JP7032593B1 - Poly (meth) acrylic acid (salt) -based water-absorbent resin and absorber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an absorbent resin in a thin absorbent article in which a liquid return amount is reduced shortly after liquid absorption. A particulate poly (meth) acrylic acid (salt) -based water-absorbent resin having the following physical property values (1) to (3): (1) Vertical liquid diffusion length. (D) is 15 mm or more (2) The ratio (L / D) of the horizontal liquid diffusion length (L) to the vertical liquid diffusion length is 0.5 to 1.5 (3) EDANA ERT460.2-02. The specified bulk density is 0.68 g / mL or more. [Selection diagram] Fig. 1

Description

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

In recent years, in sanitary materials such as disposable diapers, menstrual napkins, and incontinence pads, a water-absorbent resin as a constituent material thereof has been widely used as a water-absorbing agent from the viewpoint of body fluid absorption. Examples of such a water-absorbent resin include a hydrolyzate of a starch-acrylonitrile graft copolymer, a neutralized product of a starch-acrylic acid graft polymer, and a saponified product of a vinyl acetate-acrylic acid ester copolymer (meth). ) Acrylic acid partially neutralized polymer Cross-linked products are known, but from the viewpoint of water absorption performance, poly (meth) using (meth) acrylic acid and / or a salt thereof as the main component of the monomer. Acrylic acid (salt) -based water-absorbent resin is the most industrially produced.

The properties desired for such a poly (meth) acrylic acid (salt) -based water-absorbent resin include a non-pressurized water absorption ratio (CRC), a pressurized water absorption ratio (AAP), a water absorption rate (FSR / vortex), and a non-pressurized underpass. Many characteristics (parameters) such as liquid property, liquid permeability under pressure, impact resistance, urine resistance, fluidity, gel strength, color, particle size, etc. are known, and the same physical properties (for example, water absorption rate under no pressure) are known. ), Many regulations (parameter measurement methods) have been proposed from various viewpoints.
The water-absorbent resin, which has been developed by paying attention to these many physical properties, controls many of the above-mentioned physical properties (for example, "unpressurized water absorption ratio (CRC)", "pressurized water absorption ratio (AAP)", etc.). However, there is still a problem that it is difficult to say that it is exhibiting sufficient performance for the implementation of absorbent materials such as disposable diapers.

As a technique for exhibiting excellent performance in the practice of absorbents such as disposable diapers, Patent Document 1 discloses a method for producing a water-absorbing agent having improved wettability by hydrophilization treatment. Further, Patent Document 2 discloses a water absorbing agent having excellent liquid diffusivity. Further, Patent Document 3 discloses a water-absorbent resin which is excellent in initial water absorption rate and liquid permeability after water absorption, and a liquid in contact with an aggregate of water-absorbent resin particles easily permeates the entire aggregate.

Japanese Patent Application Laid-Open No. 11-71425 Patent 02918807 WO2020 / 122215

When the conventional water-absorbent resin is used for a thin absorbent article having a large amount of water-absorbent resin or a high concentration of water-absorbent resin, the reduction of the return amount of the liquid immediately after the liquid absorption is still insufficient. rice field.

An object to be solved by the present invention is to provide an absorbent resin in a thin absorbent article in which the amount of liquid return is reduced shortly after the liquid is absorbed.

In order to reduce the amount of liquid return shortly after liquid absorption in a thin absorbent article, the present inventors have poly (meth) acrylic acid (salt) -based water absorption having the following physical property values (1) to (3). We have found that a sex resin is suitable, and have completed the present invention.

The water-absorbent resin according to the embodiment of the present invention is a particulate poly (meth) acrylic acid (salt) -based water-absorbent resin having the following physical property values (1) to (3):
(1) The vertical liquid diffusion length (D) is 15 mm or more (2) The ratio (L / D) of the horizontal liquid diffusion length (L) to the vertical liquid diffusion length is 0.5 to 1.5.
(3) The bulk density specified by EDANA ERT460.2-02 is 0.68 g / mL or more.

(Here, the liquid diffusion length (D) in the vertical direction and the liquid diffusion length (L) in the horizontal direction are transparent containers made of acrylic resin (inner dimensions are 40 mm in the vertical direction (height) and 72 mm in the horizontal direction (width)). , Depth 3 mm) is filled with a water-absorbent resin to a height of 30 mm, and 1 mL of a 0.9% by mass sodium chloride aqueous solution at 20 ° C. colored in blue is injected from the water-absorbent resin surface in the vertical direction 1 minute later. And obtained by measuring the diffusion length in the horizontal direction)
The water-absorbent resin according to the embodiment of the present invention is agglomerates of spherical particles.

The water-absorbent resin according to the embodiment of the present invention contains water-insoluble inorganic fine particles.

The water-absorbent resin according to the embodiment of the present invention has a mass average particle size (D50) of 200 to 700 μm.

The water-absorbent resin according to the embodiment of the present invention has an AAP of 18 g / g or more.

The water-absorbent resin according to the embodiment of the present invention has a bulk density of 0.70 g / mL or more specified by EDANA ERT460.2-02.

The water-absorbent resin according to one embodiment of the present invention is obtained by reverse phase suspension polymerization in a hydrophobic organic solvent.

The polymerized hydrogel polymer is subjected to an extruder having a porous plate and extruded by the extruder to be sized.

The absorber according to the embodiment of the present invention contains the water-absorbent resin according to the embodiment of the present invention and (i) does not contain hydrophilic fibers, or (ii) the mass of the water-absorbent resin. Is 50% by mass or more of the total mass of the water-absorbent resin and the hydrophilic fiber.

According to the water-absorbent resin according to the present invention, it is possible to provide an absorbent resin in which the amount of liquid return is reduced shortly after the liquid is absorbed in the thin absorbent article. In particular, it is possible to provide an absorbent article in which the amount of liquid return is reduced shortly after the liquid is absorbed, because the thin absorbent article is excellent in liquid compatibility and quick absorption and liquid retention of the liquid.

It is a perspective view which shows the container for measuring the liquid diffusion length. It is a schematic diagram for demonstrating the measurement of the liquid diffusion length. It is the figure which looked at the absorption sheet for the liquid return amount evaluation from directly above. It is sectional drawing of the absorption sheet for evaluation of the liquid return amount.

Hereinafter, the present invention will be described while showing the best mode. It should be understood that the terms used herein are used in the meaning commonly used in the art unless otherwise noted. Accordingly, unless otherwise defined, 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, this specification (including definitions) takes precedence. The present invention is not limited to the following embodiments, and various modifications can be made 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 polymer gelling agent, which satisfies the following physical properties. That is, the physical properties of the CRC defined by ERT441.2-02 as "water swellability" is 5 g / g or more, and the Ext defined by ERT470.2-02 as "water-insoluble" is 50% by weight or less. Refers to a polymer gelling agent that satisfies the above conditions.

The water-absorbent resin can be appropriately designed according to its use, and is not particularly limited, but is preferably a hydrophilic crosslinked polymer obtained by cross-linking and polymerizing an unsaturated monomer having a carboxyl group. Further, the total amount (100% by weight) is not limited to the polymer, and a water-absorbent resin composition containing an additive or the like may be used as long as the physical properties (CRC, Ext) are satisfied.
Further, the water-absorbent resin in the present invention is not limited to the final product, but is an intermediate in the manufacturing process of the water-absorbent resin (for example, a water-containing gel-like crosslinked polymer after polymerization, a dried polymer after drying, and water absorption before surface cross-linking. (Sexual resin powder, etc.) may also be referred to, and together with the water-absorbent resin composition, all of these are collectively referred to as “water-absorbent resin”. Examples of the shape of the water-absorbent resin include sheet-like, fibrous, film-like, and particulate-like shapes, but in the present invention, particulate water-absorbent resin is preferable.

[1-2] "EDANA" and "ERT"
"EDANA" is an abbreviation for European Disposables and Nonwovens Associations, and "ERT" is a European standard (almost world standard) method for measuring water-absorbent resin (EDANA Recommended Test Method). .. In the present invention, unless otherwise specified, the physical properties of the water-absorbent resin are measured in accordance with the original ERT (revised in 2002 / publicly known literature).

[1-3] Others In the present specification, "XY" indicating a range means "X or more and Y or less".

As used herein, "ppm" means "mass ppm" unless otherwise noted.

In the present specification, "-acid (salt)" means "-acid and / or a salt thereof". "(Meta) acrylic" means "acrylic and / or methacrylic". "Poly (meth) acrylic acid (salt) -based water-absorbent resin" means a water-absorbent resin containing (meth) acrylic acid (salt) as a main component as a repeating unit, and specifically, a total simple substance used for polymerization. Of the weights (excluding the cross-linking agent), (meth) acrylic acid (salt) is preferably 50 to 100 mol%, more preferably 70 to 100 mol%, still more preferably 90 to 100 mol%, and particularly preferably substantial. A water-absorbent resin containing 100 mol%.

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

In the present specification, "weight" and "mass", "weight%" and "mass%", and "parts by weight" and "parts by mass" are treated as synonyms.

[2] Water-absorbent resin [2-1] Shape of water-absorbent resin In the present invention, the water-absorbent resin is preferably in the form of particles, specifically, in the form of amorphous crushed, spherical, football-like, or aggregate. And so on. Among them, the water-absorbent resin is preferably aggregate-like particles of primary particles (spherical particles) because the spherical shape has a large bulk density and the aggregate-like shape improves the water absorption rate. .. Here, the spherical shape includes not only a true sphere but also a substantially spherical shape having an aspect ratio of 1.0 to 1.2.

