Water absorbent composition and method for producing same

Abstract

The purpose of the present invention is to provide a method for producing a water absorbent composition which, even in cases where a water absorbent resin having a high specific surface area is subjected to surface crosslinking, maintains the surface tension of a liquid to be absorbed, has a high water absorption characteristic under pressure (for example, a high absorption against pressure (AAP) and a high interstitial water ratio under pressure), and does not inhibit the improvement of the absorption rate. The preset invention provides a method for producing a water absorbent composition that contains a water absorbent resin as a main component, the method including a step for surface crosslinking of the water absorbent resin. This method for producing a water absorbent composition is characterized in that: (1) the specific surface area of the water absorbent resin before surface crosslinking is 25 m2 / kg or more; (2) a specific peroxide and a specific organic surface crosslinking agent are used; and (3) the water vapor density in a heat treatment step is appropriately controlled.

Description

Water-absorbent composition and method for producing the same

[0001] The present invention relates to a water-absorbing agent composition containing a water-absorbing resin as a main component, and a method for producing the same.

[0002] Water-absorbent agent compositions using water-absorbent resins for the purpose of absorbing body fluids are widely used in sanitary materials such as disposable diapers, sanitary napkins, and so-called incontinence pads. Crosslinked products of partially neutralized polyacrylic acid are known as such water-absorbent resins. In recent years, as sanitary materials have become thinner, the water-absorbent agent compositions are increasingly required to have not only excellent water-absorption properties such as absorbency under no load and absorbency under load, but also a high water-absorption rate. To increase the water-absorption rate, it is necessary to increase the contact area between the liquid to be absorbed and the water-absorbent agent composition. Therefore, water-absorbent resins with high specific surface areas have been disclosed (Patent Documents 1 to 7). Furthermore, water-absorbent resins with high specific surface areas for blood absorption have been disclosed (Patent Document 8).

[0003] It is known that increasing the specific surface area improves the absorption rate of a water-absorbent resin, but on the other hand, the gel strength decreases, and therefore the water absorption properties under pressure (e.g., absorbency under pressure (AAP)) decrease, and therefore the absorption rate and the water absorption properties under pressure are in a trade-off relationship.

[0004] On the other hand, in order to increase the absorption capacity under load (AAP), it is also known to perform surface crosslinking in which a surface crosslinking agent capable of reacting with a functional group possessed by a water absorbent resin is reacted in the vicinity of the surface of the water absorbent resin to increase the crosslinking density in the vicinity of the surface of the water absorbent resin from that in the interior thereof, and various surface crosslinking techniques have also been proposed (for example, Patent Document 9).

[0005] International Publication No. WO 97 / 03114 JP 10-057805 A European Patent Application Publication No. 0872491 European Patent Application Publication No. 0937739 International Publication No. WO 99 / 03577 International Publication No. WO 2013 / 018571 International Publication No. WO 2016 / 111223 International Publication No. WO 02 / 085959 Japanese Patent No. 5128098 International Publication No. WO 2013 / 072268

[0006] As described above, various surface cross-linking techniques have been proposed to increase water absorption properties under pressure (for example, absorbency under pressure (AAP)).

[0007] On the other hand, the present inventors have noticed, while studying the absorption behavior of a water-absorbing agent composition or a water-absorbent resin having an increased specific surface area, that the absorption rate of a surface-crosslinked water-absorbent resin does not become sufficiently high as designed, even if the specific surface area is increased.

[0008] In order to increase the absorption rate, the improvement of which has been inhibited as described above, to a desired level, a treatment for further increasing the specific surface area is carried out. However, the treatment for excessively increasing the specific surface area invites a decrease in physical properties such as absorbency under pressure, which is in a trade-off relationship, and as a result, the water absorbent resin cannot obtain desired absorption characteristics, and there is a risk of problems occurring such that the performance of the sanitary material also decreases.

[0009] On the other hand, methods of hydrophilizing a water-absorbent resin without increasing the specific surface area excessively are also conceivable. As such a method, a method of disposing a hydrophilic surfactant in a water-absorbent resin is known (e.g., Patent Document 10). However, the addition of the surfactant reduces the surface tension of the absorbed liquid, which increases the amount of liquid return when the resin is used in sanitary materials such as disposable diapers and incontinence pads.

[0010] That is, an object of the present invention is to provide a method for producing a water-absorbent composition which maintains the surface tension of the absorbed liquid, has high water-absorbing properties under pressure (e.g., absorption capacity under pressure (AAP) and crevice water capacity under pressure), and does not inhibit improvement of the absorption rate, even when a water-absorbent resin having a high specific surface area is surface-crosslinked. Another object of the present invention is to provide an absorbent composition which maintains the surface tension of the absorbed liquid and has high water-absorbing properties under pressure (e.g., absorption capacity under pressure (AAP) and crevice water capacity under pressure), despite having a high specific surface area.

[0011] The present invention, which has solved the above problems, has the following configuration.

[0012] That is, one aspect of the present invention is: [1] A method for producing a water-absorbing agent composition containing a surface-crosslinked water-absorbent resin as a main component, the method including a surface-crosslinking step of the water-absorbent resin, the method for producing a water-absorbent agent composition satisfying all of the following (1) to (3): (1) The specific surface area of ​​the water-absorbent resin before surface-crosslinking is 25 m ２ / kg or more (2) The surface cross-linking step is carried out in the presence of a peroxide and an organic surface cross-linking agent capable of reacting with a carboxyl group. (3) The surface cross-linking step has a heat treatment step of carrying out a heat treatment in an atmosphere having a water vapor density of more than 0.005 g / L simultaneously with or after the addition of the organic surface cross-linking agent to the water absorbent resin. [2] In the method for producing a water absorbent agent composition according to the above [1], it is preferable that the water vapor density is 0.6 g / L or less; [3] In the method for producing a water absorbent agent composition according to the above [1] or [2], it is preferable that the shape of the water absorbent resin before surface cross-linking is an irregular crushed shape; [4] In the method for producing a water absorbent agent composition according to any one of the above [1] to [3], it is preferable that D50 (mass average particle size) of the water absorbent resin before surface cross-linking is 250 μm or more and less than 550 μm, and that a mass proportion of particles having a particle size of less than 150 μm, which are contained in the water absorbent resin before surface cross-linking, is 3 mass% or less; [5] In the method for producing a water-absorbing agent composition according to any one of the above [1] to [4], it is preferable that the method further comprises a step of polymerizing an aqueous solution of an unsaturated monomer, and that the water-absorbing resin is obtained by foaming polymerization of the aqueous solution of the unsaturated monomer; [6] In the method for producing a water-absorbing agent composition according to any one of the above [1] to [5], it is preferable that the organic surface cross-linking agent is added in a solution state to the water-absorbing resin, and that a concentration of the organic surface cross-linking agent in the surface cross-linking agent solution is 0.1 mass % or more and 60 mass % or less; [7] In the method for producing a water-absorbing agent composition according to any one of the above [1] to [6], it is preferable that the organic surface cross-linking agent is added to the water-absorbing resin from two or more addition nozzles installed in a mixer; [8] In the method for producing a water-absorbing agent composition according to any one of the above [1] to [7], it is preferable that the surface cross-linking step is performed at a differential pressure of −10 kPa or more and 0 kPa or less (slightly reduced pressure) relative to atmospheric pressure; [9] In the method for producing a water-absorbing agent composition according to any one of the above [1] to [8], the peroxide is preferably a persulfate.

[0013] Another aspect of the present invention is

[10] a water-absorbing agent composition containing a surface-crosslinked water-absorbing resin as a main component, which satisfies all of the following (1) and (2): (1) the specific surface area of ​​the water-absorbing agent composition is 25 m２ / kg or more (2) The water-absorbing agent composition has a Washburn contact angle θ2 of 72° or less when measured with a 20% by mass aqueous solution of sodium chloride.

[0014]

[11] In the water-absorbent agent composition according to the above-mentioned

[10] , it is preferable that the shape of the surface-crosslinked water-absorbent resin is an irregularly crushed shape, and the mass ratio of particles having a D50 (mass average particle size) of 250 μm or more and less than 550 μm and less than 150 μm is 3 mass% or less;

[12] In the water-absorbent agent composition according to the above-mentioned

[10] or

[11] , it is preferable that the water-absorbent agent composition has a moisture content of more than 0 mass% and 5 mass% or less;

[13] In the water-absorbent agent composition according to any one of the above-mentioned

[10] to

[12] , it is preferable that the water-absorbent agent composition has an absorption capacity against load (AAP) under a load of 4.83 kPa of 20.0 g / g or more;

[14] In the water-absorbent agent composition according to any one of the above-mentioned

[10] to

[13] , it is preferable that the water-absorbent agent composition has a crevice water capacity against load of 9.0 g / g or more;

[15] In the water-absorbing agent composition according to any one of the above

[10] to

[14] , it is preferable that the surface tension of the water-absorbing agent composition is 56 mN / m or more;

[16] In the water-absorbing agent composition according to any one of the above

[10] to

[15] , it is preferable that the Vortex of the water-absorbing agent composition is 50 seconds or less;

[17] In the water-absorbing agent composition according to any one of the above

[10] to

[16] , it is preferable that the water-absorbing agent composition further contains a peroxide decomposition product, and that the concentration of the peroxide decomposition product is higher on the surface of the water-absorbing agent composition than inside the water-absorbing agent composition;

[18] In the water-absorbing agent composition according to any one of the above

[10] to

[17] , when particles having a particle size of 300 μm or more are classified into particle group a and particles having a particle size of less than 300 μm are classified into particle group b by sieving using a JIS standard sieve with a mesh size of 300 μm, and when the amount of peroxide decomposition products present in particle group a after an impact test using a paint shaker is designated as C1 and the amount of peroxide decomposition products present in particle group b is designated as C2, it is preferable that C2-C1 is more than 0 mass % and 1 mass % or less;

[19] In the water-absorbing agent composition according to any one of the above

[10] to

[18] , it is preferable that the SFC (saline flow conductivity) of the water-absorbing agent composition is 1×10 －７ cm ３sec / g or more is preferable.

[0015] Another aspect of the present invention is

[20] an absorbent article comprising the water-absorbent agent composition according to any one of the above

[10] to

[19] .

[0016] One aspect of the present invention is a method for producing a water-absorbing agent composition containing a surface-crosslinked water-absorbent resin as a main component, the method including a surface-crosslinking step of the water-absorbent resin, wherein the method for producing the water-absorbent agent composition satisfies all of the following (1) to (3):

[0017] (1) The specific surface area of ​​the water-absorbent resin before surface cross-linking is 25 m ２ / kg or more (2) The surface cross-linking step is carried out in the presence of a peroxide and an organic surface cross-linking agent capable of reacting with a carboxyl group. (3) The surface cross-linking step has a heat treatment step of carrying out a heat treatment in an atmosphere having a water vapor density of more than 0.005 g / L simultaneously with or after adding the organic surface cross-linking agent to the water absorbent resin.

[0018] Another aspect of the present invention is a water-absorbing agent composition containing a surface-crosslinked water-absorbing resin as a main component, which satisfies all of the following (1) and (2): (1) the specific surface area of ​​the water-absorbing agent composition is 25 m ２ / kg or more (2) The water-absorbing agent composition has a Washburn contact angle θ2 of 72° or less when measured with a 20% by mass aqueous solution of sodium chloride.

[0019] The present inventors have considered that the reason why the absorption rate does not increase sufficiently as designed when the water-absorbent agent composition or water-absorbent resin having an increased specific surface area is surface-crosslinked is that the surface of the conventional water-absorbent resin is slightly hydrophobized, reducing the affinity (liquid compatibility) between the absorbed liquid and the water-absorbent resin. That is, the liquid to be absorbed is less likely to penetrate into the recesses (cavities) formed on the surface of the water-absorbent resin with a high specific surface area, and the contact area between the absorbed liquid and the water-absorbent resin surface is reduced, preventing effective use of the surface as an absorption surface that comes into contact with the absorbed liquid, thereby hindering improvement in the absorption rate. In other words, it is considered that the desired absorption rate was not obtained despite the specific surface area being increased at the expense of some degree of absorbency under load (AAP), which is in a trade-off relationship with absorption rate and crevice water retention capacity.

[0020] The present inventors have found a method for precisely controlling the affinity of a surface of a water absorbent resin with a liquid to be absorbed by paying attention to a water-absorbing agent composition and a composition of a treating agent for performing a surface cross-linking treatment of a water absorbent resin, and a dew point and a water vapor density in a heat treatment device for performing a surface cross-linking reaction, and have thus completed the present invention.

[0021] By using the method described above, it is possible to suppress surface hydrophobicity even when surface crosslinking is performed, and it is possible to provide a method for improving the affinity between the absorbed liquid and the water-absorbent resin surface, such that the absorbed liquid penetrates into the recesses (cavities) of the water-absorbent resin even in water-absorbent resins or water-absorbent agent compositions with an increased specific surface area. It is believed that the improved water absorption rate is the result of the liquid easily penetrating into the particle recesses. Furthermore, it is believed that the improved affinity between the absorbed liquid and the water-absorbent resin surface increases the amount of liquid that can be held in the gaps in the particle layer and the particle recesses, i.e., the crevice water holding capacity, and thus it is possible to obtain a water-absorbent resin with a good absorption capacity under pressure (AAP) and crevice water capacity under pressure. Furthermore, it is no longer necessary to excessively increase the specific surface area in order to increase the absorption rate, etc., and it is possible to provide a water-absorbent agent composition that is well-balanced between the absorption capacity under pressure (AAP) and crevice water capacity under pressure, which show a trade-off relationship with an increase in specific surface area.

[0022] The present inventors also focused on the contact angle as an index of the affinity between the surface of a water-absorbent resin and the liquid to be absorbed. Conventionally, contact angles of water-absorbent resins have been measured using a high-speed camera to eliminate the influence of the water-absorbent resin's tendency to absorb the desired liquid (e.g., human urine, as well as representative physiological saline and artificial urine). The water-absorbent resin is densely spread on a bed, and the liquid to be absorbed is dropped onto the bed. Images are taken at the moment of contact and analyzed to obtain the contact angle value. However, the influence of droplet deformation due to gravity cannot be eliminated at the moment the droplet falls onto and contacts the water-absorbent resin bed, and as a result, stable contact angle data cannot be obtained. On the other hand, the adoption of an improved Washburn method (penetration rate method) has enabled more accurate contact angle measurements than conventional methods.

[0023] Furthermore, the present inventors have found that by adopting an improved Washburn method, it is possible to precisely grasp and control the affinity of the water-absorbent resin surface, which varies depending on the specific surface area and the conditions of the surface cross-linking treatment, with the absorbed liquid. As a result, by preparing a water-absorbent agent composition having the above-mentioned configuration, it is possible to obtain a water-absorbent agent composition that exhibits desired absorption capacity under load (AAP), crevice water capacity under load, liquid permeability under load (SFC and GBP), etc., while maintaining a high water absorption rate.

[0024] Hereinafter, a method for producing the water-absorbing agent composition of the present invention will be described in detail, but the scope of the present invention is not limited to these descriptions, and other than the following examples, the present invention can be appropriately modified and implemented within the scope that does not impair the gist of the present invention. Specifically, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the claims, and embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0025] [1] Definition of Terms [1-1] Water-absorbent resin, water-absorbent agent composition In this specification, the term "water-absorbent resin" refers to a water-swellable, water-insoluble polymer gelling agent, generally in powder form. Furthermore, "water-swellable" refers to a CRC (absorbency without load) defined in NWSP 241.0. R2(19) of 5 g / g or more, and "water-insoluble" refers to an Ext (extractable content) defined in NWSP 270.0. R2(19) of 50 mass% or less.

[0026] The above-mentioned "water-absorbent resin" is preferably a hydrophilic cross-linked polymer obtained by cross-linking polymerizing an unsaturated monomer having a carboxyl group, but the entire amount, i.e., 100% by mass, does not need to be a cross-linked polymer, and additives and the like can be contained within a range that satisfies the performance requirements such as the CRC and Ext.

[0027] Furthermore, the above-mentioned "water-absorbent resin" may refer to "a polymer crosslinked only inside, that is, a polymer having substantially the same crosslink density inside and on the surface" or "a polymer crosslinked inside and on the surface, that is, a polymer having a relatively high crosslink density on the surface compared to the crosslink density inside."

[0028] In the present specification, the above-mentioned "polymer crosslinked only in the interior" and the above-mentioned "polymer crosslinked both in the interior and on the surface" are not distinguished in principle, and both are expressed as "water-absorbent resin". However, when it is necessary to clearly distinguish whether or not surface crosslinking has occurred, the above-mentioned "polymer crosslinked only in the interior" is expressed as "water-absorbent resin before surface crosslinking" since it is before surface crosslinking has been performed, and the above-mentioned "polymer crosslinked both in the interior and on the surface" is expressed as "water-absorbent resin after surface crosslinking" or "surface-crosslinked water-absorbent resin" since it is after surface crosslinking has been performed. Here, "before surface crosslinking" means "before adding a surface crosslinking agent" or "before starting a crosslinking reaction by heat treatment even after adding a surface crosslinking agent".

[0029] Furthermore, the above-mentioned "water-absorbing resin" may refer only to the resin component, but may also contain components other than the resin, such as additives.

[0030] In this specification, the term "water-absorbing agent composition" refers to a mixture of the above-mentioned "water-absorbing resin" and any other components than the water-absorbing resin, such as additives, which are optionally contained.

[0031] The "water-absorbent agent composition" contains a water-absorbent resin as a main component. The "main component" means that the mass ratio of the water-absorbent resin to the entire water-absorbent agent composition is preferably 50% by mass or more, and then 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, and 100% by mass or less. The "water-absorbent agent composition" also contains, as other components, preferably water, and trace components such as a polyvalent metal salt, a chelating agent, a surfactant in an amount sufficient to maintain surface tension, and a surface-modifying polymer.

[0032] [1-2] Polyacrylic acid (salt)-based water-absorbent resin In this specification, the term "polyacrylic acid (salt)-based water-absorbent resin" refers to a water-absorbent resin made from acrylic acid and / or its salt (hereinafter referred to as "acrylic acid (salt)"). In other words, the term "polyacrylic acid (salt)-based water-absorbent resin" refers to a polymer having structural units derived from acrylic acid (salt) and having a graft component as an optional component. Specifically, the polyacrylic acid (salt)-based water-absorbent resin is a polymer containing preferably 50 mol% or more, more preferably 70 mol% or more, and even more preferably 90 mol% or more, and preferably 100 mol% or less, and more preferably substantially 100 mol% of acrylic acid (salt) relative to the portion of the monomers involved in the polymerization reaction excluding the internal crosslinking agent.

[0033] [1-3] "EDANA" and "NWSP" "EDANA" is an abbreviation for European Disposables and Nonwovens Associations. Furthermore, "NWSP" is an abbreviation for Non-Woven Standard Procedure, and indicates a global standard measurement method for a water-absorbent composition or a water-absorbent resin provided by EDANA. In the present invention, unless otherwise specified, the physical properties of the water-absorbent composition or the water-absorbent resin are measured in accordance with the original NWSP (revised version in 2019). In addition, in the present invention, unless otherwise specified, the measurement method in the following examples is followed.

[0034] [1-4] CRC (NWSP 241.0.R2(19)) "CRC" is an abbreviation for Centrifuge Retention Capacity, and means the absorption capacity of a water-absorbent agent composition or a water-absorbent resin under no pressure.

[0035] [1-5] Ext (NWSP 270.0.R2(19)) "Ext" is an abbreviation for Extractables, and means the water-soluble portion of a water-absorbent agent composition or a water-absorbent resin, i.e., the amount of water-soluble components. Specifically, it refers to the amount of dissolved polymer (unit: mass %) after adding 1.0 g of a water-absorbent agent composition or a water-absorbent resin to 200 ml of a 0.9 mass % aqueous sodium chloride solution and stirring at 250 rpm for 1 hour or 16 hours. The amount of dissolved polymer is measured using pH titration. The stirring time is stated when the results are reported.

[0036] [1-6] AAP (NWSP 242.0.R2(19)) "AAP" is an abbreviation for Absorption Against Pressure, and means the absorption capacity of a water-absorbent agent composition or a water-absorbent resin under pressure.

[0037] [1-7] SFC “SFC” is an abbreviation for Saline Flow Conductivity, and is a property of the water-absorbent agent composition or the water-absorbent resin that allows the permeability of an aqueous sodium chloride solution under pressure (load of 2.07 kPa (0.3 psi)) (unit: × 10 －７ cm ３ .sec / g).

[0038]

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[0076]

[0077] "Particle size distribution" means the particle size distribution of a water-absorbent agent composition or a water-absorbent resin measured by sieve classification. Specifically, using a set of sieves with a diameter of 200 mm, sieve mesh openings of 710 μm, 600 μm, 300 μm, 150 μm, and 45 μm, and a tray, 100.0 g of the water-absorbent agent composition or the water-absorbent resin is classified for 10 minutes with a sieve shaker, and the mass of the water-absorbent agent composition or the water-absorbent resin remaining on each sieve mesh and in the tray is measured to calculate the particle size distribution (unit: mass%).

[0039] [1-9] Specific Surface Area The "specific surface area" refers to the surface area per unit mass of the water-absorbing agent composition or the water-absorbing resin (unit: m ２ / kg).

[0040] [1-10] Crevice Water Ratio Under Pressure The term "crevice water ratio under pressure" refers to the mass of crevice water per 1 g of a water-absorbent agent composition or a water-absorbent resin under a load of 0.3 psi (2.07 kPa). The term "crevice water" refers to the liquid retained between (in gaps between) gel particles when 1 g of a water-absorbent agent composition or a water-absorbent resin is swollen with a 0.69 mass % aqueous sodium chloride solution under a load of 0.3 psi (2.07 kPa) to form a layer of gel particles. In other words, the crevice water ratio under pressure is the mass (unit: g / g) of the sodium chloride aqueous solution retained in the gaps of the water-absorbent agent composition or the water-absorbent resin when the water-absorbent agent composition or the water-absorbent resin is swollen with a 0.69 mass % aqueous sodium chloride solution per 1 g of the water-absorbent agent composition or the water-absorbent resin under a load of 2.07 kPa (0.3 psi).

