Water-absorbing agent composition containing water-absorbent resin as main component, and method for producing same

Abstract

Provided is a method for producing a water-absorbing agent composition using a water-absorbent resin having a large specific surface area, the method enabling the suppression of particle size segregation in the composition due to transportation, the improvement of the fluidity of particles in the composition, and the maintenance of the water-absorbing properties of the composition at satisfactory levels.　The production method of the present invention includes a step for mixing a water-soluble fluidity improver with a water-absorbent resin under specific conditions during a surface crosslinking step or a step after the surface crosslinking step, and satisfies all of the following requirements (a) to (d): (a) the specific surface area of the water-absorbent resin is 25 m2 / kg or more; (b) when the water-soluble fluidity improver is mixed with the water-absorbent resin, the form of the water-soluble fluidity improver is an aqueous solution having a concentration of 0.01-20 mass%; (c) when the aqueous solution is added to and mixed with the water-absorbent resin, the average droplet diameter of the aqueous solution is 10 μm to 1 mm inclusive; and (d) when the aqueous solution is added to and mixed with the water-absorbent resin, the mixing power index is 70,000 or more.

Description

Water-absorbent composition containing water-absorbent resin as the main component 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. Examples of such water-absorbent resins include crosslinked products of partially neutralized polyacrylic acids. The water-absorbent agent compositions not only have excellent water-absorbing properties such as absorbency without load and absorbency under load, but also require consideration of hygiene issues and stable operation of the manufacturing equipment when used in sanitary materials.

[0003] Furthermore, in addition to the absorbency without load and the absorbency under load, the requirement for a water-absorbing resin to have a high water-absorbing speed has also increased. In recent years, water-absorbing resins with a large specific surface area have been disclosed in order to increase the water-absorbing speed (Patent Documents 1 to 7). Also, a water-absorbing resin with a large specific surface area for blood absorption has been disclosed (Patent Document 8).

[0004] International Publication No. WO 97 / 03114, Japanese Patent Application Laid-Open No. 10-057805, 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, International Publication No. WO 2005 / 075070, International Publication No. WO 2008 / 120742, International Publication No. WO 2019 / 098244, International Publication No. WO 2018 / 062539

[0005] In recent years, water-absorbent resins with large specific surface areas have been proposed with the aim of increasing the water absorption rate. A method for increasing the specific surface area has primarily been proposed by increasing the surface roughness of the water-absorbent resin (Patent Documents 1 to 7). However, increasing the surface roughness increases the likelihood of particles of the water-absorbent resin getting caught at the roughened portions, resulting in a decrease in the powder fluidity of the water-absorbent resin. Therefore, the present inventors investigated the use of surfactants (water-soluble polymers) described in Patent Documents 9 to 12 to improve the powder fluidity of water-absorbent resins with large specific surface areas and increased roughness. However, during this investigation, they discovered a problem in that particle size segregation occurred in the water-absorbent resin (water-absorbent agent composition) after the addition of a surfactant (water-soluble flowability improver) when the water-absorbent resin (water-absorbent agent composition) was transported using a feeder. Furthermore, the addition of a surfactant (water-soluble flowability improver) sometimes resulted in a decrease in the water-absorbent properties of the water-absorbent resin (water-absorbent agent composition). Therefore, there is a demand for a technology that can not only solve the above-mentioned problem of fluidity during transportation, but also maintain good water absorption properties of a water-absorbent resin (water-absorbent agent composition) while using a surfactant (a water-soluble fluidity improver).

[0006] Therefore, an object of the present invention is to provide a technology that can suppress particle size segregation during transportation in a water-absorbing agent composition (water-absorbent resin) produced using a water-absorbent resin having a large specific surface area, and further improve the fluidity of particles while maintaining good water absorption properties.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that the above-mentioned problems can be solved by adding a specific water-soluble fluidity improver to a water-absorbent resin under specific conditions in a process for producing a water-absorbent agent composition, specifically in a surface cross-linking step or in a step subsequent to the surface cross-linking step, and have thus completed the present invention.

[0008] The present invention, which has solved the above problems, has the following configuration.

[0009] That is, one aspect of the present invention is as follows: [1] A method for producing a water-absorbing agent composition containing a water-absorbent resin as a main component, the method comprising a step of preparing a monomer aqueous solution, a polymerization step, a gel-crushing step, a drying step, a crushing step, a classification step, and a surface-crosslinking step, the method comprising a step of mixing a water-soluble fluidity improver having a mass-average molecular weight of 200 or more and 50,000 or less with the water-absorbent resin in an amount of more than 0 ppm and less than 200 ppm relative to the mass of the water-absorbent resin, during the surface-crosslinking step or a step subsequent to the surface-crosslinking step, the method for producing a water-absorbing agent composition satisfies all of the following (a) to (d): (a) a specific surface area of ​​the water-absorbent resin is 25 m ２ / kg or more; (b) when the water-soluble fluidity improver is mixed with a water-absorbent resin, the form of the water-soluble fluidity improver is an aqueous solution of 0.01 mass % or more and 20 mass % or less; (c) when the aqueous solution is added to and mixed with a water-absorbent resin, the average droplet diameter of the aqueous solution is 10 μm or more and 1 mm or less; (d) when the aqueous solution is added to and mixed with a water-absorbent resin, the mixing force index defined by the following (Equation 1) is 70,000 or more.

[0010]

[0011] [2] In the production method according to the above-mentioned [1], it is preferable that the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm in the water absorbent resin after the addition of the water-soluble fluidity improver is 0.80 or less; [3] In the production method according to the above-mentioned [1] or [2], it is preferable that the water-soluble fluidity improver is one or more selected from a nonionic substance, an amphoteric substance, an anionic substance, and a cationic substance; [4] In the manufacturing method described in [3] above, it is preferable that the nonionic substance is selected from polyols, hydroxy group-modified polyols, side chain and / or terminal polyether-modified polysiloxanes, and alkylene oxide adducts of higher aliphatic amines; the amphoteric substance is selected from alkyl betaines and alkylamine oxides; the anionic substance is selected from alkyl sulfate salts, sulfate salts of higher alcohol alkylene oxide adducts, sulfonates, dicarboxylates, alkylamine diacetates, phosphate salts of higher alcohol alkylene oxide adducts, and carboxylate salts of higher alcohol alkylene oxide adducts; and the cationic substance is selected from ammonium salts; [5] In the manufacturing method described in any one of [1] to [4] above, it is preferable that the water-soluble flow improver contains at least one selected from nonionic substances; [6] In the manufacturing method according to any one of the above [1] to [5], it is preferable that the water-soluble flowability improver contains at least one selected from nonionic substances having a polyalkylene glycol chain in the molecule; [7] In the manufacturing method according to any one of the above [1] to [6], it is preferable that the water-soluble flowability improver contains at least one selected from polyols and modified products of hydroxy groups of polyols; [8] In the manufacturing method according to any one of the above [1] to [7], it is preferable that the pH of the aqueous solution is 4.5 or more; [9] In the manufacturing method according to any one of the above [1] to [8], it is preferable that the water-absorbent resin and the water-absorbing agent composition each contain 50 mass % or more of particles having a particle diameter of 300 μm or more and less than 600 μm, and that a reduction rate of a dynamic friction coefficient calculated by the following (Equation 2) is 10% or more;

[0012]

[0013] In formula 2, A: the dynamic friction coefficient of particles of the water-absorbing resin before the addition of the water-soluble fluidity improver, the particle size of which is 300 μm or more and less than 600 μm; B: the dynamic friction coefficient of particles of the water-absorbing agent composition after the addition of the water-soluble fluidity improver, the particle size of which is 300 μm or more and less than 600 μm;

[10] In the production method described in any one of the above [1] to [9], it is preferable to further add polyalkylene glycol in at least one step selected from the step of preparing the aqueous monomer solution, the step of polymerizing, and the step of pulverizing the gel, and / or between the steps;

[11] In the production method described in the above

[10] , it is preferable that the polyalkylene glycol is polyethylene glycol having a mass average molecular weight of 3,000 or less;

[12] In the production method according to the above

[10] or

[11] , the amount of the polyalkylene glycol added is preferably 0.01% by mass or more and 0.25% by mass or less, based on the total mass of the monomers contained in the aqueous monomer solution.

[0014] Another aspect of the present invention is

[13] a water-absorbing agent composition containing a water-absorbing resin as a main component, which contains a water-soluble fluidity improver, and satisfies all of the following (1) to (5): (1) the specific surface area of ​​the water-absorbing agent composition is 25 m ２ / kg or more (2) The surface tension of the water-absorbing agent composition is 56 mN / m or more (3) The flow rate of the water-absorbing agent composition is 10.0 g / s or more (4) In the water-absorbing agent composition, the mass ratio of particles having a particle size of 300 μm or more and less than 600 μm is 50 mass % or more (5) In the water-absorbing agent composition, the particles having a particle size of 300 μm or more and less than 600 μm have a dynamic friction coefficient of 0.80 or less;

[14] In the water-absorbing agent composition according to the above

[13] , it is preferable that the Vortex (water absorption rate) of the water-absorbing agent composition is 50 seconds or less;

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

[13] or

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

[16] In the water-absorbent agent composition described in any one of the above

[13] to

[15] , it is preferable that the D50 (mass average particle size) of the water-absorbent agent composition is 250 μm or more and less than 550 μm, and that the mass ratio of particles having a particle size of less than 150 μm in the water-absorbent agent composition is 3 mass % or less;

[17] In the water-absorbent agent composition described in any one of the above

[13] to

[16] , it is preferable that the AAP (absorbency against load) of the water-absorbent agent composition is 20 g / g or more.

[0015]

[18] Another aspect of the present invention is an absorbent body comprising the water-absorbing agent composition according to any one of the above items

[13] to

[17] ;

[19] In the absorbent body according to the above item

[18] , the basis weight of the pulp is 300 g / m ２

[20] In the absorbent body according to the above

[18] or

[19] , the basis weight of the water-absorbing agent composition is preferably 450 g / m or less. ２ It is preferable that:

[0016] Further, other aspects of the present invention are:

[21] an absorbent article comprising the absorbent core according to any one of the above

[18] to

[20] ;

[22] the absorbent article according to the above

[21] preferably does not contain pulp.

[0017]

[0023] Fig. 1 is a schematic diagram of a measurement section of a rheometer used when measuring a dynamic friction coefficient. In Fig. 1, 1 represents a dish (receiving section), 2 represents parallel plates, and 3 represents a water-absorbing agent composition. Fig. 2 is a schematic diagram of an electromagnetic feeder drive unit 4 used in a feed test. In Fig. 2, 4 represents the electromagnetic feeder drive unit, and 5 represents a trough.

[0018] One aspect of the present invention is a method for producing a water-absorbing agent composition containing a water-absorbent resin as a main component, the method comprising a step of preparing a monomer aqueous solution, a polymerization step, a gel-crushing step, a drying step, a pulverization step, a classification step, and a surface-crosslinking step, the method comprising a step of mixing a water-soluble fluidity improver having a mass-average molecular weight of 200 or more and 50,000 or less with the water-absorbent resin in an amount of more than 0 ppm and less than 200 ppm relative to the mass of the water-absorbent resin during the surface-crosslinking step or a step subsequent to the surface-crosslinking step, the method for producing a water-absorbing agent composition satisfying all of the following (a) to (d): (a) a specific surface area of ​​the water-absorbent resin is 25 m ２ / kg or more; (b) when the water-soluble fluidity improver is mixed with a water-absorbent resin, the form of the water-soluble fluidity improver is an aqueous solution of 0.01 mass % or more and 20 mass % or less; (c) when the aqueous solution is added to and mixed with a water-absorbent resin, the average droplet diameter of the aqueous solution is 10 μm or more and 1 mm or less; (d) when the aqueous solution is added to and mixed with a water-absorbent resin, the mixing force index defined by the above (Equation 1) is 70,000 or more.

[0019]

[0020] In this specification, the method for producing a water-absorbing agent composition having such a configuration is also simply referred to as the "production method according to the present invention" or the "method according to the present invention." Furthermore, in this specification, the "water-absorbing agent composition produced by the method according to the present invention" is also simply referred to as the "water-absorbing agent composition according to the present invention."

[0021] The present inventors have studied problems with water-absorbent agent compositions (water-absorbent resins) that arise during continuous production of sanitary materials such as disposable diapers, and have found that the supplyability (transportability) of water-absorbent agent compositions tends to be lower than in the past. In the process of studying ways to solve this problem, the present inventors have considered that the above-mentioned decrease in supplyability may be due to the use of a water-absorbent resin with a large specific surface area.

[0022] Recent sanitary materials have become thinner and lighter from a sustainability perspective, and water-absorbent resins are required to have as high a water-absorption rate as possible. To meet this demand, a technique for increasing the specific surface area of ​​a water-absorbent resin to obtain a water-absorbent resin with a high water-absorption rate has been proposed. Meanwhile, as described above, a technique for adding a surfactant (a water-soluble polymer) has been proposed to improve the supplyability (transportability) of such water-absorbent resins with a large specific surface area (Patent Documents 9 to 12). However, the present inventors have found a problem in that even if a surfactant (a water-soluble flowability improver) is added to improve the fluidity of the powder (water-absorbent resin), it may be difficult to continuously and stably supply the water-absorbent resin. More specifically, they have found that even if a surfactant (a water-soluble flowability improver) is added to a water-absorbent resin with a large specific surface area, particle size segregation occurs in the water-absorbent resin (water-absorbent agent composition) after transportation.

[0023] As a result of intensive research, the present inventors have found that there is a correlation between the continuous and stable supply of a water-absorbent resin having a large specific surface area and the kinetic friction coefficient of a water-absorbent agent composition, and have conducted extensive research into techniques for controlling the kinetic friction coefficient. As a result, they have found that in order to precisely control the kinetic friction coefficient of a water-absorbent agent composition, it is important to control the type and form of the water-soluble flowability improver to be added, as well as the conditions for adding the water-absorbent agent, and have arrived at the present invention.

[0024] In the production method according to the present invention, a water-soluble fluidity improver (sometimes simply referred to as "fluidity improver" in this specification) is added to a water-absorbent resin in the form of an aqueous solution. When the aqueous solution of the fluidity improver is added, the mixture is mixed and stirred so that the "mixing force index" defined by the above-mentioned (Equation 1) is equal to or greater than a specific value. In the production method according to the present invention, instead of individually controlling the stirring speed (peripheral speed), mixing time, and average droplet size of the aqueous solution of the fluidity improver when mixing the aqueous solution of the fluidity improver, the mixing force index defined by the above-mentioned (Equation 1) is collectively controlled and mixed so that the index is equal to or greater than a specific value. This allows the fluidity improver to be uniformly added to a water-absorbent resin having a large specific surface area. As a result, the kinetic friction coefficient of the water-absorbent agent composition is uniformly reduced without bias among particle sizes, and the fluidity of the water-absorbent agent composition becomes uniform among particle sizes, thereby making it possible to suppress the above-mentioned particle size segregation after transportation.

[0025] Furthermore, according to the production method of the present invention, as described above, the flowability improver and the water-absorbent resin are sufficiently mixed together, so that even if the amount of the flowability improver added is small, a sufficient effect of improving flowability can be obtained. Therefore, the deterioration of water absorption properties caused by the addition of the flowability improver can be suppressed, and good water absorption properties can be maintained.

[0026] According to the production method of the present invention, an aqueous solution of a fluidity improver can be uniformly added to a water-absorbent resin having a large specific surface area. That is, the fluidity improver is not unevenly added to particles having a small particle size and a large specific surface area (particles having a particle size of less than 300 μm), and the dynamic friction coefficient of particles (particles having a particle size of 300 μm or more and less than 600 μm) that account for the majority of the water-absorbent resin can be significantly reduced. In other words, significantly reducing the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm means that the fluidity of all particles is uniformly improved.

[0027] Therefore, the production method according to the present invention can provide a water-absorbing agent composition having a high water absorption rate and a small coefficient of dynamic friction for particles having a particle diameter of 300 μm or more and less than 600 μm. That is, a water-absorbing agent composition can be provided having a high water absorption rate and in which the fluidity of all water-absorbent resin particles is uniformly improved. Furthermore, by uniformly improving the fluidity, it is possible to solve the problem of particle size segregation occurring after transportation when the water-absorbing agent composition is transported by a feeder. Note that the above mechanism is speculation and does not limit the technical scope of the present invention.

[0028] 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.

[0029] Throughout this specification, unless otherwise specified, singular expressions should be understood to include the plural concept. Therefore, singular articles (e.g., "a," "an," "the," etc. in English) should be understood to include the plural concept, unless otherwise specified. Furthermore, terms used in this specification should be understood to be used in the sense commonly used in the art, unless otherwise specified. Therefore, unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification (including definitions) will prevail.

[0030] [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.

[0031] 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.

[0032] Furthermore, the above-mentioned "water-absorbent resin" may refer to "a polymer crosslinked only inside (i.e., a polymer in which the crosslink density inside and the surface are substantially the same)" or "a polymer crosslinked inside and on the surface (i.e., a polymer in which the crosslink density on the surface is relatively high compared to the crosslink density inside)."

[0033] 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 between the presence or absence of surface crosslinking, the above-mentioned "polymer crosslinked only in the interior" is expressed as "water absorbent resin before surface crosslinking" since it is before surface crosslinking, and the above-mentioned "polymer crosslinked both in the interior and on the surface" is expressed as "water absorbent resin after surface crosslinking" since it is after surface crosslinking. 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".

[0034] 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.

[0035] In this specification, the term "water-absorbing agent composition" refers to a composition containing the above-mentioned "water-absorbent resin" and "fluidity improver." The term "water-absorbing agent composition" includes both a case where a water-absorbent resin composition containing a fluidity improver is in a state ready for shipment as a final product as it is, and a case where a water-absorbent resin composition containing a fluidity improver is further subjected to any treatment.

[0036] The "water-absorbent agent composition" includes 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 50% by mass or more, where the total mass of the water-absorbent agent composition is 100% by mass. The lower limit of the mass ratio of the water-absorbent resin to the entire water-absorbent agent composition may be 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more. The upper limit is 100% by mass or less, may be less than 100% by mass, or may be 99% by mass or less. The "water-absorbent agent composition" may include water and trace components other than water as components (other components) other than the water-absorbent resin and the fluidity improver. In one embodiment, the mass ratio of the water-absorbent resin contained in the water-absorbent agent composition may be 50% by mass or more and 100% by mass or less, 60% by mass or more and less than 100% by mass, 70% by mass or more and less than 100% by mass, 80% by mass or more and less than 100% by mass, or 90% by mass or more and 99% by mass or less.

[0037] [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 of acrylic acid (salt) relative to the portion of the monomers involved in the polymerization reaction excluding the internal crosslinking agent, and preferably 100 mol% or less, more preferably substantially 100 mol%.

[0038] [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.

[0039] [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. For specific measurement methods and measurement conditions, see Examples.

[0040] [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.

[0041] [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. For specific measurement methods and conditions, see Examples.

[0042] [1-7] Specific Surface Area 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-absorbent resin (unit: m ２ / kg), and for details of the measurement conditions, see the description in the Examples.

[0043] [1-8] Others In this specification, "A and / or B" and "A and / or B" mean A, B, and combinations thereof. Furthermore, "acid (salt)" means "acid and / or its salt," and the term "(meth)acrylic" means "acrylic and / or methacrylic." Thus, for example, the term "(meth)acrylic acid" encompasses acrylic acid, methacrylic acid, and combinations thereof. Furthermore, unless otherwise specified, concentrations, % and ppm represent mass concentrations, mass % and mass ppm, respectively, and ratios are mass ratios unless otherwise specified. Furthermore, unless otherwise specified, operations and measurements of physical properties are performed at room temperature (23±2°C) and a relative humidity of 35±5% RH.

[0044] [2] Method for Producing a Water-Absorbent Agent Composition The water-absorbent agent composition obtained by the production method according to the present invention contains a water-absorbent resin as a main component, and further contains a fluidity improver. The water-absorbent resin is not particularly limited as long as it has the properties described in the above section [1-1], but is preferably a polyacrylic acid (salt)-based water-absorbent resin. That is, one embodiment of the present invention provides a method for producing a water-absorbent resin composition (water-absorbent agent composition) containing a polyacrylic acid (salt)-based water-absorbent resin and a fluidity improver. The method for producing the water-absorbent agent composition will be described in detail below.

[0045] In one embodiment, the production method of the present invention includes a step of preparing an aqueous monomer solution, a polymerization step, a gel-crushing step, a drying step, a pulverization step after drying, a classification step, a surface-crosslinking step, and a step of adding an aqueous solution of a flowability improver, and the step of adding the aqueous solution of the flowability improver is performed during the surface-crosslinking step or in a step subsequent to the surface-crosslinking step. In a preferred embodiment, the production method of the present invention includes a step of preparing an aqueous monomer solution, a polymerization step, a gel-crushing step, a drying step, a pulverization step after drying, a classification step, a surface-crosslinking step, and a step of adding an aqueous solution of the flowability improver, in this order. In addition to the steps described above, the production method of the present invention may further include a step of adding an additive, a cooling step, a rewetting step, a fine powder granulation step, a transporting step, a storage step, a packaging step, a preservation step, and the like, as necessary. Each step will be described below.

[0046] [2-1] Preparation of Aqueous Monomer Solution This step is a step of preparing an aqueous monomer solution containing a monomer serving as 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 aqueous monomer solution preferably contains one or more polymerizable internal crosslinking agents. The "main component" refers to a component in which 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. The lower limit of the content of acrylic acid (salt) relative to the monomers subjected to the polymerization reaction (excluding the internal crosslinking agent) is preferably 70 mol% or more, more preferably 90 mol% or more. The upper limit is 100 mol% or less, and may be 99 mol% or less, or may be 95 mol% or less. The preferred range of the content of acrylic acid (salt) can be a range defined by any combination selected from the above upper and lower limits. Although a slurry liquid of the monomer can be used within a range that does not affect the water absorption performance of the water-absorbing agent composition obtained as a final product, for the sake of convenience, an aqueous monomer solution will be described in this specification.

[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 lower limit of the concentration of the polymerization inhibitor in the acrylic acid is preferably 10 ppm or more, more preferably 20 ppm or more, on a mass basis. On the other hand, the upper limit is preferably 200 ppm or less, more preferably 160 ppm or less, and even more preferably 100 ppm or less. Note that the preferred range of the concentration of the polymerization inhibitor in the acrylic acid can be a range defined by any combination selected from the above 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 lower limit of the neutralization rate is preferably 10 mol% or more, more preferably 40 mol% or more, even more preferably 50 mol% or more, and particularly preferably 60 mol% or more, based on the acid groups of the monomer. On the other hand, the upper limit of the neutralization rate is preferably 90 mol% or less, more preferably 85 mol% or less, even more preferably 80 mol% or less, and particularly preferably 75 mol% or less. The preferred range of the neutralization rate can be defined by any combination selected from the upper and lower limits. 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, more preferably 10 mol % or less (lower limit: 0 mol %), 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, ethylene oxide-modified glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol acrylate, methyl meth ... 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 and the like. Furthermore, from the viewpoint of the water absorption performance of the water-absorbing agent composition or water-absorbent resin, an internal crosslinking agent having two or more polymerizable unsaturated groups is preferably 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 a (poly)alkylene glycol structure and two or more polymerizable unsaturated groups.

[0056] Specific examples of the polymerizable unsaturated group include an allyl group and a (meth)acrylate group. Of these, a (meth)acrylate group is preferred. Specific examples of the internal cross-linking agent having a (poly)alkylene glycol structure include polyethylene glycol. 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.

[0057] Examples of the internal crosslinking agent having a polymerizable unsaturated group and a (poly)alkylene glycol structure include (poly)ethylene glycol di(meth)acrylate and (poly)propylene glycol di(meth)acrylate.

[0058] The lower limit of the amount of the internal crosslinking agent used is preferably 0.0001 mol % or more, more preferably 0.001 mol % or more, and even more preferably 0.01 mol % or more, based on the monomers excluding the internal crosslinking agent. The upper limit is preferably 10 mol % or less, more preferably 5 mol % or less, and even more preferably 1 mol % or less. The 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. By using the amount within this range, it becomes easier to obtain a water-absorbing agent composition or a water-absorbent resin having desired water absorption performance (for example, suppression of an increase in water-extractable content or a decrease in absorbency).

[0059] 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.

[0060] (Substances Added to Aqueous Monomer Solution) From the viewpoint of improving the physical properties of the water-absorbent 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 such substances include hydrophilic polymers such as starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol (hereinafter sometimes referred to as "PVA"), polyacrylic acid (salts), and crosslinked polyacrylic acid (salts); carbonates, azo compounds, various types of foam-generating blowing agents, surfactants, chelating agents, and chain transfer agents. It should be noted that the surfactants and other substances added to the aqueous monomer solution are not added for the purpose of improving the fluidity of the water-absorbent agent composition.

[0061] The upper limit of 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, relative to the aqueous monomer solution. Meanwhile, the lower limit is preferably 0% by mass, more preferably more than 0% by mass. Furthermore, the upper limit of 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, relative to the aqueous monomer solution. Meanwhile, the lower limit is preferably 0% by mass or more, more preferably more than 0% by mass. The preferred ranges of the amounts of the hydrophilic polymer and the compound added can be defined by any combination selected from the upper and lower limits.

[0062] 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.

[0063] (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.

[0064] From the viewpoint of the physical properties of the water-absorbing agent composition or the water-absorbent resin, the lower limit of 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. On the other hand, the upper limit 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 the 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):

[0065] 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.

