, more preferably 5 to 800 μm, even more preferably 8 to 500 μm, even more preferably 10 to 300 μm, and still more preferably. is between 10 and 200 μm, particularly preferably between 30 μm and 100 μm.

(The bulk density)
The "bulk density" of the water absorbent resin of the present invention is measured according to the EDANA method (ERT460.2-02).

The bulk density of the water absorbent resin of the present invention is preferably 0.68 g/cm ³ to 1.00 g/cm ³ , more preferably 0.69 g/cm ³ to 0.95 g/cm ³ , most preferably 0.68 g. /cm ³ to 0.90 g/cm ³ . If the bulk density of the water-absorbing resin is 0.68 to 1.00 g/cm ³ , the rate of absorption of body fluids such as urine and blood is prevented from decreasing, making it suitable for use in high water-absorbing speed type paper diapers and the like. Suitable. In addition, the swollen gels are appropriately caught by each other, and the variation in shape retention of the gels when different loads are applied is small, so that the gels are less likely to move in the absorbent core and exhibit excellent absorption performance, which is preferable.

(surface tension)
The "surface tension" in the present invention is the surface tension of an aqueous solution when the water absorbent resin is dispersed in a 0.9% by mass sodium chloride aqueous solution, and is measured by the method described in WO2015/129917.

The surface tension of the water absorbent resin of the present invention may be 74 mN/m or less, 55 mN/m or more, preferably 60 mN/m or more, more preferably 65 mN/m or more, and still more preferably. is 70 mN/m or more. When the surface tension of the water-absorbing resin is within the above range, the swelling gel is suppressed from slipping, the gel hardly moves in the absorbent body, and excellent absorption performance is exhibited, which is preferable.

(water absorption speed)
As used herein, the "water absorption rate" of a water absorbent resin is measured according to the Vortex method of Japanese Industrial Standards JIS K 7224 (1996). That is, the water absorption rate (Vortex method) was measured by adding 2.0 g of a water-absorbent resin to 50 g of a 0.9% by mass sodium chloride aqueous solution stirred at 600 rpm with a stirrer tip, and measuring the time until the stirrer tip was covered with the test solution. refers to time.

A water-absorbing resin having an excellent "water-absorbing rate" is preferable because it quickly absorbs liquid and suppresses the amount of liquid returned when used as an absorbent article. The "water absorption rate" is 50 seconds or less, preferably 45 seconds or less, more preferably 40 seconds or less. Although the lower limit is not particularly limited, it may be 10 seconds or more, 15 seconds or more, or 20 seconds or more.

(Gel shape retention force)
In the present specification, the "gel shape retention force" of the water absorbent resin is a new load (gel shape retention) when a predetermined load is applied to the water-absorbed gel-like water absorbent resin (swollen gel). It is a physical property value. "Gel shape retention force" is measured by the method described in Examples below.

In this specification, a compression jig comprising a disk portion and a rod-shaped portion having one end connected to the center of the disk portion (the disk portion has flat surfaces on the front and back and has a diameter of 2.5 cm). , a disk shape with a thickness of 1 cm and a rod-shaped portion with a length of 5.5 cm. 10.0 mm at a speed of /min, and when the compression jig is pushed to a position of 8.0 mm, the load (N) applied to the compression jig is the gel shape retention force A, and the pushing speed is The load (N) applied to the compression jig when the compression jig is changed to 1 mm/min and measured in the same manner is defined as the gel shape retention force B.

The gel shape retention force A is preferably 6.0 N or more, more preferably 8.0 N or more, still more preferably 10.0 N or more, even more preferably 10.4 N or more, and particularly preferably 10.6 N or more. Although the upper limit is not particularly limited, it may be 15.0N or less, 14.0N or less, or 13.0N or less. The gel shape retention force B is preferably 5.0N or more, more preferably 7.0N or more, still more preferably 9.0N or more, even more preferably 9.1N, and particularly preferably 9.2N. Although the upper limit is not particularly limited, it may be 14.0N or less, 13.0N or less, or 12.0N or less.

A water absorbent resin having a high “gel shape retention force” (for example, a water absorbent resin having a gel shape retention force A of 10.0 N or more) is used as an absorbent article, especially when it is used as a thin absorbent article. Even if a load (body weight) is applied after absorbing water, the gel is less likely to be deformed or moved.

(gel transfer index)
The inventors of the present invention, while conducting research on the above-mentioned problems of the present application, found that when a load is applied locally in an absorbent article, the gel is biased inside the absorber, and the bias is caused by the gel. This is the first time that we have found that the unevenness in water absorption performance is the cause of liquid return after the second liquid absorption.

As a result of further intensive studies based on such findings, the present inventors found that the "gel transfer index", which is a new physical property value representing the change in the shape retention of the gel when different loads are applied to the swollen gel, indicates that the water absorption It was newly found that the amount of liquid returned after the second liquid absorption can be evaluated when a load (body weight) is locally applied to the elastic resin.

As used herein, "gel transfer index" is a value defined by the following formula (1);
Gel transfer index = (gel shape retention force A/gel shape retention force B) -1 (1)
A water-absorbing resin with a small "gel transfer index" suppresses deformation of the gel layer even when a load (body weight) is applied after absorbing water when used as an absorbent article, especially when used as a thin absorbent article. It is preferable because the gel is less likely to be biased in the core and the absorption performance is not impaired. The gel transfer index of the water absorbent resin of the present invention is 0.20 or less, more preferably 0.18 or less, still more preferably 0.16 or less. Although the lower limit is not particularly limited, it may indicate a negative value and may be 0.00 or more.

[3. Method for manufacturing water absorbent resin]
The method for producing the water absorbent resin of the present invention is not particularly limited as long as a water absorbent resin having desired physical properties can be obtained. Although any of the methods may be used, reversed-phase suspension polymerization is preferable because the physical properties of the water-absorbing resin can be easily controlled. Reversed-phase suspension polymerization will be described below as an example.

A method for producing a water-absorbing resin according to one embodiment of the present invention is a state in which droplets containing a monomer are dispersed or suspended in a hydrophobic organic solvent, and the monomer is polymerized to form a hydrous gel polymer. and a drying step of drying the hydrous gel polymer (for example, using an agitating dryer) to obtain a particulate dry polymer. In this specification, the "hydrous gel polymer" may be simply referred to as "hydrous gel".

As the polymerization method in the method for producing a water absorbent resin of the present invention, a reverse A water-containing gel polymer may be obtained by phase suspension polymerization, and the polymerization method may be a batch system or a continuous system. The batch type production method is a process in which an aqueous monomer solution is added or dropped into a hydrophobic organic solvent in a reactor and mixed to disperse or suspend the aqueous monomer solution, and then polymerize to obtain a water-containing It is a production method for obtaining a gel polymer. On the other hand, in the continuous production method, an aqueous monomer solution is continuously fed to a hydrophobic organic solvent in a reactor, dispersed or suspended, and then polymerized to form a water-containing gel polymer through a polymerization reaction. and the hydrophobic organic solvent are continuously discharged from the reactor. A preferred embodiment of the present invention is continuous reversed-phase suspension polymerization (continuous polymerization), and more preferably liquid-phase droplets in which an aqueous monomer solution is continuously dispersed in a hydrophobic organic solvent and polymerized. Continuous polymerization.

In the case of such a continuous production process, each operation can be continuously performed in each step and between each step, and long-term operation enables mass production, which is preferable. Continuous reverse phase suspension polymerization is also preferable from the viewpoint of the physical properties of the water absorbent resin. In the method for producing a water absorbent resin according to one embodiment of the present invention, a separation step may be provided for separating the hydrous gel polymer obtained in the polymerization step from the hydrophobic organic solvent. In the continuous production process, it is preferable to recover the hydrophobic organic solvent separated from the hydrous gel polymer in the separation step and reuse it as the hydrophobic organic solvent in the polymerization step. By adopting such a circulating production process, the amount of organic solvent used can be reduced, which is preferable in terms of production cost and waste liquid treatment.

In the continuous polymerization, the aqueous monomer solution is continuously suspended or dispersed as droplets in a hydrophobic organic solvent in a dispersing device, and the dispersion/suspension is continuously supplied to the reactor. Therefore, it is clearly distinguished from the form in which dispersion and polymerization are performed in one apparatus (batch operation, batch type). In the case of continuous operation, the operating time is preferably 1 hour or longer, more preferably 3 hours or longer, still more preferably 8 hours or longer, and even more preferably 24 hours or longer. In addition, it is usually one year or less.

The method for producing a water-absorbent resin according to one embodiment of the present invention includes any aqueous monomer solution preparation step; any dispersion step; step); an optional gel sizing step; and a drying step. Further, after the drying step, optionally, a cooling step, a pulverization step, a classification step, a surface cross-linking step, a water content (re-moistening) step, a step of adding other additives, a granulation step, a fine powder removal step, a granulation step and a fine powder A recycling process and the like can be included. In addition, it may further include a transportation process, a storage process, a packing process, a storage process, and the like.

In particular, the method for producing a water absorbent resin of the present invention preferably includes a separation step, a gel granulation step, a drying step (preferably hot air drying), and a surface cross-linking step (preferably powder surface treatment). According to the above configuration, compared to general reversed phase suspension polymerization including an azeotropic dehydration step in a hydrophobic organic solvent after polymerization and a surface cross-linking step in a dispersion system, the resulting water absorbent resin It has the advantage that the gel shape retention of is enhanced.

Hereinafter, each step that can be included in the method for producing a water absorbent resin according to one embodiment of the present invention will be described.

[3-1: Monomer aqueous solution preparation step]
The aqueous monomer solution is an aqueous solution containing a (meth)acrylic acid (salt)-based monomer, which is a raw material for the water-absorbing resin, and is dispersed or suspended in a hydrophobic organic solvent in order to perform reversed-phase suspension polymerization. solution.