[0041] By measuring the crevice water magnification under pressure, it is possible to grasp the liquid retention force between swollen gel particles, and this can be used as an index to predict the amount of rewet when the absorbent is actually used in sanitary materials such as disposable diapers.

[0042] [1-11] Others In this specification, "acid (salt)" means "acid and / or its salt", and "(meth)acrylic" means "acrylic and / or methacrylic".

[0043] [2] Manufacturing Method of Water-Absorbent Agent Composition The water-absorbent agent composition according to the present invention is a water-absorbent resin composition containing a water-absorbent resin (preferably a polyacrylic acid (salt)-based water-absorbent resin) and, if necessary, an additive. Hereinafter, a manufacturing method of the water-absorbent agent composition will be described in detail.

[0044] The method for producing a water-absorbing agent composition includes a surface-crosslinking step of a water-absorbent resin. The method for producing a water-absorbing agent composition may also include a step of preparing an aqueous monomer solution, a polymerization step, a gel-crushing step, a drying step, a pulverizing step, a classification step, or a step of adding an additive. In addition to the above-mentioned steps, the production method according to the present invention may further include a cooling step, a rewetting step, a fine powder granulation step, a transporting step, a storing step, a packaging step, a preservation step, etc., as necessary.

[0045] Each step will be described below.

[0046] [2-1] Monomer Aqueous Solution Preparation Step This step is a step of preparing a monomer aqueous solution containing a monomer, which is a raw material for the water-absorbent resin, preferably an unsaturated monomer, more preferably an unsaturated monomer having a carboxyl group, and even more preferably a monomer containing acrylic acid (salt) as a main component. The monomer aqueous solution preferably contains one or more polymerizable internal crosslinking agents. The above-mentioned "main component" means that the content of acrylic acid (salt) relative to the portion of the monomers subjected to the polymerization reaction excluding the internal crosslinking agent is 50 mol% or more (up to an upper limit of 100 mol%), preferably 70 mol% or more, more preferably 90 mol% or more, and preferably 100 mol% or less. Note that a monomer slurry can also be used within a range that does not affect the water absorption performance of the water-absorbent agent composition obtained as a final product, but for convenience, the present specification will describe a monomer aqueous solution.

[0047] (Acrylic Acid (Salt)) In the present invention, from the viewpoint of the physical properties and productivity of the water-absorbing agent composition or the water-absorbing resin, it is preferable to use a known acrylic acid (salt) as a monomer (hereinafter, sometimes referred to as a "polymerizable monomer"). Known acrylic acids contain trace amounts of components such as polymerization inhibitors and impurities. As the polymerization inhibitor, preferably, a methoxyphenol, more preferably a p-methoxyphenol, is used. From the viewpoint of the polymerizability of acrylic acid and the color tone of the water-absorbing agent composition or the water-absorbing resin, the concentration of the polymerization inhibitor in acrylic acid is preferably 10 ppm or more, more preferably 20 ppm or more, and preferably 200 ppm or less, more preferably 160 ppm or less, and even more preferably 100 ppm or less, on a mass basis. Note that the preferred range of the concentration of the polymerization inhibitor in acrylic acid can be a range defined by any combination selected from the above-mentioned upper and lower limit values.

[0048] Examples of the impurities include organic compounds such as acetic acid, propionic acid, and furfural, as well as the compounds described in U.S. Patent Application Publication No. 2008 / 0161512. Examples of acrylic acid salts include salts obtained by neutralizing the above-mentioned acrylic acid with the following basic compounds. The acrylic acid salts may be commercially available acrylic acid salts or salts obtained by neutralizing acrylic acid.

[0049] (Basic Compound) In the present invention, the term "basic compound" refers to a compound that exhibits basicity. Specific examples include sodium hydroxide. Commercially available sodium hydroxide contains heavy metals such as zinc, lead, and iron in the ppm range (by mass), so strictly speaking, it can also be referred to as a composition. In the present invention, such compositions are also considered to be included in the category of basic compounds.

[0050] Specific examples of the basic compound include carbonates and hydrogencarbonates of alkali metals, hydroxides of alkali metals, ammonia, organic amines, etc. Among them, a strongly basic compound is selected from the viewpoint of the water absorption performance of the water-absorbing agent composition or the water-absorbent resin. Therefore, hydroxides of alkali metals such as sodium, potassium, and lithium are preferred, and sodium hydroxide is more preferred. Note that the basic compound is preferably in the form of an aqueous solution from the viewpoint of ease of handling.

[0051] (Neutralization) When a salt obtained by neutralizing acrylic acid is used as the acrylic acid salt, the timing of neutralization may be any of before, during, or after polymerization, and neutralization may be performed at a plurality of times or places. Moreover, from the viewpoint of production efficiency of the water-absorbing agent composition or the water-absorbent resin, it is preferable to neutralize in a continuous manner.

[0052] When acrylic acid (salt) is used in the present invention, the neutralization rate thereof is preferably 10 mol% or more, more preferably 40 mol% or more, even more preferably 50 mol% or more, particularly preferably 60 mol% or more, and is preferably 90 mol% or less, more preferably 85 mol% or less, even more preferably 80 mol% or less, particularly preferably 75 mol% or less, based on the acid groups of the monomer. The preferred range of the neutralization rate of acrylic acid (salt) can be a range defined by any combination selected from the upper and lower limits described above. By setting the neutralization rate within this range, it becomes easier to further suppress the deterioration of the water absorption performance of the water-absorbent agent composition or the water-absorbent resin.

[0053] The above-mentioned range of the neutralization rate is applied to any of the neutralization before, during, and after the polymerization, and is also applied to the acid groups of the water-absorbing agent composition as a final product, as well as the acid groups of the water-absorbing resin.

[0054] (Other Monomers) In the present invention, monomers other than the above-mentioned acrylic acid (salts) (hereinafter referred to as "other monomers") can be used in combination with acrylic acid (salts) as needed. Specific examples of the other monomers include anionic unsaturated monomers and salts thereof, such as maleic acid, maleic anhydride, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluene sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, and 2-hydroxyethyl (meth)acryloyl phosphate; mercaptan group-containing unsaturated monomers; phenolic hydroxyl group-containing unsaturated monomers; amide group-containing unsaturated monomers, such as (meth)acrylamide, N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; and amino group-containing unsaturated monomers, such as N,N-dimethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate, and N,N-dimethylaminopropyl (meth)acrylamide. The other monomers include water-soluble or hydrophobic unsaturated monomers. When the other monomers are used, the amount of the other monomers used is preferably 30 mol % or less (lower limit: 0 mol %), more preferably 10 mol % or less, and even more preferably 5 mol % or less, based on the total amount of the monomers excluding the internal crosslinking agent.

[0055] (Internal Crosslinking Agent) In a preferred production method of the present invention, an internal crosslinking agent is used. Specific examples of the internal crosslinking agent include N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, ethylene oxide-modified glycerin tri(meth)acrylate, pentaerythritol acrylate, and the like. Examples of the internal crosslinking agent include thritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, and glycidyl (meth)acrylate. These internal crosslinking agents may be used alone or in combination of two or more. Among these internal crosslinking agents, one or more internal crosslinking agents are selected in consideration of reactivity, etc. Furthermore, from the viewpoint of the water absorption performance of the water-absorbing agent composition or water-absorbent resin, preferably an internal crosslinking agent having two or more polymerizable unsaturated groups is selected, more preferably an internal crosslinking agent having thermal decomposition properties at the drying temperature described below, and even more preferably an internal crosslinking agent having two or more polymerizable unsaturated groups with a (poly)alkylene glycol structure.

[0056] Specific examples of the polymerizable unsaturated group include allyl groups and (meth)acrylate groups. Of these, (meth)acrylate groups are preferred. Specific examples of the (poly)alkylene glycol structure include polyethylene glycol, and specific examples of internal cross-linking agents having a (poly)alkylene glycol structure include polyethylene glycol, (poly)ethylene glycol di(meth)acrylate, and (poly)propylene glycol di(meth)acrylate. The number of alkylene glycol units (hereinafter sometimes referred to as "n") is preferably 1 or more, more preferably 6 or more, and preferably 100 or less, more preferably 50 or less, even more preferably 20 or less, and particularly preferably 10 or less. The preferred range of the number of alkylene glycol units can be a range defined by any combination selected from the above upper and lower limits. In the production method according to the present invention, the number of alkylene glycol units is, for example, preferably 1 or more and 100 or less, more preferably 6 or more and 50 or less, even more preferably 6 or more and 20 or less, and particularly preferably 6 or more and 10 or less. The number of alkylene glycol units is an average of a distribution of numbers.

[0057] The amount of the internal crosslinking agent used is preferably 0.0001 mol % or more, more preferably 0.001 mol % or more, even more preferably 0.01 mol % or more, and preferably 10 mol % or less, more preferably 5 mol % or less, and even more preferably 1 mol % or less, based on the monomers excluding the internal crosslinking agent. A preferred range of the amount of the internal crosslinking agent used can be a range defined by any combination selected from the upper and lower limits. The amount of the internal crosslinking agent used is, for example, preferably 0.0001 mol % or more and 10 mol % or less, more preferably 0.001 mol % or more and 5 mol % or less, and even more preferably 0.01 mol % or more and 1 mol % or less, based on the monomers excluding the internal crosslinking agent. By using an amount within the range, an increase in the water-soluble content and a decrease in the absorbency of the water-absorbent agent composition or the water-absorbent resin are suppressed, and a water-absorbent agent composition or a water-absorbent resin having desired water absorption performance (for example, a high absorbency under load, a high crevice water capacity under load, and a high liquid permeability rate under load (SFC or GBP)) can be more easily obtained.

[0058] The internal cross-linking agent is preferably added in advance when preparing the aqueous monomer solution, and in this case, the cross-linking reaction is carried out simultaneously with the polymerization reaction. On the other hand, the polymerization reaction can be started without adding the internal cross-linking agent, and the internal cross-linking agent can be added during or after the polymerization reaction to carry out the cross-linking reaction. These methods can also be used in combination. Self-cross-linking without using an internal cross-linking agent can also be carried out.

[0059] (Substances Added to Aqueous Monomer Solution) In the present invention, from the viewpoint of improving the physical properties of the water-absorbing agent composition or the water-absorbent resin, the following substances may be added to the aqueous monomer solution at one or more points during the preparation of the aqueous monomer solution, during the polymerization reaction and the crosslinking reaction, or after the polymerization reaction and the crosslinking reaction. Specific examples of the substances include hydrophilic polymers such as starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol (hereinafter sometimes referred to as "PVA"), polyacrylic acid (salts), crosslinked polyacrylic acid (salts), carbonates, azo compounds, various foam-generating blowing agents, surfactants, chelating agents, chain transfer agents, and other compounds.

[0060] The amount of the hydrophilic polymer added is preferably 50% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less, and is preferably 0% by mass or more, more preferably more than 0% by mass, relative to the aqueous monomer solution. The amount of the compound added is preferably 5% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less, and is preferably 0% by mass or more, more preferably more than 0% by mass, relative to the aqueous monomer solution.

[0061] When a water-soluble resin or a water-absorbent resin is used as the hydrophilic polymer, a graft polymer or a water-absorbent resin composition such as a starch-acrylic acid (salt) copolymer, a PVA-acrylic acid (salt) copolymer, etc. can be obtained. These graft polymers or water-absorbent resin compositions are also included in the category of the polyacrylic acid (salt)-based water-absorbent resin according to the present invention.

[0062] (Concentration of Monomer Component) The above-mentioned substances and components (hereinafter referred to as "monomer components") are selected according to the purpose, and the amounts of the respective components are specified so as to satisfy the above-mentioned ranges, and then mixed together to prepare an aqueous monomer solution. In the present invention, in addition to preparing an aqueous solution of the monomer, a mixed solution of water and a hydrophilic solvent can also be prepared, and such a form is also considered to be an aqueous monomer solution.

[0063] From the viewpoint of the physical properties of the water-absorbing agent composition or the water-absorbent resin, the total concentration of the monomer components is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, and is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less. Note that a preferred range of the total concentration of the monomer components can be a range defined by any combination selected from the upper and lower limit values. Note that the concentration of the monomer components is calculated from the following (Formula I).

[0064] Concentration of monomer component (mass %)={(mass of monomer component) / (mass of aqueous monomer solution)}×100 (Equation I) In the above (Equation I), the "mass of aqueous monomer solution" does not include the mass of the graft component, the water-absorbent resin, and the hydrophobic organic solvent in the reversed-phase suspension polymerization.

[0065] [2-2] Polymerization step This step is a step of polymerizing the aqueous monomer solution containing the monomer containing acrylic acid (salt) as a main component and one or more polymerizable internal cross-linking agents obtained in the aqueous monomer solution preparation step, to obtain a hydrogel.

[0066] (Polymerization Initiator) In the present invention, a polymerization initiator is preferably used during polymerization. Examples of the polymerization initiator include a thermally decomposable polymerization initiator, a photodecomposable polymerization initiator, or a redox polymerization initiator used in combination with a reducing agent that promotes the decomposition of the polymerization initiator. Specific examples of the polymerization initiator include radical polymerization initiators such as sodium persulfate, potassium persulfate, ammonium persulfate, t-butyl hydroperoxide, hydrogen peroxide, and 2,2'-azobis(2-amidinopropane) dihydrochloride. One or more polymerization initiators are selected from these polymerization initiators, taking into consideration factors such as the polymerization form. Furthermore, from the viewpoints of the handleability of the polymerization initiator and the physical properties of the water-absorbing agent composition or water-absorbent resin, the polymerization initiator is preferably a peroxide or an azo compound, more preferably a peroxide, and even more preferably a persulfate. Furthermore, when an oxidizing radical polymerization initiator is used, redox polymerization may be performed in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, or L-ascorbic acid. In addition, when a peroxide is used as a polymerization initiator, the peroxide may be the same as or different from the peroxide used in the surface crosslinking step described later, but it is preferable that they are the same type (preferably, both are persulfates, more preferably sodium persulfate).

[0067] The amount of the polymerization initiator used is preferably 0.001 mol% or more, more preferably 0.01 mol% or more, and preferably 1 mol% or less, more preferably 0.5 mol% or less, and even more preferably 0.1 mol% or less, based on the monomers excluding the internal crosslinking agent. The amount of the reducing agent used is preferably 0.0001 mol% or more, more preferably 0.0005 mol% or more, and preferably 0.02 mol% or less, more preferably 0.015 mol% or less, based on the monomers excluding the internal crosslinking agent. The preferred range of the amount of the polymerization initiator or the amount of the reducing agent used can be a range defined by any combination selected from the upper and lower limits. By using an amount within this range, a water-absorbing agent composition or a water-absorbent resin having the desired water absorption performance can be more easily obtained.

[0068] In the present invention, the polymerization reaction may be initiated by irradiation with active energy rays such as radiation, electron beams, ultraviolet rays, etc. Irradiation with active energy rays may also be used in combination with the polymerization initiator.

[0069] (Polymerization Form) Examples of polymerization forms applicable to the present invention include aqueous solution polymerization, reversed-phase suspension polymerization, spray polymerization, droplet polymerization, bulk polymerization, and precipitation polymerization. Among these, from the viewpoint of ease of polymerization control and the water absorption performance of the water-absorbing agent composition or water-absorbent resin, preferably aqueous solution polymerization or reversed-phase suspension polymerization, more preferably aqueous solution polymerization, and even more preferably continuous aqueous solution polymerization is selected. Reverse-phase suspension polymerization is described in International Publication Nos. 2007 / 004529 and 2012 / 023433, etc. Examples of continuous aqueous solution polymerization include continuous belt polymerization described in U.S. Pat. Nos. 4,893,999, 6,906,159, 7,091,253, 7,741,400, 8,519,212, and JP-A-2005-36100, and continuous kneader polymerization described in U.S. Pat. No. 6,987,151, etc.

[0070] Preferred forms of the continuous aqueous solution polymerization include high-temperature initiation polymerization, high-concentration polymerization, and foam polymerization. The "high-temperature initiation polymerization" refers to a polymerization form in which the temperature of the aqueous monomer solution at the start of polymerization is preferably 30°C or higher, more preferably 35°C or higher, even more preferably 40°C or higher, and particularly preferably 50°C or higher, with the upper limit temperature being the boiling point of the aqueous monomer solution. The "high-concentration polymerization" refers to a polymerization form in which the monomer concentration at the start of polymerization is preferably 30% by mass or higher, more preferably 35% by mass or higher, even more preferably 40% by mass or higher, and particularly preferably 42% by mass or higher, with the upper limit concentration being the saturated concentration of the aqueous monomer solution. The "foam polymerization" refers to a polymerization form in which the aqueous monomer solution containing a foaming agent or bubbles is polymerized. These polymerization forms may be carried out alone, or two or more of them may be used in combination.

[0071] The foaming polymerization is one of the methods for improving the specific surface area of ​​the present invention and is one of the preferred embodiments. Examples of methods for dispersing bubbles in the foaming polymerization include: (I) a method in which gas dissolved in a monomer aqueous solution is dispersed as bubbles by reducing its solubility; (II) a method in which gas is introduced from the outside and dispersed as bubbles; and (III) a method in which a foaming agent is added to a monomer aqueous solution to cause foaming. Furthermore, the above dispersion methods may be used in combination depending on the water absorption performance of the water-absorbing agent composition or the water-absorbent resin. The bubbles dispersed in the monomer aqueous solution are trapped within the gel when the monomer aqueous solution gels as the polymerization reaction progresses, and the resulting gel has a foamed shape, which is why the term foaming polymerization is used in the present invention.

[0072] The gas dissolved in the aqueous monomer solution (I) above includes oxygen used to stabilize the monomer, inert gases such as nitrogen, carbon dioxide, and ozone, as well as mixtures of these gases.

[0073] In the case of the above method (II) of introducing a gas from the outside and dispersing it as bubbles, specific examples of the gas include oxygen, air, nitrogen, carbon dioxide, ozone, etc., and mixtures of these. Among these, from the viewpoints of polymerizability and cost, an inert gas such as nitrogen or carbon dioxide is preferably used, and more preferably nitrogen.

[0074] In the case of the method (III) of foaming by adding a blowing agent to an aqueous monomer solution, specific examples of the blowing agent include an azo compound, an organic or inorganic carbonate solution, a dispersion, or a powder having a particle size of 0.1 μm or more and 1000 μm or less, and preferably a carbonate or hydrogencarbonate such as sodium carbonate, ammonium carbonate, or magnesium carbonate. A surfactant may be used in the aqueous monomer solution containing the blowing agent or bubbles to stably maintain the bubbles.

[0075] In the methods (I) to (III) listed as the methods for dispersing bubbles in the foaming polymerization, a surfactant may be used in combination. Examples of the surfactant include anionic surfactants, nonionic surfactants, cationic surfactants, amphoteric surfactants, fluorine-containing surfactants, and organometallic surfactants. Specific examples include the surfactants described in WO 97 / 017397 and U.S. Pat. No. 6,107,358.

[0076] Although each of the above polymerization modes can be carried out in an air atmosphere, from the viewpoint of the color tone of the water-absorbing agent composition or the water-absorbent resin, it is preferable to carry out the polymerization in an inert gas atmosphere such as nitrogen or argon, and more preferably in an atmosphere with an oxygen concentration of 1% by volume or less. Incidentally, it is preferable to sufficiently replace the dissolved oxygen in the aqueous monomer solution with an inert gas, and it is more preferable to keep the amount of dissolved oxygen below 1 mg / L.

[0077] Forming a foamed hydrogel, water-absorbent resin, or water-absorbent agent composition by foaming polymerization is preferable because it increases the water absorption rate of the water-absorbent agent composition or water-absorbent resin and also facilitates immobilization of the water-absorbent agent composition to an absorbent article. The foamed shape can be confirmed by observing pores on the particle surface using an electron microscope. Examples of the pore size include pores with a diameter of 1 μm or more and 100 μm or less. The lower limit of the number of pores per particle of the water-absorbent agent composition or water-absorbent resin is preferably 1 or more, more preferably 10 or more. Meanwhile, the upper limit is preferably 10,000 or less, more preferably 1,000 or less. The pores can be controlled by the foaming polymerization. Foaming polymerization is a preferred technique for increasing the specific surface area of ​​the water-absorbent agent composition or water-absorbent resin.

[0078] [2-3] Gel Crushing Step This step is a step in which the hydrogel obtained in the polymerization step is gel-crushed to obtain a particulate hydrogel (hereinafter referred to as "particulate hydrogel"). Note that the crunching in this step is referred to as "gel crushing" to distinguish it from the "crushing" in the crushing step described below. The "gel crushing" means adjusting the hydrogel to a predetermined size using a gel crusher such as a kneader, meat chopper, or cutter mill.

[0079] With regard to the embodiments and operating conditions of gel crushing, the contents described in Japanese Patent No. 5989913 or Japanese Patent No. 6067126 also apply to the present invention. When the polymerization mode is kneader polymerization, the polymerization step and the gel crushing step are carried out simultaneously. Furthermore, when a particulate hydrogel is obtained in the polymerization step, such as reversed-phase suspension polymerization, spray polymerization, or droplet polymerization, the gel crushing step is considered to be carried out simultaneously with the polymerization step. Furthermore, by passing through the gel crushing step in the present invention, an irregularly pulverized water-absorbing agent composition or water-absorbent resin can be obtained.

[0080] The particle size of the particulate hydrogel pulverized by the gel crushing step is preferably 0.05 mm or more and 10 mm or less. If the particle size of the particulate hydrogel is too small, the physical properties of the resulting water-absorbing agent composition or water-absorbent resin may be reduced. On the other hand, if the particle size of the particulate hydrogel is too large, the particulate hydrogel may be insufficiently dried.

[0081] The D50 (mass average particle diameter) of the particulate hydrogel is preferably 50 μm or more, more preferably 100 μm or more, even more preferably 140 μm or more, and is preferably 2000 μm or less, more preferably 1500 μm or less, and even more preferably 1000 μm or less. A preferred range for the D50 (mass average particle diameter) of the particulate hydrogel can be a range defined by any combination selected from the upper and lower limits. For example, controlling the D50 (mass average particle diameter) of the particulate hydrogel within the preferred range by the following technique (B) is one technique for improving the specific surface area of ​​the water-absorbent agent composition or the water-absorbent resin, and is one preferred embodiment.

