Method for producing water absorbent, and water absorbent

Abstract

To provide a method for producing a water absorbent that makes it possible to obtain a water absorbent having reduced odor and water release after absorption while reducing the amount of fine powder generated. The production method includes a step for polymerizing a monomer aqueous solution having a neutralization rate of 50 mol% or more and drying the obtained hydrous gel. At a prescribed time, the following dicarboxylic acid (salt) is added in an amount of 100-3,000 ppm with respect to the solid content contained in the monomer aqueous solution or the solid content of the hydrous gel. (In the formula, n is 1 or 2, and R is a C8-20 linear or branched alkyl group or alkenyl group.)

Description

Method for producing water-absorbing agent and water-absorbing agent

[0001] The present invention relates to a method for producing a water-absorbing agent and a water-absorbing agent.

[0002] BACKGROUND ART A super absorbent polymer (SAP) is a water-swellable, water-insoluble polymer gelling agent, and is widely used in a variety of fields, including absorbent articles such as disposable diapers and sanitary napkins, water retention agents for agricultural and horticultural use, and industrial water-stopping agents.

[0003] Many monomers and / or hydrophilic polymers are used as raw materials for the water-absorbing resins. From the viewpoint of water absorption performance, polyacrylic acid (salt)-based water-absorbing resins using acrylic acid and / or its salt as a monomer are most widely produced industrially.

[0004] The water-absorbent resin is required to have various functions in accordance with the improvement in performance of disposable diapers, which are the main application of the water-absorbent resin. Specifically, the water-absorbent resin is required to have basic physical properties such as absorbency without load and absorbency under load.

[0005] Such a water-absorbent resin can be formed into various shapes such as a sheet, a fiber, a film, etc., and is generally formed into a powder or a particle. It is known that the water-absorbing performance, handling property, feeling of use, etc. of a powder or particle-shaped water-absorbent resin vary depending on the particle size, particle size distribution, etc. Furthermore, when a water-absorbent resin is used as an absorbent body of an absorbent article such as a disposable diaper, there is a strong market need for improvement of urinary leakage, skin rash, etc.

[0006] The main methods for producing the powdery or particulate water-absorbent resin include an aqueous solution polymerization method and a reversed-phase suspension polymerization method. Either of these production methods needs to include a drying step of the hydrogel obtained by polymerization. In the drying step, one or more of the following problems (1) to (3) may occur.

[0007] (1) The problem of reduced drying efficiency due to aggregation of the hydrogel. (2) The problem of increased load in the pulverization step after the drying step due to clumping of the dried material. (3) The problem of large amounts of fine powder being generated in the pulverization step. Fine powder (components with particle diameters of less than about 150 μm) contained in the water-absorbent resin causes a decrease in the performance of the water-absorbent resin, products containing the water-absorbent resin (water-absorbing agents), and sanitary products using the same. Therefore, the fine powder is removed from the water-absorbent resin by classification or the like during the production process, and the amount of the water-absorbing agent finally produced decreases. Therefore, when a large amount of the fine powder is generated, the productivity of the water-absorbing agent deteriorates.

[0008] One known method for solving the above problem is described in Patent Document 1. This method involves drying a hydrogel crosslinked polymer containing 0.001 to 0.5 mass % of the following compound (a) and / or compound (b):

[0009] (a) A compound having a long-chain alkyl group having 8 or more carbon atoms, one quaternary nitrogen atom, one acid group, and a betaine structure in the molecule.

[0010] (b) A compound having a long-chain alkyl group having 8 or more carbon atoms, one tertiary nitrogen atom, and one or more acid groups in the molecule.

[0011] Another known solution is the method described in Patent Document 2. This method involves drying a hydrogel crosslinked polymer in the presence of one or more additives selected from the group consisting of carboxylic acids represented by the following general formula and salts thereof.

[0012] (In the above general formula, R is a straight or branched chain alkyl group having 6 to 18 carbon atoms, or a straight or branched chain alkenyl group having 6 to 18 carbon atoms.)

[0013] International Publication No. WO2017 / 221911 Pamphlet Korean Patent Publication No. 10-2023-0062096

[0014] The compounds (a) and / or (b) are preferable compounds for solving the above problems. However, in consideration of availability and cost, there is room for consideration of alternatives that are cheaper and can be used in smaller amounts.

[0015] Furthermore, the method described in Patent Document 2 leaves room for improvement in the odor of the water-absorbing agent produced, the neutralization index of the water-absorbing agent, and / or the amount of return when the swollen water-absorbing agent is pressurized.

[0016] An object of one embodiment of the present invention is to solve the above-mentioned problems, specifically to produce a water-absorbing agent that reduces the amount of fine powder generated, reduces odor, and reduces the amount of resorption after water absorption.Another embodiment of the present invention is to provide a water-absorbing agent that reduces the content of fine powder and odor, properly controls the neutralization index, and reduces the amount of resorption.

[0017] As a result of extensive research, the present inventors have found that even a small amount of a dicarboxylic acid having a specific structure can sufficiently prevent aggregation of a hydrogel or clumping of the dried product, thereby reducing the amount of fine powder generated. Furthermore, they have found that by using a small amount of the dicarboxylic acid, the odor can be reduced and the amount of reconstitution after water absorption can be reduced. In this specification, the effect of preventing aggregation of a hydrogel and / or clumping of the dried product is referred to as "gel dispersibility."

[0018] That is, one embodiment of the present invention is a method for producing a water-absorbing agent, comprising: a polymerization step of polymerizing an aqueous monomer solution to obtain a hydrogel; and a drying step of drying the hydrogel to obtain a dried product, wherein a neutralization rate of the aqueous monomer solution is 50 mol % or more; and a dicarboxylic acid represented by the following formula (1) and / or a salt thereof is added to the aqueous monomer solution and / or the hydrogel before the completion of the drying step, and an amount of the dicarboxylic acid and / or the salt thereof is within a range of 100 to 3000 ppm with respect to a solid content contained in the aqueous monomer solution or a solid content of the hydrogel.

[0019] (In formula (1), n ​​is 1 or 2, and R is a linear or branched alkyl group having 8 to 20 carbon atoms, or a linear or branched alkenyl group having 8 to 20 carbon atoms.) Another embodiment of the present invention is a water-absorbing agent comprising a water-absorbent resin and a dicarboxylic acid represented by the following formula (1) and / or a salt thereof, wherein the content of the dicarboxylic acid represented by formula (1) and / or the salt thereof is within a range of 100 to 3000 ppm relative to the solid content of the water-absorbent resin, and the neutralization index is 15 or less.

[0020] (In formula (1), n ​​is 1 or 2, and R is a linear or branched alkyl group having 8 to 20 carbon atoms, or a linear or branched alkenyl group having 8 to 20 carbon atoms.) (The neutralization index means the number of water-absorbing agents having a neutralization rate that is 20 mol % or more lower than the average neutralization rate of the water-absorbing agents, out of 200 water-absorbing agents.)

[0021] A method for producing a water-absorbing agent according to one embodiment of the present invention can produce a water-absorbing agent that has excellent drying efficiency in the drying step, prevents the generation of a large amount of fine powder, reduces odor, and reduces the amount of resorption after water absorption. The water-absorbing agent according to one embodiment of the present invention is excellent in productivity, and also excels in reducing odor and the amount of resorption after water absorption. Furthermore, according to another embodiment of the present invention, it is possible to provide a water-absorbing agent that has a reduced fine powder content and odor, an appropriately controlled neutralization index, and a reduced amount of resorption.

[0022] An embodiment of the present invention will be described below, but the present invention is not limited thereto. The present invention is not limited to the configurations described below, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. In this specification, unless otherwise specified, "A to B" representing a numerical range means "greater than or equal to A and less than or equal to B."

[0023] [1] Definition of Terms [1-1] "Water-absorbent resin," "non-surface-crosslinked water-absorbent resin," "surface-crosslinked water-absorbent resin," "water-absorbing agent" The term "water-absorbent resin" in the present application refers to a water-swellable, water-insoluble polymer gelling agent that satisfies the following physical properties. That is, it refers to a polymer gelling agent whose water-swellability (CRC) as defined by NWSP 241.0. R2(15) is 5 g / g or more and whose water-soluble component (Ext) as defined by NWSP 270.0. R2(15) is 50 mass% or less. Furthermore, the water-absorbent resin is composed of a crosslinked polymer.

[0024] The water-absorbent resin can be designed according to its application and purpose. The water-absorbent resin is composed of a hydrophilic cross-linked polymer obtained by cross-linking an acid group-containing unsaturated monomer having a neutralizable acid group. The water-absorbent resin is preferably composed of a cross-linked polymer of an unsaturated monomer having a carboxyl group. The water-absorbent resin is not limited to a form in which the entire amount is a cross-linked polymer. As long as the physical properties (CRC, Ext) satisfy the above-mentioned numerical ranges, the water-absorbent resin may be in the form of a composition containing additives in addition to the cross-linked polymer.

[0025] The water-absorbent resin may be a “water-absorbent resin that is not surface-crosslinked” or a “surface-crosslinked water-absorbent resin.” Hereinafter, unless otherwise specified, the term “water-absorbent resin” means both a “water-absorbent resin that is not surface-crosslinked” and a “surface-crosslinked water-absorbent resin.”

[0026] The term "water-absorbing agent" in the present application refers to a water-absorbing resin in a state ready for shipment as a final product and a composition containing the water-absorbing resin. When the water-absorbing resin is shipped as a final product as it is, the water-absorbing resin becomes the "water-absorbing agent." When a composition in which an additive is added to the water-absorbing resin is shipped as a final product, the composition is called the "water-absorbing agent."

[0027] [1-2] "Hydrogel" and "Particulate Hydrogel" In this specification, "hydrogel" means a gel composed of a polymer obtained by polymerizing a monomer described later in a polymerization step described later. The "hydrogel" includes not only a hydrogel obtained by the polymerization step and not subjected to a gel crushing step described later, but also a crushed hydrogel prepared by the gel crushing step. In this specification, the crushed hydrogel obtained by the gel crushing step is also referred to as a "particulate hydrogel."

[0028] As will be described later, when kneader polymerization is performed, gel pulverization is carried out in the polymerization step. Therefore, hereinafter, the hydrogel obtained by the polymerization step employing kneader polymerization will also be referred to as a "particulate hydrogel." Furthermore, as will be described later, when reversed-phase suspension polymerization is performed, particulate gel is directly produced in the polymerization process. Therefore, hereinafter, the hydrogel obtained by the polymerization step employing reversed-phase suspension polymerization will also be referred to as a "particulate hydrogel."

[0029] [1-3] "NWSP" "NWSP" stands for "Non-Woven Standard Procedures-Edition 2015." NWSP is a standard jointly published by EDANA (European Disposables And Nonwovens Association) and INDA (Association of the Nonwoven Fabrics Industry) to standardize evaluation methods for nonwoven fabrics and their products in the United States and Europe, and includes standard measurement methods for water-absorbent resins. In this specification, the physical properties of hydrogels, water-absorbent resins, and water-absorbing agents are measured in accordance with NWSP under conditions other than those specifically described in the examples, etc.

[0030] [1-4] "Ext of water-absorbent resin" (NWSP 270.0.R2(15)) "Ext" is an abbreviation for Extractables and means the water-soluble content (amount of water-soluble components). Specifically, the Ext of a water-absorbent resin refers to the amount (unit: mass%) of a substance dissolved in the aqueous solution after 1.0 g of the water-absorbent resin is placed in 200 ml of a 0.9 mass % sodium chloride aqueous solution and stirred at 500 rpm for 16 hours. pH titration is used to measure the water-soluble content of a water-absorbent resin.

[0031] [1-5] Others In this application, the term "X to Y" indicating a range means "X or more and Y or less." Unless otherwise noted, the unit of mass "t (ton)" means "Metric ton," and "ppm" means "ppm by mass" or "ppm by weight." Furthermore, "mass" and "weight," "parts by mass" and "parts by weight," "mass %" and "weight %" are treated as synonyms. Furthermore, "~acid (salt)" means "~acid and / or its salt," and "(meth)acrylic" means "acrylic and / or methacrylic."

[0032] [2] Manufacturing Method of Water-Absorbing Agent A manufacturing method of a water-absorbing agent according to one embodiment of the present invention (hereinafter referred to as "the manufacturing method of the present invention") includes a polymerization step of polymerizing an aqueous monomer solution to obtain a hydrogel, and a drying step of drying the hydrogel to obtain a dried product, wherein a neutralization rate of the aqueous monomer solution is 50 mol % or more, and a dicarboxylic acid represented by the following formula (1) and / or a salt thereof is added to the aqueous monomer solution and / or the hydrogel by the end of the drying step, and the amount of the dicarboxylic acid and / or the salt thereof is within a range of 100 to 3000 ppm with respect to the solid content contained in the aqueous monomer solution or the solid content of the hydrogel.

[0033] (In formula (1), n ​​is 1 or 2, and R is a linear or branched alkyl group having 8 to 20 carbon atoms, or a linear or branched alkenyl group having 8 to 20 carbon atoms.) Here, the polymerization step and the drying step are part of a production step of a water-absorbing resin contained in a water-absorbing agent obtained by the production method of the present invention. Therefore, in the production method of the present invention, "until the end of the drying step" corresponds to the period during the production step of the water-absorbing resin. Therefore, the production method of the present invention includes a step of adding a dicarboxylic acid represented by formula (1) and / or a salt thereof during the production step of the water-absorbing resin. Therefore, the water-absorbing agent containing the obtained water-absorbing resin contains a dicarboxylic acid represented by formula (1) and / or a salt thereof. In this specification, the dicarboxylic acid represented by formula (1) and / or a salt thereof is treated as a constituent component of the water-absorbing agent, not as a constituent component of the water-absorbing resin. From the above, the water-absorbing agent obtained by the production method of the present invention contains the water-absorbing resin and the dicarboxylic acid represented by the formula (1) and / or a salt thereof, and is obtained as a composition containing these components.

[0034] [2-1] Addition of the dicarboxylic acid represented by the formula (1) and / or its salt In the production method of the present invention, the dicarboxylic acid represented by the formula (1) and / or its salt is added to the aqueous monomer solution and / or the hydrogel before the drying step is completed. Hereinafter, the dicarboxylic acid represented by the formula (1) and / or its salt is also referred to as the "dicarboxylic acid (salt) of the present invention."

[0035] Here, "until the completion of the drying step" refers to, for example, one or more of the aqueous monomer solution preparation step, the polymerization step, and the drying step, which will be described later. In addition, when the production method of the present invention includes the gel crushing step, "until the completion of the drying step" refers to, for example, one or more of the aqueous monomer solution preparation step, the polymerization step, the gel crushing step, and the drying step, which will be described later. The dicarboxylic acid represented by formula (1) and / or its salt may be added not only during these steps, but also between steps.

[0036] In one embodiment of the present invention, the dicarboxylic acid (salt) of the present invention can be added to the aqueous monomer solution by the following method (I) and / or (II).

[0037] (I) In the step of preparing an aqueous monomer solution described below, the dicarboxylic acid (salt) of the present invention is used as one of the raw materials for the aqueous monomer solution to prepare an aqueous monomer solution containing the dicarboxylic acid (salt) of the present invention.

[0038] (II) In the polymerization step, the dicarboxylic acid (salt) of the present invention is added to the aqueous monomer solution.

[0039] When the method (I) is carried out, the aqueous monomer solution to be subjected to the polymerization step contains the dicarboxylic acid (salt) of the present invention, and therefore the hydrogel prepared in the polymerization step contains the dicarboxylic acid (salt) of the present invention.

[0040] When the method (II) is carried out, the hydrogel prepared in the polymerization step contains the dicarboxylic acid (salt) of the present invention, as in the method (I). Therefore, the hydrogel subjected to the drying step contains the dicarboxylic acid (salt) of the present invention.

[0041] When the dicarboxylic acid (salt) of the present invention is added to the hydrogel in the gel crushing step, similarly to the case where the dicarboxylic acid (salt) of the present invention is added to the aqueous monomer solution, the resulting particulate hydrogel contains the dicarboxylic acid (salt) of the present invention. Here, the hydrogel includes the hydrogel before crushing and the hydrogel during crushing, i.e., the hydrogel in the process of being converted into the particulate hydrogel.

[0042] Furthermore, when the dicarboxylic acid (salt) of the present invention is added to the hydrogel in the drying step, the hydrogel contains the dicarboxylic acid (salt) of the present invention. Here, the hydrogel subjected to the drying step may be a hydrogel other than the particulate hydrogel, or may be the particulate hydrogel.

[0043] Therefore, in the production method of the present invention, the hydrogel containing the dicarboxylic acid (salt) of the present invention is dried in the drying step to prepare a dried product.

[0044] The dicarboxylic acid (salt) of the present invention has a hydrophobic functional group and a hydrophilic functional group. In the dicarboxylic acid (salt) of the present invention, "R" in the formula (1) functions as a hydrophobic functional group. Furthermore, when the dicarboxylic acid (salt) of the present invention is a salt, the two carboxyl groups (-COOH groups) in the formula (1) are converted into carboxylate groups (-COOH groups). － In this case, both of the two carboxyl groups may be carboxylate groups, or only one of the carboxyl groups may be a carboxylate group. Here, the carboxyl group or carboxylate group of the dicarboxylic acid (salt) of the present invention functions as a hydrophilic functional group.

[0045] The hydrogel or particulate hydrogel of the present invention contains the dicarboxylic acid (salt) of the present invention, in which the hydrophilic functional groups of the dicarboxylic acid (salt) are oriented toward the inside (interior) of the hydrogel due to their high affinity with the hydrogel, while the hydrophobic functional groups of the dicarboxylic acid (salt) are oriented toward the outside (gas phase side) of the hydrogel.

[0046] Therefore, in the drying step, the hydrogels repel each other due to the hydrophobic functional groups oriented toward the gas phase (surface) side, thereby reducing the cohesive force between the hydrogels, thereby preventing aggregation of the hydrogels and / or clumping of the dried product. In other words, when the hydrogel contains the dicarboxylic acid (salt) of the present invention, the gel dispersibility can be improved.

[0047] As a result, the production method of the present invention can improve the drying efficiency in the drying step. Furthermore, the production method of the present invention can reduce the load required for pulverization in the pulverization step after the drying step, thereby reducing the amount of fine powder generated and preventing the generation of a large amount of fine powder.

[0048] In addition, it is preferable that a larger number of the hydrophobic functional groups are oriented outward (toward the gas phase) in the surface layer portion of the hydrogel dried in the drying step, because the repulsive force between the hydrogels can be strengthened, thereby further improving the gel dispersibility. When the dicarboxylic acid (salt) of the present invention is disposed in the surface layer portion of the hydrogel dried in the drying step, a larger number of the hydrophobic functional groups can be oriented outward (toward the gas phase) of the hydrogel. Furthermore, when the dicarboxylic acid (salt) of the present invention is added to the hydrogel between the end of the polymerization step and the end of the drying step, the dicarboxylic acid (salt) of the present invention can be present in the surface layer portion of the hydrogel.

[0049] In this way, by adding the dicarboxylic acid (salt) of the present invention to the hydrogel after the completion of the polymerization step and before the completion of the drying step, the dispersibility of the gel can be further improved, and aggregation of the hydrogel and / or clumping of the dried product can be further prevented, thereby further improving the drying efficiency in the drying step and further preventing the generation of a large amount of fine powder in the pulverization step.

