Method for producing water-absorbent resin powder

Abstract

Provided is a method for producing a water-absorbing resin powder having an excellent water absorption rate. The method for producing a water-absorbing resin powder comprises a polymerization step in which an aqueous monomer solution is polymerized to obtain a hydrogel-state crosslinked polymer and a gel pulverization step in which after the polymerization step, the hydrogel-state crosslinked polymer is pulverized with a gel pulverizer to obtain a particulate hydrogel-state crosslinked polymer. The gel pulverizer is provided with an introduction opening, a discharge opening, and a main body having a plurality of built-in rotating shafts, wherein the rotating shafts each have a pulverization means. In the gel pulverization step, the hydrogel-state crosslinked polymer is continuously introduced through the introduction opening, the particulate hydrogel-state crosslinked polymer is continuously taken out through the discharge opening, the hydrogel-state crosslinked polymer to be introduced through the introduction opening has a degree of polymerization of 90 mass% or higher, the gel pulverization coefficient is 0.020-3.0 J / g·sec, and the particulate hydrogel-state crosslinked polymer discharged through the discharge opening has a mass-average particle diameter of 500 μm or less on a solid basis.

Description

Technical Field

[0001] The present invention relates to a method for producing a water-absorbent resin powder.

Background Art

[0002] A water-absorbent resin (SAP / Super Absorbent Polymer) is a water-swellable and water-insoluble polymer gelling agent, and is widely used in various fields such as absorbent articles such as paper diapers and sanitary napkins, water retention agents for agriculture and horticulture, and water-stopping agents for industry.

[0003] For the above water-absorbent resin, various monomers and hydrophilic polymers are used as raw materials thereof. From the viewpoint of water absorption performance, polyacrylic acid (salt)-based water-absorbent resins using acrylic acid and / or its salts as monomers are the most industrially produced.

[0004] With the improvement of the performance of paper diapers, which is the main application of the above water-absorbent resin, various functions (enhanced physical properties) are required. Specifically, in addition to the water absorption ratio under non-pressure and the water absorption ratio under pressure, which are basic physical properties, various physical properties such as gel strength, water-soluble components, water content, water absorption speed, liquid permeability, particle size distribution, urine resistance, antibacterial property, damage resistance, powder fluidity, deodorizing property, color resistance, low dust, and low residual monomers are required for the water-absorbent resin. In particular, in the application of sanitary products such as paper diapers, with the thinning of the product, further improvement of the water absorption speed is desired.

[0005] Typical commercial production methods of the above powdery or granular water-absorbent resin include a polymerization step, a gel pulverization (fine granulation) step performed after polymerization or simultaneously with polymerization, a drying step of the granulated gel, a pulverization step of the dried product, a classification step of the pulverized product, a recovery step of fine powder generated by pulverization and classification, and a surface crosslinking step of the water-absorbent resin powder after classification.

[0006] One method for producing superabsorbent polymers that has been proposed to date involves using a polymerization apparatus with a pulverization mechanism to simultaneously perform the polymerization process and the gel pulverization process. In this method, as the polymerization reaction of the liquid monomer progresses, the resulting water-containing gel is pulverized, and the finely granulated water-containing gel is discharged from the polymerization apparatus. Specific examples of this method, using batch-type kneaders and continuous-type kneaders, are shown in Patent Documents 1 to 3.

[0007] However, the size of the gel particles obtained with these devices is only a few millimeters to a few centimeters, which is insufficient for the current situation where further improvements in water absorption rate are required, and additional gel grinding equipment was needed. Patent document 4 proposes a method of wet grinding using a batch-type kneader or a continuous kneader to a size smaller than the gel particles that will become the product particle size of the water-absorbent resin, but this is not practical because the size of the equipment becomes excessively large.

[0008] Furthermore, in the polymerization process, the highly adhesive hydrated gel, which is undergoing monomer polymerization, is pulverized. This makes it easy for the hydrated gel to adhere to the internal components of the equipment. As the reaction progresses while the gel is attached, it solidifies, which can cause damage to the components and require more time for cleaning during maintenance. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 57-34101 [Patent Document 2] Japanese Patent Application Publication No. 60-55002 [Patent Document 3] International Publication No. 2001 / 038402 Pamphlet [Patent Document 4] Japanese Patent Application Publication No. 05-112654 [Overview of the Initiative]

[0010] In recent years, obtaining superabsorbent resins with superior water absorption rates has been particularly sought after. This requires grinding the water-containing gel to a smaller particle size than before during the gel grinding process. However, conventional kneader polymerization, which involves simultaneously performing the polymerization and gel grinding processes using a kneader with multiple shafts, has not been able to obtain water-containing gels with the desired particle size.

[0011] Therefore, an object of the present invention is to provide a water-absorbing resin with excellent water absorption rate. Another object of the present invention is to reduce residual monomers.

[0012] First, the inventors discovered that while conventional methods for obtaining a hydrated gel-like crosslinked polymer (hereinafter also referred to as "particulate hydrated gel") use an extruder (meat chopper) equipped with a porous plate (also called a die) to obtain a particulate hydrated gel, it is possible to continuously obtain a particulate hydrated gel by performing gel pulverization using a multi-screw kneader (especially a twin-screw kneader). Furthermore, they discovered that by setting the polymerization rate of the hydrated gel-like crosslinked polymer to 90% by mass or more, the gel pulverization coefficient to be between 0.02 J / g·sec and 3 J / g·sec, and the mass-average particle size of the particulate hydrated gel-like crosslinked polymer discharged from the multi-screw kneader in terms of solid content to 500 μm or less, it is possible to obtain a water-absorbing resin powder with excellent water absorption rate, thus completing the present invention.

[0013] In other words, the present invention is a method for producing a water-absorbent resin powder, comprising: a polymerization step of polymerizing an aqueous monomer solution to obtain a water-containing gel-like crosslinked polymer; and a gel grinding step of grinding the water-containing gel-like crosslinked polymer after the polymerization step using a gel grinding device to obtain a particulate water-containing gel-like crosslinked polymer, wherein the gel grinding device comprises an inlet, an outlet, and a main body containing a plurality of rotating shafts, each of which has a grinding means, and in the gel grinding step, the water-containing gel-like crosslinked polymer is continuously fed in from the inlet, and particulate water-containing gel-like crosslinked polymer is continuously taken out from the outlet, the polymerization rate of the water-containing gel-like crosslinked polymer fed into the inlet is 90% by mass or more, the gel grinding coefficient is 0.02 J / g·sec to 3 J / g·sec or less, and the mass-average particle size of the particulate water-containing gel-like crosslinked polymer discharged from the outlet is 500 μm or less in terms of solid content. [Brief explanation of the drawing]

[0014] [Figure 1] This is a partially cutaway side view showing an example of a gel grinding apparatus used in a manufacturing method according to an embodiment of the present invention. [Figure 2] Figure 1 is an enlarged view of the gel grinding device (a top view of the central part of the main body). [Figure 3] Figure 3 shows an example of the specific configuration of the rotating shaft of the gel grinding device used in the embodiment. [Figure 4] Figure 4 shows an example of the specific configuration of the rotating shaft of the gel grinding device used in the embodiment. [Figure 5] Figure 5 shows an example of the specific configuration of the rotating shaft of the gel grinding device used in the embodiment. [Figure 6] Figure 6 shows an example of the specific configuration of the rotating shaft of the gel grinding device used in the embodiment. [Figure 7] Figure 7 shows an example of the specific configuration of the rotating shaft of the gel grinding device used in the embodiment. [Figure 8] Figure 8 is a front view showing an example of a disk. [Modes for carrying out the invention]

[0015] Hereinafter, the present invention will be described in detail. However, the scope of the present invention is not limited by these descriptions, and in addition to those exemplified below, it can be appropriately modified and implemented within the scope that does not impair the gist of the present invention. Further, the present invention is not limited to the following embodiments, and various modifications are possible within the scope shown in the claims. Other embodiments obtained by appropriately combining the technical means disclosed for each of the plurality of embodiments are also included in the technical scope of the present invention.

[0016] 〔1〕Definition of terms 〔1-1〕“Water-absorbing resin” The “water-absorbing resin” in the present invention refers to a water-swellable and water-insoluble polymer gelling agent that satisfies the following physical properties. That is, it refers to a polymer gelling agent having a CRC (centrifuge retention capacity) of 5 g / g or more defined by NWSP 241.0.R2(15) as the water-swellable property, and an Ext (water-soluble content) of 50 mass% or less defined by NWSP 270.0.R2(15) as the water-insoluble property.

[0017] 〔1-2〕“Poly(meth)acrylic acid (salt)” The “poly(meth)acrylic acid (salt)” in the present invention refers to a (co)polymer of (meth)acrylic acid and / or its salt, and means a crosslinked polymer that contains a structure derived from (meth)acrylic acid and / or its salt (hereinafter also referred to as “(meth)acrylic acid (salt)”) as a repeating unit as a main component and contains a structure derived from an internal crosslinking agent as an optional component.

[0018] The above “main component” means that the usage amount (content) of (meth)acrylic acid (salt) is preferably 50 mol% to 100 mol%, more preferably 70 mol% to 100 mol%, still more preferably 90 mol% to 100 mol%, and particularly preferably substantially 100 mol% with respect to the total monomers used in the polymerization (all monomers excluding the crosslinking agent).

[0019] 〔1-3〕Definition of evaluation method "NWSP" stands for "Non-Woven Standard Procedures-Edition 2015," which was jointly published by EDANA (European Disposables And Nonwovens Association) and INDA (Association of the Nonwoven Fabrics Industry) to unify evaluation methods for nonwoven fabrics and their products in the United States and Europe, and represents a standard measurement method for superabsorbent polymers. Unless otherwise specified, the physical properties of superabsorbent polymers in this invention are measured in accordance with "Non-Woven Standard Procedures-Edition 2015." For evaluation methods not described in NWSP, measurements are performed using the methods and conditions described in the examples.

[0020] [1-3-1] “CRC” (NWSP 241.0.R2(15)) "CRC" is an abbreviation for Centrifuge Retention Capacity, and refers to the water absorption ratio (sometimes called "water absorption ratio") of a water-absorbent polymer under no pressure. Specifically, it refers to the water absorption ratio (unit: g / g) after 0.2g of water-absorbent polymer is placed in a nonwoven fabric bag, immersed in a large excess of 0.9 mass% sodium chloride aqueous solution for 30 minutes to allow free swelling, and then drained using a centrifuge (250G) for 3 minutes. For water-containing gel-like crosslinked polymers after polymerization and / or gel pulverization (hereinafter simply referred to as "water-containing gel"), 0.4g of water-containing gel is used, the measurement time is changed to 24 hours, and the CRC is determined after correcting for solid content.

[0021] [1-3-2] “Moisture Content” (NWSP 230.0.R2(15)) "Moisture Content" refers to the moisture content defined by the drying loss of the superabsorbent resin. Specifically, it refers to the value (unit: mass%) calculated from the drying loss when 4.0 g of superabsorbent resin is dried at 105°C for 3 hours. In this invention, the superabsorbent resin after drying is defined by the drying loss of 1.0 g of superabsorbent resin at 180°C for 3 hours, and the water-containing gel before drying is defined by the drying loss of 2.0 g of water-containing gel at 180°C for 24 hours.

[0022] [1-3-3] “PSD” (NWSP 220.0.R2(15)) "PSD" is an abbreviation for Particle Size Distribution, and refers to the particle size distribution of a water-absorbing resin measured by sieving classification. The mass-average particle diameter (D50) and the logarithmic standard deviation (σζ) of the particle size distribution are measured in the same manner as described in U.S. Patent No. 7,638,570. In this invention, the particle size distribution (PSD) of the particulate water-containing gel is defined by wet sieving classification using the method described later. Furthermore, the particle diameter (μm) of the particulate water-containing gel in terms of solid content is defined by the calculation method described later, based on the particle diameter (μm) of the particulate water-containing gel and its solid content (mass%).

[0023] [1-3-4] "AAP" (NWSP 242.0.R2(15)) "AAP" is an abbreviation for Absorption Against Pressure, and refers to the water absorption rate of a water-absorbent polymer under pressure. Specifically, 0.9g of water-absorbent polymer is subjected to a large excess of 0.9 mass% sodium chloride aqueous solution and subjected to 2.06 kPa (21 g / cm³) for 1 hour. 2 This refers to the water absorption ratio (unit: g / g) after swelling under a load of 0.3 kPa (approximately 49 g / cm³). In this specification, the load condition is defined as 4.83 kPa (approximately 49 g / cm³). 2 It is defined as the value measured after changing the pressure to approximately 0.7 psi.

[0024] [1-3-5] "Vortex" In this specification, "Vortex" is an index representing the water absorption rate of a superabsorbent polymer, and means the time (in seconds) required for 2 g of superabsorbent polymer to absorb 50 ml of a 0.9 mass% sodium chloride aqueous solution to a predetermined state.

[0025] [1-4] "Gel crushing" In this specification, "gel grinding" refers to an operation in which shearing and compressive forces are applied to reduce the size and increase the surface area of ​​a water-containing gel-like crosslinked polymer obtained in a polymerization process (preferably aqueous solution polymerization, unagitated aqueous solution polymerization (static aqueous solution polymerization), and particularly preferably belt polymerization) in order to facilitate drying of the polymer.

[0026] Furthermore, the shape of the resulting hydrated gel may vary depending on the type of polymerization apparatus. For example, the shape of the hydrated gel obtained by static polymerization (especially belt polymerization) is sheet-like or block-like. Here, "sheet-like" refers to a polymer with thickness in a planar direction, preferably 1 mm to 30 cm, and particularly preferably 0.5 to 10 cm. Sheet-like hydrated gels are typically obtained by belt polymerization, drum polymerization, and batch thin-film polymerization. The length and width of the sheet-like hydrated gel are appropriately determined by the size of the polymerization apparatus used. In the case of continuous polymerization (continuous belt polymerization or continuous drum polymerization), an endless sheet-like hydrated gel is obtained, and its width is the width of the belt or drum of the polymerization apparatus, preferably 0.1 to 10 m, more preferably 1 to 5 m. This endless sheet-like hydrated gel may be appropriately cut in the length direction after polymerization, or it may be further crushed into pieces of several mm to several m square after cutting. Block-like hydrated gels can be obtained by tank polymerization, etc. This block-shaped hydrated gel may be crushed into pieces of several mm to several m square as appropriate after polymerization. In contrast, in the case of kneader polymerization, the hydrated gel produced by polymerization is coarsely crushed within the same apparatus during the polymerization process, resulting in granular hydrated gel. However, the hydrated gel particles obtained by kneader polymerization have a particle size of several cm to several mm, and are not finely granulated to the level of particle size obtained in the gel crushing process according to the present invention. Furthermore, crushing the gel to the particle size obtained in the present invention by kneader polymerization would require excessively large equipment, making it impractical as an industrial manufacturing method. Therefore, such gel crushing in the polymerization process is not included in the concept of "gel crushing" in the present invention. In the present invention, the polymerization process is considered to be completed when the polymerization rate reaches the range described later. The operation of crushing a hydrated gel that has been granulated to a particle size of several cm to several mm by methods such as kneader polymerization to the particle size required in the present invention is included in the concept of "gel crushing" in the present invention.

[0027] [1-5] Others In this specification, the range "X~Y" means "greater than or equal to X, and less than or equal to Y." Unless otherwise noted, the unit of mass "t (ton)" means "metric ton," and "ppm" means "mass ppm" or "weight ppm." Furthermore, "mass" and "weight," "parts by mass" and "parts by weight," and "mass%" and "weight%" are treated as synonyms. In addition, "~acid (salt)" means "~acid and / or its salt," and "(meth)acrylic" means "acrylic and / or methacrylic."

[0028] [2] Method for producing water-absorbent resin powder The method for producing water-absorbent resin powder according to the present invention comprises a polymerization step and a gel pulverization step separate from the polymerization step. Preferably, this method further comprises a drying step, a cooling step, a pulverization step of the dried material, a classification step, a surface crosslinking step, or a sizing step after surface crosslinking. Other steps may include a monomer aqueous solution preparation step, an additive addition step, a fine powder removal step and a fine powder recycling step (fine powder recovery step), or a filling step. Furthermore, various known steps may be included depending on the purpose.

[0029] According to the manufacturing method of the present invention, a water-containing gel-like crosslinked polymer with a polymerization rate of 90% by mass or more is subjected to gel pulverization using a kneader having multiple shafts (particularly a twin-shaft kneader), and in this process the gel pulverization coefficient is controlled to be between 0.020 J / g·sec and 3.0 J / g·sec, thereby obtaining a particulate water-containing gel with a mass-average particle size d1 on a solid content basis of 500 μm or less, resulting in an absorbent resin with excellent water absorption rate and reduced residual monomers.

[0030] The following provides a detailed explanation of each step.

[0031] [2-1] Preparation of monomer aqueous solution This step is an optional step in which an aqueous solution containing monomers that will be used as raw materials for the superabsorbent resin, preferably an aqueous solution mainly containing acid group-containing unsaturated monomers (hereinafter referred to as "monomer aqueous solution"). Although a monomer slurry can also be used as long as the water absorption performance of the resulting superabsorbent resin does not decrease, for convenience, this section will describe the monomer aqueous solution.

[0032] Furthermore, the term "main component" above refers to a quantity (content) of acid group-containing unsaturated monomers that is typically 50 mol% or more, preferably 70 mol% or more, and more preferably 90 mol% or more (upper limit is 100 mol%) relative to the total amount of monomers (excluding internal crosslinking agents) used in the polymerization reaction of the water-absorbent resin.

[0033] (Acid group-containing unsaturated monomer) The acid group defined in this invention is not particularly limited, but examples include carboxyl groups, sulfone groups, and phosphoric acid groups. Examples of this acid group-containing unsaturated monomer include (meth)acrylic acid, (anhydride) maleic acid, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyltoluenesulfonic acid, vinyltoluenesulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, and 2-hydroxyethyl(meth)acryloylphosphate. From the viewpoint of water absorption performance, (meth)acrylic acid, (anhydride) maleic acid, itaconic acid, and cinnamic acid are preferred, more preferably (meth)acrylic acid, and particularly preferably acrylic acid.

[0034] (Monomers other than acid-containing unsaturated monomers) Other monomers besides acid group-containing unsaturated monomers can be any compound that can polymerize to form a water-absorbing resin. Examples 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.

[0035] (Neutralizing salt) In the present invention, a neutralized salt obtained by neutralizing some or all of the acid groups contained in an acid group-containing unsaturated monomer can be used. In this case, the salt of the acid group-containing unsaturated monomer is preferably a salt with a monovalent cation, more preferably at least one selected from alkali metal salts, ammonium salts, and amine salts, even more preferably an alkali metal salt, even more preferably at least one selected from sodium salts, lithium salts, and potassium salts, and sodium salt is particularly preferred.