[2-2] Additives contained In the present invention, the water-absorbent resin may contain additives for exhibiting various functions. Specific examples of the additive include surfactants, compounds having a phosphorus atom, oxidizing agents, organic reducing agents, water-insoluble inorganic fine particles, organic powders such as metal soaps, deodorants, antibacterial agents, pulps and thermoplastics. Examples include fibers. As the water-insoluble inorganic fine particles, the compound disclosed in "[5] Water-insoluble inorganic fine particles" of International Patent Publication No. 2011/040530 is applied to the present invention. Among these water-insoluble inorganic fine particles, particularly hydrophilic fine particles (the ratio of fine particles suspended in a colloidal state in a mixed solution of hydrophilicity (water / methanol = 70/30) described in European Patent No. 0629411 is shown. High (eg, 70% or more) or low contact angle with water (eg, 10 ° or less) as described in Patent No. 68371139, such as silica (silicon dioxide) or hydro. The inclusion of talcite improves the liquid compatibility of the water-absorbent resin (for example, water-absorbent resin particles), and is preferable because the water-absorbent resin can absorb the water-based liquid in a short time when used for an absorbent article.

The content of the water-insoluble inorganic fine particles is more preferably 0.01 to 5 parts by mass 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). Is 0.05 to 3 parts by mass, more preferably 0.1 to 1 part by mass, and particularly preferably 0.2 to 0.5 part by mass.

The water-insoluble inorganic fine particles here usually have a minute size as compared with the size of the polymer particles. For example, the average particle size of the water-insoluble inorganic fine particles may be 0.01 to 50 μm, 0.1 to 30 μm, or 1 to 20 μm. The average particle size can be measured by the pore electric resistance method or the laser diffraction / scattering method depending on the characteristics of the particles.

[2-3] CRC
"CRC" is an abbreviation for Centrifuge Retention Capacity (centrifuge holding capacity), and means the water absorption ratio of a water-absorbent resin under no pressure. In the present specification, it is measured according to the EDANA method (ERT441.2-02).

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, 44 g / g. Hereinafter, it is 42 g / g or less, 40 g / g or less, 38 g / g or less, and 36 g / g or less.

When the CRC is 30 g / g or more, the absorption amount is sufficient, and it is suitable as an absorber for absorbent articles such as disposable diapers. Further, when the CRC is 50 g / g or less, a decrease in the rate of absorbing body fluids such as urine and blood is prevented, and the CRC is suitable for use in high water absorption rate type disposable diapers and the like. The CRC value can be controlled by changing the type and amount of the internal cross-linking agent, the surface cross-linking agent, and the like.

[2-4] Ext
"Ext" is an abbreviation for Extremes (water-soluble component), and means the amount of soluble component extracted from the water-absorbent resin. The water-soluble content 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 further preferably 10% by mass or less. The lower limit is 0% by mass, 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.

When the Ext is 30% by mass or less, a decrease in the rate of absorbing body fluids such as urine and blood is prevented, and it is suitable for use in high water absorption rate type disposable diapers and the like. The value of Ext can be controlled by changing the type and amount of the polymerization initiator, the internal cross-linking agent, the surface cross-linking agent, etc., and also by using a chain transfer agent in the polymerization step.

[2-5] AAP
"AAP" is an abbreviation for Absorption Against Pressure, and means the water absorption ratio under pressure of the water-absorbent resin. In the present invention, AAP is measured according to the EDANA method (ERT442.2.2) except that the load condition is changed to 4.83 kPa (0.7 psi). Specifically, 0.9 g of a water-absorbent resin was swelled under a pressure of 4.83 kPa for 1 hour using a 0.9 mass% sodium chloride aqueous solution, and then AAP (absorption ratio under pressure) (unit: g /). g) is measured.

The AAP of the water-absorbent resin of the present invention is preferably 18 g / g or more, more preferably 20 g / g or more, still more preferably 23 g / g or more, from the viewpoint of water absorption characteristics when used as a sanitary material. The upper limit of AAP of the water-absorbent resin is not particularly limited, but is preferably 40 g / g or less.

[2-6] Moisture content The "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, still more preferably. It is 5% by mass to 13% by mass, more preferably 8% by mass to 13% by mass, and particularly preferably 10% by mass to 13% by mass. When the water content is 1% by mass to 20% by mass, a decrease in the rate of absorbing body fluids such as urine and blood is prevented, and the water content is suitable for use in high water absorption rate type disposable diapers and the like.

[2-7] Mass average particle size (D50)
The “mass average particle size (D50)” is described in “(3) Mass-Average Particle Diameter (D50) and Logimetric Standard Division” (σζ) of Division described in columns 27 and 28 of US Pat. No. 7,638,570. Measured in compliance.

The mass average particle size (D50) of the water-absorbent resin of the present invention is preferably 50 μm to 800 μm, more preferably 200 μm to 700 μm, still more preferably 250 μm to 600 μm, and particularly preferably 250 μm to 500 μm. The proportion of particles having a particle diameter of less than 45 μm is preferably 40% by mass or less, more preferably 30% by mass or less, still more preferably 20% by mass or less, and particularly preferably 10% by mass or less. When the mass average particle diameter is 200 μm or more, there is little dust and the handling is good. Further, when the mass average particle size is 700 μm or less, a decrease in the rate of absorbing body fluids such as urine and blood is prevented, and the film is suitable for use in high water absorption rate type disposable diapers and the like.

[2-8] Number average particle size When the water-absorbent resin is in the form of an aggregate of primary particles, the number average particle size of the primary particles constituting the aggregate is measured using an electron microscope. The number average particle size of the primary particles of the water-absorbent resin is preferably 5 to 800 μm, more preferably 8 to 500 μm, further preferably 10 to 300 μm, still more preferably 10 to 200 μm, and particularly preferably 30 μm to. It is 100 μm.

[2-9] Surface Tension The "surface tension" in the present invention is the surface tension of the aqueous solution when the water-absorbent resin is dispersed in a 0.9 mass% sodium chloride aqueous solution, and is described in W02015 / 129917. Measured by the method.

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, still more preferable. Is 70 mN / m or more. When the surface tension is in the above range, it is preferable because it has excellent liquid compatibility with an aqueous liquid.

[2-10] Vertical liquid diffusion length (D)
The "liquid diffusion length in the vertical direction" in the present invention is a novel physical property value for evaluating how much the aqueous liquid is diffused in the direction perpendicular to the water-absorbent resin layer, and is specifically carried out. It is a value measured by the measuring method described in the example. The liquid diffusion length in the vertical direction is measured by the method described later. A water-absorbent resin having a long liquid diffusion length in the vertical direction indicates that the water-absorbent liquid added to the water-absorbent resin has good liquid compatibility. The water-absorbent resin having good liquid compatibility can quickly take in the liquid in a thin water-absorbing sheet for light incontinence and diffuse the liquid in the water-absorbing sheet, so that the amount of liquid returned shortly after the liquid absorption can be reduced. The liquid diffusion length in the vertical direction is 15 mm or more, preferably 16 mm or more, more preferably 17 mm or more, still more preferably 18 mm or more, particularly preferably 19 mm or more, and extremely preferably 20 mm or more. The upper limit of the liquid diffusion length in the vertical direction is 30 mm or less, which is the height of the resin layer in the measurement, and the preferable range is not particularly limited, but may be 28 mm or less, 25 mm or less.

[2-11] Horizontal liquid diffusion length (L)
The "horizontal liquid diffusion length" in the present invention is a novel physical property value indicating how much the aqueous liquid is diffused horizontally with respect to the water-absorbent resin layer, and is specifically described in Examples. It is a value measured by the measuring method of. The liquid diffusion length in the horizontal direction is measured by the method described later. A water-absorbent resin having a long liquid diffusion length in the horizontal direction indicates that the water-absorbent liquid added to the water-absorbent resin has poor liquid compatibility and tends to run in the horizontal direction. If the water-absorbent resin is not well-adapted to the liquid, the water-based liquid is not quickly absorbed by the absorber, and the liquid runs on the surface of the absorber, which may cause leakage, which is not preferable. The liquid diffusion length in the horizontal direction is 72 mm or less, which is the width of the resin layer in the measurement, preferably 35 mm or less, more preferably 30 mm or less, and particularly preferably 25 mm or less. The lower limit is not particularly limited, but may be 10 mm or more and 15 mm or more.

[2-12] L / D
The “L / D” in the present invention represents the ratio of the “vertical liquid diffusion length (D)” to the above-mentioned “horizontal liquid diffusion length (L)”. The closer the L / D ratio is to 1, the more evenly the aqueous liquid is diffused to the water-absorbent resin layer, and when used as an absorber, the aqueous liquid can be diffused evenly in the absorbent body, and the liquid-absorbing liquid can be diffused. The amount of liquid returned soon afterwards can be reduced. The L / D is in the range of 0.5 to 1.5, preferably in the range of 0.6 to 1.5, more preferably in the range of 0.7 to 1.4, and particularly preferably in the range of 0.8 to 1.4. The range is extremely preferably 0.9 to 1.4.

[2-13] Bulk Density The bulk density is the volume when 100 g of the water-absorbent resin is put into an apparatus specified by the EDANA method (ERT460.2-02) and the water-absorbent resin is freely dropped and filled in a 100 mL container. The mass of the water-absorbent resin (unit: [g / mL]). The water-absorbent resin having a large bulk density is densely packed in the sheet in a thin water-absorbent sheet for light incontinence, for example, and gaps are less likely to occur between the particles of the water-absorbent resin. If a gap is formed between the particles of the water-absorbent resin, the water-based liquid invades the gap and remains without being absorbed by the water-absorbent resin, which leads to an increase in the amount of liquid returned shortly after the liquid is absorbed, which is not preferable. That is, the bulk density of the water-absorbent resin is preferably large, preferably 0.68 g / mL or more, preferably 0.69 g / mL or more, and particularly preferably 0.70 g / mL or more. The upper limit is not particularly limited, but may be 1.00 g / mL or less, 0.95 g / mL or less, and 0.90 g / mL or less.