[0066] (Polyalkylene glycol) In the step of preparing the aqueous monomer solution, polyalkylene glycol may be added. Note that in the step of preparing the aqueous monomer solution, polyalkylene glycol may also be added as an internal cross-linking agent. Furthermore, polyalkylene glycol may be added in the polymerization step and / or gel-crushing step described in detail below. That is, in one embodiment, the method for producing the water-absorbing agent composition according to the present invention preferably further includes adding polyalkylene glycol in at least one step selected from the step of preparing the aqueous monomer solution, the polymerization step, and the gel-crushing step, and / or between the steps (the step of preparing the aqueous monomer solution, the polymerization step, and the gel-crushing step).

[0067] It should be noted that the polyalkylene glycol added here is not intended to improve the fluidity of the water-absorbent resin. That is, the polyalkylene glycol added in at least one step selected from the step of preparing a monomer aqueous solution, the polymerization step, and the gel crushing step, and / or in each of the above steps, is not included in the fluidity improver of the present invention. Generally, in order to improve the fluidity of a water-absorbent resin, it is necessary to form a coating on the particle surface of the water-absorbent resin from the viewpoint of surface modification. However, when added in the above step, i.e., a step that can uniformly distribute the polyalkylene glycol inside the water-absorbent resin, almost no polyalkylene glycol is present on the particle surface of the water-absorbent resin, and the desired effect of improving fluidity cannot be obtained.

[0068] Here, the polyalkylene glycol may be added in any one of the monomer aqueous solution preparation step, the polymerization step, and the gel-pulverization step, or in any two or all of the steps. Furthermore, the polyalkylene glycol may be added between the monomer aqueous solution preparation step and the polymerization step, or between the polymerization step and the gel-pulverization step, or both, or may be added not only between these steps but also in each of the above steps. When polyalkylene glycol is added in multiple steps or between steps, the polyalkylene glycols added in each step or between steps may be the same or different. It is particularly preferred that the polyalkylene glycol be added in the monomer aqueous solution preparation step. That is, in one embodiment, the method for producing a water-absorbing agent composition according to the present invention preferably further includes adding polyalkylene glycol in the monomer aqueous solution preparation step.

[0069] The polyalkylene glycol may be a polyalkylene glycol having a structure represented by the following general formula (1):

[0070] H-(OR) ｎ In general formula (1), R is an alkylene group having 2 to 4 carbon atoms, and may be linear or branched. Furthermore, n is the average number of moles of oxyalkylene groups (—OR—) added, and is preferably 4 to 70, more preferably 4 to 50, and even more preferably 6 to 15.

[0071] In general formula (1), the oxyalkylene groups (-OR-) in one molecule may be the same or different. More specifically, examples of the polyalkylene glycol include polyethylene glycol, polypropylene glycol, polyethylene glycol-polypropylene glycol copolymer, polyethylene glycol-polypropylene glycol-polybutylene glycol copolymer, etc. These polyalkylene glycols may be used alone or in combination of two or more.

[0072] The upper limit of the mass average molecular weight of the polyalkylene glycol is preferably 3,000 or less. The lower limit of the mass average molecular weight of the polyalkylene glycol is preferably 200 or more, more preferably 300 or more, and even more preferably 400 or more. The mass average molecular weight is more preferably 2,500 or less, even more preferably 2,400 or less, and particularly preferably 2,300 or less. By setting the mass average molecular weight of the polyalkylene glycol within the above range, the water absorption performance of the water absorbent resin after surface cross-linking is improved. Here, the mass average molecular weight of the polyalkylene glycol is a value measured by gel permeation chromatography.

[0073] The polyalkylene glycol having the above-mentioned mass average molecular weight may be water-soluble. The definition of the term "water-soluble" is as described below. When the polyalkylene glycol is water-soluble, it can be added more uniformly when added in at least one step selected from the step of preparing an aqueous monomer solution, the polymerization step, and the gel-pulverization step, or between these three steps. As a result, a water-absorbent resin in which the polyalkylene glycol is uniformly present can be obtained. Furthermore, as a result, the water-absorbing performance of the water-absorbent resin after surface cross-linking is improved. Note that the polyalkylene glycol added in at least one step selected from the step of preparing an aqueous monomer solution, the polymerization step, and the gel-pulverization step, or between these three steps, does not fall under the category of the flowability improver.

[0074] In a preferred embodiment, the polyalkylene glycol may be polyethylene glycol having a mass-average molecular weight of 3,000 or less. In another embodiment, the polyalkylene glycol may be polyethylene glycol having a mass-average molecular weight of 200 to 3,000. The mass-average molecular weight of the polyethylene glycol is more preferably 300 to 2,500, even more preferably 400 to 2,400, and particularly preferably 400 to 2,300.

[0075] The amount of the polyalkylene glycol added is preferably 0.01% by mass or more and 0.25% by mass or less relative to the total mass of the monomers contained in the aqueous monomer solution. Here, the total mass of the monomers contained in the aqueous monomer solution refers to the total mass of the monomers excluding the internal crosslinking agent. The lower limit of the amount of the polyalkylene glycol added is more preferably 0.02% by mass or more, and even more preferably 0.03% by mass or more. On the other hand, the upper limit is more preferably 0.23% by mass or less, even more preferably 0.20% by mass or less, and particularly preferably 0.18% by mass or less. When two or more polyalkylene glycols are added, the amount added refers to the total amount. The preferred range of the amount of the polyalkylene glycol added can be a range defined by any combination selected from the above upper and lower limit values. Therefore, the amount of polyalkylene glycol added may be, for example, 0.02% by mass or more and 0.23% by mass or less, 0.02% by mass or more and 0.20% by mass or less, or 0.03% by mass or more and 0.18% by mass or less. Furthermore, when the polyalkylene glycol is added in a plurality of steps, that is, in at least one step selected from the step of preparing a monomer aqueous solution, the polymerization step, and the gel-pulverization step, and / or between the steps (the step of preparing a monomer aqueous solution, the polymerization step, and the gel-pulverization step), the amount added is preferably such that the total amount of the polyalkylene glycol added in each step is within the above-mentioned range. By setting the amount of polyalkylene glycol added within the above range, the water absorption performance of the water-absorbent resin after surface crosslinking is improved.

[0076] As the polyalkylene glycol, two or more polyalkylene glycols may be used. When one or two or more polyalkylene glycols are used, polyalkylene glycols having different mass average molecular weights may be used in combination.

[0077] [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.

[0078] (Polymerization Initiator) In this step, a polymerization initiator is preferably used. Examples of the polymerization initiator include a thermally decomposable polymerization initiator, a photodecomposable polymerization initiator, and a redox-based 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.

[0079] The lower limit of the amount of the polymerization initiator used is preferably 0.001 mol% or more, more preferably 0.01 mol% or more, based on the monomers excluding the internal crosslinking agent. Meanwhile, the upper limit is preferably 1 mol% or less, more preferably 0.5 mol% or less, and even more preferably 0.1 mol% or less. Furthermore, the lower limit of the amount of the reducing agent used is preferably 0.0001 mol% or more, more preferably 0.0005 mol% or more, based on the monomers excluding the internal crosslinking agent. Meanwhile, the upper limit is preferably 0.02 mol% or less, more preferably 0.015 mol% or less. The preferred ranges of the amounts of the polymerization initiator and the reducing agent used can be defined by any combination selected from the upper and lower limits. By using the amounts within these ranges, a water-absorbent agent composition or a water-absorbent resin having the desired water absorption performance can be more easily obtained.

[0080] 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.

[0081] (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.

[0082] 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.

[0083] The foaming polymerization is one of the methods for improving the specific surface area of ​​a water-absorbent agent composition or a water-absorbent resin, and is one of the preferred embodiments. Examples of the method 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 the 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-absorbent agent composition or the water-absorbent resin.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] It should be noted that surfactants that can be used in the above methods (I) to (III) do not provide the desired effect of improving the fluidity of the water-absorbent resin in this specification. In order to improve the fluidity of the water-absorbent resin, it is necessary to form a coating on the particle surface of the water-absorbent resin from the viewpoint of surface modification. However, when a surfactant is added in the polymerization step, i.e., in a step that can uniformly distribute the surfactant inside the water-absorbent resin, almost no surfactant is present on the particle surface of the water-absorbent resin, and the desired effect of improving the fluidity cannot be obtained.

[0089] 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.

[0090] 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. The upper limit is preferably 10,000 or less, more preferably 1,000 or less. The preferred range of the number of pores can be defined by any combination selected from the upper and lower limits. 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.

[0091] [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.

[0092] Regarding 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, an irregularly pulverized water-absorbing agent composition or water-absorbent resin can be obtained.

[0093] The particle diameter of the particulate hydrogel pulverized by the gel crushing step is preferably 0.05 mm or more and 10 mm or less. When the particle diameter of the particulate hydrogel is 0.05 mm or more, the physical properties of the resulting water-absorbing agent composition or water-absorbent resin become good. Furthermore, when the particle diameter of the particulate hydrogel is 10 mm or less, the particulate hydrogel can be dried efficiently.

[0094] The lower limit of D50 (mass average particle diameter) of the particulate hydrogel is preferably 50 μm or more, more preferably 100 μm or more, and even more preferably 140 μm or more. On the other hand, the upper limit is preferably 2000 μm or less, more preferably 1500 μm or less, and even more preferably 1000 μm or less. A preferred range of D50 of the particulate hydrogel can be defined by any combination selected from the upper and lower limits. For example, controlling D50 (mass average particle diameter) of the particulate hydrogel within the preferred range by the following method (B) is one method for improving the specific surface area of ​​the water-absorbing agent composition or the water-absorbent resin, and is one preferred embodiment.

[0095] 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. On the other hand, its upper limit is preferably 1.5 or less, more preferably 1.3 or less, and even more preferably 1.2 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 limit values. The smaller the value of the 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.

[0096] 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.

[0097] As the foaming polymerization of the (A) aqueous monomer solution, for example, a method of trapping dissolved gas dissolved in the aqueous monomer solution in the system when gelling is performed by high-temperature short-time polymerization to foam the solution, or a foaming polymerization method in which a surfactant is made to coexist in the aqueous monomer solution, i.e., a foaming polymerization method described in Japanese Patent No. 5647625 (specifically, for example, a method of generating bubbles 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 to be dispersed as bubbles to carry out polymerization, or a foaming polymerization method in which a foaming agent is added to the aqueous monomer solution to carry out polymerization, etc., can be adopted to increase 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 is obtained by foam polymerization of an aqueous solution of an unsaturated monomer.

[0098] In addition, as for the pulverization and granulation of the particulate hydrogel or its dried polymer (B) described above, for example, the gel pulverization process may be carried out by employing the gel pulverization method described in Japanese Patent No. 5989913 or Japanese Patent No. 6067126 or International Publication No. 2016 / 204302, and further drying the gel to obtain a water-absorbent resin having a specific surface area of ​​25 m or less. 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.

[0099] 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.

[0100] 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.

[0101] 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 paragraphs

[0257] to

[0270] of International Publication No. 2016 / 111223, which is incorporated by reference.

[0102] [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 a desired solid content to obtain a dried polymer. The 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. The lower limit of the solid content of the dried polymer is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, and particularly preferably 92% by mass or more. On the other hand, the upper limit is 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 solid content of the dried polymer can be a range defined by any combination selected from the above upper and lower limits.

[0103] 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.

[0104] The lower limit of the drying temperature in the hot air drying is preferably 100°C or higher, more preferably 150°C or higher, from the viewpoint of the color tone and drying efficiency of the water-absorbing agent composition or the water-absorbent resin. On the other hand, the upper limit is preferably 300°C or lower, more preferably 200°C or lower. 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 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.

[0105] The lower limit of the drying time in the present invention is preferably 1 minute or more, more preferably 5 minutes or more, and even more preferably 10 minutes or more. Meanwhile, the upper limit is 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 determined by any combination selected from the upper and lower limits. By setting the drying temperature and drying time within the above ranges, the physical properties of the resulting water-absorbent agent composition can be within desired ranges. Furthermore, the physical properties of the water-absorbent resin as an intermediate product can also be within desired ranges. Furthermore, when drying is performed by hot air drying, the lower limit of the hot air velocity is preferably 0.5 m / s or more. Meanwhile, the upper limit is preferably 3.0 m / s or less, more preferably 2.0 m / s or less. The preferred range of the hot air velocity can be determined by any combination selected from the upper and lower limits. Other drying conditions may be appropriately set depending on the moisture content and total mass of the particulate hydrogel to be dried, the target solid content, and the like.

[0106] [2-5] Pulverization step, classification step The pulverization step is a step of pulverizing the dried polymer obtained through the drying step. By passing through the pulverization step after drying, a water absorbent resin in an irregularly pulverized form is obtained. Furthermore, the classification step is a step of classifying the dried polymer pulverized in the pulverization step to obtain a particle size within a desired range. By passing through such steps, a water absorbent resin before surface crosslinking is obtained.

[0107] 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.

[0108] 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.

[0109] The proportion of (i) particles having a particle size of less than 150 μm (hereinafter referred to as "particles less than 150 μm") contained in the water absorbent resin after classification and before surface crosslinking is preferably 3 mass% or less, more preferably 2.5 mass% or less, and even more preferably 2 mass% or less. In addition, in continuous commercial production, it may be very difficult to reduce the proportion of particles less than 150 μm to 0 mass% from the viewpoint of production efficiency. Therefore, the proportion of particles less than 150 μm is preferably 0.1 mass% or more, more preferably 0.2 mass% or more, and even more preferably 0.3 mass% or more. In this specification, "particles having a particle size of less than 150 μm" refers to particles that pass through a sieve with an opening of 150 μm after classification using the same method as the classification method in the evaluation method of the dynamic friction coefficient in the Examples.

[0110] Furthermore, (ii) the lower limit of D50 (mass average particle diameter) is preferably 250 μm or more, more preferably 300 μm or more, and even more preferably 330 μm or more. On the other hand, the upper limit is preferably less than 550 μm, more preferably less than 500 μm, and even more preferably less than 450 μm. The preferred range of D50 (mass average particle diameter) can be a range defined by any combination selected from the upper and lower limits.

[0111] Furthermore, (iii) it is more preferable that the particle size distribution of the water absorbent resin before surface cross-linking is such that D50 (mass average particle diameter) is within the above range of (ii) and the proportion of particles less than 150 μm is within the above range of (i).

[0112] Furthermore, (iv) the lower limit of σζ (logarithmic standard deviation of particle size distribution) is preferably 0.20 or more, more preferably 0.25 or more, and even more preferably 0.27 or more. On the other hand, the upper limit is preferably 0.50 or less, more preferably 0.40 or less, and even more preferably 0.35 or less. The preferred range of σζ (logarithmic standard deviation of particle size distribution) can be a range defined by any combination selected from the upper and lower limits. The smaller the value of σζ (logarithmic standard deviation of particle size distribution), the more uniform the particle size and the less particle segregation there is, which is advantageous. 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.

[0113] 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 in 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. In addition, 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.

[0114] [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 includes 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, or the like occurs on the surface of the water-absorbent resin before the surface cross-linking step, thereby obtaining a surface-cross-linked water-absorbent resin. In the production method according to the present invention, an aqueous solution of a fluidity improver may be added in the surface cross-linking step. In this way, when the fluidity improver is added during the surface cross-linking step, the water-absorbent resin obtained after the surface cross-linking step contains the fluidity improver. Therefore, in this case, the "water-absorbent agent composition" may be the water-absorbent resin after the surface cross-linking step.

[0115] [2-6-1] Mixing step This step is a step of obtaining a humidified mixture by mixing 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. When an aqueous solution of a fluidity improver is added in the surface crosslinking step, it is preferable to add the aqueous solution in this step. Note that preferred conditions in this case are as described in [2-9] below.

[0116] (Surface Crosslinking Agent) In the present invention, it is preferable to use a surface crosslinking agent during surface crosslinking. Specific examples of the surface crosslinking agent include polyhydric alcohol compounds, amino alcohol compounds, alkylene carbonate compounds, oxazolidinone compounds, oxetane compounds, and epoxy compounds. It is preferable to use at least one surface crosslinking agent selected from these surface crosslinking agents. Furthermore, as the surface crosslinking agent, an organic surface crosslinking agent capable of forming an ester bond with a carboxyl group is preferable. Examples of the 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, for example, a carboxyl group, include surface crosslinking agents having a hydroxyl group in the molecule, such as polyhydric alcohol compounds or amino alcohol compounds, and surface crosslinking agents that generate a hydroxyl group by ring-opening, such as alkylene carbonate compounds, oxazolidinone compounds, oxetane compounds, and epoxy compounds.

[0117] More specifically, the 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, Phosphorus, 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;1,3-dioxolan-2-one, 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- alkylene carbonate compounds such as oxetane, 2-methyloxetane, 3-methyl-3-hydroxymethyloxetane, 3-ethyl-3-hydroxymethyloxetane, etc.; cyclic urea compounds; oxetane compounds such as oxetane, 2-methyloxetane, 3-methyl-3-hydroxymethyloxetane, 3-ethyl-3-hydroxymethyloxetane, etc.; amino alcohol compounds such as ethanolamine; polyvalent metal compounds such as hydroxides or chlorides of zinc, calcium, magnesium, aluminum, iron, zirconium, etc.;

[0118] Among these surface cross-linking agents, at least one surface cross-linking 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 suitable. More preferably, the surface cross-linking agent is one or more selected from the group consisting of polyhydric alcohol compounds having 3 to 6 carbon atoms and containing 2 to 3 hydroxyl groups in the molecule, epoxy compounds having 6 to 12 carbon atoms, alkylene carbonate compounds having 3 to 5 carbon atoms, and oxetane compounds having 3 to 10 carbon atoms. Furthermore, as the above-mentioned surface cross-linking agent, one or more types of surface cross-linking agents are used in consideration of their reactivity and the heating temperature in the heat treatment step. Note that the surface cross-linking step may be carried out two or more times in consideration of its effect. In this case, the second and subsequent steps may be carried out using the same surface cross-linking agent as in the first step, or may be carried out using a different surface cross-linking agent.

[0119] The lower limit of the amount of the surface crosslinking agent used is preferably 0.01 parts by mass or more relative to 100 parts by mass of the water-absorbent resin before surface crosslinking. On the other hand, the upper limit is 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. A preferred range of the amount of the surface crosslinking agent used can be a range defined by any combination selected from the upper and lower limits. By setting the amount of the 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, making it easier to obtain a water-absorbent resin or a water-absorbent agent composition with high physical properties. When multiple surface crosslinking agents are used, the amount used is the total amount thereof.

[0120] The surface cross-linking agent is preferably added to the water absorbent resin in the form of a solution, and more preferably added as an aqueous solution to the water absorbent resin before surface cross-linking. Therefore, when a fluidity improver is added in the surface cross-linking step, it is preferable that the aqueous solution of the surface cross-linking agent further contains the fluidity improver. When the surface cross-linking agent is added as an aqueous solution, the lower limit of the amount of water used is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, and even more preferably 0.5 parts by mass or more, relative to 100 parts by mass of the water absorbent resin before surface cross-linking. On the other hand, the upper limit is 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 cross-linking. Note that a preferred range of the amount of water used can be a range defined by any combination selected from the above upper and lower limit values. By setting the amount of water used within this range, the handleability of the surface cross-linking agent solution is further improved, and it becomes easier to uniformly mix the surface cross-linking agent with the water absorbent resin before surface cross-linking.

[0121]

[0043] Moreover, the lower limit of the concentration of the 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 particularly preferably 20% by mass or more. On the other hand, the upper limit is preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 45% by mass or less. Note that a preferred range of the concentration of the surface crosslinking agent can be a range defined by any combination selected from the upper and lower limit values. 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.

[0122] Furthermore, a hydrophilic organic solvent can be used in combination with the water as needed to prepare the surface cross-linking agent solution. In this case, the amount of the hydrophilic organic solvent used is preferably 5 parts by mass or less, 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 cross-linking. 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 cross-linking 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.

[0123] (Mixing method, mixing conditions) As a method for mixing the water absorbent resin before surface crosslinking with the surface crosslinking agent solution, a method in which a surface crosslinking agent solution is prepared in advance and the solution is preferably sprayed or dropped, more preferably sprayed, onto the water absorbent resin before surface crosslinking and mixed can be mentioned. In the case where a fluidity improver is added in the surface crosslinking step, it is preferable that the surface crosslinking agent solution further contains a fluidity improver. Note that preferred conditions in this case are as described in [2-9] below.

[0124] As a mixer for performing 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 is preferred. The mixer is preferably a high-speed stirring mixer, and more preferably a high-speed stirring continuous mixer. The lower limit of the rotation speed of the high-speed stirring mixer is preferably 100 rpm or more, more preferably 300 rpm or more. On the other hand, the upper limit is preferably 10,000 rpm or less, more preferably 2,000 rpm or less. The preferred range of the rotation speed of the high-speed stirring mixer can be a range specified by any combination selected from the above-mentioned upper and lower limit values.

[0125] The lower limit of the temperature of the water absorbent resin before surface crosslinking to be supplied to this step is preferably 25°C or higher, more preferably 35°C or higher, from the viewpoint of mixability with the surface crosslinking agent solution and coagulation property of the humidified mixture. On the other hand, the upper limit thereof is preferably 80°C or lower, more preferably 70°C or lower, and further preferably 60°C or lower. Furthermore, the lower limit of the mixing time is preferably 1 second or higher, more preferably 5 seconds or higher. On the other hand, the upper limit thereof is preferably 1 hour or lower, more preferably 10 minutes or lower. Note that 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.

[0126] [2-6-2] Heat Treatment Step This step is a step of applying heat 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 of the humidified mixture may involve heating the humidified mixture in a stationary state, or heating in a fluidized state using a power such as stirring, but heating under stirring is preferred in that the entire humidified mixture can be heated evenly. Specific examples of heat treatment devices that perform the heat treatment include paddle dryers, multi-fin processors, tower dryers, etc.

[0127] The lower limit of the heating temperature in this step is preferably 80° C. or higher, more preferably 90° C. or higher. On the other hand, the upper limit is preferably 250° C. or lower, more preferably 230° C. or lower. The preferred range of the heating temperature can be a range defined by any combination selected from the upper and lower limit values.

[0128] The heating time in this step is preferably 5 minutes or more, more preferably 7 minutes or more. Meanwhile, the upper limit is preferably 1.5 hours or less, more preferably 1 hour or less. The preferred range of the heating time can be determined by any combination of the upper and lower limits.

[0129] Controlling the heating temperature and heating time within the above ranges is preferable because it improves the water absorption performance of the resulting water-absorbing agent composition or the surface-crosslinked water-absorbing resin.

[0130] [2-7] Cooling step After the heat treatment (heat treatment step) in the surface cross-linking step, a cooling step may be carried out as needed. That is, the cooling step is an optional step that is provided as needed 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.

[0131] 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.

[0132] 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 less, more preferably 100° C. or less, even more preferably 90° C. or less, and particularly preferably 80° C. or less. On the other hand, the lower limit thereof is preferably 20° C. or more, more preferably 30° C. or more.

[0133] In the production method according to the present invention, an aqueous solution of a fluidity improver is added to a water-absorbent resin during the surface cross-linking step or during a step subsequent to the surface cross-linking step. Since the water-absorbent resin with a high specific surface area has a faster water-absorption rate than conventional products, it is difficult to uniformly apply the aqueous solution of the fluidity improver between particles. In addition, since the water-absorption rate of a water-absorbent resin is also affected by temperature, if an aqueous solution of the fluidity improver is added to and mixed with a water-absorbent resin at an excessively high temperature, some of the water-absorbent resin may absorb the aqueous solution of the fluidity improver. This may cause the surface of the water-absorbent resin to become sticky, which may easily generate coarse aggregated particles, and may also reduce uniformity after mixing. Therefore, particularly considering the mixability of a water-absorbent resin with a high specific surface area and an aqueous solution of the fluidity improver, it is preferable to control the temperature of the water-absorbent resin after surface cross-linking within the above range when adding the aqueous solution of the fluidity improver. By controlling the temperature within the above range, the aqueous solution of the fluidity improver can be uniformly added and mixed with the water-absorbent resin, and the dynamic friction coefficient of the water-absorbent resin particles having a particle size of 300 μm or more and less than 600 μm can be easily reduced.

[0134] 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, if the water-absorbent resin is crushed after surface cross-linking, the surface cross-linking effect decreases, so it is preferable that the shape of the water-absorbent resin before and after surface cross-linking is an irregularly crushed shape. Specifically, it is preferable that the shape of the water-absorbent resin in the surface cross-linking step of the water-absorbent resin is an irregularly crushed shape, and in consideration of the effect of adding an aqueous solution of a fluidity improver, it is also preferable that the shape of the water-absorbent resin when adding an aqueous solution of a fluidity improver in the mixing step of the aqueous solution of the fluidity improver is an irregularly crushed shape. An irregularly crushed water-absorbent resin can be obtained by crushing a hydrogel or a dried polymer.

[0135] The proportion of water-absorbent resin particles having a particle diameter of less than 150 μm (hereinafter referred to as "particles less than 150 μm") contained in the water-absorbent resin after surface cross-linking is preferably less than 3% by mass. Compared with particles having a particle diameter of 150 μm or more, particles less than 150 μm have a significantly increased specific surface area, and therefore have a faster absorption rate of aqueous liquids. Therefore, when there are many particles less than 150 μm (for example, when the mass proportion is 3% by mass or more), these particles preferentially absorb the aqueous solution of the fluidity improver. In this case, when the aqueous solution of the fluidity improver is added, the aqueous solution may be difficult to mix uniformly throughout the water-absorbent resin after surface cross-linking, or coarse particles may be generated due to aggregation of particles less than 150 μm. Therefore, in the water-absorbent resin before the aqueous solution of the fluidity improver is added, the proportion of water-absorbent resin particles having a particle diameter of less than 150 μm is preferably less than 3% by mass.