[0036] (Neutralizing agent) The neutralizing agent used to neutralize the above-mentioned acid-group-containing unsaturated monomer is not particularly limited, but inorganic salts such as sodium carbonate and ammonium carbonate, or basic substances such as sodium hydroxide, potassium hydroxide, and amine organic compounds having amino or imino groups may be appropriately selected and used. Two or more salts or basic substances may be used in combination as neutralizing agents. In this invention, unless otherwise specified, the monomer is a concept that includes the neutralized salt.

[0037] (neutralization rate) From the viewpoint of water absorption performance, the number of moles of neutralizing salt relative to the total number of moles of the acid group-containing unsaturated monomer and its neutralizing salt (hereinafter referred to as the "neutralization rate") is preferably 40 mol% or more, more preferably 40 mol% to 80 mol%, even more preferably 45 mol% to 78 mol%, and particularly preferably 50 mol% to 75 mol%.

[0038] Methods for adjusting the neutralization rate include: mixing an acid-containing unsaturated monomer with its neutralized salt; adding a known neutralizing agent to the acid-containing unsaturated monomer; and using a partially neutralized salt of an acid-containing unsaturated monomer (i.e., a mixture of the acid-containing unsaturated monomer and its neutralized salt) that has been pre-adjusted to a predetermined neutralization rate. These methods may also be combined.

[0039] The adjustment of the neutralization rate described above may be performed before the polymerization reaction of the acid group-containing unsaturated monomer begins, during the polymerization reaction of the acid group-containing unsaturated monomer, or on the hydrated gel-like crosslinked polymer obtained after the polymerization reaction of the acid group-containing unsaturated monomer is completed. Furthermore, the neutralization rate may be adjusted at one of the following stages: before the polymerization reaction begins, during the polymerization reaction, or after the polymerization reaction is completed, or it may be adjusted at multiple stages. In applications where there is a possibility of direct contact with the human body, such as absorbent articles like disposable diapers, it is preferable to adjust the neutralization rate before the polymerization reaction begins and / or during the polymerization reaction, more preferably before the polymerization reaction begins.

[0040] (Internal crosslinking agent) In a method for producing water-absorbent resin powder, an internal crosslinking agent is preferably used. The internal crosslinking agent adjusts the water absorption performance and gel strength during water absorption of the resulting water-absorbent resin.

[0041] The above-mentioned internal crosslinking agent only needs to have a total of two or more unsaturated bonds or reactive functional groups in one molecule. For example, internal crosslinking agents having multiple polymerizable unsaturated groups (that can copolymerize with monomers) in the molecule include N,N-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, and triallyl phosphate. Examples of internal crosslinking agents having multiple reactive functional groups (which can react with monomeric functional groups, e.g., carboxyl groups) within the molecule include triallylamine, polyaryloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1,4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, and polyethyleneimine (note that cyclic carbonates such as ethylene carbonate are crosslinking agents that further generate the functional group OH through reaction with carboxyl groups). Examples of internal crosslinking agents having polymerizable unsaturated groups and reactive functional groups within the molecule include glycidyl (meth)acrylate. Two or more of these may be used in combination.

[0042] Among these internal crosslinking agents, from the viewpoint of the effects of the present invention, a compound having multiple polymerization unsaturated groups in the molecule is preferred, and more preferably a compound having (poly) Oxy The compound is having alkylene structural units, and more preferably, it is having polyethylene glycol structural units.

[0043] The amount of the internal crosslinking agent used is set appropriately depending on the type of monomer and internal crosslinking agent. From the viewpoint of the gel strength of the resulting superabsorbent resin, the amount is preferably 0.001 mol% or more, more preferably 0.005 mol% or more, and even more preferably 0.01 mol% or more relative to the monomer. Furthermore, from the viewpoint of improving the water absorption performance of the superabsorbent resin, the amount is preferably 5 mol% or less, and more preferably 2 mol% or less. Note that under polymerization conditions in which the self-crosslinking reaction of the monomer is effective, the use of the internal crosslinking agent may be omitted.

[0044] (Polymerization inhibitor) The monomers used in polymerization preferably contain a small amount of polymerization inhibitor for the sake of polymerization stability. A preferred polymerization inhibitor is p-methoxyphenol. The amount of polymerization inhibitor contained in the monomer (especially acrylic acid and its salts) is usually 1 ppm to 250 ppm, preferably 10 ppm to 160 ppm, and more preferably 20 ppm to 80 ppm.

[0045] (Other substances) In the manufacturing method according to the present invention, substances exemplified below (hereinafter referred to as "other substances") may also be added to the monomer aqueous solution, to the extent that the objective of the present invention is achieved.

[0046] Other specific examples of substances include chain transfer agents such as thiols, thiolic acids, secondary alcohols, amines, and hypophosphates; foaming agents such as carbonates, bicarbonates, and azo compounds; chelating agents such as ethylenediaminetetra(methylenephosphinic acid) and its metal salts, ethylenediaminetetraacetic acid and its metal salts, and diethylenetriaminepentaacetic acid and its metal salts; hydrophilic polymers such as polyacrylic acid (salts) and their crosslinked products (e.g., recycled superabsorbent polymer powders), starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol; and fine powders. Other substances may be used individually or in combination of two or more.

[0047] The amount of other substances used is not particularly limited, but for recycled fine powder, it is 30% by mass or less relative to the monomer, and for substances other than fine powder, the total concentration of other substances is preferably 10% by mass or less relative to the monomer, more preferably 0.001% to 5% by mass, and particularly preferably 0.01% to 1% by mass.

[0048] (Monomer concentration in monomer composition (monomer aqueous solution)) In this process, the monomer concentration (=total monomer amount / (total monomer amount + total polymerization solvent amount (usually water)) in the monomer composition (monomer aqueous solution) is preferably 10% to 90% by mass, more preferably 20% to 80% by mass, even more preferably 30% to 70% by mass, and particularly preferably 40% to 60% by mass, from the viewpoint of the physical properties and productivity of the water-absorbent resin. Hereinafter, the monomer concentration may be referred to as the "monomer concentration".

[0049] (Polymerization initiator) The polymerization initiator used in the present invention is not particularly limited, as it is appropriately selected depending on the polymerization mode, etc., but examples include thermal decomposition type polymerization initiators, photodecomposition type polymerization initiators, combinations thereof, or redox type polymerization initiators used in combination with a reducing agent that promotes the decomposition of the polymerization initiator. Specifically, one or more polymerization initiators disclosed in U.S. Patent No. 7,265,190 are used. From the viewpoint of ease of handling of the polymerization initiator and the physical properties of the water-absorbent resin, peroxides or azo compounds are preferably used, more preferably peroxides, and even more preferably persulfates.

[0050] The amount of polymerization initiator used is preferably 0.001 mol% to 1 mol%, more preferably 0.001 mol% to 0.5 mol%, relative to the monomer. Furthermore, if redox polymerization is performed as necessary, the amount of reducing agent used in combination with the oxidizing agent is preferably 0.0001 mol% to 0.02 mol%, relative to the monomer.

[0051] (Dissolved oxygen content) Furthermore, the dissolved oxygen in the monomer aqueous solution before polymerization may be reduced by increasing the temperature or replacing it with an inert gas. For example, the dissolved oxygen is preferably reduced to 5 ppm or less, more preferably to 3 ppm or less, and particularly preferably to 1 ppm or less.

[0052] Furthermore, bubbles (especially the inert gas mentioned above) can be dispersed in the monomer aqueous solution. In this case, the polymerization reaction will be foam polymerization.

[0053] [2-2] Polymerization process This step involves polymerizing the monomer aqueous solution to obtain a water-containing gel-like crosslinked polymer. Preferably, this step involves obtaining a water-containing gel that is a crosslinked body mainly composed of poly(meth)acrylic acid (salt).

[0054] In addition to the method of carrying out the polymerization reaction by adding the polymerization initiator described above, there is also a method of irradiation with active energy rays such as radiation, electron beams, and ultraviolet rays. Furthermore, irradiation with active energy rays may be used in combination with the addition of a polymerization initiator.

[0055] (polymerization form) The polymerization method can be batch or continuous aqueous solution polymerization. It can also be belt polymerization or kneader polymerization. Furthermore, continuous aqueous solution polymerization is more preferred, and either continuous belt polymerization or continuous kneader polymerization can be applied. Specific polymerization methods are disclosed in U.S. Patent No. 4,893,999, No. 6,241,928, and U.S. Patent Publication No. 2005 / 215734, for example, for continuous belt polymerization, and in U.S. Patent No. 6,987,151 and No. 6710,141, for example, for continuous kneader polymerization. By employing these continuous aqueous solution polymerization methods, the production efficiency of water-absorbent resins can be improved.

[0056] Furthermore, preferred forms of the above-mentioned continuous aqueous solution polymerization include "high-temperature initiated polymerization" and "high-concentration polymerization." "High-temperature initiated polymerization" refers to a form in which polymerization is initiated at a temperature of 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 (upper limit is the boiling point) of the monomer aqueous solution. "High-concentration polymerization" refers to a form in which polymerization is carried out at a monomer concentration of 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 (upper limit is the saturation concentration). These polymerization forms can also be used in combination.

[0057] (polymerization rate of water-containing gel) The polymerization rate of the hydrated gel-like crosslinked polymer obtained in the polymerization process and fed into the inlet of the gel grinding apparatus is 90% by mass or more. Preferably, the polymerization rate of the hydrated gel-like crosslinked polymer fed into the inlet of the gel grinding apparatus is 95% by mass or more, more preferably 98% by mass or more, and particularly preferably 99% by mass or more. If gel grinding is performed when the polymerization rate is low (i.e., when the polymerization rate is less than 90% by mass) (for example, when polymerization and gel grinding are performed simultaneously, such as in kneader polymerization), a large amount of unreacted monomers contained in the ground gel particles polymerize and adhere the ground gel particles to each other, thus generating large-diameter gel particles again. Therefore, if gel grinding is performed during the polymerization process, it is necessary to grind the regenerated large-diameter gel particles again, which increases the energy required for grinding and leads to the problem of the polymerization apparatus becoming excessively large. Furthermore, if a water-containing gel with a low polymerization rate is pulverized after the polymerization process and dried while containing a large amount of unreacted monomers, the polymerization reaction will proceed during drying, and large-diameter water-containing gel particles will be regenerated from small-diameter gel particles. This can lead to problems such as a decrease in the water absorption rate of the resulting superabsorbent resin and an increase in the particle size of the dried product. While there is no particular upper limit to the polymerization rate, and 100% by mass is ideal, high polymerization rates require long polymerization times and strict polymerization conditions, which can lead to a decrease in productivity and physical properties. Therefore, an upper limit of 99.99% by mass, 99.95% by mass, even 99.9% by mass, and usually around 99.8% by mass is sufficient. Typically, the polymerization rates of water-containing gel-like crosslinked polymers obtained in the polymerization process are 90-99.99% by mass, 95-99.99% by mass, 98-99.99% by mass, 98-99.95% by mass, 98-99.9% by mass, and 98-99.8% by mass.

[0058] [2-3] Shredding process The shredding step is an optional step performed after the polymerization step and before the gel grinding step in which the hydrated gel-like crosslinked polymer is cut or roughly crushed to a size that can be fed into a gel grinding device. In particular, it is preferable to perform this shredding step when the polymerization step is belt polymerization and a sheet-like or block-like hydrated gel is obtained. Therefore, in one embodiment of the present invention, the hydrated gel-like crosslinked polymer obtained after the polymerization step is in the form of a sheet, and the invention further includes a shredding step in which the sheet-like hydrated gel-like crosslinked polymer is shredded before the gel grinding step. The means for cutting or roughly crushing the hydrated gel in the shredding step are not particularly limited, and rotary cutters, roller cutters, guillotine cutters, cutter mills, screw extruders (e.g., meat choppers, etc.) can be used, or used in combination as needed. The size to be shredded is not particularly limited as long as it can be fed into the gel grinding device described later, but the size of the hydrated gel after shredding is preferably 0.8 mm to 3 m, more preferably 1 mm to 2.5 m, and particularly preferably 1 mm to 2 m. Note that the shredding step may not be performed if the objective of the present invention is achieved.

[0059] Furthermore, in the shredding process, water and / or steam (preferably water and steam) may be supplied, similar to the specific instructions given in the gel grinding process described later. The supply of water and / or steam is carried out, for example, by supplying water and steam into the apparatus used in the shredding process. The apparatus used in the shredding process may be provided with multiple supply ports as a means of supplying water and / or steam. The specific means and preferred ranges for the method of adding steam, pressure, temperature of water and / or steam, and amount of water and / or steam supplied are the same as those described in the gel grinding process described later.

[0060] [2-4] Gel grinding process This step involves crushing the water-containing gel-like crosslinked polymer obtained in the polymerization step to obtain particulate water-containing gel-like crosslinked polymer after the polymerization step described above. In this step, the particle size of the particulate water-containing gel is adjusted to a preferred range described later. This step may be performed two or more times to obtain particulate water-containing gel with a predetermined particle size.

[0061] (Gel grinding device) In the manufacturing method according to the present invention, as shown in Figures 1 and 2, a gel grinding device having an input port, a main body containing multiple rotating shafts, and an output port is used in the gel grinding step after the polymerization step. Each rotating shaft has a grinding means. In this gel grinding device, the water-containing gel-like crosslinked polymer continuously fed into the main body from the input port is ground by the grinding means of each rotating shaft, and the particulate water-containing gel-like crosslinked polymer is continuously discharged from the output port. In this invention, the main body refers to the body portion (reference numeral 208 in Figure 1) in which the multiple rotating shafts and grinding means are installed, and is also referred to as a barrel, trough, casing, etc.

[0062] The gel grinding apparatus used in the manufacturing method according to the present invention may be vertical (where the direction of movement of the water-containing gel is vertical), horizontal, or transverse (where the direction of movement of the water-containing gel is left-right or horizontal), as long as it is a continuous type. Furthermore, vertical and transverse gel grinding apparatuses may have an inclination of 0° to 90° with respect to the horizontal direction. For example, in the case of the transverse continuous grinding apparatus shown in Figure 1, an inclination is provided as needed, and the inclination may be downward or upward from the inlet to the outlet (i.e., with respect to the direction of movement of the water-containing gel). Typically, the inclination angle is 0° to 10°, preferably 0° to 1°, and particularly preferably 0°.

[0063] In conventional manufacturing methods, when using an extruder (meat chopper) for gel pulverization, the water-containing gel is substantially pulverized near the die installed at the extrusion port, and hardly any gel pulverization occurs in the screw section involved in conveying the water-containing gel. In contrast, in the case of the gel pulverization apparatus (particularly the kneader) used in the manufacturing method according to the present invention, the water-containing gel that is introduced is continuously pulverized to a smaller particle size by the pulverizing means of the rotating shaft from the input port to the discharge port.

[0064] In detail, in this gel grinding apparatus, the water-containing gel introduced from the input port is ground to the desired particle size before being discharged from the discharge port. Therefore, unlike conventional extruders (meat choppers), this gel grinding apparatus does not require extrusion from a die, and particulate water-containing gel adjusted to the desired particle size is discharged from the discharge port. In the manufacturing method according to the present invention, by using this gel grinding apparatus, a water-absorbing resin with excellent water absorption rate can be obtained.

[0065] From the viewpoint of continuously grinding gel, it is preferable that the gel grinding apparatus has a heating means and / or a heat retention means. The heating means and / or heat retention means are not particularly limited, but from the viewpoint of preventing adhesion and aggregation of water-containing gel and particulate water-containing gel, heating means by direct heat transfer by convection and / or indirect heat transfer by heat conduction from the heated surface of the gel grinding apparatus (contact surface with water-containing gel, heat source part) heated by a heat transfer medium are preferred. More preferred heating means are vent heating for direct heat transfer and outer wall heating for indirect heat transfer.

[0066] From the viewpoint of reducing damage to the water-containing gel, the main body is preferably provided with a heating means and / or a heat retention means, more preferably a heating means, on its outer surface. An example of this heat retention means is a method of covering part or all of the outer surface of the main body (preferably 50% or more of the outer surface area, more preferably 80% or more, and especially preferably the entire surface) with an insulating material. Examples of the heating means include an electric tray, a steam tray, or a jacket heated with a heat transfer medium, all installed to cover part or all of the outer surface of the main body (preferably 50% or more of the outer surface area, more preferably 80% or more, and especially preferably the entire surface). The particle size of the particulate water-containing gel required in this invention is considerably smaller than that of conventional methods. Therefore, the adhesion and fluidity of the water-containing gel particles due to temperature changes are greater than those assumed in the conventional technology. As a result, the energy required to pulverize the water-containing gel and the cohesiveness of the pulverized gel particles can vary greatly with temperature, and as will be described later, there may be a suitable temperature range when continuously pulverizing a water-containing gel-like crosslinked polymer. By equipping the gel grinding apparatus with the above-mentioned heating and / or heat retention means, the gel grinding process can be carried out in a more favorable temperature range. Furthermore, deterioration of the quality of gel grinding due to temperature differences such as seasonal variations or day-night cycles can be avoided. In addition, it becomes possible to smoothly guide the gel grinding apparatus into stable operation during startup.

[0067] As long as the effects of the present invention are obtained, the type of grinding means that each rotating shaft has is not particularly limited. For example, various shapes of discs can be used as means that have a shearing effect on the water-containing gel. Discs may be referred to as chips, paddles, elements, kneading elements, rotors, etc. The shape of the disc is not particularly limited and can be appropriately selected from disc-shaped, approximately elliptical, approximately triangular, etc., but it is preferable that it has a major axis diameter X and a minor axis diameter Y, and more preferably is approximately elliptical. Note that the approximately elliptical shape includes not only the elliptical shape but also the barrel shape, track shape, and shapes that are deformed from parts of these (for example, the shape shown in Figure 8, which will be described later). The major axis diameter X and minor axis diameter Y will be described later. It is also possible to use discs of different shapes in combination, and their arrangement is appropriately adjusted from the viewpoint of the particle size of the target particulate water-containing gel and the energy required for grinding. In addition, arms, blades, cutting discs (CDs), etc. may be used in combination as grinding means.

[0068] For example, when each rotating shaft has a disc as a grinding means, and especially when the disc is disc-shaped or (omitted) elliptical, the ratio of the effective length L (Length, mm) inside the main body to the maximum diameter D (Diameter, mm; if multiple discs with different diameters and major axis diameters are used, the ratio of the diameter and major axis diameter of the largest disc) of the disc is defined as L / D. This L / D is preferably 5 or more, more preferably 6 or more, and even more preferably 7 or more. An L / D of 20 or less is preferable because the pressure on the water-containing gel during gel grinding is distributed throughout the main body (barrel), reducing the maximum pressure on the water-containing gel. Also, an L / D of 20 or less is preferable, and more preferably 15 or less. An L / D of 20 or less is preferable because the grinder does not become excessively large, thus suppressing an increase in equipment costs. An L / D of 5 or more and 20 or less is preferable, more preferably 6 or more and 15 or less, and even more preferably 7 or more and 15 or less. Furthermore, this effective length L refers to the axial length (total length) of the main body (barrel) portion, including the inlet and outlet, as shown in Figure 1.