As the solvent for the aqueous monomer solution, water or a mixture of water and a water-soluble organic solvent (eg, alcohol) is preferably used, and water is more preferable. When the solvent of the aqueous monomer solution is a mixture of water and a water-soluble organic solvent, the content of the water-soluble organic solvent (e.g., alcohol, etc.) is preferably 30% by mass or less, more preferably 5% by mass or less. .

The aqueous monomer solution may contain, in addition to (meth)acrylic acid, water-soluble ethylenically unsaturated monomers other than (meth)acrylic acid (other water-soluble ethylenically unsaturated monomers). Examples of other water-soluble ethylenically unsaturated monomers include (anhydrous) maleic acid, itaconic acid, cinnamic acid, vinylsulfonic acid, allyltoluenesulfonic acid, vinyltoluenesulfonic acid, styrenesulfonic acid, 2-( meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2 - Acid group-containing unsaturated monomers such as hydroxyethyl (meth)acryloyl phosphate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol mono (meth)acrylate; (meth)acrylamide, N-ethyl (meth)acrylamide, N , N-dimethyl(meth)acrylamide, Nn-propyl(meth)acrylamide, N-isopropyl(meth)acrylamide, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine, N-vinylacetamide, etc. amide group-containing unsaturated monomers; N,N-dimethylaminoethyl (meth) acrylate, N,N-dimethylaminopropyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, N,N- Amino group-containing unsaturated monomers such as diethylaminoethyl (meth)acrylate; mercapto group-containing unsaturated monomers; phenolic hydroxyl group-containing unsaturated monomers; lactam group-containing unsaturated monomers such as N-vinylpyrrolidone etc.

In consideration of the stability of the (meth)acrylic acid-based monomer and other water-soluble ethylenically unsaturated monomers (also referred to as monomer components), an aqueous monomer solution may be used if necessary. may be added with a polymerization inhibitor.

Among the monomer components, the (meth)acrylic acid-based monomer and other water-soluble ethylenically unsaturated monomers having an acid group such as a carboxyl group (hereinafter referred to as a monomer component having an acid group may be referred to as an acid group-containing unsaturated monomer) can be used as a neutralized salt in which the acid group of the monomer is neutralized. In this case, the neutralized salt of the acid group-containing unsaturated monomer is preferably a salt with a monovalent cation, and is preferably at least one salt selected from alkali metal salts, ammonium salts and amine salts. More preferably, it is an alkali metal salt, still more preferably at least one selected from sodium salt, lithium salt and potassium salt, and particularly preferably sodium salt.

From the viewpoint of the water absorption performance of the resulting water absorbent resin, the monomer component is preferably an acid group-containing unsaturated monomer and / or a salt thereof, more preferably (meth) acrylic acid (salt), (anhydrous ) At least one or more selected from maleic acid (salt), itaconic acid (salt), and cinnamic acid (salt), more preferably (meth)acrylic acid (salt), particularly preferably acrylic It is an acid (salt).

When an acid group-containing unsaturated monomer is used as a monomer, it is preferably used in combination with a neutralized salt of the acid group-containing unsaturated monomer from the viewpoint of the water absorption performance of the resulting water absorbent resin. From the viewpoint of water absorption performance, the number of moles of the neutralized salt with respect to the total number of moles of the acid group-containing unsaturated monomer and its neutralized salt (hereinafter referred to as "neutralization rate") is preferably 40 mol% or more, More preferably 40 mol % to 95 mol %, still more preferably 50 mol % to 90 mol %, even more preferably 55 mol % to 85 mol %, particularly preferably 60 mol % to 80 mol %.

In the method for producing a water absorbent resin according to one embodiment of the present invention, any one of the above-exemplified monomers may be used alone in the preparation of the aqueous monomer solution, or any two or more monomers may be used. Polymers may be used by appropriately mixing them. Further, other monomers other than the water-soluble ethylenically unsaturated monomers can be mixed as long as the object of the present invention is achieved.

In the preparation of the aqueous monomer solution, when two or more monomers are used in combination, from the viewpoint of the water absorption performance of the resulting water absorbent resin, the monomer component is, as the main component, (meth)acrylic acid (salt ) is preferably included. Containing (meth)acrylic acid (salt) as a main component means that the ratio of (meth)acrylic acid (salt) to the total amount of monomers used for polymerization is, for example, 50 mol% or more, preferably 70 mol%. Above, more preferably 80 mol % or more, still more preferably 90 mol % or more (the upper limit is 100 mol %).

In the preparation of the aqueous monomer solution, an internal cross-linking agent can be used as necessary. Examples of the internal cross-linking agent include conventionally known internal cross-linking agents having two or more polymerizable unsaturated groups or two or more reactive groups in one molecule. Examples of internal cross-linking agents include N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, Trimethylolpropane di(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, tri allyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1 , 4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, glycidyl (meth)acrylate and the like. These internal cross-linking agents may be used alone or in combination of two or more.

In the method for producing a water absorbent resin according to one embodiment of the present invention, the amount of the internal cross-linking agent to be used may be appropriately determined according to the desired physical properties of the water absorbent resin. 0.0001 to 5 mol %, more preferably 0.001 to 3 mol %, still more preferably 0.005 to 1.5 mol %, relative to the body.

Substances exemplified below (hereinafter referred to as "other substances") can also be added to the aqueous monomer solution.

Specific examples of other substances include chain transfer agents such as thiols, thiolic acids, secondary alcohols, amines, and hypophosphites; foaming agents such as carbonates, bicarbonates, azo compounds, and air bubbles; chelating agents such as metal salts of ethylenediaminetetraacetic acid and metal salts of diethylenetriaminepentaacetic acid; polyacrylic acid (salts) and crosslinked products thereof, starch, cellulose, starch-cellulose derivatives, polyvinyl alcohol and the like. Other substances may be used alone or in combination of two or more.

The amount of other substances used is not particularly limited, but the total concentration of other substances is preferably 10% by mass or less, more preferably 1 % by mass, more preferably 0.11% by mass or less. However, the total concentration (total amount) of polyacrylic acid (salt) and their crosslinked products, starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol is 30% relative to the total amount of monomers contained in the aqueous monomer solution. % by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less.

Further, in one embodiment of the present invention, the dissolved oxygen in the aqueous monomer solution may be reduced by raising the temperature or replacing it with an inert gas.

(Polymerization initiator)
A polymerization initiator may be used in the preparation of the aqueous monomer solution. If a polymerization initiator is used to prepare the aqueous monomer solution, the aqueous monomer solution may gel or increase in viscosity. (ii) the aqueous monomer solution is cooled and mixed with a polymerization initiator at a temperature lower than room temperature (20° C. or less, preferably around 0° C.), (iii) a monomer It is preferable to use a method such as subjecting the aqueous solution and the polymerization initiator to the dispersion step while line-mixing them. As the polymerization initiator, a thermal decomposition type polymerization initiator is preferably used. The thermally decomposable polymerization initiator is intended to be a compound that decomposes by heat to generate radicals. The 10-hour half-life temperature of the thermal decomposition type polymerization initiator is preferably 0° C. to 120° C., more preferably 30° C. to 100° C., still more preferably 50° C., from the viewpoint of storage stability and production efficiency of the water absorbent resin. ~80°C.

From the viewpoint of handling properties and physical properties of the water absorbent resin, the polymerization initiator is preferably a water-soluble polymerization initiator, and more preferably a water-soluble radical polymerization initiator.

Examples of water-soluble radical polymerization initiators include persulfates such as potassium persulfate, ammonium persulfate and sodium persulfate; methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl Cumyl peroxide, t-butylperoxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate, peroxides such as hydrogen peroxide; 2,2'-azobis (2-amidinopropane) ) dihydrochloride, 2,2′-azobis[2-(N-phenylamidino)propane]dihydrochloride, 2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, etc. water-soluble azo compounds; and the like. As the polymerization initiator, one of these may be used alone, or two or more of them may be used in combination. Among these, persulfates or water-soluble azo compounds are preferred, sodium persulfate, potassium persulfate, ammonium persulfate, or 2,2′-azobis(2-amidinopropane) dihydrochloride is even more preferred. Sodium persulfate is used.

The amount of the polymerization initiator to be used is appropriately set according to the type of the monomer and the polymerization initiator, and is not particularly limited, but from the viewpoint of production efficiency, it is , preferably 0.001 g/mol or more, more preferably 0.005 g/mol or more, still more preferably 0.01 g/mol or more. From the viewpoint of improving the water absorption performance of the water absorbent resin, it is preferably 2 g/mol or less, more preferably 1 g/mol or less.
Further, if necessary, other polymerization initiators such as a photodegradable polymerization initiator can be used in combination with a thermally decomposable polymerization initiator. Specific examples of the photolytic polymerization initiator include benzoin derivatives, benzyl derivatives, acetophenone derivatives, benzophenone derivatives and the like.
Further, the above thermal decomposition type polymerization initiator and reducing agent can be used together to form a redox polymerization initiator. In the redox polymerization initiator, the thermal decomposition type polymerization initiator functions as an oxidizing agent. The reducing agent used is not particularly limited, but for example, sodium sulfite, sodium hydrogen sulfite and other (bi)sulfites; reducing metal salts such as ferrous salts; L-ascorbic acid (salts), amines, etc. is mentioned.

(Concentration of monomer in aqueous monomer solution)
In the present invention, the concentration of the monomer in the aqueous monomer solution is selected according to the type of the selected monomer and hydrophobic organic solvent, etc., but in terms of production efficiency, the lower limit is preferably 10 mass. % or more, more preferably 20 mass % or more, and even more preferably 30 mass % or more. Also, the upper limit is preferably 100% by mass or less, more preferably 90% by mass or less, still more preferably 80% by mass or less, and even more preferably 70% by mass or less.

Additives such as an internal cross-linking agent, a surfactant, a density adjusting agent, a thickening agent and a chelating agent may be added to the aqueous monomer solution as long as the object of the present invention is not hindered. The type and amount of additive to be added can be appropriately selected depending on the combination of the monomer and hydrophobic organic solvent used.