[0136] Particles less than 150 μm can be appropriately adjusted by employing the same particle size adjustment method for the water-absorbent resin as in the classification step. Furthermore, the preferred form of the water-absorbent resin after surface cross-linking is the same as that described in the above section "[2-5] Crushing step, classification step." The explanations of (i) to (iv) in the section "[2-5] Crushing step, classification step" are incorporated herein. The preferred ranges of "(i) the proportion of particles less than 150 μm," "(ii) D50 (mass average particle diameter)," "(iii) D50 (mass average particle diameter) and the proportion of particles less than 150 μm," "(iv) σζ (logarithmic standard deviation of particle size distribution)," and combinations thereof are as described above. In particular, if (iv) σζ (logarithmic standard deviation of particle size distribution) is within the desired range, the variation in specific surface area between particles is small, and the variation in the absorption rate of aqueous liquid is also small, which is preferable because it facilitates uniform mixing when an aqueous solution of a flowability improver is added.

[0137] [2-8] Additives and Addition Step Thereof In the present invention, an additive other than a fluidity improver (hereinafter, also simply referred to as an "additive") may be added to one or more of the water-absorbent resin before surface-crosslinking and the water-absorbent resin after surface-crosslinking. In other words, the water-absorbent agent composition may contain an additive in addition to the water-absorbent resin and the fluidity improver.

[0138] [2-8-1] Additives Additives other than the flowability improver used in the present invention include a liquid permeability improver or a component thereof, and other additives, and these may be used alone or in combination of two or more.

[0139] (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 (excluding flowability improvers described in detail below), and inorganic fine particles can be used, and two or more types can be used in combination as necessary.

[0140] These additives may be used not for the purpose of improving liquid permeability, but to fulfill other functions such as anti-caking agents under moisture absorption, binders for water-absorbent resins, etc. When added for the purpose of other functions, they are referred to as "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.

[0141] The above-mentioned "SFC" is an abbreviation for Saline Flow Conductivity, and is the permeability of a 0.69 mass % aqueous sodium chloride solution through a water-absorbing agent composition or a water-absorbing resin under a load of 2.07 kPa.

[0142] 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.

[0143] (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 －５ mol, more preferably 0 mol or more and 1.0 × 10 －５ Less than a mole.

[0144] (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.

[0145] The lower limit of the amount of the cationic polymer added is preferably 0 parts by mass or more, more preferably more than 0 parts by mass, relative to 100 parts by mass of the water-absorbent resin. On the other hand, the upper limit 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. A preferred range of the amount of the cationic polymer added can be a range defined by any combination selected from the upper and lower limits.

[0146] (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 is preferred from the viewpoint of the effect of improving SFC and GBP.

[0147] When the primary particle diameter of the inorganic fine particles is less than 20 nm, the lower limit of the amount of the inorganic fine particles added is preferably 0 parts by mass or more, more preferably more than 0 parts by mass, relative to 100 parts by mass of the water-absorbent resin. On the other hand, the upper limit is preferably less than 1.2 parts by mass, more preferably less than 1.0 parts 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 primary particle diameter of the inorganic fine particles is 20 nm or more, the lower limit of the amount of the inorganic fine particles added is preferably 0 parts by mass or more, more preferably more than 0 parts by mass, relative to 100 parts by mass of the water-absorbent resin. On the other hand, the upper limit is 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. The preferred range of the amount of the inorganic fine particles added can be a range defined by any combination selected from the upper and lower limits.

[0148] (Other Additives) Specific examples of other additives include chelating agents, inorganic reducing agents, aromatic substances, organic reducing agents, hydroxycarboxylic acid compounds, compounds having phosphorus atoms, oxidizing agents, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, thermoplastic fibers, etc. One or more of these other additives can be used. Among these, chelating agents are preferred, and amino polycarboxylic acids or amino polyphosphates 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.

[0149] 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.

[0150] [2-8-2] Additive Addition Step The additive can be added before, after, or during at least one step selected from the monomer aqueous solution preparation step, polymerization step, gel-crushing step, drying step, pulverization step, classification step, and surface-crosslinking step. That is, in one embodiment, the production method according to the present invention may further include an additive addition step in addition to the monomer aqueous solution preparation step, polymerization step, gel-crushing step, drying step, pulverization step, classification step, surface-crosslinking step, and the step of adding an aqueous solution of a flowability improver. Preferably, the additive is added before, after, or during any of the steps subsequent to the polymerization step.

[0151] 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.

[0152] Specific examples of the apparatus used for the mixing include a stirring mixer, a cylindrical mixer, a double-walled conical mixer, a V-shaped mixer, a ribbon mixer, a screw mixer, a flow type rotary disk mixer, an airflow type mixer, a double-arm kneader, an internal mixer, a grinding kneader, a rotary mixer, a screw extruder, etc. When a stirring mixer is used, the 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.

[0153] [2-9] Aqueous Solution of Flowability Improver and Addition Step This step is a step of adding an aqueous solution of a flowability improver to a water-absorbent resin with a high specific surface area obtained through the above-mentioned steps. The step is performed during the surface cross-linking step or during a step subsequent to the surface cross-linking step. Here, "performed during a step subsequent to the surface cross-linking step" not only refers to a step of adding the aqueous solution subsequent to the surface cross-linking step, but also includes adding the aqueous solution of the flowability improver during any step subsequent to the surface cross-linking step (e.g., any step such as a cooling step, a rewetting step, or a fine powder granulation step). According to one embodiment of the present invention, by performing this step under the following specific conditions, particle size segregation due to transportation can be suppressed in a water-absorbent agent composition having a high specific surface area, and the flowability of the powder can be further improved. Furthermore, the water-absorption properties of the resulting water-absorbent agent composition are maintained favorably. Furthermore, according to another embodiment of the present invention, a water-absorbent agent composition having a high specific surface area and in which the dynamic friction coefficient of particles having a particle diameter of 300 μm or more and less than 600 μm is reduced by 10% or more can be obtained.

[0154] One aspect of the present invention is a method for producing a water-absorbing agent composition containing a water-absorbent resin as a main component, the method comprising a step of preparing a monomer aqueous solution, a polymerization step, a gel-crushing step, a drying step, a pulverization step, a classification step, and a surface-crosslinking step, the method comprising a step of mixing a water-soluble fluidity improver having a mass-average molecular weight of 200 or more and 50,000 or less with the water-absorbent resin in an amount of more than 0 ppm and less than 200 ppm relative to the mass of the water-absorbent resin during the surface-crosslinking step or a step subsequent to the surface-crosslinking step, the method for producing a water-absorbing agent composition satisfying all of the following (a) to (d): (a) a specific surface area of ​​the water-absorbent resin is 25 m ２ / kg or more; (b) when the water-soluble fluidity improver is mixed with a water-absorbent resin, the form of the water-soluble fluidity improver is an aqueous solution of 0.01 mass % or more and 20 mass % or less; (c) when the aqueous solution is added to and mixed with a water-absorbent resin, the average droplet diameter of the aqueous solution is 10 μm or more and 1 mm or less; (d) when the aqueous solution is added to and mixed with a water-absorbent resin, the mixing force index defined by the following (Equation 1) is 70,000 or more.

[0155]

[0156] [2-9-1] Water-absorbent resin In one embodiment of the present invention, an aqueous solution of a fluidity improver is added to and mixed with a water-absorbent resin. At that time, the water-absorbent resin may be a water-absorbent resin before surface cross-linking or a water-absorbent resin after surface cross-linking, but a water-absorbent resin after surface cross-linking is preferable. That is, the water-absorbent resin to which the fluidity improver is added may be a water-absorbent resin obtained through the above-mentioned surface cross-linking step (surface-cross-linked water-absorbent resin). In addition, the lower limit of the specific surface area of ​​the water-absorbent resin is 25 m ２ / kg or more. The specific surface area of ​​the water-absorbent resin is 25m ２ If the specific surface area is less than 1 / kg, a water-absorbent agent composition having a sufficient water absorption rate (Vortex) cannot be obtained. The specific surface area of ​​the water-absorbent resin used in producing the water-absorbent agent composition may be set depending on the specific surface area of ​​the desired water-absorbent agent composition. The specific surface area of ​​the water-absorbent resin is preferably as high as possible, and is preferably 26 m ２ / kg or more, more preferably 27m ２ / kg or more, more preferably 28m ２ / kg or more, and even more preferably 29m２ / kg or more, and even more preferably 30m ２ / kg or more, and particularly preferably 35m ２ / kg or more, and most preferably 36m ２ On the other hand, the upper limit is preferably 60 m ２ / kg or less, more preferably 55m ２ The preferred range of the specific surface area can be determined by any combination selected from the upper and lower limits. Therefore, the specific surface area of ​​the water-absorbent resin is, for example, 25 m ２ / kg or more 60m ２ / kg or less, ２ / kg or more 60m ２ / kg or less, ２ / kg or more 60m ２ / kg or less, ２ / kg or more 60m ２ / kg or less, ２ / kg or more 60m ２ / kg or less, ２ / kg or more 55m ２ / kg or less, ２ / kg or more 55m ２ / kg or less, ２ / kg or more 55m ２ / kg or less.

[0157] The physical properties of the water absorbent resin when an aqueous solution of a flowability improver is added preferably satisfy the respective physical properties disclosed in the above-mentioned "[2-5] Pulverization step, classification step", and for example, the above-mentioned (i) to (iv) can be applied. In particular, it is more preferable that the particle size distribution of the water absorbent resin has D50 (mass average particle diameter) within the above-mentioned range (ii), and the proportion of particles less than 150 μm within the above-mentioned range (i).

[0158] In addition to the above (a) to (d), the production method according to the present invention preferably further satisfies the following (e): (e) after the addition of the water-soluble fluidity improver, the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm in the water-absorbent resin is 0.80 or less. The dynamic friction coefficient can be measured as the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm in the water-absorbent agent composition, and is, for example, 0.10 or more and 0.80 or less, preferably 0.30 or more and 0.79 or less, more preferably 0.50 or more and 0.78 or less, even more preferably 0.60 or more and 0.77 or less, particularly preferably 0.60 or more and 0.76 or less, and most preferably 0.60 or more and 0.73 or less. For detailed measurement conditions of the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm, see the Examples.

[0159] [2-9-2] Flowability Improver In the production method according to the present invention, a flowability improver (or an aqueous solution thereof) is added to a water-absorbent resin under specific conditions. In this specification, the term "flowability improver" refers to a water-soluble component that can improve the flowability of the water-absorbent agent composition (water-absorbent resin) (reduce the coefficient of dynamic friction) by adding the agent. "Water-soluble" means that 0.1 g or more of the flowability improver dissolves in 100 g of water at 25°C. The amount of the flowability improver that dissolves in 100 g of water at 25°C is preferably 1 g or more, more preferably 5 g or more. The flowability improver is another word for the "water-soluble polymer" in the basic application of the present application, and has the same meaning. In this specification, the "water-soluble polymer" in the basic application of the present application will be referred to as a "water-soluble flowability improver" or a "flowability improver."

[0160] The lower limit of the mass average molecular weight of the fluidity improver is 200 or more, preferably 220 or more, more preferably 250 or more, and even more preferably 300 or more. If the mass average molecular weight of the fluidity improver is less than 200, it will not be possible to impart sufficient slipperiness to the water absorbent resin, making it difficult to improve the fluidity of the water absorbent agent composition during transportation. The upper limit of the mass average molecular weight of the fluidity improver is 50,000 or less, preferably 40,000 or less, more preferably 30,000 or less, even more preferably 20,000 or less, particularly preferably 10,000 or less, and most preferably 5,000 or less. If the mass average molecular weight of the fluidity improver exceeds 50,000, the viscosity of the fluidity improver itself will increase, making it difficult to add it uniformly to the water absorbent resin. The preferred range of the mass average molecular weight of the fluidity improver can be defined by any combination selected from the above upper and lower limits. Therefore, the mass average molecular weight of the flowability improver may be, for example, 200 or more and 50,000 or less, 220 or more and 40,000 or less, 220 or more and 30,000 or less, 250 or more and 20,000 or less, 300 or more and 10,000 or less, or 300 or more and 5,000 or less.

[0161] In addition, by using a flowability improver having a mass average molecular weight within the above range, the dynamic friction coefficient of particles having a particle diameter of 300 μm or more and less than 600 μm can be significantly reduced.The mass average molecular weight of the flowability improver is measured, for example, by gel permeation chromatography (hereinafter referred to as GPC) using polyethylene glycol as a standard substance.In addition, in this specification, even when referring to the molecular weight of a flowability improver with a single defined structure (for example, one with a relatively low molecular weight), it is collectively referred to as "mass average molecular weight".For the molecular weight of such a flowability improver, instead of measuring it by GPC as described above, the molecular weight can also be calculated from the chemical formula.

[0162] According to a preferred embodiment of the present invention, the water-soluble flow improver may be one or more selected from the group consisting of nonionic substances, zwitterionic substances, anionic substances, and cationic substances. In one embodiment, the nonionic substance is selected from (a) polyols, (b) hydroxyl group-modified polyols, (c) side-chain and / or terminal polyether-modified polysiloxanes, and (d) alkylene oxide adducts of higher aliphatic amines, the zwitterionic substance is selected from (e) alkyl betaines and (f) alkylamine oxides, the anionic substance is selected from (g) alkyl sulfate salts, (h) sulfate salts of higher alcohol alkylene oxide adducts, (i) sulfonates, (j) dicarboxylates, (k) alkylamine diacetates, (l) phosphate salts of higher alcohol alkylene oxide adducts, and (m) carboxylate salts of higher alcohol alkylene oxide adducts, and the cationic substance is selected from (n) ammonium salts. With this configuration, the desired effects of the present invention can be efficiently achieved.

[0163] (a) Polyols According to a preferred embodiment of the present invention, the term "polyols" refers to compounds having multiple hydroxy groups. Specific examples include polyalkylene glycols such as polyethylene glycol and polypropylene glycol, and block or random copolymers of polyethylene glycol and polypropylene glycol. The number of carbon atoms in the alkylene unit of the repeating unit in the polyalkylene glycol is preferably C1 to C6, more preferably C2 to C4, and particularly preferably C2 to C3. (In this specification, the number of carbon atoms may be represented by a number following the "C." For example, a carbon number of 1 may be represented as C1, and a carbon number of 10 may be represented as C10.) This configuration significantly reduces the coefficient of dynamic friction for particles having a particle diameter of 300 μm or more but less than 600 μm.

[0164] Polyalkylene glycols such as block copolymers or random copolymers of polyethylene glycol and polypropylene glycol are readily available on the market, and preferred examples thereof include the following products:

[0165] ・Pluronic (registered trademark, the same applies hereinafter) series manufactured by ADEKA Corporation: Pluronic L-34, Pluronic L-44, Pluronic L-64, Pluronic P-84, Pluronic P-85, Pluronic P-103, Pluronic F-68, Pluronic F-88, Pluronic F-108, Pluronic 17R-3, Pluronic 17R-4, Pluronic TR-704, Pluronic TR-913R manufactured by NOF Corporation: Pronon (registered trademark, the same applies hereinafter) #104, Pronon #204, Pronon #208, Unilube 70DP-600B, Unilube 70DP-950B manufactured by Daiichi Kogyo Seiyaku Co., Ltd. Epan (registered trademark, same below) 450, Epan 485, Epan 680, Epan 740, Epan 750, Epan 785, Epan U-103, Epan U-105, Epan U-108.

[0166] (b) Hydroxy Group-Modified Polyols According to a preferred embodiment of the present invention, the term "hydroxy group-modified polyols" refers to a compound in which one or more hydroxy groups contained in a polyol have been ester-modified and / or ether-modified. The ester and / or ether modification is preferably by a hydrocarbon group. The hydrocarbon group preferably has a carbon number of C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the carbon number is 30 or less, the hydrophobicity is not too strong, and the surface tension can be well maintained.

[0167] The hydrocarbon group is not limited to a straight chain, but may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, or an aromatic hydrocarbon group such as a phenyl group or an alkylphenyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group.

[0168] According to a preferred embodiment of the present invention, the hydroxy group-modified polyols (b) include (b-1) glycidyl-modified polyols, (b-2) alkylene oxide adducts of higher alcohols, and (b-3) alkylene oxide adducts of polyhydric alcohol fatty acid esters. (b-1) may be a compound in which at least one terminal of a (poly)alkylene glycol is modified with a glycidyl group. (b-2) may be a compound in which one terminal of a (poly)alkylene glycol is modified with a substituent having a C1 to C30 hydrocarbon group. (b-3) may be a compound in which at least one hydroxy group of a polyhydric alcohol is modified with an alkylene oxide and at least one hydroxy group of the polyhydric alcohol is modified with a substituent having a C1 to C30 hydrocarbon group via an ester bond. The polyhydric alcohol in (b-3) may be glycerin, pentaerythritol, sorbitol, sorbitan, or a sugar. With this configuration, the dynamic friction coefficient of particles having a particle diameter of 300 μm or more and less than 600 μm can be significantly reduced.

[0169] (b-1) Glycidyl-Modified Polyols Glycidyl-modified polyols are compounds in which at least one terminal of a (poly)alkylene glycol is modified with a glycidyl group. Specific examples include water-soluble polyglycidyl ethers of polyols such as water-soluble (poly)alkylene glycol diglycidyl ethers such as diethylene glycol diglycidyl ether and polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, hexanediol diglycidyl ether, glycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, and sorbitol polyglycidyl ether.

[0170] Glycidyl-modified polyols are readily available on the market, and preferred examples thereof include the following products:

[0171] Nagase ChemteX Corporation: Denacol (registered trademark, the same applies hereinafter) EX-145, Denacol EX-171, Denacol EX-211, Denacol EX-212, Denacol EX-252, Denacol EX-810, Denacol EX-811, Denacol EX-850, Denacol EX-851, Denacol EX-821, Denacol EX-830, Denacol EX-832, Denacol EX-841, Denacol EX -861, Denacol EX-911, Denacol EX-941, Denacol EX-920, Denacol EX-931, Denacol EX-313, Denacol EX-314, Denacol EX-321, Denacol EX-411, Denacol EX-421, Denacol EX-512, Denacol EX-521, Denacol EX-612, Denacol EX-614, Denacol EX-614B.

[0172] (b-2) Alkylene oxide adduct of higher alcohol The alkylene oxide adduct of higher alcohol may be a compound in which one end of a (poly)alkylene glycol is modified with a substituent having a C1 to C30 (preferably C6 to C30) hydrocarbon group. In one embodiment, the alkylene oxide adduct of higher alcohol is preferably a compound represented by the following general formula (Chemical Formula 1):

[0173]

[0174] In the above formula (Chemical Formula 1), R is a hydrocarbon group having a carbon number of C1 to C30 (preferably C6 to C30). The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (e.g., an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (e.g., a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The hydrocarbon group preferably has a carbon number of C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained. Furthermore, the hydrocarbon group is preferably a linear or branched saturated hydrocarbon group.

[0175] In the above formula, (AO) is C ｎ H ２ｎ The repeating unit (AO) in the general formula (Chemical Formula 1) can be represented by the formula [CH ２ CH ２ O], and the repeating unit may be a structure derived from ethylene oxide addition or ethylene glycol condensation.

[0176] In the above formula, a represents the number of the repeating units (AO), and when a is 2 or more, the number of carbon atoms constituting each repeating unit may be the same or different. That is, the alkylene oxide adduct of higher alcohol represented by the above general formula (Chemical Formula 1) may be a polymer having the same repeating unit (AO), or may be a block polymer or random polymer having different repeating units (AO).

[0177] In the above formula, a is preferably 1 to 1000, more preferably 2 to 500, and further preferably 2 to 300. When the number of repeating units is 1000 or less, the viscosity does not become too high, and it becomes easy to add the compound uniformly to the water-absorbent resin.

[0178] Alkylene oxide adducts of higher alcohols are readily available on the market, and preferred examples thereof include the following products:

[0179] Manufactured by Kao Corporation: Polyoxyethylene lauryl ether Emulgen (registered trademark, the same applies hereinafter) 106 (HLB=10.5), Emulgen 108 (HLB=12.1), Emulgen 109P (HLB=13.6), Emulgen 120 (HLB=15.3), Emulgen 123P (HLB=16.9), Emulgen 130K (HLB=18.1), Emulgen 147 (HLB=16.3), Emulgen 150 (HLB=18.4); Polyoxyethylene polyoxypropylene alkyl ether Emulgen MS-110 (HLB=12.7); Polyoxyethylene cetyl ether Emulgen 210P (HLB=10.7), Emulgen 220 (HLB=14.2); Polyoxyethylene stearyl ether Emulgen 320P (HLB=13.9), Emulgen 350 (HLB=17.8) Polyoxyethylene oleyl ether Emulgen 408 (HLB=10.0), Emulgen 409PV (HLB=12.0), Emulgen 420 (HLB=13.6), Emulgen 430 (HLB=16.2) Polyoxyethylene myristyl ether Emulgen 4085 (HLB=18.9) Polyoxyethylene octyldodecyl ether Emulgen 2020G-HA (HLB=13.0), Emulgen 2025G (HLB=15.7) NOF Corporation Polyoxyethylene isodecyl ether Nonion ID-203 (HLB=12.5), Nonion ID-209 (HLB=14.3) Polyoxyethylene-2-ethylhexyl ether Nonion EH-204 (HLB=11.5), Nonion EH-208 (HLB=14.6) manufactured by Nippon Nyukazai Co., Ltd. Polyoxyethylene nonylphenyl ether Newcol (registered trademark, the same applies hereinafter) 560 (HLB=10.9), Newcol 564 (HLB=12.3), Newcol 565 (HLB=13.3), Newcol 566 (HLB=14.1), Newcol 568 (HLB=15.2), Newcol 504 (HLB=16.0), Newcol 506 (HLB=17.2), Newcol 509 (HLB=18.0), Newcol 516 (HLB=18.8).

[0180] (b-3) Alkylene oxide adduct of polyhydric alcohol fatty acid ester The ethylene oxide adduct of polyhydric alcohol fatty acid ester may be a compound in which an alkylene oxide is added to at least one hydroxy group of a polyhydric alcohol, and at least one hydroxy group of the polyhydric alcohol is modified with a substituent having a C1 to C30 hydrocarbon group via an ester bond. Examples of the polyhydric alcohol include glycerin, pentaerythritol, sorbitol, sorbitan, and sugars.

[0181] Preferred examples include alkylene oxide adducts of glycerin fatty acid monoesters and alkylene oxide adducts of sorbitan fatty acid monoesters. In one embodiment, the alkylene oxide adducts of glycerin fatty acid monoesters are preferably compounds represented by the following general formula (Chemical Formula 2). Furthermore, the alkylene oxide adducts of sorbitan fatty acid monoesters include structural isomers. Therefore, in one embodiment, the alkylene oxide adducts of sorbitan fatty acid monoesters may be compounds represented by the following general formula (Chemical Formula 3) or (Chemical Formula 4).

[0182]

[0183] In the above formula (Chemical Formula 2), R is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0184] In the above formula, (A １ O) and (A ２O) is C ｎ H ２ｎ It is a repeating unit that can also be expressed as O (n is a natural number). １ O and A ２ The number of carbon atoms constituting each O (the above n) is preferably C1 to C6, more preferably C1 to C3, further preferably C2 to C3, and particularly preferably C2. That is, in the repeating unit (A １ O or A ２ O) is [CH ２ CH ２ O], and the repeating unit may be a structure derived from ethylene oxide addition or ethylene glycol condensation.

[0185] In the above formula, a and b are each the repeating unit (A １ O and A ２ When a or b is 2 or more, the number of carbon atoms constituting each repeating unit may be the same or different. １ O and A ２ In O, A １ and the number of carbon atoms constituting A ２ The number of carbon atoms constituting the alkylene oxide moiety in the general formula (Chemical Formula 2) may be the same or different. １ O or A ２ O), and １ O or A ２ The polymer may be a block polymer or a random polymer having the formula (I).

[0186] In the above formula, the sum of a and b (a+b) is preferably 1 to 1000, more preferably 2 to 500, and further preferably 2 to 300. a and b may be different from each other or may be the same. When the sum of a+b is 1000 or less, the viscosity does not become too high, and it becomes easy to add it uniformly to the water-absorbent resin.

[0187]

[0188]

[0189] In the above formulas (Chemical Formula 3 and Chemical Formula 4), R is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (e.g., an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (e.g., a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained. Furthermore, the hydrocarbon group is preferably a linear or branched saturated hydrocarbon group.

[0190] In the above formula, (A １ O), (A ２ O) and (A ３ O) are C ｎ H ２ｎ It is a repeating unit that can also be expressed as O (n is a natural number). １ O.A. ２ O and A ３ The number of carbon atoms constituting each O (the above n) is preferably C1 to C6, more preferably C1 to C3, further preferably C2 to C3, and particularly preferably C2. That is, the repeating units (A １ O.A. ２ O or A ３ O) is [CH ２ CH ２ O], and the repeating unit may be a structure derived from ethylene oxide addition or ethylene glycol condensation.

[0191] In the above formula, a to c are each the repeating units (A １ O.A. ２ O and A ３When a, b, or c is 2 or more, the number of carbon atoms constituting each repeating unit may be the same or different. １ O.A. ２ O.A. ３ In O, A １ and the number of carbon atoms constituting A ２ and the number of carbon atoms constituting A ３ The number of carbon atoms constituting the alkylene oxide moieties in the general formulae (Chemical Formula 3) and (Chemical Formula 4) may be the same or different. １ O.A. ２ O or A ３ O), and １ O.A. ２ O or A ３ The polymer may be a block polymer or a random polymer having the formula (I).