[0050] Specifically, from the viewpoint of further improving the drying efficiency and further preventing the generation of a large amount of fine powder, it is preferable to add the dicarboxylic acid (salt) of the present invention to the hydrogel during the drying step. When the production method of the present invention further includes the gel-crushing step, it is preferable to add the dicarboxylic acid (salt) of the present invention to the hydrogel after the end of the polymerization step and before the end of the gel-crushing step.

[0051] "From the end of the polymerization step to the end of the gel crushing step" means, in a case where the production method of the present invention includes the gel crushing step, after the end of the polymerization step, before the start of the gel crushing step, or during the gel crushing step.

[0052] Here, consider a case where the hydrogel to which the dicarboxylic acid (salt) of the present invention has been added is crushed in the gel crushing step. In this case, the hydrogel is crushed in the presence of the dicarboxylic acid (salt) of the present invention in the gel crushing step. This makes it possible to obtain a particulate hydrogel in which the dicarboxylic acid (salt) of the present invention is present in the surface layer portion. Furthermore, the coexistence of the dicarboxylic acid (salt) of the present invention with the hydrogel in the gel crushing step allows a larger amount of the dicarboxylic acid (salt) of the present invention to be present on the fracture surfaces newly generated in the (particulate) hydrogel in the gel crushing step. Therefore, the effect of the dicarboxylic acid (salt) can be further enhanced.

[0053] Therefore, it is more preferable that the production method of the present invention further comprises the gel-crushing step, and that the dicarboxylic acid (salt) of the present invention is added to the hydrogel after the polymerization step and before the gel-crushing step is completed, because this prevents or reduces aggregation of the particulate hydrogel during the hydrogel-crushing step, and makes it possible to obtain a water-absorbing agent with an excellent absorption rate.

[0054] The form of addition of the dicarboxylic acid (salt) of the present invention is not particularly limited, and the powdery dicarboxylic acid (salt) of the present invention may be added directly to the aqueous monomer solution or the hydrogel, or may be added in the form of a solution or dispersion, preferably a solution or dispersion.

[0055] From the viewpoint of more uniformly mixing the aqueous monomer solution or the hydrogel with the dicarboxylic acid (salt) of the present invention, the dicarboxylic acid (salt) of the present invention is more preferably added in the form of a solution.

[0056] The solution concentration when adding the dicarboxylic acid (salt) of the present invention is adjusted appropriately depending on the type of dicarboxylic acid (salt) of the present invention. When the dicarboxylic acid (salt) of the present invention is commercially available as a solution of known concentration, it may be used as is, or it may be used after adjusting the solution concentration to a desired range by dilution or the like. The solvent or diluent for the solution of the dicarboxylic acid (salt) of the present invention is not particularly limited, and is preferably water or a hydrophilic organic solvent, more preferably water. The solvent or diluent may be one type of solvent or a mixture of two or more types of solvents.

[0057] In the production method of the present invention, the amount of the dicarboxylic acid (salt) of the present invention added is within the range of 100 to 3000 ppm based on the solid content contained in the aqueous monomer solution or the solid content of the hydrogel.

[0058] Here, the solid content of the aqueous monomer solution refers to the total amount of components excluding volatile components such as water and solvents contained in the aqueous monomer solution. Specifically, it refers to the total amount of components contained in the aqueous monomer solution that will become solids in the hydrogel obtained after polymerization, including the monomer, internal cross-linking agent, polymerization initiator, and other substances listed in the "(Other substances added to the aqueous monomer solution or hydrogel)" section below. Therefore, the solid content of the aqueous monomer solution and the solid content of the hydrogel are substantially equivalent. The solid content of the aqueous monomer solution and the solid content of the hydrogel can be calculated from the amounts of raw materials, such as each monomer, used up to the polymerization step. The solid content of the hydrogel may be measured and calculated using the solid content measurement method described in the Examples.

[0059] In the production method of the present invention, the amount of the dicarboxylic acid (salt) of the present invention added is 3000 ppm or less. Therefore, the amount of odorous substances derived from the dicarboxylic acid (salt) of the present invention in the water-absorbing agent is less than when the amount added exceeds 3000 ppm. Therefore, the production method of the present invention can produce a water-absorbing agent that is excellent at reducing odors, particularly odors generated during swelling. The amount of the dicarboxylic acid (salt) added refers to a concentration calculated assuming that the entire amount is a dicarboxylic acid (acid-type compound) in which both carboxyl groups are unneutralized. For example, consider a case in which the dicarboxylic acid (salt) is a partially neutralized salt in which some of the carboxyl groups are neutralized, or a mixture of a dicarboxylic acid and a dicarboxylic acid salt. In this case, the amount of the dicarboxylic acid (salt) added refers to a concentration calculated assuming that the dicarboxylic acid (salt) to be added is a dicarboxylic acid in which all of the carboxyl groups contained in the dicarboxylic acid (salt) are unneutralized.

[0060] Furthermore, since the amount of the dicarboxylic acid (salt) of the present invention added to the water-absorbing agent is as described above, a decrease in surface tension is prevented or reduced, or the surface tension is relatively high. This reduces the amount of resorption when pressure is applied after the water-absorbing agent has absorbed water. Therefore, the manufacturing method of the present invention can manufacture a water-absorbing agent with a reduced amount of resorption after water absorption.

[0061] From the viewpoint of producing a water-absorbing agent excellent in reducing the odor and the rewet amount after absorbing water, it is preferable that the amount of the dicarboxylic acid (salt) of the present invention added is small. Specifically, the amount added is preferably 3000 ppm or less, more preferably 1000 ppm or less, and even more preferably 500 ppm or less, based on the solid content contained in the aqueous monomer solution or the solid content of the hydrogel.

[0062] On the other hand, the amount of the dicarboxylic acid (salt) of the present invention added in the production method of the present invention is 100 ppm or more based on the solid content contained in the aqueous monomer solution or the solid content of the hydrogel.

[0063] As a result, as described above, the cohesive force between the hydrogel particles is reduced in the drying step, thereby improving drying efficiency. Furthermore, the generation of a large amount of fine powder can be prevented in the pulverization step. From the viewpoints of improving drying efficiency and preventing the generation of a large amount of fine powder, a larger amount of the dicarboxylic acid (salt) of the present invention is preferred. Specifically, the amount added is preferably 100 ppm or more, more preferably 200 ppm or more, based on the solid content contained in the aqueous monomer solution or the solid content of the hydrogel.

[0064] The dicarboxylic acid (salt) of the present invention may be added two or more times before the completion of the drying step, and in that case, the total of the amounts added in each step is the amount of the dicarboxylic acid (salt) of the present invention added.

[0065] The dicarboxylic acid (salt) of the present invention is a dicarboxylic acid and / or a salt thereof represented by the above formula (1). In the above formula (1), n ​​is 1 or 2, preferably 1.

[0066] In the formula (1), R is a linear or branched alkyl group having 8 to 20 carbon atoms, or a linear or branched alkenyl group having 8 to 20 carbon atoms. Since the carbon chain of R is sufficiently long, highly hydrophobic functional groups can be oriented on the surface of the hydrogel during the drying step, and the cohesive force between the hydrogels is sufficiently reduced. Therefore, the production method of the present invention can improve the drying efficiency during the drying step and prevent the generation of a large amount of fine powder during the pulverization step. From the viewpoints of improving the drying efficiency and preventing the generation of a large amount of fine powder, the number of carbon atoms in R is preferably 12 or more, more preferably 14 or more.

[0067] On the other hand, since the carbon chain of R is not excessively long, the mobility of the dicarboxylic acid (salt) of the present invention can be sufficiently ensured. As a result, in the production method of the present invention, the dicarboxylic acid (salt) of the present invention can be sufficiently mixed with the aqueous monomer solution and / or the hydrogel, thereby solving the problem of the present invention. From this viewpoint, the number of carbon atoms in R is preferably 18 or less.

[0068] The R may be an alkyl or alkenyl having 19 or 20 carbon atoms.

[0069] Specifically, in the formula (1), when R is a linear alkyl having 8 to 20 carbon atoms, it may be, for example, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, n-octadecyl, n-nonadecyl, or n-eicosyl. When R is a linear alkenyl having 8 to 20 carbon atoms, it may be, for example, n-octenyl, n-nonenyl, n-decenyl, n-undecenyl, n-dodecenyl, n-tridecenyl, n-tetradecenyl, n-pentadecenyl, n-hexadecenyl, n-heptadecenyl, n-octadecenyl, n-nonadecenyl, or n-eicosenyl.

[0070] Furthermore, when R is a linear alkyl or alkenyl, it is more advantageous in terms of reducing the aggregation of branched particles and improving dispersibility.

[0071] The dicarboxylic acid (salt) of the present invention may be, for example, an alkali metal salt or an alkaline earth metal salt, preferably an alkali metal salt, and more preferably a potassium salt.

[0072] Specific examples of the dicarboxylic acid (salt) of the present invention include dipotassium alkenyl succinate. Examples of the dipotassium alkenyl succinate include dipotassium hexadecenyl succinate and dipotassium octadecenyl succinate. Commercially available products can also be used as the dicarboxylic acid (salt) of the present invention. Examples of the commercially available product include LATEMULL (trademark) ASK (manufactured by Kao Corporation). The dicarboxylic acid (salt) of the present invention can be used alone or in combination of two or more.

[0073] Hereinafter, each step constituting the production method of the present invention, other than the step of adding the dicarboxylic acid (salt) of the present invention, will be described.

[0074] [2-2] Step of preparing aqueous monomer solution This step is a step of preparing an aqueous solution containing a monomer that constitutes the water-absorbing resin in the water-absorbing agent to be produced. Hereinafter, the aqueous solution is also referred to as a “monomer aqueous solution.”

[0075] In addition, a slurry liquid of a monomer can also be used within a range that does not deteriorate the water absorption performance of the water-absorbing agent obtained as a final product. In addition, the "monomer aqueous solution" that can be used in the production method of the present invention can include not only an aqueous solution containing the monomer, but also a slurry liquid of the monomer.

[0076] (Monomer) In the production method of the present invention, the monomer used is a compound that can be polymerized to become a water-absorbent resin and has a neutralizable acid group, which is also called an acid group-containing monomer, as a main component. The acid group-containing monomer is not particularly limited, and examples thereof include acid group-containing unsaturated monomers or salts thereof such as (meth)acrylic 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.

[0077] Among these, from the viewpoint of the water absorption performance of the water-absorbing agent, the monomer to be used is preferably (meth)acrylic acid (salt), (anhydrous) maleic acid (salt), itaconic acid (salt), or cinnamic acid (salt), and more preferably acrylic acid (salt).

[0078] The "main component" means that the content of the acid group-containing monomer is usually 50 mol % or more, preferably 70 mol % or more, more preferably 80 mol % or more, and even more preferably 90 mol % or more (upper limit: 100 mol %), based on the total amount of monomers used in the polymerization reaction.

[0079] From the viewpoint of the water absorption performance of the water-absorbing agent, the neutralization rate of the aqueous monomer solution is 50 mol% or more, preferably 60 mol% or more, and more preferably 70 mol% or more. When the neutralization rate is 50 mol% or more, the obtained water-absorbing agent is uniformly neutralized, and as will be described later, the neutralization index of the water-absorbing agent can be set to 15 or less. Furthermore, when surface cross-linking is performed, uniform surface cross-linking can be easily performed, and the water-absorbing agent can have excellent absorption performance.

[0080] In addition, from the viewpoint of the water absorption performance of the water-absorbing agent, the neutralization rate of the aqueous monomer solution is preferably 100 mol % or less, more preferably 80 mol % or less, even more preferably 78 mol % or less, and even more preferably 75 mol % or less.

[0081] Here, the neutralization ratio of the aqueous monomer solution refers to the proportion (mol %) of neutralized acid groups when the total of neutralized and unneutralized acid groups of the acid group-containing monomer in the aqueous monomer solution is taken as 100 mol %. In this case, the salt of the neutralized acid groups is preferably at least one monovalent salt selected from alkali metal salts, ammonium salts, and amine salts. The salt is more preferably an alkali metal salt, even more preferably at least one salt selected from sodium salts, lithium salts, and potassium salts, and particularly preferably a sodium salt.

[0082] The adjustment of the neutralization rate may be carried out before the start of the polymerization reaction of the monomer, or may be carried out during the polymerization reaction of the monomer. The adjustment of the neutralization rate may be carried out at a selected stage between before the start of the polymerization reaction and during the polymerization reaction, or may be carried out at a plurality of stages. In the production method according to the present invention, the adjustment of the neutralization rate is carried out before the end of the polymerization step, as will be described later. If the step of adjusting the neutralization rate after the end of the polymerization reaction is included, the neutralization rate for each particle becomes non-uniform, resulting in the generation of portions with high and low pH, which may cause problems such as rough skin when using a sanitary material containing the water-absorbing agent.

[0083] In the production method according to the present invention, from the viewpoint of the neutralization index and water absorption performance of the water-absorbing agent, it is preferable that the neutralization rate of the aqueous monomer solution, in other words, the neutralization rate of the monomer before the start of the polymerization reaction, and the neutralization rate of the hydrogel, water-absorbent resin, or water-absorbing agent obtained after the polymerization step are substantially the same. In other words, it is preferable that the production method according to the present invention does not include a step of substantially changing the neutralization rate of the crosslinked polymer constituting the water-absorbent resin after the polymerization step. According to this production method having such a configuration, the acid groups contained in the water-absorbent resin are uniformly neutralized, thereby efficiently producing a water-absorbent agent having a neutralization index within a predetermined range. The term "substantially the same neutralization rate" means that the change in the neutralization rate of the hydrogel, water-absorbent resin, or water-absorbent agent after the polymerization step relative to the neutralization rate of the aqueous monomer solution is within 20 mol%. The change is preferably within 10 mol%, more preferably within 5 mol%, and particularly preferably within 2 mol%. The water-absorbent resin after the polymerization step includes both non-surface-crosslinked and surface-crosslinked water-absorbent resins. The neutralization rates of the hydrogel, the water-absorbent resin, and the water-absorbing agent after the polymerization step can be measured by the method described in the Examples.

[0084] In the production method of the present invention, any of the above-exemplified acid group-containing monomers may be used alone, or two or more of the acid group-containing monomers may be used in appropriate mixture. Furthermore, as long as the object of the present invention is achieved, other monomers may also be mixed. As the other monomer, a monomer capable of polymerizing with the acid group-containing monomer to form a copolymer may be used in combination with the acid group-containing monomer to prepare a hydrogel composed of such a copolymer. The other monomer is not particularly limited, and examples thereof include amide group-containing unsaturated monomers such as (meth)acrylamide, N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; amino group-containing unsaturated monomers such as N,N-dimethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide; mercapto group-containing unsaturated monomers; phenolic hydroxyl group-containing unsaturated monomers; and lactam group-containing unsaturated monomers such as N-vinylpyrrolidone. As the other monomer, one type of monomer may be used, or two or more types of monomers may be used.

[0085] (Internal Crosslinking Agent) In the method for producing a water-absorbing agent according to the present invention, the aqueous monomer solution may contain an internal crosslinking agent, if necessary. Known internal crosslinking agents can be used as the internal crosslinking agent. 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 tri(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol 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, 1,4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, and glycidyl (meth)acrylate. Among these, one or more internal crosslinking agents can be used in consideration of reactivity. Among these internal crosslinking agents, it is preferable to use a compound having two or more polymerizable unsaturated groups in one molecule.

[0086] In addition, when two or more types of the internal cross-linking agents are used in combination, the internal cross-linking structure can be changed by changing the reactivity of the functional groups. Therefore, it is preferable to select and use in combination two or more types of internal cross-linking agents having different functional groups from the compounds such as amide compounds, (meth)acrylate compounds, allyl compounds, amine compounds, imine compounds, alcohol compounds, carbonate compounds, and glycidyl compounds.

[0087] The amount of the internal crosslinking agent used is preferably 0.001 mol % to 5 mol %, more preferably 0.005 mol % to 3 mol %, based on the total amount of the monomers. When two or more internal crosslinking agents are used in combination, the amount of each internal crosslinking agent used is preferably 0.001 mol % to 5 mol %, more preferably 0.005 mol % to 3 mol %, based on the total amount of the monomers.

[0088] By setting the amount of the internal cross-linking agent used within the above range, a water-absorbing agent having desired water absorption performance can be obtained. On the other hand, if the amount used is outside the above range, there is a risk that the gel strength will decrease, resulting in an increase in the water-soluble content and / or a decrease in the water absorption capacity.

[0089] As a method for using the internal cross-linking agent, a method is preferably adopted in which the internal cross-linking agent is added in advance when preparing a monomer aqueous solution, and a cross-linking reaction (cross-linking polymerization) is carried out simultaneously with the polymerization reaction.However, the method is not limited to the above method, and a method may also be adopted in which the polymerization reaction is started without adding an internal cross-linking agent to the monomer aqueous solution, and the internal cross-linking agent is added during or after the polymerization reaction to carry out cross-linking.Furthermore, as a method for using the internal cross-linking agent, these methods may be used in combination.

[0090] (Other Substances Added to Monomer Aqueous Solution or Hydrogel) From the viewpoint of improving the physical properties of the water-absorbing agent, the following substances can be added as other substances to the monomer aqueous solution or hydrogel at one or more of the following points: during the preparation of the above-mentioned monomer aqueous solution, during crosslinking polymerization, and after crosslinking polymerization. Examples of the other substances include hydrophilic polymers such as starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol (PVA), polyacrylic acid (salt), and crosslinked polyacrylic acid (salt) compounds; foaming agents such as carbonates, azo compounds, and bubbles, surfactants, chelating agents, and compounds such as chain transfer agents. The other substances may be one type of substance or a mixture of two or more types of substances.

[0091] 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 (the lower limit is 0% by mass), relative to the aqueous monomer solution or hydrogel. 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 (the lower limit is 0% by mass), relative to the aqueous monomer solution or hydrogel.

[0092] When a water-soluble resin or a water-absorbent resin is added as the hydrophilic polymer, a graft polymer or a water-absorbent resin composition (for example, a starch-acrylic acid polymer, a PVA-acrylic acid polymer, etc.) can be obtained as a reaction product. These graft polymers and water-absorbent resin compositions are also included in the scope of the present invention.

[0093] (Monomer Concentration in Aqueous Monomer Solution) In this step, various substances described above are selected depending on the purpose, and a predetermined amount satisfying the above range is mixed with water or a mixed solvent of water and a hydrophilic solvent to prepare an aqueous monomer solution. The aqueous monomer solution is preferably a monomer solution obtained by dissolving a substance such as a monomer mainly composed of the acid group-containing monomer in water. From the viewpoint of productivity of the water-absorbing agent, the concentration of the monomer in the aqueous monomer solution is preferably 35% by mass or more, more preferably 38% by mass or more, and even more preferably 41% by mass or more, relative to the mass of the aqueous monomer solution. From the viewpoint of the physical properties of the water-absorbing agent, the concentration of the monomer in the aqueous monomer solution is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less, relative to the mass of the aqueous monomer solution. The "monomer concentration in the aqueous monomer solution" refers to the concentration of the total amount of monomer components constituting the water-absorbent resin, and does not include the internal crosslinking agent and the other substances.