[3-2: Dispersion step]
The dispersing step is a step of dispersing or suspending an aqueous monomer solution (droplets containing a monomer) in a hydrophobic organic solvent. In addition, hereinafter, when simply described as "dispersion", the concept includes suspension. In the dispersing step, more specifically, the aqueous monomer solution is added to a hydrophobic organic solvent, mixed and stirred to disperse. For example, a stirrer equipped with stirring blades (propeller blades, paddle blades, anchor blades, turbine blades, Faudler blades, ribbon blades, flat plate blades, etc.) may be used. When a stirring device having such stirring blades is used, the diameter of the dispersed droplets can be adjusted by adjusting the type, blade diameter, and number of rotations of the stirring blades, and is particularly suitable for batch-type reversed-phase suspension polymerization. can be used for Moreover, a dispersion can be obtained by the methods described in International Publication No. 2009/025235, International Publication No. 2013/018571, and the like. In the case of continuous reversed-phase suspension polymerization, the dispersing step includes continuously supplying a monomer aqueous solution and a hydrophobic organic solvent separately to a dispersing device, and dispersing the monomer dispersed in the hydrophobic organic solvent. It is preferable to make droplets comprising

Dispersing devices used in the dispersion process when conducting continuous reversed-phase suspension polymerization include spray nozzles and high-speed rotary shear type stirrers (rotary mixer type, turbo mixer type, disk type, double cylinder type, etc.), Cylindrical nozzles such as needles, orifice plates in which a large number of holes are directly provided in a plate, spray nozzles, centrifugal atomizers such as rotating wheels, etc., may be mentioned, but are not particularly limited.

(hydrophobic organic solvent)
Preferred hydrophobic organic solvents include at least one organic solvent selected from the group consisting of aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons. Specific examples include aliphatic hydrocarbons such as n-pentane, n-hexane, n-heptane and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane and decalin; benzene, toluene, xylene and the like. aromatic hydrocarbons; halogenated hydrocarbons such as chlorobenzene, bromobenzene, carbon tetrachloride, and 1,2-dichloroethane. Among these, n-hexane, n-heptane, and cyclohexane are preferred from the viewpoint of availability and quality stability. As the hydrophobic organic solvent, only one of these may be used alone, or two or more of them may be used as a mixed solvent.

In one embodiment of the present invention, if necessary, a dispersing aid such as a surfactant or polymer additive may be added to the hydrophobic organic solvent as long as the object of the present invention is not hindered. The type of dispersing aid is appropriately selected depending on the combination of the hydrophobic organic solvent and the monomers used, and usable dispersing aids include the following surfactants and polymer additives.

Specific examples of the surfactants include sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, Polyoxyethylene Alkyl Ether, Polyoxyethylene Alkylphenyl Ether, Polyoxyethylene Castor Oil, Polyoxyethylene Hydrogenated Castor Oil, Alkyl Allyl Formaldehyde Condensed Polyoxyethylene Ether, Polyoxyethylene Polyoxypropylene Block Copolymer, Polyoxyethylene Polyoxypropyl Alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkyl allyl ether phosphates etc. Among these, only one type may be used alone, or two or more types may be used in combination. As the surfactant, a polymerizable surfactant having polymerizability can also be used. Specific examples of polymerizable surfactants include compounds having the following structures.

In the formula, R ¹ and R ² are each independently hydrogen, methyl or ethyl, and n is an integer of 3-20.

Among the above surfactants, sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyethylene glycol Fatty acid esters such as fatty acid esters are preferred, and sucrose fatty acid esters having an HLB value in the range of preferably 1 to 20, more preferably 1 to 10, and even more preferably 3 to 6 are preferred.

Specific examples of the polymer additive include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene/propylene copolymer, and maleic anhydride-modified ethylene/propylene/diene terpolymer. (EPDM), maleic anhydride-modified polybutadiene, maleic anhydride/ethylene copolymer, maleic anhydride/propylene copolymer, maleic anhydride/ethylene/propylene copolymer, maleic anhydride/butadiene copolymer, polyethylene, Examples include polypropylene, ethylene/propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene/propylene copolymer, ethylene/acrylic acid copolymer, ethyl cellulose, and hydroxyethyl cellulose. Among them, 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 preferred. Among these, only one type may be used alone, or two or more types may be used in combination.

In one embodiment of the present invention, the polymer additive is preferably used as the dispersing aid, and maleic anhydride-modified ethylene/propylene copolymer is more preferably used. Moreover, the polymer additive and the surfactant may be used in combination, or the polymer additive may be used alone without using the surfactant.

The amount of the dispersing aid to be used is appropriately set according to the type of polymerization, the types of the aqueous monomer solution and the hydrophobic organic solvent, and the like. Specifically, the concentration of the dispersing aid in the hydrophobic organic solvent is preferably 0.0001 to 2% by mass, more preferably 0.0005 to 1% by mass.

[3-3. Polymerization process]
The polymerization step is a step of polymerizing droplets containing the monomer obtained in the dispersion step to obtain a hydrous gel polymer (hereinafter also simply referred to as hydrous gel).

(Reactor)
The reaction apparatus used in the polymerization step may be the dispersing apparatus used in the dispersing step as it is, or may be another apparatus. In the case of batch-type reversed-phase suspension polymerization, the apparatus used in the dispersing step can be used as it is as a reaction apparatus, which is preferable in terms of workability. When the reactor is a separate device from the dispersing device, the monomer dispersion liquid obtained in the dispersing step is supplied to the reactor.

Moreover, the shape of the reactor in which the polymerization reaction is carried out is not particularly limited, and a known reactor can be used. As described above, a stirring device that can be suitably used in the dispersion step can also be suitably used in the polymerization reaction. In the case of a continuous production method, preferably, the monomer (aqueous solution) is polymerized while moving as a droplet-like dispersed phase in a hydrophobic organic solvent, which is a continuous phase formed in the reactor. It is a shape that can be seen. Such a reactor includes, for example, a reactor in which tubular reaction tubes are arranged vertically, horizontally or spirally. In this embodiment, since the monomer (aqueous solution) is supplied to the hydrophobic organic solvent moving in the reaction section, droplets of the aqueous monomer solution do not stay and move together with the hydrophobic organic solvent. Thereby, contact between monomer reactants having different polymerization rates is suppressed.

In addition, the reactor may be equipped with temperature control means so that the continuous phase inside the reactor can be heated or cooled from the outside, if necessary.

(polymerization temperature)
The polymerization temperature, which is the reaction temperature in the polymerization step, may be appropriately set according to the type and amount of the polymerization initiator used, but is preferably 20°C to 100°C, more preferably 40°C to 90°C. If the polymerization temperature is 100° C. or lower, rapid polymerization reaction can be suppressed. In this specification, the polymerization temperature means the temperature of a hydrophobic organic solvent (hereinafter referred to as "Td") as a dispersion medium.

In the polymerization step, since the monomer (aqueous solution) is dispersed in the form of droplets in the hydrophobic organic solvent, the temperature of the aqueous monomer solution rapidly rises due to heat transfer from the hydrophobic organic solvent. . When the polymerization initiator contained in the droplets is a thermally decomposable polymerization initiator, the thermally decomposable polymerization initiator decomposes to generate radicals as the temperature rises. A polymerization reaction is initiated by the generated radicals, and a water-containing gel polymer is formed as the polymerization reaction progresses.

In the case of the continuous production method, the formed hydrogel moves inside the reactor with the moving continuous phase (hydrophobic organic solvent) and is discharged from the reactor together with the hydrophobic organic solvent forming the continuous phase.

When the aqueous monomer solution contains a thermal decomposition type polymerization initiator, the Td is preferably 70° C. or higher, more preferably 75° C. or higher, and still more preferably 80° C. or higher, from the viewpoint of the polymerization rate. be. Although the upper limit of Td is not particularly limited, it is appropriately selected from the viewpoint of safety within a range not exceeding the boiling point of the hydrophobic organic solvent forming the continuous phase.

(Multistage reversed-phase suspension polymerization)
In the method for producing an absorbent resin according to one embodiment of the present invention, multi-stage polymerization may be performed from the viewpoint of obtaining an appropriate aggregate particle size. Specifically, multi-stage polymerization can be carried out by, for example, adding the aqueous monomer solution again to carry out the polymerization reaction after the completion of the first-stage polymerization step.

(Inorganic fine particles)
In the method for producing an absorbent resin according to one embodiment of the present invention, inorganic fine particles may be added to the water-containing gel polymer during or after the polymerization from the viewpoint of obtaining an appropriate aggregate particle size. . The inorganic fine particles can also be said to be a powdery inorganic flocculant.

Examples of inorganic fine particles that can be used in the present invention include silicon dioxide, aluminum oxide, titanium dioxide, calcium phosphate, calcium carbonate, magnesium phosphate, calcium sulfate, diatomaceous earth, bentonite, zeolite, and other metal oxides. Especially preferred are silicon dioxide, aluminum oxide and titanium dioxide. Good results can be obtained when the inorganic fine particles are added in an amount of, for example, 0.001 to 1 part by mass, preferably 0.001 to 0.5 part by mass, per 100 parts by mass of the hydrous gel polymer. When the amount of the inorganic fine particles to be added is within the above range, the effect of adding the inorganic fine particles is efficiently expressed, and the influence on the water absorption performance is small, which is preferable.

[3-4. Separation process]
The separation step is a step of separating the hydrous gel polymer obtained in the polymerization step from the hydrophobic organic solvent. The type and structure of the device used in the separation step are not particularly limited, but known devices such as filtration, sedimentation, centrifugation, and compression can be used. Alternatively, the mixture may be separated from the hydrophobic organic solvent by heating under normal pressure or reduced pressure using a stirrer having stirring blades used in the polymerization step, followed by distillation. Distillation under normal pressure or reduced pressure is preferably carried out in the batch-type reversed-phase suspension polymerization.