[0192] In the above formula, the sum of a to c (a+b+c) is preferably 1 to 1000, more preferably 2 to 500, and even more preferably 2 to 300. a, b, and c may be different from each other or the same. When the sum of a+b+c is 1000 or less, the viscosity does not become too high, and it becomes easy to add it uniformly to the water-absorbent resin.

[0193] Alkylene oxide adducts of polyhydric alcohol fatty acid esters are readily available on the market, and preferred examples thereof include the following products:

[0194] Manufactured by Kao Corporation: Polyoxyethylene sorbitan monolaurate RHEODOL (registered trademark, the same applies hereinafter) TW-L120 (HLB=16.7), RHEODOL TW-L106 (HLB=13.3), RHEODOL Super TW-L120; Polyoxyethylene sorbitan monopalmitate RHEODOL TW-P120 (HLB=15.6); Polyoxyethylene sorbitan monostearate RHEODOL TW-S120V (HLB=14.9); Polyoxyethylene sorbitan tristearate RHEODOL TW-S320V (HLB=10.5); Polyoxyethylene sorbitan monooleate RHEODOL TW-O120V (HLB=15.0), RHEODOL TW-O106V (HLB=10.0); Polyoxyethylene sorbitan trioleate Rheodor TW-O320V (HLB = 11.0) manufactured by NOF Corporation Polyoxyethylene coconut fatty acid glyceryl Unigly (registered trademark, the same applies below) MK-207 (HLB = 13.0), Unigly MK-230 (HLB = 17.4).

[0195] (c) Side Chain and / or Terminal Polyether-Modified Polysiloxane According to a preferred embodiment of the present invention, "side chain and / or terminal polyether-modified polysiloxane" refers to a compound in which the side chain and / or terminal of the polysiloxane is polyether-modified. The polyether-modified site of the polysiloxane is not particularly limited, and may be the side chain of the polysiloxane, both terminals of the polysiloxane, one terminal of the polysiloxane, or both the side chain and both terminals of the polysiloxane. Examples of polyether-modified groups include polyoxyethylene groups, polyoxypropylene groups, and those having both polyoxyethylene groups and polyoxypropylene groups. This configuration significantly reduces the dynamic friction coefficient of particles having a particle diameter of 300 μm or more and less than 600 μm.

[0196] Polyether-modified polysiloxanes are readily available on the market, and preferred examples include the following products:

[0197] Shin-Etsu Chemical Co., Ltd.: KF-351A (HLB=12), KF-353 (HLB=10), KF-354L (HLB=16), KF-355A (HLB=12), KF-615A (HLB=10), KF-640 (HLB=14), KF-642 (HLB=12), KF-643 (HLB=14), KF-6011 (HLB=12); Dow Corning Toray Co., Ltd.: FZ-77 (HLB=11), L-7604 (HLB=11).

[0198] (d) Alkylene oxide adduct of higher aliphatic amine According to a preferred embodiment of the present invention, the term "alkylene oxide adduct of higher aliphatic amine" refers to a compound in which alkylene oxide is added to two hydrogen atoms on a nitrogen atom of a primary amine having a C1 to C30 hydrocarbon group. This configuration significantly reduces the coefficient of kinetic friction for particles having a particle diameter of 300 μm or more and less than 600 μm.

[0199] In one embodiment, the alkylene oxide adduct of a higher aliphatic amine is preferably a compound represented by the following general formula (Chemical Formula 5).

[0200]

[0201] In the above formula (Chemical Formula 5), ​​R is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The hydrocarbon group preferably has a carbon number of C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0202] In the above formula, (A １ O) and (A ２ O) is Cｎ H ２ｎ It is a repeating unit that can also be expressed as O (n is a natural number). １ O and A ２ The number of carbon atoms constituting each O (the above n) is preferably C1 to C6, more preferably C1 to C3, further preferably C2 to C3, and particularly preferably C2. That is, the repeating unit (A １ O or A ２ O) is [CH ２ CH ２ O], and the repeating unit may be a structure derived from ethylene oxide addition or ethylene glycol condensation.

[0203] In the above formula, a and b are each the repeating unit (A １ O and A ２ When a or b is 2 or more, the number of carbon atoms constituting each repeating unit may be the same or different. １ O and A ２ In O, A １ and the number of carbon atoms constituting A ２ The number of carbon atoms constituting the alkylene oxide moiety in the general formula (Chemical Formula 5) may be the same or different. １ O or A ２ O), and １ O or A ２ The polymer may be a block polymer or a random polymer having the formula (I).

[0204] In the above formula, the sum of a and b (a+b) is preferably 1 to 1000, more preferably 2 to 500, and further preferably 2 to 300. a and b may be different from each other or may be the same. When the sum of a+b is 1000 or less, the viscosity does not become too high, and it becomes easy to add the compound uniformly to the water-absorbent resin.

[0205] Alkylene oxide adducts of higher aliphatic amines are readily available on the market, and preferred examples thereof include the following commercial products:

[0206] Manufactured by NOF Corporation: Polyoxyethylene laurylamine Nymeen (registered trademark, the same applies hereinafter) L-207 (HLB=12.5), Polyoxyethylene alkyl coconut oil alkylamine Nymeen F-215 (HLB=15.4), Polyoxyethylene stearylamine Nymeen S-210 (HLB=12.5), Nymeen S-215 (HLB=14.5), Nymeen S-220 (HLB=15.4), Polyoxyethylene beef tallow alkylamine Nymeen T2-210 (HLB=12.5), Nymeen T2-230 (HLB=16.7), Polyoxyethylene alkyl propylene diamine Nymeen DT-208 (HLB=10.7) Manufactured by Kao Corporation Amit (registered trademark, same below) 105A (HLB=10.8), Amit 320 (HLB=15.4).

[0207] (e) Alkylbetaine According to a preferred embodiment of the present invention, "alkylbetaine" refers to a compound having a cationic group and an anionic group at non-adjacent positions in the same molecule, wherein the cationic group is a secondary to quaternary ammonium cation, and at least one of the substituents on the secondary to quaternary ammonium cation is a substituent having a C1 to C30 hydrocarbon group. This configuration significantly reduces the coefficient of kinetic friction for particles having a particle diameter of 300 μm or more and less than 600 μm. In one embodiment, the alkylbetaine is preferably a compound represented by the following general formula (Chemical Formula 6):

[0208]

[0209] In the above formula (Chemical Formula 6), R １is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0210] In the above formula, R ２ and R ３ are each independently hydrogen or a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched or cyclic saturated hydrocarbon group and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C1 to C25, and even more preferably C1 to C20. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained. R １ , R ２ and R ３ may be different or the same.

[0211] In the above formula, the structure of X is not particularly limited, except that it contains C1 or more carbon atoms.

[0212] In the above formula, the anion moiety (Z) can be a carboxylate (carboxylate anion), a sulfonate (sulfonate anion), or a phosphate (phosphate anion).

[0213] In one embodiment, the alkylbetaine may be a compound represented by the above general formula (Chemical Formula 6) or a compound (having a cationic group on the imidazolium ring) represented by the following general formula (Chemical Formula 7): An example of such a compound is commercially available Amphitol (registered trademark, the same applies hereinafter) 20YB (manufactured by Kao Corporation).

[0214]

[0215] In the above formula (Chemical Formula 7), R １ is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0216] In the above formula, R ２ is hydrogen or a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched or cyclic saturated hydrocarbon group and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C1 to C25, and even more preferably C1 to C20. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained. R １ and R ２ may be different or the same.

[0217] In the above formula, the structure of X is not particularly limited except that it contains C1 or more carbon atoms.

[0218] In the above formula, the anion moiety (Z) can be a carboxylate (carboxylate anion), a sulfonate (sulfonate anion), or a phosphate (phosphate anion).

[0219] (f) Alkylamine Oxide According to a preferred embodiment of the present invention, "alkylamine oxide" refers to a compound having a cationic group and an anionic group at adjacent positions in the same molecule, wherein the cationic group is a secondary to quaternary ammonium cation, and at least one of the substituents on the secondary to quaternary ammonium cation is a substituent having a C1 to C30 hydrocarbon group. This configuration significantly reduces the coefficient of kinetic friction for particles having a particle diameter of 300 μm or more and less than 600 μm. In one embodiment, the alkylamine oxide is preferably a compound represented by the following general formula (Chemical Formula 8):

[0220]

[0221] In the above formula (Chemical Formula 8), R １ is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0222] In the above formula, R ２ and R ３are each independently hydrogen or a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched or cyclic saturated hydrocarbon group and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C1 to C25, and even more preferably C1 to C20. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained. R １ , R ２ and R ３ may be different or the same.

[0223] In one embodiment, the alkylamine oxide as the flow improver is represented by the general formula (8) above, where R １ is a C4 to C24 linear saturated hydrocarbon group (alkyl group), and R ２ and R ３ are preferably compounds each independently a C1 to C25 linear saturated hydrocarbon group (alkyl group). １ is a C6 to C22 linear saturated hydrocarbon group (alkyl group), and R ２ and R ３ are preferably compounds each independently a C1 to C20 linear saturated hydrocarbon group (alkyl group).

[0224] Alkylbetaines and alkylamine oxides are readily available on the market, and preferred examples thereof include the following products:

[0225] Manufactured by Kao Corporation: Anhithol 20BS, Anhithol 24B (desalted product of 20BS), Anhithol 86B, Anhithol 20N, Anhithol 20YB, Anhithol 20AB, Anhithol 55AB, Anhithol 20HD Manufactured by Daiichi Kogyo Seiyaku Co., Ltd.: Amogen (registered trademark, the same applies hereinafter) S-H, Amogen K, Amogen LB-C, Amogen CB-H, Amogen HB-C, Amogen AOL Manufactured by Adeka Corporation: Adeka Anhort (registered trademark, the same applies hereinafter) PB-30L, Adeka Anhort AB-35L Manufactured by NOF Corporation: Nissan Anon (registered trademark, same hereinafter) BF, Nissan Anon BL, Nissan Anon BL-SF, Nissan Anon BDF-R, Nissan Anon BDF-SF, Nissan Anon BDC-SF, Nissan Anon BDL-SF, Nissan Anon GLM-R, Unisafe (registered trademark, same hereinafter) A-LM, Unisafe A-SM, Unisafe A-LE Manufactured by Nippon Nyukazai: Texnol (registered trademark, same hereinafter) R2 Manufactured by Toho Chemical Industry Co., Ltd.: Obazoline (registered trademark, same hereinafter) LB-SF Manufactured by New Japan Chemical Co., Ltd.: Wondamin (registered trademark, same hereinafter) OX-300.

[0226] (g) Alkyl Sulfate Salt According to a preferred embodiment of the present invention, an "alkyl sulfate salt" refers to a compound having a sulfate group (-SO 3 ) in the same molecule. ４ -). This configuration makes it possible to significantly reduce the coefficient of dynamic friction of particles having a particle diameter of 300 μm or more and less than 600 μm. In one embodiment, the alkyl sulfate ester salt is preferably a compound represented by the following general formula (Chemical Formula 9):

[0227]

[0228] In the above formula (Chemical Formula 9), R is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The hydrocarbon group preferably has a carbon number of C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0229] In the above formula, M ＋ is an alkali metal ion (Li ＋ , Na ＋ , K. ＋ etc.), which may be an ammonium ion.

[0230] Alkyl sulfate ester salts are readily available on the market, and preferred examples thereof include the following products:

[0231] Manufactured by Kao Corporation: Emar (registered trademark, same hereinafter) 2F, Latemul (registered trademark, same hereinafter) AD-25 Manufactured by Takemoto Oil & Fats Co., Ltd.: Takesurf (registered trademark, same hereinafter) A-24 Manufactured by NOF Corporation: Scintrex (registered trademark, same hereinafter) EH-R, Persoft (registered trademark, same hereinafter) SK Manufactured by Lion Specialty Chemicals Co., Ltd.: Sanol (registered trademark, same hereinafter) LM-1130.

[0232] (h) Sulfate Salt of Higher Alcohol Alkylene Oxide Adduct According to a preferred embodiment of the present invention, the term "sulfate salt of higher alcohol alkylene oxide adduct" refers to a compound in which one end of a (poly)alkylene glycol is modified with a substituent having a C1 to C30 hydrocarbon group, and the other end is a sulfate salt. This configuration significantly reduces the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm. In one embodiment, the sulfate salt of higher alcohol alkylene oxide adduct is preferably a compound represented by the following general formula (Chemical Formula 10):

[0233]

[0234] In the above formula (Chemical Formula 10), R is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The hydrocarbon group preferably has a carbon number of C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0235] In the above formula, (AO) is C ｎ H ２ｎ The repeating unit (AO) in the general formula (Chemical Formula 10) can be represented by the formula [CH ２ CH ２ O], and the repeating unit may be a structure derived from ethylene oxide addition or ethylene glycol condensation.

[0236] In the above formula, a represents the number of the repeating units (AO), and when a is 2 or more, the number of carbon atoms constituting each repeating unit may be the same or different. That is, the alkylene oxide moiety contained in the above general formula (Chemical Formula 10) may be a polymer having the same repeating unit (AO), or may be a block polymer or random polymer having different repeating units (AO).

[0237] In the above formula, a is preferably 1 to 1000, more preferably 2 to 500, and further preferably 2 to 300. When the number of repeating units is 1000 or less, the viscosity does not become too high, and it becomes easy to add the compound uniformly to the water-absorbent resin.

[0238] In the above formula, M ＋ is an alkali metal ion (Li ＋ , Na ＋ , K. ＋ ), which may be an ammonium ion.

[0239] The sulfate salts of higher alcohol alkylene oxide adducts are readily available on the market, and preferred examples thereof include the following products:

[0240] Manufactured by Kao Corporation: Sodium polyoxyethylene lauryl ether sulfate: EMAL 20C, EMAL E-27C, EMAL 270J, EMAL 20CM Manufactured by Nippon Nyukazai Co., Ltd.: Polyoxyethylene alkyl ether sulfate: NEWCOL 1020-SN, NEWCOL 2308-SF, NEWCOL 2320-SN, NEWCOL 2360-SN, NEWCOL 1305-SN, NEWCOL 1330-SF, NEWCOL 1703-SFD, NEWCOL 1525-SFC Manufactured by NOF Corporation: Sodium polyoxyethylene alkyl ether sulfate: PERSOFT EP, NISSANTRAX (registered trademark, the same applies hereinafter) K-40, NISSANTRAX K-300, PERSOFT EF, PERSOFT EDO, PERSOFT EL, PERSOFT EK.

[0241] (i) Sulfonate According to a preferred embodiment of the present invention, a "sulfonate" refers to a compound having a sulfonic acid group (-SO 3 ) in the same molecule. ３ -). This configuration makes it possible to significantly reduce the coefficient of dynamic friction of particles having a particle diameter of 300 μm or more and less than 600 μm. In one embodiment, the sulfonate is preferably a compound represented by the following general formula (Chemical Formula 11):

[0242]

[0243] In the above formula (Chemical Formula 11), R is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The hydrocarbon group preferably has a carbon number of C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0244] In the above formula, M ＋ is an alkali metal ion (Li ＋ , Na ＋ , K. ＋ ), which may be an ammonium ion.

[0245] Sulfonates are readily available on the market, and preferred examples include the following commercial products:

[0246] Manufactured by Kao Corporation: - Alkylbenzenesulfonic acid Neopelex (registered trademark, same below) GS - Sodium dodecylbenzenesulfonate Neopelex G-15, Neopelex G-25, Neopelex G-65 - Sodium alkylnaphthalenesulfonate Pelex (registered trademark, same below) NB-L - Sodium dialkyl sulfosuccinate Pelex OT-P, Pelex TR - Disodium alkylmonoamide sulfosuccinate Pelex TA - Sodium alkyldiphenyletherdisulfonate Pelex SS-L, Pelex SS-H - Sodium alkane sulfonate - Latemul PS Manufactured by Takemoto Oil & Fat Co., Ltd.: Sodium alkyldiphenyletherdisulfonate Pionin A-43-D, Takesurf A-43-NQ.

[0247] (j) Dicarboxylate According to a preferred embodiment of the present invention, a "dicarboxylate" refers to a compound having a carboxyl group (-CH) in the same molecule. ３ COO-). Note that the term "dicarboxylate" as used herein refers to compounds other than amine compounds and does not include "(k) alkylamine diacetate" described below. This configuration makes it possible to significantly reduce the coefficient of dynamic friction of particles having a particle diameter of 300 μm or more and less than 600 μm. Examples of the dicarboxylate include alkenyl succinates and acylaspartates. In one embodiment, the alkenyl succinate is preferably a compound represented by the following general formula (Chemical Formula 12). In another embodiment, the acylaspartate is preferably a compound represented by the following general formula (Chemical Formula 13).

[0248]

[0249]

[0250] In the above formulas (Chemical Formula 12) and (Chemical Formula 13), R is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The hydrocarbon group preferably has a carbon number of C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0251] In the above formula, M ＋ is an alkali metal ion (Li ＋ , Na ＋ , K. ＋ In addition, in one molecule, not only two carboxylic acid groups may form a salt (a form in which two carboxylate moieties are present), but also one carboxylic acid group alone may form a salt (a form in which one carboxylate moiety is present).

[0252] Dicarboxylic acid salts are readily available on the market, and preferred examples thereof include the following products:

[0253] Manufactured by Kao Corporation: Dipotassium alkenyl succinate Latemul ASK Manufactured by Asahi Kasei Fine Chemical Corporation: Sodium cocoyl glutamate Aminosurfact (registered trademark, same below) ACDS-L Sodium lauroyl aspartate Aminoformer (registered trademark, same below) FLDS-L.

[0254] (k) Alkylamine Diacetate According to a preferred embodiment of the present invention, the “alkylamine diacetate” is a compound having an alkyl group and two carboxyl groups (—CH ３COO-). This configuration makes it possible to significantly reduce the coefficient of dynamic friction of particles having a particle diameter of 300 μm or more and less than 600 μm. In one embodiment, the alkylamine diacetate is preferably a compound represented by the following general formula (Chemical Formula 14):

[0255]

[0256] In the above formula (Chemical Formula 14), R is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0257] In the above formula, M ＋ is an alkali metal ion (Li ＋ , Na ＋ , K. ＋ ), which may be an ammonium ion.

[0258] Alkylamine diacetates are readily available on the market, and preferred examples thereof include the following commercial products:

[0259] Manufactured by NOF Corporation: Sodium laurylaminodiacetate Nissananon LA.

[0260] (l) Phosphate Salt of Higher Alcohol Alkylene Oxide Adduct According to a preferred embodiment of the present invention, "phosphate salt of higher alcohol alkylene oxide adduct" refers to a compound in which one end of a (poly)alkylene glycol is modified with a substituent having a hydrocarbon group having 1 to 30 carbon atoms, and the other end is a phosphate salt. This configuration significantly reduces the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm. In one embodiment, the phosphate salt of higher alcohol alkylene oxide adduct is preferably a compound represented by the following general formula (Chemical Formula 15):

[0261]

[0262] In the above formula (Chemical Formula 15), R １ and R ２ are each independently a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group having a carbon number of C1 to C30 may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0263] In the above formula, a represents a repeating unit (CH ２ CH ２ O), and is preferably 1 to 1000, more preferably 2 to 500, and further preferably 2 to 300. When the number of repeating units is 1000 or less, the viscosity does not become too high, and it becomes easy to add the compound uniformly to the water-absorbent resin.

[0264] In the above formula, M ＋is an alkali metal ion (Li ＋ , Na ＋ , K. ＋ ), which may be an ammonium ion.

[0265] Phosphate salts of higher alcohol alkylene oxide adducts are readily available on the market, and preferred examples include the following products:

[0266] Manufactured by Daiichi Kogyo Seiyaku Co., Ltd.: Plysurf (registered trademark, the same applies hereinafter) A212C, Plysurf A207H, Plysurf A208S Manufactured by Takemoto Yushi Co., Ltd.: Takesurf A-72TK65, Takesurf A-7004.

[0267] (m) Carboxylate of Higher Alcohol Alkylene Oxide Adduct According to a preferred embodiment of the present invention, "carboxylate of higher alcohol alkylene oxide adduct" refers to a compound in which one end of a (poly)alkylene glycol is modified with a substituent having a C1 to C30 hydrocarbon group, and the other end is a carboxylate. This configuration significantly reduces the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm. In one embodiment, the carboxylate of higher alcohol alkylene oxide adduct is preferably a compound represented by the following general formula (Chemical Formula 16):

[0268]

[0269] In the above formula (Chemical Formula 16), R represents a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (e.g., an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (e.g., a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The hydrocarbon group preferably has a carbon number of C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained. Furthermore, the hydrocarbon group is preferably a linear or branched saturated hydrocarbon group.

[0270] In the above formula, (AO) is C ｎ H ２ｎ The repeating unit (AO) in the general formula (Chemical Formula 16) can be represented by the formula [CH ２ CH ２ O], and the repeating unit may be a structure derived from ethylene oxide addition or ethylene glycol condensation.

[0271] In the above formula, a represents the number of the repeating units (AO), and when a is 2 or more, the number of carbon atoms constituting each repeating unit may be the same or different. That is, the alkylene oxide moiety contained in the above general formula (Chemical Formula 16) may be a polymer having the same repeating unit (AO), or may be a block polymer or random polymer having different repeating units (AO).

[0272] In the above formula, a is preferably 1 to 1000, more preferably 2 to 500, and further preferably 2 to 300. When the number of repeating units is 1000 or less, the viscosity does not become too high, and it becomes easy to add the compound uniformly to the water-absorbent resin.

[0273] In the above formula, M ＋ is an alkali metal ion (Li ＋ , Na ＋ , K. ＋ ), which may be an ammonium ion.

[0274] In one embodiment, the carboxylate of a higher alcohol alkylene oxide adduct serving as a flow improver is represented by the general formula (Chemical Formula 16), where R is a C4 to C24 linear saturated hydrocarbon group (alkyl group), the number of carbon atoms constituting AO is 2 to 3 (C2 to C3), a is 2 to 500, and M ＋ In one embodiment, the carboxylate of a higher alcohol alkylene oxide adduct serving as a flow improver is a compound represented by the general formula (Chemical Formula 16), in which R is a C6 to C22 linear saturated hydrocarbon group (alkyl group), and AO is -CH ２ CH ２ O-, a is 2 to 300, M ＋ is preferably a compound that is an alkali metal ion.

[0275] Carboxylate salts of higher alcohol alkylene oxide adducts are readily available on the market, and preferred examples include the following products:

[0276] Manufactured by Kao Corporation: Polyoxyethylene lauryl ether sodium acetate Kao Akipo RLM-100NV, Kao Akipo RLM-100, Kao Akipo RLM-45NV, Kao Akipo RLM-45.

[0277] (n) Ammonium Salt According to a preferred embodiment of the present invention, "ammonium salt" refers to a compound in which at least one hydrogen atom of an ammonium salt is modified with a substituent having a C1 to C30 hydrocarbon group. This configuration significantly reduces the coefficient of dynamic friction of particles having a particle size of 300 μm or more and less than 600 μm. In one embodiment, the ammonium salt is preferably a compound represented by the following general formula (Chemical Formula 17):

[0278]

[0279] In the above formula (Chemical Formula 17), R １ is a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C2 to C28, even more preferably C3 to C26, particularly preferably C4 to C24, and most preferably C6 to C22. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained.

[0280] In the above formula, R ２ , R ３ and R ４are each independently hydrogen or a hydrocarbon group having a carbon number of C1 to C30. The hydrocarbon group may be a linear, branched or cyclic saturated hydrocarbon group and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably C1 to C30, more preferably C1 to C25, and even more preferably C1 to C20. When the hydrocarbon group has a carbon number of 30 or less, the hydrophobicity is not too strong, and the surface tension of the water-absorbing agent composition can be well maintained. R １ , R ２ , R ３ , R ４ may be different or the same.

[0281] In the above formula, N － is a counter anion of the ammonium cation, such as a halide ion, a carboxylate ion (e.g., acetate ion), a sulfonate ion, a hydroxyl ion, BF ４－ , P.F. ６－ , ClO ４－ , AsF ６－ , SbF ６－ Examples include:

[0282] Ammonium salts are readily available on the market, and preferred examples include the following products:

[0283] Manufactured by Kao Corporation: Coconutamine acetate Acetamine (registered trademark, the same applies hereinafter) 24, Stearylamine acetate Acetamine 86, Lauryltrimethylammonium chloride Courtamine (registered trademark, the same applies hereinafter) 24P, Stearyltrimethylammonium chloride Courtamine 86W, Cetyltrimethylammonium chloride Courtamine 60W, Distearyldimethylammonium chloride Courtamine D86P, Alkylbenzyldimethylammonium chloride Sanizol (registered trademark, the same applies hereinafter) C, Sanizol B-50 Manufactured by NOF Corporation: Tetradecylamine acetate Nissan Cation (registered trademark, the same applies hereinafter) MA, Dodecyltrimethylammonium chloride Nissan Cation BB, Coconut alkyltrimethylammonium chloride Nissan Cation FB, Hexadecyltrimethylammonium chloride Nissan Cation PB-300, Beef tallow alkyltrimethylammonium chloride Nissan Cation ABT2-500 - Octadecyltrimethylammonium chloride Nissan Cation AB, Nissan Cation AB-600 - Behenyltrimethylammonium chloride Nissan Cation VB-M Flake, Nissan Cation VB-F - Didecyldimethylammonium chloride Nissan Cation 2-DB-500E - Dioleyldimethylammonium chloride Nissan Cation 2-OLR - Coconut alkyldimethylbenzylammonium chloride Nissan Cation F2-50R - Tetradecyldimethylbenzylammonium chloride Nissan Cation M2-100R.