[0069] Furthermore, the effective length L (Length) inside the main unit may be, for example, 200 mm to 8000 mm, or 400 to 7000 mm. In addition, the maximum diameter D (Diameter; if multiple discs with different diameters and major axis diameters are used, the diameter and major axis diameter of the largest disc) may be, for example, 10 to 1000 mm, or 20 to 800 mm.

[0070] Furthermore, the distance (clearance) between the disc and the main body (barrel) may vary depending on the location. When the minimum clearance C is defined as the distance at which the outer circumference of the disc and the inner wall of the main body (barrel) are closest, the minimum clearance C is preferably 20% or less, more preferably 15% or less, even more preferably 10% or less, and particularly preferably 5% or less, relative to the maximum diameter D of the disc. If it is below the above upper limit, the shear force between the barrel and the disc becomes stronger during gel pulverization, resulting in good gel pulverization efficiency. Also, the minimum clearance C is preferably 0.2% or more, more preferably 0.5% or more, and even more preferably 1% or more, relative to the maximum diameter D of the disc. If it is above the above lower limit, contact between the disc and the inner wall of the main body (barrel) is suppressed, and the inclusion of metallic foreign matter due to wear is suppressed. In a preferred embodiment of the present invention, the minimum clearance C is 0.2 to 20% of the maximum diameter D of the disc.

[0071] As described later, in this gel grinding apparatus, the water-containing gel is ground to a predetermined particle size by the rotation of multiple rotating shafts, each having a grinding mechanism. The rotation speed of these multiple rotating shafts may be constant or non-constant, and is set appropriately depending on the apparatus, but is preferably in the range of 1 rpm to 1000 rpm, more preferably 3 rpm to 500 rpm, and even more preferably 5 rpm to 300 rpm. Furthermore, if the rotation speeds of each rotating shaft are different, the ratio of the rotation speeds of the other rotating shafts to the rotation speed of one rotating shaft is usually in the range of 1 to 10, and preferably in the range of 1 to 2. In addition, it is preferable that each rotating shaft is parallel.

[0072] Furthermore, when these multiple rotating shafts have discs as grinding means, the peripheral speed (V) of the disc, as defined in (Equation 1) below, may be constant or non-constant, and is set appropriately depending on the apparatus, but is preferably 0.05 m / s to 5 m / s, more preferably 0.1 m / s to 5 m / s, even more preferably 0.15 m / s to 3 m / s, and particularly preferably 0.2 m / s to 2 m / s. If it exceeds the above range, the shear force on the water-containing gel becomes excessive, which is undesirable because it causes deterioration of the physical properties of the water-containing gel particles after grinding and excessive compaction (aggregation). Also, if it falls below the above range, it is undesirable because the processing amount per unit time in the gel grinding process decreases. Furthermore, when the peripheral speeds of the discs on each rotating shaft are different, the ratio of the peripheral speed of one rotating shaft to the peripheral speed of the other rotating shafts is usually in the range of 1 to 10, and preferably in the range of 1 to 2.

[0073] Circumferential speed (V) (m / s)=πD×n / 60 (Formula 1) Here, in (Equation 1), V is the peripheral speed of the disk (unit: m / s), D is the maximum diameter of the disk (unit: m), and n is the number of rotations of the disk per unit time (unit: rpm).

[0074] Furthermore, the rotation directions of the multiple rotating shafts may be either synchronous (where each shaft rotates in the same direction) or anomalous (where each shaft rotates in opposite directions). A synchronous rotating shaft system can be expected to have self-cleaning properties, while an anomalous rotating shaft system can be expected to have a strong shearing force. The rotation direction of each rotating shaft is appropriately selected in combination with the arrangement (disk pattern) of the crushing means described later.

[0075] The gel grinding apparatus preferably has a function to supply water and / or steam to the inside of the main body. By performing gel grinding while supplying water and / or steam (preferably water and steam), a water-absorbing resin powder with a superior water absorption rate can be obtained. In addition, immediately after the start of gel grinding, the discharged particulate water-containing gel-like crosslinked polymer contains a relatively large amount of coarse particulate water-containing gel-like crosslinked polymer with a particle size of 10 mm or more. However, by performing gel grinding while supplying water and / or steam (preferably water and steam), such coarse particulate water-containing gel-like crosslinked polymer with a particle size of 10 mm or more can be reduced in a short time, and the amount of coarse gel mixed into the obtained particulate water-containing gel-like crosslinked polymer can also be reduced even in a steady state. Therefore, according to one embodiment of the present invention, water and / or steam are supplied to the inside of the main body during the gel grinding process. In another preferred embodiment, water and steam are supplied to the inside of the main body during the gel grinding process. As a means of supplying water and / or steam, the gel grinding apparatus may be provided with a plurality of water and / or steam supply ports. The location of the water and / or steam supply port is not restricted, but it is preferably installed on the side of the water-containing gel inlet. In other words, water and / or steam are supplied from near the water-containing gel inlet. Alternatively, water and steam may be supplied from different supply ports.

[0076] While there are no particular limitations on the addition of water vapor, for example, gases such as air, dry air, or nitrogen may be mixed with the water vapor and added as a mixed gas. The pressure of the added water vapor is not particularly limited, but is preferably 0.2 to 0.8 MPa, and more preferably 0.3 to 0.7 MPa. The temperature of the water and / or water vapor (including the mixed gas) is not particularly limited, but is preferably 50°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, and particularly preferably 80°C or higher. From the viewpoint of suppressing excessive heating and drying of the water-containing gel, it is preferably 200°C or lower, more preferably 170°C or lower, even more preferably 150°C or lower, even more preferably 120°C or lower, and particularly preferably 100°C or lower. In a preferred configuration, the temperature of the water and / or water vapor supplied to the inside of the main body is 50 to 200°C, 50 to 170°C, 50 to 150°C, 50 to 120°C, 60 to 120°C, 70 to 120°C, or 80 to 120°C. The temperature of the water-containing gel and particulate water-containing gel in the gel grinding device can also be adjusted by the temperature and supply amount of water and / or steam (including mixed gases) used. In this case, the steam and / or mixed gases act as a direct heat transfer medium, heating or maintaining the water-containing gel and particulate water-containing gel inside the device to a predetermined temperature. Furthermore, additives such as gel fluidizers, crosslinking agents, oxidizing agents, reducing agents, and polymerization initiators, as described later, may be added to the water and / or steam (including mixed gases).

[0077] The amount of water and / or steam supplied is preferably 0.1% to 50% by mass, more preferably 0.5% to 40% by mass, and even more preferably 1% to 30% by mass, based on the solid content mass of the water-containing gel.

[0078] The gel grinding apparatus used in the manufacturing method according to the present invention preferably has heating means and / or heat retention means on the outer surface of the main body, but a liquid heat transfer medium such as hot water or oil may be introduced into a jacket or the like installed on the outer surface of the main body, or heated gas (hot air) may be introduced as the heat transfer medium. These heat transfer mediums act as heat transfer mediums for indirect heat transfer. From the viewpoint of heating efficiency and / or heat retention efficiency of indirect heat transfer, the temperature of the heat transfer medium is preferably 50°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, and particularly preferably 80°C or higher. On the other hand, from the viewpoint of suppressing excessive heating and drying of the water-containing gel, the temperature of the heat transfer medium is preferably 200°C or lower, more preferably 170°C or lower, even more preferably 150°C or lower, even more preferably 130°C or lower, and particularly preferably 110°C or lower. Particularly preferred heat transfer mediums are hot water or steam. The temperature of the heat transfer medium may be constant, or it may be changed as appropriate during the gel grinding process.

[0079] More preferably, before the water-containing gel is introduced into the gel grinding device, the temperature inside the main body (inner surface) is heated to 50°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, and even more preferably 80°C or higher. This reduces the adhesion of the water-containing gel to the inner surface of the main body. This also further improves the water absorption rate of the resulting water-absorbing resin powder. In other words, in the manufacturing method according to the present invention, it is preferable that the inner surface of the main body is heated to the above-mentioned temperature before the water-containing gel is introduced and / or at the start of gel grinding. More preferably, the inner surface of the main body, the multiple rotating shafts, and the outer surface of the grinding means on each rotating shaft are heated to the above-mentioned temperature. On the other hand, from the viewpoint of suppressing excessive heating and drying of the water-containing gel, before the water-containing gel is introduced into the gel grinding device, the heating temperature inside the main body (inner surface) is preferably 200°C or lower, more preferably 170°C or lower, even more preferably 150°C or lower, even more preferably 130°C or lower, and particularly preferably 110°C or lower. For example, by circulating and maintaining a heat transfer medium inside the jacket provided in the main body, the temperature inside the main body (inner surface) can be adjusted to a desired range. From the viewpoint of maintaining the gel temperature during the gel grinding process, it is preferable that the temperature inside the main body (inner surface) is maintained within the above range during the gel grinding process. A preferred configuration is that, before the water-containing gel is introduced into the gel grinding device, the heating temperature inside the main body (inner surface) is 50-200°C, 50-170°C, 50-150°C, 50-130°C, 50-110°C, 60-110°C, 70-110°C, or 80-110°C.

[0080] The temperature at which the hydrated gel-like crosslinked polymer is continuously pulverized is preferably 50°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, and even more preferably 80°C or higher, as this ensures that the gel pulverization is performed smoothly and continuously.

[0081] While there is no particular upper limit to the temperature at which the hydrated gel-like crosslinked polymer is continuously pulverized, from the viewpoint of suppressing excessive heating and drying of the hydrated gel, a temperature of 200°C or lower is preferred, more preferably 170°C or lower, even more preferably 150°C or lower, even more preferably 130°C or lower, and particularly preferred 110°C or lower.

[0082] Here, for example, "continuously grinding a water-containing gel-like cross-linked polymer at 50°C or higher" means continuously grinding the water-containing gel-like cross-linked polymer while maintaining its temperature at 50°C or higher in the section shown in Figure 1(A), that is, the section from past the inlet to the outlet. In other words, "continuously grinding a water-containing gel-like cross-linked polymer using a grinding means at 50°C or higher" means "continuously grinding the water-containing gel-like cross-linked polymer using a grinding means while maintaining its temperature at 50°C or higher." For example, if the temperature T1 of the water-containing gel-like cross-linked polymer fed into the inlet of the gel grinding device is set to 50°C or higher, and the temperature of the heat transfer medium in the jacket installed outside the main body of the device is set to 50°C or higher, the temperature of the water-containing gel-like cross-linked polymer can be maintained at 50°C or higher in the section shown in Figure 1(A), and the water-containing gel-like cross-linked polymer can be continuously ground at 50°C or higher. Furthermore, even if the temperature T1 of the water-containing gel-like crosslinked polymer introduced into the inlet of the gel grinding device is 50°C or lower, the water-containing gel-like crosslinked polymer is rapidly heated by supplying high-temperature water and / or steam at the inlet, or by setting the jacket heat transfer medium temperature of the device body to a high temperature, and the device is continuously ground at a temperature of 50°C or higher in section (A).

[0083] Furthermore, in the case of large gel grinding devices, it is preferable to circulate a heat transfer medium inside multiple rotating shafts to serve as a heating and / or heat retention means. This shortens the time required to raise the temperature inside the main body when the gel grinding device is started up.

[0084] The porosity V of this gel grinding device, calculated by the following formula (Equation 2), is preferably 40% or more, more preferably 45% or more. Furthermore, the porosity V is preferably 80% or less, more preferably 70% or less, even more preferably 65% ​​or less, and particularly preferably 60% or less. The porosity V of this gel grinding device is preferably 40% or more and 80% or less, more preferably 40% or more and 70% or less, even more preferably 45% or more and 65% or less, and particularly preferably 45% or more and 60% or less.

[0085] Porosity V={(AB) / A}*100 (Formula 2) (Here, A is the internal volume (m³) of the main body portion (i.e., the portion indicated as symbol (A) in Figure 1) excluding the portions corresponding to the input and output ports in the longitudinal direction from the main body (barrel). 3 ) and B is the sum of the volumes of the multiple rotating shafts, screws and grinding means located in the part indicated by symbol (A) in Figure 1 (m 3 ) is. ) When the porosity V is below the upper limit, the pulverization of the water-containing gel becomes uniform, and the proportion of coarse particles decreases. Also, when the porosity V is above the lower limit, excessive pressure is less likely to be applied to the water-containing gel, and a decrease in productivity is less likely to occur.

[0086] In the gel pulverizing apparatus used in the manufacturing method according to the present invention, it is most preferable not to use a die (die plate), but a die may be installed at the discharge port as long as the effects of the present invention are obtained. When a die is used, it is preferable that the die opening ratio is 25% or more, more preferably 50% or more, even more preferably 60% or more, even more preferably 70% or more, and particularly preferably 80% or more. There is no particular upper limit to the opening ratio. An opening ratio of 100% is equivalent to not using a die. A die (die plate) is a plate having (multiple) through holes for discharging material from inside the main body, and is installed near the discharge port of the pulverizing apparatus. The opening ratio refers to the ratio of the total planar area of ​​all through holes to the planar area of ​​the die. The larger the opening ratio, the less likely the material inside the main body is to be blocked and the easier it is to discharge, so the effects of the present invention become more pronounced. In addition, as a mechanism for adjusting the size of the discharge port 210 in Figure 1, a plate without through holes may be installed to appropriately adjust the size of the discharge port.

[0087] Figures 1 and 2 show an example of a gel grinding apparatus 200 used in the manufacturing method according to the present invention. Figure 1 is a partially cutaway side view of the gel grinding apparatus 200, and Figure 2 is an enlarged view of the gel grinding apparatus 200 (a view of the central part of the main body from above). The basic configuration and usage method of the gel grinding apparatus 200 will be explained below using Figures 1 and 2.

[0088] As shown in the figure, the gel grinding device 200 comprises an inlet 204, a main body 208, two rotating shafts 206, an outlet 210, a drive unit 214, and a gas supply port 216. The main body 208 is also referred to as a barrel. In Figure 1, two rotating shafts 206 are provided along the direction perpendicular to the plane of the paper. The two rotating shafts 206 are parallel. The rotating shafts 206 extend in the longitudinal direction of the main body 208. One end of each rotating shaft 206 passes through the main body 208 and is connected to the drive unit 214. Although not shown, in this gel grinding device 200, the other end of each rotating shaft 206 is rotatably supported by a bearing installed behind it. In other words, the rotating shaft 206 is supported at both ends. However, the gel grinding device in the manufacturing method according to the present invention is not limited to this double-supported configuration, and may have a so-called single-supported structure without a bearing behind the outlet 210, as long as the objective of the present invention is achieved. The input port 204, gas supply port 216, and discharge port 210 are each fixed to the main body 208 and communicate with the inside of the main body 208. In Figure 1, the left-right direction is the length direction of the main body 208 and the axial direction of the rotation axis 206. Although not shown, the main body 208 has a jacket structure. The discharge port 210 may be installed on the underside of the main body as shown in Figure 1, or it may be installed at the rear of the main body. By installing it at the rear of the main body, the crushing section can be made longer even with the same size device.

[0089] Figure 2 shows a part of the main body 208 of the gel grinding device 200. Figure 2 is an enlarged view of the gel grinding device of Figure 1 (a view of the central part of the main body from above). As shown, in this gel grinding device 200, two rotating shafts 206 are built into the main body 208. The two rotating shafts 206 each have a screw 218 and a grinding means 212, starting from the one closest to the input port 204. Although not shown, a reverse screw may be installed directly in front of the end plate located behind the discharge port 210 to guide the ground material to the discharge port 210. Grinding means 212 are provided on the outer circumference of each of the two rotating shafts 206. That is, the grinding means 212 and the rotating shafts 206 are configured as separate components. In this embodiment, the rotating shaft 206 has multiple disks as the grinding means 212. The vertical direction in Figure 2 is the width direction of the main body 208. In Figure 2, the left-right direction is the length direction of the main body 208 and the axial direction of the rotation axis 206.

[0090] In one preferred embodiment of the gel grinding process using this gel grinding apparatus 200, first, the main body 208 is heated by circulating a heat transfer medium through a jacket (not shown). Then, each rotating shaft 206 is rotated by a drive device 214 (e.g., a motor). As the rotating shafts 206 rotate, the rotating shafts 206 and the multiple disks that constitute the grinding means 212 also rotate.

[0091] Next, the water-containing gel is continuously introduced into the inlet 204. At this time, water and / or steam may also be supplied to the inlet 204 simultaneously. In addition, steam and / or water may be supplied to the gas supply port 216. The water-containing gel and the main body 208 are heated by the water and / or steam and maintained at a predetermined temperature.

[0092] The water-containing gel introduced into the main body 208 comes into contact with the screw 218. Inside the main body 208, the water-containing gel moves towards the outlet 210 due to the action of the rotating screw 218. A portion of the water-containing gel is coarsely pulverized by the rotation of the screw 218.

[0093] The water-containing gel comes into contact with the crushing means 212 (i.e., multiple discs) inside the main body 208. The water-containing gel is pulverized by the shearing action of the rotating multiple discs. The water-containing gel is crushed by the transport action of the screw 218 and the shearing action of the crushing means 212 as it moves towards the discharge port 210. At the discharge port 210, particulate water-containing gel adjusted to a predetermined particle size is removed. In Figure 1, the discharge port is positioned facing downwards so that the particulate water-containing gel is removed from below, but the discharge port may also be positioned at the rear of the device so that the particulate water-containing gel is removed from the rear of the device.

[0094] The rotating shaft of the gel grinding device has multiple screws of different shapes, each having a grinding mechanism, and multiple disks. The shapes of the multiple disks may be the same or different.

[0095] Here, the shape of the disk is not limited, but examples include the shape shown in Figure 5 of Japanese Patent Publication No. 2013-139124 (flat disk, Figure 8) and the shape shown in Figure 6 of Japanese Patent Publication No. 2013-139124 (helical disk). An example of a flat disk is shown in Figure 8. Figure 8 is a front view, and the flat disk 10 has a shape in which, in a front view, the central area of ​​a substantially elliptical disk has a hole 11 through which the axis of the screw 218 passes and a flat portion 12 in the direction of the major axis. The major axis diameter X refers to the maximum length of the straight line connecting any two points passing through the center, and the minor axis diameter Y refers to the minimum length of the straight line connecting any two points passing through the center. In Figure 8, the major axis diameter X refers to the length of the straight line connecting the endpoints that are located at point-symmetrical positions on the flat portion 12. The helical disk is the same as the flat disk except that the flat portion 12 is inclined in the thickness direction. The reverse helical disc is similar to the helical disc except that the inclination of the flat section 12 is in the opposite direction to that of the helical disc.