[3-5. Gel sizing step]
A method for producing an absorbent resin according to an embodiment of the present invention may include a gel granulation step. In the gel sizing step, the water-containing gel polymer separated from the hydrophobic organic solvent in the separation step is sieved using a gel sizing device having an extrusion action part and a perforated plate. As a result, a granulated hydrogel polymer is obtained (hereinafter, the granulated hydrogel polymer is referred to as a granulated gel). Having the gel sizing step makes it easier to control the shape retention of the swollen gel of the water absorbent resin.

In one embodiment of the present invention, the hydrous gel polymer to be subjected to the gel sizing step is in the form of single spherical gel particles (primary particles) or aggregates of spherical gels (secondary particles in which primary particles are aggregated). be. Although the lower limit of the average particle size of the hydrous gel polymer is not particularly limited, it is preferably 0.01 mm or more, more preferably 0.03 mm or more, still more preferably 0.05 mm or more, and still more preferably 0.1 mm or more. Although the upper limit is not particularly limited, it is preferably 20 mm or less, more preferably 10 mm or less. In addition, in the case of a single particle shape, the particle size is referred to as a primary particle size, and in the case of an aggregate shape, the particle size of each spherical gel (primary particle) constituting the aggregate is referred to as a primary particle size. In the present invention, the average primary particle size of the primary particles constituting the hydrous gel polymer is not particularly limited, but from the viewpoint of suppressing the generation of fine powder in the drying process, it is preferably 1 to 2000 μm, more preferably 1 to 2000 μm. It is 1000 μm, more preferably 5 to 800 μm, still more preferably 8 to 500 μm, still more preferably 10 to 300 μm, particularly preferably 10 to 200 μm.

In addition, a device having a cutter may be installed before the gel sizing device having the extrusion unit and the perforated plate to crush large aggregates contained in the hydrous gel polymer.

(Water-containing gel temperature)
Although the lower limit of the temperature of the hydrous gel polymer to be subjected to the gel sizing apparatus is not particularly limited, it is preferably 80°C or higher, more preferably 90°C or higher from the viewpoint of granulation efficiency and suppression of damage to the hydrous gel. Although the upper limit of the temperature of the hydrous gel polymer when charged into the gel sizing device is not particularly limited, it is, for example, 100° C. or less.

(Gel sizing device)
As used herein, the term “gel sizing” refers to the process of extruding a hydrous gel polymer (powder wet mass) into a columnar shape from the small holes of a perforated plate so that the wet powder raw material has a substantially uniform shape and size. It is an operation to prepare a granular gel (particle size gel) having That is, by using the perforated plate, the water-containing gel in the form of coarse aggregates excessively aggregated in the separation step is pulverized, and the single-particle water-containing gel having a small particle size is appropriately aggregated. Therefore, by this step, it is possible to obtain a granulated hydrogel (regular granule gel) having a relatively uniform particle size. The granulated gel may contain monoparticulate hydrous gel.

The "gel sizing apparatus having an extrusion action part and a perforated plate" used in the gel sizing step includes an extrusion action part and a perforated plate (die or screen), and the extrusion action part is usually a perforated plate. It is not particularly limited as long as it has an extrusion member that extrudes the contents toward and can produce grains of a certain size by extruding the material from the perforated plate, for example, an extruder having a perforated plate . Alternatively, these devices may be used in series.

Furthermore, the shape of the holes of this perforated plate (die or screen) is not particularly limited, and may be arbitrarily selected from shapes suitable for use, such as perfect circles, ellipses, polygons such as hexagons, and triangles. It is possible, but from the viewpoint of sizing strength, a perfect circle or an ellipse is preferable. Although the pore diameter is not particularly limited, it is preferably 1.5 mm or less, more preferably 1.0 mm or less, and even more preferably 0.8 mm or less. When the pore diameter of the perforated plate is 1.5 mm or less, the size of the obtained granulated gel is prevented from increasing more than necessary, and the amount of fine powder generated during drying using an agitating dryer in the downstream process. can be reduced. Also, the hole diameter of the perforated plate is preferably 0.3 to 1.5 mm, more preferably 0.3 to 0.8 mm. If the hole diameter of the perforated plate is 0.3 mm or more, the extrusion can be performed efficiently.

The pore size of the perforated plate is defined as follows. First, when the hole is not a perfect circle, the geometric mean value of the short diameter and long diameter of the hole is adopted as the hole diameter. Further, when the pore diameters of the pores of the perforated plate are different, the pore diameters of all the pores are calculated, and the arithmetic mean value thereof is adopted as the pore diameter of the perforated plate. Furthermore, when the pore diameter of the perforated plate changes from the extrusion acting portion side of the perforated plate to the opposite side (the pore diameter changes in the thickness direction of the perforated plate), the value that gives the smallest pore diameter is adopted. .

In the gel sizing step, an additive may be added to the water-containing gel polymer subjected to the gel sizing step. Additives that can be added in this step include polymerization initiators, oxidizing agents, reducing agents, chelating agents, thickening agents, surfactants, cross-linking agents, acids, bases, foaming agents, organic or inorganic fine particles, polyvalent metals. Among them, additives that can control the degree of aggregation include, for example, starch, cellulose, starch-cellulose derivatives, thickeners such as polyvinyl alcohol, surfactants, fine powder of water-absorbent resin, cross-linking agents, Valence metal salts and the like are preferred.

[3-6. Drying process]
A drying process is a process of drying a water-containing gel. In the present invention, the method of drying in the drying step is not particularly limited, and conventionally known methods (e.g., azeotropic dehydration in a hydrophobic dispersion solvent) may be used. From the viewpoint of phase suspension polymerization, agitation drying or stationary drying can be employed as a more suitable technique. Among them, drying with stirring is preferable in order to maintain the particle size of the gel controlled in the gel sizing step.

A dried polymer composed of particles obtained in the main drying step can be directly used as a water absorbent resin for various uses. In addition, in the method for producing an absorbent resin according to one embodiment of the present invention, it is possible to subject the dried polymer obtained in the drying step to the surface cross-linking step described below. In this case, the dry polymer to be subjected to the surface cross-linking step, which will be described later, is also referred to as "water absorbent resin powder" for convenience.

(Additive)
Additives may be added to the hydrous gel to be subjected to the drying step as long as the effects of the present invention are not inhibited. The addition may be performed during stirring (rotation) by the rotating container and/or heating by the heating means, or may be added before the drying step (before stirring (rotation) by the rotating container and heating by the heating means). . Furthermore, it may be added in any step before the drying step. The additive can reduce excessive adhesion of the hydrous gel to each other during drying, and a water absorbent resin having an excellent water absorption rate can be obtained.

Examples of additives added to the hydrous gel include drying aids.

Specifically, from the viewpoint of industrial efficiency, it is particularly preferable to add a drying aid when handling a particulate hydrous gel with a diameter of 1 mm or less. In particular, by adding a drying aid before the drying step of the present invention, a water absorbent resin having an excellent water absorption rate can be obtained. That is, one preferred embodiment of the present invention has a step of adding a drying aid to the hydrous gel polymer.

(drying aid)
The drying aid is added for the purpose of maintaining fluidity during stirring drying in the drying process. Specific examples of drying aids include surfactants and polymer lubricants. A polymer lubricant and a surfactant may be used in combination as a drying aid. In the method for producing an absorbent resin according to one embodiment of the present invention, adhesion between hydrous gels is reduced and the hydrous gels are uniformly dried, the particle strength of the hydrous gel is not uneven, and excellent shape retention is obtained. From this point of view, it is preferable to use a drying aid.

The amount of the drying aid to be added is appropriately set according to the water content of the drying aid and the type and amount of each component contained in the drying aid (for example, gel fluidizing agent, etc.). The total amount of drying aid added in the drying step is preferably 0.001% by mass to 0.5% by mass, more preferably 0.01% by mass to 0.3% by mass, based on the solid content of the hydrous gel. , more preferably 0.02% by mass to 0.2% by mass.

In addition, when the obtained hydrogel is dried using a rotary dryer, the hydrogel is less likely to fuse and the particle size can be easily adjusted by crushing, etc., so the amount of drying aid used is reduced. be able to. In one preferred embodiment of the present invention, in the drying step, the hydrous gel polymer contains less than 0.08% by mass of a drying aid relative to the solid content of the hydrous gel polymer.

The addition of the drying aid to the hydrous gel polymer is preferably performed in a step prior to the drying step. (iii) addition to the aqueous monomer solution in the step of preparing the aqueous monomer solution; (iv) addition to the hydrophobic organic solvent in the dispersion step; Among them, addition in a step immediately before the drying step is preferable, and more specifically, addition after a step preceding the drying step (for example, gel sizing step) and before the drying step is more preferable. Furthermore, after adding the drying aid in the pre-drying step (e.g., gel sizing step), further after the pre-drying step (e.g., gel sizing step), and before the drying step. It is also a preferred form to perform addition (addition of the drying aid twice in total). In addition, the form of addition of the drying aid before the drying step is, for example, a form in which the hydrous gel polymer and the drying aid are put into the dryer; the drying aid is added to the hydrous gel polymer before the dryer. morphology and the like. Moreover, the drying aid may overlap with the surfactant and polymer additive used as the dispersing aid in the dispersing step.

Specific examples of surfactants used in drying aids include (1) sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, and sorbitol fatty acid esters. , polyoxyethylene sorbitol fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkyl allyl formaldehyde condensed polyoxyethylene ether, polyoxyethylene polyoxypropylene block Copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and polyoxy nonionic surfactants such as phosphate esters of ethylene alkyl allyl ether; (2) betaine alkyldialkylaminoacetates such as betaine capryldimethylaminoacetate, betaine lauryldimethylaminoacetate, betaine myristyldimethylaminoacetate, betaine stearyldimethylaminoacetate; Alkylamidopropyl betaine such as lauramidopropyl betaine, coconut oil fatty acid amidopropyl betaine, palm kernel oil fatty acid amidopropyl betaine, alkyl hydroxysulfobetaine such as lauryl hydroxysulfobetaine, 2-alkyl-N-carboxymethyl-N-hydroxy (3) Alkylaminodiacetic acids such as monosodium laurylaminodiacetate, potassium laurylaminodiacetate, sodium myristylaminodiacetate, etc. Examples include anionic surfactants such as monoalkali metals, and cationic surfactants such as (4) long-chain alkyldimethylaminoethyl quaternary salts. Among these, only one type may be used alone, or two or more types may be used in combination.