[3] Method for producing water-absorbent resin The method for producing the water-absorbent resin of the present invention may be any of aqueous solution polymerization, reverse phase suspension polymerization, vapor phase droplet polymerization and other polymerization methods, but the present invention From the viewpoint that it is easy to control the physical properties of the water-absorbent resin, reverse phase suspension polymerization will be described below as an example. In particular, unlike general reverse phase suspension polymerization including a co-boiling dehydration step in a hydrophobic organic solvent after polymerization and a surface cross-linking step in a dispersion system, a separation step of a reverse phase suspension polymerization gel and gel preparation are performed. A production method including a grain step, a water-containing gel drying step (preferably hot air drying), and a surface cross-linking step (preferably powder surface treatment) will be described as an example.

In the method for producing a water-absorbent resin according to an embodiment of the present invention, a water-containing gel polymer (water-containing) is obtained by polymerizing the monomer in a state where droplets containing the monomer are dispersed or suspended in a hydrophobic organic solvent. A polymerization step for obtaining a gel-like polymer) is included.

As a polymerization method, a hydrogel polymer is obtained by reverse phase suspension polymerization obtained by polymerizing the monomer in a state where droplets containing the monomer are dispersed or suspended in a liquid phase composed of a hydrophobic organic solvent. Any method may be used as long as it can be obtained, and the polymerization method may be a batch method or a continuous method. In the batch-type production method, a monomer aqueous solution is added or dropped into a hydrophobic organic solvent in a reaction apparatus and mixed to disperse or suspend the monomer aqueous solution, and then polymerization is performed to contain water. This is a production method for obtaining a gel polymer. On the other hand, in the continuous production method, a water-containing gel polymer formed by a polymerization reaction in which an aqueous monomer solution is continuously sent to a hydrophobic organic solvent in a reaction apparatus, dispersed or suspended, and then polymerized. This is a method of continuously discharging and a hydrophobic organic solvent from the reaction device. A preferred embodiment of the present invention is batch reverse phase suspension polymerization. In the method for producing a water-absorbent resin according to the present invention, a separation step may be provided to separate the hydrous gel polymer obtained in the polymerization step from the hydrophobic organic solvent.

As a method for producing the water-absorbent resin according to the embodiment of the present invention, any monomer aqueous solution preparation step; any dispersion step; polymerization step; any separation step; any gel sizing step; drying step; hydrophilicity. Includes a polymerization process. Further, after the drying step, optionally, a cooling step, a crushing step, a classification step, a surface cross-linking step, a water-containing (rewetting) step, another additive addition step, a granulation step, a fine powder removal step, a granulation step and a fine powder It can include a reuse process and the like. Further, a transportation step, a storage step, a packing step, a storage step, and the like may be further included.

Hereinafter, each step will be described.

[3-1] Monomer Aqueous Solution Preparation Step The monomer aqueous solution is an aqueous solution containing a monomer that is a raw material of a water-absorbent resin, and is dispersed or suspended in a hydrophobic organic solvent for reverse-phase suspension polymerization. It is a solution that makes it turbid.

As the solvent of the monomer aqueous solution, water or a mixture of water and a water-soluble organic solvent (for example, alcohol, etc.) is preferably used, and water is even more preferable. In the case of a mixture of water and a water-soluble organic solvent, the water-soluble organic solvent (for example, alcohol or the like) is preferably 30% by mass or less, and more preferably 5% by mass or less of the mixture.

As the monomer, (meth) acrylic acid, which is a water-soluble ethylenically unsaturated monomer, is used. Acrylic acid is particularly preferable.

In consideration of the stability of the (meth) acrylic acid, a polymerization inhibitor may be added if necessary.

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

In the production method according to the present invention, any of the above-exemplified monomers may be used alone in the preparation of the aqueous monomer solution, or any two or more kinds of monomers may be appropriately mixed and used. You may. Further, other monomers may be further mixed as long as the object of the present invention is achieved.
When two or more kinds of monomers are used in combination in the preparation of the aqueous monomer solution, it is preferable to contain (meth) acrylic acid (salt) as a main component. In this case, the ratio of (meth) acrylic acid (salt) to the entire monomer used for polymerization is usually 50 mol% or more, preferably 70 mol% or more, from the viewpoint of the water absorption performance of the obtained water-absorbent resin. It is more preferably 80 mol% or more, still more preferably 90 mol% or more (upper limit is 100 mol%).

In the preparation of the monomer aqueous solution, an internal cross-linking agent can be used, if 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 the internal cross-linking agent include N, N'-methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, and the like. 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-Butandiol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, glycidyl (meth) acrylate and the like. Only one kind of these internal cross-linking agents may be used, or two or more kinds thereof may be used.

The amount of the internal cross-linking agent used is usually 0.0001 to 5 mol%, more preferably 0.001 to 3 mol% with respect to the monomer, although it may be appropriately determined depending on the physical characteristics of the desired water-absorbent resin. , Even more preferably 0.005 to 1.5 mol%.
Further, the 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 bubbles; ethylenediamine. Chelating agents such as 4 metal salts of acetic acid, diethylenetriamine 5 metal salts of acetic acid; polyacrylic acid (salt) and cross-linked compounds thereof, starches, celluloses, starch-cellulose derivatives, polyvinyl alcohol and the like can be mentioned. Other substances may be used alone or in combination of two or more.

The amount of the other substance used is not particularly limited, but the total concentration of the other substance is preferably 10% by mass or less, more preferably 1% by mass or less, still more preferably, with respect to the monomer. Is 0.1% by mass or less. However, the total concentration of polyacrylic acid (salt) and its crosslinked product, starch, cellulose, starch-cellulose derivative, and polyvinyl alcohol is preferably 30% by mass or less with respect to the monomer, and more preferably 20. It is 1% by mass or less, and more preferably 10% by mass or less.

Further, the dissolved oxygen in the aqueous monomer solution may be reduced by raising the temperature or substituting with an inert gas.

"Initiator of polymerization"
A polymerization initiator may be used in the preparation of the monomer aqueous solution. When a polymerization initiator is used to prepare the monomer aqueous solution, gelation and viscosity increase of the monomer aqueous solution may occur. Therefore, when the polymerization initiator is added, the monomer aqueous solution is used as a hydrophobic organic solvent. Line mixing of the monomer aqueous solution and the polymerization initiator, which is performed immediately before the dispersion / suspension in the above, where the monomer aqueous solution is cooled and mixed with the polymerization initiator at a temperature lower than room temperature (20 ° C or lower, preferably around 0 ° C). At the same time, it is preferable to perform the process of subjecting to the dispersion step. As the polymerization initiator, a pyrolysis type polymerization initiator is preferably used. The pyrolysis-type polymerization initiator refers to a compound that is decomposed by heat to generate a radical, but a 10-hour half-life temperature is preferable from the viewpoint of storage stability of the pyrolysis-type polymerization initiator and production efficiency of the water-absorbent resin. A water-soluble compound having a temperature of 0 ° C to 120 ° C, more preferably 30 ° C to 100 ° C, still more preferably 50 ° C to 80 ° C is preferably used as the polymerization initiator.

From the viewpoint of handleability and physical properties of the water-absorbent resin, the water-soluble polymerization initiator is preferable, and the water-soluble radical polymerization initiator is more preferable.

Examples of the water-soluble radical polymerization initiator include persulfates such as hydrogen peroxide, ammonium persulfate, and sodium persulfate; methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, and t-butyl. Peroxides such as cumylperoxide, t-butylperoxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate, 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-based compounds; and the like. As the polymerization initiator, one of these types may be used alone, or two or more types may be used in combination. Among these, preferably a persulfate or a water-soluble azo compound, more preferably sodium persulfate, potassium persulfate, ammonium persulfate, or 2,2'-azobis (2-amidinopropane) dihydrochloride is more preferable. Sodium persulfate is used.

The amount of the thermally decomposable polymerization initiator used is appropriately set according to the type of the monomer and the polymerization initiator, and is not particularly limited, but is preferably 0 with respect to the monomer from the viewpoint of production efficiency. It is 0.01 g / mol or more, more preferably 0.005 g / mol or more, still more preferably 0.01 g / mol or more. Further, 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, it can be used in combination with other polymerization initiators such as a photodegradable polymerization initiator. Specific examples of the photodegradable polymerization initiator include benzoin derivatives, benzyl derivatives, acetophenone derivatives, benzophenone derivatives and the like.

Further, the thermal decomposition type polymerization initiator and the reducing agent can be used in combination to prepare a redox-based polymerization initiator. In the redox-based polymerization initiator, the pyrolysis-type polymerization initiator functions as an oxidizing agent. The reducing agent used is not particularly limited, but for example, (heavy) sulfite such as sodium sulfite and sodium hydrogen sulfite; reducing metal salt such as ferrous salt; L-ascorbic acid (salt), amines and the like. Can be 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 selected monomer and the type of the hydrophobic organic solvent, but the lower limit is the aqueous monomer solution in terms of production efficiency. Of the above, preferably 10% by mass or more, more preferably 20% by mass or more, still more preferably 30% by mass or more, and the upper limit is preferably 100% by mass or less, more preferably 90% by mass. It is 0% by mass or less, 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 thickener, and a chelating agent can be added to the aqueous monomer solution as long as the object of the present invention is not impaired. The type and amount of the additive can be appropriately selected depending on the combination of the monomer and the hydrophobic organic solvent used.

[3-2] Dispersion Step The dispersion step is a step of dispersing or suspending droplets containing a monomer in a hydrophobic organic solvent. In the following, when the term "dispersion" is simply used, the concept includes suspension. More specifically, the aqueous monomer solution is added to a hydrophobic organic solvent, mixed and stirred to disperse the mixture. For example, a stirring device provided with stirring blades (propeller blade, paddle blade, anchor blade, turbine blade, Faudler blade, ribbon blade, flat plate blade, etc.) may be used. When a stirring device having such a stirring blade is used, the dispersed droplet diameter can be adjusted by the type of the stirring blade, the blade diameter, and the rotation speed, which is particularly suitable for batch type reverse phase suspension polymerization. Can be used for. Further, the dispersion liquid can be obtained by the method described in International Publication No. 2009/025253, 2013/018571 and the like. In the case of continuous reverse phase suspension polymerization, in the dispersion step, the monomer aqueous solution and the hydrophobic organic solvent are continuously and separately supplied to the disperser, and the monomer is dispersed in the hydrophobic organic solvent. It is preferable to prepare a droplet containing the above.