[0096] It is preferable that the disc's minor axis diameter Y (mm) satisfies 0.2 ≤ Y / X ≤ 0.6 with respect to the disc's major axis diameter X (mm), more preferably 0.3 ≤ Y / X ≤ 0.6, and even more preferably 0.4 ≤ Y / X ≤ 0.6. When Y / X is above the lower limit, the strength of the disc is sufficient, and there is little or no risk of disc breakage even when the gel crushing strength is high. Furthermore, when Y / X is below the upper limit, the inter-axis distance is appropriate and the device size does not become too large, which is preferable in terms of device cost. If the major axis diameter and minor axis diameter of each disc are different, it is preferable that the major axis diameter and minor axis diameter of each disc satisfy the above range.

[0097] Similarly, with respect to the disk's major axis diameter X, it is preferable that the disk thickness T satisfies 0.05 ≤ T / X ≤ 1.0, more preferably 0.05 ≤ T / X ≤ 0.8, and even more preferably 0.07 ≤ T / X ≤ 0.5. When T / X is above the lower limit, the disk strength is sufficient, and there is little or no risk of disk damage even when the gel crushing strength is high. Also, when T / X is below the upper limit, the disk pattern that can be incorporated into the main body can be appropriately set, making it easier to adjust the gel crushing strength. In addition, the weight per disk does not become too heavy, and the burden of replacement work is reduced, especially when scaling up. A preferred configuration is when, with respect to the disk's major axis diameter X, the disk's minor axis diameter Y satisfies 0.2 ≤ Y / X ≤ 0.6, and the disk thickness T satisfies 0.05 ≤ T / X ≤ 1.0.

[0098] The combination of screw and disc is appropriately modified according to the physical properties of the water-containing gel, the size of the desired particulate water-containing gel, etc., referring to, for example, patent documents (Japanese Patent Publication No. 2005-35212). The symbol FS shown in Figure 3 is the feed screw, which is mainly responsible for transporting the material into the main body. The symbol RS is the reverse screw, which is installed in front of the end plate and is responsible for reversing the flow of material and guiding the material to the discharge port 210. The symbol F is the flat disc, which is mainly responsible for kneading. The symbol H is the helical disc, which is responsible for both kneading and transporting. The symbol RH is the reverse helical disc, which is responsible for kneading and returning (returning the material). Figure 3 shows, as an example, the specific configuration of the rotating shaft of the gel grinding device used in the embodiments described later. The numbers 1 to 4 shown in Figure 3 are the phase angles of each disc. In the device in Figure 3, the phase difference of each disc is set to 45 degrees. With disc 1 as the reference, disc 2 is tilted 45 degrees in the opposite direction of rotation of the axis of rotation, disc 3 is tilted 90 degrees, and disc 4 is tilted 135 degrees. With disc 2 as the reference, disc 3 is tilted 45° in the opposite direction of rotation of the axis of rotation, disc 4 is tilted 90°, and disc 1 is tilted 135°. With disc 3 as the reference, disc 4 is tilted 45° in the opposite direction of rotation of the axis of rotation, disc 1 is tilted 90° in the direction of rotation, and disc 2 is tilted 135° in the direction of rotation. With disc 4 as the reference, disc 1 is tilted 45° in the opposite direction of rotation of the axis of rotation, disc 2 is tilted 90°, and disc 3 is tilted 135°. When the phase difference between adjacent discs in the direction of travel from the inlet to the outlet is 45° in the direction of rotation of the axis of rotation, the gel is returned in the opposite direction of travel, and when the phase difference between adjacent discs in the direction of travel from the inlet to the outlet is 90°, the gel remains in place and is mixed. If the phase difference between adjacent discs in the direction of travel from the input to the output is 45° in the opposite direction to the rotation of the rotation axis, the gel is fed in the direction of travel. Therefore, when the discs are arranged 1→2→3→4→1 from the supply port to the output port, it is the feeding phase; when they are arranged 1→3→1→3 or 2→4→2→4, it is the mixing phase; and when the discs are arranged 4→3→2→1→4, it is the return phase.The type, number, and arrangement of each disc are not limited to this specific example and may be changed as appropriate depending on the physical properties of the water-containing gel, the desired size of the particulate water-containing gel, etc. Furthermore, the phase difference is not limited to 45-degree increments. For example, the phase difference may be 15°, 30°, or 60° increments. If the phase difference between adjacent discs in the direction of travel from the input to the output is greater than 0° and less than 90° in the direction opposite to the rotation direction of the rotation axis, it represents the feeding phase; if it is 90°, it represents the mixing phase; and if it is greater than 90° and less than 180°, it represents the return phase. These are not limited to this specific example.

[0099] In the present invention, it is preferable that the arrangement of the grinding means of the gel grinding apparatus includes one or more locations where the phase difference between adjacent disks in the direction of travel from the input port to the discharge port is greater than 90° and less than 180°, resulting in a return phase. When there is one or more locations where the phase difference is greater than 90° and less than 180°, there are locations that act as a return phase and dam up the water-containing gel inside the main body. Therefore, immediately after the start of gel grinding, it is possible to quickly reduce the amount of coarse particulate water-containing gel-like crosslinked polymer with a particle size of 10 mm or more contained in the particulate water-containing gel-like crosslinked polymer from the discharge port 210, thereby reducing the amount of coarse gel mixed into the resulting particulate water-containing gel-like crosslinked polymer. The number of locations in the arrangement of the grinding means of the gel grinding apparatus where the phase difference between adjacent disks is greater than 90° and less than 180° is appropriately set depending on the size of the apparatus, but for example, in order of preference, it is 2 or more, 4 or more, 5 or more, 7 or more, 10 or more, and 14 or more, and the upper limit is 30 or less and 20 or less.

[0100] Examples of gel grinding equipment include, for example, twin-screw or multi-screw kneaders. Specifically, these include twin-screw, triple-screw, quadruple-screw, or octuple-screw kneaders. From the viewpoint of production efficiency, continuous-type gel grinding equipment is preferably used. Specifically, examples of gel grinding equipment include the CKH type continuous kneader (Honda Iron Works Co., Ltd.), twin-screw extruder TEX (Japan Steel Works Ltd.), twin-screw extruder TEXαIII (Japan Steel Works Ltd.), continuous kneader (Dalton Co., Ltd.), KRC hybrid reactor (KRC HYBRID REACTER, Kurimoto Iron Works Co., Ltd.), KRC kneader (KURIMOTO-READCO CONTINUOUS KNEADER, Kurimoto Iron Works Co., Ltd.), KEX extruder (KEX EXTRUDER, Kurimoto Iron Works Co., Ltd.), and KEXD extruder (KEXD Examples include EXTRUDER (Kurimoto Iron Works Co., Ltd.), KNEADER-RUDER (Moriyama Co., Ltd.), TEX-SSG twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-CS twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-SX twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-DS twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-A twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-B twin-screw compounding extruder (Toshiba Machine Co., Ltd.), TEX-BS twin-screw compounding extruder (Toshiba Machine Co., Ltd.), and the WDR series of 4-screw and 8-screw compounding extruders (Technovel Co., Ltd.). Therefore, in a preferred embodiment of the present invention, the gel grinding device is a continuous twin-screw compounding machine.

[0101] (Gel grinding coefficient) The gel grinding coefficient represents the energy required per gram of gel per unit time in a gel grinding device, i.e., the energy required per gram of water-containing gel per unit time within the gel grinding device, and is between 0.02 J / g·sec and 3.0 J / g·sec. If the gel grinding coefficient is less than 0.02 J / g·sec, the water absorption rate is slow and / or the amount of residual monomer increases. If the gel grinding coefficient exceeds 3.0 J / g·sec, the amount of residual monomer increases.

[0102] The "time in the gel grinding apparatus" mentioned above refers to the average residence time of the hydrated gel-like crosslinked polymer in the gel grinding apparatus. The "energy per gram of gel" mentioned above refers to the gel grinding energy. In other words, the gel grinding coefficient is expressed as gel grinding energy [J / g] / average residence time [s]. The average residence time and gel grinding energy are described in detail below. When calculating the gel grinding coefficient, the gel grinding energy and average residence time are rounded to the nearest tenth of a digit, and the gel grinding coefficient is rounded to the second significant digit by rounding the third significant digit according to JIS Z8401:1999.

[0103] The gel grinding coefficient is preferably 0.030 J / g·sec or higher, more preferably 0.040 J / g·sec or higher, and even more preferably 0.050 J / g·sec or higher. From the viewpoint of improving water absorption rate, improving AAP (pressure-induced water absorption ratio), and reducing the amount of residual monomer, the gel grinding coefficient is preferably less than 2.9 J / g·sec, may be 2.0 J / g·sec or lower, or 1.5 J / g·sec or lower. The gel grinding coefficient is preferably 0.030 to 3.0 J / g·sec, more preferably 0.040 to 2.9 J / g·sec, even more preferably 0.050 to 2.0 J / g·sec, and even more preferably 0.080 to 1.5 J / g·sec.

[0104] (Gel pulverization energy) Gel Grinding Energy (GGE) is defined in International Publication No. 2011 / 126079 (corresponding to U.S. Patent Application Publication No. 2013 / 026412 and U.S. Patent Application Publication No. 2016 / 332141) and refers to the mechanical energy per unit mass (unit mass of hydrated gel) required by a gel grinding device when grinding a hydrated gel. In the present invention, since the mechanical energy applied to the hydrated gel-like crosslinked polymer is important, it is preferable to calculate the gel grinding energy by subtracting the current value when the gel grinding device is running idle. In particular, when gel grinding is performed with multiple devices, the sum of the current values ​​during idle operation becomes large, so the method of calculating by subtracting the current value during idle operation is preferable. When the gel grinding device is driven by three-phase AC power, the gel grinding energy is calculated by the following formula (1).

[0105]

number

[0106] However, in this application, the value calculated using the above formula (1), assuming that both the power factor and motor efficiency are 1, is defined as the gel grinding energy (GGE).

[0107] When the gel grinding device is driven by single-phase AC power, 3 in the above formula 1 / 2 This is calculated by changing to 1. In equation (1) above, the unit of voltage is [V], the unit of current is [A], and the unit of mass of water-containing gel is [g].

[0108] In equation (1) above, "mass of water-containing gel fed into the gel grinder per second [g / s]" refers to the value converted to [g / s] if, for example, a water-containing gel-like cross-linked polymer is continuously supplied by a quantitative feeder, and the supply rate is [t / hr].

[0109] The gel pulverization energy (GGE) is preferably 10 J / g or more, more preferably 15 J / g or more, and even more preferably 20 J / g or more. The gel pulverization energy (GGE) is preferably 250 J / g or less, more preferably 200 J / g or less, and even more preferably 150 J / g or less. The gel pulverization energy (GGE) is preferably 10 J / g or more and 250 J / g or less, more preferably 15 J / g or more and 200 J / g or less, and even more preferably 20 J / g or more and 150 J / g or less. A gel pulverization energy (GGE) above the lower limit is preferable because it allows for a reduction in the particle size of the water-containing gel particles, further improving the water absorption rate. A gel pulverization energy (GGE) below the upper limit is preferable because it suppresses an increase in equipment costs and also suppresses compaction (excessive aggregation) of gel particles, improving the water absorption rate.

[0110] The gel grinding energy (GGE) can be adjusted by, for example, the disc pattern (= adjustment of the machine's filling rate, such as the type, number, and arrangement of discs as shown in Figures 3 to 7), the amount of water / steam supplied to the gel grinding device, and the rotational speed of the rotating shaft. For example, a larger number of return phases in the disc pattern tends to increase the gel grinding energy (GGE).

[0111] (Average residence time) The average residence time of the water-containing gel-like crosslinked polymer in the gel grinding apparatus is preferably, in order of preference, 20 seconds or more, 30 seconds or more, 35 seconds or more, 40 seconds or more, 45 seconds or more, and 50 seconds or more, from the viewpoint of reducing residual monomers. The average residence time of the water-containing gel-like crosslinked polymer in the gel grinding apparatus is preferably, in order of preference, 1200 seconds or less, 1000 seconds or less, 800 seconds or less, 700 seconds or less, and 600 seconds or less, from the viewpoint of productivity and equipment cost. The average residence time of the water-containing gel-like crosslinked polymer in the gel grinding apparatus is preferably 20 seconds or more and 1200 seconds or less, preferably 30 seconds or more and 1200 seconds or less, more preferably 30 seconds or more and 1000 seconds or less, even more preferably 35 seconds or more and 800 seconds or less, particularly preferably 40 seconds or more and 700 seconds or less, and most preferably 45 seconds or more and 600 seconds or less. It is preferable that the average residence time of the water-containing gel-like crosslinked polymer in the gel grinding apparatus is above the lower limit, as this suppresses compaction (excessive aggregation) of gel particles and further improves the water absorption rate. It is also preferable that the average residence time of the water-containing gel-like crosslinked polymer in the gel grinding apparatus is below the upper limit, as this improves productivity and reduces mechanical damage to the water-containing gel.

[0112] The average residence time of a water-containing gel is adjusted by factors such as the ratio of the effective length L inside the gel grinding device to the maximum diameter D of the disc (L / D), the disc pattern (= adjustment of the machine's filling rate, such as the type, number, and arrangement of discs as shown in Figures 3 to 7), the rate at which the water-containing gel is fed, and the amount of water / steam supplied to the gel grinding device. A high L / D ratio, a slow rate of water-containing gel feeding, and a high amount of water / steam supplied to the gel grinding device tend to result in a longer average residence time.

[0113] The average residence time of the water-containing gel-like crosslinked polymer in the gel grinding apparatus shall be the value measured by the method described in the examples.

[0114] (Gel temperature) In the gel pulverization process, the temperature T1 of the water-containing gel-like crosslinked polymer introduced into the inlet of the gel pulverizer (hereinafter also referred to as "gel temperature T1 at the inlet" or simply "gel temperature T1") is preferably 50°C or higher. By controlling the gel temperature T1 in this way, it becomes easier to continuously pulverize the water-containing gel-like crosslinked polymer in a preferred form at 50°C or higher. This gel temperature T1 is preferably measured by a thermometer installed at the inlet. From the viewpoint of preventing adhesion between the pulverized water-containing gels, this gel temperature T1 is preferably 60°C or higher, more preferably 70°C or higher, and even more preferably 80°C or higher, from the viewpoint of further improving the water absorption rate of the water-absorbing resin powder. From the viewpoint of suppressing excessive drying, the gel temperature T1 is preferably 130°C or lower, more preferably 110°C or lower, even more preferably 100°C or lower, and particularly preferably 90°C or lower. For similar reasons, the gel temperature during pulverization is preferably 130°C or lower. The gel temperature T1 can be adjusted to a desired range by either keeping the water-containing gel-like crosslinked polymer, whose temperature has risen due to polymerization heat, warm when it is introduced into the gel pulverizer, or by heating the resulting water-containing gel-like crosslinked polymer.

[0115] From the viewpoint of suppressing aggregation of the pulverized water-containing gels, the temperature T2 of the particulate water-containing gel-like crosslinked polymer discharged from the gel pulverizer (hereinafter also referred to as "gel temperature T2 at the discharge port" or simply "gel temperature T2") is preferably 60°C to 140°C, more preferably 70°C to 130°C, even more preferably 80°C to 125°C and 85°C to 120°C, particularly preferably 90°C to 115°C, and most preferably 100°C to 115°C. Preferably, the temperature T2 is set to be within this temperature range, and the temperature T1 is set to be within the aforementioned temperature range. This gel temperature T2 is preferably measured by a thermometer installed at the discharge port. The gel temperature T2 can be adjusted to a desired range by appropriately adjusting the set temperature of the heating means and / or heat retention means of the gel pulverizer, and further, the residence time of the water-containing gel-like crosslinked polymer inside the gel pulverizer.

[0116] From the viewpoint of further improving the water absorption rate of the water-absorbent resin powder, it is preferable that the gel temperature T2 at the discharge port is higher than the gel temperature T1 at the input port. The difference ΔT = (T2 - T1) is preferably 5°C or higher, more preferably 8°C or higher, and even more preferably 10°C or higher. Also, the difference ΔT = (T2 - T1) is preferably 60°C or lower, more preferably 50°C or lower, even more preferably 40°C or lower, and particularly preferably 35°C or lower. Preferably, the temperature range is such that the difference ΔT = (T2 - T1) is relevant, and temperatures T1 and T2 are within the aforementioned temperature range. Note that ΔT can be adjusted to a desired range by adjusting T1 and T2 respectively as described above.

[0117] (Gel solids content) In the gel grinding process, the solid content of the water-containing gel introduced into the gel grinding apparatus (hereinafter referred to as the gel solid content) is determined by the measurement method described in the examples below. From the viewpoint of the degree of aggregation of the ground water-containing gels, the energy required for grinding, drying efficiency, and absorption performance, the gel solid content is preferably 25% to 75% by mass, more preferably 30% to 70% by mass, even more preferably 35% to 65% by mass, and particularly preferably 40% to 60% by mass.

[0118] (Gel fluidizing agent) In the manufacturing method according to the present invention, the gel fluidizer is preferably added before and / or during the gel grinding process. As a result, particulate water-containing gel containing the gel fluidizer is removed from the discharge port. That is, in a preferred embodiment of the present invention, the particulate water-containing gel-like crosslinked polymer contains the gel fluidizer. The addition of the gel fluidizer suppresses the strong adhesion or bonding between the finely ground gel particles, resulting in an improved water absorption rate of the resulting water-absorbing resin. Furthermore, the load in the crushing step during the grinding and sizing processes after the drying process, which will be described later, is reduced, resulting in a reduction in the amount of fine powder generated. From the viewpoint of ensuring that each particle of the resulting particulate water-containing gel uniformly contains the gel fluidizer, addition during the gel grinding process is more preferable, and addition at the same time as the water-containing gel is added (added from the water-containing gel inlet) is even more preferable.

[0119] The amount of gel fluidizer added is appropriately set according to the solid content of the water-containing gel or particulate water-containing gel and the type of gel fluidizer. The amount added is preferably 0.001% to 5% by mass, more preferably 0.01% to 3% by mass, even more preferably 0.02% to 2% by mass, and particularly preferably 0.03% to 1% by mass, relative to the solid content of the water-containing gel.

[0120] Examples of gel-fluidizing agents include anionic, cationic, nonionic, and amphoteric surfactants, as well as low-molecular-weight or high-molecular-weight surfactants and polymeric lubricants. Among these, surfactants are preferred.