Specific examples of polymer lubricants used as drying aids include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene/propylene copolymer, and maleic anhydride-modified ethylene/propylene/diene terpolymer. Original copolymer (EPDM), maleic anhydride-modified polybutadiene, maleic anhydride/ethylene copolymer, maleic anhydride/propylene copolymer, maleic anhydride/ethylene/propylene copolymer, maleic anhydride/butadiene copolymer Polyalkylenes such as coalescence, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, hydroxyethyl cellulose, polyethylene glycol oxide and the like. These molecular weights (weight average molecular weights) are appropriately selected in the range of preferably 2 million to 2 million, more preferably 4 million to 1 million. Among these, only one type may be used alone, or two or more types may be used in combination.

(Particle size distribution of dry polymer)
As the particle size distribution of the dried polymer obtained in the drying step, the ratio of particles having a particle diameter of 850 μm or more (the ratio of particles that did not pass through a sieve with an opening of 850 μm) is preferably 50% by mass or less, more preferably 45% by mass. % or less, more preferably 40 mass % or less. According to the method of the present embodiment, the formation of coarse particles is significantly suppressed by combining with drying using an agitating dryer, so the preferred range can be achieved. In addition, the proportion of the dry polymer having a particle diameter of 1,400 μm or more (the proportion of particles that did not pass through a sieve with an opening of 1,400 μm) is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less. is.

[3-7. Surface cross-linking step]
The water-absorbing resin powder obtained through the drying step (and any subsequent steps) is preferably surface-crosslinked with a surface-crosslinking agent. This surface cross-linking is a process of providing a high cross-linking density portion in the surface layer of the water-absorbing resin powder (a portion several tens of micrometers from the surface of the water-absorbing resin powder). Various water absorbing properties of the water absorbent resin can be improved by performing the surface cross-linking treatment. In particular, by appropriately adjusting the cross-linking density of the water absorbent resin (powder), it is possible to obtain a water absorbent resin having excellent absorption capacity under pressure. In the present invention, conventionally known surface cross-linking techniques such as surface cross-linking in a state of being dispersed in a hydrophobic organic solvent or surface cross-linking to powder in a dry state can be applied as appropriate. From the viewpoint of improving the absorption capacity under pressure of the absorbent resin, it is preferable to subject the water absorbent resin that has undergone the gel separation step and the drying step to a surface cross-linking step. In the method for producing an absorbent resin according to one embodiment of the present invention, the surface cross-linking agent used in the surface cross-linking step is distinguished from the internal cross-linking agent used in the aqueous monomer solution preparation step. It is also indicated as a "post-crosslinker".

In the method for producing an absorbent resin according to one embodiment of the present invention, the surface cross-linking step may be performed after the drying step or during the drying step. In a known surface cross-linking step, a surface cross-linking agent is generally mixed with a cross-linked polymer of a hydrous gel or a dried product thereof, and the mixture is heated to carry out a cross-linking reaction. may be provided separately after the drying step, or a surface cross-linking agent may be added in the drying step to carry out the surface cross-linking reaction and drying at the same time. Further, when the water-absorbent resin is produced by a batch-type reversed-phase suspension polymerization method, the solvent and the hydrous gel polymer can be separated by distillation in the separation step after the polymerization reaction. By adding a surface cross-linking agent even during the separation step, surface-crosslinked water absorbent resin particles can be obtained.

[3-8. Other processes]
In addition to the above-described steps, the method for producing a water-absorbent resin according to an embodiment of the present invention includes, if necessary, a cooling step, a pulverizing step, a water-containing (re-moistening) step, a step of adding other additives, and a classification step. , a sizing step, and a fines recycling step. In addition, it may further include a transportation process, a storage process, a packing process, a storage process, and the like.

(Cooling process)
In the method for producing an absorbent resin according to one embodiment of the present invention, in the optional cooling step, the particulate dried polymer obtained in the drying step is cooled using a known cooling means. , a particulate dry polymer cooled to the desired temperature can be obtained.

(Pulverization process)
The method for producing an absorbent resin according to one embodiment of the present invention preferably includes a pulverization step of pulverizing the particulate dry polymer obtained in the drying step (and any subsequent cooling step). Through the pulverization step, it is possible to obtain a water absorbent resin powder with a controlled particle size or particle size distribution.

In the pulverization step, for example, a roll mill, a hammer mill, a screw mill, a high-speed rotary pulverizer such as a pin mill, a vibration mill, a knuckle-type pulverizer, a cylindrical mixer, or the like is appropriately selected and used as a pulverization means.

(Rewetting process)
In the method for producing an absorbent resin according to one embodiment of the present invention, the optional rewetting step includes adding a polyvalent metal salt, a cationic polymer, and a chelate to the water absorbent resin particles obtained in the surface cross-linking step. addition of at least one additive selected from the group consisting of an agent, an inorganic reducing agent, and an α-hydroxycarboxylic acid compound.

In the rewetting step, the above additives are preferably added to the water absorbent resin in the form of an aqueous solution or dispersion (slurry). The additive may be added and mixed at the same time as the surface cross-linking agent solution described above.

As the rewetting step, specifically, the method described in International Patent Publication No. 2015/053372 “(2-7) Rewetting step” is also applied to the present invention.

(Other additive addition process)
In the method for producing an absorbent resin according to one embodiment of the present invention, additives other than the additives described above (other additives) can be added to add various functions to the water absorbent resin. Specific examples of such additives include surfactants, compounds having phosphorus atoms, oxidizing agents, organic reducing agents, water-insoluble inorganic fine particles, organic powders such as metal soaps, deodorants, antibacterial agents, pulp and thermoplastics. A fiber etc. are mentioned. As the water-insoluble inorganic fine particles, compounds disclosed in "[5] Water-insoluble inorganic fine particles" of International Patent Publication No. 2011/040530 are applied to the present invention. By adding these water-insoluble inorganic fine particles, the frictional force between the water-absorbent resin particles is improved, the movement of the gel in the water-absorbent core is suppressed, and the absorbent core becomes difficult to deform, which is preferable, especially silica (silicon dioxide). is preferably added. Here, the water-insoluble inorganic fine particles may also serve as the inorganic fine particles (powder inorganic coagulant) added in the polymerization step or the like, or may be added separately.

(Sizing process)
The “granule sizing step” means a step of loosening loosely agglomerated water absorbent resin through the surface cross-linking step to adjust the particle size. Note that this granule regulating step includes a fine powder removing step and a classifying step after the surface cross-linking step. The method for producing an absorbent resin according to one embodiment of the present invention preferably includes a particle size regulating step from the viewpoint of regulating the particle size of the water absorbent resin and obtaining stable water absorbing properties. A specific embodiment of the sizing step is not particularly limited, but includes, for example, a method of passing the water absorbent resin through a sieve (eg, JIS standard sieve).

(Fine powder reuse process)
The “fine powder recycling step” means a step of supplying the fine powder generated by sieving and the like in each of the above steps as it is or after granulating the fine powder to any of the steps. From the viewpoint of reducing the production loss of the water absorbent resin, the method for producing an absorbent resin according to one embodiment of the present invention preferably includes a fine powder recycling step.

[4. Uses of water absorbent resin]
In one embodiment of the present invention, an absorbent body comprising the water absorbent resin of the present invention is provided.

The use of the absorbent comprising the water-absorbent resin of the present invention is not particularly limited, but preferably includes absorbent articles such as paper diapers (for infants and adults), sanitary napkins, and incontinence pads. . In particular, when a load is applied locally to the gel layer, the gel at that location shifts and becomes unbalanced, resulting in a decrease in the efficiency of liquid absorption over multiple times. can be used as Examples of other absorbent articles include soil water retention agents, sheets for raising seedlings, seed coating materials, dew condensation prevention sheets, drip absorption materials, freshness retention materials, disposable body warmers, cooling bandanas, refrigerants, and solidification of medical waste liquids. waste soil solidification agent, water damage prevention waste liquid gelling agent, water-absorbing sandbag, emergency toilet, poultice material, thickener for cosmetics, water stop material for electrical and electronic communication cables, gasket packing, slow-release agent for fertilizer , various sustained-release agents (space disinfectants, air fresheners, etc.), pet sheets, cat litter, wound protection dressings, dew condensation prevention construction materials, oil-based moisture removers, paints, adhesives, anti-blocking agents, light Diffusing agents, matting agents, additives for decorative boards, additives for artificial marble, additives for resins such as additives for toner, and the like.

As a raw material for the absorbent body, an absorbent material such as pulp fiber can be used together with the water absorbent resin. Such absorbent materials are not limited to pulp fibers and the like, and various hydrophilic fibers (for example, pulp fibers, cotton linter crosslinked cellulose fibers, rayon, cotton, wool, acetate, vinylon, etc.) may be used. can be done.

When the absorbent body contains an absorbent material, the content (core concentration) of the water absorbent resin with respect to the total amount of the absorbent material and the water absorbent resin in the absorbent body is preferably 30% by mass to 100% by mass. %, more preferably 40% to 100% by weight, more preferably 50% to 100% by weight, even more preferably 60% to 100% by weight, particularly preferably 70% to 100% by weight, most preferably 75% by mass to 95% by mass.

By setting the core concentration of the absorbent within the above range, when the absorbent is used in the upper layer portion of the absorbent article, the absorbent article can be kept white with a clean feeling. Furthermore, since the absorber has excellent diffusibility for body fluids such as urine and blood, it is expected that the absorption amount will be improved by efficient liquid distribution.

The water absorbent resin of the present invention has an excellent gel transfer index, that is, even when a load is applied, the liquid return amount after the second time is reduced, so particularly thin absorbent articles (e.g., core concentration of 30% by mass or more).