[0094] The concentration of the monomer is calculated from the following formula (3). In the following formula (3), the mass of the monomer means the total mass of the monomer components that are polymerized to form the water-absorbent resin, and the mass of the internal crosslinking agent, and the mass of the graft component and / or the hydrophilic polymer are not included in the mass of the monomer. The mass of the aqueous monomer solution does not include the mass of the graft component and / or the hydrophilic polymer, and the mass of the hydrophobic organic solvent in the reversed-phase suspension polymerization. Therefore, the mass of the aqueous monomer solution means the total mass obtained by adding the mass of the internal crosslinking agent, the mass of the polymerization initiator described below, and the mass of water in the aqueous monomer solution to the total mass of the monomer components.

[0095] Monomer concentration (mass %)=(mass (total amount) of monomers) / (mass of aqueous monomer solution)×100 (3) [2-3] Polymerization step This step is a step of polymerizing the aqueous monomer solution obtained in the aqueous monomer solution preparation step to obtain a hydrogel.

[0096] (Polymerization initiator) The aqueous monomer solution may contain a polymerization initiator. Examples of this polymerization initiator include a thermally decomposable polymerization initiator, a photodecomposable polymerization initiator, or a redox-based polymerization initiator used in combination with a reducing agent that promotes the decomposition of these polymerization initiators. For example, polymerization initiators described in U.S. Pat. No. 7,265,190 and the like are applicable. One or more polymerization initiators are selected from these, taking into consideration the polymerization form and the like. From the viewpoints of the handleability of the polymerization initiator and the physical properties of the water-absorbing agent, preferably a peroxide or an azo compound is selected, more preferably a peroxide, and even more preferably a persulfate.

[0097] The amount of the polymerization initiator used is preferably 0.001 mol % to 1 mol %, more preferably 0.001 mol % to 0.5 mol %, based on the total amount of monomers. When the reducing agent is used, the amount thereof is preferably 0.0001 mol % to 0.02 mol %, based on the total amount of monomers. By setting the amounts of the polymerization initiator and the reducing agent used within the above ranges, a water-absorbing agent having desired water-absorbing performance can be obtained.

[0098] The polymerization reaction may be initiated by irradiation with active energy rays such as radiation, electron beams, and ultraviolet rays instead of the polymerization initiator. The active energy rays may also be used in combination with the polymerization initiator.

[0099] (Polymerization form) Examples of the polymerization form in the polymerization step include aqueous solution polymerization, reversed-phase suspension polymerization, spray polymerization, bulk polymerization, and precipitation polymerization. From the viewpoints of controllability of polymerization, water absorption performance of the water-absorbing agent, productivity, etc., aqueous solution polymerization or reversed-phase suspension polymerization is preferred as the polymerization form.

[0100] As the aqueous solution polymerization, continuous aqueous solution polymerization is more preferably selected. Specific examples of the continuous aqueous solution polymerization include continuous belt polymerization described in U.S. Pat. No. 4,893,999 and continuous kneader polymerization described in U.S. Pat. No. 6,987,151. By these continuous aqueous solution polymerizations, the water-absorbing agent can be produced with high productivity.

[0101] Preferred forms of the continuous aqueous solution polymerization include "high-temperature initiation polymerization," "high-concentration polymerization," and "foaming polymerization." These polymerization forms can be carried out alone or in combination. "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 (the upper limit is the boiling point of the aqueous monomer solution). "High-concentration polymerization" refers to a polymerization form in which the concentration of the monomer in the aqueous monomer solution 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 45% by mass or higher (the upper limit is the saturated concentration). "Foaming polymerization" refers to a polymerization form in which bubbles (particularly an inert gas, as described below) are dispersed in the aqueous monomer solution.

[0102] Reverse-phase suspension polymerization is a polymerization method in which the monomer aqueous solution is polymerized in a state where it is suspended in a hydrophobic organic solvent as droplets with a particle size of 0.01 mm to 1 mm, and can be performed in a batch or continuous manner. Reverse-phase suspension polymerization has the advantage that a hydrogel with a fine particle size is obtained simultaneously with the polymerization, thereby eliminating the need for the gel crushing step. Note that the reverse-phase suspension polymerization described in U.S. Pat. No. 4,093,776 and the like can also be applied to the present invention.

[0103] Each of the above polymerization forms can be carried out in an air atmosphere. However, from the viewpoint of preventing coloration of the obtained water-absorbing agent, the polymerization step is preferably carried out in an inert gas atmosphere such as nitrogen or argon (for example, an oxygen concentration of 1% by volume or less). More preferably, the dissolved oxygen in the aqueous monomer solution is also sufficiently replaced with an inert gas in advance. For example, before carrying out the polymerization step, a treatment for replacing the dissolved oxygen with an inert gas can be carried out so that the aqueous monomer solution has a dissolved oxygen content of less than 1 mg / L.

[0104] (Polymerization Rate) The polymerization rate of the hydrogel obtained in this step is preferably 70% by mass or more from the viewpoint of preventing or reducing excessive fusion and aggregation of the hydrogel in the drying step and improving the physical properties of the water-absorbing agent. The polymerization rate is preferably 80% by mass or more, 90% by mass or more, 95% by mass or more, 98% by mass or more, and 99% by mass or more, in that order, and most preferably 99.5% by mass or more. The theoretical upper limit of the polymerization rate is 100% by mass. By keeping the polymerization rate within the above range, aggregation of the hydrogel in the drying step and / or clumping of the dried product can be further reduced. The polymerization time can be adjusted using the polymerization rate as an indicator.

[0105] The neutralization rate of the hydrogel obtained in this step is preferably substantially the same as the neutralization rate of the aqueous monomer solution, as described above. Therefore, the neutralization rate of the hydrogel is preferably 50 mol% or more, more preferably 60 mol% or more, and even more preferably 70 mol% or more. The neutralization rate of the hydrogel is 100 mol% or less, and from the same viewpoint, preferably 80 mol% or less, more preferably 78 mol% or less, and even more preferably 75 mol% or less. When the neutralization rate is within the above range, the obtained water-absorbing agent is uniformly neutralized, and the neutralization index of the water-absorbing agent, which will be described later, can be efficiently set to 15 or less.

[0106] [2-4] Gel Crushing Step This step is a step of crushing the hydrogel obtained in the polymerization step to obtain a particulate hydrogel. The particulate hydrogel is also a type of hydrogel. The gel crushing refers to granulating the hydrogel using a gel crusher such as a kneader, meat chopper, or cutter mill. Regarding the embodiments and conditions of the gel crushing, for example, the contents described in International Publication No. 2011 / 126079 and the like can be applied to the present invention.

[0107] When kneader polymerization is selected as the polymerization form, gel pulverization is carried out in the polymerization step, and a particulate hydrogel is produced. When reversed-phase suspension polymerization is selected as the polymerization form, a particulate hydrogel is also produced in the polymerization process. Therefore, there are cases where the gel pulverization step is not carried out after the polymerization step by kneader polymerization or reversed-phase suspension polymerization. In this case, the particulate hydrogel obtained by kneader polymerization or reversed-phase suspension polymerization is directly subjected to the drying step described later. On the other hand, when the gel pulverization step is carried out, the particulate hydrogel obtained in this step is subjected to the drying step described later.

[0108] (Shape of particulate hydrogel) The shape of the hydrogel subjected to the drying step is not particularly limited. For example, when the hydrogel subjected to the drying step is a particulate hydrogel obtained through a series of steps including a gel crushing step or a particulate hydrogel obtained by kneader polymerization, both of these particulate hydrogels have a shape with a fracture surface formed by gel crushing. On the other hand, when the hydrogel subjected to the drying step is a particulate hydrogel obtained by reversed-phase suspension polymerization, the shape of the particulate hydrogel is a spherical shape or an aggregate (granulated product) thereof.

[0109] (Solid content of hydrogel) The solid content of the particulate hydrogel subjected to the drying step is preferably high from the viewpoint of drying costs. Furthermore, when the solid content is high, the dicarboxylic acid (salt) of the present invention added to the hydrogel is prevented or reduced from penetrating into the hydrogel, and is therefore arranged near the surface of the hydrogel. As a result, gel dispersibility is improved. Therefore, the solid content is preferably high from the viewpoint of improving gel dispersibility. Specifically, the solid content is preferably 40% by mass or more, more preferably 42% by mass or more, even more preferably 44% by mass or more, and particularly preferably 46% by mass or more, based on the total mass of the hydrogel subjected to the drying step.

[0110] Furthermore, from the viewpoint of gel dispersibility and reducing the load on a gel crusher when the manufacturing method of the present invention includes a gel crushing step, the solid content is preferably 70% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less.

[0111] The effects of the present invention become significant by supplying a hydrogel having a solid content within the above range to the drying step described below. Note that the "solid content of the hydrogel" refers to a value calculated from loss on drying (the change in mass when 2.0 g of a sample is dried at 180°C for 24 hours).

[0112] When measuring the solid content of the hydrogel, the water content of the hydrogel is also measured at the same time. Specifically, the solid content and water content can be measured by the methods described in the Examples.

[0113] (Solid content-equivalent mass average particle diameter of particulate hydrogel) (Solid D50) The solid content-equivalent mass average particle diameter of the particulate hydrogel subjected to the drying step (hereinafter referred to as "Solid D50") is preferably 2000 μm or less, more preferably 1000 μm or less, still more preferably 600 μm or less, particularly preferably 300 μm or less, from the viewpoint of improving drying efficiency and the water absorption rate of the water-absorbing agent. The solid content-equivalent mass average particle diameter (Solid D50) of the particulate hydrogel corresponds to the mass average particle diameter of the particulate hydrogel after drying.

[0114] In addition, from the viewpoint of the physical properties of the obtained water-absorbing agent, the Solid D50 is preferably 25 μm or more, more preferably 50 μm or more, and even more preferably 75 μm or more.

[0115] When the Solid D50 is within the above range, the gel dispersibility of the hydrogel containing the dicarboxylic acid (salt) of the present invention is improved, and the hydrogel is easily fluidized by stirring, etc., thereby improving drying efficiency. Therefore, the Solid D50 is preferably within the above range.

[0116] The Solid D50 is measured for the particulate hydrogel immediately before it is subjected to the drying step. When the production method of the present invention includes the gel crushing step, the Solid D50 is measured after the gel crushing step and before the drying step. When the particulate hydrogel is an aggregate of a plurality of particles, the Solid D50 of each particle (hereinafter referred to as "primary particle") forming the aggregate is measured.

[0117] The method for measuring the Solid D50 can be the method described in the section <Particle diameter of particulate hydrogel> in the Examples.

[0118] (Gel Temperature) The temperature of the hydrogel subjected to the drying step (hereinafter referred to as "gel temperature") is preferably 30°C or higher, more preferably 40°C or higher, 50°C or higher, and most preferably 55°C or higher, in that order.

[0119] The gel temperature is preferably 120° C. or lower, more preferably 100° C. or lower, 90° C. or lower, 80° C. or lower, and most preferably 70° C. or lower.

[0120] By setting the gel temperature within the above temperature range, the miscibility of the hydrogel with the dicarboxylic acid (salt) of the present invention, as well as the gel dispersibility and fluidity of the hydrogel are improved, so that when stirring and drying is performed in the drying step, the stirring and drying can be performed more efficiently.

[0121] The gel temperature can be measured by bringing a sensor part of a contact thermometer into contact with the hydrogel immediately before the hydrogel is subjected to the drying step. The gel temperature refers to the temperature at the time when the temperature measured by the above-mentioned method becomes stable.

[0122] As described above, when kneader polymerization is selected as the polymerization mode, gel pulverization is performed in the polymerization step. On the other hand, when the polymerization mode in the polymerization step is not kneader polymerization, the polymerization step and the gel pulverization step are separate steps, and the gel pulverization step is performed after the polymerization step.

[0123] In addition, from the viewpoint of the physical properties of the obtained water-absorbing agent, it is preferable that the gel-crushing step is carried out after the polymerization step, and it is more preferable that the gel-crushing step is carried out as a step separate from the polymerization step.

[0124] As described above, when a polymerization form other than reversed-phase suspension polymerization is adopted as the polymerization form in the polymerization step, it is preferable that the production method of the present invention includes the gel crushing step and that the dicarboxylic acid (salt) of the present invention is added to the obtained particulate hydrogel.

[0125] This allows the dicarboxylic acid (salt) of the present invention to be localized near the surface of the hydrogel to be dried in the drying step, thereby further improving the dispersibility of the gel during drying.

[0126] On the other hand, when the polymerization form in the polymerization step is reversed-phase suspension polymerization, a hydrogel having a desired particle size can be prepared in the polymerization step, and the obtained hydrogel can be directly subjected to the drying step without carrying out the gel crushing step. When the production method of the present invention does not include the gel crushing step, it is preferable to adopt reversed-phase suspension polymerization in the polymerization step and add the dicarboxylic acid (salt) of the present invention to the hydrogel after the polymerization step, from the viewpoint of improving gel dispersibility during drying. Thereby, the dicarboxylic acid (salt) of the present invention is localized near the surface of the hydrogel to be dried in the drying step, thereby further improving gel dispersibility during drying.

[0127] [2-5] Drying Step This step is a step of drying the hydrogel obtained through the above steps to obtain a dried product with a desired solid content. The "solid content" refers to a value calculated from the loss on drying (the change in mass when 1.0 g of a sample is dried at 180°C for 3 hours).

[0128] In the present invention, the solid content of the dried product is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 85% by mass to 99% by mass, particularly preferably 90% by mass to 99% by mass, and most preferably 92% by mass to 99% by mass, based on the total mass of the dried product.

[0129] (Drying Temperature and Drying Time) From the viewpoint of drying speed, the drying temperature is preferably 150° C. or higher, more preferably 160° C. or higher, even more preferably 170° C. or higher, and particularly preferably 180° C. or higher. Moreover, from the viewpoint of deterioration and discoloration of the dried product and performance of the water-absorbing agent, the drying temperature is preferably 250° C. or lower, more preferably 230° C. or lower, and even more preferably 200° C. or lower.

[0130] The drying temperature is usually determined by the temperature of the heat medium used in the drying step. For example, in the case of hot air drying, the drying temperature is determined by the temperature of the hot air. However, in the case of microwave drying or the like, which cannot be determined by the temperature of the heat medium, the drying temperature is determined by the temperature of the hydrogel subjected to the drying step. The drying temperature may be constant throughout the drying step, or may be changed as appropriate during the drying step.

[0131] The drying time is preferably 10 to 120 minutes, more preferably 20 to 90 minutes, and even more preferably 20 to 60 minutes. By drying the hydrogel in the presence of the dicarboxylic acid (salt) of the present invention, the drying time can be shortened without excessively increasing the drying temperature. Therefore, a water-absorbing agent with high physical properties that is less deteriorated by heating can be obtained.

[0132] (Drying Method) The method for drying the hydrogel is not particularly limited, and examples thereof include known methods such as conductive heat transfer drying, convective heat transfer drying (hot air drying), radiative heat transfer drying, reduced pressure drying, drying by azeotropic dehydration with a hydrophobic organic solvent, and superheated steam drying using high-temperature steam.

[0133] Furthermore, when the drying method of the hydrogel is classified based on the movement of the material to be dried (the hydrogel) during drying, there are agitation drying, static drying, material transfer type drying, hot air conveying type drying, and the like.

[0134] The agitation drying is a drying method in which the object to be dried (in the production method of the present invention, the hydrogel) is dried while being agitated in a predetermined device, and is also referred to as material agitation drying. The static drying is a method in which the object to be dried is dried in a static state in a predetermined device, and is also referred to as material static drying. The material transfer drying is a method in which the object to be dried is transferred using a conveyor or the like in air having a predetermined temperature and humidity, and the object is brought into contact with the air or the like during the transfer, thereby drying the object. The hot air transfer drying is a method in which the object to be dried is transferred using hot air and dried by the hot air during the transfer. Any method can be used as the drying method. On the other hand, agitation drying is most suitable from the viewpoint of improving drying efficiency by reducing adhesion of the hydrogel to the drying device, aggregation of the hydrogels and / or clumping of the dried product.

[0135] Therefore, the most preferred drying method is agitation drying.

[0136] During the agitation drying, adjacent particles constituting the hydrogel are successively replaced, so that certain particles successively come into contact with new particles. As a result, the hydrogel is uniformly mixed with the dicarboxylic acid (salt) of the present invention and heated by direct and / or indirect heat transfer. The dicarboxylic acid (salt) of the present invention effectively reduces aggregation of the hydrogel and / or clumping of the dried product caused by this successive contact between new particles, thereby further enhancing the drying effect of agitation. This improves drying efficiency.

[0137] With regard to the embodiments and conditions of the stirring and drying, for example, the contents described in WO 2018 / 092864, WO 2018 / 092863, etc. can be applied to the present invention.

[0138] (Drying Apparatus) The drying apparatus used in the drying step is not particularly limited as long as it can dry the hydrogel to be subjected to the drying step, and any known drying apparatus can be used. When the agitation drying is used, the drying apparatus should be an agitation type drying apparatus capable of the agitation drying, and should have a configuration in which the material to be dried in the drying apparatus is fluidized by agitation means such as an agitating blade and / or a rotating cylinder.

[0139] Examples of the agitation type drying apparatus include a single- or twin-shaft disk dryer, a single- or twin-shaft paddle dryer, a rotary dryer, a rotary kiln, and a tube dryer.

[0140] Specifically, agitation-type drying devices such as a solid air dryer (manufactured by Hosokawa Micron Corporation), a CD dryer (manufactured by Kurimoto Iron Works, Ltd.), a paddle dryer (manufactured by Nara Machinery Works, Ltd.), a steam tube dryer (manufactured by Tsukishima Kikai Co., Ltd.), a rotary kiln (manufactured by Kurimoto Iron Works, Ltd.), and a rotary dryer (manufactured by Okawara Manufacturing Co., Ltd.) can be used.

[0141] In the single-shaft or twin-shaft paddle dryer, particles are agitated by the rotation of the agitator plate, so that particles replace each other and come into frequent contact with the heat transfer surface. This allows for efficient drying. In the rotary kiln or rotary dryer, the hydrogel is placed inside a cylinder. As the cylinder rotates, the hydrogel moves inside the cylinder, replacing adjacent particles, resulting in efficient drying. In other words, the particles revolving around the cylinder move to a high position and then fall downward due to gravity. As a result, adjacent particles are replaced.

[0142] The number of the drying apparatuses may be one or two or more. A combination of multiple drying apparatuses with different specifications or configurations may be used. The agitator drying apparatus may also be used in combination with another drying apparatus that is not classified as an agitator drying apparatus. It is preferable that at least one of the multiple drying apparatuses is the agitator drying apparatus, and the type and number of drying apparatuses to be combined with this agitator drying apparatus are not limited.

[0143] When multiple drying devices are used, the timing of switching between devices can be determined using the solid content of the material to be dried as an indicator. For example, when the solid content is used as an indicator, the following configurations can be used: - Drying to a solid content of about 70% by mass in the first drying device, and then switching to a second drying device to dry to the desired solid content; - Drying to a solid content of about 70% by mass in the first drying device, further drying to a solid content of about 85% by mass in the second drying device, and then switching to a third drying device to dry to the desired solid content.

[0144] When the agitation type dryer is combined with another dryer, it is preferable to use the agitation type dryer at a stage where the solid content is low.

[0145] When the agitation drying apparatus is a mechanical agitation dryer that agitates the particulate hydrogel, which is the material to be dried, with a rotating shaft equipped with agitating blades such as an arm, a blade, a paddle, and a cut disk (CD), the rotation speed of the rotating shaft is set appropriately depending on the apparatus. The rotation speed of the rotating shaft is, for example, usually 5 rpm to 1000 rpm. In the case of a cylindrical drying apparatus such as a rotary kiln, the rotation speed of the cylinder is set appropriately depending on the type and size of the apparatus. The rotation speed of the cylinder is, for example, usually 1 rpm to 50 rpm.