[0082] As the PSD (particle size) of the particulate hydrogel, σζ (logarithmic standard deviation), which indicates the narrowness of its particle size distribution, is preferably 0.2 or more, preferably 1.5 or less, more preferably 1.3 or less, and even more preferably 1.2 or less. The smaller the value of σζ (logarithmic standard deviation of particle size distribution), the more uniform the particle size becomes, which has the advantage of allowing for more uniform drying. However, in order to make the σζ (logarithmic standard deviation of particle size distribution) less than 0.2, special operations such as particle size control during polymerization before gel crushing and classification of the particulate hydrogel after gel crushing are required, which is practically difficult to implement from the standpoint of productivity and cost.

[0083] In the present invention, the specific surface area of ​​the water-absorbent resin is 25 m ２ It is desirable to control one or more of the techniques of (A) foaming polymerization of an aqueous monomer solution, (B) pulverization and granulation of a particulate hydrogel or a dried polymer thereof, and (C) fine powder recycling so that the total weight of the hydrogel is 1 / kg or more.

[0084] As the foaming polymerization of the (A) aqueous monomer solution, for example, foaming polymerization in which a surfactant is coexisted in the aqueous monomer solution, i.e., the foaming polymerization method described in Japanese Patent No. 5647625 (specifically, for example, a method in which bubbles are generated in the aqueous monomer solution by reducing the solubility of dissolved gas in the aqueous monomer solution in the presence of a surfactant), a foaming polymerization method in which gas is introduced from the outside into the aqueous monomer solution and dispersed as bubbles to carry out polymerization, a foaming polymerization method in which a blowing agent is added to the aqueous monomer solution to carry out foaming and polymerization, or the like is employed, whereby the specific surface area of ​​the water absorbent resin can be increased to 25 m ２ / kg or more. Therefore, it is also preferable that the water-absorbent resin is obtained by foam polymerization of an aqueous solution of an unsaturated monomer.

[0085] In addition, as for granulation of the particulate hydrogel or its dried polymer (B), for example, a gel crushing process may be performed using the gel crushing method described in Japanese Patent No. 5989913, Japanese Patent No. 6067126, or International Publication No. 2016 / 204302, and further drying may be performed to increase the specific surface area of ​​the water-absorbent resin to 25 m ２ / kg or more. In addition, a water-absorbent resin having a desired specific surface area can be obtained by appropriately controlling the die hole diameter, number of holes, die thickness, amount of hot water added, rotation speed of the screw shaft, etc. of a gel crusher such as a meat chopper. The granulation may be performed on the hydrogel during polymerization, or may be performed simultaneously with drying of a finely pulverized product of the hydrogel after polymerization, or may be performed on the finely pulverized product after drying using water and / or an organic or inorganic binder. Therefore, it is also preferable to include a granulated product of a hydrogel of a water-absorbent resin or a dried product thereof.

[0086] In addition, as the above-mentioned (C) fine powder recycling, for example, the fine powder of the water absorbent resin that has passed through a sieve with an opening of 150 μm is recovered in a polymerization step, a gel crushing step, or a drying step, or the fine powder is granulated and recovered, thereby making it possible to reduce the specific surface area of ​​the water absorbent resin to 25 m ２ / kg or more. Therefore, it is also preferable that the water-absorbent resin contains recycled fine powder of the water-absorbent resin. The above-mentioned methods (A) to (C) may be carried out alone or in combination.

[0087] The specific surface area of ​​the water-absorbent resin is 25 m ２ One method for increasing the specific surface area to more than 1 / kg is to incorporate a large amount of particles with a small particle size. However, this method results in the incorporation of a large amount of particles with a small particle size, particularly fine powder that passes through a sieve with a mesh size of 150 μm. As a result, gel blocking of the resulting water-absorbing agent composition is likely to occur, which is undesirable because it reduces the liquid absorption performance and liquid permeability under pressure. Therefore, when adjusting the specific surface area using such fine powder, it is preferable to adopt the above-mentioned methods (B) and / or (C). In the present invention, it is preferable to pay sufficient attention to adjusting the particle size distribution and to carry out the adjustment method described below.

[0088] The D50 (mass average particle diameter) and σζ (logarithmic standard deviation of particle size distribution) of the particulate hydrogel are measured by the method described in WO 2016 / 111223

[0257] to

[0270] , which is incorporated by reference.

[0089] [2-4] Drying step This step is a step of drying the hydrogel and / or particulate hydrogel obtained in the polymerization step and / or gel pulverization step to the desired resin solid content to obtain a dried polymer. The resin solid content of the dried polymer is determined from the mass change when 1 g of the water-absorbent resin is heated at 180 ° C. for 3 hours, and is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, particularly preferably 92% by mass or more, and preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 97% by mass or less. The preferred range of the resin solid content of the dried polymer can be a range defined by any combination selected from the above upper and lower limits.

[0090] Specific examples of the drying method for the hydrogel and / or particulate hydrogel include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with a hydrophobic organic solvent, high-humidity drying using high-temperature water vapor, etc. Among these, from the viewpoint of drying efficiency, hot air drying is preferred, and band drying in which hot air drying is performed on a ventilated belt is more preferred.

[0091] The drying temperature in the hot air drying is preferably 100°C or higher, more preferably 150°C or higher, and preferably 300°C or lower, more preferably 200°C or lower, from the viewpoint of the color tone and drying efficiency of the water-absorbing agent composition or the water-absorbent resin. The preferred range of the drying temperature can be a range defined by any combination selected from the upper and lower limits. The drying temperature in the hot air drying is defined by the temperature of the hot air. Drying conditions other than the drying temperature, such as the hot air speed and drying time, may be appropriately set depending on the water content and total mass of the particulate hydrogel to be dried and the target resin solid content. When band drying is performed, the conditions described in WO 2006 / 100300, WO 2011 / 025012, WO 2011 / 025013, WO 2011 / 111657, etc. are appropriately applied.

[0092] The drying time in the present invention is preferably 1 minute or more, more preferably 5 minutes or more, even more preferably 10 minutes or more, and preferably 10 hours or less, more preferably 3 hours or less, and even more preferably 1 hour or less. The preferred range of the drying time can be defined by any combination selected from the upper and lower limits. By setting the drying temperature and drying time within the range, the physical properties of the obtained water-absorbing agent composition can be within a desired range. Furthermore, the physical properties of the water-absorbent resin as an intermediate product can also be within a desired range. Furthermore, when drying is performed by hot air drying, the wind speed of the hot air is preferably 0.5 m / s or more, preferably 3.0 m / s or less, more preferably 2.0 m / s or less. Note that other drying conditions may be appropriately set depending on the water content and total mass of the particulate hydrogel to be dried, the target solid content, etc.

[0093] [2-5] Pulverization step, classification step In this step, the dried polymer obtained through the drying step is pulverized in a pulverization step, and then the particle size is adjusted to a desired range in a classification step. In this step, a water absorbent resin before surface crosslinking is obtained. By passing through the pulverization step after drying, an irregularly pulverized water absorbent resin is obtained.

[0094] Specific examples of the mill used in the milling step include high-speed rotary mills such as roll mills, hammer mills, screw mills, and pin mills, as well as vibration mills, knuckle mills, and cylindrical mixers. Among these, a roll mill is preferably selected from the viewpoint of milling efficiency. A plurality of these mills can also be used in combination.

[0095] Examples of a method for adjusting the particle size in the classification step include sieve classification using a JIS standard sieve (JIS Z8801-1 (2000)) and air flow classification. Among these, sieve classification is preferably selected from the viewpoint of classification efficiency. Note that the particle size adjustment of the water-absorbing agent composition or the water-absorbent resin is not limited to being performed in the pulverization step or the classification step, and can also be performed in a polymerization step, particularly inverse phase suspension polymerization or droplet polymerization, or in other steps, for example, a granulation step or a fine powder recovery step.

[0096] The proportion of particles (i) having a particle diameter of less than 150 μm contained in the water-absorbent resin after classification and before surface crosslinking is preferably 3% by mass or less, more preferably less than 3% by mass, even more preferably 2.5% by mass or less, and even more preferably 2% by mass or less. In continuous commercial production, it may be very difficult to reduce the proportion of particles less than 150 μm to 0% by mass from the viewpoint of production efficiency. Therefore, the proportion of particles (i) having a particle diameter of less than 150 μm contained in the water-absorbent resin after classification and before surface crosslinking is preferably more than 0% by mass, more preferably 0.1% by mass or more, even more preferably 0.2% by mass or more, and even more preferably 0.3% by mass or more. The preferred range of the proportion of particles having a particle diameter of less than 150 μm can be a range defined by any combination selected from the upper and lower limit values. Therefore, the proportion of particles (i) having a particle size of less than 150 μm contained in the water absorbent resin after classification and before surface cross-linking may be, for example, more than 0 mass% and not more than 3 mass%, more than 0 mass% and not more than 3 mass%, 0.1 mass% to 2.5 mass%, 0.2 mass% to 2 mass%, or 0.3 mass% to 2 mass%. When the proportion of particles (i) having a particle size of less than 150 μm contained in the water absorbent resin after classification and before surface cross-linking is within the above range, it is preferable because it becomes easier to control the AAP (absorbency under load) and SFC (saline flow conductivity) in a more balanced manner, and it becomes easier to uniformly disperse the surface cross-linking agent, thereby improving the performance of the water absorbent agent composition.

[0097] Furthermore, the (ii) D50 (mass average particle diameter) of the water-absorbent resin before surface crosslinking after classification is preferably 250 μm or more, more preferably 300 μm or more, even more preferably 330 μm or more, and preferably less than 550 μm, more preferably less than 500 μm, and even more preferably less than 450 μm. A preferred range of the (ii) D50 (mass average particle diameter) of the water-absorbent resin before surface crosslinking after classification can be a range defined by any combination selected from the above upper and lower limit values. The (ii) D50 (mass average particle diameter) of the water-absorbent resin before surface crosslinking after classification is, for example, preferably 250 μm or more and less than 550 μm, more preferably 300 μm or more and less than 500 μm, and even more preferably 330 μm or more and less than 450 μm.

[0133] When (ii) D50 (mass average particle diameter) of the water absorbent resin after classification and before surface crosslinking is within the above-mentioned range, it becomes easier to control AAP (absorbency under pressure) and SFC (saline flow conductivity) in a more balanced manner, which is preferable.

[0098] Furthermore, (iii) it is more preferable that the particle size distribution of the water absorbent resin before surface crosslinking has a D50 (mass average particle diameter) within the range of the above (ii) and a proportion of particles less than 150 μm within the range of the above (i). In continuous commercial production, it may be very difficult to reduce the proportion of particles less than 150 μm to 0% by mass from the viewpoint of production efficiency. Therefore, it is preferably more than 0% by mass, more preferably 0.1% by mass or more, even more preferably 0.2% by mass or more, and even more preferably 0.3% by mass or more. Specifically, D50 (mass average particle diameter) of a water absorbent resin before surface crosslinking is 250 μm or more and less than 550 μm, 300 μm or more and less than 500 μm, or 330 μm or more and less than 450 μm, and a mass ratio of particles with a particle diameter of less than 150 μm contained in the water absorbent resin is more than 0 mass% and less than 3 mass%, more than 0 mass% and less than 3 mass%, 0.1 mass% or more and 2.5 mass% or less, 0.2 mass% or more and 2 mass% or less, or 0.3 mass% or more and 2 mass% or less.

[0099] Furthermore, (iv) σζ (logarithmic standard deviation of particle size distribution) is preferably 0.20 or more, more preferably 0.25 or more, even more preferably 0.27 or more, and preferably 0.50 or less, more preferably 0.40 or less, and even more preferably 0.35 or less. The preferred range of the σζ (logarithmic standard deviation of particle size distribution) can be a range defined by any combination selected from the above upper and lower limits. The smaller the value of σζ (logarithmic standard deviation of particle size distribution), the more uniform the particle size becomes, which has the advantage of reducing particle segregation. However, excessively small σζ (logarithmic standard deviation of particle size distribution) requires repeated pulverization and classification to remove coarse particles and fine particles, which may be disadvantageous in terms of productivity and cost.

[0100] The above-mentioned particle sizes, etc., i.e., (i) to (iv) above, are applied not only to the water-absorbent resin before surface cross-linking, but also to the water-absorbent resin and the water-absorbent agent composition after surface cross-linking. Therefore, it is preferable to perform a surface cross-linking treatment, i.e., a surface cross-linking step, so as to maintain the particle size within the above range adjusted for the water-absorbent resin before surface cross-linking, and it is more preferable to adjust the particle size by providing a particle size adjustment step after the surface cross-linking step. Furthermore, in the present invention, the above (i) and (iv), the above (ii) and (iv), and the above (iii) and (iv) can be arbitrarily selected and combined, and in this case, the respective preferred ranges can be arbitrarily combined.

[0101] The specific surface area of ​​the water-absorbent resin before surface crosslinking is 25 m ２ / kg or more. The specific surface area of ​​the water-absorbent resin before surface cross-linking is 25 m ２ Even when the specific surface area of ​​the water-absorbent resin before surface cross-linking is 26 m or more, the production method and water-absorbent composition of the present invention can provide a water-absorbent composition having a high water absorption rate, a high absorption capacity under pressure, and a high crevice water capacity under pressure. ２ / kg or more, 27m ２ / kg or more, 28m ２ / kg or more, 29m ２ / kg or more, 30m ２ / kg or more is preferable, and ２The upper limit of the specific surface area of ​​the water-absorbent resin before surface cross-linking is 60 m ２ / kg or less, 55m ２ / kg or less is preferred in that order. From the viewpoint of increasing the water absorption rate, the higher the specific surface area, the more desirable it is; however, if the specific surface area becomes too high, excessive foam polymerization in the polymerization step or too fine gel crushing in the gel crushing step becomes necessary, which may result in a decrease in AAP (absorbency under pressure) or SFC (saline flow conductivity). On the other hand, if the specific surface area of ​​the water-absorbent resin becomes too small, it becomes difficult to obtain a water-absorbent resin having a desired water absorption rate, which is not preferable. Note that the preferred range of the specific surface area of ​​the water-absorbent resin before surface crosslinking can be a range specified by any combination selected from the above upper and lower limit values. For example, 25 m ２ / kg or more 60m ２ / kg or less, 26m ２ / kg or more 60m ２ / kg or less, 27m ２ / kg or more 60m ２ / kg or less, 28m ２ / kg or more 60m ２ / kg or less, 29m ２ / kg or more 60m ２ / kg or less, 30m ２ / kg or more 55m ２ / kg or less, or 36m ２ / kg or more 55m ２ The specific surface area of ​​the water-absorbent resin before surface cross-linking can be controlled by the pulverization conditions of the hydrogel, etc.

[0102] The shape of the water-absorbent resin before surface crosslinking may be any of spherical, granulated, aggregated, irregularly crushed, etc., but an irregularly crushed shape is preferred in consideration of the water absorption rate of the water-absorbent resin. Here, the irregularly crushed shape refers to crushed particles having an irregular shape. The water-absorbent resin according to one embodiment of the present invention is preferably a crushed product obtained by aqueous solution polymerization. On the other hand, in cases where a crushing step is not performed, typically spherical particles or granulated products of spherical particles obtained by reverse-phase suspension polymerization or spray-droplet polymerization in which a polymerization monomer is sprayed or dropped to polymerize are not in an irregularly crushed shape. In one embodiment of the present invention, the average circularity of the water-absorbent resin before surface crosslinking is preferably 0.83 or less, more preferably 0.80 or less, and even more preferably 0.75 or less.

[0103]

[0111] [2-6] Surface cross-linking step This step is a step of providing a portion with a higher cross-linking density on the surface layer of the water absorbent resin before surface cross-linking obtained through the above-mentioned steps, and is configured to include a mixing step and a heat treatment step. In the surface cross-linking step, radical cross-linking, surface polymerization, a cross-linking reaction with a surface cross-linking agent, etc. occur on the surface of the water absorbent resin before the surface cross-linking step, and a surface cross-linked water absorbent resin is obtained.

[0104]

[0123] [2-6-1] Mixing step

[0124] This step is a step of mixing a surface crosslinking agent, preferably a solution containing a surface crosslinking agent (hereinafter referred to as "surface crosslinking agent solution"), with a water absorbent resin before surface crosslinking in a mixing device, to obtain a humidified mixture.

[0105] (Organic Surface Crosslinking Agent) In the present invention, an organic surface crosslinking agent capable of reacting with a carboxyl group during surface crosslinking is used. Specific examples of the organic surface crosslinking agent include polyhydric alcohol compounds, amino alcohols, alkylene carbonate compounds, oxazolidinone compounds, oxetane compounds, and epoxy compounds. It is preferable to use at least one organic surface crosslinking agent selected from these organic surface crosslinking agents. Furthermore, as the organic surface crosslinking agent, an organic surface crosslinking agent capable of forming an ester bond with a carboxyl group is preferred. Examples of the organic surface crosslinking agent that forms an ester bond, preferably a dehydrated ester bond, with a functional group of a polyacrylic acid (salt)-based water absorbent resin, such as a carboxyl group, include organic surface crosslinking agents having a hydroxyl group in the molecule, such as polyhydric alcohol compounds or amino alcohol compounds, and organic surface crosslinking agents that generate a hydroxyl group by ring-opening, such as alkylene carbonate compounds, oxazolidinone compounds, oxetane compounds, and epoxy compounds.

[0106] More specifically, the organic surface crosslinking agent includes ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, 1,3-propanediol, 1-methyl-1,3-propanediol, 2-methyl-1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, 2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin, polyglycerin, Serine, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanemethanol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanediol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol, mesoerythritol, D-sorbitol, sorbitol polyhydric alcohol compounds such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and glycidol; polyvalent amine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, and polyamidepolyamine, and inorganic or organic salts thereof, for example, aziridinium salts; polyvalent isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; polyvalent oxazoline compounds such as 1,2-ethylenebisoxazoline; oxazolidinone compounds such as N-acyloxazolidinone and 2-oxazolidinone;Alkylene carbonate compounds such as 1,3-dioxolan-2-one (ethylene carbonate), 4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxopan-2-one; cyclic urea compounds; oxetane compounds such as oxetane, 2-methyloxetane, 3-methyl-3-hydroxymethyloxetane, and 3-ethyl-3-hydroxymethyloxetane; and amino alcohol compounds such as ethanolamine. In addition, the organic surface crosslinking agent may be used in combination with an inorganic surface crosslinking agent such as a polyvalent metal compound such as a hydroxide or chloride of zinc, calcium, magnesium, aluminum, iron, zirconium, etc. These organic surface crosslinking agents may be used alone or in combination of two or more.

[0107] Among the above organic surface crosslinking agents, at least one organic surface crosslinking agent selected from the group consisting of polyhydric alcohol compounds, epoxy compounds, polyvalent amine compounds and their salts, oxetane compounds, and alkylene carbonate compounds is preferred, and at least one organic surface crosslinking agent selected from the group consisting of polyhydric alcohol compounds and alkylene carbonate compounds is more preferred. Preferably, the organic surface crosslinking agent is at least one selected from the group consisting of polyhydric alcohols having 3 to 8 carbon atoms (preferably 3 to 6 carbon atoms) and containing 2 to 3 hydroxyl groups in the molecule, epoxy compounds having 6 to 12 carbon atoms, alkylene carbonates having 3 to 5 carbon atoms, and oxetane compounds having 3 to 10 carbon atoms, and more preferably at least one selected from the group consisting of polyhydric alcohols having 3 to 8 carbon atoms (preferably 3 to 6 carbon atoms) and containing 2 to 3 hydroxyl groups in the molecule, and alkylene carbonates having 3 to 5 carbon atoms. In addition, as the above-mentioned organic surface crosslinking agent, one kind or two or more kinds of organic surface crosslinking agents are used in consideration of their reactivity and the heating temperature in the heat treatment step (two kinds may be used in combination). In addition, the organic surface crosslinking step may be carried out two or more times in consideration of the effect thereof, and in that case, the second or subsequent steps may be carried out using the same organic surface crosslinking agent as in the first step, or may be carried out using a different organic surface crosslinking agent.

[0108] The amount of the organic surface crosslinking agent used is preferably 0.01 parts by mass or more, preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 2 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin before surface crosslinking. The amount of the organic surface crosslinking agent used is preferably 0.01 parts by mass or more and 10 parts by mass or less, more preferably 0.01 parts by mass or more and 5 parts by mass or less, and even more preferably 0.01 parts by mass or more and 2 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin before surface crosslinking. By setting the amount of the organic surface crosslinking agent used within this range, an optimal crosslinked structure can be formed in the surface layer of the water-absorbent resin before surface crosslinking, and it becomes easier to obtain a water-absorbent resin or a water-absorbing agent composition with higher physical properties. Note that when multiple organic surface crosslinking agents are used, the amount used is the total amount thereof.

[0109] The organic surface crosslinking agent is preferably added to the water absorbent resin in a solution state (as a surface crosslinking agent solution), and more preferably added as an aqueous solution to the water absorbent resin before surface crosslinking. In this case, the amount of water used is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, even more preferably 0.5 parts by mass or more, and preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less, relative to 100 parts by mass of the water absorbent resin before surface crosslinking. A preferred range of the amount of water used can be a range defined by any combination selected from the above-mentioned upper and lower limit values. By setting the amount of water used within this range, the handleability of the surface crosslinking agent solution is further improved, and it becomes easier to uniformly mix the surface crosslinking agent with the water absorbent resin before surface crosslinking.

[0110] The concentration of the organic surface crosslinking agent in the surface crosslinking agent solution is preferably 0.1% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and preferably 60% by mass or less, more preferably 50% by mass or less, even more preferably 45% by mass or less, and even more preferably 35% by mass or less. A preferred range of the concentration of the organic surface crosslinking agent can be a range defined by any combination selected from the above-mentioned upper and lower limits. The concentration of the organic surface crosslinking agent in the surface crosslinking agent solution is, for example, preferably 0.1% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 50% by mass or less, even more preferably 15% by mass or more and 45% by mass or less, and even more preferably 15% by mass or more and 35% by mass or less. By setting the concentration of the surface crosslinking agent within the above range, an optimal crosslinked structure can be formed in the surface layer of the water absorbent resin before surface crosslinking, which has a high specific surface area, and physical properties such as water absorption performance can be improved. Note that when multiple organic surface crosslinking agents are used, the amount used is the total amount thereof.