The present invention will be described more specifically according to the following examples and comparative examples, but the present invention is not limited to these examples, and examples obtained by appropriately combining the technical means disclosed in each example. are also intended to be included in the scope of the present invention. In the examples, "parts" or "%" may be used, but "parts by mass" or "% by mass" are indicated unless otherwise specified. Moreover, unless otherwise specified, each operation is performed at room temperature (25° C.). The electrical equipment used in Examples and Comparative Examples (including physical property measurement of the water-absorbent resin) used a power source of 200 V or 100 V and 60 Hz unless otherwise specified.

(Number average particle size)
A scanning electron micrograph (SEM) of the water absorbent resin or water absorbent resin powder was taken. 50 primary particles in front of the aggregated particles were randomly selected from the photograph, and the major and minor diameters of each particle were measured and averaged to determine the primary particle size. The average value of the primary particle size of each particle was calculated, and the average value was taken as the number average particle size of the water absorbent resin.

(moisture content)
The water content of the water absorbent resin was measured according to the EDANA method (ERT430.2-02). In the measurement, the mass of the sample was changed to 1.0 g, the drying temperature was changed to 180° C., and the drying time was changed to 3 hours. Specifically, after putting 1.0 g of hydrous gel or water-absorbent resin into an aluminum cup with a bottom diameter of 50 mm, the sample (hydrous gel or water-absorbent resin) and the total mass W1 (g) of the aluminum cup are accurately measured. weighed. Next, the sample was placed in an aluminum cup and placed in an oven set at an atmospheric temperature of 180°C. After 3 hours, the sample was taken out from the oven together with the aluminum cup, and the total weight W2 (g) of the dried sample and the aluminum cup was accurately weighed. The water content α (% by mass) of the sample was obtained according to the following formula (1), where M (1.0 g) was the mass of the sample used for the main measurement.

Water content α (mass%) = {(W1-W2) / M} × 100 formula (1)
(Mass average particle size (D50))
The mass average particle diameter (D50) of the water-absorbing resin is described in US Pat. Measured according to the method described in .

(CRC)
The CRC (centrifuge retention capacity) of the water absorbent resin was measured according to the EDANA method (ERT441.2-02). Specifically, after putting 0.2 g of the water-absorbing resin in a non-woven fabric bag, it was immersed in a large excess of 0.9 mass% sodium chloride aqueous solution for 30 minutes to allow free swelling, and then centrifuged (250 G ) for 3 minutes to determine the water absorption capacity (g/g) after draining.

(AAP)
The AAP (absorption capacity under pressure) of the water absorbent resin was measured according to the EDANA method (ERT442.2-02). Note that the load condition was changed to 4.83 kPa (0.7 psi).

(ext)
The Ext (water-soluble content) of the water absorbent resin was measured according to the EDANA method (ERT470.2-02).

(The bulk density)
The bulk density of the water absorbent resin was measured according to the EDANA method (ERT460.2-02).

(surface tension)
The surface tension of the water absorbent resin was measured by the method described in WO2015/129917.

(Water absorption rate (Vortex method))
The water absorption rate of the water absorbent resin was measured according to Japanese Industrial Standard JIS K 7224 (1996).

(Gel shape retention force)
The gel shape retention force of the water absorbent resin was measured by the following method.

(1) Preparation of swollen gel A beaker (for example, mutual 2.00 g of a water-absorbing resin was put into a beaker conforming to JISR-3503 sold by Rikagaku Glass Co., Ltd. under stirring at 400 rpm.

A cylindrical Teflon (registered trademark) magnetic stirrer (length: 25 mm, diameter: 8 mm, no ring) was used for stirring. After it was confirmed that the charged water-absorbing resin swelled and the vortex on the surface of the liquid converged, stirring was stopped. Then, it was allowed to stand for 10 minutes to obtain a 20-fold swollen gel.

(2) Measurement of shape retention of swollen gel (measurement of gel shape retention A)
Autograph AG-X (manufactured by Shimadzu Corporation, trade name: Autograph AG-X) was used to measure the gel shape retention force A. The load cell observes the response of the load applied to the compression jig when the compression jig is brought into contact with the gel surface under specific conditions for the 20-fold swollen gel contained in the beaker described above. Then, the value of the load displayed on the measurement screen as the test force was defined as the gel shape retention force A (N). A more specific measuring method was as follows.

Autograph AG-X equipped with a load cell (SLBL-50N / manufactured by Shimadzu Corporation), a disc portion 1 shown in FIG. 1, and a rod portion 2 with one end connected to the center of the disc portion A jig 100 (the disk portion 1 has a flat surface on the front and back and is disk-shaped with a diameter of 2.5 cm and a thickness of 1 cm; the rod-shaped portion 2 has a length of 5.5 cm and a cross section of 0.52 cm circle) was mounted so that the flat surface of the disk portion 1 was the lower surface. Next, the position of the mounted compression jig 100 in the vertical direction was adjusted. Specifically, on the operation software (Shimadzu Autograph software Trapezium X (manufactured by Shimadzu Corporation)), click "Create new test conditions" and set the test conditions as shown below.

(Test conditions)
"system"
Test mode: Single Test type: Compression Load cell polarity: Compression Direction of movement: Down Unit: SI
Format: rounded "sensor"
Test force Channel: Test force amplifier Name: Test force Full scale: 50N
Limit: 50N (check)
Stress name: Stress Use true stress: Uncheck stroke Name: Stroke Limit: 500 mm (check)
Stroke (strain) name: Stroke (strain)
Use true strain: uncheck Displacement compensation: uncheck Displacement gauge 1
Channel: None Width meter Channel: None Other Other 1
Channel: None "Test Control"
Origin of elongation: Area 1 from the beginning
Control action: Load Control: Stroke V1: 10 cm/min
Setting between area 1 and area 2 Target value channel Stroke 10mm
Area 2
Control action: OFF
End condition Breakage setting: Uncheck all Action after test: Stop Breakage detection start point: 0.035%
Sampling time: 10msec
Preload Uncheck all "specimen"
Material: Plastic Shape: Flat plate Number of batches: 1
Number of sub batches: 1
Dimensions unit: mm
"Data Processing Items"
"Graphs" that do not need to be set
"Report" that can be set arbitrarily
Can be set arbitrarily (Observation of test force)
Observation of the test force was performed in an environment of temperature 25±2° C. and humidity 50±10%.

The compression jig 100 was placed on the measurement table so that the compression jig 100 was placed in the center of the beaker containing the 20-fold swollen gel.

Next, the compression jig 100 is lowered until the load cell connected to the compression jig 100 senses a test force of 0.01N, and the lower surface of the disk portion 1 of the compression jig 100 and the surface of the swollen gel are was brought into contact with Next, by raising the compression jig 100 by 0.05 mm, the compression jig 100 was placed at the measurement start position (a position raised vertically by 0.05 mm from the surface of the swollen gel).

Subsequently, the compression jig 100 was pushed vertically into the swollen gel by 10.0 mm at a speed of 10 cm/min.

When the compression jig 100 is pushed into a position 8.0 mm in the vertical direction from the measurement start position, the observed test force (load applied to the compression jig 100) is the gel shape retention force A (N ).

(Measurement of gel shape retention force B)
Except for changing the speed of pushing the compression jig 100 to 1 mm/min, the same operation as described above (measurement of gel shape retention force A) was performed, and the compression jig 100 was moved from the measurement start position to the 20-fold swollen gel. The test force (the load applied to the compression jig 100) observed when the gel was pushed to a position of 8.0 mm in the vertical direction was defined as the gel shape retention force B (N). The changes in the test conditions on the operation software were as follows.

(change point)
"test control"
Origin of elongation: Area 1 from the beginning
Control action: Load Control: Stroke V1: 1mm/min
Sampling time: 1sec
(gel transfer index)
A gel transfer index was calculated from the following formula (1).
Gel transfer index = (gel shape retention force A/gel shape retention force B) -1 (1)
(Measurement of liquid return amount of absorbent sheet)
(1) Fabrication of Absorbent Sheet The liquid return amount of the water absorbent resin was measured using the evaluation absorbent sheet 200 (hereinafter sometimes referred to as absorbent sheet 200) shown in FIGS. A specific procedure for producing the absorbent sheet 200 is as follows.

First, on the adhesive surface of the adhesive tape 20 (vinyl tape No. 21S manufactured by NITTO DENKO) cut to 10 cm × 10 cm, 2.75 cm from the ends of the four sides of the adhesive tape 20, the inner area (water-absorbent resin spray area 21 ) is sprinkled with 0.50 g of a water-absorbent resin 23, and a 10 cm × 10 cm nonwoven fabric 22 (air-laid nonwoven fabric, basis weight 46.6 g/m ² ) is placed on top of it, and attached to the adhesive surfaces of the four sides of the adhesive tape 20. In addition, an evaluation absorbent sheet 200 having a laminated structure (structure in which the adhesive tape 20, the water absorbent resin 23, and the nonwoven fabric 22 are laminated in this order) shown in FIG. 3 was produced.

(2) Measurement of Liquid Return Amount The prepared absorbent sheet 200 is placed on a flat table with the nonwoven fabric surface 22 facing upward, and a syringe (syringe needle gauge: 21 G) is used to insert the tip of the absorbent sheet 200 into the absorbent sheet 200. 10 mL of a 0.9% by mass sodium chloride aqueous solution at 20° C. was added over 1 minute while contacting the center of the solution. Ten minutes after the start of addition of the 0.9% by mass sodium chloride aqueous solution, a weight of Φ25 mm and a weight of 500 g was placed on the central portion of the absorbent sheet 200, and one minute later, the weight was removed. Subsequently, using a syringe, 5 mL of a 0.9% by mass sodium chloride aqueous solution at 20° C. was added over 10 seconds while the tip of the syringe was brought into contact with the central portion of the absorbent sheet 200 . Immediately after the additional addition of the 0.9% by mass sodium chloride aqueous solution is completed, 36 sheets of kitchen paper (kitchen towel manufactured by Oji Nepia Co., Ltd.) having a total weight of K1 (g) cut into 3 cm × 3 cm are stacked to absorb the Place it on the sheet 200, then place a weight of Φ25 mm and a weight of 100 g on the top of the kitchen paper, remove the kitchen paper and the weight 30 seconds after placing the kitchen paper and the weight, and remove the kitchen paper. The total weight K2 (g) of 36 sheets was measured, and the amount of liquid returned was calculated as the amount of liquid absorbed by the kitchen paper based on the following formula:
Liquid return amount (g)=K2-K1.