[0146] The drying device may also have a function of maintaining the interior of the device at normal pressure or reduced pressure. When maintaining the interior of the device at reduced pressure, the degree of reduction relative to atmospheric pressure is preferably more than 0 kPa and not more than 5 kPa, more preferably 0.1 kPa to 2 kPa, and even more preferably 0.1 kPa to 0.5 kPa.

[0147] By setting the degree of vacuum within the above range, efficient drying can be achieved, and aggregation of the hydrogel and / or clumping of the dried product can be further reduced. Note that the "degree of vacuum relative to atmospheric pressure" refers to the pressure difference from atmospheric pressure, and pressures lower than atmospheric pressure are expressed as positive (plus) values. For example, when atmospheric pressure is standard atmospheric pressure (101.3 kPa) and the degree of vacuum relative to atmospheric pressure is 10 kPa, the air pressure inside the apparatus is 101.3 kPa - 10 kPa = 91.3 kPa.

[0148] The drying device may also be provided with a function for introducing a carrier gas into the device. Examples of the carrier gas include air (including dry air), nitrogen, water vapor, and mixtures thereof. From the viewpoint of ease of handling, dry air, nitrogen, water vapor, and mixtures thereof are preferably used. The amount of the carrier gas introduced is determined arbitrarily based on the amount of water evaporation per unit time and the desired properties of the exhaust gas, such as the dew point and amount of volatile components.

[0149] A highly humid mixed gas can also be used as the carrier gas. In the present application, the highly humid mixed gas refers to a mixed gas containing water vapor. Preferably, the highly humid mixed gas is introduced into the agitation drying device and circulated for use. The temperature of the highly humid mixed gas is preferably 105°C or higher, more preferably 120°C or higher, and even more preferably 140°C or higher. The upper limit of the temperature of the highly humid mixed gas is preferably 250°C or lower, more preferably 200°C or lower. The dew point of the highly humid mixed gas is preferably 60°C or higher, more preferably 70°C or higher, even more preferably 80°C or higher, and particularly preferably 90°C or higher.

[0150] Conventionally, to prevent or reduce deterioration of the physical properties and discoloration of the water-absorbing agent due to oxygen, it was necessary to dry the hydrogel under an inert gas atmosphere such as nitrogen or argon, which resulted in increased costs. By introducing the high-humidity mixed gas into the drying device, the inside of the drying device can be kept in a low-oxygen state even if the amount of inert gas used is reduced or even without using an inert gas. As a result, improvement of the physical properties of the water-absorbing agent and prevention or reduction of discoloration are achieved. Furthermore, by introducing the high-humidity mixed gas into the drying device, the increase in costs due to the use of inert gas can be avoided. Furthermore, introducing the high-humidity mixed gas into the drying device contributes to preventing or reducing aggregation of the hydrogel during drying and / or clumping of the dried product. For these reasons, it is preferable to introduce the high-humidity mixed gas into the drying device in the drying process.

[0151] The "low-oxygen state" means a state in which the oxygen concentration in the drying device is low, and particularly means a state in which the oxygen concentration in the atmosphere in the drying chamber that houses the hydrogel is low. The oxygen concentration in the atmosphere in the drying chamber is preferably 15% by volume or less, more preferably 10% by volume or less, even more preferably 5% by volume or less, and particularly preferably 1% by volume or less. The lower limit of the oxygen concentration in the atmosphere in the drying chamber is preferably 0% by volume. Alternatively, the oxygen concentration in the atmosphere in the drying chamber may be about 0.1% by volume.

[0152] [2-6] Pulverization step, classification step This step is an optional step in which the dried product obtained in the drying step is pulverized (pulverization step), and adjusted to particles having a particle size and particle size distribution in a desired range (classification step), thereby obtaining water-absorbent resin particles described later. Note that the water-absorbent resin particles also include a water-absorbent resin that is not surface-crosslinked.

[0153] In addition, when the manufacturing method of the present invention does not include the pulverization step and the classification step, the dried product obtained in the drying step may be directly subjected to the surface cross-linking step described below as a water-absorbent resin that is not surface-cross-linked. In addition, when the manufacturing method of the present invention does not include the surface cross-linking step in addition to the pulverization step and the classification step, the dried product may be directly treated as a water-absorbent resin that constitutes a water-absorbing agent. In addition, the pulverization step differs from the gel pulverization step shown in [2-4] above in that the dried product to be pulverized has undergone a drying step.

[0154] In the production method of the present invention, the hydrogel containing the dicarboxylic acid (salt) of the present invention is dried in the drying step, so that the pulverization step and the classification step may be unnecessary. In that case, the fine powder is mainly generated in the pulverization step, so that the amount of fine powder generated can be effectively reduced. Furthermore, even when the pulverization step is performed, the load required for pulverization can be reduced, so that the amount of fine powder generated, which may cause a decrease in the performance of the obtained water-absorbing agent and products containing the water-absorbing agent, can be effectively reduced.

[0155] Examples of the equipment (crusher) used in the crushing step include high-speed rotary crushers such as roll mills, hammer mills, screw mills, and pin mills, vibration mills, knuckle-type crushers, and cylindrical mixers. A roll mill is preferably selected as the crusher because it allows for easy control of the particle size distribution of crushed particles.

[0156] The method for adjusting the particle size and particle size distribution in the classification step is not particularly limited, and preferably, sieve classification using a JIS standard sieve (JIS Z8801-1 (2000)), air classification, or the like is employed.

[0157] From the viewpoint of classification efficiency, sieve classification is more preferably adopted as a method for adjusting the particle size and particle size distribution in the classification step. More preferably, not only the dried product after the drying step but also the water-absorbing agent as a final product, the water-absorbent resin not subjected to a surface-crosslinking step, and / or the surface-crosslinked water-absorbent resin are subjected to the present steps (pulverization step, classification step) so that the particle size distribution satisfies a preferred range described below.

[0158] The particle size distribution can be appropriately adjusted not only in the present steps (pulverization step and classification step) but also in the polymerization step, particularly in the polymerization step carried out by employing reversed-phase suspension polymerization, and in the granulation step and fine powder removal step described below.

[0159] [2-7] Surface cross-linking step This step is an optional step of carrying out a cross-linking reaction in a surface layer portion of the water absorbent resin that is not surface-cross-linked. Preferably, this surface cross-linking step is a step of further providing a portion with a high cross-linking density in the surface layer portion. The surface layer portion is a portion from the surface of the water absorbent resin that is not surface-cross-linked to a depth of several tens of μm inside.

[0160] Preferably, the surface cross-linking step includes a heat treatment step. More preferably, the surface cross-linking step includes a mixing step, a heat treatment step, and a cooling step. A surface-cross-linked water-absorbent resin is obtained by performing radical cross-linking, monomer polymerization, a cross-linking reaction with a surface cross-linking agent, or the like in the surface layer portion. Note that, when the production method of the present invention includes the pulverization step and the classification step but does not include the surface cross-linking step, the non-surface-cross-linked water-absorbent resin can be treated as it is as a water-absorbent resin constituting a water-absorbing agent. Furthermore, when stirring and drying are performed in the drying step, a surface cross-linking agent may be added in the drying step, as described in International Publication Nos. 2018 / 092864 and 2018 / 092863, and the like, and the drying step and the surface cross-linking step may be performed simultaneously.

[0161] [2-7-1] Mixing step This step is a step of mixing the non-surface-crosslinked water absorbent resin that has been subjected to the drying step, and optionally the pulverization step and the classification step, and a surface crosslinking agent in a mixer to obtain a mixture.

[0162] (Moisture Content of the Non-Surface-Crosslinked Water Absorbent Resin) The moisture content of the non-surface-crosslinked water absorbent resin is preferably 20% by mass or less, more preferably 1% by mass to 15% by mass, even more preferably 1% by mass to 10% by mass, particularly preferably 1% by mass to 8% by mass, from the viewpoint of mixability with a surface crosslinking agent described later. The moisture content is measured immediately before being subjected to a mixing step.

[0163] (Surface Crosslinking Agent) As the surface crosslinking agent, known surface crosslinking agents can be applied, and for example, the surface crosslinking agents described in U.S. Patent No. 7,183,456 etc. are applied. One or more surface crosslinking agents are selected from these surface crosslinking agents in consideration of reactivity etc.

[0164] From the viewpoint of the handling property of the surface cross-linking agent and the water absorption property of the water absorbing agent, an organic compound having two or more functional groups that react with the carboxyl group of the water absorbent resin to form a covalent bond is preferably selected. Preferred surface cross-linking agents include, for example, polyhydric alcohol compounds, epoxy compounds, polyvalent amine compounds, condensates of polyvalent amine compounds and haloepoxy compounds, oxazoline compounds, oxazolidinone compounds, alkylene carbonate compounds, polyhydric glycidyl compounds, oxetane compounds, vinyl ether compounds and cyclic urea compounds.

[0165] The amount of the surface cross-linking agent used is preferably 0.001 to 10 parts by mass, more preferably 0.005 to 5 parts by mass, relative to 100 parts by mass of the non-surface-cross-linked water absorbent resin. By setting the amount of the surface cross-linking agent used within the above range, a good cross-linked structure is formed in the surface layer part of the non-surface-cross-linked water absorbent resin, and a water absorbent agent with high physical properties is finally obtained. When two or more types of surface cross-linking agents are used, the total amount thereof is preferably within the above range.

[0166] The surface crosslinking agent is preferably added as an aqueous solution (hereinafter referred to as "surface crosslinking agent solution") to the non-surface-crosslinked water absorbent resin. In this case, the mass of water in the surface crosslinking agent solution is preferably 0.1 parts by mass to 20 parts by mass, more preferably 0.3 parts by mass to 15 parts by mass, and still more preferably 0.5 parts by mass to 10 parts by mass, relative to 100 parts by mass of the non-surface-crosslinked water absorbent resin. By setting the amount of water used within the above range, the handleability of the surface crosslinking agent solution can be improved, and the surface crosslinking agent can be uniformly mixed with the non-surface-crosslinked water absorbent resin.

[0167] (Mixing method) The method of mixing the non-surface-crosslinked water absorbent resin and the surface crosslinking agent solution is not particularly limited, and a known method can be adopted. A preferred mixing method includes, for example, a method of spraying or dropping the surface crosslinking agent solution onto the non-surface-crosslinked water absorbent resin to mix them. A more preferred mixing method is a method of spraying the surface crosslinking agent solution onto the non-surface-crosslinked water absorbent resin to mix them.

[0168]

[0033] A mixer for carrying out the mixing method preferably has a large mixing power in order to uniformly and reliably mix the non-surface-crosslinked water absorbent resin and the surface crosslinking agent. As the mixer, a high-speed stirring mixer is preferable, and a high-speed stirring continuous mixer is more preferable. Specifically, the mixer is one or more selected from the group consisting of a cylindrical mixer, a double-walled conical mixer, a V-shaped mixer, a ribbon mixer, a screw mixer, a fluidized furnace rotary disk mixer, an airflow mixer, a double-arm kneader, an internal mixer, a pulverizer kneader, a rotary mixer, a screw extruder, a turbulizer, and the like.

[0169] The temperature of the non-surface-crosslinked water absorbent resin to be subjected to the mixing method is preferably 35° C. to 80° C., more preferably 35° C. to 70° C., and further preferably 35° C. to 60° C., from the viewpoint of mixability with the surface crosslinking agent solution, coagulation property of the mixture, etc. Further, the mixing time is preferably 1 second to 1 hour, and more preferably 5 seconds to 10 minutes.

[0170] [2-7-2] Heat Treatment Step This step is a step of heat treating the mixture obtained in the mixing step to obtain a surface-crosslinked water-absorbent resin.

[0171] (Heat Treatment Method) Examples of the method for heating the mixture include a method for heating the mixture in a static state and a method for heating the mixture while stirring and mixing. From the viewpoint of uniformly heating the entire mixture, it is preferable to use a method for heating the mixture while stirring and mixing. In addition, preferred examples of the apparatus used in the heat treatment step include a paddle dryer, a multi-fin processor, and a tower dryer.

[0172] The heating temperature in the heat treatment method may be set depending on the type and amount of the surface cross-linking agent used, the water absorption performance of the target water-absorbing agent, etc. The heating temperature is preferably 150°C to 250°C, more preferably 180°C to 210°C, as the temperature of the heat medium. The heating time is preferably 1 minute to 2 hours. Examples of the combination of the heating temperature and the heating time include a combination of 180°C for 0.1 to 1.5 hours and a combination of 200°C for 0.1 to 1 hour.

[0173] [2-7-3] Cooling step This step is an optional step that is performed as necessary after the heat treatment step. In this step, the water absorbent resin after the heat treatment step is forcibly cooled, and therefore, the surface cross-linking reaction proceeding in the surface layer portion thereof is quickly terminated.

[0174] (Cooling method) Examples of the method for cooling the water absorbent resin after the heat treatment step include a method for cooling the water absorbent resin after the heat treatment step in a stationary state, and a method for cooling under stirring and mixing. From the viewpoint of uniformly and quickly cooling the entire water absorbent resin after the heat treatment step, it is preferable to adopt a method for cooling under stirring and mixing as the method for cooling the water absorbent resin after the heat treatment step. Note that, preferred examples of the apparatus used in the cooling step include a paddle dryer, a multi-fin processor, and a tower dryer.

[0175] The cooling temperature in the cooling step may be set depending on the heat treatment temperature, the water absorption performance of the target water-absorbing agent, etc. The cooling temperature is preferably 40°C to 100°C, more preferably 40°C to 90°C, and even more preferably 40°C to 70°C as the temperature of the refrigerant.

[0176] [2-8] Addition step This step is an optional step carried out for the purpose of imparting various additional functions to the water-absorbing agent and / or improving the water-absorbing performance of the water-absorbing agent. In this step, various additives are added to the water-absorbing resin.

[0177] The additives are not particularly limited, and examples thereof include chelating agents, organic reducing agents, inorganic reducing agents, oxidizing agents, hydroxycarboxylic acid compounds, surfactants, compounds having phosphorus atoms, organic powders such as metal soaps, deodorants, antibacterial agents, pulp and thermoplastic fibers, polyvalent metal salts, cationic polymers, and inorganic fine particles.

[0178] From the viewpoint of the effect of addition, it is preferable that the additive is present on the surface of the water absorbent resin. Therefore, this addition step is preferably carried out simultaneously with the surface cross-linking step or after the surface cross-linking step. More preferably, the addition step is carried out after the surface cross-linking step.

[0179] [2-9] Other Steps In addition to the steps described above, at least one step selected from sizing, fine powder removal, granulation, fine powder recycling, iron removal, etc. may be added as needed. In addition, at least one step selected from transportation, storage, packaging, and keeping may also be included.

[0180] The "sizing" step includes a fine powder removal step subsequent to the surface cross-linking step, and / or a step of pulverizing and classifying the water absorbent resin when the water absorbent resin aggregates and exceeds a desired size. The "fine powder recycling" step includes a step of adding the fine powder as it is, and / or a step of forming a large hydrogel from the fine powder and adding it to any of the steps.

[0181] [2-10] Physical Properties of Water-Absorbing Agent As the physical properties of the water-absorbing agent produced by the production method of the present invention, preferred physical property values ​​of the water-absorbing agent according to one embodiment of the present invention, which will be described later, can be adopted.

[0182] [2-11] Production Method According to Another Embodiment of the Present Invention A production method according to another embodiment of the present invention is a production method for a water-absorbing agent according to the present invention, which further comprises the following constitutions (A) and (B); (A) the neutralization rate of the aqueous monomer solution is set within the preferred range described in the section “[2-2] Step of preparing aqueous monomer solution”, for example, 60 to 80 mol%; (B) the production method does not include a step of substantially changing the neutralization rate of a crosslinked polymer constituting a water-absorbent resin after the polymerization step.

[0183] A production method according to another embodiment of the present invention further comprises the above-described configurations (A) and (B), thereby appropriately controlling the neutralization index of the produced water-absorbing agent. Here, "appropriately controlling the neutralization index" means that the neutralization index is controlled to be within a preferred range described in the section "(3-7) Neutralization index of water-absorbing agents and water-absorbent resins" described later, for example, 10 or less. Therefore, the production method can produce a water-absorbing agent that prevents a large amount of fine powder from being generated, reduces odor, has an appropriately controlled neutralization index, and has a reduced rewet amount after water absorption.

[0184] [3] Water-absorbing agent A water-absorbing agent according to one embodiment of the present invention (hereinafter referred to as "water-absorbing agent of the present invention") comprises a water-absorbing resin, and a dicarboxylic acid represented by the following formula (1) and / or a salt thereof, wherein the content of the dicarboxylic acid represented by the formula (1) and / or the salt thereof is in the range of 100 to 3000 ppm relative to the solid content of the water-absorbing resin, and the neutralization index is 15 or less.

[0185] (In formula (1), n ​​is 1 or 2, and R is a linear or branched alkyl group having 8 to 20 carbon atoms, or a linear or branched alkenyl group having 8 to 20 carbon atoms.) (The neutralization index means the number of water-absorbing agents having a neutralization rate that is 20 mol % or more lower than the average neutralization rate of the water-absorbing agents, out of 200 water-absorbing agents.) The water-absorbing agent of the present invention has a content of the dicarboxylic acid and / or its salt represented by formula (1), i.e., the dicarboxylic acid (salt) of the present invention, in the range of 100 to 3000 ppm. Therefore, as described above, the amount of odorous substances that can be generated by decomposition or volatilization of the dicarboxylic acid (salt) of the present invention is reduced, and therefore the water-absorbing agent of the present invention is excellent in reducing odor, particularly in reducing odor generated during swelling. Furthermore, as described above, the amount of decrease in surface tension caused by the dicarboxylic acid (salt) of the present invention and the amount of return when pressure is applied after water absorption, which are caused by the decrease in surface tension, are reduced. Furthermore, as described above, the water-absorbing agent of the present invention has excellent productivity since the amount of fine powder generated during production is reduced.

[0186] Specifically, the content of the dicarboxylic acid (salt) of the present invention is 3000 ppm or less, more preferably 1000 ppm or less, and even more preferably 500 ppm or less, relative to the solid content of the water-absorbing resin. The lower limit of the content is 100 ppm or more, preferably 200 ppm or more. The content of the dicarboxylic acid (salt) refers to a concentration when the total amount is regarded as a dicarboxylic acid (acid-type compound) in which both carboxyl groups in one dicarboxylic acid molecule are unneutralized. The content of the dicarboxylic acid (salt) may be measured by extracting the dicarboxylic acid (salt) from the obtained water-absorbing agent and appropriately measuring it using various analytical techniques, or may be calculated from the amount of the dicarboxylic acid (salt) added in the production process.

[0187] The water-absorbing agent of the present invention is a water-absorbing agent that can be produced by the production method of the present invention. The water-absorbing resin included in the water-absorbing agent of the present invention is the non-surface-crosslinked water-absorbing resin or the surface-crosslinked water-absorbing resin.