When performing continuous reverse phase suspension polymerization, the disperser used in the dispersion step includes a spray nozzle, a high-speed rotary shear type stirrer (rotary mixer type, turbo mixer type, disk type, double cylinder type, etc.). Examples thereof include a cylindrical nozzle such as a needle, an orifice plate in which a large number of holes are directly provided in the plate, a spray nozzle, and a centrifugal atomizer such as a rotary wheel, but the present invention is 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; examples include halogenated hydrocarbons such as chlorbenzene, brombenzene, carbon tetrachloride, 1,2-dichloroethane and the like. Among these, n-hexane, n-heptane, and cyclohexane are preferable from the viewpoint of availability and quality stability. It is also possible to use it as a mixed solvent in which two or more kinds are mixed.

In the present invention, a dispersion aid such as a surfactant or a polymer additive may be added to the hydrophobic organic solvent, if necessary, as long as the object of the present invention is not impaired. The type of the dispersion aid is appropriately selected depending on the combination of the hydrophobic organic solvent and the monomer used, and examples of the dispersion aid that can be used include the following surfactants and polymer additives.

Specific examples of the surfactant include sucrose fatty acid ester, polyglycerin fatty acid ester, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene glycerin fatty acid ester, sorbitol fatty acid ester, and polyoxyethylene sorbitol fatty acid ester. Polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymer, polyoxyethylene polyoxypropyl Alkyl ether, polyethylene glycol fatty acid ester, alkyl glucoside, N-alkyl glucon amide, polyoxyethylene fatty acid amide, polyoxyethylene alkylamine, phosphoric acid ester of polyoxyethylene alkyl ether, and phosphoric acid ester of polyoxyethylene alkyl allyl ether. And so on. Of these, two or more may be used in combination. Further, a polymerizable surfactant having a polymerizable property can also be used. Specific examples of the polymerizable surfactant include compounds having the following structures.

In the formula, R ¹ and R ² are hydrogen, methyl or ethyl independently of each other, and n means an integer of 3 to 20. Among the above-mentioned surfactants, sucrose fatty acid ester, polyglycerin fatty acid ester, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene glycerin fatty acid ester, sorbitol fatty acid ester, polyoxyethylene sorbitol fatty acid ester, polyethylene glycol. Fatty acid esters such as fatty acid esters are preferable, and sucrose fatty acid esters having an HLB value in the range of 1 to 20, more preferably 1 to 10, and even more preferably 3 to 6 are preferable.

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 ternary copolymer (). EPDM), maleic anhydride-modified polybutadiene, maleic anhydride / ethylene copolymer, maleic anhydride / propylene copolymer, maleic anhydride / ethylene / propylene copolymer, maleic anhydride / butadiene copolymer, polyethylene, polypropylene , Ethylene / propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene / propylene copolymer, ethylene / acrylic acid copolymer, ethyl cellulose, hydroxyethyl cellulose and the like. Among them, from the viewpoint of dispersion stability of the monomer aqueous solution, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene / propylene copolymer, maleic anhydride / ethylene copolymer, maleic anhydride / Preferants are propylene copolymers, maleic anhydride / ethylene / propylene copolymers, polyethylene, polypropylene, ethylene / propylene copolymers, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene / propylene copolymers. Of these, two or more may be used in combination. Further, these polymer additives may be used in combination with the above-mentioned surfactant. Above all, it is preferable to use a polymer additive, and it is more preferable to use a maleic anhydride-modified ethylene / propylene copolymer. Further, in another preferred embodiment, the polymer additive is used alone without using a surfactant.

The amount of the dispersion aid used is appropriately set according to the polymerization form, the monomer aqueous solution, the type of the hydrophobic organic solvent, and the like. Specifically, the concentration of the dispersion 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 Step The polymerization step is a step of polymerizing droplets containing the monomer obtained in the dispersion step to obtain a hydrogel polymer (hereinafter, also simply referred to as hydrogel).

"Reactor"
As the reaction apparatus used in the polymerization step, the dispersion apparatus used in the dispersion step may be used as it is, or another apparatus may be used. In the case of batch type reverse phase suspension polymerization, the apparatus used in the dispersion step can be used as it is as a reaction apparatus, which is suitable in terms of workability. When the reaction device is a device different from the dispersion device, the dispersion liquid of the monomer obtained in the dispersion step is supplied to the reaction device.

Further, the shape of the reaction device in which the polymerization reaction is carried out is not particularly limited, and a known reaction device can be used. As described above, a stirrer that can be suitably used in the dispersion step can also be suitably used in the polymerization reaction. In the case of the continuous production method, the shape of the reaction device is preferably such that the monomer (aqueous solution) is a droplet-like dispersed phase in a hydrophobic organic solvent which is a continuous phase formed in the reaction device. It has a shape that allows polymerization reaction while moving. Examples of such a reaction device include a reaction device in which a tubular reaction tube is arranged in a vertical type, a horizontal type, or a spiral type. In this embodiment, since the monomer (aqueous solution) is supplied into the hydrophobic organic solvent that moves in the reaction unit, the droplets of the monomer aqueous solution move together with the hydrophobic organic solvent without staying. As a result, contact between monomer reactants having different polymerization rates is suppressed.
Further, the reaction device may be provided with a temperature adjusting means so that the continuous phase inside the reaction device 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 depending on 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 higher than 100 ° C., a rapid polymerization reaction occurs, which is not preferable. The polymerization temperature means the temperature of the hydrophobic organic solvent as a dispersion medium (hereinafter referred to as “Td”).

In the polymerization step, since the monomer (aqueous solution) is dispersed in the hydrophobic organic solvent in the form of droplets, the temperature of the monomer aqueous solution rapidly rises due to heat transfer from the hydrophobic organic solvent. .. When the polymerization initiator contained in the droplets is a thermal decomposition type polymerization initiator, the thermal decomposition type polymerization initiator is decomposed and radicals are generated as the temperature rises. Then, the polymerization reaction is started by the generated radicals, and a hydrogel is formed as the polymerization reaction progresses.

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

When the aqueous monomer solution contains a thermally decomposable polymerization initiator, the Td is preferably 70 ° C. or higher, more preferably 75 ° C. or higher, still more preferably 80 ° C. or higher from the viewpoint of the polymerization rate. be. The upper limit of Td is not particularly limited, but is appropriately selected from the viewpoint of safety as long as it does not exceed the boiling point of the hydrophobic organic solvent forming a continuous phase.

"Multi-stage reverse phase suspension polymerization"
In the production method of the present invention, multi-stage polymerization may be performed from the viewpoint of obtaining an appropriate aggregated particle size. Specifically, after the completion of the first-stage polymerization step, multi-stage polymerization can be carried out by further adding a monomer aqueous solution and carrying out a polymerization reaction.

"Inorganic fine particles"
In the production method of the present invention, inorganic fine particles may be added to the hydrogel polymer during and / or after the polymerization is completed from the viewpoint of obtaining an appropriate aggregate particle size.

Examples of the 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 soil, bentonite, zeolite, and other metal oxides. In particular, silicon dioxide, aluminum oxide, and titanium dioxide are preferable.

Good results can be obtained when the amount of the inorganic fine particles added is generally 0.001 to 1 part by weight, preferably 0.001 to 0.5 part by weight, based on 100 parts by mass of the hydrogel polymer. Within this range, the effect of adding the inorganic fine particles is efficiently exhibited, and the effect on the water absorption performance is small, which is preferable.

[3-4] Separation Step 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 apparatus used in the separation step are not particularly limited, but for example, known apparatus used for filtration, sedimentation, centrifugation, squeezing and the like can be used. Further, the mixture of the hydrous gel polymer and the hydrophobic organic solvent is heated under normal pressure or reduced pressure using a stirrer having stirring blades used in the polymerization step, and distilled to obtain the hydrophobic gel polymer and the hydrophobic organic. It may be separated from the solvent. In batch reverse phase suspension polymerization, distillation under normal pressure or reduced pressure is preferably performed.

[3-5] Gel sizing step In the gel sizing step, the hydrous gel polymer separated from the hydrophobic organic solvent in the separation step is hydrated gel using a gel sizing device having an extruding part and a porous plate. Is sized. As a result, a sized hydrogel polymer (hereinafter, the hydrogel after gel sizing is referred to as a sizing gel) can be obtained. The gel sizing step is an arbitrary step. Having a gel sizing step makes it easier to control the bulk density of the water-absorbent resin.

The hydrogel polymer used in this gel sizing step is a single particle shape of a spherical gel or an aggregate shape of a spherical gel. The lower limit of the average particle size of the hydrogel polymer is not particularly limited, but is preferably 0.01 mm or more, more preferably 0.03 mm or more, still more preferably 0.05 mm or more, still more preferably 0.1 mm or more. The upper limit is not particularly limited, but is preferably 20 mm or less, more preferably 10 mm or less. Further, in the case of a single particle shape, the particle diameter thereof is referred to as a primary particle diameter, and in the case of an aggregate shape, the particle diameter of each spherical gel constituting the aggregate is referred to as a primary particle diameter. In the present invention, the average primary particle size is not particularly limited, but is preferably 5 to 2000 μm, more preferably 5 to 1000 μm, from the viewpoint of suppressing the generation of fine particles when controlling the particle size of the final product. It is more preferably 5 to 800 μm, still more preferably 8 to 500 μm, even more preferably 10 to 300 μm, and particularly preferably 10 to 200 μm. Here, the average primary particle diameter is intended to be a number average particle diameter.