[Example 1]
A hydrous gel polymer (1) discharged from the separator 16 was obtained according to the production process shown in FIG. 1 of WO2020/067310. Here, as the dispersing device 12, the double cylindrical high-speed rotary shearing stirrer (dispersing device 12G) shown in FIG. 8 of WO 2020/067310 and used in Example 1 was used. Specifically, it was as follows.

As a preparatory step for the polymerization reaction, 21,000 g of n-heptane, which is a hydrophobic organic solvent, was introduced into the dispersing device, the reactor 14, the separating device 16, and the pipes (including joints) connecting them.

Subsequently, the liquid-sending pump 18 was operated to start circulation of the hydrophobic organic solvent at a flow rate of 300 mL/min. Furthermore, the heat exchanger 20 is operated so that the set temperature (the temperature of the hydrophobic organic solvent present in the region where the following monomer aqueous solution (1) is introduced in the reaction apparatus) is 90 ° C. A hydrophobic organic solvent was heated. Furthermore, maleic anhydride-modified ethylene/propylene copolymer (trade name: Hiwax (registered trademark) 2203A/Mitsui Chemicals, Inc.) was added as a dispersing aid in an amount of 0.11% by mass with respect to 100% by mass of the hydrophobic organic solvent. % was added.

Subsequently, sodium acrylate, acrylic acid, and ion-exchanged water are mixed, and further, polyethylene glycol diacrylate (average degree of polymerization: 9) and diethylenetriamine pentaacetic acid/pentasodium as internal cross-linking agents are blended to form a monomer. A solution containing (acrylic acid (salt)) was prepared (monomer aqueous solution preparation step).

Subsequently, the monomer aqueous solution (1) was prepared by supplying the solution containing the monomer obtained by the above operation and sodium persulfate to the mixing device 10 and mixing them. The monomer concentration (monomer concentration) of the monomer aqueous solution (1) was 43% by mass, and the monomer neutralization rate was 73 mol%. In addition, the amount of polyethylene glycol diacrylate is 0.008 mol% relative to 100 mol% of the monomer (acrylic acid (salt)), and the amount of diethylenetriamine pentaacetic acid/pentasodium is the monomer (acrylic acid (salt)). The amount of sodium persulfate was 0.12 g/mole relative to the monomer (acrylic acid (salt)).

The rotor of the dispersing device 12G (double cylindrical high-speed rotary shear type stirrer) was rotated at a rotation speed of 3600 rpm (shear rate of 2765 [1/s]). Next, the monomer aqueous solution (1) was sent to the pipe of the dispersing device at a flow rate of 40 mL/min (47.2 g/min). The supplied monomer aqueous solution (1) was dispersed in the hydrophobic organic solvent in the form of droplets by a dispersing device to obtain a dispersion (dispersing step).

The dispersion liquid obtained as described above was then supplied to the reactor 14 . Droplets composed of the aqueous monomer solution (1) in the dispersion are polymerized while falling in the reactor filled with the hydrophobic organic solvent as the continuous phase, and near the outlet of the reactor, minute droplets A spherical hydrous gel polymer (1) was obtained (polymerization step). The water-containing gel polymer (1) obtained by the above series of operations is continuously supplied from the reaction device to the separation device 16 through the joint together with the hydrophobic organic solvent, and in the separation device 16, the water-containing gel The polymer (1) and the organic solvent were separated (separation step). In the separator 16, the hydrous gel polymer (1) was an aggregate of fine spherical particles. The average particle size of spherical particles (primary particles) constituting the aggregate was 5 to 10 mm, and the average primary particle size was 80 μm.

A gel sizing device having a screw and a perforated plate with a hole diameter of 0.8 mm was previously charged with a 3.5% by mass aqueous solution of betaine lauryldimethylaminoacetate as a drying aid (the solid content of the obtained hydrous gel polymer (1) was The water-containing gel polymer (1) (gel temperature: 90° C.) added and mixed with 0.20% by mass of the gel) was added and discharged from the gel sizing apparatus to obtain a sizing gel (1) (gel sizing process). After that, 700 ppm of betaine lauryldimethylaminoacetate was added to the sizing gel (1).

Subsequently, the sizing gel (1) was dried using a rotary dryer with a heating tube (drying step). The rotary dryer has 10 heating tubes extending in the direction of the rotation axis and 2 barriers (doughnut-shaped partition plate with one circular opening in the center, opening ratio 50%). Equipped with a cylindrical rotating container (100 L volume). The ten heating tubes are spaced apart on a circumference around the axis of rotation of the rotating vessel. Furthermore, at the end of the rotary container on the outlet side, there is a doughnut-shaped partition plate (also known as discharge weir) having one circular opening (opening ratio of 24%) in the center.

First, steam of 2.7 MPa (temperature 228.1 ° C.) is introduced into each heating tube, and after heating the inside of the rotating container (defined by a contact thermometer) to over 200 ° C., the outer wall of the rotating container is also steamed. It was sufficiently heated (temperature 200° C.) by tracing. Next, the rotating container is rotated at 30 rpm so that the Froude number is 0.15, and the sizing gel (1) heated to 90° C. is supplied, dried for a drying time of 50 minutes, and the dried polymer ( 1) was obtained. During drying, the supply amount and exhaust amount of nitrogen (carrier gas; 140°C) were adjusted so that the air pressure difference inside the rotating container with respect to the outside air was -20 Pa and the exhaust dew point was 85 to 90°C.

Subsequently, the obtained dry polymer (1) is supplied to a roll mill (pulverizer) and pulverized to adjust the particle size (pulverization step), and further classified using a sieve with an opening of 150 μm, and the water absorbent resin A powder (1) was obtained. The average particle size of the water absorbent resin powder (1) was 340 μm.

0.03 parts by weight of ethylene glycol diglycidyl ether, 0.385 parts by weight of ethylene carbonate, 0.644 parts by weight of propylene glycol and 2.6 parts by weight of ion-exchanged water are added to 100 parts by weight of the obtained water absorbent resin powder (1). A surface cross-linking agent solution consisting of parts by mass was sprayed and uniformly mixed using a high-speed continuous mixer to obtain a mixture (surface cross-linking step).

After introducing the obtained mixture into a heat treatment machine adjusted to an atmospheric temperature of 195 ° C. ± 2 ° C. and performing heat treatment for 25 minutes, the powder temperature (temperature of the mixture) was forcibly cooled to 60 ° C. (Cooling step), a surface-crosslinked water absorbent resin powder (1) was obtained.

10.0 parts by mass of water is uniformly mixed with 100 parts by mass of the obtained surface-crosslinked water-absorbing resin powder (1), and the mixture is passed through a JIS standard sieve with an opening of 1000 μm for sizing (regulating). Particle step) to obtain water absorbent resin particles (1).

To 100 parts by mass of the obtained water-absorbent resin particles (1), 0.3 parts by mass of finely divided silicon dioxide (trade name: Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) is added as water-insoluble inorganic fine particles, By mixing (another additive addition step), a particulate water absorbent resin (1) was obtained. Table 1 shows the physical properties of the obtained water absorbent resin (1).

[Example 2]
A manufacturing process similar to that of Example 1 was performed. Specifically, it was as follows.

As a preparatory step for the polymerization reaction, 21,000 g of n-heptane, which is a hydrophobic organic solvent, was charged into the dispersing device, the reactor, the separating device, and the piping (including joints) connecting these devices.

Subsequently, the liquid-sending pump was operated to start circulation of the hydrophobic organic solvent at a flow rate of 240 mL/min. Furthermore, the heat exchanger is operated so that the set temperature (the temperature of the hydrophobic organic solvent present in the region where the monomer aqueous solution (2) below is introduced in the reaction apparatus) is 85 ° C. The hydrophobic organic solvent was heated. Furthermore, a sucrose fatty acid ester (trade name, S-370, manufactured by Mitsubishi Chemical Corporation, HLB = 3) as a dispersion aid is added in an amount of 0.11% by mass relative to 100% by mass of the hydrophobic organic solvent. did.

Subsequently, sodium acrylate, acrylic acid, and ion-exchanged water were mixed, and polyethylene glycol diacrylate (average degree of polymerization: 9) as an internal cross-linking agent was added to prepare a solution containing monomers.

Subsequently, the solution containing the monomer obtained by the above operation and potassium persulfate were supplied to a mixer and mixed to prepare a monomer aqueous solution (2). The aqueous monomer solution (2) had a monomer concentration of 43 mass % and a monomer neutralization rate of 75 mol %. Further, the amount of polyethylene glycol diacrylate is 0.02 mol% relative to 100 mol% of the monomer (acrylic acid (salt)), and the amount of potassium persulfate is relative to the monomer (acrylic acid (salt)). It was 0.10 g/mol.

The rotor of the dispersing device (double-cylindrical high-speed rotary shear type stirrer) was rotated at a rotation speed of 3600 rpm (shear rate of 2765 [1/s]). Next, the monomer aqueous solution (2) was sent to the pipe of the dispersing device at a flow rate of 40 mL/min (47.2 g/min). The supplied monomer aqueous solution (2) was dispersed in the hydrophobic organic solvent in the form of droplets by a dispersing device to obtain a dispersion liquid.

The dispersion obtained as described above was then fed to the reactor. Droplets composed of the aqueous monomer solution (2) in the dispersion polymerize while falling in the reactor filled with the hydrophobic organic solvent as the continuous phase, and near the discharge port of the reactor, minute droplets A spherical hydrous gel polymer (2) was obtained. The water-containing gel polymer (2) obtained by the above series of operations is continuously supplied from the reaction device to the separation device through the joint together with the hydrophobic organic solvent, and in the separation device, the water-containing gel polymer is (2) and the organic solvent were separated. In the separation device, the water-containing gel polymer (2) is an aggregate of fine spherical particles, and the average particle size of the spherical particles (primary particles) constituting the aggregate is 5 to 10 mm. was 65 μm.