[0284] The HLB of the nonionic substance is preferably not less than 11, more preferably not less than 12. When the HLB of the nonionic substance is within the above range, it can be added as an aqueous solution, and can be added uniformly to a water absorbent resin having a high specific surface area.

[0285] Here, HLB is a value calculated by the Griffin method. The HLB of a nonionic substance with an unknown HLB can be determined by the following method: Emulsify a certain type of oil with the nonionic substance whose HLB you want to determine (add a surfactant with a known HLB if necessary), then emulsify the same oil with a surfactant with a known HLB (use one with an HLB of each value), and the HLB of the nonionic substance when the emulsified states are identical is taken as the HLB of the nonionic substance.

[0286] In the production method according to the present invention, the fluidity improver added to the water-absorbent resin is preferably selected from nonionic substances, zwitterionic substances, anionic substances, and cationic substances, and more preferably selected from nonionic substances. That is, according to a preferred embodiment of the present invention, the water-soluble fluidity improver includes at least one selected from nonionic substances. Also, it is more preferred that the water-soluble fluidity improver includes at least one selected from nonionic substances having a polyalkylene glycol chain in the molecule. Furthermore, according to another preferred embodiment, the fluidity improver includes at least one selected from (a) polyols and (b) modified products of the hydroxyl groups of polyols.

[0287] According to another preferred embodiment, the flow improver comprises at least one selected from a nonionic substance, an amphoteric substance, an anionic substance, and a cationic substance, the nonionic substance is (a) a polyol or (b) a modified product of the hydroxy group of a polyol, the amphoteric substance is (f) an alkylamine oxide, the anionic substance is (m) a carboxylate salt of a higher alcohol alkylene oxide adduct, and the cationic substance is (n) an ammonium salt.

[0288] The amount of the fluidity improver added is more than 0 ppm and less than 200 ppm relative to the mass of the water-absorbent resin. By setting the amount of the fluidity improver within the above range, it is possible to suppress a decrease in the surface tension of the water-absorbent agent composition, and further to suppress a decrease in the fixed height absorption (FHA) at a height of 20 cm of the water-absorbent agent composition. By suppressing the decrease in FHA in this way, the water-absorbent capacity when pressure is applied to the absorbent body is well maintained. The preferred range of FHA, etc. will be described later.

[0289] Furthermore, some fluidity improvers improve the fluidity of the water absorbent resin when transported by a feeder, etc., i.e., improve the fluidity of the water absorbent resin in a dynamic environment, while inhibiting the water absorbent resin from starting to move from a stationary state. Even in the case where such a fluidity improver is used, by setting the amount of the fluidity improver to be added within the above range, it is possible to suitably improve the fluidity in a dynamic environment while suppressing the effect of inhibiting the water absorbent resin from starting to move from a stationary state, i.e., suppressing a decrease in the flow rate of the water absorbent agent composition.

[0290] The lower limit of the amount of the fluidity improver added is preferably 1 ppm or more, more preferably 2 ppm or more, and even more preferably 3 ppm or more, relative to the mass of the water-absorbent resin. The upper limit of the amount of the fluidity improver added is preferably 180 ppm or less, more preferably 160 ppm or less, even more preferably 150 ppm or less, and particularly preferably 80 ppm or less, relative to the mass of the water-absorbent resin. The preferred range of the amount of the fluidity improver added can be a range defined by any combination selected from the upper and lower limits. Therefore, in the production method according to the present invention, the amount of the fluidity improver added may be, for example, 1 ppm or more and 180 ppm or less, 2 ppm or more and 160 ppm or less, 3 ppm or more and 160 ppm or less, 3 ppm or more and 150 ppm or less, or 3 ppm or more and 80 ppm or less.

[0291] In the production method according to the present invention, additives may be added in the production process of a water-absorbent resin from the step of preparing an aqueous monomer solution to the step prior to the surface-crosslinking step. The additives may be a component used as a fluidity improver (the same component as the fluidity improver, and also referred to as a "water-soluble component" in this specification). In such a case, the water-soluble component added prior to the surface-crosslinking step is not included in the amount of the fluidity improver added. In other words, the amount of the fluidity improver added refers to the total amount of the fluidity improver added during the surface-crosslinking step or during steps subsequent to the surface-crosslinking step. This is because the fluidity improver added during the surface-crosslinking step or during steps subsequent to the surface-crosslinking step contributes differently to the fluidity of the water-absorbent agent composition (behavior on the surface of the water-absorbent resin) than the water-soluble component added prior to the surface-crosslinking step.

[0292] As the flowability improver, it is sufficient to add a flowability improver having the above-mentioned mass average molecular weight in the above-mentioned amount, and multiple types of flowability improvers may be used within that range. Furthermore, when using one or multiple types of flowability improvers, each flowability improver may be used in combination with one or multiple types of flowability improvers having multiple mass average molecular weights. Note that when multiple types of flowability improvers are used, the amount added refers to the total amount.

[0293] [2-9-3] Addition Step and Mixing Step In the method for producing a water-absorbing agent composition according to the present invention, the fluidity improver having the above-mentioned mass-average molecular weight may be added to the water-absorbent resin in the above-mentioned amount during the surface-crosslinking step or during a step subsequent to the surface-crosslinking step. The fluidity improver may be added only during the surface-crosslinking step, only during a step subsequent to the surface-crosslinking step, or both during the surface-crosslinking step and a step subsequent to the surface-crosslinking step. In one embodiment, the addition step of the fluidity improver is preferably performed at least during a step subsequent to the surface-crosslinking step (the fluidity improver is added after the surface-crosslinking step), and more preferably during a cooling step (the fluidity improver is added during a cooling step following the surface-crosslinking step). Adding the fluidity improver during a step subsequent to the surface-crosslinking step prevents the fluidity improver from being denatured by being heated at a high temperature.

[0294] The fluidity improver is added in the form of an aqueous solution when added to and mixed with the water-absorbent resin. Therefore, in one embodiment, it is preferable to prepare an aqueous solution of the fluidity improver in advance and then add the aqueous solution to the water-absorbent resin. In this case, the concentration of the aqueous solution is 0.01% by mass or more and 20% by mass or less, preferably 0.02% by mass or more and 15% by mass or less, more preferably 0.05% by mass or more and 10% by mass or less, and particularly preferably 0.1% by mass or more and 3% by mass or less. By setting the aqueous solution concentration within the above range, the handleability of the aqueous solution containing the fluidity improver is improved, and the fluidity improver can be uniformly mixed with the water-absorbent resin.

[0295] The lower limit of the pH of the aqueous solution of the fluidity improver is preferably 4.5 or higher, more preferably 4.6 or higher, even more preferably 5.0 or higher, and particularly preferably 5.5 or higher. When the fluidity improver contained in the aqueous solution of the fluidity improver is an ester compound containing a polyalkylene glycol chain, hydrolysis may progress due to long-term storage, exposure to high temperatures, etc. As the hydrolysis progresses, carboxylic acids are by-produced, causing a decrease in the pH of the aqueous solution of the fluidity improver. Furthermore, the hydrolysis-prone fluidity improver becomes more hydrophobic, causing problems such as insoluble residues or precipitation, which can clog spray nozzles. Controlling the lower limit of the pH of the aqueous solution of the fluidity improver within the above range is preferable because it can prevent the occurrence of the above problems. Furthermore, the upper limit of the pH of the aqueous solution of the fluidity improver is preferably 11.0 or lower, more preferably 10.5 or lower, even more preferably 10.0 or lower, and particularly preferably 9.5 or lower, from the viewpoints of suppressing deterioration of equipment that comes into contact with the aqueous solution and ensuring worker safety. The preferred pH range can be any range defined by any combination of the upper and lower limits. Thus, the pH of the aqueous solution of the flowability improver may be, for example, 4.5 to 11.0, 4.6 to 10.5, 5.0 to 10.0, or 5.5 to 9.5.

[0296] The lower limit of the amount of the aqueous solution of the fluidity improver added is preferably 0.001 parts by mass or more, more preferably 0.002 parts by mass or more, and even more preferably 0.003 parts by mass or more, relative to 100 parts by mass of the water-absorbent resin. Meanwhile, the upper limit is preferably 10 parts by mass or less, more preferably 9 parts by mass or less, and even more preferably 8 parts by mass or less. The preferred range of the amount of the fluidity improver added can be a range defined by any combination selected from the upper and lower limits. Setting the amount of the fluidity improver to 10 parts by mass or less relative to 100 parts by mass of the water-absorbent resin eliminates the need for significant drying energy for adjusting the moisture content of the water-absorbent agent composition after addition, which is advantageous in terms of production costs. Furthermore, since strong aggregation of the water-absorbent resin caused by a large amount of the fluidity improver added can be suppressed, an additional step of pulverizing the aggregates is not required. Therefore, destruction of the surface-crosslinked layer (destruction of the already formed surface-crosslinked layer) due to the addition of a pulverization step can be suppressed, and as a result, a decrease in the absorbency under pressure can be suppressed.

[0297] The aqueous solution of the fluidity improver is added to the water-absorbent resin in the form of droplets and mixed. In this case, if the average droplet diameter of the droplets exceeds 1 mm, the number of droplets per added amount of the aqueous solution of the fluidity improver decreases, resulting in a lower encounter probability with the water-absorbent resin and a lack of uniform mixing, making it impossible to uniformly improve the fluidity of the particles. As a result, the effect of reducing the dynamic friction coefficient of particles having a particle diameter of 300 μm or more but less than 600 μm is reduced. Furthermore, when an aqueous solution of the fluidity improver having droplets with an average droplet diameter exceeding 1 mm is stirred and mixed with a water-absorbent resin having a particularly large specific surface area, coarse aggregated particles are likely to occur. Therefore, the upper limit of the average droplet diameter (diameter) is 1 mm or less, preferably 0.9 mm (900 μm) or less, more preferably 0.8 mm (800 μm) or less, and even more preferably 0.5 mm (500 μm) or less. On the other hand, the smaller the droplet diameter of the aqueous solution of the fluidity improver to be added, the easier it is to uniformly mix the aqueous solution of the fluidity improver and the water-absorbent resin. However, the cost of making the droplets finer becomes too high compared to the effect obtained. Therefore, the lower limit of the average droplet diameter (diameter) is 10 μm or more, preferably 20 μm or more, more preferably 30 μm or more, and even more preferably 40 μm or more. The preferred range of the average droplet diameter can be a range defined by any combination selected from the upper and lower limits. Therefore, the average droplet diameter may be, for example, 10 μm or more and 0.9 mm or less, 20 μm or more and 0.8 mm or less, 30 μm or more and 0.5 mm or less, or 40 μm or more and 0.5 mm or less. Examples of means for adding the aqueous solution of the fluidity improver in the form of droplets include using a spray means such as a spray nozzle having a desired nozzle diameter or a straight pipe having a desired inner diameter. The average droplet diameter can be measured by taking pictures of the state during addition using a high-speed camera or the like, and is a value determined by the inner diameter of the droplet addition port in the spraying means and the spray pressure.

[0298] The temperature of the aqueous solution of the fluidity improver when adding the aqueous solution of the fluidity improver to the water-absorbent resin, specifically the temperature of the droplets, has a lower limit of preferably 20°C or higher, and an upper limit of preferably 80°C or lower, more preferably 70°C or lower, and even more preferably 60°C or lower. The preferred range of the temperature of the aqueous solution of the fluidity improver (the temperature of the droplets) can be a range defined by any combination selected from the upper and lower limits. Therefore, the temperature of the aqueous solution of the fluidity improver is preferably 20°C or higher and 80°C or lower, more preferably 20°C or higher and 70°C or lower, and even more preferably 20°C or higher and 60°C or lower. An aqueous solution of the fluidity improver adjusted to the above temperature is particularly effective in improving mixability with a water-absorbent resin having a high specific surface area. If the temperature of the aqueous solution of the fluidity improver is too high, the water in the aqueous solution of the fluidity improver evaporates, making it easier for the fluidity improver to precipitate, making uniform mixing difficult. On the other hand, if the temperature is too low, the solubility of the fluidity improver in water decreases, and the concentration of the aqueous solution of the fluidity improver that can be prepared becomes low. Therefore, when a desired flowability improver is added, the amount of water added increases, which may result in the formation of a large amount of coarse aggregated particles (product lumps) in the product, which is undesirable. Note that the temperature range of the aqueous solution is the temperature measured before being affected by the temperatures of the water-absorbent resin and the apparatus. The apparatus referred to here refers to the apparatus in which the water-absorbent resin is retained.

[0299] Although the temperature and average droplet diameter of the aqueous solution of the fluidity improver may be controlled independently, the present inventors have investigated their relationship and found that combining these factors within the preferred ranges is particularly effective in improving the mixability with water-absorbent resins having a high specific surface area. Specifically, it has been found that controlling both the temperature and average droplet diameter of the aqueous solution within the preferred ranges significantly suppresses segregation when a water-absorbent agent composition containing a water-absorbent resin and a fluidity improver is transported through a feeder. Specifically, the higher the temperature of the aqueous solution of the fluidity improver added to a water-absorbent resin having a large specific surface area, the faster the water absorption rate. Therefore, reducing the average droplet diameter is an effective means of achieving more uniform mixing. Furthermore, excessively high temperature or excessively small droplet diameter can lead to evaporation and precipitation of the aqueous solution of the fluidity improver, so it is preferable to control each of these factors within the above ranges.

[0300] The device (mixer) used to mix the aqueous solution of the flow improver preferably has a large mixing power. Specific examples of this mixer include cylindrical mixers, double-walled conical mixers, V-shaped mixers, ribbon mixers, screw mixers, rotary disk mixers, double-arm kneaders, internal mixers, grinding kneaders, rotary mixers, screw extruders, fluidized bed mixers, and airflow mixers. Devices capable of mixing by stirring are more preferred, including high-speed stirring mixers and vertical rotating disk mixers, more preferably high-speed stirring continuous mixers, and even more preferably horizontal high-speed stirring continuous mixers or vertical high-speed stirring continuous mixers. Specific examples include the Shugi Mixer (manufactured by Hosokawa Micron Corporation), Turbulizer (manufactured by Hosokawa Micron Corporation), Loedige Mixer (manufactured by Loedige Corporation), and Flow Jet Mixer (manufactured by Powder and Powtex Corporation).

[0301] Furthermore, the water absorption rate of a water-absorbent resin with a high specific surface area is faster than that of a conventional product, making it difficult to uniformly mix a trace amount of a flowability improver (less than 200 ppm) between particles. Therefore, the present inventors conducted multiple experiments by arbitrarily changing the peripheral speed, mixing time, and average droplet diameter. As a result, they found that, instead of independently controlling the peripheral speed, mixing time, and average droplet diameter, by controlling the mixing force index defined by the following (Equation 1), even when a trace amount of a flowability improver (less than 200 ppm) is added, it is possible to uniformly mix the particles of a water-absorbent resin with a high specific surface area. That is, when an aqueous solution of the flowability improver is added and mixed with the water-absorbent resin, by setting the mixing force index (defined appropriately by the following (Equation 1)) to a specific value or more, the flowability of the water-absorbent agent composition can be uniformly improved and particle size segregation after transportation can be suppressed.

[0302] [Mixing Force Index] In the production method according to the present invention, the mixing force index when an aqueous solution of a fluidity improver is added to and mixed with a water absorbent resin is calculated by the following formula (1). The mixing force index is a value calculated each time the fluidity improver is added and mixed. For example, when the fluidity improver is added and mixed both during the surface cross-linking step and during a step subsequent to the surface cross-linking step, the mixing force index refers to the mixing force index in each step. In the production method according to the present invention, either the mixing force index during the surface cross-linking step or the mixing force index during a step subsequent to the surface cross-linking step is 70,000 or more, and it is preferable that at least the mixing force index during a step subsequent to the surface cross-linking step is 70,000 or more.

[0303]

[0304] In the above formula (1), the peripheral speed (unit: m / s) refers to the peripheral speed of the stirring blade, and specifically, peripheral speed (m / s) = π × stirring blade diameter (m) × rotation speed (rpm) / 60.

[0305] The lower limit of the mixing force index is 70,000 or more, preferably 80,000 or more, more preferably 90,000 or more, even more preferably 100,000 or more, particularly preferably 500,000 or more, and most preferably 1,000,000 or more, from the viewpoint of uniformly mixing the flowability improver between particles of the water-absorbent resin with a high specific surface area. On the other hand, water-absorbent resins with a high specific surface area have more surface irregularities than conventional products, which makes them less resistant to damage and more likely to generate fine particles (fine particles) with chipped surfaces when mixed in an apparatus. Therefore, from the viewpoint of suppressing the generation of such fine particles, the upper limit of the mixing force index is preferably 6,000,000 or less, more preferably 5,000,000 or less, even more preferably 4,000,000 or less, and particularly preferably 3,000,000 or less. The preferred range of the mixing force index can be a range defined by any combination selected from the upper and lower limits. Therefore, the mixing power index may be, for example, 70,000 or more and 6,000,000 or less, 80,000 or more and 5,000,000 or less, 90,000 or more and 4,000,000 or less, 100,000 or more and 3,000,000 or less, 500,000 or more and 3,000,000 or less, or 1,000,000 or more and 3,000,000 or less.

[0306] The lower limit of the impeller diameter of the mixing device is preferably 0.01 m or more, more preferably 0.02 m or more, and even more preferably 0.03 m or more, from the viewpoint of efficient mixing and stirring. The upper limit of the impeller diameter of the mixing device is preferably 10 m or less, more preferably 5 m or less, and even more preferably 3 m or less, from the viewpoint of the size and cost of the mixing device. The preferred range of the impeller diameter of the mixing device can be a range defined by any combination selected from the upper and lower limits. Therefore, the impeller diameter of the mixing device is preferably 0.01 m or more and 10 m or less, more preferably 0.02 m or more and 5 m or less, and even more preferably 0.03 m or more and 3 m or less.

[0307] The lower limit of the rotation speed of the stirring mixer is preferably 5 rpm or more, more preferably 10 rpm or more, even more preferably 15 rpm or more, particularly preferably 100 rpm or more, and most preferably 200 rpm or more. The upper limit of the rotation speed is preferably 10,000 rpm or less, more preferably 2,000 rpm or less, even more preferably 1,000 rpm or less, and particularly preferably 500 rpm or less. The preferred range of the rotation speed can be a range defined by any combination selected from the upper and lower limits. Therefore, the rotation speed may be, for example, 5 rpm or more and 10,000 rpm or less, 10 rpm or more and 2,000 rpm or less, 15 rpm or more and 1,000 rpm or less, 100 rpm or more and 500 rpm or less, or 200 rpm or more and 500 rpm or less.

[0308] The peripheral speed (the peripheral speed of the impeller of the mixing device) may be a constant value or may change during mixing. The term "variable peripheral speed" refers to a change in peripheral speed within a single mixing device or the use of two or more mixing devices with different peripheral speeds. The lower limit of the peripheral speed of the impeller of the mixing device is preferably 0.002 m / s or more, more preferably 0.01 m / s or more, even more preferably 0.02 m / s or more, particularly preferably 0.5 m / s or more, and most preferably 1.0 m / s or more, based on the preferred ranges of the impeller diameter and rotation speed. On the other hand, the upper limit is preferably 6,000 m / s or less, more preferably 600 m / s or less, even more preferably 200 m / s or less, particularly preferably 100 m / s or less, and most preferably 50 m / s or less. The preferred range of the peripheral speed of the impeller of the mixing device can be determined by any combination selected from the upper and lower limits. Therefore, the peripheral speed of the stirring blade may be, for example, 0.002 m / s or more and 6,000 m / s or less, 0.01 m / s or more and 600 m / s or less, 0.02 m / s or more and 200 m / s or less, 0.5 m / s or more and 100 m / s or less, or 1.0 m / s or more and 50 m / s or less.

[0309] A flowability improver having a mass-average molecular weight within the above range is less likely to be incorporated into the water-absorbent resin particles than a low-molecular-weight water-soluble substance. In other words, by adding the flowability improver to the water-absorbent resin and then thoroughly mixing, the flowability improver adhering to the particle surface of one water-absorbent resin can be brought into contact with the particle surface of another water-absorbent resin. As a result, the uniformity of the flowability improver being mixed throughout the water-absorbent resin is improved. Therefore, the "mixing time" in this specification refers to the total time during which the water-absorbent resin is stirred and mixed within one hour, starting from the time when the flowability improver and the water-absorbent resin come into contact. The lower limit of the mixing time is preferably 10 seconds or more, more preferably 15 seconds or more, and even more preferably 20 seconds or more. The upper limit of the mixing time is preferably 60 minutes or less, more preferably 30 minutes or less, and even more preferably 20 minutes or less. The preferred range of the mixing time can be defined by any combination selected from the upper and lower limits. Therefore, the mixing time may be, for example, 10 seconds or more and 60 minutes or less, 15 seconds or more and 30 minutes or less, or 20 seconds or more and 20 minutes or less.

[0310] For example, when an aqueous solution of a fluidity improver is added to have an average droplet diameter E1 (mm), and is stirred and mixed at a peripheral speed C1 (m / s) for a mixing time D1 (s), and D1 is within one hour after the fluidity improver and the water absorbent resin come into contact with each other, the mixing force index is calculated by the formula, "mixing force index (unit: no) = C1 (m / s) × D1 (s) / {E1 (mm) / 1000}".

[0311] For example, when an aqueous solution of a fluidity improver is added at an average droplet diameter E2 (mm), stirred and mixed at a peripheral speed C2 (m / s) for a mixing time D2 (s), and D2 is the time from when the fluidity improver and the water absorbent resin come into contact with each other until 1 hour and 10 minutes have elapsed, the mixing force index is calculated by "mixing force index (unit: no) = C2 (m / s) × 3600 (s) / {E2 (mm) / 1000}", assuming that the time up to 1 hour (3600 seconds) is the mixing time.

[0312] For example, when an aqueous solution of a fluidity improver is added at an average droplet diameter of E3 (mm), stirred and mixed at a peripheral speed of C3 (m / s) and a mixing time of D3 (s), and then stirred and mixed at a peripheral speed of C4 (m / s) and a mixing time of D4 (s), and D3 and D4 are within one hour after the fluidity improver and the water absorbent resin come into contact with each other, the mixing force index is calculated by "mixing force index (unit: no) = {C3 (m / s) × D3 (s) + C4 (m / s) × D4 (s)} / {E3 (mm) / 1000}".

[0313] For example, an aqueous solution of a fluidity improver is added at an average droplet diameter of E4 (mm), stirred and mixed at a peripheral speed of C5 (m / s) and a mixing time of D5 (s), then stirred and mixed at a peripheral speed of C6 (m / s) and a mixing time of D6 (s), and further stirred and mixed at a peripheral speed of C7 (m / s) and a mixing time of D7 (s), and D5 and D6 are within one hour after the fluidity improver and the water absorbent resin have come into contact with each other, and D7 is one hour or more after the fluidity improver and the water absorbent resin have come into contact with each other, the mixing force index is calculated by "mixing force index (unit: no) = {C5 (m / s) × D5 (s) + C6 (m / s) × D6 (s)} / {(E4 (mm) / 1000}".

[0314] In addition, as long as it is within one hour after the fluidity improver and the water absorbent resin come into contact with each other, the stirring and mixing may be continuous or divided into multiple times, but it is preferable that the stirring is carried out at least immediately after the fluidity improver and the water absorbent resin come into contact with each other.

[0315] When the aqueous solution of the fluidity improver is added, the aqueous solution of the fluidity improver may further contain at least one selected from the group consisting of other additives such as chelating agents, plant components, antibacterial agents, and inorganic salts, which will be described later. In this case, the content of the additive is appropriately selected as needed, but it is desirable to set it to 0.001% by mass or more and 50% by mass or less of the aqueous solution of the fluidity improver. As the chelating agent, a chelating agent having high ion sequestering ability or chelating ability for Fe and Cu is preferred. Specifically, a chelating agent having a stability constant for Fe ions of 10 or more, preferably 20 or more, more preferably aminopolycarboxylic acids and salts thereof, and particularly preferably aminocarboxylic acids and salts thereof having three or more carboxyl groups are included. Specific examples of these polycarboxylic acids include diethylenetriaminepentaacetic acid, triethylenetetraaminehexaacetic acid, cyclohexane-1,2-diaminetetraacetic acid, N-hydroxyethylethylenediaminetriacetic acid, ethylene glycol diethyl ether diaminetetraacetic acid, ethylenediaminetetrapropionic acid, N-alkyl-N'-carboxymethylaspartic acid, N-algenyl-N'-carboxymethylaspartic acid, and alkali metal salts, alkaline earth metal salts, ammonium salts, or amine salts thereof. The salts may be fully neutralized, partially neutralized, or mixtures. Among these, diethylenetriaminepentaacetic acid, triethylenetetraaminehexaacetic acid, N-hydroxyethylethylenediaminetriacetic acid, and salts thereof are most preferred. The lower limit of the amount of the chelating agent used is preferably 0.00001 parts by mass or more, more preferably 0.0001 parts by mass or more, relative to 100 parts by mass of the water-absorbent resin. The upper limit is preferably 10 parts by mass or less, more preferably 1 part by mass or less. The preferred range of the amount of the chelating agent used can be a range defined by any combination selected from the upper and lower limit values.