In addition, a device having a cutter may be installed in front of the gel sizing device having the extrusion action part and the perforated plate to crush large agglomerates.

"Water-containing gel temperature"
The lower limit of the temperature of the hydrous gel to be put into the gel sizing device is not particularly limited, but is preferably 80 ° C. or higher, more preferably 90 ° C. or higher from the viewpoint of sizing efficiency and suppression of damage to the hydrous gel. The upper limit of the water-containing gel temperature at the time of charging the gel sizing device is not particularly limited, but is generally 100 ° C. or lower.

"Gel sizing device"
In the present specification, "gel sizing" means to produce grains having a substantially uniform shape and size from a wet powder-like raw material by extruding a wet mass of powder into a columnar shape from a small hole of a porous plate. It is an operation. That is, by using the porous plate, the hydrogel having the shape of coarse agglomerates excessively aggregated in the solvent separation step of the previous step is crushed, and the monoparticle-like hydrogel having a small particle size is appropriately aggregated. Will be done. Therefore, by this step, it is possible to obtain a hydrous gel (granulation gel) having a granulated shape having a relatively uniform particle size. The sizing gel may contain a single particle water-containing gel.

The "gel granulator having an extrusion action part and a porous plate" used in the gel sizing step has an extrusion action part and a porous plate (die or screen), and the extrusion action part is usually a porous plate. The device is not particularly limited as long as it has an extrusion member that extrudes and supplies the contents toward the surface and can produce grains of a certain size by extruding the material from the perforated plate (for example, an extruder). Further, these devices are connected in series. You may use it side by side.

Further, the shape of the hole of this perforated plate (die or screen) is not particularly limited, and a shape suitable for use such as a polygon such as a perfect circle, an ellipse, a hexagon, and a triangle can be arbitrarily selected. Although it is possible, a perfect circle or an ellipse is preferable from the viewpoint of sizing strength. The pore diameter is also not particularly limited, but is preferably 1.5 mm or less, more preferably 1.0 mm or less, and even more preferably 0.8 mm or less. When it is not more than such an upper limit, it is possible to prevent the size of the obtained sizing gel from increasing more than necessary, and it is possible to reduce the amount of fine powder generated when controlling the particle size of the final product. The pore diameter is preferably 0.3 to 1.5 mm, more preferably 0.3 to 0.8 mm. When the pore diameter of the perforated plate is 0.3 mm or more, it can be efficiently extruded when performing the extrusion operation. The hole diameter is defined as follows. First, when the hole is not a perfect circle, the geometric mean value of the minor axis and the major axis of the hole is adopted as the hole diameter. If the pore diameters of the pores of the perforated plate are different, the pore diameters of all the holes are calculated, and the arithmetic mean value thereof is adopted as the pore diameter of the pores of the perforated plate. Furthermore, if the pore size of the perforated plate changes from the extruding part side of the perforated plate to the opposite side (the pore diameter changes in the thickness direction of the perforated plate), the value that minimizes the pore diameter is adopted. ..

In this step, an additive may be further added. Additives that can be added in this step include polymerization initiators, oxidizing agents, reducing agents, chelating agents, thickeners, surfactants, cross-linking agents, acids, bases, foaming agents, organic or inorganic fine particles, and polyvalent metals. Examples thereof include salts, and among them, additives that can control the degree of aggregation include starch, cellulose, starch-cellulose derivative, thickener such as polyvinyl alcohol, surfactant, fine powder of water-absorbent resin, cross-linking agent, and many. Valuable metal salts and the like are preferable.

[3-6] Drying Step The drying step is a step of drying the hydrogel.

In the present invention, the drying method in the drying step may be azeotropic dehydration in a hydrophobic dispersion solvent used in the conventional reverse phase suspension polymerization, but the water-absorbent resin of the present invention may be subjected to reverse phase suspension polymerization. As a more preferable method for obtaining, instead of azeotropic dehydration, either stirring drying or static drying can be preferably adopted, but in order to maintain the particle size of the gel controlled in the gel sizing step, static drying can be adopted. Is preferable.

The dried polymer composed of particles obtained in this drying step can be directly used as a water-absorbent resin for various purposes. Further, when the water-absorbent resin is produced by this production method, the dried polymer obtained in the drying step can be subjected to the surface cross-linking step described later. In this case, the dry polymer used in the surface cross-linking step described later is also referred to as "water-absorbent resin powder" for convenience.

"Additive"
Additives may be added to the hydrogel as long as the effects of the present invention are not impaired. The addition may be carried out during heating by the heating means and / or during stirring (rotation) by the rotary container, or before the drying step (before heating by the heating means and / or before stirring (rotation) by the rotary container). good. Furthermore, it may be added in any step before the drying step. With the additive, excessive adhesion between the water-containing gels during drying can be reduced, and a water-absorbent resin having an excellent water absorption rate can be obtained.

[3-7] Hydrophilization treatment step The hydrophilization treatment step is a step of subjecting the water-absorbent resin obtained through the separation step and the drying step (and any subsequent step) to the hydrophilization treatment. Further, the hydrophilization treatment step may be performed after the surface cross-linking step described later or before the surface cross-linking step. The hydrophilization treatment is a treatment for hydrophilizing the surface of the water-absorbent resin, and means that the wettability of the surface is improved before and after the treatment. As a specific method of hydrophilization treatment, when a water-absorbent resin is produced by reverse-phase suspension polymerization, the surfactant used as a dispersant during polymerization causes hydrophobicity, so it is preferable to wash it. , It is preferable to wash with an organic solvent. By the hydrophilization treatment, the effect of the surface cross-linking treatment (increasing the absorption ratio under pressure of the water-absorbent resin and reducing the return amount) can be enhanced. In addition, since the surface of the water-absorbent resin becomes highly hydrophilic, the water absorption rate also increases.

"Organic solvent"
The organic solvent is not particularly limited as long as it can clean the dispersant, but in order to enhance the cleaning effect, it is preferable to use a solvent that does not swell the water-absorbent resin. The preferred swelling ratio of the water-absorbent resin during the treatment is less than 2 times. As the organic solvent, not only a hydrophilic organic solvent but also a hydrophobic organic solvent can be used. Specific examples of the hydrophilic organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol and t-butyl alcohol; and ketones such as acetone. Classes; ethers such as dioxane and hydrocarbons; amides such as N, N-dimethylformamide; sulfoxides such as dimethylsulfoxide; etc., as hydrophobic organic solvents, n-pentane, n-heptane, n-hexane, etc. Alicyclic hydrocarbons such as n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane, decalin; halogenated hydrocarbons such as chlorbenzene, brombenzene, carbon tetrachloride, 1,2-dichloroethane, etc. Hydrogen; aromatic hydrocarbons such as benzene, toluene, xylene, etc. can be mentioned. Of these, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-hexane and cyclohexane are preferably used.

When the water-absorbent resin is hydrophilized with an organic solvent, it is preferable that the organic solvent is brought into contact with the water-absorbent resin in a heated state, so that the absorption ratio and the return amount under pressure may be further improved. The heating temperature is preferably equal to or lower than the boiling point of the organic solvent, and is generally about 40 to 120 ° C., although it depends on the type of the organic solvent used. The hydrophilization treatment is preferably performed in a state where the water-absorbent resin is dry. Note that if the hydrophilization treatment is performed while moistened with an organic solvent or water, the water absorption rate may decrease or the water absorption ratio may decrease sharply when the cross-linking treatment near the surface is performed. .. Therefore, it is preferable to filter and dry the water-absorbent resin obtained by the reverse-phase suspension polymerization to carry out the hydrophilization treatment in a state where the polymerization solvent and water are removed.

[3-8] Surface Cross-linking Step The water-absorbent resin (for example, water-absorbent resin powder) obtained through the drying step (and any subsequent step) is preferably surface-crosslinked with a surface cross-linking agent. This surface cross-linking is a process of providing a portion having a high cross-linking density on the surface layer of the water-absorbent resin (for example, the water-absorbent resin powder) (a portion several tens of μm from the surface of the water-absorbent resin (for example, the water-absorbent resin powder)). Various water absorption characteristics can be improved by performing surface cross-linking treatment. Here, by appropriately adjusting the cross-linking density, it is possible to obtain a particularly excellent absorption ratio under pressure.

In the present invention, surface cross-linking in a state of being dispersed in a hydrophobic organic solvent performed by conventional reverse-phase suspension polymerization and surface cross-linking to a dry powder generally performed in aqueous solution polymerization are performed. Including, known surface cross-linking techniques are appropriately applied, but in the present invention, surface cross-linking to a water-absorbent resin that has undergone a gel separation step and a drying step is preferable from the viewpoint of improving the absorption ratio under pressure.

The surface cross-linking agent used in this step is also referred to as a "post-crosslinking agent" in known techniques in order to distinguish it from the internal cross-linking agent used in the monomer aqueous solution preparation step.

In the present invention, the surface cross-linking step may be performed after the above-mentioned drying step or during the drying step. In a known surface cross-linking step, a surface cross-linking agent is generally mixed with a water-containing gel cross-linking polymer or a cross-linking polymer of a dried product thereof, and the mixture is heated to carry out a cross-linking reaction. The step may be separately provided after the above-mentioned drying step, or a surface cross-linking agent may be added in the drying step to simultaneously carry out the surface cross-linking reaction and drying. Further, when the water-absorbent resin is produced by the batch-type reverse phase suspension polymerization method, the solvent and the hydrogel polymer can be separated by performing distillation in the separation step after the polymerization reaction. By adding a surface cross-linking agent even in the middle of the separation step, a surface-crosslinked water-absorbent resin (for example, water-absorbent resin particles) can be obtained.