[0111] Furthermore, a hydrophilic organic solvent can be used in combination with the water as needed to prepare the surface crosslinking agent solution. In this case, the amount of the hydrophilic organic solvent used is preferably 5 parts by mass or less (lower limit: 0 parts by mass), more preferably 3 parts by mass or less, and even more preferably 1 part by mass or less, relative to 100 parts by mass of the water absorbent resin before surface crosslinking. Specific examples of the hydrophilic organic solvent include lower alcohols such as methyl alcohol; ketones such as acetone; ethers such as dioxane; amides such as N,N-dimethylformamide; sulfoxides such as dimethyl sulfoxide; polyhydric alcohols such as ethylene glycol; and the like. However, although these hydrophilic organic solvents contribute as mixing aids for uniformly dispersing the surface crosslinking agent on the surface of the water absorbent resin, they lead to increased costs from a commercial perspective, and therefore, even when used, it is preferable to limit the amount used to as small as possible.

[0112]

[0222] In addition, various additives to be added in the following "[2-8] Additives and Addition Step thereof" can also be added to the above-mentioned surface crosslinking agent solution, or can be added separately in a mixing step, within a range of 5 parts by mass or less relative to 100 parts by mass of a water absorbent resin before surface crosslinking.

[0113] (Combined Use of Peroxide and Organic Surface Crosslinking Agent) In the present invention, a peroxide is mixed in a step subsequent to the polymerization step. The mixed peroxide, together with the organic surface crosslinking agent, undergoes a surface crosslinking reaction of the water-absorbent resin. The type of peroxide is not particularly limited, but inorganic peroxides such as hydrogen peroxide; persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; permanganates; and perchlorates, as well as organic peroxides such as cumene hydroperoxide, t-butyl hydroperoxide, di-t-butyl peroxide, benzoyl peroxide, and lauroyl peroxide, can be suitably used. One or more of these may be used in combination. Among these, inorganic peroxides are preferred, and persulfates are particularly preferred, in terms of high reactivity and excellent safety. For example, ammonium persulfate is used as a food additive in Japan.

[0114] The amount of the peroxide used (the amount of peroxide present in the surface cross-linking step, preferably the amount of peroxide added to the water absorbent resin before surface cross-linking in the surface cross-linking step) is 0.001 parts by mass or more, preferably 0.01 parts by mass or more, more preferably 0.02 parts by mass or more, particularly preferably 0.05 parts by mass or more, and 3 parts by mass or less, preferably 2 parts by mass or less, more preferably 1 part by mass or less, particularly preferably 0.5 parts by mass or less, relative to 100 parts by mass of the water absorbent resin before surface cross-linking. A preferred range of the amount of the peroxide used can be a range defined by any combination selected from the above-mentioned upper and lower limit values. The amount of the peroxide used is, for example, preferably 0.001 parts by mass or more and 3 parts by mass or less, preferably 0.01 parts by mass or more and 2 parts by mass or less, more preferably 0.02 parts by mass or more and 1 part by mass or less, particularly preferably 0.05 parts by mass or less, relative to 100 parts by mass of the water absorbent resin before surface cross-linking.

[0123] When the amount of the peroxide used is 0.001 part by mass or more, desired effects (improvement of liquid permeability and control of affinity of a surface of a water absorbent resin with a liquid to be absorbed) are likely to be obtained, and when the amount of the peroxide used is 3 parts by mass or less, deterioration of the water absorbent resin and decrease in physical properties such as an increase in water-soluble content can be suppressed.

[0115] The mass ratio of the organic surface cross-linking agent to the peroxide used in the surface cross-linking step is preferably 1:0.005 to 1:1, more preferably 1:0.01 to 1:0.8, even more preferably 1:0.03 to 1:0.5, still more preferably 1:0.05 to 1:0.5, and particularly preferably 1:0.05 to 1:0.4. When the amount of peroxide used relative to the organic surface cross-linking agent is within the above range, deterioration of the water-absorbing agent composition and deterioration of physical properties such as an increase in water-soluble content can be suppressed. Furthermore, the risk of the water-absorbing agent composition being colored brown or yellow can be suppressed, making the water-absorbing agent composition suitable for sanitary materials containing a high content of the water-absorbing agent composition.

[0116] Water may be used when mixing the peroxide and the water-absorbent resin. The amount of water used is preferably 0.5 parts by mass or more, more preferably 1.0 part by mass or more, and preferably 10 parts by mass or less, and more preferably 8 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin. A preferred range of the amount of water used can be a range defined by any combination selected from the above-mentioned upper and lower limits. The amount of water used is, for example, preferably 0.5 parts by mass or more and 10 parts by mass or less, and more preferably 1.0 parts by mass or more and 8 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin. When the amount of water used is equal to or more than the above-mentioned lower limit, uniform mixing into the entire water-absorbent resin is facilitated, and when the amount is equal to or less than the above-mentioned upper limit, deterioration of mixability such as the formation of lumps can be suppressed. Furthermore, the peroxide may be coexistent in the surface-crosslinking agent solution.

[0117] The timing of adding the peroxide is not particularly limited, and it may be added in any step after the polymerization step, as long as the peroxide and the organic surface crosslinking agent can be kept coexistent in the surface crosslinking step. The above-mentioned "after the polymerization step" means including the polymerization step. As described in the explanation of the polymerization step, some peroxides, such as sodium persulfate and potassium persulfate, are used as polymerization initiators. Furthermore, the peroxide can also function as a surface crosslinking agent.

[0118] Therefore, if the peroxide that is added in the polymerization step and remains in the reaction system without being used in the polymerization reaction is in the vicinity of the surface of the water absorbent resin, the peroxide will undergo a crosslinking reaction together with the organic surface crosslinking agent in the surface crosslinking step. Thus, the peroxide may be added only in the polymerization step. When the peroxide is added only in the polymerization step, it is preferable that the above-mentioned suitable amount of peroxide remains at the time of the surface crosslinking step.

[0119] The peroxide may be added at any time after the polymerization step, such as before the start of the drying step after the polymerization step, during the drying step, after the drying step, during or before or after the pulverization step, during or before or after the classification step, granulation or before or after the granulation step, other transport steps, intermediate storage steps (e.g., hopper storage), or surface cross-linking steps. In a preferred embodiment, the peroxide is added in the surface cross-linking step. When the peroxide is added in a step other than the polymerization step, it is preferable that the suitable amount of peroxide remains at the time of the surface cross-linking step. If the peroxide and the organic surface cross-linking agent can coexist near the surface of the water-absorbent resin in the surface cross-linking step, the peroxide and the organic surface cross-linking agent can cause a cross-linking reaction on the particle surface, thereby improving gel strength. Therefore, the liquid diffusibility and liquid permeability of the particulate water-absorbing agent composition can be improved, and the affinity of the water-absorbent resin surface with the absorbed liquid can be improved.

[0120] In the surface cross-linking step, if water is further present, mixing of the peroxide with the water absorbent resin and mixing of the organic surface cross-linking agent with the water absorbent resin are facilitated. As a result, the surface cross-linking reaction by the combined use of the peroxide and the organic surface cross-linking agent is reliably carried out, which is preferable. The amount of water used is as described above.

[0121]

[0123] As described above, if a peroxide and an organic surface crosslinking agent can coexist in the surface crosslinking step, the effect of the present invention can be obtained, but in the production method according to the present invention, it is particularly preferable to add an aqueous solution containing a peroxide and an organic surface crosslinking agent to a water absorbent resin before surface crosslinking (before the heat treatment step) in the surface crosslinking step.

[0122] In this case, since an aqueous solution containing a peroxide and an organic surface crosslinking agent is added to a water absorbent resin before surface crosslinking, the peroxide and the organic surface crosslinking agent simultaneously act as a surface crosslinking agent. Note that the "surface crosslinking agent" means a substance capable of performing surface crosslinking. As described above, if the peroxide and the organic surface crosslinking agent coexist in the surface crosslinking step, it is possible to realize improvement in affinity of the water absorbent resin surface with the absorbed liquid, and therefore the timing of adding the peroxide and the organic surface crosslinking agent do not necessarily have to be simultaneous, but by adding an aqueous solution containing a peroxide and an organic surface crosslinking agent to a water absorbent resin in the surface crosslinking step, a particularly high effect can be obtained.

[0123]

[0113] Usually, in a surface crosslinking step, a water absorbent resin and a surface crosslinking agent are mixed together, and then a heat treatment is carried out. However, when one of a peroxide or an organic surface crosslinking agent is added first, the component added first penetrates deeper into the water absorbent resin before the other is added, and as a result, there is a risk that a rate of crosslinking in the vicinity of the surface of a water absorbent resin obtained will decrease.

[0124] On the other hand, when it is added as an aqueous solution containing a peroxide and an organic surface cross-linking agent, the cross-linking reaction of both components occurs closer to the surface, and therefore, it is thought that a stronger surface cross-linked layer than before is formed, and the gel strength is improved.

[0125] In one embodiment, the production method according to the present invention is carried out by adding an aqueous solution of a peroxide after the polymerization step and before the surface cross-linking step, preferably after the drying step, and adding an aqueous solution of an organic surface cross-linking agent in the surface cross-linking step. In this case, cross-linking by the peroxide also occurs inside the particles, so that the absorption capacity under load (AAP) is slightly reduced compared with the method in which an aqueous solution containing a peroxide and an organic surface cross-linking agent is added to a water absorbent resin in the surface cross-linking step.

[0126] From this point of view, it can be said that it is more effective to perform crosslinking by peroxide closer to the surface of the water absorbent resin. Note that the above-mentioned "after the polymerization step" means including the polymerization step, and "before performing the surface crosslinking step" means not including the surface crosslinking step.

[0127] The water content of the mixture obtained by adding the aqueous solution of peroxide after the drying step is preferably 0.5% by mass or more and less than 10% by mass. When the water content is 0.5% by mass or more, uniform mixing is facilitated. When the water content is less than 10% by mass, the formation of lumps is suppressed, improving mixability and handling.

[0128] Here, a heating step may be included after the drying step and the addition of the aqueous peroxide solution and before the surface cross-linking step. The heating temperature is not particularly specified, but is preferably 40°C or higher, more preferably 50°C or higher, and preferably 200°C or lower, more preferably 150°C or lower. The preferred range of the heating temperature can be a range specified by any combination selected from the above upper and lower limit values. According to the above configuration, it is possible to easily disintegrate undesired aggregates in the mixture obtained by adding the above-mentioned aqueous peroxide solution and to improve handleability.

[0129] The surface cross-linking reaction is preferably carried out by mixing an organic surface cross-linking agent, a peroxide, and water with the water-absorbent resin before surface cross-linking, as described above, followed by heat treatment. The heat treatment temperature, depending on the surface cross-linking agent used, is preferably 130°C or higher, more preferably 150°C or higher, even more preferably 180°C or higher, and may also be preferably 300°C or lower, more preferably 250°C or lower, and even more preferably 230°C or lower. The preferred range of the heat treatment temperature can be a range defined by any combination selected from the above upper and lower limits. The heat treatment temperature is preferably 130°C or higher and 300°C or lower, more preferably 150°C or higher and 250°C or lower, and even more preferably 180°C or higher and 230°C or lower. If the heat treatment temperature is lower than 130°C, absorption properties such as absorbency under pressure and liquid permeability may not be sufficiently improved. If the heat treatment temperature exceeds 300°C, deterioration of the water-absorbent resin may occur, resulting in a decrease in various performances, so caution is required.

[0130] The heat treatment temperature is determined by the heat medium temperature, but in cases where the heat medium temperature cannot be determined, such as in the case of microwaves, it is determined by the material temperature. The heat treatment time is preferably at least 1 minute, more preferably at least 5 minutes, and preferably at most 2 hours, more preferably at most 1 hour. The preferred range of the heat treatment time can be determined by any combination selected from the upper and lower limits above.

[0131]

[0044] In addition, in order to mix a water absorbent resin and a surface crosslinking agent more uniformly, a non-crosslinkable water-soluble inorganic base other than peroxide (preferably, alkali metal salt, ammonium salt, alkali metal hydroxide, and ammonia or its hydroxide), a non-reducing alkali metal salt pH buffer (preferably, hydrogen carbonate, dihydrogen phosphate, hydrogen phosphate, etc.), further a surfactant or inorganic fine particles (fumed silica, colloidal silica, colloidal alumina, etc.) may be made to coexist when mixing a water absorbent resin and a surface crosslinking agent. The amount of these used depends on the type, particle size, etc. of the water absorbent resin, but is preferably 0.005 part by mass or more, more preferably 0.05 part by mass or more, and preferably 10 parts by mass or less, and more preferably 5 parts by mass or less, relative to 100 parts by mass of the water absorbent resin. The preferred range of the above-mentioned amount used can be a range defined by any combination selected from the above-mentioned upper and lower limit values.

[0132]

[0123] A mixing method for mixing a water absorbent resin and a surface crosslinking agent is not particularly limited, and examples thereof include a method of immersing a water absorbent resin in a hydrophilic organic solvent, and mixing a surface crosslinking agent dissolved in water and / or a hydrophilic organic solvent as necessary, and a method of spraying or dropping a surface crosslinking agent dissolved in water and / or a hydrophilic organic solvent directly onto a water absorbent resin and mixing it therewith.

[0133] (Mixing method, mixing conditions) As a method for mixing the water absorbent resin before surface crosslinking with the surface crosslinking agent solution (in some cases, a surface crosslinking agent solution containing a peroxide), a surface crosslinking agent solution is prepared in advance, and the solution is added to the water absorbent resin before surface crosslinking, preferably by spraying, dropping, or a straight rod flow, and mixed. More preferably, a method of spraying and mixing is mentioned.

[0134]

[0044] In the case of addition in the form of a straight rod flow, it is preferable to design an inner diameter of an addition nozzle so that the flow velocity of the surface crosslinking agent solution in the nozzle is preferably in the range of 1 m / s or more and 20 m / s or less, more preferably 1 m / s or more and 10 m / s or less, and further preferably 1 m / s or more and 5 m / s or less. When the flow velocity in the nozzle is 1 m / s or more, the shape is unlikely to become droplet-like and intermittent addition can be suppressed, which is preferable from the viewpoint of mixability. On the other hand, when the flow velocity in the nozzle is 20 m / s or less, it is preferable because it is possible to suppress a part of the surface crosslinking agent solution being discharged in a liquid state without being mixed with the water absorbent resin due to scattering of the surface crosslinking agent solution charged into a mixing apparatus, or a part of the surface crosslinking agent solution being scattered in the opposite direction to the traveling direction of the water absorbent resin in the mixing apparatus, which increases adhesion in the mixing apparatus and causes clogging.

[0135] In order to mix the water absorbent resin before surface cross-linking and the surface cross-linking agent solution more uniformly, it is preferable to add the surface cross-linking agent solution from a plurality of points of a mixing device. When adding from a plurality of points, the number of addition nozzles is preferably 2 or more, more preferably 5 or more, further preferably 10 or more, particularly preferably 20 or more. The upper limit of the number of addition nozzles is 100 or less. As the number of the addition nozzles increases, the water absorbent resin and the surface cross-linking agent solution are mixed more uniformly, but on the other hand, the probability of occurrence of a malfunction such as clogging of the addition nozzle during feeding increases, and it may become difficult to identify the malfunctioning addition nozzle.

[0136] As a mixer for carrying out the above-mentioned mixing, a mixer having a torque necessary for uniformly and reliably mixing the water absorbent resin before surface cross-linking and the surface cross-linking agent solution is preferred. The mixer is preferably a high-speed stirring mixer, and more preferably a high-speed stirring continuous mixer. The rotation speed of the high-speed stirring mixer is preferably 100 rpm or more, more preferably 300 rpm or more, and preferably 10,000 rpm or less, more preferably 2,000 rpm or less. A preferred range of the rotation speed can be a range specified by any combination selected from the above-mentioned upper and lower limit values.

[0137]

[0063] The temperature of the water absorbent resin before surface crosslinking to be supplied to this step is preferably 35°C or higher, preferably 80°C or lower, more preferably 70°C or lower, and further preferably 60°C or lower, from the viewpoint of mixability with a surface crosslinking agent solution and coagulation property of a humidified mixture. The mixing time is preferably 1 second or higher, more preferably 5 seconds or higher, and preferably 1 hour or lower, more preferably 10 minutes or lower. The preferred ranges of the temperature of the water absorbent resin before surface crosslinking and the mixing time can be set to ranges specified by any combination selected from the above upper and lower limit values.

[0138] [2-6-2] Heat Treatment Step (Heat Treatment Apparatus) This step is a step in which heat is applied to the humidified mixture obtained in the mixing step to cause a crosslinking reaction on the surface of the water-absorbent resin before surface crosslinking. The heat treatment is carried out simultaneously with or after the addition of the organic surface crosslinking agent to the water-absorbent resin. Preferably, the heat treatment is carried out after the addition of the organic surface crosslinking agent to the water-absorbent resin, more preferably after the addition of the organic surface crosslinking agent and peroxide to the water-absorbent resin. The heat treatment of the humidified mixture may be carried out by heating the humidified mixture in a stationary state or in a fluidized state using a power such as stirring. However, heating under stirring is preferred in that the entire humidified mixture can be heated uniformly. Specific examples of heat treatment apparatuses for carrying out the heat treatment include paddle dryers, multi-fin processors, tower dryers, etc.

[0139] When using the above heat treatment device, particularly a continuous conduction heat transfer type heat treatment device, the heat transfer area (heating area) is not particularly limited as long as it can be controlled so that the powder temperature of the mixture falls within the following temperature range. However, it is preferable that the heat transfer area is 5 m with respect to the amount of the mixture to be treated. ２ / (t / hr) or more, more preferably 10 m ２ / (t / hr) or more, preferably 100m ２ / (t / hr) or less, more preferably 50m ２ / (t / hr) or less. A preferred range of the heat transfer area can be a range defined by any combination selected from the upper and lower limits. By setting the heat transfer area within the above range, it becomes easy to control the powder temperature of the mixture and the temperature of the inner wall surface of the heat treatment device within the following temperature ranges, and stable operation becomes possible, which is preferable.

[0140] The heat treatment apparatus used in the present invention preferably includes a dryer or a heating furnace equipped with a gas supply mechanism and / or a gas exhaust mechanism (hereinafter sometimes referred to as a "gas supply / exhaust mechanism"). The atmospheric water vapor density within the heat treatment apparatus is controlled by the gas supply / exhaust mechanism. In this case, it is preferable to not only install an intake port / exhaust port, but also to adjust the amount and pressure of the gas circulating within the heat treatment apparatus using a blower or the like. Furthermore, the number of intake port / exhaust port is not limited to one each, and multiple intake port / exhaust port can be provided, taking into consideration the size of the heat treatment apparatus used and the atmospheric water vapor density.

[0141] Furthermore, the heat treatment device may be used as a plurality of heat treatment devices in which the same or different heating methods, stirring methods, gas supply methods and exhaust methods are combined.

[0142] (Water Vapor Density Inside Heat Treatment Device) The lower limit of the water vapor density inside the heat treatment device used in this heat treatment step is greater than 0.005 g / L, preferably greater than 0.05 g / L, more preferably 0.07 g / L or more, even more preferably 0.10 g / L or more, even more preferably 0.15 g / L or more, and particularly preferably 0.20 g / L or more. If the water vapor density is 0.005 g / L or less, the amount of water evaporating from the water absorbent resin is large and the evaporation rate is fast, making it impossible to obtain a water absorbent resin with excellent affinity for the desired absorbed liquid. Furthermore, the upper limit of the water vapor density inside the heat treatment device used in this heat treatment step is preferably 0.60 g / L or less, more preferably 0.55 g / L or less, and even more preferably 0.50 g / L or less. The preferred range of the water vapor density can be a range defined by any combination selected from the above upper and lower limits. The water vapor density inside the heat treatment device used in this heat treatment step is preferably more than 0.005 g / L and not more than 0.60 g / L, more preferably more than 0.05 g / L and not more than 0.60 g / L, even more preferably 0.07 g / L or more and not more than 0.60 g / L, even more preferably 0.10 g / L or more and not more than 0.60 g / L, even more preferably 0.15 g / L or more and not more than 0.55 g / L, and particularly preferably 0.20 g / L or more and not more than 0.50 g / L. Having the water vapor density below the upper limit makes it easier to control the water vapor density without placing an excessive burden on the production equipment. Details such as the conditions for measuring the water vapor density are described in the Examples.

[0143] In the present invention, the water vapor density in this step means the average water vapor density of the gas present in the space above the mixture in the heat treatment device, and is preferably measured vertically above the mixture being heated in the heating section of the heat treatment device.

[0144] In the present invention, the water vapor density is the value obtained by dividing the weight of water contained in a gas by the amount of non-condensable gas, and has the dimension of "weight / volume." Specifically, the condensable component (water) is collected and measured from the sampled gas by cooling, solvent absorption, or the like, and the amount of the remaining non-condensable gas is measured using a gas meter or the like. The water vapor density is determined by dividing the weight of the collected water by the volume of the amount of non-condensable gas converted to the standard conditions of 0°C and 1 atmosphere. In the present invention, the term "non-condensable gas" refers to a gas that is in a gaseous state under the standard conditions of 0°C and 1 atmosphere. The non-condensable gas preferably used is air or nitrogen, or a mixture of air and nitrogen.

[0145] (Operation Conditions) In order to achieve the effects of the present invention, the present invention is characterized in that the water vapor density inside the heat treatment device used in the heat treatment step is controlled to more than 0.005 g / L. The operation conditions in the heat treatment step will be described in detail below.