[0121] (Surfactants) Specifically, surfactants used in gel fluidizers include: (1) sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkyl allyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate of polyoxyethylene alkyl allyl ethers. Examples of surfactants include (2) nonionic surfactants such as esters, (3) alkyldimethylaminoacetic acid betaines such as capryldimethylaminoacetic acid betaine, lauryldimethylaminoacetic acid betaine, myristyldimethylaminoacetic acid betaine, and stearyldimethylaminoacetic acid betaine; amphoteric surfactants such as lauric acid amidopropyl betaine, coconut oil fatty acid amidopropyl betaine, palm kernel oil fatty acid amidopropyl betaine, alkyl hydroxysulfobetaine such as lauryl hydroxysulfobetaine, and alkylcarboxymethylhydroxyethylimidazolinium betaine such as 2-alkyl-N-carboxymethyl-N-hydroxyethylimidazolinium betaine, (4) anionic surfactants such as monoalkali metal alkylaminodiacetic acid such as monosodium laurylaminodiacetic acid, potassium laurylaminodiacetic acid, and sodium myristylaminodiacetic acid, and (5) cationic surfactants such as long-chain alkyldimethylaminoethyl quaternary salts. Two or more of these may be used in combination. In particular, from the viewpoint of further improving the water absorption rate of the water-absorbing resin powder, amphoteric surfactants are preferred, and alkyldimethylaminoacetic acid betaine is more preferred.

[0122] (Polymer lubricant) In the manufacturing method according to the present invention, polymer lubricants as exemplified below can be added to the monomer aqueous solution or aqueous gel, to the extent that the objective of the present invention is achieved.

[0123] Specific examples of the above polymeric lubricants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified ethylene-propylene-diene terpolymer (EPDM), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethylcellulose, ethyl hydroxyethylcellulose, polyalkylene oxides such as polyethylene glycol, and polysiloxanes modified with side chains and / or terminal polyethers. The molecular weight (weight-average molecular weight) of these is preferably in the range of 200 to 2 million, more preferably in the range of 400 to 1 million, and can be appropriately selected. Two or more of these may be used in combination.

[0124] Furthermore, these polymer lubricants may be used in combination with the above-mentioned surfactants as gel fluidizers. When surfactants and polymer lubricants are used in combination, the total amount added is appropriately set according to the polymerization form, the composition of the monomer aqueous solution, and the water content of the aqueous gel. When added to a monomer aqueous solution, the amount is set as the concentration relative to the monomer components; when added to an aqueous gel, it is set relative to its solid content; and when added to both, it is set as the sum of the above.

[0125] The total amount of surfactant and polymer lubricant added is preferably 5% by mass or less, more preferably 3% by mass or less, preferably 0.001% by mass or more, and particularly preferably 0.01% by mass or more, relative to the solid content of the aqueous gel.

[0126] (surface tension) The type and amount of gel fluidizer added are adjusted as appropriate, taking into consideration the suppression of aggregation of particulate water-containing gel during the gel grinding and drying processes. Based on the amount of return of the resulting superabsorbent resin powder in actual use in absorbent articles (diapers), it is preferable to use a type and amount of gel fluidizer that does not excessively reduce the surface tension of the superabsorbent resin in the final product. For example, the type and amount of gel fluidizer are selected so that the surface tension of the superabsorbent resin (surface tension of a dispersion of superabsorbent resin in physiological saline) is preferably 55 mN / m or higher, more preferably 60 mN / m or higher, and even more preferably 65 mN / m or higher. This surface tension is measured by the method described in WO2015 / 129917. Examples of gel fluidizers that can achieve a surface tension within this range include amphoteric surfactants.

[0127] (Other additives) During the gel grinding process, or between the gel grinding process and the subsequent drying process, the surface crosslinking agent described in [2-7-1] or other additives described in [2-10] may be added.

[0128] (Particle size of particulate water-containing gel) In the manufacturing method according to the present invention, from the viewpoint of the absorption rate of the absorbent resin powder produced, the mass-average particle diameter d1 in terms of solid content of the particulate water-containing gel-like crosslinked polymer discharged from the outlet of the gel pulverizer is 500 μm or less. If d1 exceeds 500 μm, the water absorption rate slows down (see Comparative Example 2 below). d1 is preferably 460 μm or less, more preferably 400 μm or less, and even more preferably 350 μm or less. Since a smaller d1 is preferable, its lower limit is not particularly limited, but it is usually 1 μm or more, and may be 10 μm or more, 20 μm or more, 30 μm or more, or 50 μm or more. The mass-average particle diameter d1 in terms of solid content of the particulate water-containing gel-like crosslinked polymer discharged from the outlet of the gel pulverizer can be controlled, for example, by the average residence time in the pulverizer, the gel crushing energy (GGE), etc. The shorter the average residence time, the larger d1 tends to be. Furthermore, the smaller the gel fracturing energy (GGE), the larger d1 tends to be. The mass-average particle diameter d1 of particulate water-containing gel, calculated on a solid content basis, is determined by the physical property measurement method (g) described later.

[0129] Furthermore, regarding the particle size distribution of the particulate water-containing gel, it is preferable that particles in the range of less than 150 μm, calculated on a solid content basis, constitute 10% or more by mass, more preferably 25% or more by mass, and even more preferably 40% or more by mass. Furthermore, regarding the particle size distribution of the particulate water-containing gel, it is preferable that particles in the range of less than 850 μm, calculated on a solid content basis, constitute 80% or more by mass, more preferably 85% or more by mass, even more preferably 90% or more by mass, particularly preferably 95% or more by mass, with an upper limit of 100% by mass. The logarithmic standard deviation (σζ) of the particle size distribution is 0.2 to 1.0, more preferably 0.2 to 0.8, and even more preferably 0.2 to 0.7.

[0130] (Solid content of particulate water gel) The solid content (solids) of the particulate water-containing gel discharged from the outlet of the gel pulverizer is preferably 25% to 75% by mass, more preferably 30% to 70% by mass, even more preferably 35% to 65% by mass, and particularly preferably 40% to 60% by mass. By subjecting the particulate water-containing gel with a solid content within the above range to the drying process, a granular dried product with high CRC and minimal damage due to drying (such as an increase in water-soluble content) can be obtained.

[0131] (Polymerization rate of particulate water gel) The polymerization rate of the particulate water-containing gel discharged from the gel grinding device is within the range of the polymerization rate before it was introduced into the gel grinding device, and further polymerization may be advanced in the gel grinding process. The degree of polymerization is appropriately adjusted by the heating and residence time in the gel grinding device, the amount of polymerization initiator remaining in the water-containing gel after polymerization, and the amount of any polymerization initiator added afterwards. The polymerization rate after the gel grinding process is determined by the physical property measurement method described later, similar to the polymerization rate before gel grinding. The polymerization rate of the particulate water-containing gel after gel grinding is 90% by mass or more, preferably 95% by mass or more, more preferably 98 to 99.99% by mass, and ideally 100% by mass. In particulate water-containing gels with a polymerization rate within the above range, aggregation and adhesion during drying are avoided.

[0132] [2-5] Drying process This process involves drying a particulate water-containing gel-like crosslinked polymer, preferably a particulate water-containing gel-like crosslinked polymer containing a gel fluidizing agent, to a desired solid content to obtain a dried product. The "solid content" refers to a value calculated from the loss on drying (the change in mass when 1.0 g of the sample is dried at 180°C for 3 hours).

[0133] The solid content of the dried product after the drying process is preferably 80% by mass or more, more preferably 85% to 99.8% by mass, even more preferably 90% to 99.7% by mass, even more preferably 92% to 99.5% by mass, particularly preferably 96% to 99.5% by mass, and most preferably 98% to 99.5% by mass. If the solid content after drying is excessively high, it will not only require a long drying time, but may also lead to deterioration of physical properties and discoloration after drying. If the solid content after drying is low, it may lead to a decrease in productivity in the sizing process described later and a decrease in the water absorption ratio (CRC). When the surface crosslinking process described later is carried out after the drying process, drying to the above solid content is preferable because it further improves the physical properties. The moisture content of the dried product (=100 - solid content) can be determined from the above solid content.

[0134] There are no particular limitations on the drying method used in the drying process of the present invention, and static drying, agitation drying, fluidized bed drying, etc., can be used as appropriate. In addition, various drying methods can be employed, such as heating drying, hot air drying, vacuum drying, infrared drying, microwave drying, drum dryer drying, azeotropic dehydration drying with hydrophobic organic solvents, and high-humidity drying using high-temperature steam.

[0135] (drying equipment) The drying equipment used in the drying process is not particularly limited, and one or more types such as heat transfer conduction dryers, radiant heat transfer dryers, hot air heat transfer dryers, and dielectric heating dryers can be appropriately selected. It may be a batch type or a continuous type. It may also be a direct heating type or an indirect heating type. Examples of heat transfer dryers include ventilated band type, ventilated circuit type, ventilated vertical type, parallel flow band type, ventilated tunnel type, ventilated agitation type, ventilated rotary type, fluidized bed type, and airflow type. When performing band drying (ventilated band drying) with hot air drying on a ventilated belt, the conditions described in International Publication Nos. 2006 / 100300, 2011 / 025012, 2011 / 025013, 2011 / 111657, etc., should be applied as appropriate.

[0136] While there are no particular limitations on the heating method, from the viewpoint of drying efficiency and reducing thermal damage to the water-absorbent resin, heating methods for particulate water-containing gels are preferred, which involve direct heat transfer by convection and / or indirect heat transfer by heat conduction from the heating surface of a dryer heated by a heat transfer medium (the contact surface with the particulate water-containing gel, the heat source part). More preferred heating methods are vent heating for direct heat transfer and outer wall heating and tubular heating for indirect heat transfer.

[0137] In the drying process, a gas may be introduced into the dryer. The gas is not particularly limited, but examples include air, dry air, nitrogen, water vapor, and mixtures thereof. The gas acts as a carrier gas, promoting drying by discharging the water vapor generated during drying to the outside of the dryer. Furthermore, if heated gas is used, the gas also acts as a heat transfer medium, further promoting drying. Preferably, nitrogen, water vapor, and mixtures of these with air are used. When a mixture containing water vapor (hereinafter also referred to as a high-humidity mixture) is used, the inside of the dryer becomes a low-oxygen state, suppressing oxidation and deterioration during drying. As a result, improved performance of the water-absorbing resin and low discoloration can be achieved. The direction of gas movement may be parallel, countercurrent, or a mixture of both relative to the direction of movement of the particulate water-containing gel being dried.

[0138] Drying conditions are appropriately selected depending on the type of drying apparatus and the solid content of the particulate water-containing gel, but the drying temperature is preferably 100°C to 300°C, more preferably 150°C to 250°C, even more preferably 160°C to 220°C, and particularly preferably 170°C to 200°C. If the temperature is below the above range, the drying time becomes excessively long and is uneconomical. If the temperature is above the above range, deterioration of the physical properties of the water-absorbing resin and significant discoloration occur, which is undesirable. The drying time is preferably 1 minute to 10 hours, more preferably 5 minutes to 2 hours, even more preferably 10 minutes to 120 minutes, and particularly preferably 20 minutes to 60 minutes. If the drying time is below the above range, it is necessary to raise the drying temperature excessively, which is undesirable as it deteriorates the physical properties of the water-absorbing resin and significant discoloration occurs. If the temperature is above the above range, the dryer becomes large and the processing capacity decreases, which is uneconomical.

[0139] In the present invention, agitation drying using an agitation dryer is also a preferred drying method. This agitation drying is a type of drying operation by conductive heat transfer, and it is possible to perform the drying treatment on the material to be dried preferably continuously using an indirect heating method. This has the advantage of high drying efficiency and suppression of deterioration of the physical properties of the resulting water-absorbing resin. The agitation method and form of the agitation dryer are not particularly limited, and examples include: a container rotation type dryer in which the container containing the contents itself rotates, vibrates, or oscillates; a mechanical agitation type dryer in which the contents are agitated by a rotating shaft equipped with agitation blades such as arms, blades, and paddles; a floating agitation type dryer in which the contents are suspended by a gas such as air; a flow path division type dryer in which the flow path is divided by gravity and branching plates, etc.; a high-speed shear type dryer; an impact type dryer, etc. In the drying process of the present invention, material agitation drying, in which the material to be dried is dried while being agitated, can also be suitably adopted. In the material agitation drying, adjacent materials to be dried (particulate water-containing gel) are sequentially replaced while being agitated, so that one particle comes into contact with a new particle after another. As a result, the material to be dried (particulate water-containing gel) is efficiently heated by direct and / or indirect heat transfer, and excessive aggregation and clumping of the particulate water-containing gels is suppressed. When performing agitated drying using the above-mentioned agitated dryer, the conditions described in International Publication Nos. 2018 / 092863 and 2018 / 092864 shall be applied as appropriate.

[0140] Furthermore, by adding a surface crosslinking agent, as described later, during this drying process, the drying of the particulate water-containing gel and the heat treatment process for surface crosslinking can be performed in the same process.

[0141] [2-6] Grinding and Classification Process This step involves crushing and / or classifying the dried material obtained in the above drying step to obtain a water-absorbing resin powder, preferably of a specific particle size. This step differs from the gel crushing step described in [2-4] above in that the material to be crushed has already undergone the drying step.

[0142] This process is carried out before and / or after the [2-7] surface crosslinking process, preferably before the [2-7] surface crosslinking process, and may be carried out two or more times before and after the [2-7] surface crosslinking process.

[0143] Examples of equipment (grinders) used in the grinding process of the present invention include high-speed rotary grinders such as roll mills, hammer mills, screw mills, and pin mills, as well as vibratory mills, knuckle-type grinders, cylindrical mixers, etc., which may be used in combination as needed.

[0144] (particle size) The mass-average particle size (D50) of the water-absorbent resin powder before surface crosslinking is preferably 200 μm or more, more preferably 200 μm to 600 μm, even more preferably 220 μm to 550 μm, and particularly preferably 250 μm to 500 μm, from the viewpoint of water absorption rate, water absorption ratio under pressure, etc.

[0145] Furthermore, the content of fine particles with a particle size of less than 150 μm as defined by standard sieving classification is preferable to be as low as possible, preferably 0 to 5% by weight, more preferably 0 to 3% by weight, and even more preferably 0 to 2% by weight relative to the total amount of water-absorbing resin powder.

[0146] Furthermore, the fewer the coarse particles with a particle size of 850 μm or more as defined by standard sieving classification, the better. From the viewpoint of water absorption rate and other factors, 0 to 5% by weight is preferred, 0 to 3% by weight is more preferred, and 0 to 1% by weight is even more preferred, relative to the total amount of water-absorbing resin powder.

[0147] Furthermore, the proportion of particles with a particle size of 150 μm or more and less than 850 μm is preferably 90% by weight or more, more preferably 95% by weight or more, even more preferably 98% by weight or more, and particularly preferably 99% by weight or more, relative to the total water-absorbing resin powder, in terms of water absorption rate, water absorption ratio under pressure, etc. (upper limit is 100% by weight).

[0148] [2-7] Surface crosslinking process This step involves adding a surface crosslinking agent that reacts with the functional groups (particularly carboxyl groups) of the water-absorbent resin powder to cause a crosslinking reaction, and is also referred to as a post-crosslinking step. In the manufacturing method according to the present invention, in this step, after adding the surface crosslinking agent to the water-absorbent resin powder, a crosslinking reaction is caused by heat treatment. The water-absorbent resin powder obtained by the configuration of the present invention has suppressed aggregation and adhesion between particles, so a layer of surface treatment agent is easily formed on the entire surface of each particle, and the surface treatment layer is easily maintained even if pulverization is performed after the surface crosslinking step. Therefore, the AAP (pressure-induced water absorption ratio) of the obtained water-absorbent resin tends to be high. Accordingly, a preferred embodiment of the present invention further includes a surface crosslinking step after the gel pulverization step. This step comprises a surface crosslinking agent addition step and a heat treatment step, and may optionally include a cooling step after the heat treatment step. Furthermore, various conditions described in International Publication No. 2018 / 092863 may be applied as appropriate. The surface crosslinking step can be performed, for example, simultaneously with the drying step, after the drying step, or after the pulverization and classification steps.

[0149] [2-7-1] Surface crosslinking agent addition process This process involves mixing the above-mentioned water-absorbent resin powder with a surface crosslinking agent to prepare a water-absorbent resin powder containing a surface crosslinking agent for use in the surface crosslinking process.

[0150] (Surface crosslinking agent) As the surface crosslinking agent, a surface crosslinking agent capable of reacting with multiple functional groups (preferably multiple carboxyl groups) of the water-absorbing resin is used, preferably a surface crosslinking agent capable of forming covalent or ionic bonds, and even more preferably a surface crosslinking agent capable of forming covalent bonds. Specifically, polyhydric alcohol compounds such as ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin, polyglycerin, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol, sorbitol, etc.; ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglyc Epoxy compounds such as cerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol polyglycidyl ether, glycidol, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, trimethylolpropane polyglycidyl ether, neopentyl glycol diglycidyl ether, and 1,6-hexanediol diglycidyl ether; polyhydric amine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, and polyethyleneimine, and their inorganic or organic salts; polyhydric isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; aziridine compounds such as polyaziridine; polyhydric oxazoline compounds such as 1,2-ethylenebisoxazoline, bisoxazoline, and polyoxazoline; carbonate derivatives such as urea, thiourea, guanidine, dicyandiamide, and 2-oxazolidinone;1,3-Dioxolan-2-one (ethylene carbonate), 4-Methyl-1,3-Dioxolan-2-one, 4,5-Dimethyl-1,3-Dioxolan-2-one, 4,4-Dimethyl-1,3-Dioxolan-2-one, 4-Ethyl-1,3-Dioxolan-2-one, 4-Hydroxymethyl-1,3-Dioxolan-2-one, 1,3-Dioxan-2-one, 4-Methyl-1,3-Dioxan-2-one, 4,6-Dimethyl-1,3-Dioxan-2-one, 1,3-Dioxo Examples include alkylene carbonate compounds such as pan-2-one; halo-epoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin, and their polyvalent amine adducts; oxetane compounds; silane coupling agents such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane; and polyvalent metal compounds such as hydroxides, chlorides, sulfates, nitrates, or carbonates of zinc, calcium, magnesium, aluminum, iron, and zirconium. Two or more of these may be used in combination. Among the above surface crosslinking agents, one or more selected from polyvalent alcohol compounds, polyvalent metal ions, epoxy compounds, oxazoline compounds, and alkylene carbonate compounds are preferred, and one or more selected from polyvalent metal ions, epoxy compounds, oxazoline compounds, and alkylene carbonate compounds are more preferred.

[0151] (Surface crosslinking agent solution) The amount of the surface crosslinking agent added is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 2% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0.1% by mass or less, relative to the solid content of the water-absorbent resin. The lower limit is preferably 0.001% by mass or more, more preferably 0.01% by mass or more.