[3-9] Other Steps In the method for producing a water-absorbent resin according to the present invention, in addition to the above-mentioned steps, a cooling step, a pulverization step, a water-containing (rewetting) step, and addition of other additives are required. It can include a step, a classification step, a sizing step, and a fine powder reuse step. Further, a transportation step, a storage step, a packing step, a storage step, and the like may be further included.

(Cooling process)
In the optional cooling step, the particulate dry polymer obtained in the drying step is cooled by using a known cooling means to obtain a particulate dry polymer cooled to a desired temperature. be able to.

(Crushing process)
It is preferable to go through a pulverization step of pulverizing the particulate dry polymer obtained in the drying step (and any subsequent cooling step). By undergoing a pulverization step, a water-absorbent resin powder having a controlled particle size or particle size distribution is obtained.

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

(Rewetting process)
In this step, which is arbitrarily carried out, a polyvalent metal salt, a cationic polymer, a chelating agent, an inorganic reducing agent, and an α-hydroxycarboxylic acid are added to the water-absorbent resin (for example, water-absorbent resin particles) obtained in the surface cross-linking step. This is a step of adding at least one additive selected from the group consisting of compounds.

The additive is preferably added to the water-absorbent resin (for example, water-absorbent resin particles) as an aqueous solution or a dispersion (slurry). The additive may be added and mixed at the same time as the above-mentioned surface cross-linking agent solution.

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 present invention, additives other than the above-mentioned additives can be added in order to add various functions to the water-absorbent resin. Specific examples of the additive include surfactants, compounds having a phosphorus atom, oxidizing agents, organic reducing agents, water-insoluble inorganic fine particles, organic powders such as metal soaps, deodorants, antibacterial agents, pulps and thermoplastics. Examples include fibers. As the water-insoluble inorganic fine particles, the compound disclosed in "[5] Water-insoluble inorganic fine particles" of International Patent Publication No. 2011/040530 is applied to the present invention. Of these additives, water-insoluble inorganic fine particles are preferable, and in particular, by adding hydrophilic fine particles such as silica (silicon dioxide), the water-absorbent resin (for example, water-absorbent resin particles) becomes more compatible with the liquid and has absorbability. It is preferable because it can absorb an aqueous liquid in a short time when used for an article.

(Granulation process)
The "grain sizing step" means a step of loosening the loosely aggregated water-absorbent resin through the surface cross-linking step to adjust the particle size. In addition, this granulation step includes a fine powder removal step and a classification step after the surface cross-linking step. The sizing step is preferably carried out from the viewpoint of adjusting the particle size of the water-absorbent resin and obtaining stable water-absorbing physical characteristics.

(Fine powder reuse process)
The “fine powder reuse step” means a step of supplying the fine powder generated by sieving classification or the like in each of the above steps as it is or after granulating the fine powder to any of the steps. The fine powder reuse step is preferably carried out from the viewpoint of reducing the production loss of the water-absorbent resin.

[4] Uses of the water-absorbent resin The use of the water-absorbent resin of the present invention is not particularly limited, but is preferably used as an absorber for absorbent articles such as paper diapers (for infants and adults), menstrual napkins, and incontinence pads. Can be mentioned. In particular, it can be used as an absorber for thin high-concentration paper diapers, which has had a problem of the amount of liquid returning shortly after absorbing liquid. Examples of other absorbent articles include, for example, soil water retention agents, seedling raising sheets, seed coating materials, dew condensation prevention sheets, drip absorbers, freshness-preserving materials, disposable cairo, cooling bandanas, ice packs, medical waste liquid solidification. Agents, residual soil solidifying materials, water damage prevention waste liquid gelling agents, water-absorbing soil sac, disaster simple toilets, wet cloth materials, cosmetic thickeners, water blocking materials for electrical and electronic communication cables, gasket packing, sustained-release agents for fertilizers , Various sustained-release agents (space disinfectants, fragrances, etc.), pet sheets, cat sand, wound protection dressing materials, dew condensation prevention building materials, oil moisture removers, paints, adhesives, anti-blocking agents, light Examples thereof include diffusing agents, matting agents, additives for decorative boards, additives for artificial marble, additives for resins such as additives for toner, and the like.

Further, as a raw material for the absorber, an absorbent material such as pulp fiber can be used together with the water-absorbent resin. In this case, the content (core concentration) of the water-absorbent resin in the absorber is preferably 30% by mass to 100% by mass, more preferably 40% by mass to 100% by mass, and further preferably 50% by mass to 100% by mass. %, More preferably 60% by mass to 100% by mass, particularly preferably 70% by mass to 100% by mass, and most preferably 75% by mass to 95% by mass.

By setting the core concentration in the above range, when the absorber is used for the upper layer portion of the absorbent article, the absorbent article can be kept in a clean white state. Further, since the absorber is excellent in diffusivity of body fluids such as urine and blood, efficient liquid distribution is expected to improve the absorption amount.

More specifically, the above-mentioned absorber contains the water-absorbent resin according to the embodiment of the present invention and (i) does not contain hydrophilic fibers, or (ii) the mass of the water-absorbent resin is It may be an absorber having an amount of 50% by mass or more of the total mass of the water-absorbent resin and the hydrophilic fiber.

The hydrophilic fiber is not particularly limited, and examples thereof include pulp fiber, cotton linter crosslinked cellulose fiber, rayon, cotton, wool, acetate, vinylon and the like.

When the mass of the water-absorbent resin is 50% by mass or more of the total mass of the water-absorbent resin and the hydrophilic fiber, more specifically, the mass of the water-absorbent resin is the total mass of the water-absorbent resin and the hydrophilic fiber. It may be 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 100% by mass.

The present invention will be described in more detail with reference to Examples and Comparative Examples shown below, but the present invention is not limited thereto, and can be obtained by appropriately combining the technical means described in each Example. Examples are also included in the scope of the present invention. In addition, although the display of "parts" or "%" may be used in the examples, it represents "parts by mass" or "% by mass" unless otherwise specified. Unless otherwise specified, each operation is performed at room temperature (25 ° C.). As the electrical equipment (including the measurement of the physical properties of the water-absorbent resin) used in Examples and Comparative Examples, a power supply of 60 Hz at 200 V or 100 V was used unless otherwise specified.

<Evaluation method>
[Number average particle size]
Scanning electron micrographs (SEMs) of water-absorbent resin or water-absorbent resin powder were taken. Fifty primary particles in front of the aggregated particles were randomly selected from the photographs, and the major and minor diameters of each particle were measured and averaged to obtain the primary particle diameter. The average value of the primary particle diameters of each particle was calculated, and the average value was taken as the number average particle diameter of the water-absorbent resin.

[Moisture content]
The water content was measured according to the EDANA method (ERT430.2-02). In the measurement, the mass of the sample (water-absorbent resin) 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 pouring 1.0 g of water-absorbent resin or water-absorbent resin powder into an aluminum cup having a bottom surface diameter of 50 mm, the total mass W1 (g) of the sample (water-absorbent resin or water-absorbent resin powder) and the aluminum cup. ) Was accurately weighed. Next, the sample was placed in an aluminum cup and allowed to stand in an oven set to an atmospheric temperature of 180 ° C. After 3 hours, the sample was taken out of the oven together with the aluminum cup, and the total mass W2 (g) of the dried sample and the aluminum cup was accurately weighed. When the mass of the sample (water-absorbent resin) used in this measurement was M (1.0 g), the water content α (mass%) of the sample was determined according to the following (Equation 1):
Moisture content α (mass%) = {(W1-W2) / M} × 100 formula (1).

[Mass average particle diameter (D50)]
The mass average particle size (D50) of the water-absorbent resin is determined by “(3) Mass-Average Particle Diameter (D50) and Logimetric Standard Measurement (σζ)” described in columns 27 and 28 of US Pat. No. 7,638,570. It was measured according to the method described in.

[CRC]
The CRC (centrifuge holding capacity) of the water-absorbent resin was measured according to the EDANA method (ERT441.2-02). Specifically, 0.2 g of the water-absorbent resin is placed in a bag made of a non-woven fabric, and then immersed in a large excess of 0.9 mass% sodium chloride aqueous solution for 30 minutes for free swelling, and then a centrifuge (250 G). ) For 3 minutes, the water absorption ratio (unit: g / g) after draining was determined.

[AAP]
The AAP (absorption ratio under pressure) of the water-absorbent resin was measured according to the EDANA method (ERT442.2.2). The load condition was changed to 4.83 kPa (0.7 psi).

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

[Vertical liquid diffusion length, horizontal liquid diffusion length]
The liquid diffusion length of the water-absorbent resin was measured by the procedure shown below.

FIG. 1 is a perspective view showing a container used for measuring the liquid diffusion length. Gently pour the water-absorbent resin into a transparent container 2 made of acrylic resin (inner dimensions are vertical (height) 40 mm, horizontal (width) 72 mm, depth 3 mm) to a position of 30 mm in height, and then absorb water. The resin was filled by leveling it horizontally.

Using a syringe (syringe needle gauge; 21G), 1 mL of a 0.9% by mass sodium chloride aqueous solution colored in blue at 20 ° C. was placed in the center of the upper surface (3 mm × 72 mm surface) of the water-absorbent resin filled. The injection was gently performed over 5 seconds from the arrow A) in FIG. After 1 minute, the vertical and horizontal diffusion lengths [mm] of the 0.9 mass% sodium chloride aqueous solution colored in blue at 20 ° C. were measured.

FIG. 2 is a schematic view for explaining the measurement of the liquid diffusion length, and is a view of the container 2 filled with the water-absorbent resin 1 as viewed from the side. In the colored region 3 colored by the aqueous sodium chloride solution (including the swollen portion when the water-absorbent resin 1 swells upward due to liquid injection), the longest diffusion distance in the vertical and horizontal directions is set in the vertical direction. The liquid diffusion length D and the horizontal liquid diffusion length L were set.

〔The bulk density〕
The bulk density of the water-absorbent resin of the present invention was measured according to the EDANA method (T460.2-02).