[0146] In the present invention, in order to control the water vapor density in the heat treatment device within a desired range, a predetermined gas is introduced through an intake port provided in the heat treatment device, and gas present in the space above the mixture in the heat treatment device is discharged through an exhaust port. The flow rate of the intake and exhaust airflow is not particularly limited, but is preferably at least 0.1 (Nm ３ / hr) and 10,000 (Nm ３ / hr) or less, and 5000 (Nm ３ / hr) or less is more preferable, and 3000 (Nm ３ / hr) or less is more preferable. ３ / t) or less, and 1000 (Nm ３ / t) or less is more preferable. ３ " means the volume of gas converted to standard conditions (0°C, 1 atmosphere).

[0147] The gas introduced into the heat treatment device is not particularly limited as long as it can control the water vapor density within the above range, and examples thereof include a method of using steam, dry air, nitrogen, helium, argon, and dry air. In order to control the water vapor density, a method of using water vapor generated from water contained in the water absorbent resin by heating in this step may also be mentioned.

[0148] The pressure inside the heat treatment device is preferably slightly reduced. Specifically, the differential pressure (gauge pressure) relative to atmospheric pressure is preferably −10 kPa or more and 0 kPa or less, more preferably −5 kPa or more and 0 kPa or less, and even more preferably −2 kPa or more and 0 kPa or less. Adjusting the differential pressure (gauge pressure) is one way to control the water vapor density inside the heat treatment device within a desired range, and keeping the pressure inside the heat treatment device within the above range makes it easier to control the water vapor density to more than 0.005 g / L.

[0149] In this heat treatment step, the mixture is heat-treated, and the powder temperature is preferably controlled to be within a desired temperature range by controlling the heat medium temperature of the heat treatment device, the atmospheric temperature and atmospheric dew point inside the heat treatment device, the water vapor density, the inner wall surface temperature, the heat transfer area, the residence time, etc. The powder temperature is preferably 150°C or higher, more preferably 160°C or higher, and even more preferably 170°C or higher, and preferably 250°C or lower, more preferably 240°C or lower, and even more preferably 230°C or lower. The preferred range of the powder temperature can be a range defined by any combination selected from the upper and lower limits. If the powder temperature is lower than 150°C, the covalent bond for forming a surface cross-linked layer may be insufficient. On the other hand, if the powder temperature exceeds 250°C, the water-absorbent resin is thermally deteriorated, making it impossible to obtain a desired water-absorbent resin, which is not preferable.

[0150]

[0123] The powder temperature of the mixture means the highest temperature in a reaction step, and in a continuous type, it is evaluated by a temperature of a water absorbent resin (a reaction product after heat-treating a mixture of a water absorbent resin and a surface crosslinking agent) immediately after being discharged from a heat treatment device.

[0151] The residence time in the heat treatment device (heat treatment time) is not particularly limited, but is preferably 5 minutes or more and 90 minutes or less, and more preferably 10 minutes or more and 60 minutes or less.

[0152] The water vapor density may be controlled by appropriately controlling the supply amount, discharge amount, temperature, etc. of gas in consideration of heat transfer from the inner wall surface and water absorbent resin of the heat treatment device, the amount of water vapor generated from the water absorbent resin, etc. Specifically, examples include a method in which a measuring device is installed in the heat treatment device and the gas is introduced as needed to adjust the density, or a method in which the density is adjusted by changing the discharge amount or pressure of the gas, etc. In the present invention, a plurality of control methods may be appropriately combined.

[0153] The water vapor density varies depending on the position of the heated area and the treatment time, but it is desirable to control it to be within a certain range within the heat treatment device. The term "within a certain range" means that the water vapor density is within the above range for preferably at least 50%, more preferably at least 70%, and even more preferably at least 80% of the total time of the heat treatment, and that the variation is preferably within 0.017 g / L, more preferably within 0.009 g / L, even more preferably within 0.007 g / L, and particularly preferably within 0.006 g / L.

[0154] Furthermore, if the flow rate of the airflow is within the above range, the water vapor density at an appropriate measurement point in the gas exhaust mechanism of the heat treatment device may be taken as the water vapor density inside the heat treatment device specified in the present invention.

[0155] (Atmospheric dew point and atmospheric temperature in heat treatment device) In the present invention, the atmospheric dew point and atmospheric temperature in this step refer to the average dew point and average temperature of the gas present in the space above the mixture in the heat treatment device, and are preferably measured vertically above the mixture being heated in the heating section in the heat treatment device.

[0156] It is preferable to control the lower limit of the atmospheric dew point inside the heat treatment apparatus used in this heat treatment step to more than 0° C., 10° C. or more, 20° C. or more, 30° C. or more, 40° C. or more, or 45° C. or more, and the upper limit to 100° C. or less. When the atmospheric dew point is too low, the amount of water evaporated from the water absorbent resin is large and the evaporation rate is also fast, so that there is a risk that it is not possible to obtain a water absorbent resin having excellent affinity with a desired liquid to be absorbed.

[0157] Moreover, the atmospheric temperature inside the heat treatment device used in this heat treatment step is controlled preferably to 100° C. or more and 300° C. or less, more preferably 100° C. or more and 250° C. or less, and further preferably 100° C. or more and 230° C. or less. If the atmospheric temperature is lower than 100° C., moisture evaporated from the water absorbent resin condenses in the heating device, and further the water absorbent resin adheres, which may prevent stable continuous production and may lead to a decrease in productivity and physical properties.

[0158]

[0111] [2-7] Cooling step This step is an optional step that is provided as necessary after the heat treatment step in the surface cross-linking step. This step is a step of forcibly cooling the surface-crosslinked water absorbent resin that has been subjected to the heat treatment step to a predetermined temperature, thereby quickly completing the surface cross-linking reaction.

[0159] The cooling of the water absorbent resin after the surface cross-linking may be performed in a stationary state or in a fluidized state using a power such as stirring, but cooling under stirring is preferred in that the entire water absorbent resin can be cooled uniformly. From the above viewpoint, examples of the cooling device for performing the cooling include a paddle dryer, a multi-fin processor, and a tower dryer. Note that these cooling devices can also be made to have the same specifications as the heat treatment device used in the heat treatment step. This is because they can be used as cooling devices by changing the heat medium of the heat treatment device to a refrigerant.

[0160] The cooling temperature in this step may be appropriately set depending on the heating temperature in the heat treatment step, the water absorption performance of the water-absorbing agent composition or the water-absorbent resin after surface cross-linking, etc. Specifically, the temperature of the water-absorbent resin after surface cross-linking is preferably 150° C. or lower, more preferably 100° C. or lower, even more preferably 90° C. or lower, particularly preferably 80° C. or lower, and is preferably 20° C. or higher, more preferably 30° C. or higher. A preferred range of the temperature of the water-absorbent resin after surface cross-linking can be a range specified by any combination selected from the above upper and lower limit values.

[0161] The shape of the water-absorbent resin after surface cross-linking may be any of spherical, granulated, aggregated, irregularly crushed, etc., but an irregularly crushed shape is preferred in consideration of the water absorption rate of the water-absorbent resin. Furthermore, crushing the water-absorbent resin after surface cross-linking reduces the surface cross-linking effect, so the shape of the water-absorbent resin before and after surface cross-linking is preferably an irregularly crushed shape. An irregularly crushed water-absorbent resin can be obtained by crushing a hydrogel or a dried polymer. In one embodiment of the present invention, the average circularity of the particulate water-absorbing agent composition is preferably 0.83 or less, more preferably 0.80 or less, and even more preferably 0.75 or less. In the case of an aggregate, the average circularity of the primary particles is within the above range.

[0162]

[0133] In the water absorbent resin after surface crosslinking, (i) the mass proportion of particles less than 150 µm, (ii) D50 (mass average particle diameter), (iii) D50 (mass average particle diameter) and particles less than 150 µm, (iv) σζ (logarithmic standard deviation of particle size distribution), and the combinations and suitable ranges thereof are as described above.

[0163] [2-8] Additives and Addition Steps Thereof In the present invention, additives may be added to one or more of the water-absorbent resin before surface-crosslinking, the water-absorbent resin during surface-crosslinking, and the water-absorbent resin after surface-crosslinking. In other words, the water-absorbent agent composition may contain additives in addition to the water-absorbent resin. Examples of additives include a liquid-permeability improver or a component thereof, other additives, and the like, and these may be used alone or in combination of two or more.

[0164] [2-8-1] Additives (liquid permeability improver or agent containing the same) Examples of the liquid permeability improver used in the present invention include additives having a function of improving saline flow conductivity (hereinafter referred to as "SFC") and gel bed permeability under load or no load (hereinafter referred to as "GBP") of a water-absorbent agent composition or a water-absorbent resin, and for example, at least one compound selected from polyvalent metal salts, cationic polymers, and inorganic fine particles can be used, and two or more types can be used in combination as necessary.

[0165] These additives may be used not only for the purpose of improving liquid permeability, but also to fulfill other functions such as anti-caking agents under moisture absorption, agents for controlling the flow of powder, deodorants, fragrances, and binders for water-absorbent resins. When added for the purpose of other functions, they are called "same-component agents." The amount of the liquid permeability improver or the same-component agent added is appropriately determined depending on the compound selected. When these additives are used alone or in combination with two or more types, the appropriate range of each addition amount can be appropriately selected within the ranges described below.

[0166] The above-mentioned "GBP" is an abbreviation for Gel Bed Permeability, and is the permeability of a 0.9 mass % sodium chloride aqueous solution to a water-absorbent agent composition or a water-absorbent resin under load or in free swelling, and is a value measured in accordance with the GBP test method described in International Publication No. 2005 / 016393.

[0167] (Polyvalent Metal Salt) When a polyvalent metal salt is used, the polyvalent metal cation of the polyvalent metal salt is preferably divalent or more, more preferably trivalent or more, and preferably tetravalent or less. Usable polyvalent metals include aluminum and zirconium. Therefore, examples of polyvalent metal salts that can be used in this step include aluminum lactate, zirconium lactate, aluminum sulfate, and zirconium sulfate. Among these, aluminum lactate or aluminum sulfate is more preferred, and aluminum sulfate is even more preferred, from the viewpoint of the SFC improvement effect. The amount of the polyvalent metal salt to be added is preferably 0 mol or more to 3.6 × 10 mol per 1 g of the water absorbent resin. －５ less than 1.4 × 10 moles, more preferably 0 moles or more and 1.4 × 10 moles or less －５mol, more preferably 0 mol or more and 1.0 × 10 －５ Less than a mole.

[0168] (Cationic Polymer) When a cationic polymer is used, examples of the cationic polymer include substances described in U.S. Patent No. 7,098,284. Among them, vinylamine polymers are more preferred from the viewpoint of improving SFC and GBP. The mass average molecular weight of the cationic polymer is preferably 5,000 or more and 1,000,000 or less.

[0169] The amount of the cationic polymer added is preferably 0 parts by mass or more, more preferably more than 0 parts by mass, and is preferably less than 2.5 parts by mass, more preferably less than 2.0 parts by mass, and even more preferably less than 1.0 part by mass, relative to 100 parts by mass of the water absorbent resin.

[0170] (Inorganic fine particles) When inorganic fine particles are used, examples of the inorganic fine particles include substances described in U.S. Patent No. 7,638,570. Among them, silicon dioxide (amorphous fumed silica, colloidal silica, etc.) is preferred from the viewpoint of improving SFC and GBP.

[0171] When the inorganic fine particles have a primary particle diameter of less than 20 nm, they may be added in an amount of preferably 0 part by mass or more, more preferably more than 0 part by mass, preferably less than 1.2 parts by mass, more preferably less than 1.0 part by mass, and even more preferably less than 0.5 parts by mass relative to 100 parts by mass of the water-absorbent resin. When the inorganic fine particles have a primary particle diameter of 20 nm or more, they may be added in an amount of preferably 0 part by mass or more, more preferably more than 0 part by mass, preferably less than 2.0 parts by mass, more preferably less than 1.5 parts by mass, and even more preferably less than 1.0 part by mass relative to 100 parts by mass of the water-absorbent resin.

[0172] (Other Additives) Specific examples of other additives include chelating agents, inorganic reducing agents, aromatic substances, organic reducing agents, hydroxycarboxylic acid compounds, surfactants, compounds having phosphorus atoms, oxidizing agents, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, and thermoplastic fibers. One or more of these other additives can be used. Among these, chelating agents are preferred, and aminopolycarboxylic acids or aminopolyphosphates are more preferred. Specific examples of the chelating agent include chelating agents described in JP-A-11-060975, WO 2007 / 004529, WO 2011 / 126079, WO 2012 / 023433, JP-T-2009-509722, JP-A-2005-097519, JP-A-2011-074401, JP-A-2013-076073, JP-A-2013-213083, JP-A-59-105448, JP-A-60-158861, JP-A-11-241030, and JP-A-2-41155.

[0173] Other additives, particularly chelating agents, are added or contained in an amount preferably in the range of 0.001% by mass or more and 1% by mass or less based on the monomer or water-absorbent resin.

[0174] [2-8-2] Additive Additive Additives can be added before, after, or during at least one step selected from the step of preparing an aqueous monomer solution, the polymerization step, the gel-crushing step, the drying step, the pulverization step, the classification step, and the surface-crosslinking step. Preferably, they are added before, after, or during any step subsequent to the polymerization step.

[0175] When the additive is added to a water-absorbent resin, if the additive is a liquid or a solution in an aqueous medium such as water, it is preferable to spray the liquid or solution onto the water-absorbent resin and apply sufficient torque to uniformly and reliably mix the water-absorbent resin and the additive. On the other hand, if the additive is in a solid state such as a powder, it may be dry-blended with the water-absorbent resin, or an aqueous liquid such as water may be used as a binder.

[0176] Specific examples of the apparatus used for the mixing include an agitator mixer, a cylindrical mixer, a double-walled conical mixer, a V-shaped mixer, a ribbon mixer, a screw mixer, a fluidized rotary disk mixer, an airflow mixer, a double-arm kneader, an internal mixer, a grinding kneader, a rotary mixer, a screw extruder, etc. When an agitator mixer is used, its rotation speed is preferably 5 rpm or more, more preferably 10 rpm or more, and preferably 10,000 rpm or less, more preferably 2,000 rpm or less.

[0177] [2-9] Other Steps In the present invention, in addition to the steps described above, a granulation step, a sizing step, a fine powder removal step, a fine powder recovery step, a fine powder recycling step, an iron removal step, etc. may be carried out as necessary. In addition, at least one step selected from a transportation step, a storage step, a packaging step, a keeping step, etc. may be further included.

[0178] [3] Physical Properties of the Water-Absorbent Agent Composition The water-absorbent agent composition obtained through the above-described steps becomes a final product if it is in a state ready for shipment. The water-absorbent agent composition according to the present invention is a water-absorbent agent composition containing a water-absorbent resin as a main component, and satisfies all of the following (1) and (2):

[0179] (1) The specific surface area of ​​the water-absorbing agent composition is 25 m ２ / kg or more (2) The water-absorbing agent composition has a Washburn contact angle θ2 of 72° or less when measured with a 20% by mass aqueous solution of sodium chloride.

[0180] [3-1] Specific surface area The specific surface area of ​​the water-absorbing agent composition according to the present invention is 25 m ２ By setting the specific surface area to 26 m / kg or more, a more excellent water absorption rate and crevice water retention capacity can be obtained. ２ / kg or more, 27m ２ / kg or more, 28m ２ / kg or more, 29m ２ / kg or more, 30m ２ / kg or more is preferable, and ２ The specific surface area of ​​the water-absorbing agent composition according to the present invention may be 60 m / kg or more. ２ / kg or less, 55m２ / kg or less is preferable in that order. A preferred range of the specific surface area of ​​the water-absorbent agent composition can be a range defined by any combination selected from the upper and lower limits. From the viewpoint of increasing the water absorption rate and increasing the crevice water retention capacity, the higher the specific surface area, the more desirable it is. However, if the specific surface area is too high, excessive foaming polymerization in the polymerization step or too fine gel crushing in the gel crushing step may be required, resulting in a risk of a decrease in AAP (absorbency against pressure) or SFC (salt flow conductivity). On the other hand, if the specific surface area of ​​the water-absorbent agent composition is too small, it is difficult to obtain a water-absorbent agent composition having the desired water absorption rate and crevice water retention capacity, which is not preferable. The specific surface area of ​​the water-absorbent agent composition may be, for example, 25 m ２ / kg or more 60m ２ / kg or less, 26m ２ / kg or more 60m ２ / kg or less, 27m ２ / kg or more 60m ２ / kg or less, 28m ２ / kg or more 60m ２ / kg or less, 29m ２ / kg or more 60m ２ / kg or less, 30m ２ / kg or more 55m ２ / kg or less, or 36m ２ / kg or more 55m ２ / kg or less.

[0181] In this specification, the term "specific surface area" refers to the surface area per unit mass of a water-absorbing agent composition or a water-absorbing resin (unit: m ２ / kg) and can be obtained by analyzing three-dimensional image data of the water-absorbent agent composition or the water-absorbent resin obtained using a microfocus X-ray CT system (manufactured by Shimadzu Corporation: inspeXio SMX-100CT) described below with high-speed three-dimensional analysis software (manufactured by Ratoc System Engineering Co., Ltd.: TRI / 3D-VOL-FCS64).

[0182] [3-2] Washburn contact angles θ1 and θ2 The Washburn contact angle θ1 (unit: °) of the water-absorbing agent composition according to the present invention is preferably 65° or less, 63° or less, 60° or less, 58° or less, 56° or less, 55° or less, 54° or less, 53° or less, 52° or less, and 51° or less, in this order, and is also preferably 20° or more, 22° or more, 25° or more, 28° or more, and 30° or more, in this order. A preferred range of the Washburn contact angle θ1 can be a range defined by any combination selected from the above upper and lower limit values. The Washburn contact angle θ1 (unit: °) of the water-absorbent agent composition according to the present invention may be, for example, 20° or more and 65° or less, 22° or more and 63° or less, 25° or more and 60° or less, 28° or more and 58° or less, 30° or more and 56° or less, 30° or more and 55° or less, 30° or more and 54° or less, 30° or more and 53° or less, 30° or more and 52° or less, or 30° or more and 51° or less.

[0183] The Washburn method contact angle θ2 (unit: °) of the water-absorbent agent composition according to the present invention is preferably 72° or less, 70° or less, 68° or less, 67° or less, 65° or less, 63° or less, 61° or less, or 60° or less, in this order, and is also preferably 25° or more, 27° or more, 30° or more, 33° or more, 35° or more, 40° or more, 45° or more, or 50° or more, in this order. The preferred range of the Washburn method contact angle θ2 can be a range defined by any combination selected from the upper and lower limit values ​​described above. If the Washburn method contact angle θ2 (unit: °) of the water-absorbent agent composition according to the present invention exceeds 72°, it becomes difficult to improve the water absorption characteristics under pressure (for example, AAP or crevice water multiplier under pressure) while maintaining a high water absorption rate, which is an advantage of a high specific surface area. The Washburn contact angle θ2 (unit: °) of the water-absorbent agent composition according to the present invention is preferably, for example, in the order of 25° or more and 72° or less, 27° or more and 70° or less, 30° or more and 68° or less, 33° or more and 67° or less, 35° or more and 65° or less, 40° or more and 63° or less, 45° or more and 61° or less, or 50° or more and 60° or less.

[0184] By controlling the Washburn contact angle θ2 to 72° or less, and preferably the Washburn contact angle θ1 to 65° or less, a water-absorbing agent composition having excellent affinity between the water-absorbent resin surface and the absorbed liquid and excellent crevice water retention is obtained. In terms of excellent affinity between the water-absorbent resin surface and the absorbed liquid, the smaller the Washburn contact angles θ1 and θ2, the more preferable. However, in order to achieve a Washburn contact angle of less than 20° in the case of θ1 and less than 25° in the case of θ2, excessive additional treatment or the addition of an excessive hydrophilic surfactant is required, which is not preferable from the viewpoint of economy.

[0185] The Washburn contact angle is calculated by measuring the time it takes for a liquid to rise between the powder particles from the bottom of a tube filled with the powder and the wetting load, and using the capillary constant calculated for a liquid (such as hexane or methanol) that results in a contact angle of 0°. The contact angle can be determined by analyzing penetration rate data obtained using a high-performance surface tensiometer (DyneMaster series DY-500, manufactured by Kyowa Interface Science Co., Ltd.) described below with the accompanying analysis software DYNALYZER.

[0186] [3-3] Other Properties of the Water-Absorbent Agent Composition The water-absorbent agent composition according to the present invention preferably has at least one of the following properties (a) to (k) within a suitable range.

[0187] (a) D50 (mass average particle diameter), (b) mass ratio of particles with a particle diameter of less than 150 μm, (c) σζ (logarithmic standard deviation of particle size distribution), (d) CRC (absorption capacity without load), (e) AAP (absorption capacity under load), (f) crevice water capacity under load, (g) water content, (h) SFC (salt flow conductivity), (i) Vortex (water absorption rate), (j) surface tension, (k) remaining amount of peroxide

[0188] The water-absorbing agent composition according to the present invention may have a combination of any two or more of the preferred ranges of the above physical properties (a) to (k). Preferably, it is a combination of at least the preferred ranges of (e) AAP (absorbency against pressure) and (g) water content, more preferably, it is a combination of these and a preferred range of (f) crevice water capacity against pressure, or preferably, it is a combination of the preferred ranges of (e) AAP (absorbency against pressure) and (f) crevice water capacity against pressure, even more preferably, it is a combination of these and a preferred range of (i) Vortex (water absorption rate), and even more preferably, it may have the preferred ranges of (a) to (c) in addition to these. Most preferably, it has all of the preferred ranges of (a) to (k). Specific preferred ranges of the above physical properties (a) to (k) are the ranges described for each item below.

[0189] (a) D50 (mass average particle diameter), (b) mass proportion of particles with a particle diameter of less than 150 μm, (c) σζ (logarithmic standard deviation of particle size distribution) D50 (mass average particle diameter), mass proportion of particles with a particle diameter of less than 150 μm, and σζ (logarithmic standard deviation of particle size distribution) of the water-absorbent agent composition according to the present invention may be set to the same ranges as those of the water-absorbent resin after classification and before surface-crosslinking.