[0152] The surface crosslinking agent may be added as is, but for ease of addition, it is preferable to add it as a solution dissolved in water or an organic solvent. The concentration of this surface crosslinking agent solution is preferably 1% by mass or more, more preferably 2% by mass or more. The total amount of solvent selected from water and organic solvents is preferably 0 to 10% by mass, more preferably 0.1% to 8% by mass, and even more preferably 0.5% to 5% by mass, relative to the solid content of the water-absorbing resin. When water and organic solvents are used in combination, it is preferable that water is the main component.

[0153] When added as an aqueous solution, it is preferable because the concentration of the aqueous solution can be adjusted according to the water content of the superabsorbent resin powder at the time of contact with the surface crosslinking agent. If the surface crosslinking agent does not form a solution due to its low solubility in water, it is preferable to add a hydrophilic solvent such as alcohol as appropriate to make a homogeneous solution.

[0154] [2-6-2] Heat treatment process This process involves heat-treating a water-absorbing resin powder containing a surface crosslinking agent to obtain a surface-crosslinked dried product.

[0155] (Surface crosslinking temperature) In this process, the water-absorbent resin powder containing the surface crosslinking agent is preferably heated to 100°C or higher. The preferred maximum temperature varies depending on the type of surface crosslinking agent, but is 100°C to 250°C, more preferably 120°C to 230°C, and even more preferably 150°C to 210°C.

[0156] (time) The duration of the heat treatment process should be set appropriately based on factors such as the moisture content of the material being treated, the type of surface crosslinking agent, and the thermal efficiency of the heating equipment. As a general guideline, heating should be continued until the moisture content is 10% by mass or less, which corresponds to a time range of 10 to 120 minutes, preferably 30 to 90 minutes.

[0157] (heating form) While the heating device used in the surface crosslinking process is not particularly limited, a heating device with a stirring mechanism that uses solid-to-solid contact for conductive heat transfer is preferably used, as it is less likely to cause uneven heating.

[0158] [2-7] Cooling process The manufacturing method according to the present invention preferably includes a cooling step, after the aforementioned drying step or surface crosslinking step and before the sizing step described later, in which the dried material or surface-crosslinked dried material is forcibly cooled to a desired temperature. The cooling step can be carried out using conventionally known cooling means. The cooling temperature can also be adjusted as appropriate. With respect to the cooling step (2-7) to the fine powder recycling step (2-9), the conditions disclosed in International Publication No. 2018 / 092864 may be applied as appropriate.

[0159] [2-8] Sizing process This process involves adjusting the particle size of the surface-crosslinked water-absorbent resin powder. This sizing process yields water-absorbent resin powder with more actively controlled particle size or particle size distribution.

[0160] Preferably, the sizing step includes a crushing step and / or a classification step. The crushing step is a step in which the loosely aggregated granular material after the surface treatment step is broken up with a crusher to adjust the particle size. The classification step is a step in which coarse particles and fine powder are removed from the surface-crosslinked granular material or the crushed material thereof using a classifier.

[0161] The crushing machine is not particularly limited and can include, for example, a vibratory mill, a roll granulator, a knuckle-type crusher, a roll mill, a high-speed rotary crusher (pin mill, hammer mill, screw mill), a cylindrical mixer, etc. It is preferable to use a machine that causes little damage to the dried material or surface-crosslinked dried material, and specifically, roll granulators (Matsubo Co., Ltd.), granulators (Kurimoto Iron Works Co., Ltd.), and Randell mills (Tokuju Kogyo Co., Ltd.) are examples. As a classifier, a vibratory or oscillating type sieve classifier using a sieve mesh is used.

[0162] [2-9] Fine powder recycling process The "fine powder recycling process" refers to a process in which the fine powder removed in the classification step is supplied to one of the processes either as is or after granulation. Preferably, it is a process in which the fine powder or fine powder granules are added to a process prior to the drying process for reuse. Examples of processes prior to the drying process include monomer aqueous solutions prepared in the monomer aqueous solution preparation process, hydrated gels in the process of polymerization, pulverization of hydrated gels after polymerization, and drying of particulate hydrated gels. The fine powder may be added to these processes as is, or it may be added after swelling and gelling with water or after granulation. In addition, water, crosslinking agents, binders other than water (e.g., water-soluble polymers, thermoplastic resins), polymerization initiators, reducing agents, chelating agents, color inhibitors, etc. may be added along with the fine powder.

[0163] The preferred amount of fine powder recovered is set appropriately depending on the desired particle size.

[0164] [2-10] Other processes In addition to the steps described above, the manufacturing method according to the present invention may further include, as necessary, a crushing step, a classification step, a re-wetting step, a granulation step, a transport step, a storage step, a packaging step, a custody step, and so on.

[0165] (Other additives) In addition to the surface crosslinking agents and gel fluidizers used as described above, other conventionally known components such as polymer powders (e.g., starches such as tapioca starch acetate), inorganic fine particles, dust suppressants, dried water-absorbing resins (fine powders), fluid permeability enhancers, and reducing agents (e.g., sodium sulfite) can be added as additives before or after drying.

[0166] [3] Physical properties of water-absorbent polymer powder as a product With respect to the superabsorbent resin powder obtained by the manufacturing method according to the present invention (in particular, surface-crosslinked superabsorbent resin powder is also referred to as a water absorbent), when the superabsorbent resin powder or water absorbent is used in absorbent articles, especially disposable diapers, it is desirable that at least one, preferably two or more, more preferably three or more, and even more preferably all of the physical properties listed in (3-1) to (3-5) below be controlled to a desired range. If all of the following physical properties do not meet the following ranges, the effects of the present invention may not be fully obtained.

[0167] [3-1] CRC (centrifuge holding capacity) The CRC (centrifuge capacity) of the water-absorbing resin powder (water absorbent) of the present invention is usually 5 g / g or more, preferably 15 g / g or more, more preferably 25 g / g or more, and even more preferably 30 g / g or more. There is no particular upper limit, and a higher CRC is preferable, but from the viewpoint of balancing with other physical properties, it is preferably 70 g / g or less, more preferably 50 g / g or less, and even more preferably 40 g / g or less.

[0168] If the CRC is less than 5 g / g, the absorption capacity is low, making it unsuitable as an absorbent material for absorbent products such as disposable diapers. Furthermore, if the CRC exceeds 70 g / g, the absorption rate of bodily fluids such as urine and blood decreases, making it unsuitable for use in high-absorption-rate disposable diapers. The CRC can be controlled by changing the type and amount of internal and surface crosslinking agents.

[0169] [3-2] Moisture content and solids content The water content of the water-absorbent resin powder (water absorbent) is preferably greater than 0% by mass and 20% by mass or less, more preferably 1% to 15% by mass, even more preferably 2% to 13% by mass, and particularly preferably 2% to 10% by mass. By keeping the water content within the above range, a water absorbent with excellent powder properties (e.g., fluidity, transportability, damage resistance, etc.) can be obtained. Furthermore, the solid content of the surface-crosslinked water-absorbent resin powder (water absorbent) is preferably 80% by mass or more, more preferably 85% to 99% by mass, even more preferably 87% to 99% by mass, and particularly preferably 90% to 99% by mass.

[0170] [3-3] Particle size The mass-average particle size d3 (D50) of the water-absorbent resin powder (water absorbent) is preferably 200 μm or more, more preferably 200 μm to 600 μm, even more preferably 250 μm to 550 μm, and particularly preferably 300 μm to 500 μm. The proportion of particles with a particle size of less than 150 μm is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less. The proportion of water-absorbent resin powder with a particle size greater than 850 μm is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less. This water absorbent contains particles with a particle size of 150 μm to 850 μm, preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, and particularly preferably 99% by mass or more. Ideally, it is 100% by mass. The logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.20 to 0.50, more preferably 0.25 to 0.40, and even more preferably 0.27 to 0.35.

[0171] [3-4] AAP (Water absorption ratio under pressure) The AAP (pressure-induced water absorption ratio) of the water-absorbent resin powder (water absorbent) is preferably 15 g / g or more, more preferably 20 g / g or more, even more preferably 23 g / g or more, even more preferably 24 g / g or more, particularly preferably 25 g / g or more, and most preferably 26 g / g or more. The upper limit is not particularly limited, but is preferably 35 g / g or less, and more preferably 30 g / g or less.

[0172] If the AAP is less than 15 g / g, the amount of liquid returned when pressure is applied to the absorbent material (sometimes referred to as "Re-Wet") will be high, making it unsuitable as an absorbent material for absorbent articles such as disposable diapers. Note that the AAP can be controlled by adjusting the particle size or changing the surface crosslinking agent. Furthermore, as described above, the superabsorbent resin powder obtained by the configuration of the present invention tends to have a high AAP (pressure-induced water absorption ratio) of the resulting superabsorbent resin.

[0173] [3-5] Vortex (water absorption speed) The vortex (water absorption rate) of the water-absorbing resin powder (water absorbent) is preferably 65 seconds or less, 60 seconds or less, 50 seconds or less, 40 seconds or less, 30 seconds or less, 25 seconds or less, and less than 23 seconds, in that order. The lower limit is not particularly limited, but is preferably 5 seconds or more, and more preferably 10 seconds or more.

[0174] By setting the vortex to the above range, it becomes possible to absorb a predetermined amount of liquid in a short time. When used in the absorbent material of absorbent products such as disposable diapers, the user will feel their skin wet for a shorter time, reducing discomfort and also decreasing leakage.

[0175] [3-6] Amount of residual monomer (content) The residual monomer content in the water-absorbent resin powder (water absorbent) is preferably 450 ppm or less, more preferably 430 ppm or less, and even more preferably 420 ppm or less. This form allows for low skin irritation even when the water-absorbent resin powder (water absorbent) is used in sanitary materials. The lower limit for residual monomer can be around 1 ppm. Note that the content is determined by assuming the presence of unneutralized acrylic acid, regardless of the neutralization rate.

[0176] [4] Uses of water-absorbing resin powder (water absorbent) The water-absorbing resin powder (water absorbent) of the present invention is used for applications requiring water absorption and is widely used as an absorbent material. It is also used in absorbent articles containing the absorbent material. In particular, because the water absorbent material of the present invention has excellent water absorption rate and reduces the amount of residual monomer, it is suitably used as an absorbent material suitable for sanitary articles that are attached to the surface of the body of a human or animal to absorb bodily fluids such as urine and blood (hereinafter referred to as "attachable type absorbent material"). It is also suitably used as an absorbent material suitable for sanitary articles that are placed on an object (for example, a floor, bed sheet, etc.) to absorb bodily fluids such as urine and blood of a human or animal (hereinafter referred to as "placed type absorbent material"). Examples of attachable type absorbent materials include disposable diapers, incontinence pads, and sanitary napkins, while examples of placed type absorbent materials include pet sheets, waterproof sheets for nursing care, and portable toilets for disaster relief. Examples of other absorbent articles include cat litter, drip absorbents, freshness preservatives, and condensation prevention sheets. Other uses for water-absorbing resin powder (water absorbent) include soil water retention agents, seedling sheets, seed coating materials, disposable hand warmers, cooling bandanas, ice packs, medical waste liquid solidifiers, soil solidifiers, waste liquid gelling agents to prevent water damage, water-absorbing sandbags, poultices, cosmetic thickeners, waterproofing materials for electrical and electronic materials and communication cables, gasket packing, sustained-release agents for fertilizers, various sustained-release agents (space disinfectants, fragrances, etc.), wound dressings, condensation prevention building materials, and oil water removers. Furthermore, the water absorbent of the present invention can also be used in applications where it absorbs and swells water and is mixed with resins or substrates, such as in paints, adhesives, antiblocking agents, light diffusing agents, matting agents, additives for decorative panels, additives for artificial marble, and additives for toners.

[0177] Furthermore, as a raw material for the absorbent body, absorbent materials such as pulp fibers can be used together with the water-absorbing resin powder (water absorbent). In this case, the content (core concentration) of water-absorbing resin powder (water absorbent) in the absorbent body is preferably 30% to 100% by mass.

[0178] By setting the core concentration within the above range, skin irritation and rashes during use can be suppressed when the absorbent material is used in absorbent articles. Furthermore, because the absorbent material has excellent diffusion properties for bodily fluids such as urine and blood, efficient liquid distribution is achieved, which is expected to improve the amount of absorption. [Examples]

[0179] The present invention will be described more specifically with reference to the following experimental examples, but the present invention is not limited to these descriptions, and experimental examples obtained by appropriately combining the technical means disclosed in each experimental example are also included within the scope of the present invention.

[0180] In the following, "absorbent polymer" refers to granular dried material that has undergone a drying process, surface-crosslinked granular dried material, or absorbent polymer powder and surface-crosslinked absorbent polymer powder, while "hydrated gel" refers to a hydrated gel-like crosslinked polymer or particulate hydrated gel-like crosslinked polymer that has not undergone a drying process.

[0181] Furthermore, unless otherwise noted, the electrical equipment used in the experimental examples (including the equipment for measuring the physical properties of the superabsorbent polymer) was powered by a 200V or 100V power supply at 60Hz. In addition, unless otherwise noted, the physical properties of the superabsorbent polymer and water-containing gel described below were measured under room temperature (20°C to 25°C) and relative humidity of 50%RH ± 10%.

[0182] Also, for convenience, "liter" may be written as "l" or "L," and "mass%" or "weight%" may be written as "wt%." When measuring trace components, values ​​below the detection limit are marked as ND (Non-Discretionary). It may sometimes be written as "Detected."

[0183] [Physical property measurement method] (a) CRC (centrifuge holding capacity) of superabsorbent polymer powder The CRC (centrifuge capacity) of the superabsorbent polymer powder was measured in accordance with NWSP 241.0.R2(15).

[0184] (b) Water content and solids content of the water-absorbent polymer powder The water content of the superabsorbent polymer powder was measured in accordance with NWSP 230.0.R2(15). For the measurement, the sample mass was changed to 1.0 g and the drying temperature to 180°C. The water content and solids content of the superabsorbent polymer powder were calculated from the loss during drying after 3 hours.

[0185] (c) Mass-average particle size (d3) of superabsorbent polymer powder The mass-average particle size (d3) of the water-absorbent polymer powder was measured in accordance with the method described in columns 27 and 28 of U.S. Patent No. 7,638,570.

[0186] (d) Polymerization rate of the water-containing gel 1.00 g of the sampled water-containing gel was added to 1000 g of deionized water at room temperature (at this point the polymerization reaction was substantially stopped), and after stirring at 300 rpm for 2 hours, the mixture was filtered to remove insoluble matter. The amount of monomer extracted in the filtrate obtained by the above procedure was measured using a liquid chromatograph. When the amount of monomer is taken as the amount of residual monomer m (g), the polymerization rate C (mass%) of the water-containing gel was determined according to the following formula (Equation 3). It is preferable to measure the polymerization rate immediately after sampling the water-containing gel, but if there is a time lag between sampling and measurement, it is necessary to stop the polymerization by forced cooling (contact with dry ice, liquid nitrogen, ice water, etc.).

[0187] C (mass%)=100×{1-m / (M×α / 100)} (Formula 3) However, in (Equation 3), M represents the mass (g) of the water-containing gel, and α represents the solid content (mass%) of the water-containing gel.

[0188] (e) Solid content of water gel The water content of the hydrated gel was measured in accordance with NWSP 230.0.R2(15). For the measurement, the sample mass was changed to 2.0 g, the drying temperature to 180°C, and the drying time to 24 hours. Specifically, 2.0 g of hydrated gel was placed in an aluminum cup with a base diameter of 50 mm, and the total mass W1 (g) of the sample (hydrated gel and aluminum cup) was accurately weighed. Next, the sample was placed in an oven set to an ambient temperature of 180°C. After 24 hours, the sample was removed from the oven, and the total mass W2 (g) was accurately weighed. When the mass of the hydrated gel used in this measurement was M (g), the solid content α (mass%) of the hydrated gel was determined according to the following equation (Equation 4).

[0189] α (mass%)=100-{(W1-W2) / M}×100 (Equation 4).

[0190] (f) Particle size of particulate water gel The mass-average particle size (D50) of the particulate water-containing gel was measured in accordance with the method described in WO2016 / 204302.

[0191] Specifically, 20 g of a water-containing gel (solid content α (mass%)) at a temperature of 20-25°C was added to 1000 g of a 20% sodium chloride aqueous solution containing 0.08% Emal 20C (surfactant, manufactured by Kao Corporation) (hereinafter referred to as "Emal aqueous solution") to form a dispersion, and the mixture was stirred at 300 rpm for 16 hours using a stirrer tip measuring 50 mm in length and 7 mm in diameter (a cylindrical polypropylene container with a height of 21 cm and a diameter of 8 cm, approximately 1.14 L in volume, was used).

[0192] After stirring was complete, the dispersion was poured into the center of a JIS standard sieve (inner diameter 20 cm, sieve mesh size: 8 mm / 4 mm / 2 mm / 1 mm / 0.60 mm / 0.30 mm / 0.15 mm / 0.075 mm) placed on a rotating disc. After washing the total water-containing gel onto the sieve using 100 g of Emal aqueous solution, 6000 g of Emal aqueous solution was poured from above using a shower (72 holes, liquid volume: 6.0 [L / min]) from a height of 30 cm while rotating the sieve by hand (20 rpm) over a range of 50 cm. 2The process of pouring the solution evenly over the entire sieve was repeated four times to classify the water-containing gel. The water-containing gel on the first sieve was drained for approximately 2 minutes and then weighed. The same procedure was followed for the second and subsequent sieves, and the water-containing gel remaining on each sieve after draining was weighed. The type of sieve used was changed as appropriate depending on the particle size of the water-containing gel. For example, if the particle size of the water-containing gel was fine and clogging occurred with sieves with mesh openings of 0.15 mm or 0.075 mm, a larger diameter JIS standard sieve (diameter 30 cm, mesh openings of 0.15 mm and 0.075 mm) was used.

[0193] The proportion (mass %) of the total water-containing gel was calculated from the mass of the water-containing gel remaining on each sieve using the following formula (Equation 5). The sieve opening after draining was set according to the following formula (Equation 6), and the particle size distribution of the water-containing gel was plotted on log-probability paper. The particle size corresponding to 50 mass %R on the plotted sieve was defined as the mass-average particle size (D50) of the water-containing gel with a solid content α (mass %).

[0194] X(%)=(w / W)×100 (Formula 5) R(α)(mm)=(20 / W) 1 / 3 ×r... (Formula 6) Furthermore, here, X; Mass % (%) of the water-containing gel remaining on each sieve after classification and draining. w; Mass (g) of the water-containing gel remaining on each sieve after classification and draining. W: Total mass (g) of water-containing gel remaining on each sieve after classification and draining. R(α); Sieve opening (mm) when converted to a water-containing gel with solid content α (mass%). r; This is the mesh size (mm) of the sieve used to classify a water-containing gel that has swollen in a 20% by mass sodium chloride aqueous solution.