A gel sizing device having a screw and a perforated plate with a hole diameter of 0.8 mm was previously charged with a 3.5% by mass aqueous solution of betaine lauryldimethylaminoacetate as a drying aid (the solid content of the obtained hydrous gel polymer (2) was 0.20% by mass of the polymer) was added, and the mixed hydrous gel polymer (2) (gel temperature: 90°C) was added and discharged from the gel sizing apparatus to obtain a sizing gel (2). After that, 700 ppm of betaine lauryldimethylaminoacetate was added to the sizing gel (2).

Subsequently, a drying step was performed in the same manner as in Example 1 to obtain a dry polymer (2). The obtained dry polymer (2) is supplied to a roll mill (pulverizer) and pulverized to adjust the particle size, and further classified using a sieve with an opening of 150 μm to obtain a water absorbent resin powder (2). rice field. The average particle size of the water absorbent resin powder (2) was 340 µm.

A surface cross-linking agent solution consisting of 0.015 parts by mass of ethylene glycol diglycidyl ether, 1.0 parts by mass of propylene glycol and 3.0 parts by mass of ion-exchanged water per 100 parts by mass of the obtained water absorbent resin powder (2). was sprayed and uniformly mixed using a high-speed continuous mixer to obtain a mixture.

The obtained mixture is introduced into a heat treatment machine adjusted to an ambient temperature of 195°C ± 2°C, and after heat treatment for 40 minutes, the powder temperature (temperature of the mixture) is forcibly cooled to 60°C. to obtain a surface-crosslinked water absorbent resin powder (2).

10.0 parts by mass of water is uniformly mixed with 100 parts by mass of the obtained surface-crosslinked water-absorbing resin powder (2), and the mixture is passed through a JIS standard sieve with an opening of 1000 μm for sizing (regulating). Particle step) to obtain water absorbent resin particles (2).

To 100 parts by mass of the obtained water-absorbing resin particles (2), 0.1 part by mass of finely divided silicon dioxide (trade name: Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) is added as water-insoluble inorganic fine particles, By mixing, a water absorbent resin (2) was obtained. Table 1 shows the physical properties of the obtained water absorbent resin (2).

[Comparative Example 1]
In Example 1, the polymerization process was carried out in the same manner as in Example 1, except that the amount of polyethylene glycol diacrylate as a cross-linking agent was changed to 0.007 mol% with respect to 100 mol% of the monomer (acrylic acid (salt)). , The subsequent separation step, gel granulation step, and drying step are performed to obtain a comparative dry polymer (1), which is then fed to a roll mill (pulverizer) and pulverized to adjust the particle size. It was classified using a sieve to obtain a comparative water absorbent resin powder (1). The average particle size of the comparative water absorbent resin powder (1) was 390 μm.

0.03 parts by mass of ethylene glycol diglycidyl ether, 0.385 parts by mass of ethylene carbonate, 0.644 parts by mass of propylene glycol and ion-exchanged water are added to 100 parts by mass of the obtained comparative water absorbent resin powder (1). A surface cross-linking agent solution consisting of 6 parts by mass was sprayed and uniformly mixed using a high-speed continuous mixer to obtain a mixture.

After introducing the obtained mixture into a heat treatment machine adjusted to an atmospheric temperature of 195 ° C. ± 2 ° C. and performing heat treatment for 10 minutes, the powder temperature (temperature of the mixture) was forcibly cooled to 60 ° C. , to obtain a surface-crosslinked comparative water absorbent resin powder (1).

10.0 parts by mass of water is uniformly mixed with 100 parts by mass of the obtained comparative water absorbent resin powder (1), and the particles are regulated by passing through a JIS standard sieve with an opening of 1000 μm, and the comparative water absorbent resin is (1) was obtained.

[Comparative Example 2]
In Example 1, sucrose fatty acid ester (trade name: S-370, manufactured by Mitsubishi Chemical Corporation, HLB = 3) was added at 0.11 in place of the maleic anhydride-modified ethylene/propylene copolymer as the dispersing aid. The polymerization step, subsequent separation step, gel granulation step, and drying step were carried out in the same manner as in Example 1 except that the mass % was used to obtain a comparative dry polymer (2), which was supplied to a roll mill (pulverizer). The powder was pulverized to adjust the particle size, and further classified using a sieve with an opening of 150 μm to obtain a comparative water absorbent resin powder (2).

0.03 parts by mass of ethylene glycol diglycidyl ether, 0.385 parts by mass of ethylene carbonate, 0.644 parts by mass of propylene glycol and ion-exchanged water are added to 100 parts by mass of the obtained comparative water absorbent resin powder (2). A surface cross-linking agent solution consisting of 6 parts by mass was sprayed and uniformly mixed using a high-speed continuous mixer to obtain a mixture.

After introducing the obtained mixture into a heat treatment machine adjusted to an atmospheric temperature of 195 ° C. ± 2 ° C. and performing heat treatment for 20 minutes, the powder temperature (temperature of the mixture) was forcibly cooled to 60 ° C. A surface-crosslinked comparative water absorbent resin powder (2) was obtained.

10.0 parts by mass of water is uniformly mixed with 100 parts by mass of the obtained surface-crosslinked comparative water-absorbent resin powder (2), and the mixture is passed through a JIS standard sieve with an opening of 1000 μm for sizing, Comparative water absorbent resin particles (2) were obtained.

To 100 parts by mass of the obtained comparative water absorbent resin particles (2), 0.1 part by mass of finely divided silicon dioxide (trade name: Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) is added as water-insoluble inorganic fine particles. , to obtain a comparative water absorbent resin (2). Table 1 shows the physical properties of the obtained comparative water absorbent resin (2).

From Comparative Examples 1 and 2, the water absorbent resin having a gel transfer index of 0.20 or more has a liquid return amount of more than 1.0 g after the second liquid absorption, and the liquid return is more than 1.0 g when used as an absorbent article. It occurs. On the other hand, according to Examples 1 and 2, the water absorbent resin having a local return amount of 0.20 or less has a low liquid return amount after the second liquid absorption, and does not cause liquid return when used as an absorbent article. I understand.

The water-absorbing resin composition according to one embodiment of the present invention can be used in various water-absorbing articles such as disposable diapers, sanitary napkins, incontinence products for adults (incontinence pads), sanitary materials such as pet sheets (sanitary products), and the like. Can be suitably used in.

Reference Signs List 1: Disk part 2: Rod-shaped part 20: Adhesive tape 21: Water-absorbing resin-spreading area 22: Non-woven fabric 23: Water-absorbing resin 100: Compression jig 200: Absorbent sheet for evaluation

[ Reference example 1 ]
A manufacturing process similar to that of Example 1 was performed. Specifically, it was as follows.

From Comparative Examples 1 and 2, the water absorbent resin having a gel transfer index of 0.20 or more has a liquid return amount of more than 1.0 g after the second liquid absorption, and the liquid return is more than 1.0 g when used as an absorbent article. It occurs. On the other hand, according to Example 1 , the water absorbent resin having a local return amount of 0.20 or less has a low liquid return amount after the second liquid absorption, and does not cause liquid return when used as an absorbent article. I understand.

Claims (8)

A particulate water-absorbing resin containing a crosslinked polymer containing a structural unit derived from a poly(meth)acrylic acid (salt)-based monomer, wherein the gel transfer index obtained by the following procedure is 0.20 or less. and a water-absorbent resin having a water absorption rate (Vortex method) of 50 seconds or less:
(1) Into a 100 mL beaker with a barrel diameter of 55 mm and a height of 70 mm, 38 g of a 0.9% by mass sodium chloride aqueous solution at 25° C. and 2.00 g of a water absorbent resin are added to form a swollen gel.
(2) A compression jig comprising a disk portion and a rod-shaped portion having one end connected to the center of the disk portion (the disk portion has flat surfaces on the front and back, and has a diameter of 2.5 cm and a thickness of (1 cm thick disc and 5.5 cm long bar) is placed in the center of the beaker containing the swollen gel of (1), and the circle of the compression jig is After bringing the lower surface of the plate portion into contact with the surface of the swollen gel, the compression jig was moved so that the lower surface of the disk portion was vertically raised from the surface of the swollen gel by 0.05 mm. position at the measurement start position.
(3) The compression jig arranged at the measurement start position is pushed into the swollen gel by 10.0 mm in the vertical direction at a speed of 10 cm/min.
(4) In (3), when the compression jig is pushed into a position 8.0 mm in the vertical direction from the measurement start position, the load (N) applied to the compression jig is observed, It is referred to as gel shape retention force A.
(5) The same operations as (1) to (4) are performed except that the speed of pushing the compression jig is changed to 1 mm/min, and the load (N) applied to the compression jig is observed. , gel shape retention force B.
(6) Gel transfer index for calculating the gel transfer index according to the following formula (1)=(gel shape retention force A/gel shape retention force B)−1 (1). 2. The water-absorbing resin according to claim 1, which is aggregate particles of spherical particles. 3. The water absorbent resin according to claim 1, comprising water-insoluble inorganic fine particles. The water absorbent resin according to any one of claims 1 to 3, wherein the gel shape retention force A is 10.0 N or more. The water absorbent resin according to any one of claims 1 to 4, wherein AAP is 15 g/g or more. 6. The water absorbent resin according to any one of claims 1 to 5, which is obtained by reverse phase suspension polymerization in a hydrophobic organic solvent. The water absorbent resin according to any one of claims 1 to 6, which is obtained by extruding spherical particles with an extruder having a perforated plate. It contains the water absorbent resin according to any one of claims 1 to 7 and does not contain hydrophilic fibers, or the weight of the water absorbent resin is 50 of the total weight of the water absorbent resin and the hydrophilic fibers. The absorber which is mass % or more.

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