[0190] By setting the (a) D50 (mass average particle diameter) of the water-absorbent agent composition within the above range, it is possible to control the preferable absorption properties, AAP (absorbency against pressure) and SFC (saline flow conductivity), in a more balanced manner. If the D50 (mass average particle diameter) is too small, the gel bulk density may become too high, or the preferable absorption properties, AAP and SFC, may become too low. On the other hand, if the D50 (mass average particle diameter) is too large, the coarseness of the particles of the water-absorbent agent composition becomes noticeable, and when used in absorbent articles such as disposable diapers and sanitary napkins, the feel on the skin and wearing comfort may deteriorate.

[0191] Furthermore, by setting the mass proportion of particles (b) having a particle diameter of less than 150 μm within the above range, it becomes easier to control the AAP (absorbency against pressure) and SFC (saline flow conductivity) in a more balanced manner. (b) If the mass proportion of particles having a particle diameter of less than 150 μm is too large, not only is there a risk that the AAP (absorbency against pressure), which is a desirable absorption characteristic, becomes too low, but there is also a risk that the working environment will deteriorate due to the scattering of dust in the place where the water-absorbing agent composition is handled, and that handling will become difficult due to the accumulation of fine particles inside the device, which is not preferable.

[0192] Furthermore, it is preferable that (a) the D50 (mass average particle diameter) of the water-absorbing agent composition is 250 μm or more and less than 550 μm, and (b) the mass proportion of particles with a particle diameter of less than 150 μm is 3 mass% or less. More preferably, the water-absorbing agent composition satisfies the above-mentioned range of D50 (mass average particle diameter) and also satisfies the above-mentioned range of mass proportion of particles with a particle diameter of less than 150 μm. By satisfying both of these, the above-mentioned effects are synergistically obtained. The D50 (mass average particle diameter) of the water-absorbing agent composition and the mass proportion of particles with a particle diameter of less than 150 μm are measured by the methods described in the Examples.

[0193] (d) CRC (absorbency without load) The CRC (absorbency without load) of the water-absorbing agent composition according to the present invention is preferably 25 g / g or more, preferably 40 g / g or less, more preferably 38 g / g or less, even more preferably 35 g / g or less, and particularly preferably 33 g / g or less. If the CRC (absorbency without load) is too low, the absorption capacity of the water-absorbing agent composition decreases, and the composition may not be suitable for use as an absorbent in absorbent articles such as disposable diapers and sanitary napkins. On the other hand, if the CRC (absorbency without load) is too high, the gel strength may be weakened.

[0194] (e) AAP (Absorbency Under Load) The AAP (Absorbency Under Load) of the water-absorbing agent composition according to the present invention is preferably 20.0 g / g or more, more preferably 23.0 g / g or more, even more preferably 25.5 g / g or more, more preferably 26.0 g / g or more, still more preferably 26.5 g / g or more, and is preferably 32.0 g / g or less, more preferably 31.0 g / g or less, still more preferably 30.0 g / g or less, more preferably 29.0 g / g or less. A preferred range of the AAP (Absorbency Under Load) can be a range defined by any combination selected from the above upper and lower limit values. The AAP (absorbency against load) of the water-absorbent agent composition according to the present invention is, for example, preferably 20.0 g / g or more and 32.0 g / g or less, more preferably 23.0 g / g or more and 31.0 g / g or less, more preferably 25.5 g / g or more and 30.0 g / g or less, more preferably 26.0 g / g or more and 29.0 g / g or less, and even more preferably 26.5 g / g or more and 29.0 g / g or less. By setting the AAP (absorbency against load) within the above range, the water-absorbent agent composition can absorb the liquid to be absorbed against the load even when pressure is applied to the absorbent body, and therefore the water-absorbent resin or the water-absorbent agent composition becomes suitable for use in absorbents in absorbent articles such as disposable diapers and sanitary napkins.

[0195] (f) Crevice Water Capacity Under Pressure The crevice water capacity under pressure of the water-absorbent agent composition according to the present invention is preferably 9.0 g / g or more, 9.3 g / g or more, 9.5 g / g or more, 10.0 g / g or more, 11.0 g / g or more, and 12.0 g / g or more, in this order. The upper limit is not particularly limited, but is preferably 19.0 g / g or less, and may be 18.0 g / g or less, or 17.0 g / g or less. A preferred range of the crevice water capacity under pressure can be a range defined by any combination selected from the upper and lower limits. If the crevice water capacity under pressure is less than 9.0 g / g, when the composition is used as an absorbent material for absorbent articles such as disposable diapers, body fluids such as urine and blood may not be sufficiently absorbed under pressure, which may cause liquid leakage. Therefore, the composition is not suitable for use as an absorbent material for absorbent articles such as disposable diapers. The crevice water multiplier under pressure of the water-absorbent agent composition according to the present invention is, for example, 9.0 g / g or more and 19.0 g / g or less, 9.3 g / g or more and 19.0 g / g or less, 9.5 g / g or more and 19.0 g / g or less, 10.0 g / g or more and 19.0 g / g or less, 11.0 g / g or more and 18.0 g / g or less, or 12.0 g / g or more and 17.0 g / g or less.

[0196] (g) Moisture Content The moisture content of the water-absorbing agent composition according to the present invention is preferably 5% by mass or less, more preferably 4% by mass or less, 3% by mass or less, and 2% by mass or less in that order. Also, it is preferably more than 0% by mass. If the moisture content exceeds 5% by mass, the surface of the water-absorbing agent composition begins to become sticky, the flowability as a powder decreases, and handling becomes difficult, which is not preferable.

[0197] (h) SFC (Saline Flow Conductivity) The SFC (Saline Flow Conductivity) of the water-absorbing agent composition according to the present invention is 1×10 －７ cm ３ ・sec / g or more, 3×10 －７ cm ３ ・More than sec / g, 5×10 －７ cm ３ ・sec / g or more, 10×10 －７ cm ３ ・sec / g or more, 20×10 －７ cm ３ ・sec / g or more, 25×10 －７ cm ３・sec / g or more, 30×10 －７ cm ３ The SFC (Saline Flow Conductivity) of the water-absorbing agent composition according to the present invention is preferably 120×10 －７ cm ３ ・sec / g or less, 110×10 －７ cm ３ ・sec / g or less, 100×10 －７ cm ３ ・sec / g or less, 90×10 －７ cm ３ ・sec / g or less, 80×10 －７ cm ３ ・sec / g or less, 70×10 －７ cm ３ ・sec / g or less, 60×10 －７ cm ３ ・sec / g or less, 50×10 －７ cm ３ ・sec / g or less, 40×10 －７ cm ３ The preferred range of the SFC (Saline Flow Conductivity) can be a range defined by any combination selected from the upper and lower limit values. The SFC (Saline Flow Conductivity) of the water-absorbent agent composition according to the present invention is, for example, 1×10 －７ cm ３ ・sec / g or more 120×10 －７ cm ３ ・sec / g or less, 3×10 －７ cm ３ ・sec / g over 110×10 －７ cm ３ ・sec / g or less, 5×10 －７ cm ３ ・sec / g or more 100×10 －７ cm ３ ・sec / g or less, 10×10 －７ cm ３ ・sec / g or more 90×10 －７ cm ３ ・sec / g or less, 20×10 －７ cm ３ ・sec / g or more 80×10 －７ cm ３ ・sec / g or less, 25×10 －７cm ３ ・sec / g or more 70×10 －７ cm ３ ・sec / g or less, 25×10 －７ cm ３ ・sec / g or more 60×10 －７ cm ３ ・sec / g or less, 25×10 －７ cm ３ ・sec / g or more 50×10 －７ cm ３ ・sec / g or less, 30×10 －７ cm ３ ・sec / g or more 40×10 －７ cm ３ When the SFC is equal to or higher than the lower limit, gel blocking is suppressed and deterioration of the absorption properties of the sanitary material is suppressed even when the absorbent composition is used at a high concentration in a thin sanitary material, and when the SFC is equal to or lower than the upper limit, deterioration of the CRC, which is in a trade-off relationship, is suppressed.

[0198] (i) Vortex (Water Absorption Rate) The upper limit of the vortex (water absorption rate) of the water-absorbent agent composition according to the present invention is preferably 50 seconds or less, 48 ​​seconds or less, 46 seconds or less, 44 seconds or less, 42 seconds or less, 40 seconds or less, 38 seconds or less, and 36 seconds or less, in that order. The lower limit of the vortex (water absorption rate) of the water-absorbent agent composition according to the present invention is preferably more than 10 seconds, more preferably 15 seconds or more. A preferred range of the vortex (water absorption rate) can be a range defined by any combination selected from the upper and lower limits. The vortex (water absorption rate) of the water-absorbent agent composition according to the present invention is, for example, more than 10 seconds and less than 50 seconds, more than 10 seconds and less than 48 seconds, more than 10 seconds and less than 46 seconds, more than 10 seconds and less than 44 seconds, more than 10 seconds and less than 42 seconds, more than 10 seconds and less than 40 seconds, 15 seconds or more and less than 38 seconds, or 15 seconds or more and less than 36 seconds. If the Vortex (water absorption rate) is slow (for example, longer than 50 seconds), the water-absorbing rate of the obtained water-absorbent agent composition for body fluids such as urine and blood will be slow, and there is a risk of liquid leakage, making the composition unsuitable as an absorbent for absorbent articles such as disposable diapers. Note that the Vortex (water absorption rate) can be controlled by foam polymerization, gel crushing, the production method according to the present invention, etc.

[0199] (j) Surface Tension The surface tension of the water-absorbing agent composition according to the present invention is preferably 56 mN / m or more, more preferably 58 mN / m or more, even more preferably 60 mN / m or more, and particularly preferably 65 mN / m or more. There are no particular limitations on the upper limit, but from the viewpoint of the balance with other physical properties, it is preferably 75 mN / m or less. When the surface tension is 56 mN / m or more, the amount of liquid returning when pressure is applied to the absorbent core is not too large, so that the water-absorbing agent composition is suitable for use in absorbent articles such as disposable diapers. The preferred range of the surface tension can be a range defined by any combination selected from the upper and lower limit values.

[0200] (k) Residual Peroxide Amount In the water-absorbing agent composition according to the present invention, it is preferable that a peroxide decomposition product is present on the surface, and more preferably, the concentration of the peroxide decomposition product on the surface of the water-absorbing agent composition is higher than the concentration of the peroxide decomposition product inside the water-absorbing agent composition. According to the method for producing the water-absorbing agent composition according to the present invention, the remaining peroxide component (peroxide decomposition product) is distributed more on the surface of particles of the water-absorbing agent composition than inside the particles. More specifically, according to the production method, which is one embodiment of the water-absorbing agent composition according to the present invention, the peroxide is allowed to coexist with an organic surface cross-linking agent in the surface cross-linking step, and therefore the concentration of the peroxide decomposition product on the surface of the water-absorbing agent composition is higher than the concentration of the peroxide decomposition product inside the water-absorbing agent composition, compared to when peroxide is added as a polymerization initiator only during polymerization. This configuration is preferable because it makes it easier to achieve the effects of the present invention. Note that the peroxide is decomposed into a decomposition product, for example, by heat treatment during surface cross-linking. When the peroxide is sodium persulfate, the peroxide decomposition product becomes sodium sulfate, and when the peroxide is potassium persulfate, the peroxide decomposition product becomes potassium sulfate.

[0201] The amount of residual peroxide remaining on the particle surfaces of the water-absorbing agent composition according to the present invention after decomposition is preferably more than 0% by mass, 0.005% by mass or more, 0.010% by mass or more, 0.015% by mass or more, and 0.020% by mass or more, in that order, and more preferably 1.0% by mass or less, 0.7% by mass or less, 0.35% by mass or less, and 0.1% by mass or less. A preferred range of the residual peroxide amount can be a range defined by any combination selected from the upper and lower limit values. The residual peroxide amount is, for example, more than 0% by mass to 1.0% by mass, 0.005% by mass to 0.7% by mass, 0.010% by mass to 0.35% by mass, 0.015% by mass to 0.1% by mass, or 0.020% by mass to 0.1% by mass. When the residual amount of peroxide is 0% by mass or less, i.e., when the residual amount of peroxide on the particle surface is the same as or smaller than the residual amount of peroxide inside the particle, the peroxide does not act effectively on the particle surface of the water-absorbing agent composition, and therefore the desired effect (control of affinity between the surface of the water-absorbent resin and the absorbed liquid) cannot be obtained. On the other hand, when the residual amount of peroxide exceeds 1.0% by mass, it is not preferable because it causes deterioration of the water-absorbing agent composition and decreases in physical properties such as an increase in water-soluble content.

[0202] The residual amount of peroxide can be determined by the following methods (a) to (e).

[0203] (A) A predetermined impact test is performed on the water-absorbent agent composition according to the present invention; (B) The water-absorbent agent composition that has been subjected to the impact test is sieved using a JIS standard sieve with a mesh size of 300 μm into a particle group a having a particle size of 300 μm or more and a particle group b having a particle size of less than 300 μm; (C) the content of peroxide decomposition products present in the particle group a is taken as C1; (D) the content of peroxide decomposition products present in the particle group b is taken as C2; (E) the remaining amount of peroxide is calculated by "C2 - C1".

[0204] In the impact resistance test, most of the surface portions of the water-absorbing agent composition that are scraped off become the particle group B. Therefore, it can be said that the larger C2-C1 is, the more peroxides are present on the surface of the water-absorbing agent composition.

[0205] The details of the impact test in (A) above are described in the Examples.

[0206] [3-4] Relationship between Water-Absorbent Resin and Water-Absorbent Agent Composition The amount of the water-absorbent resin contained in the water-absorbent agent composition according to the present invention is preferably 95% by mass or more, more preferably 98% by mass or more, and even more preferably 99% by mass or more, based on the total amount of the water-absorbent agent composition. Note that, when the above-mentioned various additives are contained, the amount of the water-absorbent resin contained in the water-absorbent agent composition does not become 100% by mass.

[0207] The shape of the water-absorbing agent composition according to the present invention may be any of spherical, granulated, aggregated, irregularly crushed, etc., but is preferably in an irregularly crushed shape in consideration of the water absorption rate.

[0208] [4] Uses of Water-Absorbent Agent Composition The water-absorbent agent composition according to the present invention is preferably used mainly as an absorbent or an absorbent layer (hereinafter collectively referred to as "absorbent") of absorbent articles such as disposable diapers and sanitary napkins, and more preferably used as an absorbent of absorbent articles in which a large amount is used per absorbent article.

[0209] The absorbent body refers to a particulate water-absorbing agent composition formed into a sheet, fiber, cylinder, or other shape, and is preferably formed into a sheet to form an absorbent layer. In addition to the water-absorbing agent composition of the present invention, absorbent materials such as pulp fibers, adhesives, nonwoven fabrics, etc. can also be used in combination for forming the absorbent body. In this case, the amount of the water-absorbing agent composition in the absorbent body (hereinafter referred to as "core concentration") is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, particularly preferably 80% by mass or more, and preferably 100% by mass or less. By setting the core concentration within the above range, when the absorbent body is used in an absorbent article, even if the water-absorbing agent composition gels upon absorbing urine, appropriate spaces can be formed between the gel particles.

[0210] [5] Absorbent Article The absorbent article according to the present invention includes the absorbent core and typically includes a liquid-permeable top sheet and a liquid-impermeable back sheet. Examples of the absorbent article include disposable diapers and sanitary napkins.

[0211] When the absorbent article is, for example, a disposable diaper, the disposable diaper is produced by sandwiching an absorbent body containing the water-absorbing agent composition of the present invention between a liquid-permeable top sheet located on the side that comes into contact with the skin when worn and a liquid-impermeable back sheet located on the outside when worn. The disposable diaper is further provided with members known to those skilled in the art, such as adhesive tape, for fixing the disposable diaper after being worn.

[0212] In the absorbent article according to the present invention, when the absorbent body absorbs liquid and the water-absorbent agent composition swells and gels, appropriate spaces are created between the gel particles, preventing moisture from accumulating and reducing the stuffy feeling inside the disposable diaper. Furthermore, when the water-absorbent agent composition contains a fragrance or a deodorizer, these components are suitably evaporated through the spaces. Therefore, by virtue of either or both of the above-mentioned effects, an absorbent article that is comfortable for the wearer and their caregiver can be provided. Note that the water-absorbent agent composition according to the present invention can also be suitably used in applications such as a pet urine absorbent and a urine gelling agent for portable toilets, in addition to the above-mentioned disposable diapers and sanitary napkins.

[0213] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples, and can of course be practiced with appropriate modifications within the scope of the above-mentioned and below-mentioned aims, all of which are included within the technical scope of the present invention. Furthermore, in the present invention, the measurement methods for the above-mentioned physical properties are based on the measurement methods described in the examples unless otherwise specified.

[0214] The methods for measuring each physical property are as follows. For example, when the measurement target is something other than a water-absorbent agent composition, the term "water-absorbent agent composition" in the following description is applied by replacing it with "particulate hydrogel," "water-absorbent resin before surface-crosslinking," or "water-absorbent resin after surface-crosslinking."

[0215] [D50 (mass average particle diameter), mass proportion of particles with a particle diameter of less than 150 μm, and σζ (logarithmic standard deviation of particle size distribution)] D50 (mass average particle diameter), mass proportion of particles with a particle diameter of less than 150 μm, and σζ (logarithmic standard deviation of particle size distribution) of the water-absorbing agent composition according to the present invention were measured in accordance with the measurement method described in U.S. Pat. No. 7,638,570.

[0216] [CRC (absorption capacity without load)] The CRC (absorption capacity without load) of the water-absorbent agent composition according to the present invention was measured in accordance with the EDANA method (NWSP241.0.R2(19)). Specifically, 0.2 g of the water-absorbent agent composition was placed in a nonwoven bag, and then immersed in a large excess of a 0.9 mass % aqueous sodium chloride solution for 30 minutes to allow the water-absorbent agent composition to freely swell. Thereafter, the water-absorbent agent composition was dehydrated using a centrifuge (250 G), and the absorption capacity without load (CRC) (unit: g / g) was measured.

[0217] [Absorbency Against Load (AAP)] The AAP (absorbency against load) of the water-absorbent agent composition according to the present invention was measured in accordance with the EDANA method (NWSP242.0.R2(19)). Specifically, 0.9 g of the water-absorbent agent composition was subjected to a large excess of a 0.9% by mass aqueous solution of sodium chloride for 1 hour at 4.83 kPa (49 g / cm ２ After swelling under a load of 0.7 psi, the absorbency under pressure (AAP) (unit: g / g) was measured.

[0218] [SFC (Saline Flow Conductivity)] The SFC (Saline Flow Conductivity) of the water-absorbent agent composition according to the present invention was measured in accordance with the measurement method described in US Pat. No. 5,669,894.

[0219] [Washburn Method Contact Angles θ1 and θ2] The Washburn method contact angles θ1 and θ2 of the water-absorbent agent composition according to the present invention were calculated based on the Lucas-Washburn equation (the following (Equation 1)) by measuring the rate at which the water-absorbent agent composition absorbs a sample liquid by capillary force using a surface tensiometer (DY-500 manufactured by Kyowa Interface Science Co., Ltd.) and measurement software DYNALYZER (Ver. 2.2.5.0). Note that θ2 was calculated by changing the filter paper (θ1) to a stainless steel screen (θ2) with a mesh opening of 400 mesh (36 μm), similar to the mesh used in the AAP measurement, in order to eliminate adverse effects of the water-absorbent agent composition when absorbing liquid, and the measurement principle, test procedure, etc. are the same.

[0220] (Measurement principle) The capillary phenomenon of the absorbent structure containing the water-absorbent agent composition of the present invention depends on the contact angle θ of a liquid that wets the particles or particulate matter. The contact angle θ is calculated from the following (Equation 1) and (Equation 2). When the measurement object is a powder, in order to determine the contact angle θ, a penetration rate test using two types of liquid is generally carried out.

[0221] In this specification, first, a penetration rate test was conducted using n-hexane (contact angle θ is as close to zero as possible), and "c" was calculated according to the following (Equation 2). Next, a penetration rate test was conducted using a 20 mass % sodium chloride aqueous solution adjusted to a liquid temperature of 15°C, and the "c" calculated using n-hexane and the measurement results were substituted into the following (Equation 1) to calculate the contact angle θ in the 20 mass % sodium chloride aqueous solution.

[0222]

[0223] where m: suction mass (g), t: measurement time (sec), and parameters for a 20% by mass aqueous solution of sodium chloride at 15°C are: η: absolute viscosity of the liquid (mPa s) = 1.768, ρ: density of the liquid (g / cm ３ ) = 1.150 σ: Surface tension of the liquid (mN / m) = 79.99

[0224] In addition, "m" in the above (Equation 1) ２ / t' means the speed at which a 20 mass % sodium chloride aqueous solution is sucked up into the water-absorbing agent composition by capillary force, and is the speed at which a 20 mass % sodium chloride aqueous solution is sucked up into the water-absorbing agent composition by capillary force. ２ The penetration rate was calculated from the difference.

[0225]

[0226] where m: suction mass (g) t: measurement time (sec) Parameters in n-hexane at 20°C: absolute viscosity of liquid (mPa s) = 0.320 ρ: density of liquid (g / cm ３ ) = 0.66 σ: surface tension of the liquid (mN / m) = 18.4 cos θ: contact angle of the liquid = 1 (θ of n-hexane was set to 0°)

[0227] In addition, "m" in the above (Equation 2) ２ / t' means the speed at which n-hexane is absorbed into the water-absorbing agent composition by capillary force, and is the speed at which n-hexane is absorbed into the water-absorbing agent composition by capillary force during the period from 10 seconds to 20 seconds after the start of measurement. ２ The penetration rate was calculated from the difference.

[0228] (Test Procedure: θ1) Next, the test procedure: θ1 is shown. The following operations were performed for each measurement. All consumables with part numbers are listed in the web catalog "DyneMaster Surface Tension Meter Series Options" on the Kyowa Interface Science Co., Ltd. website, and unless otherwise specified, the test procedures were performed in accordance with the video manual on the CD that came with the DyneMaster DY-500.