[0316] The lower limit of the amount of the plant component used is preferably 0 parts by mass or more, more preferably 0.001 parts by mass or more, and even more preferably 0.002 parts by mass or more, relative to 100 parts by mass of the water-absorbent resin, in order to exhibit deodorizing properties. Meanwhile, the upper limit can be preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less. The preferred range of the amount of the plant component used can be defined by any combination selected from the upper and lower limits. The plant component is preferably at least one compound selected from polyphenols, flavones and their analogues, and caffeine, and more preferably at least one compound selected from tannin, tannic acid, Chinese gallnut, gallnut, and gallic acid. The antibacterial agent may be any known antibacterial agent having antibacterial properties, such as those described in JP-A-11-267500.

[0317] [2-9-4] Curing step The mixture of the water-absorbent resin and the aqueous solution of the fluidity improver obtained by the above-mentioned series of operations is preferably subjected to a curing treatment. That is, the production method according to the present invention preferably further comprises a step of curing the mixture of the water-absorbent resin and the fluidity improver after mixing the water-absorbent resin with the fluidity improver. The "curing" refers to an operation of eliminating the wettability of the surface of the water-absorbent resin and powdering it. The "curing treatment" is a treatment of curing an object by controlling the temperature of the object to a predetermined curing temperature and maintaining that temperature state for a predetermined curing time.

[0318] In one embodiment of the present invention, a heat medium such as hot air is preferably used for the curing treatment. Furthermore, when the curing treatment is performed in a step subsequent to the surface cross-linking step, the lower limit of the heating temperature (e.g., the heat medium temperature or the material temperature) is preferably 40°C or higher, more preferably 50°C or higher, and the upper limit is preferably 150°C or lower, more preferably 140°C or lower, and particularly preferably 100°C or lower. Furthermore, the curing time within this temperature range is preferably 1 minute to 2 hours, more preferably 5 minutes to 1.5 hours, and particularly preferably 10 minutes to 1 hour. By setting the curing temperature and curing time within the above ranges, the surface state of the resulting water-absorbing agent composition can be prevented from becoming wet and becoming sticky, making it easy to handle as a powder. Furthermore, since too high a curing temperature or too long a curing time is energy-inefficient, it is preferable that the curing temperature and curing time be within the above ranges. The preferred ranges of the heating temperature and curing time (heating time) can be defined by any combination selected from the above upper and lower limits.

[0319] The addition and mixing of the aqueous solution of the fluidity improver and the subsequent curing treatment may be performed in the same apparatus or different apparatuses. Furthermore, the timing of the above-mentioned series of treatments may be during the heat treatment step in the surface crosslinking step, during the cooling step, or after the cooling step. The apparatus used is exemplified by the above-mentioned apparatus (mixing apparatus), and a heat medium such as gas or conductive heat may be adjusted so that the temperature inside the apparatus is the above-mentioned temperature. When curing, the mixture may be stirred or left to stand, i.e., unstirred, as long as the temperature and water content can be controlled within a predetermined range. When the curing treatment is performed by standing, the lower limit of the thickness of the water-absorbent resin to be laminated is preferably 1 cm or more, more preferably 5 cm or more, and even more preferably 10 cm or more. On the other hand, the upper limit may be preferably 100 cm or less, more preferably 80 cm or less, and even more preferably 70 cm or less, and then the curing treatment may be performed by laminating the water-absorbent resin. The preferred range of the thickness can be a range defined by any combination selected from the above-mentioned upper and lower limit values. The cured water-absorbing resin can be pulverized or classified as needed to obtain a water-absorbing agent composition having a desired particle size.

[0320] [2-10] 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.

[0321] [2-11] Reduction Rate of Dynamic Friction Coefficient According to the production method of the present invention, the dynamic friction coefficient of a water-absorbent resin (i.e., a water-absorbent agent composition) after adding a fluidity improver can be significantly reduced compared to the dynamic friction coefficient of the water-absorbent resin before adding the fluidity improver. Specifically, a production method for a water-absorbent agent composition can be provided, in which the reduction rate of the dynamic friction coefficient calculated by the following (Equation 2) is 10% or more.

[0322]

[0323] In formula 2, A: the kinetic friction coefficient of particles of the water-absorbent resin before the addition of the flowability improver, the particle size of which is 300 μm or more and less than 600 μm; B: the kinetic friction coefficient of particles of the water-absorbent agent composition after the addition of the flowability improver, the particle size of which is 300 μm or more and less than 600 μm. The kinetic friction coefficients A and B are measured by the method described in the Examples.

[0324] In this specification, "particles having a particle diameter of 300 μm or more and less than 600 μm" refers to particles that, after classification by the classification method in the evaluation method for the dynamic friction coefficient in the Examples, pass through a sieve with a mesh size of 600 μm but remain on a sieve with a mesh size of 300 μm.

[0325] In the manufacturing method according to the present invention, it is preferable that the reduction rate of the dynamic friction coefficient is 10% or more.

[0326] The lower limit of the dynamic friction coefficient reduction rate is preferably 12% or more, more preferably 15% or more, and particularly preferably 20% or more. On the other hand, the higher the upper limit, the better, and is not particularly limited, but is, for example, 90% or less, preferably 70% or less, more preferably 50% or less, and even more preferably 35% or less. A preferred range of the dynamic friction coefficient reduction rate can be a range defined by any combination selected from the upper and lower limit values. That is, the dynamic friction coefficient reduction rate is, for example, 10% or more and 90% or less, preferably 12% or more and 90% or less, more preferably 15% or more and 90% or less, and particularly preferably 20% or more and 90% or less.

[0327] In a preferred embodiment, the water-absorbent resin (before the flowability improver is added) and the resulting water-absorbent agent composition each contain 50% by mass or more of particles having a particle diameter of 300 μm or more and less than 600 μm, and the reduction rate of the dynamic friction coefficient calculated by the above (Equation 2) may be 10% or more. In this case, the preferred range of the reduction rate of the dynamic friction coefficient is the same as above.

[0328] [3] Physical Properties of Water-Absorbent Agent Composition The water-absorbent agent composition obtained through the above-mentioned steps becomes a final product if it is in a state ready for shipment. By the above-mentioned production method according to the present invention, a water-absorbent agent composition satisfying the following (1) to (5) can be obtained. Therefore, another aspect of the present invention provides a water-absorbent agent composition containing a water-absorbent resin as a main component, containing a flowability improver, and satisfying all of the following (1) to (5): (1) The specific surface area of ​​the water-absorbent agent composition is 25 m ２ / kg or more; (2) The surface tension of the water-absorbing agent composition is 56 mN / m or more; (3) The flow rate of the water-absorbing agent composition is 10.0 g / s or more; (4) In the water-absorbing agent composition, the mass ratio of particles having a particle diameter of 300 μm or more and less than 600 μm is 50 mass % or more; (5) In the water-absorbing agent composition, the dynamic friction coefficient of particles having a particle diameter of 300 μm or more and less than 600 μm is 0.80 or less.

[0329] [3-1] Specific Surface Area The lower limit of the specific surface area of ​​the water-absorbing agent composition according to the present invention is set to 25 m ２ By setting the specific surface area to 26 m / kg or more, a more excellent water absorption rate can be obtained by the Vortex method. ２ / kg or more, more preferably 27m ２ / kg or more, and more preferably 28m ２ / kg or more, and even more preferably 29m ２ / kg or more, and even more preferably 30m ２ / kg or more, and particularly preferably 35m ２ / kg or more, and most preferably 36m ２ On the other hand, the upper limit of the specific surface area of ​​the water-absorbing agent composition is preferably 60 m ２ / kg or less, more preferably 55m ２ The preferred range of the specific surface area of ​​the water-absorbing agent composition can be determined by any combination selected from the upper and lower limit values. Therefore, the specific surface area of ​​the water-absorbing agent composition is, for example, 25 m ２ / kg or more 60m ２ / kg or less,２ / kg or more 60m ２ / kg or less, ２ / kg or more 60m ２ / kg or less, ２ / kg or more 60m ２ / kg or less, ２ / kg or more 60m ２ / kg or less, ２ / kg or more 55m ２ / kg or less, ２ / kg or more 55m ２ / kg or less, ２ / kg or more 55m ２ From the viewpoint of increasing the water absorption rate, it is desirable that the specific surface area of ​​the water-absorbing agent composition is as high as possible, but if the specific surface area is too large, excessive foam 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 under pressure). ２ If the amount is less than 1 / kg, it is difficult to obtain a water-absorbing agent composition having a desired water absorption rate (Vortex), which is not preferable.

[0330] [3-2] Surface Tension The lower limit of the surface tension of the water-absorbent agent composition according to the present invention is 56 mN / m or more, preferably 58 mN / m or more, more preferably 60 mN / m or more, and even more preferably 65 mN / m or more. The upper limit is not particularly limited, but is preferably 75 mN / m or less in terms of balance with other physical properties. If the surface tension is less than 56 mN / m, the amount of liquid return when pressure is applied to the absorbent increases, making the water-absorbent agent composition unsuitable for use in absorbent articles such as disposable diapers. The preferred range of the surface tension of the water-absorbent agent composition can be defined by any combination selected from the upper and lower limits. Therefore, the surface tension of the water-absorbent agent composition is, for example, 56 mN / m to 75 mN / m, preferably 58 mN / m to 75 mN / m, more preferably 60 mN / m to 75 mN / m, and even more preferably 65 mN / m to 75 mN / m. The surface tension can be controlled by adding an additive after surface crosslinking, etc. For detailed conditions for measuring the surface tension, see the Examples.

[0331] [3-3] Flow Rate The lower limit of the flow rate of the water-absorbing agent composition according to the present invention is 10.0 g / s or more. The lower limit of the flow rate is preferably 10.2 g / s or more, and more preferably 10.5 g / s or more. A flow rate of less than 10.0 g / s increases the time required to supply the water-absorbing agent composition to the hopper and to supply the water-absorbing agent composition from the hopper to the feeder, which is undesirable from the viewpoint of production efficiency. The upper limit of the flow rate is preferably 20.0 g / s or less, and more preferably 15.0 g / s or less. Performing a process to increase the flow rate beyond the above range may result in poor economic efficiency. The preferred range of the flow rate may be defined by any combination selected from the above upper and lower limit values. Therefore, the flow rate is, for example, 10.0 g / s or more and 20.0 g / s or less, preferably 10.2 g / s or more and 20.0 g / s or less, and more preferably 10.5 g / s or more and 15.0 g / s or less. For detailed measurement conditions of the flow rate, see the Examples.

[0332] [3-4] Mass Proportion of Particles Having a Particle Diameter of 300 μm or More and Less than 600 μm In the water-absorbing agent composition according to the present invention, the lower limit of the mass proportion of particles having a particle diameter of 300 μm or more and less than 600 μm is 50% by mass or more, preferably 53% by mass or more, more preferably 55% by mass or more, and even more preferably 57% by mass or more. When the mass proportion is 50% by mass or more, the transport stability of the water-absorbing agent composition through a feeder is further improved. In continuous commercial production, it may be very difficult to set the mass proportion of particles less than 300 μm and the mass proportion of particles greater than 600 μm to 0% by mass, respectively, from the viewpoint of production efficiency. Therefore, the upper limit of the mass proportion of particles having a particle diameter of 300 μm or more and less than 600 μm is preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 85% by mass or less, particularly preferably 80% by mass or less, and most preferably 75% by mass or less. The preferred range of the mass proportion of particles having a size of 300 μm or more and less than 600 μm can be a range defined by any combination selected from the upper and lower limits. Therefore, the mass proportion of particles having a size of 300 μm or more and less than 600 μm is, for example, 50% by mass or more and 95% by mass or less, preferably 53% by mass or more and 90% by mass or less, more preferably 55% by mass or more and 85% by mass or less, even more preferably 57% by mass or more and 80% by mass or less, and particularly preferably 57% by mass or more and 75% by mass or less. According to the production method of the present invention, an aqueous solution of a flowability improver can be uniformly added to a water-absorbent resin having a large specific surface area. That is, the flowability improver is not unevenly added to particles having a small particle size and a large specific surface area (particles having a particle size of less than 300 μm), and the dynamic friction coefficient of particles that account for the majority of the water-absorbent agent composition, i.e., particles having a particle size of 300 μm or more and less than 600 μm, can be significantly reduced. In other words, a significant reduction in the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm means that the fluidity of all particles is uniformly improved.

[0333] [3-5] Dynamic Friction Coefficient In the water-absorbent agent composition according to the present invention, the upper limit of the dynamic friction coefficient (sometimes referred to simply as "dynamic friction coefficient" in this specification) of particles having a particle diameter of 300 μm or more and less than 600 μm is 0.80 or less, preferably 0.79 or less, more preferably 0.78 or less, even more preferably 0.77 or less, particularly preferably 0.76 or less, and most preferably 0.73 or less. The dynamic friction coefficient exceeding 0.80 means that the flowability improver is unevenly added to particles having a particle diameter of 300 μm or more and less than 600 μm. In other words, when the dynamic friction coefficient exceeds 0.80, the difference in fluidity between particles having a particle diameter of 300 μm or more and less than 600 μm and particles having a particle diameter of 300 μm or more and less than 600 μm becomes large, and as a result, particle size segregation of the water-absorbent agent composition occurs during transportation by a feeder, which is undesirable. On the other hand, the lower limit is, for example, 0.10 or more, preferably 0.30 or more, more preferably 0.50 or more, and even more preferably 0.60 or more. The preferred range of the dynamic friction coefficient of the particles having a size of 300 μm or more and less than 600 μm can be a range defined by any combination selected from the upper and lower limit values. Therefore, the dynamic friction coefficient of the particles having a size of 300 μm or more and less than 600 μm is, for example, 0.10 or more and 0.80 or less, preferably 0.30 or more and 0.79 or less, more preferably 0.50 or more and 0.78 or less, even more preferably 0.60 or more and 0.77 or less, particularly preferably 0.60 or more and 0.76 or less, and most preferably 0.60 or more and 0.73 or less. For detailed measurement conditions of the dynamic friction coefficient of the particles having a size of 300 μm or more and less than 600 μm, please refer to the Examples.

[0334] [3-6] Preferred Embodiments The water-absorbing agent composition according to the present invention preferably has at least one of the preferred ranges for the specific surface area, surface tension, flow rate, mass proportion of particles having a particle diameter of 300 μm or more and less than 600 μm, and dynamic friction coefficient, on the premise that all of the above (1) to (5) are satisfied. More preferably, the water-absorbing agent composition satisfies at least the preferred ranges for the surface tension and flow rate, on the premise that all of the above (1) to (5) are satisfied. Even more preferably, the water-absorbing agent composition satisfies at least the preferred ranges for the surface tension, flow rate, and dynamic friction coefficient, on the premise that all of the above (1) to (5) are satisfied. Particularly preferably, the water-absorbing agent composition satisfies at least the preferred ranges for the specific surface area, surface tension, flow rate, and dynamic friction coefficient, on the premise that all of the above (1) to (5) are satisfied. Most preferably, the water-absorbing agent composition satisfies all of the specific surface area, surface tension, flow rate, mass ratio of the particles, and dynamic friction coefficient, on the premise that all of the above (1) to (5) are satisfied.

[0335] More specifically, on the premise that the water-absorbing agent composition according to the present invention satisfies all of the above (1) to (5), the surface tension is preferably from 56 to 75 mN / m, more preferably from 58 to 75 mN / m, even more preferably from 60 to 75 mN / m, and particularly preferably from 65 to 75 mN / m, and the flow rate is preferably from 10.0 to 20.0 g / s, more preferably from 10.2 to 20.0 g / s, and particularly preferably from 10.5 to 15.0 g / s. In addition to the above, the water-absorbing agent composition has a coefficient of dynamic friction of preferably 0.10 or more and 0.80 or less, more preferably 0.30 or more and 0.79 or less, even more preferably 0.50 or more and 0.78 or less, still more preferably 0.60 or more and 0.77 or less, particularly preferably 0.60 or more and 0.76 or less, and most preferably 0.60 or more and 0.73 or less. In addition to the above, the water-absorbing agent composition has a specific surface area of ​​preferably 25 m ２ / kg or more 60m２ / kg or less, more preferably 26m ２ / kg or more 60m ２ / kg or less, and even more preferably 27m ２ / kg or more 60m ２ / kg or less, and even more preferably 28m ２ / kg or more 60m ２ / kg or less, and even more preferably 29m ２ / kg or more 60m ２ / kg or less, and even more preferably 30m ２ / kg or more 55m ２ / kg or less, particularly preferably 35m ２ / kg or more 55m ２ / kg or less, most preferably 36m ２ / kg or more 55m ２ In addition to the above, in the water-absorbing agent composition, the mass ratio of particles having a particle diameter of 300 μm or more and less than 600 μm is preferably 50% by mass or more and 95% by mass or less, preferably 53% by mass or more and 90% by mass or less, more preferably 55% by mass or more and 85% by mass or less, still more preferably 57% by mass or more and 80% by mass or less, and particularly preferably 57% by mass or more and 75% by mass or less.

[0336] [3-7] Other Properties of the Water-Absorbent Agent Composition It is preferable that the water-absorbent agent composition according to the present invention further has at least one of the following properties (a) to (g) within a suitable range.

[0337] (a) D50 (mass average particle size), (b) mass proportion of particles with a particle size of less than 150 μm, (c) CRC (absorbency without load), (d) AAP (absorbency under load), (e) SFC (saline flow conductivity), (f) Vortex (water absorption rate), (g) FHA (fixed height absorption at a height of 20 cm). Any two or more of the preferred ranges of the physical properties (a) to (g) above may be combined. Most preferably, all of the preferred ranges of (a) to (g) are satisfied.

[0338] (a) D50 (Mass Average Particle Diameter) The lower limit of D50 (Mass Average Particle Diameter) of the water-absorbing agent composition according to the present invention is preferably 250 μm or more, more preferably 300 μm or more, and even more preferably 330 μm or more. Meanwhile, the upper limit is preferably less than 550 μm, more preferably less than 500 μm, and even more preferably less than 450 μm. A preferred range of D50 of the water-absorbing agent composition can be defined by any combination selected from the upper and lower limits. Therefore, the D50 (Mass Average Particle Diameter) is 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. By setting the D50 (Mass Average Particle Diameter) within the above range, it is possible to control the preferred absorption characteristics, AAP (Absorbency Against Load) and Vortex (Water Absorption Rate), in a more balanced manner. When the D50 (mass average particle diameter) is 250 μm or more, the AAP (absorbency against pressure), which is a preferred absorption characteristic, can be maintained at a good value. On the other hand, when the D50 (mass average particle diameter) is less than 550 μm, the Vortex (water absorption rate), which is a preferred absorption characteristic, can be maintained at a good value. In addition, the particle coarseness of the water-absorbing agent composition becomes less noticeable, and when used in absorbent articles such as disposable diapers and sanitary napkins, the feel on the skin and wearing comfort can be maintained good. For detailed measurement conditions of the D50 (mass average particle diameter), see the Examples.

[0339] (b) Proportion of Particles with a Particle Diameter of Less than 150 μm The upper limit of the proportion of particles with a particle diameter of less than 150 μm in the water-absorbent agent composition according to the present invention is preferably 3% by mass or less, more preferably 2% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0% by mass, relative to 100% by mass of the water-absorbent agent composition. In continuous commercial production, it may be very difficult to achieve a proportion of particles with a particle diameter of less than 150 μm of 0% by mass from the viewpoint of production efficiency. Therefore, the lower limit is preferably 0.1% by mass or more, 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 with a particle diameter of less than 150 μm can be a range defined by any combination selected from the upper and lower limits. Therefore, the proportion of particles with a particle diameter of less than 150 μm is preferably 0.1% by mass or more and 3% by mass or less, more preferably 0.2% by mass or more and 2% by mass or less, and even more preferably 0.3% by mass or more and 1% by mass or less. By setting the ratio of particles of less than 150 μm within the above range, it becomes easier to control the AAP (absorbency against pressure) and Vortex (water absorption rate) in a more balanced manner. When the ratio of particles of less than 150 μm is 3 mass% or less, not only is the AAP (absorbency against pressure), which is a preferable absorption characteristic, maintained at a good value, but also it is possible to suppress the deterioration of the working environment due to scattering of dust in the place where the water-absorbent agent composition is handled and the difficulty in handling due to accumulation of fine particles in the device.

[0340] Furthermore, it is preferable that the water-absorbing agent composition satisfies the above-mentioned preferred range of D50 (mass average particle diameter) and also satisfies the above-mentioned preferred range of the proportion of particles less than 150 μm. By satisfying both of these, the above-mentioned effects are obtained synergistically. Specifically, it is preferable that the D50 (mass average particle diameter) of the water-absorbing agent composition is 250 μm or more and less than 550 μm, and that the proportion of particles in the water-absorbing agent composition having a particle diameter of less than 150 μm is 3 mass% or less. Furthermore, the preferred ranges of the D50 (mass average particle diameter) and the proportion of particles in the water-absorbing agent composition having a particle diameter of less than 150 μm are as shown in (a) and (b) above, respectively. The D50 (mass average particle diameter) and the proportion of particles less than 150 μm of the water-absorbing agent composition are measured by the methods described in the Examples.

[0341] (c) CRC (absorbency without load) The lower limit of the CRC (absorbency without load) of the water-absorbing agent composition according to the present invention is preferably 25 g / g or more. Meanwhile, the upper limit is preferably 40 g / g or less, more preferably 38 g / g or less, even more preferably 35 g / g or less, particularly preferably 32 g / g or less, and most preferably 30 g / g or less. The preferred range of the CRC can be determined by any combination selected from the upper and lower limits. If the CRC (absorbency without load) is too low, the absorbency of the water-absorbing agent composition will decrease, potentially making it unsuitable 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.

[0342] (d) AAP (Absorbency Under Load) The lower limit of the AAP (Absorbency Under Load) of the water-absorbing agent composition according to the present invention is preferably 20 g / g or more, more preferably 23 g / g or more, even more preferably 25 g / g or more, and particularly preferably 25.2 g / g or more. On the other hand, the upper limit is preferably 30 g / g or less, more preferably 28 g / g or less. The preferred range of the AAP can be defined by any combination selected from the upper and lower limits. Therefore, the AAP is preferably 20 g / g or more and 30 g / g or less, more preferably 23 g / g or more and 30 g / g or less, even more preferably 25 g / g or more and 30 g / g or less, and particularly preferably 25.2 g / g or more and 28 g / g or less. By setting the AAP (absorbency against pressure) within the above range, the amount of liquid returning to the absorbent body when pressure is applied thereto can be further reduced, and therefore the water-absorbent resin or water-absorbent agent composition is suitable for use in absorbents in absorbent articles such as disposable diapers and sanitary napkins.

[0343] (e) SFC (Saline Flow Conductivity) The lower limit of the SFC (Saline Flow Conductivity) of the water-absorbing agent composition according to the present invention is preferably 1×10 －７ cm ３ sec / g or more, more preferably 2×10 －７ cm ３ sec / g or more, more preferably 3×10 －７ cm ３sec / g or more, and even more preferably 5×10 －７ cm ３ sec / g or more, even more preferably 10 x 10 －７ cm ３ sec / g or more, and even more preferably 15×10 －７ cm ３ sec / g or more, particularly preferably 20 × 10 －７ cm ３ sec / g or more, most preferably 25 x 10 －７ cm ３ The upper limit of SFC is not particularly limited, but a higher value is preferable. For example, it is 50×10 －７ cm ３ The preferred range of the SFC can be a range defined by any combination selected from the upper and lower limits. Therefore, the SFC is preferably 1×10 －７ cm ３ ・sec / g or more 50×10 －７ cm ３ sec / g or less, more preferably 2×10 －７ cm ３ ・sec / g or more 50×10 －７ cm ３ sec / g or less, more preferably 3×10 －７ cm ３ ・sec / g or more 50×10 －７ cm ３ sec / g or less, even more preferably 5×10 －７ cm ３ ・sec / g or more 50×10 －７ cm ３ sec / g or less, even more preferably 10 x 10 －７ cm ３ ・sec / g or more 50×10 －７ cm ３ sec / g or less, even more preferably 15×10 －７ cm ３ ・sec / g or more 50×10 －７ cm ３ sec / g or less, particularly preferably 20 × 10 －７ cm ３・sec / g or more 50×10 －７ cm ３ sec / g or less, most preferably 25 x 10 －７ cm ３ ・sec / g or more 50×10 －７ cm ３ sec / g or less. For detailed conditions for measuring the SFC, see the Examples.

[0344] (f) Vortex (Water Absorption Speed) The upper limit of the Vortex (water absorption speed) of the water-absorbent agent composition according to the present invention is preferably 50 seconds or less, more preferably 48 seconds or less, even more preferably 46 seconds or less, particularly preferably 44 seconds or less, and most preferably 42 seconds or less. On the other hand, the lower limit is preferably more than 10 seconds, more preferably 15 seconds or more. The preferred range of the Vortex can be defined by any combination selected from the upper and lower limits. Therefore, the Vortex (water absorption speed) is preferably more than 10 seconds and less than 50 seconds, more preferably more than 10 seconds and less than 48 seconds, even more preferably more than 10 seconds and less than 46 seconds, particularly preferably more than 10 seconds and less than 44 seconds, and most preferably 15 seconds or more and less than 42 seconds. When the Vortex (water absorption speed) is 50 seconds or less, the resulting water-absorbent agent composition has a high absorption speed for body fluids such as urine and blood, and is suitable as an absorbent for absorbent articles such as disposable diapers. The vortex (water absorption rate) can be controlled by foam polymerization, particle size distribution, etc. For detailed measurement conditions of the vortex (water absorption rate), see the examples.