〔surface tension〕
The surface tension of the present invention was measured by the method described in WO2015 / 129917.

[Amount of liquid returned from the absorbent sheet]
(1) Preparation of Absorption Sheet FIG. 3 is a view of the absorption sheet 4 viewed from directly above, and FIG. 4 is a cross-sectional view of the absorption sheet 4. 1.0 g of water-absorbent resin 1 is sprayed on the adhesive surface of the adhesive tape 6 (NITTO DENKO vinyl tape No. 21S) cut into 10 cm x 10 cm by 1 cm from each end of each side, and a non-woven fabric of 10 cm x 10 cm. 5 (air-laid non-woven fabric, grain amount 46.6 g / m ² ) was placed and bonded to the adhesive surfaces on all four sides to prepare an evaluation absorbent sheet 4.

(2) Measurement of return amount The evaluation absorption sheet 4 is placed on a flat table, and a 0.9% by mass sodium chloride aqueous solution at 20 ° C. is placed 1 cm above the center of the evaluation absorption sheet 4 in the vertical direction. 10 mL was added over 20 seconds. Thirty seconds after the start of addition of 10 mL of the sodium chloride aqueous solution, 10 sheets of filter paper (ADVANTEC2 Φ55 mm) and a weight of 1.0 kg were placed on the center of the evaluation absorption sheet. After 10 seconds, the filter paper and the weight were removed, and the weight of the filter paper was measured, and the amount of increase in the weight of the filter paper (the weight of the liquid absorbed by the filter paper) was taken as the amount of liquid return.

[Example 1]
Take 800 g of cyclohexane in a 2000 ml four-neck separable flask equipped with a stirrer, reflux condenser, thermometer, nitrogen gas introduction tube and dropping funnel, and use sucrose fatty acid ester (DK ester (registered trademark) F-50) as a dispersant. / HLB = 6) manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd. was added and dissolved, and nitrogen gas was blown to expel the dissolved oxygen.

Separately, a monomer aqueous solution (1) consisting of 141 g of sodium acrylate, 36 g of acrylic acid, 0.022 g of N, N'-methylenebisacrylamide, and 327.65 g of ion-exchanged water was prepared in a flask. Nitrogen gas was blown in to expel the dissolved oxygen dissolved in the aqueous solution. Next, 0.95 g of a 15% aqueous solution of sodium persulfate was added to the monomer aqueous solution in the flask, and then the whole amount was added to the separable flask and dispersed by stirring at 415 rpm. Then, the bath temperature was raised to 60 ° C. to start the polymerization reaction, the bath temperature was maintained at 60 ° C. for 2 hours, the polymerization was stopped, filtration was performed by suction filtration, and the residue was air-dried overnight to contain water. A gel polymer (1) was obtained. The average primary particle size of the hydrogel polymer (1) was 100 μm.

Next, the hydrous gel polymer (1) (gel temperature: 90 ° C.) is charged into a gel sizing device having a screw and a porous plate having a pore size of 0.8 mm, and the sizing gel (gel temperature: 90 ° C.) is discharged from the gel sizing device. 1) was obtained.

The obtained sizing gel (1) was dried using a hot air dryer to obtain a dry polymer (1). The drying was carried out by spreading the sizing gel (1) on a stainless steel wire mesh having an opening of 850 μm and allowing hot air at 105 ° C. to be aerated for 40 minutes.

The obtained dry polymer (1) is added to methanol heated to 60 ° C. (10 times the weight of the dry polymer (1)), stirred for 1 hour, then filtered and dried for hydrophilization treatment. Was done.

Subsequently, the hydrophilized dry polymer (1) was supplied to a roll mill (crusher) and pulverized to adjust the particle size to obtain a water-absorbent resin powder (1). The average particle size of the water-absorbent resin powder (1) was 70 μm.

A surface cross-linking agent solution consisting of 0.1 part by mass of ethylene glycol diglycidyl ether, 1.0 part by mass of isopropyl alcohol and 3.0 parts by mass of ion-exchanged water is sprayed on 100 parts by mass of the water-absorbent resin powder (1). It was sprayed and mixed uniformly using a high speed continuous mixer.

The obtained mixture was introduced into a heat treatment machine whose atmospheric temperature was adjusted to 100 ° C., heat-treated for 40 minutes, and then the powder temperature was forcibly cooled to 60 ° C. to obtain surface-crosslinked water-absorbent resin powder. (1) was obtained.

5.0 parts by mass of water is uniformly mixed with 100 parts by mass of the surface-crosslinked water-absorbent resin powder (1), and the particles are sized by passing through a JIS standard sieve having an opening of 300 μm to prepare the water-absorbent resin. Particles (1) were obtained.

To 100 parts by mass of the obtained water-absorbent resin particles (1), 0.3 parts by mass of fine particle silicon dioxide (trade name: Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) is added and mixed to absorb water. Resin (1) was obtained. Table 1 shows the physical characteristics of the obtained water-absorbent resin (1).

[Example 2]
Take 800 g of n-heptane in a 2000 mL four-neck separable flask equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas introduction tube and a dropping funnel, and use a maleic anhydride-modified ethylene-propylene copolymer as a dispersant. Product name: High wax (registered trademark) HW2203A / manufactured by Mitsui Kagaku Co., Ltd. 0.88 g was added and dissolved, and nitrogen gas was blown to expel the dissolved oxygen.

Separately, in a flask, a single amount consisting of 127 g of sodium acrylate, 36 g of acrylic acid, 0.077 g of polyethylene glycol diacrylate (n = 9), 0.008 g of diethylenetriamine-5 acetate / 3 sodium, and 215 g of ion-exchanged water. A body aqueous solution (2) was prepared, and nitrogen gas was blown into it to expel the dissolved oxygen dissolved in the aqueous solution. Then, 1.5 g of a 15% aqueous solution of sodium persulfate was added to the monomer aqueous solution in the flask, and then the whole amount was added to the separable flask and dispersed by stirring at 230 rpm. Then, the bath temperature was raised to 60 ° C. to start the polymerization reaction, the bath temperature was maintained at 60 ° C. for 2 hours, the polymerization was stopped, filtration was performed by suction filtration, and the residue was air-dried overnight to contain water. A gel polymer (2) was obtained. The average primary particle size of the hydrogel polymer (2) was 55 μm.

Next, the hydrous gel polymer (2) (gel temperature: 90 ° C.) is charged into a gel sizing device having a screw and a porous plate having a pore size of 0.8 mm, and the sizing gel (gel temperature: 90 ° C.) is discharged from the gel sizing device. 2) was obtained.

The obtained sizing gel (2) was dried using a hot air dryer to obtain a dried polymer (2). The drying was carried out by spreading the sizing gel (2) on a stainless steel wire mesh having an opening of 850 μm and allowing hot air at 105 ° C. to be aerated for 40 minutes.

The obtained dry polymer (2) is added to methanol heated to 60 ° C. (10 times the weight of the dry polymer (2)), stirred for 1 hour, then filtered and dried for hydrophilization treatment. Was done.

Subsequently, the hydrophilized dry polymer (2) is supplied to a roll mill (crusher) and pulverized to adjust the particle size, and further classified using a sieve having an opening particle diameter of 150 μm to obtain a water-absorbent resin powder. (2) was obtained. The average particle size of the water-absorbent resin powder (2) was 310 μm.

From 0.003 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 2.6 parts by mass of ion-exchanged water with respect to 100 parts by mass of the water-absorbent resin powder (2). The surface cross-linking agent solution was sprayed and mixed uniformly using a high-speed continuous mixer.

The obtained mixture was introduced into a heat treatment machine whose atmospheric temperature was adjusted to 195 ° C. ± 2 ° C., heat-treated for 25 minutes, and then the powder temperature was forcibly cooled to 60 ° C. to absorb water crosslinked. A sex resin powder (2) was obtained.

10.0 parts by mass of water is uniformly mixed with 100 parts by mass of the surface-crosslinked water-absorbent resin powder (2), and the particles are sized by passing through a JIS standard sieve having an opening of 1000 μm to prepare the water-absorbent resin. Particle (2) was obtained.

In 100 parts by mass of the obtained water-absorbent resin particles (2), hydrotalcite (product name: DHT-6, manufactured by Kyowa Chemical Industry Co., Ltd., Mg ₆ Al ₂ (OH) ₁₆ CO _{3.4H 2} O) 0. 3 parts by mass was added and mixed to obtain a water-absorbent resin (2). Table 1 shows the physical characteristics of the obtained water-absorbent resin (2).

[Example 3]
In Example 2, for 100 parts by mass of the water-absorbent resin particles (2), 0.3 mass of fine particle silicon dioxide (trade name: Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) instead of hydrotalcite. A water-absorbent resin (3) was obtained in the same manner as in Example 2 except that the parts were added and mixed. Table 1 shows the physical characteristics of the obtained water-absorbent resin (3).

[Example 4]
Take 800 g of n-heptane in a 2000 mL four-neck separable flask equipped with a stirrer, reflux condenser, thermometer, nitrogen gas introduction tube and dropping funnel, and use sucrose fatty acid ester (S-370 / Mitsubishi Chemical) as a dispersant. HLB = 3) manufactured by Co., Ltd. was added and dissolved, and nitrogen gas was blown to expel the dissolved oxygen.

Separately, a monomer aqueous solution (3) consisting of 141 g of sodium acrylate, 36 g of acrylic acid, 0.209 g of polyethylene glycol diacrylate (n = 9), and 214 g of ion-exchanged water was prepared in a flask, and nitrogen was prepared. Gas was blown in to expel the dissolved oxygen dissolved in the aqueous solution. Next, 2.0 g of a 10% aqueous solution of potassium persulfate was added to the monomer solution in the flask, and then the entire amount was added to the separable flask and dispersed by stirring at 230 rpm. Then, the bath temperature was raised to 85 ° C. to start the polymerization reaction, the bath temperature was maintained at 85 ° C. for 2 hours, the polymerization was stopped, filtration was performed by suction filtration, and the residue was air-dried overnight to contain water. A gel polymer (3) was obtained. The average primary particle size of the hydrogel polymer (3) was 200 μm.