[0195] (g) Mass-average particle size (d1) of particulate water gel on a solid content basis In accordance with WO2016 / 204302, the particle diameter on a solid content basis (mass-average particle diameter converted to solid content for the dry particulate water-containing gel) d1 was calculated from the solid content percentage α (mass%) of the particulate water-containing gel obtained in (e) above and the mass-average particle diameter (D50) of the water-containing gel with solid content percentage α (mass%) obtained in (f) above, according to the following formula (Equation 7).

[0196] SolidD50(d1) = GelD50 × (α / 100) 1 / 3 ... (Formula 7) Furthermore, here, GelD50: Mass-average particle size (μm) of particulate water-containing gel with solid content α (mass%). α: Solid content percentage (mass) of particulate water-containing gel Solid D50 (d1): This is the mass-average particle size (μm) converted to the solid content of the water-containing gel.

[0197] (h) Vortex of superabsorbent polymer powder (water absorption time) The vortex (water absorption time) of the superabsorbent polymer powder was measured according to the following procedure. First, 0.02 parts by mass of food additive Brilliant Blue No. 1 was added to 1000 parts by mass of pre-prepared physiological saline solution (0.9% by mass sodium chloride aqueous solution), and then the liquid temperature was adjusted to 30°C.

[0198] Next, 50 ml of the above-mentioned physiological saline solution was measured into a 100 ml beaker, and 2.0 g of superabsorbent polymer powder was added while stirring at 600 rpm using a stirrer tip with a length of 40 mm and a diameter of 8 mm. The time from when the superabsorbent polymer powder was added until the powder absorbed the physiological saline solution and covered the stirrer tip was measured as Vortex (absorption time) (unit: seconds).

[0199] (i) AAP (Water absorption ratio under pressure) of water-absorbent polymer powder The AAP (pressure-induced water absorption ratio) of the water-absorbent polymer powder was measured in accordance with NWSP 242.0.R2(15). The load condition was changed to 4.83 kPa (0.7 psi) for the measurement.

[0200] (j) Average residence time The average residence time (seconds) of the water-containing gel in the gel grinding apparatus was determined according to the following method.

[0201] First, a blue-colored aqueous gel was prepared separately by polymerization in the same manner as in Example 1 described later, except that Blue No. 1 was added to the monomer aqueous solution at a concentration of 1% by mass (relative to the monomer solution). Next, the uncolored aqueous gel was fed into the gel pulverizer at a predetermined feeding speed and allowed to operate stably. Without changing the feeding speed of the aqueous gel, the blue-colored aqueous gel was fed in place of the aqueous gel for 5 seconds, and then the uncolored aqueous gel was fed at the same speed to continue gel pulverization. The point at which the blue aqueous gel was fed was set as 0 seconds, and particulate aqueous gel discharged from the gel pulverizer was sampled every 5 seconds.

[0202] A 15g sample of hydrated gel was placed in a resealable polyethylene bag (size A, manufactured by Seisan Nippon Co., Ltd., 70mm long, 50mm wide, 0.04mm thick). A 15kg weight with a square base of 80mm x 80mm was then placed on top of the bag for 5 seconds to form a sheet. During this process, any trapped air in the bag was removed. Subsequently, the b-value of the resulting sheet-like sample was measured using a spectrophotometer SZ-Σ80 COLOR MEASURING SYSTEM (manufactured by Nippon Denshoku Industries Co., Ltd.) under the measurement conditions of reflectance measurement / standard whiteboard No. 1 / 30φ light-emitting pipe. Five measurements were taken for each sample, and the average value was calculated. The weight was placed on the sample to adjust its shape after each measurement. The b-value was similarly calculated for hydrated gel samples taken every 5 seconds. The stronger the blue tint of the hydrated gel, the smaller the b-value (less than 0 and with a large absolute value). The sampling time for the water-containing gel with the strongest blue tint (minimum b value) was defined as the average residence time (min). If the gel crushing process was performed multiple times, the average residence time for each step was measured, and the sum of these was defined as the average residence time (min).

[0203] (k) Amount of remaining monomers The amount of residual monomer in the superabsorbent polymer was measured in accordance with NWSP 210.0.R2(15). Specifically, 1.0 g of superabsorbent polymer was added to 200 ml of a 0.9% by mass sodium chloride aqueous solution, stirred at 500 rpm for 1 hour using a 35 mm long stirrer tip, filtered, and the amount of monomer eluted into the filtrate was measured by high-performance liquid chromatography. The amount of residual monomer is expressed as a mass percentage of the superabsorbent polymer (unit: ppm).

[0204] (l) Evaluation of the amount of coarse gel The particulate water-containing gel was sampled from the gel grinding device at a time equal to twice the average residence time, starting from the moment the water-containing gel-like cross-linked polymer was introduced into the device through its inlet. 100 g of the sampled particulate water-containing gel was added to 5000 g of a 20% sodium chloride aqueous solution containing 0.08% by mass Emal 20C (surfactant, manufactured by Kao Corporation) (hereinafter referred to as "Emal aqueous solution") to form a dispersion, which was then stirred at 500 rpm for 16 hours using a stirrer tip measuring 50 mm in length and 7 mm in diameter. After stirring, the dispersion was placed in the center of a JIS standard sieve (inner diameter 20 cm, sieve opening 16 mm) and the number of water-containing gel particles on the sieve was measured. A value of ○ indicates 1 or fewer particulate water-containing gel particles, a value of △ indicates 2 to 3 particles, and a value of × indicates 4 or more particles.

[0205] [Example 1] (Preparation process for monomer aqueous solution) A monomer aqueous solution was prepared consisting of 300 parts by mass of acrylic acid, 100 parts by mass of a 48.5% sodium hydroxide aqueous solution, 0.65 parts by mass of polyethylene glycol diacrylate (average n number 9), 16.4 parts by mass of a 0.1% trisodium diethylenetriaminepentaacetate aqueous solution, and 273.2 parts by mass of deionized water.

[0206] Next, while continuously supplying the above monomer aqueous solution, which has been heated to 38°C, using a metering pump, 150.6 parts by mass of a 48.5% by mass sodium hydroxide aqueous solution is added. Line mixer The mixture was continuously mixed. During this process, the temperature of the monomer aqueous solution rose to 87°C due to the heat of neutralization.

[0207] (Polymerization process) Furthermore, 13.2 parts by mass of a 4% by mass sodium persulfate aqueous solution Line mixer After continuous mixing, the mixture was continuously supplied to a continuous polymerization machine having a flat polymerization belt equipped with weirs at both ends, so as to reach a thickness of 10 mm. Polymerization then occurred continuously for 3 minutes, yielding a strip-shaped hydrated gel (1a). The obtained strip-shaped hydrated gel (1a) was cut according to the processing speed and feeding interval of the gel pulverizing device described later, to obtain strip-shaped hydrated gel (1b) with a width of several cm. The polymerization rate of the strip-shaped hydrated gel (1b) was 98.5% by mass, and the solid content was 53% by mass.

[0208] (Gel grinding process) As a gel grinding device, a device equipped with a main body (barrel) containing two rotating shafts rotating in the same direction at the same rotational speed was used to grind a strip-shaped water-containing gel (1b). A feed screw (FS) and a reverse screw (RS), along with grinding means such as a flat disc (F), a helical disc (H), and a reverse helical disc (RH), were installed on each rotating shaft in the configuration shown in Figure 3. The numbers 1 to 4 shown in Figure 3 represent the phase angles of each disc. With disc 1 as the reference, 2 means it is tilted 45 degrees in the opposite direction to the rotational direction of the rotating shaft, 3 means 90 degrees, and 4 means 135 degrees. In the configuration shown in Figure 3, there was one reverse phase. The diameter of each disc was 50 mm. The ratio L / D of the effective length of the main body (effective length inside the barrel) L to the maximum diameter D of the disc was 13.5, and the porosity V was 50%. The barrel had a jacket structure and a gas supply port that penetrated the jacket and introduced water vapor into the main body.

[0209] First, a heat transfer medium was circulated inside the jacket and maintained at 105°C. Then, with the rotation speed set to 50 rpm, a strip-shaped hydrated gel (1b) heated to 80°C was fed into the inlet of the gel pulverizer at a rate of 0.25 kg / min (one strip-shaped hydrated gel (1b) every 5 seconds). At the same time as the hydrated gel (1b), a 10% by mass aqueous solution of lauryldimethylaminoacetic acid betaine and 90°C water were supplied from the inlet, and 0.6 MPa steam was supplied from the gas supply port. The amount of lauryldimethylaminoacetic acid betaine supplied as solid content was 0.08% by mass relative to the solid content of the strip-shaped hydrated gel (1b). The amount of 90°C water supplied was 14.5% by mass relative to the solid content of the strip-shaped hydrated gel (1b). The amount of 0.6 MPa steam supplied was 26.1% by mass relative to the solid content of the strip-shaped hydrated gel (1b). The gel grinding conditions are shown in Table 1. The properties of the particulate water-containing gel (1c) removed from the discharge port of the gel grinding apparatus are shown in Tables 2 and 4.

[0210] (drying process) The obtained particulate water-containing gel (1c) was dried using a hot air dryer. This dryer is equipped with a cage made of wire mesh with a mesh size of 1.2 mm (bottom size 30 cm x 20 cm). 500 g of particulate water-containing gel (1c) was spread almost uniformly on the bottom surface of the cage, and dried product (1A') was obtained by blowing 190°C hot air from below for 30 minutes.

[0211] (Crushing, classification process) The cooled and dried material (1A') was fed into a roll mill and pulverized to obtain a water-absorbent resin (1A) with a mass-average particle size (d2) of 600 μm.

[0212] (Surface crosslinking process) Next, a surface crosslinking agent solution consisting of 0.025 parts by mass of ethylene glycol diglycidyl ether, 0.3 parts by mass of ethylene carbonate, 0.5 parts by mass of propylene glycol, and 2.0 parts by mass of deionized water was sprayed onto 100 parts by mass of water-absorbent resin (1A) and mixed. This mixture was heat-treated at 200°C for 30 minutes to obtain surface-crosslinked water-absorbent resin (1B). This water-absorbent resin (1B) was fed into a roll mill and pulverized, and the components that passed through an 850 μm sieve but not through a 150 μm sieve were collected to obtain water-absorbent resin (1C). The physical properties of water-absorbent resin (1C) are shown in Table 3.

[0213] [Example 2] (Preparation process for monomer aqueous solution) A monomer aqueous solution was prepared in the same manner as in Example 1 (preparation step of monomer aqueous solution).

[0214] (Polymerization process) A strip-shaped hydrated gel (2b) was obtained in the same manner as in Example 1 (polymerization step). The polymerization rate of the strip-shaped hydrated gel (2b) was 98.5% by mass, and the solid content was 53% by mass.

[0215] (Gel grinding process) Particulate water-containing gel (2c) was obtained in the same manner as in Example 1 (gel grinding process), except that the feeding rate of the strip-shaped water-containing gel (2b) was set to 0.45 kg / min (at a rate of one strip-shaped water-containing gel (2b) every 5 seconds), the amount of 90°C water supplied was changed to 13.5% by mass relative to the solid content of the strip-shaped water-containing gel (2b), and the amount of 0.6 MPa water vapor input was changed to 14.5% by mass relative to the solid content of the strip-shaped water-containing gel (2b). The gel grinding conditions are shown in Table 1. The characteristics of the particulate water-containing gel (2c) removed from the discharge port of the gel grinding apparatus are shown in Tables 2 and 4.

[0216] (drying process) A dried product (2A') was obtained in the same manner as in Example 1 (drying step).

[0217] (Crushing, classification process) A water-absorbent resin (2A) with a mass-average particle size (d2) of 615 μm was obtained in the same manner as in Example 1 (grinding and classification process).

[0218] (Surface crosslinking process) A water-absorbent resin (2C) was obtained in the same manner as in Example 1 (surface crosslinking process). The physical properties of the surface-crosslinked water-absorbent resin (2C) are shown in Table 3.

[0219] [Example 3] (Preparation process for monomer aqueous solution) A monomer aqueous solution was prepared in the same manner as in Example 1 (preparation step of monomer aqueous solution), except that polyethylene glycol diacrylate (average n=9) was changed to 1.07 parts by mass and diethylenetriaminepentaacetate trisodium was changed to ethylenediaminetetramethylenephosphonate pentasodium.

[0220] (Polymerization process) A strip-shaped hydrated gel (3b) was obtained in the same manner as in Example 1 (polymerization step). The polymerization rate of the strip-shaped hydrated gel (3b) was 98.5% by mass, and the solid content was 53% by mass.

[0221] (Gel grinding process) In the gel grinding apparatus, feed screws (FS) and flat discs (F), which are grinding means, are installed on two rotating shafts that rotate in the same direction in the configuration shown in Figure 4 (in the configuration shown in Figure 4, there are 14 return phases), the diameter of each disc is changed to 123 mm, the ratio L / D of the effective length of the main body (effective length inside the barrel) L to the maximum diameter D of the disc is changed to 10.2, the heat transfer medium temperature of the jacket is changed to 80°C, the rotation speed is changed to 70 rpm, and strip-shaped water-containing gel (3b) is fed... Particulate hydrated gel (3c) was obtained in the same manner as in Example 1 (gel grinding process), except that the input speed was changed to 3.0 kg / min (at a rate of one strip-shaped hydrated gel (3b) every 5 seconds), the supply amount of lauryldimethylaminoacetic acid betaine was changed to 0.04% by mass relative to the solid content of the strip-shaped hydrated gel (3b), the supply amount of 90°C water was changed to 12.3% by mass relative to the solid content of the strip-shaped hydrated gel (3b), and the input amount of 0.6 MPa steam was changed to 5.3% by mass relative to the solid content of the strip-shaped hydrated gel (3b). The gel grinding conditions are shown in Table 1. The characteristics of the particulate hydrated gel (3c) taken out from the discharge port of the gel grinding device are shown in Tables 2 and 4. Note that, as shown in Figure 4, the discharge port of the gel grinding device is located at the rear of the device.

[0222] (drying process) A dried product (3A') was obtained in the same manner as in Example 1 (drying step).

[0223] (Crushing, classification process) A water-absorbent resin (3A) with a mass-average particle size (d2) of 605 μm was obtained in the same manner as in Example 1 (grinding and classification process).

[0224] (Surface crosslinking process) A water-absorbent resin (3C) was obtained in the same manner as in Example 1 (surface crosslinking process). The physical properties of the surface-crosslinked water-absorbent resin (3C) are shown in Table 3.

[0225] [Example 4] (Preparation process for monomer aqueous solution) A monomer aqueous solution was prepared in the same manner as in Example 1 (preparation step of monomer aqueous solution), except that polyethylene glycol diacrylate (average n=9) was changed to 0.85 parts by mass.

[0226] (Polymerization process) A strip-shaped hydrated gel (4b) was obtained in the same manner as in the polymerization step of Example 1. The polymerization rate of the strip-shaped hydrated gel (4b) was 98.5% by mass, and the solid content was 53% by mass.

[0227] (Gel grinding process) A feed screw (FS) and a reverse screw (RS), along with grinding means consisting of a flat disc (F), a helical disc (H), and a reverse helical disc (RH), were installed on two rotating shafts rotating in the same direction in the configuration shown in Figure 5 (in the configuration shown in Figure 5, there is one reverse phase), and a particulate water-containing gel (4c) was obtained in the same manner as in Example 2 (gel grinding process). The gel grinding conditions are shown in Table 1. The characteristics of the particulate water-containing gel (4c) removed from the discharge port of the gel grinding apparatus are shown in Tables 2 and 4.

[0228] (drying process) A dried product (4A') was obtained in the same manner as in Example 1 (drying step).

[0229] (Crushing, classification process) A water-absorbent resin (4A) with a mass-average particle size (d2) of 583 μm was obtained in the same manner as in Example 1 (grinding and classification process).

[0230] (Surface crosslinking process) A water-absorbent resin (4C) was obtained in the same manner as in Example 1 (surface crosslinking process). The physical properties of the surface-crosslinked water-absorbent resin (4C) are shown in Table 3.

[0231] [Example 5] (Preparation process for monomer aqueous solution) A monomer aqueous solution was prepared in the same manner as in Example 1 (preparation step of monomer aqueous solution), except that polyethylene glycol diacrylate (average n=9) was changed to 1.07 parts by mass.

[0232] (Polymerization process) A strip-shaped hydrated gel (5b) was obtained in the same manner as in Example 1 (polymerization step). The polymerization rate of the strip-shaped hydrated gel (5b) was 98.5% by mass, and the solid content was 53% by mass.

[0233] (Gel grinding process) In the gel grinding apparatus, feed screws (FS) and flat discs (F), which are grinding means, are installed on two rotating shafts that rotate in the same direction in the configuration shown in Figure 6 (in the configuration shown in Figure 6, there are 5 return phases), the diameter of each disc is changed to 53 mm, the ratio L / D of the effective length of the main body (effective length inside the barrel) L to the maximum diameter D of the disc is changed to 8.4, the heat transfer medium temperature of the jacket is changed to 80°C, the rotation speed is changed to 165 rpm, and strip-shaped water-containing gel (5b) is fed in. Particulate hydrated gel (5c) was obtained in the same manner as in Example 1 (gel grinding process), except that the speed was changed to 1.04 kg / min (a rate of one strip-shaped hydrated gel (5b) every 5 seconds), the supply amount of lauryldimethylaminoacetic acid betaine was changed to 0.04 mass% relative to the solid content of the strip-shaped hydrated gel (5b), the supply amount of 90°C water was changed to 26.1 mass% relative to the solid content of the strip-shaped hydrated gel (5b), and the input amount of 0.6 MPa steam was changed to 5.0 mass% relative to the solid content of the strip-shaped hydrated gel (5b). The gel grinding conditions are shown in Table 1. The characteristics of the particulate hydrated gel (5c) taken out from the discharge port of the gel grinding device are shown in Tables 2 and 4. As shown in Figure 6, the discharge port of the gel grinding device is located at the rear of the device.

[0234] (drying process) A dried product (5A') was obtained in the same manner as in Example 1 (drying step).

[0235] (Crushing, classification process) A water-absorbent resin (5A) with a mass-average particle size (d2) of 624 μm was obtained in the same manner as in Example 1 (grinding and classification process).

[0236] (Surface crosslinking process) A water-absorbent resin (5C) was obtained in the same manner as in Example 1 (surface crosslinking process). The physical properties of the surface-crosslinked water-absorbent resin (5C) are shown in Table 3.

[0237] [Example 6] (Preparation step of monomer aqueous solution) A monomer aqueous solution was prepared in the same manner as in the (Preparation step of monomer aqueous solution) of Example 1.