[0345] (g) FHA (fixed height absorption at a height of 20 cm) The lower limit of the FHA (fixed height absorption at a height of 20 cm) of the water-absorbing agent composition according to the present invention is preferably 24.0 g / g or more, more preferably 24.5 g / g or more, even more preferably 25.0 g / g or more, and particularly preferably 25.5 g / g or more. The upper limit is not particularly limited, but is preferably 30.0 g / g or less in terms of balance with other physical properties. The preferred range of the FHA can be determined by any combination selected from the upper and lower limits. When the FHA is 24.0 g / g or more, the absorbency when pressure is applied to the absorbent is improved, making the absorbent suitable for absorbent articles such as disposable diapers. The FHA can be controlled by the internal crosslinking agent, particle size, surface crosslinking agent, and / or additives added after the surface crosslinking process. For detailed measurement conditions of the FHA (fixed height absorption at a height of 20 cm), see the Examples.

[0346] [3-8] Relationship between Water-Absorbent Resin and Water-Absorbent Agent Composition The lower limit of the amount (mass percentage) of the water-absorbent resin contained in the water-absorbent agent composition according to the present invention is preferably 80% or more, more preferably 85% or more, even more preferably 90% or more, particularly preferably 95% or more, more preferably 98% or more, and even more preferably 99% or more, based on the total amount of the water-absorbent agent composition. Note that, when the various additives described above (flowability improver, other additives, and water added together with these additives) are contained, the amount of the water-absorbent resin contained in the water-absorbent agent composition does not reach 100% by mass. That is, the upper limit of the amount (mass percentage) of the water-absorbent resin contained in the water-absorbent agent composition according to the present invention may be less than 100% by mass.

[0347] 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.

[0348] [4] Uses of the Water-Absorbent Agent Composition The water-absorbent agent composition according to the present invention is preferably used mainly as an absorbent body or an absorbent layer (hereinafter collectively referred to as "absorbent body") of absorbent articles such as disposable diapers and sanitary napkins, and more preferably as an absorbent body of absorbent articles in which a large amount is used per absorbent article. That is, another aspect of the present invention provides an absorbent body containing the water-absorbent agent composition.

[0349] The absorbent body refers to a water-absorbent agent composition formed into a sheet, fiber, cylindrical, or other shape, and is preferably formed into a sheet to form an absorbent layer. In addition to the water-absorbent agent composition of the present invention, absorbent materials such as pulp fibers (pulp), adhesives, nonwoven fabrics, etc. can also be used in combination for molding. Therefore, in one embodiment, the absorbent body of the present invention further contains pulp fibers (pulp) in addition to the absorbent. In this case, the lower limit of the amount of the water-absorbent 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, and particularly preferably 80% by mass or more. Meanwhile, the upper limit is preferably 100% by mass or less. The preferred range of the core concentration can be defined by any combination selected from the upper and lower limits. By setting the core concentration within the above range, when the absorbent body is used in an absorbent article, even if the water-absorbent agent composition gels upon absorbing urine, appropriate spaces can be formed between gel particles.

[0350] In the absorbent body, the weight of the water-absorbing agent composition is 25 g / m ２ 450g / m or more ２ Preferably, 50 g / m or less ２ More than 400g / m ２ More preferably, 75 g / m or less ２ 350g / m or more ２ The following is more preferable: By setting the basis weight of the water-absorbing agent composition within the above range, when the absorbent body is used in an absorbent article, urine can be efficiently absorbed.

[0351] In the absorbent body, the basis weight of the absorbent material such as pulp (preferably pulp) is 0 g / m ２ More than 300g / m ２Preferably, 0 g / m ２ More than 250g / m ２ More preferably, 0 g / m ２ More than 200g / m ２ More preferably, 0 g / m ２ 150g / m or more ２ The following is particularly preferred. By setting the basis weight of the pulp within the above range, when the absorbent body is used in an absorbent article, the absorbent article can be made thinner. As described above, the absorbent body has a basis weight of 0 g / m2 of the absorbent material such as pulp (preferably pulp). ２ That is, in one embodiment, the absorbent body may not contain pulp.

[0352] A specific example of an absorbent body is one in which a long water-absorbent sheet having a water-absorbent agent composition (water-absorbent resin) fixed between two sheets is cut (usually cut into a rectangle having a width of about 10 cm and a length of about several tens of cm). Such absorbents are produced by cutting the long water-absorbent sheet in the disposable diaper manufacturing process, and in recent years have come to be used in the manufacture of disposable diapers (so-called SAP sheet diapers). By purchasing or manufacturing long water-absorbent sheets, disposable diaper manufacturers can simplify the disposable diaper manufacturing process and, by eliminating the use of pulp, can make disposable diapers thinner. The water-absorbent sheet has a configuration in which a water-absorbent agent composition (water-absorbent resin particles) is sandwiched and fixed between upper and lower sheets (particularly nonwoven fabric sheets). In the manufacture of disposable diapers, the long absorbent sheet is typically produced and then cut into a rectangular shape approximately 10 cm wide and several tens of cm long, which is then incorporated into the disposable diaper (see International Publication No. WO 2010 / 143635).

[0353] [5] Absorbent Articles The water-absorbent agent composition according to the present invention is suitably used in absorbent articles. That is, another aspect of the present invention is an absorbent article containing the water-absorbent agent composition. Another aspect of the present invention provides an absorbent article containing the absorbent body. Such absorbent articles usually include a liquid-permeable top sheet and a liquid-impermeable back sheet in addition to the absorbent body. Examples of absorbent articles include disposable diapers and sanitary napkins.

[0354] 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.

[0355] In one embodiment, the absorbent article preferably does not contain pulp. That is, the absorbent article is preferably a pulp-free diaper. The absorbent article may be a pulp-free diaper in which the basis weight of the water-absorbing agent composition in the absorber contained therein is within the above range.

[0356] In another embodiment, the absorbent article may further contain pulp in addition to the water-absorbent agent composition. That is, the absorbent article may be a diaper containing pulp. The absorbent article may be a diaper containing pulp, in which the basis weight of the pulp in the absorber contained therein is within the above range.

[0357] 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.

[0358] The measurement method for each physical property in the table is as follows. Each measurement was performed under conditions of room temperature (23±2°C) and relative humidity of 35±5% RH. In addition, in the following description of the measurement method for each physical property, when the measurement target is something other than a water-absorbent agent composition (for example, a particulate hydrogel, a water-absorbent resin before / after surface-crosslinking, etc.), the term "water-absorbent agent composition" in the following description is replaced with "particulate hydrogel," "water-absorbent resin before surface-crosslinking," or "water-absorbent resin after surface-crosslinking."

[0359] <Method of Measuring Physical Properties> [Dynamic Friction Coefficient] With respect to the water-absorbing agent composition according to the present invention, the dynamic friction coefficient of particles having a particle diameter of 300 μm or more and less than 600 μm was measured by the method described below.

[0360] (Classification of Water-Absorbent Agent Composition) Using two JIS standard sieves (THE IIDA TESTING SIEVE: diameter 16 cm) with mesh sizes of 600 μm and 300 μm, 100.0 g of the water-absorbent agent composition was classified. The classification was carried out by sieving for 5 minutes with a vibration classifier (IIDA SIEVE SHAKER, TYPE: ES-65 (rotation speed: 60 Hz 230 rpm, impact speed: 60 Hz 130 rpm), SER. No. 0501).

[0361] By the above classification operation, water-absorbing agent compositions having a particle size of 600 μm or more, water-absorbing agent compositions having a particle size of 300 μm or more but less than 600 μm, and water-absorbing agent compositions having a particle size of less than 300 μm were obtained.

[0362] (Measurement of shear stress by rheometer) The water-absorbing agent composition having a particle size of 300 μm or more and less than 600 μm obtained by the above classification operation was measured for shear stress by using a rheometer (MCR301, manufactured by Anton Paar) (see Fig. 1). Hereinafter, the measurement method will be described in detail with reference to Fig. 1.

[0363] 1. Dish 1 (inner diameter: 52 mm, depth: 30 mm, material: aluminum, note: the inner bottom surface was sandblasted) and parallel plate 2 (diameter: 50 mm, material: aluminum, note: the plate surface was processed into a grid pattern) were placed in the rheometer. Note that dish 1 and parallel plate 2 were thoroughly washed and dried before use. The rheometer and dish 1 were also placed so that they were strictly horizontal.

[0364] 2. The measurement temperature was set to 25° C., and the lift position (the distance between the dish 1 and the parallel plate 2) was set to 100 mm.

[0365] 3. The zero gap between the dish 1 and the parallel plate 2 was adjusted.

[0366] 4. The parallel plate 2 was lifted up to the lift position set in the operation of 2 above, and the distance between the dish 1 and the parallel plate 2 was widened to 100 mm. Then, 10.0 g of the water-absorbing agent composition having a particle diameter of 300 μm or more and less than 600 μm obtained by the classification operation was uniformly spread in the dish 1. Note that, after the zero gap adjustment operation of 3 above, the dish 1 and the parallel plate 2 were not removed from the rheometer. In other words, when the water-absorbing agent composition was spread in the dish 1, the operation was performed with the dish 1 and the parallel plate 2 still installed in the rheometer. Furthermore, when the dish 1 and the parallel plate 2 were temporarily removed from the rheometer for cleaning or the like, the zero gap adjustment was performed again when they were installed in the rheometer again, and the water-absorbing agent composition was spread without removing the dish 1 and the parallel plate 2 from the rheometer.

[0367] 5. The parallel plate 2 was lowered to the measurement start position (the position where the parallel plate 2 comes into contact with the water-absorbing agent composition spread in the dish 1).

[0368] 6. The shear stress of the water-absorbent agent composition was measured under the measurement conditions shown in Table 1 below. Measurements were made continuously under conditions (1) to (5) shown in Table 1 below, and the time required from the start of condition (1) to the end of condition (5) was 1,050 seconds. After the measurement, the parallel plates were lifted up to a lift position of 100 mm, and the powder surface of the water-absorbent agent composition (the surface that had been in contact with the parallel plates) was confirmed. If, for example, there was a partial depression on the powder surface and a part of the parallel plates was not in contact with the powder surface, the measurement was repeated. In other words, only when there was no depression on the powder surface of the water-absorbent agent composition after the measurement and the entire bottom surface of the parallel plates was in contact with the powder surface was considered to be a correct measurement.

[0369] 7. Among the shear stresses obtained in the measurements in 6 above, those measured under the conditions (3), (4) and (5) shown in Table 1 below, the average values ​​of the 41st to 60th measurement points were taken as the shear stresses (unit: Pa) of the water-absorbent agent composition under the conditions (3), (4) and (5), i.e., under normal loads of 1 N, 3 N and 5 N.

[0370] The shear stresses of the water-absorbent agent composition having a particle size of 300 μm or more and less than 600 μm obtained by the above classification operation in the present invention under normal loads of 1 N (condition (3)), 3 N (condition (4)), and 5 N (condition (5)) were measured twice using the same sample (water-absorbent agent composition) and the average values ​​were calculated. However, when the difference in the shear stress under 5 N (condition (5)) between the two measurements exceeded 150 Pa, an additional measurement was conducted, and the average value was calculated from the two measurements in which the difference was within 150 Pa.

[0371]

[0372] (Calculation of normal stress) The normal stress (unit: Pa) of the water-absorbent agent composition under the conditions (3), (4) and (5) shown in Table 1 above was calculated by the following formula (a).

[0373] Normal stress (Pa) = (normal load (N)) / (area of ​​parallel plates (m ２ )) ... (Equation a) The area of ​​the parallel plate in the above (Equation a) is 0.0252 × π (m ２ )

[0374] That is, the normal stresses of the water-absorbing agent compositions having particle sizes of 300 μm or more and less than 600 μm obtained by the above classification operation under normal loads of 1 N (condition (3)), 3 N (condition (4)), and 5 N (condition (5)) were 509 Pa, 1528 Pa, and 2546 Pa, respectively.

[0375] (Calculation of Dynamic Friction Coefficient) The dynamic friction coefficient of the water-absorbing agent composition having a particle size of 300 μm or more and less than 600 μm obtained by the above classification operation was calculated by the following method.

[0376] 1. The shear stress and normal stress of the water-absorbing agent composition under normal loads of 1N, 3N, and 5N were plotted with the shear stress on the vertical axis and the normal stress on the horizontal axis.

[0377] 2. The plot of the three points was linearly approximated by a straight line without an intercept passing through the origin, and the slope of the straight line was taken as the coefficient of kinetic friction of the water-absorbent agent composition.

[0378] In the above linear approximation, if the R-squared value of the approximation equation was less than 0.9000, the shear stress was measured again using the rheometer, and only the slope of the linear approximation for which the R-squared value was 0.9000 or more was adopted as the coefficient of dynamic friction.

[0379] [Dynamic Friction Coefficient Reduction Rate] With regard to the water-absorbing agent composition according to the present invention, the dynamic friction coefficient reduction rate (unit: %) of particles having a particle diameter of 300 μm or more and less than 600 μm was calculated by the following (Equation 2).

[0380]

[0381] In the formula 2, A is the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm of the water-absorbent resin before the flowability improver is added; B is the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm of the water-absorbent agent composition after the flowability improver is added.

[0382] The dynamic friction coefficients of A and B were measured by the above-mentioned method, and then the dynamic friction coefficient reduction rate was calculated using the above-mentioned (Equation 2).

[0383] [Feeding Test] A feeding test of the water-absorbing agent composition according to the present invention was carried out using an electromagnetic feeder drive unit 4 (series: MFS type small series, model: MUS-6, manufactured by Murakami Seiki Kosakusho Co., Ltd.) (see Fig. 2) and a controller (series: MC type, model: MC-2-2, manufactured by Murakami Seiki Kosakusho Co., Ltd.). Hereinafter, the test method will be described in detail with reference to Fig. 2.

[0384] 1. A trough 5 (total length: 20 cm, width: 7 cm, weir: 5 cm, material: SUS) was installed in an electromagnetic feeder drive unit 4.

[0385] 2. 50.0 g of the water-absorbing agent composition was placed on the trough 5 so as to be located at a position between 12 cm and 20 cm from the outlet of the trough 5 .

[0386] 3. The controller was set to 2 and the feed was started.

[0387] 4. The water-absorbing agent composition dropped from the outlet of the trough 5 was sampled between 70 seconds and 120 seconds counting from the start of feeding.

[0388] [Particle Size Change Rate] The particle size change rate (unit: %) of the water-absorbing agent composition according to the present invention was calculated by the following method.

[0389] (Classification of Water-Absorbent Agent Composition) Using two JIS standard sieves (THE IIDA TESTING SIEVE: diameter 8 cm) with mesh sizes of 600 μm and 300 μm, the water-absorbent agent composition before the above-mentioned feed test and 10.0 g of the water-absorbent agent composition sampled in the above-mentioned feed test were each classified. The classification was carried out by sieving for 5 minutes using a vibration classifier (IIDA SIEVE SHAKER, TYPE: ES-65 (rotation speed: 60 Hz 230 rpm, impact speed: 60 Hz 130 rpm), SER. No. 0501).

[0390] By the above classification operation, the water-absorbent agent composition before the above feed test and the water-absorbent agent composition sampled in the above feed test were classified into a water-absorbent agent composition having a particle size of 600 μm or more, a water-absorbent agent composition having a particle size of 300 μm or more but less than 600 μm, and a water-absorbent agent composition having a particle size of less than 300 μm, respectively.

[0391] (Calculation of Particle Size Change Rate) For each of the "water-absorbing agent composition having a particle size of 600 μm or more," "water-absorbing agent composition having a particle size of 300 μm or more and less than 600 μm," and "water-absorbing agent composition having a particle size of less than 300 μm," the particle size change rate was calculated based on the following (Equation b) to (Equation d).

[0392] (particle size change rate (mass %) of water-absorbent agent composition having a particle diameter of 600 μm or more)=α1−β1 (formula b) where, α1: mass proportion (unit: mass %) of water-absorbent agent composition having a particle diameter of 600 μm or more in the water-absorbent agent composition before the feeding test is carried out, and β1: mass proportion (unit: mass %) of water-absorbent agent composition having a particle diameter of 600 μm or more in the water-absorbent agent composition sampled in the feeding test.

[0393] (particle size change rate (mass %) of water-absorbing agent composition having a particle size of 300 μm or more and less than 600 μm)=α2−β2 (formula c) where, α2: mass proportion (unit: mass %) of water-absorbing agent composition having a particle size of 300 μm or more and less than 600 μm in the water-absorbing agent composition before the feeding test is carried out, and β2: mass proportion (unit: mass %) of water-absorbing agent composition having a particle size of 300 μm or more and less than 600 μm in the water-absorbing agent composition sampled in the feeding test.

[0394] (particle size change rate (mass %) of water-absorbent agent composition having a particle size of less than 300 μm)=α3−β3 (formula d) where, α3: mass proportion (unit: mass %) of water-absorbent agent composition having a particle size of less than 300 μm in the water-absorbent agent composition before the feeding test is carried out, and β3: mass proportion (unit: mass %) of water-absorbent agent composition having a particle size of less than 300 μm in the water-absorbent agent composition sampled in the feeding test.

[0395] [Specific Surface Area] The specific surface area of ​​the water-absorbent agent composition according to the present invention is a value found by analyzing three-dimensional image data of the water-absorbent agent composition, obtained using a microfocus X-ray CT system (inspexio SMX-100CT manufactured by Shimadzu Corporation), with high-speed three-dimensional analysis software (TRI / 3D-VOL-FCS64 manufactured by Ratoc System Engineering Co., Ltd.). Specifically, first, 1.0 g of the water-absorbent agent composition was placed in a plastic cylindrical container with a lid, having an inner diameter of about 1 cm and a height of about 5 cm, and the mixture was shaken well to ensure uniform particle size. Subsequently, a double-sided tape was attached to the bottom of the cylindrical container, which was then fixed on the sample stage of the microfocus X-ray CT system, and three-dimensional image data was obtained under the conditions shown in Table 2 below.

[0396]

[0397] Subsequently, analysis was carried out using the above-mentioned high-speed three-dimensional analysis software according to the following procedure.

[0398] 1. From the menu, select Particle Measurement > 3D Particles > Particle Separation > Giant Particle Separation.

[0399] 2. In the Binary tab on the EVC panel, select L-W, leave the W value at its default value, and increase the L value by 1 to extract a circular measurement region. This process was then applied to all slice images. The image data extracted by this operation is designated as (A).

[0400] 3. In the Binaryize tab on the EVC panel, select L-W, leave the W value at its default value, and change the L value from its default value to 37580 to extract all particles in the measurement target area. This process was then applied to all slice images. The particle image data extracted by this operation is shown as (B).

[0401] 4. Based on the particle image data (B), first, Ers Sml was selected in the Binary tab on the EVC panel to remove particles considered to be noise with a particle size of 10 voxels or less. Next, Invert was selected in the Binary tab on the EVC panel to invert the areas where particles were extracted and the areas where they were not. Next, Ers Sml was selected to remove particles considered to be noise with a particle size of 10 voxels or less. Next, Labeling was selected in the 3D tab on the EVC panel, and Volume and Max were selected to extract only the area with the largest volume. Finally, Invert was selected again in the Binary tab on the EVC panel to remove noise in the measurement target area and extract all particles with voids filled. The particle image data extracted by these operations is referred to as (C). Note that the term "void" here refers to a cavity present inside the water-absorbent resin that is not in contact with the outside world.

[0402] 5. In the L Op tab (inter-channel logical operation processing), particle image data (B) was subtracted from particle image data (C), and then Ers Sml was selected in the Binary tab on the EVC panel to remove particles with a particle size of 10 voxels or less that were considered to be noise, thereby extracting voids.

[0403] 6. Based on the particle image data (C), small particle extraction was selected on the large particle separation panel (large particle extraction was not selected), and the constriction ratio, Repair Filter Size, and Repair Mrg Sml Diameter were all set to 0, and the particles were separated and color-coded.

[0404] 7. In the 3D tab on the EVC panel, Labeling was selected, and further, coordinate values ​​(cycles) were selected and the microparticle size was set to 10, and particle separation operation was performed.

[0405] 8. From the menu, select Particle Measurement > 3D Void in Particle > Post-Separation Measurement. Next, on the post-separation measurement panel, select voxcel as the unit, remove edge particles, select surface area calculation and void calculation as the measurement items, and select the image data (A) extracted in operation 2 above as the measurement ROI, and then perform the calculation process.

[0406] By the above operation, the total surface area of ​​all particles in the measurement area (unit: mm ２ ) and apparent total volume (unit: mm ３ ), and total void volume (unit: mm ３ The apparent total volume refers to the total volume of all particles calculated assuming that there are no voids inside the particles. Furthermore, the value obtained by the image analysis was used, and the true density of the water-absorbing agent composition was calculated as 1.7 g / cm. ３ The specific surface area (unit: m) of the water-absorbing agent composition is calculated from the following (formula e). ２ / kg) was calculated.

[0407] Specific surface area=total surface area of ​​all particles / ((apparent total volume−void total volume)×1.7) (Equation e).

[0408] [ST (Surface Tension)] The ST (surface tension) of the water-absorbing agent composition according to the present invention was measured by the following method.

[0409] 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.

[0410] 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 aqueous sodium chloride 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.

[0411] [Flow Rate] The flow rate (unit: g / s) of the water-absorbent agent composition according to the present invention was measured in accordance with WSP250.3(10). Specifically, 100.0 g of the water-absorbent agent composition was placed in a funnel equipped with a damper at the bottom, the damper was opened, and the time from the start of the flow to the end of the flow was measured to calculate the amount of the water-absorbent agent composition flowing down per unit time, which was taken as the flow rate (unit: g / s).

[0412] [D50 (mass average particle diameter) and σζ (logarithmic standard deviation of particle size distribution)] D50 (mass average particle diameter) 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 paragraphs

[0245] to

[0246] of U.S. Pat. No. 7,638,570.

[0413] [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.

[0414] [SFC (Saline Flow Conductivity)] The SFC (Saline Flow Conductivity) of the water-absorbing agent composition according to the present invention (unit: × 10 －７ cm ３ The viscosity (sec / g) was measured in accordance with the measurement method described in U.S. Pat. No. 5,669,894.

[0415] Specifically, 1.500 g of the water-absorbing agent composition was uniformly placed in a container, and then the water-absorbing agent composition was immersed in artificial urine and swelled under a pressure of 2.07 kPa. The artificial urine was prepared by mixing 0.25 g of calcium chloride dihydrate, 2.0 g of potassium chloride, 0.50 g of magnesium chloride hexahydrate, 2.0 g of sodium sulfate, 0.85 g of ammonium dihydrogen phosphate, 0.15 g of diammonium hydrogen phosphate, and 994.25 g of pure water.

[0416] Sixty minutes after applying pressure, the height (cm) of the gel layer of the swollen water-absorbing agent composition was recorded. Next, a 0.69% by mass aqueous solution of sodium chloride was passed through the gel layer while the gel layer was pressurized at a pressure of 2.07 kPa. The room temperature during this process was adjusted to 20°C to 25°C. Using a balance and a computer, the amount of saline solution passing through the gel layer was recorded at 20-second intervals, and the flow rate Fs (T) of the passing saline solution was measured. The flow rate Fs (T) was calculated by dividing the mass (g) of the passing saline solution, which increased every 20 seconds, by the passage time (s). The time Ts was defined as the time at which the hydrostatic pressure of the saline solution became constant and a stable flow rate was obtained. Using data measured over a 10-minute period starting from this Ts, the flow rate Fs (T = 0) was calculated. That is, Fs (T) was plotted against time, and Fs (T = 0) was calculated based on the results obtained by the least squares method. Fs (T=0) is the initial flow rate (g / s) of saline solution passing through the gel layer, and the saline flow conductivity (SFC) was calculated by the following formula (f):

[0417] SFC = {Fs (T = 0) × L0} / (ρ × A × ΔP) ... (Equation f) where L0: height of gel layer (unit: cm) ρ: density of saline solution (unit: g / cm ３ ) A: Cross-sectional area of ​​the gel layer (unit: cm ２ ) ΔP: hydrostatic pressure applied to the gel layer (unit: dyne / cm ２ )

[0418] [CRC (absorbency without load)] The CRC (absorbency without load) of the water-absorbent agent composition according to the present invention was measured in accordance with NWSP 241.0.R2(19). Specifically, 0.2 g of the water-absorbent agent composition was placed in a nonwoven fabric 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 CRC (absorbency without load) (unit: g / g) was measured.

[0419] [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 NWSP 242.0. 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 against pressure (AAP) (unit: g / g) was measured.

[0420] [FHA (fixed height absorption value at a height of 20 cm)] The FHA (fixed height absorption value at a height of 20 cm) of the water-absorbent agent composition according to the present invention was measured in accordance with the measurement method described in paragraphs

[0104] to

[0116] of U.S. Patent Application Publication No. 2005 / 0003191.

[0421] [pH of Aqueous Solution of Flowability Improver] The pH of the aqueous solution of the flowability improver according to the present invention was measured using a portable pH meter D-71 manufactured by HORIBA while stirring the aqueous solution at 30 rpm.

[0422] <Production of water-absorbent resin> [Production Example 1] (Process for preparing aqueous monomer solution) 422.0 parts by mass of acrylic acid, 173.9 parts by mass of 48.5% by mass sodium hydroxide aqueous solution, 2.5 parts by mass of polyethylene glycol diacrylate (average molecular weight: 523), 1.3 parts by mass of 2.0% by mass diethylenetriaminepentaacetic acid trisodium aqueous solution, 0.2 parts by mass of polyethylene glycol 600 (mass average molecular weight 600, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 403.1 parts by mass of deionized water were charged into a 2 L polypropylene container and mixed to prepare an aqueous monomer solution (1'). The liquid temperature of the aqueous monomer solution (1') exceeded 40°C due to the heat of neutralization and heat of dissolution generated during the mixing process.