[0188] The water-absorbing resin constituting the water-absorbing agent of the present invention may preferably be composed of an unsaturated monomer having an acid group and / or its salt, more preferably an unsaturated monomer having a carboxyl group and / or its salt, and even more preferably a crosslinked polymer of acrylic acid and / or its salt. Hereinafter, "an unsaturated monomer having an acid group and / or its salt" will be referred to as "an acid group-containing monomer (salt)," "an unsaturated monomer having a carboxyl group and / or its salt" will be referred to as "a carboxyl group-containing monomer (salt)," and "acrylic acid and / or its salt" will be referred to as "acrylic acid (salt)." Furthermore, the water-absorbing resin may preferably contain "a crosslinked polymer having an acid group-containing monomer (salt) as the main component," more preferably "a crosslinked polymer having a carboxyl group-containing monomer (salt) as the main component," and even more preferably "a polyacrylic acid (salt)." In this application, a "crosslinked polymer having an acid group-containing monomer (salt) as its main component" refers to a crosslinked polymer that contains, as a repeating unit, a structure derived from the acid group-containing monomer (salt) as its main component, and optionally contains a structure derived from an internal crosslinking agent. In this application, a "crosslinked polymer having, as a repeating unit, a structure derived from the carboxyl group-containing monomer (salt)" refers to a crosslinked polymer that contains, as a repeating unit, a structure derived from the carboxyl group-containing monomer (salt) as its main component, and optionally contains a structure derived from an internal crosslinking agent. In this application, a "polyacrylic acid (salt)" refers to polyacrylic acid and / or a salt thereof. In particular, a "polyacrylic acid (salt)" refers to a crosslinked polymer that contains, as a repeating unit, a structure derived from acrylic acid (salt) as its main component, and optionally contains a structure derived from an internal crosslinking agent.

[0189] The "main component" refers to a monomer whose amount (content) is 50 mol% or more, preferably 70 mol% or more, more preferably 90 mol% or more, and particularly preferably substantially 100 mol% relative to the total amount of monomers involved in the polymerization reaction. Theoretically, the upper limit of the content of the main component monomer relative to the total amount of monomers involved in the polymerization reaction is 100 mol%.

[0190]

[0113] The "acid group-containing monomer (salt)", the "carboxyl group-containing monomer (salt)" or the "acrylic acid (salt)" which constitutes the water absorbent resin in one embodiment of the present invention preferably contains a monovalent salt, more preferably an alkali metal salt or an ammonium salt, further preferably an alkali metal salt, particularly preferably a sodium salt.

[0191] When the water-absorbing agent of the present invention is used for absorbents in absorbent articles such as disposable diapers, it is preferable that, among the physical properties listed below, CRC, AAP, color tone (white balance), surface tension, and water absorption rate (vortex) are controlled within desired ranges. Furthermore, after satisfying these physical properties, at least one, preferably two or more, and more preferably all of Ext (water-soluble content), PSD (particle size distribution), D50 (mass-average particle diameter), σζ (logarithmic standard deviation of particle size distribution), and Moisture Content (moisture content) are controlled within desired ranges.

[0192] It is preferable that these physical properties satisfy the following ranges in order to ensure sufficient performance in high-concentration disposable diapers. The following physical properties are measured by the methods described in the examples.

[0193] [3-1] CRC (Centrifuge Retention Capacity) of Water-Absorbing Agents and Water-Absorbing Resins (NWSP 241.0.R2(15)) "CRC" is an abbreviation for Centrifuge Retention Capacity, and means the water absorption capacity under no pressure. In this specification, CRC may also be referred to as "water absorption capacity." Specifically, the CRC of a water-absorbing agent refers to the water absorption capacity (unit: g / g) after 0.2 g of the water-absorbing agent is placed in a nonwoven fabric bag, immersed in a large excess of 0.9% by mass sodium chloride aqueous solution for 30 minutes to allow free swelling, and then drained in a centrifuge (250 G) for 3 minutes.

[0194] The CRC of the water-absorbing agent of the present invention is usually 5 g / g or more, preferably 10 g / g or more, more preferably 20 g / g or more, even more preferably 25 g / g or more, and particularly preferably 45 g / g or more. In consideration of the balance with other physical properties, the CRC is preferably 60 g / g or less, more preferably 58 g / g or less, and even more preferably 55 g / g or less.

[0195] Therefore, the preferred range of the CRC may be appropriately selected within the above range, for example, 10 g / g to 55 g / g, 25 g / g to 60 g / g, or 20 g / g to 55 g / g.

[0196] A water-absorbing agent having a CRC within the above range has a large absorption amount and a high absorption rate of body fluids such as urine and blood, and is therefore suitable as an absorbent for absorbent articles such as disposable diapers, etc. The CRC is controlled by changing the types and / or amounts of an internal cross-linking agent and a surface cross-linking agent used during production.

[0197] The CRC of the water-absorbing resin can be measured in the same manner as in the above-mentioned method for measuring the CRC of a water-absorbing agent, by replacing the water-absorbing agent with a water-absorbing resin. The CRC of the water-absorbing resin contained in the water-absorbing agent of the present invention is preferably within the same range as the CRC of the above-mentioned water-absorbing agent.

[0198] [3-2] AAP (0.3 psi) (absorption capacity under pressure) of water-absorbing agents and water-absorbing resins (NWSP 242.0.R2 (15)) "AAP" is an abbreviation for Absorption Against Pressure, and means the water absorption capacity under pressure. Specifically, the AAP of a water-absorbing agent refers to the water absorption capacity (unit: g / g) after 0.9 g of the water-absorbing agent is swollen under load for 1 hour in a large excess of a 0.9 wt% aqueous sodium chloride solution. AAP indicates the pressure conditions and is expressed as "AAP 0.7 psi (or 4.83 kPa)" and "AAP 0.3 psi (or 2.06 kPa)". The "0.7 psi" written immediately after the AAP indicates the magnitude of the pressure applied during measurement. In this application, measurements were carried out under a pressure of 2.06 kPa (0.3 psi) as in the examples described later. Also, NWSP 242.0.R2(15) refers to this as "Absorption Under Pressure," but this is essentially the same.

[0199] The AAP of the water-absorbing agent of the present invention is preferably 10 g / g or more, more preferably 20 g / g or more, and even more preferably 29 g / g or more. In addition, taking into consideration the balance with other physical properties, the AAP is preferably 50 g / g or less, more preferably 45 g / g or less, and even more preferably 40 g / g or less. As described above, in the present application, the AAP is the AAP measured under the condition of 2.06 kPa, i.e., 0.3 psi, and is a parameter generally referred to as "AAP 0.3 psi".

[0200] A water-absorbing agent having the AAP within the above range is suitable as an absorbent for absorbent articles such as disposable diapers, because the amount of liquid returning to the absorbent article is small when pressure is applied after absorbing water in the absorbent article such as disposable diapers, etc. The AAP is controlled by changing the particle size and particle size distribution of the water-absorbing agent, the surface cross-linking agent, etc.

[0201] The AAP of the water-absorbent resin can be measured in the same manner as in the above-mentioned method for measuring the AAP of a water-absorbing agent, by replacing the water-absorbing agent with a water-absorbent resin. The AAP of the water-absorbent resin contained in the water-absorbing agent of the present invention is preferably within the same range as the AAP of the above-mentioned water-absorbing agent.

[0202] [3-3] PSD (particle size distribution) (NWSP 220.0.R2(15)), D50 (mass average particle diameter) and σζ (logarithmic standard deviation of particle size distribution) of water-absorbing agents and water-absorbing resins The PSD means the particle size distribution of the water-absorbing agent, which is measured as a result of carrying out sieve classification on the water-absorbing agent. The D50 and σζ are measured by carrying out the same method as the method described in "(3) Mass average particle diameter (D50) and logarithmic standard deviation of particle size distribution" in columns 27 to 28 of U.S. Pat. No. 7,638,570 on the water-absorbing agent.

[0203] The PSD (particle size distribution), D50 (mass average particle diameter) and σζ (logarithmic standard deviation of particle size distribution) of the water-absorbing agent of the present invention are preferably within the following ranges, respectively.

[0204] The PSD, D50 and σζ of the water-absorbing resin can be measured in the same manner as in the above-mentioned method for measuring the PSD, D50 and σζ of the water-absorbing agent, by replacing the water-absorbing agent with a water-absorbing resin. The PSD, D50 and σζ of the water-absorbing resin contained in the water-absorbing agent of the present invention are preferably within the same ranges as the PSD, D50 and σζ of the water-absorbing agent shown below.

[0205] Specifically, the PSD means that the proportion of particles having a particle diameter of less than 150 μm in the water-absorbing agent of the present invention is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less, relative to the mass of all particles constituting the water-absorbing agent of the present invention. Furthermore, the PSD also means that the proportion of particles having a particle diameter of 850 μm or more in the water-absorbing agent of the present invention is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less, relative to the mass of all particles constituting the water-absorbing agent of the present invention. The lower the lower limit of the proportion of these particles, the better, in either case. 0% by mass is desirable, and approximately 0.1% by mass may also be acceptable. Having the proportion of these particles within the above range contributes to improving the physical properties of the water-absorbing agent of the present invention.

[0206] The water-absorbing agent of the present invention has a D50 (mass average particle diameter) of preferably 200 μm to 600 μm, more preferably 200 μm to 550 μm, even more preferably 250 μm to 500 μm, and particularly preferably 350 μm to 450 μm. Furthermore, the water-absorbing agent of the present invention has a σζ (logarithmic standard deviation of particle size distribution) of preferably 0.20 to 0.50, more preferably 0.25 to 0.40, and even more preferably 0.27 to 0.35.

[0207] [3-4] Moisture Content and Solid Content of Water-Absorbing Agent, Water-Absorbing Resin, and Hydrogel (NWSP 230.0.R2(15)) In the present invention, the moisture content of the water-absorbing agent is measured under the conditions in accordance with NWSP, except that the mass of the water-absorbing agent as a measurement sample is changed to 1.0 g, the drying temperature is changed to 180°C, and the drying time is changed to 24 hours. Specifically, the moisture content and solid content of the water-absorbing agent can be determined according to the description in the Examples. Furthermore, the solid content α (mass%) of the water-absorbing agent can be determined from the moisture content (mass%) of the water-absorbing agent by the following formula (3).

[0208] Solid content α (mass %) = 100 - water content (mass %) (3) Furthermore, the water content and solid content of the hydrogel refer to the water content and solid content of the hydrogel before drying. The water content and solid content of the hydrogel can be measured after the polymerization step and before the drying step. Therefore, the water content and solid content of the hydrogel may be the water content and solid content of the hydrogel obtained in the polymerization step and not subjected to the gel-crushing step, or the water content and solid content of the hydrogel obtained in the gel-crushing step, i.e., the particulate hydrogel.

[0209] In the present invention, the water content and solid content of the hydrogel are measured in the same manner as in the above-mentioned method for measuring the water content of a water-absorbing agent, except that the mass of the measurement sample is changed to about 2.0 g, the drying temperature is changed to 180°C, and the drying time is changed to 24 hours, and the other conditions comply with NWSP. The solid content of the hydrogel is also determined according to the above-mentioned formula (3).

[0210] The water content and solid content of the water-absorbing resin can be measured in the same manner as above by replacing the water-absorbing agent with the water-absorbing resin in the method for measuring the water content and solid content of the water-absorbing agent. The water content of the water-absorbing resin contained in the water-absorbing agent of the present invention is also preferably within the same range as the water content of the water-absorbing agent shown below.

[0211] The moisture content of the water-absorbing agent of the present invention is preferably more than 0 mass % and 15 mass % or less, more preferably 1 mass % to 13 mass %, even more preferably 2 mass % to 10 mass %, and particularly preferably 2 mass % to 9 mass %.

[0212] The water-absorbing agent of the present invention preferably has a moisture content within the above range, from the viewpoint of improving powder properties (e.g., flowability, transportability, damage resistance, etc.). The moisture content is controlled in a drying step, a surface cross-linking step, an adding step, etc.

[0213] [3-5] Surface tension of water-absorbing agent and water-absorbing resin The surface tension of the water-absorbing agent of the present invention is preferably 50 mN / m or more, more preferably 55 mN / m or more, and even more preferably 60 mN / m or more. When the surface tension is 50 mN / m or more, the water-absorbing agent of the present invention can further reduce the amount of return when pressure is applied after absorbing water and becoming swollen. On the other hand, the surface tension of the water-absorbing agent of the present invention may be 75 mN / m or less, and preferably 73 mN / m or less. The surface tension can be measured, for example, by the method described in the Examples.

[0214] The surface tension of the water-absorbing resin can be measured in the same manner as the surface tension of the water-absorbing agent. The surface tension of the water-absorbing resin contained in the water-absorbing agent of the present invention is also preferably within the same range as the surface tension of the water-absorbing agent.

[0215] [3-6] Water Absorption Rate (Vortex) of Water-Absorbing Agent The water absorption rate of the water-absorbing agent of the present invention is expressed, for example, as a parameter measured by a method generally referred to as the Vortex method: water absorption rate (Vortex): unit [seconds]. The smaller the value of the water absorption rate, the faster and more excellent the water absorption rate. The water absorption rate (Vortex) is preferably smaller, and the upper limit is preferably less than 25 seconds, more preferably 20 seconds or less, even more preferably 18 seconds or less, and particularly preferably 16 seconds or less. The lower limit of the water absorption rate (Vortex) is usually 5 seconds or more, preferably 10 seconds or more. The water absorption rate (Vortex) can be measured by a known method, for example, by the method described in the examples.

[0216] The water absorption rate of the water-absorbing resin can be measured in the same manner as above by replacing the water-absorbing agent with a water-absorbing resin in the method for measuring the water absorption rate of the water-absorbing agent. The water absorption rate of the water-absorbing resin contained in the water-absorbing agent of the present invention is also preferably within the same range as the water absorption rate of the above-mentioned water-absorbing agent.

[0217] [3-7] Neutralization Index of Water-Absorbing Agent and Water-Absorbing Resin In this specification, the neutralization index refers to the number of water-absorbing agents or water-absorbent resins having a neutralization rate that is 20 mol% or more lower than the average neutralization rate of the water-absorbing agent or water-absorbent resin, out of 200 particles of the water-absorbing agent or water-absorbent resin. The neutralization index of the water-absorbing agent of the present invention is 15 or less, preferably 10 or less, more preferably 5 or less, and particularly preferably 0. The higher the neutralization index, the more non-uniform the neutralization of acid groups contained in the water-absorbing agent or water-absorbent resin, i.e., the particulate hydrogel and its dried product, is. The closer to 0 the neutralization index, the more uniform the neutralization. According to the water-absorbing agent of the present invention, by having the neutralization index of 15 or less, it is possible to prevent or reduce a decrease in water-absorbing performance due to non-uniform surface treatment of the particulate hydrogel and its dried product. If the neutralization index of the water-absorbing agent of the present invention is outside the above range, the interaction between the dicarboxylic acid contained in the water-absorbing agent and the water-absorbing agent becomes non-uniform, and the desired effect may not be obtained. In this specification, the neutralization index means the number of water-absorbing agents having a neutralization rate lower than the average neutralization rate of the water-absorbing agents by 20 mol % or more among 200 water-absorbing agents. The neutralization index of the water-absorbing agent or water-absorbing resin is measured by the method described in Examples.

[0218] The neutralization index of the water-absorbing resin of the present invention is also preferably within the same range as the neutralization index of the above-mentioned water-absorbing agent.

[0219] [3-8] Neutralization Ratio of Water-Absorbing Agent and Water-Absorbent Resin The water-absorbing agent of the present invention has a neutralization index of 15 or less, and is therefore a uniformly neutralized water-absorbing agent. Therefore, the neutralization ratio of the water-absorbing agent can be set within a preferred range. Specifically, the neutralization ratio of the water-absorbing agent is preferably 50 mol% or more, more preferably 60 mol% or more, and even more preferably 70 mol% or more. Furthermore, the neutralization ratio of the water-absorbing agent is 100 mol% or less, preferably 80 mol% or less, more preferably 78 mol% or less, and even more preferably 75 mol% or less. It is preferable that the neutralization ratio of the water-absorbing agent is within the preferred range from the viewpoint of the water-absorbing performance of the water-absorbing agent.

[0220] The neutralization rate of the water-absorbing resin is preferably within the same range as the neutralization rate of the water-absorbing agent. The neutralization rates of the water-absorbing resin and the water-absorbing agent can be measured by the method described in the Examples.

[0221] [3-9] Water-absorbing agent according to another embodiment of the present invention A water-absorbing agent according to another embodiment of the present invention is a water-absorbing agent according to the present invention, which further comprises a constitution in which the neutralization index is within the appropriate range, which is the preferred range described in the section "[3-7] Neutralization index of water-absorbing agent and water-absorbing resin," for example, 10 or less. The water-absorbing agent according to another embodiment of the present invention further comprises the constitution, and thereby has better absorption performance. Therefore, the water-absorbing agent has excellent productivity due to a reduced content of fine powder, and has excellent absorption performance, and is also excellent in reducing odor and the amount of rewet after absorbing water.

[0222] [4] Uses of Water-Absorbing Agent The water-absorbing agent of the present invention is preferably used for absorbents of absorbent articles such as disposable diapers, sanitary napkins and incontinence pads, packaging supplies such as drip sheets for foods and the like, industrial water-stopping agents, and agricultural and horticultural water-retaining agents.

[0223] In particular, it can be used as an absorbent for high-concentration disposable diapers (disposable diapers in which a large amount is used per diaper) which have been problematic due to odor, coloration, etc. derived from raw materials. By using the water-absorbing agent of the present invention in the upper layer part of the absorbent, remarkable effects can be expected.

[0224] In addition to the water-absorbing agent, an absorbent material such as pulp fiber can also be used as the absorbent. In this case, the content of the water-absorbing agent in the absorbent (core concentration) is preferably 30% by mass to 100% by mass, more preferably 40% by mass to 100% by mass, even more preferably 50% by mass to 100% by mass, still more preferably 60% by mass to 100% by mass, particularly preferably 70% by mass to 100% by mass, and most preferably 75% by mass to 95% by mass, relative to the mass of the entire absorbent.

[0225] When an absorbent having a core concentration within the above range is used in the upper layer of an absorbent article, the absorbent article maintains a clean white color. Furthermore, since this absorbent article has excellent diffusibility of body fluids such as urine and blood, efficient liquid distribution is expected to improve absorption capacity.

[0226] An embodiment of the present invention may include the following inventions [1] to

[12] .

[0227] [1] A method for producing a water-absorbing agent, comprising: a polymerization step of polymerizing an aqueous monomer solution to obtain a hydrogel; and a drying step of drying the hydrogel to obtain a dried product, wherein a neutralization rate of the aqueous monomer solution is 50 mol % or more, and a dicarboxylic acid represented by the following formula (1) and / or a salt thereof is added to the aqueous monomer solution and / or the hydrogel before the completion of the drying step, and an amount of the dicarboxylic acid and / or the salt thereof added is within a range of 100 to 3000 ppm with respect to a solid content contained in the aqueous monomer solution or a solid content of the hydrogel.

[0228] (In formula (1), n ​​is 1 or 2, and R is a linear or branched alkyl group having 8 to 20 carbon atoms, or a linear or branched alkenyl group having 8 to 20 carbon atoms.) [2] The method for producing a water-absorbing agent according to [1], wherein a concentration of the monomer in the aqueous monomer solution is 35 mass % or more with respect to a mass of the aqueous monomer solution.

[0229] [3] The method for producing a water-absorbing agent according to [1] or [2], further comprising a gel crushing step of crushing the hydrogel to obtain a particulate hydrogel.