[0238] (Polymerization step) The same operation as in the (Polymerization step) of Example 1 was performed to obtain a strip-shaped water-containing gel (6a). The obtained strip-shaped water-containing gel (6a) was cut to obtain a strip-shaped water-containing gel (6b) with a width of several centimeters. Then, the obtained strip-shaped water-containing gel (6b) was put into a screw extruder for a further shredding process. As the screw extruder, a meat chopper having a screw shaft with an outer diameter of 86 mm and a perforated plate with a diameter of 100 mm, a thickness of 10 mm, 6 holes, and an aperture ratio of 30% at the tip (extrusion port) was used. With the rotational speed of the screw shaft of the meat chopper set at 115 rpm, the strip-shaped water-containing gel (6b) was supplied at 4.6 kg / min, and at the same time, warm water at 80 °C was supplied at 4.6% by mass based on the solid content of the strip-shaped water-containing gel (6b), and steam at 0.6 MPa was supplied at 4.1% by mass based on the solid content of the strip-shaped water-containing gel (6b) to obtain a shredded water-containing gel (6b'). The polymerization rate of the obtained shredded water-containing gel (6b') was 98.9% by mass, and the solid content rate was 51% by mass.

[0239] (Gel pulverization step) The same operation as in the (Gel pulverization step) of Example 2 was performed on the shredded water-containing gel (6b') to obtain a particulate water-containing gel (6c). The shredded water-containing gel (6b') was fed in portions of 37.5 g every 5 seconds so that the feeding speed was 0.45 kg / min. The gel pulverization conditions are shown in Table 1. The properties of the particulate water-containing gel (6c) taken out from the discharge port of the gel pulverization device are shown in Tables 2 and 4.

[0240] (Drying step) A dried product (6A') was obtained in the same manner as in the (Drying step) of Example 1.

[0241] (Pulverization and classification step) A water-absorbent resin (6A) with a mass average particle diameter (d2) of 605 μm was obtained in the same manner as in the (Pulverization and classification step) of Example 1.

[0242] (Surface crosslinking process) A water-absorbent resin (6C) was obtained in the same manner as in Example 1 (surface crosslinking process). The physical properties of the surface-crosslinked water-absorbent resin (6C) are shown in Table 3.

[0243] [Example 7] (Preparation process for monomer aqueous solution) A monomer aqueous solution was prepared in the same manner as in Example 3 (preparation step of monomer aqueous solution).

[0244] (Polymerization process) The same procedure as in Example 1 (polymerization step) was performed to obtain a strip-shaped hydrated gel (7a). The obtained strip-shaped hydrated gel (7a) was cut to obtain strip-shaped hydrated gel (7b) with a width of several centimeters. Subsequently, the obtained strip-shaped hydrated gel (7b) was fed into a screw extruder for a further shredding process. A meat chopper was used as the screw extruder, with an outer diameter of 86 mm on the screw shaft and a perforated plate at the tip (extrusion port) with a diameter of 100 mm, a thickness of 10 mm, 15 holes, and an opening ratio of 29%. With the screw shaft rotation speed of the meat chopper set to 115 rpm, 4.6 kg / min of strip-shaped hydrated gel (7b) was supplied, along with 80°C hot water at 4.6% by mass relative to the solid content of the strip-shaped hydrated gel (7b), and 0.6 MPa of steam at 4.1% by mass relative to the solid content of the strip-shaped hydrated gel (7b), to obtain shredded hydrated gel (7b'). The polymerization rate of the obtained shredded hydrated gel (7b') was 99.0% by mass, and the solid content was 51% by mass.

[0245] (Gel grinding process) Particulate hydrated gel (7c) was obtained by performing the same operation as in Example 3 (gel grinding step) on the shredded hydrated gel (7b'). The shredded hydrated gel (7b') was added at a rate of 0.25 kg every 5 seconds to maintain an input rate of 3.0 kg / min. The gel grinding conditions are shown in Table 1. The characteristics of the particulate hydrated gel (7c) removed from the discharge port of the gel grinding device are shown in Tables 2 and 4. As shown in Figure 4, the discharge port of the gel grinding device is located at the rear of the device.

[0246] (drying process) A dried product (7A') was obtained in the same manner as in Example 1 (drying step).

[0247] (Grinding and classification process of dried materials) A water-absorbent resin (7A) with a mass-average particle size (d2) of 610 μm was obtained in the same manner as in Example 1 (grinding and classification process).

[0248] (Surface crosslinking process) A water-absorbent resin (7C) was obtained in the same manner as in Example 1 (surface crosslinking process). The physical properties of the surface-crosslinked water-absorbent resin (7C) are shown in Table 3.

[0249] [Example 8] (Preparation process for monomer aqueous solution) A monomer aqueous solution was prepared in the same manner as in Example 1 (preparation step of monomer aqueous solution), except that polyethylene glycol diacrylate (average n=9) was changed to 0.61 parts by mass.

[0250] (Polymerization process) A strip-shaped hydrated gel (8a) was obtained by performing the same operation as in Example 1 (polymerization step). The obtained strip-shaped hydrated gel (8a) was cut to obtain strip-shaped hydrated gel (8b) with a width of several centimeters. Subsequently, the obtained strip-shaped hydrated gel (8b) was fed into a screw extruder for a further shredding process. A meat chopper was used as the screw extruder, with an outer diameter of 86 mm on the screw shaft and a perforated plate at the tip (extrusion port) with a diameter of 100 mm, a thickness of 10 mm, 18 holes, and an opening ratio of 29%. With the screw shaft rotation speed of the meat chopper set to 115 rpm, 4.6 kg / min of strip-shaped hydrated gel (8b) was supplied, along with 4.6% by mass of 80°C hot water relative to the solid content of the strip-shaped hydrated gel (8b), and 4.1% by mass of 0.6 MPa of steam relative to the solid content of the strip-shaped hydrated gel (8b), to obtain shredded hydrated gel (8b'). The polymerization rate of the obtained shredded hydrated gel (8b') was 99.0% by mass, and the solid content was 51.1% by mass.

[0251] (Gel grinding process) On two rotating shafts that rotate in the same direction, a feed screw (FS), a reverse screw (RS), and flat disks (F), helical disks (H), and reverse helical disks (RH) as pulverizing means are installed in the configuration shown in Fig. 7 (in the configuration shown in Fig. 7, the return phase is at 0 positions). The temperature of the heat medium in the jacket was changed to 80°C, the rotational speed was changed to 40 rpm, the feeding rate of the shredded water-containing gel (8b') heated to 70°C was set to 0.64 kg / min (a pace of feeding 53.33 g every 5 seconds), and except that the supply amount of water at 90°C was changed to 13.5 mass% based on the solid content of the shredded water-containing gel (8b'), and the input amount of steam at 0.6 MPa was changed to 14.5 mass% based on the solid content of the shredded water-containing gel (8b'), particulate water-containing gel (8c) was obtained in the same manner as in Example 1 (gel pulverization step). The gel pulverization conditions are shown in Table 1. The properties of the particulate water-containing gel (y8c) taken out from the discharge port of the gel pulverizer are shown in Tables 2 and 4.

[0252] (Drying step) In the same manner as in Example 1 (drying step), a dried product (8A') was obtained.

[0253] (Pulverization and classification step) In the same manner as in Example 1 (pulverization and classification step), a water-absorbing resin (8A) with a mass average particle diameter (d2) of 605 μm was obtained.

[0254] (Surface crosslinking step) In the same manner as in Example 1 (surface crosslinking step), a water-absorbing resin (8C) was obtained. The physical properties of the surface-crosslinked water-absorbing resin (8C) are shown in Table 3.

[0255] [Comparative Example 1] In Comparative Example 1, a surface-crosslinked water-absorbing resin (9C) was obtained in the same manner as in Experimental Example 3 disclosed in PCT / JP2021 / 034800. Specifically, it is as follows.

[0256] (Preparation step of monomer aqueous solution) A monomer aqueous solution was prepared in the same manner as in Example 1 (preparation step of monomer aqueous solution), except that polyethylene glycol diacrylate (average n=9) was changed to 0.61 parts by mass.

[0257] (Polymerization process) A strip-shaped hydrated gel (9b) was obtained in the same manner as in the polymerization step of Example 1.

[0258] (Gel grinding process) A feed screw (FS) and a reverse screw (RS), along with grinding means consisting of a flat disc (F), a helical disc (H), and a reverse helical disc (RH), were installed on two rotating shafts rotating in the same direction in the configuration shown in Figure 5. The rotation speed was changed to 100 rpm, the feeding rate of the strip-shaped water-containing gel (9b) was changed to 0.64 kg / min (at a rate of one strip-shaped water-containing gel (9b) every 2.5 seconds), the supply amount of 90°C water was changed to 11.8 mass% relative to the solid content of the strip-shaped water-containing gel (9b), and the input amount of 0.6 MPa water vapor was changed to 9.7 mass% relative to the solid content of the strip-shaped water-containing gel (9b). Particulate water-containing gel (9c) was obtained in the same manner as in Example 1 (Gel Grinding Process). The gel grinding conditions are shown in Table 1. The characteristics of the particulate water-containing gel (9c) taken out from the discharge port of the gel grinding apparatus are shown in Tables 2 and 4.

[0259] (drying process) A dried product (9A') was obtained in the same manner as in Example 1 (drying step).

[0260] (Crushing, classification process) The cooled and dried material (9A') was fed into a roll mill and crushed, and then classified using JIS standard sieves with mesh sizes of 850 μm and 150 μm. The component that passed through the 850 μm sieve but not through the 150 μm sieve was collected to obtain the superabsorbent polymer (9A'').

[0261] (Surface crosslinking process) Next, a surface crosslinking agent solution consisting of 0.025 parts by mass of ethylene glycol diglycidyl ether, 0.3 parts by mass of ethylene carbonate, 0.5 parts by mass of propylene glycol, and 2.0 parts by mass of deionized water was sprayed onto 100 parts by mass of the superabsorbent resin (9A'') and mixed. This mixture was heat-treated at 200°C for 30 minutes to obtain surface-crosslinked superabsorbent resin (9C). The physical properties of the surface-crosslinked superabsorbent resin (9C) are shown in Table 3.

[0262] [Comparative Example 2] In Comparative Example 2, a surface-crosslinked superabsorbent resin (10C) was obtained in the same manner as in Experimental Example 4 disclosed in PCT / JP2021 / 034800. Specifically, the procedure is as follows.

[0263] (Preparation process for monomer aqueous solution) A monomer aqueous solution was prepared in the same manner as in Example 1 (preparation step of monomer aqueous solution), except that polyethylene glycol diacrylate (average n=9) was changed to 0.61 parts by mass.

[0264] (Polymerization process) A strip-shaped water-containing gel (10b) was obtained in the same manner as in the polymerization step of Example 1.

[0265] (Gel grinding process) A feed screw (FS) and a reverse screw (RS), along with grinding means consisting of a flat disc (F), a helical disc (H), and a reverse helical disc (RH), were installed on two rotating shafts rotating in the same direction in the configuration shown in Figure 7 (in the configuration shown in Figure 7, there are 0 return phases). The jacket heat transfer medium temperature was changed to 80°C, the rotation speed to 40 rpm, and the feeding rate of the 70°C heated strip-shaped water-containing gel (10b) was changed to 0.64 kg / min (at a rate of one strip-shaped water-containing gel (10b) every 2.5 seconds). Except for the absence of water and steam supply, the process was the same as in Example 1 (gel grinding process) to obtain particulate water-containing gel (10c). The gel grinding conditions are shown in Table 1. The characteristics of the particulate water-containing gel (10c) removed from the discharge port of the gel grinding apparatus are shown in Table 2.

[0266] (drying process) A dried product (10A') was obtained in the same manner as in Example 1 (drying step).

[0267] (Crushing, classification process) A water-absorbing resin (10A'') was obtained in the same manner as in Comparative Example 1 (grinding and classification process).

[0268] (Surface crosslinking process) A surface-crosslinked superabsorbent resin (10C) was obtained in the same manner as in Comparative Example 1 (surface crosslinking process). The physical properties of the surface-crosslinked superabsorbent resin (10C) are shown in Table 3.

[0269] [Comparative Example 3] (Preparation process for monomer aqueous solution) A monomer aqueous solution was prepared in the same manner as in Example 3 (preparation step of monomer aqueous solution).

[0270] (Polymerization process) A strip-shaped hydrated gel (11b) was obtained in the same manner as in the polymerization step of Example 1.

[0271] (Gel grinding process) A feed screw (FS) and a reverse screw (RS), along with grinding means consisting of a flat disc (F), a helical disc (H), and a reverse helical disc (RH), were installed on two rotating shafts rotating in the same direction in the configuration shown in Figure 6. Particulate water-containing gel (11c) was obtained in the same manner as in Example 3 (gel grinding process), except that the feeding rate of the strip-shaped water-containing gel (11b) was changed to 1.0 kg / min (at a rate of one strip-shaped water-containing gel (11b) every 5 seconds). The gel grinding conditions are shown in Table 1. The characteristics of the particulate water-containing gel (11c) removed from the discharge port of the gel grinding apparatus are shown in Tables 2 and 4.

[0272] (drying process) A dried product (11A') was obtained in the same manner as in Example 1 (drying step).

[0273] (Crushing, classification process) A water-absorbent resin (11A) with a mass-average particle size (d2) of 608 μm was obtained in the same manner as in Example 1 (grinding and classification process).

[0274] (Surface crosslinking process) A water-absorbent resin (11C) was obtained in the same manner as in Example 1 (surface crosslinking process). The physical properties of the surface-crosslinked water-absorbent resin (11C) are shown in Table 3.

[0275] [Table 1]

[0276] [Table 2]

[0277] [Table 3]

[0278] [Table 4]

[0279] As shown above, the water-absorbing resins of the examples had a fast water absorption rate and reduced residual monomer content. In contrast, compared to the water-absorbing resins of the comparative examples, the water-absorbing resin of Comparative Example 1, which had a gel pulverization coefficient exceeding 3.0 J / g·sec, had a larger residual monomer content than the examples. The water-absorbing resin of Comparative Example 2, which had a mass-average particle diameter in terms of solid content of the particulate water-containing gel-like crosslinked polymer exceeding 500 μm, and the water-absorbing resin of Comparative Example 3, which had a gel pulverization coefficient of less than 0.020 J / g·sec, had a slower water absorption rate and a larger residual monomer content compared to the examples.

[0280] This application is based on Japanese Patent Application No. 2022-056292, filed on 30 March 2022, and Japanese Patent Application No. 2022-124526, filed on 4 August 2022, the disclosures thereof which are incorporated herein by reference in their entirety. [Industrial applicability]

[0281] The superabsorbent resin powder obtained by the present invention is suitable for use as an absorbent material in hygiene products such as disposable diapers. [Explanation of symbols]

[0282] 10. Flat Disc 11...hole 12...Flat area 200... Gel crushing device 204...Inlet 206... Rotating shaft 208...Main body (barrel) 210...Discharge port 212... Grinding means 214... Drive unit 216...Gas supply port 218... Screw

Claims

1. A polymerization step involves polymerizing an aqueous monomer solution to obtain a water-containing gel-like crosslinked polymer, After the polymerization step, the water-containing gel-like crosslinked polymer is pulverized using a gel pulverizer to obtain a particulate water-containing gel-like crosslinked polymer, It includes, The gel crushing apparatus comprises an inlet, an outlet, and a main body containing multiple rotating shafts, each of which has a crushing mechanism. In the gel pulverization step, the water-containing gel-like crosslinked polymer is continuously fed in from the inlet, and the particulate water-containing gel-like crosslinked polymer is continuously removed from the outlet. The polymerization rate of the water-containing gel-like crosslinked polymer introduced into the input port is 90% by mass or more. The gel pulverization coefficient is 0.020 J / g·sec or more and 3.0 J / g·sec or less. The mass-average particle size of the particulate water-containing gel-like crosslinked polymer discharged from the outlet is 500 μm or less, based on solid content. A method for producing a water-absorbing resin powder, wherein the water-containing gel-like crosslinked polymer is a crosslinked body mainly composed of poly(meth)acrylic acid (salt).

2. The manufacturing method according to claim 1, wherein the average residence time of the water-containing gel-like crosslinked polymer in the gel grinding apparatus is 30 seconds or more and 1200 seconds or less.

3. The manufacturing method according to claim 1 or 2, wherein the water-containing gel-like crosslinked polymer obtained after the polymerization step is in the form of a sheet, and further comprises a shredding step of shredding the sheet-like water-containing gel-like crosslinked polymer before the gel pulverization step.

4. The manufacturing method according to claim 1 or 2, wherein the particulate water-containing gel-like crosslinked polymer contains a gel fluidizing agent.

5. The manufacturing method according to claim 4, wherein the gel fluidizing agent is added to the water-containing gel-like crosslinked polymer before and / or during the gel grinding step.

6. The rotating shaft has a disk, and the ratio of the effective length L inside the main body to the maximum diameter D of the disk (or the diameter of the largest disk if multiple disks of different diameters are used), L / D is The manufacturing method according to claim 1 or 2, wherein the value is 5 or more and 20 or less.

7. The manufacturing method according to claim 6, wherein, with respect to the major axis diameter X of the disk, the minor axis diameter Y of the disk satisfies 0.2 ≤ Y / X ≤ 0.6, and the thickness T of the disk satisfies 0.05 ≤ T / X ≤ 1.

0.

8. The manufacturing method according to claim 1 or 2, wherein the porosity V of the gel grinding apparatus, calculated by the following formula, is 40% or more and 80% or less. V={(AB) / A}*100 (Here, A is the internal volume (m³) of the main body portion excluding the portion corresponding to the input port and the output port in the longitudinal direction from the main body. ３ ) and B is the sum of the volumes of the plurality of rotating shafts and the crushing means located in the main body portion excluding the portions corresponding to the input port and the discharge port in the longitudinal direction from the main body (m ３ )

9. The manufacturing method according to claim 1 or 2, wherein the arrangement of the grinding means of the gel grinding apparatus includes one or more locations where the phase difference between adjacent disks in the direction of travel from the input port to the discharge port is greater than 90° and less than 180°, resulting in a return phase.

10. The manufacturing method according to claim 1 or 2, wherein the solid content of the water-containing gel-like crosslinked polymer introduced into the input port is 25 to 75% by mass.

11. The manufacturing method according to claim 1 or 2, wherein the solid content of the particulate water-containing gel-like crosslinked polymer discharged from the outlet is 25 to 75% by mass.

12. The manufacturing method according to claim 1 or 2, wherein the water-containing gel that is introduced is continuously crushed by a crushing means having a rotating shaft from the inlet to the outlet.

13. The manufacturing method according to claim 1 or 2, further comprising a surface crosslinking step after the gel grinding step.

Images