[0423] (Polymerization Step) Next, the aqueous monomer solution (1') was cooled with stirring, and when the liquid temperature reached 40°C, 178.7 parts by mass of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added to the aqueous monomer solution (1') over approximately 20 seconds in an open-to-air state and mixed (starting the second-stage neutralization). Thus, the aqueous monomer solution (1) was prepared. At this time, the liquid temperature of the aqueous monomer solution (1) rose to approximately 78°C due to the heat of neutralization and heat of dissolution generated during the mixing process. Although precipitates were observed immediately after starting to mix the aqueous sodium hydroxide solution with the aqueous monomer solution (1'), they gradually dissolved, and the prepared aqueous monomer solution (1) became a transparent, homogeneous solution.

[0424] Next, under stirring, nitrogen gas was introduced into the aqueous monomer solution (1) using a gas filter tube (manufactured by TOP Corporation, particle number #4) under conditions of a pressure of 0.1 MPa and a flow rate of 0.1 L / min for 5 seconds. Subsequently, 18.4 parts by mass of a 4.5 mass% aqueous sodium persulfate solution was added to the aqueous monomer solution (1) into which nitrogen gas had been introduced. Thereafter, the aqueous monomer solution (1) was immediately poured into a stainless steel bat-shaped container (bottom 340 mm x 340 mm, height 25 mm, inner surface: Teflon (registered trademark) coated) in an open-to-air state. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (1) into the bat-shaped container was 65 seconds. The bat-shaped container was preheated to a surface temperature of 50°C using a hot plate (NEO HOTPLATE HI-1000 / manufactured by Iuchi Seieido Co., Ltd.) before pouring the aqueous monomer solution (1).

[0425] The polymerization reaction started within 1 minute after the aqueous monomer solution (1) was poured into the batt-shaped container. During the polymerization reaction, the polymerization of the aqueous monomer solution (1) proceeded while expanding and foaming in all directions and generating steam. The resulting polymer then shrunk to a size slightly larger than the bottom of the batt-shaped container. Two minutes after the start of the polymerization reaction, the resulting polymer, a hydrogel (1), was removed from the batt-shaped container. This series of operations was carried out in an open-to-air state.

[0426] (Gel Crushing Step) Next, the hydrogel (1) obtained by the polymerization reaction was cut into pieces each having a mass of about 60 g, and then the gel was crushed using a meat chopper (HL-G22SN, plate hole diameter 6.0 mm / manufactured by Remacom Co., Ltd.) to obtain a particulate hydrogel (1). The amount of the hydrogel (1) added was approximately 360 g / min, and in parallel with the addition of the hydrogel (1), deionized water adjusted to 90 ° C. was added to the meat chopper at a rate of 50 g / min while the gel was crushed. The D50 (mass average particle diameter) of the particulate hydrogel (1) was 390 μm.

[0427] (Drying step) Next, the particulate hydrogel (1) was spread on a wire mesh with a mesh size of 300 μm and placed in a hot air dryer. Thereafter, the particulate hydrogel (1) was dried by passing hot air at 190° C. for 30 minutes to obtain a dried polymer (1). There was no undried material in the dried polymer (1).

[0428] (Pulverization step / classification step) Next, the dried polymer (1) was put into a roll mill (WML type roll pulverizer, manufactured by Inokuchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 710 μm and 150 μm. By this operation, an irregularly pulverized water absorbent resin (1) before surface crosslinking, which passed through the sieve with a mesh size of 710 μm and remained on the sieve with a mesh size of 150 μm, was obtained.

[0429] (Surface Cross-Linking Step) Next, an aqueous solution of a surface cross-linking agent consisting of 0.2 parts by mass of 1,6-hexanediol, 0.4 parts by mass of triethylene glycol, and 3.0 parts by mass of deionized water was sprayed and added to 100 parts by mass of the water absorbent resin (1) before surface cross-linking, and the mixture was mixed uniformly. Thereafter, the obtained mixture was heat-treated at 210°C for 40 minutes, thereby carrying out surface cross-linking. Next, the mixture after the heat treatment was classified using two types of JIS standard sieves with mesh sizes of 710 μm and 150 μm. By this operation, a surface-cross-linked water absorbent resin (1) was obtained, which passed through a sieve with a mesh size of 710 μm and remained on a sieve with a mesh size of 150 μm.

[0430] [Production Example 2] A water-absorbent resin (2) was produced under the following production conditions with reference to Reference Example 1 described in Japanese Patent No. 4,926,474.

[0431] (Step of Preparing Aqueous Monomer Solution) 5.9 parts by mass of polyethylene glycol diacrylate (average molecular weight: 523) was added to 5,500 parts by mass of an aqueous solution of sodium acrylate having a neutralization rate of 75 mol% (monomer concentration: 38% by mass) and mixed to prepare an aqueous monomer solution (2).

[0432] (Polymerization and Gel Crushing Step) Next, the aqueous monomer solution (2) was degassed under a nitrogen gas atmosphere for 30 minutes.

[0433] Next, the aqueous monomer solution (2) was supplied to a reactor formed by attaching a lid to a jacketed stainless steel double-arm kneader having an internal volume of 10 L and two sigma-type blades, and the system was purged with nitrogen gas while maintaining the aqueous monomer solution (2) at 30°C. 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 (2), and polymerization started after about 1 minute. Then, polymerization was carried out at 30°C to 90°C, and 60 minutes after the start of polymerization, the particulate hydrogel (2) was taken out.

[0434] The obtained particulate hydrogel (2) was finely divided into particles having a diameter of about 5 mm.

[0435] (Drying step) Next, the particulate hydrogel (2) was spread on a wire mesh with a mesh size of 300 μm and placed in a hot air dryer. Thereafter, the particulate hydrogel (2) was dried by passing hot air at 150° C. for 90 minutes to obtain a dried polymer (2). There was no undried material in the dried polymer (2).

[0436] (Pulverization step / classification step) Next, the dried polymer (2) was put into a roll mill (WML type roll pulverizer, manufactured by Inokuchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm. By this operation, an irregularly pulverized water absorbent resin (2) before surface crosslinking, which passed through the 850 μm mesh sieve and remained on the 150 μm mesh sieve, was obtained.

[0437] (Surface Cross-Linking Step) Next, an aqueous surface cross-linking agent solution consisting of 0.03 parts by mass of ethylene glycol glycidyl ether, 0.5 parts by mass of propylene glycol, 0.3 parts by mass of 1,4-butanediol, and 3.0 parts by mass of water was sprayed and added to 100 parts by mass of the water absorbent resin (2) before surface cross-linking, and the mixture was uniformly mixed. Thereafter, the obtained mixture was heat-treated at 200°C for 45 minutes, thereby carrying out surface cross-linking. Next, the mixture after the heat treatment was classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm. By this operation, a surface-cross-linked water absorbent resin (2) was obtained which passed through a sieve with a mesh size of 850 μm and remained on a sieve with a mesh size of 150 μm.

[0438] [Production Example 3] (Process for preparing aqueous monomer solution) 422.0 parts by mass of acrylic acid, 173.9 parts by mass of 48.5% by mass sodium hydroxide aqueous solution, 2.3 parts by mass of polyethylene glycol diacrylate (average molecular weight: 523), 1.3 parts by mass of 2.0% by mass diethylenetriaminepentaacetic acid trisodium aqueous solution, 0.2 parts by mass of polyethylene glycol 600 (mass average molecular weight 600, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 403.2 parts by mass of deionized water were charged into a 2 L polypropylene container and mixed to prepare an aqueous monomer solution (3'). The liquid temperature of the aqueous monomer solution (3') exceeded 40°C due to the heat of neutralization and heat of dissolution generated during the mixing process.

[0439] (Polymerization Step) Next, the aqueous monomer solution (3') was cooled with stirring, and when the liquid temperature reached 40°C, 178.7 parts by mass of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added to the aqueous monomer solution (3') over approximately 20 seconds in an open-to-air state and mixed (starting the second-stage neutralization). Thus, an aqueous monomer solution (3) was prepared. At this time, the liquid temperature of the aqueous monomer solution (1) had risen to approximately 78°C due to the heat of neutralization and heat of dissolution generated during the mixing process. Although precipitates were observed immediately after starting to mix the aqueous sodium hydroxide solution with the aqueous monomer solution (3'), they gradually dissolved, and the prepared aqueous monomer solution (3) became a transparent, homogeneous solution.

[0440] Next, under stirring, nitrogen gas was introduced into the aqueous monomer solution (3) using a gas filter tube (manufactured by TOP Corporation, particle number #4) under conditions of a pressure of 0.1 MPa and a flow rate of 0.1 L / min for 5 seconds. Subsequently, 18.4 parts by mass of a 4.5 mass% aqueous sodium persulfate solution was added to the aqueous monomer solution (3) into which nitrogen gas had been introduced. Thereafter, the aqueous monomer solution (3) was immediately poured into a stainless steel bat-shaped container (bottom 340 mm x 340 mm, height 25 mm, inner surface: Teflon (registered trademark) coated) in an open-to-air state. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (3) into the bat-shaped container was 67 seconds. The bat-shaped container was preheated to a surface temperature of 50°C using a hot plate (NEO HOTPLATE HI-1000 / manufactured by Iuchi Seieido Co., Ltd.) before the aqueous monomer solution (3) was poured.

[0441] The polymerization reaction started within 1 minute after the aqueous monomer solution (3) was poured into the batt-shaped container. During the polymerization reaction, the polymerization of the aqueous monomer solution (3) proceeded while expanding and foaming in all directions and generating steam. The resulting polymer then shrunk to a size slightly larger than the bottom of the batt-shaped container. Two minutes after the start of the polymerization reaction, the resulting polymer, a hydrogel (3), was removed from the batt-shaped container. This series of operations was carried out in an open-to-air state.

[0442] (Gel Crushing Step) Next, the hydrogel (3) obtained by the polymerization reaction was cut into pieces each having a mass of about 60 g, and then the gel was crushed using a meat chopper (HL-G22SN, plate hole diameter 8.0 mm / manufactured by Remacom Co., Ltd.) to obtain a particulate hydrogel (3). The amount of the hydrogel (3) added was approximately 360 g / min, and in parallel with the addition of the hydrogel (3), deionized water adjusted to 90 ° C. was added to the meat chopper at a rate of 50 g / min while the gel was crushed. The D50 (mass average particle diameter) of the particulate hydrogel (3) was 700 μm.

[0443] (Drying step) Next, the particulate hydrogel (3) was spread on a wire mesh with a mesh size of 300 μm and placed in a hot air dryer. Thereafter, the particulate hydrogel (3) was dried by passing hot air at 190° C. for 30 minutes to obtain a dried polymer (3). There was no undried material in the dried polymer (3).

[0444] (Pulverization step / classification step) Next, the dried polymer (3) was put into a roll mill (WML type roll pulverizer, manufactured by Inokuchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm. By this operation, an irregularly pulverized water absorbent resin (3) before surface crosslinking, which passed through the 850 μm mesh sieve and remained on the 150 μm mesh sieve, was obtained.

[0445] (Surface cross-linking step) Next, a surface cross-linking agent aqueous solution (containing a fluidity improver. Concentration of the fluidity improver in the aqueous solution: 0.03 mass%, pH of the aqueous solution: 4.7) consisting of 0.01 part by mass (amount of fluidity improver added: 10 ppm) of an aqueous solution containing 0.2 part by mass of 1,6-hexanediol, 0.4 part by mass of triethylene glycol, and 10% by mass of polyoxyethylene (20) sorbitan monostearate (flowability improver, trade name: RHEODOL TW-S120V, manufacturer: Kao Corporation) and 3.0 parts by mass of deionized water was added to 100 parts by mass of the water absorbent resin (3) before surface cross-linking, using a straight tube having an inner diameter of 0.2 mm (average droplet diameter: 0.4 mm (400 μm)), and the mixture was mixed uniformly. The addition of the surface cross-linking agent aqueous solution was carried out while stirring the water absorbent resin (3) at a rotation speed of 450 rpm (circumferential speed: 1.34 m / s) using a three-one motor equipped with an anchor-type stirring blade (diameter: 57 mm, height: 70 mm) made of a metal rod with a diameter of 3 mm. Stirring was carried out for 10 seconds from the start of addition (mixing force index: 33,500), and then the resulting mixture was heat-treated at 210 ° C. for 40 minutes to perform surface cross-linking. Next, the mixture after the heat treatment was classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm. By this operation, a surface-crosslinked water absorbent resin (3) was obtained that passed through a sieve with a mesh size of 850 μm and remained on a sieve with a mesh size of 150 μm.

[0446] [Production Example 4] In the surface cross-linking step of Production Example 3, except that the amount of addition of an aqueous solution containing 10 mass% of polyoxyethylene (20) sorbitan monostearate (fluidity improver, trade name: RHEODOL TW-S120V, manufacturer: manufactured by Kao Corporation) was changed to 0.03 part by mass (amount of fluidity improver added: 30 ppm) to prepare a surface cross-linking agent aqueous solution (containing a fluidity improver. Concentration of the fluidity improver in the aqueous solution: 0.08 mass%, pH of the aqueous solution: 4.7), and that in the addition of the surface cross-linking agent aqueous solution, the stirring time was changed to 30 seconds counting from the start of addition (mixing power index: 100,500), a surface-cross-linked water absorbent resin (4) was obtained.

[0447] [Example 1] Into a Loedige mixer (type: M5R, manufacturer: Loedige GmbH, stirring blade diameter: 0.19 m) heated to 60°C, 750 g of the surface-crosslinked water absorbent resin (1) obtained in Production Example 1 was heated to 60°C and then added. Subsequently, while stirring the surface-crosslinked water-absorbent resin (1) at a rotation speed of 340 rpm (peripheral speed: 3.38 m / s), 0.5 parts by mass of deionized water and 0.01 parts by mass of a 10% by mass aqueous solution of polyoxyethylene (20) sorbitan monostearate (trade name: Rheodor TW-S120V, manufacturer: Kao Corporation) were added by spraying an aqueous solution (1) (amount of fluidity improver added: 10 ppm, fluidity improver concentration in the aqueous solution: 0.20% by mass, pH of the aqueous solution: 6.9) at a liquid temperature of 25 ° C. to 100 parts by mass of the surface-crosslinked water-absorbent resin (1) (specification: 1 / 4M-K008, manufacturer: Ikeuchi Co., Ltd., average droplet size: 100 μm). After stirring for 60 seconds (mixing power index: 2,028,000) counting from the start of addition, the mixture was discharged from the Lödige mixer. The obtained mixture was laminated in a layer of 5 cm and allowed to stand for 30 minutes in a ventilated hot air dryer with the atmospheric temperature set to 60° C. The cured mixture was passed through a wire mesh with an opening of 850 μm to obtain a water-absorbing agent composition (1).

[0448] [Comparative Example 1] A comparative water-absorbing agent composition (1) was obtained by carrying out the same operation as in Example 1, except that the hollow conical nozzle used for adding the aqueous solution (1) in Example 1 was changed to a syringe (volume: 12 ml, manufacturer: Terumo Corporation, average droplet diameter: 4 mm). The mixing power index in Comparative Example 1 was 50,700.

[0449] [Example 2] A water-absorbent agent composition (2) was obtained by performing the same operation as in Example 1, except that the aqueous solution (1) in Example 1 was changed to an aqueous solution (2) (amount of fluidity improver added: 50 ppm, concentration of fluidity improver in the aqueous solution: 0.91 mass %, pH of the aqueous solution: 6.9) containing 0.5 parts by mass of deionized water and 0.05 parts by mass of a 10 mass % aqueous solution of polyoxyethylene (20) sorbitan monostearate (trade name: Rheodol TWS120V, manufactured by Kao Corporation) and having a liquid temperature of 25°C.

[0450] [Comparative Example 2] The same operation as in Comparative Example 1 was carried out, except that the surface-crosslinked water absorbent resin to be used in Comparative Example 1 was changed to the surface-crosslinked water absorbent resin (2) obtained in Production Example 2, to obtain a comparative water-absorbing agent composition (2).

[0451] Example 3 Into a plastic container having a capacity of 200 mL (inner diameter: 70 mm, depth: 140 mm), 50 g of the surface-crosslinked water absorbent resin (3) obtained in Production Example 3 was placed after adjusting the temperature to 60°C. Subsequently, while stirring the surface-crosslinked water absorbent resin (3) in a plastic container at a rotation speed of 450 rpm (circumferential speed: 1.34 m / s) using a Three-One motor equipped with an anchor-type stirring blade (diameter: 57 mm, height: 70 mm) made of a metal rod having a diameter of 3 mm, 0.50 parts by mass (amount of fluidity improver added: 10 ppm) of an aqueous solution (3) containing 0.20 mass% of polyoxyethylene (20) sorbitan monostearate (trade name: Rheodol TW-S120V, manufacturer: manufactured by Kao Corporation) at a liquid temperature of 25°C (fluidity improver concentration in the aqueous solution: 0.20 mass%, pH of the aqueous solution: 6.9) was added using a straight pipe having an inner diameter of 0.2 mm (average droplet diameter: 0.4 mm (400 μm)) After stirring for 30 seconds (mixing force index: 100,500) counting from the start of addition, the mixture was discharged from the plastic container. The obtained mixture was layered in a 5 cm layer and allowed to stand for 30 minutes in a ventilated hot air dryer with an atmospheric temperature set to 60°C to harden. The hardened mixture was passed through a wire mesh with an opening of 850 µm to obtain a water-absorbing agent composition (3).

[0452] [Comparative Example 3] A comparative water-absorbing agent composition (3) was obtained by performing the same operation as in Example 3, except that the stirring time when the aqueous solution (3) was added to the surface-crosslinked water-absorbing resin (3) in Example 3 was changed to 3 seconds (mixing force index: 10,050).

[0453] [Example 4] In Example 3, the aqueous solution (3) was changed to an aqueous solution (4) containing 0.20 mass % of polyoxyethylene polyoxypropylene alkyl ether (product name: Emulgen MS-110, manufacturer: Kao Corporation) (fluidity improver concentration in the aqueous solution: 0.20 mass %, pH of the aqueous solution: 6.2) at a liquid temperature of 25°C, and the amount of the aqueous solution added was changed to 1.00 part by mass (amount of fluidity improver added: 20 ppm), and the same operation as in Example 3 was performed to obtain a water-absorbent agent composition (4).

[0454] [Example 5] A water-absorbent agent composition (5) was obtained by performing the same operation as in Example 4, except that the aqueous solution (4) was changed to an aqueous solution (5) containing 0.20 mass % of polyoxyethylene (6) lauryl ether (trade name: Emulgen 108, manufactured by Kao Corporation) and having a liquid temperature of 25°C (fluidity improver concentration in the aqueous solution: 0.20 mass %, pH of the aqueous solution: 5.3).

[0455] [Example 6] A water-absorbent agent composition (6) was obtained by performing the same operation as in Example 4, except that the aqueous solution (4) was changed to an aqueous solution (6) containing 0.20 mass % of polyoxyethylene (10) lauryl ether sodium acetate (trade name: Kao Akipo RLM-100NV, manufacturer: Kao Corporation) and having a liquid temperature of 25°C (fluidity improver concentration in the aqueous solution: 0.20 mass %, pH of the aqueous solution: 5.6).

[0456] [Example 7] A water-absorbent agent composition (7) was obtained by performing the same operation as in Example 4, except that the aqueous solution (4) was changed to an aqueous solution (7) containing 0.20 mass % of polyoxyethylene (4.5) lauryl ether sodium acetate (trade name: Kao Akipo RLM-45NV, manufacturer: Kao Corporation) and having a liquid temperature of 25°C (fluidity improver concentration in the aqueous solution: 0.20 mass %, pH of the aqueous solution: 6.2).

[0457] [Example 8] A water-absorbent agent composition (8) was obtained by performing the same operation as in Example 4, except that the aqueous solution (4) was changed to an aqueous solution (8) containing 0.20 mass % of coconut amine acetate (...

Claims

1. A method for producing a water-absorbing agent composition mainly composed of a water-absorbent resin, comprising a step of preparing a monomer aqueous solution, a polymerization step, a gel crushing step, a drying step, a crushing step, a classification step, and a surface cross-linking step, the method comprising the step of mixing a water-soluble flowability improver having a mass average molecular weight of 200 or more and 50,000 or less with the water-absorbent resin in an amount of more than 0 ppm and less than 200 ppm relative to the mass of the water-absorbent resin during the surface cross-linking step or during a step subsequent to the surface cross-linking step, the method for producing a water-absorbing agent composition satisfying all of the following (a) to (d): (a) the specific surface area of ​​the water-absorbent resin is 25 m ２ / kg or more; (b) when the water-soluble fluidity improver is mixed with a water absorbent resin, the water-soluble fluidity improver is in the form of an aqueous solution of 0.01 mass % or more and 20 mass % or less; (c) when the aqueous solution is added and mixed with a water absorbent resin, the average droplet diameter of the aqueous solution is 10 μm or more and 1 mm or less; (d) when the aqueous solution is added and mixed with a water absorbent resin, the mixing force index defined by the following (Equation 1) is 70,000 or more.

2. The method according to claim 1, wherein the dynamic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm in the water-absorbent resin after the addition of the water-soluble flowability improver is 0.80 or less.

3. The method according to claim 1, wherein the water-soluble fluidity improver is at least one selected from the group consisting of nonionic substances, amphoteric substances, anionic substances and cationic substances.

4. The method according to claim 3, wherein the nonionic substance is selected from polyols, hydroxyl group modified polyols, side chain and / or terminal polyether modified polysiloxanes, and alkylene oxide adducts of higher aliphatic amines; the zwitterionic substance is selected from alkyl betaines and alkyl amine oxides; the anionic substance is selected from alkyl sulfate salts, sulfate salts of higher alcohol alkylene oxide adducts, sulfonates, dicarboxylates, alkyl amine diacetates, phosphate salts of higher alcohol alkylene oxide adducts, and carboxylate salts of higher alcohol alkylene oxide adducts; and the cationic substance is selected from ammonium salts.

5. The method according to claim 1, wherein the water-soluble flow improver comprises at least one selected from nonionic substances.

6. The method according to claim 1, wherein the water-soluble flow improver comprises at least one selected from nonionic substances having a polyalkylene glycol chain in the molecule.

7. The method according to claim 1, wherein the water-soluble flow improver comprises at least one selected from the group consisting of polyols and modified products of hydroxyl groups of polyols.

8. The method according to claim 1, wherein the pH of the aqueous solution is 4.5 or higher.

9. The method according to claim 1, wherein the water-absorbing resin and the water-absorbing agent composition each contain 50% by mass or more of particles having a particle diameter of 300 μm or more and less than 600 μm, and a reduction rate of a dynamic friction coefficient calculated by the following (Equation 2) is 10% or more: In formula 2, A is the kinetic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm of the water-absorbing resin before the water-soluble fluidity improver is added; B is the kinetic friction coefficient of particles having a particle size of 300 μm or more and less than 600 μm of the water-absorbing agent composition after the water-soluble fluidity improver is added.

10. The method according to claim 1, further comprising adding polyalkylene glycol during at least one step selected from the step of preparing the aqueous monomer solution, the step of polymerization, and the step of gel crushing, and / or between each of the steps.

11. The method of claim 10, wherein the polyalkylene glycol is polyethylene glycol having a mass average molecular weight of 3,000 or less.

12. The method according to claim 10, wherein the amount of the polyalkylene glycol added is 0.01% by mass or more and 0.25% by mass or less based on the total mass of the monomers contained in the aqueous monomer solution.

13. A water-absorbing agent composition containing a water-absorbent resin as a main component, containing a water-soluble fluidity improver, and satisfying all of the following (1) to (5): (1) The specific surface area of ​​the water-absorbing agent composition is 25 m ２ / kg or more; (2) A surface tension of the water-absorbent agent composition is 56 mN / m or more; (3) A flow rate of the water-absorbent agent composition is 10.0 g / s or more; (4) A mass ratio of particles having a particle diameter of 300 μm or more and less than 600 μm in the water-absorbent agent composition is 50 mass % or more; (5) A dynamic friction coefficient of particles having a particle diameter of 300 μm or more and less than 600 μm in the water-absorbent agent composition is 0.80 or less.

14. The water-absorbent composition according to claim 13, wherein the water-absorbent composition has a Vortex (water absorption speed) of 50 seconds or less.

15. The water-absorbing agent composition has a saline flow conductivity (SFC) of 1×10 －７ cm ３ The water-absorbent composition according to claim 13, wherein the water-absorbent composition has a surface area of ​​0.05 -sec / g or more.

16. The water-absorbent agent composition according to claim 13, wherein D50 (mass average particle size) of said water-absorbent agent composition is 250 μm or more and less than 550 μm, and the mass ratio of particles of said water-absorbent agent composition having a particle size of less than 150 μm is 3 mass% or less.

17. The water-absorbent composition according to claim 13, wherein the water-absorbent composition has an AAP (absorbency against pressure) of 20 g / g or more.

18. An absorbent body comprising the water-absorbent composition according to any one of claims 13 to 17.

19. Pulp weight is 300g / m ２ 20. The absorbent body of claim 18, wherein:

20. The water-absorbing agent composition has a basis weight of 450 g / m ２ 20. The absorbent body of claim 18, wherein:

21. An absorbent article comprising the absorbent body according to any one of claims 18 to 20.

22. The absorbent article of claim 21, which is pulp-free.

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