[0230] [4] The method for producing a water-absorbing agent according to [3], wherein the dicarboxylic acid represented by the formula (1) and / or the salt thereof is added to the hydrogel after the polymerization step is completed and before the gel-pulverization step is completed.

[0231] [5] The method for producing a water-absorbing agent according to any one of [1] to [4], wherein the solid content of the hydrogel subjected to the drying step is 40 mass % or more with respect to the total mass of the hydrogel subjected to the drying step.

[0232] [6] The method for producing a water-absorbing agent according to any one of [1] to [5], wherein the drying step is a step of drying the hydrogel by stirring and drying.

[0233] [7] The method for producing a water-absorbing agent according to any one of [1] to [6], wherein the drying step is a step of drying the hydrogel under conditions where the drying temperature is 150°C or higher.

[0234] [8] The method for producing a water-absorbing agent according to any one of [1] to [7], wherein the surface tension of the water-absorbing agent is 50 mN / m or more.

[0235] [9] A water-absorbing agent comprising a water-absorbent resin and a dicarboxylic acid represented by the following formula (1) and / or a salt thereof, wherein the content of the dicarboxylic acid represented by the formula (1) and / or the salt thereof is within a range of 100 to 3000 ppm relative to the solid content of the water-absorbent resin, and the water-absorbing agent has a neutralization index of 15 or less.

[0236] (In formula (1), n ​​is 1 or 2, and R is a linear or branched alkyl group having 8 to 20 carbon atoms, or a linear or branched alkenyl group having 8 to 20 carbon atoms.) (The neutralization index means the number of water absorbing agents having a neutralization rate that is 20 mol % or more lower than the average neutralization rate of the water absorbing agents, out of 200 water absorbing agents.)

[10] The water absorbing agent according to [9], which has a surface tension of 50 mN / m or more.

[0237]

[11] The water-absorbing agent according to [9] or

[10] , which has a water-absorbing speed (Vortex) of physiological saline solution of less than 25 seconds.

[0238]

[12] The water-absorbing agent according to any one of [9] to

[11] , wherein the water-absorbing resin contains a crosslinked polymer containing an acid group-containing monomer (salt) as a main component.

[0239] Furthermore, one embodiment of the present invention may also include the inventions shown in [9'] to [11'] below.

[0240] [9'] A water-absorbing agent comprising a water-absorbent resin and a dicarboxylic acid represented by the following formula (1) and / or a salt thereof, wherein the content of the dicarboxylic acid represented by the formula (1) and / or the salt thereof is within a range of 100 to 3000 ppm relative to the solid content of the water-absorbent resin.

[0241] (In the formula (1), n ​​is 1 or 2, and R is a linear or branched alkyl group having 8 to 20 carbon atoms, or a linear or branched alkenyl group having 8 to 20 carbon atoms.) [10'] The water-absorbing agent according to [9], which has a surface tension of 50 mN / m or more.

[0242] [11'] The water-absorbing agent according to [9'] or [10'], wherein the water-absorbing speed (Vortex) of physiological saline is less than 25 seconds.

[0243] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0244] The present invention will be explained in more detail with reference to the following experimental examples. However, the present invention is not limited to these explanations, and experimental examples obtained by appropriately combining the technical means disclosed in each experimental example are also included in the scope of the present invention.

[0245] Hereinafter, a pulverized product obtained by pulverizing the dried product obtained in the drying step will be referred to as a "water absorbent resin powder", and particles obtained by classifying the pulverized product will be referred to as "water absorbent resin particles". Furthermore, the "water absorbent resin" includes the dried product, a non-surface-crosslinked water absorbent resin powder, a non-surface-crosslinked water absorbent resin particle, a surface-crosslinked water absorbent resin powder, and a surface-crosslinked water absorbent resin particle.

[0246] Unless otherwise noted, the electrical devices used in the experimental examples (including devices for measuring physical properties) were powered by a 60 Hz power supply of 200 V or 100 V. Furthermore, unless otherwise noted, the physical properties of the hydrogel, water-absorbent resin, and water-absorbing agent described below were measured under conditions of room temperature (20°C to 25°C) and a relative humidity of 50% RH ± 10%.

[0247] For convenience, "liter" may be expressed as "l" or "L," and "mass %" or "weight %" may be expressed as "wt%." Furthermore, mass measurements (weighing) were performed using mass meters manufactured by Sartorius, product names: AZ3102 and CPA224S.

[0248] [Measurement and Calculation of Physical Properties] The physical properties of the hydrogels, water-absorbent resins, and water-absorbing agents produced in Examples 1 to 6 and Comparative Examples 1 to 7 were measured and calculated by the following methods.

[0249] <"CRC" (NWSP 241.0.R2(15))> The CRC of the water-absorbent resin particles and the water-absorbing agent was measured in accordance with NWSP 241.0.R2(15). Specifically, 0.2 g of the water-absorbent resin particles or the water-absorbing agent was placed in a nonwoven bag, and then immersed in a large excess of a 0.9% by mass aqueous solution of sodium chloride for 30 minutes to allow free swelling, and then the water was drained using a centrifuge (250 G) for 3 minutes. The mass of the water-absorbent resin particles or the water-absorbing agent after the draining was measured, and the water absorption capacity of the water-absorbent resin particles or the water-absorbing agent was calculated and designated as the CRC (unit: g / g).

[0250] <"AAP 0.3 psi" (NWSP 242.0.R2(15))> The AAP of the water-absorbing agent was measured in accordance with NWSP 242.0.R2(15). Specifically, 0.9 g of the water-absorbing agent was dissolved in a large excess of a 0.9 wt % aqueous solution of sodium chloride for 1 hour at 2.06 kPa (21 g / cm ２ Then, the mass of the water-absorbing agent in the swollen state was measured, and the water absorption capacity was calculated and expressed as AAP (unit: g / g).

[0251] <"PSD" (NWSP 220.0.R2(15))> The PSD of a water-absorbent resin powder and a water-absorbing agent was measured in accordance with NWSP 220.0.R2(15). Specifically, the PSD was measured using a method similar to that described in "(3) Mass-average particle diameter (D50) and logarithmic standard deviation of particle size distribution" in columns 27 to 28 of U.S. Pat. No. 7,638,570.

[0252] <Amount of Fine Powder Generated (150 pass)> In the measurement of PSD, a proportion [mass %] of a mass of a water absorbent resin powder which had passed through a sieve having an opening of 150 µm relative to a total mass of a water absorbent resin powder (hereinafter, also referred to as "150 pass") was calculated.

[0253] <"Solid content rate" (NWSP 230.0.R2(15))> The solid content rate of the water-absorbent resin particles was measured in accordance with NWSP 230.0.R2(15), with the mass of the measurement sample changed to 1.0 g, the drying temperature changed to 180°C, and the drying time changed to 24 hours. The water-absorbent resin particles were used as the measurement sample. Specifically, first, the mass of approximately 1.0 g of the measurement sample was accurately weighed and designated as M (g). Next, the measurement sample M (g) was placed in an aluminum cup having a bottom diameter of 50 mm, and then the total mass W1 (g) of the cup was accurately weighed. Next, the cup was placed in an oven set to a temperature of 180°C. After 24 hours had elapsed, the cup was removed from the oven, and the total mass W2 (g) of the cup containing the measurement sample was accurately weighed. The moisture content (mass%) and solid content α (mass%) of the measurement object were calculated according to the following formulas (2) and (3).

[0254] Water content (mass%) = {(W1 - W2) / M} × 100 (2) Solid content α (mass%) = 100 - water content (mass%) (3) Furthermore, the solid content of the hydrogel obtained in the gel crushing step and immediately before being subjected to the drying step, i.e., the particulate hydrogel, was measured. Specifically, the solid content of the particulate hydrogel was measured by the same method as the method for measuring the solid content of the water-absorbing agent described above, except that the particulate hydrogel was used as a measurement sample instead of the polymeric resin particles and the amount of the measurement sample was changed to about 2.0 g.

[0255] <Water absorption rate (Vortex)> 0.02 parts by weight of Food Blue No. 1, a food additive, was added to 1,000 parts by weight of a previously prepared 0.90% by weight sodium chloride aqueous solution, and the liquid temperature was adjusted to 30 ° C. 50 ml of a blue-colored 0.90% by weight sodium chloride aqueous solution was weighed into a 100 ml beaker, and 2.00 g of water-absorbent resin particles or a water-absorbing agent was added while stirring at 600 rpm with a cylindrical stirrer having a length of 40 mm and a diameter of 8 mm, and the water absorption time (seconds) was measured. The endpoint was measured in accordance with the standard described in JIS K 7224-1996 "Explanation of the water absorption rate test method for superabsorbent resins," and the time until the water-absorbent resin particles or the water-absorbing agent absorbed the physiological saline and the test liquid covered the stirrer tip was taken as the water absorption time (seconds).

[0256] <Particle diameter of particulate hydrogel> (Particle size distribution and mass average particle diameter (Gel D50) of particulate hydrogel) The hydrogel obtained in the gel crushing step and immediately before being subjected to the drying step, i.e., the particle size distribution and mass average particle diameter (Gel D50) of the particulate hydrogel were measured by the following methods.

[0257] 20 g of particulate hydrogel (solid content α% by mass) was added to 1000 g of a 20% by mass aqueous solution of sodium chloride containing 0.08% by mass of polyoxyethylene (3) sodium lauryl sulfate (surfactant) (hereinafter referred to as "surfactant aqueous solution") to prepare a dispersion. The dispersion was stirred at 300 rpm for 16 hours using a 50 mm long x 7 mm diameter stirrer tip. The container used was a cylindrical polypropylene container (height 21 cm, diameter 8 cm, internal volume approximately 1.14 L).

[0258] After stirring the dispersion, the dispersion was poured into the center of a JIS standard sieve (diameter 21 cm, sieve openings: 8 mm / 4 mm / 2 mm / 1 mm / 0.60 mm / 0.30 mm / 0.15 mm / 0.075 mm) placed on a rotating plate. Subsequently, 100 g of an aqueous surfactant solution was used to wash out all of the particulate hydrogel remaining in the container onto the sieve. Thereafter, 6000 g of an aqueous surfactant solution was poured into the water injection range (50 cm) from a height of 30 cm above the sieve using a shower (72 holes, liquid volume: 6.0 [L / min]) while rotating the sieve by hand (20 rpm).２ ) was poured evenly over the entire sieve four times. The particulate hydrogel was classified in this manner. The classified particulate hydrogel remaining on the first sieve of the sieves was drained for about 2 minutes, and then its mass was weighed. The second and subsequent sieves were also classified in the same manner, and the classified particulate hydrogel remaining on each sieve after draining was weighed. The type of sieve was appropriately changed depending on the gel particle size of the target particulate hydrogel. For example, when the target particulate hydrogel was fine and clogging occurred on a sieve with a mesh size of 0.15 mm or 0.075 mm, it was replaced with a larger diameter JIS standard sieve (diameter 30 cm, mesh size 0.15 mm, 0.075 mm) and classified.

[0259] From the mass of the particulate hydrogel remaining on each sieve, the ratio X (mass%) of the mass of the particulate hydrogel remaining on each sieve to the total mass of the hydrogel remaining on all sieves was calculated using the following formula (5). The mesh size r (mm) of each sieve used for classification was converted based on the following formula (6). The converted mesh size R (mm) and the ratio X (mass%) of the mass of the particulate hydrogel remaining on each sieve to the total mass of the particulate hydrogel remaining on all sieves were plotted on logarithmic probability paper. The value of the mesh size corresponding to 50% by mass was read and used as the mass average particle size (Gel D50) of the particulate hydrogel.

[0260] X (%) = (w / W) x 100... (5) R (mm) = (20 / W) １／３ ×r (6) In the above formulas (5) and (6), X: mass % (%) of the particulate hydrogel remaining on each sieve after classification and draining; w: mass (g) of each of the particulate hydrogels remaining on each sieve after classification and draining; W: total mass (g) of the particulate hydrogels remaining on all sieves after classification and draining; R: sieve opening (mm) when converted into particulate hydrogel before swelling with the surfactant aqueous solution; and r: sieve opening (mm) used for classification.

[0261] (Solid content-equivalent mass average particle diameter of particulate hydrogel) (Solid D50) The solid content-equivalent mass average particle diameter (Solid D50) of the particulate hydrogel was calculated from the solid content (α) of the particulate hydrogel and the mass average particle diameter (Gel D50) of the particulate hydrogel according to the following formula (7): Solid D50 = Gel D50 × (α / 100) １／３ ... (7) In the above formula (7), Gel D50 is the mass average particle diameter (μm) of the particulate hydrogel, and α is the solid content (mass %) of the particulate hydrogel.

[0262] <Surface Tension> 50 ml of a 0.90 wt % aqueous sodium chloride solution adjusted to 23°C was placed in a thoroughly cleaned 100 ml beaker, and the surface tension of the 0.90 wt % aqueous sodium chloride solution was first measured using a surface tensiometer (High-Performance Surface Tensiometer DY-500, manufactured by Kyowa Interface Science Co., Ltd.). The measured value was 73 mN / m. It is conventionally known that the surface tension of a 0.90 wt % aqueous sodium chloride solution at 23°C is in the range of 71 mN / m to 75 mN / m. Therefore, it was confirmed that there was no problem with the measurement device used this time.

[0263] Next, a thoroughly washed cylindrical stirring bar having a length of 25 mm and 0.500 g of water-absorbent resin particles or water-absorbing agent were placed in a beaker containing 40 ml of the 0.90 wt % sodium chloride aqueous solution after the surface tension measurement, which had been adjusted to 23°C, and the mixture was stirred for 3 minutes at 350 rpm. After 3 minutes, the stirring was stopped and the mixture was allowed to stand for 3 minutes, so that the water-absorbent resin particles or the water-absorbing agent that had absorbed water were allowed to settle. Thereafter, the surface tension of the supernatant liquid in the beaker was measured by the same operation as in the measurement of the surface tension of the 0.90 wt % sodium chloride aqueous solution described above.

[0264] If the water-absorbent resin particles or the water-absorbing agent floated on the liquid surface 2 minutes after the stirring was stopped, the water-absorbent resin particles or the water-absorbing agent floating on the liquid surface were removed using a thoroughly cleaned spatula before the measurement. However, due to the fast water absorption rate or high absorption capacity of the water-absorbent resin particles or the water-absorbing agent, there may be cases where the amount of supernatant liquid necessary for measurement does not remain after the water-absorbent resin particles or the water-absorbing agent settled. In such cases, 40 ml of a 0.90 wt % sodium chloride aqueous solution was appropriately added within the minimum amount necessary for measurement to adjust the amount of supernatant liquid in the beaker, and then the surface tension was measured. Note that the surface tension was measured using a plate method using a platinum plate. The plate was thoroughly washed with deionized water and heated and washed with a gas burner before each measurement.

[0265] <Single Rewet> One gram of the water-absorbing agent was weighed and used as a measurement sample. Additionally, the weight of ten circular filter papers with a diameter of 55 mm was measured in advance and used as the "weight of the filter paper before water absorption." The measurement sample was evenly spread in a 90 mm diameter petri dish, and a predetermined amount of 0.90 wt % sodium chloride aqueous solution was poured on top and allowed to stand for five minutes. Next, the ten filter papers were placed on the absorbed measurement sample, and a 500 g cylindrical weight (bottom diameter 55 mm) was placed on top and pressed for 10 seconds. The cylindrical weight was then removed, and the filter paper was removed and its weight was measured. The amount of the sodium chloride aqueous solution used is usually set to an amount less than the CRC of the water-absorbing agent so that the water-absorbing agent to be measured can absorb the entire amount of the poured sodium chloride aqueous solution. When comparing the single rewet of two or more types of water-absorbing agents, the amount of the sodium chloride aqueous solution used in measuring the single rewet of each is usually the same. Based on this, with reference to the CRC of the water absorbing agent measured in advance, in the measurement of the Rewet alone of the water absorbing agents produced in Example 1 and Comparative Examples 2 and 3 of the present application, the amount of the sodium chloride aqueous solution used was set to 30 g.

[0266] Here, the removed filter paper had absorbed the sodium chloride aqueous solution (return water) that had seeped out of the measurement sample due to pressure. Therefore, the weight of the removed filter paper was designated as the "weight of the filter paper after water absorption." Using the measured "weight of the filter paper after water absorption" and "weight of the filter paper before water absorption," the simple Rewet was calculated according to the following formula (8). The simple Rewet refers to the amount of return of the water-absorbing agent after water absorption. Simple Rewet [g] = Weight of filter paper after water absorption [g] - Weight of filter paper before water absorption [g] (8) <Odor Evaluation> 2 g of the water-absorbing agent was weighed out and used as a sample for odor evaluation. The odor evaluation sample was placed in a 100 ml beaker, and 10 g of a 0.90 wt % sodium chloride aqueous solution was poured on top of it. The opening of the beaker was then sealed with a 10 cm square odor bag (manufactured by Omi Odor Air Service Co., Ltd.). After leaving the beaker to stand for 10 minutes, 10 evaluators evaluated the odor intensity on a 5-point scale, and the average value was used as the odor evaluation result. Note that, as shown below, the smaller the number in the evaluation results, the less odor there is and the better the result.

[0267] Furthermore, in the odor evaluation, the water absorbing agents obtained in Comparative Examples 2 and 3 were used to relatively compare the odor intensities of the target samples. Specifically, the water absorbing agent described in Comparative Example 3 does not contain dipotassium alkenyl succinate, which is one of the causes of odor, and therefore it is clear that it has a weaker odor than the water absorbing agent described in Comparative Example 2, which contains dipotassium alkenyl succinate in an amount equal to or greater than the amount specified in the present invention. Therefore, the water absorbing agent described in Comparative Example 2 was set as the standard for odor intensity "4" on a five-point scale, and the water absorbing agent described in Comparative Example 3 was set as the standard for odor intensity "2," and the odor intensity of the water absorbing agent obtained in each Example was compared to these, thereby carrying out odor evaluation. A detailed explanation of the odor intensity on a five-point scale is as follows. Odor intensity "1": The odor is weaker than that of the water absorbing agent described in Comparative Example 3. Odor intensity "2": The odor is about the same as that of the water absorbing agent described in Comparative Example 3. Odor intensity "3": The odor is stronger than that of the water absorbing agent described in Comparative Example 3, but weaker than that of the water absorbing agent described in Comparative Example 2. Odor intensity "4": Has an odor similar to that of the water absorbing agent described in Comparative Example 2. Odor intensity "5": Has a stronger odor than that of the water absorbing agent described in Comparative Example 2.

[0268] <Neutralization Index> The neutralization index of the water-absorbing agent was determined with reference to JP-A-10-101735 (European Patent Publication No. 0882502).

[0269] Specifically, 200 particles of the water-absorbing agent, classified to 250-600 μm using a JIS standard sieve, were placed in the opening of a 1.6 mm thick plastic plate with a 20 mm x 20 mm opening and a cover glass attached, as shown in FIG. 2 of JP-A-10-101735, and 0.2 ml of deionized water was added. Furthermore, 0.05 ml of a 0.1 wt % methyl red (MR) ethanol solution was added to the swollen gel using a microsyringe, and the coloring of the 200 particles was observed using a pH indicator after 10 minutes. The number of particles (out of 200) with a neutralization rate 20 mol % or more lower than the average neutralization rate of the water-absorbing agent was counted, and the neutralization index of the water-absorbing agent (corresponding to the first neutralization index described in JP-A-10-101735) was determined. The average neutralization rate was calculated from the acid group-containing monomer used as a raw material for the water-absorbing agent and the amount of neutralizer used. A water-absorbing agent having a neutralization rate 20 mol % lower than the average neutralization rate was separately synthesized, and used as a reference sample of colored particles as comparative particles. Particles having a color equivalent to that of the reference sample, or particles colored more deeply than that of the reference sample, were determined to be colored particles. The larger the neutralization index, the more non-uniform the neutralization of the water-absorbent resin powder constituting the water-absorbing agent.