Next, the hydrous gel polymer (3) (gel temperature: 90 ° C.) is charged into a gel sizing device having a screw and a porous plate having a pore size of 0.8 mm, and the sizing gel (gel temperature: 90 ° C.) is discharged from the gel sizing device. 3) was obtained.

The obtained sizing gel (3) was dried using a hot air dryer to obtain a dried polymer (3). The drying was carried out by spreading the sizing gel (3) on a stainless steel wire mesh having an opening of 850 μm and allowing hot air at 180 ° C. to be aerated for 50 minutes.

200 g of the obtained dried polymer (3) was added to 10 L of methanol heated to 60 ° C., stirred for 1 hour, filtered and dried to perform hydrophilization treatment.

Subsequently, the hydrophilized dry polymer (3) is supplied to a roll mill (crusher) and pulverized to adjust the particle size, and further classified using a sieve having an opening particle diameter of 150 μm to obtain a water-absorbent resin powder. (3) was obtained. The average particle size of the water-absorbent resin powder (3) was 350 μm.

A surface cross-linking agent solution consisting of 0.015 parts by mass of ethylene glycol diglycidyl ether, 1.0 part by mass of propylene glycol and 3.0 parts by mass of ion-exchanged water is sprayed on 100 parts by mass of the water-absorbent resin powder (3). It was sprayed and mixed uniformly using a high speed continuous mixer.

The obtained mixture was introduced into a heat treatment machine whose atmospheric temperature was adjusted to 195 ° C. ± 2 ° C., heat-treated for 40 minutes, and then the powder temperature was forcibly cooled to 60 ° C. to absorb water crosslinked. A sex resin powder (3) was obtained.

10.0 parts by mass of water is uniformly mixed with 100 parts by mass of the surface-crosslinked water-absorbent resin powder (3), and the particles are sized by passing through a JIS standard sieve having an opening of 1000 μm to prepare the water-absorbent resin. Particle (3) was obtained.

To 100 parts by mass of the obtained water-absorbent resin particles (3), 0.1 part by mass of fine particle silicon dioxide (trade name: Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) is added and mixed to absorb water. Resin (4) was obtained. Table 1 shows the physical characteristics of the obtained water-absorbent resin (4).

[Comparative Example 1]
A comparative water-absorbent resin (1) was obtained in the same manner as in Example 1 except that the hydrophilization treatment step was not carried out and the fine particle silicon dioxide was not used. Table 1 shows the physical characteristics of the obtained comparative water-absorbent resin (1).

[Comparative Example 2]
Take 500 g of n-heptane in a 2000 mL four-neck separable flask equipped with a stirrer, reflux condenser, thermometer, nitrogen gas introduction tube and dropping funnel, and 0.92 g of sucrose fatty acid ester S-370 as a dispersant. In addition, the temperature was raised to dissolve the dispersant, and then the mixture was cooled to 55 ° C.

Separately from the above, 92 g of an 80 wt% acrylic acid aqueous solution was added to a 500 mL Erlenmeyer flask. While cooling from the outside, 102.2 g of a 30 wt% sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol% of acrylic acid to prepare a partially neutralized aqueous solution of acrylic acid. Further, 50.2 g of water, 0.11 g of potassium persulfate as a polymerization initiator, and 18.4 mg of ethylene glycol diglycidyl ether as a cross-linking agent were added to prepare a monomer aqueous solution for polymerization.

The entire amount of this aqueous monomer solution for polymerization was added to the four-necked separable flask under stirring to disperse the mixture, and after the system was sufficiently replaced with nitrogen, the temperature was raised and the bath temperature was maintained at 70 ° C. Then, the polymerization reaction was carried out for 1 hour. To the water-containing gel after completion of the polymerization, 0.66 g of a 14 mass% trans-1,2-diaminocyclohexane 4-sodium acetate aqueous solution as an aminocarboxylic acid-based metal chelating agent was added under stirring. Then, water was removed from the hydrogel-like substance by azeotropic dehydration. 4.14 g of a 2% by mass ethylene glycol diglycidyl ether aqueous solution was added to the obtained gel, and water and n-heptane were further removed by distillation and dried to obtain a comparative water-absorbent resin (2). Table 1 shows the physical characteristics of the obtained comparative water-absorbent resin (2).

[Comparative Example 3]
A water-absorbent resin was obtained by aqueous solution polymerization. That is, 67.0 parts of a 37% sodium acrylate aqueous solution, 10.2 parts of acrylic acid, 0.079 parts of polyethylene glycol diacrylate (average number of ethylene oxide units 8) and 22.0 parts of ion-exchanged water are mixed to prepare a monomer aqueous solution. Prepared. Nitrogen was blown into the monomer aqueous solution in a vat to reduce the dissolved oxygen in the solution to 0.1 ppm or less.

Subsequently, the temperature of the monomer aqueous solution was adjusted to 18 ° C. under a nitrogen atmosphere, and then 0.16 parts of a 5 mass% sodium persulfate aqueous solution and 0.16 parts of a 5% 2,2'-azobis (2-amidinopropane) dihydrochloride aqueous solution 0.16. , 0.15 part of 0.5% L-ascorbic acid aqueous solution and 0.17 part of 0.35% hydrogen peroxide aqueous solution were sequentially added dropwise under stirring.

Polymerization started immediately after dropping hydrogen peroxide. After that, stirring was stopped, and after 10 minutes, the temperature of the monomer reached the peak temperature. The peak temperature was 85 ° C. Subsequently, the vat was subsequently immersed in a hot water bath at 80 ° C. and aged for 10 minutes to obtain a comparative hydrogel polymer (3).

The obtained comparative hydrogel polymer (3) was crushed with a meat chopper and then dried using a hot air dryer to obtain a comparative dry polymer (3). The drying was carried out by spreading the comparative hydrogel polymer (3) crushed by a meat chopper on a stainless steel wire mesh having an opening of 850 μm and allowing hot air at 180 ° C. to aerate for 30 minutes.

The comparative dry polymer (3) is supplied to a roll mill (crusher) to adjust the particle size, further classified into those that pass through a sieve having a mesh size of 500 μm and remain on a sieve of 105 μm, and comparatively absorb water. A sex resin powder (3) was obtained. The average particle size of the comparative water-absorbent resin powder (3) was 370 μm.

Comparative water-absorbent resin powder (3) A surface cross-linking agent solution consisting of 0.05 parts by mass of ethylene glycol diglycidyl ether, 1 part by mass of propylene glycol, 3 parts by mass of ion-exchanged water and 1 part by mass of isopropyl alcohol is sprayed on 100 parts by mass. It was sprayed and mixed uniformly using a high speed continuous mixer. The obtained mixture was introduced into a heat treatment machine whose atmospheric temperature was adjusted to 180 ° C ± 2 ° C, heat-treated for 40 minutes, and then surface-crosslinked by forcibly cooling the powder temperature to 60 ° C. A water-absorbent resin powder (3) was obtained.

A solution consisting of 45% by mass of diethylenetriamine 5 acetate 3 sodium salt 0.022 part by mass and ion exchanged water 1 part by mass was uniformly mixed with 100 parts by mass of the surface-crosslinked comparative water-absorbent resin powder (3) for comparative water absorption. Sexual resin particles (3) were obtained.

0.5 parts by mass of fine particle silicon dioxide (trade name: Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) is added and mixed with 100 parts by mass of the obtained comparative water-absorbent resin particles (3) for comparison. A water-absorbent resin (3) was obtained. Table 1 shows the physical characteristics of the obtained comparative water-absorbent resin (3).

The water-absorbent resin composition according to the embodiment of the present invention includes disposable diapers, menstrual napkins, incontinence products for adults (incontinence pads), sanitary materials such as pet sheets (sanitary products), and soil water retention for agriculture and gardening. It can be suitably used in various water-absorbent articles such as agents and industrial water-stopping agents.

1 Water-absorbent resin 2 Container 3 Colored area

Claims (7)

Particulate poly (meth) acrylic acid (salt) -based water-absorbent resin having the following physical property values (1) to (3):
(1) The vertical liquid diffusion length (D) is 15 mm or more (2) The ratio (L / D) of the horizontal liquid diffusion length (L) to the vertical liquid diffusion length is 0.5 to 1.5.
(3) The bulk density specified by EDANA ERT460.2-02 is 0.68 g / mL or more.
(Here, the liquid diffusion length (D) in the vertical direction and the liquid diffusion length (L) in the horizontal direction are transparent containers made of acrylic resin (inner dimensions are 40 mm in the vertical direction (height) and 72 mm in the horizontal direction (width)). , Depth 3 mm) is filled with a water-absorbent resin to a height of 30 mm, and 1 mL of a colored 0.9 mass% sodium chloride aqueous solution at 20 ° C. is injected from the water-absorbent resin surface, and 1 minute later, vertically and horizontally. Obtained by measuring the diffusion length in the direction) The water-absorbent resin according to claim 1, which is an aggregate-like particle of spherical particles. The water-absorbent resin according to claim 1 or 2, which comprises water-insoluble inorganic fine particles. The water-absorbent resin according to any one of 1 to 3, wherein the mass average particle diameter (D50) is 200 to 700 μm. The water-absorbent resin according to any one of claims 1 to 4, wherein the AAP is 18 g / g or more. The water-absorbent resin according to any one of claims 1 to 5, wherein the bulk density defined by EDANA ERT460.2-02 is 0.70 g / mL or more. The water-absorbent resin according to any one of claims 1 to 6 is contained, and (i) no hydrophilic fiber is contained, or (ii) the mass of the water-absorbent resin is hydrophilic with the water-absorbent resin. An absorber that is 50% by mass or more of the total mass with the sex fiber.

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