[0229] First, the following operation was carried out to determine the constant "c". Homogeneous n-hexane (Fujifilm Wako Pure Chemical Industries, special grade reagent) was prepared and adjusted to a constant temperature (20°C). Next, a measurement column was assembled using a circular powder measurement filter paper (product number 7221), a Teflon (registered trademark) column (product number 7279), and a Teflon (registered trademark) column nozzle (product number 8077). At this time, the filter paper was compressed using the attached compressor without containing the water-absorbing agent composition, to close the gap between the nozzle and the filter paper, and to align the height of the nozzle surface and the bottom of the filter paper.

[0230] Next, 3.0 g of the water-absorbing agent composition was weighed and placed in a Teflon (registered trademark) column, and then the side of the column was lightly tapped with a finger 10 times to smooth the surface of the water-absorbing agent composition without compressing it with an attached compressor. Thereafter, a column holder (product number 7175) was attached to the top of the column, and the column was hung from the top of a measuring device using a small hook (product number 6947).

[0231] Next, the test solution was prepared. The included dish (product number 1196) was thoroughly rinsed with pure water to remove substances that affect surface tension, such as sebum and surfactants, and then thoroughly washed with n-hexane three times to replace the liquid. After that, n-hexane adjusted to 20°C was poured into the dish up to the indicated line, and the dish was placed on the stage of the device.

[0232] Next, the measurement conditions were set in the measurement software in the following order:

[0233] 1) Select "Powder contact angle measurement" from "Select measurement type". 2) Select "Powder penetration rate measurement" from "Measurement settings" > "Measurement" > "Measurement". 3) In "Measurement settings" > "Measurement" > "Basic settings", input 0.03 g of liquid contact sensitivity, 60 seconds of measurement end time, and 0.1 seconds of sampling interval, select fully automatic in operation method, uncheck the valid range, and input 10.0% for both the start and end range in calculation range. 4) In "Measurement settings" > "Measurement" > "Probe settings", input 5.00 mm for the powder column radius. 5) In "Measurement settings" > "Measurement" > "Sample settings", input values ​​of the surface tension, density, and viscosity of the liquid sample, and the density of the powder sample. 6) In the lower part of "Measurement settings" > "Measurement" > "Sample settings", select "Weight" from "Porosity", "Weight", and "Filling height", and input the mass of the filled water-absorbing agent composition. 7) In "Measurement Settings" > "Measurement" > "Stage Operation Settings", enter 0.200 mm / s for both the stage ascent speed and stage descent speed, and 2.0 mm for the descent distance after measurement. 8) If necessary, in "Measurement Settings" > "Display" > "Graph Axis Settings" and "Output Settings", set the vertical and horizontal axes of the graph you want to view during and immediately after measurement, and the parameters to be output.

[0234] After the measurement is completed, a CSV file containing all the information necessary for calculation using the above (Equation 1) and (Equation 2) can be obtained.

[0235] After setting the measurement conditions, the software was used to raise the stage of the device, and after it was brought close enough to the position of the column nozzle, the measurement was started. After the measurement end time set above has elapsed, the measurement will automatically end. The square of the mass of the liquid sucked up between 10 and 20 seconds after the start of the measurement (m ２ ) was assumed to be a straight line, and a regression line was set. The slope of the regression line was defined as "m ２ The above operation was carried out three times in total, with the sample replaced each time. The average value of the three measured values ​​obtained was substituted into the above (Equation 2) to calculate the constant "c".

[0236] Next, a 20% by mass aqueous solution of sodium chloride adjusted to 15°C was prepared as a test liquid, and the same procedure as above was carried out except that n-hexane was used as the test liquid, and the results of "m ２The test liquid was not a 0.9 mass % aqueous sodium chloride solution (physiological saline) and the temperature was different from the body temperature of 36° C. in order to suppress absorption of the water-absorbent agent composition.

[0237] The constant "c" obtained by the above operation and the "m ２ The Washburn contact angle θ was calculated from " / t" using the above (Equation 1). The contact angle θ obtained by the above operation was designated as "θ1".

[0238] (Test procedure: θ2) In the above test procedure: θ1, the powder measurement filter paper (product number 7221) was changed to the mesh used in the screen of the AAP measurement cylinder, and ２ The same operation as in the above test procedure: θ1 was carried out, except that " / t" was the slope of the regression line from 0 second to 10 seconds after the start of measurement. In measuring the contact angle θ2, the following absorption mass correction was carried out in order to eliminate the influence of the water-absorbent agent composition absorbing the test liquid.

[0239] The absorption mass correction was determined as the difference between the "measured value of the suction mass" of the 20% by mass sodium chloride aqueous solution between 0 and 10 seconds after the start of measurement and the "absorption mass" of the 20% by mass sodium chloride aqueous solution between 0 and 10 seconds after the start of measurement.

[0240] The "absorbed mass" was measured by the following method.

[0241] First, in the method for measuring FSR (water absorption rate) described in WO 2009 / 016055

[0196] to

[0197] , which is incorporated by reference, the absorbing liquid was changed to 2 g of a 20 mass % aqueous sodium chloride solution at a temperature of 15°C, and the FSR (water absorption rate) of the water-absorbent agent composition was measured. Hereinafter, the FSR (water absorption rate) of the water-absorbent agent composition measured by changing the absorbing liquid to 2 g of a 20 mass % aqueous sodium chloride solution at a temperature of 15°C is referred to as FSR(20). Herein, the temperature and concentration of the aqueous sodium chloride solution were adjusted to those of the aqueous sodium chloride solution used in measuring the contact angle.

[0242] Next, the position of the mesh of the contact angle measurement column was set to a height of 0 mm, and a scale in 1 mm increments in the height direction was attached to the outer surface of the column. 3.0 g of a water-absorbent agent composition was placed in the column, and the side surface was then tapped 10 times. Thereafter, the height of the water-absorbent agent composition was read from the scale, and the mass of the water-absorbent agent composition was divided by the height of the water-absorbent agent composition to calculate the mass of the water-absorbent agent composition per 1 mm.

[0243] For each measurement, the state of absorbing the 20% by mass aqueous solution of sodium chloride was recorded on video, and the difference (mm) in the liquid height between 10 seconds and 20 seconds after the start of the measurement, which was read from the scale, was multiplied by the mass of the water-absorbent composition of 1 mm, and the result was defined as the mass (a) of the water-absorbent composition involved in the absorption.

[0244] Next, the absorbed mass for every 0.1 seconds was calculated from the mass (a) of the water-absorbent agent composition involved in the absorption and the FSR(20), and the accumulated mass from 0 seconds to 10 seconds was defined as the "absorbed mass." Specifically, the absorbed mass (x) at 0.1 seconds is a × (1 / 100) × FSR(20) × 0.1 seconds, the absorbed mass (y) at 0.2 seconds is x + a × (2 / 100) × FSR(20) × 0.1 seconds, and the absorbed mass (z) at 0.3 seconds is y + a × (3 / 100) × FSR(20) × 0.1 seconds. This method was repeated to calculate the accumulated absorbed mass from 0 seconds to 10 seconds.

[0245] The constant "c" obtained by the above operation and the m of a 20 mass% sodium chloride aqueous solution corrected for absorption mass are ２ The Washburn contact angle θ was calculated from / t using the above formula (1). The contact angle θ obtained by this operation was designated as "θ2".

[0246] [Crevice Water Capacity Under Pressure] The crevice water capacity under pressure of the water-absorbing agent composition according to the present invention was measured in accordance with the measurement method described in paragraphs

[0225] to

[0251] of WO 2016 / 111223, which is incorporated by reference.

[0247] [Specific Surface Area] The specific surface area of ​​the water-absorbing agent composition according to the present invention was measured in accordance with the measurement method described in WO 2021 / 162072.

[0248] Specifically, the three-dimensional image data of the water-absorbent agent composition or the water-absorbent resin obtained using a microfocus X-ray CT system (Shimadzu Corporation: inspeXio SMX-100CT) was analyzed with high-speed three-dimensional analysis software (Ratoc System Engineering Co., Ltd.: TRI / 3D-VOL-FCS64). The measurement conditions of the X-ray CT system and the calculation of the specific surface area (analysis by analysis software) are as described in International Publication No. 2021 / 162072

[0182] to

[0184] (U.S. Application Publication No. 2023 / 076935

[0301] to

[0327] ), which are incorporated by reference.

[0249] [Moisture Content] (WSP230.3(10)) "Moisture content" means the amount of moisture in a water-absorbing agent composition or a water-absorbing resin, which is determined by the loss on drying after drying 1 g of the water-absorbing agent composition or a water-absorbing resin at 105°C for 3 hours. WSP230.3(10) describes it as "Moisture Content," but the meaning is essentially the same.

[0250] [Vortex (Water Absorption Rate)] The vortex (water absorption rate) of the water-absorbing agent composition according to the present invention was measured in accordance with JIS K 7224 (1996) by the following procedure. First, 0.02 parts by mass of a food additive, Food Blue No. 1 (CAS No. 3844-45-9), was added to 1,000 parts by mass of physiological saline to color it, and the liquid temperature was adjusted to 30°C. This was used as a test liquid. Next, 50 mL of the test liquid was measured and placed in a 100 mL beaker, and a cylindrical stirrer with a length of 40 mm and a diameter of 8 mm was placed in the beaker, and stirring was initiated at 600 rpm. Subsequently, 2.0 g of a water-absorbent resin was placed in the test liquid during stirring, and the time until the stirrer tip was covered with the test liquid was measured and used as the water absorption rate by the vortex method.

[0251] [Surface Tension] The surface tension of the water-absorbing agent composition according to the present invention was measured by the following method.

[0252] First, 40 ml of a 0.9% by mass aqueous solution of sodium chloride adjusted to 23 to 24° C. was placed in a thoroughly washed 50 ml beaker, and the surface tension of the 0.9% by mass aqueous solution of sodium chloride was measured using a surface tensiometer (K11 automatic surface tensiometer, manufactured by KRUSS). In this measurement, the surface tension value must be within the range of 72 mN / m to 74 mN / m.

[0253] Next, after the surface tension measurement at a temperature adjusted to 23 to 24°C, a thoroughly cleaned cylindrical stirring bar with a length of 25 mm and a cross-sectional diameter of 7 mm and 0.5 g of the water-absorbing agent composition were placed in a beaker containing 40 ml of a 0.9% by mass sodium chloride aqueous solution, and the mixture was stirred at 350 rpm for 3 minutes. After 3 minutes, the stirring was stopped, and the mixture was left to stand for 2 minutes to allow the absorbed water-absorbing agent composition to settle, after which the surface tension of the supernatant was measured again by the same procedure. In this measurement, a plate method using a platinum plate was adopted, and the plate was thoroughly washed with deionized water and heated and washed with a gas burner before each measurement.

[0254] [Residual Amount of Peroxide] The amount of decomposed peroxide present on the particle surface of the water-absorbing agent composition can be measured by the following method.

[0255] (Pretreatment 1: Impact Test) 30 g of the water-absorbing agent composition and 10 g of glass beads with a diameter of 6 mm (soda-lime glass beads for precision fractional distillation filling) were placed in a glass container with a diameter of 6 cm and a height of 11 cm (manufactured by Nihon Yamamura Glass Co., Ltd. / Mayonnaise 225), and the container was sealed with a resin inner lid and an outer lid (manufactured by TOP Co., Ltd. / Product Code: 604-003 / Product Name: Inner lid and outer lid included in the Mayonnaise Bottle PP Cap Set). Thereafter, the water-absorbing agent composition was pulverized by shaking at 800 rpm using a paint shaker (manufactured by Toyo Seiki Seisakusho Co., Ltd. / Test Disperser: Product No. 488). The shaking time was adjusted for each water-absorbing agent composition to be measured so as to satisfy the following formula (3).

[0256] 0.2<(mass of particle group a) / (total mass of particle group a and particle group b)<0.3 ... formula (3) Details of the paint shaker are described in JP-A-9-235378. After the shaking, the glass beads were removed using a JIS standard sieve with 2 mm openings.

[0257] (Pretreatment 2: Sieving) The water-absorbing agent composition after pulverization obtained in the impact test of the above-mentioned pretreatment 1 was sieved using a JIS standard sieve with a mesh size of 300 μm into a particle group a having a particle size of 300 μm or more and a particle group b having a particle size of less than 300 μm. Most of the particle surface portions scraped off by the above-mentioned pulverization were considered to pass through the JIS standard sieve with a mesh size of 300 μm.

[0258] (Quantitative Determination of Peroxide) The persulfate added as the peroxide in the examples of the present application decomposes in the manufacturing process of the water-absorbing agent composition (mainly the drying process and the heat treatment process), and the remaining component (peroxide decomposition product) is present in the water-absorbing agent composition as a sulfate. For example, in the case of sodium persulfate, the remaining component is sodium sulfate. The quantitative determination of sodium sulfate will be described in detail below, but the quantitative determination of other substances can also be performed in accordance with this method.

[0259] The following configuration can be given as an ion chromatography used for the quantitative determination of sodium sulfate.

[0260] Ion chromatography system: Dionex Integration RFIC system Column: Dionex IonPac AS-18, Dionex IonPac AG-18 Sodium sulfate, the object to be measured, was dissolved in a 0.1% by mass aqueous formaldehyde solution to prepare a standard aqueous solution of sodium sulfate at an arbitrary concentration. This standard aqueous solution was analyzed using the ion chromatography configured as described above, and a calibration curve was created from the relationship between the peak area of ​​the resulting chromatogram and the concentration of sodium sulfate. Note that, since the peaks detected by ion chromatography are peaks derived from sulfate ions generated by ionization of sodium sulfate, calculations were performed assuming that the total amount of sulfate ions was sodium sulfate, and were converted by mass.

[0261] Subsequently, 0.1 g of each of the water-absorbing agent compositions of particle group a and particle group b obtained in the above pretreatment 2 was placed separately in a cylindrical polypropylene container with a capacity of approximately 250 mL, and 100 g of a 0.1 mass % formaldehyde aqueous solution was further added, followed by stirring at 500 rpm for 1 hour with a cylindrical stirrer tip with a length of 25 mm. After the stirring, the supernatant liquid containing no swollen water-absorbing agent composition was collected and filtered with a sample pretreatment filter (GL Chromatodisc, 25A, 0.2 μm, Cat. No. 5040-28502), thereby obtaining extract a and extract b to be measured.

[0262] The extract a and the extract b were subjected to quantitative analysis by the ion chromatography, and the contents C1 and C2 of sodium sulfate remaining in the water-absorbent agent compositions of the particle group a and the particle group b were determined from the calibration curves prepared in advance.

[0263] Unless otherwise noted, the raw material compounds, reaction reagents, and solvents used in the examples and comparative examples were commercially available products (e.g., products sold by Nippon Shokubai Co., Ltd.). The type of standard aqueous solution was changed depending on the remaining components. For example, in the case of potassium persulfate, the remaining component was potassium sulfate, so a potassium sulfate aqueous solution was used as the standard aqueous solution.

[0264] The content of the peroxide remaining in the water-absorbing agent composition of the particle group a and particle group b was corrected for the solid content by the following formula (4).

[0265] Water content corrected content (mass %)=content (mass %) / (charged amount of water-absorbing agent composition 0.1 g×solid content (mass %)) (Equation 4) The solid content of (Equation 4) was calculated from (Equation 5).

[0266] Solid content (mass %)=100−water content (mass %) (Equation 5) The water content of the water-absorbing agent composition was measured as described above.

[0267] [Water Vapor Density in Heat Treatment Device] In the production method according to the present invention, the water vapor density in the heat treatment device was measured by the following method. That is, it was a measurement value in the atmosphere vertically above the content (water absorbent resin) heated in the heating part of the heat treatment device, and was measured by collecting gas located within 5 cm, preferably within 3 cm, and more preferably within 1 cm above the powder surface of the content.

[0268] Examples of a method for collecting the gas include a method of storing the gas in a cylindrical container having an appropriate capacity, and a method of suctioning the gas using a pump and condensing or absorbing the organic surface crosslinking agent and water vapor (condensable components) during the process. From the viewpoint of measurement accuracy, the latter method is preferred, and an apparatus having the following configuration is preferred.

[0269] The gas collection device is preferably an apparatus having a sampling line which is a hard tube, preferably made of SUS, with an inner diameter of 1 mm or more and 10 mm or less, which is heat-resistant and chemical-resistant; a gas switching unit, preferably a heat-resistant 60,000-kcal valve; a trap unit which condenses or absorbs condensable components; a flow rate measuring unit which measures the flow rate of non-condensable gas which has passed through the trap unit; and a suction pump connected downstream of the flow rate measuring unit.

[0270] Furthermore, the section upstream of the trap is preferably designed to be able to maintain the temperature at or above the temperature of the collected gas at the inlet of the sampling line. The temperature during this maintenance is preferably 100°C or higher, and more preferably 100°C to 150°C. If blockage due to dust occurs, it is preferable to install a filter or a cyclone dust collector in the sampling line.

[0271] The flow rate of the collected gas may be such that the volume from the inlet of the sampling line to the inlet of the flow rate measuring unit divided by the flow rate of the non-condensable gas takes preferably 10 seconds or less, more preferably 5 seconds or less, and even more preferably 3 seconds or less. The water content in the collected condensate (solution of condensable components) can be determined by Karl Fischer titration or the like.

[0272] <Production of water-absorbent resin> [Production Example 1] (Preparation process of aqueous monomer solution) In a 2 L polypropylene container, 421.7 parts by mass of acrylic acid, 140.4 parts by mass of 48% sodium hydroxide aqueous solution, 2.3 parts by mass of polyethylene glycol diacrylate (PEGDA, n = 9), 1.3 parts by mass of 2.0% diethylenetriaminepentaacetic acid trisodium aqueous solution, 4.4 parts by mass of 1.0% polyoxyethylene (20) sorbitan monostearate (manufactured by Kao Corporation) aqueous solution, and 390.3 parts by mass of deionized water were added and mixed to prepare an aqueous monomer solution (S1'). The deionized water was preheated to 40 ° C.

[0273] (Polymerization step) Subsequently, the aqueous monomer solution (S1') was cooled with stirring, and when the liquid temperature reached 39°C, 211.9 parts by mass of a 48% by mass aqueous sodium hydroxide solution was added to the aqueous monomer solution (S1') over a period of about 20 seconds in an open-to-air state, and mixed to prepare an aqueous monomer solution (S1). At this time, the temperature of the aqueous monomer solution (S1) had risen to about 81°C due to the heat of neutralization and heat of dissolution generated during the mixing process.

[0274] Next, nitrogen gas was introduced into the stirred aqueous monomer solution (S1) using a Kinoshita glass bowl filter (Kinoshita Rika Kogyo Co., Ltd.: Filter Particle No. 4) under conditions of a pressure of 0.1 MPa and a flow rate of 0.1 L / min for 10 seconds, and then 17.6 parts by mass of a 4.0 mass% aqueous sodium persulfate solution was added, stirred for an additional approximately 5 seconds, and poured into a stainless steel bat-shaped container (bottom 340 x 340 mm, height 25 mm, inner surface: Teflon (registered trademark) coated) in an open-to-air state. Note that the time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (S1) into the bat-shaped container was 55 seconds, and the bat-shaped container was heated using a hot plate (Iuchi Seieido Co., Ltd.: NEO HOTPLATE HI-1000) until the surface temperature reached 40°C. The polymerization reaction started 59 seconds after the aqueous monomer solution (S1) was poured into the bat-shaped container. The polymerization reaction proceeded while expanding and foaming in all directions upward while generating steam, and then the solution shrunk to a size slightly larger than the bottom of the bat-shaped container. The polymerization reaction (expansion and contraction) was completed within about 1 minute. The hydrogel (S1) was removed 3 minutes after the start of the polymerization reaction.

[0275] (Gel crushing step) Next, the hydrogel (S1) was cut to an appropriate size, and then the tip had a perforated plate with a diameter of 100 mm, a hole diameter of 6.4 mm, 83 holes, an opening rate of 34%, and a thickness of 10 mm. The outer diameter of the screw shaft was 86 mm, the rotation speed of the screw shaft was 130 rpm, and the inner diameter of the casing was 88 mm. The gel was crushed to obtain a particulate hydrogel (S1). The mass average particle diameter of the obtained particulate hydrogel (S1) was 360 μm.

[0276] (Drying step) Next, the particulate hydrogel (S1) was spread on a wire mesh with an opening of 300 μm and placed in a hot air dryer. Thereafter, the particulate hydrogel (S1) was dried by passing hot air at 190° C. for 30 minutes to obtain a dried polymer (S1).

[0277] (Pulverization step / classification step) Subsequently, the dried polymer (S1) was put into a roll mill (manufactured by Inokuchi Giken Co., Ltd.: WML type roll pulverizer) and pulverized, and then classified using two types of JIS standard sieves with openings of 710 μm and 150 μm, thereby obtaining an irregularly pulverized water absorbent resin (S1) before surface crosslinking. The obtained water absorbent resin (S1) before surface crosslinking had a CRC of 33.0 g / g and a specific surface area of ​​41 m ２ / kg, D50 was 390 μm, and the mass proportion of particles with a particle size of less than 150 μm was 2.6 mass%. The physical properties of the water absorbent resin (S1) before surface crosslinking are also shown in Tables 1-1 and 2-1.

[0278] (Surface cross-linking step) Next, a surface cross-linking agent solution comprising 0.18 parts by mass of 1,6-hexanediol, 0.4 parts by mass of triethylene glycol, 0.01 parts by mass of a 10.0% by mass aqueous solution of polyoxyethylene (20) sorbitan monostearate, and 3.0 parts by mass of deionized water was added to 100 parts by mass of the water absorbent resin (S1) before surface cross-linking from two positions using a high-speed stirring continuous mixer, and mixed uniformly, to obtain a mixture (S1).

[0279] Subsequently, the mixture (S1) was heat-treated with stirring for 30 minutes in a heat treatment device (heat medium temperature 205°C) in which the atmospheric temperature was adjusted to 180°C and the gauge pressure in the heat treatment device was adjusted to a water vapor density of 0.42 g / L. Thereafter, the mixture (S1) was crushed until it passed through a JIS standard sieve with an opening of 850 µm, thereby obtaining a surface-crosslinked water absorbent resin (S1).

[0280] [Comparative Example 1] The water-absorbent resin (S1) before surface cross-linking in Production Example 1 was used as a water-absorbent agent composition (C1) obtained in Comparative Example 1. The conditions in Comparative Example 1 are shown in Table 1-2, and the physical properties of the water-absorbent agent composition (C1) are shown in Table 1-3.