[0270] <Neutralization Rate> (Preparation of Measurement Solution) A 0.90% by mass sodium chloride aqueous solution was prepared as physiological saline. 200.0 g of the physiological saline was weighed into a 250 mL plastic container with a lid (diameter 6 cm x height 9 cm). Next, 1.00 g of a water-absorbing agent or water-absorbent resin was added to the physiological saline and stirred at 500 rpm for 16 hours using a magnetic stirrer with a diameter of 8 mm and a length of 25 mm to obtain an extract. This extract was filtered using a sheet of filter paper (manufactured by Advantec Toyo Co., Ltd., product name: JIS P 3801, No. 2, thickness: 0.26 mm, retention particle size: 5 μm), and 50.0 g of the resulting filtrate was weighed out and used as the measurement solution. When measuring the neutralization rate of hydrogel particles, a measurement solution was prepared in the same manner using 2.0 g of hydrogel particles instead of the water-absorbing agent or water-absorbent resin.

[0271] (Measurement and Calculation of Neutralization Rate) First, physiological saline was titrated with 0.1 N aqueous sodium hydroxide solution to pH 10, and then titrated with 0.1 N hydrochloric acid to pH 2.7 to obtain blank titers (referred to as [bNaOH] and [bHCl], respectively). Next, the same titration procedure was performed on the measurement solution to obtain titers (referred to as [NaOH] and [HCl], respectively). From the obtained titers, the neutralization rate was calculated according to the following formula.

[0272] [Example 1] In the examples and comparative examples, the monomer is composed of acrylic acid and an acrylate salt produced by neutralizing acrylic acid, and the sum of the contents of these is the monomer content when calculating the monomer concentration.

[0273] (Step of Preparing Aqueous Monomer Solution) The following substances were charged into a 2-liter polypropylene container and mixed to prepare an aqueous monomer solution (a1): 422.2 g (5.86 mol) of acrylic acid, 15.01 g (0.049 mol % relative to the monomer) of a 10% by mass aqueous polyethylene glycol diacrylate solution (molecular weight 523) as an internal crosslinking agent, 25.80 g (0.0010 mol % relative to the monomer) of a 0.1% by mass aqueous solution of trisodium diethylenetriaminepentaacetic acid (DTPA 3Na), 173.9 g of a 48.5% by mass aqueous solution of sodium hydroxide, and 368.1 g of deionized water (ion-exchanged water).

[0274] Next, the aqueous monomer solution (a1) was cooled with stirring. When the temperature (liquid temperature) of the aqueous monomer solution (a1) reached 42.5°C, 178.7 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to prepare an aqueous monomer solution (a2). At this time, the temperature of the aqueous monomer solution (a2) rose to 78.5°C due to heat of neutralization.

[0275] (Polymerization Step) Next, while stirring the aqueous monomer solution (a2), 16.5 g of a 4.5 mass% aqueous sodium persulfate solution (0.053 mol% relative to the monomer) was added to the aqueous monomer solution (a2) to obtain an aqueous monomer solution (a'2). Thereafter, the aqueous monomer solution (a'2) (monomer concentration 43 mass%, neutralization rate of acrylic acid 73 mol%) was immediately poured into a stainless steel bat-shaped container (bottom 340 x 340 mm, height 25 mm, inner surface; Teflon (registered trademark) coated) in an open-to-air system. The time from adding the 48.5 mass% aqueous sodium hydroxide solution to the aqueous monomer solution (a1) to pouring the aqueous monomer solution (a'2) into the bat-shaped container was 1 minute. The vat-shaped container was preheated to a surface temperature of 50°C using a hot plate (NEO HOTPLATE HI-1000 / Iuchi Seieido Co., Ltd.), and heating was continued until the end of the polymerization. The time from when the temperature of the aqueous monomer solution (a'2) began to rise (from the start of polymerization) until it reached its maximum temperature (over 100°C) was 10 seconds. A hydrogel (b1) was obtained by the above polymerization reaction.

[0276] (Gel crushing step) The hydrogel (b1) was put into a screw extruder and gel crushing was carried out. As the screw extruder, a meat chopper was used, in which the outer diameter of the screw shaft was 86 mm and a die diameter plate having a diameter of 100 mm, a thickness of 10 mm, and a diameter of 6.4 mm was installed at the tip (extrusion port). Gel crushing (first gel crushing) was carried out while supplying hot water at 80 ° C., water vapor, a 5% by mass aqueous solution of dipotassium alkenyl succinate (manufactured by Kao Corporation, Latemul ASK), and a 2.5% by mass aqueous solution of hydrogen peroxide simultaneously with the hydrogel (b1). As a result, a first particulate hydrogel precursor was obtained.

[0277] Here, the LATEMURU ASK was an aqueous solution of a mixture of dipotassium hexadecenyl succinate and dipotassium octadecenyl succinate. The hot water at 80°C was supplied at 1 mass%, the water vapor at 1 mass%, the dipotassium alkenyl succinate at 0.040 mass% (concentration in terms of alkenyl succinic acid: 0.033 mass%), and the hydrogen peroxide at 0.10 mass% relative to the solid content of the hydrogel (b1).

[0278] Subsequently, using the same method as the first gel crushing, the first particulate hydrogel precursor obtained by the first gel crushing was further gel-crushed (second gel crushing) to obtain a second particulate hydrogel precursor. However, in the second gel crushing, the addition of the dipotassium alkenyl succinate aqueous solution and the hydrogen peroxide aqueous solution was not performed in the first gel crushing. Next, using the same method as the second gel crushing, the second particulate hydrogel precursor was further gel-crushed (third gel crushing) to obtain a particulate hydrogel (c1). The temperature of the hydrogel (c1) was 95 ° C.

[0279] (Drying step of particulate hydrogel) The particulate hydrogel (c1) was dried using a rotary heater equipped with heating tubes. This drying device was equipped with a cylindrical rotary vessel (volume 35 L) having 10 heating tubes extending in the direction of the rotation axis. First, steam of 2.7 MPa (temperature 228.1 ° C) was introduced into each heating tube, and the interior of the rotary vessel (determined by a contact thermometer) was preheated to above 200 ° C., and then the outer wall of the rotary vessel was also sufficiently heated by tracing. Next, the particulate hydrogel (c1) was introduced, and the rotary vessel was rotated so that the Froude number Fr was 0.07. Carrier air at 140 ° C. was supplied into the rotary vessel at 72 L / hr, and drying was performed for 40 minutes. After drying, the dried product was collected from the outlet of the drying device. The collected dried product was designated as dried product (A1). The solid content of the dried product (A1) was 98.6 mass%.

[0280] (Pulverization and classification process of dried product) The dried product (A1) was allowed to cool. The cooled dried product (A1) was supplied to a roll mill with an inter-roll clearance adjusted to 0.3 mm and pulverized to obtain a water-absorbent resin powder (B1). The water-absorbent resin powder (B1) was classified using JIS standard sieves with openings of 850 μm and 150 μm, and the component that passed through the 850 μm sieve but did not pass through the 150 μm sieve was collected to obtain a water-absorbent resin particle (C1).

[0281] (Surface cross-linking step) A surface cross-linking agent solution composed of 0.035 parts by weight of ethylene glycol diglycidyl ether, 1.5 parts by weight of propylene glycol, and 3.5 parts by weight of deionized water was sprayed onto 100 parts by weight of the water-absorbent resin particles (C1), and mixed to obtain a mixture. The obtained mixture was heat-treated at 100°C for 30 minutes to obtain surface-cross-linked water-absorbent resin particles. The surface-cross-linked water-absorbent resin particles were designated as water-absorbing agent (D1).

[0282] Example 2 (Step of Preparing Aqueous Monomer Solution, Polymerization Step) The same operations as in the (step of preparing an aqueous monomer solution) and (polymerization step) of Example 1 were carried out to obtain a hydrogel (b1).

[0283] (Gel crushing step) A particulate hydrogel (c2) was obtained by the same operation as in (Gel crushing step) of Example 1, except that the amount of the aqueous dipotassium alkenyl succinate solution used in the first gel crushing step was changed so that the dipotassium alkenyl succinate was 0.020% by mass (concentration in terms of alkenyl succinic acid: 0.016% by mass) relative to the solid content of the hydrogel (b1).

[0284] (Drying step of particulate hydrogel) A dried product (A2) was obtained by the same operation as in (Drying step of particulate hydrogel) of Example 1, except that the particulate hydrogel (c2) was used instead of the particulate hydrogel (c1). The solid content of the dried product (A2) was 98.2 mass%.

[0285] (Pulverization and classification step of dried material) Except for using the dried material (A2) instead of the dried material (A1), the same operation as in (Pulverization and classification step of dried material) of Example 1 was carried out to obtain a water absorbent resin powder (B2) and water absorbent resin particles (C2). The water absorbent resin particles (C2) were used as they were as a water absorbing agent (D2).

[0286] Example 3 (Preparation of Aqueous Monomer Solution) A monomer aqueous solution (a3) ​​was prepared by the same procedure as in the preparation of the aqueous monomer solution (a1) in Example 1, except that the amount of deionized water (ion-exchanged water) used was changed from 368.1 g to 366.0 g. Subsequently, a monomer aqueous solution (a4) was prepared by the same procedure as in the preparation of the aqueous monomer solution (a2) in Example 1, except that the aqueous monomer solution (a3) ​​was used instead of the aqueous monomer solution (a1). During the preparation of the aqueous monomer solution (a4), the temperature of the aqueous monomer solution (a4) rose to 78.5°C due to heat of neutralization, as with the aqueous monomer solution (a2).

[0287] (Polymerization step) Next, 2.1 g of a 5% by mass aqueous solution of dipotassium alkenyl succinate (Latemul ASK, manufactured by Kao Corporation) and 16.5 g of a 4.5% by mass aqueous solution of sodium persulfate were added to the stirred aqueous solution of monomer (a4) to obtain an aqueous solution of monomer (a'4). The 2.1 g of Latemul ASK corresponds to 0.020% by mass of dipotassium alkenyl succinate (concentration of 0.016% by mass in terms of alkenyl succinic acid) relative to the solid content of the aqueous solution of monomer (a4). Furthermore, the 16.5 g of the 4.5% by mass aqueous solution of sodium persulfate corresponds to 0.053 mol% relative to the monomer. The aqueous solution of monomer (a'4) had a monomer concentration of 43% by mass and a neutralization rate of acrylic acid of 73 mol%.

[0288] A hydrous gel (b3) was obtained by carrying out the same operation as that for producing the hydrous gel (b1) in the vat-type vessel in the (polymerization step) of Example 1, except that the aqueous monomer solution (a'4) was used instead of the aqueous monomer solution (a'2). Note that, as in Example 1, the time from when the temperature of the aqueous monomer solution (a'4) began to rise (from the start of polymerization) until it reached the maximum temperature (over 100°C) was 10 seconds.

[0289] (Gel crushing step) The same operation as in Example 1 (gel crushing step) was carried out, except that the hydrous gel (b1) was replaced with the hydrous gel (b3), and the dipotassium alkenyl succinate aqueous solution was not used in the first gel crushing, to obtain a particulate hydrous gel (c3). Subsequently, the particulate hydrous gel (c3) was placed in an OK bag (Okura Kogyo Co., Ltd.), and a 5% by mass aqueous solution of dipotassium alkenyl succinate (manufactured by Kao Corporation, Latemul ASK) was added and mixed to a concentration of 0.020% by mass (equivalent to 0.016% by mass of alkenyl succinic acid) relative to the solid content of the particulate hydrous gel (c3). As a result, a particulate hydrous gel (c'3) was obtained.

[0290] (Drying step of particulate hydrogel) A dried product (A3) was obtained by the same procedure as in (Drying step of particulate hydrogel) of Example 1, except that the particulate hydrogel (c'3) was used instead of the particulate hydrogel (c1). The solid content of the dried product (A3) was 98.3 mass%.

[0291] (Pulverization and classification step of dried material) Except for using the dried material (A3) instead of the dried material (A1), the same operation as in (Pulverization and classification step of dried material) of Example 1 was carried out to obtain a water absorbent resin powder (B3) and water absorbent resin particles (C3). The water absorbent resin particles (C3) were used as they were as a water absorbing agent (D3).

[0292] Example 4 (Step of Preparing Aqueous Monomer Solution, Polymerization Step) The same operations as in the (step of preparing an aqueous monomer solution) and (polymerization step) of Example 1 were carried out to obtain a hydrogel (b1).

[0293] (Gel Crushing Step) The hydrogel (b1) was crushed using a KRC Kneader (manufactured by Kurimoto Iron Works), a twin-screw kneader equipped with a main body (barrel) incorporating two rotating shafts rotating in the same direction, to obtain a particulate hydrogel (c4). Each rotating shaft was provided with a circular disk. The barrel had a jacket structure, and a gas inlet penetrating the jacket was provided to introduce water vapor into the main body.

[0294] First, a heat medium at 105 ° C. was circulated inside the jacket, and the temperature inside the main body (barrel) was maintained at 105 ° C. Thereafter, the rotation speed was set to 50 rpm, and the hydrogel (b1) was fed into the inlet of the twin-screw kneader. At that time, water at 90 ° C. and a dipotassium alkenyl succinate aqueous solution having a concentration of 5% by mass (manufactured by Kao Corporation, Latemul ASK) were supplied simultaneously with the hydrogel (b1). Subsequently, a hydrogen peroxide aqueous solution having a concentration of 2.5% by mass was supplied from the inlet, and further, water vapor at 0.6 MPa was supplied from the gas inlet. The amount of water supplied at 90 ° C. was 11.8% by mass relative to the solid content of the hydrogel (b1). The amount of water supplied at 0.6 MPa was 9.7% by mass relative to the solid content of the hydrogel (b1). The dipotassium alkenyl succinate and hydrogen peroxide were supplied at 0.040% by mass (concentration of alkenyl succinic acid: 0.033% by mass) and 0.10% by mass, respectively, relative to the solid content of the hydrogel (b1). The diameter D of the disk used for gel crushing was 50 mm, and the minimum clearance between the barrel and the disk was 1 mm (2% of the disk diameter D).

[0295] (Drying step of particulate hydrogel) A dried product (A4) was obtained by the same operation as in (Drying step of particulate hydrogel) of Example 1, except that the particulate hydrogel (c4) was used instead of the particulate hydrogel (c1). The solid content of the dried product (A4) was 98.6 mass%.

[0296] (Pulverization and classification step of dried material) Except for using the dried material (A4) instead of the dried material (A1), the same operation as in (Pulverization and classification step of dried material) of Example 1 was carried out to obtain a water absorbent resin powder (B4) and a water absorbent resin particle (C4). The water absorbent resin particle (C4) was used as a water absorbing agent (D4) as it was.

[0297] [Example 5] (Step of preparing aqueous monomer solution, polymerization step, gel crushing step) The same operations as in Example 1 (step of preparing aqueous monomer solution), (polymerization step), and (gel crushing step) were performed to obtain a particulate hydrogel (c1).

[0298] (Step of drying particulate hydrogel) The particulate hydrogel (c1) was dried using a 5 L capacity agitator dryer (manufactured by Kurimoto Iron Works Co., Ltd.; CD dryer, model CD-80). After adjusting the jacket temperature of the dryer to 180°C and the rotation speed of the agitator plate of the dryer to 30 rpm, the particulate hydrogel (c1) was charged into the dryer and dried for 30 minutes to obtain a dried product (A5). The solid content of the dried product (A5) was 98.8% by mass.

[0299] (Pulverization and classification step of dried product) Except for using the dried product (A5) instead of the dried product (A1), the same operation as in (Pulverization and classification step of dried product) of Example 1 was carried out to obtain a water absorbent resin powder (B5) and a water absorbent resin particle (C5). The water absorbent resin particle (C5) was used as a water absorbing agent (D5) as it was.

[0300] Example 6 (Step of Preparing Aqueous Monomer Solution, Polymerization Step) The same operations as in the (step of preparing an aqueous monomer solution) and (polymerization step) of Example 1 were carried out to obtain a hydrogel (b1).

[0301] (Gel crushing step) Except for not using the aqueous solution of dipotassium alkenyl succinate, the same operation as in Example 4 (gel crushing step) was carried out to obtain a particulate hydrogel (c6). Subsequently, the particulate hydrogel (c6) was placed in an OK bag (Okura Kogyo Co., Ltd.), and a 5% by mass aqueous solution of dipotassium alkenyl succinate (manufactured by Kao Corporation, Latemul ASK) was added and mixed so that the dipotassium alkenyl succinate was 0.040% by mass (equivalent to 0.033% by mass of alkenyl succinic acid) relative to the solid content of the particulate hydrogel (c6). As a result, a particulate hydrogel (c'6) was obtained.

[0302] (Drying step of particulate hydrogel) The same operation as in (Drying step of particulate hydrogel) of Example 1 was carried out except that the particulate hydrogel (c'6) was used instead of the particulate hydrogel (c1), to obtain a dried product (A6).

[0303] (Pulverization and classification step of dried material) Except for using the dried material (A6) instead of the dried material (A1), the same operation as in (Pulverization and classification step of dried material) of Example 1 was carried out to obtain a water absorbent resin powder (B6) and water absorbent resin particles (C6). The water absorbent resin particles (C6) were used as they were as a water absorbing agent (D6).

[0304] Comparative Example 1 (Step of Preparing Aqueous Monomer Solution, Polymerization Step) The same operations as in the (step of preparing an aqueous monomer solution) and (polymerization step) of Example 1 were carried out to obtain a hydrogel (b1).

[0305] (Gel crushing step) A particulate hydrogel (cc1) was obtained by the same operation as in (Gel crushing step) of Example 1, except that the amount of the aqueous dipotassium alkenyl succinate solution used in the first gel crushing was changed so that the dipotassium alkenyl succinate was 0.005% by mass (equivalent to 0.004% by mass of alkenyl succinic acid) relative to the solid content of the hydrogel (b1).

[0306] (Drying step of particulate hydrous gel) Drying was carried out in the same manner as in Example 1 (drying step of particulate hydrous gel), except that particulate hydrous gel (cc1) was used instead of particulate hydrous gel (c1). As a result, the particulate hydrous gel (cc1) aggregated in the dryer, and undried material was generated. In addition, some of the obtained dried material became agglomerated. The agglomerated dried material was so hard that it could not be crushed to about several mm by human hand alone, and when subjected to crushing using the same roll mill under the same conditions as in Example 1, the roll could not bite into the dried material, and it could not be crushed.

[0307] Comparative Example 2 (Step of Preparing Aqueous Monomer Solution, Polymerization Step) The same operations as in the (step of preparing an aqueous monomer solution) and (polymerization step) of Example 1 were carried out to obtain a hydrogel (b1).

[0308] (Gel crushing step) A particulate hydrogel (cc2) was obtained by the same operation as in (Gel crushing step) of Example 1, except that the amount of the aqueous dipotassium alkenyl succinate solution used in the first gel crushing was changed so that the dipotassium alkenyl succinate was 0.40% by mass (equivalent to 0.33% by mass of alkenyl succinic acid) relative to the solid content of the hydrogel (b1).

[0309] (Drying step of particulate hydrogel) A dried product (cA2) was obtained by the same procedure as in (Drying step of particulate hydrogel) of Example 1, except that the particulate hydrogel (cc2) was used instead of the particulate hydrogel (c1). The solid content of the dried product (cA2) was 98.7% by mass.

[0310] (Pulverization and Classification Step of Dried Material)

[0134] The same operation as in (Pulverization and Classification Step of Dried Material) of Example 1 was performed, except that the dried material (cA2) was used instead of the dried material (A1), so as to obtain a water absorbent resin powder (cB2) and a water absorbent resin particle (cC2).

[0311] (Surface cross-linking step) Except for using the water-absorbent resin particles (cC2) instead of the water-absorbent resin particles (C1), the same operation as in (Surface cross-linking step) of Example 1 was carried out to obtain surface-cross-linked water-absorbent resin particles. The water-absorbent resin particles were designated as a water-absorbing agent (cD2).

[0312] Comparative Example 3 (Step of Preparing Aqueous Monomer Solution, Polymerization Step) The same operations as in the (step of preparing an aqueous monomer solution) and (polymerization step) of Example 1 were carried out to obtain a hydrogel (b1).

[0313] (Gel Crushing Step) A particulate hydrogel (cc3) was obtained by carrying out the same operation as in (Gel Crushing Step) of Example 1, except that the aqueous dipotassium alkenyl succinate solution was not used in the first gel crushing.

[0314] (Drying step of particulate hydrogel) The obtained particulate hydrogel (cc3) was dried using a hot air dryer. This dryer was equipped with a basket (bottom size 30 cm x 20 cm) made of a wire mesh with a mesh size of 1.2 mm. 500 g of particulate hydrogel (cc1) was spread almost uniformly on the bottom of the basket, and hot air at 190 ° C. was blown from below for 30 minutes to obtain a dried product (cA3). The dried product (cA3) was in the shape of a single block.

[0315] (Crushing and Classification of Dried Product) The dried product (cA3) was cooled by standing. The cooled dried product (cA3) was crushed to particle sizes of about several mm using only human hands without using any tools.

[0316]

[0223] The same operation as in (Crushing and classifying step of dried product) of Example 1 was performed except that the crushed dried product (cA3) was used instead of the dried product (A1), so as to obtain a water absorbent resin powder (cB3) and a water absorbent resin particle (cC3).

[0317] (Surface cross-linking step) Except for using the water-absorbent resin particles (cC3) instead of the water-absorbent resin particles (C1), the same operation as in (Surface cross-linking step) of Example 1 was carried out to obtain surface-cross-linked water-absorbent resin particles. The water-absorbent resin particles were designated as a water-absorbing agent (cD3).

[0318] [Comparative Example 4] (Step of preparing aqueous monomer solution, polymerization step, gel crushing step, and step of drying particulate hydrogel) The same operations as those in Comparative Example 3 (step of preparing aqueous monomer solution, polymerization step), (gel crushing step), and (step of drying particulate hydrogel) were performed to obtain a dried product (cA3).

[0319] (Pulverization and classification step of dried material) Except for changing the clearance between the rolls to 0.4 mm, the same operation as in (Pulverization and classification step of dried material) of Comparative Example 3 was performed to obtain a water-absorbent resin powder (cB4) and water-absorbent resin particles (cC4). The water-absorbent resin particles (cC4) were used as they were as a water-absorbing agent (cD4).

[0320] Comparative Example 5 (Step of Preparing Aqueous Monomer Solution, Polymerization Step) The same operations as in the (step of preparing an aqueous monomer solution) and (polymerization step) of Example 1 were carried out to obtain a hydrogel (b1).

[0321] (Gel Crushing Step) A particulate hydrogel (cc5) was obtained by carrying out the same operation as in (Gel Crushing Step) of Example 1, except for the matters shown in the following (i).

[0322] (i) In the first gel crushing, instead of the dipotassium alkenyl succinate aqueous solution, a potassium oleate aqueous solution (OS Soap, manufactured by Kao Corporation) having a concentration of 5% by mass was used so that the potassium oleate content was 0.20% by mass (concentration in terms of oleic acid: 0.18% by mass) relative to the solid content of the hydrous gel (b1).

[0323] (Drying step of particulate hydrous gel) Drying was performed in the same manner as in Example 1 (drying step of particulate hydrous gel), except that particulate hydrous gel (cc5) was used instead of particulate hydrous gel (c1). As a result, the particulate hydrous gel (cc5) aggregated in the dryer, and undried material was generated. In addition, some of the obtained dried material became agglomerated. The agglomerated dried material was so hard that it could not be crushed to about several mm by human hand alone, and when subjected to crushing using the same roll mill under the same conditions as in Example 1, the roll could not bite the dried material, and it could not be crushed.

[0324] Comparative Example 6 (Step of Preparing Aqueous Monomer Solution, Polymerization Step) The same operations as in the (step of preparing an aqueous monomer solution) and (polymerization step) of Example 1 were carried out to obtain a hydrogel (b1).

[0325] (Gel Crushing Step) A particulate hydrogel (cc6) was obtained by carrying out the same operation as in (Gel Crushing Step) of Example 1, except for the matters described in the following (ii).

[0326] (ii) In the first gel crushing, a 5% by mass aqueous solution of sodium lauryl glycol carboxylate (Viewlite (trademark) SHAA, manufactured by Sanyo Chemical Industries, Ltd.) was used in place of the aqueous dipotassium alkenyl succinate solution, so that the sodium lauryl glycol carboxylate was 0.20% by mass (concentration in terms of lauryl glycol carboxylic acid: 0.18% by mass) relative to the solid content of the hydrous gel (b1).

[0327] (Drying step of particulate hydrous gel) Drying was performed in the same manner as in Example 1 (drying step of particulate hydrous gel), except that particulate hydrous gel (cc6) was used instead of particulate hydrous gel (c1). As a result, the particulate hydrous gel (cc5) aggregated in the dryer, and undried material was generated. In addition, the obtained dried material was agglomerated. The agglomerated dried material was so hard that it could not be crushed to about several mm by human hand alone, and when subjected to pulverization using the same roll mill under the same conditions as in Example 1, the roll could not bite into the dried material, and it could not be pulverized.

[0328] Comparative Example 7 (Preparation step of aqueous monomer solution, polymerization step) 200 g of acrylic acid, 0.73 g of N,N'-methylenebisacrylamide, and 587 g of deionized water were placed in a 1 L reaction vessel, and the contents were stirred while maintaining the temperature at 20°C to prepare an aqueous monomer solution (a5). Subsequently, nitrogen gas was introduced into the aqueous monomer solution (a5) until the dissolved oxygen concentration in the aqueous monomer solution (a5) was less than 0.02%, and then, under a sealed condition, 2.44 g of a 1% by mass aqueous hydrogen peroxide solution, 2.92 g of a 0.2% by mass aqueous ascorbic acid solution, and 6.82 g of a 2% by mass aqueous 2,2-azobisaminodipropane dihydrochloride solution were added to the aqueous monomer solution (a5) and stirred. After allowing to stand, heat generation was observed, and after the temperature inside the reaction vessel reached approximately 85°C, the reaction vessel was subsequently heated at 60°C for 3 hours to obtain a hydrogel-like crosslinked polymer (b7).

[0329] (Gel Crushing Step) The obtained hydrogel-like crosslinked polymer (b7) was chopped into blocks of 3 to 5 cm square. The block-shaped hydrogel-like crosslinked polymer (b7) was fed into a screw extruder, and gel crushing was carried out. The screw extruder was a meat chopper with an outer diameter of the screw shaft of 86 mm, and a die plate of 100 mm diameter, 10 mm thickness, and 6.4 mm diameter installed at the tip (extrusion port). Gel crushing (first gel crushing) was carried out while simultaneously feeding the hydrogel (b7) with a 5% by mass aqueous solution of dipotassium alkenyl succinate (manufactured by Kao Corporation, Latemul ASK) and a 20% by mass aqueous solution of sodium hydroxide.

[0330] Here, the dipotassium alkenylsuccinate was a mixture of dipotassium hexadecenylsuccinate and dipotassium octadecenylsuccinate. The dipotassium alkenylsuccinate was supplied in an amount of 0.040% by mass (concentration in terms of alkenylsuccinic acid: 0.033% by mass) relative to the solid content of the hydrous gel-like crosslinked polymer (b7), and the aqueous sodium hydroxide solution was supplied in an amount sufficient to neutralize 73 mol% of the carboxyl groups in the hydrous gel-like crosslinked polymer (b7).

[0331] Subsequently, the addition of the aqueous dipotassium alkenyl succinate solution and the aqueous sodium hydroxide solution was not performed, and the gel was further crushed (second gel crushing) under the same conditions as the first gel crushing to obtain a particulate hydrogel (cc7). The temperature of the particulate hydrogel (cc7) discharged from the screw extruder, i.e., the particulate hydrogel (cc7) immediately after the second gel crushing, was 90°C.

[0332] (Step of drying particulate hydrogel) The obtained particulate hydrogel (cc7) was dried using a hot air dryer. This dryer was equipped with a basket (bottom size 30 cm x 20 cm) made of a wire mesh with a mesh size of 1.2 mm. 500 g of particulate hydrogel (cc7) was spread almost uniformly on the bottom of the basket, and hot air at 190°C was blown from below for 30 minutes to obtain a dried product (cA7). The dried product (cA7) was in the shape of a single block.

[0333] (Crushing and Classification of Dried Product) The dried product (cA7) was cooled by standing. The cooled dried product (cA7) was crushed to particle sizes of about several mm using only human hands without using any tools.

[0334]

[0223] The same operation as in (Crushing and classifying step of dried material) of Example 1 was performed except that the crushed dried material (cA7) was used instead of the dried material (A1), so as to obtain a water absorbent resin powder (cB7) and a water absorbent resin particle (cC7).

[0335] (Surface cross-linking step) Except for using the water-absorbent resin particles (cC7) instead of the water-absorbent resin particles (C1), the same operation as in (Surface cross-linking step) of Example 1 was carried out to obtain surface-cross-linked water-absorbent resin particles. The water-absorbent resin particles were designated as a water-absorbing agent (cD7).

[0336] [Results] The structural formulas of dipotassium alkenyl succinate used in Examples 1 to 6 and Comparative Examples 1 and 2, potassium oleate used in Comparative Example 5, and sodium lauryl glycol carboxylate used in Comparative Example 6 are shown in Table 1 below.

[0337] Furthermore, the physical properties of the particulate hydrogels (hydrogels to be subjected to the drying step), water-absorbent resin powders, water-absorbent resin particles, and water-absorbing agents obtained in Examples 1 to 6 and Comparative Examples 1 to 6 are shown in the following Tables 2 and 3.

[0338] As shown in Table 1, dipotassium alkenyl succinate corresponds to the dicarboxylic acid (salt) of the present invention. On the other hand, potassium oleate and sodium lauryl glycol carboxylate do not correspond to the dicarboxylic acid (salt) of the present invention.

[0339] From the descriptions of Examples 1 to 6, the methods for producing the water-absorbing agents of Examples 1 to 6 satisfy all of the following requirements (A) to (D).

[0340] (A) The method includes a polymerization step of polymerizing an aqueous monomer solution to obtain a hydrogel, and a drying step of drying the hydrogel to obtain a dried product.

[0341] (B) The neutralization rate of the aqueous monomer solution is 50 mol % or more.

[0342] (C) The dicarboxylic acid (salt) of the present invention is added to the aqueous monomer solution and / or the hydrogel before the completion of the drying step.

[0343] (D) The amount of the dicarboxylic acid (salt) of the present invention added is within the range of 100 to 3000 ppm based on the solid content of the aqueous monomer solution or the solid content of the hydrogel.

[0344] Therefore, the manufacturing methods of the water-absorbing agents of Examples 1 to 6 correspond to the manufacturing method of the present invention. On the other hand, the manufacturing methods of the water-absorbing agents of Comparative Examples 1 to 7 do not satisfy at least one of the requirements (B), (C), and (D) above. Therefore, these manufacturing methods do not correspond to the manufacturing method of the present invention.

[0345] Furthermore, the water-absorbing agents (D1) to (D6) produced in Examples 1 to 6 further contain the dicarboxylic acid (salt) of the present invention, since the dicarboxylic acid (salt) of the present invention is added to the water-absorbing resin during production. Furthermore, the solid content of the water-absorbing resin is substantially the same as the solid content contained in the aqueous monomer solution during production or the solid content of the hydrogel.

[0346] Therefore, the content of the dicarboxylic acid (salt) of the present invention is within the range of 100 to 3000 ppm relative to the solid content of the water-absorbent resin. Therefore, water-absorbing agents (D1) to (D6) correspond to the water-absorbing agent of the present invention. On the other hand, water-absorbing agents (cD2) to (cD4) produced in Comparative Examples 2 to 4 do not contain the dicarboxylic acid (salt) of the present invention, or the content thereof is outside the range of 100 to 3000 ppm. Therefore, water-absorbing agents (cD2) to (cD4) do not correspond to the water-absorbing agent of the present invention. Furthermore, the water-absorbing agent (cD7) produced in Comparative Example 7 has a neutralization index exceeding 15, and does not correspond to the water-absorbing agent of the present invention.

[0347] As shown in Table 2, in Examples 1 to 6, the amount of "150 passes", which indicates the amount of fine powder generated during production, is smaller than in Comparative Examples 2 to 4 and 7. This indicates that in Examples 1 to 6, the generation of a large amount of fine powder is prevented.

[0348] Furthermore, as shown in Tables 2 and 3, the surface tension of the water-absorbing agent (D1) produced in Example 1 and the surface tension of the water-absorbing resins (C2) to (C6) produced in Examples 2 to 6, i.e., the water-absorbing agents (D2) to (D6), are as high as 50 mN / m or more.

[0349] Therefore, it is considered that the water-absorbing agents (D1) to (D6) have a small simplex Rewet, which represents the amount of rewet when a water-absorbing agent in a swollen state is pressurized. A small simplex Rewet of the water-absorbing agent means that the amount of rewet of the water-absorbing agent after absorbing water is small, and the water-absorbing agent is excellent in reducing the amount of rewet after absorbing water. In fact, as shown in Table 3, the water-absorbing agent (D1) produced in Example 1 had a smaller simplex Rewet than the water-absorbing agents (cD2), (cD3), and (cD7) produced in Comparative Examples 2, 3, and 7, and was shown to be excellent in reducing the amount of rewet after absorbing water.

[0350] In addition, as shown in Table 3, the water-absorbing agent (D1) produced in Example 1 also obtained good results in the odor evaluation, demonstrating that it is also excellent in odor reduction.

[0351] From the above, it was found that the manufacturing method of the present invention can manufacture a water-absorbing agent that reduces odor and the amount of resorption after water absorption while preventing a large amount of fine powder from being generated. Furthermore, it was found that the water-absorbing agent of the present invention is excellent in productivity and also excellent in reducing odor and the amount of resorption after water absorption.

[0352] Furthermore, as shown in Table 3, the water-absorbing agent (D1) produced in Example 1 and the water-absorbing agents (cD2) and (cD3) produced in Comparative Examples 2 and 3 had a neutralization index of 15 or less. On the other hand, the water-absorbing agent (cD7) produced in Comparative Example 7 had a neutralization index of more than 15. Furthermore, the water-absorbing agents (D1), (cD2) and (cD3) had higher AAP (0.3 psi) than the water-absorbing agent (cD7).

[0353] Therefore, it is considered that the water-absorbing performance of the water-absorbing agent (cD7), such as the absorbency against pressure (AAP), was reduced because the surface treatment of the particulate hydrogel and its dried product became non-uniform due to the neutralization index exceeding 15. On the other hand, it is considered that the water-absorbing agents (D1), (cD2) and (cD3) avoided the reduction in the water-absorbing performance.

[0354] Therefore, it was found that in the water-absorbing agent of the present invention, when the neutralization index is 15 or less, the deterioration of the water-absorbing performance of AAP and the like is avoided, and the water-absorbing performance is further improved.

[0355] The production method according to the present invention can be used to produce, with good productivity, a water-absorbing agent that is excellent in reducing odor and reducing the amount of rewetted after absorbing water.

Claims

1. A method for producing a water-absorbing agent, comprising: a polymerization step of polymerizing an aqueous monomer solution to obtain a hydrogel; and a drying step of drying the hydrogel to obtain a dried product, wherein a neutralization rate of the aqueous monomer solution is 50 mol % or more, and a dicarboxylic acid represented by the following formula (1) and / or a salt thereof is added to the aqueous monomer solution and / or the hydrogel before the completion of the drying step, and the amount of the dicarboxylic acid and / or the salt thereof is within a range of 100 to 3000 ppm relative to the solid content contained in the aqueous monomer solution or the solid content of the hydrogel. (In formula (1), n ​​is 1 or 2, and R is a linear or branched alkyl group having 8 to 20 carbon atoms, or a linear or branched alkenyl group having 8 to 20 carbon atoms.) 2. The method for producing a water-absorbing agent according to claim 1, wherein the concentration of the monomer in the aqueous monomer solution is 35% by mass or more relative to the mass of the aqueous monomer solution.

3. The method for producing a water-absorbing agent according to claim 1 or 2, further comprising a gel crushing step of crushing the hydrogel to obtain a particulate hydrogel.

4. The method for producing a water-absorbing agent according to claim 3, wherein the dicarboxylic acid represented by the formula (1) and / or its salt is added to the hydrogel after the polymerization step is completed and before the gel-pulverization step is completed.

5. The method for producing a water-absorbing agent according to any one of claims 1 to 4, wherein the solid content of the hydrogel subjected to the drying step is 40 mass % or more relative to the total mass of the hydrogel subjected to the drying step.

6. The method for producing a water-absorbing agent according to any one of claims 1 to 5, wherein the drying step is a step of drying the hydrogel by stirring and drying.

7. The method for producing a water-absorbing agent according to any one of claims 1 to 6, wherein the drying step is a step of drying the hydrogel under conditions where the drying temperature is 150°C or higher.

8. The method for producing a water-absorbing agent according to any one of claims 1 to 7, wherein the surface tension of the water-absorbing agent is 50 mN / m or more.

9. A water-absorbing agent comprising a water-absorbent resin and a dicarboxylic acid represented by the following formula (1) and / or a salt thereof, wherein the content of the dicarboxylic acid represented by the formula (1) and / or the salt thereof is within a range of 100 to 3000 ppm relative to the solid content of the water-absorbent resin, and the neutralization index is 15 or less. (In formula (1), n ​​is 1 or 2, and R is a linear or branched alkyl group having 8 to 20 carbon atoms, or a linear or branched alkenyl group having 8 to 20 carbon atoms.) (The neutralization index means the number of water-absorbing agents having a neutralization rate that is 20 mol % or more lower than the average neutralization rate of the water-absorbing agents, out of 200 water-absorbing agents.) 10. The water-absorbing agent according to claim 9, which has a surface tension of 50 mN / m or more.

11. The water-absorbing agent according to claim 9 or 10, which has a water-absorbing speed (Vortex) for physiological saline solution of less than 25 seconds.

12. The water-absorbing agent according to any one of claims 9 to 11, wherein the water-absorbing resin contains a crosslinked polymer containing an acid group-containing monomer (salt) as a main component.

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