[0281] [Comparative Example 2] The surface-crosslinked water-absorbent resin (S1) in Production Example 1 was used as a surface-crosslinked water-absorbent agent composition (C2) obtained in Comparative Example 2. The conditions in Comparative Example 2 are shown in Table 1-2, and the physical properties of the water-absorbent agent composition (C2) are shown in Table 1-3.

[0282] [Example 1] In Comparative Example 2, except that the surface cross-linking agent solution was changed to a mixed solution containing 0.18 parts by mass of 1,6-hexanediol, 0.4 parts by mass of triethylene glycol, 0.01 parts by mass of a 10.0% by mass aqueous solution of polyoxyethylene (20) sorbitan monostearate, 0.2 parts by mass of sodium persulfate, and 3.0 parts by mass of deionized water relative to 100 parts by mass of the water absorbent resin (S1) before surface cross-linking, a surface-cross-linked water absorbent resin (1) was obtained by the same operation as in Comparative Example 2. The surface-cross-linked water absorbent resin (1) was designated as a water-absorbent agent composition (1). The conditions in Example 1 are shown in Table 1-2, and the physical properties of the water-absorbent agent composition (1) are shown in Table 1-3, respectively.

[0283] [Example 2] The same operation as in Example 1 was carried out, except that the gauge pressure in the heat treatment device was adjusted to adjust the water vapor density of the heat treatment device to 0.13 g / L, to obtain a surface-crosslinked water-absorbent resin (2). The surface-crosslinked water-absorbent resin (2) was designated as a water-absorbent agent composition (2). The conditions in Example 2 are shown in Table 1-2, and the physical properties of the water-absorbent agent composition (2) are shown in Table 1-3.

[0284] [Comparative Example 3] A surface-crosslinked water-absorbent resin (C3) was obtained by the same operation as in Comparative Example 2, except that the gauge pressure in the heat treatment device was adjusted to adjust the water vapor density to 0.13 g / L in Comparative Example 2. The surface-crosslinked water-absorbent resin (C3) was designated as a water-absorbent agent composition (C3). The conditions in Comparative Example 3 are shown in Table 1-2, and the physical properties of the water-absorbent agent composition (C3) are shown in Table 1-3.

[0285]

[0286]

[0287]

[0288] (Summary) From the results of the water-absorbent agent compositions of Example 1 and Comparative Example 2, which were conducted under the same conditions except for the addition of peroxide in the surface cross-linking step, the water-absorbent agent composition of Example 1, which had a contact angle θ2 of 72° or less, had a lower Vortex and a higher AAP and crevice water multiplier under pressure than the water-absorbent agent composition of Comparative Example 2. Similarly, from the results of the water-absorbent agent compositions of Example 2 and Comparative Example 3, which were conducted under the same conditions except for the addition of peroxide in the surface cross-linking step, the water-absorbent agent composition of Example 2, which had a contact angle θ2 of 72° or less, had a lower Vortex and a higher AAP and crevice water multiplier under pressure than the water-absorbent agent composition of Comparative Example 3. Furthermore, from the value of the peroxide residue on the surface of the water-absorbent agent composition, it can be seen that the peroxide added in the surface treatment step remained on the particle surface and was functioning. Note that, since surface treatment was not conducted in Comparative Example 1, the values ​​of AAP and crevice water multiplier under pressure were very small and therefore not measured.

[0289] [Comparative Example 4] In the surface cross-linking step of Production Example 1, the surface cross-linking agent solution was changed to a surface cross-linking agent solution consisting of 0.4 parts by mass of 1,4-butanediol, 0.6 parts by mass of propylene glycol, 0.01 parts by mass of a 10.0% by mass aqueous solution of polyoxyethylene (20) sorbitan monostearate, 0.1 parts by mass of sodium persulfate, and 2.7 parts by mass of deionized water, relative to 100 parts by mass of the water-absorbent resin (S1) before surface cross-linking, and further, in a heat treatment device (heat medium temperature 200 ° C) in which the atmospheric temperature was adjusted to 180 ° C and the gauge pressure in the heat treatment device was adjusted to a water vapor density of 0.05 g / L, the mixture (S1) was heat-treated while stirring for 30 minutes, The same operation as in Production Example 1 was performed to obtain a water-absorbent agent composition (C4) after surface cross-linking. The conditions in Comparative Example 4 are shown in Table 2-2, and the physical properties of the water-absorbent agent composition (C4) are shown in Table 2-3, respectively.

[0290] [Comparative Example 5] The same operation as in Example 1 was carried out, except that the gauge pressure in the heat treatment apparatus in Example 1 was adjusted to adjust the water vapor density to 0.05 g / L, to obtain a surface-crosslinked water-absorbent resin (C5). The surface-crosslinked water-absorbent resin (C5) was designated as a water-absorbent agent composition (C5). The conditions in Comparative Example 5 are shown in Table 2-2, and the physical properties of the water-absorbent agent composition (C5) are shown in Table 2-3.

[0291] [Example 3] In Comparative Example 4, the same operation as in Comparative Example 4 was carried out, except that the gauge pressure in the heat treatment device was adjusted to adjust the water vapor density of the heat treatment device to 0.42 g / L, to obtain a surface-crosslinked water-absorbent resin (3). The surface-crosslinked water-absorbent resin (3) was designated as a water-absorbent agent composition (3). The conditions in Example 3 are shown in Table 2-2, and the physical properties of the water-absorbent agent composition (3) are shown in Table 2-3, respectively.

[0292] [Example 4] The same operation as in Example 1 was carried out, except that the gauge pressure in the heat treatment device was adjusted to adjust the water vapor density of the heat treatment device to 0.51 g / L, to obtain a surface-crosslinked water-absorbent resin (4). The surface-crosslinked water-absorbent resin (4) was designated as a water-absorbent agent composition (4). The conditions in Example 4 are shown in Table 2-2, and the physical properties of the water-absorbent agent composition (4) are shown in Table 2-3.

[0293]

[0294]

[0295]

[0296] (Summary) From the results of the water-absorbent agent compositions of Examples 1, 2, and 4 and Comparative Example 5, and Example 3 and Comparative Example 4, which were under the same conditions except for the water vapor density in the surface cross-linking step, the water-absorbent agent compositions of Examples in which the contact angle θ2 was 72° or less had low Vortex, and high AAP and crevice water multiplier under pressure.

[0297] [Production Example 2] According to Reference Example 1 described in Japanese Patent No. 4926474, a water-absorbent resin (S2) before surface cross-linking and a water-absorbent resin (S2) after the surface cross-linking step were obtained.

[0298] (Step of Preparing Aqueous Monomer Solution) 15.0 parts by mass of polyethylene glycol diacrylate (average molecular weight: 523) was added to and mixed with 5,500 parts by mass of an aqueous sodium acrylate solution having a neutralization rate of 75 mol% (monomer concentration: 38% by mass), to prepare an aqueous monomer solution (S2).

[0299] (Polymerization step, gel crushing step) The aqueous monomer solution (S2) was degassed for 30 minutes under a nitrogen gas atmosphere, and then the aqueous monomer solution (S2) was introduced into a reactor formed by attaching a lid to a jacketed stainless steel double-arm kneader having two sigma-type blades and an internal volume of 10 L. The atmosphere in the reactor was replaced with nitrogen gas while maintaining the temperature of the aqueous monomer solution (S2) at 30°C.

[0300] Subsequently, 2.46 parts by mass of sodium persulfate and 0.10 parts by mass of L-ascorbic acid were added while stirring the aqueous monomer solution (S2) in the reactor, and the polymerization reaction started after about 1 minute. The polymerization reaction was continued at 30°C to 90°C, and 60 minutes after the start of the polymerization reaction, the particulate hydrogel (S2) in the reactor contents was removed. The particulate hydrogel (S2) was fragmented into particles of about 5 mm.

[0301] (Drying step) The particulate hydrogel (S2) was placed on a wire mesh with an opening of 300 μm and placed in a hot air dryer. Thereafter, the particulate hydrogel (S2) was dried by passing hot air at 150 ° C. for 90 minutes to obtain a dried polymer (S2). Note that there was no undried material in the dried polymer (S2).

[0302] (Pulverization step, classification step) Next, the dried polymer (S2) was put into a roll mill (WML type roll pulverizer, manufactured by Inokuchi Giken Co., Ltd.) and pulverized. Thereafter, it was classified using JIS standard sieves with mesh sizes of 850 μm and 150 μm. By this classification operation, an irregularly pulverized water absorbent resin (S2) before surface crosslinking having a particle diameter of 150 μm or more and less than 850 μm was obtained. The water absorbent resin (S2) had a CRC of 33 g / g and a specific surface area of ​​22 m ２ / kg, D50 (mass average particle diameter) was 410 μm, and the mass proportion of particles less than 150 μm was 2.2 mass%. The physical properties of the water absorbent resin (S2) before surface crosslinking are also shown in Table 3-1.

[0303] (Surface cross-linking step) Subsequently, a surface cross-linking agent solution composed of 0.18 part by mass of 1,6-hexanediol, 0.4 part by mass of triethylene glycol, 0.2 part by mass of sodium persulfate, and 3.0 parts by mass of deionized water was added to 100 parts by mass of the water absorbent resin (S2) before surface cross-linking from two positions using a high-speed stirring continuous mixer, and mixed uniformly, to obtain a mixture (S2).

[0304] Subsequently, the mixture (S2) was heat-treated with stirring for 30 minutes in a heat treatment device (heat medium temperature 205°C) in which the atmospheric temperature was adjusted to 180°C and the gauge pressure in the heat treatment device was adjusted to a water vapor density of 0.42 g / L. Thereafter, the mixture (S2) was crushed until it passed through a JIS standard sieve with an opening of 850 µm, thereby obtaining a surface-crosslinked water absorbent resin (S2).

[0305] [Comparative Example 6] The water-absorbent resin (S2) after surface cross-linking in Production Example 2 was used as a water-absorbent agent composition (C6) obtained in Comparative Example 6. The conditions in Comparative Example 6 are shown in Table 3-2, and the physical properties of the water-absorbent agent composition (C6) are shown in Table 3-3.

[0306] [Production Example 3] The same operation as in Production Example 1 was carried out except that the amount of polyethylene glycol diacrylate was changed to 2.5 parts by mass and the hole diameter of the perforated plate of the meat chopper was changed to 9.5 mm, to obtain an irregularly crushed water-absorbent resin (S3) before surface crosslinking. The mass-average particle diameter of the particulate hydrogel (S3) obtained during the production was 700 μm, and the water-absorbent resin (S3) before surface crosslinking was irregularly crushed, had a CRC of 33 g / g, and a specific surface area of ​​27 m ２ / kg, D50 (mass average particle size) was 390 μm, and the mass proportion of particles smaller than 150 μm was 2.5 mass%.

[0307] Subsequently, a surface crosslinking agent solution composed of 0.18 part by mass of 1,6-hexanediol, 0.4 part by mass of triethylene glycol, 0.2 part by mass of sodium persulfate, and 3.0 parts by mass of deionized water was added to 100 parts by mass of the water absorbent resin (S3) before surface crosslinking from two positions using a high-speed stirring continuous mixer, and mixed uniformly, to obtain a mixture (S3).

[0308] Subsequently, the mixture (S3) was heat-treated with stirring for 30 minutes in a heat treatment device (heat medium temperature 205°C) in which the atmospheric temperature was adjusted to 180°C and the gauge pressure in the heat treatment device was adjusted to a water vapor density of 0.42 g / L. Thereafter, the mixture (S3) was crushed until it passed through a JIS standard sieve with an opening of 850 µm, thereby obtaining a surface-crosslinked water absorbent resin (S3).

[0309] [Example 5] The water-absorbent resin (S3) after surface cross-linking in Production Example 3 was used as a water-absorbent agent composition (5) obtained in Example 5. The conditions in Example 5 are shown in Table 3-2, and the physical properties of the water-absorbent agent composition (5) are shown in Table 3-3.

[0310] [Example 6] In Production Example 3, except that the surface cross-linking agent solution was changed to a mixed solution containing 0.7 parts by mass of propylene glycol, 0.4 parts by mass of ethylene carbonate, 0.2 parts by mass of sodium persulfate, and 2.9 parts by mass of deionized water relative to 100 parts by mass of the water absorbent resin (S3) before surface cross-linking, the same operation as in Production Example 3 was performed to obtain a water absorbent resin (6) after surface cross-linking. The water absorbent resin (6) after surface cross-linking was designated as a water absorbent agent composition (6). The conditions in Example 6 are shown in Table 3-2, and the physical properties of the water absorbent agent composition (6) are shown in Table 3-3, respectively.

[0311] [Comparative Example 7] The same operation as in Example 6 was carried out, except that the surface cross-linking agent solution in Example 6 was changed to a mixed solution containing 0.7 parts by mass of propylene glycol, 0.4 parts by mass of ethylene carbonate, and 2.9 parts by mass of deionized water relative to 100 parts by mass of the water absorbent resin (S3) before surface cross-linking, to obtain a water absorbent resin (C7) after surface cross-linking. The water absorbent resin (C7) after surface cross-linking was designated as a water absorbent agent composition (C7). The conditions in Comparative Example 7 are shown in Table 3-2, and the physical properties of the water absorbent agent composition (C7) are shown in Table 3-3, respectively.

[0312] [Example 7] The same operation as in Production Example 3 was carried out, except that the surface cross-linking agent solution in Production Example 3 was changed to a mixed solution containing 1.1 parts by mass of ethylene carbonate, 0.2 parts by mass of sodium persulfate, and 2.9 parts by mass of deionized water relative to 100 parts by mass of the water absorbent resin (S3) before surface cross-linking, to obtain a water absorbent resin (7) after surface cross-linking. The water absorbent resin (7) after surface cross-linking was designated as a water absorbent agent composition (7). The conditions in Example 7 are shown in Table 3-2, and the physical properties of the water absorbent agent composition (7) are shown in Table 3-3, respectively.

[0313] [Example 8] The same operation as in Production Example 3 was carried out, except that the surface cross-linking agent solution in Production Example 3 was changed to a mixed solution containing 0.8 parts by mass of glycerin, 0.2 parts by mass of sodium persulfate, and 3.0 parts by mass of deionized water relative to 100 parts by mass of the water absorbent resin (S3) before surface cross-linking, to obtain a water absorbent resin (8) after surface cross-linking. The water absorbent resin (8) after surface cross-linking was designated as a water absorbent agent composition (8). The conditions in Example 8 are shown in Table 3-2, and the physical properties of the water absorbent agent composition (8) are shown in Table 3-3, respectively.

[0314] [Example 9] The same operation as in Example 8 was carried out, except that the temperature of the heat medium in the heat treatment device was changed to 200°C, to obtain a surface-crosslinked water-absorbent resin (9). The surface-crosslinked water-absorbent resin (9) was designated as a water-absorbent agent composition (9). The conditions in Example 9 are shown in Table 3-2, and the physical properties of the water-absorbent agent composition (6) are shown in Table 3-3.

[0315] [Comparative Example 8] In Example 8, the surface cross-linking agent solution was changed to a mixed solution consisting of 0.8 parts by mass of glycerin and 3.0 parts by mass of deionized water relative to 100 parts by mass of the water absorbent resin (S3) before surface cross-linking, and the heat medium temperature of the heat treatment device was changed to 200°C, and the same operation as in Example 8 was performed to obtain a water absorbent resin (C8) after surface cross-linking. The water absorbent resin (C8) after surface cross-linking was designated as a water absorbent agent composition (C8). The conditions in Comparative Example 8 are shown in Table 3-2, and the physical properties of the water absorbent agent composition (C8) are shown in Table 3-3, respectively.

[0316]

[0317]

[0318]

[0319] (Summary) From the results of the water-absorbent agent compositions of Comparative Example 6 and Example 5, which were performed under the same conditions except for the difference in the specific surface area of ​​the water-absorbent resin, the water-absorbent agent composition of Example 5, in which the contact angle θ2 was 72° or less, had a lower Vortex, and a higher AAP and crevice water multiplier under pressure than the water-absorbent agent composition of Comparative Example 6.

[0320] Furthermore, from the results of the water-absorbent agent compositions of Example 6, Comparative Example 7, Example 9, and Comparative Example 8, which were under the same conditions except for the addition of peroxide in the surface cross-linking step, the water-absorbent agent compositions of the Examples, in which the contact angle θ2 was 72° or less, had lower Vortex and higher AAP and crevice water multiplier under pressure than the water-absorbent agent compositions of the Comparative Examples. Furthermore, from the values ​​of the amount of peroxide remaining on the surface of the water-absorbent agent composition, it is clear that the peroxide added in the surface treatment step remained on the particle surface and acted. Furthermore, the water-absorbent agent compositions of Example 7 and Example 8, in which different types of surface cross-linking agents were used, also had lower Vortex and higher AAP and crevice water multiplier under pressure.

[0321] The water-absorbent agent compositions (1) to (9) had a Washburn contact angle θ2 of 72° or less. It is believed that the production method of the present application improved the affinity between the absorbed liquid and the surface of the water-absorbent agent composition, and that the liquid became more easily permeable to cavities formed on the surface of the water-absorbent resin, thereby improving the crevice water multiplier under pressure. This reduces the amount of liquid returning to absorbent articles such as disposable diapers.

[0322] Moreover, the amount of peroxide remaining on the surface of the water-absorbing agent composition indicates that the peroxide added in the surface treatment step remains on the particle surface and continues to function.

[0323] The water-absorbent agent composition according to the present invention can be suitably used for, for example, disposable diapers. The water-absorbent agent composition according to the present invention can also be suitably used for various applications such as absorbent articles other than disposable diapers (sanitary napkins, incontinence pads, etc.), soil water-retaining agents for agricultural and horticultural use, and industrial water-stopping agents.

[0324] This application is based on Japanese Patent Application No. 2023-193270, filed on November 13, 2023, the disclosure of which is hereby incorporated by reference in its entirety.

Claims

1. A method for producing a water-absorbing agent composition containing a surface-crosslinked water-absorbing resin as a main component, the method including a surface-crosslinking step of the water-absorbing resin, the method for producing the water-absorbing agent composition satisfying all of the following (1) to (3): (1) The specific surface area of ​​the water-absorbent resin before surface cross-linking is 25 m ２ / kg or more (2) The surface cross-linking step is carried out in the presence of a peroxide and an organic surface cross-linking agent capable of reacting with a carboxyl group. (3) The surface cross-linking step has a heat treatment step of carrying out a heat treatment in an atmosphere having a water vapor density of more than 0.005 g / L simultaneously with or after the addition of the organic surface cross-linking agent to the water absorbent resin.

2. The method according to claim 1, wherein the water vapor density is 0.60 g / L or less.

3. The method according to claim 1, wherein the water-absorbent resin before surface cross-linking is in an irregularly pulverized shape.

4. The manufacturing method according to claim 1, wherein the D50 (mass average particle diameter) of the water absorbent resin before surface crosslinking is 250 μm or more and less than 550 μm, and the mass ratio of particles having a particle diameter of less than 150 μm contained in the water absorbent resin before surface crosslinking is 3 mass% or less.

5. The method according to claim 1, further comprising a step of polymerizing an aqueous monomer solution, wherein the water-absorbing resin is obtained by foaming polymerization of the aqueous monomer solution.

6. The method according to claim 1, wherein the organic surface cross-linking agent is added to the water absorbent resin in a solution state, and the concentration of the organic surface cross-linking agent in the surface cross-linking agent solution is 0.1% by mass or more and 60% by mass or less.

7. The method according to claim 1, wherein the organic surface cross-linking agent is added to the water-absorbent resin through two or more addition nozzles installed in a mixer.

8. The method according to claim 1, wherein the surface cross-linking step is carried out under a slightly reduced pressure of -10 kPa or more and 0 kPa or less as a pressure difference with respect to atmospheric pressure.

9. The method of claim 1, wherein the peroxide is a persulfate.

10. A water-absorbing agent composition containing a surface-crosslinked water-absorbing resin as a main component, which satisfies all of the following (1) and (2): (1) The specific surface area of ​​the water-absorbing agent composition is 25 m ２ / kg or more (2) The water-absorbing agent composition has a Washburn contact angle θ2 of 72° or less, as measured with a 20% by mass aqueous solution of sodium chloride.

11. The water-absorbent composition according to claim 10, wherein the shape of the surface-crosslinked water-absorbent resin is irregularly pulverized, and the mass ratio of particles having a D50 (mass average particle size) of 250 μm or more and less than 550 μm, and less than 150 μm, is 3 mass % or less.

12. The water-absorbent composition according to claim 10, wherein the absorbency against pressure AAP of said water-absorbent composition is 20.0 g / g or more at a load of 4.83 kPa.

13. The water-absorbent composition according to claim 10, wherein the water-absorbent composition has a crevice water ratio under pressure of 9.0 g / g or more.

14. The water-absorbent agent composition according to claim 10, wherein the surface tension of the water-absorbent agent composition is 56 mN / m or more.

15. The water-absorbent composition according to claim 10, wherein the vortex of the water-absorbent composition is 50 seconds or less.

16. The water-absorbent composition according to claim 10, further comprising a peroxide decomposition product, the concentration of the peroxide decomposition product being higher on the surface of the water-absorbent composition than inside the water-absorbent composition.

17. A water-absorbent composition according to claim 10, wherein, when particles having a particle size of 300 μm or more are classified into particle group a and particles having a particle size of less than 300 μm are classified into particle group b by sieving using a JIS standard sieve with a mesh size of 300 μm, and when the amount of peroxide decomposition products present in particle group a is C1 and the amount of peroxide decomposition products present in particle group b is C2 after an impact test using a paint shaker is performed on the water-absorbent composition, C2-C1 is more than 0 mass % and 1 mass % or less.

18. The water-absorbent composition has a saline flow conductivity (SFC) of 1×10 －７ cm ３ The water-absorbent composition according to claim 10, wherein the water-absorbent composition has a water-absorbent strength of at least 1.0 sec / g.

19. An absorbent article comprising the water-absorbent agent composition according to any one of claims 10 to 18.

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